Stress–thermooxidative aging behavior of polyamide 6

The long‐term stress–thermooxidative aging behavior of polyamide 6 (PA6) was studied in terms of the creep behavior, mechanical properties, chemical structure, crystallization, and orientation behavior. During aging, a thermooxidation reaction occurred, which included molecular chain degradation and crosslinking, in PA6. Meanwhile, when the samples were subjected to stress, crystallization, orientation, and chain scission were induced. In the initial stages of aging, the stress‐induced crystallization and orientation dominated; this resulted in an increase in the creep deformation, mechanical strength, crystallinity, and orientation factor. Molecular degradation and chain scission dominated in the subsequent aging process and resulted in a decrease of the mechanical strength, reduced viscosity, crystallinity, and orientation factor and an increase in the formation of oxide and peroxide products. The stress may have promoted the chain scission of PA6 during thermal aging and resulted in a decrease in the reduced viscosity and an increase in the carboxylic acid concentration. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Study on the long‐term thermal‐oxidative aging behavior of polyamide 6

The long‐term thermal‐oxidative aging behavior of polyamide 6 (PA6) was studied by comparison with the stabilized sample in this work. The variation of mechanical properties of the pure and the stabilized samples of PA6 with aging time at 110°C, 130°C, and 150°C were investigated, respectively. The aging mechanism of PA6 under heat and oxygen was studied in terms of the reduced viscosity, crystallization behavior, dynamic mechanical behavior, and chemical composition through the methods of polarized light microscopy (PLM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X‐ray photoelectron energy spectrum (XPS), and so on. The results indicated that at the initial stage of aging, the molecular crosslinking reaction of PA6 dominated resulting in the increase of the mechanical strength, reduced viscosity, and the glass transition temperature of the sample. And the molecular degradation dominated in the subsequent aging process resulting in the decrease of the melting temperature, the increase of the crystallinity, and the formation of the oxides and peroxides products. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

New insight into the thermal-oxidative stability of polyamide 6: A comparison investigation on the effect of hindered amine and CuI/KI

Polyamide 6 (PA6) and stabilized PA6 with two different thermal stabilizers, (ie, metal salts CuI/KI and hindered amine KYN818 [1,3-benzendicarboxamide,N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)]) were separately prepared by melt blending and then aged for different time. The effects of aging temperature and aging time on crystallization behaviors of PA6 and stabilized PA6 were systematically investigated by differential scanning calorimetry and X-ray diffraction. The variations of melting temperature, crystallinity and 2θ values suggest that aging under high temperature will accelerate the formation of crystals initially and then the degradation of molecular chains. The yellowness index and maximum decomposition temperature of stabilized PA6 are higher than that of PA6, which is dominated by different aging mechanisms and properties of stabilizers. In addition, the viscosity of all systems is increased at the preliminary stage, which is caused by the crosslinking network and the post-polycondensation. The results of dynamic mechanical analysis showed that the glass transition temperature (T_g) decreases because the degradation of molecular chains during the aging process. Moreover, the carbonyl index for stabilized PA6 is lower than PA6, indicating the degradation is hindered by stabilizers. A comparison of the stabilization performance between CuI/KI and KYN818 was investigated to differentiate the long-term stabilizing efficiency.     

UV老化对ZnO填充PA6/LGF复合材料性能的影响

用熔融共混法制备出长玻璃纤维(LGF)含量为30%的LGF增强尼龙6(PA6)/ZnO(PA6/LGF/ZnO)复合材料,并采用氙灯紫外(UV)老化法研究了ZnO含量为0%~8%及UV老化时间为0~800 h对复合材料力学性能和结晶性能的影响。结果表明,经UV辐照后,PA6/LGF/ZnO复合材料的拉伸强度升高,韧性下降,其中当ZnO添加量为6%时,复合材料在800 h老化后的拉伸强度和缺口冲击强度保留率较未添加ZnO的复合材料分别提高了5.39%和4.98%。SEM分析表明在UV老化过程中,LGF与PA6基体之间的界面并未受到明显破坏,老化主要集中发生在PA6基体上。随着老化时间的延长,PA6/LGF复合材料中的PA6出现UV交联而使其结晶度从26.19%降低至20.70%;对于ZnO含量为6%的PA6/LGF/ZnO复合材料,老化800 h后其结晶度仅从22.92%下降至21.34%,PA6/LGF/ZnO复合材料整体上的结晶性能趋于稳定。     

Changes in crosslinking structure of thermo-oxidative aging chloroprene rubber and its effect on strain-induced crystallization

Chloroprene rubber (CR) is frequently quoted as an example of strain-induced crystallizing elastomer. Many researchers have focused on studying the aging mechanism and mechanical properties of aged CR, little has been done on the effect of crosslinking structure on strain-induced crystallization (SIC). The present paper related systematic investigations combining the changes of crosslinking structure and its influence on SIC. It was firstly investigated based on FTIR, crosslinking density test and GPC. The results showed that the molecular chains were oxidized in the early stage of aging, but the crosslinking reaction still dominated. At the same time, sol content and polydispersity decreased. In the later stage of aging, CR was oxidized, resulted in severe molecular chains fracture and an increase in the free sol content and polydispersity. The impact of aging on SIC under tensile condition was further investigated by the tensile tests. An obvious stress-hardening was found for unaged CR, but not in aged CR. The crystallinity and microcrystalline region orientation tests showed that oxidation, crosslinking by double bonds and dechlorination during aging were not conducive to the orientation of molecular chains during stretching. It is the aging process that has an adverse effect on the SIC of CR.     

Effect of electron-beam radiation on thermoplastic composites

Abstract

The effect of electron beam (EB) radiation on carbon fibre reinforced (CF) thermoplastic (PBT, PPS, PA) composites was investigated. To clarify whether crosslinking could take place without or only with the presence of a crosslinking agent, special attention was paid to the incorporation of this agent into polymer sheets with a carbon fibre content of 50% by volume. The thermal and mechanical properties of the materials before and after exposure under different irradiation doses were evaluated. For materials based on PBT, PPS and PA46, no significant changes in properties after irradiation could be observed. However, CF/PA66 exhibited some changes in the presence of a crosslinking agent after irradiation, which could be related to an irradiation-induced crosslinking reaction. The effect of irradiation on the flexural properties was insignificant but an improvement in the creep behaviour was observed. Non-reinforced PA66 plates were also manufactured and a gel content measurement indicated that crosslinking was successfully induced. Additional studies allowed the changes in the polymer due to this crosslink to be quantified.     

Effect of high-energy radiation on the uniaxial tensile creep behaviour of ultra-high molecular weight linear polyethylene

The tensile creep (and other tensile) properties of ultra-high molecular weight polyethylene (UHMW PE) have been determined before and after electron beam irradiation and compared with similar results on normal molecular weight high-density polyethylene (NMW PE). In both polymers, irradiation increases the tensile modulus and the yield stress whilst reducing creep. The major effects occur over the first 20 MRad irradiation dose, though creep strain continues to diminish with dose in UHMW PE up to 64 MRad. Most of the effects can be attributed to crosslinking in the amorphous phase, though the rise in yield stress seems to require crosslinking in the crystalline phase, and the initial rise in modulus in UHMW PE seems to reflect a rise in crystallinity. Comparison with other polymers shows that the creep behaviour of UHMW PE remains relatively poor, even after irradiation. The improvements obtained may, however, be significant in applications where creep resistance is of secondary importance compared with, say, impact and wear resistance, in which UHMW PE excels.     

Study on the creep behavior of polypropylene

The creep behavior and creep failure law of polypropylene (PP) were investigated by using a multifunctional stress‐aging testing machine under different aging environmental conditions (temperature, UV, and stress). Photoinduced changes in samples were studied using gel permeation chromatography and X‐ray photoelectron spectrometer. Surface morphologies were also observed by scanning electron microscopy. It was found that there is a critical failure strain (ε_crit) for PP during the creep course. Once the creep deformation exceeds the ε_crit, creep failure of PP takes place very rapidly. The value of ε_crit is independent of the tensile stress and UV irradiation, whereas it is only affected by the temperature and the nature of the PP, such as molecular weight and molecular structure. With increasing temperature, the value of ε_crit increases gradually. In addition, the creep rate of PP increases rapidly with increasing tensile stress and temperature as well as under irradiation with UV light. This study may provide a new way to predict the service lifetime of PP. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Thermal and wide angle X‐ray analysis of chemically and radiation‐crosslinked low and high density polyethylenes

Low and high density polyethylenes (PE) were crosslinked by two methods, namely, chemically by use of different amounts of tert‐butyl cumyl peroxide (BCUP) and by irradiation with different doses of electron beam. A comparison between the effects of these two types of crosslinking on crystalline structure, crystallinity, crystallization, and melting behaviors of PE was made by wide angle X‐ray diffraction and DSC techniques. Analysis of the DSC first heating cycle revealed that the chemically induced crosslinking, which took place at melt state, hindered the crystallization process and decreased the degree of crystallinity, as well as the size of crystals. Although the radiation‐induced crosslinking, which took place at solid state, had no significant influence on crystalline region, rather, it only increased the melting temperature to some extent. However, during DSC cooling cycle, the crystallization temperature showed a prominent decrease with increasing irradiation dose. The wide angle X‐ray scattering analysis supported these findings. The crystallinity and crystallite size of chemically crosslinked PE decreased with increasing peroxide content, whereas the irradiation‐crosslinked PE did not show any change in these parameters. As compared with HDPE, LDPE was more prone to crosslinking (more gel content) owing to the presence of tertiary carbon atoms and branching as well as owing to its being more amorphous in nature. HDPE, with its higher crystalline content, showed relatively less tendency toward crosslinking especially by way of irradiation at solid state. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3264–3271, 2006     

Influence of thermooxidative aging on the static and dynamic mechanical properties of long‐glass‐fiber‐reinforced polyamide 6 composites

The static and dynamic mechanical properties, thermal behaviors, and morphology of pure long‐glass‐fiber‐reinforced samples [polyamide 6 (PA6)/long glass fiber (LGF)] with different thermal exposure times at 160°C were studied by comparison with stabilized samples in this study. The aging mechanism of the PA6/LGF samples under heat and oxygen was studied with the methods of thermal Fourier transform infrared (FTIR), differential scanning calorimetry, dynamic mechanical analysis, scanning electron microscopy (SEM), and so on. The results indicate that the static mechanical strength, melting temperature, and crystallization temperature decreased because of the decomposition of the macromolecular chain of PA6 resin and the debonding of the interface between the glass fibers and matrix. The glass‐transition temperature and crystallinity also increased and decreased, respectively, after aging. The macromolecular chain decomposition dominated in the subsequent aging process; this resulted in many sharp and brittle microcracks appearing on the surfaces of the aged samples, as shown by SEM and the FTIR spectra. The existence of stabilizers endowed the PA6/LGF composites with better retention of static and dynamic mechanical properties. The reason was that the metal ions of the copper salt antioxidant acted as an anti‐aging catalyst in the reinforced PA6 system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39594.     

改性纤维素纳米晶对聚酰胺6结晶行为的影响

通过表面基团反应制备了异氰酸酯改性纤维素纳米晶（IPDI?CNC)，将其与聚酰胺6（PA6）共混后研究了有机纳米晶对PA6结晶行为的影响。利用差示扫描量热仪（DSC）对比分析了纤维素纳米晶（CNC）和IPDI?CNC对PA6的结晶度、结晶动力学参数及结晶速率的影响。结果表明，由于IPDI的引入，IPDI?CNC在PA6中的分散性更好，可以有效提高PA6的结晶度；采用 Jeziorny法研究非等温结晶过程，发现IPDI?CNC起到了异相成核作用，提高了PA6的结晶速度和结晶度；由于PA6的结晶能力提高了，IPDI?CNC的加入使PA6的吸水率降低了71.4 %。     

UV irradiation and aging effects on nanoscale mechanical properties of ultra high molecular weight polyethylene for biomedical implants

Abstract

As a result of its relatively high strength, high chemical resistance, low creep and low wear rate, ultra high molecular weight polyethylene (UHMWPE) has been widely used as the 'soft' articulating surface for total hip and knee arthroplasty. However, for long term artificial joint replacements, accelerated wear as a result of aging of UHMWPE is one of the most important problems that can lead to joint failure. Therefore, the present work is focused on investigating the effects of thermal and serum aging and UV irradiation dose on the nanomechanical properties (elastic modulus, hardness and visco-elasticity) of UHMWPE (type GUR410) specimens under different deformation rates. The continuous stiffness measurement (CSM) nanoindentation technique is used in the present work to measure the nanomechanical properties. The results show a considerable increase in the nanomechanical properties with increasing deformation rate. It is also demonstrated that the nanomechanical properties of the thermally and serum aged UHMWPE specimens decrease compared to the virgin specimens, while their visco-elastic behaviour increases. For the UV irradiated specimens, the nanohardness and nano-elastic modulus show an increase with irradiation dose especially for small penetration depths. Moreover, a considerable decrease in visco-elastic behaviour was observed for the UV irradiated specimens as a result of the crosslinking effect of the radiation.     

PA6/类水滑石纳米复合材料的制备与性能研究

将自制的双甲氧基苯甲酸根离子插层的类水滑石(MgAl-PMOBA-LDHs)分散到己内酰胺（CL）水溶液中,利用CL原位水解开环聚合制备了聚酰胺6（PA6）/MgAl-PMOBA-LDHs纳米复合材料,并通过透射电子显微镜、X射线衍射仪、紫外吸收光谱仪、热分析等对其性能进行了表征。结果表明,MgAl-PMOBA-LDHs 在复合材料中呈纳米级均匀分散,少量均匀分散的MgAl-PMOBA-LDHs 使复合材料对波长为320~380 nm的紫外线吸收效果明显;少量均匀分散的MgAl-PMOBA-LDHs 起到了很好的异相成核剂的作用,不仅提高了PA6的结晶温度,而且也提高了其结晶度;部分板层剥离的MgAl-PMOBA-LDHs 明显有助于PA6 的γ 晶型的产生。     

On‐line opto‐viscoelastic analysis of polypropylene fibres using multiple‐beam Fizeau fringes in transmission and a modified creep device

A creep device attached to an automated multiple‐beam Fizeau system in transmission was modified with a designed digital ruler. This device allows on‐line measurements of fibre length during creep experiments in terms of an analogue voltage value. The influence of sustained stress values on creep deformation and optical properties (n^||, n^? and Δn) for polypropylene (PP) fibres was studied interferometrically. The opto‐viscoelastic properties of PP fibres were determined for three different values of constant applied stress of 11.536, 18.717 and 25.905 MPa, at room temperature. Also, the variations of the cross‐sectional area and Poisson's ratio were studied during creep extensions. The compliance curves were obtained as a function of both time and applied stresses. Empirical formulae are suggested to describe the creep compliance curves for PP fibres, and the constants of these formulae were determined and described at each applied stress. A Kelvin chain was used to model the mechanical behaviour of the PP fibres under study. The effect of strain on the mean refractive indices, orientation function density and crystallinity was investigated as a result of the recorded data. Microinterferograms are given for illustration. The modified creep device with the designed digital ruler enables one to obtain instantaneous automatic accurate recording of fibre length values during creep experiments. Calculation of refractive indices, orientation function and crystallinity shows a difference in material behaviour at small stresses from that at higher stresses which may be attributed to different strain rates caused by different stresses. Copyright © 2010 Society of Chemical Industry     

The effect of γ radiation on the physical structure and mechanical properties of ultrahigh molecular weight polyethylene

Ultrahigh molecular weight polyethylene has been irradiated using a cobalt 60 source to give received dose between zeor and 50 Mrad. Irradiacted specimens were subjected to tensile characterization, dynamic mechanical analysis, and differential scanning calorimetry. Changes in tensile and dynamic mechnaical properties following irradiation arise from both molecular rearrangement and from increased crystallinity following scissionof long interlamella tie chains. The effects of post-irradiation aging on mechanical properties are associated with increasing crystallinity resulting from decomposition of metastable groups formed in the amorphous region during irradiation. Irradiated materials have been subject to sinusoidal stressing between 0.275 and 0.55 of yield stress for 100,000 cycies, and changes in mechnaical and physical properties measured. Increased resistance to creep during stressing was observed with the irradiated materials, behavior which is consistent with previsouly observed changes in crystallinity and crosslink density. Overall property changes measured following stressing were small compared with those induced by the initial irradiation.     

Crystallization behavior of melt-spun poly(vinyl alcohol) fibers during drawing process

The crystallization behavior of melt-spun poly(vinyl alcohol) (PVA) fibers during hot drawing process was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and computer controlled electronic universal testing machine. The effects of drawing temperature and drawing speed on the crystallinity and the stress induced crystallization of PVA fibers were discussed. The results showed that the crystallization process of PVA fibers during hot drawing presented three stages: initial stage, stress induced crystallization stage and slowly increasing stage. And PVA fibers with high crystallinity can be obtained by properly increasing the drawing temperature and drawing speed, especially when the drawing temperature and speed were 453 K and 100 mm/min respectively. The stress induced crystallinity of PVA fibers during drawing process was the difference between the crystallinity of PVA fibers after drawing and after only heat treatment. At the low drawing speed, i.e. 50 mm/min, due to the strong molecular movement and orientation relaxation under high temperature, the effect of stress induced crystallization weakens with the increase of drawing temperature, the ratio between stress induced crystallinity and thermal induced crystallinity changed from 8.7%:0%(393 K) to 1.7%:5% (453 K). While at the high drawing speeds, i.e. 100 mm/min and 500 mm/min, with the decrease of available orientation relaxation time, the stress induced crystallization plays an important role during the drawing process, the ratio between stress induced crystallinity and thermal induced crystallinity were 8%:2.9% and 10.2%:0.5% at 453 K respectively.     

Characterization of crystalline structure and free volume of polyamide 6/nitrile rubber elastomer thermoplastic vulcanizates: Effect of the processing additives

Blends of polyamide 6 and nitrile rubber (PA6/NBR) dynamically vulcanized may generate innovative products for special purposes where both high temperature and chemical resistance are key factors. In this investigation, we show that the crystalline nature of the PA6 can be controlled in terms of its morphological aspects (degree of crystallinity, crystal size, and structure) as a consequence of the presence of NBR and processing additives. Our results indicate that this crystalline control is dependent on the plasticization caused by the processing additives. Furthermore, imide‐like linkage formation was favored in the presence of ethylene‐co‐vinyl acetate (EVA)‐g‐maleic anhydride, resulting in changes in the molecular mobility of the PA6 matrix, crystallization parameters, and viscoelastic properties when compared to the others EVA additives. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45576.     

PA6/LiC1复合材料结构分析与性能研究

采用熔融共混挤出的方法,制备了尼龙(PA)6/LiCl复合材料,研究了不同含量的LiC1对PA6的结晶结构及性能的影响.傅立叶变换红外光谱、差示扫描量热、X射线衍射分析及力学性能测试结果表明,LiC1中“+和PA6间产生了络合作用,破坏了PA6中原有的氢键,降低了PA6的分子链的规整排列,但该络合作用使PA6分子链间产...     

Physicochemical properties of irradiated and loaded LDPE films with polyfunctional monomers

Polyfunctional monomers (PFMs), namely, trimethylol propane trimethacrylate (TMPTMA), trimethylol propane triacrylate, ethylene glycol dimethacrylate, and diethylene glycol diacrylate were blended with low‐density polyethylene (LDPE) and exposed to different doses of EB irradiation. Fourier transform infrared and ultraviolet and UV–vis spectroscopy of the unirradiated, irradiated, unloaded, and PFMs‐loaded LDPE films were studied under various irradiation doses up to 300 kGy. The degree of crosslinking and oxidative degradation, as measured by the spectroscopic parameters, were dependent on both the irradiation dose and the type of loaded PFMs. For all of the loaded monomers, the extent of crosslinking increased at different rates as a function of irradiation dose. TMPTMA monomer was the most efficient in enhancing the crosslinking of LDPE films compared to the other loaded monomers. However, the unloaded LDPE film showed the least extent of crosslinking. In addition, the EB‐radiation‐induced changes, such as trans‐vinylene formation, a decrease in vinyl and vinylidene unsaturation; and carbonyl double‐bond formation and change in crystallinity were correlated. The importance of these results on the prediction of the role of polyfunctional monomers in the production of crosslinked polymers is discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2025–2035, 2003     

Study on the influence of electron beam irradiation on the thermal,mechanical, and rheological properties of ethylene‐octene copolymer with high comonomer content

Ethylene‐octene copolymer (EOC) was irradiated using electron beam irradiation at different dosages (30, 60, 90, and 120 kGy). Effect of irradiation dosage on thermal and mechanical properties was studied. When compared to low density polyethylene, EOC exhibited higher degree of crosslinking reflected in increased gel content, higher elastic modulus (G′), and lower tan δ obtained by rheology measurement at 150°C. Crosslinking caused improvement in high‐temperature creep and room temperature and also elevated temperature elastic properties. Differential scanning calorimetry revealed that e‐beam irradiation has caused a gradual reduction in crystallinity and a presence of a fraction with higher melting temperature. In the case of EOC, as the extent of crosslinking increased, stress at break showed an increasing trend whereas irradiation dosage had an inverse effect on elongation at break. Radiation dosage has positive effect on thermal stability estimated by thermogravimetric analysis. After 30 min of thermal degradation at 220°C, slightly higher C?O peak for crosslinked sample was found by Fourier transform infrared spectroscopy while for room temperature samples no C?O peak was detected. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013