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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to foam plastic sheeting, and a process for its
manufacture, that possesses paper-like characteristics.
2. Description of the Prior Art
Foamed plastic has found wide acceptance as a versatile and relatively
inexpensive material. It is light-weight, highly compressible, has
excellent insulating properties, lends itself readily to molding into
products with good eye-appeal and can be formed into rigid structures
having strength and cushioning properties which surpass those of paper
products for many uses.
One area in which foamed plastic has been found to be deficient when
compared to paper is that of flexibility. Foam sheeting, although it can
be thermoformed into an almost infinite variety of shapes and
configurations, normally tends to rupture upon being folded or bent. By
addressing itself to, and solving, such a problem, the present invention
provides a foam product that greatly increases the scope of applications
in which foamed plastics may be used to advantage.
SUMMARY OF THE INVENTION
The present invention comprises a flexible, dead-foldable foamed plastic
sheet having a compressed cellular structure and being further
characterized by having in close proximity to at least one of its two
major surfaces a quantity of fractured compressed cells sufficient in
number such that the sheet is rendered dead-foldable. The compressed sheet
hereinafter disclosed has both appearance and folding characteristics
approaching or superior to those of paper and cardboard products while
retaining the more desirable insulating and cushioning characteristics of
foam and even improving on the tensile properties of the original foam
sheet. The present invention also encompasses a process useful for making
such flexible sheet material.
In accordance with this invention, a foam plastic sheet, e.g. polystyrene
foam, is sandwiched between surfaces, at least one of which is textured
(e.g., screen like), placed in a press or other suitable compressing means
and compressed to reduce the thickness of the sheet while simultaneously
fracturing a substantial portion of the cells of said foam sheet at one or
both surfaces of said sheet. Thus, the purpose of this procedure is at
least threefold:
(1) to compress the cell structure of the foam;
(2) to partially destroy the resilience and surface character of the foam;
and
(3) to provide means whereby the compressed gaseous matter trapped within
the foam structure can escape (e.g., by virtue of the channels formed by
the screen-like structure penetrating the surface of said foam sheet).
In specific embodiments, the compression step may be effected by placing
the foam sheet between surfaces having a screen-like character, between
surfaces having numerous small, needle-like protrusions, between surfaces
having numerous closely spaced protrusions or grooves, combinations of
such devices, or with a smooth or decoratively embossing surface against
one or both sides of the foam sheet.
Following the above compression step and when desired to obtain a sheet
with a smoother surface and further compressed, the foam sheet can be
subjected to a second compression between two smooth surfaces to further
densify, or reduce the thickness of the foam sheet and also to restore
some of the smooth character to the surface of the sheet.
Foam sheet processed in the aforedescribed manner possesses greatly
improved folding characteristics. The extent to which it approaches any
particular grade of paper or cardboard in appearance and character depends
largely upon the pressure used in each of the respective stages of
compression, when two compression steps are used, and upon the spacing of
the protrusions in the initial compression. Additionally, the product of
this process, particularly when two stages of compression are used,
possesses excellent paper-like surface characteristics. It may be written
upon with ink or pencil, typed on or imprinted with a stamp or printing
press. Using suitable adhesives, it may be laminated to foil, plastic film
or any other desired material and decorative or informative labels may be
similarly attached.
The mechanical properties (including tensile strength, modulus, break
strength and percent elongation) of compressed foam sheet as embodied
herein are also enhanced relative to the foam sheet in its original form.
BRIEF DESCRIPTION OF THE DRAWINGS
To facilitate understanding of this invention reference is made in the
description of the embodiments to the drawings in which:
FIG. 1 is a perspective view of an apparatus assembly useful for carrying
out the process of this invention;
FIG. 2 is a side elevation of the assembly of FIG. 1;
FIGS. 3, 4 and 5 are sections of the primary roller of the assembly of FIG.
1 showing alternate surface texture patterns useful in the practice of the
invention and foam sheeting after being compressed thereby;
FIGS. 6 and 6A are diagrammatic views showing another embodiment of the
process of this invention wherein flat screen-like plates are used in the
initial compression step and arrows are used symbolically to indicate the
compression means; and
FIGS. 7 and 7A are diagrammatic views of the embodiment of FIG. 6 wherein
flat smooth plates are used in the secondary compression step and arrows
are again used symbolically to indicate the compression means.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one of the preferred embodiments the process of this invention is
carried out by forming a sandwich comprising a sheet of foam plastic
material contacted on top and bottom by a flat plate and applying suitable
pressure thereon, as illustrated in the drawings by FIGS. 6, 6A, 7 and 7A.
More specifically, a sheet of foam material 12 is placed between two
textured plates 13a and 13b, herein shown as having a screen-like
structure but other designs having closely spaced protrusions and/or
grooves as may occur to one skilled in the art and will accomplish the
intended purpose may be substituted therefor. The resultant sandwich
(i.e.--textured plate 13a, foam sheet 12 and textured plate 13b) is then
compressed by suitable means, illustrated symbolically in FIG. 6A by
arrows 14.
After this initial compression step the textured plates 13a and 13b are
removed, leaving foam material 15 which has a thickness less than that of
the original foam sheet 12 and contains impressions of the surface pattern
of plates 13a and 13b. This foam material 15 may then be placed between
plates 16a and 16b, if a smoother or thinner sheet is desired, said plates
having smooth surfaces and the resulting sandwich compressed by suitable
means as illustrated in FIG. 7A by arrows 17. In this second compression
step the foam material is further reduced in thickness and also the depth
and intensity of the surface impressions left by 13a and 13b are reduced,
thereby restoring a degree of smoothness to the surface of the foam sheet.
The product resulting from this process will be thinner and more dense than
the starting foam sheet 12, will have relatively smooth surface character
and will be highly flexible and foldable as well as possessing improved
mechanical strength.
Foam sheet material processed in the described manner is capable of being
dead-folded, which is to say that the material may be creased and folded
back upon itself and the foam sheet in the immediate vicinity of the fold
will thereafter remain substantially in the folded configuration. The
surface of such folded sheet will not rupture merely by virtue of its
being folded, as would the surface of a conventional compressed or
uncompressed foamed plastic sheet if one were to attempt to fold such
conventional sheet back upon itself. The present material is thereby
suitable for folding into cartons, box liners, bags, wrappers for various
commodities, etc., or for use as a flat sheet in such applications
traditionally filled by paper as typewriting and printing medium,
envelopes, stationery, and so forth.
An embodiment of the process of this invention is further illustrated by
FIGS. 1 and 2 in the drawings. Referring to FIG. 1, foam plastic sheet 2
is fed from supply source 1, which may be a foam extrusion device, a roll
of previously formed foam material or any other suitable source of foam
plastic material, to a pair of counter-rotating nip rollers 3a and 3b.
Foam sheet 2, having a thickness of 10 mils to 200 mils and preferably 10
mils to 100 mils and a density of about 0.02 gm. per cubic centimeter to
about 1.0 gm. per cubic centimeter and preferably 0.050 gm. per cubic
centimeter to 0.25 gm. per cubic centimeter, is passed between nip rollers
3a and 3b, the surfaces of which comprise a screen-like structure, and is
initially compressed or reduced to desired thickness (preferably 3 mils to
25 mils) by means of a predetermined pressure applied on foam sheet 2 by
rollers 3a and 3b via a suitable (e.g., piston type) load producing means
8 (see FIG. 2).
The textured surfaces of rollers 3a and 3b partially penetrate the surface
of foam sheet 2 (see FIG. 5) at the same time that it is being compressed
by said rollers, thereby effecting surface cell fracture and facilitating
the escape of some of the gaseous material contained within the now
compressed internal cell structure of the foam sheet. The rupture of at
least a substantial portion of the compressed cells provides a maximum
reduction in the thickness of the foam sheet as it passes between the
rollers and prevents the sheet material from returning to its original
thickness after the applied pressure is removed. The degree of thickness
reduction imparted to the foam sheet in this step will depend upon several
factors, primarily the pressure applied between the nip rollers and the
amount of cell rupture, and these factors may be varied to achieve
end-product characteristics as desired.
Upon leaving rollers 3a and 3b the foam material 4, reduced in thickness
from that of foam sheet 2 and containing impressions of the surface
pattern of rollers 3a and 3b, may be passed between counter-rotating
smooth surface nip rollers 5a and 5b. Rollers 5a and 5b press against foam
4 with a predetermined force applied via a suitable load producing means 9
(see FIG. 2), further reducing the thickness of the foam material and also
reducing the depth and intensity of the impressions left by 3a and 3b
thereby restoring a desired degree of smoothness to the surface of the
foam sheet. The amount of additional thickness reduction achieved over the
initial compression step can be substantial and will depend primarily upon
the pressure applied between 5a and 5b.
The thinned, flexible, relatively smooth product 6, having a thickness
which is preferably in the range of 2 mils to 20 mils, is then wound on
take-up roll 17 for transport and storage. Alternately, product 6 may be
fed directly to appropriate subsequent process means where it is cut or
otherwise formed into a desired end product.
Load producing means 8 and 9 are shown as functioning with rollers 3a and
5a, respectively, and comprising a piston-like construction for
illustrative purposes only. They may be attached to any combination of
rollers and be of any construction which would accomplish the intended
functions as set forth in the preferred embodiments.
The pattern of protrusion, grooves, indentations, etc. used on the rollers
in the primary compression step may be of any design to accomplish the
desired amount of cell rupture. The drawings illustrate, as embodiments,
four sample patterns which may be used in the practice of this invention:
FIG. 1 shows a pattern of rectangular, screen-like protrusions; FIG. 3
shows a similar gridwork which is diagonally disposed on the roller; FIG.
4 shows a pattern of closely spaced needle-like protrusions; and FIG. 5
illustrates a series of closely spaced ridge-like structures. Other
patterns which may occur to one skilled in the art may be substituted for
those without departing from the spirit and scope of this invention.
In practice of this invention, one or more of the rollers, for instance 3a
or 5a, may be replaced with another roller designed to impart a purely
decorative embossing on one side of the foam sheet to achieve some
particular and desired effect. One such decorative effect would be to give
the product an alligator skin-like texture, but many other designs are
possible and will be obvious to those skilled in the art.
EXAMPLES
In a plurality or runs, as set forth in Table I, polystyrene foam sheets
ranging in average thickness from 90 mils to 121 mils were in individual
runs sandwiched between two screens of defined mesh size. The sandwich was
then placed in a press and initially compressed at 800-5,000 psi. The
initially compressed polystyrene foam sheets, upon removal from the press,
were then placed between two smooth surface plates and compressed again,
this time at 4,775 psi. The following Table I tabulates in detail for each
run the thickness of the starting polystyrene foam sheet and of the foam
sheet after each compression step, the mesh size of the screen assembly
for the initial compression step, the pressure employed for each
compression step and the percent change in thickness resulting from each
step.
TABLE I
__________________________________________________________________________
Primary Compression Secondary Compression
Step Step
Average Initial
Mesh Average Sheet
% Average Sheet
%
Run No.
Thickness Size
PSI Thickness
Change
PSI Thickness
Change
__________________________________________________________________________
1 91.8 mils
20 378 34.3 mils
62.6 4775
18.8 mils
79.5
2 100.3 mils
20 796 35.6 mils
64.6 4775
15.3 mils
84.7
3 107.8 mils
20 1,591
25.8 mils
76.1 4775
15.3 mils
85.8
4 115.3 mils
20 3,182
24.4 mils
78.5 4775
15.0 mils
87.0
5 91.0 mils
20 3,182
21.8 mils
76.0 4775
14.0 mils
84.6
6 90.5 mils
20 4,775
22.6 mils
75.0 4775
13.3 mils
85.3
7 117.0 mils
40 1,591
29.3 mils
75.0 4775
18.2 mils
84.4
8 117.8 mils
40 2,387
21.6 mils
81.7 4775
14.4 mils
87.8
9 102.3 mils
40 3,182
25.3 mils
75.3 4775
12.3 mils
88.0
10 102.3 mils
40 4,775
15.1 mils
85.2 4775
12.1 mils
88.2
11 121.0 mils
60 1,591
35.0 mils
71.1 4775
19.8 mils
83.0
12 96.3 mils
60 2,387
12.9 mils
86.6 4775
11.4 mils
88.2
13 94.0 mils
60 3,182
11.9 mils
87.3 4775
10.6 mils
88.7
14 107.8 mils
60 4,775
13.8 mils
87.2 4775
12.4 mils
88.5
15 114.0 mils
80 3,182
27.7 mils
75.7 4775
16.6 mils
85.4
16 115.0 mils
80 3,978
26.0 mils
77.4 4775
17.1 mils
85.1
17 91.3 mils
80 4,775
10.9 mils
88.1 4775
10.2 mils
88.9
18 110.3 mils
smooth
1,591
56.3 mils
49.0 4775
31.6 mils
71.3
19 94.0 mils
surface
3,182
40.4 mils
57.0 4775
26.8 mils
71.5
20 106.0 mils
plates
4,775
34.8 mils
67.2 4775
22.5 mils
78.8
I
__________________________________________________________________________
The data shows the effect of varying the pressure used in the initial
compression step for any given mesh size and also the effect obtained from
varying the mesh size used in that same step. As can be seen, each mesh
size has a minimum pressure which must be applied to obtain maximum
compression, but the application of substantially greater pressure beyond
that minimum point produces little additional benefit. As the mesh size is
increased (i.e.--the openings become smaller) the minimum pressure applied
to achieve optimum compression is also increased.
Run Nos. 18, 19 and 20 are included for purposes of comparison. In these
runs the screens used in the initial compression step were removed and
replaced with flat plates so that the end product was the result of two
pressings between flat plates. As can be seen from the data, even at the
highest pressures the reduction in thickness was less than that obtained
by practice of this invention. In addition, the compressed foam sheet
obtained from these runs was rigid and brittle, unlike the compressed
sheets obtained from Run Nos. 1 through 17 which were bendable and
foldable without rupturing.
These runs were all conducted at ambient temperature and, since the amount
of compression resulting from any given combination of mesh size and
pressure is a function of the temperature of the foam sheet, it must be
realized that the degree of compression shown in Table I is a reflection
only of the operating parameters used and is in no way limiting. It is
contemplated that as the temperature is increased the optimum pressure
required for compression at any given mesh size will decrease. One of the
preferred embodiments places the process of this invention directly on the
production line after the extruder and it is to be expected that the foam
sheet will be at elevated temperature. The process of this invention is,
in fact, useful at temperatures up to and including the glass transition
temperature of the foam plastic material.
Table II set forth data on the results of standard mechanical tests carried
out on the products from the noted runs of Table I. The testing methods
were as follows:
______________________________________
Tensile Modulus A.S.T.M. D-638
Break Strength A.S.T.M. D-638
% Elongation A.S.T.M. D-638
______________________________________
TABLE II
__________________________________________________________________________
Tensile Modulus
Break Strength
Elongation at
.times.
10.sup.4
psi
.times.
10.sup.3
psi
Break, %
No. MD TD 45.degree.
MD TD 45.degree.
MD TD 45.degree.
__________________________________________________________________________
Control
(uncompressed foam)
0.97
0.59
0.66
0.17
0.14
0.16
4 4 4
2 4.73
2.10
2.11
1.35
0.72
0.69
9 6 6
10 1.60
0.80
1.18
0.39
0.26
0.28
4 4 4
11 3.85
1.25
2.76
0.85
0.40
0.70
6 11 9
12 3.79
2.44
2.37
0.81
0.54
0.56
5 8 9
13 4.56
2.24
2.53
0.79
0.49
0.60
4 7 9
14 2.45
0.95
1.19
0.70
0.40
0.40
7 6 6
15 1.34
0.91
1.07
0.46
0.35
0.38
8 8 9
17 2.72
0.66
2.27
0.69
0.31
0.64
6 9 8
__________________________________________________________________________
All of the tested samples exhibited increased break strength and tensile
modulus over the results obtained for the sample of uncompressed foam.
With the exception of Run No. 10, all of the samples also showed increased
elasticity (% elongation at break) over the uncompressed foam. While Run
No. 10 gave the same value for elongation as did the control sample, there
was no indication of loss of elasticity.
Table III demonstrates the effect of compressing foam material of different
thicknesses using identical conditions for compression. Primary
compression for all samples was carried out using a 60 mesh screen at 3180
psi. Secondary compression was between smooth plates at 4,800 psi.
TABLE III
______________________________________
Average
Starting Final Final
Thickness Density Thick- Thickness
Density
Sheet Mils gms/cc ness Mils
Change, %
gms/cc
______________________________________
A 50 .114 13.5 73.0 .423
B 81 .074 20.0 75.3 .300
C 98 .124 49.0 50.0 .248
D 78 .098 13.0 83.3 .588
E 90 .051 10.0 88.8 .459
______________________________________
Table IV shows the results of standard mechanical tests conducted on the
materials of Table III. The testing methods were as follows:
______________________________________
Tear Strength Trouser Tear (50"/min.)
Tensile Strength ASTM
D-638
Tensile Modulus ASTM
D-638
______________________________________
TABLE IV
______________________________________
Density Thick- Tear St.
Sample (gm/cc) ness 50"/min.
Tensile St.
Modulus
Foam I F (mils)
(gm/mil)
(psi) psi .times. 10.sup.3
______________________________________
A MD .114 .423 13.5 2.0 2850 75
TD 6.0 1080 25
B MD .074 .300 20.0 3.7 1280 39
TD 4.4 630 17
C MD .124 .248 50.0 4.7 1260 50
TD 4.3 930 30
D MD .098 .588 13.0 3.0 2430 86
TD 8.4 1330 66
E MD .051 .459 10.0 2.3 2400 93
TD 3.9 2050 51
______________________________________
MD = Machine Direction
TD = Transverse Direction
I = Initial
F = Final?
Although the present invention has been described with reference to the
preferred embodiments, it should be understood that modifications and
variations may be resorted to without departing from the spirit and scope
of the invention, as those skilled in the art will readily understand.
* * * * *
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