Papers are commonly described as materials with smooth and flat surface that have an even structure. However, microscopic view reveals that paper materials structure is far from homogeneous. In fact, paper is complex structure that consists of network formed by fibers originated from wood or similar to wood sources. Moreover, filler particles (usually kaolin, calcium carbonate or other mineral components), various papermaking chemicals and residual raw material components, like lignin, are present in a paper material structure. Pores filled with air occupy free space between fibres. Some paper materials, especially printing papers, are coated with one or several thin layers of mineral pigments (usually kaolin, calcium carbonate, other minerals or mixture of these pigments).depending on the end-use purpose in order to obtain desired properties of final product or to reduce the raw material costs by substituting high quality chemical pulp by cheap mechanical pulp or recycled pulp in the middle layer of the paperboard. Therefore, various paper materials can be regarded as very heterogeneous composite products where the wood or wood-like fibres are the main constituents of the network (Piili, 2009).
The main constituent of the paper network, the wood fibres, have much longer length than their width. Therefore, a paper sheet is typically thin and at a macroscopic scale the fibre network in paper reminds of a flat 2D network. However, when the pores between fibres are taken into consideration, fibre network can be understood as a 3D network in a microscopic scale, where the network structure is filled with pores and air voids, as shown in Figure 1 (Niskanen, 1998; Piili, 2009).
Figure 1. Typical fibre 3D network: top side of paper material (Piili, 2009).
This special structure of paper materials, i.e. 3D fibre network, has a strong effect on the optical properties of paper materials and consequently also on the interactions between laser beam and paper material (Niskanen, 1998).
2.1.1 Cellulose structure
The elemental composition of cellulose consists of 44-45 % of carbon, 6.0-6.5 % of hydrogen and 48.5-50 % of oxygen. The empirical formula of cellulose is C6H10O5. The chain-like macromolecular structure of the cellulose molecule has been generally accepted (Figure 2) (Krassig, 1993).
Figure 2. Structure of the cellulose molecule (Krassig, 1993).
The structure of cellulose can be described as a long chain polymer molecule consisted of repetitive glycoside residues. The glucose base units are linked together by one, 4- -glucosidic bonds formed between the carbon atoms C (1) and C (4) of bordering glucose units. The -glucosidic link requires that the plane of the pyranose ring of each second glucose unit along the molecular chain is turned around the C1 - C4 axis by 180 with respect to the glucose units lying in between. This means that cellulose is actually one, 4- -polyacetal of cellobiose with a repeating length of 1.3 nm (Krassig, 1993).
At the both ends of cellulose molecule, the terminal hydroxyl groups are present. These two hydroxyl groups are different in their nature. The C1 hydroxyl group, shown on the Figure 2 at left side of molecule, is an aldehyde hydrate group with reducing activity. They originate from the formation of the pyranose ring through an intermolecular hemiacetal reaction. The C (4) hydroxyl group on the right side of the cellulose molecule is an alcoholic hydroxyl and consequently non-reducing (Krassig, 1993).
2.1.2 Hemicelluloses
Hemicelluloses are low-molecular polysaccharides, which are contained in plant cell walls as well as cellulose and lignin. Most of hemicelluloses differ from cellulose better solubility in alkaline solutions and their ability to hydrolyse in water. Hemicelluloses in plants are the backbone of the
construction material. The content of hemicelluloses in wood and others wood-based materials is 13-43 % (Sharkov and Kuibina, 1972). Although hemicellulose is usually considered as structural polysaccharides, it includes a few other plant polymers such as the arabinogalactans, among them (Timell, 1967).
2.1.3 Lignin
Lignin is a natural, amorphous, three-dimensional, polyphenolic polymer, which is built up of phenyl propane units. Most of the lignin is concentrated in the middle lamella (the space between the cells). The biological role of lignin is to participate in forming cell walls in living plants along with the cellulose and hemicelluloses. It serves the purpose to bind fibers together. The remaining part is located throughout the secondary cell wall. Here lignin interpenetrates and encrusts the cellulose fibers and the hemicellulose (Timell, 1967; Papp et al., 2004).
2.1.4 Paper material/Fibres
Basically, natural fibers have a hollow cross section structure. Never-dried fibers are almost completely uncollapsed (Figure 3) (Page, 1967).
Figure 3. Different degrees of fiber collapse: (A) original fiber, (B) partially collapsed fiber, and (C, D) completely collapsed fiber (Jayme amd Hunger, 1961).
Hollow structure of the wood fibers is collapsed in paper/cardboard making process. During drying both the fiber cross section and the fiber cell wall respectively collapse and contract. Thus, the cell walls also contribute in light scattering and act as optical boundaries (Jayme amd Hunger, 1961).
The effects of laser treatment on cellulosic material (in terms of behaviour under laser treatment) can be studied also from materials such as cotton that has the chemical structure close to that of paper material. Study done by Chow et al. studied surface morphology of cotton fibres after interaction with CO2 laser beam. It was revealed that surface was severely influenced by laser beam what resulted in pores, cracks and fragments. Observations were made on the fibre surface using scanning electron microscopy (SEM) technology. Using the results of Fourier transform infrared spectroscopy – Attenuated total reflectance (FTIR-ATR) analysis, it can be said that laser irradiation induced thermal degradation. Amount of oxidation products, such as carbonyl and carboxyl groups, was higher in areas treated with laser beam. This was also further supported by X-ray photoelectron spectroscopy (XPS) analysis which revealed changes in the elemental
composition and content of carbon and oxygen after irradiation with laser beam. This clearly indicated that the chemical composition of the surface of the laser-treated cotton fabric was modified (Chow et al., 2011).
2.1.5 Paper products and their classifications
Generally paper products can be classified into two major groups: mechanical pulp dominating grades and chemical pulp dominating groups. Thus the classification is maid according to the origin of raw material (Paulapuro, 2000).
Mechanical pulp dominating paper grades generally can contain 25 – 100 % mechanical pulp, but usually more than 50 %, and chemical pulp is added in order to increase strength properties and improve runability. Minerals are used as fillers and/or as coating. Mechanical pulp dominating paper grades comprise various newsprint grades, supercalandered (SC) papers and coated mechanical paper grades (Paulapuro, 2000).
Chemical pulp dominating grades are uncoated or coated fine paper grades with maximum mechanical fiber content of 10 %. Generally, these paper grades contain only traces or no mechanical pulp and 5 – 25 % fillers (Paulapuro, 2000).