A paradigm shift: the new role of heterogeneity and interactions.
This is a uniquely interesting time for the coatings industry, not only as a result of the attractiveness of these new technologies but, rather, because what is really changing is the fundamental way in which problems are approached. A new paradigm and a whole new perspective are emerging.
COHESION: A VERY IMPORTANT PROPERTY
One of the most important properties of a coating film, indeed almost the quintessence of being a film, is cohesion. Cohesion is most obviously manifested when the film is deformed. If applied tension is plotted versus deformation, as in Figure 1, it can be seen that the stretching is reversible at small deformations--the film returns to its original state when the tension is released. The slope of the graph in the linear behavior of this section is the modulus of elasticity of the film, which gives an idea of its rigidity: the more resistance it offers to deformation, the harder it is.
When the deformation exceeds the elastic limit and becomes irreversible, the behavior begins to be plastic or viscous. In this case, there is a variation in the slope. Smaller increments of tension are needed to produce the deformation. At a certain point the film finally breaks; the two coordinates at this moment correspond to the elongation at break and the tensile strength, which are the maximum elongation and tension that the film supports before breaking. In addition, the area under the curve is the work of rupture, which is proportional to the abrasion resistance.
THE COHESIVE BEHAVIOR OF COATINGS MATERIALS
As is shown in Figure 2, some materials offer resistance to deformation but cannot elongate very much before breaking: such materials are hard and brittle. On the other hand some materials are easily deformed even at low applied stress. If they resist elongation, they are soft and tough; if not, they are soft and weak. On the other hand, if they show a steep curve and resist deformation they are hard and strong. The best behavior shows a steep initial response that extends toward great elongations; such materials are hard and tough.
Different general behaviors are required for different types of coatings according to the functions that they must fulfill. As is depicted in Figure 3, in general, an exterior wood or a roof coating is required to withstand high elongations due to the deformation of the substrate. On the other hand, industrial coatings must have high resistance to deformation, whereas synthetics and latex coatings generally exhibit intermediate behaviors. The major challenges for the coatings formulator are, for example, developing wood and roof coatings that can support deformation but that do not have too low a modulus of elasticity (and therefore poor drying characteristics) and, on the other hand, formulating industrial coatings that have a high modulus but that also resist large deformations.
If a material is hard and brittle, then changing the conditions, such as by increasing the temperature, adding plasticizers, diminishing the [T.sub.g] of the components, or reducing the deformation speed will tend to increase flexibility. For example, if the temperature exceeds that of the [T.sub.g], the behavior will be similar to that shown on the bottom curve in Figure 3. If the modulus is increased, that is to say, if we move toward the left of the graph, the hardness will increase and the drying time and the tendency towards blocking will be improved. The modulus of elasticity and the work of rupture regulate the drying time, scratch and abrasion resistance, and general mechanical behavior--many important properties of a coatings film.
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Modulus of elasticity is, in fact, a key to formulation. In addition to regulating the mechanical properties, it is closely related to the initial coefficient of diffusion which regulates the amount of water or chemicals that can enter a film. As a result, it has an enormous influence on water, chemical, and blister resistance. In addition, blister resistance depends on the modulus of elasticity not only because of its effect on diffusion of aggressive materials; the mechanism of blister formation itself requires the deformation of the film which is also regulated by its modulus of elasticity. Similarly, modulus also influences the delamination rate. In Figure 4 we can see how important this influence is; the logarithm of [D.sub.o] falls steeply from [10.sup.-5] to [10.sup.-15] with [T.sub.g] or with the modulus of elasticity, which is closely related.
We can compare the cohesion properties of different synthetic organic materials by viewing them on a plane, where the most brittle with least cohesion are placed at the bottom, with cohesion increasing upwards, as in Figure 5.
In this way we can see that polycarbonate (of bisphenyl A) and nylon, have the greatest cohesion, while hydrocarbon resins and polystyrene show the least and the rest are located in intermediate positions, with epoxies toward the hard side and polyurethanes toward the flexible side. This behavior difference is usually explained in terms of the solubility parameter theory, because its square is proportional to the cohesive energy density.
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In comparing the values of tensile strengths in MPa and of modulus of elasticity in GPa of polycarbonate and nylon, as in Table 1, it can be seen that they have similar tensile strengths, whereas the modulus of elasticity of the polycarbonate is somewhat greater.
NATURE IS WISE AND CRAFTY
If the tensile strength and modulus of synthetic materials are compared with those of biological materials of roughly similar composition, such as polycarbonate with wood which are both compounds of carbon, hydrogen and oxygen (Table 2), we see that the tensile strength of wood is twice as large and the modulus of elasticity is almost five times as large as that of polycarbonate. Similarly, tendon which is based on polyamides, displays nearly double the tensile strength and about the same modulus of elasticity as Nylon 6. In addition, bone has nearly double the tensile strength and ten times greater the modulus of elasticity. Thus, the properties of the synthetic materials could be dramatically improved. In addition, new theoretical tools are necessary, because conventional ones such as the solubility parameters theory, fail to explain these differences.
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It is in this difference between natural and synthetic materials that we find the most interesting work, not only in coatings technology, but also in science in general.
If we analyze the characteristics of the biological materials, we see that they have (1):
(1) Properties that vary in response to performance requirements, that is, they are able to adapt to external variable conditions; they are intelligent.
(2) Capacity of self-repair: they can regenerate if they are damaged.
(3) Heterogeneous phases, with important and differentiated interphases from materials of very dissimilar characteristics, usually hard and soft such as wood, which is a composition of flexible cellulose and more rigid lignin; skin or tendon, a composition of firm collagen and rubbery elastin, and bone which, in addition, has very hard inorganic inclusions.
(4) Precise hierarchical organization of the heterogeneous phases in a great range of length scales.
Points (1) and (2) are potentially sources of great innovations in coatings as is demonstrated by the increasing interest in smart and self-repairing coatings. Nevertheless, the focus of this article is not on these two very interesting routes of investigation, instead, I will concentrate in the last two properties.
Hierarchical structures" are assemblages of molecular units or their aggregates that are embedded or intertwined with other phases, which in turn are similarly organized at increasing size levels." (2)
If we take a look at the structure of a tree from the scale of Angstroms up to meters as in Figure 6 we see that it starts from parallel polymer chains of cellulose that form amorphous and crystalline dominions. These chains are made up of hemi-cellulose, pectins, and glycoproteins forming microfibrils, that is, crystalline cellulose in an amorphous matrix, with this structure forming part of another larger-scale structure as well. Think about a weave that is forming another weave in a superior instance and, for that reason is hierarchic. Macrofibrils form the cellular walls, which in the dimension of the meter, form the trunk of the tree, etc.
In contrast to this composition of properties conferred by the presence of organized heterogeneous phases in biological materials, the classic technology of coatings has considered homogeneity as the technical ideal of theory and conventional practice. Homogeneity, regularity, uniformity, absolute compatibility, and the like have often been obsessively sought. Coatings had to be as compatible as possible, all the molecules had to be equal, (3) with monomers distributed in the most regular form possible and, in addition, if they were particles of emulsions they had to be as similar in size as possible. The films had to be the most homogenous, everything distributed as uniformly possible, with components of maximum regularity.
THE NEW PERSPECTIVE
The new perspective instead seeks to obtain those extra properties found with biological materials, by creating heterogeneous domains. Even more important, it seeks to organize them in a very precise manner to benefit from interactions: thus achieving properties through synergism that are not obtained from the pure components alone.
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When formulating from the homogeneous perspective of the rule of mixtures, linear behavior is obtained. As is shown below, with a heterogeneous perspective, a nonlinear behavior of interactions, with improved properties, is possible.
THE EFFECT OF USUAL AGGREGATES
As coatings technologists we know that although homogeneity has been almost a compulsion, there is nothing more heterogeneous than a coatings film, not least because of the use of pigments and extenders.
If the modulus of elasticity of a film is measured as the amount of pigment is increased, and the values are plotted against the PVC, a graph similar to Figure 7a is obtained. The modulus increases steeply with increasing PVC, and the increase is steeper as the aspect ratio of the pigment increases (fibrous > lamellar > spherical).
Comparing the stress-strain diagrams of materials with increasing amounts of pigment with that of the unmodified material, as shown in Figure 7b, it can be seen that it is possible for a soft and flexible material to be modified to one with a considerably increased modulus. Even though the modulus is increased, some of the mechanical resistance is conserved until the aggregate amount of pigments and extenders exceeds the critical PVC, beyond which the material becomes brittle. That is to say, there is a way to reinforce the film.
THE EMERGENCE OF NANOTECHNOLOGY
Unfortunately, adding pigments and extenders does not change only the modulus of elasticity. Other properties such as transparency and gloss are also affected as can be seen in Figure 8, following the classic curve of Asbeck and Van Loo. If gloss and transparency are not to be affected, it is necessary to add particles or create heterogeneous dominions that are smaller than 0.1 microns, i.e., of the order of nanometers. On the other hand, as permeability and tendency to corrosion are also increased, it is necessary to reinforce the interface and, more importantly, to organize the heterogeneous phases. In addition, if particles of the order of less than 100 nm are involved, the specific surface is very large, and the increase of viscosity can be substantial. Taken together, these observations suggest that, in order to take advantage of the reinforcement that extenders and pigments produce in a coatings film, a technology of inclusions is required. Since we are dealing with materials of particle sizes on the order of the nanometers, in fact it should be a nanotechnology of the inclusions, and as the materials that compose heterogeneous phases are called composites, then, in fact, we are dealing with nanocomposites. Nanotechnology, the new wave of knowledge, is now considered a new scientific and technological revolution, on a level with some of the greatest innovations.
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THE GREAT VALUE OF INTERACTIONS
Materials engineering, which benefits from properties of traditional materials, characterizes composites in the following way:
(1) They are multi-phase materials, i.e., they are constituted by several phases.
(2) The phases are separated by distinguishable and differentiated interfaces: a continuous phase called a matrix and dispersed phases or inclusions (the added materials).
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(3) The components are chemically and physically dissimilar, for example, highly elastic and highly plastic.
(4) They display properties that cannot be obtained from the single individual components alone; that is, there is interaction between the components.
We can see that composites have nearly the same characteristics as biological materials. In general, composites consist of a matrix or continuous phase and some disperse phase materials added to increase the modulus of elasticity or the resistance or elongation of a material.
In recently emerging technologies, this is achieved through optimization of interactions across phase boundaries. Experts in the design of experiments have emphasized the importance of these synergistic interactions to such a degree that they are considered the "hidden gold" of formulation and they have developed tools to discover them. (4-7)
WHAT FILM REINFORCING REALLY IS AND HOW IT IS ACCOMPLISHED
As we have seen, coatings films, are strong and resistant when they simultaneously display high values of modulus of elasticity and elongation or, equally, high values of the work of rupture. Thus, reinforcing a film involves increasing both properties at the same time. For example, an exterior wood coating or a roofing compound must support the dimensional changes of the substrate and requires high elongations, which are associated, as a general problem of formulation, with softness and poor drying in synthetic systems and blocking in water-based ones. In this case, the modulus of elasticity should be increased. From a classic perspective, however, this is generally done at the expense of elongation loss, as seen in the behavior of the dotted lines shown in Figure 9. The aim of the new perspective is to obtain the behavior of the superior curve or the maximum that can be reached via interaction between the different phases.
HOW MATERIALS SUPPORT STRESS
When a tension is exerted, polymer materials flow, i.e., they become deformed in an irreversible way. This takes place by means of a mechanism which, following much controversy, became the basis for the Nobel Prize awarded to de Gennes in 1991. When the molecular chains in polymeric materials reach a certain critical molecular weight [M.sub.c] (a very interesting situation in itself, because many things happen at this stage, changing properties dramatically), they become as entangled as a plate of spaghetti, as shown in Figure 10. If one end is pulled, the only possible movement for the chain is to slide throughout its length like a reptile, due to the restrictions imposed by the neighboring chains. For this reason this movement is called reptation. In this way material flows and relaxes tensions. So, to increase the modulus of elasticity, the movement of reptation should be restricted. Namely, heterogeneous domains where the reptation is restricted should be created, either by means of chemical bonds that restrict the movement of the polymer chains, by means of the formation of crystalline dominions, by important secondary forces, or by steric hindrance. This is the strategy followed, for example, in rubber technology where particles of carbon black are added to the latex. Of course, carbon black, by its chemical constitution, has a more restricted reptation compared to that of the latex molecules. Similarly, in concrete, sand particles with higher modulus are added to the cement and glass fibers are added to polymers to increase their resistance, that is to say, to reinforce them. It is important to point out that when the strategy of adding particles to reinforce the film is followed, it is necessary to verify that a good interaction between the phases takes place at the interface to ensure an efficient stress transfer.
In fact, today it can be said that all the materials that display the characteristics of being hard and resistant at the same time are composites; it is almost a necessary and sufficient condition.
The opposite case occurs when reinforcing means strengthening or increasing the mechanical resistance or elongation of a film that is hard and brittle as is frequently required in industrial coatings. From a classical perspective, this is done generally at the expense of a loss of hardness and modulus of elasticity. To reach the behavior of the superior curve in Figure 11 through the interaction of phases, the inverse direction should be followed: promoting dominions of favored relaxation, such as through reptation which is an important method, but not the only one. Another mechanism, for instance, called debonding, occurs when certain extenders are dispersed in a rigid polymer. Depending on the nature of the extender and the polymer, the particles separate at the interface when the material is deformed and this mechanism operates as a center of relaxation. Following this strategy, soft particles for example, micronized rubber particles are added to brittle polymers to favor reptation or certain extenders are added to produce debonding.
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TAKING ADVANTAGE OF HETEROGENEITY
We will now concentrate on how the modulus of elasticity is affected when a heterogeneous phase is created in a system. In Figure 12 the values of the modulus of the pure matrix and of the material of the inclusion are represented at both ends of a graph, together with those of the composites as a function of the volume fraction of the inclusion. There are two cases that theoretically appear as limits, within which all the real cases are located.
The first and most common case is a polymer to which inclusions of greater modulus of elasticity are added, as shown on the lower curve. This shows the increase of modulus with the PVC by adding extenders as we saw previously. The curve is a parabola and the modulus of elasticity of the composite depends on those of the pure components and the volumetric fractions of the composite. This limit case corresponds theoretically to the one of particle inclusions that are perfect spheres in a perfectly viscous matrix with a modulus of elasticity higher than that of the continuous phase. This system behaves in the same way as two springs placed in a series using different spring constants. When a tension is exerted it will be the same in both materials but they will deform in different ways. Roughly, one material will function by increasing the modulus and the other by relaxing the tensions that take place.
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The other limit case is that of a matrix of high modulus of elasticity and a dispersed phase of lower modulus of elasticity, added in small amounts--in small volumetric fractions--or the case where the dispersed material is constituted by very long fibers that are perfectly interlaced throughout the material. In this case, the modulus of the composite will depend linearly on the modulus and the volumetric fractions of the components. This model corresponds to that of two springs in parallel. Both materials support the same deformation but different tensions, and the modulus is practically determined by that of the hardest material.
The example of the upper curve of Figure 12 is a maximum desired goal to be obtained. It is the new technological obsession because we can get otherwise unobtainable properties: in the deformation of the more conventional case of the lower limit with a cross-sectional orientation of spring elements, if a crack develops, in general the crack will propagate through the most fragile material all along the thickness of the film. However, in the other case, which is oriented throughout the direction of the tension, the integrity of the film will not be affected even if one of the particles of the inclusion is broken, since there are very many others. In addition, the most rigid material transmits rigidity throughout the film. Here, the new concept of bicontinuity plays a very important role, resembling the ideal of a laminar film of parallel layers just mentioned. As seen in Figure 13, it is theoretically possible to design a material that can be continuous in two constituent phases at the same time. Again, if a section is broken, the integrity is not affected and, in addition, the tensions relax locally in the most plastic material, with greater possibilities of reptation. What is also very important is that, in addition, much higher chemical resistances can be obtained, since the material of greater modulus is longitudinally continuous and creates a barrier to the passage of damaging materials.
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THE PROPERTIES OF COMPOSITES
The properties of composites depend on their constituent phases, their modulus of elasticity, their elongation or relaxation contributions, the relative amounts of matrix and dispersed phases, and, particularly important, the geometry of the dispersed phases: the size, form (spherical, fibrous, laminar, etc.), distribution and orientation of the inclusions, and characteristics of the interface. The distribution and orientation are of vital importance because, depending on how they are organized, the maximum possible behavior of the superior limit will be reached.
There are four types of materials: ceramics, metals, polymers, and composites. Ceramics are all the inorganic materials that are not metals. It can be seen in Figure 14 that ceramics display the highest values of modulus of elasticity, a little greater than those of metals. Both have a modulus of elasticity considerably greater than polymers, whose values extend from [10.sup.-3] GPa to 10 GPa, while ceramics and metals may have modulus from the highest levels of polymers to values 100 times greater. The resulting composites have properties that approach those of materials with greater modulus, generally at lower costs.
NEW TECHNOLOGICAL STRATEGIES IN COATINGS
The new strategies to improve the properties of coatings films consist of producing microphases either with ceramic particles or between polymer particles. Although much work has been done with metallic inclusions to take advantage of their electrical and magnetic properties, here we will concentrate specifically on non-metallic phases.
One strategy of creating heterogeneous domains can be the one that, in principle, is the task of our technology and that we would all initially follow--physical inclusion, or adding another material.
Another strategy is to begin with a completely homogenous material and produce the heterogeneity during the drying process, i.e., to produce a phase separation.
The third one, which could be called structuration, is a mixed strategy, that is to say, beginning with a certain constitution of phases that are additionally organized during the drying of the film.
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THE INCLUSION OF CERAMICS
Consider some examples of the first strategy--including ceramic particles. On one hand, we have the case of a physical inclusion as with the employment of hydrosols. They are water dispersions of ceramic particles, usually silica, of size ranging from 10-60 nm. Due to the size of the particles and the exposed surface, the most difficult point of the technology is dispersion stability. For the dispersion to remain stable, an electrostatic stabilization should be established; particles should be charged, usually negatively. This technology is used, for example, to modify latex systems to significantly increase their chemical resistance, washability, and hardness. When the film dries, the emulsion coalesces and the ceramic particles lose stability since the pH drops. In this way, they form inorganic domains within the coalesced organic domains.
In the second strategy, we start off with a completely homogenous system and produce heterogeneity during drying. In this case, for instance, organic-inorganic hybrid molecules are used: part of the molecule is organic and part is inorganic. They are called ceramers because hybrid materials, partly ceramic and partly polymeric, are formed during drying. This type of hybrid molecule can be reacted either with oligomers in the organic part or with more inorganic precursors. During cure, inorganic and organic heterogeneous domains are formed by the reaction of the corresponding parts. By precisely organizing the structures formed in the scheme previously mentioned, very high hardness and abrasion resistance could be obtained because, according to the relative amount of the components, a reinforced polymer or a very resistant glass is produced. With this technology it is possible to achieve in relatively low temperature processes properties resembling those of tiles or, possibly, superior properties because the materials could have even greater mechanical resistance. Another intermediate strategy is nano-encapsulation, a very new process of structuration that was recently introduced internationally. When trying to include ceramic particles as large as 10 to 60 nm, for example of silica, in an organic matrix, we find a limit in viscosity due to the size and energy of the exposed surface of particles. This problem was solved by reacting the surface of the silica with an organic part that can then be cured with acrylic oligomers via UV. In this way, abrasion resistance can be multiplied by 10 times. Through the encapsulation of particles up to 30% of inorganic modification could be added. It is important to keep in mind the vital importance that organization has in this new perspective.
THE CONSTRUCTION OF PHASES IN POLYMERS
Diverse strategies also exist for nanocomposites of heterogeneous polymeric domains. One of them is the blending of different polymers and/or latex. The constituents could be unimodal, (of different nature but with similar particle sizes) or polymodal, (mixtures with different modulus of elasticity and different sizes). The latter is the most interesting one because it allows a better organization of phases, and consequently, better properties and results. An example of this strategy is the impressive work by Eckersley and Helmer (8) that received the 1996 Roon Award. Their work demonstrated, as can be seen in Table 3, that when formulating from the perspective of a conventional homogenous emulsion, the MFFT (minimum film formation temperature) of the latex must be around room temperature to get the required tensile strength and elongation. The best blocking resistance that is possible to be obtained is a not very good, and in addition, coalescing solvents are needed. Alternatives, such as UV curing, for example, are expensive.
They saw that by maintaining the global composition of this polymer but separating it into two different polymers--a very soft one of MFFT of 1[degrees]C and a very hard one with a [T.sub.g] of 62[degrees]C but with very different particle sizes they could obtain films with similar mechanical properties. However, these films would have an MFFT of 6[degrees]C with the best block resistance ratings, no loss of tensile strength, and improved elongation, as is seen in Table 2.
The difference in size results, after coalescence, in a more suitable type of organization which can be studied by means of the percolation theory. Percolation theory studies how dispersed systems formed by a random process interconnect geometrically.
The percolation theory has been fundamental, among other things, in the new theories on the critical PVC that were published by Floyd and Holsworth (9) years ago in a very influential paper. This is a route that, as we see, is tending more and more toward the ideal organization, one of bicontinuity.
Another strategy is one of structured dispersions, where the dispersed globules already contain a certain heterogeneity of phases which form microdomains distributed within the globule: on the periphery, at random, the well-known core-shell and inverted core-shell, etc. so as to have, after coalescence, a precise distribution of phases which produces a pre-determined interaction of properties. For instance, if we observe a microphotograph of a coalesced film of a core-shell that has a soft core of polybutyl acrylate and a hard shell of polymethyl methacrylate, we see that, here also, we are slowly approaching the previously mentioned ideal of bicontinuity. What it is indeed necessary to do is to create hard domains and domains of relaxation and to produce such an organization that will take maximum advantage of interactions.
Another method involves organic-organic hybrids such as interpenetrating polymer networks (IPN) where, two networks are interlaced in such a way that reptation is practically impossible (Figure 15) and the displacement of networks that can be made is minimal. In this case, as shown in Figure 16, the effect of the interaction is such that the superior limit is even surpassed, because the modulus of elasticity obtained by interpenetrating, for instance, a polyurethane network with an epoxy one, still increases much more with respect to that of the linear relation between the pure materials. In addition, here it is possible to obtain compatible polymer compositions that would not normally be possible by direct blending. This is obtained by means of several techniques; one of them is to produce a polymer and then to impregnate it with monomers that react only with themselves so that the structure of the diagram is formed. This is used mainly in industrial systems where conventional polymers are developed that continue to crosslink while baking and the properties of the components are, in this way, greatly improved through the creation of microdomains with minimum reptation. A related structure can be seen with the semi-interpenetrating polymer networks (SIPN) where there is a three-dimensional crosslinked network that is impregnated with thermoplastic polymer molecules as is seen at the bottom of Figure 15. These are more common and many hybrids of this type are now available on the market, as for instance, the polyurethane-acrylic materials that have a polyurethane structure impregnated with acrylic molecules, and may even form an acrylic shell able to coalesce. This results in a film that is reinforced through centers of higher modulus and very restricted reptation. With this, the properties of polyurethanes are obtained, but at prices approaching those of acrylics.
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Another approach focusus on hyperbranched and dendritic polymers or dendrimers that can have the forms shown in Figure 17. Branching is obtained by successive reactions that produce different layers that can be either in the form of a branch or have a more symmetrical distribution. The surface of these molecules has very different characteristics from the interior, for example, the interior has lower density because most of the material is accumulated in the surface. The reptation is also very restricted here. This technology is used to modify systems for powder coatings that offer considerably lower process temperatures and improved properties. They display such valuable properties as a very low viscosity, due to a low chain entanglement, and high crosslink density due to the large amount of functional groups in the periphery of the molecule.
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From this perspective the possibility of manipulating the construction of heterogeneous materials to achieve ideal properties is practically infinite. For example, we can employ multiple strategies like the ones described to simultaneously produce hard crystalline structures of tubular or fibrous types interlaced with soft amorphous regions. Such complex structures tend toward the hierarchical and complex organizations of biological materials.
The strategy of organizing heterogeneity is slowly expanding: not only in the sense of discontinuity, to exploit all the possibilities of interactions of phases, but also in the sense of irregularity as, for instance, that of the distribution of monomers in a polymer chain, as in block copolymers, with superior properties that can be composed by varying the monomers and blocks types, the volume fractions, the molecular weights, etc. and in the sense of composing nonuniformity, as the bimodality of molecular weight or particle size distributions.
A NEW TECHNOLOGICAL PARADIGM
What these new technologies have in common is a new way to see problems. In effect they constitute a new paradigm in the sense of the concept developed by Thomas Kuhn. (10) A paradigm is the set of theories and methods established and shared by scientists and their institutions that determines what the problems are and how they must be understood. If we compare the new and the old paradigms of our industry, we would see that with the classic technology we follow the logic of the compound, in the sense of a chemical compound, that has an identity given by a certain determined structure, from the perspective of homogeneity. If discontinuity exists, the interface is regarded merely as a surface of separation between the phases. Nanotechnology, however, is a new perspective that is much more than just a nano-scale: it is nano-scale plus organization. Much of the efforts arising in the research centers and the companies are now led by the logic of the composite, of the interaction of phases, that contributes to composable properties and interaction of variables rather than the more restricted one of the compound. This perspective involves heterogeneity, where the interface is not merely a surface of separation between the phases, but rather it is a phase in its own right, so that two concepts phase boundary are needed, interface and interphase. We have seen the importance of the interphase when we spoke of debonding; there is another related concept, transcrystallinity, which will be mentioned below.
NEW AND OLD TOOLS
The classical tools and assumptions used in our technology including, for example, conventional differential equations, as employed in the Fick laws, the equations of Kubelka-Munk, and the laws of thermodynamics as used in the solubility parameters, adhesion and wetting theories, are limited by the range of validity of the assumptions from which they arise. This does not mean they are not valid. For example, for a problem to be treated via differential equations in the classical way it is necessary to deal with functions that are continuous and infinitely differentiable, that is to say, that the dynamics are the same throughout the entire space and throughout the entire process which, obviously, is rarely the case in a drying film or even in a can. (11) In addition, the laws of thermodynamics and those derived from them offer statistical descriptions. Even the molecular branch of thermodynamics, i.e., statistical thermodynamics, assumes a statistical description and the differentiation and specificity of interactions are lost. For example, in the theory of solubility parameters the contribution from all the functional groups is averaged equally throughout the molecule. Thus it is effectively assumed that the molecule as a whole interacts with another molecule as another whole. What is lost with this is the possibility of describing the locality of organization and preferential interactions. Today, many new tools are complementing the classical ones such as iterative laws, collections of trajectories, percolation theory, etc.
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NANOTECHNOLOGIST WITHOUT KNOWING IT
This new paradigm, as theoretical as it might seem, is strong and valuable and is not only a theory to decorate the universities. It has great practical utility, and, if we take a closer look at the well-known technologies that we have been using, in some ways we have been like Moliere's character, nanotechnologists without knowing it--we have been applying some principles of this paradigm unconsciously. In other words, the theory does not always come up to the level of the experimentation. An example is the plastic pigments made from styrene emulsions with a very high [T.sub.g], (around 100[degrees]C). These pigments were used, and in some cases still are, to improve the hiding power and the washability of latex coatings. What, unfortunately, was not always realized was that the modulus of elasticity could also be increased and the MFFT and the tendency to the blocking could also be improved. Most important, the coalescent system that was added may have destroyed heterogeneity that was valuable to maintain. Also, the possibility of geometrical organization of phases, as mentioned previously in the case of polymodality, was disregarded.
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Another daily example is one of high polymer alkyds. By the 1960s, around the same time in which the plastic pigments were developed, Bobalek et al. published a work in the Journal of Applied Polymer Science. The authors, by means of a special process that they called high polymers, obtained alkyds that were much more condensed and featured important reductions of drying times and higher hardness. In addition, as verified in Table 4, the resistance to sodium hydroxide, the delamination time, increased 24x which confirms that an increase of the modulus of elasticity not only improves the mechanical properties but also the chemical resistance. According to the new paradigm's perspective what could be happening was that dispersed microgels were obtained (in fact, nano-gels) which, since they were very condensed, had a more restricted reptation, and thus produced the observed effects.
Finally, where we have been nanotechnologists in the most unconscious way possible is in the case of self-assembly (or liquid crystalline) polymers, which develop heterogeneity and liquid crystalline organization by themselves. In the microphotograph on the left of Figure 18, a crystalline formation of spherical symmetry can be observed, which is called spherulitic geometry or structure. In the diagram on the right, a scheme of that organization is shown. It occurs frequently in polyurethanes, since the urethane groups have the interactivity and the spatial configuration to generate the aggregation of functional groups to form a kind of microfibril whose modulus of elasticity is considerably greater than the most flexible part of the molecule (e.g., polyethyleneglycol). In this case, there are crystalline regions in an amorphous matrix and the film is crossed by a network of microfibrils in such a way that if we stretch the film there is greater resistance than might otherwise be expected due to the reinforcing action of those microfibrils. If some break, the most flexible segments relax the tension by reptation, showing a behavior more in line with the upper limit. This type of organization accounts for the fact that polyurethane technologies, even waterborne ones, can reach hardnesses of more than 2H together with elongations higher than 300%.
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Besides the formation of crystalline structures in the mass, there is another phenomenon that could be used to regulate properties even more--that of transcrystallinity. In the microphotograph of Figure 19, it is possible to compare the spherulitic crystallinity in the mass and the different one that takes place next to the fibers, separating the two phases. This phenomenon, which can be promoted or eliminated, can modify properties enormously.
Science, as well as technology, is entering a new dimension of complexity. (12) Rather than assuming a simple, homogeneous reality that can be described by a simple trajectory, the model of essentially heterogeneous and adaptable complex biological systems is showing its value in producing more useful materials. As never before, breathtaking innovations are appearing in the coatings technology by taking advantage of the very simple concepts that we have described.
The old perspective, although it proved useful in the past, might now operate as what Gaston Bachelard (13) called an "epistemological obstacle" to a better understanding of the complexity of the coatings phenomena.
We have to be prepared with new concepts and tools to be able to uncover the simple but very effective benefits that heterogeneity and interactions can give.
Table 1--Strength and Modulus of Synthetic Materials Tensile Strength (MPa) Modulus (GPa) Polycarbonate 64 2.3 Nylon 6 70 1.8 Table 2--Strength and Modulus of Biological Materials Tensile Strength (MPa) Modulus (GPa) Wood 130 11 Tendon 125 1.8 Bone 135 18 Table 3--Advantages of Heterogeneity in Latex Formulation (a) MFFT ([degrees]C) Blocking TS (psi) Elongation % Homogeneous 25 7 1200 290 SL70/HS30 3 8 1070 510 SL60/HS40 6 9-10 1050 390 SL50/HS50 7 10 820 270 (a) Taken from reference 8, S.T. Eckersley and B.J. Helmer, 1997, by permission of the FSCT. Table 4--Comparative Properties of Conventional vs High Polymer Alkyds Conventional Alkyd High Polymer Alkyd Dust free time 1:20 0:15 Tack free time 3:10 1:10 Sward hardness (14 days) 14 20 NaOH resistance (denude time) 2:00 48:00 (a) Reprinted from J. Applied Polym. Sci., Bobalek et al., 8, 645 (1964) with permission of John Wiley & Sons, Inc.
(1) Tirrell, D.A., Aksay, I., Baer, E., Calvert, P.D., Cappelllo, J., Dimarzio, E.A., Evans, E.A., Fessler, J.H., Hoffman, J.D., Jaffe, M., Mayer, G., Mow, V.C., Waiwright, S.A., "Hierarchical Structures in Biology as a Guide for New Materials Technology," Committee on Synthetic Hierarchical Structures, National Materials Advisory Board, National Research Council, National Academy Press, Washington D.C., 1994, p. 18.
(2) Ibid., p. 1.
(3) Jones, F.N., "Targets for Resin Development in the New Millennium," JOURNAL OF COATINGS TECHNOLOGY, 72, No. 909, 153 (2000).
(4) Anderson, M.J. and Whitcomb, P.J., "Optimization of Paint Formulations Made Easy with Computer-Aided Design of Experiments for Mixtures," JOURNAL OF COATINGS TECHNOLOGY, 68, No. 855, 75 (1996).
(5) Whitcomb, P., "Computer-Aided Tools for Optimal Mixture Design," Paint & Coatings Ind., November 1999.
(6) Anderson, M. and Whitcomb, P., "DOE Simplified, Practical Tools for Effective Experimentation," Productivity Inc., 2000.
(7) Anderson, M.J. and Whitcomb, P.J., "Computer-Aided Design Tools for Optimal Mixture," Paint & Coatings Ind., November 2001.
(8) Eckersley, S.T. and Helmer, B.J., "Mechanistic Considerations of Particle Size Effects on Film Properties of Hard/Soft Latex Blends," JOURNAL OF COATINGS TECHNOLOGY, 69, No. 864, 97 (1997).
(9) Floyd, F.L. and Holsworth, R.M., "CPVC as Point of Phase Inversion in Latex Paints," JOURNAL OF COATINGS TECHNOLOGY, 64, No. 806, 65 (1992).
(10) Kuhn, T., The Structure of Scientific Revolutions, University of Chicago Press, 1962.
(11) Nicolis, G. and Prigogine, I., "Exploring Complexity," W.H. Freeman & Co, 1989, especially [section]6.2, Materials Science, pp. 219-223.
(12) It is interesting to mention that well-known, common phenomenon of our technology, Benard cells formation, is one of the most paradigmatic examples of the concept of complexity of the new physics.
(13) Bachelard, G., The Formation of the Scientific Mind, Clinamen Press Ltd., 2002.
by Abel Kivilevich*
Presented at the SATER Congress, Buenos Aires, Argentina, November 2002.
*Products Design and Development Consultant, Neuquen 504-5D-11, 1405 Buenos Aires, Argentina; firstname.lastname@example.org.
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