Poly discoloration

Whereas nonionic ethylene oxide adducts discolor badly on contact with sodium hydroxide, phosphate derivatives of these nonionics exhibit good color stability even under these conditions. But in the presence of strong acids poly-oxyethylated phosphate esters undergo hydrolysis to the base nonionic and phosphoric acid. However, the free surface-active acids by themselves show little tendency to hydrolyze. They have a pH value of 2 in aqueous solution. 

Poly(ethylene terephthalate), (PET), is a thermoplastic polymer widely used in the production of fibers and films on exposure to near ultraviolet light, PET fibers tend to lose their elasticity and break easily PET films become discolored, brittle and develop crazed surfaces. Such deterioration in properties has been attributed to photochemical reactions initiated by the... 

ISO 305, Plastics - Determination of thermal stability of poly(vinyl chloride), related chlorine-containing homopolymers and copolymers and their compounds - Discoloration method, 1990. 

Poly[(a-carboxymethyl)ethyl isocyanide] may be saponified with sodium hydroxide in methanol at room temperature in 5 days or at reflux in 20hrs, attended by discoloration. Aqueous solutions of the isolated salt do not show viscosities expected of polyanions. Attempt to isolate the free polycarboxylic acid by acidification is accompanied by decarboxylation, which is to be expected on the basis of its chemical structure (55). 

The thermal stability of poly(vinyl chloride) is improved greatly by the in situ polymerization of butadiene or by reaction with preformed cis-1,4-polybutadiene using a diethyl-aluminum chloride-cobalt compound catalyst system. The improved thermal stability at 3-10% add-on is manifested by greatly reduced discoloration when the modified poly-(vinyl chloride) is compression molded at 200°C in air in the absence of a stabilizer, hydrogen chloride evolution at 180°C is retarded, and the temperature for the onset of HCl evolution and the peak decomposition temperature (DTA) increase, i.e. 260°-280°C and 290°-325° C, respectively, compared with 240°-260°C and 260°-280°C for the unmodified homopolymer, in the absence of stabilizer. The grafting reaction may be carried out on suspension, emulsion, or bulk polymerized poly(vinyl chloride) with little or no change in the glass transition temperature. 

This hypothesis has been confirmed by the greatly improved thermal stability of PVC as a result of the formation of a graft copolymer of d -l,4-polybutadiene onto poly (vinyl chloride). The improved thermal stability is demonstrated by the almost total absence of discoloration on molding the graft copolymer into a film at 200°C in air, the reduced rate of dehydrochlorination on heating in an inert atmosphere at 180°C, and higher onset and peak temperatures for hydrogen chloride evolution as determined by differential thermal analysis. 

Polyesterification. High molecular weight linear polyester resins, such as poly(ethylene terephthalate) (PET), poly(propylene terephthalate) (PPT), and poly(butylene terephthalate) (PBT), can be produced by either transesterification of dimethyl terephthalate (DMT) with an excess of the corresponding diol or by direct esterification of terephthalic acid (TPA). Tetraalkyl titanates, such as TYZOR TPT or —TYZOR TBT, have been found to be excellent catalysts for either of these reactions. However, in the case of PET, the residual titanate catalyst reacts with trace quantities of aldehydic impurities produced in the polymerization process to generate a yellow discoloration of the polymer (468,469). In the case of PPT and PBT, where the color of polymer is not as critical, organic titanates are the catalyst of choice because of their greater reactivity than antimony or tin (470). Numerous processing variations have been described in the literature to minimize formation of tetrahydrofuran in the PBT process (471—472). 

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). 

Journal of Vinyl and Additive Technology 3, No.2, June 1997, p.107-11 REVERSIBLE DISCOLORATION EFFECTS IN THE PHOTO AGEING OF POLY(VINYL CHLORIDE)... 

Monomers Not Polymerizable by Plasma Initiation. When styrene and a-methy1styrene were subjected to plasma treatment, the monomers became yellowish and only trace amounts of insoluble films were formed. The discoloration was intensified and extensive formation of dark films were observed if carbon tetrachloride was added as the solvent. No post-polymerization was detectable for these monomers. Generally styrene and a-methylstyrene readily undergo thermal polymerization. However, no thermal polymerization was possible for these monomers after having been subjected to plasma treatment for one minute or less. It has been demonstrated from the emission spectra of glow discharge plasma of benzene (6) and its derivatives (7 ) that most of the reaction intermediates are phenyl or benzyl radicals which subsequently form a variety of compounds such as acetylene, methylacetylene, allene, fulvene, biphenyl, poly(p-phenylenes) and so forth. It is possible that styrene and a-methylstyrene also behave similarly, so that species from the monomer plasma are poor initiators for polymerization. 

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. 

The photodegradation of poly(2,6-dimethyl-l,4-phenylene oxide), 1, has received considerable attention both in industrial and in academic laboratories. Workers have observed that when poly-(phenylene oxide) films are exposed to light of wavelengths greater than 300 nm in the presence of oxygen, considerable discoloration and crosslinking occur accompanied by the appearance of carbonyl and hydroxyl bands in the infrared spectrum (2-5). Most workers in the field have ascribed these results to a hydroperoxide-mediated free radical oxidation of the benzylic methyl groups (Scheme I). 

Multiblock copolymers based on poly(a-methylstyrene) also show significantly better oxidative thermal stability than the block copolymers based on polystyrene. Thus, polystyrene-polydimethyldisiloxane multiblock copolymers lose half of their tensile strength after 80 hours with considerable yellowing at 150°C in air, but corresponding materials based on poly( -methylstyrene) show no discoloration or loss in tensile properties under the same conditions. 

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