Optimization of the fabrication conditions and effects of multi-walled carbon nanotubes on the tensile properties of various glass fibers/unsaturated polyester resin composites

2.2.1 Matrix modification

According to our previous results (50), a higher tensile strength and higher dispersion quality can be obtained when mixing MWCNTs and UPR at a high temperature. In this study, MWCNTs were mixed with UPR by a Hot and Stir machine (HY-HS11, DS Global, Gyeonggi, South Korea) at 60°C and 2000 rpm for 1 h. Then, the mixing temperature was reduced to optimum initial curing temperature range. This step is very important because the curing time will be very short at a high temperature (Section 3.1), which may obstruct the fabrication procedure.

2.2.2 Composite structure fabrication

Generally, the composite laminate was fabricated with four layers of the fiber and matrix using a roller in a hand lay-up fabrication method. The ratio of fiber and matrix was kept as a constant (Table 1) to survey the effect of fiber changes, vacuum and adding MWCNTs. Here, the CSM weight fraction was 0.25 (25%) and the woven weight fraction was 0.5 (0.5%). Meanwhile, the neat or modified UPR was quickly mixed at an optimum hardener ratio (1 wt.%) for 30 s by hand. After fabrication, the curing process was started at a certain temperature (from 25 to 35°C) for 24 h. Then, an oven (Hanyang Scientific equipment, Seoul, South Korea) was used for the post-curing process at 80°C for 2 h to remove air voids and improve cross-linking of the matrix and fiber. Finally, five tensile specimens in rectangular shape were cut by a diamond cutter (The University of Ulsan, Ulsan, South Korea) for each plate (the length was 200 mm, the width was 20 mm, the thickness was varied in each plate) without a tab.

In the case of applying a vacuum, a bleeder (Jet Korea Aerospace Industry, Gyeoungsangnam, South Korea), peel ply (Airtech, CA, USA) and a vacuum bag (Airtech, CA, USA) were used for packing samples and mold after laminating. The high vacuum pump (W2V10, Woosung Automa, Gyeonggi, South Korea) was connected by a hose to assist fabrication process for 5 h to ensure UPR cured fully.

2.3.1 Curing temperature measurement

A thermometer (FLUKE 568, Fluke corporation, WA, USA) was set at the center of the mixture to monitor the exothermic temperature of the chemical reaction between UPR and MEKP. In this study, the initial temperature was investigated based on the exothermic temperature and curing time. Twenty grams of UPR was poured into a jar and then, the jar was moved to a refrigerator in this case, 10.9°C and an oven at other different temperatures (28.5, 35.2 and 45.5°C) for a few minutes to ensure a uniform temperature of the UPR. Next, 1 wt.% of MEKP was added into the jar and mixed for 30 s. Finally, a thermometer connected to a computer was used for data recording.

2.3.2 Density measurement

The densities of the pure UPR obtained at different initial curing temperatures and the composite samples were measured using an AND (GF-200, Precision Weighting Balances, MA, USA) apparatus at atmospheric pressure and 22°C.

2.3.3 Thermal gravimetric analysis (TGA)

The thermal characteristics were evaluated using a TGA Q50 (V20.13, TA instruments, DE, USA) in a nitrogen environment (40 ml/min balance purge flow). The temperature was maintained under isothermal conditions for 5 min before it was increased to 800°C at a rate of 10°C/min. Roughly 10 mg samples were prepared to evaluate the effects of the initial curing temperature, fiber and vacuum.

2.3.4 Tension test

The tension test was conducted using a universal testing machine (DTU-900MHN, Daekyung Tech and Testers, Incheon, South Korea) at a test speed of 2 mm/min according to ASTM D 3099. An extensometer with a 50 mm gauge length was used. The tensile properties were analyzed to evaluate the effects of the making conditions, fiber changes, fabrication method and MWCNTs on the composite structures.

3.1.1 Curing behavior of unsaturated polyester resin

Resin modification was chosen as a simple method to improve the mechanical properties of the composite structure. In fact, the forming procedure of the composite structure can be naturally described by curing of the main binder (polyester). As a result, the geometry of the structure can be generated completely at the finishing time of curing. Actually, the curing behavior of UPR can be strongly accelerated by many factors such as the hardener content, temperature and other conditions (3). There are several reports detailing curing process results (51), (52), (53), where 1 wt.% of hardener was demonstrated as the optimum value for UPR due to the higher compressive strength and lower exothermic temperature (50).

Figure 1 shows the curing behaviors of UPR for different initial temperatures. Generally, the viscosity of UPR increased during the chemical reaction between the polyester and the hardener while the temperature increased simultaneously. Gelation formation occurred until the mixture was converted fully to the solid stage. It is clear that the total curing time at higher initial curing temperatures is shorter than at lower temperatures. The maximum exothermic temperature generated was 165.5°C at an initial temperature of 45.5°C with the shortest time (594 s) to reach the maximum temperature. On the other hand, the lowest initial curing temperature (10.9°C) had the smallest exothermic temperature (124.8°C) and took the longest time (3561 s) to reach the maximum temperature. With initial temperatures of 28.5°C and 35.2°C, the exothermic temperatures were 132.6°C and 138.9°C and the times to reach these peaks were 1980 s and 1687 s, respectively. The differences of the exothermic temperatures and curing times were not significantly different for initial temperatures in the range of 28.5°C–35.2°C. In comparison with other results we can see that, lower temperature (30°C) indicated a lower rate of reaction (51), but higher tensile properties were received (53). Based on this, the reason for cooling down the mixture from the high temperature before pouring the hardener into UPR in Section 2.2.1 can be explained. Indeed, the exothermic temperature was slightly reduced in both cases of initial temperatures of 28.5°C and 35.2°C because the chemical reaction was not immediately held after mixing the hardener. Therefore, the mixture was automatically cooled down because the heat was transferred to the room temperature. However, in the case of an initial curing temperature of 10.9°C (lower than room temperature), the exothermic temperature nonlinearly increased at the beginning. After curing, the temperatures in all cases were reduced similarly to room temperature (25°C) after different times. This means that the cooling rate also changed depending on the initial curing temperature.

Figure 1:

Curing behavior for different initial temperatures.

In another study, Zhang et al. (54) found the maximum exothermic temperature and the time to reach the peak temperature of another thermosetting resin (epoxy) were approximately 175°C and 150 min, respectively. In comparison with the current result, epoxy has a much lower curing rate that means the solidification procedure is much longer than UPR. It can result in some limitations of fabrication methods using a vacuum for UPR such as vacuum bagging and resin infusion methods because the resin flow cannot transfer after the gelation time. Consequently, a vacuum should be applied as soon as possible to ensure UPR completely filled into fibers. It is also very important to consider thoroughly the curing time in mass production using UPR and a vacuum.

3.1.2 Density of unsaturated polyester resin

UPR is a thermosetting resin. Thus, theoretically, the volume of the sample should shrink after curing (55). This was confirmed in the value of the density of UPR obtained in this study. The samples from curing behavior investigation were reused to figure out density and TGA. In the liquid stage, the density of UPR is 1.15 (g/cm³) (Table 2) and it is changed to 1.22 (g/cm³) in the solid stage (Table 3). The same mass of UPR was obtained in both the liquid and solid stages but the density changed, which means the volume was smaller in the solid stage. Interestingly, for all initial curing temperatures, the density of UPR was almost the same. Thus, the volume shrinkage of polyester does not depend on the initial curing temperature.

3.1.3 Thermo-gravimetric evaluation of unsaturated polyester resin

There are some valuable information that can be provided by TGA measurements, such as the filler content of materials, composition of multi-component materials, decomposition kinetics, moisture and volatile content. Table 4 shows the behavior of UPR under elevated temperatures. It is clear that the degradation procedure was similar for the different samples. The degradation started at about 287°C with ~5.7% mass loss and most of the UPR was gone at around 541.2°C (~1.1% mass remained). The small amount of mass reduction before degradation may be due to water absorption and/or volatile contents, while the mass retention after degradation (up to 800°C) is possibly due to the impurities of UPR.

3.1.4 Tensile properties of composite structures

From Section 3.1.1 to Section 3.1.3, we have concluded that the optimum curing temperature should be started approximately in the range of 25–35°C. In addition, Belloul et al. (53) showed the different optimum temperatures of resin (30°C) and composite (40°C) based on their tensile properties. Practically, the initial curing temperatures were indicated by the probe of the thermocouple at the center of samples in the previous section that may be a little bit different from the surrounding temperatures. Therefore, composite structures were fabricated to verify the effect of initial curing temperature range on the tensile properties. A CSM/woven/CSM/woven sample was fabricated as the stacking sequence of the composite structure by hand lay-up. The initial curing temperatures were 22°C (MC1), 30°C (MC2) and 32°C (MC3). The tensile properties of materials are shown in Figure 2. It is clear that tensile properties of composite materials are also sensitive to environmental temperature. Higher tensile strength (20.88%) and elastic modulus (18.16%) were obtained when initial curing temperature rose from 22°C to 32°C.

Figure 2:

Tensile properties of CSM/woven/UPR for the different making conditions.

3.2.1 Thermo-gravimetric of composite structures

Glass fibers are the main reinforcement components for composite materials in this study. The CSM has random directions and shorter lengths of fibers, while woven has orthogonal structures and longer fiber lengths. Therefore, the thermal gravimetric analysis was carried out to examine the thermal resistance with the appearance of different fibers. Figure 3 (solid lines) indicates a higher thermal resistance of composite materials due to the addition of glass fibers. Similarly, Budai et al. (56) also reported that the decomposition of UPR is delayed by adding glass fibers. Besides, the degradation rates of the materials are described by the dashed lines. The woven structure was the toughest candidate in this fiber group, as demonstrated by its highest onset degradation temperature and the lowest rate of decomposition. That means the longer fiber and specific fiber structures in woven really affect on the thermal resistance of composite materials. In addition, the residual masses remaining at high temperatures (from 500 to 800°C) were different in each case. For example, at 700°C, the mass retentions of UPR, UPR/CSM, UPR/woven, and UPR/combination of fibers were about 1.27%, 23.42%, 46.51% and 33.83%, respectively. The parallel nature of both the solid and dashed lines in the high temperature range (500–800°C) also indicates good thermal stability of the fibers. The mass retention represents the exact fiber content of composite structures. In comparison to the input values in Table 1, the mass of the fibers was conserved, except for the small amounts of moisture and volatiles. It can be concluded that the chemical compositions of CSM and woven are almost similar and the thermal degradation is slightly dependent on the fiber structure and significantly dependent on the fiber concentration in composite materials.

Figure 3:

Effect of fibers on the thermo-gravimetric behavior of the composite structures.

3.2.2 Tensile properties of composite structures

According to the rule of mixtures (3), the mechanical properties of a fiber composite can be calculated based on volume fractions. Thus, the 2 times higher weight fraction of woven than CSM (Table 1) that can be estimated that mechanical properties of woven composite relatively are 2 times higher than CSM composite. In fact, the ultimate tensile strength and elastic modulus of CSM/UPR were 87.92 MPa and 7.09 GPa, respectively (Figure 4). Meanwhile, the tensile strength of woven/UPR was almost 2.8 times higher than that of the CSM composite, but Young’s modulus of the woven composite was only 17.84% higher than that of CSM/UPR. Even though both fibers have almost similar chemical compositions (Section 3.2.1), woven composite has much higher mechanical properties than CSM that can be concluded about the higher fiber length (57) and orthotropic fiber arrangement of woven.

Figure 4:

Tensile properties of composite structures with different of fiber components.

Practically, woven has higher surface density than CSM (Table 2), but there are some gaps on the surface of layer and fibers are bundled together (Figure 5). In comparison, woven may absorb less resin than CSM. In composite structures, CSM should be fabricated with a greater amount of UPR to ensure that the resin is fully filled into the fibers. As a consequence, woven fiber fraction is always higher than CSM that can result in a heavier product. In addition, woven has the native waviness surface that can be formed a rough surface composite structures. A composite laminate was fabricated by combining both of them as a CSM/woven/CSM/woven structure to obtain the best function of fibers. As a result, the combination of fibers can obtain acceptable tensile properties at Figure 4. The CSM/woven/CSM/woven composite had a 79.02% higher ultimate tensile strength and a 8.54% higher elastic modulus than the CSM composite.

Figure 5:

The different glass fiber types.

(A) CSM and (B) woven.

3.3.1 Density of composite structures

The vacuum was used after hand lay-up fabrication, which was know as the hybrid fabrication method (58). Specific structures were fabricated by this method to achieve higher density (lower void content) and mechanical properties. A vacuum is useful in isolating samples with the surrounding environment and removing the unnecessary resin from samples to bleeder, peel ply and even to the hose. In comparison to the compression molding method, a vacuum may have less of an effect on the mechanical properties but it is a simple, more flexible and convenient method with a variety of product geometries. In fact, the effect of a vacuum is strongly dependent on the polymerization process. A vacuum may not have much influence if it is applied after the transition formation period because the polymer cannot be transferred at much higher viscosity. Fortunately, the density of composite structures increased due to the longer curing time employed based on the optimum fabrication conditions. As can be observed in Figure 6, the density increased by 7.09%, 10.51% and 11.63% in CSM/UPR, woven/UPR and combination/UPR, respectively. The density of CSM and woven were 2.494 and 2.546 g/cm³ in the experiment. They are much higher than the density of composite structures and UPR. Therefore, the reason for increasing density in Figure 6 may be attributed to the removing of a significant amount of UPR.

Figure 6:

Effect of a vacuum on the density of composite structures.

3.3.2 Thermo-gravimetric behavior of the composite structures

The contents of the fibers in the composite structures with and without a vacuum are presented in Figure 7. It confirms that a vacuum has influenced on removing UPR in the composite structure from the TGA results. For instance, around the onset of degradation, a vacuum can reduce the mass loss by 1.41%, 2.18% and 2.49% for CSM, woven and fiber combination, respectively. Especially, more than twice the CMS and fiber combination contents were obtained, while 60.09% woven fiber was obtained with heating up to 800°C.

Figure 7:

Effect of a vacuum on the thermo-gravimetric behavior of the composite structures.

It also can be seen that the mass loss rates are lower under a vacuum following the slope of curves at Figure 7 that means the better thermal stability of composite structures. If the mass retention represents the input fiber weight fraction, then it is recommended that less of UPR can be used for composites fabrication under the certain vacuum conditions. The almost linear increment of mass retention from CSM, woven and their combination can emphasize that thermal resistance and decomposition of composite structure are mainly dependent on the utilized fiber ratios. The little higher UPR removing in CSM composite displayed the better effect from the vacuum pressure because woven is stiffer under the bundled form and UPR may be confined between woven gaps.

3.3.3 Tensile properties of composite structures

When a vacuum is applied, the vacuum flow can pull-out the extra UPR and the vacuum pressure can generate compression condition. As a consequence, the cross-sectional area of composite structures can be decreased. Therefore, the tensile properties of the composite structures could be increased proportionally in the elastic region (58), (59). Generally, the ultimate tensile strength and elastic modulus of composite structures were positively influenced by a vacuum (Figure 8A and B). Indeed, the tensile strengths of CSM/UPR, woven/UPR and combination/UPR increased from 87.92 to 125.36 MPa, from 351.67 to 402.96 MPa and from 190.26 to 258.14 MPa, respectively. Furthermore, the elastic modulus also increased by 38.63% in CSM/UPR, 24.31% in woven/UPR and 42.46% in combination/UPR. These results can be also attributed to the continuous vacuum pressure which can remove most of the flaws and voids of the samples.

Figure 8:

Effect of vacuum on the tensile properties of composite structures.

(A) Tensile strength and (B) elastic modulus.