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Water-borne acrylic emulsion paints
Progress
in Organic
Coatings, 5 (1977) 79 - 96 S.A., Lausanne - Printed in the Netherlands
0 Elsevier Sequoia
WATER-BORNE
SWARAJ AB
ACRYLIC
EMULSION
79
PAINTS*
PAUL**
Wiih. Becker,
Research
Department,
S-102
70, Stockholm
9 {Sweden)
Contents Introduction, Types
79
of water-borne
Properties
systems,
required in emulsion
80 gloss paints, 81
Dependence of coating properties 4.1 Thermoplastic coatings, 82 4.2 Thermosetting coatings, 87 Dependence of coating 5.1 Influence of latex 5.2 Influence of latex 5.3 Influence of water Summary,
composition,
properties on latex composition, particle size, 90 stabilizing system, 93 as prime solvent, 94
82
90
94
Acknowledgements, References,
on polymer
35
95
1. Introduction
The earliest types of house paints consisted mainly of lead or zinc oxide in linseed or tung oil. The main problems associated with such systems were: pigment agglomeration, poor pigment wetting, long drying times, higher tendency of dirt retention and chalking, and large number of coats (min. 3 coats) required to achieve good hiding. In the 1930’s oils were replaced by alkyds as vehicles and they were used in graphic arts, interior enamels and industrial fields. AIkyds were used in house paints in the 1950’s for improved colour and gloss retention, but such paints could not be marketed before 1960. This was due to two reasons: firstly, the use of zinc oxide as a pigment resulted in widespread intercoat peeling problems, and secondly, the knowledge of organic mildew inhibitors and the control of rheological properties *Presented as part of the doctorate course on Emulsion Ins22te of Technology, Stockholm, 28th January 1977. Previous address: Department of Polymer Technology, Technology, S-100 44, Stockholm 70.
Polymerization The Royal
at the Royal
Institute
of
80
of the paints were insufficient. Probably the greatest progress in house paints in the last 50 years has been the development of latex-based paints. Interior flat wall paints based on styrene-butadiene latexes were introduced in the late 1940’s and immediately became very popular. Semi-gloss latex paints were first commercialized around the 1960’s, as the latex available in the early years just could not seem to carry the formulas to the higher levels of adhesion, water resistance, hardness and other features that were required for the advances in trade sales paints. Recently, the increasingly restrictive anti-pollution legislation, which regulates the maximum amount of organic solvents that can be emitted into the atmosphere, has generated a renewed interest in water-borne systems. Apart from this, elimination of fire hazards, potential economy of water as a solvent and the shortage of solvents due to the oil crisis may, in future, also favour water-borne systems. The initial acceptance of latex paints was due to their easy application, clean-up and low levels of residual odour. More specifically, interest in aqueous acrylic polymers is higher as compared with poly(viny1 acetate) (PVAc) and other systems because of their high and inherent durability. Higher gloss values, in the case of acrylates, is attained due to two reasons: firstly, they have higher values of refractive index as compared with PVAc (viz. refractive indices of poly(buty1 methacrylate) (PBMA) and poly(methacrylate) (PMA) are 1.483 and 1.479) and secondly, they give rise to latexes with smaller particle size due to the more hydrophobic character of the acrylic copolymers than PVAc. 2. Types of water-borne systems Water-borne acrylic polymers can be placed into three classes [ 1, 23 : (a) aqueous dispersions or emulsions, (b) colloidal or water “solubilized” dispersions, and (c) water-reducibles. TABLE
1
Comparison of the three classes of water-borne acrylics Property
Aqueous
Appearance
Opaque Exhibits
dispersion
light scattering
Colloidal
dispersion
Water-reducible
Translucent Exhibits light scattering,
Clear No light scattering -
Particle si2.e
20.1 pm
About
Self-crowding capacity constant, K
- 1.9
1.0 + 0
0
Molecular weight
1 million
20,000 - 200,000
20,000 - 50,000
Low, independent of polymer mol wt.
More viscositysensitive, somewhat dependent on mol. wt.
Viscosity very dependent on polymer mol. wt.
Viscosity
20 - 100 nm
81
These three types vary significantly in physical and mechanical properties and thus provide a considerable formulation range for coatings
chemists. The main differences between the three classes are summarized in Tables 1 and 2 [l] . It is obvious from Table 2 that colloidal dispersions [3 - 51 are hybrids of the aqueous dispersions and water-reducible polymers. Table 3 summarizes some of the functional monomers which are used in the synthesis of watersolubilized acrylic copolymers [l] . Table 2 also shows some of the application differences between the three classes. The reasons for such differences have been discussed elsewhere [l] . In this review, only the emulsion type of binders will be considered
3. Properties
required in emulsion gloss paints
The properties required for an emulsion-based paint can be summarized as shown in Table 4. Table 5 shows an inter-relation between the various film properties, the polymer type and the paint formulations 163.
TABLE
2
Application
characteristics
of the three classes of water-borne
acrylics
Colloidal dispersion
Water-reducible
High
Intermediate
Low
Excellent
Excellent
Very good
Resistance props.
Excellent
Good - excellent
Fair - good
Viscosity control
Requires external thickeners
Thickened by addn. of cosolvent
Governed by polymer mol. wt.
MFT restraint
Hard films, require coalescence
Forms hard films with min. coalescence
None
Property
Aqueous dispersion
Solid content at application viscosity Durability
Formulation
Complex
Intermediate
Simple
Pigment dispersibility
Poor
Good - excellent
Excellent
Application difficulties*
Many
Some
Few
Specular gloss
Low
More like waterreducible systems
High
*Ease of clean-up, foaming, method of application, problems (flow, levelling).
substrate wetting and rheological
82 TABLE
3
Functional
monomers
Acidic Methacrylic Acrylic
acid
acid
Itaconic
acid
Maleic anhydride
TABLE
useful for preparation
acrylics
Basic
Non-ionic
Dimethylaminoethyl methacrylate
Methacrylamide
t-Butylaminoethyl methacrylate
Acrylamide
Diethylaminoethyl acrylate
Hydroxypropyl methacrylate
2-Vinylpyridine
Hydroxyethyl acrylate
4
Properties
required
in emulsion
gloss paints
Odour/toxicity
Balanced rheology
3. 4. 5. 6. 7. 8.
of water-solubilized
Ease of application (structure/non-drip) Good flow Sag resistance Shelf stability Compatibility with the pigment Good lapping Coalescence at 5 “C Drying at high R.H. Good re-coating
9.
Good
gloss
10. Good build 11. Good colour 12. Good colour retention 13. Good opacity 14. Block resistance 15. Mar/abrasion resistance 16. 17. 18. 19.
Stain resistance Water resistance Adhesion to old paint films Exterior durability
As it is not possible to cover in this review all the aspects which influence properties of the emulsion-based paints, only a correlation of the film properties with the polymer and the latex composition will be discussed. the coating
4. Dependence of coating properties on polymer composition Development of stiitable methods for the preparation of the acrylic monomers [ 7 - 10 ] has provided the acrylic polymer chemist with an unusual freedom in designing a polymer to meet a set of end-use requirements. Based on the polymer composition, acrylic latexes can be placed into two distinct classes: 4. I Thermoplastic coatings This represents a class of polymers whose hardness varies with temperature, Le. above Tg they behave like rubber. Acrylic polymers used in coatings
83 TABLE
5
Relationships
between
formulation
and film properties Additive
Solvent
TiOg (Type/level)
Polymer Type
Mol. Wt.
Part. Size Dist.
Coalescent
Other
Sol. Resin
Other
Gloss
X
X
X
X
X
X
X
X
Flow
X
X
X
X
X
X
X
X
X
X
X
X
Interactions
Application/ wet-edge Film formation
X
X
Dryingire-coat Opacity
X
X
X
X
X
X
X
X
X
X
X
X
X
x
X
ThermoplastJ hardness
X
X
X
X
X
Durability
X
X
cost
X
X
primarily consist of poly(methacrylates) turally be represented as follows:
X
X
X
and poly(acrylates)
X
X
which can struc-
y43 I: +CH2-
7
j;;
+CH2-
7
3,
4:=O
F=O
P R
P R
poly(methacrylate)
poly( acrylate)
It is obvious from the structures that the coating properties are strongly influenced by the following three factors: (a) The presence of the CH3 group or H atom on the a-carbon atom. (b) The length of the ester side group, R. (c) The presence of the functionality in the ester side group. (a) The presence of a methyl group in the cy-position results in a hindrance to the segmental rotation of the polymer backbone. As a result, poly(methacrylates) are invariably harder, less extensible polymers than the corresponding acrylates, as shown in Table 6 [ 111. (b) Table 6 also illustrates the influence of the ester group chain length on the tensile strength and the elongation of the polymers. Decrease in tensile strength is attributed to the increase in the sementaIl rotations in the side chains due to an increase in the free volume of the polymers. Thus the
84 TABLE 6 Properties of poly(methacrylates)
and poly(acrylates) Elongation
Polymethacrylates Alkyl group of the ester
Tensile strength
(p.s.i.)
Methyl
9000
4
Ethyl
5000
7
Butyl
1000
230
1000
750
Ethyl
33
1800
Butyl
3
2000
(70)
Polyacrylates Alkyl group of the ester Methyl
TABLE 7 Contribution of monomer types to the film properties Film property
Contributing
Ekterior durability Hardness
Methacryiates
monomers and acrylates
Methyl methacrylate Styrene Methacrylic and acrylic acid
Flexibi1it.y
Ethyl acrylate Butyl acrylate 2-Ethylhexyl acrylate
Stain resistance
Short chain methacrylates and acrylates
Water resistance
Methyl methacrylate Higher methacrylates
Mar resistance
Methacrylamide Acrylonitrile
Solvent and grease resistance
Acrylonitrile Methacrylamide Methacrylic acid
and acrylates
copolymers are more common in commercial applications than the homopolymers because a wide range of strength and flexibility can be achieved by choosing a suitable combination of comonomers. (c) As it has been shown earlier [ll] that the specific film properties depend on the functionality of the ester side chain, a polymer chemist can often prepare a tailor-made polymer to achieve the defined requirements such as flexibility, adhesion, hardness, etc. A correlation between the film properties and the type of monomers has been shown in Table 7 111 J .
85 TABLE
8
A correlation between minimum film forming temperature temperature ( Tg) for polar and non-polar polymers
Tg (“C)
KHN*
18
25
2.9
21
10
1.25
Type of polymer
MFT
55 % Ethyl acrylate + 45 7% methyl methacrylate 50% Styrene
+ 50%
butyl acrylate
Standard styrene-butadiene *Knoop
Hardness Number,
18
latex a standard
(RIFT) and glass transition
(“C)
0.38
test.
A further understanding of the relation between the macroscopic film properties of an acrylic coating and the polymer composition has become possible due to the successful application of the two parameters, viz. the glass transition temperature Tg and the solubility parameter 6. The glass transition temperature Tg is defined as that temperature, or more precisely that temperature region, at which significant segmental rotation in the backbone and/or side chains occurs. As the temperature passes through this region, the properties of the polymer film change dramatically, i.e. above Ts the film becomes rubbery, flexible and softer because the polymer segments can respond to stresses impressed on them. Since the same molecular processes determine the tensile strength and extensibility of the polymers, a close correlation between Tg and the mechanical properties of the polymers has been found (cfi Table 6). In the case of emulsion paints, an alternative term, the “minimum film-forming temperature” (MFT), is generally used. MFT represents the minimum temperature at which film coalescence occurs. Although no quantitative reiation between the Tg and the MFT exists, MFT is always found to be lower than Tg for most of the acrylates due to the plasticizing influence of water and emulsifier during film coalescence. The driving forces for polymer deformation during film coalescence, resulting in a continuous film, arises either from the surface tension [ 121 or capillary forces [ 131 which increase with decreasing particle size as a result of drying or from “autohesion”, the mutual inter-diffusion of free polymer chain ends across the particle-particle interface in the coalesced film. It has been shown by Protzman et al. [ 14] that in the case of non-polar polymers such as styrene-acrylate, MFT is higher than the Tg values. Such a correlation between MFT and TZ and thus their influence on film hardness has been shown both for the polar and the non-polar polymers in Table 8 1141. The higher value of MFT for styrene-acrylate (non-polar type) polymer shown in the table was supposed to be due to the stabilizer system. With the styrene-butadiene system, a sodium lauryl sulphate would probably cause a much higher surface charge with consequent charge repulsion effects than a polar acrylic copolymer, with which the stabilizer would be more compatible. A relationship between the MFT and Tukon hardness and thereby its influence on the film properties can be summarized as shown in Fig. 1 [ 151.
86
l0
20
30
f.0
50
60
70
90 80 MFT( “Cl
Fig. 1. Relationship between MFT and Tukon hardness and their influence on the paint properties. x Trade sales house paints for bare wood, 8 floor paints, 0 board coat:qs, n shingle coatings. Pigment binding, softness and flexibility, adhesion to substr?Lt3, blocking tendency and dirt pick-up of the paint films increases with the direction of the arrow.
Thus it is obvious that by understanding the factors influencing Tg and RIFT of an acrylic polymer, it is possible to predict the variety of properties of the actual coating systems [ 16 - 191. The solu bility parameter, 5, invented by Hildebrand [ 201 and applied to polymers by Burrell [ 21,223 , has also proved to be invaluable in understanding, correlating and predicting the solubility and compatibility of acrylic polymers, which in turn influences the gloss and the solvent a< .zk on the paint films. The 6 value is derived from the regular solution theory, which assumes that the entropy of mixing the components of a solution is the same as for an ideal solution, i.e. mixing is a random process, and that the enthalpy of mixing can be calculated from the model by making simplifyiqg assumptions about the nature of the molecular interactions involved. The iheory predicts that a polymer will be miscible with another polymer or solvent if the enthalpy of mixing of the two materials is essentially zero. This condition is guaranteed if the solubility parameters of the components are equal. Small 1233 has given a method for estimating the 6 value of polymers_ He assigned a “molar attraction constant” to the various organic groupings in the polymers, and thus from a knowledge of the polymer’s structural formula and density, 6 has been calculated as follows:
where d = polymer density, 17 = molecular weight of the repeating unit, EG= sum of molar attraction constants for each organic grouping in repeating units. Baaed on the 6 value of the solvent and the polymer, solvent attack on the coatings can be predicted, e.g. polymers containing long alkyl side chains such as poly(n-but@ acrylate) (~5= 8.7) are likely to have good resistance to water (6 = 23.4) and ethanol (6 = 12.7), whereas the polar polymers such as poly(acrylonitrile) (6 = 12.5) are predicted to show good resistance to attack
8’7
by aliphatic hydrocarbons (mineral spirit, 6 = 7.0). The knowledge of 6 values is also essential to obtain glossy films and reasonable evaporation rates of the coalescing solvents, wet-edge time, etc. Durability properties also depend on the polymer composition of the coating materials. Acrylics generally have excellent durability. They resist discoloration when exposed to elevated temperatures and are not easily attacked by acids or bases. This is mainly due to the chemical inertness of the C-C single bonds of the polymer backbone as compared with ether or amide linkages. The ester side chains can be hydrolysed with difficulty, and if such an attack occurs it does not result in the scission of the polymer backbone and so the film maintains its integrity. General durability characteristics of acrylic homopolymers are summarized in Table 9 [If ] . TABLE 9 General durability characteristics of acrylic homopolymers Ester
Methacrylate
Acrylate
Methyl
Very good
Poor
Ethyl
Excellent
Fair
i-Butyl
Excellent
Good
n-Butyl
Excellent
Excellent
2-Ethylhexyl
Very good
The general superiority of the methacrylates over the acrylates is due to the absence of a tertiary hydrogen atom in the former, which hinders the formatio? of a free-radical intermediate by hydrogen abstraction such as -(CH2-C-C02R)-, which in turn results in chain scission. The increasing durability with increasing ester chain length, as shown in Table 9, is supposed to result from the increasing flexibility and hydrophobicity of the paints, which increases with longer ester groups. The greater flexibtiity renders more dimensional stability to the paint film with the dimensions of the substrate change. It has been shown [24] that the durability of the copolymers plasticized to have the same TE decreases in the following order: BMA = MMA > MMA/BA > MMA/EA. In addition to chemical inertness, acrylics show superior durability due to the fact that the polymers are transparent to the spectral region between 3500 A and 3000 a, which is the most photochemically active region of the solar spectrum. Modification of acrylics by polymers or pigments which absorb in this region, e.g. alkyds or TiO 2, invariably reduces the exterior durability of the coatings. 4.2 Thermose t ting coatings The coalescence mechanism of the film formation in’ the thermoplastic emulsions limits the range of hardness that can be designed into the polymer
88
backbone. This limited seriously the use of emulsion-based paints for industrial purposes, which was essential in order to obtain improved block and print resistance. The introduction of thermosetting acrylic emulsions [25 29,4] has overcome these problems. The basic concepts of these types of emulsions were not new, but were only a logical extension of the solventbased thermosetting chemistry. The oldest thermosetting vehicle used even now is porcelain enamel, which is an aqueous dispersion of silica, sand and other componenB fused to produce a hard, ceramic finish. These high quality finishes are still extensively used in bath-tubs and household appliances such as range tops and hot water heaters, where extreme heat and chemical resistance is required. However, the expense, lack of flexibility, poor adhesion except to few substrates, and the intense heat required to fuse the finish, limits the use of porcelain to a few applications, and these are mainly replaced by thermosetting acrylate coatings. In the thermosetting types, reaction functionality is copolymerized into the polymer backbone, and, after application, the cross-linking reaction is thermally activated in situ between the comonomers (internal type) or blended reactants (external type). The different types of thermosetting systems used commercially are shown in Table 10.
TABLE
10
Differenttypesof commercially availablethermoseLting systems Pendant groups
Cross-linking
1. -CH20H
Al koxy aminoplast, e.g. melamineformaldehyde. benzoguanamine-formaldehyde and isocyanates
Acids 127 ]
Epoxy
Basic
Hydroxyl
agents
Catalysts
0 2.
-&-OH
resins
Carboxyl 3.
-Ch$-,CHg 0
Carboxyfic polymers, amines [ 281 and hydroxy compounds [ 28, 291
Acidic-MAA, AA (internal), NH&l, PTSA Basic-DMAEMA (internal), quaternary amm. compounds
Epoxy resins Alkoxy aminoplasts Carboxyl-containing
Acidic-AA or MAA (int.) H3P04, PTSA, morpholine salt of PTSA
EPOXY
4.
:: -CNH_CH2OH -$NH_CH20R 0 Acrylamide deriv.
AA -
Acrylic
DMAEMA
-
Acid, MAA
Dimethyl
-
Methacrylic
Aminoethyl
polymers -
Acid,
Methacrylate.
PTSA
-
p-Toluene
Sulfonic
Acid,
89
Recently, some new’ thermosetting systems based on aminimide [ 301, miscellaneous nitrogen-containing compounds such as aziridine, pyrrolidone, sulphonium derivatives, acrolein and siloxanes have been reported 1313 . Aminimide types are interesting due to being heat-activated isocyanate precursors. However, their commercial use for industrial purposes cannot yet be exploited due to the unavailability of the monomers on the commercial scale. Typical monomers, comprising mainly a thermosetting acrylic resin, can be summarized as shown in Table 11. It has been shown in the literature [ 17 3 that in the case of thermosetting coatings the film hardness can, apart from the polymer composition, also be controlled by the degree of cross-linking in the polymer films. Such a relationship between the film hardness and the degree of cross-linking has been reported earlier [ll] , and is shown in Fig. 2. It is obvious from the figure that such cross-links have no effect on the hardness of the films below Tg of the polymeric films. A comparison of the relative performance of the different acrylic thermosetting systems and the conventional alkyd/melamine systems has been shown in Table 12 [32] . According to this table, the acrylic thermosetting systems provide a better control of the end properties and the crosslinking reaction conditions than the conventional alkyd/melamine types. TABLE
11
Typical monomers used in the preparation of a thermosetting acrylic resin Contribution to film properties
Monomers Methyl methacrylate (MMA) Styrene Vinyltoluene Acrylonitrile (AN)
Hardness
Ethyl
acrylates
Butyl
and
2-Ethylhexyi Butyl maleate
I
Flexibility
methacrylates
Acrylamide Butoxymethylacrylamide
Hydroxyalkyl acrylates
Cross-links
Glycidyl acrylates Acrylic acid Acrylic acid (AA) Methacrylic acid M-AA) Maleic anhydride (MA)
Cure accelerator
90
Temperalure
Fig. 2. Hardness-temperature relationships for polymers of varying Tg and degree of crosslinking. (a) Soft polymer, highly cross-linked; (b) soft polymer, slightly cross-linked; (c) soft polymer, no cross-linking; (d) medium hard polymer, highly cross-linked; (e) medium hard polymer, no cross-linking. TABLE
12
Comparison of the performance with conventional al kyd.s
properties
Type
Curing temperature
Alkyd-melamine Acid-epoxy Acrylic-melamine Methylol-amide Urethane
250 375 250 325 -80
of different
(“P)
1 = Best, 4 = worst. All values relative to acrylic
thermosetting
acrylic
polymers
Toughness
Exterior durability
Chemical resistance
2 1 3 2 1
4 3 1 2 3
3 1 2 2 2
polymers.
5. Dependence of coating properties on latex composition Latexes are mainly composed of polymer particles dispersed in water and the stabilizer system which consists mainly of emulsifiers and thickeners. Most of the application problems, mentioned earlier, in the case of latex paints arise mainly from the latex particle size and the use of water as the primary solvent [Z] . The former involves rheological problems such as flow and levelling, film formation at low temperatures, gloss and the water resistance of the films, while the latter gives rise to problems relating substrate wetting and pigment wetting. 5. I In fZuence of latex particle size The main problems arising from the latex particle size we rheological problems. The poor rheology in the case of latex paints results from a sudden increase in viscosity after the critical coalescent concentration has been reached as compared with the gradual increase in the case of a solution polymer. Apart from rheological problems, this sudden viscosity rise causes
91
Solids Fig. 3. Viscosity dependence on the total solids of the binders for the water-borne systems. I Water-solubilizable dispersion, II aqueous emulsions, III water-reducible.
2 IU
!L-
_SJiO '30 2
2* aJ
5:
I
II
T-“-i=
24%
z
a i2
St
so 100200
Shear Stress
popping
I
20
.lZ
20
Stress(dynes,cm2)X,~3
Fig, 5. Dependence of paint’s high shear viscosity II 70% 0.63 pm latex + 30% 0.103 I_crnlatex.
or solvent
Lo
lx
(dynes,crn2)
Fig. 4. Dependence of paint’s low shear viscosity II 70% 0.63 flrn latex + 30% 0.103 pm latex.
blistering
l&?c I1
60
5
due
to water
on latex particle size. I 0.63
pm latex,
on latex particle size. I 0.63 Pm latex,
encapsulation
in the case
of
thermosetting emulsions. Dependence of the viscosity on solids of the waterborne acrylic resin is illustrated in Fig. 3 [11] . Several studies relating to the rheological behaviour of emulsion-based paints have been reported earlier in the literature [33 - 411. Painting comprises mainly two steps, brushing and drying, and each step involves different shear rate ranges. In order to find a quantitative correlation between the paint film behaviour and the rheological properties of the paints, accurate viscosity measurements are essential at different shear rate ranges. References [34 - 401 describe different methods used to study the rheological behaviour of paint films. From a comparative study of the different methods used for rheological studies [37, 403, it has been concluded that although the drawdown bar method [33] and determination of viscosity recovery rate using Brookfield-Wells and plate and cone viscometers [35] are rather suitable, the visual method is the best. Bowel1 1421 showed how the paint rheology depends on the latex particle size at low and high shear rates (Figs. 4 and 5). Figure 5 shows that paint I has a higher brush drag due to its larger high shear viscosity, although these shear rates are not as high as those encountered in brushing. Figure 4 shows that paint I has a lower viscosity at low shear rates corresponding to those involved in flow and levelling, thus yielding good flow properties_ The use of suitable thickeners has also been exploited to affect the rheological properties 143 - 451 when the particle size of the
92 latex could not be varied in the directions desired by the formulator. Such a dependence of the rheological properties on the latex particle size or on the presence of thickeners can be supposed to be due to the “stand-off” distance, which influences the relaxation time for the instantaneous viscosity recovery at low shear rates, after the cessation of high shear conditions and the yield stress for paint deformation. Among the various types of thickeners used, cellulose ethers and poly(viny1 alcohol) of moderate molecular weight and low hydrolysis grade were found to be most satisfactory. Cellulose ethers gave good stable viscosities but usually poor flow, whereas poly(viny1 alcohol) gave very good flow and levelling but could not be used so often due to poor borax stability. The latex particle size has also been found to affect film formation at low temperatures [42], as shown in Table 13, and stain resistance. The film formation has only been determined qualitatively, by touching the films. The gloss of the latex paints has also been found to be very poor as compared with the other water-borne systems, because the latex paints result in less smooth surfaces due to their particle size. The particle size of the latexes has also been shown to influence the water uptake by the polymer films [46], as shown in Fig. 6. The low water uptake with increasing latex particle size was supposed to be due to the higher film uniformity in the case of films resulting from large particle size latexes as compared with the small particle size latexes. Higher film uniformity, in the former case, depends on the larger “standoff” distances than in the latter, due to containing a lower number of latex particles per cm3 which in turn results in longer relaxation times for levelling of films. TABLE 13 Dependence
of low temperature
Particle size (m) 0.630 70%
touch-up
on the latex particle size
50 “F
60 “F
‘70 “F
80 “F
Failed
Failed
Failed
Borderline
Failed
Borderline
Passed
Passed
0.630
30% 0.103
yroo 0
.ZSO b s 60 5 40 z 20
2
E
4 6 6 30 Immersion Timetdays)
Fig. 6. Dependence of water uptake on latex particle size. -0-d = 0.56 E.tm.
--a-d
= 11 pm,
93
5.2 Influence crf latex stabilizing system In order to obtain mechanical and storage stability
in latexes,
most of
the latexes contain stabilizers such as emulsifiers or protecting colloids. These stabilizers have also been shown to influence gloss and water resistance of the films due to their incompatibility with the polymers and the latex instability during film drying respectively. Snuparek [47 ] has shown that the water absorption of the latex films decreases with the increasing latex stabilization either by the post-addition of non-ionic emulsifiers or by the presence of carboxyl groups ir the polymers. Some of the results relating the water uptake to the emulsifier and carboxylic group content are illustrated in Figs. 7 - 9 147). A decrease in the water uptake with increasing amounts of the emulsifier and carboxylic groups content has been supposed to be due to the increased latex stabilization during film formation which in turn hinders premature flocculation and results in smoother films. The decrease in water uptake by the polymer films containing the same amount of -COOH groups at higher pH is supposed to be due to the higher extent of dissociation of the carboxylic groups in the polymers compared with that at lower pH values, thus resulting in an increased latex stabilization.
10
L1IIII 2 6
I 6
102
*I xl2
-1, 10 6 10 2 6 Immersion
0 n 6 10 10 2 Time (days)
Fig. 7. Dependenceof water uptake by the films on the amount of post-added non-ionic emulsifiersand carboxylic group content in the polymer under alkaline conditions. 0 0% acrylic acid (AA), 0 1% AA, 0 3% AA, V 4% AA.
l0
2
6
10 2
6
10 2 Immersion
Fig. 8. Dependence 03%AA, v4%AA.
IO 2
6
6
10
Time (days)
of water uptake
under acidic conditi&ns.
0
1%
AA,
A 2%
A&
Immersion
Time
(days)
Fig. 9. Dependence of water uptake of films deposited from latexes made alkaline with NH3 after preparation. 0 1% AA, * 2% AA, q 3% AA, V 4% AA.
5.3 In fiuence of water as prime solvent
Substrate and pigment wetting are the main problems arising from water as the prime solvent, which in turn influences the gloss and the film adhesion. Zisman 1481 has postulated that in order to wet a substrate, the critical surface tension (‘yC) of the coating should be lower than the 7C of the substrate_ The poor substrate wetting is due to the high surface tension of water (-72 dyn/cm) as compared with organic solvents (-30 dyn/cm). It has been shown 123 that the yC is reIated to the solubility parameter of the polymer by YC =
3.61 x Gpoiymer
where 6 is the solubility parameter of the polymer, which can be estimated by Small’s method as mentioned earlier. yC values for some of the polymers have earlier been reported by Swell [49]. Thus it is obvious from the above discussion that one way of improving wetting characteristics is by reducing the surface tension of the latex paints. But, because too low values of surface tension are impractical due to foaming problems, water-miscible cosolvents or wetting agents ahematively may be beneficial.
Summary Future problems in the use of water-borne acrylic emulsion paints can be summarized as follows: (1) As most of the adhesion problems [50] of latex paints arise from the water-sensitivity of the paint films due to the presence of water-sensitive emulsifiers and stabilizers, the development of latexes containing very small amounts of emulsifiers or containing emulsifiers which are reactive on drying so that they lose their water solubihty is required. (2) Since foaming in latex paints is also a big problem, latexes with high surface tension are needed. As shown earlier, surface tension affects the wetting of pigments and substrates; there is a need for the use of higher levels
95
of glycols to improve the wet-edge time by developing less water-sensitive and tougher latex films. (3) The development of latexes which give tougher and more resistant films than those available today is also needed.
Acknowledgements The author is indebted to Prof. B. R&rby for reading the manuscript and for valuable comments, and also to the directors of AB Wilh. Beckers for permission to publish this paper. References 1 W. H. Brendley, Jr. and T. H. Haag, ACS Meeting Preprints, 32(2) (Aug. 1972) 350. Coatings and Coating Processes, 2 J. I;. Gardon and J. W. Prane (Eds.), Non-polluting Plenum Press, New York, 1973, p. 5. W. H. Brendley and E. C. Carl, Paint Varn. Prod., 63 (3) (1973) 23. M. R. Yunaska and A. Mercurio, Pigm. Resin Technol., 3(10) (1974) 6. Northwestern Sot. for Coatings Technol., J. Paint Technol., 46 (1974) 57. R. J. Woodbridge, Polym. Paint Colour J., (30 June 1976) 537. L. S. Luskin and R. J. Myers, Encyclopedia of Polymer Science and Technology, Vol. 1. Wiley, New York, 1964, pp. 177 - 444. Acrylic Esters, Reinhold, New York, 1954. 8 E. H. Riddle, Monomeric Corp., Park Ridge, 1965. 9 M. Sittig, Acrylic Acid and Esters, Noyes Development and Appli10 R. H. Yocum, Functional Monomers, Their Preparation, Polymerization cation, Vois. 1 and 2, Marcel Dekker, New York, 1973. 11 W. H. Brendley, Paint Varn. Prod., 63 (July 1973) 19. 108. 12 R. E. Dilon, L. A. Matheson and E. B. Bradford, J. Colloid Sci., 6 (1951) 423. 13 G. L. Brown, J. Polym. Sci., 22 (1956) 14 T. F. Protzman and G. L. Brown, J. Appi. Polym. Sci., 4 (1960) 81. and T. E. Purcell, J. Paint Technol., 44 (1972) 86. 15 R. P. Hopkins, E. W. Lewandowski 131. 16 H. Burrell, Off. Dig., 34 (1962) 86. 17 M. Akay, S. J. Bryan and E. F. T. White, J. Oil Colour Chem. Assoc., 56 (1973) 549. 18 F. N. Kelley and F. Bueche, J. Polym. Sci., 50 (1961) 19 A. Mercurio, Off. Dig., 987 (1961). 20 J. L. Hildebrand, J. M. Prausnitz and R. L. Scott, Regular and Related Solutions, Van Nostrand Reinhold Co., New York, 1970. 197. 21 H. Burrell, J. Paint Technol., 40 (1968) 728. 22 H. Burrell, Off. Dig., 27 (1955) 23 J. Small, Appl. Cbem., 3 (1953) 71. for Maximum Weatherability, 24 R. E. Harren and A. Mercurio, Acrylic Coatings-Design Chicago Coating Symposium, April 1974. 25 D. H. Klein and W. J. Elm, J. Paint Technol., 45 (Jan. 1973) 68. 1009. 26 K. E. Piggot, J. Oil Colour Chem. Assoc., 46 (1963) 27 W. J. Blank and H. L. Hensley, J. Paint Technol., 46 (June 1974) 46. 21. 28 I. Metil, Mod. Paint Coat., 65(4) (1975) (1972) 29 M. Yoshino, M. Shibata, M. Tanaka and M. Sakai, J. Paint Technol., 44(564) 116. 30 W. J. Mckillip, B. M. Culbertson, G. M. Gynn and P. J. Menardi, Ind. Eng. Chem. Prod. Res. Dev., 13 (1974) 197.
96 31 H. Warson, ACS Div. Polym. Chem. Repr., 169th Meeting, Chicago, 1975. 32 W. H. Brendiey, G. V. Calder and L. A. Wetzel, in J. K. Craver and R. W. Tess (Eds.), Appl. Poiym. Sci., Org. Coat. Piast. Chem., ACS, 1975, p. 859. 33 J. S. Dodge, J. Paint Technol., 44 (1972) 72. 34 R. W. Kreider, Off. Dig., 36 (1964) 1244. 35 T. C. Patton, Off. Dig., 36 (1964) 745. 36 A. Quach and C. M. Hansen, J. Paint Technol., 46 (1974) 40. 37 A. Quach, Ind. Eng. Chem. Prod. Res. Dev., 12 (1973) 110. 38 C. M. Hansen, ibid.. 11 (1972) 426. 39 M. Camina and D. Pvl. Howell, J. Oil Colour Chem. ASSOC., 55 (1972) 929. 40 New York Sot. for Paint Technology, J. Paint Technol., 46 (1974) 31. 41 I. M. Kreiger, Adv. Colloid Interface Sci., 3 (1972) 111. 42 S. T. Bowell, in J. K. Craver and R. W. Tess (Eds.) Appl. Polym. Sci., Org. Coat. Plast. Chem., ACS, 1975, p. 597. 43 J. E. Glass, J. Oil Coiour Chem. Assoc., 58 (1975) 169. 44 W. C. Arney and J. E. Glass, ibid., 59 (1976) 372. 45 N. Sarkar and R. H. Lalk, J. Paint Technol., 55 (1972) 929. 46 J. Snuparek, J. Oil Colour Chem. Assoc., 59 (1976) 19. 47 J. Snuparek, ibid., 55 (1972) 1007. 48 W. Zisman, Ind. Eng. Chem., 55 (10) (1963) 19. 49 J. Swell, Mod. P&t., 66 (June 1971). 50 G. G. Schurr, in J. K. Craver and R. W. Tess (Eds.), Appl. Polym. Sci., Org. Coat. Plast. Chem., ACS, 1975, p. 548.
in Organic
Coatings, 5 (1977) 79 - 96 S.A., Lausanne - Printed in the Netherlands
0 Elsevier Sequoia
WATER-BORNE
SWARAJ AB
ACRYLIC
EMULSION
79
PAINTS*
PAUL**
Wiih. Becker,
Research
Department,
S-102
70, Stockholm
9 {Sweden)
Contents Introduction, Types
79
of water-borne
Properties
systems,
required in emulsion
80 gloss paints, 81
Dependence of coating properties 4.1 Thermoplastic coatings, 82 4.2 Thermosetting coatings, 87 Dependence of coating 5.1 Influence of latex 5.2 Influence of latex 5.3 Influence of water Summary,
composition,
properties on latex composition, particle size, 90 stabilizing system, 93 as prime solvent, 94
82
90
94
Acknowledgements, References,
on polymer
35
95
1. Introduction
The earliest types of house paints consisted mainly of lead or zinc oxide in linseed or tung oil. The main problems associated with such systems were: pigment agglomeration, poor pigment wetting, long drying times, higher tendency of dirt retention and chalking, and large number of coats (min. 3 coats) required to achieve good hiding. In the 1930’s oils were replaced by alkyds as vehicles and they were used in graphic arts, interior enamels and industrial fields. AIkyds were used in house paints in the 1950’s for improved colour and gloss retention, but such paints could not be marketed before 1960. This was due to two reasons: firstly, the use of zinc oxide as a pigment resulted in widespread intercoat peeling problems, and secondly, the knowledge of organic mildew inhibitors and the control of rheological properties *Presented as part of the doctorate course on Emulsion Ins22te of Technology, Stockholm, 28th January 1977. Previous address: Department of Polymer Technology, Technology, S-100 44, Stockholm 70.
Polymerization The Royal
at the Royal
Institute
of
80
of the paints were insufficient. Probably the greatest progress in house paints in the last 50 years has been the development of latex-based paints. Interior flat wall paints based on styrene-butadiene latexes were introduced in the late 1940’s and immediately became very popular. Semi-gloss latex paints were first commercialized around the 1960’s, as the latex available in the early years just could not seem to carry the formulas to the higher levels of adhesion, water resistance, hardness and other features that were required for the advances in trade sales paints. Recently, the increasingly restrictive anti-pollution legislation, which regulates the maximum amount of organic solvents that can be emitted into the atmosphere, has generated a renewed interest in water-borne systems. Apart from this, elimination of fire hazards, potential economy of water as a solvent and the shortage of solvents due to the oil crisis may, in future, also favour water-borne systems. The initial acceptance of latex paints was due to their easy application, clean-up and low levels of residual odour. More specifically, interest in aqueous acrylic polymers is higher as compared with poly(viny1 acetate) (PVAc) and other systems because of their high and inherent durability. Higher gloss values, in the case of acrylates, is attained due to two reasons: firstly, they have higher values of refractive index as compared with PVAc (viz. refractive indices of poly(buty1 methacrylate) (PBMA) and poly(methacrylate) (PMA) are 1.483 and 1.479) and secondly, they give rise to latexes with smaller particle size due to the more hydrophobic character of the acrylic copolymers than PVAc. 2. Types of water-borne systems Water-borne acrylic polymers can be placed into three classes [ 1, 23 : (a) aqueous dispersions or emulsions, (b) colloidal or water “solubilized” dispersions, and (c) water-reducibles. TABLE
1
Comparison of the three classes of water-borne acrylics Property
Aqueous
Appearance
Opaque Exhibits
dispersion
light scattering
Colloidal
dispersion
Water-reducible
Translucent Exhibits light scattering,
Clear No light scattering -
Particle si2.e
20.1 pm
About
Self-crowding capacity constant, K
- 1.9
1.0 + 0
0
Molecular weight
1 million
20,000 - 200,000
20,000 - 50,000
Low, independent of polymer mol wt.
More viscositysensitive, somewhat dependent on mol. wt.
Viscosity very dependent on polymer mol. wt.
Viscosity
20 - 100 nm
81
These three types vary significantly in physical and mechanical properties and thus provide a considerable formulation range for coatings
chemists. The main differences between the three classes are summarized in Tables 1 and 2 [l] . It is obvious from Table 2 that colloidal dispersions [3 - 51 are hybrids of the aqueous dispersions and water-reducible polymers. Table 3 summarizes some of the functional monomers which are used in the synthesis of watersolubilized acrylic copolymers [l] . Table 2 also shows some of the application differences between the three classes. The reasons for such differences have been discussed elsewhere [l] . In this review, only the emulsion type of binders will be considered
3. Properties
required in emulsion gloss paints
The properties required for an emulsion-based paint can be summarized as shown in Table 4. Table 5 shows an inter-relation between the various film properties, the polymer type and the paint formulations 163.
TABLE
2
Application
characteristics
of the three classes of water-borne
acrylics
Colloidal dispersion
Water-reducible
High
Intermediate
Low
Excellent
Excellent
Very good
Resistance props.
Excellent
Good - excellent
Fair - good
Viscosity control
Requires external thickeners
Thickened by addn. of cosolvent
Governed by polymer mol. wt.
MFT restraint
Hard films, require coalescence
Forms hard films with min. coalescence
None
Property
Aqueous dispersion
Solid content at application viscosity Durability
Formulation
Complex
Intermediate
Simple
Pigment dispersibility
Poor
Good - excellent
Excellent
Application difficulties*
Many
Some
Few
Specular gloss
Low
More like waterreducible systems
High
*Ease of clean-up, foaming, method of application, problems (flow, levelling).
substrate wetting and rheological
82 TABLE
3
Functional
monomers
Acidic Methacrylic Acrylic
acid
acid
Itaconic
acid
Maleic anhydride
TABLE
useful for preparation
acrylics
Basic
Non-ionic
Dimethylaminoethyl methacrylate
Methacrylamide
t-Butylaminoethyl methacrylate
Acrylamide
Diethylaminoethyl acrylate
Hydroxypropyl methacrylate
2-Vinylpyridine
Hydroxyethyl acrylate
4
Properties
required
in emulsion
gloss paints
Odour/toxicity
Balanced rheology
3. 4. 5. 6. 7. 8.
of water-solubilized
Ease of application (structure/non-drip) Good flow Sag resistance Shelf stability Compatibility with the pigment Good lapping Coalescence at 5 “C Drying at high R.H. Good re-coating
9.
Good
gloss
10. Good build 11. Good colour 12. Good colour retention 13. Good opacity 14. Block resistance 15. Mar/abrasion resistance 16. 17. 18. 19.
Stain resistance Water resistance Adhesion to old paint films Exterior durability
As it is not possible to cover in this review all the aspects which influence properties of the emulsion-based paints, only a correlation of the film properties with the polymer and the latex composition will be discussed. the coating
4. Dependence of coating properties on polymer composition Development of stiitable methods for the preparation of the acrylic monomers [ 7 - 10 ] has provided the acrylic polymer chemist with an unusual freedom in designing a polymer to meet a set of end-use requirements. Based on the polymer composition, acrylic latexes can be placed into two distinct classes: 4. I Thermoplastic coatings This represents a class of polymers whose hardness varies with temperature, Le. above Tg they behave like rubber. Acrylic polymers used in coatings
83 TABLE
5
Relationships
between
formulation
and film properties Additive
Solvent
TiOg (Type/level)
Polymer Type
Mol. Wt.
Part. Size Dist.
Coalescent
Other
Sol. Resin
Other
Gloss
X
X
X
X
X
X
X
X
Flow
X
X
X
X
X
X
X
X
X
X
X
X
Interactions
Application/ wet-edge Film formation
X
X
Dryingire-coat Opacity
X
X
X
X
X
X
X
X
X
X
X
X
X
x
X
ThermoplastJ hardness
X
X
X
X
X
Durability
X
X
cost
X
X
primarily consist of poly(methacrylates) turally be represented as follows:
X
X
X
and poly(acrylates)
X
X
which can struc-
y43 I: +CH2-
7
j;;
+CH2-
7
3,
4:=O
F=O
P R
P R
poly(methacrylate)
poly( acrylate)
It is obvious from the structures that the coating properties are strongly influenced by the following three factors: (a) The presence of the CH3 group or H atom on the a-carbon atom. (b) The length of the ester side group, R. (c) The presence of the functionality in the ester side group. (a) The presence of a methyl group in the cy-position results in a hindrance to the segmental rotation of the polymer backbone. As a result, poly(methacrylates) are invariably harder, less extensible polymers than the corresponding acrylates, as shown in Table 6 [ 111. (b) Table 6 also illustrates the influence of the ester group chain length on the tensile strength and the elongation of the polymers. Decrease in tensile strength is attributed to the increase in the sementaIl rotations in the side chains due to an increase in the free volume of the polymers. Thus the
84 TABLE 6 Properties of poly(methacrylates)
and poly(acrylates) Elongation
Polymethacrylates Alkyl group of the ester
Tensile strength
(p.s.i.)
Methyl
9000
4
Ethyl
5000
7
Butyl
1000
230
1000
750
Ethyl
33
1800
Butyl
3
2000
(70)
Polyacrylates Alkyl group of the ester Methyl
TABLE 7 Contribution of monomer types to the film properties Film property
Contributing
Ekterior durability Hardness
Methacryiates
monomers and acrylates
Methyl methacrylate Styrene Methacrylic and acrylic acid
Flexibi1it.y
Ethyl acrylate Butyl acrylate 2-Ethylhexyl acrylate
Stain resistance
Short chain methacrylates and acrylates
Water resistance
Methyl methacrylate Higher methacrylates
Mar resistance
Methacrylamide Acrylonitrile
Solvent and grease resistance
Acrylonitrile Methacrylamide Methacrylic acid
and acrylates
copolymers are more common in commercial applications than the homopolymers because a wide range of strength and flexibility can be achieved by choosing a suitable combination of comonomers. (c) As it has been shown earlier [ll] that the specific film properties depend on the functionality of the ester side chain, a polymer chemist can often prepare a tailor-made polymer to achieve the defined requirements such as flexibility, adhesion, hardness, etc. A correlation between the film properties and the type of monomers has been shown in Table 7 111 J .
85 TABLE
8
A correlation between minimum film forming temperature temperature ( Tg) for polar and non-polar polymers
Tg (“C)
KHN*
18
25
2.9
21
10
1.25
Type of polymer
MFT
55 % Ethyl acrylate + 45 7% methyl methacrylate 50% Styrene
+ 50%
butyl acrylate
Standard styrene-butadiene *Knoop
Hardness Number,
18
latex a standard
(RIFT) and glass transition
(“C)
0.38
test.
A further understanding of the relation between the macroscopic film properties of an acrylic coating and the polymer composition has become possible due to the successful application of the two parameters, viz. the glass transition temperature Tg and the solubility parameter 6. The glass transition temperature Tg is defined as that temperature, or more precisely that temperature region, at which significant segmental rotation in the backbone and/or side chains occurs. As the temperature passes through this region, the properties of the polymer film change dramatically, i.e. above Ts the film becomes rubbery, flexible and softer because the polymer segments can respond to stresses impressed on them. Since the same molecular processes determine the tensile strength and extensibility of the polymers, a close correlation between Tg and the mechanical properties of the polymers has been found (cfi Table 6). In the case of emulsion paints, an alternative term, the “minimum film-forming temperature” (MFT), is generally used. MFT represents the minimum temperature at which film coalescence occurs. Although no quantitative reiation between the Tg and the MFT exists, MFT is always found to be lower than Tg for most of the acrylates due to the plasticizing influence of water and emulsifier during film coalescence. The driving forces for polymer deformation during film coalescence, resulting in a continuous film, arises either from the surface tension [ 121 or capillary forces [ 131 which increase with decreasing particle size as a result of drying or from “autohesion”, the mutual inter-diffusion of free polymer chain ends across the particle-particle interface in the coalesced film. It has been shown by Protzman et al. [ 14] that in the case of non-polar polymers such as styrene-acrylate, MFT is higher than the Tg values. Such a correlation between MFT and TZ and thus their influence on film hardness has been shown both for the polar and the non-polar polymers in Table 8 1141. The higher value of MFT for styrene-acrylate (non-polar type) polymer shown in the table was supposed to be due to the stabilizer system. With the styrene-butadiene system, a sodium lauryl sulphate would probably cause a much higher surface charge with consequent charge repulsion effects than a polar acrylic copolymer, with which the stabilizer would be more compatible. A relationship between the MFT and Tukon hardness and thereby its influence on the film properties can be summarized as shown in Fig. 1 [ 151.
86
l0
20
30
f.0
50
60
70
90 80 MFT( “Cl
Fig. 1. Relationship between MFT and Tukon hardness and their influence on the paint properties. x Trade sales house paints for bare wood, 8 floor paints, 0 board coat:qs, n shingle coatings. Pigment binding, softness and flexibility, adhesion to substr?Lt3, blocking tendency and dirt pick-up of the paint films increases with the direction of the arrow.
Thus it is obvious that by understanding the factors influencing Tg and RIFT of an acrylic polymer, it is possible to predict the variety of properties of the actual coating systems [ 16 - 191. The solu bility parameter, 5, invented by Hildebrand [ 201 and applied to polymers by Burrell [ 21,223 , has also proved to be invaluable in understanding, correlating and predicting the solubility and compatibility of acrylic polymers, which in turn influences the gloss and the solvent a< .zk on the paint films. The 6 value is derived from the regular solution theory, which assumes that the entropy of mixing the components of a solution is the same as for an ideal solution, i.e. mixing is a random process, and that the enthalpy of mixing can be calculated from the model by making simplifyiqg assumptions about the nature of the molecular interactions involved. The iheory predicts that a polymer will be miscible with another polymer or solvent if the enthalpy of mixing of the two materials is essentially zero. This condition is guaranteed if the solubility parameters of the components are equal. Small 1233 has given a method for estimating the 6 value of polymers_ He assigned a “molar attraction constant” to the various organic groupings in the polymers, and thus from a knowledge of the polymer’s structural formula and density, 6 has been calculated as follows:
where d = polymer density, 17 = molecular weight of the repeating unit, EG= sum of molar attraction constants for each organic grouping in repeating units. Baaed on the 6 value of the solvent and the polymer, solvent attack on the coatings can be predicted, e.g. polymers containing long alkyl side chains such as poly(n-but@ acrylate) (~5= 8.7) are likely to have good resistance to water (6 = 23.4) and ethanol (6 = 12.7), whereas the polar polymers such as poly(acrylonitrile) (6 = 12.5) are predicted to show good resistance to attack
8’7
by aliphatic hydrocarbons (mineral spirit, 6 = 7.0). The knowledge of 6 values is also essential to obtain glossy films and reasonable evaporation rates of the coalescing solvents, wet-edge time, etc. Durability properties also depend on the polymer composition of the coating materials. Acrylics generally have excellent durability. They resist discoloration when exposed to elevated temperatures and are not easily attacked by acids or bases. This is mainly due to the chemical inertness of the C-C single bonds of the polymer backbone as compared with ether or amide linkages. The ester side chains can be hydrolysed with difficulty, and if such an attack occurs it does not result in the scission of the polymer backbone and so the film maintains its integrity. General durability characteristics of acrylic homopolymers are summarized in Table 9 [If ] . TABLE 9 General durability characteristics of acrylic homopolymers Ester
Methacrylate
Acrylate
Methyl
Very good
Poor
Ethyl
Excellent
Fair
i-Butyl
Excellent
Good
n-Butyl
Excellent
Excellent
2-Ethylhexyl
Very good
The general superiority of the methacrylates over the acrylates is due to the absence of a tertiary hydrogen atom in the former, which hinders the formatio? of a free-radical intermediate by hydrogen abstraction such as -(CH2-C-C02R)-, which in turn results in chain scission. The increasing durability with increasing ester chain length, as shown in Table 9, is supposed to result from the increasing flexibility and hydrophobicity of the paints, which increases with longer ester groups. The greater flexibtiity renders more dimensional stability to the paint film with the dimensions of the substrate change. It has been shown [24] that the durability of the copolymers plasticized to have the same TE decreases in the following order: BMA = MMA > MMA/BA > MMA/EA. In addition to chemical inertness, acrylics show superior durability due to the fact that the polymers are transparent to the spectral region between 3500 A and 3000 a, which is the most photochemically active region of the solar spectrum. Modification of acrylics by polymers or pigments which absorb in this region, e.g. alkyds or TiO 2, invariably reduces the exterior durability of the coatings. 4.2 Thermose t ting coatings The coalescence mechanism of the film formation in’ the thermoplastic emulsions limits the range of hardness that can be designed into the polymer
88
backbone. This limited seriously the use of emulsion-based paints for industrial purposes, which was essential in order to obtain improved block and print resistance. The introduction of thermosetting acrylic emulsions [25 29,4] has overcome these problems. The basic concepts of these types of emulsions were not new, but were only a logical extension of the solventbased thermosetting chemistry. The oldest thermosetting vehicle used even now is porcelain enamel, which is an aqueous dispersion of silica, sand and other componenB fused to produce a hard, ceramic finish. These high quality finishes are still extensively used in bath-tubs and household appliances such as range tops and hot water heaters, where extreme heat and chemical resistance is required. However, the expense, lack of flexibility, poor adhesion except to few substrates, and the intense heat required to fuse the finish, limits the use of porcelain to a few applications, and these are mainly replaced by thermosetting acrylate coatings. In the thermosetting types, reaction functionality is copolymerized into the polymer backbone, and, after application, the cross-linking reaction is thermally activated in situ between the comonomers (internal type) or blended reactants (external type). The different types of thermosetting systems used commercially are shown in Table 10.
TABLE
10
Differenttypesof commercially availablethermoseLting systems Pendant groups
Cross-linking
1. -CH20H
Al koxy aminoplast, e.g. melamineformaldehyde. benzoguanamine-formaldehyde and isocyanates
Acids 127 ]
Epoxy
Basic
Hydroxyl
agents
Catalysts
0 2.
-&-OH
resins
Carboxyl 3.
-Ch$-,CHg 0
Carboxyfic polymers, amines [ 281 and hydroxy compounds [ 28, 291
Acidic-MAA, AA (internal), NH&l, PTSA Basic-DMAEMA (internal), quaternary amm. compounds
Epoxy resins Alkoxy aminoplasts Carboxyl-containing
Acidic-AA or MAA (int.) H3P04, PTSA, morpholine salt of PTSA
EPOXY
4.
:: -CNH_CH2OH -$NH_CH20R 0 Acrylamide deriv.
AA -
Acrylic
DMAEMA
-
Acid, MAA
Dimethyl
-
Methacrylic
Aminoethyl
polymers -
Acid,
Methacrylate.
PTSA
-
p-Toluene
Sulfonic
Acid,
89
Recently, some new’ thermosetting systems based on aminimide [ 301, miscellaneous nitrogen-containing compounds such as aziridine, pyrrolidone, sulphonium derivatives, acrolein and siloxanes have been reported 1313 . Aminimide types are interesting due to being heat-activated isocyanate precursors. However, their commercial use for industrial purposes cannot yet be exploited due to the unavailability of the monomers on the commercial scale. Typical monomers, comprising mainly a thermosetting acrylic resin, can be summarized as shown in Table 11. It has been shown in the literature [ 17 3 that in the case of thermosetting coatings the film hardness can, apart from the polymer composition, also be controlled by the degree of cross-linking in the polymer films. Such a relationship between the film hardness and the degree of cross-linking has been reported earlier [ll] , and is shown in Fig. 2. It is obvious from the figure that such cross-links have no effect on the hardness of the films below Tg of the polymeric films. A comparison of the relative performance of the different acrylic thermosetting systems and the conventional alkyd/melamine systems has been shown in Table 12 [32] . According to this table, the acrylic thermosetting systems provide a better control of the end properties and the crosslinking reaction conditions than the conventional alkyd/melamine types. TABLE
11
Typical monomers used in the preparation of a thermosetting acrylic resin Contribution to film properties
Monomers Methyl methacrylate (MMA) Styrene Vinyltoluene Acrylonitrile (AN)
Hardness
Ethyl
acrylates
Butyl
and
2-Ethylhexyi Butyl maleate
I
Flexibility
methacrylates
Acrylamide Butoxymethylacrylamide
Hydroxyalkyl acrylates
Cross-links
Glycidyl acrylates Acrylic acid Acrylic acid (AA) Methacrylic acid M-AA) Maleic anhydride (MA)
Cure accelerator
90
Temperalure
Fig. 2. Hardness-temperature relationships for polymers of varying Tg and degree of crosslinking. (a) Soft polymer, highly cross-linked; (b) soft polymer, slightly cross-linked; (c) soft polymer, no cross-linking; (d) medium hard polymer, highly cross-linked; (e) medium hard polymer, no cross-linking. TABLE
12
Comparison of the performance with conventional al kyd.s
properties
Type
Curing temperature
Alkyd-melamine Acid-epoxy Acrylic-melamine Methylol-amide Urethane
250 375 250 325 -80
of different
(“P)
1 = Best, 4 = worst. All values relative to acrylic
thermosetting
acrylic
polymers
Toughness
Exterior durability
Chemical resistance
2 1 3 2 1
4 3 1 2 3
3 1 2 2 2
polymers.
5. Dependence of coating properties on latex composition Latexes are mainly composed of polymer particles dispersed in water and the stabilizer system which consists mainly of emulsifiers and thickeners. Most of the application problems, mentioned earlier, in the case of latex paints arise mainly from the latex particle size and the use of water as the primary solvent [Z] . The former involves rheological problems such as flow and levelling, film formation at low temperatures, gloss and the water resistance of the films, while the latter gives rise to problems relating substrate wetting and pigment wetting. 5. I In fZuence of latex particle size The main problems arising from the latex particle size we rheological problems. The poor rheology in the case of latex paints results from a sudden increase in viscosity after the critical coalescent concentration has been reached as compared with the gradual increase in the case of a solution polymer. Apart from rheological problems, this sudden viscosity rise causes
91
Solids Fig. 3. Viscosity dependence on the total solids of the binders for the water-borne systems. I Water-solubilizable dispersion, II aqueous emulsions, III water-reducible.
2 IU
!L-
_SJiO '30 2
2* aJ
5:
I
II
T-“-i=
24%
z
a i2
St
so 100200
Shear Stress
popping
I
20
.lZ
20
Stress(dynes,cm2)X,~3
Fig, 5. Dependence of paint’s high shear viscosity II 70% 0.63 pm latex + 30% 0.103 I_crnlatex.
or solvent
Lo
lx
(dynes,crn2)
Fig. 4. Dependence of paint’s low shear viscosity II 70% 0.63 flrn latex + 30% 0.103 pm latex.
blistering
l&?c I1
60
5
due
to water
on latex particle size. I 0.63
pm latex,
on latex particle size. I 0.63 Pm latex,
encapsulation
in the case
of
thermosetting emulsions. Dependence of the viscosity on solids of the waterborne acrylic resin is illustrated in Fig. 3 [11] . Several studies relating to the rheological behaviour of emulsion-based paints have been reported earlier in the literature [33 - 411. Painting comprises mainly two steps, brushing and drying, and each step involves different shear rate ranges. In order to find a quantitative correlation between the paint film behaviour and the rheological properties of the paints, accurate viscosity measurements are essential at different shear rate ranges. References [34 - 401 describe different methods used to study the rheological behaviour of paint films. From a comparative study of the different methods used for rheological studies [37, 403, it has been concluded that although the drawdown bar method [33] and determination of viscosity recovery rate using Brookfield-Wells and plate and cone viscometers [35] are rather suitable, the visual method is the best. Bowel1 1421 showed how the paint rheology depends on the latex particle size at low and high shear rates (Figs. 4 and 5). Figure 5 shows that paint I has a higher brush drag due to its larger high shear viscosity, although these shear rates are not as high as those encountered in brushing. Figure 4 shows that paint I has a lower viscosity at low shear rates corresponding to those involved in flow and levelling, thus yielding good flow properties_ The use of suitable thickeners has also been exploited to affect the rheological properties 143 - 451 when the particle size of the
92 latex could not be varied in the directions desired by the formulator. Such a dependence of the rheological properties on the latex particle size or on the presence of thickeners can be supposed to be due to the “stand-off” distance, which influences the relaxation time for the instantaneous viscosity recovery at low shear rates, after the cessation of high shear conditions and the yield stress for paint deformation. Among the various types of thickeners used, cellulose ethers and poly(viny1 alcohol) of moderate molecular weight and low hydrolysis grade were found to be most satisfactory. Cellulose ethers gave good stable viscosities but usually poor flow, whereas poly(viny1 alcohol) gave very good flow and levelling but could not be used so often due to poor borax stability. The latex particle size has also been found to affect film formation at low temperatures [42], as shown in Table 13, and stain resistance. The film formation has only been determined qualitatively, by touching the films. The gloss of the latex paints has also been found to be very poor as compared with the other water-borne systems, because the latex paints result in less smooth surfaces due to their particle size. The particle size of the latexes has also been shown to influence the water uptake by the polymer films [46], as shown in Fig. 6. The low water uptake with increasing latex particle size was supposed to be due to the higher film uniformity in the case of films resulting from large particle size latexes as compared with the small particle size latexes. Higher film uniformity, in the former case, depends on the larger “standoff” distances than in the latter, due to containing a lower number of latex particles per cm3 which in turn results in longer relaxation times for levelling of films. TABLE 13 Dependence
of low temperature
Particle size (m) 0.630 70%
touch-up
on the latex particle size
50 “F
60 “F
‘70 “F
80 “F
Failed
Failed
Failed
Borderline
Failed
Borderline
Passed
Passed
0.630
30% 0.103
yroo 0
.ZSO b s 60 5 40 z 20
2
E
4 6 6 30 Immersion Timetdays)
Fig. 6. Dependence of water uptake on latex particle size. -0-d = 0.56 E.tm.
--a-d
= 11 pm,
93
5.2 Influence crf latex stabilizing system In order to obtain mechanical and storage stability
in latexes,
most of
the latexes contain stabilizers such as emulsifiers or protecting colloids. These stabilizers have also been shown to influence gloss and water resistance of the films due to their incompatibility with the polymers and the latex instability during film drying respectively. Snuparek [47 ] has shown that the water absorption of the latex films decreases with the increasing latex stabilization either by the post-addition of non-ionic emulsifiers or by the presence of carboxyl groups ir the polymers. Some of the results relating the water uptake to the emulsifier and carboxylic group content are illustrated in Figs. 7 - 9 147). A decrease in the water uptake with increasing amounts of the emulsifier and carboxylic groups content has been supposed to be due to the increased latex stabilization during film formation which in turn hinders premature flocculation and results in smoother films. The decrease in water uptake by the polymer films containing the same amount of -COOH groups at higher pH is supposed to be due to the higher extent of dissociation of the carboxylic groups in the polymers compared with that at lower pH values, thus resulting in an increased latex stabilization.
10
L1IIII 2 6
I 6
102
*I xl2
-1, 10 6 10 2 6 Immersion
0 n 6 10 10 2 Time (days)
Fig. 7. Dependenceof water uptake by the films on the amount of post-added non-ionic emulsifiersand carboxylic group content in the polymer under alkaline conditions. 0 0% acrylic acid (AA), 0 1% AA, 0 3% AA, V 4% AA.
l0
2
6
10 2
6
10 2 Immersion
Fig. 8. Dependence 03%AA, v4%AA.
IO 2
6
6
10
Time (days)
of water uptake
under acidic conditi&ns.
0
1%
AA,
A 2%
A&
Immersion
Time
(days)
Fig. 9. Dependence of water uptake of films deposited from latexes made alkaline with NH3 after preparation. 0 1% AA, * 2% AA, q 3% AA, V 4% AA.
5.3 In fiuence of water as prime solvent
Substrate and pigment wetting are the main problems arising from water as the prime solvent, which in turn influences the gloss and the film adhesion. Zisman 1481 has postulated that in order to wet a substrate, the critical surface tension (‘yC) of the coating should be lower than the 7C of the substrate_ The poor substrate wetting is due to the high surface tension of water (-72 dyn/cm) as compared with organic solvents (-30 dyn/cm). It has been shown 123 that the yC is reIated to the solubility parameter of the polymer by YC =
3.61 x Gpoiymer
where 6 is the solubility parameter of the polymer, which can be estimated by Small’s method as mentioned earlier. yC values for some of the polymers have earlier been reported by Swell [49]. Thus it is obvious from the above discussion that one way of improving wetting characteristics is by reducing the surface tension of the latex paints. But, because too low values of surface tension are impractical due to foaming problems, water-miscible cosolvents or wetting agents ahematively may be beneficial.
Summary Future problems in the use of water-borne acrylic emulsion paints can be summarized as follows: (1) As most of the adhesion problems [50] of latex paints arise from the water-sensitivity of the paint films due to the presence of water-sensitive emulsifiers and stabilizers, the development of latexes containing very small amounts of emulsifiers or containing emulsifiers which are reactive on drying so that they lose their water solubihty is required. (2) Since foaming in latex paints is also a big problem, latexes with high surface tension are needed. As shown earlier, surface tension affects the wetting of pigments and substrates; there is a need for the use of higher levels
95
of glycols to improve the wet-edge time by developing less water-sensitive and tougher latex films. (3) The development of latexes which give tougher and more resistant films than those available today is also needed.
Acknowledgements The author is indebted to Prof. B. R&rby for reading the manuscript and for valuable comments, and also to the directors of AB Wilh. Beckers for permission to publish this paper. References 1 W. H. Brendley, Jr. and T. H. Haag, ACS Meeting Preprints, 32(2) (Aug. 1972) 350. Coatings and Coating Processes, 2 J. I;. Gardon and J. W. Prane (Eds.), Non-polluting Plenum Press, New York, 1973, p. 5. W. H. Brendley and E. C. Carl, Paint Varn. Prod., 63 (3) (1973) 23. M. R. Yunaska and A. Mercurio, Pigm. Resin Technol., 3(10) (1974) 6. Northwestern Sot. for Coatings Technol., J. Paint Technol., 46 (1974) 57. R. J. Woodbridge, Polym. Paint Colour J., (30 June 1976) 537. L. S. Luskin and R. J. Myers, Encyclopedia of Polymer Science and Technology, Vol. 1. Wiley, New York, 1964, pp. 177 - 444. Acrylic Esters, Reinhold, New York, 1954. 8 E. H. Riddle, Monomeric Corp., Park Ridge, 1965. 9 M. Sittig, Acrylic Acid and Esters, Noyes Development and Appli10 R. H. Yocum, Functional Monomers, Their Preparation, Polymerization cation, Vois. 1 and 2, Marcel Dekker, New York, 1973. 11 W. H. Brendley, Paint Varn. Prod., 63 (July 1973) 19. 108. 12 R. E. Dilon, L. A. Matheson and E. B. Bradford, J. Colloid Sci., 6 (1951) 423. 13 G. L. Brown, J. Polym. Sci., 22 (1956) 14 T. F. Protzman and G. L. Brown, J. Appi. Polym. Sci., 4 (1960) 81. and T. E. Purcell, J. Paint Technol., 44 (1972) 86. 15 R. P. Hopkins, E. W. Lewandowski 131. 16 H. Burrell, Off. Dig., 34 (1962) 86. 17 M. Akay, S. J. Bryan and E. F. T. White, J. Oil Colour Chem. Assoc., 56 (1973) 549. 18 F. N. Kelley and F. Bueche, J. Polym. Sci., 50 (1961) 19 A. Mercurio, Off. Dig., 987 (1961). 20 J. L. Hildebrand, J. M. Prausnitz and R. L. Scott, Regular and Related Solutions, Van Nostrand Reinhold Co., New York, 1970. 197. 21 H. Burrell, J. Paint Technol., 40 (1968) 728. 22 H. Burrell, Off. Dig., 27 (1955) 23 J. Small, Appl. Cbem., 3 (1953) 71. for Maximum Weatherability, 24 R. E. Harren and A. Mercurio, Acrylic Coatings-Design Chicago Coating Symposium, April 1974. 25 D. H. Klein and W. J. Elm, J. Paint Technol., 45 (Jan. 1973) 68. 1009. 26 K. E. Piggot, J. Oil Colour Chem. Assoc., 46 (1963) 27 W. J. Blank and H. L. Hensley, J. Paint Technol., 46 (June 1974) 46. 21. 28 I. Metil, Mod. Paint Coat., 65(4) (1975) (1972) 29 M. Yoshino, M. Shibata, M. Tanaka and M. Sakai, J. Paint Technol., 44(564) 116. 30 W. J. Mckillip, B. M. Culbertson, G. M. Gynn and P. J. Menardi, Ind. Eng. Chem. Prod. Res. Dev., 13 (1974) 197.
96 31 H. Warson, ACS Div. Polym. Chem. Repr., 169th Meeting, Chicago, 1975. 32 W. H. Brendiey, G. V. Calder and L. A. Wetzel, in J. K. Craver and R. W. Tess (Eds.), Appl. Poiym. Sci., Org. Coat. Piast. Chem., ACS, 1975, p. 859. 33 J. S. Dodge, J. Paint Technol., 44 (1972) 72. 34 R. W. Kreider, Off. Dig., 36 (1964) 1244. 35 T. C. Patton, Off. Dig., 36 (1964) 745. 36 A. Quach and C. M. Hansen, J. Paint Technol., 46 (1974) 40. 37 A. Quach, Ind. Eng. Chem. Prod. Res. Dev., 12 (1973) 110. 38 C. M. Hansen, ibid.. 11 (1972) 426. 39 M. Camina and D. Pvl. Howell, J. Oil Colour Chem. ASSOC., 55 (1972) 929. 40 New York Sot. for Paint Technology, J. Paint Technol., 46 (1974) 31. 41 I. M. Kreiger, Adv. Colloid Interface Sci., 3 (1972) 111. 42 S. T. Bowell, in J. K. Craver and R. W. Tess (Eds.) Appl. Polym. Sci., Org. Coat. Plast. Chem., ACS, 1975, p. 597. 43 J. E. Glass, J. Oil Coiour Chem. Assoc., 58 (1975) 169. 44 W. C. Arney and J. E. Glass, ibid., 59 (1976) 372. 45 N. Sarkar and R. H. Lalk, J. Paint Technol., 55 (1972) 929. 46 J. Snuparek, J. Oil Colour Chem. Assoc., 59 (1976) 19. 47 J. Snuparek, ibid., 55 (1972) 1007. 48 W. Zisman, Ind. Eng. Chem., 55 (10) (1963) 19. 49 J. Swell, Mod. P&t., 66 (June 1971). 50 G. G. Schurr, in J. K. Craver and R. W. Tess (Eds.), Appl. Polym. Sci., Org. Coat. Plast. Chem., ACS, 1975, p. 548.