Data/hora: 21/09/2024 04:14:03

Biblioteca(s): 	Embrapa Florestas.	
Data corrente: 	06/06/1995	
Data da última atualização: 	25/08/2009	
Autoria: 	TOMASELLI, I.	
Título: 	The influence of high-temperature drying on some physical and mechanical properties of Pinus radiata D. Don.	
Ano de publicação: 	1977	
Fonte/Imprenta: 	1977.	
Páginas: 	264 f.	
Idioma: 	Inglês	
Notas: 	Thesis (Doctor of Philosophy) - University of Melbourne, Australia.	
Conteúdo: 	Drying small specimens of Pinus radiatta at temperatures of 140 and 180 generally exhibitis three stages,comfirming the findings of past reseachers.Nevertheless it seems that there are some points of past to befurther investigated,especially in the second stage of drying,where mass transfer from the surface to the airsteam also appears to act as a partial controlling factors,in adiction tho the reconized princopal and secondary controlling factors- heat transfer from the surface of the wood to the (interior)surface of evaporation,and heat transfer from the air to the surface.The drying times for high temperature schedules obtanied in this experiment were very short when compared with the normal temperature schedule.Surface darkening was observed form all high-temperature drying schedules,and the colour change was more evident inadult wood than in juvenile wood .Surface darkening generally disappeared after dressing,except when the material was dried to a very low moisture content,were tha dark appearence was observed also in the interior of the boards.Shrinkage increassed with temperature,especially foradult wood boards is more affected by temperature than is thickness,inpractice the dirension will be further reduced by dressing because of the presence of the wary surface.The surface of some adult wood boardswas found to be wavy with higher in early wood,and this seens to be associated with colapse.Density was a general not affected when drying was carried out 140 or at 180c to a final moisture content of 10.The highetest reduction was found when juvenile wood was dried at 180 and 5m/s to a final moisture content of 5%.Equilibrium moisture content for a givem ambient condition was found to be a lower for all bords dried at high temeprature when compared with normally dried matched controls.Based on the reduction of equilibrium moisture content and on Stam's hypotesis subsequent sweling and shrinkage of the material when dryingto a final moisture content of 10%,and around 30% less when drying to a final moisture content of 10%.The highest reduction was found when juvenile wood was dried at 180c 5m/s to a final moisture content of 5%.The reduction in hygroscopisity is not uniform all trought the thickness,i.e the centre part normally retaind a higher equilibrium moisture content.This fact is considered important especially for properties that are determined at centre of specimem,as for example shear streght When adjustament of moisture condition,an "over adjustament"wilkl generraly be applied for the high-temperature-dried material in the case ofshear,if this adjustment is based on the average equilibrium moisture of the shear line moisture content at the shear line.The information provided by the study of internal temperature distribution of the wood helps in the explanation of most findings of this research.For the comparison of the wood deying in diferrent condition,using graphs such asthose presented in this thesis seems to be more.effective than the tradicional plotting.Based on the fact that internal temperature of the wood mainly dependson the mouisture content of the wood,when drying at high temperature,the internal of the wood might be used in the future as tool, to indicate when driyng should be termined .Because juvenile wood dries first in a mixed stack of boards,owing to the lower initialmoisture content the internal temperature increases to higher levels,and a consequence most of the reductions in streght properties are observed in juvenile wood were accentuated by the presence of internal checks.Internal checks seem to be the main problem in drying Pinus radiatta at high temperature,especially in thicker material.After driyng to a final average moisture content of 10%,internal cheks were mainly observed in juvenile wood,but after driyng to an a average misture content of 5%,severe internal checks were also observed in juvenile wood,butafter drying to an average moisture content of 5%,severe internal checks were also observed in adult wood.The over-heating caused by drying to a low moisture content,associated with internal checks observed both adult and juvenile wood,caused very significant reduction in shear streght and in work to maximum load.Drying 50mm thinck Pinus radiata bords at 180c and 5m/s of air velocity to a final moisture content of 5%caused a meam reduction of 30% in radicial shear streght for juvenile and adult wood.Work to maximum load reduced more than 40% for juvenilewood .The reduction found for bords 50mm thick,dried to a final moisture content of 5%,might not be a problem in the case of thinner bords,for example 25mm thick,where the incidence of internal checks was foundto be significantly lower.When driyng at any of the experimental schedules to a final moisture content of 10%,work to maximum load seems to be affected especially in the case of juvenile wood.Reductions even higher than 20%might be expected.The other properties are in general notaffected.Increases are also obtanied when drying at high temperature,even when the drying is conducted to a final moisture content of 5%.These increases are practically only observed when comparing the properties of material conditioned at the same ambient atmosfere without corrections for differences im equilibrium moisture content.Comparing the strenght properties of material dried at high temperature with the matched material dried at high temperature with the matched material dried normally,after adjustement of the strenght values for a moisture content of 12%,these increases are normaly replaced by non-significant changes or decreses,indicating that detrimental effects of high temperature may at times be present but for practical purposes they are to the lower equilibrium moisture content.The least affected property seems tobe compresion parralel to the grain,and increases were generally observed,even when drying to a very low moiture content.On the other hand,drying at 140c and 10m/s of air velocity presented  better results thandrying at same temperature and 5m/s of air velocity,It seems that although the internal temperature increses when higher air velocity is used,the higher air velocity.It is believed that air velocities as high as 10m/s could be used during the intial stages of drying,but it seems that it should be reduced at the final stages of drying'were the beneficial influence of the air velocity on the rate of drying is lees significant,while its undesirable influence on the internal temperature becames important.This would contribute to minimeze possible detrimentaleffects on strenght properties.It must stresed that in experiment the bords were not restrained during dryiping,and althogh there is evidence that the effect of restraint on the general pattern here presented would be small,no garantee is given that the preent results are fully applicable in the case of material restrained during drying.It is recognized that further investigation might be a important especially for juvenile wood.As a general conclusion in can be said that drying at temperatures as high as 180c seems not the reduce the streght properties of Pinus radiatta bords providing that:(a)and apropriate air velocity is used (in this experimet 5m/s with 180c and 10m/s with 140c)(b)the wood is not dried to a very low moisture content (probably not lower than 10%).	
Palavras-Chave: 	Propriedade físico-mecânica.	
Thesagro: 	Pinus Radiata.	
Thesaurus Nal: 	mechanical properties.	
Categoria do assunto: 	--	
Registro original: 	Embrapa Florestas (CNPF)	
Exemplares: 	CNPF - (29898-1) - ADC - TS - -- - TS0135 -  - TS0135 - DIS