1.HPL material system
HPL consists of layered cellulose fibre sheets (paper) which are impregnated with heat-curing resins. Impregnation is the process of completely saturating dry paper with a liquid resin. The top layer is impregnated with melamine resin and features decorative colours or printed designs. The core layers are impregnated with phenolic or melamine resins, both of which are condensation resins. Melamine resins are produced by a chemical reaction between melamine and aldehydes and are categorised as aminoplasts. Full hardening through polycondensation turns these resins into thermosetting plastics. Melamine resins harden to form transparent, light-fast, scratch- resistant and hard coatings. They are therefore ideal as surface layers for HPL. Phenolic resins are produced from phenols and aldehydes through polycondensation. After pressing, they produce a relatively elastic, thermoset core layer together with the kraft paper. The addition of heat (temperatures over 120 °C) under high pressure (at least 5 MPa) causes the resins to flow and then harden, bonding the paper layers into a homogeneous material. This creates a material with a sealed surface and a bulk density of ≥ 1.35 g/cm³. The kraft paper used for the core layers (80 – 300 g/m²) comes from a sustainably managed forest or from recycled materials. The paper is unbleached, has a high grammage and a strong absorption capacity for impregnation resins. Decorative paper (50 – 160 g/m²), consisting of high-quality cellulose fibres, is used for the HPL top layer. This paper is bleached so it can be dyed or printed (e.g. wood, stone or custom patterns). Overlay paper (15 – 80 g/m²) is bleached transparent paper with a high resin absorption capacity. It is used to protect the printed or dyed decorative papers and improves abrasion resistance.
The properties of HPL can be modified by adding additional layers (metals, barrier papers, etc.).
