Polymers degradation discoloration

Fig. 4 Effect of hindered phenol concentration on extent of discoloration of extruded PP (Reproduced with kind permission of Polym Degrad Stab, 1999, 64 145)...

Some polymers show discoloration as well as reduction of the mechanical properties (e.g. aromatic polyesters, aromatic polyamides, polycarbonate, polyurethanes, poly (phenylene oxide, polysulphone), others show only a deterioration of the mechanical properties (polypropylene, cotton) or mainly yellowing (wool, poly(vinyl chloride)). This degradation may be less pronounced when an ultraviolet absorber is incorporated into the polymer. The role of the UV-absorbers (usually o-hydroxybenzophenones or o-hydroxyphenylbenzotriazoles) is to absorb the radiation in the 300-400 nm region and dissipate the energy in a manner harmless to the material to be protected. UV-protection of polymers can be well achieved by the use of additives (e.g. nickel chelates) that, by a transfer of excitation energy, are capable of quenching electronically excited states of impurities (e.g. carbonyl groups) present in the polymer (e.g. polypropylene). 

Chemical breakdown usually involves oxidative chain reactions that cause embrittlement of semicrystalline polymers and discoloration of poly(vinyl chloride) and polymers with aromatic groups. The reactions are complicated by the presence of transient intermediates and by rates that depend on minute concentrations of molecular defects, impurities and additives. They also depend on several important piiysi-cal factors outlined in this brief overview of polyolefin degradation, two of these factors, the transfer of excitation energy and the transport of products and protectants, play a major role in stabilization processes. 

Hollande, S. and Laurent, J.L. (1997) Study of discoloring change in PVC, plasticizer and plasticized PVC hlms, Polym. Degrad. Stab., 55, 141-145. 

The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21-23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the ester-interchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. 

Heat stabilizers belong to one of the two major classes primary heat stabihzers and secondary heat stabilizers. When heated, chlorinated vinyl resins liberate HCl which causes further polymer degradation and discoloration. Primary heat stabilizers fimction both by retarding this dehydrochlorination and by reacting with liberated HCl to delay progressive degradation. 

On exposure to heat and/or tight the allylic chlorine atoms in the PVC structure break down to release hydrogen chloride. This leads to discoloration and polymer degradation. Hydrogen chloride can also combine with moisture to form hydrochloric acid, which could corrode processing equipment. It is the function of the heat... 

Polymer degradation is a frequent problem in extrusion. Degradation usually manifests itself as discoloration, loss of volatile components (smoking), or loss of mechanical properties. According to the mode of initiation, the following types of degradation can be distinguished ... 

Polymer degradation can result in discoloration and other associated visual defects (such as gels and black specks), inconsistent flow characteristics and a general deterioration in physical properties. Molecular weight can increase as a result of crosslinking reactions or decrease as a result of chain scission reactions. 

Pospisil J, Habicher WD, Pilar J, Nespurek S, Kuthan J, Piringer GO, Zweifel H. Discoloration of polymers by phenolic antioxidants. Polym Degrad Stab 2002 77(3) 531-8. 

Yatchigo S, Sasaki M, Ishii T, Tanaka S. Studies on polymer stabilizers part HI— prevention of NO gas discoloration with a new antioxidant. Polym" Degrad Stab 1992 37 99-106. 

Pandey, K.K. A note on the influence of extractives on the photo-discoloration and photo-degradation of wood. Polym. Degrad. Stab. 87(2), 375-379 (2005)... 

Chang, H.T., Chang, S.T. Correlation between softwood discoloration induced by accelerated lightfastness testing and by indoor exposure. Polym. Degrad. Stab. 72, 361-365 (2001)... 

To cite a couple of examples of polymer degradation, polyethylene, if exposed to high temperatm-es in an oxygen atmosphere, suffers an impairment of its mechanical properties by becoming brittle, and the utility of poly(vinyl chloride) is limited because this material may become discolored when exposed to high temperatures, although such environments may not affect its mechanical characteristics. 

J. Pospisil, W.-D. Habicher, J. Pilar, S. NeSphrek, J. Kuthan, G.-O. Piringer, H. Zweifel. Discoloration of polymers by phenolic antioxidants. Polymer Degradation and Stability 77(3), 531-538 (2002). 

The manner in which a thermoplastic material is affected by exposure to a given chemical depends on a number of variables. First and foremost is whether the thermoplastic reacts with that chemical. It may be completely impervious to that chemical, no matter what. Or it may be unaffected at low temperature, but affected by exposure at high temperature. Then there is the relative concentration of the chemical, whether the exposure is constant or intermittent, and the duration of the exposure. Finally, there is the chemical mechanism involved. Is the chemical acting as a plasticizer, and if so, is it a reversible action, or permanent Is the chemical causing an oxidation reaction, polymer degradation, or simply a discoloration of the surface, etc. ... 

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