Polyols hydrolytic stability

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. 

Polyesters are another important class of polyols. There are many polyester types used, so a generic structure is shown in Scheme 4.4. They are often based on adipic acid and either ethylene glycol (ethylene adipates) or 1,4-butanediol (butylene adipates). Polyesters, because of the polar carbonyl groups, contribute more to intermolecular forces, and physical properties such as tear and impact resistance are often improved by using them. They are also utilized for their solvent and acid resistance and light stability. Relatively poor hydrolytic stability is... 

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. 

In addition, polyester polyols are made by the reaction of caprolactone with diols. Poly(caprolactone diols) are used in the manufacture of thermoplastic polyurethane elastomers with improved hydrolytic stability. The hydrolytic stability of the poly(caprolactone diol)-derived TPUs is comparable to TPUs based on the more expensive long-chain diol adipates. Polycthcr/polycstcr polyol hybrids arc syndicsized from low molecular weight polyester diols, which are extended with propylene oxide. 

Thermal, oxidative and hydrolytic stability. Organic esters and PAO inhibited lubricant base stocks resist oxidative and thermal degradation better than petroleum-based oil petroleum (121 °C), PAO (121-177°C), diesters (149-177°C), and polyol esters (177-218°C). 

The hydroxyl terminated polybutadienes lead to PU with physico-mechanical properties significantly lower than those of PU based on polyether or polyester polyols. The nonpolar polymeric chain and the extraordinary hydrophobicity mean that hydroxy terminated polybutadienes are used for special applications, due to their excellent electrical insulation properties which are equal or superior to epoxies or silicone elastomer systems. The hydrolytic stability of PU elastomers derived from hydroxyl terminated polybutadiene is superior to the majority of other types of PU. Thus, some specific applications of... 

The hydrolytic resistance of a polyurethane depends strongly on the nature of the oligo-polyol chain. As a general rule, very hydrophobic chains and water repellent polyols give polyurethanes with excellent hydrolytic stability. The relative order of the hydrolysis resistance of polyurethanes function of oligo-polyol nature is ... 

It is considered that the polyurethanes based on oleochemical polyols, dimer acids and dimer alcohols, PTHF and PC-polyols lead to polyurethanes with excellent hydrolytic stability. Polycaprolactone (PCL) polyols and poly (butylene adipate) lead to polyurethanes with good hydrolytic stability, but use of poly (diethylene glycol adipate) give polyurethanes with poor hydrolytic resistance. 

Pavlova, M., Oraganova, M. and Novakov, P. (1985). Hydrolytic stability and protective properties of polyurethaner oligomers based on polyester/ether/polyols. Polymer, 26 (11), 1901-1905. 

Features Sat. aliphatic polyester polyol with all primary hydroxyl functionality good hydrolytic stability for outdoor applies. 

Chem. Descrip. Dimer acid-based polyester polyol Uses Modifier (increases flexibility, impact resist., toughness, hydrolytic stability) for industrial coatings (automotive, coil, textile, leather, wood lacquers) building block for PU elastomers, primers, sealants, elastic coatings... 

Classification Thermoplastic elastomer Definition Polymer consistg. of hard segments embedded in a soft, elastomeric polyol phase exc. abrasion/impact resist. high str. wide working temp, range good hydrolytic stability two types polyether- or polyester-based Properties Solid... 

Castor oil-based polyurethane resin is used to obtain graphite composite as an electrode material. The 60% graphite (w/w) composite exhibits good mechanical and appropriate electric resistance and offers ease of preparation and surface renovation. The polyurethanes of soybean oil-based polyol with glass reinforced composites exhibit mechanical properties comparable with those based on petrochemical polyol. The oxidative, thermal and hydrolytic stability of soybean oil-based composites are superior to those of petrochemical polyol. All the results indicated that a polyurethane matrix based on soybean oil is a preferable alternative to petrochemical polyurethanes in glass reinforced composites. 

The following polyols are well established in the urethane adhesive field (i) polyesters, often preferred due to their combination of cohesive and adhesive properties (ii) polyethers, selected when hydrolytic stability and resistance to bacterial attack are predominant requirements (iii) poly(caprolactone) glycol (see Fig. 8.2), which offers a compromise in properties between the polyesters and polyethers and (iv) poly(butadiene)-based glycol (see Fig. 8.2). 

Polyesters do vary in their resistance to hydrolysis and this variability is reflected in the hydrolytic stability of the polyurethanes. Sometimes it is possible to perform hydrolysis tests on the polyols which give good correlation with the stability of final polyurethane. One such method is the hydrolysis of the polyester by sodium hydroxide solution at 100°C. 

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 choice of which alkylene oxide is used has several effects. The choice of oxides effects the reaction rate of the alkylene oxide addition to the sucrose molecule. It also has an effect on the physical cheuracteristies of the resulting polyol and plays a major role on the physical properties of the final rigid urethcuie foam. Table I shows the effects of using ethylene oxide, propylene oxide, and mixtures of the two oxides. Butylene oxide offers an advantage over the other oxides in the area of hydrolytic stability. However, butylene oxide is not widely used because it commcuids a high price eind cannot be justi-... 

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. 

Polyols are selected on the basis of cost, rate of esterification, stability during high temperature processing, functionality, and hydrolytic stability of their esters. The most widely used diol is NPG, and the most widely used triol is TMP. A comparison of results of testing films of coatings made with a series of polyesters from several polyols is given in Reference 167. Coatings based on CHDM polyesters cross-linked with MF resins showed the best hydrolytic stability. 

Polyester polyols are based on saturated aliphatic or aromatic carboxylic acids and diols or mixtures of diols. The carboxylic acid of choice is adipic acid because of its favorable cost/performance ratio. For elastomers, linear polyester polyols of molecular weight of ca 2000 are preferred. Branched polyester polyols, formulated from higher fimctional glycols, are used for foam and coating applications. Polyester polyol-derived polyurethanes have a lower hydrolytic stability... 

Park SH, Chung ID, Hartwig A, Kim BK. Hydrolytic stability and physical properties of waterborne polyurethane based on hydrolytically stable polyol. Colloids Surf A Physicochem Eng Aspects 2007 305(1-3) 126-31. 

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