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PHYSICAL PROPERTIES OF GLASS ENAMELS FOR MULTI-TRIP BEVERAGE BOTTLES

The technical requirements of glass enamels for the ACL decoration of Multi-Trip Beverage Bottles are clearly defined by the international beverage companies. The requirements covering chemical resistance, physical resistance and thermal expansion may be summarised as follows:

Chemical Resistance

General guidelines are laid out for the determination of chemical resistance of glass enamels as follows:

Acid Resistance - Measurement of Lead and Cadmium released from the fired enamel surface in accordance with the standard DIN 51031. Fired ware is immersed in a solution of 4% Acetic Acid for 24 hours at 24 + 2°C. Also, visual test to assess staining of enamel surface using 10% Citric Acid solution for 15 minutes at room temperature as described in test DTM No. 78 of 12 May 1992.

Alkali Resistance - Determination of weight loss from fired ware after immersion in a solution of 9.1% Sodium Hydroxide and 0.9% Tri-Sodium Phosphate at 88 + 1°C for a pre-determined time. Detailed test method is described in ASTM: C675-85. Also, visual test as described in test DTM Number 63 of 12 May 1992.

Sulphide Resistance - Assessment of damage to a fired surface in accordance with the standard ASTM C777-84. Fired ware is immersed in a saturated solution of Hydrogen Sulphide at room temperature. Also, test using Sodium Sulphide and Acetic Acid as described in test DTM Number 142 of 13 May 1992.

Detergent Resistance - Assessment of damage to Super Soilax commercial dishwater detergent at 95°C for 24 hours as described in test DTM No. 75 of 12 May 1992.

Physical Resistance

Logos on multi-trip bottles should have a scuff resistance at least equal to that of the bottle itself. Where bottles come into contact with each other (during filling, washing processes and whilst in use), point of contact damage is inevitable. When the logo is no longer legible, or is deemed unacceptably scuffed, it is withdrawn from service.

If damage to enamel decoration is significantly greater than damage to glass (on a similar undecorated point of contact), the enamel may be deemed to lack physical resistance. It should also be determined as to whether the enamel itself is at fault or whether the decorating lehr firing conditions and/or the printing application contributes to any apparent "softness".

Although firing temperature may be generally given at 600 to 640°C, the firing cycle determines the precise "soak" period and peak temperature required to achieve optimum chemical and physical resistance and to mature the enamel to give best colour and gloss.

Thermal Expansion

As a general rule, thermal expansion of the enamels should be slightly lower than that of the glass substrate. A typical soda-lime glass has a coefficient of thermal expansion of 87 x 10-7. Enamels, therefore, should ideally be 84+2 x 10.7 in order to ensure that the enamels remain in tension relative to the glass. If enamels are significantly outside this range, ie, significantly below 82 or even above the expansion of the glass, the resultant stress can cause excessively low bursting pressures and/or breakages during filling and handling. If the printing and firing conditions deviate from established parameters, the compounded effect can result in very high losses.

Accurate determination of enamel expansion (as described in DTM Number 79 of 10 September 1992 and ASTM C978).

Enamel suppliers/manufacturers must certify that the thermal expansion of their products lies within the agreed range to suit the substrate.

Lithium

The presence of lithium in beverage bottle enamels can, under certain firing conditions, create high levels of residual stress in the glass around the label area. This problem became so acute some years ago that the beverage companies ruled that no enamel used to decorate their carbonated drink products should contain more than 30 parts per million (0.003%) of lithium.

Enamel manufacturers should certify this for all products designated for beverage bottle decoration.