Heterogeneous oxidative degradation in irradiated polymers

In the presence of oxygen, polymeric materials may undergo diffusion-limited heterogeneous degradation with significant oxidation occurring only near the surfaces. It is important in studying polymer degradation (and in designing accelerated aging experiments) to be able to determine under what conditions heterogeneous degradation occurs, and to estimate the extent of significant oxidative penetration under those conditions. We describe here the use of several techniques for identification of oxidative degradation gradients. A rapid determination of oxidation depth is accomplished by optical examination of metallographically polished cross-sectioned samples; oxidized and nonoxidized regions are distinguished by differences in surface reflectivity. A more detailed determination of the shapes of degradation gradients is accomplished by performing a series of sensitive determinations of relative hardness changes across the surface of cross-sectioned, polished samples. Typical oxidation depths for the commercial polymers examined are on the order of fractions of millimeters over a dose-rate range of 10⁴–10⁶ rad/h. Significant variations among different materials are found, as would be expected given differences in oxygen consumption and permeation rates. A detailed example is given of the tensile property behavior of a Viton material over a range of dose rates where the degradation is seen to change from strongly heterogeneous at high dose rates to homogeneous as the dose rate is lowered. Degradation differences in this material are very pronounced: at high dose rates the polymer undergoes primarily crosslinking to give a hard brittle material, whereas under lower dose rates, where oxygen permeation is complete, the polymer undergoes predominantly scission to yield a soft, easily stretchable material.     

Kinetics of thermal decomposition of PVC/fly ash composites

In this study, the kinetics and mechanisms of thermal degradation of Poly Vinyl Chloride (PVC) composites reinforced with class-F fly ash are studied experimentally and numerically using Flynn–Wall model. The addition of fly ash to the polymer matrix results in a decrease in the primary degradation temperature and an increase in the secondary degradation temperature. The metal oxides in the fly ash act as acid absorbers, which results in the destabilization of PVC during its dehydrochlorination process. However, they also react with the chlorine free radicals, which prevents the formation of HCl during degradation. In addition, it is observed that calcium and iron oxides, present in fly ash, are more reactive to the chlorine radicals rather than the silicon and aluminum oxides. The effect of fly ash chemical composition on the degradation of PVC composites was studied by comparing the thermal properties of composites containing two different classes of fly ashes, class-F and class-C, at similar levels. Thermal stability of the composites is found to be dependent on the chemical composition of fly ash. Higher dehydrochlorination rate is observed in the case of composites filled with class-F fly ash than those reinforced with class-C fly ash.     

A study on the degradation of dye-sensitized solar cells irradiated by two different dose rates of γ-rays

This work mainly investigates the radiation effects and the damage mechanism on the dye-sensitized solar cells (DSSCs) irradiated by two dose rates of ⁶⁰Co γ-rays. Experimental results reveal that the performance of DSSC decreases significantly after irradiation, and the degradation rates of the short circuit current and the maximum power in the high dose rate both outweigh those in the low dose rate at the same adsorbed dose. The γ radiation effects on fluorine-doped tin oxide (FTO) and electrolyte were also studied respectively to investigate the degradation rates of DSSC at different dose rates. The results indicate that the degradation rates are closely relative to concentration of γ irradiation-induced defect on FTO.     

Greffage radiochimique de l'acrylonitrile dans la masse de films de PTFE

Acrylonitrole can be grafted in the bulk of PTFE films provided the monomer is allowed to diffuse into the grafted zones. For a given radiation dose, the amount of grafting is higher the lower the dose-rate. Even higher grafting efficiencies are reached under conditions of discontinuous irradiations, the reacting mixture being stored in the dark in a thermostat between successive irradiations. The grafting yield is higher the higher the storage temperature. The bulk grafting involves a progressive swelling of the grafted polyacrylonitrile branches by acrylonitrile monomer via a dipole-dipole association of the -CN groups of the monomer with the -CN groups in the polymer.     

Thermal expansion of irradiated polytetrafluoroethylene

The thermal expansion coefficient of gamma-irradiated Polytetrafluoroethylene (PTFE) has been measured in the temperature range 80–340 K by using a three-terminal capacitance technique. The samples are irradiated in air at room temperature with gamma rays from a Co⁶⁰ source at a dose rate of 0.26 Mrad/h. The change in crystallinity is measured by an x-ray technique. The expansion coefficient is found to increase with radiation dose below 140 K owing to the predominant effect of degradation. Above 140 K, the expansion coefficient is found to decrease with radiation dose because of the enhanced crystallinity. The temperatures of the two first-order phase transitions are also found to shift to lower temperatures owing to degradation of PTFE.     

Predictive aging results in radiation environments

We have previously derived a time-temperature-dose rate superposition methodology, which, when applicable, can be used to predict polymer degradation versus dose rate, temperature and exposure time. This methodology results in predictive capabilities at the low dose rates and long time periods appropriate, for instance, to ambient nuclear power plant environments. The methodology was successfully applied to several polymeric cable materials and then verified for two of the materials by comparisons of the model predictions with 12 year, low-dose-rate aging data on these materials from a nuclear environment. In this paper, we provide a more detailed discussion of the methodology and apply it to data obtained on a number of additional nuclear power plant cable insulation (a hypalon, a silicone rubber and two ethylene-tetrafluoroethylenes) and jacket (a hypalon) materials. We then show that the predicted, low-dose-rate results for our materials are in excellent agreement with long-term (7–9 year) low-dose-rate results recently obtained for the same material types actually aged under bnuclear power plant conditions. Based on a combination of the modelling and long-term results, we find indications of reasonably similar degradation responses among several different commercial formulations for each of the following “generic” materials: hypalon, ethylene-tetrafluoroethylene, silicone rubber and PVC. If such “generic” behavior can be further substantiated through modelling and long-term results on additional formulations, predictions of cable life for other commercial materials of the same generic types would be greatly facilitated.     

含羧酸稀土光敏剂降解PVC塑料的可降解研究

以羧酸共生稀土作为PVC塑料紫外光氧化降解的光敏剂,采用人工加速老化实验、户外曝晒实验等方法进行降解试验,并用红外光谱,紫外光谱和凝胶色谱等方法对含羧酸共生稀土光敏剂可降解PVC塑料的可降解性能进行了研究。初步探讨了羧酸共生稀土敏化PVC膜光氧化降解的作用机理。结果表明,羧酸共生稀土(如La、Ce、Pr等)对PVC分子的结构具有明显的光敏化降解作用,并具有抑制PVC在光照过程中发生交联的作用。     

Thermal and mechanical behaviour of flexible poly(vinyl chloride) mixed with some saturated polyesters

Four saturated polyesters poly(hexamethylene adipate), poly(ethylene adipate), poly(hexamethylene terephthalate) and poly(ethylene terephthalate) were prepared. The resulting materials were characterized by IR and ¹H NMR, end group analysis and gel permeation chromatography. The effect of blending these polyesters (5 and 10%) with poly(vinyl chloride) (PVC) in the melt was investigated in terms of changes in the thermal behaviour of PVC by studying the weight loss after 50 min at 180 °C, colour changes of the blend before and after aging for one week at 90 °C, the variation in glass transition temperature and the initial decomposition temperature. The results gave proof for the stabilizing role played by the investigated polyesters against the thermal degradation of PVC. The best results are obtained when PVC is mixed with 5% aliphatic polyesters rather than with aromatic ones. This is well illustrated not only from the increase in the initial decomposition temperature (IDT), but also from the decrease of % weight loss and from the lower extent of discolouration of PVC, which is a demand for the application of the polymer. It was also found that blending PVC with 5% of the four investigated polyesters before and after aging for one week at 90 °C gave better mechanical properties even than that of the unaged PVC blank.     

硬质聚氯乙烯自然老化的变色行为与分子结构变化

以硬质聚氯乙烯(PVC)为研究对象,通过色差实验、FTIR、UV等方法探究了PVC在湿热、辐射低的琼海和暖温、干旱且辐射高的若羌地区自然老化的变色行为与分子结构变化.结果表明,太阳辐射是影响PVC自然老化过程中变色行为和分子结构变化的主要环境因素,PVC在干旱、辐射高的若羌地区比在湿热、辐射低的琼海地区更易变黄和变红,而在两地变暗的程度相近;这是因为随着自然老化时间的延长,PVC逐渐发生脱除氯化氢反应和氧化反应,在干旱且辐射高的若羌,更易发生脱除氯化氢反应,而在湿热且辐射低的琼海,脱除氯化氢反应和氧化反应的速度相近,在若羌老化的PVC样品生成了更多的共轭长度(n)≥8的C=C长共轭序列所致.     

Co-pyrolysis behaviors and kinetics of plastics–biomass blends through thermogravimetric analysis

Co-pyrolysis behaviors of plastics–biomass blends were investigated using a thermogravimetric (TG) analysis from room temperature to 873 K with a heating rate of 5–40 K min^?1 in an inert atmosphere. The selected biomass sample was sawdust of pine wood (WS). Polyvinyl chloride (PVC), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) were selected as plastic samples. The difference of mass loss between experimental and theoretical ones (calculated as arithmetic sums of those from each separated component) was used as a criterion of synergetic effect. The experimental results indicated that a significant synergetic effect existed during the high-temperature region of plastics and WS co-pyrolysis process, specially, the dehydrochlorination reaction of PVC and the degradation of hemicellulose and cellulose in the WS during the co-pyrolysis process showed synergetic effect, as well as the reaction of plastics (LDPE, HDPE, and PP) and WS. Based on the TG data with different heating rates, the kinetics parameters, especially activation energy, were calculated using the Friedman method. The activation energy of plastics, WS, and their blends were from 92.8 to 359.5 kJ mol^?1. The activation energy of the PVC–WS blends was at a range of 180.2–254.5 kJ mol^?1 in the second stages. The activation energies range of LDPE–WS, HDPE–WS, and PP–WS blends were 164.5–229.6, 213.2–234.3, and 198.4–263.6 kJ mol^?1, respectively.     

Preparation and degradation of head-to-head PVC

The course of the chlorination reaction of cis-1,4-polybutadiene is dependent on the choice of solvent. When methylene chloride is used, a pure addition reaction of chlorine leads to a polymer with the structure of head-to-head, tail-to-tail PVC. The thermal stability of the head-to-head PVC polymer has been studied by thermal volatilization analysis, thermogravimetry, and evolved gas analysis for hydrogen chloride, and the changes in the ultraviolet (UV) spectrum of the polymer during degradation have been investigated. The head-to-head polymer has a lower threshold temperature of degradation than normal PVC, but reaches its maximum rate of degradation at a higher temperature for powder samples of the polymer under programmed heating conditions. Blends of head-to-head PVC with poly(methyl methacrylate) have also been degraded, and the presence of the head-to-head polymers, like that of normal PVC, results in depolymerization of the PMMA as soon as the dehydrochlorination reaction commences. The mechanism of degradation of head-to-head PVC is discussed.     

等规聚丙烯在γ射线辐照下的链结构和结晶行为

利用凝胶渗透色谱(GPC)、傅里叶变换红外光谱(FTIR)和示差扫描量热(DSC)等手段对不同剂量γ射线辐照后等规聚丙烯(iPP)的分子链结构及结晶行为的变化进行了研究.结果表明,γ射线辐照使iPP的分子量下降,并在其分子链中产生羟基和羰基等极性基团,从而影响其结晶行为.在非等温结晶过程中,当辐照剂量≤50 kGy时,iPP的热结晶温度略有升高;增大辐照剂量,iPP的热结晶温度明显降低.iPP的熔融温度则随辐照剂量的增大而降低,且分裂成双峰.利用Avrami方程研究了辐照前后iPP的等温结晶动力学,发现辐照前后样品的Avrami指数n都在3左右,表明iPP的结晶遵循异相成核机理,且不受辐照剂量和等温结晶温度的影响,但总结晶速率随等温结晶温度和辐照剂量的升高而逐渐减小.探讨了iPP经过γ射线辐照后,分子链断裂、链结构变化和结晶速率之间的关系.     

Automatic equipment for studying the processes of low-temperature dehydrochlorination of chlorine-containing polymers

A multichannel, highly sensitive installation for the study of the kinetics of decomposition of halogen-containing polymers under automatic conditions has been described.Hydrogen halide produced by polymer decomposition is recorded using mercury-halide ion-selective electrodes, the results being processed by computer.Low-temperature (313–403 K) degradation of poly(vinyl chloride) has been studied. At the glass transition temperature (358 K) the Arrhenius parameters for the degradation of PVC change. A dependence between the segmental mobility of PVC macromolecules and the rate of polymer dehydrochlorination is demonstrated.     

Monitoring the moisture-related degradation of ethylene propylene rubber cable by electrical and SEM methods

Several electrical approaches were used to monitor the electrical degradation of ethylene propylene rubber (EPR) cable under (1) high temperature water submerged for long-term, (2) room temperature water submerged after thermal and radiation aging, and (3) loss-of-coolant accident simulation conditions. The moisture-related degradation behavior was investigated by the correlation of electrical measurements, moisture content analysis, and scanning electron microscope (SEM) observation. The results indicate that void size and density of insulation material strongly affect the material's water resistant capability and electrical degradation. Also, the void size of insulation material after moisture-related degradation is much bigger than that after thermal and radiation aging, and the former causes material swelling. Combining insulation resistance and dissipation factor condition monitoring (CM) techniques can provide useful means for monitoring cable degradation while the cable is exposed to moisture and high temperature/radiation environment. The Arrhenius model is applied together with appropriate acceptance criteria obtained by insulation resistance measurement to calculate cable remaining life. According to test results and the remaining life calculation, it is shown that the moisture-related degradation of EPR cable can be early assessed by insulation resistance measurement, which is a non-destructive prognostic CM technique.     

Radiation effects on the thermal degradation of poly(vinyl chloride) and poly(vinyl alcohol)

Radiation effects on the formation of conjugated double bonds in the thermal degradation of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) were investigated. Thin films of PVC and PVA were either irradiated with γ-rays at ambient temperature (pre-irradiation) and then subjected to thermal treatment, or irradiated at elevated temperatures (in situ irradiation). An extensive enhancement of the thermal degradation was observed for the pre-irradiation of the PVC films, which was more effective than the effect of the in situ irradiation at the same absorption dose. For the PVA degradation, however, the effect of the in situ irradiation was larger than that of the pre-irradiation. The results were explained and related mechanisms were discussed based on radiation-induced chemical reactions and their individual contributions to the thermal degradation behaviors of the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3089–3095, 1998     

质子辐照对有机硅树脂增强甲基硅橡胶的性能损伤及机理

硅橡胶具有优良的电绝缘性以及耐高低温、耐电晕、耐化学腐蚀、耐大气老化、耐臭氧和耐辐射等性能，近年来在航天领域得到了广泛应用．在宇宙空间的带电粒子如质子、电子等的辐照作用下，硅橡胶会发生性能退化，其质损率上升，产生析气现象，从而直接影响航天器的可靠性和寿命．自80年代以来，人们就对不同辐照源及不同粒子注量对硅橡胶材料的影响进行了研究，但直到近几年才将研究热点转向辐照场中硅橡胶的性能退化及微观结构的演变规律，其辐照环境大多集中于γ射线、Si^＋和F^＋离子、He^2＋离子等．对于空间环境下带电粒子辐照对硅橡胶材料的影响，国外由于相应的研究工作保密性强，报道甚少；国内哈尔滨工业大学则对我国自行研制的白炭黑增强空间级甲基硅橡胶质子辐照作用后的性能与微观结构的变化做了较为深入的研究。     

Radiation curing of methacryloyloxyalkyl carbonates

The results of studying the radiation curing of methacryloyloxyalkyl carbonates (MC), a new type of oligomer, are presented. These oligomers are notable for their high rates of radiation curing. The radiation yield G(–M) is 2 × 10⁵ for MC and 2 × 10⁴ for triethylene glycol dimethyacrylate. The polymerization rate of MC appeared to be proportional, independent of the conversion degree, to the irradiation dose rate in the power of 0.9–1.0 (for dose rates ranging from 0.4 to 15 rad/s). In regard to the temperature dependence of the polymerization rate of MC for small conversion degrees, two temperature regions with different values of effective activation energy (18–20 kcal/mol and 2 kcal/mol, respectively) were observed. When an irradiated MC sample is being unfrozen, its polymerization occus in the region of devitrification (220–240°K). As distinct from mass polymerization, in the polymerization of MC solutions in acetone and benzol the mobility of growing chains increases so that the bimolecular termination becomes possible and the limiting conversion of double bonds is derived. Rather small irradiation doses necessary for curing MC and the proportionality of the radiation-induced polymerization rate to the dose rate make these oligomers valuable for various industrial applications.     

Photooxidation of PVC

Pure and peroxided PVC films were irradiated under monochromatic ultraviolet light between 300 and 400 nm. The degradation state of PVC is measured by means of carbonyl absorption in the infrared spectra and polyene content by UV spectroscopy. The initial rate of carbonyl formation depends on the wavelengths and oxidized impurities content. Peroxided PVC is oxidized faster than pure PVC with wavelengths above 320 nm. Two hazardous ranges of wavelengths have been detected: 300–320 and 350–370 nm. Good protection of PVC against UV radiation can be assured by the exclusion of wavelengths under 380 nm.     

Peroxide induced crosslinking and degradation of polyvinyl chloride

Peroxide induced crosslinking and degradation of polyvinyl chloride (PVC) were experimentally investigated using an on-line electron spin resonance (ESR) spectroscopy technique. The reaction variables included temperature, peroxide type and concentration. A single line ESR spectrum was observed with its peak-to-peak width decreasing during the reaction. The mechanism involved in the reaction was elucidated based on the radical information. The radical concentration versus reaction time profile exhibited two distinct regions: the chemically initiated reaction continued by the thermal initiation. The addition of peroxide induced and significantly enhanced the thermal initiated crosslinking and degradation. The radical concentration data coupled with the extent of dehydrochlorination gave an estimate of the rate constant of polyene propagation. A significant decrease of the rate constant was observed during the reaction. The gel content and swelling ratio were also measured to provide additional information to the reaction process. The initial gelation rate increased with the increase of temperature and/or peroxide concentration. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 851–860, 1998