Mechanism of Autocatalysis in the Thermal Dehydrochlorination of Poly(vinyl chloride)

Abstract

Autocatalysis during the thermal dehydrochlorination of poly(vinyl chloride) (PVC) is shown to be a free-radical process that converts the ordinary monomer units of the polymer into chloroallylic structures that have low thermal stabilities. In the first stage of dehydrochlorination, conjugated polyene sequences are created by a nonfree-radical route. They react with HCl to give cation monoradicals and/or excited cation diradicals. One or both of these species, or other radicals formed from them, can then abstract methylene hydrogen in order to produce new radicals that are also carbon-centered. These are converted by chlorine-atom β scission into the chloroallylic segments, which start the growth of new polyenes in the usual (nonradical) way. At 180 °C in solid PVC, autocatalysis was inhibited by free-radical scavengers (a hindered phenol, triphenylmethane, and metallic mercury) but greatly enhanced by an increased concentration of HCl when all-trans-β-carotene, a model for PVC polyene sequences, was introduced simultaneously. When they were subjected to autocatalytic conditions, other model compounds gave products that apparently resulted from the abstraction of hydrogen by free-radical intermediates.

Heat stabilising flexible PVC with layered double hydroxide derivatives

“…It proceeds as an autocatalytic dehydrochlorination of the PVC backbone with the liberated hydrogen chloride assuming a catalytic role [9]. The mechanism of autocatalysis involves a free-radical process that converts ordinary polymer repeat units into chloroallylic structures of low thermal stability [10]. Apart from liberating a corrosive gas, this degradation reaction also results in the formation of polyene sequences of -conjugated double bonds along the polymer chain.…”

Investigating the influence of conduit residues on polyurethane plates

“…It is widely accepted that PVC dehydrochlorination involves a gradual reaction between chlorine and hydrogen atom neighbors in the polymer chain, comprising a double bond between the carbon atoms in positions in which the two atoms were originally connected, forming a structure of allylic chlorine with another chlorine atom of the polymer chain [11,33,35,39] . Complete combustion leads to the formation of hydrochloric acid, carbon dioxide and water.…”

“…Therefore, to meet the requirements of the NBR 9178:2015 [48] , a flame-retardant additive should be included in the formulation [11,51] .…”

“…It should withstand temperatures from -5 °C to 60 °C for 24 hours without showing any deformity, not allowing the passage of electric current above 100 mA. PVC conduit was chosen as the focus of this research because the main properties of the material encompass thermal insulation, low flammability, and self-extinguishing flame [6,[10][11][12] .…”

The effect of magnesium hydroxide, hydromagnesite and layered double hydroxide on the heat stability and fire performance of plasticized poly(vinyl chloride)

“…In view of these complications it is convenient to model the degradation using the single step reaction approximation (39)(40)(41)(42)(43). A central feature of PVC degradation is its autocatalytic nature (18,19). This implies that an autocatalytic reaction mechanism (44, 45) should be considered, e.g.…”