Wood Technology

1. Submitted to:- Dr. MEENU SOOD
Submitted by:- Tapan
Adhikari,Phd 1st sem,
F-2019-15-D
Dept. of Medicinal &
Aromatic Plants

2. Why study polymers ?
• Wood is a polymer
• All adhesives are polymers
• Necessary to understand the interactions between
the wood and adhesives in order to understand the
nature and properties of various bonds.

3. What is a Polymer?
• Like a molecular necklace:
– joined to form polymer
– beads are monomer
– 1,000 to 1,000,000 monomers per chain

4. What are polymers ?
•Polymers are very high molecular weight organics compounds
formed principally of C,H, N, O and others.
•A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A
•Examples are
–Wood
•cellulose(polysaccharide, polymer of a type of sugar)
•hemicellulose
•lignin(complex polymer of alcohol substituted benzene)
–natural rubber(polyisoprene)
–Teflon(polytetraflourine)
–polypropylene rope

5. • Monomer: Low molecular weight compound that can be
connected together to give a polymer
• Oligomer: Short polymer chain
• Copolymer: polymer made up of 2 or more monomers
– Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B
– Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B
– Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B
– Graft : AAAAAAAA
B
B

6. Cellulose
• Cellulose is an organic compound with the formula
(C6H10O5)n, a polysaccharide consisting of a linear chain of
several hundred to over ten thousand β(1→4) linked D-
glucose units.
• Cellulose is the structural component of the primary cell wall
of green plants.About 33% of all plant matter is cellulose (the
cellulose content of cotton fiber is 90%, that of wood is 40–
50% and that of dried hemp is approximately 45%.
• Cellulose is a straight chain polymer: unlike starch, no coiling
or branching occurs.

7. Hemicellulose
• Heteropolymer of pentose (xylose and arabinose)
and hexoses (glucose, galactose, mannose) and
sugar acids. Hemicelluloses are also sugar
polymers but different from cellulose because they
are:
– Branched little polymers
– Contain some molecules other than sugars.
– Much smaller than cellulose as they are made up
of between 50-300 sugars
– There are lots of varieties of hemicelluloses.
– Not very resistant to chemical attack – many
easily break down to simple sugars

8. LIGNIN
• Heteropolmers consisting of three
hydroxycinnamyl alcohol monomers (C9) differing
in their degree of methoxylation: p-coumaryl,
coniferyl and sinapyl alcohols
• Softwood lignins are rich in coniferyl alcohol (90%)
• Hardwood lignins are made of about equal
amounts of coniferyl and synapyl alcohols
• Grasses consist of coniferyl and synaply alcohol
and significant amounts of p-coumary lalcohol (10-
20%)

9. • No sugars in it
• Nature’s glue – very similar to phenolic resin
used in plywood. Holds cellulose and
hemicelluloses together
• Second most plentiful natural material
• Must be removed or weakened to separate
fibers; turn wood to pulp

10. How do monomers become
polymers ?
Monomers react with each other
by :
–condensation reaction(step
reaction)
–addition reaction(chain reaction)

11. Polymer Configurations
A polymer can have any of three basic molecular shapes. The
shape is determined by the functionality of the monomers which
make up the polymer. The three configurations are:

12. Physical Properties
Stretch
Linear Polymer
The chains can be stretched, which causes
them to flow past each other. When released,
the polymer will not return to its original form.
Stretch
Cross-Linked Polymer
The cross-links hold the chains together.
When released, the polymer will return to it's
original form.
Relax

13. Types of Polymers
• Polymer Classifications
– Thermoset: cross-linked polymer that cannot be melted (tires,
rubber bands)
– Thermoplastic: Meltable plastic
– Elastomers: Polymers that stretch and then return to their
original form.
– Thermoplastic elastomers: Elastic polymers that can be
melted (soles of tennis shoes)
• Polymer Families
– Polyolefins: made from olefin (alkene) monomers
– Polyesters, Amides, Urethanes, etc.: monomers linked by
ester, amide, urethane or other functional groups
– Natural Polymers: Polysaccharides, DNA, proteins

14. Thermoplastics

15. Thermosets

16. Polymer Synthesis
• There are two major classes of polymer formation mechanisms
– Addition polymerization: The polymer grows by sequential
addition of monomers to a reactive site
• Chain growth is linear
• Maximum molecular weight is obtained early in the reaction
– Step-Growth polymerization: Monomers react together to
make small oligomers. Small oligomers make bigger ones, and
big oligomers react to give polymers.
• Chain growth is exponential
• Maximum molecular weight is obtained late in the reaction

17. Surface chemistry
Chemistry of Adhesion
Adhesive Bond Formation-
• Adhesives wet, flow, and set to a solid during bond formation.
• The transformation from liquid adhesive to solid bond can he achieved in a
number of ways.
• The transformation of liquid monomers to solid adhesive involves an
increase in molecular size and molecular weight through polymerization.

18. Adhesives for wood
• The most widely used adhesive types are:
• Urea-formaldehyde (UF) resins,
• Melamine-formaldehyde(MF) resins,
• Phenol-formaldehyde (PF) resins,
• Diisocyanates, Polyisocyanates, polymers and copolymers of vinyl acetate,
and polyamides.

19. These are all predominantly thermo-setting resin systems
Phenolic Resin Adhesives
• PF resin adhesives are low molecular weight prepolymers formed from phenol and
formaldehyde and are of the thermosetting type. The polymerization is controlled
by the acid or alkaline conditions, i.e., pH. at which it is conducted.
Urea-Formaldehyde and Melamine-Formaldehyde Resin
• Urea (NH2CONH2) reacts with formaldehyde similarly to phenol to produce
methylol derivatives then condense further to yield a cross-linked network. At a
mole ratio of 1.5-2 mol of formaldehyde to urea and a pH of 7.5, a mixture of the
monomethylol, dimethylol, trimethylol, and tetramethylol ureas are formed.

20. Isocyanate-Based Adhesives
• Generally diisocyanates or poly-isocyanates are used in the
preparation of polyurethane adhesives. The urethane link (-O-
C(O)-NH-) is formed in the reaction of an alcohol with an
isocyanate (-N = C = O) group: Thus, when diisocyanates are
reacted with dihydroxy compounds (diols) a polyurethane is
formed, which is composed of repeating urethane links:
HO-R-OH + 0 = C-N = R'-N = C = 0 — w-O-R-O-C(O)-
NH-R'-NH-C(O)

21. Tannin-Based Adhesives
• Condensed tannins—obtained as extracts of the barks of trees
such as wattle, hemlock, or pine consist of flavonoid units that
have undergone varying degrees of condensation.
• The phenolic rings of tannins are important to adhesive
chemistry because they have reactive sites available for
condensation with formaldehyde.
• Wattle extracts, rich in the resorcinolic tannins, have been
exploited more commercially than tannins of the phloro-
glucinolic type obtained from pine trees.

22. Thermoplastic Adhesives
Hot melt Adhesives
• Thermoplastic polymers, are essentially linear polymers that soften and
flow when heated and solidify on cooling.
• Therefore, many of them are suitable for use as hot-melt adhesives.
Because the cooling occurs quickly and there is no curing reaction
involved, fast assembly on a mass scale, as in paperback book binding, is
possible. Low molecular weight polyethylene, (CH2-CH)n, is used widely
as a hot melt adhesive

23. Chemistry of Weathering and Protection
Property Changes During Weathering
Chemical Changes:
• Weathering degrades and solubilizes lignin. Cellulose appears to be affected
considerably less, except for the top surface layer of the wood.
• The UV-degradation process is initiated by the formation of free radicals and
presumably begins with oxidation of phenolic hydroxyl .
• This degradation process results in a decrease in methoxyl and lignin content
and an increase in acidity and carboxvl concentration of wood substance.
• These photochemical changes are enhanced more by moisture than by heat.

24. Colour Changes
• The colour of wood exposed to the outdoors is affected very
rapidly.
• Generally, all woods change toward a yellow to brown due to
the chemical breakdown (photooxidation) of lignin and wood
extractives.
• This yellowing or browning occurs after only several months
of exposure in sunny, warm climates. Woods rich in
extractives may become bleached before the browning
becomes observable.

25. Physical Changes
• Weathering of the wood surface due to the combined action of light and
water causes surface darkening and leads to formation of macroscopic to
microscopic intercellular and intracellular cracks or checks.
• Strength of cell wall bonds is lost near the wood surface. As weathering
continues, rainwater washes out degraded portions and further erosion
takes place.
• Because of the different types of wood tissue on the surface, erosion and
checking differ in intensity, and the wood surface becomes increasingly
uneven.
• Hardwoods erode more slowly than do softwoods.