Heat expandable microspheres

Table 21.1 The effect of foaming PP with heat expandable microspheres on its thermal...

Table 21.2 Density of five common thermoplastics upon injection molding into 5 mm thick plaques with simultaneous foaming using different heat expandable microspheres.

Polystyrene-Epoxy Syntactic Foam. Hollow polystyrene microspheres are produced by heating expandable polystyrene (in other words gas-filled spheres, e.g., propane or butane-filled polystyrene) of microscopic size. The expandable polystyrene microspheres may be added to the epoxy-resin formulation, and the exothermic heat (or the heat during oven cure) can be employed for the expansion. In this manner, foams having densities as low as 80 kg/m (5 Ib/ft ) may be developed. 

Grade composed of poly(vinyhdene chloride) copolymer is less resistant to heat and solvent. Micropearl F-30, F-50, F-80, FlOO - expandable microspheres available in wet and dry forms. These microspheres are marketed for Matsumoto Yushi-Seiyaku, Co., Ltd. Japan... 

Expandable microsphere Thermoplastic microspheres are droplets of hquid hydrocarbon encapsulated in a shell of a thermoplastic polymer. When exposed to heat, the shell softens and the hydrocarbon gasifies, and the microsphere expands firom, typically, 12 to 40 pm and the density drops from 1000 to 30-40 kg/m. The microspheres can be used either as a form of blowing agent, or may be supphed in expanded form for use as a lightweight filler. The activating temperature of mold and material is lOOC (212F). 

Microspheres made from polymers or reactive oligomers (RO s) are manufactured by thermally treating sprayed solutions or emulsions. A solution of any film-forming polymer can be used. When an RO is used the solvent has to be evaporated, the sprayed monolithic particles are then heated to expand the gas or vapor within the particle, and the final microspheres are hardened 22,23). 

On the other hand, thermoplastic-based hollow microspheres can be prepared by heating thermoplastics containing low-boiling-point solvents. One example is polystyrene hollow microspheres. In the first stage, expandable polystyrene powder is prepared, e.g., polystyrene powder containing propane, butane or pentane is prepared by emulsion polymerization. The powder is then exposed to steam for expansion to form hollow microspheres. 

Close temperature control is required to obtain reproducible results, because the expansion of the polystyrene is a function of temperature. The polystyrene microspheres may be pre-expanded by steam or radiant heat and then added to the mold, heated in advance to a convenient temperature, such as 60°C. The mixed epoxy resin is then poured over the pre-expanded beads and allowed to cure. In this case, a limited amount of further expansion will occur as a result of exotherm or cure temperature to permit complete mold fill (10). A typical formulation is shown below ... 

Microspheres ml-kr9- sfirs (1894) n. Tiny, hollow spheres of glass or plastic used as fillers to impart low density to plastics, such plastics being known as syntactic foams. Plastics used to make microspheres include phenohc, epoxy and a co-polymer of vinylidene chloride and acrylonitrile. The last contains a heat-activated blowing agent that expands the spheres either before their incorporation into a matrix polymer or afterward. The co-polymer spheres impart better mechanical properties to the matrix than do the glass or epoxy microspheres. 

Another approach is to use thermoplastic microspheres encapsulating a gas, in unexpanded or pre-expanded form. When heated (usually at about 100°C), the... 

When MEH is heated, it is obvious that microspheres naturally expand against matrix and both matrix and microspheres would permanently deform if deformation is sufficiently high. Consequently, compressive residual stresses around microspheres would be created when cooled down. In order to confirm the compressive residual stresses in matrix, thin sections of MEH were examined and the residual stresses/strains were indeed found to exist around the microspheres, as shown in Figure 3.30. Both cross-polarized and nonpolarized images, under a transmission optical microscope, of thin section taken from the midplane of a MEH specimen with a microsphere content of 20% are shown in Figure 3.30. 

Perlite por- lit [F, fr. perle pearl] (1833) A siliceous lava which, when heated to 720—1,090°G, expands to 10—20 times its original volume, forming tiny, hollow, spherical bubbles. Perlite is much used as an ingredient of lightweight concrete and as a density-lowering filler for plastics. See also Microspheres. 

---