Study On The New And High Efficient Thermo-stabilizers Of Polyoxymethylene

Due to its special molecular structure, polyoxymethylene (POM) has the poorest thermal stability among the polymer materials, and the thermal stabilization technology is substantially important for POM in industrial application. This work is focused on the development of the new and high efficient thermo-stabilizers to improve the thermo-stability of POM.During the polymerization, cationic initiators like Lewis acids are often used to copolymerize trioxides with cyclic ethers and acetals, which maybe still active in the crude polymer after polymerization, and do a negative part in initiating POM and result in chain breaking. Besides, the crude POM copolymers downstream of polymerization step still contain unstable end groups (hemiformal acetals), which prohibit direct use or processing of polymers. Therefore, the study on the post-processing unit is substantially important for the application of POM. In this work, amine treatment was applied to stabilize POM, which can deactivate Lewis acid arising from the residual catalyst and promote the decomposition of the unstable end groups of POM due to its weak alkali property. The active hydrogen on its molecule can even absorb the formaldehyde and formic acid generated from oxidized formaldehyde. The effects of the different kinds of amine compounds and their treatment technique on the thermo-stability of POM were investigated, which indicated that the thermo-stability of POM can be improved remarkably by applying triethanolamine as the amine treatment agent through melting processing, and this technique is simple and easy to be realized. The effect of triethanolamine on the thermo-stability of POM was further studied by the measurement of isothermal weight loss analysis, isothermal weight loss rate analysis, and formaldehyde emission amount analysis, multiple processing and long-term aging analyses. Compared with the untreated POM, the POM treated with 0.2wt% triethanolamine maintained higher thermal stability and mechanical properties during multiple processing and long-term aging. Due to its high boiling point and hydroxyl groups on its molecule, triethanolamine had high thermo-stability during processing and strong interfacial bonding with POM. The triethanolamine melting treatment was carried out during the extruding processing, which was simple and high efficient, and easy to be conducted.At present, the low-molecule weight compounds such as melamine (MA) and dicyandiamide are commonly used as the main formaldehyde absorbents of POM in industry. But these kinds of low-molecule formaldehyde absorbent are easy to volatilize during high-temperature melting process which result in the loss of the compound, reduction of the thermo-stabilizing efficiency and formation of the mould deposit, seriously affecting the surface quality and production efficiency of POM. Excessive use of such compounds even leads to the decrease of the mechanical property of POM. In this work, a new type of high-molecule formaldehyde absorbent-- hexamethylenediamine formaldehyde polycondensate (HF) was prepared by the condensation polymerization of hexamethylenediamine and formaldehyde. The effect of the synthetic technique including formaldehyde / hexamethylenediamine molar ratio, reaction temperature and dispersant dosage on the thermo-stability of POM was studied. The HF polycondensate with slightly branched long chain, small particle size, high melting points, high thermal stability and high thermo-stabilization efficiency was obtained. The thermal stabilization effect of HF on POM was studied by the measurements of isothermal weight loss of analysis, isothermal weight loss rate analysis, TGA analysis, balance torque analysis and formaldehyde emission amount analysis, which showed that HF with multiple-functions as formaldehyde & formic acid absorbent, initiator-deactivator and stabilizer of the end groups, can replace MA to enhance the thermo-stability of POM. The crystallization nucleation effect of HF on POM was studied by the methods of polarized light microscopy (PLM) and differential scanning calorimetry (DSC), which showed that the addition of HF can restrain the formation of large spherulites, and make POM crystal grain more fine, spherulites size decrease substantially, the number of crystal cells increase greatly, and made the crystallization and melting temperature rise, the crystallization half-peak width decrease and crystallinity increase. The isothermal crystallization kinetics was analyzed by Avrami equation. Compared with the virgin POM, the addition of nucleating agent HF can improve the Avrami exponent (n) and the crystallization rate constant (k), reduce the half-time of isothermal crystallization (t_1/2) and the time corresponding to the maximum rate of crystallization t_p, indicating the remarkable nucleation effect of HF on POM with the improvement of the crystallization growth rate and being more close to perfect spherulites. The investigation of mechanical property showed that the addition of HF would not do harm to the mechanical property of POM, on the contrary, it improved impact toughness of POM to a certain degree, which also attributed to the improvement of the crystallization property of POM.Due to its excellent chemical and thermal stability, and high specific surface area layer structure, montmorillonite (MMT) was applied to compound with POM. The effect of MMT with different kinds of surfactant on the thermo-stability of POM was studied, which showed that MMT-S2 had great compatibility with POM, and when the content was 0.2wt%, the P0M/MMT-S2 performed the best thermo-stability. These results also indicated that POM is different from other polymers, because only a relative small amount of MMT can enhance the thermo-stability of POM efficiently, while the excessive use of MMT may lead to the degradation of POM. In this work, the POM / montmorillonite nanocomposites were prepared by melt, solution, pan milling and in situ intercalation blending respectively. The XRD analysis showed that compared with the pure organo-MMT, the X-ray data of POM/ MMT composites prepared by different ways shift to smaller angle and there is an increase of the d-spacing for the intercalated MMT, suggesting that some degree of POM molecules intercalaration into the MMT layers, which is substantially important for the uniform dispersion of MMT in POM matrix. For the POM/MMT composites prepared by pan milling 18 times showed no diffraction peaks of ordered layer structure, indicating that the layer structure of MMT had been destroyed and exfoliated. The investigation of crystallization effect of MMT-S2 on POM showed that the addition of MMT-S2 result in the restrain of large spherulites formation, more fine of crystal grain, increase of crystallinity, the rise of crystallization and melting temperature, and improvement of the crystallization growth rate of POM. The in situ intercalated P0M/MMT-S2 had the highest tensile strength and notched charpy impact strength at the 0.3wt% content of MMT-S2.