Study On Degradation And Stabilization Of Polyoxymethylene

The effects of the stabilizers including copolyamide (COPA) polyamide-6 (PAX poly( ethylene-co-vinyl alcohol) (EVOH) as well as COPA/EVOH on the thermal stability of Polyoxymethylene (POM) have been respectively evaluated by using isothermal degradation at 220℃ and thermogravimetric analysis (TGA). The results show that stabilizers perform very well under nitrogen atmosphere, and the degradation temperatures for POM can be obviously raised by extremely low amounts (0.2%) of the stabilizers. While in air atmosphere the stabilizers capable of consuming formaldehyde coming from POM degradation effectively reduce the weight loss rate of POM at 220℃. When the contents of COPA PA increase to 1.2%, the weight loss are no more than a half of the blank POM (without stabilizers). Compared with single-composition stabilizers such as COPA PA and EVOH, COPA/EVOH as compound stabilizers can achieve better effects in stabilization both in nitrogen and air condition, particularly the stabilizing system of POM/COPA/EVOH48 shows the best thermal stability.The kinetic analysis of the main degradation for POM containing various stabilizers are made from TG cures by Coats-Redfernd method, showing remarkable difference in mechanisms depending on measurement atmosphere and stabilizers. The reaction order for blank POM is 0.8 in nitrogen and 1.3 in air In the presence of COPA PA EVOH, reaction order sharply declines in nitrogen and approaches 0, which presents a zero-order kinetics for POMs containing single-composition stabilizer, however POM/COPA/EVOHs with reaction order of 1.0 follow the first-order kinetic relationship. While in air all the POMs show similar mechanisms of the first-order degradation, in which reaction orders fluctuate between 1.1 and 1.3. Analysis also indicate that activation energy of degradation in compound-stabilizer systems are considerably greater than others, among these POM/COPA/EVOH48 has the highest activation energy reaching220.19 KJmol-1 in nitrogen which is three times the value of blank POM (77.74 KJmol-1), and 530.37 KJmol-1 in air which also exceeds the blank POM (354.59KJmol-1) by fifty percent.Thermal degradation behaviors of POM are further studied in nitrogen by simultaneous thermogravimetry and Fourier transform infrafed spectroscopy (TG-FTIR). Two primary processes of degradation are definitely determined by the production of formaldehyde and carbon monoxide collected from successive FTIR spectrum. Firstly the POM tends to split off formaldehyde starting at unstable chain-ends, and then random chain scission occurs at elevated temperature. The degradation processes of POM/COPA/EVOH are consistent with the blank POM, but compound stabilizer can effectively inhibit degradation behavior, greatly improve the degradation temperature of POM in the initial stage, consequently enhance the degradation resistance for POM.Thermooxidative degradation of POMs are investigated under processing temperatures (180-240) by isothermal TG and FTIR analysis of POM residue. The blank POM becomes extremely instable as temperature increases, indicating that a rise in temperature has a highly accelerating effect on degradation. The degradation rate constant for the blank POM is 9.31 X 10-3 min-1 at 240, 100 times higher than that of 180 (6.6x10-5 min-1) , while at 230 the life time (at the level of 5% weight loss) is rather shorter than 3 minutes. According to the analysis, the first-order kinetic relationships are obtained from thermal oxidation at 220 for all the POMs at conversion within 5%. And degradation rate constant decreases considerably by adding stabilizers, especially compound stabilizers. During thermooxidation of POM, the IR spectrum shows typical absorption at 1735cm-1 assigned to the formation of carbonyl group (C=O) which proves the cleavage of carbon-oxygen bond initiated by free-radical mechanism in the presence of oxygen.