Module 2. Packaging materials

Lesson 5

5.1 Introduction

Glass generally refers to hard, brittle, transparent material, such as those used for windows, many bottles, or eyewear. Glass is one of the most important packaging materials because of its high barrier and see-through properties. In the technical sense, glass is an inorganic substance formed from a mixture of sand (73%), sodium oxide (13%), and calcium oxide (12%), with a proportion of broken glass or culler (15 to 30% of total weight). Many glasses contain silica as their main component and glass former.

5.1.1 What is glass?

Glass is a mixture in which all the constituent atoms have been persuaded by heating to line up in to a random but rigid net wall in which each silica atom is linked to ‘4’ oxygen atoms and then to other silica atoms, both atoms of sodium and calcium distributed in holes in the network.

5.1.2 Approximate composition of glass

Natural White Glass:
  • Silica (SiO2) – 73%
  • Lime (CaO) – 11%
  • Soda (Na2O) – 13-14%
  • Alumina (Al2O3) – 1.7%
  • Magnesia (MgO) – 0.3%
  • Potash (K2O) – 0.3-0.4%
  • Iron oxide (Fe2O3) 0.05%
  • Sulphur Trioxide (SO3) – 0.19%
The content of soda decides the cost of the glass.

To colour the glass appropriate materials are added for example:
  • Green colour – Chromium and Iron,
  • Amber – Iron, Sulphur and carbon
  • Opal – Fluorine
The principal raw materials required for manufacture of glass are sand lime stone and soda ash. Screen printing, ceramic spray and plastic coating are used for decoration or printing on the glass.

5.2 Characteristics of Glass Containers

Glass containers have several characteristics that make them ideal for food and beverage packaging:

1. They are impervious to moisture, gases, odors, and microorganisms.

2. They are inert and do not react with or migrate into food products.

3. They have filling speeds comparable to those of cans.

4. They are suitable for heat processing when hermetically sealed.

5. They are transparent to microwaves.

6. They are reusable and recyclable.

7. They are resealable.

8. They are transparent and display the contents.

9. They can be molded into a variety of shapes and colors.

10. They are perceived by the customer to add value to the product.

11. They are rigid and allow stacking without container damage.

12. They can be printed on directly or by using paper labels.

The main disadvantages of glass as a packaging container are:

1. Higher weight and hence higher transportation costs than other types of packaging containers.

2. Lower resistance than other materials to fractures, scratches, and thermal shock.

3. More variable dimensions than other containers.

4. Potentially serious hazards arise from glass splinters or fragments in foods.

5. Permeability to UV light.

This latter problem can be overcome by incorporating various oxides, sulphides, or selenides to color glass and block out the incident UV radiation.

5.3 Types of Glass Containers

Commonly used glass containers are bottles, jars, tumblers and jugs, carboys, vials, and ampoules.

1. Bottles account for the bulk of glass containers. They are made in various shapes and sizes (from 100 ml to 4 lit) and are characterized by a round neck that is much narrower than the body. This facilitates pouring of contents and allows attachment of suitable closures such as screw-type or snap-on caps or cork plugs. Fruit juices and drinks like flavoured/sterilized flavoured milk, milk beverages etc. are often packaged in bottles. The glass bottles used in Dairy industry are heat resistant bottles. But, they can withstand 50°C temperature difference during heating and only 30°C temperature difference during cooling, which is important during in-bottle sterilization of milk.

2. Jars are wide mouthed bottles with no neck, and this affords easy access to the product. They are used for liquid, viscous, solid, and semisolid products such as fruit pieces, sauces, and tomato pastes. They are closed in a similar manner to bottles, but with larger closures.

3. Tumblers are similar to jars but without a neck and a “finish” for the end closure. They are shaped like a drinking glass and are used for such products as jams and jellies.

4. Jugs are large sized bottles with carrying handles. They are used to package wine and institutional, industrial, and household products.

5. Carboys are large globular wicker-covered glass bottles for holding acids or other corrosive liquids.

6. Vials and ampoules are small, thin-walled glass containers. They are mainly used in the pharmaceutical industry for drugs and in the food industry for small quantities of very expensive ingredients, such as flavors.

7. Food products packed in glass: Baby foods, malted milk foods, sterilized flavoured milk, beer, soft drinks, meat/fish products, fruits and vegetable products.

5.4 Glass Container Production

Broadly, modern glass container factories are three-part operations: the batch house, the hot end, and the cold end. The batch house handles the raw materials; the hot end handles the manufacture, the furnaces, annealing ovens, and forming machines; and the cold end handles the product-inspection and packaging equipment.

5.4.1 Glass container forming process

1. There are, currently, two primary methods of making a glass container —
  • The blow and blow method: The glass first is blown from below, into the blank moulds, to create a parison, or pre-container. The parison is then flipped over into a final mould, where a final blow blows the glass out, in to the mould, to make the final container shape.
  • The press and blow method: The parison is formed with a metal plunger, which pushes the glass out, into the blank mould. The process then continues as before, with the parison being transferred to the mould, and the glass being blown out into the mould.
2. When heated to a high temperature (1500°C), the raw materials liquefy.

3. Specific amounts of molten glass or gobs are shaped in a parison mold by the blow-and-blow process or the press-and-blow process.

4. The glass is then annealed at ~ 540°C to remove stresses and cooled under carefully controlled conditions to prevent distortion or fracturing.

5. In all cases a stream of molten glass, at its plastic temperature (1050°C-1500°C), is cut with a shearing blade to form a cylinder of glass, called a gob.

6. Both processes start with the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds.

5.5 Considerations for Selection of Glass Containers

1. When selecting a glass container for a food it is, important to be careful in choosing dimensions and finish so that the correct volume will be available, the product can be easily filled and dispensed, and a proper closure can be selected. 'Finish' refers to the type and dimension of neck and mouth of the containers, i.e. thread, lug, friction, snap-cap, roll-on etc. There are many standard finishes.

2. Other important factors in selecting a glass container for food are its colour which can influence the type of light reaching the food and its ability to resist thermal shock. Some glasses cannot withstand sudden changes in temperature i.e. filling a hot product into a container and then plunging it into cold water. Special glasses are available for this purpose.

3. Though total quantity of glass used is steadily increasing, its market share in packaging has been diminishing due to the entry of newer and cheaper materials like plastics.

4. Of late, the rediscovered virtue of recyclability of low weight and scratch resistant glass is likely to bring back its lost glory in food packaging.

5. Recent developments in West have given rise to light weight scratch resistant glass bottles and researches are on for making unbreakable bottles.

6. Novel features like vacuum ring; press-on and twist-off lug caps are being increasingly used.

7. In India, though glass is used in a wide variety of single use packaging applications, the technology needs updating and the light weight bottle is yet a dream.

8. The economic advantage of the returnable glass bottle as in case of soft drinks is still unbeatable.

Last modified: Thursday, 8 November 2012, 5:32 AM