This invention pertains to glass bottle coating formulations and more particularly to those based on random ethylene copolymers, containing lower alkyl acrylate, acrylic acid and lower monoalkyl acrylate moieties in addition to ethylene moieties, together with minor amounts of adhesion promoters and heat stabilizers.
Carbonated beverages packaged in glass bottles are potentially hazardous because of the internal gas pressure. This internal gas pressure can explode defective bottles producing flying glass fragments which may injure occupants of the immediate vicinity. The larger carbonated beverage bottles, that is, those containing over 16 ounces are more prone to accidental explosion than smaller bottles.
Carbonated beverage bottles have been coated with various plastic materials in an attempt to ameliorate this dangerous condition. For example, a jacket of polystyrene foam has been used for protection of such bottles from impact thereby lessening breakage. However, the polystyrene jacket does not retain glass fragments upon breakage and therefore does not qualify as a shatter resistant bottle, the goal sought by the bottling industry.
An ethylene/vinyl acetate copolymer has been used on light-weight non-returnable carbonated beverage bottles to improve shatter resistance but such coatings are opaque and rough and require a thickness of at least 15 mils for glass containment.
Salts of ethylene/methacrylic acid polymers have been used for bottle coatings but are also limited to disposable or one trip bottles because the polymer coating does not stand the caustic treatment used in washing and sterilizing the returned bottles before reuse.
In general the following requirements exist for commercially acceptable polymer coatings for carbonated beverage bottles:
1. Fragment retention of beverage bottles containing 60 psig carbon dioxide internal pressure.
2. Appearance must be clear and have a smooth texture.
3. The polymer coated surface must provide good lubricity and label adhesion.
4. The polymer coating must be abrasion resistant and retain a good appearance under all use conditions.
5. The polymer coatings must be compatible with the environment.
Of the above requirements fragment retention is by far the most important and critical. One procedure used by bottle manufacturers comprises charging polymer coating bottles with 4 volumes of carbon dioxide, i.e., 60 psig and dropping them from a height of 4 feet in a horizontal position onto a steel plate embedded in concrete. The drop results in side wall failure generally in the impact area. A scatter index is calculated for 24 bottles by multiplying the percent glass found in concentric zones measured in feet from the point of impact. The percent retention is calculated as the percent glass retained within the distance of three feet from the point of impact. The desired FIGURE is greater than 80 percent.
Another test comprises dropping the pressurized bottles in a vertical position and measures the number of tears inflicted on a 4 mil polyethylene film surrounding the drop zone at a distance of 12 inches therefrom by flying glass particles.
Appearance is important to permit the consumer to inspect the contents of the coated bottle and to permit decorative printing directly on the bottle thus eliminating separate labeling operations at a bottling plant.