Wood Physics & Wood Materials

The microfibril angle (MFA) refers to the angle between the longitudinal axis of a cell and the direction of the cellulose microfibrils in the secondary cell wall of wood fibers and tracheids. Microfibrils are bundles of cellulose molecules that provide strength and flexibility to the cell wall.

The importance of the microfibril angle lies in its strong influence on the mechanical properties of wood:

• Strength: A lower microfibril angle, where the microfibrils are more aligned with the longitudinal axis of the cell, tends to increase the tensile strength of the wood.
• Stiffness: higher MFA leads to lower stiffness (more flexible)
• Shrinkage and Swelling

Resin is a type of substance produced by many plants, particularly coniferous trees. It's a viscous compound that is secreted in response to injury to the plant, such as a break in the bark, and it's used to seal up wounds and prevent the invasion of insects and pathogens.

Pits (ger. Tüpfel) refer to small openings in the cell wall that allow for water and nutrients to be transported between cells.

• Simple Pits: These are essentially just holes in the cell wall, allowing for direct communication between the adjacent cells. They're typically found in longitudinal cells, like fibers, which are responsible for giving the wood its strength.

• Bordered Pits: These are more complex structures, found in tracheids and vessel elements, which are involved in water transport. A bordered pit has a central hole (or aperture) surrounded by a doughnut-shaped border. This structure allows for the movement of water, while also having a safety mechanism to prevent air bubbles (which can block water transport) from spreading throughout the tree.

Longitudinal direction
0.1 - 0.2% per 1% change in moisture content

Radial direction
2 - 4%

Tangential direction
3 - 8%

Factors

• degree of swelling and shrinkage
  • density
  • chemical composition of the cell wall
    (extractives, volume fraction of hemicellulose and lignin)
• swelling and shrinkage anisotropy
  • density
  • fiber orientation
  • microfibril angle (tension/compression/juvenile wood)

a fibrillation process

Examples:

• Grinding (Masuko Grinder)
• Homogenizing (e.g. Microfluidizer, APV Homogenizer)
• Cryocrushing (Liquid N₂)

particle-based products

• production of the particles --> drying --> gluing
  --> sheet formation --> pre-press --> press
• typical density profiles:
  • homogeneous
  • clear differentiation
  • little differentiation

fiber-based products

• wet process (high-density fiberboards, insulation boards)
• dry process (MDF, nowadays also for insulation boards)
• typical density profiles:
  • medium density, homogeneous
  • medium density, distinct density differences
  • high-density fiberboard
    • smooth side --> highest
    • middle --> lowest
    • wire side --> higher
• applications:
  • roof and wall panels
  • insulation panels (low density)
  • flooring panels (high density)
  • furniture (MDF)

Cellulose - Hemicellulose - Lignin

Spruce
40.4% - 31.1% - 28%

Beech
43.3% - 31.8% - 24.4%

Hardwoods are evolutionary younger and have reached a higher level of specialization in their wood structure.

Softwood - tissue structure

• Tracheids (~90%)
  • structural parameters of one cell type are varied according to the function
  • functions
    • water transport (earlywood tracheids)
    • mechanical function (latewood tracheids)
  • parameters
    • cell wall thickness
    • lumina size
• parenchyma cells
  • mainly in the wood rays
  • storage function
  • resin production
  • production of extractives
  • living cells until heartwood formation

Hardwood - tissue structure

• specific cell types for each function
• vessels --> water transport
• fibers/tracheids --> mechanical function
• ray cells more prominent than in softwoods

Density and Hardness

• Softwoods --> generally softer (they grow faster)
• Hardwoods --> generally denser (exceptions e.g. balsa) --> more resilient

Leaves

• most softwoods are coniferous
• most hardwoods are deciduous and broad-leaved (except: spruce)

Reproductive Structures

• softwoods --> cones (gymnosperms)
• hardwoods --> flowers/fruits (angiosperms)

Reaction wood is a type of wood formed by a tree in response to environmental stress or gravitational pull to restore its vertical orientation or to support branches. It has a different structure, chemical composition, and mechanical properties compared to normal wood.

• Tension wood (hardwood trees/angiosperms)
  • pulls the tree back toward its original position
  • higher proportion of cellulose
  • lower microfibril angle (~ 0°)
     
• Compression wood (softwood trees/gymnosperms)
  • pushes the tree back to its original position
  • compression stresses are generated during cell differentiation
  • cells are more rounded, have thicker cell walls rich in lignin
    higher microfibril angle (~ 30-50°)
  • lack S3-layer

Juvenile wood, also known as core wood or inner wood, is the wood that forms near the center of a tree during its early growth years.

• higher microfibril angle
• lower density
• generally less stiff and lower strength

Mature or adult wood, also known as outer wood, is the wood that a tree produces after the juvenile phase.

False heartwood, also known as discolored heartwood or incipient heartwood, is a type of wood that forms in some tree species when the living sapwood near the center of the tree begins to darken in color, typically due to fungal infection or other forms of stress (environmental factors). Only the lumina of the cells are impregnated with extractives and there is no increase in durability.

Deciduous trees lose their leaves during a particular season, typically autumn in temperate climates, to conserve water during the dry or cold season. Evergreen trees, on the other hand, retain their leaves year-round.

Evergreen (generally softwoods)

• Most species of conifers (e.g., pine, hemlock, spruce, and fir), but not all (e.g., larch)
• live oak, holly, and "ancient" gymnosperms such as cycads
• Many woody plants from frost-free climates
• Rainforest trees
• All Eucalypts
• Clubmosses and relatives
• Most Bamboos

Deciduous (generally hardwoods)

• many broadleafs (oak, ash, birch, beech, maple)

Angiosperms, also known as flowering plants, whose seeds are produced inside a protective structure called a fruit, which develops from the fertilized ovule after flowering.

• typically broad-leaved
• no conifers
• most decidiuous and some evergreens
• oak, maple, birch, beech, ash

Gymnosperms, from the Greek gymnos "naked" and sperma "seed".
The seeds are not enclosed in a fruit. They are often formed in and released from cones, which is why gymnosperms are sometimes referred to as conifers.

• all conifers
• most evergreens
• larch, spruce, pine, fir

• wood is a hygroscopic, porous material with interconnected capillaries of a wide range of diameters
• water movements in the microsystem (cell wall) and macrosystem (cell lumina)
• cell walls take up water (vapor) from the air (sorption)
• cell lumina transport liquid water

Free water --> Macrosystem (cell lumina)

Bound water --> Microsystem (cell wall)

Combination of:

• capillary transport
• Diffusion (Fick's laws)

Diffusion rate depends on the following:

• density
• temperature
• moisture content
• cutting direction

The pressure in a capillary depends on its diameter
the smaller the diameter --> the higher the pressure

Chemisorption

• Formation of a monomolecular layer
• almost no swelling
• heat release
• ~0.6% moisture content, rel. humidity below 20%

Adsorption/Physiosorption

• formation of a molecular multilayer
• less heat release
• ~6-15% moisture content, rel. humidity below 75%

Capillary condensation

• saturation pressure in a micro-capillary is lower than at a plane surface or a macro-capillary
• water vapor is condensing
• ranges of adsorption and capillary condensation are overlapping (transition zone)
• ~15% moisture content to FSP, rel. humidity 75-100%

Impact factors:

• previous history
• chemistry of the cell wall (hemicellulose > lignin)
• mechanical loading (compressive loading --> slower sorption)
• thermal pre-treatment (higher temperature --> higher sorption)
• impregnation (chemical pre-treatment)
  salt impregnation --> higher sorption
• extractives

Bulk density \(\rho_u\)

Oven-dry density \(\rho_{dtr}\)

"Raumdichtezahl" \(R\)

cell wall density \(\rho_r\)

Drying method

moisture content  \(u = \frac{m_u - m_{dtr}}{m_{dtr}} \cdot 100 \: [\%]\)

Resistance test

Making use of the correlation between wood moisture content and electrical resistance.

in \(\left[ kg/m^3 \right]\)

spruce
470

douglas fir
530

larch
500 - 560

pine
350 - 850

maple
620 - 750

oak
600 -900

birch (european)
670

ash (european)
710

beech
700 - 900

ebony
1100 - 1300

balsa
110 - 140

\(E_T < E_R << E_L\)

softwood:

1 : 1.7 : 20

hardwood

1 : 1.7 : 13

Impact factors

• moisture content
• microfibril angle

in \(\left[ N/mm^2 \right]\)

\(E_L -- E_T -- E_R\)

Spruce
10'000 -- 450 -- 800

Pine
11'000 -- 500 -- 1000

Oak
13'000 -- 920 -- 1580

Azobe/Bongossi
17'000 -- 2060 -- 3230

in \(\left[ N/mm^2 \right]\)

\(G_{LR} -- G_{RT} -- G_{IT}\)

Spruce
600 -- 40 -- 650

Pine
- -- 70 -- 680

Oak
1080 -- 400 -- 800

\(\begin{bmatrix}\varepsilon_1 \\ \varepsilon_2 \\ \varepsilon_3 \\\gamma_{23} \\ \gamma_{13} \\ \gamma_{12} \\\end{bmatrix}=\begin{bmatrix}\frac{1}{E_1} & -\frac{\mu_{21}}{E_2} & -\frac{\mu_{31}}{E_3} & 0 & 0 & 0 \\-\frac{\mu_{12}}{E_1} & \frac{1}{E_2} & -\frac{\mu_{32}}{E_3} & 0 & 0 & 0 \\-\frac{\mu_{13}}{E_1} & -\frac{\mu_{23}}{E_2} & \frac{1}{E_3} & 0 & 0 & 0 \\0 & 0 & 0 & \frac{1}{G_{23}} & 0 & 0 \\0 & 0 & 0 & 0 & \frac{1}{G_{13}} & 0 \\0 & 0 & 0 & 0 & 0 & \frac{1}{G_{12}} \\\end{bmatrix}\cdot\begin{bmatrix}\sigma_1 \\ \sigma_2 \\ \sigma_3 \\\tau_{23} \\ \tau_{13} \\ \tau_{12} \\\end{bmatrix}\)

3 E-Moduli

3 Shear Moduli

6 Poisson's ratios

\(H_B = \frac{2 \cdot F}{\pi D \left( D - \sqrt{D^2 - d^2} \right)} \quad \left[ \frac{N}{mm^2} \right]\)

F - Force to indent a polished steel ball into the sample

D - Diameter of the ball

d - Diameter of the indent

is technically used for stress grading

creep

stress relaxation

creep strength

a) idealized creep curve

b) creep curves for different stress levels

The asymptote indicates the stress level that an element can take "forever".

(total) strain = elastic + viscoelastic + mechano-sorptive + plastic

Superimposition of creep and swelling

Strong influence of sample cross-section

Tensile loading and adsorption

Sample length increase due to creep plus increase due to swelling

Compressive loading and adsorption

Sample length decreases due to creep but increases due to swelling

The mechano-sorptive effect refers to the increased creep behavior in wood that occurs under changing moisture conditions.

This swelling or shrinking, combined with mechanical stress, can lead to significantly increased deformation, or creep, over time.

Chemical Modification

Remove the matrix component lignin while retaining the inherent cellulose hierarchical architecture.

Enhancing the mechanical properties of wood by chemical modification such as delignification.

delignification --> cellulose scaffolds (shapeable and high cell wall porosity at wet state)

--> eases functionalization and densification to high fibre volume fractions

Densified cellulose scaffolds possess excellent mechanical properties in dry state

but exposure to water or high humidity drastically diminishes the mechanical performance

--> Cellulose scaffolds need to be impregnated with a matrix system

for example via impregnation of epoxy resin and a subsequent densification

currently testing different bio-based matrix systems

DW = delignified wood

DWRP = delignified wood-reinforced polymer (epoxy)

GFRP = glass fiber-reinforced polymer (epoxy)