A Hardness Tester for Rubber1 - American Chemical

The weight of the iron core and the zero pull of the solenoid are balanced by a counter weight on the tipper. Moreover, we have assumed that the condu...
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perceptibly warmer than when working normally. Serertheless, we run the cell in series with a 60-watt lamp as a protection on the lead which does not pass through the coil. The weight of the iron core and the zero pull of the solenoid are balanced by a counter weight on the tipper. Moreover, we have assumed that the conductivity of the water feeding the Venturi pump and thence the cell is of a constant and negligible order. I n the Great Lakes region of America this is a safe assumption. Where, however, the water supply is not so pure and is variable during the day, it is necessary t o replace the counterbalance weight by another iron core actuated by a solenoid and compensating cell fed with a second stream of water. Only the added conductivity conferred by the indicator vapor will then be recorded. This double scheme is capable of an interesting application. Dilute solutions of two gaseous materials such as ammonia and sulfur dioxide may be mixed, a saturator and detector

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being coupled with each cell. I n series with one is an absorption tower carrying alkali, in the other a wash bottle holding sulfuric acid. The one chain allows ammonia only to pass, the other, sulfur dioxide. Thus, perfect balance of the tipper is secured during variation of the load (Figure 8). We are using the scheme successfully for other reactions and have considered it advisable to seek patent protection. It is not possible to give definite figures with regard to performance. If the mixing pot is too small or too large. or the water stream feeding the conductivity cell is out of adjustment, the tipper will oscillate too far in each directiou and cause alternate over- and under-neutralization. With correct adjustment, which is very easy to obtain, the effluent. colored with phenolphthalein for acid-alkali reactions, or permanganate with oxidation-reduction reactions, will fluetuate between colorless and faintly pink. Caugnt in a large tank, the effluent of several hours is as nearly neutral as the product of a laboratory titration.

A Hardness Tester for Rubber' Erle C. Zimmerman and R. W. Brown THEFIRESTOXE TIREA N D RUBBERCOMPANY, A K R O N ,OHIO

ARIOUS forms of instruments have long been in use for measuring the hardness of vulcanized rubber. This use has undoubtedly been more or less extensive, but hardness has been rarely mentioned in the literature on rubber. Some of the earlier instruments were not direct-indicating, could not be operated by hand, and required the preparation of special samples. The first hand-operated instruments were a step in simplification, but were objectionable because of a low degree of accuracy and sensitivity. In attempting to overcome the objectionable features of these instruments, an instrument of improved design was developed u-hich has been i s use for several years and found to answer all the requirements satisfactorily.


Development of Penetrometer

Several instruments constructed to measure the hardness of vulcanized rubber are shown in Figure 1. The one a t the extreme left was originally developed for nieaquring plaqtic flow, but was later found adaptable in measuring hardness. The two instruments in the center of the illustration were developed in an attempt t o secure a portable instrument for the practical measurement of hardness. but they are limited somewhat in accuracy. The instrument at the right of the illustration, with its spring cover at extreme right, represents the first attempt by the authors to correct the difficulties experienced with previous instruments. All these instruments operate on the same fundamental principle-i. e., the application of a load upon a point arranged t o penetrate the rubber, the hardness of which is being determined, with an instrument dial indicating the amount of penetration. Obviously, to secure accuracy the applied load must be kept constant and a t a predetermined value. Also, the shape of the penetration point must be accurately maintained. In designing their instrument, the authors chose values of 1, 2, and 3 pounds, or 500, 1000, and 1500 grams, standardized the penetration point as a semi-sphere, 8/61 inch or 1.25 mm. radius, and made provision to measure the pene1 Presented before the Division of Rubber Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927.

tration in thousandths of an inch or hundredths of a millimeter with 0.100 inch or 2.50 mm. maximum penetration. These values were selected from a number of different value, tried out and found to cover satisfactorily the range of hardness ordinarily encountered in vulcanized rubber compounds. Developments to date have resulted in instruments whose readings repeat to a high order of accuracy, and are who1 y interchangeable. Means for readily checking the zero reading and for determining the load on the penetration point greatly assisted in securing uniformity between instruments. In the actual construction of such instruments, it has been found necessary to make the spring relatively long so that the change in load between minimum and maximum penetration is negligibly small. This feature constitutes one of the very serious faults with the early instruments. The improvement made in this respect is shown in the in




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Zinc oxide stock Zinc oxide stock Zinc oxide stock Solid tire I~eeumalictire tread Pneiirniitie tire tread Pnriumntic tire tread

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impruved Form of Penetrometer

Stocks of high permanent set show abnoriiial penetration because the point of the instrument slowly penetrates the rubber after the initial reading has been taken. Many stocks high in gas black or softener will show this behavior. Under these conditions it is difficult to obta.in a definite penetration rending, and it is necessary to wait a few seconds before taking the reading. The fact that the effect of excessive permanent set or "logginess" in a rubber compound can be detected readily on n penetrometer makes this instrument of still further value to the rubber technologist. Use of Penetrometer in Testing Vulcanized Rubber The portable and mounted penetrometers are both useful instrument,s for the rubhrr teahnologist. The hand instrument, is particularly useful in measuring h a r d n e s s of pneumatic-tirc t r e a d s . With rare exceptions, tire sections are so


Figure 4-Cross

Kp./sq. ~ m . 9.84 1'47 219.3 231 248.5 197 52.7 287 73.8 225 126.5 232 19.3 28.0 30.3 46.0

Section 01 Penefromefer

thick that the effect of thickness on the penetration value is ncgligible. The hand instrument may also be used convenientiy for solid tires and miscellaneous molded articles such as shock insulators, motor supports, heels, soling, etc. In nu case is it necessary to cut rubber products and prepare special samples for penetration tests. The penetrometer cnn be operated with great speed, it being possible to make as many as fifteen tests per minute so that a large volume of product can be tested in a short time. The mounted instrument is, of course, more accurate than the portilhle instrument, and it serves well for lnboratory control work or resarch compounding. A mukiple-cavity mold can be used to advantage in making slabs of uniform thickness. As pointed out previously, these slabs should be a t least 0.25 inch (6 mm.) and preferably 0.5 inch (12 mm.) thick. The mounted instrument can also be used for smallrubber nrticles having parallel surfaces. The penetrometer is equipped normally with a 3-ponnd (1500-gram) spring whioh is suitable for all types of rubber stock from hard rubber down t o fairly soft rubher. The lighter springs may be used to advantage for very soft stacks.