# Green and Efficient Processing of Wood with Supercritical CO2: A Review
## 3 Wood Impregnation
### Wood Preservation with Biocides  
Wood easily decays and discolors in humid environments. Wood preservation is used to improve its antibacterial capacity and its resistance to wood-decaying fungi, wood-bluing fungi, molds, and wood-damaging insects, which extends the service life of wood products [63; 64; 65]. The one of primary wood preservation methods is to add preservatives in the wood through impregnation [66]. However, it is difficult to impregnate the inner part of the wood with preservatives by conventional impregnation due to the large fluid resistance and poor permeability of wood [67]. At present, wood impregnation is treated by water-based or hydrocarbon solvents [57]. The uses of water-based solvents in wood impregnation leads to volume expansion of wood and dimensional stability cannot be restored immediately [57]. The use of hydrocarbon solvents has a negative effect on the environment due to VOCs emission. The idea of wood preservation with scCO\({}_{2}\) was carried out due to properties of CO\({}_{2}\) in a supercritical state that was mentioned before. [PERSON] et al. [57] exploited the timber impregnation with the sufficient solubility (2.32 g/L-5.07 g/L) of the insecticide permethrin using scCO\({}_{2}\) in 50 \({}^{\circ}\)C and 15 MPa. Even though the maximum solubility (30.07 g/L) was achieved at 30 MPa, the lowest effective pressure is always adopted in the business, which can reduce the cost. A methanol co-solvent improves the solubility of permethrin in scCO\({}_{2}\). After decompression, the insecticide loses its solubility and deposits within the wood, thus protecting wood. There is a higher insecticide retention value and better protection under scCO\({}_{2}\) compared to a light organic solvent. [PERSON] et al. [50] used organic fungicides (tebuconazole, propiconazole, and IPBC) for wood impregnation using different pressures and temperatures of scCO\({}_{2}\). They concluded that the proportion of fungicide in the wood and CO\({}_{2}\) had a significant effect on the fungicide deposition. The fungicide solubility increased with the density of the CO\({}_{2}\). Fungicide deposition occurred due to the loss of fungicide solubility during decompression and because a large amount of fungicide was adsorbed by the wood during impregnation. There are vast differences in impregnation efficiency between softwood and hardwood due to large differences in wood structure. Moreover, artificial boards also have particular impregnation characteristics under scCO\({}_{2}\) treatment. [PERSON] et al. [68] and [PERSON] [58] studied five kinds of wood-based composite materials (medium-density fiberboard (MDF), hardwood plywood, softwood plywood, particleboard, and oriented strand board) with 3-iodo-2-propynyl butylcarbamate (IPBC) preservative under scCO\({}_{2}\) treatment. The results showed that IPBC improved the preservation of these wood-based composite materials. Furthermore, the treatment largely depended on the temperature, pressure, fungicide dosage, and wood-based composite material [69]. Particleboard, MDF, and hardwood plywood achieved the best preservation at 55 \({}^{\circ}\)C and 11.7 MPa, while softwood, plywood, and OSB showed the best performance at 35 \({}^{\circ}\)C and 9.81 MPa. [PERSON] et al. [70] treated Douglas-fir and radiata pine (_Pinus radiata_ D. Don) with cyproconazole using scCO\({}_{2}\) impregnation, and the results showed that treatment conditions led to different distributions of fungicides. Most studies indicate fungicide can move in the wood structure by decompression in the vessel, because pressure gradient force preservatives loaded the water to flow. However, decompression can easily lead to water passage blockage, which results in the uneven deposition of fungicides, making it difficult to process wood [55; 71]. In the study [70], they adopted temperature-induced biocide deposition at the end of the cycle, and the results showed that this method reduced the consumption of fungicide and more efficiently retained fungicides with minimal line clogging.