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Roles and evidence of wood flour as an antibacterial promoter for triclosan-filled poly(lactic acid)
Composites: Part B 43 (2012) 2730–2737
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Roles and evidence of wood flour as an antibacterial promoter for triclosan-filled poly(lactic acid) C. Prapruddivongs, N. Sombatsompop ⇑ Polymer Processing and Flow (P-PROF) Research Group, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi (KMUTT), 126 Pracha-Uthit, Thongkru, Bangmod, Bangkok 10140, Thailand
a r t i c l e
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Article history: Received 26 April 2011 Received in revised form 26 August 2011 Accepted 30 August 2011 Available online 3 May 2012 Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Thermal analysis E. Extrusion
a b s t r a c t Mechanical and antibacterial performances of poly(lactic acid) (PLA) were assessed as a function of triclosan and wood fillers under a wide range of contact times using Gram-negative Escherichia coli (E. coli, ATCC 25922) as testing bacteria. Water contact angle (WCA) measurement, Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were performed to characterize the PLA, its blend with triclosan, and its composite with wood filler, and used to substantiate the antibacterial performance results. The experimental results suggested that introduction of wood flour changed the mechanical properties of PLA, whereas triclosan had no definite effect on the mechanical properties. E. coli growth appeared to increase with contact time for PLA, but to decrease with contact time for the wood/PLA composites. Based on the results and quantitative evidence, it was proposed that the wood flour acted as an ‘‘antibacterial promoter’’ for triclosan blended wood/PLA composites, which facilitated triclosan migration onto the wood/PLA composite surfaces to kill the bacteria. The molecular interactions between PLA, triclosan and wood were quantitatively characterized experimentally by CAM, DSC and FTIR. The change in hydrophilicity by the presence of wood filler was found to be the main reason for the improved antibacterial activity of triclosan in the wood/PLA composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Poly(lactic acid) (PLA) is a biodegradable and aliphatic polyester obtained from agricultural plants such as sugarcane and corn starch; one of its most widespread uses is in food packaging applications. In such applications, PLA products are required to be classified as highly hygienic or antimicrobial. Methods to obtain antimicrobial packaging products containing PLA as well as other polymers are usually carried out through incorporating or blending of antimicrobial agents, coating an antimicrobial substance onto a PLA surface, or immobilizing antimicrobials to polymers by ion or covalent linkages [1]. However, the effectiveness of these methods is dependent upon the molecular structure of the polymers used, the type and concentration of antibacterial agents, and the processing conditions [2]. Based on the available literature, there have been a number studies of the anti-microbial performance of PLA in various forms, including neat, blended and composite, and using different types of antimicrobial agents including nisin, Nisaplin, and nano-silver, under a wide range of testing conditions. Jin and Zhang [3] studied the effect of nisin on the antimicrobial properties of PLA film
⇑ Corresponding author. Tel.: +66 2 470 8645; fax: +66 2 470 8647. E-mail address: [email protected] (N. Sombatsompop). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.032
against foodborne pathogens using plate count agar, and found that nisin/PLA films could inhibit Gram-positive Listeria monocytogenes and Salmonella enteritidis, but showed less antimicrobial efficiency for Gram-negative Escherichia coli. At high temperature (up to 433 K) nisin/PLA and NisaplinÒ/PLA composites effectively inhibit bacteria growth in the presence of pectin [4]. However, it was noted that the added pectin resulted in decreases in the mechanical properties of the nisin/PLA and NisaplinÒ/PLA composites. Use of nano-silver for antimicrobial performance in polymeric materials has also interested scientists and technologists for creating value-added PLA products. Nano-silver (Ag), which was incorporated into PLA fibrous membranes for tissue engineering scaffold applications, could kill both E. coli and Staphylococcus aureus [5]; but the effectiveness of the Ag+ released from silver zeolite/ PLA composite films to kill the bacteria was enhanced by increasing the test temperature, media polarity and crystallinity level of PLA composite films [6]. Wood plastic composite (WPC) is a relatively new material which is manufactured by blending thermoplastics and wood fillers. WPC products have increasingly become attractive because of cost savings, good mechanical properties, better dimensional stabilities, and environmental issues, and have many potential uses and applications, such as window and door components, packaging, handrails, pallets, and structural members in decks and small beams. Most thermoplastics used include polyethylene (PE) [7],
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polypropylene (PP) [8], and poly(vinyl chloride) (PVC) [9], due to easy handling and wide processing windows. However, a survey of recent literature has indicated that experimental data and information on wood flour (WF) and PLA composites in WPC applications are rare, possibly due to processing difficulties and poor mechanical properties of the final products as a result of molecular incompatibilities between PLA and wood [10,11]. Lee et al. [12] found that the addition of WF in PLA plastic lowered the glass transition (Tg) and crystalline temperatures (Tc) of the PLA, and also decreased the tensile strength. Similar behavior was also observed for toughness properties [13], although the deficiency could be partially remedied by chemical treatment of the wood surface [13,14]. Having studied the available published research, we found that none of the published studies have investigated the antimicrobial activities of wood flour/PLA composites. It was therefore proposed in this work that wood/PLA composites cannot only make the PLA more environmentally friendly, but also result in a higher tendency toward biodegradability after their service life. In other words, wood/PLA packaging products should be hygienic, have the ability to inhibit the growth of bacteria during service, and should quickly biodegrade after use. It was assumed that the presence of hydrophilic wood should result in a faster biodegradable reaction of PLA, since PLA is usually chemically biodegraded via hydrolysis [15–17] in the presence of water molecules: i.e. the higher the water absorption, the greater the biodegradation rate of PLA. Our studies on the properties of wood/PLA composites are twofold: one concerning their mechanical and antibacterial properties, and the other their biodegradable properties. However, the present work focused only on the effect of wood flour on the mechanical and antibacterial properties of PLA. Triclosan (2,4,40 -trichloro-20 hydroxydiphenylether) was selected and used as an antibacterial agent; as previously found, it had the ability to retard bacterial growth in PVC thermoplastic [18]. The effect of varying contact time with E. coli was also of interest. The mechanical properties were studied in terms of tensile and impact properties. A number of structural, thermal and surface characterizations – including Fourier transform infrared spectroscopy, differential scanning calorimetry, and contact angle measurement – were performed to explain and substantiate the antibacterial performance evaluations. 2. Experimental 2.1. Materials and chemicals Poly(lactic acid) 2002D (NatureWorks, USA) was used in this work, having a specific gravity of 1.24 and a melt flow rate of 4. Triclosan (2,4,40 -trichloro-20 -hydroxydiphenylether), 24 USP, was used as an antibacterial agent (Koventure Co., Ltd., Thailand) in the form of white powder having a melting temperature range of 56–58 °C and a decomposition temperature above 280 °C. The specifications for triclosan, including suppliers, grades, physical and thermal properties, and chemical structure, can be found elsewhere [18]. Wood flour with an average particle size of 100–300 lm was used (V.P. Wood Co. Ltd., Bangkok, Thailand). N-2(aminoethyl)-3-aminopropyltrimethoxysilane (KBM 603, Shin-Etsu Chemical Co. Ltd., Japan) was used as a coupling agent for chemical surface treatment of wood flour. Gram-negative E. coli (E. coli, ATCC 25922) was used as the testing bacteria.
before blending with triclosan by the high-speed mixer. (All the material formulations used in this work are given in Table 1.) All components were then melt-blended using a twin screw extruder (Polylab-Rheomex CTW 100P, Haake, Germany). The blending temperature profiles from feed to die zones were 170, 180, 180, 180 and 170 °C, respectively, using a 50 rpm screw rotating speed to produce pellets of triclosan/PLA blends and triclosan/wood/PLA composites. The pellets were then dried in an oven at 70 °C overnight before being compression-molded to produce test pieces in the form of flat films having thicknesses of 1 mm and 3 mm for antibacterial and mechanical property testing, respectively. The mold temperature used was 170 °C, with preheating and holding times of 5 and 3 min, respectively, before cooling down to ambient temperature; compression pressure was 150 kg/cm2. For antibacterial testing, film specimens 1 mm thick were made in rectangular pieces 2.5 5.0 cm2 [2]. For mechanical property testing, specimens 3 mm thick were produced, according to ASTM D638-8 and D625-06a, for tensile and impact strength testing, respectively.
2.3. Mechanical properties A universal testing machine was used to determine the tensile properties, in accordance with ASTM D638-8. The notched Izod impact strength was measured by an impact testing machine, following ASTM D625-06a. All reported mechanical property data were averaged from at least nine independent experiments.
2.4. Antibacterial activity evaluations Plate count agar (PCA) was used for evaluating the antibacterial performances of PLA, PLA blend with triclosan, and PLA composite with wood flour, in accordance with ASTM E2149 (2001) test method. E. coli was inoculated in 5 ml liquid nutrient broth (NB) at 37 °C for 24 h. Optical density values (OD) of inocula were measured using UV-Vis spectroscopy (Hach DR/4000, USA). Each inoculum was diluted to OD of 0.1 in growing media peptone solution (prepared by 1 g/L peptone, at pH of 6.8–7.2). Two pieces of 2.5 5 cm2 film specimens were put in flasks loaded with OD of 0.1 peptone solution. The flasks were shaken using a reciprocal shaker at a shaking speed of 100–120 rpm at 37 °C ± 0.5 °C for 60, 120, 180 and 240 min, respectively. Contact time – i.e. the time the triclosan and PLA or wood/PLA composites were shaken in flasks filled with E. coli in peptone solution (OD 0.1) – varied from 60 to 240 min. A tenfold serial dilution was applied for bacteria colony counting (usually in a range of 30–300 colonies) [2]. For each contact time, 100 lL of the bacteria solution was placed on agar in sterilized Petri dishes. The inoculated plates were then held for 24 h at 37 °C ± 0.5 °C. Finally, the living cell bacteria colonies were carefully counted for evaluating the antibacterial activity.
Table 1 Materials and composite formulations. Materials
PLA (wt.%)
Wood flour (wt.%)
Triclosan (wt.%)
PLA
100 99.5 99.0 98.5 95.0 94.5 94.0 93.5 90.0 89.5 89.0 88.5
– – – – 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0
– 0.5 1.0 1.5 – 0.5 1.0 1.5 – 0.5 1.0 1.5
5%WF/PLA
2.2. Specimen preparation Wood flour was chemically treated with the silane coupling agent at 1.0% wood flour using a high-speed mixer; then the treated wood flour was dried in an oven at 80 °C for 3 d to remove all moisture content. PLA was first dried in an oven at 70 °C overnight
10%WF/PLA
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Fig. 1. Mechanical properties of PLA and wood flour/PLA composites in terms of: (a) Young’s modulus and tensile strength; (b) elongation at break and impact strength.
The percent reduction of bacteria colony-forming-units (CFU) was calculated using the following equation:
%R ¼
AB 100 A
ð1Þ
where R is the decrease of bacteria (%), A is the average amount of bacterial colonies from composites without triclosan for a given contact time (CFU/ml), Bis the average amount of bacterial colonies from composites incorporated with triclosan for a given contact time (CFU/ml) 2.5. Characterizations Fourier transform infrared spectroscopy (FTIR; Spectrum Spotlight 300, PerkinElmer, USA) was utilized with an attenuated total reflectance mode to examine molecular interactions between PLA, triclosan and wood flour. Differential scanning calorimetry (DSC; DSC822, MettlerToledo, USA) was used to monitor the physical and thermal properties of PLA, triclosan and wood flour. Each sample was first heated from 30 to 200 °C, cooled down to 30 °C and then re-heated to 200 °C. The heating and cooling rate were 10 °C/ min under nitrogen. The glass transition temperature (Tg) and melting temperature (Tm) were determined. Percentage of crystallinity (Xc) was also obtained via the DSC curves given by the following equation:
X C ð%Þ ¼
DHm 100 w DH0m
ð2Þ
where DHm is the enthalpy of the sample, DH0m is the enthalpy of fusion for 100% crystalline PLA (93.7 J/g), w is the weight fraction of PLA in composites.
Fig. 2. Mechanical properties of PLA and wood flour/PLA composites with different triclosan loadings: (a) Young’s modulus; (b) tensile strength; (c) elongation at break; (d) impact strength.
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3. Results and discussion 3.1. Mechanical properties The mechanical properties for the poly(lactic acid) (PLA) filled with different concentrations of wood flour are given in Fig. 1 in terms of Young’s modulus, tensile strength, elongation at break, and impact strength. It can be seen that as the wood content increased, the Young’s modulus increased, while the tensile strength, elongation at break, and impact strength decreased. The increase in the modulus was due to the fact that the wood had greater rigidities than the PLA. This effect was usually found with the thermoplastics that had lower rigidities than the wood [7,8,19]. The decreases in ultimate tensile and impact properties were caused by incompatibilities between the wood and the PLA [11,12,14], the effect being more pronounced for higher wood contents. The effect of triclosan loading on the mechanical properties of neat PLA and wood/PLA composites is shown in Fig. 2a–d. It can be seen that the changes in all mechanical properties as a function of triclosan loading had no definite trend under the given experimental errors. In order to facilitate the understanding of the effect of triclosan, the mechanical property results were reported in range form as given in Table 2, showing the ranges of mechanical property changes after adding triclosan at 0.5–1.5 wt.%. It was found that the changes in Young’s modulus (DE), tensile strength (DTS), elongation at break (DB), and impact strength (DIS) of the PLA and wood/PLA composites ranged 0.02–0.06 GPa, 3.35–4.54 MPa, 1.25–2.44%, and 0.21–0.54 102 J/mm2), respectively, for the addition of wood contents of 5–10 wt.% and triclosan loadings of 0.5–1.5 wt.%. This implies that the addition of triclosan had a slight effect on the mechanical properties of PLA. The effect of wood addition on the mechanical properties was much more pronounced than that of the triclosan addition. Therefore any changes in antibacterial performance, which will be discussed later, would not be primarily caused by triclosan addition, especially in the case of wood/PLA composites. 3.2. Antibacterial activity
Fig. 3. Viable cell count of E. coli with triclosan loadings of 0.5–1.5 wt.% for different contact time different contact times: (a) PLA; (b) 5.0% wood flour/PLA composites; (c) 10.0% wood flour/PLA composites.
Water contact angle (WCA) studies were performed to quantify the change in surface energy, surface chemistry and roughness of PLA blends with triclosan and composites with wood flour. Deionized water was dropped onto PLA blend and composite surfaces, and then the contact angle was measured using a contact angle goniometer (model 100-00, Ramé-hart Instrument Co., USA). In the developing step, 5 times/100 lL droplets were dropped on specimen surfaces using three independent samples, and the average contact angle values were then reported.
The quantitative results for antibacterial performances of PLA and wood/PLA composites are given in Fig. 3, which shows a viable colony count for E. coli under different triclosan concentrations and contact times. It was observed that for neat PLA (Fig. 3a), as the triclosan content increased the viable cell count decreased for all contact times, suggesting that triclosan had the ability to retard bacterial growth. However, when the contact time was increased the bacteria continued to grow, but at a slower rate in the case of PLA with the presence of triclosan. For wood/PLA composites (Fig. 3b and c), increasing the triclosan content reduced the viable cell count. This effect was more pronounced when compared with the neat PLA, as mentioned earlier. The effect of contact time on the changes in viable cell count for wood/PLA composites did not exhibit the same trend as observed for the neat PLA; with increased contact time, the viable cell count for wood/PLA composites tended to decrease. This phenomenon was clearly seen for high triclosan loadings of 1.0–1.5 wt.%. The antibacterial performance could also be viewed in terms of percent bacteria reduction, the calculations for which were already given in Eq. (1). Table 3 shows bacterial reduction percentages for PLA and wood/PLA composites with different triclosan contents and contact times. Similar to the results in Fig. 3, the higher the triclosan concentration and contact time, the greater the percent of bacterial reduction. Evidently the wood particles acted as an antibacterial promoter for triclosan-based wood/ PLA composites, i.e. the wood particles allowed more triclosan to migrate onto the wood/PLA composite surfaces. The explanation
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Table 2 Effect of triclosan content on the changes in mechanical properties of PLA and wood flour/PLA composites. Materials
PLA 5%WF/PLA 10%WF/PLA
Young’s modulus (E)(GPa) for triclosan loadings of 0.5–1.5 wt.%
Tensile strength (ST)(MPa) for triclosan loadings of 0.5–1.5 wt.%
Elongation at break(B)(%) for triclosan loadings of 0.5–1.5 wt.%
Impact strength (IS) (x103J/mm2) for triclosan loadings of 0.5–1.5 wt.%
Max–Min
DE
Max–Min
DST
Max–Min
DB
Max–Min
DIS
0.88–0.90 0.94–0.88 0.96–1.01
0.02 0.06 0.05
49.48–54.02 45.88–49.23 41.77–45.84
4.54 3.35 4.07
7.78–10.22 7.56–8.81 5.67–8.00
2.44 1.25 2.33
4.05–4.26 3.24–3.78 2.36–2.86
0.21 0.54 0.5
Table 3 Percent bacteria reductions for PLA and wood flour/PLA composites with triclosan loadings of 0.5–1.5 wt.% for different contact times. Materials
PLA
5%WF/PLA
10%WF/PLA
Contact time (min.)
60 120 180 240 60 120 180 240 60 120 180 240
Percent bacteria reduction (%) 0.5% Tricolsan
1.0% Tricolsan
1.5% Tricolsan
14.86 53.16 62.95 74.92 10.77 49.58 45.74 54.95 21.67 63.27 64.20 63.68
9.46 63.16 63.86 78.81 24.62 77.73 81.91 88.16 48.33 74.83 82.72 88.05
17.57 65.26 74.11 83.40 32.31 87.39 83.69 91.65 40.00 86.39 96.71 96.78
Table 4 Water contact angle of PLA and 10% WF/PLA incorporated with triclosan loadings of 0 and 1.5 wt.%. Material
Triclosan content (%)
Contact angle (°)
PLA
0.0
58.6
1.5
68.8
0.0
70.8
1.5
64.8
10%WF/PLA
proposed in this work is that introducing the wood particles, which are highly hydrophilic in nature [9,20], into the triclosan-filled PLA makes the PLA become more hydrophilic; as a result, more water molecules can be absorbed into/onto the triclosan/wood/PLA surfaces. The water uptake or absorption would then facilitate the triclosan migration onto the PLA surface to kill the bacteria. 3.3. Quantitative evidence for the role of wood flour as an antibacterial promoter in triclosan-filled wood/PLA composites It is essential to prove the proposed mechanism and explanation for the role of wood flour as an antibacterial promoter, as discussed earlier, through independent physical and structural
Wettability characteristic
characterizations. If triclosan migration on the PLA surface by the assistance of wood flour had occurred, one would expect to observe chemical and physical changes on the wood/PLA composite surfaces. In this respect, water contact angle (WCA) measurement, Fourier transform infrared spectroscopy, and differential scanning calorimetry were performed. Table 4 shows the contact angle values for PLA and PLA with triclosan or wood additive. For neat PLA, the addition of triclosan or wood had significant effects on its hydrophilicity: the contact angles of PLA appeared to increase from 58.6° to 68.8° and from 58.6° to 70.8°, respectively. This indicates that the PLA became less hydrophilic due to the presence of triclosan. The changes in the hydrophilicity of PLA by the addition of triclosan are related to molecular interactions between the two
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Fig. 4. Intermolecular hydrogen bonds of PLA with triclosan.
substances, as illustrated in Fig. 4. Intermolecular hydrogen bonds are presumed to exist between PLA and triclosan. If these interactions occur, the initial hydrophilicity of PLA would be expected to decrease. A similar phenomenon probably applies for the composite of PLA and wood. However, when 1.5% triclosan was added into the wood/PLA composites (5% and 10% wood contents), the contact angle value for the wood/PLA was found to decrease from 70.8° to 64.8°. This can be explained using a proposed interactive model, as given in Fig. 5. It is thought that triclosan is likely to have a preference with PLA via hydrogen bonding. The wood particles in the triclosan/PLA system would enable the wood/PLA composites to absorb more water molecules onto the triclosan/wood/PLA surfaces; this absorbed moisture would allow triclosan migration onto the PLA specimen surfaces due to a diffusion process, and eventually resulted in the decreases in contact angle value. The diffusibility of triclosan to kill bacteria has been found by Silapasorn et al. [18] and Chung et al. [21], who suggested that the efficiency of triclosan for killing bacteria incorporated in vinyl thermoplastics was
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dependent on its diffusion ability in the media. The results shown in Figs. 4 and 5 can be used to support the viable cell count results in Fig. 3 for triclosan/wood/PLA composites as compared with triclosan/PLA. Table 5 shows the effect of triclosan content on hydrophilicity (water contact angle) for PLA and wood/PLA composites. It can be seen that although the presence of triclosan decreased the hydrophilicity of the PLA and wood/PLA composites, its varying content had very little effect on the change in hydrophilicity. This may be because the increased triclosan content was probably distributed and/or dispersed within the bulk PLA and wood/PLA composites, rather than on the sample surfaces where the contact angles were being measured. The interactions between PLA, triclosan and wood, as seen in the contact angle results, can be substantiated by the FTIR analysis results in Fig. 6, showing FTIR spectra for PLA, triclosan, and PLA with 1.5 wt.% triclosan. It can be seen that the FTIR spectrum for triclosan/PLA was very similar to that for neat PLA. This was to be expected, since the triclosan content added was very small compared to the bulk PLA. However, a slight change could be observed in the carbonyl functional (C@O) peak of PLA at a wavenumber of 1753 cm1. The carbonyl peak became broadened and split into two small peaks (at wavenumbers of 1753 and 1746 cm1). These changes in the FTIR spectra indicated the intermolecular interaction between PLA and triclosan (Fig. 5). Fig. 6b shows FTIR spectra for PLA, wood, and PLA with 10 wt.% wood content. It can be seen that the FTIR spectrum for wood/PLA was similar to that for neat PLA, with the presence of a hydroxyl group (AOH) at a wavenumber of 3341 cm1. Unlike triclosan in PLA, the additional hydroxyl peak occurred due to the reasonable amount of wood flour added to the PLA. Finally, it was interesting to note that the FTIR spectrum for the triclosan/wood/PLA sample in Fig. 6c exhibited a mixture of the triclosan/PLA and wood/PLA spectra. This represents the broadening and splitting of the carbonyl peak at wavenumbers of 1753 and 1746 cm1 from the triclosan/PLA interaction, and the hydroxyl peak at a wavenumber of 3341 cm1 from the wood part. It appears that the contact angle results in Table 4, the molecular interactions in Fig. 4, and the FTIR results in Fig. 6 correspond well
Fig. 5. A possible model of moisture regain by presence of wood in triclosan/PLA blends: (a) without wood; (b) with wood.
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C. Prapruddivongs, N. Sombatsompop / Composites: Part B 43 (2012) 2730–2737 Table 5 Water contact angle of PLA, 5% WF/PLA and 10% WF/PLA incorporated with triclosan loadings of 0–1.5 wt.%. Samples
Triclosan content (%)
Contact angle (°)
PLA
0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5
58.6 68.2 68.4 68.8 69.3 60.1 64.7 61.7 70.8 58.2 63.3 64.8
5%WF/PLA
10%WF/PLA
Fig. 6. FTIR spectra for PLA and wood/PLA composites filled with triclosan: (a) PLA, triclosan, and triclosan/PLA; (b) PLA, wood flour, and wood flour/PLA composites; (c) PLA and triclosan/wood flour/PLA composites.
to the antibacterial activity results in Fig. 3, in accordance with the proposed concept of the role of wood flour as an antibacterial promoter. More evidence highlighting the role of wood flour as an antibacterial promoter was from DSC analysis, which showed crystallization and melting behavior in terms of the glass-transition (Tg) and melting (Tm) temperatures, and the degree of crystallinity (Xc). The DSC results for neat PLA, triclosan/PLA, wood/PLA, and triclosan/
wood/PLA composites are given in Fig. 7. It was found that the degree of crystallinity of PLA was unaffected by triclosan, but increased from 7.9% to 36.3% by the presence of 10% wood. The Tg and Tm of PLA were approximately 60–62 °C and 152–153 °C, respectively, and did not change with the addition of triclosan. Although the addition of triclosan had no effect on the Tm change, the Tm peak characteristics were affected by the presence of 10 wt.% wood. Similar behavior was also found in the case of bamboo fiber/PLA composites [22]. The Tm for the wood/PLA composite exhibited a double melting peak, the first melting peak being around 151 °C and the second melting peak around 157–159 °C. These two melting peaks were probably associated with the recrystallization of PLA due to the presence of wood. This was because the wood particles caused imperfections of the PLA crystals. Upon heating during the DSC test, a recrystallization process of the PLA occurred, in order to form perfect and stable crystals. This recrystallization phenomenon could be confirmed by the appearance of exothermic peaks around 90–135 °C for the PLA samples with wood, which eventually caused the occurrence of the double melting peak: one being for PLA crystallization, and the other for PLA recrystallization due to the presence of wood. This kind of recrystallization phenomenon of PLA due to highly hydrophilic fiber was also evidenced by Shi et al. [22]. In relation to physical properties, it is likely that the double melting peak was responsible for the increases in the degree of crystallinity from 7.9% to 36.3% by the presence of 10% wood. The increases in crystallinity level due to the presence of wood particles were also suggested in the case of wood/PP composites, as a result of transcrystalline structures formed on the wood surfaces [23]. It was essential to relate the thermal behavior with the antibacterial performance of PLA by the addition of triclosan and wood. Park et al. [24] documented that thermoplastics with a higher degree of crystallinity would have lower antibacterial activities due to the difficulty of the antibacterial agent in penetrating through the crystalline phase as compared with the amorphous structure. However, this characteristic was not the case in this work. The increases in the degree of crystallinity due to the addition of wood obtained in this work did not increase the rigidities of the triclosan/PLA to that extent. This was clear evidence that the increase in the tensile modulus with increasing wood flour content was relatively small (see Fig. 1). In other words, the increasing effect on the rigidity of triclosan/PLA is probably suppressed by the increasing hydrophilic effect when wood flour is added to the triclosan/ PLA blend (Table 4 and Fig. 5). This was why the antibacterial performance still improved, although the crystallinity and rigidity of the triclosan/PLA had slightly increased with the added wood flour. It can therefore be concluded that the hydrophilic effect played a more important role than the physical and thermal properties in the antibacterial activity of triclosan in the PLA and wood/PLA composites.
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Fig. 7. DSC thermogram for PLA, wood flour/PLA, triclosan/PLA and triclosan/wood flour/PLA composites.
4. Conclusions In this work, triclosan and wood were introduced into PLA, and the mechanical and antibacterial performance were assessed and discussed. The experimental results suggested that the mechanical properties of PLA were affected by the addition of wood. Young’s modulus was found to increase; and the tensile strength, elongation at break, and impact strength decreased with increasing wood content. The mechanical properties as a function of triclosan loading had no definite trend due to the small amount of triclosan added. Bacterial growth as a function of contact time in neat PLA was different from that in that in wood/PLA composites. It was proposed in this work that the presence of wood particles could promote the antibacterial performance of triclosan/PLA blends, which was substantiated by WCA, FTIR and DSC characterizations. The hydrophilic effect played a more important role in the antibacterial activity of triclosan in the wood/PLA composites. Acknowledgments The authors would like to thank the Office of the Higher Education Commission (OHEC) under the National Research University Program, and the Thailand Research Fund (TRF Research Senior Scholar; RTA5280008) for financial co-support throughout this work. References [1] Pradeep T, Jain P. Potential of silver nanoparticle-coated polyurethane foam as an antibacterial water filter. Biotechnol Bioeng 2005;90:59–63.
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[2] Chammanee P, Sombatsompop K, Kositchaiyong A, Sombatsompop N. Effects of anti-bacterial agents, sample preparation and contact time on anti-bacterial efficacy in MDPE film. J Macromol Sci Phys 2009;48:755–65. [3] Jin T, Zhang H. Biodegradable polylactic acid polymer with Nisin for use in antimicrobial food packaging. J Food Sci 2008;73:127–34. [4] Liu L, Jin T, Finkenstadt V, Liu C, Cooke P, Coffin D, et al. Antimicrobial packaging materials from poly(lactic acid) incorporated with pectin-NisaplinÒ microparticles. Chem Chem Technol 2009;3:221–30. [5] Li L, Li Y, Li J, Yao L, Mak AFT, Ko F, et al. Antibacterial properties of nanosilver PLLA fibrous membranes. J Nanomate 2009 [Article ID 168041:1-5]. [6] Fernandez A, Soriano E, Hernandez-Munoz P, Gavara R. Migration of antimicrobial silver from composites of polylactide with silver zeolites. J Food Sci 2010;75:186–93. [7] Faruk O, Matuana LM. Nanoclay reinforced HDPE as a matrix for wood–plastic composites. Compos Sci Technol 2008;68:2073–7. [8] Bledzki A, Faruk O. Injection moulded microcellular wood fibre–polypropylene composites. Composites A 2006;37:1358–67. [9] Pattamasattayasonti N, Chaochanchaikul K, Rosarpitak V, Sombatsompop N. Effects of uv-weathering aging and CeO2 based coating layer on mechanical and structural changes in wood/PVC composites. J Vinyl Addit Technol 2011;17:9–16. [10] Finkenstadta VL, Tisserat B. Poly(lactic acid) and osage orange wood fiber composites for agricultural mulch films. Ind Crops Prod 2010;31:316–20. [11] Sykacek E, Schlager W, Mundigler N. Compatibility of softwood slour and commercial biopolymers in injection molding. Polym Compos 2010;31:443–51. [12] Lee SY, Kang IA, Doh GH, Yoon HG, Park BD, Wu Q. Thermal and mechanical properties of wood flour/talc-filled polylactic acid composites: effect of filler content and coupling treatment. J Therm Compos Mater 2008;21:209–23. [13] Pilla S, Gong S, O’Neill E, Rowell MR, Krzysik AM. Polylactide-pine wood flour composites. Polym Eng Sci 2008;48:578–87. [14] Petinakis E, Yu L, Edward G, Dean K, Liu H, Scully AD. Effect of matrix–particle interfacial adhesion on the mechanical properties of poly(lactic acid)/woodflour micro-composites. J Polym Environ 2009;17:83–94. [15] Van Nostrum CF, Veldhuis TFJ, Bos GW, Hennink WE. Hydrolytic degradation of oligo(lactic acid): a kinetic and mechanistic study. Polymer 2004;45:6779–87. [16] Wiggins JS, Hassan MK, Mauritz KA, Storey RF. Hydrolytic degradation of poly(D,L-lactide) as a function of end group: carboxylic acid VS hydroxyl. Polymer 2006;47:1960–9. [17] Tsuji H, Miyauchi S. Poly(l-lactide): VI effects of crystallinity on enzymatic hydrolysis of poly(l-lactide) without free amorphous region. Polym Degrad Stab 2001;71:415–24. [18] Silapasorn K, Sombatsompop K, Kositchaiyong A, Wimolmala E, Markpin T, Sombatsompop N. Effect of chemical structure of thermoplastics on antibacterial activity and physical diffusion of triclosan doped in vinyl thermoplastics and their composites with CaCO3. J Appl Polym Sci 2011;121:253–61. [19] Sombatsompop N, Yotinwattanakumtorn C, Thongpin C. Influences of types and concentrations of maleic anhydride grafted polypropylene and impact modifiers on mechanical properties of PP/wood sawdust composites. J Appl Polym Sci 2005;97:475–84. [20] Sombatsompop N, Chaochanchaikul K. Effect of moisture content on mechanical properties, thermal and structural stability, and extrudate texture of PVC/wood sawdust composites. Polym Int 2004;53:1210–8. [21] Chung D, Papadakis SE, Yam KL. Evaluation of a polymer coating containing triclosan as the antimicrobial layer for a packaging film. Int J Food Sci Technol 2003;38:165–9. [22] Shi QF, Mou HY, Gao L, Yang J, Guo WH. Double-melting behavior of bamboo fiber/talc/poly(lactic acid) composites. J Polym Environ 2010;18:567–75. [23] Sombatsompop N, Kositchaiyong A, Wimolmala E. Experimental analysis of temperature and crystallinity profiles of wood sawdust/PP composites during cooling. J Appl Polym Sci 2006;102:1896–905. [24] Park ES, Moon WS, Song MJ, Kim MN, Chung KH, Yoon JS. Antimicrobial activity of phenol and benzoic acid derivatives. Int J Biodeter Biodegrad 2001;47:209–14.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Roles and evidence of wood flour as an antibacterial promoter for triclosan-filled poly(lactic acid) C. Prapruddivongs, N. Sombatsompop ⇑ Polymer Processing and Flow (P-PROF) Research Group, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi (KMUTT), 126 Pracha-Uthit, Thongkru, Bangmod, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 26 April 2011 Received in revised form 26 August 2011 Accepted 30 August 2011 Available online 3 May 2012 Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties D. Thermal analysis E. Extrusion
a b s t r a c t Mechanical and antibacterial performances of poly(lactic acid) (PLA) were assessed as a function of triclosan and wood fillers under a wide range of contact times using Gram-negative Escherichia coli (E. coli, ATCC 25922) as testing bacteria. Water contact angle (WCA) measurement, Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were performed to characterize the PLA, its blend with triclosan, and its composite with wood filler, and used to substantiate the antibacterial performance results. The experimental results suggested that introduction of wood flour changed the mechanical properties of PLA, whereas triclosan had no definite effect on the mechanical properties. E. coli growth appeared to increase with contact time for PLA, but to decrease with contact time for the wood/PLA composites. Based on the results and quantitative evidence, it was proposed that the wood flour acted as an ‘‘antibacterial promoter’’ for triclosan blended wood/PLA composites, which facilitated triclosan migration onto the wood/PLA composite surfaces to kill the bacteria. The molecular interactions between PLA, triclosan and wood were quantitatively characterized experimentally by CAM, DSC and FTIR. The change in hydrophilicity by the presence of wood filler was found to be the main reason for the improved antibacterial activity of triclosan in the wood/PLA composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Poly(lactic acid) (PLA) is a biodegradable and aliphatic polyester obtained from agricultural plants such as sugarcane and corn starch; one of its most widespread uses is in food packaging applications. In such applications, PLA products are required to be classified as highly hygienic or antimicrobial. Methods to obtain antimicrobial packaging products containing PLA as well as other polymers are usually carried out through incorporating or blending of antimicrobial agents, coating an antimicrobial substance onto a PLA surface, or immobilizing antimicrobials to polymers by ion or covalent linkages [1]. However, the effectiveness of these methods is dependent upon the molecular structure of the polymers used, the type and concentration of antibacterial agents, and the processing conditions [2]. Based on the available literature, there have been a number studies of the anti-microbial performance of PLA in various forms, including neat, blended and composite, and using different types of antimicrobial agents including nisin, Nisaplin, and nano-silver, under a wide range of testing conditions. Jin and Zhang [3] studied the effect of nisin on the antimicrobial properties of PLA film
⇑ Corresponding author. Tel.: +66 2 470 8645; fax: +66 2 470 8647. E-mail address: [email protected] (N. Sombatsompop). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.032
against foodborne pathogens using plate count agar, and found that nisin/PLA films could inhibit Gram-positive Listeria monocytogenes and Salmonella enteritidis, but showed less antimicrobial efficiency for Gram-negative Escherichia coli. At high temperature (up to 433 K) nisin/PLA and NisaplinÒ/PLA composites effectively inhibit bacteria growth in the presence of pectin [4]. However, it was noted that the added pectin resulted in decreases in the mechanical properties of the nisin/PLA and NisaplinÒ/PLA composites. Use of nano-silver for antimicrobial performance in polymeric materials has also interested scientists and technologists for creating value-added PLA products. Nano-silver (Ag), which was incorporated into PLA fibrous membranes for tissue engineering scaffold applications, could kill both E. coli and Staphylococcus aureus [5]; but the effectiveness of the Ag+ released from silver zeolite/ PLA composite films to kill the bacteria was enhanced by increasing the test temperature, media polarity and crystallinity level of PLA composite films [6]. Wood plastic composite (WPC) is a relatively new material which is manufactured by blending thermoplastics and wood fillers. WPC products have increasingly become attractive because of cost savings, good mechanical properties, better dimensional stabilities, and environmental issues, and have many potential uses and applications, such as window and door components, packaging, handrails, pallets, and structural members in decks and small beams. Most thermoplastics used include polyethylene (PE) [7],
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polypropylene (PP) [8], and poly(vinyl chloride) (PVC) [9], due to easy handling and wide processing windows. However, a survey of recent literature has indicated that experimental data and information on wood flour (WF) and PLA composites in WPC applications are rare, possibly due to processing difficulties and poor mechanical properties of the final products as a result of molecular incompatibilities between PLA and wood [10,11]. Lee et al. [12] found that the addition of WF in PLA plastic lowered the glass transition (Tg) and crystalline temperatures (Tc) of the PLA, and also decreased the tensile strength. Similar behavior was also observed for toughness properties [13], although the deficiency could be partially remedied by chemical treatment of the wood surface [13,14]. Having studied the available published research, we found that none of the published studies have investigated the antimicrobial activities of wood flour/PLA composites. It was therefore proposed in this work that wood/PLA composites cannot only make the PLA more environmentally friendly, but also result in a higher tendency toward biodegradability after their service life. In other words, wood/PLA packaging products should be hygienic, have the ability to inhibit the growth of bacteria during service, and should quickly biodegrade after use. It was assumed that the presence of hydrophilic wood should result in a faster biodegradable reaction of PLA, since PLA is usually chemically biodegraded via hydrolysis [15–17] in the presence of water molecules: i.e. the higher the water absorption, the greater the biodegradation rate of PLA. Our studies on the properties of wood/PLA composites are twofold: one concerning their mechanical and antibacterial properties, and the other their biodegradable properties. However, the present work focused only on the effect of wood flour on the mechanical and antibacterial properties of PLA. Triclosan (2,4,40 -trichloro-20 hydroxydiphenylether) was selected and used as an antibacterial agent; as previously found, it had the ability to retard bacterial growth in PVC thermoplastic [18]. The effect of varying contact time with E. coli was also of interest. The mechanical properties were studied in terms of tensile and impact properties. A number of structural, thermal and surface characterizations – including Fourier transform infrared spectroscopy, differential scanning calorimetry, and contact angle measurement – were performed to explain and substantiate the antibacterial performance evaluations. 2. Experimental 2.1. Materials and chemicals Poly(lactic acid) 2002D (NatureWorks, USA) was used in this work, having a specific gravity of 1.24 and a melt flow rate of 4. Triclosan (2,4,40 -trichloro-20 -hydroxydiphenylether), 24 USP, was used as an antibacterial agent (Koventure Co., Ltd., Thailand) in the form of white powder having a melting temperature range of 56–58 °C and a decomposition temperature above 280 °C. The specifications for triclosan, including suppliers, grades, physical and thermal properties, and chemical structure, can be found elsewhere [18]. Wood flour with an average particle size of 100–300 lm was used (V.P. Wood Co. Ltd., Bangkok, Thailand). N-2(aminoethyl)-3-aminopropyltrimethoxysilane (KBM 603, Shin-Etsu Chemical Co. Ltd., Japan) was used as a coupling agent for chemical surface treatment of wood flour. Gram-negative E. coli (E. coli, ATCC 25922) was used as the testing bacteria.
before blending with triclosan by the high-speed mixer. (All the material formulations used in this work are given in Table 1.) All components were then melt-blended using a twin screw extruder (Polylab-Rheomex CTW 100P, Haake, Germany). The blending temperature profiles from feed to die zones were 170, 180, 180, 180 and 170 °C, respectively, using a 50 rpm screw rotating speed to produce pellets of triclosan/PLA blends and triclosan/wood/PLA composites. The pellets were then dried in an oven at 70 °C overnight before being compression-molded to produce test pieces in the form of flat films having thicknesses of 1 mm and 3 mm for antibacterial and mechanical property testing, respectively. The mold temperature used was 170 °C, with preheating and holding times of 5 and 3 min, respectively, before cooling down to ambient temperature; compression pressure was 150 kg/cm2. For antibacterial testing, film specimens 1 mm thick were made in rectangular pieces 2.5 5.0 cm2 [2]. For mechanical property testing, specimens 3 mm thick were produced, according to ASTM D638-8 and D625-06a, for tensile and impact strength testing, respectively.
2.3. Mechanical properties A universal testing machine was used to determine the tensile properties, in accordance with ASTM D638-8. The notched Izod impact strength was measured by an impact testing machine, following ASTM D625-06a. All reported mechanical property data were averaged from at least nine independent experiments.
2.4. Antibacterial activity evaluations Plate count agar (PCA) was used for evaluating the antibacterial performances of PLA, PLA blend with triclosan, and PLA composite with wood flour, in accordance with ASTM E2149 (2001) test method. E. coli was inoculated in 5 ml liquid nutrient broth (NB) at 37 °C for 24 h. Optical density values (OD) of inocula were measured using UV-Vis spectroscopy (Hach DR/4000, USA). Each inoculum was diluted to OD of 0.1 in growing media peptone solution (prepared by 1 g/L peptone, at pH of 6.8–7.2). Two pieces of 2.5 5 cm2 film specimens were put in flasks loaded with OD of 0.1 peptone solution. The flasks were shaken using a reciprocal shaker at a shaking speed of 100–120 rpm at 37 °C ± 0.5 °C for 60, 120, 180 and 240 min, respectively. Contact time – i.e. the time the triclosan and PLA or wood/PLA composites were shaken in flasks filled with E. coli in peptone solution (OD 0.1) – varied from 60 to 240 min. A tenfold serial dilution was applied for bacteria colony counting (usually in a range of 30–300 colonies) [2]. For each contact time, 100 lL of the bacteria solution was placed on agar in sterilized Petri dishes. The inoculated plates were then held for 24 h at 37 °C ± 0.5 °C. Finally, the living cell bacteria colonies were carefully counted for evaluating the antibacterial activity.
Table 1 Materials and composite formulations. Materials
PLA (wt.%)
Wood flour (wt.%)
Triclosan (wt.%)
PLA
100 99.5 99.0 98.5 95.0 94.5 94.0 93.5 90.0 89.5 89.0 88.5
– – – – 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0
– 0.5 1.0 1.5 – 0.5 1.0 1.5 – 0.5 1.0 1.5
5%WF/PLA
2.2. Specimen preparation Wood flour was chemically treated with the silane coupling agent at 1.0% wood flour using a high-speed mixer; then the treated wood flour was dried in an oven at 80 °C for 3 d to remove all moisture content. PLA was first dried in an oven at 70 °C overnight
10%WF/PLA
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Fig. 1. Mechanical properties of PLA and wood flour/PLA composites in terms of: (a) Young’s modulus and tensile strength; (b) elongation at break and impact strength.
The percent reduction of bacteria colony-forming-units (CFU) was calculated using the following equation:
%R ¼
AB 100 A
ð1Þ
where R is the decrease of bacteria (%), A is the average amount of bacterial colonies from composites without triclosan for a given contact time (CFU/ml), Bis the average amount of bacterial colonies from composites incorporated with triclosan for a given contact time (CFU/ml) 2.5. Characterizations Fourier transform infrared spectroscopy (FTIR; Spectrum Spotlight 300, PerkinElmer, USA) was utilized with an attenuated total reflectance mode to examine molecular interactions between PLA, triclosan and wood flour. Differential scanning calorimetry (DSC; DSC822, MettlerToledo, USA) was used to monitor the physical and thermal properties of PLA, triclosan and wood flour. Each sample was first heated from 30 to 200 °C, cooled down to 30 °C and then re-heated to 200 °C. The heating and cooling rate were 10 °C/ min under nitrogen. The glass transition temperature (Tg) and melting temperature (Tm) were determined. Percentage of crystallinity (Xc) was also obtained via the DSC curves given by the following equation:
X C ð%Þ ¼
DHm 100 w DH0m
ð2Þ
where DHm is the enthalpy of the sample, DH0m is the enthalpy of fusion for 100% crystalline PLA (93.7 J/g), w is the weight fraction of PLA in composites.
Fig. 2. Mechanical properties of PLA and wood flour/PLA composites with different triclosan loadings: (a) Young’s modulus; (b) tensile strength; (c) elongation at break; (d) impact strength.
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3. Results and discussion 3.1. Mechanical properties The mechanical properties for the poly(lactic acid) (PLA) filled with different concentrations of wood flour are given in Fig. 1 in terms of Young’s modulus, tensile strength, elongation at break, and impact strength. It can be seen that as the wood content increased, the Young’s modulus increased, while the tensile strength, elongation at break, and impact strength decreased. The increase in the modulus was due to the fact that the wood had greater rigidities than the PLA. This effect was usually found with the thermoplastics that had lower rigidities than the wood [7,8,19]. The decreases in ultimate tensile and impact properties were caused by incompatibilities between the wood and the PLA [11,12,14], the effect being more pronounced for higher wood contents. The effect of triclosan loading on the mechanical properties of neat PLA and wood/PLA composites is shown in Fig. 2a–d. It can be seen that the changes in all mechanical properties as a function of triclosan loading had no definite trend under the given experimental errors. In order to facilitate the understanding of the effect of triclosan, the mechanical property results were reported in range form as given in Table 2, showing the ranges of mechanical property changes after adding triclosan at 0.5–1.5 wt.%. It was found that the changes in Young’s modulus (DE), tensile strength (DTS), elongation at break (DB), and impact strength (DIS) of the PLA and wood/PLA composites ranged 0.02–0.06 GPa, 3.35–4.54 MPa, 1.25–2.44%, and 0.21–0.54 102 J/mm2), respectively, for the addition of wood contents of 5–10 wt.% and triclosan loadings of 0.5–1.5 wt.%. This implies that the addition of triclosan had a slight effect on the mechanical properties of PLA. The effect of wood addition on the mechanical properties was much more pronounced than that of the triclosan addition. Therefore any changes in antibacterial performance, which will be discussed later, would not be primarily caused by triclosan addition, especially in the case of wood/PLA composites. 3.2. Antibacterial activity
Fig. 3. Viable cell count of E. coli with triclosan loadings of 0.5–1.5 wt.% for different contact time different contact times: (a) PLA; (b) 5.0% wood flour/PLA composites; (c) 10.0% wood flour/PLA composites.
Water contact angle (WCA) studies were performed to quantify the change in surface energy, surface chemistry and roughness of PLA blends with triclosan and composites with wood flour. Deionized water was dropped onto PLA blend and composite surfaces, and then the contact angle was measured using a contact angle goniometer (model 100-00, Ramé-hart Instrument Co., USA). In the developing step, 5 times/100 lL droplets were dropped on specimen surfaces using three independent samples, and the average contact angle values were then reported.
The quantitative results for antibacterial performances of PLA and wood/PLA composites are given in Fig. 3, which shows a viable colony count for E. coli under different triclosan concentrations and contact times. It was observed that for neat PLA (Fig. 3a), as the triclosan content increased the viable cell count decreased for all contact times, suggesting that triclosan had the ability to retard bacterial growth. However, when the contact time was increased the bacteria continued to grow, but at a slower rate in the case of PLA with the presence of triclosan. For wood/PLA composites (Fig. 3b and c), increasing the triclosan content reduced the viable cell count. This effect was more pronounced when compared with the neat PLA, as mentioned earlier. The effect of contact time on the changes in viable cell count for wood/PLA composites did not exhibit the same trend as observed for the neat PLA; with increased contact time, the viable cell count for wood/PLA composites tended to decrease. This phenomenon was clearly seen for high triclosan loadings of 1.0–1.5 wt.%. The antibacterial performance could also be viewed in terms of percent bacteria reduction, the calculations for which were already given in Eq. (1). Table 3 shows bacterial reduction percentages for PLA and wood/PLA composites with different triclosan contents and contact times. Similar to the results in Fig. 3, the higher the triclosan concentration and contact time, the greater the percent of bacterial reduction. Evidently the wood particles acted as an antibacterial promoter for triclosan-based wood/ PLA composites, i.e. the wood particles allowed more triclosan to migrate onto the wood/PLA composite surfaces. The explanation
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Table 2 Effect of triclosan content on the changes in mechanical properties of PLA and wood flour/PLA composites. Materials
PLA 5%WF/PLA 10%WF/PLA
Young’s modulus (E)(GPa) for triclosan loadings of 0.5–1.5 wt.%
Tensile strength (ST)(MPa) for triclosan loadings of 0.5–1.5 wt.%
Elongation at break(B)(%) for triclosan loadings of 0.5–1.5 wt.%
Impact strength (IS) (x103J/mm2) for triclosan loadings of 0.5–1.5 wt.%
Max–Min
DE
Max–Min
DST
Max–Min
DB
Max–Min
DIS
0.88–0.90 0.94–0.88 0.96–1.01
0.02 0.06 0.05
49.48–54.02 45.88–49.23 41.77–45.84
4.54 3.35 4.07
7.78–10.22 7.56–8.81 5.67–8.00
2.44 1.25 2.33
4.05–4.26 3.24–3.78 2.36–2.86
0.21 0.54 0.5
Table 3 Percent bacteria reductions for PLA and wood flour/PLA composites with triclosan loadings of 0.5–1.5 wt.% for different contact times. Materials
PLA
5%WF/PLA
10%WF/PLA
Contact time (min.)
60 120 180 240 60 120 180 240 60 120 180 240
Percent bacteria reduction (%) 0.5% Tricolsan
1.0% Tricolsan
1.5% Tricolsan
14.86 53.16 62.95 74.92 10.77 49.58 45.74 54.95 21.67 63.27 64.20 63.68
9.46 63.16 63.86 78.81 24.62 77.73 81.91 88.16 48.33 74.83 82.72 88.05
17.57 65.26 74.11 83.40 32.31 87.39 83.69 91.65 40.00 86.39 96.71 96.78
Table 4 Water contact angle of PLA and 10% WF/PLA incorporated with triclosan loadings of 0 and 1.5 wt.%. Material
Triclosan content (%)
Contact angle (°)
PLA
0.0
58.6
1.5
68.8
0.0
70.8
1.5
64.8
10%WF/PLA
proposed in this work is that introducing the wood particles, which are highly hydrophilic in nature [9,20], into the triclosan-filled PLA makes the PLA become more hydrophilic; as a result, more water molecules can be absorbed into/onto the triclosan/wood/PLA surfaces. The water uptake or absorption would then facilitate the triclosan migration onto the PLA surface to kill the bacteria. 3.3. Quantitative evidence for the role of wood flour as an antibacterial promoter in triclosan-filled wood/PLA composites It is essential to prove the proposed mechanism and explanation for the role of wood flour as an antibacterial promoter, as discussed earlier, through independent physical and structural
Wettability characteristic
characterizations. If triclosan migration on the PLA surface by the assistance of wood flour had occurred, one would expect to observe chemical and physical changes on the wood/PLA composite surfaces. In this respect, water contact angle (WCA) measurement, Fourier transform infrared spectroscopy, and differential scanning calorimetry were performed. Table 4 shows the contact angle values for PLA and PLA with triclosan or wood additive. For neat PLA, the addition of triclosan or wood had significant effects on its hydrophilicity: the contact angles of PLA appeared to increase from 58.6° to 68.8° and from 58.6° to 70.8°, respectively. This indicates that the PLA became less hydrophilic due to the presence of triclosan. The changes in the hydrophilicity of PLA by the addition of triclosan are related to molecular interactions between the two
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Fig. 4. Intermolecular hydrogen bonds of PLA with triclosan.
substances, as illustrated in Fig. 4. Intermolecular hydrogen bonds are presumed to exist between PLA and triclosan. If these interactions occur, the initial hydrophilicity of PLA would be expected to decrease. A similar phenomenon probably applies for the composite of PLA and wood. However, when 1.5% triclosan was added into the wood/PLA composites (5% and 10% wood contents), the contact angle value for the wood/PLA was found to decrease from 70.8° to 64.8°. This can be explained using a proposed interactive model, as given in Fig. 5. It is thought that triclosan is likely to have a preference with PLA via hydrogen bonding. The wood particles in the triclosan/PLA system would enable the wood/PLA composites to absorb more water molecules onto the triclosan/wood/PLA surfaces; this absorbed moisture would allow triclosan migration onto the PLA specimen surfaces due to a diffusion process, and eventually resulted in the decreases in contact angle value. The diffusibility of triclosan to kill bacteria has been found by Silapasorn et al. [18] and Chung et al. [21], who suggested that the efficiency of triclosan for killing bacteria incorporated in vinyl thermoplastics was
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dependent on its diffusion ability in the media. The results shown in Figs. 4 and 5 can be used to support the viable cell count results in Fig. 3 for triclosan/wood/PLA composites as compared with triclosan/PLA. Table 5 shows the effect of triclosan content on hydrophilicity (water contact angle) for PLA and wood/PLA composites. It can be seen that although the presence of triclosan decreased the hydrophilicity of the PLA and wood/PLA composites, its varying content had very little effect on the change in hydrophilicity. This may be because the increased triclosan content was probably distributed and/or dispersed within the bulk PLA and wood/PLA composites, rather than on the sample surfaces where the contact angles were being measured. The interactions between PLA, triclosan and wood, as seen in the contact angle results, can be substantiated by the FTIR analysis results in Fig. 6, showing FTIR spectra for PLA, triclosan, and PLA with 1.5 wt.% triclosan. It can be seen that the FTIR spectrum for triclosan/PLA was very similar to that for neat PLA. This was to be expected, since the triclosan content added was very small compared to the bulk PLA. However, a slight change could be observed in the carbonyl functional (C@O) peak of PLA at a wavenumber of 1753 cm1. The carbonyl peak became broadened and split into two small peaks (at wavenumbers of 1753 and 1746 cm1). These changes in the FTIR spectra indicated the intermolecular interaction between PLA and triclosan (Fig. 5). Fig. 6b shows FTIR spectra for PLA, wood, and PLA with 10 wt.% wood content. It can be seen that the FTIR spectrum for wood/PLA was similar to that for neat PLA, with the presence of a hydroxyl group (AOH) at a wavenumber of 3341 cm1. Unlike triclosan in PLA, the additional hydroxyl peak occurred due to the reasonable amount of wood flour added to the PLA. Finally, it was interesting to note that the FTIR spectrum for the triclosan/wood/PLA sample in Fig. 6c exhibited a mixture of the triclosan/PLA and wood/PLA spectra. This represents the broadening and splitting of the carbonyl peak at wavenumbers of 1753 and 1746 cm1 from the triclosan/PLA interaction, and the hydroxyl peak at a wavenumber of 3341 cm1 from the wood part. It appears that the contact angle results in Table 4, the molecular interactions in Fig. 4, and the FTIR results in Fig. 6 correspond well
Fig. 5. A possible model of moisture regain by presence of wood in triclosan/PLA blends: (a) without wood; (b) with wood.
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C. Prapruddivongs, N. Sombatsompop / Composites: Part B 43 (2012) 2730–2737 Table 5 Water contact angle of PLA, 5% WF/PLA and 10% WF/PLA incorporated with triclosan loadings of 0–1.5 wt.%. Samples
Triclosan content (%)
Contact angle (°)
PLA
0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5
58.6 68.2 68.4 68.8 69.3 60.1 64.7 61.7 70.8 58.2 63.3 64.8
5%WF/PLA
10%WF/PLA
Fig. 6. FTIR spectra for PLA and wood/PLA composites filled with triclosan: (a) PLA, triclosan, and triclosan/PLA; (b) PLA, wood flour, and wood flour/PLA composites; (c) PLA and triclosan/wood flour/PLA composites.
to the antibacterial activity results in Fig. 3, in accordance with the proposed concept of the role of wood flour as an antibacterial promoter. More evidence highlighting the role of wood flour as an antibacterial promoter was from DSC analysis, which showed crystallization and melting behavior in terms of the glass-transition (Tg) and melting (Tm) temperatures, and the degree of crystallinity (Xc). The DSC results for neat PLA, triclosan/PLA, wood/PLA, and triclosan/
wood/PLA composites are given in Fig. 7. It was found that the degree of crystallinity of PLA was unaffected by triclosan, but increased from 7.9% to 36.3% by the presence of 10% wood. The Tg and Tm of PLA were approximately 60–62 °C and 152–153 °C, respectively, and did not change with the addition of triclosan. Although the addition of triclosan had no effect on the Tm change, the Tm peak characteristics were affected by the presence of 10 wt.% wood. Similar behavior was also found in the case of bamboo fiber/PLA composites [22]. The Tm for the wood/PLA composite exhibited a double melting peak, the first melting peak being around 151 °C and the second melting peak around 157–159 °C. These two melting peaks were probably associated with the recrystallization of PLA due to the presence of wood. This was because the wood particles caused imperfections of the PLA crystals. Upon heating during the DSC test, a recrystallization process of the PLA occurred, in order to form perfect and stable crystals. This recrystallization phenomenon could be confirmed by the appearance of exothermic peaks around 90–135 °C for the PLA samples with wood, which eventually caused the occurrence of the double melting peak: one being for PLA crystallization, and the other for PLA recrystallization due to the presence of wood. This kind of recrystallization phenomenon of PLA due to highly hydrophilic fiber was also evidenced by Shi et al. [22]. In relation to physical properties, it is likely that the double melting peak was responsible for the increases in the degree of crystallinity from 7.9% to 36.3% by the presence of 10% wood. The increases in crystallinity level due to the presence of wood particles were also suggested in the case of wood/PP composites, as a result of transcrystalline structures formed on the wood surfaces [23]. It was essential to relate the thermal behavior with the antibacterial performance of PLA by the addition of triclosan and wood. Park et al. [24] documented that thermoplastics with a higher degree of crystallinity would have lower antibacterial activities due to the difficulty of the antibacterial agent in penetrating through the crystalline phase as compared with the amorphous structure. However, this characteristic was not the case in this work. The increases in the degree of crystallinity due to the addition of wood obtained in this work did not increase the rigidities of the triclosan/PLA to that extent. This was clear evidence that the increase in the tensile modulus with increasing wood flour content was relatively small (see Fig. 1). In other words, the increasing effect on the rigidity of triclosan/PLA is probably suppressed by the increasing hydrophilic effect when wood flour is added to the triclosan/ PLA blend (Table 4 and Fig. 5). This was why the antibacterial performance still improved, although the crystallinity and rigidity of the triclosan/PLA had slightly increased with the added wood flour. It can therefore be concluded that the hydrophilic effect played a more important role than the physical and thermal properties in the antibacterial activity of triclosan in the PLA and wood/PLA composites.
C. Prapruddivongs, N. Sombatsompop / Composites: Part B 43 (2012) 2730–2737
Fig. 7. DSC thermogram for PLA, wood flour/PLA, triclosan/PLA and triclosan/wood flour/PLA composites.
4. Conclusions In this work, triclosan and wood were introduced into PLA, and the mechanical and antibacterial performance were assessed and discussed. The experimental results suggested that the mechanical properties of PLA were affected by the addition of wood. Young’s modulus was found to increase; and the tensile strength, elongation at break, and impact strength decreased with increasing wood content. The mechanical properties as a function of triclosan loading had no definite trend due to the small amount of triclosan added. Bacterial growth as a function of contact time in neat PLA was different from that in that in wood/PLA composites. It was proposed in this work that the presence of wood particles could promote the antibacterial performance of triclosan/PLA blends, which was substantiated by WCA, FTIR and DSC characterizations. The hydrophilic effect played a more important role in the antibacterial activity of triclosan in the wood/PLA composites. Acknowledgments The authors would like to thank the Office of the Higher Education Commission (OHEC) under the National Research University Program, and the Thailand Research Fund (TRF Research Senior Scholar; RTA5280008) for financial co-support throughout this work. References [1] Pradeep T, Jain P. Potential of silver nanoparticle-coated polyurethane foam as an antibacterial water filter. Biotechnol Bioeng 2005;90:59–63.
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[2] Chammanee P, Sombatsompop K, Kositchaiyong A, Sombatsompop N. Effects of anti-bacterial agents, sample preparation and contact time on anti-bacterial efficacy in MDPE film. J Macromol Sci Phys 2009;48:755–65. [3] Jin T, Zhang H. Biodegradable polylactic acid polymer with Nisin for use in antimicrobial food packaging. J Food Sci 2008;73:127–34. [4] Liu L, Jin T, Finkenstadt V, Liu C, Cooke P, Coffin D, et al. Antimicrobial packaging materials from poly(lactic acid) incorporated with pectin-NisaplinÒ microparticles. Chem Chem Technol 2009;3:221–30. [5] Li L, Li Y, Li J, Yao L, Mak AFT, Ko F, et al. Antibacterial properties of nanosilver PLLA fibrous membranes. J Nanomate 2009 [Article ID 168041:1-5]. [6] Fernandez A, Soriano E, Hernandez-Munoz P, Gavara R. Migration of antimicrobial silver from composites of polylactide with silver zeolites. J Food Sci 2010;75:186–93. [7] Faruk O, Matuana LM. Nanoclay reinforced HDPE as a matrix for wood–plastic composites. Compos Sci Technol 2008;68:2073–7. [8] Bledzki A, Faruk O. Injection moulded microcellular wood fibre–polypropylene composites. Composites A 2006;37:1358–67. [9] Pattamasattayasonti N, Chaochanchaikul K, Rosarpitak V, Sombatsompop N. Effects of uv-weathering aging and CeO2 based coating layer on mechanical and structural changes in wood/PVC composites. J Vinyl Addit Technol 2011;17:9–16. [10] Finkenstadta VL, Tisserat B. Poly(lactic acid) and osage orange wood fiber composites for agricultural mulch films. Ind Crops Prod 2010;31:316–20. [11] Sykacek E, Schlager W, Mundigler N. Compatibility of softwood slour and commercial biopolymers in injection molding. Polym Compos 2010;31:443–51. [12] Lee SY, Kang IA, Doh GH, Yoon HG, Park BD, Wu Q. Thermal and mechanical properties of wood flour/talc-filled polylactic acid composites: effect of filler content and coupling treatment. J Therm Compos Mater 2008;21:209–23. [13] Pilla S, Gong S, O’Neill E, Rowell MR, Krzysik AM. Polylactide-pine wood flour composites. Polym Eng Sci 2008;48:578–87. [14] Petinakis E, Yu L, Edward G, Dean K, Liu H, Scully AD. Effect of matrix–particle interfacial adhesion on the mechanical properties of poly(lactic acid)/woodflour micro-composites. J Polym Environ 2009;17:83–94. [15] Van Nostrum CF, Veldhuis TFJ, Bos GW, Hennink WE. Hydrolytic degradation of oligo(lactic acid): a kinetic and mechanistic study. Polymer 2004;45:6779–87. [16] Wiggins JS, Hassan MK, Mauritz KA, Storey RF. Hydrolytic degradation of poly(D,L-lactide) as a function of end group: carboxylic acid VS hydroxyl. Polymer 2006;47:1960–9. [17] Tsuji H, Miyauchi S. Poly(l-lactide): VI effects of crystallinity on enzymatic hydrolysis of poly(l-lactide) without free amorphous region. Polym Degrad Stab 2001;71:415–24. [18] Silapasorn K, Sombatsompop K, Kositchaiyong A, Wimolmala E, Markpin T, Sombatsompop N. Effect of chemical structure of thermoplastics on antibacterial activity and physical diffusion of triclosan doped in vinyl thermoplastics and their composites with CaCO3. J Appl Polym Sci 2011;121:253–61. [19] Sombatsompop N, Yotinwattanakumtorn C, Thongpin C. Influences of types and concentrations of maleic anhydride grafted polypropylene and impact modifiers on mechanical properties of PP/wood sawdust composites. J Appl Polym Sci 2005;97:475–84. [20] Sombatsompop N, Chaochanchaikul K. Effect of moisture content on mechanical properties, thermal and structural stability, and extrudate texture of PVC/wood sawdust composites. Polym Int 2004;53:1210–8. [21] Chung D, Papadakis SE, Yam KL. Evaluation of a polymer coating containing triclosan as the antimicrobial layer for a packaging film. Int J Food Sci Technol 2003;38:165–9. [22] Shi QF, Mou HY, Gao L, Yang J, Guo WH. Double-melting behavior of bamboo fiber/talc/poly(lactic acid) composites. J Polym Environ 2010;18:567–75. [23] Sombatsompop N, Kositchaiyong A, Wimolmala E. Experimental analysis of temperature and crystallinity profiles of wood sawdust/PP composites during cooling. J Appl Polym Sci 2006;102:1896–905. [24] Park ES, Moon WS, Song MJ, Kim MN, Chung KH, Yoon JS. Antimicrobial activity of phenol and benzoic acid derivatives. Int J Biodeter Biodegrad 2001;47:209–14.