CA1213790A - Process for barrier coating polyethylene terephthalate containers with a copolymer of vinylidene chloride - Google Patents

Abstract

PROCESS FOR BARRIER COATING POLYETHYLENE
TEREPHTHALATE CONTAINERS WITH A COPOLYMER
OF VINYLIDENE CHLORIDE

Abstract of the Disclosure A process for providing polyethylene terephthalate containers with a gas-impermeable coating of a copolymer of vinylidene chloride is disclosed. The process includes locating the container to be coated in close proximity to one or more airless spray nozzles and impacting the outside surface of the container with an atomized spray of an aqueous polyvinylidene chloride dispersion from the nozzles. The impacting force of the polyvinylidene chloride dispersion on the surface of the containers is sufficient to initiate uniform coalescence of the copolymer on the surface of the container to form a uniform coating thereon. The coating on the container is then dried in a controlled atmosphere to remove the water from the coating without distorting the container. The dried coating is smooth, uniform and uniformly transparent. In operation, the overspray can be collected and returned to achieve greater than 95%
material efficiency. The process can be carried out in a continuous manner to provide a continuously moving series of containers with a uniformly transparent, gas-impermeable polyvinylidene chloride coating at production rates suitable for commercial applications.

Description

13~'3~7 l 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, . _ ~Z~L3~
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.

~' ~ 37~C) 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 O .-s ~.

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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 ~:~ lb/~. ~

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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~
. ' .~ ._-~ q 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-* - trade mark - !

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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 ~1 -10- ' 1 ~ .

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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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1~13~3i0 ~ 1 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 , ~ ._-' ~

O, ~137,go 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 -14- ~ ~

~ lZ137~0 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 -15- , .'~ ' .. , . ' .

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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.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A polyethylene terephthalate substrate having a smooth, uniform, uniformly transparent, gas-impermeable barrier coating of a copolymer of vinylidene chloride thereon made by the process comprising the steps of:
(a) providing a substrate of polyethylene terephtha-late;
(b) impacting said substrate with an atomized airless spray of an aqueous dispersion of a copolymer of vinylidene chloride with a force sufficient to initiate uniform coal-escence of said copolymer on the substrate to form said coating on said substrate; and (c) drying said coating on said substrate by removing the water therefrom while controlling the rate of removal of the water from the coating.

2. A polyethylene terephthalate bottle having a smooth, uniform, uniformly transparent, substantially crack and craze-free polymer coating on the outside surface thereof, said coating having a gas-impermeability such that a bottle having an internal pressurization of 60 psig loses 9 psig or less pressurization over a 16-week period at 25°C made by the process comprising the steps of:
(a) providing a bottle of polyethylene terephthalate;
(b) impacting said bottle with an atomized airless spray of an aqueous dispersion of a copolymer of vinylidene chloride with a force sufficient to initiate uniform coal-escence of said copolymer on said surface to form said coating thereon;

(c) maintaining the relative humidity in the area surrounding said bottle at 20% or greater during impacting of said bottle with said copolymer spray;
(d) drying said coating on said bottle while main-taining the area surrounding said bottle at 50-90% relative humidity and at a temperature and rate such that said coating is dried to a substantially tack-free condition without distoration of the bottle.

3. A substrate having a smooth uniform, uniformly transparent gas barrier coating of a coploymer of vinylidene chloride thereon made by the process comprising the steps of:
(a) providing a substrate with a surface to be coated;
(b) impacting said surface with an atomized airless spray of an aqueous dispersion of a copolymer of vinylidene chloride; and (c) heating said substrate to form a substantially uniform polymer coating thereon.