Studies On Color Stability Of Polyoxymethylene Copolymer

The color stability of polyoxymethylene (POM) is important to the application of the resin. The studies on the discoloration mechanism of POM and the improvement of the color stability are essential to the commercial development of acetal resins.The effects of different additives on the discoloration of POM were examined. The experimental results revealed that the additives in resin are the key factors which result in discoloration of POM. The aging experiments at both ambient temperature and 140°C have shown that the POM resins containing different additives could be discolored; the resin which contains antioxidant Irganox1010 undergo the biggest color changing. The yellow index of POM containing Irganox1010 alone was 9.04 after extrusion and 14.62 after staying at ambient temperature for 20days. After 7 days thermal aging, the yellow index could increase to 24.25. The acetal resins itself did not undergo discoloration even though it will be degraded and powdered after 2 days thermal aging. In addition to the antioxidant, other additives which are usually used in POM, such as CaSt and EBS, have also influences on the discoloration of POM.When the additive formulation which discolor the POM resin was applied to the polyethylene, the formulation did not change color of PE during age. Therefore, it can be concluded that the discoloration of CO-POM under ambient temperature is closely related to the characteristic inclusion of formaldehyde, formic acid and metal ions in POM.resin.In order to decrease the effect of metal ions, the metal deactivator was formulated into additives for POM. The result showed that when Mg(0H)2 is used as acidic scavenger, the color stability of POM was improved by adding 0.01% metal deactivator HOSTANOX OSP1 or 0.05% MD697 and HOSTANOX OSP1 exhibit a better performance than MD697. However, for the formulations in which CaSt, molecular sieve CT-200 are used as acid scavenges, the metal deactivator couldn't enhance color stability of POM.The chloroform extraction of aged POM which contains 0.3% Irganox1010 was analyzed by NMR. The results showed that the oxidation and sacrifice of Irganox1010 was the key factor of the discoloration of CO-POM and after adding 0.01% boric acid into the system, the concentration of the substance which contribute to discoloration obviously reduced.The discoloration of POM could be suppressed by careful combination of Irganox1010 and other additives. When Melamine/PA6 and CT-200 were choosed as formadehyde scavenger and acid scavenger respectively and glycerol mono stearate was used as lubricant, the POM had a better color stability.It was found that the addition of a little amount of H3BO3 into resin could decreased the yellow index of POM to 6.94, even the resin were aged at ambient temperature for 20 days and 7 days for thermal aging ,the yellow index was remained at 8.55 and 16.85 respectively. However, by adding 0.01% NaOH into the acetal resin, the yellow index of POM increased to 15.38 after extrusion. After treated at ambient temperature for 20 days and 7 days thermal aging ,the yellow index continously increased to 16.94 and 57.76 respectively.The application of secondary antioxidant 619 can also improve the color stability of CO-POM. Increasing the concentration of acid scavenger in the additive formulation would benefit stability of POM.The additive formulation based on Irganox1010, which had excellent color stabilization, was successful developed..The dispersion of additive in the resin have also the effect on the discoloration of POM. When the additive was mixed intensively at high speed, the additives were well dispersed in the acetal resin and become more efficient. Both color stability and thermal stability was better than the one mixed at lower speed.