2.3 - Blowing agents

Cellular polyurethanes are manufactured by using blowing agents to form gas bubbles in the polymerizing reaction mixture. The oldest and most common blowing agent is water, which reacts with the isocyanates liberating carbon gas and forming polyurea rigid structures. The auxiliary blowing agents (ABAs) are low boiling point compounds, also used in foam formulation. The auxiliary blowing agents function is to absorb the heat from exothermic reactions; vaporizing, and providing additional gas, useful in expanding foam to a lower density. With the evidences that the chlorofluorocarbons (CFCs) are responsible for ozone depletion, many other products have been studied as alternatives being taken into account the toxicity, inflammability, environmental impact, cost and physical properties. The definitive substitution of CFCs affects different segments of the industry differently.

2.3.1 - Flexible foams

For the different segments of the industry involved in the production of slabstock flexible foams (Chapter 3), moulded flexible foams (Chapter 4) and semi-rigid foams, the elimination of CFCs as auxiliary blowing agent represents an additional minimum cost. The most common option is water, which reacts with the isocyanates, forming carbon gas and polyurea rigid structures. To reduce the density and the hardness of the slabstock foams, the auxiliary blowing agents (ABAs) may be used to aid in attaining densities and softness not obtainable with conventional water-isocyanate blowing chemistry. In slabstock flexible foam production, methylene chloride is one of the most popular products, but it suffers restrictions in certain areas: European countries, for example. Acetone is also used successfully. However, some precautions should be taken, due to its inflammability. Another alternative is the use of liquid carbon dioxide as blowing agent in continuous and discontinuous slabstock flexible foam processes. In PU molded systems, mainly integral skin foams (Chapter 4), CFCs were initially substituted by HCFCs, like HCFC-141b (not totally innocuous to ozone depletion) and other systems based in pentanes, or even by water as the only blowing agent.

2.3.2 - Rigid foams

The rigid foams for thermal insulation need the use of ABAs (Table 2.7) like CFCs, HCFCs, pentanes, HFCs, etc, to minimize their thermal conductivity (Chapter 5.4.3). These gases are kept in the closed cells of the PU rigid foams, and are responsible for their excellent insulating properties. Due to environmental problems, CFCs were phased out, and nowadays, we no longer have any universal blowing agents. The most common alternatives for the rigid foam production are: the use of water as the only blowing agent (resulting in smaller insulating properties); and the use of auxiliary blowing agents (ABAs) like chlorofluorohydrocarbons (HCFCs), or hydrocarbons such as pentanes, and hydrofluorocarbons (HFCs). The substitution of CFCs for the others ABAs happened in different ways, in different regions of the world. The choice depended on: thermal conductivity, foam quality, inflammability, cost, and end use. In Europe and Japan, CFC-11 was substituted by HCFC-141b or pentanes. In the USA, the most common option was HCFC-141b, due to uncertainties regarding the properties, environmental consequences, and inflammability of the rigid foam produced with pentanes. In Latin America, Middle East, Africa and Asia, CFC-11 is still used. The tendency, however, is to eliminate its use gradually. According the Montreal Protocol, the CFCs phase out is scheduled to 2010.

CFC-11
CCl3F

HCFC-22
CHClF2

HCFC-141b, the leading and most versatile CFC-11 substitute was introduced around a decade ago, and its introduction was relatively easy. In 2001, the world consumption of HCFC-141b was close to 135 thousand tons, mainly due to its low thermal conductivity. However, regarding the ozone depletion and global warming, HCFCs are not totally safe, and according to the Montreal Protocol, they will phase out in 2003 for European Union and United States and 2040 for developing countries. Two other HCFCs, HCFC-22, HCFC-142b and their blends have also been commercially used as foam blowing agents. Their applications have been limited because of their low boiling points. Nevertheless, because they have lower ozone depletion potentials (ODP) than HCFC-141b, lately, they have gained in importance.

HFC-134a 
CF3CFH2

 HFC-245fa 
CF3CH2CF2H

Other zero ODP options include hydrocarbons like pentanes and HFCs (Table 2.7b). Pentanes, especially the cycle-pentane that possesses smaller thermal conductivity, are cheaper, and represent a very attractive alternative since there are established process conditions adapted to these inflammable products. Cyclopentane has become the accepted blowing agent for the appliance industry in Europe. Besides its attractive environmental data, cyclopentane offers competitive processing and performance properties. Its boiling point is somewhat higher than it is for CFC-11 or HCFC-141b, what causes a minimal effect on rise profile, but together with its relatively good solubility in the polyols reduces blowing agent losses during processing. Cyclopentane has vapor thermal conductivity lower than HFC-134a, and aging studies showed that it remains in the cells. The disadvantages of cyclopentane are its flammability and its plasticizing effect on the polymer matrix, which demand appropriate safety precautions and higher densities compared to the water/CFC-11 co-blown systems.

The two most promising HFCs are HFC-134a and HFC-365mfc (scheduled to be commercialized in 2003). HFC-134a is used as blowing agent and as gas for compressors and can become a substitute of HCFC-141b in the USA. Though it possesses low solubility in polyols that limits its amounts in a lot of formulations. HFCs are zero ODP, but they contribute to global warming and, for that reason, they were included in the Kioto Protocol. Besides these, other different inflammable and non-flammable mixtures of different types of AEAs, have also been tested in the PU rigid foams production.

2.4 - Surfactants