Presentation on theme: "Lesson 5 2014. Lesson 5 2014 Our goal is, that after this lesson, students are able to recognize the key criteria for selecting polymers and are able."— Presentation transcript:
Our goal is, that after this lesson, students are able to recognize the key criteria for selecting polymers and are able to use different tools to support the systematic material selection process for proper selection of polymers.
Special materials properties affecting to proper selection of polymers: 1. Glass transition temperature 2. Shape of the stress-strain curve 3. Viscoelastic behavior 4. Creeping strength and heat deflection temperature 5. Fatigue strength and grazing 6. Impact strength and brittleness temperature 7. Ageing -sunlight, chemicals 8.Stress cracking - residual stresses due to manufacturing - environmental reasons (e.g. some chemicals)
Fracture mechanisms of polymers Both ductile and brittle fracture are possible. Brittle fracture is favored at lower temperatures, higher strain rates, and at stress concentrators Brittle to ductile transition often occurs with increasing temperature The third “fracture mechanism is called “crazing “…
Crazing occurs when localized regions yield, forming microvoids inside polymer chain structure. Fibrillar bridges or fibrils are formed around and between voids. Crazing absorbs fracture energy and increases fracture toughness Fibrils in polymer chains Microvoids Fibrils in polymer chains Microvoids Strain
Relative elongation [%] Stress [MPa] Linear or non- linear plastic deformation Reduction of the cross- section area Plastic deformation Ultimate tensile strength Yeld strength Viscoelasticity: Viscoelastic behavior is determined by rate of strain: elastic for rapidly applied stress, viscous for slowly applied stress!
POLYMERS POLYMERIZATIONCHARACTERISTICS OF POLYMERS (TYPES) THERMOPLASTIC CONDENSATION POLYMERIZATION THERMOSEPTIC TYPES OF POLYMERCHAINS ADDITION POLYMERIZATION ELASTOMERS CARBON- HYDROGEN POLYMERS (PE) CARBON-CHAIN POLYMERS (PTFE) HETEROCHAIN POLYMERS (PA) POLYMER CONSTRUCTIONS WITH THE AROMATIC RINGS IN THE CHAIN (Kevlar) AMIDI-GROUP
Polymerization 1. Addition is a chain-reaction, where monomer units are attached one at a time. E.g. PVC. 2. Condensation is a step reaction, which produce the mer units. Usually there is small by-product that is later eliminated. E.g. PA. Note: Polymers manufactured with condensation polymerization absorb easily water, which can damage their structure relatively soon!
Effects of the chemical structure on the polymers` properties Structure and bonding of mers Molecular structure Stereo isometric forms Double bonded (c=c) Number of monomers Construction of the polymer chain Single bonded (c-c) Co- polymers LINEAR CROSS- LINKED Aromatic rings Isotactic Syndiotactic Eutactic TACTICITY Heterotactic Atactic BRANCHED DENSITY Low- density LD High- density HD Medium- density MD Linear low- density LDD Ultra high- molecular weight (UHMW) Homo- polymers
Bonding between the atoms Bonding energy kJ/mol C-C350 C-H410 C-F440 C-Cl330 C-O350 C-S260 C-N290 N-N160 N-H390 O-H460 C = C810 C = O715 C = N615 AFFECTS OF BONDING BETWEEN MERS
N N O O H H n H OH CHEMICAL STRUCTURE OF KEVLAR AROMATIC RING
CH 3 CH 3 CH 3 METHYLENE GROUPS HIGH STRENGTH OF THE STRUCTURE CH 3 CH 3 CH 3 HIGH STIFFNESS AND RIGIDITY OF THE STRUCTURE CH 3 CH 3 CH 3 METHYLENE GROUPS AFFECTS OF STEREOISOMETRIC FORMS (TACTICITY)
Density classification PropertyLDPELLDPEHDPE Mass (g/cm³) 0,92- 0,93 0,922- 0,926 0,95- 0,96 Tensile strenght (GPa)6,2-17,312,4-20,0 20,0- 37,3 Elongnation to rupture %550-600600-80020-120 AFFECTS OF DENSITY
ASPECTS AFFECTING THE CRITICAL TEMPERATURE OF POLYMERS Creeping strength at the specific temperature Decomposition temperature of the polymer chain Melting point Required temperature during the manufacturing process Fatigue strength at the specific temperature Maximum operating temperature Heat deflection temperature (load is specified) Glass transition temperature Viscoelastic behavior related to temperature and impact forces Brittleness temperature Polymer degradation due to overheating Minimum operating temperature
E (Modulus of elasticity) T glass transition T melting T (Temperature) Glassy state Leathery state Rubbery flow Liquid flow Modulus of elasticity of polymers depending on temperature Rigid state Viscoelasticity : -glass at low temperatures -rubber at intermediate temperatures -viscous liquid at high temperatures. Viscoelastic behavior is determined by rate of strain (elastic for rapidly applied stress, viscous for slowly applied stress)
Examples of glass transition temperatures for some polymers
STRESS [MPa) TIME NEEDED TO FRACTURE [h] TEMPERATURE 23ºC 70ºC 100ºC 1 100 10000 GREEPING STRENGTH Many polymers susceptible to time-dependent deformation under constant load – viscoelastic creep Creep may be significant even at room temperature and under moderately low stresses (below yield strength).
IMPACT STRENGTH ULTIMATE TENSILE STRENGTH MAX. OPERATING TEMPERATURE PI MIN. OPERATING TEMPERATURE PC PC+ glass-fiber UTILIZATION OF FOUR-FIELD ANALYSIS FOR POLYMERS’ SELECTION PC+ glass-fiber Rejected area
RESISTANCE AGAINST ALCALINE AGENTS RESISTANCE AGAINST ACID AGENTS RESISTANCE AGAINST ORGANIC SOLVENTS 1 WATER ABSORBTION PTFE PI UTILIZATION OF FOUR-FIELD ANALYSIS FOR POLYMERS’ SELECTION Required area
UTILIZATION OFCOBWEB-ANALYSIS FOR POLYMERS’ SELECTION
Applications from mechanical engineerig: Polymer gears: High Performance Polymers (PEEK,PES,PI) Harsh loading conditions Polyasetal POM Good fatigue strength Polyamide PA Good adhesive wear resistance Phenol polymers, e.g. PF Cost-effectiveness Sliding bearings: Polyamide PA, Polyethylene PE, Teflon (small friction coefficient with adjacent steel components) The properties of polymers can be improved by reinforcing the matrix (carbon, aramid or other fibers) or by surface treatments (e.g. MoS2)
Remember the manufacturability aspects! Polymer Shrinkage during extrusion into mold %