4 Paper III: Influence of curing conditions on properties of melamine modified wood
4.2.1 Determination of minimum curing time and temperature
The analysis with DSC (200 F3 Maia, NETZSCH GmbH, Selb Germany) was used to record the dependency of the degree of conversion (degree of curing) on temperature and time of the curing process. Oven-dry beech wood specimens were impregnated and weighed directly after impregnation to determine the solution uptake (SU). Afterwards cylindrical specimens were die-cut, weighed and stored at -18 °C until curing and subsequent testing.
Figure 4: Preparation of melamine treated beech specimens for DSC analysis
The specimens (Figure 4) of MMF treated beech wood were cured in a laboratory drying oven at set temperatures for specific durations (Table 6).
Table 5: Overview of the solid content (SC) of the impregnation solution [%], size of the specimens for treatment (mm³), and type of specimens (solid specimens or wood particles) for analysis of the different curing series.
List of the applied tests for each curing series.
Series Applied tests SC
[%]
Size of specimens for treatment (rad x tan x lon) [mm³]
Specimen type for analysis Minimum thermal
requirements
DSC 50 4 x 10 x 65 (impregnation);
4 x 4.5 (curing, DSC)
Solid specimens
Material 3-point bending 19 10 x 10 x 180 Solid specimens
properties Nitrogen fixation 25 x 25 x 10 Wood particles
FA emission Solid specimens
Formaldehyde FA content 19 25 x 25 x 10 Wood particles
properties FA emission Solid specimens
Content of free FA Solid specimens
Nitrogen fixation Wood particles
Material and Methods
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Table 6: Parameters of the curing processes for the determination of the minimum thermal requirements via DSC: Curing temperature [°C] and - duration [h]. Subsequent analysis of the residual reactivity via DSC.
Temperature [°C] Duration [h]
90 0.1, 0.2, 0.4, 0.8, 1.7, 3.3, 6.7, 27, 48, 107
90 0.1, 0.2, 0.4, 0.8, 1.7, 3.3, 6.3, 13, 19, 91
105 0.1, 0.2, 0.4, 0.8, 1.7, 3.3, 6.7
110 0.1, 0.2, 0.4, 0.8, 1.7
120 0.1, 0.2, 0.4, 0.8
The specimens consisted of wood, water and resin but only the resin released measurable reaction energy. Therefore, the mass of the specimens was corrected to only account for the mass of the resin (Eq. 1).
Mi – M0 – Mw = Mr (1)
Mr = Mass of resin [g]
Mi = Mass after impregnation and die-cut [g]
M0 = Dry mass before treatment [g]
Mw = Mass of water [g]
After curing the specimens were placed in high pressure, gold plated steel crucibles (30 µl) which were sealed and subjected to a temperature gradient ranging from 20 °C to 180 °C at a heating rate of 10 °C/min. The enthalpy changes were recorded and analyzed for the onset- and peak temperature and enthalpy integral (H) with the NETZSCH Proteus Thermal Analysis 5.2.0 program.Onset - and peak temperature were used to describe the reaction. The onset temperature is defined as the intersection of the tangents of the peak and the extrapolated baseline. It is defined as the temperature at which the reaction starts to accelerate and subsequently proceeds without external energy input. The peak temperature is defined by the maximum thermal activity (heat flow) of the reaction. Uncured specimens were analyzed in the DSC to determine the full reaction enthalpy. As described by Kandelbauer et al.
(2009b) the calculated degree of conversion is the ratio of the enthalpy of the cured and uncured specimens (Eq. 2). To determine the influence of wood on the curing reaction of MMF, the reaction kinetics of the pure stock solution (75 % SC) of the resin was analyzed in the DSC with the same temperature program as mentioned above.
Hα [%] = (Huncured - Hcured) / Huncured *100 (2)
Hα = Degree of conversion [%]
Huncured = Enthalpy integral of the uncured specimen [kJ/g]
Hcured = Enthalpy integral of the cured specimen [kJ/g]
Paper III: Influence of curing conditions on properties of melamine modified wood
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The information about the reaction time and temperature derived from the DSC measurements was used to lay out the following curing series.
4.2.2 The influence of curing time, temperature and relative humidity on work in bending, nitrogen fixation and formaldehyde emission
To determine the influence of the curing parameters on the material properties, the process parameters were varied individually. Curing took place in a laboratory oven (XVC305 UNOX S.p.A., Padova Italy) with the capability to control the temperature as well as the relative humidity (RH). Based on the DSC results, the applied curing parameters were 90 °C, 105 °C and 120 °C for 4, 24 and 48 hours. The RH (% steam) during curing was varied between 0 %, 40 %, 80 % and 100 %. The percentage of steam resulted in different RH levels at different temperatures. For economic reasons, this was only applied to curing processes lasting 24 h (Table 7).
Table 7: Parameters of the curing processes for the determination of the material properties and the formaldehyde properties. Temperature [°C], duration of curing [h] and the relative humidity of the curing process [% steam]
4.2.2.1 Determination of work in bending
The work in bending was measured in a three-point bending test DIN 52 186 (1978) on a universal testing machine (Z010 Zwick/Roell, Ulm Germany). The results were analyzed with testXpert II (Zwick/Roell, Ulm Germany). 15 replicates per group were used, for the dimensions and solid content of the impregnation solution see Table 5.
4.2.2.2 Determination of nitrogen fixation
The results of curing nitrogen containing resins such as MMF can be controlled by the nitrogen fixation (NF). The content of nitrogen remaining in the sample after an extraction method is considered to be fixated. The extraction method in this study was leaching of the specimens based on DIN EN 84 (1997).
Material and Methods
26
Entire specimens were leached and afterwards ground up in a cutting mill with a 2 mm sieve (SM 100 by RETSCH, Haan Germany) for subsequent nitrogen content analysis. The Kjeldahl method (1883) was used to determine the nitrogen content of the sample. The wood particles were subjected to the nitrogen analysis (block digestion system SBS 850, steam distillation D 1000 and back titration TS 10, FoodALYT system OMNILAB, Bremen Germany). The NF was calculated as the ratio of nitrogen content of leached and unleached samples (Eq. 3).
FixN [%] = (Nunleached – Nleached) / Nunleached *100 (3) FixN = Nitrogen fixation [%]
Nunleached = Nitrogen content unleached sample [%]
Nleached = Nitrogen content leached sample [%]
4.2.2.3 Determination of formaldehyde emission
The FA emissions of the specimens after curing were measured based on the EN 717-3 (1996). Four specimens were placed in each flask containing 50 ml demineralized water. Three flasks per curing process were placed in an oven at 40 °C for 24 h. The formaldehyde concentration of the solution was photometrically determined (Specord 205, Analytik Jena AG, Jena Germany) using the acetyl acetone method.