US2136150A - Noncurling coated abrasive product and method of making the same - Google Patents

Description

Patented Nov. 8, 1938 UNITED STATES NONCURLING COATED ABRASIVE PRODUCT AND METHOD OF MAKING THE SAME Nicholas Ewing Oglesby, Troy, N. Y., assignor to Behr-Mannin g Corporation, Troy,

N. Y., a

corporation of Massachusetts No Drawing.

Application November 17, 1934, Se-

rial No. 753,546. Renewed October 12, 1937 14 Claims.

This invention relates to coated abrasives, and more particularly to a new method of producing coated abrasive articles and the product resulting therefrom. The primary object of the invention is the production of coated abrasive products, such as sheets, strips, discs, and the like, which are characterized by a reduction or elimination oi the usual tendency to curl or cup after the articles have been completed.

w Heretofore, in the manufacture of coated abrasives, such as sandpaper, it has been a common trouble to experience what is known as cupping of the coated abrasive sheets and belts. By cupping it is meant that the belt or sheet, as the 15 case may be, does not lie flat after fabrication, but curls or cups on an axis either parallel or at right angles to the machine direction of the paper. This cupping or curling may occur: 1. With the sand coating on the convex surface,

2. With the sand coating on the concave surface.

Generally speaking, what is lmown as closed coats, or tight (closely arranged) grain coatings, curl with the sand on the convex surface, whereas open coatings, or coatings where there is considerable space between the grains, curl with the abrasive grains on the concave surface. In either event, serious difilculty arises in using the product since it is normally desired to sand a flat surface.

In the case of a belt with a sand concave curl, the tendency is to cut into the work deeply on the edges of the belt and to out less deeply with the center of the belt. It the sand is on the convex surface, the situation is reversed, a deep cup being produced in the center of the belt and a more shallow out being produced on the edges 01' the belt. Not only is this true, but breakage of belts often can be traced to curling of the coated belt.

In cases 0! sheets that are used for hand sanding operations, it is also desirable to have a flat surface, the handling of a curled sheet being very inconvenient and resulting in decreased production as well as a poorer quality or work.

In the manufacture of coated abrasives, it is common practice to apply a coat of adhesive, generally animal glue, to a paper backing (for belt use, normally cylinder paper) and then to apply a coat oi abrasive grains, such, for instance, as

Garnet or Alundum, partially dry the first adhesive coat and then apply a second or sizing coat of adhesive. After the second adhesive coat is applied, the coated web is dried and taken down in the form of rolls for subsequent slitting into belts or cutting into sheets.

The curling of the belts or the sheets, as the case may be, is caused by unequal expansion or contraction of the paper backing on the one hand and the sand-glue coat, on the other hand, when exposed to variable humidities. Both glue and 5 paper absorb and lose moisture according to the changes in the humidity of the atmosphere. There is associated with this gain or loss of moisture, as the case may be, an expansion or contraction of the paper and of the glue. The 10 abrasive grains do not change their dimensions with humidity. It the coated abrasive web is flat 'at some humidity such as 55% relative humidity and a temperature of F., and the product is subsequently exposed to an atmosphere of 30% 15 relative humidity at 70 F., both the paper and the glue lose moisture and contract. If the gluesand mixture contracts more than the paper, a sand concave curl will result, whereas it the paper contracts more than the sand-glue coat, a sand 20 convex curl will result.

I have discovered that in order to produce a product which will be flat at all reasonable humidities, it is necessary that the paper and the sand-glue coat contract or expand substantially 25 the same amount, with loss or gain of moisture.

In other words, the paper and the sand-glue coat should have substantially the same expansioncontraction characteristic under similar changes in the moisture content of the surrounding atso mosphere. I

In the case of closed coats, that is, coats with I closely spaced grains, the paper heretofore used tends to contract more than the sand-glue coat with a drop in moisture content resulting from a drop in humidity, with the result that a sand convex curl is produced. Likewise, in the case of very open coatings, that is coatings with only about half of the paper covered with grain, the glue-sand coat tends to contract more than the 40 paper heretofore used, with loss of moisture, so that a sand concave curl is produced.

It might seem a simple matter to adjust the sand coat, that is, make a coating of sand somewhere between the closed and open coats, so as to 45 match the expansion. and contraction oi the paper, and thus produce a non-curling sheet. This can be done and a sand-glue coat can be made to match practically any commercial paper in existence in expansion characteristics, but the to product produced is not suitable for the variable sanding operations that must be performed. Di!- ierent types of coatings are needed for difierent abrading operations. For instance, hard wood or metal generally should be sanded with a tight 5! orclosecoatoigrainsoastoresistwearing away or the abrasive edges and give the desired production per belt or sheet oi coated abrasives. On the contrary, soft, greasy leather should be sanded with an open coating oi sand so as to leave plenty of clearance between the grains and prevent premature filling of the coated abrasive by the soft bulky material that is removed, failure in such cases occurring as a result of filling between and over the grains. Thus, the many uses to which coated abrasives are put in the art requires a wide variety of different types 01' coatings, and no single type 01' paper backing is suitable i'or all.

It is a primary object 01' the present invention to produce a complete line of coated abrasives having greatly improved non-curling characteristics. This object is accomplished by my new method of controlling the expansion-contraction characteristics of the paper backing sheet under moisture content changes, and relating those characteristics to the corresponding characteristic of the particular type oi adhesive abrasive coating desired.

In the case of coated abrasive belts. strong rope papers made on a cylinder paper machine are normally used to meet the requirements for high longitudinal tensile strength and good tearing strength and to prevent the destruction of the belt on the machine. been used to some extent and it is common practitle to employ some jute in the furnish. Various mixtures of rope, wood and jute are also used.

In the manufacture of paper especially on a' cylinder machine as heretofore used, the fibres are oriented with the long direction of the fibre predominantly parallel to the running direction of the cylinder machine. The result is a relatively high length strength of the paper as compared with the cross strength of the papers.

I have discovered that, when an individual fibre expands or contracts with gain or loss or moisture, the resulting change in its dimensions is very largely a change in the diameter or short direction of the fibre, the length direction bein changed to a much smaller degree. I have determined that it is the sum total or expansions and contractions of the individual fibres that largely governs the expansion and contraction characteristics of the paper. Thus, the proportion of transversely disposed fibres to longitudinally disposed fibres in the sheet not only controls the cross to length tensile strength ratio, but also, the ratio between transverse and iongitudinal contraction and expansion characteristics of the sheet under changes in moisture content, such as may result from changes in the humidity of the surrounding atmosphere. As the proportion of cross fibres and relative transverse tensile strength is increased, the relative transverse contraction and expansion characteristic of the sheet is decreased.

If we consider, i'or example, lb. rope cylinder paper, length strength usually varies from 130 to lbs. per inch of width of paper, whereas the cross strength, per inch of width of paper, will vary from about 50 to 75 lbs. Fourdrinier papers have a much lower length strength and a somewhat increased relative cross strength. Such Fourclrinier papers are not, however, generally so suitable for belt work because of their low length tensile strengths.

If we consider any specific paper, either rope or Fourdrinier, the orientation of the fibres is roughly indicated by the length and cross ten- Long wood fibres have sile strengths. Cylinder papers as made in the past for coated abrasive work have had a cross to length tensile strength ratio falling between .31 and .52. Generally speaking, closed coats of sandpapers with a ratio of about .45 to .52 have only a moderate degree of curl. With open coatings, the curl may be quite bad, sand concave. With closed sand coats, papers with a cross to length ratio of .31 to .40 curl quite badly, sand convex. With open coatings, the tendency to curl sand concave is only moderate for papers showing a cross to length ratio of .31 to .37.

In addition to the cross to length relation, the expansion-contraction characteristics are controlled, somewhat, by the density of the paper. A less dense paper will expand and contract and curl less than a more dense paper, provided the paper is not so porous that substantial penetration oi the adhesive into the paper occurs. This means that for a given basis weight of paper curling, will be decreased by using a relatively thick (high caliper) paper instead or a thin (low caliper) paper. Thus, 130 lb. rope cylinder paper with a caliper of 11.5 mils will cup less than 130 lb. cylinder paper with a caliper oi! 9.0 mils. Generally speaking, the thick papers have a relatively rough surface and are not, therefore,

so suitable for fine grits of coated abrasives. For this reason, it is advisable to still further increasethe cross to length tensile ratio and obtain a flat product with the smoother and more dense paper.

Heretofore, the importance of control of fibre orientation has not been appreciated in the coated abrasive art. Moreover, even accidentally strong cylinder paper has not existed which met the requirements for belts and other severe machine operations, with a cross to length strength ratio in excess of .52; also, there has not been used in the paper backed abrasive industry, a coated abrasive paper backing with a cross to length ratio below .31. As a result of my discoveries and by the use of suitable equipment, such as a Bonbert Cylinder Machine, of the type disclosed in United States Patent 1,924,154, it is now possible to control the cross to length strength ratios within wide limits and produce backings tor coated abrasives which have the desired cross to length tensile ratios and, as a result, the desired transverse and longitudinal expansion and contraction characteristics under changes in moisture content.

Desirable cross to length strength ratios for closed coats of sandpaper intended for use as belts or machine covers, have been found to be preferably from .52 to .65, although in some cases there is an advantage in using a paper with a cross to length ratio in excess or .65. For very-open sand coatings for the same uses, a desirable ratio range of cross to length strength is .25 to .31.

It should be understood that the curl found objectionable for belts, drum covers and similar mechanical operations is the curl about an axis parallel with the running direction of the belt or cover, as the case may be. This is true since the belts or covers are handled in roll form, the roll being wound around an axis perpendicular to the long direction of the belt or cover. When belts or covers are made and placed upon the machine, cupping or curling about an axis perpendicular to the running direction is prevented by the tension that is applied to the belt or cover.

Thus, it is of maximum importance to control the cross fibre content of the paper and the re- 15 suiting cross tensile strength, and transverse expansion and contraction characteristic of the sheet for belt use, so that the latter characteristic corresponds substantially with the expansion and contraction characteristic of the particular glue-sand coat to be used in the finished belt. As stated above, the longitudinal fibre content, the resulting length tensile strength, and longitudinal expansion-contraction characteris tic are not of particular importance in belt use, because of the tension under which such devices operate. The requirement for longitudinal tensile strength is satisfied, if the paper is strong enough to withstand the tension under which it is used. Control of the cross fibre content and transverse expansion and contraction, to correspond to expansion and contraction of the glue-sand coat will prevent curling of the belt about an axis parallel to the length of the belt, which is the important consideration.

When we consider coated abrasives in sheet form, sometimes known as ream paper, the material is not necessarily kept under tension in use. As a matter of fact many of these sheets are used primarily for hand operations and no tension is applied. Likewise, the sheet may be used in any direction and there is no requirement for excess strength in one direction to the exclusion of strength in the other direction. For such uses as this, a superior product is produced by having equal strengths in all directions.

If we make an open coat sheet out of 8. cylinder paper with a low cross to length ratio, the sheet curls badly, sand concave, about an axis perpendicular to the machine direction of the original paper backing. Since the requirements for tensile strength are not so severe, so-called Fourdrinier papers are used in many cases for goods that are to be cut into sheet form. In the case of Fourdrinier paper, the balance between cross to length strength is better, ratios as high as .67 occurring in the case of Fourdrinier papers.

Such paper has not, however, eliminated the.

curling of sheet form coated abrasives. A common form of sheet is 9" x 11''. As will be apparent, it is impossible to match the expansion tion of this paper with a definite sand-glue coat.

As previously explained, no single sand weight coating will meet all the requirements of the trade. I have found that from the commercial standpoint, the most satisfactory solution for the sheet problem is to produce an appropriate backing with, roughly, cross to length ratio of 1, and then use the sand coat best suited for the operation in question. It is true that the product will often tend to curl on one axis or the other, or to form a segment of a hemispherical surface. The curl, however, on the average, is less than is the case if the paper has unbalanced structure which in one direction gives a greater expansion or contraction and in the other a lower expansion or contraction with changes in humidity, than is the case in either direction if the paper mcking has a balanced structure. This can be easily understood. Many of these sheets approach squares or in any case, have the long and short dimensions of the same order of magnitude. A curl on either axis may, therefore, occur with substantially equal facility, and the nearest approach to a flat sheet is one with a balanced structure of the backing having an equal expansion and contraction in the two directions with changes in humidity, since the alternate is a backing with one direction having a higher and the other direction a lower rate of change with changes in humidity than is the case in either direction with the balanced structure which has in both directions, equal but intermediate rates of change with humidity.

A balanced structure of backing for hand sheets has the further advantage of a uniform strength in all directions. With hand sheets the direction of rubbing in use is largely a matter of chance, hence ease of breakage is more often dependent upon the weakest direction of the paper. With a balanced structure of paper, the strength in any direction is less than that of the stronger direction of unbalanced paper, but is appreciably greater than that of the weaker direction.

It is a common practice in the coated abrasive industry to form a backing by combining cloth and paper with an adhesive such as animal glue. I have found that the expansion characteristics of the paper with changes in humidity very largely determine the curl of coated abrasives made with this combination backing, in which the abrasive and adhesive coating is applied to either the paper or the cloth side of the combination. It has been found that a combination backing with improved curling characteristics is produced by selecting paper in accordance with the principles heretofore laid down for selecting paper backings. Where I refer to "a paper backing" in the specification and claims, I intend to cover a cloth paper combination backing, as well as a backing consisting solely of paper.

It is also common practice to use as a backing for coated abrasives, especially for discs that are to be used in leveling welds and projecting edges in metal work, a backing consisting either of vulcanized fibre or of vulcanized fibre combined with cloth, with an adhesive such as animal glue. By vulcanized fibre" which I intend to include by the broad term "paper", is meant, a paper of commercially pure cellulose usually made of rags or oihighly purified cellulosesuch as alpha fibre, which has been treated with zinc chloride, sulphuric acid, calcium sulphocyanate or similar solutions well known in the art, to produce a highly hydrated, relatively stifl, and dense product. I have found that discs made by coating either fibre or fibre-cloth combination, have a much smaller tendency to curl or cup if the fibre has a high cross to length tensile strength ratio. The ideal backing is formed by using fibre with a cross to length strength tensile ratio of substantially 1. Not only is such a ratio advantageous in preventing curl but since the material is used in the form of a disc, the requirements for tensile strength are the same in all directions. It has further been determined that the higher the cross to length. tensile strength ratio of the paper used for making the vulcanized fibre, the higher the same ratio for the finished vulcanized fibre. While it is very difiicult to produce a vulcanized fibre with a cross to length ratio equal to 1, it is possible to more closely approach this ratio than has been the case in the past. Heretofore, it has not been the practice to make vulcanized fibre with a cross to length tensile ratio in excess of .52, but by using paper with a high cross to length strength ratio, it has been found possible to produce vulcanized fibre with a ratio well in excess of .52. Where I refer to a vulcanized fibre backing in the specification and claims, I intend to cover a cloth-vulcanized fibre combination backing as well as a backing consisting solely of vulcanized fibre.

In cases where a synthetic phenolic resin or other relatively non-hygroscopic adhesive is used to replace the relatively hygroscopic adhesives, such as animal glues, the paper, combination, or fibre backing expands and contracts to an appreciable extent, 1. e., substantially the usual extent, with changes in humidity, but the adhesive changes in dimensions to only a negligible extent with changes in humidity. In such cases, it is advantageous to use as a backing, or in making combination backing, paper or fibre with the lowest rate of change in dimensions with humidlty (highest cross to length tensile ratio), consistent with other requirements, 1. e., breaking tensile strength in length direction for belts and the like.

From the point of view of both method and article, the present invention has an important bearing on coated abrasives which are made in accordance with the recently developed electrostatic and magnetic methods of depositing and orienting the abrasive grains upon an adhesively coated backing sheet. In these methods great pains are taken accurately and carefully to orient the grains on the sheet, in predetermined relation to each other, and with their major axes perpendicular to the sheet. Obviously, some of the advantages which might be expected to resuit from such orientation or the grains are lost, if their position with respect to the sheet and the work is subsequently changed. When an abrasive sheet curls in accordance with changes in atmospheric conditions, the relation of the grains to one another and to the backing sheet is necessarily altered. Such curling of the sheet and the changing of the angles of the grains is undesirable in specially oriented sandpaper articles.

Thus, the present invention, which results in a non-curling sandpaper, improves the product made by the recently developed electro-static and magnetic depositing and orienting means, because it results in maintaining the orientation, and the predetermined positioning of the abrasive grains accomplished by such methods.

Where cross to length tensile strength ratio is used in these specifications, it should be understood that this ratio is obtained by dividing the cross tensile strength by the length tensile strength. The length direction is what is known as the machine direction. The cross tensile strength is obtained by testing 1" strips of paper cut perpendicular to the machine direction. The length tensile is obtained by testing I" strips which are cut parallel to the machine direction. All tests are made at a relative humidity of 65% and a temperature of 70 F., after the paper has been conditioned at this humidity and temperature for at least two hours. A paper testing machine, motor driven, such as a Scott machine, is used. At least three samples are used for each determination, The testing machine is motor driven. The distance between jaws of the testing machine is 5", and the speed of travel of the testing machine .is from 7 to 8 inches per minute. Samples that break in the jaws of the testing machine should be rejected. Results used as cross and length tensile strengths respectively, are the average of three tests in each case.

It should also be understood that where the term cylinder paper" is used, the paper may be formed on one cylinder to produce a one ply cylinder paper, or on two or more cylinders to produce a multiple ply cylinder paper.

I claim:--

1. A coated abrasive comprising a paper backing and an adhesive abrasive coating thereon, said paper backing having its fibres so selectively oriented that the selected resultant ratio of its length strength to its cross strength is such as to make the expansion and contraction differential between the paper and the ahesive abrasive coating in the presence of humidity changes so predeterminedly small that the coated abrasive is rendered substantially completely non-curling when exposed to variable humidities.

2. An open coated abrasive article comprising a paper backing and an adhesive abrasive coating theeron, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is such as to make the expansion and contraction difierential between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is rendered substantially completely non-curling when exposed to variable humidities.

3. A close coated abrasive article comprising a paper backing and an adhesive abrasive coating thereon, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is such as to make the expansion and contraction difierential between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is rendered substantially completely non-curling when exposed to variable humidities.

4. An abrasive belt comprising a paper backing and an adhesive abrasive coating thereon, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is such as to make the expansion and contraction differential transversely of the belt between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is substantially completely non-curling transversely of the belt when exposed to variable humidities.

5. A coated abrasive comprising a paper backing and an adhesive abrasive coating thereon, said paper backing having its fibres oriented to produce substantial equality in cross to length strength ratio and in cross to length ratio of expansion-contraction characteristics under humidity changes, said composite adhesive abrasive coat having an expansion-contraction characteristic under humidity changes which is equal in all directions so that the expansion and contraction diilerential between the paper and the adhesive abrasive coating in the presence of humidity changes is so small that the coated abrasive is renderedrelatively non-curling when exposed to variable humidities.

6. A coated abrasive article such as an elongated sandpaper beit comprising a backing sheet of cylinder paper and a close coat of abrasive grains adhesively united thereto, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is within the range of substantially 0.52 to 0.05 and is such as to make the expansion and contraction differential transversely of the belt between the paper and theadhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is relatively non-curling transversely of the belt when exposed to variable humidities.

'l. A coated abrasive article such as an elongated sandpaper belt comprising a backing sheet of paper and an open coat of abrasive grains adhesively united thereto, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is within the range of substantially 0.25 to 0.31 and is such as to make the expansion and contraction differential transversely of the belt between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is relatively non-curling transversely of the belt when exposed to variable humidities.

B. A coated abrasive comprising a paper backing and an adhesive abrasive coating thereon, said paper backing having its fibres so oriented that the ratio of its length strength to its cross strength is within the range of substantially 0.70 and 1.0 and having transverse and longitudinal contraction and expansion characteristics which proportionately approach equality so as to make the expansion and contraction diilerential between the paper and the adhesive abrasive coating in the presence oi humidity changes so small that the coated abrasive is rendered relatively non-curling when exposed to variable humldities.

9. A coated abrasive having a paper backing comprising vulcanized fibre and an adhesive abrasive coating thereon, said paper backing having its fibres so oriented that the ratio 01 its length strength to its cross strength is in excess of 0.52 andis such as to make the and contraction diii'erential between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is relatively free from curling when exposed to variable humiditles.

10. A coated abrasive comprising a paper backing and an adhesive abrasive coating thereon, said adhesive being non-hygroscopic. the paper backing having its fibres so oriented that the ratio 01' its length strength to its cross strength is substantially within the range of 0.10 and 1.0 and is such as to make the expansion and contraction diflerentlal between the paper and the adhesive abrasive coating in the presence of humidity changes so small that the coated abrasive is relatively free from curling in all directions when exposed.to variable humidities.

11. A coated abrasive comprising a paper backing and a synthetic resin adhesive abrasive coating thereon, said adhesive being non-hygroscopic, the paper backing having its fibres so oriented that the ratio of its lengthstrength to its cross strength is substantially within the range of 0.70 and 1.0 and is such as to make the expansion and contraction diflerential between the paper and the adhesive-abrasive coating in the presence of humidity changes so small that the coated abrasive is relatively free from curling in all directions when exposed to variable humidities.

12. A coated abrasive comprising a paper backing and a synthetic resin adhesive abrasive coating thereon, said paper backing having its fibres so selectively oriented that the selected resultant ratio of its length strength to its cross strength is such as to make the expansion and contraction diil'erential between the paper and the adhesiveabrasive coating in the presence of humidity changes so predeterminedly small that the coated abrasive is rendered substantially completely noncurling when exposed to variable humiditles.

13. An open coated abrasive article comprising a paper baclnng and an adhesive-abrasive coating thereon, said paper backing having its nbres so oriented that the ratio of its length strength to its cross strength is within the range of substantially 0.25 to 0.31 and is such as to make the expansion and contraction differential in the cross direction between the paperand the adhesive-abrasive coating in the presence oi humidity changes so small that the coated abrasive is rendered substantially completely non-curling on an axis parallelto the machine direction when exposed to variable humidities.

14. Aclosecoatedabrasivearticle i a paperbackingofcylinderpaperandanadhesiveabrasive coating thereon, said paper backing havingitsflbressoorientedthattheratiootits lengthstrengthtoits crossstrengthiswlthinthe rangeofsubstantiallymfiiito 0.65andissuchas to make the expansion and contraction dflierentialinthecrossdirectionbetweenthepaperand adhesive-abrasive coating in the presence 01' humiditychangessoamallthatthecoatedabrasive is rendered substantially completely non-curling on an axis parallel to the machine direction when exposed to variable humiditles.

NICHOLAS EWING OGLEBY.