Regenerability

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RECYCLABLE PERFORMING FOAM

Expanding plastics means achieving lightweight products.
This also means low consumption of all the materials used in their production (crude oil, etc.) and low energy consumption when using the finished products.

LIFE CYCLE

ciclo di vita polimeri

100% RIGENERABILITY

Proxital uses expansion technologies that do not chemically alter raw materials. This allows total regenerability of products at the end of their useful life. Foam processing waste is also totally regenerable. In this regard, Proxital offers an ad hoc service.
To allow your company to free itself from the wastage associated with the processing
of polyethylene and polypropylene foams, Proxital offers you a solution to the
problem
in accordance with the CONAI regulation and Italian Environmental
Code152/2006.

Proxital staff are on hand to provide further information about the service offered.

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From waste...
...to Quality HDPE

RECYCLABLE AND REGENERABLE POLYETHYLENE FOAM

Polymer foams are consistently present on the marketplace thanks to the wide range of different applications they can fulfil. More specifically, thermoplastic polymer foams play a key role in industrial manufacturing, in which polyethylene is progressively gaining market share. The purpose of this article is to illustrate how conventional polyethylene foam can be treated at the end of its life.

1. The history of polyethylene foam

The first natural and synthetic latex foams were developed in the 1920’s. Polystyrene, a thermoplastic polymer, was only invented in the 1940’s.

The market for thermoplastic polymers has grown after the Second World War thanks to the easy availability of unsaturated hydrocarbons (butane, propylene, ethylene, butadiene, etc.), which were by-products of new petrol refining techniques. In the 1960’s various cell-forming techniques were developed to transform thermoplastic polymers into flexible and semi-flexible foams. This is also when the first polyethylene foams began to appear.

Initially, polyethylene was made into high density foam, using nitrogen as an expansion agent. Later, HCFC gases were also used to create lower density products. Technological developments such as the dual extrusion screw allowed to improve the manufacturing extrusion process at continuous cycle.

The Montreal Treaty of 1989 ended the use of CFC and HCFC gases as expansion agents, leading to the poptimization of alternative technologies, especially chemical expansion.

Today the flexible foam market is dominated by polyurethane and polyolefin, the most important of them is polyethylene. Its foams are available in numerous varieties, both in terms of manufacturing methods and chemical-physical properties.

2. Types of polyethylene foam

LDPE polymer is made of branched chains, with branches as long as 100 units of ethylene. Despite the large branches, the chains maintain their capacity to align themselves into ordered structures that allows LDPE to crystallise up to 50%. The remaining amorphous portion still allows the chains and spherulites to run in the polymer fluid, giving the material its characteristic ductility at room temperature and vitreous transition temperature of – 90°C.

For some applications, a more elastic behaviour may be desirable and in this case the polymer can be crosslinked by arranging the fluid structure into a static reticule. In this way the thermoplastic polymer of LDPE is chemically transformed into an ordinary thermosetting polymer.

Polyethylene foams are a gaseous dispersion in a low density polyethylene matrix (LDPE). The solid matrix delimits the cells, which can be either open or closed. In the first case, the gas can move freely inside the solid, while in the second case its passage is almost completely obstructed by the cell walls. A “chemical” or “physical” expansion is used to achieve this type of dispersion.

Typically, conventional foam (non-crosslinked) is produced by physical expansion in extrusion process. In this case, a compressed gas is injected directly into the molten polymer. HCFC gases were used for this purpose in the past, but in the 1990’s these were replaced with environment-friendly substances. Atmospheric gases (CO2, N2, He, air) or volatile liquids such as aliphatic hydrocarbons (usually butane or pentane) can be used. The expansion agent is typically found in a supercritical state at the extrusion temperature, and is therefore finely blended with the polymer. At the moment of extrusion, the pressure drop causes a sudden change of state to gaseous form. The gas collects around the nucleation centres created by nucleating compounds that were specially added to the mixture. The gas therefore forms bubbles in the molten material, while it cool rapidly till to fall below its softening temperature. At this point, the gas remains trapped in closed cells. This process allows to obtain very low density foams up to 14 Kg/m3, which corresponds to a reduction in density of 98%.

3. Properties

Low density polyethylene is one of the most versatile polymers available on the market. It is most commonly used in the production of packing films, but also for the injection moulding of rigid objects and the extrusion of foams. Some of the more technical applications include the manufacture of corrosion-resistant recipients and surfaces, weldable components and components that need to be both flexible and resilient.

LDPE has a variable density between 910 and 940 Kg/m3. It is chemically inert at ambient temperature and thermally stable up to 80°C. It is highly ductile and resilient, with an excellent level of resistance to acids, alcohols, bases and ethers and a good level of resistance to all organic compounds, with the exception of halogenated hydrocarbons. It is non-toxic, odourless and resistant to moulds and fungus.

Once transformed into foam, polyethylene maintains its chemical properties, with the advantage of a reduction in density from 50% to over 98%. Polyethylene foams are used in many filed for his wide range of solutions.

The foam can achieve a density of less than 14 Kg/m3, reduced heat conductivity and low dynamic rigidity, even maintaining good resilience and toughness. The mechanical features depend largely by the type and density of the foam.

In the packing materials industry, PE foam is usually used at densities between 14 Kg/m3 and 40 Kg/m3, whilst EPS is used at densities between 14 Kg/m3 up to over 600 Kg/m3.

Conventional PE foam

EPS

Resilience

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Toughness

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Elasticity

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Elastic modulus

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Compressive strength

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Tensile strength

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Thermal conductivity

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Noise reduction

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Vapour transmission

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Water impermeability

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Dimensional stability

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Resistance to chemical agents

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Recyclability

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Cost effectiveness

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Resilience: measures the capacity to absorb energy in a short time period, i.e. the ability to resist impact without breaking. Polyethylene foam is a ductile material at ambient temperature, it does not break.

Toughness: measures the capacity to absorb energy over a prolonged time period, i.e. the ability to plastically deform before breaking. The molecular structure of polymers gives them an excellent level of toughness and resistance to stress. The behaviour of foams, however, depends greatly on their density. EPS typically has a more fragile behaviour than EPE.

Elasticity: indicates the material’s capacity to deform elastically in response to a force and to regain its original 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 will have a good level of elasticity. Elastic deformation is reversible, i.e. it allows the material to regain its original shape.

Elastic modulus: measures the resistance of the material to elastic deformation. PE foam is a soft material and therefore the correlation between applied force and resulting deformation is dependent on the force itself. Under traction it has a behaviour similar to crosslinked foam, whilst under compression it demonstrates an exponential trend in relation to the force because its elastic modulus is dependent on the compression of gas in the cells. In this case the elastic modulus grows in line with the growth of the applied force.

Compressive strength: measures the material’s resistance to compression stress. Conventional polyethylene foam has an exponential behaviour as we have already seen, whilst EPS offers a linear response and it also resist to small deformations.

Tensile strenght: measures the material’s resistance to traction.

Thermal conductivity: indicates the material’s capacity to prevent heat transmission.

Noise reduction: indicates the material’s capacity to prevent sound transmission.

Vapour transmission: Indicates the material’s capacity to slow down vapour transmission. Closed-cell PE foam is significantly greater than open-cell ones.

Water impermeability: indicates the material’s capacity to prevent the absorption and passage of water. Also in this case, closed-cell foams are greater, but the difference is negligible. In any case, physical PE foam has the advantage of a particularly water repellent surface.

Dimensional stability: Indicates the material’s capacity to withstand thermal stress without deforming.

Resistance to chemical agents: Indicates the material’s capacity to preserve itself and remain unchanged when exposed to corrosive substances or solvents. PE is a material with a high degree of chemical inertness, which makes it highly resistant. EPS on the other hand is sensitive to many organic solvents.

Recyclability: Indicates the ability to reuse the material when it reaches the end of its useful life. Conventional PE foam is easy to recycle and can be used to make films, foams and other LDPE products.

4. Life cycle

4.1 Raw materials

LDPE is made in a tubular reactor or autoclave to achieve the free radical polymerization of ethylene. Ethylene itself is made by steam cracking of refined petroleum.

tabella consumi

Feedstock energy is a concept in addition to the input/output tables of the Life Cycle Inventory methodology; it is meant to facilitate the interpretation of resource use. Since the backbone of polymers is generally 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 a fuel.

4.2 Processing

The LDPE expansion process involves polymer melting (the melting point varies from 110°C to 120°C) and extrusion with an expansion agent. This is an environmental-friendly process that does not require water, does not release toxic gases into the atmosphere (see paragraph 2), and requires only a modest amount of electrical power: 0.6 KWh/Kg equivalent to 2.2 MJ

4.3 Handling at the end of useful life

When a plastic product reaches the end of its life cycle, it can be disposed of in one of the following ways:

  • Recycling
  • Incineration
  • Landfill

Recycling

Polyethylene is one of the easiest plastics to recycle, it just needs to be melted and then it can be extruded and moulded to make new products. Whilst EPS recycling campaigns are sporadic, LDPE recycling is diffused and well tested. Conventional foam that is not crosslinked, is chemically equivalent to non-expanded LDPE, and is therefore recycled in the same way.

trattamenti fine vita

To recycle LDPE, it needs to be shredded, cleaned (if it comes from non-secure outside sources), melted and extruded into pellets. These pellets can then be extruded, expanded or moulded into new products. The plant required for this process therefore comprises of:

  • Shredder
  • Single-screw extruder
  • Pellet former

With a melting point of between 105°C and 115°C, LDPE recycling is economical not only in terms of plant requirements, but also in terms of process.

Conventional foam is nothing more than LDPE with small quantities of additives used for extrusion and it can therefore by recycled just as easily. The quantity of additives in the material varies from between 2% and 6% in weight. It is therefore easy to mix it with virgin material to bring the impact of the additives down to tolerable levels.

Taking care to select the obtained regenerated material, it can be use in quantities up to 40% for the production of new foam or up to 80% in less sensitive productions such as the extrusion of films.

By recycling the material, it is possible to regenerating the raw materials simply at energy costs for melting and transforming it into pellets (Regeneration process). Whilst virgin LDPE requires over 25 MJ of energy, regenerated material can be made using less than 3 MJ, whilst the material’s feedstock energy is preserved. In addition to reducing the use of raw materials, recycling also allows for a considerable energy saving on product’s life cycle.

The market value of the recycled material is between 30% and 40% of the cost of virgin material.

Energy recovery

Polyethylene has a similar calorific power to diesel fuel, and is therefore one of the most valuable plastics for waste-to-energy processes.

(MJ/Kg) LDPE 44,6 PS 41,9 Diesel 44,8

Landfill

Among the disposal options for polyethylene, landfill is certainly the least desirable. The material’s excellent chemical inertness makes it highly resistant to biodegradation. When not suitably compressed, foam also occupies an excessive volume in relation to its mass comparing other types of waste.

5. Environmental impact

In conclusion, conventional not crosslinked polyethylene foam can reasonably be defined as being recyclable and regenerable , independently of the expansion method used. But it is important to remember that crosslinked foam is not recyclable.

The environmental impact of foam is very low, considering that its production does not require water, it only releases gases into the atmosphere that have either very low or nil environmental risk, it reduces the consumption of raw materials to a minimum, and requires very little energy. In addition, the product is completely recyclable at the end of its life.

FOUNDS POR FESR 2014/2020

Il Bando Regionale POR FESR 2014/2020 Azione 3.1.1 ha reso possibile per l’azienda Proxital Srl  la riduzione dell’uso di materie prime nel ciclo produttivo dell’Azienda, attraverso la riduzione degli scarti di produzione ottenibile con:

  • diminuzione dei cambi di set-up della macchina al variare del tipo di produzione da eseguire;
  • riciclaggio degli scarti di produzione attraverso la rigenerazione per il loro riutilizzo nel processo di lavorazione.

L’investimento, nello specifico, è consistito nell’acquisto di:

  • Macchina avvolgitrice (ideata e progettata sulle specifiche esigenze dell’Azienda), capace di elaborare altezze di rotoli superiori alle macchine standard (fino a 3 m).
  • Trituratore e impianto di granulazione, per il riciclo (rigenerazione) degli scarti di produzione.
  • Impianto pneumatico automatizzato, per il trasporto degli scarti di lavorazione nel reparto di rigenerazione.

L’agevolazione, concessa dalla Regione Veneto, ammonta ad euro 67.500,00.

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