Materials are more significant in our life more than we concern. Every segment in our life was related with materials, such as transportation, housing, clothing, communication, recreation, and food production. The most current research focused on nanocomposites. Materials can be classified into four groups, which were metals, ceramics, polymers, and composites. They were differentiated by their mechanical properties, electrical, magnetic optical and thermal behaviors, chemical stability and other physical properties such as density.
Many of our modern technologies require materials with unusual combinations of properties that cannot be met by solely one type of materials. Advanced materials usually are compiled by two different types of materials bonded together with one as matrix and another as fillers. Those types of materials are named as composite. There are several types of composite and the common one is polymer-matrix composites. In general, a composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases so that a better combination of properties is realized. According to this principle of combined action, better property combinations are fashioned by the judicious combination of two or more distinct materials. Polymer composite is progressing rapidly because of constant requests for stronger, lighter, and less expensive materials to meet the demands of industrial consumers. Commonly, the efficiency of reinforcing fillers in the matrix is inversely proportional to the size of fillers and proportional to the ratio of blending in the matrix. Therefore, the geometry of the filler is an important factor in the blending of composites. When the surface to volume ratio of the filler was greater, the blending result will become more effective. Looking into the theoretical point of view, smaller materials with a high aspect ratio will provide the most effective reinforcing fillers (Chou et al., 2002).
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Besides that, the dispersion of fillers in the polymer matrix must be taken into consideration. The more homogenous the blending and mono-layer dispersion is, the better is the reinforcing role and other benefits, such as barrier properties (Chou et al., 2002).
With the unusual properties of polymer composites, it has become an important commercial material. Materials with synergistic properties are selected to produce composites with adapted properties. However, there are limitations in optimizing the traditional micrometer-scale composite fillers properties. Lately, there are some new inventions to overcome the limitations of traditional micrometer-scale polymer composites, which is nanoscale filled polymer composites where the filler was < 100 nm in at least one dimension.
The research and development on nanofilled polymer increased substantially in recent year. Firstly, it is due to the unique combination of properties that observed from blending in some polymer nanocomposites. Besides that, the interaction of the nanocomposites was improved by the in-situ polymerization and it will enhance the effectiveness of the filler in composites. The significant development of nanoparticles in the chemical processing was another reason of further research in nanocomposites.
In recent years, practically everything became "nano" even materials which are around for more than a hundred years such as carbon black. Nanocomposites can be deliberate as solid structures with nanometer-scale dimensional repeat gap between the different parts that make up the structure. Generally, nanocomposites materials will have different mechanical, electrical, optical, electrochemical, catalytic, and structural properties compare to the specific component.
There are a lot of hard works in progress to develop high-performance materials that have potential properties for engineering applications such as aerospace materials, automobiles, etc. Although, ceramics were brittle, relatively stiff, strong in compression more than tension, hard, chemically inert and poor conductors of electricity and heat. But when in applications, they create many unsolved tribulations; among them were low facture toughness and strength, lack of mechanical properties at high temperatures, and poor resistance to creep, fatigue, and thermal shocks. In order to solve these problems, ceramics were shaped in nano-scaled to become filler in polymers.
It is important to take note that nanoparticulate loading will have significant property improvements with very low filler loading levels compare to traditional microparticle filler that required higher loading levels to achieve desired result. Besides that this change can result in significant weight reductions of materials to have similar capabilities, greater strength for similar structure dimensions and, for barrier applications, it will increase the barrier performance with similar material thickness (Oscar, 2007). Other than that, the sizes of micrometer-fillers are similar to the critical crack size and it will cause early failure while nanofillers are an order of smaller scale and it can prevent the early failure. Then the nanofillers lead to nanocomposites with enhanced ductility and toughness (Schadler et al, 2007).
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The most important improvements from the nanocomposites materials were mechanical property. Those improvements create major interest in numerous automotive and general/industrial applications. Other applications that nanocomposites have potential use are engine covers, door handles, and intake manifolds and timing belt covers. The general applications that currently being considered include usage as impellers and blades for vacuum cleaners, power tool housings, mower hoods and covers for portable electronic equipment such as mobile phones, pagers etc. For example, the clay nanocomposite already brought many innovations in technologically important domains for industrial world such as, packaging industry, automotive and aerospace industries (Bhuvanesh et al, 2006).
Commonly, low quantities of nanometric-sized particles were added into reinforced polymers to form polymer nanocomposites (Belanger, 2006). From the last 20 years, the commercial research organizations and academic laboratories started developing polymer nanocomposites (Hussain et al., 2006). Until nowadays, the commercial interest on nanocomposites has focused on thermoplastics and leak in thermosets polymer.
There are three common methods used to enhance polymers with nanofillers to produce nanocomposites, namely the melt compounding, in-situ polymerization and the solvent method. Melt compounding or processing is the simplest method and most widely used for produce composites. It was done all together when the nanofillers and polymer is being processed through an extruder, injection molder, or other processing machine. For in-situ polymerization, polymerization will be carried out with adding filler into liquid monomer. Lastly for solvent method, fillers are added into polymer solution by using solvent such as toluene, chloroform and acetonitrile to merge the polymer and filler molecules (Alyssa, 2005).
The nanocomposites was capable of achieving the unique combinations of properties and high potential for successful commercial development so the study of nanocomposites was increased dramatically. The small size of nanofillers can lead the unique properties of the particles themselves. Other than effect of size to the filler properties, it will lead to a large interfacial area between matrix and filler in the composites. The interfacial area will affect the degree of interaction between the filler and the polymer, and then it manipulates the materials properties. For that reason, it was very important to investigate on technique to control the interface before developing polymer nanocomposites.
2.3 Particulate Filled Thermoplastic Composite
There are a lot of polymeric materials that contain fillers in particle types. Examples of fillers are glass beads, silica flour, carbon nanotubes, and clay. Those fillers can be considered as discontinuous fibers which there have lengths that equivalent with their diameter. The main reasons for using fillers are (Xanthos, 1996; Rothon, 2002; Pukanszky, 2005):
Cost reduction of materials
Improving and controlling of processing characteristics
Control of thermal expansion
Improvement in mechanical properties
Ceramics are usually associated with "mixed" bonding - a combination of covalent, ionic, and sometimes metallic. Ceramics are the materials that have more tendencies to beoame brittle, relatively stiff, and hard, so it were stronger in compression compared to tension. Ceramics can be categorized as domestic ceramics, natural ceramics, engineering ceramics, glasses and glass ceramics, and electronic materials. Engineering ceramics are widely used as the cutting edges of tools because of their hardness and abrasion resistance (Auerkari, 1996).
Basically, engineering ceramics are pure compounds; in general there have oxides, nitrides, carbides, borides or silicides. In several types of engineering ceramics, alumina is one type of them. In the alumina itself, it's got eight types of shape and only one of them was stable, that is Î±-alumina. The most structural shapes are made from Î±-alumina which has a spherical. There have various grade of structural alumina based on different impurity concentrations and different levels of porosity (Liu et al., 2009).
The applications for alumina are well-known, and it includes ceramic materials, catalyst support for high temperature operations, polishing or lapping abrasive, and for a plurality of other uses (Liu et al., 2009). Alumina is used for those applications because of some important properties. Those properties are high compression strength and hardness; resistant to abrasion, thermal shock, and chemical attack in wide range of chemical at elevated temperatures; high thermal conductivity, degree of refractoriness, dielectric strength, and electrical resistively, transparent to microwave radio frequencies, low neutron cross section capture area, and raw material availability (Auerkari, 1996; Shehata et al., 2009).
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Alumina is attractive for engineering applications due to the chemical and thermal stability, good strength, thermal and electrical insulation characteristics and also the availability of materials (Auerkari, 1996). Some significant properties of alumina for structural application are its Young's modulus, tensile strength, compressive strength, and its strength at elevated temperatures.
There are a numbers of different types of alumina. Normally alumina was categorized according to their manufacturing process, crystal structure (hardness), and sizing or classification process. Before alumina formed the stable phase, which is alpha phase, alumina will go through transition aluminas (such as c, k, g, d, h, q). The stability of alumina was depending on the oxygen sublattice structure and the distribution of aluminium ions in tetrahedral and octahedral interstitial sites. For the transition alumina, oxygen atoms were arrange in face-centred cubic (fcc) arrangement while aluminium in both octahedral and tetrahedral sites. While as in alpha alumina, the oxygen sublattice is hexagonal-close-packed (hcp) structured with 2/3 of octahedral sites occupied with aluminium (Bournaza et al. 2009).
Within all the alumina, alpha phase is the hardest and densest form of alumina. Î±-alumina is buildup of the colorless spherical crystals with specific gravity about 4.0. It was insoluble in water and organic solvents but slightly in strong acids and bases. Besides that, Î³-alumina is composed of colorless crystals with specific gravity about 3.6. It can be transformed to Î±-alumina at temperature 1,100 - 1,200Â°C, so it is also considered as intermediate phases of alumina (Liu et al., 2009; Yan et al., 2010).
2.4 Low-Density Polyethylene
The primary component by volume in a polymer nanocomposite is the polymer matrix. Polymers have a wide range of properties that are heavily dependent upon their chemistry, molecular structure, and processing history. Besides that, polymer was widely available in market. Accordingly, all of these factors influence the definition of a general mechanism for polymer nanocomposite mechanical properties.
Polyethylene (PE) is a thermoplastic commodity made by the chemical industry and heavily used in consumer products. It has been discovered in 1933 and involved into two form which is low density polyethylene (LDPE) and high density polyethylene (HDPE). It was the most widely used polymer. The uniqueness of polyethylene compared to other polymers is their chemical resistance, durability, flexibility, and the most important is it cheap (Osswald and Menges, 2003). The limitation of polyethylene applications are their low strength and low heat resistance compared to 'engineering' polymers, and also their mechanical properties, which degrade when exposed to certain environments such as ultraviolet (UV) radiation from sunlight (Vasile and Pascu, 2005). Polyethylene can be classified into several categories based on density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight (Peacock, 2000).
LDPE also known as high pressure PE because it was produced at pressures ranging from 1,500 - 3,000 bars and temperature 150 - 250Â°C. Traditionally, it is defined as products with density between 0.915 - 0.935 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure well (Gregory, 2009; Peacock, 2000).
The mechanical properties of LDPE is somewhere between rigid polymers such and soft polymers. It has good toughness and pliability over a moderately wide temperature range. Besides that, it also was a viscoelastic material and it is ductile at temperature below 0Â°C. Due to the wieldable and machinable characteristics of LDPE, it has wide range of applications, such as shrink wrap, stretch film, coatings for paper milk cartons, hot and cold beverage cups, container lids, toys, and squeezable bottles (Vasile and Pascu, 2005).
2.5 Sol-gel method (Preparation of nano Î±-alumina)
There are two categories of method to synthesize nano alumina, namely the physical and chemical methods. Physical methods include mechanical milling, laser ablation, and flame spray. Chemical methods include sol-gel processing, solution combustion decomposition and vapour deposition (Tok et al., 2006).
Sol-gel method is one of the fastest develop existing chemistry. The main advantage of this method is it offers an alternative approach to conventional production of ceramics of various properties and applications (Administrator, 2004). Sol-gel is defined by the stable suspension of colloidal solid particles or polymers in a liquid 'sol' and porous, three-dimensional, continuous solid network surrounding a continuous liquid phases, 'gel' (Wright and Sommerdijk, 2001).
Sol-gel method consists of two steps. First is 'sol' then transformed to 'gel'. In ceramic synthesis, there are two different sol-gel routes and it depends on the gel structure. Those two sol-gel routes are: (a) particulate gel which uses a network of colloidal particles, and (b) polymeric gel that uses an array of polymeric chains. Due to the effectiveness of polymeric gel route for ceramic, this route was chosen to prepare the nano Î±-alumina.
The significant advantage of sol-gel processing of ceramic powders is that homogeneous compositions can be prepared at temperatures lower than required for conventional powder processes. Furthermore, the reactants used in sol-gel processing are available in very high purities, which allow the formation of high-purity powders of crystalline ceramics.
Commonly, the appropriate metal alkoxides hydrolyzed to oxides. There areseveral advantages of using metal alkoxides as precursors. The utmost is the alkoxides can be prepared easily and commercial available. Interaction of alkoxides with water yields precipitates of hydroxides, hydrates, and oxides. The precipitate particle sizes were usually range between 0.01 to 1 Î¼m, depending on the hydrolysis conditions. So, nanoparticles are easily produced in this method.
There are three basic steps in the sol-gel process. These are conversion of the sol to a gel by hydrolysis and condensation reactions. Then gel is then converted into oxide by drying and firing. After gelation, the gel usually consists of a weak interconnected network of small liquid-filled pores. Gelation time can vary from seconds to several days. In the gel form, it may contain only about 5 vol% of the oxide. Liquid in the gel was removed by drying and firing to form the particles. The dried gel is calcined and fully convereds to oxide. Adequate drying rate has to be developed to avoid cracking (Carter and Norton, 2007).
2.6 Ultrasonication of Nano alumina (Pre-preparation of filler)
Engineered nanoparticles (<100 nm) are synthesized to achieve unique physical and chemical properties and functionalities (Jiang et al., 2008). In the process synthesizing nanoparticles, it was very important to understand the colloidal properties in order to successfully manufacture it (Saltiel et al., 2004).
So, the main problem that has to be solved for the nanoparticles handling is their agglomeration. Nanoparticles tend to agglomerate between themselves and having size larger than nanometer scale. Material performance of particle will decrease when the particle agglomerated. When the particle was larger than nanometer scale, voids will formed and act as preferential sites for crack initiation and failure. Particles, especially in the nano regime tend to agglomerate, or cluster, due to the dominant intermolecular Van der Waals interactions between them. To solve the agglomeration problem, dispersion needs to be done.
Dispersion degree strongly influences the rheological properties of nanosized particles containing suspension and, as a consequence, the quality of the deposited film. There are several types of dispersion method and the most common was using ultrasonication. To produce good particle dispersion in a solution, a change in the inter-particle interfacial region by either chemical or physical methods is necessary. The chemical effect occurs when the particles contain adsorbed surfactants or macro-molecules to form electrostatic or steric interference within the inter-particles.
During the ultrasonication process, particles will be dispersed using ultrasonic waves and the particles will move in constant Brownian motion. When two particles approach each other, energies (zeta potential) exist between the particles will determine whether the particle will separate or agglomerate. Basically, attraction energy between the particles will cause the particle agglomeration and repulsive energy will lead particles apart (Lin, 2008). The appropriate sound waves that applied in ultrasonication process will attract electrostatic forces. This makes ultrasound an effective means for the dispersing and deagglomeration but also for the milling and fine grinding of micro-sized and sub micro-size particles.
The main difficulties of incorporate inorganic nanofillers in polyethylene are dispersion quality. This is due to the hydrophobic nature of polyethylene, which gives rise to a significant problem in enhancing adhesion between the 'hydrophilic' nanofiller and the matrix and filler. From Hu and Dai (2003), it was found that the anionic surfactant sodiul dodecyl sulfate (SDS) will modify the alumina surface to hydrophobic. With the adding of anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), at the pH ~7, it leads to an increase in the electrostatic repulsion force between particles, and reduced agglomeration (Wang et al., 2009).
2.7 Alumina determination with spectroscopy
Determination of alpha alumina content in alumina by X-ray diffraction (XRD) is one of the oldest industrial instrumental methods of analysis. It is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials. (PANalytical)
Aluminum forms sesqui-oxide with rhombohedral (corundum) structure. In this structure the metal atom is octahedrally coordinated with oxygen atoms.
Figure 2.1: The corundum structure (Al, small sphere; O, large sphere) (Feret et al., 2000).
Table 2.1: Alumina (corundum)-major XRD reflections (JCPDS-ICDD file no. 46-1212) (Feret et al., 2000.)
Cu K Î±1
Co K Î±2
The corundum structure can be visualized as layers of hexagonal close-packed oxygen atoms with small Al atoms in two-thirds of the octahedrally coordinated holes between the oxygen atoms. The atomic positions consist of 12 aluminum atoms and 18 oxygen atoms. The unit cell dimensions are: a = b = 4.7588 Å and c = 12.992 Å. The most important alpha reflections observed on a diffractogram are listed in Table 2.1.
Interference of alpha peaks by strong peaks belonging to a sub-alpha family was also considered. Shown in Table 2.2 are those peaks of sub-alpha compounds that have relative intensities above 50%. Almost all sub-alpha compounds can affect the 1.405 Å alpha peak. Beta alumina may also influence the 1.374 Å reflection of the alpha phase. Reflections other than beta have little chance of being present in highly calcined aluminas; hence their potential interference with alpha reflections is negligible.
Table 2.2: Interference of alpha peak (Å) by strong sub-alpha peaks (Å). (Feret et al., 2000.)
2.8 Properties Testing in Polymer Composite
Before working on materials, it is important to understand key properties. In the progress of research and development, several testing were necessary to identify the properties of materials that developed.
Almost any compound having covalent bonds, whether organic or inorganic, absorbs various frequencies of electromagnetic radiation in the infrared region of the electromagnetic spectrum. A molecule absorbs only selected frequencies of infrared radiation. The absorption of infrared radiation corresponds to energy changes with the order of 8 to 40 kJ/mole. Radiation in this energy range corresponds to the range encompassing the stretching and bending vibrational frequencies of the bonds on most covalent molecules. Since every type of bond has a different natural frequency of vibration, and since of the same type of bond in two different compounds are in two slightly different environments, no two molecules of different structure have exactly the same infrared absorption pattern, or infrared spectrum.
Mechanical properties are defined as measure of a material's ability to carry or resist mechanical forces or loads. Under applied forces, materials behavior measured with mechanical testing such as strength, hardness, toughness, elasticity, plasticity, brittleness, ductility, and malleability. Mechanical behavior of material reflects the relationship between its responses to an applied load.
Ultimate tensile properties are often used for the characterization of polymers and polymer composites. These characteristics are comparatively easy to measure, yet they furnish important information on the deformation and failure behavior of polymers. From the tensile testing, stress - strain diagram will produced, which it can used to determine other properties. The information that can be getting from tensile test is stress and strain, elastic modulus, yield strength, and some others.
Flexural strength also known as modulus of rupture, bend strength, or fracture strength. It's referred to the maximum bending stress in tension at failure or ability to resist deformation under load. For the material that didn't crack, the flexural stress is called the flexural yield strength. Usually flexure testing was done on relatively flexible materials such as polymers, wood and composites.
Impact testing of materials is a method to measure the ability of a sample to absorb energy before breaking. It also defined as ability of material to resist the sudden high rate loading. Impact performance is one of the major concerns in many polymeric material applications. Below or above some critical temperature, materials will experience difference types of failure and amounts of energy absorption. It was because material will become brittle or ductile at certain critical temperature. Brittle materials need little energy to start a crack, little more to propagate to failure. Highly ductile materials require high energy load to initiate and propagate the crack. In many applications, impact behavior of the materials is a critical measure of the service life and involves perplexing problems of product safety and liability (Söver et al. 2009).
From Lauke (2008), the addition of a rigid particle component to a macroscopic ductile polymer matrix leads to an embrittlement of the resulting composite. Then, the stiffness or Young's modulus can be improved by adding rigid inorganic particles since it was having higher stiffness compared to polymer matrices. However, strength strongly depends on the stress transfer between the particles and matrix. These composites show a tendency to a reduced necking behavior with increasing particle volume fraction compared to the bulk matrix. From Jordan et al. (2008), it reported that with addition of calcium carbonate nanoparticle in polypropylene matrix, the elastic modulus was increased compare to pure matrix. However, if fracture toughness is considered the behaviour is more complex and an increase of fracture toughness with increasing volume fraction of hard particles can be observed.
The mechanical properties of particulate-polymer composites depend strongly on the particle size, component properties, structure, particle-matrix interface adhesion and particle loading (Fu et al. 2008; Móczó and Pukánszky, 2008). Although the structure of particulate filled polymers is usually thought to be very simple, often structure related phenomena determine their properties. The most important structure related phenomena are homogeneity, the attrition of the filler or reinforcement, aggregation, and the orientation of anisotropic particles (Móczó and Pukánszky, 2008).
Interface interactions significantly influence the properties of heterogeneous polymer systems (Pukánszky, 1990). In reality the adhesion between the components is never zero, due to the secondary forces acting on the surface. The surface free energy of fillers and reinforcements usually will high and results strong interaction between filler and matrix. The tensile properties depend on the interfacial area and on the strength of the interaction. The characteristics of the matrix strongly influence the effect of the filler on composite properties. The reinforcing effect of the filler increases with decreasing matrix stiffness.
From the Móczó and Pukánszky, 2008, that nano-particle reinforced polymers show very similar energy dissipation mechanisms as particles on the micro-scale do. This is surprising because expecting the tininess of nano-particles may influence the polymer structure elements and should be able to cause complete new mechanisms or suppress micro-composites. Besides that, molecular forces play an important role, which cause attractions and repulsions of particles. These forces determine the network structure of nanoparticles and finally the fracture processes on that level (Móczó and Pukánszky, 2008).
Interfacial debonding determines the initiation and development of the damage process. However, after particle debonding certain regions of matrix may be under higher elongations causing synergism between debonding and higher contributions of energy dissipation due to plastic yielding compared to dissipation without previous debonding (Nielsen and Landel, 1994).
From the Fu et al. 2008, thermoplastics filled with rigid particulates reported a significant decrease of fracture toughness compared to the neat polymers. There are, however, several studies that show toughness increase with introduction of rigid particles in polypropylene and polyethylene. Enhancement of impact properties of some pseudo-ductile polymers by the introduction of inorganic particles has also been achieved (Fu et al. 2008).
There are some factors that will affect the impact strength of specimens, such as notches, temperature and orientation. The impact strength of a notched specimen is less than that of an unnotched one. It is because notches are stress concentration and most of the deformation occurs at the tip of notches. In the notches specimens, impact strength was calculated from energy to propagate the crack. While the unnotched specimens, the energy to initiate a crack need to take account. Temperature during usage was significant because the impact strength will increase along with the increment of temperature. Besides that, crystallinity generally decreases the impact strength of polymers that have a Tg well above the test temperature (Nielsen and Landel, 1994). The impact strength is largely determined by the dewetting and crazing phenomena. After the dewetting takes place, the nature of the stress concentration changes so as to tend to produce cracks or crazing at the equator of the particles.
Morphology is the study of form comprising shape, size, and structure. It is important for materials studies. For nanomaterials, morphology has special significance since form, in this case, dictates physical and chemical properties. Unlike bulk materials, properties of nanomaterials are strongly corrected to shape. This shape is attained during growth through a self-assembling process dictated by the interplay of size and molecular interactions. When the nanomaterials been process became nanocomposites, the properties are controlled not only by morphology of individual nanomaterials, but also by the nature of interactions, which, in turn, is determined by the distribution of the nanomaterials in matrix (Sanyal et al., 2002).
The most common morphology test was scanning electron microscopy (SEM). Normally, SEM images were taken to confirm the mode of failure of tensile and impact sample. From the images, the interaction between the filler and matrix can be observed.
Dynamic Mechanical Analysis (DMA)
DMA is an excellent tool to characterize the viscoelastic properties of polymer composites. A better understanding of the dynamic mechanical properties of the composite will help to define structure/property relationships and subsequently to relate these properties to the product final performance (Yang et al. 2007). In DMA, a mechanical modulus is determined as a function of temperature, frequency, and amplitudes. It is a technique that does not require a lot of specialized training to use for material characterization.
Basically, this technique used to evaluate the mechanical response of a polymer as it is deformed under periodic stress. Besides that it also characterize the viscoelastic and rheological properties of polymers such as modulus and strength, viscosity, damping characteristics, low and high temperature behavior, viscoelastic behavior, compliance, stress relaxation, creep, gelation, projection of material behavior, and polymer lifetime prediction (Treny and Duperray, 1997; Hanschke, 2001).
For polymer materials, DMA is widely used to obtain the loss factor, which is linked with damping properties. The information can obtained from DMA includes major transitions as well as secondary and tertiary transitions not readily identifiable by other methods. It also allows characterization of bulk properties directly affecting material performance.
Glass transition temperature (Tg) is an important tool used to modify physical properties of polymer molecules. The technique also measures the modulus (stiffness) and damping (energy dissipation) properties of materials as they are deformed under periodic stress. Such measurements provide quantitative and qualitative information about the performance of the materials. In polymer composites, interfaces between fillers and the matrix play a major role in damping. DMA can be used to evaluate elastomers, viscous thermoset liquids, composite coatings, and adhesives, and materials that exhibit time, frequency, and temperature effects on mechanical properties because of their viscoelastic behavior. From the Chrissafis et al. (2009), the stiffness of HDPE was increased with the adding of silica nanocomposite, which indicates that the rigid particle will enhance the polymer. It is believed that it due to increase in the stiffness of the matrix with the reinforcing effect imparted by the nanoparticles that allowed a greater degree of stress transfer at the interface (Vladimirov et al. 2006).
When using materials, normally it will involve with stress applications and temperature changes. So when selecting of materials, it was important to understand the thermal responses of materials to various temperatures. To understand the response of the material to the heat application, thermal properties of material need to determine. For example, when a solid is exposed to high temperature condition, it will absorb heat, and its temperature will increase and its dimensions may change. Heat energy in the materials will transport to the cooler region if the temperature gradients exist, and ultimately, the material may melt. In practical utilization of solids, heat capacity, thermal expansion, and thermal conductivity are the most important properties.
In polymer study, temperature changes are very important because polymer was highly sensitive to the temperature changes. At too high temperature, polymer will degrade and affect the properties of polymer. If at too low temperature, polymer will not flow and brittle. So, thermal study on polymeric material is important. The thermal and thermochemical properties of polymer were focus on treatments of crystallization, melting and glass-transition phenomena. Crystallization is the process by which, upon cooling, an ordered solid phase is produced from a liquid melt having a highly random molecular structure. The melting transformation is the reverse process of crystallization, and it occurs when the polymers heated. The glass-transition phenomenon occurs with amorphous or noncrystallizable polymers which, when cooled from a liquid melt, become rigid solids yet retain the disordered molecular arrangement that is characteristic of the liquid state; consequently, they may be considered frozen liquids (or amorphous solids). It's happen due to the reduction in motion of large segments of molecular chain with decreasing temperature.
Previously, thermal analysis defined as a group of techniques in which a physical property of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature program. Commonly, in determining the thermal analysis of a material, it will through four processes based on the temperature. With the increasing of the temperature, sample will through heating, melting, oxidation and then decomposition. The techniques of thermal analysis include differential scanning calorimetry (DSC) for measuring heat flow and thermogravimetric analysis (TGA) used for mass difference, and some others.
In thermogravimetric, the amount and rate of change in the weight of a sample measured as a function of temperature or time in a controlled atmosphere. Normally, the change in mass was due to sublimation, evaporation, decomposition, and chemical reaction, magnetic or electrical transformations. TGA measurement is performed in a defined atmosphere under inert or oxidizing conditions. During the test, the sample mass was measured continuously using a very sensitive electronic balance. Interfering buoyancy and drag force effects are compensated by blank curve subtraction.
Besides measuring the mass changes, TGA also is an ideal method for compositional analysis. In the curve of weight change versus heating or cooling of TGA, the composition of compound such as volatile compounds, polymers, fillers or ash can be analyzed. The temperature range in which the polymer pyrolyzes yields valuable qualitative information because decomposition kinetics differs from polymer to polymer.
Generally, TGA will provide information about the temperature and course of decomposition reactions in inert atmosphere as well as on combustion profiles in air or oxygen. For polyethylene, it decomposes into a large number of paraffinic and olefinic compounds without residue (Chrissafis et al. 2009).
The lifetime or shelf life of a polymer can be estimated from the kinetic data. When the TGA performed at different heating rates, the activation energy of a thermal event could be determined (Ozawa, 1965). As the heating rate increased, the thermogravimetric changes occurred at higher temperatures. With plotting the logarithm of the heating rate or scan speed against the reciprocal of the absolute temperature at the same conversion or weight loss percentage, the linear correction can be achieved. The slope was directly proportional to the activation energy and known constant. It was assumed that the initial thermogravimetric decomposition curve obeyed first-order kinetics. With using the relationships between temperature and conversion levels, the rate constants and pre-exponential factors could be calculated.
A differential scanning calorimeter measures the amount of energy absorbed or released by a sample as it is heated, cooled, or hold at a constant temperature and to a reference pan subjected to the same temperature program. Heat flow corresponds to a transmitted power and is therefore measured in milliwatts. The energy transmitted corresponds to the change in enthalpy change is said to be endothermic. When the sample releases energy, the process is said to be exothermic.
DSC measurements present information on thermal effects, which it can observed as peaks and characterized by their enthalpy change and temperature range. Examples of such thermal effects are melting, crystallization, solid-solid transitions, and chemical reactions. Besides that, from DSC specific heat capacity also can be obtained and this means thermal effects measured by DSC cause by a change of heat capacity. Sahebian et al. (2007) reported that nano-sized calcium carbonate in HDPE/CaCO3 nanocomposites presents a significant effect on crystallinity, crystallization rates, melting point and heat of melting of HDPE. Such a behavior, was also, observed by using SiO2 and MMT where the crystallization rates in both nanocomposites are higher, compared to neat HDPE, but the degree of crystallinity was reduced. So, it concludes that nanoparticles can decrease the degree of crystallinity even though it can increase the crystallization rate of materials (Chrissafis et al. 2009).
Since polymerization processes are mainly exothermic reactions in which each additional chain formation step generates a defined amount of heat, the reaction process can be monitored using DSC directly and continuously.