Polymeric materials rigid foam

The difunctional N-cyanourea compounds were found to polymerize into different polymeric materials at different temperatures. At room temperature, a linear polymer was obtained either from the polymerization of a di-N-cyanourea monomer or directly from the mixture containing a diisocyanate and cyanamide. At elevated temperature (>100°C), the di-N-cyanourea monomer, or the mixture of a diisocyanate and cyanamide, cross-linked to a rigid foam or flexible material, depending on the structure of the monomer. 

Examples are given of the use of extruded PE foam sections in conjunction with other polymeric materials in the production of triangular, semicircular, rectangular and trapezoidal profiles, e.g. beam-like structures in which the rigid support may be of GRP. The foam can be bonded with thermally-fusible adhesives or two-sided self adhesive tape affixed to the foam prior to fabricating. A standard range of the foam prodncts is available, but other types can be tailor-made. 

A number of "crude" isocyanates (polymeric isocyanates), undistilled grades of MDI and TDI, are available in the market. Some of these products, such as various forms of crude MDI, have a functionality varying between 2 and 3. They have a lower reactivity and a lower vapor pressure than the corresponding pure isocyanate. They have found extensive use in one-shot rigid foams. However, they are also employed in coating, sealant, and adhesive applications. The Upjohn Chemical Division has published an excellent bulletin on the use and precautions in handling isocyanates, polyurethanes, and related materials (13A). 

These foams can then be extended into the area of flame-retardant materials, where methyl oleate-polyesters were used as polyols in the synthesis of silicon-containing polyurethanes [89]. Despite not strictly being foams, methyl oleate, soybean and sunflower oils have also been investigated to produce semi-rigid flame retardant materials [90]. In this instance, they were brominated, acrylated and then radically copolymerized with styrene to form the polymeric material. 

PURs include a wide range of materials ranging from thermoplastic elastomers to flexible as well as rigid foams. Polyurethanes are commonly formed upon reaction of isocyanates with alcohols (see Figure 8), in order to form polymeric structures both reaction partners have to contain at least two reactive functions (diisocyanates or diols). 

These materials are often used in a partially polymerized form, known as prepolymers. These are reacted with glycols to form high-molecular-weight polymers, or with mixtures containing controlled amounts of water to form rigid foams (note that water reacts with isocyanates to form CO2, which is generated in situ to form the foam). R and R may be aromatic or aliphatic, to form aromatic, aliphatic, or mixed polyurethanes. Also, different polyurethane prepolymers are produced from diisocyanates and diamines, giving rise initially to a urea bond (NH —CO—NH—) instead of urethane ... 

Foamed polymeric materials have several inherent features that combine to make them economically important. Any foamed material is a good heat insulator by virtue of the low conductivity of the gas, usually air, contained in the system. In a rigid foam, the gas contained in the cells may not be exchanged with air for a long time. The thermal conductivities of some common gases in W/m °C at 27°C are as follows air, 0.0262 n-pentane, 0.0144 nitrogen, 0.024 and carbon dioxide, 0.0146. Despite the low conductivity of many chlorofluorocarbons at around 0.012 W/m °C,... 

Metastable foams have a persistence lasting from a few minutes to months. They are stabilized at the liquid-gas interface by the presence of amphiphilic and/or polymeric materials that retard the loss or drainage of liquid from the area between bubbles, or form a somewhat rigid, mechanically strong bilayer that maintains the foam stmcture. Because the stabilizing structure of the metastable foams is fluid, it can be disrupted by a number of factors such as vibration, dust particles, evaporation, pressure, and other environmental changes. Even the most stable of... 

Coefficient of Linear Thermal Expansion. The coefficients of linear thermal expansion of polymers are higher than those for most rigid materials at ambient temperatures because of the supercooled-liquid nature of the polymeric state, and this applies to the cellular state as well. Variation of this property with density and temperature has been reported for polystyrene foams (202) and for foams in general (22). When cellular polymers are used as components of large stmctures, the coefficient of thermal expansion must be considered carefully because of its magnitude compared with those of most nonpolymeric stmctural materials (203). 

---