US2239828A - Coated abrasive - Google Patents

Description

April W41. N. E. OGLESBY I 3 COATED ABRAS IVE Filed Feb 6, 1957 2 Sheets-Sheet l wwyszozquzszss zzw $9 3 mewvm M ii ' April 29, 1941. QGLESBY 2,239,828

COATED ABRAS IVE Filed Feb. 6. 1937 H Milli 2 Sheets-Sheet 2.

Patented Apr. 29, 1941 UNITED -STATES PATENT OFFICE COATED ABRASIVE Nicholas E. Ogiesby, Troy, N. Y., assignor to Behr- Manning Corporation, tion of Massachusetts Application February 6, 1937, Serial No. 124,507 3 Claims. "(01. 51-293) This invention relates to the manufacture of coated abrasives, particularly coated abrasives made with a synthetic resinous binder.

One object of this invention is to provide a binder for coated abrasives which is resistant to moisture changes and which is substantially as eflicient when used under conditions of very high humidity or very low humidity as when used under conditions of average humidity.

It is a further object of this invention to provide a binder for coated abrasives which is heat resistant and which is resistant to the combined influences of heat and moisture.

It is a further object of this invention to provide a coated abrasive made with a resinous binder which is an efficient abrading device.

It is another object of this invention to provide coated abrasives in which the abrasive grains are held by a strong binder so that a substantial portion of the grains will be above the level 01' the adhesive binder and free to engage the work.

It is a further object of this invention to maintain the abrasive grains in substantially their initially applied positions.

Still another object of the present invention is the provision of a relatively rapid and commercially practical method for the manufacture of coated abrasives with one or more of the advantages mentioned.

The invention accordingly consists in the features of construction, combinations of elements, arrangements of parts, and in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the appended claims.

It is to be understood that by coated abrasives I mean abrasives which include a backing of paper, cloth or of combination as set forth in co-pending application, Oglesby, Reilly and Gilbert, Serial No. 124,506, filed February 6, 1937 and upon one or both sides of which there is adhered a surface of abrasive grits as for example, flint, garnet, corundum, emery, artificially fused aluminum oxide and silicon carbide.

The present invention is related to the inventions set forth in my co-pending applications, Serial Nos. 124,504 and 124,505, filed February 6, 1937. These applications describe the use of a highly viscous resinous adhesive as the means of bonding the abrasive grains to the backing;

In my co-pending application, Serial No. 124,504, filed February 6, 1937, I have described Troy, N. Y., a corporaaldehyde resins, the

the manufacture of coated abrasives witha synthetic resinous adhesive, in which 1) A viscous resin is employed (2) The consistency of the resin at the time of and after grit application is critically controlled, and I (3) The coated abrasive is subjected to critical-drying and curing conditions to harden the resin. I

By reference to the said co-pending application, it will be found that a, relatively long time is required for the curing operation in order to avoid flow of the coating and displacement of the abr'asivegrains from their initially applied positions. Among other things the present invention includes the operation of the process and the production of theproduct disclosed in the said co-pending application at a greater rate of speed than would otherwise be the case where orientation is maintained and/or substantial flow of the adhesive coating is avoided.

The phenal-aldehyde resins are eflicient binders for my purpose, but I may also use the cresol urea formaldehyde resins and the glyptal resins.

It is known that various catalysts and variations in the quantities of catalysts have an eilect upon the curing rate of resins, as for instance, the phenol formaldehyde resins. If excessive quantities of alkali are used to catalyze the condensation of the resin, the reaction becomes impossible to control. Likewise, if excessive quantitles of alkali are present, it is diflicult to heat the required quantities of resin for the coated abrasive process without having the resin con verted too rapidly in the adhesive heating kettle or other heating device. As made and delivered to the coating machine for my purpose, these resins are quite viscous. They cannot be stored warm for any great length of timehence the necessity fora means of heating large quantities of the resin for use in the coating operation. It is impractical to take a resin which has been treated with catalyst to give a very rapid rate of cure and bring it tothe required temperature for the coatingapplication without having an excessive change in the viscosity of the resin and in some cases Without having the resin go over into an unusable jelly state. The problem presented is therefore to produce a resin which can be stored and later heated to the required temperature and viscosity for coating in the sandpaper art, and to later cure this resin after application at a rate which will not make'the over about '70 to '75 minutes to about 120 .variation, 2. part manufacture of coated abrasives with a resinous bond either costly or complicated.

I have discovered that this problem can be commercially solved and that certain disadvantages of the process as heretofore carried out, can be eliminated by making the resins as disclosed in co-pending application Serial No. 124,504. filed February 6, 1937, and then adding materials which speed up the curing process after the resin has been brought to the required coating temperature and before it has been applied to the backing by conventional coating rolls, or other methods.

A number of resinous materials may be prepared which are suitable for use in the present invention and the following will serve as' illustrations RESIN Example I Heat together to 100-110 C. while stirring, phenol,- 100 parts by weight, and sodium hydroxide (solid) .86 part by weight, and hold at 100-110 C. for minutes to dissolve the sodium hydroxide. Then cool to 50 C. and add slowly while stirring, paraforrnaldehyde, parts by weight.

While stirring raise the temperature slowly 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 or all of the formaldehyde may be replaced with hexamethylene tetramine.

Rnsm

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

Heat together to 100110 C. while stirring, phenol, 100 parts by weight, and sodium hydroxide (solid) .86 part 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.

Apply a slight vacuum (about /2" to 1") and raise the/temperature slowly over one to one and one-quarter hours to 100 C. Raise vacuum to 10-15" until temperature is about C. Hold at this temperature under vacuum until cooled sample shows desired viscosity. Then cool to 35 C.

Since the viscosities required in any particular case will vary with the grit size, one convenient method of providing the various viscosities necessary in a sandpaper factory is to blend resins of different viscosities, e. g. one a much more viscous resin than the other. Thus, 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 125-1'l5 F., the exact temperature depending upon the grit size and other conditions 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 em- Cal ployed. but is not 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 may be materially lowered so that it can be satisfactorily applied with coating rolls. After the adhesive coating is applied to the back ing, 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 towards the bottoms of the loops in the festoons. Furthermore, it will be appreciated that still more highly condensed resins than would otherwise be feasible can be used by thinning with a solven Where it is desired to prepare a resin thinned with solvent the following will serve as an example:

Rasm

Example III Mix phenol parts by weight, formalin 187 parts by weight, and barium hydroxide 6 parts by weight, and heat for four hours at (in-65 C. Apply 25-28" vacuum and heat to C. Hold under vacuum at this temperature until 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.

Resin Example IV tory results for some uses may beobtained with urea resin prepared as follows: Add sodium carbonate solution to 2.25 moles of formaldehyde till a pH between 6 and '7 is obtained, bringing to a reflux, slowly adding one mole of urea dissolved in hot water, and adding formic acid till a pH of 4.? is reached. A formic acid-sodium formate buffer is also added at this point. The mixture is refluxed for 30 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 may 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 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 sufliciently fluid or may be rendered so by use of a resin solvent or by heating as to be spreadable, as for instance, by calender rolls or is otherwise capable of formation on the backing or support as a continuous coat or film of uniform. 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 the required viscosity or consistency at the time of and after grit application. Prior to grit application the resinous film of adhesive 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.

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 gritswithin 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 resi'nous adhesive may be increased by a drop in temperature or loss in solvent or a combination of these methods, or by other convenient means. The drop in temperature maybe 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. applied positions, and it therefore becomes advisable 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 favorable orientation of grains 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, the coated web may be changed in direction and moved into the curing room, i'estooned 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. 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 greater the viscosity of the resinous film the higher may be the initial curing temperature and it becomes a problem of the first importance to to an adhesive film with such a high viscosity that the abrasive grains can be supported at atemperature 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 illustrationsof coated abrasives as given of the manufacture hereafter.

A critical drying cycle, such as is required in my process, extends over a relatively long period of time, and may include a number of difierent temperatures. Over such a period of time a relatively slow fiow of coating or a slow motion of grains in their initially one hundredths of an inch in 15 minutes.

grains within the coating may permit displacement of the adhesive coating or of the abrasive grains within the coating and such flow or dislocation of grains may not be readily detectable during a short period of time, with the result that the product may be damaged materially before such a slow process of damage becomes evident and can be corrected. Where the product is damaged by such a slow process, such damage results from the existence of conditions wherein the viscosity of the adhesive is too low. Since the drying cycle is critical, I find it desirable to apply a test to determine the critical viscosity 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 drying conditions as herein described without any visible substantial flow of adhesive or dislocation of the grits from their initially applied positions, .and the initial drying 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 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 further been 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 that it 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 necessary to compromise between the rate of flow permitted and the rate homical.

So long as the temperature is not higher than 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 temshould be no material loss of orientation and the average motion of the grains towards the botv of curing found commercially eco- .but constant conditions may tom of the sample should not exceed six one hundredths of an inch.

For the initial test to determine possible loss of orientation or flow of the coating, any reasonable be taken. If the flow 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 used 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 because of flow of the adhesive coating over the backing which may sometimes occur as for instance in case of surface hardening of the adhesive even though there is not' an excessive loss of orientation. After the correct initial drying conditions are determined, these conditions may be maintained 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 should be applied. It should be appreciated that repeated increases in temperature with a fiow 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 theinitial 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 to keep 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 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 to permit any measureable fiow 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 vertical 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 temperature, the viscosity of the adhesive used to trap the grains must be increased to a point that will decrease the fiow at the initial curing cycle to or below six one hundredths of an inch for the first 15 minutes. Furthermore, where 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 must be kept 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 theadhesive at the time of trapping the grains must be increased 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.

For a given grit size and a given thickness or 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 difficult to prevent change of position of the larger grit sizes than the smaller grit sizes and that in the case of the larger grit sizes either a more viscous adhesive is used or a lower initial curing temperature is used or both a higher viscosity of the resinous adhesive 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 thinner films of resins of the same viscosity.

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 flow 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 fiow, and 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 fiow characteristics of the resin are dependent upon the film thickness as well as the viscosityoi the adhesive. Furthermore, the flow 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 must be adjusted to obtain the final desired flow characteristics of the resin and limited motion of the grains within the adhesive, irrespective of other influences which are properties of the specific adhesive used, the backing used and the abrasive grains used. This means that in any case after grit application, the viscosity of the particular adhesive used should under the conditions of use be great enough to prevent substantial fiow of the adhesive coating or substantial dislocation of the grits from their initially applied positions.

The use of orientation methods in the manuiacture of coated abrasives has increased materially the efficiency of such products and it is within the province of the present invention to further improve the art by the 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 eflicient product.

As one form of the present invention, the process includes dispersion with orientation by means of an electrical field as described in the patents to Elmer C. Schacht, 2,027,307 and 2,027,309, January 7, 1936. A preferred form of using the process of electrical dispersion and orientation described in the above patents 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-distantfrom each other, with their longer axes substantially perpendicular to the backing.

This orientation for many u es ives the most eflicient pergl meiflfiggal abrasive prodw y, orientation methods of coating '1 in so far as the favorable orientation brought about, for example, by the field (electrostatic or magnetic) is not maintained, and it is a feature of this invention that the difficulties incident to taking advantage of orientation practice are effectively overcome and that the positioning of the grits is preserved because the syntheticresin used has a viscosity, i. e. body strength and jellylike character to support the grains against flow on the one hand 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 size that will be coated. While, for convenience,

a variety of compositions and viscosities may be used in applying the adhesive to the backing, the visco ty of the adhesive at the time of or immediately after the sand is applied is critical. This critical viscosity may also be attained if de: sired by partial curing of the adhesive after application to the backing, by evaporation of solvent, or by subjecting the adhesive to a reduced temperature, but before the abrasive grains are applied. Under various conditions considerable variations may be made in the distance between the adhesive application and the grit application and there may be, 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 backings 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 atmosphere can vary individually within wide limits, but for a particular grit size, the viscosity or jelly strength of the adhesive must not fall below a critical value after sand application as explained herein.v

The drying and curing temperatures employed to a phenol formaldehyde resin binder.

about 300 F. The temperature of the resin at the time of application varies from about F. to about 175 F. The grits are sometimes applied in heated condition and have a temperature of about to F.

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 graph illustrating the effectiveness of the present invention.

Figure 2 is a diagrammatic illustration of one type of apparatus which is suitable for carrying out my invention, and

Figure 3 is a diagrammatic view of a modification. Y

Referring to Figure 1, curve I is a control ouring curve in which no accelerating agent is added Cur 2 represents the same resinto w e y e tetram" as been added. In both cases coating is grit #24 aluminous oxide on a fibre-cloth combination backing. In each case the curing operation has been carried out at the highest rate consistent with maintaining the grains in substantially their initially applied positions and consistent with the prevention of excessive flow of the resinous adhesive over the coated abrasive backing. It will be noted that the total time of cure has been reduced from about 37 /2 hours to a little less than 12 hours by the use of 5% of hexamethylene tetramine in the resin ,whichwasmade by blending resins prepared in accordance with Resin Examples I and II.

Referring to Figure 2, the numeral Ill denotes a roll of backing material of cloth, paper, or a combination as set forth in the application of Oglesby, Reilly and Gilbert, filed February 6, 1937, Serial Number 124,506. The making machine includes coating rolls II, the upper of which is preferably of steel and the lower of rubber. The lower roll rotates in an adhesive coating trough l2 containing the adhesive 13 and which is jacketed as shown to receive a heating or cooling medium; the making coat is applied as the backing passes between the coating rolls H. The numeral l4 represents a glue or adhesive kettle or container equipped with a steam jacket for heating the adhesive and with an agitator for stirring the adhesive, and I5 and I6 represent outlet pipes from the kettle through which the adhesive is allowed to flow past valves 11 and I8 to small kettles or chambers I9 and 20.

Small kettles l9 and 20 are equipped with a steam jacket as shown for heating or maintaining the temperature of the adhesive. These small kettles are equipped with inlets 2| and 22 through which measured quantities of the accelerating agents may be added. Outlet pipes 23 and 24 lead from the small kettles and it is through these that the adhesive flows past valves 25 and 26 respectively to the coating trough [2 of the sandpaper making machine. Adhesive trough l2 as explained is equipped with a jacket for heating the adhesive or maintaining its temperature as required. A conventional sand hopper is indicated at 2'! from which abrasive grains 28 are fed to the adhesively coated backing l0. After grit application the 'web passes over nip rolls 29 which may be used t extend through a range of from about 75 F. to 75 press the abrasive grains into the adhesive coating. A sand drum 30 may be used to further press the grains into-the adhesive if desired. The numeral 3| represents a sandpaper rattler which of hexais used to knock the loose grains away from the abrasively coated web, so that only those grains that are well anchored will be retained. The suction drum 32 constitutes means by which the web is pulled through the sandpaper coating machine. It is to be understood that the apparatus just described is preferred, but other means of carrying out the method may be employed.

I will describe by way of illustration of the invention, a suitable process for coating #24 aluminous oxide. A fabric backing may be selected from various types of paper, properly treated cloth and a combination of cloth and paper or of cloth and vulcanized fibre. For the manufacture of #24 fibre backed discs with a resinous binder, a preferred backing is described in co-pending application of Oglesby, Reilly and Gilbert, Serial No. 124,506, filed February 6, 1937. While I may coat either side of the combinat o I usually prefer to coat the cloth side. s give or the coating of grit #24 aluminous oxide is illustrated by Resin Example I or by a mixture of Resin Example I andnsq Example 11 in case it is desired to use a viscosity slightly lower than that of Example I.

The process of coating will be understood by reference to Figure 2 wherein the numeral in denotes a roll of vulcanized fibre and cloth combined as described in the aforementioned copending application which is preferred, or of any other suitable backing material. The backing material is threaded through the machine as indicated in Figure 2, and is pulled through the Irving h" nvwn H v. Am q, Th asses over the idler rolls and through the coating rolls H where a coat of adhesive i3 is applied The web then passes over idler rolls to a position beneath sand hopper 21 where a coating of abrasive grains is allowed to drop continuously on the moving adhesively coated web so as to give an excess of abrasive grains over and above the quantity of grains required to fully coat the web. The web then passes through nip rolls 29 where the abrasive grains are pressed into the adhesive. From nip rolls 29 the web passes around sand drum 3!] where the grains are still further pressed into the adhesive. From sand drum 30 the web passes over rattler 3i to suction drum 32. From suction drum 32 the web is conveyed by any convenient means to a suitable 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. 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 to A; or more of their volumes exposed above the average level of the adhesive binder.

The adhesive l3 used in coating box [2 is first heated in adhesive heating kettle H to a suitable coating temperature as for instance 150-160 F. When the adhesive has been brought to the required temperature, a portion of this adhesive is run, for example, to small kettle i9. Through inlet 2| an accelerating curing agent is added and the agent is well mixed by means of the agitator while the temperature is maintained at that required for coating by the jacket around @Jilhe-ITOWGTEI by the alcohol. curing chamber as for instance a room equipped v/r/igiinhanging the adhesiv coated web in the For this reason, a substantial the small kettle which may be used either for heating by steam or for cooling by water. After the accelerating agent has been thoroughly mixed with the resinous adhesive, the adhesive is allowed to flow through valve 25 to coating trough l2. In the same way the resinous adhesive is allowed to flow into small kettle 20 and is there mixed with an accelerating curing agent. The resin from small kettle I9 is used as required in i0 coating trough 12 while another small batch of resin with accelerating curing agent is prepared in small kettle 20, and the process is continued using alternately resin with accelerating curing agent from small kettles l9 and 20. 5 A suitable accelerating curing agent for this type of resin is hexamethylene tetramine and a suitable quantity of this material is from about 1% to about 5%, according to the extent to which it is desired to speed up the cure. Often 2 /2% will be found to give good results in commercial practice. The use of this quantity of hexamethylene tetramine gives a material decrease in the time required for the cure and this ty of the addition agent has no deleterious bond. In practice we have found that the use tion of the sand sizing coat of adhesive frM about 38 hours to about 16 hours.

Commercial hexamethylene tetramine is usually purchased in the form of fine powder. When an attempt is made to disperse this powder in the resin difficulty is encountered in that the powder tends to form c enot readily dispersed in and wetted by the resin. While this is true, by suitable mechanical mixing equipment, it is possible and feasible to disperse the powdered hexamethylene tetramine in the resins used by way of illustration. It has been found that the hexamethylene tetramine can be more readily added and dispersed in the resin if the powdered hexamethylene tetramine is first wetted with a small quantity of a solvent for the resin such as ethyl alcohol. Where this method of wetting hexamethylene tetramine is used, about 10 pounds of powder may be wetted with about gallon of ethyl alcohol, stirrin r. as to insure wett' n dual particles If a more fluid paste is desired, larger quantities of alcohol may be used but it is generally advantageous to use only the minimum quantity of alcohol required for proper dispersion of the powder in the particular equipment available. In some cases a paste of hexamethylene tetramine wet with water may be added.

While the foregoing method of adding powder may often be used to advantage, a pre;;rred method of adding small quantities of hexamethylene tetramine to heated resins of the class given by way of illustration, is to dissolve the hexamethylene tetramine in water to form a saturated solution and then add the solution to the heated resin. Hexamethylene tetramine is very soluble in water, a saturated solution of hexamethylene tetramine in water containing 46 grams of hexamethylene tetramine to grams of solution at ordinary temperatures. The resins given by way of illustration having been dehydrated in the process of manufacture, will readily alxsorb sufficient water to permit the addition of the quantities of hexamethylene tetramine usually required in the practice of this invention.

It should be understood that after the addition effect upd P ysical properties of the adhesive of hexamethylene tetramine, the resin is highly reactive and should not be held at an elevated temperature for any extended time before use thereof; otherwise, the-re will be a rapid increase in viscosity and finally a conversion of the resin to an unusuable jelly-like condition.

By mixing hexamethylene tetramine and the resin in small kettles l9 and 20 it is feasible to carry out a large scale coating operation without at any time having to hold for an extended length of time, resin to which has been added hexamethylene tetramine.

In some cases, the hexamethylene tetramine solution is continuously metered into the flow of resin to the coating trough l2. Referring to Figure 3,

and accelerating agent are mixed in the mixing device 35 and fed by a conduit 39 extending therefrom and having a valve 40, to the adhesive trough l2. While such a method is feasible, it is usually less accurate than the form of apparatus illustrated in Figure 2 and Figure 2 is, therefore, a preferred form of apparatus for carrying out this process.

A suitable drying cycle, after application of the grits, for the adhesive, grits, and the method described by way of illustration has been found to be- 105 F. for 1 hour 110 F. for Lhour 115 F. for 1 hour 120 F. for 1 hoflr 125 F. for 1 hour 130 F. for 2 hours In any specific case the minimum curing rate may be determined by the flow test hereinbefore described.

The goods should then be cooled to a temperature of around 70-80 F. to render the adhesive less tacky and taken down in Jumbo rolls or other convenient form if desired or the goods may be carried directly to a sand sizing apparatus and sized. If taken down in the form of rolls, the rolls are returned and mounted on a suitable bundle stand or'other means in front of a conventional coated abrasive sizing machine. The sizing resin is'heated to the required temperature and otherwise prepared in an adhesive heating kettle such as is illustrated at H, in Figure 2. After being brought to the required condition, the adhesive is allowed to flow to small 9 and 20, Figure 2, and there mixed with accelerating curing agent. The adhesive is then allowed to flow alternately from small adhesive kettles such as H! and 20 to the coating trough of the sandpaper sizing machine. A suitable quantity of hexamethylene tetramine for the sizing coat is 2 of the weight to the resin used, but more or less may be used within limits, according to the results desired.

A satisfactory temperature for application of the sizing adhesive used in this example is 140-150 F. A satisfactory adhesive for the sizing operation may be prepared by taking 60 parts by weight of the viscous resin prepared as illus- I and mixing with 40 7 trated in Resin, Example I parts by weight of less viscous resin illustrated in Resin Example II. The viscosity of the resin may be varied as desired by varying the relative percentages of resin from Resin Example I and from Resin Example 11. After the required siz- The cycle may be conveniently controlled by the test heretofore explained, with the object initially applied positions and preventing any material flow of the coat. After the critically controlled drying cycle has been carried as high as a temperature of 130 F. and held there for about three hours, the web may be cooled to a temperature of around F. to reduce the tacki- 3 hours at 70 C. 5 hours at 80 C.

then gradually raise the temperature to C. so that a temperature of 100 C. is reached within hour. Hold at 100 C. for one hour. Raise during the next hour to a temperature of C. and hold at 120 C. for one hour. During the next hour raise gradually to C. and then hold at 140 C. for one hour. During the next hour raise temperature gradually to C. and then hold for three hours. 011 the heat, open the oven and allow the strips to cool. then removed and cut into conventional disc shapes or any other form Likewise the rate of cure can be speeded up by adding small quantities of paraform as 8 fol-instance, about of paraform may be added in small kettles l9 and 20 of Figure 2.

While usually tougher resins are produced by alkaline catalysts or alkaline curing agents, acid curing agents may sometimes be used to advantage. A suitable example of an acid catalyst is oxalic acid. Oxalic acid may be added as a saturated solution of oxalic acid in water, or in some cases a. solution of oxalic acid in alcohol may be added. When used with the resins of Examples I and 11, the free alkali of the resin may be neutralized with hydrochloric acid and about .6% oxalic acid may be added. The rate of cure at low temperatures is speeded up materially by this addition of oxalic acid. Phosphoric acid and 0th:- er suitable acids may be used. It is, however, a disadvantage of the acid catalysts that they sometimes have a. deleterious effect on the strength of the cellulosic backing and for this reason the acid catalysts are generally not pre ferred accelerating curing agents.

In the foregoing coated abrasive example this invention has been described in detail in connection with coating of coarse abrasive grain, the coarse abrasive grains being difficult to handle without dislocation from their initially applied positions. It is a particular feature of the present invention that the resinous adhesives of the type given by way of illustration may be more quickly brought to an immobile condition at a low temperature, thus rendering more practical the maintenance of the grits in their initially applied positions, while at the same time providing a more economical commercial rate of cure.

The present invention is advantageous in connection with maintaining the orientation of grits applied by orienting methods as, for example, the electro-st-atic methods disclosed in the patents to Elmer C. Schacht. Number 0-80 aluminous oxide abrasive paper is a good illustration of a product in which a high degree of orientation is desirable, and I will now describe the use of the present invention in connection with the manufacture of this item.

A suitable backing is 130 lb. cylinder paper. A suitable adhesive may be prepared by taking 50% of Resin Example I and 50% of Resin Example II. The resins may be heated and prepared as illustrated in Figure 2. A suitable accelerating curing agent as, for instance, 2 hexamethylene tetramine is added as heretofore explained in connection with Figure 3. The adhesive is applied as illustrated in Figure 2, a suitable temperature for the application of the resin being 150 to 160 F. After adhesive application and prior to grit application, there will be an increase in viscosity as, for instance, due to a drop in temperature or a loss of water from the resin. A correct coating viscosity is preferably adjusted in accordance with my copendlng application Serial $10,124,504, and preferably a pre-treatment is carried out. in accordance with the said copending application 124,505.

The abrasive grits are then applied by one of the electro-static orienting methods, but I prefer to use a method in which the adheslvely coated backing with the face downward is disposed above a belt carrying the abrasive grains as heretofore explained. After the abrasive grains are applied, there is preferably an in? crease in viscosity of the resinous adhesive while the web is still traveling in a horizontal position as, for instance, by drop in temperature or by loss of solvent applied as a pre-treatment of the adhesive coat. The abrasiveiy coated web is then delivered to a drying room and subjected to 'a critical drying cycle, the drying cycle being -c0ated web has one and one-half to two hours, is carried out to ties of the adhesive, and then form a coating controlled by the test heretofore explained.

It will be found that the cure of the adhesive at low temperatures, whereby orientation is maintained can be carried out much more rapidly. In other words, I may either use a lower temperature and secure the desired commercial rate of cure or I may use the same temperature as that employed where the present invention is not used and secure a more rapid rate of cure whereby there is either a more complete maintenance of orientation at the same rate of cure or the same degree of maintenance of orientation at a more rapid rate of cure.

been cured at progressively increasing temperatures there is preferably given an intermediate heating treatment at a relatively high temperature as, for instance, an intermediate heating at about iii) C. for about a half an hour. The web is then subjected to a sand sizing treatment. A suitable resin for the sand sizing operation is Resin Example 11 or preferably Resin Example II, thickened with an amount of Resin Example I which depends to some extent upon the amounts! size that it is desired to apply, coated abrasives being given different degrees of sizing according to the intended use of the product. The sand sizing adhesive is applied on a sand sizing machine equipped with two coating rolls. The accelerating curing agent as, for example, 2/2% hexamethylene tetramine, is added to the sizing adhesive as heretofore explained in connection with Figure 2. The size may be conveniently applied at a temperature of about 150 F.

After application of sand sizing adhesive, the coated web is again conveyed to a sand paper drying room where heat is applied under critical conditions to prevent flow of adhesive coating. A final high temperature curing as, for instance, at a temperature of to C. for

develop high tensile strength and toughness of the resinous adhesive.

The present invention is particularly useful in connection with the use of heat hardening adhesives. which must be cured by a critically controlled and progressively rising drying cycle which includes a final high. temperature treatment. However, this invention is useful in connection with any application where it is desired to heat large quantities of adhesive in a relatively unreactive condition so that there will be no excessive change in the physical properof the adhesive on a backing in a highly reactive form whereby it may be more readily cured.

In connection with the examples given by way of illustration, 2. method'has been disclosed wherein the accelerating agent is added immediately before the adhesive film is formed on the backing. Other variations of this invention may be employed and similar results may be obtained, the illustrations given being preferred methods. I may first form a resinous coating on the backing sheet and then add an accelerating curing agent as, for instance, by spraying a solution or blowing a powdered form of the accelerator, e. g. hexamethylene tetramine onto the surface of the adhesive coating after which the coating may be further treated as by allowing time for diffusing of the accelerat- After the ing agent into the adhesive film or by heating to hasten the diifusion and increase the viscosity of the adhesive film by further curing prior to grit application in which latter case, the resinous adhesive may be applied in a less viscous form than would otherwise be the case. Furthermore, after the abrasively coated web has been subjected to a partial or complete curing operation, I may spray an accelerating curing agent or blow a powdered form of accelerating curing agent onto and between the grits before application of the sizing adhesive, whereby the rate of cure of the sizing adhesive and of the making adhesive, if the same has not been previously completely cured, is hastened. Where it is desired to practice both a treatment or pre-treatment of the film as disclosed in my copending applications Serial No. 124,504 and 124,505, filed February 6, 1937, and the use of an accelerating curing agent, I may combine the two steps, as for example, I may apply a solution of oxalic acid in alcohol to a previously formed coat of viscous adhesive thereby creating a surface zone of lower viscosity and at the same time adding an accelerating curing agent. Furthermore, where it is desired to apply heat to increase the rate of diffusion of the accelerating curing agent, as for example, where the accelerating curing agent is added as a powder, into the adhesive, and where at the same time I wish to create a surface zone of the adhesive which is more fluid than the bottom zone of the adhesive, I may use the same heating means to accomplish both purposes. Altemately I may mix an accelerating agent with a thin form of resin or a resin solution and apply the same to a previously formed viscous adhesive layer, thereby combinin the steps of adding an accelerating curing agent and creating a surface zone of lower viscosity. Furthermore, I may apply a solvent containing an accelerating curing agent, a thin resin containing an accelerating curing agent or a solution of a resin containing an accelerating curing agent H grains "of an abrasivcly coated web prior to the sizing operation whereby the steps of pre-wetting the grains and adding the acceleratingcuring agent to the size are combined, and then apply the size.

The term accelerating curing agent" specification is not to be confused with catalysts inthis ful use of catalysts in resin forming reactions as, for instance, in the manufacture of "Bakelite" depends upon the use of catalysts of a nature and in a quantity that will not make the reaction uncontrollable, that is, too rapid, whereby the resin cannot be manufactured with controlled properties permitting successful application and later hardening. By accelerating curing agent, I mean an agent or a quantity of an agent over and above that considered feasible for use in the manufacture of, or heating of, or handling of resins which is applied to the adhesive either immediately before, concurrently with, or subsequent to the formation of the adhesive film, whereby the adhesive is brought to a more reactive condition than has heretofore been considered feasible in the manufacture of coated abrasives.

Although I have described the method in considerable detail, 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 sandpaper which comprises heating a batch of a highly viscous synthetic resin adhesive to reduce its viscosity to substantially a coatable state, while the resin is in this state, mixing an accelerating curing agent therewith which promotes substantially immediate thlckening of the resin at the coating temperature, then while maintaining the temperature so that the viscosity of the mixture is capable of picking up a desired grit weight, coating the same upon a backing and applying the abrasive grit, and then maintaining the temperature of the abrasive coated web so that the viscosity of the coating will increase rapidly enough to retain the grit in its initially applied positions and prevent the grit during the curing of the resin due to the thickening of the resin and the catalytic action of the accelerating agent.

2. The method in accordance with claim 1 wherein while the resin is in its coatable state successive small portions thereof are mixed with the curing accelerating agent.

3. The method in accordance with claim 1 wherein the mixing of the resin with the accelera' curing agent and the coating of the mixture upon a backing are substantially continuous.

NICHOLAS E. OGLESBY.

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