Degradation & Stabilization of Polymers - PowerPoint PPT Presentation

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Degradation & Stabilization of Polymers

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  1. Degradation & Stabilization of Polymers

  2. Resin Identification Code The Society of the Plastics Industry, Inc. (SPI) introduced its resin identification coding system in 1988 at the urging of recyclers around the country

  3. Plastics A Plastic is... .. a material that contains as an essential ingredient, an organic high molecular weight polymer, is solid and rigid in its finished state, and at some stage in its manufacture or its processing into a finished article, can be shaped by flow.

  4. Production of polymer-based products Primary Resources Basic Petrochemical End Products Polymer Materials Plastics Ethylene Propylene Styrene Vinyl Chloride Butadiene Cyclohexane Acetylene HDPE LDPE LLDPE PP PVC ABS PA Acetal PC PUR PBT etc. Crude Oil Elastomer Fibers Natural Gas Adhesives + Coatings

  5. Polymer Definition A Chemical compound formed by many monomers linking to form larger molecules that contain repeating structural units. Mono-one Mer-unit Monomer ------------------------ ------------------------ Polymer Molecule Poly-many

  6. Polymer Families Materials Plastics (Polymers) Thermosets Thermoplastics

  7. Polymer Families Thermoplastics Plastics capable of softening and flowing when heated, hardening when cooled, and softening when reheated - REVERSIBLE PROCESS Thermosets Plastics which become permanently rigid when heated and cooled - IRREVERSIBLE PROCESS

  8. Polymer Families Materials Plastics (Polymers) Thermosets Thermoplastics Engineering Commodity

  9. Polymer Families Engineering and Commodity • Corrosion Resistance • Thermal/Electrical Resistance • Practical Toughness and Stiffness • Light Weight Engineering • High Temperature Resistance • Flame Resistance

  10. Polymer Families Plastics Thermoplastics Thermosets High Performance Commodity Engineering Amorphous Crystalline Amorphous Amorphous Crystalline Crystalline Blends PMMA PVC PS PE PP ABS PC/ABS ASA PC MPPO PEI PEEK PPS PBT PA POM PC/PBT PPO/PA ABS/PA

  11. Engineering Plastics Five EP Polyamide - PA Polycarbonate - PC Polyoxymethylene - POM Poly(butylene terephthalate) - PBT modified Poly(1.4-phenylene oxide) – mPPO Poly(phenylene sulfide) – PPS SIX…


  13. History of Major Plastics

  14. Polymer Morphology • Amorphous • Crystalline Refers to the Structure of the Polymer Material

  15. Polymer Morphology Amorphous Resins

  16. Polymer Morphology Crystalline Resins

  17. Polymer Morphology Crystalline Polymers are Actually Semi - Crystalline Regions of Crystallinity in an Otherwise Amorphous Mass

  18. Polymer Morphology Amorphous Crystalline Broad Softening Range Sharp Melting Point

  19. Polymer Morphology Amorphous Polymers: • Are Structural Below the Glass Transition Temperature (TG)and Rubbery Above It • Rely on Physical Entanglements of the Molecular Chains for Structural Properties BelowTG

  20. Polymer Morphology Glass Transition Temperature (TG) TG Glassy Rubbery Raise Temperature of Polymer Both amorphous and crystalline polymers exhibit a glass transition temperature.

  21. Adding Heat Increases Space Between Molecular Chains Polymer Morphology Model of Amorphous Polymers Locked Entanglements Stiff Flow Easier Flow T G Raise Temperature of Polymer Raise Temperature of Polymer

  22. Polymer Morphology Model of Crystalline Polymers T Rigid Solid Soft Solid Flows Easily TM G Raise Temperature of Polymer Adding heat increases space between molecular chains but crystalline structure prevents flow.

  23. T G Polymer Morphology Amorphous Polymer Modulus (Stiffness) Temperature

  24. Polymer Morphology Crystalline Polymer TG TM Modulus Glassy State (Stiffness) Glass Transition Leathery Region Rubbery Plateau Liquid Flow Temperature

  25. Amorphous Crystalline Polymer Morphology Amorphous vs. Crystalline Modulus (Stiffness) Amorphous TG Crystalline TG Crystalline TM Temperature

  26. Polymer Softening Range Crystalline Materials Have a Sharp Melting Point TG TM Solid (glassy) Stiff Flow (rubbery) Flows Easily Temperature

  27. Polymer Flow Characteristics • Adding Heat to a Polymer Melt will Increase Flow • Adding Too Much Heat or Heating for Too Long May Cause Degradation • It is Important to Know the Processing Temperature Range for Each Plastic to Make Good Parts

  28. Polymer Amorphous Processing Range TG Degradation Processing Temperature Range Raise Temperature of Polymer

  29. Polymer Crystalline Processing Range TM TG Degradation Processing Temperature Range Raise Temperature of Polymer

  30. Tg= Glass transition temperature Tm = melting temperature Each processing step causes degradation, a result of the combined action of shear, heat and oxygen.

  31. Modes of initiation (Degradation) • Thermal* • Photo (light induced)* • Chemical • Mechanical • Biological • High Energy Radiation *Will be discussed

  32. What is degradation • In practice, any change of the polymer properties relative to the initial, desirable properties is called degradation. In this sense, "degradation" is a generic term for any number of reactions which are possible in a polymer. • These reactions, in turn, lead to a change in the physical and optical properties of the polymer.

  33. Some of these properties include:Tensile StrengthBrittlenessImpact strengthToughnessDrawabilityAdhesive strengthElastic modulusMelt viscosityHardnessSoftening temperatureGloss

  34. Tensile strength is important for a material that is going to be stretched or under tension TENSILE STRENGTH - Tensile strength is defined as the force required to break the specimen or cause complete separation of constituents in a linear direction. ELONGATION - Elongation is defined as the distance (in percent) the specimen will stretch from its original size to the point atwhich it breaks.

  35. Calculation • 1. Tensile Strength = Max Load / Cross-sectional area of test specimen • 2. The displacement (stretching) of a due to the imposed force • % Elongation = (DL / L ) x 100 (L = originallength of test specimen) • 3. Modulus =The ratio of stress to strain in the elastic region • Modulus of Elasticity = Stress / Strain (Young Modulus) • The modulus is the slope of the stress-strain curve. If the modulus large (corresponding to steep angle of the curve), the material resists deformation strongly. Such materials are said to be Stiff. TheNumber average molecular weightMn, Weight average molecular weightMw, and the most fundamental characteristic of a polymer its molecular weight distribution.MWD These values are important, since molecular weight and molecular weight distributionaffect many of the characteristic physical properties of a polymer.

  36. Toughness If one measures the area underneath the stress-strain curve, colored red in the graph below, the number you get is something we call toughness. Toughness is really a measure of the energy a sample can absorb before it breaks. Think about it, if the height of the triangle in the plot is strength, and the base of the triangle is strain, then the area is proportional to strength times strain. Since strength is proportional to the force needed to break the sample, and strain is measured in units of distance (the distance the sample is stretched), then strength times strain is proportional is force times distance, and as we remember from physics, force times distance is energy.

  37. In general a higher molecular weight increases all of these properties. The reason is primarily explained by entanglement. Higher molecular weights imply longer polymer chains and longer polymer chains imply more entanglement. *Theimpact toughness is reduced by a broad MWD. *The impact toughness is generally increased by increasing molecular weight up to the point where embrittlement becomes important. In geneal the ultimate tensile strength and elongation, brittle temperature, and softening point will be affected adversely by a decrease in molecular weight. The relative magnitude of the effect will depend on the initial molecular weight. This is because most properties become independent of molecular weight when the degree of polymerization is greater than 700-800.

  38. Melt Flow (Index or Rate)(MFI) The Melt Flow Rate (MFR) as defined by ASTM D-1238, defines a polymers flow in terms of the number of grams extruded in 10 minutes at standard conditions, using specific geometric, temperature, and rate conditions. At the end of the specified time, the melt strand is cut off, weighted, and the MFR is calculated. The material with high viscosity will have a low MFR, and vise-versa. Physical Properties of Polymer MFI Molecular Weight

  39. Most commercial plastics are manufactured by processes involving chain polymerization, polyaddition, or polycondensation reactions. These processes are generally controlled to produce individual polymer molecules with defined Molecular weight (or molecular weight distribution) Degree of branching, and Composition Once the initial product of these processes is exposed to further shear stress, heat, light, air, water, radiation or mechanical loading, chemical reactions start in the polymer which have the net result of changing the chemical composition and themolecular weight of the polymer. !! Goal: Keep molecular architecture intact !!

  40. MECHANISTIC ASPECT OF POLYMER DEGRADATION Mode of Initiation Chain scission RandomChain scission (Hydrocarbons) Thermal Norrish Type (I, II) chainscission (SSR) Photochemical Enzymatic attack of peptide and glucoside (SSR) Chemical Side Chain elimination Solvolysis of ester linkage (SSR) Chemical Thermal Elimination of HCl (PVC) (CR) Thermal Depolymerization Autooxidation Thermal, Photochemical, Mechanical, chemical Cross-linking SSR= Single step reaction CR = Chain reaction

  41. Yellowness index Mn, Mw

  42. MFI Viscosity Mn, Mw

  43. Depolymerization Typical for Poly Methyl methacrylate and mainly forother acrylate based polymers

  44. Cross-linking In general, chain scisson will cause an initial hardening and rise in tensile strength. Viscosity MFI Flexibility Polymer network

  45. With the influence of heat, shear, oxygen or light, the polymer backbone can react via free radicals reactions. These reactions are very complex and can lead to numerous species depending on the nature of the radicals and the polymer structure. Polyethylene or Polypropylene can react very differently. In presence of radical, Polyethylene generates macroradicals having tendency to recombine generally - but not always - to branching and even gelling. In film extrusion, where optical properties are important, this phenomena is called "fish eyes" or unmelts.