Fibrous materials used in corrugated board manufacture
There are four main categories in which the fibrous materials can be
placed, three of these categories comprise the lining materials and the
fourth is the material that form the corrugations. These four categories
are as follows:
- Kraft (K)
- Test (T1, T2 & T3)
- Chip (C, BDC & T4)
- Fluting (FR & S/C)
Although there are variations within each group they will nevertheless conform to the general definitions of that group.
Kraft liner represents the top grade material in terms of appearance and physical properties used in the corrugated industry. These liners are in general made almost entirely from softwood pulp (B.S. 3130). In the UK all the kraft liner has to be imported, kraft liner is available in various forms i.e. unbleached kraft, bleached kraft, mottled kraft and white top kraft. Fully bleached kraft is rarely used these days. In the recent pass the specification for kraft have also been loosened, so that a small percentage (up to 13% in brown kraft) of recycled material can also now be included. This usually takes the form of re-pulped kraft liner or corrugated box waste. This waste is applied to the base ply of the liner. There are two advantages to this, namely it conserves fibre and closes up the sheet, decreasing it's porosity, which is and aid when used on any machines which pick up the board via vacuum suction cups.
Mottled Kraft and white lined kraft can contain up to 8% of waste, which is added for the same reasons as it is in the brown kraft liners. It is not possible to add larger percentages of waste, as this could affect the whiteness of the sheet. Bleaching of the top ply for these materials is of the oxygen type, mainly for environmental reasons.
pH levels - 5.5 to 7.
Test (T1, T2, T3)
The most common type of Test liner is a homogeneous material manufactured from 100% chosen grades of waste, e.g. from box plant off cuts and supermarket empty boxes. Obviously, the higher the proportions of kraft in the waste, the higher grade of test liner that is produced.
Although the surface finish of this type of Test liner can be good it is not quite up to the high standard of Kraft liner. Test liner is sized (with cooked corn starch) to a level similar to Kraft, but it is more prone to cracking. The compression component of Test liner approaches that for kraft, but the Burst level is considerably lower. Test liner is widely used as an alternative to Kraft for the inner lining of corrugated board but it can also be used as an outer liner when appropriate. Care should be used in using Test on both facings of a case, if the pack is likely to be subjected to a high or continually changing moisture environment.
A special Test liner with an identical surface finish to Kraft is also made, this incorporates a Kraft outer layer and is used when a similar appearance to Kraft liner is desired. This is known as duplex Test. Test 1 is not a duplex Test. The outer surface of the Test liner can also be produced in a range of colours (White, Mottled etc.).
pH levels - 5.5 to 6.
Chip/Brown Dyed Chip (C, BDC, T4)
Chip liners are manufactured from lower grades of waste paper and are unsized. Their use is normally restricted to application as an inner liner and/or centre liner in double wall board. It can also be used throughout in the manufacture of fittings, as appropriate.
pH levels - 6.5.
Semi chemical Fluting mediums
Semi chemical fluting is manufactured from hardwood pulp, with the inclusion of up to 35% of waste or other fibre. The waste material is added to allow a greater choice of types of wood sources to be used, plus conserve wood fibre.
Waste based Fluting mediums 100% waste based materials, chemically reinforced with corn starch.
pH levels - 6.5.
Generally the pH level of board itself is neutral, since we use a
slightly alkerline starch.
Other Materials used in Corrugated board manufacture
Corrugator adhesive is made from a corn based starch formula.
Manufacture's Joint Adhesive
Manufacture's Joint (side seam) Adhesive, on glued cases is usually
either Polyvinyl acetate emulsion or Hot melt adhesive. Their components
should comply with US F & DR CFR 175.105.