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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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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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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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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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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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The oxidation of cumene proceeds by a free radical

mechanism, of which the following are the more important

reactions:

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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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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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Other phenols:

Cresol

Xylenols

Resorcrinol

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Raw Materials (Formaldehyde)

The most common route for the preparation of

formaldehyde is the following:

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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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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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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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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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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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Several types of resins:

Resols resinsNovolak resin

Surface coating resin

Casting resin

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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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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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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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Novolak Resin

The resin is then discharged and cooled to give a hard,brittle solid.

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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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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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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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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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Cross-linking of the following:

Resols resins

Novolak resin

Surface coating resin

Casting resin

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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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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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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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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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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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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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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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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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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

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