Thermal Degradation of Chlorocarbon and Fluorocarbon Polymers

The thermal instability of poly (vinyl chloride) is a cause for concern commercially and has, therefore, stimulated extensive investigations. The overall process of degradation is complex and still not completely resolved. Some of the questions that remain are: 1. Is the overall process of degradation ionic or free radical in nature? 2. What is the exact mechanism of initiation? It is easilly observable that the degradation is accompanied by evolution of HCl and blackening of the polymer. Also, at elevated temperatures, poly (vinyl chloride) not only gives off hydrogen chloride, but this dehydrochlorination is also accompanied by rapid depolymerization [483]. The rate of decomposition decreases with increasing molecular weight and is highest in oxygen and lowest in helium atmosphere. It was claimed that HCl exerts no catalytic effect upon the rate of decomposition [483]. This contrasts with evidence that was presented earlier that HCl does accelerate thermal degradation [484]. The earlier evidence was obtained from autocatalytic decmposition of films. Based on that eveidence, it was hypothesized that the chloride ion acts as a base (supporting an ionic mechanism of decomposition) [484]:

process, but may or may not be essential to the depolymerization reaction itself, once the process has been started [485]. The above shown ionic mechanism was disputed in a study where ESR signals were recorded during thermal decomposition of the polymer at elevated temperatures [486]. This strongly supports a free-radical mechanism. A free-radical mechanism was proposed earlier by Bamford and Feuton [487]. This mechanism illustrates the formation of hydrochloric acid and is based on rupture of carbon–chlorine bonds:

Initiation:

Propagation:

Subsequent interaction of macromolecular radicals leads to cross-linking. Many additional investigations demonstrated that thermal decompositions of vinyl polymers with pendant electronegative groups that we can designate as X, result, after elimination of HX and formation of macromolecular residues, with polyene structures [488]. In addition to that, at higher temperatures the polyene sequences that form rearrange into a large number of aromatic hydrocarbons [489–492]. Formation of polyenes is common to poly (vinyl chloride), poly (vinyl bromide), poly (vinyl alcohol), and poly (vinyl acetate) [493]. The polyene structures partly decompose at formation due to bond stress and molecular reorganization processes. The stresses result from formation of conjugated structures along the polymeric chains. When the chains fracture, the remaining portions of polyene sequences form aromatic compounds [47]. It was postulated that the process initially involves reactions of enone groups, which are present in poly (vinyl chloride) as anomalies, with chlorine units of a neighboring polymeric chain [494, 495]:

The first step is formation of a dihydropyran ring. This is followed by a retro-Diels-Alder splitting and leads to regeneration of the a,b-unsaturated ketone and to formation of a double bond in the poly (vinyl chloride) molecule. The double bond initiates a subsequent elimination of hydrogen chloride from that molecule [495]. An intramolecular initiation process that explains constant rate of dehydrochlorination was also proposed [496]:

It was pointed out that poly (vinyl chloride) is, in a sense, its own worst enemy, in that all the structural defects that are known to contribute appreciably to its thermal instability. They are formed in the polymer molecules during the process of polymerization by routes involving hydrogen abstraction from the polymer backbone [584].

Gupper et al. used micro Raman spectroscopy to study thermal degradation of poly(vinyl chloride) containing various additives [586]. They observed a linear increase in conjugated sequences in the process of dehydrochlorination. Carty and coworkers investigated thermal decomposition of chlorinated poly(vinyl chloride) [496]. The thermal decomposition of pure chlorinated poly(vinyl chloride) (without stabilizer or lubricant) was studied by dynamic thermogravimetric analysis at heating rates from 5 to 100C/min in atmospheres of nitrogen, air, and oxygen. In each case, a two-step decomposition was observed, similar to that for poly(vinyl chloride) where dehydrochlorination is followed by pyrolysis/oxidation of the carbonaceous residue. The rate of dehydrochlorination was dependent on atmosphere, occur ring slightly slower in nitrogen than in air, and slightly more quickly in oxygen than in air. The decomposition of the residual char was clearly dependent on the conditions that it formed in. Under dynamic conditions, chars formed at high heating rates appeared more resistant to oxidative degra dation than those formed more slowly. However, when chars were formed by heating at different rates and then held at 500C, thechar formed at the slowestheating rate was the slowest to be oxidized. The uptake of oxygen by the char appears to be rate limiting. At low heating rates, char oxide is similar in both air and oxygen. As the heating rate is raised, the rate of mass loss of char in air becomes progressively closer to that in nitrogen until at 100C/mm they are almost identical. The rates of thermal decompositions of poly (vinylidine chloride) s were shown to depend upon the method by which the polymers were prepared [497]. Those that were formed from very pure monomers by mass polymerization are most stable. Polymers prepared by emulsion polymerization, on the other hand, degrade fastest. The mechanism of degradation of poly (vinylidine chloride) was proposed to be as follows [498–500]:

1. Hydrochloric acid is eliminated in a chain reaction.

2. Conjugated sequences condense to form cross-linked structures. Some support for the above mechanism came from stepwise heating studies of poly (vinylidine chloride) [501]. Although polytetrafluoroethylene has the reputation for being quite stable thermally, it does degrade at elevated temperatures. The polymer upon pyrolysis yields almost 100% of monomer. The mechanism is believed to be free-radical unzipping of the chains until the entire chain is consumed. This can be illustrated as follows [457]. Initially, the chain ruptures:

this is followed by formation of monomers:

Polychlorotrifluoroethylene is less stable thermally than polytetrafluoro-ethylene. It yields as much as 86.0% volatiles in 30 h at 331.8C[11]. These volatiles contain large amounts of monomer. A mechanism that resembles the postulated degradation mechanism of polytetrafluoroethylene was proposed [457].

Unlike polytetrafluoroethylene, polyvinyl fluoride, poly (vinylidine fluoride), and poly tri fluoroethylene yield primarily on heating HF [457]. Among these three, poly (vinylidine fluoride) yields larger amounts of HF than do the other two polymers with an accompanying formation of double bonds.

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العتبة العباسية المقدسة تعلن تأجيل مؤتمر معالم الحياة الطيبة عند أهل البيت (عليهم السلام) الدولي الثاني

قسم العلاقات العامة يقيم أمسية رمضانية ثقافية لطلبة الأقسام الداخلية في جامعة بابل

قسم الشؤون الفكرية ينظم ختمة قرآنية رمضانية في دولة تشاد بإفريقيا

ضمن مشروع بيوت النور.. المجمع العلمي ينظّم أمسية قرآنية رمضانية في كربلاء

المزيد

الآخبار الصحية

الماء المملح.. فائدة حقيقية أم "صيحة زائفة"؟

عادة بسيطة تدعم الصحة وتحافظ على الوزن المثالي

الغذاء النباتي يقلل مخاطر السرطان.. مع استثناء مهم

الحليب قليل أم كامل الدسم.. أيهما أفضل؟

المزيد

الآخبار التكنلوجية

أنثروبيك تتحدى البنتاغون.. وترامب يأمر بوقف التعامل معها

ميزة جديدة من "إنستغرام" لحماية المراهقين من الانتحار

الأولى من نوعها.. "تقنية مذهلة" في هاتف سامسونغ الجديد

لماذا تشعر المرأة بالألم أكثر من الرجل؟.. دراسة تجيب

المزيد

مواضيع ذات صلة

Degradation of Polymeric Materials by Ionizing Radiation

Oxidation of Chain-Growth Polymers

Oxidative Degradation of Polymers

Thermal Degradation of Cellulosic Materials

Thermal Degradation of Polyimides, Polyoxidiazoles, and Polyquinoxalines

Thermal Degradation of Aromatic Polysulfones

Thermal Degradation of Epoxy Resins

Thermal Degradation of Polyamides

Thermal Degradation of Polyesters

Thermal Degradation of Poly (Vinyl Acetate)

Thermal Degradation of Methacrylic and Acrylic Polymers

Thermal Degradation of Polyolefins and of Polymers from Conjugated Dienes