Fibre reinforcements John Summerscales ACMC University of Plymouth

Glossary of fibre/textile terms Fibre/textile terms are defined at: o S324A9%20FibreGlossary.htm S324A9%20FibreGlossary.htm

Principal fibres basalt, boron carbon fibres glass fibres rigid-rod polymers (aramid and PBX fibres) o e.g. Kevlar, Twaron polyethylene fibres o e.g. Dyneema, Spectra natural fibres o flax, hemp, jute, kenaf, sisal surface treatments on fibres

Griffith crack theory Alan Griffith (1920) studied strengths of glass rods and fibres fibre strength becomes markedly higher as fibre diameter decreases to ~10 micrometres critical stress above which cracks of a given size will spontaneously propagate. critical stress level is higher for small cracks. AG’s very fine fibres were strong because cracks in them would be very small. AG’s work was the key to present understanding of brittle fracture in all materials. the strength of the modern fibreglass industry is "a fitting memorial to his pioneering efforts".

Glass fibres A: high alkali grade o originally made from window glass C: chemical resistance or corrosion grade o for acid environments D: low dielectric o good transparency to radar: Quartz glass E: electrical insulation grade o E = most common reinforcement grade (E ~70 GPa) L: high lead content for radiation absorption M: high modulus grade R: reinforcement grade o European equivalent of S-glass S: high strength grade (a common variant is S2-glass) o fibre with higher Young’s modulus and temperature resistance o significantly more expensive than E-glass

Glass-forming oxides Oxide% in E-glass% in S-glassEffect on Fibre Properties SiO very low thermal expansion Na 2 Otrace high thermal expansion, moisture sensitivity K2OK2O-- Li 2 O--high thermal expansion, moisture sensitivity CaO17.5traceresistance to water, acids and alkalis MgO4.510resistance to water, acids and alkalis B2O3B2O3 8.0tracelow thermal expansion Al 2 O improved chemical durability Fe 2 O 3 trace green colouration ZnO--chemical durability PbO--increased density and brilliance (light transmission) and high thermal expansion BaO--high density and improved chemical durability TiO 2 improved chemical durability especially for alkali F2F2 trace

beware! Glass fibres: beware! Handling fibres causes damage o salts on the skin can displace bonding ions from the glass structural network o oil and grease on the skin transfer to fibre and act as release agents Health and safety issues o Commercial fibres should NOT be respirable as diameter is > 5 μm

Surface finish (known as “size”) protect fibre surfaces from damage lubricate fibres during mechanical handling impart anti-static properties bind fibres together for easy processing coupling agent promotes interfacial bond

Carbon fibres natural graphite has o Young’s modulus of GPa in-plane o Young’s modulus of 30 GPa through plane carbon fibre o turbostratic layered structure of contiguous benzene rings o a single layer of graphite = graphene. standard (high strain/high strength) fibres o E > 210 GPa (E is equivalent to steel) high-modulus (HM-) fibres o E > 350 GPa o when E>400 GPa incorrectly called “graphite fibre” in USA

Carbon fibres precursor materials are: o polyacrylonitrile (PAN) o pitch, and o rayon (regenerated cellulose) and lignin manufacturing imposes orientation by: o spinning of polymer to fibre o stretching polymer precursor o graphitisation (pyrolysis) under tensile stress o HM fibres pyrolysed at >1650°C

beware! Carbon fibres: beware! as fibre modulus rises, strain to failure falls carbon fibres conduct electricity longitudinal coefficient of thermal expansion of carbon fibres is slightly negative o this effect increases in magnitude with increasing modulus

Rigid rod polymers: aramid aramid is derived from poly aryl amide commercial reinforcements fibres are: o Kevlar (DuPont) reinforcement,  molecule is poly(para-phenylene tere-phthalamide) [PPTA] o Twaron (Akzo) reinforcement o Nomex (DuPont) for paper and honeycombs  molecule is poly(meta-phenylene iso-phthalamide)

Aramid fibres FibreCharacterE (GPa) σ' (GPa) ε' (%) Kevlar 29 high-toughness, high-strength, intermediate modulus for tire cord Kevlar 49 high modulus, high-strength for composite reinforcement Kevlar 149 ultra-high modulus recently introduced

beware! Aramid fibres: beware! very low resistance to axial compression o typically ~20% of corresponding tensile strength o poor transverse properties o low longitudinal shear modulus fibres break into small fibrils (fibres within the fibre) o fibrils from rod-like structure of liquid crystal precursor fibres are hygroscopic o they absorb water fibre surfaces degrade in ultraviolet (UV) light.

Rigid-rod polymer fibres aramid (PPTA) aramid chemical structure alternates o aromatic (aryl) benzene rings, and o the amide (CONH) group. PBX: poly benz[x]azole C O N N C O H

PBI PBO PBT PBX rigid rod polymers S N H O

Polyethylene fibres made from UHMWPE (ultra-high molecular weight polyethylene) trade names o Dyneema (DSM), and o Spectra (Allied Corporation) excellent modulus and strength-to-weight properties (similar to aramid) lower density than aramid o weight specific properties are superior (almost match those of HM carbon fibres?)

beware! Polyethylene fibres: beware! fibres melt at ~150°C fibre surface is effective release agent

Natural fibres reinforcement mostly uses the structural fibres from plant stems (bast fibres) the fibres most used are o temperate zone: flax, hemp o Tropical zone: jute, kenaf and sisal MATS324: topic dealt with in separate lecture MATS231: natural fibre less than ideal when wet

Summary density o aramid (1.44) < carbon ( ) < glass (2.56) modulus of standard fibre is o glass (70 GPa) < aramid (140 GPa) < carbon (210 GPa) strength of synthetic reinforcement fibres o usually ~ 1 GPa (if not virgin fibre) toughness o carbon (brittle) < glass < aramid (tough) beware!:beware!: each fibre has different problems