The expanded plastic in Physical Foam does not undergo any chemical alteration since the expansion process is totally “physical”, natural, unlike crosslinked foam which instead are chemically treated and therefore the final product obtained is not recyclable. Physical Foam, on the other hand, is 100% recyclable and regenerable since the Polypropylene (PP) and Polyethylene (PE) polymer maintains their structure intact even after the physical expansion process, regardless of the degree of density adopted for the different products.
Polyolefin polymeric foams can boast an impressive presence on the market, thanks to the vastness of their possible applications. In particular, expanded thermoplastic polymers play a leading role in industrial manufacturing, where the increasingly common polyolefins (polyethylene and polypropylene) are gradually gaining market.
The first natural “foam” is the wood itself, which is by its nature micro-alveolate. 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 World War II thanks to the easy availability of unsaturated hydrocarbons (butene, propylene, ethylene, butadiene, etc.) derived from the new oil refining methods. Since the 1960s, therefore, various cellulation methods have been experimented with 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, the polyethylene was expanded to high density using nitrogen as an expanding 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 productions in a continuous cycle, giving a strong impetus to the diffusion of the material.
The 1989 Montreal Treaty interrupted the consolidated use of CFC and HCFC gases as blowing agents, stimulating the refinement of available alternative technologies, and in particular chemical expansion.
Today the flexible foam market is dominated by polyurethanes and polyolefins, the most important of which by far is 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 greatest use in the production of packaging films, but is also used in the injection molding of rigid objects and in the extrusion of expanded foams. Among the most technical applications are those for corrosion-resistant containers and surfaces, 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 retains its chemical properties, with the advantage of a reduced density from 50% to over 98%. Polyethylene foams are used in various sectors due to the wide range of solutions that can be obtained.
The foam can reach densities of less than 15 kg / m3, a reduced thermal conductivity and a 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 variable densities from 14 Kg / m3 up to over 600 Kg / m3.
Resistance to chemicals
0 = Reference
+ = Improvement
Resilience: absorbs energy in a short period of time, meaning it 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 very much 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 closed cells. As long as the cells are intact, the material has a good elastic behavior. 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 reticulated, while in compression it shows an exponential trend of stress, since its elastic modulus depends on the compression of the gas in the cells. In this case the elastic modulus grows 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 wide resistance to traction stress.
Thermal conductivity: prevents the transmission of heat.
Noise reduction: prevents sound transmission.
Steam transmission: slows down the steam transmission. Closed cell polyethylene foam is far superior to open cell foam.
Water impermeability: prevents the absorption and passage of water. Also in this case, closed cell expansions 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 deforming.
Resistance to chemical agents: It is preserved and remains unaltered 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 polyethylene foam is easily recyclable for the production of films, foams and other LDPE products.
LDPE is produced in a tubular reactor or in an autoclave for free radical polymerization of ethylene. Ethylene in turn is produced by steam cracking refined oil.
The energy of raw materials is a concept in addition to the input / output tables of the life cycle inventory methodology; is designed to facilitate the interpretation of the use of resources. Since the backbone of polymers are generally hydrocarbon chains, the plastics industry defines the energy of raw materials as the portion of resource input that ends up in the polymer rather than being used as a fuel.
The LDPE expansion process involves melting the polymer (melting point varying between 110 ° C and 120 ° C) and extrusion with a blowing agent. It is a process with low environmental impact, 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 non-cross-linked foam is chemically equivalent to non-expanded LDPE, and is therefore recycled in the same way.
To recycle LDPE, simply grind it, clean it (if coming from unsafe external sources), melt it and extrude it into pellets to be used for extrusion and expansion or molding of new products. The plant necessary for the process therefore consists of:
- Single screw extruder
With a melting temperature between 105 ° C and 115 ° C, recycling of LDPE is economical not only in terms of plant requirements, but also in the process.
Conventional foam is nothing more than LDPE added with reduced quantities of other substances useful for extrusion, so it can be recycled just as easily. The amount 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 product, 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 a percentage of up to 80%. Other applications for recycled LDPE are pharmaceutical packaging, electrical cable coating, piping and injection molded items.
By recycling the material it is possible to recover the raw material at the only 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 energy of feedstock of the material is preserved. The recycling of the material, in addition to reducing the use of raw materials, also allows significant energy savings on the life cycle of the product.
The environmental impact of non-cross-linked expanded polyethylene and polypropylene is very moderate. The production does not require water, releases only gases that are of very low or zero environmental risk into the atmosphere, minimizes the consumption of raw materials, and requires modest amounts of energy. Furthermore, the product is completely recyclable at the end of its life.
Reducing the density of the product from 19 to 15.5 kg / m3 (i.e. from 19 to 15.5 gr / m2 for 1 mm thick reference products) leads to a minimum reduction of 18% in the use of raw materials and of electricity 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.
Funds Por Fesr 2014/2020
The Regional Call POR FESR 2014/2020 Action 3.1.1 made it possible for the Proxital Srl company to reduce the use of raw materials in the company’s production cycle, through the reduction of production waste obtainable with:
- reduction of machine set-up changes when the type of production to be performed varies;
recycling of production waste through regeneration for their reuse in the manufacturing process.
- Specifically, the investment consisted in the purchase of:
- Wrapping machine (conceived and designed on the specific needs of the company), capable of processing roll heights higher than standard machines (up to 3 m);
Shredder and granulation plant, for the regeneration of production waste;
Automated pneumatic system, for the transport of processing waste in the regeneration department.
The subsidy granted by the Veneto Region amounts to € 67,500.00.