Composites- Thermoplastic Matrices Flashcards
What aspects of thermoplastics’ chemical and physical structure influences their performance?
Structure of the repeating unit (steric and electronic influences).
Higher molecular weight gives improved properties.
Entangled chains in amorphous thermoplastics give improved properties.
Aligned chains in semi-crystalline thermoplastics give improved properties
Price to pay for improving properties of thermoplastic matrices
Processing is more difficult for ones with better properties
Polypropylene
Low cost, reasonable modulus. YM 1.5-2GPa. Tensile strength >20MPa. Tg<0. Processing temperature about 200°C. Is a commodity thermoplastic. Performance relatively low with its aliphatic structure and single C-C bonds along the chain. Increased crystallinity improves properties and solvent resistance. Used in automotive industry as short glass fibre filled PP stampable sheet like GMT (Glass Mat Thermoplastic). Also machine parts, household items.
Polyamides
Slightly higher cost and modulus than PP. YM 2-4 GPa. Tensile strength >40MPa. Tg about 50°C. Processing T about 200°C. Contain amide groups in the main chain (-NH-C(=O)-). Aka nylons. Nylon 6 or 6,6 or 11 or 12 all used as matrices. Extensive H bonding between chains. Excellent wear resistance so used in intake manifolds, housings, gears, bearings, bushings, sprockets. Water and other solvents can disrupt can disrupt H bonding, very susceptible to wet or hot-wet conditions. Effect minimised when combined with short glass fibres.
Polysulfones
Slightly higher cost, higher modulus and high T performance. YM >2.5GPa. TS>60MPa. Tg>170. PT>300. Contain sulfone group (-SO2-) with S=O bonds not on backbone. Backbone phenylene groups give excellent performance. Inflexible sulfone group (steric hindrance) means ether groups (-O-) used on backbone otherwise too stiff to process. Strong, stiff, tough, chemical and thermal resistance
Similar alternatives to polysulfone (PSU)
Polyethersulfone (PES) only has SO2, phenyl and ether groups on backbone and not another C(CH2)2 group and has better heat resistance. Polyphenylene sulfone (phenyl ring and -SO2- only) is too stiff to be useful. Polyphenylene sulfide (PPS) used instead which just has sulfide group(-S-) not SO2.
Thermoplastic polyimides
Much higher cost, higher modulus, high T performance. YM >GPa. TS>70MPa. Tg>200. PT>300. Contain imide linkages (-C(=O)N-C(=O)-) with links to phenyl groups from the two Cs and N. Performance better or equal to polysulfones. Better heat and solvent resistance. T>300 can be tolerated for short periods. Polyetherimide (PEI)
Polyketones
Much higher cost, higher modulus, highest T performance. YM >3.8GPa. TS>100MPa. Tg>140. PT>350. Contain ketone groups (-C(=O)-) and phenyl groups. Performance better or equal to polysulfones. Heat resistance not as good as polyimides as >250 can be tolerated for short periods. Polyetheretherketone (PEEK) has ether, benzene, ketone, benzene, ether, benzene. Polyetherketoneketone PEKK has ether, benzene, ketone, benzene, ketone, benzene. Stiff, strong, tough, thermal and chemical resistance. Expensive and hard processing
Processing amorphous thermoplastics
PSU, PES, PEI. Heating leads to disentanglement of polymer chains and gradual change from rigid solid to viscous liquid. Process above Tg. E.g for PSU (Tg=190) process at 300
Processing semi-crystalline thermoplastics
PP, nylons, PEEK. Heating results in melting of crystalline phase to give viscous amorphous liquid. Process above melting temperature Tm. E.g for PEEK (Tg=143, Tm=343) process at 400
What temperature does standard polymer processing equipment go up to?
About 250°C
In what form are thermoplastics generally obtained?
Fully polymerised in the form of powder or granules from the manufacturer
What does high thermoplastic melt viscosity mean for manufacturing?
Means combination with reinforcements is difficult. Need to use high pressure moulding techniques like extrusion or injection moulding. Use short flow distances. Processing using solvents possible but less popular recently due to VOC regulations
Pultrusion
Fibres and polymer pulled through a heated die producing continuous products that are coated in the polymer. Product generally has a constant cross-section such as a flat, a tube or a beam. Impregnation of the fibres can be difficult but can be used as a way of making moulding chip (later processed) with long fibres.
Extrusion and injection moulding
Very similar to that if infilled polymers. Requires use of short fibres so they don’t block flow channels. Techniques rely on high shear in polymer to melt and homogenise it. Putting fibres into the moulded results in them being broken up but also distributed within the melt. Careful mould design allows alignment of fibres in different direction to reinforce component as desired. Properties limited by amount of attrition fibres experience
