Polymer blends and composites from renewable resources
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 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(β-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 β-hydroxybutyrate with β-hydroxyvalerate [5], [6]. This family of materials, known as poly(3-hydroxybutyric acid-co-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], [8], [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.
Section snippets
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.
Aliphatic polyester blends
Aliphatic polyesters have been recognized for their biodegradability and susceptibility to hydrolytic degradation. Examples of this group are PLAs, which also have the advantage of controllable crystallinity and hydrophilicity, and therefore overall degradation rate [40], [41], [42], [43], [44], [45]. Another family of polyesters being studied widely are poly(hydroxyalkanoate)s (PHAs) that occur in nature. They are produced by a wide variety of micro-organisms as an internal carbon and energy
Blends of hydrophobic and hydrophilic polymers
Most natural polymers are hydrophilic materials since they contain either hydroxyl or polar groups. On the other hand, most synthetic biodegradable polymers, especially the aliphatic polyesters, are hydrophobic or sensitive to moisture. Blending these two kinds of polymers together is of significant interest, since it could lead to the development of a new range of biodegradable polymeric materials.
Multilayer composites
Natural polymers such as starch and protein are potential alternatives to petroleum-based polymers for a number of applications. An inherent problem with the use of natural polymers (e.g. thermoplastic starch) as biodegradable materials is their sensitivity to moisture. One approach toward solving this problem is to laminate thermoplastic starches with water-resistant, biodegradable polymers. Of the different techniques for lamination, co-extrusion or multilayer extrusion has many advantages,
Fiber-reinforced composites
Fiber-reinforced plastics have successfully proven their value in various applications because of their excellent specific properties, e.g. high strength and stiffness, and low weight. Fiber-reinforced composites have been discussed and reviewed by many authors previously, and many books are available. In that respect, natural fibers are of basic interest since they not only have the functional capability to substitute the widely used glass fibers, but they also have advantages from the point
Novel nanocomposites
Since the pioneering work by the Toyota group [129], [130], [131], polymer nanocomposites have attracted an increasing amount of attention. Polymer nanocomposites, in particular polymer/silicate nanocomposites, have been shown to exhibit significant improvements in properties compared with pure polymers or conventional composites [132]. The inclusion of well-dispersed nanosilicates in polymers has led to a vast array of changes in properties, including increased storage modulus, increased
Concluding remarks
The study and utilization of natural polymers is an ancient science. The use of these 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 the structure and properties. However, there is still a long way to go in research to obtain ideal polymeric blends and composites from renewable
Acknowledgments
The authors would like to thank Dr. Stuart Batman for pre-reading and useful comments. The authors from SCUT, China would like to acknowledge the research funds NFSC (50540420129); DHE (104148) and GNSF (NO.05200617).
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