Foamed cure time

Processing of rigid foams from two part formulations involves combining measured quantities of the polyisocyanate with a polyhydroxyl such that there are no or limited reactive isocyanate functional groups. Moisture is not required to complete the cure. Once the reactants are combined the mixture is poured into a form where expansion and polymerization take place simultaneously. Cure times are usually very fast, on the order of minutes. 

The type of polymer obtained depends on factors such as the pH and temperature of reaction, the ratio of melamine to formaldehyde, and the type of catalyst employed. For decorative laminates, melamine-formaldehyde is prepared by reacting melamine in stainless steel kettles under reflux, alkaline conditions with 37% to 46% formaldehyde in aqueous solution. The reaction temperatures used vary from 80 to 100°C and are maintained until the condensation has reached the desired end point—that is, reacted sufficiently but still water-soluble. The end point is checked by measurements of viscosity, cure time, and water tolerance. Depending on the type of laminate to be produced, other constituents (surfactants, plasticizers, release and anti-foam agents) normally are added to the base resin before impregnation of the surface papers. It is common practice also at this stage to adjust the pH by adding acid catalysts. 

For the reaction of TDI with a polyether triol, bismuth or lead compounds can also be used. However, tin catalysts are preferred mainly because of their slight odor and the low amounts required to achieve high reaction rates. Carboxylic acid salts of calcium, cobalt, lead, manganese, zinc, and zirconium are employed as cocatalysts with tertiary amines, tin compounds, and tin—amine combinations. Carboxylic acid salts reduce cure time of rigid foam products. Organic mercury compounds are used in cast elastomers and in RIM systems to extend cream time, ie, the time between mixing of all ingredients and the onset of creamy appearance. 

The mold is closed before the foam begins to rise, and after the two minutes cure time, the part is demolded. 

Chemical system and specimen (1) Foam density (g/cc) (2) Water content after soaking %) (3) Compressive strength (psi) (4) E, initial tangent modulus of elasticity (psi) (5) Curing time (days)... 

When the open cavity is filled, slowly foaming mixtures are used because the mold has to have enough time to close. In the past, the so-called closing time was brought down to less than four seconds due to the use of constantly improving motion drives. Neverless, the cure times are longer than the cures time for the closed mold injection method of PUR systems. 

Ex. 1. Tuftane film is ideally suited for bonding emblems, numerals, and letters to many fabrics by heat and pressure alone. It also flame-bonds well to both polyester- and polyether-urethane foams at commercial bonding speeds. Since it contains no volatiles it does not require cure times as do solvent- or water-based adhesive systems. All Tuftane films can be adhered thermally by hot bar, thermal impulse, ultrasonic, or dielectric methods over a wide range of temperatures. Adhesive lamination to many substrates is possible by the heated drum, curing oven, or multiple can methods. Fabric bonds made with Tuftane are strong and withstand laundering and dry cleaning. 

---