HYDROPHOBICALLY SIZED FIBROUS WEB AND A METHOD FOR THE PREPARATION OF A SIZED WEB LAYER

The invention relates to a hydrophobically sized fibrous web layer， methods for the preparation of a fibrous web or a fibre-based coating， a multilayer board product having at least a middle layer formed of said fibrous web， as well as use of a heat-sensitive surfactant for said methods and products. According to the method microfibrillated cellulose (MFC) and hydrophobic size are brought to a foam with water and the heat- sensitive surfactant， the foam is supplied to a forming fabric of a paper or board machine， dewatered by suction of air through the forming fabric， and dried to a web product. Alternatively the foam may be supplied onto a premade fibrous web and dried to form a coating layer. To prevent gradual degradation of the hydrophobic sizing the hydrophilic functionality of the surfactant contained in the web is finally destroyed by heating. The method uses e.g. AKD-based surfactants with a hydrophilic moiety which is decomposed by heat. If pulp of a greater fibre length， such as CTMP， is also included， a high bulk in combination with a high Scott bond value is achieved， to provide improved wet and dry tensile strength for the paper and board products.