Oligomer polymer foams

Polymer foams is a term used to describe a system consisting of a gas dispersed in solid polymers. They represent a group of important materials that have wide industrial application and combine properties intrinsic to ordinary polymers with very low density and 

The physicochemical features of the processes of formation, stabilisation and solidification of foams are best studied for a polymer foam from urea-formaldehyde resins. That is why the urea polymer foams are used here below to exemplify the principles of optimisation of the technology for production of polymer foam materials. 

The initial oligomer as an aqueous solution is obtained from the reaction of urea and formaldehyde at 100°C and pH = 5.8-6 [130]. The process of polycondensation occurs in the presence of acidic catalyst and yields a tri-dimensional polymer, releasing water and formaldehyde [131]. Surfactants are added as foaming agent to the initial composition for the formation of urea polymer foams [125,130]. Various additives are employed to improve the sanitary properties of these plastics. For example, ammonium carbonate reduces the content of free formaldehyde, while addition of carbonates of alkaline metals inhibits corrosion [125]. 

The formation and preservation of the uniform cell structure of either the solid polymer or frozen foam depend on the properties of the initial foam, i.e. on its expansion ratio and stability. On the one hand, the former two processes are determined mainly by the rates of drainage and internal foam collapse. On the other hand, however, they are affected by the rate of polymerisation and the increase in strength of the foam material. That is why optimisation of the processing properties of polymer foams can be achieved by decreasing both the rate of microsyneresis and drainage, and the rate of internal foam collapse. Along with that precise control of the rate of solidification, reduction of the water content and introduction of plastifiers are needed. 

The kinetics studies of solidification of urea-formaldehyde resins have shown [24] that at low solidifier (phosphoric acid) concentrations, in the range from 0.3 to 0.75%, a prolonged induction period of solidification is observed, lasting for ca. 30 min. The optimum rate of solidification is achieved at acid concentrations higher than 1-1.5%, but the process is strongly delayed at resin concentrations lower that 5-10%. 

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A direct relation between the polymer foams density (respectively, the expansion ratio) and the oligomer content in it has been established [114], Addition of water soluble polymers (polyvinyl alcohol, carboxymethyl cellulose, etc.) led to a reduction in resin consumption without deteriorating the quality of the solid foam obtained. Polymer foams of density 5-7 kg m3 were produced when the oligomer consumption was not more than 15-20% with respect to the solution volume. 

Only isotropic gas-filled polymers based on RO will be wnsidered. Anisotropic (integral) and syntactic foams based on RO are discussed in general problems of the physical chemistry and technology of polymer foams based on high polymers and reactive oligomers are dealt with in ... 

Shutov, F. A. Foamed Polymers Based on Reactive Oligomers, Vol. 39, pp. 1-64. 

Shimidzu, I. Cooperative Actions in the Nucleophile-Containing Polymers. Vol. 23, pp. 55-102. Shutov, F. A. Foamed Polymers Based on Reactive Oligomers, Vol. 39, pp. 1-64. 

Syntactic foamed plastics (from the Greek ovvxa C, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, microcapsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder1,2). 

Almost none of the cited papers dealing with water absorption by syntactic foams deals with the mechanism by which water or other small molecules penetrate into these materials. Filyanov et al.148,149) undertook one of the first attempts using the ED-20 epoxy oligomer—glass microsphere system. Water absorption by a filled polymer is known to depend on the sorptive properties of the binder, the stability of... 

Finally, oligomers in reactive processing to produce foamed plastics and elastomers for various industrial purposes. These materials have many of the valuable properties of conventional polymers, with the addition of low density and high heat, electrical, and sound insulation characteristics.23... 

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