Post —by Gautam Shah
Paper Finishing Operations account for 20% or more of the cost of paper (raw cellulose to reusable paper conversion). A finishing process improvises the quality, or may give birth to a very different product. Many of the finishing processes are carried out at a different plants’ locations, along with size conversion. Such plant processes are sometimes called off machine processes, because these are carried out, away from the paper production plant.
Papers need to be conditioned before and after finishing materials and techniques are applied. The object of conditioning is to eliminate some of the undesirable side effects that have been acquired during paper making process. It is a maturing process having two principal objects.
■ To rid the paper of stresses that may have been incurred during the paper making process, and that may otherwise be released during subsequent treatments.
■ To stabilize the moisture content evenly across the width and along the machine direction, to a desired level.
● Curls are caused by the different rates of shrinkage or expansion of the two faces of the paper sheet. Curls arise from the web due to differences in fiber orientation between the top and bottom of the sheet, and also due the difference in the distribution of fines.
● Cockle is caused by differences in internal stresses in a sheet. Generally this is the result of unequal drying, due to localized variations in moisture and substance contents.
There are TWO distinct types of paper conversion. One is referred to as wet converting, in which paper in roll form is coated, impregnated, and laminated with various applied materials to improve properties for special purposes. The second is referred to as dry converting, in which paper in roll form is converted into such items as bags, envelopes, boxes, small rolls, and packs of sheets.
Calendaring is a process of pressing between two smooth chilled rolls, or plates to produce a smooth-finish, known as machine finish. Calendaring, also compacts the paper-mass and create a glossy finish or desired texture on the surface. Special papers are given additional treatments. Paper is subjected to a further calendaring process -super-calendaring, under great pressure between metal and paper-covered rolls. Coated paper, such as that used for fine half-tone reproduction, is sized with clay or glue and calendared.
Papers are coated by many different materials and through equally varied techniques. Lacquer or polymer coated papers are used where moisture resistance is required. Plastic papers have coatings or layers of pre formed polymeric films. These are generally moisture and dirt resistant, and are used for packaging, children’s books etc. Wax coated papers are used for disposable dishes and ice-cream containers. Carbon papers have a coating of wax impregnated with carbon black pigment. Thermal papers have static properties suitable for achieving a printout in xerox or fax like machines. Cheque or security papers have special starch coatings, which can be seen in UV light. Electrical winding papers are coated with phenol formaldehyde varnish. Laminates have printed design papers sandwiched between a paper substrate (layers of papers bonded to gather with phenolic compounds) and film or coating of melamine compound. Blue print papers have ferric azo compounds that are light sensitive but can be stabilized with ammonia. Photographic papers are super quality, ultra bleached, super calendared and low level PH, papers, coated with silver compounds.
Papers have been coated to improve the surface for better reproduction of printed images for over 100 years. The introduction of half-tone and colour printing has created a strong demand for coated paper. Coatings are applied to paper to achieve uniformity of surface for printing inks, lacquers, and the like, to obtain printed images without blemishes visible to the eye. Coated papers help to enhance opacity, glossy to matt finishes, and to achieve economy in the weight and composition of base paper stock by the upgrading effect of coating.
Co-extrusion-coating process, is a relatively new development in the application of functional coatings. It is used to apply polyethylene and such extrudable thermo-plastic materials to paper, or paper boards. Polyethylene resin has ideal properties for use with packaging paper, being waterproof, resistant to grease, water vapour, and gases, highly stable, flexible in heat sealing, and free from odour and toxicity. The hot extruded film is stretched and combined with paper between a pair of rolls, one of which is rubber-covered pressure roll and the other a water-cooled, chromium-plated steel roll. The amalgamation between the plastic film and the paper is perfect and a permanent bond is created.