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BACKGROUND OF THE INVENTION
Tapes having pressure-sensitive adhesive layers exceeding 0.1-0.2 mm in
thickness tend to be difficult and expensive to manufacture and to have
low shear strength. For applications requiring greater thickness,
foam-backed pressure-sensitive adhesive tapes such as disclosed in
Canadian Pat. No. 747,341 are often employed. However, the porous nature
of the foam involves a number of problems such as a tendency to wick
liquids. The elastic memory of some foams tends to cause them to lift from
low spots on rough or uneven surfaces. Foam layers of less than about 1.0
mm are difficult to manufacture and hence rather expensive.
Brochman U.S. Pat. No. 3,565,247 was in part directed to the problem of
economically producing pressure-sensitive adhesive layers exceeding 0.1 mm
in thickness, disclosing a microcellular adhesive layer which is both a
foam and a pressure-sensitive adhesive. When compressed to half its
original thickness, the microcellular adhesive layer typically shows less
than 5 percent recovery. Apparently the surfaces at the opposite sides of
each cell adhere to each other to inhibit recovery. Such tapes are not on
the market.
OTHER PRIOR ART
Belgian Pat. No. 675,420 which was published May 16, 1966, concerns process
and apparatus for making conventional pressure-sensitive adhesive tape of
acrylate copolymers such as were earlier disclosed in U.S. Pat. No. Re.
24,906 (Ulrich). In the process of the Belgian patent, a mixture of
monomers is coated onto a backing sheet and then polymerized in situ to a
pressure-sensitive adhesive state. The polymerization may be initiated by
ultraviolet light or by heat if the mixture includes a heat-activatable
polymerization initiator. A typical monomer mixture which is polymerizable
by ultraviolet light comprises 90 parts ethyl-hexyl acrylate, 5 parts
acrylic acid and 5 parts polyvinylisobutylic acid. While the Belgian
patent does not mention the thickness of its pressure-sensitive adhesive
layers, the layers of the examples have coating weights of 10-50
g/m.sup.2, from which it can be deduced that the thicknesses were about
0.01 to 0.05 mm. The Belgian technique should permit somewhat greater
thicknesses, although when using ultraviolet light, the maximum thickness
would be limited by the transparency of the polymerizable coating.
An article by Blackley and Sheikh, "Particulate Reinforcement of
Polyacrylate Elastomers", Rubber Chemistry and Technology, Vol. 48 (1975),
pages 819-859, concerns the effect of fine-particle filler in a
polyacrylate elastomer matrix. Specifically, glass beads were dispersed
into a prepolymer syrup of poly(ethyl acrylate) dissolved in ethyl
acrylate, benzoin and ethylene glycol dimethacrylate which in sheet form
was subjected to ultraviolet light to complete the polymerization. Average
diameter of the glass beads was about 3 micrometers (page 823).
A large number of patents relating to the manufacture of glass microbubbles
suggest their use as fillers for a variety of materials. After making such
a suggestion, Yeatch U.S. Pat. No. 2,797,201 states: "The particles may
also be adhered together, using various techniques or binders, to produce
a cellular type material of the nature of plastic foam and expanded
plastics, for use as thermal, electrical and sound insulation material
board, plaster board, gaskets, seals . . . " (col. 10, lines 35 ff.)
Jonnes U.S. Pat. No. 3,524,794 discloses a fluid sealing gasket comprising
a vulcanized elastomer and glass microbubbles occupying about 50% of the
volume of the gasket. The preferred thickness of the gasket is about one
millimeter.
Erwin U.S. Pat. No. 3,314,838 discloses a spreadable liquid composition
comprising a pressure-sensitive adhesive, a volatile liquid vehicle and
glass microbubbles. The composition may be spread onto a substrate such as
a billboard and allowed to dry to provide a coating as shown in the
drawing wherein the diameter of a typical microbubble exceeds the average
thickness of the pressure-sensitive adhesive layer. Flexible sheet
material which is laid over the coating contacts only the tips of its
"goose-flesh" surface and hence can be slid into precise position. After
it is in position, pressure sufficient to break the microbubbles is
applied to force the surface of the sheet into full contact with the
adhesive. Implicit in the Erwin disclosure is the concept of applying the
adhesive to the flexible sheet material instead of to the billboard or
other substrate.
THE PRESENT INVENTION
The present invention provides what is believed to be the first truly
economical tape having a pressure-sensitive adhesive layer of 0.2 to 1.0
mm in thickness. However, the novel tape can be economically produced at
thicknesses as small as 0.1 mm and as great as 2.5 mm or more.
The pressure-sensitive adhesive layer of the novel tape offers good
resistance to both peel and shear forces and also possesses an
extraordinary combination of properties, e.g., being fairly elastic under
briefly applied stresses but having very low elasticity after stress is
maintained for a period of time. When pressed against a rough surface, the
adhesive flows into and remains in intimate contact with minute contours
after the pressure is removed. For example, the ultimate softness of the
adhesive permits the novel tape to be used to mount a decorative metal
medallion to cover round-head screws, because the adhesive can flow around
the screw heads to make permanent contact with the underlying surface. In
contrast, the elasticity of a typical foam-backed pressure-sensitive
adhesive tape would tend to lift it from the underlying surface.
Typically, the adhesive layer of a pressure-sensitive adhesive tape of the
present invention, when tested at a thickness of 3 mm, has a Shore 00
hardness of at least 50 at one second and at most 30 at 30 minutes.
In short, the tape of the invention comprises a pressure-sensitive adhesive
layer consisting essentially of a polymeric pressure-sensitive adhesive
matrix and glass microbubbles having a specific gravity not exceeding 1.0
(measured in bulk) dispersed throughout the matrix. Although the novel
tape has the physical appearance of a foam-backed pressure-sensitive
adhesive tape, its polymeric matrix is substantially free from voids
except for the hollow cavities of the individual microbubbles. In tests on
a number of tapes of the invention, the pressure-sensitive adhesive layer
showed no water absorption.
The novel tape can be made in essentially the same manner as the tape of
the aforementioned Belgian Pat. No. 675,420 except that glass microbubbles
are dispersed into the polymerizable mixture before it is coated out.
Ideally the polymerization is initiated by ultraviolet light, in which
event both the polymerizable mixture and the microbubbles must be
reasonably transmissive of ultraviolet light. The ultraviolet transparency
is enhanced if the walls of the microbubbles are thin. Furthermore, glass
microbubbles having thinner walls tend to be less expensive on a volume
basis. Hence, their specific gravity is preferably less than 0.2, ideally
less than 0.1.
Instead of employing ultraviolet light, the matrix may contain a
heat-activatable polymerization initiator and hence be polymerized by
heat. This permits one to use microbubbles which are opaque to ultraviolet
light, but the process may be slower and thus more expensive than
polymerization by ultraviolet light.
The matrix may be coated onto and polymerized against a backing sheet which
has a low-adhesion surface from which the adhesive layer is readily
removable or onto a backing sheet to which it remains permanently adhered,
e.g., aluminum or steel foil, crepe paper or a plastic film such as
cellulose acetate or biaxially-oriented polyethylene terephthalate film.
The average diameter of the glass microbubbles should be 10-200
micrometers. Microbubbles of smaller average diameter would tend to be
unduly expensive, whereas it would be difficult to coat out a
polymerizable mixture containing microbubbles of larger average diameter.
Preferably the average diameter of the microbubbles is within the range of
20 to 80 micrometers. The glass microbubbles should comprise 20-65 volume
percent of the pressure-sensitive adhesive layer. It would be unduly
difficult to try to make a coherent void-free coating at higher
percentages, whereas the advantages of the invention would not be
significantly realized at less than 20 volume percent of the microbubbles.
Preferably 45-55 volume percent of the pressure-sensitive adhesive layer
comprises glass microbubbles.
The thickness of the pressure-sensitiveadhesive layer should exceed three
times the average diameter of the microbubbles and twice the diameter of
substantially every microbubble. This allows the microbubbles to migrate
within the adhesive under applied pressure instead of breaking, and the
adhesive can flow into intimate contact with rough or uneven surfaces,
while retaining its foam-like character. Optimum performance in this
respect is attained if the thickness of the pressure-sensitive adhesive
layer exceeds seven times the average diameter of the microbubbles.
When the polymerizable mixture has a viscosity of less than 1000 cps. prior
to addition of the microbubbles, it is desirable to employ a thixotropic
agent such as fumed silica to keep the microbubbles uniformly dispersed.
Even in the presence of a thixotropic agent, it is desirable after storage
to stir the mixture immediately prior to coating it out to insure uniform
dispersion of the microbubbles.
Because the glass microbubbles are uniformly dispersed throughout the
polymerizable mixture before it is coated out and are small relative to
the thickness of the coating, the exposed surface of the resultant
pressure-sensitive adhesive tape tends to be smooth and can be expected to
have a root-mean-square surface roughness not exceeding 8 micrometers.
When the exposed surface of the adhesive layer is covered with a temporary
low-adhesion protective web, it will in time take on the contour of the
protective web. If that contour is rough, the adhesive layer will have a
rough surface after removal of the protective web but will quickly conform
under pressure to substrates to which it is applied to form strong
adhesive bonds.
Where it is desired to adhere the novel tape to a surface to which its
pressure-sensitive adhesive layer would not form a strong bond, it may be
desirable to apply to one or both of its faces of its microbubble-filled
adhesive layer a layer of unfilled pressure-sensitive adhesive which is
especially selected for its adhesion to that surface. For example, strong
bonds to certain automotive paint surfaces can be attained only by
pressure-sensitive adhesives which cannot be polymerized by ultraviolet
light. Typically the root-mean-square roughness of the exposed face of the
unfilled layer is less than 5 micrometers.
THE DRAWING
The single FIGURE of the drawing is a chart on a semi-log scale showing the
hardness of the pressure-sensitive adhesive layers of two tapes of the
invention as a function of time.
In the following examples, all parts are by weight except where otherwise
indicated.
EXAMPLE 1
In a 1000-ml stainless steel beaker, 294 grams of isooctyl acrylate and 6
grams of acrylic acid were blended with a 3-blade propeller mixer at 500
rpm. To this blend was added 0.75 gram of benzoin ethyl ether with
stirring until dissolved. Nine grams of fumed silica ("Cab-o-sil" M-5) was
blended in until a uniform dispersion was achieved, about 10 minutes. The
stirring rate was increased to 1500 rpm, and 33.3 grams of glass
microbubbles were added. The microbubbles had a specific gravity of 0.07
(measured in bulk--true value 0.11) and were 20-150 micrometers in
diameter (average 55 micrometers).
This material was knife-coated to a thickness of 0.75 mm onto a paper
backing sheet having a low-adhesion silicone surface. The coating was
placed under an ultraviolet lamp (Sylvania FR 40BL-235) at a distance of
15 cm in a nitrogen atmosphere containing a maximum of 150 ppm of oxygen.
After five minutes, the coating was fully polymerized without any
noticeable shrinkage. Its surface was smooth and free from wrinkles. The
resultant pressure-sensitive adhesive tape had the appearance and feel of
a foam-backed pressure-sensitive adhesive tape.
EXAMPLES 2-3
Additional tapes of the present invention were prepared in the same manner
as in Example 1 using the following materials to form the
microbubble-filled pressure-sensitive adhesive layer, except that the
UV-polymerizable mixture of Example 3 was coated to a thickness of 1.0 mm.
______________________________________
Example
Parts by weight 2 3
______________________________________
Isooctyl acrylate 67.09 80
Acrylic acid 11.84 20
Benzoin ethyl ether 0.19 0.25
Polyvinyl ethyl ether
(Union Carbide EDBM grade)
2.24 3
Hydrocarbon-type tackifier
(Hercules XPS 541) 7.45 --
Wetting agent ("Triton" X-100)
3.73 15
Fumed silica 1.5 7
Glass microbubbles of Example 1
5.96 5
______________________________________
To each face of the microbubble-filled pressure-sensitive adhesive layer of
Example 3 was laminated an unfilled pressure-sensitive acrylate copolymer
adhesive layer of the type disclosed in U.S. Pat. No. Re. 24,906 having a
thickness of 0.075 mm and carried by a paper backing sheet having a
low-adhesion surface. One unfilled layer and the microbubble-filled layer
of Example 3 were passed face-to-face between a rubber and a steel roll
which was heated to 120.degree. C. The pressure applied by the rolls was
275 kPa. Then the backing sheet of the microbubble-filled layer was peeled
away to expose its other face for lamination to a second strip of the
unfilled layer. The resultant laminated tape of Example 3 was employed in
the testing reported below.
The microbubbles occupied the following volume percentages of the
pressure-sensitive adhesive layers, exclusive of the unfilled surface
layers of the tape of Example 3:
______________________________________
Adhesive layer of Example
1 2 3
______________________________________
Volume % microbubbles
47 35 27
______________________________________
Testing
Tape of Example
1 2 3
______________________________________
T-peel in g/cm of width
(ASTM D-1876-72)
3200 4450 5320
Shear strength at 21.degree. C.
aluminum backing to stain-
less steel, 2.54 .times. 1.27 cm,
500 g 1000 g 1000 g
time to failure
20 min. <10,000 min.
<10,000 min.
______________________________________
Strips of the tape 2.5 cm in width were stretched 300% (12.5 cm to 50 cm)
while supported in water and released after 24 hours. The following table
reports the degree of stretching 5 seconds after removal of the stress and
24 hours later.
______________________________________
Tape of Example
1 2 3
______________________________________
% stretched after
5 seconds 260% 258% 285%
24 hours 217% 230% 260%
______________________________________
Another set of strips 2.5 cm in width was stretched 100% (from 12.5 cm to
25 cm) in air and released after 10 seconds. The following table reports
the lengths 30 seconds after removal of the stress.
______________________________________
Tape of Example
1 2 3
______________________________________
Recovered to
14 cm 16 cm 13 cm
______________________________________
The laminated tape of Example 3 was especially useful for adhering metal
medallions to embossed vinyl fabric of the type used for car tops. Its
pressure-sensitive adhesives flowed under pressure into the contours of
the fabric and showed no tendency to work loose.
To provide specimens of sufficient thickness for testing the hardness of
the adhesive layer, two strips of the tape were laid face-to-face and
laminated together using a hard rubber roll under hand pressure. Then one
backing sheet was peeled off to laminate a third strip and its backing
sheet was peeled off to laminate a fourth strip. The total thickness of 3
mm insured that the hardness readings would not be unduly affected by the
underlying table of the hardness tester.
Then using a Shore Hardness Tester on the 00 scale, readings were taken at
various periods of time after the probe first contacted the adhesive. The
readings were:
______________________________________
Elapsed Time Shore 00 Hardness
in Minutes Example 1 Example 2
______________________________________
0.017 56 --
0.05 49 56
0.083 43 --
0.1 -- 50
0.17 38 --
0.2 -- 46
0.25 35 --
0.5 28 42
1.0 20 39
2.0 13 36
3.0 9 34
6.0 4 --
9.0 2 --
10.0 -- 30
20.0 0 --
28.0 -- 26
45.0 -- 25
90.0 -- 23
______________________________________
The drawing shows these readings on a semi-log scale, curve 1 for Example 1
and curve 2 for Example 2. The adhesives of both Examples 1 and 2 were
reasonably elastic under briefly applied stresses but much softer under
prolonged stress. The adhesive of Example 1 responded to prolonged stress
as if it were virtually dead-soft. In each case, if the probe were applied
only for a few seconds and removed, the mark of the probe would quickly
disappear. At the conclusion of each test, the mark of the probe remained
essentially unchanged after 24 hours, indicating that the adhesive had
essentially taken a permanent set.
EXAMPLE 4
The following ingredients were blended and coated onto a low-adhesion
backing essentially by the procedure of Example 1:
100 g of 90/10 isooctyl acrylate/acrylic acid partially reacted to a
viscosity of 1000-1200 cps. and containing 0.25 g benzoin ethyl ether and
0.1 g of a crosslinking agent,
10 g of an unreacted mixture of 90 parts isooctyl acrylate and 10 parts
acrylic acid,
7 g of glass microbubbles of Example 1.
The coatings were transferred to a biaxially-oriented polyethylene
terephthalate backing 0.075 mm in thickness and slit to a width of 1.27 cm
for testing, with results as follows:
______________________________________
Shear Resistance
180.degree. Peel Failure
at 500 g and at from glass at
65.degree. C. from stain-
30 cm/minute and
Adhesion less steel 21.degree. C.
thickness
(Minutes) (g/cm of width)
______________________________________
0.125 mm 910 1280
0.5 mm 1088 2624
______________________________________
In each test, a hard-rubber roller having a mass of 6.8 kg was employed in
making the bonds to the test surfaces. In the peel test, the tape then
remained in contact with the glass surface for 24 hours at ordinary room
temperature before testing.
EXAMPLES 5-6
Two pressure-sensitive adhesive tapes were prepared as in Example 1 except
that the average diameter of the microbubbles was 63 micrometers, the
ratio of isooctyl acrylate to acrylic acid was 87.5:12.5, and the
thickness of the polymerized adhesive layer was 1.0 mm. One of the tapes
[Example 5] was unmodified. To the exposed face of the other [Example 6]
was laminated an unfilled pressure-sensitive adhesive layer of the
copolymer of 90 parts isooctyl acrylate and 10 parts acrylic acid, the
thickness of which was 50 micrometers.
Using a proficorder, the root-mean-square roughness of exposed faces of the
tapes was:
Example 5 (unmodified): 3.8 micrometers
Example 6 (laminated): 1.8 micrometers
Each of the tapes showed essentially no moisture pickup after being
immersed in water for 24 hours at ordinary room temperature.
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