Method Of Internally Coating Rigid Or Semi-rigid Plastic Containers

Abstract

A method for spin coating the interior of a generally rigid or semi-rigid plastic hollow article, such as a container or parison, with a synthetic organic resinous material to provide a protective coating therein. The method comprises placing a quantity of a dispersion of synthetic organic resinous material, preferably a latex or solution, in a hollow article, spinning the article at high speed thereby causing centrifugal force to distribute and hold a coating of the dispersion uniformly on the interior surfaces of the same and then heating the coating while continuing to spin the article. The result of the spin coating method is an article with a substantially uniform interior protective film providing improved article characteristics such as gas and liquid barrier properties, color and appearance, taste and flavor protection and the like.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of coating hollow articles, with particular reference to spin coating the interior of plastic containers or parisons, for forming the same, with a dispersion of synthetic organic resinous material which will enhance the end use characteristics of the containers or parisons.

2. Description of the Prior Art

Coating the interior walls of a container with a protective film is well known. Generally the methods used in the past to internally coat plastic, metal and paper containers are dipping, brushing or spraying. Also, in the metal can and paper container coating methods of the prior art, the coatings may be either sprayed or brushed into the containers as they are rotated or placed in the bottom of the containers and forced up the walls of the same utilizing the centrifugal force generated by a subsequent rotational operation. The containers are then removed from the coating apparatus and dried in conventional hot air or infrared ovens.

One of the primary problems associated with the prior art coating methods, including spin coating, is their inability to form uniform coatings without coating sag substantially in excess of 0.1 mil in thickness on the walls of a container without the addition of thickeners which are detrimental to the properties of the resulting film. Another problem is their inability, when used to coat a latex, to completely wet-out and maintain a wetted surface of a nonporous substrate such as a plastic without the addition of substantial amounts of wetting agents which are also detrimental to the properties of the resulting film.

SUMMARY

In general, the present invention provides a method for uniformly spin coating the interior walls of generally rigid or semi-rigid plastic hollow articles such as a container or parison, for forming the same, with a synthetic organic resinous material, hereinafter commonly referred to as plastic material, which will result in an article having an interior protective coating providing improved characteristics. The method comprises placing, preferably spraying or brushing, a quantity of a dispersion of plastic material in a hollow article, spinning the same at a high speed thereby causing centrifugal force to distribute and/or hold a coating of the dispersion uniformly on the interior wall surfaces of the article and then heating the coating to dry or fuse the same while continuing to spin the article. The dispersion of synthetic organic resinous material may be a latex, emulsion, suspension, solution or the like but is preferably a latex or suspension. The article is spun at a high rate of speed of at least 100 r.p.m., preferably from about 1,000 to 5,000 r.p.m., with the resulting centrifugal force distributing and/or holding the dispersion uniformly on the walls of the article. The coating may be dried or fused by inserting an infrared or other like heating element into the article, blowing hot air into the article, dielectric heating, microwave heating or other known heating means or combinations thereof while continuing to spin the same with the coating being held to the container walls by the centrifugal force. The drying time of the coating is dependent on thickness of coating, amount and type of heating and the like. Application of multiple layers of coating, including adhesive or tie coatings applied before the protective coatings, fall within the scope of the invention.

The plastic dispersion is preferably a latex or suspension of saran, polyvinyl chloride, polyamide, olefins, acrylonitrile, epoxy or other like plastic resins or copolymers or blends thereof which will form the desired protective coating in the parison or container. The hollow articles may be thermoformed by extrusion, injection molding, vacuum forming, blow molding and the like from polymers such as polystyrene, polypropylene, polyethylene, acrylonitrile butadiene styrene, styrene acrylonitrile, vinyl chloride, arylonitrile or like high structural strength polymers or copolymers or blends thereof which will not, in most cases, meet the necessary requirements for the products contained therein. The polymers in the article walls may be multiaxially molecular oriented or unoriented depending on desired end product. A more detailed description of various useful polymers will be discussed later.

The method described herein is directed towards, but not limited to, the fabrication of a pressurized or nonpressurized container for packaging food products, oil and greases, beverages, medicine and the like where the protective coating helps provide improved container characteristics such as a barrier to oxygen, carbon dioxide, nitrogen, water or water vapor and organic vapor or liquid transmission into or out of the container.

In addition to the above gas and liquid barrier properties, improved container characteristics such as color and appearance, taste and flavor protection, stress crack and chemical resistance and the like may also be achieved. The method herein described provides many advantages over more conventional prior art coating methods such as a symmetrically uniform coating thickness without coating sag, improved adhesion of the coating to the container or parison wall substrate due to forced wetting, reduced need for wetting agents, thickeners or other additives in the coating formulation, a more continuous layer of coating with substantially fewer pinholes therein due to a high degree of coalescence of the wet coating during spinning of the container or parison and a reduced drying time of the coating as, for example, where the polymer in a latex or suspension is of higher density than the suspending agents, such as a saran latex, the polymer is forced to the container or parison wall leaving the suspending agent at the heated surface during spinning of the container where it can be more easily removed by drying.

As previously noted, the parisons coated by the method herein described may be used to form containers having an interior protective coating. Beneficially, coated containers can be formed from coated parisons by conventional blow molding. However, an additional advantage is obtained by combining the spin coating method with a blow molding operation to form an internally coated container. By blow molding a container contemporaneously with and immediately after the spin coating of a parison, the heat used to dry the coating can be also used to soften the inside parison walls prior to the actual blowing of the container. The interior heat in the spin coated parison need only be retained while additional external heat is applied to soften the parison before the container blowing step. By using this sequence of steps, the time required to heat parisons for blow molding can be substantially reduced.

Accordingly, it is an object of the present invention to provide a new and improved method of coating the interior walls of rigid or semi-rigid plastic hollow articles such as parisons or containers. Another object of the present invention is to provide a method of spin coating the interior walls of hollow articles with a plastic material having protective properties. A further object of the present invention is to provide a method for coating the interior walls of articles which will result in a substantially uniform coating thickness without coating sag, improve adhesion of coatings to article wall substrates and provide a more continuous layer of coating with a substantially fewer number of pinholes therethrough. A still further object of the present invention is to provide a combined method for spin coating the interior of parisons with protective coatings and then blow molding said coated parisons into coated containers utilizing the heat for drying or fusing the coatings in the parisons to help soften the parisons for subsequent blowing of the containers. Other objects of the present invention will be apparent to those skilled in the art from the specification, drawing and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention are shown in the accompanying drawings where:

FIG. 1 is an exaggerated cross-sectional view of a cup shaped container for food products and the like constructed according to the principles of the present invention;

FIG. 2 is a side elevation view, with portions broken away, of a parison which may be used to make a container and which is constructed according to the principles of the present invention;

FIG. 3 is a view like FIG. 1 only showing a modified bottle shaped container;

FIG. 4 is a schematic representation of the sequence of steps followed in accordance with the principles of the present invention showing spin coating of a container similar to that shown in FIG. 1; and

FIG. 5 is a schematic representation of the sequence of steps followed in accordance with the principles of the present invention showing spin coating of a parison similar to that shown in FIG. 2 and then blow molding of the same into an internally coated container utilizing the heat for drying or fusing the coating to help soften the parison prior to blowing the container.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description illustrates the manner in which the principles of the invention are applied but are not to be construed as limiting the scope of the invention.

More specifically referring to FIGS. 1-3, plastic containers 10 and 30 and parison 20 illustrate the resulting interior coating obtained from the spin coating method herein described. Specifically, FIG. 1 illustrates a plastic cup-shaped container 10 having a side wall 12 and a bottom wall 14. Container 10 is formed from a high structural strength thermoplastic material 16. A plastic coating 18 is bonded to interior of walls 12 and 14. FIG. 2 illustrates a parison 20 used to blow mold a container, similar to that shown in FIG. 3, having a thermoplastic wall 22 to which a coating 24, similar to coating 18 shown in FIG. 1, is bonded. FIG. 3 illustrates a bottle shaped container 30 having a side wall 32, a bottom wall 34 and a neck portion 36 which is formed from a thermoplastic material 38. A coating 39, similar to coating 18 shown in FIG. 1, is bonded to the interior of walls 32 and 34 and the neck portion 36.

Referring now to FIGS. 4 and 5, the steps of the method utilized in the present invention are illustrated. Specifically in FIG. 4, step 1 illustrates the selection of a container, similar to that shown in FIG. 1, which is then spun and sprayed on the interior walls with a synthetic organic resinous dispersion, as shown in step 2. Step 3 of FIG. 4 illustrates the drying or fusion of the distributed coating while being held to the container walls by the centrifugal force with an infrared heating element which is inserted into the container while continuing to spin the same. Step 4 illustrates the resulting container with an internal protective coating adhered to the walls of the container. Steps 1 and 2 of FIG. 5 illustrate the selection and spin coating of a parison, similar to that shown in FIG. 2, following the same procedure represented by FIG. 4. In step 3, the parison is held by a heated mandrel to retain the drying or fusion heat within the parison and is externally heated in an oven utilizing any known heating means. Step 4 of FIG. 5 illustrates the subsequent blow molding of the internally coated parison and step 5 illustrates the resulting internally coated blown container which is similar to the container shown in FIG. 3.

For a more complete understanding of the nature and scope of the invention and to better demonstrate its advantages, reference may now be had to the following detailed examples thereof.

Example 1

To illustrate the method herein described, a plastic cup-shaped container, like container 10 shown in FIG. 1, having a wall thickness of about 50 mils, a diameter of about 31/4 inches and a height of about 4 inches formed of a general purpose polystyrene material was selected. A variable speed motor fitted with a spindle wihch holds the container was used to spin the container. The container was placed in the spindle and 3 grams of a saran latex poured into the same. The container was then spun for 10 seconds at about 5,000 r.p.m. with the excess latex allowed to flow out the mouth of the container. While continuing to spin the container, a 600 watt infrared heating element was inserted into the container for 10 seconds to dry the latex. The dried coating thickness was a uniform 0.4 mils on the side wall and 0.1 mils on the bottom wall.

The saran latex used in this example was formulated with about 57 weight percent solids and 2 weight percent wetting agent. The saran component comprises about 80 weight percent of a copolymer of vinylidene chloride with the balance being acrylonitrile and methyl methacrylate copolymers. The wetting agent is of the type generally referred to as an alkylphenoxypoly(oxyethylene)ethanol having the general formula RC.sub.6 H.sub.4 O(CH.sub.2 CH.sub.2 O).sub.n CH.sub.2 CH.sub.2 OH in which R may be C.sub.8 H.sub.17 or a higher homolog.

The coating on the container side wall demonstrates very good adhesion and could be removed with a 1/2 inch wide strip of No. 810 Scotch tape only after three attempts in the same area of the wall. The dried film exhibited complete wet-out on the container walls.

For comparative purposes, a flat sheet of a similar general purpose polystyrene material was coated with the same saran latex using a wire wound rod and then dried for 10 seconds with a 600 watt infrared heating element. The dried film exhibited incomplete wetting out on the flat sheet. A 0.4 mil thick portion of the coating was easily removed with tape from the sheet with only one attempt. The difference in adhesion and wet-out were attributed to the forced wetting of the walls of the container when the spin coating method herein described was used.

Example 2

To demonstrate the uniformity and lack of coating sag achieved by the spin coating method, a cup-shaped container similar to that described in Example 1 was spin coated at 1,800 r.p.m. The same saran latex and spinning and drying times were used. The resulting dried coating on the interior side wall of the container was a uniform 0.4 mil thick. A second container was spin coated following the same procedure except that the spinning step was discontinued before the coating was dried with the infrared heating element. The resulting dried coating on the side wall of the container exhibited severe coating sag and nonuniformity.

Example 3

To demonstrate the improved barrier properties of a container coated with the method herein described, a cup-shaped container, like container 10 in FIG. 1, having a wall thickness of about 40 mils, a diameter of about 31/2 inches and a height of about 31/2 inches formed of a high density polyethylene material was selected. The same procedures and equipment used in Example 1 are employed here. Three grams of a saran latex was placed in the container, the container was spun at about 5,000 r.p.m. for 10 seconds and then dried for 15 seconds while continuing to spin the container.

The high density polyethylene forming the container walls was polymerized by Ziegler catalysts and had a density of about 0.959, a melt index of about 0.5 and a flexual modulus of about 170,000. The saran latex used here was the same as that used in Example 1 except it was formulated with about 4 weight percent wetting agent.

The resulting dried coating was a uniform 0.3 mil thick on the side wall of the container. The average oxygen transmission rate determined by mass spectrometer analysis of two samples taken from the side wall of the container was found to be 0.63 cubic centimeter per 100 square inches per day at atmospheric pressure and 23.degree.C. The average oxygen transmission rate, tested in the same manner, of four 40 mil thick samples taken from the side wall of a similar uncoated container was found to be 2.78 cubic centimeters per 100 square inches per day at atmospheric pressure and 23.degree.C.

Example 4

To illustrate the method of spin coating and blow molding of a parison, like the parison with a tapered side wall shown in FIG. 5, into an internally coated container, like container 30 of FIG. 3, an injection molded, closed end, multiaxially oriented, general purpose polystyrene parison was selected which had a wall thickness of about 40 mils, a base diameter of 11/4 inches and an upper diameter just below the neck of 11/2 inches and an overall height of 41/8 inches. The same procedures and equipment used in Example 1 are employed here. Two grams of a saran latex was poured into the parison, the parison was spun for 2 seconds at 5,000 r.p.m. and then a 500 watt infrared heating element was inserted into the parison for 4 seconds to dry the latex while continuing to spin the parison. A second coating of the latex was applied over the first coating using the same conditions and procedures as described above. The dried coating had an evenly distributed thickness of 0.4 mil at the bottom and 0.6 mil at the top of the parison with the average being 0.5 mil.

The saran latex used in this example was formulated with about 50 weight percent solids and 2 weight percent wetting agent. The saran component comprised about 92 weight percent of a copolymer of vinylidene chloride with the balance being an acrylonitrile copolymer. The wetting agent was the same general type employed in Example 1.

The coated parison was then blow molded using conventional equipment. To heat soften the parison, it was placed on a mandrel, similar to that shown in FIG. 5, which had a surface, temperature of 220.degree.F. External heating was accomplished by placing the parison and mandrel for 14 seconds in an oven heated by radiant heating elements having a surface temperature of 1,000.degree.F. The parison was then placed in a mold and blown into a container, like container 30 shown in FIG. 3, with an air pressure of 60 p.s.i. for a period of 5 seconds. The dwell time of the container in the mold was 3 seconds. The blow molded container had an average side wall thickness of about 10 mils, a body diameter of 3 3/16 inches, a neck diameter of 15/8 inches and height of 43/8 inches. The coating on the side wall of the container was reduced in thickness proportionally with the reduction in thickness of the side wall substrate and had an average thickness of about 0.125 mils.

Examination of the blown container showed that it exhibited the characteristic increased toughness of a multiaxially molecular oriented container. Also, the coating had sufficient adhesion to the container wall substrate to flow therewith without any detachment when the container was blown. The coating in the blown container was continuous and pinhole free as determined by placing a mineral spirits and red dye solution into coated and uncoated blown containers for a period of one-half hour. The uncoated general purpose polystyrene containers were stained red whereas the coated containers remained unchanged. The average oxygen transmission rate determined by mass spectrometer analysis of three samples taken from the side wall of the coated containers was found to be 0.93 cubic centimeter per hundred square inches per day at atmospheric pressure and 23.degree.C. The average oxygen transmission rate, tested in the same manner, of three samples taken from the side walls of similar uncoated containers is found to be 23.0 cubic centimeters per hundred square inches per day at atmospheric pressure and 23.degree.C.

It is emphasized that a wide variety of materials may be employed in coating containers in accordance with the present invention. Particularly suited polymers are vinylidene chloride, vinyl chloride, acrylonitrile, vinylidene fluoride and/or combined mixtures thereof. Especially advantageous and beneficial are compositions of vinylidene chloride polymers, wherein the polymers contain at least about 70 weight percent vinylidene chloride, the remainder being one or more olefinically unsaturated monomers copolymerizable therewith. Suitable vinylidene chloride polymers are prepared utilizing such comonomers as methyl, ethyl, isobutyl, butyl, octyl and 2-ethylhexyl acrylates and methacrylate; phenyl methacrylate, cyclohexyl methacrylate, p-cyclohexylphenyl methacrylate, chloroethyl methacrylate, p-cyclohexylphenyl methacrylate, chloroethyl methacrylate, 2-nitro-2-methylpropyl methacrylate, and the corresponding esters of acrylic acid, methyl alphachloro-acrylate, octyl alphachloroacrylate, methyl isopropenyl ketone, acrylonitrile, methacrylonitrile, methyl vinyl ketone, vinyl chloride, vinyl acetate, vinyl propionate, vinyl chloroacetate, vinyl bromide, styrene, vinyl naphthalene, ethyl vinyl ether, N-vinyl phthalimide, N-vinyl succinamide, N-vinyl carbazole, isopropenyl acetate, acrylamide, methacrylamide or monoalkyl substitution products thereof, phenyl vinyl ketone, diethyl fumarate, diethyl maleate, methylene diethyl malenate, dichlorovinylidene fluoride, dimethyl itaconate, diethyl itaconate, dibutyl itaconate, vinyl pyridine, maleic anhydride and allyl glycidyl ether. It is also frequently advantageous and beneficial to incorporate therein a minor portion of a plasticizer and a heat and/or light stabilizer. Other compositions which may be used with benefit in coatings in accordance with the present invention are vinyl chloride polymers which contain a predominant amount of vinyl chloride therein. Fluorocarbon polymers, fluorohydrocarbon polymers and fluorohalohydrocarbon polymers may also be used with benefit. Such materials as polyvinylidene fluoride, chlorinated polyethylene and polymers of such materials as vinylidene fluoride, vinylidene fluoride and chlorotrifluoroethylene, chlorotrifluoroethylene and vinylidene fluoride, chlorotrifluoroethylene and vinyl chloride, chlorotrifluoroethylene-vinylidene fluoride and tetrafluoroethylene and the like might also be useful. Generally for economic reasons, the vinylidene chloride polymers are employed, as they are most readily available at a low cost.

It is conceivable that a variety of article configurations such as a cup-shape, frustoconical shape, cylindrical shape and the like, which are rotationally symmetrical and which have any desired wall thickness and plastic composition, may be internally coated by the method herein disclosed. Also, the dried interior coating thickness has a preferred range of about 0.1 to 1.0 mils, but can be varied considerably by changing the coating formulation and the spinning rate and time used.

Thus, while certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

Claims

1. A method for internally coating a generally rigid or semi-rigid plastic container or parison type hollow article comprising:

a. placing a quantity of a dispersion of synthetic organic resinous material into said article;

b. spinning said article at a rate of speed sufficient to distribute and hold a coating of said dispersion uniformly on interior wall surfaces of said article; and

c. heating said coating sufficient to dry or fuse the same while continuing

2. The method of claim 1 wherein said article is formed of a polymer which

3. The method of claim 1 wherein said article is a parison which is blow molded into an internally coated container contemporaneously with and immediately after said heating step c thereby utilizing said drying or fusion heat from step c to help soften said parison before blow molding

4. The method of claim 3 wherein said blown container has a wall substrate

5. The method of claim 1 wherein said dry coating has gas and liquid

6. The method of claim 5 wherein said dispersion comprises a latex the major resin component of which is a copolymer having copolymerized therein at least about 70 weight percent vinylidene chloride and a remainder of

7. The method of claim 5 wherein said dispersion comprises a suspension the major resin component of which is a copolymer having copolymerized therein at least 70 weight percent vinylidene chloride and a remainder of one or

8. The method of claim 5 wherein said dispersion comprises an emulsion the

9. The method of claim 5 wherein said dispersion comprises a suspension the

10. The method of claim 5 wherein said dispersion comprises a latex the

11. The method of claim 5 wherein said dispersion comprises a suspension

12. The method of claim 1 wherein said heating of said coating is accomplished by inserting an infrared heating element or other like

13. The method of claim 1 wherein said heating of said coating is

14. The method of claim 1 wherein said heating of said coating is

15. The method of claim 1 wherein said heating of said coating is

16. The method of claim 1 wherein said dry coating has a range from about

17. The method of claim 1 wherein said parison or container is spun at a

18. The method of claim 1 wherein said placing said dispersion into said article is accomplished by spraying.