Reduction of styrene in the environment and the workplace

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Styrene (C₈H₈, CAS #100-42-5) monomer is a Volatile Organic Compound (VOC) which is an ideal cross linking agent for the unsaturated polyester resins used in commercial fibre-reinforced polymer matrix composites.  More than one million tonnes of fibreglass and carbonfibre composites are manufactured each year in each of North America, Europe and Asia.  Alternative monomers have been investigated, but none can match the combined performance and cost-effectiveness of styrene.  Styrene evaporates from exposed uncured resin surfaces into the atmosphere during processing until the resin is fully cured.

The styrene monomer can impact on local air quality [1] with an odour threshold 0.08 [2] to 0.32 [3] parts per million (ppm) and a Photochemical Smog environmental impact classification factor (relative to ethene) of 0.412 [4].

Styrene is implicated in a number of health issues [5].  The United States National Toxicology Program Report on Carcinogens 13th Edition states that “Styrene is reasonably anticipated to be a human carcinogen".  That analysis is disputed by Rhomberg et al [6] and Collins et al [7].  Other health effects include acute toxicity, skin and eye irritation and repeat dose toxicity.  It is anticipated that styrene will be subject to a rigorous assessment in the context of the REACH (Registration, Evaluation and Authorisation of Chemicals) legislative system.  Table 1 summarises the styrene odour and time weighted average occupational exposure levels (OEL).

Plymouth University was a prime mover in the introduction of closed mould technologies for composites manufacture during the 1990s and latterly vacuum infusion processes for larger structures [17, 18].  However, many mouldings have a cosmetic resin-rich surface finish (gel coat) which is applied to the open mould tool and allowed to partially cure before the reinforcement lamination takes place.  Conventional hand gel-coating technology has been demonstrated to have significantly higher levels of styrene emission than two new closed mould processes [19, 20] developed in the FP7 InGeCt project lead by Plymouth University.  For the open mould process, the average styrene levels were in the range 28-70 ppm.  The two closed mould technologies significantly reduce the measured styrene levels to lie in the range 0.2-0.4 ppm.  The new processes offer a reduction in average styrene emission levels of >98% (worst new/best old) with obvious benefits for worker health and the reduction of environmental burdens.

Forthcoming styrene risk assessments by the European Union, monitored by European Chemical Industry Council (CEFIC) for the chemical industry, are expected to be the major driver in the near future regarding styrene classification, labelling and use.  Reduced styrene levels in the workplace are expected to improve retention of workforce personnel, minimise release to the environment and reduce odour at the factory boundary.  The European Styrene Producers Association has recently [Faes, 9] recommended that Occupational Exposure Standards should be reduced to a time-weighted average (TWA) of 20 ppm which is unlikely to be achievable without in-mould gel-coating.  Dependent on the REACH analysis, it may become essential to implement in-mould gel-coating to achieve styrene levels "as low as reasonably practicable" (ALARP) and permit the economic continuation of the polyester resin composites industry.

References (Plymouth University authors in bold font)

1. J Park, L Lee, H Byun, S Ham, I Lee, J Park, K Rhie, Y Lee, J Yeom, P Tsai, and C Yoon, A study of the volatile organic compound emissions at the stacks of laboratory fume hoods in a university campus, Journal of Cleaner Production, 2013, 66, 10-18.
2. J May, Solvent odor thresholds for the evaluation of solvent odors in the atmosphere, Staub-Reinhalt, 1966, 26(9), 385-389.
3. JE Amoore and E Hautala, Odor as an aid to chemical safety: odor thresholds compared with threshold limit values and volatilities for 214 industrial chemicals in air and water dilution, Journal of Applied Toxicology, 1983, 3(6), 272-290.
4. Styrene: Part 1 – Environment, European Union Risk Assessment Report, European Chemicals Bureau Institute for Health and Consumer Protection Priority List 1 volume 27, European Commission Joint Research Centre report EUR 20541 EN, 2002.
5. J Challenor and D Wright, Aggression in boat builders: a search for altered mood states in boat builders exposed to styrene, Occupational Medicine, 2000, 50(3), 185-192.
6. LR Rhomberg, JE Goodman and RL Prueitt, The weight of evidence does not support the listing of styrene as “Reasonably Anticipated to be a Human Carcinogen” in NTP's Twelfth Report on Carcinogens, Human and Ecological Risk Assessment, 2013, 19(1), 4-27.
7. JJ Collins, KM Bodner and JS Bus, Cancer mortality of workers exposed to styrene in the US reinforced plastics and composite industry, Epidemiology, March 2013, 24(2), 195-203.
8. BT Åström, Manufacturing of Polymer Composites, Chapman and Hall, London, 1997. ISBN 0-412-81960-0.  PU CSH Library.
9. E Faes , Styrene Industry Recommends Occupational Exposure Limits, Styrene Producers Association letter, undated circa 2011.
10. Norsodyne® 6417 Version Unsaturated Polyester Resin Technical Data Sheet, Cray Valley, Stallingborough - Lincolnshire, 2005.
11. HSE information sheet: Assessing and controlling styrene levels during contact moulding of fibre-reinforced plastic (FRP) products, Health and Safety Executive Plastics Processing Sheet 14, Sudbury - Suffolk, 2003.
12. Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs) - Styrene, Centers for Disease Control and Prevention, Atlanta - Georgia, 1994
13. Styrene IDLH Documentation, Centers for Disease Control and Prevention, 16 August 1996.
14. Styrene monomer, in AIHA Hygienic guide series. American Industrial Hygiene Association, Akron OH, 1959.
15. D Hammond, A Garcia and HA Feng, Occupational exposure to styrene vapor in a fiber-reinforced wind blade manufacturing plant, Annals of Occupational Hygiene, 2011, 55(6), 591-600.
16. RRD Stewart, HC Dodd, ED Baretta and AW Schaffer, Human exposure to styrene vapor, Archives of Environmental Health, 1968, 16(5), 656-662.
17. J Summerscales and TJ Searle, Low-pressure (vacuum infusion) techniques for moulding large composite structures, Proc I Mech. Eng. Part L: Journal of Materials: Design and Applications, 2005, 219(1), 45-58.
18. J Summerscales, Resin infusion under flexible tooling (RIFT), In L. Nicolais, A. Borzacchiello, S.M. Lee (Eds.), Encyclopedia of Composites (second ed.), John Wiley and Sons, Hoboken NJ, 2012, pp. 2648–2658.
19. W Rogers, CP Hoppins, ZJ Gombos and J Summerscales, In-mould gel-coating of polymer composites: a review, Journal of Cleaner Production, 2014, 70, 282-291.
20. C Di Tomasso, ZJ Gombos and J Summerscales, Styrene emissions during gel-coating of composites, Journal of Cleaner Production, 2014, 83, 317-328.