Madhav Sonkusare

Natural Fibre Composites Research

A controlled comparison of vacuum-infused flax and hemp epoxy laminates at MIT Manipal, covering mechanical, thermal, fire and acoustic properties. Published in two journal papers with Sohan Kumar Y.

RoleResearcher and co-author

  • 2 papers in Sci (MDPI), 2026
  • First author · thermal and acoustic paper
  • Flax vs hemp · vacuum infusion
Scanning electron micrograph of the natural fibres

The research

We compared two natural-fibre epoxy laminates using the same treatment, resin, fibre orientation and manufacturing process.

Manufacturing

Alkali-treated flax and hemp fabrics were vacuum-infused with the same epoxy system. Five unidirectional plies were cured under vacuum at room temperature.

  • 5 unidirectional plies
  • 5 wt.% NaOH · 30 min
  • 24 h ambient cure
  • ≈3.5 mm thickness
FIG. 01VACUUM-ASSISTED RESIN INFUSION
Vacuum bagInfusion meshPeel plyPly 5Ply 4Ply 3Ply 2Ply 1Glass mould
5 unidirectional pliesLayer spacing schematic
  1. Two unidirectional natural-fibre fabrics laid one over the other01FabricsUnidirectional flax (beige) and hemp (pale) fabrics, 300 GSM.
  2. Natural-fibre fabric soaking in brown alkali solution in a steel bowl02Alkali treatment30 min in 5 wt.% NaOH, then neutralised in 25% acetic acid.
  3. Flax preform under a vacuum bag with the inlet, outlet and fibre direction marked03Vacuum infusionFive plies [0]5 under peel ply, mesh and bag; resin drawn from inlet to outlet.
  4. A cured natural-fibre epoxy laminate plate held in one hand04Cured plate24 h at room temperature under vacuum, then demoulded. About 3.5 mm thick.
  5. An abrasive waterjet cutting machine with its control computer05Waterjet cuttingSpecimens cut by abrasive waterjet to each test standard.Photo from the published paper
  6. Ten cut tensile specimens in two rows on a white board06SpecimensTensile coupons, 250 × 25 mm, to ASTM D3039.

Results

Flax was stiffer and stronger in bending. Hemp absorbed more impact, held more interlaminar shear and lost mass later on heating. Both burned to the same UL 94 HB class.

FIG. 02Flax / epoxyHemp / epoxy

Mechanicalfive specimens

  • Tensile strengthMPa92 ± 587 ± 1
  • Tensile modulusGPa5.6 ± 0.125.2 ± 0.1
  • Flexural strengthMPa116 ± 3102 ± 4
  • Flexural modulusGPa5.9 ± 0.14.9 ± 0.1
  • Notched Charpy impactkJ/m²5.7 ± 0.310.6 ± 0.8
  • Interlaminar shear strengthMPa12.9 ± 0.614.3 ± 0.3
  • Vickers hardnessHV18.5 ± 0.817.4 ± 0.6

Thermalthree specimens

  • Temperature at 5% mass loss°C207 ± 3222 ± 3
  • Glass transition · second heating°C65.3 ± 0.571.9 ± 0.9
  • Char residue at 800 °Cwt.%9.29 ± 0.1510.22 ± 0.11

Firethree specimens · lower is better

  • Horizontal burning ratemm/min26.58 ± 0.6824.31 ± 0.68
  • Both: UL 94 HB
  • Both: oxygen index 21.54 vol.%
  • Brittle failure in tension, bending and shear
Scanning electron micrograph of a flax/epoxy fracture surface at 100 times, with pulled-out fibre ends and cavities
Flax / epoxy · SEM 100× · Clean pulled-out fibres and empty cavities: weaker fibre–matrix bonding.
Scanning electron micrograph of a hemp/epoxy fracture surface at 100 times, with resin-coated fibres in a dense fracture zone
Hemp / epoxy · SEM 100× · Resin-coated fibres in a dense fracture zone: stronger bonding.

Mean ± standard deviation, published rounding. Mechanical: Sci 8(6) 133 ↗, sections 3.2–3.4. Thermal and fire: Sci 8(9) 261 ↗, Tables 2, 3 and 5. Tensile strength difference not significant (p = 0.133). TGA in nitrogen at 10 °C/min; mass-loss temperatures are not service ratings.

Sound

Hemp blocked a little more sound on average, but the lead changes with frequency, so neither laminate is better across the whole range.

FIG. 03 · TRANSMISSION LOSSFlax / epoxyHemp / epoxy
Transmission loss in decibels against frequency, 63 to 6300 Hz. Both laminates rise from a few decibels at low frequency to over 20 dB above 3 kHz; hemp is mostly above flax, and the two cross in places.Averaged band · 250–2000 Hz631252505001k2k4k05101520253035Frequency (Hz)dB
Flax · mean 250–2000 Hz11.56 dBHemp · mean 250–2000 Hz13.22 dB

Normal-incidence transmission loss, ASTM E2611, impedance tube.

Findings & scope

Under the tested conditions, flax had higher flexural values; hemp had higher impact and interlaminar shear values. Thermal and acoustic responses also differed, while both shared the same HB fire classification.

Gallery

Circular specimen cut by water jet for the impedance tube

Links

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