Grape Stems and Bottle Gourd Cellulose Turn Epoxy Into a Tougher, Safer Composite

Agricultural waste is quietly becoming one of the most promising raw materials in modern materials science, and a new study published in Polymer Bulletin adds a striking example to the growing list. Researchers in India have taken two byproducts that would normally be discarded—grape stems left over from vineyards and processing facilities, and stems from the bottle gourd plant, Lagenaria siceraria—and converted them into high-performance reinforcements for epoxy composites. The team, led by G. Gokilakrishnan of Sri Eshwar College of Engineering in Coimbatore, demonstrated that with careful chemical treatment and precise formulation, these humble plant materials can transform brittle, flammable epoxy into a multifunctional engineering material with potential uses ranging from laptop back covers and acoustic wall tiles to wheelchair trays, automotive components, and construction panels.

The appeal of natural fiber reinforcements lies in a combination of properties that synthetic alternatives struggle to match. Plant fibers offer a high strength-to-weight ratio, low density, low cost, and renewability, making them attractive for industries under pressure to reduce their environmental footprint. Stem-derived fibers are particularly interesting because of their high cellulose content, which directly contributes to load-bearing capacity in polymer composites. Grape stems, belonging to the Vitaceae family, contain roughly 37 to 42 percent cellulose, while bottle gourd stems are even richer, at approximately 60 to 65 percent. The research team exploited both: chopped grape stem fibers served as the primary structural reinforcement, while cellulose particles extracted from bottle gourd stems acted as a secondary filler designed to plug gaps in the microstructure and enhance thermal and fire performance.

Preparing the reinforcements required a sequence of well-established but carefully controlled chemical processes. Grape stems were first cleaned and immersed in water for six days to allow natural microbial retting, a process in which microorganisms break down lignin and pectin, the sticky compounds binding the fiber bundles together. The softened tissue was then mechanically processed to release the fibers, which were dried at 120 degrees Celsius for two hours and chopped into lengths of roughly five to eight millimeters. The bottle gourd stems followed a more elaborate route: after four days of water soaking and milling, the ground material was treated in a heated alkaline bath of 3.5 weight percent sodium hydroxide at 90 degrees Celsius to strip out lignin and hemicellulose, then hydrolyzed in 2.5 normal hydrochloric acid at 80 degrees Celsius, and finally bleached with 1.5 weight percent hydrogen peroxide to remove residual lignin. Optical microscopy of the resulting cellulose revealed granular, irregularly shaped particles of roughly 50 micrometers, consistent with microcrystalline cellulose.

A critical step in making these natural materials work inside a hydrophobic polymer was silane surface modification. Both the chopped fibers and the cellulose particles were immersed for two hours in a solution of 3-aminopropyltrimethoxysilane, or 3-APTMS, at one weight percent in a 90:10 ethanol-water mixture adjusted to pH 4 with acetic acid. The chemistry here is elegant: during treatment, silanol groups formed on the silane molecule condense with the hydroxyl groups on the cellulose surfaces of the fibers, creating strong covalent silicon-oxygen-cellulose bonds. Meanwhile, the amine end of the silanized reinforcement reacts covalently with the epoxy rings of the polymer matrix during curing. This dual bonding mechanism bridges the naturally incompatible hydrophilic fiber and hydrophobic resin, dramatically improving interfacial adhesion, stress transfer, and dispersion while suppressing the agglomeration that plagues untreated fillers.

The composites themselves were fabricated by a manual layup method. Epoxy resin LY556 and hardener HY951 were mixed at a 10:1 weight ratio and stirred at 500 revolutions per minute, after which the silane-treated fibers and cellulose were gradually added and blended for another fifteen minutes. The mixture was cast into a steel mold coated with release agent, cured for twenty-four hours at room temperature, and post-cured at 115 degrees Celsius for two hours to complete the polymer network. The researchers produced a series of formulations containing a fixed 30 volume percent of chopped grape stem fiber with cellulose contents ranging from 1 to 9 volume percent, alongside a fiber-only composite and a neat epoxy reference, and then subjected all of them to a battery of standardized tests covering tensile, flexural, impact, and hardness behavior, dry sliding wear, flammability, and thermal conductivity.

The mechanical results revealed a clear optimum. Neat epoxy was the weakest material across the board, and adding silane-treated fiber alone lifted tensile strength by 36 percent, flexural strength by 9.4 percent, impact energy by 15.7 percent, and hardness by 7.5 percent. Introducing cellulose pushed performance further: from 1 to 5 volume percent filler, properties climbed steadily, peaking in the BFF3 composite at a tensile strength of 137.6 megapascals, a flexural strength of 157.6 megapascals, an impact energy of 3.82 joules, and a 16.4 percent hardness gain over the fiber-only material. Beyond that point, however, the trend reversed. At 7 and 9 volume percent cellulose, tensile, flexural, and impact strength all declined, a classic signature of filler agglomeration, inefficient stress transfer, and matrix discontinuity at high loading. Hardness alone continued to rise, reaching 99 Shore D at 9 volume percent, thanks to denser packing and greater resistance to surface deformation.

Scanning electron microscopy of fractured specimens explained these outcomes in vivid detail. The neat epoxy reference showed numerous air pockets that acted as structural weak points, disrupting load transfer and promoting premature failure in an already brittle matrix. The fiber-reinforced composite displayed typical failure modes including fiber breakage, fiber splitting, and fiber-matrix delamination, but the improved adhesion from silane treatment delayed crack initiation and propagation. Most tellingly, the composite with 5 volume percent cellulose exhibited the most uniform particle distribution of any sample, with the silane layer preventing agglomeration and minimizing stress concentration zones. This microscopic homogeneity translated directly into the highest mechanical strength among all the reinforced formulations, confirming that interface quality, not just filler quantity, governs composite performance.

Where the higher filler loadings lost ground mechanically, they excelled functionally. In dry sliding wear tests on a pin-on-disc tribometer at 20 newtons, 2 meters per second, and a 1000-meter sliding distance, neat epoxy fared worst with a specific wear rate of 0.60 cubic millimeters per newton-meter and a coefficient of friction of 0.90. Every reinforced formulation improved on this, with wear rates falling progressively from 0.57 down to 0.39 cubic millimeters per newton-meter and friction coefficients dropping from 0.81 to 0.56 as cellulose content rose. The 9 volume percent composite, BFF5, was the clear winner. The researchers attribute this to increased toughness and stiffness from the reinforcements, chemical bonding that prevents fiber pull-out, and more even stress distribution that suppresses localized wear damage. Thermal conductivity followed the same upward trajectory, climbing from 0.351 watts per meter-kelvin for neat epoxy to 0.475 watts per meter-kelvin for BFF5, as fine cellulose particles filled the interstitial spaces between fibers and silane bridges reduced interfacial thermal resistance, enabling efficient phonon transport through interconnected conduction pathways.

Flammability results added a further dimension to the multifunctional story. In horizontal burning tests, none of the composites exhibited flaming drips or ignited the underlying cotton indicator, and all resisted flame spread better than neat epoxy, which burned at 6.25 millimeters per minute. The BFF5 composite achieved the slowest flame spread rate of 8.33 millimeters per minute, with the intermediate formulations falling in between at 6.93 to 8.01 millimeters per minute. The team credits the uniform silane layer on the reinforcements with acting as a protective barrier that limits heat and oxygen penetration, while enhanced interfacial cross-linking during curing produces a compact, coherent char layer on flame exposure that insulates the underlying material. Reduced micro-cracking and delamination also helped the composites maintain structural integrity longer under thermal loading.

Taken together, the findings sketch a compelling picture of waste-to-value engineering. A single composite family built from vineyard discards and gourd stems can be tuned for strength at moderate filler levels or for wear resistance, fire safety, heat conduction, and hardness at higher loadings, allowing manufacturers to select formulations matched to specific applications. The study also underscores a broader lesson in composite design: the interplay between reinforcement architecture, filler loading, and interfacial chemistry determines whether added material helps or hurts. With silane treatment unlocking covalent bonds between plant cellulose and synthetic resin, agricultural residues that once rotted in landfills may soon find themselves in laptop lids, wall tiles, wheelchair trays, and car interiors—quiet proof that the future of high-performance materials may grow on vines and in gardens.

Subject of Research: Epoxy composites reinforced with grape stem fibers and bottle gourd stem cellulose

Article Title: Mechanical, wear, flammability, and thermal conductivity behaviour of epoxy composites reinforced with bottle gourd stem cellulose and chopped grape stem fibers

Article References: Gokilakrishnan, G., Mahendran, G., Gobu, N., & Narasimharaj, V. (2026). Mechanical, wear, flammability, and thermal conductivity behaviour of epoxy composites reinforced with bottle gourd stem cellulose and chopped grape stem fibers. Polymer Bulletin, 83(12), Article 657. https://doi.org/10.1007/s00289-026-06714-w

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06714-w

Keywords: epoxy composites, natural fibers, grape stem fibers, bottle gourd cellulose, silane treatment, mechanical properties, wear resistance, flammability, thermal conductivity, biocomposites, cellulose filler, polymer composites