Polyurethane flexible foams chemistry and fabrication
Flexible polyurethane foam is the largest volume application for polyurethanes and the largest category of cellular polymeric materials . The discussions in prior chapters relating to foam properties and chemistry apply to flexible foams from a fundamental standpoint. Polyurethane chemistry is flexible enough and can incorporate enough components that the potential variability in chemistry and process makes drawing hard rules about foam formulation a fruitless task. In fact, the properties of a particular foam can often be achieved by numerous combinations of foam components. Despite this challenge, it is still possible to generally discuss concepts and guidelines for foam production and methods of making foams at various scales. Furthermore, even though there are many ways to make foam, there are many fewer ways to describe the product, its performance, and the physical paths that the reactants take to the final product. This chapter will broadly define design concepts for making polyurethane flexible foams, physical concepts relevant to the formation of foam from polyurethane components, and physical descriptors of the product. Discussion of the building blocks can be found in Chapter 2, the chemical reactions that occur during foaming in Chapter 3, theoretical concepts in polymer formation and foam structure in Chapter 4, and analysis of polyurethane foams in Chapter 5.
MAKING POLYURETHANE FOAMS
Industrial production of foams is a highly optimized and reproducible operation. The volumes of chemicals consumed in a commercial-scale plant can be startlingly large, and a large plant operating at capacity can be a supply chain challenge. The production of foam after receipt of raw materials subsequently becomes an exercise in accurate mixing and pouring. The necessary requirement is to perform these mixing operations in the right amounts, in the right order, and at the right time. When done correctly, the physical processes that result in foam can be relied upon and reproduced such that a stopwatch may not detect variability between repetitions. Thus, the first step to making a foam is to assemble all the necessary reagents in sufficient amounts and accurately dispense them into a reaction volume.
When making flexible foam, calculation of the correct mass to mix of a particular ingredient is normally based on the equivalent weight of the component rather than the molecular weight of the component . The equivalent weight is simply the molecular weight of the component divided by its functionality. In setting formulation, the stoichiometry is set to match the equivalence of the isocyanate side with the total equivalence of the isocyanate-reactive side because the materials to be mixed may have quite different functionalities, but controlling stoichiometry is a necessary condition of getting a good result. The use of equivalence minimizes the likelihood of stoichiometry errors, though they do still occur.
As mentioned earlier, there are many desired flexible foam applications and a defined recipe for making each one. Flexible foams can be produced for applications such as mattresses that may be cut from large blocks into defined shapes for the application. These foams are often called slabstock foams since the foams are made as enormous slabs from which the desired product is cut. Alternatively, the foam can be poured into a predefined mold to form a custom shape. Such foams are often found in furniture cushions and automotive seating and for obvious reasons commonly called molded foams. The distinctions between slabstock and molded foams form a large enough class boundary that their formulations can be distinguished by basic rules.