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MIGRATION OF STYRENE FROM UNSATURATED POLYESTER RESINS

Introduction

Unsaturated polyester (UP) resins are versatile plastics used a wide variety of applications. Although not

a primary food packaging material, UP resins is used in water pipes, vats and work surfaces. In these

applications, UP resins have the potential for either continuous or repeated contact with food or water. In

this report, recommendations are provided to processors and converters of UP resins, to reduce the levels

of styrene migration from UP resins in compliance with anticipated legislation from the Food Contact

Directive and the Water Framework Directive.

Experimental

Four commercially available UP resin based materials were subjected to a number of post cure conditions

as described in Table 1.

Material Post cure conditions applied

SMC none

8h @ 140C

1h @ 170C

2h @ 170C

Solid surface > 4 weeks @ 20C

8h @ 80C

8h @ 120C

Gelcoated Flat Sheeting None

Flat sheeting 8h @ 80C

Flat sheeting 8h @ 120C

GRP Pipe (resin-rich liner) None

8h @ 80C

8h @ 110C

Table 1: Post cure conditions applied to UP resin based materials

Styrene migration into three food simulants, 3% acetic acid, 10% ethanol and olive oil, was determined

using single sided migration cells. The exposure conditions for the migration measurements were 2 hours

at 70C and 40 days at 40C. The residual styrene levels for all samples were measured to assess the

possible correlation between migration of styrene into the stimulants and the residual styrene content in

the material. In cases where the material was gel-coated or had a resin-rich surface then this surface wasthe simulant contact surface.

Results

The lowest migration results for all conditions were observed in the flat sheeting samples and even

though the gel coat surface, which is resin-rich, was the contact surface the effect of post cure had little

effect on the styrene migration results. This suggests that the thin material is almost completely cured

during the oven curing manufacturing process. The residual volatile tests confirmed the high level of cure

present in the as received material.

The room temperature cured solid surface material was expected to contain a high level of residual

styrene and, hence, high migration levels, which are associated with non post cured unsaturated polyester

resin. However, after moderate postcure at 80C the residual styrene content and styrene migration levels

were very low.

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The relatively high residual styrene content and styrene migration levels for the as received SMC were

surprising for material, which is produced using high temperature moulding conditions. However, the

short cycle times and the low thermal conductivity of SMC can, in relatively thick material, result in the

bulk of the material being less well cured than the surfaces hence, penetration of simulant into the bulk

material at a molecular level or via imperfections, such as pinholes, could result in the extraction of

styrene from the bulk material. High temperature post cure reduced both residual styrene content and

styrene migration to very low levels.

The glass reinforced unsaturated polyester resin (GRP) pipe samples gave the most variable data with

high migration levels even when post cured at elevated temperature. However, the residual styrene levels

were low for the post cured samples. The probable explanation for these unexpected results with the pipe

samples is that the resin rich surface could adversely affect the migration result since the resin content in

such a layer is high compared with the bulk sample and the other materials tested. It is also possible that

a long time delay (days or weeks) between manufacture and post cure could result in a less than fully

cured surface layer. The low residual styrene content result from the post cured samples of pipe, which

contradicts the high styrene migration levels, occurs because the residual styrene is measured for thewhole sample and not simply for the liner.

Conclusions

Apart from the pipe sample, the correlation between styrene migration and residual styrene content is

sufficiently encouraging to speculate that by simply measuring the residual styrene level of food contact

products manufactured from unsaturated polyester resin an assessment of an acceptable styrene migration

level can be made. In cases where the material is not homogeneous, the residual styrene levels in the food

contact surface will need to be assessed, rather than carrying out a test on the bulk material.

From the SMC, flat sheeting and solid surface test data, it would be reasonable to deduce that residual

styrene levels below 0.0005% would result in styrene migration levels into the three simulants of below

10 ppb. At residual styrene levels below 0.025% then it is likely that styrene migration will be below 200

ppb. The data is too limited to produce accurate correlation curves; however, a general conclusion is that

many components currently manufactured in GRP for food contact applications will require elevated

temperature cure or post cure, often for several hours depending upon their thickness, for the migration

levels of styrene to be considered sufficiently low enough for the intended applications.