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BACKGROUND OF THE INVENTION
This invention relates to non-skid surface compositions applied to the
surface of a material to increase the surface friction. More particularly,
this invention is directed to silica- or alumina-containing non-skid
surface compositions applied to paper products such as paperboard and
linerboard used in the manufacture of, for example, corrugated boxes,
folding carton stock, communication paper packaging, and containers.
Corrugated boxes and containers, as well as other paper packaging products,
are used in the shipment of a wide variety of goods. In many instances,
the boxes and containers are stacked for storage and shipment, and the
toppling of a stack may cause damage to the contents thereof.
Accordingly, it is desirable for the surfaces of paperboard and linerboard
containers and boxes to have a high surface friction so that the stacked
boxes will be less likely to shift and topple during handling. However,
achieving a sufficiently high surface friction on paperboard products has
been made more difficult in recent years because of the trend toward the
use of recycled fiber in paper and liner board products, in some instances
up to 100% recycled fibers may be used. The use of large percentages of
recycled fibers in the finished paper has led to increasing problems in
the handling of the paper, both in the mill and at converters, because of
its increased slipperiness. This loss in the coefficient of friction is
due to the shorter fibers which result from the additional processing
required by recycling as compared to virgin paper fibers, as well as from
contaminants introduced with the used paper. These contaminants include
dirt, wax, cold set and hot melt adhesives, defoamers, water proofing, and
other special coatings, etc. Various paper fiber processing techniques
have been employed to remove some of these contaminants prior to paper
formation but have had only marginal success in removing the wax-like
contaminants which cause the paper to become slippery.
Thus, the combination of shorter recycled fibers and wax-like contaminants
have had a detrimental effect on the finished linerboard coefficient of
friction, or slip-angle, as it is commonly measured by the TAPPI methods
T815-OM90 and T815-OM92. For instance, virgin linerboard will generally
have TAPPI-method slip-angles of between 17.degree. and 25.degree..
Typical linerboard made with some of recycled fiber will have slip-angles
of from 15.degree. to 20.degree., and some boards having a high percentage
of recycled fibers, and their concomitant contaminants, can have
slip-angles of between 10.degree. and 15.degree.. Slip-angles of less than
20.degree. have been found to be the cause of several paper and container
handling problems.
One method used in the past to increase the surface friction of paperboard
products has been to apply frictionizing agents to the exterior surfaces
of the boxes and containers, typically by spraying the surfaces of the
boxes with a frictionizing agent in a carrier or solvent. Such
frictionizing agents increase the slip angle (a measure of surface
friction) of the boxes.
For example, Carstens, U.S. Pat. Nos. 4,418,111 and 4,452,723, teach the
use of a combination of an aqueous suspension of colloidal silica and urea
as a non-skid agent. Payne et al, U.S. Pat. No. 4,980,024, teach the use
of an aqueous antiskid composition which includes silica sol, 2 percent
glycerine, and 5 percent polyacrylamide binder.
However, such prior art compositions have presented problems relating to
application and clean up of the compositions. Accordingly, the need still
exists in the art for a non-skid surface composition which is easy to
apply and clean up, which is nontoxic and noncorrosive, and which does not
affect the ability to recycle the paper to which it is coated.
SUMMARY OF THE INVENTION
The present invention meets that need by providing a non-skid surface
composition for paper and paperboard products which is easy to apply and
to clean up, which is noncorrosive and nontoxic, and which increases the
coefficient of friction on the surfaces of products coated with the
composition. Further, the composition does not interfere with the ability
to recycle the paper to which it has been coated. The present invention
also provides a coated cellulosic product and method of manufacture.
In accordance with one aspect of the present invention, a non-skid surface
composition is provided and consists essentially of an aqueous suspension
glycerine with at least one material selected from the group consisting of
colloidal silica and alumina. Preferably, the colloidal silica or alumina
has a mean particle size in the range of from about 5 to 150 nm to provide
enhanced frictional characteristics to the surfaces of coated products.
In the aqueous suspension, the ratio of colloidal silica or alumina to
glycerine is between about 1:1 to 5:1, and preferably about 2:1.
Typically, the colloidal silica or alumina comprises from about 5 to 35%
by weight based on the total (wet) weight of the composition, and the
glycerine comprises from about 1 to 13% by weight based on the total (wet)
weight of the composition, with the balance of the composition being
water. However, depending upon the particular application, the solids
content of the composition may be varied. No other components such as
binders are needed. Optionally, the composition may include an additional
polyol having the formula HOCH.sub.2 (CHOH).sub.n CH.sub.2 OH, where n is
a number from 1 to 4, such as, for example, sorbitol, or a sugar such as,
for example, sucrose or glucose.
The present invention also is directed to cellulosic products such as
paperboard, linerboard, and corrugated board which are formed into boxes,
bags, and other containers. By coating the surfaces of such products with
the composition of the present invention, the coefficient of friction and
angle of skid of these products is enhanced to provide for secure stacking
and handling of these products. Also provided is a method of making such
coated products which includes the steps of applying the composition to
the surfaces of the products from an aqueous suspension and then
evaporating the water from the coating.
Accordingly, it is a feature of the present invention to provide a non-skid
surface composition for paper and paperboard products which is easy to
apply and to clean up, which is noncorrosive and nontoxic, and which
increases the coefficient of friction on the surfaces of products coated
with the composition. This, and other features and advantages of the
present invention will become apparent from the following detailed
description and the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the present invention, the glycerine may be combined
with a silica or alumina sol or colloidal suspension of silica or alumina
in any convenient manner. Thus, the glycerine may be dissolved in water
and then combined with the colloidal silica or alumina sol and the
concentrations of the components adjusted by proper dilution.
Alternatively, the glycerine may be added directly to the sol. A typical
non-skid treatment employed by many paper mills consists of spraying a
dilute aqueous dispersion. of colloidal silica on the surface of the paper
product. The colloidal silica solutions are frequently commercially
supplied as 50% by weight dispersions. In use, these dispersions are
further diluted by the end user as much as 5:1 or 6:1, resulting in a
sprayable composition containing less than 10% solids (dry basis). It has
been found that when the present invention is used, a lower concentration
of colloidal silica or alumina may be employed to obtain equivalent or
higher slip-angles than when colloidal silica or alumina alone is used.
The composition of the present invention also provides good retention of
the coefficient of friction and thus the slip angle for coated surfaces
even after repeated slides such as would be encountered in actual use. The
composition of the present invention is effective on both virgin paper
fibers as well as recycled products.
While various combinations of silica and glycerine have proved suitable, it
has been found that a silica or alumina to glycerine weight ratio (dry
basis) of between about 1:1 to about 5:1 is most effective. While greater
amounts of glycerine may be used (for a ratio of less than 1:1),
performance of the composition is not further enhanced and added cost
becomes a factor. The most preferred ratio, however, is between about 1:1
to 2:1.
Mean particle sizes of the colloidal silica or alumina should fall within
the range of from about 5 to 150 nanometers. The preferred mean particle
size comprises a range of 20 to approximately 50 nanometers. Colloidal
silica and alumina are available commercially from a number of suppliers
including Nalco Chemical Company, Du Pont, and Vinings Industries, Inc.
We have found that the present invention is effective with colloidal silica
or alumina present in the composition in the range of from about 5 to 35%
by weight, based on the total (wet) weight of the composition. Thus, the
amount of colloidal silica or alumina present in the composition may vary
widely depending upon the preference of the end user and the method of
application of the composition to the paper product. Typical application
methods include spraying, roll coating, sponge application, or use of a
doctor blade.
We have also found that the amount of glycerine present in the composition
should be between about 1 to 13% by weight, based on the total (wet)
weight of the composition. Glycerine may be added to the composition in
its substantially pure, dehydrated state, or can be added as its hydrated
form, glycerol. The glycerine acts as both a humectant and a binder for
the colloidal silica or alumina. No additional or separate binders need-to
be added to the composition. That is, where a sufficient amount of
glycerine is present in the composition as we have taught, the colloidal
silica or alumina particles will remain in soluble form and not completely
crystalize and harden because of the water retained by the glycerine. Yet,
the particles will be firmly held to the surface of the paper product and
function to increase the skid angle of papers coated with the composition.
One major disadvantage of prior art silica/urea non-skid compositions was
that those compositions would dry on the hands of workers and on the
application equipment used and would present significant clean-up problems
including skin irritation. The composition of the present invention in its
preferred form never completely dries out, remains soluble, and thus is
readily cleaned from skin and applicators with just water. We have found
that where the percentage of glycerine in the composition is at the upper
end of the above-disclosed range, or where the glycerine to silica or
alumina ratio approaches 1:1, the composition does not dry out. Thus,
there is no expensive down time for laborious cleaning of equipment and no
clogging of spray tips used in many spray applicators currently used in
the industry. However, the use of lesser percentages of glycerine within
the above-disclosed range will still provide a product which increases the
skid angle of paper products to which it has been applied.
Further, where the preferred method of application is by spraying, the
composition of the present invention can be applied more evenly to paper
surfaces because it will not clog spray tips. Smaller diameter spray tips
may be used to apply higher solids content compositions. This decreases
warpage problems with the paper and requires less water to be evaporated
during drying.
The composition of the present invention is relatively insensitive to pH
changes. Thus, the pH of the composition may be adjusted to insure that
there will be no adverse interaction of the composition with inks printed
on the paper products which are coated. The composition of the present
invention is nontoxic and noncorrosive. Further, it does not affect the
ability of the coated paper products to be recycled.
Other optional components may be added to the composition. For example, an
additional polyol having the formula HOCH.sub.2 (CHOH).sub.n CH.sub.2 OH,
where n is a number from 1 to 4, such as, for example, sorbitol may be
added in amount of from about 0 to 10% by weight, based on the total (wet)
weight of the composition. Such a polyol may be in addition to the
glycerine in the composition or as a partial replacement for the
glycerine. Alternatively, a sugar such as, for example, sucrose or glucose
may be added to the composition. Small amounts (less than 0.5%) of
bactericides or the like may also be added.
In order that the invention may be more readily understood, reference is
made to the following examples, which are intended to be illustrative of
the invention, but are not intended to be limiting in scope.
EXAMPLE 1
A colloidal silica/glycerine composition was prepared to yield the
following concentrations (percentage by wet weight):
______________________________________
Silica.sup.1
33.50%
Glycerine
8.59%
Proxel .RTM..sup.2
0.10%
Water 57.81%
______________________________________
.sup.1 50% solids colloidal suspension
.sup.2 Bactericide, from ICI Americas, Inc.
The composition provided a 2:1 weight ratio (dry basis) of silica to
glycerine.
The composition was coated using a blade onto a relatively smooth piece of
42 pound kraft liner board. The liner board was cut into two pieces and
the coated surfaces were placed face to face in the same machine
direction. The angle of skid between the two coated pieces was measured
using the TAPPI procedure T815 OM-92. The results are reported below, with
successive test numbers representing successive tests to determine not
only the angle of skid but also whether the angle would drop off with
repeated skid testing.
______________________________________
Test Run # Angle of Skid
______________________________________
1 34.0.degree.
2 32.0
3 32.5
4 31.5
5 30.0
6 32.0
7 32.0
8 32.0
9 32.0
10 32.5
______________________________________
The average angle of skid was 32.05, with a standard deviation (.sigma.-1)
of 0.98. The decrease in angle of skid from first to last test run was
1.5.degree.. Such a decrease was well within statistical variation to be
able to infer that there was no significant drop in the coefficient of
friction on the liner board surface.
EXAMPLE 2
A second colloidal silica/glycerine composition was prepared to yield the
following concentrations (percentage by wet weight):
______________________________________
Silica.sup.1
25.00%
Glycerine
13.02%
Proxel .RTM..sup.2
0.10%
Water 61.88%
______________________________________
.sup.1 50% solids colloidal suspension
.sup.2 Bactericide, from ICI Americas, Inc.
The composition provided a 1:1 weight ratio (dry basis) of silica to
glycerine. When the water was evaporated from a small portion of the
sample, the solids (silica and glycerine) remained in liquid form, and the
silica did not crystallize or harden.
The composition was coated and then tested as described in Example 1. The
results are reported below, with successive test numbers representing
successive tests.
______________________________________
Test Run # Angle of Skid
______________________________________
1 34.5.degree.
2 32.5
3 33.0
4 32.5
5 33.0
6 33.5
7 32.5
8 33.0
9 31.5
10 31.5
______________________________________
The average angle of skid was 32.75, with a standard deviation (.sigma.-1)
of 0.89. The decrease in angle of skid from first to last test run was
3.0.degree..
While certain representative embodiments and details have been shown for
purposes of illustrating the invention, it will be apparent to those
skilled in the art that various changes in the methods and apparatus
disclosed herein may be made without departing from the scope of the
invention, which is defined in the appended claims.
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