Stabilization strategies for weatherable PU.

Polyurethanes prepared from aromatic diisocyanates are susceptible to degradation due to exposure to thermal or ultraviolet light conditions. Such degradation results in a loss of physical and mechanical properties and an increase in discoloration of the PUR article (ref. 1). The use of appropriate stabilization systems can extend the useful service life of urethane articles. Selection of appropriate stabilization systems is a function of the production conditions, end-use environments and knowledge of fundamental degradation mechanisms of the polyurethane components. Polyurethane substrates such as adhesives, sealants, elastomers and fibers may require a multi-component stabilization approach in order to reduce the effects of both oxidative and photolytically induced degradation.

The loss of polyurethane properties has been attributed to degradation of both the polyol and the urethane components. The polyols are susceptible to degradation by free radical pathways that are induced by thermal or UV exposure. Under UV-irradiation conditions, oxygen uptake experiments have been used to follow the oxidative conversion of the alkylidene portions of polyether and polyester based polyols to hydroperoxides (refs. 2 and 3). The use of primary antioxidants suppresses the formation of free radical species and limits the hydroperoxides in polyols, both during storage and during conversion into urethane articles.

When the urethane component is based on aromatic diisocyanates, in particular those using diphenylmethane diisocyanates (MDI), the polymer is quite susceptible to photolytic discoloration and degradation. Aromatic polyurethanes and their primary photo-degradation products are reported to be sensitive to terrestrial light in the wavelength regions from cat 300 nm to beyond 400 nm. The yellowing of some aromatic polyurethanes is attributed to two mechanistic pathways. The first pathway involves an excitation of the aromatic chromophore to produce a photo-Fries product, such as a primary aromatic amine, at wavelengths up to about 340 nm. The second pathway involves further photooxidation of the initial photo-Fries products, at wavelengths of 300 nm to 400 nm, to produce ultimately hydroperoxide-containing and quinone-diimide type structures (refs. 3 and 4).

In addition to discoloration problems, the loss of useful physical properties from aromatic polyurethanes due to photodegradation is problematic. For example, the delamination of an acrylic melamine coating from a RIM polyurethane substrate was reportedly due to photochemically induced degradation of the underlying urethane groups. After accelerated weathering by QUV exposure for only 40 hours, degradation of the coated RIM article resulted in both an orange discoloration and formation of polyol fragments (ref. 5). Thus the RIM polyurethane appeared to degrade rapidly under light exposure, given the rapid color change and chain cleavage.

Different classes of stabilizers find utility in enhancing the UV-stability of glycol and urethane polymer subunits (ref. 6).

* Hindered phenols are well known for their efficacy as primary, "chain-breaking" antioxidants and sequestering of free radical pathways of degradation.

* Hindered amines based on 2,2,6,6-tetramethylpiperidinyl structures have been successfully utilized as light stabilizers to stabilize polyurethanes against photooxidative degradation (ref. 7). One proposed mechanism for HALS stabilization is shown in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

* Ultraviolet light absorbers act to dissipate UV light by intramolecular dissipation as heat. We have found that the structure of UV absorbers can be altered to obtain compounds that offer unique stabilizing potential especially for polyurethanes sensitive to long wavelength UV-light. Some examples for the application of these stabilizer classes are given below.

Experimental

Preparation of RIM type, MDI-based polyurethanes.

A solution of polyol, butanediol, catalyst and surfactant was prepared and deaerated under vacuum. The solution was then cooled, and MDI was added at once with vigorous stirring for cat 30 seconds. The polyurethane solution was poured into polystyrene petri dishes, then cured in vacuo at 50[degrees]C for one hour, then at ambient temperature overnight, stored in the dark. The PUR disks were removed and mounted for accelerated weathering exposure. At intervals the disk was removed from the Weather-ometer, measured for color change, and returned immediately to the weatherometer. An Atlas Ci65A Xenon Weather-ometer was operated under dry conditions, continuous light, black panel temperature 63[degrees]C, irradiance 0.35 watt/[M.sup.2]. Yellowness Index (YI) color was measured on an ACS CS-5 Chroma-Sensor spectrophotometer per ASTM D1925.

Preparation of PUR elastomer films.

A 20 wt.% masterbatch of PUR elastomer was prepared in methyl ethylketone, containing one weight percent of Irganox 245 as antioxidant. Portions of the elastomer solution containing the light stabilizers were coated onto release paper, stored in a hood for 24 hours, then oven cured at 100[degrees]C for one hour. Films were mounted for accelerated weathering exposure (dry xenon). YI color change was measured on an ACS Spectrophotometer per ASTM E-313.

Preparation of TPU films

The TPU was a commercial product. Additives were dissolved into a 25 weight percent TPU solution in N,N-dimethylformamide. Films were cast and solvent removed in vacuo to obtain films of cat 30 micron thickness. Films were mounted for accelerated weathering exposure (spray xenon) and YI color was measured at stated intervals.

Results and discussion

Light stabilization of a RIM-type, MDI-based polyurethane

A series of MDI based polyurethanes was prepared to evaluate the efficacy of UV absorbers and HALS to retard discoloration. The typical formulation is given in table 1.

Table 1 - MDI polyurethane formulation

Reagent                            Parts
Voranol 4701 polyether polygol       87
1,4-butanediol                       13
Dabco T-12                          0.1
Sag-47 surfactant                   0.1
Isonate 143L p-MDI                   49

The change in color results for two series of MDI-based polyurethanes subjected to accelerated weathering are shown in figures 2 and 3. The highest overall discoloration occurred in those polyurethanes containing either an o-hydroxyphenyl triazine (HPT) or o-hydroxy benzophenone (BP) as UVAs. Benzotriazoles provided a lower overall change in discoloration. Moreover the Tinuvin 327 appeared to provide favorably lower overall discoloration compared to Tinuvin 328. It is postulated that the bathochromic shift seen in the Tinuvin 327 spectrum, versus Tinuvin 328, provides enhanced stabilization of the aromatic polyurethane against the longer UV wavelength light. Both compounds Tinuvin 327 and UV-1 can screen light beyond 380 nm and into the visible region, which is not achievable with conventional UV absorbers. Conversely, the HPTs and BPs are generally blue-shifted relative to benzotriazoles, and this may account for their inferior performance in this polyurethane system. The comparative UV spectra for several of the UV absorbers is illustrated in figure 4.

[Figures 2 to 4 ILLUSTRATION OMITTED]

The performance of a developmental UV absorber and UVA-HALS combinations is shown in figure 3. The develop mental UV-1 provides for an even lower overall color change than other commercial W absorbers. The combination of a HALS with a WA slightly outperforms the UVA alone. It is postulated that the HALS is sequestering photo-induced free radicals, a complimentary mechanism to the W absorber.

PUR elastomers.

PUR elastomers are utilized in adhesive formulations with ultimate end use in consumer applications such as shoe soles. Preventing discoloration is typically required to maintain visual aesthetics of the article or retention of physical properties. The efficacy of several UVA-HALS systems to protect a polyester-based polyurethane is shown in figure 5. The change in YI color after 500 hours of XAW exposure is shown for films of a polyester polyol-based PUR used in adhesives. The combination of a WA plus a HALS clearly outperformed the WA alone for minimizing discoloration. While UV-1 slightly outperformed a conventional benzotriazole, the best performance was observed for the combination of UVW-1 plus Tinuvin 765.

[Figure 5 ILLUSTRATION OMITTED]

Thermoplastic polyurethanes (TPUs)

Thermoplastic polyurethane elastomers (TPU) are based on MDI, a linear polyether or polyester polyol and glycol chain extenders such as 1,4-butanediol. TPUs exhibit elastomeric properties while being processible as a conventional thermoplastic. Proper UV stabilization can be effective in reducing the extent of discoloration of TPUs during accelerated weathering exposure.

The rapid discoloration of an unstabilized TPU film is evident as depicted in figure 6. Various combinations of UV absorber with HALS are effective in reducing the rate of discoloration during accelerated weathering. We observed that out to about 200 hours of XAW weathering, the combination of HALS with WA gave a lower rate of discoloration than using a UVA alone. After 200 hours of XAW, all the stabilized formulations underwent discoloration at roughly the same rate. We are continuing to work to understand the causes of the apparent changes in discoloration rates. In addition, we are evaluating the developmental UV-1 in other MDI-based TPUs both for retention of low color and mechanical properties.

[Figure 6 ILLUSTRATION OMITTED]

Conclusions

Various classes of polyurethane substrates have been observed to undergo rapid discoloration in the absence of adequate stabilization. Our efforts have focused in this article on MDI based polyurethanes which are taught in the literature to be particularly sensitive to near UV wavelength light. It has been shown in some cases that the combination of a UV absorber with a hindered amine light stabilizer is more effective at reducing the extent of discoloration than the use of a UV absorber alone. It is postulated that the hindered amine acts as a free radical scavenger while the UV absorber transforms UV energy into less harmful heat energy. For some MDI systems, a benzotriazole UV absorber was found to be more effective at reducing overall discoloration than a triazine or o-hydroxybenzophenone. This may be due to the bathochromic absorbance shift of the benzotriazole versus the other two classes of compounds. A new, developmental UV absorber was found in some cases to provide improved color stability versus commercial UV absorber systems. Ongoing evaluations of this developmental chemistry include its efficacy in providing color stability and its influence on retention of mechanical properties in various polyurethane systems.

References

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