I S D U S T R I A L A S D ESGISEERISG CHEII.IISTRY
292
T’ol. 19. s o . 2
Deterioration of Mineral Oils’*’ I-Mechanism
of Oxidation and Action of Negative Catalysts as Determined by a Dynamic Method By R. T. Haslam and Per K. Frolich
DBPARTMBNT OF CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAXBRIDGB, MASS.
ROM a practical point of view the oxidation of mineral oils deserves considerable attention. The products of oxidation, fatty and naphthenic acids and complicated condensation and polymerization products known as sludge, may seriously affect the essential properties of the oil. Probably the most detrimental effect of this type is encountered in the case of transformer oils, where the sludge deposits on the coolest parts of the transformer, thus lowering the efficiency of heat interchange. Simultaneously a considerable decrease in dielectric constant of the oil is caused by the water liberated during the oxidation. Thus, 0.06 per cent of water lowers the dielectric constant about 50 per cent. Concerning the mechanism of oxidation of hydrocarbons little information is available, and this lack of knowledge is pointed out by brook^.^ The researches performed in England and germ an^,^ during the war, do not throw much light on the problem. From experiments on transformer oils several investigators5 have found that copper catalyzes the oxidation, and Staeger5suggests that the reaction may proceed in two different ways-by the intermediate formation of peroxides, where unsaturated compounds are present, or by the direct oxidation of saturated compounds. I n general, the opinion is held that the oxidation of oils is
F
Oxidation Test and Methods of Analysis The highly refined Parke-Davis oil was chosen for the initial studies on oxidation in order to reduce the amount of impurities whose presence might influence the rather complicated reactions. An accelerated oxidation test was used, making it possible to obtain a reasonable rate of oxidation without the use of excessive temperatures. The standard method employed consisted of bubbling dry oxygen at a rate of 10 liters per hour through 75-gram samples of oil contained in large test tubes submerged in an oil bath. The temperature of the oil samples was kept constant either a t 130” or a t 140” C. Samples were withdrawn a t intervals for analysis. The acid resulting from the oxidation was determined by titration with 0.1 N potassium hydroxide6 and calculated as per cent oleic acid. On account of the small samples available the viscosity was measured by the procedure of Lang,’ the time of outflow being observed through a glass tube. I n computing the values given below, the viscosity of unoxidized Parke-Davis oil was chosen as unity. Effect of Negative Catalysts on Oxidation A number of substances, some of which are known to exert an anticatalytic effect on various oxidation reactions,8
Figure 2-Effect
an autocatalytic process, the products of reaction being catalysts for the further oxidation of the hydrocarbons.
of Negative C a t a l y s t s on Oxidation. 130’
* 1’
C.
Temperature,
have been tested to see whether they are also negative catalysts for the oxidation of mineral oils. As will become apparent from results reported later in this paper, the acidity of this purified oil may be considered a good indication of the progress of oxidation. I n Figure 1 is plotted per cent acid against time of oxidation for pure Parke-Davis oil and for oil to which was added a typical negative catalyst, diphenylamine. The two sets of curves
Presented under the title “Mechanism 1 Received August 28, 1926. of Dynamic Oxidation and Action of Negative Catalysts” before the Division of Petroleum Chemistry a t the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 5 t o 11, 1926. 2 The experimental work reported in this paper was carried out during the period June, 1923, t o February, 1924. 8 “The Non-Benzenoid Hydrocarbons,” Chemical Catalog Co., Inc., I Griffin, “Technical Methods of Analysis,” p. 242, McGraw-Hill Book New York, 1926. Co., New York, 1921. 4 Fischer, Ges. Abhandl. Kenntnis Kohle, 4 , Berlin (1919). 1 Walker, Lewis, and McAdams, “Principles of Chemical Engineer5 Staeger, Helv. Chirn. Acla, 4, 62, 386 (1923); Waters, THISJOURNAL, ing,” p. 80, McGraw-Hill Book Co., Kew York, 1923. 13, 901 (1921); Kissling, Chem.-Ztg., 30, 932 (1906); 31, 328 (1907); 32, 8 Moureu and Dufraisse, Chem. Reu , 3, 113 (1926). 938 (1908); 33, 521 (1909).
ISDUSTRIAL AND EXGINEERIiVG CHEMISTRY
February, 1927
show that the method of oxidation employed gives entirely satisfactory results as far as reproducibility of individual runs is concerned. Figure 2 gives the oxidation curves obtained with 0.01 per cent of various substances added to the oil. As the point a t which oxidation starts is difficult to asrertain, the almost identically shaped curves are best compared a t a certain x-ell-defined acidity, say 0.25 per cent, acid. I n Table I yarious organic compounds investigated are listed in order of increasing inhibiting effect. IO
8
J
z 0 u 4
s 2
0
2
4 6 8 HOURS OF OXIOATON
IO
I2
F i g u r e 3-Effect of Positive Ca:alysts on Oxidation. T e m p e r a t u r e , 130 * lo C.
From Figure 3 it is seen that ferric oxide and various salts of copper are positive catalysts for the oxidation. The effect of a positive catalyst may be counterbalrinced to a certain extent by a negative catalyst when present simultaneously. Thus, while metallic copper caused oxidation to start 1 hour earlier, the presence of 0.01 and 0.05 per cent diphenylamine together with the copper resulted in 2.5 and 3 hours' incubation, respectively. On the other hand, when compared with the data in Table I, these figures show that the negatiye catalytic effect of diphenylamine is lowered considerably in the presence of copper. T a b l e I-Effect
catalytic action, The substance added may be (1) oxidized and thus lose its protective power; (2) evaporated directly without being subject to oxidation; (3) partly destroyed by oxidation, partly removed by evaporation-i. e., a combination of (1) and (2); and (4) lessened in activity, as a result of reactions caused by heat-for instance, intramolecular condensation or polymerization in the catalyst itself or reaction between catalyst and oil. Furthermore, it is possible that ( 5 ) a positive catalyst is gradually built up in the system. The elaborate experiments made to decide which of these mechanisms was involved necessitated runs where oil samples containing different catalysts were subjected to heat alone for various lengths of time, and runs in which nitrogen was substituted for oxygen and later again superseded by oxygen. The results of this work may be summarized thus: p-Aminophenol is a typical example of a negative cata!yst that is removed by evaporation (Type 2 ) . Diphenylamine is mainly destroyed by heat (Type 4). Diphenylhydrazlne is partly removed by evaporation and partly destroyed by oxidation (Type 3).
6
a
293
of C a t a l y s t s on Oxidation of Parke-Davis Oil a t 130' C. COMPOUND
DELAYI N OXIDATION CAUSEDBY 0.01 GRAM CATALYST
Hours Phenyl isocyanide +Toluidine 1,2,3-XyIidine Quinoline Diphenyl Aniline Tolidine Phenylhydrazine Hydroquinone Oxanilide Diphenylguanidine B- A-aphth ylamine Ethyl-a-naphthylamine Methyl-a-naphthylamine a-Xaphthylamine p-Aminophenol Diphenylamine Phenyl-a-naphthylamine Diphenylhydrazine (unsymmetricalj
-1a
-0.5 -0.5 -0.5 -0.5 0 0 0 0 0 1 3 3 4.5 4.5
5 5.5 11 18
a The minus sign indicates positive c a t a l y s i s i . e., oxidation starts earlier than for the pure oil.
Behavior of C a t a l y s t d u r i n g Oxidation
All the curves in Figure 2 run nearly parallel. The reaction, when once started, always proceeds a t the same rate whether the oxidation starts a t once or has been delayed for a certain length of time by a negative catalyst. Without going into a discussion of the mechanism of negative catalysis, which is but little understood, various explanations may be given for the cessation of the negative
From an organic chemical point of view these various explanations for the behavior of the different catalysts seem quite sound, since compounds so different in chemical and physical nature mould not be expected to behave alike. With 0.01 per cent, p aminophenol oxidation za is delayed for 5 hours. I n one run new addi84 tions were made every 2 hours and the reaction 2o had not started even after 12 hours, a t which time the experiment g 16 was discontinued. On z the other h a n d , t h e 4 ,2 p e r i o d of incubation .* c a u s e d by diphenyl' amine was not materially altered by the addition of further amounts of this compound, presumably because the 8 16 24 32 46 new portion was added '0 HOURS OF OXIDATION t o o late-i. e., after F i g u r e &Oxidation Curves f o r Oil w i t h oxidation had actually and- w i t h t u t Catalysts. T e m p e r a t u r e , 130' * 1" G . commenced. Effect of Different Variables o n Oxidation
The susceptibility to oxidation is somewhat dependent upon the previous history of the oil. Thus, oil from tin containers oxidized more rapidly than samples taken from glass containers, and the older the sample the more readily it oxidized. Blank experiments were therefore made whenever oil from a new container was taken into use.g Prolonged heating made the oil more susceptible to oxidation. Thus, when a sample of oil was preheated with nitrogen for 24 hours a t the temperature of the oxidation test (130' C.), the period of incubation caused by diphenylhydrazine was reduced from 18 to 6 hours. Oxygen saturated with water vapor a t room temperature gave the same rate of oxidation as did dry oxygen. This is contrary to what has been observed in the drying of linseed 0i1.3 Table I1 shows a comparison between the oxidizing power of oxygen and air. At the start the rate of oxidation with One-gallon tin cans of oil, supplied by the Eastern Drug Company, were used for all the experiments reported in this paper.
294
INDUSTRIAL A N D ENGINEERING CHEMISTRY
bery's method of manufacturing artificial asphalt by oxidation of petroleum residues.3
oxygen is about five times that with air, corresponding to the fivefold increase in partial pressure. As the reaction proceeds, however, the effect of oxygen becomes more marked.
Distribution of Oxygen between Acid and Other Products of Oxidation
Table 11-Oxidation of Parke-Davis Oil w i t h Oxygen a n d Air a t 130' C. Hours of run 4 6 8 10 Ratio of acid formed with oxygen to acid formed with air 5.1 6.1 6.7 7.8
Evidently the increase in viscosity during oxidation is due to condensation or polymerization reactions resulting in the formation of more complicated molecules. The parallelism between rise in acidity and rise in viscosity therefore suggests that i t is some product of oxidation which is subject to condensation or polymerization. Determination of total oxygen by ultimate analysis of samples of both oxidized and unoxidized Parke-Davis oil show this to be true. Thus from the data in Table I11 it is concluded that only a fraction of the oxygen is present in the oil in the form of acid, the remainder being bound in some other form.
As might be anticipated, an increase in temperature results in increased rate of oxidation. Change in Viscosity during Oxidation
The S-shaped oxidation curves in Figure 4, obtained over a period of 40 hours, are typical for so-called autoxidation reactions. The curves for both pure oil and for oil containing 0.01 per cent diphenylamine reach about 26 per cent acid asymptotically, while the sample with 0.05 per cent diphenylamine does not get above 19 per cent acid. Up to about 16 per cent acid the three curves run parallel. The viscosities of the same three oil samples are plotted against time in Figure 5 . The striking similarity between the acidity and the viscosity curves at once becomes apparent. I n no case does the viscosity commence to increase until after the acidity has started to build up. After oxidation has once started, the viscosity increases in much the same way as does the acidity, but more rapidly. Thus, by plotting
VOl. 19, s o . 2
Table 111-Ultimate
Analyses of Unoxidized a n d Oxidized ParkeDavis Oil OXYGEN
SAMPLE
CARBONHYDROGEN acid
Per cent Unoxidized oil 85.7 Oxidized oil 76.7 Oxidized oil 77.2 a
Per cent 13.1 10.9 11.6
Per cent 1.2" 12.4 11.2
RATIO OXYGEN A S ACID TOTAL OXYGEN
P e r cent 0.0 3.54 1.95
...
0.29 0.17
Oxygen determined by difference.
The fact that formation of acid and increase in viscosity seem to set in simultaneously further suggests that substantial oxidation does not start until an increase in acidity can be detected. I n order to investigate this point a 13-hour run was made in the presence of diphenylhydrazine, which apparently does not give any acid until about the eighteenth hour. This oil, together with a sample oxidized for the same length of time without a catalyst, was analyzed for both acid and total oxygen. I n Table IV is also given the analysis of the pure oil before heating or oxidizing. About one per cent of oxygen (as determined by difference) is present in pure Parke-Davis oil. No additional oxygen could be detected in the sample in which the formation of acid had been inhibited by diphenylhydrazine. It must be concluded, therefore, that the two processes of oxidation to acid and to asphaltic bodies take place almost concurrently. Furthermore, the change in color points to the same conclusion. The perfectly colorless oil always turned faintly yellow a t the time when acid formation could be detected. As the oxidation proceeded the color grew darker and darker, but in no case did the coloration appear before acid was formed. Table IV-Ultimate Analyses of Unoxidized Oxidized a n d CatalystTreated Parke-Davis Oil, t h e Last S a m p l e ?aken durihg Incubation ~~
~
~
SAMPLE Figure +Change in Liscosity w i t h Time for Three Oil Samples f r o m Figure 4
the logarithm of viscosity against per cent acidity a straight line results (Figure 6). This relation no longer holds true after the acidity has come close to its constant value-i. e., when the upper bend of the oxidation curve is reached. From then on the viscosity increases at an enormous rate, while the acidity barely changes. The rise in viscosity is accelerated considerably more than the increase in acidity as the temperature is raised. This is apparent from the change in slope of the curves for the logarithm of viscosity versus per cent acidity obtained at different temperatures. The fact that condensation reactions rather than development of acidity predominate a t higher temperature is made use of in Byerley and Ma-
Unoxidized oil 13 hours' oxidation run, no catalyst 13 hours' oxidation run with 0 . 0 1 per cent diphenylhvdrazine
CARBON HYDROGEN
OXYGEN Total
As acid
Per cent
Per cent 85.7
Per cent 13.1
Per cent 1.20
84.9
12.6
2.5
0 57
85.7
13.3
1.0
0 0
0.0
~~
a Oxygen determined by difference.
That the condensation or polymerization products are not built up directly from initially formed acids was proved by experiments in which partly oxidized samples were subjected to heat in the absence of oxygen. No change in acidity or viscosity was observed. The same observation was made when nitrogen was substituted for oxygen after oxidation had started, the other conditions of the accelerated oxidation test being maintained constant. It is not possible, however, to conclude from these experiments whether or
February, 1927
1,VDUSTRIAL ,4ND ENGINEERING CHEMISTRY
not asphaltic bodies are also formed by condensation and polymerization of compounds which may ensue from further oxidation of the acids. Extensive tests showed that aldehydes are not stable in oxidized oils. (1) All samples of heated oils, with or without oxygen, both with and without the catalyst present, showed negative tests with ammoniacal silver nitrate solutions, indicating the absence of aldehydes. ( 2 ) A small amount of acetaldehyde was added t o these samples and they were again tested for aldehydes as above, with a negative result. (3) Two drops of acetaldehyde in alcohol solution gave gocd silver mirror test, showing the sensitivity of the above reaction in the absence of oil. (4) Two drops of acetaldehyde with 1 cc of pure oil which had not been oxidized or heated gave a good silver mirror with ammoniacal silver nitrate. ( 5 ) Five grams of pure oil in an Erlenmeyer flask, not heated or oxidized, with 50 cc. alcohol and 5 drops of acetaldehyde, gave a silver reaction, and showed the presence of the aldehyde. (6) Five grams of pure oil in a n Erlenmeyer flask were heated, and oxidized for 8 hours and the acid formed neutralized. Fifty cubic centimeters of alcohol and 5 drops of acetaldehyde were added and a negative aldehyde test obtained, but the oil turned a little darker than the solution in ( 5 ) . ( 7 ) Experiment (ij),repeated but not neutralized, gave same resiilts as (6). ( 8 ) Oil which had been previously heated was placed in an Erlenmeyer flask and the oil was oxidized and neutralized. Five drops of acetaldehyde which had also been neutralized were then added. After mixing and heating, the oil was titrated t c see if any acid had been formed but none was found.
295
certain conclusions as t o the mechanism of the oxidation process. 1-The start of oxidation is manifested by change in color and by increase in acidity and viscosity. A change in one of these properties is invariably connected with a corresponding change in the others. While the dynamic method did not give consistent results for lubricating and transformer oils, i t was shown that any one of the tests for acidity, viscosity, coloration, or gum formation would serve equally well as an indication of the progress of oxidation in one set of experiments. 2-The rate of oxidation increases with elevation of temperature and is about five times as rapid with pure oxygen as it is with air. The susceptibility to oxidation is somewhat dependent on the previous history of the oil 1
0
800
0
0
!
,
,
!
!
,
!
,
1
i
1
l
1
l
1
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-,
""I
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Aldehydes, when added in the absence of oxygen, disappear immediately without the formation of a proportional amount of acid, but with a darkening of the color of the oil. Hence, aldehydes, supposedly necessary intermediate products of oxidation of hydrocarbons, are, as soon as formed, condensed into asphaltic compounds or further oxidized to acids by excess oxygen. Xlechanism of Oxidation Process
On the basis of Bone's hydroxylation10 theory and the data here presented it seems safe to represent the oxidation of mineral oils by the following scheme: h ydrocarbons-+alcohols-aldehydes ketones
J.
naphthenic and fatty acids
J
condensation and polymerization products (asphaltic bodies)
As a further possibility may be mentioned the continued oxidation of the acids followed by condensation and polymerization of the resulting products into asphaltic bodies. The role played by intermediate acids in the formation of asphaltic compounds has been emphasized by Marcusson. According t o Bonelo alcohols are the first products of oxidation of hydrocarbons. The initially formed alcohol, however, is more susceptible to oxidation than the hydrocarbon itself3 and further oxidation to aldehyde occurs immediately. Hence, the effect of the negative catalyst is to delay the initial step in the above scheme-i. e., the formation of alcohol. This conclusion is justified in view of the fact that no oxygen is taken up by the oil as long as the catalyst remains active. Conclusions
A detailed study of' the oxidation by a dynamic method of highly refined (Parke-Davis) mineral oil has made possible 10 Haslam and Russell, "Fuels and Their Combustion," p. IS1, McGrawHill Book Co.. N e w York, 1926. '1 Z . ange?,'. C h e m . , 29, 346, 3-19 (1916); Miil. kgl. .liiilerialpvufun,osnml, 36, 209 f l O l S ) .
0
5
15
10 a/a
20
25
30
ACID IN aiL
Figure 6-Variation of Viscosity w i t h Acidity for Three Oil Samples from Figures 4 and 5
and is increased by preheating the oil even in an atmosphere of nitrogen. Moisture does not affect the rate of oxidation a t the temperature of the accelerated test (130" C.). 3--Ultimate analyses indicate that no oxygen is taken up by the oil until the oxidation can be detected by either one of the above variables. 4-Only a fraction of the oxygen is present in the oil combined as acids, the remainder being in the form of asphaltic bodies resulting from condensation (polymerization) of intermediate products of oxidation. With increasing temperature the viscosity increases more rapidly than does the acidity, probably because relatively more condensation takes place. 5-Aldehydes are not stable in the oxidized oil, but are immediately condensed to higher molecular compounds or are further oxidized to acids. 6-Acids do not form asphaltic products directly-i. e., without further oxidation. The viscosity and acidity of the oil do not change when an oxidized sample is maintained in the absence of oxygen even at a temperature of 130" C.
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I-VDUSTRIAL A N D ENGINEERING CHEMISTRY
7-A number of organic compounds have been found to act as inhibitors of the oxidation; others have been found to act as positive catalysts or not to affect the process at all. I n the case of negative catalysis, the oil apparently remains unchanged during a period of incubation. After this period, the oxidation proceeds as for the pure oil. 8-On the basis of the above conclusions a scheme for the mechanism of the oxidation of mineral oils is proposed. g-The stage of the oxidation which is affected by nggative
Vol. 19, KO.2
catalysts is shown to be the initial oxidation of hydrocarbons to alcohol. Acknowledgment
Thanks are due the Standard Oil Company of h’ew Jersey for permission to publish these results. Acknowledgment is made of the assistance rendered by G- Calingaert and J. Teppema in obtaining some of the experimental results.
Some Methods of Studying Cord Tire Fabric‘ By F. W. Stavely and N. A. Shepard FIRESTONE TIRE& RUBBERC O . , AKRON,OHIO
Hysteresis loss and flexing life are used as a measure Cord break in pounds and N THE early development elongation a t break are not of fatigue in tire cords. Hysteresis loss shows that the of pneumatic tires, the necessarily sufficient for the original properties of the cord are not maintained. The principal fabric used in proper evaluation of a fabric. flexing test subjects the cord to repeated stresses and in tire construction was squareGurney and Davis4 have emthe event of cord fabric failure in tire service is of value w o v e n . It was not until phasized t h a t “cord elasticity, as a means of determining the desirability of changes in or ability to return after stressabout 1915-16 that the ading t o its original dimensions, is cord construction and of developing methods of imvantage of the present type not necessarily involved in tenproving the flexing life of a given type of cord. of cord fabric construction sile strength and elongation a t Improvement in flexing life of a given cord may be was definitely e s t a b l i s h e d. rupture.” All of the above inbrought about by impregnation with rubber cements The cord tire fabric differs vestigators also called attention containing compounding and vulcanizing ingredients. to the fact that failure of a cord from the square-woven fabric For uniform impregnation with maximum penetration, or fabric in tire service follows in that the cross threads (filler from the cumulative effect of the solvent or liquid carrying the rubber should readily threads or weft) have been f r e q u e n t 1y applied stresses. wet the cotton cord. When well impregnated each of n e a r l y e l i m i n a t e d . The Gurney and Davis place some the strands constituting a cord is completely covered warp threads or cords in cord emphasis on the effect of permawith rubber, as can be demonstrated by subjecting fabric are so spaced that there nent set in a cord, for they call single strands to the action of sulfuric acid. “Road attention to the fact that the are from 202 to 26 ends per greater the ratio of percentage tests” indicate that in the event of fabric failure inch while there are only 2 to elongation to percentage set a t the mileage of certain tires can be increased by increas6 weft or filler threads per a given load, the better the reing the flexing life of the fabric. inch. The cord itself may covery for a given elongation. consist of as manv as fifteen Hysteresis as a Fatigue Test strands, twisted into a single unit, while the filler thread consists of a single strand. This makes a very loosely conIt should be pointed out, in any study of cord elongation structed fabric, consisting of cords that are held together and permanent set, that the relationship of elongation (stretch by the small and sparsely distributed filler (weft) threads. and elasticity) to permanent set (lost stretch) during the The introduction of the cord fabric eliminated the chafing first cycle in which a cord is subjected to a given load or series and sawing action of the ply yarns which cross each other of loads is quite different from that of the subsequent cycles. in square-woven fabric and also made it possible to insulate While this is true with many materials it is very pronounced the cords completely from each other with rubber. This in cotton cords since these consist of a Iarge number of cotton change was fundamental in improving the quality of the tire fibers twisted together, I n Figure 1 are found the hysteresis carcass and made it possible to double and even triple the loops of the first and second cycles during which the same life of the pneumatic tire. cord5 was subjected to a series of loads. A maximum load of Methods of studying cord tire fabric have received little 4.99 kg. (11 pounds) was used, since in some cases greater attention in technical publications, regardless of the im- loads caused the cord to break before two cycles were comportance of this material in tire construction. pleted. The change in elongation with each increment in Considerable credit is due to King and Truesdalea and also load was measured by means of a steel rule. The weights to Gurney and Davis4 for their recent attempts t o focus attention were applied by hand and remained on the cord for 2 minutes on the testing and construction of cord tire fabrics. The former before the elongation was measured. Zero elongation of have pointed out the striking difference in the stress-strain dia- the cord was taken at a load of 20 grams. The cord was grams of cotton yarn and mild steel. They have also emphasized the probability of a relationship between the permanent set and prevented from untwisting by means of a rubber band, one end of which was attached to the load and the other end to fatigue of a cotton cord. the upright of the stand supporting the cord. The curve 1 Presented before t h e Division of Rubber Chemistry a t t h e 72nd representing the second cycle was obtained by correcting for Meeting of t h e American Chemical Society, Philadelphia, Pa., September the permanent set (lost stretch) of the cord after the first 5 t o 11, 1926. cycle. Although the original length of the cord under con2 T h e number of cords per inch in cord fabric should not be confused sideration was 25 cm. (at a load of 20 grams), after the first with t h e yarn size. A 23/5/3 or 13/3,”3 cord fabric, in which the yarn size is 23 or 13, respectively, may for example contain 20 t o 26 ends per cycle had been completed and the cord was again subjected inch. to the initial load of 20 grams, it was found that the length
I
Textile Recorder, 41, 88 (April), 86, 99 (June), 103 (July) (1923). +India Rubber J., 70, 267 (1928).
5
Carded Egyptian (23/8/3); ply twist, 17, cord twist,
7’/2
