Biomedical Engineering Reference
In-Depth Information
(2) Solubility
Hydroxyls and many polar groups exist in the lignin structure, resulting in strong
intramolecular and intermolecular hydrogen bonds, and making the intrinsic lignin
insoluble in any solvent. Condensation or degradation make the separated lignin able
to be divided into soluble lignin and insoluble lignin; the former has an amorphous
structure, and the latter is the morphological structure of the raw material fibers. The
presence of phenolic hydroxyl and carboxyl makes the lignin able to be dissolved
in alkaline solution. Separated Brauns lignin and organosolv lignin can be dissolved
in dioxane, DMSO, methanol, ethanol, acetone, methyl cellosolve, and pyridine.
Alkali lignin and lignosulfonate usually can be dissolved in a dilute alkali, water,
and salt solution. Brauns lignin, phenol lignin, and many organosolv lignins can be
completely dissolved in dioxane. Acid lignin is not soluble in any solvents. The best
solvents for most separated lignin are acetyl bromide and hexafluoroisopropanol in
acetic acid [
29
].
(3) Thermal properties
Lignin is an amorphous thermoplastic polymer. It has slight friability under high
temperature and cannot form film in solution. It also has glassy transfer properties.
Under the glassy transfer temperature, lignin is in the solid glass phase; it begins
to move when it is above the glassy transfer temperature. The lignin is softened to
become sticky and has adhesive force. The glassy transfer temperature of separated
lignin varies with the raw materials, separation method, molecular weight, and water
content. The softening temperature of absolutely dried lignin ranges from 127 to
129
ı
C, which remarkably decreased with increased water content, indicating that
water acts as a plasticizer in lignin. The higher the lignin molecular weight is, the
higher the softening point is. For example, for lignins with a M
W
of 85,000 and
4,300, the softening points, respectively, are 176 and 127
ı
C[
29
].
(4) Relative density
The relative density of lignin is roughly between 1.35 and 1.50. Values vary with the
liquid for measurement if measured by water. The relative density of sulfuric acid
lignin isolated from pine is 1.451 and is 1.436 if measured by benzene. The relative
density of dioxane lignin is 1.33 when measured by water at 20
ı
C and is 1.391
by dioxane. Lignin prepared by different methods has different relative densities,
such as the relative density of pine glycol lignin is 1.362, but it is 1.348 for pine
hydrochloride lignin [
29
].
(5) Color
Intrinsic lignin is a white or nearly colorless substance; the color of lignin we can
see is the result of the separation and preparation process. For example, the color
of lignin isolated by Brauns and named after him is light cream, and the colors of
acid lignin, copper ammonia lignin, and periodate lignin vary from fawn to deep tan
[
29
].
