**Recycling of Expanded Polystyrene Using Natural Solvents**

Kazuyuki Hattori

Chapter 2. Hydrothermal Depolymerization of Polyesters and Polycarbonate in the Presence of Ammo‐

In this chapter, methods for the chemical recycling of poly(ethylene terephthalate) PET, poly(ethylene naphthalate) PEN and polycarbonate, PC using hydrothermal conversion of the polymers in aqueous ammonia and amine solutions are reviewed. This process is attractive due to highly selective conversion to monomers, easy separation of monomers in resultant aqueous solutions, and relatively mild reaction conditions, i.e., temperature lower than the melting point of polymer and low concentration of ammo‐

Chapter 3. Chemical and Thermo-chemical Recycling of Polymers from Waste Electrical and Electronic

This chapter provides a critical review on the chemical and thermo-chemical methods proposed and/or applied, during mainly the last decade, on the recycling of polymers from waste electrical and electronic equipment (WEEE). Recycling methods such as the dissolution-reprecipitation and pyrolysis are pre‐ sented. Special emphasis is given in three different polymers, i.e. polycarbonate (PC), high impact poly‐ styrene (HIPS) and Acrylonitrile-Butadiene-Styrene (ABS) which are commonly found in WEEE. The state-of-the-art of the chemical and thermo-chemical recycling methods of these polymers is illustrated, emphasizing to environmentally friendly techniques, such the use of microwave irradiation instead of

In this chapter, the significance of composting composites and nanocomposites based on bio-based pol‐ ymers used in various applications to reduce the amount of solid waste in landfills is presented. Fur‐ thermore, composting methods to produce compostable materials and international standard test

Chapter 5. Potential for Introduction of Preservative Treated Wood in Wood Waste Recycling Streams

Recycling of woody debris promises to remove a substantial volume from the waste stream, thereby prolonging the useful life of the limited landfill capacity. One potential issue with these recycling pro‐ grams is the potential for contamination of the recycling stream with treated wood. Treated wood is supposed to be either reused in a similar application or, if that cannot be done, disposed of in a munici‐ pal solid waste facility. However, it can sometimes be difficult to distinguish treated wood from other materials and varying amounts of treated wood are entering the waste stream. This chapter outlines methods for assessing volumes of treated wood in the recycling stream, examines the potential risks of

I want to express my sincere thanks to all the contributors who provided their expertise and enthusiasm to this project and InTech for making this work possible. I would like also to thank my wife Maria and the sons Savvas, Diamantis and Yiannis for their patience and the time deprived them during the prepa‐

> **Dimitris S. Achilias** Associate Professor Department of Chemistry

> > Greece

Aristotle University of Thessaloniki

Chapter 4. Compostable Polymers and Nanocomposites - A big chance for the Planet Earth.

methods for evaluation of the above mentioned materials are illustrated.

this material and then identifies possible methods for excluding these materials.

ration of my chapter and the book editing. I dedicate this book to them.

nia and Amines

VIII Preface

nia or amines.

Equipment

conventional heating.

and its Prevention

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59156

### **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 dis‐ solving 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.

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**11** Table 9 "*kJ/mol*" in the heading is not italic.

**12** Table 9 "*kJ/mol*" in the heading is not italic.

**12** Table 9 In the footnote b, 10 and 14. [10] and [14].

**13** 10 *Mn M*n (Subscript "n" is not italic)

**12** 11 low a *E*<sup>a</sup> low an *E*<sup>a</sup>

**15** 10 *p*-cymene *p*-Cymene

Figure 1. Material recycling system of EPS. **Figure 1.** Material recycling system of EPS.

#### **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 mono‐ terpenes 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 *Abies sachalinensis* and *Eucalyptus* species, in which the growth is comparatively fast, may be suitable biomass because they are not utilized effectively at present and contain many monoterpenes. Table 1 summa‐ rizes some liquid monoterpenes and terpenoids selected from the viewpoint of content rate in their leaf oils [11-13]. Both are significantly different. *d*-Limonene is much contained in *Abies sachalinensis*, but a little in *Eucalyptus*. The largest amount of bornyl acetate in *Abies sachalinen‐ sis* is not contained in *Eucalyptus*. In contrast, 1,8-cineole occurs abundantly in *Eucalyptus*, whereas does not occur at all in *Abies sachalinensis*.

2

First, some structural isomers and analogues of *d*-limonene, as shown in Figure 2, were studied on the dissolving power for PS [10]. The experimental method is as follows. A known weight of a small piece of commercial PS film with a number-average molecular weight (M¯ n) of 1.2 × 105 was put in 0.5 mL of each terpene at 50 °C, and the behavior of PS was observed by a polarizing microscopy under crossed nicols. The dissolution was judged from the disappear‐ ance of birefringence of the PS piece. The additional piece, if necessary, was put after complete dissolution was achieved. In Table 2, the dissolving power of the terpenes is listed as the weight of the PS dissolved per 100 g of each terpene. All these terpenes are capable of dissolving more than 120 g of PS per 100 g of them. The values are greater than that of toluene, which is one of the petroleum-based solvents for PS. These six terpenes except for *p*-cymene are structural isomers with different locations of a C=C bond, so they would have similar dissolving power

4


**Table 1.** Components in the leaf oils of *Abies sachalinensis* and *Eucalyptus.*

one another. This result led to a relationship between the structure and dissolving power that the position of a C=C bond does not affect the dissolving power greatly. The solubility of PS in *p*-cymene is remarkably higher than that in other terpenes, because *p*-cymene is, as described later, an aromatics that has a similar chemical structure to PS.

Table 2. Solubility of PS in several monoterpenes at 50 °C. Solvent Solubility (g/ 100 g · solvent)





b) One of the petroleum-based solvents was used for comparison.

a

Figure 2. Structure of *d*-limonene and its some isomers and analogues. **Figure 2.** Structure of *d*-limonene and its some isomers and analogues.

a) Cited from reference 10.

2

M¯ n) of 1.2 ×

**11** Table 9 "*kJ/mol*" in the heading is not italic.

**12** Table 9 "*kJ/mol*" in the heading is not italic.

Melting

Grinding

2 Recycling Materials Based on Environmentally Friendly Techniques

Dissolving

whereas does not occur at all in *Abies sachalinensis*.

105

Waste EPS

**Figure 1.** Material recycling system of EPS.

**12** Table 9 In the footnote b, 10 and 14. [10] and [14].

**13** 10 *Mn M*n (Subscript "n" is not italic)

**12** 11 low a *E*<sup>a</sup> low an *E*<sup>a</sup>

**15** 10 *p*-cymene *p*-Cymene

Small pieces

Volume reduction

Figure 1. Material recycling system of EPS.

**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 mono‐ terpenes 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 *Abies sachalinensis* and *Eucalyptus* species, in which the growth is comparatively fast, may be suitable biomass because they are not utilized effectively at present and contain many monoterpenes. Table 1 summa‐ rizes some liquid monoterpenes and terpenoids selected from the viewpoint of content rate in their leaf oils [11-13]. Both are significantly different. *d*-Limonene is much contained in *Abies sachalinensis*, but a little in *Eucalyptus*. The largest amount of bornyl acetate in *Abies sachalinen‐ sis* is not contained in *Eucalyptus*. In contrast, 1,8-cineole occurs abundantly in *Eucalyptus*,

First, some structural isomers and analogues of *d*-limonene, as shown in Figure 2, were studied on the dissolving power for PS [10]. The experimental method is as follows. A known weight

 was put in 0.5 mL of each terpene at 50 °C, and the behavior of PS was observed by a polarizing microscopy under crossed nicols. The dissolution was judged from the disappear‐ ance of birefringence of the PS piece. The additional piece, if necessary, was put after complete dissolution was achieved. In Table 2, the dissolving power of the terpenes is listed as the weight of the PS dissolved per 100 g of each terpene. All these terpenes are capable of dissolving more than 120 g of PS per 100 g of them. The values are greater than that of toluene, which is one of the petroleum-based solvents for PS. These six terpenes except for *p*-cymene are structural isomers with different locations of a C=C bond, so they would have similar dissolving power

of a small piece of commercial PS film with a number-average molecular weight (

• Petroleum-based solvents

• Bio-based solvents (Natural solvents)

Solvent recovery

Cooling

Mixing with other materials

Reshaping

Ingot Products

Building materials


a) Cited from reference [10].

b) One of the petroleum-based solvents was used for comparison.

**Table 2.** Solubility of PS in several monoterpenes at 50 °C.

As shown in Table 1, there is a considerable amount of 1,8-cineole in *Eucalyptus* leaf oil. Therefore, the next investigation of the dissolving power of natural solvents for PS went to 1,8 cineole and some related oxygen-containing terpenoids [10,14]. Figure 3 and Table 3 represent the chemical structure of the terpenoids and their dissolving power for PS, respectively. As shown in Table 1, there is a considerable amount of 1,8-cineole in *Eucalyptus* leaf oil. Therefore, the next investigation of the dissolving power of natural solvents for PS went to 1,8-cineole and some related oxygen-containing terpenoids [10,14]. Figure 3 and Table 3 represent

Figure 3. Structure of 1,8-cineole and some oxygen-containing terpenoids. **Figure 3.** Structure of 1,8-cineole and some oxygen-containing terpenoids.

a) Cited from refereces 10 and 14.

solvent) than those in terpinene-4-ol and

Table 3. Solubility of PS in several oxygen-containing terpenoids at 50 °C. Solvent Solubility (g/ 100 g · solvent) a 1,8-Cineole 55 Terpinene-4-ol 39 -Terpineol 41 2-*p*-Cymenol 105 Generally, a non-polar molecule such as PS does not interact with a polar solvent. Terpinene-4 ol and *α*-terpineol have such a high polar moiety as a hydroxyl group, hence, the solubilities of PS in them (ca. 40 g/ 100 g⋅solvent) are lower than those in the corresponding terpinene and terpinolene without a hydroxyl group (ca. 130 g/ 100 g⋅solvent, Table 2). The oxygen of 1,8 cineole is adopted to not a hydroxyl group, but an ether group. It is suggested that the higher solubility of PS in 1,8-cineole (55 g/ 100 g solvent) than those in terpinene-4-ol and *α*-terpineol is ascribed to the lower polarity of an ether group compared to a hydroxyl group. The high

the chemical structure of the terpenoids and their dissolving power for PS, respectively. Generally, a

have such a high polar moiety as a hydroxyl group, hence, the solubilities of PS in them (ca. 40 g/ 100

g · solvent) are lower than those in the corresponding terpinene and terpinolene without a hydroxyl group (ca. 130 g/ 100 g · solvent, Table 2). The oxygen of 1,8-cineole is adopted to not a hydroxyl

group, but an ether group. It is suggested that the higher solubility of PS in 1,8-cineole (55 g/ 100 g ·


Geranyl acetate 174

non-polar molecule such as PS does not interact with a polar solvent. Terpinene-4-ol and

5


6


solvent), in spite of possessing a hydroxyl group, may be due to the presence of an aromatic ring as **Table 3.** Solubility of PS in several oxygen-containing terpenoids at 50 °C.

**Solvent Solubility (g/ 100 g ⋅ solvent)<sup>a</sup>**

As shown in Table 1, there is a considerable amount of 1,8-cineole in *Eucalyptus* leaf oil. Therefore, the next investigation of the dissolving power of natural solvents for PS went to 1,8 cineole and some related oxygen-containing terpenoids [10,14]. Figure 3 and Table 3 represent the chemical structure of the terpenoids and their dissolving power for PS, respectively.

As shown in Table 1, there is a considerable amount of 1,8-cineole in *Eucalyptus* leaf oil.

1,8-cineole and some related oxygen-containing terpenoids [10,14]. Figure 3 and Table 3 represent

HO

Table 3. Solubility of PS in several oxygen-containing terpenoids at 50 °C.

Generally, a non-polar molecule such as PS does not interact with a polar solvent. Terpinene-4 ol and *α*-terpineol have such a high polar moiety as a hydroxyl group, hence, the solubilities of PS in them (ca. 40 g/ 100 g⋅solvent) are lower than those in the corresponding terpinene and terpinolene without a hydroxyl group (ca. 130 g/ 100 g⋅solvent, Table 2). The oxygen of 1,8 cineole is adopted to not a hydroxyl group, but an ether group. It is suggested that the higher solubility of PS in 1,8-cineole (55 g/ 100 g solvent) than those in terpinene-4-ol and *α*-terpineol is ascribed to the lower polarity of an ether group compared to a hydroxyl group. The high

Figure 3. Structure of 1,8-cineole and some oxygen-containing terpenoids.

OH

Solvent Solubility (g/ 100 g · solvent)

the chemical structure of the terpenoids and their dissolving power for PS, respectively. Generally, a

have such a high polar moiety as a hydroxyl group, hence, the solubilities of PS in them (ca. 40 g/ 100

g · solvent) are lower than those in the corresponding terpinene and terpinolene without a hydroxyl group (ca. 130 g/ 100 g · solvent, Table 2). The oxygen of 1,8-cineole is adopted to not a hydroxyl

group, but an ether group. It is suggested that the higher solubility of PS in 1,8-cineole (55 g/ 100 g ·


1,8-Cineole 55 Terpinene-4-ol 39


non-polar molecule such as PS does not interact with a polar solvent. Terpinene-4-ol and

**Figure 3.** Structure of 1,8-cineole and some oxygen-containing terpenoids.

solvent) than those in terpinene-4-ol and

a) Cited from refereces 10 and 14.

a

AcO


Therefore, the next investigation of the dissolving power of natural solvents for PS went to

5


*α* -Terpinene 130 *γ*-Terpinene 131 *d*-Limonene 127 Terpinolene 125 *α* -Phellandrene 125 *β* -Phellandrene 122 *p*-Cymene 212 Tolueneb 117

4 Recycling Materials Based on Environmentally Friendly Techniques

b) One of the petroleum-based solvents was used for comparison.

**Table 2.** Solubility of PS in several monoterpenes at 50 °C.

1,8-Cineole Terpinene-4-ol

OH <sup>O</sup>

a) Cited from reference [10].

dissolving power of 2-*p*-cymenol (105 g/ 100 g⋅solvent), in spite of possessing a hydroxyl group, may be due to the presence of an aromatic ring as mentioned above. mentioned above.

**Figure 4.** EPS shrunk by *α*-terpinene (a) and geranyl acetate (b) [10].

Figure 4. EPS shrunk by

Geranyl acetate shows highest dissolving power of 174 g per 100 g of it. Figure 4 demonstrates the appearance of dissolving EPS by -terpinene (a) and geranyl acetate (b) [10]. Geranyl acetate is apparently more powerful than -terpinene concerning the ability to shrink EPS. It seems that the high dissolving power of geranyl acetate is based on its flexible linear structure, which is more accessible to the inside of bulk PS compared with the cyclic terpenes in Table 2. Therefore, the dissolving power of several acyclic monoterpenes was studied for the confirmation of that. Geranyl Geranyl acetate shows highest dissolving power of 174 g per 100 g of it. Figure 4 demon‐ strates the appearance of dissolving EPS by *α*-terpinene (a) and geranyl acetate (b) [10]. Ger‐ anyl acetate is apparently more powerful than *α*-terpinene concerning the ability to shrink EPS. It seems that the high dissolving power of geranyl acetate is based on its flexible linear structure, which is more accessible to the inside of bulk PS compared with the cyclic ter‐ penes in Table 2. Therefore, the dissolving power of several acyclic monoterpenes was stud‐ ied for the confirmation of that. Geranyl acetate, citronellyl acetate, and myrcene are found in the essential oils of *Picea* genus and others [11], and citral and citronellal are components of citrus oils [15]. As shown in Table 4, geranyl acetone, geranyl formate, and citronellyl ace‐ tate have similar dissolving power as high as geranyl acetate has.


acetate, citronellyl acetate, and myrcene are found in the essential oils of *Picea* genus and others [11],

and citral and citronellal are components of citrus oils [15]. As shown in Table 4, geranyl acetone, geranyl formate, and citronellyl acetate have similar dissolving power as high as geranyl acetate has.

**Figure 5.** Structure of several acyclic terpenes and terpenoids.


Figure 5. Structure of several acyclic terpenes and terpenoids.

**Table 4.** Solubility of PS in several acyclic terpenoids at 50 °C.

7 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 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. 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

diene is probably not so flexible as to penetrate it into PS matrix. These results indicate clearly that

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

A series of these systematic experimental results causes one fundamental question: how much dissolving power do the essential oils themselves have? *Abies* oil can be easily prepared by refluxing for 6 h in water and subsequent steam distillation of the leaves of *Abies sachalinen‐ sis* [14]. *Eucalyptus* oil is commercially available from Tokyo Chemical Industry, Inc., Japan. The solubilities of PS in the *Abies* and *Eucalyptus* oils were 85 g and 96 g per 100 g of them [14], respectively, as shown in Table 5. According to the reports of Yatagai et al. [11,12], *Abies* leaf oil contains 27% of bornyl acetate and 23% of pinenes whose structure and dissolving power are as follows. dissolving power do the essential oils themselves have? *Abies* oil can be easily prepared by refluxing for 6 h in water and subsequent steam distillation of the leaves of *Abies sachalinensis* [14]. *Eucalyptus* oil is commercially available from Tokyo Chemical Industry, Inc., Japan. The solubilities of PS in the *Abies* and *Eucalyptus* oils were 85 g and 96 g per 100 g of them [14], respectively, as shown in Table 5. According to the reports of Yatagai et al. [11,12], *Abies* leaf oil contains 27% of bornyl acetate and 23% of pinenes whose structure and dissolving power are as follows. The

Bornyl acetate



8

Figure 6. Structure of bornyl acetate and pinenes. **Figure 6.** Structure of bornyl acetate and pinenes.

7

Table 4. Solubility of PS in several acyclic terpenoids at 50 °C.

Figure 5. Structure of several acyclic terpenes and terpenoids.

*dl*-Citronellal

Geranyl acetone *dl*-Citronellyl acetate

Geranyl formate

O

H O

H

O

a

Myrcene

AcO

Solvent Solubility (g/ 100 g · solvent)

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

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.

Geranyl acetone 160 Geranyl formate 175 Citronellyl acetate 156 Citral 109 Citronellal 125 Myrcene 101

**Solvent Solubility (g/ 100 g ⋅ solvent)<sup>a</sup>**

Geranyl acetone 160 Geranyl formate 175 Citronellyl acetate 156 Citral 109 Citronellal 125 Myrcene 101

a) Partly cited from reference 10.

**Table 4.** Solubility of PS in several acyclic terpenoids at 50 °C.

a) Partly cited from reference [10].

Citral

**Figure 5.** Structure of several acyclic terpenes and terpenoids.

H

O

6 Recycling Materials Based on Environmentally Friendly Techniques

AcO


**Table 5.** Solubility of PS in essential oils and several bicyclic terpenes at 50 °C.

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 *Abies* leaf oil containing approximately 50% of these three terpenes in total does not have so high dissolving power for PS. Since *Eucalyptus* oil also contains such bicyclic terpenes as 30% of 1,8-cineole and 38% of *α*-pinene, it is not a very strong solvent for PS itself. However, both oils still have dissolving power of nearly 100 g for PS per 100 g of them, so that they will be a favorable solvent for PS recycling.

#### **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 (*δ*) of a substance is defined as:

$$
\delta = \sqrt{\frac{E\_{\text{coh}}}{V}} \tag{1}
$$

where *E*coh and *V* are the cohesive energy (=vaporization energy) and molar volume of the substance, respectively. The *V* is calculated from the molecular weight and density of the substance. The *E*coh can be obtained experimentally for a volatile substance, but is usually derived from theoretical approach. Hansen [17] considered that *E*coh is consisting of three types of energies derived from the following interaction forces:

$$E\_{\rm coh} = E\_{\rm d} + E\_{\rm p} + E\_{\rm h} \tag{2}$$

where *E*d, *E*p, and *E*<sup>h</sup> are the energy of dispersion forces, polar forces, and hydrogen bonding, respectively. Then, Equation (1) is modified using the corresponding solubility parameter components, *δ* d, *δ* p, and *δ* h, to each force as follows:

$$\delta = \sqrt{\delta\_{\rm d}^{2} + \delta\_{\rm p}^{2} + \delta\_{\rm h}^{2}} \tag{3}$$

Taking account of these intermolecular interactions, Hoftyzer and Krevelen [19] expressed their components such as:

$$\delta\_{\mathbf{d}} = \frac{\sum F\_{\mathbf{d}\_i}}{V}, \delta\_{\mathbf{p}} = \frac{\sqrt{\sum F\_{\mathbf{p}\_i}^2}}{V}, \text{and } \delta\_{\mathbf{h}} = \sqrt{\frac{\sum E\_{\mathbf{h}\_i}}{V}} \tag{4}$$

where F di , F pi , and Eh i are the parameter of dispersion forces, polar forces, and hydrogen bonding, respectively, reflecting the contribution of structural groups of the substance. Among


the group contribution parameters established by Hoftyzer and Krevelen [20], those related to terpenes are shown in Table 6.

a) Cited from reference [20].

**3. Relationship between solubility parameter and dissolving power of**

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

> coh *E V*

where *E*coh and *V* are the cohesive energy (=vaporization energy) and molar volume of the substance, respectively. The *V* is calculated from the molecular weight and density of the substance. The *E*coh can be obtained experimentally for a volatile substance, but is usually derived from theoretical approach. Hansen [17] considered that *E*coh is consisting of three types

where *E*d, *E*p, and *E*<sup>h</sup> are the energy of dispersion forces, polar forces, and hydrogen bonding, respectively. Then, Equation (1) is modified using the corresponding solubility parameter

dph

Taking account of these intermolecular interactions, Hoftyzer and Krevelen [19] expressed

d h p dp h *VV V*

bonding, respectively, reflecting the contribution of structural groups of the substance. Among

å å <sup>å</sup> == = 2 , , and *<sup>i</sup> i i*

 d

ddd

d

dd

= (1)

=++ coh d p h *E EEE* (2)

= ++ <sup>222</sup> (3)

*F E <sup>F</sup>* (4)

are the parameter of dispersion forces, polar forces, and hydrogen

d

**monoterpenes**

parameter (*δ*) of a substance is defined as:

8 Recycling Materials Based on Environmentally Friendly Techniques

of energies derived from the following interaction forces:

components, *δ* d, *δ* p, and *δ* h, to each force as follows:

their components such as:

where F di

, F pi

, and Eh i b) If two identical polar groups are present in a symmetrical position, the value of *δ* p must be multiplied.

**Table 6.** Group contribution parameters related to terpenes.


**Table 7.** Group contribution parameters of geranyl acetate.

According to Table 6, the group contribution parameters of geranyl acetate are calculated as shown in Table 7. Since the molecular weight (*MW*) and density (*d*) of geranyl acetate are 196.29 g/mol and 0.909 g/cm3 , respectively, the molar volume *V* is estimated to 2.159×10−4 m3 /mol. Therefore, the solubility parameter components are:

d

$$
\begin{aligned}
\boldsymbol{\delta}\_{\text{d}} &= \frac{\boldsymbol{\Sigma} \, \boldsymbol{F}\_{\text{d}\_{\text{r}}}}{V} = \frac{3.42 \, \text{J}^{1/2} \cdot \text{m}^{3/2} \cdot \text{mol}^{-1}}{2.159 \times 10^{-4} \, \text{m}^3 \cdot \text{mol}^{-1}} = 15.8 \, \text{MPa}^{1/2}, \\\\
\boldsymbol{\delta}\_{\text{p}} &= \frac{\sqrt{\sum \boldsymbol{F}\_{\text{p}\_{i}}^{2}}}{V} = \frac{0.490 \, \text{J}^{1/2} \cdot \text{m}^{3/2} \cdot \text{mol}^{-1}}{2.159 \times 10^{-4} \, \text{m}^3 \cdot \text{mol}^{-1}} = 2.27 \, \text{MPa}^{1/2}, \text{ and} \\\\
\boldsymbol{\delta}\_{\text{h}} &= \sqrt{\frac{\sum \boldsymbol{E}\_{\text{h}\_{i}}}{V}} = \sqrt{\frac{7000 \, \text{J} \cdot \text{mol}^{-1}}{2.159 \times 10^{-4} \, \text{m}^3 \cdot \text{mol}^{-1}}} = 5.69 \, \text{MPa}^{1/2}.
\end{aligned}
$$

3 -1

m mol

From these components, the solubility parameter of geranyl acetate is found:

*i*

*V*

2.159 10

$$
\delta = \sqrt{\delta\_\mathrm{d}^2 + \delta\_\mathrm{p}^2 + \delta\_\mathrm{h}^2} = 16.9 \text{ MPa}^{1/2} \text{ J}
$$


The calculated *δ* values of all the terpenes from Table 1 to Table 5 are shown, together with the *MW* and *d*, in Table 8. The *δ* of PS is calculated to be 14.5 MPa1/2 from the structure of a repeating unit. Referring to Table 8, the *δ* values of seven terpenes from *α*-terpinene to *p*cymene are very close (14.7−15.7 MPa1/2), especially the *δ* of *p*-cymene is almost the same (14.6 MPa1/2), to that of PS. This fact is in good agreement with the experimental results in Table 2 that these terpenes, particularly *p*-cymene, dissolve a lot of PS. Although 1,8-cineole and four terpenes from the lower row in Table 8 have similar *δ* values to that of PS, their dissolving powers for PS are low. The reason for such low dissolving powers might be attributable to a steric effect as mentioned above. Hence, it is concluded that a solubility parameter is not universal because it cannot reflect the steric effect of a solvent molecule upon the *δ*. According to the same reason, the *δ* value cannot explain the high dissolving powers of three acyclic terpenoids, geranyl acetate, geranyl formate, and citronellyl acetate. The terpenoids of the alcohols and aldehydes have a reasonable relationship between the *δ* value and dissolving power.



a) Partly cited from references [10] and [14].

3/2 1 <sup>d</sup>

1/2

<sup>p</sup> 3/2 1

9 10

.490 J m mol

 <sup>å</sup> × × <sup>=</sup> == ´ × <sup>2</sup> 1/2

<sup>1</sup> <sup>h</sup>

2.159 10

From these components, the solubility parameter of geranyl acetate is found:

dph = d d + dd

4

d 4 3.42 2.1

> 0 2.15

*i E V*

**Terpenes** *MW d (g/cm3*

*α* -Terpinene 136.24 0.838 14.9 *γ*-Terpinene 136.24 0.853 15.2 *d*-Limonene 136.24 0.840 15.2 Terpinolene 136.24 0.863 15.7 *α* -Phellandrene 136.24 0.846 14.7 *β* -Phellandrene 136.24 0.850 15.0 *p*-Cymene 134.22 0.857 14.6

*F V*

*i F V*

d

10 Recycling Materials Based on Environmentally Friendly Techniques

d

p

d

power.

 J m mol m mol

 <sup>å</sup> × × <sup>=</sup> ´ = = <sup>×</sup>

3 -1


3 -1

m mol


59 10 , *<sup>i</sup>*

3 -1

m mol

000 J mol

 <sup>å</sup> <sup>×</sup> <sup>=</sup> ´ × = = 1/2 <sup>h</sup> <sup>4</sup> 5.69 MPa . <sup>7</sup>

=+<sup>222</sup> 1/2 16.9 MPa .

The calculated *δ* values of all the terpenes from Table 1 to Table 5 are shown, together with the *MW* and *d*, in Table 8. The *δ* of PS is calculated to be 14.5 MPa1/2 from the structure of a repeating unit. Referring to Table 8, the *δ* values of seven terpenes from *α*-terpinene to *p*cymene are very close (14.7−15.7 MPa1/2), especially the *δ* of *p*-cymene is almost the same (14.6 MPa1/2), to that of PS. This fact is in good agreement with the experimental results in Table 2 that these terpenes, particularly *p*-cymene, dissolve a lot of PS. Although 1,8-cineole and four terpenes from the lower row in Table 8 have similar *δ* values to that of PS, their dissolving powers for PS are low. The reason for such low dissolving powers might be attributable to a steric effect as mentioned above. Hence, it is concluded that a solubility parameter is not universal because it cannot reflect the steric effect of a solvent molecule upon the *δ*. According to the same reason, the *δ* value cannot explain the high dissolving powers of three acyclic terpenoids, geranyl acetate, geranyl formate, and citronellyl acetate. The terpenoids of the alcohols and aldehydes have a reasonable relationship between the *δ* value and dissolving


1/2

/2

*) δ (MPa1/2)*

*a*

1

2.27 MPa , and

15.8 MPa

**Table 8.** Solubility parameter of some terpenes calculated by the Hoftyzer and Krevelen method.

#### **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.



a) The average of five times measurements.

b) Partly cited from references [10] and [14].

c) Insoluble.

**Table 9.** Dissolution time and apparent activation of (*E*a) for the dissolution of PS in the terpenes.

Therefore, the dissolution time of PS in each terpene was measured at several different temperatures, and then the apparent activation energy (*E*a) of dissolution was evaluated [10,14]. The experimental results are shown in Table 9. Here, the dissolution time means a time required for the dissolution of 2.30 mg of a PS disk in 0.5 mL of a terpene at each temperature. The *E*a is estimated from the slope of an Arrhenius plot of the logarithm of dissolution time versus the inverse of dissolution temperature. Limonene and its isomers have similar low *E*<sup>a</sup> of ca. 20−25 kJ/mol one another. A group of the subsequent low an *E*a of 25−35 kJ/mol is the acyclic terpenes except for aldehydes in Figure 5. The dissolution rate of this group is relatively fast. The *E*as of *Abies* leaf oil and *Eucalyptus* oil are 34 and 39 kJ/mol, respectively. The alcohols of terpinene-4-ol, *α*-terpineol, and 2-*p*-cymenol have almost 50 kJ/mol or higher of *E*a. The order of *E*a agrees with that of dissolving power for PS well. These results on *E*a suggest that terpinene-4-ol, 2-*p*-cymenol, bornyl acetate, and *α*-pinene are not suitable for practical use as a solvent for PS recycling due to their long dissolution time even though they dissolve PS. To increase the dissolution rate of PS, Noguchi et al. attempted the addition of ethanol to limonene [6]. Although ethanol is not a solvent for PS, a small amount of ethanol gives the viscosity of the PS solution to lower. This method will be effective when the terpenes have a considerable high dissolving power for PS and a high viscosity of the PS solution prevents PS from diffusing in the solution.

### **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 M¯ n of the PS recovered slightly decreased from 1.2×105 to 1.0×105 , and polydispersity of the molecular weight distribution increases from 2.5 to 3.1 [10]. This means that small degradation of PS occurs during steam distillation process. However, in other petroleum-based solvents, further degradation takes place owing to the oxidative scission of PS chains by air [21]. Most terpenes have C=C groups that inhibit PS from oxidative decom‐ position by self-oxidation of the C=C groups. The PS recycled from limonene solutions has almost the same elastic modulus and glass transition temperature [8], indicating that it retains original mechanical properties.

#### **6. Conclusion**

**Terpenes**

a) The average of five times measurements. b) Partly cited from references [10] and [14].

12 Recycling Materials Based on Environmentally Friendly Techniques

c) Insoluble.

**Dissolution Timea**

Terpinene-4-ol –c 4,430 1,810 950 610 59.0 *α* -Terpineol 3,025 1,289 715 418 344 47.7 2-*p*-Cymenol 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 *α* -Pinene –c 1,860 852 600 503 38.5 *β*-Pinene 3,213 690 366 242 142 63.5

**Table 9.** Dissolution time and apparent activation of (*E*a) for the dissolution of PS in the terpenes.

Therefore, the dissolution time of PS in each terpene was measured at several different temperatures, and then the apparent activation energy (*E*a) of dissolution was evaluated [10,14]. The experimental results are shown in Table 9. Here, the dissolution time means a time required for the dissolution of 2.30 mg of a PS disk in 0.5 mL of a terpene at each temperature. The *E*a is estimated from the slope of an Arrhenius plot of the logarithm of dissolution time versus the inverse of dissolution temperature. Limonene and its isomers have similar low *E*<sup>a</sup> of ca. 20−25 kJ/mol one another. A group of the subsequent low an *E*a of 25−35 kJ/mol is the acyclic terpenes except for aldehydes in Figure 5. The dissolution rate of this group is relatively fast. The *E*as of *Abies* leaf oil and *Eucalyptus* oil are 34 and 39 kJ/mol, respectively. The alcohols of terpinene-4-ol, *α*-terpineol, and 2-*p*-cymenol have almost 50 kJ/mol or higher of *E*a. The order of *E*a agrees with that of dissolving power for PS well. These results on *E*a suggest that terpinene-4-ol, 2-*p*-cymenol, bornyl acetate, and *α*-pinene are not suitable for practical use as a solvent for PS recycling due to their long dissolution time even though they dissolve PS. To increase the dissolution rate of PS, Noguchi et al. attempted the addition of ethanol to limonene [6]. Although ethanol is not a solvent for PS, a small amount of ethanol gives the viscosity of the PS solution to lower. This method will be effective when the terpenes have a considerable

**30 °C 40 °C 50 °C 60 °C 70 °C**

 **(sec)**

*Ea (kJ/mol)b*

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 *Abies sachalinensis* and *Eucalyptus* species are of fast-growing and the leaf oils contain many mono‐ terpenes, they will be useful biomass for the solvent of PS recycling.

#### **Acknowledgements**

Some terpenes were kindly gifted from Tokyo Chemical Industry, Inc., and Toyotama International, Inc. The author gratefully acknowledges both companies.

#### **Author details**

Kazuyuki Hattori\*

Address all correspondence to: hattori@chem.kitami-it.ac.jp

Department of Biological and Environmental Chemistry, Kitami Institute of Technology, Koen-cho, Kitami, Japan

#### **References**


[10] Hattori K, Naito S, Yamauchi K, Nakatani H, Yoshida T, Saito S, Aoyama M, Miya‐ koshi T. Solubilization of Polystyrene into Monoterpenes. Advances in Polymer Technology 2008; 27(1) 35-39.

**Acknowledgements**

14 Recycling Materials Based on Environmentally Friendly Techniques

**Author details**

Kazuyuki Hattori\*

**References**

Koen-cho, Kitami, Japan

p657-678.

ers; 1992.

[4] Moore LA. US Patent 5,300,267, 1994.

[5] Nagamatsu T. Japanese Patent 10-219024, 1998.

Technology and Science 1998; 11(1) 39-44.

of Food Science 1969; 34(6) 610-611.

Packaging Technology and Science 1998; 11(1) 19-27.

Some terpenes were kindly gifted from Tokyo Chemical Industry, Inc., and Toyotama

Department of Biological and Environmental Chemistry, Kitami Institute of Technology,

[1] United Nations Statics Division. http://unstats.un.org/unsd/databases.htm.

[2] Khait K. Recycling, Plastics. In: Kroschwitz JI. (ed.) Encyclopedia of Polymer Science and Technology, 3rd ed. vol. 7, Hoboken, New Jersey: John Wiley & Sons; 2003.

[3] Ehrig RJ., editor. Plastics Recycling Products and Processes. Munich: Hanser Publish‐

[6] Noguchi T, Miyashita M, Inagaki Y, Watanabe H. A New Recycling System for Ex‐ panded Polystyrene using a Natural Solvent. Part 1. A New Recycling Technique.

[7] Noguchi T, Inagaki Y, Miyashita M, Watanabe H. A New Recycling System for Ex‐ panded Polystyrene using a Natural Solvent. Part 2. Development of a Prototype

[8] Noguchi T, Tomita H, Satake K, Watanabe H. A New Recycling System for Expand‐ ed Polystyrene using a Natural Solvent. Part 3. Life Cycle Assessment. Packaging

[9] Coleman RL, Lund ED, Moshonas MG. Composition of Orange Essence Oil. Journal

Production System. Packaging Technology and Science 1998; 11(1) 29-37.

International, Inc. The author gratefully acknowledges both companies.

Address all correspondence to: hattori@chem.kitami-it.ac.jp

