U.S. patent number RE29,272 [Application Number 05/694,046] was granted by the patent office on 1977-06-21 for water resistant corrugated paperboard.
This patent grant is currently assigned to Westvaco Corporation. Invention is credited to Harold L. Hintz, Joseph T. Webb.
United States Patent |
RE29,272 |
Hintz , et al. |
June 21, 1977 |
Water resistant corrugated paperboard
Abstract
A water resistant corrugated paperboard material is prepared
wherein the paperboard components (medium and liner or liners) are
each coated on both sides with a thermoplastic film which acts as a
water barrier and a water resistant adhesive for bonding the
components together. The corrugated medium is also pretreated with
an internal size treatment to prevent edge wicking, and, for the
purpose of corrugating the medium without deleteriously affecting
the thermoplastic film, the corrugating rolls are preheated to a
temperature slightly less than the melting point of the
thermoplastic film while a lubricating material is simultaneously
applied to either the corrugating rolls or the medium at the
corrugating nip.
Inventors: |
Hintz; Harold L. (Charleston,
SC), Webb; Joseph T. (Charleston, SC) |
Assignee: |
Westvaco Corporation (New York,
NY)
|
Family
ID: |
26956471 |
Appl.
No.: |
05/694,046 |
Filed: |
June 7, 1976 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
273835 |
Jul 21, 1972 |
03849224 |
Nov 19, 1974 |
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Current U.S.
Class: |
156/208; 156/210;
427/411; 427/326; 428/182 |
Current CPC
Class: |
B31F
1/2804 (20130101); Y10T 156/1021 (20150115); Y10T
156/1025 (20150115); Y10T 428/24694 (20150115) |
Current International
Class: |
B31F
1/28 (20060101); B31F 1/20 (20060101); B31F
001/22 () |
Field of
Search: |
;156/205,208,210
;106/238 ;162/164 ;428/179,182,184,186,171 ;427/326,411 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Casey, Pulp and Paper, Second Ed., 1960, pp. 1052-1057..
|
Primary Examiner: Simmons; David A.
Claims
We claim:
1. In the process of manufacturing water resistant corrugated
paperboard, consisting of at least one linerboard component having
both sides thereof coated with a film of thermoplastic material,
and at least one corrugating medium component having both sides
thereof coated with a film of thermoplastic material, the steps
of:
a. pretreating the said corrugating medium component .Iadd., before
both sides thereof are coated with the film of thermoplastic
material, .Iaddend.with an internal size treatment selected from
the group consisting of 0.75- 1.0% rosin with about 1.5% alum, 0.2%
stearic anhydride with 0.3-0.4% alum, or 0.2% succinic anhydride
with 0.3-0.4% alum to inhibit the water wicking tendencies
thereof;
b. applying a direct pre-heat treatment to one side of the said
coated and sized corrugating medium prior to entering the
corrugating nip of a corrugating machine;
c. applying an indirect pre-heat treatment to the opposite side of
the said coated and sized corrugating medium by heating at least
one of the corrugating rolls of the corrugating machine;
d. applying a lubricating treatment in the form of a water spray or
a water spray containing detergent to the corrugating nip of the
corrugating machine;
e. applying a direct pre-heat treatment to one side of the said
coated linerboard component prior to entering the bonding nip of a
corrugating machine;
f. applying an indirect pre-heat treatment to the opposite side of
the said coated linerboard component by heating the pressure roll
of the corrugating machine;
g. applying an additional direct heating treatment at the bonding
nip of the corrugating machine to render the thermoplastic film
surface on one of the components tacky and bondable;
h. pressing the tacky and bondable surface of one of the components
into contact with the other of said components at the bonding nip;
and,
i. cooling the laminate thus formed to allow the bond to set.
2. The process of claim 1 wherein the thermoplastic material is a
thermoplastic polyolefin.
3. The process of claim 2 wherein the thermoplastic polyolefin is
polyethylene.
4. The process of claim 3 wherein the polyethylene film thickness
is in the range of from 0.75 - 1.0 mil thick.
5. The process of claim 4 wherein the direct pre-heat treatments
for both the corrugated medium components and the linerboard
component are in the range of 200.degree. F.
6. The process of claim 5 wherein the temperature of at least one
of the corrugating rolls is in the range of from
150.degree.-215.degree. F.
7. The process of claim 6 wherein the lubricating treatment is
applied directly to both sides of the corrugated medium.
8. The process of claim 7 wherein the lubricating treatment is also
applied to the corrugating rolls of the corrugating machine.
9. The process of claim .[.10.]. .Iadd.8 .Iaddend.wherein the
additional direct heating treatment is of a temperature in excess
of 450.degree. F.
10. The process of claim 9 wherein subsequent components of the
laminate are bonded to the laminate formed by pre-heating the
bonding surfaces of at least two of the components to a temperature
in the range of about 200.degree. F. and applying additional direct
heating treatments in excess of 450.degree. F. at the bonding nip
to render the polyethylene film surface on one of the components
tacky and bondable.
Description
SUMMARY OF INVENTION
The present invention relates generally to an improvement in
corrugated paperboard and specifically to a water resistant
corrugated paperboard packaging material and method for producing
the packaging material.
Corrugated paperboard is widely used in the packaging industry
where substantial container strength and an economical construction
is desired. However, the water absorption and moisture penetration
characteristics of conventionally manufactured corrugated
paperboard have made it unusable for containers which are subjected
to either high humidity or direct water cooling and/or ice
containing conditions. For instance, although some attempts have
been made to use conventional corrugated paperboard as the material
of construction for iced containers, which use ice as a
refrigerant, the water produced by the melting ice soaks into the
paperboard so rapidly and produces such a weakening effect on the
containers, that complete collapse of the containers frequently
occurs before the product reaches its destination. Similarly
containers which are subjected to hydrocooling conditions, where
water is sprayed directly onto the container to maintain the
freshness of the product packaged therein, also suffer
deleteriously from the effects of water penetration and absorption.
In addition, shipment and storage conditions frequently subject
corrugated paperboard containers to high humidity conditions or
even exposure to rain or to snow, over extended periods of time.
Under such conditions, even if the container does not weaken to the
point of collapse, it is still ineffective to protect the contents
of the container from the penetration of moisture through the walls
of the container.
In an effort to overcome the susceptibility of corrugated
paperboard containers to structural weakening and failure upon
contact with water or water vapor, it has previously been suggested
to coat the paperboard liners or medium, or both, with a water
resistant sizing or coating material. Other suggestions have been
directed toward the application of a thermoplastic film to one or
both of the paperboard liners or medium especially as taught by
Canadian Pat. No. 879,643. Unfortunately, none of these previously
offered solutions serve to produce a completely satisfactory
product. In the case of the water resistant sizing or coating
technique, the problems encountered have included both an
unsatisfactory penetration of the water proof material into the
paperboard itself, or, a cracking of the water resistant film when
the paperboard blanks are folded and set-up. With respect to the
thermoplastic film laminated products, the high cost of production
and the difficulty in corrugating the thermoplastic coated medium,
or in obtaining a good bond between the thermoplastic coated medium
and/or the liner or liners on the machine, except at slow speeds,
have been the detrimental factors. In addition, no satisfactory
means has yet been developed for preventing moisture from wicking
into the edges of thermoplastic coated paperboard material and
producing a devastating weakening effect to the otherwise strong
material.
However, by the present invention, each of the limitations and
disadvantages of the prior art methods have been overcome by
providing an improved corrugated paperboard and method of
manufacturing corrugated paperboard to achieve a water resistant
corrugated paperboard having superior WVTR (water vapor
transmission rate) properties, excellent rigid when wet
characteristics, and good runnability on the machine. Broadly
stated, the present invention comprises a method of constructing an
improved water resistant corrugated paperboard, which method
employs a thermoplastic coating on both sides of the corrugated
medium and the liner or liners. Accordingly, the thermoplastic
coating serves as both the water barrier and the water resistant
adhesive for bonding the corrugated paperboard together. In
addition, the present invention also contemplates the use of a
novel size treatment for the corrugated medium to inhibit any
wicking of water from the edges through the coated medium. And
finally, the invention further incorporates a combined heating and
lubricating step at the corrugator to insure proper corrugation of
the thermoplastic coated medium without cracking or, otherwise
deleteriously affecting the thermoplastic coating.
In an illustrative embodiment, the product produced by the present
invention may be described more specifically as a water resistant
rigid when wet corrugated paperboard material wherein the
paperboard components (medium and liner or liners) are each coated
on both sides with a thermoplastic polyolefin such as polyethlene.
The polyolefin coating on both sides of the medium and liner board
is preferably in the range of 0.75 - 1.0 mil thick, or even
thicker, and serves as a water barrier and as a water-resistant
adhesive for bonding the components together. The corrugated
paperboard product is produced on conventional corrugating
equipment, except that in the process of the present invention,
wicking of water or other moisture from the edges through the
linerboard and medium is controlled by a special synthetic or rosin
sizing treatment to both the linerboard and the medium. In
addition, for the purpose of carrying out the process of the
invention, i.e., the making of the novel water resistant corrugated
paperboard product described herein, the corrugating rolls on the
corrugating machine are preferably heated to a temperature slightly
below the melting point of the polyolefin coating while
simultaneously a lubricating material is applied to either the
surface of the corrugating rolls, or to both sides of the medium at
the entrance to the corrugating nip. When polyethylene is used as
the preferred polyolefin, the corrugating rolls are preferably
heated to a temperature of between 150.degree.-215.degree. F., and
the preferred lubricating material consists of either a fine plain
water spray or a water spray containing a mild detergent.
Thus it may be stated that the novelty in the present invention as
regards the product produced lies in the use of a thermoplastic
polyolefin coating of at least 0.75 mil thick on both sides of both
the medium and the linerboard, in combination with a size treatment
to the corrugating medium consisting of by weight either at least
0.2% synthetic size, in the form of a stearic anhydride or succinic
anhydride emulsion with 0.3 - 0.4% alum, or 0.75 - 1.0% rosin size
with up to 1.5% alum. Since corrugating medium is normally unsized,
to enable the medium to pick up the usual starch adhesive when
forming conventional corrugated paperboard, the mere fact of
applying a size treatment to the corrugating medium itself,
according to the present invention, represents a departure from the
conventional manufacture of corrugated paperboard. Moreover, the
preferred sizing applied to the medium according to the present
invention is of a higher level than is normally used even on
linerboard. Conventionally, linerboard is internally sized with a
rosin/alum system containing up to 0.5% rosin. Accordingly, the
sizing treatment of the present invention serves to contribute to
the surprising results obtained with the corrugated paperboard
produced, particularly as regards strength retention after a 24
hour water soak test, since the sizing treatment prevents water
from wicking through the unprotected edges of the corrugated medium
and linerboard, and through holes or cracks that might develop in
the polyolefin film coating. In addition, the considerable increase
in the waterproofness of the corrugated paperboard produced herein
may be attributed to the thickness of the polyolefin film applied
which serves to control the overall WVTR of the board. Moreover,
since the crush strength of corrugated paperboard is greatly
reduced by moisture absorption, the thickness of the polyolefin
film is very significant with respect to the ultimate strength of
the board after soaking.
With respect to the process for producing the improved water
resistant corrugated paperboard according to the present invention,
it was discovered that a sheet of sized, polyolefin coated
corrugating medium, with a film thickness in excess of 0.75 mil on
each side, could not be readily corrugated on conventional
equipment without the addition of two significant steps. First, it
was found that flutes could not be formed in the film coated medium
without heating the medium to a temperature slightly less than the
melting temperature of the film. This was accomplished by directly
heating one surface of the medium and indirectly heating the other
surface of the medium by applying heat to one of the corrugating
rolls. Secondly, it was found that even with the heated medium,
cracks still tended to develop in the film until a lubricating
material was applied to the medium at the corrugating nip. Thus it
is believed that the novel steps introduced into the art by the
present invention, as regards the process described, constitute the
limitations whereby the film coated corrugating medium is heated to
a temperature to render the thermoplastic film soft, and, a fine
spray of lubricating water is applied to the film coated medium or
to the corrugating rolls themselves at the nip of the corrugating
rolls.
For the purpose of bonding the polyolefin coated medium and
linerboards, it was only necessary to apply sufficient heat to one
or both of the polyolefin films to render the surface thereof tacky
just prior to the bonding nips. In this manner, the components were
heat sealed to one another to form the almost completely water
resistant paperboard material disclosed herein.
DESCRIPTION OF DRAWING
FIG. 1 shows schematically the method of manufacturing single face
paperboard according to the present invention;
FIG. 2 shows schematically the method of manufacturing double face
paperboard according to the present invention; and,
FIG. 3 is a view in cross-section of the water resistant corrugated
paperboard produced according to the present invention.
DETAILED DESCRIPTION
The present invention is directed to an improved water resistant
corrugated paperboard material manufactured in a heat sealing
operation from thermoplastic coated paperboard components wherein
the thermoplastic coating is preferably a thermoplastic polyolefin
material having a film thickness on the order of from 0.75 - 1.0
mil thick. The linerboard component used, in addition to being
precoated both sides with a polyolefin such as polyethylene, is of
conventional linerboard material. However, the corrugating medium
preferred is non-conventional in that prior to coating the
corrugating medium on both sides with a polyolefin film such as
polyethylene, the paperboard material from which the corrugating
medium is made is specially treated with a size treatment comprised
of either at least a 0.2% synthetic size, in the form of a stearic
anhydride or succinic anhydride emulsion in the presence of from
0.3 - 0.4% alum, or a rosin size of from 0.75 - 1.0% rosin in the
presence of about 1.5% alum. In addition, the thickness of the
polyethylene film coating preferred is at least 0.75 mil or
thicker, thus yielding a water resistant corrugated paperboard
material having excellent rigid when wet characteristics. The
polyethylene film is preferably extruded simultaneously on both
sides of the medium (before corrugating) and the linerboard
material in a single pass through an extruder, although two passes
coating one side at a time could be used. The polyethylene film on
both sides of the medium and the linerboard material serves as an
excellent water barrier giving good WVTR properties to the
components, and as a water resistant adhesive for bonding the
component elements of the corrugated paperboard together. The
synthetic or rosin size treatment to the corrugating medium further
serves to insure a high retention of strength to the corrugated
paperboard by preventing wicking of water into the medium from the
edges of the paperboard, or moisture penetration through the
linerboards into the medium under severe humidity or water soak
conditions. Furthermore, even though the rosin size treatment is
considered the preferred treatment, it is believed that other
sizing materials such as asphaltic emulsions, alkylketenedimers,
waxes and fatty acids could be used with similar success.
The invention is also directed to a novel method of manufacturing
water resistant corrugated paperboard wherein the paperboard
components consist of one or more linerboard members and at least
one corrugated medium, each of which is coated on both sides with a
polyolefin film such as polyethylene. The method of manufacture for
a single facer operation comprises the preliminary step of applying
to both sides of a linerboard component and a sized corrugating
medium component a thermoplastic film of polyethylene or the like
having a thickness in excess of 0.75 mil. The polyethylene coated
corrugating medium is then conducted to a corrugating section of a
conventional corrugating machine where the pre-coated medium is
preferably slightly pre-heated and passed over a heated corrugator
roll and into contact with another heated corrugator roll, in the
presence of a lubricating water spray shower at the nip of the
corrugator rolls, to form corrugations in the medium without
cracking or otherwise deleteriously affecting the polyethylene film
on the medium. Subsequently, the corrugated medium and the single
face linerboard are adhered to one another by applying to either
the linerboard or corrugated medium, at the bonding surface, a
direct heating treatment to render the surface of one or both of
the components tacky or slightly melted. At the same time, both the
corrugated medium and the single face linerboard are applied with
an indirect heat treatment, through one of the corrugating rolls
and through the pressure roll, respectively, to assist in the
bonding operation. The heat applied indirectly to the linerboard
and the corrugated medium, through the pressure roll and the
corrugating roll respectively, is critical since too much heat
causes the coated linerboard or medium to adhere to the pressure
roll or corrugating roll and too little heat tends to conduct the
required heat for bonding away from the bonding nip.
Similarly, for a double facer operation, the single face board
taken from the above described operation would be bonded to another
polyethylene coated linerboard component in substantially the same
way. For example, the single face corrugated board taken from the
previous operation would be applied with a direct heat source on
its fluted surface and the second polyethylene coated linerboard
would be directly heated as before to a temperature slightly less
than the melting point of the polyethylene (less than 200.degree.
F.) to render the surfaces tacky and bondable. Finally, in the
presence of an additional direct heating source at the double face
combiner station, the single face corrugated board and the double
face linerboard would be bonded together and then passed between a
series of cold plates or belts to allow the different components to
be completely bonded into the water resistant corrugated paperboard
material described herein.
Referring now more particularly to FIG. 1 of the drawing herein, a
schematic illustration of the preferred means for manufacturing
single face corrugated paperboard according to the present
invention is shown in FIG. 1, a web 10 of linerboard is shown as
being unwound from a roll 11. The linerboard material 10 as
previously described is of a conventional type, except that for the
purposes of the present invention, both sides thereof have been
previously coated with a film of a thermoplastic polyolefin
material such as polyethylene. As stated hereinbefore, the
thickness of the film of polyethylene is preferably at least 0.75
mil, but may be between 1.0 - 5.0 mils in thickness depending upon
the type of duty intended for the final product.
After being unwound from the roll 11, the coated linerboard web 10
is then given a pre-heat treatment by the heater 19 prior to being
conducted to the pressure roll 28 as shown. The pre-heat treatment
at 19 is intended only to condition and instigate a slight
softening of the outside film of polyethylene on the linerboard web
10 prior to entering the bonding nip at 18. Accordingly the
temperature at the pre-heater 19 is only on the order of
200.degree. F. for a typical polyethylene coating.
Meanwhile, still referring to FIG. 1, there is also illustrated a
roll 13 of corrugating medium 12 which, as previously pointed out,
is decidedly unconventional. For the purposes of the present
invention, the corrugating medium 12, in addition to being
pre-coated both sides with a film of polyethylene or the like, as
in the case of the linerboard just described, also includes another
special treatment to render it not susceptible to the edge wicking
of moisture in any form. Namely, the corrugating medium 12 includes
a size treatment which may comprise either at least a 0.2%
synthetic size in the form of a stearic anhydride or succinic
anhydride emulsion with from 0.3 - 0.4% alum, or a rosin size of
from 0.75 - 1.0% rosin in the presence of about 1.5% alum. The
preferred size treatment comprises a wet end internal treatment,
although it is believed that a surface size or gate roll size
treatment would achieve similar results providing the latter two
treatments were able to obtain good penetration of the size
material into the medium. Accordingly, in view of the fact that
conventional corrugating medium is normally unsized, .Iadd.the
.Iaddend.treatment described is believed to be one of the key
factors in the surprising success of the present invention as will
be further illustrated later on.
However, for the purpose of carrying out the present invention, the
size treated and pre-coated medium component 12 is conducted to the
corrugating section 20 of the corrugating machine in a
substantially conventional manner except that, prior to reaching
the corrugating roll 23, the outside surface of the polyethylene
coated medium 12 is given a direct pre-heat treatment by the
pre-heater 17. In the case of a polyethylene coated medium 12, the
pre-heater 17 would be set at a temperature of around 200.degree.
F. since its purpose is only to slightly soften the outside film
surface of the medium 12 preparatory to entering the corrugating
nip 20. If other thermoplastic coatings were used on the medium 12,
the temperature of the pre-heater 17 would be adjusted
accordingly.
At the corrugating section 20, the medium 12 is passed around a
heated corrugating roll 23 and then between the roll 23 and another
heated corrugating roll 22. Each of these rolls 22, 23 are
preferably internally heated to a temperature slightly less than
the melting point of the polyethylene film, or in the range of from
150.degree. - 200.degree. F., depending upon the speed of
operation. The temperature applied internally through the
corrugating rolls 22, 23 is critical as pointed out hereinbefore
since it must be high enough to permit the coated medium 12 to be
corrugated yet low enough to prevent the coating from sticking to
the corrugating rolls. Thus it may be seen that both surfaces of
the pre-coated medium are heated, by the pre-heater 17 and the
corrugating roll 23, prior to entering the corrugating nip 20 to
render the medium 12 pliable and susceptible to being corrugated
without cracking or otherwise deleteriously affecting the
thermoplastic coating.
However, as pointed out hereinbefore, the fact that the medium is
heated before corrugating is not a sufficient preparation for
successfully corrugating a pre-coated medium having either a high
coat weight (i.e., a thick coating in excess of 0.75 - 1.0 mil), or
at a high speed. Accordingly, a pair of lubricating sprays 24, 25
are added to the corrugating section .[.on.]. 20 to either spray
both sides of the medium 12 before entering .[.79.]. the
corrugating nip 20, or to spray directly onto the corrugating rolls
22, 23.
FIG. 1 illustrates a spray nozzle 24 adjusted to lubricate either
the inside surface of the medium 12 or directly onto the roll 23.
Similarly, a spray nozzle 25 is also shown in FIG. 1 which
lubricates the outside surface of the medium 12 or the roll 22. In
this manner, the heated corrugating rolls 22, 23 allow the medium
to be bent in forming the flutes and the lubricant, in the form of
either a fine mist of plain or detergent containing water, acts as
a means for reducing friction as the coated medium 12 is forced
between the corrugating rolls 22, 23. It should further be
appreciated that only a schematic view of the invention is
illustrated, and for a full scale model a shower pipe or a
plurality of nozzles would be needed throughout the width of the
corrugating section 20.
After passing between the nip of the corrugating rolls 22, 23, the
now corrugated medium 12 is shown as being bonded to the single
face linerboard 10 at the combining station 18. To accomplish this
step, the bonding surface of the linerboard 10, as previously
stated, is preheated at 19 to a temperature of about 200.degree.
F., and brought into nip engagement with the indirectly heated
corrugated medium 12 between the internally heated (150.degree. -
200.degree. F.) pressure roll 28 and the corrugating roll 22.
Again, as pointed out hereinbefore, the temperature of the pressure
roll 28 must not be too high or the coated linerboard will tend to
adhere to the roll. Moreover, the temperature of the pressure roll
28 cannot be too low or the heat necessary for bonding will be
conducted away from the bonding nip. To assist in the bond and
insure that the proper surface of the corrugated medium becomes
tacky, additional direct heat is applied to the bonding surface of
the medium 12 and/or the linerboard 10 by the heating means 29. For
the purposes of the present invention, it has been found that a
hot, air burner having a capacity of at least .[.5000,000.].
.Iadd.500,000 .Iaddend.Btu/hr. connected to a large blower, is
sufficient to produce a hot air stream on the order of 450.degree.
F. and thus insure a good bond between the medium and the
linerboard at respectable machine speeds (in excess of 150
ft./min.) After the combiner station, the now single face
corrugated paperboard is passed around a first guide roller 30 and
over a second guide roller 31 and onto an endless conveyor 32 upon
which the single face board is accumulated for the purpose of
cooling the board and allowing the bond to become completely
set.
FIG. 2 illustrates a preferred method for making double face
corrugated paperboard from the single face board obtained from FIG.
1. In this view, the single face board accumulated on the endless
conveyor 32 is conveyed over a guide roller 33 past a preheat
station 34 and into a nip created between the cold plates or belts
at 37, 38. Meanwhile, the linerboard 14 is stored on a roll 15 and
as before, with the linerboard 10, has coated on both sides thereof
a film of a thermoplastic material such as polyethylene. The
linerboard 14 is accordingly passed around a guide roller 39 and
past a pre-heat station 36 and into the nip at 37, 38. At each
pre-heat station 34, 36, the bonding surfaces of the fluted medium
12 and the linerboard 14 are each pre-heated to about 200.degree. F
where polyethylene film is used, and for the purpose of rendering
the surfaces tacky or melted enough to become bonded, a second
heating means 35 is arranged to provide direct heating treatments
in excess of 450.degree. F to the fluted medium and/or liner
thereby .Iadd.insuring .Iaddend.a good bond. Finally, the double
face corrugated paperboard is then permitted to cool between the
cooling plates or belts 37, 38 until the bond sets.
In order to demonstrate the surprising results obtained as regards
the wet strength retention of the corrugated paperboard produced in
accordance with the present invention, tests were conducted wherein
boxes were constructed from different kinds of corrugated
paperboard and subjected to top-to-bottom compression strength and
24 hours water soak strength retention measurements. For the
purpose of comparison, the boxes were constructed from a
conventional, nontreated corrugated paperboard referred to as
"Domestic," a cascade wax treated corrugated paperboard wherein the
box blanks were coated with wax on each side in a curtain coater,
and referred to as "Cascade Waxed;" a wax-impregnated curtain
coated corrugated paperboard product called "Cote-A-Cor 600;" and,
the polyethylene coated product of the present invention referred
to as "Polyboard." The "Polyboard" sample had a 0.75 - 1.0 mil
thick film of polyethylene applied to each side of each component.
In each case, the basis weight of the different paperboard
components and the corrugated paperboard constructions were
substantially the same except that for the Cote-A-Cor 600 box and
the Cascade Waxed box, the fluted medium was C-flute and for the
other examples, the fluted medium was A-flute. For the purpose of
explanation, the number of flutes per linear foot is greatest with
C-flute, and the height of the flutes between the two facings is
greatest with A-flute. Therefore, it might be expected that C-flute
corrugated paperboard would be somewhat stronger than A-flute
corrugated paperboard. Some of the sample boxes were tested for
their top-to-bottom compression strength under standard atmospheric
conditions, and other samples were subjected to a 24 hour water
soak condition and then tested for top-to-bottom compression. Based
on the test described, the following data was obtained.
TABLE I ______________________________________ Top-to-Bottom
Compression Strength and % Strength Retention
______________________________________ Paperboard Std. Cond. 24
Hour Soak Strength Grade (lbs.) (lbs.) Ret. %
______________________________________ Polyboard 596 483 81.0
Cascade Waxed 713 91 12.8 Cote-A-Cor 600 872 82 9.4 Domestic 578
Delaminated -- ______________________________________
For the samples tested, the load the boxes were able to withstand
under standard conditions varied depending upon the kind of fluted
medium used as explained before. However, the amount of strength
retained by each box after the 24 hour water soak treatment was
directly related to the water absorption characteristics of the
paperboard material used. In the case of the box constructed from
Domestic .[.paperboards.]. .Iadd.paperboard.Iaddend., water
absorption was immediately observed when the box was submerged in
the water tank and delamination occurred. For the box constructed
from Cote-A-Cor 600 and the Cascade Waxed box, the water absorption
was not immediate upon dunking, but the wicking of the fluted
medium plus the moisture absorbed through the linerboard to the
fluted medium in each case was severe enough to permit internal
collapse of the paperboard material when load was applied after the
24 hour soak. In the case of the Polyboard boxes, the water
absorption was very low, and significantly, there was practically
no visible wicking of the size treated corrugated medium compared
with the other boxes, the loss in strength of the Polyboard box
after soaking was only slight, as noted in the data, and the loss
that did occur was directly related to fine cracks or pin holes in
the polyethylene coating applied to the components.
In order to further demonstrate the dramatic retention of wet
strength achieved by applying a size treatment to the
conventionally unsized corrugated medium a second test was
conducted wherein corrugated paperboard samples were prepared using
various types of medium. As before, conventionally sized linerboard
was used, coated on both sides with a 0.75 - 1.0 mil thick film of
polyethylene, and this linerboard was bonded to different examples
of corrugated medium also coated on both sides with a 0.75 - 1.0
mil thick film of polyethylene. The different corrugated mediums
chosen were a typical unsized medium, a medium sized with rosin in
the preferred manner set forth herein (0.75 - 1.0% rosin with 1.5%
alum), a medium sized with a wet strength resin (urea
formaldehyde), and a medium fabricated from conventional linerboard
material. The results of the test are shown below and illustrate
the surprising wet strength retention that could be expected by
sizing the corrugating medium in accordance with the present
invention.
TABLE II ______________________________________ Top-to-Bottom
Compression Strength and % Strength Retention
______________________________________ Std. 24 hr. Water Strength
Paperboard Cond. Soak Abs. Ret. Grade (lbs.) (lbs.) % %
______________________________________ Polyboard 594 141 55.4 23.7
(unsized med.) Polyboard 620 186 62.2 30.0 (w/s med.) Polyboard 550
174 19.2 31.6 (linerboard med.) Polyboard 540 397 18.5 73.5 (sized
med.) ______________________________________
Thus it may be seen from the results set forth above that when a
regular (unsized) corrugated medium was used, the compression
strength after soaking for 24 hours was only .[.2.37.].
.Iadd.23.7.Iaddend.% of its original value. The addition of a wet
strength resin to the medium raised the strength retention only to
30%, and, even with conventionally sized linerboard as the medium,
the wet strength retention amounted only to 31.6%. Since linerboard
is conventionally sized, it could have been expected that with the
linerboard as the medium, the strength retention would have been
higher. Later observations found that the Polyboard constructed
using linerboard as the corrugated medium were not properly coated
in the polyethylene extruder. Hence, this problem was believed to
be a contributing factor in the low strength retention found with
the linerboard corrugated medium. However, the only dramatic
increase in wet strength retention was obtained when the corrugated
medium was treated according to the present invention.
Other experiments illustrated that polyethylene was not the only
material that could be used to coat the paperboard components. When
linerboard and corrugating medium having a hot melt adhesive
coating were tested, the moisture absorption and strength retention
of these elements were found to be similar to the preferred
Polyboard product described herein.
In addition, the use of a hot melt adhesive to coat the paperboard
components would offer an excellent means for squaring and bonding
the manufacturers joint of carton blanks made from the corrugated
paperboard disclosed. Other methods of joining the carton blanks
made from the water resistant paperboard of the present invention
would consist of stitching, the use of cold setting adhesives, or a
simple heat seal treatment. Finally, because of the superior wet
strength retention and improved dry strength of the thermoplastic
coated paperboard described herein, it is conceivable that the
paperboard components used could be of a lower basis weight
material than that normally required for a given purpose. Thus,
cost savings in raw material could be achieved with the novel
corrugated paperboard material described herein.
It may be seen that the water resistant corrugated paperboard
described herein is capable of obtaining and retaining excellent
rigid when wet strength characteristics heretofore unattainable by
any competitive product. In fact, the corrugated paperboard
disclosed, with a sized medium as preferred, exhibits the best
rigid when wet performance of any corrugated paperboard ever
tested.
Thus, from the foregoing discussion, it should be apparent that a
substantial improvement in water resistant corrugated paperboard
has been achieved by the present invention. Accordingly, even
though only a schematic showing of the details of the present
invention have been presented for the purposes of description, it
should be apparent that certain modifications and innovations could
be added to the disclosure without departing from the principles of
the invention set forth. Therefore, what is intended to be covered
by Letters Patent is embraced in the following claims.
* * * * *