Three-dimensional articles such as trays and plates are manufactured from two-dimensional sheet of paperboard or cardboard by thermopressing or deep-drawing. In order to adapt to the shaping operation the board is forced to folds or wrinkles, appearing as score lines located at the corners of a rectangular tray, or are divided along the periphery if the tray or plate has a circular or oval shape. Such articles are used for packaging of food or as disposable tableware.
A typical packaging board has a triple-layer structure, in which a middle layer of chemi-thermomechanical pulp (CTMP) is sandwiched between two outer layers of chemical pulp. As the board sheet is shaped into a three-dimensional configuration, the highest stress is subjected to the spots which are forced into folds or wrinkles. The resulting problem is that due to its stiffness and limited stretching ability the board sheet risks cracking at the spots of maximal stress.
A known remedy to the cracking problem is increasing the bulk of the fibrous sheet material. EP 1160379 B1 describes a press-moldable mono- or multilayer base paper for packing containers, which may comprise an intermediate low-density (high bulk) layer between two outer high-density layers. To achieve the increased bulk the reference teaches addition of heat-expanding microcapsules as a foaming agent to the pulp slurry used for making the low-density layer. As the base paper is passed through hot water the foaming agent will cause foaming as the volatile expanding agent is released, and the foamed structure of reduced density is preserved as the base paper is dried. Compressibility of the low-density layer of the base paper in the thickness direction is 10% or more, bringing about improved moldability and reduced cracking.
Another foaming technique aimed at increasing the bulk of a fibrous sheet is foam forming, in which the pulp is turned into a foamed suspension as it is fed from a headbox to a forming fabric of a paper or board machine. Characteristic for foam forming is that the bulk is higher but the tensile index is lower. A bulkier structure is more porous, which brings about the lower tensile index. Foam forming requires use of a surfactant, which affects both the dry and the wet tensile strength of the sheet negatively. Such tensile strength loss is believed to be due to the surfactants adsorbing to the fibres and thus hindering hydrogen bonding between the fibres.
The foam forming technique has found use particularly in the making of tissue paper. Otherwise the inferior strength properties as compared to standard wet forming, as well as inferior Scott bond and elastic modulus have deterred use of foam forming for other kinds of papermaking. However, WO 2013/160553 teaches manufacture of paper or board, in which microfibrillated cellulose (MFC) is blended with pulp of a higher fibre length and turned to a fibrous web by use of foam forming. Especially a middle layer with an increased bulk is thereby produced for a multilayer board. MFC is purposed to build bridges between longer fibres and thereby lend the resulting paper or board an increased strength. The technique is said to be applicable for folding boxboard and several other paper and board products.
Another approach for utilizing foam in the manufacture of shaped products is described in WO 2015/036659. According to this reference natural and synthetic fibres are turned to an aqueous foamed suspension, which is fed into a mould and dried to a fibrous product such as a three-dimensional package, with a corresponding shape. By feeding different foamed suspensions at multiple steps the mould can be used to make products having a multilayer wall structure.