specific problems in friction, lubrication, and adhesion. Lubricating oils are used widely by the textile fiber producers to reduce friction and wear during the spinning and fiber processing operations. T h e same (or other) lubricating oils are used as ‘.finishes” by the fabricator of textiles in order to achieve a number of desirable qualities, including tightness of weave, resistance to \vear, and a suitable .‘handle.” If it were not for the lubricant on the surface of each fiber, woven textiles would wear very rapidly during use as a result of sliding or fretting of one fiber over another. LVhen such textiles are cleaned n i t h solvents or detergents, some of the lubricant is removed; when the oil supply is depleted, rapidly increasing rates of wear result. O u r research points to the possibility of decreasing the fiber friction adequately by incorporating in the polymer (before spinning the filament) a small proportion of a surface-active fluorocarbon compound. such as described here; the resulting textiles Ibould have self-renewing, dry-film lubricating properties as \vel1 as built-in resistance to detergents and dry cleaning treatments. However, research will be required to ascertain to what extent such a n additive ~ o u l dinterfere with obtaining other desired fiber properties, such as optimum tensile strength and dye receptiveness. Many polymeric solids manufactured in thin sheets exhibit excessive friction and self-adhesion, and therefore surface coatings or other surface treatments are used to cope with the difficulties. Such problems occurred in the development of cellophane and Mylar. ,4 small concentration of a suitable fluorocarbon derivative included in the bulk phase of many plastics, either before or during extrusion, may be an effective solution to such problems. Although the necessary fluorocarbon derivatives are neTv and probably will always be highpriced organic materials, because of the high cost of the fluorocarbon moiety. they might be so efficient as to be economically accept able. Bearings and gears molded from nylon, fluorocarbons, or polyoxymethylene are common objectives of commerce. Polymeric solids having even more outstanding mechanical and
other physical properties exist but exhibit much higher friction and self-adhesion; therefore, they are not used in such applications. Proper selection of the polymer and the means of incorporating the fluorochemical addition agent may make it possible to diminish adequately the dry friction and selfadhesion of the plastic with only a small sacrifice in other physical properties. Acknowledgment
JVe are grateful to the Chemical Division of Minnesota Mining and Manufacturing Co., the Organic Chemical Department of E. I. du Pont de Nemours & Co., and the Pennsalt Chemicals Corp. for making samples of their research fluorochemicals available for this investigation. Literature Cited
(1) Bernett, M. K., Jarvis, N. L., Zisman, \V. A.: J . Phys. Chem. 66, 328 (1962). (2) Bowers, R. C., Clinton, i V . C., Zisman, FV. A., 2nd. Eng. Chem. 46, 2416 (1954). (3) Bowers, R. C., Clinton, W. C., Zisman, W.A., J . Appl. Phys. 24, 1066 (1953). (4) Bowers, R. C., Clinton, LV. C., Zisman, W. A., Lubrication Eng. 9, 204 (1953) ; :Mod. Plastics 31, 131 (1954). ( 5 ) Faurote, P. D., Henderson, C. M., Murphy, C. M., O’Rear, J. G., Ravner, H., Ind. Eng. Chem. 48, 445 (1956). (6) Goodzeit, C. L., Hunnicutt, R. P., Roach, A. E., Trans. Am. Soc. Mech. Engrs. 78, 1669 (1956). (7) Hauptschein, M.: Braid, M., Lawlor, F. E., J . Am. Chem. Soc. 79. 6248 11957). (8) Jam&, S . L.,Fgx, R. B., Zisman. TV. A., Adcan. Chem. Ser., No. 43, 317 (1964). (9) Jarbis, N. L., Zisman, W.4..J . Phys. Chem. 63, 727 (1959). (10) Zbzd., 64, 150 (1960). (11) Zbzd., p. 157. (12) O’Rear. J. G.. Snierroski. P. J.. Naval Res. Lab.. NRL Rent. ‘ 5795 (July 18, 1962). (13) Zbzd., to be published. J . Phys. Chem. 64, 519 (1960). (14) Shafrin, E. G., Zisman. W. .4.. (15) Tabor, D., .‘The Hardness of Metals,” Oxford University Press, London, 1950. (16) Zisman, If.A, Adoan. Chem. Ser., KO. 43, 1 (1964). V
I
RECEIVED for review December 7, 1964 ACCEPTED March 18, 1965
STRUCTURE PROPERTY RELATIONSHIPS IN POLYURETHANE ELASTOMERS PREPARED BY A ONE-STEP REACTION K.
W. R A U S C H , J R . , A N D A . A .
R . S A Y I G H
Carwin Research Laboratories, The Upjohn Co., North Haven, Conn.
first elastomeric polyurethanes, called modified polyesters, \vex the reaction product of an essentially linear polyester and excess diisocyanate (2). The excess isocyanate \vas made to react a t elevated temperatures to form dimers, trimers, and allophonates and to effect a cure through crosslinking. Later Tvater, steam, or water of hydration of salts was used as a curing agent, but the carbon dioxide produced formed undesirable bubbles in the polymer. These bubbles were eliminated by the use of polyols, polyamines, or both as curing agents ( 6 ) . LYhen glycols (8) or diamines (7) were added in amounts less than that equivalent to the isocyanate excess in the modified polyester, a cure occurred a t elevated THE
92
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
temperatures by the reaction of the remaining isocyanate with itself, with the urethane (from glycol), or with the urea (from diamine). The “curing” reactions occurred very slowly a t ambient temperatures; therefore, the polymers were partly cured upon prolonged storage or exposure to air. T o overcome this cure problem, stable elastomer gums were made by (1) modifying polyesters (5, 72), polyester amides (5),or polyaliphatic glycol (3) with a deficiency of diisocyanate, and (2) making a diisocyanate-modified polyester (70) or polycaproester (4) react with glycol or diamine in excess of the free isocyanate. These gums were then cured by the addition of excess diisocyanate.
Substantially linear, hydroxy-terminated elastomers were prepared, to study their effect on the chemical structure of the starting materials, the ratio of diisocyanate to polymeric diol, the excess glycol, and the method of reaction. Asymmetry and steric hindrance in the diisocyanates adversely affected elastomer properties. Decreasing the density of or increasing the separation of strong cohesive functional groups had less adverse effects. These factors interfere with chain orientation, crystallization, and the cohesive forces, all of which are responsible for the strength of elastomers. Little difference in pro rties was observed between elastomers prepared from pure diphenylmethane-4,4'-diisocyanate (4,4'-MDij Ljlxdthose prepared The elastomers prepared by the mu, +,esirableone-step from a 90: 10 mixture of 4,4'-MDI and 2,4'-MDI. method generally have greater tensile strength and ultimate elongation and a highel liielting point than analogs prepared b y the prepolymer method. By balancing reaction conditions, usefur And stable thermoplastic elastomers with a wide range of physical properties can be prepared from the 90: 10 mixture of the two isomeric diphenylmethane diisocyanates.
-
T h e reaction of isocyanate-modified polyester with a n amount of glycol equal to or slightly greater than the free isocj-anate gave a thermoplastic material having properties inferior to those of the cross-linked polymers (6). Recently, hoivever, polyurethanes of the noncross-linked type having good physical properties were developed ( 7 7). Polyurethanes of linear polyesters or polyethers in which the amount of glycol used is approximately stoichiometric are of interest because they can be thermoformed by extrusion or injection molding. Polyurethanes in which glycol is deficient (isocyanate-terminated elastomers) must be postcured, while those in which the glycol is in excess (hydroxyl-terminated elastomers) need not be. Pigott e t al. (9) examined the effect of the chemical structure of the starting materials on the properties of isocyanateterminated elastomers. No such study has been reported for the hy-droxyl-terminated elastomers. These polymers possess characteristics Jvhich render them of value as both research and industrial materials. For example, the polymer strength, a result of entanglement, orientation, and physical cohesive forces between chains, is sensitive to structural changes in the components, and thereby facilitates study of the latter; the terminal hydroxyl groups are relatively insensitive to air and moisture, alloxring the polymers to be stored for a long time with no change in properties; and the elastomers are easily formed into test samples and solutions for analysis.
which time the temperature rose to 80' to 95' C., the agitation was stopped, the vacuum broken, and the reaction mass poured into an aluminum pan. T h e mixture was placed in an oven equipped with a nitrogen purge and finally heated at 110' C. for 3 hours. An essentially complete reaction was shown to be obtained with M D I during the 3-hour cure by a rate study in which isocyanato-group absorption a t 4.45 microns was measured in an infrared spectrophotometer. After 11/2 hours a t 100' C. the concentration of isocyanate was about 0.3%, which represented a 98% completion of reaction. Below this concentration, the measurement was inaccurate ; however, extrapolation of the data in Figure 1 indicates a 99.9% completion after 3 hours. Portions of the casting so obtained were compression molded a t 180' C. into 6- x 6-inch sheets, 0.025 to 0.035 inch thick. After they were aged one week, the sheets were cut into pieces for testing.
Preparation
Z
Polyurethane elastomers can be made by two methods : prepolymer and one-step. I n the prepolymer method, which has been used and studied more widely, the polymeric diol and the diisocyanate react to form a n isocyanate-terminated prepolymer, which is then extended or cured by poly01 or polyamine addition. In the one-step method all the components are mixed in one reaction. This latter process is faster, easier, and more reproducible and can be used to best advantage where the reaction rates of the diol components with the diisocyanate are comparable. Therefore, the elastomers for this study were prepared by the one-step method. Procedure
T h e equipment consisted of a glass 500-ml. resin flask with a thermometer, agitator, vent with a check valve, and vacuum and nitrogen adapter. An elastomer was prepared by weighing the required amount of predried polyester or polyether and glycol into the resin flask, draiving a vacuum of 3 to 5 mm. on the flask as the temperature of the mixture was raised to about 60' C., breaking the vacuum with nitrogen, and quickly adding liquid isocyanate to the mixture, immediately re-evacuating the reaction flask, and agitating. After 11/2 to 2 minutes of degassing, during
100 51
80 73 60
0 40 30
20
10 9 8
7
6 5
L
4
4
3
u
2
1 0 9 0 8
07
06 05
0 4 0 3
0 2
0 1
.
Figure 1 Reaction rate of diphenylmethane-4,4'-diisocyanate with polycaprolactone and butanediol at 100" C. Determined b y infrared absorption
VOL. 4
NO. 2
JUNE 1 9 6 5
93
Table 1.
Effect of Glycol Chain Lengtha
Set at Die C Modulus, Modulus, Ult. Rupture, Tear, P.S.I. P.S.I. Elong., % % PLI Poly(ethy1ene Adipate)-10% Propylene (PEPA), M.W. = 1040 765 1565 950 25 500 705 1150 1035 50 385 735 1125 1015 20 380 790 1170 1105 50 388
Ult. Tensile, P.S. I.
700%
300%
Shore Hardness A
Melt
79 81 82 81
215 200 200 187
1,3-Propanediol 1,4-Butanediol 1,5-Pentanediol 1,6-Hexanediol
7245 5650 6320 5745
1,3-PropanedioI 1,4-Butanediol 1,5-Pentanediol 1,6-Hexanediol
5125 6410 4000 3795
675 655 595 570
Polycaprolactone (PCL), M.W. = 1050 1280 805 20 1050 45 1000 890 980 60 835 1030 75
375 365 325 314
77 81 77 78
21 5 197 145
5795 5840 4830 2260
885 765 71 5 710
Polytetramethylene Glycol (PTMG), M.W. = 808 1540 845 50 1270 930 45 1115 965 50 980 830 140
412 407 362 303
81 82 82 83
189 172 145 130
3310 3875
965 780
Polytrimethylene Glycol (PTriMG), M.W. = 658 695 65 830 45
230 375
84 83
150
1,3-Propanediol 1,4-Butanediol a
NCOIOH ratio = 2.0.
MDI-90 (Carwinate-125M)used in this study.
Excess equivalent glycol
=
6800 8410 8820
935 925 750
610
834 1365
3.5 7 28
140
2%.
Table II. Effect of Isocyanate-Polyester Ratio" Shore Ult. 700% Tensile, Modulus, Ult. Set at Die C Hardness P.S.I. P.S.I. Elong., yo Rupture, % Tear, PLI A Poly(ethy1ene Adipate)-10% Propylene (PEPA), M.W. = 980-1,3-Propanediol
Eq. NCO/Eq. O H (PEPA) 1.8/1 .O 2.0/1.0 2.5/1.0
100
37 1 485 560
Melt
Ted"p.,
c.
Tg, "C.
72 77 88
165 205 210
-9 -7 -6
77 81 87
217 184 193
-23 -16 -9
Polycaprolactone (PCL,), M.W. = 1050-1,3-Propanediol Eq. NCO/Eq. O H (PCL) 2.0/1.0 2.5/1.0 3.0/1.0 a
675 850 1415
5125 5245 6900
805 735 620
MDI-90 (Carwinate-125M)used in this study.
70Excess equiv. glycol OH 11 2 0
a
375 460 590
Excess equivalent glycol = 2%.
Table 111.
.
Ult Tensile, P.S.I.
20 28 35
Effect of Glycol Excessa
700%
Ult. Set at Die C Modulus, Rupture, yo Tear, PLI P.S.I. Elong., yo Polycaprolactone (PCL), M.W. = 1050-1,3-Propanediol
530 620 575
1195 1415 1360
3055 6900 6930
64 35 29
455 590 595
Shore Hardness A
89 87 85
Table IV.
Comparison of One-Step vs. Prepol Systemsa
100%
One-step Prepol a
94
Ult. Tensile, Modulus, Ult. Set at Die C Shore P.S.I. P. s.I. Elong., yo Rupture, Tear, PLI Hardness A Poly(ethy1ene Adipate)-1070 Propylene (PEPA), M.W. = 980-1,3-Propanediol 8820 1365 750 28 560 88 7000 1420 655 590 86
8335 4445
Polycaprolactone (PCL), M.W. = 1050-1,4-Butanediol 15 428 867 780 390 740 700 12
MDI-SO (Carwinate-125M)used in this study.
174 193 195
NCOIOH ratio = 3.0.
MDI-90 (Carwinate-125M)used in this study.
One-step Prepol
Melt Temp., "C.
NCOIOH ratio
=
I&EC P R O D U C T RESEARCH A N D D E V E L O P M E N T
2.5. Excess equivalent glycol
=
2%.
84 83
Melt Temp., "C.
210 164 193 142
Effect of Isomer Content in Diisocyanate
Table V.
Ult. Tensile, P.S. I.
100%
Modulus, P.S.I.
Ult. Elong., %
Set at Rupture,
%
Die C Tear, PLI
Shore Hardness A
Melto Temp., C.
84 84 82 78 20
183 193 145 129 4'-diisocyanate (2,4 '-MDI) 3,3 '-Dimethyldiphenylmethane-4,4 '-diisocyanate (DMMDI), Carwinate-1 39D Diphenylmethane-4,4 '-diisocyanate (MDI), obtained by a series of crystallizations and distillations of mixed isomers Carwinate-125M
Results
Effect of Glycol Chain Length and Polymeric Diol Structure. Several linear aliphatic glycols were formulated with four polymeric diols, and various physical properties of the resulting elastomers were measured. Each of the glycols and the polymeric diols was used in the same equivalent proportions throughout. The results are presented in Table I. The following conclusions can be drawn from these results: 1. I n this type of system, approximately equivalent properties are obtained using any given alkanediol, whether the polymeric diol is a poly(ethy1ene adipate) of m.w. 1250 (est.), a polycaprolactone of m.w. 1000, a polytetramethylene glycol of m.w. 800, or a polytrimethylene glycol of m.w. 500 (est.). T h e increased strength derived from higher concentrations of urethane in the polymers from polymeric diols of lower molecular weight is obviously offset by the loss of strength from the following: T h e absence of hydrogen bonding between ester groups and urethane hydrogens in the polyether diols. T h e distance between ester groups. T h e cohesive energy between ester groups in the chain is greater in poly(ethy1ene adipate) than in polycaprolactone because of greater distance between ester groups in the latter. High concentration of flexible ether groups (polytrimethylene glycol gives the weakest elastomer). 2. A4study of the variation in polymer properties obtained by using different glycols with any given polymeric diol again emphasized that increasing the distance bet\\een urethane groups (in this case by using longer chain glycols) decreases the density of cohesive force between chains. This effect is coupled with the ”odd-even’’ effect of chain fit (7). T h e “zigzag” pattern in tensile strengths and elongations is particularly evident in the polyester elastomer.
Effect of Isocyanate-Polyester Ratio. T h e polyurethane molecule is composed of long, low-melting, flexible polyester chains joined by high-melting, rigid, concentrated urethane areas. Increasing the isocyanate-polyester ratio increases the concentration of high-melting, rigid areas of the chain, and thereby affects the physical properties of the elastomer. The over-all results are greater strength, lower elongation, higher melt temperature, and increased hardness (Table 11). The greater strength and lower elongation can be discussed in terms of the glass transition temperature, Tg. Studies have shown that Tg increases as the urethane concentration increases (13), most probably as a result of increased intermolecular hydrogen bonding. This relationship was confirmed by Clash-Berg torsional modulus measurements in which Tg (Table 11) is taken to be the temperature a t the inflection point of the modulus-temperature curve. T h e tensile strength and elongation of a polyurethane are related to the difference between the test temperature and Tg (74). Thus a n increase in the isocyanate-polyester ratio produces a n increased urethane concentration, hence a higher Tg. At a smaller temperature difference, the resulting elastomer has higher tensile strength and lower elongation. Effect of Glycol Excess. Use of excess glycol reduces the molecular weight of the polymer. T h e shorter chains in the polymer, being less entangled and having less total force acting on them, slip more easily. Excess glycol results, as Table I11 shows, in lower tensile properties and higher set. As the excess glycol approaches zero, the change in molecular weight is minimal. Hence, polymer properties do not significantly differ between 0 and 2 7 , excess. Effect of One-Step US. Prepolymer Methods. T h e main difference between polymers prepared by the prepolymer and the one-step methods involves chain build-up. An elastomer
M O L E P E R CENT DMMF’
Figure 4. Effect of composition upon physical properties of elastomers containing poly(ethy1ene adipate)10% propylene, propanediol, MDI-90, and DMMDI VOL. 4
NO. 2
JUNE 1 9 6 5
97
I2
11 I
I
D l
............... . ......
( i , ] ) j l l l l l 1 1
I
1
1
1
1I
Figure 6. Effect of composition upon physical properties of elastomers containing polycaprolactone, butanediol, 4,4’MDI, and DMMDI
produced by the prepolymer method is statistically more regular in the chain sequence of polyester-diisocyanate-glycoldiisocyanate-polyester, while a n elastomer produced by the one-step method, assuming the polyester and the glycol to be of equal activity, has a more random sequence. The data in Table I V show three consistent differences betv+een one-step and prepolymer elastomers : tensile strength, ultimate elongation, and melt temperature, the latter difference being the most outstanding. An explanation could lie in a higher order of crystallinity in the one-step polymers. The prepolymers, as they are extended by the glycol, build u p molecular weight a t a rapid rate; thus, the molecules become entangled and immobilized before any order can be established6 The one-step polymers begin with the slightly favored reaction of glycol and diisocyanate which produces highly crystalline, mobile chain elements. Therefore, order can be established before extended polymer growth has occurred. These areas of crystallinity, acting as crosslinks, increase the tensile strength of the one-step elastomers. Although the test sheets are molded a t high temperatures (180’ C.), the melting point of the one-step elastomer is still 30’ C. higher. Complete random disorder is not attained during short exposure to temperatures below the melting point; thus, the crystalline order persists in the one-step polymer. ISOMERS. Effect of Isocyanate Structure. DIISOCYANATE Diphenylmethane diisocyanate contains two principal isomers : the 4,4’-isomer which predominates and the 2,4’-isomer. The effects of this latter isomer upon the properties of the elastomer were studied by making several polymers from prepared mixtures of the two isomers and from production material of mixed isomers (Table V and Figure 2).
98
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
The change in polymer properties, observed as the concentration of the 2,4’-isomer increases, can best be explained in terms of the glass transition temperature, Tg. T h e 2,4’isomer produces asymmetry in the polymer chain. The result is a decrease in Tg,accompanied by a decrease in the tensile properties a t ambient temperatures. An elastomer prepared from pure 2,4’-isomer is low-melting (below 100” C.), soft, and without significant physical properties. METHYLSUBSTITUTION IiX DIPHENYLMETHANE-4,4’-DIISOCYANATE. The effect of methyl substitution on the aromatic ring ortho to the isocyanato group was studied by preparing elastomers from mixtures of diphenylmethane diisocyanate ’-diisocya(MDI-90) and 3,3 ’-dimethyldiphenylmethane-4,4 nate. The liquid solid phase chart for this mixture is presented in Figure 3. Elastomer properties are given in Table V I for two systems: polyester (Figure 4) and polycaproester (Figure 5 ) . Also shown is the 4,4’-MDI system (Figure 6). Physical strength, hardness, and melt temperature of the elastomer decrease as the concentration of methyl-substituted isocyanate increases. Hydrogen bonding of the urethane group is the major contributor to the over-all strength of the polymer. The o-methyl substitution separates the urethane group from the adjacent chains: thereby weakening the bond strength. The result is a weaker and softer polymer. Because methyl-substituted isocyanates have greatly reduced reactivity, it \vas thought that the standard 3-hour cure a t 110’ C. was inadequate. T o confirm this, a duplicate elastomer of 100% 3,3 ’-dimethyldiphenylmethane-4,4’-diisocyanate was cured for 30 hours a t 110” C. The properties of this elastomer are in parentheses below the standard results in Table VI. Additional chain extension occurs during the longer cure and thus increases the strength of the polymer. The final properties are still greatly inferior to those of the nonsubstituted diphenylmethane diisocyanate elastomers. COMBINED EFFORTS O F ISOhlERS A S D METHYL SUBSTITUTION. Table VI1 summarizes the physical properties of elastomers containing varying quantities of 4,4’-MDI, 2,4’-MDI, and DMMDI. Literature Cited
(1) Bayer, O., An,pw. Chem. 59, 257 (1947). (2) Rayer, O., Muller, E., Petersen, S., Piepenbrink, H. F., Windemuth, E., Rubber Chem. Technol. 23, 812-35 (1950). (3) Hill, F. B., Young, C. A., Selson, J. A., Arnold, R. G., Znd. Eng. Chem. 48, 927-9 (1956). (4) Hostettler, F.? U. S. Patent 2,933,477 (April 19, 1960). (5) Mastin, L. G., Seeger, N. V., Zbid., 2,625,535 (Jan. 13, 1953). (6) Muller, E.: Bayer, 0.; Petersen, S., Piepenbrink, H. F., Schmidt, F., il’einbrenner, E., Rubber Chem. Technol. 26, 493-509 (1953). (7) Muller, K. E., Petersen, S., U. S. Patent 2,778,810 (Sept. 23, 1952). (8) Muller, K. E., Schmidt, F. W., Weinbrenner, E. W., Piepenbrink, H. F., Zbid., 2,729,618 (Jan. 3, 1956). (9) Pigott, K. A,, Frye, B. F., Allen, K. R., Steingiser, S., Dans, W. C., Saunders, J. H., J . Chem. Eng. Data 5, 391-5 (1960). (IO) Schmidt, F. i V . , Muller, K. E., U. S. Patent 2,621,166 (Dec. 9, 1952). (11) Schollenberger, C. S., Zbid.: 2,871,218 (Jan. 27, 1959). (12) Seeger, N. V., Mastin, T. G., Fauser, E. E., Farson, F. S., Finelli, A. F., Sinclair, E. A,, Znd. Eng. Chem. 45, 2538-42 (1953). (13) Smith, T. L., Magnusson, A. B., J . A$@. Polymer Sci. 5 (14), 218-32 (1961). (14) Smith, T. L., Magnusson, A. B., J . Polymer Sci. 42, 391-416 (1960). for review November 12, 1964 RECEIVED ACCEPTEDMarch 31, 1965
