102 



THE INDIA RUBBER WORLD 



November 1, 1920 



(2) Calendered stocks have higher tensile strength and lower 

 ultimate elongation with the grain than across it. 



(3) Previous stretching (see Bulletin No. 38, Bureau of 

 Standards) increases tensile strength. See Tables VIII and IX. 



That plastic material is present is indicated by: 



(1) Test pieces in which a set has been developed tend to 

 recover their original length. Set decreases with time after 

 release. Pure gum recovers in 8 hours 75 per cent of the original 

 set (measured after 10 minutes). See Table X. 



(2) Schwartz (Schidrowitz, "Rubber," page 241) pointed out 



that elongation under constant load follows the equation : 



J- = o + 6 log t 



when .r zz elongation at the end of an interval of time t 



a — elongation at the end of the first minute 



b ^ a constant depending on the plastic flow of the stock. 



EXTENSION 



Fig. 8 shows (x — a) plotted against log. / for a heavily 

 loaded black stock under 25 pounds load. The curve is a straight 

 line which shows that the equation actually holds. 



This indicates that rubber consists in part of a plastic sub- 

 stance which may be regarded as a supercooled liquid which 

 probably forms a matrix for the elastic fibers. 



(3) That the set is due to plastic material is indicated by 

 the fact that it is decidedly increased when a material known 

 to be plastic, as for example mineral rubber, is added. 



Returning to the hypothesis that rubber consists of plastic 

 material and elastic fibers, it is recognized that the colloidal 

 aggregates (CioH,e)i, doubtless vary considerably in size. The 

 chief difference between plastic and elastic matter would appear 

 to be in the size of the aggregate. 



Vulcanization produces a profound change in the properties of 

 rubber. In the uncured condition it is weak and plastic. Prop- 

 erly vulcanized, it is strong, elastic and resistant to repeated 

 flexing. The polymerizing influence of sulphur is well known in 

 organic chemistry. 



TABLE VII 



Vakiation of Tensile Strength and Ultimate Elongation with Direc- 

 tion OF Grain 



Stock ' No. 1 No. 2 No. 3 

 Tensile Strength 



Longitudinal 2.730 lbs. sq. in. 9:.S lbs. sq. in. 10.550 lbs. sq. in. 



Transverse 2.675 lbs. sq. in. 625 lbs. sq. in. 3,160 lbs. sq. in. 



Eloneation Ult. 



Lcnpitudinal 630 per cent 90 per cent 



Transverse 640 per cent 210 per cent 



Cheap tread Cheap mechanical Asbestos packing 



Is it not possible that the chief effect of vulcanization is the 

 locking up or polymerization of these colloidal aggregates? 

 Plastic material probably also unites with sulphur but since it is 

 composed of much smaller aggregates the effect is not so marked. 

 If this is the case we may regard vulcanized rubber as consisting 

 essentially of a vast network of very fine fibers linked up and 

 strengthened in some way by sulphur. This network extends 

 throughout the plastic materials present and also completely sur- 

 rounds and incloses any filling material that may be present in 

 the stock. It is to this network that the writer attributes the 

 desirable properties of rubber such as its strength, its elasticity, 

 its resistance to repeated flexing and its ability to be compounded. 

 TABLE VIII 



Stock No. I No. 2 No. 3 No. 4 No. 5 No. 6 



Tensile strength, lbs. sq. in. ^,„ 



Single stretch 2470 1740 990 1710 750 930 



Repeated stretch 2610 1960 1180 1790 790 920 



Ultimate elongation, per cent. 



Single stretch 645 665 510 460 430 375 



Repeated stretch 765 780 645 555 440 465 



(Bureau of Standards, Bulletin No. 38, page 41, Table 6.) 



Carrying this picture farther let us consider an ordinary fishing 

 net. Empty, it can be stretched out to a rather great length. 

 But when it is full of fish it is no longer possible to alter the 

 shape to anything like the previous extent. The fish act as 

 struts and keep the sides of the net distended. If you will 

 imagine many nets closely interwoven and embedded in plastic 

 material, you have the writer's conception of rubber. When 

 compounding material is introduced the net is distended and 



tliere is a strut action which prevents ready change of position. 



Examples of similar conditions are found in reinforced con- 

 crete and in the mordanting or weighing of fabric. In the 

 second case we have merely interposed particles among the fibers 

 of the cloth in such a way that they are no longer free to move. 

 In other words we have wedged them into position, consequently, 

 the fabric has become stiff^cr, less pliable, and its tensile strength 

 has been greatly increased. 



Applying the same idea to a compounded stock it would ap- 

 pear that the increase in tensile strength of a stock produced 

 by compounding ingredients is due to two principal effects : 



First, that filling material so distends the network 

 reinforcement cf rubber that the stock becomes stiffer, less 

 stretch}- and its tensile strength as measured on the area at 

 rest is increased because a greater area is presented at break. 



The second and more important effect is due to the influence 

 of the compounding ingredient on the closeness of weave of this 

 net. Rubber doubtless contains colloidal aggregates of different 

 lengths. When a coarse compounding ingredient is added, only 

 the long fibers become effective in constructing the network 

 around the particles. Consequently the resulting stock has a 

 loose weave. It tears readily, the ultimate elongation is not 

 greatly influenced and the tensile strength of the rubber has not 

 been increased. A typical representative of this class is ground 



barytes. 



TABLE IX 



Effect of Repeated Stretching and Suspension Under Load on Tensile 

 Strength and Ultimate Elongation 



Stock: Compounded rather heavily; chiefly with gas black. 



Tensile strength, lbs. per sq. in.: 



Single stretch 2960 



Stretched twice to 75 per cent breaking elongation 3200 



Suspended 135 hrs. under 25 lbs. load 3950 



Ultimate elongation per cent.: 



Single stretch 585 



Stretched twice to 75 per cent breaking elongation 675 



Suspended 135 hrs. under 25 lbs. load 475 



When the compounding material is very finely divided, the 

 short colloidal aggregates also become effective in looping up 

 the particles of filler. The more finely divided the ingredient 

 the more fibers that are rendered effective. In this case the 

 network reinforcement is closely woven and contains the maxi- 

 mum number of loops, each of which is more or less wedged 

 and anchored in place bv the particle it incloses. The resulting 

 stock is close grained. It does not tear easily and has a high 

 tensile strength. Gas black is the best example of this type. 

 It has perhaps the finest state of division of all compounding 

 ingredients known to date and its effect on rubber is more 

 marked than that of any other filler. Its tensile strength and 

 tensile product values, when corrected back to the actual volume 

 of rubber present, are exceptionally high. It produces, when 

 properly handled, a closer grain than can be obtained with any 

 other material. Zinc oxide is a close second to black in point 

 of fineness and from a compounding standpoint its position is 

 admittedly the same. 



TABLE X 

 Decrease of Set with Time 



A B C D E F G 



Per Per Per Per Per Per Per 

 Cent Cent Cent Cent Cent Cent Cent 

 8 19 10 26 24 34 3/ 



6 18 8 26 22 31 34 



4 17 8 24 20 30 33 



4 17 8 24 20 30 33 



4 16 6 24 20 29 33 



2 16 6 23 20 28 32 



2 15 6 22 19 27 30 



2 15 6 22 18 27 30 



Time After Release 



10 minutes 

 20 minutes 

 40 minutes 



1 hour . . . . 



2 hours . . . 

 4 hours . . . 

 6 hours . . . 

 8 hours . . . 



Recovery in 8 hours 6 4 4 4 6 7 7 



Recovery based on set in 10 , „ 



minutes 75 21 40 15.4 25 20.6 19 



Method — Stretched to 75 per cent the breaking stretch (ultimate elongation) 

 held 10 minutes. Released and measured at intervals after release. 



Stocks: A— pure gum; lbs. B— cheap friction; C— friction; D— low 

 specific gravity tread; E — high specific gravity tread; F — cheap tread; G— 

 mechanical. 



The author wishes to thank Messrs. C. W. Bedford, E. L. 

 Davies, and Dr. W. K. Lewis for many ideas and suggestions 

 which have been incorporated into tliis paper. 



