13 Surface Coatings, Plasticisers and Rubbers

Surface Coatings, Plasticisers and Rubbers 741

These resins are produced by reacting a polyhydric alcohol, usually glycerol,

with a polybasic acid, usually phthalic acid and the fatty acids of various oils

such as linseed oil, soya bean oil and tung oil. These oils are triglycerides of the

type shown in Figure 25.30. R1, R2 and R3 usually contain unsaturated

groupings. The alkyd resins would thus have structural units, such as is shown in

Figure 25.31.

R,



R

2



I



I

c=o c=o

I

I

0

0

I

I

CH,-



R3



I

I

0

I



c=o



CH - CH2



Figure 25.30



In modem manufacturing methods the oil is sometimes reacted directly with

the glycerol to form a monoglyceride and this is then reacted with the acid to

form the alkyd resin. When the resulting surface coating is applied to the

substrate the molecules are substantially linear. However, in the presence of

certain 'driers' such as lead soaps there is oxidative cross-linking via the

unsaturated group in the side chain and the resin hardens.



I



0



I

Long Chain Containing

Unsaturated Groups



/'='



Figure 25.31



The alkyd resins are of value because of their comparatively low cost,

durability, flexibility, gloss retention and reasonable heat resistance. Alkyd resins

modified with rosin, phenolic resin, epoxy resins and monomers such as styrene

are of current commercial importance.I3.l4

The hardening process involved with the alkyd resins, because it involves air

oxidation, is only satisfactory with thin coating films. This limitation has been

circumvented by the use of unsaturated polyester resins for coating applications.

These may be considered as developments of the laminating resins discussed in

Section 25.2. The latter resins were, however, not altogether satisfactory because

the phenomenon of air inhibition of cure (see page 683) became serious when the

resin was used in an application where there was such a high surface/volume

ratio. The use of additives such as hydrogenated castor oil and paraffin wax in

quantities up to 0.5% is helpful but leads to two limitations. Firstly a thin layer

of wax is formed at the resin-substrate interface, weakening the adhesive bond.

Secondly the hardened film has a dull appearance which may require buffing if

a gloss effect is needed.



742 Polyesters

This has led to chemical modification of the polyesters, in particular the

introduction of allyl ether groups into the resins. Amongst the monomers figuring

prominently in the literature are allyl glyceryl ether I, trimethylolpropane diallyl

ether I1 ( I , 1-diallyloxypropanol) and pentaerythritol triallyl ether I11 (2,2,2-triallyloxyethanol), as shown in Figure 25.32.

HO . CH, . C H . CH, . O . CH, . C H = C H ,



1



0 . CH, . C H = CH,



/

C H , . CH, . C -0.



CH, . C H = CH,



\OH

(11)



0 . CH, . C H = CH,



HO . CH, . C



/

-0.



CH, . C H = CH,



\o.



C H , . c H = CH,



(111)

Figure 25.32



The resins are commonly cured by the use of peroxide with or without cobalt

accelerators, depending on whether the hardening is to be carried out at room

temperature or at some elevated temperature. Electron irradiation curing, which

can be completed within a few seconds, has, however, been introduced for

coatings on large flat surfaces such as plywood, chipboard and metal panels.

Whilst the unsaturated polyester coatings show high hardness, excellent clarity

and good abrasion and chemical resistance their prime advantage is the ability to

produce thick coatings. The main application area is in wood coating such as

bedroom furniture and television cabinets and for steel coating in, for example,

boat and bridge structures.

Low molecular weight liquid polyester resins are useful as plasticisers,

particularly for PVC, where they are less volatile and have greater resistance to

extraction by water than monomeric plasticisers. Examples of such plasticisers

are poly(propy1ene adipate) and poly(propy1ene sebacate). In some cases

monobasic acids such as lauric acid are used to control the molecular weight.

Cross-linkable rubbery polyesters have been produced but are now no longer

produced. Rubbery polyester-amides were introduced by IC1 under the trade

name Vulcaprene as a leathercloth material but later were used primarily as

leather adhesives and as flexible coatings for rubber goods. A typical polymer

may be made by condensing ethylene glycol, adipic acid and ethanolamine to a

wax with a molecular weight of about 5000.

HO*(CH,),*OH



+ HOOC*(CH2)4*COOH+ NHz*(CH2)2*OH

-----+ ~O(CH2)2OOC(CH2)4CONH(CH2)20~



Reviews



+



CH,O



-



wCO.NHw



CH2



i



743



+ H20



wC0.NFigure 25.33



The chain length of the polymer is then increased by reacting the wax via the

hydroxyl, amino or acid end groups with a di-isocyanate such as hexamethylenedi-isocyanate (see Chapter 27 for the appropriate reaction).

The rubbers were vulcanised by formaldehyde donors reacting across

-NH-groups

in the main chain (Figure 25.33).



References

1. BERZELIUS, I., Rap. Ann. Progr. sei. Physq., 26 (1847)

2. LIJSSAC, I. G., and PELOUZE, J., Ann., 7, 40 ( 1 833)

3. BRYDSON, J. A,, and WELCH, c. w., Plastics (London), 21, 282 (1956)

4. scorr, K . A., and GALE, G. M . , Rubber and Plastics Research Association of Great Britain,

Research Report 132 (September 1964)

5. MORGAN, P., Glass Reinforced Plastics, Iliffe, London, 3rd Edn (1961)

6. SONNERBORN, R. H., Fibreglass Reinforced Plastics, Reinhold, New York (1954)

7. DE DAN[. A ., Glass Fibre Reinforced Phstics, Newnes, London (1960)

8. LAWRENCE, J. R., Polyester Resins, Reinhold, New York (1960)

9. HAGEN, H., Glasfaserverstarkte Kunstoffe, Springer-Verlag, Berlin (1961)

10. RAECH, H., Allylic Resins and Monomers, Reinhold, New York (1965)

11. GREATREX, J. L., and HAYNES, I. E., Brit. Plastics, 35, 340 (1962)

12. WATSON, M. T., Soc. Plastics Engrs, 1083 (October 1961)

13. MARTENS, c. R., Alkyd Resins, Reinhold, New York (1961)

14. PATTON, T. e., Alkyd Resin Technology, Interscience, New York (1962)

15. GOODMAN, I., and RHYS, J. A., Polyesters, Vol. I: Saturated Polymers, IIiffe, London (1965)



Bibliography

Polyesters and their Applications, Reinhold, New York (1956)

Glass Fibre Reinforced Plasrics, Newnes, London (1960)

GOODMAN, I., and RHYS, I. A., Polyesters, Vol. I: Saturated Polymers, IIiffe, London (1965)

HAGEN, H., Glasfaserverstarkte Kunstoffe, Springer-Verlag, Berlin (1 961)

HILL, R., Fibres from Synthetic Polymers, Elsevier, Amsterdam (1 953)

LAWRENCE, J. R., Po[yester Resins, Reinhold, New York (1960)

MARTENS, c. R., Alkyd Resins, Reinhold, New York (1961)

MORGAN, P., Glass Reinforced Plastics, IIiffe, London, 3rd Edn (1961)

PARKYN, E . , LAMB, F., and C L I F r O N , B. v., Polyesters, vo[, 2: Unsaturated Polyesters and Polyester

Plasticisers, Iliffe, London (1967)

PATTON, T. c., Alkyd Resin Technology, Interscience, New York (1962)

PETUKHOV, B. v., The Technology of Polyester Fibres, Pergamon, Oxford (1963)

RAECH, H., Allylic Resins and Monomers, Reinhold, New York (1965)

SONNERBORN, R. H., Fibreglass Reinforced Plastics, Reinhold, New York (1956)

WEATHERHEAD, R. G., FRP Technology, Applied Science, London (1980)

B J o R K s r E N , J.,



DE DAN[, A.,



Reviews

and KAINMULLER, T., KUnStOffe, 80 (lo), 1121 (1990)

Kunstofle, 77, 1004-9 (1987)

CLAUSS, J. and MITCHELL, K., KUnStOffe,86, 1506-1508 (1996)

DIETRICH,H. I., ZANDER, K. and LEHNERT. G., Kunstoffe, 86, 1510-1512 (1996)

FISCHER, w., GEHRKE. J. E., and REMPEL, D., Kunstoffe, 70, 650-5 (1980)

SCHIK, I. P., Kunstoffe, 70, 1004-9 ( 1 9 8 7 )

BREITENFELLNER, E



CAESAR, H. M.,



26



Epoxide Resins



26.1



INTRODUCTION



The epoxide resins (also widely known as epoxy resins and, occasionally, as

ethoxyline resins) are characterised by the possession of more than one 1,2epoxy group (I) per molecule. This group may lie within the body of the molecule

but is usually terminal.



/"\



-CH-CH-



(1)



The three-membered epoxy ring is highly strained and is reactive to many

substances, particularly by with proton donors, so that reactions of the following

schematic form can occur:



/"\



-CH-CH-



OH



+ HX



I



X



-CH-CH-



Such reactions allow chain extension andfor cross-linking to occur without the

elimination of small molecules such as water, i.e. they react by a rearrangement

polymerisation type of reaction. In consequence these materials exhibit a lower

curing shrinkage than many other types of thermosetting plastics.

There is, quite clearly, scope or a very wide range of epoxy resins. The nonepoxy part of the molecule may be aliphatic, cycloaliphatic or highly aromatic

hydrocarbon or it may be non-hydrocarbon and possibly polar. It may contain

unsaturation. Similar remarks also apply to the chain extension/cross-linking

agents, so that cross-linked products of great diversity may be obtained. In

practice, however, the commercial scene is dominated by the reaction products of

bis-phenol A and epichlorohydrin, which have some 80-90% of the market

share.

744



Preparation of Resins from Bis-phenol A



745



The commercial interest in epoxide (epoxy) resins was first made apparent by

the publication of German Patent 676 117 by I G Farben' in 1939 which

described liquid polyepoxides. In 1943 P. Castan* filed US Patent 2 324 483,

covering the curing of the resins with dibasic acids. This important process was

subsequently exploited by the Ciba Company. A later patent of Castan3 covered

the hardening of epoxide resins with alkaline catalysts used in the range 0.1-5%

This patent, however, became of somewhat restricted value as the important

amine hardeners are usually used in quantities higher than 5%.

In the early stage of their development the epoxy resins were used almost

entirely for surface coating and developments in this field are to a large extent

due to the works of S.O. Greenlee and described in a number of patents. These

included work on the modification of epoxy resins with glycerol4, the

esterifiction of the higher molecular weight materials with drying oil acidss and

reactions with phenolic6 and amino resin^.^

Before World War I1 the cost of the intermediates for the these resins (in most

cases epichlorohydrin and bis-phenol A) would have prevented the polymers

from becoming of commercial importance. Subsequent improvements in the

methods of producing these intermediates and improved techniques of polymerisation have, however, led to wide commercial acceptance.

By the beginning of the 1980s world capacity for epoxide resins reached about

600 000 tonnes per annum but at this time plant utilisation was only about

50-60%. Thus with a global consumption of about 10 million tonnes per annum

for thermosetting plastics, epoxide resins had a share of about 3%. Western

Europe and the USA each had about 40% of the market and Japan a little over

10%. This situation has not greatly changed since then; but by the late 1990s the

world market for epoxide resins had risen to about 750 000 t.p.a.

About half of epoxide resin production is used for surface coating applications,

with the rest divided approximately equally between electronic applications

(particularly for printed circuit boards and encapsulation), the building sector and

miscellaneous uses. In tonnage terms consumption of epoxide-fibre laminates is

only about one-tenth that of polyester laminates, but in terms of value it is much

greater.

Whilst the properties of the cross-linked resins depend very greatly on the

curing system used and on the type of resin, the most characteristic properties

of commercial materials are their toughness, low shrinkage on cure, high

adhesion to many substrates, good alkali resistance and versatility in

formulation.



26.2



PREPARATION OF RESINS FROM BIS-PHENOL A



The first, and still the most important, commercial epoxide resins are reaction

products of bis-phenol A and epichlorhydrin. Other types of epoxide resins were

introduced in the late 1950s and early 1960s, prepared by epoxidising

unsaturated structures. These materials will be dealt with in Section 26.4. The

bis-phenol A is prepared by reaction of the acetone and phenol (Figure 26.1).

Since both phenol and acetone are available and the bis-phenol A is easy to

manufacture, this intermediate is comparatively inexpensive. This is one of the

reasons why it has been the preferred dihydric phenol employed in epoxide resins

manufacture. Since most epoxide resins are of low molecular weight and because



746 Epoxide Resins



+



Em-@



c=o

I

I



+



@-OH



Figure 26.1



colour is not particularly critical the degree of purity of the bis-phenol A does not

have to be so great as when used in the polycarbonate resins. Bis-phenol A with

a melting point of 153°C is considered adequate for the most applications whilst

less pure materials may often be employed.

Epichlorohydrin, the more expensive compound is derived from propylene by

the sequence of reactions shown in Figure 26.2.



CH,=CH

II



+ Cl*+



CH2=CH

I

I



+ HCl



CH,



CH -CH

2-



CH**Cl



Propylene



Allyl Chloride



+ H20/C12 -----+ Cl-CCH~-CH(OH)-CH2-CCI



I

CHZCl



Dichlorohydrin



Figure 26.2



It will noticed that the initial steps correspond with those used in the

manufacture of glycerol. The material is available commercially at 98% purity

and is a colourless mobile liquid.

Many of the commercial liquid resins consist essentially of the low molecular

weight diglycidyl ether of bis-phenol A together with small quantities of higher

molecular weight polymers. The formation of the diglycidyl ether is believed to

occur in the manner shown in Figure 26.3, the hydrochloric acid released

reacting with the caustic soda to form sodium chloride.

Although it would appear, at first glance, that diglycidyl ether would be

prepared by a molar ratio of 2: 1 epichlorohydrin-bis-phenol A, probability

considerations indicate that some higher molecular weight species will be

produced. Experimentally it is in fact found that when a 2: 1 ratio is employed, the

yield of the diglycidyl ether is less than 10%. Therefore in practice two to three



Preparation of Resins from Bis-phenol A

CH



/o\



+ HO+C----@



c~--CH,-CH-CH,



I

I



OH



141



/o\



+ CH,-CH-CH~CI



CH,

CH,



OH



C1-CH2-



C!H-CH>--



0



OH



- ($- L- @I



I



0-CH,-CH-CHrCl



CH,



CH,



/o\



% CH~--CH-CH,--O-@-~--@-



I



/o\



0-CH,-CH-CH,



+ 2HC1



Figure 26.3



times the stoichiometric quantity of epichlorhydrin may be employed. A typical

laboratory scale preparation' is as follows:

' 1 mole (228g) of bis-phenol A is dissolved in 4 moles (370g) of epichlorohydrin and

the mixture heated to 105-110°C under an atmosphere of nitrogen. The solution is

continuously stirred for 16 hours while 80g (2 moles) of sodium hydroxide in the form

of 30% aqueous solution is added dropwise. A rate of addition is maintained such that

reaction mixture remains at a pH which is insufficient to colour phenolpthalein. The



resulting organic layer is separated, dried with sodium sulphate and may then be

fractionally distilled under vacuum.'

The diglycidyl ether has a molecular weight of 340. Many of the well-known

commercial liquid glycidyl ether resins have average molecular weights in the

range 340-400 and it is therefore obvious that these materials are composed

largely of the diglycidyl ether.

Higher molecular weight products may be obtained by reducing the amount of

excess epichlorohydrin and reacting the more strongly alkaline conditions which

favour reaction of the epoxide groups with bis-phenol A. If the diglycidyl ether

is considered as a diepoxide and represented as

0



0



/ \



/ \



CH,-CH



-R-CH



-CH,



this will react with further hydroxyl groups, as shown in Figure 26.4.

It will be observed that in these cases hydroxyl groups will be formed along

the chain of the molecule. The general formulae for glycidyl ether resins may

thus be represented by the structure shown in Figure 26.5.



NaOH



---+

Figure 26.4



I



R-CH-CH2-



0



I



5



I



V



6,



v-v-v

&



Preparation of Resins from Bis-phenol A



749



When n = 0, the product is the diglycidyl ether, and the molecular weight is

340. When n = 10 molecular weight is about 3000. Since commercial resins

seldom have average molecular weights exceeding 4000 it will be realised that in

the uncured stage the epoxy resins are polymers with a low degree of

polymerisation.

Table 26.1 shows the effect of varying the reactant ratios on the molecular

weight of the epoxide resins.'



Table 26.1 Effect of reactant ratios on molecular weights

Mol. rutio

epichlorohydrin/

bis-phenol A



1



Mol. ratio

NaOHI

epichlorohydrin



I



Softening

point

("C)



Molecular

weight



Epoxide

equivalent



EP0.V

groups per

molecule



43

84

90

100



45 1

79 1

802

1133

1420



3 14

592

730

862

1176



1.39

1.34

1.10

I .32

1.21



I



I



2.0

1.4

1.33

1.25

1.2



1

I



1.1

1.3

1.3

1.3

1.3



112



It is important that care should be taken to remove residual caustic soda and

other contaminates when preparing the higher molecular weight resins and in

order to avoid the difficulty of washing highly viscous materials these resins may

be prepared by a two-stage process.

This involves first the preparation of lower molecular weight polymers with a

degree of polymerisation of about three. These are then reacted with bis-phenol

A in the presence of a suitable polymerisation catalyst such that the reaction takes

place without the evolution of by-products."

The epoxide resins of the glycidyl ether type are usually characterised by six

parameters :

Resins viscosity (of liquid resin)

Epoxide equivalent.

Hydroxyl equivalent.

Average molecular weight (and molecular weight distribution).

(5) Melting point (of solid resin).

(6) Heat distortion temperature (deflection temperature under load) of cured

resin.

(1)

(2)

(3)

(4)



Resin viscosity is an important property to consider in handling the resins. It

depends on the molecular weight, molecular weight distribution, chemical

constitution of the resin and presence of any modifiers or diluents. Since even the

diglycidyl ethers are highly viscous materials with viscosities of about 40-100

poise at room temperature it will be appreciated that the handling of such viscous

resins can present serious problems.

The epoxide equivalent is a measure of the amount of epoxy groups. This is the

weight of resin (in grammes) containing 1 gramme chemical equivalent epoxy.

For a pure diglycidyl ether with two epoxy groups per molecule the epoxide



750 Epoxide Resins

equivalent will be half the molecular weight (i.e. epoxide equivalent = 170). The

epoxy equivalent is determined by reacting a known quantity of resin with

hydrochloric acid and measuring the unconsumed acid by back titration. The

reaction involved is

OH



0



/ \



-CH-CH,



I



+ HCI --NCH-CH,-CI



It is possible to correlate epoxy equivalent for a given class of resin with infrared

absorption data.

The hydroxyl equivalent is the weight of resin containing one equivalent

weight of hydroxyl groups. It may be determined by many techniques but

normally by reacting the resin with acetyl chloride.

The molecular weight and molecular weight distribution may be determined

by conventional techniques. As the resins are of comparatively low molecular

weight it is possible to measure this by ebullioscopic and by end-group analysis

techniques.

It is useful to measure the melting point of the solid resins. This can be done

either by the ring and ball technique or by Durrans mercury method. In the latter

method a known weight of resin is melted in a test tube of fixed dimensions. The

resin is then cooled and it solidifies. A known weight of clean mercury is then

poured on to the top of the resin and the whole assembly heated, at a fixed rate,

until the resin melts and the mercury runs through the resin. The temperature at

which this occurs is taken as the melting point.

The ASTM heat distortion temperature (deflection temperature under load) test

may be used to characterise a resin. Resins must, however, be compared using

identical hardeners and curing conditions.

Typical data for some commercial glycidyl ether resins are given in Table

26.2.

Table 26.2

Average



Mol.



wt.



Epoxide

equivalent



1



Viscosity cP

at 25°C



I



350-400

450

700

950

1400

2900

3800



175-210

225-290

300-375

450-525

870-1025

1650-2050

2400-4000



I



Melting point

O C (Durrans)



I



4-10000



-



I

~



I



-



40-50

64-76

95-105

125- 132

145-155



I



Solid resins have been prepared having a very closely controlled molecular

weight distribution." These resins melt sharply to give low-viscosity liquids. It

is possible to use larger amounts of filler with the resin with a consequent

reduction in cost and coefficient of expansion, so that such resins are useful in

casting operations.



Curing of Glycidyl Ether Resins



751



26.3 CURING OF GLYCIDYL ETHER RESINS

The cross-linking of epoxy resins may be carried out either through the epoxy

groups or the hydroxy groups. Two types of curing agent may also be

distinguished, catalytic systems and polyfunctional cross-linking agents that link

the epoxide resin molecules together. Some systems used may involve both the

catalytic and cross-linking systems.

Whilst the curing mechanisms may be quite complex and the cured resins too

intractable for conventional analysis some indication of the mechanisms involved

has been achieved using model systems.

It has been shown in the course of this work'* that the reactivity of the epoxy

ring is enhanced by the presence of the ether linkage separated from it by a

methylene link.

0



/ \



0-



CH2-CH -CH,-



The epoxy ring may then be readily attacked not only by active hydrogen and

available ions but even by tertiary amines. For example, with the latter it is

believed that the reaction mechanism is as follows :

0



R,N



/ \



+ CH,-CHw



R,N@-CH,-CHw

I



This ion may then open up a new epoxy group generating another ion which

can in turn react with a further epoxy group.

0



-CH2-CHm



I



0e



+



/ \



CH2-CHw



-



w C H 2 - CH w



I



0-CH-



I



00



Since this reaction may occur at both ends of the molecule (in case of glycidyl

ether resins) a cross-linked structure will be built up.

The overall reaction is complicated by the fact that the epoxy group,

particularly when catalysed, will react with hydroxyl groups. Such groups may

be present due to the following circumstances :

(1) They will be present in the higher molecular weight homologues of the

diglycidyl ether of bis-phenol A.

(2) They may be introduced by the curing agent or modifier.

(3) They will be formed as epoxy rings are opened during cure.

(4) In unreacted phenol-type materials they are present as impurities.