ebb 427 (4) hazizan
TRANSCRIPT
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EBB 427 Application and Technology of
Engineering Polymers (Second Half)
Dr. Hazizan Md Akil
School of Materials and Mineral Resources Engineering
Engineering Campus, USM.
PHENOL FORMALDEHYDHE
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Phenol-Formaldehyde Polymers
Introduction
Phenol-formaldehyde polymers formed by the interaction
of phenol, or mixture of phenols, and formaldehyde
Commercial materials are most commonly based on
phenol itself, other phenols such as cresols, xylenols and
resorcinol are used to a limited extent
It may be noted that several aldehydes other than
formaldehyde have been used to prepare phenolic
polymers but none has attained appreciable commercial
significance.
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Phenol-Formaldehyde Polymers
Introduction
Phenol-formaldehyde reaction was first recorded in 1872
by A. Beyer.
The principal current uses include thermosetting moulding
powders (which are widely used in such applications as
general purpose electrical mouldings, heated appliance
components and automotive parts, laminates (which areextensively used for printed circuit boards and for the core
of decorative laminates, adhesives, binders and surface
coating.
Phenol formaldehyde polymers are also of significance inthat they were the first wholly synthetic polymers to be
utilised.
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
Phenol occurs in coal tar and at one time this source
satisfied commercial demands.
Nowadays, majority of phenol is synthesized from
benzene.
Firstly, benzene is alkylated with propylene to give
isopropyl benzene (cumene)
In vapour or liquid phase
In vapour, propylene and excess benze is passed over the
catalyst of phosphoric acid at 250C at 2.5 MPa.
The exit gases are fractionated
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
Cumene is taken off
Unreacted benzene is recycled.
Conversion is restricted to about 15% in order to limit the
formation of diisopropylbenzenes
In liquid phase alkylation, the catalyst is aluminiumchloride
Reaction is carried out at 50-100C at slightly above
atmospheric pressure.
Cumene is recovered by distillation Unreacted benzene is recycled
Up to 50% of benzene is converted per pass.
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
In the second step in the synthesis of phenol, cumene is
oxidised to cumene hydroperoxide with air.
The reaction is carried out either in an anhydrous system
or in the presence of a small amount of water.
The reaction mixture is maintained at pH7 by the addition
of small quatities of alkali
The conversion of cumene is restricted to 35-50% per
pass to reduce by-product formation.
Unreacted cumene is removed by distillation
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
The oxidation of cumene proceeds by a free radical
mechanism, of which the following are the more important
reactions:
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
In the third step in the synthesis of phenol, the cumene
hydroperoxide undergoes cleavage to phenol and acetone
This is usually accomplish by feeding the hydroperoxide,
with sulphuric acid as catalyst, continuously into previously
decomposed material maintained at about 50-92C.
The product is then neutralized and fractionated by
distillation.
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
In addition to phenol and aceton, there are obtained minor
amounts of other products such as acetophenone, a-
methylstyrene and phenyl dimethyl carbinol
The yield of phenol on benzene is about 85%
The economics of the process are bound up with the value
of the co-product, acetone.
Phenol is colourless crystalline solid, m.p. 41C.
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Phenol-Formaldehyde Polymers
Raw Materials (Phenol)
Other phenols:
Cresol
Xylenols
Resorcrinol
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Phenol-Formaldehyde Polymers
Raw Materials (Formaldehyde)
The most common route for the preparation of
formaldehyde is the following:
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Phenol-Formaldehyde Polymers
Raw Materials (Formaldehyde)
Methanol is prepared by the reaction of carbon monoxide
and hydrogen
In older plants in which a promoted zinc oxide catalyst is
utilised, reaction conditions are 300-400C and about
30MPa.
In newer plants a copper-based catalyst is employed
This allows the use of milder conditions, namely 200-
300C and 5-10 MPa
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Phenol-Formaldehyde Polymers
Raw Materials (Formaldehyde)
Methanol is condensed out and unreacted gases, with
fresh make-up gas, recycled to converters.
In the second stage, methanol is oxidised to formaldehyde
In one process a mixture of methanol vapour and air is
passed over a catalyst of molybdenum oxide promoted
with iron at 350-450C
The exit gas are scrubbed with water and the
formaldehyde is isolated as an aqueous solution
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Phenol-Formaldehyde Polymers
Raw Materials (Formaldehyde)
Formaldehyde is a colourless gas with a pungent odour,
b.p. -19
Formaldehyde is commercially most commonly available
as an aqueous solution, known as formalin.
Generally, formalin contains 40%w/v (37% w/w)
formaldehyde and about 8% methanol which act as
stabiliser, retarding polymerization and preventing
precipitation of insoluble polymers
In aqueous solution, monomeric formaldehyde is mainly in
the form of methylene glycol rather than in the free state
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Phenol-Formaldehyde Polymers
Raw Materials (Formaldehyde)
The preparation of high molecular weight
polyformaldehyde requires extremely pure formaldehyde.
Formaldehyde obtained directly from formalin often
contains impurities such as water, methanol and formic
acid and is not suitable.
Thus, trioxan and paraformaldehyde which can be
obtained in a state of high purity, are convinient sources.
It may be noted that the production of polyformaldehyde
consumes a very minor proportion of the total output of
formaldehyde
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Phenol-Formaldehyde Polymers
Preparation of Resin
Polymerization is normally carried out in two separateoperations
The first operation involves the formation of low molecular
weight fusible, soluble resin
The second operation involves curing reactions which leadto the cross-linked product
Various types of initial low molecular weight resins are
produced commercially
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Phenol-Formaldehyde Polymers
Several types of resins:
Resols resinsNovolak resin
Surface coating resin
Casting resin
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Phenol-Formaldehyde Polymers
Resol Resin
Interaction between a phenol with a molar excess offormaldehyde (1:1.5-2) under alkaline solution
Used mainly in preparation of adhesives, binders and
laminates
Typically, reaction is carried out in a stirred reactor,jacketed for heating and cooling
Reactor is also fitted with condensor for refluxing
A mixture of phenol, formalin and ammonia (1-3% wt of
phenol) is heated under reflux at 100C for 0.25 to 1 hour Water is then removed by distillation under reduced
pressure
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Phenol-Formaldehyde Polymers
Resol Resin
Distillation continued until cooled sample of the residualresin has a melting point of 45C-50C.
The resin is quickly discharged and cooled to give a hard,
brittle solid.
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Phenol-Formaldehyde Polymers
Novolak Resin
Novalak (or novalacs) are normally prepared by theinteraction of a molar excess of phenol with formaldehyde
(commonly 1.25:1) under acidic condition
The reaction is commonly carried out batch-wise in a
reactor of the type described in the resol resin.
A mixture of phenol, formalin and acid (usually oxalic acid-
0.5-2% weight of phenol) is heated under reflux at about
100C
Heating is continued for 2-4 hours
Water is then distilled, usually at atmospheric pressure,
until a cooled sample of the residual resin has a melting
point in the range of 65-75C.
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Phenol-Formaldehyde Polymers
Novolak Resin
The resin is then discharged and cooled to give a hard,brittle solid.
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Phenol-Formaldehyde Polymers
Surface Coating Resin
The use of straight resol and novolak resin derived fromphenol in surface coating is very limited, mainly because of
the brittleness of the film.
Further, vegetable oil cannot be used as plasticiser since
they are incompatible with the resin.
However, there are a number of ways in which oil-solubility
can be achieved and large quantities of phenolic resins
suitable for surface coatings are produced such as:
Resin modification
Use of substituted phenols
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Phenol-Formaldehyde Polymers
Surface Coating Resin
Resin modification Simple resol is heated at 150C-300C with an excess
(10:1 by weight) of rosin (abietic acid)
Use of substituted phenols
Both resol and novolak resins prepared from the phenoldescribed earlier are soluble in drying oils
Typically, the resin is heated with the oil (about equal
weight) at 150-200C for 0.5 hours
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Phenol-Formaldehyde Polymers
Casting Resin
Phenolic casting resins were at one time importantmaterials, being used for umbrella handles, artificial
jewellery, knobs and such objects where decorative effects
are required
Casting resin are prepared by the interaction of phenolwith large molar excess of formaldehyde (commonly about
1:2.3).
In a typical process, a mixture of phenol, formalin and
sodium hydroxide is heated at 70C for 3 hours.
Water is then removed by distillation under reduced
pressure
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Phenol-Formaldehyde Polymers
Casting Resin
The resulting resin is mixed with strong acid, e.g.benzenesupphonic acid, immediately poured into a mould
and allowed to harden either at room temperature or at 60-
80C
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Phenol-Formaldehyde Polymers
Cross-linking of the following:
Resols resins
Novolak resin
Surface coating resin
Casting resin
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Phenol-Formaldehyde Polymers
Cross-linking of resol resin
Resol which are produced under alkaline conditions, are
generally neutralized or made slightly acid before cure is
carried out.
Network polymers are then obtained simply by heating
The cure of resol is extremely complex, involving a number
of competing reactions.
Each of which may be influenced by reaction conditions
and it is not easy to unravel precisely what takes place.
To summarise, it may be said that the network polymer
obtained from resol is composed principally of phenolic
nuclei joined by methylene groups but there is possibility of
other types of linkages
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Phenol-Formaldehyde Polymers
Cross-linking of resol resin
The nature and extent of linkages depends on the nature
of the resol and the conditions of cure.
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Phenol-Formaldehyde Polymers
Cross-linking of Novolak resin
The conversion of novolaks into network polymers can be
accomplish only after the addition of a cross-linking agent.
Although novolaks can be cross-linked by reaction with
additional formaldehyde or with paraform,
hexamethylenetetramine (HMTA) is almost invariably used
for this purpose.
The mechanism of curing is not fully understood
On the basis of several works, it is possible that the
primary reaction between a novolak and HMTA leads to a
complex structure containing secondary and tertiary aminelinks.
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Phenol-Formaldehyde Polymers
Cross-linking of Novolak resin
On further heating, most of these links break down to give
methylene links and links may be formed and may account
for the characteristic brown colour of the cured material.
To summarise, it may be said that the network polymer
obtained from HMTA-cured is composed of phenolic nuclei
joined mainly by methylene groups with small numbers ofvarious nitrogen-containing links.
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Phenol-Formaldehyde Polymers
Cross-linking of casting resin
Casting resins have a resol-type structure but contain a
greater proportion of methylol groups
It may therefore be anticipated that an appreciable amount
of ether links are produced when a casting resin is cured.
Since castings are normally cured at relatively low
temperatures these ether links are likely to persist in the
final product
It may be noted that it is possible to prepare castings
which are colourless, whereas resols cured at 150C are
dark coloured
Ph l F ld h d P l
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Phenol-Formaldehyde Polymers
Cross-linking of casting resin
This observation is in accordance with the suggestion that
it is the thermal decomposition of ether links into quinone
methides which leads to colour formation in resols.
Similar to resol-type resin.
Ph l F ld h d P l
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Phenol-Formaldehyde Polymers
Properties of cross-linked polymers
Crosslinked phenolic are rigid, infusible and insoluble
The mechanical properties of the network polymers are
considerably influenced by the incorporation of fillers.
This point is illustrated in Table 14.1.
The polymers have good thermal and heat stability,showing little weight loss up to about 200C
Since the polymer is polar, its electrical insulating
properties are not outstanding but they are adequate for
many purposes.
Ph l F ld h d P l
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Phenol-Formaldehyde Polymers
Properties of cross-linked polymers
Also, phenolics have relatively poor tracking resistance
under conditions of high humidity, i.e. there is a tendency
to form a conductive path through carbonization along the
surface of material situated between two metal electrodes
Cured phenol-formaldehyde polymers are very resistant to
most chemical reagents
They are unaffected by all ordinary organic solvents and
water, although the presence of cellulosic fillers results in
water absorption and consequent swelling.
Ph l F ld h d P l
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Phenol-Formaldehyde Polymers
Properties of cross-linked polymers
The polymers are dissolved slowly with decomposition by
boiling phenols such as -naphthol
Simple phenol-formaldehyde materials are readily attacked
by aqueous sodium hydroxide but cresol and xylenol
based polymers are more resistant
The polymers are resistant to acids except formic acid and
strong oxidising acids but the presence of cellulosic fibres
increases the sensitivity of mouldings towards acids