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TECHNOLOGY 51
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strength, particularly with porous materials such as wood, Flickr
paper, and textiles, and are often used in applications such
as packaging and wood industry.
However, challenges include susceptibility to moisture and

mildew resistance, which can weaken their performance, and
variability in adhesive properties based on the source and
processing methods.
Ongoing research aims to enhance their water resistance
and bonding capabilities through chemical modifications.
amination.
Lignin-based Adhesives However, despite these approaches, health risks and
Owing to its abundance in nature, high industrial production environmental concerns related to green chemistry and
yield, phenolic nature, and low cost, technical lignin, a main sustainability remain. Therefore, the use of fully green bio-

by-product of pulp and paper industry, is an excellent candidate crosslinking agents for lignin is crucial in advancing lignin to
to produce bio-based adhesives as well as bio resin to replace bio-based adhesives as sustainable alternatives.
conventional formaldehyde-based adhesives.
Nowadays, a huge amount of technical lignin is being Tannins
wasted or burned for energy consumption, although it has a Tannins are another set of polyphenolic compounds present
great potential of building thermosetting bio-based adhesives. in various plants, particularly in bark, wood, leaves, and fruits.
In recent years, approximately 100 million tonnes/year They are known for their astringent properties and are

of technical lignin comes from the pulp and paper industry, classified into two main groups: hydrolysable and condensed
showing that it is a highly available raw biomaterial for bio- tannins. Tannins’ common sources include oak, chestnut,
based adhesives. quebracho, and various tree barks.
However, transition to lignin-based bio-adhesives brings Tannins from renewable resources provide wood adhesives
some challenges of finding effective lignin extraction and with sustainability, low toxicity, biodegradability, bonding strength,
fractionation methods from biomass, improving poor reactivity thermal stability and water resistance, which makes them an
for crosslinking, high chemical heterogeneity, as well as environmentally friendly alternative to synthetic formaldehyde-
unfavourable odour and colour. Among them, poor reactivity based adhesives for wood industry.
of lignin has received significant attention from researchers As non-toxic natural polyphenols, the condensed and

around the world. hydrolysable tannins are used for tanning hides to obtain
In recent years, several reaction schemes, including synthetic leather as a commercial commodity chemical and for a growing
and natural crosslinkers, have been applied to enhance lignin number of other industrial applications of mainly substituting
adhesion capacity, but this challenge is still under investigation. petroleum-based products.
Researchers have made many attempts to enhance lignin Some challenges and limitations for tannins as wood
crosslinking for bio-based wood adhesives, using various adhesives include their variability from sources, standardised
crosslinking agents, such as glyoxal, formaldehyde, phenol, formulations, and curing mechanism which is crucial for

and isocyanates, as well as modifying lignin through various improving their performance. FDM
techniques such as hydroxymethylation, glyoxalation, and ENQUIRY NO. 3202