The present invention relates to a process of plastic coating (viplatura) of containers in glass, particularly jars for containing food products in general, to be subjected to a heat treatment, and to the containers obtained in this way.

The terms "glass containers" are here intended to identify the glass containers with lid/capsule known in the art of food preserving/storing, such as for example jar, pot- food, hermetic vase, having twist-off capsule or glass lid, and the like. The term "plastic coating" (viplatura) is understood to mean the coating of various types of objects and of hollow bodies in glass with plastic materials of different thickness, colour and tactile effect.

There are essentially two purposes of the plastic coating:

- protection and safety

decorative.

Protection and safety

In the case of impact which is so violent as to cause breakage of the jar, the plastic coating with which it is covered succeeds not only in retaining the splinters of glass and the liquid contained in the jar, but also protects operatives or users of the product against wounds or cuts.

The plastic coating confers a rubber effect which facilitates the grip on the jar, preventing it from slipping also in conditions of low temperatures and with the presence of water, humidity, condensation or dripping of the product contained.

Decorative

By being able to cover the various jars and containers in glass with any colour that may be requested, it is possible to obtain from standard jars different and personalised presentations. The covering can be shiny, transparent or opaque.

Special effects can be obtained such as: fluorescence or the change in colour as a function of the temperature to which the coating is subjected.

This coating technology has been applied for several years, mainly in the chemicals, pharmaceuticals and cosmetics sectors, and consists of dipping of the container to be coated in a bath of plastic material, in particular PVC (polyvinyl chloride) and subsequent "baking" in an oven so as to obtain the end characteristics required.

The Applicant, who operates mainly in the sector of packaging in glass jars of food products in general, which have to be subjected to a heat treatment, for example pasteurization, hot filling, sterilization or the like, thought to adopt the plastic coating technology in order to exploit the advantages of "protection" and "decoration" disclosed previously, typical of this technology.

However, it has been noticed that the classic process of plastic coating, consisting substantially of dipping the container in the bath of plastic material and subsequent baking, did not give the desired results, in that the plastic coating, in particular PVC, tended to become detached from the container. This effect is due mainly to the heat treatment to which the container is subjected after filling, and is unacceptable for the sale of the product.

Canadian patent application no. 973760 describes a plastic coating of glass bottles which is performed by applying to the bottle a first layer of PVC polymer or the like, and a subsequent layer of a polymer, dissolved in solvent, having greater mechanical strength such as for example polyurethane, in order to increase the resistance of the glass to the internal pressure and prevent the scattering of pieces of glass in a more effective manner in the case of accidental breakages of the bottle.

However this application does not teach how to avoid the detaching of the PVC from the glass when the container is subjected to heat treatments for conserving the foods (e.g. pasteurization, boiling, sterilization and the like) after having been filled with the food, since the object of this application is bottles for fizzy drinks which notoriously are never subjected to heat treatments after having been filled. The object of the present invention is that of avoiding the disadvantages stated above.

More particularly, an object of the invention is that of providing a process of plastic coating for containers in glass of food products to be subjected to heat treatment, which avoids the detaching of the plastic coating from the container.

Another object of the invention is that of providing such a plastic coating process which is simple and economical to produce. These and other objects are achieved by the process according to the invention which has the features as per claim 1.

Substantially, the process of plastic coating of a glass container, according to the invention, consisting of immersing the container in a bath containing a suspension of polyvinyl chloride (PVC) and of subjecting it to polymerization in a baking oven, provides a pretreatment consisting of applying, to the container, a layer of polyurethane primer in aqueous suspension and of drying the container in an oven to eliminate the traces of water. Advantageous embodiments of the invention are disclosed by the dependent claims.

The features of the process of plastic coating of containers in glass, particularly for food products to be subjected to heat treatment, of the invention, and of the containers obtained with this process, may be understood more clearly from the following description of a non-limiting example of embodiment of the invention.

According to a preferred embodiment of the invention the jars intended to contain food products, which must be subsequently subjected to a heat treatment of conservation typical of foods (for example pasteurization, boiling, sterilization and the like), are inserted in special stainless steel supports hooked to a transport chain and restrained by internal springs which provide for compressing the mouth of the jar against an upper plate, again in stainless steel, in order to avoid internal contamination of the jar, with the total closure of the mouth during processing. Before the actual plastic coating treatment, which can be considered in itself known and which will be described here below, on the jars a layer of glue is applied, through dipping or spraying, that is a polyurethane primer in aqueous suspension, and the jars are made to transit in a drying oven in order to eliminate the traces of water present in the primer and on them. Having performed this preliminary treatment which, as will be seen, is fundamental for the obtaining of commercially acceptable products, the supports with the jars are made to move forwards towards a bath which contains a suspension of PVC (polyvinyl chloride), where a robot picks up the supports with the jars from the transport chain and immerses them in the tank with the PVC suspension.

After the immersion the robot hangs up the supports with the jars again on the chain of the system which, proceeding, allows the jars coated with the PVC to drip, eliminating the excess material. The material, by dripping, falls onto the conveyor belts which return it into the dipping bath.

After the dripping the supports, with the jars inserted, are rotated through 180° to avoid the formation of drips on the base of the jar. The rotation of the supports and of the jars allows therefore the distension of the material on the base of the jars, eliminating the possibility of the formation of a base that is not perfectly flat.

Having undergone this latter operation (rotation through 180°) the supports, with the jars with the mouth turned downwards, enter a baking and polymerization oven, where they stay for approximately 15/20 minutes until polymerization is completed.

At the exit of the oven the jars are removed from the supports, checked visually and, if perfect, sent for packaging. In the case wherein the pouring of the PVC is too close to the threading of the neck of the jar, they are sent for trimming where the excess part is removed.

In the normal production of the plastic coating the coating of the hollow bodies in glass, performed through immersion in the PVC suspension, and after the polymerization in the oven, remains adherent to the glass through cohesion, and can therefore easily be detached/peeled and allows the entry of water between the glass and the coating. This effect is even more accentuated for jars intended to contain food products which must subsequently undergo a heat treatment, for example pasteurization, at a temperature of approximately 80°C in a damp environment.

In fact, from tests carried out, it was noted that due to the high temperature, with the traditional plastic coating process, a softening of the PVC occurred with creation of bubbles between the glass and the coating, a phenomenon which is accentuated by the presence of humidity, making the jars unusable and aesthetically unacceptable.

With the above pretreatment, consisting of the application of a primer and subsequent drying in the oven, adhesivization is achieved of the PVC on the glass, also creating a diaphragm impermeable to humidity and keeping the PVC perfectly adherent to the glass.

The adhesion of the PVC to the glass achieved in this way constitutes a cooperation between the container in glass and the outer coating. In fact, the jar retains the coating at high temperature during the heat treatment and the coating, at ambient temperature, protects the glass on every other occasion, performing an accident prevention function during production, and of breakage prevention during transport or display to the public.

In fact, it is unexpected that the polyurethane, used in the Canadian patent application no. 973760 as protective top coat for PVC, was found to be effective as primer in order to increase the adhesion of the PVC to the glass.

The plastic coating likewise protects the container against the action of external atmospheric agents, giving it a new aesthetic appearance and a secure grip for the user.

As plastic coating the product with trade name PURASOL is conveniently used, which is a dispersion of polyvinyl chloride (PVC) in phthalates-free plasticizer, having the following properties: Chemical and physical properties

Decomposition temperature > 250°C Solubility in water at 20°C Insoluble

Odour Slight

Physical state Pasty

Vapour tension (150°C) < 1 mbar

Density (20°C) 1.2g/cm3

Flash point > 200°C

Ignition temperature > 350°C

Mechanical properties

Appearance smooth or porous

Colour as requested

Specific weight 1.19 g/cc at 25°C

Shore A hardness from 30 to 90 approximately Breaking load 90 kg/cm2

Elongation at break 150% / 350%

Cold torsion -30C

Young's modulus 5.7 Mill. Pascal

Tearing 28.2 Newton/mm

Kerosene extraction 2.5% weight after 96 h at 23°C As primer for the pretreatment of glass containers the product with trade name TEXANE WP 115 is conveniently used, which is a polyurethane in aqueous dispersion having the following chemical and physical properties:

Appearance Liquid/Milky

Colour White

- Odour Characteristic

pH 7.5

Initial boiling point 100°C

Solubility in water Soluble In practice the process of the present invention allows glass containers for foods to be obtained whose plastic coating is highly resistant to the heat treatments to which foods have to be subjected, once inserted in the same container.

Naturally the invention is not limited to the particular embodiment described previously, but numerous detail changes may be made thereto, within the reach of the person skilled in the art, without thereby departing from the scope of the same invention, as defined by the appended claims.