The glass transition, dehydration, crosslinking and degradation of isolated lignin with different water contents was measured by TOPEM®. The separation of the total heat flow into reversing and non-reversing components proved to be essential for identifying different thermal events. The apparent activation energy of the glass transition of dry and wet lignin was determined from the frequency dependence of the glass transition by means of multifrequency analysis using TOPEM®. This allows conclusions to be drawn about the influence of water on molecular interactions in lignin.
Introduction
Lignin together with cellulose is one of the main constituents of wood. It is an integral part of the cell walls of plants and is responsible for its mechanical properties through the process of hardening (lignification). Lignins are complex natural polymers that form randomized three-dimensional networks. The thermal softening of lignin is largely responsible for the viscoelastic properties of wood [1, 2]. The kinetics of the glass transition of lignin has been studied on wood samples using differential thermal analysis (DTA) [6] or dynamic mechanical analysis (DMA) [3–5].
However, the kinetics of the glass transition of isolated lignin has not been determined up until now. This could be due to its powdery consistency, which makes it more difficult to perform DMA measurements.
In this study, we describe measurements of the glass transition of isolated lignin using temperature-modulated DSC (TOPEM®) to investigate the influence of water and thermal aging on behavior at the glass transition.
The frequency dependence of the glass transition was measured by multifrequency analysis using TOPEM®. This allowed us to determine the apparent activation energy of dehydrated lignin and lignin with sorbed water and to draw conclusions about changes in molecular interactions.
The chemical reactions in lignin are also discussed.
Experimental details
Samples
The Alcell lignin (AL Lignin) was obtained from Repap Technologies Inc. The material consists of a mixture of wood containing 50% maple, 35% birch, and 15% poplar. The lignin was isolated using a so-called ethanol based...
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Conclusions
The molecular mobility, cooperative relaxation processes, and chemical reactions of isolated lignin have been studied using a multifrequency stochastic temperature-modulated DSC technique (TOPEM®) and thermogravimetric analysis (TGA).
TOPEM® allows the heat flow to be separated into reversing and non-reversing components. The reversing heat flow permits conclusions to be drawn about reversible relaxation processes (glass transition). The non-reversing component records the measurement of thermal events that cannot follow small temperature changes, for example dehydration, crosslinking reactions and thermal degradation. An exothermic chemical reaction was detected at temperatures above the glass transition process. This could correspond to the possible formation of intermolecular compounds. This was confirmed by dynamic rheometry [12, 13]. The reaction is not observed when the sample is thermally pretreated above the glass transition before the measurement.
With wet lignin, two glass transitions were observed because there are regions with and without sorbed water. Three thermal events were identified in addition to the glass transitions, namely dehydration, intermolecular crosslinking and thermal degradation.
The dynamic relaxation process is frequency dependent. The glass transition of isolated lignin is not a simple softening process but rather a cooperative relaxation process [14]. The apparent activation energy was determined from the frequency dependence of the heat capacity at the glass transition.
Comparison of the activation energies of dry and water-sorbed lignin shows that the activation energy of wet lignin is much lower than that of dry lignin. This indicates that the molecular mobility of the lignin chains is higher in the presence of water. The increase in molecular mobility results in reduced thermal stability.
Relaxation Phenomena of Isolated Lignin Measured by TOPEM® | Thermal Analysis Application No. UC353 | Application published in METTLER TOLEDO Thermal Analysis UserCom 35