The main objective of this review is to describe some of the important topics related to the use of marine and protective coatings for anticorrosive purposes. In this context, “protective” refers to coatings for containers, offshore constructions, wi
This article describes the applications of lasers in coating deposition processes. After an introduction concerning the types and principal characteristics of the laser and the emitted light beams, a description of the mechanism of interaction betwee
Kinetic spray process has been applied to various industrial fields such as automotive, aviation, and defense industries due to its availability to produce high-performing coating layer. However, since the properties of kinetic-sprayed coating layer
Fire is a serious threat to people and the structures they build. There is a continuous development of newer methods and materials to prevent its effects on them. Nowadays, a lot of attention is being paid in the design of public and commercial build
Metal dusting is a catastrophic form of carburization attack that takes place in carbon-supersaturated gaseous atmospheres, and is most commonly encountered in steam reforming processes such as the production of hydrogen or syngas for ammonia, Fische
Many modern optical instruments require the use of high-quality antireflecting coatings. Singleand multilayer homogeneous films are mainly used for this purpose. However, an alternative line is rapidly developed at present, which is devoted to the de
Unusual properties of quasicrystals—a low friction coefficient; high hardness, wear resistance, and oxidation; and low electrical and thermal conductivities—make quasicrystalline materials promising for practical applications. Methods of fabrication
Phosphating is one of the most widely used surface treatments of steels and aluminum due to its low-cost, easy mass production, good corrosion resistance and good adhesion with paint. Many researchers have tried to expand applications of the phosphat
Yttria-stabilized zirconia and α-alumina films were prepared by laser chemical vapor deposition at deposition rates of several hundred micrometers per hour. Moreover, the structural oxide coatings by laser chemical vapor deposition are reviewed. The
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J. Coat. Technol. Res. https://doi.org/10.1007/s11998-017-9996-4
Self-stratifying coatings: a review S. Zahedi, D. Zaarei, S. R. Ghaffarian
S. Zahedi, D. Zaarei (&) Polymer Department, Engineering Faculty, South Tehran Branch, Islamic Azad University, Tehran, Islamic Republic of Iran e-mail: [email protected] S. R. Ghaffarian, Polymer Engineering Department, Amir Kabir University, Tehran, Islamic Republic of Iran
Introduction A self-stratifying coating (SSC) is a multiresinous component that consists of two or more layers, with each layer serving a specific function, which spontaneously stratify after application on the substrate and provide an undercoat and a finishing coat in one operation.1 In the present study, the SSC and its theories together with the effects of different factors on the layers location in the coating systems as well as the main factors such as physical and surface chemistry of ingredients have been studied and taken into consideration. Recent important activities and findings in this field have been presented.
Stratification theories Phase separation/solubility parameter/density differences There are different theories that account for the selfstratification phenomenon.2 Funke, in early times as a pioneer, attempted to explain it as a result of density differences among the layers.3 The effects of viscosity and surface energy on self-stratification of some coating materials were studied by him. Based on this point of view, one common requirement in all stratified coating was the incompatibility of resins. Verkholantsev4 mainly concentrated on phase separation of the two different polymers in these systems. He aimed at finding a relationship between phase diagrams of ingredients (two solvents plus two polymers) and the separation/stratification of the whole system. Misev5 used Hansen solubility parameters (HSP) of resin pairs and solvents to predict the final outcome of the coating. It was conclusively claimed
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that immiscibility was a requirement for stratification, yet was not sufficient. Therefore, some additional requirements like suitable viscosity and surface free energy are required to obtain stratified coating.6 It has been stated that two binders can be incompatible when the compatibility parameter β is higher than 0.07 J · cm−3.7 b ¼ ðd1 d2 Þ
δ1 and δ2 are the Hildebrand solubility parameters of the two resins. Experiments were performed in order to evaluate the possible applicability of the Hansen solubility parameter (HSP) concept in designing stratifying coatings and for the characterization of the degree of incompatibility of resin combinations. The HSP concept was developed to predict the solubility of a resin in a solvent and the parameters of which are determined by measuring experimentally the solubility of a resin in 50 solvents at a concentration of 10%.8–10 The concept assumes that the solubility of a solvent can be best described by means of three parameters: δd (dispersion forces), δp (polar forces), and δH (hydrogen bonding forces), while the solubility of a polymer can be described by these three parameters together with the radius R of a sphere in the three-dimensional solution space. Upon verifying the solubility parameters of a great number of commercial resins, it was concluded that the rules of the HSP concept were largely followed, and more importantly, at much higher concentration than 10%. In the concept, the solubility parameters of a solvent mixture are assumed to be linear, dependent on the volume fractions of the parameters of the solvents. Nevertheless, upon verifying the solubility of commercial resins in a mixture of a solvent and a nonsolvent, it was concluded that the solubility limits would mostly occur at lower nonsolvent content than the one predicted by the concept, while the solubility in this case is also largely dependent on the resin concentration. Phase separation of two resins after evaporation of common solvents can be expected to occur if the resins have different solubility parameters. It was investigated whether or not the degree of incompatibility of two resins could be characterized by means of the extent of overlap of their solubility spheres. Fig. 1 shows the solubility spheres of two resins. R1 is the radius of smaller sphere or polymer. R2 is the radius of bigger sphere or polymer. The extent of overlap factor for two polymers or resins was defined as: V¼
C100 4 3 3 pR1
The parameter C as solubility volume shared by two resins (polymers) is:
Fig. 1: Hansen solubility spheres for two resins11
δh δd Fig. 2: Incompatibility of two resins characterized by the extent of overlap of their Hansen solubility parameter spheres7
1 C ¼ p½P21 ð3R1 P1 Þ þ P22 ð3R2 P2 Þ 3 In Fig. 2, incompatibility of two resins has been indicated by the extent of overlap of their Hansen solubility parameter spheres in three-dimensional systems. The parameter V can vary from 0 with no overlap to 100 at complete overlap indicating complete incompatible and compatible, respectively. It was observed that at V > 90, almost all films were translucent, indicating compatibility of resins. At V < 90, almost all films were not translucent indicating incompatibility. Moreover, the structure of films appeared to be dependent the value V: at 70 < V < 90, the films have a matt homogeneous structure, while at V < 70, the structure was much coarser, with a trend that particles become coarser at lower values of V. In this case, the structure appeared also to be dependent on the type of the solvent used. The conclusion drawn is that the degree of incompatibility of two resins can be characterized reasonably by the extent to which their solubility spheres overlaps.7 However, the likelihood of predicting the behavior of a pair of resins with accuracy
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only from the data is negligible. Although 67% of stratifying systems are found in the group V < 55%, this group also contains a high number of nonstratifying systems.11
2. 3. 4.
Kinetics aspects (effect of evaporation rate on selfstratification)
Generally, it should be possible to accomplish stratification of two incompatible resins in a mixture of two solvents by preferential evaporation of one of the solvents. To this objective, the solvent with the highest evaporation rate must be a solvent for both resins, while the solvent with the lowest evaporation rate must be a solvent for one of the resins and a nonsolvent for the other resins.7 In Table 1, the compatibility of two resins in different solvents, with various relative solvent evaporation rates, has been depicted. Also, the V factor and the ranking for stratification were obtained.7 In Table 1, we can also see that at V = 55.4%, the highest level of stratification has been reached. Surface tension relations Carr focused on surface free energy and developed a theory to predict the stratification.12 According to his theory, in a system containing two polymers, the following conditions need to be satisfied in order to obtain a stratified coating: 1.
System separation into two phases during the drying process
The interfacial tension between the two liquid phases, large enough to stop the second phase from being dispersed in the first phase. Spontaneously wetting the substrate by first layer (base coat). The lowest possible total surface and interfacial surface free energy of the required layer sequence. Migration of the binder with the lowest surface tension to the air interface during the stratification process while the binder with higher surface tension is forming the primer layer, although the requirements with respect to surface energy and interfacial energy of particular system are somewhat more complicated.13 Based on these conditions, Fig. 3 is evocative of the schematic condition of SSC.
According to the introduced concepts in Fig. 3, the relations 1–3, known as the stratification conditions, were developed: kS1 kS2 k12 0
kS1 k1 kS2 þ k2[0
kS kS2 k12 k1[0
In relations 1–3, S means substrate and 1 and 2 refer to polymer components. The interfacial free energy of each pair is shown by 12, S1, and S2. These are the signs of surface free energy. According to different research results, the pair epoxy/acrylic resin has been chosen for their surface free energy and stratified in suitable conditions.15
λs2 Fig. 3: Schematic of self-stratified coatings14
Research trends in self-stratifying coatings In 1989, through research coordinated by the Paint Research Association (PRA),16 a model for predicting stratification which is based on selected physical parameters of the component polymers was developed. They claimed that an SSC could be formulated from currently available resins and polymers. Based on their findings, medium and high solid solution coatings suitable for commercial exploitation are likely to be heterogeneous liquid in liquid dispersions, where each phase contains the appropriate pigments separately dispersed within it. Their claim was that fundamentals were based on polymer solubility, surface tension relationships and kinetic aspects of film formation. The main aim of the EC-funded BRITE project on self-stratifying coatings was to develop suitable guidelines for the selection of suitable components for the formulation and application of self-stratifying coatings.7,12 This project was funded by the European community and European coatings and resins manufacturers, aimed at formulation of different SSC. As per the results of this research, a theory of stratification was developed for films comprising binary mixtures of polymers in solution and tested for a wide range of polymer mixtures. Regarding environmental protection, a preliminary study was also implemented pivoting around the established principles of formulating waterborne stratifying coating from aqueous polymer dispersions. Their performance was then evaluated under industrial spray application conditions. The mechanism of formation of stratified films in the experimental compositions was described in terms of the solubility relationships of the polymer mixtures, the surface tensions of the separate phases and the kinetic aspects of solvent evaporation from paint films. Principal component analysis of the experimental data generated quantitative equations for calculating the probability of stratification from the physical properties of polymers. Based on this research, for medium to high solids application, the coatings were liquid in liquid emulsions rather than
homogeneous solutions. The problem of reproducibly locating pigments in the two layers was not fully resolved in this research.17 Mezger in 1992 revealed that in mixtures of conventional homogeneous resins, surface and interfacial tensions were considered to be the driving forces for self-stratification,18 leading to films with separation patterns. The main issue in that work was using block copolymer binders and an interpenetrating network of co-continuous phases with homogenous layers as film surfaces. They claimed that the block copolymer binder resins offered quite a number of attractive features for surface coating, ranging from new mechanical properties profiles through possibilities of adhesion tailoring to a diminished sensitivity of the film. In 1995, Zahedi et al.1 studied phase separation of polymers to produce self-stratifying coatings. They worked on an epoxy acrylic system and applied the coatings on glass substrates. Based on theoretical aspects and results obtained from experimental work, phase separation did not occur because surface tension and kinetics of reactions were not strong enough to cause stratification. In 1995, research was performed by Vink and Bots7 with attention to the incompatibility of the resins using solubility spheres in the Hansen solubility parameter (HSP) concept. In 1996, Benjamin et al.11 formulated liquid pigmented coatings containing two different resins which spontaneously stratified after application to a metallic substrate. They found that stratification was dependent upon the solubility of polymer pairs in common solvents, the occurrence of phase separation, and subsequent layer formation. In systems where one of the SSC resins was two component (2 K), the effect of crosslinking agent rate is very important in stratification process.11,19 In Table 2, the effect of hardener on the stratification process can be found. According to this research, it was proven that the resin pairs with polar hardeners are likely to form the best stratifying system.11 In 2014, Mahato et al.19 performed research on the role of curing agents on the self-stratification behavior of a pigmented epoxy resin-chlorinated rubber blend. Chlorinated rubber was pigmented with rutile grade titanium dioxide, and epoxy resin was pigmented with red iron oxide. Different types of curing agents on the stratification behavior of pigmented chlorinated rubber/epoxy resin were studied which show that stratification occurs only with polyamide resin hardener used as epoxy curing agent. There are some points to note when using the two component resins in SSC coatings. For instance, in reactive catalysts for the reaction between hydroxylated compounds and polyisocyanides (in polyurethanes) and also the reaction between polyepoxide and polyamine, the occurrence of a gradient concentration and stratification can be completely hindered. The main reason for this phenomenon is that the full system became gelatinous and crosslinked rapidly, impeding the coalescence of the spherical particles of the separated phase into
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Table 2: Effect of three kinds of hardeners in various pairs of resins on stratification11 Resin 2
Stratification: 0 = no stratification; 4 = full stratification
larger domains and their migration due to the steep increase in viscosity of the medium.20 Research by Jamil Baghdachi et al. in 2015 was carried out regarding the crosslinking of two component systems. They showed that even low surface energy polymers can be immobilized and forced to reside in the bulk of the vehicle. The multilayered coatings require complex application and curing processes. The traditional coating systems not only need more time for application but also consume excessive amounts of energy and manpower for film formation. It would be desirable to decrease the number of layers for better performance.2 In 2004, Abbasian et al.14 investigated the factors affecting stratification phenomenon in epoxy–acrylic coatings. According to the obtained data, they concluded that stratification mechanism was not diffusion-dominated and it might be convectiondominated. Changes of solubility parameters during evaporation of solvents serve as a factor that influences self-stratification in these types of coatings. Ewa Langer et al. in 2009 conducted research on this subject.21 Properties of compositions at different stages of solvent evaporation were characterized by their solubility parameters and surface tension. Both parameters were calculated from the weight fractions of particular components determined by gas chromatography (GC). Changes of solubility parameters and surface tension values observed during the evaporation process were similar, both for the mixture of
solvents and for the solution of epoxy/acrylic resins in the same solvent mixture. In other research, Mezzenga et al.22 examined amine cured epoxy systems blended with various reactive epoxidized dendritic hyper branched polymer modifiers. They developed a procedure for calculating solubility parameters of epoxy resin during polymerization and a model for predicting free energy of mixing being a function of mixture composition, temperature, and a degree of polymerization. This model corresponds well with the phenomenon of the chemically induced phase separation (CIPS).23 Differences in surface tension of coatings containing epoxy and acrylic resins may be utilized to predict the course of solvent evaporation as well as the ability of the system of self-stratification. The more significant differences in surface tension of both resins are the less similar course of evaporation and the higher ability to stratify. Temperature has been introduced as another important factor for stratification effect.20 In any case, the solvent which is appropriate for the thermoplastic resin must not evaporate too quickly to allow for a moderate gradual increase in viscosity of the medium rather than a steep one. Acceleration by heat curing (80°C) gives, in general, and in spite of faster solvent evaporation, films with higher gloss than those obtained by drying at ambient temperature. Nevertheless, if stratification is evident, some spherical inclusions of the upper layer will remain in the lower paint layer.
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agents in preparations of paints very often had a rather deleterious effect on the self-stratification behavior. Similarly, the addition of leveling agent into the coating formulation did not modify the surface aspect but led to a rather detrimental effect on the phase separation.7 In 1995, Vink and Bots7 formulated stratifying paints of two pigmented binders, while the pretreatment of the pigments with particular adhesion promoters could favor stratification considerably. During film formation of good stratifying paints, some processes occur which are comparable to those resulting in the formation of Bernard cells. Thus, factors affecting the pigment floating process also affect stratification. The presence of pigments with different particle size was a necessary condition for migration. On the contrary, additives like surfactants and antifloating agents, which prevent the floating process, have a deleterious effect on the process of stratification. Table 6 shows the effect of two types of surface treatments on pigment migration. In this research, IBTMEO (isobutyl trimethoxysilane) and AMEO (aminopropyltriethoxysilane) were employed as surface treatment for Bayferrox and Kronos LKR pigments, respectively. It must be noted that the resins used were Epikote and Alloprene and perfect migration of pigments in right layers occurred.11 In Table 7, the effect of various surface treatments on stratification for titanium dioxide has been demonstrated. The best result was yield at V = 40.2% and also with surface treatment of titanium dioxide with AMEO. As depicted in Table 8, the effect of different types of epoxy resins and solvents in stratification has been demonstrated. It has been found that when the molecular weights of epoxy resins increase, better stratification is obtained. Also, with a decrease in the overlap factor of two resins, incompatibility and a higher degree of stratification occurred.
Pigmented self-stratifying coatings The presence of the pigments in SSC compositions will affect the stratification behavior. Good stratifying paints of two pigmented binders could be formulated, while the pretreatment of the pigments with particular adhesion promoters could favor stratification considerably.11 It was found that pigmentation had no adverse effects on stratification and, to the contrary, it seems to favor it.20 Benjamin et al.,11 successfully worked on the SSC systems containing a protective pigment in the adjacent layer. The most successful systems were those in which a protective pigment was added to the primer resin located at the substrate interface. Such systems could be usefully employed industrially in pigmented base coat/clear topcoat type application. Addition of a second pigment to improve durability of SSC system has been proven to be far more difficult. In some cases, resins still exhibited stratification, while the pigments spread through both layers. Degrees of success were achieved utilizing a silane-coated titanium dioxide pigment which could be located in the topcoat resin. Table 3 shows the effect of the pigment on selfstratification of resinous phases in an experimental work by Benjamin et al.11 Table 3 indicates that changing the resin type via an increase in molecular weight leads to a higher degree of stratification. Table 4 shows that with the use of proper surface treatment of titanium dioxide, the migration of pigment was improved and pigment was located at the right place.24 TiO2 with AMEO (aminopropyltriethoxysilane) as a surface treatment, at a level of 13 wt% (based on pigment), demonstrated better migration in SSC system. It has been claimed that the presence of additives such as dispersing and wetting agents has deleterious effects on stratification phenomena (Table 5).7 The presence of additives in SSC’s formulations on plastic substrate could affect the stratification.20 Addition of wetting and dispersing
Table 3: Addition of two pigments in self-stratify coating system11 Resin 1 Epikote Epikote Epikote Epikote Epikote Epikote Epikote Epikote Epikote Epikote Epikote Epikote
* Pigment located through whole film, ** some evidence of pigment separation, *** pigment remains totally in one resin
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Table 4: Addition of AMEO-coated R-SM2 to resin11 Resin 2
Lumiflon LF 200 Lumiflon LF 200 Lumiflon LF 200 Neocryl B 700 Neocryl B 700 Neocryl B 700
TiO2 TiO2 TiO2 TiO2 TiO2 TiO2
coated coated coated coated coated coated
with with with with with with
Resin 1 AMEO AMEO AMEO AMEO AMEO AMEO
Epikote Epikote Epikote Epikote Epikote Epikote
828 903 1001 828 903 1001
1 4 4 1 3 3
** *** *** * * **
* Pigment located through whole film, ** some evidence of pigment separation, *** pigment remains totally in one resin
Table 5: Stratification of systems of Epikote/Alloprene R10 as TiO2 in MEK-MIAK as a function of additive7 Ist combination of Epikote/Fe2O3 additive None None None None Disperbyk Disperbyk Byk 300 Byk 300 Lactimon Lactimon Resiflow Byk 307
2nd combination of Alloprene R10/TiO2 additive
Ranking of stratification
None Lactimon or Byk 307 Byk 300 Disperbyk or Resiflow None, Lactimon, or Byk 300 Disperbyk or Resiflow None, Lactimon, or Byk 300 Disperbyk or Resiflow None, Lactimon, Resiflow, or Byk 300 Disperbyk None, Lactimon, Byk 300, Disperbyk, or Resiflow
1 2 3 4 4 6 2/3 4 4 6 6 2
Disperbyk: alkylammonium salt of polycarboxylic (dispersant, BYK) BYK 300/307 polyether modified dimethylpolysiloxane, 52% in xylene/Bu OH and pure, respectively (surface active agents, BYK): Lactimon: alkylammonium salt of copolymer of polycarboxylic acid and polysiloxane (flow control BYK) Resiflow W50: polyacrylate (flow control agent, Worlee)
Table 6: Stratification of systems in MEK containing 10% silane-treated pigments7 1st combination
Models for prediction self-stratification of coatings UNIFAC model The Unique Functional-Group Activity Coefficients (UNIFAC) model can predict phase behavior, vapor pressure, evaporation rates, and surface tensions for systems containing two polymers or resins in a solvent mixture. The prediction of phase separation during solvent evaporation from a thin film is satisfactory in
most cases especially when the differences in molecular weight between two polymers are not too large. Calculation of the properties of mixtures needs knowledge of the activity coefficient of different components. UNIFAC model, which is based on the group contribution principle, uses a table of group interaction parameters estimated from experimental data, to calculate activity coefficients. According to the group contribution principle, properties of a molecule can be calculated by summing the properties of the functional groups constituting the molecule.15
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Table 7: Stratification of combinations in MIBK of Epikote 1001-/IBTMEO-treated Bayferrox 140M with physically drying binders containing TiO2(Kronos LKR 10%) treated with different adhesion promoters7 2nd combination Resin
A 189 A174, A186, A187 or AMEO A189 or AMEO A174, A186, A187 A174, A186, A187, A189 or AMEO AMEO A174, A186, A187 or A189 AMEO, A186, A189 A174 or A187 AMEO A 189 A 187 A 186 A 174 A174, A186, A187, A189 or AMEO A 174 or A189 A186, A187 or AMEO
Table 8: Stratification of pigmented systems of Epikote/Alloprene R10 as a function of the type of epoxy resin and solvent7 1st combination of Epikote/Fe2O3 Epikote 828 1001 1004 1007 1009 1002 1004 1009 1009 1009 1009
* 1 = the highest degree of stratification 6 = the lowest degree of stratification
Surface energy variation with concentration of two polymers This model can be used to predict which resin combinations will give rise to self-stratifying systems, due to surface energy/concentration relationship of the pure resins in solution and assuming that systems have phases that are separated.15
Computer model for self-stratification During the work done on self-stratification by the participant’s laboratories, in the frame of BRITEEURUM contract, enormous amounts of data were gathered. The volume of information was so vast that right at the onset it was decided to have statistical processing of these data with the aim of building
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empirical predictive models. In that project, the formulation of coatings for steel was undertaken by PRA (UK) and TNO (Netherland). In addition, coatings for plastic and wood substrates were the responsibility of CoRI (Belgium) and EOLAS (Ireland), respectively. FPL (Germany) studied the principles governing the formulation of aqueous stratifying coatings. DTI (Denmark) evaluated the performance of the coatings when applied by industrial spraying procedure. NIF (Denmark), the originators of Hansen solubility parameters, initially provided a theoretical basis for polymer solubility and phase behavior of binary mixtures of polymer in solution. Unfortunately, NIF went into liquidation, but the key personnel formed a new private company EnPro, which was subcontracted to complete the theoretical studies.17 Finally, CERIPEC Institute in France analyzed the data generated by the other partners to produce this computer model for predicting stratification behavior.25 A decision was made to build this model by taking into consideration only thermodynamic properties. Kinetic properties were left out due to difficulty in assessing them for each data set. This limitation was important since if thermodynamic conditions were favorable, the kinetics of the process could disrupt correct stratification. Thus, the empirical modeling approach presented takes into account results for formulations, having vividly shown various degrees of stratification or no stratification at all. With the assumption that kinetic parameters play only a minor role, data used for modeling were those obtained on solvent-based formulations from PRA, EOLAS, TNO, and CoRI together with stratification degree of the obtained films. TNO presented a large number of results using a wide range of additives and pigments. Due to the fact that physical and chemical information on additives and surface treatments of pigments was not available, it was decided to leave those components out of the scope of modeling. Statistical analyses were performed, using various methods of MODULAD and SPAD (systeme portable d Analyzes de Don-nees). Construction of data tables was done with Quattro Pro (Borland) allowing the creation of ASCII files which can be used by MODULAD methods.25
Industrial Application of SSC It seems that industrial application of self-stratify coatings was introduced by Verkholantsev in 2003. He offered a tool to formulate industrial and special SSC with outstanding performance. Self-stratifying compositions capable of producing multicoat film structures by one-coat application can be formulated as liquid or powder coatings, suitable for many industrial applications particularly in anticorrosion and antifouling coating systems.26
Corrosion Resistance of SSC Westcott et al. demonstrated a hybrid coating consisting of both a stratified layer that incorporates corrosion inhibiting ions and a silicate barrier layer.27 The stratified layer was applied to the aluminum alloy using the layer-by-layer (LBL) assembly (or electrostatic self-assembly) method and comprised alternating layers of charged polyelectrolyte and nanometer-thickness clay platelets. These nanocomposite coatings could be doped with corrosion inhibiting ions and allow lateral diffusion of those ions through the coating to have high throwing power. The LBL coating was applied under organic–inorganic hybrid coating of organically modified silicates (ORMOSILs) to provide additional barrier protection and bind to primers and paints. These types of hybrid coatings demonstrated superior corrosion protection compared to hexavalent chromium coatings. Schematic of these coating is shown in Fig. 4. In 2011, Langer et al.28 studied barrier and active anticorrosive pigments in SSC. The results of their investigations indicated that, in salt fog test, SSC demonstrated comparable or even better corrosion protection properties in comparison with classical twolayered systems containing the same resins and pigments. The best adhesion to substrate, interlayer cohesion, and pigment distribution were obtained in SSC systems containing chrome oxide green and micaceous iron oxide (MIO). The FTIR–ATR spectrum of pigmented system has been obtained and is shown in Fig. 5. The cross section of stratified pigmented resinous phases is shown in Figs. 6 and 7. In 2013, Chen29 developed self-stratified two phase coating systems based on difference in surface tension mechanism. Epoxide and acrylic were chosen as the two phases given that epoxide could provide excellent adhesion to the metal substrate and excellent corrosion resistance due to the secondary hydroxyl group, while acrylic copolymers could provide better flexibility, dura-
Clay layer Active corrosion inhibitors Aluminum substrate Fig. 4: Layers of clay platelets, polyelectrolytes, and active corrosion inhibitors are assembled on the aluminum substrate and coated with an ORMOSIL gel27
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Fig. 5: ATR–FTIR spectrum of stratified coatings in pigmented systems28
Fig. 6: Cross section of coating containing chrome oxide green and lamellar iron oxide (MIO)28
Fig. 7: Cross section of coating containing titanium dioxide and lamellar iron oxide (MIO)28
bility, and resistance to ultraviolet degradation. In these experiments, a series of low, medium, and high molecular weight acrylic copolymers with different 2,2,2 trifluoroethylmethacrylate concentrations were successfully synthesized and confirmed by FTIR, H, C, F NMR, GPC, and mass spectrometry. Bisphenol-based epoxide was successfully modified with tetraethyl orthosilicate oligomer, which was characterized by FTIR and mass spectrometry. The results revealed that acrylic copolymer with low molecular weight and low surface tension would improve the stratification. Also, tetraethyl orthosilicate oligomer-modified epoxy improved the stratification. Self-stratification in antimicrobial and antifouling coatings In 2011, Rajan et al30 studied the effect of formulation variables on fouling-release performance of stratified
siloxane–polyurethane coatings. The effects of formulation variables, such as type of polyol, solvent type and solvent content, and coating application method, on the surface properties of siloxane–polyurethane fouling-release coating were explored. Fouling-release coating allowed the easy removal of marine organisms from a ship’s hull via the application of a shear force to the surface. Self-stratified siloxane–polyurethane coating was a new approach to a tough fouling-release coatings system. Different siloxane–polyurethane coatings were formulated and applied using drawdown and drop-casting methods. The resultant coatings were tested for surface energy using contact angle measurements. The performance of the majority of the formulated coatings was found to be better than those of standard commercial antifouling coatings. The fouling-release performance was still maintained when the siloxane molecular weight and content were changed. The barnacle adhesion for 20% siloxane by weight was as low as 0.07 MPa, compared to the commercial silicone fouling-release coatings. Nevertheless, the tough and durable siloxane–polyurethane SSCs are promising candidates for marine foulingrelease coatings. M.B. Yagci et al. worked on self-stratifying antimicrobial polyurethane coatings in 2011.31 In this work, antimicrobial polyurethane coatings were prepared via a self-stratification approach. Self-stratification was confirmed by dynamic contact angle analyses and Xray photo electron spectroscopy. The final films showed strong antimicrobial activity against both Gram-positive staphylococcus aureus and Gram-negative Escherichia coli type bacteria. In 2015, Zhao et al. studied a self-stratified antimicrobial acrylic coating via one-step UV curing system. They designed and synthesized a novel quaternary ammonium methacrylate compound (QAC-2) bearing a perfluoroalkyl tail on one end and an acrylic moiety on the other. Through one-step UV curing of QAC-2 and methyl methacrylate (MMA) with ethylene glycol dimethacrylate (EGDMA) as the crosslinker, they obtained crosslinked coatings with excellent antimicrobial properties, as shown by the total kill against both Gram-negative E.coli and Gram-positive Staphylococcus epidermidis at a QAC-2 concentration as low as 0.06 mol% (around 0.4 wt.%) relative to MMA, which was substantially lower than the QAC amount required in the coatings containing QACs with a hydrocarbon tail.32
Evaluation methods for stratification Usually, FTIR–ATR, SEM and FTIR, H, C, F NMR, GPC, and mass spectrometry have been used to obtain information on the polymer composition of the upper and lower surfaces of the stratified films.14,29 Measurement of surface tension has been used as a prediction of self-stratification of layers.33,34
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Validation of stratification was accomplished using drop shape analysis (DSA) for surface energy measurements. Moreover, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) have been employed for determining differences in composition and identifying chemical composition on the surfaces, respectively. In SSC, two phases have also been characterized with field emission scanning electron microscopy (FESEM) with an attached X-ray energy dispersive system (EDS).29 In pigmented SSC, the pigment distribution coefficient can be calculated based on the color difference between the upper and bottom layer as well as by means of the XRF method.28 In addition, the appearance of both film surfaces was characterized by color measurements applying a Minolta tristimulus meter.7
New approaches in SSC Recently, SSCs containing three or more layers have been noticed. Three layers of SSC have been studied by Liu et al.35 In 2014, they prepared self-stratifying epoxy–polyacrylate emulsions with three-layer core shell structures using a two-stage emulsion polymerization procedure. The films prepared from the emulsions showed signs of self-stratification. The core was prepared as epoxy emulsion by employing phase inversion; the inner shell was formed by the aggregation of the epoxy resin and the polyacrylate at the seeded polymerization stage, and the outer shell was formed at the growth stage of the polymerization of acrylate monomers. The film prepared from the emulsions exhibited self-stratification. This strategy might be applicable to the preparation of multilayers of selfstratifying coatings in future works (Fig. 8).
EP SEEDED POLYMERIZATION
Fig. 8: Schematic illustration of the procedure for preparing three-layer core shells35
One of the goals in this field to transfer selfstratification technology to the other areas. Multilayered films from a single application have benefits in many surface coatings systems as well as other related areas. For instance, adhesives, where two dissimilar materials are to be bonded together, could greatly benefit from the advantages of self-stratification. Similarly, printing inks, where different interfaces are required to have different characteristics, could benefit from this approach. Self-stratifying intumescent coating is the current new project started in France in 2014.21 Moreover, there was another project named “self-stratifying in epoxy/silicone coating” by Beaugendre et al. at Lille University in France.23 In this research, innovative selfstratifying coatings based on epoxy/silicone blend have been developed and applied on plastic substrate (polycarbonate). The perfect self-stratification of this system was evidenced by microscopic analysis coupled with X-ray mappings. The influence of solvents and curing agents of the stratification process has been investigated. Most of the new patents in this field are assigned by automobile companies. It seems that the use of pigmented SSC will be a priority in future studies.36–39
Conclusion The feasibility of self-stratification which is of great use in the coating industries for theoretical and experimental point of view was reviewed. Practical and industrial aspects of stratification in coating systems were investigated. It seems that there was no major obstacle to applying self-stratifying coatings in actual applications. Self-stratifying coatings can be developed from aqueous polymer dispersions to powder coatings and high solids. Nevertheless, tailoring application to different substrates offers chances to achieve new interesting properties. Further study of the relationship between surface treatments of pigments and their affinity to different polymers is required. The migration behavior of pigments in these coatings is of interest and needs to be studied further. Also, using nanopigments and their effect on SSC is another interesting subject that requires more studies.
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