Crosslinking flexible foams

The benefits of the Sioplas moisture crosslinking process for the manufacture of crosslinked PE foam and the end-uses of such foams are discussed. The process involves grafting of alkoxysilanes onto ethylene homo- or copolymers to provide moisture crosslinkable polymers suitable for the manufacture of flexible, rigid, open-ceUed, closed-cell or very low density foams. 11 refs. 

A polyol of high MW (3000-6500 daltons) and of low functionality, of around 2-3 hydroxyl groups/mol, if reacted with a diisocyanate, leads to a low crosslinked, flexible polyurethane structure. This structure is characteristic of flexible polyurethane foams. Because the resulting structure is a crosslinked one, the MW of the resulting polyurethane is infinite in value. Only linear polyurethanes have a finite and determinable MW. 

Crude MDI and PAPI are especially used in highly crosslinked polyurethanes, such as rigid polyurethane foams. Polymeric MDI have lower vapour pressures than TDI. Mixtures of TDI with polymeric MDI are also used in many applications, (e.g., in high resilience flexible foams). Aliphatic diisocyanates have a much lower reactivity than aromatic isocyanates. The most important aliphatic diisocyanates are presented in Figure 2.5 [1-3, 6, 23-25] ... 

From high MW triols or low branched oligo-polyols (MW = 3000-6500 daltons) polyethers, polyesters, filled polyols (polymer polyols), are obtained elastic PU with a low degree of crosslinking (flexible and semiflexible foams, coatings etc). 

Flame retardant flexible foams are very difficult to obtain due to the low crosslink density, low aromaticity, open cell structure and long polyolic aliphatic chains. Generally, flame retardants flexible foams are produced with additive flame retardants, for example with powdered melamine + tris (2-chloropropyl) phosphate [14]. 

However, until recently, the principal application of polyurethanes has been as in situ-polymerized rigid and flexible foams used for insulation and cushioning, respectively.— — Unlike the phenolic, urea and furan mortars, in which the high polymer formation is the result of the chain extension and crosslinking of a prepolymer, the final polyurethane product is the result of a reaction between hydroxyl and isocyanate groups as shown by the following equation ... 

Stamicarbon BV CRT technology - high-pressure clean tubular reactor, nonfouling reactor, constant pressure and temperature profile, peroxides as initiators, low-cost production, short residence times. Total licensed capacity 1.8 MMT/y. LDPE (density 918-930 kg/m MFI = 0.2-70 g/10 min) and EVA (10 wt% VA) copolymers for injection moulding, sheets, flexible pipes, cable sheathings, blow moulding, crosslinking, and foam, etc. 

Some polyurethane materials (e.g., flexible foams with a small degree of crosslinking or monolithic thermoplastic polyurethanes) can be processed by methods typical of thermoplastic polymers. Compression molding in a hydraulic press is most often used to this purpose. At high temperature and pressure, the crosslinks are broken and the material flows in the mold. In this way,... 

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

Injected parts transparent, filled to a greater or lesser degree, possibly crosslinked and/or foamed in microcellular form for tyres (light uses), kneepads, footwear, soles, sports and technical items, buoys, flexible toys, protective caps. .. 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

By crosslinking, the viscosity of EVA at high temperatures is increased and the individual cells are kept in a stable condition without rupture or agglomeration. A low density microcellular foam can be thus obtained. By selecting the vinyl acetate content, the EVA foam is flexible and highly resilient with easy coloring and adherent to other materials. The application is used widely in shoe soles, sandals and cushion materials. 

Orlando, FL, 7th-llth May, 2000, paper 651 CHEMICAL CROSSLINKING OF FLEXIBLE PVC FOAM FORMULATIONS Ibarra-Gomez R Yanez-Flores I G Rodriguez-Femandez O S Gilbert M Coahuila,Centro de Investigacion en Quimica Aplicada Loughborough,University (SPE)... 

The product is called l,l,l—Trimethylolpropane (TMP) and is used mainly in the manufacture of triols by adduction with propylene oxide (PO), for flexible polyurethane foams and of synthetic lubricants by esterification with fatty acids. The allyl ethers of trimethylolpropane are used as crosslinking agents, for example, in acrylic resin systems. The world-wide capacity for TMP production is in an increasing trend. 

In Figure 1.9 one can see an hypothetical crosslinked structure of a flexible polyurethane foam resulting from an oligo-triol of MW of 3000-6500 daltons and a diisocyanate [3-16, 20]. 

---