A new liquid carbodiimide hydrolytic stabilizer for ester polyurethanes and TPUs.

The objectives of this article are to briefly 1) discuss the historical overview of carbodiimides, 2) study carbodiimide chemistry and 3) illustrate what makes carbodiimide chemistry unique for polyurethane elastomers and for thermoplastic urethane elastomer applications. During these discussions, the article will make claims as to how these carbodiimide additives make a better castable and millable polyester polyurethane perform in the highly engineered atmosphere in which they are placed.

Carbodiimide chemistry originated in the early 1850s through the Hinterberger's lead oxide catalyst work. Both Zinin and Biziro in the 1860s expanded Hinterberger's work through the use of a thiourea derivative. However, Weith in the early 1870s correctly characterized and formulated the carbodiimides prepared by both Hinterberger and Biziro (ref. 1).

Carbodiimide chemistry research interest focused as a tool to aid nucleotide and peptide synthesis and as permease inhibition. Industrial interest did not reappear until the advent of the new synthetic polymers in the early 1930-50s. The major research for carbodiimides occurred through the direction of German researchers Zetzsche and E. Schmidt. Further interest picked up with Balon's synthetic efforts at DuPont resulting in his patents in 1956-58 (ref. 2).

A 1965 U.S. patent issued to Neumann, Holtschmidt and Fischer orchestrated the use for "Stabilization of polyesters with polycarbodiimide (ref. 3)." A year later, several Rubber Chemistry and Technology articles dealt with the hydrolytic stability/instability of urethane elastomers in various environments. The studies initiated by Ossefort and Testroet (ref. 4), and Magnus, Dunleavy and Critchfield (ref. 5) spearheaded the efforts to bring carbodiimide chemistry back into favor.

Three years later, a paper by Schollenberger and Stewart (ref. 6) demonstrated the important role carbodiimides play in hydrolytically stabilizing polyester thermoplastic polyurethanes produced from high acid number polyester polyols. This was further substantiated in an unpublished internal Bayer AG article by Neumann, Holtschmidt and Kallert, who demonstrated the aging and hydrolytic stability of polyester elastomers containing carbodiimides.

Chemistry

Chemistry structures of industrially used carbodiimides have the R - N = C = N--R basic structure. The common reactions which occur with the carbodiimides, - N = C = N--, occur as addition reactions. The R groups attached to the carbodiimide functionality determine selectively what type of addition reactions occur and under what kinetically driven conditions.

With the aid of the R groups, carbodiimides selectively act as acid and water scavengers. Through these reactions, they convert and neutralize both water and acids into nonhazardous urea structures as illustrated in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

Monomeric and polymeric carbodiimide structures are commercially available. The monomeric carbodiimides have a high degree of application in stabilizing not only castable polyurethane polyester elastomer, but also monofilament polyethylene terephthalates. The uniqueness of the molecule with its specific steric isopropyl groups allows it to selectively react with water and also with low molecular organic and inorganic acid moieties. In addition to the selectivity of the isopropyl group side chains, it also helps to stabilize the carbodiimide molecule from a shelf storage life stand point (figure 2).

[Figure 2 ILLUSTRATION OMITTED]

In the case of polymeric carbodiimides, the same specific steric isopropyl groups are utilized to selectively react with water and also low molecular organic and inorganic acid moieties at the high processing temperatures and application temperatures. In the new oligimeric carbodiimide, the steric groups are a larger bulk molecular entity. The generic structure for the oligimeric carbodiimide is illustrated in figure 3.

[Figure 3 ILLUSTRATION OMITTED]

However, the reactivity at ambient temperatures is considerably slower for the polymeric carbodiimides as opposed to the monomeric carbodiimides. The ambient temperature reactivity rates for carbodiimides are: Monomeric carbodiimide [is greater than] new oligimeric carbodiimide [is greater than] polymeric carbodiimides.

The polymeric carbodiimides find their application forte in higher temperature engineering plastics. For example, the residual carboxylic acid groups in a polyethyleneterephthalate (PET) undergo reactions with the carbodiimides at elevated reaction temperatures of several hundred degrees during the melt mix in the extruder. In order to preserve the excess carbodiimide in the extruder mix, the melt is extruded through a die to a monofilament and then cooled to room temperature by passing the fiber through a cold water bath.

The water/or acid reaction is important when one takes in to account the effects leveled upon polyester elastomers. It is well known (refs. 7-10) that under water/or acid exposure conditions, polyesters tend to hydrolyze and undergo an inversion of the polyester formation, i.e. polycondensation. The reactions shown in figure 4 demonstrate the polycondensation reaction, and then the saponifaction reactions by water and by acid.

[Figure 4 ILLUSTRATION OMITTED]

Experimental

One of the big challenges of the monomeric carbodiimide is the insoluble nature of the material in diol matrixes and some polyester blends. This was always the case for 1,4 butanediol where the monomeric material oiled the surface layer and would not disperse properly.

In the laboratory, an interesting experience developed in trying to solubilize the new liquid carbodiimide in a 1,4 butanediol matrix. Certainly the observable solubility improved by moving the 1,4 butanediol solution temperatures upward from ambient conditions to those of 70-80 [degrees] C. However, without continuous agitation, the 1,4 butanediol/ liquid carbodiimide material would start to precipitate out of solution. This occurred even when both materials were at 70-80 [degrees] C at the point of addition of 1-5% liquid carbodiimide to 1.4 butanediol. The only way to insure even distribution of the liquid carbodiimide in the 1,4 butanediol was to vigorously stir the material and create a dispersion in the butanediol matrix.

However, again in the laboratory, a pleasant surprise occurred when we ran solubility tests of the liquid carbodiimide in ethylene glycol, a shorter chain diol than 1,4 butanediol. The liquid carbodiimide was readily soluble in the ethylene glycol at the working levels of 1-5%, which previously was examined for the 1,4 butanediol extender.

Table 1 illustrates the solubility benefit of the new oligomeric carbodiimide compared to the monomeric carbodiimide.

Table 1 - solubility of monomeric carbodiimide versus new oligimeric carbodiimide

Matrix                Monomeric     New oligomeric

Desmophen 2000        Soluble       Soluble
Desmophen 2000 KS     Soluble       Soluble
1,4 Butanediol        Insoluble     Dispersible at
                       oily layer    70-80 [degrees] C
Ethylene glycol       Insoluble     Soluble at
                       oily layer    min. 5-10%
Propylene carbonate   --            25%
                                    50%
Trimethylolpropane    Insoluble     Dispersion at
                                     ~77 [degrees] C
Multranol 4012        --            Soluble at 10%

Water                 Insoluble     Soluble
Ethanol               Soluble       Soluble
Acetone               Soluble       --
Benzoflex             Insoluble     Soluble
 (Phlatate ester)

In order to aid control of addition of the new liquid carbodiimide to extruder throats for stabilizing thermoplastic polyurethanes (TPU), a viscosity versus temperature curve was generated in the laboratory. In figure 5, the curve illustrates the region of addition ease for both ester polyurethane elastomers, as well as ester thermoplastic polyurethanes.

[Figure 5 ILLUSTRATION OMITTED]

Due to the aliphatic character of the new liquid carbodiimide, there is less ultraviolet light degradation than the aromatic monomeric and polymeric carbodiimides.

Results and discussion

The traditional application for monomeric carbodiimides is as a hydrolytic stabilizer for cellular Vulkollan in the jounce bumper automotive application. Other areas ideally suited for monomeric carbodiimide use are in polyester pipeline pigs for the oil patch industry; large polyester elastomeric rollers; and in polyester adhesives for peel test improvement.

In the case of the cellular Vulkollan (NDI chemistry) application, the new liquid carbodiimide compared favorably with the traditional monomeric carbodiimide. The series of 80 [degrees] C water aging graphs (figures 6-9) illustrate the comparisons quite well.

[Figures 6-9 ILLUSTRATION OMITTED]

In figure 6, Shore A hardness values illustrate the new liquid carbodiimide loading levels at 1% and 2% versus comparable amounts of monomeric carbodiimide water aged over 14 days. However, notice at the 2% loading level for the new liquid carbodiimide, the initial Shore A hardness value is some three points lower than the 1% loading level and both loading levels for the monomeric carbodiimide. This difference continues across the duration of the 14 day 80 [degrees] C water aging study. At the 2% loading level for the new liquid carbodiimide, it almost acts like a plasticizer. This certainly occurs from practical experience with the monomeric carbodiimide at loading levels above 2%.

Figure 7 illustrates tensile strength comparisons for both products at the same 1% and 2% loading levels in the polymer. Clearly the 1% loading level of the new liquid carbodiimide runs parallel with the 1% loading level of monomeric carbodiimide. In the comparison of the 1% loading levels, the new liquid carbodiimide runs a slight improvement in the 80 [degrees] C water aging, starting at day three, and shows further improvement in the widened gap at the end of 14 days.

[Figure 7 ILLUSTRATION OMITTED]

Figures 8 and 9 demonstrate the elongation at break for the polymer aged in 80 [degrees] C water. Figure 8 covers the loading levels of 1% and 2% for both carbodiimide products over the 14 day comparison period. In that comparison, the 1% and 2% monomeric carbodiimide loaded polymers tend to increase to a maximum and then drop off starting at 11 days into the aging test. In comparing the new liquid carbodiimide at 1% and 2% loading levels, they also reach a maximum at 11 days and then drop off very slightly at the fourteenth day. However, the decreased elongation at break values drop off considerably less with the new liquid carbodiimide than the monomeric carbodiimide.

In figure 9, the 80 [degrees] C water aging study doubles from 14 days to 28 days. Also, an intermediate loading level of 1.6% for the new liquid carbodiimide shows improvement over the 2% loading level studied in figure 8. In both the 1% and the 1.6% loading levels of the new liquid carbodiimide, the aging study is a marked improvement over the 1% monomeric carbodiimide stabilized polymer.

Conclusions

The new liquid oligimeric carbodiimide provides a product which is soluble and easier to introduce into the polymer matrix. This product provides a more user-friendly means of introduction into the "B-side" of the two component polymer mix. This circumvents the straggle with a third stream introduction procedure or an introduction into the isocyanate, except for special cases as in butanediol or butanediol blends.

As a result of ease of introduction of the new liquid carbodiimide, two new applications were commercialized this year. One application involved improved introduction and hydrolytic stabilization of the new liquid carbodiimide versus the monomeric carbodiimide in small print rolls.

The other application involved a replacement of a reactive oxizolidine product and a processing improvement to stabilize a motor shaft coupling mount. In this application, the new liquid carbodiimide gave a more consistent performing polymer hydrolytically.

[Figures 3-6 ILLUSTRATION OMITTED]

Acknowledgements

"Peroxide curing of millable polyurethane" is based on a paper given at the May, 1998 Rubber Division meeting. "A new liquid carbodiimide hydrolytic stabilizer for ester polyurethanes and TPUs" is based on a paper given at the April, 1999 Rubber Division meeting. "An introduction to the chemistry of polyurethane rubbers" is based on a paper given at the April, 1999 Rubber Division meeting.

References:

(1.) Kuzer, Frederick and K. Douraghi-Zadeh. (1967), "Advances in the chemistry of carbodiimides," Chemical Reviews, 2:107-152.

(2.) Allesandro, Rocco Thomas. (1977), "The mass spectrometry of allyl and aryl carbodiimides," St. Johns University, New York, Ph.D. Thesis, pp. 1-265.

(3.) Neumann, W., H. Holtschmidt, J. Peter and P. Fischer. (1965), "Stabilization of polyesters with polycarbodiimides," U.S. Patent No. 3,193,522.

(4.) Ossefort, Z.T. and F.B. Testroet. (1966), "Hydrolytic stability of urethane elastomers," Rubber Chemistry and Technology, 39(4):1308-1,327.

(5.) Magnus, G., R.A. Dunleavy and F.E. Critchfield. (1966), "Stability of urethane elastomers in water, dry air and moist air environments," Rubber Chemistry and Technology, 39 (4):1,328-1,337.

(6.) Schollenberger, C.S. and F.D. Stewart. (1971), "Thermoplastic polyurethane hydrolysis stability," J. Elastoplastics, 3:28-56.

(7.) Brown, Daniel W., Robert E. Lowry and Leslie E. Smith. (1981), "Kinetics of the reaction between polyester acid and carbodiimide in dry polyester diols and in a polyester polyurethane, "Macromolecules, 14:659-663.

(8.) Brown, Daniel W., Robert E. Lowry and Leslie E. Smith. (1982), "Hydrolytic degradation of polyester polyurethanes containing carbodiimides, " Macromolecules, 15:453-458.

(9.) Mc Afee, E.R. "Sterically hindered carbodiimides: Use of selective agents for hydrolytic protection of ester and imide groups -- review of past, present and future, "paper presented at 34th Annual Society of Plastic Industry's Polyurethane Technical/Marketing Conference, October 21-24, 1992.

(10.) Oertel, Gunter, et. al. Polyurethane Handbook, 2nd Ed. Carl Hanser Verlag, (1994), pp. 402-403, 431.3