In: Chemistry
Describe the stage of polymerization through which a phenolic resin passes to become thermoset.
In aqueous solution, formaldehyde exists in equilibrium with
methylene glycol.
Depending on the pH of the catalyst, these monomers react to form
one of two general resin types: NOVOLAC RESINS and RESOL
RESINS.
Novolac Resins
An acidic catalyst and a molar excess of phenol to formaldehyde
are conditions used to make novolac resins. The following
simplified chemistry illustrates the wide range of polymers
possible. The initial reaction is between methylene glycol and
phenol.The reaction continues with
additional phenol, and splitting off of water.
The reaction creates a methylene bridge at either the ortho
position or the para position of the phenolic aromatic rings. The
"rule of thumb" is that the para position is approximately twice as
reactive as the ortho position, but there are twice as many ortho
sites (two per phenol molecule) so the fractions of ortho-ortho,
para-para and ortho-para bridges are approximately equal.
Branching occurs because reaction can occur at any of three sites
on each ring. As the reaction continues, the random orientations
and branching quickly result in an extremely complex mixture of
polymers of different sizes and structures. The reaction stops when
the formaldehyde reactant is exhausted, often leaving up to 10% of
un-reacted phenol. Distillation of the molten resin during
manufacturing removes the excess phenol and water.
The final novolac resin is unable to react further without the
addition of a cross-linking agent. Plenco novolac resins come with
and without a curing agent. The resins having the curing agent
incorporated cure or "thermoset" to the desired degree when
processed by the customer.
Because an additional agent is required to complete the resin's
cure, the industry commonly refers to novolac resins as "two-stage"
or "two-step" products. The most common phenolic resin
cross-linking agent is hexamethylenetetramine, also known as hexa,
hexamine, or HMTA. Ground and blended with the resin, hexa serves
as a convenient source of formaldehyde when heated to molding and
curing temperatures. A special attribute of hexa is that it reacts
directly with resin and phenol without producing appreciable
amounts of free formaldehyde. Hexa cures the resin by further
linking and polymerizing the molecules to an infusible state. Due
to the bond angles and multiple reaction sites involved in the
reaction chemistry, the resulting polymer is not a long straight
chain but rather a complex three-dimensional polymer network of
extreme molecular weight. This tightly cured bonding network of
aromatic phenolics accounts for the cured materials' hardness, and
heat and solvent resistant properties.
Certain catalysts can affect the orientations of the methylene
linkages. Catalysts that preferably promote ortho-ortho linkages
tend to preserve the more reactive para positions:
Novolac resins made with these catalysts tend to cure more rapidly
than the standard randomly linked resins. Novolac resins are
amorphous (not crystalline) thermoplastics. As they are most
typically used, they are solid at room temperature and will soften
and flow between 150° and 220°F (65°C - 105°C). The number average
molecular weight (Mn) of a standard phenol novolac resin is between
250 and 900. As the molecular weight of phenol is 94 grams per
mole, a Mn of 500 corresponds to a resin where the average polymer
size in the entire distribution of polymers is five linked phenol
rings. Novolac resins are soluble in many polar organic solvents
(e.g., alcohols, acetone), but not in water.
Characteristics
Bonding Strength
The primary use of phenolic resin is as a bonding agent.
Phenolic resin effortlessly penetrates and adheres to the structure
of many organic and inorganic fillers and reinforcements, which
makes it an ideal candidate for various end uses. A brief thermal
exposure to complete the cross-linking or "thermoset" process
results in attainment of final properties. The unique ability of
phenolic resin to "wet out" and to cross-link throughout the
fillers and reinforcements provides the means to engineer the
desired mechanical, thermal, and chemically resistant
properties.
Applications benefiting from the hardness, and heat and chemical
resistance properties afforded phenolic resins include abrasive
grinding wheels, friction linings, refractory products, and other
molded parts used in high temperature or aggressive environments.
For years, phenolic resin's exceptional compatibility with
cellulose fillers has been used to great benefit for particleboard,
plywood, hardboard, oriented strand board, substrates for melamine
laminates and decking applications. Composites for demanding
applications such as on oil platforms, missile components, and heat
shields are produced using phenolic resins along with process
technologies such as resin transfer molding (RTM), pultrusion, or
filament winding.
Liquid phenolic resins penetrate and saturate paper and other
substrates to provide good mechanical strength, electrical
properties, or filtration capabilities. Typical examples of these
applications include NEMA electrical laminates, decorative
laminates, clutch and transmission papers, and filtration
products.
High Temperature Performance
A key characteristic of thermoset phenolic resin is its ability to withstand high temperature under mechanical load with minimal deformation or creep. In other words, cured phenolic resin provides the rigidity necessary to maintain structural integrity and dimensional stability even under severe conditions. For this reason, phenolic resin binders meet the challenges of high temperature environments in demanding applications such as refractory, friction, foundry and aerospace products. Examples of applications that take special advantage of the dimensional stability of phenolic molding compounds are natural gas valves, automotive brake pistons, pulleys, and hydraulic and water pump housings and seals.