Wood modification with different types of silicon compounds

In the present study silicon containing formulations were investigated for their applicability in solid wood modification. Scots pine (Pinus sylvestris) and beech (Fagus sylvatica) were modified with selected silicon containing systems, e. g. amino silicone micro- and macro- emulsions, quaternary ammonium silicones, silane/siloxane, silicone-wax and silicate formulations. Water based types of solutions and micro- emulsions containing functional groups were most favorable for full impregnation processes, whereas macro-emulsions are better situated for surface applications. However, the modification of wood with silicones can not be considered as wood modification in the classical sense (Mai & Militz, 2005). Silicones containing a higher degree of functionalisation with amino-groups increased the resistance against fungal degradation. Micro and macro emulsions of silicones showed, depending on the number of amino-groups, enhanced resistance against white rot fungi but were more effective against brown rot decay. Further studies revealed a second promising class of silicones, quaternary amino-silicon compounds exhibiting considerable protection against several staining fungi. Amino-silicones or silicone- quat formulations inhibited the colonization and discoloration by moulds and blue stain organisms. A delay in decay of modified scots pine samples, most probably due to a lag in colonization, was observed for quaternary ammonium- silicones exposed to soil bed testing. A combination of analytical techniques revealed that all silicone based systems penetrate into the wood matrix, but penetration and deposition was limited due to the pore sizes in wood and the particle sizes of silicon formulations. The dimensional stabilisation with micro- emulsions of amino silicones (SMI) was improved up to 35%. Silicon was mainly deposited in the cell walls in the case of SMI and 3- isocyanatopropyltriethoxysilane (IPTES). The water vapour absorption of silicone wood composites was unchanged or slightly decreased compared to the untreated wood. However, silicone modification of wood showed good protection against liquid water uptake. The alteration and wash out effects of silicone from modified wood and the loss of the specific hydrophobic character was low. The long term protection of silicone modified wood panels against liquid water uptake was confirmed in outside weathering tests. Artificial weathering emphasized a protection from fast discoloration and revealed a delay in crack development. NMR-relaxometry showed that different types of bound water occur in the cell wall after treatment of wood with micro-emulsion systems. Mechanical properties such as the modulus of elasticity were found to be unchanged for all tested silicon formulations whereas the Janka hardness of scots pine increased up to 70% after treatment with methyl hydrogen siloxane. The paintability of the treated material was poor and represents a challenge for further investigations. The fire protection of silicon modified wood was moderate but less efficient compared to conventional fire retardant agents. The introduction of silicones in solid wood modification can be seen as an exciting approach to replace existing preservatives for wood used in hazard class 3 applications and to enhance the service life of wooden products. For the future it is assumed that a range of products will appear on the market that are based on the described silicone technologies.