Cast Elastomers from Ethylene Glycol and 1,4-Butane Diol Curatives. K. Hillshafer, J. Westfall and S. Stefanova Stepan Company April, 2015

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1 Cast Elastomers from Ethylene Glycol and 1,4-Butane Diol Curatives K. Hillshafer, J. Westfall and S. Stefanova Stepan Company April, 2015 Abstract The major curative for hot cast elastomers from methylene diphenyl diisocyanate prepolymers is generally 1,4-butane diol due to the symmetry and crystallinity of the hard segments created as these materials react. Ethylene glycol, however, is a comparatively inexpensive alternative to 1,4-butane diol which reacts with the same prepolymers to form hard segments with much higher melting points and comparatively better critical phase separation. In this study, the advantages and differences in hard segment melting point, reactivity and physical properties of hot cast elastomers cured with ethylene glycol and 1,4- butane diol are examined. Introduction The use of 1,4-butane diol (BDO) as a prepolymer curative for polyurethane elastomers is extensive and well-documented. 1 3 The symmetry of BDO, when reacted with 4,4 - diphenylmethane diisocyanate (MDI), generally produces crystalline materials that become hard domains in two phase elastomers. Blackwell et. al. have previously characterized this phenomena with x-ray diffraction crystallography. 4 6 When BDO is replaced with ethylene glycol (EG), however, higher hard segment melting points have been observed via Differential Scanning Calorimetry (DSC). 7 This is to be expected, since at the melting point of a substance, T melt, the Gibbs free energy, G melt, depends upon the enthalpy, H melt and entropy S melt :

2 G melt = H melt T melt S melt (1). Because G melt is positive (nonspontaneous) at temperatures below a melting point and negative (spontaneous) above a melting point, the continuous function (1) is subject to the Intermediate Value Theorem and accordingly must be zero at the melting point. Then at the melting point of a substance, (1) simplifies to T melt = H melt S melt (2). Within a family of similar materials, the enthalpy is usually similar, so the numerator in (2) is essentially constant. Entropy is governed by Boltzman s landmark relationship between entropy, the probability of finding a molecule in a particular arrangement,, and Boltzmann s constant, k B, as so that at a melting point, S = k B ln(ω) (3), T melt = ~Constant k B ln(ω) (4). The value of is related to the degrees of freedom a molecule has which in turn is related to the number of atoms in the molecule considered. Since polyurethanes from EG and MDI have less atoms and degrees of freedom for conformations than the analogous BDOpolyurethanes, the denominator in (4) is smaller for EG-extended polyurethane elastomers versus the analogous BD-polyurethanes making T melt higher for EG-extended prepolymers. The proceeding result is influenced by a number of particulars and the media (or soft segment) in which polymerization and hard segment formation develops is of significant influence. Specifically, a less polar polyol medium should support the formation of isolated (phase segregated) polar and crystalline hard segments as in the case of shorter and more polar

3 carbamates of EG versus BDO. Whereas BDO prevails at the industry standard for a glycol chain extender, it was of interest to see if EG could be advantageous in certain instances over BDO as different polyols are used for soft segment formulation. Experimental STEPANPOL PC-1040P-55 (an ~ 55 mg KOH/g OH value ethylene-butylene adipate, Stepan), STEPANPOL PC-102P-56 (nominally 56 mg KOH/g OHv butylene-adipate), VORANOL N (typical 56 mg KOH/g OH value polypropylene glycol, Dow Chemical), MONDUR M (4,4 -diphenyl methane diisocyanate, Bayer), BDO (1,4-butane diol, Aldrich), EG (ethylene glycol, Aldrich) and BiCAT 8 (organometallic bismuth catalyst, Shepherd Chemical) were all used as received. The procedure to prepare each prepolymer was to react predetermined amounts of premelted MDI and a polyol at ca. 70 C for two hours. The resulting products were stored to cool under a nitrogen blanket and then analyzed for weight percent of available isocyanate content (%NCO) by the standard, indirect di-n-butyl amine method (ASTM D 2572). Prepolymers were later reacted in 3 weight percent excess with a curative, either BDO (45 g/eq.) or EG (31 g/eq.), by hand mixing at room temperature followed by the addition of ~ 0.1% by weight of BiCAT 8 latent catalyst. Brief stirring was resumed and the blend was subsequently cast into a suitable room temperature mold and pressed at 120 C (Table I). After one hour, elastomeric plaques were demolded and allowed to equilibrate for at least one week at 25 C and 50% relative humidity before testing.

4 Table I Elastomer formulations to produce plaques for study Reference Polyol Curative Prepolymer % NCO 15BDO1040 PC-1040P-55 BDO EG1040 PC-1040P-55 EG BDO1040 PC-1040P-55 BDO 10 10EG1040 PC-1040P-55 EG 10 5BDO1040 PC-1040P-55 BDO 5 5EG1040 PC-1040P-55 EG 5 15BDOPPG PPG-2000 BDO EGPPG PPG-2000 EG BDOPPG PPG-2000 BDO 10 10EGPPG PPG-2000 EG 10 15BDO102 PC-102P-55 BDO EG102 PC-102P-55 EG BDO102 PC-102P-55 BDO EG102 PC-102P-55 EG Dog bone parts were stamped from plaques to measure tensile strength, elongation, and modulus (ASTM D 638, Type I). Heat sag resistance was obtained by suspending horizontally 1 X 6 rectangular strips of elastomer samples and measuring the droop after placement in a 250 F oven for 30 minutes. The remaining uncut plaque portions were analyzed for Shore A and D hardness (ASTM D 2240) and characterized by Differential Scanning Calorimetry (DSC; TA Instruments Discovery Trios Software v ) with data collected from -60 C to 250 C at the rate of 10 C/minute. Results & Discussion The studies as outlined in Table I were conducted in three groups as distinguished by the polyol used to prepare prepolymers. The data analysis was to review the physical property and DSC results pairwise; that is, comparison of prepolymers identical in both %NCO and polyol backbone against the glycol extender used to make final elastomeric plaques. No qualitative differences in reactivity between EG- and BDO-cured elastomers were noted while hand mixing.

5 For all results, the pairwise measured modulus was either statistically the same or trivially different. Furthermore, BDO-extended prepolymers generally gave slightly higher Shore A and D hardness values. Beyond those properties, the project focus was tensile strength and ultimate elongation where higher values were regarded as better or improvements, heat sag where less (lower values) was considered better and hard segment melting points as interpreted from thermal transitions in DSC plots. Table II lists the analysis results of plaques prepared by curing prepolymers from MDI and STEPANPOL PC-1040P-55, an ethylene-butylene adipate. Table II Elastomeric plaque analysis from an ethylene-butylene adipate soft segment Reference Hardness Shore A/D Tensile Strength, psi Elongation, % Modulus, psi Heat Sag, cm 250 /30 min. Thermal Transition, C 15BDO 65/ EG 63/ BDO 61/ ND* 10EG 60/ ND 5BDO 55/ ND 5EG 54/ ND *No thermal transition detected The percent of the standard deviation relative to the average of the data, often called the coefficient of variation or relative standard deviation, was < 3% for all tensile, elongation and modulus results. The differences in these results in Table II within equivalent % NCO prepolymers cured with EG and BDO are therefore statistically significant: BDO-extended elastomers outperformed the EG-extended products in tensile and elongation. Heat sag data were single measurements, so the data again show better results with BDO over EG by simple inspection. The heat sag results for the elastomers from cured 5% NCO prepolymers were all at the limit of no resistance, so study of the 5% NCO systems was suspended for the remainder of

6 the project. Note also the higher thermal transition (interpreted as a melting point) for the EGcured prepolymer versus the BDO analogue as expected from prior thermodynamic reasoning. The lower percent NCO elastomers failed to exhibit significant thermal activity, presumably because too few hard segments were formed in those cases. Accordingly, DSC measurements of 10 and 5% NCO prepolymers were omitted for the remainder of this study. Of significance in Table II was the higher melting point of the 15% NCO ethylenebutylene adipate cured from EG versus BDO (243 and 234 C, respectively) whereas the properties seemed inferior in the former case. This indicates higher-melting hard segments were forming from the EG-extended elastomer, but not attended by the expected property improvements. Differences in phase-segregation could be an explanation: high melting hard segments may not align (or crystallize) sufficiently in a particular medium (a prepolymer). Poor phase segregation could be altered by simply changing the selection of the polyol used to make a prepolymer, keeping the %NCO and isocyanate type invariant. Table III gives the analogous results of Table II for the elastomers prepared by extending MDI prepolymers from a 2,000 molecular weight polypropylene glycol (PPG), a polyether polyol.

7 Table III Elastomeric plaque analysis from a polypropylene glycol soft segment Reference Hardness Shore A Tensile Strength, psi Elongation, % Modulus, psi Heat Sag, cm 250 /30 min. Thermal Transition, C 15BDO 65/ EG 63/ BDO 60/ ND 10EG 61/ ND Table III shows improved properties for EG over BDO when the polyol soft segment was changed in the 15% NCO prepolymer series; these differences appear statistically significant. In the 10% NCO series, where less hard segments are formed, from extending with less glycol, this was not observed. Also, the relative relationship of the thermal transitions is the same as with the ethylene-butlyene adipate (EG gives the higher value), but these values were higher than the polyester series. The only deliberate difference between these two series of elastomers was the soft segment choice, supporting the concept that polyol influences the curative necessary to develop optimal properties. From this, it may be possible to change the performance of the chain extender in a cast elastomer by again changing the soft segment. With the ability to vary the monomers used, this is more easily accomplished with polyesters over polyethers. The third set of experiments was to change the polyester soft segment from the original ethylenebutylene adipate to a butylene adipate (STEPANPOL PC-102P-56). The reasoning was in exchanging from an ethylene-butylene adipate to a polypropylene glycol, the polyol (and likely prepolymer) polarity was reduced, allowing polar hard segments to phase segregate more efficiently forming more hard domains. This is supported by the DSC transition temperatures: since the same hard segments are likely formed (EG-MDI and BDO-MDI), but in a less polar

8 Tensile, psi (PPG) medium, the segments phase segregate more efficiently, resulting in less freezing point depression, or higher observed thermal transitions. Table IV shows the comparative evaluations of this ester swap. Table IV Elastomeric plaque analysis from a butylene adipate soft segment Reference Hardness Shore A/D Tensile Strength, psi Elongation, % Modulus, psi Heat Sag, 250 /30 min. Thermal Transition, C 15BDO 96/ EG 92/ BDO 92/ ND 10EG 90/ ND The tensile strength results for both EG- and BDO-cured elastomers in Table IV are now nearly identical. Remarkable, though, is the lower heat sag value for the EG versus the BDO extended elastomer. Again, the thermal transitions moderate between the previous values. Figure I is a graphical display of the tensile data for the 10 and 15% prepolymers of Tables II IV extended from BDO and EG Plot of 10 and 15% NCO tensile data by prepolymer type and curative Prepolymer % NCO by weight EG 1040 BDO 1040 EG PPG BDO PPG EG 102 BDO 102 Figure I. Colors group prepolymers from the same polyols; dashed lines are where BDO was used as the curative

9 In Figure I, the effects on tensile strength of changing soft segments (polyol) and curatives are displayed for comparison. For EG as a curative, an ethylene-butylene adipate matrix does not develop as high a tensile strength relative to a BDO curative. With the PPGbased prepolymer, the situation in reversed. In a purely BDO-adipate matrix, EG is seen as equivalent to a BDO curative. These conclusions are all with 95% (two-tailed) confidence. Remarkably, however, was the improvement (reduction) in heat sag with the 15% NCO butlylene adipate-based prepolymer when EG was used as the curative. Although curing 10% prepolymers with either diol curative gave lackluster sag resistance, the 15% NCO prepolymers gave far better heat sag resistance versus the elastomers based on ethylene-butylene adipate or a PPG. To rationalize this behavior, the solubility parameter, δ 2, may be estimated for repeating units of polyols and hard segments from the substance density, repeating unit molecular weight, M and factors for the i components, F i,that make up the repeating units: δ 2 = ρ F i M (5). From a soft segment aspect, STEPANPOL PC-1040P-55 ( 2 calculated = 9.00 (cal/cc) 1/2 ) may be viewed as more polar than a PPG matrix (6.77 (cal/cc) 1/2 ) and STEPANPOL PC-102P-56 (8.89 (cal/cc) 1/2 ), an adipate from BDO only, is in between. Similarly, the solubility parameters for the EG- and BDO-MDI hard segments may also be figured and compared with the soft segment results (Figure II):

10 Soft Segment: STEPANPOL PC-1040P-55 (EG-BDO Adipate) 9.0 STEPANPOL PC 102P-55 (BDO Adipate ) 8.89 Hard Segment: PPG (Propylene Oxide) 6.77 EG-MDI 12.8 BDO-MDI 10.7 Decreasing Polarity 13 2 (cal/cc 3 ) 1/2, calculated 5 Figure II. Relative positions of soft and hard segments by calculated solubility parameter Polyols of lower polarity may provide a media where higher polarity hard segments may effectively agglomerate to form hard domains. Larger difference in these two polarities creates a better phase segregation during the polymerization process and therefore higher properties like tensile and elongation and less heat sag. Better phase segregation also relates to higher crystalline melting points, an absence of freezing point depression, as seen in the DSC data. We have shown that with ethylene-butylene adipate prepolymers, BDO is a superior curative to EG in our tests. However, this is not the case as the polyol or soft segment is changed. Lower polarity in a curing media (polyol) tends to favor EG, especially for tensile and heat sag properties with higher isocyanate content. Current industry trends favor EG over BDO

11 economically. Regulatory pressures may cause restrictions on obtaining BDO in the future, making EG a more attractive alternative when possible. References 1. Oertel, G. (1993), Polyurethane Handbook, 2 nd edition, Hanser Publishers, New York, pp. 389, Szycher, M. (2013). Szycher s Handbook of Polyurethanes. 2 nd edition, CRC Press, FL, p Woods, G. (1987). The ICI Polyurethanes Book. Wiley, New York, p Blackwell, J. and Lee, CD (1983). Hard-Segment Domain Sizes in MDI/Diol Polyurethane Elastomers. J. Poly. Sci., 21, Androsch. R. Blackwell, J. Chvalun, S. N., Festel, G., Eisenbach, C.D. (1997). X-Ray investigation of the structure of polyurethena elastomers based on 1,5-napthalene diisocyanate. Acta Polymer, 48, Blackwell, J. (1983). The Structure of Plyurethane Elastomers. Case Western Reserve University Publication, Turner, Robert B. and Morgan, Roy E., Reaction injection molded polyurethanes having particular flexural modulus factors and at least two thermal transition temperatures in a particular range, US Patent 4,246,363, filed June 19, 1989, issued January 21, 1981 Acknowledgements Appreciation is in order for Ms. Stella Stefanova for part preparation and DSC analysis. Mr. Tom Johnson completed the physical testing.