Composites Flashcards
What are composites?
• Materials composed of more than one base material – usually,
matrix and reinforcement
Reinforcement may consist of long or short fibres, platelets,
particles etc
• Long fibres: (i.e. carbon fibre / glass fibre) spars/masts
• Short fibres: injection moulded tennis rackets
• Platelets: some types of car filler, resin ornaments.
What are the biomedical applications of composites
Bradley Hip
• Consists of a metal core with
CFR-PEEK outer layer
HAPEX
• HAPEX is an artificial bone analogue composite
made from HA (hydroxyapatite) and polyethylene
• HAPEX is used for orthopaedic implants like
tympanic (middle ear) bones
What are Matrix Systems?
Polymeric Matrices (bioresorbable and non-bioresorbable)
Mostly thermoplastics
- Polysulfone
- Poly-ether-ether-ketone (PEEK)
- Polyethylenes (UHMWPE and HDPE)
- Poly-tetra-fluoro-ethylene (PTFE)
- Poly(methylmethacrylate)(PMMA)
- Polylactic acid (PLA)
- Poly(lactic-co-glycolic acid) (PLGA)
- Polycaprolactone (PCL)
- Hydrogels
Fibre Reinforcement
Length is much greater than cross-section
• Much more mechanically effective than particles
• Usually anisotropic due to fibre orientation
• Customisation to match application
Applications in Biomaterials:
• Arterial prostheses
• Intervertebral discs
• Fixation plates and Nails
Reinforcing Systems - Polymer fibres
Aramid (Aromatic Polyamide, Liquid Crystal Polymer)
• Kevlar®, Nomex®, (both by Dupont)
• Twaron® (Teijin/Twaron, Japan)
• Light, stiff, strong
• Absorb moisture (a disadvantage)
• Poor compressive strength
• Applications: Dentistry, ligament prostheses
Reinforcing Systems – Carbon Fibre
- Lightweight, High Strength, Radiolucent
- BUT – Poor Shear Strength
Examples:
• Short fibre Carbon reinforced UHMWPE in orthopaedic applications
• Aimed to increase longevity of bearing surfaces
• Osteolysis and failure of tibial inserts in knee prostheses
Scaffolding device for ligament repair
• Performed poorly, permanent wear debris in joint
Reinforcing Systems - Polymer fibres - UHMWPE
UHMWPE
• Spectra (Honeywell), Dyneema (DSM, Heerlen), Toyobo
(Toyobo, Japan)
• High modulus, strength, light weight
• Do not absorb water
• BUT adhere poorly to matrix, hence performance is not
fully realised
• Applications: Dentistry (reinforcing acrylic resins),
intervertebral disc prostheses, ligament augmentation
Reinforcing Systems - Polymer fibres - PET
PET (Poly-ethylene-terephthalate) • Dacron® • Used as fabrics for arterial grafts in cardiovascular surgery • Proposed applications • Artificial tendons and ligaments • Ligament augmentation (fibres alone or as composites) • Soft tissue prosthetics • Intervertebral discs • Plastic surgery
Reinforcing Systems - Polymer fibres - Biodegradable Polymers
Biodegradable polymers
• PLA, PGA, and co-polymers
• Used as reinforcement and self – reinforcement (due to variation
in crystallinity and molecular weight)
Applications
• Ligament reconstruction (as fibres)
• Scaffolds for tissue engineering (as fibres reinforcing tissues)
• Biodegradable intramedullary pins and plates (as composites)
• Biodegradable scaffolds for bone regeneration
Reinforcing Systems - Ceramics
Usually as particulate reinforcement
• Calcium Phosphates (tri-calcium phosphate and hydroxyapatite)
• Bioactive (integrate with tissue)
• HAPEX (HA-PE composite, marketed by Smith & Nephew)
• Also used: Al, Zn phosphates, Bioglass and glass-ceramics,
bone mineral
Reinforcing Systems - Glasses
Commercial glasses (S-glass, E-glass fibres)
• High strength/weight ratio
• Good dimensional stability,
• Resistant to heat, cold, moisture, corrosion
• Low cost
Applications
• Orthopaedics (hip stems, with carbon fibre)
• Dentistry
Reinforcing Systems - Glasses - Bio-Glass
Bio-Glass (45S5 glass)
• Bioactive (takes years to resorb)
• Composed of SiO2 Na2O, CaO and P2O5
Advantages
• Highly bioactive
• FDA approved and commercially available
• Tensile modulus similar to bone (30-35 GPa)
Disadvantages • Mechanical weakness (UTS 40-60 MPa) • Low fracture resistance • Limited to low load applications • Also, S53P4 available and clinically used
Reinforcing Systems - Glasses - Resorbable Phosphate-based glass
Resorbable Phosphate-based glass
• E = 48 GPa (compare to 72 GPa for s-glass)
• UTS = 500 MPa (4 GPa for s-glass)
• Used as reinforcement for bioresorbable polymers
to produce fully resorbable composites
Dental Composites
• Dental composite resins have replaced both types of materials used in anterior and posterior teeth restoration
• Silver and gold amalgams were used for posterior teeth restoration
• Anterior teeth used to be restored with acrylic resins and synthetic cements which were cosmetically more attractive than the metal amalgams but had a shorter life
• The inclusions were normally barium glass or silica. The inclusions
improved the stiffness and wear resistance
• The matrix was an additional reaction product of bis (4-hydroxyphenol), dimethylmethane, and glycidyl methacrylate called BIS-GMA
Dental Composites - Notes
• The use of colloidal silica allowed the composite to be polished,
which reduced wear and plaque accumulation
• However it can only be used at low wt % compared with other fillers
since it tends to increase the viscosity of the mix too much
NB:
• A 75 wt% filler corresponds to approximately 50% volume
• Notice the contraction…this is an issue for all dental fillings leading to problems associated with saliva and bacteria at the interface edges
• Other problems include creep during the first three hours when the stiffness increases by a factor 2.5 – 4. The other major problem is wear
Strength of Metals vs Composites
Observations • Unidirectional carbon has a higher modulus than steel (in fibre direction) • CFRP has an even more spectacular specific modulus • GRP possesses only moderate stiffness
Composite Fabrication
Common methods • Hand lay-up • Spray up • Compression moulding • Resin transfer moulding • Injection moulding • Filament winding • Pultrusion
Fabrication in situ (e.g. bone cement)
In-situ manufacture
• Dental restorative composites
• Particle-reinforced bone cement
Open mould processes
Mostly not appropriate for biomedical applications Possible methods: • Vacuum-bag • Autoclave • Filament • Winding
Closed mould processes
Compression moulding
• Pre-preg (fibre and matrix) heated in
closed two piece mould under pressure
Injection moulding
• Fibre-matrix mix injected into mould at
elevated temperature and pressure
Continuous pultrusion
• Continuous fibres impregnated with resin
in bath then drawn through heated die
Composite Principles – short fibres
• Random or aligned (sometimes both, due to flow effects)
• Usually assumed to be randomly oriented in centre, and tangential
to flow direction at edge in injection moulding
Assumptions: • Fibres adhere well to matrix: perfect transmission of force • Fibres are randomly aligned; edge effects are insignificant • Fibres are uniformly distributed
Approximation:
• Material is modelled as a series of “plies” of
aligned fibres
Composite Principles – long fibres
• Strong along fibre axis
• Weak across it (and in many cases, fibres are poor in
shear)
Choice of Reinforcement
• Stiffness?
• Toughness?
• Energy absorption
Composite principles
Volume Fractions - the percentage of the composite that can theoretically be taken up by the reinforcement, however in practice this is often much lower than the theoretical value
Short and Long Fibre Composites
‘Short’ fibres are not as effective as ‘long’ or continuous fibres
• Load transfer mechanism results in end effects which may reduce
the fibre stress
• Alignment is difficult to control
• Volume fraction is lower, particularly for randomly-oriented short
fibres
Short fibres are more often used with thermoplastic resins
Injection moulding can lead to considerable fibre damage and
reduction in length
