Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
1. Field of the Invention
This invention relates to the coating of polyethyle~sterephthelate substrates with a copolymer of polyvinylidene
chloride to provide the substrates with a gas-impermeable
barrier coating and, more particularly, to an airless spray
coating process for providing the surface of polyethylene
terephthalate containers with a high quality, uniformly trans-
parent barrier coating to substantially reduce or prevent the
passage of gases through the walls of the containers.
2. Description of the Prior Art
Plastic containers for beverages made of polyethylen~
terephthalate (commonly referred to as PET bottles or con-
tainers) have become popular for a number of reasons including
their light weight; their strength and capacity to hold bever-
age&, including carbonated beverages such as soft drinks and
cola~ their laek of toxicity and the economies of materials anf~
methods by which the containers can be manufactured. Typically
these containers are made by a process called "blow moldingn in
which a preform or parison is heated and stretched both axiallv
and radially by air pressure in a mold to the desired shape of
the container. Such biaxially oriented PET containers are
strong and have good resistance to creep, i.e., they maintain
their dimensions even under the internal pressure caused by
gases in the liquid inslde the bottles. Moreover, the con~
tainers are relatively thin walled, and henc~ are lightwei~ht
but, nevertheless, are capable of withstanding without undu~
dlstortion over the deRlred sh~lf llfe of tha produat the
internal pressure exerted by a carbonated liquid~ such ag so~t
drinks and colas, . _
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However, a major problem with such thin-walled PET
containers are that they are permeable to gases such as
carbon dioxide and oxygen. That is, with PET containers,
these gases are capable of migrating or passing through
the wall of the container due to the pressure differential
between the gas inside and the pressure outside of the
container, Thus, in the case of bottles containing carbonated
liquids the pressurizing carbon dioxide in the liquid which is
typically at a pressure on the order of 60-75 pounds per
square inch gauge (psig) can migrate through the walls of the
container and be released. This migration of carbon dioxide
takes place over a period of time. As a result, the carbonated
liquid gradually loses its carbon dioxide; and, when the bottle
is opened, the beverage lacks carbonation or is what is common-
ly referred to as being "flat". Conversely, PET containers
are permeable to oxygen which permits the oxygen in room air
to migrate through the walls and into the container which
can cause spoilage of certain comestibles contained in the
containers which are subject to deterioration by the presence
of oxygen. This then affects the flavor and quality of the
container contents.
At present, one commercial manufacturer and bottler
of carbonated soft drinks requires that the loss of pressure
in PET bottles at room temperature 23C 50% r.h. over a
sixteen week period be no more than 15%, e.g., no more than
9 psig starting from 60 psig. This is referred to as the
"shelf life" of -the bottle, i.e., how long the bottle and -
its contents can be held prior to sale without unacceptable
deterioration of product quality. With uncoated PET bo-ttles,
in some cases, the time required to distribute the bottles
to the point of sale alone can exceed this shelf life for up
to one-half of the United States.
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The problem of gas permeability in PET bottles or
containers i9 particularly severe where the contalner i~ rela-
tlvely smallS and, as a result, the ratio of the sur~ace area oFthe contalner to the volume of the contents ls larger than wlth
larger containers. An example of such a contalner is a lt2
llter slze contalner, whlch is a desirable slze for carbonated
liquids such a soft drinks and coias~
For the foregoing reasons, prior workers in the art
have found lt desirable to provide PET containers with a layer
of material which has a low vapor and gas permeability which
thus provides a coating or barrier on the surface of the con-
tainers to prevent the passage of gases therethrough. One
material which,has been employed by prior art workers to provids
such a barrier coating is a copolymer of vinylidene chloride
(commonly referred to as PVDC). This material is a polymer
which may be applied as a liquid and thereafter dried to form
the desired barrier coating. Variou~ techniques have been
employed to apply barrier coatings of PVDC including the coatin~
of PET preforms prior to blow molding and roll coating of the
surface of blow molded PET containers.
Although PVDC has been successfully applied to the
surface of PET containers by such methods as roller coating,
such a process is not particularly efficient or economical in
that it does not lend itself to high speed production rates.
That is, in industry, PET bottles are produced at a rate of 700
to 1800 bottles per minute. Thus, an efficient and economic
coating process should provide the PET bottle with a PVDC
coating at a rate of 300 bottles per minute or greater. Tho
co~t of equipment to satisfy this production rate by roller -~ _-
coating is inordinately high.
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It is known in industry that spray coating i~ an
efficient and high speed method oE applying coating materials i:.
a liquid form to substrates. Uowever, it has been found that
wh~n PET bottles are 3pray coated with PVDC according to con~en
tional ~pray coating techniques the resulting coating is very
non-uniform and is not uniformly transparent such that it
di~torts the surface appearance of the bottle and thus i9
totally commercially unacceptable. Moreover, the pressure
losses from such coated containers are ~nacceptably high. Tha_
is, in today' 9 commercial applications, the PVDC barrier coatir.
on PET containers must be highly uniform, fimooth, clear, uni-
formly transparent, glossy, not subject to delamination, and nc_
cracked or crazed as well as substantially inpermeable to gaq
migratiop. Otherwise, the coated container .i9 simply unusable
commercially. Prior to the present invention, a process ha~ no~
been available to spray coat PET containers with PVDC which
produces barrier coatings meeting these requirements.
Sununary_of the_Invention
The present invention has overcome the problem of
applying PVDC barrier coatings on PET containers by providing a
pray coating process which results in PET containers having a
~ubstantially gas-impermeable, clear, smooth, uniformly tr~n -
parent PVDC barrier coating having a high gloss which does not
contain cracks or crazing. This process is carried out by
sirless spraying of PET containers with an aqueous dispersion o~
PVDC and thus is amenable to high speed productlor. proce~a~
wlth high coating efficiencies.
~ ccording to the proces~ of the pre~ent invention, a
PET container at room temperature is located ~n clo~e proximity . _
to one or more airless spray nozzles through which is sprayed an
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¦ aqueous dispersion of PVDC such that the outside surface of th3
¦ container i9 lmpacted with an atomized airles~ 3pray of the
aqueouq dispersion of PVDC to provide the outside surface of -.ho
contalner with a barrier coating of PVDC. The coating i9 then
dried to remove the water therefrom without distortinq the PET~
- Preferably, the drying of the coating is carried out at a
controlled humidity and temperature to prevent too rapid a
removal of water from the coating. A presently preferred
environment for drying of the coating is 20 to 90~ relative
humidity and a temperature of 170-175F. The drying time is
short enough to keep the temperature of the container below
about its 140P distortion temperature but yet long enough to
dry the coating to a substantially tack-free condition. Tha
resulting coating is highly uniform, smooth, clear, uniformly
tran~parent, glo~sy, not subject to delamination, and i9 not
cracked or crazed. Moreover, the coating is substantially
ga3 impermeable and meets the ~shelf life~ standard of no more
than a 15~ loss of pressurQ over a sixteen week period re~erre~
to above.
Although it cannot be stated conclu~_vely, it ia
believed that the close proximity of the surface of the bottle
to the airless spray no~zle in combination with the pressure of
-the spray causes a sufficiently high impact force of the PVDC
coating material with the surface of the container~to initiate
~uniform coalescence of the PVDC barrier coating material to for~
a uniform coating of the copolymar on the ~urfacu of the aon~
L taine~.
The practice of tha prHser,t inv~ntlen thu~ pr~vld~
alaar, unlformly transparent PVDC barrier coating on PET con- _~
tainers. The PVDC coating material i~ applied to a thickne~s -¦
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sufficient to meet the requirement that the loss of pressure
from the container be less than or equal to 9 psig beginning
from 60 psiy over 16 weeks or more with the containers being
held at 23C (73F), 50% r.h. It has been reported in a
paper authored by Phillip T. DeLassus, Donald L. Clarke, and
Ted Cosse of the Dow Chemical Co., of Midland, Michigan
entitled "Saran Coatings on PET Bottles: Application,
Permanance and Recycle" that a PVDC coating having a thickness
in the range of about .1 to .2 mils (about 2 1/2 to 5 microns)
is sufficient to meet such a specification. A presently
preferred range of coating thicknesses is about 2 1/2 to 12
microns and preferably about 8 to 9 microns.
In operation, the~present invention is amenable to
the coating of containers either in a batch process or in a
continuous process where a line of continuously moving
containers are coated and dried. Moreover, alternative means
can be provided for exposing the outside surface of the
containers to be coated to the airless spray of PVDC coating
material. One means is to rotate the container in front of
one or more airless spray nozzles to achieve complete coating
of the outside surface to be coated. Another method is to
have a number of nozzles oriented such that the total outside
surface area of the container to be coated is impacted by
the material without rotation of the container.
Among the many advantages of the present invention is
that it admits of a highly efficient and relatively high
production rate process for applying PVDC coatings to PET
bottles such as by moving a line of PET containers through
a continuous coater at coating rates of 300 bottles per
minute or greater. This operation is carried out inside of
an enclosure where
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overspray is collected and returned to be repumped to the sp~ay
nozzles with 95~ transfer efficiency. The resulting coatir~3
are substantially gas impermeable, clear, smooth, uniformly
transparent, and do not contain any cracking or crazing and are
not subject to delamination. All in all, the present invent on .
provides a process for spray coating PET substrates with PVDC
barrier coatings to provide coatings having superior physical
properties, which process can be carried out at production ra '3s
suitable for commercial applications.
Other objects and advantages of the present invention
will become apparent from the following detailed description,
reference being had to the accompanying drawing.
Brief Description of the Drawinqs
Fig. 1 is a photograph of an experimental apparat~s
showing the coating of a PET bottle according to the present
invention.
Fig. 2 i5 a photograph ~imilar to Fig. 1 showing th~
PET bottle 15 seconds after coating and before drying of the
coating.
Fig. 3 is another photograph of the same experimen-tal
apparatus shown in Yigs. 1 and 2 but showing coating of a PET
bottle with the bottle spaced from the spray nozzle,
Fig. 4 is a photograph comparing the appearance of
bottles coated according to the methods shown in Figs. 1 and 2
and that shown in Fig. ~.
Detailed Descri~tion of the Invention
In its general aspect, the process contemplate~ the
airless spray coating of PET containers or bottles at room
temperature with aqueous dispersions of a polyvinylidene
chloride copolymer. As used herein, the term "dispersion~
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encompasses an emulsion, solution or latex and denotes a ~7
disperslon of a polymer, e.g., on the order of 1000 to 2000
Angstroms in si~e, dispersed in a continuous phase consisting
essentially of water. Typically, the percentage of polymer
solids in the dispersion is on-the order of 40 to 60% solids by
weight. Examples of such a copolymer emulsion suitable for use
in the present invention are ~ARAN* sold by ~. R. Grace ~
Company, Chemical Division, Baltimore, Maryland; Dow XD30563.2
sold by Dow Chemical Company, Midland, Michigan; Morton Serfene
2011 sold by Morton Chemical Company, Crystal Lake, Illinois;
and Union P-931, sold by Union Chemical Division of the Union
Oil Company, Anaheim, California. Some material compositions may
have a surface tension such that wetting of the substrate is
difficult. In such instances, pretreatment by methods known by
those skilled in the art including flame treatment and corona
discharge will enhance wetting. The coating is applied to the
exterior of the PET containers by positioning the containers in
cloqe proximity to one or more airless spray rlozzles and impact-
ing the surface of the containers with ~n atomized spray of the
dispersion ejected from the airless spray nozzles. It is
desirable to maintain the relative humidity in the area of the
container being coated at greater than 20~. This may be accom-
plished, for example, by spraying the walls of the coating
chamber with water or by injecting steam into the coating area
through one or more nozzles. In continuous coaters where the
overspray is collected and repumped to the no2~1es, water would
dilute the coating material. Thus, it is desirable to spray the
emulsion itself against walls of the chamber or into thc coatinc
area in addition to spraying the bottles during the coating
operation to maintain the desired relative humidity in the
enclosure without dilution of the PVDC coating material.
Maintaining the relative humidity above 20~ keeps the coating
from drying too quickly in the coat.ing enclosure and thus
prevents the formation of microcracks in the cQatingO Micro-
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cracks can cause non-transparency of the coating and provide
avenues for the migration of gases through the coating. Micxo-
cracks thus are to be avoided.
During the coating operation the bottles may be
rotated, e.y., at speeds of 500 rpm, to insure complete covera-
~of the outside surface of the bottles with the liquid coating
material being sprayed from one or more fixed spray nozzles.
Alternatively, the nozzles could be mounted on movable arms sl_h
that they could be moved to cover the surface of a series o~
non-rotating bottles. Still further, a number of fixed nozzles
pointed in different directions could be used again to achieve
complete exposure of the bottle surface to be coated to the
liquid spray.
Whatever the apparatus employed, it is critical to
achieving high quality, uniformly transparent PVDC coatings on
PET bottles that the PVDC spray contact the bottle with a force
sufficient to initiate uniform coalescence of the polymer to
form a uniform coating having the desired properties recited
above. In an airless spray application system, it has been
fo~nd that the impacting force of the liquid spray on the bottl:
surface is a function of the airless pressure, nozzle size,
rotational speed of the bottle, if any, and the spacing distance
of the bottle surface to be coated from the nozzle surface. All
other variables being equal, it has been found that by locating
~he bottles physically in close proximity to the nozzles that
excellent results can be achieved.
This discovery is demonstrated by and can be further
appreciated from the following example.
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Refer~ing to Fig. 1, a 1~2 liter bottle 10 was mounted
vertically on a spindle 12 which extended into a spray coating
chamber 14. The bottle 10 was held at its open end by threading ¦
the cap end of the bottle 10 into an end cap 16 mounted on the
end of the spindle 12. Two airless spray nozzles 18 and 20 were ¦
mounted in the wall of the spray coating chamber 14. These
nozzles were two 6/12 nozzles, Part No. 710244 manufactured by
Nordson Corporation of Amherst, Ohio. These nozzles operate at ¦ l~
.06 gallons per minute ~as measured with a water flow rate of ¦ ;
500 psig) and produce a 12-inch wide fan 10 inches away from the
nozzles. The nozæles were operated without restrictors. The
upper nozzle 18 was pointed 10 below the horizontal and the
lower nozzle 20 was pointed 8~ above the horizontal such that
the noz21e openings were spaced vertically one ~rom another
~bout 4 1~2". This arrangement produced a strip of coating
application area about 1 inch wide from top to bottom of the
bottles, which were about 7 inches in height, with an overlap of ¦
about 1 inch at the middle of the bottle. The bottles 10 were
rotated at 500 rpm by rotating the spindle 12, and the nozzles
18 and 20 were actuated 200 milliseconds for application of the ¦ I
spray coating material. I jl
To demonstrate the effect of locating the bottles in
cloae proximity to the noz~les, a series of tests were run wi-th ¦
bottles spaced various distances from the nozzles. Fig. 1 shows ¦
the bottle being sprayed located at a distance of 2 1/2 inches
from the nozzles, which is within the practice of the present ¦
invention, using W. R. Grace NoO 820 PVDC emulsion~ a pressure
of 0 psig, 200 millisecond exposure, and 500 rpm rot~tion
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Fig. 2 shows the bottle 15 ssconds after coating and
before drying of the coating.
Fig. 3 shows a second bottle 22 located 4 l/2 inche~
from the nozzles 18 and 20 during the coating operation, all
other conditions being the same. Comparing Fig. l to Fig. 3,
the impact of the atomized spray material on the surface of the
bottle 10 in Fig. 1 was significant compared to that shown in
Fig. 3. That is, in Fig. 1, the spray could be characterized a-
a vigorous "scruhbing" or "washing" of the surface of the bottle
10, while in the arrangement shown in Fig. 3, the bottle surface
was e~posed to what wa~ closer to a soft mist.
~ Figs. 1-3 visually demonstrate the differiDg effect o:
locnting the bottle to be coated in close proximlty to the
nozzle such that the surface i~ actually impacted with the
airless spray as opposed to locating it a distance away whsre,
although the spray contacts the bottle surface, there i~ in-
sufficient impacting force or shear to intiate uniform coales-
cence of the polymer coating.
The results of various test runs comparing the surfac~
appearance of 1/2 liter bottles coated at different distances i~
set forth in the Table below. In each case the coating was
dried by convection by continuing rotation of the bottle o~er a
hot plate~
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r rJ rJ r~ rl r~ rl rJ ~ !_
u 0 rJ rJ rJ rJ r~ r~ r~rJ r~
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Referring to Table I, it may be seen that test samples
A and B which were located in relatively close proximity to the
spray nozzles, i.e., at about 2 1/2 inches, had excellent,
uniformly transparent, PVDC coatings which were superior in
appearance and uniformity. Sample C, also located at 2 1/2 i
inches from the nozzle had a slightly poorer appearance which is
attributable to the substantially lower nozzle pressure and thus
lower impacting force of the spray as compared to Samples A and
B. All had good coating weights. For a 1/2 liter bottle, the
area to be coated is about 55 square inches. The density of the
PVDC material was about 1.6. Uniformly applied, a 400 mg
coating thu~s translates to a thickness of about ~3 microns which
is within the scope of the present invention.
When the bottles were moved away from the nozzles as
in Samples D-I the coating quality became progressively worse.
For example, comparing Sample A with Sample G, the
nozzle pressures and exposure times were the same, but Sample A
which was located 2 1/2 inches from the nozzle had a superior
coating while Sample G located 6 1/2 inches from the nozzles was
unacceptable. It should be recognized that any appearance belo~
a 9 is not commercially ~cceptable. Thus, Sample D, which was
located 4 1/2 inches from the nozzle (a location illustrated by
Fig. 3) was commercially unacceptable even though coated at the
same nozzle pressure and exposure tlme as Sample A and having
relatively good coating weight.
In summary, the foregoing Table shows that sample
bottles located 2 1/2 inches from the nozzles operating at
pressures from 350 to 750 psig showed excellent to superior
results. Sample bottles displaced from the nozzles 4 1/2 to 6
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1~2 inches had vastly inferior coatings which would be com~er
clally unacceptable in terms of coating quality.
In explanation of the~e results, it i8 believed that
when the bottle is located in close proximity to the dirles~
spray nozzle that the force of the airless spray of material ;~
impacting on the bottle surface is greatest. It i9 believed
that this force creates a shear on the polymer coating material
a~ it impacts the 3urface of the bottle which i3 believed to be
critical to the initiation of uniform coalescence of the polymer
particles which in turn is critical to achieving a uniform F
polymer coating. The action of the spray on the bottle can be
variou~ly described a9 "hydraulic scrubbing~ or a "shearing" i~
action; but, nevertheless, the impacting of the coating on the ,~
~urface of the bottle has been found critical to achieving the ~'
results achieved by the present invention.
~he importance of coating quality can be appreciated
by referring to Fig. 4 wherein two 1/2 liter bottles are com-
pared side by side. The bottle on the left was coated at a
distance of 2 1/2 inches from the nozzle while the bottle on the
right was located at a distance of 4 1/2 inchesO The letter "A~
is located behind each bottle such that the viewer mu3t look
through the bottle to see the letter. As i9 clearly apparent, ,~
the bottle on the left has a highly uniformly transparent
coating while that on the right has a coating which i9 occluded ~'l
and non-uniform and one that i9 commercially unacceptable. 1ll
As stated above, it will be appreciated that the range ~_¦
of distances at which the bottle can be placed ~i9 a functlon of 1~1
noz~le si~e, the pressure of the spray, the coating time and tm
rotational ~peed of the bottle. However, it ha~ been found ~ 'r
crit~cal that the relatlon of these variable~ to the distance
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the bottles are spaced away from the spray nozzle be such that
the force of the spray on the bottles is sufficient to initiate
uniform coalescence of the polymer coating material. J
In an operation, various systems for coating bottles
may be envisioned. The present invention contemplates both a
batch coating operation wherein bottles are impacted with the
airless spray of coating material while being continuou~ly
rotated, as well as a continuous coating operation wherein a
conti.nuously moving line of containers is coated as it move~
along the length of an enclosed coating chamber which i9 open a:.
the ends to permit the entrance to and exit of the bottles from
the chamber. In either case, it is desirable to collect the
overspray and return it to the system to achieve a material
efficiency greater than about 95~. The desired high humidity ~.
the coating area may be accomplished by spraying the PVDC ¦
emulsion on the walls of the coater or into the coatinq area.
Vapor losses in the enclosure may be m~de up through addition of
vapors from sources other than those used for spraying the
bottles.
After coating, the coating is dried by removing the
water therefrom. It is important that the rate of drying or .
evaporation of the water from the coating be controlled to
prevent any flashing or quick drying of the coating. Such
flashing or quick drying would cause what is known as ~mud
cracks" in the coating which would seriously detract from the
uniform transparency of the coating as well a~ provide avenue~
for migration of the gases through the coating. Evaporation is . _.
controlled by drying the coated containers, for example, by hot
air convection drying in a tunnel or chamber in which a relative
humidity in the range of 20-90~ is maintained to retard quick
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evaporation of the water. A drying tunnel at a temperature on
the order of 170 to 175F could be employed with the bottles
passing there through at a rate such that the temperature of the
bottles stays below the distortion temperature of the PET
material, which is about 140F, while the bottles emerge from
the tunnel with the coating in a substantially tack-free condi-
tion.
As stated above, in either a batch coating system or a
continuous coating system the bottles can be rotated in front of
fixed airless spray noæzles. We have found that by locating the
bottles with the centerline of the nozzles offset from the
centerline of the bottles in a direction away from the direction
of rotation of the bottles such that the bottles in effect
rotate into the impacting spray that a greater impacting force
of the spray on the bottles can be achieved resulting in improved
coating quality.
Although the present invention has been described in
reference to its applicability to coating PET substrates and,
particularly, PET bottles or containers with PVDC, there is no
reason theoretically why it cannot be expanded to other systems
or substrates where it is desired to provide a substantially
smooth, uniform polyvinylidene chloride coating to the surface
of a substrate for gas-barrier purposes or for other purposes,
such as, for example, to polycarbonate, polypropylene and
polyvinyl chloride substrates.
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