Orientation of polyoxymethylene by plane strain compression and rolling with side constraints

Abstract

In this paper, the authors describe the application of the constrained rolling process to produce highly oriented polyacetal bars with enhanced mechanical properties. The focus is on two commercial grades of polyoxymethylene Delrin 100 and Tarnoform 300. Tarnoform, unlike Delrin, is a copolymer. The structure and properties of the samples rolled to different compression ratios are described.     

Physical and mechanical characterization of oriented polyoxymethylene produced by die-drawing and hydrostatic extrusion

The mechanical properties and structure of uniaxially oriented polyoxymethylene (POM) produced by two solid-state processes, hydrostatic extrusion and die-drawing, are compared. In the former process there is no net component of tensile stress whereas in the latter case the sample is subjected to axial tensile stresses at the die-exit. The tensile nature of the stresses in die-drawing causes void formation and growth in the oriented sample whereas, in the case of hydrostatic extrusion, voids are suppressed due to the compressive stress fields. The mechanical properties of the oriented samples are compared together with relevant structural data, and their differences discussed.     

Role of primary amine in polyoxymethylene (POM)/bentonite nanocomposite formation

Polyoxymethylene (POM)/organo-modified bentonite nanocomposites are successfully prepared by melt intercalation method of which a primary ammonium salt is an effective surfactant, as evidenced from improvement in mechanical and gas barrier properties. Nanocomposite structures analyzed by XRD and TEM show mixed nanostructure of flocculation and exfoliation (flocculated/exfoliated nanocomposite) when primary ammonium-treated bentonite is used, whereas the quaternary ammonium-treated bentonites induce the mixture of intercalation and flocculation (intercalated/flocculated nanocomposite). The incorporation of organo-modified bentonite gives an effect on crystallization by generating numerous nucleating sites, especially in the case of bentonite with primary ammonium surfactant. The nanocomposites obtained exhibit improvement in flexural strength, flexural modulus, and elongation at break. The thermal degradation temperature is decreased by 40 °C, whereas the oxygen barrier is increased by 50%, as compared to neat POM.     

Structural changes in isothermal crystallization process of polyoxymethylene investigated by time-resolved FTIR, SAXS and WAXS measurements

Structural evolution in the isothermal crystallization process of polyoxymethylene from the molten state has been investigated by carrying out the time-resolved measurements of infrared spectra and synchrotron small angle X-ray scattering (SAXS) and wide angle X-ray scattering. In case of isothermal crystallization at 130 °C, for example, the infrared bands intrinsic of folded chain crystal (FCC) morphology appeared at first, and then the bands of extended chain crystal (ECC) morphology were detected with time delay of ca. 150 s. In the SAXS experiment at 130 °C, the lamellar stacking structure of the long period of ca. 15 nm was observed at first, which changed rapidly to ca. 12 nm in a short time. The SAXS peak with the long period of ca. 6 nm started to appear with a time delay of ca. 150 s after the initial lamellae appeared and coexisted with the initially-observed 12 nm peak. Judging from the timing to detect these characteristic infrared and SAXS signals, a good correspondence was found to exist between the stacked lamellar structure of 12 nm long period and FCC morphology and between the structure of 6 nm long period and ECC morphology. The quantitative analysis was made for the SAXS data on the basis of the lamellar insertion model combined with the paracrystalline theory of the second-kind of disorder. The following structural evolution was deduced from all these results. Immediately after the temperature jump from the melt to 130 °C, the stacked lamellar structure of FCC morphology was generated at first. New lamellae were formed from the amorphous region in between the originally-existing lamellae about 150 s later, where the random chain segments bridging the adjacent lamellae were extended to form the taut tie chains, giving infrared bands of ECC morphology. This inserted lamellar structure of 6 nm long period coexisted at a population of ca. 6% with the initially-formed lamellar stacking structure of 12 nm long period. When the experiment was made at 150 °C, only the formation of stacked lamellar structure of FCC morphology was observed and the insertion of new lamella did not occur.     

Plastic deformation and damage of polyoxymethylene in the large strain range at elevated temperatures

The deformation behaviour of polyoxymethylene has been studied in plane strain compression at temperatures from 120 °C up to 165 °C and in uniaxial tension and simple shear at 160 °C for strain rates from 10⁻⁴ to 1 s⁻¹. In uniaxial tension the stress-strain behaviour was determined by a novel video-controlled testing system. The measurements showed that there was a very significant evolution of volumetric strain, indicating that damage mechanisms play a key role in the plastic deformation behaviour.All tests showed similar deformation stages with a short region of visco-elastic behaviour followed by a rounded yield point. The von Mises equivalent yield stress for these tests showed a linear relationship with logarithmic strain rate, suggestive of an Eyring type thermally activated process. After yielding, all stress-strain curves showed a long plastic deformation regime, which in shear occurred at constant stress. In plane strain compression there was also only a very small increase in stress, in contrast to uniaxial tension where very significant strain hardening was observed at high strains, which is attributed to the onset of structural changes.     

Formation kinetics of polyoxymethylene dimethyl ethers from methylal and trioxane with little water

Polyoxymethylene dimethyl ethers(CH_3-O-(CH_2 O)n-CH_3, PODE_n, n1) can be added to diesel to reduce air pollution caused by incomplete combustion. Driven by this need, a cost-effective and efficient synthetic route is presented and investigated by this work for the production of PODEn, which are formed from methylal and trioxane with low water content(10 %(mass)) over HZSM-5 catalyst in a batch slurry autoclave at the temperature from 353.15 K to 393.15 K. The reaction rate laws including the PODEnand byproducts of polyoxymethylene hemiformals(HF_n), polyoxymethylene glycols(MG), methanol(Me OH) and methyl formate(MF) are developed. The rate constants of propagation step(k_6) and depolymerization step(k_(-6)) are assumed independent on polymerization degree of PODE_n. The rate of reaction is proportional to the content of the catalyst(W_(cat)) for the catalytic reaction within the scope of the study. All kinetic parameters were estimated by combining genetic algorithm and least square regression to fit experimental data well. This work is valuable for process optimization and reactor design.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Controllable synthesis of polyoxymethylene dimethyl ethers by ionic liquids encapsulated in mesoporous SBA-16

The promising combustion and emission properties of polyoxymethylene dimethyl ethers(PODE_n) are of significant interest. However, the synthesis of PODE_n products with desired chain lengths is still a problem facing synthetic PODE_n. Herein, a series of unique IL@SBA-16-Cx solid catalysts are prepared by encapsulation of ionic liquids(ILs) within the nanocage of SBA-16 through a silylation method. The structure of the encapsulated catalyst was characterized by UV–vis spectra, Fourier transform infrared(FT-IR),N_2 adsorption–desorption isotherms, Powder X-ray diffraction(XRD), Transmission electron microscopy(TEM) and Elemental analysis. The encapsulated catalysts show similar catalytic activity to the homogeneous counterparts and display higher selectivity to the targeted PODE_(3–5) products than their homogeneous counterparts in the synthesis of PODE_n from methanol(MeOH) and trioxymethylene(TOM). The encapsulated catalysts exhibit a superior PODE_(3–5) selectivity and could be the promising catalysts for PODE_n synthetic reaction.     

Effects of microvoiding on tensile deformation and fracture of polyoxymethylene

Abstract

Interrupted tensile tests were performed on different notch profile polyoxymethylene specimens at 160°C primarily in order to provide sample material for morphological and structural examinations. Scanning electron microscopy and small angle X-ray scattering were employed to examine the contribution of microvoids and other forms of damage to the large scale tensile deformation behaviour. It was established that the nucleation of spherical microvoids occurs during tensile deformation in the pre-yielding region. It was also concluded that voiding is well established by the yield point, while the formation of new microvoids is a continuous process during subsequent deformation. The interrupted tests clearly demonstrated the different stages in the formation, growth and coalescence of voids to form a continuous fracture path leading to fibrillation of the material and ultimately to fracture. Simple finite element models were used to predict the evolution of stress triaxiality with increasing deformation in the notched specimens.     

Thermal and spectroscopic analysis of worn polyoxymethylene surfaces and wear debris explaining degradation and polymerisation mechanisms

Degradation and polymerization of polyoxymethylene homopolymer (POM-H) surfaces after sliding at 8 to 150 MPa and 0.005 m/s over a total sliding distance of 3000 m is investigated by using thermal analysis (DSC, TGA, DTA) and Raman spectroscopy of worn surfaces or wear debris. There is mainly mechanical interaction and slight softening at 8 MPa (relatively high friction, low wear), softening at 16 to 55 MPa (decreasing friction and high wear) and finally melting at 150 MPa (very low friction, overload wear). At low contact pressures, wear debris remains amorphous and degradation of noncrystallised material during sliding manifests in broadening of the melting peak below the melting temperature. Degradation of C–O–C due to chain scission and radical reactions into CH₃ end groups are illustrated by Raman spectra. It is confirmed that the debris has long resident times and the maximum polymer surface temperature (T* = 93°C) is below the crystallisation temperature. At intermediate contact pressures, crystallisation results in a polymer fraction with higher thermal resistance. From the calculated temperatures T* = 120 to 150°C, crystallisation is beneficial for coherent transfer with larger particle sizes. At high contact pressures, the wear debris is immediately removed from the contact interface due to melting (T* = 200°C) and has thermal properties similar to the bulk material. There is no reaction between the debris in the interface, resulting in a thick polymer transfer film.     

Structural changes in non-isothermal crystallization process of melt-cooled polyoxymethylene[II] evolution of lamellar stacking structure derived from SAXS and WAXS data analysis

Structural change occurring in the cooling process of polyoxymethylene from the molten state has been investigated by carrying out the temperature dependent measurements of small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). In the SAXS experiment the generation of lamellar stacking structure with long period of ca. 14 nm was detected at first and then the new lamellae were inserted in between the already-existing lamellae to give the long period of 7 nm below 140 °C. The SAXS data were analyzed on the basis of lamellar insertion model by taking into account the second kind of paracrystalline disorder about the lamellar stacking mode. The thus obtained results were combined with the previously published infrared spectral data, and the structural change was deduced concretely. The generation of taut tie chains passing through the adjacent lamellae was proposed, which could reasonably explain the observation of infrared bands characteristic of extended-chain-crystal-like morphology.     

Structural changes in non-isothermal crystallization process of melt-cooled polyoxymethylene. [I] Detection of infrared bands characteristic of folded and extended chain crystal morphologies and extraction of a lamellar stacking model

Structural change in the crystallization process of polyoxymethylene (POM) cooled from the molten state has been investigated by the measurements of infrared spectra and small-angle (SAXS) and wide-angle X-ray scatterings (WAXS). When the melt was cooled slowly, the infrared bands characteristic of a folded chain crystal (FCC) were observed to appear around 156 °C. Below 140 °C, the infrared bands intrinsic of an extended chain crystal (ECC) were detected to increase in intensity. In the SAXS measurement, the peak (L₁) corresponding to a stacked lamellar structure with the long period of ca. 14 nm was found to grow in parallel to the growth of infrared FCC bands. In the temperature region of the observation of infrared ECC bands, the new peak (L₂) of long period of ca. 7 nm was found to appear and the intensity exchange occurred between the L₁ and L₂ peaks, that is, with decreasing temperature the L₂ peak increased the intensity and its height became comparable to the L₁ peak height. By combining all these experimental data, a model to illustrate the formation process of lamellar stacking structure has been presented. After the appearance of stacked lamellar structure of 14 nm long period from the melt, new lamellae are created in between the already existing lamellae and the long period changes to the half value, 7 nm. Some of molecular chain stems in a lamella are speculated to pass through the adjacent lamellae to form a bundle of fully extended taut tie chains, which are considered to be observed as the infrared bands characteristic of ECC morphology. Although the POM samples used in this experiment may contain small amount of low-molecular-weight macrocyclic component, it was not plausible judging from the various experimental data to assign the secondarily observed 7 nm SAXS peak to the repeating period originating from the stacked structure of macrocyclic compounds.     

Assessment of rolling resistance models in discrete element simulations

Particulate systems are of interest in many disciplines. They are often investigated using the discrete element method because of its capability to investigate particulate systems at the individual particle scale. To model the interaction between two particles and between a particle and a boundary, conventional discrete element models use springs and dampers in both the normal and tangential directions. The significance of particle rotation has been highlighted in both numerical studies and physical experiments. Several researchers have attempted to incorporate a rotational torque to account for the rolling resistance or rolling friction by developing different models. This paper presents a review of the commonly used models for rolling resistance and proposes a more general model. These models are classified into four categories according to their key characteristics. The robustness of these models in reproducing rolling resistance effects arising from different physical situations was assessed by using several benchmarking test cases. The proposed model can be seen to be more general and suitable for modelling problems involving both dynamic and pseudo-static regimes. An example simulation of the formation of a 2D sandpile is also shown. For simplicity, all formulations and examples are presented in 2D form, though the general conclusions are also applicable to 3D systems.     

Study on the prediction technique of the wear amount of polyoxymethylene

The influence of the sliding velocity and atmospheric temperature on the specific wear amount of polyoxymethylene was evaluated with the ring‐on‐ring apparatus. The specific wear amount showed the variation of S‐character type with maximum value, according to the change in the sliding velocity and the atmospheric temperature. When those results are plotted according to the sliding surface temperature, they have been found to be arranged as the same dependence. The specific wear amount of polyoxymethylene was assumed to be proportional to the product of actual contact area and reciprocal of the fracture energy. Then, the temperature dependency of the product of the fracture energy, calculated from stress–strain curve, and the actual contact area, calculated from Hertz's equation with elastic modulus, was able to explain the sliding surface temperature dependency of the specific wear amount well. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4212–4218, 2006     

共聚尼龙增韧改性聚甲醛的研究

本文采用共聚尼龙，用机械共混的方法对聚甲醛进行增韧改性，并对共混物的形态，结构与力学性能进行测试与分析。研究发现，在ＰＯＭ／ＣＯＰＡ共混体系中，存在着氢键的相互作用，且氢键的相对强度随ＣＯＰＡ含量的增加而增加。ＣＯＰＡ的加入，使得ＰＯＭ的熔融温度Ｔｍ和结晶温度Ｔｃ均增加，ＣＯＰＡ含量在１０％以下质量范围内，ＣＯＰＡ对ＰＯＭ具有增韧改性作用。     

轧制油乳液液珠大小和运动速度测量的可视化

利用自行设计的微型可视化反应器对轧制油／水乳液体系进行了研究。轧制油／水体系中油珠在水中分散良好，外形为球形，其直径约为5～20μm。在时间差为1s的时间段内油珠的运动速率单调变化，而从总体上看，随着静置时间的延长，液珠运动速率减小，变化幅度趋缓。本实验装置为单个微米级粒子大小与运动速度的测量提供了一种新的、可视化的方法。     

异戊橡胶在低温压缩应变下的结晶性能研究

研究了温度、压缩应变和时间3个主要因素对异戊橡胶结晶的影响。结果表明,在0℃及-25℃下,胶料低温结晶时的饱和压缩应变为40%;在-40℃下,胶料低温结晶时的临界压缩应变为35%左右。在0℃下,胶料低温结晶发生较为明显的时间区间为2-4 h;在-25℃下,胶料低温结晶发生较为明显的时间区间为4-8 h。在-40℃下,压缩变形量在30%以下将会延迟胶料低温结晶现象的发生,压缩变形量在40%以上时将会诱导低温结晶现象的发生。     

Kinetic modeling of gamma-aminobutyric acid production by Lactobacillus brevis based on pH-dependent model and rolling correction

Gamma-aminobutyric acid (GABA) is a natural non-protein functional amino acid, which has potential for fermentation industrial production by Lactobacillus brevis. This work investigated the batch fermentation process and developed a kinetic model based on substrate restrictive model established by experimental data from L₂₅(5⁶) orthogonal experiments. In this study, the OD₆₀₀ value of fermentation broth was fixed to constant after reaching its maximum because the microorganism death showed no effect on the enzyme activity of glutamate decarboxylase (GAD). As pH is one of the key parameters in fermentation process, a pH-dependent kinetic model based on radial basis function was developed to enhance the practicality of the model. Furthermore, as to decrease the deviations between the simulated curves and the experimental data, the rolling correction strategy with OD₆₀₀ values that was measured in real-time was introduced into this work to modify the model. Finally, the accuracy of the rolling corrected and pH-dependent model was validated by good fitness between the simulated curves and data of the initial batch fermentation (pH 5.2). As a result, this pH-dependent kinetic model revealed that the optimal pH for biomass growth is 5.6-5.7 and for GABA production is about 5, respectively. Therefore, the developed model is practical and convenient for the instruction of GABA fermentation production, and it has instructive significance for the industrial scale.     

Thermal Degradation of Polyoxymethylene Evaluated with FTIR and Spectrophotometry

Thermooxidative degradation at and beyond processing temperatures of a polyoxymethylene copolymer has been progressively accelerated to carbonisation through oven-storage aging. The quantification of such degradation was carried out through FTIR spectrography and colour spectrophotometry methods. The state of thermo-oxidation was monitored through the FTIR spectra corresponding to the carbonyl aldehyde group (O=CH) and methylene bending (–CH₂), as well as the chromaticity changes experienced on the sample surface. The results showed that the FTIR bands suggested are a reliable indicator during the initial conditions of degradation, however, the spectrophotometry becomes more appropriate method for more aggressive conditions leading to carbonisation.     

胺处理聚甲醛的热稳定化研究

针对聚甲醛由于合成中残留催化剂及不稳定端基导致其热稳定较差的问题,采用胺对聚甲醛进行处理,并考察了胺种类、胺处理工艺及水用量对聚甲醛热稳定性能影响;采用等温热质量损失率、等温热质量损失速率、甲醛释放量分析等方法对三乙醇胺对POM的热稳定作用进行了进一步研究。结果表明：以三乙醇胺为胺处理剂、采用熔融后处理法可有效提高聚甲醛热稳定性能,三乙醇胺最佳用量为0.2％。