1. Field of the Invention
The present invention relates to a novel polymeric material, which can be reused by decomposition and re-synthesis. Also, the present invention relates to a novel process for decomposition-repolymerization of a polymeric material, and to a novel system of material circulation.
2. Related Background Art
Through the years, various novel materials, which are useful in everyday life and industry, have been developed successively by coal chemical techniques and by petrochemical techniques. Typical examples include plastic materials, such as polyethylene, polypropylene, polystyrene and polyvinyl chloride, and rubbers such as polyisoprene and polybutadiene. Recently, resin materials having unique properties have been Developed, such as polyimide resins having excellent heat resistance and high impact strength, and entirely-aromatic liquid crystalline polymers.
However, such polymers are seldom reused. The polymers, after disposal as waste materials, will remain in the environment to impose a heavy burden thereon on a global scale. Waste materials from industrial activities and everyday life are becoming serious as social problems, because of the shortage of dumping sites, undesirable generation of dioxins on incineration, increase in carbon dioxide concentration in the air, and so forth. At the moment, development of materials and products in consideration of the global environment is being anticipated in connection with the carbon dioxide gas emission quotas and waste materials. It is considered to be necessary to develop a technique, which minimizes the consumption of the global resources to maintain the global environment.
In recent years, to meet the above problems, techniques have been developed for reuse of polymeric materials: for example, reuse of plastic parts after simple washing and reworking of used resins for other uses of different added value; a material recycling technique, such as molding of a used resin and a virgin resin in a sandwich state, as described Japanese Patent Publication No. 6-24739; chemical recycling techniques, such as decomposition of a used resin into a monomer after being cut into pieces; and a thermal recycling technique, such as use of waste resin as fuel.
However, of such material recycling techniques, the reuse technique is limited to using the parts in the same capacity. The material recycling technique has problems with the stability of the properties due to the deterioration of the material, guarantee of the products, deterioration of appearance, and so forth. The use of this technique is limited practically to lower grade articles. The chemical recycling technique is limited in the kinds of applicable materials and has problems with the monomer yield. Also, a large amount of energy is required to decompose the polymer into the monomer. The thermal recycling technique has problems of the combustion heat inherent to the material and reduction of carbon dioxide generation. Thus, no recycling technique meets the requirements of the market. To meet these requirements, a novel resin, which can be regenerated using a small amount of energy, and a novel technique for regenerating the resin are wanted.
The resins are classified roughly into condensation polymerization type polymers and addition polymerization type polymers. The condensation polymerization type polymers, which are typified by polyamides, are readily depolymerized at the condensation sites by an acid or base, whereas the addition polymerization type polymers, such as polystyrene, require a large amount of energy for depolymeization in an inert gas atmosphere under a high temperature.
Moreover, the decomposition products contain a mixture of dimers, trimers, tetramers, and so forth, in addition to the monomers, as described in T. Sawaguchi et al., Polym. Int. 49, 921 (2000). From the mixture, only the polymerizable monomer should be isolated, and a large amount of energy is necessary for the isolation. The yield of the recovered monomer is also important. Some of the addition polymerization polymers, such as polypropylene, cannot readily be depolymerized by the above-mentioned method.
Besides, decomposition of plastics using water or carbon dioxide in a supercritical state of a high temperature and a high pressure has been investigated, as described in Japanese Patent Laid-Open No. 8-72058. Such a method is not regarded to be the best from the standpoint of the large-scale treatment and the large amount of energy to be inputted. Therefore, universally applicable techniques are needed.
At the moment, the materials and products, which meet the global environmental problems, such as carbon dioxide gas emission quotas and waste problems, are needed. On the other hand, a reduction in consumption is required to maintain the natural resources.
The conventional addition type polymers, which are synthesized by a monomer addition reaction, can be regenerated for the material circulation only by chemical decomposition into the monomer.
Otherwise, a polymer can be formed from lower polymer molecules shorter in length than the practical polymer molecules as a kind of chemical parts (hereinafter the lower polymer being referred to as a “polymer” occasionally) by introducing a functional group into the parts for linkage-and-decomposition, and synthesizing the polymer from these parts. The polymer after use as an article, such as a molded product, can be returned to the original chemical parts by breaking the linkage between the parts. The recovered parts can be formed again into a polymer by linking the parts together.
Specifically, a polymer that has two condensable functional groups, and a molecule that has two functional groups capable of linking with the above condensable functional groups to serve as a coupler for the polymer, are employed.
An example is provided below using a styrene polymer. A styrene polymer having a carboxyl group on each end of the molecule is employed as the two-functional polymer, and butanediol is employed as the coupling molecule having two functional groups linkable with the above functional groups of the polymer. These compounds are linked together by dehydration condensation in the presence of an acid catalyst to form a high polymer having a structure of successive linkage of the styrene polymer and the butanediol. Each of the styrene polymer moieties and butandiol moieties are linked by ester linkage.
The ester linkage can be broken by a hydrolysis reaction into the original styrene polymer having a carboxyl group at the respective ends and butanediol. Thus, the reaction is reversible. The compounds thus obtained can be converted into a high molecular styrene polymer by the same polycondensation reaction as above repeatedly without limitation of the repetition time.