Thermo-oxidative Degradation Of Polymers Research Articles

Insights into the Mechanism and Kinetics of Thermo-Oxidative Degradation of HFPE High Performance Polymer.

The growing requisite for materials having high thermo-oxidative stability makes the design and development of high performance materials an active area of research. Fluorination of the polymer backbone is a widely applied strategy to improve various properties of the polymer, most importantly the thermo-oxidative stability. Many of these fluorinated polymers are known to have thermo-oxidative stability up to 700 K. However, for space and aerospace applications, it is important to improve its thermo-oxidative stability beyond 700 K. Molecular-level details of the thermo-oxidative degradation of such polymers can provide vital information to improve the polymer. In this spirit, we have applied quantum mechanical and microkinetic analysis to scrutinize the mechanism and kinetics of the thermo-oxidative degradation of a fluorinated polymer with phenylethenyl end-cap, HFPE. This study gives an insight into the thermo-oxidative degradation of HFPE and explains most of the experimental observations on the thermo-oxidative degradation of this polymer. Thermolysis of C-CF3 bond in the dianhydride component (6FDA) of HFPE is found to be the rate-determining step of the degradation. Reaction pathways that are responsible for the experimentally observed weight loss of the polymer is also scrutinized. On the basis of these results, we propose a modification of HFPE polymer to improve its thermo-oxidative stability.

Control over the Process of Thermo-Oxidative Degradation of Polymers in a Solution

The article discusses the process of low molecular rubber synthesis by the method of thermo-oxidative degradation as a controlled object. It was found that the control parameters affecting the degradation rate are the following: concentration of reaction initiator, temperature and mass concentration of the polymer. The main control parameters are the following: degree of degradation, quality of the resulting polymer and time of reaction. It was found that for effective control the process temperature and concentration of the initiator are to be stabilized in the vicinity of the predetermined value, and mass concentration is determined by dosage of initial components. In this case, the kinetics of the degradation process can be described as a linear function of time. I.e., the degree of degradation is linearly related to duration of reaction. Then, defining the initial quality of the polymer (its average molecular weight), the degradation process is performed till the time the predetermined value of degradation is reached. Since the initiator is consumed during the degradation, in order to stabilize its concentration, it is proposed to add the initiator into the reaction medium. We obtained a mathematical description that describes the kinetics of the process during continuous and divided feeding of the initiator, and dependency for calculating initiator feed rate. It has been shown that if the initiator is fed in a divided manner, linearity of degradation kinetics over time is ensured, which simplifies the technology of the process. Parameters of the approximating linear dependence of the kinetics of degradation have been determined. It has been shown that, even at the initial stage, the kinetics of degradation steadily tends to the linear asymptote. The asymptote's tilt angle depends on the constant of degradation rate (and consequently in the temperature of reaction) and on the initial concentration of the initiator. Degradation kinetics offset by the Y-axis depends on the initial molar concentration of the polymer. Thus, it is possible to control degradation rate by adjusting initial polymer concentration.

ICTAC Kinetics Committee recommendations for collecting experimental thermal analysis data for kinetic computations

The present recommendations have been developed by the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC). The recommendations offer guidance for obtaining kinetic data that are adequate to the actual kinetics of various processes, including thermal decomposition of inorganic solids; thermal and thermo-oxidative degradation of polymers and organics; reactions of solids with gases; polymerization and crosslinking; crystallization of polymers and inorganics; hazardous processes. The recommendations focus on kinetic measurements performed by means of thermal analysis methods such as thermogravimetry (TG) or thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). The objective of these recommendations is to assist a non-expert with collecting adequate kinetic data by properly selecting the samples and measurement conditions.

Thermo-oxidative Degradation of Polymers Containing Phosphorus in the Main Chain

In the present study the thermal behavior of some polyphosphonates and polyphosphates was studied. The polymers were synthesized by the inverse phase-transfer catalysis method. The thermal behavior was tested on a Perkin-Elmer DIAMOND device, in a dynamic air atmosphere and with heating rates of 5, 7, 10 and 12 °C min-1. By inspecting the thermogravimetric curves, a remarkable thermal stability up to 200 °C was observed. Between 240 and 380 °C, an endothermic decomposition occurred, with the tendency to a maximum at the end of the process. The end of the thermo-degradation was connected with an oxidative process. A kinetic analysis of the thermogravimetric/differential thermogravimetric results by means of the Friedman, Flynn—Wall—Ozawa, and non-parametric kinetic methods is the basis of discussions on the influence of the molecular architecture on the thermal behavior.

Antioxidative Activity of Carbon Nanotube and Nanofiber

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have electron affinities similar to those of fullerenes C60 and C70 and they are therefore capable of acting as radical scavengers in free radical chain reactions, including polym- erisation and the thermo-oxidative degradation of polymers. It is assumed that the CNTs and CNFs used as integral part of polymer composites are able to exhibit an antioxidant effect in these materials because of their radical accepting capacity. To examine this presumption the antioxidative activity of original and purified commercial multiwall carbon nanotube MWCNT and carbon nanofibre of platelet structure CNF-PL has been studied by means of a model oxidation reaction of cumene initiated (2,2'-azobisisobutyronitrile, AIBN) in liquid phase. This model reaction was designed to simulate the thermo-oxidative processes in carbon-chain polymers and allows comparison and transfer of obtained results to a polymer system. Kinetic measurements of oxidation rates showed that the effect of inhibition for the model oxidative reaction in the pres- ence of the original and purified MWCNT and CNF-PL strongly depends on the presence of metals (Co, Fe) in the nanoparticles. Rates of oxidation Wo2 (CNT;CNF) observed for the unrefined samples are result of the two competing rates - rate of inhibition Winh.(CNT; CNF) caused by structures of the CNT or CNF and the rates of initiation Wi(M) due to the follow- ing interaction: ROOH + M (Co;Fe) i.e, Wo2 ~ W inh (CNT;CNF) + Wi(M). The effective rate constants for the addition of cumyl radicals (R� ) to MWCNT and CNF-PL have been determined. These constants reduced to the same concentration (0.5wt.%) and temperature (60 o C) units have magnitudes: k1(MWCNT) (MWCNT) = (2.8 ± 0.3) � 10 4 s -1 and k1(CNF) (CNF) = (6.0 ± 1.0) � 10 3 s -1 . Thus, the effective rate constant, reflecting the antioxidative activity for the CNT, is five times higher than that for the CNF, is about equal to the rate constant for HAS Chimassorb 2020: k1(Chim.2020)(Chim. 2020) = (2.2 ± 0.3) � 10 4 s -1 , is ten times less than that for the HAS Chimassorb 119FL: k1(Chim.119FL)(Chim. 119FL) = (2.8 ± 0.3) � 10 5 s -1 and is about forty times less than that for the case of fullerene C60: k1(C60)(C60)(353K) = (1.2 ± 0.2) � 10 6 s -1 .

Review on the thermo-oxidative degradation of polymers during processing and in service

AbstractDuring processing as well as in service life, polymers degrade. For the majority of polymers this is due to oxidative processes. In this review the mechanism of the thermo-oxidative degradation of the most important (unstabilized) polymers as well as the factors determining the degradation rates are discussed.

Antioxidant Properties of Multiwall Carbon Nanotubes: First Measurements Using a Model Oxidative Reaction

Carbon nanotubes (CNTs) have electron affinities similar to those of fullerenes and they are therefore capable of acting as radical traps in free radical chain reactions, including polymerisation and the thermo-oxidative degradation of polymers. It is proposed that the CNTs used as integral part of polymer composites are able to interrupt chain propagation, leading to an antioxidant effect in these materials. This is because of their radical accepting capacity. To support this postulate, the antioxidative activity of commercial carbon nanotubes (denoted by CNT-MW(1b)) has been studied by means of a model reaction, namely the oxidation of cumene initiated (2,2'-azobisisobutyronitrile, AIBN) in the liquid phase. This reaction was designed to simulate the thermo-oxidative processes in carbon-chain polymers and it allows for the comparison and transfer of results to a polymer system. Measurements of oxidation rates showed that the inhibition of the model oxidative reaction in the presence of the original CNT-MW (1b) is very weak over the concentrations range up to 0.05 wt.%. Increasing the CNT-MW (1b) concentration above 0.05 wt. % facilitates catalytical action of the metal (Co) originally contained in the commercial CNT-MW (1b). In general, the rate of oxidation Wo2 depends on two competing reaction rates. These are the rate of inhibition Winh.(CNT) caused by the structure of the CNT and the rate of initiation Wi(Co) due to the interaction ROOH + M (Co) i.e. Wo2 ~ W inh (CNT) + Wi(Co). Nevertheless, experimental explorations under the condition Winh (CNT) >> Wi(Co) allow us to assess the inhibiting parameters of the CNT- MW. The effective rate constant for the addition of cumyl R· radicals to CNT-MW (1b) was determined to be k [CNT](333–353 K) = (4.0 ± 2.0)·103 s−1. This is an order of magnitude smaller than the constant for their addition to fullerene C60. The model kinetic data obtained specify the limits of rational use of the carbon nanotubes in polymer composites.

Effect of micron and nano-grade titanium dioxides on the efficiency of hindered piperidine stabilizers in a model oxidative reaction

Polymer additives often show many side reactions during the ageing of polymers which change the useful life-time of the materials. In this regard nano-grade titanium dioxide additives with high surface areas and nano-particle sizes appeared in recent years to be used in polymers where the side reactions have been found to markedly influence mechanisms of ageing. The interaction between titanium dioxide pigments and stabilizers therefore is proposed as a field of great importance. In this paper the influence of nano and micron particle grade anatase and rutile titanium dioxide pigments on the efficiency of a hindered amine stabilizer, Chimassorb 119FL and Chimassorb 119 has been investigated in the system comprising the model radical reaction of cumene initiated (2,2′-azobisisobutyronitrile, AIBN) oxidation. This model reaction was designed to simulate the thermo-oxidative processes in polymers. Kinetic measurements of oxidation rates in the absence of the pigments showed strong retarding activity of the stabilizers under conditions of the model experiments. The rates depend linearly on the reciprocal square root of the concentration of the stabilizers over a sufficiently wide range thereby fitting the mechanism of addition of cumylalkyl R radicals to the Chimassorb molecules. The rate constants for the addition of cumyl R radicals to Chimassorb 119FL and Chimassorb 119 were determined to be k (333 K) = (1.4 ± 0.2) × 10 8 and (1.2 ± 0.2) × 10 8 M −1 s −1, respectively. Hence the Chimassorb 119 family is relatively powerful retarders of thermal oxidation. Further measurements of oxidation rates in the presence of titanium dioxide particles showed that there is a significant reduction in the retarding action of the stabilizers in the presence of titanium dioxides. It occurs as a result of the initiating action of titanium additives producing an additional concentration of free radicals and inducing an additional oxidation rate which may weaken the initial inhibiting efficiency of the added hindered piperidine stabilizers. The titanium nano-samples added at a 1-wt.% into the oxidized condensed system are able to completely decrease the initial inhibiting efficiency of the stabilizers. In terms of the degree of the sensitising action the titanium dioxides can be ordered as: nano-rutile > nano-anatase treated hydroxyapatite > nano-anatase untreated > micro-anatase > micro-rutile. The behaviour of titanium dioxide particles incorporated with hindered piperidine HAS stabilizers in condensed systems may thus be used for the assessment of their performance during the thermo-oxidative degradation of polymers.

Some methodological problems concerning the kinetic analysis of non-isothermal data for thermal and thermo-oxidative degradation of polymers and polymeric materials

The procedures used in some very recent works for evaluation of the kinetic parameters from non-isothermal data corresponding to thermal and thermo-oxidative degradation of polymers and polymeric materials are critically analysed. It is pointed out that only the use of a set of conversion degree vs. temperature curves recorded at different heating rates can give reliable information on the degradation mechanism and in some suitable cases, can lead to the true kinetic parameters. A general algorithm for evaluation of the kinetic parameters from the non-isothermal data is suggested. The application of this algorithm for a set of simulated non-isothermal data has been shown that when the activation energy is independent of the conversion, the invariant kinetic parameters method associated with the criterion of independence of kinetic parameters on the heating rate is recommended for evaluation of the kinetic triplet (activation energy, pre-exponential factor, conversion function).

An influence of micron and nano-particle titanium dioxides on the efficiency of antioxidant Irganox 1010 in a model oxidative reaction

The influence of nano and micron particle grade anatase and rutile titanium dioxide pigments on the efficiency of the phenolic antioxidant Irganox 1010 has been studied using the model radical reaction of initiated (AIBN) cumene oxidation. This low-molecular reaction serves for simulating the thermo-oxidative processes of carbon-chain polymers and allows a comparison of the results to a polymer system. Kinetic measurements of induction periods in the presence of the pigments clearly showed that there was a reduction in the inhibiting activity of Irganox 1010 under conditions of the model experiments. It has been shown that stages of chain propagation and termination of the inhibited oxidation were not changed while the rate of initiation is substantially increased. The interaction between pigments and cumyl hydroperoxide as a new generator of free radicals leading to an increase in the initial initiation rate has been proposed and proved by experiments without the initiator AIBN. On the susceptibility towards hydroperoxides the titanium dioxides can be ordered as nano-rutile > nano-anatase > micro-anatase > micro-rutile and follows the relationship with decreasing surface area. The behaviour of titanium dioxide pigments incorporated with phenolic antioxidants in condensed systems should be considered in the assessment of their performance during the thermo-oxidative degradation of polymers.

Investigation of Fullerenes as High Temperature Stabilizers of Poly(methyl methacrylate) and Polystyrene

Investigation has been made on the effects of fullerenes C60 and C70 on the degradation of PMMA and PS under helium and oxygen by a DSC method. The dependences of the temperature of the onset of the thermal and thermo-oxidative degradation of the polymers on concentration of C60 and C70 have been obtained. The temperature limits of effective inhibition of the polymers by fullerenes have been determined. The temperature limit depends on the chemical structure of polymer, namely it is considerably greater in the degradation of more stable polymer. In the thermo-oxidative degradation of the polymers with addition of fullerenes, the temperature limit of inhibition of less stable PS is much less and of more stable PMMA is considerably greater than that in the presence of well-known antioxidants. It was concluded that fullerenes are more effective high temperature inhibitors of the thermo-oxidative degradation of PMMA than well-known antioxidants. The suggestion was made that the thermo-oxidative degradation of polymers initiates the oxidation of fullerenes.

The evaluation of the non-isothermal kinetic parameters of the thermal and thermo-oxidative degradation of polymers and polymeric materials: its use and abuse

The methods of evaluation of the non-isothermal kinetic parameters of the thermal and thermo-oxidative degradations of polymers and polymeric materials are critically analyzed. It is pointed out that only the use of a set of thermogravimetric curves recorded at different heating rates can give reliable information on the degradation mechanism and in some suitable cases, can lead to the real kinetic parameters.

Thermo-oxidative degradation of polymers in presence of organocadmium compounds

The use of the method of “non-chain” inhibition for stabilization of PMMA, PS and butyl rubber by organocadmium compounds has been explored. It is shown that the latter are more effective for non-chain inhibition of thermo-oxidative degradation as compared with a dispersion of metallic cadmium.

Mechanism of inhibition of thermal and thermo-oxidative degradation of polymers by polyarylene alkylenes and polymeric hydrocarbons

Abstract To explain the mechanism of inhibition of thermo-oxidative degradation of polymers by polyarylene alkylenes, it was assumed that complexes are formed by charge transfer between radicals formed during degradation and aromatic nuclei of the polyarylene alkylenes. The relation is discussed between the electron-donor ability of various polyarylene alkylenes and their inhibiting activity. It was shown that poly-1-vinylnaphthalene, poly-9-vinylanthracene, poly-N-vinylcarbazole also inhibit degradation. It was concluded that it is not the methylene groups of the chain, but aromatic nuclei which form charge transfer complexes with radicals, that are responsible for the inhibiting effect of polyarylene alkylenes.