关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究毕业论文外文资料翻译.docx

1、关于混有二氧化钛的氧化镁在PVC门窗型材中的可用性研究 毕业论文外文资料翻译外文资料翻译 A study on usability of magnesium oxide with titanium dioxide in PVC door and window profiles1. IntroductionPolyvinyl chloride is among the most widely used synthetic organic polymer materials. Plasticized polyvinyl chloride compositions are widely encoun

2、tered as, for instance,vinyl sheet goods 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

3、 radiation in the ultraviolet (UV) 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

4、radiation that does reach the earths 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 a

5、rcs, and various sun-lamps can emit 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 bon

6、ds present in polymer structures. Not all the polymers are equally affected by UV radiation, and some have a degree of resistance, otably polymethyl methacrylates and fluorocarbons. Others, that in their pure forms could be expected to be resistant to UV, are degraded because of contaminants present

7、 that act as sites for UV energy absorption.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

8、so that polymer degradation results. 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 i

9、ncreasing discoloration. In the course 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 ass

10、umed that the damage, articularly 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 electromagne

11、tic radiation in the UV region, is occurred according to the following reactions: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 qualiti

12、es, 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 ro

13、utes. 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 be

14、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 set

15、s 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 range

16、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 in m

17、ost 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 to t

18、he 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 and pr

19、opagate 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.

20、 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 polymer from heat degradation. Among the most widely used ultraviolet stabilizer

21、s is titanium dioxide pigment. Filling a polyvinyl chloride composition with this pigment substantially reduces the effective depth of penetration of ultraviolet light into the surface of an article formed from such a composition.Mohamed et al. pointed out that barbituric acid and thiobarbituric aci

22、d are nontoxic organics, thermally stable materials of high melting point. Both contain active methylene groups, and can act as H-donor through their enolic hydrogen groups, which can intervene with the radical species derived from the thermal degradation of PVC. They investigated the possibility of

23、 using barbituric acid and its thioanalogue as thermal stabilizers for rigid PVC.The effective stabilization often requires a combination of antioxidant system in which complementary overlap of different mechanistic pathways involved. This act often referred to as synergism, is the motivation for th

24、e use of admixing composition of dibutyltin maleate and trinitro and its ester homologues. The stabilization agents of dibutyltin maleate and trinitro esters could retard somewhat the photodegradation of PVC. It is hoped that the total stabilizing effect of this admixed system should be greater than

25、 the sum of the individual effects when PVC is subjected to an environment where the effects of heat and UV are combined. Turoti et al. investigated the effect of the stabilizing action of admixed mixtures of dibutyltin maleate and trinitro and its ester homologues on polyvinyl chloride exposed to n

26、atural atmosphere. In their study, the degradation and stabilization reactions were monitored by color formation, tensile strength and elongation at break, reduced viscosity as well as determination of time to embrittlement. It is observed that the stabilized PVC sample has an effective reduction in

27、 degradation reactions.Titanium dioxide is by far the most important of white inorganic pigments and possesses all-round suitability. While rutile titanium dioxide is highly reflective at visible wavelengths, it is also highly absorptive at ultraviolet wavelengths. However, although titanium dioxide

28、 is a highly effective ultraviolet light stabilizer for polyvinyl chloride compositions, it does have several serious drawbacks. An important disadvantage is the cost of titanium dioxide which has historically tended to be high compared with filler or extender pigments such as calcium carbonate and

29、talc. Another significant disadvantage of using titanium dioxide as an ultraviolet stabilizer in unplasticized polyvinyl chloride compositions is that historically titanium dioxide has been periodically in short supply.The relatively high cost of titanium dioxide is an especially significant disadva

30、ntage for the manufacture of articles for exterior use from unplasticized polyvinyl chloride compositions because such articles must often have substantially greater dimensions, for structural reasons than the effective penetration depth of ultraviolet light in the articles. Thus, it is highly desir

31、able to able to reduce the level of titanium dioxide in such a composition without experiencing an accompanying increase in the rate of degradation and reduction in service life. Although it seems to decrease the level of titanium dioxide in the PVC composition will tend to increase the effective pe

32、netration depth of ultraviolet length and will consequently accelerate the degradation of the PVC, the experimental observations do not support such an expectation. Since PVC compositions consist generally of from about 0.55 parts by weight of rutile titanium dioxide per hundred parts by weight of t

33、he polyvinyl chloride, there is no guarantee for the bulk of titanium dioxide to locate near the external surfaces of articles exposed to sun lights.In this study, usability of magnesium oxide with titanium dioxide in the PVC compositions for forming of the exterior articles such as door and window profile is investigated in terms of determining di