Abstract – The incorporation of nanomaterials in the polymer matrix is considered to be a highly effective approach to enhance the mechanical properties of infill material. In this paper, nanocomposites of epoxy with 0.1 wt% graphene nanoplatelets (GnPs) were experimentally investigated to evaluate their behaviour regarding neat epoxy resin. The mechanical tests such as compressive test, tensile test, flexural test and lap shear test were conducted. GnPs were dispersed using a sonication process followed by three-roll mill method to ensure an optimum enhancement in the properties of polymer matrices can be achieved. The experimental results clearly show an improvement in the strength and Young’s modulus especially for tensile, flexural and lap shear test with additional of GnPs as nanofiller. The presence of GnPs as an additive has a significant reinforcement effect and has succeeded in increasing the ductility of the grout thus reducing its brittle behaviour. This proves that the performance of graphene-based grout is reliably expectable and capable to minimize sudden rupture.
Keywords: Infill material, epoxy grout, graphene nanoplatelets, three-roll mill
In the petrochemical industry, pipelines play a critical role in transporting crude oil and gas for both offshore and onshore operations. Carbon steel is a most suitable material for pipes due to its efficiency, cost-effective and safest ways for oil and gas transportation over a long distance 1–3. As in-service duration increases, these pipes are frequently exposed to severe environmental conditions, thus induce corrosion, cracking, and wear 4. For many reasons, a structural element might need strengthening or repair and pipelines are not exceptional and any of these reasons are equally applicable to pipelines although the primary reason is often corrosion. Existing steel pipelines may experience metal loss (internal or external) caused by erosion or corrosion damage mechanisms leading to a reduction of pipe thickness and subsequently decreases its strength 5.
The fibre-reinforced polymer (FRP) composites method is the most recent repair technique used for repairing pipes that have been corroded. Recently, fibre-reinforced polymer (FRP)-based repair systems are found to be a promising candidate for rehabilitation of corroded steel pipeline structures because of its high strength, stiffness, and corrosion resistance 6 and it has already been proven an effective tool for the repair of damaged structures 7–9. With improved innovations and technology, the pipeline industry benefited from the continued development of composite materials. The future trend will likely focus on optimizing the design of composite repair system and there are efforts in reducing or eliminating the used composite wrap in the pipeline repair system. This aforementioned goal can be achieved by strengthening the infill material of composite repair system as the effectiveness of pipeline repair systems largely depends on the performance of the grout 7, 10.
Previously, there are little efforts to improve infill material due to the function of the current infill material is only limited to fill the damaged section and provide a smooth surface for the composite wrap. However, infill material plays a key role in transferring the load from the pipe to the composite repair and increases the load resistance of the structure. It means that when the infill material fails to transfer the load, the attached fibres fail to reinforce the structure 11. Therefore, the additional filler can be used as a way to improve the infill material. Nevertheless, type of material to be selected as an additive is limited because the amount of infill material used in the composite repair system is in small quantity. Additionally, there are several factors that need to be considered to effectively improve the infill material properties by adding nanomaterial as filler. These factors include the dispersion of nanomaterial in the polymeric metric and the optimum nanomaterial contents needed as a function of the expected enhancement.
Graphene nanomaterial was selected as additional filler due to its superior properties. It is considered ideal reinforcing agents for polymers and they have been widely used to enhance the mechanical, thermal and electrical properties of epoxy polymers 12, 13. Recently, energy storage 14, 15, supercapacitor 16, 17, and composite coating 18 have become the most-mentioned topics regarding the uses of graphene and there has been a shift towards sensor and biosensor 12, 19, 20. The abovementioned statement is evidencing the growing potential of graphene-based material for distinctive applications. In addition to that, graphene has already shown promising results in the field of polymer composites. Previous studies have reported that graphene gave significant enhancement on the mechanical and thermal properties of reinforced metal matrix composites 21, 22 and polymeric composites 23, 24. These remarkable improved properties have made graphene nanomaterials a popular reinforcement candidates in improving the properties of existing materials to produce new high-performance composites.
Based on some available literature, it is possible to identify and modify the properties of infill materials which are compatible with other repair elements to enhance its properties. Previous research conducted by 25, 26 used graphene nanomaterial to enhance the properties of infill material in the composite repair system, but the results are not as expected as graphene is known to give a significant improvement in mechanical properties. This is probably due to inhomogeneous dispersion process and preparation technique. The proper dispersion can ensure the nanomaterials are able to enhance the properties of infill material. The unique nanocomposite effects can only be effective if the nanoparticles are well dispersed in the surrounding polymer matrix 27.
In the current practice of pipeline repair using composite material, infill material play a significant role as a load transfer medium for the pipe and outer shell of the composite. It is essential to characterize the mechanical properties of epoxy grouts as stand-alone material to determine their efficiency as infill materials in pipeline composite repair system. If the performance of infill material can be improved then it may increase repair efficiency and provide secondary protection to the pipeline system. Hence, this paper will study the potential of nanomaterial which is graphene nanoplatelets in improving the mechanical properties of infill material to be used in pipeline composite repair system.
II. EXPERIMENTAL WORK
In this research, commercially available epoxy grout was selected to be used based on a combination of modified epoxy resins and hardener. This epoxy resin is the most commonly used resin for grouting and filling in a construction application. Nano-based material that will be included as an additive or additional filler is graphene nanoplatelets (GnPs). The GnPs has an average thickness of approximately 0.68-3.41 nm and particle diameter is 1– 4 ?m with >99.5 wt% carbon content with the appearance of black/ grey powder.
B. Graphene nanoplatelets dispersion
A weighted amount of as-received GnPs was prepared at the desired concentrations. First, the GnPs were pre-dispersed in an acetone solution for 45 minutes using Hielscher ultrasound sonicator and were left to evaporate for 24 hours at room temperature.
After that, GnP was added to a weight amount of resin and were manually mixed using stirrer until the GnPs was completely dissolved. The epoxy/GnP mixture was further dispersed using a three roll mill calendering (EXAKT 80E Advanced Technologies GmbH) to achieved homogeneous dispersion as shown in Figure 1. The epoxy/GnP mixture is inserted into the gap between the feed roller and centre roller and transported to the third roller as shown in Figure 2. The dwell time of graphene suspension on the roll was approximately 1 minute while graphene was dispersed in the resin by enormous shear forces resulting from the rollers turning at a speed ratio of 9:3:1.
The calendaring process was applied for four consecutive times for each batch and the time required for each mill-rolling cycle was approximately 10 minutes. The detail parameters of three roll mill process such as the gap size between the roller and the speed (represent the lowest speed) were tabulated in Table 1. Figure 3 shows the outcome of the dispersion process using three-roll mill machine. The final product had the appearance of a homogeneous, well-dispersed mixture.