Radiation Physics and Chemistry 67 (2003) 415–419

Phase separation during radiation crosslinking of unsaturatedpolyester resin

Irina Puci!c*, Franjo Ranogajec

Rudjer Bo$skovi!c Institute, P.O. Box 180, Zagreb 10002, Croatia

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.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Unsaturated polyester resin; Radiation-initiated crosslinking; Phase separation; DSC; FTIR; Thickness

1. 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 com-

patible 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

(o1%) (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 determi-

nation 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 advan-

tage 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

*Corresponding author. Tel.: +385-1-45-61-166; fax: +385-

1-45-61-166.

E-mail address: pucic@rudjer.irb.hr (I. Puci!c).

0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0969-806X(03)00077-X

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.

2. 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 (Puci!c, 1995). The spacing between inner and

outer cylinder was 1.5 and 6 mm. Crosslinking was

initiated by 60Co g-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 from gels and analysed; at highest

dose in 1.5 mm thick sample only gel was found.

Residual reactivity of each phase was measured in

aluminium sample pans using Perkin–Elmer DSC 7 at

heating rate of 5�C/min. Infrared spectra of samples

were recorded on Perkin–Elmer FTIR 2000, samples

were mixed with KBr and pressed into pellets. Spectra

were analysed using Galactic 5.0 software. DSC traces

and spectra of uncured resin were also recorded.

3. 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, while unreacted resin and liquid parts showed

wide double exotherms. The exotherm of uncured resin

had a shoulder at about 150�C while the higher-

temperature maximum was in the same range as in gels.

The shape of reaction exotherm of liquid parts of

partially cured samples was similar to that of uncured

resin but higher-temperature maximum was less pro-

nounced and the reaction heat was lower. Sample

thickness did not influence the shape of reaction

exotherms.

Double reaction peaks or peaks with shoulders are

often detected in DSC traces during non-isothermal

crosslinking of UPE resins. Kubota (1975) investigated

crosslinking of poly(dipropylene glycol fumarate) in-

itiated by tert-butyl perbenzoate with different reactive

diluents, styrene, monochloro styrene, t-butyl styrene

and vinyl toluene, and found two distinct exotherms: a

greater one with a maximum between 130�C and 140�C,

while the second small exotherm was detected between

160�C and 190�C. Reaction heats and temperatures of

both peaks depended on reactive diluent but the author

proposed that second exotherm was probably due to the

initiator decomposition, stating that polyester homo-

polymerization should have a maximum at about

200�C. The proof that higher-temperature exotherm is

in fact caused by homopolymerization of polyester

unsaturations can be seen from further Kubotas (1975)

experiments where addition of styrene to the resin

reduced the high-temperature exotherm. Different in-

itiation mechanisms or promoter effects are often

supposed to cause the double reaction exotherms

(Martin, 2001; Yang, 2002). It has to be stressed that

in such cases both maximums appear at lower tempera-

tures than in our experiments and since there is no

initiator or promoter in our system the measured events

are caused by resin itself.

-5

-4

-3

-2

-1

0

1

2

3

4

5

50 100 150 200 250 300T ˚C

H m

W

uncured resin

gel 6 mm

gel 1.5 mm

liquid phase 1.5 mm

liquid phase 6 mm

Fig. 1. DSC traces of uncured resin, and residual reactivity of

liquid phases and gels of resin samples 1.5 and 6 mm thick cured

to 4.45 kGy are shown.

I. Puci!c, F. Ranogajec / Radiation Physics and Chemistry 67 (2003) 415–419416

Some authors assigned the higher-temperature

exotherm to homopolymerization of styrene (Lu, 2001,

1998) or polyester (Avella, 1985). Although homopoly-

merization of polyester is still considered not to be very

likely in presence of styrene, this approach should be

reviewed by taking the role of microgels into account.

The formation and subsequent microgel shrinkage

further reduces diffusion of styrene inside the coil so

many polyester double bonds remain unreacted and

their homopolymerization is to be expected if the

reaction system is heated to high enough temperature.

Based on this, we assign lower-temperature part of the

exotherm maximum of liquid phase between 115�C and

145�C to styrene–polyester copolymerization and styr-

ene homopolymerization and we assign the higher-

temperature part of the exotherm in liquid phase and

single reaction exotherm in gels at about 180–200�C to

polyester homopolymerization. No low-temperature

process in gels implies that all the styrene had previously

reacted.

Further insight into changes related to phase separa-

tion and thickness effects was gained by FTIR spectro-

scopy. All the spectra were normalized to the polyester

absorption at 1730 cm�1 and are shown in Fig. 2.

Absorptions were assigned according to Paauw (1991,

1993) and Delahaye et al. (1998). The spectra of liquid

phases of partially cured resin samples mutually differ

very little and those differences are not related to total

dose or sample thickness. Styrene absorptions at 1495,

778 and 701 cm�1 are less pronounced than in spectra of

uncured resin indicating that significant part of styrene

has already been included in gel. In FTIR spectra of

2000

1953

1879

164

816

31

160

1 1578

1495 14

50

138

3

1293

1260

1158

1072

980

912

778

744

701

554

1.5 mm 4.5 kGy

1.5 mm 6.7 kGy

6 mm 6.7 kGy

6 mm 4.5 kGy

8.9 kGy

6.7 kGy

4.5 kGy

1800 1600 1400 1200 1000 800 600 400

uncured resin

gel 1.5 mm

gel 6 mm

liquid phases

v cm-1

1730

8.9 kGy

6.7 kGy4.5 kGy

Fig. 2. FTIR spectra of unreacted UP resin, liquid and gel phases of partially cured polyester samples 1.5 and 6 mm thick.

I. Puci!c, F. Ranogajec / Radiation Physics and Chemistry 67 (2003) 415–419 417

gels, the increase in absorption intensity of phenyl ring

vibration at 701 cm�1 with dose confirms that styrene

concentration increased. At the same time, styrene vinyl

absorption at 778 cm�1 decreased meaning that styrene

either copolymerized with polyester or homopolymer-

ized and can be seen from the weak absorption at

762 cm�1 (Urban, 1991) that emerges at highest doses.

This confirmed the assumption that phase separation

during crosslinking of UPE resin was caused by

difference in chemical composition of phases. The higher

styrene concentration in gels that, according to 778 cm�1

absorption, has reacted and the temperature of the

exotherm maximum indicating that only polyester

unsaturations remained unreacted after initial radiation

crosslinking, can be explained by microgel formation.

Microgels have high concentration of unreacted polye-

ster double bonds inside, while most of the styrene forms

a layer on their surface. That layer very likely includes

some short polystyrene chains linked to polyester core.

Such structures can be viewed as building blocks of gel.

When gels are reheated during residual reactivity

determination by DSC, polyester double bonds homo-

polymerize and cause high-temperature exotherm. Since

great part of styrene of unreacted resin get included

in gel, concentration of polyester in liquid phase

increased and because of its incompatibility with

styrene layer on microgels, phase separation occurred.

As the dose increased, polyester coils from liquid phase

become included in gel, probably by reacting with

styrene layer and in the end the whole sample turns

to gel.

Almost no influence of sample thickness or dose on

reaction heats of liquid phase was detected but in gel

phase both residual heat and styrene concentration

strongly depended on dose as seen in Fig. 3. In 6 mm

thick samples, the residual heat was significantly higher

at the lowest dose but lower at the highest dose and the

concentration of reacted styrene (seen from the ratio of

absorption intensities at 701–778 cm�1) showed the same

power dependence on dose, while in 1.5 mm thick

samples those dependences were linear. Such beha-

viour is due to heat generated during crosslinking

that in thicker samples cannot be dissipated into the

environment. Although radiation crosslinking was

performed at 21�C, local temperature increase led

to decrease of viscosity and increase of styrene diffusion

into the polyester coils. Huang (1993) found that at

higher-temperatures microgels tend not to rapidly

overlap resulting in greater number of microgels

and increase in polyester conversion which can

explain appearance of gel at higher dose in thicker

samples.

4. 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 the same temperature range

as the higher-temperature exotherm of liquid phase

was detected. Higher concentration of styrene in gels

seen in FTIR spectra can be explained by the formation

of styrene and polystyrene layer on microgels that form

the gel, but with unreacted polyester unsaturations

inside.

The course of reaction depended on sample thickness–

residual reactivity and styrene concentration of 6 mm

gels showed power dependence on dose while in 1.5 mm

thick samples the dependence was linear. The heat

generated by crosslinking influenced microgel formation

and shifted the appearance of gel to higher doses in

thicker samples.

-80

-60

-40

-20

0

2.00 4.00 6.00 8.00 10.00D kGy

H m

W

0.1

0.3

0.5

0.7

0.9

A70

0

DSC 1.5 mm

DSC 6 mm

IR 1.5 mm

IR 6 mm

Fig. 3. Dose dependence of residual reaction heats and

700 cm�1 styrene absorptions (A700) of 1.5 and 6 mm thick gel

phases of partially cured polyester samples.

I. Puci!c, F. Ranogajec / Radiation Physics and Chemistry 67 (2003) 415–419418

References

Avella, M., Martuscelli, E., Mazzola, M., 1985. Kinetic study of

the cure reaction of unsaturated polyester resins. J. Therm.

Anal. 30, 1359–1366.

Bergmark, P., Flodin, P., 1987. Structure of the carbon chain

polymer in unsaturated polyesters. Polymer 28, 1662–1666.

Day, D.R., 1994. Cure characterization of thick SMC parts

using dielectric and finite difference analysis. J. Reinf. Plast.

Compos. 13 (10), 918–926.

Delahaye, N., Marais, S., Saiter, J.M., Metayer, M., 1998.

Characterization of unsaturated polyester resin cured with

styrene. J. Appl. Polym. Sci. 67 (4), 695–703.

Dobo, J., 1985. Some features of radiation processing in the

plastics industry. Radiat. Phys. Chem. 26 (5), 555–558.

Hsu, C.P., Lee, L.J., 1993a. Free-radical crosslinking copoly-

merization of styrene unsaturated polyester resins.1.

Phase Separation Microgel Formation Polymer. 34 (21),

4496–4505.

Hsu, C.P., Lee, L.J., 1993b. Free-radical crosslinking copoly-

merization of styrene unsaturated polyester resins.3. Kinet.

Gelation Mech. Polymer. 34 (21), 4516–4523.

Huang, Y.J., Chen, C.J., 1993. Curing of unsaturated polyester

resins-effects of comonomer composition. III. Medium

temperature reactions. J. Appl. Polym. Sci. 48, 151–165.

Kubota, H., 1975. Curing of highly reactive polyester resin

under pressure: kinetic studies by differential scanning

calorimetry. J. Appl. Polym. Sci. 19, 2279–2297.

Lu, M.G., Kim, S.W., 2001. Unsaturated polyester resin based

on recycled pet: preparation and curing behavior. J. Appl.

Polym. Sci. 80 (7), 1052–1057.

Lu, M.G., Shim, M.J., Kim, S.W., 1998. Curing behavior of an

unsaturated polyester system analysed by avrami equation.

Thermochim. Acta 323, 37–42.

Martin, J.L., 1999. Kinetic analysis of an asymmetrical DSC

peak in the curing of an unsaturated polyester resin

catalysed with MEKP and cobalt octoate. Polymer 40

(12), 3451–3462.

Paauw, M., Pizzi, A., 1991. A FTIR analysis method for

simple unsaturated polyesters. J. Appl. Polym. Sci. 42,

1377–1384.

Paauw, M., Pizzi, A., 1993. Completion of unsaturated

polyesters analysis by FTIR. J. Appl. Polym. Sci. 48,

931–934.

Puci!c, I., Ranogajec, F., 1995. DC-electrical conductivity as a

method for monitoring radiation curing of unsaturated

polyester resins, 1. Measurement conditions and compar-

ison with extraction analysis data. Radiat. Phys. Chem. 46

(3), 365–370.

Urban, M.W., Gaboury, S.R., Provder, T., 1991. Spectro-

scopic evidence for styrene homopolymerization during

polyester/styrene crosslinking. Polym. Commun. 32 (6),

171–175.

Yang, H., Lee, L.J., 2002. Effects on resin chemistry on redox

polymerization of unsaturated polyester resins. J. Appl.

Polym. Sci. 84 (1), 211–227.

I. Puci!c, F. Ranogajec / Radiation Physics and Chemistry 67 (2003) 415–419 419