Thermal oxidation and molding feasibility of cycloolefin copolymers (COCs) with high glass transition temperature

https://doi.org/10.1016/j.polymdegradstab.2005.10.012Get rights and content

Abstract

Cycloolefin copolymers (COCs) with high glass transition temperature (Tg = 203 °C) have been synthesized and pelletized by extrusion molding. However, their colors change from transparent to yellow during extrusion molding because of thermal oxidation and generation of alkene groups. We have successfully blended several antioxidants (Irganox 1010, Irgafos 168, Irganox HP2225 and Irganox HP2921) into lab-made COCs to avoid the discoloration. The experimental results show that Irganox HP2921 is the best antioxidant among the antioxidants used and can effectively not only suppress thermal oxidation but also eliminate the color stain.

Introduction

Since cycloolefin copolymers (COCs) exhibit excellent optoelectronic and chemical properties such as low birefringence, low water-absorbance, low specific gravity, high transparency, and high chemical resistance, they have attracted much attention for many years and are applied for optical lenses, medical as well as food containers, optical discs, and liquid crystal displays (LCDs) [1], [2], [3]. Nevertheless, COCs with high thermal resistance and their molding feasibility are less studied.

In this paper, we have copolymerized norbornene and ethylene to form poly(norbornene ethylene) with high glass transition temperature (Tg > 200 °C) in the presence of metallocenes as catalysts. However, the discoloration of lab-made COCs takes place after pelletization by extrusion molding owing to thermal oxidation and production of alkene groups. To prevent the thermal oxidation and color stain, several antioxidants (Irganox 1010, Irgafos 168, Irganox HP2225, and Irganox HP2921) have been successfully introduced into lab-made COCs.

Section snippets

Instruments

Infrared (IR) spectra, ultraviolet/visible (UV/vis) spectra, and differential scanning calorimetry (DSC) results were recorded on a Perkin–Elmer RX-1 FT-IR System, a HITACHI U-3300, and a TA DSC Q100, respectively. Furthermore, molecular weight and the yellow indices b (CIE LAB system) were measured by a Waters Alliance GPCV2000 and a Nippon Denshoku SA 2000, respectively. We executed the pelletization by a twin-screw extruder (APV Chemical Machinery MP 2015).

Materials

Lab-made COCs were synthesized

Discoloration problem

Since COCs have been successfully synthesized, we pelletized them by extrusion molding at 260 °C. Unfortunately, the pellet was yellow rather than transparent. Therefore, we tried to measure its DSC, UV/vis, and IR spectra to understand why the color changed.

As shown in Fig. 2, lab-made COCs exhibit high thermal resistance and its Tg can reach 203 °C in nitrogen. However, oxidation occurs at 195–250 °C (Tmax = 239 °C) while they are under air as shown in Fig. 2. This reveals that oxygen in the air

Conclusions

COCs with high Tg have been successfully prepared. Although their color contamination occurs after extrusion molding owing to the thermal oxidation and production of lkene groups, their molding feasibility can be improved by the addition of appropriate antioxidants, which can suppress thermo-degradation.

Acknowledgements

Financial support by ITRI D342XT2100 is highly appreciated.

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