Components in the leaf oils of
1. Introduction
The recycling of natural resources and waste products is the most important process in the concept of green chemistry. Recently, the utilization of biomass has been a significant topic, whereas the recycling of petroleum resources must receive similar attention. Expanded polystyrene (EPS) is widely used in packing and building materials and for electrical and thermal insulation owing to the light weight and low thermal and electrical conductivities. The porosity of EPS is very high such as 98% of the apparent volume is porous. At present, over 2 million tons of EPS are produced in the world per year [1], and the rate of the material recycling is relatively high among commodity plastics [2].
For the recycling of EPS, melting [2,3] or solvent treatment [4,5] is required to reduce the volume and to be reshaped subsequently, as illustrated in Figure 1. The melting process is simple, but brings about some chemical degradation and cannot avoid debasing the quality of the original polystyrene (PS), so the solvent treatment is, in many respects, more desirable for an effective recycling system. Although there are various solvents for PS, for example, hydrocarbons, alkyl halides, aromatics, esters, and ketones, petroleum-based solvents are not favorable to the global environment. Limonene, which is a component of citrous oils, was derived from the above concept, and it is a pioneer of natural solvents for EPS [6-8]. Lately, the recycling of EPS using limonene has been realized in practical use with a semi-industrial scale, however, peel corresponding to approximately 1,000 oranges is necessary to extract 100 mL of limonene [9]. Except for limonene, there is few report on the natural solvents for EPS. This chapter is mainly focused on the dissolution of PS in naturally abundant monoterpenes including limonene, particularly, the relationship between the chemical structure and dissolving power for PS. In addition, the properties of the PS recycled by using these solvents are also described, compared with those of the original PS.
2. Naturally occurring monoterpenes and their dissolving power for PS
Hattori et al. [10] paid attention to the fact that, as limonene is one of terpenes, other monoterpenes and terpenoids are expected to dissolve PS as well. Terpene is a biomolecular hydrocarbon whose structural backbone possesses an isoprene unit. Corresponding to the number of an isoprene unit, they are called monoterpene (C10), sesquiterpene (C15), diterpene (C20), sesterterpene (C25), and so forth. Many monoterpenes are liquid at room temperature and main components of essential oils. In particular, the leaf oils of
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Bornyl acetate | 27.0 | 0 |
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22.6 | 3.1 |
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15.6 | 0 |
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13.3 | 37.9 |
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9.7 | 0.5 |
Myrcene | 1.9 | 0.4 |
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0.4 | 2.9 |
1,8-Cineole | 0 | 29.9 |
First, some structural isomers and analogues of
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130 |
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131 |
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127 |
Terpinolene | 125 |
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125 |
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122 |
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212 |
Tolueneb | 117 |
As shown in Table 1, there is a considerable amount of 1,8-cineole in
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1,8-Cineole | 55 |
Terpinene-4-ol | 39 |
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41 |
2- |
105 |
Geranyl acetate | 174 |
Generally, a non-polar molecule such as PS does not interact with a polar solvent. Terpinene-4-ol and
Geranyl acetate shows highest dissolving power of 174 g per 100 g of it. Figure 4 demonstrates the appearance of dissolving EPS by
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Geranyl acetone | 160 |
Geranyl formate | 175 |
Citronellyl acetate | 156 |
Citral | 109 |
Citronellal | 125 |
Myrcene | 101 |
These values are higher than those of typical cyclic monoterpenes in Table 2. The relatively low dissolving power of citral and citronellal compared with acyclic esters would be due to the occurrence of the terminal aldehyde group of a polar moiety that causes the reduction of accessibility to the hydrophobic matrix of PS. Unexpectedly, myrcene does not show very high dissolving power of 101 g per 100 g of it although it is a non-polar hydrocarbon. The structure of the terminal conjugated diene is probably not so flexible as to penetrate it into PS matrix. These results indicate clearly that flexible linear terpenes have higher dissolving power for PS than cyclic terpenes have.
A series of these systematic experimental results causes one fundamental question: how much dissolving power do the essential oils themselves have?
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85 |
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96 |
Bornyl acetate | 67 |
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44 |
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48 |
The solubilities of PS in bornyl acetate and both pinenes are less than half of those in limonene isomers. Bornyl acetate and the pinenes have a bulky bicyclic structure, which is likely to be disadvantageous to penetrate into PS. As a result, the
3. Relationship between solubility parameter and dissolving power of monoterpenes
As a general standard for the judgment that a given solute is soluble or insoluble in a solvent, there is a method to compare the "solubility parameter" of the solute with the solvent. Hildebrand first devised the theory of this concept [16], and afterward Hansen [17], Barton [18], and Hoftyzer and Krevelen [19,20] et al. have developed this theory. The solubility parameter (
where
where
Taking account of these intermolecular interactions, Hoftyzer and Krevelen [19] expressed their components such as:
where
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−CH3 | 0.42 | 0 | 0 |
−CH2− | 0.27 | 0 | 0 |
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0.08 | 0 | 0 |
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−0.07 | 0 | 0 |
=CH2 | 0.40 | 0 | 0 |
=CH− | 0.20 | 0 | 0 |
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0.07 | 0 | 0 |
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−CH3×4 | 1.68 | 0 | 0 |
−CH2−×3 | 0.81 | 0 | 0 |
=CH−×2 | 0.40 | 0 | 0 |
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0.14 | 0 | 0 |
−COO− | 0.39 | 0.49 | 7000 |
Sum | 3.42 | 0.49 | 7000 |
According to Table 6, the group contribution parameters of geranyl acetate are calculated as shown in Table 7. Since the molecular weight (
From these components, the solubility parameter of geranyl acetate is found:
The calculated
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136.24 | 0.838 | 14.9 |
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136.24 | 0.853 | 15.2 |
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136.24 | 0.840 | 15.2 |
Terpinolene | 136.24 | 0.863 | 15.7 |
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136.24 | 0.846 | 14.7 |
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136.24 | 0.850 | 15.0 |
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134.22 | 0.857 | 14.6 |
1,8-Cineole | 154.25 | 0.923 | 15.0 |
Terpinene-4-ol | 154.25 | 0.927 | 19.1 |
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154.25 | 0.934 | 19.2 |
2- |
150.22 | 0.976 | 19.4 |
Geranyl acetate | 196.29 | 0.909 | 16.9 |
Geranyl acetone | 194.32 | 0.873 | 16.8 |
Geranyl formate | 182.29 | 0.908 | 16.9 |
Citronellyl acetate | 198.31 | 0.890 | 16.8 |
Citral | 152.24 | 0.890 | 17.8 |
Citronellal | 154.25 | 0.855 | 17.4 |
Myrcene | 136.24 | 0.794 | 15.9 |
Bornyl acetate | 196.29 | 0.980 | 15.8 |
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136.24 | 0.859 | 13.6 |
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136.24 | 0.874 | 14.2 |
4. Dissolution rate of PS in monoterpenes
When the recycling efficiency of PS is being considered, not only dissolving power but also dissolution rate is one of the important factors on evaluating the performance of a solvent.
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545 | 401 | 334 | 262 | 208 | 20.3 |
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496 | 359 | 289 | 240 | 196 | 19.7 |
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519 | 471 | 375 | 283 | 200 | 20.7 |
Terpinolene | 525 | 425 | 365 | 301 | 248 | 16.0 |
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390 | 321 | 235 | 166 | 125 | 25.2 |
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263 | 191 | 147 | 114 | 90 | 23.1 |
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215 | 149 | 109 | 85 | 66 | 25.1 |
1,8-Cineole | 4,480 | 1,390 | 626 | 478 | 302 | 56.3 |
Terpinene-4-ol | –c | 4,430 | 1,810 | 950 | 610 | 59.0 |
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3,025 | 1,289 | 715 | 418 | 344 | 47.7 |
2- |
11,458 | 3,830 | 1,991 | 829 | 403 | 71.2 |
Geranyl acetate | 719 | 543 | 493 | 424 | 269 | 19.1 |
Geranyl acetone | 748 | 505 | 451 | 323 | 211 | 25.7 |
Geranyl formate | 628 | 527 | 325 | 253 | 152 | 30.7 |
Citronellyl acetate | 869 | 507 | 411 | 292 | 265 | 25.5 |
Citral | 1,168 | 712 | 490 | 347 | 230 | 34.3 |
Citronellal | 597 | 380 | 290 | 231 | 150 | 28.2 |
Myrcene | 435 | 297 | 200 | 165 | 117 | 27.9 |
Bornyl acetate | 14,900 | 3,660 | 1,590 | 862 | 558 | 69.8 |
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–c | 1,860 | 852 | 600 | 503 | 38.5 |
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3,213 | 690 | 366 | 242 | 142 | 63.5 |
Therefore, the dissolution time of PS in each terpene was measured at several different temperatures, and then the apparent activation energy (
5. Recovery of PS and natural solvents, and physical properties of the recycled PS
Currently, it entails a high cost to gather natural solvents such as essential oils for the recycling of waste EPS, so that the recovery and reuse of the solvent are required. In addition, the properties and performance of the recycled PS are important. Terpenes and PS can be simply recovered by steam distillation of a solution of PS in terpenes; a typical example is as follows. A 10% solution of PS in geranyl acetate is subjected to steam distillation to recover 98% of the geranyl acetate used. The
6. Conclusion
The essential oil in plants and its main components, terpenes and terpenoids, are good solvent for PS. EPS is recyclable by using those natural solvents in place of petroleum-based ones. The dissolving power of terpenes for PS strongly depends on their chemical structure. Basically, terpenes of which solubility parameter is close to that of PS dissolve much PS as predicted from the theory, as well as the dissolution rate is high as that of toluene, a petroleum-based solvent. In oxygen-containing terpenes, the ethers and esters show higher dissolving power than the alcohols according to the rule of solubility parameter. However, even though the solubility parameter is close to that of PS, acyclic terpenes have higher dissolving power compared to cyclic ones and bicyclic terpenes show relatively low dissolving power and dissolution rate for PS. These findings enable the judgment whether a certain terpene is suitable for the solvent of PS recycling from the chemical structure. The PS recovered by means of steam distillation of a solution of PS in terpenes shows slightly reduced molecular weight, but almost the same mechanical properties, compared to the original PS. Such reduction of molecular weight can be minimized by steam distillation under nitrogen atmosphere. Since
Acknowledgments
Some terpenes were kindly gifted from Tokyo Chemical Industry, Inc., and Toyotama International, Inc. The author gratefully acknowledges both companies.
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