Thursday 7 July 2016

How to use 2-Part PU (Polyurethane) Foam

This product is absolutely ideal for filling cavities and for increasing buoyancy! It's strong, rigid, and incredibly simple to use. In this post, we'll tell you a bit more about PU Expanding Foam and also share a video we made a few weeks ago.
Producing the foam

Use two plastic cups to measure out an equal volume of each part, then pour both components into a clean mixing container. Ensure you stir thoroughly, but work quickly - the mixture will begin to thicken in just 25-35 seconds, reaching its maximum rise in about 150 seconds. 
The uncured mix is highly adhesive and will bond to most materials. While the mixture is foaming, take care not to inhale the fumes. 

Immediately after mixing, the liquid foam is poured into the cavity to be filled, then left to foam and cure. It will reach a sticky condition in about 4 minutes, and will be completely cured in about half an hour. 
If you are filling a deep cavity, it's a good idea to pour in a series of layers (around 25mm each), leaving the foam to cure between pours. This is because the exotherm (internal heat) generated during the curing process increases with greater volumes of foam, and this can cause problems.
Each liquid kilo of polyurethane yields approximately 0.08m2 (1 cu.ft.) of rigid foam.


PU foam is supplied in several pack sizes and it is necessary to calculate the volume of the foam required so as the correct pack size can be selected to minimise wastage. 

This can be done by measuring the cavity and simply multiplying length by width by height (in metres) and dividing by 0.0283. 

This calculation will give the total amount of foam required in kg. 

Example:A buoyancy tank measuring, 0.5m (length) x 0.5m (width) x 0.5m (height) = 0.13m3. 

Divide this figure by 0.0283 = 4.59 

Therefore the total liquid foam required is 4.59kg (1 x 5kg pack)


0.0283m3 (1 cu.ft) of hollow buoyancy tank provides 28kg of positive buoyancy in fresh water and about 30kg in sea water. Polyurethane foam gives about 27kg per m3 of buoyancy. The calculations on buoyancy are complex since many materials have a buoyancy factor that has to be deducted from the total theoretical requirement. 

As a very rough guide (including crew, motor and equipment) the following buoyancy guidelines can be used: 
2.4m dinghy requires 70kg, approx. 0.08m3
3.6m runabout requires 198kg approx. 0.23m3
4.2m sailing dinghy requires 88kg approx. 0.10m3
6m sailing cruiser requires 830kg approx. 0.93m3

Benefits and Applications

The foam imparts considerable stiffness with only a minimal increase in weight and is used widely by boat-builders for filling masts and booms where extra rigidity is required. Polyurethane foam is extremely buoyant - just one kilogram of mix will produce enough foam to support about 27 kg in water. To produce enough buoyancy chambers on most small craft, the foam mix is poured into previously prepared box sections - i.e., under the seats. On canoes, the mix is simply poured into the bow and stern sections, then covered with “bulkheads” of fibreglass.

The foam can also be used (with a suitable release agent) to produce lightweight moulded items. Wood, fibreglass or metal moulds can be used and are produced in the same way as those for fibreglass laminating. However, due to the pressure generated by the foam, the mould needs to be considerably stronger than is normally the case, and with a split mould, the separate parts must be securely bolted together. A typical use for this technique is in the construction of sailboard hulls although it should only be employed in conjunction with a suitable laminated shell which has been specially designed to provide the necessary strength. The foam mix, being designed to produce maximum buoyancy, is of comparatively low density and lacks any great inherent strength. Apart from the permanent buoyancy the foam provides it also has good thermal insulation properties as well as reducing noise levels. Polyurethane foams are resistant to benzine, diesel oils, acids and alkalis in normal concentrations.

Polyurethane foam blanks are available ready-formed for use as surfboards or sailboard hulls. The blanks are shaped to individual requirements by the user, sanded down and sheathed in a fibreglass skin. They are constructed from white foam which, whilst retaining a reasonable degree of buoyancy, gives greater strength and rigidity.

Other uses for PU Expanding Foam include acoustic and thermal insulation, rapid prototype shapes in fibreglass, and lightweight, foamed shapes produced in wood, fibreglass or metal moulds.


You may find that insufficient foam has been generated (see our video!) This could be due to: incorrect proportions of components; inadequate mixing or low working temperature (optimum working temperature is 20ºC). Do not use below 10ºC, or with a restricted mould.

Storage and Safe Handling
The two components will remain stable for at least six months, if stored in sealed cans in dry conditions. Storage temperatures should not exceed 20ºC. Contamination of the materials by moisture must be avoided. 
The Part B compound is an isocyanate - care must be taken to avoid contact with the skin or eyes. You should also avoid breathing the fumes. Always work in a well ventilated room or in the open air. If the foam mix is in contact with the skin or eyes, flush IMMEDIATELY, with copious amount of water. If irritations persists, consult a doctor. Medical aid should also be obtained if excessive inhalation occurs or if any material is swallowed. Always exercise care when opening cans, as pressure can build up in the container.
If the product catches on fire, use only a dry powder extinguisher.
Any spillage should be soaked up in rags or cotton waste, and the contaminated area sprinkled with sand or sawdust. This should be left for at least ten minutes, then swept up and the area washed. The waste should be soaked in water before disposal. When dealing with a large spillage, protective clothing and respirators should be worn because of the danger of fumes.

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