Discoloration of PVC

It is the absorption of blue wavelengths in the white light spectrum by these polyene sequences that make degraded PVC surfaces appear yellow. Surface discoloration of PVC cladding (siding) and window frames exposed to sunlight is well known (Edge et al., 2010) and is a result of this reaction. 

Temperature, humidity, precipitation, and atmospheric pollution have significant influence on PVC weathering behavior. The border climate at the surface has significant influence on the discoloration of PVC surfaces moist surfaces yellow less quickly [86]. 

The materials of construction, from the cupboard to the fan, should be inorganic and resistant to attack by perchloric acid. For the cupboard itself suitable materials include stainless steel of types, 316 or 317, solid epoxy resin, and rigid PVC. Stainless steel has been popular for this application as it is easy to form, weld, and polish. It is, however, attacked by the acid, which causes discoloration of the metal surface and the formation of iron(III) perchlorate, which can be explosive. Ductwork, separate from other extract systems, is usually made from stainless steel or plastic materials. Fire regulations may preclude the use of plastic ductwork or require it to be sheathed in an outer casing of metal or GRP. The fan casing and impeller can both be made of plastic. 

Figure 10. Kinetics of the discoloration of a 2 mm plate of stabilized PVC, with or without a 70 ym epoxy acrylate coating upon QUV aging at 40°C...

PVdC-coated PVC increases the sheet s water-vapor permeability resistance by a factor of 5-10. The coating is applied at different loadings per square meter of PVC sheet. The coating is applied on one side and usually faces the product and lidding material. Stability studies will establish whether interactions with the product are taking place. This is usually indicated by discoloration of the blister or the product. 

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. 

Discoloration. When PVC, free of plasticizer or stabilizer, is compression molded in air at 200°C under adequate pressure (c.g., 500-6000 psig), thermal degradation results in discoloration to a pink to brown colored film. When the modified PVC containing as little as 3% of grafted cis-1,4-polybutadiene was pressed under the same conditions, the resultant film was essentially colorless or no more than faintly discolored. 

Consequently, reduced or retarded hydrogen chloride evolution is a more reliable measure of the thermal stability of PVC. When the unmodified polymer was pressed to a film in air at 200°C on a black iron or untreated steel plate, the film was deeply colored, and the metal surface beneath the polymer film was corroded and became covered with a film of rust. In contrast, under the same conditions, the pressed film from the graft copolymer was very slightly discolored, and the metal surface remained free of corrosion and rust even after two years exposure to the atmosphere. 

To improve the thermal stability of PVC, it is common to add stabilizers, generally metalloorganic compounds, in concentrations of 1-5 parts per 100 parts of PVC. Although the cis-1,4-polybutadiene grafted PVC, in the absence of an added stabilizer, yielded essentially colorless or only faintly discolored films, completely colorless films were obtained when the conventional stabilizers were added in concentrations of 0.1-0.3 parts per 100 parts of modified PVC. Organotin stabilizers were not necessary and, in some cases, actually resulted in greater color development than when they were absent. 

A further indication of the enhanced thermal stabilization inherent in the grafted polymer was the reduction in the concentration of conventional stabilizer required to prevent discoloration of an unmodified PVC, when the latter was blended with the graft copolymer or the mixture of graft copolymer and unmodified PVC resulting from the heterogeneous grafting reaction. 

Diepoxyethylbenzene was first prepared by Everett and Kon (3) as a liquid which evidently was not pure because analysis showed a difference of 4% in the carbon content. Our product was a white crystalline mass with melting point 79°C. and boiling point 95°C. at 0.06 mm. Catalytic hydrogenation gave the theoretical absorption. The stabilizing effect in PVC mixtures was about 30% of dibutyltin laurate—i.e., 3.3 times more of the stabilizer is necessary to obtain the same discoloration as dibutyltin laurate in a plasticized mixture of PVC with 30% dioctylph-thalate at 200°C. for 20 minutes. 1,3,5-Triacetylbenzene was prepared by the two methods mentioned above. 

All copolymers were purified carefully from unreacted lead undecylenate by extraction with suitable solvents. Figure 6 shows the results of dehydrochlorination of some samples at 170 °C. As can be seen, the rate of dehydrochlorination is not only distinctly lower compared with that of PVC, but there is, initially, also a certain period during which no hydrogen chloride is evolved. Although the sample, prepared with 6 wt. % lead salt in the monomer mixture, contains only about 0.3 mole % lead salt in the polymer, it exhibits a remarkable thermal stability at 170 °C. almost no dehydrochlorination occurs within the first 2 hours. This is, therefore, a genuine case of internal stabilization, which finds expression also in the color of the polymer. Discoloration of the samples is substantially less than that of unstabilized PVC the polymer containing 1 mole % lead salt is still nearly colorless after 2 hours of heat treatment. 

The polyenes formed in these reactions cause discoloration through their absorption in the UV and visible spectra. Conventional formulations of PVC become perceptibly yellow at 5 kCy and deep brown at 25 kGy. Radiation-stable formulations of PVC usually incorporate free radical scavengers to compensate for the reactivity but often still require the inclusion of optical brighteners or blue tints to disguise the discoloration. 

R-22 to R-23). Sodium stearate appears to be quite efficient in preventing discoloration of air-irradiated PVC whereas the samples containing organo-tin stabilizers turn brown at rather low doses [Chapiro, 1962]. 

Poly(vinyl halides) (PVC etc.) and PoIy(vinyl alcohol) (PVA).—A series of papers has appeared concerned with the photo-dehydrochlorination, discoloration, and sensitized photolysis of model compounds of PVC,819 and its copolymers.220 The photolysis of poly(vinyl bromide)221 and a trifluorochloroethylene-vinylidene fluoride copolymer228 has been described. The photo-oxidative destruction of PVA has been reported in a series of papers.223... 

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