Puu-0.3240

Wood Products: Properties & Performance

Wood-based composites: non-structural(“wood-based panels”)

27th January 2014

Contents

• What is a wood-based composite?• Wood-based composite products• Why wood-based composites?• Market size• Drivers• Products & characteristics• Manufacture & structure• Product properties & performance• Value adding• Applications

What is a wood-based composite?

• A material composed of wood particles, ranging in size from fibres to lamellae (including particles, strands and veneers), bonded together with an adhesive.

• Typically panel products

• Properties range from “non/semi-structural” to “structural”

• Typically used in the construction and furniture industries

• Products manufactured in many regions Worldwide

• Around 40 million cubic metres produced in European Union annually

What is a wood-based composite?

• Broadly defined as either structural or non-structural• Structural wood-based composites include products

such as:– Glue laminated lumber (glulam)– Laminated veneer lumber (LVL)– Plywood– Oriented Strand Board (OSB)– And others…..

• Whereas non-structural wood-based composites include:– Particleboard (“chipboard”)– Medium density fibreboard (MDF)– Soft-board & hardboard

LOG

Gluelam

LVL

Plywood

OSB

Par

ticle

boar

dMDF

Sawnwood

Why wood-based composites?

Drivers: technical

• Wood available only in limited shapes and sizes – panel products can be made to a range of dimensions

• Create new properties in the material. Wood is highly anisotropic, wood-based composites can be “engineered” to have tailored properties

• Improve dimensional stability (e.g. plywood)• Improve machining properties (e.g. MDF) • Reduce variability in properties by removing or

randomising defects• Visual improvement through lamination; a greater range of

finishes possible• Good strength/stiffness to weight ratios – good in

applications and reduces transportation costs

Drivers: commercial

• Utilise small dimension wood and poor quality material• Effective use of “waste” resources (sawdust, shavings,

the “urban forest”)• Use non-wood lignocellulosic resources• Value added opportunities

General issues

• Raw material; trends towards poorer quality, fast grown species

• Competition for the forest resource from energy and other uses (e.g. recreation)

• Good way of utilising recycled or reclaimed wood• Non-wood raw materials being used (cereal straws)

– Wheat straw, kenaf etc• Formaldehyde from the binder (adhesive): a big issue• Volatile Organic Compounds (VOCs) in general

Consumption of wood-based composites

Wood Based Panel Consumption 2004

0

20000

40000

60000

80000

100000

120000

140000

160000

WPB PB MDF Plywood OSB

Product

Co

nsu

mp

tio

n (

x1000 m

3)

Europe

World

(Source: UNECE Forest Products Statistics: http://www.unece.org/forests/fpm/onlinedata.html)

Non-structural wood-based composites

Particleboard: characteristics

• Largest production volume worldwide• Manufactured from particles of wood (or non-wood resources) that

have been “chipped” from larger pieces of wood• Chips typically have dimensions of several millimetres or 10s of

millimetres. Core material of coarser material (larger chips)• Usually bonded with Urea Formaldehyde (UF) resins, or resins

“fortified” with melamine to make them more moisture resistant (MUF)

• Mainly used for furniture, worktops, DIY (Do-it-yourself), flooring, roofing and general construction

• Particleboard density typically in the region of 650-750 kg/m3

Particleboard (chipboard)• Particleboard arose due to scarcity of veneer and the desire to

utilise waste materials such as sawdust and planer shavings etc• It was first produced in Germany in the 1940s• Rapid growth followed• Switch to virgin wood for the manufacture of chips so as to give

better performance• Ironically, there has been a switch back to recovered & waste wood

with the desire to reduce waste and improve resource efficiency. Now, a significant amount of raw material is waste, or recovered wood (e.g. Kronospan PB mill in Chrik, N .Wales, U.K. uses a high proportion of recycled wood in its manufacture)

• Probably no coincidence that production of PB and other wood-based composites arose at the time when waterproof glues such as urea formaldehyde and phenol formaldehyde became widely available

Environmental credentials

• “Kronospan chipboard comprises 70% recycled fibre and 30% virgin FSC traceable timber – diverting 300,000 tonnes of waste from landfill per year. The product has also been given a carbon-positive award by The Blue Green Carbon Company. Our chipboard exceeds all the relevant UK and European standards; we manufacture via a quality system accredited to BS EN ISO 9002:2000; all Kronospan chipboard is independently certified as low formaldehyde, meeting the European E1 standard”

Kronospan corporate literature (http://www.kronospan.co.uk/)

Chipboard

Structure of particleboard

• Particles of mm dimensions

• Typically 3 layers – surface or face material of fine particles

• Density profile developed during manufacture for improved properties in bending and a harder surface

Applications for particleboard

• Applications:– Examples include: general building work, joinery, upholstered

furniture, furniture carcass, veneering, foiling.

• Application brochure

MDF (LDF and HDF)• MDF, LDF (Low Density Fibreboard) and HDF (High Density

Fibreboard) are dry-formed fibre boards (as distinct from hardboard)• It is a relatively “new” product, with significant growth since the

1980’s• Manufactured from wood fibres that have been produced in a

pressurised refining process (TMP of CTMP)• Wood fibres are typically 1-3 mm long and 20 µm in diameter• Usually bonded with UF or MDI (methylene diphenyl diisocyanate)

resins• Produces a very “fine-scale” and homogeneous product, suitable for

a variety of applications• Can be worked, turned and laminated – used for furniture, fittings,

partitions, wall-linings, mouldings, flooring and general construction

MDF: characteristics

• MDF – Medium Density Fibreboard– Average density: 700 – 800 kgm-3 (typical target 780 kgm-3)– Core density: 600 – 700 kgm-3

– Face density: 1000 – 1100 kgm-3

• Generic term for a range of dry formed fibreboards all produced in a similar way:– HDF: Above 800 kgm-3

– LDF: Below 650 kgm-3

– ULDF: Below 550 kgm-3

• Produced in a range of thicknesses from “thin” (<2 mm) up to 60 mm

Medium Density Fibreboard

(Source: Emeri Enterprise Co.,Ltd.)

MDF

• First made when fibre produced for wet formed hardboard was processed as particleboard

• Good machining and strength properties ensured that MDF rapidly became popular

• Technical developments such as “blowline blending” in the 1980’s spurred production

• Competition with particleboard manufacture has led to developments in both sectors

Other wood-based panels

• Other fibre boards– Insulation boards (softboard)– Hardboard

• Cement bonded boards

Properties & performance of wood-based panels

How are wood-based composites typically made?

• Wood particles, an adhesive (and additives such as wax) are mixed together and pressed together to consolidate– Wood raw material prepared (e.g. debarking)– Converted (size reduction) into veneers, strands, chips or fibres– Dried– Gluing (typically with Urea Formaldehyde or Phenol

Formaldehyde) and application of additives– Forming or lay-up– Hot pressing to compact the particles together and cure the resin– Finishing and value adding

Structure, properties & performance

• Wood-based composites are heterogeneous materials, in other words they are composites

• Their properties are affected by:• Raw material characteristics

– Strength, chemical composition, density

• Interaction between particles– Particle to particle bonding

• Structural organisation of constituents– Size an shape of the particles– Orientation of particles– Packing– Relative proportions of the constituents

Process

Basic properties

Particles & resin

Ideal properties of a particle

What are the requirements for the particle?• WBCs needs to support mechanical loads. Loads on the board are

transferred via the resin bonds to the individual particles (by analogy with synthetic composites: the particle is the “reinforcement” and the resin forms the “matrix”)

Ideally, therefore, the particles should:• Be inherently strong, stiff and free of defects to sustain mechanical

loads without breaking• Possess a suitable geometry (size & shape) for good contact with

neighbouring particles so as to promote effective load transfer• Have surface characteristics that are compatible with resin binders

for a strong glue-line bond or “interface”

Particle• Inherent properties:

– Largely controlled by raw material (e.g. species, tissue), but influenced by processing

– Ultimately depends on chemistry and ultrastructure

• Geometry:– Controlled by processing and

the type of raw material used– High L/W ratio preferable (e.g.

promotes multiple numerous bonding opportunities)

• Surface:– Largely influenced by raw

material and processing

Resin

• Sufficient cohesive strength to transfer loads from particle to particle

• Sufficiently flexible to absorb some movement between particles

• Sufficiently compatible with the particle surface to form a good bond - adhesion

• Able to wet (“spread”) over the particle for good distribution

• Retention of properties under adverse conditions

Interaction between constituents

Interface, microstructure & micromechanics

Bonding

• Stress transfer between resin and particles takes place across the interface (boundary) or interphase (boundary zone) between the two

• Model:– Strong enough to transmit

stresses– Durable so as not to break

down under adverse environmental conditions

• Development of interface or interphase depends upon– Surface characteristics of

particle– Resin properties

Ideal (micro)structure

• Resin forms “bridges” between particles • More bonds promote improved stress transfer

(particle geometry)

High aspect ratio particlesFrequent particle-particle bonding

Low aspect ratio particlesFewer particle-particle bonds

Factors affecting properties and performance

Resin distribution

• Resin distribution on particle – resin droplet size

• Large number of small resin bonds versus small number of large bonds

Compaction & bond geometry

Compaction to promote

particle-particle contact

Deformation in particle increases area for bonding

Structure

Alignment, packing arrangement and relative proportions of constituents

Particle orientation

• Introducing a preferred orientation to the particles alters the elastic & strength properties of the panel

Particle packing arrangement and density

• Packing arrangement influenced by the size, shape and properties of the particles

• Good packing promotes efficient use of material

Panel properties

• Panel products are frequently used in situations where good flexural or shear properties are needed

• Panel flexural stiffness properties are proportional to density

• Panel flexural stiffness also affected by board thickness

• Make panels thicker or denser…..

Through thickness density profile

• Average MDF density in the range 500 to 800 kg/m3. Typically around 750 kg/m3

• “High” density surface (~1100 kg/m3)

• “Low” density core (~700 kg/m3)

How does this relate to panel products?

• High aspect ratio strands are used in OSB to promote flexural properties

• The surface of MDF fibre important in the development of panel properties. Surface properties largely controlled by refining process

• Resin distribution on fibre, particles and strands, important for the development of properties

• Veneers in plywood and laminated veneer lumber are oriented to maximise certain mechanical properties

• Development of the through thickness (vertical) density profile important for panel properties

Properties & standards

• Main physical tests:– Modulus or elasticity (MOE)/modulus of rigidity (MOR)– Thickness swell (2h and 24 h)– Internal bond strength (IBS)

• (Transverse tensile strength)

• Main QA test made on panel boards

– Formaldehyde emissions

• Measures of:– Adhesive bond strength– Panel stiffness and strength– Sensitivity to moisture

Resources

• Manufacturers:– Kronospan– Egger (http://www.egger.com/UK_en/index.htm) – Norboard (http://www.norbord.co.uk/)

• European Panels Federation (http://www.europanels.org/)

• Wood-based Panels: An introduction for specialists• Wood-based panels international