Degradation, hydrolytic

Hydrolysis can cause degradation via internal or external fiber or filament reactions. Geotextiles manufactured using certain resins are particularly affected when the immersion fiquid has very high (pH 10) or very low (pH 3) alkalinity. 

Halse et al. (1987) listed trends in degradation behavior insofar as loss of strength is concerned. Extremely high pH values can affect some polyesters whereas extremely low pH values can be harsh on some polyesters and polyamides. It is important that the polyester resin used for permanent geotextile applications has a high molecular weight (eg, 25,000) and a low carboxyl end group concentration (eg, 30). These effects were further described by Hsuan et al. (1993) and Hsieh et al. (2004). 

In case there is concern, the candidate geotextile should be incubated in water with the prevailing pH levels at 20°C and 50°C for at least 120 days and then tested for changes in strength and elongation. For a baseline comparison, it is important to have a complete parallel set of samples incubated in distilled water (pH = 7) at the same temperatures. 

Not all polymers are prone to hydrolytic degradation. For example the backbone of poly(olefin)s is essentially inert to hydrolytic degradation. However, poly(ester)s and poly(amide)s are more or less sensitive to hydrolytic degradation. 

Poly(lactic acid)s and their copolymers belong to the family of aliphatic polyesters therefore, their ester groups are hydrolytically degraded in the presence of water according to the following reaction  

Simple esters such as ethyl acetate can be considered to be created by the elimination of water from an alcohol and an organic acid, and the reaction as being fully reversible. In reality, the reaction between acetic acid and ethanol is a complex bimolecular process, and in the absence of a catalyst this occurs very slowly, i.e., ethanol is too weak a nucleophile to add readily to the carbonyl double bond of acetic acid. If a strong acid is present as a catalyst it should protonate the acetic acid to yield a carbonium ion, which is sufficiently electrophilic to react with the ethanol molecule. 

Obviously, the addition of the acid catalyst will only serve to increase the rate at which equilibrium between ester/water on the one hand and acid/alcohol on the other is reached to drive the reaction one will need to remove water from the system or start with a large excess of alcohol. 

The reaction may be reversed by use of the same (or different) acid catalyst and excess water, but in most organic chemistry approaches to this it is preferred to utilise base catalysis to hydrolyse an ester. 

The last step in the base hydrolysis of an ester is proton transfer from the carboxylic acid molecule to the alkoxide ion. This reaction is virtually irreversible. From a practical standpoint, base hydrolysis is a more useful process as, with for example sodium hydroxide, the acid is now in its salt form. This means that the products can be separated much more easily. 

These features of the chemistry of esters need to be taken into consideration when discussing similar reactions in polyesters. 

FIGURE 9.14 DSC thermograms of solution-spun fibers after drawing at various temperatures (glass transition temperature and various melting peaks are evident) [78]. 

Copolymerization of L-lactide with other analogous cyclic lactones, such as DL-lactide, D-lactide, glycolide, or 8-capro-lactone, produces polymers with relatively random distribution of comonomers. The Tg of PL A copolymers decreases proportionally to the content of the glycolide or 8-caprolac-tone comonomer to some extent. Moreover, the presence of stereochemical defects in PLLA reduces rate of crystallization, and extent of crystallization of the resulting 

The degradation kinetics of PLLA is largely affected by its crystallinity. As is known, degradation of PLA proceeds via hydrolysis, which is in turn controlled by the water diffusion in the free volume amorphous phase. In addition to crystallinity, other factors such as molecular weight, surface/vol-ume ratio, purity, and chain orientation can greatly affect degradation kinetics [7]. 

Migliaresi et al. [83] compared the hydrolytic degradation of different molecular weight and crystallinity PLLAs, obtained by quenching or annealing treatments. 

This degradation-induced crystallization is a very important drawback for the biomedical use of PLLA as an implant 

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Commercial condensed phosphoric acids are mixtures of linear polyphosphoric acids made by the thermal process either direcdy or as a by-product of heat recovery. Wet-process acid may also be concentrated to - 70% P2O5 by evaporation. Liaear phosphoric acids are strongly hygroscopic and undergo viscosity changes and hydrolysis to less complex forms when exposed to moist air. Upon dissolution ia excess water, hydrolytic degradation to phosphoric acid occurs the hydrolysis rate is highly temperature-dependent. At 25°C, the half-life for the formation of phosphoric acid from the condensed forms is several days, whereas at 100°C the half-life is a matter of minutes. 

The hexahydrate is formed by the addition of anhydrous STP to water or by the hydrolysis of sodium trimetaphosphate [7785-84-4] (STMP), (NaP02)3, in alkaline media. The hexahydrate is stable at room temperature but undergoes rapid hydrolytic degradation to pyro- and orthophosphate upon drying. 

Fig. 10. Nomograph for estimating the rate of hydrolytic degradation of pyrophosphate and tripolyphosphate (tetramethyl ammonium salts) (27). For...

Like P—O—C linkages, P—O—P linkages are susceptible to hydrolytic degradation. Scrambling or interchange usually occurs for phosphoms oxyesters at temperatures and acidities lower than those required for the carbon esters but greater than those for the sulfur esters. 

Hydrolytically degradable plastic is a degradable plastic in which the degradation results from hydrolysis. 

It has also been found that there can be interactions between hydrolytic degradation and photochemical degradation. Especially in the case of melamine-formaldehyde cross-linked systems, photochemical effects on hydrolysis have been observed. 

Cellulose may be degraded by a number of environments. For example, acid-catalysed hydrolytic degradation will eventually lead to glucose by rupture of the l,4-(3-glucosidic linkages. Intermediate products may also be obtained for which the general term hydrocellulose has been given. 

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

Abrasion-resistant duties may involve abrasion in an aqueous phase or abrasion by dry particulate materials. The selection of the polyurethane type is most important to obtain the best results. Polyester-based polyurethanes perform best in dry abrasion due to their low hysteresis properties and excellent resistance to cut initiation and propagation. However, polyester polyurethanes are susceptible to hydrolytic degradation, and therefore polyether polyurethanes are normally used for aqueous abrasion duties. 

These materials, when exposed to continuous high humidity, especially in the presence of an electrical field, hydrolyze into the acid and alcohol precursors from which they are made. The acid plus water present make a conductive material that will cause the material to short the electrical circuit. The process by which the decomposition of the TS polyester takes place is very gradual at first and then accelerates so that extended testing of the material is necessary to be sure that the particular polyester composition used is resistant to hydrolytic degradation. 

Reliable data in the literature for the stress versus strain properties of composite propints are exceedingly difficult to find. Since the binder chemical properties and curing additions are susceptible in many cases to hydrolytic degradation, the exact formulations under test should be specified. Additionally, the binder to oxidizer adhesion properties are dependent upon particle size distribution used in the pro-pint. This should be specified and in almost all literature sources unearthed, it remained unknown. As some of these data show, the manner of conducting the test and control of such... 

We have studied the extractant behavior of a series of compounds containing the carbamoylmethylphosphoryl (CMP) moiety in which the basicity of the phosphoryl group and the steric bulk of the substituent group are varied (10,LL). These studies have led to the development of extractants which have combinations of substituent groups that impart to the resultant molecule improved ability to extract Am(III) from nitric acid and to withstand hydrolytic degradation. At the same time good selectivity of actinides over most fission products and favorable solubility properties on actinide loading are maintained (11). 

Caliceti P, Veronese FM, Marsilio F, Lora S, Seraglia R, and Traldi P. Fast atom bombardment in the structural identification of intermediates in the hydrolytic degradation of polyphosphazenes. Org Mass Spectrom, 1992, 27, 1199-1202. 

We have already seen how water solubility and hydrolytic degradability can be built into the carrier macromolecule by the use of specific side groups. Here we will review an additional way in which drug molecules have been Linked to polyphosphazenes—by coordination. 

The protein collagen undergoes hydrolytic degradation to gelatin in a manner which indicates the presence of a small minority of comparatively easily hydrolyzable bonds. Scatchard, Oncley, Williams, and Brown concluded that these are regularly spaced at intervals of about 1200 units in the collagen molecule. 

Figure 2 Hydrolytic degradation of poly(phosphoester-urethanes) based on TDI.

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