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ROBERT J. ATHEY Elastomers Laboratory, E. I. du Pont de Nemours & Co., Wilmington, Del.
Chemical Compounding of Liquid Urethane Elastomers
THE properties for which urethane elastomers have become known EXCELLEST
are related to the unusual chemical and structural features of the polymer itself. T h e advent of liquid urethane polymers has afforded the chemist a n opportunity to study the effect of structural changes in the polymer on the physical properties of the resulting elastomeric products. This, in turn, has given the rubber chemist a new dimension : compounding by chemical means rather than by physical methods. T h e amount of chain-extending agent, reaction temperature, and molecular weight of the liquid polymer segments influences vulcanizate properties of urethane elastomers. Hardness and toughness of the vulcanizate can be increased if monomeric toluene diisocyanate is added together with sufficient diamine to react with it as well as the polymer. I n the work described here, the influence of these four variables was studied using vulcanizates prepared from polytetramethylene ether glycol and 2,4-toluene diisocyanate (TDI). Three polymers were developed which offer the rubber industry wide latitude in designing new compounds. Each liquid polymer was warmed to 100" C. and degassed under vacuum. Supplementary TDI was added when required, and the resulting solution mixed thoroughly. The solution was brought to curing temperature and mixed well with melted diamine (Moca). Where pot life was under 5 minutes, the mixed compound was poured into preheated molds, allowed to gel, and press-cured. The pressure forced air bubbles from the molding during cure. Otherwise, the compounds were degassed after final mixing, poured into molds, and oven cured. Before testing, pieces were stored for at least one week at 75" F. and SO'% relative humidity.
Limits of Variables Studied
Variables
Limits
Apparent mol. wt. of starting liquid polymer' 850-7000 Cure temp., O C. 70-140 Toluene diisocyanate levelb 0-2 0 DiamineClevel, % of theoreticald 33-100 a Calculated using 42 /% NCO X 2 X 100, where 42 is mol. wt. of the isocyanate group. The value is multiplied by 2 because the liquid polymers are difunctional. * P.p.h. of liquid polymer. ,Mota, 4,4'-methylenebis-(o-chloroaniline), E. I. du Pont de Nemours & Co., Inc. Theoretical amount is that required to react with all isocyanate in the liquid polymer/TDI mixture.
branching developed in the final vulcanizate. Stocks made using less than the theoretical amount of diamine exhibit reduced compression set, higher modulus, and reduced swelling in solvent when compared with vulcanizates made with the stoichiometric quantity of diamine. Excess isocyanate in such a system cannot react to form urea linkages because of diamine deficiency. I t is believed to react instead with a hydrogen atom of a substituted urea group to yield a substituted biuret structure.
T h e biuret comprises a chain branch point because it is trifunctional. I n a given system, concentration of biuret groups is increased as the curing temperature is raised, resulting in a more highly branched structure. As the apparent molecular weight of the liquid polymer decreases, modulus, tear strength, hardness. and stiffness increase, but resistance to Method A compression set and impact resistance decrease (Figure 1). Hardness and tear strength exhibit similar curves. 4s the apparent molecular weight changes from 850 to 7000, 1007, modulus drops from 3600 to 400 p.s.i., hardness from 78 to 23 Shore D, tear strength from 190 to 20 p.l.i., and compression set from 407, to 5%. The change in properties is more gradual in the high molecular weight range. Most of the properties tested, except tear strength, exhibit a maximum or minimum a t some diamine level between 33 and 1007, of the theoretical amount (Figure 2). Modulus at 100% and hardness of the 1300 molecular weight polymer show maximum values of 1900 p.s.i., and 52 Shore D, respectively
Physical Tests Used
Test Tensile strength, Modulus Elongation Tear strength Compression set Impact resistance Flexural modulus Abrasion resistance
ASTM Method D-412 D-470 D-395,Method A D-256 D-797 D-394
Remarks Speed, 1 inch/min. Correlates well with hand tear a/r-in. diam. pellets; 600 lb. press.; unit loading, 1350 p.s.i.; most too hard for Method B test Notched Izod test Temp., 25" C. Nat. Bur. Standards
Discussion T h e amount of diamine present to react with the -NCO terminations on the liquid polymer block appears to control the degree of cross-linking or chain
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Range o f Vulcanizate Properties Obtained 100% modulus, p.s.i. Hardness, Shore A Hardness, Shore D Tear strength, p.1.i. Compression set, % Impact resistance, ft.-lb./in.
400-6000 65-99+ 25-78 20-200 2-80 1.5-20+
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Figure 1. Decreasing liquid polymer molecular weight increases vulcanizate hardness and compression set
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Figure 2. Effect of changing diamine level i s similar for all polymers
VOL. 52, NO. 7
JULY 1960
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a t about 80% of theoretical and decrease o n either side of this level. Resistance to iMethod A compression set is relatively constant at about 10% over the Moca range, but goes through a slight maximum at about 5070 of theory of Moca. Behavior of these properties indicates that a small amount of biuret branching is required for the development of maximum modulus and hardness. The optimum degree of branching occurs when 807, of theoretical amount of Moca is used. Tear strength seems to be related only to degree of cross-linking, because it increases as branching decreases. Of the four variables tested, cure temperature had the least effect. Increasing the temperature of cure from 70' to 100' C. increases modulus and hardness but decreases tear strength. A continuing increase to 140' C. results in slightly softer stocks which have lower modulus and further decreased tear strength. compression set is influenced only slightly by curing temperature changes (Figure 3). Higher curing temperatures increase the proportion of biuret branchinga t least 100' C. is needed for the substituted biuret groups to form, which i n turn are needed for maximum modulus and hardness. At lower temperatures, very little biuret formation occurs. A t 140' C., it is believed that the proportion of biuret branching increases to the point where the orderly structure of the vulcanizate is disrupted, and softer, more flexible materials are formed. This effect is similar to that of using low Moca concentrations. The most tearresistant vulcanizates result from 70' C. cures, where the proportion of biuret branching is a t a minimum.
C U R E TEMR,*C.
Figure 3. Curing a t 100" C. provides the best balance of physical properties
The curves in Figure 4 (no T D I added) show that the hardest materials which can be prepared from a given polymer using Moca as a curing agent, are obtained a t 100' C., using SO% of theory of Moca. The modulus contours of each polymer closely resemble the hardness curves. The point of highest modulus occurs at a temperature of 100' C., and at 80% Moca. The change in hardness and modulus with increasing Moca level is more pronounced at 140' C. than a t 70' C. The dependence of compression set on cure temperature, Moca level, and molecular weight was complex. At low molecular weight. a drop in diamine level from 100 to 3 3 7 , causes a significant drop in compression set from over Three Polymers Have Been Developed
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Interactions
Interactions among the four variables were determined statistically. Fourdimensional plots are actually required for a four variable system, but because such a system cannot be easily represented graphically, triordinate plots were used. One variable in each case was held constant.
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Adiprene L-100 100 Adiprene L-167 100 LD-213 100 Moca 11 20 25 100 100 70-80 Mix. temp., O C. 3/100 1/100 1/100 Cure, hr./' C. 1007, modulus, p.s.i. 1000 1900 3500 Tensile strength, p s i . 4500" 4500" 7500 Elongation, % 450 350 280 Hardness, Shore A 90 95 99 42 50 78 Hardness, Shore D 150 190 Tear strength, p.1.i. 60 Comp. set, % 22 hr./ 70' C., 1350 p.s.i. 7 8 40 Flexural modulus, p.s.i. 8000 15000 90000 Impact resist., ft.-lb./in. F F 6 Abrasion resistance 175 275 380 Compression deflection, 5%, p.s.i. 450 750 6000 F = flexible. 5 Tested at 40 in.,'min.
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50 to l O 7 ~ ,and is relatively insensitive to cure temperature. At higher molecular weights, Moca level has little effect, except to cause a minimum of 570 at the 50 to 60% level us. 10% at both Moca extremes. Increased cure temperatures generally caused increased compression set a t the higher molecular weights, undoubtedly because softer vulcanizates resulted. As an example of the utility of chemical compounding, the lowest molecular weight polymer, containing no added T D I and cured with SOYGof the theoretical amount of Moca at 100' C., has a hardness of 78, Shore D, a 100% modulus of 3600 p.s.i., split tear strength of 190 p . L , and a compression set of 45%. By changing the conditions to 66% of theory of Moca and using a curing temperature of 120' C. a stock is obtained which exhibits a hardness of about 60 Shore D, a modulus of 3000 p s i . , tear strength of 75 p.l.i., and compression set of 25%. Thus, selecting the compound properly permits nearly any combination of properties. As a result of this work, three polymers have been developed. Adiprene L-100 is useful for covering the conventional rubber range up to 90 Shore A ( I ) . Adiprene L-167 is useful for preparing hard rubber stocks up to 95 Shore A (approximately 50, Shore D). The third polymer, LD-213 extends the available hardness range upward from 50 to 75-80, Shore D. It yields vulcanizates which may be called elastoplastic because they have properties intermediate between those of hard elastomers and structural plastics. The vulcanizates prepared from the starting polymer of 7000 apparent molecular weight exhibited low tensile and tear strength. This material was not considered for further development. The hard vulcanizates are being investigated for potting and encapsulation, as chemical and abrasion resistant coatings, and for the preparation of molded goods. They are promising as replacements for plastics and metals in gears, bearings, ball joint liners, heel lifts, and solid industrial tires.
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Adding toluene diisocyanate before vulcanization increases modulus, hardness, and resistance to compression set for all polymers
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Figure 4. Maximum hardness and modulus are obtained with 80% Moco a t 100' C.
Maximum tear strength occurs a t a low cure temperature (70' C.) and a high Moca (1 00%) level
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited (1) Rubber Age 85, 77 (April 1959). RECEIVED for review June 29, 1959 ACCEPTED May 3, 1960 Division of Rubber Chemistry, ACS, Los Angeles Calif., May 1959.
