Softwood

In general, softwood lignin predominantly contains guaiacyl units (90–95%) linked together with ether and carbon-to-carbon bonds while lignin derived from hardwood has an equivalent amount of guaiacyl and syringyl units with a negligible amount of p-hydroxyphenyl [63].

From: Lignin-Based Materials for Biomedical Applications , 2021

Sustainability of timber, wood and bamboo in construction

M. Asif , in Sustainability of Construction Materials, 2009

2.2 Softwood and hardwood

Wood can be broadly classified into two main groups: softwoods and hardwoods. The terms 'softwood' and 'hardwood' do not indicate softness or hardness of particular timbers. Some hardwoods are actually softer and lighter than softwoods. Mountain-grown Douglas Fir, for example, produces an extremely hard wood although it is classified as a softwood, and Balsawood is classified as a hardwood although it is very soft. Softwood and hardwood normally differ from each other in terms of the botanic structure of the wood. The dominant feature separating hardwoods from softwoods is the presence of pores or vessels in the former. Softwood and hardwood forests are not uniformly distributed in the world – the Northern Hemisphere contains mostly softwood forests and the Southern Hemisphere mostly hardwoods as shown in Table 2.1. 1

Table 2.1. Regional distribution of softwood and hardwood forests in the world 1

Region Softwood forest (%) Hardwood forest (%)
Africa 0.2 10.9
Asia 5 19.5
Central America 1.5 2.3
Europe 8.2 4.3
CIS* 53 13.6
North America 30.5 13.4
South America 0.8 32
Oceania 0.8 4
World (total) 100 100
*
Confederation of the Independent States of the former Soviet Union.

2.2.1 Softwood

Softwoods are conifers and normally have needle-like leaves. They generally have lower densities and are often light in colour. Softwoods usually grow quicker than hardwoods and are cheaper, softer and easier to work. Common examples of softwood include: pine, fir, spruce, larch and cedar.

2.2.2 Hardwood

Hardwoods generally have broad leaves and often have dark-coloured wood. They normally have higher densities and thicker cell walls than softwoods. There are a much greater number of hardwood species than there are softwoods. Some examples of hardwood include: oak, ash, elm, beech, birch and teak.

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Development of biobased wood polymer nanocomposites: industrial applications, market, and future trends

Moon Mandal , ... Tarun K. Maji , in Handbook of Polymer Nanocomposites for Industrial Applications, 2021

22.3.1.1 Softwoods

Softwoods are gymnosperms or conifers with naked seeds (not exposed in the ovary of the flower). They are nonporous and do not contain vessels. They are usually cone-bearing with needle- or scale-like evergreen leaves. Most softwoods keep their needles all year but larches and bald cypresses lose their needles during the autumn or winter. Generally softwoods grow in a pyramid shape: small at the top and broadening as they go down [39]. Examples of softwoods are cedar, pine, fir, spruce, hemlock, tamarack, and redwood. The cellular structure of softwoods is shown in Fig. 22.2.

Figure 22.2. Cellular structure of softwood (white pine). AR, Annual rings; BP, border pit; FWR, fusiform wood rays; HRD, horizontal resin duct; RR, edge grain; S, earlywood; SM, latewood; SP, simple pits; TG, flat grain; TR, tracheid; TT, end grain; VRO, vertical resin duct; WR, wood rays.

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Wood: Sawn Materials

P.R. Blankenhorn , M.S.H. Bhuiyan , in Reference Module in Materials Science and Materials Engineering, 2016

3.1.2 Softwood lumber grades

Softwood nonstructural grade categories include appearance, factory (shop) including softwood finished products, and industrial clears. Softwood structural grades and hardwood factory, dimension, and finish product grades are more standardized than softwood nonstructural grades. This may be associated with the large number of different softwood regions in the USA, each one with separate grading agencies promulgating softwood grading rules.

Softwood appearance lumber grades are used for interior walls, molding, siding, architectural woodwork, trim, and other special applications. The two general categories are finish and selects. Finish grade names depend on the grading agency, and include letter combinations such as B and better, C, C and better, and D, or names such as superior and prime. These grades may be further categorized as flat grain, vertical grain, or mixed grain.

Factory (shop) grades of softwood lumber are grades of lumber that are cut and used in remanufactured wood products. There are many grades and characteristics associated with factory grades of softwood lumber and the different softwood grading agencies. Utilization of a softwood factory grade will depend on the grade and the softwood grading agency promulgating the different factory grades. Typical grades within each factory grade product classification are select, No. 1, No. 2, and No. 3. Softwood factory grade names are too numerous to categorize and summarize.

Industrial clears are grades that have excellent appearance, wood properties, and finishing characteristics. Grading is based on the best face, and the grades are high-quality lumber used for products such as cabinets and doors. There are usually three grades in this category.

Softwood finished market product grades include similar items to the hardwood finished market products. However, some softwood grading agencies have grades for additional products such as ladder, pole, pencil, tank stock, scaffold plank, pipe material, and siding. As with the hardwood finished market products, each product has a subset of grades.

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Wood: Future Availability

J.L. Bowyer , in Encyclopedia of Materials: Science and Technology, 2001

(a) Natural forests

Softwood species have long comprised the majority of industrial wood used annually worldwide. Within natural forests currently (i.e., in 2000), these species occur in greatest volume in Russia, North America (primarily the USA and Canada), and in northern Europe. The current net annual growth (all growth minus losses from fire, insects, disease, and decay) of softwood species in nonreserved forests of North America is very close to being in balance with the annual harvest. Moreover, pressures to reduce forest harvest levels are high and increasing. On the other hand, the softwood forests of northern and western Europe are currently increasing in volume (i.e., growth exceeds harvest), and despite antiharvest sentiment in some segments of society, it appears that modest increases in annual softwood harvests may be possible. The greatest potential for increased softwood harvests is in Russia, and in Siberia and in the Russian Far East in particular. These forests currently contain over half of the large softwood timber in the world, and political and economic instability in recent decades has brought harvest activity to a near standstill in vast regions. When the situation will stabilize so as to bring about a greater level of management and harvesting activity in these forests is an open question; in the longer term, however, there is little doubt that Russian forests will contribute more than they do now to the world's wood supply ( Sedjo and Lyon 1990).

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Pulp and Paper: Wood Sources

J.R. Dillen , ... M.F. Hamza , in Reference Module in Materials Science and Materials Engineering, 2016

2.1 Softwood

Softwoods are the preferred raw material for 'strong' papers, primarily because of the length and slenderness of its fibers. In other papers it is typically used as a 'reinforcement' component. Low-density softwoods (various spruces, firs, scots pine, lodgepole pine, etc.) in which thin-walled fibers normally are predominant are preferred for papers with high demands for bonding-related strength characteristics, such as tensile, burst, and surface strength. Denser softwoods, such as the southern pines (slash pine, loblolly pine, longleaf pine, and shortleaf pine) and some US western species, such as the Douglas fir, give more tear strength and bulk, which are essential when paper with a high bending stiffness is required. The radiata pine, which is increasingly cultivated in South America, South Africa, and New Zealand, holds an intermediate position.

Production of lumber for sawmills traditionally has been and still is the primary objective of forestry based on softwoods. Pulpwood extracted from softwood forests emanates mostly from thinnings and from the tops of mature trees. Consequently, pulpwood normally contains a larger share of juvenile wood than the average of a mature tree. Juvenile wood is characterized by having slightly smaller fibers and a somewhat lower density than the average of a mature tree.

In mechanical pulping, softwoods, especially spruce, are the preferred raw material, since most hardwoods are too dense. Exceptions in this respect are Populus species (aspen, cottonwood, etc.)

In mechanical pulps, the original wood fiber characteristics of different wood species are not retained to the same extent, since in most of these processes a major fraction of the fibers is broken down to smaller elements or fragments. Fragmentation is very dependent on wood density, and low-density wood normally will yield a higher pulp quality.

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INTERFACE BOND CHARACTERISTICS BETWEEN WOOD FIBRES AND A CEMENT MATRIX

M. Guadalupe SIERRA BELTRAN , Erik SCHLANGEN , in Brittle Matrix Composites 9, 2009

Wood

Softwood fibres from spruce, larch and pine are used in this study. Fibres from softwood have a more simple form without vessels compared to hardwood fibres and the fibres are also longer (3 to 5 mm average) and thicker (up to 45 µm). Softwood was found by Blankenhorn et al. [ 1] to be less inhibiting to cement setting than hardwood is. For larch and spruce and pine two different pulping procedures have been used. To obtain bundles of fibres out of lumber pieces of larch and spruce blocks of 1×1x2 cm have been cooked following a neutral sulphite semi-chemical (NSSC) pulping procedure as described by Walker [2] after which the bundles were manually taken apart. The bundles from pine on the other hand were prepared out of a veneer sheet which was first cut into pieces of about 2×3 cm and then cut into bundles using a microtome. Due to the difference in pulping procedure the pine bundles have a uniform rectangular section while the spruce and larch bundles cross sections vary in dimensions and shape. All bundles were cut at length 10 mm. The bundles have a tensile strength of 700 to 1000 MPa, and Modulus of elasticity of 25 to 40 GPa according to Sierra and Schlangen [3]. From now on these bundles will be referred as fibres.

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Wood microstructure – A cellular composite

M.P. Ansell , in Wood Composites, 2015

1.2.1 Softwoods and hardwoods

Softwoods are made up of two types of cells, tracheids and rays (see Figures 1.5 and 15.1). The two major functions of tracheids involve supporting the mass of the tree and transporting water and mineral salts from the roots up the stem (Barnett and Bonham, 2004). Around 90% of cells in softwoods are tracheids, which are aligned parallel with the trunk (longitudinally) and hence allow the vertical transportation of fluids whilst also acting as the primary structural elements. In contrast, ray cells are located in the radial–longitudinal (RL) plane and ensure radial movement of water and minerals between the tracheids (Dinwoodie, 2000) as well as storing starchy material. Rays are the sole means of translocating products of photosynthesis from the inner bark into the tree as well as storage.

Figure 1.5. SEM images of Scots pine (Pinus sylvestris) softwood. (a) 3D sections with a complete annual ring within the cross-section and (b) tracheid width is ~   30   μm and bordered pit openings are visible on the radial–longitudinal section.

The pit openings imaged in Figure 1.5b allow the movement of moisture from tracheid to tracheid. Many of these openings are termed bordered pits and contain cellulose membranes which act as valves and control the passage of moisture in response to internal pressure (Choat et al., 2008). Bordered pits, whilst allowing bi-directional flow, act as stop valves when there are sudden differences in pressure which are caused by breaks and embolisms in the water column. Once closed, bordered pits do not reopen. The tracheids in softwoods range in length from 2 to 4   mm so the presence of pits is essential to allow water to pass from cell to cell as it passes from the ground to the leaves in the process of transpiration.

Due to the thin walls and large lumen of the cellular material, earlywood is less dense than the latewood and is responsible for conducting water up the stem (Desch and Dinwoodie, 1996). The latewood is produced later in the season and due to its thicker walls and smaller lumens, it is responsible for supporting and strengthening the tree. The earlywood-to-latewood ratio within an annual ring is known to vary from year to year depending on the climate and growing conditions, in turn affecting the mechanical properties of the wood.

In evolutionary terms, hardwoods are much younger than softwoods and their cellular structure is more complex (Figure 1.6). The longitudinal cellular elements include fibres and tracheids together with much larger vessels. In oak (Figure 1.6a), the vessels develop in the earlywood in concentric rings and the structure is termed ring porous. In species such as beech, the vessels are randomly distributed and the structure is termed diffuse porous. In hardwoods, vessel size in often a function of water potential. Large vessels soon form embolisms as water potential falls during the growing season, hence the need for formation of smaller vessels. Where water potential is low, all trees have small diameter conducting elements. The radial cells occupy a much larger volume in hardwoods, sometimes as high as 50% (Figure 1.6b), but they perform the same role as in softwoods.

Figure 1.6. SEM images of English oak (Quercus robur) hardwood. (a) 3D sections with a complete annual ring within the cross-section including large ring porous vessels and (b) cross-section, illustrating variation in the diameter of longitudinal cells, and tangential longitudinal section, containing a high proportion of medullary ray cells.

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Natural fibres for paper and packaging

Raphaël Passas , in Handbook of Natural Fibres (Second Edition), 2020

18.2.1.1 Softwood fibres

Softwood structure is quite simple because there are only three kinds of cells. Tracheids, which are called long fibres by the pulp and papermakers, have two functions. They conduct the sap in the tree but they also ensure the mechanical characteristics of the wood. They are arranged in the longitudinal direction most of the time. At their surface, they have some pits allowing the sap to go from one tracheid to the other (bordered pits) and from one tracheid to parenchyma cells (cross-field pits). To distinguish early wood from late wood it is necessary to care about tracheid width and about cell wall thickness (cf. Table 18.2). Early wood tracheids have a larger lumen and thin wall fibre in order to transport a large amount of sap. For some species, horizontal tracheids could be present (e.g. Pinus Sylvestris). Rays are in radial direction. They are composed of superimposed parenchyma cells. These could be associated with horizontal tracheids for some species. Parenchyma cells allow the sap conduction in the radial direction and they can store nutrients. Resin canals are not always present. They are oriented either longitudinally or radially and they communicate with each other (Isenberg, 1963). Fig. 18.4 summarises schematically the softwood structure.

Table 18.2. Fibere width and cell wall thickness of softwood.

Species Fib width (μm) Cell wall thickness (μm)
Earlywood Latewood Earlywood Latewood
Pine 30–39 25–28 2.8–3.4 3.5–4.0
South Pine 40–60 25–33 3.0–4.5 7.0–10

Figure 18.4. Schematic softwood structure (source: Smook, 1992).

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Timber

Peter A. Claisse , in Civil Engineering Materials, 2016

33.2.3 Forests that have been planted and are replanted after cutting

Most softwood is produced in this way. When these forests are harvested, there is little environmental impact, except for the effect of transport, etc. Trees take most carbon dioxide from the atmosphere when they are young, and some old trees actually emit methane that is a powerful greenhouse gas. If the trees are harvested, and the timber is used in a structure, the carbon dioxide is removed from the atmosphere and not returned (as it would be, if the timber was burnt or decayed naturally). The impact of softwood use on the environment/greenhouse effect is therefore probably beneficial.

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