FEATURES
structure . As the wood absorbs and releases moisture , it swells and shrinks and results in internal stresses that build up inside the wood structure . Unless the process of swelling and shrinking is inhibited by the correct protection and maintenance thereof , the wood will start to split and crack . This not only adversely affects the aesthetic appearance of timber but may also weaken its structural integrity .
“ Protection of timber against natural weathering requires penetrating water repellent wood sealers that have been specifically formulated for exterior applications . In most cases a combination of wood preservation to prevent biological attack and wood protection against weathering is required to fully ensure the long-term durability of timber used under exterior conditions ,” says Conradie .
He adds , “ Fire is the most destructive enemy of wood and it can completely destroy timber structures within minutes , especially when hot and windy conditions exist . For effective protection and control of potential fire damage and destruction , timber needs to be pressure treated with an acceptable fire retardant product . The destructive nature of fire can only be effectively controlled and slowed down by proper pressure impregnation of timber with an effective fire retardant .”
The various hazards which wood material will be subjected to has enormous bearing on the extent to which wood preservation will be effective , notes Breedt . For example , a piece of wood kept continually dry inside a building is subject to a much lower hazard than a piece embedded in the ground . The hazard level determines the required intensity of the key wood preservation factors , namely the toxicity of the preservative chemical , its fixing characteristics , the penetration required and the retention required .
“ For example , interior timbers may only require protection against wood borers , necessitating a simple borer-specific , unfixed chemical-like boron . It also may require penetration only in the outer sapwood , as well as a retention level of a very low order . However , for ground contact hazard , a heavy duty preservative such as a Chromated Copper Arsenate ( CCA ) or Creosote is required , with continuous penetration to a significant depth and with relatively high retentions ,” explains Breedt .
BEST PRACTICE IN WOOD PRESERVATION
The most common and oldest form of wood preservation has been the impregnation process , and more commonly pressure processes , ranging from low to high pressure processes that may or may not include vacuum cycles during the processes . These processes use mainly biocidal active ingredients ranging from heavy industrial-type oil-based preservatives to water-based metallic inorganic preservatives as well as water- or solvent-based organic preservatives .
“ Of late there has been some developments – mainly internationally – in modification processes both thermal and chemical ( not biocidal ) that render timber as a non-food source for decay fungi and wood destroying insects ; however , these modification processes vary in its extent of application and also its impact on the structural and mechanical properties of timber ,” says Breedt .
He continues , “ The process of traditional wood preservation has not changed that much , apart from a few new adoptions , due to the changes in the preservative types used , with a noticeable move to more environmentally friendlier preservatives . The criticality of end application , conditions and effectiveness of preservatives will , in many cases , determine the types used . The most significant change in methods have been the advent of modification processes as an alternative to pressure impregnation processes ; however , these methods are still very much applied in a niche market for now due to availability , price , as well as long-term efficacy in different exposure conditions .
“ The effectiveness of all wood preservatives depends on the penetration achieved and the retention of preservative in the penetrated zones of the timber commensurate to the use class , exposure and end application . These are dependant of selecting timber that is permeable and susceptible to the preservation and importantly prepared correctly , seasoned as high moisture content is a limiting and restricting factor when timber is preserved .
“ To effectively protect timber against biological attack , wood preservation products and the appropriate application techniques are required . The only way to avoid and totally prevent biological attack on timber is to use timber that has been pre-treated by a proper pressure impregnation or diffusion treatment process with approved wood preservatives such as CCA or borates . However , if not pre-treated with a wood preservative , wood borer , termite and fungal attack on timber can also be effectively controlled by application of brush-on biocides ,” says Conradie .
As for the coatings side of timber preservation , newer generation hard wax oil products have really come into their own . “ We have seen a huge increase in the demand for hard wax oils versus the usual surface coatings that have been used historically such as varnishes and penetrating oils . This is predominantly due to the maintenance required for both options . It is very important to periodically inspect timbers used externally and maintain them if or when needed ,” says Carel Steenkamp , channel manager of Rubio Monocoat South Africa .
Steenkamp says these hard wax oil products will stain , seal and protect timber in a single layer and can also lend a decorative effect as they incorporate colour pigmentation . The pigmentation used can add UV protection to the surface , and as such , slows down cell degradation , extending the life of the timber even further . “ Additional protection can be
34 AUGUST / SEPTEMBER 2018 //