00An experimental study on the mechanical properties of epoxy/silica micro-composite
Taha Kamal (MM-025), M. Talha Bin Yasin (MM-030) and Ali Shamroz (MM-044)
Department of Materials Engineering, NED University Of Engineering and Technology, Karachi
In this article, the mechanical properties of epoxy/silica microcomposites are discussed which were explored during the experiment. Thermo?mechanically durable industrial polymer nano-composites have great demand as structural components 9. Epoxy resin is a polar thermosetting polymer that is widely employed in different branches of industry and everyday life, due to their stable physical and chemical properties 7. Thermosetting polymer such as epoxy is a very crucial engineered adhesive 1. These micro-composites are used as insulators for outdoor applications. The use of additives is a very common method to enhance the properties of composite 10. Fillers tend to improve toughness and are able to work at higher temperatures 17. An epoxy resin and a hardener were used in the experiment along with silica particles as a filler material.
In order to determine the exact size of silica, laser particle analyzer was used. Hardness was measured with the help of Shore Hardness Tester. In order to measure the tensile strength, Universal Testing Machine (UTM) was used. For measuring the glass transition temperature (Tg), dynamic Differential Scanning Calorimetry (DSC) analysis was performed
Keywords: epoxy, hardener, silica, epoxy/silica composite, mechanical properties
Outdoor insulating bodies that were previously made from traditional ceramic materials like porcelain and glass are now being made from epoxy insulating materials because of their unique properties. Many researchers have proved that if the fillers are added in the sizes that are nano or micro into polymer matrix can incontrovertibly lift up the thermal and mechanical properties 15. It has been reported that silica sand nano and micro particles have significant properties when added to polymers and improve strength, flexibility and durability 16. The influence of silica particle loading on epoxy is evident in the results of electrical parameters like dielectric strength, arc resistance and tracking resistance 5. It has been found that particle volume, shape, and size all has influence on characteristics and properties of composites 13. They are light in weight, repellant to water and are pollution-resistant as compared to the ceramic materials. Generally, 65-80% of micro sized silica is induced into the epoxy matrix in order to satisfy the dimensional stability of heavy electrical equipment. Different ratio of these fillers and epoxy gives different properties. 14
Polymer composites gives higher strength and resistance to corrosion. These epoxy/silica microcomposites do not work well in polluted and humid environments. This is because the conductive contaminants form a layer on the insulating bodies and reduce the insulation performances such as surface leakage current, arcing and flashover. This problem can be solved by the use of water repellents, like silicone oils, that are introduced into the epoxy system to increase the hydrophobicity. Composites with increased mass density have higher probability of including structural imperfections such as voids, which degrade both the electrical and thermal properties 8. In this study, the mechanical properties of epoxy/silica microcomposites were determined. Specimens were prepared with the help of moulds in which micro-sized silica was mixed with epoxy resin and hardener. Mechanical and thermal properties of a polymer is identified. 12
Epoxy and hardener were used in the experiment. Micro-sized silica was used as inorganic filler. The net quantity of silica was 500 grams and was in the form of a white, crystalline powder. The average particle size according to the Laser Particle Analysis was 174 µm. In order to prepare the epoxy/silica microcomposite, silica (5, 10 and 15 wt %) was homogenously mixed with the epoxy and hardener.
Figure 1: Particle size analysis of silica particles
Three specimens were prepared with the following configurations. The amount of epoxy and hardener added were kept same in all samples in the ratio 1:0.8.
Table SEQ Table * ARABIC 1: Amount and wt % of silica added in epoxy matrix
Sample Weight percentage of silica /wt% Amount of silica added /g
A 5 25
B 10 50
C 15 75
First, measured volumes of epoxy and hardener were poured into a beaker and were evenly mixed by using a stirrer. Then micro-sized silica was weighed by using a weighing balance and then it was transferred into the beaker. Epoxy, curing agent i-e hardener and silica were uniformly blended together for about 10-15 minutes. A thick, yellow-coloured paste was formed.
This paste was then poured evenly into a rectangular, air-tight plastic box with dimensions as 220x160x85 mm. 48 hours were given for drying after which a hard, rigid sheet of microcomposite was produced with a size of 220x160x10 mm. The same technique was applied to prepare 3 different samples comprising of 5, 10 and 15 wt % silica. Hacksaw and hand grinder was used to cut further samples from the sheet, which were then grinded and finished, of standard sizes in order to perform different tests.
We performed mechanical and thermal tests on our micro sized silica based epoxy composites to check the properties upon different compositions of silica that are given below:
Dynamic Differential Scanning Calorimetry (DSC)
Tensile testing, also known as tension testing 2, is a fundamental materials science and engineering test in which a sample is subjected to a controlled tension until failure. Properties that are directly measured via a tensile test are ultimate tensile strength, breaking strength, maximum elongation and reduction in area 3.
Tensile test was performed under the recommendations of ASTM D638-14 with the help of Universal Testing Machine (UTM). The sample was flat with the dimensions of 160x20x5 mm. Equipment required for the test included the UTM, extensometer, tensile grips and software for data acquisition. The method was simple. First, the sample was loaded into the tensile grips. The extensometer was attached to the sample. Then, the grips were separated at a constant rate of speed. The time targeted from the beginning of test to the rupture should be from 30 seconds to 5 minutes. The test ends when the sample is fractured. A graph is generated the software.
The results of the test are as under:
Graph 1: Tensile test on varying compositions of silica. 0%, 5%, 10%, 15% respectively
On studying the tensile results it can be concluded that the epoxy composite containing 0% silica by weight has the maximum tensile strength as compared to 5%, 10% and 15% respectively while the 5% silica containing epoxy matrix has the highest yield strength than that of other compositions. The 0% micro sized silica based composite will show the maximum elongation throughout the test. The specimen containing 15% silica by wt is also showing elongation but that is still less than that of 0% and 5%, and also has the lowest yield strength than other compositions.
As the amount of silica increases in to the epoxy matrix the tensile strength decreases. The less the amount of silica in epoxy by weight the higher will be the tensile strength and vice versa.
As the silica content in the epoxy matrix increases the probability of porosity also increases that’s why the tensile strength is decreasing due to the increased porosity level in the structure of composite.
Table SEQ Table * ARABIC 2: Results for tensile strength obtained from figure 1
Silica content in epoxy matrix
by wt % Tensile strength
DYNAMIC DIFFERENTIAL SCANNING CALORIMETRY (DSC)
One of the most widely used techniques to measure glass transition temperatures (Tg), melting points (Tm), and heat capacities is Differential Scanning Calorimetry (DSC). This method uses two small identical sample holders; one contains the test sample in a sealed small aluminum pan and the other contains an empty reference pan. The temperatures of the two samples are measured with two identical platinum-resistance thermistors. The differential power needed to maintain the two pans at the same temperature during the heating cycle is then measured as a function of temperature.4
To measure the glass transition temperature DSC was performed as follows:
Cured epoxy was weighed exactly about 2.88-5 mg approx. The glass transition temperature, Tg, was measured at a heating rates of 10 °C/min, where all the samples were pre-cured at 100 °C.
The DSC curves obtained after the experiment are as under:
Graph 2: DSC curve for epoxy containing 5% silica
Graph 3: DSC curve for epoxy containing 10% silica
Graph 4: DSC curve for epoxy containing 15% silica
Graph 5: Combined DSC curves obtained for 5%, 10%, and 15% silica content
The glass transition temperature, Tg, of epoxy composite is calculated by tangent method. Two straight lines were drawn before and after the peak at the slope region in order to find out the transition temperature. The intersections point of the lines the glass transition temperature of the composite.
Graph 6: Glass transition temperatures obtained via tangent method
From graph 6, it can be easily interpreted that the epoxy containing 15% silica content has the highest glass transition temperature than that of 5% and 10% respectively. It can be seen that if we increase the silica content from 5% the tg for the epoxy composite decreases till 10% and then increases again on increasing the silica content.
Table 3: Glass transition temperature of epoxy containing 5%, 10% and 15% silica
Silica content in epoxy matrix
by wt % Glass transition temperature
Hardness is not a fundamental property of a material, yet hardness testing is considered a useful quality?control tool. Many properties are predicted from hardness values when combined with additional information such as alloy composition. The following is a list of such properties: resistance to abrasives or wear, resistance to plastic deformation, modulus of elasticity, yield strength, ductility, and fracture toughness. Some of these properties, such as yield strength, have numerical relationships with hardness values, whereas others such as fracture toughness are based on observations of cracks surrounding the indentations. 6
Hardness test was carried out by the Shore Hardness Tester under the recommendations of ASTM D2240. The geometry of sample was flat and the dimensions were 3x3x7 mm. D scale was used as the samples were hard. The indenter exerted a load of 4000 grams on the sample and the hardness was calculated when the indenter comes in contact with the sample firmly.
Table 4: Hardness results obtained from shore D testing
Silica content in epoxy matrix
by wt % Hardness
After the study of these results, the hardness of the specimens is quite contradicting. An epoxy resin with 0% silica has high hardness as compared to 5%, 10% and 15% silica. The reason behind this can be the porosity or agglomeration of silica particles in to the epoxy matrix. Hardness is inversely proportional to silica content. As the silica content increases the hardness decreases.
According to this investigational study, silica was used as filler in epoxy matrix. Epoxy, curing agent and silica (5, 10 and 15 wt %) with a size of 173 µm were all homogenized together to prepare a microcomposite. Then certain mechanical tests were performed. Maximum values of hardness, tensile strength and Tg were 70 shore D, 36.63 MPa, 63 °C respectively. The tensile strength of the composite is decreasing on increasing the silica content in to the matrix due to the presence of porosity because of high concentration of silica in the structure. The reason behind this is the strong intermolecular attractions between epoxy matrix and silica such as covalent and hydrogen bonding
Overall this study shows that the incorporation of silica as matrix filler results in improvement on mechanical properties, up to a limiting value. Above this limit decreased performance is expected as a result of silica agglomeration in the matrix .Thus, it is concluded that silica /epoxy microcomposites reinforced with silica give higher mechanical performance than conventional ceramic materials.
We, the students of Materials Engineering Department, would like to pay our sincere gratitude to the respected teacher, Miss Ambreen, the lab attendants and the whole laboratory staff, who guided us in completing this project. This project not only broaden our views related to composites but also helped us in acquiring sound knowledge in this field.