# Influence of the Extraction Medium of Tannins from Eucalyptus Bark on the Properties of Rigid Tannin–Furfuryl Alcohol Foams

## TL;DR

This study explores how different extraction methods from eucalyptus bark affect the properties of tannin-based foams, aiming to develop sustainable alternatives to petroleum-based materials.

## Contribution

The study links tannin extraction chemistry directly to foam performance, offering insights into sustainable biobased material development.

## Key findings

- Alkaline extraction (NaOH) produced foams with higher compressive strength and smaller, denser cells.
- Aqueous extraction preserved antioxidant activity but slightly reduced fire resistance in foams.
- Extraction medium significantly influenced tannin chemical profiles and foam properties.

## Abstract

Eucalyptus bark is an abundant forestry residue in Brazil
and a
promising renewable source of condensed tannins for advanced material
applications. Among them, tannin–furfuryl alcohol foams stand
out as sustainable alternatives to petroleum-based foams due to their
intrinsic fire resistance, low density, and high porosity. The performance
of such foams is strongly influenced by the chemical profile of the
tannins, which depends on the extraction medium. This study investigated
tannin extraction from eucalyptus bark using distilled water, sodium
hydroxide (NaOH), and sodium bisulfite (NaHSO3), and evaluated
their impact on the structural, antioxidant, and functional properties
of the extracts and on the performance of the resulting foams. Extraction
yields were highest with NaOH (>50% for some clones), moderate
with
NaHSO3, and lowest with water. However, aqueous extracts
showed superior antioxidant activity, whereas NaOH extracts displayed
enhanced ferric-reducing capacity despite lower phenolic content.
FTIR analyses confirmed solvent-dependent differences in hydroxyl,
carbonyl, and aromatic groups. The foams produced exhibited distinct
physical and mechanical behaviors. NaOH-derived foams showed the highest
compressive strength (∼280 kPa) and smaller, denser cells,
while NaHSO3-derived foams were lightweight and highly
porous but structurally weaker. Aqueous extracts yielded intermediate
strength but higher water uptake due to hydroxyl-rich compounds. All
formulations maintained thermal stability above 400 °C and displayed
self-extinguishing behavior under flame exposure. Nonetheless, incorporation
of aqueous extracts slightly reduced fire resistance compared to control
foams. These findings highlight the pivotal role of the extraction
medium in determining tannin reactivity and, consequently, foam properties.
Alkaline extraction favored mechanical performance, while aqueous
extraction preserved phenolic-rich fractions with stronger antioxidant
activity. This study links extraction chemistry directly to material
performance. It contributes to the valorization of forestry residues
into biobased foams, offering sustainable alternatives to synthetic
polymeric materials.