Polyamides hydrolysis, resistance

Polyamides absorb water and are sensitive to it. Special grades are marketed for their hydrolysis resistance. 

Hydrolysis resistant polyamide-610 compositions comprising glass fibers, copper, and nucleating agent. The nucleating agent is carbonblack/talc mixture. ... 

The improvement in hydrolysis resistance by crosslinking is shown in Figure 5.264 for TPU and in Figure 5.295 for polyethylene. Figure 5.265 iiiustrates improvement in stress-cracking resistance by crosslinking in polyamide 6. 

At room temperature, polyamide 11 absorbs a maximum of 1.9% moisture after water storage, and polyamide 12 absorbs a maximum of 1.5%. Thanks to their low water absorption, polyamide 11 and 12 exhibit higher hydrolysis resistance than polyamide 6 and 66, which is especially advantageous in neutral and alkaline aqueous media, even in boiling water. The hydrolysis resistance of polyamide 11 and 12 is thus satisfactory for most applications. Resistance is limited in acidic... 

Hydrolysis resistance was investigated based on ISO 13628-2 [973] and API 17J [974] in demineralized water with and without carbon dioxide at 10 bar and at three different temperatures. Figure 5.338 shows the results Both polyamides exhibited chemical aging phenomena at 100 and 120 °C [923]. 

Transparent polyamide exhibits good to very good hydrolysis resistance in hot water up to 95 °C. Homopolymeric transparent polyamide will develop clouding under long-term contact with water at temperatures above 80 °C (see Table 5.57). 

It is estimated that the polyamide market penetration rate in air intake manifolds was 65% in 2000. This is forecast to increase to around 85% by 2005. Material suppliers are continuing to develop grades with improved hydrolysis resistance, which enables them to withstand continuous contact with hot water and glycols. 

The pelargonic acid by-product is already a useful item of commerce, making the overall process a commercial possibiUty. The 13-carbon polyamides appear to have many of the properties of nylon-11, nylon-12, or nylon-12,12 toughness, moisture resistance, dimensional stabiUty, increased resistance to hydrolysis, moderate melt point, and melt processibiUty. Thus, these nylons could be useful in similar markets, eg, automotive parts, coatings, fibers, or films. Properties for nylon-13,13 are = 56 (7 and = 183 (7 (179). 

Of the transparent polyamides the Grilamid material has the lowest density and lowest water absorption. It is also claimed to have the best resistance to hydrolysis, whilst transparency is unaffected by long-term exposure to boiling water. Tbe properties of Trogamid T and Grilamid TR55 are compared in Table 18.11. 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

Polyamides (nylons) The main types of nylon are oil and petrol resistant, but on the other hand susceptible to high water absorption and to hydrolysis. There are a few solvents such as phenol, cresol and formic acid. Special grades include a water-soluble nylon, amorphous copolymers and low molecular weight grades used in conjunction with epoxide resins. Transparent amorphous polyamides are also now available. 

Potyimides obtained by reacting pyromellitic dianhydride with aromatic amines can have ladder-like structures, and commercial materials are available which may be used to temperatures in excess of 300°C. They are, however, somewhat difficult to process and modified polymers such as the polyamide-imides are slightly more processable, but with some loss of heat resistance. One disadvantage of polyimides is their limited resistance to hydrolysis, and they may crack in aqueous environments above 100°C. 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

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. 

Many properties of polyamides are attributable to the formation of hydrogen bonds between the NH and CO groups of neighboring macromolecules. This is evidenced by their solubility in special solvents (sulfuric acid, formic acid, m-cresol), their high melting points (even when made from aliphatic components), and their resistance to hydrolysis. In addition, polyamides with a regular chain structure crystallize very readily. 

Aside textile and packaging applications the use of PET (Poly(ethylene Terephthalate) for structural applications is rather limited compared to equivalent polymers such as polyamides. Two main reasons can be given. Firstly, the high sensitivity of PET toward hydrolysis and its slow crystallisation kinetics constrain its processing. Secondly, its low glass transition temperature constrains its use if amorphous, whereas its weak impact resistance if semicrystalline constrains its use when crystallised. The industrial objective of this work deals with the latter of these points increasing the impact resistance of semi-crystalline PET. 

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