The impact of log pre-heating on birch veneer surface quality, bond formation and plywood performance

Anti Rohumaa

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Bond formation and performance in veneer-based products is highly dependent on veneer quality, which in turn can be affected by processing factors including log pre-heating conditions. Hence, it has been shown that the heating of logs enhance many veneer and plywood properties, the mechanisms behind the bond strength increase is not fully understood. Therefore, the main focus of this dissertation was to investigate the effects of the pre-heating process on the surface properties of veneer and its potential to improve bond quality. Additionally the applicability of commonly used test methods to evaluate the effect of pre-heating temperature on surface properties and bonding quality were assessed. This series of studies clearly show that lathe check depth and pulling direction have a strong influence on the measured shear strengths of plywood tested according to SFS-EN 314, even though this is a test for adhesive bonding and not intended for measuring wood quality. Therefore, controlling lathe checks properties could result in much lower variability in wood bonding tests allowing researchers to more easily identify other factors influencing bond quality, e.g. felling season and pre-heating temperature. This study also indicated that log pre-heating temperature not only causes the softening of material in the rotary peeling process, but also causes irreversible changes in the wood material which subsequently has an effect on bond strength development. Results show that the temperature history of the log prior rotary peeling is an important factor which contributes to veneer properties and bond strength. Furthermore, the interpretation of adhesion data obtained by ABES should be treated with caution in situations where the veneer properties and history are not fully known. Assuming that veneer is a homogenous substrate might lead to a misinterpretation of results obtained by ABES. The results of the study show that winter felled birch logs are most susceptible to the effects of pre-heating temperature and heating birch at low temperature leads to remarkable variation in the results caused only by seasonal variation. When paying attention to the aforementioned issues, ABES was found to be very useful for evaluating the impact of veneer production variables or the wood properties on the bonding process. The advantage of the surface integrity measurements technique developed in this study is that it evaluates the particles loosely attached to surface, which could act as a locus of failure in the bondline and cannot be evaluated by roughness measurement techniques. Finally, while the bonding quality of plywood is driven by the uncontrolled effects of lathe checks, further development of bonding and new adhesive concepts will be complicated.
Translated title of the contributionHaudonnan vaikutus koivuviilun pinnan laatuun, liimasauman muodostumiseen ja vanerin liimasauman lujuuteen
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Hughes, Mark, Supervising Professor
  • Frihart, Charles R., Thesis Advisor, External person
  • Hunt, Christopher G., Thesis Advisor, External person
Publisher
Print ISBNs978-952-60-6927-2
Electronic ISBNs978-952-60-6926-5
Publication statusPublished - 2016
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • ABES
  • bond quality
  • lathe checks
  • plywood
  • pre-heating
  • surface integrity
  • veneer

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