Heterogeneous Latices as Binders in Porous Particle Structures M. RIGDAHL, 1 L. LASON, 1 R. HAGEN, 1 O. K ARLSSON, 2 B. WESSLE ´ N 2 1 Swedish Pulp and Paper Research Institute ( STFI ) , Box 5604, S-114 86 Stockholm, Sweden 2 Lund Institute of Technology, Department of Chemical Engineering, Box 124, S-221 00 Lund, Sweden Received 22 April 1996; accepted 19 August 1996 ABSTRACT: Heterogeneous carboxylated styrene-butadiene ( S / Bu ) latices were pre- pared by a two-stage polymerization process, using three seeds of polystyrene with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu-ratio of 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. It has been found that the morphology of the films made from these latices influenced the modulus in the rubbery region of these films. The heterogeneous latices were used as binders in porous structures based on micron-sized kaolin particles. Such structures are typically employed as paper coatings. Polyester substrates were coated with aqueous suspensions containing the kaolin particles and the heterogeneous latex. The coatings were dried at room temperature, which corresponds to the rubbery region of the latex films. It was found that a higher modulus ( which is determined here by the morphology of the latex film) in the rubbery region of the films was associated with coating layers with higher porosity, greater light scattering ability, and higher coating gloss. This is interpreted as being the result of a retarded shrinkage of the coating layers during the drying of these structures due to the increase in modulus of the latex films.  1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 661–670, 1997 Key words: binders; latex; minerals; paper coating; porosity INTRODUCTION In order to provide the required cohesive prop- erties of the structure, the binder must have a Coatings to improve the surface characteristics rather low film-forming temperature, normally of and the print quality of paper and board are po- the order of 0–25°C. Films of these latices are rous structures built up of micron-sized mineral then quite ductile at room temperature, which is particles. A synthetic polymer latex is, in most beneficial for the surface strength of the coated cases, used to ensure a sufficient cohesion of this material, e.g., the danger of coating rupture dur- structure and also to anchor the thin coating layer ing subsequent printing on the paper is reduced. 3 to the paper substrate. Styrene-butadiene ( S / Bu ) A polymer with high ductility and a rather low latices are very common for this application. Young’s modulus is thus favorable for the surface These latices are normally carboxylated by incor- strength of the coated material. On the other porating small amounts ( up to 10% ) of vinyl acids, hand, the use of a polymeric binder with a lower which improve the adhesion between the latex modulus improves the surface strength at the ex- and the mineral pigment or the paper substrate. 1,2 pense of the bending stiffness, which is another important property of the coated product. In order to achieve a proper balance between the surface Correspondence to: M. Rigdahl  1997 John Wiley & Sons, Inc. CCC 0021-8995/97 / 050661-10 strength and the stiffness properties of the coated 661