3.1. Density and Moisture Content
Density and moisture content (below fiber saturation) of native wooden specimens have a strong influence on their elasto-mechanical properties [23
]. In this study, density and moisture content were examined based on the impact bending specimens.
shows the oven-dry density of the specimens. Since the specimens were sorted according to their oven-dry density before modification, the maximum deviation from the mean oven-dry density within each group was only ±15% for tensile specimens and ±5% for impact bending specimens.
Material moisture content (
decreased with increasing intensity of the modification (Figure 1
a). This was to be expected as the additional weight due to the modification changed the basis on which the MC is calculated. Reduced equilibrium moisture content
, however, avoids this effect by calculating the moisture content on the basis of the dry weight before modification [5
]). In contrast to
decreased only after modification with either one of the phenol-resins (Figure 1
values varied between 8.9% and 12.8%, an influence of the moisture content on the examined elasto-mechanical properties cannot be eliminated.
Hosseinpourpia et al. [24
] stated that modification with a phenol-resin (molecular weight of 191 g/mol) decreased
stronger than modification with MF. It is therefore assumed that the molecular weight and therefore the cell wall penetration, irrespective of the resin, has a great influence on the penetration and thus on wood properties. Xie [25
] reported a slightly increased
also after modification with DMDHEU.
3.4. Influence of (Low) Concentration
The influence of the solution concentration was evident in all examined elasto-mechanical properties and for each modification agent. The higher the concentration, the stronger was the reduction. As stated by Bollmus [20
] for DMDHEU, this study affirmed the strong influence of chemical modifications, especially on impact bending strength, and work to maximum load in traction already at a concentrations as low as 0.5%.
As the curing temperature was the same regardless of the modification concentration, it thus might be a possible explanation for the strong influence of the low concentration modification on the elasto-mechanical properties. Fengel and Wegener [31
] as well as Nicholas and Williams [32
] state that under certain circumstances (duration, pressure, moisture content) temperatures above 100 °C might lead to mass losses and reduced elasto-mechanical properties. Thus, it was assumed that the curing temperature of 120 °C, applied for 24 h, could have caused slight decreases in elasto-mechanical properties. However, it is highly unlikely that this curing process caused a decrease of up to 47% of impact bending strength (Table 3
). Further investigations should be conducted on the strong influence of very low modification concentrations on elasto-mechanical properties of wood.
3.5. Influence of Modification Agents
Reasons for reduced elasto-mechanical properties of chemically modified wood have been discussed widely [23
]. In which manner, a modification agent affects the elasto-mechanical properties of wood is determined by various factors. The most important factors are: penetration in to the cell wall, cross-linking, pH value, and formaldehyde content. To assess the influence of each factor separately is very difficult, and as the factors might inter- or counteract, the topic is very complex.
One reason for the strong influence of Phenol1 on the elasto-mechanical properties already at a low concentration could be its low molecular weight compared to Phenol2 and MF. This enables the resin to penetrate into the cell wall through nano-pores, whereas only a fraction of those modification systems with a higher molecular weight was able to enter the cell wall [15
]. However, at a concentration of 0.5%, even if the modification system penetrated the cell wall completely, a reduction as strong as measured in this study was probably not solely due to the modification agent.
Penetration of the cell wall is a requirement for the modification agent to be able to affect
of wood. Lower
of specimens modified with phenol-formaldehyde resins, especially Phenol1 at 20%, could have had a positive influence on elasto-mechanical properties [20
]. However, the results of this study did not show a positive effect of a lower
on elasto-mechanical properties as specimens modified with Phenol1 exhibited strong reductions of properties just like specimens modified with other modification agents.
During curing, the molecules of the modification agent build a network. If reactions between the modification agent and wood cell wall polymers take place, this is called cross-linking. Cross-linking restricts the flexibility of the wood and thus decreases the ability to relieve mechanic stress through strain [10
]. It has been discussed whether the modification agents applied in this study are able to cross-link with the cell wall [6
]. However, it is rather unlikely that cross-linking occurs at a concentration as low as 0.5%. Even if no cross-linking takes place, the rigid structure of the cured resin itself stiffens the wood to some extent [5
]. Since embrittlement of chemically modified wood is one of the biggest drawbacks, further investigations regarding cross-linking are necessary.
The pH-values of the modification agents at initial composition varied (Table 1
). The pH-values at the concentrations used for modification were not measured, but it was assumed that they varied between the modifications agents as well. Depending on the pH-value in combination with temperature, different chemical reactions take place [1
]. Which milieu occurred during the modification and how it affected the elasto-mechanical properties [10
] should be subject to further investigations. As DMDHEU was the only modification agent with an acidic pH value, this could be a reason for the fact that it caused the strongest reductions of all elasto-mechanical properties examined. It is known that treatment with high or low pH values has an effect on wood properties.
All of the modification systems applied in this study contained formaldehyde (methanal, CH2O) at different concentrations. Formaldehyde tends to form networks as it is bifunctional [41
]. According to Burmester [42
] as well as Rowell [43
], treatment of wood with formaldehyde alone leads to a considerable embrittlement of the wood. Reasons for this are cross-linking and hydrolysis of cellulose. A higher formaldehyde content of DMDHEU compared to the other modification agents could be a reason for the strong reduction of elasto-mechanical properties of wood modified with DMDHEU. Research in the past focused on formaldehyde containing resins, which have the advantage of faster and more complete curing. However, more recent developments show [44
] that even without formaldehyde, some resins do show good potential for wood modification.