CHAPTER 2
LITERATURE REVIEW
This study involves variety of testing, methods and material. The elaboration of previous study need to be done as to collect information in such a wide area such as technical data, process and the nature of the material itself. All the data, information and analysis done by previous study need to be reviewed as to ensure its significances to the study. The information taken from books, journals, articles, web site, thesis and proceeding is cited in reference due to respect to the author.
2.1 The nature of composite material
Oxford dictionary defined composite as a material consisting of different parts or material [1]. Polymer composite consists of two or more distinct parts, distinct material phase that combined together to form new material consist of properties that cannot be achieve by single material acting alone.
Newer materials and composites that have both environmental and economic benefits are being developed for applications in the automotive, building, furniture and packaging industries. Agro and forest resources have always played an important role in the composite and plastics industry. The earliest phenolic products were filled with wood flour to reduced the production cost and improve the processibility of the resin. Enormous quantities of agro-wastes have subsequently been used as fillers in thermosetting composite [2] as it would eventually reduce cost and alter the properties needs for end-application.
Polymer composite may be made from either thermosetting or thermoplastic plastic that act as matrices that bind the filler together. Filler used in composite can be either fibre and/or particulate form. Recent development uses bio-based filler in polymer composite. Bio-filler was introduced in the polymer composite as a re-useable material that enhances cost effectiveness to the processibility and eventually to the overall performance of the polymer composite itself. By introduced filler in polymeric material, the properties of the composite can be alter and modified into something that may be useful to the end product and thus end user.
Intrinsically fillers can be divided into two types which are reactive and non-reactive. Reactive filler will react with the environment. A good example of reactive filler is gibbsite (alumina hydroxide) which react with both strong acid and basic substance such as oxygen. Gibbste also loses its water of crystallization around 200 0C. Other filler exhibit similar behaviour to a greater or lesser extent [3]. The silicate minerals such as kaolin, mica, talc, and quartz are in chemically terms is virtually inert which can only be affected by very strong acids and alkalis. The carbonate minerals such as and hydroxide minerals are very reactive to acids [4].
The interaction between filler or fibre with its polymer matrix is the major aspect one should consider in the development of polymer composite. The interfacial area between polymer and filler is the key to polymer composite. Good contact interface between filler and polymer can introduce excellent and desired mechanical and thermal properties. The study of interface between filler and polymer is exceptionally wide. The interaction between the constituent elements in filler may sometimes determine its crystallinity, which then dictates all the intrinsic properties of the filler in the polymer composite.
The filler properties of polymer composite is wide; from reinforce, unreinforced, bio-degradable, fire retardant and much more [3]. The common modification in applying filler or fibre into polymer composite is to modified the mechanical properties such as incorporating reinforce filler or fibre. This method is widely use in composite industries to produce mechanically strong component for aircraft, marine and automotive industries.
The use of filler in particulate form to be incorporated into polymer matrix is widely study. Researchers around the world are currently looking for a better, cost effective, easy and abundance wastes bio-based filler to substitute synthetic and conventional filler. But one must remember the use of particulate filler in polymer composite need special consideration before it can be use in the industries. The things must be consider are the surface chemistry of the filler, the interaction, cost, chemistry, composition and impurities, density or specific gravity, optical properties, thermal properties, thermal conductivity, shape and size of the particle and much more [5].
2.2 Chicken eggshell (Es)
Chicken eggshell (Es) is the brown in colour, solid, hard layer of the outer of a chicken egg. Es is a well known waste material in the world. Statistic have been made to estimate the Es wastes that has been dispose to the landfill in Malaysia alone which is approximately 28260 tonne/year for population of 13.2 million adult aged 18 to 59 years old [6]. United States Department of Agriculture grades eggs by the interior quality of the egg and the appearance and condition of the eggshell such as its thickness, size and its surface or morphology [7]. Es is an aviculture byproduct which has been listed worldwide as one of the worst environmental problems, especially in those countries where the egg product industry is well developed. In the U.S. alone, about 150,000 tonne of this material is disposed in landfills yearly [16].
2.2.1 Chicken eggshell as bio-based filler in polymer composite
Newer materials and composites that have both environmental and economic benefits are being developed for applications in the automotive, building, furniture and packaging industries. Agro and forest resources have always played an important role in the plastics industry. Enormous quantities of agro-wastes such as wood flour, keratin fibre, coconut husk, and much more have subsequently been used as fillers in thermosetting composite [8].
In this study, chicken eggshell (Es) is study to determine its suitability as filler to be used in thermosetting composites. Es provides rigidity to the polymer composite based on eggshell's crystal structure nature. Patricio Toro et al. (Sept 2006) found that eggshell/polypropylene composite made of chicken eggshell has slightly higher crystallinity than similar polypropylene (PP) composite made of commercial calcium carbonate (Differential Scanning Calorimetry measurement gave 48% of crystallinity as compared with 46% of commercial calcium carbonate) [9]. Study has been conducted shows that the natural CaCO3 in seashell, hen's eggshell and other natural shell has significantly high modulus and mechanical properties than synthetic CaCO3 which derived from quarried source [11]. Es was suitable enough to be used as filler in bio-based polymer composite [10] because of its characteristics which qualifying eggshell as a good candidate for bulk quantity, inexpensive, lightweight and low load-bearing composite application.
2.2.2 Composition of chicken eggshell
Eggshell (Es) typically consists of ceramic materials constituted by a three-layered structure, namely the cuticle on the outer surface, a spongy (calcareous) layer for the second layer and an inner lamellar (or mammillary) layer for the last layer. There are three different stages during eggshell formation can be categorized, namely initial, fast growth and termination. Eggshell deposition starts with calcium carbonate (CaCO3) spherulits nucleating on the eggshell membranesand the growth continues until adjacent spherulits fuse together at the initial stage. Then, columnar crystals (palisades) materialize from the spherules during the fast growth stage. Columnar crystal growth proceeds until eggshell calcification is terminated with the deposition of the cuticle layer at the final termination stage. Figure 2.1 shows the cross section texture of the final hen eggshell. Calcite crystals forming the eggshell precipitate in the uterine fluid, an acellular (Noncellular; a living entity without cell/s) environment containing ionized calcium and bicarbonate greatly in excess of the solubility product of calcite, as well as the native and soluble organic precursor of the shell matrix [4].
Eggshell organic matrix is composed by proteins, glycoprotein and proteoglycans in the calcified layers and by different types of collagens in the eggshell membranes. The spongy and mammillary layers form a matrix composed of protein fibres bonded to calcite (calcium carbonate) crystal [12]. Eggshell is composed mainly of a mineral part (approximately 94% by weight) made of columnar calcite crystals of CaCO3 and a pervading organic matrix (protein) as a remaining material. CaCO3 can be found naturally in a form of polymorphs as aragonite, calcite and vaterite (µ-CaCO3). The trigonal crystal structure of calcite is the most common can be found and the most stable compared to aragonite and vaterite. Vaterite and aragonite is a metastable phase of calcium carbonate at ambient conditions at the surface of the earth. As it is less stable than either calcite, vaterite has a higher solubility than either of these phases. Once vaterite is exposed to water, it converts to calcite (at low temperature) or aragonite (at high temperature: ~60°C). However, vaterite does occur naturally in mineral springs, organic tissue, gallstones, and urinary calculi [30]. A. Herna´ ndez-Herna´ ndez et al. also found three groups of molecules have been found in Es; ubiquitous components such as osteopontin and clusteria, egg white proteins such as ovalbumin, lysozyme, ovotransferrin, and organic constituents unique to the process of shell calcification [10]. The last group comprises dermatan and keratan proteoglycans as well as ovocleidins and ovocalyxins [13, 14].
Figure 2.1: Cross-sectional SEM micrograph through the chicken eggshell [18].
An eggshell is the outer layer of the egg has 9 - 12% of the total weight off the chicken egg [12]. Table 2.1 showed the approximation of the percentage of eggshell constituent.
Table 2.1: Eggshell constituents based on percentage [14]
Constituents
Percentage, %
Calcium Carbonate, CaCO3
94
Organic Matter
4
Magnesium Carbonate, MgCO3
1
Calcium Phosphate, Ca3(PO4)2
1
Figure 2.2: Cross-section of chicken egg [15]
The composition of CaCO3 that was found in eggshell is shown by Fourier Transform Infrared Spectroscopy (FTIR) in the region 875 to 712 cm-1 which should be associated with the in-plane deformation and out-plane deformation modes respectively. The infrared spectrum of the absorbance of the eggshell flakes was obtained between 4000 and 400 cm-1, using an average of 32 scanning at 4 cm-1 resolution [12, 14]. The FTIR spectroscopy is a useful tool in the molecular characterisation of inorganic species such as CaCO3.
The elemental composition of Es has been reported to be about 98.2% calcium, 0.9% magnesium and 0.9% phosphorus (present in shell as phosphate) [18]. The Es is composite structure of CaCO3 in association with organic substance. Calcium (Ca), magnesium (Mg), and sodium (Na) are major inorganic constituents of the Es. The distribution of Ca is not homogeneous throughout the Es thickness or can be further elaborated that this phenomenon is more pronounced after hatching indicating the consumption of the inner layers contents by the embryo during its development. On the other hand, Mg and Na concentration in the inner part of the Es before hatching are higher than after hatching for the same reasoning. The previous study have be done to use Laser Induced Breakdown Spectroscopy (LIBS) technique to study the elemental composition of Es before and after hatching and concluded that an increasing Mg content is significant to the increasing of the Es rigidity and hardness. The study revealed that by using LIBS technique, magnesium concentration in the eggshell is decreasing after hatching by about 60% in the average with respect to its values before hatching and Na on the other hand ecrease by about 30% of the hatching [18]. Figure 2.2 and 2.3 shows the LIBS spectrum of the eggshell before and after hatching process.
Figure 2.2: Emission spectrum of the eggshell showing the Mg spectra before and after hatching
Figure 2.3: LIBS spectrum of the eggshell showing Na intensity before and after hatching.
On the other hand, the concentration of Ca increased by almost 90% after hatching process. The increase of Ca can be interpreted by the depletion of the mammillary layer during the incubation period as the embryo consumes the elements and calcium in the inner layer for its development [18].
2.2.3 Physical and chemical properties of chicken eggshell
Chicken eggshell (Es) is an industrial waste of by-product containing approximately 94% of calcium carbonate (CaCO3) and approximately 5-6% of organic material such as type X collagen, sulphated polysaccharides, and other proteins which cause its disposal constitutes a heavy environmental hazard [16].
The term eggshell (Es) is define for the outer covering layer of a hard-shelled of an egg. The colour of Es can be varies from dark brown to creamy white. The observation made by the previous study proven that Es contain several layers. This is proven by the outer shell shows dark colour and the inner shows white colour. Es has relatively lower density compared to mineral calcium carbonate according to standard test method of measuring density ASTM 679 which is 0.4236 g/cm3 compared with 0.4670 g/cm3 of commercial calcium carbonate or 0.4581 g/cm3 of talc [9]. Previous study conducted shows that Es filler gives slightly higher crystallinity value compared to mineral CaCO3. Differential Scanning Calorimetry (DSC) analysis showed that eggshell/polypropylene composite have higher crystallinity of 48% compared to mineral CaCO3/polypropylene or talc/polypropylene composite which gives only 46% crystallinity measurement [10] which lead to increasing of rigidity and stiffness to the polymer composite at the same time reduce the density of the polymer composite. In summary, eggshell/polypropylene composite gives higher tensile modulus that CaCO3/polypropylene composite which shows better reinforcement than composites with traditional CaCO3 filler.
Es strength is an important factor to minimize the incidence of Es breakage in egg producing industry. The amount of mineral consists in Es shows linear relationship to the Es strength. The organic matrix may also exert some control on the biochemical properties of the eggshell by influencing the fabric or microstructure in the mineralization processes. Previous reports show that various macromolecules in the calcified Es and shell's membranes, which are composed of collagens, glycosaminoglycans (GAGs), proteoglycans (PGs) and diverse proteins, are involved in controlling eggshell mineralization [20]. GAGs consisted of a repeating disaccharide unit contain sulphate ester and carboxylate group. This is important for the binding of cations during the process of mineralization, so they are assumed to be involved in the binding of calcium ions and in regulation of the mineralization processes [21-23]. Y.W Ha et al. found that the concentrations of uronic acid and sulfated GAGs were positively correlated with breaking strength in shell membrane and negatively correlated in calcified shell (spongy matrix). The calcified shell weight increased with shell strength, but the absolute amount of uronic acid and sulphated GAGs was not correlated each other. Also, the weight of shell membranes was almost constant, while the amount of uronic acid and sulphated GAGs increased with shell strength. In conclusion, the GAGs content in eggshell membranes has significant correlation with the Es breaking strength [24].
The surface of Es consists of numerous evenly distributed funnel-shaped pore canals with submicron-scaled mouth forming connection passages between the hard outer shell and the cuticle (a foamy layer of protein on the outer surface) [25]. This funnel-shaped pore canals permits the transfer of gasses through the Es. Carbon dioxide (CO2) and moisture pass through the pores and being replaced from the outside gasses such as oxygen (O2) and the large end of egg will form a small air cell within a few minutes after laid [26]. These gases provide constant support needed by the egg during hatching process. The cuticle thickness on the eggs of domestic hens varies from 0.5 to 12.8 um [27] over the surface of the same egg and has an effective life span of 96 hours after oviposition (Oviposition is the process of laying eggs) [28]. Theoretically the cuticle subserves a number of diverse functions, varying from reducing water loss to the first lines of defence against bacterial penetration by blocking the external surface of pores. The cuticle consists of 85-87% protein, 3.5-4.4% carbohydrates, 2.5-3.5% fat, and 3.5% ash [29].
Any form of damage or defect on the Es will increases the risk of penetration by microorganisms [31]. Since the pioneering study of egg microbiology, a great many research have shown that the chicken egg is reinforced with chemical and physical defences against microorganisms [32]. Infection of the outer surface of an Es is potently the first step in transshell infection. Many studies have shown that the shell acquires a broad range of contaminants through contact with nest material [32]. Interestingly, every layer and part of Es plays its role to defence against external attacks that can harm the egg inside.
2.3 Polyester
Polyester was broadly classified into unsaturated and saturated polymers. These two broad divisions subdivided as follows [40]:
Unsaturated polyester
Laminating and Casting Resins. These were based on dibasic acids and dihydric alcohols. The polyester unit formed must be capable of copolymerizing with a vinyl-type monomer, thereby yielding a vinyl-polyester copolymer or simply cured polyester having a thermosetting polymer structures
Alkyds. In general, it is the same as laminating and casting resin although the glyptal (coating surface) types are modified with oils or fatty acids. This term used to describe a group of thermosetting moulding materials based on the reaction of a dihydric alcohol with unsaturated acid such as maleic and terephthalic. A vinyl type monomer was also need to speed up the cross-linking and curing in compression and resin transfer moulding process.
Saturated polyester
Fibres and films. This type of polyester was based on the reaction of terephthalic acid with ethylene glycol and was a linear polymer, high molecular weight which does not undergo any cross-linking reactions.
Plasticizers. These were polyester was completely saturated and normally called as polymeric plasticizer.
However, in this research, Thermosetting unsaturated polyester is the resin that would be further discuss as it is one of the main material used for this research.
2.3 Thermosetting unsaturated polyester (PEs)
Thermosetting unsaturated polyester (PEs) is the most widely used resin system, particularly in the marine industry. PEs is among the more versatile synthetic polymer in that can be find widely in commercial use such as fibers, plastics, and coatings. There are approximately 2.2 billion kg/year of PEs used around the globe in the manufacture of a wide assortment of products such as sinks, shower stalls, pipes, gratings, and high performance components for boats, buses, trucks, trailers and automobiles. The majority of yachts, dinghies and workboats were built in polyester-based resin system because of the properties of the polyester which offers good mechanical properties, high processibility, and high chemical resistance, although they often undergo undesirably high volume shrinkage [34]. PEs has wide range of grades that are suitable for almost every manufacturing process such as hand lay-up, spray-up, casting, filling, resin transfer moulding, resin infusion, vacuum injection, continuous lamination, filament winding, and pultrusion. These versatile resins are composed of a family of materials that are made from aromatic acids which have benzene ring (isophthalic, terephthalic, and phthalic anhydride), reacted with a glycol (typically propylene glycol) or mixture of glycols, and maleic anhydride. The polymer thus generated is diluted with styrene monomer to adjust the viscosity and form the final resin composition [35].
The common methods of synthesizing simple esters as used to make polyesters. These include direct esterification (1), transesterification (2) and the reaction of alcohol with acyl chloride (3) or anhydrides (4) [37].
RCO2H + R'OH ↔ RCO2R' + H2O (1)
RCO2R" + R'OH ↔ RCO2R" + R'OH (2)
RCOCL + R'OH → RCO2R' + HCL (3)
(RCO)2O + R'OH → RCO2R' + RCO2H (4)
PEs is unsaturated polymer which capable of being cured from liquid state to solid state when subjected to the certain condition. These resins are styrene-based, flammable and catalyzed when combined with Methyl Ethyl Ketone Peroxide (MEKP) [34]. The unsaturated part of the polyester resin means that the backbone of the polymer itself has double bond. In chemistry, the reaction of base with acid produces salt. Similarly, in organic chemistry the reaction of an alcohol with organic acid will produces ester and water [15].
2.3.1 Types of thermosetting unsaturated polyester (PEs)
Thermosetting polyester can be divided into three difference types. These three types of resin differ from one another in term of their molecules arrangement, properties and their synthesis process. This sub chapter would discuss all three types of different thermosetting polyester.
Ortho-phthalic polyester
Ortho-phthalic polyester is known for its rigid, low reactivity, unpromoted, non-thioxotropic and low viscosity. This type of thermosetting polyester is commonly used for fibre reinforced plastic (FRP) and general purpose gel-coat products because this resin system is inexpensive and offer relatively good mechanical and thermal properties. The Ortho resins (made from phthalic anhydride) are the most inexpensive class of resin and are used when the structural and corrosion requirements of the part are low [36]. Recent research and development introduce the usage of 2-Methyl-1, 3-propanediol (trade name: MPDiol) to production of thermosetting polyester to improve in processing and its final properties. The synthesis of ortho-phthalic polyester using MPDiol is done via one pot one stage reaction which requires no added catalyst [35].
Figure 2.4: Ortho-phthalic polyester chemical structure
Iso-phthalic polyester
Iso-phthalic polyester is perfect for making dimensionally stable polyester mould, corrosive service part fabrication, and as a durable repair material for tank linings. Iso-phthalic polyester features a high molecular weight and crosslink density and offers good corrosion resistance and blister resistance in a variety of aqueous and acidic media [14]. The Iso-phthalic resins have very good structural and anti-corrosion properties but with their relatively high cost are used only in demanding applications [35]. The preparation of the Iso resin is a very straightforward one pot two stage reaction. With the relatively high reactivity of the Isophthalic acid and the MPDiol in the reaction, it is not necessary to add a catalyst. The PEs resin produced has low color and good cure reactivity. The thermal and mechanical properties of the polymer produced are as expected to exceed those conventional synthesis method [35].
Figure 2.5: Iso-phthalic (meta-phthalic) polyester chemical structure
Terephthalic polyester
The Terephthalic resins are currently made in small volumes and are considered a specialty resin. The main reason for the limited availability of Terephthalic resins is the difficulty in making these resins from terephthalic acid (TPA) and propylene glycol (PG). Even though PG is the predominant glycol used in producing all types of PEs's other glycols are used including neopentyl glycol (NPG), diethylene glycol (DEG), and ethylene glycol (EG). Each of these glycols when used in production of a PEs resin makes a contribution to the final set of polymer characteristics, including HDT, water uptake, strength, weatherability, and much more. When using MPDiol, preparation of the terephthalate resin is one pot two stage reaction. Terephthalic acid (TPA) is a very insoluble and non reactive material. Even when using a reactive glycol like MPDiol and taking advantage of its high boiling point, best results are obtained by using a small amount (approximately 100 ppm) of catalyst in the reaction. This esterification can be completed with no catalyst present, but addition of an organo tin oxide catalyst is the most efficient method to complete the reaction. The finished resin is a low color material with high curing reactivity. The thermal and mechanical properties of the PEs are excellent yielding a resin with properties that are comparable to those of an Iso type resin [35].
Figure 2.6: Terephthalic (para-phthalic) polyester chemical structure
2.3.2 Synthesizing and preparation of thermosetting unsaturated polyester (PEs)
Previous study shows vast method and process in synthesizing thermosetting polyester (PEs). In general chemistry, all the process have similar process which involving nucleophilic addition to carbonyl group, addition that help and facilitated by the polar nature of the carbon-oxygen double bond in which the ability of the carbonyl oxygen atom to be assume a formal negative charge, and the planar configuration of the trigonal carbon that minimizes steric interference [37]. The general mechanism may be written as in (5) where Y = OH, OR', CL or OCOR.
(5)
Less traditional methods are also used in polyester manufacturing such as acidolysis (6) which is a variation of transesterification used primarily to make aromatic polyester; the reaction of carboxylic acids with epoxides (7), nucleophilic displacement (8), and ring-opening reactions of cyclic esters will lead to formation of linear polyester [37].
RCO2H + R'CO2R" ↔ RCO2R" + R'CO2H (6)
RCO2H + CH2OCH2 → RCO2CH2CH2OH (7)
RCO2- + R'Br → RCO2R' + Br - (8)
Ring-opening polymerization of cyclic ester or lactones is about cationic, anionic and complex coordination initiators [38]. The process of cationic polymerization appears instantly to involve an intermediate acylium ion with each propagation step proceeding with acyl-oxygen bond cleavage (9) (Z+ is initiator). The mechanism of anionic polymerization of lactones is by nucleophilic addition to the carbonyl group followed by acyl-oxygen cleavage which is analogous to ordinary ester saponification in which lead to propagation through alkoxide ion ends (10) (B- is initiator). The example polymerization by using complex coordination catalysis is ring-opening polymerization of ε-caprolactone using dibutylzinc-triisobutyl-aluminum system (11).
(9)
(10)
(11)
In the other hand, hyperbranched polyester was discovered by previous study. Hyperbranched polyester has significantly good solubility, low viscosity and high degree of functionality. The high solubility derived from the dendritic branching which prevents close chain packing. Hyperbranched polyester can be synthesized in a single, uncontrolled, divergent polymerization reaction [39]. AxB-type monomers are required where x is greater than one.
Thermosetting unsaturated polyester (PEs) is also produced by using three-stages mechanism reaction of α,β-ethylenically unsaturated polyesters and monomers [41]. The production of PEs uses unsaturated dicarboxylic acids or anhydrides which is contains four or five carbon atom such as maleic or citraconic acid anhydride and fumaric acid. Hydrogenated phthalic acids or their anhydrides preferably used for the production of PEs are tetra- and hexahydrophthalic acid, methylated tetra- and hexahydrophthalic acid, hexachloroendomethylene tetrahydrophthalic acid or the anhydrides of above mentioned compound. Tetra- and hexahydrophthalic acid and their anhydrides are particularly preferred. The first stage of producing PEs involving direct esterification of tetrahydrophthalic acid anhydride, ethylene glycol, triethylene glycol, trimethylol diisocyanate and water. The components were heated under nitrogen 160 0C for two hours. The temperature was then increased to 230 0C for nine hours. During this period, the esterification process is carried out. The melt components were then cooled to 160 0C. The second stage is maleic acid anhydrides were added and the mixture was subsequently heated under nitrogen 200 0C for two hours and then cooled at 110 0C to form 65% by weight solution in styrene containing 003% by weight based on melt of di-ter-butyl quinine. The third stage is tolylene diisocynate was stirred into the styrene based solution cooled to 70 0C and the mixture was kept at 75 0C for three hours. After the process, the isocyanate radicals (NCO) content had fallen to 0.05% and the product was cooled to room temperature and produced 65% by weight of PEs [41].
The preparation of PEs from maleic acid anhydride and carboxylic acid was previously developed. This synthesis was said to produce 10% increase of polyester chains which lead to increasing of cross-linking density of the copolymerization-cured resin. According to the invention, polyesterification is initiated with the introduction of maleic acid anhydride into the reaction vessel in an amount less than the amount desired in the prescription by 1% to 30%. The preparation of PEs by polycondensation of maleic acid anhydride with saturated dicarboxylic acid and nearly or equivalent amount of diols by reserving 1% to 30% of the prescribed amount of maleic acid anhydride at the beginning of the reaction and subsequently adding it in total or in two or more increments to the reaction mixture during polyesterification at the stage when the acid number of molecular weight of the polyester has reached about 50 to 250 or 300 to 1500. After the subsequently added portions(s) of maleic acid anhydride, acting as catalyst has been introduced; the polyesterification is continued until the desired molecular weight is obtained [42].
