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Corona treatment of polystyrene (PS) sheets for refrigerators

Polystyrene (PS) sheets are used to produce refrigerator cells, doors and inner door panels for refrigerators. Solid sheets with thickness varying from 2mm to 6mm have a better reaction to expanded polyurethane foam if they undergo the corona treatment.

How PS sheets are produced

  1. The sheets are produced with a flat-head extrusion process, at the end of which they are cut into individual measured pieces.
  2. Afterwards, a thermoforming process (moulds used at high temperatures) determines the desired shape: cell, door or inner door panel.
  3. The parts obtained are then filled with expanded polyurethane foam. A layer of expanded polyurethane is sprayed on the external part of the thermoformed piece in order to create an insulating layer that will guarantee that the proper temperatures are maintained.

Although PS is a material with quite a high natural surface tension that lends itself to adhere well to polyurethane foams, the thermoforming process, which acts at high temperatures and changes the shape and thickness of the sheet, stresses the material and decreases its surface tension. To optimise the foaming process, therefore, the polystyrene (PS) needs to undergo the corona treatment.

Corona treatment to favour expanded polyurethane adhesion

The foaming process is a determining factor in thermal insulation and affects the quality of the refrigerator. The polyurethane foam must be applied evenly on the entire outer surface of the cell or the door and, most importantly, it must remain well-anchored over time, withstanding the different temperatures to which it is subjected.

If the polyurethane foam detaches, it can create an air bubble which, little by little, causes a loss of insulation as it expands, in addition to it being an aesthetic flaw in the refrigerator.

The corona treatment increases the surface tension of the sheet, making it more receptive to firmly anchor the polyurethane foam. The surface treatment must be done:

  • when the material is produced by extrusion, before cutting the sheets into the different shapes
  • without interfering with the surface tension of the side opposite the one being foamed (visible inside the refrigerator).

During the subsequent thermoforming process, the material will undergo a loss of surface tension that can even exceed 10 points.

Why use Cartonplast?

Ferrarini & Benelli recommend using high-efficiency corona treatments that are able to reach treatment levels of over 56 dyne/cm. This way, at the end of thermoforming:

  • the surface tension will still be sufficient for firm anchoring
  • you can use a lower thickness of polyurethane foam with the same result, allowing you to save on materials.

The CARTONPLAST solution uses special electrodes and specific power dosing to avoid increasing the surface tension of the non-foamed side (counter-treatment), which makes it easier for the inside of the refrigerator to retain droplets of water that then freeze (the exact opposite of the "no frost" effect highly advertised by manufacturers).