J. R. White

The role of physical structure and morphology in the photodegradation behaviour of polypropylene

Abstract Photo-oxidatative degradation of isotactic polypropylene (PP) has been examined in samples made by injection and compression moulding and using different moulding conditions. Samples were exposed to ultraviolet radiation (UV) in the laboratory for periods of up to 48 weeks. The extent of chemical degradation was assessed by gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR); the specimens were tested in tension and the fracture surfaces were inspected by scanning electron microscopy. The processing pathways defined the structural characteristics of the starting materials, as determined by light microscopy and X-ray diffraction. The investigation conducted here indicates that the fractional crystallinity is the main structural parameter controlling the rate of degradation of polypropylene. The mechanical properties of degraded PP were shown to depend not only on the extent of chemical degradation but also on the character of the polymer physical structure, especially the spherulite size. With most samples studied here a partial recovery in tensile properties was observed after 6–9 weeks exposure. Analysis of the fracture surfaces showed that improvement in the tensile properties coincided with the appearance of a ductile zone inside the surface embrittled layer, indicating that the cracks in the surface layer were arrested on reaching the ductile material in the interior. Surface cracks were formed spontaneously after 9 weeks of UV exposure and the pattern and concentration of these cracks also depends on the processing type and conditions. In injection moulded samples, surface cracks were of the form of circular arcs radiating from the injection gate and they were correlated with the flow lines generated during mould filling.

Weathering of polymers: mechanisms of degradation and stabilization, testing strategies and modelling

The weathering of polymers is reviewed with attention concentrated on the mechanisms of degradation and stabilization, the methods of testing weatherability, and the predictive modelling of weathering behaviour. An introduction to the chemical mechanisms of degradation and stabilization is given and reference made to some of the many reviews available in the literature. Significant emphasis is placed here on engineering aspects, such as the way that weathering influences fracture mechanisms. The difficulties associated with relating accelerated laboratory tests with outdoor service behaviour are discussed. The complexities of the degradation processes limit modelling of the rate of degradation to rather specific systems and the predictions cannot be generalized.

Crystallization and melting behaviour of photodegraded polypropylene — I. Chemi-crystallization

Abstract An investigation has been conducted into the effects of photodegradation on the crystallinity and melting behaviour of isotactic polypropylene (PP). PP samples having different structural characteristics were prepared and exposed to ultraviolet radiation (u.v.) in the laboratory for periods of up to 48 weeks. The changes in crystallinity during exposure were followed by X-ray diffraction and differential scanning calorimetry (d.s.c.), whereas the chemical degradation of the specimens sampled was evaluated by gel permeation chromatography (g.p.c.) and Fourier transform infrared spectroscopy (f.t.i.r.). An increase in fractional crystallinity during u.v. exposure was noted for all types of samples studied, and the gain in crystallinity was usually between 6% and 7% and was virtually independent of the initial structure of the polymer. Measurements conducted at different depths within the test bars indicated that the fractional crystallinity increased during u.v. exposure due to crystal growth using molecule segments released by the scission of molecules (probably taut molecules). This process, called chemi-crystallization, is restricted by the chemical defects introduced into the molecules by the photodegradation. Possible mechanisms for the chemi-crystallization process are discussed. The melting thermograms of most types of samples exhibited single peaks with melting range increasing with exposure time. The broadening of the melting thermograms occurred during the period between the commencement and the completion of the chemi-crystallization. Secondary crystallization proceeded much more slowly in the interior of the test bars because of the limited oxygen supply, but the final crystallinity was the same as the plateau value, which was attained more rapidly when the material in the interior was exposed by machining away the surface prior to u.v. exposure.

Crystallization and melting behaviour of photodegraded polypropylene — II. Re-crystallization of degraded molecules

Abstract Isotactic polypropylene bars were exposed to ultraviolet radiation (UV) in the laboratory for periods up to 48 weeks causing a reduction in molecular size and the build-up of chemical groups like carbonyls and hydroperoxides. The specimens were re-crystallized from the melt under isothermal and non-isothermal conditions and investigations were conducted by differential scanning calorimetry (DSC), X-ray diffraction, and light microscopy. At first the fractional crystallinity of the re-crystallized materials increased with exposure as the result of decreasing molecular size, but for longer exposures the fractional crystallinity decreased because of the increase in concentration of chemical impurities. In highly degraded specimens, the presence of γ-phase crystals was detected. Kinetic studies revealed that the rates of nucleation and growth may be affected differently by photodegradation, and that the degraded molecules crystallized much faster than the unexposed material when their rates of crystallization under the same supercooling are compared. Double melting peaks were observed in DSC thermograms and were shown to be due to re-organization during heating, but in some cases the segregation of highly defective molecules is the major reason for peak doubling. Molecular segregation also changes the spherulite morphology during crystallization, as revealed by polarized light microscopy.

Photodegradation of polypropylene containing a nucleating agent

The effect of including 0.5 wt % talc on the photodegradation behavior of polypropylene (PP) was investigated in injection-molded samples exposed to ultraviolet radiation (UV) in the laboratory for periods of up to 24 weeks. The structure of the talc-nucleated samples was characterized by X-ray diffraction and light microscopy and compared with that of nonnucleated PP, and the information was used to explain the differences in their photodegradation behavior. Measurements of the extent of chemical degradation were made by Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography ( GPC ), and they showed that there is no significant effect of a nucleating agent on the kinetics of photooxidation. PP bars containing a nucleating agent showed a larger reduction in mechanical properties with UV exposure and a partial recovery in properties after prolonged exposure was observed with both types of samples. An increase in crystallinity during UV exposure (chemicrystallization) was detected by X-ray diffraction and differential scanning calorimetry (DSC) and the dependence with exposure time was similar in nucleated and nonnucleated samples. DSC was also used to determine the melting behavior and transition temperatures of the specimens during exposure and after recrystallization from the melt.

Analysis of chain‐scission and crosslinking rates in the photo‐oxidation of polystyrene

A study of the chain-scission and crosslinking rates in polystyrene photodegraded in the laboratory with fluorescent tubes (UVA-340) was made using GPC molecular weight distributions. The analysis was based on the assumption that scission and crosslinking occur randomly and employed a Monte Carlo procedure to compute the changes in molecular weight distribution for chosen values of scission and crosslinking rates and compared the computed profiles with measurements made on the photodegraded samples. Results were obtained for three different exposures and at various depths within 3.2-mm-thick bars. The scission/crosslinking ratio, λ, was between 3 and 8 for all samples measured in this study. The lowest values of λ were found near the exposed surface and the highest near the bar center. Both scission and crosslinking rates were much lower in the interior, presumably the result of oxygen starvation. Some bars were exposed while loaded to 10 MNm−2 in uniaxial tension. The stress appeared to increase the reaction rates somewhat near the surface and to depress the rates in the interior correspondingly.

Effect of stabilizer and pigment on photo-degradation depth profiles in polypropylene

Photo-degradation depth profiles in 3 mm thick polypropylene injection mouldings containing (i) hindered amine light stabilizer (HALS); (ii) titanium dioxide (TiO2) pigment; and (iii) both HALS and TiO2, have been compared with those obtained with similar mouldings containing no ultraviolet (UV) stabilizer or pigment. Laboratory UV exposures produced steep degradation profiles in unstabilized samples with very little degradation occurring near the centre even after prolonged exposure. In samples stabilized by HALS the degradation was almost uniform through the thickness. At the surface, the level of degradation in the stabilized bars was much less than in unstabilized bars whereas in the centre the degradation was much more than in unstabilized bars. This is attributed to diffusion-controlled oxidation. The presence of TiO2 confined degradation to a thin region close to the exposed surface.

Effect of tensile stress on chain scission and crosslinking during photo-oxidation of polypropylene

Abstract Scission and crosslink concentrations have been determined in polypropylene samples using gel permeation chromatography molecular weight distributions and computer simulations. Measurements have been made at different depths within injection moulded bars 3 mm thick after 4 weeks ultraviolet (UV) exposure. The scission and crosslink concentrations were found to be 0.025 and 0.006 mol/kg respectively at the surface and 0.004 and 0.001 mol/kg respectively at the centre. The values at the unexposed face were similar to those at the exposed face. Samples exposed to UV while loaded in uniaxial tension (10 MN/m 2 ) showed markedly increased chain scission but little change in the crosslinking rate.

Photo-oxidation of polystyrene under load

The effect of a tensile stress on the rate of photo-oxidation of polystyrene was investigated. Molecular-weight measurements showed that tensile stress accelerates molecular scission in injection-moulded bars exposed to ultraviolet irradiation. Changes in the residual-stress distribution were observed during the exposure but they were not sufficient to reverse the sense of the residual stresses, and the residual stress near the surface remained compressive. Depth profiling indicated that the degradation process was oxygen-diffusion limited and an approximate analysis of the kinetics gave an activation energy for degradation of 4.2 kJm−2, which is also consistent with a diffusion process.

STRESS-ACCELERATED PHOTO-OXIDATION OF POLYPROPYLENE AND GLASS-FIBRE-REINFORCED POLYPROPYLENE

Abstract The effect of tensile stress on the rate of photo-oxidation of polypropylene has been investigated. Molecular weight measurements have shown that tensile stress accelerates molecular scission in injection-moulded bars exposed to ultraviolet irradiation. Corresponding to this a small drop in the tensile strength was observed. No change was found in the failure mechanism that could be attributed to the tensile stress present during irradiation. Depthprofiling indicated that the degradation process was oxygen diffusion-limited. The presence of glass-fibre-reinforcement reduced the sensitivity of the material to the application of tensile stress during ultraviolet irradiation.