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 - Brown Kraft)
- TEST (T - Brown Test)
- CHIP (C)
- FLUTING (WB - Waste Based & SC - Semi-chem)
Although there are variations within each group they
will nevertheless conform to the general definitions of that group. Liners
are usually specified by their weight, either in Grams per square meter
or Pounds per hundred square feet. In the US specify materials by their
burst values is still quite common.
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). 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 fiber 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.
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
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. The outer surface of the Test liner can also be produced
in a range of colors (White, Mottled etc.).
pH levels - 5.5 to 6.
Chip/Brown Dyed Chip (C)
Chip liners (sometimes called Test 4) are manufactured
from lower grades of waste paper and are unsized. Their use is normally
restricted to application as an inner liner and/or center liner in double
wall board. It can also be used throughout in the manufacture of fittings,
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 fiber. The waste
material is added to allow a greater choice of types of wood sources to
be used, plus conserve wood fiber.
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 alkaline starch.
Other Materials used in Corrugated board manufacture
Corrugator adhesive is made from a corn based starch formula.
Printing inks used in flexographic post printing are water based.
Manufacture's Joint Adhesive
Manufacture's Joint (side seam) Adhesive, on glued cases
is a Polyvinyl acetate emulsion, made from components that comply with
US F & DR CFR 175.105