Food Packaging Technology 3(2+1)

Lesson- 12 Glass Containers

12.0 Introduction

Glass is inert with respect to foods, transparent and impermeable to vapours, gases and oils. Because of the smooth internal surface of glass containers, they can be washed and sterilized and used as multitrip containers, e.g. milk and beer bottles. However, glass containers are relatively heavy compared to their metal or plastic counterparts, susceptible to mechanical damage and cannot tolerate rapid changes in temperature (low thermal shock resistance).Broken glass in a food area is an obvious hazard. The two main types of glass container used in food packaging are bottles, which have narrow necks, and jars and pots, which have wide openings. Glass closures are not common today, but were once popular as screw action stoppers with rubber washers and sprung metal fittings for pressurized bottles, e.g. for carbonated beverages, and vacuumised jars, e.g. for heat preserved fruits and vegetables. Ground glass friction fitting stoppers were used for storage jars, e.g. for confectionery. Glass bottles are widely used for beers, wines, spirits, liqueurs, soft drinks and mineral waters. Within these categories of food and drinks, the products range from dry powders and granules to liquids, some of which are carbonated and packed under pressure, and products which are heat sterilized.

 12.1. Glass container manufacture

12.1.1. Melting

Glass is melted in a furnace at temperatures of around 1350°C (2462°F) and is homogenized in the melting process, producing a bubble-free liquid. The molten glass is then allowed to flow through a temperature controlled channel (forehearth) to the forming machine, where it arrives via the feeder at the correct temperature to

 suit the container to be produced. For general containers suitable for foods and carbonated beverages, this would be in the region of 1100°C (2012°F).

12.1.2. Container forming

In the feeder the molten glass is extruded through an orifice of known diameter at a predetermined rate and is cropped into a solid cylindrical shape. The cylinder of glass is known in the trade as a gob and is equivalent in weight to the container to be produced. The gob is allowed to free-fall through a series of deflectors into the forming machine, also known as the IS or individual section machine, where it enters the parison. The parison comprises a neck finish mould and a parison mould, mounted in an inverted position. The parison is formed by either pressing or blowing the gob to the shape of the parison mould. The parison is then reinverted, placed into the final mould and blown out to the shape of the final mould, from where it emerges at a temperature of approximately 650°C (1200°F).

 Figure 12.1: The blow and blow forming process (courtesy of Rock ware Glass)

A container is said to have been produced by either the press and blow or blow and blow process. In general terms, the press and blow process is used for jars and the blow and blow process for bottles. An alternative, for lightweight bottles, is the narrow neck press and blow process. The press and blow process is generally best suited to produce jars with a neck finish size of ≥35mm (≥1.25”); the other two processes are more suited to produce bottles with a neck finish size of ≤35mm (≤1.25”).

Figure 12.2: The wide mouth press and blow forming process (courtesy of Rockware Glass)

The narrow neck press and blow process offers better control of the glass distribution than the blow and blow process, allowing weight savings in the region of 30% to be made.

Figure 12.3: The narrow neck press and blow forming process (courtesy of Rockware Glass)

Glass containers become weaker with use, due to abrasion of the outer surface as a result of container to container contact or contact with other surfaces. Treating the surface with compounds of titanium or tin and replacement of the sodium ions at the surface with potassium ions can reduce this problem. The resistance of glass containers to sudden changes in temperature is reduced as the thickness of the glass increases. Thus, when designing glass containers which are to be subjected to heating or cooling, e.g. when the product is to be sterilized or pasteurized in its bottle or jar, or if the container is to be hot-filled with product, a compromise has to be achieved between their mechanical strength and thermal shock resistance. Heating and cooling should be carried out relatively slowly to avoid thermal damage to glass containers.

Glass containers are sealed by compressing a resilient disc, ring or plug against the sealing surface of the container and maintaining it in the compressed condition by means of a retaining cap. The resilient material may be cork, rubber or plastic. The cap is made of metal or plastic. The cap may be screwed on, crimped on or pushed in or onto the finish of the container. Roll-on caps are used as tamper-evident closures. Different closures are effective when: (a) the pressure inside the container is close to atmospheric pressure (normal seal), (b) the pressure inside the container is less than that outside (vacuum seal), (c) the pressure inside the container is higher than that outside (pressure seal). Pressure seals are necessary when packaging carbonated drinks. Single trip glass containers are used for liquids such as some beers, soft drinks, wines, sauces, salad dressings and vinegars and for dry foods such as coffee and milk powders. Multitrip containers are used for pasteurized milk, some beers and soft drinks. Products heated in glass containers include sterilized milk, beer, fruit juices and pickled vegetables.

 12.2. Surface treatments

After forming, surface treatment is applied to the container in two stages: hot end and cold end treatment, respectively.

12.2.1. Hot end treatment

The purpose of hot end surface treatment is to prevent surface damage whilst the bottle is still hot and to help maintain the strength of the container. The most common coating material deposited is tin oxide, although derivatives of titanium are also used. This treatment tends to generate high friction surfaces; to overcome this problem, a lubricant is added

12.2.2. Cold end treatment

The second surface treatment is applied once the container has been annealed. Annealing is a process which reduces the residual strain in the container that has been introduced in the forming process. The purpose of the cold end treatment is to create a lubricated surface that does not break down under the influence of pressure or water, and aids the flow of containers through a high speed filling line. Application is by aqueous spray or vapor, care being taken to prevent entry of the spray into the container, the most commonly used lubricants being derivatives of polyester waxes or polyethylene. The surface tension resulting from this treatment can be measured by using Dynes indicating pens. Labelling compatibility should be discussed with either the adhesive supplier or the adhesive label supplier depending on the type of label to be used.

12.3 Inspection and quality

Quality assurance needs are defined and incorporated into the specification of the glass container at the design stage and by, consistency in manufacture, thereby meeting the needs of packing, distribution and use. Quality control, on the other hand, comprises the procedures, including on-line inspection, sampling and test methods used to control the process and assess conformity with the specification. The techniques used can broadly be defined as chemical, physical and visual.

Chemical testing by spectrophotometry, flame photometry and X-ray fluorescence is used to check raw materials and the finished glass. Small changes in the proportions and purity of raw materials can have a significant effect on processing and physical properties.

Physical tests include checking dimensional tolerances, tests for colour, impact strength, thermal shock resistance and internal pressure strength. Visual tests check for defects that can be seen. Visual inspection on manufacturing and packing lines is assisted today by automatic monitoring systems; Infrared cameras can be used in a system to examine containers directly after formation.