Improving the processability of expandable polystyrene beads with glycerol esters

Martijn Jansen, Applications Scientist at Corbion

From food trays and disposable cups to electrical goods packaging and building insulation, expandable polystyrene (EPS) has a wide variety of uses. The value of the global EPS market is expected to reach $17.7 billion (£14 billion) by 2020, as its excellent water resistance, high stiffness and relatively light weight make it an increasingly popular choice for plastics manufacturers across several industries. The largest regional market is currently Asia-Pacific, representing almost half of global demand for EPS. However, with the Middle East predicted to become the world’s fastest growing construction market, the need for low density EPS products for use in insulation material is likely to grow exponentially.

With high-end applications come high expectations, and EPS producers are expected to deliver low density after expansion, a homogeneous foam structure, good mechanical strength at higher temperature and no water uptake by the foamed beads. In order to achieve the required performance of the EPS foam in a timely and cost-effective manner, foam manufacturers are looking for a short maturation period, fast moulding cycle times and easy mould release.

To reach the desired foaming and final application properties of EPS, processing aids are typically required. Offering a range of polymer additives for the manufacture of EPS beads via a suspension polymerisation process, Corbion attended Arabplast for the first time this year to meet potential customers in the Middle East. Here, Martijn Jansen, application scientist, focuses on the advantages offered by glycerol esters as anti-static agents and processing aids. He explains why a small particle size and consistent distribution of the particles is particularly important.

A sticky problem

If left untreated, the surfaces of EPS particles have poor processing characteristics. For example, EPS beads tend to acquire various quantities of static charge. This can result in the agglomeration of beads, which may hamper the screening process and cause flowability problems. The presence of electrostatic charges in the pre-foamed particles may also lead to inhomogeneous filling of moulds during the final step of the expansion process.

Published literature suggests that glycerol esters can be used as anti-static agents and processing aids for EPS beads. For example, Corbion’s PATIONIC 909 is a glycerol monoester and can be added prior to the pre-expansion stage to reduce the agglomeration of EPS beads. The polar hydroxyl groups from the glycerol moieties are sensitive to moisture in the air and this results in the dissipation of the static electric charges. Meanwhile, PATIONIC 919, a glycerol triester, facilitates the merging of the EPS beads during the final expansion step in the mould and thus contributes to a homogeneous foam. The use of PATIONIC 919 will also result in an easier release of the EPS-foam from the mould.

Customer test feedback confirms that PATIONIC glycerol esters are particularly well-suited to EPS beads, due to the consistent distribution and very small size of the additive particles. The narrow particle size distribution of both PATIONIC 909 and 919 is thought to result in a more homogeneous layer around the EPS bead, so that the effectiveness of both additives on the EPS foam quality and the final foaming process is increased.

The chemistry explained

Glycerol esters are combinations of glycerine and fatty acids, which occur naturally as triglycerides in vegetable oils and animal fats. The chemical nature of the fatty acids (i.e. the chain lengths and level of saturation) determine the final properties of the glycerol esters, including migration in polymers, lubrication properties, volatility, shelf-life, crystallinity and surfactant activity. Glycerol stearates are predominantly based on stearic acid and have a solid appearance at room temperature, due to high levels of saturated fatty acids. Other glycerol esters have high levels of unsaturated fatty acids, such as oleic acid, and the resulting glycerol oleates are semi-solid or liquid at room temperature.

The majority of industrial applications for glycerol esters require mono or mono/di esters for optimum performance. The naturally occurring triester undergoes glycerolysis and, after excess glycerine is removed, the final product of this chemical reaction is the mono/diester and a small amount of triester. Distillation can then be used to further refine to a 95% monoester product.

To obtain high levels of saturation, the triesters that have been used in the production of mono/diesters can be chemically hardened by hydrogenation.

Expanding solutions

When selecting an anti-static agent, it is important to strike the right balance between cost, quality and performance. Glycerol esters typically offer advantages over other polymer additives such as

quaternary ammonium salts, alkylphosphates and ethoxylated-propoxylated fatty alcohols. For example, glycerol ester products can be produced directly from vegetable oils as Corbion does, without the need to prepare separate reactants or use more complex chemistry that produces waste materials.

As to the future, Corbion’s polymer chemists and applications engineers are currently partnering with EPS producers to identify additional ways in which its high quality polymer additives can act as processing aids throughout the foaming process. This may eventually result in new polymer additive products to further optimize the final EPS properties, as Corbion continues to work closely with customers to overcome their specific processing challenges.