Puu : CELLULOSE CHEMISTRY Natural fibre composites

Puu-23.6080: CELLULOSE CHEMISTRY Natural fibre composites Mark Hughes 23rd June 2016 Outline • A short history of natural fibre composites • Natura...
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Puu-23.6080: CELLULOSE CHEMISTRY

Natural fibre composites Mark Hughes 23rd June 2016

Outline • A short history of natural fibre composites • Natural fibres for composites • Composite properties: toughness

Outline • A short history of natural fibre composites • Natural fibres for composites • Composite properties: toughness

Gordon Aerolite • Development began in 1936 with work undertaken by De Bruyne to utilise cotton fabric, as reinforcement in phenolic mouldings • A composite consisting of unidirectionally aligned unbleached flax thread impregnated with phenolic resin • A number of prototype aircraft structural components were produced from Gordon Aerolite. One of the first of these was an aircraft wing spar!

Gordon Aerolite • Produced by laying-up strips of resin impregnated unbleached flax yarn to form a cross-ply laminate structure, or unidirectional bar or strip of material • Heated and pressed to form the composite • Around 75% by volume was fibre, held together with resin which formed the remaining 25% • Ultimate tensile strength and Young’s modulus of longitudinally loaded material were around 480 MPa and 48 GPa respectively. (These values have rarely been matched in more recent research) Cross-ply laminate (similar construction to plywood)

Other early natural fibre reinforced composites • Henry Ford first raised the possibility of using hemp fibre reinforced soybean resin in cars! • The body panels of the Trabant were produced from cotton reinforced unsaturated polyester resin • Development of “cold cure” resins and glass fibre in the 1940s led to a decline in natural fibre composites until the 1990s

Current range of natural fibre composites • Current natural fibre composites fall into two broad categories: • Wood Plastic Composites (WPCs)

– Wood fibre or another agricultural fibre combined with a plastic such as polyethylene or polypropylene (or a range of other polymers

• Natural fibre reinforced composites

– Range of natural fibres grown specifically for their good properties, combined with a polymer resin or plastic to form a composite material

• Broad classification and the distinction between the types is not always so clear

Wood Plastic Composites - WPCs • Now in commercial production in many countries, especially in N. America • Europe slow to take-off, but now strong interest. E.g. UPM-Kymmene “ProFi” decking launched on the European market in the last decade • Applications are mainly in the construction sector, where they can replace materials such as treated timber

Natural fibre reinforced composites • Automotive components • Most manufacturers nowadays incorporate several kilos (5-10) of natural fibre reinforced polymer matrix composites in their cars • In excess of 50 k tonnes per year in EU • Technical advantages include lower density, giving rise to weight savings, and the replacement of higher embodied energy materials, such as glass fibre

Outline • A short history of natural fibre composites • Natural fibres for composites • Composite properties: toughness

(Source: Müssig & Slootmaker 2010)

(Source: Müssig & Slootmaker 2010)

Bast fibre • Bast fibres, found in the outer portion, or bark of the stem, have very good potential in view of their excellent mechanical properties • Extraction of the “technical” fibre involves several steps: – Retting – Scutching (decortication) – Hackling (cleaning)

• The process of fibre production is still principally “agricultural”, rather than industrial

Flax (water) retting

Cloney Farm, Knocknacarry around 1914 (Source: http://homepage.ntlworld.com/sean_quinn/cdun/knock.htm )

Industrial hemp (Cannabis sativa)

Extraction of bast fibres

(Source: Fischer & Müssig 2010)

Technical fibres • Bast fibres are “bundles” or groups of individual cells (“ultimate” fibres)

(Source: Fischer & Müssig 2010)

Morphological characteristics

(Source: Müssig et al 2010)

Morphological characteristics

(Source: Müssig et al 2010)

Fibre, processing, defects and composites • Natural fibres are inherently variable in structure, giving rise to uneven straining, with strain concentrations forming around features such as pits or nodes • Fibres are susceptible to damage through the formation of kinkbands and micro-compressive defects • This happens during growth, especially when the plant is subject to stress (wind, drought) • Or when the fibres are processed mechanically during decortication • Or when the fibres are processed chemically (e.g. to improve fibrematrix adhesion) • Fibre strength is reduced (can be significant) • Defects affect the micromechanics

Processing induced defects

(Hänninen et al, 2012)

Defects increase susceptibility to chemical degradation

(Hänninen et al, 2012)

Outline • A short history of natural fibre composites • Natural fibres for composites • Composite properties: toughness

Toughness: essential for engineering materials • Toughness may be regarded as the resistance a material possesses to the propagation of cracks or crack-like defects which might ultimately lead to failure • Cracks may, for example, be macroscopic, ‘stress raisers’ such as bolted joints, or sharp changes in section, or alternatively preexisting crack-like defects in the material itself. These cracks result in localised stress concentrations, the magnitude of which depend upon the size and shape of the crack

Stress concentrations • If the stress concentrations are high enough, the material in the vicinity of the crack-tip may fail. Under certain conditions, a crack may propagate catastrophically, leading to sudden failure of the material • The crack-tip may, therefore, be viewed as a mechanism whereby local stresses in the material are raised sufficiently for fracture to occur

Energy absorption • However, for the crack to propagate, it must be energetically favourable for it to do so • The energy to drive the crack forward is provided by the release of stored strain energy in the material, together with any external work done by the loading system • Therefore, a material that possesses mechanisms whereby significant amounts of energy can be absorbed as the crack advances or if, by some contrivance, the stress concentration at the crack-tip can be relieved, then the material is likely to be tough

Energy absorption • Brittle materials such as glass have little means of energy absorption or crack-blunting and hence fail in a catastrophic manner, exhibiting low fracture energies of around 0.01 kJ/m2 • Ductile metals such as mild steel, on the other hand, absorb large quantities of energy by plastic deformation. Typically, tough engineering materials such as steel exhibit fracture energies of around 100 kJ/m2

Toughness of natural fibre reinforced composites • At equivalent fibre volume fraction toughness is an order of magnitude lower

Charpy impact strength (kJ m-2)

80

60

hemp reinforcement hemp trendline jute reinforcement CSM reinforcement jute trendline

40

20

0 0

10

20

30

40

fibre volume fraction (%)

Comparison of un-notched Charpy impact strength. Jute, hemp and C.S.M. glass fibre reinforced laminates

50

Stress ahead of the crack-tip

(Cook & Gordon 1964)

Cracks and interfaces Interface

A fibre reinforced composite contains multiple interfaces (Cook and Gordon, 1964)

The “Cook-Gordon” crack stopping/blunting/deflection mechanism • Stress field ahead of advancing crack, “opens up” an interface • Transverse stress about 20% of axial stress

(Cook and Gordon, 1964)

Energy absorbing processes in composites • Several energy absorbing mechanisms have been identified. These are: – – – –

Matrix deformation and fracture Fibre fracture Interfacial debonding Frictional sliding and fibre pull-out

• The contribution that each of the energy absorbing mechanisms makes to the overall toughness of the composite varies, depending upon the composite system involved and the properties of the phases

Matrix deformation & fracture • With brittle thermoset polymers, the contribution from matrix deformation and fracture to the overall fracture energy of the composite is likely to be small. Typically, fracture energies of thermosetting polymers are of the order of 0.1 kJ/m2 • The fracture energy of thermoplastic polymers such polypropylene, or polyethylene are greater

Fibre fracture • Brittle fibres such as glass exhibit very low fracture energies of the order 0.01 kJ/m2. The contribution to the overall work of fracture of the composite is likely, therefore, to be small • Wood fibres can, however, exhibit high works of fracture. It has been observed that wood fibres can deform in a ‘pseudoplastic’ manner, due to the microfibril angle in the S2 layer. This results in shear failure in the fibre cell wall, leading to energy absorption. This mechanism is believed to account fro up to 90% of the work of fracture of wood across the grain (10-30 kJ/m2)

Fibre fracture • The work of fracture of the cell wall material (i.e. not including the plastic deformation – the so-called “intrinsic toughness”) has been reported to be between