F A C T 1
Paper Characteristics (properties of paper)
There are many kinds of paper, to differientate the grades, it is descibed with the result of the measurement of the paper. Underneath follows an overvieuw of the different paper characteristics. Paper Characteristics are measured by 23 °C en 50% relative moisture of the air.
Ash content is the amount of inorganic filler or coating of paper, including calcium carbonate, china clay and titanium dioxide, added during paper-making to increase such paper properties as brightness, whiteness, or opacity. In many occasions ash content is determined on oven dried samples which are heated, this leaves only the inorganic fraction of the material measured as the ash content, which is expressed as a percentage (%) of the original oven dried weight.
The weight or substance per unit area is obviously fundamental in paper and paper board products. The Basis weight of paper is the weight per unit area. This can be expressed as the weight in grams per square meter (GSM or g/M2), pounds per 1000 sq. ft. or weight in kg’s or pounds per ream (500 sheets) of a specific size. REAM WEIGHT is a common term to signify the weight of a lot or batch of paper. Control of basis weight is important as all other properties are affected. Variations in moisture content in paper affects the grammage.
Brightness, Whiteness and Colour
Brightness is defined as the percentage reflectance of blue light only at a wavelength of 457 nm. Whiteness refers to the extent that paper diffusely reflects light of all wave lengths throughout the visible spectrum. Whiteness is an appearance term. Colour is an aesthetic value. Colour may appear different when viewed under a different light source. Brightness is an arbitrarily defined, but carefully standardised, blue reflectance that is used throughout the pulp and paper industry for the control of mill processes and in certain types of research and development programs. Brightness is not whiteness. However, the brightness values of the pulps and pigments going into the paper provide an excellent measure of the maximum whiteness that can be achieved with proper tinting. The colour of paper, like of other materials, depends in a complicated way on the characteristics of the observer and a number of physical factors such as the spectral energy distribution of the illuminant, the geometry of illuminating and viewing, the nature and extent of the surround and the optical characteristics of the paper itself.
Bulk is a term used to indicate volume or thickness in relation to weight. It is the reciprocal of density (weight per unit volume). It is calculated from caliper and basis weight. Sheet bulk relates to all other sheet properties. Decrease the bulk or in other words increase the density, and the sheet gets smoother, glossier, less opaque, darker, lower in strength etc.
An important consequence of the absorption and de-absorption of moisture by paper is the change in dimension that usually accompanies changes in moisture content. Such changes in dimension may seriously affect register in printing processes and interfere with the use of such items as tabulating cards. Uneven dimensional changes cause undesirable cockling and curling. Dimensional changes in paper originate in the swelling and contraction of the individual fibres. It has been observed that cellulosic fibres swell in diameter from 15 to 20% in passing from the dry condition to the fibre saturation point. It is impossible to be precise about the degree of this swelling because paper-making fibres differ considerably in this property, and because the irregular cross-section of fibres creates difficulty in defining diameter. Change that occurs in the dimensions of paper with variation in the moisture content is an important consideration in the use of paper. All papers expand with increased moisture content and contract with decreased moisture content, but the rate and extent of changes vary with different papers.