August 1. 191/".] 



THE INDIA RUBBER WORLD 



649 



pared after completion of the scientific tests made without 

 knowledge of the tirst results. 



Empiric judging of rubber depends largely on color, smell 

 and stretch. These tests at best are only roughly approximate, 

 although the rubber expert by means of them can distinguish 

 marked differences in quality. It is a mistake to presuppose 

 that mechanical properties of the rubber when vulcanized run 

 parallel to color, smell and stretch in the raw state. Variations 

 in physical condition do not permit hand pulled tests nor quanti- 

 tative expression of value. The influence of temperature varia- 

 tions are also marked. 



Smoked sheet cannot be judged better by rule of thumb than 

 crepe. Its color denotes only the degree of smoking. Sheet is 

 easier judged as to its mechanical properties than crepe. 



Black rubber presents the most difficulties for empiric judgment 

 as it cannot be subjected to a hand test. 



Scientific methods obviate these objections because each sample 

 is suitably prepared and tested quantitatively. The 137 samples 

 under investigation were examined after the followirg scheme: 



A. Chemical Analysis : 



1. Moisture. 



2. Resin. 



3. Ash. 



4. Nitrogen calculated as albumen. 



5. Rubber. 



6. Acetic degree. 



B. ViSCGSITV XUMBER. 



C. VULCANIZATIOX AND PHYSICAL TeSTS : 



1. Tensile strength. 



2. Elongation. 



3. Permanent set after 24 hours at 400 per cent stretch meas- 

 ured after si.x hours' rest. 



4. Temporary set measured directly after release from 400 

 per cent stretch. 



5. Determination of load necessary to stretch sample 400 per 

 cent (kilograms per square centimeter). 



6. Difference of the load necessary to stretch sample 400 per 

 cent and that required for the last of five successive 400 per cent 

 stretchings. 



7. Elasticity or rebound. 



8. Coefficient of vulcanization. 



Concerning the importance and correlation of these deter- 

 minations, it is concluded: (1) Chemical analysis is not sufficient 

 alone for judgment of the rubber quality. Properties of rubber 

 are chiefly determined by the physical nature of rubber and the 

 rubber molecule. The quantitative chemical differences are not 

 enough to account for the large differences found in the physical 

 properties ; (2) in general a high viscosity indicates good mechan- 

 ical properties of the rubber after vulcanization. However, the 

 opposite must be assumed with some reservation, since in 

 studying the relation between the viscosity and tensile strength 

 it appeared that various samples with a low viscosity had a 

 very high tensile strength. The samples that exhibited this 

 relation were almost exclusively smoked sheet. This phenomenon 

 is caused by the fact that it is very difficult to dissolve smoked 

 sheet in benzol completely. The dissolved part has a low 

 viscosity and presumably contributes little to the excellent quali- 

 ties shown by the sample after vulcanization, while the very 

 considerable amount of imdissolved rubber apparently is the 

 most valuable part of the sample and most probably causes 

 the good properties after vulcanization. 



The VoorJichfingsdienst (Information Service) has taken 

 the position that rubber which possesses good mechanical proper- 

 ties under certain normal vulcanization conditions, is better than 

 that which may show these good properties only after pro- 

 longed vulcanization. It thus had to base judgment of the 

 samples sent in on a determination of the mechanical qualities 

 and of the coefficient of vulcanization of the rubber vulcanized 

 according to a standard method.' 



Test samples for vulcanization were prepared with seven and 

 one-half per cent of sulphur and were vulcanized one and one- 

 half hours at three and one-half atmospheres. 



The tensile strength, permanent set after stretching 400 per 

 cent and the coefficient of vulcanization were taken as quantities 

 suitable for classifying the samples. These quantities are closely 

 related. Thus, high tensile strength generally accompanies high 

 coefficient of vulcanization and low permanent set. Elongation 

 at break also indicates the quality of the rubber ; of samples 

 equally loaded, that is, the best which has the highest elonga- 

 tion at break. 



The remainder of the physical tests made were set aside as 

 practically valueless for the end in view. 



One of the most important points demonstrated by this 

 investigation is the lack of uniformity in First latex rubber. The 

 greatest divergencies were found in viscosity, tensile strength, 

 permanent set and the coefficient of vulcanization. The causes 

 of this lack of uniformity and its prevention is one of the most 

 important problems of the rubber industry. 



The rubbers investigated were classified as follows : 



Breaking strain Coefficient of 



Class. (kilograms per sq. c. m.) Permanent set. vulcanization. 



I 135 k. g. and over Maximum 5% Minimum S 



II 120 k. R. to 135 k. g. 5% to 7% Minimum 4 



III 90 k. g. to 120 k. g. 7% to 12% Minimum 3 



IV 80 k. g. to 90 k. g. Over 12% Below 3 



V Below 80 k. g. Over 12% Below 3 



The division of the 137 samples among the different classes is 

 indicated below : 



Class. Sheet. Crepe. Blanket. 



I 17* 3 



H 12 21 3 



Til 7 51 2 



IV 11 1 



V 9 



Totals 36 95 6 



* One sample unsmoked. 



These figures demonstrate clearly that, in general, smoked sheet 

 is better than crepe. However, there are samples of crepe that 

 are equal to smoked sheet as far as mechanical properties are 

 concerned. The number of samples of blanket is too small to 

 permit a decisive conclusion but the figures would indicate that 

 blanket is inferior in quality to smoked sheet and often also to 

 crepe. 



The empiric judgment of the rubbers by the contributors 

 varied considerably in method. Seven contributors only, out of 

 IS, sent in their judgment. Some arranged the rubbers in 

 groups, others graded them and one sent the market values. From 

 the last method it is seen that according to the prevailing system 

 inferior crepe is valued as highly as the best crepe. 



Lack of uniformity in plantation rubbers reveals itself chiefly 

 in the difference in rapidity of vulcanization. Two rubbers out- 

 wardly of absolutely equal value may produce entirely different 

 results after vulcanization. Empiric judgment cannot tell how 

 a rubber will behave during vulcanization, hence the estimate 

 of value is liable to be incorrect. This causes much uncer- 

 tainty and disappointment to the manufacturer. 



Another disadvantage which will be removed by the introduc- 

 tion of scientific methods of inspection is the putting together of 

 equal lots of rubber coming from different plantations and pos- 

 sessing widely divergent properties. 



The main advantages of the scientific system of grading rub- 

 ber will be, (1) relatively better prices, especially of classes II 

 and III, in comparison with those of hard fine Para; (2) im- 

 provement of the product of a good many estates, the managers 

 of which grope in the dark at present ; (3) increased use of 

 plantation rubber, certain qualities of which are already as good, 

 if not better than hard fine Para. 



INFLUENCE OF PIGMENTS ON RUBBER. 



In an article on pigments in rubber ("Journal of Industrial and 

 Engineering Chemistry," July, 1917) Dr. Maximilian Toch states 

 the nature and chemical effects on rubber of active compounding 

 ingredients or "reinforcing pigments," such as antimony sul- 



