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.