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
SMC
Post cure conditions applied
none
8h @ 140°C
1h @ 170°C
2h @ 170°C
Solid surface
> 4 weeks @ 20°C
8h @ 80°C
8h @ 120°C
Gelcoated Flat Sheeting
Flat sheeting
Flat sheeting
None
8h @ 80°C
8h @ 120°C
GRP Pipe (resin-rich liner)
None
8h @ 80°C
8h @ 110°C
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 70°C and 40 days at 40°C. 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 was
the 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 80ºC the residual styrene content and styrene migration levels
were very low.
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Cefic 6th February 2004
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 the
whole 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.
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Cefic 6th February 2004