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 degradationpolymer, however, better manufacturing practices have improved the economics of producing mono￾mers 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 commer￾cialization of PHB. The brittleness of PHB was improved through copolymerization of b-hydroxy￾butyrate with b-hydroxyvalerate [5,6]. This family of materials, known as poly(3-hydroxybutyric acid￾co-3-hydroxyvaleric acid) (PHBV), was first com￾mercialized 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 poly￾mers 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 improve￾ment 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 degrada￾tion, 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 some￾times 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 classifica￾tion. 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 intra￾cellular communication; proteins function as struc￾tural 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