Resistance to hydrolysis

In discussing the effect of moisture on the performance of aluminum/epoxy bonds, Orman and Kerr suggested that the irreversible losses in bond strengths observed can be explained in terms of the stress-induced hydrolysis of primary chemical bonds. The evidence obtained indicated that, with the exception of the boundary layer, such stress-induced reactions do not readily occur at temperatures below about 90°C. 

The effects of high humidity on the dynamic mechanical properties and thermal transitions of a commercial nylon-epoxy adhesive have been reported by Butt and Cotter. Exposure of the cured adhesive to 43°C and 97% RH for times ranging from 142 h to 2,040 h resulted in a substantial decrease (as much as 82%) in the complex dynamic tensile modulus. Some of the results of these authors are shown in Table I. The thermal transitions 

In a similar study, Brewis et al observed the same type of reductions in the mechanical properties of the adhesive. They demonstrated that the reduction in strength of single lap joints on exposure to moisture was linearly related to the fractional water content of the joints. On saturation with water the joints were weakened by 44%, whereas films of the adhesive were weakened by as much as 74%. It was pointed out that the lack of correlation between the bulk and adhesive joint strength was due to the different stress distributions in the two situations. They concluded that the principal mechanism of loss of adhesive joint strength was by diffusion-controlled water plasticization of the adhesive. There is an apparent discrepency between this work, in which water has a reversible effect on joint strength, and that of Butt and Cotter, in which the effect is irreversible. This will be discussed further in Section III.B. 

There is evidence to suggest that exposure to humid environments, at temperatures above about 60°C can produce permanent damage in epoxy adhesives.Apicella and coworkers have explained this in terms of the formation of microcavities produced by clusters of water molecules. This mechanism of damage is expected to be evident in thermoset resins in which the rigid crosslinked structure does not allow the matrix to relax after microvoid formation. 

Another effect of solvent absorption by an adhesive is that the fracture toughness can be increased. Mizutani and Iwatsu have measured the fatigue crack growth of a DGEBA epoxy, cured with triethylenetetramine, in air and in the presence of various solvents. Their results show that the fatigue 

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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). 

The recovery of fiber from broke (off-specification paper or trim produced in the paper mill) is compHcated by high levels of urea—formaldehyde and melamine—formaldehyde wet-strength resin. The urea resins present a lesser problem than the melamine resins because they cure slower and are not as resistant to hydrolysis. Broke from either resin treatment may be reclaimed by hot acidic repulping. Even the melamine resin is hydrolyzed rapidly under acidic conditions at high temperature. The cellulose is far more resistant and is not harmed if the acid is neutralized as soon as repulping is complete. 

Unlike the 2-aLkyl-2-imida2olines, this stmcture is stable and resistant to hydrolysis. After ring cleavage, reaction with sodium chloroacetate yields linear products ... 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

The sulfides are associated like the oxides, but to a lesser degree. They are crystalline, sharp-melting, soluble ia many organic solvents, and resistant to hydrolysis. Most are cycHc trimers (106). 

The orange-red titanium acetylacetone chelates are soluble in common solvents. These compounds are coordinately saturated (coordination number equals 6) and thus much more resistant to hydrolysis than the parent alkoxides (coordination number 4). The alkoxy groups are the moieties removed by hydrolysis. The initial product of hydrolysis is beheved to be the bis-hydroxy bis-acetylacetone titanate, (HO)2Ti(acac)2, which oligomerizes to a... 

The hydrolysis of phosphites is retarded by the addition of a small amount of a base such as triethanolamine. A more effective approach is the use of hindered phenols for esterification. Relatively good resistance to hydrolysis is shown by two esters derived from hindered phenols tris(2,4-di-/ / butylphenyl)phosphite [31570-04-4] (25) and tetrakis(2,4-di-/ /f-butylphenyl)4,4 -biphenylenediphosphonite [38613-77-3] (26). The hindered fluorophosphite [118337-09-0] (27) has excellent resistance to hydrolysis. 

The Group 4—6 carbides are thermodynamically very stable, exhibiting high heats of formation, great hardness, elevated melting points, and resistance to hydrolysis by weak acids. At the same time, these compounds have values of electrical conductivity. Hall coefficients, magnetic susceptibiUty, and heat capacity in the range of metals (7). 

Chemical Properties. Neopentanoic acid [75-98-9] undergoes reactions typical of carboxyUc acids. Reactions often proceed less readily than with straight-chain acids because of the steric hindrance around the carbonyl group. However, this steric hindrance at the a-carbon results ia derivatives that are typically more resistant to hydrolysis and oxidation. 

Desalination membranes with improved, rigid, and stable surfaces have been prepared from cellulose acetate propionate (170). These films are generally more resistant to hydrolysis than those from cellulose acetate. 

In secondary operations, where chemicals are injected into hydrocarbon formations in conjunction with a chemical flooding process, polyamines are used to reduce the loss of injected chemicals to the formation by adsorption and precipitation (312). TEPA and other ethyleneamines are used with water-soluble polymeric thickeners in water—flood petroleum recovery operations to stabilize viscosity, mobiUty, and pH while imparting resistance to hydrolysis (313). 

Although imidazolinones are usually resistant to hydrolysis, oxazolinone rings are often easily opened. In acid-catalyzed reactions of this type, water converts azlactones (181) into a-acylamino-a,/3-uhsaturated acids (182) (77AHC(21)175). 1,3,4-Oxadiazolinones are readily opened by hot water to give hydrazine carboxylic acids which undergo decarboxylation. 

Although poly(vinyl fluoride) resembles PVC in its low water absorption, resistance to hydrolysis, insolubility in common solvents at room temperature and a tendency to split off hydrogen halides at elevated temperatures, it has a much greater tendency to crystallise. This is because the fluorine atom (c.f. the chlorine atom) is sufficiently small to allow molecules to pack in the same way as polythene. 

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. 

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. 

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. 

With a Tg of 273°C and excellent resistance to hydrolysis these films may be used continually at temperatures up to 160°C and for short periods up to 260°C. Their excellent electrical insulation characteristics are well maintained up to temperatures quite close to the Tg as illustrated in Figure 21.12. Some other typical properties are given in Table 21.7. 

In certain cases resistance to hydrolysis and solvents (particularly oils) may also be required. 

Thermoplastic polyurethane elastomers have now been available for many years (and were described in the first edition of this book). The adipate polyester-based materials have outstanding abrasion and tear resistance as well as very good resistance to oils and oxidative degradation. The polyether-based materials are more noted for their resistance to hydrolysis and fungal attack. Rather specialised polymers based on polycaprolactone (Section 25.11) may be considered as premium grade materials with good all round properties. 

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. 

Urea is sufficiently important as an additive to PF resins for OSB to warrant some discussion. It has had a large favorable economie impact on the OSB industry. When used, it is generally added after the polymerization is complete. Thus, it is not part of the polymer and does not have any direet effect on polymer resistance to hydrolysis, as might be expected if it was part of the polymer backbone. Under alkaline pH conditions, urea-formaldehyde adducts do not polymerize at a rate that is significant compared to the PF polymerization therefore, the urea does not participate signifieantly in the euring proeess of the PF, despite the faet that it is present during the cure. Since urea is not present in the cured PF polymer per se, it does not detract from the durability of the polymer. Despite this, it is possible to see redueed OSB durability as a result of formulated urea if its use has led to actual PF polymer application rates that are too low. 

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. 

Trifluoromethyl groups are very resistant to hydrolysis, unless they are allylic or benzylic, or vicinal to a carbon linked to hydrogen. In the last case, elimination of hydrogen fluonde leads to the formation of a difluoromethylene group which is key to additional reactions... 

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