Thermal-degradation

Thermal degradation does not occur until the temperature is so high that primary chemical bonds are separated. It begins typically at temperatures around 150-200 °C and the rate of degradation increases as the temperature increases. Pioneering work in this field was done by Madorsky and Straus (1954-1961), who found that some polymers (poly (methyl methacrylate), poly(oc-methylstyrene) and poly (tetrafluoroethylene)) mainly form back their monomers upon heating, while others (like polyethylene) yield a great many decomposition products. 

The types of polymer degradation can be divided into three general categories chain depolymerisation, random scission and substituent reactions. 

For many polymers thermal degradation is characterised by the breaking of the weakest bond and is consequently determined by a bond dissociation energy. Since the change in entropy is of the same order of magnitude in almost all dissociation reactions, it may be 

In Fig. 21.1 the dissociation energy of the weakest bond of the same polymers, supplemented with the data of a number of radical initiators (peroxides and azo compounds) is plotted against the most characteristic index of the heat resistance, viz. the temperature of half decomposition (Ta4/2). The relationship is evident, though not sufficiently accurate for a reliable estimate of T yi- 

The process of thermal decomposition or pyrolysis is characterised by a number of experimental indicators  

Thermal degradation in multiphase polymeric systems is a chemical process that generally takes place either through polymer molecules/long-chain radical species or through reaction involving small-molecule radical species that are produced in one phase and subsequently diffuse into other phase. The thermal degradation behaviour of polystyrene (PS) [6] and polypropylene (PP) [7, 8] can be profoundly influenced by the presence of a second polymer. 

Thermal degradation takes place in three stages. Up to 300°C the polymer releases only some water and remaining traces of phenol and formaldehyde. Decomposition starts above 300°C, when water, carbon monoxide, carbon dioxide, methane, phenol, cresols, and xylenols are expelled. The third stage begins above 600°C, again involving the release of water, carbon dioxides, methane, benzene, toluene, phenol, cresols, and xylenols. 

Thermal decompositions have been studied most effectively by mass spectroscopic thermal analysis, thermogravimetric analysis, and electrical conductivity. Several analytical characterizations of phenolic resins have recently been reported, making use of a variety of properties, including expansion coefficients, specific heat capacity, ultrasonic properties, dipole moments, and laser light scattering. Recently, high-temperature properties of reinforced phenolic components have been studied by Goetzel.  

The thermal stability and degradation rate of PLA and the formation rate, total yield and enantiomeric fractions of LA depend on a variety of factors (i) the molecular structure, (ii) the type and concentration of initiator, co-initiator, catalyst, and additive, (iii) the concentration of LA and water in PLA, and (iv) the method and reaction conditions including the pressure of the surrounding gas, temperature, and time [157, 273-278]. 

As stated above, the hydrolytic degradation resistance of PLA-based materials was enhanced by stereocomplex formation. However, even in the molten state (i.e. above the I m of the stereocomplex crystallites), the PLLA/PDLA blend has a higher thermal resistance than neat PLLA or PDLA [275,276]. 

Dozens of environmental microbes have been reported to degrade PLLA or PDLLA [189]. The PLLA and PDLLA in the environment are assumed to initially undergo chemical hydrolytic degradation followed by the bioassimilation of degraded low molecular weight lactic acid oligomers and monomers [288]. The mineralization of PLLA by microbes to form CO2 occurs in soils [289] and compost [290]. The microbes in compost induce degradation 

PVC s molecular structure results in weak thermal resistance therefore, the addition of thermostabilizers during processing is unavoidable [530]. The main function of PVC-stabilizers is to delay dehydrochlorination and the subsequent formation of polyene sequences. 

The presence of plasticizers, such as chloroparafflns and/or phosphoric acid esters, can have a negative effect on thermal resistance [560]. 

Barium and zinc compounds and Calcium and zinc compounds 

Lead salts and lead soaps and Trlbaslc lead sulfate. 

PVC stabilized only with Ca/Zn or Ba/Zn soaps exhibits unsatisfactory long-term stability and poor initial color in processing. Co-stabilizers improve the initial color of these PVC compounds under thermo-mechanical load and its iong-term thermal stability. All potential co-stabilizers, such as phosphites, poiyoiefins, dihydropyridine, and epoxidized soybean oii, are measured against the market ieader, Rhodiastab 50, a /S-diketone. This product has woridwide approvai for food contact and is indispensable to obtain good initiai coior when Ba/Zn or Ca/Zn systems are used [561]. 

As the first aromatic polyester of high commercial practicality, PET has been studied extensively. Results of the earliest studies, from 1950 until the late 1960s [1-5], provided (on paper at least) a viable explanation of the features of PET thermal degradation under oxygen-free conditions. 

From the high polymeric nature of PET, and the known synthetic route for its manufacture (formation of bis(hydroxyethyl terephthalate) followed by condensation of same), it can be deduced that in the as-produced polymer by far the predominant structure will be A. B will be the next highest, although much less than A and C will constitute only a very low initial level. PET stability towards all forms of degradation is very much dependent on their being a low content of the carboxyl end-group. 

From these early studies, it was considered that thermal degradation of PET does not involve a radical (homolytic) pathway, at least at the temperatures generally encountered by this polymer. The initial 

In the solid state such reactions may or may not proceed but, depending on the conformation of the affected segment of the polyester chain, it is assumed that a PET melt exhibits no order and that the necessary positioning of the atoms in the chain will occur through random motions of a freely rotating polymer chain. Reactions may thus be posited to occur as follows  

It is immediately obvious from the above speculations that degradation via this route will result in a rapid increase in the number of acid end groups present. For unit B, the vinyl alcohol produced is unstable, and most probably will immediately rearrange to acetaldehyde, i.e.. 

The bond dissociation energy of phenylene-oxy bonds is not very different from that of oxy-methylenebonds. Accordingly, the two-step decrease in weight of hexafluoroisopropylidene-unit-containing poly(formal)s, especially Bisphenol AF poly(formal) (7), is not brought about by a different reaction mechanism from 

Importance of these processes for the quality of plasticized products leads one to ejqtect that a large body of information is available on the subject. It is shown below that there are substantial gaps in our present knowledge. 

Source Data from Madorsky and Straus (1961), Arnold (1979), Van Krevelen and Nijenhuis (2009) 

T is the temperature of initial decomposition, i.e., this is the temperature at which the loss of weight during heating is just measurable is the temperature of half decomposition, i.e., this is the temperature at which the loss of weight during pyrolysis reaches 50 % of its initial value 

The sensitivity of polymers to temperature variations is determined by several factors such as the residual content of impurities (peroxide or catalyst) remaining after manufacture. 

The thermal degradation of different types of EVA was carried out at three temperatures (150,175, and 200 °C) in an isothermal oven [46]. A change in crystallinity is the first process that takes place in the early stage of heating. This is followed by other phenomena, the most important of which is the oxidation of the polymer matrix starting from 235 °C (Table 3.17). 

Pipes of HOPE were exposed to chlorinated water at elevated temperatures. The materials were stabilised with hindered phenols and phosphites [47]. Measurement of the oxidation induction time 

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To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

The thermal degradation of mixtures of the common automotive plastics polypropylene, ABS, PVC, and polyurethane can produce low molecular weight chemicals (57). Composition of the blend affected reaction rates. Sequential thermolysis and gasification of commingled plastics found in other waste streams to produce a syngas containing primarily carbon monoxide and hydrogen has been reported (58). 

Polymer T,°C TJC Softening temperature,°C Thermal degradation temperature,°C... 

The question as to whether a flame retardant operates mainly by a condensed-phase mechanism or mainly by a vapor-phase mechanism is especially comphcated in the case of the haloalkyl phosphoms esters. A number of these compounds can volatilize undecomposed or undergo some thermal degradation to release volatile halogenated hydrocarbons (37). The intact compounds or these halogenated hydrocarbons are plausible flame inhibitors. At the same time, thek phosphoms content may remain at least in part as relatively nonvolatile phosphoms acids which are plausible condensed-phase flame retardants (38). There is no evidence for the occasionally postulated formation of phosphoms haUdes. Some evidence has been presented that the endothermic vaporization and heat capacity of the intact chloroalkyl phosphates may be a main part of thek action (39,40). 

Over the years animal studies have repeatedly shown that perfluorinated inert fluids are nonirritating to the eyes and skin and practically nontoxic by ingestion, inhalation, or intraperitoneal injection (17,22). Thermal degradation can produce toxic decomposition products including perfluoroisobutene which has a reported LC q of 0.5 ppm (6 hr exposure in rats) (31). This decomposition generally requires temperatures above 200°C. 

Chemical Properties. Vacuum thermal degradation of PTFE results in monomer formation. The degradation is a first-order reaction (82). Mass spectroscopic analysis shows that degradation begins at ca 440°C, peaks at 540°C, and continues until 590°C (83). 

Standard thermoplastic processing techniques can be used to fabricate FEP. Thermal degradation must be avoided, and a homogeneous stmcture and good surface quaUty must be maintained. 

More recent work reports the onset of thermal degradation at lower temperatures and provides a clearer picture of the role of oxygen (73—75). In the presence of oxygen, backbone oxidation and subsequent cleavage reactions initiate decomposition. In the absence of oxygen, dehydrofluorination eventually occurs, but at significantly higher temperatures. 

The self-ignition temperature of PVF film is 390°C. The limiting oxygen iadex (LOI) for PVF is 22.6% (98), which can be raised to 30% ia antimony oxide-modified film (99). Hydrogen fluoride and a mixture of aromatic and aUphatic hydrocarbons (100) are generated from the thermal degradation of PVF. Toxicity studies, ie, survival and time to iacapacitation, of polymers, ceUulosics (101,102), and airplane iaterior materials (103) expose... 

Concentration and Aroma Recovery. Concentration of juice from deciduous fmit is best carried out using an evaporator that causes as httle thermal degradation as possible and that permits recovery of volatile materials important to the aroma of the fresh fmit, ie, essence. Evaporators that use a high temperature for a short time and operate under a vacuum, such as the APV Crepaco falling film plate evaporator or the Alfa Laval centrifugal... 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Temperatures should not exceed 60°C for the Type I resins, and 40°C for Type II and acryflc resins. Thermal degradation and the loss of functional groups occur when these temperatures are exceeded. Elimination of siUca from the resin bed is further improved by preheating the bed with warm water before injecting the NaOH solution. 

In steel-on-steel lubrication with a zinc dialkyl dithiophosphate additive, a complex surface paste appears to form first of zinc particles and iron dithiophosphate. The iron dithiophosphate then thermally degrades to a brown surface film of ZnS, ZnO, FeO, plus some iron and zinc... 

Thermal, Thermooxidative, and Photooxidative Degradation. LLDPE is relatively stable to heat. Thermal degradation starts at temperatures above 250°C and results in a gradual decrease of molecular weight and the formation of double bonds in polymer chains. At temperatures above 450°C, LLDPE is pyrolyzed with the formation of isoalkanes and olefins. 

Polymer Chemical description Ionic state Thermal degrad-ation, °C AppHcation Limitations... 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Plasticizer molecules can undergo thermal degradation at high temperatures. Esters based on the more branched alcohol isomers are more susceptible to such degradation. This can, however, be offset by the incorporation of an antioxidant, and plasticizer esters for cable appHcations frequently contain a small amount of an antioxidant such as bisphenol A. 

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