The role of lactic acid polymers into wood is to reinforce the lignocellulosic matrix. Therefore, the favourite polymerization protocol is the one which leads to the highest in-situ polymerization degree.
In the polymers manufacturing industry, especially PLA manufacturing, designing a polymer with a significant polymerization degree in an efficient process makes the use of chemical catalysts unavoidable.
As care must be taken not to expose wood to severe temperature, catalyst addition is a possibility to help polymerization in soft conditions. A lot of well suited chemical catalysts can be found in the literature. The assumption of their efficiency is done by dispersing them separately into the oligomers, without being impregnated into the wood matrix. The as – prepared mixtures are vacuum heated and their polymerization degree assessed. Sulfuric acid and Tin (II) ethyloxanoate have proved themselves to be the most efficient catalysts. Indeed, starting from the oligomers product, a second heating step leads to a polymerization degree of 40 with 0.6% sulphuric acid (120°C / 1h) and 24 with 5% Tin (II) ethyloxanoate (150°C / 1h).
A commercially available PLA shows a density of around 70’000 g. mol-1. Because of physical restrictions, this is obviously not achievable into the lignocellulosic matrix. All the more that polymerization conditions have to be soft enough to avoid wood degradation.
Following the in-situ polymerization by mean of Thermo-Mechanical Analysis (TMA) gives valuable information about the influence of the components the ones on the others. Impregnated beech samples (20mm longitudinal x 0.6mm radial x 5mm tangential) have sustained a three points bending test within a temperature increase of 200 °C (from 25 to 225°C with a rate of 2°C. mn-1). As can be seen in Figures 1 and 2, a critical temperature of about 112°C induces a significant decrease of the Young Modulus.
In the same temperature range (from around 60°C to around 112°C), the Young Modulus decrease is related to a strong deformation. This is due to the oligomers viscosity decrease, which leads to a deeper penetration in the matrix. This is translated by a noticeable wood softening. From 112°C to around 180°C, Young Modulus increases, while deformation is reduced. This is due to the oligomers polymerization which reinforces the structure.
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Fig. 2. Deformation of impregnated beech veneer sample compared to beech control sample
To focus on the softening temperature, TMA bending tests have been carried out under five isothermal conditions (Figure 3). This reveals that from 120°C, the first Young Modulus strong decrease step is followed by a compensating increase step.
On the basis of these observations, the treatment process has been set up (Figure 4).
Fig. 4. Composite manufacture processing
