Microcellular natural rubber using supercritical CO2 technology

J. of Supercritical Fluids 149 (2019) 70–78

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Microcellular natural rubber using supercritical CO2 technology W. Tessanan a , P. Phinyocheep a,∗ , P. Daniel b , A. Gibaud b a

Department of Chemistry, Faculty of Science, Mahidol University, Rama VI Road, Payathai, Bangkok 10400, Thailand Institut des Molécules et des Matériaux du Mans (IMMM), UMR CNRS 6283, Le Mans Université, Faculté des Sciences et Technologie, Bd O. Messiaen, 72085 Le Mans, Cedex 09, France b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Microcellular natural rubber is prepared using supercritical CO2 technology. • Cell size and size distribution drop with increasing saturation pressure and time. • Increment of saturation temperature enhances cell size and size distribution. • Crosslinking level of rubber affects on reducing cell size and size distribution.

a r t i c l e

i n f o

Article history: Received 29 December 2018 Received in revised form 12 March 2019 Accepted 28 March 2019 Available online 29 March 2019 Keywords: Batch foaming Cell size distribution Microcellular Natural rubber Supercritical CO2 state

a b s t r a c t Microcellular natural rubber (NR) prepared using a supercritical CO2 technology is a promising alternative to replace traditional foaming agent due to environmental concern. Crucial parameters for the foaming process including CO2 saturation time (30, 60, 90, 120, 180) min, pressure (0, 8.5, 10.5, 12.5) MPa, temperature (45, 55, 65, 85) ◦ C, and crosslinking characteristics of NR (pre-vulcanized time at 15 min and 30 min) were orderly conducted. The results obtained showed a decrement of average cell size (less than 10 ␮m), cell size distribution, and expansion ratio depending on an enhancement of saturation time and pressure. However, the increment of saturation temperature affected the increase in average cell size, cell size distribution, and expansion ratio. In case of the crosslinking behavior, an escalation of the pre-vulcanized time of rubber influenced the decrement in average cell size, cell size distribution and expansion ratio resulting from an elevation of matrix crosslinking. © 2019 Published by Elsevier B.V.

1. Introduction Over the past decades, cellular polymers have been extensively used in many fields, such as transportation, packaging, construction, industry, and agriculture, because of the outstanding features of these advanced materials, such as saving material and light

∗ Corresponding author. E-mail address: [email protected] (P. Phinyocheep). https://doi.org/10.1016/j.supflu.2019.03.022 0896-8446/© 2019 Published by Elsevier B.V.

weight [1–3], thermal insulation [4,5], absorption [6,7], and energy damping [8,9]. In recent years, the growing attention in the supercritical CO2 , as an environmentally friendly alternative foaming agent, has been considered as an ideal and favorable promising way for development of the cellular structural polymer [10,11]. Several studies have explained and discussed the foaming of polymeric materials with this technology for design the polymeric foam used in widespread applications, including special engineering plastic foam, electronic devices, sound and thermal insulators, biomedical engineering and so forth [5,12–34].

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Theoretically, the supercritical CO2 is a substance, in which both temperature and pressure are over the critical point [35]. Its specific behavior exhibits the common aspect between gas and liquid state [36]. The gas-like viscosity makes it has high diffusion rate, while its liquid-like density allows efficiently excellent solubility power. Hence, it has been enormously used to be a solvent and a plasticizer in varieties of application [28,37]. The supercritical CO2 has been taken mainly for producing the cellular materials because it is easy to reach supercritical state (critical temperature, Tc =31 ◦ C and critical pressure, Pc =7.38 MPa) and merely remove off the polymeric material, as well as low cost and green technique [38]. More importantly, the parameters in the foaming process can be precisely controlled to achieve the desired cellular structure [33,39]. Natural rubber (NR), known as a green elastomer, has been extensively obtained from Hevea brasiliensis rubber tree. It composes mainly of the cis-1,4-polyisoprene segment along polymeric chain [40]. NR exhibits many well-known properties, such as high elasticity, excellent strength and biodegradability. It is also considered a green and renewable material [41]. Microcellular NR or NR foam is one of the expanded rubber materials with cell size smaller than 10 ␮m [42]. It has been attracted enormous attention because of having superior performance properties, such as excellent elasticity, lightweight, sound absorption, and good thermal insulation. These unique characteristics make it been employed in latex foam mattress, shoe insoles and wheelchair cushion, thermal insulation, energy absorption and structural appliances [43,44]. Industrially and commercially available natural rubber-based foam have mainly manufactured via two pathways depending on starting rubber form including NR latex as a liquid form [45], and dry NR as a solid form [46]. Most of the cases, chemical blowing agents have been used for fabrication of NR or elastomeric foam. Several types of chemical blowing agents, which have commercially been employed, include oxydibenzenesulfonyl hydrazide (OBSH), azodicarbonamide (ADC) [47], and sodium bicarbonate (NaHCO3 ) [48]. Nevertheless, the concept of chemical blowing agents used in both forms of NR (latex and dry rubber) still has severe drawback from chemical residue remained in the rubber cellular products, leading to toxicity to human and extensive environmental issue for some applications [49]. Furthermore, the chemical foaming agent produces large and uncontrolled bubble sizes, in which the large cellular diameter may decrease mechanical properties, hence limited applications [27]. Up to now, there has been a little attention to prepare cellular elastomeric materials using supercritical CO2 foaming technique since an extra parameter of crosslink characteristic of the elastomer has to be considered in addition to supercritical CO2 variable conditions [19,22,28]. The balance of cross-linking behavior and cellular growth derived from CO2 saturation has to be considered significantly for production of rubber foam using a green supercritical CO2 foaming agent. No previous literature has been reported on the preparation of microcellular NR through this approach. The use of supercritical CO2 technology will be environmentally friendly in using non-hazardous chemicals and also the use of waste CO2 produced from industries will be also additionally solved. As a consequence, this study aims mainly at investigating the use of supercritical CO2 technology for preparation of microcellular material derived from natural rubber. The critical foaming parameters including CO2 saturation time, pressure, and temperature were substantially investigated. Additionally, crosslinking characteristic conducting from the pre-vulcanized time of rubber was also assessed.

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Table 1 Formulation of natural rubber latex compounding used in the study. Ingredients

%TSC*

Amounts (phr)

High ammonia NR (HANR) latex Zinc oxide (ZnO) dispersion Zinc diethyl dithiocarbamate (ZDC) dispersion Zinc 2-mercaptobenzothiazole (ZMBT) dispersion Sulphur dispersion

60 50 50 50 50

100 3 0.5 0.5 1.5

*

TSC = Total solid content.

Fig. 1. Schematic diagram of a batch foaming equipment used in this study.

2. Material and methods 2.1. Materials Commercial high ammonia concentrated natural rubber (HANR) latex containing 60% dried rubber content (DRC) was provided by Thai rubber latex Co. Ltd., Thailand). 50% Sulfur (S) suspension was used as a vulcanizing agent. 50% zinc-2-mercaptobenzothiazole (ZMBT) and 50% zinc diethyl dithiocarbamate (ZDC) were employed as an accelerator. 50% Zinc oxide (ZnO) was used as an activator. All of the chemical curing agents were commercial grade and supplied by Lucky four Co., Ltd., Thailand. Carbon dioxide (CO2 ) gas (purity: 99.9% w/w) was purchased from Linde, Germany. 2.2. Latex compounding Firstly, HANR latex was stabilized with Tergitol (non-ionic surfactant) by 60 min stirring time. Subsequently, all of the aqueous curatives, which were commonly prepared by a ball milling, were mixed with HANR latex by 60 min mixing time conforming to ASTM D1076: 2010. Formulation of all ingredients was presented in Table 1. NR latex compound was poured into a glass plate and dried under ambient temperature for seven days. The dried rubber sheet was partially vulcanized at 100 ◦ C with a series of time (15 min and 35 min) using a hot oven. This process was depicted as the pre-curing step or pre-vulcanized step. 2.3. Preparation of microcellular natural rubber The pre-vulcanized NR sheet was cut into a small rectangular specimen with a dimension of 5 mm × 15 mm × 2 mm. Later, it was placed into a high pressure autoclave (Separex-France Cellule 60 mL) equipped with a CO2 tank and then immediately fed with CO2 into the autoclave. A schematic diagram of a batch foaming equipment used in this study was displayed in Fig. 1. At the end of the foaming process, the pressure within the autoclave was rapidly

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Fig. 2. Schematic diagram for preparation of natural rubber foam.

released to atmospheric pressure. And then, the expanded rubber specimen was immediately taken out the autoclave and placed it into a hot oven at 150 ◦ C for 20 min for post-vulcanization process, and this stage was defined as the post-vulcanized step. The procedure and mechanism diagram for preparation of cellular NR is shown in Fig. 2 that was modified from the technique of Hong et al. [28] and Liao et al. [22]. Three rubber samples were used for preparation of microcellular rubber for each CO2 saturation condition. The system of this study was orderly carried out with given foaming conditions as follows: CO2 saturation time at (30, 60, 90, 120, 180) min, saturation pressure at (0, 8.5, 10.5, 12.5) MPa, and saturation temperature at (45, 55, 65, 85) ◦ C. Additionally, NR sheet without foaming process was used as a control sample to successfully confirm preparation of microcellular rubber by this technology. 2.4. Characterization 2.4.1. Microcellular structural morphology Morphologies of cellular rubber were observed using a scanning electron microscope (SEM) (JEOL JSM-6510LV) with an accelerating voltage of 15 kV. All of the samples were generally immersed in liquid nitrogen around 30 min before cracking. The obtained cryogenic fractured specimen was then sputter-coated uniformly with gold before SEM observation. 2.4.2. Average cell size and cell size distribution Average cell size and cell size distribution of samples were determined from SEM micrographs operated with specific software of JEOL Company (SemAfore 5.2). 100 different cells were measured. Three SEM images in each CO2 saturation condition were investigated for reproducibility purpose. 2.4.3. Volume expansion ratio The expansion volume of microcellular natural rubber was determined from NR density before and after the foaming process. The density was measured using water displacement approach, based on the ASTM D792. The volume expansion ratio (VER) can be calculated by Eq. (1) [50]. Volume expansion ratio(VER) =

 f

(1)

Where  and f are the densities of NR without foaming and NR foam, respectively. Density of materials in this study was determined using a densimeter (MD-200S). 3. Results and discussion Microcellular materials obtained by supercritical CO2 , a green physical blowing agent, has received increasing attention since it will not concern the environmental issue compared with the use of chemical blowing agents. In addition, it is practically possible

to control cellular morphology (cell size and cell size distribution) of thermoplastic material by adjusting process parameters as the following: (a) CO2 saturation time which is the contact time for CO2 saturation during foaming. (b) CO2 saturation pressure which is the pressure of supercritical CO2 during the foaming process (c) CO2 saturation temperature which is the temperature of the system during foaming. In the case of rubbery material, the crosslinking characteristic is, additionally important parameter concerned for the foaming process since it has not only important role to the final properties but also the cell size characteristic. All these four parameters will be carefully investigated. The crosslinked or prevulcanized NR rubber sheet with 15 min pre-vulcanized time was firstly used to study the CO2 parameters and 20 min post-cure time. Considering the SEM micrographs (Fig. 3), the morphological appearance of the NR sheet revealed the occurrence of microcellular structure after the introduction of supercritical fluid state of CO2 when compared with NR compound without foaming process (Fig. 3a). It can be proposed the successful preparation of microcellular natural rubber using supercritical CO2 technology. Moreover, cellular morphology showed the irregular shape possibly due to the elastic property of the partially crosslinked rubber. 3.1. Effect of CO2 saturation time Generally, saturation time is one of vital parameters for the foaming process using supercritical CO2 technology. In this study, the equilibrium of saturation time or soaking time for CO2 distribution and dissolving into NR matrix was firstly determined before studying on the other parameters. The condition of this part was performed at given saturation pressure and temperature (P =10.5 MPa and T =55 ◦ C) using 15 min pre-vulcanized rubber sheet. Saturation time was conducted in the range of (30–180) min. The data of average cell size measured from SEM image (Fig. 4) was listed in Table 2, showing reproducibility of the microcellular rubber by having low standard deviation (low uncertainty value). It can be seen in Table 2 and Fig. 4k that the average cell size became a decrement in diameter from 17.31 ␮m to (9.28, 8.99, 8.32, 8.93) ␮m while the volume expansion ratio (Fig. 4l) tended to reduce with an enhancement of soaking time from 30 min to (60, 90, 120, 180) min, respectively. In term of the size distribution, Fig. 4f–j revealed obviously that the cell size distribution is narrower with an escalation of saturation time. Consequently, it could be attributed to an increment of soaking time that promoted the degree of CO2 sorption in NR matrix resulting on the enhancement of cell nucleation site and the cell growth is limited under the restricted space of crosslinked NR sheet, leading to the formation of smaller cell size [31]. Furthermore, it might be noted that there is no significant difference in the cell size after 60 min soaking time. This revealed that the CO2 sorption is reached the equilibrium after 60 min and, therefore, this duration time was chosen as an optimum period for study on the other foaming conditions. Nevertheless, coalescent bubbles were

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Fig. 3. SEM micrographs of (a) NR before foaming process and (b) microcellular NR prepared by supercritical CO2 technology.

Fig. 4. SEM micrographs and cell size distributions of microcellular natural rubber prepared with different saturation time: (a, f) 30 min, (b, g) 60 min, (c, h) 90 min, (d, i) 120 min and (e, j) 180 min, as well as (k) cell size and (l) expansion ratio (P =10.5 MPa and T =55 ◦ C).

Table 2 Cell size of microcellular NR prepared by varying CO2 saturation time. Cell size (␮m) Sample

NR foam-30min NR foam-60min NR foam-90min NR foam-120min NR foam-180min

Sample 1

Sample 2

Sample 3

Average

SD

Average

SD

Average

SD

17.32 9.29 8.99 8.26 8.94

2.29 0.99 1.16 1.59 1.12

17.20 9.28 9.00 8.32 8.92

2.13 1.00 1.02 1.29 1.09

17.43 9.25 8.97 8.38 8.93

1.42 1.01 1.17 1.57 1.21

Average cell size (␮m)

Reproducibility

17.32 9.28 8.99 8.32 8.93

0.13 0.02 0.03 0.07 0.01

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also found from the fusion of adjacent small cell foams to form a larger cellular structure. Compared with the use of chemical blowing agent, the cell diameter of bubbles prepared by this technique with varying CO2 saturation time is still smaller than 20 ␮m. These results seemed to be possible for development of NR foam with considerably high mechanical properties [27]. 3.2. Effect of CO2 saturation pressure Supercritical CO2 saturation pressure is also one of the crucial influence parameters for controllable cellular microstructure. The morphology of foams has revealed in several reports, which is consistent with not only the classical homogeneous nucleation theory [51–53] but also the classical heterogeneous nucleation theory [14,54–56]. In this research part, the processing condition (T =55 ◦ C and t =60 min) was substantially chosen and performed at various pressures (0, 8.5, 10.5, 12.5) MPa. The size of bubbles measured from three SEM images of different CO2 saturation pressure are summarized in Table 3. The results disclosed clearly a low standard deviation (low uncertainty) of cell size, confirming the reproducibility of this technology. The results from SEM images (Fig. 5) shown in Table 3 displayed a gradual decrement of average cell size from 13.84 ␮m to (9.35 and 7.08) ␮m by increasing saturation pressure from 8.5 MPa to (10.5 and 12.5) MPa, respectively. The volume expansion ratio of the rubber sheets tended to be also reduced with an escalation of saturation pressure (Fig. 5h). Moreover, cell size distribution was relatively narrower with an introduction of higher saturation pressure (Fig. 3d–f). These results could be theoretically explained by the classical nucleation theory in cases of homogeneous nucleation theory [51–53] and heterogeneous nucleation theory [14,54–56]. In the light way of homogeneous nucleation theory as expressed by Colton and Suh [57], the homogeneous nucleation (NHOM ) is presented in Eq. (2) and the barrier energy or activation energy for nucleation (G∗ ) is displayed in Eq. (3). NHOM = C0 f0 exp G∗ =

−G∗ KT

16 3 3(Ps − P0 )2

(2) (3)

Where C0 is gas molecule density (cm3 /g), f0 is frequency constant of the clustering gas molecules around the nuclei, K is the Boltzmann constant, T is the temperature,  is the surface formation energy, Ps is the saturation pressure (Pa), and P0 is the environmental pressure (Pa).As seen on theory, CO2 sorption level tends to be higher with the increment of pressure. The relationship between the two equations above exhibits that G∗ lowered with the introduction of higher saturation pressure resulting in an increment in NHOM . It can be simply explained that the activation energy for nucleation process was reduced with increasing pressure, leading to a higher nucleation density. With a large amount of cell nuclei generated and simultaneously grew in the restricted area, therefore, the size of bubble is quite smaller and more uniform [30]. In the case of heterogeneous nucleation theory, the solid-state curative agents in the NR matrix should be focused. It might be acted as the nucleating agents that can significantly enhance the heterogeneous nucleation site for fabrication of well-defined microcellular NR in the foaming process by supercritical CO2 technology. In accordance with classical heterogeneous nucleation, the existence of nucleating agents or impurities, which is similar to an island, could induce a mass of CO2 aggregation on its surface. The change of local stress between the surface of island phase and matrix can be easily occur during the releasing pressure process and, subsequently, persuade the formation of a large amount of

nucleation sites, increasing enormously the nucleation rate. Apparently, a large number of cell nuclei over the limited space restrained the cell growth resulting on the fabrication of smaller cell size with a much more uniform cell size distribution [13,14,56]. Fig. 6 proposed the schematic diagram of the nucleation process during foaming of microcellular NR. However, it is also found some coalescence from adjacent bubbles resulting in an occurrence of prominent cellular within the rubber foam. Furthermore, considered on the size of bubble with varying saturation pressure, this technique showed the formation of cell diameter less than 20 ␮m. Generally, NR foam prepared by the use chemical blowing agent has average cell size more than 100 ␮m both in the preparation of rubber foam from the latex form [45] and the solid form [48]. 3.3. Effect of CO2 saturation temperature Saturation temperature is another parameter concerning during the foaming process using a supercritical CO2 system. In this section, 15 min pre-vulcanized rubber was chosen as the previous studied foaming process and proceeded at a given saturation pressure and time (P =10.5 MPa and t =60 min). Saturation temperature was orderly varied at (45, 55, 65, 85) ◦ C, respectively. SEM images in Fig. 7 revealed obviously the formation of larger cellular size, non-uniform cell, as well as some cellular coalescence with an increment of CO2 saturation temperature. Table 4 listed the data of cell size and standard deviation of microcellular rubbers measured from SEM images. The average cell size tended to be increased from 6.27 ␮m to (9.38, 10.62, 16.52) ␮m with the increment of saturation temperature from 45 ◦ C to (55, 65, 85) ◦ C, respectively. Furthermore, the volume expansion ratio showed the increment trend with an enhancement of saturation temperature (Fig. 7j). The representatively higher temperature provided wider cell size distribution than lower temperature (Fig. 7e–h). These results could be understandably described by the classical nucleation theory [15,16]. The diffusivity (D) of CO2 could account for cell nucleation rate at a given saturation temperature due to CO2 diffusivity is a function of saturation temperature. It can be ascribed by Arrhenius relation using the following Eq. (4) [16].

 E  D

D = D0 exp −

RT

(4)

Where D0 is the diffusion coefficient constant, ED is the activation energy for diffusion of gas in a polymer, R is the gas constant, and T is the saturation temperature. As already mentioned in the equation, an increasing in temperature influences an increment of CO2 diffusivity throughout the polymer matrix. The solubility of CO2 in polymer matrix was declined and the generation of cell nucleation site is importantly limited. Therefore, it was a much more space for cell growth leading to the formation of bigger cell size and cell size distribution. Moreover, it could explain this phenomenon in such a way of viscoelastic change on polymeric materials. There is a general reduction on the viscosity and melt strength of polymer with an escalation of temperature. Consequently, the resistance of polymer matrix to the cell growth is decreased that would lead to the bigger cell size and size distribution with high expansion ratio [22,58,59]. However, this technique with varying saturation temperature, the cell size is still smaller than 20 ␮m with good reproducibility manner when compared with the use of chemical blowing that the cell size is higher than 100 ␮m [45,48]. 3.4. Effect of the pre-vulcanized time of rubber The degree of curing or crosslinking of the rubber materials might influence on the penetration and swelling behavior of the

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Table 3 Cell size of microcellular NR prepared by varying CO2 saturation pressure. Cell size (␮m) Sample

NR foam-8.5MPa NR foam-10.5MPa NR foam-12.5MPa

Sample 1

Sample 2

Sample 3

Average

SD

Average

SD

Average

SD

13.69 9.38 7.18

1.94 1.08 0.75

13.95 9.31 7.08

1.91 1.11 0.62

13.84 9.35 6.99

1.81 1.04 0.71

Average cell size (␮m)

Reproducibility

13.84 9.35 7.08

0.12 0.09 0.09

Fig. 5. SEM micrographs and cell size distributions of microcellular natural rubber prepared with different saturation pressure: (a, d) 8.5 MPa, (b, e) 10.5 MPa, and (c, f) 12.5 MPa, as well as (g) cell size and (h) expansion ratio (T =55 ◦ C and t =60 min).

Table 4 Cell size of microcellular NR prepared by varying CO2 saturation temperature. Cell size (␮m) Sample

NR foam-45C NR foam-55C NR foam-65C NR foam-85C

Sample 1

Sample 2

Sample 3

Average

SD

Average

SD

Average

SD

6.27 9.35 10.91 16.60

0.80 1.06 1.26 2.36

6.23 9.42 10.62 16.52

0.77 1.03 1.14 2.31

6.31 9.38 10.55 16.43

0.69 1.08 1.13 2.33

supercritical CO2 . Theoretically, the higher curing or vulcanizing time of the rubber sheet in general leads to higher crosslink density [28]. This part involving the study of the different pre-vulcanized time of rubber (15 min and 35 min) on the structural characteristic of microcellular rubber proceeded at 12.5 MPa with 45 ◦ C

Average cell size (␮m)

Reproducibility

6.27 9.38 10.62 16.52

0.04 0.06 0.19 0.10

for 120 min. The data of average cell size measured from SEM images was summarized in Table 5, showing reproducibility of the microcellular rubber with low standard deviation (low uncertainty value). Fig. 8 showed SEM micrographs, cell size and size distribution, as well as the volume expansion ratio of natural rub-

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Table 5 Cell size of microcellular NR prepared by varying pre-vulcanized time of rubber. Cell size (␮m) Sample

Precured NR-15min Precured NR-35min

Sample 1

Sample 2

Sample 3

Average

SD

Average

SD

Average

SD

7.61 6.28

0.92 0.75

7.69 6.24

1.09 0.74

7.78 6.32

1.08 0.69

Fig. 6. Proposed schematic diagram showing the nucleation process during foaming of microcellular NR.

Average cell size (␮m)

Reproducibility

7.69 6.28

0.08 0.04

of microcellular rubber proceeded with the 35 min pre-vulcanized time was smaller than the 15 min pre-vulcanized time. This result is consistent with many works in the literatures explaining to the crosslinking behavior that could induce cell nucleation via stress variation around cross-linked segment [15,21]. Cell density, thus, is enhanced with an increment in pre-vulcanized time [22]. In our studied condition, the cell diameter size of microcellular NR is still less than 20 ␮m with varying degree of crosslinking when compared with NR foam prepared from the use of chemical blowing agent having cell size larger than 100 ␮m [45,48]. 4. Conclusions

ber foam with different pre-vulcanized time of rubber sheets. The experimental result found methodically that the average cell size decreased from 7.69 ␮m to 6.28 ␮m while the volume expansion ratio showed the decrement trend with the increment of prevulcanized time from 15 min to 35 min. The 35 min pre-vulcanized rubber (Fig. 6d) exhibited quite narrower of cell size distribution than the 15 min pre-vulcanized rubber (Fig. 6c). It could be theoretically described that higher pre-vulcanized time provides higher crosslink density that made the rubber stiffer and higher matrix strength than lower pre-vulcanized time, leading to be difficulty in the expansion of the matrix and bubbles. Therefore, the bubble size

Microcellular natural rubber could be successfully prepared using supercritical CO2 technology. The essential parameters for the foaming through supercritical CO2 including saturation tine, pressure and temperature are substantially conducted. Moreover, crosslinking characteristic varied on the pre-vulcanized time of rubber was also assessed in this work. The obtained results revealed that an increment of CO2 saturation tine and pressure could decrease average cell size and the volume expansion ratio, and significantly reduce of cell size distribution. Whereas, an enhancement of CO2 saturation temperature influenced an increment of average

Fig. 7. SEM micrographs and cell size distributions of microcellular natural rubber prepared with different saturation temperature: (a, e) 45 ◦ C, (b, f) 55 ◦ C, (c, g) 65 ◦ C, and (d, h) 85 ◦ C, as well as (i) cell size and (j) expansion ratio (P =10.5 MPa and t =60 min).

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Fig. 8. SEM micrographs and cell size distributions of microcellular natural rubber prepared with different pre-vulcanized time of rubber: (a, c) 15 min and (b, d) 35 min, as well as (e) cell size and (f) expansion ratio (P =12.5 MPa, T =45 ◦ C and t =120 min).

cell size, the volume expansion ratio, and range of cell size distribution. In such a way of crosslinking characteristic, the higher pre-vulcanized time provided smaller average cell size, lower volume expansion ratio and relative narrower cell size distribution. However, the mechanical properties of the microcellular rubber have to be further investigated. The results obtained can pave the way for a new foam preparation technology which can be very much beneficial as the process is considered reproducibility and environmentally friendly in using non-hazardous chemicals in addition to the use of waste CO2 produced from industries.

Acknowledgments The authors would like to thank partial support from an ERASMUS+ grant 2015-1-FR01-KA107-014841 between Mahidol University and Le Mans University. The scholarship of Science Achievement Scholarship of Thailand (SAST) to W.Tessanan is very much appreciated.

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