Ispitivanje mikrovlačne čvrstoće drva – pregled praktičnih aspekata metodologije

Microtensile testing is a specific and delicate variant of standard tensile testing and is performed on small samples. Often also referred to as “thin strip” method, the testing of thin veneers is described in this paper with particular aspects of its field and scope of application, along with comments on reliability and variability of results. Experimental guidelines of the precise technique are presented as well. The method consists of preparation of microtomed longitudinal wood sections, which may be treated or exposed to various conditions, agents or weathering, followed by tensile testing in a large number of replicas. Therefore, the reduction in size of samples shortens and/or facilitates the testing. Testing at zero span (the jaws of testing instrument being initially in contact) reflects to a greater extent the mechanical properties of the cellulose component, while the finite (usually 10 mm) span test yields more information about matrix properties i.e. lignin intercellular material and the degree of fibre bonding. This method has some shortcomings related to the fact that practical applicability is restricted to a small number of species, and that great skill is required to prepare the material, execute the testing and interpret the results. The method may be time consuming and technically demanding, depending on species and type of experiment. However, further analyses of test strips (like colour measurements, chemical analytical testing, biodeterioration studies, etc.) render the method useful for a multi-aspect approach to specific studies on wood.Mikrovlačno je ispitivanje varijanta standardnoga vlačnog ispitivanja provedeno na malim probama. Često nazivano „metodom tankih listića“, ispitivanje čvrstoće na vlak mikrotomiranih vrlo tankih furnira u ovom se članku opisuje uz posebno razmatranje polja primjene te metode, kao i uz komentare o pouzdanosti i varijabilnosti rezultata. Članak također predstavlja eksperimentalne detalje i naputke za provođenje metode visoke preciznosti. Prednost te metode očituje se u činjenici da je longitudinalne tanke mikrotomirane odječke moguće zaštićivati različitim sredstvima (primjerice, impregnirati sredstvima za površinsku zaštitu), potom izlagati različitim utjecajima starenja i, konačno, podvrgnuti ispitivanju čvrstoće na vlak. Male dimenzije individualnih proba omogućuju ispitivanja na velikom uzorku. Ispitivanja na nultome početnom rasponu ispitnih čeljusti uvelike odražavaju mehanička svojstva celuloze, dok ispitivanja na određenom (najčešće 10-milimetarskom) rasponu daju informacije o ukupnim svojstvima materijala – ne samo o svojstvima mikrofibrila nego i o svojstvima veziva (lignina i hemiceluloze). Postupci izbora, pripreme, mikrotomiranja, uzorkovanja, izlaganja i ispitivanja opisani u ovom radu ključni su za postizanje velike pouzdanosti (niskih koeficijenata varijacije), kako pri ponavljanju, tako i pri obnavljanju ispitivanja. Pravilno je mikrotomiranje, pak, ključan korak u procesu pripreme materijala, dok kontrola debljine osigurava kvalitetu pripremljenog materijala, a time i pouzdane i točne rezultate. Nedostatak opisane metode jest to što je praktična primjenjivost smanjena na mali broj vrsta drva. Nadalje, metoda zahtijeva veliku vještinu i pozornost operatera pri pripremi materijala, provođenju ispitivanja i interpretaciji rezultata. Metoda je tehnički zahtjevna, a pokusi mogu biti dugotrajni. Unatoč tome, primjena te metode relativno je brz i pouzdan način ocjene kemijskih i strukturnih promjena, kao i ispitivanja biološke razgradnje tankih slojeva drva

Ispitivanje mikrovlačne čvrstoće drva – utjecaj svojstava materijala, izlaganja i uvjeta ispitivanja na fotodegradaciju

This paper presents the effects of properties of tested material and exposure conditions on the final result of testing. These include density, uniformity of ring width, number of rings and latewood portions, as well as light source, presence of water and duration of exposure. Influences of these parameters in testing of several softwood species after exposure to different natural and artificial photodegradation regimes were monitored by means of changes in microtensile properties. The main findings indicate that comparisons between various species should be made taking into account average material properties, predominantly latewood portion. The fact that strength loss changes follow the same pattern during different exposure conditions indicates that there is no difference in the nature of degradation process in various weathering machines. This forms a basis for the sound comparison between the artificial and natural weathering regimes.U radu se opisuje istraživanje utjecaja svojstava ispitivanog materijala i uvjeta izlaganja drva na konačni rezultat ispitivanja fotodegradacije njegova površinskog sloja. Ispitivana svojstva materijala obuhvaćaju gustoću drva, ujednačenost širine goda, broj godova i udio kasnog drva, utjecaj izvora zračenja, postojanje vode i duljinu izlaganja vremenskim utjecajima. Mjerenje mikrovlačne čvrstoće u smjeru drvnih vlakana ispitano je na više vrsta drva nakon prirodnoga i ubrzanih umjetnih izlaganja vremenskim utjecajima. Rezultati pokazuju kako je za valjanu usporedbu različitih vrsta drva potrebno odabrati materijal prosječne gustoće, posebno pazeći na udio kasnog drva. Činjenica da je gubitak čvrstoće tijekom različitih vrsta izlaganja podjednak upućuje na zaključak kako nema bitne razlike u prirodi procesa površinske degradacije ovisno o vrsti izvora zračenja. To je osnova za valjanu usporedbu rezultata dobivenih izlaganjem u različitim prirodnim i umjetnim uvjetima

Application of the ESEM Technique in Wood Research: Part I. Optimization of Imaging Parameters and Working Conditions

A study using the ESEM (Environmental Scanning Electron Microscopy) technique was performed on wood objects in order to assess the particular advantages, possibilities, and limitations of this microscopic tool. In contrast to conventional high vacuum SEM, in ESEM specimens can be investigated in a gaseous atmosphere, usually of water vapor. This enables the observation of non-conductive, polymeric, composite, and porous materials (such as wood) in their natural state, without drying, evacuating, or sputtering them with a layer of carbon or metal. Further advantages include observations in a wide range of temperatures (-15° to 1000°C), conduction of dynamic processes such as condensation, freezing, and thawing of the specimen during observation, or mechanical testing.The imaging quality of ESEM for natural samples, however, is inferior to that of conventional SEM, and the specimens are liable to beam damage. The process of acquiring an image in ESEM is more complex than in SEM, demanding the optimization of a number of interacting parameters. These include the physical conditions of the specimen, conditions of the chamber environment, and electronic parameters of the formation and optimization of the image.The work on the ESEM can be performed through several operational modes that offer various sets of environmental and imaging conditions. This article presents guidance for assessment of influential operating parameters and their selection for the optimization of the ESEM work with wood

Effect of treatments with 1,3-dimethylol-4,5-dihydroxy-ethyleneurea (DMDHEU) on the tensile properties of wood

The thin-veneer strip technique was applied to investigate the modifying effects of 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) on the tensile strength of wood. Pinewood veneers treated with solutions of DMDHEU alone and in combination with magnesium chloride (MgCl2) as a catalyst showed considerable strength losses of up to 50% in zero-span and up to 70% in finite-span testing modes. The higher strength losses in the latter case are due to hemicellulose degradation, which cannot be assessed in zero-span testing. Strength loss observed after treatment with DMDHEU and MgCl2 was approximately as high as the sum of the strength losses determined after individual treatments with MgCl2 or DMDHEU. Micrographs of veneers after finite-span testing revealed that catalysed DMDHEU treatment changed the predominant failure mode from interfibre fracture (in controls) to intrafibre fracture. The mechanism of strength loss resulting from treatment with DMDHEU is discusse

Dimenzijska stabilnost podnih obloga od pregrijanog drva

Heat treated wood (HTW) is successfully applied for floorings due to its better moisture resistance, increased dimensional stability, and uniform colour change to darker, brownish colours. The aim of this work was to define the hygroscopic range and equilibrium moisture content at ambient conditions of heat treated wood of two wood species – ash and beech. Material was treated at two temperature levels, 190 and 210 °C, and the properties were compared with native wood. The reduction in dimensional changes is expressed by volumetric shrinking and Anti Shrink Efficiency (ASE). Additionally, parquet elements were made out of such HTW, oil-impregnated and waxed, and subsequently tested for water vapour and liquid water permeability. Shrinking gradients of HTW were not reduced in comparison with native beech wood, but the absolute reduction in water uptake resulted in cca 50 % lower EMC values and up to cca 60 % improved ASE values. Surface treatment further improved the hygroscopic properties of HTW.Pregrijano se drvo uspješno primjenjuje za podne obloge zahvaljujući smanjenoj higroskopnosti, boljoj dimenzijskoj stabilnosti te ravnomjernoj promjeni boje u tamnije smeđe tonove. Cilj je ovog rada utvrditi higroskopski raspon i ravnotežni sadržaj vode pri sobnim uvjetima za dvije vrste pregrijanog drva za parket – za jasenovinu i bukovinu. Uzorci su tretirani pri dvije temperaturne razine – 190 i 210 °C, a svojstva uspoređena s nativnim drvom. Smanjenje dimenzijskih promjena izraženo je kao poboljšanje dimenzijske stabilnosti (engl. Anti Shrink Efficiency – ASE). Nadalje, od proba pregrijanog drva načinjene su parketne daščice, tretirane parketnim uljem i voskom, te testirane na vodoupojnost i paropropusnost. Koeficijenti utezanja pregrijanog drva nisu smanjeni u usporedbi s nativnim, ali je apsolutno smanjenje vodoupojnosti za 50% rezultiralo povećanjem dimenzijske stabilnosti za 60%. Površinska je obrada dodatno poboljšala higroskopna svojstva pregrijanog drva

Dimenzijska stabilnost podnih obloga od pregrijanog drva

Heat treated wood (HTW) is successfully applied for floorings due to its better moisture resistance, increased dimensional stability, and uniform colour change to darker, brownish colours. The aim of this work was to define the hygroscopic range and equilibrium moisture content at ambient conditions of heat treated wood of two wood species – ash and beech. Material was treated at two temperature levels, 190 and 210 °C, and the properties were compared with native wood. The reduction in dimensional changes is expressed by volumetric shrinking and Anti Shrink Efficiency (ASE). Additionally, parquet elements were made out of such HTW, oil-impregnated and waxed, and subsequently tested for water vapour and liquid water permeability. Shrinking gradients of HTW were not reduced in comparison with native beech wood, but the absolute reduction in water uptake resulted in cca 50 % lower EMC values and up to cca 60 % improved ASE values. Surface treatment further improved the hygroscopic properties of HTW.Pregrijano se drvo uspješno primjenjuje za podne obloge zahvaljujući smanjenoj higroskopnosti, boljoj dimenzijskoj stabilnosti te ravnomjernoj promjeni boje u tamnije smeđe tonove. Cilj je ovog rada utvrditi higroskopski raspon i ravnotežni sadržaj vode pri sobnim uvjetima za dvije vrste pregrijanog drva za parket – za jasenovinu i bukovinu. Uzorci su tretirani pri dvije temperaturne razine – 190 i 210 °C, a svojstva uspoređena s nativnim drvom. Smanjenje dimenzijskih promjena izraženo je kao poboljšanje dimenzijske stabilnosti (engl. Anti Shrink Efficiency – ASE). Nadalje, od proba pregrijanog drva načinjene su parketne daščice, tretirane parketnim uljem i voskom, te testirane na vodoupojnost i paropropusnost. Koeficijenti utezanja pregrijanog drva nisu smanjeni u usporedbi s nativnim, ali je apsolutno smanjenje vodoupojnosti za 50% rezultiralo povećanjem dimenzijske stabilnosti za 60%. Površinska je obrada dodatno poboljšala higroskopna svojstva pregrijanog drva

Methods for improving building wood properties

Posljednjih godina intenzivno se radi na razvoju metoda poboljšanja svojstava i trajnosti drva i sustava prevlaka-drvo. Svojstva drva poput dimenzionalne stabilnosti, vodoodbojnosti, otpornosti prema biološkoj razgradnji te otpornosti prema ultraljubičastoj i vidljivoj svjetlosti, pa čak i mehanička svojstva drva mogu se poboljšati modifikacijskim (toplinskim i kemijskim) postupcima. Nadalje, uvođenjem novih materijala za površinsku obradu s nanočesticama može se povećati vodoodbojnost, a dodatno i otpornost prema ultraljubičastom zračenju. Postupci toplinske modifikacije i acetilacija u inozemstvu se već komercijalno primjenjuju, dok se djelotvornost ostalih pokusnih postupaka, kao što je obrada površine nano premazima, tek treba potvrditi u praktičnoj primjeni. Zbog poboljšanih svojstava drvo ostaje konkurentan materijal u graditeljstvu, posebno za drvena pročelja, ograde, prozore i vrata, vrtni namještaj, ali i kao materijal za podove i namještaj u interijeru.The development of methods for improving wood properties and durability of wood and wood-coating system has been intensified lately. Wood properties such as dimensional stability, water repellency, biological resistance, lightfastness in ultraviolet (UV) and visible spectrum, and even mechanical properties, can be improved by modification methods, such as heat treatments and chemical modifications. Furthermore, the application of new finishing materials which incorporate nano-sized particles may lead to improved hydrophobicity and resistance to UV radiation. Heat treatments and acetylation are currently being commercially applied in EU, while the efficacy of other experimental modification methods, such as nano-coating finishing, seek final practical affirmation. Due to improved natural properties, wood still remains a competitive building material, particularly in applications for wooden claddings, fences, joinery, garden furniture, as well as for interior furniture and flooring