Chemical degradation of polyurethane.
There are many ways polyurethanes can chemically degrade. Some of these are:
* Solvolysis Hydrolysis is defined as the reaction with water. Thermolysis reactions are those which occur due to heat. Oxidation involves the reaction with oxygen. It can be initiated with heat (thermooxidation) or by light (photooxidation). Photolysis are the reactions caused by the interactions with light. Pyrolysis is considered those reactions which occur due to burning. There are also chemical degradations caused by the attack of microorganisms. These are called microbial degradations. Attack on polyurethanes by solvents, alcohols for example, can cause a degradation referred to as solvolysis. Complete papers could be and have been written on each. This article will review the first four: Hydrolysis, thermolysis, oxidation and photolysis.
First, let's examine the bonds this article will be referring to throughout (figure 1). Typically a diisocyanate and a polyol are reacted to prepare a polyurethane prepolymer. The polyols are usually either a polyether or polyester. That prepolymer then is cured with a diamine or a diol, resulting in either a urea or urethane. Upon postcuring, additional isocyanate can react with the urea to give biuret or with the urethane to give allophanate. The bonds subject to the various chemical degradations are highlighted for easy identification.
The three bonds most susceptible to hydrolytic degradation are the ester, urea and urethan (figure 2). The ester reverts to the acid and alcohol. This acid further catalyzes ester hydrolysis. This reaction then becomes autocatalytic. Because of the autocatalytic nature of ester hydrolysis it is the most prevalent. The urea bond hydrolyzes to a carbamic acid and an amine. The carbamic acid normally isn't stable and typically undergoes further reaction. The urethane, although somewhat less susceptible, undergoes hydrolysis to yield a carbamic acid and an alcohol.
Comparing various polyurethane systems, we can see in figure 3 that polyester-TDI MBOCA systems hydrolyze quite rapidly, two to four times faster than polyether-TDI MBOCA systems. Using the same polyester in an MDI-BD system we can see the polyester also degrades more rapidly than the analogous polyether system. We can also see the influence of environment on the degradation. The polyester in an MDI-BD system was enhanced to a greater resistance than that of the polyether TDI MBOCA systems (ref. 1). Another important environmental influence on hydrolysis is temperature. At 50 [Degrees] C the tensile half life of a polyester-TDI-MBOCA system may be four or five months while that of a polyether-TDI-MBOCA appears to be almost two years. At 70 [Degrees] C, however, these half lives fall to two weeks and five weeks, respectively. And at 100 [Degrees] C they become a matter of days.
As we have seen, polyesters do not fare well in hydrolysis situations. If a polyester must be used in a wet environment, carbodiimides help in prolonging their longevity. Carbodiimides act as acid scavengers. As seen in figure 4, the acid and carbodiimide react to form an intermediate which rearranges to give an N-acyl urea. This consumes the acid which can no longer catalyze the hydrolysis.
Figure 5 shows the marked increase in life span of this polyester with a 2% addition of polycarbodiimide. It should be remembered, however, that the carbodiimide is being consumed and eventually will be totally used up resulting in the onset of catastrophic hydrolysis.
Heat can cause degradation of polyurethane. The onset of allophanate dissociation is around 100-120 [Degrees] C. For biurets it is around 115 to 125 [Degrees] C. These reactions are dissociations and somewhat reversible. They give back the urethane or urea from which they were formed. The urethane begins its thermal degradation around 140 to 160 [Degrees] C which is prior to the urea which is about 160-200 [Degrees] C. Since the urethane group degrades before the urea, it is these degradation reactions I would like to review. The urethane can dissociate to the isocyanate and polyol from which it was formed. This reaction is reversible as long as the isocyanate is not lost to a side reaction. The second reaction produces a primary amine and an olefin. The third reaction produces a secondary amine. Since these latter reactions generate [CO.sub.2] which is lost as a gas they are irreversible. I will delve into the mechanism of these two reactions.
If we look at the urethane bond as pictured in figure 6, we see the aliphatic character of the polyol from which urethane was formed. Rewriting this slightly differently we can see how cleavage of the oxygen to the first [CH.sub.2] group and association of one hydrogen on the second [CH.sub.2] group would lead to the carbamic acid and an olefin (figure 7). The carbamic acid decomposes to give a primary amine and a [CO.sub.2].
If we do the same for mechanism B (figure 8) writing the urethane structure to include the first [CH.sub.2] group of the polyol, we can re-write it to visualize how the secondary amine would be formed. Cleavage of the oxygen [CH.sub.2] bond with association of the [CH.sub.2] hydrogen to the NH group would force cleavage of the nitrogen carbonyl carbon bond splitting out [CO.sub.2]. This would result in the secondary amine.
Again, which of these thermodegradation reactions takes place and to what extent depends on the structure of the urethane, the reacting conditions and the environment.
Once again, oxidation is simply that degradation which occurs due to the reaction with oxygen. Oxidations may be heat initiated or light initiated. Heat initiated oxidation is called thermooxidation and light initiated oxidation is called photooxidation. I would like to split this subject into these two parts, thermooxidation and photooxidation, and discuss each separately. In the discussion of photooxidation I will include photolysis reactions since they are closely related.
Previously we saw the ester to be the weak link in hydrolysis. Now it is the ether that is the weak link in thermooxidation (figure 9). Thermooxidation proceeds via a radical mechanism. Heat causes a hydrogen extraction at a carbon alpha to the ether linkage. This radical is subject to oxygen addition and forms a peroxide radical. The peroxide radical then extracts another hydrogen from along the backbone to form a hydroperoxide. The hydroperoxide radical then decomposes to form an oxide radical and the hydroxyl free radical.
The oxide radical will cleave at either of two places (figure 10). One, it may cleave at the carbon bond adjacent to the oxide radical. If so, formates are formed. If, on the other hand, cleavage is at the carbon-oxygen bond, aldehydes are formed. The order of stability of polyethers to thermooxidation is: PTMG types are more stable than polyethylene oxide glycols, which are in turn more stable than polypropyleneoxide glycols.
The exact mechanism of photolytic degradations is unsure. Photooxidation is believed to take place in MDI and TDI aromatic urethane via a quinoid route. The urethane bridge oxidizes to the quinone-imide structure as seen for MDI in figure 11. This structure is a strong chromophore resulting in the yellowing of urethanes. Further oxidation produces the diquinone-imide structure which is amber in color and is responsible in part for the browning of urethanes. To prevent the discoloring anti-quinoid or non-quinoid structures must be used.
A second scheme in the photolysis of polyurethane is the scission of the urethane bond. There are two possible bonds to cleave as seen in figure 12. The nitrogen to carbon bond can cleave to result in an amino radical and a formate radical. The formate radical will liberate [CO.sub.2] and an alkyl radical will result. Should the carbon to oxygen bond cleave, a carbamyl radical and an alkoxy radical will be formed. The carbamyl radical will decompose to generate the amino radical and CO. The net result of these urethane scission reactions then is three radicals (amino, alkyl and alkoxy) which will undergo further reactions.
Those further reactions are seen in figure 13. In equation 1, two amino radicals react to form an intermediate which in turn reacts with the alkoxy radical to form diazo products. These are again chromophoric materials which are to blame for polyurethanes turning brown in sunlight. The second reaction demonstrates how olefins are formed in these processes. The third reaction is an oxidation process wherein aldehydes are produced. In equation 4 the alkoxy radical undergoes scission to produce formaldehyde and another alkyl radical which may then be used in equations 2 or 3.
A third photo degradative process is known as the Photo Fries rearrangement. A sequence of this reaction for a TDI based urethane can be seen in figure 14. This rearranged product undergoes additional degradation to colored AZO products.
There are stabilizers for these radical producing processes. Certain families of compounds known as antioxidants and UV stabilizers have been shown to be effective in inhibiting degradation in polyurethanes. Hindered phenols and aromatic amine compounds act as radical chain terminators. Thioethers and phosphites are peroxide decomposers. Any of these compounds will disrupt the degradation process. Benzotriazoles, certain hindered amines and benzophenones will absorb the UV light and use its energy in a non-destructive sequence.
It can be shown that combinations of these compounds act synergistically. Typically a UV stabilizer such as benzotriazole used in conjunction with an antioxidant such as a hindered phenol will inhibit discoloring and property loss for longer periods of time than either one used alone.
In summary, this article has reviewed chemical polyurethane degradations including hydrolysis, thermolysis, thermooxidation, photooxidation and photolysis. It has shown that there are stabilizers which can help improve the longevity of a polyurethane in use. It has also inferred that selecting the right urethane for a given use is highly important.
References [1.] Pentz, W.J., Krawiec, R.G. "Hydrolytic stability of polyurethanes," Rubber Age, Vol. 107, No. 12, Dec. 1975, p. 39. [2.] Athey, R.J. "Watix resistance of liquid urethane vulcanizate," Rubber Age, Vol. 96, No. 5, Feb. 1965, p. 705. [3.] Schollenberger, C.S., Stewart, F.D., "Thermoplastic polyurethane hydrolysis stability," Advances in Urethane Service and Technology, Vol. 1, Chapter 4. [4.] Matuszak, M.L., Frisch, K.C., Reegen, S.L., "Hydrolysis of lineos polyurethanes and model monocarbamates," Journal of Polymer Science, Polymer Chemistry Edition, Vol. 11, No. 7, July 1973, p. 1683. [5.] Saunders, J.H., Frisch, K.C., "Polyurethanes chemistry and technology, part 1 chemistry," Interscience Publishers, New York, NY, (1964), pp. 106, 107. [6.] Fabris, H.J., "Thermal and oxidative stability of urethanes," Advances in Urethane Science and Technology, Vol. 6, p. 17380. [7.] Mathur, C.N., Kresta, J.E., "Thermooxidation and stabilization of urethane and urethane-urea block copolymers," Polymer Science Technology (Plenum), 26 (Polymer Addit.), p. 135-153. [8.] Osawa, Z., "Photodegradation and stabilization of polyurethanes," Dev. Polym. Photochem., 3, Chapter 6,209. [9.] Rek, V., Bravar, M., "Ultraviolet degradation of polyester based polyurethane," J. of Elastomers and Plastics, Vol. 15, January (1983), p. 33-42. [10.] Hoyle, C.E., Kim, K., "Photolysis of aromatic diisocyanate based polyurethanes in solution," J. Polymer Science, Part A: Polymer Chemistry, Vol. 24, p. 1880-1894.
PHOTO : Figure 1 - chemical reactions - polyurethane preparation
PHOTO : Figure 2 - hydrolysis reactions
PHOTO : Figure 3 - polyurethane hydrolysis
PHOTO : Figure 4 - carbodiimide stabilization of polyester polyurethane
PHOTO : Figure 5 - effect of polycarbodiimide on polyester polyurethane
PHOTO : Figure 6 - thermostability of polyurethane
PHOTO : Figure 7 - thermodegradation mechanism (A)
PHOTO : Figure 8 - thermodegradation mechanism (B)
PHOTO : Figure 9 - thermooxidation
PHOTO : Figure 10 - thermooxidation - oxide cleavage
PHOTO : Figure 11 - photolysis scheme 1 - photooxidation
PHOTO : Figure 12 - photolysis scheme 2 - urethane scission
PHOTO : Figure 13 - photolysis scheme 2 B - reactions of radicals
PHOTO : Figure 14 - photolysis scheme 3 - photo-fries
|Printer friendly Cite/link Email Feedback|
|Date:||Sep 1, 1990|
|Previous Article:||Let's do it right.|
|Next Article:||Polyurethane applications for the vibrating needle curemeter.|