Water-based film formation.
Initially water and cosolvent evaporate during what is known as the wet stage. Here, the latex particles and any pigment particles become increasingly concentrated and go from being well separated to eventually touching. The controlling factors are temperature, relative humidity, volatility of the solvent, and activity coefficient of the solvent(s) in the water. At some point, the solids increase enough to "touch" and latex particle compaction and deformation occurs. At this stage, all of the remaining solvent has been forced into the polymeric binder, softening it. In a very real and measurable sense, the[T.sub.g] of the film has been suppressed and lowered from the [T.sub.g] of the "dry" polymer. The dynamics of the solvent loss from the developing film, and the subsequent increase in [T.sub.g]with time, is described in the paper previously mentioned.
The [T.sub.g] of the solvent is very important and is summarized in Table 1. Water in the polymer will lower the apparent [T.sub.g] of the polymer and aid in film formation. Since the [T.sub.g] of water cannot be measured, the clear to cloudy transition on a minimum film forming temperature (MFFT) bar is used. "It is hypothesized that this transition also closely corresponds to the point where particle deformation is reasonably complete. A reduction of the MFFT below the of the [T.sub.g] polymer is due to the hydroplasticization effect of water." (1) Hoy modified the Fox-Flory Equation (2) to take into account the effect of both film forming aids and water on the T[.sub.g] of a polymer. In Hoy's treatment, the volume fraction of the solvent con tributes to the [T.sub.g] (by lowering it), just as any soft mono mer does to the polymer [T.sub.g]. The noticeable difference is that as the solvent leaves the film, the [T.sub.g] rises with time.
Although cosolvents are used in coatings to lower the MFFT and aid in film formation, they are volatile. This is important as water evaporation is both temperature and humidity dependant, while cosolvent evaporation is primarily temperature dependant. In high humidity conditions, water can evaporate slower than cosolvents, leading to poor film formation.
To maximize the properties of a film (hardness, chemical resistance, water resistance, etc.) all of the cosolvent needs to leave the film. Basically, Taylor and Klots have shown that there are two primary mechanisms by which solvent leaves a film after compaction and deformation. Initially, diffusion of solvent through the film is fast (because the overall [T.sub.g] is low compared to the ambient temperature). The rate limiting step is the evaporation of the cosolvent from the surface of the film. The rate of solvent loss is therefore still volatility driven, i.e., faster evaporating solvents still continue to come out faster than slower evaporating solvents. Air velocity still impacts the mass transfer of solvent to the air. Eventually, however, the [T.sub.g]rises enough that the diffusion of the solvent through the film becomes the rate limiting step. The final dry [T.sub.g] of the polymer and the air temperature play the main roles in determining how much solvent remains in the film at that point. Any remaining solvent leaves by a diffusion-controlled mechanism. Air velocity would not affect the solvent loss during this latter diffusion-controlled state.
Importantly, during this whole time, polymer chains have been inter-diffusing, leading to the development of a mechanically coherent film. Taylor and Klots postulated that if the cosolvent lowered [T.sub.g] of the polymer was more than 50[degrees]C below ambient temperature, that "interparticle chain diffusion is essentially complete before the onset of diffusion control." (1) Not every application, like architectural coatings, needs full inter-diffusion in order for absolute property development.
We thank J.W. Taylor and T.D. Klots for their article and recommend reading it for further knowledge.
(1.) Taylor, J.W.and Klots, T.D., "An Applied Approach to Film Formation: The Glass Transition Temperature Evolution of Plasticized Latex Films," international Waterbome, High Solids, and Powder Coatings Symposium Proc., 2002.
(2.) Hoy, K.J., "Estimating Effectiveness of Latex Coalescing Aids," J. Paint Technol., 45 (579), 51-56 (1973).
Table 1-Solvent Properties Solvent Relative Evaporation [T.sub.g] Rate n-butyl actetate=1 ([degrees]C) Ethylene glycol 0.078 -124 n-butyl ether (EB) Diethylene glycol 0.0035 -108 n-butyl ether (DB) Propylene glycol 0.213 -118 n-propyl ether(PnP) Propylene glycol 0.09 -116 n-butyl ether(PnB) Dipropylene glycol 0.012 -104 n-propyl ether(DPnP) Dipropylene glycol 0.0050 -104 n-butyl ether (DPnB) Propylene glycol 0.0012 -96 phenyl ether(PPh) Butyoxy ethyl acetate 0.03 -112 (EB Acetate) Dipropylene glycol 0.03 -106 methyl ether (DPM) Butyoxy ethoxy ethyl 0.002 -100 acetate (DB acetate) Dibutyl phthalate 0.000008 -87 Ester of 2,2,4 trimethyl 0.00165 -84 1,3 pentane diol (Texanol[TM]) (a) (a) Texanol is a registered trademark of Eastman Chemical Company.
By Mike Praw and Timothy D. Klots, BASF Corporation
Mike Praw and Timothy K/ots, BASF Corporation; firstname.lastname@example.org
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|Title Annotation:||FORMULATOR'S CORNER|
|Author:||Praw, Mike; Klots, Timothy D.|
|Date:||May 1, 2011|
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