Polyurethane Foams

Polyurethanes comprise a family of highly adjustable polymer materials. Their application spans a wide range from consumer to industrial segments.

The greatest advantage of Polyurethane chemistry is that the two compounds polyol and isocyanates which are in liquid form react in the presence of selected catalysts, stabilisers etc to produce solid Polyurethane foams (PUF).

Rigid PUF with excellent structural properties is available in density ranging from 10 to 1000 kg/m³. Rigid Polyurethane having densities greater than 200 kg/m³ are as wood substitutes and as structural foams for various applications.

The two reactants can also be mixed at high velocity to produce a spray of fine droplets and can be sprayed using a spray-gun to produce foam. Polyurethane spray system is widely used in construction to be applied on horizontal or vertical surfaces.

Rigid Polyurethane foams have manifold advantages. The thermal conductivity (K) of rigid Polyurethane of 45 mm thickness is 0.019 w/mt., while that of expanded Polystyrene is 0.04 (for an 80 mm thickness) and of glass wool is 0.055 for a 100 mm thickness. This indicates that with a lesser thickness we can obtain better insulation with PUF. They provide choice in varying density, for depending on the requirements, the density of right PU foam can be varied to incorporate better insulation properties. Easy production and simplified design ability to withstand molten bitumen light weight and reduced manual usage are other plus points. PUF facilitates higher usage volume so as to give outside dimension to cite an example, a freezer of 100 mm conventional insulation. It has a usable volume of about 330 litres while for using that rigid PU foam the insulation thickness is reduced to 55 to give a usable volume of 450 litres with 35 percent volume gain.

Rigid PU foam is a universal material for construction industry owning to its unsurpassed low K-factor (thermal conductivity). It is used for making insulation boards. Flat roof with food weatherability and extremely low water absorption have become essential in the construction industry. Rigid PU Foams with their closed cell nature show excellent water resistance. For Board cut from bunstock, rigid PU blocks are used, with roofing felt, bitumen, glass and aluminium foil as facings. These insulation boards can be installed either with square edges or with overlapping edges. They are also used for seam insulation where the reaction mixture is poured in the cavity and allowed to foam and the overflowing PU foam is then cut.

The use of rigid PU foam reduces heat loss and PUF insulted floors retain a comfortable temperature ceilings of basement and the upper most floors can be insulated with rigid PU foams.

As sandwich elements

The versatility of PU in general allows the rigid PU Foam to be produced by moulding the foam with appropriate facing or by adhering the facings to the finished foam. The choice of facings depends on several factors such as application technique, cost, stability, weatherability, flammability etc. Elements with glass reinforced polyester facings are frequently used for cooling closed walls and for cold storage building. Sometimes even sheets of ABS, PVC or PS are used as lining. Facing made of particle board, plaster board or asbestos are used for elements for façade roofs and prefabricated houses.

Rigid PU Foam can be fixed into a prefabricated moulded assembly. Here porous concrete plaster board can be used on both sides and for piping. Ventilation ducts can be installed between the facings to facilitate easy finishing. During foaming any pressure developed due to reaction can be off set by supporting the mould.

The spray method makes the production of foam layer possible in any thickness on vertical or horizontal surfaces, even on ceilings. Its main application is in the insulation of roofs and walls. An important advantage of this procedure is that ceiling and insulation are one process and the construction of a roof is a simplified one. The rigid PU foam is resistant to weather. It also resists sparks and radiant heat.

Windows, plain scale doors and elaborated entrance doors are being manufactured regularly by many PU foamers. The foam product is malleable and can be fitted with hinges, locks and other hardware even more easily than the natural wood product. Excessive testing shows that the product will not wrap, crack or deteriorate. It will hold paint or can be prefinished in the mould to any desired colour.

Although in the West, rigid polyurethane is widely used in India, its usage is only minimal. But this is picking up, at present the polyol requirements for such application are indigenously available with us.

Typical Properties of Polyurethane Foam

Property Rating Benefit Thermal insulation Highest Space Rigidity High Structural Adhesion High Manufacturing Dimensional stability High Maintenance Density Low Handling Water Vapour Transmission Low Construction

Equivalent thickness required for same degree of insulation

860 mm common bricks 80 mm concrete blocks 140 mm softwood 65 mm fibreboard 50 mm cork 45 mm mineral wool 40 mm polystyrene 25 mm polyurethane

           

Thermal Conductivity

Material W/moK Kcal/mhoC Btu in./ft2hoF Mineral Wool 0.030-0.032 0.030-0.032 0.24-0.26 Glass Fibre 0.033-0.037 0.029-0.032 0.23-0.26 Cork 0.038-0.043 0.034-0.038 0.27-0.30 Expanded Polystyrene 0.032-0.037 0.028-0.032 0.22-0.26 Unaged Rigid Polyurethane Foam 0.017-0.020 0.015-0.017 0.12-0.14 Aged Rigid Polyurethane Foam 0.022-0.025 0.019-0.022 0.15-0.17

Physical Properties

Typical physical property ranges for diisocyanate based foams at an overall density of 32 kg/m³ (2 lb/ft³)

* Compression strength BS 4370 120-210 kpa 1.2-2.1 kgf/cm2 17.30 lb/in2 * Dimensional stability BS 4370 Not more than 2% linear change between 30oC and 70oC * water vapour transmission BS 4370 (permeability at 38oC and 0-88% relative humidity) 13.35 ugm/Nh 0.041-0.110 g/m 24 h mmHg 2.5-6.7 per inches

* These tests are equivalent to ISO 844, ISO 2796, ISO 1663 respectively.

The “Guidelines for the Use of Flammable Materials in the Building Industry” applicable in the Federal Republic of Germany determines that building materials having classification B3 according to DIN 4102 (“leicht entflammbar”) must not be used. Depending on risk situation, products of classification B2 (“normal entflammbar”) or B1 (“schwer entflammbar”) are accepted. Polyurethane rigid foams having B2 and B1 classifications have been available for a long time. The tendency for materials to char during a fire was attained with various products through modification with Polyisocyanurate and this had an especially favourable effect in reducing fire risk.

A number of tests were performed to evaluate the behaviour of polyurethane rigid foam under real life fire conditions. A study performed by the International Isocyanate Institute, Inc., is of particular interest. It evaluated corrugated steel metal in a roof configuration which was covered with polyurethane rigid foam on the outside for insulation. Another evaluation was performed by CNO in the Netherlands, which included different insulation materials for comparison was designed to determine how such roofs behave in case a fire breaks out in the interior of a building. Polyurethane rigid foam passed this test particularly well because of its heat stability and tendency to form a char (41).