Oligomeric foams

On the other hand, for the understanding of the importance of the achievements made in solving specific problems (in our case relating to oligomeric foams) and in the advance of polymer science as a whole, one should use the inductive method. Therefore, this survey is concluded by a discussion of the trends and perspectives of development of all gas-filled polymers. 

The ever increasing demand for foamed polymers stimulates research aimed at the extension of the raw material base and variety of these materials. In this connection it is necessary to consider the types of oligomeric foams which have not yet left the laboratories , but which, in our opinion, are very promising for the further development of the industry of gas-filled polymers. 

In order to prepare wood substitutes and materials with open-cell structures, the so-called water-filled oligomeric foams are used These foams are obtained by mixing the OFM-monomeric composition with water until an emulsion is formed which is hardened after the addition of an initiator and activator. As a result, a white rigid material is obtained which is a spatial network copolymer with uniformly distributed micro-inclusions of water (2—5 nm). Optimal strength is reached at 50-60% of water, although the amount of water may be as high as 90%. At optimal water concentration, the cell walls withstand cryolitic destruction till 34 K. At 7 mass % of water the apparent denaty of the material reaches 250—9(X) kg/m and it resembles natural wood in appearance and in some properties. 

Among promising oligomeric foams one should also mention 2-pyranyl foams for a discussion of the chemistry, technology and properties of these materials see Finally, several works concerned with the preparation of foams from cumaron-indene and aniline-formaldehyde oligomers have been reported ... 

In this review foams on the basis of SIN systems and polymer-oligomeric systems are referred to as multicomponent oligomeric foams. 

A discussion of the formation chemistry and the molecular mechanisms responsible for the properties of multicomponent oligomeric foams is not the subject of this survey. We only emphasize here the complexity of these problems which is connected with the morphology of gasfilled polymers (see the six levels of structural organization, Chap. 3.2). 

In our opinion, multicomponent oligomeric foams provide a variety of chemical-tedinological and technical-economical possibilities which let expect practical utilization in the near future. 

New Data on the Morphology of Oligomeric Foams 5.1 TIk Quantitative Concept of Uniformity... 

The existence of microcells in the structure of oligomeric foams was confirmed by studies on other types of oligomeric foams phenolic polyurethane and urethane-phenolic multicomponent oligomeric foams ... 

Accordii to these results the following important statement can be made the structure of oligomeric foams is a matrix system of thin polymeric films with two groups of cells, diarply differing in size and number and enclosed into one anotiier. 

Several other experimental findings support the existence of a microceliular structure in oligomeric foams. Thus, Oween and Denis ) observed an anomalous pattern (in the expression of the authors) for certain types of silicone surfactants the liquid foam system consists of gas bubbles the sizes of which differ by several orders of magnitude. The possibility of formation of very small gas bubbles after a marked reduction of the surface tension coefficient in poly-lurethane formulations has been reported by Dubyaga and Tarakanov ). 

Hence, all the above-mentioned data fit into a quite consistent system of new notions about the morphology of oligomeric foams. 

However, the detection of microcells in the structure of several oligomeric foams on the one hand, and the measured values of the specific surface on the other hand, require a re-examination of our notions about the degree of dispersion in these materials. 

As shown above, the minimal of microcells is in the range of 10 jum (R < 1000 A) and S in the range of 10 cm. Thus, recent data on the structure of oligomeric foams, at least those described above, allow assignment of these foams to high or fine dispersion systems. 

The assignment of ol meric foams to fine dispersion systems is of considerable importance since it enables to approach the study and modification of structure by using the concepts of dispersion media. The advantages of such an approach are evident if one takes into account the variety of ideas and methods developed in the study of high dispersion systems of non-polymeric nature and the first results of the application of this approach to oligomeric foams 1. 

Considerable information has been accumulated regarding the relationdiip between the portion of open cells and the technical properties of oligomeric foams 83-85) xhere are considerably less data available on the relationship between the amount of open cells and other morphological parameters of RO foams, for example apparent density. Reticular foams based on RO are not considered in this survey because the open-cell structure is created by secondary processing of finished products and not during foaming. 

In the early 1960 s Saunders and Frisch proposed a colloidal-chemical mechanism of open-cell formation in oligomeric foams. Later, Rossmy et al. formulated a physical mechanism of cell opening due to the effect of water vapor see Chap. 5.3). The data presented in this section explain the formation of open cells on the basis of morphological factors taking into account the type of packing of gas bubbles in oligomeric foams. 

Numerous experimental data on dielectric permeability e and dielectric losses tg 6 of oligomeric foams at various frequencies have been collected. For all plastic foams the foBowing linear relationship exists between e and 7 (Fig. 17) ... 

Fig. 17. Dependence of the dielectric permeability e and of the tangent of dielectric loss angle tg8 on the apparent density of rigid oligomeric foams. Epoxide - PE-8 (1) and PE-9 (2) polyurethane - PPU-204 (3), PPU-305 A (4) andPPU-307 (5) )...

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