CHEM-E0120: An Introduction to Wood Properties and Wood Products Wood, water and dimensional stability

Transcription

1 CHEM-E0120: An Introduction to Wood Properties and Wood Products Wood, water and dimensional stability Mark Hughes 3 rd October 2016

2 Objectives Changes in relative humidity The kinetics of sorption and sorption isotherms Dimensional changes in wood (movement) Swelling & shrinkage anisotropy Impact on wood products

3 The internal climate of a building (Unpublished data)

4 Variations in RH

5 RH & EMC (Source: Dinwoodie, 2000)

6 Sorption kinetics RH MC (Hill et al. 2010)

7 Moisture gradients Since moisture transport processes are not instantaneous, (e.g. limited by the permeability of the material and diffusion), there will be a gradient of moisture from the surface inwards This will have practical significance: internal moisture induced stresses causing e.g. checking and its effect on the buffering capacity (we ll talk about this later) of wood

8 Dimensional changes at the cell-wall level Water molecules may bond with free OH-groups in wood but may also break existing OH-bonding among/between wood constituents During drying these bonds are able to reform As water molecules have a finite size, they squeeze the cell wall polymers apart, thus leading to swelling As the cellulose core is highly crystalline, most of the dimensional changes taking place at the microfibrillar level result in transverse swelling, rather than a change in length (Image:

9 Swelling and shrinkage at the microfibrillar level (Source: Desch & Dinwoodie, 1981)

10 Swelling & shrinkage at the microfibrillar level (M.Hughes ) Lignin/hemicellulose matrix Cellulose chains in microfibril core Axial view Longitudinal view Longitudinal view in the dry, unswollen state Water molecules cannot penetrate the tightly hydrogen-bonded crystalline core of the microfibril, so swelling in the longitudinal sense due to moisture is limited to amorphous regions

11 Swelling Dry Partially saturated f.s.p. (fully saturated) H 2 O L T0 T1 T2 T2 > T1 > T0 L, the length of chains remain unaltered (M.Hughes )

12 Swelling Dry Partially saturated f.s.p. (fully saturated) (M.Hughes )

13 Anisotropic behaviour Microfibrils are not aligned parallel to the fibre axis but at a discreet angle to the axis Microfibrils in the S2 layer (dominant layer) are at angles of degrees from fibre axis. So, the main dimensional change is in the transverse direction, but some component in longitudinal direction

14 10 degree microfibril angle (M.Hughes )

15 30 degree microfibril angle (M.Hughes )

16 90 degree microfibril angle (M.Hughes )

17 Swelling direction vs. microfibril angle (Source Dinwoodie, 2000)

18 Anisotropic behaviour At the cell wall level adsorption / desorption leads to dimensional changes Which direction is influenced the most: radial/tangential/longitudinal? Changes are greatest in the transverse directions (>> longitudinal) Tangential dimensional changes are approximately twice as great as radial What causes this difference?

19 Difference between radial and tangential Due to: movement Restraint of the rays (in the radial direction) Differences in the thickness of the middle lamella between the tangential and radial planes Differences in the degree of lignification of the radial and tangential cell walls Small differences in the MFA between the tangential and radial cell walls Alternation of earlywood and latewood in the radial plane (thought to be the main contributor). i.e. the late wood and early wood either act in series or in parallel

20 Anisotropy in movement Shrinkage (%) on drying from green to 12% MC (Dinwoodie, 2000) Botanical name Chlorophora excelsa Tectona grandis Pinus strobus Picea abies Pinus sylvestris Tsuga heterophylla Quercus robur Fagus sylvatica Commercial name Iroko Teak Yellow pine Whitewood Redwood Western hemlock European oak European beech Transverse Tangential 1,0 1,2 1,8 1,5 2,2 1,9 2,5 3,2 Radial 0,5 0,7 0,9 0,7 1,0 0,9 1,5 1,7 Longitudinal <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 Approx. x2 difference

21 Dimensional changes Shrinkage / swelling expressed as a percentage is given by: S Ls Linitial,, L(%) 100% L R T initial S L s L initial is the swelling (or shrinkage) in the Radial, Tangential or Longitudinal sense (or any other dimension, such as thickness in board products) is the dimension after swelling or shrinkage is the dimension before swelling or shrinkage

22 Dimensional changes (Source: Dinwoodie, 2000)

23 Dimensional changes: moisture induced strain Shrinkage / swelling expressed as a percentage is given by: S Ls Linitial,, L(%) 100% L R T initial S L s L initial is the swelling (or shrinkage) in the Radial, Tangential or Longitudinal sense (or any other dimension, such as thickness in board products) is the dimension after swelling or shrinkage is the dimension before swelling or shrinkage

24 Hooke s law E L L L 0 0 E is Young s modulus is stress L L 0 Example (softwood): Young s modulus perpendicular to the grain ~ 1 GPa Strain in moving from 90% to 60% RH ~ 1,5 % (moisture induced strain) Moisture induced stress is then 15 MPa Tensile strength perpendicular to the grain ~ 3,7 MPa! is strain is extended length is original length S Ls Linitial,, L(%) 100% L R T initial

25 Impacts of dimensional changes Differential moisture induced stains can lead to stresses in e.g. glued joints Swelling pressure in board products can lead to delamination Cracking of paint films or other coatings Dimensional instability in solid wood and wood products Can be controlled by the proper design of wooden products/structures (!!) and/or for example wood modification (e.g. thermal modification)

26 Crack (M.Hughes )

27 Failure of paint film (M.Hughes )

28 Moisture induced delamination (of MDF) (M.Hughes )

29 Movement Dimensional changes are in response to a loss/gain of bound water i.e. dimensional changes occur below f.s.p. Movement is the dimensional change resulting from changes in MC below the fibre saturation point, i.e. loss of or gain of water by the cell wall This occurs in response to changes in RH

30 Movement classes Represent the sum of the radial and tangential movements of changing RH from 90% to 60% at 25 o C (Adapted from Desch & Dinwoodie, 1981) Small <3.0 Medium Abura Ash (Eur.) Ash (Jap.) Afara Elm Beech Af. Mahogany Kapur Birch Af. Walnut Keruing Ekki Agba Lime Holly Balsa Maple, rock Ramin Guarea Oak (Eur), oak (jap.) Iroko Poplar Jelutong Sapele Mahogany (Central Am.) Sycamore Makoré Utile Obche Walnut (Eur.) Teak Large >4.5 Douglas fir Parana pine Norway spruce Pitch pine W. hemlock Radiata pine Yellow pine Scots pine

31 Other impacts of adsorption/desorption Wood absorbs moisture from the air during high RH times and releases it during drier periods Wood indoors stabilises RH reducing peaks This is termed buffering

32 Changes in mechanical properties Some properties affected more than others The percentage change in the modulus of rupture (MOR) of Scots pine, for example, per 1% change in MC is: 4.2% (in the range 6-10% MC) 3.3% (in the range 12-16% MC) 2.4% (in the range 20-24% MC) Therefore MOR will change by approximately 25% when the EMC changes from 12 to 20%

33 Failure of coatings & films No failure Strain condition F c ; Explanation β is smaller than F c and ; this means that the stress in the coating is not large enough to cause either cohesive failure or loss of adhesion. Cracking F c ; β is larger than F c but smaller than ; hence the coating will adhere to the wood but fail cohesive in the coating. This means that the coating cracks Flaking F c ; β is both larger than and F c; therefore the coating can both fail cohesive and adhesively. We will observe flaking from the surface. Peeling F c ; β is smaller than F c but larger than ; this means that the coating will not fail by cohesively but adhesively. This will be observed as spontaneously detachment of the coating from the wood (peeling). Blistering F c ; ( W M ) β is smaller than F c but β+w M larger than W M represent the work from the vapour pressure coming from the wood. This means that the coating will lose its adhesion from the wood but will not fail cohesively, and blistering occurs. Wood cracking F W < W ; W > F c This case actually represents failure starting in the wood surface. If the hygroscopic strain at the wood surface (β W) exceeds the cohesive strength of the wood (F W), the wood itself will crack. These cracks might propagate into the coating, up to the surface where it will be seen as coating cracks. Strain energy (ß) adhesion (γ) and cohesion (F c ) (de Meijer 2002)

34 References and further reading Desch, H.E. and Dinwoodie, J.M. (1981). Timber: Its Structure, Properties, and Utilisation, Sixth edition. Macmillan, London; New York De Meijer, M., Militz, H Moisture transport in coated wood. Part 2: Influence of coating type, film thickness, wood species, temperature and moisture gradient on kinetics of sorption and dimensional change. Holz als Roh- und Werkstoff 58: Dinwoodie, J.M. (2000). Timber: Its nature and behaviour Engelund, E.T., Thygesen, L. G., Svensson, S. and Hill, C.A.S. (2013). A critical discussion of the physics of wood-water interactions. Wood Sci Technol 47: Hill, CAS, Norton, A and Newman, G (2010) The Water Vapor Sorption Behavior of Flax Fibers Analysis Using the Parallel Exponential Kinetics Model and Determination of the Activation Energies of Sorption. Journal of Applied Polymer Science, 116(4):

35 Additional points

36 Volumetric changes Volumetric shrinkage / swelling expressed as a percentage is given by: Vs Vinitial VS (%) 100% V initial VS is the volumetric swelling (or shrinkage) V s is the volume after swelling (or shrinkage) V initial is the volume before swelling (or shrinkage)

37 Water absorption Water absorption expressed as a percentage is given by: Absorption W W W soaked initial initial 100% W soaked is the weight (g) after soaking W initial is the initial weight (g) before soaking