TPU from aliphatic diisocyanates.

In recent years, thermoplastic polyurethane resin (TPU) usage has grown at a very high rate. The desirable combination of excellent mechanical properties over a wide temperature range, paintability, compatibility with other plastics and recyclability make TPUs ideal candidates for many demanding applications. In addition, there are increasing demands for TPU resins that not only have excellent properties but are also resistant to color changes in an environment under constant radiation exposure from sunlight or artificial sources. For these applications, aliphatic diisocyanate-based TPUs are the natural answer. Aliphatic TPUs are commonly prepared from either methylene-bis(4-cyclohexylisocyanate) ([H.sub.12]MDI) or isophorone diisocyanate (IPDI). In addition to similar morphology/property relationships like those in the methylene (diphenyldiisocyanate) (MDI) based elastomers, the existence of steric isomers of both [H.sub.12]MDI and IPDI further complicates the phase morphology of aliphatic diisocyanate-based TPUs.

Thermoplastic polyurethanes are block copolymers consisting of hard segments derived from diisocyanates, a low molecular weight chain extender, and soft segments that are mainly polyols. Among the factors that affect the morphology of TPUs (refs. 1-7), solubility parameter and molecular weight of the polyol are probably the two most commonly used to design desirable TPU compositions (refs. 3 and 8). For the two typical polyols that are used to make TPU today, polyester and poly(tetramethylene oxide) glycol (PTMEG), polyester polyols afford smaller differences in the solubility parameters between soft segments and hard segments. Therefore, polyester polyols can be used at wider molecular weight ranges (650 - 4,000). On the other hand, good TPUs can be prepared typically only from PTMEG with molecular weight of 2,000 or below.

However, a new generation of oxyalkylene polyols (PPG), manufactured using an organometallic catalyst, provides a nice alternative to prepare TPU with polyether backbones. These PPGs have very low unsaturation and monol contents (table 1). They not only can afford high molecular weight TPUs with excellent properties, their more favorable solubility parameter (ref. 8) as compared to PTMEG also allows the possibility of preparing low hardness polyether TPUs using higher molecular weight PPGs.

Table 1 - polyol properties

Polyol   Molecular    Unsaturation   % EO   Primary OH
        weight (Mw)     (meq/g)                (%)
  A       2,000          0.006       24.8      82.1
  B       4,000          0.011       19.2      88.7
  C       7,000          0.017        9.0      70.0
  D      11,300          0.019        3.8      82.2
  E       4,000          0.061       21.1      84.8 To date, some interesting studies on aliphatic thermoplastic polyurethanes (ATPU) derived from [H.sub.12]MDI with varied steric isomer ratios using either polyester polyols or PTMEG were reported (refs. 9-11). However, similar work using IPDI with different steric isomer ratios was not found in the literature. In this study, we have prepared various ATPUs based on the new generation of PPGs using both [H.sub.12]MDI and IPDI with different steric isomer ratios. The results of this study not only provide an interesting comparison to the previous studies that were based on [H.sub.12]MDI and polyester polyols or PTMEGs, but also provide further illustrations on the relationships between the phase morphology of a TPU and its properties.

Experimental

The different isomer ratios of [H.sub.12]MDI were prepared by fractional crystallization. The different isomer ratios of IPDI were prepared by fractional distillation.

The physical properties of the polyoxyalkylene glycol used in this study are summarized in table 1. ATPUs with different hard segment levels were made with [H.sub.12]MDI and IPDI using either 1,4-butanediol or ethylene glycol as the extender. The hard segment is defined as the chain extender-isocyanate reaction product, and the soft segment is defined as the polyol-isocyanate reaction product. An NCO/OH index of 1.01 was maintained throughout.

TPU preparation

The polyol was charged into a resin flask, heated to 105-110[degrees]C and vacuum dried for 1.5-2.0 hrs. The dried polyol was charged into another resin flask along with the appropriate amount of BDO chain extender; primary antioxidant, 0.4 wt. %; secondary antioxidant, 0.1 wt. %; and lubricant, 0.5 wt. %. The mixture was vacuum dried for 1-2.0 hrs. at 85[degrees]C. To prepare the one shot casting, the hot polyol (80-90[degrees]C) was weighed into a plastic beaker, the appropriate amount of tin catalyst (0.01-0.01 wt.) was added, followed by the calculated amount of 80-90[degrees]C [H.sub.12]MDI. The mixture was rapidly stirred until it started to gel (15-30 sec.) and then was poured into a Teflon coated pie pan. The cured casting was granulated, vacuum dried at 100[degrees]C, and compression molded into plaques in aluminum molds on a Carver laboratory press. TPUs based on IPDI were prepared in a similar manner.

The TPUs were tested on an Instron using standard ASTM procedures. The molecular weights were determined by gel permeation chromatography (GPC) using polystyrene standards in N,N-dimethylformamide as the solvent. Dynamic mechanical spectra were determined using a Rheometrics RDA H instrument.

Results and discussion

There are three possible steric isomers of [H.sub.12]MDI, the trans,trans, the cis,trans and the cis,cis (figure 1). Among them, the trans,trans has the most linear structure. Therefore, it is recognized that TPU hard segments derived from trans,trans [H.sub.12]MDI have the best packing order. Typically, [H.sub.12]MDI with different isomer ratios are referred to by their trans,trans isomer contents. The [H.sub.12]MDI commercially available today contains 20% trans,trans isomer ([H.sub.12]MDI20). [H.sub.12]MDI with other isomer ratios can be prepared using a variety of methods. We prepared two other [H.sub.12]MDI samples [H.sub.12]MDI-11 and [H.sub.12]MDI-48) using fractional crystallization methods. For this study, we also prepared a series of TPUs that have the same hard segment content, using the three [H.sub.12]MDI samples [H.sub.12]MDI-11, [H.sub.12]MDI-20 and [H.sub.12]MDI-48). The polyols used were the new PPGs that have low unsaturation and low monol contents. The physical properties of some examples are summarized in table 2. The results shown in this table demonstrate quite nicely the effect of both the molecular weight of the polyol and the trans,trans isomer content of the [H.sub.12]MDI on the physical properties and phase morphology of the resulting TPU.

[TABULAR DATA 2 OMITTED]

First of all, when the hard segment content of a TPU was kept constant, the elastomers based on 2,000 molecular weight polyol had higher hardness values than those based on 4,000 molecular weight polyol. However, higher hardness does not necessarily mean that these elastomers will also have higher modulus or tensile strength. When [H.sub.12]MDI with 20% trans,trans isomer ratio ([H.sub.12]MDI-20) was used, the hard segments had little packing order. Therefore, the soft segments of the elastomers based on 2,000 molecular weight PPG were quite miscible with the hard segment. As expected, this miscibility between soft and hard segments was reduced when the soft segment molecular weight was increased. This change in phase miscibility was clearly demonstrated by comparing the tan [sigma] position and peak width of the polyurethanes based on 2,000 and 4,000 molecular weight (figures 2 and 3). On the other hand, when [H.sub.12]MDI with 48% trans,trans isomer ratio ([H.sub.12]MDI-48) was used, the hard segments were highly ordered, therefore, even for those based on 2,000 molecular weight PPG the hard and the soft segments were quite phase segregated. Again, these changes in phase morphology can be demonstrated by their tan 8 curves (figures 2 and 3). The higher degree of phase mixing in sample A-2000-20 and sample B-4000-11 (2) means that in these samples, more hard segments are soluble in the soft segments, and the resulting elastomers have much lower ultimate tensile strength.

It is interesting to note that when the more linear [H.sub.12]MDI-48 was used for the hard segments, elastomers based on either 2,000 or 4,000 molecular weight (sample A-2000-48 sample B-4000-48) were found to have similar phase morphology (figure 4), and their properties were quite similar. However, at very high strain (>500%), stress of the elastomer based on 2,000 molecular weight polyol (A-2000-48) increased at a much faster rate than the elastomer based on 4,000 molecular weight (B-4000-48) (table 2). In earlier studies using the infrared dichroism technique, it has been demonstrated by comparing the orientation functions that when a TPU was under stress, the soft segments align parallel to the applied stress at all strain levels (refs. 12 and 13). At high strain level the elastomeric soft segments begin to uncoil and extend. Therefore, it is quite reasonable to assume that shorter soft segments at high strain level would probably have a higher degree of extension that requires higher external force to achieve the same strain level. This higher force requirement results in higher tensile strength at high elongation.

When comparing elastomers based on the same polyol but using [H.sub.12]MDI with different isomer ratios (figure 3), [H.sub.12]MDI with the highest trans,trans isomer ratio always afforded elastomers with higher storage modulus in the rubbery plateau region and better dimensional stability at higher temperatures. For the TPUs based on 2,000 molecular weight PPG, the difference in [H.sub.12]MDI isomer ratio has a significant influence on the phase miscibility. Therefore, the difference in isocyanate isomer ratios also causes significant difference in rubber plateau storage modulus, thermal dimensional stability, and tan [sigma] peak position and peak width (figure 2). On the other hand, elastomers based on 4,000 molecular weight PPG are already quite phase segregated, even when [H.sub.12]MDI with a very low trans,trans isomer content was used in the hard segments. A further increase in the trans, trans isomer content was found not to have a large effect on the tan [sigma] peak position or the storage modulus of the elastomers (figure 3).

Consistent with earlier studies on other TPUs based on either aromatic diisocyanates or IPDI (refs. 6 and 14), these results also demonstrate that 4,000 molecular weight PPG is a better choice than 2,000 molecular weight PPG to prepare [H.sub.12]MDI-based aliphatic thermoplastic polyurethane elastomers with hardness below Shore 85A. For higher hardness TPU, the balance between solubility parameters and polyol molecular weight still requires the use of PPG with 2,000 molecular weight or below to provide the most desirable morphology.

As mentioned earlier, the introduction of a new generation of PPGs that have very low unsaturation and monol contents makes it possible to prepare TPU with high molecular weight and good properties that are useful for these comparative studies. The comparison of properties of aromatic diisocyanate-based TPU was reported before (refs. 6 and 15). In a similar manner, [H.sub.12]MDI-20 based TPU using either conventional PPG or the new generation PPG with 4,000 molecular weight were prepared and compared. The comparison of the physical properties is summarized in table 3, and that of the dynamic mechanical properties is shown in figure 5. These results clearly demonstrated that the new PPG-based TPU has a much higher molecular weight than the traditional PPG based elastomer. For properties that are not affected by the polyurethane molecular weights, such as the glass transition temperature and phase miscibility (as indicated by the peak position and peak width of the tan [sigma] curve), both elastomers are quite similar. However, for those properties that are highly dependent on polyurethane molecular weights, such as the tensile strength, tear strength and elongation, the elastomer based on the new PPG is much better by a large margin than that based on the traditional PPG. Furthermore, higher molecular weight and lesser amount of monofunctional fragments also allow the new PPG-based TPUs to have better thermal stability as indicated by the extended rubber plateau.

Table 3

Property                 B-4000-20    E-4000-20
[H.sub.12]MDI, % t,t         18.8         18.8
Split, KN/m                 16.29        10.68
Die C, KN/m                 60.60        33.10
Tensile strength, MPa       18.50         4.93
Modulus, MPa
100%                         4.82          2.16
200%                         7.08          3.93
300%                         8.26          4.73
Elongation                    783           280
Shore hardness                76A           64A
Wt. average MW            293,900       150,500

One of the unique advantages of using these new PPGs as building blocks for TPU is the possibility to prepare a good, softer TPU using higher molecular weight PPGs. For example, we have prepared good ATPUs with lower than Shore 50A hardness using 7,000 and 11,000 molecular weight PPGs. The physical and dynamical properties of these two TPUs are shown in table 4 and figure 6. As these results show, both TPUs have excellent molecular weights and mechanical properties. The length and flatness of the rubbery plateau region of both materials also indicate that they maintain excellent elastomeric properties over a wide temperature range.

Property             C-7000-20       D-11,300-20
[H.sub.12]MDI, % t,t         18.8           18.8
Split, KN/m                  8.93           8.93
Die C, KN/m                 26.44          26.80
Tensile strength, MPa        4.03           3.95
Modulus, MPa
100%                         1.25           1.20
200%                         2.12           2.02
300%                         2.89           2.92
Elongation                    420            473
Shore hardness                50A            48A
Wt. average MW            213,900        256,100

IPDI also has steric isomers. However, unlike [H.sub.12]MDI, the difference in molecule linearity between the cis isomer and the trans isomer is not very obvious, as shown in figure 1. Therefore, the polyurethane elastomers derived from IPDI with different isomer ratios may have property differences similar to those observed in [H.sub.12]MDI based TPU. The current commercially available IPDI has cis/trans isomer ratio of about 75/25. We have prepared two more IPDI samples that have different cis/trans isomer ratios (50/50, 34/66) and then, in turn, used them to prepare aliphatic thermoplastic polyurethanes.

A comparison of the physical properties of these IPDI based TPUs is summarized in table 5 and their dynamic mechanical properties are shown in figure 7. Quite interestingly, we have observed that when the isomer distribution between cis and trans isomers of the IPDI used changes between high cis to equal cis-trans to high trans, the properties of the TPU go through a minimum. Although, the difference in linearity between cis and trans isomer is not large, as shown in figure 1, higher hard segment ordering is still achieved by the closeness of the molecules with the same steric arrangement. The resulting hard segments with better ordering probably explain why IPDI with either higher cis or higher trans isomer concentration affords a better TPU than those derived from IPDI with an even cis/trans isomer distribution.

Property                  B-4000-75     B-4000-50     B-4000-34
IPDI, c,t                    75/25         50/50         34/66
Split, KN/m                  19.97         18.91         17.37
Die C, KN/m                  68.48         55.52         83.89
Tensile strength, MPa        19.12         14.22         23.22
Modulus, MPa
100%                          4.18          3.29          3.98
200%                          6.33          5.28          5.76
300%                          8.29          6.99          7.28
Elongation                     665           650           835
Shore hardness                 82A           80A           85A
Wt. average MW             610,100       348,900       628,700

Conclusion

High molecular weight aliphatic thermoplastic polyurethane with excellent physical properties can be prepared from a new generation of poly(oxyalkylene oxide) glycol (PPG) that have very low unsaturation and monol content. Due to more favorable phase morphology, lower hardness TPUs can be prepared using higher molecular weight PPGs. The difference in the trans,trans isomer content of a [H.sub.12]MDI has a more pronounced effect on the properties of the TPU with a higher degree of hard/soft segment miscibility. The two isomers (cis or trans) in IPDI have similar non-linear structures. These isomers by themselves do not affect the physical properties of a TPU in either direction. However, higher content of either isomer will afford TPUs with better properties.

Acknowledgements

"TPU from aliphatic diisocyanates" is based on a paper presented at the October, 1994 meeting of the Polyurethane Division of SPI.

"A new noise absorbing, vibration damping PU" is based on a paper presented at the October, 1994 meeting of the Rubber Division.

[Figures 1 to 7 ILLUSTRATION OMITTED]

References

(1.) G.A. Senich and W.J. MacKnight, Multiphase Polymers, Chapter 6, S.L. Cooper and G.M. Estes, ed., American Chemical Society 1979, Washington, D.C. (2.) C.S. Paik Sung, T.W. Smith, C.B. Hu and N.H. Sung, Macromol., 12, 528 (1979). (3.) K.K. Hwang, S.S.B. Lin, S.Y. Tsay and S.L. Cooper, Polymer, 25, 947 (1984). (4.) R.W. Seymour, A.E. Allegrezza, Jr. and S.L. Cooper, Macromol, 6, 896 (1973). (5.) A.V. Tobolsky and H.F. Mark, Polymer Science and Material, Wiley-Intersciences, 1971, New York. (6.) A.T. Chen, R.R. Wells, C.P. Smith, J.W. Reisch, M.M. Emmet and J.M. O'Connor, Proceedings, Polyurethane World Congress 1993, SPI, 388 (1993). (7.) C.S. Paik Sung, C.B. Hu and C.S. Hu, Polymer Preprints, 19, 679, 686, 692 (1978). (8.) Y.P. Chang and G.L. Wilkes, J. Polym. Sci., 13, 455 1975). (9.) C.A. Byrne, D.P. Mack and J.M. Sloan, Rubber Chem. Tech., 58, 985 (1985). (10.) J.W. Dieter and C.A. Byrne, Polym. Eng & Sci., 27, 673 (1987). (11.) S.D. Seneker, L Born, H.G. Schmelzer, C.D. Eisenback and K. Fischer, Colloid Polym. Sci. 270, 543 (1992). (12.) R. W. Seymour and S.L Cooper, Rubber Chem. & Tech., 47, 19 (1977). (13.) J.C West and S.L Cooper, J. Polym. Sci. Polym. Sym., 60, 127 (1977). (14.) C.P. Smith, A.T. Chen and J.M. O'Connor, Proceedings, Antec 93, SPE, 1271 (1993). (15.) C.P. Smith, J. W. Reisch and J.M. O'Connor, J. Elast. & Plastics, 24, 306 (1992).