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Laminated Veneer Products: Shape Stability and Effect of Enhanced Formability on Bond-Line Strength
Linnaeus University, Faculty of Technology, Department of Forestry and Wood Technology. (Skog och träteknik)ORCID iD: 0000-0001-8644-4099
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns two aspects of the manufacture of laminated veneer products (LVPs). The first aspect is related to the possible improvement of the shape stability of LVPs, and the second has its starting point in the modification of the veneer for enhanced formability as well as the question of whether and how these modifications affect the bond-line strength.

LVPs are veneers bonded with adhesive into predetermined shapes, mostly for the production of furniture and interior fittings. Since any deviation from the intended shape is a problem for both manufacturer and customer, various studies have sought to evaluate the influence of different materials and process parameters on shape stability. Parameters studied have included wood species (beech and birch), an adhesive system based on urea formaldehyde, the adhesive distribution on the veneer, climate, moisture content and fibre orientations of the veneers, as well as the orientation of the individual veneers in a multiply.

Manufacturers of LVPs must consider some basic facts about wood in orders adequately to provide shape-stable LVPs to customers. Wood emits and absorbs moisture in relation to the surrounding climate, and this can lead to shrinkage and swelling. Such moisture induced movements differ in the wood’s different directions, and the magnitude is specific for the species. A thorough understanding of this is the basis for achieving shape-stable LVPs.

Symmetry is defined in this thesis such that the veneer properties are balanced in the laminate. This means that opposite veneers on either side of the centre veneer have similar characteristic. An LVP will become distorted if the veneers are asymmetrically oriented before the press. Deviation from the desired shape can be small immediately after the pressing, but it may increase significantly with moisture content (MC) variations. Asymmetry may result when veneers with different fibre orientations are included in the laminate or when the veneers are placed asymmetrically. It may also occur if veneers with different MCs are bonded together asymmetrically. One aggravating factor is that the lathe checks that are introduced when the veneers are peeled or sliced from the log affect the shape stability. In 3-ply crosswise-oriented plywood, the veneer surfaces on which the lathe checks occur should be oriented in the same way for high shape stability.

Based on existing knowledge, the production of shape-stable LVPs requires that the veneers are conditioned to a uniform MC and sorted with regard to fibre orientation and the side with lathe checks before bonding. End-user climates should govern the MC of the veneers and the moisture added with the adhesive during the process. Straight-grain veneers and symmetry should always be the goal.

Moulding can cause stretching, i.e. strain, of the veneers depending on the curvature of the mould. To prevent the veneers from rupture, there are various ways to strengthen the veneers particularly in the transverse direction in which the veneer is weakest. However, tests have shown that these pre-treatments of veneers for enhanced formability can prevent the adhesive from penetrating the wood surface. It is therefore important to confirm that the pre-treatment does not affect the bond-line strength. 

Abstract [sv]

Denna avhandling berör två områden inom tillverkning av plan- och formpressade fanerprodukter. Det första avser formstabiliteten hos dessa och om det är möjligt att förbättra densamma. Det andra har sin utgångspunkt i modifiering av faner för ökad formbarhet och huruvida dessa modifieringar påverkar limfogens styrka.

Plan och formpressade fanerprodukter består av faner som sammanfogats med lim till en förutbestämd form. Metoden används främst för tillverkning av möbler och inredningar. Avvikelse från avsedd form hos produkterna är ett stort problem för både tillverkare och kunder. Orsakerna till avvikelser i form kan relateras till både materialet och processen men också till hur produkten används. Studier har genomförts för att utvärdera inverkan av olika material- och processparametrar på formstabiliteten hos några utvalda produkter. De undersökta parametrarna var träslag (rödbok och björk), limsystem (baserade på urea- formaldehyd), limspridning, klimat, jämviktsfuktkvot, fiberorintering hos faner samt orienteringen av faner i den specifika produkten.

För att kunden till skiktlimmade produkter ska erhålla en formstabil vara krävs att tillverkarna tar hänsyn till grundläggande fakta om trä. Detta material avger och tar upp fukt i förhållande till omgivande klimat, vilket innebär att trä krymper och sväller. Detta sker dessutom med varierande magnitud i olika riktningar av träet. Denna variation är även träslagsspecifik. Att ha en förståelse för detta beteende är grunden för tillverkning av formstabila träprodukter.

Symmetri definieras i denna avhandling med att faneregenskaperna är balanserade i laminatet. Detta innebär att det motsatta faneret på vardera sidan av det mittersta faneret i laminatet har liknande egenskaper. Om faneren sammanläggs och pressas på ett asymmetriskt sätt blir produkterna skeva eller kupade. Avvikelsen från önskad form kan vara liten direkt efter pressningen men kan öka avsevärt i samband med fuktkvotsvariationer. Asymmetri kan uppstå genom att faner med avvikande fiberorientering ingår i sammanläggningen eller att faneren läggs asymmetriskt. Det kan även uppstå om faner med olika jämviktsfuktkvot sammanlimmas på ett icke symmetriskt sätt. En försvårande faktor är att även de sprickor som uppstår när faneren tillverkas genom svarvning eller knivskärning inverkar på formstabiliteten. I en korsvis limmad tre-lagers plywood ska fanerens sida med sprickor vara orienterad åt samma håll för hög formstabilitet.

För att uppnå formstabila produkter bör faneren vara konditionerade till en enhetlig jämviktsfuktkvot och vara rätfibriga, dvs. att fiberriktningen sammanfaller parallellt med ytornas kanter. Som nämnts tidigare bör även fanerets sida med sprickor beaktas. Slutanvändarens, det vill säga kundens, klimat bör styra fanerens måljämviktsfuktkvot som tillsammans med tillförd fukt från limmet påverkar produktens jämviktsfuktkvot i tillverkningsprocessen. Dessutom ska läggningen vara symmetrisk.

Vid formpressning kan faneren utsättas för sådana sträckningar, dvs. töjningar, att bristningar uppkommer. Därför finns det olika sätt att förstärka faneren främst i transversell riktning där faneren är som svagast. Detta kan ske genom olika bearbetningar och/eller pålimningar. Dessa förbehandlingar av faner för förbättrad formbarhet kan dock hindra limmet från att tränga in i träytan vilket tester visat. Därför är det viktigt att kontrollera att förbehandlingen inte påverkar limfogens styrka. 

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2016. , 120 p.
Series
Linnaeus University Dissertations, 247/2016
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
URN: urn:nbn:se:lnu:diva-52014Libris ID: 19403599ISBN: 978-91-88357-13-7 (print)OAI: oai:DiVA.org:lnu-52014DiVA: diva2:918075
Public defence
2016-05-02, Weber, Hus K, Växjö, 13:00 (English)
Opponent
Supervisors
Available from: 2016-04-11 Created: 2016-04-09 Last updated: 2016-11-22Bibliographically approved
List of papers
1. Shape stability of laminated veneer products: an experimental study of the influence on distortion of some material and process parameters
Open this publication in new window or tab >>Shape stability of laminated veneer products: an experimental study of the influence on distortion of some material and process parameters
2013 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 8, no 3, 198-211 p.Article in journal (Refereed) Published
Abstract [en]

Laminated bending of veneers is a common used and important process for manufacture of components primarily for furniture and interior purposes. According to the use, such products are in general very sensitive to variations in the intended shape, i.e. distortion can be of great problem and more or less destroy the use of the final product. The most critical mode of distortion is twist, but also other changes in shape may be of interest to keep low. The causes to distortion of laminated bended veneer products can be of material (veneer, adhesive and the combination of these), design, processes and climate nature and there is a challenge to know which parameters which are of major importance for distortion, both directly after moulding and when the products are in use.

In this study, the influence of type of UF-adhesive hardener, i.e. liquid or powder, water content of adhesive, adhesive distribution, variation of moisture content of glued veneers, and fibre orientation of veneers, on twist and position for a 3D-veneer construction (a chair seat shell) has been studied. Distortion, i.e. twist and position, has been determined directly after moulding and after moisture cycling. The moisture cycling was to simulate and accelerate conditions that the shells are subjected in use. The aim of the work is to study how the above mentioned material and process parameters influence the shape stability of the products.

The results show that the climate, i.e. how a certain level of temperature and relative humidity influence the moisture content of the moulded product, has a clear impact on the distortion of the product in use. An increase in moisture content results in a significant increase in distortion and vice versa. The level to which the moulded products distort during climate variation can be controlled through controlling material, design and process parameter during moulding. Of the studied parameter mentioned above, a deviation in fibre orientation of the veneers in the moulded assembly is the most critical parameter to have under control to minimize distortion. The fibre deviation mainly results in an increase in twist. A high moisture content of a veneer vis-à-vis the rest of the veneers in the assembly before moulding, will result in increased position and twist of the moulded product in use. The difference of moisture content between veneers and the position of veneers with high moisture content in the assembly will influence the level of distortion. Other studied parameter also influences the distortion to a lesser extent and can in these cases be related to the moisture distribution in the mouldings.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2013
Keyword
twist, spring-back, moulding, wood, laminated veneer products
National Category
Forest Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-19141 (URN)10.1080/17480272.2013.803501 (DOI)2-s2.0-84883560185 (Scopus ID)
Available from: 2012-05-31 Created: 2012-05-31 Last updated: 2017-12-07Bibliographically approved
2. Shape stability of laminated veneer products: How to decrease the negative effects of fibre deviation?
Open this publication in new window or tab >>Shape stability of laminated veneer products: How to decrease the negative effects of fibre deviation?
2013 (English)In: Forest Products Society (FPS)  67th International Convention, Madison: Forest Products Society , 2013Conference paper, Published paper (Refereed)
Abstract [en]

A shortcoming of the laminated bending process is that the products may become distorted after moulding and during use. Annually, significant financial losses have incurred in the furniture and interior design industries as a result of distorted products. In this study, we have examined the influence of deviation of fibre orientation of individual veneers on distortion of a moulded shell to find ways to improve shape stability of laminated veneer products.

Ninety cross-laminated shells, consisting of 7 veneers of Birch (Betula pubescens Ehrh.) with a total thickness of 3.6 mm, were studied. The in-plane dimensions of the veneer were 400x660 mm. All the veneers were straight-grained, but to simulate deviation in fibre orientation some of the individual veneers were oriented 7 degrees relative to the main orientation of the other veneers in the laminate. Distortion was determined directly after moulding and after storage in a changing relative humidity.

The results show the well-known fact that deviation of fibre orientation of the veneers in the laminate influences the shape stability of the product. The results from this study, however, also show how the placement of the abnormal veneers in the laminated veneer products influences the degree of distortions. From this basic knowledge some improvements for production of laminated veneer products were suggested.

Place, publisher, year, edition, pages
Madison: Forest Products Society, 2013
Keyword
thermo hydro mechanical processing, bending, veneer, moulding, wood
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-27435 (URN)
Conference
Forest Products Society (FPS) 67th International Convention, June 9-11, Austin, Texas, USA
Available from: 2013-07-05 Created: 2013-07-05 Last updated: 2016-04-27Bibliographically approved
3. Influence of veneer orientation on shape stability of plane laminated veneer products
Open this publication in new window or tab >>Influence of veneer orientation on shape stability of plane laminated veneer products
2014 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 9, no 4, 224-232 p.Article in journal (Refereed) Published
Abstract [en]

One of the most important quality aspects of a laminated veneer product is its shape stability under changing relative humidity (RH). This study aimed to establish an understanding of how the orientation of individual veneers in the laminate, i.e., orientation according to fibre orientation and orientation of the loose (the side with ‘lathe checks’) or tight side of the veneer, affects the shape stability. Three-ply laminates from peeled veneers of beech (Fagus sylvatica L.) were studied. The four types of laminate were the following: loose sides of all veneers in the same direction (cross and parallel centre ply) and loose sides of the outer veneers facing inward (cross and parallel centre ply). Four replicates of each type yielded 16 samples. The samples were exposed to RH cycling at 20% and 85% RH at 20°C, and the shapes of the samples were determined. The shape stability was influenced by the veneer orientation. Laminations with the middle veneer perpendicular to the top and bottom veneer (cross-laminated) showed the best shape stability, especially when the loose sides of the veneers were oriented the same direction. In parallel-laminated veneers, the laminates with opposite directions of the loose sides in the two outermost veneers showed the best shape stability. The major explanation of the behaviour of the laminates is that the loose side expanded more than the tight side from the dry to the humid climate, which was shown by optical 3D deformation analysis (ARAMISTM). After RH cycling, the laminates with cross plies showed visible surface checks only when the tight side was facing outwards.

Keyword
adhesive, distortion, lamination, plywood, thermo-hydro-mechanical processing, THM, wood
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-34541 (URN)10.1080/17480272.2014.919022 (DOI)2-s2.0-84911408156 (Scopus ID)
Available from: 2014-06-02 Created: 2014-06-02 Last updated: 2017-12-05Bibliographically approved
4. Shape stability of laminated veneer products: a review – defining and achieving shape stability
Open this publication in new window or tab >>Shape stability of laminated veneer products: a review – defining and achieving shape stability
2015 (English)In: International Wood Products Journal, ISSN 2042-6445, E-ISSN 2042-6453, Vol. 6, no 2, 89-95 p.Article in journal (Refereed) Published
Abstract [en]

Laminated veneer products (LVPs) consist of veneers bonded together with an adhesive into a predetermined shape. Deviation from the intended shape is a problem for both manufacturers and customers of the final products. The shape stability of LVP depends on a variety of material and process parameters, and this review identifies several important factors that influence shape stability, the purpose being to find ways of decreasing distortion and improving shape stability so that product quality can be improved.

The review ends to the following statements:

The fibre orientation of the veneer strongly influences the shape stability of LVP. The products can show considerable distortion (particularly twist) if the fibre orientation of the veneer is oriented in an unsuitable way in the assembly before moulding.

The effect of fibre orientation on shape stability can be very small directly after moulding, but it can increase considerably when the moulded product is subjected to a change in moisture content (MC). In general, a change in moisture leads to distortion. Moisture changes alone, however, result in a controlled distortion.

The orientation of the loose and tight sides of the veneers in LVP affects the shape stability.

An asymmetrical construction, coupled with different levels of moisture in the different veneers in an assembly before moulding, will result in poor shape stability of the product.

The choice of wood species affects shape stability. Beech shows more distortion and poorer shape stability than birch.

Based on existing knowledge to produce shape-stable LVP, the veneers should be conditioned to uniform MC and sorted with regard to fibre orientation and loose or tight side of the veneer before bonding. End-user climates should govern the veneers’ MC and added moisture from the adhesive during the process. Straight grain veneers and symmetry should always be sought.

This review reveals several areas that need to be further clarified in order to achieve shape-stable LVP, for example the influence of adhesive, the distribution of pressure, temperature, stresses and strains during moulding and the development of numerical methods to better predict the final shape.

Keyword
Fibre angle, Moisture content, Relative humidity, Symmetry, Wood
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-43592 (URN)10.1179/2042645315Y.0000000004 (DOI)2-s2.0-84940938537 (Scopus ID)
Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2017-12-04Bibliographically approved
5. The effect of veneer modification on the bond-line strength in laminated veneer products
Open this publication in new window or tab >>The effect of veneer modification on the bond-line strength in laminated veneer products
2015 (English)In: Pro Ligno, ISSN 1841-4737, E-ISSN 2069-7430, Vol. 11, no 4, 43-49 p.Article in journal (Refereed) Published
Abstract [en]

A major problem in the manufacture of three-dimensional laminated veneer products (LVP) isdamage due to stretching and/or buckling of the veneer. To reduce or eliminate this problem, veneerdensification or adding a strengthening layer to the veneer can be an alternative. To study how veneermodification influences the veneer-to-adhesive bond strength, three methods of modification werestudied in relation to an unmodified reference veneer: (1) densified veneer, (2) veneer pre-bonded withpaper and hot melt adhesive (HMA), (3) veneer pre-bonded with non-woven polypropylene (NW)fabric glued to the veneer (a) with a urea formaldehyde (UF) adhesive, (b) with a mixture of UF andpolyvinyl acetate (PVAc) adhesive, and (c) with a PVAc adhesive. Densification, pre-bonding withpaper, and NW with UF/PVAc adhesive mixture resulted in no or only a slight decrease in strength ofthe bond-line compared to the reference. NW glued with UF or PVAc adhesive showed a considerablereduction in the strength of the bond-line. The climatic cycling had no significant influence on the bondstrength.

Place, publisher, year, edition, pages
Brasov, Romania: Transilvania University, 2015
Keyword
densification; HMA; pre-bonding, PVAc; strengthening; THM processing; UF; wood
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-47309 (URN)
Conference
The International Conference Wood Science and Engineering in the Third Millennium (ICWSE 2015), Brasov, Romania, Nov. 5-7
Available from: 2015-11-16 Created: 2015-11-16 Last updated: 2017-12-01Bibliographically approved

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