Film Formation in Waterborne Coatings - American Chemical Society

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Chapter 16

Small-Angle Neutron Scattering Studies of Composite Latex Film Structure 1

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Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch016

Y. Chevalier , M . Hidalgo , J.-Y. Cavaillé , and B. Cabane 1

Laboratoire des Matériaux Organiques à Propriétés Spécifiques, Centre National de la Recherche Scientifique, B.P. 24, 69390 Vernaison, France Centre de Recherches sur les Macromolécules Végétales, Centre National de la Recherche Scientifique, B.P. 53, 38041 Grenoble Cedex 09, France Service de Chimie Moléculaire and Unité Mixte Rhône-Poulenc, Centre National de la Recherche Scientifique, CE Saclay, 91191 Gif sur Yvette, France 2

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The structure of composite films made of high T polystyrene (PS) nodules dispersed in a low T polybutylacrylate (PBuA) matrix was studied by means of small angle neutron scattering. For films cast from mixtures of PS and PBuA latexes, segregation of PS particles leads to dense clusters of PS particles in a PBuA continuous medium. This segregation has the main features of a phase separation. For films cast from two-stage (core-shell) particles, this segregation phenomenon is prevented, depending on the coverage of the PS core by the PBuA shell. Upon annealing the films above the T of PS, extensive coalescence of PS particles occurred when large contacts were already present in the dry film at room temperature, whereas coalescence could be prevented when PS particles were taken apart by the presence of a PBuA shell. The extent of coalescence had a strong influence on the films mechanical properties. g

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In waterborne coatings applications, polymer films can be obtained by drying polymer particles colloidal suspensions. During the drying process, particles have to approach each other, stick, and finally get deformed from spherical to polyhedral as water evaporates (J-3). Particles made of polymers of low glass transition temperature (T ) are thus required in such a film formation process but the final (dry)filmsmechanical properties are poor. A mechanical properties reinforcement can be obtained when solid particles are mixed with the low T polymer ones. Those solid particles can be either inorganic or organic when a high T polymer is used. In this latter case, a further structural transformation of the dry films can be obtained by heating the films above the glass transition temperature of the high T polymer. The weak bonds between solid particles at their points of contact are then strengthened by polymer g

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Current address: Elf-Atochem, 95 rue Danton, 92300 Levallois-Perret, France 0097-6156/96/0648-0244$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

16. CHEVALIER ET AL.

SANS Studies of Composite Latex Film Structure

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mterdiffusion (coalescence), which results in a further enhancement of elastic modulus (4-6). A large variety of mechanical properties reinforcements was observed, depending on high T polymer particles radii, surface chemistry or volume fraction(46). Efficient reinforcement can be achieved when the high T polymer particles form a continuous percolating network into the low T polymer matrix. In this context, the structure of the high T polymer particles dispersion in the dry film is a key parameter that one wishes to be able to control This is the motivation for performing systematic studies about the structure of dry composite films made of solid particles dispersed into a low T polymer matrix. In the present work, the structure of films made of glassy polystyrene (PS) particles in a soft polybutylacrylate (PBuA) matrix was studied by means of small angle neutron scattering (SANS). %

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Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch016

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Materials and Methods Materials. The PS/PBuA films were obtained by drying aqueous latex dispersions. The PS and PBuA polymers were brought into the films either by mixing PS and PBuA homopolymer latex particles before drying, or by drying aqueous suspensions of composite PS/PBuA particles. PBuA coalescence occurs during the drying process when two PBuA domains contact each other because of the low T of PBuA (-50°C). The two film formation processes are summarized in Figure 1. It has been shown that efficient mechanical reinforcement could be achieved with mixtures above a PS particle content of about 30% volume fraction (4). The structure of films having two different PS contents were then studied: 10% and 45% PS, below and above this percolation threshold respectively. The PS and PBuA particle diameters were 45 nm and 80 nm respectively, as measured by quasi-elastic light scattering. For composite particles the PS volume fraction was 40%, and two different soft shells were studied. The first one (sample A ) was a pure PBuA shell which gave imperfect core-shell morphologies (half-moon like) and the second type of shell (sample B) was made of a copolymer of butyl acrylate and 10% methacrylic acid (MA) which gave a better coverage of the PS core by the soft shell (Figure 2). Those particles were prepared in two steps: PS emulsion polymerisation gave a PS seed of 120 nm diameter and the PBuA or PBuA-coMA shell was obtained in a second polymerisation step (7). %

Small Angle Neutron Scattering on Latex Films. SANS experiments were carried out with the D l l diffiactometer (8) at the high flux reactor of the Institut LaueLangevin (DLL) at Grenoble. Data reduction was performed with standard procedures available as program packages at the ILL, giving after incoherent background subtraction the differential scattering cross-section άσ/άΩ = I(q) (9). The scattering vector domain observed was 10" Â" < q < 0.1 Â" , which defined the length scale explored in real space as 10 A > d > 10 Â. The PS particle diameters are well in this range, so that both the PS particle structure (radius and surface area) and ordering (PS-PS correlations) could be studied by this way (10). A good contrast between PS and PBuA could be obtained with natural isotopic compositions. No deuterated materials were necessary. The scattering length densities of different polymers are the sum of the scattering lengths of nuclei b divided by the molecular volume V, ρ = (ΣΖ>ί)/Κ The contrasts Ap = \p- pp AI of 3

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In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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FILM FORMATION IN WATERBORNE COATINGS

DRYING

Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch016

MIXING AND DRYING PBuA particles suspension in water

PS particles suspension in water

mixing

COMPOSITE

PARTICLES PS/PBuA particles suspension in water

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Dispersion of PS particles in a PBuA matrix

Figure 1. Film formation processes either from mixtures of PS and PBuA particles or from core-shell composite PS/PBuA particles.

sample A

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Figure 2. Schematic structure of composite particles of samples A and B.

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

16.

SANS Studies of Composite Latex Film Structure

CHEVALIER ET AL.

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hydrogenated PS, PBuA and of the zvvdtterionic emulsifier used in emulsion polymerisation are given in Table I. The scattering length density of deuterated polystyrene is also given for comparison. Table I. Scattering length densities of compounds found in dry films Compound PBuA^H) PS( H) PS( H) Emulsifier

Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch016

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Scattering length density ρ (cm ) 0.66 10 1.40 10 6.42 10 0.08 10

Contrast 2

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0.74 10 5.76 10 0.58 10

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It can be seen in Table I that the contrast between the emulsifier and the PBuA is of the same order of magnitude as that between PS and PBuA. Thus, the elimination of the emulsifier is very important, any emulsifier remaining after the latex washing procedures can give very strong scattering if it is segregated from the polymer into lumps of large size. An emulsifier which strongly adsorbs onto the polymer particles is necessary for obtaining monodisperse PS particles of small size (D < 100 nm), but its strong adsorption makes it difficult to completely wash out. The PBuA particles used in PS-PBuA mixtures were prepared using sodium dodecylsulfate (SDS) as an emulsifier because it is easy to remove. A larger particle diameter and size polydispersity is not a problem since PBuA particles coalesce into a continuous matrix during film formation at room temperature. It has already been observed that the zwitterionic emulsifier dodecyl(mnethyknmioniopropylsulfonate (C H "N(CH )2-(CH )3-S03 ) which was used for emulsion polymerisation (77) did give such a strong scattering at low angles in dry films of homogeneous poly(butylacrylate-co-styrene) (2,5). 12

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Results and Discussion Films Obtained at Room Temperature from Mixtures of PS and PBuA Particles. The SANS data obtained for dry films with 10% and 45% PS contents are shown in Figure 3 on a Log-Log scale. Going from the high-^ region (high resolution) to the low-