Investigation of synthetic and natural cork fiber laminate polymer composite bending characteristicsABSTRACT
Cork is becoming recognized as a sustainable and renewable material owing to its low density, superior acoustic and thermal insulation properties, energy absorption capabilities, and environmental friendliness. Sustainability has catalyzed innovation in kayak production and several other industries. This project sought to develop an environmentally sustainable "sandwich" composite material including a cork agglomerate core, natural fiber facings, and bio-based epoxy resin. This study investigated several material combinations and produced laminate plates with a cork core, flax fiber skins (100–500 g/m2), and bio-epoxy resin. The composite was meticulously produced using vacuum pressing to ensure its integrity and functionality. The laminate plates were then subjected to extensive testing for mechanical qualities. Bending tests demonstrated favorable force-displacement characteristics for composites. Impact testing at 2.9 m/s recorded energy absorption of up to 1.99 J. The evaluation of several specimen orientations enabled a comprehensive assessment of composite performance. In bending tests, 500 g/m2 flax fiber laminates achieved a maximum load of 65 N and a displacement of 5.5 mm, comparable to standard fiberglass composites which recorded 60 N and 4 mm. Impact tests demonstrated that the cork core effectively absorbs energy, making it appropriate for rigorous applications. Density correlations indicated that flax laminates with a mass of 500 g/m2 exhibited the highest strength.
Keywords:
Cork; Natural fibers; Sandwich composites; Mechanical testing; Bio-based materials
1. INTRODUCTION
The pursuit of sustainable, organic materials has transitioned across sectors as stakeholders and consumers strive for more eco-friendly options. Natural fiber reinforced polymer composites (FRPs) have emerged as a potential eco-friendly alternative, exhibiting mechanical qualities equivalent to synthetic FRPs, along with advantages such as recyclability, low density, and biocompatibility. While the utility of natural FRPs in engineering has been confirmed across multiple sectors, including automotive and construction, their use in the sporting goods and recreation industry remains notably restricted, despite the significant environmental impact of this sector. Automobile manufacturers have been prudent in maintaining the stringent mechanical standards of the sports sector; nevertheless, the rising demand and substantial improvements in enabling materials now provide a chance to reevaluate bio-based composites for sports applications [1]. Cork is a fascinating and adaptable natural material that is well suited to provide solutions for both sustainability and mechanical performance in creative sports goods applications. Cork is derived from cork oak trees, which are renewable resources. It is gathered during the bark stripping process of trees that are trimmed in cork forests. It exhibits advantageous properties, including a virtually insignificant Poisson’s ratio, strong damage resistance, exceptional viscoelastic energy dissipation, low density, acoustic and thermal insulation, fire resistance, and significant hydrophobicity. Corks used as wine stoppers demonstrate their dependability and durability, as shown by centuries of practical application. These characteristics significantly influence sports applications like kayaks, which endure dynamic loads, harsh weather conditions, and stringent safety standards. Utilizing it as a core material in sandwich constructions has resulted in enhanced mechanical capability due to the synergistic interaction with skins such as natural fibers [2]. Composites consist of high-stiffness and high-strength skins bonded to a lightweight core, providing comparable strength-to-weight ratios, energy absorption, and, crucially, dimensional stability. Certain cover materials consist of metals, glass fibers, and carbon; some skins use glass fibers and carbon, while cores include foams, honeycombs, and wood. Natural fibers derived from flax, hemp, and jute are plant-based, highly renewable, and biodegradable materials that have a specific strength comparable to that of glass fibers. Plant-derived epoxy resins exhibit reduced pollutant emissions compared to conventional petroleum-based epoxy systems. Utilizing the existing unused natural skins and resins in conjunction with cork cores may enable the creation of entirely recyclable sandwich composites that meet athletic performance standards [3]. While researchers have demonstrated the feasibility of utilizing natural sandwich composites in the automotive, aerospace, and construction industries, their application in the sporting goods sector has not garnered adequate attention, despite the significant environmental impact associated with the production of sporting goods. Sporting goods are one of the major global sectors, valued at $648 billion, and the rapid product cycles and disposability of these items have resulted in heightened waste and pollution. For example, the manufacture of athletic equipment yields an estimated 10 million tons of fiberglass globally each year, based on market analysis. The imperative mechanical requirements and safety considerations in the design of athletic equipment have restricted the use of natural alternatives deemed less robust than the established synthetic materials, such as fiberglass. These concepts must be surmounted by thorough mechanical validation [4].
From a sporting application standpoint, k Moses’ kayaks exemplify the pioneering use of natural composites, specifically cork floating elements, which are historically utilized for flotation, combined with sophisticated advanced composite manufacturing techniques. The worldwide manufacturing and sales rate of kayaks is increasing by over 5% annually, resulting in corresponding environmental problems associated with fiberglass kayaks. Furthermore, because to the semi-custom characteristics of kayak production, there exists the potential for the large-scale integration of novel materials without disrupting existing supply networks. Consequently, the development of a “drop-in” sustainable sandwich composite with mechanical performance equivalent to traditional kayak materials should significantly improve environmental sustainability in sports products [5]. The quest for sustainable materials has become more vital, particularly due to the heightened awareness of environmental conservation among consumers and enterprises. This work investigates the challenges associated with natural fiber-reinforced polymer composites (FRPs) in sports goods, with the objective of demonstrating that these composites possess mechanical properties comparable to those of typical synthetic FRPs. This research is wholly innovative as it investigates the feasibility of using cork, an exceptionally unique and sustainable material, to fabricate sandwich composites with natural fiber skins that demonstrate mechanical properties akin to those of traditional composites. The use of cork in sports products represents a significant paradigm shift from conventional materials, indicating a change in the environmental effect of the athletic industry. This study aims to explore the feasibility of using bio-based sandwich constructions including a cork core and natural fiber skins for the manufacture of sports goods. In applications such as kayaks, these composites are seen to provide sufficient mechanical strength, environmental compliance, and the safety criteria anticipated for sports equipment [6]. The objective of conducting thorough mechanical characterizations, including bending and impact evaluations, is to validate the capability of these natural composites and their potential to compete with fiberglass/epoxy composites if required. This work seeks to enhance knowledge in sustainable material solutions and to promote the adoption and expansion of sustainable materials in the sporting goods industry, yielding improved environmental and consumer benefits.
2. MATERIALS AND EXPERIMENTAL METHODS
The samples included six sandwich composite laminates using cork agglomerate cores, natural fiber facings, and an epoxy matrix. The material of major importance was 3 mm Core Cork NL10, with a density of 120 kg/m3, procured from Amorim Cork Composites. Skins used flax fiber textiles with a density range of 100–500 g/m2, woven from flax fiber and fiberglass with a density of 500 g/ m2, procured from Easy Composites. Other epoxy resins were Sika epoxy, 330 epoxy, and Super Sap CLR bio-based epoxy from Entropy Resins. Laminate plates measuring 500 × 500 mm were produced by a meticulous hand layup and vacuum bagging procedure. The cork core was engineered to match the plate thickness, but the fiber skins were created to exceed the dimensions of the plate significantly. The composition of cork consists of cells, which confer advantageous thermal conductivity and enough compressive strength, making it suitable for thermal insulation applications involving compressive stresses. Certain cork composites have compressive strength ranging from 1 to 26 MPa and flexural strength from 0.5 to 4.0 MPa. An effective penetration procedure included the consecutive application of an epoxy resin mixture onto the waxed mold. The roller was used to ensure the resin disperses evenly and equally throughout the cloth. The cork core was essential in generating a robust sandwich structure between the impregnated fiber skins. Ultimately, it was anticipated that the whole laminate assembly would be enveloped by a porous release fabric to regulate the curing process. This apparatus was subsequently evacuated meticulously at a vacuum pressure of 0.781 bar. Epoxy curing is an exothermic reaction, signifying that heat is generated during the curing process of the epoxy resin system. The curing of epoxy resin necessitates a temperature range of 72–74°F (22–24°C) and a relative humidity of 40–60%. A decrease of 1 degree below 72°F (22°C) diminishes the resin curing duration by 15–25%, contingent upon the resin’s mass. It is recommended to vacuum degas the combined resin and hardener to remove air bubbles generated during the mixing process. The combined epoxy must be subjected to a vacuum pressure not exceeding −80 kPa for about 10 minutes to get optimal results. At the moment of tipping, the epoxy may be vacuum degassed, then poured into the mold, and then placed in a pressure chamber for the whole curing duration. This aid minimizes any residual bubbles to sizes imperceptible to the human eye and ensures uniform filling of all mold holes. These may be referred to as epoxy formulations, in which the curing period under pressure varies with temperature. The final flood layer of epoxy requires a minimum drying period of 72 hours (3 days), particularly for substantial projects like epoxy resin tabletops or bar tops, and should be applied at a temperature of 75–80°F (24–27°C). Epoxy will continue to cure and achieve optimum strength and durability within two weeks at appropriate temperature.
These plates include various material compositions and construction options, serving as the foundation for the comprehensive assessment of their mechanical properties and performance metrics. The first process included using CAD software named SolidWorks to create the specimens’ designs. Once the design was completed, the CAD file could be immediately input into the software used by the machine. The requisite parameters in this program included a cutting speed of 2 m/min and a cutting pressure of 360 Bar. For example, there was unnecessary to use an abrasive surface to facilitate the cutting operation, since the sandwich material is thin and readily manipulated. The plates have subsequently been positioned on the machine’s grate, due to its low weight, an additional metal plate was required to be affixed above the sandwich to prevent movement during cutting. Upon proper alignment of the plates and integration of the design file into the software, the machine commenced operation and generated the specimens with precision according to the specified cuts. The methodical design of the specimens and the used water jet cutting procedures ensured the reproducibility of the samples for further testing and assessment. Numerous instances occurred when freshly cut samples were propelled to locations where the equipment was set to traverse, and they were then extracted utilizing a stick. Upon completion of sample cutting, all specimens were accurately labeled to facilitate the subsequent recording of their details. This labeling process was thorough, enabling each specimen to be easily linked to its corresponding Plate and the specific manufacturing circumstances of its preparation. Furthermore, the samples were weighed to determine their individual weights. The weighing procedure was a crucial stage in data collecting, enabling the precise assessment of the weight characteristics of the samples intended for material examination and evaluation.
Subsequently, three samples from each Plate were used for length measurements. This stage aimed to get the average mass value per unit area. Figure 1 below illustrates the sample of individual weight measurements. The appendices are organized to enhance the readers’ comprehension of this research and provide further information. Samples were extracted from the plates using the water jet cutting procedure, measuring 20 × 150 mm, in accordance with ASTM C393 flexural testing standards [7]. Bending tests used a 10 kN Shimadzu machine with support rods of 8 mm in diameter and a loading nose with a diameter of 10 mm, operated at a crosshead speed of 6 mm/min. Five identical specimens of each laminate type were evaluated, both with the laminate volume fraction oriented normal to the skin plane and in the inverted orientation. Charpy V-notch impact testing was conducted with a Ray-Ran pendulum impact tester with impact arms weighing 0.476 kg and 0.119 kg, both released from a consistent height to achieve a velocity of 2.9 m/s. Five repetitions per arm were conducted for each laminate and orientation. Energy absorption was assessed based on the specimen’s thickness, breadth, and the heights of the post-impact arms seen.
3. RESULTS AND DISCUSSIONS
The force-displacement curves shown in Figure 2 exhibit the characteristics of the test specimens under typical loading conditions as described. The curves clearly indicate markedly different mechanical characteristics among the several plates. Figure 3 succinctly summarizes the test findings, emphasizing two critical parameters: maximum load in certain circumstances and displacement at the specific point of interest. This summary graph clearly illustrates the association between load and the fiber density used in the composite plates. It suggests that an increase in fiber density correlates with an increase in the applied load, therefore achieving the maximum strength and stiffness determined by the material’s design. The choice of fiber density considerably influences the mechanical properties of the composite plates. The subsequent study will elaborate on these findings, addressing material selection and application for these composites, while also proposing potential avenues for enhancing the design and optimization of composite structures. This will detail how these individual plate compositions influence their mechanical properties and suitability for various real-world applications.
The force-displacement findings are shown below, derived by testing the specimens in an inverted orientation, as seen in Figure 4. These statistics are essential in comprehending how the specimens exhibited varying mechanical properties when subjected to inverted loading. The differences in force-displacement hysteresis between specimens tested in normal and inverted orientations demonstrate that the structure and orientation of the plates greatly influence the mechanical behavior of the specimens. This indicates the need of elucidating how these factors influence the properties of composite structures, hence enhancing insights into their applied usefulness and adaptability.
The impact tests performed on all samples with the 0.119 kg mass arm demonstrate uniform failure mechanisms. The lower mass arm delivered less energy upon contact, resulting in a less vigorous test that caused little damage to the test specimens. However, the values are neither provided nor addressed here, since this lower-mass arm was evaluated on fifty samples, and breakages in these samples have little significance in real-world situations, as shown by the current research. The following data pertain to the Charpy impact tests conducted with a 0.476 kg mass arm positioned in the standard orientation. Only the standard position was evaluated on Plates 5 and 6, which had identical faces, and their statistics were presented accordingly. The data tables illustrate the average values of impact resistance measured in kJ/m2 by the testing apparatus and the average absorbed energy quantified in joules. The absorbed energy values were calculated using the formula from the ISO 179 standard [8].
A bar graph showing the impact resistance of different plates, labelled Plate 1 through Plate 6, is shown in Figure 5. The impact resistance is shown in kilojoules per square meter (kJ/m2) on the graph, with the energy absorbed shown in joules on the y-axis. Plate 1 has the greatest impact resistance, measuring 30 kJ/m2, according to the graph. Plate 2, which has an impact resistance of 25 kJ/m2, comes closely behind. The impact resistance of the remaining plates then steadily declines; Plate 6 has the lowest impact resistance, at 3 kJ/m2. The impact resistance of the plates and the energy absorbed are shown to be directly related in the graph. This suggests that plates with greater impact resistance have a greater capacity to absorb energy before failing. In conclusion, Plate 1 is the most impact-resistant, with Plate 2 following closely after. Plate 6 is the least impact-resistant of the subsequent plates, which show progressively decreasing levels of impact resistance [9].
The graphic in Figure 6 shows how much energy is absorbed by different plates, all of which have an impact resistance. This usage comprises six plates, designated as Plate 1, Plate 2, Plate 3, Plate 4, Plate 5, and Plate 6. The graph above indicates that Plate 1 has withstood up to 30 J/m2 of energy. It has been shown that Plate 2, which has the second-highest impact resistance among the plates, can absorb 25 J/ m2. The remaining plates’ impact resistance steadily decreases, with Plate 6 recording the lowest impact or blow energy of just 3 J/ m2 [10]. The aforementioned graph also shows a clear relationship between the impact resistance and the amount of energy absorbed by the plates. This suggests that plates with high levels of toughness, as determined by their impact strength, have a greater capacity to endure energy before failing. It is evident from the accompanying graph that Plate 1 > Plate 2 is the order in which the impact resistance of the Plates is organized. Impact resistance varies among the other plates, with Plate 6 having the lowest resistance [11,12,13]. These kinds of comparisons will allow us to determine which sandwiches in each comparison are the most and least advantageous in the next section [14,15,16,17,18]. This section concentrates on the test using the arm with the largest mass applied in the impact testing since the arm with the least mass did not shatter any samples. Because the sandwiches have two quite distinct faces, it is possible to determine which face is more resistant to bending and impact. Stated differently, this study allows for a relatively easy evaluation of the textiles used as face materials. Sandwiches 5 and 6 [12, 13] are not included in this study; only sandwiches with the distinct faces are included.
Figure 7 illustrates how the most representative sample from each case is used in this research to produce the Force-Displacement graphs for each instance under examination. We may see all of the Plates’ curves, as the comparison graph above illustrates. It is evident from Plate 1 that the specimen that was tested in the upside-down position experiences more bending strength. The highest force on the first graph is 245 N, but the maximum force on the second graph is 250 N. Similarly, the greatest displacement on the first graph is 3 mm, whereas the maximum displacement on the second graph is 4 mm. Despite this, the graphs might be quite near to one another. With regard to Plate 2, it is feasible to see how the two samples’ behaviors and failure mechanisms vary from one another. The load lowers rapidly once the curve reaches its maximum load in the normal position test. Little gradual failures in the inverted test happen as a result of the fibers on its faces rupturing one after another [19]. The force-displacement graphs of Profile 4 exhibit very small differences in maximum values, with a difference of around 10N and a displacement difference of almost 10 mm. The two curves have a very good connection, as seen in Plate 3. The failure zone, where samples tested incorrectly may withstand more displacement and force than those tested correctly, is where significant variations are only seen [20, 21]. The greatest presentation of the two distinct rock faces is seen in Plate 4. This plate exhibits a force variation of 6 N and a displacement variation of 10 mm, indicating that it can withstand higher loads in its usual position. The failure of this sandwich is thought to have resulted from inadequate energy of 1.99J in both normal and inverted configurations, as throughout the testing none of the Plate 1 specimens broke. Consequently, it exhibits positive impact behavior [22]. Figure 8a and 8b make it evident that there is excellent agreement between the impact behaviors of Plate 2’s two sides. One test item, nevertheless, used less energy than the others while in a typical posture. It is still fragile since it was only seen once and is hence not particularly entailing. With regard to Plate 3, it can be said that there is no discernible difference since both places of this plate respond to strikes from the test arm in the same way. The graph, which shows a better capacity of samples in an inverted way as the lowest point of energy resistance meets the average test point in a standard position, relates to the changes found with Plate 4. Since the only thing separating the sandwiches of Plates 2 and 3 is the kind of resin used, a comparison might be made to see how this specific element influences the behavior of the Plates, particularly when subjected to bending and impact loads. Depending on the findings that are obtained, the analysis graphs for the Plates 2 and 3 tests will seem like this.
Force graphs of plates 2 and 3 (a) displacement and absorbed energy (b) displacement and absorbed energy (inverted).
Based on the aforementioned figures, it is likely that Plate number 2 in both of the examined locations had the sample with the greatest flexural resistance out of all the samples. In contrast, the failure mechanism does not change when the samples are positioned in an inverted posture. In this scenario, the plate specimen containing the bio-resin failed gradually rather than all at once. Considering the specifics of impact testing and the methods available for estimating absorbed energy levels, the following inference may be made: they are comparable. When compared to sika resin, there is a tendency to have a minor edge, nevertheless. Another similarity is that, in contrast to the readings for Plate 3, the values for Plate 2 have shown to be more erratic. Figure 8 shows that there is a noticeable strength differential between Plate 2 and the other plates. However, there is one thing that should be brought up. But as can be seen in Figure 8(a), Plate 2’s curve has a completely different tendency from the other three. This is because, although the other three have fibers that weigh only 200 and 100 g each, Plate 2 contains fibers that weigh 500 and 400 g.
Figure 9 shows that since the curves associated with Plates 5 and 6 are symmetrical, they were left alone. Therefore, when exposed to efforts on one side or the other, only curves whose Plates had distinct faces and presumably different behaviors were integrated. Upon examining the graph, it is evident that sample 2 has an advantage over sample 4. Once again, as Figure 9(b) illustrates, the primary cause is thought to be the much reduced blanket density on Plate 4 compared to Plate 2.
4. CONCLUSIONS
This study presented novel natural cork cored natural fiber sandwich composites that might be utilized to build kayak boats more sustainably than synthetic fiberglass composites. After conducting mechanical testing with 3-point bending and Charpy impact on laminates with different fiber/resin orientations and combinations, cork, flax fiber skins, and bio-based epoxy resin were confirmed to be appropriate core materials. Among the important conclusions drawn from the research that were carried out are; Further examinations comparing specimens with flax fiber surface densities of 800 and 600 g/m2 revealed that the heavier laminates were once again more impact- and flexibility-resistant. In this instance, the petroleum-based conventional epoxy performed marginally worse than the bio-based acrylic epoxy resin, but it showed a marginally greater bending strength, making the epoxy a perfect fit for usage in laminate manufacturing. Additionally, the impact test verified that the cork core’s ability to prevent damage was sufficient to withstand dynamic stress. This suggests that designing a single-face material with the perfect configuration is not feasible; as a result, application-specific testing and development tailored to the prioritized features should be feasible. The flax fiber faces coated at 500 g/m2 with bio-epoxy showed mechanical properties comparable to the fiberglass reference, making it a strong competitor. For example, these laminates demonstrated 1.5–2 J impact energy absorption and peak bending loads of 60–65 N without failure—levels that were on par with industry-standard fiberglass laminates. We also benefited much from the cost and weight exercises. The natural fiber goods made by the competition were more expensive and heavier than fiberglass due to weight considerations. Conversely, the designs using 100-200 g/m2 flax had a less weight gain as compared to fiberglass and a lower cost premium when compared to flax with a higher density. In general, as they may be more effective, continuous fiber reinforced polymer matrix composites may be employed for components that do not need to be stiff. The results of this research suggest that natural composites based on cork and flax fibers might be a suitable replacement for synthetic equivalents in the context of high-performance sport equipment, such as kayaks. It is imperative that further study be done on natural cork core composites since they have comparable mechanics and additional attributes like extended life duration and energy dissipation. The organic resins and fibers are continually improving, and the price is decreasing. This site’s content will be helpful in advising and encouraging the sporting goods sector to use environmentally friendly composite materials.
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