Poly(lactic acid) (PLA) is one of the most produced and processed bio-polymers. It is produced from lactic acid monomers polycondensation. Lactic acid can be synthesized from fossile resources or extracted from corn and produced by biotechnological means, which gives the natural origin of PLA. Polycondensation of aqueous lactic acid into PLA is mostly achieved by Ring Opening Polymerization (ROP): the first step is a direct polycondensation leading to oligomers. This step is limited by a complicated elimination of the water produced. In the second step, the oligomers are depolimerized into lactide cycles. The last
step consists in opening the cycles to build long PLA polymer chains. Residual monomers which did not react are then recycled and reused. The only first step of direct polycondensation has been considered to prepare the wood treatment product.
Solid wood impregnation can be achieved in two steps. The first step consists in making the acid treatment penetrate the cells. This is achieved under vacuum. The second step consists in making the acid mono – and/or oligomers polymerize into the wood matrix.
To help polymerization, aqueous acid dilution must be first dehydrated and oligomerized. Indeed, water would be an obstacle to the in-situ polymerization step. On the other side, if oligomerization can help the in-situ polymerization step, attention must be paid to the oligomers viscosity, as the perfect case would be to achieve not only the lumen but also the cell wall penetration. It has been shown that an oligomerization of aqueous lactic acid (88% in water), leading to a mean polymerization degree of 2 to 3, is a good compromise. Such a product possesses a viscosity of 750 cP, and can be obtained by vacuum dehydration at 120°C for 1 hour. This gives stable oligomers, which can be stored in close flasks at room temperature for several weeks.
Wood matrix must be dryed until anhydrous state before the impregnation step. Removing as much water molecules as possible is also necessary to free the hydroxyle grafting sites. Samples of pine sapwood (Pinus sylvestris) and beech (Fagus sylvatica) are used as reference species.
The impregnation process begins with a vacuum step where most of the air contained in wood is extracted. Then, lactic acid oligomers are introduced in the container until covering wood samples. The container stays some hours in low pressure. Alternation of low and atmospheric pressure is conducted for one day. This process is expected to allow most of the air contained in wood to be replaced by oligomers. The chemical affinity between oligomers and wood constituents in the cell walls leads then to the wall penetration.
Following this protocol, high impregnation yields can be reached. Indeed, up to 70 % of mass uptake is noticed on beech and 120 % on pine sapwood. According to wood and lignocellulosics mean densities, and following the simplified formula (1), it seems that up to 80 % of wood cellular vacuum (beech and pine sapwood respectively) can be filled with oligomers. This is information of a quite good affinity between lactic acid oligomers and solid wood.
with P : wood porosity
p0 : anhydrous wood density plcm lignocellulosic material density » 1530 kg. m-3 Lactic acid oligomers final polymerization is then achieved by oven-heating. Checking the impregnation yield evolution along heating time gives valuable information about the in – situ polymerization rate. Indeed, water is the polycondensation by-product gradually eliminated.