PIGMENTATION OF ACRYLIC RESINS 579

The Pigmentation of Acrylic Resins* J. R. TAYLOR AND H. FOSTER

BP Chemicals ( U K ) Ltd. BP Plastics Department, Hayes Road, Sully, Penarth, Cluniorgan

Acrylic resins may be divided into two main categories-solvent-borne and water-borne systems-and these have k e n further subdivided into two groups, namely solutions and dispersions. Problems associated with the pigmentation of acrylic resins are discussed. and methods of assessing the wetting properties of the system are mentioned. The use of wetting and dispersing agents is examined. and the concept of the hydrophilic-lyophilic balance (HLB) value is investigated in this context. Pigmentation is one important factor that must be considered in assessing the exterior durability of acrylic resins, certain types of which are ideal media for metallic finishes because of their excellent polishing and weathering properties. The phenomenon of geometric rnetamerism

associated with metallic finishes is discussed.

Over the last ten years acrylic resins have become more widely accepted for use in surface coatings. Arising from their intro- duction, many new applications of surface-coating materials have bten possible, e.g. the new gloss latex paints are based on acrylic polymers. Besides such novel applications, acrylic resins have in some instances replaced existing materials, e.g. thermoplastic acrylic resins are used instead of nitrocellulose in car finishes and alkyd-melamine systems have been replaced by thermosetting acrylic resins in both domestic-appliance and autobody enamels.

The pigmentation of the various classes (Table 1) of both aqueous and solvent-thinned acrylic resins will be considered.

TABLE 1 Acrylic Resin Systemst

Solvent-borne Water-borne Solutions Dispersions Solutions Dispersions

'Conventional' Organosols and Water-soluble Water-dispersible acrylic mins 'dispersyrners' acrylic resins latex resins

Both TPA TPA at present Usually TSA Usually TPA and TSA

(latices)

TPA Thermoplastic acrylics TSA Thermosetting ncrylicr

Solvent-borne Solution Polymers These resins may be subdivided into two groups, viz. thermo-

plastic and thermosetting resins. The former are popular in the car-finishing industry because of their excellent polishing and reflow properties. These properties are described later. The main disadvantage of this type of resin is its high molecular weight, resulting in a low build up of film, necessitating the application of several coats to give a satisfactory final film thickness.

Thennosetting acrylic resins may be self-crosslinking, but usually require the addition of a crosslinking agent, e.g. an epoxy or amino resin.

GENWlAL PROBLEMS ASSOCIATED WITH PIGMENTATION OF SOLVENT- SOLUBLE SYSTEMS

The theory of the pigment-wetting properties of various oils, acids, alcohols and numerous types of resins has been thoroughly covered in the literature.

The following are known to affect the pigmentation of any solvent-based solution polymer:

(1) Pigment-wetting properties of the medium (2) Cohesive-mergy density of the solvents (3) Dispersionstability of the system (4) Chemical interaction between pigment and medium ( 5 ) Use of dispersing and wetting agents.

In choosing resins and pigments for a particular application, the weathering properties of the pigment-resin systems may also have to be taken into account. It may be of interest to consider some of the above properties in more detail.

Paper presented on behalf of the Oil and Colou! Chemists' Association t The bash of thc design of thwmorctting acrylic msin sys1cms is aiven in an addendum

Pigment-wetting Properties of the Medium The ability of a medium to wet a pigment can be generally

related to the work required to replace the pigment-air interface by a pigment-medium interface, and can be expressed in terms of interfacial contact angle and tension, or the derived functions work of adhesion or adhesion tension. These functions depend not only on the nature of the pigment surface, but also on the chemical constitution of the medium, and in particular on the number and type of polar groups present.

It is generally accepted that polar groups in the medium greatly improve its pigment-wetting properties, especially in relation to pigments, such as titanium dioxide (C.1. Pigment White 6), that have polar surfaces. The effect depends on the proportion and type of polar groups, and solvent-borne acrylic resins are usually less efficient wetting media than more polar molecules, such as alkyds and polyesters. For this reason, polar groups from, for example, carboxyl-containing monomers are sometimes intro- duced into thermoplastic acrylic resins.

Numerous methods are available for the determination of the degree of dispersion of a pigment in a particular medium. Sheppard and Cope (I) give the following examples:

( I ) Hegman gauge (2) Measurement of specific surface, using the Evans Electro-

(3) Microscopic examination of particle size (4) Tinting strength ( 5 ) Opacity of a tilm (6) Reflectance and spectrophotometric readings (7) Centrifugal particle-size analysis (8) Rheological techniques, including Daniel flow-point measure-

selenium Photoextinction Sedimentometer

ments.

Cohesive-energy Density of the Solvents Pigment agglomerates, both organic and inorganic, can be

broken down by specific solvents, e.g. n i t roparahs and n- butanol. Cleavage is caused by solvents that have a high cohesive- energy density, which is the latent heat of evaporation of the liquid at the temperature concerned minus the work done in expanding the vapour against the surrounding pressure, expressed in terms of unit volume of liquid. It can be considered as a measure of the ease of diffusion of the liquid into its surroundings.

This theory has been applied with success to the milling of alkyd paints containing Prussian Blue (C.I. Pigment Blue 27), and could no doubt be extended to the dispersion of pigments in acrylic media.

Dispersion Stability This property does not necessarily accompany that of good

initial dispersion. Two main mechanisms of stabilisation are recognised-that due

to repulsion of electrical charges and that due to the adsorbed layer. In general, electrical stabilisation occurs mainly in aqueous

580 JSDC DECEMBERi1969; TAYLOR AND FOSTER

and other highly polar or ionised systems, whereas adsorbed- laycr etfects predoniinate in non-aqueous liquids of low polarity, such as are Ibund in solvent-based acrylic-polymer solutions.

Studies of the adsorption of carboxyl-hydroxyl-terminated polyesters on to titanium dioxide have indicated that much lower dispersion stabilities are obtained with the hydroxyl-terminated polyesters than with polyesters containing carboxyl and hydroxyl or carboxyl terniinal groups. This result was confirmed with solutions of long-chain fatty acids and alcohols, even though adsorption occurred to almost comparable extents. The presence or trace amounts of water completely inhibited adsorption of f l i t 14 alcohols, although it may increase adsorption of fatty acids on surface-treated titanium dioxide.

I f these ideas are applied to the pigment dispersion of hydroxyl- atcd acrylic resins, it should follow that the dispersion stability of these resins depends on the number of free carboxyl groups introduced into thc resin chain. There are therefore two reasons for the introduction of carboxyl-containing monomers, viz.

( I ) to catalyse the reaction betwcen the acrylic resin and the

(7) to improve the pigment-wetting propertics and the stability

The elrcct on pigment-wetting properties and stability of the presence of dilTercnt molecular species in an acrylic polymer has n o t bcen completcly elucidated, but in polyesters the attachment of carboxyl groups to low-molecular-weight molecules greatly assists wctting o f the surface of titanium dioxide pigments but givcs poor stability, since wetted pigment particles are allowed to approach too closely in systems that are not stabilised by an ionic mechanism.

Chenticol Itmruciion beriveen Pigtneni iind Meiliiiiti

It is usual in the paint industry to avoid using pigment-media combinations that react chemically. The best known instance is that of the reactivity of basic pigments, e.g. zinc oxide (C.1. Pigment White 4), with acidic media and alkyd or acrylic resins of high acid value. In such cases, gelation or 'livering' can occur.

Use of Dispersing and Wetting Agents These agents are usually surface-active agents, and numerous

types are available to the paint formulator. There has been a tendency to use these materials indiscriminately, but a more scicntific itpproach to their incorporation into a paint formulation has been studied. The conccpt of the HLB (hydrophilic-lyophilic balance) value and its use in aiding the correct choice of wctting agents for the dispersion of pigments in thermosetting acrylic resins is of interest. By this concept, surfactants are classified according to their relative affinity for water or oil. Thus com- pounds having a low HLB value are lyophilic, whereas those with a high value ;ire hydrophilic. Pascal and Reig (2) have stated that an optinium HLH value exists for a given pigment system which is independent o f the type or nature of the dispersing medium.

One method of calculating the HLB value of a non-ionic bit r f x t an t is shown below:

amino resin

of the resulting dispersion.

IOOH 5( H -1- L ) HLRValue -

where t i is the 1 1 1 o l ~ ~ ~ l i i r weight o f the hydrophilic portion of the stirI';ictaiit and 1, thc molecular weight of the lyophilic portion. For non-ionic surfxtiints, the HLB value is 0-20.

One meihod of applying this concept involves taking a group of chemically similar surfactants covering the range of HLB values. These are then used in the paint systems under investigation and the HLB value of the most eRcient surfactant is found. A series of chemically different surfactants having this given value is then examined, and the best wetting agent for the particular pigment combination is selected.

Work has bcen carried out at our laboratories using the Tcxofor range of surfactants. The paint systems examined were

based on acrylamide acrylic resins crosslinked with epoxy resins and the pigment combination was rutile titanium dioxide and Carbon Black (Lamp Black 700) (C.I. Pigment Black 6) (280:lO by wt). The results indicate that a surfactant having an HLB value of approximately 10 gave optimum dispersion for this system (Figure I ) .

-- 1 ~- -~ P

I

5 10

HLB value

ASTM Ratings

A Striation 10 nono B Colour development 9 ver slight

on rub up 8 stir& 6 definite 4 moderate 2 moderartheavy 0 heavy

Figtire I--Eflect of H L B valiie of siirfuctant on dispersion of acrylic enamel

SOME ASPECTS OF PIGMENTATION OF AUTOBODY ENAMELS BASED ON HYDROXYLATED THERMOSETTING ACRYLIC RESINS

One of the many properties demanded of a thermosetting acrylic enamel by the motor-car manufacturer is the absence of colour change in the enamel after the addition of an acid catalyst to enable low-temperature spot repairs to damaged coatings to be carried out during fitting of the engine, trim, etc., during pro- duction. In addition, when respraying is unnecessary, only a minimum colour change must occur when the paint film is sanded to remove imperfections and then polished to its initial gloss. Many factors affect the performance of enamels in acid- catalysed repair and polishing tests, including the method of manufacture of the paint, the pigmentation, the choice of catalyst and the age of the paint after the addition of acid catalyst before it is sprayed on to the damaged area.

In the manufacture of autobody enamels, various methods may be used to disperse the pigment into the resin. They include ball milling, sand milling, triple-roll milling and high-speed cavitation. Work carried out in our technical service laboratories with high- speed ball mills (Steel Cowlishaw high-speed mill), using thermo- setting hydroxylated acrylic and melamine resin systems, has led to the following conclusions:

(1) Dispersion should preferably be carried out in the acrylic resin. Generally, melamine resins gave unsatisfactory results, but it is possible by modification of the constitution of the melamine polymer to obtain resins that give stable dispersions.

(2) Wetting agents should be avoided where possible, since these gave generally inferior results with respect to colour change on polishing of the film and on acid-catalysed repair, owing to non- uniformity of colour throughout the thickness of the film, despite the improved rate of wetting of the pigment during the manu- facture of the paint.

(3) The formulation of pastel-coloured enamels often requires the use of a mixture of pigments together with titania pigments. and successful dispersion can be obtained by codispersing all the pigments, followed by only minor tinting to the exact colour required.

(4) The stability of the pigment dispersion achieved in the enamel is more important than the degree of dispersion, and hence pigments that have been specially treated to prevent flocculation of the dispersed particles should be used.

( 5 ) The addition of a small quantity of a special antifloat additive, e.g. silicone-treated calcium carbonate, to the paint during milling eliminated flotation.

PIGMENTATION OF ACRYLIC RESINS 581

(6 ) Butyl hydrogen maleate was the most satisfactory acid catalyst examined. Other acids, such as phthalic acid, caused a greater change of colour of the catalysed paint film.

General Conclusions relating to A rid-raralyved Repnir The degree of colour change that can occur when an acid

catalyst is added to the paint may vary as the paint is matured up to approximately m e week.

With most pigments, the colour change' in low-temperature stoving (8@ I 0 0 C) became less pronounced on ageing owing to stabilisation of the system. but there were exceptions, such as an amine-treated rutile titania, whose colour changed considerably even on ageing of the paint. In practice, acid-catalysed enamels are not normally used once they have aged beyond the period of one working day.

The gloss of an acid-catalysed portion of an autobody finish is a very important property and should be virtually identical with that of the standard material.

Different levels of acid catalyst were examined to find the most satisfactory balance between gloss retention and hardness after acid-catalysed repair. The results are shown in Table 2.

TABLE 2 Gloss and Hardness of Aeid-catalysed White Enamels Based on

a Reflow Hydroxyacrylic Resin Pigment Tioxide R-CR3 Pigment : binder ratio 0.8:1 Acrylic: melamine resins ratio (on solids) 7:3

Melamine 20" Gloss of 2 0 Gloss or Pencil uncatalysed calalysed enamel hardness

enamel 1.5% 37; 1.5:; 3 %

catalyst* catalyst* catalyst* catalyst* At 100 83 42 H 2H B: loo 100 100 H B H

*B.P. Catalyst 'XIOO' [hulyl hydrogen maleate (42.5 parts by wt); dibutyl mnlcntc (42.5); n-butatiol ( I 5.0)] vol./wt based on enamel at a viscosity 01-32 s (Ford 4 cup, 80" F )

tPartially compatible with reflow acrylic resin in the rtoved film :Completely compatible wizh reflow acrylic resin in the slovcd film

The Daniel flow-point curves of the two melamine resins refer- red to in Table 2 are shown in Figiire 2, which confirms the antici- pated results, i.e. that the melamine resin with the better pigment- wetting properties gave higher gloss readings both in the uncata- lysed and in the catalysed enamels.

Solids content of melamine resin solution 0 Melamina Resin A f - ) Melamine Resin 6

Figrrrc 2 -Daniel flon-pin1 ciirve.T for Melamine Resins A and B

Daniel flow-point curves are prepared from data obtained as

Solutions of the resin under test are made up to solids contents of 5 , 10, 20 and 30% by addition of appropriate solvcni. These solutions are added in turn from a burette to a known weight of the specified pigment (20 g for titanium dioxide) contained in a beaker. The pigment and resin solution are thoroughly kneaded during the titration. until a consistency is obtained which allows a drop of the mix to just flow from the end of the mixing rod. This is the end-point, and similar points are obtained for each solution. These titration results are plotted against the solids content used.

fOllOlNS:

The vehicle solution giving the lowest reading is the one capable of dispersing the largest quantity of pigment.

Coinonomer Conrposition o f Acrylic Resins The comonomer composition of the acrylic resin affects the

pigment-dispersing properties; resins having high styrene con- Lents gave infcrior results owing to thc relatively non-polar nature of the resin.

Acrylic resins with high styrene contents were generally more prone to become paler in colour on acid-catalysed repair, whereas acrylic resins with lower styrcne contents became darker. By suitable choice of monomers. excellent results on acid-catalysed repair were obtained. In fact, hy mixing resins that became paler and with those that became darker, the desired minimum colour change could be obtained. The change in colour on polishing was less affected by the change in comonomcr content of the resin.

Effect of Different Carbon Block Pigtirents Carbon Black pigments of difTerent particle-size distributions

and oxygen contents were codispersed with an untreated rutile titanium dioxide in a hydroxyacrylic resin. Carbon Blacks with a high oxygen content and large average particle size had generally the best wetting properties of the pigments tested. In acid- catalysed repair enamels, flotation effects became more pro- nounced. No clear correlation could be found between the ease of dispersion of the pigments and the degree of colour change on acid-catalysed repair. I n addition, there was no apparent relation between change in colour on acid-catalysed repair and the oxygen content and average particle size of the various Carbon Blacks used with the thermosetting acrylic resins investigated.

Effect of Difcrent Crudes of Titaniirnr Dioxide in Pastel-cobiired Enuniels

Commercial grades of titanium dioxide pigments are usually treated with alumina and silica, and in some instances are coated with an organic additivc such as an amine or polyol to improve the dispersion characteristics of the pigment and give improved colour retention and weathering properties.

In pastel colours the degree of colour development will be influenced by both the particle size and the treatment to which thc surface of the titania has been subjected, and. if an acid catalyst such as phthalic acid or butyl hydrogen maleate is added to accelerate the cure of an autobody enamel, flocculation of one or more of the pigments may be increased, resulting in a change of colour of the catalysed finish.

In our work, it was interesting to note that a titania pigment with an organic coating tinted to a grey with a Carbon Black pigment gave only a very slight colour change (compared with an untreated grade) on addition of acid catalyst. but in a cream enamel containing a small quantity of a yellow iron oxide (C.I. Pigment Yellow 42), the treated titania gave the more severe colour change.

It isevident, therefore, that each pigment combination must be examined in its own right with a given resin.

Flocciilation, Flooding orrd Flotation Much has been published on these three phenomena over the

past two decades. In particular Crow1 (3, 4). Daniel (5) and Bell (6) have made comprehensive studies.

Flocciilation is a process whereby previously dispersed pigment particles form loose aggregates or flocculates that can be redispersed by shear and will subsequently re-form on standing.

Floodhg is the uniform increase in concentration of one or more pigments at the surface of a paint film during the transition from the liquid to the solid state after application.

Flotrition is used to describe the effect when the colour change is uneven, e.g. streaky or tramlined. This is usually associated

582 JSDC DECEMBER 1969; TAYLOR AND FOSTER

Figure 3-Photomicrograph of cross-section of pastel blue acrylic enamel film containing flocculation-resistant pigment

Figure &Photomicrograph of cross-section of pastel blue acrylic enamel firm contuining untreated pigment

PIGMENTATION OF ACRYLIC RESINS 583

with the formation of Benard cells, which often appear as a honeycomb pattern on the surface of a paint film. If shear is applied during the application of the film the cells may become elongated or striated.

These three defects can usually be overcome. The problem of flocculation may be reduced by selecting flocculation-resistant pigments. Phthalocyanine Blues (C.I. Pigment Blue IS) are susceptible to this defect; Figures 3 and 4 are photomicrographs of cross-sections of pastel blue enamels based on two types of Phthalocyanine Blue mixed with titania. The Phthalocyanine Blue used in the enamel in Figure 3 was resistant to flocculation.

Flooding and flotation can usually be overcome by the use of small quantities (10-30 p.p.m.) of a silicone fluid (M.S. 200) or by the incorporation of a silicone-treated calcium carbonate in the mill charge. It is essential to eliminate flooding during a reflow process, otherwise a difference in colour will appear between reflowed and untouched areas of the finish. This process will be described later.

REFLOW 1HERMOSETII"JG ACRYLIC RESINS

Thermoplastic acrylic resins have been established for some time as suitable media for surface-coating enamels in the auto- motive industry. One of their advantages is the ability to reflow during the so-called bake-sand-bake process outlined below. Until recently it was assumed that, because of the chemical nature of thermosetting acrylic resins, they could not be used in such processes, since, after the initial stoving period, flow would be prevented by excessive crosslinking of the reacting acrylic and melamine resins. However, they may now be used in reflow systems (7).

Two thermosetting reflow processes have been evaluated by car manufacturers, namely (1 ) the bake-sand-bake and (2) the bake-sand-respray-bake techniques. In both of these, the enamel coat is sprayed and initially baked at approximately 80-90"C, at which stage the solvent evaporates with very little crosslinking, so that the coating remains thermoplastic. The paint film must then be hard enough to permit defects to be sanded out with abrasive paper without excessive clogging. Subsequently, the film is wiped with a tack-rag and, in the bake-sand-bake process, immediately restoved at 140°C. During this final stoving, the paint film must soften sufficiently to allow reflow to occur, thus obliterating the sanding marks. In the bake-sand-respray-bake process, the sanded areas are wiped with a tack-rag and re- sprayed with a second coat of the enamel without any masking of the remainder of the car body. The paint is then given a final stoving at a higher temperature than that used for the initial bake. During the final stoving period, all dry spray from the respray operation must be completely accepted by the first coat of enamel, so that both coats are virtually indistinguishable. The reflow characteristics of a thermosetting enamel can be affected not only by the acrylic resin but also by the nature of the melamine resin used for crosslinking. Mutual compatibility and pigment- wetting properties of both acrylic resin and melamine resin are believed to have an important bearing on reflow properties.

Choice of Pigment It was found that certain pigments had an adverse effect on the

reflow properties of enamels. Seven grades of titanium dioxide pigments having various surface coatings and particle-size distributions were examined and, of these, only four gave satis- factory reflow properties. These enamels were tested to a typical specification of a motorcar manufacturer. The grades of titania that allowed good reflow properties and gave satisfactory uniform films were the easily dispersible types and gave higher gloss read- ings than the samples yielding inferior reflow properties. Addition of small amounts of Carbon Black pigments led to a marked impairment in the reflow properties of white enamels. An indica- tion of the quantity of a carbon of fine particle size which could

be incorporated in white enamels based on a coated rutile titanium dioxide pigment without impairing the reflow properties is given in Figure 5.

L

0 2 0 4 0 b0 0 0

[Pigment, yo by wt of total pigment

Figure 5-Effect of Carbon Black tinter on reflow of enamel based on titania

The graph indicates that reflow is impaired by the addition of a very small proportion of this particular Carbon Black. In practice, a coarser particle size would normally be used for tinting white enamels.

Figure 6 shows the effect on reflow properties of the type of titanium dioxide used in grey enamels, each tinted with identical quantities of Carbon Black. The three enamels were each tested as follows:

The reflow enamel was sprayed on to a primed metal panel and stoved for 22 min at 100°C. The right-hand half of the panel was wet-sanded using 5Wgrade silicone carbide paper and deionised water. The panel was wiped to remove all traces of debris. The bottom one-third of the panel was then masked and the enamel was sprayed on to the panel in a single pass so that a band of overspray particles appeared across the unmasked area. The masking was removed and the panel stoved for 12 min at 138°C. The reflow properties were assessed on the sanded area on the bottom one-third of the panel, and the overspray acceptance was visually assessed near the top of the panel. The sharpness of the image of a reflected fluorescent strip light indicates the degree of reflow and overspray acceptance obtained.

METALLIC FINISHES

The increasing use of metallic finishes in automotive enamels is well known and is possibly the reason for the widespread acceptance of hydroxylated acrylic finishes, which technically provide an ideal carrier for non-leafing grades of aluminium pigments (C.I. Pigment Metal 1). Metallic enamels hased on acrylic resins possess excellent gloss, colour retention, resistance to acid spotting and good durability and in these respects are usually more acceptable than enamels containing alkyd-melamine resins.

Incorporation of the metallic particles into the acrylic medium presents no particular problem. The flake aluminium is usually supplied as a paste in a hydrocarbon solvent and dispersion is readily achieved by mixing in a high-speed stirrer.

A difficulty often encountered with metallic finishes is one of application. If this is not carried out under precisely controlled conditions, poor orientation of the metallic flakes will occur, resulting in a poor metallic pattern with a dark patchy finish and inferior gloss. A typical distribution of aluminium flakes in a metallic film is shown in Figure 7, a cross-sectional photomicro- graph of a metallic film.

584 JSDC DECEMBER 1969; TAYLOR AND FOSTER

A Tioxide R-CR B Tioxide R-CRZ C Tioxide R-CR3

Figure 6-Photograph of refiowed grey enamels, showing efect of grade of titoniu

Figure 7-Plloto~icrograph of cross-section of ntetallic film

PIGMENTATION OF ACRYLIC RESINS 585

Ideally the lamellar metallic particles should all be in planes parallel to the surface of the film.

Geometric Melanierisni

A phenomenon associated with some metallic finishes is the change in colour observed when the painted object is viewed from different angles. This effect-occasionally referred to as ‘Ep’ or ‘flop’-is sometimes observed in polychromatic h i shes con- taining flake-like metallic particles and other pigments (8, 9).

Although this phenomenon is not peculiar to acrylic finishes, it is of particular interest to the manufacturer of hydroxylated resins or acrylic enamels, since these are particularly suitable media for use in polychromatic car finishes because of their outstanding durability and polishing properties. The manufac- turer of an acryllic metallic enamel must, therefore, be aware of ‘flip’ when formulating his paint, since there must obviously be some degree of control of this effect when coating mass-produced articles or carrying out spot repairs on large objects such as motor cars.

Metamerism is the change in colour of an object viewed under different light sources. The startling change in colour of certain painted objects when moved from daylight into fluorescent lighting is a familiar example of this phenomenon. Geometric metamerism is a similar effect, but occurs when a single light source is moved with respect to the observer. Alternatively, the object may move past a stationary light source, but in so doing the observer’s eye will move, and thus the angle subtended by the light path from source to observer via the moving object will also change.

A simple way of exemplifying ‘flip’ when it occurs is to view the panel directly and in a mirror. The difference in colour between the panel and its reflected image in the mirror is an indication of the amount of ‘flip’ obtained, and is due to the difference in the direction of white light travelling from a light source to the panel, with respect to the observer, in the first instance by direct observation and in the second by observation in the mirror. In other words, we are first observing the panel by viewing in the same direction as the incident beam of light, and then in almost the opposite direction.

Figure 8 is a light diagram that offers a simple explanation of the phenomenon. White pigments scatter light of all wavelengths in the visual spectrum, whereas black pigments absorb light of all wavelengths. Other pigments transmit, reflect, and absorb light to different degrees.

__ Aluminium flake particles

2 Red pigment particles Green pi;ment particles

Figure 8 -Proposed theory of geometric metamerism

The two pigments used in the present work were (1) An organic green pigment [Monolite Fast Green Y3 (ICI)],

which transmits green light and absorbs light of other wavelengths (2) A red oxide [Turkey Red Oxide R22 (C.I. Pigment Red (IOZ)),

which scatters red light and absorbs light of other coloun. If the effects of the green and red pigments are considered

independently in the presence of aluminium flakes such as Alcoa 2250, the panel will appear green when viewed towards the light source and red when viewed from the opposite direction.

In our laboratories we have studied the geometric metamerism of metallic enamel films using a spectrogoniophotometer. This

type of instrument has been described in detail by h o f (10). The curves shown in Figures 9 and 10 were obtained from a metallic film containing green and red pigments in addition to .metallic- flake particles. Incident lights of wavelengths 580-720 nm (red) and 510-590 nm (green) were used which approximated as near as possible to those that gave the maximum reflectance of the red and transmittance of the green pigments used.

Full-scale deflection. Ye

- Red light A = 575600 nm (approx.)] - - - Green li;ht \A = 510-590 nm)

Figure 9-Spectrogoniophotometric curves of panel exhibiting negligible ‘pip’

Full-scale deflection. Yo

- Red light - - - Green light

Figure 10-Spectrogoniophotometric curves of panel exhibiting pronounced ‘pip’

DURABILITY OF HYDROXYLATED ACRYLIC ENAMELS

One of the outstanding advantages of thermosetting hydroxy- acrylic enamels is their outside durability in metallic enamels compared with corresponding alkyd-melamine finishes, as illustrated in Figure 11.

[..I ---u.LJ J m 1966 3 6 9 12 1s I P

Exposure period. months x -x Acrylic-melamine washed x- - - X Acrylic-melamine unwashed a--A Alkyd-melamine washed A- - -A Alkyd-melamine unwashed

Figure I I-Florida weathering of hydroxylated acrylic and alkyd resins it1 metallic enamels

JSDC DECEMBER 1969; TAYLOR AND FOSTER

In solid colours the durability of exposed hydroxyacrylic enamel films is influenced by the pigment used. The comonomer composition of acrylic resins has been shown to have a significant effect on the weathering properties of the films (11). Various aspects of the light fastness of pigment-media combinations have been covered in the literature (12, 13).

PIGMENTATION OF ACRYLATED ALKYDS

Alkyds based on a fatty acid or vegetable oil reacted with pentaerythritol and phthalic anhydride do not normally cause difficulty in pigmentation because of their highly polar nature. The incorporation of acrylate or styrene monomers during the manufacture of the alkyd often gives rise to comparatively poor wetting properties.

Conversely, the pigment-wetting properties of an acrylic polymer may conveniently be improved by introducing alkyd or polyester molecules, either by esterification or by copolymerisa- tion. Improved dispersion has been obtained (Figure 12).

J 4 8 I 2 16 2 0

Millin; time. h Acrylrtcd Alkyd --- Coconut Alk d e@- Hydroxpcryric

figure 12.- Dispersion properties of an acrylated alkyd in grey enamels (.pigmentation: titanium dioxide and Carbon Black) (no wetting agents)

Solvent-borne Dispersions ORGANOSOLS

These materials are dispersions of powdered resins in a non- solvent liquid continuous phase, which is usually a volatile organic liquid. These systems are analogous to water-borne latices, and the film-forming mechanism is one of removal of the liquid phase on stoving. leading to coalescence of the polymer particles. Vinyl chloride polymers and copolymers have been mainly used so far, but other polymers have been investigated, including acrylic copolymers, which may find increasing use in car and domestic-appliance finishes.

DISI’ERSYMERS

Recently Cook (14) described work carried out with non- aqueous polymer dispersions as new surface-coating materials. These have been termed dispersymers and are similar in some respects to aqueous dispersions such as thermoplastic-polymer emulsions. These new resins are graft polymers with a solids content of about 80%. The monomers described include methyl methacrylate, which was graft-polymerised on to crepe rubber, the disperse phase being petroleum ether or white spirit to give latex-like products during polymerisation. The requirements for pigmentation of these resins have not been published.

Thermoplastic dispersymers give paints with a much higher build up of film than those based on normal thermoplastic-resin solutions.

The absence of large quantities of strong solvent in these new paints reduces the difficulties of solvent retention in the film. Metallic finishes suitable for the automotive industry have been formulated. A second type of dispersymer is based on a thermo- setting resin.

Advantages given by these resins include the possibility of paints with a high solids content and resistance to hydrocarbons. Resins satisfactory for metallic and coloured paints have been examined.

Water-borne Dispersions ACRYLIC EMULSION PAINTS: GENERAL CONSIDERATIONS

The film formation of pigmented acrylic copolymer latices and copolymers with vinyl acetate has been studied by Rednap (15). The latices contain colloid and surfactants to give a stable product during transportation, storage and pigmentation, but some further additions are usually necessary during pigmentation.

A decorative emulsion paint (pastel colour) usually contains rutile titanium dioxide together with mixtures of an organic dye or a metal oxide or hydroxide pigment and extender. A yellow emulsion-paint formulation can often include the use of an acetoacetarylamide pigment [e.g. Hansa Yellow (C.I. Pigment Yellow I)] or basic iron oxide pigment to give the required colour. Difficulties caused by recrystallisation of the organic pigment can be experienced owing to the solubility of some pigments in unreacted monomer, if present in the latex, or coalescing solvent used in the paint formulation. This can give a colour change during storage of the paint. It is essential, therefore, that the correct choice of pigment and surfactant is made during stabilisa- tion of the paint. Dispersions of organic pigments in selected surfactants for use as tinters are now available in order to maintain the maximum colour stability in the paint.

Bondy (16) has referred to the use of the HLB-value system for the surfactant in the latex formulation to ensure optimum stability. Suitable HLB values for latices containing various copolymers are given in Table 3.

TABLE 3 Optimum HLB Value for Various Ingredients of a Latex Paint

Paint ingredient Recommended HLB value of surfactant for optimum stability

Latices Copolymers with ethyl acrylate 15-17 Styrene-acrylate copolymers 12-1 5

Inorganic oxides 18-20 Pigments

Organic pigment, e.g. Phthalocyanine approx. 14 Blue or Hansa Yellow

It can be seen that the HLB values required for pigment and extenders for the most satisfactory dispersion and stabilisation may vary for each ingredient over a considerable range. I t is therefore not always possible to have a surfactant system that has the exact HLB value suitable for all the pigments in the paint and this may lead to partial flocculation of one of the pigments.

These difficulties are particularly noticeable in deep-tone colours owing to the hydrophobic nature of the organic pigment and the hydrophilic nature of the extenders used in paint formula- tions. The emulsion-paint formulation also requires the use of a thickening agent, which isusually acolloid of fairly high molecular weight, such as sodium carboxymethylcellulose or hydroxyethyl- cellulose, to give acceptable rheological properties for application. The mode of incorporation of these colloids into an aqueous pigment paste is important, since, if the amount of surfactant present is low, large quantities of the colloid may be adsorbed on the pigment surface and the latex may become unstable. If extra surfactant is added before the colloid, the adsorption of the latter may be small, but flocculation could take place. A variation in the mode of production of the paint, therefore, can have a profound bearing on its physical properties.

PIGMENTATION OF ACRYLIC RESINS 587

If too little high-molecular-weight thickening agent of high affinity for the pigment is used, flocculation, syneresis and loss of opacity may occur owing to adsorption of the colloid on the pigment. More stable paints can often be prepared by the use of an aqueous mixture of pigments in which a portion of the sur- factants and colloids is present to wet the pigment and extender in the initial stages of manufacture.

Water-borne Solutions The pigmentation of water-soluble resins has been studied by

various workers (15, 17). Much of this work has been associated with the electrodeposition process as applied to stoving systems. Acrylic resins have not been employed in any quantity in this type of application. There may be, however, an increased interest in this type of resin for white and pastel-coloured finishes.

Air-drying water-soluble resins have also been studied, but little use has been made of acrylic resins in this type of system.

Conclusions Acrylic resins may be conveniently divided into four groups,

according to the type of solvent in which they are carried, and according to whether the combination so formed is a true solution or a dispersion.

In general, the problems associated with the pigmentation of oleoresinous and alkyd media are similar to those associated with acrylic resins. In many cases the surface structure of an acrylic resin is closely allied to that of an alkyd resin, in that the same chemical groups are present, e.g. carboxyl and hydroxyl groups are common to both alkyd and hydroxylated acrylic resins.

In addition to the pigment-wetting properties of the acrylic resin, it may be necessary to take into account the pigment- wetting properties of the crosslinking resin, which may have an important bearing on the gloss and stability of the final product.

In considering a suitable type of pigmentation for an acrylic enamel, it is often important to consider the outside-durability characteristics, and, in this respect, acrylic resins are ideal media for metallic finishes based on aluminium flakes. This type of finish presents additional problems for the colour matcher because of its tendency to exhibit geometric metamerism.

The introduction of acrylic monomers into latex paints has not as yet led to any particular pigmentation problems beyond those already associated with latex systems. The HLB-value concept has proved to be a useful method of categorising surface- active agents as grinding aids both in latex and in conventional paint systems. * * *

We thank our colleague Mr P. J. Fry for his assistance in compiling this paper. (MS. received 5 May 1969)

References I Sheppard, 1. R., and Cope, G., J . Oil Col. Chem. Assocn, 46 (1963)

2 Pascal, R. H., and Reig. F. L., Of. Dig., 36 (1964) 475. 3 Crowl, V. T., J . Oil Col. Chem. Assocn, 50 (1967) 1023. 4 Idem, ibid., 46 (1963) 169. 5 Daniel, F. K., Fatipec, 7 (1964) 280. 6 Bell. S . H., ibid., 7 (1964) 74. 7 Taylor, J. R., and Foster, H . , J . Oil Col. Chem. Assocn, 51 (1968)

8 Johnston, R. M., Color Engng, 5 (1967) 42. 9 Davidson, H . R.. ibid., 3 (1965) 22.

I0 Loof, H. J., Paint Tech., 38 (1966) 632. I 1 Taylor, J. R.,'and Price, T. I., 1. Oil Col. Chem. Assocn. 50 (1967)

12 Gates. A. P.. and Patterson. D.. ibid.. 50 (1967) 1008.

220.

975.

139. , - - - - I

13 VesCe: V. C.; ~ofi i i i , 31 i19S9i 142. 14 Cook, C., h i n t , Oil& Col. J . , 154 (1968) 990. I S Rednan E. F., J. Oil Col. Chert!. Assocn. 49 (1966) 1023. 16 Bondy; C., ibid.. 49 (1966) 1045. 17 Taker, L., and Taylor, J. R., ibid., 48 (1965) 462; 49 (1966) 756.

ADDENDUM Basis of Design of Thermosetting Acrylic Resins

Table 4 shows typical monomers which may be present in thermosetting acrylic resins. A very common combination of monomers for the copolymer is styrene with alkyl acrylates and the effect of varying their ratio is shown in Figure 13.

TABLE 4 Typical Monomers That May Be Present in a Thermosetting

Acrylic Resin Monomer

Methyl methacrylate Styrene Vinyltoluene Acrylonitrile

Contribution

Hardness

lacrylates and methacrylates Ethyl Butyl 2-Ethylhexyl J Butyl maleate

Crosslinkages

Acrylamide Butoxymethylacrylamide Hydroxyalkyl acrylates Glycidyl acrylates Acrylic acid

Acrylic acid Methacrylic acid Maleic anhydride

Cure acceleration I

increase t--

Durability ?

Flexibility Amino-resin

compatibility

increase *--- Acrylate

Hardness increase +

increase Styrene - +

Figure I3-Eflect of copolymer composition upon physical properties

As the styrene content increases: ( i ) Cost falls

(i i) Hardness increases (iii) Chemical resistance increases ( i v ) Amino-resin compatibility decreases

As the acrylate content increases: ( i ) Flexibility increases

(i i) Exterior durability increases (within limits).

This is a pattern that applies whatever the crosslinking monomers or groups employed, be they hydroxyl, acrylamide, carboxyl, etc., although the optimum ratios will vary from system to system.

588 JSDC DECEMBER 1969; TAYLOR AND FOSTER

Discussion

Dr W. SHAW: Pigment printing pastes are generally oil-in- water or water-in-oil emulsions. Water-based dispersions, not containing solvent in emulsion form, give duller colours in use than the eniulsion pastes. Is this inevitable or can improvements in this respect be made?

Mr TAYLOR: The formation of the ink film from an oil-in- water or water-in-oil emulsion system is different from that of a straight oil or resin solution, since with the first two types the pigment has to be wetted by the oil during the evaporation of the water phase. It is well known that the refractive index and wetting properties of the oil will modify the colour and brightness of the pigment when coated by the oil. The addition of an organic solvent will often reduce the viscosity of the oil used and will also assist in the wetting of the pigment and removal of water from the pigment surface, particularly if the pigment is organic and there- fore hydrophobic in nature.

It is possible to increase the dispersion and wetting of the pig- ment surface by pretreatment, making it more hydrophobic in character. These treatments are numerous and are normally carried out by the pigment manufacturer and include the use of rosin soaps or siloxanes. The addition of suitable wetting agents will increase the efficiency of the flushing of the pigment into the oil phase during film formation.

Water-based dispersions, such as those based on poly(viny1 alcohol), acrylates and poly(viny1 acetatetacrylate copolymers, being in the form of particles of the copolymers dispersed in water, do not coat the surface of the pigment as easily as that of

an oil emulsion, since coalescence of the polymer particle must take place at the time of phase change during the evaporation of the water from the film. The use of organic solvents reduces the viscosity of the polymer particles and the solvent, acting as a coalescing agent, increases the ease of wetting of the pigment and therefore gives better dispersion of the pigment and hence an apparent deeper tone. Improvements to the pigments for this type of system are similar to those given above for the emulsion type.

Mr D. G. DUFF: In evaluating the HLB value of a surfactant, how is the molecular weight of the ‘hydrophobic portion’ estimated ?

Mr TAYLOR: In the paper by Pascal and Reig (2) the Atlas HLB System is mentioned. The HLB value requires that the molecular weight and type are known for the emulsifier or wetting agent being considered.

If, for example, we consider an emulsifier stated to be a poly- oxyethylene sorbitan monolaurate and having an HLB value calculated to be 16.7, the actual value is obtained as follows:

The molecular weight of the emulsifying agent = sorbitan (164) t lauric acid (200) t 20 mol. ethylene oxide (880) - water of esterifica- tion (18) = 1226.

The molecular weight of the hydrophilic portion-sorbitan + ethylene oxide (164+880)= 1044.

The HLB value of the emulsifying agent is therefore

- x ’ 1044 - x 100 = 17.0 5 1226

By analytical means, the HLB value obtained by the manufacturers was 16.7.