Study On The Polymeric Modification Of The Formaldehyde Absorbent For Polyoxymethylene

The formaldehyde absorbent is a substantially important assist-thermostabilizer for polyoxymethylene(POM). At present, the low-molecule compounds such as melamine (MA) and dicyandiamide are commonly used as the main formaldehye absorbent of POM in industry due to their high thermo-stabilizing efficiency. But these kinds of low-molecule formaldehye absorbent are easy to volatilize during high-temperature melting process resulting 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 thesis, the polymeric modification of formaldehyde absorbent for POM was studied by the following methods, in order to improve the thermo-stability of formaldehyde absorbent, reduce its volatilization, and improve its thermal stabilization effect on POM.Polyamide (PA) with high molecule-weight and thermostability, can be used as the formaldehye absorbent of POM by the addition reaction of the active hydrogen on the amine group of PA molecule to formaldehye. The thermal stabilization effect of COPA and HOPA on POM were studied by the measurements of isothermal weight loss analysis at 220℃, isothermal weight loss rate analysis at 222℃, non-isothermal gravity analysis (TGA) and balance torque analysis, which showed that with the increase of COPA or HOPA content, the thermal weight loss and thermal weight loss rate of POM decreased, thecharacteristic thermal degradation temperature increased, the maximal thermal weight loss rate decreased and the thermal stability of POM was improved effectively. The non-isothermal degradation kinetics evaluated from TGA by the Coats-Redfern method showed that the HOPA-modified POM had higher activation energy (E) and frequency factor (A) than the virgin POM. The multiple processing measurements indicated that HOPA can remarkably improve the thermal stabilization of POM. The increase extent of the yellow index (YI), the melt index (MI) and the decrease extent of notched charpy impact strength, elongation at break are lower than that of the virgin POM after five times of extrusion, indicating that addition of HOPA reduced the tendency of POM toward molecular-weight degradation and yellowing under the application of the combined effect of high mechanical shear stress and high temperature during melt processing.The melt temperature of COPA and HOPA is a little higher than POM, when POM cooled from melt, COPA or HOPA dispersing in POM crystallize before POM, and can act as the crystal nucleus for POM crystallization. The crystallization nucleation effect of COPA and HOPA on POM was studied by the methods of polarized light microscopy (PLM) and differential scanning calorimetry (DSC), which showed that the addition of COPA or HOPA can restrain the formation of large spherulites, and make POM crystal grain more fine. The nucleation effect of COPA on POM was not so remarkable, and HOPA made the crystallization and melting temperature rise, crystallization growth rate and crystallinity (Xc) increase. The isothermal crystallization kinetics was analysed by Avrami equation. Compared with the virgin POM, the addition of nucleating agent HOPA can improve the Avrami exponent (n) and the crystallization rate constant (k), reduce the half-time of isothermal crystallization Ua and the time corresponding to the maximum rate of crystallization tp, indicating the remarkable nucleation effect of HOPA on POM with the improvement of the crystallization growth rate of POM and being more perfect of the spherulites. The investigation of mechanical property showed that HOPA can improve the impact toughness andelongation rate at break, and reduce the molding shrinkage rate to some extent.Based on the above research, the synergetic effect of HOPA and MA on the thermal stability of POM were studied. The thermal stability of POM compounded with the complex stabilizer HOPA/MA was better than that of the single used stabilizer HOPA or MA with equal content. The modified POM with HOPA/MA had higher activation energy and frequency factor than the virgin POM. Partly replace of MA by HOPA, the impact toughness of POM increased, and the general performance and product quality of POM can be improved effectively. This research achievement has passed through the production experiment.MA was polymeric modified by condensation reacting with formaldehyde to form the melamine formaldehyde polycondensate (MF) with high molecule-weight and comparatively high processing thermo-stability. The balance between the cross-linkage degree of MF and the remaining content of active hydrogen atoms on MF molecules, which can react with formaldehyde produced by POM decomposition as the formaldehyde absorbent of POM, can be determined by the reacting molar ratio of MA and formaldehyde. The thermal stabilization effect of MF on POM was studied by the measurements of isothermal weight loss analysis at 220 °C, isothermal weight loss rate analysis at 222 °C, TGA analysis and balance torque analysis, which showed that MF had almost the equal thermal stabilization to MA on POM. The multiple processing and long-term thermal stability at high temperature measurements indicated that MF even showed higher thermal stabilization on POM than MA. The particle size of the product can be controlled by the types and concentrations of the dispersion agents, which was substantially important for the mechanical property, especially the impact toughness of POM, and the finely divided, cross-linked, and water insoluble MF polycondensates were obtained. The crystallization nucleation effect of MF on POM was studied by the methods of polarized light microscopy (PLM) and differential scanning calorimetry (DSC), which showed that the addition ofMF can make POM crystal grain more fine, and the spherulite interface became blur. Moreover, the impact strength was improved after five times of extrusion, due to the better dispersion of MF in POM matrix and better nucleation effect of MF on POM.