Self-Crosslinking Surfactant-Free Acrylic Dispersions for High-Performance Coatings Applications
Acrylic dispersions have found widespread use in modern coatings technology. They have excellent durability, which makes them suitable for indoor and outdoor decorative paints, and can be formulated into high-resistance coatings for industrial uses. Generally, for high-performance coatings in a variety of applications, several key properties are of importance.
- Hardness and scratch resistance
- Anti-blocking (for stacking recently coated substrates)
- Resistance against household chemicals and grease
- Outdoor durability and UV resistance
- Flexibility and toughness
Especially high glass-transition temperature (Tg) and high molecular-weight (Mw) acrylic dispersions can achieve very good application properties, but high levels of coalescents are required for good film formation. With the ongoing drive toward more environmentally friendly coatings, the aim is to reduce the coalescent demand further with zero VOCs as the ultimate goal.
In recent years, several options have been investigated and published aimed at combining the good properties of high-Tg polymer backbones with good film formation. Some useful techniques that are practiced include the following.
- The use of hydrophilic low-Mw polymer backbones.
- The use of sequential polymerization to create phase-separated particle morphologies.
- The use of crosslinking techniques.
Hydrophilic Low-Molecular-Weight The first method is the use of a very hydrophilic and, in some cases, low-Mw polymer backbone. If a sufficient amount of acrylic or methacrylic acid is incorporated into the backbone, these dispersions can be either alkaline soluble or alkaline swellable;4-5 in both cases, the minimum film-formation temperature (MFT) and the coalescent demand are significantly reduced. The main reason for this is that the high hydrophilicity enables water to act as a plasticizing agent. The neutralized acid groups are ionic, which will make water molecules strongly adsorb onto the polymer chains. This adsorbed water will reduce the Tg of the polymer chains and, hence, improve their mobility. This will automatically lead to improved film formation by more rapid and efficient chain entanglement. When the neutralizing amines evaporate during or after film formation the ionic groups will disappear and the resulting film may even be somewhat water resistant.
To maintain a workable balance between viscosity and solids content, low-Mw polymers or oligomers are used. Since low-Mw polymers in general are not very resistant toward water and chemicals, and hydrophilic backbone polymers have poor water resistance, they are often combined with some form of crosslinking after or during film formation. This can be self-crosslinking or two-component crosslinking, and will upgrade the resistance properties6-7 significantly.
One particularly interesting crosslinking concept for acrylic oligomers with a high acid content is the use of polyaziridines, which will lead to a reduction of the acid group concentration in the final film.8 Clearly, the poor resistance properties are the most important drawback of this approach.
Using Sequential Polymerization to CreateAnother method for obtaining the desired balance of properties at a low coalescent demand is the use of phase separated polymers,9-10 where there is a significant difference in Tg between core and shell. In such particles, the low-Tg part contributes to good film formation, while the high-Tg part gives the desired good resistance, blocking and hardness properties. A range of different morphologies can be obtained depending on the kind of polymerization strategy that is applied, the choice of monomers, polymer viscosity, the molecular weight of the polymers, and the degree of grafting between the phases.9,11,12 This may vary from small domains inside a particle to core-shell, or inverted core-shell, but can also be raspberry-like structures, or completely phase-separated morphologies (see Figure 2).
Phase-Separated Particle Morphologies
It has been found that core shell particles with soft shells (low Tg) and hard cores (high Tg) have lower MFTs over the average compositions and have a reduced coalescent need. In general, morphologies with a hard shell have very good block resistance, but MFTs tend to be much higher. These multi-phase systems were compared with blends of hard and soft dispersions, and were found to have superior properties.13 Because of the improved film formation of the multi-phase particle emulsions, better properties are generally obtained. Films have fewer irregularities and defects, yielding a smoother surface and hence higher gloss and better optical transparency. The chemical resistance of films cast from such emulsions is moderate due to their thermoplastic nature.
Although this is a very useful and widely used technique, there is still room for improvement. The high-Tg phase in these systems very often hampers the film formation, also due to its very high molecular weight. Performance could be greatly improved if the high-Tg polymer phase (which is the most resistant and hardest part of the system) would entangle properly and would contribute to the matrix of the film.
An additional disadvantage of phase-separated emulsions is that they require high concentrations of surfactant, just as normal emulsion polymers. These concentrations are normally in the range of 1-3% by weight. It has been found that this surfactant can contribute negatively to film appearance and film properties. One of the problems that can occur is surfactant exudation, which will be discussed.
Applying Crosslinking to Improve Chemical ResistanceIt is clear that while a good MFT-hardness balance can be achieved, both approaches also yield the severe disadvantage of inferior chemical resistance. As mentioned, these can be overcome by post crosslinking either the low-Mw polymer or the low-Tg polymer. Crosslinking can be effective in two ways: chemical reaction between polymer chains following interdiffusion and crosslinking at the interfaces of the particles. In the first case, a coherently formed film is fixed and the molecular weight of the chains present in this film is increased. In the second case, two particles are chemically bound by crosslinking during or just after film formation. Formation of a chemical link between particles has an effect on properties similar to entanglement between polymer chains, but can take place on a much shorter time scale.
A variety of crosslinking reactions that can be used for acrylic dispersions have been extensively described in past years. There are so-called two-component crosslinking systems where one of the crosslinking components is added just before application of the dispersion. Alternatively, there are self-crosslinking (one pot) systems where all reactive components are present and long-term storage stable. The crosslinking reaction can be triggered by the evaporation of water upon drying, a change of pH, or by curing at elevated temperature, where the crosslinking reaction is faster, or reactive groups are de-blocked. Examples of suitable crosslinking systems are the reaction of azeridines with acid groups on the polymer backbone, the reaction of OH functionality on the backbone with post added isocyanates14 or melamines,15 the reaction of amines with epoxy functionality16 where either can be on the polymer backbone, the auto-oxidation of incorporated fatty acid groups,17,18 the self condensation of alkoxy-silane functionality,19 the self condensation of n-methylolacrylamide,6,7 metal-ion coordination with backbone functional groups such as acetoacetoxy6 groups or acid groups, and the reaction of acetoacetoxy groups with amines20,21 or acetoacetoxy groups with unsaturated groups in a Michael reaction.22
The Role of the SurfactantSurfactants play a crucial role in emulsion polymerization. They are required for emulsification of the monomers, formation of micelles as polymerization loci and colloidal stabilization of the polymer particles. In addition, surfactants reduce the surface tension of the resulting dispersion, which is required for substrate wetting and for film formation. Due to the presence of surfactants, the dispersion can be formulated into a coating because the surfactants will avoid destabilization of the dispersion on addition of formulation components. In fact, they may even aid in effective mixing of these components and the dispersion.
On the other hand, surfactants are in most cases water soluble and mobile components in the film. They have a tendency to cluster together or migrate (see Figure 3), either to the film-air interface or the film-substrate interface.23 In Figure 4, surfactant exudation to the surface of an acrylic dispersion film can be observed, where the exuded surfactant crystallized at the polymer air interface. The surfactants can in this way seriously affect the water sensitivity of the film, as well as the adhesion characteristics.
Additionally, surfactants that remain on the interfaces of the polymer particles will create hydrophilic channels through the film, which may cause undesirable effects such as water transport through the film to the substrate.
Several options have been studied to reduce the negative effects of surfactants while maintaining the positive aspects, such as the use of copolymerizable surfactants23,24 and polymeric surfactants. Copolymerizable surfactants chemically bond to the polymer particles and cannot migrate through the film or to the surface freely and as a result films from acrylic dispersions prepared with copolymerizable surfactants should have better water-resistance and adhesion properties.
There are, however, some issues associated with the use of copolymerizable surfactants. The reactive groups on the surfactants are often not of the same reactivity as the reactive groups of the monomers used, which may lead to poor incorporation or homopolymerization of the surfactants in the water phase, which is not desired. And if incorporation is very good the resulting dispersion can be very high in surface tension because there is no free surfactant left, which impairs wetting, leveling and formulation of the dispersion. In addition, to promote good copolymerization with the acrylic monomers ideally, the reactive group of the surfactant is in the hydrophobe of the surfactant, and such surfactants are hardly commercially available.
Alternatively, polymeric surfactants can be used.25 These are much less mobile than conventional surfactants and are usually very strongly absorbed to the particle surface because of their multiplicity of hydrophobes. So again they will not easily form clusters or migrate to the surface of a film. These polymeric surfactants can be prepared by a variety of methods including bulk polymerization, suspension polymerization or emulsion polymerization.
It is also possible to prepare surfactant-free acrylic dispersions, which are solely stabilized by the anionic initiator (persulphate) end groups,26 but in general only low-solids systems can be prepared this way. The solids can be raised by incorporation of ionic monomers, but in general the resulting systems have low tolerance toward the addition of formulating component.
Another route to obtaining surfactant-free dispersions is the preparation of self-dispersing polymers in organic solvents, which are subsequently dispersed into water.27 After dispersing, the organic solvents are distilled off to yield a so-called secondary dispersion or artificial latex. Here the dispersing groups are most usually acid groups that have been neutralized to salts by the addition of an amine. The ionized polymers have molecular weights of 10-50K, are surface active and are known to be useful in the stabilization of hydrophobic polymers. In this case, core-shell particle morphologies are created where the ionized polymer forms the shell, and the hydrophobic polymer forms the core of the particle. The ionized polymer in this case functions as the stabilizer for the hydrophobic core.
Upon film formation of such secondary dispersions, the matrix of the film will consist of the ionized polymers and hydrophobic particles are dispersed in this matrix. Since the ionized polymers are usually low in molecular weight and contain fairly high levels of copolymerized and ionized acid the matrix of the film will tend to be the weakest part of the system with regard to sensitivity to water and toughness of the film. The low Mw of the ionized polymers will aid the coalescence process and entanglement of the polymer chains to build strength in the film. Development of properties such as toughness and resistances will be more rapid than for a comparable high-molecular-weight dispersion.
Results and DiscussionThis article discusses the use of polymeric self-crosslinking stabilizers for the preparation of surfactant-free acrylic dispersions with a core shell morphology.
The core consists of normal high-Mw hydrophobic acrylic polymer. One of the purposes of the shell is to stabilize the core against flocculation and coagulation. To do this, the shell is made up from high Tg hydrophilic alkaline-soluble polymer chains, which are modified with hydrophobic components to make them highly surface active (from here on such polymers will be referred to as polymeric surfactants).
Polymeric Self-Crosslinking SurfactantIn Figure 5, the surface tension of aqueous solutions of such polymeric surfactants is compared to that of polymethacrylic acid and a commercially available hydrophilic alkaline soluble acrylic oligomer.
As can be observed, at very low concentration the surface tension of the polymeric surfactant solution is quickly reduced and equilibrates. The final surface tension is approximately 43 mN/m, while for polymethacrylic acid this is >60 mN/m. This is likely caused by the multiple hydrophobic groups that a polymeric surfactant contains, which causes the polymer chain to stretch along the surface of the air water interface.
As can also be observed, just using a commercially available hydrophilic alkaline polymer is far less effective in reducing the surface tension.
To demonstrate that the self-crosslinking polymeric surfactants are indeed effective stabilizers for the preparation of acrylic dispersions, a REACT-IR experiment was performed. The REACT-IR probe detects small particle size (monomer swollen) particles and water dissolved monomers. Large monomer droplets are outside the detection range of the REACT-IR probe. Monomer is added to a solution of self-crosslinking polymeric surfactant in which the probe is immersed. As shown in Figure 6, the IR peaks from the monomer appear and grow during a period of about 10 minutes after addition of the monomers after which an equilibrium is reached. In the left plot, they peak at a wave number around 1,200. This same peak is plotted in intensity vs. time on the right. Some baseline shift will occur due to temperature variations during the reaction. The change in this peak indicates that monomer is being absorbed into the self-crosslinking polymeric surfactant aggregates until an equilibrium is reached. A visual check at this stage confirms that no large monomer droplets remain and all monomer is emulsified. When initiator is added at this point, conversion of the monomer is almost instantaneous, with a large temperature exotherm. This is especially clear in the intensity vs. time plot on the right. This suggests that the polymerization mechanism has strong resemblance to that of a mini emulsion polymerization.
The resulting dispersion is stable and generally has a very low particle size, which confirms that these self-crosslinking polymeric surfactants are very efficient stabilizers.
To verify that a core shell structure is obtained, the dispersion was analyzed with TEM. The self-crosslinking polymeric surfactant was modified so it could be selectively stained vs. the core polymer. Figure 7 shows that the stained self-crosslinking polymeric surfactant (the dark-colored material) is present in the particle shell and in the water phase.
The Effect on Film FormationAs described previously, this hard hydrophilic self-crosslinking polymeric surfactant will have good film formation since it is an alkaline-soluble polymer. Entanglement, however, is not really applicable in the self-crosslinking polymeric surfactants, since their molecular weights are just below the entanglement Mw. The result of this is that a very fast mixing of the low-Mw hard hydrophilic polymeric surfactant will occur. This is followed by a crosslinking reaction leading to a crosslinked continuous hard phase containing discontinuous soft domains. The formation of crosslinks at the particle interface will give an even better buildup of film strength than polymer-chain entanglement would, and generally in a much shorter time scale. By focusing crosslinking at the particle interface it is used most effectively. Many of the crosslinking techniques are quite expensive, and should be used in the most efficient way possible. Precrosslinking should be avoided as much as possible, since this will negatively affect entanglement during film formation.
Alkaline-soluble systems on their own generally lack sufficient resistance properties to be of practical use. To overcome this problem, polymeric surfactants were used as stabilizers in a subsequent emulsion polymerization. This way, core shell particles are created, which have the polymeric surfactant as the shell, and a high-Mw resistant polymer in the core (see Figure 8).
Relatively small quantities of polymeric stabilizer are required for effective stabilization of the core. By introducing the same self-crosslinking mechanism in the core polymer, performance of the system can be upgraded even further. Now not only does the polymeric dispersant crosslink with itself to form a crosslinked matrix in the resulting film, but this matrix is also grafted onto the resistant high-Mw cores of the particles. This effect is demonstrated in Table 1.
As can be observed, crosslinking of the core polymer alone has no effect on the hardness MFT balance, compared to no crosslinking at all. If just the polymeric surfactant is self crosslinking the hardness MFT balance is improved. But the best result is achieved if both the polymer and the polymeric surfactant crosslink.
In effect, a core shell polymer with a film forming hard shell polymer and a high Mw, resistant core is created, where the hard component will form the matrix of the film and will then crosslink with itself and the core polymer (see Figure 9). As well as the core, the shell will contribute to the overall hardness, toughness, and resistance of the final film.
No surfactant will be used and hence no surfactant exudation will take place, as can be observed in Figure 10, and no hydrophilic channels to the film can be created.
The photo on the right in Figure 10 is a phase image of a film cast from a dispersion containing a self-crosslinking polymeric surfactant, which was recorded in AFM tapping mode. The light areas represent hard material and the dark areas represent soft material.
It can be observed that the surface of the film is mainly hard material, which confirms that the surface of the film is covered with the polymeric crosslinked surfactant. This is beneficial for hardness and anti-blocking. In the photo on the left, the height differences are depicted. It can be seen that the surface is quite smooth and individual particles can no longer be observed, confirming that film formation was efficient.
Investigating Film Formation and Film Properties of Model DispersionsIn the system under investigation, there are several design parameters that can be adjusted to optimize application properties.
- Ratio between shell material and core material,
- Tg of both phases,
- Hydrophilicity of the core, and
- Acid value of the shell phase.
The Tg of the high-Mw core and the ratio between high Tg shell and the core obviously affected surface hardness, as can be seen in Table 2.
From the first two entries in the table, it is clear that the Tg of the core has a huge effect on surface hardness as well as on MFT. Both these effects are, however, reduced on increase of the shell:core ratio to 1:1. It is apparent that for a system comprising equal amounts of core and shell material a good hardness/MFT balance can be achieved even when the Tg of the high-Mw core is very low.
Acid value did not have a significant effect on surface hardness (see Table 3), which can be expected since the effect of the difference in acid concentration on theoretical Tg was compensated for by other monomers. Since the acid value concentration will influence the degree at which water can act as a plasticiser it is, however, expected to affect MFT of these emulsions.
While the surface hardness values seem to be independent from acid value, MFT was indeed affected by the acid value. Especially at high shell:core ratio (entries 7 and 8) MFT was significantly reduced with increasing acid value while maintaining good surface hardness. This can partly be explained with increasing water plastization and partly by taking into account that at higher shell:core ratio smaller particles will be formed. It is well known that smaller particles tend to give lower MFTs than larger particles with the same composition. While particle sizes for the polymer emulsions with a shell:core ratio of 1:4 were found to be in the range of 250-400 nm, at a shell:core ratio of 1:1 the particle sizes were between 40 and 70 nm.
Entry 7 especially yielded a very good balance between MFT and surface hardness: 15?C and 121, respectively. Reducing the acid value of the shell led to only a marginal increase in surface hardness, while the MFT increased by 20?C.
These results show that shell:core ratio, Tg of the high-Mw core, and acid value of the shell can be useful tools to control the balance between film formation on the one hand and film properties on the other.
For most applications, anti-blocking is a required property. Normally, an easy way of introducing anti-blocking is to increase the Tg of the film, which will, however, cause poor film formation properties. For our model emulsions with a shell:core ratio of 1:1, good film formation could be combined with excellent blocking properties. Table 4 shows the blocking properties of model emulsions as a function of chemical composition of the core.
For both core compositions, very good blocking and early blocking results were obtained. Interestingly, these good blocking properties were obtained for films containing 50% by weight of very low-Tg material. Table 4 shows that at these low core Tg good surface hardness properties were found and acceptable MFTs. In particular, the polymer having a core containing methyl methacrylate gave a very interesting set of properties with an MFT of only 15 degC, high surface hardness and excellent blocking properties.
Application in High-Performance CoatingsAs demonstrated, variation of the design parameters can have a big impact on the application properties. When the design is optimized for application in industrial wood coatings the following properties can be achieved (see Table 5). Because of the absence of surfactant, the system will be low foaming. The dispersion has a very low particle size, which results in excellent film formation and high transparency. An additional benefit of the very low particle size is the almost translucent optical appearance of the binder, which is comparable to traditional solventborne coatings. Also wood wetting, which is an important property for industrial wood coatings, is excellent, because of the good wetting potential of the self-crosslinking polymeric surfactant and the small particle size, which makes penetration into wood pores efficient.
By modifying some of the design parameters, a surfactant-free self-crosslinking acrylic dispersion for joinery can be created. In this application especially, flexibility is of importance combined with good anti-blocking properties (see Table 6). For achieving good outdoor durability the coating will have to be flexible enough to compensate for the changing wood dimensions which are weather dependent. The coatings need to be fast drying and should be applicable by a variety of application techniques, such as spraying, dipping etc. Again these dispersions have a very low particle size and the desired translucent appearance.
These coatings can be formulated and pigmented, and will still retain all properties. When formulated at 18% PVC, the flexibility is still >80%, which is sufficient to achieve good outdoor durability.
While this article focused on high-Tg shells and low-Tg cores, the use of low-Tg shells combined with high-Tg cores can lead to very interesting systems for a variety of end uses.
ConclusionThe use of self-crosslinking polymeric surfactants is an effective route for the preparation of surfactant-free acrylic dispersions. These dispersions do not show any of the disadvantages associated with conventional surfactants and yet are efficiently stabilized and easy to formulate. High-performance acrylic dispersions can be prepared when self-crosslinking polymeric surfactant of high Tg are used for preparation of core shell particles where the self-crosslinking polymeric surfactant is the shell, stabilizing a high-Mw self-crosslinking core polymer. Upon film formation a high Tg crosslinked matrix is formed containing high-Mw resistant core particles. These systems have an excellent hardness/MFT balance and require little or no coalescents for film formation. They have excellent anti-blocking properties and good resistances to chemicals and water. Such dispersions show promise for a wide area of coatings and ink applications.28
The authors would like to thank Danny Visser, Harriet van der Sande, Dorina van Haeringen, Anton Peters and Yvonne Smak for their contributions to this work.
For more information on acrylic dispersions, contact A. J. P. B¿ckmann, NeoResins, Sluisweg 12, PO Box 123, 5140AC Waalwijk, The Netherlands, phone +31(0)416689797; fax +31(0)416689922; or Circle Number 77.
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28 This technology is protected by several patents, owned by NeoResins.