The first natural "expanded" is the wood itself, which is by its nature micro-honeycomb. At an industrial level, however, the first natural and synthetic latex foams were developed in the 1920s, but polystyrene, a thermoplastic polymer, was expanded only in the 1940s.

The use of thermoplastic polymers exploded after the Second World War thanks to the easy availability of unsaturated hydrocarbons (butene, propylene, ethylene, butadiene, etc.) derived from new oil refining methods. Since the 1960s, therefore, various cellular methods were experimented to transform thermoplastic polymers into flexible and semi-flexible foams, and it is precisely at this time that the first productions of expanded polyethylene date back.

Initially polyethylene was expanded to high density using nitrogen as blowing agent. Later, HCFC gases were also used to achieve lower densities. Technological developments such as the double extrusion screw made it possible to improve the extruded production in a continuous cycle, giving a strong impulse to the diffusion of the material.

The Montreal Treaty of 1989 ended the established use of CFC and HCFC gases as blowing agents, stimulating the refinement of available alternative technologies, and in particular chemical blowing.

Today the flexible foam market is dominated by polyurethanes and polyolefins, the most important of which is by far polyethylene. Its foams are available in numerous varieties, both in terms of production method and chemical-physical characteristics.

Low density polyethylene is one of the most versatile polymers on the market. It finds its main use in the production of packaging films, but it is also used in the injection molding of rigid objects and in the extrusion of expanded foams. Among the more technical applications is that for containers and surfaces resistant to corrosion, weldable components, components for which flexibility and resilience are required.

LDPE has a variable density between 910 and 940 Kg/m3, is chemically inert at room temperature, is thermally stable up to 80°C, is very ductile and resilient, offers excellent resistance to acids, alcohols, bases and ethers, and a good resistance to all organic compounds with the exception of halogenated hydrocarbons. It is non-toxic, odorless and resistant to mold and mildew.

Once expanded, polyethylene maintains its chemical properties, with the advantage of a density reduced from 50% to over 98%. Polyethylene foams are used in the most varied sectors due to the wide range of obtainable solutions.

The foam can reach densities of less than 15 kg/m3, reduced thermal conductivity and low dynamic stiffness, while maintaining good resilience and toughness. The mechanical characteristics largely depend on the type of foam and the density.

In the packaging sector, expanded PE is usually used with densities between 14 Kg/m3 and 40 Kg/m3, while EPS with densities ranging from 14 Kg/m3 up to over 600 Kg/m3.

Resilience: Absorbs energy in a short period of time, i.e. resists shocks. Expanded polyethylene is a ductile material at room temperature, it does not crack.

Toughness: absorbs energy over a prolonged period of time, i.e. it deforms plastically before breaking. The molecular conformation of the polymers gives them excellent toughness and resistance to fatigue, but the behavior of the foams depends a lot on their density. Typically EPS has a more brittle behavior than EPE.

Elasticity: it deforms elastically in response to an effort and regains its shape when it ceases. Conventional foams can take advantage of the compression of the gas contained in the closed cells. As long as the cells are intact, the material has good elastic behaviour. An elastic deformation is reversible, i.e. it allows the material to recover its original shape.

Elastic modulus: Expanded polyethylene is generally a soft material, for which the correlation between applied stress and resulting deformation is dependent on the stress itself. In traction it has a behavior close to that of the lattice, while in compression it shows an exponential trend of the stress, since its elastic modulus depends on the compression of the gas in the cells. In this case the elastic modulus increases linearly as the applied stress increases.

Compressive strength: Conventional expanded polyethylene has an exponential behavior, while EPS offers a linear response, and resists even small deformations.

Tensile Strength: It has a broad resistance to tensile stress.

Thermal conductivity: prevents the transmission of heat.

Noise Reduction: Prevents sound transmission.

Vapor Transmission: Slows down vapor transmission. Closed cell polyethylene foam is far superior to open cell foams.

Impermeability to water: prevents the absorption and passage of water. Also in this case the closed cell foams are superior, but the difference is negligible. However, physical expanded polyethylene has the advantage of a particularly hydrophobic surface.

Dimensional stability: resists thermal stress without deformation.

Resistance to chemical agents: It is preserved and remains unchanged when placed in contact with corrosive substances or solvents. Expanded polyethylene is a material with a great chemical inertness, and therefore it is very resistant. EPS, on the other hand, is sensitive to many organic solvents.

Recyclability: It is possible to reuse the material when the product reaches the end of its life. Conventional expanded polyethylene is easily recyclable for the production of LDPE films, foams and other manufactured products.

LDPE is produced in a tubular reactor or in an autoclave by free radical polymerization of ethylene. Ethylene in turn is produced by steam cracking of refined petroleum.

The energy of raw materials is a concept in addition to the input / output tables of the life cycle inventory methodology; it is designed to facilitate the interpretation of the use of resources. Since the backbone of polymers are generally the hydrocarbon chains, the plastics industry defines feedstock energy as the portion of resource input that ends up in the polymer rather than being used as fuel.

The LDPE expansion process involves melting the polymer (melting point variable between 110°C and 120°C) and extrusion with a blowing agent. It is a low environmental impact process, which does not require water, does not emit harmful gases into the atmosphere and requires a modest amount of electricity: 0.6 KWh/Kg equivalent to 2.2 MJ.

Polyethylene is one of the most easily recyclable plastics, because it is sufficient to melt it to be able to extrude and print new products. While EPS recycling campaigns are sporadic, LDPE recycling is widespread and well-proven. Conventional, uncrosslinked foam is chemically equivalent to unexpanded LDPE, and is therefore recycled in the same way.

To recycle LDPE, it is sufficient to grind it, clean it (if it comes from unsafe external sources), melt it and extrude it into pellets to be used for extrusion and the expansion or molding of new products. The plant necessary for the process therefore consists of:

• Shredder
• Single screw extruder
• Pelletizer

With a melt temperature between 105°C and 115°C, LDPE recycling is economical not only in plant requirements, but also in the process.

Conventional foam is nothing more than LDPE added with small quantities of other substances useful for extrusion, therefore it can be recycled just as easily. The quantity of additives present in the material varies between 2% and 6% by weight, and therefore it is easy to mix it with virgin material until the impact of the fillers is tolerable.

Taking care to count the additives present in the regenerated material, it can be used in quantities up to 25% for the extrusion of new foam. For applications less sensitive to the purity of the raw material, such as film extrusion, it is possible to use it in percentages up to 80%. Other applications of recycled LDPE are drug packaging, electrical cable sheathing, piping and injection molded objects.

By recycling the material it is possible to recover the raw material at only the energy cost of melting and pelletizing the waste: while the production of virgin LDPE requires over 25 MJ of energy, it is possible to obtain regenerated material using less than 3 MJ, while the feedstock energy of the material is retained. The recycling of materials, in addition to reducing the use of raw materials, also allows for significant energy savings over the life cycle of the product.

The environmental impact of non-crosslinked expanded polyethylene and polypropylene is very moderate. The production does not require water, only releases into the atmosphere gases that have very little or no environmental risk, minimizes the consumption of raw materials, and requires modest amounts of energy. Furthermore, the product is fully recyclable at the end of its life.

The reduction in product density from 19 to 15.5 kg/m3 (or from 19 to 15.5 gr/m2 for 1 mm thick reference products) results in a minimum 18% reduction in the use of raw materials and of electrical energy necessary for the production of the foam.

From another point of view, for the same kg of product placed on the market, 18% more products were packaged with an equivalent reduction in the levels of co2 generated per unit of product.