Matrices and commodity composites Flashcards
(29 cards)
Required resin system properties
Good mechanical properties
Good adhesive properties
Good toughness properties
Good resistance to environmental degradation
Role of matrix in composite material
transfer load between reinforcement fibres/particles and to protect the reinforcement from the environment
Ideal stress / strain curve for a resin system:
- High ultimate strength
- High stiffness (indicated by the initial gradient)
- High strain to failure
Resin is initially stiff but at same time will not suffer from
brittle failure
Additives and particulate fillers are combined with resin to:
enhance mechanical properties
improve processability,
reduce shrinkage and improve dimensional stability,
enhance UV resistance
reduce costs.
Types of Resin Reinforcement: Nanoscale
nano-clay platelets
carbon nanotubes
Graphene
define toughness
a measure of a material’s resistance to crack
propagation Stress / strain curve of resin system provides some indication of the material’s toughness. More deformation before failure → tougher / more crack-resistant
Why is high adhesion between resin and reinforcement fibres necessary for any resin system
Ensures loads are transferred efficiently and prevents cracking or fibre / resin debonding when stressed
Fibres are treated after manufacture with
coating or ‘size’ to protect them and to
enhance adhesion with resin.
name two classifications of polymers
thermosets
thermoplastics
Curing process of thermosets
Liquid-Open time Open time (i.e. working time) - portion of the cure time, after mixing, that the resin/hardener mixture remains liquid (suitable for infusion)
Gel-Initial cure
Mixture passes into initial cure phase - begins to gel or “kickoff.” Epoxy no longer workable, progresses from a tacky, gel consistency to firmness of hard rubber
Solid-Final cure
The epoxy mixture has cured to a solid state
The speed of reaction in a Thermosets curing process depends on:
Speed of reaction depends on:
(a) Types of chemicals (resin + hardener)
(b) Temperature: high T increases reaction speed (influenced by both ambient T + exothermic reaction)
Note that the exothermic reaction is significant, thus, thick parts can cure much faster than thin parts – be careful!
Which forms of polymers typically make up Thermosets
Crosslinked polymers
Network polymers
Describe thermosets: (properties, bonds, examples)
Thermosets start soft and harden when heated. This is a
permanent and non-repeatable process
During initial heat treatment covalent cross-links are formed
between polymer chains that anchor chains together
Usually 10 to 50% of chain mer units are cross-linked
Rubbers, epoxies, phenolics, polyester and vinyl ester resins are thermosetting
Describe Thermoplastics: (properties, bonds, examples)
Thermoplasts soften when heated (and eventually melt) and harden when cooled, a process that can be repeated – much like chocolate!!
Thermal energy of molecules diminishes restraining forces of secondary bonds - molecules become easier to move when a stress is applied
Most linear polymers are thermoplastic e.g. polyethylene, polypropylene, polystyrene, polycarbonate, nylon
Which forms of polymers typically make up Thermoplastics
Linear polymers
Branched polymers
Typical Resins systems in automotive (thermosets)
Polyester
Vinyl ester
Phenolic
Epoxy
Typical Resins systems in automotive (thermoplasts)
Polypropylene
Nylon
PEEK
Thermosets versus Thermoplastics
Thermosets
- better creep resistance
- higher glass transition temperature
- lower Coefficient of Thermal Expansion (CTE)
- reduced shrinkage and warp during moulding
- better moisture and chemical resistance
Thermoplastics
- higher strain to failure
- tougher than thermosets
- more familiar to automotive designers
- more flexible (snap-fit applications)
- re-meltable so can be welded (no adhesives or fasteners)
- recyclable
What stages can reinforcement and matrix be combined at in the manufacture process?
• Matrix and reinforcement can be combined before part
manufacture (pre-impregnated or prepreg)
• Matrix and reinforcement can be combined during part
manufacture using liquid moulding techniques (precursor materials tend to be cheaper)
Pre-impregnated Commodity Composites
Thermosetting composites consisting of chopped fibres with a polyester, vinyl ester or epoxy matrix
- Bulk Moulding Compound (BMC)
- Sheet Moulding Compound (SMC)
Thermoplastic composites consisting of chopped fibres
- Long Fibre Reinforced Thermoplastic (LFT)
- Glass Reinforced Thermoplastic (GMT)
Bulk Moulding Compound (BMC)
• Bulk Moulding Compound (BMC);
thermosetting polymer blend of fibre
reinforcement and fillers
• Forms a viscous, ‘puttylike’
compound for compression or
injection moulding
• Bulk Moulding Compound (BMC) is
highly filled and reinforced with short
glass fibres between 10% and 30%
by weight
• Glass length typically between 0.8
and 12.5mm
Compression Moulding of BMC
Fast process & complex parts possible
Putty-like consistency means low stresses and inexpensive
tooling (compared to metals)
Sheet Moulding Compound (SMC)
- Sheet Molding Compound (SMC) similar to BMC
- Glass length is slightly longer than BMC - between 12.5 and 25mm
- Used in compression molding - often of larger parts - where higher mechanical strengths need to be achieved
- Glass reinforcement is between 10% and 60% by weight
Production of Sheet Moulding Compound
SMC consists of a long glass fibres squashed between 2
sheets of thermosetting polyester
Compression Moulding of SMC
Fast process & complex parts possible
Low stresses so, again, inexpensive tooling compared to forming sheet metals
Better mechanical properties than BMC parts
