Note: Descriptions are shown in the official language in which they were submitted.
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BII-AYER ADHESIVE S~STEM FOR REINFORCED STRAPPING TAPE
Backqround of the Invention
The present invention relates to a bilayer adhesive
system for use in natural rubber adhesive tapes and
particularly for use in a reinforced strapping tape ~or use
in the appliance packaging industry.
The suitability of any adhesive tape ~or use in a
particular application is a function of several factors.
Properties of the adhesive such as peel strength, tensile,
tack and the like are important factors, but other properties
of the tape and its construction also affect its usefulness
lncluding the flexibility, softness and elastomeric character
of the adhesive layer.
Reinforced, pressure sensitive adhesive tapes
(PSAT) have been used in the appliance packaging industry for
many years. Typically, such tapes are constructed from a
backing substrate having, on its surface, a layer of
reinforcing strands and a pressure sensitive adhesive (PSA),
particularly a natural rubber-based pressure sensitive
adhesive (NRPSA). These reinforced, natural rubber adhesive
tapes can be applied to a variety of appliance surfaces
(adherends) with little or no surface preparation. However,
one important disadvantage of these products in particular
and natural rubber pressure sensitive adhesive tapes in
general is their relatively high manu~acturing cost. Due to
the relatively high coating weights associated with
reinforced, strapping tapes with NRPSAs and because natural
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rubber adhesives must be cast from a solvent at relatively
low solids, the adhesive must be coated at slow line speeds
(e.g., about 150 fpm) to avoid breaching the lower explosive
level (~E~) of solvent vapor in the drying ovens of typical
coating lines. While one solution to this problem is to coat
the natural rubber adhesive in multiple layers in tandem,
this increases processing e~uipment costs, e.g. multiple
coating heads and/or additional dryer capacity.
Attempts have been made to construct tapes using
hot melt pressure sensitive adhesives (~MPSA). Typically,
reinforced strapping tapes are constructed by applying to a
backing support two adhesive layers, e.g., an acrylic
laminating pressure sensitive adhesive layer and a top hot
melt pressure sensitive layer. The acrylic layer, which is
typically deposited from an emulsion, bonds the reinforcing
strands to the backing substrate. The hot melt pressure
sensitive adhesive is applied as a top coat which fills the
spaces between the reinforcing strands and ensures an
uninterrupted adhesive layer atop the strands. While the raw
material cost o~ the reinforced hot melt adhesive is, on
average, higher than that of the natural rubber-based
reinforced pressure sensitive adhesive, less of it is needed
(typically 30 lbs. per 3000 ~t2 vs. 48 lbs.). Furthermore, it
can be applied in tandem with the acrylic emulsion adhesive
at relatively high line speeds, when compared to solvent cast
natural rubber adhesives (typically 600 fpm vs. 150 fpm) at a
lower processing cost which translates into a lower total
cost than the natural rubber-based tape.
One disadvantage with hot melt pressure sensitive
adhesive tape is that it re~uires greater care to apply to
the adherend, i.e., it requires a dust-~ree, grease-free
surface and careful rub-down. Also, the hot melt pressure
sensitive adhesive does not exhibit consistently high wet-
tack and adhesion to and transfer-free removal from all of
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the adherends encountered in the appliance packaging
industry.
Accordingly, there is a need in the industry for a
pressure sensitive adhesive tape and, more parti.cularly, for
a reinforced strapping tape which exhibits the adhesion wet-
tack characteristics of natural rubber adhesives and which
can be produced at a lower total cost.
Summary of the Invention
It has now been found that the robust adhesive
properties of a natural rubber adhesive can be provided with
reduced manufacturing cost if a bilayer adhesive system is
employed which includes a layer of natural rubber adhesive
atop a thermoplastic foundation layer. By using natural
rubber adhesive, robust adhesive properties are provided in
the tape while the thermoplastic foundation layer provides
internal body and structure while obviating the need to cast
a thick natural rubber adhesive layer from a solvent, The
thermoplastic foundation layer can be formulated to provide a
bilayer structure which mimics the dynamic mechanical
properties of the natural rubber based adhesive layer or to
improve over the properties and function of a tape having a
pressure sensitive adhesive monolayer. Thus, replacing the
portion of the natural rubber adhesive layer adjacent the
substrate has the advantages of both reducing manufacturing
cost and introducing an additional degree of freedom into the
design of a natural rubber adhesive tape that enables further
optimization of tape function.
Accordingly, a pressure sensitive adhesive tape in
accordance with the present invention is constructed of a
support having a first surface and a second surface;
a layer of a release agent provided on said first
surface of the support, i.e., the ~ackside of the tape;
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a foundation layer containing a hot melt
thermoplastic coating provided on said second sur~ace, i.e.,
the front side of the tape; and
a layer containing a natural rubber pressure
sensitive adhesive overlying said layer of said thermoplastic
resin.
In accordance with a particular embodiment of the
invention, a reinforced pressure sensitive adhesive strapping
tape is provided containing a support having a first surface
and a second surface;
a layer containing a release agent on the ~ir~t
surface of the support;
a first adhesive layer containing a laminating
adhesive on the second surface of the support;
a layer of reinforcing strands on the top of the
first adhe~ive layer;
a second adhesive layer containing a hot melt
pressure sensitive adhesive on top of the rein~orcing
strands; and
a third adhesive layer containing a natural rubber
pressure sensitive adhesive on top of the second adhesive
layer.
Brief Description of the Drawinq
Fig. 1 is a cross-sectional schematic view of a
reinforced, pressure sensitive adhesive tape in accordance
with one embodiment of the present invention.
Figs. 2 and 5 are graphs of G versus temperature
~or a bilayer adhesive in accordance with the invention
(curve Inv.), a natural rubber adhesive layer (NR), and a
layer of an HMPSA (curve HM).
Figs. 3 and 6 are graphs of G versus temperature
for a bilayer adhesive in accordance with the invention
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(curve Inv.), a natural rubber adheRive layer (~R), and a
layer of an HMPSA (curve HM).
Figs. 4 and 7 are graph~ of tan delta versus
temperature for a bilayer adhesive in accordance with the
invention (curve Inv.), a natural rubber adhesive layer (NR),
and a layer o~ an HMPSA ~curve HM).
Detailed Descri~tion o~ the Invention
The term "hot melt" as used herein refers to a
composition that is essentially solventless which is coated
at an elevated temperature and cooled to set the coating. A
reinforced strapping tape 10 in accordance with one
embodiment of the present invention is illustrated in Fig. 1
and comprises a support 12 having on one surface thereof a
release agent 14 and on the opposite side thereof a layer o~
a laminating adhesive 16. A layer of reinforcing strands 18
is laid on the surface of the laminating adhesive layer 16
and a thermoplastic composition such as a hot melt pressure
sensitive adhesive 20 is applied over the reinforcing layer
such that it surrounds and penetrates the spaces among the
reinforcing strands 18 and provides a continuous adhesive
layer overlying the reinforcing strands 18. A layer of
natural rubber adhesive 22 is coated on top of the hot melt
pressure sensitive adhesive 20. Since only a thin layer o~
the natural rubber pressure sensitive adhesive 22 is applied,
the level of solvent vapor liberated while coating the
natural rubber adhesive is drastically reduced allowing the
natural rubber adhesive 22 to be applied at speeds higher
than can be employed using a natural rubber adhesive alone in
the absence o~ a hot melt pressure sensitive adhesive.
The backing support of the present reinforced
strapping tape may be any of the materials commonly used in
the industry and typically includes biaxially oriented
polypropylene (BOPP), polyethylene terephthalate (PET),
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machine direction oriented polypropylene (MOPP), paper,
polyethylene, and the like. Preferably, the backing support
is polyethylene terephthalate or biaxially oriented
polypropylene.
Any conventional release agent can be used to
provide the release properties. Representative examples of
the release agents useful in the present invention include
polyvinyl octadecyl carbamate (PVODC); a Cl4-Cl8 fatty acid
coordinated with a transition metal such as chromium; the
reaction product of a di-~-organo functional dimethylsiloxane
oligomer having a terminal active hydrogen, a carboxyl group-
bearing monomer, and a diisocyanate; and terpolymers o~
stearyl methacrylate, acrylonitrile and vinyl pyrrolidone.
Typically, the release agent is cast from a solvent system
such as toluene, isopropylalcohol, water, or mixtures thereof
in which the release agent is present at about 1-7%. In a
preferred aspect of the invention, 0.5-3% PVODC in toluene is
used to apply the release agent. The coverage of the release
agent will vary with its selection and the adhesive which it
faces. Coverages o~ about 1 to 5 g/cm2 wet coating are
common.
In accordance with the invention, a thermoplastic
foundation layer i8 provided in the tape which replaces a
portion o~ the thickness of the natural rubber adhesive. A
particularly convenient thermoplastic layer is a layer
containing a thermoplastic rubber. It has been found that
the tape can be optimized for use in certain applications by
modifying the properties of the thermoplastic foundation
layer. For example, in certain applications, a softer, lower
Tg foundation layer may be required while in others a more
rigid, higher foundation layer may be desired.
While the thermoplastic layer need not be an
adhesive, hot melt adhesive compositions have been found to
be particularly convenient to use as the foundation layer.
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Hot melt adhesives are generally thermoplastic materials
which contain as essential constituents a thermoplastic
polymer and a tacki~ier. Hot melt adhesives normally are
homogeneous mixtures and are manufactured by compolln~l ng,
i.e., melt blending the thermoplastic polymer with the
tackifier and antioxidants and/or other stabilizing
additives. The tackifier can be used in an amount of about
80 to 300 parts per lO0 parts of the thermoplastic polymer.
The molten mixture then is coated on a substrate.
Typically the thermoplastic foundation layer is
formed from a composition containing a thermoplastic resin
wherein the resin is an A-B-A block copolymer, where A
represents a thermoplastic polystyrene and B represents a
rubber midblock of polyisoprene, polybutadiene, or
poly(ethylene/butylene). Useful thermoplastic rubbers are
available from Shell Development Co., ~ouston, Texas, under
the tradename Kraton D or Kraton G or from Dexco Polymers
under the tradename Vector 4113. Typically, the foundation
layer is formulated by blending the thermoplastic resin with
a tackifying resin so as to provide a composition which melts
at a temperature of about 130~C to 200~C and has viscosity of
less than 100,000 cps at 500~F.
In a preferred aspect of the invention, the hot
melt pressure sensitive adhesive includes a styrene-isoprene-
styrene block copolymer (S-I-S) system a tackifier resin such
as a blend of an aliphatic olefin-derived resin and a
polyterpene resin, naphthenic or paraffinic oil, and
antioxidants. These ingredients are processed through an
extruder at high shear to melt all the ingredients into a
homogeneous mixture. A preferred HMPSA contains about 100
parts of Vector 4113 S-I-S block copolymer, about 80-300
parts of an aliphatic olefin-derived resin blended with a
polyterpene resin, 5-30 parts of a naphthenic oil and 1 part
of an antioxidant.
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While the invention is described above with respect
to the use of an HMPSA as the ~oundation layer, it will be
appreciated that other hot melt thermoplastic compositions
could also be used including compositions in which the
thermoplastic resin is an ethylene vinylacetate copolymer, a
polyester or a polyamide. One approach which can be used in
formulating a thermoplastic elastomeric composition for use
as the foundation layer, is to design a formulation which,
when coupled with a natural rubber adhesive layer mimics the
dynamic mechanical properties of a natural rubber adhesive
layer alone. In accordance with one embodiment, the
thermoplastic foundation layer is formulated so as to provide
a bilayer structure which has G and, still more preferably,
G' and G' versus temperature profiles which are substantially
similar to a natural rubber adhesive layer over a temperature
range of about -20~ to 110~C.
Figures 2-4 illustrate the correlation in dynamic
mechanical properties as represented by G , G and tan delta
versus temperature curves that is obtained in the bilayer
structure in accordance with one embodiment of this
invention. Fig. 2 is a plot o~ G (dyne/cm2) versus
temperature for a natural rubber adhesive layer (curve NR), a
bilayer in accordance with the invention of an HMPSA
~oundation layer and a natural rubber overcoat where the
HMPSA contains 100 parts o~ the tacki~ying resin in 100 parts
SIS block copolymer (curve Inv.) and a HMPSA layer (curve
HM). While the curves for the bilayer and natural rubber
layers are closely aligned, the HMPSA curve is not. Figs. 3
and 4 show the corresponding curves for G and tan delta
where the curve HM is for the HMPSA composition alone, the NR
curve is the natural rubber adhesive composition alone and
the Inv. curve is the bilayer.
Figs. 5-7 are curves for G, G and tan delta for
a natural rubber adhesive layer (curve NR), an HMPSA layer
containing 200 parts tacki~ying resin per 100 parts SIS block
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copolymer (curve HM) and a bilayer (curve Inv.) of the two in
accordance within the invention. It will be not:ed that there
is close correspondence in the NR and Inv. curves for G' and
G and that there is substantial correspondence in tan delta.
In the table below, 90~ and 160~ static peel test results are
provided for the individual natural rubber and HMPSA layers
and the bilayer of the invention. The static results con~irm
the DMA results and show that the bilayer behaves similar to
a natural rubber adhesive layer.
Figure 7
Adhesive ~ystem90~ Static Peel160~ Static Peel
Natural Rubber 10/16" 12/16"
BiLayer (100/100)15/16" 22/16"
HMPSA (100/100) 43/16" X>3~
The preferred coat weight of the thermoplastic
foundation layer i8 about 10-40 lbs/ream (3000 ~t2). The
preferred coating method is a hot melt slot orifice coater
where the hot melt pressure sensitive adhesive is metered
through a thin slot and deposited onto the support.
The natural rubber-based pressure sensitive
adhesive is a natural rubber compounded with a tackifying
resin, an antioxidant and a crosslinking additive. The
natural rubber-based pressure sensitlve adhesive is typically
cast from toluene at about 20-50% concentration. To prepare
the adhesive, natural rubber elastomer is processed in a
Banbury to reduce the molecular weight of the elastomer.
Activators such as zinc oxide and extenders such as clay and
calcium carbonate are added to the elastomer in the Banbury
in a conventional manner. The elastomer containing the
extenders and activators is typically processed to a Mooney
viscosity of about 42, at 212~F after one minute warm-up and
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four minutes running time. One hundred parts o~ the
processed elastomer, about 60 to 300 parts of a tackifying
resin such as polyterpene resin or a rosin ester, and about 1
to 2 parts of antioxidant are processed in a Baker Perkins
and, thereafter, about 4 to 16 parts of a crosslinker such as
phenol-formaldehyde resin are added. Toluene is added at the
end of the mixing cycle to impart the proper viscosity for
coating onto the web structure. The preferred coat weight of
the natural rubber adhesive is about 10-40 lbs/ream (3000
ft2). The natural rubber-based pressure sensitive adhesive is
pumped to the coating line and coated by knife over roll,
rever~e roll, or slot die coating method.
The natural rubber adhe~ive layer and thermoplastic
foundation layer compositions include one or more compatible
tackifying agents which are utilized in an effective amount
to promote good adhesion when used in the adhesive layer and
to provide the desired dynamic mechanical properties when
used in the foundation layer. Various tackifying resins can
be utilized which are generally well known to the art and
described in the literature. These resins generally include
rosin and its derivatives and various hydrocarbon resins.
The rosin group comprises rosins, modified rosins and their
various derivative~ such as esters. The hydrocarbon resin
group comprises polyterpenes, synthetic hydrocarbon resins,
and various modified or special rosins which are primarily
phenolics. Examples of specific rosin tackifiers include gum
rosin, wood rosin, tall oil rosin, and the like. Such rosins
are generally a mixture of organic acids called rosin acids.
The various rosin acids can be reacted with a variety of
alcohol to form esters. Examples of specific rosin
tackifiers include glycerine rosin ester, e.g., Flocal 85,
manufactured by Hercules, Inc.; hydrogenerated
pentaerythritol ester, e.g., Pentalyn H, manufactured by
Hercules, Inc.; hydrogenated glycerine ester, e.g.,
Staybelite Ester lG, manufactured by Hercules, Inc.; modified
tall oil rosin, e.g., Sylvatac RX, manufactured by Sylvachem
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Corp.; polymerized rosin such as Sylvatac 95, manufactured by
Sylvachem Coxp ., and rosin ester such as Zonester 85,
manufactured by Arizona Chemical Co.
= , Hydrocarbon tackifier resins are low molecular
weight polymers derived from crude monomer streams.
Hydrocarbon re5in streams can be classified as containing
primarily aromatic, aliphatic, and diene (cyclic olefin)
monomers. Polymerization of such streams is generally
carried out using a Lewis acid catalyst or by a ~ree-radical
process using heat and pressure. The aromatic hydrocarbon
resins generally contain aromatic petroleum resins and resins
from coal tar, commonly called coumarone-indene resins. The
various aliphatic hydrocarbon resins are produced from light,
so called carbon-5 petroleum fractions wherein the principal
monomers are Ci8 and trans-piperylene. Other hydrocarbon
resins include mixed aliphatic-aromatic resins as well as
terpene resins. Tackifier resins are described in more
detail in the Handbook of Pressure-Sensi tive Adhesive
Technology, edited by Donatas Satas, Van Nostrand Rhinehold
~ompany, 1982, Chapter 16, pages 353-369.
Conventional reinforcing strands can be used to
prepare the tape. The reinforcing strands are typically
~iberglass, nylon, polyester or polyamide such as Kevlar
available from DuPont. Preferably, the reinforcing strands
are fiberglass strands such as G-150 fiberglass strands. In
a preferred aspect of the invention, the reinforcing strands
are applied to the tape so that they are oriented in the
machine direction.
The function of the laminating adhesive is to bind
the rein~orcing strands to the support. Any conventional
laminating adhesive can be used to adhere the reinforcing
strands to the support and, in particular, pressure-sensitive
adhesives are useful. Both wet laying and dry laying
techniques can be used in assembling the strands with the
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W097l29625 PCT~S97/02478
support. Representative examples of this adhesive include
acrylic emulsion adhesives such as a butylacrylate,
ethylacrylate, or 2-ethylhexylacrylate pressure-sensitive
adhesives or pressure sensitive adhesives containing a
combination thereof. In one embodiment of the invention, the
laminating resin is an acrylic emulsion sold under the
designation Rhobond PS-90 or Rhobond PS-99 from Rohm and
Haas. The laminating adhesive is applied in an amount of 2
to 15 lbs/ream
In a typical example of the reinforced strapping
tape of the present invention, a 1.42 mil polyethylene
terephthatlate (PET) backing is coated on one surface with a
molecular layer of polyvinyl octadecyl carbamate (PVODC) and
on the opposite surface with 5 dry lbs/ream of acrylic
emulsion a &esive. Thirty ends per inch (epi) of G-150
fiberglass strands are applied to the top of the acrylic
layer and then overcoated with 25 lbs/ream of hot melt
pressure sensitive adhesive at 600 fpm. The hot melt
pressure sensitive adhesive is then coated with 20 dry
lbs/ream of a natural rubber-based pressure sensitive
adhesive in a second machine pass at 375 fpm, wherein the
natural rubber-based pressure sensitive adhesive is
covalently bonded to the styrene-isoprene-styrene elastomer
ln the hot melt pressure sensitive adhesive.
Having described the invention in detail and by
reference to the particular embodiments thereof, it will be
apparent that numerous modifications and variations are
possible without departing from the scope of the invention as
defined by the following claims.
What is claimed is:
