Grades And Properties Of Paper

The Material Paper: a Survey

Introduction
The term paper refers to sheet material that is made essentially from fibers of plant origin. The characteristic feature of this nonhomogeneous material is its fiber network, which is usually arranged in layers containing pores of varying size. The properties of paper differ substantially from those of the fiber resources. These differences are illustrated in Table 11.1, which shows some selected material prop¬erties of wood, as the most important fiber source, in comparison with paper.

 As a result of the physical and chemical properties and the highly ordered state of wood fibers, wood is unsuitable for direct production of sheet material which could serve as writing material. The construction of paper, i. e., sheet material with properties that are optimal for writing, printing or packaging, is possible only after the break¬down of wood into its elemental fiber building blocks, their modification in the pulping process, and their controlled bonding in the papermaking process.

11.1 The Material Paper: a Survey
can be made with an extremely wide range of properties by varying the construc¬tion parameters: type of wood, pulping process, stock preparation, and papermak¬ing process. This is illustrated in Table 11.2.
The wide range of paper and board properties that result from the construction parameters mentioned above can be increased still further by the use of additives such as mineral and chemical aids. For instance, the smoothness and brightness of printing paper can be increased by addition of inorganic pigments (fillers)

[1]. Many special demands made on paper can be met by using suitable chemical additives, e. g., wet strength by using wet-strength additives or printability by using sizing agents [2]. However, paper produced with all these aids will never meet all conceivable requirements at a time. In each case only a few paper properties that are particularly important for the intended purpose can be optimized, as far as the other properties are concerned, concessions must be made.

Material Properties
The specific characteristic features of paper vary widely, depending on the type of paper. Nevertheless, there are a number of properties that are characteristic of all papers. These are inhomogeneity, hygroscopicity, anisotropy, and viscoelasticity.

Paper is an inhomogeneous material made from homogeneous elements: fibers, fillers, and air-filled pores. The homogeneous regions extend over a few microme¬ters (a few millimeters in the longitudinal fiber direction), corresponding to the characteristic dimensions of these particles.
A sheet of paper also exhibits other inhomogeneities, which can have character¬istic dimensions of a few millimeters or centimeters. These are known as cloudi¬ness and result from undesired fiber flocculation in the sheet forming process [3]. In this way, variations in caliper and density develop, and this leads to correspond¬ing variations in transparency, the “clouds”. Finally, the production process can also give rise to inhomogeneities, e. g., due to stock pulsations in the headbox and in the sheet forming section [4]. Only betaradiographic measurements can give exact information about local mass differences. The existence of inhomogeneities must be taken into account in paper testing, e. g., by choosing the appropriate size of sample.

The most important characteristic of paper is its hygroscopicity, i. e., its ability to absorb or release moisture, depending on the ambient climate, until an equilib¬rium is reached. It is significant whether the state of equilibrium is established by absorption or desorption of water [5]. The equilibrium moisture contents of some types of paper at various levels of relative humidity are listed in Table 11.3.
As a result of the hygroscopicity of paper, physical paper properties, such as sheet dimensions, basis weight, tensile strength, strain to rupture, folding strength, bending stiffness, etc., are dependent on the ambient conditions [6]. With a change in the ambient climate, the full attainment of a new equilibrium mois¬ture content of paper is a process which takes several hours. However, the change in the physical properties of the paper starts almost without delay and occurs very quickly at the beginning and slows down later. For this reason, paper should be processed in rooms in which the climatic conditions are favorable for the partic¬ular paper property required. The paper must be in equilibrium with the climate in the room and the climatic conditions must be kept largely constant. The testing of paper requires the establishment of testing climatic conditions (standard climate: 23 °C, 50 % relative humidity, cf. Section 11.2.2.1).
Paper is an anisotropic material with regard to many physical properties. This anisotropy is due to the anisotropic properties of the individual fibers, which result from the fibrillated microstructure of the fiber rather than the fiber shape [7]. As a result of the fibrillated structure, the fiber can accept, for example, high tensile forces in the direction of the fiber axis with low elongation; however, even small tensile forces acting perpendicular to the fiber axis cause high elongations.

Detailed information on all different paper grades is here.

During drying the fiber shrinks in the axial direction by only about 1 to 2 %. In comparison, shrinkage in the direction perpendicular to the fiber axis (radial direc¬tion) reaches about 30 %. If the fibers in a paper sheet were completely randomly oriented sheet shrinkage in all directions would be equal and mainly governed by the low longitudinal fiber shrinkage. The more the fibers in a sheet are oriented in one direction the higher sheet shrinkage will occur in the direction perpendicular to the main fiber orientation.

This is due to the increased influence of the larger radial fiber shrinkage. The fibers in a paper produced on a paper machine are not aligned randomly, the machine direction is usually preferred (anisotropy of fiber orientation). Furthermore the fiber mat is passed through the paper machine un¬der tension, which prevents free shrinkage of the fibers during drying, mainly in the machine direction. Restraining forces in the cross machine direction are lower and nonuniform across the width, being smaller at the edges. This results in a nonuniform cross machine profile of shrinkage.
Both fiber alignment and shrinkage restraints are responsible for the anisotropy of the moisture expansion of finished paper, which is generally far lower in the machine direction than in the cross machine direction, the latter being nonuni¬form across the width. Both the above factors also affect the load – deformation properties (strain-to-stress) of paper, which are therefore also anisotropic (cf. Table (11.2)).

Detailed information on all different paper grades is here.

Many papers exhibit an anisotropy with respect to their composition in the nor¬mal direction (z-direction). The process technology of production on the four¬drinier wire is responsible for this phenomenon. On the wire side, substances (fillers and fines) that can pass through the wire are washed out, while on the other side, they are retained by the fiber mat. This results in different smoothness prop¬erties on the two sides of the paper, also known as the two-sidedness of paper. Two-sidedness of paper can be minimized by twin wire formers. A further anisotropy in the z-direction results from frozen stresses during nonsymmetrical drying of the web at the top and bottom sides. As a result of the in-plane anisotropy of the sheet, the machine direction must be considered and marked when taking samples for testing anistropic properties, such as strength. Two-sidedness is important in the testing of printability, for example. Therefore top and bottom sides have to be marked as well.
Finally, paper has viscoelastic properties [8, 9], i. e., it can be elastic like a solid or viscous like a thick liquid. The viscoelasticity is also a result of the superimposition of the properties of the individual fibers and those of the fiber network. The vis¬cous flow of the individual fibers is caused by the sliding of fibrils at high tensile loads. The individual fibers exhibit elastic behavior at low tensile loads. In the fiber structure of paper, the sliding of fibers at fiber intersections results in a flow effect and the paper undergoes plastic deformation. A characteristic feature of viscoelas¬ticity is the dependence of the onset of flow on the loading rate. Small loads acting over long periods of time result in flow, whereas large intermittently acting loads cause only elastic deformation. Therefore, in the determination of the tensile strength of paper, the time is fixed within which the tensile stress applied causes the paper to break.
 
Summary
The material paper has essentially four characteristic features: inhomogeneity, hy¬groscopicity, anisotropy, and viscoelasticity. These features must be taken into con¬sideration in the testing of paper (cf. Table (11.4)). The magnitude of these features depends on the type of fiber, the fiber raw material, the pulping process, and on the process techniques used in stock preparation and papermaking. Consequently, there are a large number of variables in papermaking and many degrees of free¬dom are involved in the adjustment of the desired paper properties. Therefore, several different paths can usually be followed to obtain papers with comparable technical properties. This is of importance in the standardization of paper proper¬ties. Only the technical characteristics may be stipulated in each case, but not the process used to realize these characteristics, otherwise, there would be the danger of excluding new technologies and impeding technical progress.

Detailed information on all different paper grades is here.