Hydrolysis Resistant Polyester Polyols

Alkyl carbonates are relatively labile concerning the hydrolysis reaction. Surprisingly, polycarbonate polyols give PU that are extremely resistant to hydrolysis, superior to those PU derived from polyesters based on adipic acid and diethylene glycol. The explanation of this paradox, mentioned before, is that between the hydrolysis products of polycarbonate polyols, acidic groups which are able to further catalyse hydrolysis reactions are not formed. The products of polyester polyol hydrolysis are diacids and glycols. The products of polycarbonate polyols hydrolysis are carbon dioxide (a gas which is eliminated easily) and glycols [76] ... 

The addition of carbodiimides, e.g. Staboxal PCD, which is supplied by Bayer, to poly(ester-urethanes) is one of the most effective ways to stabilize them against hydrolysis. Also the polyurethane structure can be tailored for better hydrolysis resistance through elimination or reduction of the ester groups present in the polyol. Hydrolysis resistance increases in the order of ether > polycaprolactone > polyester. Satrastab, developed by SATRA (Shoe and Allied Trades Research Association, Kettering, England), is also claimed to be an effective hydrolysis stabilizer for formulated polyurethanes in poromeric footwear materials. 

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. 

When reacting either ethylene carbonate or propylene carbonate with an aliphatic diamine, a polyurethane can be produced (Figure 2.14). Polyethylene ether carbonate) diols (Harris et al., 1990), when fabricated into polyurethanes using MDI and BDO, produce elastomers that have polyester polyol features. This was shown using 13C NMR. The structure gives rise to potential for a very high virtual cross-linking density. These carbonate-derived polyesters have superior hydrolysis resistance compared to the traditional materials. 

Soft segments One of the two phases which make up polyurethane. The soft segment is composed of long-chain polyether or polyester polyols. The soft segment controls many of the polyurethane properties such as tensile and tear strength, hydrolysis and chemical resistance, glass transition temperature, and flexibility. 

Polyesters make tough and wear-resistant urethanes. The one major drawback is the hydrolysis at the ester grouping. The hydrolysis can either be acid or alkali promoted. In more neutral conditions, the major breakdown product is normally adipic acid that catalyses further attack. The normal approach is to use carbodiimides to block further breakdown. Polyols made using polypropylene carbonate produce polyurethanes with polyester characteristics but with enhanced hydrolysis resistance. 

Polyether Polyols. The major polyols for preparing various urethane foams are polyether polyols. Polyester polyols are used only in specific applications. The advantages of polyether polyols are choice of functionality and equivalent weight the viscosities are lower than those of conventional polyesters production costs are cheaper than for aliphatic polyesters and resulting foams are hydrolysis-resistant. 

Polyester polyols have an intrinsic defect they are liable to hydrolyse under high humidity/temperature conditions. To prevent the hydrolysis of polyester-based polyurethanes a worldwide research effort, led to the synthesis of polyester polyols with improved hydrolysis resistance [1, 6-8, 12, 13]. 

The hydrolysis susceptibility of a polyester or a polyester-based PU depends on the concentration of ester bonds, on the polyester polyol acidity, on the hydrophobicity of the glycol or dicarboxylic acid used for polyester synthesis, and on the steric hindrance around the ester groups. Low concentration of the ester bonds, low polyester acidity, high comonomer hydrophobicity and high steric hindrance around the ester groups confer hydrolysis resistance to the polyester-based PU. 

The intensive research done in the last few years proved that the hydrophobicity of the glycol or of the dicarboxylic acid used for polyester polyol synthesis is one of the most important parameters to improve the hydrolysis resistance of polyester-based PU. 

Figure 8.2 Diols with hydrophobic groups used in hydrolysis resistant polyester polyol...

PCL polyols are used to produce hydrolysis and solvent resistant PU elastomers which are flexible at lower temperatures. A characteristic of these special polyester polyols is their uniform and fast reactivity due to the 100% primary terminal hydroxyl groups. A characteristic of elastic polyurethanes, based on PCL polyols, is the clickability and superior tear resistance. 

Chem. Descrip. Highly branched hydroxyl-terminated polyester polyol Uses Polyester for hard, durable exterior coatings and one-shot castables Features Lowm.w. for reaction with isocyanates, alkylated melamines, and other hydroxyl reactive curatives hydrolysis, heal, and chemical resistance, high hardness Rucoflex S-101 [Bayer]... 

Chem. Descrip. Sat., aliphatic, linear, hydroxyl-terminated polyester polyol Uses Polyester for sol n. coatings and adhesives, thermoplastic elastomers, castable prepolymers, and one-shot castables Features Offers aging, hydrolysis resistance, and solubility in ketone soivs., and strong, resilient elastomers melts at low temps. 

Some other specialised polyester polyols, like polycarbonate and polycaprolactone, possess superior hydrolytic resistance. Aliphatic polyhydrocarbon polyol such as hydroxyl terminated polybutadiene (HTPB), is advantageous in many respects. These substances are resistant to acidic or basic hydrolysis, possess good adhesion and can be used where low polarity and good electrical insulation are required. 

The specialty class of polyols includes poly(butadiene) and polycarbonate polyols. The poly(butadiene) polyols most commonly used in urethane adhesives have functionalities from 1.8 to 2.3 and contain the three isomers (x, y and z) shown in Table 2. Newer variants of poly(butadiene) polyols include a 90% 1,2 product, as well as hydrogenated versions, which produce a saturated hydrocarbon chain [28]. Poly(butadiene) polyols have an all-hydrocarbon backbone, producing a relatively low surface energy material, outstanding moisture resistance, and low vapor transmission values. Aromatic polycarbonate polyols are solids at room temperature. Aliphatic polycarbonate polyols are viscous liquids and are used to obtain adhesion to polar substrates, yet these polyols have better hydrolysis properties than do most polyesters. 

Polycaprolactone (PCL) polyols, due to the presence of a relatively long repeated hydrophobic segment -(CH2)5- are recognised as polyesters which lead to polyurethanes with good resistance to hydrolysis [7, 16]. 

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. 

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