To start with, polyethylene is a rigid plastic, but it can be foamed in different ways. We at NMC Cellfoam use foamed polyethylenes almost exclusively.
To manufacture foamed polyethylene plastics we primarily use low-density polyethylene (PE-LD), cross-linking and foaming agents as well as various additives, such as fireproofing and colouring agents, that improve the properties of the final product and expand its range of application.
The foaming can be conducted mechanically, chemically or physically. Foaming agents help to form the foam-like structure. In the manufacturing process of plastic foams, chemical foaming agents produce gases as they react, while physical foaming agents turn into gas during the process. The typical densities of the resulting cellular plastics range from 24 to 300 kg/m3.
Polyethylene cellular plastics are either cross-linked or non-cross-linked materials, which affects their mechanical properties as well as their surface quality and recyclability.
Chemically cross-linked XLPE cellular plastics
In extrusion, the material mixture is plasticated and shaped into a thin string through an extrusion head in a continuous process, after which the polymer chains are cross-linked and the material swells, or foams, in the oven. The end result is a foamed polyethylene material that can be converted into rolls or sheets, for example.
Chemically cross-linked cellular plastic can also be manufactured as batches of larger sheets using a method where the material mixture is poured into a mould where it foams and swells. A shell forms around the blank created in the mould, which is then removed prior to use.
XLPE cellular plastic has improved mechanical properties over non-cross-linked cellular plastic, and typically its surface is rougher and its cellular structure slightly larger than in physically cross-linked plastics. The material is manufactured as 3–15 mm thick layers using extrusion, and thicker materials are achieved through layering and thermal linking. When manufactured in batches, the plastic foam sheets can be over 100 mm thick. XLPE cellular plastics are used for thermal insulation, product packaging, sealing and in personal protective equipment, for example.
Physically cross-linked IXPE cellular plastics
The manufacturing process of physically cross-linked cellular plastic contains several stages, and the cross-linking is typically conducted by irradiation, which produces a different and smaller cellular structure for the plastic material. This typically improves the tensile properties of the material and results in smoother and better-looking surfaces.
IXPE cellular plastic can be manufactured at thicknesses of under 0.5 mm, which is significantly thinner than XLPE plastics. Physically cross-linked polyethylene is also used for thermal insulation, food and product packaging, sealing and in personal protective equipment.
Cross-linked Zotefoam XLPE cellular plastics made with high pressure
Extruded and cross-linked sheets are foamed with nitrogen in Zotefoam’s unique manufacturing process. The sheets are first saturated with nitrogen in autoclaves using high pressure and heat. In the second stage the saturated sheets are foamed in the autoclaves using low pressure and high heat. This flexible manufacturing process enables the use of a wide variety of plastic raw materials, and the end result is an extremely consistent and fine cellular plastic structure.
The autoclave process only allows for the manufacture of sheet-like parts, and the process creates a shell on the surface of the sheets that is then removed. The manufactured sheets can then be joined together in post-processes to form thicker sheets or rolls.
Zotefoam cellular plastics are clean and extremely consistent high-grade products. Their mechanical features are excellent, and when using them one can often opt for a more lightweight foam plastic than with traditional chemically cross-linked foam plastics. Cross-linked Zotefoam polyethylene cellular plastic is used for thermal insulation, product packaging, sealing, personal protective equipment and in healthcare products that require special cleanliness and good disinfection properties.
Non-cross-linked EPE cellular plastic
Non-cross-linked polyethylene cellular plastic has the weakest mechanical properties, and it is the most affordable material. It also has the simplest manufacturing process, as extrusion foaming using temperature and pressure changes and an agent such as liquid butane (C4H10), for example, creates the final shape for the material immediately as it leaves the extrusion head. The process can be used to manufacture thicker materials, and the cell size is typically larger than in cross-linked cellular plastics.
The benefit of EPE cellular plastic is its excellent recyclability, as the material can be turned back into raw material and used again for the manufacturing process. It is often used as padding in product packaging and in various flotation devices thanks to its buoyancy.
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