Warren J Grigsby*, Hank Kroese and Elizabeth A. Dunningham

Characterisation of pore size distributions in variously dried Pinusradiata. Analysis by thermoporosimetry

Warren J Grigsby*, Hank Kroese and Elizabeth A. Dunningham

Bioproduct Development, Scion, Private Bag 3020, Rotorua 3010, New Zealand

SUPPLEMENTARY MATERIAL

OUTLINE FOR THE Procedure for pore size distribution analysis

Differential Scanning Calorimetry (DSC)

DSC analysis was undertaken exactly as that described by Park et al. (2006)using a Thermal Analysis Q1000 DSCinstrument.At least six replicateaccurately weighed specimens (±0.01 mg)were used for each sample. For those samples having undergone drying, specimens cut from each wood blockwere immersed in water for 5 minutes, blotted with tissue and then weighed into DSC pans as described by (Diesteet al. 2009). All specimens were placed in encapsulated aluminium Hermetic pans.

A DSC temperature step program shown in Table 1(Park et al. 2006) was utilised where each specimen was first cooled and equilibrated to -30°C. After equilibration, the specimen was heated to -20°C at 1°C/min and then allowed to equilibrate at this temperature before continuing the next heating step to -15°C. The remaining temperature steps are outlined in Table 1 and give a thermogram as that in Figure 1.The moisture content of each DSC specimen was subsequently determined by placing a pin hole in the DSC pan lid and then oven drying (105°C) to constant weight.

Estimation of Pore Size

The pore size diameter at each melting temperature was calculated from the Gibbs-Thomson equation (Equation 1) to give a range of pore sizes (400 to 2.6 nm) for the temperature range of -0.1°C to -15°C, respectively (Park et al. 2006). T0 is the melting temperature of water (273.15 K), ls is the surface energy at the ice-water interface (12.1 mJ/m2),  and Hf are the density and specific heat of fusion of freezing bound water, respectively (1000 kg/m3, 334 J/g), D is the diameter of the pore and T the melting temperature depression (K).

(Equation 1.)

Table 1. DSC temperature step ramp used to determine enthalpy changes offreezing bound water in wood cell wall pores

Step* / DSC Temperature
1 / Equilibrate at -30.0°C
2 / Ramp 1.0°C/min to -20.0°C
3 / Ramp 1.0°C/min to -15.0°C
4 / Ramp 1.0°C/min to -10.0°C
5 / Ramp 1.0°C/min to -6.0°C
6 / Ramp 1.0°C/min to -4.0°C
7 / Ramp 1.0°C/min to -2.0°C
8 / Ramp 1.0°C/min to -1.5°C
9 / Ramp 1.0°C/min to -1.1°C
10 / Ramp 1.0°C/min to -0.8°C
11 / Ramp 1.0°C/min to -0.5°C
12 / Ramp 1.0°C/min to -0.2°C
13 / Ramp 1.0°C/min to -0.1°C

* At completion of each step the sample was equilibrated at the stated temperature, before proceeding to the next heating step (Park et al. 2006).

Figure 1.Example DSC thermogram of green wood (earlywood) using the temperature step program.

Determination of bound water and the respective conversion to relative sample pore size distributionswas undertaken according to the methodology described by Park et al.(2006). DSC thermograms using time as the x-axis (Figure 2) were integrated to give the enthalpy of melting (Table 2) for each temperature step.The mass of freezing water (mFW, mg) for each DSC specimen was calculated from the Hpeak (J/g), the integrated area of the endothermic peak, the specific heat of melting of pure water (333.6 J/g) and mass of the sample (m, mg) using equation 2 (Diesteet al. 2009).

(Equation 2.)

Figure 2.DSC thermogram of green wood (latewood) using the temperature step program with time as x-axis.

The amounts of bound water may be converted to g/g using the mass of total water present, providing a percentage of water present in each pore size (for example see Table 2, Figure 3). In this work the bound freezing water is also expressed as a ratio of total bound freezing water to provide comparative pore size distributions, being independent of total water in samples (Figure 4).

Figure 3. Pore size distributions as bound water per total water (g/g) for radiata pine green sapwood.

Table 2. Calculation of melting enthalpies and freezing bound water for representative green wood earlywood (EW) and latewood (LW) specimens of radiata pine sapwood

Heating
Step
(C) / Pore
Size
(nm) / Enthalpy of melting
(Jg-1) / Mass of freezing water
(mFW, mg) / Bound Water per Total Water
(mFW/mTW, g/g)
EW / LW / EW / LW / EW / LW
-15 / 2.6 / 11.2 / 13.8 / 0.097 / 0.133 / 0.047 / 0.071
-10 / 4 / 12.0 / 14.3 / 0.104 / 0.139 / 0.051 / 0.074
-6 / 6.6 / 10.2 / 12.2 / 0.088 / 0.118 / 0.043 / 0.063
-4 / 9.9 / 5.8 / 7.2 / 0.050 / 0.069 / 0.024 / 0.037
-2 / 19.8 / 7.0 / 7.6 / 0.061 / 0.074 / 0.030 / 0.039
-1.5 / 26.4 / 4.4 / 3.7 / 0.038 / 0.036 / 0.019 / 0.019
-1.1 / 36 / 13.7 / 8.7 / 0.118 / 0.084 / 0.058 / 0.045
-0.8 / 49.5 / 0.3 / 4.2 / 0.003 / 0.040 / 0.001 / 0.022
-0.5 / 79.2 / 4.1 / 4.6 / 0.036 / 0.045 / 0.017 / 0.024
-0.2 / 198 / 2.0 / 1.3 / 0.017 / 0.013 / 0.008 / 0.007
-0.1 / 396 / 0.0 / 0.0 / 0 / 0.000 / 0.000 / 0.000
Sum mFW (mg) / 0.611 / 0.751
Sample (mg) / 2.88 / 3.23
Sample o.d. (mg) / 0.83 / 1.36
Sample MC (%) / 247 / 138

Figure 4.Pore size distributions as bound water per total bound water (g/g) for green wood.

References

1. Park, S, Venditti, RA, Jameel, H, Pawlak, JJ,(2006). "Changes in pore size distribution during the drying of cellulose fibers as measured by differential scanning calorimetry." Carbohydrate Polymers 66: 97-103.
2. Dieste A, Krause, A, Mai C, Sebe G, Grelier S, Militz H, (2009). "Modification of Fagussylvatica L. with 1,3-dimethylol-4,5-dihydroxy ethylene urea (DMDHEU). Part 2: Pore size distribution determined by differential scanning calorimetry." Holzforschung 63: 89-93.