July 1, 1917.] 



THE INDIA RUBBER WORLD 



583 



A Method for the Valuation of Washed and Dried Rubber. 



By Frederick Daiincrth, Ph.D., and Roscoc M. Gage. 



[Dr. Dannerth and R. M. Gage submit this a,s a )iiethod upon H'hieh they have spent much time and which has given satisfac- 

 tory results. They do not claim that it is the only tnethod and im-itc comment and criticism. — The Editor.] 



PAKT I.— GENERAL. 



IN the preparation of specifications for rubber goods, an ex- 

 tensive discussion has taken place concerning the use of such 

 words as Para rubber, plantation rubber and Hevea rubber, 

 because of the fact that it is impossible to distinguish these rub- 

 bers in the finished, vulcanized rubber goods. Some contend 

 that it is more important that the product comply with certain 

 physical or chemical tests. With this in mind, one of the authors 

 has recommended the use of the phrase "pure fine Para rubber 

 or its equivalent," so that it will be possible to use plantation 

 rubber in those cases where experience shows that it is good. 

 This in turn makes it necessary to define the term "equivalent," 

 and after a careful consideration of all the elements involved 

 the authors have adopted the following definition of this phrase : 



"In specifications, a rubber shall be considered the equivalent 

 of pure fine Para rubber, if, when tested in the following man- 

 ner, according to a standard recipe, it will show a tensile strength 

 at least equal to that of pure fine Para rubber." 



(1) The standard recipe for compounding the Para rubber 

 and the rubber which is to be compared with it shall contain the 

 following ingredients : 



Washed and dried rubber 50 per cent 



Zinc oxide 37 per cent 



Lead monoxide (litliarge) 10 per cent 



Sulphur 3 per cent 



100 per cent 



(2) The mineral powders shall not contain more than 0.5 

 per cent moisture and must be of such fineness that they can be 

 washed through a 200-mesh sieve, leaving not more than 0.5 per 

 cent of coarse particles. But all the particles must be fine enough 

 to pass through a 100-mesh sieve. 



(3) The rubber is to be broken down on a mixing mill, the 

 mineral powders are to be added and the whole batch is to be 

 mixed as quickly as possible. 



(4) Each rubber compound is to be vulcanized in such a 

 manner as to secure a maximum tensile strength, provided, 

 however, that the test piece shall stretch from 2 to 14 inches 

 without rupturing; tlie object being to provide a minimum 

 elongation, so that the operator is enabled to vulcanize the speci- 

 men to the greatest possible strength under that condition. 



(5) Method of testing. The tensile strength test is to be 

 carried out on a Scott testing machine or a machine of similar 

 type, power driven. The vulcanized test specimen at least six 

 inches in length is scribed with bench marks two inches apart. 

 The jaws of the testing machine are adjusted so that the maxi- 

 mum distance between them is four inches. The test specimen 

 is placed in the jaws and the machine is operated so that the 

 jaws separate at the rate of 20 inches per minute. No test will 

 be considered unless the test specimen breaks between the bench 

 marks. 



(6) The stretch is carried out simultaneously with the ten- 

 sile strength test. In order to fulfill the requirements of the 

 official test, the bench marks must be at least 14 inches apart 

 before the test specimen breaks. 



(7) In order to obtain the record for the highest possible 

 tensile strength, at least three samples of the rubber compound 

 shall be vulcanized in each instance, and in case of dispute five 

 samples shall be tested. 



(8) Tensile strength shall in all cases be calculated from the 

 cross sectional area of the original sample. 



(9j Tlie rubber compound shall be vulcanized in a steam 

 press in the form of a sheet not greater than % inch in thick- 

 ness, and the test specimen shall be cut from this sheet by 

 means of a die. 



(10) The mineral powders used in the recipe are preferably 

 dried in vacuum, at a temperature five degrees above the boil- 

 ing point of water in vacuo. They shall contain' not more than 

 0.5 per cent of moisture. 



The authors are of the opinion that this method could be 

 adapted to advantage for the valuation of washed and dried rub- 

 ber for special conditions. For example, a compound might be 

 prepared containing 40 per cent rubber, 47 per cent zinc 

 oxide, and another might be prepared containing 30 per cent 

 rubber, 57 per cent zinc oxide, the percentage of the other in- 

 gredients being so adjusted as to total 100 per cent. 



It will be noted that this method has been so devised as to 

 be of the most general applicability. Xo provision has there- 

 fore been made for determining such properties of the com- 

 pound as: (1) aging test; (2) resistance to abrasion; (3) re- 

 sistance to steam under pressure ; (4) resistance to oil or vola- 

 tile solvents ; (5) dielectric properties ; (6) resistance to dry 

 heat; (7) flexibility at low temperatures. 



It is now generally recognized that these tests can be carried 

 out only on specific compounds which are prepared in the fac- 

 tory for certain rubber products. The}- are, in other words, 

 "specific tests." 



THE MINER.^L POWDERS. 



The fineness of the mineral powders used in the recipes is 

 preferably determined by the following method, adapted from 

 the standard cement test. Wire cloth for standard sieves for 

 rubber pigments shall be woven (not twilled), from brass, 

 bronze or other suitable wire, and mounted without distortion 

 on frames, not less than 1.5 inches below the top of the frame. 

 A circular sieve, 8 inches in diameter, is preferred. A standard 

 200-mesh sieve is one having nominally a 0.0029 inch opening 

 and 200 wires per inch, standardized by the United States Bureau 

 of Standards, and conforming to those requirements. 



The test shall be made with 50 grams of the mineral pow- 

 der. The sieve shall first be thoroughly cleaned. The mineral 

 powder shall be placed on the 200-mesh sieve. The powder shall 

 be well distributed over the sieve and shall then be gently 

 spray^ed with a jet of water such as would emanate from the 

 water wash bottle commonly used by chemists. The sieve 

 should be turned frequently. The operation should continue 

 until not inore than O.OS gram passes through in one minute of 

 continuous sieving. The fineness shall be determined from the 

 weight of the residue on the sieve expressed as a percentage of 

 the weight of the original material. 



The percentage of the residue shall not exceed 0.5 per cent, 

 but all particles must be fine enough to pass through a 100- 

 mesh sieve. 



PAHT II.— EXPERIMENTAL. 



.'Ml the tests enumerated in this paper are based on batches 

 of compound weighing 500 grams each. The time consumed for 

 the several preliminary operations was ; 



Minutes. 



1. Weighing out one batch 10 



2. Milling or mixing one batch 20 



3. Allowance for resting after milling 45 



4. Placing in the press, vulcanizing for 30 minutes and 



removing from the press : total 45 



5. ."Mlowance for resting after vulcanizint' 210 



