Note: Descriptions are shown in the official language in which they were submitted.
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I4OT D-Bx,rZ FLEXOGRAPHIC ZNK COMPOSITIONS P,ND METHODS OF PREPARING SA1vIE
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part of Application United States Serial
No.
10/034,004, filed December 28, 2001.
FIELD OF THE INVENTION
io This invention relates to coating compositions. More particularly, the
invention
relates to solvent free solid hot melt coating compositions for use in hot
melt coating
processes.
BACKGROUND OF THE INVENTION
Conventional coating compositions typically require organic solvents or water
for
their application. The use of solvents is well known to be linked with a
variety of
disadvantages, both for the manufacturer as well as the coater. For example.
the
danger of fire, explosion, odor, and environmental pollution exists with
solvent based
coating compositions. Further, such coating compositions also lead to a
lessening of the
coating quality due to the presence of residual solvent, which, for example
causes a
decrease in the blocking temperature. Also, only certain maximum coating
speeds can
be achieved which are determined by the rate of evaporation of the solvent or
water
employed, and which cannot be sufficiently increased by modifying construction
of the
coating apparatus.
The problems of environmental pollution by solvents have led to the
development
of solvent free coating compositions which contain reactive components that
are cured
or hardened at elevated temperatures or by incident radiation. In particular,
reactive
monomers and/or oligomers have been used as vehicles for coating compositions.
Such reactive coating compositions when coated are hardened on a substrate by
heat,
ultraviolet light or electron radiation in a very short time to form a film.
While such
solvent-free coating compositions have eliminated pollution by solvents, the
hardened
coating compositions frequently contain residual unreacted monomers which can
lead
to contamination in such applications as food packaging.
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Hot melt coating compositions of the present invention are a new development
based on the recent successful development of hot melt printing inks.
Illustrative of the
development of hot melt printing inks are the gravure and flexographic
printing
processes employing solvent free inks, which are solid at room temperature but
molten
at printing temperatures, such as have been disclosed in U.S. Patent
4,066,585. The
disclosed inks comprise a pigment and a thermoplastic binder having a
softening point
between 90 C and 160 C. The binder comprises a synthetic polyamide resin or
synthetic polyesteramide resin, each resin being the condensation product of
(1) an acid
component comprising a dimerized fatty acid and a monocarboxylic acid and (2)
an
lo amine component comprising a diamine and, in the case of the polyesteramide
resin,
additionally comprising a diol and/or alkanolamine.
While advances have been made in hot melt ink technology, there remains a
need in the general coating arts for hot melt coating compositions which are
solvent free
and which do not contain residual unreacted monomers upon curing. Further,
there
exits a need for a coating which is solid (100%) at room temperature. There is
also a
need for a clear composition having a good moisture-vapor-transmission (MVTR)
without using volatile solvents and other toxic ingredients or applying film
lamination.
Lastly, there is a need for a coating which has good adhesion to a variety of
substrates
such as paper, clay, coated board, film and foil.
SUMMARY OF THE INVENTION
It has now been found that the above objectives can be realized by employing a
solvent free coating composition comprising:
(A) a solid linear alcohol at room temperature;
(B) a thermoplastic binder; and
(C) a wax;
wherein, the coating composition, which is solid at room temperature, has a
melting point of at least about 75 C, and when heated to a temperature between
about
90 C and about 135 C forms a coating composition which has a coating viscosity
between about 100 cps and about 1200 cps.
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The present invention also provides a method of preparing a solvent free
coating
composition having a linear alcohol which is a solid at room temperature, a
thermoplastic binder, and a wax. When heated to between about 90 C and about
135 C the solvent free coating composition has a coating viscosity between
about 100
cps and about 1200 cps.
The present invention also provides a method for preparing a hot melt
flexographic printing coating composition by preparing a pigment dispersion
and
mixing it with a linear alcohol which is a solid at room temperature, a
thermoplastic
binder, a wax and optionally, a solid plasticizer to form a homogenous solvent
free
coating composition having a viscosity of between about 100 cps and about 1200
cps at
a temperature of between about 90 C and about 135 C.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a novel solvent free coating composition
which is
solid at room temperature and to a method of coating using this coating
composition in
a molten state, e.g., at coating temperatures of about 90 C and higher. As
used herein
the term "solid" is intended to mean that the physical state of a designated
component is
solid at ambient room temperature, i.e., the component has a melting point or
a
softening temperature substantially above ambient room temperature. The
solvent free
coating compositions of the invention are substantially free of condensation
polymers or
any other such component which would change the essential character of the
coating
composition.
Alcohol
The solid linear alcohol in the invention functions as a dispersing medium to
maintain a uniform suspension. Suitable linear alcohols include but are not
limited to
fully saturated, long-chain linear alcohols having a melting point of about 75
C or
greater and a number-average molecular weight (Mn) of about 350 or greater.
Preferably, the linear alcohols have a Mn between about 350 and about 750 and
a
melting point between about 75 C and about 110 C. Preferably, the solid linear
alcohol
is a C14 alcohol such as Unilin alcohols which are products of the Baker
Petrolite Corp.
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Unilin alcohols are fully saturated, long-chain linear alcohols having
average carbon
chain lengths up to C50 with the primary hydroxy function of the alcohol
distributed
among all of the carbon chain lengths. A particularly preferred solid linear
alcohol is
Unilin 500 alcohol which has a Mn of 550; a melting point of 99 C, a hydroxyl
number
of 83 mg KOH/g sample, and a viscosity at 149 C of 5.5 cps.
Thermoplastic Binder
Thermoplastic binders for use in the present invention should have a softening
point of about 70 C. and therefore be solid at ambient temperature. Suitable
io thermoplastic binders include but are not limited to ethylene copolymers,
hydrocarbon
resins or a combination thereof.
In one embodiment of the invention the thermoplastic binder is a copolymer of
ethylene with either acrylic acid or vinyl acetate. In a preferred embodiment,
the solid
thermoplastic binder is poly(ethylene-acrylic acid) which has a Mettler Drop
Pt. (ASTM
D-3954) of about 90 C to about 105 C; a Brookfield viscosity at 140 C of about
550 cps
to about 650 CPS; and an acid number between about 40 and about 120. A
particularly
preferred poly(ethylene-acrylic acid) of this type is A-C 5120 copolymer of
ethylene-
acrylic acid which is marketed by Honeywell (formerly Allied Signal Inc.),
Specialty
Chemicals, Morristown, NJ. Another preferred thermoplastic binder is A-C 5120
copolymer which is a poly(ethylene-acrylic acid) having a Mettler Drop Pt. of
92 C, a
Brookfield viscosity at 140 C of 650 cps; and an acid number of 120. In an
added
embodiment of this invention, the vinyl thermoplastic binder is poly(ethylene-
vinyl
acetate) in which the copolymer contains between about 15 wt.% to about 50 wt.
% of
vinyl acetate. Poly(ethylene-vinyl acetate) copolymers of this type have Melt
Indexes
ranging from about 8 to about 2500 and a softening point (Ring and Ball, ASTM
E28
hereinafter identified as R&B") ranging from about 74 C to about 150 C.
Poly(ethylene-
vinyl acetate) copolymers of this type which are used in the following
examples are
marketed by E.I. duPont deNemours and Co. as Elvax EthyleneNA copolymers and
by Elf Atochem as Evatane EthyleneNA copolymers.
In another embodiment of this invention, the thermoplastic binder is a
hydrocarbon resin. Typical hydrocarbon resins which are useful in formulating
the
coating compositions of this invention include but are not limited to Escorez
5380 (R&B
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softening point 85 C), Escorez 210, (R&B softening point 94 C), Escorez 5400
(R&B
softening point 100-106 C), and Escorez 5600 (R&B softening point 100-106 C).
Wax
Suitable waxes for use in the invention are non-volatile at coating operating
temperatures and have low melt viscosities. Such waxes or mixture of waxes
provide a
proper degree of toughness and flexibility to the applied coating composition
for the
intended application. Typical waxes for use in the present invention include
but are not
limited to highly branched hydrocarbon waxes, polyethylene homopolymer waxes,
io oxidized polyethylene waxes (such as E-2020 from Baker Petrolite), animal
waxes
(such as spermaceti wax), vegetable waxes (such as rice bran wax, carnuba wax
and
candilla wax), and combinations thereof. Preferred waxes are highly branched
hydrocarbon waxes which typically have a viscosity at 99 C of about 1-400 cps
and
preferably a viscosity at 99 C of about 1-20 cps.
Waxes which have these properties include but are not limited to the animal
wax
spermaceti wax, which is a complex cetyl ester mixture and has a melting point
of 45 C
- 49 C, and Rosswax 3009 which is marketed by Frank B. Ross Co. Inc., Jersey
City,
N.J., and has a low molecular weight polyethylene derived from high density
polyethylene, has a Mn of 1000 - 1200; a molecular weight distribution in the
range of
about 1.0 to about 2.0; a melting point (Drop D-127) of 82 C - 104 C; and an
extremely
low melt viscosity. A particularly preferred wax of this type is VYBARO 253
polymer
marketed by Baker Petrolite. VYBARO 253 polymer, which is used in the
following
examples, is a highly branched hydrocarbon which has a number average
molecular
weight (Mn) of 520 (by vapor pressure osmometry); a softening point (ASTM D36)
of
about 67 C; and a viscosity at 99 C of about 6 cps (ASTM D3236).
Solid Plasticizer
The present invention may also include one or more solid plasticizers.
Suitable
solid plasticizers include, but are not intended to be limited to, solid
esters of benzoic
acid, phthalic acids and aliphatic/cycloaliphatic acids with melting points
above 40 C
and which are substantially non-volatile at coating press operating
temperatures, or
combinations thereof. A preferred solid plasticizer is dicyclohexylphthalate.
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Method of Preparation
The solvent free coating composition of the present invention is prepared by
first
mixing a linear alcohol which is solid at room temperature, if present a solid
plasticizer,
a thermoplastic binders and waxes in a container at a temperature of at least
90 C to
form a homogeneous molten coating composition which is allowed to cool to room
temperature to form the solvent free coating composition of the present
invention. The
coating mixture may be heated at a temperature of at least 110 C, preferable
at least
120 C. The coating mixture may be heated for a period of a few hours,
preferably
between one to four hours, more preferably between two to three hours. The
container
for heating may be, but is not limited to, a metal can or aluminum pan.
Typically, the solvent free coating composition may be cooled to room
temperature to form a solid coating composition which can be packaged and
stored for
later use in hot melt coating.
In a preferred embodiment of this invention, the solid coating composition
comprises: an ethylene-acrylic acid copolymer, a linear alcohol which is a
solid at room
temperature, dicyclohexylphthalate, and a highly branched hydrocarbon wax.
Preferably, the ethylene-acrylic acid copolymer is A-CO 5120 copolymer of
ethylene-
acrylic acid identified above, the solid linear alcohol is Unilin0 550 alcohol
identified
above, and the highly branched hydrocarbon wax is VYBARO 253 polymer
identified
above. The solvent free coating compositions of this embodiment are
particularly useful
in hot melt coating on a variety of substrates giving applied coating
compositions with
very good adhesion and gloss.
In another preferred embodiment of this invention, the solid coating
composition
comprises a pigment; an ethylene-vinyl acetate copolymer, a solid linear
alcohol at
room temperature, dicyclohexylphthalate, and a polyethylene homopolymer wax.
Preferably, the ethylene-vinyl acetate copolymer is ElvaxO 40W ethylene-vinyl
acetate
copolymer (containing 40 wt.% of vinyl acetate, and has a Melt Index of about
52 and a
softening point of about 104 C (ring & ball ASTME28)), the solid linear
alcohol is
Unilin0 550 alcohol identified above, and the polyethylene homopolymer wax is
Rosswax 3009 identified above. The solvent free coating compositions of this
embodiment are particularly useful in hot melt coating on a variety of
polyester
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substrates and treated polyethylene substrates, providing applied coating
compositions
with very good adhesion and gloss.
Each of the preferred solvent free coating compositions has a melting point of
about 75 C or greater, and when heated to a temperature between about 90 C and
about 135 C, forms a molten coating composition which has a viscosity between
about
100 cps and about 1200 cps, preferably between about 100 cps and about 700
cps.
The solvent free coating composition of the present invention is free of
volatile
io solvents while also avoiding the problem of unreacted residual monomers
which can
migrate into the substrate on which the coating is applied, such as food
packaging. The
solvent free coating composition is also solid and in its clear embodiment,
has a good
MVTR of less than 2 grams/100 in2/day at various temperatures. The solvent
free
coating composition of the present invention also has good adhesion to a
variety of
substrates such as paper, clay, coated board, film and foil.
The solvent free coating compositions as prepared herein are used in hot melt
coatings. It is broken into small pieces and placed into a heated coating
composition
reservoir where it is melted and maintained slightly above its melting point,
i.e., brought
to a temperature between about 90 C and about 135 C to form a molten coating
composition which has a viscosity between about 100 cps and about 1200 cps.
The
molten coating composition is then applied to a heated anilox roller in
operational
contact with the surface of a heated coating element, and printed from the
surface of the
flexographic plate onto a substrate such as conventional print stock,
polymeric films,
metal sheets, and the like. The use of the solvent free coating compositions
of the
invention in hot melt coating is more fully described in the following
Examples. The
coating substrate may be selected from a variety of flexible films and papers
including
but not limited to polypropylene film with both sides corona treated,
polypropylene film
with both sides acrylic coated, polypropylene film with both sides PVDC
coated,
chemically treated polyester film, corona treated polyester film, PVDC coated
polyester
film, aluminum foil, and paper products such as coated paper, cardboard,
corrugated
paper, and the like The substrate may be at room temperature or may be pre-
heated
before coating, and optionally cooled by chill rollers after coating.
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The linear alcohol may be present in the coating composition in an amount of 5-
40 wt.%, preferably about 10-30 wt.%. The thermoplastic binder may be present
in an
amount of 35 - 65 wt. %, preferably about 45-60 wt.%. The wax may be present
in an
amount of 5-50 wt.%, preferably about 30-40 wt.%. The solid plasticizer may be
present in an amount of 3-15 wt.%, preferably about 5-10 wt.%.
The solvent free coating compositions of this invention will now be
illustrated by
the following Examples, which are not intended to be in any way limiting.
Example I
The solvent free coating composition of the present invention was prepared
with
the following ingredients set forth in Table 1 below:
Table 1
Component Grams
Unilin wax X-1 152 (C>14 alcohol 10
and homopolymer)
AC 5120 (Allied Signal) (an 55
ethylene-acrylic acid copolymer)
VBAR 253 polymer (paraffin Wax) 35
TOTAL 100
All three ingredients were added and maintained at a temperature of 125 C for
a
period of 3-4 hours until all materials were melted. The melted materials were
mixed at
high speed for 5-10 minutes and poured into a container to solidify at room
temperature.
The viscosity of the coating composition was measured at 85 cps at 120 C. The
viscosity was determined at the designated temperature using a Carri-Med
AR1000
Rheometer. A graphical representation of the viscosity of the coating at
varying
temperatures is set forth in Graph 1 below:
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0:5000
y
Kot Me1t Clear. (:oariuu_
0.4000
.r. 83 C
0.3000
~..
0.2000 Iso-eeS
_14TCF'S;.
=122 CPS
} l0I CfS
0.1000
-85 ~ES ~
Q
8Q:0~ 90.0 100.0- T1-10,0- L20-:0- Q0.0
temperature: (Deg:G):
The hot melt clear coating composition sample was tested by ASTM standard
method on "Mocon" instrument model # DL 100 at various temperatures and
thickness
to measure Moisture-Vapor-Transmission-Rate (MVTR) value as indicated in Table
2
below.
Table 2
Temperature Thickness Mocon readings*
(g/100 sq.in./day)
115 C 0.5 ml Over Range
115 C 1.0 ml 0.4050
115 C 1.5 ml 0.3361
130 C 0.5 ml 1.3355
130 C 0.75 ml 0.5749
130 C 1.0 ml 0.4050
130 C 1.5 ml 0.9046
* reading is the average of two measurements.
The above MVTR values (less than 2 grams/100 sq.in./day) achieved with the
coating composition meets the ASTM standard.
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Those skilled in the art having the benefit of the teachings of the present
invention as hereinabove set forth, can effect numerous modifications thereto.
These
modifications are to be construed as being encompassed within the scope of the
present invention as set forth in the appended claims.
