Carbamate end-capped poly(oxymethylene) copolymer with enhanced thermal stability prepared by reactive extrusion

Carbamate end-capped poly(oxymethylene) (POM) copolymer was prepared by reactive extrusion and its stability has been studied by means of thermogravimetric analysis (TGA) both in air and in nitrogen, content of formaldehyde generated (C.F.G) and isothermal weight loss. 4, 4′-diphenylmethane diisocyanate (MDI) was used as an end-cap agent to react with POM molecules during extrusion. MFI results and FTIR spectra indicate that MDI did react with POM, which replaced the labile hydroxyl chain ends with thermally more stable carbamate structures and at the same time increased the molecular weight due to chain extension or possible branching. At low MDI contents (≤1 wt%), end-capping dominated the procedure while chain extension or possible branching occurred more frequently with the increase of MDI, resulting in the continuous reduction of MFI. The presence of branched or crosslinked structure has been verified by the huge difference between weight-average molecular weights deduced from two different methods and melt flow ratio. Side reactions also occurred during extrusion, including the self-polymerization of MDI and reactions between MDI and active-hydrogen containing compounds. According to TGA and C.F.G results, crude M90 extruded with MDI showed superior thermal stability to either original crude M90 or commercial POMs. Similar stabilization effect can be found in commercial M90 and commercial M270 but was less prominent because of their inherently better stability. Such stabilization effect depends largely on end-capping, although some of it should attribute to the roles of formaldehyde absorbent the carbamate structures and side products played. Thermal stability of crude M90 with 1 wt% MDI was further improved through the addition of stabilizer package. After end-capping, the mechanical properties of the POM still remained at an acceptable level.