INDUSTRIAL AND ENGINEERING CHEMISTRY
2416
Connors, J. S.,and I l i l l e r . .A .J. P c ~ w ~Processing, c ? ~ 5, 29 (1950). Gordon, .J. A.. Peiidei~ni LIW..25, .i-74, T i , 80, 81, 83, 86
(ly. Hutchiiisoii. A. J . L.. U. S. I’atent 2,177,068 (1939). .Jones. G . 13.. Langs,ioeii, .I., Seuiiiann, AI. 31. C.. and Zomlefer, , (’hem., 9, 125 (19441, , a n d I3lolilrl. C’. I,..P P t i i E f b g . . 22, No. 6, C-37
Kruger. 11. (I., and .\[u , .J. I-lon xirfacee by t 8 h c x sliding process. The initial and the constant find values of p s and the corresponding coriat,arit value of I*k are givcw in Table 1. If the slider is made to imverse the same pat,h a second tirw in the same direction without removal of deposits or abrasion oi the surfaces between traverscs, the valuei: of p8 :tnd j i k remain essentially constant throughout the run. The resulting values of and pk tire also given in Table I. Both viiliies are less than the
November 1954
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
'rABLE
I.
cOEFFICIENTS O F FRICTIOX FOR N Y L O N ON r\JYLoh'
- Static Coefficient of Fi,iction Lubricant Dry Water Ethylene glycol Glycerol n-Decane n-Hexadecane Perfluorolube oil Perfluorotrihexvlamine
Source Eastman, practical U.S.P.
Connecticut Hard Rubber Co. Eastman, white label Du Pont, b.p. 130°-1500 C./lO mm. Minnesota Mininn and Mfa. . Co. .(research sample) n-Xonyl alcohol Eastman, white label Pelargonic m i d Eastman, white label Oleic acid Eastern Regional Research Laboratory, U. 8. Dept. Agriculture Stearic acid in hexadecane (approx. Acid and solvent, Bastman, white label 0.1 wt. %) pri-n-Octadecylamine in hexadecane Smine, Minnesota Mining and Mfe. Co.; solvent, Eastman, white label (approx. 0.1 wt %) Perflnorolauric acid in decane (ap- Acid, Minnesota Mining and Mfe. Co: solvent, Connecticut Hard Rubber Co.' prox. 10-5 mole 7%) Dow Corning Corp., viscosity 50 os. a t Polyinethylsiloxane DC 500
".
9, x0 P '
Polymethylphenylsiloxane DC 610 Polymethylphenylsiloxane DC 550
Daw Corning Corp. (low phenyl), >iscosity 50 os. a t 25' C. Dow Corning Corp. (medium phenyl), viscosity 100 os. a t 25' C.
va!ue finally obtained a t the end of the first traverse over the clean aurface. I n this experiment the friction was further reduced since both the slider and plane surface had previously been work hardened or oriented by the first traverse. If the same slider (without being cleaned or reabraded) is made to traverse a clean, newly abraded area, the stick-slip values are nearly constant throughout the run, and the values of pa and u h are practically equal to the the final values obtained with the two freshly abraded surfaces. The sliding of steel on nylon produced smooth rather than intermittent sliding with values of p s and ~h of 0.37 and 0.34, respevtively. However, pk gradually increased with sliding distance and approached 0.37. The results of measuring friction under the three conditions dewribed were found to be practically identical. Previous studieb of steel sliding on polyethylene demonstrated that the steel slider became covered with an o r i e n t d film of polyethylene ( 6 , 6). It is believed that there occurred a similar but lesser transfer of oriented polymer with steel sliding on nylon. The effect of the transfer of polymer on the friction is not readily measured in these experiments because the coefficient of friction for oriented nylon on a steel ball against nylon is nearly equal to the coefficient for a clean steel ball on nylon. The coefficient of friction bctween two unlubricated materials may be approximated by thP ratio of the shear strength, s, to the yield pressure, p , of the softer material, provided that during the sliding the shear occurs nithin the bulk of the softer inaterial rather thsn a t the junctions of the asperities (18). This requires that the strength of the junctions be greater than the shear stiength of the softer material. Other considerations are the difference between the apparent area and the real area in computing.the yield pressure and the difference between the shear strength measured with a bulk piece and the actual shear strength under the conditions existing during the friction measurements. The s / p ratio for nylon is 0.6. This would be the expected static coefficient of friction for both steel on nylon and nylon on nylon if the shearing occurred in the bulk of the plastic and if the real yield pressure and actual shear strength could be measured accurately. Since the measured values of pe (0.37 and 0.46, iespectively) are much lower than the predicted value for each combination, it is likely that there is a t least, some shearing a t the junctions, LUBRICANTS FOR NYLON SURFACES
When stick-slip frictional motion occurs, the relative rate of movement between the slider and the plate during the slip may be of the order of a few centimeters per second, whereas the velocity of the hydraulically driven table holding the plane speci-
New Constant 0.46 0.52 0.58
(fii.)
0.60 0.70 0.35
0.36 0.37 0.34 0.58 0.43
Used slider repeated track 0.42 0.48 0.56 0.35 0.30 0.29 0.45 0.40
05.5 0.60 0.5;
0.38 0.31 0 29
0.32 0.28 0.23
New Initial 0.76 0.66 0.70 0.55
0 65
On
2417
Kinetic Coefficient of Friction ( p k-J New slider on new track Used slider constant vali~e repeated trark 0 35 0.37 0.33 0 30 0.19 0.18 0.19 0 19 0.27 0 24 0.18 0.16 0.24 0.21 0.20 0 19 0.16 0.14 0.13
0.15 0.12 0.11
0.50
0.22
0.17
0.13
0.12
0 63
0.30
0.24
0.20
0.16
0.60
0.33
0.30
0.26
0.24
0 68
0.43
0.40
0.17
0.16
0 70
0 36
0.32
0 17
0.15
0 65
0 35
0 31
0 20
0.18
men is only 0.01 em. per second. Therefore, in the experiments with lubricated surfaces, uh may include a hydrodynamic component, ( 3 ) . Under these circumstances the value of pk will be lower than for conditions of bounda,ry friction; since this introduces an additional variable, I . ~ B will not be as significant as the coefficient of stat,ic friction. No liquid used to lubricate the nylon-nylon combination was able to eliminate the stick-slip motion. Consequently, t.he following discussion of the friction of nylon on nylon will be concerned primarily with the statio coefficient of friction rather than with pk. It has been generally found that a useful boundary lubricant inust be able t o adsorb either chemic,ally or physically a t the surface and thereby decrease the real area of contact and the adhesion of the two rubbing surfaces. The classic results by Ha8rdy( 1 4 ) on thc decrease of p8 with increased chain length in a honiologous series of saturated straight-chain alcohoh, straight-chain fatty acids, and straight-chain paraffins in the lubrication of metals has been modified in one respect by Bowden and associates (3). I n each homologous series there is a steady decrease in p8 with increasing molecular weight until usapproaches a lower limit. For the lubrication of steel the minimum number of carbon at.oms per molecule necessary to obtain the lowest value of pa in the series was 20 for the paraffins, 15 for the alcohols. and 6 for the acids. Fundamentally this occurs because the total value of the van der Waals cohesive forces between the long hydrocarbon chains of adjacent molecules of paraffin-type boundary films can be of as much importance as the adhesion to the metal in reducing friction ( I , 7). .ks the temperature is raised toward the melting point for a solid lubricant, the lateral adhesion i p dirninished, the degree of condensittion of the film decreases, and eventually a sharp increase in friction results. This effect is especially prominent with metallic salts of fatty acids where there is an additional gain because the melting point of the salt may be much above that of the arid (92). I n the frictional behavior of solutions of polar compounds, the effect of the solvent on the value of k s is often of secondary importance, since small concentrations of certain types of long-chain polar soluter are nearly as effective as the pure solute ( 1 , 7 , I S ) . Inasmuch as dry clean nylon has large coefficient,s of friction against nylon or steel, it is evident that a suitable lubricant will often be advantageous. Hence, liquids were sought which would adsorb and spread on nylon. Most desirable would be those able to form adsorbed boundary lubricating films without attacking the bulk plastic. Liquids to which nylon is essentially inert include nearly all organic acids, carbon disulfide, halogenated hydrocarbons, solutions of alkalies (sodium hydroxide) and soaps, gasoline, white
2418
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. COEFFICIENTSOF FRICTION FOR STEEL ON
0
Lubricant Dry Fater Ethylene glycol Glycerol n-Decane n-Hexadeoane Perfluorolube oil0 Perfluorotrihexylaminea n-Sonyl alcohola Pelargonic acid Oleic acid Stearic acid in hexadecane (approx. 0.1 w t . %) pri-n-Octadecylamine in hexadecane (itpprox. 0.1 wt. %) Perfluorolaurio aoid in decane (approx. 10 - - E mole %I Polyniethylsiloxane D C 500a Polymethylphenylsiloxane DC 610O Polyrnethylphenylsiloxane DC 550" Stick-slip occurs.
~ Y L O X
PS
pk
0.37 0 23 0.20 0.23 0 34 0.30 0.30 0.31 0 23 0.17 0.15 0 13
0.310.19 0.16 0.18 0 30 0.26 0.19 0.14 0.12 0.12 0.08 0.08
0.17
0.14
0.30 0.19 0.17 0.21
0.28 0.12 0.13 0.17
mineral oil, benzene, aldehydes, ketones, and alcohols. It ie degraded by inorganic acids such as hydrochloric, sulfuric, and nitric and is soluble in formic acid, phenol, ni-cresol, cresylic acid! and xylenol. h study of the JTetting properties of nylon (10) has shown that liquids with surface tensions below approximately 42 to 46 dynes per cm. at 20' C. are all able to spread on nylon. Hence, any of a wide variety of liquids such as aliphatic hydrocarbons, perfluorocarbons, or silicones can form a satisfactory fluid base for the development of liquid or grease lubricants for nylon. Good evidence was found for hydrogen bonding between the amide groups in the surface of nylon with water, glycerol, formamide, and thiodiglycol (10). Therefore, long-chain amphipathic molccules with polar groups suitable for hydyogen bonding with the amide groups in the surface of nylon should be effective in adsorbing as a monolayer on nylon and therefore might be useful as boundary lubricants. This led t,o frictional experiments on nylon wet with compounds such as long-chain alcohols, acids, amines, and amides and t,heir svlutioris in various liquids. However, one would expect nylon to be more difficult to lubricate than metals, since the adsorption sites on nylon (the amide groups) are much farther apart than the cross-mtional diameter of these paraffin-chain molecules. Therefore, it should be difficult to form on nylon a solid or condensed adsorbed film of such polar molecules. One mould expect the adsorbed polar molecules to be very much tilted away from the normal but not as much as 90' unless the number of carbon atoms per niolecule is less than the number of -CR2-groups between successivcl amide groups in the polymer chain-i.e., less than six for the nylon made from adipic acid. Pure ethylene glycol, glycerol, water, and n-hexadecane were tried also, to see how the hydrogel] bonding and nonbonding liquids would behave. The compounds used in this study were derived from the s o u r ~ e s indicated in Table I. I n general, the friction nieasurmients were riot sensitive to impurities of the same molecular type and same order of volatility as the liquid under study. It was impera,tive, however, to avoid the presence of more ~tronglyadsorbable impurities, especially in nonpolar liquids Each liquid used was passed a t least, once through a vertical column of an appropriate adsorbent to eliminate this type impurity. Thus, hexadecane and decane were percolated through sucocssive layers of activated alumina and silica gel, perfluorolube oil (a perfluorinated kerosine) through activated alumina, perfluorotrihcsylamine and the silicones through Florid, and the glycerol through Florid in a dry atmosphere. Ethylene glycol, n-nonyl alcohol, and pelargonic acid were each used as received. Oleic acid was purified by fractional crystallization five times. The liquid phase was retained, and the melting point range v a s 17" to 22' C. with a neutral equivalent of 287. The acid was further purified by gradually raising the temperatmurefrom 25' to 137' C. under a preswre of 0.3 mm. to distill off the materials of low boiling point
Vol. 45, No. 1:
The perfluorolauric acid was prepared as described by Ilaie arid associates (16). Stearic acid was recrystallized twice from ethyl alcohol and washed with ether: it had n melting Doint of 68 7 " to 69.2" C. Measurements of the boundary frict,ion of nylon lu bricatetl with these substances were made at the same temperature arid with the same normal load as those used in observing dry friction. The nylon and steel surfaces were cleaned as before. A check for cleanliness mas made by a dry frict)ion measurement; a minimum value of p 8 of 0.37 for a steel slider on the nylon dish was t,he criterion of adequate cleanliness. Lubricant was the]! applied to the nylon disk with a clean platinum dipper; no lubri.. cant rvas applied to the slider. After a few minutes, the frictioi. measurements for steel on nylori were started; imniediatcly aftc.; that, measurements wero made on nylon ~lidingon nylon using the same plane nylon specimens. All the lubricants tried on the nylon-nylon combination wtm* characterized by intermittent or st,iclr-slip motion. The sa,me procedure? used in the dry friction niessurements vvere used, ani? the same general trends were observed. Thus the initial valuc?r of fiLgand U & for two freshly abraded surfaces were high, and the! decreased with sliding distance to constant values. Repeated tracking, rithout reabrading or recleaning of either surface, produced lower friction than the final constant values obtained at the end of the first run, The latter values could be reproduced by making a t,hird run using the same slider (without cleaning it) over a freshly abraded surEa.1.e. The resulting values nf for each condition a,re given in Table I. p s and I n discussing the relative effectiveness of the lubricants for t,he nylon-nylon combination the data of greatest interest are thow measured after i,he two freshly abmdoti for the constant value of surfaces had slid 1 em.; this was much more reproducible th:iri the initid value of fi8. h l l three lubricants in solutjion arc h t e r lubricants than t,hoii ~olvents. Stearic acid caused a, significa~ntly lower valuc oi ua than did octadecylamine, although both have the same H i i phatic hydrocarbon chain. This deinonst8rates that the bonds better than the amine to the arnidc groups of nylon, fluorolauric acid caused B higher vdue of L., than did stearic: itoitl I yet there is good evidence ( l o ) that it adsorbs on nylon by hydrogen bonding just as does it,earic avid. This differenixc. i r , friction may be explained in terms of this weaker forces of coiicsioii i &he-CF,-groupe of neighboring molecules than b ~ tween the -.-.-C11~- groups. The difference in the cohesiorin.l effect. is apparent,ly more important, in detprniining the decreasr in boundary friotion than the difference in the number of carboi: atoms This explains why pelargonic acid is slightly better a boundary lubricant than perfluorolauric acid, even though there are three fepver carbon at,oms per inolecule in t,he former. Seithei hesadecane nor decane is a good boundary lubricant because QY the highly uncondensed and nearly nomidherent film adsorbed 011 the nylon. The small differenm in friction between pelargonic. and oleic acids indicates that because of the double bond in thr cis configuration in the latter, there is a loss in intermolecu1a.r cohesion in the film despite the greater number of -CH*group?. Pelargonic acid is a more effective boundary lubricant than nnonyl alcohol. This is reasonable since the acid group shoultl hydrogel1 bond more strongly than the alcohol group to each a,mide group of nylon. Ethylenc glycol and water are two of the poorest lubricants for nylon. The molecules of these two liquids are small anti various obvervations have shovrn that they permeate into the. aurfacae of nylon (10, 17). Glycerol, with one -CII(OH) unit more than ethylene glycol is much more viscous and a much better lubricant. This may be due tu film adsorption of the glycserol on nylon to give some lowering of friclion. It is an intcrwting fact that, Hardy ( l 4 ) reported a similar relative situation f o r the boundary friction of glycol and glycerine on bismuth. Perfluorotrihexylamine caused a relatively high value of u s .
__
November 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
The short aliphatic chain and the weak cohesive forces between the --CF$- groups in adjoining chains contribute to reducing its effectiveness. Also, the lack of an amino hydrogen atom in the tertiary amine prevents hydrogen bonding. Perfluorolube oil caused a high static coefficient of friction as was to be expected in view of its inability tjo adsorb (10) and the weak cohesive forces between -CF2groups. The silicones are not effective bounditry lubricants because they are unable to react with or adsorb on the surface of the nylon. Some of the liquids studied raised pa values above the value for dry nylon, although a t the beginning of the run these values were not higher. It is presumed that this occurs because the presence of the liquid either prevents orientation or work hardening of the nylon. All these liquids caused a large reduction in the values of pb. This is evidence again of some contribution from hydrodynamic lubrication during the measurements of ph, The measured values of p8 and p l ~for lubricated steel againfit riylon are given in Table 11. Only perfluorolube oil, perfluorotrihexylamine, n-nonyl alcohol, and the silicones, caused stickdip motion. For Qhereasons indicated, under the conditions of slow sliding used in these tests both pa and pic measure the boundary friction for all the other lubricants. The relative order of uk is t,he same as the relative order of p 3 , For the liquids in Table I1 which caused stick-slip motion, the values of uk are always so much lower than the values of p g that it must be concluded that there is a hydrodynamic contribution to the value of ph, The static friction for steel against nylon is much less for each lubricant than the static friction for nylon against nylon. This is because of the lower coefficient of dry friction of the former and because steel is easier to lubricate. Since the adsorption sites are much closer together on steel than on nylon, much more closely packed adsorbed films are possible. Fatty acids were again the most effective compounds in reducing friction. I n the presence of small concentrat,ions of water, the fatty acids react with iron and many other oxidizable metals to form condensed, high melting films of metallic soap ( 3 ) . It is for this reason that the fatty acids are much bet'ter lubricants than the alcohols, which are only able to adsorb physically. Thus pelargonic acid was definitely more effective than nonyl alcohol in lubricating steel against nylon, and it was very much better than decane. Pure hexadecane gave slightly lower friction than decane since it has a longer chain. Perfluorokerosine and perfluorotrihexylamine were again ineffective lubricants for reasons outlined earlier in this paper. The addition of perfluorolauric acid caused only a slight decrease in the value of M~ of decane; possibly this is because the solution was too dilute (Table I). Ethylene glycol and water exhibited much lower values of rut with steel against nylon than with the nylon-nylon combination; this may be because permeability and doftening of the plastic is less a factor in lubricating the former combination. Silicones reduce p a to about one half of the dry friction value for the combination steel on nylon. This is surprising because silicones are poor lubricants for both nylon against nylon and steel against steel. When some of the silicones are wed to lubricate steel on steel the friction is higher than the dry friction (11 ). Experimental evidence demonstrated that the silicones would not adsorb as oriented films on a nylon surface. Therefore, the lower friction of steel on nylon must be caused by adsorption of the silicone on the steel. This decreases the adhesion of steel to nylon and increases the amount of sliding a t the steel-nylon interface. DISCUSSION
Sylon is difficult, to lubricate because the adsorption sites are so far apart that adsorption of a sufficiently condensed film is prevented. The steel-nylon combination is easier to lubricate because a close-packed film can be formed on the steel by any long-chain aliphatic polar material able to adsorb. Thus the static friction for nylon on nylon is always larger than it is for
2419
steel on nylon. This mechanism should also apply to many other metals; hence, low friction would be expected between most metals and nylon in the presence of a polar lubricant. It is concluded that the lateral forces between the adjacent adsorbed molecules of the lubricant are also important in determining their ability to reduce friction of plastics, as demonstrated by the lower friction with increased chain length of acids. Perfluorinated hydrocarbons are poorer boundary lubricants than the analogous hydrocarbon; probably this is because of the weak adsorption to the nylon and the weaker lateral forces. Evidently the base liquid used to develop an oil or grease for lubricating nylon should have a high boiling point and must not attack the plastic. Good examples of suitable liquids are mineral oils, halogenated hydrocarbons, esters, and silicones. The best additives are the long-chain fatty acids, alcohols, or amines. Since these are effective through adsorption of the polar groups by the metal or by the amide groups in nylon, they are more effective in lubricating metals rubbing against nylon than nylon against nylon. Under the best conditions, boundary friction of metals against nylon may be made as low as 0.08 to 0.15. It is difficult to compare the published results on the friction of plastics since the results are dependent upon the methods of cleaning and preparing specimens as well as the type of measuring apparatus. Of the previous work cited, only the tests of Shooter and Tabor (18) were made under conditions similar to those used for the present experiments. The agreement is fairly good for both steel on nylon and for nylon on nylon, if their results are assumed to be kinetic values. Although the coefficient of friction is not, in general, dependent on the load, in the load range used by Lincoln 116),
