Thermal stabilization of poly vinyl

The presence of catalyst residues, such as alkali hydroxide or alkali acetate, a by-product of the hydrolysis reaction, is known to decrease the thermal stability of poly(vinyl alcohol). Transforming these compounds into mote inert compounds and removal through washing are both methods that have been pursued. The use of mineral acids such as sulfuric acid (258), phosphoric acid (259), and OfXv o-phosphotic acid (260) has been reported as means for achieving increased thermal stability of the resulting poly(vinyl alcohol). 

For example, by using 90 parts of vinyl chloride and 10 parts of vinyl acetate, the random copolymer formed has the toughness of poly (vinyl chloride), thermal stability of poly (vinyl acetate) and solubility akin to poly (vinyl acetate). These combination of properties makes it useful as a paint. 

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

Thermal Stabilization of Poly(vinyl chloride) through Graft Copolymerization of cw-1,4-Polybutadiene... 

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. 

The low thermal stability of poly(vinyl chloride) (PVC) has been ascribed to the pressure of low amounts of anomalous structures with... 

The low chemical and thermal stability of poly(vinyl ketones) leads to a sensitivity to degradation reactions. Poly(methyl isopropenyl ketone) lost water at about 250 °C, to yield glassy, red, non-crosslinked products. It was proposed that an intramolecular aldol... 

Under Applied Centrifugal Force D422 Thermal Stability of Poly (Vinyl Chloride) (PVC)... 

CdO is used in connection with the stabilization of poly(vinyl chloride). This is discussed below in more detail. It also finds application in modifying the thermal properties of teflon and some rubbers. CdS is used in some smoke detectors, in lasers and in phosphors. The cadmium(II) halides are important as catalysts and are also used in pyrotechnics. Cadmium borates of the general type (Cd0)x(B203), are also used as phosphors. CdS04 is employed in the Weston cell, which is important as a voltage standard.137... 

Most thermal stabilizers for poly (vinyl chloride) are metal salts of carboxylic acids or mercaptans. The commonly used metals are cadmium, barium, zinc, lead, calcium, and dibutyltin. Originally it was assumed the metal salts act as scavengers for hydrogen chloride. However, Frye and Horst (7, 8) found evidence for the introduction of ester groups in the polymer from metal carboxylate stabilizers, which led them to postulate that thermal stabilizers function by substituting the unstable chlorine atoms with the ligands of the stabilizer to yield derivatives which are more thermally stable than the original chloride. 

This paper presents the results of a study of the reactions of several thermal stabilizers for poly (vinyl chloride) with an allylic chloride model and a tertiary chloride model. The findings of this study provide considerable insight into the mechanism of stabilizer action. 

Allylic Chloride vs. tert-Chloride Reactivity. There is some question in the literature as to whether the allylic chloride moiety or ferf-chloride group is more responsible for the thermal instability of poly (vinyl chloride) (I, 2). To shed some light on this problem we compared the relative reactivities at 100 °C. in chlorobenzene of 4-chloro-2-pentene and 2-chloro-2-methylbutane with dibutyltin -mercaptopropionate. Data are summarized in Table I. The half-time for the reaction of the allylic chloride with the stabilizer mercaptide group was less than 15 minutes, whereas the half-time for the tert-chloride was nearly 20 times longer. The greater reactivity of the allyl chloride suggests that it is the more important functionality in polymer degradation. However, these results on rates of chlorine substitution are not necessarily an exact measure of thermal instability. 

Poly(vinyl ketones) such as poly(ethylene-a//-carbon monoxide) CAS 111190-67-1, poly(methyl vinyl ketone) CAS 25038-87-3, and poly(methyl isopropenyl ketone) CAS 25988-32-3, also have practical applications. For example, poly(ethylene-a/f-carbon monoxide) is used in photodegradable plastics and in various copolymers. Several studies were reported regarding the thermal stability of these polymers. It has been shown that poly(ethylene-a/f-carbon monoxide) decomposes upon heating with chain scission generating small molecular weight alkenes and ketones. Some literature reports discussing the thermal decomposition of poly(vinyl ketones) are summarized in Table 6.5.5 [13]. 

Most mercaptides are not available as commercial products, since large-scale applications of these chemical compounds are quite limited (only mercaptides of tin and antimony are used industrially as thermal stabilizers for poly(vinyl chloride) [de Sousa et al., 2000 Qu L. et al., 2002]). However, these compounds can be synthesized in a very simple way. Owing to their low water solubility, mercaptides can precipitate by reacting thiols (or thiophenols) with aqueous solutions of the corresponding metal salts. In addition to the well-known mercaptides of mercury, lead, zinc, and copper, many others, such as mercaptides of silver, gold, platinum, palladium, iridium, nickel, iron, cobalt, antimony, bismuth, and cadmium, have been prepared. 

Benaniba, M.T., Belhaneche-Bensemra, N. and Gelbard, G. (2001) Stabilizing effect of epoxidized sunflower oil on the thermal degradation of poly(vinyl chloride). Polymer Degradation and Stability, 74(3), 501-505. 

G. Ayrey, B. C. Head, and R. C. Pollet, The thermal dehydrochlorination and stabilization of poly(vinyl chloride), Macromol. Revs. 8, 1 (1974). 

Example 5.18 Suppression of the Thermal Dehydrochlorination of Poly (Vinyl Chloride) by Addition of Stabilizers... 

The stabilization of poly(vinyl chloride) against light has been reviewed by Wirth and Andreas. Detailed mechanistic studies have indicated the importance of peroxides in the process of photo-oxidation. It was suggested that protection could be successfully achieved by exclusion of radiation of A < 380 nm. E.s.r. examination of irradiated samples demonstrated the intervention of peroxides in the mechanisms with the ultimate formation of carbonyl groups which caused chain scission by Norrish cleavage. Photo-oxidation of samples of poly(vinyl chloride) modified by incorporation of acrylonitrile-butadiene-styrene, methyl methacrylate-butadiene-styrene, and methyl methacrylate-acrylonitrile-butadiene-styrene copolymers has been investigated. Discolouration was accelerated by the presence of the modifiers. Thermal pre-treatment accelerated photo-induced decomposition. Mechanical properties were also examined, and scanning electron microscopy showed surface defects due to decomposition of the modifier. ... 

The thermal and thermooxidative stability of poly(vinyl methyl ether) (PVME) was described in Section 3.4.2. PVME was a component in some... 

Ivan B. Thermal degradation and stabilization of poly (vinyl chloride). In Jimenez A, Zaikov GE, enters. Polymer analysis and degradation. Huntington, NY Nova Science Publishers, Inc 2000. p. 91-104. 

Bensemra N, Van Hoang T, Michel A, Bartholin M, Guyot A. Thermal dehydrochlorination and stabilization of poly(vinyl chloride) in solution. Part III zinc and calcium stearates as stabilizers. Polym Degrad Stab 1989 24(1) 33-50. 

F. Gong, M. Feng, C. Zhao, S. Zhang, and M. Yang. Thermal properties of poly (vinyl chloride)/montmorillonite nanocomposites. Polymer Degradation and Stability, 84 (2004), 289-294. 

Sionkowska et al. [46] farther studied the photochemical stability of poly(vinyl pyrrolidone) in the presence of 1, 3 and 5 % collagen irradiated at X, = 254 nm in air for 24 h and characterized the samples by by UV-Vis and FTIR spectroscopy and thermal analysis (TGA, DTG). It was observed that PVP was thermally and photo-chemically less stable in the presence of collagen. Collagen led to ciosslinking of PVP and increased its polarity during irradiation, indicating surface photo-oxidation. 

The side effects due to degradation can be alleviated through a precise knowledge of the mechanism involved. For instance, the autocatalytic thermal dehydrochlorination of poly(vinyl chloride) and poly(vinylidene chloride) suggests that basic additives can weU stabilize them and prevent their degradation by neutralizing the HCl gradually formed. 

Fig. 24. Change in resistance with thermal aging of poly(vinyl chloride). A and B indicate different stabilizer systems.

Boboli, E., Industrial methods for preparation of organotin compounds for production of thermal stabilizers for poly(vinyl chloride) and other chlorinated polymers," Przem. Chem. 377-6 (I967) CA 68j 87769. 

Previous work by this research group has shown that organic thiols containing one or more carboxylate ester functions (ester thiols) are remarkably effective as thermal stabilizers for poly(vinyl chloride) (PVC). Some prototypical examples of these thiols are compounds 1-9, where R = 2-ethylhexyl and R = 3-heptyl [1-9]. 

In the case of Poly(vinyl acetate) -Iodine complex, it has been reported that the complex suddenly disappears when the temperature is above the Tg of the polymer [57, 58]. In the PVA-Iodine complex, the amount of complex begins to decrease at the 20 °C transition, followed by a gradual decrease in a wide span of temperature. It should be remarked that this 20 °C transition, demonstrated by the beginning of the decrease in the amount of the complex in PVA films may be the same as that observed by Yokota et aL [26] in aqueous PVA/iodine-KI boric acid solutions. They studied the thermal stability of the blue complex in the solution to find that dissociation of polyiodine begins at 10-15 °C. Although our case is that of PVA bulk without boric add, while their case is that of solution with boric add, the difference is not substantial because the complex is... 

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. 

---