Degrading polymers

Other aspects of stabilization of acetal resins are briefly discussed under processing and fabrication. Reference 15 provides a more detailed discussion of the mechanism of polymer degradation. 

Boron also reacts with hydroxyl-containing polymers such as cellulose. When exposed to a flame the boron and hydroxyl groups form a glassy ester that coats the substrate and reduces polymer degradation. A similar type of action has been observed in the boron—alumina tfihydrate system. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Acid Chloride Reaction. In situations where the reactants are sensitive to high temperature or the polymer degrades before the melt poiat is reached, the acid chloride route is often used to produce the polyamide (47). The basic reaction ia the presence of a base, B , is as follows ... 

J. E. Rabek, Polymer Degradation Mechanisms and Experimental Methods, Chapman and HaU, London, 1995, pp. 296—306. 

W. Schnabel, Polymer Degradation, Hanser Publishers, Munich, Germany, 1981, pp. 154—177. 

Polym. Degrad. Stabil 45(2) (1994), papers from the ArdlUPAC International Sjmposium on Macromolecules, Montreal, Canada. 

The metabohc rate of poly(ester—amide) where x = Q has been studied in rats using carbon-14 labeled polymer. This study indicates that polymer degradation occurs as a result of hydrolysis of the ester linkages whereas the amide linkages remain relatively stable in vivo. Most of the radioactivity is excreted by urine in the form of unchanged amidediol monomer, the polymer hydrolysis product (51). 

R. Mazzeo, Polymer Degradation andits Prevention Especially DuringMixing, at the MCA Meeting of the 147th Rubber Division, Philadelphia, Pa., May 2—5,1995, American Chemical Society, Washington, D.C. 

Antiozonants (qv) prevent or reduce polymer degradation by the active ozone molecule. Some antioxidant compounds, such as the /)i7n7-phenylenediamines, are excellent as antiozonants (36). The protection by these compounds is thought to be either a reaction with the ozone before it can react with the surface of the mbber or an aid in reuniting chains severed by ozone (37). 

N. Grassie, ed.. Developments in Polymer Degradation, Vols. 1 and 2, AppHed Science PubHshers, London, 1977. 

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

Most polymer degradation caused by the absorption of uv light results from radical-initiated autoxidation. 

Fig. 3. Mechanisms for polymer degradation. The illustration is a schematic representation of three degradation mechanisms I, cleavage of cross-links II, hydrolysis, ionisa tion, or protonation of pendent groups III, backbone cleavage. Actual biodegradation may be a combination of these mechanisms.

Functional dyes (1) of many types are important photochemical sensitizers for oxidation, polymerization, (polymer) degradation, isomerization, and photodynamic therapy. Often, dye stmctures from several classes of materials can fulfiH a similar technological need, and reviewing several dye stmctures... 

The principal mechanism of polymer degradation during aging is the acid-catalyzed cleavage of the ether linkage in the backbone. The acid acceptor. 

---