Stability of Polyurethanes

Polyurethanes are susceptible to degradation which can be initiated by several means, including photolytic, thermal, oxidative, and solvolytic. With polyurethane adhesives, photodegradation is typically not a problem, since it is uncommon for the adhesive to be exposed to direct light. The other three forms of degradation, however, do pose problems. The factors affecting degradation and the means by which they may be inhibited have been well documented by a number of authors.  

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A study was done measuring the thermal oxidative stability of polyurethanes made from PPG polyols, varying the isocyanate curative. Oxygen absorption was... 

Based on the values of cohesive energies and dissociation energies of the bonds involved in the polyurethane structure, it may be possible to establish the following relative order regarding the thermal stability of polyurethanes function of the oligo-polyol structure ... 

As a general rule the thermal stability of polyurethanes is directly linked with the mobility of polymeric chains. Low mobility, crosslinked polyurethane structures, based on high functionality polyols are more thermostable than the high mobility, low crosslinked... 

Javni, I. Z.S. Petrovic A. Guo R. Fuller. Thermal stability of polyurethanes based on vegetable oils. /. Appl. Polym. Sci. 2000, 77, 1723-1734. 

Hergenrother, R.W., H.D. Wabers, and S.L. Cooper, Effect of hard segment chemistry and strain on the stability of polyurethanes in vivo biostability. Biomaterials, 1993, 14, 449 58. 

Lights stability of polyurethanes depends to a large extent on their chemical structure, and both components (Le., isocyanate and polyol) have an influence. Polyurethanes based on aliphatic isocyanates and polyester diols show the best light stability if yellowing is considered, whereas polyurethanes based on aromatic isocyanates and polyether diols are worst in this respect. 

Z. Ozawa, Photodegradation and Stabilization of Polyurethanes, in N.S. Allen (ed.). Developments in Polymer Photochemistry — 3, Applied Science, London (1982), Chapter 6, p. 209. 

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. 

Cellulose nanocrystals interacted with both soft and hard segments, enhancing stiffness and thermal stability of polyurethane. Cellulose nanocrystals act as nucleating agents in PU. ... 

EFFECT OF DIISOCYANATE STRUCTURE ON THE RELATIVE THERMAL STABILITY OF POLYURETHANE ELASTOMERS... 

Increase in crosslink density, type of crosslinking and introduction of isocyanurate ring structures in the polymer-chain backbone has a strong beneficial effect on the thermal stability of polyurethanes and is discussed later in this chapter. It is known that thermal stability increases with increasing isocyanate content of urethane elastomers, and in addition to the formation of the thermally stable isocyanurate rings, stability is influenced by the different types of urethane-based groups formed, as shown in Table 3.9a. 

The way in which the chemical structure of the various chain extenders of Table 3.16 influences the thermal stability of polyurethane elastomers based on the molecule Capa 225/CHDI/chain extender in the molar ratio 1 2 1, respectively, is given in the following figures and tables. For example, Fig. 3.7 shows the dynamic mechanical thermal properties of a series of the polyurethane elastomers in which the variable is the chain extender. The temperature at which the value of the storage modulus (log E ) changes significantly is considered to indicate the limit of thermal stability of the polyurethane elastomers. 

The temperature at which the first transition takes place above room temperature is considered to indicate the limit of thermal stability of polyurethanes. From Table 3.20 the following order of decreasing transition temperatures which represent decreasing thermal stability is observed ... 

COMPARISON OF HYDROLYTIC STABILITY OF POLYURETHANE ELASTOMERS BASED ON POLYETHER AND POLYCAPROLACTONE... 

Barikani, M. Hepburn, C. (1987). The relative thermal stability of polyurethane elastomers. 3 Influence of chain extender system. J. Cell. Polym., 6(3), 47-66. 

As already mentioned, the hydrolytic stability of polyurethanes is influenced by the nature of the macroglycol used in their preparation. Polyesters prepared from sterically hindered glycols, such as neopentyl glycol (18), and a long-chain diacid, or aromatic diacid, for example, terphthalic acid (19), will maximize moisture stability. Polyester polyols have been developed which are claimed to be similar in hydrolytic stability to PTMO polyurethanes. Increasing the crosslink density in polyester polyurethanes has been shown to improve hydrolytic stability as well however, this may not always be a viable solution, since the crosslink density can have a profound influence on other properties of the polymers as already described. 

Figure 3.328. Variation in time of chemical stability of polyurethane in water [ 909 ] 1) unstressed 2) stressed at 9.7 MPa 3) aged for 200 hr and subsequently stressed at 9.7 MPa 4) stressed at 9,7 MPa with concomitant ageing.

Figure 3.330. Variation in time of chemical stability of polyurethane in soln. 10% H SO. (a) At different efforts [909] 1) o = 0 2) 7.0 3) 9.7 4) 12.6 5) 14.3 Mpa and (b) after accelerated ageing 1) unstressed 2) static fatigued 3) firstly aged and subsequent supposed to static fatigue 4) static fatigued and to concomitant accelerated aging.

Hearon K, Smith SE, Maher CA, Wilson TS, Maitland DJ. The effect of free radical inhibitor on the sensitized radiation crosslinking and thermal processing stabilization of polyurethane shape memory polymers. Radiat Phys Chem 2013 83 111-21. 

Resistance to various environmental factors, such as heat, light and humidity, is one of the most important properties of materials. Stability of polyurethanes under the influence of these factors varies with the structure of the material. Thus we will discuss just the stability of the urethane group and a few typical chemical groups in urethane materials, such as ether or ester. 

Hydrolytic stability of polyurethanes depends on the composition and concentration of the weakest groups. Polyesterurethanes have considerably lower resistance to hydrolysis than polyether or polybutadiene based polyurethanes. Hydrolysis of ester groups is catalyzed by acid groups formed as the product of hydrolysis. Therefore, an efficient way of slowing down the process is to block the acid formed, which is usually carried out by adding carbodiimides. Such solutions are temporary since after the consumption of all carbodiimide the process accelerates again. 

Foti, S., Maravigna, P., and Montaudo, G. (1982). Effects of N-methyl substitution on the thermal stability of polyurethanes and polyureas. Polymer Degradation and Stability, 4 287-292. 

Thapliyal, B. P., Chandra, R. (1990). Advances in photodegradation and stabilization of polyurethanes. Progress in Polymer Science, 15(5), 735-750. 

Thermal stability of polyurethane elastomers based on a produet of the anionic copolymerization of butadiene and isoprene in the ratio of 80 20 and isoprene was first studied by DSC. The preferred eonditions (temperature of the isothermal segment and oxygen eonsumption) were revealed to determine the oxidation induction time of this type of materials. The effeet of Irganox 1010, Evemox 10, Songnox 1010 and 1010 Chinox stabilizers on the oxidation induction time has been studied. 

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