The present invention relates to a method for preparing a thermoplastic item having a single layer or a bi- layer, the thermoplastic item obtained with the method and the uses of the item as an internal cladding for electrical domestic appliances, in particular as a inner door for refrigerators or as a refrigerating cell.

Thermoplastic items obtained by extrusion are normally used for a vast range of applications, for example for forming inner doors for refrigerators and refrigerating cells and, in general, plastic components for electrical domestic appliances, shower trays and components for showers, parts of vehicles, food packaging, etc.

Manufacturers of thermoplastic items are constantly researching for production methods that enable obtaining items having a low environmental impact and which enable savings to be made on production costs.

Items having a low environmental impact are, for example, those articles produced starting from raw materials that have been recycled, or which can be obtained by using smaller quantities of raw materials. In the sector of plastic materials, and in particular, in the sector of electrical domestic appliances, there still exists a great need to identify technologies which enable obtaining items that are characterised by a positive environmental impact; for example, lighter items which require the use of a smaller quantity of raw materials with respect to the quantity normally used and therefore enabling a reduction in energy use, with a consequent reduction in production costs.

The main problem encountered in the production of lower- density items in terms of material is how to succeed in obtaining a product that maintains the same characteristics of mechanical resistance, chemical resistance, elastic modulus and hardness required for the desired uses.

This problem is solved with the use of a production method of a thermoplastic item and the item obtained using the process, as delineated in the accompanying claims.

The present invention relates to a method for production of a thermoplastic item, comprising steps of:

a) mixing a thermoplastic polymer with a mixture of an expanding chemical agent and a carrier;

b) hot-extruding the mixture;

c) preparing a sheet having a thickness comprised between 0.5 and 3 mm by calendering the extruded mixture ;

d) subjecting the calendered sheet to heat-forming at a temperature comprised between 120 °C and 250 °C, preferably between 140 °C and 180 °C, in order to obtain a desired shape thereof.

The process is characterised in that it uses, as a mixture of . an expanding chemical agent and a carrier, a mixture of citric acid and polyethylene. In this mixture, the polyethylene is the carrier and the citric acid is the expanding agent.

The mixture preferably comprises from 40% to 80%, preferably from 40% to 60%, of citric acid and from 20% to 60% of polyethylene, preferably from 40% to 60%. The percentages to which reference is made are percentage volume .

In a preferred embodiment, the citric acid and the polyethylene are in a 1:1 mixture. The ratio between the citric acid and the polyethylene is a volume ratio.

More preferably, the expanding chemical agent is Hydrocerol 593 produced by the company Clariant.

The thermoplastic polymer used as a starting material is preferably selected from among the range of monovinylidene aromatic polymers (also known as styrene polymers) ; more preferably the styrene polymer is selected from among: general purpose polystyrene (GPPS) , high impact polystyrene (HIPS) , acrylonitrile butadiene styrene (ABS) and acrylonitrile styrene resin (SAN) .

The most preferred styrene polymer is high impact polystyrene (HIPS) .

The thermoplastic polymer can be virgin and/or recycled. Preferably between 30% and 100% in weight of virgin thermoplastic polymer is used, and from 0% to 70% in weight of recycled thermoplastic polymer.

The expanding chemical agent is added to the starting mixture in quantities comprised of between 0.5% and 3%, preferably between 1% and 2% in weight.

In a preferred embodiment, apart from the thermoplastic polymer and the mixture of an expanding chemical agent and a carrier, a colorant is added, for example titanium dioxide, in a quantity of from 1.5% to 4% in weight, preferably from 2% to 2.5% in weight, such as to obtain the classic white colouring of plastic components for electrical domestic appliances or other applications.

After mixing the raw materials in the batcher, the mixture thus-obtained enters the extruder at ambient temperature and is heated to a temperature comprised between 150 °C and 230 °C, preferably between 150 °C and 200 °C, while it is pushed by the extruding screw, which has a variable-volume profile. The heating is done by means of resistances.

The mixture, pushed by the extruding screw, exits from the drawing head and enters a calender, which gives the sheet the desired thickness. The preferred thickness for the applications of the invention is comprised between 1 and 2 mm.

When the mixture exits from the drawing head of the extruder, before entering the calender, by effect of the mixture of an expanding chemical agent and a carrier, it undergoes an expansion which enables a reduction in density of not greater than 30%, preferably not greater than 20% in weight with respect to a non-expanded item made of the same material.

The reduction in density is preferably comprised between 14% and 18% in weight, such as to guarantee the same mechanical performance as an item made of the same polymer material but not expanded.

During calendering the sheet undergoes a cooling to a temperature of less than 100 °C, preferably to a temperature comprised between 80 and 95 °C.

The thus-calendered sheet can then optionally be further subjected to cooling, then to be subjected to a corona treatment before being cut to the desired dimension; corona treatment is a surface treatment of plastic, normally used in the sector of electrical domestic appliances such as to give the thermoplastic item a surface finishing that is such as to enable a filler material which will subsequently come into contact with the surface of the sheet (such as for example a polyurethane foam or a different insulating material) to adhere optimally thereto.

The heat-forming of the thermoplastic item is done by heating the sheet to a preferred temperature comprised between 140 °C and 180 °C, preferably between 170 °C and 180 °C. The sheet is then placed in contact with the mould having the desired shape, for example the shape of an inner door or a refrigerating cell.

The die is preferably micro-perforated to enable the thermoplastic item to adhere to the mould by application of a vacuum. The application of the vacuum determines a cooling of the thermoplastic item, causing the plastifying and/or the complete solidification thereof. The thermoplastic item thus formed can optionally be finished and coupled to the support (for example, to the door of a refrigerator) by application of a material having adhesive properties, for example polyurethane foam.

A further object of the present invention is an expanded single-layer thermoplastic item obtainable with the above-described process.

The thermoplastic item is characterised by a single layer of expanded polymer material (preferably polystyrene, more preferably high-impact polystyrene) , with a reduction in density of not greater than 30%, preferably not greater than 20% in weight with respect to an item of the non-expanded material. The density of the expanded single layer is preferably comprised between 0.5 e 1 g/mL, preferably between 0.7 and 0.9 g/mL. The thickness of the single-layer is comprised between 0.5 e 3 mm, preferably between 1 and 2 mm. Preferably the thickness is not uniform and varies within the described intervals.

In a further embodiment, the thermoplastic item can be a bilayer made up of two different polymer materials, or of the same material.

The thermoplastic polymers used for forming the two layers correspond to those used or producing the single- layer item, i.e. they are monovinylidene aromatic polymers (also known as styrene polymers), preferably selected from among various combinations of GPPS, HIPS, ABS and SAN.

In a preferred embodiment, the bilayer comprises two layers of the same polymer material, preferably two layers of polystyrene, more preferably high-impact polystyrene .

The process for obtaining the bilayer item corresponds to the preparation method of the single-layer in which a further co-lamination passage is present, or a co- extrusion step cl) before the calendering of the first layer. The material which is deposited on the first expanded layer to form a second layer has aesthetic functions and is preferably not expanded.

In a case of a co-lamination step, the co-lamination is preferably performed at a temperature comprised between 50 °C and 90 °C, while in a case of a co-extrusion step, the co-extrusion is performed at a temperature of between 180 °C and 220 °C.

The heat-forming of the bi-layer as illustrated above is then performed.

In a preferred embodiment the total thickness of the bi- layer corresponds to the thickness of a single-layer as described above. _. The second layer preferably has a thickness comprised between 20 and 80 micron, preferably between 50 and 60 micron, i.e. a thickness which practically does not have any effect on the total thickness of the item.

The single-layer or bi-layer item of the invention can be used for various applications, preferably for internal linings of electrical domestic appliances, for example refrigerators and washing machines. A particularly preferred application relates to the use of the thermoplastic item as an inner door for refrigerators or as a refrigerating cell.

EXAMPLE OF PREPARATION OF AN INNER DOOR FOR A REFRIGERATOR OR A REFRIGERATING CELL.

The following are inserted in a gravimetric batcher: virgin high-impact polystyrol 50%, recycled high-impact polystyrol (regrind) up to 50%, 1.5%-2.5% titanium dioxide, 1.4-1.6%, in particular 1.6% of expanding agent Hydrocerol 593 (diluted version of Hydrocerol 591). The percentages to which reference is made are weight percentages .

After mixing the raw materials in the batcher, the mixture thus obtained enters the extruder, at ambient temperature, at the start of the plastifying screw; the screw has a variable-volume profile; the mixture is heated while it passes in the screw at a mean temperature of about 200 °C, by means of resistances.

The mixture exits the drawing head and is crushed by a calender which gives the sheet a thickness of about 1.6- 1.7 mm. When the mixture exits from the drawing head before entering the calender, by effect of the expanding agent, it undergoes an expansion that causes the density to diminish by 16% in weight; the sheet exits from the calender cooled to 95 °C.

The calendered sheet is further cooled, subjected to corona treatment and then cut.

The heat-forming of the inner door or the refrigerating cell is done by heating the sheet to a temperature T of 140-180 °C, preferably 160-165 °C. The sheet is thus rested on a mould which has the shape of the inner door or the refrigerating cell; a vacuum is applied to the mould which is transmitted to the plastic by means of the micro-perforations; in this way the plastic adheres to the mould and cools as the mould is cold and the plastifying and/or the complete solidification is attained.

The same process was repeated using 1.4-1.6%, in particular 1.5 and 1.6% of Hydrocerol 593 and virgin high-impact polystyrene up to 100%; 40% of virgin high- impact polystyrene, up to 60% of recycled high-impact polystyrene and 1.4-1.6%, in particular 1.6% of Hydrocerol 593. The percentages to which reference is made are percentages in weight.

The following table reports the results obtained for the inner doors and refrigerating cells in terms of reduction of weight and density. 100% virgin 100% virgin 50% 60% olysty ene polystyrene recycled recycled

1.5% 1.6% polystyrene/50% polystyrene/40%

Hydrocerol Hydrocerol virgin virgin

593 593 polystyrene polystyrene

1.6% 1.6%

Hydrocerol Hydrocerol

593 593

Density 0.89 Density 0.88 Density 0.86 Density 0.84 g/mL g/mL g/mL g/mL

Weight Weight Weight Weight

reduction 16% reduction 17% reduction 19% reduction 21%

All the inner doors and refrigerating cells obtained with the method of the invention have demonstrated good surface finishing, a good degree of expansion and thus of weight reduction, and a good distribution of porosity .

The mechanical properties and properties of resistance to the onslaught of any chemical elements are comparable to the inner doors and refrigerating cells obtained with non-expanded polystyrene.