A summary of the different types of flexible films is as follows:
Plain cellulose is a glossy transparent film which is odorless, tasteless and biodegradable (within approximately 100 days). It is tough and puncture resistant, although it tears easily. However, it is not heat sealable and the dimensions and permeability of the film vary with changes in humidity. It is used for foods that do not require a complete moisture or gas barrier.
Polypropylene is a clear glossy film with a high strength and is puncture resistance. It has moderate permeability to moisture, gases and odors, which is not affected by changes in humidity. It stretches, although less than polyethylene.
Low-density polyethylene is heat sealable, inert, odor free and shrinks when heated. It is a good moisture barrier but has a relatively high gas permeability, sensitivity to oils and poor odor resistance. It is less expensive than most films and is therefore widely used.
High-density polyethylene is stronger, thicker, less flexible and more brittle than low-density polyethylene and has lower permeability to gases and moisture. It has higher softening temperature (12º C) and can therefore be heat sterilized. Sacks made from 0.03 – 0.15mm high-density polyethylene have a high tear strength, penetration resistance and seal strength. They are waterproof and chemically resistant and are used instead of paper sacks.
Polystyrene is a brittle clear sparkling film which has high gas permeability. Polyvinylidene chloride is very strong and is therefore used in thin films. It has very low gas and water vapor permeabilities and is heat shrinkable and heat sealable. However, it has a brown tint which limits its use in some applications. Nylon has good mechanical properties a wide temperature range (from 60 to 200°C). However, the films are expensive to produce, they require high temperatures to form a heat seal, and the permeability changes at different storage humidities.
Films are coated with other polymers or aluminum to improve the barrier properties or to import heat sealability. For example, nitrocellulose is coated on one side of cellulose film to provide a moisture barrier but to retain oxygen permeability. A nitrocellulose coating on both sides of the film improves the barrier to oxygen, moisture and odors and enables the film to be heat sealed when broad seals are used. A coating of vinyl chloride or vinyl acetate gives a stiffer film which has intermediate permeability. Sleeves of this material are tough, stretchable and permeable to air, smoke and moisture. They are used, for example, for packaging meats before smoking and cooking.
A thin coating of aluminum produces a very good barrier to oils, gases, moisture, odors and light.
Lamination of two or more films improves the appearance, barrier properties or mechanical strength of a package.
This is the simultaneous extrusion of two or more layers of different polymers. Coextruded films have three main advantages over other types of film:
- They have very high barrier properties, similar to laminates but produced at a lower cost.
- They are thinner than laminates and are therefore easier to use on filling equipment.
- The layers do not separate.
Paper & Cardboard
‘Sulphate’ paper is strong and hence used for paper sacks for flour, sugar, fruits and vegetables. ‘Sulphite’ paper is lighter and weaker and is used for grocery bags and sweet wrappers, as an inner liner for biscuits and in laminations. Greaseproof paper is sulphite paper made resistant to oils and fats, for meat and dairy products. Glassine is a greaseproof sulphite paper which is given a high gloss. It is resistant to water when dry but loses its resistance once it becomes wet. Tissue paper is a soft paper used for example to protect fruits against dust and bruising.
Many papers are also treated with wax to provide a moisture barrier and allow the paper to be heat sealed. However, a simple wax coating is easily damaged by folding or by abrasive foods. This is overcome by laminating the wax between layers of paper and/or polyethylene. Waxed papers are used for bread wrappers and inner liners for cereal cartons.
Boards are made in a similar way to paper but are thicker to protect foods from mechanical damage. The main characteristics of board are thickness, stiffness, the ability to crease without cracking and the degree of whiteness. White board is suitable for contact with food and is often coated with polyethylene, polyvinyl chloride or wax for heat sealability. It is used for ice cream, chocolate and frozen food cartons. Chipboard is made from recycled paper and is not used in contact with foods (for example the outer cartons for tea and cereals). It is often lined with white board to improve the appearance and strength. Other types include paperboard and moulded paperboard trays (for example egg cartons).
Further information can be obtained about local costs and availability of different types of packaging from the following institutions:
Packaging Institute of the Philippines
Unit 725, 7/F Cityland Shaw Tower
St. Francis Ave. cor. Shaw Blvard.
Tel: (02) 687-3051, 687-3143
Association of Flexible Packaging Manufacturers of the Philippines
c/o Flexo Manufacturing Corporation, 275 Mabini St., Caloocan City
Tel: (02) 809-8489
Fax: (02) 809-1435