Binders and mineral materials are among the main components of paper. Binders, which come in the form of aqueous dispersions or suspensions of latex, impart the necessary cohesion to all of the components involved in manufacturing said paper. Mineral materials, such as calcium carbonate, make it possible to improve the quality of the sheet of paper, particularly with regard to its optical properties.
The concept of “hybrid” or “self-binding” particles appeared several years ago. These are individual solid particles in which particles of mineral materials and latex particles are intimately bound. The internal cohesion forces are such that the resulting particles have an excellent mechanical stability, and may be implemented directly in the manufacture of paper.
If this is done, the logistical problems related to handling two products, binders and mineral materials, are avoided, and unwanted physical and chemical interactions related to mixing these two products are also reduced when they are implemented separately. To that end, one may refer to the document “Physical and Chemical Modifications in latex binders and their effects on the coating colour rheology” (Advanced Coating Fundamentals Symposium, San Diego, Calif., USA, May 4-5, 2001, pp 108-123), which demonstrates the harmful influence that mixing a sytrene-butadiene latex binder with calcium carbonate has on the rheology of a paper coating slip.
The methods for manufacturing such particles involve at least one step of grinding mineral materials in the presence of binders. Grinding refers to an operation that leads to a reduction in the size of the particles: the size of the hybrid particles that are formed is smaller than the size of the initial mineral material particles. Several descriptions and variants in this matter can now be found in documents WO 2006 008657, WO 2006 128814 and WO 2008 139292.
Document WO 2006 008657 describes a method for co-grinding inorganic materials in an aqueous medium, in the presence of binders. The self-binding nature is measured using a crush test performed on samples taken from the solid particles that result from the step of co-grinding. Out of the 6 examples illustrating the invention, 4 of them (examples 1, 3, 5, and 6) implement 2 binders, including an acrylic ethylene-acid copolymer (Polygen™ WE 4 sold by BASF™). Furthermore, only tests 5 and 6 lead to end concentrations of calcium carbonate greater than 50% of their total weight; in such a case, a wetting agent and an acrylic dispersing agent must then be added to the suspension resulting from the grinding.
Document WO 2006 128814 focuses on the impact of such hybrid particles on the final properties of the sheet of paper: it particularly states that the optical properties of the sheet, such as its opacity, are improved. It describes the manufacturing of these particles by means of a method of grinding calcium carbonate in water in the presence of binders, particularly with Polygen™ WE 4 (in 3 of the 4 tests). On the other hand, none of the resulting aqueous suspensions have a solids content of mineral materials greater than 30% of their weight.
Document WO 2008 139292, meanwhile, discloses a method implementing a step of grinding a binder in the presence of a mineral material, followed by adding an inverse emulsion; the tests illustrate the implementation of Polygen™ WE 4, in order to grind a mixture of calcium carbonate with binder at a concentration of 20% solids content, with the product obtained being 40% concentrated.