Thermoplastic microspheres

The development of hollow thermoplastic microspheres has offered the prospect of lower density plastisols with superior physical properties and appearance than have been attainable by other known techniques. Such microspheres have become available, having, for example, diameters on the order of 10 to 200 im, most often 20 to 100 pm, and densities of 0.06 to 0.02 g/cm, or even as low as 0.015 g/cm. The thermoplastic material of which the microspheres are formed is typically a polyvinylidene chloride or a copolymer of polyvinylidene chloride with other vinyl or acrylic monomers, such as acrylonitrile, butylene, and the like. The hollow microspheres contain a physical blowing agent, typically a lower alkane, most often a butane or pentane, or their mixtures. A variety of such materials are commercially available. 

Method of piastisoi manufacture and use determines final product density. Making a piastisoi composition comprising a thermoplastic resin, a plasticizer resin, inert filler, and hollow thermoplastic microspheres should consider the following points ... 

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). 

Ahmad, M. (2001) Flexible vinyl resiliency property enhancement with hollow thermoplastic microspheres. J. Vinyl Addit. TechnoL, 7 (3), 156—161. 

Ahmad, M. (2004) Thermoplastic microspheres as foaming agents for wood plastic composites. Proceedings of the WPG 2004 Conference, Wood Plastics Composite Org., September 2004, Vieima, Austria. 

Several companies (Expancel, Kureha) have developed hollow thermoplastic microspheres (containing a volatile liquid, usually isopentane or isobutane) that can be expanded to make foams for use in wallcoverings and shoe parts. They can produce fine-cellular foams by rotational moulding. 

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

Expandable microspheres can also be used. They are thermoplastic microspheres encapsulating a gas. They are used for foamed items and for weight reduction. The recent developments in microspheres include Ecosphere hollow glass microballons from Emerson Cuming for highly filled PP. 

Ahmed, M, Thermoplastic Microspheres as Blowing Agents for Wood Filled Plastics. Wood-Plastic Composites 2004, Vieima, Austria (2004)... 

Spray methods for obtaining microspheres have a number of disadvantages. High temperatures and bulky and complicated equipment are needed moreover they pose fire and explosion hazards. Low temperature methods, such as using emulsified thermoplast solutions, saturated polyester resins in liquid heat carriers, or suspension polymerization, are preferable1,3S). 

The methods used to increase the water resistance of a glass microsphere foam are basically those applied to glass-reinforced plastics, filled thermoplasts, and elastomers, viz. hydrophobic adhesion compounds are added to binder and microsphere dressing 147). The compounds added are alkyl alkoxysilane derivatives, amino or epoxy alkoxysilanes for epoxy and phenolic resins, vinyl or methacryloxy alkoxysilanes for polyester resins. The dressing agents used are aminoethoxysilanes (y-aminopropyl-... 

The matrix is considered to be the binder for the microspheres. Typical matrix materials include (a) thermosetting resins such as epoxy resins, unsaturated polyesters, vinyl esters, phenolics, polyurethanes, and silicones (b) thermoplastic resins such as polyethylene, polystyrene, polyvinyl chloride (c) asphalt and (d) gypsiun and cement. 

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. 

These thermoplastic resins are used by mixing in the melt with thermosetting microspheres. 

Thermosetting-matrix resins consist of two-component liquid systems which can be easily blended with hollow microspheres at room temperature. In contrast, thermoplastic-matrix resins must be melted for blending with thermosetting hollow microspheres. The following are examples of syntactic foams. 

Unexpanded microspheres 820, 643, 551, 461, 051, 053, 054, 091,092 hollow particles with thermoplastic shell encapsulating a gas available in wet (WU) and dry (DU) form. The grade numbers signify materials which have different particle diameter, expansion rate, solvent resistance, and temperature of expansiom... 

Expancel . [Expancel] Expandable thermoplastic hollow microspheres used in printing inks, paper and board, nowoven materials, body fillers, paints, explosives... 

Ski boots use a wide variety of plastics adapted for low temperatures. Thermoplastic polyurethane/ABS blends and plyamides are used for the outer shell of ski boots and modified polyethylene terephthalate for the binding. Polyurethane (PU) foam is often used to line the boots. Microsphere fillers may be incorporated into foam-lined boots to add further thermal insulation to the wax binder. The antivibrational characteristics of PU foams have also led to their use in ski fittings. A sandwich construction of polyurethane elastomer and aluminum alloy has been fitted between the ski and the binding to reduce shock and vibration. 

Also investigating surface textiles electrodes, Pylatiuk et al. (2009) compared five different conductive materials three types of silicone rubbers loaded with carbon or other nanoparticles, silver-coated polyamide yams and a flexible thermoplastic elastomer loaded with silver-coated glass microspheres. The results of the test electrodes were compared with those of a standard Ag/AgCl gel electrode. It was found that the silicone mbbers and the coated polyamide yams gave results comparable to the reference gel electrodes. The nanoparticle-loaded silicone mbber gave very favourable results with the ability to be used dry and a signal to noise ratio better than the gel electrode reference. 

Cellulose is the most abundant polysaccharide on Earth, with several thousand D-glucose units in a polymer chain (Figure 21.6). It is the main structural component of the cell wall of green plants. Cotton contains almost 90% of cellulose, whereas wood and dried hemp contain 50% and 45%, respectively. The first cellulose-based thermoplastic polymer was manufactured in 1870 and the first chemical synthesis was done by Kobayashi and Shoda in 1992 [130]. Solubility of cellulose in water depends on its chain length and it is degradable by enzymatic reaction [131]. Cellulose is easy to machine to form various shapes such as textiles, microsphere, sponges, and membranes. 

Fig. 3.2. A glass sphere of radius 10 m, glued onto the cantilever. The microspheres can be attached to the cantilever by nsing an optical microscope and a precise 3D micropositioning system. A thermoplastic glue is used for attaching.

Organic spheres are predominantly polymeric, consisting of synthetic or natural polymers. The field of polymeric nano- and microparticles is vast, comprising, for instance, latex particles for coatings, hollow particles for syntactic foams, and microcapsules for foaming and additive release. In addition, there are core-shell microbeads and coated polymeric particles, where the particles can exhibit multiple functionalities, thanks to the individual features of their different layers 1]. As fillers in thermosets and thermoplastics, hollow microspheres and expandable microcapsules are among the most frequently used in commercial applications. 

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

Jonsson, L. (2005) Expandable microspheres as foaming agents in thermoplastics, diermosets and elastomers. Proceedings of the Advances in Plastics Technology Conference, Inst, for Plastics Processing, November 2005, Katowice, Poland. 

Kawaguchi, Y. and Oishi, T. (2004) Synthesis and properties of thermoplastic expandable microspheres the relation between crosslinking density and expandable property. J. Appl. Polym. Sci., 93, 505-512. 

The lactide/glycolide bioresorbable polymers are thermoplastics which can be processed by many methods, including fibre spinning, extrusion, and injection moulding, which means they can be fabricated into a variety of wound closure items (e.g. sutures), implantable devices (e.g. bone plates, bone screws), and drug delivery systems, which include microspheres, fibres, films, rods and others. 

These foams can be defined as composites consisting of hollow microspheres and a polymeric matrix. This one is made of a thermosetting (PU, PIR, PF, EP, silicone or unsaturated polyester) or of a thermoplastic (PE, PP, PVC, PS, polyimide) [56]. The microspheres can be made of silica, glass, carbon, ceramics or polymers such as PS, PE, PP, polyamide (PA), polymethyl methacrylate (PMMA), divinyl benzene (DVB)-maleic anhydride, and so on [56-58]. The diameter of the tiny hollow spheres is 300 mm or less [35]. They contain an inert gas such as nitrogen or a CFC. The properties of these syntactic foams depend on matrix type, microsphere type (and the contained gas), ratio matrix to microspheres, curing process, production technology. Syntactic foams can be made in combination with the conventional ones. Such a complex composite can be formnlated into a mouldable mass then shaped or pressed into cavities. 

The reinforcement used with plastics, both thermosetting resins and thermoplastics, is usually a fibre or filament, used either on its own, or in mixtures. Non-fibrous materials can also be used in some cases. Reinforcing fillers are also used, including glass flakes, mica platelets, fibrous and finely divided minerals, and hollow and solid glass microspheres. 

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