5.3.1 Polyurethane foam
One of the best commercially available choices of insulation material for fishing vessels is polyurethane foam. It has good thermal insulating properties, low moisture-vapour permeability, high resistance to water absorption, relatively high mechanical strength and low density. In addition, it is relatively easy and economical to install. The main features of polyurethane foams are shown in Table 5.1.
Polyurethane foam is effective as an insulator because it has a high proportion (90 percent minimum) of non-connected closed microcells, filled with inert gas. Until recently, the inert gas most commonly used in polyurethane foams was R-11 (trichlorofluoromethane). However, the Montreal Protocol on Substances that Deplete the Ozone Layer has called for the phasing out of the use of CFCs such as R-11. Replacement foaming agents are being investigated at the present time, with hydrocarbons, hydrofluorocarbons and inert gases such as carbon dioxide being developed as substitutes.
The main ways polyurethane foams can be applied and used are as rigid boards/ slabs and pre-formed pipes, which can be manufactured in various shapes and sizes. The main applications of these types of foam are in chill rooms, ice stores and cold stores. Structural sandwich panels incorporating slabs of foam can be produced for prefabricated refrigerated stores.
Foam can also be produced in situ by a variety of means, as follows:
It can be poured in place. This involves mixing the chemicals either manually or by mechanical means and pouring in open moulds or spaces where insulation is required. The mixture creates a foam and solidifies. If necessary, the solidified foam can be cut to the required size or shape.
It can be sprayed directly onto a solid surface using guns that mix and atomize the foam as it is being applied. For example, fish holds or tanks can be sprayed directly on the outside surface and inaccessible areas may be sprayed on and built up without the need of moulds. The foam will adhere to itself and most metals, wood and other materials. It can also be injected into a cavity (e.g. it can be used for moulded insulated boxes). Spray and injection techniques are becoming the most widely used for the installation of rigid polyurethane foam in ships and fishing vessels.
In frothing, the mixture of chemicals is dispensed partially pre-expanded, like an aerosol cream. Appropriate equipment, including an extra blowing agent, is required for immediate pre-expansion. The final phase of expansion takes place as the chemical reaction reaches completion. This technique is used when rigid foams/panels with a high strength-weight ratio are required.