3.2.1. Resin Control Panels
Figure 3 presents a load–deflection curve of the flexural test for both control resin panels. Noticeably, the behavior of these resins is different. For instance, at the same test condition, the CA panels are more ductile and did not have abrupt failure, whereas the CS panels were more brittle and exhibited abrupt failure. The CA panels continued flexing during the test until the test was manually stopped. After the load was removed, with some time, the shape of the CA panels returned to their nearly original shape (
Figure 4).
The flexural behavior of UPE resin is heavily reliant on the styrene content. Sanchez et al. (2000) reported that a 6% styrene content in UPE resin resulted in more ductile behavior when compared to UPE resin with 38% styrene content. Similar flexural behavior was observed from the current study where the CS panels have significantly higher modulus compared to the CA panels, which had no styrene content.
Similar to flexural behavior, the CS panels have better tensile properties than CA panels, with CS panels having four times higher tensile load at failure than CA panels (
Figure 5). Both resins have small elongation showing their brittle properties; however, CA panels have lower modulus and shorter elongation if compared to CS panels.
The lower mechanical properties of CA panels may be due to the higher molecular weight of AESO (≈1260 g/mol), which is approximately 12 times higher than styrene (104 g/mol). C=C bonds in AESO have large steric hindrance, which limits mobility as well as limits the potential for crosslinking with UPE [
20]. In contrast, the C=C bonds in styrene are more accessible for crosslinking, thus promoting a more rigid polymer structure. This limitation may result in lower crosslink density between AESO and UPE than styrene and UPE, thus contributing to the lower mechanical properties of the CA panels. Entanglements between polymer chains will increase the modulus, and greater molecular weight increases the probability of entanglement, thus suggesting the potential for AESO to form a more rigid polymer than styrene. The distance between entanglements or crosslinks will greatly affect the modulus, and this information is lacking in the current study. However, crosslinking is more effective than entanglements to enhance modulus given otherwise consistent polymer morphology [
32,
33]. The styrene–UPE system is far more likely to crosslink than the AESO–UPE system [
20].
3.2.2. Grape Cane Fiber-Reinforced UPE Resin Composites
Regardless of the resin type, there was no statistical difference between the panels in all cases of grape cane fiber-reinforced UPE resin composites, with the exception of AO tensile modulus. In general, among the three panel types, when the control samples were excluded, AW panels exhibited the highest mean in tensile and flexural strength, while SO panels displayed the highest mean in tensile and flexural moduli (
Table 3).
Assuming adequate adhesion, adding reinforcement material to a resin matrix typically improves the overall performance of the composite. In some cases, fiber addition increases the modulus but reduces the strength [
34]. The mechanical properties of AESO-based grape cane panels were improved compared to CA specimens, as displayed in
Table 4. For instance, the greatest improvement was observed in both tensile and flexural moduli for AO and AW panels (
Table 4). For instance, AO-N1 panels had a 625% and 633% increase for the tensile and flexural moduli, respectively, compared to control specimens; while AW-N1 panels had approximately 775% and 667% increase of tensile and flexural moduli, respectively, when compared to CA panels.
Comparing AO and AW panels without control samples shows that the AW panels had a slightly higher mean for both tensile and flexural properties (
Table 3); however, there was no statistical difference except for tensile modulus. No treatment concentration effects on the properties of these two panels were observed.
Figure 6 shows an example of load–deflection curves of the flexural and tensile tests for AESO-based panels with and without fiber reinforcement (CA, AW, AO). Based on this example, adding the whole bark fibers drastically improved the flexural properties. Whole bark fibers and outer bark fibers had a similar effect on the tensile properties.
The AW panels were fabricated from a mixture of the outer and inner bark of grape cane (
Figure 1). Although these fibers originated from the same area (bark area), they have distinctive morphological cell units and different chemistry [
29,
35,
36,
37]. Due to these differences, one would expect that the fibers may have different behavior under compression during composite fabrication. For instance, the combination of these fibers in the composite panels appears to complement each other in terms of having better packing. Better fiber packing in the AW panels may be due to the difference in the cross-section and the texture of these fibers [
37], resulting in better overall properties, depending on the NaOH treatment.
Adding grape cane fibers in styrene-based resin panels resulted in less or equal values for tensile and flexural properties, respectively (
Table 5). Styrene–UPE resin is a commercial product with a wide range of applications, including automotive, marine, and construction. It offers high mechanical properties and durability. The reduction in flexural properties upon the addition of grape cane fiber may be due to the weak interface adhesion between fiber and the resin. No treatment concentration effects were observed in SO panels, except for flexural strength (
Table 2). The finding showed that N1 may be sufficient to fabricate panels with better overall performances.
Figure 7a,b show examples of load–deformation curves for flexural and tensile tests of styrene-based panels (CS and SO). CS panels had approximately two times more deflection for the flexural test and had approximately the same elongation as the SO panels for the tensile test. According to
Table 2, when grape cane fibers were added to the styrene-based panels, the mechanical properties were comparable to the control panels. The findings show that using grape cane fibers in the composite fabrication reduces the resin content, but there is no improvement of properties.
In general, the grape cane composites had relatively similar mechanical performance to other PFRP products reported in the literature but lower performance than comparable GFRP products [
38,
39,
40]. Past studies have explored underutilized agricultural wastes, such as rice husks, bagasse, vakka, banana, and pineapple leaves [
40,
41,
42,
43] and other non-wood fibers, such as kenaf, bamboo, Napier grass fiber, and jute [
20,
39,
41,
44] in composite applications. Composites made with commercial non-wood fibers have higher overall composite properties when compared with fibers from underutilized agricultural wastes [
20,
41,
45]. For instance, untreated kenaf fiber-reinforced styrene–UPE resin panels had values of approximately 105 MPa and 4.5 GPa for tensile strength and tensile modulus, respectively [
20]. In comparison, alfa fiber composite panels had a tensile strength and tensile modulus of 17.5 MPa and 0.4 GPa, respectively [
13]. Another study reinforced the polyester resin with Napier grass fibers and reported the tensile strength and tensile modulus of 13 MPa and 1.5 GPa, respectively [
44]. For panels with glass fibers, the tensile strength and tensile modulus were 110 MPa and 6.5 GPa, respectively [
38]. Although the findings from the current study did not exceed the performance of GFRP or PFRP made from commercial non-wood fibers, when compared to PFRP from underutilized fibers, the grape cane fibers exhibited better properties.