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HFA-Projekt: Suche nach möglichen Alternativen für Kreosot zur Imprägnierung von Bahnschwellen aus Holz

Die Holzforschung Austria (HFA) betreut das Forschungsprojekt „Bahnschwelle 2020“. Ziel ist es, Alternativen für das derzeit wegen der guten Langzeiterfahrungen noch unverzichtbare Schutzmittel Kreosot (Steinkohlenteeröl) zur Imprägnierung von Bahnschwellen aus Holz zu finden.

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Impregnation processes

The single Rueping process is used primarily with the wood types pine, larch and oak. Only impregnation oil (creosote) is used. After moving the wood into the impregnation tank a preliminary air pressure is produced, the amount of which (1.5 bar to 5.0 bar absolute pressure) is determined by factors such as the amount of impregnation necessary from the sapwood right to the edge of the heartwood and by the quantity of impregnation oil to be applied. This preliminary air pressure must be maintained for a minimum of 15 minutes. Subsequently, while maintaining the preliminary air pressure, the impregnation tank is flooded with impregnation oil at a temperature ranging from 110 °C to a maximum of 120 °C. The impregnation oil which is cooling down in the impregnation tank must be quickly heated to a temperature of 110 °C +/-10 °C. This temperature must be maintained throughout the entire pressure phase, the duration of which depends on the type of wood.

When using the single Rueping process, after establishing the preliminary air pressure it is possible to include a heat conditioning phase for the wood under preliminary air pressure conditions, in the form of an impregnation oil bath of varying length, which follows the pressure phase. The heat conditioning phase has the effect of raising the impregnation temperature (lower viscosity), so it improves the penetration of the impregnation oil into the wood. The pressure is raised to a minimum of 8 bar. The strength of the oil pressure and the duration of the oil pressure phase are determined by the impregnation necessary for the entire sapwood to the edge of the heartwood, as well as the quantity of impregnation oil to be applied, taking the type of wood, wood composition and wood dimensions into account. The method comes to an end with the reduction of the tank to atmospheric pressure, the return of the impregnation oil to the storage vessel, and the production of the highest final vacuum possible, which is to be maintained for a specific time.

The improved double RUEPING process is the process of preference for deciduous woods such as beech, and is likewise used with an impregnation oil (creosote). They are impregnated lightly over the entire cross section in the direction of the axis. In practice it is a combination of two single RÜPING impregnations, whereby in the first part of the combination, the impregnation pressure phase – which follows the adjustment of the preliminary air pressure (between 1.5 and 5 bar absolute pressure) and the filling of the impregnation tank while maintaining the preliminary air pressure – is shortened to 1 hour.

In the tank-vacuum pressure method the wood moved into the impregnation tank is initially ventilated at an absolute pressure of a maximum of 0.10 bar (pre-vacuum), and maintained for a minimum of 30 minutes. Subsequently, while maintaining the vacuum, the tank is flooded and a pressure of a minimum of 8 bar generated. The pressure phase should be maintained for a minimum of 45 minutes. The impregnation is now continued as long as it takes until the wood doesn’t absorb any meaningful amounts of wood protection liquid anymore (in two back-to-back cycles within 15 minutes, the duration of pressure not more than 5 litres per m3 of wood = end phase control). After the pressure phase usually a concluding vacuum of 10 to 20 minutes is carried out.