US2184896A - Coated abrasive - Google Patents

Description

Dec. 26, 1939.

N. E. oGLEsBY 2,184,896

COATED ABRASIVE 2 Sheets-Sheet 1 Original Filed Feb. 6, 1937 Mbho/as 1 09/6559 Dec. 26, 1939.

E. OGLESBY COATED ABRASIVE 2 Sheets-Sheet 2 Original Filed Feb. 6, 1937 ,A/wh-ozm 55 0 1635 Patented Dec. 26, 1939' UNITED v STATES COATED ABRASIVE Nicholas E. Oglesby, Troy, N. Y., assignor to Behr- Manning Corporation, Troy, N. Y., a corporation of Massachusetts Application February 6, 1937, Serial No. 124,504

Renewed November 22, 1939 Claims." (01. 51-278) This invention relates to coated abrasives and includes an improved product and process of manufacture. Coated abrasives usually comprise a backing of paper, cloth or other suitable sup- 3 port and an abrasive surface of grits such as emery, alundum, flint, garnet, corundum, slicon carbide, etc., adherently united to one or both sides thereof.

In its broadest aspect the invention comprises using synthetic resin adhesives for bonding the grits to the support.

I have discovered that synthetic resins form satisfactory adhesives if (1) a viscous resin is employed; (2) the viscosity of 'the resin at the time of and after grit application is critically controlled, and (3) the coated abrasive is subjected to critical drying and curing conditions.

I have developed an economical process of manufacturing a complete range of the commonly used grit sizes and the product has, among other advantages, those enumerated hereinafter.

The resinous bond is developed to have a high tensile strength, resistance to heat, resistance to moisture and resistance to a combination of heat and moisture.

Furthermore, my resinous bond is characterized by hardness, rigidity and. toughness.

By virtue of the high tensile strength, hardness, toughness and rigidity of my binders, it is possible to hold the abrasive grains firmly in place so that the abrasive cutting edges or points are in rigid contact with the work and are relatively free from turning over or turning their points or edges back from the work when the abrasive article is in use, as would be the case if my binder were too flexible or too soft. It will therefore be clear that my binder is relatively inflexible as compared with the binder used in the product known to the trade as waterproof sandpaper. While my binder may be classed as relatively inflexible, it is possessed of sufllcient toughness to prevent disintegration of the binder or dislocation of the grains from the binder before they have performed their abrasive functions under the impacts encountered in actual use of the abrasive article, even though the work is rough, hard and tough. Furthermore, my bond, while relatively inflexible, is possessed of sufllcient toughness to withstand mechanical flexing of the coated abrasive article so as to break the bond and impart flexibility to the abrasive article without material injury to the bond between the grits and the binder and is possessed of sumcient brittleness to permit breaking by a mechanical flexing operation without material injury to the union between the grit layer and the backing.

It should be appreciated that coated abrasives are often referred to as flexiblesheet abrasives. In the case of coated abrasives, known in the 5 trade as waterproof sandpaper, flexibility has been imparted to or retained in the sheet by virtue of the inherent flexibility of the binder. In the case of conventional coated abrasives made with a glutinous binder, it has been the custom to impart flexibility to the coated sheet by a mechanical breaking of the adhesive-abrasive coat. The requirements of flexibility are different for different classes of products, and the breaking of the adhesive has been regulated 15 in accordance with the intended use of the product. Several methods are known in thecoated abrasive art for mechanical flexing of sheets made with a relatively inflexible binder such as glue. In one method of flexing, the web is pulled 20 under tension over a bar with its back against the bar edge, the width of the edge of the bar being varied according to the distance desired between breaks, the narrow edged bars giving the closest breaks. Sometimes the web is pulled 25 over a bar in such a way as to produce breaks which are substantially perpendicular to the length direction of the coated web, while in other cases, the breaks are at about an angle of 45'to the length direction of the coated web.

Another method of mechanical flexing of coated abrasives consists of pulling the coated web between a rubber roll and a small steel roller to which pressure is appliedto push the web into the rubber and form regular breaks in theadhesive abrasive coating.

As stated before, while my adhesive has high tensile strength as, for instance, 8,000 to 14,000 lbs. per square inch or more, and while it is rel-- atively tough, it is sufficiently inflexible and 40 brittle, that is, of the nature to permit mechanical flexing without material injury to the union between the grits and the adhesive or the union between the adhesive and the reinforcing back- The tensile strength, hardness and toughness of my bond is not seriously impaired by temperatures up to 500 F. which are frequently encounv tered in industrial uses, that is, the resin bond does not break down at these high temperatures, thereby extending the usefulness of the coated abrasive beyond a point heretofore reached. Again the tensile strength, toughness and hardness of the bond is not appreciablyaflfected by varies with its components and/or the amount of curing but in general the adhesive bond for coated abrasives prepared in accordance with this invention has a tensile strength in excess of 6,000 lbs. per square inch, usually about 8,000 to 14,000. lbs. or higher, and an elongation at rupture of about 2-4%, such tensile strength and elongation values representing anadvanced stage of cure with the resinous adhesives used by way of illustration, as will be hereinafter more fully explained. By tensile strength I mean the strength of a corresponding film of the adhesive cured in the same manner but without the abrasive grains. It should be understood that while the tensile strength values given herein are measured at 70 F. and 50% relative humidity, these values are not materially altered by practical humidity changes or most temperatures encountered.

A further advantage of my process and product is that the abrasive grains are retained in substantially their initially applied positions which may vary with the intended use, a grinding disc requiring a heavier and more compact coating than a coated abrasive article intended for woodworking, leather work and the like. In many such cases the new electrostatic orientation methods are used and it is an important phase\ of this invention that I am able to maintain a very high degree of the initial orientation brought about by the use of the electrostatic methods.

A further advantage of my process and product is that the grains may be uniformly distributed and kept separated so as to prevent bunching or lumping of the grains, such bunching or lumping being undesirable from the standpoint of the amount of work that the abrasive article can do and particularly objectionable from the standpoint of the finish obtained.

I produce coated abrasive articles in which the top surfaces, edges and points of the grains are relatively'free from resinous adhesive, such freedom from resinous adhesive being conducive to a free-cutting coated abrasive. In this connection, it is an important and distinguishing characteristic of my product that a substantial proportion of the abrasive grains have about onethird to one-half or more of their volume above the level of the surrounding adhesive bond.

I prefer to carry out my process so as-to leave a continuous film of resinous adhesive underneath the abrasive grains, such a film generally serving to support the grains rigidly and preventing their penetration into the softer supporting backing which is usually of cellulosic origin.

Furthermore, my adhesive bond is resistant to the action of lubricating oils, petroleum solvents, parafiin or similar lubricating. materials that are frequently used in sanding.

Various types of artificial resins having the required characteristics maybe used in the practice of this invention. Condensation products of phenol, urea, or cresol, and formaldehyde or hexamethylene tetramine are quite satisfactory as well as alkyd resins and.are mentioned as "representative.

I prefer a relatively non-flexible synthetic resin since I have discovered that more eflic ient coated abrasives can be produced from phenolic resins by the omission of flexifying agents such as have heretofore been proposed, namely, glycerol, starch, dextrin, sugar and various proteins, castor oil and shellac. Ihave found that the addition of these foreign bodies is more often deleterious, e. g., in reducing tensile strength, than beneficial, and that the harder unplasticized resins for most uses produce products which are superior to the plasticized resins where plasticizers progosed in-the prior art and known to me are use In the past there has been no displacement of glutinous binders with synthetic resinous binders and I attribute the lack of success of the resinous binders in a large degree to the failure to develop the optimum properties in resinous binders.

As an illustration of the properties that may be developed in synthetic resinous binders and how 1 heating of a film of this liquid resin results in what is known as a jelly stage. Continued heating increases the firmness or rigidity of the jelly and the jelly is finally transformed into a deflnitely solid condition. This solid is very brittle or tender at ordinary temperatures and if -dry may be powdered. The phenol formaldehyde resin is entirely unsuited for my purpose while either in the firm jelly-like state or the tender solid condition. Relatively prolonged heating at low temperatures such as 165 F. will not develop the toughness, hardness, and tensile strength required for the best coated abrasive bond. The tender solid may, however, be converted into a hard but tough film by a relatively short period of heating, say about 2-4 hours at about -l50 C., and I find it is in this final condition that the, resin is very hard and tough, but relatively inflexible, and is then a very.eflicient binder for coated abrasive use.

I attribute the failure of prior attempts to make coated abrasives with phenol formaldehyde resins in no small degree to the fact that in some cases the resin has been cured only to a very firm jelly and in this condition it has a certain 'amount of flexibility and is relatively free from brittleness. It is, however, too weak and soft and lacks the hardness and toughness desired for my abrasive bond. In other cases a plasticizer has been added to the resin to overcome thethe flexibility of the resin provided the resin is given a final high temperature treatment to thoroughly complete its cure. In this case the glycerine loses much of its plasticizing effect and there is some deleterious effect on the hardness and tensile strength of the bond.

With any particular resin some compromise in properties may be required. I have found, however, that by using a phenol formaldehyde or other resin as herein set forth, without any plasticizer, very efllcient coated abrasives can be made, provided the resin is cured to a tough but hard and relatively inflexible condition.

The use of viscous adhesives in accordance with this invention is associated with relatively little penetration of the backing. Since there is little penetration of the backing, there is little modification of the backing itself by the adhesive bond, and I am, therefore, able to use less flexible binders than have hitherto beenconsidered feasible where there has been considerable penetration of the backing.

The urea formaldehyde resin used by way of illustration also passes through the stages of thin liquid, viscous liquid, jelly, tender solid and relatively tough solid condition. While the urea resins may be cured somewhat more readily than the phenol formaldehyde resins, I find that a.

relatively high final temperature (about 125 C. or higher) is desirable in producing the optimum tensile strength, toughness and hardness for this type of resin. The glyptal resins produced by the condensation of glycerine and phthalic anhydride or similar bodies behave in an analogous manner but each resin requires particular study and treatment to develop the most favorable properties.

The following are typical examples of satisfactory resins:-

Resin Example I Heat together to 100-110 C. while stirring, phenol, 100 parts by weight, and sodium hydroxide (solid) .86 parts by weight, and hold at 100-110 C. for 15 minutes to dissolve the sodium hydroxide. Then cool to C.. and add slowly while stirring, paraformaldehyde, 30 parts by weight.

While stirring raise the temperature slowly over about 70 to '75 minutes to about 120 C. and hold at 100-120 C. for about half an hour or until cooled sample is of desired viscosity. The mass is not allowed to boil so that the reaction will not become violent and uncontrollable.

Instead of using'paraformaldehyde as illustrated in Example I, I may also use formaldehyde and secure equivalent results. In still another variation, a part or all of the formaldehyde may be replaced with hexamethylene tetramine.

Resin EmampleII A satisfactory method of preparing a resin with lower viscosity than Example I is as follows:

Heat together to -110 C. while stirring phenol, 100 parts by weight, and sodium hydroxide (solid), .86 parts by weight, and hold at 100- C. for 15 minutes to dissolve the sodium sary in a sandpaper factory is to blend resins of diflerent viscosities, e. g., one a much more viscous resin than'the other. Thus, any two resins can then be mixed as required to obtain the desired viscosity for a particular grit size.

. The resins prepared as heretofore described are applied to the paper or other backing with conventional coating rolls and are preferably applied warm, a suitable temperature range for application being -175 F., the exact temperature depending upon the grit size and other con ditions of operation such as the hardness of the rubber coating roll and the viscosity of the adhesive used.

In the practice of this invention with the foregoing resins, a solvent may sometimes be employed, but is not usually preferred, since its use adds an otherwise avoidable cost.

There are some cases, however, where the use of a solvent'is advantageous in preserving the orientation of the grain. This is true since by the addition of volatile solvents the viscosity of the adhesive maybe materially lowered so that it can be satisfactorily applied with coating rolls. After the adhesive. coating is applied to the backing, rapid loss of solvent from the thin film of adhesive gives a rapid increase in the viscosity and setting rate of the adhesive coating. This rapid setting is desirable in order that the grains may be held in their original oriented positions, and as a further means of overcoming the loss of grain orientation by flowing, and to prevent the coating of adhesive and abrasive from flowing from the tops toward the bottoms of the loops in the festoons. Furthermore, it will be appreciated that still more viscous resins than would otherwise be feasible can be used by thinning with a solvent.

Where it is desired to prepare a resin thinned with solvent the following will serve as an example:

Resin Example III Mix phenol 100 parts by weight, formalin 187 parts by weight, and barium hydroxide 6 parts byweight, and heat for four hours at 60-65 C. Apply 25-28" vacuum and heat to 110 C. Hold under vacuum at this temperature until a drop in ice water is brittle. Then add alcohol (denatured) 40 parts by weight to dissolve the condensation product and cool the mass.

If a lower viscosity is desired, more solvent may be added or conversely less solvent may be used for higher viscosities.

The resin itself when made in accordance with Resin Example III if substantially completely dehydrated is a solid at ordinary temperatures. Furthermore, a resin made in accordance with Resin Example I if substantially completely de hydrated is a solid or tough jelly at ordinary temperatures. Although used with a solvent, the resin of Resin Example III is remarkably free of penetration into the base at coating concentrations and, therefore, when this resin is used, there is no objectionable brittlizing due to impregnation. Likewise, other resinous adhesives used by way of illustration are relatively free of objectionable penetration into the paper backings.

Resin Example IV While I prefer the phenol formaldehyde resins to the urea formaldehyde resins, fairly satisfactory results for some uses may be obtained with urea resin prepared as follows: Add sodium carbonate solution to 2.25 mols of formaldehyde till a pH between 6 and '7 is obtained, bringing to a reflux, slowly adding one mol of urea dissolved in hot water, and adding formic acid till a pH of 4.7 is reached. A formic acid sodium formate buffer is also. added at this point. The mixture is refluxed for minutes and dehydrated under a vacuum to a viscous syrup. Resins of a wide range of reactivity can be made by proper adjustment of the hydrogen-ion concentration of the final syrup. Resin so prepared does not have very good keeping qualities and should be used soon after preparation. It may be kept a somewhat longer time if refrigerated. Mixtures or blends of the urea resin may be formed with the aforementioned phenolic resins and form a satisfactory binder.

Where the term viscous resin is used, I refer to the condition of the resin at the time of grit application and immediately thereafter. At the time the resinous adhesive is applied to the backing, it is sufficiently fluid or may be rendered so by use of a resin solvent or by heating as to be spreadable, as for instance, by calendar rolls or is otherwise capable of formation on the backing or support as a continuous coat or film of uniform 4 required thickness. It will generally be found economical to apply a viscous coating of resin to the reinforcing backing but the viscosity of the resinous coating may be changed in many ways prior to grit application, provided the resinous film has therequired viscosity or consistency at the time of and after grit application. Prior to grit application the resinous film of adhesive on the backing may be increased in viscosity by loss of solvent, by drop in temperature, by further condensation or curing of the resin, or by any combination of these means.

The terms viscous and viscosity are used in this specification to denote a property of adhesive which resists flow. It is recognized that my process involves at different stages, matter in a state of viscous flow, matter in a state of plastic fiow and matter in a state without fiow. For the purpose of this specification a resin may be so viscous or have so high a viscosity that it has an infinitesimal rate of flow or even no flow; a viscous adhesive may be a liquid, a jelly or a solid which will be considered as matter of different degrees of viscosity. It should be appreciated that the present specification deals particularly with films of adhesive as used in coated abrasives and that the flow characteristics of the resin are dependent upon the film thickness as well as the viscosity of the adhesive. Furthermore, the fio-w characteristics of the film are infiuenced by the adhesive forces between the backing and the adhesive on the one hand, and between the grains' and the adhesive on the other hand. Flow characteristics also depend to some 'extent upon the surface tension of the film. In the use of any specific adhesive the viscosity is adjusted to obtain the final desired fiow characteristics of the resin and limited motion of the grains within the adhesive, irrespective of other influences which are properties of the spestantial dislocation of the grits from their initially applied positions.

At the time of grit deposition, the resinous film should have the highest possible viscosity consistent with wetting and trapping of the grains as hereinafter explained, and after grit deposition the viscosity or consistency of the resinous adhesive should be such that there will be substantially no motion of the grits within the resinous coating or flow of the resinous coating on the supporting backing. As a further means of preventing movement of the applied grains until the viscosity of the resinous adhesive has been increased, the web may be carried substantially horizontally and the viscosity of the resinous adhesive may be increased by a drop in temperature or loss in solvent or a combination of these methods or by other convenient means.

I The drop in temperature may be brought about by exposure of a film at higher temperatures to lower room temperatures or by positively applied refrigeration.

At the time of grit deposition the resin should have the highest viscosity consistent with picking up the required weight of abrasive grains, but in some cases this viscosity will not be high enough to support the grains in their initially applied positions and it therefore becomes ad visable to increase the viscosity to prevent motion of the grains, it being understood that the motion is relatively slow, especially in the horizontal position, thus rendering feasible the freezing of the grains in their favorable positions, i. e., before there has been any material movement of the grains from their initially applied positions.

Once the viscosity of the resinous adhesive is at or above a critical point, i. e., at or above a point high enough to prevent material flow of the resinous coating and/or material motion of the grains within the resinous coating, the coated web may be changed in direction and moved into the curing room, festooned and cured in any way as desired so long as conditions are not brought about which, would bring the viscosity of the resin below this critical value. In lieu of festooning, a drying tunnel with suitable conveyors may be used. The initial curing conditions are therefore intimately connected with and dependent upon the viscosity of the resinous film. The initial curing conditions must be such that the viscosity of the resin will not be lowered below the critical point required to maintain the grains in substantially their original positions. The

such a high viscosity that the abrasive grains can be supported at a temperature high enough to produce a commercially feasible rate of cure of the resin. Methods by which this problem is solved will be clear from the detailed illustrations of the manufacture of coated abrasives as given hereafter.

It is dimcult to measure the viscosity or jelly strength of an adhesive at the time of sand application in any direct manner so an indirect measurement becomes necessary for proper control of the process. I have discovered, for example, that in order to maintain the grits substantially in their initially applied positions and prevent flow of the adhesive coating. with the resinous adhesives used by way of illustrations herein, the adhesive should, preferably at the initial drying conditions, have so high a jelly strength and so great a viscosity that it will not pick up a full close coat of abrasive grains when about excess of abrasive grains at ordinary temperature (80 F. or less) are allowed to fall about 18" and strike the adhesively coated backing and the excess grains are then given an opportunity to drop out of the adhesive coat within 5 seconds after application. By being at the initial drying conditions I mean at the initial drying temperature, at the initial drying humidity and having the same solvent content (when solvent is used) and degree of cure as under the conditions that 'will exist when the goods are brought into a non-horizontal position and kept non-horizontal for an appreciable time interval as for example for curing. Likewise the adhesive when at the initial drying conditions should not pick up afull close coat of abrasive grains at ordinary temperatures when the grains are applied counter to gravity.

It should be appreciated that the test as to whether or not a resinous coat of adhesive will pick up a full close coat of grains is a method of measuring the condition of the resin. This same method of measuring the condition of the resinous film is also suitable for use where it is desired to manufacture open coated abrasives, the viscosity for an erficient open coating of any grit number being at least as high or higher than for the same grit number of closed coat.

In referring herein to a full close coat of abrasive grains, I mean a weight up to the minimum of the Coated Abrasive Association standard for close coatings. In case of alundum (fused aluminum oxide) these weights are as follows:

Pounds/sandpaper ream 10/0-400 1.8 9/0-320 2.6 8/0-280 4.1 '7/0-240 5.1 6/0-220 7.2 5/0-180 8.8 4/0-150 10.2 3/0-120 12.3 2/0-100 I 15.8 0-80 20.5 1/2-60 ..V 27.5 1430 36.4 l -40 45.6 2-36 54.9 2%,30 66.4 3-24 '77.? 3 -20 95 4 to which no grit has been applied can be stamped out conveniently by means of a die and sand may then be dropped from about 18 onto the adhesively coated sample as heretofore explained. It is recognized that the presence or absence of grit may have some influence on the condition of the adhesive as it reaches the initial drying position but the slight difference which exists between a sample to which grit has been applied and one to which grit has not been applied, is not such as would vitiate the value of the test. Not only may this test he applied to determine the condition of the resinous adhesive when it reaches the initial drying conditions but it is also of value at points intermediate between the sand application and the initial drying position. It is desirable that the resinous film be brought ,to a condition that will not pick up a full close crease the viscosity as rapidly as feasible to a point that will not pick up a full close coat of sand. As explained herein, the viscosity may be increased rapidly after sand application .by using adhesives at elevated temperatures or by using adhesive containing a volatile solvent or by using warm sand or by any combination of these methods.

. After the abrasive grains have been applied to the viscous resinous adhesive and the coated web has been delivered to a drying tunnel .or drying room, as for instance a room provided for festooning and heating in festoon form under controlled conditions, the coated web is further processed by carrying out a critical curing cycle. The initial curing temperature and the other ouring conditions, such as for instance humidity, must be such that at no time will the viscosity of the resinous adhesive drop below the critical value required to prevent flow of the resinous coating and material motion of the grains within the resinous coating. For economical reasons it will usually be desirable to use the highest rate of curing consistent with other conditions; hence, it will usually befound desirable to use the highest possible initial temperature consistent with other requirements but conditions must be maintained during the drying cycle so that the increase in viscosity of the resin due to theresin condensing reaction or due to loss of residual solvent, or to both, is at a rate equal to or greater than the decrease in viscosity due to the progressive rise brought about in the curing temperature.

While for a given grit size and a given thickness of resinous coating on a given backing the initial drying temperature and other conditions are dependent upon the viscosity of the resinous adhesive, it should be appreciated that it is more diificult to prevent motion of the larger grit sizes than the smaller grit sizes, and that in the case of the largergrit sizes, either a more viscous adhesive is used or a lower initial curing temperature is used or both a higher viscosity of the resin and a lower initial temperature are employed. It should be further appreciated that for a given viscosity of resinous film there will be a greater tendency for thick or heavy coatings of the resinous film to flow than would be the case with a thinner film of resin of the same viscosity.

A critical curing cycle, such as is required in my process, extends over a relatively long period of time, and may include a number of different temperatures. Over such a period of time a relatively slow flow of coating or a slow motion of grains within the coating may permit displacement of the adhesive coating or of the abrasive v.

grains within the coating and such flow or dislocation of grains may not be readily detectable I during a short period of time, with the result cosity of the adhesive bond during the process of curing especially where such data is not available from previous runs with the same adhesive under identical or less favorable conditions. The product is delivered to the initial curing conditions as herein described without any visible substantial fiow of adhesive or dislocation of the grits from their initially applied positions, and the initial curing conditions are so adjusted that the average motion of the grains from the tops of a vertical web towards the bottom will not exceed six one hundredths of an inch in minutes. It has been determined experimentally that at a given set of conditions, that is as for example, at constant temperature and humidity, the greater part of the flow of the coating towards the bottoms of the loops and loss of orientation due to the grains turning over, occurs within the first 15 minutes. It has been further determined, that an average motion of grains from the tops towards the bottoms of the festoons of not to exceed six one hundredths of an inch in 15 minutes, insures the maintenance of a large part of the initially attained orientation of grains.

It should be appreciated thatit is desirable to keep the loss of orientation and the flow of the coating at a minimum and that in general in the practice of my invention the average motion of the grains may be considerably less than six one hundredths of an inch-for the first 15 minutes. Since, however, it is desirable, for reasons of economy, to cure the coated abrasive as rapidly as possible, it sometimes becomes advisable to compromise between the rate of flow permitted and the rate of curing found commercially economical. So long as the temperature is not higher tha can be endured by the operator, the motion of grains from the tops towards the bottoms of the festoons and the loss of orientation may be conveniently determined with suitable supports, a suitable microscope, suitable illumination and a fixed reference scale, both the scale and the grains being observed through the microscope.

Many methods of making the required measurements will occur to those skilled in the art. In case the temperature is above that which can be endured by the operator, an electric oven equipped with a clear glass front and with a temperature control may be conveniently used. By use of a suitable microscope, scale and proper illumination, the sample may be viewed through the front glass of the oven. A sizeable sample as for instance, 5" x 3", is placed in the oven as by hanging from a support so that the abrasiveadhesive coating is visible through the glass front of the oven. Attention is fixed upon a few representative grains within the field of vision, and during a 15 minute period of observation there should be no material loss of orientation and the average motion of the grains towards the bottom of the sample should not exceed six one hundredths of an inch.

For the initial test to determine possible loss of orientation or fiow of the coating, any reasonable but constant conditions may be taken. If the fiow of the coating during the test period is too great, conditions should be brought about which will increase the viscosity of the adhesive coating. If there is little or no motion, conditions may be brought about which would decrease the viscosity of the adhesive, as for instance, an increase in temperature which would increase the rate of cure of the resin. By the use of this test it is possible to'determine the conditions of cure that may be permitted without substantial loss of orientation or dislocation of the grains from their initially applied positions, as for instance, because of flow of the adhesive coating over the backing which may sometimes occur in case of surface hardening of the adhesive even though there is not an excessive loss of orientation. After the correct initial cur-- ing conditions are determined, these conditions may bemaintained for any desired period of time without material flow of the adhesive or loss of orientation of the grains. The rate of cure depends upon the resinous adhesive used and the initial temperature which it has been found feasible to use. After partial curing at the initial conditions, it is usually desirable to increase the temperature to increase the rate of cure. Before raising the curing temperature or otherwise bringing about conditions that might lower the viscosity of the adhesive, a test to determine the extent of flow or loss of orientation at the new conditions may be applied. Itshould be appreciated that repeated increases in temperature with even a flow of somewhat less than six one hundredths of an inch would eventually damage the coating to a material degree. For reasons of economy it is usually advisable to use the highest temperature possible for the initial curing conditions and then raise the temperature gradually so that the flow of the resin and loss of orientation will be negligible with succeeding increments of temperature. In running a test to determine the temperature increment that is permissible without material injury to the coating, it is advisable tokeep the average motion of the grains from the tops towards the bottoms of the festoons at a rate of not to exceed one one hundredth of an inch for the first 15 minute interval at the new curing conditions. In practice, it will be found that after relatively few temperature increments, the rate of cure will have been hastened to a point that a commercial rate of cure will be feasible without having any measurable flow of the coating or loss of orientation with succeeding temperature increments.

The test heretofore described for controlling the critical drying cycle by measuring the average motion of grains away from the top and towards the bottom of a substantially vertically hanging abrasively coated strip is also useful in controlling the critical viscosity of the adhesive used to trap the grains in the coating operation. If the average motion of the grains from the top towards the bottom of the vertically suspended strip exceeds six one hundredths of an inch at the lowest commercially feasible initial curing cycle, the viscosity of the adhesive used to trap the grains must be increased to a point that will decrease the flow at the initial curing cycle to or below six one hundredths of an inch for the first 15 minutes. any equipment condition wherein the web is brought into a substantially vertical position for an appreciable interval of time between the sand application and the drying chamber exists, the average motion of the grains from the tops towards the bottoms of the suspended loops or other forms of suspended webs should bekept at a rate not to exceed six one hundredths of an inch for any 15 minute interval of time. If the average motion of the grain is greater than six one hundredths of an inch as measured by this test, the viscosity of the adhesive at the time of trapping the grains must be increased Furthermore, where,

to bring the flow down to or below this value, or the viscosity of the adhesive must be rapidly increased immediately after the grains are trapped as for instance by cooling or by solvent removal.

In the case of the coarser grit numbers such as coated abrasive, Example II hereinafter disclosed, it has been found that a low initial curing temperature such as in the neighborhood of about 100 F. to 110 F. is required. The manner in which this curing temperature is progressively increased will be'clear from the detailed example hereinafter given.

In the case of a medium grit given by way of illustration as coated abrasive, Example I, for the manufacture of #0 aluminous oxide abrasive paper, the viscosity or body of the resin is rapidly increased by loss of solvent at a relatively low temperature such as about 75 F. Due to this rapid increase in viscocity by loss of solvent, it becomes possible to quickly reach a curing temperature of 160 F., thus illustrating the dependence of the initial drying conditions and therate at which the curing temperature can be increased, upon the viscosity or consistency of the applied film of adhesive and the rate at which this viscosity can be increased.

The curing of the resin required after grit application but before the sand sizing coat of resin is added will vary with the grit size, the resinous adhesive used, the type of coating used, as for example, electrostatically oriented or not, and the manner in whichit is intended to apply the sizing coat of adhesive as well as the nature of the sizing coat of adhesive used. The curing of the coated web prior to application of the sand sizing adhesive must, however, be such that the sand size can be applied and the resinous bond be thereafter completely cured without flow of the resinous adhesive or a material movement of the particles from their initially deposited positions. In the case of a coarse grit relatively compact coating, designated as coated abrasive, Example II, an initial curing temperature after application of the sand size is given as around 100 F. In this case the making coat of resinous adhesive at the time of sand size application is still in a condition that it is materially softened by the sand sizing adhesive; hence the necessity for the critical drying conditions after the use of the sand size given by way of illustration.

In the case of coated abrasive Example I the making coat of resinous adhesive has been brought to a condition where it is not materially softened by the sand sizing resin; hence after the loss of alcohol from the sizing adhesive the temperature can be quickly raised to 160 F.

Since the making coat of adhesive which anchors the abrasive grains in place is not softened during the sizing operation, there is little danger of dislocating the grains from their initial favorable positions, and the fundamental requirement is to maintain conditions wherein the sizing coat of adhesive will not itself flow over and through the grains, whereby the sizing coat would be light at the tops and heavy at the bottoms of the loops, or otherwise lose its uniformity of distribution.

While I have used relatively low initial curing temperatures to prevent flow of the resinous coating and/or maintain orientation of the abrasive grits, it should be appreciated that the curing temperature is progressively increased and that a final high curing temperature is used to develop the high tensile strength, hardness and toughness hereinbefore described. While this high final curing temperature will depend upon the nature of the resinous material used, I have found it desirable to use temperatures of about 140-150 C. in the case of the phenol formaldehyde resins given by way of illustration.

Not only is a low initial temperature required to prevent flow of the resin coating and movement of the grits from their initially disposed locations but I have further found that the most efficient coated. abrasives are made when the adhesive bond uniting the grains to each other and to the reinforcing backing is dense and continuous and free from bubbles, air pockets, vapor pockets and the like. I have found'that a low initial curing temperature and a controlled progressive increase of temperatures prevents the formation of bubbles and pockets which may result from (1) the expansion of air; (2) the liberation of water, ammonia or other gas or vapor in the condensation process and (3) the too rapid evolution of solvent vapor where solvents are used in coating the resinous adhesive. If the initial curing temperature is too high, bubbles,

pockets and the like are formed and these imperfections materially decrease the efiiciency of the coated abrasive article. a

The use of orientation methods in the manufacture of coated abrasives has increased materially the efficiency of such products andit is the province of the present invention to further improve the art bythe utilization of synthetic resins of the highly viscous types, as binders. I have discovered that such resins insure, in accordance with this invention, that the initial favorable orientation of the grits is substantially preserved throughout the subsequent manufacturing operations of the coated abrasive, resulting in a highly efficient product. I have produced for the first time a line of coated abrasive having all the common grit sizes and having the grits in their most efficient oriented positions.

As one form of the present invention, the process includes dispersionwith orientation by means of an electrical field as described in the patents to Elmer C. Schacht, 2,027,307 and 2,027,309, J anuary 7, 1936. A preferred form of using the process of electrical dispersion and orientation described in the above patents is referred to hereafter and comprises an operation wherein the adhesively coated backing with the adhesive face downward is disposed above a belt carrying the abrasive grains, and the two are passed simultaneously and in close proximity between electrodes constituting a magnetic or electrostatic field. The effect of the field is to raise the grains against gravity, and space them approximately equi-distant from each other, with their longer axes substantially perpendicular to the backing.

This orientation for many uses gives the most efficient performance in the final abrasive product. Obviously, orientation methods of coating fail insofar as the favorable orientation brought about, for example, by the field (electrostatic or magnetic) is not maintained, and it is a particular feature of this invention that the difiiculties incident to taking advantage of orientation practice are effectively overcome and that the positioningof the grits is preserved because the synthetic resin used has a viscosity, 1. e., body strength and jelly-like character to support the grains against flow on the onehand and turning over on the other.

It is to be understood that various elevated temperatures may be used as an aid in applying the various resins to the reinforcing backings prior to grit application. Also, in some cases, solvents may be used, depending upon the conditions of operation and the particular type of resinous body being used, as well as upon the grit s'ze that will be coated. While, for convenience, a variety of compositions and. viscosities may be used in applying the adhesive to the backing, the viscosity of the adheive at the time of or immediately after the sand is applied is critical. This critical viscosity may also be attained if desired by partial curing of the adhesive, evaporation of solvent, or reduction in temperature after ap plication t the backing, but before the abrasive grains are applied. Under various conditions con iderable variations may be made in the distance between the adhesive application and the grit application and 'there maybe, coincident with these variations in distance, wide variations in the temperature and humidity of the atmosphere between the point of adhesive application and grit application. Due to the variations of backlngs' used, there is a great variation in heat capacity of the backing and therefore in the ability of the backing to change the temperature of the adhesive applied. The condition of the adhesive applied, the distance from adhesive application to sand application and the temperature of the atmo"phere can vary individually within wide limits, but for a particular grit size, the viscosity or jelly strength of the adhesive must not be below a critical value after sand application and until cured, as explained herein.

The drying and curing temperatures employed extend through a range of from about 75 F. to about 300 F. The temperature of the resin at the time of application varies from about 75 F. to about 175 F. The grits are sometimes applied in heated condition and have a temperature of about 100 to 160 F.

Repre"entative safe drying cycles are given by way of illustration in coated abrasive Examples I and II. The curing cycles given are based upon experience with the specific adhesive used in each case. I prefer, however, to control my critical drying cycle by the means of the flow-test hereinbefore described and by using this test, I am enabled to secure the highest rate of cure consistent with maintaining the quality of the goods.

In order that the invention may be more clearly understood, reference is had to the accompanying drawings in connection with which, representative processes will be described in detail.

In the drawings:

Figure 1.is a diagrammatic view illustrating one form of the process in which orientation of the particles is accomplished.

Figure 2 is a similar diagrammatic view of another form of the process in which orientation of the particles may or may not be employed.

Figure 3 is a top elevation of a strip of coated abrasive made in accordance with the present invention and Figure 4 is a sectional view of coated abrasive made in accordance with the present invention and in which the grits have been oriented.

The application of the grits may take place with any conventional method and produce a susure and teaching of the aforesaid Schacht pat-' ents.

Coated abrasive-Example I In the following example, describing one process of the invention, I use a resin prepared according to Example III, containing approximately solids and 25% solvent, such as alcohol. However, a re=in of proper viscosity, such as in Resin Examples I or II and containing no solvent or a mixture of resins obtained by blending Resin Examples I and II, may be used or resins prepared according to Resin Example IV or alkydresins may be used. Various sizes of grit may be used, but in the present example I will describe the process for making #0-80 aluminous oxide abrasive paper, the grit being medium size.

Referring to Figure 1, there is provided a roll it of suitable paper, cloth or other fabric backing material. 'In many cases, a roll of 130 pound rope cylinder paper, as is commonly used for coated abrasives, will be found desirable. The numeral ll represents coating rolls for application of the adhesive. The lower roll II is preferably made of rubber or some other resilient material, while the upper roll may be either of steel if desired. The numeral l2 represents a conventional glue box with a jacket for heating or cooling, into which the lower coating roll II dips so as to transfer the resinous adhesive I3 to the paper backing l0. The numeral l4 represents a suitable storage means for the resinous adhesive and is preferably equipped with an agitator. end a jacket for either heating or cooling the adhe"ive by steam. or water as the case may be. The numeral I5 is a suitable outlet pipe running from the adhesive container I4 to the glue trough 12. A valve I6 is located near the glue trough for control of the flow of adhesive from the adhesive container M to the glue trough I 2.

Conventional electrodes A and B are used in making oriented electrostatic coatings by the upward propulsion or counter-to-gravity method as previously described.

The numeral I'l represents a hopper to hold the abrasive grains l8, which hopper is preferably equipped with heating coils (not shown) for warming the sand if desired. The numeral l9 denotes a belt used to carry the abrasive grains [8 between the electrodes A and B.

The abrasively coated paper is manufactured by passing the paper web i0 through the coating rolls II where adhesive I3 is applied at an appropriate temperature. In the present example,

a temperature of 120-130" F. has been found satisfactory. The adhesively coated web l0 then passes from the coating rolls ll over suitable idler rolls 20, so as to be brought between electrodes A and B, above and in close proximity to the moving belt l9 carrying the grits. At the same time abrasive grains l8 heated to about 158 F. in the hopper H are applied to the belt l9 which carries the grains between electrodes A and Band below the adhesively coated Web 10. The electrostatic field between electrodes A and B raises the grains Hi from the belt IE to the downwardly facing adhesively coated surface of the web I0, where they are, by virtue of the control of viscosity of the adhesive, retained in an oriented position with their major axes prepond-erantly perpendicular to the plane of the backing 10. The coated abrasive web, while traveling, is then, if necessary, treated, e. g. by evaporation of solvent or by cooling or by both, to increase the viscosity of or rigidify the resin bond, and passes over suitable idler rolls 2|, preferably horizontally, for the period of time a, leases immediately following grit application under the tension created by a suction drum 2!. 1he period is generally the time required :for the resin to increase in viscosity to a point where the grains will be supported when the web is in a non-horizontal position. From the suction drum 22, the abrasively coated web is then carried by well known methods to a suitable drying room which may be a sandpaper drying room with festoons and-means of supplying heat to the room containing the festoons.

A suitable heatingi. e., drying and curing cycle falling'withincritical conditions, for this medium-sized grit, and the resin used by way of illustration, is one hour or more at room temperature, namely, about 75 F. and then eighteen hours at 160 F. followed by an intermediate heating step of one-half hour at 140 C.

By intermediate heating I mean that in some cases, where a sizing coat of a synthetic resin, preferably one of those herein mentioned, is applied to the coated abrasive, I find it desirable either to partially or completely cure the making coat before the sizing operation. That is to say, after grit application the making coat is'subjected to a progressively increasing temperature such as will not reduce the viscosity of the bond below the critical value and cause material flow of the coat or dislocating motion of the abrasive grains, and is then for purposes of the present example given an intermediate heating at a temperature required for condensing the resin to a point beyond the tender or brittle condition as for example, atemperature of about 140 C. for a period of approximately A;-% of dislocation of the abrasive grains either by sof- I .tening or cracking of the making coat.

After the heating cycle is completed, a second or sizing coat of an appropriate resin is applied, as by spraying or rolling. Preferably a suitable resin for sizing is prepared from Resin Example III by diluting to a solids content of about 55% with ethyl alcohol. The sizing coat may be satisfactorily applied at a temperature of around 70-80 F.

After the sizing operation the coated web is again taken to a sandpaper drying room and festooned. A satisfactory drying cycle for this particular product after the sizing operation is one hour at room temperature, about 75 F., then eighteen hours at 160 F., followed by one and three-quarter hours heating at 140 C.

It will be appreciated that many variations can be made in the process provided the essential and critical conditions are maintained. The viscosity of the adhesive during and immediately after sand application, as well as the temperature cycle provided for drying and curing the time a drop in temperature immediately after coating which would maintain the desired orientat-ion.

It is also important that the correct quantity esive be applied by coating rolls II, and this depends upon the pressure exerted by the coating rolls, the hardness of the coating rolls, the diameter of thecoating rollsand the viscosity of the material applied to the backing.

There are limits to the viscosityiof the adhesive that can be handled by any commercially practical coating rolls, but the abhesive cited by way of illustration operates satisfactorily for grit #-80 alundum when coating with the average conventional sandpaper coating rolls under conditions cited in this example.

Between coating rolls II' and electrodes A and B, alcohol isevaporated and the resin becomes progressively thicker. Obviously the amount of alcohol evaporated depends upon the temperature and time period to which the adhesively .coated web is exposed, which, in turn, are governed (1) by the distance between coating rolls II and electrodes A and B, and (2) the speed at which'the' machine is operated. Satisfactory speeds range from 30 feetto 200 feet per minute and a satisfactory span for ordinary temperatures '01 the coating room, that is, about 70-80 F. and a speed of 30' feet, would be about 9 feet. Obviously, many arrangements of the variables involved can be made, so as to deliver the correct viscosity of adhesive between the electrodes A and B. For instance, higher speeds require longer spans, while lower speeds require shorter spans and higher temperatures require shorter spans in case the viscosity is increased primarily by the evaporation of solvent, while lower temperatures decrease the rate of evapora'-- tion'of solvent.v Where the viscosity is increased primarily by a drop in temperature, lower sur. rounding temperatures will require shorter spans and higher surrounding temperatures will require longer spans. It will be evident that in many cases both temperature drop and loss of solvent may be used simultaneously for increasing the viscosity.

.In the present illustration an adhesive phenolic resin has been used which, before the addition of alcohol in its preparation, is brittle in ice water. The cure of this resin in the resin-making process has been advanced and, by evaporation of alcohol from the adhesive, a point is reached where higher initial drying and curing tempera? tures may be more quickly applied without loss of orientation.

Where required, the step of increasing viscosity, e. g., the cooling step immediately subsequent to grit application, preferably takes place at a temperature and for a suflicient time period to cool the continuously moving abrasively coated ,web

and changes direction, as well as-when it is ar ranged upon festoons in the drying room, no flow of the bond and the grits'will take place, such as might produce'clumps and non-uniformity in the abrasive surface. t

This cooling step is a preferred method of operation, but in some cases, as for instance where there is substantial loss of solvent, the resin bond is rendered sufliciently immobile without positively applied cooling.

Coated abrasive-Exampie II aluminous oxide. A fabric backing may be selected from various types of paper, properly treated cloth, and combinations of cloth and paper. or of cloth and vulcanized fibre. For. the manufacture of #24 fibre backing discs with a resinous binder, a preferred backing is described in a co-pending application of Oglesby, Reilly and Gilbert, Serial No. 124,506, filed Feb. 6, 1937. While either side of a combination backing may be coated I usually prefer to coat the cloth side where cloth constitutes one of the outside laminae. A suitable adhesive for the coating of grit #24 aluminous oxide is illustrated by Resin Example I or by a mixture of Resin Example I and Resin Example H.

The preferred backing consists of one lamina of vulcanized fibre of about 10 mills in thickness and weighing about 200 lbs. per paper ream to which is adhesively united a de-sized drill cloth under conditions which prevent blisters and separations of the lamina. A suitable adhesive for the laminating operation is Resin Example I or Resin Example I thinned with Resin Example II. After laminating the web is preferably partially cured to a highly adhesive but relatively flexible condition.

Vulcanized fibre is a term applied to a hard, horny paper-like material which is generally produced by manufacturing paper from cotton rag stock and then subjecting the rag paper to an action known as vulcanizing which consists of an appropriate treatment with sulphuric acid or more commonly with a solution of zinc chloride, whereby the cellulose is more or less gelatinized but generally without complete disintegration of the individual fibres. After the gelatinizing action, the web is washed thoroughly to remove all traces of sulphuric acid or zinc chloride as the case may be, as the presence of even minute quantities of these materials will result in a rapid deterioration of the product as, for example, loss of strength,

The combination web prepared as described and as given in further detail in the said co-pending application is removed from the curing racks and is ready for coating. While either side of the web may be coated, I generally prefer to coat the cloth side of the combination.

The process of coating will be clearly understood by reference to Figure 2, wherein the numeral l denotes a roll of vulcanized fibre and cloth combined as described in the aforesaid co-pending application, which is preferred, or any other suitable backing material. The numeral [2 indicates an adhesive coating trough into which the lower roll ll dips and should be equipped with a jacket for heating or cooling its contents. Thenumeial l3 denotes the adhesive in coating box l2. A suitable adhesive storage means is shown at l4 for the resinous adhesive, and is preferably provided with an agitator and a jacket for heating or cooling of the adhesive by steam or water, as the case may be. A convenient delivery tube l5 extends from storage means I 4 to adhesive box l2 and should contain a valve l6 located near the adhesive box [2 for control of the flow of adhesive from the: storage means l4 to the adhesive box l2.

The numeral l1 represents a conventional sand hopper for application of sand to the adhesively coated web ID. The sand hopper should preferably be equipped with heating coils (not shown) for proper heating and control of the temperature of the abrasive grains l8.

A combination of three rolls 23 is used to press the abrasive grains into the adhesive coating and there is provided a still larger roll 24 around which the coated web III may pass in order to push the abrasive grain further into the ad hesive. A conventional rattler 25, used to dislocate any abrasive grains which are not well embedded in the adhesive, acts upon the web which is drawn through the machine by a suction drum 22.

The present illustration is useful for producing heavy metal cutting discs and, in such case, it is less essential for the abrasive grains to have their major axes perpendicular to the plane of the backing material. It is important, however, that these grains be evenly distributed and that they be uniformly placed into and spaced within the adhesive.

As an aid to even spacing of the grains, it is sometimes advantageous to have the abrasive grains pass from hopper I! through an electrostatic field provided by one of the overhead methods described in the aforesaid Schacht patents before the grain strikes the adhesively coated backing. The electrostatic field serves to space the grains more evenly so that they make a more perfect pattern on the heavily coated backing and this favorable spacing so produced persists in many cases after the abrasive grains are rolled in as at 23 and 24.

In this case, an excess of abrasive grains is applied to the adhesively coated backing and the applied pressure during rolling is against the excess grains. For this reason, a substantial number of the grits will be oriented with their longer axes at an angle of 45 to 90 with the reinforcing backing. Thus, a substantial proportion of the grains have their sharp points or edges free to engage the work. A substantial percentage of the abrasive grains have-from A; to or more of their volumes exposed above the average level of the adhesive binder.

In the operation of the process, the web passes over suitable idler rolls and is brought between coating rolls H, where the desired coating of adhesive I3 is applied.

A suitable coating temperature is 150-160 F., and the adhesive is brought to this temperature prior to application to the backing III.

A suitable weight of making adhesive for this particular case is about 22 pounds of adhesive per sandpaper ream.

The adhesively coated web passes from coating rolls ll over suitable idler rolls and comes beneath the sand or grits whence the sand or grits are applied. In this particular case,.satisfactory results may be obtained by applying the sand or grits at a temperature of 150 F. After the sand is applied in excess of that required for the coating, the adhesively coated web carrying some excess sand, continuously passes into the nip rolls 23,where the sand is pushed into the adhesive. The coated Web then passes around sand drum 24, where there is a further pushing in of the abrasive grain. From sand drum 24 the web passes by rattler 25 where the loosely anchored grains are removed by light tapping of the rattler blades against the uncoated side of the backing and are allowed to fall back to sand drum 24, where they are gradually dissipated around the ends of the drum. The coated web then passes over suction drum 22 which is used to pull the web through the machine and from there, by any conventional means, is continuously conveyed to a sandpaper drying room where the coated web aieasoc is festooned and heat is appliedthrough a critically controlled curing and drying cycle.

Since, in this example grit #24 alundum is being used, which is a very coarse, heavy grit and hence subject to flow and dislocation of both grains and adhesive after coating, the maintenance of the bond at a critically controlled viscosity at the time of sand application and thereafter (cooling the coated abrasive web as 1 required to harden the bond and preserve orientation), and the use of a critical drying and curing cycle affords a very advantageous process and produces a high quality product.

For the adhesive and method used, by way of 15 illustration, a satisfactory drying cycle has been found to be 105 F., for two hours; 110 F. for one and three-quarter hoursf 115 F. for two hours; 120 F. for two hours; 125 F.'for two hours and 130 F. for four hours. The goods 20 should then be cooled to a temperature of around 70 or 80 F., to render the adhesive. less tacky and are then taken down in jumbo form or other conventional ways, or if desired the goods may be carried directly to the sizing apparatus and 25 sized.

Preferably, the product in the form of rolls is returned and mounted on a suitable bundle stand or other means in front of a conventional coated abrasive sizing machine. The sizing resin is pre- 30 pared in a conventional coated abrasive adhesive kettle, such as illustrated at I! in Figure 2, and run to the sizing trough of the sizing machine. A satisfactory temperature for application of the sizing adhesive used in this case is 140-150 F.

tion may be prepared by taking 60 parts by weight of the viscous resin prepared as illustrated in Resin Example I and mixing with 40 parts by weight of the less viscous resin illustrated in Resin Example II. For many applications a satisfactory weight of the sizing material to be applied is 27 pounds per sandpaper ream. After the sizing coat has been applied,- as described, on conventional sandpaper equipment, the sand sized web is then delivered by the usual means to a sandpaper drying room where it is festooned and where a heat cycle is applied. This cycle is critical to preserve the uniform distribution of the grains or orientation, the sized goods, as explained, being subject to flow with consequent dislocation of the adhesive and the abrasive grains. is applied at a temperature of 140-150 F. in the example cited and it will be further noted that the initial drying condition is 100 F. There is, therefore, a decided drop in temperature after .application of the sizing coat. Associated with this drop in temperature, is the customary increase in viscosity. In the present illustration, a satisfactory drying cycle within the drying room is as follows: 100 F.-six and one-quarter hours; 105 F.flve hours; 110 F.-five and onequarter hours; 115 F.four and one-half hours; 120 F.-five hours; 125 F.-five V and onequarter hours and 130 F.--ten and one-half hours. After this drying cycle has been completed the goods should be cooled to a temperature of around 95 F., to reduce the tackiness of the adhesive so that the goods can be taken down in the form of rolls. Excessive cooling-should be avoided as otherwise the goods will become too brittle for satisfactory taking down and winding up in rolls.

The goods, taken from the sandpaper drying room in the form of rolls, are then cut into satis- A satisfactory adhesive for the sizing opera-" It will be noted that the sizing adhesive factory lengths or strips for a subsequent drying operation to finally cure the resin and develop tensile strength. It has been found that a drying cycle of any commercially economical duration at relatively low temperatures will not produce the final cure of this resin which is required for maximum cutting efficiency and that the cut of the coated material is greatly increased by a. high temperature drying cycle now to be described.

The strips cut from the rolls of coated goods are conveniently cured to the final condition by placing them on suitably arranged shelves within a baking oven which can be regulated within critical limits to obtain the desired temperatures and temperature control. A satisfactory final drying cycle for the strips is as follows: eight hours-'I0 C.; eight hours-80 C.;,then gradually raise the temperature to 100 C..so that a temperature of 100C. is reached within one hour; hold at 100 C. for one hour; raise during the next hour to a temperature of 120 C. and hold at 120 C. for one hour; during the next hour raise gradually to 140 C. and then hold at 140 C. for one hour; during the next hour raise gradually to 150 C. and then hold for three and one-half hours. Turn off the heat, open the oven and allow the strips to cool. The strips are then removed and are cut into conventional disc shapes or any other form that it is desired to produce as an abrasive article.

The products of each of the foregoing examples have the high tensile strength, grit arrangement with a substantial portion of the grains above the adhesive and other physical characteristcs above described.

The product obtained by orientation, as with electrostatic means'as described in connection with Coated Abrasive Example ,I, is shown in Figure 4. It will be noted that a substantial volume of abrasive particles is made available for cutting; i. e., is above the average level of the surrounding bond; that the bond is dense and continuous, and that there is a continuous' film of adhesive 13 between the abrasive grains and the adjacent surface of the backing, as shown at 26.

While the curing of synthetic resins, used by way of illustration, to a final hard, insoluble, in-

fusible state is accompanied by some contraction in the volume of the resin it has been found by careful examination of the finished product made according to this invention that there has been no material displacement of the grains from their original positions in such way as to adversely affeet the initial location of the grain.

Where very viscous phenolic resins are used, and especially if hardening of the surface occurs, the grit applied may not readily become embedded in and be held by the adhesive and where this condition exists and as an aid to wetting and proper trapping of the sand in its oriented position, I have used hot sand, which softens the resin immediately adjacent to it and is conducive to immediate bonding. This method can be used with satisfaction-in many cases but I usually prefer to heat or soften the top surface of the resin coating by a source of radiant heat immediately prior to the application of the sand or use accelerating curing agent which may be of acid or alkaline character, but preferably alkaline, such as a suitable percentage (1 -5%) of hexamethylene tetramine. A peculiar difficulty arises in the use of an accelerating curing agent or of a fast curing resin, since large quantities of the resin must be heated to a coatable condition before application to the backing. If the resin is formulated initially to react as rapidly as desired in the coated abrasive structure, the rate of curing isso rapid that a problemof heating the resin, e.g., a phenolic resin to a coatable condition without excessive condensation and resultant thickening, arises. Furthermore, if the accelerating curing agent is added to the resin at the time of manufacture, the stability of the resin in storage is materially decreased even at ordinary temperatures.

I have found that the inherent difiiculty of using a fast-curing resin in coated abrasives may be overcome by several methods which will be used by way of illustration: In the first case, the unaccelerated or relatively slow-curing resin binder is first heated to a coatable state whereupon the curing accelerating agent is added and the heated mixture is applied to the backing before the viscosity of the resinous adhesive has in creased to a point where it would not be coatable. This may be accomplished by adding the accelerating agent in suitable quantities to relatively small quantities of the binder. Altemately, by suitable arrangement, I am able to continuously add a carefully metered stream of the accelerating agent to the heated resin on its way to the coating trough.

I may also add accelerating agents to relatively small batches of a thinner form of resin, heat the resin to bring it to the desired viscosity and coat within a very short period of time, the size of the batch being regulated according to the rate of use of the resin. This is usually uneconomical and commercially not preferred.

The foregoing three methods presuppose the usual practice of making the resin, storing it and later using it. While not in the usual case desirable, the same purpose may be accomplished by manufacturing small batches of resin and after the manufacturing process has been well advanced, adding the accelerating agent and coating with the resin when it has reached its optimum viscosity consistent with spreadability and adhesiveness, as described in the previous case, but this method is also uneconomical andsimilarly not preferred. The step of using an accelerating cure is more fully explained in my copending application, Serial No. 124,507 filed Feb. 6. 1937 and constitutes optional procedure.

I am aware that abrasive articles of the socalled flexible type and containing synthetic resin binders have heretofore been proposed and that the incorporation of synthetic resin in a glue bond has also been suggested. So far as I am aware, no one has heretofore recognized the critical factors hereinbefore disclosed with respect to the control of the resin before, at and after its application to the backing and the necessity for accurate control of these critical factors in order to obtain and maintain the desired grit disposition and final product characteristics hereinbefore described.

In referring herein to sand or grit, I mean abrasive grains and the grains may be of any desired character as described herein.

By curing of a resin, I mean all or any part of the operation carried out to convert the resin from the physical condition in which it is coated to the reinforcing backing, to the strong, tough, final condition required for an eflicient abrading device. By curing, therefore, I refer to removal of the solvent with only slight chemical change in the resin; to the removal of little solvent with decided chemical change in the resin or to that part of the hardening process which consists essentially of a chemical reaction and in which little, if any, solvent is removed, such products as may be removed being formed in the chemical reaction associated with hardening of the resin.

Wherever products made in accordance with the present invention show an objectionable degree of sand concave curl, that is, a curl in which the abrasive is on the concave surface, this condition is materially improved by mechanical flexing of the coated abrasive as heretofore described. A preferred method of correcting sand concave-curl and producing instead thereof a relatively fiat or sand convex coated abrasive is disclosed in co-pending application of Oglesby and Strain filed concurrently herewith, Serial No. 124,508, filed Feb. 6, 1937. A preferred form of carrying out the invention described in the said co-pending application consists of a controlled shrinkage of the backing after the final curing of the coated abrasive to produce a coated abrasive which is either flat or shows a controlled and desirable degree of sand convex-curl.

A convenient method of shrinking the backing in accordance with the method of the said application consists of treating the backing with a controlled amount of water or other suitable shrinking liquid, allowing the liquid to penetrate the backing and subsequently drying, preferably while the web is held substantially flat or while the web is in the form of a roll whereby curling during the drying process is prevented.

Although I have described the method in considerable details, it will be understood that the apparatus illustrated and the details of the method disclosed are merely illustrative and that such variations in the method and apparatus are within the invention as come within the scope of the appended claims.

I claim:

1. The method of making coated abrasives comprising forming on a backing a binder layer of a synthetic resinous adhesive, controlling the viscosity of the resinous adhesive whereby the abrasive grits are properly trapped, wetted and supported, and while the adhesive is in such condition, applying a coat of abrasive grains, controlling the viscosity of the binder to prevent substantial flow thereof or movement of the grains from their init ally applied positions, delivering the said coated web to a drying chamber, and subjecting the web to an initial temperature in the drying chamber such that there will be no substantial flow of adhesive or dislocation of the grains, advancing the cure of the resin under conditions that will avoid flow and substantial movement or the grits by raising the temperature in such a way that the increase in viscosity of, the binder will be at a rate equal to or greater than the decrease in viscosity brought about by the temperature increase, finally curing the binder at temperatures high enough to develop tensile strengths substantially above 6,000 pounds per square inch and whereby the resinous binder.

becomes tough enough to hold the grains rigidly in place under impacts encountered in use of the abrasive, but is sufliciently brittle to permit mechanical flexing by cracking the bond without. material injury to the union between the grains and the bond.

2. The method of making coated abrasives comprising forming on a backing a binder layer of flowable synthetic resinous adhesive, increasing the viscosity of the resinous adhesive whereby the abrasive grits are properly trapped, wetted and supported, and while the adhesive is in such condition applying a coat of abrasive grains, con trolling the viscosity of the binder to prevent substantial flow thereof or movement of the grains from their initially applied positions, delivering the said coated web to a drying chamber and supporting it vertically therein and subjecting the web to an initial temperature in the dry ing chamber such that there will be no substantial flow of adhesive or dislocation of the grains, advancing the cure of the resin under conditions that will avoid flow or substantial movement of the grits by raising the temperature in such a way that the increase in viscosity oithe binder due to curing will be substantially at a rate equal to. or greater 1 than the decrease in viscosity brought about by the temperature increase, finally curing the binder at temperatures high enough to develop tensile strengths substantially above 6,000 pounds per square inch and whereby the resinous binder becomes tough-enough to hold the grains rigidly in place under impacts encountered in use of the abrasive but is sufliciently brittle to permit mechanical flexing by cracking the bond without material injury to the union between the grains and the bond.

3. The method of making coated abrasives comprising forming on a backing a binder layer of synthetic resinous adhesive, controlling the viscosity of the resinous adhesive whereby the abrasive grits are properly wetted and trapped and supported in their initially applied positions, applying a coat of abrasive grains thereto, delivering the coated web to a drying chamber, subjecting the web to increasing temperatures controlled to advance the cure of the resin without producing substantial movements of the grits,

continuing the cure under such conditions until the resinous adhesive has been advanced in cure but has not been cured to the tender brittle condition, removing the web from the drying chamber and applying a coat of size over the abrasive grains, controlling the temperature and viscosity of the coated web to prevent substantial flow of the adhesives or movement of the grains from 5 their initially applied positions, delivering the web to a drying chamber, subjecting the web to increasing temperatures controlled to advance. the cure of the resin without producing substan-. tial movements of the grits and finally curing the 70 resin at high temperatures for a time period sufficient to develop a tensile strength of substantially above 6,000 pounds per square inch in the bond, and form a tough bond holding the grains rigidlyin place but which is sufficiently brittle 75 to permit mechanical flexing by cracking the supporting it vertically therein, subjecting the web to increasing temperatures controlled to advance the cure of the resin without producing substantial movements of the grits, continuing the cure under such conditions until the resinous adhesive has been advanced in cure but has not been cured to the tender brittle condition, removing the we from the drying chamber and applying a coat of resinous adhesive over the abrasive grains, controlling the temperature and viscosity of the coated web to prevent substantial flow of the adhesives or movement of the grains from their initially applied positions, delivering the web to a drying chamber and supporting it vertically therein, subjecting the web to increasing temperatures controlled to advance the cure of the resin without producing substantial move-- ments of the grits and finally curing the resin at high temperatures for a time period sufficient to develop a tensile strength of substantially above 6,000 pounds per square inch in the bond, and form a tough bond holding the grains rigidly in place but which is sufliciently brittle to permit mechanical flexing by cracking the bond without serious injury to the union between the grits and the bond.

5. The method of making coated abrasives having a backing, an adhesive layer of synthetic resin and a layer of grit embedded therein which comprises forming a layer of synthetic resin bond on the backing and maintaining the same of a viscosity to wet, trap and support the desired grit weight, while the adhesive on said backing has such viscosity, applying a layer of grit thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity, such as might result in displacement of the grains, delivering the said coated web to a drying chamber, subjecting the abrasive coated web to increasing temperatures controlled to'advance the cure of the resin without producing substantial movements of the grits, advancing the cure of the bond to a point where it is beyond the tender brittle stage and is insoluble in a sizing coat, removing the web from the drying chamber and applying a sizing coat, delivering the web to a drying chamber, subjecting the web to increasing temperatures controlled to advance the cure-of the resin without producing substantial flow of the sizing coat and finally curing both the binder coat of adhesive andthe sizing coat of adhesive at high temperatures for a time period suflicient to develop a tensile strength in the adhesives substantially above 6,000 pounds per square inch thereby forming a tough bond which holds the grains rigidly in place but which is sufliciently brittle to permit mechanical flexing by cracking the bond without serious injury to the union between the grains and the bond.

6. The method of making coated abrasives having a backing, an adhesive layer of synthetic resin and a layer of grit imbedded therein which comprises forming a layer of the synthetic resin bond on the backing and maintaining the same of a viscosity to wet, trap and support the desired grit weight, while the adhesive on said backing has such viscosity, applying a layer of grit thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity, such as might result in displacement of the grains, delivering the said coated web to a drying chamber and supporting it vertically therein, subjecting the abrasive coated web to increasing temperatures controlled to advance the cure of theresin without producing substantial movements of the grits, advancing the cure of the bond to a point where: it is beyond the tender brittle stage and is insoluble in a sizing coat of a resinous adhesive, removing the web from the drying chamber and applying a sizing coat of resinous adhesive, delivering the web to a drying chamber and supporting it vertically therein, subjecting the web to increasing temperatures controlled to advance the cure of the resin without producing substantial flow of the sizing coat and finally curing both the binder coat of adhesive and the sizing coat of adhesive at high temperatures for a time period sufficient to develop a tensile strength in the adhesives substantially above 6,000 pounds per square inch thereby forming a tough bond which holds the grains rigidly in place but which is sufl'iciently brittle to permit mechanical flexing by cracking the bond without serious injury to the union between the grains and the bond.

'7. The method of making coated abrasives comprising forming on a backing a binder layer of a synthetic resinous adhesive, maintaining the same at a viscosity suitable to trap and support the desired grit weight, and while the adhesive on said backing has such viscosity, applying a layer of abrasive grains thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity, such as might result in displacement of the grains, delivering the said coated web to a drying chamber, subjecting the web to an initial curing temperature of not in excess of 130 F. and at which temperature there will be no substantial flow of adhesive or dislocation of the grains, advancing the cure of the resin under conditions that will avoid flow and substantial movement of the grits by raising the temperature in such a way that the increase in viscosity of the binder due to curing will be at a rate equal to or greater than the decrease in viscosity brought about by the temperature in crease, finally curing the binder at temperatures of not less than about 140 C. to develop a tensile strength of substantially above 6,000 pounds per square inch in the bond and whereby the resinous binder becomes tough enough to hold the grains rigidly in place under impacts encountered in use of the abrasive but is sufificiently brittle to permit mechanical flexing by cracking the bond without material injury to the union between the grains and the bond.

8. The method of making coated abrasives comprising forming on a backing a binder layer of a phenolic resinous adhesive, maintaining the same at a viscosity suitable to trap and support the desired grit weight, and while the adhesive on said backing has such viscosity, applying a layer of abrasive grains thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity, such as might result in displacement of the grains, delivering the said coated web to a drying chamber, subjecting the web to an initial curing temperature of not in excess of F. and at which temperature there will be no substantial flow of adhesive or dislocation of the grains, advancing the cure of the resin under conditions that will avoid flow and substantial movement of the grits by raising the temperature in such a way that the increase in viscosity of the binder due to curing will be at a rate equal to or greater than the decrease in viscosity brought about by the temperature increase, finally curing the binder at temperatures of not less than about C. to develop a tensile strength of substantially above 6,000 pounds per square inch in the bond and whereby the resinous binder becomes tough enough to hold the grains rigidly in place under impacts encountered in use of the abrasive but is sufficiently brittle to permit mechanical flexing by cracking the bond without material injury to the union between the grains and the bond.

9. The method .of making coated abrasives having a backing, an adhesive layer of synthetic resin and layer of grit imbedded therein which comprises forming a layer of the synthetic resin bond on the backing and maintaining the same of a viscosity to wet, trap and support the desired grit weight, While the adhesive on said backing has such viscosity, applying an oriented coating of abrasive grits thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity such as might result in displacement of the grains, delivering the said coated web to a drying chamber and supporting it vertically therein, subjecting the abrasive coated web to increasing temperatures con trolled to advance the cure of the resin without producing substantial movements of the grits, advancing the cure of the bond to a point where it is beyond the tender brittle stage and is insoluble in a sizing coat of a resinous adhesive, removing the web from the drying chamber and applying a sizing coat of resinous adhesive, delivering the web to a drying chamber and supporting it vertically therein, subjecting the web to increasing temperatures controlled to advance the cure of the resin without producing substantial flow of the sizing coat and finally curing both the binder coat of adhesive and the sizing coat of adhesive at high temperatures for a time period suflicient to develop a tensile strength in the adhesives substantially above 6,000 pounds per square inch thereby forming a tough bond which holds the grains rigidly in place but which is sufliciently brittle to permit mechanical flexing by cracking the bond without serious injury to the union between the grains and the bond.

10. The method of making coated abrasives having a backing, an adhesive layer of synthetic resin, and a layer of grit embedded therein which comprises applying the synthetic resin bond to the backing and maintaining the same of a viscosity to wet and trap the desired grit weight, while the adhesive on said backing has such viscosity applying an oriented coating of abrasive grits thereto, controlling the viscosity of the resin bond after grit application to avoid any decrease of viscosity such as might result in displacement of the grains, delivering the said coated Web to a drying chamber and supporting it vertically therein, subjecting the abrasive coated web to increasing temperatures controlled to advance the cure of the resin without producing substantial movements of the grits, the increase in viscosity of the binder due to curing being at a rate substantially equal to or greater than the decrease in viscosity brought about by the temperature increase, and finally curing the resin greases at high temperatures for-a time period sufllcient to develop a tensile strength substantially above 6,000 pounds per square inch and form a tough bond holding the grains rigidly in place but sufliciently brittle to permit mechanical flexingwithout substantial injury to the union between the grains and the bond. v

11. A method of making coated abrasives comprising forming on a backing a layer of a synthetic resin binder, applying a layer of grit to said binder and increasing the viscosity of the binder to a point such that when the coated abrasive is placed in a vertical position the average motion of the grains from the top to the bottom of the coated abrasives will not exceed 96 of an inch for any fifteen minute interval of time, curing the resin by increasing the curing temperatures and controlling the-temperatures so that upon each increase in temperature and while the coated abrasive is vertically extended the average motion of the grains from the top to the bottom ,of th coated abrasive will not be greater than substantially A of an inch for the first fifteen minute interval at the new curing conditions, and finally curing the resin at high temperatures for a time period suflicient to ad-' -vance the cure of the resin beyond the tender brittle stage.

12. The method of making coated abrasives comprising forming on a backing a layer of a synthetic resin binder, applying a layer of grit to said binder and increasing the viscosity of the binder to a point such that when the coated abrasive is placed in a vertical position the average motion of the grains from the top to the bottom of the coated abrasives will not exceed H of an inch ior'any fliteenminute interval of time, curing the resin by increasing the curing temperatures and controlling the temperatures so that upon each increase in temperature and while the coated abrasive is vertically extended the average motion of the grains from the top to the bottom of the coated abrasive will not be greater than substantially $5 of an inch for the first fliteen minute interval at the new curing conditions, and finally curing the resin at high temperatures for a time period sufllcient to develop a tensile strength substantially above 6,000 pounds per square inch and form a tough bond holding the grains rigidly in place but which, is

sufllciently brittle to permit mechanical flexing without substantial injury to the union between the grains and the bond. 7

13. A coated abrasive comprising a cellulosic backing having a layer of grit bonded thereto by a layer 01 synthetic substantially unplasticized resin cured to an inflexible state, at least one third of the volume of the grits of tne grit layer being above the average level of the resinous co adhesive, the bond between the. grains extending under and forming a ,stratumbetween the bottoms of the grains and the backing and the backing having a substantial portion of its thickness unpenetrated by and unmodified by said resinous adhesive, said resin bond being cured for such time periods and temperatures as to have the characteristics of being water-resistant, hard, rigidly holding the grains in place and having a high tensile strength substantially above 6,000 pounds per square inch and being tough enough to prevent disintegration of the binder or dislocation of the grains from the binder, under impacts encountered in use but being sufllciently brittle to permit mechanical flexing in which the bond-is cracked to impart unpenetrated by and unmodified by said resinous adhesive, said resin bond being cured for such time periods and temperatures as to have the characteristics of being water-resistant, hard, rigidly holding the grains in place and havin a high tensile strength substantially above 6,000 pounds per square inch and being tough enough to prevent disintegration of the binder or dislocation of the grains from the binder under impacts encountered in use but being sumclently brittle to permit mechanical flexing in which the bond is cracked to impart flexibility to the abraslve articles without serious injury to the bond;

- 15. A coated abrasive comprising a cellulosic backing having a layer of grit bonded thereto by a layer of cured phenol-aldehyde resin, at least one third ofthe volume of the grits of the grit layer being above the average level of the resinous adhesive, the resinous adhesive consisting of two layers, the bottom layer extending under the grains and forming a stratum between the bottoms of the grains and the backing, the top layer of adhesive being applied over the grains embedded in the bottom layer and extending down between the grains and uniting with the bottom layer of adhesive, the backing having a substantialportion of its thickness unpenetrated by and unmodified by said resinous adhesive, said resin bond being cured for such time periods and temperatures as to have the characteristics of being water-resistant, hard, rigidly holding the grains in place and having a high tensile strength substantially above 6,000 pounds per square inch and being tough enough to prevent disintegration of the binder or dislocation of the grains from the binder under impacts encountered in use but being sufliciently brittle to permit mechanical flexing in which the bond is cracked to impart flexibility to the abrasive articles without serious injury to the bond.

. NICHOLAS E. OGLISBY.

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