Adhesion of Rubber and Textiles EFFECT OF AMOUNT OF SPUN STAPLE YARN IN TEXTILE
E. M. Borroff, R. S . Khot, and W. C. Wake Reeearch Association of British Rwbber Manufacturers, Crogdon, England
HE first paper of this
the conditions which, in the bonding of two fabric layers by B rubber cement, lead t o cohesive or ad- hesive failure. In general, cohesion is poorer than adhesion for unvulcaniaed rubbers, but is improved by vulcanization and can be improved by the addi- tion of resins OP cross-
* T series (8) investigated
The work reported forms part of a general program on the adhesion of natural and synthetic rubbers to textiles. Included are improvements of test procedure and the use of improved test procedure to separate mechanical from specific factors in textile adhesion with the ultimate aim of designing textiles to give improved adhesion.
The most important result is the establishing of the role played by the projecting fiber end. This is embedded in the rubber and must either be pulled out or broken off in order to separate the rubber from the textile. The work shows that the strength of the bond formed is a function of the number of fiber ends protruding from the textile and of the number broken off in the rubber.
polar or one is polar and the other contains polarizable double bonds, then the spe- cific adhesion is relatively high. In a rubber-fabric article, therefore, some spe- cific adhesion exists between the two materials, but this earlier work has shown that factors which may be termed mechanical are the more important.
The exact nature of the linking agenta such as poly- A convenient means of obtaining good adhesion of mechanical effect on which isocyanates. adhesion to fabric so much
The second paper ( 1 ) the construction of the textile with a small percentage of depends formed the subject was concerned with the spun staple, the fibers of which have high individual of the fourth paper of this investigation of specific and tensile strength. A continuous filament warp and a series (1). A number of mechanical adhesion. Me- suitable arrangement of picks of continuous filament and fabrics were specially woven chanical adhesion is that spun staple yarns will give the required adhesion. The from spun staple and from
continuous filament viscose, arising from the inter- use of sateen weaves with spun staple on the weft face, penetration or locking to- which has also to be proofed, enables fabrics with good acetate rayon, and nylon.
Adhesion experiments with gether of the makrial and adhesion characteristics also to have the sheen charac- natural and with synthetic the adhesive. Specific ad- teristic of continuous filament materials.
hesion is independent of the polar rubbers showed that form of the surface rand can the adhesion was greatest be considered '60 arise from the intermolecular attraction due to with spun staple nylon and least with acetate rayon. The nylon the electronic dispersion forces or t o molecular dipoles or in- fiber possessed the greatest breaking strength and acetate rayon the duced dipoles. In this investigation two main series of experi- smallest breaking strength, and this correlated with the fact that ments were made. fragments of fibers were left embedded in the,rubber after the
In the first the chemical nature of the surface of a cotton duck rubber-to-fabric bond was broken and could be recovered from was modified by acetylation and nitration a t two different levels, the rubber.
rubber to fabric is suggested by the work. This involves
and by reacting with a quaternary ammonium salt to give a non- polar surface consisting of long hydrocarbon chains. Experi- ments were also made to ascertain the effect of rigorous drying compared with normal conditions. From this first series of ex- periments with treated cotton duck i t was concluded that altera- tion in the polarity of the cloth does not influence the adhesion by an amount sufficient to be noticeable, unless the polarity is sub- stantially reduced by coating the surface with a waxy envelope. In this case the adhesion of the polar rubbers (polychloroprene and butadiene-acrylonitrile copolymer) only is affected, being reduced somewhat. All the chemical treatments, however, weakened the fabric somewhat and this general weakening correlates with a slight general reduction in adhesion. This observation became cxplicable only in the light of later work.
The second series of experiments concerned the adhesion of rub- bers to smooth foils of material similar to that formed by chemical treatment of the cotton duck-i.e., the f i s t series of experiments was planned with the mechanical factor maintained constant but present and the specific factor altered and the second series with-a virtual absence of the mechanical factor but the same variation of specific factor. This second series of experiments showed clearly that in the case of rubbers and cellulose derivatives or some other materials in sheet form, differences in the magnitude of specific adhesion are obtained according to whether or not dipoles are present. If both faces are completely apolar--e.g., natural rubber on poly thenethe specific adhesion is relatively low; if both are
The picture which has thus been built up of this mechanical adhesion is one in which the projecting fiber ends from the spun yarn are embedded in the rubber spread on the fabric, so that the adhesion arises not so much from the penetration of the rubber into the fabric as the penetration of fibers of the fabric into the rubber.
There are two factors which are, therefore, of importance in de- termining the bond strength-vie., the number of fibers projecting into the rubber and the strength with which they are held. This paper is concerned primarily with the influence of the former factor, but i t is worth while to consider briefly the strength with which the fibers are held in the rubber.
Rubber shrinks during the vulcanization process, and if the projecting textile fiber is considered as a cylinder i t is obvious that circumferential forces on the cylinder will result from the shrinkage. The forces preventing the withdrawal of the cylinder from the rubber, in so far as contact between cylinder and rubber is not perfect, will be increased by the extra loading induced by the circumferential forces in the same way that frictional forces are proportionate to the applied load. If molecular contact be- tween rubber and the cylinder of textile material is made, then the force required to withdraw the cylinder will depend on the intermolecular forces and hence is a manifestation of specific ad- hesion. The improvement of contact brought about by the shrinkage of the rubber is likely to be of more importance with irregular fibers than regular fibers. By working with nylon, which
440 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 43, No. 2
gives regular fibers of uniform circular cross section, differences due to the degree of cure and other factors which are likely to affect the shrinkage of rubber onto the fiber are minimized.
Test TeehnPqne Used Direct Tension Test. The investigation reported here used the
direct tension test technique developed by two of the authors and first reported elsewhere (1).
Pairs of close-grained hard wood cylinders are taken and circles of fabric about 3 inches in diameter are cemented to them. The fabric is pleated back and wired into groove provided. Usually one, but occasionally two, tacks are hammered home and the rubber in the form of a dough in benzene is applied with a spatula to the surfaces, which are combined after evaporation of the sol- vent. They are then held together rigid during vulcanization. They are tested by fracturing the bond in a suitable testing ma- chine. After fracture, the rubber is stripped from the face to which it still adheres, its thickness is measured, and its exact area is determined by tracing on to squared paper, the area of the tack head(s) being deducted. A photograph is reproduced from the earlier paper illustrating the test piece (Figure 1).
COURTESY INSTITUTION OF T H E RUBBER INDUSTRY
Figure 1. Direct Tension Test Pieces
In the course of this investigation it was found that the fiber ends remaining in the rubber after fracture of the rubber-textile bond could be recovered and their numbers estimated in the following way:
The rubber was swollen in benzene to facilitate its removal from the fabric to which it was still adherent without damage to the fiber ends of that piece of fabric, and the piece of vulcanized rub- ber obtained was then heated in p-dichlorobenzene under condi- tions of free access of air, so that it dissolved. The solution was diluted and filtered the fiber fragments being washed with sol- vent, dried, and w2ghed. A reasonable proportion, usually be- tween 300 and 400, was counted under a low power microscope and weighed and the total number was then calculated from the total weight.
Effect of Thickness of Rubber and Rate of Separation. One of the advantages claimed for the direct tension test is its relative insensitivity to thickness of the rubber film compared with the extreme sensitivity of a stripping test, and it was thought that this lack of sensitivity to thickness would also result in a lack of sensitivity to the rate of separation of the pieces. This lack of sensitivity has been confirmed with the figures given in Table I, although the figures taken together can be used to demonstrate that a real though small effect actually exists in a direction op-
Table I. Effect of Thickness of Rubber and Rate of Separation on Adhesion to Fabric
Testing Machine, Thickness of Recorded Load, Inches/Min. Rubber4 Lb./Sq. Inch
13 I 195, 205, 210 I1 195, 190, 183
I11 198, 196, 182 25 I 179, 188, 193
I1 188, 198, 199 I11 182, 153,*183
I 204, 195, 188 184, 194, 188
I11 181, 171, 178
a I. Approx. 0.03 inch. 11. Approx. 0.06 inch. 111. Approx. 0.09 inch.
posite to that observed in the case of a stripping test. Fabric E was used in these experiments.
Examination of the figures of Table I by analysis of variance, set out in TableII, shows that both rate of testing and thickness of the rubber layer have a significant effect but that there is no inter- action between these effects.
Table PI. Analysis of Variance, Showing Significance of Thickness and Rate Effects
Source of Variance Squares Freedom Variance Ratio cance Between rates 508.22 2 254.11 2 . 9 P = 0.05 Between thicknesses 1042.88 2 521.44 5.94 P = 0.01 Interaction 606.89 1i}22 l51.72\ Residual
Sum of Degrees of Variance Signifi-
1328.66 73.5 i88 .0 -- 3486.65 26
It is important, therefore, to consider the magnitudes of the effect of rate of test and of thickness of test sample separately.
The standard deviation due to random experimental error is given by the analysis as 9.4 pounds and the one result marked in Table I by an asterisk, therefore, differs significantly from its replicates. The mean loads in pounds for the different thick- nesses, ignoring the effect of rate of test and omitting the marked figure, are given in Table 111, together with a second series of similar figures in which the rates were not accurately known arid could not, therefore, be subjected to analysis for the effect of rate. The agreement in absolute value between the two series of results is fortuitous. These latter figures are given in the table as series 11.
Table 111. Effect of Thickness of Rubber on Adhesion to Fabric
Thickness of Rubbera
I I1 I11
a Same as in Table I.
Mean Load Series I Series I1
195 191 184
200 190 186
The thickness of rubber studied was twice, four times, and six times the average length of the fiber ends protruding from the surface of the fabric. The measured distances before vulcaniza- tion approximated 0.029, 0.058, and 0.088 inch, respectively, al- though there must have been variations of thickness betweeen replicates nominally identical. Table I11 shows the effect to be small, even though analysis of variance has established its real existence. Doubling the thickness of the rubber layer lowers the recorded bond adhesion by 2% in one case and 5% in the other. Trebling the thickness causes a drop of 5 and 7.5%. As recorded above, the standard deviation for a single observation is about 5%, so that the thickness effect will not be significant unless a number of test pieces are examined. Working in duplicate or triplicate, the effect of doubling the thickness can be ignored. Whereas the thickness effect in this series of experiments was significant a t the 0.01 level, the rate effect is shown by Table I1 to be significant only at the 0.05 level. This significance of the dif- ference between the loads recorded for different rates arises from a difference between the lowest rate used (13 inches per minute) and the two other rates, which are indistinguishable. The drop in load recorded between the slowest rate (mean load 195 pounds per square inch) and the other two rates (mean load 188 pounds per square inch) is only a few per cent and would not normally be of any significance. However, the effect does exist, though not nearly to the same magnitude as with stripping tests, and it is in the opposite direction.
February 1951 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 441
Nambers ef Eiber Ends Projmting from Fabrio Snrfaces
The principal aim of the work reported in this paper was to use the test technique described above, in an investigation designed to supplement the work reported in the fourth paper of the series by experiments with a series of fabrics woven with the same yarn in which the numbers of fibers projecting from the surface varied over a wide range. This was achieved by using various weaves described below and actually counting the number of projecting
Distonce between gloss ploter
R=Regi w n of focus o f microscope
Figure 2. Counting of Fiber Ends Projecting from Fabric Surfaces
fiber ends. The number of fiber ends projecting from the surface of a fabric does not follow the nominal amount of spun yarn used in its construction. This is due to a number of factors, including the cover factor of the weave, the nature of the weave, and the numbers of ends and picks per inch. The differences are most marked by comparing fabrics B and C of Table IV with fabrics D, E, and F, or by comparing the warp and weft faces of the same fabric-namely, B and E, or C and F, where the expected 3 t o 1 and 4 to 1 ratios in the number of protruding fiber ends are obvi- ously not realized.
There are a number of technical difEioulties in counting the number of fiber ends projecting from a surface, and several methods were tried before the one described was finally adopted. For the reasons set out below it is important to realize that the numbers quoted are an estimate of the number per unit area, and although t...