Note: Descriptions are shown in the official language in which they were submitted.
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METHOD O~ ~MINATIN~ MUhTIPLE ~AYERS
This invention concerns the field of bonding
together multiple layers to form a laminate and, more
particularly, a method of bonding wherein an adhesive
impregnated scrim is preheated and then placed between the
5layers of material to be bonded.
Bonding of laminates by using a film of adhesive
that activates under heat to become either tacky or liquid
and flowable to form the bond is well known in the prior
art. The resultant heat aativated bond is mechanical or
chemical, or both. The technique of heat activating a film
of adhesive is used in many industrial applications and,
particularly, in situations where the resultant laminate is
molded to form such items as interior panels on vehicles.
Examples of patents dealing with the lamination of multiple
layers using some form of thermally activated adhesion
between the layers are U.S. Patent Nos: 3,996,082;
~,221,619; 4,500,59~; 4,571,279; ~,5~8,~58; 4,71~,681; and
~,731,276.
All of the above referenced patents are similar
in that the adhesive is heated after it is placed between
the layers to be laminated; in other words, the entire
laminate sandwich structure must be heated to activate the
adhesive. Hence, the activation temperature of the
adhesive must, necessarily, be low enough that the heating
will cause no harm to any of the materials to be laminated.
Furthermore, this prior art technology has other
shortcominys. I'here can be problems getting enough heat
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through the laminates (which are often formed of thermally
insulating materials) to melt the adhesive layer. A
typical method of bonding is the use of hot dies (molds and
platens); which need much expensive energy to become
sufficiently hot. The dies are expensive, and the cycle
times are, typically, quite long. These long cycle times
result in high capital investment per unit of capacity.
For contoured parts, the laminates are molded by
use of heated dies. However, this results in high stress
areas on the contoured parts. To mold these highly
stressed parts, they must be held in the die until cool
down (for a thermoplastic material) or thermoset (for a
thermosetting material) occurs. The necessity of cooling
down the highly contoured parts causes excessive dwell time
in the expensive dies, further exacerbating the problems
noted in the prior paragraph. Some systems inject a
cooling gas into the die, but such dies must be designed to
withstand the stresses caused by the temperature cycliny,
and are, hence, even more expensive~ Furthermore, highly
contoured parts cannot be efficiently done on a production
basis with current adhesive films due to these problems.
~ he combination of the viscosity of the molten
adhesive combined with the long, hot dwell cycles under
pressure in the prior art often causes migration of the
molten adhesive into porous substrates. This migration
results in adhesive starvation at bond lines and resultant
poor bonds in reproducible and non-reproducible modes, and
is also detrimental to the porous substrate.
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The larye molds necessary ~o ~orm large items
have inherent problems o~ hot and cold spots, temperature
variations of 250 F. from one part o~ the platen to another
are normal. Furthermore, pressure differences from one
section of the platen to another are also normal due to
size variations in the substrate layers and die
misalignment. These conditions magnify the problems noted
in the preceding paragraphs.
It is known to heat the substrates inside the
dies by injection of superheated dry steam. While this
system offers a solution to some o~ the problems noted
above, such as migration, a new complication is introduced
to the system; the injected superhea-ted dry steam must be
kept clean.
Furthermore, in the prior art, the melt
temperature of the adhesive must be low enough not to cause
damage to the substrates. This requirement imposes a
severe constraint on both the types of substrates that may
be used, as well as the adhesive itself.
The method and device disclosed and claimed
herein are designed to overcome the problems of the prior
art noted above. Applicant's invention is a preheatable
scrim which is insertable between the layers of materials
to be laminated. The scrim comprises an adhesive layer and
a carrier in contact with the adhesive layer. The
adhesive is selected so that it exhibits activation
properties having a time/temperature window. To this end,
the adhesive selected will be normally amorphous in
structure, but should also have a degree of crystalinity so
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that it will set and hold a bond quickly. ~lternatively,
the adhesive may be one that becomes "tacky" when heated to
its activation temperature, or it may be one that has a
sharp melting poink which is substantially higher than the
temperatures at which the layer of materials begin to
degrade.
Also disclosed is a method of using the scrim to
bond the layers of material together to form a laminate.
The scrim is preheated to the activation temperature of the
adhesive. By "activation temperature" is m~ant the
temperature at which the adhesive becomes liguid, semi
liquid or tacky. When it has reached the activation
temperature, the preheated scrim is what is referred to in
the art as "bond ready." The bond ready scrim is then
positioned between the layers to be laminaked to form a
sandwich laminate. Sufficient pressure is applied so that
the sandwich components are brought into intimate ~ontact
with the bond ready scrim and/or eah other. The
components achieve intimate contact before the scrim cools
off and is no longer bond ready. Depending on the shape,
the materials and the adhesive used, this intimate conkact
may be maintained ~or as short a time as a fraction of a
second, such as would occur in a nip roll in a continuous
operation, to as long a time as 180 seconds as miyht occur
in piecewor~.
The adhesive coating on the carrier may be layers
of several components. For a continuous carrier, such as
film or foil, an adhesive layer will be disposed on both
surfaces o~ the carrier. Multiple layers of different
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adhesives are used for bonding dissimilar substrates that
require different chemistries or mechanisms for specific
adhesion to each surface. Furthermore, more layers may be
interposed between the multiple layers of adhesives to fill
gaps therebetween or for cost reduction.
The following detailed description may best be
understood by reference to the following drawing in which:
FIGURE 1 is a perspective view of the
preheatable scrim of the present invention;
FIGURE 2 is perspective view of a laminate
formed with the preheatable scrim of Figure 1, with the top
layer shown partially peeled away to expose the scrim;
FIGURE 3 shows a method of forming the
preheatable scrim of the present invention; and
FIGURE 4 shows an alternative method for forming
the preheatable scrim.
Throughout the following detailed description,
like reference numerals are used to refer to the same
elements shown in multiple figures of the drawing.
Referring now to the drawing, and in particular to Figure
1, there is shown a preheatable scrim 10 of the present
invention. The scrim 10 is formed of a carrier 12, here
shown in the form of a web formed of woven strands 13 of a
material such as fiberglass. It is impregnatingly coated
with an adhesive 14, preferably in a film form.
The adhesive 14 is selected to have certain
characteristics. The adhesive, itself, can have a sharp
melting point which is substantially higher than the
degradation temperatures of the layers 16, 18 ~shown in
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Figure 2) which are to be bonded together. By "degra~ation
temperature" is meant the temperature at which the
properties or structure of a material are adversely and
irreversibly altered. The adhesive 14 may be either a
thermoplastic or thermosetting polymeric or resinous
system. After the layers 16, 1~ are laminated together by
means of preheatable scrim 10, the bonds formed by the
adhesive should not deform or yield under loads of lo grams
or more per square inch of film at temperatures of 160~ F.
or higher, when tested according to ASTM 816, procedure 21,
the protocol of which is incorporated by reference herein.
It is contemplated that the thickne~s of adhesive 1~ will
range between approximately 0.0005 to 0.2500 inches.
During the bonding process, the adhesive should either form
bonds which do not deform or yield under loads of 10 grams
per square inch at temperatures of 1600 F. or higher,
exhibit a melt viscosity of between 0.100 ~o 5,000,000
poise during cure, or have bonds which e~hibit a
progression of cure after initiation thereof at a reduced
temperature or over time as demonstrated by an increased
softening point. A thermoplastic adhesive should have a
melt viscosity of between 0.100 to 5,000,000 poise th~ough
a range of at least 10O F. within 100O F. above its
softening point as determined by the ASTM 816, procedure 21
softening point test. It has been found that moderately
good results can be obtained wi~h Dow~ adhesive film No.
899, which is a copolymer of ethylene and acrylic acid.
Other suitable adhesive types could include, for example:
ethylene-vinyl acetate, polypropylene; phenolic; polyamide,
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polyethylene, acrylic; polyurekhane and styrene-butadiene
systems.
Although depicted in the form of a web, the
carrier 12 can comprise a non-woven, knitted, or continuous
material. For example, it could be a flimsy piece of
continuous substrate structurally similar to facial tissue.
It must be able to withstand the temperature and mechanical
stresses of the coating process without significant changes
in physical properties. After coating with adhesive, the
carrier 12 must carry the adhesive through the flexing and
bonding processes without adversely affecting the resulting
bonded assembly. The carrier 12 is not designed to lend
structural support to the bond line, although it may have
that incidental function. The carrier 12 functions to
carry the adhesive, not to reinforce the bond.
It is contemplated that, if formed in the web
structure shown in Figures 1 and 2, the carrier 12 will
have from as few as one strand 13 every four inches to as
many as 200 strands 13 per linear inch in both directions
for conventional two directional construction. If the
carrier 12 is cross-woven, the number of strands 13 should
not exceed 400 strands per square inch in all directions.
Each individual strand 13 has a diameter less than or equal
to 0.200 inches. The same diameter requirements apply for
strands of non-woven carriers, but the number of strands 13
per square inch can be as high as 4,000.
For many applications, such as automobile
interiors, the strands 13 of carrier 12 should be spaced in
a range of between 6 to 16 per linear inch. It has been
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determined that such spacing will result in layers 16-18
which bond together with no adhesive gap~ larger than 1/8
inch in diameter.
The carrier 12 may be impregnated with the
adhesive 14 by any of a number o~ well known methods. For
example, as shown in Figure 3, the carrier 12 may be passed
through a bath 20 of liquid adhesive, and subsequently
passed over a roll coater 22 or a knife (not shown) to
evenly coat the carrier 12 then cooled down or dried to
form the scrim 10. Alternatively, as shown in Figure 4,
carrier 12 and adhesive 14 may be continuously supplied via
rollers 30,32. The carrier 12 and the adhesive 14 are then
pressed together via heated roller 38 to form the scrim 10.
If carrier 12 is continuous film or foil, another adhesive
14 will be contacted with the opposite surface o~ carrier
12 to ~orm a three-layer sandwich which is then hot rolled
to form scrim 10. Doubtless, othsr variations may occur
to those skilled in the art.
As shown in Figure 2, the preheatable scrim 10
may be used to bond together layers lG-18 to form laminate
19. To accomplish this, the coated scrim 10 is fixtured
into a frame (not shown) and put in a heat chamber (not
shown~ where the adhesive 14 is brouyht to a temperature
of, typically, between 1600 to 6900 F. for bondiny. The
means for heating the scrim could include, for example,
convection, induction, infrared, ultrasonic, microwave and
flame heating. The optimum temperature will vary for each
type of adhesive coating, but all will fall into the
indicated range. The bond ready scrim 10 is indexed into
a holding position between layers i6, 18 and sufficient
pressure is applied so that the sandwiched components are
brought into intimate contact with the bond ready scrim 10
and/or each other. The pressure may be applied by, for
example, a roller, a die, a mold, a platen, etc. The
pressure on the sandwiched components is maintained until
they all achieve intimate contact before the preheated
scrim lO cools off and is no longer bond ready. In some
applications, the intimate contact is maintained for only
10 a fraction of a second; this would be the case ~or
continuous operations in which the pressure is provided by
a nip roller. Conversely, in some piecework applications,
the intimate contact may be maintained for two or three
minutes. In any case, the layers 16,18 will act as a heat
15 sink, helping the adhesive to cool rapidly. If layers
16,18 have a lower melting point than adhesive 14, they
will not de~orm since they do not have to be heated.
Since the scrim 10 is preheated before insertion
between layers 16,18 the dwell time within the mold or d:Le
20 is much reduced. Furthermore, the layers of material 16-18
do not need to be heatad. Hence, a much wider variety of
materials may be laminated together by the method disclosed
herein since heat sensitive materials may be selected.
Furthermore, since the layers 16-18 arP not heated, the
25 cooling time for the laminate is much shorter. Again, this
considerably reduces dwell time. Also, because the scrim
is heated, rather than the sandwiched components,
heatin~ is much more uniform and the problem of hot and
cold spots is virtually eliminated.
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Hence, by usiny the preheatable scrim and
laminating method disclosed herein, considerable savings
may be realized in the laminating process. Reduced dwell
times results in shorter cycle times. For example, a prior
art cycle time o~ 30 to 1~0 seconds may be reduced to lo
seconds or less. Far more laminate can be produced per
unit of time. Moreover, the equipment to produce the
laminate can be made much simpler since, for molded
components, the molds or dies, themselves, no longer need
to be heated, and do not need to be provided with the
capability of injecting superheated dry steam and/or
cooling gases. Furthermore, the range of materials which
can be laminated by this process is considerably enlarged.
While the preheatable scrim and method o~ the
present invention have been described with re~erence to
certain exemplifications and embodiments, the invention is
not limited to the particular exemplifications and
embodiments disclosed. Doubtless, other variations in
desiyn may occur to those skilled in the art without
departiny ~rom the scope o~ the invention. The true scope
of the present invention is limited only by the claims
appended hereto.
