Release Liners - Paper
Paper is made from trees by removing the bark and chipping the wood into small pieces and dispersing in water to separate the fibres. Purely mechanical fibre separation leaves resins such as lignin within the mix and to produce a strong even paper it is normal to digest the wood mix in a chemical mix of sodium hydroxide and sodium sulphide, which is much more efficient at separating the fibres to make pulp. The process is often referred to as the Kraft Process and the paper is kraft paper, or simply Kraft. The main paper types for silicone coating are glassines, claycoated papers, PE coated papers, greaseproofs and vegetable parchments.
Glassine is the common name for supercalendered kraft (SCK), and involves a smoothing and compressing operation of a surface sized (or sealed) chemical paper (kraft). The smoothing and compression is achieved by passing the paper through a stack of alternating hard and soft rollers. The supercalendering improves the surface hold-out of the paper allowing more silicone to remain on the surface, and the compression gives a paper which is not going to compress further when being die cut giving a resilient surface for a clean cut. Another feature of the paper is a higher degree of transparency which allows a photocell to detect the position of a label and gives good label registration.
Claycoated papers (CCKs) are kraft papers which have a layer of kaolin coated on 1 or 2 sides to give a smooth surface for printing of silicone coating. Print CCKs are normally formulated to allow ink solvents to be absorbed into the coating and are of limited use for silicone coating. The clay surface is quite hard and survives die cutting well, so is often used for sheet labels for laser printing. The open back of a one side CCK is easy to rehumidify and allows sheet flatness.
PE coated papers are normally a LDPE extrusion coated paper, PE weights in the range of 10-25gms are normal and give a good degree of surface smoothness, which is passed on to the adhesive. A high gloss adhesive tends to wet out well and becomes invisible in use, so this type of paper is good for graphics laminates where an overlaminating film should not distract from the photographs covered by adhesive “silvering” caused by imperfect contact with the photographs, trapping small air bubbles.
2side PE coated papers have the added bonus of being dimensionally stable in high or low humidity conditions. Changes in humidity cause paper to expand or contract and if laminated to another material this could cause creasing, or small tunnels (piping) which could allow airborne dirt onto the adhesive.
Greaseproofs and parchments are mainly used for bakery purposes because they tend to be grease resistant and heat resistant.
To find out more about the range of release liners we manufacture using paper please contact us with your specific enquiry. contact us