1、外文资料翻译 A study on usability of magnesium oxide with titanium dioxide in PVC door and window profiles 1. Introduction Polyvinyl chloride is among the most widely used synthetic organic polymer materials. Plasticized polyvinyl chloride compositions are widely encountered as, for instance,vinyl sheet g
2、oods and as objects formed from plastisols.Polyvinyl chloride is commercially available in a variety of grades, some of which are suitable for preparing rigid,plasticizer-free compositions for extrusion . For plastics, prolonged exposure to the suns electromagnetic radiation in the ultraviolet (UV)
3、region can lead to photooxdiation and degradation of physical properties, often manifested by color change and embrittlement. Similarly,the UV component of ordinary fluorescent lighting can degrade polymers and many of the additives used with them. The effective UV radiation that does reach the eart
4、hs surface extends from about 290400 nm. This range happens to include the highest energy component UV band, and the segment around 300 nm, which is the most distractive to plastics. Some man-made high-energy radiation sources mercury arc lamps, xenon arcs, carbon arcs, and various sun-lamps can emi
5、t radiation at wave lengths below 290 nm and these can degrade plastics even more severely than natural sun light. Hence, they are often used for accelerated testing of plastics. The energy content of UV radiation in the 290400 nm can rupture most of the chemical bonds present in polymer structures.
6、 Not all the polymers are equally affected by UV radiation, and some have a degree of resistance, otablypolymethylmethacrylates and fluorocarbons. Others, that in their pure forms could be expected to be resistant to UV, are degraded because of contaminants present that act as sites for UV energy ab
7、sorption. Absorption of radiation energy by polymer produces molecular excitations: if the level of absorbed energy is high enough, it can activate a chemical reaction whereby internal bonds (carbon to carbon, carbon to hydrogen, carbon to halogen, etc.) are broken so that polymer degradation result
8、s. PVC is damaged by dehydrochlorination (release of hydrogen chloride), autooxidation and echanochemical chain scission. This degradation is caused by the simultaneous sequence of these reactions. Dehydrochlorination, prevailing reaction during processing,leads to increasing discoloration. In the c
9、ourse of the proceeding degradation the physical properties are also changed in the direction of increasing embrittlement. PVC of ideal constitution should be thermally stable, which was concluded from investigations with model substances. Therefore, it has to be assumed that the damage, articularly
10、 the dehydrochlorinations, starts from sites of the macromolecule with labile chlorinecarbon bonds. PVC can be degraded by heat and sun lights. The release of hydrogen chloride, which is the indication of PVC degradation in prolonged exposure to the suns electromagnetic radiation in the UV region, i
11、s occurred according to the following reactions: n H C lCHCHC H C lCH nHe a tn )( 2 The color of PVC-based article is changed from yellow to black according to degrees of the degradation. Once the reaction has started, polymers quickly and progressively experience changes in appearance: surface qual
12、ities, gloss,chalking, color, electrical properties, tensile strength and elongation； and can reach the end points of embrittlement and total disintegration. The degradation of polymers exposed to UV, often described as photodegradation and frequently identified as photooxidation, can follow various
13、 routes. By absorbing UV radiation directly, a polymer molecule can reach a high-energy excited state where it becomes unstable. If the excess energy can be dissipated in a fashion that does not affect the molecule by making it phosphoresce or fluoresce, or by converting the energy to heat that can
14、be carried away, or by transferring the energy to another molecule, photochemical reaction does not started and thus, polymer degradation will not happen. However, such actions occur only rarely, since most polymers cannot dispose of the excitation energy without undergoing a chemical reaction that
15、sets off a degradative process. In theory, many pure polymers should not absorb UV radiation, and thus, not be subject to photodegradation. However, in practice the most polymers contain impurities such as carbonyl or carboxy groups or hydroperoxides that readily absorb radiation in the 290400 nm ra
16、nge causing them to break down. Thus, generating sites within the polymer structure where chemical reactions can be initiated and propagated by free radicals. The active groups may be unavoidably present as a result of reactions that occur during polymerization. Similarly, metallic ions are present
17、in most polymers as residues from polymerization catalysts, or as constituents of compounding additives such as heat stabilizers, antioxidants, colorants, fillers and others. The metal ions are highly receptive to the absorption of UV radiation, and are efficient in transferring the absorbed energy
18、to the polymer molecules around them, thus, they act as photo-sensitizers and can promote degradation at the same time that they perform their desired functions. Another contributor to photodegradation of polymers is oxygen, which helps any free radicals that may be liberated by the UV to initiate a
19、nd propagate oxidation of the polymer, hence, the term photooxidation. Polyvinyl chloride suffers from poor heat stability. Its degradation occurs by autocatalytic dehydrochlorination initiated at the labile sites in the polymer chains. This leads to severe discoloration and loss of mechanical properties. The dehydrochlorination most probably proceeds by a chain mechanism involving radical intermediates. Various defect sites in PVC are branching. Inorganic and organic thermal stabilizers are commonly added to protect the
