3. MATERIALS TO BE USED IN FISH BOXES AND CONTAINERS - PROPERTIES AND SUITABILITY

3.1 Introduction
3.2 Properties of Materials Used for Fish Boxes and Insulated Containers

3.2.1 Plastic
3.2.2 Aluminium alloys
3.2.3 Wood
3.2.4 Expanded polystyrene
3.2.5 Fibre board

3.1 Introduction

In manufacturing of fish boxes and containers, a great number of different materials have been used. The main categories of materials are metals or alloys, natural materials like wood, bamboo, fibre board, etc., and synthetic materials (plastics).

The most common materials used in manufacturing fish boxes and insulated fish containers are wood, aluminium alloys, HD-polyethylene (high-density polyethylene), expanded polystyrene ("styropor") and solid fibre board. Even if other materials such as steel, bamboo, etc., are in common use in many developing countries, the properties of these materials are not very suitable. Steel is very corrosive and is three times denser than aluminium. Bamboo, as it is normally used, is a material which presents hygiene problems. This description of materials therefore mostly covers the common ones mentioned above.

Materials are rated on their cost, ease of constructing suitable containers and physical and chemical proportion.

3.2 Properties of Materials Used for Fish Boxes and Insulated Containers

3.2.1 Plastic

Fish boxes and containers made from HD-polyethylene are superior to those made from other materials, but they are rather expensive. The fabrication is inexpensive, but the material carries a relatively high cost (depending on oil prices). Production must be carried out in large numbers to get a price which competes with other materials. The expected life is 5-7 years when properly handled. Life is longer for boxes designed to facilitate stocking.

The material is impervious to liquids which facilitates cleaning and reduces opportunities for bacterial growth. However, the material is easily changed with static electricity which can draw dust when stored for long time. The temperature limitations are +100 °C and -40°C, but it must be realized that the material becomes brittle at low temperatures. Therefore it is not suitable as packing material for freezing.

Evaluation of physical and chemical properties of HD-polyethylene:

• Weight: The density of HD-polyethylene i8 low compared with many other materials.
• Corrosion: The material is resistant to rust and corrosion.
• Insulation effect: HD-polyethylene 15 a poor conductor of heat and provides a better insulation than metals.
• Workability- fabrication: HD-polyethylene is adapted to mass production methods and can be produced in large quantities at low manufacturing costs. On the other hand complexity of production methods reduces the opportunity to produce fish boxes without professional skills and equipment.
• Noise: The material makes some noise when it is handled but is less noisy than some materials such as metals.
• Toxity and resistance to chemicals: The material is not toxic and transfers no smell or taste to the contents. It is resistant to oil and grease and cleaning chemical.
• Surface: The surface is smooth and easy to clean.
• Colours: Products in a wide range of colours are available.
• Strength: HD-polyethylene has only medium strength and has disadvantages with respect to mechanical damage.
• Repairability: Broken boxes and containers are generally not repairable and must usually be replaced.

3.2.2 Aluminium alloys

Fish boxes and containers made from aluminium alloys are expensive. They are manufactured in forms suitable both for stacking and nesting. and are well suited for freezing of fish.

Physical and chemical properties to be considered:

• Weight: Aluminium is known for its lightness compared with other metals (approx. one third of the weight of steel).
• Corrosion: Generally the corrosion resistance of aluminium is very good. Some foods react with aluminium. especially shellfish. (A protective lining/coating is recommended to prevent contact in such cases) .
• Insulation effect: Aluminium has higher heat conductivity than other materials used for manufacturing fish boxes and containers. The insulation effect is therefore very poor. Faster freezing. however. is an advantage of aluminium.
• Workability-fabrication: Aluminium has good workability and is easily formed. Mass production is preferable.
• Strength: Aluminium boxes have a high mechanical strength.
• Toxity: Aluminium is non-toxic and is recommendable in direct contact with most foods but can reach with some shellfish.
• Surface: The surface is smooth. and is easily cleaned. although care must be taken with alkaline detergents and sanitizers.
• Repairability: Fish boxes and containers made from aluminium alloys are not easily mended. but in certain cases it is possible to repair them.
• Durability: Expected life is 8-10 years.
• Scrap value: When no longer fit for use. the aluminium fish boxes and containers have some scrap value.
• Noise: Fish boxes and containers made from aluminium are very noisy when handled empty.

3. 2. 3 Wood

Compared with modern materials as aluminium alloys and HD-polyethylene. wooden fish boxes and containers are rather cheap. -and are used mostly in less industria1ized countries.

Wooden boxes can be used on vessels. in chill stores. in processing plants and in transport on trains and lorries. The capacity ranges from 20 to 150 kg.

Chemical and physical properties:

• Permeability: unsealed wood can absorb 15-20% water.
• Workability-fabrication: Fish boxes and containers made from wood can usually be produced near the plant in any quantity required.
• Repairability: Easy to repair.
• Toxity: Wood is a non-toxic material.
• Insulation effects: Wood gives a limited insulating effect which is lessened when wet.
• Surface: Wood has a surface that is difficult to clean thoroughly. Dealing with paint helps the cleaning. Unpainted wooden boxes are difficult to clean because of porous and rough surfaces.
• Durability: Wood has a short life-time (up to 1-2 years).
• Weight: The weight of the fish box will differ with the amount of water absorbed.

Disadvantages of wooden boxes

Difficult to clean. if handled roughly, they are easily damaged (mostly caused by inherent defects in the wood, poor design and poor nailing).

3.2.4 Expanded polystyrene

Chemical and physical properties:

• Strength: The mechanical strength depends on the fusion process when made, but the material has less strength than other materials previously mentioned.
• Repairability: If the box is broken it cannot be repaired.
• Insulation effects: The material has superior insulation properties.
• Weight: The material is very light (down to 1/4 of wood) .

3.2.5 Fibre board

Boxes made of massive fibre board are primarily used as non returnable units. The fibre board is often coated with polyethylene inside and outside. These boxes are weak compared to wood, plastic and aluminium if subject to pressure.

List of properties to be considered:

• Strength: Boxes made from fibre board can sustain most types of handling except stacking without special arrangement.
• Insulation effects: The insulation properties of fibre board are comparable with plastic.
• Weight: The material is rather light (between wood and expanded polystyrene).

Table 3 Characteristics of various materials used in making fish boxes

Table 4 Physical properties of materials used in making fish boxes

Note 1: Figures given will represent some material types in use. Due to great variation in materials they must only be looked upon as indicative and a means to compare different materials.