INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1926
Theoretically, if this view is correct, the percentage of nitrogen in the gas should decrease as the digestion progresses and the soluble gaseous nitrogen has been liberated t o a large extent in an effort t o maintain equilibrium. This actual decrease has been found in the tanks a t UrbanaChampaign, as can be seen from the data submitted. Summary
1-Gases from foaming Imhoff tanks contain more carbon dioxide, less nitrogen, and substantially the same amount of methane as compared with gases from nonfciaming tanks.
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2-The relation of gas composition to foaming is a problem requiring the evaluation of such factors as surface tension, type of biological action, and rate of gas evolution. 3-The amount and heating value of gas available from Imhoff tanks is sufficient t o warrant its collection and use for fuel purposes. 4-The higher amounts of nitrogen present in gases from new tanks can be explained, in part, by the “de-solution” of nitrogen dissolved in the sewage in the digestion chamber to establish equilibrium with gas bubbles containing no nitrogen.
A New Mechanical Test for Rubber Insulation’ By C. L. Hippensteel BELLTELEPHONE LABORATORIES. INC.,NEWYORK,N. Y.
ETHODS f o r t h e
the insulation tube is so small This paper discusses the development of a rapid routhat the familiar elongated p h y s i c a l tJesting of tine test which will numerically express the ability of dumb-bell specimen cannot be r u b b e r t i r e comthe rubber insulation to resist cutting by the conductor obtained from it, even though pounds and mechanical rubat the points of support and to resist cracking at points the insulation is first slit, reber goods have been developed of extreme flexure. Up to the present time no one test moved from the conductor, to a relatively satisfactory of that nature has been described. This test is not yet and laid flat. In the case of state in recent years. Definite fully developed but gives sufficient promise to justify smaller wires such as are used s t a n d a r d s of clualitv have the hone that it will merit a fuller reDort at a later time. i n t e l e p h o n e installations, been set largely-in terms of modified forms of test specitensile strength, elongation under stress, permanent set, and resistance to wear. These are mens are used, such as tubes of the insulating material from properly taken as the principal criteria of quality, especially which the conductor has been removed or a tangential specimen in the case of tires, which occupy a very prominent place in sliced from one side of the wire. However, such specimens the rubber industry. Less attention has been given to the are not entirely satisfactory, as they do not have the enlarged mechanical testing of the rubber insulation of aerial wire. cross section a t the ends which is essential to satisfactory The service conditions and requirements for rubber insulation gripping in the tensile machine. This is especially true for aerial wire are obviously different from those for me- of highly distensible materials, and such a method is not chanical rubber goods, tires, etc., which are subjected to commonly regarded as applicable to compounds containing constant wear, repeated stressings, high pressure, and ex- more than 30 per cent of raw rubber, With lower grade posure to heat, oil, or water. materials the results are subject to considerable variations Generally speaking, the important requirements of in- because of the small cross section of the specimen and the sulation are continuity to insure against any cixternal elec- low strength of the materials. Also, the preparation of such trical contacts; centralization of the conductor in the in- specimens involves a large element of personal skill, and sulating material, especially in the case of thin-walled in- slight imperfections in the specimen may cause misleading sulation; resistance to cutting of the wire through the in- results. Furthermore, the time required to prepare the sulation where it rests against glass or porcelain insulators specimens, make the measurements, and calculate the results or where one wire crosses another in a twist; resistance into specific values is considerable. Nevertheless, the results to cracking when bent; and particularly resistance to aging of the tensile tests on modified forms of test specimens may in which climate plays an important part. Often special be of considerable value. However, .it is not always clear requirements, both mechanical and electrical, are set in just how such results may be related to the performance particular cases. The serviceability of rubber insulation of insulated wire under certain special conditions of commay be considerably affected by other elements of the finished pression which the insulation is required to withstand in wire. Thus, such insulation is commonly protected from service, such as the tendency of the conductor to cut through injury by a textile braid, and, in the case of outside wire, the insulation. the braid also carries a wax weather-proofing compound, The Bell System, large users of rubber-covered aerial which protects the insulation to a considerable degree from wire, employ a more rapid test suitable for inspection purthe action of light and oxygen. On the other hand, the poses as a criterion of the ability of the rubber insulation waxes of the weather-proofing compound may tend to diffuse sufficiently to resist cutting by the underlying conductor. into the insulation, thereby softening it and diminishing its This test, called a “penetration test” (sometimes known as resistance to cutting. The test described in this paper de- a “hardness test”), is as follows: The rubber insulation shall pends on this resistance to cutting. resist being cut by a taut steel wire of definite size resting on the insulation a t right angles to the conductor when a Usual Mechanical Tests specified load is applied for a specified time. Establishment Tensile strength tests are often made on test strips of the of electrical contact is used to indicate complete penetration wire insulation. Unless the wire or cable is very large, of the cutting wire. To judge whether or not the insulation is too brittle for satisfactory service, it is given a wrap test. 1 Presented before the joint meeting of the Division of Rubber Chemistry That is, the wire, after the braid is removed, is wrapped and the Akron Section of the American Chemical Society, Akron, Ohio, about itself three times, left for 16 to 24 hours, and rewound February 22 and 23, 1926.
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INDUSTRIAL A N D ENGINEERISG CHEMISTRY
again in the reverse direction. The insulation is required to show no signs of rupture. The penetration and wrap tests are not entirely satisfactory. Neither test affords a numerical measure of the relative quality of one sample as compared with another, and consequently we are unable to detect reliably samples which may be close to failure and which would probably fail after a relatively short life, owing to the gradual deterioration due to natural aging processes. The final answer is obtainable only after a period of aging during which the penetration test is applied a t set intervals. An endeavor was made to provide a means of expressing the results of the penetration test in numerical terms, either by noting the time required to cut through the insulation under a given load or by determining the load required to cut through in a given time. I n both cases a high degree of inconsistency mas observed. The inconsistencies are attributed partly to actual variations in the physical properties of the material underlying the wire in successive repetitions of the test. Thus, one of the inherent defects of this penetration test is that the sample submitted to test is very small, being limited to rubber lying immediately under and about the cutting wire. Consequently, variations in resistance to penetration may be due to single filler particles or aggregates of filler particles forming hard spots, or to soft spots caused by localized high concentration of wax or other softeners. Also,. part of the quantitative inconsistency of the penetration test is due to failure to make a positive electrical contact, even when the rubber wall is cut, on account of the presence of reaction products of the insulating material with the metal.
Vol. 18, No. 4
when in service. The samples can be prepared and the tests made in a reasonably short time, and, therefore, it seems feasible as a routine inspection test. This test has the additional merit that it can be applied to the insulation in place on the wire rather than to a fragment of the insulation. The first tests were made on an available testing machine on which means were provided for applying and measuring compressive loads and changes in thickness. But this machine, not especially suited to this purpose, made the method cumbersome. Therefore, a self-recording machine
Proposed Test A 2-inch length of insulated wire, preferably after removal of the braid if there is any, is compressed between steel blocks with parallel plane faces which are made to approach each ohher gradually at a uniform rate. Means are provided for reading or recording the pressure on the insulation and its thickness simultaneously. I n making tests of this sort a very characteristic performance is observed. As the blocks or jaws approach each other the insulation is compressed around the conductor into an elliptical form, thus subjecting the thin films of rubber between the wire and the faces of the jaws to a stress which ultimately causes rupture. During this process the pressure rises at first slowly, then rapidly till the film ruptures and the conductor is suddenly exposed. At this point the pressure drops off sharply and indicates that rupture has occurred. If the insulation does not fail on both sides a t the same time, ill be two maxima of this sort. The compressive then there w load a t the maximum point is a numerical measure of the ability of the insulating compound to resist cutting by the conductor. Values of as much as 1100 to 1200 pounds have been recorded for certain medium grade insulations. It is found also that the insulating film is very thin a t the time of rupture in the case of a high-grade resilient compound, but in the case of a brittle compound the rupture occurs before the rubber has undergone much reduction of thickness. This relation between thickness a t break and brittleness of the compound seems to afford a measure of the ability of the insulating compound to stand severe bending. Unfortunately, there have been available very few samples of wire which failed in a wrap test, but in those few cases the thickness a t rupture was much greater than normal. By this method a much greater portion of insulation is tested than by the penetration test, and in a way which is believed to be closely comparable with the conditions of strain occurring in the insulation a t points of support, etc.,
Figure 1
was sought with which the tests could be performed with a minimum of time and effort. Moreover, it was considered desirable for inspection purposes to have a light-weight machine which could be carried about by hand. The machine finally adopted is pictured in Figure l. The load is applied by means of the screw through a heavy compression spring inside the cylinder to the upper jaw. The deflection of the spring under load causes a proportional rotation of the cylinder by means of a rack and gear. The simultaneous changes in load and thickness can therefore be followed during a compression test by the line produced by the recording needle on an appropriately arranged chart, such as is illustrated in Figure 2. I n fact, a sort of stress-strain curve is plotted. I n making a test the chart is placed in a fixed position on the cylinder, such that when the jaws are just closed on a bare conductor without pressure the needle indicates zero load and thickness. A 0.033-inch movement of the recording needle vertically indicates a change of 0.001 inch in thickness of the insulation under compression. A movement of 0.3 inch horizontally on the surface of the rotating cylinder indicates a change in pressure of 100 pounds. Rupture of the insulation is indicated by a sudden drop of the needle, such as will be observed in Curve B. The cross indicates where rupture occurred. After compressing a sample in one direction a test may well be made on an adjoining section after rotating the wire
IAVDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1926
through an angle of 90 degrees. Divergence between the two curves tends to indicate the degree of eccentricity of the conductor with respect to insulation. Figure 2 shows typical curves that are obtained for different classes of rubber or other insulating compounds. Curve A is typical of a highly distensible, high-grade rubber compound. Curve B is typical of the average run of rubbercovered aerial wire compounds, such as have been tested in the work to be described later. Curve C is typical of a brittle compound, and Curve D shows the type of curve which may be expected if a soft, semiplastic rubber compound I , ,
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is quite comparable with that attainable in other physical tests of rubber compounds. Application
As a practical test of the utility of the machine and method, the compression test has been consistently applied over a period of 4 or 5 months to regular current receipts of samples of a certain type of mire, furnished by seven suppliers for inspection purposes. The insulation has a nominal wall thickness of l/s2 inch. The conductor is a 17-gage wire (0.045 inch diameter). The compounding formulas for the insulation are unknown to the writer, but represent medium grade insulation. I n addition to applying the compression test to the freshly received samples, the accelerated Geer aging test has been applied to them for 7 days and the compression test repeated thereafter. Natural aging tests are also being conducted, followed by compression tests a t regular intervals. Some typical results obtained with the product of three suppliers are shown in Table 11. Table I1
THICKNESS AT RUPTURE-INCHES After 7 Reduction After 7 Change in days inload days thickness Geer on aging Geer on aging aging Per cent As rec'd aging Per cent 830 0.030 0.028 640 0.019 0.017 740 7.9 0.023 0.023 0.0
L04D 4T RUPTURE
MASLFACTURER As rec'd Max. 950 Figure 2
or cold-flowing insulating composition other than rubber is tested. I n the last case there is no point of sudden rupture, but the insulating material is gradually pushed away from the conductor. The consistency of results when repeated tests are made on adjacent sections of the same wire is shown in Table I. Table I Load a t rupture Expt. 1 2
Lbs.
1240 1200 1180 1270 1180 1190 1160 1230 1180 1140 1197 i5.5
k?
A
(Mas. Vin. iv.
B
66Y
825
0.033 0.019 0.027
0.030 0.016 0,024
11.1
0.028 600 580 Vin 410 420 0.017 kv. 502 495 1.0 0.022 The nearest value to this one was 700 Ibs.
0.030 0.017 0.022
0.0
1
1170 900 1020
1050 720 863
15.4
(Mas.
I
C a
Conclusion Thickness at rupture Inches 0.0185 0.018 0.018 0.0175 0.019 0.0195 0.0185 0.019 0.0195 0.020 0.0187 16.7
Much of the observed variation is probably due to nonuniformity in thickness of the insulation. The agreement
On the whole, the results have been consistent and intelligible, but the machine in its present form has some mechanical defects, and a redesign is contemplated. Better insurance of mechanical stability over long periods of use of the machine and increased sensitivity as regards measurement of thickness a t rupture are desirable. It should be understood that the values for load a t rupture will vary, not only with the quality of the compound, but also with the dimensions of the wire, so that ultimately suitable standards will have to be set for different sizes of wire to be inspected.
Program of the National Committee on Wood Utilization Secretary Hoover, chairman of the National Committee on Wood Utilization, has called a meeting of the committee for April 28 in Washington, D. C., t o consider a wide range of suggestions for undertakings in the interest of better utilization of wood and wood products. This committee, which was established by direction of President Coolidge, is composed of important consumers, distributors, and manufacturers of wood and wood products. The vice chairman is William 13. Greeley, Forester of the United States. The aim of the committee is t o promote a more efficient utilization of wood, and its scope covers practically every phase of manufacture, distribution, and consumption. It has adopted as its slogan, "Utilize wood and save our forest." According t o Axel H. Oxholm, director of the committee, proper utilization of the greatest portion of every tree felled would mean an extension of the life of our forest resources until our second-growth forests mature. The failure properly to utilize our forest products has caused a heavy drain on our timber stands. Heretofore, taking the country as a whole, we have marketed from 25 t o 35 per cent of the standing tree. This has been largely because most consumers still adhere to the specifications and the quality require-
ments which pevailed when timber was cheap and plentiful. Convinced t h a t every part of the tree can eventually be used either as lumber or manufactured wood products, the chief aim of the committee will be to create such conditions that the market will absorb this production. I n order to do this, its work will extend into the manufacture of lumber, pulp, paper, wood chemicals, naval stores, charcoal, composition board, and other by-product possibilities. The broader standardization of grades and sizes, trade extension, educational efforts, and the application of scientific findings to production and marketing of wood products will be considered. The development of the program will be undertaken by subcommittees composed of leading technical and practical men in each special field. The majority of the manufacturers of lumber and wood products have indicated t h a t they are eager to cooperate in this movement. The committee has also the active cooperation of the Forest Products Laboratory of the Bureau of Standards. Scientific facts developed by these and other institutions will be made the basis of the committee's recommendations, and the public will receive impartial and reliable information in regard to improvements in wood-using practices.
