An investigation into shrinkage and collapse behaviour of Eucalyptus grandis and Eucalyptus grandis x urophylla wood

Thesis (MScFor)--Stellenbosch University, 2020.

ENGLISH ABSTRACT: Only 1 % of land is used for forestry in South Africa and the country might experience a shortage of structural softwood sawn timber in the near future. Despite this, South Africa actually produces more Eucalyptus logs than the local processing industry can process, hence end up exporting large volumes of Eucalyptus chips to foreign countries. Additionally, on average, the annual increment of South African E. grandis is almost twice that of South African Pinus species. Recently, the green processing of Eucalyptus lumber into engineered wood products has been investigated. However, the variation of dimensional changes during the drying process due to shrinkage and collapse make the efficient processing of Eucalyptus lumber a challenge. A better understanding of the shrinkage and collapse behaviour of Eucalyptus grandis and Eucalyptus grandis X urophylla might enable improved lumber separation and processing. The objectives of the study were to profile the variations of (1) collapse and (2) shrinkage (a) radially and (b) along the height of trees and to explore the relationships they have with basic physical properties of the tree. The hypothesis was that the non-uniform dimensional changes found in the end products of Eucalyptus grandis are due to these variations. Seventy trees of Eucalyptus grandis and Eucalyptus grandis X urophylla were collected from Tzaneen, Limpopo, South Africa. The trees were selected based on age, genetic improvement, as well as an additional splitting trial: 10x trees young (8 yrs) and genetically improved, 10x trees young (7 yrs) and genetically unimproved, 10x trees mature (13 yrs) and genetically improved, 10x trees mature (13 yrs) and genetically unimproved, 20x trees from a splitting trial (17 yrs) and an older trial (24 yrs). All trees were E. grandis except the mature groups which were E. grandis X urophylla. Disks that were later cut into wedges were removed from four heights (1.3 m, 6.4 m, 11.46 m, and 16.52m) from each tree. Two logs of 4.2 m were removed from two heights, processed into boards, and kiln dried. On the boards collapse was visually assessed and shrinkage measurements were taken before and after drying. On the wedge samples shrinkage, collapse, density, moisture content, permeability, extractives contents and heartwood/sapwood ratio were measured. All the samples and measurements were taken so that they give the radial variation between centre (pith), transition zone and outer wood (sapwood). An important finding in this study was that the properties measured on small wedge samples differed significantly from what was observed on sawn boards. This was true for both collapse and shrinkage - the properties on which this study focussed. On boards, collapse varied depending on group, log position and the radial position of the boards. The radial position of the boards had a highly significant effect on board collapse, with collapse generally decreasing from pith to bark. There were first order interactions between all the factors and board collapse behaviour was not consistent throughout groups and log positions. In terms of the causal factors, there were weak but significant correlations between collapse and extractives content, density, and permeability (although these results should be viewed with caution since it was obtained from the small wedge samples). A very interesting result is the weak correlation between radial and tangential collapse on small wedge samples showing the clear directional nature of collapse. On boards, shrinkage varied depending on group, log position and the radial position of the boards. Shrinkage in the thickness direction was higher in the centre boards and decreased towards the outer boards. This was due to the effect of collapse on the thickness direction as collapse is very size sensitive and no matter the direction (radial or tangential) the wider side experienced the most collapse. In the width direction, the shrinkage followed the normal pattern increasing from centre boards towards the outer boards. The width direction shrinkage had almost similar trends with the results found on wedges in both directions (radial and tangential). Twist, bow and cup also varied significantly radially and along the height. However, the magnitude of these was generally far better than the requirements of national standards. Bow had a similar trend as shrinkage in the width direction. The trends for twist and cup were not consistent. Some of the results obtained from small disk samples were inconclusive and contradicted what was observed in boards. It is recommended that future studies focus on obtaining a better understanding on the effect of specimen size. Although trends observed from the small samples were interesting, the practical significance is limited if the results cannot be related to observations in industrial size boards.

AFRIKAANSE OPSOMMING: Slegs 1% van die landarea in Suid-Afrika word vir bosbou gebruik, en die land sal moontlik binne die nabye toekoms 'n tekort aan strukturele sagtehout hê. Desondanks produseer SuidAfrika in werklikheid meer Eucalyptus-stompe as wat die plaaslike verwerkingsbedryf kan verwerk, en sodoende word daar groot hoeveelhede Eucalyptus-spaanders na die buiteland uitgevoer. Daarbenewens is die jaarlikse groei van Suid-Afrikaanse E. grandis gemiddeld byna twee keer meer as die Suid-Afrikaanse Pinus-spesies. Onlangs is die groenbewerking van Eucalyptus-hout in houtprodukte ondersoek. Die variasie van dimensionele veranderinge tydens die droogproses as gevolg van inkrimping en ineenstorting maak die doeltreffende verwerking van Eucalyptus-hout egter 'n uitdaging. 'n Beter begrip van die krimp- en ineenstortingsgedrag van Eucalyptus grandis en Eucalyptus grandis X urophylla kan verbeterde skeiding en verwerking van hout moontlik maak. Die doelwitte van die studie was om die variasies van (1) ineenstorting en (2) inkrimping (a) radiaal en (b) langs die hoogte van bome te beskryf en om die verwantskappe wat hulle het met die basiese fisiese eienskappe van die boom te ondersoek. Die hipotese was dat die nieeenvormige dimensionele veranderinge in die eindprodukte van Eucalyptus grandis te wyte was aan hierdie variasies. Sewentig bome van Eucalyptus grandis en Eucalyptus grandis X urophylla is van Tzaneen, Limpopo, Suid-Afrika versamel. Die bome is geselekteer op grond van ouderdom, genetiese verbetering, sowel as 'n addisionele splitsingsproewe: 10x bome jonk (8 jaar) en geneties verbeter, 10x bome jonk (7 jaar) en geneties onverbeterd, 10x bome volwasse (13 jaar) en geneties verbeter, 10x bome volwasse (13 jaar) en geneties onverbeterd, 20x bome vanaf 'n splitsingsproef (17 jaar) en 'n ouer proef (24 jaar). Al die bome was E. grandis, behalwe die volwasse groepe wat E. grandis X urophylla was. Diskette wat later in wiggies gesny is, is van vier hoogtes (1,3 m, 6,4 m, 11,46 m en 16,52 m) van elke boom verwyder. Twee houtblokke van 4,2 m is van twee hoogtes verwyder, in planke verwerk en oond gedroog. Op die planke is ineenstorting visueel beoordeel en krimpmetings is voor en na droging gedoen. Op die wigmonsters is krimping, ineenstorting, digtheid, voginhoud, deurdringbaarheid, inhoud van uittreksels en hart- / saphoutverhouding gemeet. Al die monsters en metings is geneem sodat dit die radiale variasie tussen middel (“pith”), oorgangsone en spinthout (“sapwood”) gee. 'n Belangrike bevinding in hierdie studie was dat die eienskappe wat op klein wigmonsters gemeet is, beduidend verskil van wat op saagplanke waargeneem is. Dit geld vir beide ineenstorting en krimping - die eienskappe waarop hierdie studie gefokus het. Op die planke het die ineenstorting verskil, afhangende van die groep, stomp-posisie en die radiale posisie van die planke. Die radiale posisie van die planke het 'n baie belangrike uitwerking op die ineenstorting van die hout gehad, en die ineenstorting het meestal van pit tot bas afgeneem. Daar was eerste orde interaksies tussen al die faktore, en die gedrag van die direksie-ineenstorting was nie konsekwent in groepe en stompposisies nie. Wat die oorsaaklike faktore betref, was daar swak, maar beduidende korrelasies tussen ineenstorting en ekstraksieinhoud, digtheid en deurlaatbaarheid (alhoewel hierdie resultate met omsigtigheid beskou moet word, aangesien dit uit die klein wigmonsters verkry is). 'n Baie interessante resultaat is die swak korrelasie tussen radiale en tangensiële ineenstorting op klein wigmonsters wat die duidelike rigtingverwante aard van ineenstorting toon. Afhangend van groep, stompposisie en die radiale posisie van die planke het krimping gewissel. Krimping in die dikterigting was hoër in die middelborde en neem af na die buitenste planke. Dit was te danke aan die effek van ineenstorting op die dikterigting, aangesien ineenstorting baie sensitief is vir die grootte en dit maak nie saak die rigting (radiaal of tangensiëel) wat die breër kant die meeste ineenstorting ervaar het nie. In die breedterigting volg die inkrimping die normale patroon wat van die middelborde na die buitenste planke toeneem. Die inkrimping van die breedterigting het byna soortgelyke neigings gehad met die resultate wat op albei rigtings se wiggies gevind is (radiaal en raaklynig). Draai, boog en koptrek het ook aansienlik radiaal en langs die hoogte gevarieer. Die omvang hiervan was egter oor die algemeen baie beter as die vereistes van nasionale standaarde. Boog het 'n soortgelyke neiging gehad as krimp in die breedterigting. Die neigings vir draai en koptrek was nie konsekwent nie. Sommige van die resultate wat op klein skyfmonsters verkry is, was onoortuigend en weerspreek dit wat in planke gesien is. Dit word aanbeveel dat toekomstige studies fokus op die verkryging van 'n beter begrip van die grootte van die monster. Alhoewel tendense wat by die klein monsters waargeneem is, interessant was, is die praktiese belang daarvan beperk as die resultate nie verband hou met waarnemings in industriële grootte planke nie.

Masters