The last few decades have seen significant advances in the use of composite materials in structural applications. There is no doubt that in engineering circles, composites have revolutionized traditional design concepts and made possible a unique range of new and exciting possibilities as durable materials for construction.
“A composite material, although itself composed of other materials, can be considered a new material that has the characteristic properties that come from its components, processing and microstructure.” (1) Materials that are readily known as composites are metals. , Glass fibers and laminated wood. Paper is a composite material and its compositional nature is a major factor in its wide variety.2 In each type of fiber there are differences in cell length, fibril angle, cellulose content, lignin content, and extractive chemicals. This variety is produced in the same tree due to factors such as early wood versus late wood, young wood versus mature wood, and poplar wood versus heartwood, with additional effects from bark and other contaminants. The compound is also found in fibers such as flax, cotton, grasses, hemp and jute. Most papers are made up of a combination of several types of fibers. Additional changes are due to changes in cooking (sulfate, sulfite, soda), bleaching, and refining (methods of modifying the physical properties of the fibers, such as threshing). In addition to the changes that result from differences in the composition of the pulp, the paper-making process causes changes. Using the same paste but changing the variables of the paper machine can produce papers with very different physical properties. For example, changing the speed of the machine changes the orientation of the fiber, resulting in different resistance properties, especially in the direction of the machine and the cross-section of the sheet. Different wet press conditions change the values of density, porosity, optical, bonding and strength. Calendars affect surface and strength properties. Although the chemistry of several articles can be the same, the physical properties can be quite different, resulting in very different papers.
Composite variables are divided into two types: varnish (dough composition) and molding (production). Furniture variables include types of refinements, sources of fibers and chemicals, dimensions of fibers (length, width and angle of fibrils), furniture composition (ratio of fibers), sizes, fillers and coatings. Formation variables include impact rate, fiber direction, base weight, wet press, drying, fiber distribution, and calendering. Figure 1 is an outline of the many equipping and shaping variables that contribute to the composite structure of the paper. The following discussion of just one of the formation variables, the direction of the paper, should show the complexity of the paper as a composite structure. The main factors controlling the direction of the paper are the orientation of the fiber (how the fibers are placed in the sheet), bonding and drying conditions. Three directions are defined: machine or MD direction, cross machine direction or CD, and off-plane direction or ZD direction. Paper is an anisotropic material: it has different properties depending on the direction of the sheet being tested. There are 9 elastic modulus measurements that can be made along three different axes. 5, 6, 7 Most measurements are made in the MD or CD direction and the two results are significantly different.