Experimental studies on pyrolysis and partial oxidation of polyethylene (PE) in supercritical water were conducted by use of batch reactors at a reaction temperature of 420°C and reaction time of 30 min. In the case of pyrolysis, PE decomposition was enhanced with increasing water density. Since the effect of supercritical water on pyrolysis itself was found to be not significant through the basic studies using n-hexadecane, the enhancement of PE decomposition by supercritical water was considered to be due to dissolution of high molecular weight hydrocarbons into supercritical water and diffusion of water into the molten PE phase. For partial oxidation, the yield of partial oxidation products, such as CO, CO₂, alcohols, aldehydes, and ketones, increased with increasing water density of supercritical water. In addition, the yield of H₂ and n-alkane also increased with increasing water density. The partial oxidation of PE was influenced by the area of the interface between the molten PE phase and water-O₂ phase. The reaction rate of partial oxidation of PE increased with an increase of the interfacial area. Through the studies, PE decomposition in supercritical water was found to be controlled by the changes of phase behavior and mass transfer with temperature and pressure.

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Applications of supercritical fluid chromatography (SFC) to the analysis of plant polyprenols are described. By the use of SFC, chromatographic resolution of polyprenol homologs and their geometric isomers was improved markedly as compared with that obtained by the use of conventional high-performance liquid chromatography (HPLC). Under optimized SFC conditions, individual homologs from 10 mer to 100 mer were separated. It was also possible to isolate each geometric isomer of polypernol homologs from 13 mer to 20 mer by the fractionation using SFC. The chain-length distributions of polyprenol samples determined by SFC agreed essentially with those determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). SFC analysis of polyprenols extracted from the leaf, root and seed coat of a rubber-producing plant, Eucommia ulmoides Oliver (E. ulmoides), has revealed the presence of all-trans polyprenols with degrees of polymerizations larger than 10, for the first time in nature. The all-trans polyprenols had broad distributions of chain-lengths leading to high molecular weight, which suggests that the polyprenols should act as biosynthesis intermediates for the high molecular weight trans-1, 4-polyisoprene in this plant. The chain-length distributions of the trans polyprenols from the leaf, root and seed coat differed from each other, suggesting the presence of site-specific control mechanisms of chain-termination. On the other hand, cispolyprenols have been found to occur in all parts of E. ulmoides. They had narrow chain-length distributions similar to those of dolichols.

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Previously, we reported that nanocrystals of organic and polymeric compounds can easily be obtained by reprecipitation from their solution. The reprecipitation method, however, is not applicable to compounds that are hard to dissolve in conventional solvents. To address this issue, an advanced method, i. e. the supercritical fluid reprecipitation (SCFR) method has been investigated; the nanocrystallization of difficult-to-dissolve compounds such as phthalocyanine and fullerenes has become possible. In the case of titanylphthalocyanine, the γform-nanocrystals 50 nm in size could be prepared by using supercritical acetone in SCFR method. The performance of the organic photoconductors was evaluated. When C₆₀ was subjected to this technique, the nanocrystals with size of about 40 nm could be fabricated in the dispersion liquid. The optical properties were found to be dependent on the crystal size in a similar manner to that for other π-conjugated organic nanocrystals. The nanocrystallization of polydiacetylene and others were also possible using SCFR method. The results were compared with those obtained by the conventional reprecipitation method.

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To clarify the characteristics of cracking polyethylene in supercritical water, thermal cracking and supercritical water cracking experiments were conducted. In comparing the species, yields and structures of the cracked products, consideration was given to the characteristics of the degradation mechanism. How supercritical water contributes to the degradation of polyethylene was also considered. It was found that supercritical water cracking, as compared to thermal cracking, is characterized by high oil yields and suppressed coke production. The result of an experiment using D₂O as a tracer to crack polyethylene indicated that the hydrogen in the supercritical water is captured into the oil (a cracking product) and the donation of hydrogen from the supercritical water to the oil increases with a higher water fill rate for the reactor volume. The hydrogen donation mechanism from the supercritical water to the oil was found to include some prominent reaction stages: lower alkenes (propylene, etc.) produced from the cracking of polyethylene are converted into secondary alcohols (2-propanol, etc.); and the hydrogen released in the oxidation of secondary alcohols into ketones (2-propanone, etc.) is donated to the cracked product. Additionally, in the hydrogenation degradation reaction, it was clarified that supercritical water donates hydrogen to the active ends of the molecule produced by the cracking of polyethylene and such donation allows the molecule produced to be stable; hydrogen donation is limited to a specific site within the methylene chain; and the higher fill rate has a more suppressive effect on the formation of gaseous cracked products. This is because the hydrogen donation capability of supercritical water with the higher fill rate is superior to that of the lower fill rate and the active ends of the molecule are more easily stabilized, suppressing the progress of the reactions.

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The hydrolysis activity of Candida rugosa lipase (lipase OF) treated with supercritical CO₂ was measured. The activity of the lipase, which was treated for one hour at 308.15 K and 19.61 MPa, is approximately 2.5 times higher than that of the untreated lipase. Changes in the content of proteins in the lipase and also in the size of the lipase particle after treatment were investigated through fluorescence emission spectra meter and scanning electron microscopy respectively. The size reduction of particles and the purification of the enzyme might be the main reasons for the increase in Candida rugosa lipase's activity.

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Synthesis of ε-caprolactam from caprolactone with ammonia was carried out in supercritical water (SCW: T374°C, P>22.1 MPa). A main reaction pathway for the formation of ε-caprolactam was elucidated on the basis of a series of reactions of 1) ε-caprolactone, 2) hexanol + ammonia, 3) hexanoic acid + ammonia, and 4) hexanoic acid + hexanamide in SCW. The results clearly showed the following three steps of reactions for the formation of ε-caprolactam. The first step is the ring cleavage of ε-caprolactone to yield 6-hydroxyhexanoic acid. The second step is the dehydration between 6-hydroxyhexanoic acid and ammonia to form 6-hydroxyhexanamide. The final step is the intramolecular dehydration of 6-hydroxyhexanamide to produce ε-caprolactam. The effects of reaction temperature, water density, and ammonia concentration were also studied. At 380°C and 38 MPa (water density 0.5g/cm³), the yield of ε-caprolactam from ε-caprolactone and ammonia at a ratio 1: 5 increased with increasing reaction time and reached 79.2% in 60min.

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For development of a process of recovering chemicals from biomass by using supercritical water as a reactor solvent, a mechanistic study has been conducted for the reaction of ^D-glucose and ^D-fructose in sub-and supercritical water. A series of experiments was conducted over wide ranges of reaction temperature (350-450°C), pressure (25-40MPa), and reaction times (0.02-1.02s) with a flow type rector. As a result, we found that the contribution of retro-aldol condensation became predminonant at higher temperatures, while that of dehydration reaction (formation of furan derivatives) became significant at lower temperatures. In supercritical condition, retro-aldol condensation became essential for reducing reaction pressure, against the dehydration reaction.

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The decomposition and debromination treatment of tetrabrominated bisphenol A type epoxy resin (brominated epoxy resin), which is a flame-resistant polymer, were investigated using sub- and supercritical water. At 300°C, 25 MPa, and 30 min of reaction time, the noncatalytic hydrolysis of brominated epoxy resin inhibited the carbonization of resin, and decomposed it to water-soluble products including phenol and tetrahydrofuran (THF) -soluble products which had relatively heavy molecular weight. More than 98% of the bromine in the brominated epoxy resin was transported into water using a decomposition reaction. As compared with the pyrolysis of brominated epoxy resin, in the hydrolysis using subcritical water the carbonization of resin was inhibited, the decomposition of resin proceeded, and abstraction of bromine from the resin was accelerated.

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Poiy (p-phenyleneterephthalamide) was decomposed using supercritical water in a batch reactor at temperatures ranging from 300 to 600°C and reaction times ranging from 10 to 60min. Reaction products were identified with a gas chromatograph-mass spectrometer (GCMS) and high performance liquid chromatography (HPLC). Poly (p-phenyleneterephthalamide) decomposed well in supercritical water and formed mainly p-phenylenediamine, benzene, and carbon dioxide. The p-phenylenediamine yield reached its theoretical value. Alkalis or oxygen as an additive increased the yield of the compounds. When the aromatic polyamide was decomposed in a flow reactor the main products were p-phenylenediamine and benzoic acid. These results show that mass-scale feedstock recycling of polyamide is possible.

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In this article, we report the decomposition of silane-crosslinked polyethylene (silane-XLPE) by supercritical water and supercritical methanol for material recycling. By supercritical water, XLPE decomposed to polyethylene (PE) wax. By supercritical methanol over 340°C, XLPE was also turned into wax. However, we obtained thermoplastic polyethylene from silane-XLPE after it was heated in supercritical methanol at 300-340°C and 8-10 MPa for 30 min. The number-average molecular weight of PE was about 40000, which was equal to that of the raw material of silane-XLPE. The NMR and FT-IR spectra of PE further indicated that the crosslinking element (-Si-O-bond) was decomposed to -Si-O-H or -Si-O-CH₃ preferentially. Moreover, the gel fraction was 0%. These data indicate that only the crosslinking element is decomposed and the PE main chain still remains. This technique can be used for recycling silane-XLPE.

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Poly (methyl methacrylate) (PMMA) could be infused with Ti (O-i-C₃H₇) ₄ by supercritical carbon dioxide. Ti (O-i-C₃H₇) ₄ was infused into the surface of PMMA at low temperatures, and diffused into the deep area by increasing the pressure at a constant temperature. It became clear that Ti (O-i-C₃H₇) ₄ reacts to produce TiO₂·XH₂O clusters. The diameter of clusters, the thickness of the layer diffusing the cluster, and the area ratio of cross section of the cluster to the unit area of the layer showed maximum values at a near critical temperature, and they became large by increasing the pressure. They were shown to be controlled by the temperature and pressure.

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In the investigation of polymer-supercritical fluid systems, it is particularly important to study the sorption and plasticization effect of supercritical fluids. In this work, the sorption and plasticization effect of supercritical carbon dioxide in poly (ethylene glycol) (PEG) and poly (ethylene oxide) (PEO) were directly measured with a new in situ high-pressure thermogravimetry-differential thermal analysis (TGDTA) based on magnetic suspension. The sorption of carbon dioxide in molten PEG and PEO increased almost linearly with pressure. Henry's constant values, K_p, were evaluated using the experimental data. A linear expression between ln (1/K_p) and (T_c/T) ² was shown for PEG/CO₂ and PEO/CO₂ systems. Compressed carbon dioxide gas depressed the melting points of PEG and PEO. The melting point of PEG and PEO decreases linearly with pressure of carbon dioxide, giving a dT_m, /d_p value of-2.04°C/MPa and -1.24°C/MPa, respectively.

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