Injection time directly depends upon the gate solidification time. The gate solidifi­cation time is the time when the material stops flowing due to its solidification at the cavity gate. Injection time consists of filling time and holding time. The gate sealing time is determined by measuring the weight of the product at regular intervals while increasing the injection time till the weight of the product becomes constant. If the injection time is less, means the holding pressure is removed before the material is solidified the material in the mold cavities would flow back due to high pressure in the mold cavity leading to various defects like voids, sink, war page and dimension­al mismatch. Therefore in order to optimize the injection time, holding time should be reduced gradually till the optimum hold time is achieved (till gate solidification).

8.3.1.2 INJECTION PRESSURE

Injection pressure consists of the filling pressure and the holding pressure. The fill­ing pressure during injection molding is usually more than the holding pressure. It should be able to inject 90 to 95% of the shot volume into the mold cavity. Ideally the filling pressure should be the maximum pressure that can be achieved without flashing but high pressure would lead to increased stresses in the molded part. Hold­ing pressure is important to overcome the shrinkage during low temperature solidi­fication of the crystalline polymer. The holding pressure should be set according to the material shrinkage properties. High holding pressure also reduces the void and sink defects, but it should be regulated carefully as high holding pressure also leads to burr formation.

MINH-TAN TON-THAT, TRI-DUNG NGO, and BOUCHERVILLE

ABSTRACT

The incorporation of renewable resources in composite materials is a viable means to reduce environmental impact and support sustainability develop­ment in the composites industry. Cellulosic fiber polymer composites have received very much attraction for different industrial applications because of its low density and its renewable ability. However, the uses of cellulosic fibers in the composite are limited in many applications that require fire re­sistance due to their flammability and their low thermal resistance.

This chapter reports an innovative and sustainable treatment approach to retard the burning of cellulosic fibers for composite production in which a minimum amount of nontoxic and low cost inorganic chemicals have been used. Different types of reacting minerals and different treatment parameters have been investigated in order to determine the most cost-effective treat­ment solution. The cellulosic fibers obtained from this approach become self-extinguished while there is no negative effect on fiber strength. The composite with the treated cellulosic fibers also shows their good fire resis­tance with minor effect on the mechanical properties. Thus this solution will open the door for the use of the cellulosic fibers in composites for applica­tions where fire resistance is an important issue, particularly in aerospace, transportation, and construction.

“Fire Resistance Cellulosic Fibers for Biocomposites” by Minh Tan Ton-That and Tri Dung Ngo was originally published with the National Research Council of Canada (© by the authors).

11.1 INTRODUCTION

Cellulosic materials (natural and synthetic) in different forms (fiber, film, powder, particle, pellet, chip, etc.) at different sizes (nano, micro or macro) are often flam­mable and have low thermal resistance. They can be burned and also can spread the fire in the presence of oxygen. Thus, their use either in direct or nondirect form is limited in applications requiring fire resistance. Due to their flammability, the use of cellulosic materials in polymer composites is also limited in certain applications.

Cellulosic materials are treated with different flame retardants depending on the application, for example in furniture, textiles or composites. The most com­monly used flame retardants are based on halogen, phosphorous, boron, ammonium, graphite, alkaline-earth metallic compounds or mixtures thereof. To improve fire re­sistance of organic polymer composites, the incorporation of flame retardants based on halogen, phosphorous, metallic hydroxide (magnesium hydroxide, aluminum hydroxide, calcium hydroxide, layer double hydroxide), metallic oxide (antimony oxide, boron oxide), silicate (clay, talc), etc., in the polymer matrix has been widely used.

Among the compounds listed above, halogen based flame retardants are well known to be the most efficient as they can be used at a low concentration in the final composition thus limiting their impact on other properties of the product. How­ever, halogen compounds are considered to be harmful to the environment. Boron compounds are supposed to be efficient, however, they tend to be washed off due to their good solubility in water. Less harmful flame retardants based on phospho­rous, graphite or alkaline-earth metallic compounds are much less efficient, thus a large amount of those additives must be used in the formulation. The use of flame retardant incorporated in a polymer matrix alone does not satisfactorily resolve the flammability problem in cellulose-polymer composites, especially when the con­centration of cellulose is quite significant in the formulation of the composite.

It is generally known that metal hydroxides, including barium hydroxide, can be used as a flame retardant for cellulosic materials1-4 and for polymer materials.5 Fur­ther, Herndon6 used a flame retardant composition for cellulosic material compris­ing sodium hydroxide and a metal salt of boron among other ingredients. The metal salt of boron is defines as borax, which is a sodium tetraborate. De Lissa7 suggested a flame-proofing composition comprising potassium hydroxide and/or potassium carbonate and possible a small amount of sodium hydroxide and/or sodium carbon­ate and may include another potassium salt. Musselman8 proposed inorganic addi­tives to impart flame resistance to polymers. The additives include hydroxides and metal salts that evolve gas. One such metal salt is barium chloride dihydrate. The use of a mixture of a polycondensate of a halogenated phenol and an alkaline earth metal halide in a flame retarding composition has also been suggested.9

Flame retardant compositions in which ancillary flame retardant additives may be used alone or in combination, such as metal hydroxides and metal salts, including alkaline earth metal salts, has also been reported10.

Fukuba11 discloses the use of “alkali compounds” for use in flame resistant plas­ter board. The “alkali compounds” are defined as at least one of an alkali metal hy­droxide, alkali metal salt, alkaline earth metal hydroxide or alkaline earth metal salt. It is preferred to use a mixture of alkali metal salts and alkaline earth metal salts, for example a mixture of sodium and calcium formate.

Yan demonstrate the use of a flame retardant composition which initially in­volves the step of making magnesium hydroxide from the reaction of magnesium sulfate and sodium hydroxide.12

It is known that treatment of cellulosic materials with alkaline earth metal car­bonates (e. g., barium carbonate) imparts fire resistance to the cellulosic material13. Here, the alkaline earth metal carbonate is applied to the cellulosic material by first coating the cellulosic material with an alkaline earth metal chloride and then treating the so-coated material with sodium carbonate. It is also known to use both a clay and a metal hydroxide in a fire retarding composition comprising a polymer material.1415

However, there is no disclosure treating a cellulosic material with an aqueous reaction mixture of an alkali metal hydroxide and alkaline earth metal salt simulta­neously with or shortly after mixing the alkali metal hydroxide with alkaline earth metal salts.

There remains a need for an environmentally friendlier, effective approach to producing fire-resistant cellulosic materials. This chapter presents an innovative method for improving fire resistance of cellulosic materials, especially when the cellulosic material is to be used in polymer composites, which is simple, cost-effec­tive and environmentally friendly.

Lignocellulosic fibers are biomaterials composed of cellulose, hemicellulose, and lignin. The content of the components depends on the wood species. Table 14.3 shows a general chemical composition (cellulose, hemicelluloses and lignin con­tent) as well as the fiber length in hardwoods and softwoods.

In general, hardwoods species present slightly higher cellulose content and sig­nificantly lower lignin content. Hemicellulose content between the two is variable depending on the wood species.

The hollow cellulose fibrils are heterogeneously embedded within a matrix of hemicellulose and lignin. Bonds between carbohydrates (hemicellulose and cellu­lose) and lignin and between hemicellulose and cellulose are present within the ce­menting matrix. The forming cellulose-hemicellulose network comprises the main structural component of the fiber cell. On the other hand, lignin-carbohydrate bonds (presented as benzyl esters, benzyl ethers, and phenyl glycosides) increase the stiff­ness of the cellulose-hemicellulose composite.

With respect to structure, the fibrils possess a thin primary wall (S1) first formed during cell wall biogenesis that is engirded by a larger, more voluminous secondary wall (S2) made up of three layers with a middle layer determining the load-bearing capacity of the wood fiber. This middle layer can be characterized as a rope-like structure of helically wound microfibrils from long chained cellulose macromol­ecules. An important property controlling the ultimate mechanical strength of the wood cell architecture is the angle betwixt the fiber axis and the microbrils known as the MFA (microfibril angle) that spans a gamut of values depending on the wood species.

From the view of sustainable development, the new materials associated with re­newable source, low toxicity, high performance and environmental biodegradability

after disposal are enormously explored. The concerns over new materials from re­newable resource have recently increased because of the economic consequences of depleting petroleum resources, the demands from industrialists and customer for high performance lightweight low-cost materials and the environmental reg­ulations.1,2 From biomass, polymers can be obtained as native biopolymers, raw materials for monomers and bio-engineered biopolymers. Polysaccharides such as cellulose, starch, chitosan/chitin, etc. are the abundantly available biopolymers on the planet earth. They are replacing the materials for many industrial applications where synthetic polymers have been materials of choice, traditionally. As the na­tive biopolymers are not conventionally processable, research efforts have been fo­cused on the processing and meeting the requirements of particular applications. For packaging, the polymeric materials must exhibit flexibility, transparency, water and gas barrier properties, biodegradability (after disposal) antimicrobial, thermal and mechanical properties whereas surface adhesion (hydrophilicity), biocompatibility, biodegradability and dimensional stability.

Since natural biopolymers exhibit poor mechanical and thermal properties and processability they are very often blended with synthetic polymers synthetic biopolymers such as, polylactic acid and polycaprolactone and reinforced with par­ticulates.3 With the recent breakthroughs on nanoscience and nanotechnology, which allow tuning the materials properties at nano-scale level, the biobased nanocompos­ites are explored as renewable biomaterials.4,13 Hence, this chapter focuses on the recent developments in biobased nanocomposites based on chitosan and cellulose nanocrystals where both matrix and filler that are biologically renewable (Fig. 16.1).

The packaging materials based on polymer nanocomposites are recently pre­ferred not only for extending the shelf-life of food products but also for improving

the quality of food by acting as a carrier of some active substances such as antioxi­dants and antimicrobials.14 Owing to their biocompatibility, they are also explored for biomedical applications (as drug delivery system, wound dressing and biore­sorbable materials and low cytotoxic scaffolds for tissue engineering).1518

Spiders coat their webs with sticky, adhesive substances that have been reported to facilitate prey capture. In orb weavers, the aggregate gland has been characterized as the structure that extrudes aqueous glue that coats the spiral capture threads. Chemical analysis of the aqueous glue solution has revealed high concentrations of organic compounds related to neurotransmitters, small peptides, free amino acids, low concentrations of inorganic salts and glycoproteins.55 Two cDNAs encoding glycoproteins have been reported in the literature and proposed to be major con­stituents of the glue droplets found on spiral capture threads; these products are named Aggregate Spider Glue 1 (ASG1) and Aggregate Spider Glue (ASG2).56 In cob-weaver spiders, data are emerging to suggest a different functional role for the aggregate gland (Fig. 1.1). In part, this could be somewhat anticipated and hypoth­esized because cob-weavers lack spiral capture silk. In cob-weavers the aggregate gland has been demonstrated to secrete two distinct proteins that are constituents of connection joints in three-dimensional webs, which are structures that glue scaffold­ing fibers together.57 These products, named Aggregate Gland Silk Factor 1 (AgSF1) and Aggregate Gland Silk Factor 2 (AgSF2), have markedly divergent protein ar­chitectures as well as are highly distinctive relative to protein sequences from tradi­tional fibroins. AgSF2, a 40-kDa nonglycosylated protein, has novel internal amino acid block repeats with the consensus sequence Asn-Val-Asn-Val-Asn (NVNVN), whereas AgSF1 contains pentameric Gln-Pro-Gly Ser-Gly (QPGSG) iterations that are similar to modular elements with mammalian elastin. AgSF1 has the potential to self-assemble into fibers and X-ray diffraction of synthetic threads reveals the pres­ence of noncrystalline domains that resemble classical rubber networks.57

All the bio-based materials used in this study (PGPE, GPE, and TA) are water-solu­ble and hydrophilic substances. The average number of epoxy groups per molecule of PGPE and GPE is 4.1 and 2.0, respectively. Because the viscosity of GPE (150 cps at 25 °C) was much lower than that of PGPE (1300 cps at 25 °C), a mixture of GPE, TA, and WF can be compounded without solvent. However, it was necessary to add a solvent in case of a mixture of PGPE, TA, and WF. Since some precipitate was liberated when aqueous solutions of PGPE and TA were mixed, ethanol was used as a mixing solvent. The mixture of PGPE, TA, and WF or GPE, TA, and WF was cured at the condition of 160 °C for 3 h with epoxy/hydroxy ratio of 1/1, at which most balanced thermal and mechanical properties were attained for the cured products of GPE and TA in the previous section.22 Although the biocomposites with WF content higher than 70 wt.% can be prepared, the obtained composites became brittle and the surface was rough. Figure 4.5 shows FE-SEM photographs of the fractured surfaces of PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF composites with WF contents of 60 and 70 wt.%. It appeared that WF is tightly incorporated into the crosslinked epoxy resins and their interfacial adhesion is good. This result may be attributed to the fact that the polyphenol moiety of TA and lignocellulose moiety of WF resemble each other. There are some voids on the fractured surface of PGPE/ TA(1/1)/WF, probably generated during the evaporation of ethanol when compared with GPE/TA/WF.

Figures 4.6 and 4.7 show the temperature dependency of storage modulus (E’) and tan 8 for PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF measured by DMA, respec­tively. The E’ at the rubbery plateau region over 80 °C for all the composites was much higher than that of control cured resins, suggesting a superior reinforcement effect due to the wood fibers. The tan 8 peak temperature related to Eg for the com­posites was a little lower than that of the corresponding neat resins. The reason is not clear, but it is thought that some components of WF react with the epoxy resins, and/or that WF disturbs the crosslinking reaction.

FIGURE 4.7 DMA curves of GPE/TA(1/1) and GPE/TA(1/1)/WF biocomposites.22

Figure 4.8 shows typical TGA curves of GPE/TA(1/1), GPE/TA(1/1)/WF60 and WF. Since the thermal decomposition temperature of WF was lower than that of GPE/TA(1/1), the GPE/TA(1/1)/WF composite exhibited two-step thermo-degra­dation. The 5% weight loss temperatures of all the composites are summarized in Table 4.4. Consequently, the 5% weight loss temperatures of all the composites were lower than those of the corresponding cured neat resins.

Figures 4.9 and 4.10 show the relationship between tensile properties and fiber content for PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF, respectively. Although ten­sile modulus (4.3 GPa) of PGPE/TA(1/1)/WF was much higher than that of PGPE/ TA(1/1) (2.7 GPa), the tensile strength of the composite was lower than that of PGPE/TA(1/1). On the other hand, both the tensile modulus and strength of GPE/ TA(1/1)/WF were much higher than those of GPE/TA(1/1) (2.4 GPa and 37 MPa). Those values increased with WF content, became maximal values (5.1 GPa and 51 MPa) at WF content 60 wt.%, and were lowered at 70 wt.%. In general, although the tensile modulus of polymer/plant fiber biocomposites is higher than the control polymer, the strength is rather lower because of a poor interfacial adhesion. It is noteworthy that the tensile strength of GPE/TA is improved by the addition of WF without any interfacial modification. This result should be attributed to the superior interfacial adhesion between GPE/TA and WF. As the reason that the tensile strength of PGPE/TA(1/1)/WF did not increase, the following factors are considered. As ten­sile strength of PGPE/TA(1/1) is much higher than that of GPE/TA(1/1), the in­terfacial adhesion strength between PGPE/TA(1/1) and WF is not higher than the strength of PGPE/TA(1/1). The structural defects due to some voids are observed as was shown in Fig. 4.5. Also, the fact that the tensile modulus and strength of both PGPE/TA(1/1)/WF and GPE/TA(1/1)/WF composites with WF content 70 wt.% are lower than those of the composites with 60 wt.% suggests that the packing of ma­trix resin between the WF particles is relatively insufficient for composites with 70 wt.%.

□ PGPE/TA(1/1) О PGPE/TA(1/1 )/WF50 I I PGPE/TA(1/1)/WF60 PGPE/TA(1/1)/WF70

As the tan 5 peak temperature of GPE/TA(1/1)/WF was lower than that of GPE/ TA(1/1) (Fig. 4.7), the epoxy/hydroxy ratio appropriate for GPE/TA/WF compos­ites was investigated. Table 4.4 summarizes the tan 5 peak temperature of GPE/ TA and GPE/TA/WF60 prepared at epoxy/hydroxy ratios from 1/0.6 to 1/1.2. In case of the control GPE/TA, the tan 5 peak temperature related to Tg increased with decreasing epoxy/hydroxy ratio. Considering that all of the three hydroxy groups of PG moiety of TA are hard to react with epoxy groups of GPE, it is supposed that an actual stoichiometric epoxy/hydroxy ratio should be lower than 1/1. Although GPE/TA(1/1.2)/WF60 exhibited the highest tan 5 peak temperature among the GPE/ TA/WF60 composites in a similar manner to the control resins, GPE/TA(1/0.8)/ WF60 had a little higher tan 5 peak temperature than GPE/TA(1/1)/WF60. In case of GPE/TA(1/0.8)/WF60, there is a possibility that the excess epoxy groups of GPE reacted with the hydroxy groups in WF at the curing temperature of 160 °C. As a
result, GPE/TA(1/0.8)/WF60 had almost the same tan 5 peak temperature as GPE/ TA(1/0.8).

Table 4.4 also summarizes 5% weight loss temperature of GPE/TA and GPE/ TA/WF60 prepared at epoxy/hydroxy ratios from 1.0/0.6 to 1.0/1.2. Regarding the control GPE/TA, the 5% weight loss temperature a little decreased with decreas­ing epoxy/hydroxy ratio. As TA itself has the lowest 5% weight loss temperature (285 °C), the presence of unreacted TA moiety in the GPE/TA with a higher TA con­tent caused a decrease of the 5% weight loss temperature. In case of the composites with WF content 60 wt.%, GPE/TA(1/0.8)/WF60 exhibited the highest 5% weight loss temperature among the GPE/TA/WF composites.

Figure 4.11 shows the tensile properties of GPE/TA/WF60 composites prepared at various epoxy/hydroxy ratios. The GPE/TA(1/0.8)/WF60 showed the highest tensile modulus (5.22 GPa), strength (54.9 MPa) and elongation at break (1.35%), indicating that the best ratio of epoxy/hydroxy is ca. 1/0.8. In case of polypropylene (PP)/WF composites, it is known that the preparation of the PP/WF composite with WF content higher than 50 wt.% is not easy, and that the addition of maleic anhy­dride-grafted polypropylene (MAH-PP) improves the tensile properties. The tensile modulus and strength of PP/MAH-PP/WF (45/5/50) composite are reported to be 4.55 GPa and 40.4 MPa, respectively.64 It is also known that that tensile modulus and strength of high-density polyethylene (HDPE)/WF (35/65) composite are 2.6 GPa and 15.6 MPa, and those of HDPE/poly(ethylene-cobutyl acrylate-co-maleic anhy — dride)/WF (32.5/2.5/65) are 2.5 GPa and 18.6 MPa, respectively.65 The GPE/TA/ WF composites in which neither modifier nor compatibilizer is added have higher tensile modulus and strength than these petroleum-based plastics/WF composites.

A. R. MOHANTY and S. FATIMA

6.1 INTRODUCTION

With growing concerns towards the environment, designers are looking forward to the use of environmental friendly materials for product development. Though the aim is not to replace completely the metals or synthetic materials by these environ­ment friendly materials. For the former have high strength and long life. However in few applications the use of composites made up of a combination of natural fiber based materials and a binder have many advantages. Like they are environmental friendly, less weight, economical and in few places are also available abundantly in nature. Natural based fiber materials like flax, ramie, hemp, banana fiber, coir, jute, cotton can be used to produce biocomposites which have many industrial ap­plications. These biocomposites are being used to manufacture machinery enclo­sures, furniture, highway crash barriers, geo-textiles, as reinforcement in concrete, apparels, bags, yarns, carpets and building materials. However, they have a strong potential to be used for noise control applications as well.

Human beings are uncomfortable at exposure to high sound levels for a long duration. Such undesired sound are known as noise. Thus designers of machines, building architects, city planners, automobile designers and environmentalist have to take due consideration in design of machines, building, city planning and so forth, so that the human being is comfortable in the respective situations. Moreover due to stiff global competition among manufacturers, everyone is striving to make their products quieter than the other. Keeping the above in consideration, this chapter introduces to the basics of sound along with the associated terminology. Then it fo­cuses on the physical, mechanical, thermal and acoustical properties of such materi­als. Followed by a description of their use in various applications like architectural acoustics, home appliances, machinery enclosure and automobiles for noise control.

For noise control in machineries, many a times a lined enclosure is placed around a machine to reduce the radiated noise. Enclosures are used around IC engines, gen­erator sets, compressors, blowers, punching press, etc. The walls of the enclosures are usually made out of thick steel sheets and are lined with sound absorbing materi­als of a certain thickness, which can withstand high temperature. Once enclosures are put around the machine, the temperature inside the enclosure increases which may affect the components of the machinery in particular the lubricants, which are used in bearings, etc. Thus usually enclosures for machines, which run continu­ously are provided with some sort of inlet air and exhaust airports. Since the ports on the enclosures have openings, it is recommended that an inline inlet and exhaust silencer is provided at the ports for bringing in fresh air and expelling out the warm air. Sometimes in few applications when the exhaust ports are not available cooling arrangement of the inlet air by a chiller is made. The inlet and exhaust silencers in such enclosure are of the dissipative type with jute-based lined ducts. Figure 6.25 shows a view of an IC engine-dynamometer setup covered by a perforated faced 50 mm jute fiber lined enclosure. The noise in the test cell reduced by around 10 dB by using such a jute-lined enclosure.

Nowadays, many investigations are conducted regarding the development and char­acterization of biopolymers since conventional synthetic plastic materials are resis­tant to microbial attack and biodegradation.17,18 Among all biopolymers, starch has been considered as one of the most promising one due to its easy availability, biode­gradability, lower cost and renewability.19 Starches are commonly used biopolymers. Starches are the major form of stored carbohydrate in plants such as corn, wheat, rice, and potatoes. They are hydrophilic polymers that natively exist in the form of discrete and partially crystalline microscopic granules which are held together by an extended micellar network of associated molecules.8 Starches are composed of both linear and branched polysaccharides well known as amylose and amylopec — tin, respectively (Fig. 9.7). Native starches contain about 70-85% amylopectin and 15-30% amylose. The ratio of amylose and amylopectin in starches varies with the botanical origin.20 The Amylopectin is mainly responsible for the crystallinity of the starch granules. Starch granules exhibit hydrophilic properties and strong intermo­lecular connection through hydrogen bonding formed by hydroxyl groups on the granule surface.21

Amylose molecules consist of 200-20,000 glucose units, which form a helix as a result of the bond angles between the glucose units. Amylopectin is a highly branched polymer containing short side chains of 30 glucose units attached to every 20-30 glucose units along the chain. Amylopectin molecules may contain up to 2 million glucose units.22 Starches from various sources are chemically similar and their granules are heterogeneous with respect to their size, shape, and molecular constituents. Proportion of the polysaccharides amylose and amylopectin become the most critical criteria that determine starch behavior.23,24 Most amylose molecules (molecular weight ~105-106 Da) are consisted of (1 —>4) linked a-D-glucopyranosyl units and formed in linear chain. But, few molecules are branched to some extent by (1—6) a-linkages.25,26 Amylose molecules can vary in their molecular weight distribution and in their degree of polymerization (DP) which will affect to their solution viscosity during processing, and their retrogradation/recrystallization be­havior, which is important for product performance. Meanwhile, amylopectin is the highly branched polysaccharide component of starch that consists of hundreds of short chains formed of a-D-glucopyranosyl residues with (1—4) linkages. These are interlinked by (1—6)-a-linkages, from 5 to 6% of which occur at the branch points. As a result, the amylopectin shows the high molecular weight (107-109 Da) and its intrinsic viscosity is very low (120-190 mL/g) because of its extensively branched

molecular structure.25,26

Starches are highly potential candidates for developing sustainable materi­als, for it is simply generated from carbon dioxide and water by photosynthesis in plants.21,27,29 However, they display poor melt processability and are highly water soluble and hence, they are difficult to process and are brittle. The presence of nu­merous intermolecular hydrogen bonds affects processability of starches.2030 Thus, for application purposes, starches need a plasticizer to render them processable. Plasticizers such as water, glycerol and sorbitol assist in increasing the starch flow and also decrease the glass transition temperature and melting point of starch.82031 Starch can be transformed into thermoplastic-like material, when the molecular in­teractions are disrupted by using plasticizers under specific conditions. The heat­ing of starch granules in the presence of plasticizers yield a nonirreversible transi­tion and swelling of amorphous areas.20 The process of disrupting starch molecular structure is known as gelatinizing and the plasticized starch is called thermoplastic starch (TPS). It is vital to note that starches are not real thermoplastic but they act as synthetic plastic in the presence of plasticizers (water, glycerol, sorbitol, etc.) at high temperature. The various properties of thermoplastic starch product such as mechanical strength, water solubility and water absorption can be prepared by alter­ing the moisture/plasticizer content, amylose/amylopectin ratio of raw material and the temperature and pressure in the extruder (Mohanty et al., 2000).32 Plasticizers are the most important material to increase the flexibility and processibility of TPS. There are large number of researches that were performed on the plasticization of TPS using glycerol 33, sorbitol 34, urea and formamide35, dimethyl sulfoxide36and low molecular weight sugars 37. The properties of TPS also depend a lot on moisture. As water has a plasticizing power, the materials behavior changes according to the relative humidity of the air through a sorption-desorption mechanism.38The factors that greatly influence the final morphology of TPS are composition, mixing time, temperature, shear and elongation rate of the operation.20

TPS alone can be used for the production of useful products but the moisture sensitivity of starch limits its usage in many commercial applications. To enhance the properties of starch, various physical or chemical modifications of starch such as blending, derivation and graft copolymerization are employed.21 Blending of TPS with PHA, PLA, and PCL produce 100% biodegradable materials. The aim of blending low cost starch with completely degradable polyester is to lower the cost of the latter while maintaining other significant properties at an acceptable level.21-39-40

11.4.2.1 PF/FLAX FIBER COMPOSITES

Vertical VC-2 burning test was conducted on the PF composites and the results are shown in Table 11.7. Sample PF-C2 is a comparative example of a PF composite containing untreated flax fiber. PF-C2-21/P2 is PF composite containing flax fibers treated with a bi-component aluminum-containing system. Burning time or burning length are the time or the length it takes for the fire stop after the external ignition flame was removed. Thus, a shorter burning time and length indicate a more fire resistant material. PF is a thermoset resin which itself has considerable resistance to fire. Thus, in the PF composites flax fiber mainly contributes to burning the com­posite specimen. As is evident from Table 11.8, fire-resistant flax fibers of this ap­proach provide a tremendously significant greater resistance to burning in the PF matrix than untreated flax fibers that allows the obtained composite with the treated flax fibers to be classified as self-extinguished (Fig. 11.6).

Figure 11.6 illustrates the remains of the PF composite flax fabric after VC-2 burning test. The PF composite with nontreated flax fabric has burned very much before the fire stops. However, the PF composite with treated flax behaves very dif­ferently, the fire stops very shortly after the torque was removed.

The SEM observation of the fracture composite specimens demonstrates a good fiber-matrix in both the untreated and treated flax composites. Figure 11.7 illustrates the resin sticks on the fracture fibers in both, untreated and treated fiber after ten­sile test. Thus, the treatment does not cause any harm to the fiber-matrix interface, which determines the mechanical properties.

Flexural properties of the PF/flax fiber composite samples are shown in Table 11.8. It is evident that PF composites containing fibers treated with a bi-component aluminum-containing system have comparable flexural properties with the refer­ence if standard deviation is taken into account. It is in coherent with the SEM observation.