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gelatin biomass composites
Composites: Part A 43 (2012) 45–52
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Preparation and properties of sisal microfibril/gelatin biomass composites Xuejing Zheng, Jie Liu, Ying Pei, Junwei Li, Keyong Tang ⇑ College of Materials Science and Engineering, Zhengzhou University, Henan 450052, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 2 July 2011 Accepted 30 August 2011 Available online 6 September 2011 Keywords: A. Fibers A. Polymer–matrix composites (PMCs) B. Physical properties
a b s t r a c t In this work, the natural sisal fibers were fibrillated by enzyme hydrolysis or mechanical disintegration into microfibrils with a width of 5–10 lm and different aspect ratios. The sisal microfibrils or microfibril mats were added into the gelatin to prepare biomass composites, by solvent-casting or solution impregnation techniques, respectively. The morphology, mechanical properties, biodegradation property, and water adsorption behaviors of the composites were investigated. It was found that the tensile strength of the composites was dramatically increased with the addition of sisal microfibrils. The degradation ratio of the composites decreased continuously with increasing the sisal fibril content. The addition of sisal microfibrils decreased the water uptake at equilibrium and the water diffusion coefficient. Scanning electron microscopy characterization showed that the sisal microfibrils were very well embedded in the gelatin matrix, showing a good interfacial adhesion. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The ever growing global environmental awareness and societal concern, high rate of depletion of petroleum resources, new environmental regulations, concepts of sustainability and low-carbon economy have triggered the search for new products and processes that are compatible with the environment [1–7]. The exhaustive use of petroleum based resources has initiated the efforts to develop biodegradable materials [8–14]. Biomass composites from plant derived fibers (natural/biofibers) and crop/bioderived plastic (biopolymer/bioplastic) are eco-friendly and such composites are termed as ‘‘green composites’’ [1]. The major attractions about green composites are that they are environmentally friendly, fully degradable and sustainable. Besides, at the end of their life, they can be easily disposed of or composted without harming the environment [15]. The use of biodegradable and environment-friendly plant-based ‘‘lignocellulosic’’ fibers has been a natural choice for reinforcing polymers to make them ‘‘greener’’ [16–25]. The growing interest in lignocellulosic fibers is mainly due to their economical production with few requirements for equipment and low specific weight, which results in a higher specific strength and stiffness, compared to glass reinforced composites. They also present safer handling and working conditions compared to synthetic reinforcements. Plant fibers are nonabrasive to mixing and molding equipment, which can contribute to significant cost reductions. The most interesting
⇑ Corresponding author. Tel.: +86 371 67763216; fax: +86 371 67780050. E-mail address: [email protected] (K. Tang). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.08.024
aspect about natural plant fibers is their positive environmental impact. In many of the reported research works about plant-based composites, the plant fibers are treated with alkaline solutions to remove the non-cellulose substances and then added into the polymer matrix. However, the reinforcing effect of the plant fibers cannot be fully realized in this way. It is known that natural plant fibers have a complex hierarchical structure. A single plant fiber (usually 100–200 lm in width) is actually a bundle of fibrils. One fibril is composed of many microfibrils, and one microfibril is composed of hundreds and thousands of cellulose chains. Therefore, plant fiber has complex hierarchical structure. Natural fibers can be processed in different ways to yield reinforcing elements with different mechanical properties [26]. The elastic modulus of bulk natural fibers such as wood is about 10 GPa. Cellulose fiber with the moduli up to 40 GPa can be separated from wood, for instance, by chemical pulping processes. Such cellulose fibers can be further subdivided by hydrolysis followed by mechanical disintegration into microfibrils with an elastic modulus of 70 GPa. Theoretical calculations of the elastic moduli of cellulose chains have given values of up to 250 GPa, however, there is no technology available to separate these from microfibrils. It can be seen that if the natural plant fibers are fibrillated into microfibrils with smaller diameters, a drastic reinforcing effect can be obtained [27,28]. In this work, the natural sisal fibers were fibrillated into microfibrils by alkaline treatment followed by enzyme hydrolysis or mechanical disintegration. Sisal microfibrils with width around 5–10 lm and with different aspect ratios were obtained. The sisal microfibrils were added in the form of microfibrils or microfibril mat into gelatin solution to prepare biomass composites by
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techniques of solvent-casting and solution impregnation, respectively. The mechanical properties, water adsorption behavior, and biodegradation property were investigated. The morphology of the composites was characterized by using scanning electron microscopy. 2. Materials and methods
2.6. Mechanical test The samples were conditioned at RH 65% for at least 1 week to constant weight. Mechanical testes of the dumbbell samples were measured with a universal tensile tester, CMT5140. The tests were carried out at a crosshead speed of 50 mm/min. The average of at least five tests was reported. Table 1 shows the short name of the samples used in this paper.
2.1. Materials 2.7. Water adsorption behavior Gelatin granules (biochemical reagent) were supplied by Tianjin Chemicals Co., Tianjin, China. Natural sisal fibers with a diameter of 100–200 lm were purchased from Guangxi Sisal Fiber Co., Guangxi, China. Dialdehyde starch (medical grade) was purchased from Tai’an Jinshan Modified Starch Co., Shandong, China. Other chemical agents were all analytically pure. 2.2. Gelatin modification The gelatin was soaked in deionized distilled water at 40 °C for 45 min. The mixture was then heated to 60 °C until the solution became homogeneous. Solution with concentration of 20% gelatin was obtained. Poly(ethylene glycol) (PEG 400), content of 20% based on dry gelatin, was added as a plasticizing agent. The gelatin was crosslinked with dialdehyde starch (DAS, 7.4% based on dry gelatin) or glutaraldehyde (GTA, 1.5% based on dry gelatin) at 60 °C and pH 8–9. Gelatin modified with DAS and GTA is denoted as DAS-Gel and GTA-Gel, respectively. 2.3. Alkali treatment of sisal fibers The chopped sisal fibers and a 10% NaOH solution were put into a beaker and stirred for 2 h at room temperature. Afterward, the sisal fibers were washed thoroughly with water to remove NaOH and dried at 50 °C in a vacuum oven. 2.4. Fibrillation of sisal fibers The fibrillation of sisal fibers were carried out with two methods. Method 1: a mount of b-glucanase was added in NaAc/HAc buffer solution (pH 4.8) to prepare an enzyme solution with concentration of 0.25%. The alkali-treated sisal fibers were hydrolyzed in the enzyme solution at 50 °C for 1 h under continuous agitation. Sisal microfibrils with diameters of 5–10 lm and aspect ratio of 5–30 were obtained. Method 2: the alkali-treated sisal fibers were put in a stirrer and disintegrated mechanically with a stirring rate of 20,000 rpm. Sisal microfibrils with diameters of 5–10 lm and aspect ratios larger than 100 were obtained. 2.5. Preparation of sisal microfibril/gelatin biomass composites The sisal microfibrils with short aspect ratios were added into the modified gelatin solution and mixed under agitation. Then the mixture were poured into a self-made polypropylene mold and dried in air at room temperature. This method is called as solvent-casting technique. The composite prepared in this way is denoted as SMF/Gel biomass composite. The sisal microfibrils with long aspect ratios were mixed with water to make a mat by negative-pressure filtration. Put the sisal microfibrils mat in the mold, and pour the modified gelatin solution into the mold. The composites were dried in air at room temperature. This method is called as solution impregnation technique. A composite with high microfibril content can be prepared by this method. The composite prepared is denoted as SMFm/Gel biomass composites.
The water adsorption kinetics of the composites was determined. Rectangular specimens sized 10 mm 10 mm with a thickness of 0.6 mm were prepared. The samples were conditioned at 25 °C in a desiccator containing P2O5 to a constant weight. The samples were weighed accurately, noted as W0. Then the samples were put in a desiccator containing a supersaturated salt solution of CuSO45H2O to give a RH of 98%. The weight of the samples at the adsorption time of t was noted as Wt and the weight at adsorption equilibrium was noted as We. The amount of water adsorbed at equilibrium was calculated as:
Me ð%Þ ¼
We W0 100 W0
ð1Þ
The mean water uptake of each sample was calculated for various adsorption times (t). The mass of water adsorbed at time t, (Wt–W0), can be expressed as follows:
" # 1 Wt W0 X 8 Dð2n þ 1Þ2 p2 t ¼ exp 2 2 We 4L2 n¼0 ð2n þ 1Þ p
ð2Þ
where We is the weight adsorbed at equilibrium; 2L, the thickness of the film; and D, the diffusion coefficient [2]. At a short time, Eq. (2) can be written as:
1=2 Wt W0 2 D ¼ t1=2 L p We
ð3Þ
At (Wt–W0)/We 6 0.5, the error in using Eq. (3) instead of Eq. (2) to determine the diffusion coefficient is on the order of 0.1% [29]. 2.8. Biodegradation property The biodegradation behavior of the composites was studied with a method similar to that described in Ref. [30]. The weight of the composites was measured (W0). The degradation buffer was prepared from phosphate buffer saline (PBS), pH adjusted to 5.2 with acetic acid, and 0.1 w/v% hen egg white (lysozyme) was added. The samples were immersed in the degradation buffer and incubated at 37 °C for 12 h. After 12 h, the samples were removed, washed with deionized water, dried and re-weighed W1. The degradation ratio was calculated as following:
DRð%Þ ¼
W0 W1 100 W0
ð4Þ
where DR is the biodegradation percentage, W0 is the weight at zero biodegradation time, and W1 is the weight at the degradation time Table 1 Sample name notation. Short name
Matrix
Reinforcing agent
SMF/Gel SMFm/Gel SMF/DAS-Gel SMFm/DAS-Gel SMF/GTA-Gel SMFm/GTA-Gel
Gelatin Gelatin DAS modified DAS modified GTA modified GTA modified
Sisal Sisal Sisal Sisal Sisal Sisal
gelatin gelatin gelatin gelatin
microfibril microfibril mat microfibril microfibril mat microfibril microfibril mat
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of 12 h. Each degradation experiment was repeated three times and the average was reported. 2.9. Scanning electron microscopy characterization The surfaces or fracture surfaces of the samples were examined by scanning electron microscopy (SEM, FEI-quanta 200, The Netherlands). The facture surfaces were got by immersing and breaking the samples in liquid nitrogen, or by tensile fracture tests. The surface was coated with a thin layer of gold prior to SEM examination. 3. Results and discussion 3.1. Morphology of sisal microfibrils and the composites Scheme 1. Possible hydrogen bonding formed between sisal fiber and gelatin.
gelatin
Transmittance/%
Fig. 1 shows the morphology of the natural sisal fiber and the sisal microfibrils fibrillated. The diameter of the natural sisal fiber is about 100 lm and the surface is rough (Fig. 1a). The enzyme hydrolysis process leads to effective fiber fibrillation, breaking down the fiber bundle into smaller fibrils with width of around 5–10 lm and aspect ratio of 5–30 (Fig. 1b). The sisal microfibrils prepared by mechanical disintegration is thin and long, with smooth surface. The sample is around 5–10 lm in width with the length of more than 1 mm. Therefore, the aspect ratio is larger than 100 (Fig. 1c). It is interesting to see that the mechanical fibrillation results in microfibrils with substantially larger aspect ratio compared with the enzyme fibrillation. This may be the result that mechanical fibrillation tends to cleavage along the longitude of the cellulose fibers. When subjected to enzyme hydrolysis, however, cellulose fibers undergo transverse cleavage along the amorphous regions and results in a rod-like material with a relatively low aspect ratio. Meanwhile, the disintegration of cellulose fibers goes inevitably with the decomposition or degradation during enzyme fibrillation. Since each anhydroglucose unit contains three free hydroxy groups, there are large amount of hydroxy groups in cellulose macromolecules. Gelatin, a protein, contains abundant hydrophilic groups on the chains, such as amido, hydroxyl and carboxyl. Therefore, a great number of strong hydrogen bonding can be formed between the hydroxy groups in cellulose and the hydrophilic sites in gelatin, as described in Scheme 1. Due to the strong intermolecular interaction, the compatibility between sisal microfibril and gelatin is expected to be good. To analyze the interaction between the two phases, FTIR characterization was carried out for pure gelatin and sisal microfibril/gelatin composite (50 wt.%/50 wt.%) and the results are shown in Fig. 2. A broad absorption band at 3421 cm1 for pure gelatin is assigned to the –NH stretching coupled with the hydrogen bonding. When gelatin is composed with
1245
3421
composite
1238 3322 4000 3500 3000 2500 2000 1500 1000
500
-1
wavenumber /cm
Fig. 2. FTIR spectra of gelatin and SMF/Gel composite.
sisal microfibril, this peak shifts to lower wavenumber at 3322 cm1. Moreover, the peak at 1245 cm1 for –NH bending in pure gelatin shifts to 1238 cm1 in the composite. These results indicate that strong interaction exists between gelatin and sisal microfibrils.
Fig. 1. SEM surface morphology of sisal microfibrils. (a) Natural sisal fiber, (b) sisal microfibrils by enzyme hydrolysis with short aspect ratios, (c) sisal microfibrils by mechanical disintegration, with long aspect ratios to make mats.
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(a) SMF/Gel
(c) SMF/DAS-Gel
(e) SMFm/DAS-Gel
(b) SMFm/Gel
(d) SMF/GTA-Gel
(f) SMFm/GTA-Gel
Fig. 3. SEM images of cross-sectional morphology of SMF and SMFm reinforced gelatin biomass composites.
To investigate the dispersion of sisal microfibrils in gelatin matrix, biomass composites have been prepared and the crosssectional morphology is shown in Fig. 3. It is seen that sisal microfibrils are tightly covered by gelatin, showing a very good interfacial adhesion, as shown in Fig. 3a. When gelatin is crosslinked with DAS or GTA, the SMF and SMFm is also well dispersed in the matrix, however the dispersion is less uniform due to the high viscous crosslinked matrix compared with the uncrosslinked one (Fig. 3c–f). 3.2. Mechanical properties Fig. 4 shows the tensile strength of the composites. The line tagged by –j– represents the tensile strength of sisal fiber/gelatin composite, in which the natural sisal fibers were not fibrillated other than ordinary NaOH treatment. Alkaline treatment removes most of the non-cellulose substances but basically remains the original diameters of the sisal fibers. It is seen that the tensile strength
decreases with the increase of sisal fibers content, and then increases slightly. When the sisal fibers are fibrillated into microfibrils, however, the tensile strength of the composites increases drastically. When 15 wt.% sisal microfibrils are incorporated, the tensile strength of SMF/DAS-Gel is increased from 5.3 MPa to 7.9 MPa, an increase of 49%; while the tensile strength of SMF/GTAGel is increased from 5.4 MPa to 7.9 MPa, an increase of 46%. At the sisal microfibril content of higher than 15 wt.%, however, the tensile strength of the composites decreases gradually. It may be that the sisal microfibrils aggregate at high concentration and cannot be dispersed uniformly in the system. This is apparently not in favor of the tensile strength increase. The improvement of tensile strength with sisal microfibril mat is even more significant. For SMFm/DAS-Gel, the tensile strength is increased from 5.3 MPa to 15.3 MPa with the addition of 17.9 wt.% sisal microfibril mat, nearly three times of the unfilled DAS-Gel matrix. For SMFm/ GTA-Gel, the tensile strength is increased from 5.4 MPa to 14.8 MPa with 16.9 wt.% sisal microfibril mat, an increase of
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Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/DAS-Gel SMFm/DAS-Gel
Sisal fiber/Gel (naturla sisal fibers treated with NaOH) SMF/GTA-Gel SMFm/GTA-Gel
16
Tensile strength (MPa)
Tensile strength (MPa)
16
12
8
4
12
8
4
0
0 0
5
10
15
20
0
25
5
10
15
20
25
Sisal content (wt%)
Sisal content (wt%)
Fig. 4. The tensile strength of SMF/Gel and SMFm/Gel biomass composites.
250 Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/DAS-Gel SMFm/DAS-Gel
200 160 120 80 40
Elongation at break (%)
Elongation at break (%)
240
Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/GTA-Gel SMFm/GTA-Gel
200 150 100 50 0
0 0
5
10
15
20
25
Sisal content (wt%)
0
5
10
15
20
25
Sisal content (wt%)
Fig. 5. The elongation at break of sisal fibril reinforced gelatin biomass composites.
174%. It is seen that with large aspect ratio (>100), the stress can be transferred effectively from gelatin to the reinforcing agent due to the good fibril–matrix interface adhesion. The sisal microfibril mat can bear the load effectively. The microfibrils can also prevent the crackle from propagating. These will give rise to an obvious improvement in mechanical properties. However, the addition of sisal fibrils decreases the elongation at break of the biomass composites, as shown in Fig. 5. The results indicate that although the sisal microfibrils can bear the load efficiently, the deformation ability is limited. The composites trends to be brittle. Fig. 6 shows the tensile fracture surface morphology of sisal microfibril mat/gelatin biomass composites. It can be seen that when sisal microfibrils are pulled out, a fair amount of polymer residue remains on the microfibril. The good wetting of the reinforcing agent by the matrix again confirms the good compatibility as the result of the strong interaction between the two phases. When the composite undergoes load, the stress can be transferred effectively from the matrix to the reinforcing agent. The sisal microfibrils can bear the stress efficiently, and therefore, increases the tensile strength of the composites dramatically. 3.3. Biodegradation performance A polymer composite is expected to be biodegradable but with a desirable life. As a protein, gelatin provides a suitable substrate for many organisms and is sensitive to the attack from bacteria and microorganisms. The degradation behavior of sisal microfibril/gelatin composites in PBS containing lysozyme was investigated. The results are illustrated in Fig. 7. It is found that the degradation ratio of all the composites investigated decreases continuously with the increase of sisal microfibril content. It indicates that the addition of
sisal microfibrils improves the anti-biodegradation performance. This is favorable for improving the service life of the composites. Meanwhile, if the sisal microfibrils are woven into a mat, the degradation ratio is even lower than that of the corresponding SMF/ Gel composites. It is worthy to note that the degradation ratio is much lower for the DAS crosslinked system than that for the GTA crosslinked one. It may be that, due to the high relative molecular weight of DAS, the DAS crosslinked gelatin is also a blend of DAS and gelatin. Attributed to the reactivity of its dialdehyde functional groups, DAS shows a more significant antibacterial activity than pure gelatin. The antibacterial activity of DAS endows the composites with good anti-biodegradation ability. 3.4. Water adsorption behavior Gelatin and cellulose have many hydrophilic groups on the macromolecular chains. The hydrophilic performance of the composites influences the physical properties. Therefore, the investigation of the water adsorption behavior of the composites is important. Fig. 8 shows the water adsorption curves of the biomass composites versus time at the relative humidity of 98%. For comparison, the adsorption curve of the original gelatin with identical crosslinking agent and PEG 400 contents is also included in the figure. From Fig. 8 it is seen that all adsorption curves are characterized by three zones: fast adsorption zone at short time (t < 40 h), a rapid increase in water uptake occurs; sequential slow adsorption zone at extended time (40 h < t < 100 h), the water uptake increases gradually; and the plateau at long time (t > 100 h), the adsorption rate hardly changes, corresponding to an adsorption equilibrium.
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Fig. 6. SEM images for tensile fracture surface of sisal microfibril/gelatin biomass composites. (a) SMFm/DAS-Gel, (b) SMFm/GTA-Gel, (c and d) are the higher magnification images of selected areas in (a and b).
SMF/DAS-Gel SMFm/DAS-Gel
26 24 22 20 18
Degredation ratio (wt%)
Degredation ratio (wt%)
33
(a)
28
(b)
SMF/GTA-Gel SMFm/GTA-Gel
32 31 30 29 28
0
5
10
15
20
25
Sisal microfibrils content (wt%)
0
5
10
15
20
25
Sisal microfibrils content (wt%)
Fig. 7. The degradation properties of SMF/Gel and SMFm/Gel biomass composites.
The water uptake at equilibrium for gelatin and the biomass composites as a function of sisal microfibrils content is plotted in Fig. 9. It is observed that unfilled DAS-Gel and GTA-Gel absorbs around 70% and 73% water, respectively. The water uptake at equilibrium decreases with increasing the sisal microfibrils content, regardless in the form of SMF or SMFm. Therefore, the presence of sisal microfibrils within the composites decreases the water sensitivity and reduces the swelling. This phenomenon can be ascribed to the reduction in the hydrophilicity of the biomass composites. The interaction between gelatin and sisal microfibrils consumes some of the amino groups of gelatin and the hydroxyl groups of cellulose molecular chains, and this reduces the hydrophilic tendency of the composites and depresses the water uptake through capillary action at the interface.
The water diffusivity or diffusion coefficient, D, of water in the sisal microfibrils reinforced gelatin composites was estimated using Eq. (3). The plots of (Mt–M0)/Me as a function of (t/L)1/2 were performed for all the compositions and for (Mt–M0)/Me 6 0.5. The diffusion coefficients were calculated from the slope of these plots and the results are shown in Fig. 10. The unfilled gelatin displays the highest diffusion coefficient. For the biomass composites with DAS crosslinked gelatin as matrix, adding SMF results in a decrease of D value from 11.57 1010 for unfilled gelatin to 8.34 1010 for the 25 wt.% filled system, while adding SMFm results in a D value to 10.07 1010 for the 21.3 wt.% filled one. For the biomass composites with GTA crosslinked gelatin as matrix, adding SMF results in a decrease of D value from 11.62 1010 for unfilled gelatin to 8.07 1010 for the 25 wt.% filled system, while adding
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80
80
(a) SMF/DAS-Gel Water uptake / %
Water uptake / %
(b) SMFm/DAS-Gel
70
70 60 50 40 content of SMF 0 5% 10% 15% 20% 25%
30 20 10 0 0
50
100
150
200
60 50 40 content of SMFm 0 4.4% 8.6% 11.2% 17.9% 21.3%
30 20 10 0
250
0
300
50
100
80
(c) SMF/GTA-Gel
200
250
300
(d) SMFm/GTA-Gel
80 70
60 50 40 content of SMF 0 5% 10% 15% 20% 25%
30 20 10 0 0
50
100
150
200
250
Water uptake / %
70
Water uptake/%
150
t/h
t/h
60 50 40 content of SMFm 0 4.8% 7.5% 10.7% 16.9% 20.3%
30 20 10 0 0
300
50
100
150
200
250
300
t/h
t/h
Water uptake at equilibrium / %
75 SMF/DAS-Gel SMFm/DAS-Gel
70 65 60 55 50 0
5
10
15
20
Water uptake at equilibrium / %
Fig. 8. Water adsorption versus time of sisal fibril/gelatin biomass composites at RH 98%.
75 SMF/GTA-Gel SMFm/GTA-Gel
70 65 60 55 50
25
5
0
Sisal microfibril content / wt%
10
15
20
25
Sisal microfibril content / wt%
-9
-9
1.2x10
SMF/DAS-Gel SMFm/DAS-Gel
-9
1.1x10
-9
1.0x10
-10
9.0x10
-10
8.0x10
-10
7.0x10
0
5
10
15
20
Sisal microfibril content / wt%
25
1.2x10
Water diffusion coefficient / %
2
Water diffusion coefficient / cm .s
-1
Fig. 9. Water uptake at equilibrium of sisal microfibril/gelatin biomass composites at RH 98%.
SMF/GTA-Gel SMFm/GTA-Gel
-9
1.1x10
-9
1.0x10
-10
9.0x10
-10
8.0x10
-10
7.0x10
0
5
10
15
20
Sisal microfibril content / wt%
Fig. 10. Water diffusion coefficient of sisal microfibril/gelatin biomass composites at RH 98%.
25
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SMFm results in a D value to 9.02 1010 for the 20.3 wt.% filled one. It is clear that the addition of sisal microfibrils decreases the water diffusion coefficient of the composites. The incorporation of sisal microfibrils tends to stabilize the gelatin matrix when it is submitted to environment with great moisture. 4. Conclusions The sisal fibers can be fibrillated into microfibrils with different aspect ratios by alkaline treatment followed by enzyme hydrolysis or mechanical disintegration. Sisal microfibrils and gelatin has good compatibility due to the strong intermolecular interaction between them. The incorporation of sisal microfibrils greatly improves the mechanical properties, anti-biodegradable property and decreases the water adsorption ability of the composites. When the sisal microfibrils are used in a form of a mat, the reinforcing effect is more drastic. Acknowledgment The financial support of this work by the National Natural Science Foundation of China (Grant No. 20604026) and Henan Scientific & Technological Department (No. 092300410130) is gratefully acknowledged. References [1] Netravali AN, Chabba S. Composites get greener. Materials today; 2003 April 22–29. [2] Lu Y, Weng L, Zhang L. Morphology and properties of soy protein isolate thermoplastics reinforced with chitin whiskers. Biomacromolecules 2004;5:1046–51. [3] Juntaro J, Pommet M, Kalinka G, Mantalaris A, Shaffer MSP, Bismarck A. Creating hierarchical structures in renewable composites by attaching bacterial cellulose onto sisal fibers. Adv Mater 2008;20:3122–6. [4] Mohanty AK, Misra M, Drzal LT. Natural fibers, biopolymers, and biomass composites. CRC Press; 2005. [5] Reddy N, Yang Y. Biofibers from agricultural byproducts for industrial applications. Trends Biotechnol 2005;23(1):22–7. [6] Duchemin BJC, Newman RH, Staiger MP. Structure-property relationship of allcellulose composites. Compos Sci Technol 2009;69(7–8):1225–30. [7] Bledzki AK, Manun AA, Jaszkiewicz A, Erdmann K. Polypropylene composites with enzyme modified abaca fibre. Compos Sci Technol 2010;70(5):854–60. [8] Liu L, Yu J, Cheng L, Qu W. Mechanical properties of poly(butylene succinate) (PBS) biocomposites reinforced with surface modified jute fibre. Compos Part A: Appl Sci Manuf 2009;40(5):669–74. [9] Soykeabkaew N, Nishino T, Peijs T. All-cellulose composites of regenerated cellulose fibres by surface selective dissolution. Compos Part A: Appl Sci Manuf 2009;40(4):321–8.
[10] Summerscales J, Dissanayake NPJ, Virk AS, Hall W. A review of bast fibres and their composites. Part 1 – Fibres as reinforcements. Compos Part A: Appl Sci Manuf 2010;41(10):1329–35. [11] Summerscales J, Dissanayake NPJ, Virk AS, Hall W. A review of bast fibres and their composites. Part 2 – Composites. Compos Part A: Appl Sci Manuf 2010;41(10):1336–44. [12] Duigou AL, Pillin I, Bournaud A, Davies P, Baley C. Effect of recycling on mechanical behaviour of biocompostable flax/poly(L-lactide) composites. Compos Part A: Appl Sci Manuf 2008;39(9):1471–8. [13] Sreekumar PA, Gopalakrishnan P, Leblanc N, Saiter JM. Effect of glycerol and short sisal fibers on the viscoelastic behavior of wheat flour based thermoplastic. Compos Part A: Appl Manuf 2010;41(8):991–6. [14] Lee BH, Kim HS, Lee S, Kim HJ, Dorgan JR. Bio-composites of kenaf fibers in polylactide: role of improved interfacial adhesion in the carding process. Compos Sci Technol 2009;69(15–16):2573–9. [15] John MJ, Thomas S. Biofibres and biomass composites. Carbohydr Polym 2008;71:343–64. [16] Li Y, Mai YW, Ye L. Sisal fibre and its composites: a review of recent developments. Compos Sci Technol 2000;60:2037–55. [17] Li X, Tabil LG, Panigrahi S. Chemical treatments of natural fiber for use in natural fiber-reinforced composites: a review. J Polym Environ 2007;15:25–33. [18] Nakayama A, Kakugo A, Gong JP, Osada Y, Takai M, Erata T. High mechanical strength double-network hydrogel with bacterial cellulose. Adv Funct Mater 2004;14(11):1124–8. [19] Lee SH, Wang S. Biodegradable polymer/bamboo fiber biomass composite with bio-based coupling agent. Composites: Part A 2006;37:80–91. [20] Wang L, Kumar R, Zhang L. Investigation into hemp fiber- and whiskerreinforced soy protein composites. Front Chem China 2009;4(3):313–20. [21] Reddy N, Yang Y. Properties and potential applications of natural cellulose fibers from cornhusks. Green Chem 2005;7:190–5. [22] Alix S, Marais S, Morvan C, Lebrun L. Biocomposite materials from flax plants: preparation and properties. Compos Part A: Appl Sci Manuf 2008;39(12):1793–801. [23] Huang X, Netravali A. Biodegradable green composites made using bamboo micro/nano-fibrils and chemically modified soy protein resin. Compos Sci Technol 2009;69(7–8):1009–15. [24] Chiellini E, Cinelli P, Fernandes EG, Kenawy ERS, Lazzeri A. Gelatin-based blends and composites. Morphological and thermal mechanical characterization. Biomacromolecules 2001;2:806–11. [25] Bledzki AK, Jaszkiewicz. Mechanical performance of biocomposites based on PLA and PHBV reinforced with natural fibres-A comparative study to PP. Compos Sci Technol 2010;70(12):1687–96. [26] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Prog Polym Sci 1999;24:221–74. [27] Zimmermann T, Pohler E, Geiger T. Cellulose fibrils for polymer reinforcement. Adv Eng Mater 2004;6(9):754–961. [28] Pkk M, Ankerfors M, Kosonen H, Nyknen A, Ahola S, Sterberg M, et al. Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels. Biomacromolecules 2007;8(6):1934–41. [29] Angles MN, Dufresne A. Plasticized starch/tunicin whiskers nanocomposites 1. Structural analysis. Macromolecules 2000;33:8344–53. [30] Haroun AA, Gamal-Eldeen A, Harding DRK. Preparation, characterization and in vitro biological study of biomimetic three-dimensional gelatin– montmorillonite/cellulose scaffold for tissue engineering. J Mater Sci: Mater Med 2009;20:2527–40.
Contents lists available at SciVerse ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Preparation and properties of sisal microfibril/gelatin biomass composites Xuejing Zheng, Jie Liu, Ying Pei, Junwei Li, Keyong Tang ⇑ College of Materials Science and Engineering, Zhengzhou University, Henan 450052, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 2 July 2011 Accepted 30 August 2011 Available online 6 September 2011 Keywords: A. Fibers A. Polymer–matrix composites (PMCs) B. Physical properties
a b s t r a c t In this work, the natural sisal fibers were fibrillated by enzyme hydrolysis or mechanical disintegration into microfibrils with a width of 5–10 lm and different aspect ratios. The sisal microfibrils or microfibril mats were added into the gelatin to prepare biomass composites, by solvent-casting or solution impregnation techniques, respectively. The morphology, mechanical properties, biodegradation property, and water adsorption behaviors of the composites were investigated. It was found that the tensile strength of the composites was dramatically increased with the addition of sisal microfibrils. The degradation ratio of the composites decreased continuously with increasing the sisal fibril content. The addition of sisal microfibrils decreased the water uptake at equilibrium and the water diffusion coefficient. Scanning electron microscopy characterization showed that the sisal microfibrils were very well embedded in the gelatin matrix, showing a good interfacial adhesion. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The ever growing global environmental awareness and societal concern, high rate of depletion of petroleum resources, new environmental regulations, concepts of sustainability and low-carbon economy have triggered the search for new products and processes that are compatible with the environment [1–7]. The exhaustive use of petroleum based resources has initiated the efforts to develop biodegradable materials [8–14]. Biomass composites from plant derived fibers (natural/biofibers) and crop/bioderived plastic (biopolymer/bioplastic) are eco-friendly and such composites are termed as ‘‘green composites’’ [1]. The major attractions about green composites are that they are environmentally friendly, fully degradable and sustainable. Besides, at the end of their life, they can be easily disposed of or composted without harming the environment [15]. The use of biodegradable and environment-friendly plant-based ‘‘lignocellulosic’’ fibers has been a natural choice for reinforcing polymers to make them ‘‘greener’’ [16–25]. The growing interest in lignocellulosic fibers is mainly due to their economical production with few requirements for equipment and low specific weight, which results in a higher specific strength and stiffness, compared to glass reinforced composites. They also present safer handling and working conditions compared to synthetic reinforcements. Plant fibers are nonabrasive to mixing and molding equipment, which can contribute to significant cost reductions. The most interesting
⇑ Corresponding author. Tel.: +86 371 67763216; fax: +86 371 67780050. E-mail address: [email protected] (K. Tang). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.08.024
aspect about natural plant fibers is their positive environmental impact. In many of the reported research works about plant-based composites, the plant fibers are treated with alkaline solutions to remove the non-cellulose substances and then added into the polymer matrix. However, the reinforcing effect of the plant fibers cannot be fully realized in this way. It is known that natural plant fibers have a complex hierarchical structure. A single plant fiber (usually 100–200 lm in width) is actually a bundle of fibrils. One fibril is composed of many microfibrils, and one microfibril is composed of hundreds and thousands of cellulose chains. Therefore, plant fiber has complex hierarchical structure. Natural fibers can be processed in different ways to yield reinforcing elements with different mechanical properties [26]. The elastic modulus of bulk natural fibers such as wood is about 10 GPa. Cellulose fiber with the moduli up to 40 GPa can be separated from wood, for instance, by chemical pulping processes. Such cellulose fibers can be further subdivided by hydrolysis followed by mechanical disintegration into microfibrils with an elastic modulus of 70 GPa. Theoretical calculations of the elastic moduli of cellulose chains have given values of up to 250 GPa, however, there is no technology available to separate these from microfibrils. It can be seen that if the natural plant fibers are fibrillated into microfibrils with smaller diameters, a drastic reinforcing effect can be obtained [27,28]. In this work, the natural sisal fibers were fibrillated into microfibrils by alkaline treatment followed by enzyme hydrolysis or mechanical disintegration. Sisal microfibrils with width around 5–10 lm and with different aspect ratios were obtained. The sisal microfibrils were added in the form of microfibrils or microfibril mat into gelatin solution to prepare biomass composites by
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techniques of solvent-casting and solution impregnation, respectively. The mechanical properties, water adsorption behavior, and biodegradation property were investigated. The morphology of the composites was characterized by using scanning electron microscopy. 2. Materials and methods
2.6. Mechanical test The samples were conditioned at RH 65% for at least 1 week to constant weight. Mechanical testes of the dumbbell samples were measured with a universal tensile tester, CMT5140. The tests were carried out at a crosshead speed of 50 mm/min. The average of at least five tests was reported. Table 1 shows the short name of the samples used in this paper.
2.1. Materials 2.7. Water adsorption behavior Gelatin granules (biochemical reagent) were supplied by Tianjin Chemicals Co., Tianjin, China. Natural sisal fibers with a diameter of 100–200 lm were purchased from Guangxi Sisal Fiber Co., Guangxi, China. Dialdehyde starch (medical grade) was purchased from Tai’an Jinshan Modified Starch Co., Shandong, China. Other chemical agents were all analytically pure. 2.2. Gelatin modification The gelatin was soaked in deionized distilled water at 40 °C for 45 min. The mixture was then heated to 60 °C until the solution became homogeneous. Solution with concentration of 20% gelatin was obtained. Poly(ethylene glycol) (PEG 400), content of 20% based on dry gelatin, was added as a plasticizing agent. The gelatin was crosslinked with dialdehyde starch (DAS, 7.4% based on dry gelatin) or glutaraldehyde (GTA, 1.5% based on dry gelatin) at 60 °C and pH 8–9. Gelatin modified with DAS and GTA is denoted as DAS-Gel and GTA-Gel, respectively. 2.3. Alkali treatment of sisal fibers The chopped sisal fibers and a 10% NaOH solution were put into a beaker and stirred for 2 h at room temperature. Afterward, the sisal fibers were washed thoroughly with water to remove NaOH and dried at 50 °C in a vacuum oven. 2.4. Fibrillation of sisal fibers The fibrillation of sisal fibers were carried out with two methods. Method 1: a mount of b-glucanase was added in NaAc/HAc buffer solution (pH 4.8) to prepare an enzyme solution with concentration of 0.25%. The alkali-treated sisal fibers were hydrolyzed in the enzyme solution at 50 °C for 1 h under continuous agitation. Sisal microfibrils with diameters of 5–10 lm and aspect ratio of 5–30 were obtained. Method 2: the alkali-treated sisal fibers were put in a stirrer and disintegrated mechanically with a stirring rate of 20,000 rpm. Sisal microfibrils with diameters of 5–10 lm and aspect ratios larger than 100 were obtained. 2.5. Preparation of sisal microfibril/gelatin biomass composites The sisal microfibrils with short aspect ratios were added into the modified gelatin solution and mixed under agitation. Then the mixture were poured into a self-made polypropylene mold and dried in air at room temperature. This method is called as solvent-casting technique. The composite prepared in this way is denoted as SMF/Gel biomass composite. The sisal microfibrils with long aspect ratios were mixed with water to make a mat by negative-pressure filtration. Put the sisal microfibrils mat in the mold, and pour the modified gelatin solution into the mold. The composites were dried in air at room temperature. This method is called as solution impregnation technique. A composite with high microfibril content can be prepared by this method. The composite prepared is denoted as SMFm/Gel biomass composites.
The water adsorption kinetics of the composites was determined. Rectangular specimens sized 10 mm 10 mm with a thickness of 0.6 mm were prepared. The samples were conditioned at 25 °C in a desiccator containing P2O5 to a constant weight. The samples were weighed accurately, noted as W0. Then the samples were put in a desiccator containing a supersaturated salt solution of CuSO45H2O to give a RH of 98%. The weight of the samples at the adsorption time of t was noted as Wt and the weight at adsorption equilibrium was noted as We. The amount of water adsorbed at equilibrium was calculated as:
Me ð%Þ ¼
We W0 100 W0
ð1Þ
The mean water uptake of each sample was calculated for various adsorption times (t). The mass of water adsorbed at time t, (Wt–W0), can be expressed as follows:
" # 1 Wt W0 X 8 Dð2n þ 1Þ2 p2 t ¼ exp 2 2 We 4L2 n¼0 ð2n þ 1Þ p
ð2Þ
where We is the weight adsorbed at equilibrium; 2L, the thickness of the film; and D, the diffusion coefficient [2]. At a short time, Eq. (2) can be written as:
1=2 Wt W0 2 D ¼ t1=2 L p We
ð3Þ
At (Wt–W0)/We 6 0.5, the error in using Eq. (3) instead of Eq. (2) to determine the diffusion coefficient is on the order of 0.1% [29]. 2.8. Biodegradation property The biodegradation behavior of the composites was studied with a method similar to that described in Ref. [30]. The weight of the composites was measured (W0). The degradation buffer was prepared from phosphate buffer saline (PBS), pH adjusted to 5.2 with acetic acid, and 0.1 w/v% hen egg white (lysozyme) was added. The samples were immersed in the degradation buffer and incubated at 37 °C for 12 h. After 12 h, the samples were removed, washed with deionized water, dried and re-weighed W1. The degradation ratio was calculated as following:
DRð%Þ ¼
W0 W1 100 W0
ð4Þ
where DR is the biodegradation percentage, W0 is the weight at zero biodegradation time, and W1 is the weight at the degradation time Table 1 Sample name notation. Short name
Matrix
Reinforcing agent
SMF/Gel SMFm/Gel SMF/DAS-Gel SMFm/DAS-Gel SMF/GTA-Gel SMFm/GTA-Gel
Gelatin Gelatin DAS modified DAS modified GTA modified GTA modified
Sisal Sisal Sisal Sisal Sisal Sisal
gelatin gelatin gelatin gelatin
microfibril microfibril mat microfibril microfibril mat microfibril microfibril mat
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of 12 h. Each degradation experiment was repeated three times and the average was reported. 2.9. Scanning electron microscopy characterization The surfaces or fracture surfaces of the samples were examined by scanning electron microscopy (SEM, FEI-quanta 200, The Netherlands). The facture surfaces were got by immersing and breaking the samples in liquid nitrogen, or by tensile fracture tests. The surface was coated with a thin layer of gold prior to SEM examination. 3. Results and discussion 3.1. Morphology of sisal microfibrils and the composites Scheme 1. Possible hydrogen bonding formed between sisal fiber and gelatin.
gelatin
Transmittance/%
Fig. 1 shows the morphology of the natural sisal fiber and the sisal microfibrils fibrillated. The diameter of the natural sisal fiber is about 100 lm and the surface is rough (Fig. 1a). The enzyme hydrolysis process leads to effective fiber fibrillation, breaking down the fiber bundle into smaller fibrils with width of around 5–10 lm and aspect ratio of 5–30 (Fig. 1b). The sisal microfibrils prepared by mechanical disintegration is thin and long, with smooth surface. The sample is around 5–10 lm in width with the length of more than 1 mm. Therefore, the aspect ratio is larger than 100 (Fig. 1c). It is interesting to see that the mechanical fibrillation results in microfibrils with substantially larger aspect ratio compared with the enzyme fibrillation. This may be the result that mechanical fibrillation tends to cleavage along the longitude of the cellulose fibers. When subjected to enzyme hydrolysis, however, cellulose fibers undergo transverse cleavage along the amorphous regions and results in a rod-like material with a relatively low aspect ratio. Meanwhile, the disintegration of cellulose fibers goes inevitably with the decomposition or degradation during enzyme fibrillation. Since each anhydroglucose unit contains three free hydroxy groups, there are large amount of hydroxy groups in cellulose macromolecules. Gelatin, a protein, contains abundant hydrophilic groups on the chains, such as amido, hydroxyl and carboxyl. Therefore, a great number of strong hydrogen bonding can be formed between the hydroxy groups in cellulose and the hydrophilic sites in gelatin, as described in Scheme 1. Due to the strong intermolecular interaction, the compatibility between sisal microfibril and gelatin is expected to be good. To analyze the interaction between the two phases, FTIR characterization was carried out for pure gelatin and sisal microfibril/gelatin composite (50 wt.%/50 wt.%) and the results are shown in Fig. 2. A broad absorption band at 3421 cm1 for pure gelatin is assigned to the –NH stretching coupled with the hydrogen bonding. When gelatin is composed with
1245
3421
composite
1238 3322 4000 3500 3000 2500 2000 1500 1000
500
-1
wavenumber /cm
Fig. 2. FTIR spectra of gelatin and SMF/Gel composite.
sisal microfibril, this peak shifts to lower wavenumber at 3322 cm1. Moreover, the peak at 1245 cm1 for –NH bending in pure gelatin shifts to 1238 cm1 in the composite. These results indicate that strong interaction exists between gelatin and sisal microfibrils.
Fig. 1. SEM surface morphology of sisal microfibrils. (a) Natural sisal fiber, (b) sisal microfibrils by enzyme hydrolysis with short aspect ratios, (c) sisal microfibrils by mechanical disintegration, with long aspect ratios to make mats.
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(a) SMF/Gel
(c) SMF/DAS-Gel
(e) SMFm/DAS-Gel
(b) SMFm/Gel
(d) SMF/GTA-Gel
(f) SMFm/GTA-Gel
Fig. 3. SEM images of cross-sectional morphology of SMF and SMFm reinforced gelatin biomass composites.
To investigate the dispersion of sisal microfibrils in gelatin matrix, biomass composites have been prepared and the crosssectional morphology is shown in Fig. 3. It is seen that sisal microfibrils are tightly covered by gelatin, showing a very good interfacial adhesion, as shown in Fig. 3a. When gelatin is crosslinked with DAS or GTA, the SMF and SMFm is also well dispersed in the matrix, however the dispersion is less uniform due to the high viscous crosslinked matrix compared with the uncrosslinked one (Fig. 3c–f). 3.2. Mechanical properties Fig. 4 shows the tensile strength of the composites. The line tagged by –j– represents the tensile strength of sisal fiber/gelatin composite, in which the natural sisal fibers were not fibrillated other than ordinary NaOH treatment. Alkaline treatment removes most of the non-cellulose substances but basically remains the original diameters of the sisal fibers. It is seen that the tensile strength
decreases with the increase of sisal fibers content, and then increases slightly. When the sisal fibers are fibrillated into microfibrils, however, the tensile strength of the composites increases drastically. When 15 wt.% sisal microfibrils are incorporated, the tensile strength of SMF/DAS-Gel is increased from 5.3 MPa to 7.9 MPa, an increase of 49%; while the tensile strength of SMF/GTAGel is increased from 5.4 MPa to 7.9 MPa, an increase of 46%. At the sisal microfibril content of higher than 15 wt.%, however, the tensile strength of the composites decreases gradually. It may be that the sisal microfibrils aggregate at high concentration and cannot be dispersed uniformly in the system. This is apparently not in favor of the tensile strength increase. The improvement of tensile strength with sisal microfibril mat is even more significant. For SMFm/DAS-Gel, the tensile strength is increased from 5.3 MPa to 15.3 MPa with the addition of 17.9 wt.% sisal microfibril mat, nearly three times of the unfilled DAS-Gel matrix. For SMFm/ GTA-Gel, the tensile strength is increased from 5.4 MPa to 14.8 MPa with 16.9 wt.% sisal microfibril mat, an increase of
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Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/DAS-Gel SMFm/DAS-Gel
Sisal fiber/Gel (naturla sisal fibers treated with NaOH) SMF/GTA-Gel SMFm/GTA-Gel
16
Tensile strength (MPa)
Tensile strength (MPa)
16
12
8
4
12
8
4
0
0 0
5
10
15
20
0
25
5
10
15
20
25
Sisal content (wt%)
Sisal content (wt%)
Fig. 4. The tensile strength of SMF/Gel and SMFm/Gel biomass composites.
250 Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/DAS-Gel SMFm/DAS-Gel
200 160 120 80 40
Elongation at break (%)
Elongation at break (%)
240
Sisal siber/Gel (natural sisal fiber treated with NaOH) SMF/GTA-Gel SMFm/GTA-Gel
200 150 100 50 0
0 0
5
10
15
20
25
Sisal content (wt%)
0
5
10
15
20
25
Sisal content (wt%)
Fig. 5. The elongation at break of sisal fibril reinforced gelatin biomass composites.
174%. It is seen that with large aspect ratio (>100), the stress can be transferred effectively from gelatin to the reinforcing agent due to the good fibril–matrix interface adhesion. The sisal microfibril mat can bear the load effectively. The microfibrils can also prevent the crackle from propagating. These will give rise to an obvious improvement in mechanical properties. However, the addition of sisal fibrils decreases the elongation at break of the biomass composites, as shown in Fig. 5. The results indicate that although the sisal microfibrils can bear the load efficiently, the deformation ability is limited. The composites trends to be brittle. Fig. 6 shows the tensile fracture surface morphology of sisal microfibril mat/gelatin biomass composites. It can be seen that when sisal microfibrils are pulled out, a fair amount of polymer residue remains on the microfibril. The good wetting of the reinforcing agent by the matrix again confirms the good compatibility as the result of the strong interaction between the two phases. When the composite undergoes load, the stress can be transferred effectively from the matrix to the reinforcing agent. The sisal microfibrils can bear the stress efficiently, and therefore, increases the tensile strength of the composites dramatically. 3.3. Biodegradation performance A polymer composite is expected to be biodegradable but with a desirable life. As a protein, gelatin provides a suitable substrate for many organisms and is sensitive to the attack from bacteria and microorganisms. The degradation behavior of sisal microfibril/gelatin composites in PBS containing lysozyme was investigated. The results are illustrated in Fig. 7. It is found that the degradation ratio of all the composites investigated decreases continuously with the increase of sisal microfibril content. It indicates that the addition of
sisal microfibrils improves the anti-biodegradation performance. This is favorable for improving the service life of the composites. Meanwhile, if the sisal microfibrils are woven into a mat, the degradation ratio is even lower than that of the corresponding SMF/ Gel composites. It is worthy to note that the degradation ratio is much lower for the DAS crosslinked system than that for the GTA crosslinked one. It may be that, due to the high relative molecular weight of DAS, the DAS crosslinked gelatin is also a blend of DAS and gelatin. Attributed to the reactivity of its dialdehyde functional groups, DAS shows a more significant antibacterial activity than pure gelatin. The antibacterial activity of DAS endows the composites with good anti-biodegradation ability. 3.4. Water adsorption behavior Gelatin and cellulose have many hydrophilic groups on the macromolecular chains. The hydrophilic performance of the composites influences the physical properties. Therefore, the investigation of the water adsorption behavior of the composites is important. Fig. 8 shows the water adsorption curves of the biomass composites versus time at the relative humidity of 98%. For comparison, the adsorption curve of the original gelatin with identical crosslinking agent and PEG 400 contents is also included in the figure. From Fig. 8 it is seen that all adsorption curves are characterized by three zones: fast adsorption zone at short time (t < 40 h), a rapid increase in water uptake occurs; sequential slow adsorption zone at extended time (40 h < t < 100 h), the water uptake increases gradually; and the plateau at long time (t > 100 h), the adsorption rate hardly changes, corresponding to an adsorption equilibrium.
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Fig. 6. SEM images for tensile fracture surface of sisal microfibril/gelatin biomass composites. (a) SMFm/DAS-Gel, (b) SMFm/GTA-Gel, (c and d) are the higher magnification images of selected areas in (a and b).
SMF/DAS-Gel SMFm/DAS-Gel
26 24 22 20 18
Degredation ratio (wt%)
Degredation ratio (wt%)
33
(a)
28
(b)
SMF/GTA-Gel SMFm/GTA-Gel
32 31 30 29 28
0
5
10
15
20
25
Sisal microfibrils content (wt%)
0
5
10
15
20
25
Sisal microfibrils content (wt%)
Fig. 7. The degradation properties of SMF/Gel and SMFm/Gel biomass composites.
The water uptake at equilibrium for gelatin and the biomass composites as a function of sisal microfibrils content is plotted in Fig. 9. It is observed that unfilled DAS-Gel and GTA-Gel absorbs around 70% and 73% water, respectively. The water uptake at equilibrium decreases with increasing the sisal microfibrils content, regardless in the form of SMF or SMFm. Therefore, the presence of sisal microfibrils within the composites decreases the water sensitivity and reduces the swelling. This phenomenon can be ascribed to the reduction in the hydrophilicity of the biomass composites. The interaction between gelatin and sisal microfibrils consumes some of the amino groups of gelatin and the hydroxyl groups of cellulose molecular chains, and this reduces the hydrophilic tendency of the composites and depresses the water uptake through capillary action at the interface.
The water diffusivity or diffusion coefficient, D, of water in the sisal microfibrils reinforced gelatin composites was estimated using Eq. (3). The plots of (Mt–M0)/Me as a function of (t/L)1/2 were performed for all the compositions and for (Mt–M0)/Me 6 0.5. The diffusion coefficients were calculated from the slope of these plots and the results are shown in Fig. 10. The unfilled gelatin displays the highest diffusion coefficient. For the biomass composites with DAS crosslinked gelatin as matrix, adding SMF results in a decrease of D value from 11.57 1010 for unfilled gelatin to 8.34 1010 for the 25 wt.% filled system, while adding SMFm results in a D value to 10.07 1010 for the 21.3 wt.% filled one. For the biomass composites with GTA crosslinked gelatin as matrix, adding SMF results in a decrease of D value from 11.62 1010 for unfilled gelatin to 8.07 1010 for the 25 wt.% filled system, while adding
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80
80
(a) SMF/DAS-Gel Water uptake / %
Water uptake / %
(b) SMFm/DAS-Gel
70
70 60 50 40 content of SMF 0 5% 10% 15% 20% 25%
30 20 10 0 0
50
100
150
200
60 50 40 content of SMFm 0 4.4% 8.6% 11.2% 17.9% 21.3%
30 20 10 0
250
0
300
50
100
80
(c) SMF/GTA-Gel
200
250
300
(d) SMFm/GTA-Gel
80 70
60 50 40 content of SMF 0 5% 10% 15% 20% 25%
30 20 10 0 0
50
100
150
200
250
Water uptake / %
70
Water uptake/%
150
t/h
t/h
60 50 40 content of SMFm 0 4.8% 7.5% 10.7% 16.9% 20.3%
30 20 10 0 0
300
50
100
150
200
250
300
t/h
t/h
Water uptake at equilibrium / %
75 SMF/DAS-Gel SMFm/DAS-Gel
70 65 60 55 50 0
5
10
15
20
Water uptake at equilibrium / %
Fig. 8. Water adsorption versus time of sisal fibril/gelatin biomass composites at RH 98%.
75 SMF/GTA-Gel SMFm/GTA-Gel
70 65 60 55 50
25
5
0
Sisal microfibril content / wt%
10
15
20
25
Sisal microfibril content / wt%
-9
-9
1.2x10
SMF/DAS-Gel SMFm/DAS-Gel
-9
1.1x10
-9
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Fig. 9. Water uptake at equilibrium of sisal microfibril/gelatin biomass composites at RH 98%.
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Fig. 10. Water diffusion coefficient of sisal microfibril/gelatin biomass composites at RH 98%.
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X. Zheng et al. / Composites: Part A 43 (2012) 45–52
SMFm results in a D value to 9.02 1010 for the 20.3 wt.% filled one. It is clear that the addition of sisal microfibrils decreases the water diffusion coefficient of the composites. The incorporation of sisal microfibrils tends to stabilize the gelatin matrix when it is submitted to environment with great moisture. 4. Conclusions The sisal fibers can be fibrillated into microfibrils with different aspect ratios by alkaline treatment followed by enzyme hydrolysis or mechanical disintegration. Sisal microfibrils and gelatin has good compatibility due to the strong intermolecular interaction between them. The incorporation of sisal microfibrils greatly improves the mechanical properties, anti-biodegradable property and decreases the water adsorption ability of the composites. When the sisal microfibrils are used in a form of a mat, the reinforcing effect is more drastic. Acknowledgment The financial support of this work by the National Natural Science Foundation of China (Grant No. 20604026) and Henan Scientific & Technological Department (No. 092300410130) is gratefully acknowledged. References [1] Netravali AN, Chabba S. Composites get greener. Materials today; 2003 April 22–29. [2] Lu Y, Weng L, Zhang L. Morphology and properties of soy protein isolate thermoplastics reinforced with chitin whiskers. Biomacromolecules 2004;5:1046–51. [3] Juntaro J, Pommet M, Kalinka G, Mantalaris A, Shaffer MSP, Bismarck A. Creating hierarchical structures in renewable composites by attaching bacterial cellulose onto sisal fibers. Adv Mater 2008;20:3122–6. [4] Mohanty AK, Misra M, Drzal LT. Natural fibers, biopolymers, and biomass composites. CRC Press; 2005. [5] Reddy N, Yang Y. Biofibers from agricultural byproducts for industrial applications. Trends Biotechnol 2005;23(1):22–7. [6] Duchemin BJC, Newman RH, Staiger MP. Structure-property relationship of allcellulose composites. Compos Sci Technol 2009;69(7–8):1225–30. [7] Bledzki AK, Manun AA, Jaszkiewicz A, Erdmann K. Polypropylene composites with enzyme modified abaca fibre. Compos Sci Technol 2010;70(5):854–60. [8] Liu L, Yu J, Cheng L, Qu W. Mechanical properties of poly(butylene succinate) (PBS) biocomposites reinforced with surface modified jute fibre. Compos Part A: Appl Sci Manuf 2009;40(5):669–74. [9] Soykeabkaew N, Nishino T, Peijs T. All-cellulose composites of regenerated cellulose fibres by surface selective dissolution. Compos Part A: Appl Sci Manuf 2009;40(4):321–8.
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