Available online at www.sciencedirect.com SCIENCE ODIRECT. ELSEVIER Prog.Polm.si.31(200576-602 www.elsevier.com/locate/ppolysi Polymer blends and composites from renewable resources Long Yu"-b.*,Katherine Dean",Lin Lib alth scientific and industrial Research Ora tion.CMIT.Melbour Abstrac This article review grad ble polymers,various oped ov rom t uch as polylactic acid; d (3)polymer rom mic and the technology of sed to improve the adhesion between 2006 Elsevier Ltd.All rights reserved. Keyd Polymer:Blend:Composite:Renewable resource:Biodegradable Contents duction 32 7000 PLA/PHB blends 4. drophilic polymer aut r.C ealth Scientific and Industrial Research Organization,CMIT.Melbourne.Vic.3169.Australia
Prog. Polym. Sci. 31 (2006) 576–602 Polymer blends and composites from renewable resources Long Yua,b,, Katherine Deana , Lin Lib a Commonwealth Scientific and Industrial Research Organization, CMIT, Melbourne, Vic. 3169, Australia b Centre for Polymer from Renewable Resources, School of Food and Light Industrial Engineering, South China University of Technology, Guangzhou, China Received 22 July 2005; received in revised form 17 March 2006; accepted 23 March 2006 Available online 6 June 2006 Abstract This article reviews recent advances in polymer blends and composites from renewable resources, and introduces a number of potential applications for this material class. In order to overcome disadvantages such as poor mechanical properties of polymers from renewable resources, or to offset the high price of synthetic biodegradable polymers, various blends and composites have been developed over the last decade. The progress of blends from three kinds of polymers from renewable resources—(1) natural polymers, such as starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid; and (3) polymers from microbial fermentation, such as polyhydroxybutyrate—are described with an emphasis on potential applications. The hydrophilic character of natural polymers has contributed to the successful development of environmentally friendly composites, as most natural fibers and nanoclays are also hydrophilic in nature. Compatibilizers and the technology of reactive extrusion are used to improve the interfacial adhesion between natural and synthetic polymers. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polymer; Blend; Composite; Renewable resource; Biodegradable Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 2. Natural polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578 2.1. Melt processed blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 2.2. Aqueous blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580 3. Aliphatic polyester blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582 3.1. Blends of PLA family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582 3.2. Blends of PHA family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584 3.3. PLA/PHB blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585 3.4. Other polyester blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 4. Blends of hydrophobic and hydrophilic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 ARTICLE IN PRESS www.elsevier.com/locate/ppolysci 0079-6700/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2006.03.002 Corresponding author. Commonwealth Scientific and Industrial Research Organization, CMIT, Melbourne, Vic. 3169, Australia. Tel.: +61 3 9545 2797; fax: +61 3 9544 1128. E-mail address: long.yu@csiro.au (L. Yu)
L.Yu et al Prog.Polym ScL 31(2006)576-602 4.1.Starch/PLA blends 4.1.1 ued for starch/PLA blends. 4. ch/PHB ble 43 PHB/cellulose derivative blends 9 g 5. Multi Multilayer extrusion 99 6 etwen multi-layers 6.1.Starch reinforced with cellulose fibers 6.1.l Hot press molding and foaming .593 63 oldin 504 6.2 Other starch/cellulose composites 594 Aliphatic polyester reinforced with natural fibers 7. 9 p 96 Protein/nanoclay composites ng rem 9 References 1.Introduction at of oil.Moder vide erful tool tracted an incr nt levels,and last two decades,predominantly due to two major reasons:firstly environmental concerns,and sec and properties.These new levels of understanding ondly the realization that our petroleum resources bring opportunities to develop materials fo are finite. rally,polymers from renewable nev resource: into thre so mean uch as starch.proter onment in los (3) ers from microhial fer nd the polyhydroxybutyrate (PHB).Like numerous other petroleum-based polymers.many properties of limits their application.Another limitation of many PFRR can also be improved through blending and natural polymers is their lower softening tempera ture study and utilization of natural polymers is development o syntheti pol mers using ch as pape natura and ples,st develop th ability of petroleum at rd is PLA. biochemical inertness of petroleum-based products from ag icultural products and is readily biodegrad- have proven disastrous for the natural polymers able.Lactide is a cyclic dimer prepared by the market.It is only after a lapse of almost 50 years controlled depolymerization of lactic acid,which in that the significance of eco-friendly materials has turn can be obtained by the fermentation of corn. been realized once again.These ancient materials sugar cane,sugar beat 1,2.PLA is not a nev
4.1. Starch/PLA blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 4.1.1. Compatiblizers used for starch/PLA blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587 4.1.2. Reactive blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 4.2. Starch/PHB blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 4.3. PHB/cellulose derivative blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 4.4. Chitosan/PLA blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 4.5. PHB/chitosan and PHB/chitin blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590 5. Multilayer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 5.1. Multilayer extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 5.2. Interfaces between multi-layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 6. Fiber-reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592 6.1. Starch reinforced with cellulose fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 6.1.1. Hot press molding and foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 6.1.2. Extrusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.1.3. Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.2. Other starch/cellulose composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594 6.3. Aliphatic polyester reinforced with natural fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595 7. Novel nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595 7.1. Starch/nanoclay composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596 7.2. Protein/nanoclay composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596 7.3. PLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 8. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598 1. Introduction Polymers from renewable resources have attracted an increasing amount of attention over the last two decades, predominantly due to two major reasons: firstly environmental concerns, and secondly the realization that our petroleum resources are finite. Generally, polymers from renewable resources (PFRR) can be classified into three groups: (1) natural polymers, such as starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid (PLA); and (3) polymers from microbial fermentation, such as polyhydroxybutyrate (PHB). Like numerous other petroleum-based polymers, many properties of PFRR can also be improved through blending and composite formation. The study and utilization of natural polymers is an ancient science. Typical examples, such as paper, silk, skin and bone arts, can be easily found in museums around the world. However, the availability of petroleum at a lower cost and the biochemical inertness of petroleum-based products have proven disastrous for the natural polymers market. It is only after a lapse of almost 50 years that the significance of eco-friendly materials has been realized once again. These ancient materials have rapidly evolved over the last decade, primarily due to the issue of the environment and the shortage of oil. Modern technologies provide powerful tools to elucidate microstructures at different levels, and to understand the relationships between structures and properties. These new levels of understanding bring opportunities to develop materials for new applications. The inherent biodegradability of natural polymers also means that it is important to control the environment in which the polymers are used, to prevent premature degradation. For example, the water solubility of many natural polymers raises their degradability and the speed of degradation, however, this moisture sensitivity limits their application. Another limitation of many natural polymers is their lower softening temperature. The development of synthetic polymers using monomers from natural resources provides a new direction to develop biodegradable polymers from renewable resources. One of the most promising polymers in this regard is PLA, because it is made from agricultural products and is readily biodegradable. Lactide is a cyclic dimer prepared by the controlled depolymerization of lactic acid, which in turn can be obtained by the fermentation of corn, sugar cane, sugar beat [1,2]. PLA is not a new ARTICLE IN PRESS L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 577
578 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 or o the pactie ve improved ch PLA d r could is at the fo front of the emerging biodegradable This new nanoc ositeshowed dramatic plastics industries In nature.a special group of polyesters is content.The reinforcement with filler is particularly produced by a wide variety of micro-organisms as important for polymers from renewable resources an internal carbon and energy storage,as part of since most of them have the disadvantages of lower their survival mechanism.Poly(B-hydroxybutyrate) and lower modulus (PHB)wa s190 the re, hydrophil a10 of mos nat polymers offers 20 The energy crisis of the 1970s was an incentive to Many natural polymers are hydrophilic and some seek naturally occurring substitutes for synthetic of them are water soluble water solubility raises plastics.which sped up the research and commer degradability and increases the speed of degrada cialization of PHB.The brittleness of PHB was on.however this moisture sensitivity limits their improved through copolymerization of B-hydrox Blends and multilayers of natura polymers wi rn a D(PHBV) s can als new low-cost products v of PHBV is still the n naior ba The ne blends and composites are extending usage the utilization of polymers from renewable resource Like most polymers from petroleum,polymers into new value-added products. from renewable resources are rarely used by themselves.In fact,the history of composites from 2.Natural polymer blends onger Wide range the ark n rom rushe s.p d rdin ve ewabl material.During the them.such as starc opium war more than 1000 vears ago.the Chinese actively used in products today while many other built their castles to defend against invaders using a remain underutilized.Natural polymers can some kind of mineral particle-reinforced composite made times be classified according to their physical from gluten rice,sugar,calcium carbonate and character. For example.starch and cellulose are sand classifie into different groups,but they ers are del enc mate emica mecha pol小ysac prope egetabl polymers surveyec seed-hair fibers.Cellulose ralpolymers perform a diverse set of the maior substance obtained from y functions in their native setting For example and applications for cellulose fiber-reinforced poly polysaccharides function in membranes and intra mers have again come to the forefront with th cellular communication;proteins function as struc- focus on renewable raw materials 7-9.Hydrophilic ural materals and catalysts:and lipids function as cellulose fibers are very compatible with mos energy stores I0.Nature can provide a impressive natural polymers. The rei array ol poryme I to be fibers is a perfect example in fiber coatigs,ge ilms of biodeg on in the ers oba ned fron es is thei polymeric materials.The interest in new nanoscale dominant hydrophilic character.fast degradation
polymer, however, better manufacturing practices have improved the economics of producing monomers from agricultural feedstocks, and as such PLA is at the forefront of the emerging biodegradable plastics industries. In nature, a special group of polyesters is produced by a wide variety of micro-organisms as an internal carbon and energy storage, as part of their survival mechanism. Poly(b-hydroxybutyrate) (PHB) was first mentioned in the scientific literature as early as 1901 and detailed studies begin in 1925 [3,4]. Over the next 30 years, PHB inclusion bodies were studied primarily as an academic curiosity. The energy crisis of the 1970s was an incentive to seek naturally occurring substitutes for synthetic plastics, which sped up the research and commercialization of PHB. The brittleness of PHB was improved through copolymerization of b-hydroxybutyrate with b-hydroxyvalerate [5,6]. This family of materials, known as poly(3-hydroxybutyric acidco-3-hydroxyvaleric acid) (PHBV), was first commercialized in 1990 by ICI. However, the high price of PHBV is still the major barrier to its wide spread usage. Like most polymers from petroleum, polymers from renewable resources are rarely used by themselves. In fact, the history of composites from renewable resources is far longer than conventional polymers. In the biblical Book of Exodus, Moses’s mother built the ark from rushes, pitch and slime— a kind of fiber-reinforced composite, according to the modern classification of material. During the opium war more than 1000 years ago, the Chinese built their castles to defend against invaders using a kind of mineral particle-reinforced composite made from gluten rice, sugar, calcium carbonate and sand. Fibers are widely used in polymeric materials to improve mechanical properties. Vegetable fibers (e.g. cotton, flax, hemp, jute) can generally be classified as bast, leaf or seed-hair fibers. Cellulose is the major substance obtained from vegetable fibers, and applications for cellulose fiber-reinforced polymers have again come to the forefront with the focus on renewable raw materials [7–9]. Hydrophilic cellulose fibers are very compatible with most natural polymers. The reinforcement of starch with cellulose fibers is a perfect example of PFRR composites. The reinforcement of polymers using fillers is common in the production and processing of polymeric materials. The interest in new nanoscale fillers has rapidly grown in the last two decades, since it was discovered that a nanostructure could be built from a polymer and a layered nanoclay. This new nanocomposite showed dramatic improvement in mechanical properties with low filler content. The reinforcement with filler is particularly important for polymers from renewable resources, since most of them have the disadvantages of lower softening temperatures and lower modulus. Furthermore, the hydrophilic behavior of most natural polymers offers a significant advantage, since it provides a compatible interface with the nanoclay. Many natural polymers are hydrophilic and some of them are water soluble. Water solubility raises degradability and increases the speed of degradation, however, this moisture sensitivity limits their application. Blends and multilayers of natural polymers with other kinds of PFRR can be used to improve their properties. Blends can also aid in the development of new low-cost products with better performance. These new blends and composites are extending the utilization of polymers from renewable resource into new value-added products. 2. Natural polymer blends Wide ranges of naturally occurring polymers derived from renewable resources are available for various materials applications [10,11]. Some of them, such as starch, cellulose and rubber, are actively used in products today, while many others remain underutilized. Natural polymers can sometimes be classified according to their physical character. For example, starch and cellulose are classified into different groups, but they are both polysaccharides according to chemical classification. Table 1 lists some natural polymers surveyed by Kaplan [10]. These natural polymers perform a diverse set of functions in their native setting. For example, polysaccharides function in membranes and intracellular communication; proteins function as structural materials and catalysts; and lipids function as energy stores [10]. Nature can provide an impressive array of polymers that have the potential to be used in fibers, adhesives, coatings, gels, foams, films, thermoplastics and thermoset resins. One of the main disadvantages of biodegradable polymers obtained from renewable sources is their dominant hydrophilic character, fast degradation ARTICLE IN PRESS 578 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602
L.Yu et al Prog.Polym Sel.31 (2006)576-602 579 List of natural polymers arch.cellulose.pectin.konjac.alginate, serum,albumin.collagen/gelatine,silks,resilin.polylysine.polyamino acids.poly(y-glutamic acid),elastin. s,surfactants,emulsan overwhelming abundance and its a ewa [131.Ho itself propciple.the erties of natural p rs can be eplac significantly improved by blending with synthetic mostwater soluble.difficult to process and brittle polymers.Polymer blending is a well-used technique when used without the addition of a plasticizer.In whenever modification of properties is required addition,its mechanical properties are very sensitive because it uses conventional technology at low cos to moisture content.Blending two or more chemi The usual objective for preparing a novel blend of not to change the pr ercom components dras ally,but er has blended h starch for on the e 1970s and 1980s.n app with various polyolefins were developed.However. on gelatinized starch nds ann and 1 4trans these blends were not biodegradable and thus the (gutta percha)for food packaging or biomedical advantage of using a biodegradable polysaccharide applications.Components are mixed to an adequate was lost.In this section,polymer blends only from degree of dispersion by thermal pressing.A series o natural raw materials are discussed blends of gutta percha with gelatinized tarch.with are water and without plasticizers or compatibilizers. wa water has s a solvent,di t to pres plas s121 processing ma ty f gutta peren polysaccharides are the main tue The and wate polymers.their interaction with water and with each other in a water medium give the structure-property intermediate values between the two components relationships in these materials.An analysis of the Carvalho et al.I151 studied the blending of starch glass transition temperature and thermal profile with natural rubber.Thermoplastic starch/natura gives one of the best illustrations of the role of water nds were prepared using natural er-reinforced e of ere prepared in intens tch mixer detail in a later section dispersion of the starc matrix was homoge neous because of the presence of 2.1.Melt processed blends the aqueous medium,with rubber particles ranging in size from 2 to &um.The results revealed a Starch is one of the most promising natural reduction in modulus and tensile strength,making polymers because of its inherent biodegradability. the blends less brittle than thermoplastic starch
rate and, in some cases, unsatisfactory mechanical properties, particularly under wet environments. In principle, the properties of natural polymers can be significantly improved by blending with synthetic polymers. Polymer blending is a well-used technique whenever modification of properties is required, because it uses conventional technology at low cost. The usual objective for preparing a novel blend of two or more polymers is not to change the properties of the components drastically, but to capitalize on the maximum possible performance of the blend. In the 1970s and 1980s, numerous blends of starch with various polyolefins were developed. However, these blends were not biodegradable, and thus the advantage of using a biodegradable polysaccharide was lost. In this section, polymer blends only from natural raw materials are discussed. Since the majority of natural polymers are water soluble, water has been used as a solvent, dispersion medium and plasticizer in the processing of many natural polymer blends [12]. Since proteins and polysaccharides are the main constituents of natural polymers, their interaction with water and with each other in a water medium give the structure–property relationships in these materials. An analysis of the glass transition temperature and thermal profile gives one of the best illustrations of the role of water in natural polymers. Natural fiber-reinforced composites are one of the successful examples and will be discussed in detail in a later section. 2.1. Melt processed blends Starch is one of the most promising natural polymers because of its inherent biodegradability, overwhelming abundance and its annual renewal [13]. However, by itself, pure starch is not a good choice to replace petrochemical-based plastics. It is mostly water soluble, difficult to process and brittle when used without the addition of a plasticizer. In addition, its mechanical properties are very sensitive to moisture content. Blending two or more chemically and physically dissimilar natural polymers has shown potential to overcome these difficulties. Natural rubber has been blended with starch for a number of different applications. Arvanitoyannis et al. [14] reported on biodegradable blends based on gelatinized starch and 1,4-transpolyisoprene (gutta percha) for food packaging or biomedical applications. Components are mixed to an adequate degree of dispersion by thermal pressing. A series of blends of gutta percha with gelatinized starch, with and without plasticizers or compatibilizers, was prepared in an attempt to preserve the excellent biocompatibility of gutta percha. A low amount of plasticizer was incorporated into the blends to improve mechanical properties. The gas and water permeability values of the blends were found to be intermediate values between the two components. Carvalho et al. [15] studied the blending of starch with natural rubber. Thermoplastic starch/natural rubber polymer blends were prepared using natural latex and cornstarch. The blends were prepared in an intensive batch mixer at 150 1C, with the natural rubber content varying from 2.5% to 20%. The dispersion of rubber in the thermoplastic starch matrix was homogeneous because of the presence of the aqueous medium, with rubber particles ranging in size from 2 to 8 mm. The results revealed a reduction in modulus and tensile strength, making the blends less brittle than thermoplastic starch ARTICLE IN PRESS Table 1 List of natural polymers Polysaccharides Plant/algal: starch, cellulose, pectin, konjac, alginate, caragreenan, gums Animal: hyluronic acid Fungal: pulluan, elsinan, scleroglucan Bacterial: chitin, chitosan, levan, xanthan, polygalactosamine, curdlan, gellan, dextran Proteins Soy, zein, wheat gluten, casein, serum, albumin, collagen/gelatine, silks, resilin, polylysine, polyamino acids, poly(g-glutamic acid), elastin, polyarginyl–polyaspartic acid Lipids/surfactants Acetoglycerides, waxes, surfactants, emulsan Speciality polymers Lignin, shellac, natural rubber L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602 579
580 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 alone.Phase separation in some was depc ntent made e additio 2.2.Aqueous blends of rubber possible.The addition of rubber was. however.limited by phase separation,the appear. Many natural polymers cannot be melt processed. ance of which depended on the glycerol content either because they degrade on or before melting Scanning electron microscopy (SEM)showed good (softening)or because they are designed to incorpo- rate substances that do not stand high temperature h starch ma h drugs,etc.).Fo form withou any kind of nu ion.More are usually biocom atible and the presence of the non-rubher constituents of the non-cytotoxic due to their similarity with living latex was not only responsible for insuring the tissues.Biopolymers are an important source of latex stability,but also for improving the compati- material with a high chemical versatility and with bility between the thermoplastic starch and the high potential to be used in a range of biomedical natural rubbe the hoth the r phases Finally,glycerol seemed to applications [1,19.A great variety of materials contribute derived ces ha of om natural sour al and st3h、 abundant biopolymers on earth.S Since of the variety of biofibe ers such as lig of civilization,the excellent chemical and physical losic natural fibers [20.21].Starch-based polymers properties of these materials have made them a present enormous potential for wide used in the useful component in many applications.However. biomedical field,as these natural polymers are blends of starch and protein have not showi totally biodegradable and inexpensive when com to other bio vailabl he to d egradable polymers av od and rc 20 protein/starch interaction during ties that make them suitable for in established a kinetic model for starch gelatiniza wide array of biomedical applications,ranging from tion and the effect of starch/protein interactions bone replacement to engineering of tissue scaffolds Matveev et al.[12]studied the effect of water on the and drug-delivery systems. glass transition of protein,polysaccharides and Starch-based thermoplastic hydrogels for use as blend considering inter-macromolecular hydrogen one cement ts or drug-delivery carriers hav an et develope ugh b d th bicate 26- Thermoplastic polysaccharides such as cellulose radical 2 5.actetate were polymerization with methyl methacrylate and/or an acrylic acid monomer.The polymerization was simultaneously onto polysaccharides,hydroxy-func- initiated by a redox system consisting of benzoyl tionalized plasticize and also peroxide and 4-dimethlyaminobenzyl alcohol at lov hydroxy-functional fillers.Organosolv lignin,cellu odegradable characte we dified。 polymers poly( ma. sult of me ko ramic con nd h lapatite [32 the excellent compatibility of nolv(. aprolactone)
alone. Phase separation was observed in some compositions, which was dependent on rubber and plasticizer content (glycerol). Increasing the plasticizer content made the addition of higher amounts of rubber possible. The addition of rubber was, however, limited by phase separation, the appearance of which depended on the glycerol content. Scanning electron microscopy (SEM) showed good dispersion of the natural rubber in the continuous phase of the thermoplastic starch matrix. The process employed in this investigation called upon the use of both starch and latex in their natural form, without any kind of purification. Moreover, the presence of the non-rubber constituents of the latex was not only responsible for insuring the latex stability, but also for improving the compatibility between the thermoplastic starch and the natural rubber phases. Finally, glycerol seemed to contribute to both the plasticization of the starch and to the improvement of the starch/rubber interface. Cellulose and starch are some of the most abundant biopolymers on earth. Since the origin of civilization, the excellent chemical and physical properties of these materials have made them a useful component in many applications. However, blends of starch and protein have not shown significant promise in biodegradable materials applications to date. The system of starch/protein, however, has been studied in food field [12,16]. Kokini et al. [16] studied starch conversion and protein/starch interaction during processing, and established a kinetic model for starch gelatinization and the effect of starch/protein interactions. Matveev et al. [12] studied the effect of water on the glass transition of protein, polysaccharides and blends, considering inter-macromolecular hydrogen and dipole–dipole interactions. Warth et al. [17] used the technology of reactive extrusion to develop starch/cellulose acetate blends. Thermoplastic polysaccharides such as cellulose- 2,5-actetate were produced by means of reactive processing technology that grafted cyclic lactones simultaneously onto polysaccharides, hydroxy-functionalized plasticizer, and optionally also onto hydroxy-functional fillers. Organosolv lignin, cellulose, starch and chitin were added for reinforcement of the polymer blends. Compatibility between oligolactone-modified cellulose acetate and fillers were markedly improved when fillers were added during the reactive extrusion process. As a result of the excellent compatibility of poly(e-caprolactone) with numerous polymers, it has been possible to prepare a wide range of new polymer blends. 2.2. Aqueous blends Many natural polymers cannot be melt processed, either because they degrade on or before melting (softening) or because they are designed to incorporate substances that do not stand high temperature (proteins, drugs, etc.). For these examples, aqueous blending is the preferred technology, particularly in biomedical applications. Natural polymers are usually biocompatible and non-cytotoxic due to their similarity with living tissues. Biopolymers are an important source of material with a high chemical versatility and with high potential to be used in a range of biomedical applications [18,19]. A great variety of materials derived from natural sources have been studied and proposed for different biomedical uses, namely polysaccharides (starch, alginate, chitin/chitosan) or protein (soy, collagen, fibrin gel) and, as reinforcement, a variety of biofibers such as lignocellulosic natural fibers [20,21]. Starch-based polymers present enormous potential for wide used in the biomedical field, as these natural polymers are totally biodegradable and inexpensive when compared to other biodegradable polymers available [22–25]. Aqueous blends of soluble starch and cellulose acetate have been studied intensively [26–29] because these blends have a range of properties that make them suitable for use in a wide array of biomedical applications, ranging from bone replacement to engineering of tissue scaffolds and drug-delivery systems. Starch-based thermoplastic hydrogels for use as bone cements or drug-delivery carriers have been developed through blending starch with cellulose acetate [26–28]. Pereira et al. [26] reported on biodegradable hydrogels, based on cornstarch/ cellulose acetate blends, produced by free-radical polymerization with methyl methacrylate and/or an acrylic acid monomer. The polymerization was initiated by a redox system consisting of benzoyl peroxide and 4-dimethlyaminobenzyl alcohol at low temperature. Utilizing the biodegradable character of starch-based blends, with the biostability of the acrylic polymers poly(methyl methylacrylate (PMMA) and poly(acrylic acid) [30,31] used as the matrix of these systems, and the incorporation of the well-known ceramic compound hydroxylapatite [32], Espigares et al. [27] developed partially biodegradable ARTICLE IN PRESS 580 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602