Action of laccase on mechanical softwood pulps
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.
Provided by VTT Technical Research Centre of Finland