Introduction
The above quote alludes to the pliability and the malleability of plastic. Plastic is defined by the online version of the Merriam-Webster Dictionary as a synthetic material which is comprised of a variety of organic polymers which can be moulded into a shape while soft, and then set into a rigid, yet slightly flexible form. Glass fibre reinforced plastic (GFRP) is plastic which has been reinforced or strengthened by the use of glass fibres. This composite material has been used in the construction of large-scale items such as luxury boats, missiles, and tanks, and small-scale items, such as tubs, fans, hoods and scrubbers [8]. GFRP have been the material of choice to design and construct such items because of it has been demonstrated to have significant properties such as indicated by (but not limited to) the following: chemical resistance, being lightweight, maintaining high levels of stiffness when force is being exerted. Indeed, it can be argued that although tensile forces can go beyond the tolerance levels of the fibres which would lead to the fibres themselves fracturing, and cause material failure, glass fibre reinforced plastic (GFRP) is an effective and durable composite material because glass fibres in thermoplastics can be used to complement certain design programs and it has been effective in the use of constructing large-scale items, such as luxury boats and chemical plants since it has the ability to resist deforming forces when the fibres of the polymers are parallel to the force that is being exerted on it.
Production of Composites (the Pultrusion Process)
Pultrusion is a manufacturing process which was invented by W. Brandt Goldsworthy in 1951. It is a manufacturing process which involves the production of continuous lengths or rolls of reinforced polymer structural shapes with constant cross sections [14, 8]. Raw materials are a liquid resin mixture, which contains resin, fillers and other additives. It may also contain flexible textile fibres, such as glass fibres [14, 8]. It is important that a continuous roll is made during this process which is pulled instead of pushed, as opposed to the extrusion process [14, 8]. The material goes through the tension roller before going through the resin impregnator and becomes soaked by the resin [14, 8]. It then passes through the die and heat source [14, 8]. The pull mechanism pulls it before it is set hard as a fibre reinforced polymer [8].
In standard extrusion process the reinforcement materials like glass fibres are woven or braided strands impregnated with resin [14, 8]. This is process is then most likely followed by a separate preforming system. The glass fibres are then pulled through a heated stationary die where the resin undergoes polymerisation [14, 8]. The impregnation is either done by pulling the glass fibre reinforced composite through a bath or by injecting resin into an injection chamber which is usually linked to the heated stationary die [14, 8].
According to Wolff [14], there have been recent advancements in the production of GFRP which enables GFRP to be produced within a “reduced overall development” time. Wolf describes the process as a thermoplastic pultrusion process which uses “commingled glass/polypropylene fibre roving” [14]. This process involves the addition of a “multi-filament extrusion process” to the glass manufacturing process [14].
Wolff [14] explains that the Glass Polyroving process is effective because the commingled glass permits the thermoplastic resin to be “predistributed” throughout the fibre matrix. In addition, this process enabled the reduction of preliminary costs and development time of including more “in-situ extrusion resin” delivery equipment to processing line [14]. Furthermore, the process allowed the glass fibre concentrations to be set at “60% and 75%” by weight [14].
Properties of GFRP (Advantages and Limitations)
There are many reasons why a designer or builder will choose GFRP to be implemented in their design. This composite material has various qualities or properties. Some of these qualities include the following: it has properties which allow it to be resistant to chemicals; it has stiffness; it allows the designer to design freely; it is lightweight; it has good electrical insulating properties and it has the ability to retain its dimensional stability across variety of temperature ranges [6].
Furthermore, most designers and builders appreciate the fact that the GFRP’s strength can be adjusted to complement a particular design program by the additives of composites such as Zinc Sulphide (ZnS) and Titanium Oxide (TiO2). For instance, Deogonda and Chalwa [5] have demonstrated that when ZnS and TiO2 are added as filler material “[t]ensile, [b]ending, and [i]mpact strength” then the strength of the GFRP increased. The researchers also noted that zinc sulphide filled composite demonstrated more “tensile load” when compared to unfilled and titanium oxide filled composites [5]. However, it was determined that the “impact toughness value” for GFRP composites filled with ZnS and TiO2 was less than “unfilled glass composites” [5]. This was the case since ZnS and TiO2 made the GFRP more brittle and harder [5].
The limitations of GFRP include not being able to withstand pressure or force which is exerted perpendicular to its fibres rather than parallel to them. Correia et al. [4], noted in their study the performance of GFRP profiles when placed in four different environments. These environments included: immersion in water at 20°C; condensation of water at 60°C; the exposure to the accelerated weathering QUV equipment; and the exposure to accelerated weathering Xenon-arc equipment. These researchers discovered that the GFRP used in infrastructural applications experienced significant strain when exposed to moisture and this situation was worsened when the temperature was increased [4]. On the other hand, the experiments involving the use of the QUV equipment and the Xenon-arc showed that UV radiation did not have a significant impact on the “mechanical properties” of the GFRP profile used in infrastructural applications [4].
Types of Glass Fibres
There are two main categories or classifications of glass fibres. These are the general-purpose fibres and premium special purpose fibres [13]. Wallenberger, Watson, and Li [13] explain that over ninety percent of glass fibres belong to the general purpose fibres category, and are a part of the “E-glass varieties.” The second category, the premium special purpose fibres, comprises of the following glass fibres: ECR-glass, hollow fibres, bicomponent fibres, and trilobal fibres. These premium special purpose fibres are less commonly used. The ones that are more widely used include the following: D-glass, C-glass, A-glass and S-class.
The D-glass has the best electrical properties but lacks in mechanical properties when compared to the S-glass and E-glass [12]. C-glass is high in properties which make it chemically resistant [12]. The A-glass has properties which allow it to be chemically resistant, but lacks electrical properties [12]. On the other hand, S-glass is primarily created to be used in mechanical applications [12].
The E-glass varieties, which belong in the general purpose category, are optimized to have high electrical insulating properties while being resistant to resistant to attacks from water [12]. Wallenberger, Watson, and Li [13] note that E-glass fibres are considered general purpose fibres because they offer “useful strength at low cost.”
Wallenberger, Watson, and Li [13] explain that there are two variants of the boron-containing E-glass which are commercially produced. The first variant has the following formula: SiO2-Al2O3-CaO-MgO [13]. The other boron-containing E-glass has the following chemical composition: SiO2-Al2O3-CaO [13]. Wallenberger, Watson, and Li [13] explain that there is another commercially produced boron-free E-glass variant which is derived from the formula used to produced the first boron-containing E-glass variant that was mentioned previously.
Applications and Uses of Glass Fibres
Glass fibre reinforced plastics are used in a variety of applications in various industries and fields. Designers and builders appreciate this composite material, and choose to use this material compared to most other types of composite materials. GFRP is used in the following contexts: structural applications (such as the construction of buildings and bridges); maritime applications (such as the building of luxury yachts, sailing yachts, and competition kayaks); and in automotive applications.
GFRP is the perfect material to be used in the construction of edifices or buildings. This is because it can be used in the strengthening of solid surfaces for kitchens and bathrooms as well as cast synthetic marble for these areas [6, 7]. It is also suitable for the construction of entryways, beams, columns, roof tiles as well as slabs of buildings and bridges [6, 7]. The material can be moulded into very complex or complicated shapes [7]. This material has been used to build Caesar’s Palace in Las Vegas, Nevada, Hollywood Casino in Louisiana, and the Great Wall of Waters at the Atlantis Resort in Bahamas.
Glass fibre is a better material than aluminium, for instance, for the creation of automotive engines. This is the case since the use of the glass fibres translates into a sixty percent reduction in weight of the vehicle, better aerodynamics and surface quality of the automobile, and a reduction in the use of components by combining parts and forms into simpler shapes [12, 6, 7].
GFRP is a popularly used material by in the maritime industry due to its capabilities of resisting water damage. Glass fibres have been used in the building of the hulls and decks of luxury yachts and boats [10, 6, 12]. It has also been used in the building of sailing kayaks, and sailing yachts [10, 6].
Aging Process of Glass Fibres in Water and Salt Water
According to a study conducted by Renaud and Greenwood [11], it was discovered that a brand of E-CR glass (Advantex® glass) and traditional E-glass were able to provide improved performance when exposed to environmental conditions involving the use of tap water, deionized water, and saltwater. Furthermore, the study concluded that the least aggressive material for the types of glass fibres used to conduct the study was the salt water [11]. This was then followed by tap water and deionized water [11].
Moreover, the study conducted by Boisseau, Davies, and Thiebaud [3] concluded that the “residual failure stresses” in bending decrease when the water retention increases when placed in salt or seawater environmental conditions. This is especially true for composites reinforced with E-glass fibre [3]. Boisseau, Davies, and Thiebaud [3] noted that HP-glass fibres showed improved performance compared to composites reinforced with E-glass fibres. However, the study documented a reduction in strength when placed in saltwater, and had impact on the aging process [3]. On the other hand, the aging process can be reversed after the glass fibres were dried [3].
Mechanical Characterization of the Glass Fibre Using Young’s Modulus to Tensile Modulus
There have been several studies which have been conducted to determine the mechanical characterization of glass fibres. These studies have used various methods, including Young's modulus, to determine the tensile modulus of glass fibres. One of these studies which would be examined carefully is one conducted by Pardini and Manhani [9]. This study arrived at the conclusion that the tensile strength for glass fibres is within the range of 1.83.0 GPa (average 2.38 GPa). In addition, there were two procedures used in the evaluation of the Young’s modulus. These were the ASTM Standard and the rigidity method. It was discovered by using the ASTM Standard that the Young’s modulus for glass fibres is ~76 GPa [9]. However, by using the rigidity method it was found that a Young’s modulus of 50 GPa was calculated [9]. Therefore, based on these findings, it can be concluded that glass fibres have an average tensile strength of the glass fibres although the rigidity method underestimates the tensile strength of glass fibres.
References
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3. Boisseau, A, Davies, P, Thiébaud, F, 2011 Sea water ageing of composites for ocean energy conversion systems: influence of glass fibre type on static behaviour. Published on-line in Applied Composite Materials, doi:10.1007/s10443-011-9219-6
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9. Pardini, LC & Manhani, LG 2002, “Influence of the testing gage length on the strength, Young's modulus and Weibull modulus of carbon fibres and glass fibres”, Materials Research, vol. 5 no. 4, pp. 411-420. Available from: SCIELO. [27 October 2014].
10. Plessis, H 2010, Fibreglass boats, 5th edn, Adlard Coles Nautical, London.
11. Renaud, C.M., Greenwood, M. E.: Effect of Glass Fibres and Environments on Long-Term Durability of GFRP Composites. Owens Corning Composites Publications (2009)
12. Types of Fibre Reinforcement, n.d. Available from: http://www.automateddynamics.com/article/thermoplastic-composite-basics/types-of-fiber-reinforcement. [27 October 2014].
13. Wallenberger, F, Watson, J, & Li, H, 2001, Glass Fibres, ASM International, Ohio.
14. Wolff, R 2011, Thermoplastic pultrusion process using commingled glass/polypropylene roving, Fibreglass Industries, Inc, Amsterdam, NY.
