As a method for imparting a plurality of functions to an acrylic polymer, a method is known of emulsion-polymerizing a plurality of layers having different functions in multiple steps.
For example, as an agent for modifying the impact-resistant of an epoxy resin, there is a report on a core-shell type powdery polymer having an inner layer composed of a (meth)acrylate polymer having a glass transition temperature of −30° C. or lower and an outer layer composed of a (meth)acrylate polymer having a glass transition temperature of 70° C. or higher (patent document 1).
Further, as a vinyl chloride sol alternate material, (meth)acrylic polymer particles are known composed of a core layer giving the flexibility of a coated film and a shell layer suppressing contact of the polymer of the core layer to a plasticizer, thereby giving storage stability. For example, there is a report on a core-shell type acrylic polymer particle composed of a core part and a shell part each having a specific solubility parameter and a specific glass transition temperature (patent document 2). Moreover, there is a report on a method of adding a polymerization inhibitor in the course of a polymerization step to produce an emulsion containing polymers having different particle sizes (patent document 3).
In the method described in patent document 1, however, if the mass ratio of the shell component in the polymer particle is reduced, it becomes difficult to form a particle covered uniformly by the shell layer. It is believed that this difficulty is caused since a polymerization reaction progresses sequentially from a part of contact of the monomer of the shell component to the polymer of the core. When the amount of the core component is 80% by mass or more, it becomes difficult to coat the core component completely by the shell layer, and storage stability thereof tends to be insufficient. If the shell component is used in an amount necessary for uniform coating of the core component, the proportion of the core component decreases, leading to a polymer in which the effect of modifying impact resistance as the original object is low.
Also in the method described in patent document 2, if the mass ratio of the shell part in the polymer particle is reduced, it becomes difficult to form a sufficient coat by the shell component. Thus, if the mass ratio of the core component is increased to obtain a coated film giving satisfactory flexibility, it becomes difficult to completely coat the core, and storage stability thereof tends to be insufficient. Moreover, if the shell component is used in an amount necessary for uniform coating of the core component, the flexibility of the coated film lowers.
The method described in patent document 3 is a method for obtaining a dispersion containing two polymer particles having different physical properties, and is not a method in which a polymer of a core component is coated uniformly by a small amount of a polymer of a shell component and the performances of the core component and the shell component are allowed to manifest.
In the conventional methods, as described above, it is difficult to uniformly coat a polymer of a core component by a small amount of a polymer of a shell component and to manifest the performances of the core component and the shell component in a good balance.