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Biodegradable poly(lactic acid)-based hybrid coating materials for food packaging films with gas barrier properties
Journal of Industrial and Engineering Chemistry 18 (2012) 1063–1068
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Biodegradable poly(lactic acid)-based hybrid coating materials for food packaging films with gas barrier properties Gree Bang, Seong Woo Kim * Department of Chemical Engineering, Kyonggi University, 94-6 Yiui-dong Yeongtong-gu, Suwon, Kyonggi-do 443-760, South Korea
A R T I C L E I N F O
Article history: Received 28 July 2011 Accepted 13 December 2011 Available online 21 December 2011 Keywords: Biodegradable PLA Sol–gel Hybrid Coating Barrier properties
A B S T R A C T
The inclusion of biodegradable poly(lactic acid) (PLA) as an organic component into the inorganic silica networks was attempted to prepare environmentally friendly hybrid coating materials with improved gas barrier properties by using sol–gel method. The PLA film obtained by melt extrusion casting process was used as a substrate for coating with prepared PLA/SiO2 hybrids. Interfacial attraction between the organic and the inorganic phases in the hybrid was promoted by employing 3-isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase interaction, morphologies, crystallization behavior, and optical transparencies for the prepared hybrids were investigated not only to evaluate the phase compatibility, but also to present an evidence for the gas permeation behavior through the hybrid coated PLA film. The incorporation of the silica component at appropriate level of content substantially increased the resistance to gas permeation. The films retained high transparency, with optical transmittance of over 92%, and showed oxygen and water vapor barrier properties improved by 69.7% and 45.7%, respectively, over those of neat PLA film. Aging process improved the barrier properties of the hybrid coatings due to created dense network structures. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Packaging materials for food and medicine require high resistances to oxygen and water vapor permeation to ensure products do not deteriorate during storage and handling [1,2]. Polymeric packaging materials with high barrier properties are usually produced with multilayered structures by coextrusion or lamination process [3,4]. These multilayer films include various nondegradable polymer resins, such as polyolefin, polyamide, poly(ethylene terephthalate), and ethylene-vinyl alcohol copolymers. Since nondegradable polymer resins synthesized from petrochemicals can cause environmental problems after their useful life, ecologically safe, biodegradable polymers have been used for the application of short-term storage packaging films [5,6]. A variety of biodegradable polymer materials have been developed and commercialized to replace conventional nondegradable polymer resins. Biopolymers for use as biodegradable packaging synthesized from petrochemicals or renewable resources include aliphatic thermoplastic polyesters, such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(e-caprolactone)
* Corresponding author. Tel.: +82 31 249 9787; fax: +82 31 257 0161. E-mail address: [email protected] (S.W. Kim).
(PCL), and poly(hydroxyl butyrate) [7]. Among these, PLA synthesized from renewable resources has attracted much attention as it can achieve excellent mechanical properties, biodegradability, and biocompatibility, at competitive cost [8–10]. Despite its advantages, crystalline PLA shows a limitation for the application of gas barrier films to be used for the food or medical packaging materials, as it has relatively low resistance to oxygen and water vapor permeation compared with conventional nondegradable polymer resins [11]. Therefore, much work has been performed to improve the gas permeation resistance of biodegradable PLA resins by combining it with inorganic materials. The most frequently used approach is the introduction of nanoscale, organically modified, layered silicates into polymer matrices to prepare biodegradable nanocomposites [12–15]. Gas barrier, mechanical, and thermal properties can be improved upon the addition of small amounts of nanosized clay, due to the homogeneous dispersion of intercalated or exfoliated layered silicate platelets in the continuous matrix. However, clay loadings above a critical content can result in poor dispersion of silicate layers that exhibit intercalated clay tactoids with large domain size, which ultimately deteriorate the physical properties of the nanocomposites. Hence, this nanotechnology approach using nanoclay has a limitation in increasing the barrier properties to the level required as the high barrier packaging materials.
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.12.004
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An alternative approach to improve gas barrier properties involves incorporating nanostructured silica components synthesized from inorganic precursors into organic polymer phases using sol–gel technology. Organic–inorganic hybrid coatings of various nondegradable polymer substrate films has been reported to improve gas barrier properties [16–19]. In these hybrid coating materials, the nondegradable polymer resins such as poly(vinyl alcohol), poly(vinyl acetate), poly(vinylidene chloride), and poly(ethylene-co-vinyl alcohol) were applied as the organic components. However, such nondegradable polymer-based hybrid coatings would severely limit the environmental benefits of using biodegradable polymer substrates. Therefore the preparation of biodegradable polymer-based hybrid coatings that can be combined with biopolymer substrate films has been reported for less environmentally damaging biodegradable food packaging films [11]. This work reports the incorporation of biodegradable PLA resin as an organic polymer into inorganic silica networks to prepare biodegradable organic–inorganic hybrid materials with high gas barrier properties that could be coated onto PLA substrate films. The PLA/SiO2 hybrid materials were synthesized by sol–gel process using tetraethoxysilane (TEOS) as an inorganic precursor and 3isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase microstructures, thermal properties, and crystallization behaviors of the resulting hybrid materials were investigated by SEM, DSC, and XRD. The gas barrier properties of the biodegradable hybrid-coated PLA films were assessed by measuring the permeabilities of oxygen and water vapor through the coated films. 2. Experimental 2.1. Materials and preparation Tetraethoxysilane (TEOS, Acros Organics, 98%) and 3-isocyanatopropyl-triethoxysilane (IPTES, Aldrich, 95%) were used as inorganic silicate precursor and silane coupling agent, respectively. Poly(lactic-acid) (PLA, 2002D, NatureWorks Co. Ltd.) was used as the organic polymer in the PLA/SiO2 hybrid materials. Hydrocholoric acid (HCl, Samchun Chemical, 37 wt%) was used as a catalyst. Tetrahydrofuran (THF, Ducsan, Korea) was used as a cosolvent to facilitate hydrolysis during the sol–gel reactions by achieving miscibility between TEOS and water. PLA substrate film with thickness of ca. 50 mm was prepared by casting film process using a single screw extruder with an attached T-die (D-5489, Plastik Paschinenbau Co., Germany). The PLA grade (2002D) was that used for the preparation of the hybrids. Strong adhesion between the hybrid coating layer and the substrate was assured due to use of the same PLA resin in each layer. The PLA/SiO2 hybrid coating sol was prepared by stirring a mixture of TEOS, distilled water, and THF with a molar composition ratio of 1:2:6 for 3 h to partially hydrolyze the TEOS under the acid catalyst. The initial pH of the TEOS solution was adjusted to 2.0 by the addition of 2 M hydrochloric (HCl) acid solution. 5 wt% PLA solution with homogeneous dispersion was prepared by dissolving PLA resin in a good solvent of THF. The PLA resin was modified by adding the silane coupling agent, IPTES, to the homogenized PLA solution with stirring for 3 h at 60 8C. This modified PLA solution and the partially hydrolyzed TEOS solution were then mixed and stirred vigorously for 2 h at room temperature to yield PLA/SiO2 hybrid coating sol. PLA/SiO2 hybrids were prepared with various inorganic silicate precursor contents (TEOS: 0.01, 0.02, 0.03, 0.04, 0.06, 0.08 mol). In all hybrid samples, the amounts of added IPTES silane coupling agent and PLA resin were fixed at 0.001 mol and 1 g, respectively. The hybrid coating sols were applied to PLA films using a bar coater (R.D.S. #12). The coated films were dried at 60 8C for 24 h in
a drying oven, and then aged at room temperature for 28 days. In addition to coated films, hybrid gels were also obtained by casting the hybrid sols onto Petri dishes covered with polyimide film and drying for 15 days at room temperature. The gel samples were covered with paper filters to prevent contamination from impurities in the air. All the dried samples were stored in a desiccator to prevent the effects of moisture prior to performing characterization. 2.2. Characterization Phase attractions between the inorganic silicate and the organic PLA polymer in the hybrids were examined by Fourier transform infrared spectroscopy (FT-IR, JASCO-430, Jasco Co., Japan). Samples were diluted with KBr before FT-IR measurement by mixing the hybrid gels with KBr powder and pressing them into pellets less than 1 mm thick. A hybrid gel to KBr weight ratio of 200:1 was used. Crystallization in the PLA/SiO2 hybrids was examined by Xray diffractometer (XRD, 3D Max-C, Rigaku Co., Japan) with Cu Ka radiation operated at 40 kV and 30 mA. Samples were scanned in the range of 1–308 at a scanning rate of 1.08/min. Differential scanning calorimetry (DSC, N536-0003, Perkim Elmer Co., USA) was used to measure the hybrids’ thermal properties. Samples were initially heated from room temperature to 200 8C at 10 8C/ min, at which they were held for 5 min to remove their previous thermal history. They were then cooled to room temperature at a fast cooling rate of 50 8C/min to become completely amorphous. Slower heating at 2 8C/min was then conducted to observe cold crystallization behavior. Morphologies of fractured surfaces of the hybrid materials were observed by field-emission scanning electron microscopy (FE-SEM, JSM-6500F, Jeol Co., Japan). The optical transparencies of hybrid coating films were measured by visible spectrophotometry (Optizen 1412V, Mecasys Co., Korea) over the visible wavelengths of 400–800 nm. Oxygen permeabilities of the coated films were assessed by apparatus made in the laboratory according to the ASTM D3985. Water vapor permeability was measured by a water vapor transmission rate tester (Permatran-W3/33MA, Mocon Co., USA).
3. Results and discussion 3.1. FTIR analysis In the organic–inorganic hybrid materials, the phase attraction between two phases has been considered as a crucial factor to the production of hybrid materials with high performance. Particularly, in nanostructured hybrid materials synthesized via sol–gel method using a polymer resin system as an organic component, the phase morphology in association with the physical properties of the hybrid materials is greatly influenced by phase interaction between the polymer chain segments and the dispersed inorganic domains. Strong interfacial attraction can suppress microphase separation in the materials, favoring phases with stable and homogeneously dispersed microstructures. Various silane coupling agents containing reactive functional groups, such as 3-glycidoxypropyltrimethoxysilane (GPTMS), 3isocyanatopropyltriethoxysilane (IPTES), aminopropyltriethoxysilane (APTES), and methacroxypropyltrimethoxysilane (MPTMS), have been used to promote interfacial adhesion between organic and inorganic phases in hybrid materials. If the organic polymer contains hydroxyl groups (–OH), they can form strong covalent bonds with IPTES through its highly reactive isocyanate (N5 5C5 5O) groups, resulting in the formation of urethane groups. Therefore, IPTES was used here as a silane coupling agent to promote interfacial adhesion through strong covalent bonding between its
G. Bang, S.W. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 1063–1068
Fig. 1. Comparison of FT-IR spectra for the PLA/SiO2 hybrid without IPTES and that with IPTES.
isocyanate groups and hydroxyl groups pendent on the ends of the PLA chains. Fig. 1 shows FTIR spectra of PLA/SiO2 hybrids with and without IPTES. The presence of IPTES resulted in characteristic peak corresponding to amine groups (–NH) at 1530–1575 cm1 due to chemical reaction between isocyanate group of IPTES and hydroxyl group from PLA molecule. This observed peak indicates that strong interfacial adhesion between the organic PLA phase and the inorganic silica network was achieved during sol–gel processing. This reaction mechanism has been previously observed in EVOH/ SiO2 hybrid coating materials with addition of IPTES, in which the organic EVOH copolymer contained hydroxyl groups in its vinyl alcohol units [19]. 3.2. Crystallization behavior The physical and mechanical properties of crystalline polymers depend on the molecular morphology, which is controlled by crystallization [20]. Particularly, gas barrier properties are significantly influenced by both the degree of crystallinity and the crystalline structure developed in the semicrystalline polymer,
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which affect the rate of transport of gas molecules through the polymer. In this study, therefore, we investigated the crystallization behavior of the PLA polymer which is combined with silica network in the hybrid. Fig. 2 shows DSC results of neat PLA and three PLA/SiO2 hybrids with various silica contents. These traces were registered during second heating run with a slow heating rate, exhibiting cold crystallization, which was started from completely amorphous solid state. Neat PLA showed one melting endothermic peak at ca. 149 8C; the samples with silica showed two distinct melting peaks, which were attributed to the melting of less ordered crystals, characterized by thinner crystalline lamella, at lower temperature followed by the melting of more perfect crystals after recrystallization during heating [20,21]. Similar melting behavior has also been observed in PLA/clay nanocomposites [10] and plasticized PLA [9]. The melting temperatures (Tm) determined from the binary endothermic peaks for the PLA hybrids was observed to be almost invariant with increasing silica content. On the other hand, the inclusion of silica in the PLA matrices up to 64.3 wt%, corresponding to 0.03 mol TEOS, was found to decrease the crystallization temperature (Tc) of the PLA, which was determined from the exothermic peak obtained during heating at 2 8C/min between 80 and 130 8C. The silica contents in the hybrid were calculated assuming 100% conversion from the TEOS precursor to the silicate network. The Tc of PLA resin in the hybrid was reduced by ca. 16 8C compared with that of neat PLA by the incorporation of silica at 37.5 wt%, corresponding to 0.01 mol silica precursor TEOS. In this hybrid, nanosized silica particles formed from sol–gel reactions appeared to act as effective nucleating agents, and thus reduced the cold crystallization temperature of PLA through promoting overall crystallization rate, determined from nucleation and crystal growth rates of the PLA. Similar crystallization behavior of PLA in PLA/clay nanocomposites has also been reported [21–23]. It was believed that the nanoclay with large surface area, which is capable of providing more nucleating sites, efficiently enhanced the crystallization rate and subsequently reduced the crystallization temperature of PLA. The measured melting and crystallization temperatures, and calculated degree of crystallinity (Xc) are listed in Table 1. The degree of crystallinity of PLA in the hybrids was determined from the following equation:
X c ð%Þ ¼
DHf;s 100 DHf;PLA ð1 W silica Þ
where DHf,s is the enthalpy of fusion of sample; DHf,PLA refers to the enthalpy of 100% crystalline PLA, which was set as 93.6 J/g, and Wsilica is the mass fraction of silica component [21]. The incorporation of small amount of silica particles at 37.5 wt% in the PLA slightly increased the degree of crystallinity of PLA due to enhanced crystallization process achieved by the dispersed nucleating silica nanoparticles. However, the presence of silica particles with excess amount exhibited the reverse effect on the trend of degree of crystallinity as increasing silica content. This was likely because higher silica contents restrict the PLA’s molecular chain mobility, which is necessary for its regular packing into crystal lattices, due to increased interaction sites between the organic PLA and the inorganic silica phase, ultimately retarding crystallization. Table 1 DSC data of neat PLA and PLA/SiO2 hybrids with different silica content.
Fig. 2. DSC heating thermograms for the neat PLA and PLA/SiO2 hybrids with different TEOS contents.
Sample
Silica content (wt%)
Tc (8C)
Tm1 (8C)
Tm2 (8C)
Xc (%)
Neat PLA Hybrid-0.01 Hybrid-0.03 Hybrid-0.06
0 37.5 64.3 78.3
118.8 102.4 95.7 100.8
149.5 146.2 146.8 147.7
– 154.7 153.8 154.3
22.33 22.55 18.49 14.81
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silica contents showed reduced peak intensities, including area, due to decreased crystallinity of the PLA, as observed in the previous DSC results. 3.3. Nanostructure morphology
Fig. 3. X-ray diffraction patterns for the neat PLA and PLA/SiO2 hybrids with different TEOS contents of 0.01 and 0.02 mol.
Fig. 3 shows X-ray diffraction patterns of neat PLA and the hybrids. Neat PLA showed the most intense peak (the main peak) at 2u = 16.78 from (2 0 0) and (1 1 0) reflections; less intense peaks were observed at 2u = 14.8, 19.1, and 22.48 corresponding to (0 1 0), (2 0 3), and (0 1 5) reflections, respectively [20,24]. The silica-containing hybrids showed peaks slightly shifted to lower 2u values. This indicates that the presence of silica particle caused slight modification of crystalline structure of PLA matrix, basically maintaining the a-form structure. The hybrid samples with higher
In general, the mechanical, optical, thermal, and barrier properties of the organic–inorganic hybrid materials produced by sol–gel method are greatly influenced by the phase morphology, which depends on the various factors such as concentration of each component, phase attraction between two phases, and processing condition. Phase morphologies of fractured gels were observed to examine the dispersion of the nanostructured silica particles in the PLA matrices. Fig. 4(a)–(c) shows SEM micrograph of hybrids with TEOS contents of 0.01, 0.03, and 0.06 mol; the IPTES content was maintained at 0.001 mol in all the samples. The hybrid gels with lower TEOS contents (0.01 and 0.03 mol) contained homogeneously and uniformly dispersed silica particles with average sizes of below 60 nm in the PLA matrices. They also showed no phase separation. However, excess addition of TEOS precursor (0.06 mol) resulted in large silica clusters due to physical coalescence among the silica nanoparticles or chemical condensation between residual silanols on the silica surface, which led to partial phase separation. The hybrid with 0.03 mol TEOS and without IPTES silane coupling agent shows distinct microphase separation and the formation of silica clusters larger than 100 nm (Fig. 4(d)). These may have been caused by weak interactions between the organic PLA and the inorganic silicate network. The variation of morphology with composition was similar to that previously reported for EVOH/SiO2 hybrids, in which the phase compatibility was also controlled by IPTES silane coupling agent that could form covalent bonds through chemical reactions with hydroxyl groups in the organic EVOH phase.
Fig. 4. Phase morphology of IPTES (0.001 mol) added PLA/SiO2 hybrids with various TEOS contents of (a) 0.01, (b) 0.03, (c) 0.06 mol, and (d) no IPTES added hybrid with TEOS content of 0.03 mol.
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3.5. Barrier properties
Fig. 5. Light transmittance of the non-coated neat PLA film and various PLA/SiO2 hybrids coated PLA films.
3.4. Optical transparency of the hybrid coated films Fig. 5 shows relative light transmittances with respect to visible light wavelength for the various hybrid coated films. The film with 0.03 mol TEOS (corresponding to 64.3 wt% silica content) was highly transparent, transmitting over 92.5% of incident visible light range of 500–800 nm, showing comparable transparency to the neat PLA film. This may have originated from its more homogeneous and finer microstructure with dispersed silica nanoparticles. In the case of both hybrid coated film samples with addition of 0.01 and 0.06 mol TEOS, however, a deterioration in optical transparency was observed, exhibiting similar light transmittance rate for both of them. In the sample with least TEOS (0.01 mol, corresponding to 37.5 wt% silica), the discrepancy in the degree of crystallinity and crystal size of the organic PLA phase in the hybrid coating layers and the PLA substrate increased light scattering in the two-layer structure with different refractive indexes, which reduced optical transmittance. Excess TEOS (0.06 mol, corresponding to 78.3 wt% silica) resulted in large silica clusters that also reduced optical transmittance through increased light scattering.
Fig. 6. Oxygen and water vapor permeabilities of the PLA/SiO2 hybrids coated films as a function of TEOS content.
Fig. 6 shows measured oxygen and water vapor permeabilities of the hybrid coated PLA film with respect to TEOS inorganic precursor content. For all the hybrids with different TEOS contents, the silane coupling agent, IPTES, was added in the same amount of 0.001 mol. As seen in the figure, the oxygen and water vapor permeation behavior through hybrid coated film was shown to be very similar. In the range of TEOS contents up to 0.03 mol corresponding to silica content of 64.3 wt%, both oxygen and water vapor permeabilities were dramatically reduced with increasing TEOS content, which was due to increased amount of the inorganic silica with superior barrier property. When 0.03 mol TEOS was added in the hybrid, the oxygen and water vapor barrier properties of the hybrid coated PLA film were improved by 69.7% and 45.7%, respectively, as compared with pure PLA film, exhibiting oxygen and water vapor permeabilities of 16.03 cc mm/m2 day atm and 8.80 g/m2 day, respectively. These remarkable improvements in both oxygen and water vapor barrier properties were attributable to a stable homogeneous microstructure with uniformly dispersed silica particles of nano-scale dimensions, which did not show microphase separation in the hybrids. In the range of TEOS content above 0.03 mol, on the other hand, both oxygen and water vapor permeabilities were observed to be almost invariant with incorporated amount of silica component; the incorporation of the inorganic silica component with an excess amount (about 65 wt% silica in current study) revealed no more contribution to the enhancement in the barrier property of hybrid coatings. This result may be due to the partial microphase separation in the hybrid coating layer and the lack of interfacial adhesion between the coating layer and the PLA substrate. The effect of IPTES content on the oxygen barrier property of the hybrid coated film was also investigated. Fig. 7 shows the oxygen permeabilities of PLA films coated with hybrids containing 0.01 and 0.02 mol TEOS with respect to IPTES content. Oxygen permeation varied similarly with IPTES content, irrespective of TEOS content. Film with IPTES contents of up to 0.002 mol showed significantly reduced oxygen permeability due to improved interactions between the organic and the inorganic components in the PLA/SiO2 hybrid. However increased oxygen permeability, corresponding to the deterioration in the barrier property, was observed in films with IPTES contents above 0.003 mol. This may be due to the fact that with addition of excess amount of IPTES
Fig. 8. The effect of aging time on the oxygen barrier property of the PLA films coated by hybrids with 0.03 and 0.06 mol TEOS content.
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Fig. 7. Oxygen permeability of the PLA/SiO2 hybrids coated films as a function of IPTES content.
silane coupling agent, the organic molecules in the IPTES has a greater tendency of hindering the formation of siloxane(Si–O–Si) groups through condensation reactions during sol–gel process, and leads to less-dense inorganic silica networks and increased gas permeation rate. These results show that the silane coupling agent could control interfacial attraction between the organic and the inorganic phases and also affect the inorganic network structure created from sol–gel reactions. Fig. 8 shows the effect of aging on the oxygen permeability of PLA films coated with hybrids with 0.03 and 0.06 mol TEOS. All of coated film samples exhibited improvement in oxygen barrier property with increasing of aging time. As shown in the figure, the oxygen permeabilities dramatically decreased with increased aging up to 21 days, after which the permeabilities slightly reduced, achieving equilibrium within 28 days. Therefore, the above results for oxygen and water vapor permeability were of films aged for 28 days. After deposition of the hybrid sols onto the PLA substrate, the coated thin liquid film gradually solidified into a thin coating layer via gelation of the dispersed inorganic silica phase and crystallization of the PLA resin during drying as the solvent was removed from the hybrid sol. The crosslink density of the silica networks in the hybrid solids is generally low due to insufficient curing during fast coating and drying. Insufficient curing can be compensated by aging that involves continuing physical and chemical changes while the films are stored at ambient temperature [25]. The results here show that aging improved the barrier properties of PLA/SiO2 hybrid coated film by increasing inorganic silicate networks units in the hybrid. 4. Conclusions Environmentally less damaging hybrid coating materials with low gas permeability were prepared by incorporating biodegrad-
able organic PLA resin into inorganic silicate with superior barrier characteristics via sol–gel process. The resulting PLA/SiO2 hybrids showed improved phase compatibility with addition of IPTES silane coupling agent capable of creating covalent bonds between the organic and the inorganic phases. This strong bonding resulted in homogeneous microstructures with silica nanoparticles dispersed in the PLA matrix. Cold crystallization behavior of the PLA polymer phase embedded in the hybrid were found to be significantly influenced by the presence of nanosized silica particles produced via sol–gel reaction; the crystallization rate, the degree of crystallinity, and the crystalline structure of the semi-crystalline PLA depended on the incorporated silica content. The hybrid films were highly transparent, achieving visible light transmittances of over 92%. Their gas barrier properties were substantially improved up to appropriate level of content for the silica (64.3 wt% corresponding to 0.03 mol TEOS) and silane coupling agent (0.002 mol IPTES). However, addition of silica component and IPTES with excess amount exhibited no more contribution or reverse effect in the enhancement of the barrier properties. In addition, it was revealed that aging time was significant coating process parameter to produce hybrid coated PLA film with high gas barrier property for the application of food packaging films.
Acknowledgment This work was supported by Kyonggi University Research Grant 2010.
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Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Biodegradable poly(lactic acid)-based hybrid coating materials for food packaging films with gas barrier properties Gree Bang, Seong Woo Kim * Department of Chemical Engineering, Kyonggi University, 94-6 Yiui-dong Yeongtong-gu, Suwon, Kyonggi-do 443-760, South Korea
A R T I C L E I N F O
Article history: Received 28 July 2011 Accepted 13 December 2011 Available online 21 December 2011 Keywords: Biodegradable PLA Sol–gel Hybrid Coating Barrier properties
A B S T R A C T
The inclusion of biodegradable poly(lactic acid) (PLA) as an organic component into the inorganic silica networks was attempted to prepare environmentally friendly hybrid coating materials with improved gas barrier properties by using sol–gel method. The PLA film obtained by melt extrusion casting process was used as a substrate for coating with prepared PLA/SiO2 hybrids. Interfacial attraction between the organic and the inorganic phases in the hybrid was promoted by employing 3-isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase interaction, morphologies, crystallization behavior, and optical transparencies for the prepared hybrids were investigated not only to evaluate the phase compatibility, but also to present an evidence for the gas permeation behavior through the hybrid coated PLA film. The incorporation of the silica component at appropriate level of content substantially increased the resistance to gas permeation. The films retained high transparency, with optical transmittance of over 92%, and showed oxygen and water vapor barrier properties improved by 69.7% and 45.7%, respectively, over those of neat PLA film. Aging process improved the barrier properties of the hybrid coatings due to created dense network structures. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Packaging materials for food and medicine require high resistances to oxygen and water vapor permeation to ensure products do not deteriorate during storage and handling [1,2]. Polymeric packaging materials with high barrier properties are usually produced with multilayered structures by coextrusion or lamination process [3,4]. These multilayer films include various nondegradable polymer resins, such as polyolefin, polyamide, poly(ethylene terephthalate), and ethylene-vinyl alcohol copolymers. Since nondegradable polymer resins synthesized from petrochemicals can cause environmental problems after their useful life, ecologically safe, biodegradable polymers have been used for the application of short-term storage packaging films [5,6]. A variety of biodegradable polymer materials have been developed and commercialized to replace conventional nondegradable polymer resins. Biopolymers for use as biodegradable packaging synthesized from petrochemicals or renewable resources include aliphatic thermoplastic polyesters, such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(e-caprolactone)
* Corresponding author. Tel.: +82 31 249 9787; fax: +82 31 257 0161. E-mail address: [email protected] (S.W. Kim).
(PCL), and poly(hydroxyl butyrate) [7]. Among these, PLA synthesized from renewable resources has attracted much attention as it can achieve excellent mechanical properties, biodegradability, and biocompatibility, at competitive cost [8–10]. Despite its advantages, crystalline PLA shows a limitation for the application of gas barrier films to be used for the food or medical packaging materials, as it has relatively low resistance to oxygen and water vapor permeation compared with conventional nondegradable polymer resins [11]. Therefore, much work has been performed to improve the gas permeation resistance of biodegradable PLA resins by combining it with inorganic materials. The most frequently used approach is the introduction of nanoscale, organically modified, layered silicates into polymer matrices to prepare biodegradable nanocomposites [12–15]. Gas barrier, mechanical, and thermal properties can be improved upon the addition of small amounts of nanosized clay, due to the homogeneous dispersion of intercalated or exfoliated layered silicate platelets in the continuous matrix. However, clay loadings above a critical content can result in poor dispersion of silicate layers that exhibit intercalated clay tactoids with large domain size, which ultimately deteriorate the physical properties of the nanocomposites. Hence, this nanotechnology approach using nanoclay has a limitation in increasing the barrier properties to the level required as the high barrier packaging materials.
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.12.004
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An alternative approach to improve gas barrier properties involves incorporating nanostructured silica components synthesized from inorganic precursors into organic polymer phases using sol–gel technology. Organic–inorganic hybrid coatings of various nondegradable polymer substrate films has been reported to improve gas barrier properties [16–19]. In these hybrid coating materials, the nondegradable polymer resins such as poly(vinyl alcohol), poly(vinyl acetate), poly(vinylidene chloride), and poly(ethylene-co-vinyl alcohol) were applied as the organic components. However, such nondegradable polymer-based hybrid coatings would severely limit the environmental benefits of using biodegradable polymer substrates. Therefore the preparation of biodegradable polymer-based hybrid coatings that can be combined with biopolymer substrate films has been reported for less environmentally damaging biodegradable food packaging films [11]. This work reports the incorporation of biodegradable PLA resin as an organic polymer into inorganic silica networks to prepare biodegradable organic–inorganic hybrid materials with high gas barrier properties that could be coated onto PLA substrate films. The PLA/SiO2 hybrid materials were synthesized by sol–gel process using tetraethoxysilane (TEOS) as an inorganic precursor and 3isocyanatopropyltriethoxysilane (IPTES) as a silane coupling agent. Phase microstructures, thermal properties, and crystallization behaviors of the resulting hybrid materials were investigated by SEM, DSC, and XRD. The gas barrier properties of the biodegradable hybrid-coated PLA films were assessed by measuring the permeabilities of oxygen and water vapor through the coated films. 2. Experimental 2.1. Materials and preparation Tetraethoxysilane (TEOS, Acros Organics, 98%) and 3-isocyanatopropyl-triethoxysilane (IPTES, Aldrich, 95%) were used as inorganic silicate precursor and silane coupling agent, respectively. Poly(lactic-acid) (PLA, 2002D, NatureWorks Co. Ltd.) was used as the organic polymer in the PLA/SiO2 hybrid materials. Hydrocholoric acid (HCl, Samchun Chemical, 37 wt%) was used as a catalyst. Tetrahydrofuran (THF, Ducsan, Korea) was used as a cosolvent to facilitate hydrolysis during the sol–gel reactions by achieving miscibility between TEOS and water. PLA substrate film with thickness of ca. 50 mm was prepared by casting film process using a single screw extruder with an attached T-die (D-5489, Plastik Paschinenbau Co., Germany). The PLA grade (2002D) was that used for the preparation of the hybrids. Strong adhesion between the hybrid coating layer and the substrate was assured due to use of the same PLA resin in each layer. The PLA/SiO2 hybrid coating sol was prepared by stirring a mixture of TEOS, distilled water, and THF with a molar composition ratio of 1:2:6 for 3 h to partially hydrolyze the TEOS under the acid catalyst. The initial pH of the TEOS solution was adjusted to 2.0 by the addition of 2 M hydrochloric (HCl) acid solution. 5 wt% PLA solution with homogeneous dispersion was prepared by dissolving PLA resin in a good solvent of THF. The PLA resin was modified by adding the silane coupling agent, IPTES, to the homogenized PLA solution with stirring for 3 h at 60 8C. This modified PLA solution and the partially hydrolyzed TEOS solution were then mixed and stirred vigorously for 2 h at room temperature to yield PLA/SiO2 hybrid coating sol. PLA/SiO2 hybrids were prepared with various inorganic silicate precursor contents (TEOS: 0.01, 0.02, 0.03, 0.04, 0.06, 0.08 mol). In all hybrid samples, the amounts of added IPTES silane coupling agent and PLA resin were fixed at 0.001 mol and 1 g, respectively. The hybrid coating sols were applied to PLA films using a bar coater (R.D.S. #12). The coated films were dried at 60 8C for 24 h in
a drying oven, and then aged at room temperature for 28 days. In addition to coated films, hybrid gels were also obtained by casting the hybrid sols onto Petri dishes covered with polyimide film and drying for 15 days at room temperature. The gel samples were covered with paper filters to prevent contamination from impurities in the air. All the dried samples were stored in a desiccator to prevent the effects of moisture prior to performing characterization. 2.2. Characterization Phase attractions between the inorganic silicate and the organic PLA polymer in the hybrids were examined by Fourier transform infrared spectroscopy (FT-IR, JASCO-430, Jasco Co., Japan). Samples were diluted with KBr before FT-IR measurement by mixing the hybrid gels with KBr powder and pressing them into pellets less than 1 mm thick. A hybrid gel to KBr weight ratio of 200:1 was used. Crystallization in the PLA/SiO2 hybrids was examined by Xray diffractometer (XRD, 3D Max-C, Rigaku Co., Japan) with Cu Ka radiation operated at 40 kV and 30 mA. Samples were scanned in the range of 1–308 at a scanning rate of 1.08/min. Differential scanning calorimetry (DSC, N536-0003, Perkim Elmer Co., USA) was used to measure the hybrids’ thermal properties. Samples were initially heated from room temperature to 200 8C at 10 8C/ min, at which they were held for 5 min to remove their previous thermal history. They were then cooled to room temperature at a fast cooling rate of 50 8C/min to become completely amorphous. Slower heating at 2 8C/min was then conducted to observe cold crystallization behavior. Morphologies of fractured surfaces of the hybrid materials were observed by field-emission scanning electron microscopy (FE-SEM, JSM-6500F, Jeol Co., Japan). The optical transparencies of hybrid coating films were measured by visible spectrophotometry (Optizen 1412V, Mecasys Co., Korea) over the visible wavelengths of 400–800 nm. Oxygen permeabilities of the coated films were assessed by apparatus made in the laboratory according to the ASTM D3985. Water vapor permeability was measured by a water vapor transmission rate tester (Permatran-W3/33MA, Mocon Co., USA).
3. Results and discussion 3.1. FTIR analysis In the organic–inorganic hybrid materials, the phase attraction between two phases has been considered as a crucial factor to the production of hybrid materials with high performance. Particularly, in nanostructured hybrid materials synthesized via sol–gel method using a polymer resin system as an organic component, the phase morphology in association with the physical properties of the hybrid materials is greatly influenced by phase interaction between the polymer chain segments and the dispersed inorganic domains. Strong interfacial attraction can suppress microphase separation in the materials, favoring phases with stable and homogeneously dispersed microstructures. Various silane coupling agents containing reactive functional groups, such as 3-glycidoxypropyltrimethoxysilane (GPTMS), 3isocyanatopropyltriethoxysilane (IPTES), aminopropyltriethoxysilane (APTES), and methacroxypropyltrimethoxysilane (MPTMS), have been used to promote interfacial adhesion between organic and inorganic phases in hybrid materials. If the organic polymer contains hydroxyl groups (–OH), they can form strong covalent bonds with IPTES through its highly reactive isocyanate (N5 5C5 5O) groups, resulting in the formation of urethane groups. Therefore, IPTES was used here as a silane coupling agent to promote interfacial adhesion through strong covalent bonding between its
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Fig. 1. Comparison of FT-IR spectra for the PLA/SiO2 hybrid without IPTES and that with IPTES.
isocyanate groups and hydroxyl groups pendent on the ends of the PLA chains. Fig. 1 shows FTIR spectra of PLA/SiO2 hybrids with and without IPTES. The presence of IPTES resulted in characteristic peak corresponding to amine groups (–NH) at 1530–1575 cm1 due to chemical reaction between isocyanate group of IPTES and hydroxyl group from PLA molecule. This observed peak indicates that strong interfacial adhesion between the organic PLA phase and the inorganic silica network was achieved during sol–gel processing. This reaction mechanism has been previously observed in EVOH/ SiO2 hybrid coating materials with addition of IPTES, in which the organic EVOH copolymer contained hydroxyl groups in its vinyl alcohol units [19]. 3.2. Crystallization behavior The physical and mechanical properties of crystalline polymers depend on the molecular morphology, which is controlled by crystallization [20]. Particularly, gas barrier properties are significantly influenced by both the degree of crystallinity and the crystalline structure developed in the semicrystalline polymer,
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which affect the rate of transport of gas molecules through the polymer. In this study, therefore, we investigated the crystallization behavior of the PLA polymer which is combined with silica network in the hybrid. Fig. 2 shows DSC results of neat PLA and three PLA/SiO2 hybrids with various silica contents. These traces were registered during second heating run with a slow heating rate, exhibiting cold crystallization, which was started from completely amorphous solid state. Neat PLA showed one melting endothermic peak at ca. 149 8C; the samples with silica showed two distinct melting peaks, which were attributed to the melting of less ordered crystals, characterized by thinner crystalline lamella, at lower temperature followed by the melting of more perfect crystals after recrystallization during heating [20,21]. Similar melting behavior has also been observed in PLA/clay nanocomposites [10] and plasticized PLA [9]. The melting temperatures (Tm) determined from the binary endothermic peaks for the PLA hybrids was observed to be almost invariant with increasing silica content. On the other hand, the inclusion of silica in the PLA matrices up to 64.3 wt%, corresponding to 0.03 mol TEOS, was found to decrease the crystallization temperature (Tc) of the PLA, which was determined from the exothermic peak obtained during heating at 2 8C/min between 80 and 130 8C. The silica contents in the hybrid were calculated assuming 100% conversion from the TEOS precursor to the silicate network. The Tc of PLA resin in the hybrid was reduced by ca. 16 8C compared with that of neat PLA by the incorporation of silica at 37.5 wt%, corresponding to 0.01 mol silica precursor TEOS. In this hybrid, nanosized silica particles formed from sol–gel reactions appeared to act as effective nucleating agents, and thus reduced the cold crystallization temperature of PLA through promoting overall crystallization rate, determined from nucleation and crystal growth rates of the PLA. Similar crystallization behavior of PLA in PLA/clay nanocomposites has also been reported [21–23]. It was believed that the nanoclay with large surface area, which is capable of providing more nucleating sites, efficiently enhanced the crystallization rate and subsequently reduced the crystallization temperature of PLA. The measured melting and crystallization temperatures, and calculated degree of crystallinity (Xc) are listed in Table 1. The degree of crystallinity of PLA in the hybrids was determined from the following equation:
X c ð%Þ ¼
DHf;s 100 DHf;PLA ð1 W silica Þ
where DHf,s is the enthalpy of fusion of sample; DHf,PLA refers to the enthalpy of 100% crystalline PLA, which was set as 93.6 J/g, and Wsilica is the mass fraction of silica component [21]. The incorporation of small amount of silica particles at 37.5 wt% in the PLA slightly increased the degree of crystallinity of PLA due to enhanced crystallization process achieved by the dispersed nucleating silica nanoparticles. However, the presence of silica particles with excess amount exhibited the reverse effect on the trend of degree of crystallinity as increasing silica content. This was likely because higher silica contents restrict the PLA’s molecular chain mobility, which is necessary for its regular packing into crystal lattices, due to increased interaction sites between the organic PLA and the inorganic silica phase, ultimately retarding crystallization. Table 1 DSC data of neat PLA and PLA/SiO2 hybrids with different silica content.
Fig. 2. DSC heating thermograms for the neat PLA and PLA/SiO2 hybrids with different TEOS contents.
Sample
Silica content (wt%)
Tc (8C)
Tm1 (8C)
Tm2 (8C)
Xc (%)
Neat PLA Hybrid-0.01 Hybrid-0.03 Hybrid-0.06
0 37.5 64.3 78.3
118.8 102.4 95.7 100.8
149.5 146.2 146.8 147.7
– 154.7 153.8 154.3
22.33 22.55 18.49 14.81
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silica contents showed reduced peak intensities, including area, due to decreased crystallinity of the PLA, as observed in the previous DSC results. 3.3. Nanostructure morphology
Fig. 3. X-ray diffraction patterns for the neat PLA and PLA/SiO2 hybrids with different TEOS contents of 0.01 and 0.02 mol.
Fig. 3 shows X-ray diffraction patterns of neat PLA and the hybrids. Neat PLA showed the most intense peak (the main peak) at 2u = 16.78 from (2 0 0) and (1 1 0) reflections; less intense peaks were observed at 2u = 14.8, 19.1, and 22.48 corresponding to (0 1 0), (2 0 3), and (0 1 5) reflections, respectively [20,24]. The silica-containing hybrids showed peaks slightly shifted to lower 2u values. This indicates that the presence of silica particle caused slight modification of crystalline structure of PLA matrix, basically maintaining the a-form structure. The hybrid samples with higher
In general, the mechanical, optical, thermal, and barrier properties of the organic–inorganic hybrid materials produced by sol–gel method are greatly influenced by the phase morphology, which depends on the various factors such as concentration of each component, phase attraction between two phases, and processing condition. Phase morphologies of fractured gels were observed to examine the dispersion of the nanostructured silica particles in the PLA matrices. Fig. 4(a)–(c) shows SEM micrograph of hybrids with TEOS contents of 0.01, 0.03, and 0.06 mol; the IPTES content was maintained at 0.001 mol in all the samples. The hybrid gels with lower TEOS contents (0.01 and 0.03 mol) contained homogeneously and uniformly dispersed silica particles with average sizes of below 60 nm in the PLA matrices. They also showed no phase separation. However, excess addition of TEOS precursor (0.06 mol) resulted in large silica clusters due to physical coalescence among the silica nanoparticles or chemical condensation between residual silanols on the silica surface, which led to partial phase separation. The hybrid with 0.03 mol TEOS and without IPTES silane coupling agent shows distinct microphase separation and the formation of silica clusters larger than 100 nm (Fig. 4(d)). These may have been caused by weak interactions between the organic PLA and the inorganic silicate network. The variation of morphology with composition was similar to that previously reported for EVOH/SiO2 hybrids, in which the phase compatibility was also controlled by IPTES silane coupling agent that could form covalent bonds through chemical reactions with hydroxyl groups in the organic EVOH phase.
Fig. 4. Phase morphology of IPTES (0.001 mol) added PLA/SiO2 hybrids with various TEOS contents of (a) 0.01, (b) 0.03, (c) 0.06 mol, and (d) no IPTES added hybrid with TEOS content of 0.03 mol.
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3.5. Barrier properties
Fig. 5. Light transmittance of the non-coated neat PLA film and various PLA/SiO2 hybrids coated PLA films.
3.4. Optical transparency of the hybrid coated films Fig. 5 shows relative light transmittances with respect to visible light wavelength for the various hybrid coated films. The film with 0.03 mol TEOS (corresponding to 64.3 wt% silica content) was highly transparent, transmitting over 92.5% of incident visible light range of 500–800 nm, showing comparable transparency to the neat PLA film. This may have originated from its more homogeneous and finer microstructure with dispersed silica nanoparticles. In the case of both hybrid coated film samples with addition of 0.01 and 0.06 mol TEOS, however, a deterioration in optical transparency was observed, exhibiting similar light transmittance rate for both of them. In the sample with least TEOS (0.01 mol, corresponding to 37.5 wt% silica), the discrepancy in the degree of crystallinity and crystal size of the organic PLA phase in the hybrid coating layers and the PLA substrate increased light scattering in the two-layer structure with different refractive indexes, which reduced optical transmittance. Excess TEOS (0.06 mol, corresponding to 78.3 wt% silica) resulted in large silica clusters that also reduced optical transmittance through increased light scattering.
Fig. 6. Oxygen and water vapor permeabilities of the PLA/SiO2 hybrids coated films as a function of TEOS content.
Fig. 6 shows measured oxygen and water vapor permeabilities of the hybrid coated PLA film with respect to TEOS inorganic precursor content. For all the hybrids with different TEOS contents, the silane coupling agent, IPTES, was added in the same amount of 0.001 mol. As seen in the figure, the oxygen and water vapor permeation behavior through hybrid coated film was shown to be very similar. In the range of TEOS contents up to 0.03 mol corresponding to silica content of 64.3 wt%, both oxygen and water vapor permeabilities were dramatically reduced with increasing TEOS content, which was due to increased amount of the inorganic silica with superior barrier property. When 0.03 mol TEOS was added in the hybrid, the oxygen and water vapor barrier properties of the hybrid coated PLA film were improved by 69.7% and 45.7%, respectively, as compared with pure PLA film, exhibiting oxygen and water vapor permeabilities of 16.03 cc mm/m2 day atm and 8.80 g/m2 day, respectively. These remarkable improvements in both oxygen and water vapor barrier properties were attributable to a stable homogeneous microstructure with uniformly dispersed silica particles of nano-scale dimensions, which did not show microphase separation in the hybrids. In the range of TEOS content above 0.03 mol, on the other hand, both oxygen and water vapor permeabilities were observed to be almost invariant with incorporated amount of silica component; the incorporation of the inorganic silica component with an excess amount (about 65 wt% silica in current study) revealed no more contribution to the enhancement in the barrier property of hybrid coatings. This result may be due to the partial microphase separation in the hybrid coating layer and the lack of interfacial adhesion between the coating layer and the PLA substrate. The effect of IPTES content on the oxygen barrier property of the hybrid coated film was also investigated. Fig. 7 shows the oxygen permeabilities of PLA films coated with hybrids containing 0.01 and 0.02 mol TEOS with respect to IPTES content. Oxygen permeation varied similarly with IPTES content, irrespective of TEOS content. Film with IPTES contents of up to 0.002 mol showed significantly reduced oxygen permeability due to improved interactions between the organic and the inorganic components in the PLA/SiO2 hybrid. However increased oxygen permeability, corresponding to the deterioration in the barrier property, was observed in films with IPTES contents above 0.003 mol. This may be due to the fact that with addition of excess amount of IPTES
Fig. 8. The effect of aging time on the oxygen barrier property of the PLA films coated by hybrids with 0.03 and 0.06 mol TEOS content.
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Fig. 7. Oxygen permeability of the PLA/SiO2 hybrids coated films as a function of IPTES content.
silane coupling agent, the organic molecules in the IPTES has a greater tendency of hindering the formation of siloxane(Si–O–Si) groups through condensation reactions during sol–gel process, and leads to less-dense inorganic silica networks and increased gas permeation rate. These results show that the silane coupling agent could control interfacial attraction between the organic and the inorganic phases and also affect the inorganic network structure created from sol–gel reactions. Fig. 8 shows the effect of aging on the oxygen permeability of PLA films coated with hybrids with 0.03 and 0.06 mol TEOS. All of coated film samples exhibited improvement in oxygen barrier property with increasing of aging time. As shown in the figure, the oxygen permeabilities dramatically decreased with increased aging up to 21 days, after which the permeabilities slightly reduced, achieving equilibrium within 28 days. Therefore, the above results for oxygen and water vapor permeability were of films aged for 28 days. After deposition of the hybrid sols onto the PLA substrate, the coated thin liquid film gradually solidified into a thin coating layer via gelation of the dispersed inorganic silica phase and crystallization of the PLA resin during drying as the solvent was removed from the hybrid sol. The crosslink density of the silica networks in the hybrid solids is generally low due to insufficient curing during fast coating and drying. Insufficient curing can be compensated by aging that involves continuing physical and chemical changes while the films are stored at ambient temperature [25]. The results here show that aging improved the barrier properties of PLA/SiO2 hybrid coated film by increasing inorganic silicate networks units in the hybrid. 4. Conclusions Environmentally less damaging hybrid coating materials with low gas permeability were prepared by incorporating biodegrad-
able organic PLA resin into inorganic silicate with superior barrier characteristics via sol–gel process. The resulting PLA/SiO2 hybrids showed improved phase compatibility with addition of IPTES silane coupling agent capable of creating covalent bonds between the organic and the inorganic phases. This strong bonding resulted in homogeneous microstructures with silica nanoparticles dispersed in the PLA matrix. Cold crystallization behavior of the PLA polymer phase embedded in the hybrid were found to be significantly influenced by the presence of nanosized silica particles produced via sol–gel reaction; the crystallization rate, the degree of crystallinity, and the crystalline structure of the semi-crystalline PLA depended on the incorporated silica content. The hybrid films were highly transparent, achieving visible light transmittances of over 92%. Their gas barrier properties were substantially improved up to appropriate level of content for the silica (64.3 wt% corresponding to 0.03 mol TEOS) and silane coupling agent (0.002 mol IPTES). However, addition of silica component and IPTES with excess amount exhibited no more contribution or reverse effect in the enhancement of the barrier properties. In addition, it was revealed that aging time was significant coating process parameter to produce hybrid coated PLA film with high gas barrier property for the application of food packaging films.
Acknowledgment This work was supported by Kyonggi University Research Grant 2010.
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