New protection technology to substitute creosote for the protection of railway sleepers, timber bridges, and utility poles

Creosote is one of the oldest and most effective wood preservatives, mainly used in heavy-duty applications outdoors with high demand to safety and service life, such as railway sleepers, utility poles, and timber bridges. Creosote however is controversial within the European Commission due to its toxicity and its approval for future use is questionable. The project CreoSub aimed at the development of alternative protection systems that show a better health and safety profile than creosote. Besides its strong biocidal effect, creosote confers hydrophobicity, which counteracts crack formation and thereby reduces the risk of infestation by wood destroying microorganisms; this dual function is special for a wood preservative and makes great demands on finding substitutes. In this regard, the performance of tall oil impregnated wood samples was analyzed in CreoSub that had been exposed in field trials for a decade. The results revealed that raw tall oil or tall oil derivatives alone without the addition of biocides do not provide sufficient protection in heavy-duty applications outdoors; consequently, the main project focus was on oily systems containing inorganic and organic biocides. Laboratory decay tests gave initial indications for the required minimum uptakes of the new systems to sufficiently protect Scots pine and European beech against wood destroying fungi. These must be verified outdoors and will therefore be successively supplemented by data from field tests in Norway, Germany, and USA during the next years. Though the samples have been exposed and evaluated for approximately two years by the project end, it is too early to draw reliable conclusions. The project partners will therefore follow up the tests. Besides the investigations on biocidal efficacy, the ability of the preservatives to protect wood from crack formation has been being investigated under accelerated weathering conditions. Preliminary results of the ongoing test indicate that the new oil borne systems are as effective as creosote. In addition to the small samples in laboratory and field tests, Norwegian utilities are planning to erect 120 poles at two sites in coastal and interior Norway right after the project end to establish profound documentation on the performance of two of the most promising new preservatives under real-use conditions. The poles will be frequently inspected until 2037. The driving force behind the legislative movement to ban creosote are health and environmental concerns. In this context, the stability of the preservatives against water leaching is of particular interest. Oily products showed less leaching of copper than water-borne products in laboratory tests. The development of new preservatives includes to define optimum retention levels, i.e., a minimum level due to efficacy reasons and an upper limit due to economic and environmental reasons. This task was successfully addressed in a WP on impregnation optimization. Investigations on the influence of the new protection systems on mechanics did not reveal any negative impact on the modulus of elasticity, 3-point bending strength, and impact bending strength. Moreover, drilling patterns to improve impregnability were optimized with respect to mechanics. Electrical conductivity is an important material property for railway sleepers and utility poles regarding signaling and safety, respectively. Investigations in CreoSub did not show any increase in conductivity due to the new preservatives. Other aspects as metal corrosion and gluability are currently investigated. CreoSub included life cycle analyses to assess the impact of sleepers, poles, and glulam made of different materials on global warming and toxicity. Wooden poles impregnated with the new preservatives have a significantly lower impact on global warming than poles made of steel, composite, or concrete reinforced with recycled steel. Steel poles have clearly the highest effect on human toxicity, mainly due to outputs during production. Also used as sleepers, steel has a stronger impact on global warming and human toxicity than impregnated pine, beech, or oak. The same applies to composite sleepers. Concrete, however, shows similar impacts on toxicity and global warming as wood in the LCA on utility poles and railway sleepers. An LCA on timber bridge elements compared the environmental impacts of different types of double-impregnated glulam with each other. Overall, the impacts of the three products were in the same range. The comparison between creosote and the new preservatives shows similar performances in all LCA. An important difference however is creosote's classification as a probable human carcinogen, which is mainly based on results on skin cancer. In an LCA, however, carcinogenicity related to direct skin contact is not addressed. The impact category toxicity is only based on emissions to air, soil and water, which in turn are considered as very difficult to assess.

Creosote oil is one of the oldest industrially used and most effective wood preservatives. During the last years, the European Commission has restricted the use of creosote due to environmental and health concerns. An approval for using creosote after 201 8 is very questionable. The probable ban of creosote oils together with earlier banning of CCA (copper, chromium, arsenic) and other chromium containing wood preservatives in Europe will not facilitate any alternative wood protection system to the market for heavy-duty applications outdoors like railway sleepers, timber bridges, and utility poles.The probable ban of creosote will hit the wood industry and the users of creosote-treated wood products hard. Alternative products are not market-ready yet. The current lack of alternative protection systems implies that entire key markets for wood, namely railway sleepers, utility poles and timber bridges, are likely to be lost to non-renewable materials. In CreoSub, a new protection technology will be developed from laboratory to industrial scale.The new protection technology will most likely have a better environmental, health and safety profile than creosote. Moreover, most of the protection systems in CreoSub are based on CTO that is mainly used as a renewab le energy source today. The use of CTO in wood protection systems is assumedly a more environmentally and economically sustainable way than to combust it for energy generation.A viable technology developed in CreoSub is not only aimed to be suitable for r ailway sleepers, utility poles, and timber bridge components, i.e., products with high requirements to public safety and technical service-life in Use Class 3 (above ground) and Use Class 4 (in ground contact). Protection systems showing good performance in CreoSub might also be suitable for the protection of products in marine applications (Use Class 5) like piles and dolphins in salt water.