Is hardwood harder than the softwood?
You may have heard about different types of trees? One of the distinctions comes through the definition of hardwood and softwood. In fact, distinguishing these types by measuring hardness is not the finest idea, since wood shows a great variability in hardness between the species. The best way to distinguish them is by observing leaves. Hardwoods have large leaves which they lose every autumn, with some exceptions, as usual. They reproduce by flowers. Such trees are also called deciduous. Perfect examples of hardwood trees are birch, maple, oak, eucalyptus (evergreen, actually), chestnut and poplar.
Coniferous forests. Photo by: Johannes Plenio. / Deciduous forest. Photo by: Valentin Sabau
Softwood trees are, on the other hand, coniferous with spruce and pine being the most popular examples. Softwood trees are often taller and grow faster than hardwood. This is the reason they are the largest source of timber on Earth. In Europe, they predominantly grow in the northern hemisphere making Nordic regions rich in forest and related industries.
What if you get two chunks of wood. Is it possible to distinguish them?
Often yes. They softwood has a more porous structure and, therefore, feels rougher when you rub it. A spongier structure is the reason why softwoods are indeed often lighter than hardwoods, and this is where “hard” and “soft” originate from. We tend to associate hardness with density.
Oak. Photo by: Anja Osenberg / Spruce. Photo by: Ilona Freiburg
What about pulp fibers? Is there a remarkable difference between them depending on the source?
Well, here it more complicated since fibers may have undergone a rough treatment on the way from trees to pulp or can even be recycled. Furthermore, a great variability in fiber morphology makes the visual comparison very difficult. However, on average, softwood fibers are considerably longer (1-3 mm as compared to 0.6-1 mm in hardwoods) and somewhat thicker than hardwood. The shape factor plays a role in producing the paper grades. Hardwood fibers tend to give a smoother surface. Hardwood contains large vessel cells which are specialized in transporting the water in the tree. These vessels have such a characteristic shape for each type of tree, that it is sometimes possible to identify the source of pulp by looking at the vessel cells in it.
The most popular hardwood used in paper-making is eucalyptus, which can grow extremely fast in an appropriate climate and thus can compete with softwoods.
Softwood pulp (optical microscopy and SEM)
Artem Kulachenko, KTH Royal Institute of Technology
Leader of WP3, Paper and packaging, & supervisor of IRP8