Post 210 –by Gautam Shah
A paper is valued for its quality, form and suitability for a purpose. There are many factors that determine these. The raw material composition, manufacturing and post processing methods mark the papers’ quality, strength, feel and appearance. The form chiefly relates to the post production processes and treatments and its size conversion. The paper becomes suitable for a purpose after various treatments, shape forming and combinative make-up with other materials.
One of the most important factors of paper raw materials is the fibre quality, its length and integrity. Chemical pulps in general have higher fiber length compared to semi chemical pulp and mechanical pulp, when made from same wood. Mechanical pulping, though has greater yield, it damages or shortens the fibre. Next in importance is the proportion of rag versus the cellulose. Higher rag content provides greater strength and better quality.
Paper made from unbeaten or inadequately beaten fibres is characterized by a lack of tensile strength, by high porosity, high absorbency, full opacity, by its irregular surface and in many cases by a wild uneven formation. A sheet from a well-beaten stock, exhibits greater mechanical strength, a higher density, a measure of grease-proof, translucency and in extreme cases transparency, a smoother surface and a more regular formation. A waste recycled pulp apparently has lower capacity to swell compared to a virgin-pulp due to prime `beating’. Pulp from better quality recycled waste is used for newsprint paper and straw board or packing carton paper. Card board and card have small amount of rags. Writing papers have better rag content that is contain substantial linen or cotton fibres.
Grease-proof (resistant to oil and fat penetration) and ‘glassive’ grades require a paper as dense and homogeneous as possible. This is achieved by reducing the length of fibres. On the other hand craft paper, where strength and anti tear properties are important, fiber shortening is minimized.
Water Absorbency of a paper depends on the quality of the pulp and also on the degree and method of sizing. Papers are internally sized (i.e. sized during pulp formation stage), tub-sized or surface-sized after a paper formation stage. Sizing materials include glues, casein, starch, rosin, gelatin and many synthetic polymeric emulsions and suspensions.
PH Value of a Paper determines permanency of a paper, i.e. deterioration of a paper due to ageing. Papers with a tendency to yellow and become brittle have acidic content. Documents, account books, art work prints, maps, survey sheets, require acid free papers. Addition of alkaline substances like calcium or a magnesium carbonate can reduce acidity of a paper, but also increase the weight of a paper, and make it impermeable and smooth.
Density of a Paper is also governed by many other factors, such as, quality of pulp, sizing, pressing, post manufacturing treatments like calendaring.
Hardness and Softness of Paper are related to many factors such as the constituents of the pulp, grain orientation, pressing, calendaring, sizing, loading substances, presence of residual chemicals (bleaching and optical brightening agents, acids). Brittleness occurs due to over handling of pulp, presence of residual chemicals and the moisture content. Papers often are treated with silicone emulsions, soaps, to provide a soft feel.
Optical Properties include opacity, brightness, gloss and colour. Opacity of a paper relates to legibility of an image through a paper or from the backside imprint on the paper. Tracings have very smooth grain structure with show through, achieved by very through beating of pulp or by impregnation with oil or waxy substances. While some papers are mildly translucent or have strike-through properties (legibility of an image printed on the back face). Opacity is often a desired property of copying, printing, writing papers. Opacity of a paper is achieved by loading substances, calendering processes, impregnation, coatings, sizing, dyeing etc.
Brightness shows as the degree to which white or near-white papers and paper-board can reflect the light of the blue end of the spectrum.
Gloss, Glare, Finish, and Smoothness, are used in describing the surface characteristics of paper. Smoothness refers to the absence of surface irregularities under visual and feel conditions. Gloss refers to surface lustre. Glare, is used for a more intense reflection and often an unpleasant effect of high gloss. Calendaring and coating, are important paper-treating methods that affect gloss.
Colour of paper at natural level is dull or off-white to bleached or super white. The most common way to impart colour to paper is to add soluble dyes or coloured pigment to the paper stock. Direct dyes with a natural affinity for cellulose fibre are highly absorbed, even from dilute water solution. Basic dyes have a high affinity for ground-wood and unbleached pulps.
This article in continuation of earlier articles on same subject >>