Foams
Its formulation is based on polyols with a low hydroxyl number (OH) combined with isocyanates with a low content of functional groups (NCO), linked to special propellants and an exactly measured amount of water. The formula is stoichiometrically designed to achieve a fast-curing material (foamed or not) with a density between 10 and 80 kg/m³.
The most used applications in the flexible polyurethanes segment are the manufacture of mattresses, pillows and seats for both the home industry (chairs and armchairs) and the industrial industry (seats and armchairs for cars, buses, etc.). In this segment, foams between 18-50 kg/m are usually used. Other applications occur in the packaging industry, in which foams are used as anti-impact protectors for packaging of delicate parts. Their main characteristic is that they are open cells and have low density (12-15 kg/m³).
Rigid polyurethanes with a density of 30-60 kg/m³ are widely used as thermal insulators. Its main applications are the thermal insulation of refrigerators (refrigerators), freezers (freezers), cold rooms and sheets for warehouses and industrial roofs (foam molded within a matrix). They are also used as projected thermal insulation for already built industrial sheds (for example to insulate farms where chickens, pigs, etc. are raised).
The thermal insulation capacity of polyurethane is due to the gas trapped in the closed cells of the polymer framework.
A variety of rigid polyurethanes are PIR polyurethanes, which thanks to their better behavior against fire are used in linings of pipes that conduct fluids at high temperatures in extremely humid areas. Its main characteristic is the urea nature of the polymer.
A variety of rigid polyurethanes are spray polyurethanes, which are high reaction rate formulations, used in coatings subject to the force of gravity, such as building insulation, storage tanks, and even tubes or pipes.
Another variety of rigid polyurethanes are those used to make imitation wood pieces, with densities ranging between 100-250 kg/m³. There are also formulations with higher density (up to 800 kg/m³) commonly called "duromers" for the production of structural parts, such as industrial machine casings, accessories for coaches, etc.
Reactivity can be observed in a simple visual inspection and, in the case of foams, is divided into the following times, measured in seconds:.
The isocyanate and the polyol, when mixed, cause a series of chemical reactions that lead to bonds of urethanes, polyurethanes, allophanates, modified ureas, cyanates, prepolymers, etc. In total, there are about 17 simultaneous chemical reactions, in which the catalyst package causes it to take one preferred direction or another.
An exotherm is generated that can raise the temperature to more than 100 °C, which causes the propellant in solution in the polyol to become a gas. The reaction of isocyanate with water generates carbon dioxide. Due to the heat generated, part of the water turns into steam. All this causes the mixture to expand, forming small cells after gelation or creaming. Although the CO cells are part of the cross-linking, they are intermixed with those containing fluorocarbons for dimensional stability purposes.
Some polyols contain anti-flame components that make them flame retardant. In some countries the use of this component is mandatory for certain applications, and they are classified under safety standards.
The cells are formed as the thread time is reached, ending at the Tack free time.
The propellants are ecologically modified fluorocarbons such as R-141 B"), R-245FA, or cyclopentane, which comply with the Montreal Protocol for the preservation of the atmospheric ozone layer. Evidently, water and, to a lesser extent, carbon dioxide are also used. Freon-11 (R-11"), as well as other organochlorines, were discarded years ago due to their impact on the ozone layer.
At the end of the chemical reaction, the polyurethane foam contains millions of irregular cells, which – depending on the formulation used – are what ultimately give it the characteristics of thermal insulation, resilience, acoustics, etc.
Dimensional stability is a very important aspect in the quality of the foam formed: many times it has happened that poorly balanced polyol formulas, excess water, or poor polyol/isocyanate mixtures produce a contraction of the polymer, buckling and losing its shape. The polyol/isocyanate mixture must be stoichiometrically balanced. In general the mixture is 10% above the stoichiometric for greater safety; A greater mixture of polyol and less isocyanate leads to soft and unstable foams, while an excess of isocyanates leads to urea foams (PIR polyurethanes).
The polyurethane industry moves millions of dollars/euros around the world, and specialists in the subject are scarce and highly valued.
The main market for rigid polyurethane is the thermal insulation industry (refrigerators, etc.); secondly, the flexible polyurethane industries (mattresses, seats, etc.).
A smaller percentage is used for molding automobile parts, vehicle parts, decoration elements, etc.
A polyurethane foam has a heat transfer coefficient of approximately 0.0183 BTU heat transfer units.
Source: IRAM Standard 11601. (Argentina).
Due to the excellent ease of their synthesis and their relationship of mechanical and insulating properties, rigid polyurethanes are used in the refrigeration, insulation, furniture, etc. industries[13].