Phenol formaldehyde resins are synthetic polymers obtained by the reaction of phenol or substituted
phenol with formaldehyde.
Phenolic resins are mainly used in the production of circuit boards.
They are better known however for the production of molded products including pool balls, laboratory
countertops, and as coatings and adhesives.
In the form of Bakelite, they are the earliest commercial synthetic resin.
Formation and structure Phenol-formaldehyde resins, as a group, are
formed by a step-growth polymerization reaction that can be either acid- or base-catalysed.
Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene
glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature
and pH.
Phenol reacts with formaldehyde at the ortho and para sites allowing up to 3 units of formaldehyde
to attach to the ring.
The initial reaction in all cases involves the formation of a hydroxymethyl phenol:
HOC6H5 + CH2O → HOC6H4CH2OH The hydroxymethyl group is capable of reacting
with either another free ortho or para site, or with another hydroxymethyl group.
The first reaction gives a methylene bridge, and the second forms an ether bridge:
HOC6H4CH2OH + HOC6H5 →2CH2 + H2O The diphenol2CH2 is called bisphenol F, which
is an important monomer in the production of epoxy resins.
Bisphenol-F can further link generating tri- and tetra-and higher phenol oligomers.
2 HOC6H4CH2OH →2O + H2O Novolacs
Novolacs are phenol-formaldehyde resins with a formaldehyde to phenol molar ratio of less
than one.
The polymerization is brought to completion using acid-catalysis such as oxalic acid,
hydrochloric acid or sulfonate acids.
The phenol units are mainly linked by methylene and/or ether groups.
Novolacs are commonly used as photoresists.
See also photolithography.
The molecular weights are in the low thousands, corresponding to about 10–20 phenol units.
Hexamethylenetetramine or "hexamine" is a hardener added to crosslink novolac.
At a temperature >90 °C, it forms methylene and dimethylene amino bridges.
Resoles Base-catalysed phenol-formaldehyde resins
are made with a formaldehyde to phenol ratio of greater than one.
These resins are called resoles.
Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on
the resin to be formed, and are then heated.
The first part of the reaction, at around 70 °C, forms a thick reddish-brown tacky
material, which is rich in hydroxymethyl and benzylic ether groups.
The rate of the base-catalysed reaction initially increases with pH, and reaches a maximum at
about pH = 10.
The reactive species is the phenoxide anion formed by deprotonation of phenol.
The negative charge is delocalised over the aromatic ring, activating sites 2, 4 and 6,
which then react with the formaldehyde.
Being thermosets, hydroxymethyl phenols will crosslink on heating to around 120 °C to
form methylene and methyl ether bridges through the elimination of water molecules.
At this point the resin is a 3-dimensional network, which is typical of polymerised phenolic
resins.
The high crosslinking gives this type of phenolic resin its hardness, good thermal stability,
and chemical imperviousness.
Resols are referred to as "one step" resins as they cure without a cross linker unlike
novolacs, a "two step" resin.
Resols are major polymeric resin materials widely used for gluing and bonding building
materials.
Exterior plywood, oriented strand boards, engineered laminated composite lumber are
typical applications.
Crosslinking and the phenol/formaldehyde ratio When the molar ratio of formaldehyde:phenol
reaches one, in theory every phenol is linked together via methylene bridges, generating
one single molecule, and the system is entirely crosslinked.
This is why novolacs do not harden without the addition of a crosslinking agent, and
why resols with the formula F:P >1 will.
Applications Phenolic resins are found in myriad industrial
products.
Phenolic laminates are made by impregnating one or more layers of a base material such
as paper, fiberglass or cotton with phenolic resin and laminating the resin-saturated base
material under heat and pressure.
The resin fully polymerizes during this process.
The base material choice depends on the intended application of the finished product.
Paper phenolics are used in manufacturing electrical components such as punch-through
boards and household laminates.
Glass phenolics are particularly well suited for use in the high speed bearing market.
Phenolic micro-balloons are used for density control.
Snooker balls as well as balls from many table-based ball games are also made from phenol formaldehyde
resin.
The binding agent in normal brake pads, brake shoes and clutch disks are phenolic resin.
Synthetic resin bonded paper, made from phenolic resin and paper, is used to make countertops.
Phenolic resins are also used for making exterior plywood commonly known as WBP Plywood because
Phenolic resins have no melting point but only a decomposing point in the temperature
zone of 220 degree Celsius & above.
Phenolic resin is used as a binder in loudspeaker driver suspension components which are made
of cloth.
Sometimes people select phenolic resin parts because their coefficient of thermal expansion
closely matches that of the aluminum used for other parts of a system, as in early computer
systems and Duramold.
The Dutch forger Han van Meegeren mixed phenol formaldehyde with his oil paints before baking
the finished canvas in order to fake the drying out of the paint over the centuries.
Trade names Maritex Aquarius is a very high performance
bearing material made using novolac resin, fibre reinforcement and added lubricants for
momentary dry run situations.
It is also used dry at slow speeds and high pressures, showing very low wear rates of
shaft or bearing.
Bakelite is made from phenolic resin and wood flour.
Novotext is cotton fibre-reinforced phenolic, using randomly oriented fibres.
Oasis is "an open-celled phenolic foam that readily absorbs water and is used as a base
for flower arrangements."
Biodegradation Phenol-formaldehyde is degraded by the white
rot fungus Phanerochaete chrysosporium See also
Urea-formaldehyde References
External links No longer available from the previously linked
source.