Polycarbonate hydrolysis, resistance

Hydrolysis studies compared a polycarbonate urethane with a poly(tetramethyl-ene adipate) urethane and a polyether urethane based on PTMEG. After 2 weeks in 80°C water, the polycarbonate urethane had the best retention of tensile properties [92], Polycarbonates can hydrolyze, although the mechanism of hydrolysis is not acid-catalyzed, as in the case of the polyesters. Polycarbonate polyurethanes have better hydrolysis resistance than do standard adipate polyurethanes, by virtue of the highest retention of tensile properties. It is interesting to note in the study that the PTMEG-based urethanes, renowned for excellent hydrolysis resistance, had lower retention of physical properties than did the polycarbonate urethanes. 

It is speculated that the water does not diffuse as easily into the polycarbonate as into the PTMEG urethane, making the polycarbonate more resistant to plasticization by water and subsequent hydrolysis. The tensile properties of the PTMEG polyurethane eventually returned to nearly the original tensile properties, if the material was allowed to dry at room temperature for 2 weeks. This observation lends credence to the idea that the PTMEG urethane was plasticized by water. 

Polybutadienes, polycaprolactones, polycarbonates, and amine-terminated polyethers (ATPEs) are shown in Scheme 4.4 as examples of other commercially available polyols. They are all specialty materials, used in situations where specific property profiles are required. For example, ATPEs are utilized in spray-applied elastomers where fast-reacting, high-molecular-weight polyamines give quick gel times and rapid viscosity buildup. Polycarbonates are used for implantation devices because polyuredtanes based on them perform best in this very demanding environment. Polycaprolactones and polybutadienes may be chosen for applications which require exceptional light stability, hydrolysis resistance, and/or low-temperature flexibility. 

Polyesters produce tough, oil-resistant polyurethanes, with the major drawback being lower hydrolysis resistance compared to polyurethanes made using polyethers. The two newer groups of polyesters (polycaprolac-tone- and polycarbonate-based) both have better resistance to hydrolysis. Their toughness is very close to the basic polyester polyurethanes. Their disadvantage is cost. 

The polyester polyurethanes based on polycarbonate diols and the intrinsic hydrolysis resistance of these special kind of polyurethanes were discussed before (Section 8.1). 

In one of the early comprehensive review articles on the properties of polyarylates, Bier [28] mentioned that the polyarylate biased on Bisphenol A exhibited attractive electrical and mechanical properties. Injection mold-ability was considered to be a limitation, but extrusion was possible. Bier stated that the hydrolysis resistance was poor (in the same range as polycarbonates based on Bisphenol A). 

Organic silicon compounds are also used alone. In contrast to phosphites, they do not reduce hydrolysis resistance in polycarbonates. 

Hydrolysis resistance is better in polyarylates than in polycarbonate. Figure 5.342 left. It increases with increasing content of ester structures. However, it does not achieve the resistance of polysulfone [723]. 

Polyadipates, polycarbonate diols, and polyethers based on long-chain diols as components of the TPU molecule result in higher hydrolysis resistance. Esters with hydrophobic diol and an acid with low dissociation constant are especially well protected against the influence of hydrolysis. Long-chain diols are thus more hydrolysis resistant than short-chain diols [86]. 

Blends of PBTP and polycarbonate (PC), marketed by General Electric (USA) under the name Xenoy, overcome some of the deficiencies of PC, in particular, improving the processing behaviour, hydrolysis resistance and low-temperature impact performance. The amorphous regions of the PBTP appear to be miscible with the wholly amorphous PC, with the glass transition of the 50 50 blend at 100°C being intermediate between that of PC (150°C), and that of PBTP (35-40°C). The PBTP crystallizes to a small extent. The impact behaviour is improved by the inclusion of a small quantity of acrylic rubber. A demanding application for the 50 50 blend, with added rubber, is the... 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

The chemical resistance of polyester materials is well recognised to be limited because of the comparative ease of hydrolysis of the ester groups. Whereas this ease of hydrolysis was also observed in aliphatic polycarbonates produced by... 

Aliphatic polycarbonates have few characteristics which make them potentially valuable materials but study of various aromatic polycarbonates is instructive even if not of immediate commercial significance. Although bisphenol A polycarbonates still show the best all-round properties other carbonic ester polymers have been prepared which are outstandingly good in one or two specific properties. For example, some materials have better heat resistance, some have better resistance to hydrolysis, some have greater solvent resistance whilst others are less permeable to gases. 

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. 

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. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

In general, polycarbonate resins have fair chemical resistance to aqueous solutions of acids or bases, as well as to fats and oils, Chemical attack hy amines or ammonium hydroxide occurs, however, and aliphatic and aromatic hydrocarbons promote crazing of stressed molded samples, BPA polycarbonate has excellent resistance to hydrolysis. 

Because of the low solubility of water in the resin, BPA polycarbonates arc inherently resistant to aqueous acid and base, although strong nucleophilic bases can cause, hydrolysis,... 

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] ... 

As an immediate consequence, the resistance to hydrolysis makes the hexanediol-polycarbonates and the resulting polyurethanes suitable for a long useful life. The specific... 

The hydrolysis of esters or carbonates of tertiary alcohols have also been employed to degrade resist polymer backbone, with a view to making them soluble in the developer. In particular, Frechet et al. prepared resists based on thermally depolymerizable polycarbonates, such as the structure shown in Scheme 7.46, which exhibit great thermal lability and undergo multiple... 

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