Abstract
During recent years, the traditional pulp and papermaking
business in Europe has been striving to find new viable
applications for wood fibres. The target has been to
improve the value and properties of traditional fibres
and fibre products and to find new applications for wood
fibres that would support much-needed growth in the
industry. At the same time, interest in using renewable
materials in new applications has increased. However, the
natural properties of the fibres limit their use in many
applications. Fibre functionalization, i.e. bonding of
new compounds to the fibres, is a method to produce
fibres with altered properties.
An interesting option is targeted modification of fibre
surface lignin via enzymatic radical formation with
oxidative enzymes. The highly reactive radicals generated
on the fibre surface can be utilised in the bonding of
new compounds. In order to exploit the laccase-based
functionalization method, deep understanding of factors
affecting the formation of phenoxy radicals in fibres is
needed. Furthermore, factors affecting the degree of
bonding need to be clarified. The main aim of this thesis
was to elucidate the effects of laccase treatments on
softwood TMPs and their fractions. Furthermore, potential
utilisation of the radicals formed by laccase-catalysed
oxidation in fibre functionalization was assessed.
The studied laccases were found to be reactive with the
studied TMPs and their fractions. The degree of oxidation
of TMP was found to be influenced by the presence of
dissolved and colloidal substances (DCS). However, the
results did not confirm the previously suggested role of
DCS in the laccase-catalysed oxidation of fibre-bound
lignin.
Laccase appeared to be able to catalyse the oxidation of
free fatty and resin acids. The type of chemical linkages
present in fatty and resin acids was found to define the
effect of laccase. It seems that laccases can be used to
oxidise fatty acids with several double bonds and resin
acids with conjugated double bonds.
Laccase treatment of milled wood lignin (MWL) was not
found to decrease the amount of total phenols in lignin,
whereas the amount of conjugated phenols in lignin was
found to increase. It was concluded that the effects of
laccase on low- molecular mass substrates, such as
lignans, are different to those on the more complex
lignin. Apparently, in larger lignin structures, the
formed radicals can delocalise into the structure.
Two types of radicals can be detected after laccase
treatments in wood fibres, i.e. "short-living" radicals
that can only be detected immediately after the laccase
treatment and stable, "long-living" radicals that can be
detected in dried samples even days after the treatment.
The stable radicals detected in dry samples represent
only a small part of the originally generated radicals.
The formed radicals should be utilised in bonding of the
new compounds within an appropriate short time after
activation, before the radicals are delocalised in the
structure.
Bleaching of TMP affects the amount and the stability of
radicals formed in the laccase-catalysed oxidation. More
radicals were generated in the laccase-catalysed
oxidation on bleached TMP than on unbleached TMP.
Peroxide bleaching was found to cause changes in surface
chemistry so that "long-living" radicals could only be
detected in the fines fraction. This might indicate that
the possible levels of modification of unbleached and
bleached fines and fibres are different.
Bonding of 3-hydroxytyramine hydrochloride to TMP could
be demonstrated, which suggests that compounds containing
functional groups can be bonded to wood fibres via
laccase-catalysed oxidation of surface lignin. Even
though the laccase-aided fibre functionalization method
is limited to lignin-rich pulps, its potential is
remarkable. It has been shown that the method can be used
to create completely new properties in lignin-containing
fibres.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 21 Aug 2014 |
Publisher | |
Print ISBNs | 978-951-38-8269-3 |
Electronic ISBNs | 978-951-38-8270-9 |
Publication status | Published - 2014 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- fibre activation
- fibre functionalization
- surface modification
- oxidative enzymes
- laccase
- lignin
- TMP