Carlos A. Giudice and Andrea M. Pereyra*
In the first instance it is appropriate to analyze what happens when an unlimited amount of binder is systematically added to a limited amount of dry pigment powder: in the initial system the air occupies the interstices between the pigment particles and in the final condition only the binder is present.
These two extreme points represent the range of the PVC scale (pigment concentration by volume): 100% for the pigment alone and 0% for the binder only. Between these two extreme situations (all pigment and all binder) it is obvious that a first stage is observed in which the binder progressively displaces air from the interstices between the particles, existing pigment, binder and air.
Finally, a point is reached where the air is absolutely dislodged and the binder completely completes the empty spaces between the densely arranged particles. This single point is called critical volume pigment concentration (CPVC).
A subsequent incorporation of binder leads to a second stage in which the pigment particles are separated from each other (the initial dense packing and therefore the consequent contact between particles disappears), with an increasing distance as more binder is incorporated. Finally, and in the final state, only the binder is fundamentally present.
PVC values lower than CPVC ensure that the binder occupies all interstices but values higher than the aforementioned generate empty spaces that are occupied by air (the binder is insufficient). Certainly, the value at which these dramatic changes take place is called critical since the properties of the system, particularly the dry film state, are abruptly modified.
In the case of emulsion paints, the mechanism by which a wet film evolves to the dry state is totally different from that presented by solvent-based paints. The latter makes it necessary to introduce a modified concept of CPVC for latices, Figures 1 and 2.
The dispersion of pigment in emulsions and the way in which the drying stage occurs are unique and different from those of the solvent type. Emulsions are not solutions of a binder in a solvent but concentrated suspensions of discrete, spherical, relatively sticky particles of a resinous material in water. Drying a latex (removal of water) leads to resinous particles adhering strongly to each other due to increasing surface tension due to the decrease in distances between particles of the copolymer. Water loss can happen either by its evaporation into the atmosphere and/or by the absorption of a porous substrate.
The shrinkage of the film involved in this stage generates coalescence (cold melting of the resinous particles) around the pigment/charge particles, which leads to a more or less densely packed arrangement. Pigment compaction to achieve a high CPVC value is achieved mainly by plastic deformation of the copolymer particles, usually assisted by a coalescing agent.
The CPVC value of a latex should be adjusted to as high a percentage level as possible, as this implies a significant binding power of polymer dispersion and consequently a significant reduction in costs.
In the case of solvent-based paints, the vehicle (liquid solution) surrounds the pigment particles dispersed in the paint; during drying the system becomes more viscous and the binder flows around the particles during almost all this stage; a significant volumetric contraction of the film is observed.
Latice CPVC: influence of variables
Glass transition temperature Tg
The ability of emulsions to form film depends on the thermoplastic properties of the copolymer. Coalescence takes place only at temperatures above the Tg glass transition temperature. At temperatures below the value of Tg, the polymer is in a vitreous state, so coalescence is not possible. For this reason, hard and soft polymers have, respectively, high and low temperatures Tg.
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The copolymerization of hard and soft monomers allows to select the desired Tg value. However, low values, although they allow coalescence without any auxiliary means, generate films of reduced mechanical resistance (wet abrasion, washability, etc.); the latter improves with the increase of the content of the emulsion in the formulation but increasing the costs in parallel.
The Tg values of the emulsion are usually indicated for non-pigmented polymer dispersions; the incorporation of pigments and fillers, depending on each formulation, increases the glass transition temperature.
The influence of Tg on CPVC for latices is as follows: hard polymers have lower critical pigment level values than soft polymers, as the latter have greater plastic deformation and are therefore more adept at forming a more densely compacted film.
Average particle size
The small latex particles move more easily than those of larger diameter and also reach, in the state of coalescence, a greater contact between them leading to form a more densely packed continuous medium (the dispersed phase corresponds to the pigment and charge particles). In summary, smaller diameter polymer particles exhibit a greater ability to penetrate the interstices of pigment/charge system particles, leading to higher CPVC formulations.
This last property is also manifested when latex is applied both on aged paint films that have a slight chalking and on porous substrates; as a result, latices based on small polymer dispersions promote better adhesion and also generate brighter films. However, it should be mentioned that in critical situations (surfaces with abundant chalking) it is common to mix outdoor latex with 10/30% of an alkyd vehicle to ensure good penetration and therefore satisfactory adhesion to the substrate.
The chemical nature and particularly the average size of the polymeric particles, for similar compositions of the remaining components, can influence the value of the CPVC up to about 15 percentage points.
Efficiency and porosity of binders
The CPVC for latex paints is always lower than that for equivalent solvent-based paints. This means that a larger volume of latex polymers (Vlatex) than the corresponding volume of soluble resin a solvent (Vsol) is required to fill the interstices of a given amount of pigment/charge arranged in a dense packing.
The efficiency of the binder is usually defined by the Vsol / Vlatex ratio and is always less than the unit (efficiency 1 corresponds to flax oil). For its part, the porosity index represents the missing binder fraction in the interstices of the pigment/charge particles; the binder fraction in the empty space for a given PVC value (higher than the critical) is lower for a latex than for a soluble resin since the latex film is more porous.
The glass transition temperature of the copolymer, the average size of the polymer particles and the type and quantity of coalescing agent together influence the efficiency and porosity of the binders.
Coalescing agents
Polymer dispersions with high Tg need external plasticization for the homogeneous formation of the film; A suitable coalescing agent is essentially a transient plasticizer that facilitates plastic flow and elastic deformation of latex particles. Non-volatile plasticizers provide a permanent effect, but like soft polymers, they decrease the mechanical properties of the film. In short, a coalescing agent improves film formation over a wide temperature range and prevents the presence in the final film of soft polymer particles.
The effectiveness of coalescing agents can be very varied; in general, it depends fundamentally on the ability to superficially dissolve the polymer particles, the rapid evaporation after the coalescence is finished and the null or negligible influence on the stability of the emulsion. The optimal type and amount should be determined in laboratory tests. Polymer dispersions with increasing Tg require higher percentage levels of coalescing agent to achieve maximum CPVC.
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However, an excess of coalescing agent leads to lower CPVC values, presumably due to premature coalescence due to excessive softening of the polymer particles that promotes the formation of aggregates and consequently a porous structure.
Influence of pigments
The type of pigment and in many cases those of the same chemical nature that were made with different surface treatments have a strong influence on CPVC. The pigment/charge ratio should not be less than 30/70; for outdoors, the high proportions of loads (reduced refractive index) produce a photochemical discoloration of the binder and then, in many cases, also an excessive chalking.
In general, and considering the dry film thicknesses per layer of latex that are usually applied, pigments with a higher oil absorption rate (smaller average particle diameter) lead to lower CPVC values for a type and content of charges.
The influence of the type of pigment, the surface treatment and the average size of the particles of the same, for a given type of load, can influence up to 10 points on the critical value of the PVC.
Influence of loads
The formulation of emulsion-type paints generally includes mixtures of loads, which must be optimized from a technical and economic point of view.
Nodular loads (natural and precipitated calcium carbonate, dolomite, etc.) have a strong influence on the position of the CPVC; the decrease in particle size (increase in the specific area and consequently in the oil absorption rate) leads to a decrease in CPVC. This can vary up to 15% or more depending on the type and chemical nature of the charge.
Extremely fine particle loads lead to very low critical PVC values; in practice they are not used alone or in large quantities due to their tendency to form cracks and porosity too high.
Completely different results are observed with a laminar charge (micaceous iron oxide, talc, mica, kaolin, etc.); it increases the value of CPVC perhaps because it distributes pigments efficiently and provides very dense compaction. In addition, the presence of overlapping sheets on the surface of the film is likely. In this case, a high oil absorption value leads to reduced CPVC values.
The loads, because they occupy the first place from the quantitative point of view in an emulsion type paint, are of vital importance to bring the CPVC to a high value and consequently to formulate the product with a PVC at a sufficient distance from the CPVC.
Position of the CPVC
The usual façade paints in the trade are formulated with very varied PVC values; many of them exceed the critical value and consequently significantly increase the risk of crack formation. The laboratory evaluation of the aforementioned critical value is therefore of strong interest.
The position of the CPVC can be determined through different properties of the dry film such as density, adhesion, porosity, blistering, wet abrasion resistance, gloss, covering power, etc. In general, the records of the property studied are plotted according to the values of the PVC. The experimental determination of the CPVC in some cases, is carried out by adjusting the curves to a polynomial of the form y = A+Bx+Cx2+Dx3 by means of a regression method and calculating the root of the second-order derivative (inflection point). In other cases, tangents are drawn to the curves, in the adjacencies of the CPVC, resulting in the intersection of the same the critical value estimated according to the property considered.
Cracks in the film
The cracks considered in this article correspond to those that form during the stage of film drying ("mud cracking" or "cracking") and therefore the cracks that may appear after exposure to the weather and the aging of the films are not involved.
Drying a latex (water removal) leads to an approximation of the pigment particles by concentration of the system components; these spaces act as r-radius capillaries. The latter generates the excess pressure P, coming from the surface tension (P = 2 g / r). In CPVC, the excess remaining pressure is maximal and responsible for the formation of cracks in the film.
To evaluate the formation of cracks, deaerated (bubble-free) latex paint is applied on a sheet of steel with a suitable device that allows to obtain increasing thicknesses of film. After drying under laboratory conditions (20 5 ºC and 60 10 % relative humidity) for 48 hours, with the help of a magnifying glass the front or boundary line in which the cracks appear is determined; a dry film thickness gauge allows you to define the corresponding value in that area.
Permissible limits are based on practical criteria; so for example in latex paints for interiors a satisfactory value is 400 m while for exteriors it is 800 m; cracks produced in thicknesses greater than those mentioned are considered acceptable, Figures 3 and 4. {mospagebreak}
Formulation: crack removal
The formation of cracks in the films of paints based on polymer dispersions is due to the fact that during the drying of the same (coalescence) tensions are generated spontaneously of different magnitude. The latter depends on the formulation variables, the raw materials used and the way the paints are made (dispersion efficiency).
For experimental visualization of film stresses, paints should be applied on thin polyvinyl chloride substrates in different thicknesses, increasing depending on the estimated stress.
After the application is finished, the panels must be trimmed to eliminate uncoated areas or areas with variable thicknesses. These panels are arranged horizontally on a smooth surface, in an environment with controlled humidity and temperature.
After a certain time (during the coalescence phase), the panels usually begin to curve in the direction of their transverse axis and then are arranged on one of their longitudinal edges. The thickness of the film and its formation speed are significant variables for the time in which the deformation of the panel develops, while the radius of curvature depends fundamentally on the residual stress of the dry film.
This method for the determination of the stress of the film is also suitable for the evaluation of the CPVC: the tension (curvature of the film) presents a maximum in the critical value of the PVC, a gradual decline is observed for both increasing and decreasing values of the latter.
Consequently, high film stress can lead to cracks, even for low thicknesses, in paints formulated with PVC values slightly lower and higher than the critical one, in addition to those located in the area of the aforementioned value.
The elimination of cracks or rather that they are formed in very high film thicknesses (high quality products), is achieved with very high CPVC paints in order to formulate latices with PVC values located prudently distant from the critical point but equally high (economic paints).
Products with significant CPVC are obtained considering the aforementioned variables: glass transition temperature of the emulsion, average size of the polymer particles, efficiency and porosity of the binders, type and quantity of coalescing agents and characteristics of pigments and fillers (specific area, nature of the surface, size and shape of the associated particles after dispersion, etc.).
The optimal formulation of emulsion paints allows to achieve simultaneously a high efficiency and economy. High quality paints for interiors exhibit low gloss (matte), high dry and wet covering power, ease of painting, perfect leveling without runoff, very good resistance to wet abrasion and no tendency to the formation of cracks, among other important properties. Experiences indicate that by an adequate selection of raw materials in terms of type and percentage level in the composition, CPVC can be increased to high values (for example 60%) and therefore also PVC (20% above the critical value) Thus it is possible to reduce costs without affecting quality: crack formation usually takes place at dry film thicknesses significantly above 400 m.
On the other hand, the economical emulsion type paints for interiors have a great commercial presence, they are formulated with low pigmentation (7 / 8% by weight) but with high PVC which forces to adjust the CPVC to a value as high as possible. Frequent formulations have a CPVC close to 60% and a PVC of approximately 85% (the separation is then approximately 25%). The formation of cracks of these extremely economical paints is observed at acceptable values (above 400 m), with good resistance to wet abrasion and even acceptable covering power.
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With regard to paints for facades, these are distinguished by their excellent weathering behavior, by their high resistance to wet abrasion and by not forming cracks in film thicknesses less than 800 m. Overall, the solids content is higher than indoors (approximately 60%). The CPVC can reach a value close to 65% and therefore the PVC can oscillate in 55%, that is, a difference of approximately 10% with which the film tension is reduced and the possibility of crack formation is minimal at high thicknesses. These LOWER PVC values than those cited for indoor latices lead to a higher percentage level of binder in the formulation in order to give it greater resistance to the film but at a higher cost.
Thanks
The author thanks CONICET (National Council for Scientific and Technical Research) and UTN-FRLP (Universidad Tecnológica Nacional Facultad Regional La Plata).
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*Cidepint. [email protected]
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