Cellulose (2015) 22:2629–2639 DOI 10.1007/s10570-015-0684-1
ORIGINAL PAPER
Mechanical reinforcement of cellulose nanocrystals on biodegradable microcellular foams with melt-compounding process Ning Lin . Youli Chen . Fei Hu . Jin Huang
Received: 13 May 2015 / Accepted: 11 June 2015 / Published online: 25 June 2015 Ó Springer Science+Business Media Dordrecht 2015
Abstract The polymeric foamed composites were developed from the biodegradable poly(butylene succinate) (PBS) reinforced by the biomass-based cellulose nanocrystals (CNC) via the melt-compounding treatment. As the highly-crystalline and rigid nanoparticles, the presence of CNC in the polyester matrix can simultaneously enhance the flexural strength and flexural modulus of the foamed composites. With the addition of 5 wt% CNC, the flexural strength and modulus of the PBS foamed composite increased by 50 and 62.9 % in comparison with those of the neat foamed material. Furthermore, the introduction of the CNC significantly affected the cells morphology, structure and stability during the foaming process, which facilitated the increase of the cell density and the homogeneous cell size and distribution of the foamed composites. With the addition of 5 wt% azodicarbonamide as the chemical blowing agent and 5 wt% CNC as the bionanofillers, the foamed composite showed the increased cell density of 7.1 9 105 cell/ cm3, which was 69.1 % higher than that of the neat
N. Lin Y. Chen F. Hu J. Huang (&) School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China e-mail:
[email protected] J. Huang School of Chemistry and Chemical Engineering, Southwest University, No.2 Tiansheng Road, BeiBei District, Chongqing 400715, China
foamed material. The mechanical enhancement of the foamed composites was attributed to the nanoreinforcement of the CNC served as the stress transferring phase, and meanwhile the promising improvement on the cells structure and stability for the foamed composites was ascribed to the effect of the CNC acted as the nucleation component. Keywords Cellulose nanocrystals Mechanical reinforcement Microcellular foam Meltcompounding
Introduction Driven by a growing concern on the environmental pollution, there has been an explosion of interest in the use of biomass as the source of renewable materials during recent years. Cellulose is reported as the most abundant natural polymer resource produced in the biosphere (from the plants, animals and even bacterial) with an annual production estimated to be over 7.5 9 1010 tons (Habibi 2014). Typically isolated by the acid hydrolysis from cellulose fibers, cellulose nanocrystal (CNC) is a kind of highly-crystalline and rod-like nanoparticle, which possesses numerous advantages in the ‘‘green’’ materials including the carbon neutrality, sustainability, recyclability and nontoxicity. Impressive mechanical properties, nanoscale dimensions and high aspect ratio of CNC make it an ideal candidate for the reinforcement of polymeric
123
2630
materials to replace the conventional fillers, such as glass fibers or inorganic nanoparticles (Dufresne 2012). Since the first report on the use of CNC as the reinforcing nanofillers to the polymeric matrix (Favier et al. 1995), a large of research efforts have been put on the production and development of high performance and/or multifunctional CNC-reinforced polymeric composites (Lee et al. 2014). In the case of the well dispersion of the nanoparticles in polymeric matrices, CNC have been proved to the significant enhancement for the mechanical modulus and strength of the composites even at the low loading levels (Pei et al. 2011; Visakh et al. 2012). Polymeric foam is the low-density material containing gaseous voids in the microstructure, which may be produced by the physical and/or chemical blowing agents during the foaming process. Different from the bulk polymer, the porous structure existing in the polymeric foams endows the special properties for the materials, which can be widely used in the insulation, absorbents, cushioning and packaging (Forest et al. 2015). In order to develop the highvalue polymeric foams, the researchers attempted to combine the nanoparticles and foaming technology to produce the foamed composite materials with the expected lightweight, high strength and multifunctional properties (Lee et al. 2005). Two significant issues on the development of the foamed composites are commonly emphasized, which involves the selection of polymeric matrix and the blending fillers. Traditionally, the polymeric matrices for the preparation of the foamed materials mainly include the polymers of poly(vinyl chloride) (Wu et al. 2009), polyolefin (Naguib et al. 2005), polystyrene (Pushpadass et al. 2008) and polyurethane (Chen et al. 2014). However, these traditional polymers are generally obtained from the petrochemical feed stocks, which are not environmental-friendly materials due to their low biodegradability. Therefore, the biodegradable polymers recently drew much attention for the development of novel foamed materials, such as poly(e-caprolactone) (PCL), poly(lactic acid) (PLA), poly(hydroxy butyrate) (PHB), poly(butylene succinate) (PBS) etc. (Jeon et al. 2013). A small amount of well-dispersed fillers in the polymeric foams may serve as the nucleation sites to facilitate the bubble nucleation during the foaming process, which will effectively affect the cell size and cell density for the foamed composites. Furthermore,
123
Cellulose (2015) 22:2629–2639
the presence of the fillers can significantly enhance the thermal stability and mechanical properties of the foamed materials, which is beneficial to their practical application. Therefore, besides the polymeric matrix, the filler used in the foamed composites is another critical factor for the structure and properties of the materials. Diverse inorganic and organic fillers have been reported in the preparation of the foamed composites, for instance calcium carbonate (Chen et al. 2013), nanosilica (Gong et al. 2012), carbon nanofiber (Shen et al. 2005), carbon nanotube (Lim et al. 2011), clay (Wong et al. 2013), montmorillonite (Zhou et al. 2014), lignin (Xue et al. 2014) and wood fibers (Faruk et al. 2007). As a rigid biomass-based nanoparticle, CNC can be expected to be the promising nanofiller to enhance the performance and provide the possible nucleation effect for the foamed composites. However, only several studies introduced the CNC in the development of the polymeric foamed composites, which involved the reports on the preparation of the polyurethane/CNC (Li et al. 2013) and PCL/CNC (Mi et al. 2014) foamed composites. The polymeric matrix used in this study, PBS, is a thermoplastic and biodegradable aliphatic polyester generally synthesized by a polycondensation reaction of 1,4-butanediol and succinic acid, which can also be prepared by biological fermentation process from the agricultural crops containing cellulose, glucose and lactose (Hwang et al. 2012). Derived from the promising biodegradability and melting processability, PBS is widely used as the food packaging materials and mulch films by the industrial meltprocessing technique. This study is an attempt to introduce CNC as the reinforcing bionanofillers into polyester matrix for the production of the foamed composites via the melt-compounding process. With the purpose of investigating the morphological effect and mechanical reinforcement of rigid CNC, the foamed composites were prepared containing different concentrations of chemical blowing agent (from 3 to 6 wt%) and various loading levels of CNC (from 0 to 10 wt%). The presence of CNC at the moderate loading level can significantly improve the flexural strength and modulus, and meanwhile promote the structural stability and increase of the cell density for the PBS-based microcellular foams. From the results of this study, the nanoreinforcing effects of CNC to the PBS-based composites and microcellular foams were investigated, and the possible role of CNC to provide
Cellulose (2015) 22:2629–2639
the increased nucleating sites for the formation of stable cells in the foamed composites was discussed. Ultimately, with the improved performance and controlled structure, the biodegradable foamed composites from the PBS and CNC developed in this study were expected to have the increasing applicability for the substitution of some current foamed polymeric materials.
Experimental Materials Commercial PBS pellets was purchased from Anqing Hexing Chemical Co., Ltd (Anhui, China), with the density of 1.26 g/cm3 and the number average molar weight of 18 9 104 Da. Cotton linter was supplied by Hubei Chemical Fiber Corporation (Xiangfan, China). Azodicarbonamide (AC) was purchased from Shanghai Heritage Corporation (Shanghai, China), and used as the chemical blowing agent. Sulfuric acid, ammonium hydroxide, zinc oxide (ZnO) and other analytical-grade reagents were purchased from Shenshi Chemical Corporation (Wuhan, china), and used without the further purification. Extraction of cellulose nanocrystals from cotton linter The isolation of CNC was performed on the hydrolysis of cotton fibers with the sulfuric acid according to our previous report (Lin et al. 2011a, b). Briefly, the cotton linter was dispersed in 30 % (v/v) H2SO4 solution, and stirred constantly at 60 °C for 6 h. Disordered and paracrystalline regions of cellulose fibers were expected to be hydrolyzed, whereas crystalline regions that have the higher resistance to acid attack remained intact. After the hydrolysis, the suspension was washed by successive centrifugation with water. The suspension was then dialyzed overnight against the distilled water, and finally loose powders of CNCs were obtained with the freeze-drying treatment. Preparation of the PBS/CNC composites with the melt-compounding process The melt-compounding mixture of the PBS and CNC components was carried out in the internal mixer
2631
(Changzhou Suyan Science and Technology Co., Anhui, China) with the preheating of the two rotors at 120 °C and the rotating speed of 72 rpm for 15 min. The blending mixtures of the PBS/CNC with various CNC concentrations (from 0 to 10 wt%) were prepared after the melt-compounding treatment. Then, the mixture was compression-molded with the R-302 hot-press machine (Qien Development Technology Co., Wuhan, china) as the plate at 120 °C under the pressure of 10 MPa for 10 min. With the air cooling of the plates to about 30 °C, the PBS/CNC composites were obtained with the thickness of about 2 mm and the length and width of 60 mm 9 13 mm. Preparation of the PBS/CNC–AC foamed composites with the melt-compounding process With the similar procedure as the preparation of the PBS/CNC composites, the mixing of the PBS, CNC, AC (the foaming agent), and ZnO (the blowing promoter) was performed on the internal mixer at 120 °C with the rotating speed of 72 rpm for 15 min. The blending mixture was compression-molded with the hot-press machine as the foamed plates at 167 °C under the pressure of 10 MPa for 10 min. With the decomposition reaction of the AC component, there was the foaming process during the hot-compression treatment derived from the release of N2 and CO2 gas molecules in the composites. In order to investigate the effects of the AC and CNC components on the microstructure and property of the foamed materials, three series of the foamed materials (including the PBS–AC foamed materials, the PBS/CNC3–AC foamed composites, and the PBS/CNC–AC5 foamed composites) were prepared with the change of the AC concentrations at 3, 4, 5, 6 wt% and the presence of various CNC loading levels from 0 to 10 wt%. All the foamed composites were coded as PBS/CNCx–ACy. The x and y represent the contents of the nanofiller (CNC) and the blowing agent (AC) in the foamed composites. Characterization and analysis A small amount of CNC was ultrasonically dispersed in water, and negatively stained with uranyl acetate (2 wt%) in the ethanol solution. Transmission electron microscopy (TEM) was performed on an H-7000FA electron microscope at 75 kV to observe the
123
2632
Cellulose (2015) 22:2629–2639
morphology of CNC. The average length and diameter of CNC were measured from the TEM images with more than one hundred individual nanoparticles. All the foamed composites were frozen and quenched off in the liquid nitrogen. The fracture surface of the samples were sputtered with the gold and then observed with the X-650 scanning electron microscope instrument (SEM). of the foamed composites The average cells size (D) was calculated from the SEM images with at least 100 cells in the central of the images. Statistical measurement was performed by the software image-pro 8. The void fraction (Vf) was determined by the densities of the foamed material and the unfoamed material, as calculated according to the Eq. (1), where qf is the density of the foamed materials and q represents the density of the pure PBS polymer (1.26 g/cm3). Vf ¼ 1
qf q
ð1Þ
The cell density (N0, cells/cm3) is defined as the number of cells per unit volume for the foamed polymer, which can be calculated by Eq. (2). The parameters of n, M and A represent the number of cells, the magnification factor and the area (cm2) of the micrograph images (Matuana et al. 2009). N0 ¼
nM 2 A
1:5
1 1 Vf
ð2Þ
The mechanical properties of the foamed composites, including the flexural strength and modulus, were measured on a CMT6503 universal testing machine following the procedure of GB/8812-88. The flexural rate of the experiments was 1 mm/min, and the maximum flexural strain of the foamed composites was controlled as 3.51 %. Before the measurements, all the samples were kept at 35 % humidity for 7 days. A mean value of five replicates from each sample was taken. Differential scanning calorimetry analysis (DSC) was used to investigate the thermal property of the foamed composites, which were analyzed on a DSCQ200 instrument under the nitrogen atmosphere in the range of -70 to 150 °C at the heating or cooling rate of 20 °C/min. X-ray diffraction analysis (XRD) was performed on a D/max-2500 X-ray diffractometer for the foamed composites and the CNC powders with Cu Ka1 radiation
123
(k = 0.154 nm) in the range of 2h = 5°–40° using the fixed time mode of a step interval of 0.02°. Results and discussion Morphological analysis of CNC and appearance of the foamed composites The morphology and dimension of CNC were investigated by TEM in the aqueous suspension, as shown in Fig. 1. Derived from the cotton linter, CNC typically exhibited the rod-like morphology with the length of 200–300 nm and the diameter of 10–20 nm (image A). On the basis of the higher magnification (image B), individual nanocrystal can be observed with the length of 184.0 nm and the diameter of 10.7 nm, which can be calculated the aspect ratio of CNC as about 17 from the average length and diameter. Figure 2 showed the photos of the surface appearance (image A) and the cross-sectional appearance (image B) of the PBS/CNC5–AC5 foamed composite. With the hot-compression treatment and the foaming process, the foamed composite possessed the smooth surface and the microcellular structure of crosssection from the PBS polymeric matrix. Effect of the foaming agent (AC) concentration on the structure of the foamed composites In the polymeric foam, the foaming process usually includes the cell nucleation, cell growth and cell stabilization with the effect of the foaming agent. The cell nucleation occurs at the initiation sites during the polymer melting, which contains the release of the supersaturated gases from the blowing agent. Then, the cells gradually grow and reach their critical sizes. The gas evolves with the decomposition of the blowing agent diffusing into the cell, and finally it stabilizes (closed-cell) or ruptures (pen-cell) (PopIliev et al. 2003). The blowing agent used in this study is AC, which can release the N2 and CO2 gas molecules at the decomposition temperature of 167 °C. The foaming agent concentration is one of the key factors for the structure and properties of the foamed materials. In order to determine the optimal foaming agent (AC) concentration, the foamed composites with the presence of four AC concentrations (3,
Cellulose (2015) 22:2629–2639
2633
Fig. 1 TEM images of CNC at different observation scales Fig. 2 Photos of the PBS/ CNC5–AC5 foamed composite with a surface images, b cross-sectional images; the chemical structures of the PBS and AC
4, 5, 6 wt%) were prepared without or with the addition of CNC. To avoid the possible microstructure destruction from the presence of excess nanoparticles, the CNC with the low loading level (3 wt%) was introduced in the foamed composites with the various AC concentrations. As shown in Fig. 3, the PBS–AC4 and PBS–AC5 foamed materials with the presence of the 4 and 5 wt% AC foaming agents (images B and C) exhibited the even and well-distributed cells with the spherical morphology. However, it can be observed that the formation of cells with the uneven sizes on the structure of the PBS–AC3 foamed material (image A) especially with the presence of some extremely small cells. Furthermore, plenty of the oval-shaped cells with large scales appeared on the structure of the PBS– AC6 foamed material (image D), which can be attributed to the formation of the excess gas and possible merger in the polymer melting from the superfluous addition of the 6 wt% AC foaming agent. Significantly, from images A0 to D0 , the addition of rigid nanoparticles (CNC) in polymer microcellular
foams commonly promote the smaller and more sable, even-distribution cells in the foamed composites in comparison with the foamed materials without the addition of CNC. According to the interface nucleation theory (Forest et al. 2015), the gas molecules prefer to be nucleated at the interface of solid phase and the melting phase, which indicates that the presence of solid particles in the polymeric foaming process can be favor to the heterogeneous nucleation and stable formation of microcellular gas pores. The average sizes and cell densities of the foamed materials were statistically measured by the image-pro 8.0 software, as shown in Fig. 4. Because of the formation of higher contents of gas, the bulk densities of the foamed materials decreased with the increase of the AC foaming agent concentrations. In comparison with the neat PBS foamed materials, the addition of 3 wt% CNC effectively decreased the cell size and increased the cell density of the foamed composites. The presence of highly-crystalline nanocrystals can produce the solid interface and play the role of the bubble
123
2634
Cellulose (2015) 22:2629–2639
Fig. 3 The neat PBS foamed materials containing the AC foaming agent of a 3 wt%, b 4 wt%, c 5 wt%, d 6 wt% (scale bar 200 lm); and the PBS/CNC3 foamed composites (with the
CNC loading level of 3 wt%) containing the AC foaming agent of a0 3 wt%, b0 4 wt%, c0 5 wt%, d0 6 wt% (scale bar 200 lm)
heterogeneous nucleation for the formation of the increased nucleation sites, which can promote the structural and scale stability of the gas nucleation and cell growth in the polymeric foamed composites.
possible formation of percolating network from the hydrogen-bonding interaction of CNC. Attributed to the presence of the gas molecules from the foaming effect, the mechanical properties of the foamed materials were commonly lower than those of the unfoamed materials, such as the comparison between the neat PBS material (the flexural strength and flexural modulus of 9.2 and 398 MPa) and the neat PBS–AC5 foamed material (the flexural strength and flexural modulus of 4.2 and 210 MPa). However, the introduction of rigid nanoparticles CNC can still provide the significant enhancing effect to the PBSbased microcellular foams. In particular, with the addition of 5 wt% CNC, the flexural strength was advanced by 50 % and the flexural modulus was improved by 62.9 % for the PBS/CNC5–AC5 foamed composite compared with the neat PBS–AC5 foamed material. In the structure of the foamed composites, rigid CNC can serve as the stress transferring phase, and meanwhile act as the nucleation agent to improve the effective stress area, which was beneficial to the enhancement of the mechanical properties of the foamed composites. When the CNC loading levels were higher than 5 wt%, the flexural modulus of the
Effect of the CNC concentration on the mechanical property and microstructure of the foamed composites Figure 5 showed the effects of CNC on the mechanical properties of the PBS-based composites (image A) and microcellular foams (image B) including the flexural strength and flexural modulus. Both flexural strength and flexural modulus of the PBS/CNC composites gradually enhanced with the increase of the CNC loading levels. The flexural strength and flexural modulus of the PBS/CNC10 composite reached to the maximum values of 14.3 and 720 MPa, which were advanced by 55.4 and 80.9 % in comparison with the neat PBS material. The enhancement of the flexural strength of the PBS/CNC composites can be ascribed to the effect of rigid CNC served as stress transferring phase, and the reinforcement of the flexural modulus of the PBS/CNC composites can be attributed to the
123
Cellulose (2015) 22:2629–2639
2635
Fig. 5 The effects of the CNC contents on the flexural strength and modulus of the PBS/CNC composites and the PBS/CNC– AC5 foamed composites Fig. 4 The effects of the AC blowing agent with various concentrations on the cell size (a) and cell density (b) of the neat PBS foamed material and the PBS/CNC3–AC foamed composites: 3 wt% (open square), 4 wt% (open circle), 5 wt% (filled diamond) and 6 wt% (open triangle)
foamed composites slightly increased, while the flexural strength of the foamed composites started to decrease (PBS/CNC7–AC5 and PBS/CNC10–AC5). The presence of the superfluous nanoparticles may cause the microphase separation or the collapse of the cellular structure in the systems, which will result in the stress defect for the inferior mechanical properties of the foamed composites. The value of 5 wt% CNC as the nanoreinforcing filler was the optimal loading level, which can simultaneously improve the flexural strength and flexural modulus of the PBS foamed composites. To investigate the microstructure of the PBS/CNC composites and PBS/CNC–AC5 microcellular foams, the fracture morphologies of the materials were observed with SEM. As the comparison, the SEM images of the unfoamed composites with the addition
of different contents of CNC were shown in Fig. 6a–d. The neat PBS material presented a striated and smooth fractured surface (image A). With the addition of 3 and 5 wt% CNC, the fracture morphologies of the composites exhibited the slightly coarse surface but maintained the intact and continuous structure (images B and C), which indicated the homogeneous dispersion of nanoparticles at these loading levels in the composites by the melt-compounding treatment. However, there was the obvious change in the fractured morphology of the PBS/CNC10 composite (image D), which showed the highly coarse surface and even some aggregates on the microstructure in comparison with the neat PBS material. The introduction of superfluous CNC may induce the microphase separation of the PBS-based composites. As proved before, the excess gas in the polymeric matrix is hardly to form the stable and regular cellular structure, which may lead to the stress defects and reduce the practical use of the foamed materials. Therefore, the AC foaming agent concentration was
123
2636
Fig. 6 SEM micrographs of the fracture surfaces of the composites: a PBS, b PBS/CNC3, c PBS/CNC5, d PBS/ CNC10 (scale bar 20 lm); and the cross-section of the foamed
controlled as 5 wt% with the introduction of different loading levels of CNC in the PBS/CNC–AC5 foamed composites. Figure 6e–h showed the cells morphologies of the foamed composites affected by the CNC contents. In comparison with the neat PBS–AC5 foamed material (image E), the PBS/CNC5–AC5 foamed composite (image G) exhibited the smaller cell sizes and more homogeneous cell distribution, while some cells with the larger sizes or irregular shapes were observed in the fracture morphologies of the PBS/CNC3–AC5 and PBS/CNC10–AC5 foamed composites (images F and H). As shown in Fig. 7, specific measurements on the average cell size (CZ, lm) and cell density (CD, cell/ cm3) of the foamed composites were statistically performed from the SEM images with at least 100 cells in the central of the images. The average cell size and cell density of the neat PBS–AC5 foamed material were 274 lm and 4.2 9 105 cell/cm3. With the increase of the CNC contents, the average cell size of the foamed composites gradually decreased and the cell density of the foamed composites increased contrarily. The PBS/CNC5–AC5 microcellular foam possessed the highest cell density of 7.1 9 105 cell/
123
Cellulose (2015) 22:2629–2639
composites: e PBS–AC5, f PBS/CNC3–AC5, g PBS/CNC5– AC5, h PBS/CNC10–AC5 (scale bar 200 lm)
Fig. 7 The effects of the CNC contents on the cell size and cell density of the PBS/CNC–AC5 foamed composites
cm3, and the lowest average cell size of 187 lm. The presence of 5 wt% CNC nanoparticles can provide more nucleating sites for the formation of the gas cells, which promoted the 69.1 % increase of the cell density in comparison with the neat PBS foam. It was worth noting that higher contents of CNC (7 and 10 wt%) may block the expansion of the gas molecules, and the separated layer will induce the possible merger or
Cellulose (2015) 22:2629–2639
collapse of the formed cells. Associated with the SEM image in Fig. 6, the increase of the cell sizes and reduction of the cell density for the PBS/CNC10–AC5 foamed composite can be proved by the observation of the irregular cell shapes and uneven cells distribution. The crystalline property of the foamed composites The crystalline property of the PBS-based composites and microcellular foams affected by the introduction of highly-crystalline CNC was investigated by the XRD and DSC analysis. The XRD patterns of the PBS/ CNC–AC5 foamed composites and CNC were shown in Fig. 8. The diffraction peaks of 2h angles at about 16.4°, 22.6° and 34.4° on the CNC pattern were assigned to the typical reflection planes of cellulose I 002 and 040 respectively (Lin et al. 2011a, b). 101, According to the Segal method (Segal et al. 1959), the crystallinity index of CNC was calculated as 84.4 %, which indicated the highly-crystalline structure of CNC. PBS is a kind of semi-crystalline polyester, which exhibited the main characteristic crystalline peaks located at about 19.0° and 23.0° of 2h. The similar diffraction patterns appeared on the patterns of the PBS/CNC–AC5 foamed composites, which suggested the preservation of crystalline structure of the PBS polymeric matrix after the introduction of the
2637
CNC nanofillers. However, due to the overlap of the main peak at 23.0° on the patterns, it was difficult to distinguish the change of crystalline property from the PBS component in the foamed composites. DSC measurements were performed to investigate the thermal property and further analyze the crystalline property of the formed composites. The data of the glass transition temperature at midpoint (Tg,mid), heat-capacity increment (DCp), melting temperature (Tm), heat of fusion (DHm), and crystallinity index (vc) associated with the PBS fraction from the PBS/CNC composites and the PBS/CNC–AC5 microcellular foams were summarized in Table 1. In comparison with the neat PBS material and the neat PBS–AC5 foamed material, the presence of CNC provided the less impact on the thermal properties of both PBSbased composites and microcellular foams, involving the slight changes of the Tg,mid (about 2–4 °C) and Tm (about 1–2 °C). The weak influence to the thermal properties of the materials indicated the maintenance of the continuous structure of the materials when incorporating the hydrophilic nanofillers into the hydrophobic matrix during the melt-compounding treatment. However, the introduction of highly-crystalline nanoparticles into the semi-crystalline polymeric matrix inevitably caused the change of the crystalline property of the materials. Based on the transformation of the DHm values from the different loading levels of CNC, the crystallinity index (vc) of the PBS component in the foamed composites can be calculated according to the following equation: vc ¼ DHm =xDHm
Fig. 8 XRD patterns of a CNC, b PBS–AC5, c PBS/CNC3– AC5, d PBS/CNC5–AC5, e PBS/CNC10–AC5
ð3Þ
where DHm = 110.3 J/g is the melting enthalpy of the 100 % crystalline PBS, and x represents the weight fraction of PBS in the foamed composites. Commonly, for both the composites and microcellular foams, the presence of rigid CNC induced the slight increase of the crystallinity indices associated with the PBS component, which can be attributed to the motion restriction of the polymeric chains from rigid nanoparticles. This result was consistent with the recent report on the crystallinity increase of the PLAbased composites with the reinforcement of cellulose nanofibrils (Fujisawa et al. 2014). Interestingly, the crystallinity indices of the microcellular foams were significantly higher (about 5–10 %) than those of the unfoamed composites, which suggested the effects on the possible hindrance or organization from the gas
123
2638 Table 1 DSC data of the PBS/CNC composites and the PBS/CNC–AC5 foamed composites including the glass transition temperature at midpoint (Tg,mid), heatcapacity increment (DCp), melting temperature (Tm), heat enthalpy (DHm) and the calculated crystallinity index (vc)
Cellulose (2015) 22:2629–2639
Samples
Tg,mid (°C)
Tm1 (°C)
Tm2 (°C)
DHm (J/g)
vc (%)
PBS
-35.5
0.26
92.6
104.4
35.5
32.2
PBS/CNC1
-37.5
0.43
94.0
104.4
35.9
32.9
PBS/CNC3
-39.0
0.60
91.9
103.7
38.2
35.7
PBS/CNC5
-38.2
0.39
92.7
104.5
36.1
34.5
PBS/CNC10
-37.5
0.33
92.5
103.3
31.3
31.5
PBS–AC5
-31.5
0.22
91.9
105.3
42.8
40.8
PBS/CNC1–AC5
-34.6
0.18
92.9
104.3
39.2
37.8
PBS/CNC3–AC5 PBS/CNC5–AC5
-35.4 -34.0
0.32 0.22
93.6 92.6
104.1 104.4
42.6 43.1
42.0 43.4
PBS/CNC10–AC5
-35.3
0.17
92.4
104.5
34.6
36.9
molecules to the PBS polymeric chains during the foaming process. The PBS/CNC5–AC5 foamed composite with the addition of 5 wt% CNC possessed the highest vc value of 43.4 %, which was consistent with the significant enhancement of CNC to the mechanical properties of the material. Superfluous introduction of rigid nanoparticles (10 wt% CNC) may lead to the slight incompatibility and even microphase separation of the composites and foams, which induced the reduction of the heat of fusion and crystallinity index of the materials.
Conclusion As the highly-crystalline and rigid nanoparticles, CNC were introduced in the biodegradable polyester for the preparation of the composites and microcellular foams. Rod-like CNC provided the significant reinforcement to improve the flexural strength and modulus of the PBSbased composites and microcellular foams. Particularly, with the addition of 5 wt% CNC, the flexural strength and modulus were advanced by 50 and 62.9 % for the foamed composite in comparison with the neat foamed material. Besides the mechanical enhancement of the CNC to the foamed composites, the presence of these rigid nanoparticles can act as the additional nucleation sites for the promotion of the cell formation and growth during the foaming process, which was favor to the structural stability and distribution of the cells in the foamed composites. With the introduction of 5 wt% AC as the chemical blowing agent and 5 wt% CNC as the bionanofillers, the PBS/CNC5–AC5 foamed composite showed the increased cell density of 7.1 9 105 cell/cm3, which was 69.1 % higher than
123
DCp [J/(g K)]
that of the neat PBS–AC5 foamed material. The potential application of the biodegradable foamed composites developed in this study from the ‘‘green’’ polyester polymer and biomass nanoparticles may involve the fields of the insulation, absorbents, cushioning and packaging. Further investigation on the practical uses of this foamed composite will be performed in the future study. Acknowledgments This work was supported by National Natural Science Foundation of China (51373131), Project of New Century Excellent Talents of Ministry of Education of China (NCET-11-0686), and Fundamental Research Funds for the Central Universities (Self-Determined and Innovative Research Funds of WUT, 2014-II-009).
References Chen J, Sun X, Turng LS, Liu T, Yuan WK (2013) Investigation of crystallization behavior of solid and microcellular injection molded polypropylene/nano-calcium carbonate composites using carbon dioxide as a blowing agent. J Cell Plast 49:459–475. doi:10.1177/0021955X13488400 Chen MJ, Chen CR, Tan Y, Huang JQ, Wang XL, Chen L, Wang YZ (2014) Inherently flame-retardant flexible polyurethane foam with low content of phosphorus-containing crosslinking agent. Ind Eng Chem Res 53:1160–1171. doi:10. 1021/ie4036753 Dufresne A (2012) Nanocellulose: from nature to high performance tailored materials. Walter de Gruyter, Germany Faruk O, Bledzki AK, Matuana LM (2007) Microcellular foamed wood-plastic composites by different processes: a review. Macromol Mater Eng 292:113–127. doi:10.1002/ mame.200600406 Favier V, Canova GR, Cavaille´ JY, Chanzy H, Dufresne A, Gauthier C (1995) Nanocomposite materials from latex and cellulose whiskers. Polym Adv Technol 6:351–355. doi:10.1002/pat.1995.220060514 Forest C, Chaumont P, Cassagnau P, Swoboda B, Sonntag P (2015) Polymer nano-foams for insulating applications
Cellulose (2015) 22:2629–2639 prepared from CO2 foaming. Prog Polym Sci 41:122–145. doi:10.1016/j.progpolymsci.2014.07.001 Fujisawa S, Zhang J, Saito T, Iwata T, Isogai A (2014) Cellulose nanofibrils as templates for the design of poly (l-lactide)nucleating surfaces. Polymer 55:2937–2942. doi:10.1016/ j.polymer.2014.04.019 Gong W, Liu KJ, Zhang C, Zhu JH, He L (2012) Foaming behavior and mechanical properties of microcellular PP/ SiO2 composites. Int Polym Proc 27:181–186. doi:10. 3139/217.2458 Habibi Y (2014) Key advances in the chemical modification of nanocelluloses. Chem Soc Rev 43:1519–1542. doi:10. 1039/C3CS60204D Hwang SY, Yoo ES, Im SS (2012) The synthesis of copolymers, blends and composites based on poly (butylene succinate). Polym J 44:1179–1190. doi:10.1038/pj.2012.157 Jeon B, Kim HK, Cha SW, Lee SJ, Han MS, Lee KS (2013) Microcellular foam processing of biodegradable polymers—review. Int J Precis Eng Manuf 14:679–690. doi:10. 1007/s12541-013-0092-0 Lee LJ, Zeng C, Cao X, Han X, Shen J, Xu G (2005) Polymer nanocomposite foams. Compos Sci Technol 65:2344–2363. doi:10.1016/j.compscitech.2005.06.016 Lee KY, Aitoma¨ki Y, Berglund LA, Oksman K, Bismarck A (2014) On the use of nanocellulose as reinforcement in polymer matrix composites. Compos Sci Technol 105:15–27. doi:10.1016/j.compscitech.2014.08.032 Li S, Li C, Li C, Yan M, Wu Y, Cao J, He S (2013) Fabrication of nano-crystalline cellulose with phosphoric acid and its full application in a modified polyurethane foam. Polym Degrad Stabil 98:1940–1944. doi:10.1016/j. polymdegradstab.2013.06.017 Lim SK, Lee SI, Jang SG, Lee KH, Choi HJ, Chin IJ (2011) Synthetic aliphatic biodegradable poly (butylene succinate)/MWNT nanocomposite foams and their physical characteristics. J Macromol Sci Phys 50:1171–1184. doi:10.1080/00222348.2010.503119 Lin N, Huang J, Chang PR, Feng J, Yu J (2011a) Surface acetylation of cellulose nanocrystal and its reinforcing function in poly (lactic acid). Carbohydr Polym 83:1834–1842. doi:10.1016/j.carbpol.2010.10.047 Lin N, Yu J, Chang PR, Li J, Huang J (2011b) Poly (butylene succinate)-based biocomposites filled with polysaccharide nanocrystals: structure and properties. Polym Compos 32:472–482. doi:10.1002/pc.21066 Matuana LM, Faruk O, Diaz CA (2009) Cell morphology of extrusion foamed poly (lactic acid) using endothermic chemical foaming agent. Bioresour Technol 100:5947– 5954. doi:10.1016/j.biortech.2009.06.063
2639 Mi HY, Jing X, Peng J, Salick MR, Peng XF, Turng LS (2014) Poly (e-caprolactone) (PCL)/cellulose nano-crystal (CNC) nanocomposites and foams. Cellulose 21:2727–2741. doi:10.1007/s10570-014-0327-y Naguib HE, Park CB, Song SW (2005) Effect of supercritical gas on crystallization of linear and branched polypropylene resins with foaming additives. Ind Eng Chem Res 44:6685–6691. doi:10.1021/ie0489608 Pei A, Malho JM, Ruokolainen J, Zhou Q, Berglund LA (2011) Strong nanocomposite reinforcement effects in polyurethane elastomer with low volume fraction of cellulose nanocrystals. Macromolecules 44:4422–4427. doi:10. 1021/ma200318k Pop-Iliev R, Liu F, Liu G, Park CB (2003) Rotational foam molding of polypropylene with control of melt strength. Adv Polym Technol 22:280–296. doi:10.1002/adv.10056 Pushpadass HA, Weber RW, Hanna MA (2008) Expansion, morphological, and mechanical properties of starch–polystyrene foams containing various additives. Ind Eng Chem Res 47:4736–4742. doi:10.1021/ie071049h Segal LGJMA, Creely JJ, Martin AE, Conrad CM (1959) An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Text Res J 29:786–794. doi:10.1177/004051755902901003 Shen J, Zeng C, Lee LJ (2005) Synthesis of polystyrene–carbon nanofibers nanocomposite foams. Polymer 46:5218–5224. doi:10.1016/j.polymer.2005.04.010 Visakh PM, Thomas S, Oksman K, Mathew AP (2012) Crosslinked natural rubber nanocomposites reinforced with cellulose whiskers isolated from bamboo waste: processing and mechanical/thermal properties. Compos A 43:735–741. doi:10.1016/j.compositesa.2011.12.015 Wong A, Wijnands SF, Kuboki T, Park CB (2013) Mechanisms of nanoclay-enhanced plastic foaming processes: effects of nanoclay intercalation and exfoliation. J Nanopart Res 15:1–15. doi:10.1007/s11051-013-1815-y Wu Q, Zhou N, Zhan D (2009) Effect of processing parameters and vibrating field on poly (vinyl chloride) microcellular foam morphology. Polym Plast Technol Eng 48:851–859. doi:10.1080/03602550902994912 Xue BL, Wen JL, Sun RC (2014) Lignin-based rigid polyurethane foam reinforced with pulp fiber: synthesis and characterization. ACS Sustain Chem Eng 2:1474–1480. doi:10.1021/sc5001226 Zhou J, Yao Z, Zhou C, Wei D, Li S (2014) Mechanical properties of PLA/PBS foamed composites reinforced by organophilic montmorillonite. J Appl Polym Sci 131:40773. doi:10.1002/app.40773
123
