Chemistry of Wood
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a.. Chemical Breakdown
a.. Low Molecular Weight Substances
a.. ash
b.. organic extractives
a.. High Molecular Weight Substances
a.. Polysaccharides
b.. cellulose
c.. polyoses
d.. Lignin
Trees
a.. Contain only 1-5% low molecular weight substances
b.. 65-75% of the remaining 95-99% consists of polysaccharides
c.. 25-35% is lignin
Distribution of Carbon
a.. vast majority is found in cellulosic plants
Cellulose
a.. 50% of wood weight
b.. b-D-polyglucose with high molecular weight
c.. Main structural component of cell wall
d.. Chemically identical in hardwoods and softwoods
e.. same no matter where it is obtained from
f.. Straight chain or linear molecule
Polyoses
a.. 20-25% of wood weight
b.. Not chemically bonded to cellulose
c.. covalently bonded to lignin
d.. Composed of 5 sugars
e.. arabinose, galactose, glucose, mannose, xylose
f.. Contain some uronic acids
g.. carboxylic acids
h.. Branched chain molecules
i.. Lower molecular weight than cellulose
j.. Composition is different in hardwoods and softwoods
Lignin
a.. Made of base phenylpropane units
b.. No repeating structure
c.. amorphous
d.. Concentrated in lamella
e.. Formed last during cell development
f.. Covalently bonded to polyoses
g.. Different in hardwoods and softwoods
Low Molecular Weight Substances
a.. 3-5% by weight of wood material
b.. Exert strong influences on pulp properties and processing
c.. Organic materials
a.. known as extractives
b.. very sticky
c.. controlled by reducing tackiness
d.. examples include:
a.. aromatic compounds
b.. tanning compounds
c.. stilbenes
d.. lignans
e.. flavonoids
f.. terpenes
g.. aliphatic acids
h.. alcohols
a.. Inorganic Materials
a.. found in ray parenchyma
b.. only small percent of wood weight (<1%)
c.. come from the soil
d.. examples include:
a.. potassium
b.. calcium
c.. magnesium
Isolation Techniques
a.. Extractives
a.. steam distillation to isolate volatiles, terpenes
b.. solvent extraction for fatty acids, resin acids, waxes, and
tannins
c.. water extraction for carbohydrates, proteins, and inorganic salts
b.. Inorganic Material
a.. incineration at 600-800oC (ashing)
c.. Cellulose, Lignin, and Polyoses
a.. Holocellulose
a.. product obtained after selective removal of lignin
b.. very low residual lignin content
c.. minimal loss of polysaccharide
d.. minimal oxidative and hydrolytic degradation of cellulose
e.. Methods for obtaining holocellulose
a.. chlorination and extraction
b.. acidified sodium chlorite
b.. Cellulose
a.. may be obtained two ways
a.. separation from holocellulose
b.. direct isolation from wood
b.. cellulose, since it is not bonded to other substances, is not
chemically modified after isolation from wood
a.. a-cellulose
a.. wood cellulose that is insoluble in strong NaOH (17.5%)
b.. b-cellulose
a.. soluble in strong NaOH, but precipitates when neutralized
c.. g-cellulose
a.. material remaining soluble after neutralization
d.. Direct Isolation Methods
a.. nitric acid in ethanol
b.. acidified acetyl-acetone and dioxane
c.. Cl2 and NO2 in DMSO or SO2 in DMSO
a.. Lignin
a.. lignin may be obtained in two ways
a.. lignin residue
b.. lignin dissolution
b.. problem with lignin isolation methods - product obtained is not
the same as the material in the wood
a.. Acid Lignins
a.. wood treated with sulfuric (68-72%) or hydrochloric acid
b.. gives "condensed lignin"
c.. highly modified in structure and properties
b.. Milled Wood Lignin
a.. ball milling followed by extraction with dioxane
b.. closest material to actual lignin
c.. Enzymatic Lignin
a.. ball milling followed by treatment with cellulase
b.. substance which attacks and eats cellulose
d.. Autohydrolysis
a.. steam treatment
e.. Lignins from pulping processes
a.. all highly modified lignins
a.. lignin sulfonates
b.. alkali lignin
c.. Kraft lignin
f.. Lignin Determination Methods
a.. Direct
a.. Acid Insoluble (Klason Lignin)
a.. strong acid + wood
b.. carbohydrates are solubilized
c.. lignin becomes insoluble
b.. Fourier Transform Infrared (FTIR)
a.. Indirect
b.. don't measure lignin specifically
c.. measure material left after dissolution of lignin from
holocellulose
a.. Kappa Number
b..
a.. KMnO4 oxidizes the lignin
b.. assumptions:
a.. the carbohydrates are not oxidized and therefore consume
no KMnO4
b.. the unconsumed KMnO4 reacts with KI
c.. thiosulfate reacts with free iodine and changes it back
to iodide
c.. not good for mechanical pulps
d.. carbohydrates left in the normal state will react with
KMnO4
e.. for Kappa Number < 70 there is a linear relationship with
acid lignin
c.. Roe Number
a.. uses chlorine (Cl2)
b.. not used very much anymore
d.. Hypo Number
e.. uses hypochlorite (NaHClO)
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