414 



THE INDIA RUBBER WORLD 



[May 1, 1913. 



6. The influence of previous stretching on strength and 

 elongation is often so marked that it should be taken into 

 consideration. 



In testing for set and strength it is convenient to use the same 

 test pieces for both determinations, but by so doing, the stretch- 

 ing vkrhich a specimen receives in connection with the measure- 

 ment for set, has a tendency to increase not only its ultimate 

 elongation but also its strength. This increase in strength ap- 

 pears to he more pronounced in high grade rubbers than in com- 

 pounds of low quality. It is rather surprising that specifications 

 do not always state the methods to be followed in this case. 



7. There appears to be a marked difference in the methods fol- 

 lowed in determining "set." Although it is well known tliat the 

 recovery of rubber immediately after release is very rapid, it is 

 not unusual to see specifications that require the set to be meas- 

 ured "at once" and the specimen remarked in preparation for a 

 second stretch and mea.surcment for set. Now since the straight 

 specimen does not lend itself readily to the usual methods of 

 taking autographic records and since it is very difficult to meas- 

 ure the set immediately after the release except by autographic 

 means, much more uniform results could lie secured by 

 measurinsj the first set after a definite number of sec- 



onds, sa\ fifteen or twenty, have been allowed to elapse. 



A further objection to the practice of attempting to measure 

 and remark the specimens immediately after release, is that a 

 variation of only a few seconds in the time of remarking is suffi- 

 cient to influence the second set very materially. So great is 

 this influence in fact, that if the specified limits for set were in- 

 sisted upon, material would often be accepted or rejected accord- 

 ing to the quickness with which the operator was able to remark 

 the test pieces after the first set. 



8. A variation within reasonable limits of the speed at which 

 rubljer is stretched does not affect the strength and ultimate 

 elongation to a very great extent, still its influence is generally 

 considered to be sufficient to justify uniformity in the rate of 

 stretching, Whether it is more desirable to maintain a uniform 

 speed, or to increase the load at a uniform rate, is a point on 

 which there are differences of opinion. I believe that stretching 

 at uniform speed is the method usually adopted in this country. 



In conclusioti I would state that my aim has been to deal 

 principally with those points which are most essential to the 

 development of uniform methods of testing, no attempt having 

 been made to treat the genera! subject of physical testing in any- 

 thing like an exhaustive manner. 



Lithopone and Oxide of Zinc in the Rubber Industry. 



By (7. C. Stouts and Cilbcrl Rigg_. 

 A PAPER PREPARED FOR THE THIRD IXTF.RX 



THE zinc oxide of commerce is a white amorphous powder, 

 the purity of which depends entirely on the character of 

 the ore from which it is produced. Zinc ores, with the 

 exception of those from Franklin Furnace. New Jersey, contain 

 considerable lead and usually cadmium. For the production of 

 a white oxide, suitable for rubber compounds, the presence of 

 these constituents except in very small quantities is inadmissible, 

 especially when the object is to make as white a product as 

 possible. The necessity of avoiding the presence of lead and 

 cadmium, which are the only volatile constituents of the ores 

 other than zinc, insures unusual regularity in the composition 

 of oxide of good color. For example, the oxide made by the 

 New Jersey Zinc Co. contains upward of 99 per cent, of zinc 

 oxide. The principal impurities are small quantities of carbon- 

 dioxide and sulphur trioxide. 



Zinc oxide is made by two processes, known respectively as 

 the direct, or American, and the indirect, or French. In the 

 direct process the zinc is driven off from the ore 1)y the reducing 

 action of carbon at a high temperature, and the zinc vapor 

 burned direct to oxide. In the indirect process the ore is first 

 smelted to produce metallic zinc, or spelter, and this metal is 

 then burned to oxide. The latter process is naturally more costly 

 than the former, and the products made by the two differ in 

 certain material points. 



Direct Procf..ss Oxide: The grades used in rubber com- 

 pounding are "XX" (red brand) and "Special" (red brand). 

 The difference between the two is mainly one of color, the 

 Special being more desirable when the whiteness of the com- 

 pound is important. 



Indirect Pfocess Oxide: These are graded as "Red Seal." 

 "Green Seal" and "White Seal." They are of a purer and 

 brighter white than direct process oxides. The Green Seal 

 oxide is smoother and of a more perfect white than the Red 

 Seal. The white Seal is a comparatively new product, and is 

 characterized liy its remarkable lightness and bulkiness. While 

 Green Seal and Red Seal oxides are packed 300 pounds to the 

 barrel. White Seal can only be packed 150 pounds to the barrel. 

 The exact meaning of this difference will 1.; discussed later in 

 this paper. 



of the New Jersey Zinc Company. 

 \TIOXAL RUBBER CONFERENCE, BUT NOT READ. 



Chemical and Physical Properties of Oxide of Zinc : In 

 general, the first essential of a zinc oxide for rubber purposes 

 is, as indicated above, purity. The presence of serious amounts 

 of metals that yield colored sulphides is entirely inadmissible, 

 as during the process of vulcanization these sulphides are 

 formed and discolor the product. Further, in rubber com- 

 pounds it is in the first degree desirable that the compounder 

 should know exactly what he is using if he is to get good re- 

 sults. Uniformity of composition, therefore, is very desirable. 



The determination of zinc is a rather difficult matter, and 

 the analytical error involved is frequently of such magnitude in 

 comparison with the amount of impurity present as to make the 

 determination valueless. Even in experienced hands the error 

 is rarely less than 0.25 per cent., which, taken in connection with 

 a zinc oxide content of over 99 per cent., obviously vitiates any 

 attempt to determine the amount of impurity present by differ- 

 ence. 



We believe that users of oxide will be well advised to confine 

 their analytical work to simply testing for adulteration and to 

 ascertaining the suitability of an oxide for a particular purpose 

 by a practical compounding test in the laboratory, rather than 

 to spend much time and effort in attempting to determine the 

 zinc contents of the oxide. This mode of procedure we under- 

 stand is in vogue in the majority of ruliber laboratories. 



Zinc oxide should dissolve in dilute hydrochloric acid without 

 effervescence and without smell, and should leave no appreciable 

 residue. On the addition of ammonium chloride to the solution 

 and then an excess of ammonium hydrate and ammonium caf- 

 bonate, the liquid should remain perfectly clear, or show at 

 most a slight cloudiness. Care is needed in carrying out this 

 test, as a considerable excess of ammonia and ammonium car- 

 bonate is necessary to dissolve the carbonate of zinc first formed. 

 If the solution in hydrochloric acid has been performed in a 

 test tube, it is desirable to transfer the liquid to a beaker before 

 proceeding with the addition of ammonium chloride, etc. 



Having regard to the nature of tlie process by whicli zinc 

 oxide is made, positive results, as for example, a residue in- 

 solulilc in hydrochloric acid, indicate either a leaded oxide, 

 wliich is unsuitable for rulilier purposes, or wilful adulteration. 



