Essential Oils from Citrus reticulata cv. Shatangju Peel: Optimization of Hydrodistillation Extraction by Response Surface Methodology and Evaluation of Their Specific Adhesive Effect to Polystyrene.

Essential oil (Eo) from the Citrus reticulata peel has been widely used, and its adhesive effect on polystyrene (PS) was found accidentally. To analyze the essential oils of C. reticulata cv. Shatangju (CrspEos), the extraction of these oils by steam distillation was optimized using the response surface method. The chemical composition of CrspEos was analyzed by gas chromatography-mass spectrometry. Then, the adhesive effect of these essential oils on PS was evaluated. The adhesion area, the influence of adhesion on the thickness of the essential oil on the materials, the effect of adhesion on the transmittance of PS, the strength of adhesion point, and specificity of adhesion were determined. The optimum extraction conditions resulting in the extraction yield of 47.37 μL g-1 were a ratio of liquid-to-solid of 8.94:1, a soaking time of 199.45 min, and an extraction time of 138.71 min. The major component in the essential oils was d-limonene (56.66%), followed by myrcene (6.62%). CrspEos presented a specific adherence effect on PS without influencing the thickness and transmittance of PS but with stronger tenacity than the parent material. CrspEos can be used as an environmentally friendly specific adhesive for PS.

Introduction

Polystyrene (PS) has excellent optical properties, with a transmittance of 88–92% and a refractive index of 1.59–1.60. This polymer can pass through all wavelengths of visible light. PS is easy to process and form, transparent, inexpensive, rigid, insulative, and good printing applications. This material is widely used as optical chemical instrument parts in lighting instruction, lampshades in daily decoration, a transparent film in packaging, and various meter shell and capacitor dielectric layer in the electrical appliance industry.[1−3] Similar to other plastic products, damaged PS products can be repaired using an adhesive.

Adhesives are derived from synthetic and natural resources, but synthetic adhesives still dominate this market. The preference for more environmentally friendly, naturally derived materials is driven by the strict government regulations to reduce and even eliminate materials that are slightly toxic and enhance consumer awareness. The majority of reported natural adhesives are for wood and cellulose, including tannin, lignin, protein, carbohydrate, and unsaturated oil. Only two seed oil derivatives have been translated recently as wood adhesives. For example, linseed oil has been applied to prepare a resin that can be used as an adhesive or surface-coating material.[4,5] No naturally derived adhesive but two specific synthetic adhesives have been reported for PS. One adhesive contains isoamyl acetate, chloroform, acetone, polystyrene waste (w/w/w/w, 60:13:20:7), while the other type consists of polyethylene (high boiling substance at 40–60 °C), isooctyl acetate, chloroform, and polystyrene (w/w/w/w, 50:20:13:7).[6]

The essential oils (Eos) in orange peel have various activities with an orange flavor. These Eos are colorless or slightly yellow and account for 1.06–4.62% of the total weight of the dry peel.[7] The extraction, chemical composition, and biological activity of the Eos from Citrus reticulata Blanco and Fortunella margarita (Lour.) Swingle peels have been reported. Liu et al.8 optimized the extraction of Eos using response surface methodology (RSM). The results showed an oil yield of 2.14% in a liquid-to-material ratio of 8.4:1 (mL g–1), sodium chloride concentration of 5.3%, and distillation time of 3.5 h. A total of 39 different components had been identified by gas chromatography–mass spectrometry (GC–MS) analysis.[8] With the use of CO2 supercritical fluid extraction, Mira et al. found that temperature and pressure could affect the Eos composition.[9] Hosni analyzed the components of the products from four types of citrus peels, and the results showed that monoterpenoid hydrocarbons accounted for 97.59–99.3%, which included 92.52–97.3% of limonene and 1.37–1.82% of β-pinene.[7] Wang reported that the Eos from orange peel can be used to recover expanded PS (EPS).[10] Limonene, which is a component of citrus Eos, is a pioneer of natural solvents for EPS.[11−13]d-Limonene, with the function of recycling PS, is used to prepare nanocomposite polymers. This component is also used as a monomer that undergoes UV-catalyzed thiol-ene polymerization reactions with polythiol comonomers to produce polymeric products consisting of precipitated PS phases dispersed throughout the elastomeric poly (thioether) networks.[14]

The Eos from the peel of C. reticulata cv. Shatangju (Crsp) is rarely reported. In this study, the response surface method was used to optimize the extraction of the Eos from Crsp (CrspEos) by steam distillation. The chemical composition and amount of CrspEos were analyzed by gas chromatography–mass spectrometry (GC–MS). The adhesive effect of CrspEos on PS was also evaluated.

Results and Discussion

Single-Factor Experiment

The ratio of liquid-to-solid, soaking time, and extraction time were selected as targets in the single-factor experiments. The other variables were controlled, and the effects of each factor on the extraction of CrspEos were determined.

With the ratio of liquid-to-solid of 6:1–10:1, soaking time of 0 min, and the extraction time of 60 min, it was found that as the ratio of liquid-to-solid increased, the yield of Eos increased gradually, reached 45.00 ± 1.11 μL g–1 at 9:1, and then decreased (Figure a). Therefore, the optimum ratio of liquid-to-solid was set as 9:1.

Figure 1

Effects of different factors on the extraction of CrspEos: (a) liquid-to-solid ratio; (b) soaking time (min); and (c) extraction time (min). Data are shown as mean ± standard deviation (SD) (n = 3).

Soaking time was also an influencing factor in the extraction of Eos. Figure b shows the results when the ratio of liquid-to-solid and extraction time were controlled at 7:1 and 60 min, respectively; therefore the soaking time was set in the range of 0–240 min. The extraction yield of CrspEos (YCrspEos) significantly increased to 37.81 ± 0.81 μL g–1 when the soaking time was increased from 0 to 180 min but decreased from 180 to 240 min.

Different extraction times were set to 30, 60, 90, 120, and 150 min with the ratio of liquid-to-solid of 7:1 and the soaking time of 0 min. YCrspEos was enhanced obviously to 46.80 ± 1.51 μL g–1 within the extraction time range of 30–120 min and then decreased (Figure c).

RSM Analysis

The designed and experimental data of the ratio of liquid-to-solid (X1), soaking time (X2), and extraction time (X3) are shown in Table . The 20 experimental runs, including 8 (23) factor points, 6 extra points (star points), and 6 center points, are also listed in Table . Six replicates of the center point were used to evaluate the experimental errors and the repeatability of the calculation method.[15] The quadratic polynomial regression model of the three factors was generated as followswhere Y is the yield of CrspEos (μL g–1), and X1, X2, and X3 are the coded variables for the ratio of liquid-to-solid, soaking time, and extraction time, respectively.

Table 1

Central Composite Design Matrix of the Three Variables—Liquid-to-Solid Ratio (X1), Soaking Time (X2), and Extraction Time (X3)—for the Extraction of CrspEos

				extraction yield (μL g–1)
no.	X1aR	X2bS	X3cE	predicted	experimental
1	–1	1	1	45.85	46.00
2	–1	1	–1	40.06	40.67
3	–1	–1	1	38.87	39.33
4	–1	–1	–1	36.58	36.67
5	1	1	1	42.70	43.33
6	1	1	–1	40.75	41.00
7	1	–1	1	41.55	41.67
8	1	–1	–1	43.09	43.67
9	1.682	0	0	43.26	42.67
10	–1.682	0	0	40.43	40.00
11	0	1.682	0	40.96	40.33
12	0	–1.682	0	37.06	36.67
13	0	0	1.682	45.46	45.00
14	0	0	–1.682	41.89	41.33
15	0	0	0	46.86	46.67
16	0	0	0	46.86	46.33
17	0	0	0	46.86	46.67
18	0	0	0	46.86	47.00
19	0	0	0	46.86	47.67
20	0	0	0	46.86	47.00

atio of liquid-to-solid (mL g–1); the five levels (−1.682, −1, 0, 1, 1.682) correspond to 7.32:1, 8:1, 9:1, 10:1, and 10.68:1, respectively.

oaking time (min); the five levels (−1.682, −1, 0, 1, 1.682) correspond to 79.09, 120, 180, 240, and 280.91, respectively.

xtraction time (min); the five levels (−1.682, −1, 0, 1, 1.682) correspond to 69.55, 90, 120, 150, and 170.45, respectively.

The findings from the analysis of variance (ANOVA) of the experimental results are shown in Table . The values of F (61.76) and P < 0.0001 indicated that the quadratic regression equation was ideal, and the model was highly significant. The linear coefficients (X1, X2, and X3), interaction coefficients (X1X2, X1X3, and X2X3), and quadratic coefficients (X12, X22, and X32) had significant effects (P < 0.01). The influence of the conditions on the CrspEos yield followed the order of soaking time (X2, min), extraction time (X3, min), and the ratio of liquid-to-solid (X1). The P-value of the lack of fit was 0.1349 (>0.05), indicating that the experimental data adapted to the model.

Table 2

Variance Analysis of the Extracted Equation of CrspEos

source	sum of squares	df	mean square	F value	P-value prob > F
model	225.1686	9	25.01873	61.7612	<0.0001	significant
X1	9.667599	1	9.667599	23.86542	0.0006
X2	18.31503	1	18.31503	45.21245	<0.0001
X3	15.3786	1	15.3786	37.96358	0.0001
X1X2	17.0528	1	17.0528	42.09651	<0.0001
X1X3	7.33445	1	7.33445	18.10581	0.0017
X2X3	6.125	1	6.125	15.12016	0.0030
X12	45.35916	1	45.35916	111.9735	<0.0001
X22	111.0908	1	111.0908	274.2386	<0.0001
X32	18.30775	1	18.30775	45.19448	<0.0001
residual	4.050882	10	0.405088
lack of fit	3.007882	5	0.601576	2.883875	0.1349	not significant
pure error	1.043	5	0.2086
cor total	229.2195	19
R2	0.982327
adj R2	0.966422
pred R2	0.891967
adeq precision	22.8436	C·V%	1.48

The residual-standard values in Figure were all in ±3 intervals, suggesting that the model was consistent with the experimental data with no error recorded.[16] The adequate precision was 22.8436, which is more than 4, indicating an adequate signal and the suitability of this model to be applied in the navigation-design space. Thus, this model can be used to navigate the design space. The “Pred R-Squared” of 0.8920 reasonably agreed with the “Adj R-Squared” of 0.9664. The two values indicated the total variability of the model response, and the determined value of the coefficient indicated the model accuracy.

Figure 2

Studentized residuals and the predicted response plot.

Figure shows the response surface (three-dimensional (3D)) for YCrspEos as a function of various ratios of liquid-to-solid and soaking times at a fixed extraction time of 138.73 min. YCrspEos increased rapidly when the ratio of liquid-to-solid was increased from 8 to 9 but decreased rapidly when the ratio of liquid-to-solid increased beyond 9. Moreover, YCrspEos increased with increased soaking time from 120 to 210 min and then decreased from 210 to 240 min. In theory, soaking can lead to the expansion of the tissue cells and enlargement of the intercellular space. The phenomena accelerate the dynamic exchange of the internal and external liquids of the cells and facilitate the extraction of the Eos.[17] When the soaking time is too long, pectin and other components in the raw materials will be dissolved, resulting in the emulsifying effect, thereby inhibiting the distillation of Eos. These events will lead to a decrease in the oil yield.[18] The 3D response surface plots in Figure show the effects of the ratio of liquid-to-solid and the extraction time. It was found that YCrspEos improved first with the increase of the ratio of liquid-to-solid from 8 to 9 or the extraction time from 90 to 138 min, but did not increase significantly with further increase in either parameter. Increasing the ratio of liquid-to-solid is favorable for the diffusion and dissolution of the Eos, and the mass transfer rate also increases. When the ratio of liquid-to-solid is too high, the heating time will be longer and the amount of Eos generated at the same time will be reduced, and energy will be wasted.[19] Increasing the soaking time or extraction time enhanced YCrspEos in the initial stage of the reaction (Figure ). According to Ahmed, further increase in the extraction time increases the oil yield, but the oil quality deteriorates beyond 3 h.[20] The optimized conditions for the experiment were the ratio of liquid-to-solid of 8.94, soaking time of 199.49 min, and extraction time of 138.76 min. An additional run was conducted to evaluate the accuracy of the model. Under the optimized extraction conditions, the predicted and actual values (mean of three measurements) of the Eos yield were 47.35 and 47.37 μL g–1, respectively. The agreement between the predicted and actual values indicated that the model was adequate for the extraction process.

Figure 3

Response surface (3D) showing the effects of the ratio of liquid-to-solid vs soaking time (min) on the yield of CrspEos.

Figure 4

Response surface (3D) showing the effects of the ratio of liquid-to-solid vs extraction time (min) on the yield of CrspEos.

Figure 5

Response surface (3D) showing the effects of soaking time (min) vs extraction time (min) on the yield of CrspEos.

Eos Yield and Characterization

As shown in Figure S1, the components of CrspEos were analyzed by GC–MS, and 26 components were identified. The Eos mostly consisted of terpenoid and aldehyde groups, including d-limonene (56.66%), myrcene (6.62%), γ-terpinene (5.69%), linalool (3.13%), m-cymene (3.04%), and β-citronellol (2.76%). These six components accounted for 78.1% of the total Eos (Table ). The chemical constituents of the Eos from the orange peel were analyzed by GC–MS. The terpenoid and aldehyde groups were the main components of the Eos from the orange peel, but other components varied slightly.[8,21] This result may be due to the fact that the oranges used were obtained from different places and production seasons.

Table 3

Relative Percentage Composition of CrspEos

no.	compounds	RIaR	molecular formula	area (%)
1	α-thujene	924	C10H16	0.13
2	(+)-α-pinene	948	C10H16	1.41
3	sabinene	984	C10H16	1.07
4	β-pinene	985	C10H16	0.47
5	myrcene	993	C10H16	6.62
6	octanal	995	C8H16O	1.85
7	α-terpinene	998	C10H16	0.67
8	m-cymene	1025	C10H14	3.04
9	d-limonene	1028	C10H16	56.66
10	nd	1033	ND	0.41
11	β-cis-ocimene	1040	C10H16	0.77
12	γ-terpinene	1056	C10H16	5.69
13	terpinolene	1083	C10H16	0.99
14	linalool	1090	C10H18O	3.13
15	1-nonanal	1095	C9H18O	0.67
16	β-terpineol	1123	C10H18O	0.92
17	citronellal	1125	C10H18O	1.57
18	terpinen-4-ol	1170	C10H18O	1.55
19	α-terpineol	1175	C10H18O	1.55
20	dihydrocarveol	1177	C10H18O	0.50
21	trans-carveol	1189	C10H16O	1.27
22	decanal	1192	C10H20O	2.20
23	1-p-menthen-9-al	1217	C10H16O	0.46
24	lemonol	1221	C10H18O	0.94
25	β-citronellol	1231	C10H20O	2.76
26	l(-)-carvone	1254	C10H14O	1.06
27	perillaldehyde	1255	C10H14O	1.65

etention indices determined on the Rxi-5 column, using the homologous series of n-alkanes (C8–C14).

nd: not determined.

Adhesive Effect of CrspEos on PS

The adhesive effect of CrspEos on PS was discovered accidentally during an experiment when the CrspEos was dropped on the surface of the PS cover of 96-well plates (the smooth surface). To investigate in detail the adhesive effect, the adhesion area of the Eos, influence of adhesion on the thickness of the Eos on the materials, effect of adhesion on the transmittance of PS, strength of adhesion point, and specificity of CrspEos adhesion were measured.

As shown in Figure , the adhesion area was decreased from 159.51 ± 8.03 to 81.12 ± 8.13 mm2 μL–1 Eos with increasing volume of the Eos from 1 to 5 μL. The strain increased from 23.29 ± 0.68 to 31.46 ± 0.53 N, and the larger adhesion of the joint was accompanied with more adhesion strength. The CrspEos could adhere to PS by dissolving. This process can be regarded as immersional wetting in interface chemistry. The surface Gibbs function (ΔG) represents the driving force of the immersional wetting process. A smaller value of ΔG indicates an easier increase in the immersional wetting area. The contact angle θ increased with the enhancement of the liquid volume, and the diminished value of cos θ led to an increase in ΔG. Thus, the adhesion point per unit volume was reduced. Second, the CrspEos dissolved in the PS increased the viscosity, and the poor fluidity reduced the adhesion area per unit volume.[22] With the increase of the volume of CrspEos, the total adhesion area increased, the bonding area became stronger, and the damage of strain increased. The adhesion point was equivalent or had more strength than the parent material as the joint was all destroyed outside the adhesion point by the strain, which illustrated that the strength was sufficient. Some cloudiness was observed at the adhesion point after it was destroyed by external forces (Figure ).

Figure 6

Relationship of adhesion area of CrspEos, the strain for destroying the adhesion points, and the volume of CrspEos.

Figure 7

Damage of polystyrene adhesion points with different volumes of CrspEos by the strain. The adhesion point was adhered by (a) 1 μL of CrspEos; (b) 2 μL of CrspEos; (c) 3 μL of CrspEos; (d) 4 μL of CrspEos; and (e) 5 μL of CrspEos.

To evaluate the influence of adhesion on the thickness of the Eos, the adhesive failure and the change in the thickness were measured. The data can be used as a reference value to ensure the adhesion affect and avoid the squeeze-out effect. The change in the thickness on the PS before and after adhesion is shown in Table . The thickness was not influenced after adhesion with 1, 2, 3, 4, and 5 μL of CrspEos. The thickness of tA1 + tA2 was consistent with that of tA1:A2 (P > 0.05). Although, visual damage was observed at the interface between the Eos and PS, no significant difference was found in the material thickness before and after adhesion. Table shows the transmittance of visible light (wavelengths in the range of 400–800 nm) through the PS material before and after adhesion. T3 was indistinguishable from the theoretical value T2. This phenomenon indicated that the transmittance was not influenced by the adhesive effect (P > 0.01).

Table 4

Thickness of PS before and after Adhesiona

CrspEos (μL)	tA1 + tA2 (mm)	tA1:A2 (mm)	P = 0.37
1	2.42 ± 0.12	2.46 ± 0.12	P = 0.69
2	2.35 ± 0.22	2.43 ± 0.19	P = 0.66
3	2.33 ± 0.24	2.31 ± 0.27	P = 0.93
4	2.39 ± 0.18	2.33 ± 0.16	P = 0.75
5	2.38 ± 0.31	2.36 ± 0.24	P = 0.58

The P-value represented the difference of the thickness of PS before and after adhesion; when P > 0.05, the difference was not statistically significant. t is the thickness of the PS material (A1, A2, and A1:A2).

Table 5

Transmittance (T) of PS before and after Adhesion

wavelength (nm)	T1	T2	T3
400	90.77 ± 1.31b	82.40 ± 2.36b	82.83 ± 2.65b
420	91.73 ± 1.29b	84.16 ± 2.36b	84.40 ± 2.59b
440	92.17 ± 1.31b	84.96 ± 2.40b	85.37 ± 2.71b
460	92.90 ± 1.28b	86.31 ± 2.37b	86.43 ± 2.63b
480	93.00 ± 1.32b	86.50 ± 2.45b	86.77 ± 2.42b
500	93.00 ± 1.35b	86.50 ± 2.50b	86.87 ± 2.59b
520	93.03 ± 1.29b	86.56 ± 2.39b	87.13 ± 2.44b
540	93.03 ± 1.36b	86.56 ± 2.52b	87.30 ± 2.31b
560	92.93 ± 1.34b	86.38 ± 2.49b	87.47 ± 2.25b
580	92.87 ± 1.45b	86.26 ± 2.68b	87.77 ± 2.25b
600	92.97 ± 1.62b	86.45 ± 2.99b	88.17 ± 2.27b
620	92.97 ± 1.71bc	86.45 ± 3.16b	88.57 ± 2.35bc
640	93.10 ± 1.74bc	86.70 ± 3.23b	88.93 ± 2.39bc
660	92.67 ± 2.32bc	85.91 ± 4.28b	89.43 ± 2.29bc
680	92.47 ± 2.28bc	85.54 ± 4.20b	89.27 ± 2.18bc
700	92.90 ± 2.19bc	86.34 ± 4.06b	89.63 ± 2.03bc
720	93.67 ± 2.12bc	87.76 ± 3.96b	90.30 ± 1.97bc
740	95.40 ± 2.04b	91.04 ± 3.88b	92.03 ± 1.96b
760	95.70 ± 1.95b	91.61 ± 3.71b	92.23 ± 2.03b
780	95.60 ± 1.93b	91.42 ± 3.67b	92.10 ± 2.07b
800	95.33 ± 1.86b	90.91 ± 3.52b	91.90 ± 2.07b

T1 is the transmittance of A; T2 is the theoretical transmittance of the A1:A2; theoretical value was calculated based on the value of transmittance of PS according to the Lambert–Beer law, and T3 is the transmittance of the A1:A2.

Means in the same row with different superscript lowercase letters differ significantly (P < 0.05).

The adhesion of CrspEos on the different plastic materials, including poly(ethylene terephthalate) (PET), high-density polyethylene (HDPE), poly(vinyl chloride) (PVC), low-density polyethylene (LDPE), polypropylene (PP), PS, and polycarbonates (PC), were investigated. The adhesion effect presented only in the group of PS/PS illustrated that CrspEos could specifically adhere to PS. The structures of the main components in CrspEos and the precursor of PS, and phenylethylene are shown in Figure . The similar structure may have caused the specificity of CrspEos adhesion. Hattori et al. found that d-limonene, myrcene, γ-terpinene, and m-cymene have dissolving power of nearly 127, 101, 130, and 212 g for PS per 100 g of the component, respectively.[23] Wang et al. reported that Eos from the orange peel can be used to recover the PS foam.[10]

Figure 8

Structure of polystyrene and the main components of CrspEos.

Given that PS is prone to turbidity and yellowing because of the influence of light or dust, adding appropriate antioxidants as anti-aging agents when preparing optical components of PS is necessary.[24] CrspEos has been reported to exhibit excellent antioxidant activities.[21] The reported PS adhesives contained different proportions of restricted chemical compositions. For example, acetone (category III) is listed in the Catalogue of Precursor Chemicals in the United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances,[25] and trichloromethane (category II) is listed in the Catalogue of Precursor Chemicals in the Regulations for the Administration of Precursors and Chemicals used in the Production of Narcotic Drugs and Psychotropic Substances.[26] In this paper, the adhesive effect of CrspEos on PS has no effect on the thickness, transmittance, and specific adhesion, indicating that the plant-derived CrspEos can be used as a candidate of environmentally friendly specific adhesive for PS with potential as an anti-aging agent. According to GB/T 2943-2008, CrspEos can be classified as a vegetable glue.[27]

Materials and Methods

Materials and Reagents

C. reticulate cv. Shatangju, which was produced in Shatangkeng Village Guangxi, was purchased from the campus supermarket and identified by the Department of Pharmacy, School of Pharmacy, Jilin University. The Crsp was dried in the shade for one week at room temperature and smashed with a medicinal pulverizer (particle size, <0.5 mm). All reagents and solvents were of analytical grade. The plastic materials for adhering included poly(ethylene terephthalate) (PET), high-density polyethylene (HDPE), poly(vinyl chloride) (PVC), low-density polyethylene (LDPE), polypropylene (PP), PS, and polycarbonates (PCs).

Experimental Design

The ratio of liquid-to-solid (X1), soaking time (X2, min), and extraction time (X3, min) were selected as single factors to investigate their effects on the extraction yield of CrspEos. The three factors were assumed to be independent of each other. The Eos from dried Crsp (30 g) were extracted by hydrodistillation using a Clevenger-type apparatus. The experimental conditions are listed in Table . CsrpEos were collected and stored in the dark at 4 °C. The yield was calculated as followswhere Y(v/w), VEos, and WCrsp are the Eos yield (v/w, μL g–1), the volume of Eos (μL), and the weight of Csrp (g), respectively.

Table 6

Extraction Conditions of the Single-Element Test

	ratio of liquid-to-solid	soaking time (min)	extraction time (min)
series 1	6:1, 7:1, 8:1, 9:1, 10:1	0	60
series 2	7:1	0, 60, 120, 180, 240	60
series 3	7:1	0	30, 60, 90, 120, 150

Based on the single-factor experiment and according to the central composite design experimental principle,[28] a series of Eos extraction experiments was designed using 30 g of Crsp. A response surface analysis with a three-factor–five-level scheme was formulated using Design Expert 8.0.6.1 (Stat-Ease, Inc, Minneapolis) based on the response of the CrspEos. The independent variables were coded at five levels (−1.682, −1, 0, 1, 1.682). The generalized second-order polynomial model used in the response surface analysis is as followswhere Y is the experimental response (yield of CrspEos); B0 is a constant; B1, B2, and B3 are the linear regression coefficients; B11, B22, and B33 are the quadratic regression coefficients; B12, B13, and B23 are the interactive regression coefficients of the model; and X1, X2, and X3 are the noncoded values for the ratio of liquid-to-solid, soaking time, and extraction time, respectively

Determination of the Chemical Composition of the Eos by GC–MS

GC–MS was performed as previously reported.[29] A GC–MS instrument (Shimadzu GCMS-QP-2010 plus) was employed with the following conditions: Rxi-5MS, 30 m × 0.25 mm × 0.25 μm; helium (99.999%), carrier gas; and flow rate, 1.0 mL min–1. The heating program was set to column temperature at 50 °C (held for 3 min), increased to 100 °C at 50 °C min–1 to 200 °C at 8 °C min–1 and to 290 °C at 100 °C min–1, which was maintained for 10 min. The temperatures of the injector and the interface were 280 and 230 °C, respectively, and the pressure was 117.6 kPa. The Eos components were determined by comparing the Kovats index and mass spectra of known components in the mass spectrometry database (NIST).[30] The relative amount of the compounds was computed from the flame ionization detector (FID) peak areas without applying the FID response factor correction. This value was expressed as a percentage.

Adhesive Effect of CrspEos on PS

Two PS material plates (12.5 cm × 8.5 cm, A1 and A2) were squeezed together for 5 min after the addition of 1–5 μL of Eos between them. Adhesion was as soft as possible to prevent bubbles between the two PS plates. The adhesion plate (A1:A2) was used to measure the adhesion area of the Eos, the influence of adhesion on the thickness of the Eos on the materials, the effect of adhesion on light transmittance, the strength of adhesion point, and the specificity of the Eos adhesion.

Calculation of Adhesion Area of the Eos

The adhesion area (S) formed by the CrspEos on the PS material plates was summed depending on the area of the circle (S1) and the middle figure of four circles (S2) on the PS material. The area of the different volumes of Eos was calculated as followswhere S is the adhesion area of the Eos; d is the diameter of the circle of the 96-well plates; S1 is the area of the circle; S2 is the area between the four circles; and n1 and n2 are the numbers of S1 and S2, respectively.

Influence of Adhesion on the Thickness of Eos on the Materials

The thickness (t) values of the two PS materials (tA1 and tA2) were measured using a Vernier caliper, and then the values were added. After adhesion, the thickness of the PS materials (tA1:A2) was measured. Five points on the material were randomly selected for measurements. The differences between the values of tA1+t and t were analyzed.

Effect of Adhesion on the Transmittance of PS

The transmittance, which follows the Lambert–Beer law, every 20 nm in the visible-light region (400–800 nm) of A1, A2, and A1:A2 was measured using a spectrophotometer. The transmittance was calculated as followswhere T1 is the transmittance of A; T2 is the transmittance of the A1:A2; E is the absorptivity; C is the concentration; and l is the thickness of the PS material.

Strength of Adhesion

The strength of the A1:A2 joints was evaluated according the methods of GB/T2791-1995[31] and GB/T2790-1995[32] with slight modifications. Distilled water (v) was added into a beaker (with known weight, m0), which was connected to A of A1:A2, until the connecting part A2 was separated from A1. The strain (F) that could be endured by the A1:A2 joints was calculated as followswhere v and m0 represent the volume of the distilled water when the A1:A2 joint was separated and the weight of beaker, respectively; ρ is the density of the distilled water; and g is the gravitational acceleration.

Specificity of the Adhesion of CrspEos

To investigate the specificity of the adhesion of CrspEos on PS, six kinds of common plastic materials, namely, PET, HDPE, PVC, LDPE, PP, and PCs, were employed to replace PS.

Statistical Analysis

Adhesion experiments were performed in triplicate. The results were expressed as mean ± standard deviation. The experimental data were statistically tested by ANOVA by performing Duncan’s multiple-range tests. SPSS statistical software version 19.0 was used, and P < 0.05 indicated that the effect was significant.

Conclusions

The optimum conditions for extracting CrspEos by steam distillation were as follows: the ratio of liquid-to-solid of 8.94:1, soaking time of 199.45 min, and extraction time of 138.71 min, which resulted in the extraction yield of 47.37 μL g–1. A total of 26 components were detected by GC–MS, and the Eos were composed mostly of terpenoid and aldehyde groups, including d-limonene (56.66%), myrcene (6.62%), γ-terpinene (5.69%), linalool (3.13%), m-cymene (3.04%), and β-citronellol (2.76%). CrspEos displayed specific adherence effect without affecting the thickness and transmittance of PS but with stronger tenacity than the raw material. CrspEos can be used as an environmentally friendly specific adhesive for PS.