Matrix polymer: amine-cured epoxy
The epoxy resin and amine hardener were supplied by Hexion. Amine-cured epoxy was prepared by mixing reagents Epikote Resin RIMR135TM and Epikure Curing Agent RIMH137TM stoichiometrically, in a ratio of 100:30 by weight. The mixture was degassed in a vacuum chamber for 30 minutes to remove bubbles. The density of the polymer (ߩ) was 1.1 g/cm3.
Resin and hardener system consisted of the following compounds by composition: 63 wt%
Bisphenol A diglycidyl ether (DGEBA; CAS 1675-54-3; number average molecular weight ≤ 700); 14 wt% 1,6-hexanediol diglycidyl ether (HDDGE; CAS 16096-31-4); 14 wt% poly(oxypropylene)diamine (POPA; CAS 9046-10-0; molecular weight 230); and 9 wt% isophorondiamine (IPDA; CAS 2855-13-2).
Chemical structures of these compounds are shown in Figure 2.1.
Figure 2.1. Molecular structures of epoxy and hardener components: (A) DGEBA monomer; (B) DGEBA oligomer (n = 1–2); (C) HDDGE; (D) POPA; (E) IPDA.
The dogbone and rectangular sample steel molds, shown in Figure 2.2, were prepared using computer numerical control (CNC) machining for casting the epoxy into the required geometry.
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Figure 2.2. Steel molds for casting epoxy (left) dogbones and (right) rectangular samples.
Degassed resin was cast into the respective mold, followed by curing at room temperature for 24 hours and post-curing in an air oven at 80 °C for 16 hours. Full cure was achieved [3]. After samples were post-cured, the polymer samples were removed from the mold’s grooves and cut into the desired length with a vertical bandsaw. Sample preparation was followed by grinding with sandpaper (FEPA P60, grain size 269 μm). The resin molds allowed making rectangular DMTA (40 mm x 7 mm x 2 mm) and dogbone-shaped (200 mm x 30 mm x 2 mm with 20 mm width in the narrowest part) specimens according to ISO 6721 and ISO 527 [129,130]. In both cases, the prepared molds allowed sufficient width control within a tolerance of 5%. In order to get samples to the right thickness and enable sufficient thickness control, a metal holder for grinding was prepared and used, as shown in Figure 2.3.
The desired thickness was obtained using grinding and polishing machine Jean Wirtz PHOENIX 2000 and SiC grinding discs (FEPA P500, grain size 30 μm). Exicator grease was used to enable sufficient adhesion of the sample with the holder. The sufficient thickness control, correct length and width were ensured within a 5% tolerance. Dogbone-shaped epoxy samples used in static tension and fatigue tests were prepared in a similar way.
The geometry of dogbone specimens equipped with Tokyo Sokki Kenkyujo strain gauges (gauge length of 6 mm) are shown in Figure 2.4. The specified dimensions from ISO 6721 and ISO 527 [129,130]
were achieved within 5% tolerance. The placement of strain gauges as shown in Figure 2.4 allowed to measure strains in both the direction of the applied load and the direction normal to it, thus enabling the calculation of Poisson’s ratio.
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Figure 2.3. Steel sample holder for grinding rectangular epoxy samples.
Figure 2.4. Geometry of dogbone specimens used for static tensile and fatigue tests. The placement of strain gauges is indicated.
Reinforcement material: R-glass fibers
A typical glass fiber used for marine and oil & gas applications was selected. Boron-free and fluorine-free high strength, high modulus 3B HiPer-TexTM W2020 R-glass fiber bundles and stitch-bonded mats were used. These are classified as high-strength, high modulus R-glass (defined by an international standard ISO 2078 [131]). The material was the same in both cases (bundles and mats) and possessed the same properties. An average fiber diameter was 17 ± 2 μm [132]. The density of glass (ߩ) was 2.54 g/cm3 [132]. A single bundle had about 4098 fibers [52]. The specific surface area of glass fibers was determined to be 0.09 m2/g from geometrical considerations as a product of number, circumference and length of the fibers [52]. Specific surface area determined with Brunauer-Emmett-Teller (BET) of sized and bare glass fibers was 0.180 and 0.084 m2/g, respectively [22]. Bare glass fibers were obtained by desizing glass fibers via heat cleaning. It should also be noted, that the heat cleaning might have had an effect on the density and the chemical nature of the surface layer of glass fibers, which could affect the initial dissolution of the desized glass fibers (this would affect ONLY the bare fibers). R-glass fiber bundles were used for dissolution experiments and fiber bundle tensile tests, while R-glass fiber mats
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were used for making the laminates. A typical glass fiber Young’s modulus value was taken from literature (72.4 GPa) [47]. This value was used throughout this work. However, later it was found that the modulus of the studied R-glass was slightly higher (86 – 89 GPa), according to the most recent datasheet by 3B [133]. This, however, did not affect the results significantly and did not change the conclusions at all. All fibers used throughout this work were sized, unless otherwise stated, and are shown in Figures 2.5 and 2.6.
Figure 2.5. R-glass fiber bundles.
Figure 2.6. Micrograph of R-glass fiber bundles taken with a digital microscope Keyence VHX6000.
17 Sizing & the sizing-rich composite interphase
The sizing is a multi-component coating that results in the formation of the composite interphase during the manufacture of GFRPs [11]. This microconstituent has a proprietary composition. However, it is known that typical sizings consist of about five various chemicals [76,77]. Furthermore, it is known, that the sizing contains an organofunctional silane commonly referred to as a coupling agent [78-80].
This class of chemicals can be considered the most important in the glass fiber sizing, as it is the main component that promotes adhesion and stress-transfer between the polymer matrix and the fiber [11].
It also provides improvements in interphase strength and hygrothermal resistance of the composite interphase [80,133]. The silane coupling agents have the general structure [X-Si(-O-R)3] where R is a methyl or ethyl group and X is a reactive group in respect to the polymer. When applied to fibers, it is first hydrolysed to a silanol in presence of water. It is unstable and further condenses onto the fibers by producing a siloxane network, which then partially becomes covalently bonded to the glass fiber surface. During the composite manufacture, the X reactive groups of the silane may still be available to react with the thermosetting polymer, leading to a strong network bridging between the fiber and the matrix polymer [11]. The most common coupling agents are silane compounds [70]. According to a size formulation patent review by Thomason and specifically a patent EP2540683A1 by Piret, Masson and Luc of 3B, the coupling agent in the studied W2020 sizing was an epoxysilane [76,77]. Usually sizings contain about 10 wt% of the coupling agent [88].
The composition of the sizing also consists of a number of multi-purpose components, such as a film former which, holds the filaments together in a strand and protects the filaments from damage through fiber-fiber contact. Film formers are as closely compatible to the polymer matrix as possible.
Epoxies, such as in this case [70], are very common film formers [78]. Usually sizings contain about 70-80 wt% of the film former [88].
The sizing may also contain cationic or non-ionic lubricants, that reduce fiber-fiber abrasion, or other additives, such as antistatic agents, emulsifiers, chopping aids, wetting agents or surfactants, and antioxidants [11].
The exact composition of the sizing used in this study was not known, but based on technical details on the given R-glass fibers elsewhere [70], it is assumed that the sizing is based on the general characteristics described above. The results obtained are compatible with this assumption.
Glass fiber-reinforced composite laminates
Composite laminates were prepared via vacuum-assisted resin transfer molding (VARTM) using the same curing and post-curing procedure as for the polymer. The constituent glass fibers and matrix polymer materials were the same, as described before. The composite laminates were cut into rectangular bars and subsequently into composite plates with dimensions 20 mm x 20 mm x 1 mm (Papers I, IV and VI) and 50 mm x 50 mm x 1.5 mm (Paper VIII) with fibers oriented parallel (C1 plates) and normal (C3 plates) to the large face of the plate, respectively, as shown in Figure 2.7. The thickness was adjusted within 5% tolerance using grinding and polishing machine Jean Wirtz PHOENIX 2000 and SiC discs (FEPA P500, grain size 30 μm). The specified dimensions were achieved within 5%
tolerance.
Figure 2.7. Composite plate configurations: (left) C1 and (right) C3.
18 Reagents and other chemicals
The distilled water (resistivity 0.5-1.0 MΩ·cm) was used for conditioning of the epoxy, glass fibers and composite samples. It was produced using the water purification system Aquatron A4000. The pH of the distilled water was 5.650 ± 0.010, being lower than neutral due to dissolved CO2 from atmosphere in equilibrium.
IUPAC standard buffer solutions made by Radiometer analytical were used for studying the effect of pH on kinetics of GF and GFRP dissolution. The solutions of pH 1.679 ± 0.010, 4.005 ± 0.010, 5.650 ± 0.010, 7.000 ± 0.010 and 10.012 ± 0.010 were used. All of the samples were put dry into the water solutions, meaning that they were all saturated at respective pH and temperature.