Wood and Fiber Fundamentals

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Wood and Fiber Fundamentals

• Composition of wood
• What exactly is wood?
• Carbohydrate (composed of Carbon, Hydrogen, and
  Oxygen)
• Elemental composition (dry wood)
• 49.5 C
• 6 H
• 43.5 O
• Very much NOT homogeneous

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Base Composition of Wood
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Wood and Fiber Fundamentals

• Composition of wood
• Four major classes of compounds
• Cellulose (45)
• Hemicellulose (25 - 35)
• Lignin (20 - 30)
• Extractives (2-8)
• (Composition varies between hardwoods and
  softwoods)
• Of these, paper and paperboard products are
  mostly composed of cellulose

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Wood and Fiber Fundamentals

• Cellulose
• Linear polymer of the sugar glucose
• Glucose C6H12O6
• Polymer of sugar poly-sugar or
• polysaccharide
• Glucose is extremely important biochemically as
  a basic energy source (biological fuel)
• Table sugar sucrose
• Sucrose disaccharide composed of glucose (C6)
  and fructose (C5)

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Wood and Fiber Fundamentals

• Cellulose
• Ring structures hooked end to end in pairs

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Wood and Fiber Fundamentals

• Cellulose
• Cellulose (C6H10O5)n
• n degree of polymerization (i.e. how many
  monomer units linked together)
• Wood n 10,000
• Paper fibers n 600-1500
• Starch has identical chemical formula
• (C6H10O5)n
• Difference is in linkage hooking glucose units
  together

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Wood and Fiber Fundamentals

• Cellulose
• linear structure repeated
• at all levels
• final result is macroscopic
• fiber

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Wood and Fiber Fundamentals

• 2. Hemicellulose
• Polysaccharides like cellulose, but
• Variety of monomer units (i.e. C5 and C6 sugars)
• Much smaller polymers (i.e. lower values of n)
• Branched instead of linear like cellulose
• Very hydrophilic (water-loving) and swellable
  in water
• Important role in bonding cellulose fibers
  together in a sheet
• Need to be removed completely for production of
  dissolving pulp

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Wood and Fiber Fundamentals

• 2. Hemicellulose

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Wood and Fiber Fundamentals

• 3. Lignin
• Glue that holds together cellulose fibers in wood
• Amorphous (i.e. non-crystalline) polymer which
  also provides water resistance to wood
• Kraft pulping lignin burned as a high-value
  energy source (Kraft recovery process)
• Sulfite pulping Lignosulfonate chemical
  by-products can be produced
• road binders, cattle feed, dispersants, drilling
  mud additives

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Wood and Fiber Fundamentals

• 3. Lignin
• Very complicated chemical structure
• Basic chemical building block is the phenyl
  propane unit

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Wood and Fiber Fundamentals

• Extractives
• Represent what we commonly call pitch
• extractives means extractable or soluble in an
  organic solvent such as chloroform
• Provide insect and rot resistance to the tree
• Mixture of
• Resin acids (e.g. abietic acid or rosins)
• triple-ring structures
• Fatty acids (e.g. stearic acid)
• C18 long chain hydrocarbons

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Extractives Chemical Structure
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Wood and Fiber Fundamentals

• Extractives
• Provide starting material for chemical byproducts
  such as turpentine and tall oil
• Naval Stores industry
• Pitch becomes highly concentrated in stumps of
  pine trees
• Pine stumps harvested for pitch content
• Rosin for paper sizing agent and anti-friction
  aid

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Crude Tall Oil Recovery
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Wood and Fiber Fundamentals
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Hardwood v. Softwood Fiber Chemistry

• Chemical differences between hardwood and
  softwood fibers
• hemicellulose content (or amount)
• HWs 35 SWs 25
• Lignin content (or amount)
• HWs 20 SWs 25-30
• hemicellulose and lignin chemistry
• specific forms of extractives (chemical
  structure)
• Cedar trees hydroxyketones which are toxic to
  wood destroying fungi (thujaplicins)

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Lignin Alcohol Building Blocks
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Wood and Fiber Fundamentals
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Wood and Fiber Fundamentals

• Thus, wood is basically a natural composite
  material with cellulose fibers as the reinforcing
  material, and lignin as the continuous matrix
  material
• Amazingly good strength, water resistance, and in
  some cases rot resistance (i.e. cedar)
• Provides an abundant source of cellulose fibers
  for papermaking, usually as a by product of
  lumber manufacture

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Wood and Fiber Fundamentals

• Fiber structure and anatomy
• General Fiber structure
• Composed of layers or walls
• Varying microfibril orientation in each wall
• Walls are mostly cellulose and hemicellulose
• Some lignin in outer walls
• End Result amazingly strong and flexible
  structure

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Wood and Fiber Fundamentals

• P primary wall
• S1 1st secondary layer
• S2 2nd secondary layer
• S3 3rd secondary layer
• L warty layer
• S2 layer is bulk of fiber mass
• ML middle lamella (lignin)
• Forms plywood type of structure

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Wood and Fiber Fundamentals

• Microstructure of fiber showing layers and linear
  construction elements

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Wood and Fiber Fundamentals

• Fiber characteristics hardwoods v. softwoods
• Hardwood trees
• Red alder
• Maple
• Birch
• Aspen
• Softwood trees
• Douglas fir
• Western hemlock
• Western red cedar
• Ponderosa pine
• Lodgepole pine

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Wood and Fiber Fundamentals

• Hardwood fibers
• Libriform fibers shorter, thinner, finer
• Vessel elements usually detrimental to paper
  quality
• In general, presence of HW results in a smoother
  sheet with better printing qualities
• Softwood fibers
• Trachied fibers longer, wider, coarser
• NO vessel elements
• In general, presence of SW results in a stronger,
  more porous sheet

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Wood and Fiber Fundamentals
HW vs.. SW fiber types and dimensions Libriform
fibers 10-20 ?m thick Trachied fibers 20-40 ?m
thick
1- 2 mm
HW
4 - 7 mm
SW
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Wood and Fiber Fundamentals

• Distinguishing fiber characteristics
• What is unique about a fiber?
• High length to diameter (L/D) ratio gt 100
• Wall collapsibility or conformability
• Indicated by the fiber coarseness value
• High coarseness value low collapsibility (e.g.
  most SW fibers)
• Low coarseness value high collapsibility (e.g
  most HW fibers)

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Wood and Fiber Fundamentals
Fiber Dimensions
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Wood and Fiber Fundamentals

• Earlywood vs. Latewood Fibers
• EW fibers grow in Spring
• LW fibers grow in Summer or early Fall
• Causes existence of tree growth rings
• Major difference fiber wall thickness

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Wood and Fiber Fundamentals
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Wood and Fiber Fundamentals

• Effect of fiber wall thickness on sheet structure
  and bonding
• Collapsibility of fiber wall is key

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Wood and Fiber Fundamentals

• SEM of Edge of Paper Sheet

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Wood and Fiber Fundamentals

• Fiber-fiber bonding, or
• what exactly holds together fibers in a sheet of
  paper?
• Cellulose contains many hydroxyl (-OH) groups
• These groups can form hydrogen bonds (which also
  form in water)
• Makes cellulose fiber self-bonding to itself

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Wood and Fiber Fundamentals

• Hydrogen Bonding in Paper