1,721,044 research outputs found
Volatile organic compounds and formaldehyde in nature, wood and wood based panels
No full textThe emission of terpene compounds from vegetation is subject to seasonal and diurnal variations. Due to oxidation of terpene compounds simple aldehydes like formaldehyde can be formed. Insofar formaldehyde is an ubiquitous chemical. Due to its high reactivity it has a short half-life time. Wood and wood-based panels emit a low, but still detectable, amount of formaldehyde. The emission depends on exogenic (temperature, relative humidity, air exchange level) and endogenic (wood species, binder level, binder type, production conditions, etc.) factors. With the aging of boards formaldehyde release declines tremendously to reach very low level. Nevertheless, with low fuming binders wood-based panels with formaldehyde emission close to that of untreated wood can be prepared. Non-formaldehyde volatile organic compounds (VOCs) can be released from wood and wood-based panels. The emission rate depends on the wood species as well as on the boundary conditions (drying, storage, etc.). In the case of wood-based panels it depends also on the production factors as well as on the storage conditions. Methods to assess VOCs have been developed and regulations regarding the limits of emission values are under way
Significance of wood extractives for wood bonding
No full textWood contains primary extractives, which are present in all woods, and secondary extractives, which are confined in certain wood species. Extractives in wood play a major role in wood-bonding processes, as they can contribute to or determine the bonding relevant properties of wood such as acidity and wettability. Therefore, extractives play an immanent role in bonding of wood chips and wood fibres with common synthetic adhesives such as urea-formaldehyde-resins (UF-resins) and phenol-formaldehyde-resins (PF-resins). Extractives of high acidity accelerate the curing of acid curing UF-resins and decelerate bonding with alkaline hardening PF-resins. Water-soluble extractives like free sugars are detrimental for bonding of wood with cement. Polyphenolic extractives (tannins) can be used as a binder in the wood-based industry. Additionally, extractives in wood can react with formaldehyde and reduce the formaldehyde emission of wood-based panels. Moreover, some wood extractives are volatile organic compounds (VOC) and insofar also relevant to the emission of VOC from wood and wood-based panels
On the penetrability of various glues in chips from pine sapwood and heartwood
No full textInvestigation of the penetrability of various glues [urea-formaldehyde resin (UF-resin), phenol-formaldehyde resin (PF-resin), melamine-urea-phenol-formaldehyd made of pine sapwood and heartwood revealed: the various glues penetrate chips from pine sapwood regardless of tree age, height of the sample studied and surface structure deeper than in those made of heartwood. The various glues more deeply penetrate chips from sapwood and heartwood of the 48-year-old pine than chips from sapwood and heartwood of the 124-year-old pine. The penetrability of various glues in chips from sapwood and heartwood of the 124-year-old pine increases with increasing height of the sample studied. Moreover, glues penetrate sanded surfaces of chips from sapwood and heartwood of the 48- und the 124-year-old pine to a lesser extent than unsanded chip surfaces
Influence of different climatic conditions on the roughness of uncoated medium density fiberboards (MDF)
No full textThe surface roughness is primarily a function of the raw material properties, but other factors like type and amount of resin, press cycle, sanding and moisture content of the boards may also affect the roughness and other surface properties. In this study the effect of the equilibrium moisture content on medium density fiberboards (MDF) surfaces was evaluated using different raw materials in the surface layers as well as different binders. The statistical analysis confirmed that the relationship between equilibrium moisture content and average roughness is a complex function of interactions between many variables
On the release of volatile acids from wood-based panels – chemical aspects –
No full textWood-based panels release different amounts of volatile organic acids, which depend, among other factors, on the binder used. The volatile acids released from the boards can be measured using the flask method following the principle of measuring the formaldehyde release according to EN 717-3. Particleboards bonded with alkaline curing phenol-formaldehyde resins (PF-boards) release higher amounts of acetic and lower amounts of formic acid compared to those boards bonded by acid curing urea-formaldehyde reins (UF-resins) or binders based on polymeric diphenylemethandi-isocyanates (PMDI). This has been explained by the equilibrium reaction between sodium hydroxide in PF-resins and volatile acids, which leaves less free formic acid than free acetic acid to emanate from the boards. Increasing the binder content in the boards also leads to an increase in the over all amount of sodium hydroxide in the boards. Accordingly, less free acids are left, which could be released from the boards. Fibres made by the chemo-thermo-mechanical process (CTMP) using sodium hydroxide and sodium sulphite as pulping agents release less formic acid than those made by the thermo-mechanical pulping (TMP). This behaviour was also attributed to the equilibrium reaction between the alkali in CTMP-boards and the volatile acids
Preliminary research on the influence of anatomical wood section on gluability of sapwood and heartwood from pine (Pinus sylvestris L.)
No full textDie Scherfestigkeitswerte von UF-Harz-gebundenem Kiefernsplint- und -kernholz sind in Abhängigkeit von der anatomischen Richtung in der Tabelle 2 zusammengestellt. Daraus geht hervor:1. Die Scherfestigkeit der Klötzchenpaare aus dem Splintholz ist höher als die des Kernholzes in den entsprechenden anato-mischen Richtungen.</p
EFECTO DE LA HUMEDAD DE EQUILIBRIO EN LA RUGOSIDAD DE TABLEROS DE FIBRAS DE DENSIDAD MEDIA (MDF)
No full textLa rugosidad superficial es primariamente una función de las propiedades de la materia prima, pero otros factores como la cantidad y el tipo de resina, ciclo de prensado, lijado y contenido de humedad de los tableros pueden afectar la rugosidad y otras propiedades de la superficie. En este estudio los efectos del contenido de humedad de equilibro en superficies de tableros de densidad media (MDF), fueron evaluados usando diferentes materias primas en sus superficies como también diferentes adhesivos. El análisis estadístico confirma que la relación entre el contenido de humedad de equilibrio y la rugosidad media es una compleja función de interacciones entre muchas variables, pero donde tiene un papel relevante desde el punto de vista práctico el tipo de adhesivo
On the responsiveness of hardened UF-resins of different molar ratio towards ammonia fumigation
Full text linkFumigation of two cross-linked UF-resins of molar ratio U:F 1:2.1 and 1:1.4 with ammonia reduces their formaldehyde release. The reduction seems to be more excessive in case of resin with molar ratio U:F 1:2.1. Ammonia fumigation also increases the nitrogen content of the hardened resins depending on their molar ratio U:F. Resin of the molar ratio U:F 1:2.1 is more receptive to ammonia than that of U:F 1:1.4. Moreover, the pH-value of aqueous extractives of the resins increases due to the ammonia treatment depending on the molar ratio U:F of the resin. Resins with high formaldehyde content are more responsive to ammonia fumigation
On the acidity of wood from indigenous trees
No full textThe acidity of wood chips from beech, oak, pine and spruce was evaluated by measuring the pH-value of cold water extractives, the buffering capacity of the extractives towards alkali. Moreover, the amount of volatile acids (formic and acetic acid) was also estimated using the flask method procedure used for the determination of formaldehyde release. The results reveal that extracts of chips from different wood species show different pH-values and a widely differing buffering capacity of the extractives. Moreover, it was found that the emission of formic acid is much higher from softwoods (pine and spruce) compared to hardwoods (beech and oak). The nature of volatile acids in wood seems to impact the shape of the titration curves of the wood extractives
- …
