Mechanical Properties
The mechanical properties of molded foam depend primarily on density. Generally strength characteristics increase with density, however the cushioning characteristics of EPS 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 foam packaging industry has developed typical cushioning curves for use by designers. Shock cushioning properties of molded foam 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 -17C and 43C.
Density
Packaging density must be considered when choosing the correct level of cushioning needed for the job. In the preliminary design stages cushion curves developed from dynamic drop testing are used to determine the correct package configuration - foam thickness and density - to adequately protective the product. By varying the density, thickness and shape of the molded foam, the designer can meet the protection requirements of a wide range of delicate products.
Dimensional Stability
Dimensional stability is another important characteristic. It represents the ability of a material to retain its original shape or size in varying environmental conditions of use and exposure to changes in temperature and/or relative humidity. Molded foam offers exceptional dimensional stability, remaining virtually unaffected within a wedge range of ambient factors. The maximum dimensional change of EPS foam can be expected to be less than 2%, which puts EPS in accordance with ASTM Test Method D2126.
Thermal Insulation
For construction insulation applications the polystyrene foam industry has developed test data as reported in ASTMC 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.
Water Absorption and Vapor Transmission
Moisture resistance is the ability of a material to prevent water from entering its structure and eroding its mechanical properties. The cellular structure of molded foam 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 foam 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 molded EPS, ARCEL or R-MER
Chemical Resistance
Water and aqueous solutions of salts and alkalis do not affect EPS, ARCEL or R-MER, while many organic solvents are not compatible. This should be taken into consideration when selecting adhesives, labels and coatings for direct application to the product. All substances of unknown composition should be tested for compatibility. Exposing molded foam to the substance at 120 - 140 F may carry out accelerated testing. UV radiation has a slight effect on molded polystyrene. It causes superficial yellowing and friability, but does not otherwise affect its physical properties. |
