Vinyl Based Nano-composites - Term Paper Example

Vinyl based Nanocomposites Nanocomposites have been used in recent times for various structural and industrial applications. Extensive research into the modification of polymer materials through the introduction of nanoparticles has allowed for the production of materials for a wide variety of industrial applications and functional requirements. Various nanoparticles such as clay, silica and metals have been used for developing the nanostructures so as to enhance their mechanical, electrical, thermal, chemical and optical properties besides providing more stability through their use. The method of preparation of nanocomposites is also important for providing the final product with a desired set of properties. In this paper, a number of nanoparticles will be discussed along with their associated method of incorporation into the polymer matrix to produce the final nanocomposite. Likewise, the study will also discuss the preparation of a vinyl ester nanocomposite using Silicon Carbide as the nanoparticle to highlight many of its improved properties that are validated through related tests. Introduction The use of hybrids made from inorganic substances of a layered structure like clays has been a subject of elaborate research. However, the subject is experiencing a resurgence both in terms of research and industrial activity due to the numerous properties that nanocomposites stand to provide. Several variables associated with materials, that can be controlled, can have a profound influence both on the properties and the structure of the nanocomposite such as the kind of the clay used, the kind of pre-treatment, the polymer component chosen and the manner in which the nanocomposite incorporates the polymer. Likewise, the relation between the nanocomposite and the polymer material used is dictated by the processing method chosen and the purpose for production (whether the user is a special processor or integrated manufacturer). The next section provides a detailed overview of some of the materials used in nanocomposites today and the methods available to process them. Literature Review Clays Clays are often naturally found minerals and are thus exposed to varying degrees of constitutions in a natural manner. In fact, the purity associated with the clay can significantly affect the properties of the nanocomposite. Most clays are categorized as alumina-silicates that comprise a sheet based structure. These silicates consist of alumina (Octahedral) bonded with silica (Tetrahedral) in a number of combinations [2]. Consider the experiment by Beall and Tsipurski [13] who used a 1:2 arrangement between the octahedral and tetrahedral to produce smectite clays, which includes the commonly found montmorillonite. In certain cases, they sometimes found the aluminium in the crystal structure to have been replaced by metals like magnesium. The chemical composition of the material is important as the clay sheets contain a charge along the surface as well as the edges, with counter-ions present in the inter-layer spacing balancing this charge (Becker, 2005). The thickness of these platelets (layers) was found to be in the range of 1 nm, which provided high aspect ratios between 100-1500. The weight of the platelets is greater than typical polymers, which is often represented incorrectly when describing clay based nanocomposites. Further, studies by Fisher and Koster [14] on using layered minerals demonstrated that these platelets possess a certain degree of flexibility and have very large surface areas in the range of several hundred m2/gram. Most clays have also been found to possess ion exchange properties, providing them a vibrant hydrophilic property and making them incompatible with many of the polymer varieties. As such, one of the prerequisites for creating polymer-clay nanocomposites is to alter the polarity of the clay to make it organophillic. This has been achieved through an ion exchange process using an alkyl-ammonium ion. In the case of montmorillonite, the cations of sodium are exchanged by using ADA (12-aminododcanoic acid) (Ovidko, 2004). Na+-CLAY + HO2C-R-NH3+Cl- .HO2C-R-NH3+-CLAY + NaCl Kornmann [15], who conducted extensive studies on the synthesis and characteristics of nanocomposites, has attributed the ion exchange process as having a profound impact on the production of the nanocomposite. Although the procedures adopted so far have largely increased the manufacturing cost, the clay material is relatively cheaper without any restrictions on supply. Besides Montmorillonite, other varieties like hectorites and synthetic clays such as hydroalcite are also commonly used depending on the specific properties required. Synthetic processing The synthetic process of manufacturing a nanocomposite is based on the choice of the final material either as an intercalated or as an exfoliated hybrid. In the case of the former, the organic component is introduced in the gap between the clay’s layers in order to expand the spacing between the layers. Okamoto et al. [16] have further shown that the layers maintain a spatial relationship amongst them. When considering an exfoliated structure, the layers are completely separated and distributed everywhere within the organic matrix. There is also a third option involving dispersion of the clay particles (known as tactoids) throughout the polymer matrix, although this method represents a simple utility of clay as a filler material. Factors influencing clay-based hybrid formation In recent times, extensive studies by Lagaly [17] have been conducted identify the factors that influence the production of specific organo-clay hybrids explained above. As clay-based nanocomposites provide several improvements in various properties, it is also necessary to understand the effect on de-lamination of clay. These include the exchange capacity, polarity of the medium of reaction and the chemical properties of the cations within the layers. Through modifications of the surface polarity, these cations facilitate an introduction of polymer precursors into the interlayer space that is favourable from a thermodynamic perspective. The extent of de-lamination is also dependent on the polarity of the clay. Clays that are positively charged (like hydrotalcite), the modification of the cation is replaced by using an inexpensive anionic surfactant. A number of such modifications can be used based on numerous considerations such as the polymer type, interactions of the ion-dipole, coupling agents (silane) and block co-polymers. The intercalation of small molecules like dodecyl-pyrrolidone into the clay matrial is an example of ion dipole interactions. Morgan [3] proved that the displacement of the molecules driven entropically provides a way to introduce the polymer. Unwanted interactions of the edges with the polymer molecules can be overridden with silane coupling agents that modify such edges. These can be utilized with the organo-clay treated by the cation. This combination of clays with a polymer can be substituted by using a block co-polymer [3]. A block co-polymer includes two individual components, each of which is compatible with the clay and polymer molecules. The component compatible with clay is hydrophilic in nature while the one compatible with the polymer is a hydrophobic block. Care should be taken to restrict the block length to a length that is not too high, which guarantees a high output of exfoliation. Incorporation of the Polymer Gupta [4] has stated on the basis of elaborate studies polymer nanocomposites that the selection of the modified clay is essential to ensure that an effective level of penetration has been achieved by the polymer into the spacing between the layers of the clay, thereby producing either the intercalated or the exfoliated product. The development of the chemical technology in this context is undoubtedly in progress to identify newer methods. The incorporation can be done by using the polymer itself or through the monomer, giving the polymer-clay nanocomposite. The monomer method of incorporation has been established as the most successful technique by Haque et al. [18], although such systems come with certain limits of applicability. Polymer introduction is achieved through melt blending and solution blending. The former method relies on shear to de-laminate the clay and is less effective for in-situ processes in generating exfoliated nancomposites. A number of materials including thermo-plasts and thermo-sets have been used for incorporation into nanocomposites. These include Nylons, Epoxy-resins, EVA (Ethylene-vinyl acetate), Polyimides, and polyimides. Synthesis and properties of various nanocomposites The current section will discuss some of the commonly used particles to produce nanocompisites. Organic/Clay Nanocomposites The class of organic nanocomposites has been developed for the aviation industry which requires lightweight, transparent and durable materials for related applications. The products belonging to this class comprise organic hybrid matrices that ate shaped as platelets whose size is in the range of a few nanometres thick and measure several hundred nanometres in terms of length. Organic nanocomposites possess a high aspect ratio along with a large surface area. If dispersed properly within the polymer matrix at a weight ratio of 1:5, the material becomes suitable for making coatings and films which have a number of uses for industrial applications. In comparison to a natural polymer, analysis by Wang and Pinnavaia [19] has proven that an organic nanocomposite presents several major improvements in strengthness, tear, modulus, radiation, resistance to fire, gaseous permeability and a high optical transparency. The production of organic nanocomposites generally comprises three different steps [5]. In the first step, an organic matrix based resin is prepared and is an oligomer containing an alkoxy-silane and a cross-linked group such as phenul-ethynyl. Likewise, a clay solution prepared through the dispersion of the layered clay particles in a solvent. The substrate is then combined and subjected to ultrasound and high-shear mixing to achieve the required level f intercalation between the stacked layers. During the second step, the mixed clay solution is mixed with the organic resin solution and subjected to further shaking. The hydroxyl part of the organic resin reacts with the hydroxyl on the surfaces as well as the exfoliated edges of the clay particles, leading to the formation of hydrogen bonds that strengthen the exfoliation present besides the development of high shear within the material. The last stage is a film casting process, which begins by casting the material on a glass plate or any other smooth and dry surface. This spread solution is allowed to become dry to produce a tack free layer, which is further dried by passing heated air above it. This thermal process condenses the remaining silane groups, producing a network of exfoliated particles staked into layers. The material is further consolidated through additional cross-linking through the thermal process (Okamoto, 2009). It is also possible to manufacture oriented films through shear and fibre spinning methods. Carbon black nanocomposites As mentioned before, nanocomposites show remarkably different properties than bulk polymer due to the smaller filler and increase in the surface area. The works of Joseph [7] ascertain that properties of the composite can change dramatically based on the geometric shape, dispersion state, size of the particles and their distribution. Moreover, nanocomposites exhibit enhanced mechanical properties, permeability to water, hydrocarbons and water, increased resistance to corrosion and chemicals and special changes i electrical, magnetic and optical characteristics. Agarwal and Broutman [20] have studied the impact of nanofillers in polymer nanocomposites, wherein the glass transition and the relaxation tendencies of the polymer matrix have been studied thoroughly. In many of these cases, the surface conditions and dispersion of the nanoparticles also play crucial roles in the several properties of the nanocomposite. In the case of carbon black nanocomposites, [20] have discussed the added difficulty due to the involvement of the nanoparticles in the cross linking phenomenon, which changes the nature of curing and its results in comparison to nanocomposites generated using other materials. Epoxy resins, the materials used for nanocomposites of this category, are electrical insulators. To facilitate a dissipation of the electrostatic charge and provide anti-static properties to the material, [20] have used conductive nanoparticles like carbon are dispersed in the matrix. This renders the polymer nanocomposite conductive provided the filler concentration attains a critical value at which the electrical conductivity shows a remarkable improvement (referred to as the percolation threshold). Balancing the conductivity with the required mechanical properties is the biggest challenge in the case of using filled polymer nanocomposites. Metallic powder nanocomposites Polymer nanocomposites represent a section of hybrid materials that consist of a matrix made up of a polymer material and an inorganic compound that has at least one of its dimensions in the range of a nanometre. Since the discovery of nanoparticles in the 1970s, the synthesis of such particles from metals has become one of the active fields of material science given the wide range of properties that can be configured up to the nanometre level. Such nanomaterials are a hybrid composition between organic matter and transition metals. In some cases, researchers like Smith [8] have shown that metals can be substituted with rare oxides of metals such as Iron and Cerium. Metallic nanocomposites are unique as they provide relevant mechanical, electrical and chemical properties. The properties of polymer based composites have largely been influenced by the size, composition, shape and the concentration of the inorganic component within the matrix. Ratna [9] has reduced the particle size to the nano-level which had a profound influence on the macroscopic properties of the composite due to a breakdown of the mixture theory. This breakdown results due to the degree of interfacial zones that gain prominence relative to the bulk behaviour. Metallic nanocomposites find application in areas such as biosensors, optics, membranes and retardants. The hybrid in this class of nanocomposites is usually synthesized using water soluble polyaniline acid as the organic base and Fe3O4 in water as the inorganic component. The magnetic Fe3O4 has a diameter of 10-12 nm. Given the dispersible nature of the inorganic hybrid, they are suitable for coating glass and metal substrates. Vinyl Ester Nanocomposites A number of nanoparticles are used in the developing nanocomposites. These nanoparticles can include anything from silica and silica based clays to polymeric nano-fibres and tubes. Further, several organic compounds have been brought into use in recent years and have been adapted to various composite bases such as epoxy, polyesters and vinyl ester through the efforts of Onovo [10]. Amongst all such materials used, vinyl ester nanocomposites have gained special attention given their wide range of applicability. So far, most of the work on polymer nanocomposites involves layered silicon embedded within the polymer matrix and very little is known about reinforced composites. To demonstrate the special properties of Vinyl ester nanocomposites, the paper will consider a related production process as an example which will use ceramic materials with covalent properties like Silicon Carbide (SiC) as the nanoparticle due to its superior mechanical, thermal and chemical properties. The low viscosity and low cost associated with vinyl ester fibres coupled with their good curing levels at room temperature has led to an extensive use of this combination when designing such nanocomposites. The SiC nanoparticles described in this example have a diameter in the range of 30 nm. Significance of Vinyl Ester Resins Knauth [21] has used matrix material such as Derakane Momentum in the epoxy vinyl ester, which has an excellent strength due to a hybrid molecular structure. Knauth [21] has also attributed this choice due to the fact that vinyl esters are are resistant to most solvents and chemicals besides having good resistance to heat due to the aromatic rings present in their structure. The ester groups have two double carbon bonds (C=C) at the extremities of the Chain, which contributes to the high reactivity of the vinyl ester resin. Besides using Silicon Carbide as the nanoparticle, [21] has also used an ester functional silane, gamma-methaacryloxy-trimethoxy-silane (MPS), as a coupling agent, which facilitates the formation of a stable link between the organo-functional group of the ester and the hydrolysable groups, leading to silanol formation that bonds on the SiC surface. Usually, there are 3 reactive ends (silanols –Si-OH) per molecule, although the reactive sites on the SiC surface are not more than one for every MPS molecule. The bonding leads to a linkage of the form –Si-O-Si-, thereby exhibiting good wet strength and flexural characteristics. In short, substances such as MPS help bind the SiC to the vinyl ester matrix through a series of covalent bonds, resulting in a very strong interfacial bond. Thermal properties The figure below shows a micrograph prepared by Thomas [11] that demonstrates the distributrion of the SiC nanoparticles at different temperatures (100K and 250K). The figure indicates a good distribution of a industry grade vinyl ester nanocomposite. The readings were taken by [21] by heating the SiC nanoparticles in air and nitrogen to temperatures of 1000 degrees Celsius through a process known as a Thermogravimetric analysis (TGA). The TGA provides an insight into the level of moisture absorption at the SiC surface. When heated at 10°C/min, the vinyl ester nanocomposite showed a maximum reduction in weight at 200°C as shown below. The lost moisture was realized from the endothermic condensation of water from the Si-OH bonds and also from water absorbed by the hydrophilic surface of SiC. The results indicate that moisture increased the particl’s affinity and is difficult to remove once immersed into the organic agglomerates. [21] observed some weight loss beginning at around 600°C and attributed it to the oxidation of the carbon present. This shows the composites highly resistant nature in high temperatures. Viscosity Surface treatment is an important step in the production of vinyl ester nanocomposites. Without this step, Lamm [12] has observed that SiC nanoparticles have a larger viscosity over the SiC particles. Further, the viscosity of the SiC based vinyl ester attains the critical value of 1500 cP. The effect of the curing process can be evaluated through the Differential Scanning Calorimetry (DSC) method, which can project the kinetics of the curing processes. The figure below demonstrates that with rising SiC concentration, the exotherm peak shifts towards higher temperatures, indicating that the prensence of SiC nanoparticles slows down the reaction. Thorough analysis by Becker [1] has established the relationship between the viscosity of the matrix to be proportional to the particle concentration and inversely proportional to the particle size. The kinetic nature depends both on the filler type and the content of the system. SiC, due to its high surface energy and area, associates with the hydrophilic part of the vinyl ester molecule thereby reducing the mobility of the polymer. This produces more viscosity and delays the influence of curing. Dispersion The photographs shown below were taken from dispersion tests conducted by Joseph [7] and indicate the level of dispersion of the untreated SiC particles within the vinyl ester resin. The resolution of the photograph at the microscopic level was restricted by the wavelength of visible light (at 0.5 µm). The optical verification indicates that homogenous composition of the vinyl ester SiC composite is possible through sonication and extensive ultrasonic agitation. [7] also observed that addition of MPS as described previously led to better dispersion. The size of the average agglomerated particle was found to be 0.35 with MPS and 0.64 µm without MPS. The figure below shows the related differences between both versions. Flexural properties Similar studies and subsequent tests by Mouritz [5] indicate that the Young’s modulus of SiC is around 392-697 GPa, which means it is 100 times stiffer in comparison to vinyl ester resin. Further, the modulus increases with an increase in the SiC volume fraction. According to [5], changes in the interface and the quality of dispersion did not have any significant influence on the Young’s modulus for the material and the strength was enhanced with the addition of MPS. This increase is attributed to better dispersion and strong interfacial bonding between the SiC nanoparticle and the vinyl ester due to the MPS. Higher loading of particles led to larger and loosely bonded nanoparticle complexes, creating concentrations of stress and decreasing the strength. Further, in continuance with his research on SiC based nanocomposites, Becker [1] analyzed the fracture mechanism which showed a failure in the case of the clean resin upon reaching a critical value. The photographs below represent the fracture plane that contains hackle marks along the direction of propagation of the crack. The fracture patterns of the resin contain the SiC nanoparticles demonstrates a different mechanism altogether as shown. Shear deformation and the matrix crazing are also observed at the tip of the crack whereas the initiation site is located at the SiC sub-surface. The SiC nanoparticles can also be seen to be de-bonded from the vinyl ester based matrix. Conclusion The paper has considered a number of nanocomposites, related techniques of production and the various properties associated with such materials to highlight the several advantages that this class of products has got to offer to the industry. A nanocomposite is simply a better structured and highly bonded matrix of a polymer, whose properties have been improved through the addition of a filler particle known as a nanoparticle. The choice of a particular substance as a suitable nanoparticle is dependent on its size and chemical affinity with the host material. The study initially focused on clay based nanocomposites to highlight the role played by organic compounds in the bond formation that enhances several mechanical and electrical properties of the polymer material. Nanocomposites have a larger surface area than their regular counterparts and can thus be used for purposes of coating and as paints in the most effective manner. A number of nanoparticles have been studied as part of this paper, including clay, carbon black and metallic powder. A common aspect of nanocomposite production is that it involves a thorough process that must be duly followed in order to facilitate proper cross linking up to the nanometre level. It is this sheer ability of a nanocomposite to hold together in tougher conditions that allows it to be used for specific applications such as making films etc. The paper has also utilized the example of a vinyl ester nanocomposite with Silicon Carbide as the nanoparticle to demonstrate the various changes in the properties of the nanocomposite through a series of tests. Despite a high content of moisture, the resulting nanocomposite shows a high resistance to extreme temperatures and explains its suitability for use in high temperature environments. The concentration of the SiC has a direct impact on the viscosity and flexural properties of the composite signifying its importance in today’s industry that is becoming more and more sophisticated in terms of products and requirements. Keywords Nanocomposite Polymer Nanoparticle Silica Organic clay silicates silane hydrophillic smectite montmorillonite organophillic hectorites Entropy Polyimides Nanometre Silicon Carbide Vinyl Ester MPS Thermal Flexural Thermogravimetric analysis agglomerate viscosity Calorimetry Dispersion References 1. Becker (2005), Inorganic polymeric nanocomposites and membranes. New York: Springer. 2. Ovidko (2004), Mechanical properties of nanostructured materials and nanocomposites: symposium held December 1-5, 2003, Boston, Massachusetts, U.S.A. Volume 791 of Materials Research Society symposia proceedings. Materials Research Society. 3. Morgan (2007), Flame retardant polymer nanocomposites. London: Wiley. 4. Gupta (2009), Polymer Nanocomposites Handbook. London: CRC Press. 5. Mouritz (2006), Fire Properties of Polymer Composite Materials. New York: Springer. 6. Okamoto (20039), Polymer/Layered Silicate Nanocomposites. Berlin: Rapra Publishing. 7. Joseph (2006), Polymer nanocomposites: processing, characterization, and applications. New York: McGraw Hill. 8. Smith (1999), Structure and properties of vinyl ester nanocomposites . Cornell University. 9. Ratna (2007), Epoxy Composites: Impact Resistance and Flame Retardancy. Delhi: Rapra Publishing. 10. Onovo (2008), Vinyl ester nanocomposite syntactic-structural foams. Pittsburg University Press. 11. Thomas (2008), Polymer Nanocomposite Research Advances. London: Nova Publishers. 12. Lamm (2007), Flammability and mechanical properties of cast molded vinyl ester nanocomposites. Pittsburg State University. 13. Beall and Tsipurski (1998), Nanocomposites Produced Utilizing a Novel Clay Surface Modification. San Francisco. 14. Fisher, Gielgens, and Koster (1999), Nanocomposites from Polymers and Layered Minerals. Acta Polym. 15. Kornmann (2004), Synthesis and characteristics of Thermoset-Clay Nanocomposites. Found at URL: http://www.mb.luth.se/a_mpp/mpp-staff/Xavier.kommann/intoduction.pdf 16. Okamoto, Morita, and Hideyuki (2001), Polymer, New York: Wiley. 17. Lagaly (1999), Introduction:from clay mineral-polymer interactions to clay mineral – polymer nanocomposites. Applied Clay Sci. 18. Haque, Hossain, Dean and Shamsuzzoha (2002), S2-glass Fiber Reinforced Polymer Nanocomposites: Manufacturing, Structures, Thermal and Mechanical Properties. Journal of Composite Materials. 19. Wang and Pinnavaia (1998), Hybrid Organic-Inorganic nanocomposites: Exfoliation and Magadiite Nanolayers in an Elastomeric Epoxy. Chem. Mater. 20. Agarwal and Broutman (2005), Analysis and Performance of Fiber Composites. London: John Wiley & Sons. 21. Knauth (2007), Nanocomposites: ionic conducting materials and structural spectroscopies. New York: Springer. Read More