Dehydrochlorination Stabilizers

Even in the presence of the antioxidants and dehydrochlorination stabilizers, the rate is eight times faster than that of uncrossed polymer at 150°C in nitrogen and three times faster at 180 C in oxygen (Figure 3). 

Dehydrochlorination stabilizers serve mainly as an additive for poly(vinyl chloride) (PVC). PVC is sensitive to dehydrochlorination. 

Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

Copolymers of vinyl chloride with 2-10% propylene became available in the USA in 1973 (Sta-Flow by Air Products and Chemicals Inc.). Compared with the vinyl chloride homopolymer these copolymers have a reduced tendency to dehydrochlorination and thus improved heat stability. This is of importance both in application and during processing, and one particular claim made for these products is their ease of moulding. 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

Acid acceptor. This is the main function of metal oxides in CR adhesive formulations. Upon age, small amounts of hydrochloric acid are released which may cause discolouration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergistically in the stabilization of solvent-borne polychloroprene adhesives against dehydrochlorination. 

The parent five-membered nitronate having no substituent at the 3-position was too unstable to be isolated. However, 3-substituted derivatives were highly stabilized. Especially, the 3-ethyl derivatives having a terminal electron-withdrawing substituent are readily available by the dehydrochlorination of 3-chloro-l-nitropropane in the presence of electron-deficient alkenes. It was our delight that the reaction of 3-al-kyl-substituted five-membered nitronates was also successfully catalyzed by R,R-DBFOX/Ph-Ni(SbFg)2 complex to at room temperature. This reaction was highly endo-selective (cisjtrans= 91 9) and enantioselective for the endo cycloadduct (92% ee). 

Starnes and Plitz [51], in their studies of PVC stabilization with di( -butyl)tin bis ( -dodecyl mercaptide) or with mixtures of the mercaptide and di( -butyl)tin dichloride, found that the rate of dehydrochlorination was inversely related to the amount of sulphur content of approximately 0.9%. The following mechanism of allyl chloride substitution was proposed [Eq. (12)]. 

Organic metal salts have frequently failed to produce an appreciable chemical stabilization effect, either during dehydrochlorination induction periods or in later decomposition stages. While this does not rule out the occurrence of Frye and Horst substitution reactions, it does suggest that these reactions may not be responsible for the observed retardation of color developments [126-128]. 

Naqvi [134] has proposed an alternative model to the Frye and Horst mechanism for the degradation and stabilization of PVC. At room temperature, PVC is well below its glass transition temperature (about 81°C). The low thermal stability of the polymer may be due to the presence of undesirable concentrations of like-poles in the more or less frozen matrix with strong dipoles. Such concentrations, randomly distributed in the polymer matrix, may be considered to constitute weak or high energy spots in the polymer, the possible sites of initiation of thermal dehydrochlorination. 

Thermal stabilities of modified PVC samples acet-oxylated to varying degrees (reaction temperature 46°C) were determined [45]. Rate of thermal dehydrochlorination at 1% degradation was taken as a measure of thermal stability. The log of the degradation rate is plotted against the acetate content of the polymer in Fig. 2. 

Wirth and Andreas [141] studied the effect of octyl-tin chlorides on the thermal stability of PVC. They were found to significantly retard the dehydrochlorination in the following order ... 

This shows a direct relationship of nonpolar /i-alkyl groups with the stabilizing effectiveness of the tin compound. The larger the number of nonpolar -alkyl groups attached to tin, the more effective it is in retarding dehydrochlorination. 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

Both methyl acrylate and butyl acrylate have been used to prepare vinylidene chloride copolymers with sufficient stability to permit thermal processing. The presence of alkyl acrylate units in the polymer mainchain limits the size of vinylidene chloride sequences and thus the propagation of degradative dehydrochlorination. More importantly it lowers the melt... 

Chlorine-containing polymers such as poly(vinyl chloride) PVC undergo an autocatalytic dehydrochlorination reaction under the influence of elevated temperature and UV radiation. Since the HCl originating from the dehydro chlorination of the PVC chains is believed to sustain this autocatalytic process, stabilizers that irreversibly bond HCl can thus inhibit the degradation. Heavy metal compounds such as cadmium stearate or lead stearate are currently used for this purpose. However, alternatives are required due to environmental problems associated with the use of heavy metals. Indeed, the largest current application of LDH materials is in the polymer industry, mainly to stabilize PVC [3,229-232]. 

The effect of charge delocalization en route to the activated complex is the result of the relatively nonpolar micellar environment compared to bulk water, charges in the micellar pseudophase are less stabilized by interactions with their environment (cf. stabilization of developing charges by the electrostatically non-neutral environment for (pseudo) unimolecular reactions). This effect was found for the dehydro-bromination reaction of 2-(p-nitrophenyl) ethyl bromide and the dehydrochlorination of 1,1,1 -trichloro-2,2-bis(p-chlorophenyl)ethane. ... 

Suppression of the Thermal Dehydrochlorination of Poiy(vinyl Chloride) by Addition of Stabilizers... 

---