Polymer Degradation and Stability (v.67, #3)

Fire hazards and some common polymers by D.J. Irvine; J.A. McCluskey; I.M. Robinson (383-396).
The annual fire statistics for the UK shows the pattern of injury and loss due to fire. Drawing on the current knowledge of fire safety engineering, the types of hazard commonly encountered in fires are described, including thermal parameters, smoke and toxic gas evolution. Fire tests based on established Standard tests measure only a limited range of material behaviours to a fire. Using the growing discipline of fire science, a better description of the intrinsic fire hazards for each material can be made. In a series of model calculations, some of the more commonly encountered polymers used in the construction industry are compared in their fundamental behaviour to fire using a fire science approach. From this, a simple ranking of material behaviour across a range of constant external heat fluxes can be made for each major fire hazard. The inclusion of data necessary for fire safety engineering to the growing number of polymer property databases is a readily available means to help promote fire safety through good engineering practice.

Flexible polyether polyurethane foam based on toluene diisocyanate and polyether polyol has been liquefied by dimethyl phosphonate (CH3O)2P(O)H at 160°C. The degraded products have been studied by 1H, 13C and 31P NMR spectroscopy and shown to be phosphorus-containing oligomers.

Some composites containing a cross-linked blend of Neoprene and “chlorobutyl rubber” have been artificially aged at temperatures of 35 and 80°C for up to 3 months. Overall rate constants for their degradation at 387°C, before and after the ageing, have been measured by pyrolysis-GC–MS in order to assess whether the ageing has had any effect on their thermal stability. The results show that the low-temperature ageing treatment has caused no detectable deterioration in the thermal stability of the composites as measured at the higher temperature. The mean value of the first order rate constant, determined from a reproducibility test using 21 data points, is 0.0205±0.0036 s−1 at 387°C (68% confidence limit). The scatter is attributed to macro inhomogeneity in the composition of the samples. With respect to the different batch samples, the mean values of the specific rates of degradation of the two samples were found to lie within the standard deviation of the overall scatter. The effects of the artificial ageing have also been assessed by comparing the resolved gas chromatograms of the many products obtained from the 387°C pyrolyses of the control and the aged samples. These chromatograms are virtually identical, and this suggests that not only is the thermal stability unaffected, but also that the detailed mechanisms of the degradation have not changed with ageing. Mass spectra and mass chromatograms have also been examined in order to assess the general structures present within the wide variety of degradation products. High-yield low molecular weight components include isomeric butenes (butene-1, butene-2, and 2-methyl-propene-1), but it is difficult to distinguish between these because of the similarities in their mass spectra. There is also evidence for oligomeric butenes and many other aliphatic hydrocarbons (both saturated and unsaturated, linear and cyclic) with molecular weights up to several hundred. Other products include hydrogen chloride and chlorinated hydrocarbons, plus a few other hetero-compounds which can be related to known minor additives in the composites. Hydrogen chloride has also been established as a product at its detection limit when 100 μg of the chlorobutyl rubber component alone were heated at 100°C for 10 s, despite the fact that this material contains less than 2 wt% of chlorine.

Atactic and syndiotactic 1,2-polybutadiene, atactic 3,4-polyisoprene and poly(4-methyl-1,3-pentadiene)1,2 syndiotactic, having respectively vinyl, methylvinyl and dimethylvinyl pendant groups have been ozonized. The reaction products and the reaction speed toward ozone attack in solution have been studied by FT-IR spectroscopy. The tacticity seems to have no influence on either the reaction products or the reaction speed with ozone. The main product formed by ozonization of syndiotactic 1,2-polybutadiene and of poly(4-methyl-1,3-pentadiene)1,2 syndiotactic is a poly(acrolein) or poly(propenal). Thus each vinyl or dimethylvinyl monomer unit can be mainly converted into an aldehyde pendant group. In the case of atactic 3,4-polyisoprene, ozonization allows changing the methylvinyl group into a methylketone pendant group. Ozonization of atactic 3,4-polyisoprene and also poly(4-methyl-1,3-pentadiene)1,2 syndiotactic, introduces some epoxy groups into the vinyl double bonds as a secondary reaction. The reaction speed with ozone was found to be faster with the most alkyl-substituted double bond in agreement with a general rule. Thus the reaction speed is from the fastest to the slowest as follows: [poly(4-methyl-1,3-pentadiene)1,2 syndiotactic]>[3,4-polyisoprene]>[1,2-polybutadiene]. Ozonization of vinyl polymers could be a way for introducing oxygenated functional groups which can then be used as sites for crosslinking these polymers to prepare new polymer networks.

Thermal degradation and ageing of segmented polyamides by Subiman Ghosh; D Khastgir; Anil K Bhowmick; P.G Mukunda (427-436).
The degradation behaviour and kinetics of degradation of segmented polyamides with varying block lengths have been studied by non-isothermal thermogravimetry in air and nitrogen and by infrared spectroscopy. In air, all the polymers show two stage decomposition, whereas in nitrogen, the decomposition occurs in a single stage. In both atmospheres the degradation, however, follows first order kinetics. The infrared spectroscopic analysis of the degraded products reveals that the decomposition occurs in the polyether linkage followed by polyamide hard block. The mechanism of degradation is of course a complex one. In the case of thermooxidative degradation, a decrease in hard block molecular weight has a great influence on the activation energy values. The effect of ageing on the mechanical properties of these polymers has also been studied. On ageing, it is observed that both tensile strength and elongation at break drop sharply in the initial stage. After this initial drop, both properties show a marginal change with time and temperature of ageing. Retention of physical properties is better with high hard block molecular weight polymers.
Keywords: Segmented polyamides; Block copolymer; Degradation; Thermogravimetric analysis; Ageing; Block molecular weight;

Study of the degradation of fire-retarded PP/PE copolymers using DTA/TGA coupled with FTIR by J.-P Gibert; J.-M Lopez Cuesta; A Bergeret; A Crespy (437-447).
A PP/PE copolymer was successively flame retarded using Mg(OH)2, then using brominated trimethylphenyl indane associated with Sb2O3 (Br/Sb), and finally using blends of equal weights of this last combination with Mg(OH)2 or talc-containing non-hydrated fillers. Decompositions of pure and additive-containing copolymer were studied by DTA/TGA coupled with FTIR. A good correlation exists between the maxima of Gram–Schmidt curves and the derivatives of TGA curves. The coupling of techniques shows that the incorporation of the Br/Sb flame retardant limits strong exothermic phenomena due to sample ignition. In the case of Mg(OH)2 associated with Br/Sb, the decomposition of the hydrated mineral occurs at a lower temperature than the reaction between brominated trimethylphenyl indane and Sb2O3. This delays the action of Br/Sb flame retardant towards higher temperatures, improving the thermal stability of the polymer. A good agreement is also found between DTA/TGA-FTIR conclusions and fire resistance tests carried out on standardized samples. When magnesium hydroxide is replaced by the fillers, the interest in using a pure talc, which appeared in fire resistance tests, is not strongly confirmed by DTA/TGA-FTIR. This discrepancy may be ascribed to the reduced influence of mass diffusion phenomena due to the small weight of the sample used in thermal analysis experiments.

Isotactic polypropylene was irradiated under vacuum in a complete set of experimental conditions. The influence of the irradiation parameters, total absorbed dose, D, and dose rate, I, was analysed in order to verify the theoretical predictions of a simple kinetic model, already presented (Sarcinelli L, Valenza A, Spadaro G. Polymer 1997;38:2307), based on the rates of the main reactions occurring during irradiation, i.e. β-scission, addition to double bonds and termination. The concentration of free radicals and double bonds formed during irradiation is detected together with a comparison to their dependence on the irradiation parameters. A quantitative determination of the “inversion curve”, forecasted by the model, i.e. an implicit function of D and I, which gives the dose values enhancing chain branching and molecular weight increase at fixed dose rate and vice versa, was also performed.

Polycaprolactone microparticles and their biodegradation by D.R Chen; J.Z Bei; S.G Wang (455-459).
Polycaprolactone (PCL) microparticles were prepared by an emulsification-solvent evaporation technique using various stabilizers such as gelatin, hydrophobically modified polyacrylamide derivative (PAM) and poly(vinyl alcohol) (PVA). It was shown that the particle size distribution of PCL microparticles was 13.4±4.7 (μm) in diameter with gelatin as stabilizer. The degradation behavior of the PCL microparticles was determined and compared with that of PCL film at pH 7.4 at 37±1°C with and without lipase. The shape of PCL had no obvious effect on its degradation rate. It suggested that homogeneous degradation dominated the process. The degradation rate of PCL microparticles was enhanced in the presence of lipase enzyme. The degree of crystallinity of PCL microparticles increased with degradation, proving preferential degradation in amorphous domains of the PCL microparticles.
Keywords: Polycaprolactone; Microparticles; Biodegradation; Crystallinity; Lipase;

The photochemical stability and photodegradation pathways of poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) and copolymers of 1H,1H,2H,2H-perfluorodecyl methacrylate with 2-ethylhexyl methacrylate (XFDMA–EHMA) have been investigated under artificial solar light irradiation. The extent of degradation was assessed by weight loss and gel content determination, size exclusion chromatography and FTIR spectroscopy. PTFEMA exposed up to 2000 h showed only molecular changes due to a low extent of chain scission. The two XFDMA–EHMA copolymers underwent much more degradation, with extensive cross-linking, formation of low-molecular weight products, and oxidation reactions on the chains. The behaviour of the copolymers is controlled by the reactivity of the EHMA units, as was shown by comparison with results obtained on degradation of PEHMA homopolymer.
Keywords: Coatings; Photodegradation; Acrylic polymers; Fluorinated coatings;

Photoageing at λ>300 nm (SEPAP, WOM, Xenotest), natural ageing (Bandol) and thermal ageing (100 and 150°C) were performed on ethylene–propylene–diene monomer (EPDM) rubber designed for automotive applications. The thermal ageing study was concentrated on a formulation designed for engine applications (peroxide crosslinked) and photoageing concerned a formulation for outdoor applications (sulfur vulcanized). Analyses were based both on infrared spectroscopy (surface analysis and profile by successive abrasion) and on physical properties analysis (dynamic mechanical thermal analysis (DMTA) and tensile tests). Both methods were complementary to describe the two competitive phenomena involved in ageing of elastomers (oxidation and crosslinking).
Keywords: EPDM; Elastomer; Photo-oxidation; Thermo-oxidation; Natural ageing; ATR–FTIR spectroscopy; Mechanical analysis;

Thermal degradation mechanisms of polystyrene, polymethacrylonitrile, and their random copolymers have been investigated by flash pyrolysis by Py-GC–MS and by programmed temperature TG–FTIR. The thermal degradation mechanisms of the copolymers are discussed in terms of the competition between depolymerization and back-biting reaction on the basis of bond dissociation energies of C–C and C–H bonds (Q C–C and Q C–H) in the polymer chains. The activation energy of pyrolysis (ΔE a) obtained by Ozawa's plot and Q C–C values was increased with the content of methacrylonitrile units in the copolymer chain, although the onset temperatures of loss of sample mass in TG curves shifted to the lower temperature region. These phenomena have been explained in terms of the change of the ratio of depolymerization to back-biting reaction depending on chemical structure and degradation temperature.

Preparation and thermal study of polymers derived from acrylamide by Maria Elisa S.R.e Silva; Eider R. Dutra; Valdir Mano; José C. Machado (491-495).
Polyacrylamide (PAA) and some of its N-alkyl substituted derivatives have been synthesised and characterised using thermal analysis. Thermal stability was studied by thermogravimetry (TG) and differential scanning calorimetry (DSC) was used to determine the glass transition temperatures (T g). The results showed an increase in degradation temperature (T d) as the hydrogens of the amide nitrogen were replaced by alkyl groups. An opposite tendency was verified for T g values. The observed increase in T d values is discussed in terms of the formation of an intermediate ion in the degradation, while the decrease in T g values is discussed in terms of polymer–polymer interactions such as hydrogen bonds.
Keywords: Polyacrylamide; Thermal analysis; DSC; TG;

Various amides and model compounds derived from phenylalanine and succinic acid have been degraded in abiotic and biotic conditions as a first step in the study of the mechanism, rate and extend of mineralization of polyesteramides. These compounds have been incubated in the presence of pure strains of micro-organisms isolated from an industrial compost for household refuse. The rate of substrate disappearance has been measured by UV absorption spectroscopy or determined from oxygen consumption data as a function of time. Attention has been paid to the configuration of the amino-acid and to the nature of the terminal groups. It has been shown that the decay of such compounds is due to the presence of amino-acid and not to the nature of the end groups. N-Succinyl-l-phenylalanine HOOC-CH(CH2-Ph)-NH-CO-(CH2)2-COOH has been demonstrated to be very rapidly mineralized while the corresponding dimeric compound N,N′-Succinyl-l,l-phenylalanine HOOC-CH(CH2-Ph)-NH-CO-(CH2)2-CO-NH-CH(CH2-Ph)-COOH was much more slowly degraded, most probably because of its larger size. The difference between the biodegradation processes mediated by micro-organisms and by isolated enzymes is emphasized.

Two quantitative methods have been used to follow the biodegradation of a polyester-amide containing l-phenylalanine units in the presence of a pure strain of micro-organisms isolated from an industial compost for household refuse. Weight loss and oxygen consumption have been measured as a function of time. They respectively monitor the main chain hydrolysis into small soluble fragments and the mineralization of these fragments. The residual solid and the soluble fractions have been characterized by GPC, FTIR and NMR. Specific hydrolysis has been observed at the ester site. It has also been shown that the rate of mineralization of the polymer is limited by the rate of assimilation of the fragments. When pure enzymes are the biological agents, the hydrolysis step only occurs. The hydrolysis products identified in the presence of papain are the same as those released in the presence of the micro-organisms used in this work.

It is suggested that the chemiluminescence from the thermal oxidation of polypropylene arises predominantly from the decomposition of associated hydroperoxides. Measurements of the chemiluminescence response to heating/cooling cycles applied to polypropylene oxidation in the induction period and in an advanced stage of the process is a novel approach to the extrapolation of the oxidation course to the lower temperature region.
Keywords: Chemiluminescence; Polypropylene; Peroxides; Decomposition of peroxidic structures;

The thermal degradation of poly(tetrahydrofuran) (PTHF) shows many features in common with its homologues poly(ethylene oxide) (PEO) and poly(trimethylene oxide), the main degradation processes being accounted for by homolysis of the C–O backbone bonds followed by further reactions of the radicals thus formed. The thermal stability of PTHF and its complexes with LiBr and LiI are in the order PTHF>PTHF/LiBr>>PTHF/LiI. The marked instability of the LiI complex is due in part to participation of I° radicals in promoting decomposition reactions. Pure PTHF is much less stable in air than in N2 while the stabilities of the complexes in air are similar to their stabilities under N2, so that the order of stability to thermal oxidation is PTHF<PTHF/LiI<PTHF/LiBr. The stabilising influence of the salts against oxidation of PTHF is explained on the basis of the mechanisms proposed previously for other polyethers.

A kinetic analysis of thermal degradation of polymers using a dynamic method by Jin Woo Park; Sea Cheon Oh; Hae Pyeong Lee; Hee Taik Kim; Kyong Ok Yoo (535-540).
A new method of kinetic analysis using a dynamic model which accounts for the thermal degradation of the polymer at any time, is proposed. The method was applied to predict the thermal degradation of high-density polyethylene (HDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) by a conventional nonisothermal thermogravimetric technique at several heating rates between 10 and 50°C/min. In addition, the various analytical methods reported in the literature were used in comparison with the kinetic analysis results obtained from the dynamic method. It was found that the dynamic method gave a reliable value of kinetic parameters, and the apparent activation energy and overall reaction order of HDPE were larger than those of LDPE and LLDPE.
Keywords: Thermal degradation; Kinetic analysis; Dynamic method; Polyethylene;

A MOPAC calculation is performed to explain the new ortho-substitutent effect of phenols proposed by the authors; an α-hydrogen of an ortho-substituent being used to regenerate a phenol from the phenoxy radical. The benzyl radical derived from the intramolecular hydrogen transfer of a phenoxy radical is found to be as stable as, or more stable than, the phenoxy radical. The regenerated phenol also is strikingly more stable than a quinoid type compound formed by the reaction of a phenoxy radical with another peroxy radical. Thus, the new ortho-substituent effect mentioned above can also be well explained by this work.
Keywords: Antioxidant; Hydrogen transfer; Phenols; Substituent effect; MOPAC calculation;

Thermal, oxidative and radiation stability of polyimides I. Bismaleimidoethane and different diamine-based polyimides by Katarina Popov-Pergal; Miroslav Pergal; Dragan Babić; Milena Marinović-Cincović; Renata Jovanović (547-552).
The syntheses of 1,2-ethane-bis-maleamidoacid, N,N′-1,2-bis-maleimidoethane (BME), and polyimide resins based on BME and four different diamines are given. The thermal and thermooxidative stability of the resins was studied by thermogravimetry (TG) following parameters such as temperatures for defined weight loss and initial decomposition rate, i.e. shape of TG and first derivative TG curves. Samples have been irradiated and their radiation stability evaluated on the basis of the thermal and thermo-oxidative behavior of the irradiated samples. The thermal behaviors have been analyzed and the resin based on the diamine with the benzene ring showed the best thermal stability. Radiation in the applied dose range showed an increase of thermal stability.

Aryl-substituted dithianes and dithiolanes as process stabilizers for polyolefins by John M. Zenner; Robert E. Lee; Vincenzo Malatesta (553-561).
New process stabilizers for polyolefins have been identified. 2-Aryl-1,3-dithianes, 2-aryl-1,3-dithiolanes and derivatives easily undergo H-atom abstraction, yielding a persistent benzyl-type free radical, and thus act as chain breaking stabilizers for polyolefins. Synergism with phenolic antioxidants and control of color development has been observed.

Behaviour of carbon black pigments as excited state quenchers in LDPE by Norman S. Allen; Jose M. Pena; Michele Edge; Christopher M. Liauw (563-566).
A total of eight different carbon black pigments have been incorporated into low density polyethylene at 0.2% w/w concentration and their ability to operate as effective singlet and triplet quenchers examined using luminescence analysis. All pigments effectively reduced the fluorescence and phosphorescence emission intensities. A large variability was observed in the triplet lifetime quenching, the effect varying with the pigment type. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. The implications of carbon black pigments to operate as “super quenchers” under practical concentration conditions is discussed.