Adhesive system

1. Presented by:
Prabhat Tiwari
Ph.D (Agroforestry)

2. Overview of Topics
 Introduction
 Adhesive uses
 Formaldehyde Adhesives
 Isocyanates
 Epoxides
 Acrylics
 Ethylene Vinyl Acetate (EVA)
 Poly Vinyl Acetate (PVA)
 Biobased Adhesives
 Construction Adhesives
 Other Adhesives
 Conclusion

3. Introduction
 The properties of an adhesive need to not only match the
needs of the bonded assembly in its end use, but also need
to be compatible with the substrate and the bonding process
conditions
 Adhesives need to be compatible with the bonding
conditions used commercially
 For manufacturing bonded products, low-cost and rapid
setting of the adhesive are important factors

4.  To understand the application, setting, and performance
of adhesives
Knowledge of general polymer chemistry and
polymer properties is essential
POLYMER FORMATION
Aspects of polymers that need to be considered
include use,class, type, and size
Contd………….

5. SELF-ADHESION
 Wood can self-adhere, but generally adhesives are
needed to give sufficient strength
 The forces working against good self-adhesion are
The roughness of the surface and
The lack of mobility of the wood components
 For good adhesion, the two surfaces have to be brought
into contact at the molecular level
 Hemicellulose more readily forms hydrogen bonds to
bond the adjoining fibers, while lignin more readily forms
chemical bonds

6. Adhesives Uses
From: The Woodhandbook (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch09.pdf)

7. FORMALDEHYDE ADHESIVES
 This adhesives involve reactions of formaldehyde with
phenol, resorcinol, urea, melamine, or mixtures
 The reactions can involve three steps:
1) Formaldehyde reaction with a nucleophilic center of the
comonomer to form a hydroxymethylderivative
2) Condensation of two of these hydroxymethyl groups to form
a bismethylene ether group,with loss of a water molecule
3) Elimination of formaldehyde from the bismethylene ether to
form a methylene bridge

9. Urea-Formaldehyde
 An amino resin that is the polymeric condensation product of a
reaction with formaldehyde and urea.
 Most panel boards worldwide are made with UF
 Advantages:
 Water solubility prior to cure, hard, flame resistant, good thermal
stability, can be tailored to a wide range of curing conditions,
inexpensive compared to other resins
 Disadvantages:
 Bond durability – caused by hydrolysis of the aminomethylenic
bond
 Emits formaldehyde
 Wax is usually added to wood products to increase water resistance
of the end product

10. Urea-Formaldehyde Chemistry

11. Melamine-Formaldehyde
 Include MF (melamine-formaldehyde) and MUF (melamine-
urea-formaldehyde)
 Similar to UF, MF is formed by a condensation of melamine to
formaldehyde. The amino group in melamine reacts completely
with formaldehyde groups leading to complete methylolation. Up
to six formaldehyde molecules may be attached (see Pizzi 1994).
 Advantages
 More durable than UF, lower formaldehyde emissions, high
tack with low viscosity (important for fiberboard), cure over
a wide range of pH
 Disadvantages
 More expensive than UF, less durable than phenol
formaldehyde

12. Melamine-Formaldehyde Chemistry

13. Phenol Formaldehyde
 Polycondensation product of phenol and formaldehyde
 First commercial polymer (bake-lite) and still of large
commercial importance
 Used in structural and external panels (OSB, Plywood,
Parallam)
 Advantages
 Non-conductive, heat resistant, water resistance, moderately
inexpensive
 Disadvantages
 Emits formaldehyde, brittle, distinctive redish-brown color, much
more expensive than UF, need a higher temperature and longer
cure time than UF or MUF

14. Phenol Formaldehyde Chemistry

15. Resorcinol Formaldehyde
 Resorcinol formaldehyde may be a combination of
resorcinol and PF resins. They are two-part systems that
are mixed with a catalyst to cure at room temperature.
They are primarily used in laminated beams, finger joints,
and structural applications.
 Advantages
 Very resistant to moisture, strong bonds, long-term
durability
 Disadvantages
 Can have long curing times, expensive, reddish-brown
color

16. Resorcinol Formaldehyde Chemistry

17. Adhesive Durability
This schematic illustrates the relative rate of degradation for some wood adhesives.
From: The Woodhandbook (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch09.pdf)

18. Isocyanates
 Primary reaction is isocyanate and water to form an amine
and subsequently a poly urea
 Used in structural, exterior panels that are strong and
moisture resistant
 Advantages
 100% solids, no formaldehyde, wets wood better than PF, does
not introduce excess moisture, durable and strong bonds
 Disadvantages
 Much more expensive than formaldehyde based adhesives,
sensitizing agent

19. Types of Isocyanate adhesives
1) Polymeric Diphenylmethane Di-isocyanate Adhesives
2) Emulsion Polymer Isocyanate Adhesives
3) Polyurethane Adhesives

20. Polymeric Diphenylmethane Di-isocyanate Adhesives
 The polymeric diphenylmethane diisocyanate (pMDI) is used
mainly for production of the core of oriented strandboard and
wood bonding
 It is a mixture of the monomeric diphenylmethane diisocyanate
and methylene-bridged polyaromatic polyisocyanates
 The only known adhesive that works well for the bonding of
strawboard
Advantages- High moisture content, low temperatures, low
polarity and forming bonds
Disadvantages- Extra cost, safety issues, limited use

21. Emulsion Polymer Isocyanate Adhesives
Emulsion polymer isocyanates are generally two-part adhesives
that are mixed prior to use and have been used for panel bonding
and bonding of oriented strandboard in engineered wood
products, bonding of plastics to wood surfaces
The emulsion allows higher molecular weight polymers to be
used while keeping a low solution viscosity for ease of application
Advantage- form fairly durable bonds; good water-resistance and
also have ability to bond plastics and other non-wood substrates
Disadvantage- Higher cost and the need to mix the two
components

22. Polyurethane Adhesives
 Polyurethanes can be either one- or two-component systems
in two component adhesives has an isocyanate portion and an
isocyanate-reactive portion Good mixing of these two
components just prior to bonding is critical
Advantages- good strength, flexibility, impact resistance, and
ability to bond many substrate
Disadvantage- Wood failure in some applications, health
problems and low molecular weight and excessive volatility

23. Epoxides
 Many different chemicals that consist of an epoxide ring that reacts
with an amine or free radical to cure with time, heat, or ionizing
radiation. Usually, epoxies are cured as two part systems with a resin
and a hardener.
 Advantages
 Good adhesion to a wide range of materials depending on formulation, a
wide range of formulations are available, reacts completely with little to no
VOC emissions, moisture resistant
 Disadvantages
 Not a lot of information of the efficacy and durability of wood bonds, some
components of common epoxies are carcinogens (bisphenol-A), expensive
 Current uses include repairing glulams, molded wooden boats, bonding
wood to other materials
Image from: http://en.wikipedia.org/wiki/File:Epoxide_generic.png

24. Acrylics
 Many different chemicals that consist of an acrylate or methacrylate
group that reacts with a hardener or free radical to cure with time, heat,
or ionizing radiation.
 Advantages
 Good adhesion to a wide range of materials depending on formulation, a
wide range of formulations are available, moisture resistant, heat resistant,
methacrylates make a stiffer, but often harder to cure bond
 Disadvantages
 Not a lot of information of the efficacy and durability of wood bonds, may
off-gas harmful vapors, are more expensive
 Currently, acrylics have very limited use in wood, but there has been a
lot working on impregnating wood for flooring, counter tops and other
application. There is a large potential for its utility in radiation cure in
wood products.

25. Ethylene Vinyl Acetate (EVA)
 A common, nontoxic, thermoplastic (hot melt) adhesive used
in edge-banding, packaging, paper and plastic overlays,
patching, and furniture assemble. It bonds rapidly and can fill
gaps.
 Advantages
 Non-toxic, easy application, moisture resistant, inexpensive
 Disadvantages
 Low strength, poor creep performance, poor thermal stability,
low penetration, requires special equipment

26. Poly (vinyl) Acetate (PVA)
 Solvent loss system that is usually water based that can be cured
at room temperature under pressure. Solvent loss systems need
intimate contact by applying pressure to form adhesive bonds.
 Advantages
 Cheap, high dry strength, non-toxic (can be used in food contact
applications), can be combined with cross-linking agents and
catalysts to increase durability, dries clear to varying color
 Disadvantages
 Low moisture resistance (cross-linking improves this, but makes it
more toxic and expensive), low heat resistance
 Used in many furniture, molding, doors and architectural
applications. It is commonly referred to as carpenters or wood
glue

27. BIOBASED ADHESIVES
The most common bio-based adhesives are protein-based
Tannins and lignin adhesives tend to have good moisture
resistance and are not readily attacked by microorganisms
Carbohydrates adhesives have not found much utility in wood
bonding
Starch is widely used in the construction of corrugated board
but generally lacks the strength and water resistance
The cellulosic adhesives suffer from loss of strength under wet
conditions and support the growth of microorganisms

28. Protein Based
 Made from proteins from animals, blood, casein (milk), and soy
 Usually mixed with water and lime and most commonly cure at
room temperature
 New soy adhesives are being combined with formaldehyde based
adhesives to reduce VOC’s and use a renewable feedstock
 Advantages
 High dry strength, good thermal resistivity
 Disadvantages
 Poor moisture and biological resistance
 Still used in interior doors and furniture because of good fire
performance

29. Tannin Adhesives
 Tannins are polyhydroxypolyphenolics that occur in many
plant species
 Tannins have been used as adhesives in South Africa,
Australia, Zimbabwe, Chile, Argentina, Brazil, and New
Zealand
Advantage- More reactive than phenol and used in composite
(particleboard and medium density fiberboard) production,
laminate and finger joint bonding, and for damp-ply resistant
corrugated cardboard
Disadvantage- High viscosity, limited availability, and
inconsistent source

30. Lignin Adhesives
 Lignins are phenolic derivatives, they are very different
from tannins
 They constitute 24–33% of the woody substance in
softwoods and 16–24% in hardwoods
Uses- Lignin has been used with PF resins
Advantage– Available in large quantities at low cost,
completely aromatic
Disadvantage- They are much slower in their reaction , have
only a few phenolic rings, no polyhydroxy phenyl rings

31. CONSTRUCTION ADHESIVES
 These are large segment of the adhesive market, with their
use for attachment of floor and wall coverings, and in assembly
of buildings
 Construction adhesives are usually elastomers
 The use of an adhesive can give extra rigidity to the
structure
Advantages- Good strength, high molecular weight
Disadvantage- Embrittlement and fracture

32. OTHER ADHESIVES
 Contact adhesives have been used for bonding of plastic
laminates to wood
 Contact adhesives are mainly used in bonding plastic
laminates to particleboard for countertops and furniture
 Film adhesives involve either partially cured adhesives or
adhesives applied onto a carrier suchas a fiberglass mat or tissue
paper
 These are used where applying a liquid adhesive may be
difficult, such as in bonding of very thin wood veneers

33. CONCLUSION
 Wood structure has so many variables in the different species, cell
structures within a species, and spatial scales of examination that it is
hard to model the process, despite these problems, many adhesives
have been developed that are stronger and some are even more
durable than the wood itself
 Many functional adhesives have been developed that allow a wide
variety of woods and wood pieces to be glued together in a useful and
cost-effective manner
 The area that is best understood is the chemistry of the adhesives,
even though there are aspects, such as the effects of the composite
processing dynamics, that need to be more thoroughly researched