The Interaction Anhydride Modified Wood with Water. Prof. Callum Hill Centre for Timber Engineering Napier University

Transcription

1 Establishing Timber as a preferred material for construction COST FP0802 VIENNA MAY 2009 The Interaction Anhydride Modified Wood with Water Prof. Callum Hill Centre for Timber Engineering Napier University June 09

2 Acetylation O O C H 3 C C OH + O O CH 3 + H 3 C C CH 3 COOH O The Centre for Timber Engineering, Napier University Slide 2 of 30

3 Acetylation As WPG increases the cell wall swells This is manifested as macroscopic swelling of the cell wall But the relationship between the two is not simple The Centre for Timber Engineering, Napier University Slide 3 of 30

4 The Centre for Timber Engineering, Napier University Slide 4 of 30

5 The Centre for Timber Engineering, Napier University Slide 5 of 30

6 The Centre for Timber Engineering, Napier University Slide 6 of 30

7 The Centre for Timber Engineering, Napier University Slide 7 of 30

8 Measuring cell wall volume Use helium pycnometry Must use dry samples Can use to determine volume swelling due to chemical modification Cannot use to determine cell wall volume swelling due to presence of moisture The Centre for Timber Engineering, Napier University Slide 8 of 30

9 Theoretical volume change Roger Rowell used a value for density of acetyl group in the cell wall of g cm -3, which is the density of acetic acid Question 1: is this a good idea or not? Theoretical volume change can be calculated as: (W mod W unmod )/1.049 cm 3 The Centre for Timber Engineering, Napier University Slide 9 of 30

10 Corsican pine (He pyc) Measured Volume Change (cm 3 ) Theoretical Volume Change (cm 3 ) The Centre for Timber Engineering, Napier University Slide 10 of 30

11 Corsican pine (external) Measured Volume Change (cm 3 ) Theoretical Volume Change (cm 3 ) The Centre for Timber Engineering, Napier University Slide 11 of 30

12 Question 2 What is the reason for reduction in cell wall moisture content due to modification with acetic anhydride? Is it because of loss of hydrophilic sorption sites (OH groups)? Is it because of cell wall bulking? The Centre for Timber Engineering, Napier University Slide 12 of 30

13 EMC (%) OH Substitution? Acetic Hexanoic Unmodified O 19.5% WPG FSP C O CH O O C C H 2 H 2 C C H 2 H 2 C CH RH (%) The Centre for Timber Engineering, Napier University Slide 13 of 30

14 Conclusion EMC reduction is not due to OH substitution But a word of warning EMC is reduced, in part, because the wood weighs more Therefore need to determine EMC R Which is the moisture content based upon the cell wall mass (not cell wall mass plus adduct) The Centre for Timber Engineering, Napier University Slide 14 of 30

15 EMC or EMC R (%) EMC and EMC R Unmodified Acetic EMC Acetic EMC R Hexanoic EMC Hexanoic EMC R RH (%) The Centre for Timber Engineering, Napier University Slide 15 of 30

16 Question If OH substitution is not the reason for a reduction in EMC is it cell wall bulking? We need to know how much volume the acetyl group occupies in the cell wall and we cannot get this by measuring external dimensional changes We have to use helium pycnometry The Centre for Timber Engineering, Napier University Slide 16 of 30

17 Molar volume This is the volume occupied by one mole of adduct Equivalent to the volume occupied by one acetyl group (divide molar volume by Avagadro s number) Should not change with WPG Molar volume = [(Vmod Vunmod)*MW] / (Wmod Wunmod) The Centre for Timber Engineering, Napier University Slide 17 of 30

18 Molar volume Corsican pine Molar Volume (cm 3 mol -1 ) WPG (%) The Centre for Timber Engineering, Napier University Slide 18 of 30

19 Conclusion Molar volume appears to vary with WPG (theoretically it should be constant) Molar volume has a value between 30 and 40 cm 3 mol -1 The Centre for Timber Engineering, Napier University Slide 19 of 30

20 Principle WATER-SATURATED CELL WALL VOLUME Volume of cell wall occupied by water at saturation Volume available for water Cell wall volume occupied by bonded reagent NB THESE VOLUMES CANNOT BE DETERMINED BY MEASURING THE EXTERNAL DIMENSIONS OF THE SAMPLE OVEN-DRY CELL WALL VOLUME The Centre for Timber Engineering, Napier University Slide 20 of 30

21 FSP and WPG sorption isotherm Fibre saturation point (%) UNMODIFIED ACETIC PROPIONIC BUTYRIC VALERIC HEXANOIC b 40cm 3 mol -1 30cm 3 mol Weight percentage gain (%) The Centre for Timber Engineering, Napier University Slide 21 of 30

22 Conclusion Either cell wall bulking is not responsible for the reduction in FSP Or We are not measuring FSP when we project adsorption isotherms to 100% RH Stamm discussed this issue at length many years ago Projection to 100% RH does NOT give FSP! So why do so many people persist in doing this? The Centre for Timber Engineering, Napier University Slide 22 of 30

23 FSP Generally, projection methods give FSP values of 25 30% What do we mean by FSP? Cell wall MC at saturation How do we know we have achieved saturation? With solute exclusion we know we have saturated the cell wall Solute exclusion gives FSP of 40% The Centre for Timber Engineering, Napier University Slide 23 of 30

24 Solute exclusion Probe molecule Inaccessible water The Centre for Timber Engineering, Napier University Slide 24 of 30

25 Solute exclusion - unmodified Accessibility (ml/g) Probe Diameter (Angstroms) The Centre for Timber Engineering, Napier University Slide 25 of 30

26 Solute exclusion 13% WPG Accessibility (ml/g) Probe Diameter (Angstroms) The Centre for Timber Engineering, Napier University Slide 26 of 30

27 Solute exclusion 21% WPG Accessibility (ml/g) Probe Diameter (Angstroms) The Centre for Timber Engineering, Napier University Slide 27 of 30

28 FSP and WPG solute exclusion FSP (%) UNMODIFIED ACETIC HEXANOIC cm 3 /mol cm 3 /mol WPG (%) The Centre for Timber Engineering, Napier University Slide 28 of 30

29 Conclusion Volume occupied by anhydride molecule in cell wall is no longer available to water molecules This reduces FSP as determined by solute exclusion FSP as determined by projection of adsorption isotherm does not obey this behaviour The Centre for Timber Engineering, Napier University Slide 29 of 30

30 What is FSP? Moisture content (%) Desorption from saturation Desorption Adsorption Relative humidity (%) The Centre for Timber Engineering, Napier University Slide 30 of 30

31 The Centre for Timber Engineering is based at Napier University in Edinburgh. It aims to provide a support base to establish timber as a preferred material for construction through: Undergraduate and Postgraduate Programmes Continuing Professional Development Courses Applied and Strategic Research Expert Advice, Consultancy and Knowledge Transfer The Centre for Timber Engineering, Napier University Slide 31 of 30