Hydrolytic stability of urethane

In order to address these issues, a brief discussion of thermal, oxidative, and hydrolytic stability of urethanes will be offered, so as to aid the adhesion scientist in designing a urethane adhesive with the desired durability. 

There appear to be conflicting reports regarding the degradation of urethanes. For example, some urethanes are reported to have relatively poor hydrolysis resistance and good biodegradability [77], while other urethanes are reported to be so hydrolytically stable that they have been successfully used as an artificial heart [78]. Both reports are correct. It will be shown that the thermal, oxidative, and hydrolytic stability of urethanes can be controlled, to some degree, by the choice of raw materials used to make the urethane. 

Effect of acid number on the hydrolytic stability of urethanes made from polyftetramethylene adipate)... 

Figure 15.13 illustrates the hydrolytic stability of various polymeric materials, determined by a hardness measurement after exposure to high-RH aging. A period of 30 days in the 100°C, 95 percent RH test environment corresponds approximately to a period from 2 to 4 years in a hot, humid climate such as that of southeast Asia. The hydrolytic stability of urethane potting compounds was not believed to be a problem until it resulted in the failure of many potted electronic devices that were noticed first during the military action in Vietnam in the 1960s. 

Gahlmer, F. H. and Nleske, F. W., "Hydrolytic Stability of Urethane and Polyacrylate Elastomers in Humid Environments," Urethanes In Elastomers and Coatings (articles from the Journal of Elastoplastlcs), pp. 6-20, Technomlc Publ. Co., Inc., Westport (1973). 

Ossefort, Z. T., and Testroet, F. B., Hydrolytic Stability of Urethane Elastomers, 89th Meeting, Rubber Division, American Chemical Society, San Francisco, Calif. May 4, 1966. Also, Rubber Chemistry and TechnoL, 39, (4), Part 2, 1308 (September 1966). 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

Polyurethanes are sensitive to strong acids, strong alkalis, aromatics, alcohols, hot water, hot moist air and saturated steam. The hydrolytic stability of polyurethanes in applications must be considered carefully. However, polyurethanes are resistant to weak acids, weak alkalis, ozone, oxygen, mineral grease, oils and petroleum. There are doubts for the oxidation stability of polytetramethylene ether glycol based polyurethanes. Polycarbonate urethane is a promising substitute with good oxidation stability. 

This esterification does not require the use of catalysts, though catalysts based on the organic-metallics of titanium or zirconium, e.g. tetrabutyl titanate, are often used to reduce reaction times. However, the trace residues of some of these catalysts can have adverse effects on the hydrolytic stability of the final urethane elastomer. Typical polyester structures are shown in Table 1.3. 

The use of catalysts in the manufacture of urethane elastomers is to be treated with caution as the hydrolytic stability of the final product is usually adversely affected, especially when it is polyester-based. Trace amounts of catalysts are usually present in raw-material manufactured polyesters and polyethers and these must be considered by either their removal or neutralization, otherwise the balance between chain extension, crosslinking and hydrolytic stability will be disturbed. 

It is possible to make some general comments on the relative hydrolytic stability of different polyurethane elastomer structures. The three principal linear polyol series used in synthesizing urethane elastomers can, for hydrolytic stability, be ranked in the following order ... 

The use of hexamethylene diisocyanate and 4-isocyanatomethyl butanoate has the advantage of high reactivity of the isocyanate function and high hydrolytic stability of the urethane group compared with the ester bond. 

Santerre J, Labow R. The effect of hard segment size on the hydrolytic stability of polyether-urea-urethanes when exposed to cholesterol esterase. J Biomed Mater Res 1997 36(2) 223-32. 

The rate of reversion or hydrolytic instability depends on the chemical stracture of the base adhesive, the type and amount of catalyst used, and the flexibility of the adhesive. Certain chemical linkages, such as ester, urethane, amide, and urea, can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride-cured epoxies. Generally, amine-cured epoxies offer better hydrolytic stability than anhydride-cured types. Figure 7.34 illustrates the hydrolytic stability of various polymeric materials determined by a hardness measurement Reversion is usually much faster in flexible materials because water permeates more easily. 

Besides of hydrolytic stabilizers, the hydrolytic stability of poly-(urethane)s can be tailored by the structure of tire backbone. 

L. Fambri, A. Penati, and J. Kolarik, S5mthesis and hydrolytic stability of model polyfester urethane ureas), Angewandte Makromolekulare Chemie, 228(1) 201-219,1995. 

Such polyurethanes have excellent hydrolytic stability compared to water-reducible polyesters and superior abrasion resistance. In view of the importance of developing low solvent emission coatings, considerable effort is being devoted to new types of water-borne urethane resins (62,63). 

Biodegradable poly(phosphoester-urethanes) containing bisglycophosphite as the chain extender were synthesized. Methylene bis-4-phenyl isocyanate (MDI) and toluene diisocyanate (TDI) were initially used as diisocyanates. Since there was a concern that the degradation products could be toxic, the ethyl 2,6-diisocyanatohexanoate (LDI) was synthesized and replaced the MDI (or TDI). The hydrolytic stability and solubility of these polymers were tested. Preliminary release studies of 5-fluorouracil from MDI based poly(phosphoester-urethane) and methotrexate from LDI based poly(phosphoester-urethane) are also reported. 

Uses. Neopentyl glycol is used extensively as a chemical intermediate in the manufacture of polyester resins (see Alkyd resins), polyurethane polyols (see Urethane POLYMERS), synthetic lubricants, polymeric plasticizers (qv), and other polymers. It imparts a combination of desirable properties to properly formulated esterification products, including low color, good weathering and chemical resistance, and improved thermal and hydrolytic stability. 

The chemical structure of the prepolymer influences its chemical resistance. As a result of their structure, polyesters have inherently better oil resistance but lower hydrolytic stability. The ether groups in the polyether urethanes... 

Polyurethane liner pads employed in naval missile launch systems are exposed to high humidities during storage and occasionally to sea water after their installation. In order to determine their durability towards water, a detailed study was made of their hydrolytic stability. Even though both polyether and polyester based urethanes can give the physical properties required for this application, only the polyether urethanes were studied since they display much superior resistance to hydrolysis (1, 2, 3, ). 

Accelerated hydrolytic stability tests Indicate that the polyether-based urethane employed In this study In the preparation of the liner pads can successfully withstand contlntious Immersion In water at 3S C for 24 years. Under such an exposure, the pads are expected to lose approximately 20 percent of their original compressive strength. 

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