Note: Descriptions are shown in the official language in which they were submitted.
J
METHOD OF INTRODUCING AN INTEGRAL
THERMO-BONDED LAYER INTO THE
SURF~CE OF A THERMOFORMED SUBSTRATE
Field of the Invention
The invention relates to the introduction of integral
thermo-bonded layers into the surface of a thermoformed
thermoplastic material. More particularly the invention
relates to introducing an integral thermo-bonded clear or
colored surface into a thermoplastic article that is shaped
and formed in thermoforming processes from a thermoplastic
sheet substrate and a coating composition formulated for
bonding to a specific sheet material in the thermo-bonding
processes disclosed herein.
Backqround of the Invention
Thermoplastic parts are being used to replace metal in
different applications where their weight and cost are
lower than various metals that couId be used. Plastic
parts that are molded or thermoformed can be coated after
being formed with various types of coatings, but typically
these coatings will not adhere well to the plastic
substrate and eventually flake off or are easily abraded
off. There is also some difficulty obtaining ~uniform
coverage on odd shapes. ~,~
Clear or colored, decorative or protect:ive discrete~
coatings are cast from solvent-based coatings onto the
surface of objects made of thermoplastic materials. Such
coatings have conventionally been introduced by applying
solvent-based paints or coating compositions onto the
surface of preformed thermoplastic objects. The coating
layers are discrete coatings which means the paint, after ;
solvent volatilization, creates a coating that~ rests upon
the surface of the thermoplastic substrate without intimate
:
s
bonding between the polymers of the substrate and the
coating composition. Such paints have been used to form
coatings on a variety of materials made in a variety of
thermoplastic processes such as injection molding,
thermoforming, blow molding, etc.
In conventional thermoforming processes, sheet
thermoplastic material is cut to a predetermined size,
introduced into a thermoforming machine, is heated and
~ormed into a desired shape or geometry. Once the
thermoformed article is shaped, coatings have commonly been
applied through spray-on, brush-on, curtain coating, or
other application technology. Such conventional technology
using typical solvent-based paint formulations form
discrete coatings on the surface of the thermoformed
objects. Such paint formulations contain in a solvent base
common polymeric bonding agents, pigments and other common
paint ingredients. The formation of such coatings is
desirable in order to provide an attractive colored
appearance, informational legends, or protection from
scratches or other mechanical insult. Such coatings are
not intimately bonded or integral with the underlying
thermoplastic matrix. Such solvent-based paints have been
used for many years. However, the discrete coatings
obtained from these paints commonly are not sufficiently
resistant to chip, scratch or other coating removal
mechanisms such as weathering. Such discrete coatings are
easily removed in day-to-day use through relatively minor~
mechanical impact from day-to-day wear and tear.
Brief Discussion of the Prior Art
A number of solutions to the coatings problem have
been attempted. The texture of the surface of the
underlying thermoplastic substrate has been altered through
mechanical abrasion, chemical treatment, etc. to introduce
areas of incre~sed adhesion to the integral coatings.
Additives have been added to the coating to attempt to
' '
J.')/J
increase the discrete layer bond strength to the underlying
substrate. The use of curing chemicals such as epoxy
compositions, polyurethane compositions, aminoplast resins,
phenoplast resins, etc. have been introduced into the
coatings to increase cohesiveness in the coating and
adhesivity to the underlying thermoplastic substrate.
Grunwald et al, U.S. Pat. Nos. 3,793,106 and 3,798,096
teach forming a thermoplastic layer on an aluminum surface,
removing the aluminum and applying coatings onto the
receptive revealed plastic surface. Grunwald et al relies
on the tendency of the newly revealed surface produced by
removing the aluminum substrate to be highly receptive to
the addition of coatings to increase bond strength.
8uxton, U.S. Pat. No. 3,788,961 teaches a method of
producing a plastic coating on an article by placing the
plastic article in an electrolyte in an electrophoretic
cell. The electrolyte includes a codispersion of a plastic
material and a finely divided particuIate solid material
that through the passage of an electric current forms a
thick coating layer of plastic and solid particulate on the
article.
Neumann, U.S. Pat. No. 3,839,129 teaches the formation
of reflective surfaces on molded objects from foil
materials by forming a laminate comprising substrate metal
layer and protective film wherein the film is used in in-
mold molding processes by introducing a metallic layer
containing a foil and a thermoplastic substrate.
Grunwald et al, U.S. Pat. No. 3,864,147 teaches a
process for modifying the surface of a polymer substrate to
improve bonding capacity to coatings such as metal films,
paints and inks. The procedure involves laminating a
sacrificial metal foil onto the surface, chemically
removing the foil and simultaneously developing a network
of microscopic fissures and cracks which improves the
tendency of the surface to bond to coatings, films, paints
~ J~i3J J
and inks.
Stengle et al, U.S. Pat~ Nos. 3,868,343 and 3,935,346
teach a process and compositions for coating polymeric
substrates such as polycarbonates and acrylics with a
curable material that provides a hard abrasion resistant,
mar resistant, chemical resistant and acetone resistant
adhered coating. Stengle uses an alkyl alcohol, melamine
formaldehyde, and condensation products that are blended
with organopolysiloxanes which cure to form an adherent
coating.
Summary of the Invention
We have found that integral, mechanically bonded
coatings can be formed in thermoplastic processes by
applying to sheet-like thermoplastic materials, a coating
composition comprising a polymeric material compatible with
the underlying thermoplastic substrate along with a solvent
and a desired powdered pigment or other coating material.
After the solvent is removed, the compatible thermoplastic
polymer becomes plastio or softens and fuses during a
thermoforming step to form an integral coating mechanically
melt-bonded and intimately introduced into the
thermoplastic surface by the thermoforming thermal process.
We have further found ~ that compatible thermoplastic
polymeric materials are most active in forming such
integral coatings during the thermoforming step.- Non-
compatible materials fail to provide sufficient melt mixing
or other interaction, at the coating/thermoplastic surface~ f
interface, during thermoforming to form an integral
coating. ~y compatible thermoplastic we mean materials
that a e compositionally the same as or sufficiently
similar to the thermoplastic substrate to form a compatible
melt mixture.
In somewhat sreater detail, compatible thermoplastics
for use in the coating compositions of the invention are
typically chemically similar to the polymer in the
J ~3 rJ
thermoplastic substrate. In other words, in a coating
composition prepared for a polyvinyl chloride substrate, a
polyvinyl chloride polymer or copolymer will be dispersed
or dissolved in the solvent phase in conjunction with the
pigment or other coating material. In the coating of a
polystyrene material, a polystyrene polymer or copolymer
will be introduced into the coating material, etc. While
the selection of chemically similar polymers for the
coating and substrate is the most straightforward method of
finding compatible coatings for the substrate, chemically
dissimilar polymeric materials can be found that are
compatible with the underlying substrates through
procedures discussed below.
The term thermo-bonding or thermo-bonded means in
thermoforming processes, a coating applied to a
thermoplastic sheet is bonded to the sheet through a
thermal process and the coating becomes an integral layer
by thermal action resulting in a fusing of coating polymer
and substrate polymers.
The term integral coating means a coating that during
the thermoforming process becomes intimately bonded and
mechanically fused at the interface between the coating
material and the underlying substrate through a melt fusion
process in which the coating is no longer separate from the
underlying substrate.
The term discrete coating layer means a coating layer
that is merely surface to surface joined to the underlying
substrate, with no melt fusion occurring. However,
depending on the nature of the substrate, some mechanical
bonding can be present, but there is little or no intimate
involved fusion of the coating layer and substrate.
By compatible we mean with respect to two polymeric
materials, that the materials when blended tend to form a
homogeneous mixed melt that has no tendency to phase or
separate.
.:
, ' ` '
.: ' : ',
2~ ;
Detailed Description of the Invention
The invention resides in a method for introducing a
thermo-bonded coating into the surface of a thermo~ormed
substrate. The coating method of the invention produces
coated thermoplastic substrates that are hard, abrasion
resistant, and solvent resistant.
Thermo-Forminq Processes
Thermo-forming is a process for converting a plastic
sheet into parts, e.g., a tray for packaging meat, egg
cartons, etc. There are three basic methods o~
thermoforming: vacuum, pressure forming (compressed air),
and mechanical.
The vacuum forming process is the most popular
thermoforming method. A thermoplastic sheet is clamped in
a frame and is brought close to radiant electric heaters.
The sheet is softened to a formable condition and is then
moved to and down over a mold. The molten resin is sucked
against the mold by vacuum which quick~y- removes the air
between the mold and the sheet. The plastic sheet is held
against the mold until it cools below the heat distortion
temperature. Excess plastic is then trimmed from the part
and is recycled. The hotter the mold and the faster the
vacuum/air pressure the better material distribution will
be. However, exceeding the required mold temperature
should be avoided.
Pressure forming or compressed air is used any time
pressures greater than atmospheric (14.7 psi) are required.
More pressure is used to obtain better detail, closer
tolerances, faster cooling cycles, more strain-free parts,
better distribution of sheet material and tighter
tolerances. Common pressures used are about 50 psi. In
free pressure forming, a hot plastic sheet is sealed over a
blow box so only the periphery of the sheet is in contact
with any tooling. Compressed~air is injected into the box,
pressurizing the sheet into the desired configuration. The
, ' ~
' : ~ ,.
?~ J,r~ J ,'J
bubble height OL the plastic can be controlled with a
photocell or microswitch, timed or ~eyeballed". Normal
blowing pressures are ~rom 20 to 120 psi.
In the mechanical forming process, there is no vacuum
or compressed air to move the plastic sheet. The forces
necessary to move the sheet are applied by mechanical or
manual stretching, bending, compressing, stamping or a
pressure blanket. One technique is stretch forming, in
which the hot plastic sheet is stretched mechanically or by
hand over or around a mold and clamped in place for
cooling. Another technique is using matched molds in which
the heated plastic sheet is compression molded between two
matching molds. Foams, along with filled and fiber
reinforced materials, are frequently processed this way.
Another technique, strip heating, is the easiest method for
forming along a straight line. Using this method, it is
possible to thermoform parts and then, as a post operation,
strip form sections to obtain a more uniform wall thickness
and smaller beginning blank size. This process lends
itself to fast production of simple containers, store
display fixtures, furniture, and many industrial items.
Thermo-forminq ~eat Requirements
During the forming process temperature and vacuum
and/or compressed air are critical factors. Any variation
in temperature of the hot plastic sheet will greatly affect
the "hot strength" or elasticity (tensile) of the plastic. :~
Under normal conditions it is essential that the sheet~~
material be heated very uniformly throughout. With this
type of heat, the faster the vacuum the better will be the
material distribution as the sheet does not have a chance
to cool off as it is being formed. This produces a minimum
of internal stress and will supply finished parts with the
best possible physical properties. When pressure forming
is used and the material is moved even faster than by
vacuum, the material distribut1on will be better and the
'
i
: :', ' , .
,
.
J .~",,: ~;J ii.-'J
-- 8 --
parts even more stress-free. All thermoplastic materials
have specific processing temperatures. Table I shows the
various temperature ranges for some thermoplastic
materials.
.
.
. .
': : ' ~' ' ' `, ` ' `'
- ~
Jf
~ .1 ~ ~ ~ ~ ~ ~ ~ ~ N N ~[:1 ~`1 N ~ 1~ ~ t_
.. ~ C.) c~ co co o ~o 1` CO :) CO ~O ~) O 00 (~
E. o ~ ~ ~ ,1 ~ t`~ ,~ ~ ~ ,1 ,1 ~ ~ ~ ,
~:1
L,
~: . OOO OOO OOO OO~ OIl~OOOO
L l4 ~ D o~ o N ~ ) m ~ ~D ~ ~ In
P O ~ ~7 ~ ~ ~ ~ ~ r~ ~ ~ ~ ~ ~
~D ~r
~ ~ I
~ I a~ ~r t~ ~ ~ ~ a~ D
Ll C, ~ U~ 1` t` 'r C~ ~r ~1 ~ ~ ~ U'l O ~ ~r CO
f O r-l ~ 1 r~ ~ ~ ~ (~
I o ~n
U ~I C ^ L . O O O O Ltl Lî O O O In 11) 0 O U'~ O O O O
Z O O C~ a) ~ o r~ o o u~ r~ o l--o ~n co o ~:
d Z ~ ~ ~ o ~ ~ ~ ~ N~ ~f ~f NN~ ~ E~
W I 1 ~f c~f ~ ~ In r u~ ~ ~ ~ ~ oo , ~ ~n ~r co t~ a~
~ IU ~f ~ ~ ~ ~ ~N ~
_ r~ _ O r-l r-l rl rl r-l r- r 1 N N rl rl rl rl N r~ N,--I r~ ~1
~ 'If I
~ a, l ~
:~ ~,1 E . o o u~ o Ln o u~ o o o o o o u~ m o m o ~
Hl ~ S.l 1 ~1 Ctf Ct`f N r~ r ~~ ~f ~ ~ co Ctf ~I r~ f O
~ O ~ O NN~ ~N~ N~ ~ NNN N~N~NN ~f
~Zl U V
m cn v~ . ~ ~ r~ ~ O t~N~f U~
d ~ ~ U NN~ ~N~ N~ ~ NNN N~N ~ O rl
E ~ C . O rl rl rl rl r-l rl rl N N rl r~ rl r~ ~I r-l N ~ ~1
P~ U~
~ g If
~rf :~ O E. . o o o o o Lr o Ln n o o Ln In ~ o o o o
Z O ~ ~ ~ ~f ~, o ~ ~, ~ ~ N ~ ~f~f~ ~f~N~ N
~ O NN~ NN~ N~ ~ NNN N~N~NN
O If
~ ~) C
C ~f I 1 ~ rl ~ ~ ~ ~ ~ O Nf~ ~fN ~
~ I ~ a~ t~ ~ ~ t~ ~ r~ o rl ~ f CO ~ ~f C~f ~ U~f ~ ~f
P; C ¦ o ~ N N rl r-l ~v
P: la L I rl
r-l E In o ul m Ltl c o o o o o I Ln u~ u~ o o NIW ,r_
O I ~ ~ ~ f ~ ~ ~ O rl ~f ~ ¦ ~ N~fO~flO ~~
~ ¦ o r-l rl rl r~l rl t~ rl ~r ~ rl rl r-l r-l ~1 r-l rl rl r-l I
I
_ a) ~ I
1~ 1 ~ ~ E. I E~
E. E~ 1 C ~v ~:: ~, O I ~,
h ~ O O a, ~ a, .
I a, ~ ~ ~ rl ~ ¦ a~ r-1 ~: I 1-l ..
N 1 ~ ~ a,~ c rl ~ ~v ~v I
U o I E~ ~f ~ I a, ~
P ~ c u u, u~ ~ a, :~ ~ I rl 4-1 C ~ ~ v~ I rv
P~ ~ I ~ ,1 ~1 ~ rl I f v ~ 0: E4 ~1 ~rl I llf
r-l ~ I ~ ,~ ~if J-\ ~If a, u~ >1 '1 ~, Q,:a~ w - ~ ~ I ,~
~f ~f U ~v ~ v ~ ~ u~ S ~ u~ C 10 u~ f I a,
.~ ".~ 1 x ~ I "~ c c~ ~ ~ ~a --, c ~ ~ ~ I v
~1 (~ r-l r-i Cl ~i ¦ llf r--l a, a) r-l: /If a, ,1 Q~ rl ~17 llf O rl r~
a.J ~f ~ ~ ~v< ~4 ~ v~ >1 )-I r~l >1 ~ I -
" tn a, ~ s~: ~, r-l r-l r-l r-l r-l . O rl r-l r~ I v~
m u u u ~, I o o o o ~ I o o ~, a, - ~ I a,
:~ ~¢ d ~ m ~, ~ ~ ~ P~ ~:
.
.
1 :: :
.
.
,
. .
.
: ,
.
~ ~3 ~
-- 10 --
The various temperature ranges of Table I are
explained as follows:
Mold and Set Temperature: The set temperature is the
temperature at which the thermoplastic sheet hardens and
can be safely taken from the mold. This is generally
defined as the Heat Distortion Temperature at 66 psi (455
kPa). The closer the Mold Temperature is to the Set
Temperature, without exceeding it, the less you will
encounter internal stress problems in the part. For a more
rapid cycle time, if post shrinkage is encountered, post
cooling fixtures can be used so that parts may be pulled
early.
Lower Processing Limit: This column shows the lowest
possible temperature for the sheet before it is completely
formed. Material formed at or below this limit will have
severely increased internal stress that later can cause
warpage, lower impact strength and other poorer physical
properties - another reason for rapid vacuum or forming
pressure. The least amount of internal stress is obtained
by a hot mold, hot sheet, and very rapid vacuum and/or
compressed air.
Orienting Temperatures: Biaxially orienting the
molecular structure of the thermoplastic sheet
approximately 275 to 300% at these temperatures and then
cooling greatly enhances properties, such as impact and
tensile strength. Careful matching of heating, rate of ~
stretch, mechanical stresses, etc. are required to achieve~
maximum results. When thermoforming oriented material,
good clamping of the sheet must be used. The sheet is-
heated as usual to its proper forming temperature and
thermoformed. The hot forming temperatures do not realign
the molecular structure; therefore, the better properties
of the oriented sheet are carried into the finished part.
Normal Forming Temperature: This is the~ temperature
which the sheet should reach for proper ~orming conditions
'
-
~ J ,
under normal circumstances. The core (interior) o~ the
sheet must be at this temperature. The normal ~orming
temperature is determined by heating the sheet to the
highest temperature at which it still has enough hot
strength or elasticity to be handled, yet below the
degrading temperature.
Upper Limit: The Upper Limit is the temperature at
which the thermoplastic sheet begins to degrade or
decompose. It is crucial to ensure that the sheet
temperature stays less than this amount. When using
radiant heat the sheet surface temperature should be
carefully monitored to avoid degradation while waiting for
the "core" of the material to reach forming temperature.
These limits can be exceeded, if for a short time only,
with a minimum of impairment to the sheet properties.
Thermoforming Machines
There are many types of thermoforming machines having
different features which can be used in-the thermoforming
process. Two of the more common machines are roll fed in-
line machines and sheet-fed pressure machines. The
machines generally have 50 psi compressed air and 29" of Hg
vacuum available as a standard. In some plants, extrusion
of sheet and vacuum-forming are integrated into a
continuous process. Timers are utilized to control the
length of the heating and cooling periods, which depend
upon the composition and sheet thickness of the
thermoplastic material. For deep molds with considerable~
surface area, stretching the molten plastic to fit the mold
can be difficult. Xowever, the molten sheet can be
stretched by one of several methods prior to contact with
the mold. Molds can be made of polished wood or
thermosetting resins, but more frequently they are made of
aluminum.
Thermoplastic Th rmoforminq Materials ~
Thermoplastic materials can be repeatedly softened by
.
~ ,
. . . . ~
~,} ~ J
- 12 -
elevated heating and hardened by cooling. These resins are
all linear, with many having slightly branched polymers.
They consist of long molecules and each may have side
chains or molecular groups not attached (not crosslinked).
Newly developed thermoplastics can process as usual, but at
the end of the process are crosslinked using special
techniques (e.g., nucleating agent, hot mold above forming
temperature, etc.). Thermoplastic materials can also be
crosslinked by radiation which turns them into either an
undeveloped or full thermoset which can greatly improve the
physical properties of the thermoformed part.
Any thermoplastic resin that can be extruded or
calendered into sheet or film can be thermoformed.
However, those with low hot strength at the forming
temperature may be very difficult to form. Sheet and film
can be produced by extrusion, co-extrusion, continuous
casting, extrusion casting, calendering, compression
molding, autoclave and press laminating.
There are two phases of thermoplastics - amorphous and
crystalline. In the amorphous phase, thermoplastics are
devoid of crystallinity and have no definite order.
Amorphous materials have a randomly ordered molecular
structure, having behavior very similar to a very viscous,
inelastic liquid. Upon heating, an amorphous sheet
gradually softens and eventually acquires the
characteristics of a liquid, but without a definite point
of transition from solid to liquid state. Amorphous resins~-
normally have better hot strength characteristics than
crystalline resins and as a result form more easily. These
resins usually require less energy to bring them to forming
temperature and to cool than crystalline resins, but
amorphous resins are never as easy-flowing as crystalline
resins. When cooled, amorphous resins do not reach a
totally "non-flowing" solid state and, there~ore, have a
tendency toward "creep" or "movement" with age when a load
~J ~ J .) ",~ '.J rJ
- 13 -
is applied. The following plastics are amorphous: ~BS
(acrylonitrile-butadiene-styrene), styrene, vinyl, acrylic,
the cellulosics, and polycarbonates.
In the crystalline phase, thermoplastics have a very
orderly group of molecules. Crystalline thermoplastic
molecules have a natural tendency to line up in rigid,
precise, highly-ordered structures like a chain link fence.
This gives these resins good stiffness and low creep. Most
of the crystalline materials used in thermoforming are also
partly amorphous (e.g., polypropylene normally is about 65%
crystalline and 35~ amorphous). Unlike amorphous plastics,
when crystalline sheet is heated it remains very stiff
until it reaches the glass transition (Tg) temperature, the
minimum forming temperature of the sheet, at which point
the plastic softens. As the sheet continues to become
hotter it rapidly becomes more fluid. The next condition
to occur is the ideal forming temperature. Unfortunately,
with most crystalline materials this is only a very few
degrees below the melt temperature. Consequently, a lot of
these resins have to be "coldl' formed at the "orienting"
temperature (see Table I) or a little bit above, allowing
an excessive amount of internal stresses causing the
lowering of the heat distortion point, warpage, less impact
strength, etc. This is why these materials are very
difficult to thermoform. However, the polypropylene resin
suppliers have made tremendous chemistry advances only r
recently to correct this problem. There are now several
excellent grades of crystalline thermoplastics that exhibit
very good hot strength at the regular forming temperature
(about 330 F.). The following are crystalline
thermoplastic materials: nylon, polyethylene,
polypropylene, polyphenylene sulfide, and acetal.
According to the method of the invention, a coating
composition is applied to a surface of a thermoplastic
substrate sheet. The coating composition employs in a
:
s ~ 2 ? ' r~ f "~
- 14 -
solvent base a pigment and a thermoplastic solution grade
polymer, copolymer, or terpolymer that is compatible with
the thermoplastic substrate sheet. The polymer used in the
coating is specifically chosen to be one that will thermo-
fuse with the underlying thermoplastic substrate. When the
coating composition is initially applied to the
thermoplastic substrate, a discrete, wet coating is formed
on the substrate having a thickness of between about 1-15
mils. This discrete coating layer can be formed by curtain
coating, spraying, or roll coating the coating composition
onto the surface of the thermoplastic substrate. After
being dried or allowing for solvent flash off, the discrete
coating has a thickness of between about 0.2-4 mils. The
coated thermoplastic substrate is then heated to the
thermoforming temperature of the thermoplastic substrate
and the polymeric component of the coating composition at
which point the coating and the substrate are sufficiently
soft to promote melt fusion resulting in an integral
coating. When the thermoforming temperature is reached,
the plastic substrate and coating composition thermally
fuse forming an integral coating that is mechanically
multi-bonded and intimately introduced into the surface of
the substrate by the shear force on the bond line caused by
stretching forces. Compatible polymeric materials are most
active in forming such integral coatings during the
thermoforming step. Non-compatible materials fail to r'
provide sufficient melt fusion during thermoforming to form
an integral coating.
Typically compatible thermoplastics for use in the
coating composition of the invention are chemically similar
to the polymer used in the thermoplastic substrate. For
example, in a coating composition prepared for a polyvinyl
chloride substrate, a polyvinyl chloride polymer will be
dispersed or dissolved in the solvent phase in conjunction
with the pigment or other coating material. While the
'' ~ ':: ' .' -
' '' ' ~ :; ~: . . .. ... ... ... .
~ J ~j ';3^;~
-- 15 --
selection of chemically similar polymers for the coatingand substrate is the most straightforward method of finding
compatible coatings for the substrate, chemically
dissimilar polymeric materials can be used that are
compatible with the underlying substrate as discussed
below.
The thermoplastic substrate used in the thermoforming
prGcess of the invention can include amorphous resins such
as vinyl polymers, copolymers and mixtures thereof. These
polymers can include polyvinyl chloride (PVC), polystyrene,
and acrylic resins such as polymethyl methacrylate. These
particular resins are useful because they soften but do not
sag when heated. Sagging causes thinning of the resin and
a sagged sheet may have more surface area than the mold,
resu~ting in folds and areas of double thickness.
Copolymers that can be employed in the substrate are ABS,
ABS/polycarbonate, ABS/vinyl blend, acrylate-modi~ied
styrene-acrylonitrile, acrylic/vinyl blend, polyetherimide
copolymer, polyvinylidene chloride/vinyl chloride,
acrylonitrile copolymer, and acrylonitrile-ethylene-
styrene. Cellulosics can also be employed in the invention
such as CAB (cellulose-acetate-butyrate) and cellulose
propionate. Other suitable materials are Teflon
(fluorinated-ethylene-propylene), Surlyn, butadiene-
styrene, nylon, polycarbonate, polyester, polyether
sulfone, polyolefins, polyphenylene oxide, polyphenylene
sulfide, polysulfone, and polyurethane. These polymeric
materials have an average molecular weight of between about
2,000 and about 350,000 and are preferably within the range
of from about 25,000 to about 250,000. A preferred polymer
is PVC having an average molecular weight of about 50,000
to 350,000. The above-mentioned polymers are commercially
available and methods for their preparation are well known
in the art.
The compatible thermoplastic polymer of the coating
,
.
2 3~JJ,J.j.,
- 16 -
composition can include the above mentioned polymers,
copolymers and terpolymers or mixtures thereof. Preferred
polymers for the coating composition include PVC,
polystyrene, acrylic resins such as polymethyl
methacrylate, and the copolymer of ABS. The compatible
thermoplastic polymer can also include chemically
dissimilar polymers that are still compatible with the
underlying substrate. These polymers can include
chlorinated rubber, chlorinated polyethylene, and
chlorinated polyvinyl chloride. The weight average
molecular weight of these polymers is from about 20,000 to
about 250,000 and is preferably within the range of from
about 25,000 to about 150,000. A preferred polymer for the
coating composition is PVC having a molecular weight
average of about 25,000 to about 50,000.
There are many types of pigments that can be
incorporated into the coating composition of the invention
to provide various desired colors. These include both
inorganic and organic pigments. Inorganic pigments that
can be used include white opaque pigments such as titanium
dioxide (TiO2), zinc oxide, zinc sulfide, antimony
trioxide, etc.; brown and red pigments such as iron oxide,
cuprous oxide, cadmium sulfoselenides, etc; orange and
yellow pigments such as those derived from lead chromate,
lead sulfate, lead/molybdate, zinc chromate, cadmium,
barium chromate, etc.; green pigments such as chrome oxide,
copper acetate, cobalt zinc-alumina, etc.; blue pigments
such as complex iron potassium or sodium cyanides, lead
sulfate-lead sulfide-carbon, cobalt aluminate, etc. Black
pigments that can be used are elemental carbon, graphite,
black iron oxides, etc. Metallic powder and flake can also
be used as pigments in the invention and include aluminum
flake, bronze powders from various metals, lead powder,
zinc dust, gold and silver as well as alloy powders.
Organic pigments can be used in the invention such as
`
:
~ ~3f ~ r ~ i
organic dyestuffs. Most pigments made from organic dyes
include an extender or substrate as a means of obtaining
the desirable physical properties of a pigment which are
not present in a large proportion of the organic dyes used
for ma~ing pigments. Even those dyes having the physical
properties of a pigment are often diluted with extenders to
obtain specific physical properties as well as to make them
less costly. A great many natural or synthetic organic
dyestuffs can be made into pigments by fixing them on an
inert base pigment. Useful organic dyestuffs include
yellow chloronitroaniline derivative, paranitraniline-
beta-naphthol derivative (red), dinitraniline-beta-naphthol
derivative (orange), azo compound derivatives,
anthraquinones, indigoid derivatives, arylides of
hydroxynaphthoic acid, arylides of acetoacetic acid,
pyrazolone derivatives, etc.
One preferred inorganic pigment is flaking grade
aluminum available as Standard Paste No. 6205 from
Silberline Manufacturing. Using this pigment results in
paint finishes having a smooth texture with a
characteristic bright metallic color. Paints made from
this pigment at concentrations of about 2 pounds per gallon
give the best all-around durability and appearance. Other
preferred pigments include various organic and inorganic
pigments dispersed in dioctyl phthalate (DOP). These are
available as Stan-Tone~ PC colors from Harwick Chemical r
Corporation. Preferred pigments for the invention are
those that have good heat and light stability. The process
of the invention also allows for the use of some very
expensive light stable organic pigments that are not
usually economically practical and would not be considered
for compounding into the plastic substrate. However, it is
economically feasible to use these pigments in a thin
coating over the surface of a thermoplastic substrate by
using the method of the invention.
. . ..
.
:
.
?~ f '~ J
The solvent employed in the coating composition helps
to soften the plastic surface of the substrate providing a
lower fusing temperature of the substrate and coating. The
solvent choice is dependent on the particular thermoplastic
resin's solubility characteristics. Solvents having
solubility parameters similar to the resin need to be
chosen for use in the invention and a true solvent will
produce a clear solution. Diluent solvents may also be
employed so long as the resin shows good tolerance for the
solvent. Too much diluent can cause precipitation o~ the
resin, cloudiness, or greatly increased viscosity. The
primary solvent or solvents used in the invention need to
be the highest boiling solvents so that they are the last
to leave the coated substrate upon heating. For economy
purposes the amount of diluent is maximiæed to a point
before precipitation or polymer incompatibility.
Solvents that can be used in the invention are higher
boiling ketones such as cyclohexanone, methyl ethyl ketone
(MEK), methyl amyl ketone (MAK), methyl isobutyl ketone
(MIBK), or methyl isoamyl ketone (MIAK). Other solvents
that can be used are dimethylformamide (DMF), chlorinated
solvents, acetates, toluene, or xylene.
Optional ingredients in the coating composition can
include stabilizers, plasticizers, and flow control
additives. Stabilizers for the thermoplastic material that
are useful in the invention are organotins such as
Thermalite 31 and 813 from M&T Chemical, and Plastholl ESO
(epoxized soy oil~. There are many plasticizers available
in the industry which provide lower melting points,
improved fusing of coating materials, improved overall
gloss and better workability of the coating. Plasticizers
that are useful include triaryl phosphate available as
Kronitex~ 100 from FMC Corp., alkyl benzyl phthalate
available as Santicizer 261, and DOP. Flow control
additives are used before thermoforming in applying the wet
.
. .
,
.
2 ~ 2 ~J ~
-- 19 --
coating to the underlying plastic substrate. A useful
additive is a xylene solution of ethyl acrylate and 2-
ethylhexyl acrylate copolymer available as a 50~ solution
of ModaFlow~ resin modifier in xylene (Monsanto Co.).
Dispersants and defoamers are also useful in the
invention. One such dispersant which is used to disperse
pigments for improved color, as for TiO2, is an anionic
surfactant consisting of 70% active sodium dioctyl
sulfosuccinate available as Drewfax~ 0007 (Drew Chemical
Corp.). Antifoaming agents that can be used are silicone
defoamers such as Dow Corning~ Antifoam A compound.
Therefore, the coatiny composition of the invention
comprises about 60 to 85~, preferably about 65 to 75 ~ of a
solvent; about 15-30~, preferably about 20-25% o~ a
thermoplastic resin; about 3-20~, preferably about 5-10% of
a pigment; about 0.05-1.0~, preferably about 0.2-0.7~ of a
stabilizer; about 0-9%, preferably about 2-6~ of a
plasticizer; about 0-1.2~, preferably about 0.2-0.6% of a
flow control additive.
The coating composition has a viscosity of about 70
cps (centipoise) to 1,000 cps, and preferably about 100 to
300 cps.
The coated thermoplastic substrate can be heated in a
thermoforming or moldinq process by radient heat rods, oven
heating or microwave heating. The higher the temperature
(to the upper limit), the more complete the fusing process ,r_
will be. The temperature required for fusing the coating
onto any given thermoplastic will depend on the softening
point of the thermoplastic substrate and; the polymer used
in the coating. For example an a~luminum containing coating
composition can be fused to rigid PVC sheets at as low as
about 200 F. Typical temperatures during the
thermoforming process are about 260-300 F. (See Table I.)
Examples
Of particular value is the coating of thermoplastic
:: :
,
, . '- ' ' :' ' ~ . ' ''' ~ ' . " ,
. - .: : : ~
2~ i.;J
- 20 -
parts with an aluminum colored paint containing a flaking
or non-flaking grade aluminum pigment. Three similar
coatings (Examples 1-3) have been applied to a rigid PVC
sheet by spray, and after allowed to dry, thermoformed into
various shapes. The resulting aluminum color is bright,
glossy, and matches very well to polished aluminum. Even
though the parts are stretched during the thermoforming
process, the coating remains uniform across the plastic
part. The aluminum colored coating provides a high degree
of weather resistance because of the high reflectivity of
aluminum flake, preventing degradation of the coating and
underlying thermoplastic substrate from heat and UV
radiation.
The polymers chosen for the coatings in Examples 1-3
are copolymers of PVC which have exceptional weather
resistance, and when used with the aluminum pigment, they
will have many fold times the serviceability in exterior
applications than other colored PVC parts. PVC plastic
will typically fade in color, chalk r and lose physical
properties after exposure to exterior weathering. Because
of the protective shield of these durable coatings, the PVC
plastic will retain its physical properties and appearance
even after long term exterior exposure. If these coatings
become scratched or stained in service r they can easily be
recoated on the job site by brushing or spraying the
coating over the existing surface, with the newly applied r
coating fusing onto the existing coated surface.
,
~ J~
- 21 -
Table I I
Aluminum Coatinq Composition
Examples 1-3
Percentage
Ranqe
Solvent 60-85%
Resin 15-30%
Stabilizers (0.5-4 phr) 0.05-1.0%
Aluminum Paste (25-75 phr) (65% 6-30%
Aluminum Flake in Solvent)
Plasticizer (0-25 phr) 0-6%
Flow Control Additives (0-5 phr) 0-1.2
Example 1
Xylene 42.0
Methyl Ethyl Ketone (MEK) 22.0
FPC-471 Vinyl Resin (PVC, maleate ester) 20.0
Plastholl ~SO (epoxy soy oil stabilizer for PVC) 0.3
Thermalite 813 (PVC heat stable organotin) 0.1
Aluminum Flake 6205 (50 phr) 15.4
Anti-Terra U (flow control aqent) 0.2
30.4 solids, 200-250 cps 100.00
I
ExamPle ?
Xylene 35.2
MEK 35.0
VAGH Vinyl Resin (PVC, vinyl acrylate, vinyl 8.33 ~ ~
alcohol)
VMCH Vinyl Resin (PVC, vinyl acrylate, maleic a . 33
acid)
Plastholl ESO ~ 0.25 ~ -
Thermalite 813 0.08
Aluminum Flake 6205 (50 ~hr~ 12.81
25.3~ solids,.900-960 cps 100.00
:: ~ : : : : : :
:: - :
, . ~ . ' : ~ ' .' ....... ' ' -. ; - ' ' '
:~. -: . ,
- ~2 -
Example 3
Xylene 3~.2
MEK 34.0
FPC-471 Vinyl Resin 20.0
Plastholl ESO 0-3
Thermalite 31 0.1
Tricresyl Phosphate ~10 phr) 2.0
Anti-Terra U 0.2
Alcon ~luminum Paste 465 (30 Phr) 9.2
100.00
Of particular interest for the aluminum colored PVC
plastic parts is its usage on fitting covers and jacketing
for insulated pipes and tanks. These aluminum colored
fitting covers match up very well with aluminum jacketing,
or the aluminum jacketing can be replaced entirely with the
aluminum colored plastic, providing the same pleasing
appearance and long term durability at a much lower cost.
The aluminum coating provides improved chemical resistance
of the plastic part, as well as keeping the temperature of
the surface low for improved thermal efficiency when used
over insulated cold piping. The impact resistance of the
plastic is improved as much as 50% due to the reinforcing
property of the coating. Flammability of the plastic is
also reduced due to the heat reflecting property of the
aluminum coating slowing the spread of flames.
Testinq of ExamDles 1-3
The coatings in~Examples 1-3 were applied to rigid PVC
at 1, 2, and 3 mils dry film thickness by spray and air
dried. The sheets were then thermoformed into fitting
covers which provided a uniform colored coating with
exceptional abrasion resistance that was impossible to
separate off the plastic substrate.
The coated PVC sheet (20 to 25 mils in thickness) was
bent 180 back on itseli without any ioss of adhesion or
.
.
': ' ' '
.
-
.
- 23 -
color in the crease mark. The coated PVC was also exposed
to -40 F. and 350 F. with no adverse effects. The coated
PVC plastic sheet was also bent to fracture at -40 F. with
no adverse effects seen on the coated surface. Typically,
a coating would crack and/or flake off under such tests if
it was not fused to the surface of the PVC.
The same aluminum coated PVC sheets at 25 mils
thickness were impacted with a Gardener reverse impact
tester, with the aluminum coating facing the impacter. The
uncoated PVC sheets had an impact resistance of 23 inch-
lbs. while the coated PVC sheets had an impact resistance
of 30-40 inch-lbs. depending on the dry film thickness of
the coating (1, 2, and 3 mils tested). There was no
disbondment of the coating in the impacted area from these
tests.
The following Examples (4-5) are coating compositions
prepared with pigments other than aluminum.
Example 4
Xylene 30.8
MEK 30,0
Cyclohexanone 10.0
Plastholl ESO 0.3
Drewfax 0007 (Dispersant) 0.3
VYHH Vinyl Resin (PVC, vinyl acrylate) 20.0
TiO2
Tinuvin 292 (UV-light absorber/stabilizer) 0.2 ;~
Multiflow (Leveling-gloss Agent) 0.4
Stan-Tone~ 40 PC-03 Blue Paste 3.0
100.00
.
'
~i}~3~';~.;.',
- 24 -
Example 5
Methyl Isobutyl Ketone (MIBK)25.0
Xylene 5.0
Kronitex 100 (phosphate plasticizer) 1.9
Drewplus 0007 (dispersant aid for TiO2 0.2
sulf anionic)
Plastholl ESO 0-3
FPC-471 Vinyl Resin 19.0
Uritane OR-600 (TiO2) 5.0
Shear at low speed until all resin dissolved, then high
shear to Hegman 8
Methyl Isoamyl Ketone (MIAK)10.0
Multiflow 0.4
Anti-foam A (silicone) 0.02
40 PC-03 Blue 1.10
25 PC-04 Red 2.30
Xylene 29.78
Blend uniEorm 100.00
Wt. Solids: 30.0% +/- 1%
Volume Solids: 20.3%
Viscosity 2/50/77: 127 cps +/- 15 cps
Wt./gal. 7.93 lbs. +/- 0.2
Coverage Rate 0.5 mils dry, 650 ft /gal.
Examples 6-9 are aluminum coating compositions
prepared with various polymers that were applied to
diEferent types of thermoplastic substrates.
.
~J i~ '?J .~ r'j /~
- 25 -
Example 6
PVC Copolyme r /Aluminum
MIBK 26.7
Xylene 5.0
Kronitex 100 1.8
Plastholl ESO 0.3
FPC-471 Vinyl Resin 18.0
Blend until all vinyl dissolved
~ultiflow 0.4
Alcon Aluminum Paste 465 9.0
MIAK 9 0
Xylene 29.8
Blend uniform 100.0
Wt. Solids: 26.2
Volume Solids: 16.8
Viscosity 2/50/75 F. 90-120 cps
Wt./gal. 7.8 +/- 0~2 lbs. ~,
Coverage Rate 535 ft2/gal. for 3 mils wet,
0.5 mils dry
Example 7
Acrylic Resin/Aluminum
Xylene 39.6 ^~
MEK 20.0
~IAK 10.0
Multiflow 0.4
Elvacite Resin 2009 (Methyl Methacrylate) 20.0
Alcon Aluminum Paste 465 10.0
Blend Uniform 100.0
. ~
- :
-: .
2 ~
- 26 -
Viscosity: 2/50/75 F. 90-100 cps
Wt. Solids: 28.7~
Volume Solids: 20.3%
Wt./Gal.: 7.82 lbs.
Coverage Rate: 400-600 ft.2/gal., 0.5-0.8 mils dry
_am~les 8 and 9:
Hiqh Stvrene CoPolymers/Aluminum
Example 8 Example 9
Xylene 57.6 57.6
MIAK 10.0 10.0
Multiflow 0.4 0.4
Kronitex 100 2.0 2.0
Aluminum Flake 6205 10.0 10.0
Pliolite AC High Styrene- 20~0 --
Acrylate Resin
Pliolite S5B High Styrene- -- 20.0
Butadiene Resin
100 . O 100 . O
Viscosity 3/50/75 165 cps 162 cps
Wt. Solids 28.7~ 28.7%
Wt./Gal. 7.81 lbs. 7.82 lbs.
~olume Solids 22.2% 22.0%
Coverage Rate ft2/gal. 400-600 400-600
Table III summarizes the results of applying the~
coatings of Examples 6-9 to different thermoplastic
substrates using the method of the invention.
As Table II shows, fusion between the coating and
plastic substrate can be achieved when the resin used in
the coating is very similar to the plastic substrate, or
highly compatible with it. On the other hand, where there
was a dissimilarity between~ the substrate and coating
polymers, fusion was not achieved. ~
:
~ '
'
,i, ,1 :'' ,-
` .
2~ 3
Table III
Alternate Coating/Plastic Systems
Compatibility*/Adhesion**/~:~usinq***
Plastic Substrate
Coatina Acrylic ABS Polystyrene PVC
Example 6 Good/ Poor/ Poor/ Excellent/
PVC Good/ Good/ Poor/ Excellent/
Copolymer None None None Fused
Example 7 Excellent/ Excellent/ Good/ Excellent/
Acrylic Excellent/ Excellent/ Good/ Excellent/
Resin Fused Fused None Fused
Example 8 Poor/ Unknown/ Excellent/ Fair/
High Excellent/ Excellent/ Excellent/ Good/
Sytrene- None None Fused None
Acrylic
Copolymer
Example 9 Poor/ Unknown/ Excellent/ Fair/
High Excellent/ Excellent/ Excellent/ Good/
Styrene- None None Fused None
Butadiene
Copolymer
_________________
* Compatibility of resin used in coating with plastic, or~
the ability to blend the resin into the plastic to
modify its properties.
** The degree of adhesion of the applied coating to the
plastic after 10 minutes in the oven at the processing
temperature of given plastic (350 F. for~acrylic, 300
F. for all others).
*** The fusing of the coating to the plasti~ surface so ît
cannot be scraped or lifted off after 10 minut~es in the
.
- 28 -
oven at the processing temperature.
Testinq of Examples 6~9
Determination of whether or not the coating
compositions of Examples 6-9 actually fused to the plastic
substrate was determined by three methods, which were:
1. Scratching the coating through to the plastic
substrate with a sharp metal object in an attempt to lift
it off of the surface. Where the coating had fused to the
surface, it was impossible to flake off any of the coating.
The coating could only be removed by scratching down
through the coating and the surface of the plastic so that
a deep scratch was left in the plastic, with no lifting of
the coating on either side of the scratch.
2. Coated samples were placed in boiling water for 4
hours and then evaluated for blistering, flaking, cracking,
or loss of adhesion. Of the coatings that fused there was
no evidence of loss of adhesion, but in most cases it
appeared to be only tougher and more difficult to scratch.
3. Coated samples were tested by the Gardner Falling
Ball Method, impacting on the back side of the plastic.
There was no loss of adhesion of the coating in any of the
Examples where the coating fused to the surface of the
plastic sheet even though it was impacted to the shattering
point.
Other resins may also fuse to a given plastic even if
they are not similar to the plastic if they prove to have
good compatibility and are thermoplastic. This is--
evidently the case with acrylic resin (in Example 7) fusing
to the ABS and PVC plastic sheets (see Table III). Other
resin possibilities dissimilar to the plastic substrate for
use in coating compositions include chlorinated rubber,
chlorinated polyethylene, or chlorinated polyvinyl
chloride.
While the invention has been described and fully
explained in the detailed description of the specification
,
.
- : . ' ~ `, , '' '
. .
, , ,
.
.. .
s~
- 29 -
and preferred embodiments, many embodiments of the
invention can be made without departing from the spirit and
scope of the invention.
:
:
: ~ . . `
- . : . ~ . . .
: ,
' ' : ' :
., ~ ,. .
