Composite agglomerates, commonly known as “white agglomerates”, is made from the agglomeration of cork granules that can be mixed with other materials such as rubber, plastic, asphalt, cement, plaster, casein, resins, glues, giving rise to composite agglomerates, thus obtaining a wide variety of products.
Granulation: The cork used in the production of construction and decoration materials comes from the first and second bark stripped from the cork oak, which is not used for stoppers, and cork leftovers, shavings, pieces and waste from other industries. The cork is ground, with the granules taking on a range of sizes (usually above 0.25 mm and below 22.4 mm) and densities (usually 70-90kg/m3). The granules are obtained using a number of different types of mill, according to the material to be ground and the type of granulate required.
After grinding, the granules are normally cleaned, followed by drying using forced circulation of hot air by means of rotating driers, giving the granulate the required moisture content.
Agglomeration: These granules are the raw material for agglomerates, which are created by means of a process of agglutination of granules with a specific size and density using pressure, heat and a binder, depending on the product and application required.
Following automatic or manual dosage, the mixing of the granules with the binders and, if used, other additional components, is achieved by means of a mechanical process – blade or helical mixers. The density resulting from this process will depend on the intended purpose. For example, agglomerates for decorative purposes commonly have densities of 200 to 350 kg/m3 and use fine to medium granule sizes; for floorings the density is 450 kg/m3 to 600 kg/m3, while for expansion joints (cork is used between rigid elements – such as concrete – as an acoustic and thermal insulant) medium calibre granules are used and density varies between 250 and 350 kg/m3. This agglutination is achieved using synthetic polyurethane, phenolic and melamine resins or vegetable-based resins.
Colouration: During the agglomeration stage, the granules may also be coloured with pigments, which can be of a number of different colours.
Moulding: The granules and resins are then placed in moulds, usually made of metal and rectangular in shape – though for cork rolls cylindrical moulds are used -, after which the moulds are pressed. They are then placed in a heating chamber, which may be an oven (using temperatures between 110 and 150ºC for a period of 4 to 22 hours) or high-frequency systems which are either continuous (tunnels) or discontinuous (the moulds used are made of fibreglass and can be ready for handling in 3-4 minutes).
Next, the agglomerate is unmoulded and subjected to cooling/stabilisation, resulting in an agglomerate block suitable for cutting into sheets. Another moulding system can be used, called the blanket system. In this case the granules, binders and agents are mixed and then spread out on a conveyor belt and when passing through a heated plate press at a temperature of 120-180ºC, they are glued together resulting in a single sheet of the desired thickness.
Sanding: The next stage is sanding, which allows fine-tuning of the thickness and texture of the cork sheet. The sheets are then cut into a rectangular or square shape and checked for size and squareness.
Decoration: Once the sheets are produced, the different types of decorative materials and coverings are created. These can be made using a single sheet, by overlaying several sheets of agglomerate or laminated natural cork, or by combining agglomerate with other materials such as wood. The sheets are glued together with the aid of rollers or presses. The completed sheets can be given a range of surface finishes: varnish, wax, paint or covered with particles, for example PVC. Some companies offer the possibility to print on the cork (Digital Printing) a drawing, a photo, a pattern. Imagination is the limit.
Selection: Finally, manual/visual selection/rejection takes place to eliminate any defects which may have occurred (such as broken corners, defective varnishing, etc).
The product is then packaged and stored.
Manufacture of cork fabric and paper: These products are obtained by lamination of sheets of natural or agglomerated cork. The process of granulation and agglomeration In this case is similar to that for composite agglomerates. The laminated sheets are very thin sheets (usually with a thickness of 50-500mm) glued to fabric, textiles or paper. This application highlights the texture and veins of cork which can be detected by touch. They are produced in various patterns and colours and used for a variety of purposes.