The glossary you have been waiting for
Opacity, bulk and smoothness – the list goes on. There is a lot to think about in the world of paper and a lot of different terms to describe the material’s properties. Finding the right solution for your need is largely based on us understanding what you want to achieve with your product. That in turn is much easier if we speak the same language, so here's a quick guide to the universal language of paper.
What is the difference between LWU, SC and LWC paper?
LWU paper is an uncoated paper in which no extra additives have been used. The LWU paper has been pressed through a calender to achieve a smoother surface. SC paper is a supercalendered uncoated paper – which means that it has undergone extra calendering (polishing). The paper also contains a large amount of filler (pigment) to increase the smoothness and improve the printing properties. LWC paper has an added surface layer comprising clay and binder, applied by a special coating unit.
Wood-containing or wood-free paper
The term wood-free paper doesn’t means that no wood is involved. The chemical process for the manufacture of pulp for wood-free paper uses around 50 per cent wood. The benefit of wood-free paper is high brightness, but it does have poorer opacity. When making a wood-containing paper, the majority of the raw material in the pulp production is wood, amounting to around 98 per cent. Since a wood-containing paper contains a great deal of lignin, a substance in the wood’s cell walls, it yellows more quickly when exposed to daylight. However, wood-containing paper also has many benefits, such as stiffness, thickness, cost-effectiveness and environmental positivity.
Steam, heat and pressure can be used to change the paper’s surface properties and give it more gloss. The paper is pressed between two hot rollers under high pressure with added steam. One roller is steel and the other is made from a plastic polymer. The calendering process is not unlike ironing a shirt.
L*, a* and b*
The a* and b* values describe the shade of the paper. a* corresponds to the red/green scale and b* corresponds to the blue/yellow scale. The higher the a* value, the redder the paper. The higher the b* value, the yellower the paper. The key is often to find a balance between green/red and blue/yellow, in order to produce a paper that is felt to be easy on the eye, while also looking white. The L* value defines the brightness of the paper.
The paper’s bulk describes how voluminous the paper is and is measured in cm3/g. Two different papers can have the same basis weight but different bulk, which means that the paper with the lower bulk has usually been compressed (calendered) more in production. The lower the bulk, the thinner the paper. Paper made from fresh fibre has advantages in terms of bulk, since the fibre is more voluminous than recovered fibre which has already been used one or more times in paper production. The opacity is also better in a paper with high bulk.
A paper’s thickness depends a great deal on the type of raw material in the paper. Fresh fibre offers better scope to produce a thicker paper compared with recovered fibre. Thickness is measured in micrometers (µm), commonly known as microns, and it is worked out by multiplying the basis weight and the bulk.
Gloss can describe both the paper gloss and the print gloss. The paper gloss is the natural shine that the paper has after passing through the paper machine. The higher the value, the glossier the paper. Excellent print gloss can be achieved by using the right ink on the right paper.
This describes the smoothness or roughness of the paper’s surface. Smoothness is measured either in µm according to the method Parker Print Surf, PPS , or in ml/min using the Bendtsen method. In both cases, a lower value means a smoother surface. The smoother the surface, the better the potential for good print results. The more even the paper, the greater the opportunity for detailed image reproduction that closely resembles the original. The halftone dots are bigger on a paper with an uneven surface. This is because the printing inks flow out into the irregularities. This is called dot gain. The effect is a darker printed result and images that can look rather fuzzy. The number of shades falls and the transitions between different colour tones become less clear. Using too high a screen frequency can, in the worst case, cause clogging in the print, where the darker sections of the images soot up and become black.
Opacity is a measure of the paper’s transparency. Different paper grades have a different opacity. This is because they have a differing capacity to absorb and disperse light. Opacity is an important parameter, as you don’t want what is printed on one side of a page to show through on the other. In a glossy/calendered paper, the paper base is thinner and more transparent.
The surface is smoother, which also increases its transparency. Opacity deteriorates further during the printing process,
particularly when printing dark, high-coverage surfaces. This is because the oily ink seeps down into the paper and makes it more transparent. Uncoated, glossy, white paper in low basis weights can thus be particularly problematic when it comes to opacity. The opacity can be adjusted via the quantity and type of pulp, filler, calendering and the brightness of the paper. The approximate values for the opacity of printing paper are 89–97%.
Other words that are worth knowing about
Cellulose fibre: The fibre that paper pulp is made from. Extracted from the wood’s cells.
Lignin: A substance in paper manufacturing that comes from the cell walls of wood. The lignin makes paper yellow when exposed to daylight.
Stock: A blend of primarily paper pulp and water.
Headbox: Where the stock is fed into the paper machine.
Wire: A fine mesh in the paper machine that dewaters the stock.
Fibre direction: The direction in which the majority of fibres lie.
Basis weight: The weight of the paper. Measured in grams per square metre, g/m²
Permanence: How well a paper retains its properties over time.
Dimensional stability: How the paper changes along its length and width depending on moisture levels. The greater the change from the original size, the poorer the dimensional stability.