The Sulzer EPS process represents a unique, patented process for the continuous production of EPS micro pellets by melt impregnation. The process includes a proprietary combination of dosing devices, static mixer(s), cooler(s) and an underwater pelletiser. The Sulzer EPS process takes advantage of static mixer technology for the dispersion and dissolution of a pentane blowing agent into the polymer melt as well as the admixing of heat-sensitive additives.
The technological benefits of the process include: formulation flexibility through excellent dispersion of pigments, efficient temperature and shear control throughout the process, the ability to recycle excess impregnated material and the production of EPS micro pellets with a narrow size distribution. EPS micro pellets from the Sulzer EPS process can be processed and converted into foamed parts, applying the same processes as for conventionally-produced EPS beads.
The industrial polymerisation of styrene into polystyrene (PS) was developed in the early 20th century, and was followed by the development of expandable polystyrene particles (EPS) approximately 50 years later.
EPS is a particle foam prepared by expanding PS beads that contain a physical blowing agent, usually pentane or n-butane. Steam heating causes the beads to expand and the final shaped part is achieved by moulding the pre-expanded beads with steam and pressure.
The typical chain of production involves EPS resin suppliers that produce polystyrene resin granulates impregnated with the blowing agent in large scale industrial facilities and EPS moulders who manufacture the end products like packaging, cups and construction materials according to customer specifications.
The classical EPS production process developed in the 1950s consists of the batch polymerisation of styrene monomer in a suspension medium, typically water, with a stabilisation agent. Radical polymerisation reaction occurs in the monomer-rich droplets and vigorous stirring and the stabilisation agent are used to control the droplet distribution.
At approximately 66% reaction conversion, a physical blowing agent is introduced into the reaction vessel under pressure. The polymerisation reaction continues in the presence of the blowing agent to allow for the impregnation of the blowing agent into the suspended polystyrene particles. Impregnation is controlled through the careful adjustment of the process parameters, which include stirring parameters, reaction time, and temperature.
Subsequently, the reaction is stopped and the vessel is cooled and depressurised. The suspension process produces particles with a distribution of diameters between 10μm and several millimetres. The suspension product needs to be washed, sieved and coated before being employed for moulding applications. Even though modern suspension lines produce much narrower distributions than in the past, the product is still sieved into between three and five fractions that are sold separately.
Commercial EPS applications require beads with diameters preferably in the range of 0.4-1.6mm. A small fraction of off-grade material with particle sizes being smaller or larger than any commercially useable grades is always produced by suspension processes. In addition to this, the key drawback from the suspension process is the required use of a suspension medium, the need to wash the EPS particles after collection and restrictions on the additives that can be added during the reaction. Additives, such as pigments or other fillers, must be either water or monomer soluble to allow for addition before or during the polymerisation reaction.
Sulzer EPS melt impregnation process
In the 1990s the need for an alternative process to produce EPS became apparent with the demand from converters to apply pigments to their moulded products and with the development of impact-strengthened EPS. These materials could not easily be prepared by the suspension process due to the non-solubility of the additives in the sensitive suspension formulations and process.
In 1994 Sulzer Chemtech responded to this demand by introducing a process for the production of EPS based on melt impregnation with static mixers. This process offers the possibility of economically producing EPS with a formulation flexibility that is not easily obtained in the suspension polymerisation process.
The Sulzer EPS process for the continuous melt impregnation of a polymer with a physical blowing agent and various additives consists of a proprietary combination of dosing devices, static mixers, coolers and an underwater pelletiser (See Figure 1).1,2 The process takes advantage of proprietary static mixer technology for the dispersion and dissolution of the pentane blowing agent into the polymer melt as well as the admixing of heat-sensitive additives.
In comparison to an extruder, static mixers have a uniform, low shear field over the cross-section and allow for efficient heat removal, thus avoiding degradation of the polymer resin or heat-sensitive additives. The product quality is consistent and can easily be controlled, as the additives and the blowing agent are directly injected into the melt (See Figure 2). Sulzer Chemtech offers the EPS process with capacities that range from pilot scales to large scale industrial plants.
Benefits of the Sulzer EPS process
The Sulzer EPS process is a continuous process resulting in continuous product quality. In comparison to the classical suspension process, the Sulzer EPS process additionally offers the customer processing flexibility, environmental benefits and energy-saving possibilities:
1. The raw material for the process is polystyrene, although suitable grades of GPPS, HIPS and recycled polystyrene materials may also be employed;
2. A wide range of additives (also pigments) can be dispersed in the polymer matrix, thereby enabling production of EPS with improved insulation values;
3. Narrow EPS micro pellet size distribution resulting in minimal waste production and no need to sieve the product;
4. Negligible process water consumption as compared to the classical suspension process;
5. Possibility to directly recycle waste impregnated pellets and/or beads;
6. Economical, compact and easy operation;
7. High innovation potential (alternative polymers, blowing agents, additives) for in-house development of individual recipes for future applications.
Dispersion: Pigments for thermal insulation
Three mechanisms contribute to the thermal conductivity of EPS: conduction, convection and radiation. The thermal insulative properties of EPS foam are impacted by the structure and contents of the foam, that is the foam density, choice of blowing agent and cell size distribution.
The advantages of EPS foam as insulation material over competing insulation solutions, such as mineral wool, polyurethane, and extruded polystyrene (XPS), are its low density and low cost. As the density of the EPS foam decreases, the share of infrared radiation passing through the material strongly increases. Therefore, pigments such as graphite, carbon or aluminium particles are added to the EPS to absorb and/or reflect infrared radiation and thus improve thermal insulation.
With the addition of pigments, the insulative property of low-density EPS can reach the same level as with EPS with a density four times higher. The optimised mixing technology of the Sulzer EPS process, as well as the right combination of additives in the process, leads to an improved dispersion of the pigments in the final product. The improved dispersion in the Sulzer process results in a lower additive consumption that is required to meet similar insulative effects as compared to other processes (See Figure 3).
Flexiblity: Use of HBCD alternatives
EPS foam, unless formulated with flame retardants, does not fulfil common building codes with respect to flame spread and smoke development. Therefore an additional flame barrier must be employed in the construction of the foam, otherwise the foam must be impregnated with a suitable flame retardant.
The most commonly used flame retardant for EPS is hexabromocyclododecane (HBCD), a highly-brominated flame retardant. HBCD has been the flame retardant of choice in EPS for several decades due to its high efficiency. Recently, however, the toxicity and the environmental impact of HBCD has become a concern. It has been shown that HBCD bioaccumulates and biomagnifies and several environmental protection agencies have now put the chemical on their lists of concerns. This development has prompted the EPS industry to start the search for a substitute for HBCD. Due to the efficient temperature and shear control, the Sulzer EPS production process is so flexible as to allow the use of any new flame retardant currently in development.
Simplified Sulzer EPS process
Using the in-depth process expertise from more than a decade of EPS process development, Sulzer Chemtech has developed a second-generation EPS process suited to small scale production (See Figure 4). Using the combination of a twin-screw extruder, in which PS, the blowing agent and all of the required additives are compounded, as well as Sulzer's proprietary melt coolers, Sulzer has designed a simplified manufacturing unit that is attractive for smaller capacities of about 500–3000kg/hr. This extruder process, the result of a joint development with the German extruder manufacturer Coperion, allows for the economic production of EPS specialities, even on scales adapted to the requirements of larger converters.
Sulzer EPS Process development: 100% recycled EPS
Sulzer Chemtech is continuously working to meet anticipated market demand and is presently developing a simplified one-step, direct, recycle process for post-commercial and post-consumer waste EPS. The basic concept includes the compacting of post-commercial and post-consumer PS foam with a blend of nucleating agents and other additives.
The resin melt is impregnated with a physical blowing agent, in this case a mixture of n- and iso-pentane, the additives and blowing agent are dispersed using static mixer(s). The melt is cooled with a mixer/cooler and the mixture is subsequently pelletised using an underwater pelletiser (See Figure 5). Additionally, a second feed stream can be included into the design to allow for the addition of temperature-sensitive additives, such as flame retardants.
In a feasibility study, EPS is produced from 100% recycled post-commercial and post-consumer PS foam containing flame retardant; the material is compacted into PS, nucleating agents are admixed and the melt is impregnated with a physical blowing agent. Static mixers are used to evenly disperse both the nucleating material and the blowing agent throughout the resin system. Subsequently EPS granulate is produced. The structural and mechanical properties of the resultant foam are found to be equivalent to those produced from virgin general purpose polystyrene (GPPS) using the Sulzer EPS process.
References
1. EP0668139, "Process for preparing expandable plastic granules," Sulzer Chemtech Ltd.
2. EP1925418, "Method and device for manufacturing polymer particles," Sulzer Chemtech Ltd.
