Effects of alkaline and acid solutions on glass/epoxy composites
Introduction
The interest in glass-reinforced plastics (GRP) components for highly corrosive environments, as an alternative to stainless or coated steel ones, is becoming common. They offer an attractive potential for reducing the weight, as consequence of their high specific strength and stiffness, competitive cost, good static and dynamic properties, good resistance to corrosion and simplified fabrication.
Nowadays, composite pipes are largely used in the chemical industry, building and infrastructures [1]. On the other hand the use of the GRP tanks in hydrometallurgical process plants or other components is becoming common [2]. However, GRP tanks and pipes may be degraded due to abrasion, change in brittleness or hardness, delamination or separation of fibre from matrix and degradation of matrix due to high speed flow of hard particles, cyclic loading and unloading of tanks, diffusion of acid solutions and so on [2].
The effect of alkaline and acid solutions on the GRP mechanical properties can be found in open literature, but the studies presented are not sufficient to establish a full knowledge of this subject. Mahmoud et al. [3] shows that the change in the flexural strength, hardness and Charpy impact resistance depends upon the type of acids and the period of immersion. For example, relatively to HCl, flexural strength was found to be insensitive until 30 days of immersion and, after this period, a decrease can be observed around 10%. In terms of hardness, they showed that the Barcol hardness of the polyester drops around 15% after 90 days of exposure. However, for external pipe surface the hardness was found to be insensitive until 30 days of immersion, while for internal surface this phenomenon was verified until 60 days. In terms of Charpy impact resistance a slight decrease, around 5%, can be observed until 60 days of immersion and in last 30 days (between 60 and 90 days) a significant drop of 10% occurs. Combining the HCl effect with temperature, all properties above mentioned decreased significantly. Polyester and bisphenol A epoxy vinyl ester resins were exposed to two different acidic solutions (1M H2SO4, Co spent electrolyte), at two different temperatures (25 °C, 75 °C) and for two exposure durations (1 week, 4 weeks) by Banna et al. [2]. They concluded that the polyester resin had lower modulus values when exposed to higher temperature solutions or higher exposure duration compared to the bisphenol A epoxy vinyl ester resin. For both resins the average hardness increased after 2 weeks of exposition and then decreased after 4 weeks exposure (but still higher than the unexposed). Finally, the microstructure of the polyester degraded more under acid and higher temperature exposure as indicated by increased surface roughness, cracks and diffusion of sulphur into the cracks. On the other hand, Stamenovic et al. [1] studied the effect of alkaline and acid solutions on the tensile properties of glass-polyester composites. They concluded that the alkaline solution decreases the tensile properties (ultimate tensile strength and modulus) and this tendency increases with the pH value. Concerning the acid solutions, they increase the tensile properties and this tendency was more relevant when the pH value decreases. For both solutions, Stamenovic et al. [1] concluded that the changes observed on the tensile properties are proportional with the exposure time (number of days in liquid). The effects of sulphuric acid concentration and the sequential layup of glass fibre reinforcements on the diffusion behaviour glass/epoxy laminates were studied by Pai et al. [4]. The results showed that composite specimens with chopped strand mat as the skins layers exhibited higher weight gain than those with woven roving mat as the skin layers. Material degradation is more pronounced with the increase of sulphuric acid concentration, which can be explained by hydrolytic dissolving of the matrix in contact with this acid [1], [4]. Degradation studies were carried out in different solvents like 10% NaOH, 1 N HCl and 10% NaCl by Sindhu et al. [5] and its influence on mechanical properties was analysed. It was observed that the tensile strength and the modulus (E) increases in acid environments and decreases for the other solvents. Finally, several studies were performed on GRP under stress corrosion cracking conditions [6], [7], [8], [9], [10], [11]. According to Kawada and Srivastava [11] stress corrosion cracking in GRP occurs as a result of a combination of loads and exposure to a corrosive environment. Sharp cracks initiate and propagate through the material as a direct consequence of the weakening of the glass fibres by the acid. The strength of the fibre reduces dramatically as a result of diffusion of acid and chemical attack of the fibre surface at the crack tip, which causes a highly planar fracture with a much reduced failure stress.
The aim of this work is study the flexural and low velocity impact response of a glass fibre/epoxy composite after immersion in hydrochloric acid (HCl) and sodium hydroxide (NaOH). Intends to increase the knowledge of the material degradation by two different solutions, an acid and an alkaline solution, in terms of flexural and impact strength. The bending test was selected because, according to Banna et al. [2], is the most sensitive to the change of exposure conditions. On the other hand, impact damage is considered the primary cause of in-service delamination in composites, which are very dangerous because they have severe effects on the performance of those materials [12], [13], [14], [15], [16], [17]. This subject, low velocity impact associated with highly corrosive environments, is not reported in bibliography yet and the low velocity impact is the most serious problem, given the difficulty of its visual detection [18], [19].
Section snippets
Material and experimental procedure
Composite laminates were prepared in the laboratory from glass fibre Prepreg TEXIPREG® ET443 (EE190 ET443 Glass Fabric PREPREG from SEAL, Legnano, Italy) and processed in agreement with the manufacturer recommendations, using the autoclave/vacuum-bag moulding process. The laminates were manufactured with the stacking sequence [452, 902, −452, 02]s. The processing setup consisted of several steps: make the hermetic bag and apply 0.05 MPa vacuum; heat up to 125 °C at a 3–5 °C/min rate; apply a
Bending loads
The material degradation, in terms of static properties, was analysed by bending tests. Banna et al. [2] studied the degradation of composite materials using tensile and bending tests and they concluded that the last one (bending tests) was the most sensitive to the change of exposure conditions. Fig. 1 shows a representative example of the load versus flexural displacement curves for control samples and samples exposed at HCl and NaOH solutions during 36 days. These curves show, also, the
Conclusions
This work studied the flexural and low velocity impact response of a glass fibre/epoxy composite after immersion in hydrochloric acid (HCl) and sodium hydroxide (NaOH).
It was concluded that the corrosive environmental affects significantly the flexural strength and flexural modulus. The exposure time was determinant on the mechanical properties degradation. The alkaline solution shows to be more aggressive than the acid solution, promoting the lowest flexural properties. Complementary tests
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