Mark J. Kirwan Paper and Paperboard Packaging Technology
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CORRUGATED FIBREBOARD PACKAGING
327
(calliper) of the board is the main criterion for preserving the stiffness of the
board, it is an important measurement of the degradation of corrugated board
(see Nordman et al., 1978; Azens, 1985).
Different crush levels of corrugated board were obtained by crushing the board
sample between two rotating cylinders. This pre-crushing was done in the range of
0–60% by various settings of the pressure rolls. Thickness (calliper) was measured
before and after crushing under standard pressure (20kPa) and at higher pressures:
80 kPa for C and A flutes and 150kPa for B flute. Thickness (calliper) decreases
dramatically after 20% pre-crushing (see Figure 11.15).
To guarantee the non-degradation of the corrugated board, in order to control
calliper during manufacture in the plant, and to retain the ultimate strength and
cushioning properties of the board, a gauge so-called ‘Differential Micrometer’,
has been developed from the same thickness (calliper) loss measurement concept.
The differential micrometer indicates the difference in corrugated board calliper
(thickness) when measured at two different pressures (20kPa and 80 or 150kPa).
A recommendation to the manufacturing plants is that the difference must not
exceed 10% of the flute height.
In order to evaluate properly the performance of soft board, the hardness of
the corrugated board is the other important criterion. Hardness is the resistance
of the board in the early stages of the FCT (ISO 3035) as illustrated on the
load–compression curves in Figure 11.16. Contrary to FCT values, hardness is
more sensitive to calliper nips (roll pressure) and consequently, the extent of
crushing. Hardness relates to the real strength of the flutes whereas FCT is
a late yield point.
In summary, hardness and softness are quantified values of what is generally
described as the ‘softness’ property of the corrugated board.
Extent of pre-crushing (%)
Residual calliper (%)
0 204060 80
50
60
70
80
90
100
110
Calliper
Figure 11.15 Impact of pre-crushing on the final calliper.
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
Flute bonding and score cracking
The pin-adhesion test (PAT) is used to evaluate the bonding between the fluting
and the liners. It measures the force, in newtons per metre, required to separate the
fluting from the liner by TAPPI method T 821 pm81. Pins are inserted between the
flutes and the force required to break down the adhesion is measured as indicated
in Figure 11.17.
Figure 11.16 Flat crush detailed curves.
Figure 11.17 Pin-adhesion test.
CORRUGATED FIBREBOARD PACKAGING
329
As shown in Figure 11.18 the glue bonding of the tips of the fluting to the liner
is very important for the performance of the corrugated structure throughout its
life in packing, storage and distribution.
Single-face bonding is quite different to the bonding on the double backer due
to a difference in the morphology of the adhesive deposit between single face and
double backer. In single-face bonding, the adhesive is distributed on each side of
the flute tip. In the double backer, the adhesive deposit is placed on top of the tip.
This difference in position comes from the difference in the technology used for
the application of the tip of the fluting to the liner. Figure 11.18 shows the double
backer bonding position. The average starch adhesive in a B or C flute is varying
between 6 and 9g/m
2
for the two places.
Score cracking effects frequently occur in the corrugated plants while converting
the corrugated board sheets. Strain–stresses can occur in folding and erecting the
case in some conditions. During the converting of the corrugated board, moisture
is one of the key parameters to influence the score cracking behaviour (see paper
properties below).
This effect can occur when the moisture content of the board is high and it can
also occur when it is low.
Several series of measurements have indicated that the optimum moisture
content is approximately 8%. Figure 11.19 illustrates this point.
In Figure 11.20, the difference between the three papers – referenced Test Liner
No. 3, Test Liner No. 2 and Kraft Liner – is the burst index level. For the first paper,
the burst index is below 2.5kPa(g/m
2
)
−1
while this property is below 3kPa(g/m
2
)
−1
for the second paper and above 4.0kPa (g/m
2
)
−1
for the third.
Figure 11.18 Photograph of bonding seen by microscope (double backer).
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
Moisture resistance
The sensitivity of the paper to moisture variations is very important. The
procedure for moisture measurement is by a gravimetric moisture analysis.
This procedure is used to determine the moisture content of a sample of the
corrugated board. It is achieved by the removal of water from a weighed sample
in an oven.
Figure 11.19 Score cracking variation with paper moisture.
Figure 11.20 Score cracking variation for different lining papers.
CORRUGATED FIBREBOARD PACKAGING
331
The corrugated board sampling is defined in the Standards and the calculation
of moisture content is described below:
Weight of moisture = wet sample − dry sample
The moisture at the end of a corrugator for B or C flute board must be in the range
of 7–7.5%.
Paper properties
The papers used in corrugated board are made from wood fibres which are used to
produce paper. Recycled fibre from recovered paper and board is a major source
of fibre for the corrugating industry. Papers are defined by their structural prop-
erties, their mechanical properties and their moisture sensitivities. The basic properties
of paper as a component of the corrugated board are described in Table 11.4.
For corrugated board, the liners comprise unbleached and bleached fibres (virgin
or recycled) and for fluting medium, recycled or semi-chemical fibres are used.
The basic packaging-paper classification was defined by the European Association
‘Groupement Ondulé’ in the year 1990. This classification corresponds to two classes of
liner (test liner and kraft) and two selections for the fluting (semi-chemical and recycled).
Test liner is a paper predominately comprising recycled fibre. Table 11.5
presents the European classification. Table 11.6 presents the kraft liner range for
% Moisture =
weight of moisture 100×
weight of wet sample
------------------------------------------------------------
Table 11.4 Basic benchmarking table of paper characteristics
Note: IBT = internal bond test. This is the energetic load, or force, required to separate the
layers of a paper in the Z direction (thickness), i.e. perpendicular to the paper surface, several
test methods are available (ISO, TAPPI and Scott Ply Bond).
Properties Characterization Critical element for
Nature of fibres Virgin or recycled Price
Grammage Basis weight (g/m
2
) Weight reduction
Thickness Microns Structure
Sheet density Porosity, IBT Cohesion
Strengths Burst, stiffness and compression Performance, equivalence
Surface Smoothness, brightness, gloss . . . Printing
Orientation Machine or cross direction
(MD, CD)
Stability
Moisture sensitivity Cobb, repellency Curl, warp, lifetime
Table 11.5 Test liners range – 3 levels, with Test liner 1 being the best
Note: SCT
CD
is the test value for the CD of the liner.
Test liner 1 Test liner 2 Test liner 3
Grammage (g/m
2
) 125 150 200 125 150 200 125 150 200
Burst index 3.0 3.0 2.9 2.5 2.5 2.4 2.0 2.0 1.8
SCT
CD
value (kN/m) 2.15 2.60 3.40 1.90 2.35 3.00 1.65 2.05 2.70
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
both unbleached and white top (bleached). Fluting is classified by the CMT
30
index class (Table 11.7).
In order to provide the basis for satisfactory corrugated box board performance,
it is very important to know the relative orientation of the papers because the
strength properties depend on the orientation: MD and CD.
The variation of characteristics for one Kraft Liner, 140 g/m
2
, is presented in
the graphs in Figure 11.21a, b.
Paper and corrugated board humidity
The natural paper and board sorption is illustrated in Figure 11.22 (Poustis &
Vidal, 1994).
In the sampling procedure, to evaluate the paper moisture, seven or eight
outer layers of the paper reel are always removed before the specimen is selected,
Figure 11.23.
The take-up of moisture occurs more through the unprotected reel end rather
than through the body of the reel, Figure 11.24.
Table 11.6 Kraft liner range – 2 types (bleached and unbleached)
and 2 levels of grammage
Burst index ISO brightness
Grammage (g/m
2
) <250 >250 —
Unbleached >3.5 >2.9 No brightness specified
White top >3.5 >2.9 >70%
Table 11.7 Fluting range
Fluting medium CMT
30
index
Semi-chemical >1.8
Recycled >1.6
Burst index
MD/CD
4.5
4.0
3.5
3.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0
MD/CD
SCT (kN/m)
0
1
2
3
4
5
6
7
1.0 1.5 2.0 2.5 3.0 3.5 4.0
SCT
MD
SCT
CD
(a) (b)
Figure 11.21 Strength and orientation of papers (a & b).
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11.2.2 Corrugated fibreboard manufacture
Corrugated fibreboard boxes are manufactured in a corrugated board plant or in
a sheet feeder plant. The corrugated board plant consists of the corrugator which
produces flat sheets of corrugated fibreboard and of the converting equipment
where the corrugated sheet is converted into corrugated board boxes by printing,
cutting, scoring (creasing) and gluing (or possibly, taping or stitching). These
latter processes are known as the converting operations.
Corrugated fibreboard in sheet form is also sent to smaller factories known as
sheet feeder plants, where they are converted into packaging for short run length
orders, quick deliveries and for very specific local markets.
The production of the corrugated board is carried out in several stages in-line.
•
Production of the single face corrugated board – Figure 11.25: The fluting
medium is conditioned with heat and steam to make it pliable enough to accept
and retain the shape of the fluting. The fluting shape is pressed into the medium
0 50 100
0
4
8
12
16
Relative humidity (%)
Paper moisture (%)
Figure 11.22 Paper and moisture equilibrium.
0
0
5
10
15
10 20 30 40 50
Distance (cm)
Moisture content (%)
Figure 11.23 Typical moisture distribution from the outside to the inside of a reel.
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
either using two profiled rolls or, in the ‘fingerless’ process, by forming the fluting
medium on one profiled roll using vacuum. After the corrugation of the fluting
medium, starch adhesive is applied to the tips of the corrugations and the medium
is combined with the liner which has also been conditioned to bring it to the
same temperature and moisture content as that of the fluting medium.
•
A second liner is applied at the double backer to produce single wall, or double
face, corrugated board – Figure 11.26.
0
0
5
10
15
50 100 150 200
Distance (cm)
Moisture content (%)
Exposed end
Protected end
Figure 11.24 Typical moisture distribution from end to end of reel.
Pre-heater inner liner
Pre-heater fluting
Fluting
paper
Single face
web
Inner liner
Corrugated rolls
Figure 11.25 Production of single face corrugated board.
CORRUGATED FIBREBOARD PACKAGING
335
•
After passing through a drying section, the board is matured and cooled before
being slit to the required width and cut to the required length. Scores (creases)
may also be applied to the board in the MD of the corrugator.
The key parameters of the process are:
•
fluting roll profile (A, B, C, etc.)
•
roll temperature and pressure
•
viscosity of the adhesive
•
relative speed of the pre-heated inner liner and the fluting medium
•
pre-heat conditions for the liner and fluting medium.
The speed of the single facer is more than that of the double backer, and the excess
board accumulates in a bridge system between the two lining stations balancing
the difference in speed.
In order to make a double wall corrugated board, the machine would incorp-
orate additional sections as shown in Figure 11.27. In this process, two single face
corrugated webs are formed in Zone A. The flutes of Single Face 1 are glued to the
liner of Single Face 2 in Zone B. A liner for the flutes of Single Face 2 is applied
to the flutes of Single Face 2 and the combined board is dried between heating
plates in Zone C.
Starch adhesive preparation is an important element for the orderly running of
the process. Essentially, a corrugating starch adhesive is a four-component system.
It consists of a carrier or cooked starch component, a raw starch component,
caustic soda and borax, all prepared in water. All ingredients are mixed together in
a ‘kitchen’.
The carrier (cooked starch) component carries the raw starch component to the
tips of the fluting so that when heat energy is supplied, the raw starch swells
Figure 11.26 Production of single wall (double face) corrugated board. (Reproduced, with
permission, from The Institute of Packaging.)
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PAPER AND PAPERBOARD PACKAGING TECHNOLOGY
‘in situ’, in the situation where sufficient water is present in the glue line to adhere
to the tip of the medium to the liner. The caustic soda helps prepare the carrier for
its ‘duties’ and determines the initial gel point or swelling point of the raw starch
component.
Borax (hydrated sodium borate) is added in the form of penta-hydrate or deca-
hydrate, i.e. hydrated to the extent of 5 or 10 molecules of water of crystallization,
respectively. The primary function of the caustic/borax treatment is to bring about
extensive chemical changes in the starch structure, which modify its physical
properties.
The action of the borax causes the starch to become a more highly branched
polymer chain with higher viscosity and tack. It contributes the necessary rheological
properties for efficient adhesive roll pickup and transfer to the tips of the formed
fluting.
At this point, one may as well ask, ‘What are the essential characteristics of
a good, commercial corrugating starch adhesive?’ There are four basic adhesive
characteristics that a starch system must deliver:
•
reproducibility from batch to batch
•
viscosity stability during storage and circulation to the combining machine
•
proper rheological or filming properties to permit the desired pickup and
transfer of adhesive from applicator roll to the flute tip of the corrugating
medium
•
deep, penetrating adhesive bonding consistent with the needs of production
and downstream operations.
If any of the above elements are missing, then the adhesive cannot be classified as
a good commercial corrugating adhesive.
The flat sheets of corrugated board coming off the corrugator are transferred to
the converting operations where the main machines are printer/slotters, die cutters
and folder gluers, depending on the corrugated product being produced.
Single face N° 1
Single face N°
1
Single face N°
2
Single face N°
2
Double wall starch
application
Heating
plates
Slitter
scores
Rotary knives
Linear LinearStarch Fluting
(Zone A) (Zone B) (Zone C)
Figure 11.27 Production of double wall corrugated board.