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In this review, application of biocomposites in packaging industry, food packaging exclusively, was studied as a replacement in comparison to conventional petrochemical material such as PVC, Expanded Polystyrene (EPS) and so on. This review has focused on improvement of mechanical and barrier properties that is the common defect of natural composites. In whole journals you will see different methods of preparing composite material in order to improving properties. Starch based films have poor mechanical behavior and barrier properties. Addition of fibers such as wheat barn, sugarcane fiber, corn fiber and clay particles usually improve some properties. But these additives may have an inverse effect sometimes that should be overcome by changing the type or amount of fibers, more additives like plasticizers (glycerol and sorbitol). Inclusion of corn fiber in potato starch decreased strength and flexibility of the film. Addition of PVA (poly vinyl alcohol) powder improved that reduction properly. Nanoclay particles had significant effects in most cases. A proper dispersion of clay in matrix can improve the properties drastically. Generally, when nanoparticles are used as filler in composite material, dispersion uniformity will be crucial. XRD (X-ray diffraction) is one of the most important tests in nanocomposites which determine the uniformity of nanofillers in matrix.
Keywords: food packaging, biodegradable composites, barrier properties, biocomposite films, nanocomposite films.
We consume plastic products more than 200 million tones currently, with almost 5% increment each year. This statistic indicates that high dependence of plastic goods to crude oil. Increment of oil price influences on plastic industry directly. It is crucial to employ other alternative material as a replacement instead of petrochemical based raw material such as polyethylene, polypropylene, polyvinylchloride, and so on. These petrochemical based materials are used as a result of their good mechanical characteristics (tensile strength and modulus), thermal resistance, barrier properties to moisture and gas, and high availability at fairly low cost. But currently their application should be limited because they are not biodegradable and they pollute the environment. Large quantities of plastic materials that use in packaging industry are not recyclable because they are often impure, dirty with foods leftover, and make ecological problems. As a consequence biodegradability is not only a functional requirement but also an important environmental attribute.
Alves et al.  stated that the most materials that use for packaging products are petrochemical based. Petrochemical based polymers are used because of their good availability, large quantity, and reasonable cost. They prepare proper functionality and mechanical properties such as tensile and tear strength and good barrier properties to gas and heat but they pollute the earth. Scientists are studying biodegradable composite as a proper replacement. The main issue in biocomposites will be the mechanical and barrier properties which should be focused more.
Alves et al.  studied water vapor permeability (WVP) and gas barrier properties. The experiment showed that WVP was dependent on driving force but inclusion of 5%wt mica could decrease water permeability about 20 % with higher applied driving force. Avella  (2005) investigated montmorillonic (clay) dispersing homogeneously in different starch-based polymers by melt processing techniques. The results show improvement in modulus and tensile strength.
Cannarsi et al.  studied the use of biodegradable composites for meat packaging. In this case beef steaks packaged in three films separately. PVC, a film made of starch and polyester and a mixture of three different biodegradable polyesters. Packaged steaks were stored for more than 6 days in two different thermal conditions (4 and 15 C) to simulate actual storage temperature and thermal abuse. No significant differences were observed using these three types of films. Therefore biodegradable films can be a good replacement for PVC films as a result of solving petrochemical films degradation.
Silvestre et al.  examined new generation of packaging materials as a replacement for conventional petrochemical packaging materials. Nanotechnology application offer improved packaging materials with higher performances. These new packaging should provide food safety. There are large numbers of food products under investigation these days. Food packaging materials need some special characteristics to provide an acceptable packaging. The most important issues that are mentioned in food packaging include moisture and gas permeability and water absorption which cause food spoilage. Moreover mechanical properties that determine strength and stiffness of the packaging film which are essential to preserve food from deformation and damage.
Starch Based Composite Films
Cinelli et al.  stated that products made of EPS (Expanded Polystyrene) are not recyclable. They studied foam plate based on potato starch, corn fibers and PVA by casting. Addition of fibers to starch decrease mechanical properties. After use of PVA they had an improvement in strength, flexibility and water resistance. 88 % of PVA (powder) in the mixture made better the reduction in tensile happened by addition corn fibers.
Cyras et al.  studied different fractions of nanoclays (2, 3, 5% wt) were studied in potato starch with glycerol as a plasticizer. Addition of nanoclays increased heat resistance and reduced water absorption as well in a condition with 75% of humidity. They presented a drastic improvement in Young's modulus (about 500% with 5wt% of clay) duo to the strong interfacial interaction between matrix and clay particles (170 GPa). Figure 1 shows an optical micrograph for diffractions of nanoclays in potato starch.
Fig.1. Optical micrograph of the films of starch (a) and the composites with: 2 wt% (b), 3 wt% (c) and 5 wt% (d) of MMT, Cyras et al.  (2008).
Famá et al.  studied the effects of wheat bran in different fraction (1.5, 13.5, 27.1 mg/ g matrix) as reinforcement in cassava starch and glycerol & potassium sorbate as plasticizers. Adding fibers increased glycerol-rich glass transition temperature (Tg). Increasing of filler didn't affect on matrix density. Addition of wheat bran improved mechanical properties, reduced WVP, enhanced film hardening and decreased moisture content.
Vercelheze et al.  studied the biodegradable trays made of cassava starch / sugarcane fibers and Na-montmorillonite (Na-MMT) and the effects of contents on mechanical and microstructure properties. Inclusion of fibers and Na-MMT decreased the density and made trays less rigid. Results showed an increase in moisture content of the foam at 75% humidity. The foam after immersing in water during 1 minute had a significant water absorption more than 50% that is because of porosity and lower density.
Soykeabkaew et al.  studied starch/jute and bacterial composites that provided by casting method. Fiber nature and the amount of fibers in matrix affect on film reinforcement. The maximum amount of jute is 60% of starch weight and the cellulose maximum amount is 50% wt. Strong interaction between starch and fibers and good dispersion of fibers improved mechanical properties largely. The films made of starch and bacterial cellulose (40-50% wt) had superior mechanical properties than composites including jute fibers. Composites with 50% wt cellulose had averaged Young's modulus, 2.6 GPa and tensile strength, 58 GPa which are 106 and 20 fold more than pure starch films respectively. The films including bacterial cellulose had better water permeability and thermal stability than films including jute fibers. Bacterial cellulose in starch increase degradation due to improved thermal stability.
Chang et al.  stated that chitin nanoparticles (CNP) reinforcements have better effects on composite properties than typical chitin particles. They investigated combination of potato starch plasticized with glycerol and CNP by casting method. They observed CNP dispersion in matrix uniformly, good interaction between matrix and filler, improved Tg, tensile strength and water vapor permeability at low content of CNP in CNP/GPS composite. Addition of higher amount of filler (more than 5% wt) in potato starch had inverse effects on properties.
Kristo and Biliaderis.  examined nanocrystals of starch as reinforcing agent in pllulan matrix besides sorbitol as a plasticizer. Nanocrystals are prepared from waxy maize starch with acid hydrolysis at 35C. They investigated water absorption and water vapor barrier of nanocomposites including different fractions of starch nanocrystals (0-40% w/w). Addition of filler decreased water absorption whereas WVP was constant up to 20% w/w of filler in the composite then came down with more inclusion of filler. Thermal tests showed that glass transition temperature increased with further addition of filler that can be because of limitation in mobility of matrix chains as a result of strong interaction between starch particles and interaction between starch particles and matrix. Increasing of nanocrystals enhanced the Young's modulus and the tensile strength significantly, but decreased the elongation at break in samples tested in different relative humidity from 43% to 75% RH.
Dias et al.  studied films composed of rice flour in comparison to rice starch based films. Rice flour has a good availability with low cost and it's believed to be a proper replacement for rice starch after improvements. SEM analysis showed a compact structure for both films. In this case biodegradable films based on rice starch and rice flour with plasticizing of glycerol and sorbitol investigated in terms of mechanical and barrier properties. Both starch and flour based films revealed the same mechanical properties whereas WVP of flour based films were two times as much as starch based films. Films plasticized with sorbitol were more rigid and less permeable to water, while glycerol plasticized films were more flexible and have less water vapor barrier.
Prachayawarakorn et al.  studied TPRS (thermo plastic rice starch) based composites improved by cotton fibers or LDPE (low-density polyethylene). Both cotton fibers and LDPE improved tensile strength and Young's modulus considerably. Water absorption reduction with addition of cotton fibers and LDPE was clearly obvious.
Glycerol Plasticized Films
Mahecha et al.  examined films based on achira flour in conditions that affect on mechanical and barrier properties of the film. They investigated drastic items which can optimize the properties of the film. Some alternatives such as Cg (glycerol concentration), Tp, Ts and RH were assessed. The optimized magnitude of each item was 17g for Cg in 100 g of flour, 90 â-¦C for Tp (process temperature), 44.8 â-¦C for Ts (drying temperature), and 36.4% for RH (relative humidity). The files prepared in these conditions revealed high strength 7 MPa, low solubility 38.3% and.
Ghasemlou et al.  stated edible kefiran films prepared with addition of plasticizers (glycerol and sorbitol) at different fractions (10, 15, and 25% w/w) by casting. Mechanical properties, WVP and glass transition temperature were assessed. Higher amount of glycerol enhanced flexibility and water vapor permeability. Glycerol showed better influences on plasticizer for kefiran films than sorbitol.
Chang et al.  investigated effects of cellulose nanoparticles as filler on glycerol plasticized starch based composite. GPS/CN composite formed by casting method was examined in tensile strength and WVP. Experiments showed that the tensile strength improved with addition of CN. Tensile strength enhanced from 3.15 to 10.98 MPa due to increasing CN content in matrix form 0 to 5 wt.%. WVP decreased from 5.75 _ 10_10 to 3.43 _ 10_10gm_1 s_1 Pa_1. These improvements in properties may be caused by a good interaction between filler and matrix because of their same polysaccharide structure.
Nanoclay Composite Films
Muller et al.  studied effects of nanoclay particles on mechanical properties and WVP of starch based films. They investigated pure and reinforced Cassava starch films where prepared with extrusion and thermo pressing steps. They added nanoclay using glycerol and without glycerol. Water vapor barrier increased with increasing of nanoclay in matrix and it was more significant in glycerol plasticized film. The inclusion of nanoclay in cassava starch enhanced the tensile strength and made the films more rigid where this improvement was more obvious in films that nanoclay was added using glycerol.
Rihm  examined different fraction of clay (0, 2.5, 5, 10, 15 g clay/ 100 g agar). Clay content affect on film properties significantly. SEM results showed that the highest tensile strength of film in 10% clay content. Further inclusion of clay (more than 10%) had a diverse effect on properties. WVP, water absorption and solubility decreased with increasing the amount of clay that is due to strong interaction between matrix and filler. Figure 2 shows SEM results of clay contents in agar based composite.
Fig.2. Scanning electron micrographs of agar and agar/Cloisite Na+ nanocomposite films prepared with different content of nanoclay, Rihm (2011).
Souza et al.  stated tensile strength and barrier properties (WVP & Oxygen permeability) of cassava starch based composite films with different glycerol and nanoclays incorporation methods. Analysis showed that methods of incorporation did not affect on properties whereas glycerol content improved tensile and barrier properties drastically. They investigated that lower content of glycerol indicate better properties that films with higher glycerol content. Then cassava film was prepared with inclusion of nanoclays and analyzed. The results presented significant improved properties for both nanoclays and glycerol content.
Chung et al.  studied the nanoclays in starch based films. They noticed that a proper dispersion of clay in matrix can improve the properties drastically. Nanoparticles improved modulus and tensile strength of starch and did not influence on elongation capacity. Addition of 5% wt clay in matrix with a good dispersion increased modulus up to 65% and strength up to 30% in comparison to pure starch film. More inclusion of clay had a reverse result in properties improvements.
Dias et al.  examined the barrier and mechanical properties on composite films based on rice flour with and without cellulose fibers made by casting process. Glycerol and sorbitol also used as plasticizers. The film include of fibers showed lower permeability in comparison with pure rice flour. Fibers improved the film's mechanical properties with higher tensile strength. Sorbitol in the film made it more rigid while glycerol didn't affect on deformation capacity.
Müller et al.  stated mechanical properties of glycerol plasticized starch films with inclusion of cellulose fibers as reinforcing agent. They prepared a solution of cassava starch (3%) and addition of different filler fraction (0, 0.10, 0.30 and 0.50 g of fiber/ g of starch). Experiments done in different relative humidity showed that the increasing of fiber content in starch enhances tensile strength and make the film more rigid. More over addition of cellulose fiber improved the stability of starch based film in higher relative humidity.
Saxena et al.  examined water vapor permeability in biopolymers reinforced with cellulosic whiskers. In this study they prepared xylan based film with cellulosic whiskers. The experiments represented that addition of 10% whiskers in xylan, reduced the water permeability up to 74% than pure xylan. In comparison to xylan film reinforced with 10% softwood fiber, xylan film reinforced with cellulosic whiskers had a 362% water barrier improvement.
Martins et al.  examined bacterial and vegetable cellulose as reinforcement in cornstarch based composites. They used glycerol and water for plasticizing. Several special tests (TGA, XRD, DMA, tensile tests, SEM, and water sorption) were applied on both vegetable and bacterial fiber composite materials. Dispersion of fiber and strong interaction between the cellulose fibers and cornstarch composites with bacterial cellulose content had better mechanical behavior than vegetable cellulose content composite. Elongation at break decreased from 144% to 24% and 48% for bacterial and vegetable cellulose content respectively. Figure 3 shows scanning electron micrograph images of bacterial and vegetable cellulose in cornstarch based composite.
Fig.3. SEM micrograph of vegetable and bacterial celluloses and the fragile fractured surface of TPS filled with VC and BC: (A) vegetable cellulose; (B) bacterial cellulose; (C) and (D) TPS/VC composites (5 wt %); (E) and (F) TPS/BC composites (5 wt. %), Martins et al. (2009).
Poly Lactic Alcohol (PLA) Based Films
Cheng et al.  stated effects of chicken feather as reinforcing agent in composites with poly lactic acid (matrix). Different amount of chicken feather were used to prepare specimens for mechanical and thermal tests. Composites were prepared with extruding method with 2, 5, 8 and 10% wt content of CFF. Feather reinforced composites showed higher tensile strength than pure PLA and the optimum magnitude was 4.2GPa for 5% wt feather content. SEM morphology revealed a well dispersion of feather in matrix. Thermogravimetric analysis (TGA) indicated thermal stability increment in feather composite in comparison to pure poly lactic acid composites.
Jamshidian et al.  studied improvement of mechanical modulus, tensile strength and water vapor permeability of PLA reinforced by AT (Î±- tocopherol) and AP (ascorbyl palminate). Experiments showed that these two reinforcing material do not affect on tensile strength and modulus as they don't influence on water vapor permeability drastically. Finally AP is not recommended in poly lactic acid for modification of packaging films. As shown in Figure 4, on adding the AP to PLA film, AP crystals appeared on the PLA surface and made a branching structure.
Fig.4. AP crystals in PLA-AP films containing 0.1% (a, b), 0.3% (c, d), 0.6% (e, f) and 1% (g, h) AP, images were taken at ambient temperature, Jamshidian et al. (2012).
Rhim et al.  stated PLA composites and their WVP and tensile strength with inclusion of different types of nanoclays (Cloisite Na, Cloisite 30B and Cloisite 20A). Pure PLA was assessed and indicate 50.45 MPa, 3% elongation at brake 1.8 Ã- 10^-11 gm/m2 WVP. Elongation at brake and tensile strength had 11-17% and 10-20 reduction with 5% content of clay in PLA depending on the type of clay used. Water vapor permeability decreased 6-33% with addition of clay. Cloisite 20A decreased WVP more that the other types and showed the least decrement in tensile.
Murariu et al.  studied poly lactic acid (PLA) and expanded graphite (EG) blend. EG was added in PLA using different methods and thermo mechanical and fire-retardant properties were investigated. Increasing of EG content in PLA composites enhanced rigidity, Young's modulus and storage modulus significantly. EG in PLA exhibited proper thermal stability without change in glass transition and melting temperatures. Purification of expanded graphite improved mechanical properties. Inclusion of EG in PLA had a drastic effect on fire retardant whereas decreased rate of heat release significantly.
More Cases of Biodegradable Composites
Azeredo  stated that the use of biopolymers is limited due to their poor barrier and mechanical properties that should be improved by addition of reinforcing materials. The interaction between matrix and fillers are so important. Nanocomposites are the products that use filers with at least one nanoscale dimensions as reinforcement. As shown in figure 5, nanoparticles have larger surface area than microscale counterparts that offer a much better matrix-filler interfacial interaction and performance.
Fig.5. Types of composite derived from interaction between clays and polymers: (a) phase-separated microcomposite; (b) intercalated nanocomposite and (c) exfoliated nanocomposite, Azeredo (2009).
Sapalidis et al.  stated that PVA/ Zostera flakes with up to 20 wt% showed better thermal resistance rather than pure PVA. The water sorption capacity didn't differ but gas barrier improved. With higher filler contents (more than 30%), they found a reduction of mechanical properties. Finally algal fibers don't affect the integrity of the film.
Arvanitoyannisa et al.  examined of low-density polyethylene (LDPE) and starch (rice or potato) by extruding method and different fraction of water as plasticizer. Mechanical and barrier properties were studied. They noticed that high content of starch more than 30% w/w decreased mechanical properties but improved gas permeability and water vapor transmission. A few theoretical and pragmatic experiments for mechanical and gas barrier properties designed and the deviation between these two experiments was expressed. Finally the rate of biodegradability also increased with increased starch content.
Arvanitoyannisa et al.  stated low density polyethylene and wheat starch and other starch, in different condition of humidity. Before storage barrier and mechanical properties of film were examined. After some theoretical and practical experiments they explained observed deviations. Some pragmatic models showed better mechanical properties. Addition of more than 30% of starch affect adversely on mechanical behaviors for LDPE and starch films while increased transmission of WV(water vapor) and permeability of gases. Degradability of several films of LDPE and starch with changes in their mechanical characteristics were assessed.
Ghanbarzadeh et al.  studied whey protein films and whey protein laminated by zein protein film separately. Olive oil and glycerol were added as plasticizers to improve mechanical and barrier properties. Laminated film showed superior UTS (ultimate tensile strength) than the single layer whey protein films (260% for whey-zein-glycerol and 200% for whey-zein-olive oil films). Laminated whey film exhibited UTS 2-3 times higher than single layer film. Moreover laminated film showed improved barrier properties in comparison to single whey protein film and inferior barrier properties than single zein film. Laminating increased the barrier properties 180% in glycerol plasticized and 200% in olive oil plasticized films.
Kristo et al.  stated biocomposite films consist of pullulan and sodium caseinate with blend and bilayer types. Mechanical and water barrier properties and water sorption were investigated with different weight ratio. The 1/3 and 3/1 weight ratio of P/S in both type (blend and bilayer) didn't affect on moisture sorption and WVP as well. P/SC ratio increasing exhibited a decrease in YM (E) and tensile strength and showed an increase in elongation at break. It means that P content causes flexibility and SC content imparts stiffness to the film. Beeswax lamination had a good improvement of WV barrier, whereas it didn't affect drastically on tensile strength, modulus and elongation at break.
Wang et al.  studied carrot puree as composite base with carboxyl-methyl cellulose (CMC), corn starch and gelatin. Glycerol was utilized as plasticizing agent. Hydrocolloids and plasticizer investigated for their effects on properties of the film. CMC and gelatin contents did not drastically influenced on film elongation at break, oxygen permeability and WVP but significantly increased tensile strength (TS). Corn starch increased tensile strength and water vapor permeability whereas didn't affect film elongation at break and oxygen permeability. An increase in glycerol content declined the tensile strength and enhanced elongation at break (p < 0.05), oxygen and water vapor permeability.
Yang and Paulson  stated presence of beeswax and stearic-palmitic acids (S-P) blend in gellan films separately. They examined gellan/lipid films with different fraction of lipid contents (0-25%) for mechanical behavior, barrier properties (WVP) and opacity. Inclusion of lipids into gellan enhanced WVP drastically but reduced mechanical properties and made the films less clear. Beeswax had better effects on WVP and indicated better mechanical behavior in comparison to S-P acids. The tensile strength of film with beeswax (14.3%) was investigated with changes in water activity. Increment of water activity reduced tensile strength and modulus but didn't affect on elongation at break. Reduction rate of composite tensile strength was less obvious than films without lipids. It seems that lipids make gellan films less moisture sensitive.
Although the properties of conventional material in packaging industry are superior in many aspects but the issue of earth pollution that is the matter of controversy these days make scientists to focus on biodegradable raw material more. All are investigating the improvement of mechanical and barrier properties of natural composites. In all papers you find different methods of preparing composite material in order to improving properties. Starch based films have poor mechanical behavior and barrier properties. Biodegradable plastics are used because of their degradation in short time and at lower temperature (degradation temperature) than chemical plastics. Main manufacturers are incoming the market, material costs are falling drastically, and performances and processibility are improving significantly. In addition, new standards in use of biodegradable materials are helping to enhance market growth. Edible and biodegradable films which are prepared with some compounds (composite) have been made to have the advantages of the complementary useful properties of the special basic materials, and to overcome their individual drawbacks. Most composite films studied to date combine one or several polyester (or lipid) compounds with a hydrocolloid-based one. The future of edible and/or biodegradable materials is therefore probably dependent on the development of applications where some of their preferential properties (such as gas selectivity) are enhanced, or on the development of composite materials.
The authors would like to acknowledge Universiti Putra Malaysia for the support in writing this paper.