Expanded polystyrene (EPS) is a generic term for polystyrene and styrene copolymers that are expanded into a variety of useful products. Expanded Polystyrene is supplied to molders in the form of a polystyrene bead; Expanded Polystyrene beads are loaded with a blowing agent, usually pentane, and other chemical agents and additives that give the beads expansion vibrancy and allow them to be processed and molded into low-density foam articles; Expanded Polystyrene is comprised of 90 percent air. The shock absorbing properties and other qualities of Expanded Polystyrene foam, combined with its low-cost, high insulating properties, custom moldability and ease of processing make it a popular packaging material.
End Use Applications
For more than 50 years, the effectiveness of Expanded Polystyrene has been proven in numerous packaging applications used by a wide variety of industries, consumer product manufacturers and catalogue and shipping companies. Lightweight Expanded Polystyrene is ideal for these packaging applications due to its physical properties, in particular its cushioning characteristics, dimensional stability and its thermal and moisture resistance. Custom-molded Expanded Polystyrene interior packaging has been highly effective in protecting sensitive electronic components, consumer goods and office equipment; its moldability allows interior packaging components to hold products snugly in place. High insulating properties have made Expanded Polystyrene a popular choice in the food packaging, medical and pharmaceutical industries. Expanded Polystyrene is also used to protect a myriad of other products used for component assembly, during internal distribution and storage and delivery to the end user.
Because Expanded Polystyrene can be molded into virtually any shape or size, it is well suited to automated production lines. End caps, rails and other interior Expanded Polystyrene packaging pieces can be customized to accommodate the needs of automated integrated production systems; Expanded Polystyrene interior packaging pieces can be quickly and efficiently put into place via automated procedures during the packaging assembly process.
A key benefit of Expanded Polystyrene is that it is recyclable. Expanded Polystyrene materials can be reprocessed and molded into new packaging products or durable goods. Formal Expanded Polystyrene recycling programs have been established in several countries throughout the world.
Engineered for Optimal Performance
Regional Expanded Polystyrene molders utilize a multi-stage production process to expand and mold the beads into Expanded Polystyrene products. They use one of two different processes to expand Expanded Polystyrene heads: continuous pre-expansion and batch pre-expansion.
The mechanical properties of Expanded Polystyrene foam depend primarily on density, as illustrated in Table 1. Generally, strength characteristics increase with density, however the cushioning characteristics of Expanded Polystyrene foam packaging are affected by the geometry of the molded part and, to a lesser extent, by bead size and processing conditions, as well as density. This unique characteristic allows a packaging engineer to fine-tune cushioning performance by simple processing changes, without the need to redesign or retool.
For shock cushioning, the Expanded Polystyrene packaging industry has developed typical cushioning curves for use by designers of Expanded Polystyrene transport packaging. Shock cushioning properties of Expanded Polystyrene are not significantly affected by change in temperature. Recent studies conducted at San Jose State University, Packaging Program, have shown that the optimum performance characteristics of Expanded Polystyrene are not affected by changes between -17 C and 43 C. Packaging engineers should regard the following data as an accurate representation of the performance of Expanded Polystyrene foam.
Packaging density must be considered when choosing the correct level of cushioning needed for the job. In the preliminary design states, cushion curves developed from dynamic drop testing are used to determine the correct package configuration - foam thickness and density - to adequately protect the product. By varying the density, thickness and shape of the Expanded Polystyrene foam, the designer can meet the protection requirements of a wide range of delicate products.
Dimensional stability is another important characteristic of Expanded Polystyrene foam. It represents the ability of a material to retain its original shape or size in varying environmental conditions. Different plastic polymers vary in their reaction to the conditions of use and exposure to changes in temperature and/or relative humidity. Some shrink, some expand and some are unaffected. Expanded Polystyrene offers exceptional dimensional stability, remaining virtually unaffected within a wide range of ambient factors. The maximum dimensional change of Expanded Polystyrene foam can be expected to be less than 2% which puts Expanded Polystyrene in accordance with ASTM Test Method D2126.
For construction insulation applications the Polystyrene Foam industry has developed test data as reported in ASTM C 578 Standard Specification for Rigid Cellular Polystyrene Thermal Insulation. This standard addresses the physical properties and performance characteristics of EPS foam as it relates to thermal insulation in construction applications. There has been no need to develop such a formal document for the packaging industry. EPS is an effective, economical packaging material for produce, pharmaceuticals and other perishables, when these items must be shipped and stored in temperature controlled environments. The uniform, closed cellular structure of EPS is highly resistant to heat flow. The thermal conductivity (k factor) of EPS packaging varies with density and exposure temperature, as shown in Table 2 .
Water Absorption and Vapor Transmission
Moisture resistance is the ability of a packaging material to prevent water from entering its structure and eroding its mechanical properties. The cellular structure of Expanded Polystyrene is essentially water resistant and provides zero capillarity. However, Expanded Polystyrene may absorb moisture when it is completely immersed, due to the fine interstitial channels between the molded beads. While molded Expanded Polystyrene is nearly impervious to liquid water, it is moderately permeable to water vapor under pressure differentials. Vapor permeability is determined by both density and thickness. Generally, neither water nor water vapors affect the mechanical properties of Expanded Polystyrene. See Table 3.
Water and aqueous solutions of salts and alkalis do not affect expanded polystyrene. Most organic solvents are not compatible with Expanded Polystyrene. This should be taken into consideration when selecting adhesives, labels and coatings for direct applications to the product. All substances of unknown composition should be tested for compatibility. Accelerated testing may be carried out by exposing molded polystyrene to the substance at 120 - 140 F. UV radiation has a slight effect on molded polystyrene. It causes superficial yellowing and friability, but does not otherwise effect its physical properties.
The volume resistively of molded polystyrene within the 1.25 - 2.5 pef density range, conditioned at 73 F and 50% r.h. is 4x10 ohm-cm. The dielectric strength is approximately 2KV/mm. At frequencies up to 400 MHz, the perceptivity is 1.02 - 1.04 with a loss factor less than 5 x 10 and less tat 3 x 10 at 400 MHz. Molded Expanded Polystyrene can be treated with anti-static agents to comply with electronic industry and military packaging specifications.
When Properly Engineered there is no substitute for Expanded Polystyrene
The fundamental objectives of transport packaging materials are to preserve and protect a product from damage, through the manufacturing process all the way to delivery to the consumer. Choosing the right packaging material requires a balance of many factors, including ease of handling and storage, weight, cushioning characteristics, manufacturing efficiency, ease of identification, customer requirements, cost and more.
Expanded Polystyrene protective packaging offers a broad range of physical properties to allow packaging designers to meet the many challenges of protection and distribution. These properties, in combination with appropriate engineering design considerations, provide the design flexibility required to create truly cost effective protective packaging.
Information taken from AFPR | Alliance of Foam Packaging Recyclers | Technical Bulletin
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