Resistance to hydrolytic attack

Polyurethane elastomers are exceptionally tough, abrasion resistant, and resist attack by oil. The polyester types (AU) are susceptible to hydrolytic attack at above ambient temperatures, and certain polyester thermoplastic polyurethanes have been known to stress crack in cable jacketing applications when in contact with water at ambient temperatures this latter effect has sometimes, incorrectly, been ascribed to fungal attack. Polyether types (EU) are far more resistant to hydrolytic attack. Certain polyurethanes can be attacked by UV light, the resistance to this agency primarily being determined by the isocyanate used. 

Stationary phase technology has also seen significant improvements over the past years. The silica base material is nowadays often a hybrid material, synthesized from tetraalkoxysilanes and functionalized trialkoxysilanes, for example, methyl-trimethoxysilane (MTMS). The introduction of alkyl-trialkoxysilanes into the silica backbone makes the material more resistant to hydrolytic attack and also improves their separation behavior for basic analytes.30 C18 (= octadecylsilane) stationary phases are still the materials typically used in environmental analysis, and the enormous choice of materials with gradually different properties allows columns to be selected that are particularly well suited to a given separation task.31 Reversed phase separations with materials of shorter alkylsilane chain length (C8, C4, and Cl) are less frequently used. 

In addition, polymer B Is reported to provide superior resistance to hydrolytic attack compared to other polyester based polyurethanes. 

Factors such as fiber diameter, compactness, and the difficult solubility of collagen would seem of importance in determining such properties as resistance to hydrolytic attack by enzymes. This inertness is probably not entirely derived from the chemistry of collagen but depends also on physical factors which prevent an enzyme from gaining access to its substrate. Pepsin, acting under acid conditions on swollen fibrils, is able to cause degradation (195), and increased temperature or physical maceration facilitates even the attack by trypsin in neutral or slightly alkaline environments (194, 200). 

Poly(vinyl fluoride) is moderately crystalline. The crystal melting point, Tm, is approximately 200 °C. The high molecular weight polymers dissolve in dimethylformamide and in tetramethyl urea at temperatures above 100 °C. The polymer is very resistant to hydrolytic attack. It does, however, lose HF at elevated temperatures. 

An example of a specialty aromatic polyamide is a fluorinated polyanude. It was prepared in an attempt to form a polymer with superior heat stability and resistance to hydrolytic attacks ... 

Acetals generally exhibit poor resistance to hydrolytic attack. Some, however, are much more resistant than others, depending upon the glycol. The following formal was reported to exhibit good hydrolytic stability under both acidic and basic conditions ... 

The constitution of the polyester and the presence of suitable stabilizers are of decisive importance for resistance to hydrolytic attack in polyester-based polyurethane. 

PGA. Due to the presence of -CH3 side groups, PLA is more hydrophobic than PGA, and is more resistant to hydrolytic attack than PGA. For most appUcalions the (L) isomer of lactic add is chosen because it is preferentially metabolized in the body. The main disadvantage of PLA is its brittleness and poor thermal stabihty. 

Available evidence suggests that arylamide bonds generally are quite resistant to hydrolytic attack in plant tissues. A notable exception, however, is the substituted propionanilide propanil (2), which is rapidly hydrolyzed in resistant rice plants but not in susceptible weeds such as Echinochloa crus-galli (Figure 10.2). Hydrolysis of propanil is mediated by a specific... 

Chlorinated paraffins are relatively inert and exhibit excellent resistance to chemical attack and are hydrolytically stable. They are soluble in chlorinated solvents, aromatic hydrocarbons, esters, ketones, and ethers but only moderately soluble in ahphatic hydrocarbons and virtually insoluble in water and lower alcohols. 

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. 

The organic structures holding the ions, porphyrins, are themselves quite resistant to decomposition. They are not open to hydrolytic attack but note that they are sensitive to oxidation. 

The most important area for PTHF and THF-alkylene oxides copolymers remains the PU elastomers area (including elastomeric fibres), the PTHF and the high THF content copolymers confer on the resulting PU elastomers specific properties, such as excellent hydrolytic stability, resiliency, low temperature stability, elasticity at lower temperatures, and resistance to fungus attack [7, 35, 36, 37]. 

The sensitivity to hydrolysis is a key issue in many applications. The ester bond in 4GT-PTMO copolymers is sensitive to hydrolysis however, it is fairly protected since most of the ester is contained in a crystalline structure. The addition of a small amount (1-2%) of a hindered aromatic polycarbodiimide substantially increases the lifetime of this material in the presence of hot water or steam (Brown et al., 1974). Polyurethanes are susceptible to hydrolytic attack, especially those with polyester soft segments. However, polyester soft segment polyurethanes are generally more resistant to oils, organic solvents, and thermal degradation. lonomers will swell when exposed to water in fact, a commercial hydrated perfluorosulfonic ionomer (Nation) is used as a membrane separator in chlor-alkali cells. Styrene-diene copolymers and polyolefin TPEs are insensitive to water. 

Uses Soft segment in formation of elastomers such as thermoplastic urethanes, polyether esters, polyether amides, coatings, adhesives and sealants, casting resins, and urethane foams Features Produces elastomeric end-prods, with hydrolytic stability, resist. to microbial attack, low temp, flexibility and elasticity Properties APHA 40 max. wax-like solid sol. in most org. solvs. barely sol. in water m.w. 650 sp.gr. 0.977 (40 C) acid no. 0.05 hyd. no. 166.2-179.5 flash pt. 215 C 0.025% water Storage Hygroscopic protect from moisture and air dry nitrogen blanket should be applied to open containers before resealing Polythix M [Poly-Resyn]... 

Features Mfg. of prods, having exc. hydrolytic stability and resist, to chem. attack... 

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