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Heterophase and self-stratifying polymer coatings
PROGRESS INORGANIC COATINGS ELSEVIER
Progressin Organic Coatings 26 ( 1995 ) 3 1-52
Heterophase and self-stratifying polymer coatings V.V. Verkholantsev Tambour Ltd., POB 2238, A!&o 24121, Israel Received 5
February 1995;revised 16 February 1995;accepted2 1February I995
Abstract
Film-forming compositions based on incompatible polymer blends, including curable epoxy resins, produce coatings with a heterophase polymer matrix structure. Composed as homophase solutions with use of common solvents, they become heterophase films due to evaporation of solvents during application or during the film-forming process. When phase separation takes place during curing reactions, it results in a fine heterophase polymer structure. Under certain conditions, heterophase liquid polymer blend compositions form non-homogeneous-in-layer (self-stratified) coatings. Phase separation in binders, which were initially homophase due to the presence of common solvents, can also terminate with self-stratification. A few driving forces for stratifying processes have been suggested, particularly of surface tension gradients resulting from non-uniform solvent evaporation, and a difference in contraction of separated phases. Possible mechanisms and conditions for their applicability are discussed in conjunction with resultant heterophase polymer structure. A number of application areas are suggested, where polymer-polymer heterophase or self-stratifying polymer structures could give advantages in terms of service properties in comparison to homophase-matrix coatings. Keywords: Polymer blends; Heterophase
films; Self-stratified
coatings; Surface tension gradients
1. Introduction
Polymer-polymer incompatibility and heterophases in materials are not considered adequately in many polymer technologies. Technologies of plastics, films, membranes and other polymer materials exploit heterophase structures successfully to aid their properties [ 1,2]. Instead, with the exception of a few examples, coatings technology avoids any incompatibility between binders. About two decades since W. Funke and collaborators published their works on selfstratifying coatings [ 3-6 1, this approach has attracted attention of several reviewers and researchers [ 7-121 as an opportunity to produce coatings having multilayer structures by one-coat application of a single paint composition. 0300-9440/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .SSDIO300-9440(95)00552-P
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It was suggested that a properly selected incompatible polymer blend, being melted on a substrate (in the case of powder paints) or dissolved in solvents (solvent-based technology) to produce two-phase liquid composition in the course of film forming processes, should have the capacity for self-stratification and formation of a double or multilayer film. Each layer has to be formed with a polymer component of this incompatible polymer blend, driven by difference in the free surface energy. So, a heterophase polymer blend, being converted into a liquid on a substrate, has been suggested to form a self-stratifying coating. Unfortunately, the difference in free energy between two states is a potential which can provide stability or instability to the system but which cannot serve as a driving force for material flux or flow (until some gradients of surface or other forces are present). Coagulation mechanisms of the film formation process [ 131 can be applied to waterbased compositions containing a mixture of two polymer emulsions (latexes). The mixture can provide a non-homogeneous-in-layer coating as a result of the difference in the colloid stability and, correspondingly, in the coagulation rates on drying of the two emulsions. Electrodeposition process with use of mixed dispersions [ 141 may also yield non-homogeneous-in-layer coatings. Produced in this way coatings are hardly identified as ‘self stratified (since electriadagulation cannot be referred to as ‘internal forces’). Misev [9] has suggested a different mechanism of self-stratification, perhaps more convenient for the traditional coatings technology, where a homophase of binders is considered preferable. His idea was to convert an initially homophase binder, based on a blend of partially compatible resins having one thermoset polymer partner into a two-phase system due to the development of crosslinking reactions. This neglects miscibility limits for polymers and provides contraction and self-stratification. This author has also suggested a handsome terminology, identifying the separating and stratifying polymer phase as a ‘stratifier’ and the matrix phase as a ‘carrier’. From a physicochemical standpoint, in order to provide stratification, any contraction force needs a gradient through the film thickness. Otherwise, phase separation process in a viscous liquid has to terminate with formation of an emulsion, which needs another driving force for formation of layers and their displacement in the desirable order. Compositions based on epoxy/thermoplastic incompatible resin blends were studied in a series of works [ 15-171 as binders capable of producing either polymer-polymer heterophases or simultaneously heterophase and self-stratifying polymer structures. It was also reported [ 18-191 for epoxy-based paints that heterophase and self-stratifying approaches were utilized to improve coating properties such as weather resistance, and mechanical and adhesion characteristics. Although this has been introduced in a few paints (as powder [6], solvent-based [ 10,111 and water-based coatings [ 20,211) , the self-stratification concept is still consistent with a number of contradictions and unclear aspects. (i) The self-stratification concept considers that a desirable coating structure is the primary goal whereas the traditional approach is based on the coating properties. Particularly, when the self-stratification concept was applied to solvent-based epoxy coatings [ 10,11,22], it was found that not all incompatible polymer blends capable of producing heterophase or self-stratifying coatings are able to satisfy the technical demands. (ii) From the standpoint of polymer theory, the interface between incompatible polymers should exhibit a lack of adhesion durability, at least, for two reasons.
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First, considering an equilibrium interface between two polymers, the adhesion durability of the interfacial contact should drop with the increase in the difference in polarity between contacting polymers, i.e. adhesion durability should be higher between compatible and lower between incompatible polymers. Secondly, this situation becomes even worse in the case of film-forming processes in multilayer drying films (non-equilibrium interface). Since normally the shrinkage due to the release of solvents (or formation of crosslinked networks) is different in each polymer layer, the development of residual stresses at interfaces may be significant. A sharp selfstratifying process may result in an easily splittable film [ 221. (iii) Two main types of self-stratifiedcoating structures are obtained in practice, either sharply separated into two homophase layers films, or partially separated with the matrix polymer layer remaining as the heterophase. (iv) It remains unclear which type of polymer structure, non-homogeneous-in-layer or microheterophase, contributes more substantially to the advanced properties of the selfstratifying coatings. Extended service life, which is reportedly inherent to the epoxy-based self-stratified coatings [ lo,11 1, may be sometimes associated with their heterophase polymer structure, gained thanks to incompatibility of the thermoplastic modifier with the matrix-forming epoxy resin, rather than with their multilayer structures. Incomplete phase separation during film forming processes enables heterophase structures to seal micro cracks formed in coatings on long-term exposition to the light, alternating temperatures, vibrations etc., and to maintain compact coherence of the polymer matrix. The most favorable composite coating structure was found to be one formed in the course of incomplete phase separation process and moderately developed self-stratification [ 10, I I]. The latter allows the separated polymer layer to keep its ‘roots’ in the matrix phase, to prevent disbonding. Particularly, for lo@-120 pm DFT coatings, it means formation of roughly 5-10 pm of a separated top layer. Being added to a well-controlled heterogeneity of the matrix phase, and provided with crosslinkage in both layers, this coating structure can give a substantial improvement in the coating characteristics. This paper is aimed to assemble some characteristic data and observations, obtained in our other work on heterophase and self-stratifying epoxy ambient cured paints. An attempt is made to give a better understanding of the self-stratification process in liquid paints, focusing on a phase separation of polymer solutions, driving forces and mechanisms of selfstratification.
2. Solutions of incompatible
polymer blends and their phase behavior
Any development in self-stratifying coatings should start with the selection of suitable polymer partners, taking into consideration their technical properties in the coating, their compatibility and other characteristics. When adapting to conventional solvent-based technology, we should first evaluate the behavior of the selected incompatible polymers in the presence of paint solvents. In order to be able to control the amount and properties of separated phases, one needs, as the
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simplest system, a composition of two incompatible polymers and two solvents, having different volatility and affinity for the selected polymers [ 10,111. Phase separation is a process driven by intermolecular forces, and the formation of binodal phases occurs due to the minimization of the free energy of mixing. Thermodynamics of polymer solutions and blends [ 23,241 allows predictable calculations of phase behavior (separation condition, composition for binodal phases etc.) for relatively simple compositions such as two- or three-component polymer solutions. For practical needs, especially in the case of technical polymers and multicomponent systems, it is advisable to use experimental data regarding phase behavior of polymer blends and solutions. The term ‘experimental diagram’ means that a binodal curve (or surface) is drawn through the points obtained by analyses of equilibrium compositions of the separated liquid layers. 2.1. Phase diagrams Typical quatemary (2 polymers + 2 solvents) experimental phase diagrams are presented in Fig. 1 for a few incompatible polymer blends and selected solvents. Gel permeation chromatography (GPC) and chemical analyses were used to characterize the composition of binodal solutions in three- and four-component systems [ lo]. Any point of a phase diagram expresses an equilibrium composition, and its position with regard to a binodal surface reflects its state as either a homophase solution, a metastable solution (ready to separate), or a two-phase system. The position of a spinodal surface, which cannot be obtained from this experiment, is shown marginally. All compositions inside the binodal surface correspond to non-stable polymer-polymer emulsions, which are inclined to separate into layers until they remain liquid. For example, point B represents a liquid heterophase (two-phase) multicomponent polymer binder, i.e. polymer-polymer emulsion. Such an emulsion can be obtained either by immediate mixing of rather concentrated polymer solutions (for example, by blending of three-component solutions P and Q) , or by evaporation of solvents from initially homophase solution A. The second case is associated with the process of phase separation, when the point of composition crosses the binodal and then the spinodal borderline and with the production of a polymerpolymer emulsion. Both phases of this emulsion have a binodal composition (points X and Z are connected with a tie-line and belong to two different branches of the binodal surfaces). They contain all four components, but in different ratios, Amount of each phase can be calculated through XB/BZ ratio. The opposite direction of the phase transformation (B -+ A) is also possible. A polymer-polymer emulsion can be transformed back into a homophase solution with the addition of a true solvent. In order to produce coatings from an incompatible polymer blend, we may select a binder as an initially homophase system (more or less a diluted polymer solution, point A) or as a two-phase liquid, a polymer-polymer emulsion (point B) . 2.2. Phase separation Phase separation in a polymer solution, caused by the change in its composition or temperature, which corresponds to a shift into a heterophase region on its phase diagram,
fig.
1. phase diagrams for experimental quaternary systems: (a) solid epoxy resin-chlonnated poly(ethylene)-propylene glycol methyl ether (PGME)-xylene. 1. emulsions or gels): !b) solid epoxy resinconcentration region of homophase solutions; 2. metastable solutions; 3, labial state (unstable solutions, polymer-polymer acrylic resin_xylene-PGhE; (c) solid epoxy resin-chlorinated poly( ethylene)-xylene-butyl alcohol; (d) solid epoxy resin-perchlorovinyl resin-acetone-xylene.
Propylene GlYCol, Methyl Ether
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may proceed according to either a neucleogenic (nucleation and particle growth) or a spinodal mechanism. When phase separation occurs as a result of a relatively fast change in the parameters of state (for instance, due to evaporation of highly volatile solvents from a developed solution/air interface), it proceeds via a spinodal mechanism [ 23,241, producing a jelly system, comprised of a network of two interpenetrating phases. The time scale for the phase separation process during the film-forming process can be done in terms of the Lifshitz-Slesov concept [ 251. The time, r, needed for the phase inhomogeneities of radius r to appear under the action of over saturation ( p), can be evaluated according to the equation: 1 0-P -=r2 7 where D is the diffusion coefficient. Assuming for a polymer solution D= lo-” cm2/s [ 261 and /3= 0.2, the production of the non-homogeneities of r = 0.1 pm takes 50 s (and = 80 min when the solution becomes more concentrated and D = lo-l3 cm2/s). This means that the phase separation process proceeds fast enough in non-viscous dilute polymer solutions. If the opposite situation occurs, the formation of heterophase coating structures will last for a longer period and can be interrupted by a solidification or curing reaction. If a phase diagram for the given polymer blend composition is unknown, one can operate with practical tests, i.e. observing the results of blending of polymer solutions. 2.3. Experimental observations of phase behavior As an example, Table 1 presents some results obtained by blending a commercial solid epoxy resin with a few thermoplastic solutions. All selected parts, being mixed, produce Table 1 Phase behavior of selected commercial polymers in blended solutions Thermoplastic resins (TR)
TR/SER’ ratio (wt.%)
Total solids content in heterophase mixtures (%)
True solvents
Solids content in solutions, achieved homogeneity
Chlorinated rubber (50% solution)
36164
63
MBK
48
Chlorinated PE (40% solution)
2lll3
59
MEK
35
Vinyl resin (30% solution)
22118
56
MEK
42
Acrylic resin (30% solution)
22118
56
MIBK
42
’ Solid epoxy resin (SER), 10% sol. + solutions of thermoplastic resins.
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heterophase, milky and sometimes pearlescent, viscous or jelly-type masses, which should be identified as non-stable polymer-polymer emulsions. They can be converted back into clear homophase solutions by addition of some solvents [methyl ethyl ketone (MEK), emulmethyl-iso-butyl kethone (MIBK) etc. J Finally 5647% solids polymer-polymer sions become homophase solutions, when diluted to 3548% solids. In practice, epoxy resins are cured with hardeners. Addition of 20% (on epoxy resin ) of a poly( amino)-amide hardener converts all homophase solutions into two-phase polymer-polymer emulsions. They demand for more solvents to be restored as homophase solutions. Summing up, we may say that the incompatible polymer blend technology offers a few different types of film-forming compositions. First, we may use heterophase polymer-polymer emulsions, obtained by immediate mixing of individual homophase solutions of incompatible polymers. These emulsions can be involved into film forming processes either per se (producing heterophase polymer films), or being first transformed into a homophase solution with a solvent, which is common for both polymers, and then producing more or less controlled heterophase structures (heterogeneity and phase composition of the separated phases may be controlled by changing the amount or the nature of solvents, or by use of additives). We may also separate these polymer-polymer emulsions into binodal solutions and use these compositions to produce heterophase polymer structures [ 171. A binodal solution can be subjected to dilution with either a true solvent (producing more stable polymer solution), or with a non-solvent, which converts it into a polymer-polymer emulsion. Finally, we may combine binodal solutions in any ratio, forming polymer-polymer emulsions of the desirable compositions. Some of these compositions can be subjected to self-stratification during film-forming processes. Thus, the solutions of incompatible polymer blends allow the formation of homophase, heterophase and separated-in-layers coating structures.
3. Molecular weight distribution of incompatible polymer blends in separated phases From GPC analysis, the molecular weight distribution appears different in original and binodal solutions. The data, presented in Table 2, reflects this difference for solutions of a blend of a solid epoxy resin with an acrylic resin. In this binodal phase, enriched with epoxy resin (lower layer), this polymer is represented mainly by the higher molecular weight fractions and the lower molecular weight fractions are relegated to the acrylic-enriched binodal phase (upper layer). Similar results were obtained with binodal solutions of epoxy/chlorinated PE (Table 3) and epoxy/vinyl resin blends. The difference becomes negligible (below the limit of GPC evaluation), when the epoxy/thermoplastic ratio is decreased to lo/90 wt.%. Non-adequate distribution between separated solutions of incompatible polymers, having a wide molecular weight distribution, is reported in the literature [ 271. This can be explained by the fact that due to a kinetic effect, the thermodynamic equilibrium is impeded. In heterophase and self-stratifying coatings technology this phenomena should be taken into consideration. Particularly, in an epoxy/thermoplastic coating, formed through the
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Table 2 Phase separation and molecular Layer
Enriching resin
weight distribution Total non-volatiles (wt.%)
of a solid epoxy/acrylic Epoxy/Acrylic ratio (wt.%)
resin blend in binodal solutions
Molecular weight distribution (%) 970
1400
2500
3500
Upper (84%)
acrylic
38.2
51/49
16
29
29
26
Lower (16%)
epoxy
49.5
9317
9
24
33
34
15
29
29
27
original epoxy resin
Solid epoxy resin (CIBA Geigy), epoxy equivalent = 500; acrylic resin (Paraloid B66, Rohm and Haas); main solvent, xylene; epoxy/acrylic ratio 57:43 wt.% and 66:45 ~01%.
Table 3 Content of epoxy species (wt.%) Epoxy species (MW
In original resin (%)
in separated binodal solutions
Epoxy resin/chlorinated PE weight ratio 30170
70/30
90/10
1’
2b
1
2
1
2
17 30 28 25
8 24 33 35
22 31 25 22
13 28 30 29
18 29 28 26
15 29 29 27
88
12
83
17
67
33
Resin content in the layer (wt.%) Epoxy resin Chlorinated PE
27 73
90 10
61 39
95 5
86 14
96 4
Solids (wt.%)
28
40
28
33
30
31
870 1400 2500 3500
15 29 29 27
Volume fraction of the layer (%)
Original composition: solid epoxy resin/chlorinated wt. a 1, Top layers (all chlorinated PE-enriched) b2, Lower layers (all epoxy-enriched).
PE, blended as 30% solutions in xylene/MPA=50/50
phase separation process, the thermoplastic-enriched molecular weight epoxy species (thereby promoting structure with a minimal content of epoxy resin in phase will act as a scavenger for the lower molecular
by
phase has to be a collector of the lower the formation of a crosslinked polymer the total blend). The epoxy-enriched weight vinyl species.
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4. Heterophase coating structures
The filling of organic coatings with inorganic pigments and extenders is a traditional approach to their structural modification. Assuming an average particle size for pigments and extenders in the range of OS-3 pm for PVC 30% this corresponds to the magnitude of specific interface (S,,) = l-6 M2/cm3. These figures may be compared to the specific surface of a polymer-polymer interface, which could be achieved in the course of the phase decomposition process. An estimation of a minimal size of heterophasity in polymers is presumed as 300 A [ 241, but even taking 1000 A (0.1 pm) and assuming the volume fraction of an incoherent phase as 50%, S,, will come to 30 M*/cm”, i.e. one order of magnitude higher. The phase decomposition process, when overlapped with evaporation of solvents, creates a severe difficulty for the quantitative prediction of the resultant polymer structure. In the course of application and film formation, an initial polymer solution simultaneously undergoes mass and heat transfer, surface phenomena (interference with a substrate and air, adsorption on internal interface with subsequent change in adsorption equilibrium) and sometimes also chemical reactions. When a homophase solution of an incompatible polymer blend is used as a binder, the type of phase separation process during the application and film formation is important for the consequent heterophase polymer structure [ 23,241. Nucleogenic mechanism used to be terminated with the formation of a polymer-polymer emulsion, whereas a spinodal type phase decomposition process creates a special capillary or interpenetrated phase structure. Both of these types of two-phase systems are unstable until the remaining liquid (or pseudoplastic, gel phases) manifesting a trend to coalescence, contraction, phase segregation, etc.
Fig. 2. SEM micrographs of film cross sections (solid epoxy/perchlorovinyl resin blends) : (a) formed from a solution 30% solids, epoxy resin/perchlorovinyl resin weight ratio 50/50; (b) from a binodal solution, polymer/ polymer weight ratio 87/ 13.
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Fig. 2 illustrates the results of two different mechanisms used to form heterophase structures from solutions of an incompatible polymer blend: first, due to a phase decomposition process in a liquid polymer solution, which gives a rather rough heterophase structure (in Fig. 2(a) the incoherent phase is enriched with epoxy resin, and the coherent phase with perchlorovinyl resin) ; and secondly, as a result of phase separation in a solid film, which was formed first as a homophase film from a binodal solution and then converted into a heterophase state in the course of the curing process (Fig. 2(b) ) .
5. Self-stratifying coatings 5.1. Driving forces for stratification Self-stratification can be described as a phase separation, directed in the coating axes, particularly as a normal to the coating interfaces. ‘Ibis process needs some driving force applied during the film-forming process to a still liquid heterophase binder in order to separate phases between the top and the lower layers of the film. Intermolecular forces provide phase separation and formation of heterophase polymer structure through a diffusion process. However, they are unable to place the separated phases in an assigned direction, which is necessary for self-stratification. For this the binder needs a separate driving force. This function can be performed by an interfacial tension gradient, by the film contraction force and possibly by some other gradients. We may define the forces, oriented in the coating coordinates, as the forces of destination. A comprehensive review of internal forces in the drying films has been presented in the paper [ 281. 5.2. Self-stratification mechanisms In previous publications a dozen driving forces and mechanisms of self-stratification have been discussed as applicable to various binders and types of paints, based on the incompatible polymer blends: (a) mixed polymer ‘melts’ (powder and solvent free paints, low VOC stoving compositions), (b) binders, based on mixtures of polymer dispersions (solvent or waterborne paints on mixed emulsions), (c) solventborne paint compositions, based on polymer-polymer emulsions (composed as 2-pack paints), (d) solventborne binders, based on initially homophase solutions of incompatible polymer blends in volatile solvents, convertible into a heterophase state and capable of selfstratification during film forming processes. 5.2.1, Mechanisms driven by gravitation (systems (a, b, c) 16,lo] It features a temperature above Tr of both polymers. Complete and sharp stratification can be performed with a hardener-free composition, or in the case, where the curing reactions proceed well above Tp
V. V. Verkholantsev / Progress in Organic Coatings 26 (I 995) 31-52
Fig. 3. SEM micrographs blend, 70/30 wt.%). (a)
of cross sections of completely
stratified polymer films (epoxy/poly(siloxane)
41
resin
double layer structure; (b) interfacialborder; (c) a ‘large’particle of siloxane-enriched phase,retained in epoxy-enrichedlayer.
Driven by gravitation, self-stratification in melted incompatible polymer blends can proceed until the complete phase separation and formation of a double layer polymer structure (Fig. 3 (a) ) with sharp interface (Fig. 3 (b) ) Kinetic restrictions prevent stratification of even the larger particles (Fig. 3(c) ). Complete and sharp stratification can be performed only with hardener-free composition. Rough time scale estimation of the process can be done in terms of the Stokes sedimentation relation, where an expression for the velocity of sedimentation (V,) of the dispersed phase particles scales as:
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Fig. 4. SEM micrographs of adhesion surface of a film (epoxy/vinyl resin blend, 50/50 wt.%) and delaminated
from a glass substrate under cooling (liquid nitrogen).
where A p is the relative density of liquid polymers or phases of polymer-polymer emulsions, 17is the viscosity of the coherent phase, and r is the particle radius. Even when the A p value is sufficiently high, the temperature must be well above the Tf of both polymers in order to obtain a low viscosity. In practice, a film of HI-100 pm formed from an incompatible solid epoxy/vinyl resin blend, added with a plasticizer, demands l-3 h heating at 120 “C for a complete stratification.
5.2.2. Selective wetting mechanisms {systems a, c, d) [8,12,29] This type of self-stratification process is due to the selective wetting of a substrate by one phase of a two-phase liquid. Its features are sensitivity to the nature of substrate, demands low volatile solvents, non-stable polymer-polymer emulsion and lower viscosity of the stratifying phase. This mechanism can produce local stratification. This mechanism is suitable for controlling adhesion durability and surface properties in relatively thin stoving coatings. As shown in Fig. 4 which represents a SEM micrograph of the surface of a film that has been formed on and then delaminated from a glass substrate under the action of thermal stresses, created by cooling under liquid nitrogen. This interface exhibits heterophase polymer structure. The darker epoxy-enriched ‘stains’ are formed from the particles of the separated phase which have spread over the substrate surface thanks to selective wetting.
5.2.3. Different coagulation These mechanisms are substrate during electro- or the penetration into a porous
or penetration rates mechanisms (systems b, c) [21,30,31] associated with the difference in the rates of adagulation on a chemideposition between the two separated phases or during substrate.
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5.2.4. Pigment wetting mechanism (systems a, c, d) [32] This follows the following steps: (a) phase separation if system C is used; (b) selective wetting of pigment particles with one polymer phase; (c) particle coalescence to form a gel structure; (d) contraction of the latter accompanied by separation to form a sandwich structure. 5.2.5. Mechanisms driven by suface tension gradients 5.2.5.1. Capillaryjow (systems c, d) [IO]. This mechanism may occur in incompatible polymer blend solutions which are selected near to the two-phase concentration region and diluted with the solvent blend with each component chosen according to its volatility and affinity to both polymers. Being applied to a substrate in conditions of intensive evaporation of solvents (for instance, by spray), the binder will be shifted into the labial region of the phase diagram (see Fig. 1). Undergoing phase separation according to the spinodal mechanism, this binder will form an interpenetrating network of separated liquid phases [ 33,341. The driving forces for the capillary flow will appear as a result of the non-uniform evaporation of solvents (the Marangony Effect). Interfacial tension between binodal polymer solutions is quite low [ 161. But the nonequilibrium condition for phase separation allows only approximate identification of these phases with binodal solutions. The non-equilibrium conditions of phase separation may create a substantially higher interface tension for each phase providing a distinct potential for the capillary flow. Due to the highly-developed interface, this structure possesses substantial interfacial free energy. An expression for a net surface force (F) gained due to the surface tension gradient V y (local cooling or local concentration variation), can be written in the form [ 33-371: F= -41rr=Vy
(3)
From this equation,
the velocity of a liquid flow (VJ can be expressed as:
r.Vy
v,a -
(4) 77
where r is the average radius of capillaries. The stratification rate, therefore, depends on the interfacial surface tension gradient, on parameters of a capillary structure and on flow characteristics. From the colloid standpoint, any liquid heterophase three-dimensional network has to experience a trend to be spontaneously transformed into emulsion (as to the structure with less interfacial surface). Such ‘selfemulsification’ suspends the self-stratification, making the drying film structure no longer ‘transferable’ for a flux of liquid. A criterion (0) was offered to predict the direction of emulsification (i.e. as to which phase tends to be the incoherent phase). This criterion contains a few characteristic parameters of both phases: the viscosity ( q), the surface activity A ) and the concentration (C) of surface-active species (the phase volumes are assumed to be equal) [ 381: D w/o=
77”
-
A,
.-
%v A,
cw
-
Co
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(b)
(a) Top
Interphase
Fig. 5. Capillary stratification mechanism (schematic presentation): (a) phase decomposed structure (interpenetrating network); (b) capillary system, capillary flow and self-emulsification process; (c) top surface, covered with a stratified phase.
where the phases are identified as more polar (w) and less polar (0). The emulsion ‘polarin-less polar’ type forms when D < 1 and vice versa. From this equation one may conclude that the stratifying phase always has to possess some higher ability to keep coherence than the matrix (carrier) phase. The stratifier phase should be provided with a lower viscosity, a higher phase volume and with a surface-active component (additive, modifier, etc.), which could effectively stabilize the interface from the side of the stratifying phase. For this, the enrichment with a lower molecular weight polymer and a lower volatile solvent, along with lower total polarity, can be recommended for the stratifying phase. A controversial situation (‘self-emulsification against self-stratification’) is shown in Fig. 5. In order to provide a better flow, the stratifying phase has to separate as a diluted solution. Since this phase will also form the incoherent phase, its polymer volume content has to be higher than the coherent phase in order to avoid porosity in the final, solvent-free polymer structure. 5.2.5.2. Benard cells (systems b, c, d) [34,35]. Driven by the surface tension gradients, gained due to the evaporation of solvents [ 221 and applied to the coherent phase of a low viscous polymer-polymer emulsion (pre-formed or formed on a substrate during the phase separation according to the nucleogenic mecha-
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nism) , this process results in the transportation of the lightest polymer particles to the top interface of the coating. With application of this mechanism, self-stratifying coatings can be formed using a substantial amount of highly volatile organic solvent and normally with a relatively thin and clear top coating layer [ 8,9], 5.2.6. Phase contraction mechanism (systems c, d) [29,39] This mechanism is due to minimizing the free interfacial energy in the liquid-gel systems by diminishing the inter-facial surface. It is similar to coalescence, but proceeds in the systems with the spatial interfaces. The mechanism is equally applicable to initially homo- and heterophase compositions. When properly controlled, it provides efficient stratification and, at the same time, a sufficiently firm and developed interface between the separated polymer layers. Its features are long-lasting heterophase gel structures, low viscosity of the stratifying phase, and dependence of the resultant polymer structure on the nature of the substrate. Fig. 6 describes schematically this approach to the non-homogeneity-in-layer. When a liquid heterophase binder of selected composition is applied under certain conditions to a solid substrate, it first forms an interpenetrating phase network (Fig. 6(a) ) . Fig. 6(b) shows that owing to the difference in composition and volatility of solvents, both phases experience different degrees of contraction. This produces a driving force for the phase separation which acts as a destination force if one phase is ‘fixed’ on a substrate due to the selective
A
A
(Stratified c-
/
Phase <--
(--
c---
-p ('
--X
-
I-
A
,--
-
--
Fig. 6. Contraction stratification mechanism: (a) interpenetrating phase network; (b) prevailing the phase 1; (c) stratified structure. 1 and 2, polymer phases; 3, attachment to a substrate.
contraction
of
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V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
Fig. 7. SEM micrographs of film cross-sections (fractured under liquid nitrogen): (a) double layer structure (epoxy / poly (siloxane) resin blend, the upper part of = 100 pm DFT coating) ; (b) filled with Al flake coating (epoxy/poly(siloxane) resin blend); (c) heterophasenon-stratifiedepoxykinyl film (50/50 wt.%); (d) epoxy/ vinyl film, heated at 120 T.
wetting, or attached to the top interface (as a result of solidification of a drying surface layer). Contraction-bound self-stratification process in a drying film cannot gain much intensity in the cases when either viscosity increases rapidly, or self-emulsification takes place in the early step of the film-forming process. Fig. 7 shows examples of films formed using solvent-based epoxy-poly( siloxane) and epoxy/vinyl resin compositions. The first blend (Fig. 7(a) ) forms a film with a clear stratified top layer of 5-7 pm, enriched with siloxane resin. Pigmented composition reveals similar ability for stratification (Fig. 7(b) ), producing a pigment-free top layer.
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
1
,x
/
Matrix
Phase /
Stratified
I
41
Phase
I
I
I
k=-+ r( Stratification I1 ’ Formation I Polymer
b
/
i
I
’
Time -
Secondary Phase Separation
of Heterophase Structure
Fig. 8. Scheme for time-dependent transformation in phase decomposing liquid polymer films: 1, phase decomposition process produces two liquid phases; 2. capillary structure, capillary flow and self-emulsification; 3. contraction of phase structures; 4, development of crosslinking reactions.
An epoxy/vinyl blend, due to the domination of the self-emulsification over the selfstratification, produces top-to-bottom uniform, heterophase films with an epoxy-enriched incoherent phase (Fig. 7(c) ) . In the absence of a hardener, this structure undergoes contraction at 120°C (Fig. 7 (d) ) forming a double layer of almost completely stratified polymer structure. A certain part of the separated phase (epoxy-rich) is incorporated into the vinylrich phase. Fig. 8 provides a uniform scheme for mechanisms 5 and 6 as they develop with time.
6. Registration of stratification A number of ways can be suggested to express self-stratification in the coatings: (i) in terms of concentration, where the difference in concentration of a selected polymer in the top and the bottom coating layers, as well as the difference between its average (calculated from composition) and measured concentration on the selected coating interface, or by the characterizing of a polymer concentration through the film thickness; (ii) in terms of thickness, comparing the thickness of the layers, having different polymer structures or composition; and (iii) in terms of properties, by comparison of selected properties (hardness, chemical resistance [ 7,101 etc.) for both coating interfaces. Scanning electron microscopy with X-ray analysis (SEM EDS) can be successfully applied to delaminated coatings, providing information about the concentration of each
48
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
Table 4 SEM EDS analysis of epoxy/vinyl and epoxylsiloxane coatings delaminated from a glass substrate ( = 80 pm DF-U WI Polymer blend
Film interface
Cl or Si content (%) x 150
Epoxy/vinyl
Air Substrate
Epoxy/siloxane
Air Substrate
1.36 0.81
x 2000
In binder
1.51 1.21
1.25
1.27 0.86
0.95
polymer on both interfaces (see Table 4). Similar information is available from the results of infrared spectroscopy (ATR FTIR) with a prism for film surface analysis using partially attenuated total reflectance [40]. As for example, for epoxy/vinyl self-stratified coating, applied by spray on a foil and delaminated, the absorption bands of vinyl resin are twice as intense on the top interface as on the adhesion layer. The situation with epoxy absorption bands is the inverse (Fig. 9). Both methods are unable to evaluate the inner coating structures. SEM characterization and analysis of cross-sections, obtained particularly by a fragile cut (sometimes together Vinyl Resin-Rich Upper Layer Epoxy-Rich Bottom Layer /////////////////////// Substrate
Fig.9. ATRFTIRfilm surface analysis of a self-stratified epoxy/vinyl resin coating (75 125 wt.%, 120 pm DFT)
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995’) 31-52
49
with a glass substrate), can give information about both non-homogeneous-in-layer and heterophase polymer structures, especially when this has been combined with microprobe analysis [ IO,1 11. Light microscopy of film cross-sections is applicable for rough stratified structures.
7. Coating properties associated with heterophase and self-stratified structures Heterophase polymer-polymer structures generate the following advantages for coatings [ 4 I-451 : improved mechanical properties, including a successful balance between hardness and flexibility; elimination of internal stresses, which may occur during the filmforming process as well as during the coating service; and reduced permeability. According to the reviewers [ 7-101, self-stratification in polymer coatings allows an improvement in adhesion durability (thereby contributing to corrosion protection) or penetration into porous substrate, and upgrading chemical durability, light and weather resistance, wear-resistance, surface slip, etc. Besides polymer-polymer ratio and curing reaction, commercial coating compositions allow a number of ways of controlling heterophase polymer structures. The most important approaches are the introduction of surface additives (polymer or oligomer surfactants) and the variation of solvents and application conditions. As minor additives, some siliconic [ 10,l l] or fluorine-containing oligomers [ 121 are capable of substantially accelerating the phase separation process with a moderate or negligible influence on the composition of equilibrium solutions. In contrast, solvents, in order to be effective, have to be introduced in significant amounts, thereby changing substantially an equilibrium distribution of components. If one can control heterophase coating structures, one can obtain a series of materials of almost equal composition, but with substantially different technical properties. Table 5 compares the vapour permeability of a conventional solvent-based epoxy paint (having a uniform, homophase polymer matrix), an experimental heterophase matrix-based epoxy (45-92) and a coating which combines a bulk heterophase structure enriched with vinyl resin (2-92). Paints contain similar pigments and extenders (PVC 22%). One may point out that polymer-polymer heterogeneity decreases permeability for these coatings. Silicone modifier (stratification promoter), being introduced in the ready for application paint, effects negatively the isolation properties of both homophase and heterophase-matrix coatings (probably by inducing a rougher polymer structure). Being introduced into a stratifying composition, the same additive renders the coating less permeable.
8. Fields of application At present solvent-based self-stratifying coatings have a limited use in general application fields, since they demand the use of substantial amounts of volatile organic solvent and quite precise control of composition and application conditions. When these limitations are not critical, one can exploit advantages of self-stratification in the following situations:
50
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
Table 5 Vapour permeability of selected coatings (free films, 80-85 pm DFP) Paint
Vapour permeability (mg/cm’/day) With modifier a
Without modifier lb
2
1
2
Standard 2-component epoxy paint
0.75
0.69
1.02
1.06
45-92 heterophase
0.31
0.40
0.59
0.47
2-92 heterophase and stratified
1.31
0.72
0.98
0.66
’Modifier (silicone oligomer) promotes self-stratification. b 1: dried at ambient temperature, 14 d; 2: dried and heated at 60 “C, 48 h.
(i) Coil-coating technology. Solvent-based compositions are still widely used here, and their surface gradients and selective wetting can occur sufficiently for self-stratification. (ii) Anticorrosive paints for heavy-duty coatings or rusted surfaces. The stratified to a substrate phase, which evolves as enriched with anticorrosive additive (rust converter, inhibitor, etc.) as well as with wetting agent, adhesion promoter, etc., extends corrosion protection especially for rusty or marginally prepared substrates [ 221. (iii) Stoving enamels. With self-stratification these paints could strike a better balance between adhesion and surface properties for one-coat applications. (iv) Emulsion-type compositions for wood can exploit the porous substrate for stratification due to the difference in penetration for the coherent and incoherent phases [ 211. (v) Decorative coatings. (vi) Cavitation damping marine paints [ 181. (vii) Wear-resistant coatings [ 291, (viii) Coatings that combine the weather resistance of acrylic or poly (urethane) top coats with the protective properties of epoxy-enriched adhesion layers [ 10,111.
9. Conclusions
Polymer-polymer composite and self-stratification approaches, based on the use of binders consisting of incompatible polymer blends are available for various types of paints, exploiting both homophase and heterophase liquid binders. Phase consideration, particularly based on the build and analyses of phase diagrams for solutions of incompatible polymer blends, allows some prediction of their behavior during the film-forming processes. Being applied to a solid substrate at room conditions, these binders release solvents and form heterophase polymer-solvent gel type structures.
V.V. Verkholantsev/Progress
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51
The solid films that they form afterwards may have various heterophase and sometimes self-stratified polymer structures depending on a number of factors, including the polymerpolymer ratio, solids content, viscosity, relative affinity and volatility of solvents, amount and nature of hardener, etc. A few mechanisms of the formation of heterophase and self-stratified coatings are discussed. The interface between sharply separated incompatible polymers suffers from a lack of interlayer adhesion. In order to produce a favorable polymer structure, that combines heterophasity with a moderate self-stratification, a film-forming composition has to contain a substantial amount of organic solvents. An incomplete separation of the binders, based on incompatible polymer blend, into the layers enriched with different polymers, may be utilized in a few areas of coating application. Particularly, epoxy/thermoplastic binders provide a situation where self-stratified and microheterophase polymer structures are produced simultaneously. This gives an opportunity for simultaneous improvement of structure-dependent coating characteristics bound to both interfaces and ‘bulk’ part of a coating, when even a single paint material is applied in one coat.
Acknowledgements The author is indebted to M. Flavian for his regular attention and discussions and to M. Tabachnik, D. Raban and Y. Shaff for their contribution to experimental work and preparation of the manuscript.
References [ I ] L.A. Utraki and R.A. Weiss (eds.), MultiphasePolymers: Blendrandionomers, American Chemical Society, Washington, DC, 1989. [Z] C.B. Bucknall, Toughened Plastics, Applied Science, London, 1977. [ 3 ] W. Funke, J. Oil Colour Chem. Assoc., 59 ( 1976) 506. [4] W. Funke, Ind-Lukier-Bet.. 44 (8) (1976) 305. [ 51 M. Schmitthenner and W. Funke, Dtsch. Farben-2.. 30 ( 1976) 506. [6] H. Murase and W. Funke, XVFATIPEC Congress Book, II 1980, p, 387. [7] V.V. Verkholantsev, Prog. Org. Coat., 13 (198.5) 71. [8] C. Carr, J. Oil Colour Chem. Assoc., 73 ( 1990) 403. [9] T.A. Misev, J. Coat. Technol., 63 (1991) 23. [IO] V.V. Verkholantsev, Prog. Org. Coat., I8 (1990) 43. [ 111V.V. Verkholantsev, J. Coat. TechnoI., 64 ( 1992) 5 1. [ 121 S. Benjamin, C. Cam and D.J. Walbridge, Proc. PRA 2nd Int. Conf: Fluorine in Coatings, Sept. 1994, Paper 6. [ 131 V. Bobochidze, Non homogeneous-in-layer coatings, based on polyurethane dispersion, Ph.D. Thesis, Coating Research Institute, Moscow, 1984 (in Russian). [ 141 LA. Krilova, S.V. Sazonova and N.I. Morozova, Prog. Org. Coat., 21 ( 1992). [ 151 L.S. Strekachinskaya et al., Visokomolec. Soed., 268. (1984) 770. [ 161 V. Verkholantsev and V.V. Krilova, Visokomolec. Soed., 30 (1988) 1653.
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[ 171 V.V. Krilova and V. Verkholantsev, Lakokras. Mater. Ikh Primen., 2 ( 1987) 36. [ 181 Yu.B. Shleomenson, Yu.E. Zobachev, A.M. Petrov and V.V. Verkholantsev, Lakokras. Mater. Ikh Primen., 2 (1981) 32. [ 191 V.V. Krilova, 1.1.Kainova and V.V. Verkholantsev, Lakokras. Mater. Ikh Primen. 4 (1984) 32. [20] MC. Langdon, Surf: Coar. lat. (JOCCA), 77 (6) (1994) 262. [21] A. Holland, XXI FATIPEC Congress Book, II (1992) 207. [22] V.V. Verkholantsev and M. Flavian, 12th Corrosion Congress Book, Houston, 7X, 1993, Vol. I, pp. 99-
113. 0. Olabisi, L.M. Robeson and M.T. Shaw, Polymer-Polymer Miscibility, Academic Press, London, 1979. Ju. Lipatov, Colloid Chemisrry of Polymers. Naukova, Dumka, Kiev, 1984. I.M. Lifshitz and V.V. Slesiv, J. Exp. Theor. Phys. (USSR), 35 (1958) 399. A.E. Tchalykh, Polymer Diflusion, Chomoa, Moscow, 1987. C.C. Piccardi and J. Borrajo, Polymer Inr.. 32 (1993) 241. R.C. Chan and D.L. Feke, J. Coat. Technol.. 65 (1993) 53. [29] V.V. Verkholantsev, V.D. Babajanz and Yu.A. Mulin, Lakokras. Marer. Ikh Primen., 6 ( 1981) 26. [30] Th. Mezger, Prog. Org. Coat, 20 (1992) 353. [31] V.V. Verkholantsev, V.V. Bobohidze and G.V. Rudnaja, Studies in rhe Field of the Synrhesis, Modification ana’Physico-chemistry of Paint Systems, NIITEIM, Moscow, 1981, pp. 79-84. [ 321 L.S. Strekatchinskayaand V.V. Verkholantsev, Preparation and Properries of Coarings, NIITEIM, Moscow, 1982, pp. 29-31. [33] Sun Suk Lee and Sung ChuI Kim, Polym. Eng. Sci., 33 (1993) 598. [34] J. Thomson, Phil. Mag. Ser., 4 (10) ( 1955) 330. [35] S. Wu, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. [36] P. Caries, A.M. Cazabat and E. Kolb, ColloidSurf: A: Physicochem. Eng. Aspects, 79 (1993) 65. [ 371 L.E. Striven and C.V. Sterling, Narure, 187 l-1960) 186. [38] V.V. Verkholantsev and IV. Shvaikovskaya, Colloid J. USSR, 49 (1987) 1178. [39] K.-H.W. Reichert and H. Murase, Drsch. Farben-Z., 29 (1975) 254. [40] V.V. Bobohidze and V.V. Verkholantsev, Bumazn. Prom., (10) (1980) 22. [41] V. Verkholantsev, Lakokras. Mater. Ikh Prim., 4 (1990) 12. [42] I. Frischinger and S. Dirlikov, Waterborne Higher-Solids and Powder Coatings Symp. Book, New Orleans, 1992, pp. 463-472. [43] B. Geisler and F.N. Kelley, J. Appl. Polym. Sci., 54 (2) (1994) 177. [44] L. Soos, Polym. Paint Colour J.. 183 (1993) 490. [4S] C. Carr, S. Benjamin and D.J. Walbridge, Eur. Coat J., 4 (1995) 262.
[23] [24] [25] [26] [27] [28]
Progressin Organic Coatings 26 ( 1995 ) 3 1-52
Heterophase and self-stratifying polymer coatings V.V. Verkholantsev Tambour Ltd., POB 2238, A!&o 24121, Israel Received 5
February 1995;revised 16 February 1995;accepted2 1February I995
Abstract
Film-forming compositions based on incompatible polymer blends, including curable epoxy resins, produce coatings with a heterophase polymer matrix structure. Composed as homophase solutions with use of common solvents, they become heterophase films due to evaporation of solvents during application or during the film-forming process. When phase separation takes place during curing reactions, it results in a fine heterophase polymer structure. Under certain conditions, heterophase liquid polymer blend compositions form non-homogeneous-in-layer (self-stratified) coatings. Phase separation in binders, which were initially homophase due to the presence of common solvents, can also terminate with self-stratification. A few driving forces for stratifying processes have been suggested, particularly of surface tension gradients resulting from non-uniform solvent evaporation, and a difference in contraction of separated phases. Possible mechanisms and conditions for their applicability are discussed in conjunction with resultant heterophase polymer structure. A number of application areas are suggested, where polymer-polymer heterophase or self-stratifying polymer structures could give advantages in terms of service properties in comparison to homophase-matrix coatings. Keywords: Polymer blends; Heterophase
films; Self-stratified
coatings; Surface tension gradients
1. Introduction
Polymer-polymer incompatibility and heterophases in materials are not considered adequately in many polymer technologies. Technologies of plastics, films, membranes and other polymer materials exploit heterophase structures successfully to aid their properties [ 1,2]. Instead, with the exception of a few examples, coatings technology avoids any incompatibility between binders. About two decades since W. Funke and collaborators published their works on selfstratifying coatings [ 3-6 1, this approach has attracted attention of several reviewers and researchers [ 7-121 as an opportunity to produce coatings having multilayer structures by one-coat application of a single paint composition. 0300-9440/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .SSDIO300-9440(95)00552-P
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V. V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
It was suggested that a properly selected incompatible polymer blend, being melted on a substrate (in the case of powder paints) or dissolved in solvents (solvent-based technology) to produce two-phase liquid composition in the course of film forming processes, should have the capacity for self-stratification and formation of a double or multilayer film. Each layer has to be formed with a polymer component of this incompatible polymer blend, driven by difference in the free surface energy. So, a heterophase polymer blend, being converted into a liquid on a substrate, has been suggested to form a self-stratifying coating. Unfortunately, the difference in free energy between two states is a potential which can provide stability or instability to the system but which cannot serve as a driving force for material flux or flow (until some gradients of surface or other forces are present). Coagulation mechanisms of the film formation process [ 131 can be applied to waterbased compositions containing a mixture of two polymer emulsions (latexes). The mixture can provide a non-homogeneous-in-layer coating as a result of the difference in the colloid stability and, correspondingly, in the coagulation rates on drying of the two emulsions. Electrodeposition process with use of mixed dispersions [ 141 may also yield non-homogeneous-in-layer coatings. Produced in this way coatings are hardly identified as ‘self stratified (since electriadagulation cannot be referred to as ‘internal forces’). Misev [9] has suggested a different mechanism of self-stratification, perhaps more convenient for the traditional coatings technology, where a homophase of binders is considered preferable. His idea was to convert an initially homophase binder, based on a blend of partially compatible resins having one thermoset polymer partner into a two-phase system due to the development of crosslinking reactions. This neglects miscibility limits for polymers and provides contraction and self-stratification. This author has also suggested a handsome terminology, identifying the separating and stratifying polymer phase as a ‘stratifier’ and the matrix phase as a ‘carrier’. From a physicochemical standpoint, in order to provide stratification, any contraction force needs a gradient through the film thickness. Otherwise, phase separation process in a viscous liquid has to terminate with formation of an emulsion, which needs another driving force for formation of layers and their displacement in the desirable order. Compositions based on epoxy/thermoplastic incompatible resin blends were studied in a series of works [ 15-171 as binders capable of producing either polymer-polymer heterophases or simultaneously heterophase and self-stratifying polymer structures. It was also reported [ 18-191 for epoxy-based paints that heterophase and self-stratifying approaches were utilized to improve coating properties such as weather resistance, and mechanical and adhesion characteristics. Although this has been introduced in a few paints (as powder [6], solvent-based [ 10,111 and water-based coatings [ 20,211) , the self-stratification concept is still consistent with a number of contradictions and unclear aspects. (i) The self-stratification concept considers that a desirable coating structure is the primary goal whereas the traditional approach is based on the coating properties. Particularly, when the self-stratification concept was applied to solvent-based epoxy coatings [ 10,11,22], it was found that not all incompatible polymer blends capable of producing heterophase or self-stratifying coatings are able to satisfy the technical demands. (ii) From the standpoint of polymer theory, the interface between incompatible polymers should exhibit a lack of adhesion durability, at least, for two reasons.
V.V. Verkholantsev/Progress
in Organic Coatings 26 (1995) 31-52
13
First, considering an equilibrium interface between two polymers, the adhesion durability of the interfacial contact should drop with the increase in the difference in polarity between contacting polymers, i.e. adhesion durability should be higher between compatible and lower between incompatible polymers. Secondly, this situation becomes even worse in the case of film-forming processes in multilayer drying films (non-equilibrium interface). Since normally the shrinkage due to the release of solvents (or formation of crosslinked networks) is different in each polymer layer, the development of residual stresses at interfaces may be significant. A sharp selfstratifying process may result in an easily splittable film [ 221. (iii) Two main types of self-stratifiedcoating structures are obtained in practice, either sharply separated into two homophase layers films, or partially separated with the matrix polymer layer remaining as the heterophase. (iv) It remains unclear which type of polymer structure, non-homogeneous-in-layer or microheterophase, contributes more substantially to the advanced properties of the selfstratifying coatings. Extended service life, which is reportedly inherent to the epoxy-based self-stratified coatings [ lo,11 1, may be sometimes associated with their heterophase polymer structure, gained thanks to incompatibility of the thermoplastic modifier with the matrix-forming epoxy resin, rather than with their multilayer structures. Incomplete phase separation during film forming processes enables heterophase structures to seal micro cracks formed in coatings on long-term exposition to the light, alternating temperatures, vibrations etc., and to maintain compact coherence of the polymer matrix. The most favorable composite coating structure was found to be one formed in the course of incomplete phase separation process and moderately developed self-stratification [ 10, I I]. The latter allows the separated polymer layer to keep its ‘roots’ in the matrix phase, to prevent disbonding. Particularly, for lo@-120 pm DFT coatings, it means formation of roughly 5-10 pm of a separated top layer. Being added to a well-controlled heterogeneity of the matrix phase, and provided with crosslinkage in both layers, this coating structure can give a substantial improvement in the coating characteristics. This paper is aimed to assemble some characteristic data and observations, obtained in our other work on heterophase and self-stratifying epoxy ambient cured paints. An attempt is made to give a better understanding of the self-stratification process in liquid paints, focusing on a phase separation of polymer solutions, driving forces and mechanisms of selfstratification.
2. Solutions of incompatible
polymer blends and their phase behavior
Any development in self-stratifying coatings should start with the selection of suitable polymer partners, taking into consideration their technical properties in the coating, their compatibility and other characteristics. When adapting to conventional solvent-based technology, we should first evaluate the behavior of the selected incompatible polymers in the presence of paint solvents. In order to be able to control the amount and properties of separated phases, one needs, as the
34
V.V. Verkholantsev/Progress
in Organic Coatings 26 (1995) 31-52
simplest system, a composition of two incompatible polymers and two solvents, having different volatility and affinity for the selected polymers [ 10,111. Phase separation is a process driven by intermolecular forces, and the formation of binodal phases occurs due to the minimization of the free energy of mixing. Thermodynamics of polymer solutions and blends [ 23,241 allows predictable calculations of phase behavior (separation condition, composition for binodal phases etc.) for relatively simple compositions such as two- or three-component polymer solutions. For practical needs, especially in the case of technical polymers and multicomponent systems, it is advisable to use experimental data regarding phase behavior of polymer blends and solutions. The term ‘experimental diagram’ means that a binodal curve (or surface) is drawn through the points obtained by analyses of equilibrium compositions of the separated liquid layers. 2.1. Phase diagrams Typical quatemary (2 polymers + 2 solvents) experimental phase diagrams are presented in Fig. 1 for a few incompatible polymer blends and selected solvents. Gel permeation chromatography (GPC) and chemical analyses were used to characterize the composition of binodal solutions in three- and four-component systems [ lo]. Any point of a phase diagram expresses an equilibrium composition, and its position with regard to a binodal surface reflects its state as either a homophase solution, a metastable solution (ready to separate), or a two-phase system. The position of a spinodal surface, which cannot be obtained from this experiment, is shown marginally. All compositions inside the binodal surface correspond to non-stable polymer-polymer emulsions, which are inclined to separate into layers until they remain liquid. For example, point B represents a liquid heterophase (two-phase) multicomponent polymer binder, i.e. polymer-polymer emulsion. Such an emulsion can be obtained either by immediate mixing of rather concentrated polymer solutions (for example, by blending of three-component solutions P and Q) , or by evaporation of solvents from initially homophase solution A. The second case is associated with the process of phase separation, when the point of composition crosses the binodal and then the spinodal borderline and with the production of a polymerpolymer emulsion. Both phases of this emulsion have a binodal composition (points X and Z are connected with a tie-line and belong to two different branches of the binodal surfaces). They contain all four components, but in different ratios, Amount of each phase can be calculated through XB/BZ ratio. The opposite direction of the phase transformation (B -+ A) is also possible. A polymer-polymer emulsion can be transformed back into a homophase solution with the addition of a true solvent. In order to produce coatings from an incompatible polymer blend, we may select a binder as an initially homophase system (more or less a diluted polymer solution, point A) or as a two-phase liquid, a polymer-polymer emulsion (point B) . 2.2. Phase separation Phase separation in a polymer solution, caused by the change in its composition or temperature, which corresponds to a shift into a heterophase region on its phase diagram,
fig.
1. phase diagrams for experimental quaternary systems: (a) solid epoxy resin-chlonnated poly(ethylene)-propylene glycol methyl ether (PGME)-xylene. 1. emulsions or gels): !b) solid epoxy resinconcentration region of homophase solutions; 2. metastable solutions; 3, labial state (unstable solutions, polymer-polymer acrylic resin_xylene-PGhE; (c) solid epoxy resin-chlorinated poly( ethylene)-xylene-butyl alcohol; (d) solid epoxy resin-perchlorovinyl resin-acetone-xylene.
Propylene GlYCol, Methyl Ether
V.V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
36
may proceed according to either a neucleogenic (nucleation and particle growth) or a spinodal mechanism. When phase separation occurs as a result of a relatively fast change in the parameters of state (for instance, due to evaporation of highly volatile solvents from a developed solution/air interface), it proceeds via a spinodal mechanism [ 23,241, producing a jelly system, comprised of a network of two interpenetrating phases. The time scale for the phase separation process during the film-forming process can be done in terms of the Lifshitz-Slesov concept [ 251. The time, r, needed for the phase inhomogeneities of radius r to appear under the action of over saturation ( p), can be evaluated according to the equation: 1 0-P -=r2 7 where D is the diffusion coefficient. Assuming for a polymer solution D= lo-” cm2/s [ 261 and /3= 0.2, the production of the non-homogeneities of r = 0.1 pm takes 50 s (and = 80 min when the solution becomes more concentrated and D = lo-l3 cm2/s). This means that the phase separation process proceeds fast enough in non-viscous dilute polymer solutions. If the opposite situation occurs, the formation of heterophase coating structures will last for a longer period and can be interrupted by a solidification or curing reaction. If a phase diagram for the given polymer blend composition is unknown, one can operate with practical tests, i.e. observing the results of blending of polymer solutions. 2.3. Experimental observations of phase behavior As an example, Table 1 presents some results obtained by blending a commercial solid epoxy resin with a few thermoplastic solutions. All selected parts, being mixed, produce Table 1 Phase behavior of selected commercial polymers in blended solutions Thermoplastic resins (TR)
TR/SER’ ratio (wt.%)
Total solids content in heterophase mixtures (%)
True solvents
Solids content in solutions, achieved homogeneity
Chlorinated rubber (50% solution)
36164
63
MBK
48
Chlorinated PE (40% solution)
2lll3
59
MEK
35
Vinyl resin (30% solution)
22118
56
MEK
42
Acrylic resin (30% solution)
22118
56
MIBK
42
’ Solid epoxy resin (SER), 10% sol. + solutions of thermoplastic resins.
V. V. VerkholantsevlProgress
in Organic Coatings 26 (1995) 31-52
37
heterophase, milky and sometimes pearlescent, viscous or jelly-type masses, which should be identified as non-stable polymer-polymer emulsions. They can be converted back into clear homophase solutions by addition of some solvents [methyl ethyl ketone (MEK), emulmethyl-iso-butyl kethone (MIBK) etc. J Finally 5647% solids polymer-polymer sions become homophase solutions, when diluted to 3548% solids. In practice, epoxy resins are cured with hardeners. Addition of 20% (on epoxy resin ) of a poly( amino)-amide hardener converts all homophase solutions into two-phase polymer-polymer emulsions. They demand for more solvents to be restored as homophase solutions. Summing up, we may say that the incompatible polymer blend technology offers a few different types of film-forming compositions. First, we may use heterophase polymer-polymer emulsions, obtained by immediate mixing of individual homophase solutions of incompatible polymers. These emulsions can be involved into film forming processes either per se (producing heterophase polymer films), or being first transformed into a homophase solution with a solvent, which is common for both polymers, and then producing more or less controlled heterophase structures (heterogeneity and phase composition of the separated phases may be controlled by changing the amount or the nature of solvents, or by use of additives). We may also separate these polymer-polymer emulsions into binodal solutions and use these compositions to produce heterophase polymer structures [ 171. A binodal solution can be subjected to dilution with either a true solvent (producing more stable polymer solution), or with a non-solvent, which converts it into a polymer-polymer emulsion. Finally, we may combine binodal solutions in any ratio, forming polymer-polymer emulsions of the desirable compositions. Some of these compositions can be subjected to self-stratification during film-forming processes. Thus, the solutions of incompatible polymer blends allow the formation of homophase, heterophase and separated-in-layers coating structures.
3. Molecular weight distribution of incompatible polymer blends in separated phases From GPC analysis, the molecular weight distribution appears different in original and binodal solutions. The data, presented in Table 2, reflects this difference for solutions of a blend of a solid epoxy resin with an acrylic resin. In this binodal phase, enriched with epoxy resin (lower layer), this polymer is represented mainly by the higher molecular weight fractions and the lower molecular weight fractions are relegated to the acrylic-enriched binodal phase (upper layer). Similar results were obtained with binodal solutions of epoxy/chlorinated PE (Table 3) and epoxy/vinyl resin blends. The difference becomes negligible (below the limit of GPC evaluation), when the epoxy/thermoplastic ratio is decreased to lo/90 wt.%. Non-adequate distribution between separated solutions of incompatible polymers, having a wide molecular weight distribution, is reported in the literature [ 271. This can be explained by the fact that due to a kinetic effect, the thermodynamic equilibrium is impeded. In heterophase and self-stratifying coatings technology this phenomena should be taken into consideration. Particularly, in an epoxy/thermoplastic coating, formed through the
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
38
Table 2 Phase separation and molecular Layer
Enriching resin
weight distribution Total non-volatiles (wt.%)
of a solid epoxy/acrylic Epoxy/Acrylic ratio (wt.%)
resin blend in binodal solutions
Molecular weight distribution (%) 970
1400
2500
3500
Upper (84%)
acrylic
38.2
51/49
16
29
29
26
Lower (16%)
epoxy
49.5
9317
9
24
33
34
15
29
29
27
original epoxy resin
Solid epoxy resin (CIBA Geigy), epoxy equivalent = 500; acrylic resin (Paraloid B66, Rohm and Haas); main solvent, xylene; epoxy/acrylic ratio 57:43 wt.% and 66:45 ~01%.
Table 3 Content of epoxy species (wt.%) Epoxy species (MW
In original resin (%)
in separated binodal solutions
Epoxy resin/chlorinated PE weight ratio 30170
70/30
90/10
1’
2b
1
2
1
2
17 30 28 25
8 24 33 35
22 31 25 22
13 28 30 29
18 29 28 26
15 29 29 27
88
12
83
17
67
33
Resin content in the layer (wt.%) Epoxy resin Chlorinated PE
27 73
90 10
61 39
95 5
86 14
96 4
Solids (wt.%)
28
40
28
33
30
31
870 1400 2500 3500
15 29 29 27
Volume fraction of the layer (%)
Original composition: solid epoxy resin/chlorinated wt. a 1, Top layers (all chlorinated PE-enriched) b2, Lower layers (all epoxy-enriched).
PE, blended as 30% solutions in xylene/MPA=50/50
phase separation process, the thermoplastic-enriched molecular weight epoxy species (thereby promoting structure with a minimal content of epoxy resin in phase will act as a scavenger for the lower molecular
by
phase has to be a collector of the lower the formation of a crosslinked polymer the total blend). The epoxy-enriched weight vinyl species.
V. V. Verkholantsev / Progress in Orpnic
Coa!ings 26 (I 99.5) 31-52
39
4. Heterophase coating structures
The filling of organic coatings with inorganic pigments and extenders is a traditional approach to their structural modification. Assuming an average particle size for pigments and extenders in the range of OS-3 pm for PVC 30% this corresponds to the magnitude of specific interface (S,,) = l-6 M2/cm3. These figures may be compared to the specific surface of a polymer-polymer interface, which could be achieved in the course of the phase decomposition process. An estimation of a minimal size of heterophasity in polymers is presumed as 300 A [ 241, but even taking 1000 A (0.1 pm) and assuming the volume fraction of an incoherent phase as 50%, S,, will come to 30 M*/cm”, i.e. one order of magnitude higher. The phase decomposition process, when overlapped with evaporation of solvents, creates a severe difficulty for the quantitative prediction of the resultant polymer structure. In the course of application and film formation, an initial polymer solution simultaneously undergoes mass and heat transfer, surface phenomena (interference with a substrate and air, adsorption on internal interface with subsequent change in adsorption equilibrium) and sometimes also chemical reactions. When a homophase solution of an incompatible polymer blend is used as a binder, the type of phase separation process during the application and film formation is important for the consequent heterophase polymer structure [ 23,241. Nucleogenic mechanism used to be terminated with the formation of a polymer-polymer emulsion, whereas a spinodal type phase decomposition process creates a special capillary or interpenetrated phase structure. Both of these types of two-phase systems are unstable until the remaining liquid (or pseudoplastic, gel phases) manifesting a trend to coalescence, contraction, phase segregation, etc.
Fig. 2. SEM micrographs of film cross sections (solid epoxy/perchlorovinyl resin blends) : (a) formed from a solution 30% solids, epoxy resin/perchlorovinyl resin weight ratio 50/50; (b) from a binodal solution, polymer/ polymer weight ratio 87/ 13.
40
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
Fig. 2 illustrates the results of two different mechanisms used to form heterophase structures from solutions of an incompatible polymer blend: first, due to a phase decomposition process in a liquid polymer solution, which gives a rather rough heterophase structure (in Fig. 2(a) the incoherent phase is enriched with epoxy resin, and the coherent phase with perchlorovinyl resin) ; and secondly, as a result of phase separation in a solid film, which was formed first as a homophase film from a binodal solution and then converted into a heterophase state in the course of the curing process (Fig. 2(b) ) .
5. Self-stratifying coatings 5.1. Driving forces for stratification Self-stratification can be described as a phase separation, directed in the coating axes, particularly as a normal to the coating interfaces. ‘Ibis process needs some driving force applied during the film-forming process to a still liquid heterophase binder in order to separate phases between the top and the lower layers of the film. Intermolecular forces provide phase separation and formation of heterophase polymer structure through a diffusion process. However, they are unable to place the separated phases in an assigned direction, which is necessary for self-stratification. For this the binder needs a separate driving force. This function can be performed by an interfacial tension gradient, by the film contraction force and possibly by some other gradients. We may define the forces, oriented in the coating coordinates, as the forces of destination. A comprehensive review of internal forces in the drying films has been presented in the paper [ 281. 5.2. Self-stratification mechanisms In previous publications a dozen driving forces and mechanisms of self-stratification have been discussed as applicable to various binders and types of paints, based on the incompatible polymer blends: (a) mixed polymer ‘melts’ (powder and solvent free paints, low VOC stoving compositions), (b) binders, based on mixtures of polymer dispersions (solvent or waterborne paints on mixed emulsions), (c) solventborne paint compositions, based on polymer-polymer emulsions (composed as 2-pack paints), (d) solventborne binders, based on initially homophase solutions of incompatible polymer blends in volatile solvents, convertible into a heterophase state and capable of selfstratification during film forming processes. 5.2.1, Mechanisms driven by gravitation (systems (a, b, c) 16,lo] It features a temperature above Tr of both polymers. Complete and sharp stratification can be performed with a hardener-free composition, or in the case, where the curing reactions proceed well above Tp
V. V. Verkholantsev / Progress in Organic Coatings 26 (I 995) 31-52
Fig. 3. SEM micrographs blend, 70/30 wt.%). (a)
of cross sections of completely
stratified polymer films (epoxy/poly(siloxane)
41
resin
double layer structure; (b) interfacialborder; (c) a ‘large’particle of siloxane-enriched phase,retained in epoxy-enrichedlayer.
Driven by gravitation, self-stratification in melted incompatible polymer blends can proceed until the complete phase separation and formation of a double layer polymer structure (Fig. 3 (a) ) with sharp interface (Fig. 3 (b) ) Kinetic restrictions prevent stratification of even the larger particles (Fig. 3(c) ). Complete and sharp stratification can be performed only with hardener-free composition. Rough time scale estimation of the process can be done in terms of the Stokes sedimentation relation, where an expression for the velocity of sedimentation (V,) of the dispersed phase particles scales as:
42
V.V. Verkholantsev/Progress
in Organic Coatings 26 (199.5) 31-52
Fig. 4. SEM micrographs of adhesion surface of a film (epoxy/vinyl resin blend, 50/50 wt.%) and delaminated
from a glass substrate under cooling (liquid nitrogen).
where A p is the relative density of liquid polymers or phases of polymer-polymer emulsions, 17is the viscosity of the coherent phase, and r is the particle radius. Even when the A p value is sufficiently high, the temperature must be well above the Tf of both polymers in order to obtain a low viscosity. In practice, a film of HI-100 pm formed from an incompatible solid epoxy/vinyl resin blend, added with a plasticizer, demands l-3 h heating at 120 “C for a complete stratification.
5.2.2. Selective wetting mechanisms {systems a, c, d) [8,12,29] This type of self-stratification process is due to the selective wetting of a substrate by one phase of a two-phase liquid. Its features are sensitivity to the nature of substrate, demands low volatile solvents, non-stable polymer-polymer emulsion and lower viscosity of the stratifying phase. This mechanism can produce local stratification. This mechanism is suitable for controlling adhesion durability and surface properties in relatively thin stoving coatings. As shown in Fig. 4 which represents a SEM micrograph of the surface of a film that has been formed on and then delaminated from a glass substrate under the action of thermal stresses, created by cooling under liquid nitrogen. This interface exhibits heterophase polymer structure. The darker epoxy-enriched ‘stains’ are formed from the particles of the separated phase which have spread over the substrate surface thanks to selective wetting.
5.2.3. Different coagulation These mechanisms are substrate during electro- or the penetration into a porous
or penetration rates mechanisms (systems b, c) [21,30,31] associated with the difference in the rates of adagulation on a chemideposition between the two separated phases or during substrate.
V. V. Verkholantsev
/ Progress
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Coatings 26 (I 995/ 31-52
43
5.2.4. Pigment wetting mechanism (systems a, c, d) [32] This follows the following steps: (a) phase separation if system C is used; (b) selective wetting of pigment particles with one polymer phase; (c) particle coalescence to form a gel structure; (d) contraction of the latter accompanied by separation to form a sandwich structure. 5.2.5. Mechanisms driven by suface tension gradients 5.2.5.1. Capillaryjow (systems c, d) [IO]. This mechanism may occur in incompatible polymer blend solutions which are selected near to the two-phase concentration region and diluted with the solvent blend with each component chosen according to its volatility and affinity to both polymers. Being applied to a substrate in conditions of intensive evaporation of solvents (for instance, by spray), the binder will be shifted into the labial region of the phase diagram (see Fig. 1). Undergoing phase separation according to the spinodal mechanism, this binder will form an interpenetrating network of separated liquid phases [ 33,341. The driving forces for the capillary flow will appear as a result of the non-uniform evaporation of solvents (the Marangony Effect). Interfacial tension between binodal polymer solutions is quite low [ 161. But the nonequilibrium condition for phase separation allows only approximate identification of these phases with binodal solutions. The non-equilibrium conditions of phase separation may create a substantially higher interface tension for each phase providing a distinct potential for the capillary flow. Due to the highly-developed interface, this structure possesses substantial interfacial free energy. An expression for a net surface force (F) gained due to the surface tension gradient V y (local cooling or local concentration variation), can be written in the form [ 33-371: F= -41rr=Vy
(3)
From this equation,
the velocity of a liquid flow (VJ can be expressed as:
r.Vy
v,a -
(4) 77
where r is the average radius of capillaries. The stratification rate, therefore, depends on the interfacial surface tension gradient, on parameters of a capillary structure and on flow characteristics. From the colloid standpoint, any liquid heterophase three-dimensional network has to experience a trend to be spontaneously transformed into emulsion (as to the structure with less interfacial surface). Such ‘selfemulsification’ suspends the self-stratification, making the drying film structure no longer ‘transferable’ for a flux of liquid. A criterion (0) was offered to predict the direction of emulsification (i.e. as to which phase tends to be the incoherent phase). This criterion contains a few characteristic parameters of both phases: the viscosity ( q), the surface activity A ) and the concentration (C) of surface-active species (the phase volumes are assumed to be equal) [ 381: D w/o=
77”
-
A,
.-
%v A,
cw
-
Co
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
(b)
(a) Top
Interphase
Fig. 5. Capillary stratification mechanism (schematic presentation): (a) phase decomposed structure (interpenetrating network); (b) capillary system, capillary flow and self-emulsification process; (c) top surface, covered with a stratified phase.
where the phases are identified as more polar (w) and less polar (0). The emulsion ‘polarin-less polar’ type forms when D < 1 and vice versa. From this equation one may conclude that the stratifying phase always has to possess some higher ability to keep coherence than the matrix (carrier) phase. The stratifier phase should be provided with a lower viscosity, a higher phase volume and with a surface-active component (additive, modifier, etc.), which could effectively stabilize the interface from the side of the stratifying phase. For this, the enrichment with a lower molecular weight polymer and a lower volatile solvent, along with lower total polarity, can be recommended for the stratifying phase. A controversial situation (‘self-emulsification against self-stratification’) is shown in Fig. 5. In order to provide a better flow, the stratifying phase has to separate as a diluted solution. Since this phase will also form the incoherent phase, its polymer volume content has to be higher than the coherent phase in order to avoid porosity in the final, solvent-free polymer structure. 5.2.5.2. Benard cells (systems b, c, d) [34,35]. Driven by the surface tension gradients, gained due to the evaporation of solvents [ 221 and applied to the coherent phase of a low viscous polymer-polymer emulsion (pre-formed or formed on a substrate during the phase separation according to the nucleogenic mecha-
V. V. Verkholantsev / Progress in Organic Coatings 26 (I 995) 31-52
45
nism) , this process results in the transportation of the lightest polymer particles to the top interface of the coating. With application of this mechanism, self-stratifying coatings can be formed using a substantial amount of highly volatile organic solvent and normally with a relatively thin and clear top coating layer [ 8,9], 5.2.6. Phase contraction mechanism (systems c, d) [29,39] This mechanism is due to minimizing the free interfacial energy in the liquid-gel systems by diminishing the inter-facial surface. It is similar to coalescence, but proceeds in the systems with the spatial interfaces. The mechanism is equally applicable to initially homo- and heterophase compositions. When properly controlled, it provides efficient stratification and, at the same time, a sufficiently firm and developed interface between the separated polymer layers. Its features are long-lasting heterophase gel structures, low viscosity of the stratifying phase, and dependence of the resultant polymer structure on the nature of the substrate. Fig. 6 describes schematically this approach to the non-homogeneity-in-layer. When a liquid heterophase binder of selected composition is applied under certain conditions to a solid substrate, it first forms an interpenetrating phase network (Fig. 6(a) ) . Fig. 6(b) shows that owing to the difference in composition and volatility of solvents, both phases experience different degrees of contraction. This produces a driving force for the phase separation which acts as a destination force if one phase is ‘fixed’ on a substrate due to the selective
A
A
(Stratified c-
/
Phase <--
(--
c---
-p ('
--X
-
I-
A
,--
-
--
Fig. 6. Contraction stratification mechanism: (a) interpenetrating phase network; (b) prevailing the phase 1; (c) stratified structure. 1 and 2, polymer phases; 3, attachment to a substrate.
contraction
of
46
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
Fig. 7. SEM micrographs of film cross-sections (fractured under liquid nitrogen): (a) double layer structure (epoxy / poly (siloxane) resin blend, the upper part of = 100 pm DFT coating) ; (b) filled with Al flake coating (epoxy/poly(siloxane) resin blend); (c) heterophasenon-stratifiedepoxykinyl film (50/50 wt.%); (d) epoxy/ vinyl film, heated at 120 T.
wetting, or attached to the top interface (as a result of solidification of a drying surface layer). Contraction-bound self-stratification process in a drying film cannot gain much intensity in the cases when either viscosity increases rapidly, or self-emulsification takes place in the early step of the film-forming process. Fig. 7 shows examples of films formed using solvent-based epoxy-poly( siloxane) and epoxy/vinyl resin compositions. The first blend (Fig. 7(a) ) forms a film with a clear stratified top layer of 5-7 pm, enriched with siloxane resin. Pigmented composition reveals similar ability for stratification (Fig. 7(b) ), producing a pigment-free top layer.
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
1
,x
/
Matrix
Phase /
Stratified
I
41
Phase
I
I
I
k=-+ r( Stratification I1 ’ Formation I Polymer
b
/
i
I
’
Time -
Secondary Phase Separation
of Heterophase Structure
Fig. 8. Scheme for time-dependent transformation in phase decomposing liquid polymer films: 1, phase decomposition process produces two liquid phases; 2. capillary structure, capillary flow and self-emulsification; 3. contraction of phase structures; 4, development of crosslinking reactions.
An epoxy/vinyl blend, due to the domination of the self-emulsification over the selfstratification, produces top-to-bottom uniform, heterophase films with an epoxy-enriched incoherent phase (Fig. 7(c) ) . In the absence of a hardener, this structure undergoes contraction at 120°C (Fig. 7 (d) ) forming a double layer of almost completely stratified polymer structure. A certain part of the separated phase (epoxy-rich) is incorporated into the vinylrich phase. Fig. 8 provides a uniform scheme for mechanisms 5 and 6 as they develop with time.
6. Registration of stratification A number of ways can be suggested to express self-stratification in the coatings: (i) in terms of concentration, where the difference in concentration of a selected polymer in the top and the bottom coating layers, as well as the difference between its average (calculated from composition) and measured concentration on the selected coating interface, or by the characterizing of a polymer concentration through the film thickness; (ii) in terms of thickness, comparing the thickness of the layers, having different polymer structures or composition; and (iii) in terms of properties, by comparison of selected properties (hardness, chemical resistance [ 7,101 etc.) for both coating interfaces. Scanning electron microscopy with X-ray analysis (SEM EDS) can be successfully applied to delaminated coatings, providing information about the concentration of each
48
V.V. Verkholantsev/ Progress in Organic Coatings 26 (1995) 31-52
Table 4 SEM EDS analysis of epoxy/vinyl and epoxylsiloxane coatings delaminated from a glass substrate ( = 80 pm DF-U WI Polymer blend
Film interface
Cl or Si content (%) x 150
Epoxy/vinyl
Air Substrate
Epoxy/siloxane
Air Substrate
1.36 0.81
x 2000
In binder
1.51 1.21
1.25
1.27 0.86
0.95
polymer on both interfaces (see Table 4). Similar information is available from the results of infrared spectroscopy (ATR FTIR) with a prism for film surface analysis using partially attenuated total reflectance [40]. As for example, for epoxy/vinyl self-stratified coating, applied by spray on a foil and delaminated, the absorption bands of vinyl resin are twice as intense on the top interface as on the adhesion layer. The situation with epoxy absorption bands is the inverse (Fig. 9). Both methods are unable to evaluate the inner coating structures. SEM characterization and analysis of cross-sections, obtained particularly by a fragile cut (sometimes together Vinyl Resin-Rich Upper Layer Epoxy-Rich Bottom Layer /////////////////////// Substrate
Fig.9. ATRFTIRfilm surface analysis of a self-stratified epoxy/vinyl resin coating (75 125 wt.%, 120 pm DFT)
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995’) 31-52
49
with a glass substrate), can give information about both non-homogeneous-in-layer and heterophase polymer structures, especially when this has been combined with microprobe analysis [ IO,1 11. Light microscopy of film cross-sections is applicable for rough stratified structures.
7. Coating properties associated with heterophase and self-stratified structures Heterophase polymer-polymer structures generate the following advantages for coatings [ 4 I-451 : improved mechanical properties, including a successful balance between hardness and flexibility; elimination of internal stresses, which may occur during the filmforming process as well as during the coating service; and reduced permeability. According to the reviewers [ 7-101, self-stratification in polymer coatings allows an improvement in adhesion durability (thereby contributing to corrosion protection) or penetration into porous substrate, and upgrading chemical durability, light and weather resistance, wear-resistance, surface slip, etc. Besides polymer-polymer ratio and curing reaction, commercial coating compositions allow a number of ways of controlling heterophase polymer structures. The most important approaches are the introduction of surface additives (polymer or oligomer surfactants) and the variation of solvents and application conditions. As minor additives, some siliconic [ 10,l l] or fluorine-containing oligomers [ 121 are capable of substantially accelerating the phase separation process with a moderate or negligible influence on the composition of equilibrium solutions. In contrast, solvents, in order to be effective, have to be introduced in significant amounts, thereby changing substantially an equilibrium distribution of components. If one can control heterophase coating structures, one can obtain a series of materials of almost equal composition, but with substantially different technical properties. Table 5 compares the vapour permeability of a conventional solvent-based epoxy paint (having a uniform, homophase polymer matrix), an experimental heterophase matrix-based epoxy (45-92) and a coating which combines a bulk heterophase structure enriched with vinyl resin (2-92). Paints contain similar pigments and extenders (PVC 22%). One may point out that polymer-polymer heterogeneity decreases permeability for these coatings. Silicone modifier (stratification promoter), being introduced in the ready for application paint, effects negatively the isolation properties of both homophase and heterophase-matrix coatings (probably by inducing a rougher polymer structure). Being introduced into a stratifying composition, the same additive renders the coating less permeable.
8. Fields of application At present solvent-based self-stratifying coatings have a limited use in general application fields, since they demand the use of substantial amounts of volatile organic solvent and quite precise control of composition and application conditions. When these limitations are not critical, one can exploit advantages of self-stratification in the following situations:
50
V. V. Verkholantsev / Progress in Organic Coatings 26 (1995) 31-52
Table 5 Vapour permeability of selected coatings (free films, 80-85 pm DFP) Paint
Vapour permeability (mg/cm’/day) With modifier a
Without modifier lb
2
1
2
Standard 2-component epoxy paint
0.75
0.69
1.02
1.06
45-92 heterophase
0.31
0.40
0.59
0.47
2-92 heterophase and stratified
1.31
0.72
0.98
0.66
’Modifier (silicone oligomer) promotes self-stratification. b 1: dried at ambient temperature, 14 d; 2: dried and heated at 60 “C, 48 h.
(i) Coil-coating technology. Solvent-based compositions are still widely used here, and their surface gradients and selective wetting can occur sufficiently for self-stratification. (ii) Anticorrosive paints for heavy-duty coatings or rusted surfaces. The stratified to a substrate phase, which evolves as enriched with anticorrosive additive (rust converter, inhibitor, etc.) as well as with wetting agent, adhesion promoter, etc., extends corrosion protection especially for rusty or marginally prepared substrates [ 221. (iii) Stoving enamels. With self-stratification these paints could strike a better balance between adhesion and surface properties for one-coat applications. (iv) Emulsion-type compositions for wood can exploit the porous substrate for stratification due to the difference in penetration for the coherent and incoherent phases [ 211. (v) Decorative coatings. (vi) Cavitation damping marine paints [ 181. (vii) Wear-resistant coatings [ 291, (viii) Coatings that combine the weather resistance of acrylic or poly (urethane) top coats with the protective properties of epoxy-enriched adhesion layers [ 10,111.
9. Conclusions
Polymer-polymer composite and self-stratification approaches, based on the use of binders consisting of incompatible polymer blends are available for various types of paints, exploiting both homophase and heterophase liquid binders. Phase consideration, particularly based on the build and analyses of phase diagrams for solutions of incompatible polymer blends, allows some prediction of their behavior during the film-forming processes. Being applied to a solid substrate at room conditions, these binders release solvents and form heterophase polymer-solvent gel type structures.
V.V. Verkholantsev/Progress
in Organic Coatings 26 (199.5) 31-52
51
The solid films that they form afterwards may have various heterophase and sometimes self-stratified polymer structures depending on a number of factors, including the polymerpolymer ratio, solids content, viscosity, relative affinity and volatility of solvents, amount and nature of hardener, etc. A few mechanisms of the formation of heterophase and self-stratified coatings are discussed. The interface between sharply separated incompatible polymers suffers from a lack of interlayer adhesion. In order to produce a favorable polymer structure, that combines heterophasity with a moderate self-stratification, a film-forming composition has to contain a substantial amount of organic solvents. An incomplete separation of the binders, based on incompatible polymer blend, into the layers enriched with different polymers, may be utilized in a few areas of coating application. Particularly, epoxy/thermoplastic binders provide a situation where self-stratified and microheterophase polymer structures are produced simultaneously. This gives an opportunity for simultaneous improvement of structure-dependent coating characteristics bound to both interfaces and ‘bulk’ part of a coating, when even a single paint material is applied in one coat.
Acknowledgements The author is indebted to M. Flavian for his regular attention and discussions and to M. Tabachnik, D. Raban and Y. Shaff for their contribution to experimental work and preparation of the manuscript.
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[23] [24] [25] [26] [27] [28]