Old Jute Weakens Recycled Denim Epoxy Composites, Study Finds

What if the secret to greener construction materials is already hanging in the back of your closet? A team of researchers in Bangladesh has taken that question to the laboratory, and the answer they found is more nuanced than most sustainability headlines would suggest. In a study published in Results in Engineering, Robiul Hossen, Main Uddin Apu, and colleagues systematically investigated what happens when recycled denim fabric, recovered from jeans-manufacturing waste, is hybridized with untreated short jute fibers inside an epoxy matrix. Their central finding is striking: the jute, far from strengthening the composite, actually undermined it across nearly every mechanical measure, a result that carries important implications for anyone hoping to spin textile waste into structural materials.

The motivation for the work is grounded in one of the largest waste streams on the planet. The global textile sector generates more than 92 million tons of waste annually, and denim, composed primarily of cotton cellulose fibers, makes up a significant share of both post-consumer and post-industrial textile refuse. While recycling initiatives have improved diversion rates in recent years, a substantial fraction of this material is still landfilled, contributing to soil contamination, groundwater pollution, and greenhouse gas emissions. Recycling textile waste is technically difficult because fabrics are heterogeneous, often blended, and laced with chemical additives from processing. Yet embedding waste textile fibers into polymer matrices has emerged as a promising strategy to transform low-value waste into functional materials for structural, thermal, and acoustic applications.

Denim has particular appeal as a reinforcement. Its woven structure, high fiber content, and residual mechanical integrity allow recycled denim composites to achieve competitive performance compared with some conventional glass-fiber-reinforced systems, while offering clear sustainability advantages. Jute, meanwhile, is one of the most widely available and cost-effective natural fibers in the world, prized for its favorable stiffness and tensile strength among bast fibers. Hybridizing the two waste streams seemed like an obvious win. But there was a catch the researchers deliberately chose to confront: untreated jute fibers carry a surface burden of waxes, lignin, hemicellulose, and pectins that inhibit effective wetting and bonding with hydrophobic epoxy matrices. Chemical treatments such as alkali treatment, silane coupling, and acetylation can fix this problem, but they add cost, processing complexity, and chemical usage that erode the environmental logic of natural fiber composites in the first place.

The experimental design was elegantly simple. The team fabricated two laminates by hand lay-up, each containing identical amounts of epoxy resin, hardener, and recycled denim. The denim, a heavyweight 3/1 twill cotton fabric with an areal density of 300 to 400 grams per square meter, came from pre-consumer offcuts at jeans factories and was used as received, without washing or treatment. The jute, by contrast, was recovered from used and deteriorated jute bags, washed, solar-dried for two hours, and cut into short fibers roughly five to ten millimeters long. Five denim layers were stacked in each mold; in the hybrid version, 5.80 grams of untreated jute, about 1.7 percent of the laminate by weight, was first mixed into the epoxy resin and brushed between the denim plies. The laminates were compressed under a 30-kilogram load, cured for 72 hours at ambient conditions, and post-cured at 60 degrees Celsius for one hour, yielding final thicknesses of approximately 4.1 millimeters.

The mechanical results tell a story of good intentions colliding with interfacial chemistry. In tensile testing according to ASTM D3039, the denim-only composite achieved an ultimate tensile strength of 31.4 plus or minus 0.2 megapascals at a strain of 10.5 percent, exhibiting continuous strain hardening as the woven cotton yarns progressively straightened and reoriented under load. The jute-containing hybrid plateaued at just 25.2 plus or minus 0.3 megapascals, fracturing at less than half the strain. That represents a 19.5 percent reduction in strength and a 59 percent reduction in strain at maximum stress. The energy penalty was even more dramatic: numerical integration of the stress-strain curves showed the denim composite absorbed 2.69 megajoules per cubic meter up to peak stress, while the hybrid managed only 0.80, a roughly 70 percent loss in tensile energy absorption. Initial stiffness, by contrast, was nearly identical between the two materials, at around 1.3 gigapascals, indicating the jute only revealed its destructive influence once significant deformation began.

Flexural and impact testing confirmed the same pattern. In three-point bending, flexural strength fell from 67.8 plus or minus 3.4 megapascals for the denim composite to 55.5 plus or minus 4.6 megapascals for the hybrid, an 18 percent drop, while flexural modulus declined 13 percent, from 2,843 to 2,482 megapascals. Unnotched Charpy impact tests told the most sobering story: impact strength plummeted 33 percent, from 8.30 to 5.60 kilojoules per square meter. Poor adhesion between the untreated jute and the epoxy creates weak zones that act as stress concentrators under sudden loading, facilitating crack initiation, fiber pull-out, and premature fracture. The researchers note that in bending, the compressive half of the section and the denim layup partially mask the weak interface, whereas in pure tension the poorly bonded jute phase governs failure, which explains why the energy loss was so much larger in tensile loading.

Scanning electron microscopy of the fractured surfaces provided direct, visually compelling evidence for the mechanism. The denim-epoxy composite showed fibers well embedded in the matrix with good interfacial interaction, with matrix residue clinging to the cotton yarns and indicating effective load transfer. The hybrid composite told a different story: inadequate fiber wetting, extensive fiber pull-out, microvoids, and clean jute fiber surfaces with visible gaps at the interface, the microscopic fingerprints of debonding at low stress. The pulled-out jute appeared as split technical-fiber bundles, a phenomenon known as fibrillation, in which failure proceeds by separating elementary fibers held together by a pectin- and lignin-rich middle lamella rather than by fracturing the fiber itself. Because bundle splitting and pull-out dissipate little energy, these morphological features explain the early plateau in the tensile curves, the severe loss of energy absorption, and the depressed impact resistance of the hybrid material.

Interestingly, the thermal and moisture results were not uniformly negative for the jute hybrid. Thermogravimetric analysis showed both composites exhibited comparable degradation profiles, with the principal mass-loss region between 300 and 450 degrees Celsius and maximum decomposition rates at approximately 365 degrees Celsius for both. Differential scanning calorimetry revealed a main endothermic decomposition peak at 388.1 degrees Celsius for the denim composite and a slightly lower peak at 381.3 degrees Celsius for the hybrid, consistent with the earlier onset of hemicellulose decomposition in the jute phase. Notably, the hybrid retained 8.3 percent char residue at 600 degrees Celsius, while the denim-only composite was almost fully volatilized. Even more surprising, the hybrid absorbed less water, 5.60 percent versus 6.39 percent, and took longer to saturate, 384 hours versus 288 hours. The researchers attribute this to the jute-filled resin occupying the inter-yarn channels that otherwise act as wicking pathways in the denim-only laminate, rather than to the jute surface chemistry itself.

The practical upshot is a clear engineering directive rather than a dead end. The denim-epoxy composite, with its balanced property set, is well suited for low-to-moderate load-bearing, non-safety-critical applications such as furniture panels, interior partitions, automotive trim, packaging inserts, and equipment casings. The jute hybrid, with lower strength but slower water uptake and reduced porosity, may find a niche in indoor panels for humid environments where mechanical demands are modest. But the study’s most important contribution may be its cautionary message for the green materials movement: untreated jute fibers behaved as defect sites rather than reinforcements, and the literature the authors compiled shows that alkali, silane, or acetylation treatments can substantially improve jute-epoxy performance. Until surface treatment is incorporated, the authors conclude, the promise of denim-jute hybrid composites will remain unfulfilled, a reminder that in sustainable materials engineering, chemistry at the interface matters as much as the sustainability of the supply chain.

Subject of Research: The effect of untreated jute fiber incorporation on the mechanical, thermal, and hygroscopic properties of recycled denim-epoxy hybrid composites

Article Title: Effect of untreated jute fiber incorporation on the properties of recycled denim–epoxy hybrid composites

Article References: Hossen, R., Apu, M. U., Neha, T. R., Ahasan, E., Islam, M. R., & Mim, J. J. (2026). Effect of untreated jute fiber incorporation on the properties of recycled denim–epoxy hybrid composites. Results in Engineering, 32, Article 112828. https://doi.org/10.1016/j.rineng.2026.112828

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.112828

Keywords: Effect, untreated, jute, fiber, incorporation, properties, recycled, denim, epoxy, hybrid, composites, scientific research