Keywords

1 Introduction

Confinement effects due to the presence of hard nanoparticles in a polymer matrix have a profound effect on polymer \({T}_{g}\)[1]. So far the nano-confinement effect has been studied from casting films out of a polymer solution in the presence of inorganic nanoparticles [1]. The interactions between the polymer chain segments and/or the end groups of the chain play an important role, leading to either an increase (for interaction) or a decrease in (repulsive forces) Tg. We demonstrate the effect of end groups in determining the Tg via a simple equation to work out the concentration of nanoparticles for maximum \({T}_{g}\) in a composite for a given polymer. Using polystyrene-silica system with various end groups and polyvinyl alcohol-silica nanocomposite with interacting groups present throughout the polymer chain, we demonstrate that loading of nanoparticles for maximum \({T}_{g}\) is governed mostly by interactions of chain ends with the nanoparticle surface.

2 Methods

For the polyvinyl alcohol system with silica we used aqueous solution of PVA of different molecular weights and cast films by drying For the polystyrene-silica combinations we used solutions in THF to cast the film [2]. We also studied whether similar Tg jumps can also be observed in latex systems. For the latex systems we used composite of latex and silica and analyzed the Tg using Dynamic Mechanical Analysis and Differential Scanning Calorimetry. Finally, a recipe for water-borne coatings that introduces nano-silica particles in the formulation according to the experimental composition that gave maximum Tg was prepared. The corresponding coatings films were tested for gloss, scratch resistance and other typical paint film tests.

3 Results and Discussion

We observed the Tg jump phenomena due to the nano-confinement effect in PVA-silica, polystyrene-silica and latex-silica systems. We studied water based PVA system in depth under controlled conditions to establish the relationship between nanoparticle size and polymer molecular weight. A typical result when used silica nanoparticle (Fig. 1).

Fig. 1.
figure 1

Effect of silica loading on Tg (deduced from dynamic mechanical analysis) in PVA-silica composite prepared in water. Composites prepared in water with a solution of PVA (degree of polymerization of 2000) and suspension of silica nanoparticles (average diameter -24 nm).

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For the first time the nano-confinement effect was also shown to occur in a latex system by studying the composites formed by latex and varying amount of silica nanoparticles A typical example of Tg effect in latex-silica composite system is shown in Fig. 2.

Fig. 2.
figure 2

Effect of Tg (deduced from dynamic mechanical analysis) on silica loading in Sty, HEA, MMA and MAA latex-silica composite prepared in water (silica nanoparticles average diameter -10 nm).

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The Tg increase in the latex system would mean that during the film formation process, at the stage where the particle deformation and polymer diffusions happens, the polymer chains are starting to interact with the surface of the inorganic particles through hydrogen bonding between hydroxyl groups on the surface and acid groups on the latex. This effect was evident in a non-tacky film obtained when we used a low Tg latex (15°C) in the final formulation.

Surprisingly all the Tg effects showed an optimum at a certain loading of the nanoparticles (Figs. 1 and 2). This optimum loading corresponding to the highest Tg depends on the particle size of the nanoparticles as well as the molecular weight of the polymers and their end groups.

We were able to derive an equation that predicts the location of this optimal loading.

For the latex systems, the resulting coatings performed like their regular counterparts, with the difference that no coalescing agents/plasticizers were added and instead a lower Tg latex was utilized (with the regular co-stabilizing acidic comonomers) and a low loading of commercial inorganic nanoparticles. The crucial factor is to select the optimum loading of these inorganic nanoparticles which can be predicted via our model but would otherwise be relatively difficult to establish experimentally.

The question might be asked why this effect does not play a role in regular coating formulations with an abundance of inorganic (pigment and filler) particles. The answer lies in the size of these particles, representing a much smaller surface area than the typical nanoparticles used in these studies, being in the 10–50 nm range whereas the pigment particles are approximately 10 times bigger and the fillers are at least 100 times bigger. To test whether this Tg effect finally translates to the preparation of SOC free water-based paint formulations we prepared coating formulations with low Tg latex and bench marked its performance with commercial formulations. We did not see any notable difference in performance in terms of color, gloss, tackiness or visual defects and the coating performed well in a one-year outdoor testing.

4 Conclusion

The nanoconfinement effect can be observed in films cast from solutions. We have shown that there is a strong influence on the loading of nanoparticles where this Tg jump is highest. The location depends on the combination of nanoparticles and polymer and the molecular weight as well as the end groups of the polymers. For the first time we were also able to show that the same effect can be utilized in water-borne coatings, leading to the elimination of coalescing agents/plasticizers and NoVOC water-borne coatings.