CHEM-E2105. Wood and Wood Products

Transcription

1 CHEM-E2105 Wood and Wood Products Wood-water relationships I Mark Hughes 31 st January 2017

2 How does water affect wood? Dimensional changes: Initial shrinkage from green conditions Warping and other unwanted distortions Small changes in response to fluctuations in relative humidity ( movement ) Changes in both short- and long-term mechanical properties: Strength, stiffness and toughness Mechanosorptive properties Susceptibility to biodeterioration: Decreased risk of attack with lower moisture content Suitable conditions for some decay/fungus: C, MC 35-50% <20% thought to be a safe threshold MC

3 Today s topics 1. States of water in wood 2. Moisture content 3. Fibre Saturation Point 4. Equilibrium Moisture Content 5. Sorption Silicone nanofilaments coating on wood surface creating water repellent surface. Image:

4 1. States of water in wood

5 Water in wood Green wood (i.e. wood that is in the native state, having never been dried) contains both free water and bound water Free water is liquid water present in the void spaces within wood, i.e. the cell lumen. Free water is not chemically associated with the cell wall polymers and therefore does not influence mechanical properties Bound water is intimately associated with the cell wall polymers through hydrogen bonding with accessible hydroxyl (- OH) groups on the cell wall polymers (amorphous cellulose, hemicelluloses and lignin) Has been further categorized as freezing and non-freezing bound water. Recently it has been considered that only non-freezing bound water and free water are present in solid wood Image: f533d73d5428/13_05/toolbox13_05/unit9_selecting_timber/section4_seasoning/lesson3_the_drying_process.htm

6 Free water & bound water Cell-wall fully saturated, lumen filled with water ( green ) 2. Above 25-30% moisture content cell cavities contain some water -> free water on the cell wall surface. 3. At the fibre saturation point (see later slides), no remaining free water, but cell-wall remains fully saturated (i.e. only bound water) 4. Water in cell-wall below the fully saturated point, cell-walls lose bound water and shrink. (Image:

7 2. Moisture content

8 Wood & water The amount of water in wood is known simply as the moisture content (MC) Moisture content does itself does not describe the state of the water in wood, though certain inferences can be made depending upon the MC In the hydroscopic range, where only bound water is present in wood, the MC at equilibrium with the ambient relative humidity (RH) is termed the equilibrium moisture content (or EMC)

9 Moisture content The total amount of water in wood is known as the moisture content. This can be bound water or a combination of free water and bound water Moisture content is generally expressed in terms of the oven dry mass of wood Oven dry means dried at >100 o C until constant weight (usually at 103 o C) M init M od MC% 100% M MC is the moisture content expressed as a percentage od M init is the initial mass of the sample M od is the oven dry mass of the sample

10 MC of green wood Green wood has a MC often exceeding 100%. The MC of air-dried wood is somewhere in the region of 8-18% depending on the environmental conditions (Source: Dinwoodie, 2000)

11 Measurement of MC in practice Various methods, e.g.: Gravimetric methods (by weighing) Moisture meters electrical devices that measure the conductivity of wood or capacitance (proportional to the amount of water) Works well within a certain range (typically 5-20%) but at the extremes the measurements become inaccurate (Upper image: (Lower image:

12 3. Fibre saturation point

13 Free water, bound water and fibre saturation point A. Cell wall fully saturated, lumen filled with water ( green ) B. Thin film of free water remaining on lumen surface C. No remaining free water, but the cell wall is fully saturated D. Water in cell wall below the fully saturated point (A) (C) (B) (D)

14 Fibre saturation point ( f.s.p. ) The point at which the bound water is at a maximum and no free water remains Fibre saturation point is a concept (as it is impossible to see or measure directly the point at which there is no free water and only bound water exists) The moisture content at f.s.p. was generally thought to be in the region of 30% (very difficult to measure experimentally), corresponding with a change in properties (e.g. mechanical) Solute exclusion or measurement of EMC at RH approaching 100% (>99.9%) gives f.s.p. values around 10% higher, i.e. 40% The last 10% probably has little effect on mechanical properties, accounting for the differences

15 MC and mechanical properties bound free (Source: Dinwoodie, 2000)

16 FSP and mechanical properties FSP bound free (Source: Dinwoodie, 2000)

17 Wood and water Wood is a hygroscopic material, i.e. it will attract moisture from the surroundings When dry wood gets wet, it swells, leading to sticking doors, drawers and peeling paintwork etc Conversely, when wood dries, it shrinks, distorts and cracks But it is only loss/gain of bound water that affects the dimensions of wood Wood-based products such as particleboard, medium density fibreboard (MDF) and plywood also respond to moisture, but the effect is also complicated by their own structure

18 Importance of drying Wood is usually dried to bring the MC close to the final MC that it will equilibrate to whilst in service and so will undergo smaller dimensional changes For examples, at 65% RH, 20 o C, the MC is around 12% To reduce the MC below a level at which biological attack will be favoured (generally regarded to be around 20% MC) Note: drying does not entirely eliminate dimensional changes!

19 4. Equilibrium moisture content

20 Equilibrium moisture content Equilibrium moisture content, or EMC, is the moisture content that wood reaches when it is placed in certain conditions of temperature and relative humidity. In other word the wood reaches equilibrium with its surroundings EMC is not only dependent on the RH but also sorption history (see later slide) Most wood properties are dependent upon moisture content. It is therefore very important to measure the properties under standard conditions of relative humidity and temperature (generally 65% RH and 20 o C temperature). This is defined in various Standards

21 Relative humidity Relative humidity (RH) is the term used to describe the amount of water vapour that exists in a gaseous mixture of air and water vapour It is the ratio of the partial pressure of water vapour in the mixture to the saturated vapour pressure of water at a given temperature and is dependent upon temperature

22 5. Sorption

23 Relative humidity & EMC Example water vapour sorption measurement of Scots pine at 20 C (Image: Kristiina Laine)

24 Influence of moisture history

25 Source: Dinwoodie, 2000 Relationship between RH, MC and temperature Warm air can hold much more water than cold air

26 Bound water: why does wood attract water? Water is a polar molecule. Because of its polarity, it is attracted to the polar hydroxyl ( OH) groups in the cell wall polymers (mainly in the amorphous regions) of wood and forms hydrogen bonds The hydrogen bonds bind the water to the wood hence the term bound water Hydrogen bonds are relatively strong (but only a fraction of that of the covalent bonds that bind the glucose units of the cellulose chain together) and so the association between wood and water is relatively strong Remember that the extensive inter- and intra-molecular hydrogen bonding in the wood cell wall polymers also accounts for the crystal structure of cellulose and helps control the structure of wood

27 Hydrogen bonding Hydrogen bonds form when polar molecules or moieties (parts of the molecule) are in close proximity The polarity in the case of OH (hydroxyl) groups arise because the hydrogen atom is attached to an oxygen atom which is slightly electronegative, causing a dipole OH groups are present in abundance in the cell wall polymers of wood: hemicelluloses possess the most, followed by cellulose and lignin. Known as sorption sites Estimates that 1-2 H 2 O molecules attach to each sorption site at f.s.p. Hydrogen bonding should not to be confused with the strong chemical (covalent) bonding that hold the atoms in the polymers together Hydrogen bond Bond energies, e.g.: H-bond: 21 kj/mol C-O bond: 358 kj/mol (Image:

28 Inter- and intra- molecular hydrogen bonding Intra-molecular H- bonding within one cellulose chain Inter-molecular H- bonding between neighboring cellulose chains

29 Cellulose

30 Hemicelluloses

31 Lignin

32 Where do the water molecules go? Water is attracted to accessible hydroxyl groups in the amorphous regions of wood: At the microfibrillar level this means the disordered regions of cellulose and hemicellulose surrounding the crystalline cellulose core Example visualisation of the crystalline parts of cellulose microfibrils Disordered / amorphous Ordered / crystalline

33 Some further thoughts Water molecules are small! So cell wall accessibility is an issue; Hydroxyl groups in crystalline cellulose are mainly involved in inter- and intra- molecular bonding and are therefore not able to interact with water; As hemicelluloses are branched molecules, they are only slightly crystalline. They are more accessible and therefore hydroscopic; Lignin is rather more hydrophobic Water molecules are small! ~3.2Å (3.2 x m), 0.32 nm!

34 20 Moisture content (%) Total Data Multilayer Monolayer Relative humidity (%) (Source: C.A.S. Hill)

35 (Image: Laine, 2014: based on Salmén 1990)

36 Relative humidity and EMC RH changes with temperature and other climatic factors and can range widely from <30% to >90% even in house interiors Wood will try to reach equilibrium with those surroundings (Source: Desch & Dinwoodie, 1981)

37 Rate of sorption Moisture transport results from: Vapor diffusion in a porous system Sorption Diffusion of bound water Many factors affect the rate. (Figure from: Engelund et al, 2013)

38 References and further reading Desch, H.E. and Dinwoodie, J.M. (1981). Timber: Its Structure, Properties, and Utilisation, Sixth edition. Macmillan, London; New York 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: