Deterioration hydrolytic

Foams prepared from phenol—formaldehyde and urea—formaldehyde resins are the only commercial foams that are significantly affected by water (22). Polyurethane foams exhibit a deterioration of properties when subjected to a combination of light, moisture, and heat aging polyester-based foam shows much less hydrolytic stabUity than polyether-based foam (50,199). 

Most practical cements contain Mg " which is less strongly bound to the polyacrylate than Zn (Gregor, Luttinger Loebl, 1955a). Magnesium oxide forms a paste with PAA which sets to a plastic mass this is not hydrolytically stable, for when placed in water it swells and softens (Hornsby, 1977 Smith, 1982a). Moreover, if ZnO powder contains more than 10% MgO, the resultant cement deteriorates under oral conditions. 

Abiotic spoilage is produced by different physical and chemical changes such as hydrolytic action of enzymes, oxidation of fats, breakdown of proteins, and a browning reaction between proteins and sugars. However, in this chapter we focus on microbial deterioration and their effects on bioactive compounds. 

Hydrolytic degradation of polymers is still the main reason for the occurrence of faults in processing. This form of degradation commonly causes a reduction of the IV, associated with a deterioration in the mechanical properties, particularly the tensile strength. Therefore, it is important to ensure sufficient drying of the raw materials. Drying is a crucial prerequisite of any polyester processing... 

A few animals (especially ruminants and termites) are able to metabolize cellulose, but even these animals depend on appropriate microorganisms in their intestinal tracts to hydrolyze the -1,4 links other animals, including man, cannot utilize cellulose as food because they lack the necessary hydrolytic enzymes. However, such enzymes are distributed widely in nature. In fact, deterioration of cellulose materials —textiles, paper, and wood —by enzymatic degradation (such as by dry rot) is an economic problem that is not yet adequately solved. Efforts to turn this to advantage through enzymatic hydrolysis of cellulose to glucose for practical food production have not been very successful (see Section 25-12). 

Pancreatic and malt amylases gradually lose their activity in the aqueous dispersions in which they act. As above noted, there is good evidence that this is due to a destructive hydrolysis of the enzyme. The destructive action of water upon enzyme is less pronounced in the presence of substrate, probably because the combination of enzyme with substrate serves to some extent to protect the enzyme from hydrolysis. It is less rapid in solutions of commercial pancreatin and in water extracts of malt than it is in solutions of purified pancreatic and malt amylases, doubtless because of the presence in the former of substances which are products of protein hydrolysis (proteoses, peptones, polypeptids, amino acids) and whose presence therefore tends to retard further protein hydrolysis and thus to protect the enzyme protein from hydrolytic destruction, or at least to diminish the rate at which such deterioration of the enzyme occurs. 

Increased HA levels in the bloodstream decrease immune competence.153 Various mechanisms have been invoked. An HA coating around circulating lymphocytes may prevent ligand access to lymphocyte surface receptors.95 96 154 155 The increased HA may represent one of the mechanisms for the immunosuppression in the fetus. The reappearance of high levels of HA in old age may be one of the mechanisms of the deterioration of the immune system in the elderly. The increasing levels of HA with aging may be a reflection of the deterioration of hydrolytic reactions, including the hyaluronidases that maintain the steady state of HA. This is a far more likely mechanism than an increase in HA synthase activity. 

Celluloses are similar to other linear polymeric materials in that they can possess one-dimensional order within an individual chain as well as three-dimensional order within an aggregate of chains. Increments in the levels of order occur during the isolation of native celluloses and also as a result of exposure to conditions that promote molecular mobility, such as elevated temperatures and immersion in plasticizing fluids. These increments generally result in embrittlement of the cellulosic materials. Similar effects are expected to occur upon aging of cellulosic textiles and papers over extended periods, and may be accelerated by hydrolytic cleavage of cellulosic chains. The implications of these effects for conservation practices, both with respect to recovery of function as well as in the assessment of deterioration, are reviewed. 

Approximately 150 different amino acid residues have been reported in proteins (1 5). At least half of these could undergo chemical deteriorations under the conditions of stress usually encountered. Many of these deteriorative reactions involve hydrolytic scissions, not only of peptide bonds but of the many different nonprotein substances added covalently to proteins postribosomally. These susceptible side chain groups are indole, phenoxy, thioether, amino, imidazole, sulfhydryl, and derivatives of serine and threonine (such as 0-glycosyl or O-phosphoryl), the disulfides of cystine, and, of course, the amides (such as asparagine and glutamine). With strong acid or alkali, other residues, such as serine and threonine, also are less stable. 

There are, of course, many carbonyl compounds formed by hydrolytic or oxidative deteriorations of lipid constituents, and most of these are potentially capable of entering into Mai Hard reactions with proteins. One such product is reputedly malon-aldehyde (26) (Figure 10). 

Acid-catalyzed hydrolytic degradation of cellulose proceeds according to the principles of chemical kinetics. Nonetheless, concepts of kinetics have not been widely applied in the literature concerning the conservation of cellulosic materials. Thirty years ago, McBurney (I) provided an excellent exposition of this subject. We will review the subject in the light of developments since that time (2) and will present examples from the literature and from our own work to illustrate ways in which an analysis of the kinetics of chain scission can help conservators better understand the deterioration of cellulose-based materials. 

Although the concepts outlined in this chapter are particularly appropriate for the interpretation of hydrolytic deterioration of cellulose, they show promise as an aid in the interpretation of thermal, photochemical, photolytic, and enzymatic degradation as well. Equations 3 and 4 are generally applicable to the scissioning process in linear polymers (33, 34). 

Lipase activity results in hydrolytic rancidity. There is little or no change in flavor of the bran with an increase in FFA (5). Lipoxygenase activity, however, increases with the presence of FFA resulting in oxidative rancidity (36). It is oxidative deterioration that is responsible for the flavor and odor of rancid rice bran. 

The concern over the hydrolytic stability of the CTB-Cu salt that was raised by Fig. 5 was confirmed by exposure of the PSVP/CTB-Cu blend to the laboratory environment for several days. Fig. 6 shows that with time there was aj apparent reversion to carboxylic acid functionality (>1700 cm ) and a reduction In carboxylate functionality (ca. 1600 cm ). Surprisingly, though, a limited study revealed no noticeable deterioration of the mechanical properties of the PSVP-CTB-Cu blende after exposure to the atmosphere. 

Coolants. Underhood fluids also include water and antifreeze solutions. Physical properties of glass-reinforced plastics of all types deteriorate, sometimes drasticaFly, upon exposure to water at elevated temperatures. Bair and Miner of Bell Laboratories have recently studied the hydrolytic stability of PPS and glass-reinforced PPS using a calorimetric metnod (6)- The results which they obtained agree well with our data. (Figure 6)... 

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