Properties of cotton cloth

Cotton fabric (CF) reinforced geopolymer composites are fabricated with fibre loadings of 4.5, 6.2 and 8.3 wt%. Results show that flexural strength, flexural modulus, impact strength, hardness and fracture toughness are increased as the fibre content increased. The ultimate mechanical properties were achieved with a fibre content of 8.3 wt%. The effect of water absorption on mechanical and physical properties of CF reinforced geopolymer composites is also investigated. The magnitude of maximum water uptake and diffusion coefficient is increased with an increase in fibre content. Flexural strength, modulus, impact strength, hardness and fracture toughness values are decreased as a result of water absorption. Scanning electron microscopy (SEM) is used to characterise the microstructure and failure mechanisms of dry and wet cotton fibre reinforced geopolymer composites.

Keywords

• Geopolymer composites;
• Microstructures;
• Mechanical properties;
• Water absorption

1. Introduction

Geopolymers are aluminosilicate inorganic polymers formed by polymerisation of aluminosilicates with alkaline solutions. Geopolymers have several desirable attributes which include good mechanical properties and durability . They are environmentally friendly, being derived from natural materials, and because they can be prepared at room temperature they do not emit the high levels of carbon dioxide associated with the preparation of Portland cement and . However, despite their many desirable attributes such as relatively high strength, elastic modulus and low shrinkage, geopolymers suffer from brittle failure like most ceramics. This limitation may be readily overcome with fibre reinforcement as in high performance polymer–matrix composites. Hitherto, the most common fibre reinforcements used in geopolymer composites have been based on carbon, basalt, glass and polyvinyl alcohol fibres, , and .

Current concerns over the environment and climate change have given rise to an increasing interest in replacing the synthetic fibres currently used in geopolymer composites or other brittle matrices with natural plant fibres and . The advantages of natural plant fibres over traditional glass fibres are low density, low cost, biodegradability, acceptable specific properties, less wear during processing and low energy consumption during extraction. The wide variety of natural fibres available locally is an added benefit to manufacturers the composites and .

Natural fibres have a few disadvantages when used as reinforcements, such as higher moisture absorption which brings about dimensional changes thus leading to micro-cracking and poor thermal stability. The moisture absorption by the composites containing natural fibres had several adverse effects on their properties and affected their long-term performance. Water absorption can lead to swelling of the fibre, forming voids and micro-cracks at the fibre–matrix interface region which may result in a reduction of the mechanical properties and dimensional stability of composites, and . Several studies in the use of natural fibre reinforced polymeric composites have reported that water molecules act as a plasticiser agent in the composite material, which normally leads to a decrease in the mechanical properties of the composites after water absorption, and .

Moisture diffusion in composites may degrade mechanical properties by three different mechanisms and . The first mechanism involves the diffusion of water molecules inside the micro gaps between polymer chains. The second mechanism involves capillary transport into gaps and flaws at interfaces between fibre and matrix. The third mechanism involves swelling effects which propagate microcracks in the matrix. In general, moisture diffusion in a composite depends on factors such as volume of fibre, voids, viscosity of matrix, humidity and temperature.

In order to promote the wider use of such materials in high-performance applications, it is essential to consider the effect of moisture absorption and water uptake on their physical and mechanical properties. However, according to the best knowledge of authors, no research was reported about the effect of water absorption on the mechanical properties of cotton fabric (CF) reinforced geopolymer composites. In this research, CF-reinforced geopolymer composites with different fibre contents (4.5, 6.2 and 8.3 wt%) have been successfully fabricated. The effect of fibre content on the mechanical properties has been investigated in terms of flexural strength, modulus, impact strength, hardness and fracture toughness. The effect of water absorption on the mechanical properties of composites has also been studied as a function of fibre content. Scanning electron microscopy (SEM) has been used to investigate the morphology, micro-structure and failure mechanisms of CF-reinforced geopolymer composites.

2.1. Materials

CF of 30 cm × 7.5 cm was used as a reinforcing material for the fabrication of geopolymer composites. The chemical composition and the physical properties of CF are shown in and respectively . Low calcium fly-ash (ASTM class F), collected from the Collie power station in Western Australia, was used as the source material of the geopolymer matrix. The chemical compositions of fly-ash are shown in . The alkaline activator for geopolymerisation was a combination of sodium hydroxide solution and sodium silicate grade D solution. Sodium hydroxide flakes of 98% purity were used to prepare the sodium hydroxide solution. The chemical composition of sodium silicate solution was 14.7% Na2O, 29.4% SiO2 and 55.9% water by weight.

Table 1.

Chemical analysis of cotton.