Method of manufacturing corrugated board using a thermal radiant energy source

Corrugated paperboards are prepared for corrugation and adhesive bonding of mediums and liners by exposing the mediums liners and adhesives to radiant energy from a thermal source ranging from 1100.degree. C. to 2300.degree. C. and emitting a dominant wavelength in the range of 2.1 to 1.0 microns. The speed of adhesive bond development is increased. Polymeric adhesives may be used as alternatives to conventional starch based adhesives.

FIELD OF THE INVENTION 
This invention relates to an improved method of bonding liner boards to 
corrugated mediums in the manufacture of corrugated board for box 
manufacture. 
BACKGROUND OF THE INVENTION 
Conventionally, in applying liners to corrugated medium the first liner is 
applied at the single facer to form a single faced board and this is 
achieved by applying adhesive to the flute tips while the medium is 
contained on the corrugating roll and then applying the liner under 
pressure and heat. The combination of heat from the corrugating roll and 
pressure roll and mechanical pressure itself forms the first liner to 
medium bond. Subsequently, the single face board is passed to a double 
facer or double backer where adhesive is applied to flute tips and the 
board passed over a series of steam heated platens to bond the second 
liner. Pressure is also applied by a transport belt on top of the board 
and a series of small rollers riding on that belt. Travel of the double 
face board through the double facer or double backer is assisted by 
sandwich belts after the steam heated platens. 
The size of the plant and the length of the production floor is quite long 
in conventional plant for producing corrugated board. The heating platens 
are of extensive length to ensure sufficient curing of the adhesive bond 
as the board passes over the platens at speeds of up to 300 to 400 meters 
per minute. 
Another characteristic of conventional corrugated board is that of 
"pressure lines" which result from the pressure applied by the pressure 
roll at the single facer. This is compounded by "washboarding" which is 
the effect of the adhesive, when drying, drawing the liner out of its 
linearity. These effects make the board unsuitable for high quality 
printing on that side. 
SUMMARY OF THE INVENTION 
It is an object of this invention to improve the quality of corrugated 
board and decrease the size of the plant required to produce board at 
commercially acceptable speeds. 
To this end the present invention provides a method of forming corrugated 
paperboard in which at least one liner is applied to the corrugated medium 
using low mechanical pressure and the adhesive bonding is achieved with 
the application of radiant energy at a wavelength for which water, paper 
and adhesive have a low absorption coefficient. Preferably the wavelength 
is in the near infrared region of 1.0 to 2.1 microns more preferably 1.0 
to 1.5 microns. 
The selection of the wavelength range is critical to this invention. 
Radiant heaters have been proposed previously for curing adhesives in 
corrugated board and laminates but the wavelengths used have resulted in 
absorption of the radiant energy near the surface of the paperboard and 
the heat input into the adhesive layer has been by conduction through the 
paper. This did not radically improve the speed of bond development and 
there was little advantage compared to the use of conventional methods of 
heat input. For example U.S. Pat. Nos. 4,169,007 and 4,589,944 propose the 
use of I-R radiation in heating corrugated board and adhesives but without 
avoiding use of pressure or significantly reducing the curing time. U.S. 
Pat. No. 4,169,007 discloses the use of radiation with wavelengths within 
the range 2.6 to 3.5 microns for making single face board but without the 
aforesaid advantages of this invention. 
Tabulated data supplied by IR equipment suppliers, show that the percentage 
energy radiated within the range 1.0 to 1.5 microns and 2.5 to 3.5 microns 
varies with the peak wavelength transmission as follows. 
______________________________________ 
Source % Energy 
Peak Temperature 
% Energy in 1.0-1.5 
in 2.5 to 3.5 
wavelength 
.degree.C. micron wavelength 
micron range 
______________________________________ 
3.04 675 1.0 17.5 
2.64 816 2.2 23.8 
2.35 954 4.0 24.5 
1.75 1370 13.6 21.5 
1.50 1650 19.4 18.2 
1.23 2065 23.5 13.6 
1.05 2480 28.9 9.2 
______________________________________ 
An infra-red absorption spectrum, as supplied by the IR lamp suppliers, for 
a typical paper sheet of 185 gsm basis weight, shows that in the 2.5 to 
3.5 micron wavelength range only 5% of the incident radiation is 
transmitted and 95% is absorbed to generate heat in the paper. In the 1.0 
to 2.1 micron wavelength range 41.0% of the incident radiation is 
transmitted. Based on these figures, 18.2% of the radiant energy is then 
available for heating and gelling the starch glue, using the preferred 1.0 
to 2.1 micron wavelength range, whereas only 0.7% of the radiant energy is 
available for this purpose in the 2.5 to 3.5 micron range which is the 
range recommended in U.S. Pat. No. 4,169,007. 
Our use of IR is to effectively utilise the transmitted element. 
It is a feature of thermal radiation as described by the laws of 
Stefan-Boltzmann and Wien that the radiant emittance of the source of 
thermal energy is proportional to the fourth power of the source 
temperature and that the energy is emitted over a range of wavelengths. 
The dominant wavelength is inversely proportional to the source 
temperature. 
Infra-red radiation from a thermal source ranging in temperature from 
1100.degree. C. to 2300.degree. C. with peak wavelengths ranging between 
2.1 and 1.0 microns is the preferred radiation for effectively bonding 
liners and mediums. At these wavelengths paper, water and adhesives have a 
low co-efficient of absorption with the energy absorbed increasing with 
increasing thickness of paper and adhesive film. The radiant thermal 
energy therefore penetrates deep into the paper structure and into the 
adhesive film or bead leading to rapid temperature rise of the adhesive 
and throughout the paper structure and the adhesive. This results in rapid 
gelling of adhesive and therefore rapid bond development. The high 
concentration of water in the adhesive film or bead in particular 
contributes to a high energy absorption and temperature rise in the glue. 
Totally transmitted radiation is reflected back through the paper layers 
and the adhesive film or beads by the metal surface against which the 
paper rests or by reflector plates situated opposite the source of radiant 
energy. 
At 1100.degree. C. to 2300.degree. C. temperature range, the radiant 
emittance of the source is sufficiently high to effectively bond the 
product components at required commercially viable operating speeds to 
make practicable the use of small radiating areas required by the 
constraints of the commercial corrugator design. 
It is preferred that the I-R radiators are disposed axially along the 
rollers and not tangentially. The amount of energy for bonding may be 
reduced significantly by locating the lamps axially and parallel to the 
sheet surface. Such an arrangement allows the bank of I-R lamps to conform 
to the sheet which passes past the I-R banks. Such conformance results in 
higher efficiency and has reduced the amount of energy used in bonding by 
60%. The facility to conform to curved surfaces allows concentration of 
the IR beam and application to areas where space is a limitation. 
Preferably the air pressure at the face of the board adjacent the radiant 
energy source should be as close to ambient pressure as possible. 
In another aspect of this invention Infra-red radiation is used instead of 
steam heated rollers to prepare mediums for corrugation. 
The traditional method of heating and softening the mediums to assist in 
their conforming to the flute profiles of the corrugating rolls and 
maintaining that conformance is to heat the rolls by steam injection into 
roll bodies. With the use of high intensity infra-red energy applied to 
the mediums before they enter the corrugating labyrinth equivalent heating 
and softening of the sheets can be obtained. The high intensity short 
wavelength unit has a tuning range 1.0 to 2.5 mcirons which is used to 
optimise the heating and softening of variable thickness paper sheets. 
Another advantage is a significant, if not total, reduction in the use of 
steam for heating corrugating roll bodies and avoiding the need to 
manufacture the corrugating rolls as pressure vessels. This avoidance also 
increases the rigidity of the roll bodies and trunnions so reducing roll 
vibration. 
The use of high intensity electrical energy does involve installation of 
additional transformer capacity, cabling and switchgear. 
Conventionally starch or starch based water soluble adhesives have been 
used for bonding corrugated boards. Starch adhesives are quite acceptable 
for use with the present invention and curing times are greatly decreased. 
However, polymeric adhesives are also suitable and provide an adequate 
bond comparable or superior to that of starch based adhesives. 
The various aspects of this invention are applicable to conventional 
corrugated board manufacture but is also applicable to the production of 
unconventional board having two intermediate corrugated mediums bonded at 
their flute tips. This type of board and methods of manufacture are 
described in European Patent Specifications 213957, 279609 and 
Applications 88311884.6 and 89903961.4.

In the conventional single facer of FIG. 1 a medium 1 is corrugated between 
the corrugating rolls 4 and 5. Adhesive is applied to the flutes of medium 
1 at adhesive applicator station 6 and a liner board 2 is then passed 
around pressure roller 7 to press the liner 2 onto the fluted medium 1 to 
form the single faced board 3. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The product of this process exhibits the pressure lines. 
In FIG. 2 the medium 1 is passed in the usual way through the nip of 
corrugating rolls 10 and 11 and subsequently adhesives applied to the 
flutes of medium 1 by the adhesive applicator 12. The medium 1 is heated 
by an I-R radiator 14 of this invention prior to entering the corrugating 
labrynth between corrugating rolls 10 and 11. 
A liner board 2 is passed over the S-wrap rollers 13 to contact and lightly 
adhere to the fluted medium 1. The adhesive bond between the medium 1 and 
liner 2 is bonded by using an I-R heater 14 of this invention which has a 
face shaped to conform to the surface of the liner as it passes over the 
corrugating roll 11. The heater 14 extends axially parallel to the roll 
11. The single faced board 3 is thus formed without the application of 
excess pressure or the creation of pressure lines in the single faced 
board. 
In FIG. 3 the production of unconventional board is illustrated in which 
the two mediums 22 and 23 are bonded together and the liner boards 21 and 
24 respectively are in turn bonded to the joined mediums as disclosed in 
European Patent 213957. 
The two mediums 22 and 23 are corrugated on corrugating rolls 33 and 34 and 
31 and 32 respectively. I-R radiators 44 which are axially aligned and 
shaped to conform to the surface of the corrugating rollers, are disposed 
adjacent corrugators 31 and 33 to preheat and soften the mediums 23 and 22 
respectively prior to corrugation. As medium 23 passes over the roller 32 
adhesive is applied by adhesive station 35. Mediums 22 and 23 are then 
bonded together at their flute tips by passing through the nip of 
synchronised rollers 32 and 34 and the joined mediums are then passed onto 
the carrier roll 37. Adhesive is applied to the outer medium 22 at the 
adhesive station 38 and the liner 21 is brought into low pressure contact 
with the mediums by feeding through a series of feed rolls including 
S-wrap rolls 39. 
After applying the liner the mediums and liner are then carried by carrier 
roll 37 past the radiant energy station 44 which is axially aligned above 
the carrier roll 37, The I-R radiators 44 are shaped to conform to the 
contour of the roll 37. 
The adhesive used by adhesive applicator 12 in FIG. 2 or adhesive 
applicators 32 and 38 in FIG. 3 may be starch or a polymeric adhesive. 
Alternatively thermoplastic film capable of bonding paperboard may be fed 
between the nip of corrugating rolls 10 and 11 in FIG. 2 or 32 and 34 in 
FIG. 3 and subsequently subjected to I-R radiation to melt the film into 
an adhesive layer. Similarly a polymer film may be fed with the liner 
board 2 in FIG. 2 or liner 21 in FIG. 3 and thus do away with the adhesive 
stations. 
It should be noted that the apparatus as shown in FIG. 3 can be used to 
make conventional corrugated board by simply not using corrugating rollers 
31 and 32. 
The radiant energy sources 14 in FIG. 2 and 44 in FIG. 3 are incandescent 
sources at 2100.degree. C. emitting in a wavelength range centred at 1.2 
.mu.m. The surface of the carrier roll 37 acts as a reflector. 
The I-R lamps are preferably located at a distance of 30-40 mm from the 
surface of the liner being bonded. Cooling air may be supplied to the 
lamps to prolong lamp life and provide rapid cooling down and should have 
an exit air pressure in the range zero to 70 mm water gauge. 
Zero pressure I-R units or "closed face" bonding units which draw in air at 
the operating face are an alternative which are judged to be equally 
effective or more effective than the I-R units cooled by air forced from 
within the units and discharging onto the sheet. 
After leaving the carrier roll 37 adhesive is applied to exposed medium 23 
and a liner 24 is applied to the single faced double flute structure by 
roll 42, The liner 24 is brought to the roll 42 over a conventional array 
of rollers including S-wrap feed rollers 41. 
The adhesive bond is formed by use of the radiant heaters 44 as described 
above. 
In the case of the second liners a reflector plate 45 is used to reflect 
radiation back on to the corrugated board, Preferably an air cushion is 
created over the surface of the reflector plate 45 to protect the 
reflective coating from abrasion by the corrugated board. 
At speeds of 80 to 100 meters per minute bonding times of less than a 
second can be achieved compared to conventional bonding times of up to 4 
seconds at the double backer station. 
By the use of this invention the liner medium bonds do not exhibit 
"pressure lines" or "wash boarding" and both sides of the double wall 
board are suitable for printing. 
The key process advantages are: 
1. Absence of the steam heated platen unit which occupies extensive floor 
space on the conventional line. 
2. Absence of the "bridge" between single facer and double backer which 
requires transfer belts and facilities to assist feeding to the double 
backer. Both advantages 1 and 2 allow a much smaller floor area to be 
used. 
3. Reduced losses as a conventional machine stoppage results in board 
remaining in the steam platen area and overheating to lose required 
quality. 
4. Reduced adhesive usage compared to conventional double backers.