Sago Waste Transformed Into Lightweight Polyester Composites in New Study

In the sago-processing heartlands of Aceh, Indonesia, mountains of soft, starchy residue pile up behind mills that turn tropical palm trunks into one of Southeast Asia’s staple foods. Indonesia grows roughly 85 percent of the world’s sago plantations and produced 385,905 tonnes of sago starch in 2022, yet most of the fibrous pith left over after extraction is simply discarded. A new open-access study in Environmental Challenges argues that this overlooked stream of biomass could become a low-cost filler for unsaturated polyester resin, the same cheap, fast-curing plastic that local fishermen use to patch their boats. The catch, the researchers found, is that the more sago pith waste you pack into the resin, the more the composite’s internal structure works against it.

The research team, led by Luthfi and colleagues, fabricated composites containing five different volume fractions of sago pith waste, from 20 to 40 percent, in a commercial orthophthalic unsaturated polyester resin identified as SHCP 3315, cured with 1.5 weight percent methyl ethyl ketone peroxide hardener. Before any mixing began, the raw waste collected from facilities in Lhokseumawe had to be cleaned. The material was washed repeatedly in plain water, stirred and squeezed to strip away residual soil and sand, then sun-dried for five to seven days. Moisture content was verified with a gravimetric oven method at 60 degrees Celsius, and only batches drying below 2 percent moisture were accepted. During drying, the pith gradually shifted from white to light brown, a visual signature of moisture loss and natural oxidation.

Because dried sago pith tends to clump into soft agglomerates that could seed weak spots in a finished panel, the team sieved the material through a 20-mesh screen, capping particle size at roughly 0.841 millimeters. Notably, they skipped the glycerol plasticization used in earlier sago-waste studies, deliberately choosing a simpler and cheaper route. The apparent density of the sieved particles was estimated at 1.4286 grams per cubic centimeter using a simple ethanol displacement calculation, a value the authors candidly flag as an estimate rather than a validated true density, since no calibrated pycnometer or degassing was employed. That honesty matters, because the same number feeds into the theoretical density and porosity calculations that frame the entire mechanical analysis.

The composites themselves were mixed by hand, homogenized with a motorized mixer, and cast into 300 by 300 by 10 millimeter molds lined with aluminum foil, curing at room temperature for at least 24 hours. Three formulations, at 20, 30, and 40 percent sago pith waste, served most physical and thermal tests, while intermediate compositions at 25 and 35 percent were fabricated specifically to sharpen the mechanical trend analysis. Density measurements told the first part of the story: experimental density fell steadily from 1.08435 grams per cubic centimeter at 20 percent filler to 1.00232 at 40 percent, while estimated porosity climbed from 8.86 percent to 19.78 percent. In other words, more sago means a lighter panel, but also one riddled with more internal voids, as the resin increasingly struggles to wet and penetrate the growing mass of particles.

Flexural performance followed the same downward slope. In modified three-point bending tests on 200 by 15 by 10 millimeter specimens over a 160 millimeter span, average bending strength dropped from 3.87 kilograms-force per square millimeter for the 20 percent composite to 2.92 for the 40 percent version, and the maximum bending moment fell from 1.611 to 1.219 kilograms-force meters. A one-way analysis of variance confirmed these differences were statistically significant, with F values of 20.68 for moment and 19.06 for stress and p-values in the millionths. Strain at failure, intriguingly, showed a rising trend with more filler, from 1.19 to 1.35 percent, but the ANOVA returned p equal to 0.265, so the authors correctly decline to claim that sago content meaningfully changes how far the material bends before it breaks.

Scanning electron microscopy of the fractured surfaces explains why the strength falls. Micrographs of the 20 percent composite revealed residual sago starch granules roughly 20 to 30 micrometers across, cracked polyester matrix, and irregular voids with torn edges. Clean particle pull-out and interfacial gaps pointed to weak adhesion between the untreated lignocellulosic filler and the resin, meaning stress is not transferred efficiently across the interface. To separate failure damage from manufacturing defects, the team also imaged carefully cut, unfractured cross-sections, where voids appeared smoother and near-spherical, the classic fingerprint of air bubbles entrapped during mixing and molding. The 40 percent composite showed all of these features in greater abundance, consistent with its highest porosity, and the authors conclude that interfacial optimization, such as surface treatment of the particles, is the clearest path to better performance.

Fourier transform infrared spectroscopy added the chemical dimension. Using potassium bromide pellets, the team found all composites dominated by the polyester’s ester carbonyl stretch near 1730 inverse centimeters, with a broad hydroxyl band around 3533 to 3539 inverse centimeters growing more pronounced as sago content rose, reflecting the cellulose, hemicellulose, and absorbed water carried in with the biomass. Crucially, no new absorption bands unique to the composites appeared, indicating that sago pith and polyester interact primarily through physical rather than covalent chemical bonding. That chemical aloofness at the interface dovetails neatly with the pull-out features seen under the electron microscope.

Thermal analysis rounded out the picture. Differential scanning calorimetry showed the glass transition temperature sliding from 58 degrees Celsius at 20 percent sago to 54 at 30 percent and 50 at 40 percent, a shift the authors attribute to changes in crosslink density, interfacial region volume, and free volume when bio-filler particles enter the network. Because cured unsaturated polyester is a crosslinked thermoset, the broad high-temperature endotherms, which shifted from 165 to 200 degrees Celsius and grew in enthalpy from 3.74 to 5.86 joules per gram, reflect relaxation and thermally activated transitions rather than any true melting. Thermogravimetric analysis under nitrogen revealed multi-stage degradation: negligible moisture loss below 150 degrees Celsius, a 4 to 6 percent stage linked to hemicellulose decomposition up to 330 degrees, and a massive degradation stage between 331 and 490 degrees where more than 80 percent of the mass vanished. Final residues at 600 degrees Celsius ranged from 6.57 to 7.44 percent, with the 40 percent composite retaining the most, likely carbonaceous char from lignin plus natural mineral ash, though the authors caution the trend is not systematic.

Taken together, the study delivers a sober but genuinely useful verdict: sago pith waste can indeed be incorporated into unsaturated polyester as a renewable, nearly free filler, lightening the material while leaving the resin’s chemistry intact, but rising filler content drags in porosity and interfacial defects that erode bending strength. The authors frame the work as the integrating chapter of a broader program that previously examined tensile, acoustic, and thermal-conductivity behavior separately, and they point squarely at future work on particle-size optimization, improved processing, and surface chemical modification of the waste as the levers that could turn an agricultural liability into panels for insulation, construction, and other value-added uses. For Indonesia’s sago mills, the residue currently rotting behind the factory may be one sieve, one mixer, and one better interface away from a second life.

Subject of Research: Characterization of sago pith waste-filled unsaturated polyester composites

Article Title: The analysis of sago pith waste – unsaturated polyester composites by mechanical, SEM, FTIR, DSC, and TGA tests

Article References: Luthfi, Azhar, Jagodang, H., Riyadhsyah, T., Rihayat, T., & Setiawan, A. (2026). The analysis of sago pith waste – unsaturated polyester composites by mechanical, SEM, FTIR, DSC, and TGA tests. Environmental Challenges, 25, Article 101681. https://doi.org/10.1016/j.envc.2026.101681

Image Credits: AI Generated

DOI: Not provided

Keywords: sago pith waste, unsaturated polyester resin, natural fiber composites, flexural strength, porosity, FTIR, DSC, TGA, SEM, biomass waste utilization, Indonesia, thermal degradation