Phase separation during radiation crosslinking of unsaturated polyester resin

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Abstract

Phase separation during radiation-initiated crosslinking of unsaturated polyester resin was studied. Residual reactivity of liquid phases and gels of partially cured samples was determined by DSC. Uncured resin and liquid phases showed double reaction exotherm, gels had a single maximum that corresponded to higher-temperature maximum of liquid parts. The lower-temperature process was attributed to styrene–polyester copolymerization. At higher temperatures, polyester unsaturations that remained unreacted due to microgel formation homopolymerized. FTIR revealed different composition of phases. In thicker samples, reaction heat influenced microgel formation causing delayed appearance of gel and faster increase in conversion.

Introduction

The difference in chemical nature of unsaturated polyesters (UPE) and monomers used as crosslinking agents (also called reactive diluents) like styrene has significant influence on the course of crosslinking reaction. Macroscopically, these components are compatible in a limited concentration range that depends on polyester composition. On microscopic level, UPEs are only partially swelled by styrene so concentration of monomer inside polyester coil is lower compared to its surrounding by about an order of magnitude. As the reaction starts, partially crosslinked polyester coils form spherical structures, microgels, responsible for phase separation that starts at very low reaction conversion (<1%) (Huang, 1993). Polyester-rich and styrene-rich phases are formed, the reaction courses and conversions being different in each phase. Hsu et al. (1993a) used optical microscopy to observe formation of microgels and by ESR measurements (Hsu et al., 1993b) showed that the rate of increase of the concentration of stable radicals was significantly higher in polyester-rich phase because lower mobility of polyester chains prevents termination. If enough energy is brought into such system, for example during residual reactivity determination by DSC, steric hindrances will be subdued and homopolymerization of unreacted polyester double bonds is to be expected.

The diffusion limits mixing of chemical initiators with resin components; so initiation occurs mostly in styrene-rich phase while inside polyester coil, initiation is scarce increasing the differences in reaction courses. Such effects can be avoided by radiation initiation that is homogenous throughout the system. Its another advantage is that it can be performed at any temperature and can be interrupted at a chosen reaction time so the system can be analysed at selected reaction stages. In spite of those advantages, to our best knowledge, radiation initiation has not been applied to study the phase separation during crosslinking of UPE resins and it is one of the purposes of our investigation.

Another goal of our experiments was to investigate the effect of sample thickness on reaction course. The crosslinking of UPEs is highly exothermic and the thermal conductivity of system is very poor (Day, 1994). Because of this, in thicker samples significant increase in local temperature occurs. Bergmark (1987) determined that during isothermal reaction at 80°C the temperature in the middle of the sample of dimensions 20×40×40 mm3 increased steeply to 250°C. The temperature increase during radiation-initiated reaction was not so great (Dobo, 1985), but still significant and its influence on reaction extent in thicker samples is to be expected.

Section snippets

Experimental

Commercial UPE resin was supplied by “Chromos tvornica smola” Zagreb. It is composed of polyester based mostly on propylene glycol and maleic anhydride with about 30% of styrene. Samples were irradiated to selected doses in aluminium cylindrical cells with Teflon spacers (Pucić, 1995). The spacing between inner and outer cylinder was 1.5 and 6 mm. Crosslinking was initiated by 60Co γ-radiation at dose rate of 0.56 kGy/h at 21°C to doses of 2.2, 4.5, 6.7 and 8.9 kGy. Liquid parts were separated

Results and discussion

As the crosslinking of UPE resin reached certain extent, resulting gel became insoluble in the unreacted resin and the phase separation occurred. At doses below 4 kGy whole sample is liquid, at higher doses gel is formed so remaining liquid part could be separated from gel. Two different types of behaviour can be seen in Fig. 1 where DSC traces of uncured resin and liquid and gel phases of selected partially cured samples are shown. Gels had one distinct reaction peak between 180°C and 200°C,

Conclusions

Phase separation was detected during radiation-initiated crosslinking of UPE resin due to microgel formation. Lower-temperature part of double reaction exotherm of liquid parts of partially cured samples was caused by copolymerization of styrene and polyester and styrene homopolymerization. Its maximum (or shoulder in uncured resin) corresponded to styrene–polyester reaction, while higher-temperature exotherm resulted from polyester homopolymerization. In gels only one exotherm that appeared in

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