Mechanical and physical properties of ultraoriented polyoxymethylene produced by microwave heating drawing

The properties of ultra-oriented polyoxymethylene tubes produced by drawing under microwave heating have been assessed by mechanical testing, optical microscopy, scanning electron microscopy, X-ray analysis, birefringence and differential scanning calorimetry. The highest Young's modulus of 58 GPa was obtained at room temperature (77 GPa at ?150°C) at a draw ratio of 33. The maximum tensile strength was 1.7 GPa at a draw ratio of 26. The nonuniformity of Young's modulus in a radial direction has been compared with the nonuniformity of the birefringence and heat of fusion.     

Morphological changes of wide polyoxymethylene rod during a microwave heating drawing process

The microwave heating drawing process for producing a polyoxymethylene (POM) rod (2.5 mm in diameter) with a sonic modulus of 40 GPa has been analysed by investigating the changes in both orientation and thermal properties during drawing. During the initial crystalline deformation in the necking region, the lamellae are oriented perpendicular to the draw direction and are then unfolded into microfibrils. The crystalline orientation function reaches a high value (0.988) at a draw ratio of 6 immediately after necking. In the advanced ultra-drawing stage, the Young modulus increases gradually with increasing amorphous orientation. At the same time, the orientation distribution in the radial direction is caused by the temperature distribution induced in the radial direction of the rod. It is noted that fine adjustments of ambient temperature and microwave power are required to get ultra-high-modulus POM rods over 40 GPa with large crosssections.     

Production and properties of highly oriented polyoxymethylene by die‐drawing

Highly oriented monofilaments were produced by a high‐temperature die‐drawing process followed by tensile drawing. It was shown that a successful high‐speed process required high‐quality melt‐extruded rod. The mechanical properties and structure of the die‐drawn products were investigated by means of tensile and bending tests, dynamic mechanical measurements, DSC, and X‐ray diffraction. The bending modulus and the tensile strength increased with increasing draw ratio. It was also observed that at high draw ratios the γ‐dispersion peak in the dynamic mechanical tan δ curve, which is associated with main chain micro‐Brownian motion in the amorphous regions, diminishes, implying that these chains become taut. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1268–1278, 2003     

Micro‐structure and characteristics of highly oriented polyoxymethylene obtained by press and biaxial drawing

The physical structure of oriented polyoxymethylene (POM) drawn in two steps by press drawing and subsequent simultaneous biaxial drawing was studied. The degree of crystallinity, the orientation, and the tensile modulus depending on draw ratio were analyzed. By the press drawing method, the draw ratio reached 6.0, the tensile modulus was 4.5 GPa, and the degree of crystallinity decreased from 70% to 65%. By the two‐step drawing of press drawing followed by subsequent simultaneous biaxial drawing, the tensile modulus reached 11 GPa when the draw ratio was 14 times, and the degree of crystallinity increased from 71% to 76–80%, but it did not depend on the draw ratio, and showed effectively a constant value (average 77%). The physical structural properties, such as the degree of crystallinity, the orientation of the crystal, and the lamella structure, were analyzed by SEM, X‐ray diffraction, and other methods. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 835–844, 2006     

Young modulus and degree of crystallization of highly‐elongated polyoxymethylene

The physical structure of polyoxymethylene (POM) drawn into two steps by a press and a simultaneous biaxial drawing machine was studied and the drawing dependency on the degree of the crystallization, the orientation, and the modulus were analyzed. The stretching ratio by the press reached 6.0 and the tensile modulus of elasticity increased from 2.5 to 4.5 GPa. However, the degree of crystallization decreased slightly. The rupture elongation increased in the lower drawing region and it peaked when the drawing ratio was 1.7. The film stretched by 2 times was drawn by the biaxial drawing machine. The high tensile modulus of elasticity was obtained and the maximum value was 11.5 GPa at 14 times of the drawing ratio. The lamella structure of POM was supposed to loosen and become oriented to the drawing axis ambiguously by the first drawing. The lamella was highly oriented by the second stretching procedure. The tensile strength and the elongation as well as the modulus were analyzed as a function of the degree of the stretching and the crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1223–1227, 2006     

Preparation of ultrahigh modulus isotactic polypropylene by means of zone drawing

Highly oriented isotactic polypropylene with ultrahigh modulus was prepared by means of the zone drawing method proposed by Kiho and Asai. This method features heating a limited portion of the sample. The maximum value of modulus attained was 15 GPa, which corresponds to about half the crystal modulus along the molecular axis evaluated by the x-ray method. Such an ultrahigh modulus was obtained for a sample of medium molecular weight by inducing necking in the heater and subsequently cooling the sample at lower temperature. The loss tangent peak with the initiation of micro-Brownian motion of amorphous chains became progressively smaller and broadened more at higher temperatures with increasing draw ratio. Orientation function of the crystal c-axis increased with draw ratio λ in the region below λ = 9, but reached a constant value, 0.995, in the region above λ = 9. The modulus increased with draw ratio in all ranges of the draw ratio.     

Bonding adhesive joints with radio-frequency dielectric heating

The feasibility of using radio-frequency (RF) dielectric heating to cure thermoset adhesives has been evaluated. Thermoset and thermoplastic polymer panels have been bonded to steel using conventional one-and two-part epoxy and two-part urethane adhesives. Process cycle times for adhesive cure using RF heating were about 20 to 60 seconds, compared with about 20 to 30 minutes for the same materials using conventional oven-cure methods. Thermoset substrates bonded included glass fibre-reinforced composite panels based on sheet moulding compound (SMC) and resin transfer moulding styrene-vinylester (RTM). Thermoplastics such as polycarbonate, polyarylate, Noryl, ABS and polymethylmethacrylate were also successfully bonded. RF bonding experiments were performed by preparing and testing lap-shear joints as well as by joining a large test structure with a 25 mm x 1.25 m bondline. Bonding of painted steel to SMC composite, SMC to SMC and steel to RTM composite were also accomplished using the RF dielectric heating process.     

Study of the production of ultra-high modulus polyoxymethylene by tensile drawing at high temperatures

The tensile drawing behaviour of polyoxymethylene has been studied, with particular reference to the production of ultra-high modulus oriented material. The influence of molecular weight and draw temperature, and the comparative effectiveness of single-stage and two-stage drawing processes have also been examined. In addition to the molecular weight range it appears that both draw temperature and drawing rate must be specified within very narrow limits, if ultra-high modulus material is to be produced. It is tentatively suggested that this is because effective high draw requires a suitable coincidence of rate processes involving both the crystalline and non-crystalline regions of the polymer.     

聚甲醛生产技术的发展

介绍了聚甲醛的生产现状、生产方法,指出了我国聚甲醛生产上存在的问题。     

粒度对陶瓷粉体介电损耗及微波加热性能的影响

微波加热与传统加热不同,微波加热是向被加热物质内部辐射微波电磁波,推动偶极子运动,使之相互碰撞、摩擦而生热,是通过电磁场中介质损耗而引起的体加热,因此加热性能取决于物质的介质损耗。本文将通过对碳酸钙和石英两种陶瓷粉体的研究来探讨其粒度对粉体介电损耗及微波加热性能的影响。     

Generation of orthorhombic polyoxymethylene in a cationic polymerization system of trioxane

Orthorhombic polyoxymethylene, which was originally found two decades ago in Italy in a polymerization system of aqueous formaldehyde, has been rediscovered in a cationic polymerization system of trioxane designed to grow needle-like polyoxymethylene single crystals. Besides the single crystals obtained in the liquid phase, it has been known that a thin film bearing a number of particles, up to ～ 1 mm diameter, is formed as a by-product on the inner wall in contact with the gaseous phase. X-ray analysis has now revealed that the particles consist essentially of the orthorhombic crystalline form. The morphology has been investigated and the growth mechanism briefly discussed.     

聚甲醛的生产、性能改良及应用

简要介绍了聚甲醛的物化特性及广泛用途,并对聚甲醛的改性研究现状及研究方向做了总结,浅述了我国聚甲醛的生产现状、生产企业及发展前景,并对未来聚甲醛行业的发展进行了探讨。     

聚甲醛纤维的发展与应用

聚甲醛具有优良的综合性能,广泛应用于包括化学纤维在内的许多领域。在未来一段时期内,聚甲醛将会得到更快的发展。介绍了聚甲醛纤维的发展历程及其各种制备方法,并阐述了聚甲醛纤维的力学性能、耐化学腐蚀性能,且对其应用领域进行了分析。     

热塑性聚氨酯弹性体共混增韧聚甲醛的研究

聚甲醛（POM）共混增韧是高分子学术界迄今未能很好解决的一个世界性难题，本文采用热塑性聚氨酯弹性体（TPU），应用双螺杆挤出机采用熔融共混的方法对POM增韧改性进行了研究，对POM/TPU共混体系的力学性能、流动性能和形态进行了测试分析。结果表明：随着TPU用量的增加，共混体系的冲击强度（I）和断裂伸长率（ψ）随之增加，即韧性增加，而拉伸强度（σ）、弯曲强度（σw）、弯曲模量（Ew）和硬度（H）随之降低，即刚性下降。     

Phonons and heat capacity of polyoxymethylene

Using Wilson's GF matrix method as modified by Higgs for an infinite system, we report normal modes and their dispersion for polyoxymethylene using the Urey‐Bradley force field. Heat capacity has been evaluated as a function of temperature from dispersion curves via density‐of‐states. The results obtained agree well with the experimental data reported in the literature. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Nonisothermal crystallization kinetics of polyoxymethylene/montmorillonite nanocomposite

The nonisothermal crystallization kinetics of polyoxymethylene (POM), polyoxymethylene/Na–montmorillonite (POM/Na–MMT), and polyoxymethylene/organic–montmorillonite (POM/organ–MMT) nanocomposites were investigated by differential scanning calorimetry at various cooling rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of POM/Na–MMT and POM/organ–MMT nanocomposites. The difference in the values of the exponent n between POM and POM/montmorillonite nanocomposites suggests that the nonisothermal crystallization of POM/Na–MMT and POM/organ–MMT nanocomposites corresponds to a tridimensional growth with heterogeneous nucleation. The values of half‐time and the parameter Z_c, which characterizes the kinetics of nonisothermal crystallization, show that the crystallization rate of either POM/Na–MMT or POM/organ–MMT nanocomposite is faster than that of virgin POM at a given cooling rate. The activation energies were evaluated by the Kissinger method and were 387.0, 330.3, and 328.6 kJ/mol for the nonisothermal crystallization of POM, POM/Na–MMT nanocomposite, and POM/organ–MMT nanocomposite, respectively. POM/montmorillonite nanocomposite can be as easily fabricated as the original polyoxymethylene, considering that the addition of montmorillonite, either Na–montmorillonite or organ–montmorillonite, may accelerate the overall nonisothermal crystallization process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2281–2289, 2001     

Prospects for the application of dielectric heating processes in the pre‐treatment of oilseeds

In some branches of industry dielectric heating processes are used in a wide range of different applications like drying of agricultural products and textiles or disinfection processes in the food processing and pharmaceutical industry. This report shows potential uses of this technology for thermal pre‐treatment of oilseeds. Therefore, the basic principles of the dielectric heating mechanism and some resulting characteristics are presented in comparison to conventional heating systems. Special aspects of the application of microwaves and high‐frequency energy for thermal pre‐treatment of oilseeds are also presented. Finally, some scenarios for imaginable applications of this technology in the European oil milling industry are discussed.     

Thermooxidative ageing of polyoxymethylene,part 1: Chemical aspects

The thermal oxidation of unstabilised POM homopolymer has been studied at 90, 110, and 130°C under various oxygen pressures ranging from 0 to 2.0 MPa by gravimetry and IR spectrometry. This latter reveals essentially the formation of formate (F) groups and alcohols in smaller quantities. The formate growth and mass loss rates increase pseudo hyperbolically with oxygen pressure, the asymptotic level being reached at pressures up to 0.5 MPa. These results lead to propose a mechanistic scheme of radical chain oxidation in which depolymerization is initiated by hydroperoxide unimolecular decomposition, one formate and one chain scission being formed per decomposition event whereas the kinetic chain length of the depolymerization zip is of the order of 100. A kinetic model derived from the mechanistic scheme, taking into account substrate consumption, simulates correctly the shape of kinetic curves and the effect of temperature and oxygen pressure. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

分子筛催化剂对聚缩醛二甲醚合成的影响

本研究以甲醇及甲缩醛为原料、酸性分子筛为催化剂合成聚缩醛二甲醚(PODEn),考察了ZSM-5分子筛的硅铝比、磷改性、粒径尺寸等因素对反应的影响。结果表明,Si/Al为50、P2O5含量为6 wt.%、粒径尺寸为5nm的ZSM-5分子筛催化剂活性较佳,三聚甲醛的反应转化率达到95.2%,PODEn(n=2～4)的选择性为58.0%。     

Thermooxidative aging of polyoxymethylene,part 2: Embrittlement mechanisms

The thermal oxidation at 130°C of polyoxymethylene homopolymer (H) and copolymer (C) samples of close initial molar masses has been studied by gravimetry, rheometry, X ray scattering, and tensile testing. Both samples undergo random chain scission and depolymerization. Their crystallinity ratio increases, whereas their long period decreases. All these changes are faster for H than for C. Tensile tests reveal that there is no significant change of behavior law except for the ultimate strain, which decreases abruptly when the weight average molar mass reaches a value of the order of 70 kg mol^?1 for H and 90 kg mol^?1 for C. At these molar masses values, the entanglement network of the amorphous phase has undergone only small damagement. In contrast, a chemi‐crystallization process has induced significant morphological changes, especially a decrease of the interlamellar thickness. It is suggested that this latter phenomenon could be responsible for embrittlement. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009