150 



THE INDIA RUBBER WORLD 



[Uecembki 



properly by measuring the increase in elongation corresponding 

 to two suitable points on the load coordinate as follows : 

 E,-E 



Tvpe (Conventional = 



2.5 

 Where E = extension per cent at 60O grams per square milli- 

 meter and E, is thai at 1040 grams. 



The extensions corresponding to the loads stated for various 

 types for the correct cure in each case were found to be as 

 follows : 



"Correct" cure extensions at 600 grams and 1,040 grams 

 llimetcr (mixing ICO of rubber to 8 of sulphur). 



Extension, Per Cent. 



600 Grams per 104O Grams per 



A glance at the above table and Figure 2 will reveal the fact 

 which is of fundamental importance, namely, that the lower (or 

 better) the type figure (i. e., the more rapid the decrease in ex- 

 tension with increasing tension towards the end of the curve), the 

 less rapid is the decrease in the initial portion. In other words, 

 a curve representing a rubber A which is flatter in type than rub- 

 ber B, will lie for the whole of its course above the latter at the 

 correct cure (t. e., where a = &). As confirmation in this con- 

 nection it is interesting to note that de Vries and Hollendorn§, in 

 a paper from the Central Station at Buitenzorg (which has 

 adopted our method of determining "type"), found that "the 

 maximum of tensile strength is found higher on the paper the 

 flatter (smaller) the 'slope,' that is, the better type or quality as 

 determined by this propert}'." These authors found a range of 

 variation in different grades of plantation rubbers of 36 to S3. 



Load (Grams Per Squ 



The same authors in a previous paper'l found that "the perma- 

 nent set for one standard state of cure shows a close relationship 

 with the property called 'type.' by Schidrowitz. ... As the 

 'type' in ordinary routine testing of raw rubber can be determined 



§ "Journal of the Society of Chemical Industry," 1917. page 1261. 

 II "Journal of the Society of Chemical Industry," 1917, page 1261. 



in the same operation as tensile strength, a separate test for per- 

 manent set, when using the above-named mixture of rubber and 

 sulphur only, is superfluous for ordinary samples. Only in the 

 case of rubber prepared by special methods a deviation from the 

 above given relationship may occur and lead to important con- 

 clusions." 



.\ consideration of the above will demonstrate the fallacy of 

 measurinR the quality of a rubber by the resistance to stretch 

 method, which consists in determining the load necessary to pro- 

 duce a given extension at a single point, and assumes that the 

 greater the load the better the rubber. For rubbers of different 

 "types," the reverse is actually the case. Further, it is obvious 

 that the application of this method for determining state of cure, 

 as suggested H, may give fallacious results. 



NON-VARIATION OF TYPE. 



The authors have demonstrated that the constancy of a.n or 

 "type" is not subject to variation by experimental conditions, 

 citing particularly variations in curing time; mixing by different 

 operators and machines ; curing in different vulcanizers ; method 

 of heating; overworking the rubber mixing and variations of 

 "rest" or "storage" after mixing. 



THE COBKECT CURE. 



We have shown that the curve in any series of cures which we 

 select as the "correct" cure possesses certain attributes which 

 represent (theoretically) an ideal balance of properties. It is the 

 curve in which a-b, that is to say, the toughness or tenacity equals 

 the limit of extension. It is obvious that for purposes of com- 

 parative evaluation of crude rubber, a certain definite curing stan- 

 dard must be selected. 



There is a great and undeniable advantage in a method which 

 not only indicates quite clearly which curve represents the "cor- 

 rect" or standard cure, but which in addition, in the case of a 

 cure which is not correct, indicates quite clearly what the correct 

 cure should be. 



Some examples are given below of "correct cures" obtained on 

 the basis of a single preliminary cure. The rubbers consisted of 

 various plantation grades (smoked sheet, pale crepe, brown 

 crepe, and scrap). Some rubbers of an experimental type are also 

 included. 



E.xAMPi.Es or "Correct Cures," Obtained on Basis of One Preliminary 



Cuke. 

 (Preliminary Cure in Each Case = 2 Hours, Except Where Otherwise 

 Stated.) 



Preliminary Cure. "Correct Cure." 



r Figures. ^ ,■ — -Figures. ^ 



Number. B. S. E. Time. B. S. E. 



p 1.592 9.95 1 hr. 1,786 10.03 



M 1,288 9.86 2J^ hrs. 1,554 9.85 



F. .'.' 1,521 10.94 3 hrs. 1,850 10.28 



F 1 581 10.78 4 hrs. 1,960 10.68 



F' 2 798 10.46 3J^ hrs. 1,260 10.12 



f' 3 840 10.35 3K hrs. 1.094 10.08 



N ..;; 1.484 11,34 2J^ hrs. 1,515 10.72 



F. 4 1,219 11.00 3 hrs.5" 1,886 10.40 



F 5 1,204 11,00 3 hrs. 1.313 9.82 



31 A. VIII 2,380 9.81 IJ^ hrs. 2,345 10.50 



31 S. VII 1,341 9.33 l-^hrs. 1.554 9.86 



36 S. VII 1,198 11.2 2}4 hrs. 1,642 10.57 



77 S XI 476 10.0 3^ hrs. 1,700 10.40 



79 S XI 460 9.58 35i hrs. 1.850 10.50 



C 48 1,660 10.2 l^hTS. 1.900 10,40 



38 S X 1,262 10.48 3 hrs. 1,579 10.20 



W 332 (3 h.) 2,058 9.27 2 hrs. 1,947 10.30 



W. 333 (3 h.) 1,730 9.62 2J^ hrs. 1,770 lO.lO 



W. 336 (3 h.) 2,072 9,64 254hrs. 2.188 10.53 



W. 338 (3 h.) 2.184 10.20 2'A hrs. 2,000 10.85 



842a (2'A h) 2,083 9.54 l^hrs. 2.160 10.75 



841a (1 "4 h.) 1,960 10.30 1 hr. 2.016 10.68 



W. 344 (2^ h.) 1,477 9,8 3^ hrs. 1.540 9.55 



W. 352 (354 h.) 2,140 9.95 2J^ hrs. 1.880 10.45 



W. 354 (2^ h.) 1,568 10.80 2J4 hrs. 1.946 10.36 



Note. — B.S = Breaking strain (lb. per sq. inch), E, = Elongation at 

 breaking (taking original length =; 1). 



CHEMICAL PATENTS. 



THE UNITED STATES. 



ELASTIC Composition comprising coarse rubber and a composi- 

 tion consisting of long-fibered cotton and high-grade rubber 

 friction and skim; an alkaline earth; an oxide of one of the 

 ][ Kratz and Flower. The India Rubber World. Tune 1, 1919. page 485. 



