1 / 21

Chemical and Physical Properties Chapter 5

Chemical and Physical Properties Chapter 5. Professor Joe Greene CSU, CHICO. MFGT 041. Chapter 5 Objectives. Objectives Thermal Properties (energy inputs, thermal stability temperature, glass transition and melting temp) Weathering (UV degradation and oxidation)

jorn
Télécharger la présentation

Chemical and Physical Properties Chapter 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chemical and Physical PropertiesChapter 5 Professor Joe Greene CSU, CHICO MFGT 041

  2. Chapter 5 Objectives • Objectives • Thermal Properties (energy inputs, thermal stability temperature, glass transition and melting temp) • Weathering (UV degradation and oxidation) • Chemical resistivity and solubility • Permeability • Electrical Properties • Optical Properties • Flamability

  3. Thermal Properties • Plastics properties are affected by mechanical forces (Chap 4) as well as environmental exposure to heat, UV, moisture, salt sprays, solvents. • Energy Inputs • Thermal or UV can cause • Degradation or burning which breaks the covalent bonds • Softening or thermal transitions break hydrogen bonds and untangle polymer chains • Key thermal transitions are • Melting temperature: polymer becomes amorphous • Glass Transition temperature: glassy state to rubbery state

  4. Form of Polymers Melt Temp Tm Rubbery Increasing Temp Tg Glassy Polymer Form • Thermoplastic Material: A material that is solid, that possesses significant elasticity at room temperature and turns into a viscous liquid-like material at some higher temperature. The process is reversible • Polymer Form as a function of temperature • Glassy: Solid-like form, rigid, and hard • Rubbery: Soft solid form, flexible, and elastic • Melt: Liquid-like form, fluid, elastic

  5. Glass Transition Temperature, Tg • Glass Transition Temperature, Tg: The temperature by which: • Below the temperature the material is in animmobile(rigid) configuration • Above the temperature the material is in a mobile (flexible) configuration • Transition is called “Glass Transition” because the properties below it are similar to ordinary glass. • Transition range is not one temperature but a range over a relatively narrow range (10 degrees). Tg is not precisely measured, but is a very important characteristic. • Tg applies to all polymers (amorphous, crystalline, rubbers, thermosets, fibers, etc.)

  6. Glass Transition Temperature, Tg Amorphous Modulus (Pa) or (psi) Vol. Crystalline Tg Tg Tg -50C 50C 100C 150C 200C 250C -50C 50C 100C 150C 200C 250C Temperature Temperature • Glass Transition Temperature, Tg: Defined as • the temperature wherein a significant the loss of modulus (or stiffness) occurs • the temperature at which significant loss of volume occurs

  7. Thermal Stability Temperature Char Melt Amorphous Vol. Leathery Crystalline Hard, Stiff Hard, Stiff Tg Tm Tchar -50C 50C 100C 150C 200C 250C • Maximum use temperature • Rule of thumb: Plastic material should not be used at temperatures above 75% of Tg. • Example: Tg of ABS is 100°C. Then the maximum use application for the ABS pipe should be 75°C • Figure 5.1 • Amorphous Materials • Melt, rubbery, stiff • Have a reported Tg • Crystalline materials • Melt, stiff • Have a reported Tm, Tg is not usually used • Themoset Materials • Have a Tg where they lose modulus Temperature

  8. Crystalline Polymers Tg -50C 50C 100C 150C 200C 250C Temperature • Tg: Affected by Crystallinity level • High Crystallinity Level = high Tg • Low Crystallinity Level = low Tg Modulus (Pa) or (psi) High Crystallinity Medium Crystallinity Low Crystallinity Tg

  9. Thermal Properties • Table 3.2 Thermal Properties of Selected Plastics

  10. Additives • Environmental effects can be mitigated with the use of additives • Antioxidants: Oxidation of plastics involves oxygen in a series of chemical reaction that break the bonds of the polymer and reducing the molecular weight down into a powder. • Primary antioxidants work to stop or terminate oxidation reactions • Secondary antioxidants work to netralize reactive materials that cause oxidation • Susceptible Materials: PP and PE oxidize readily • Major types • Phenolic • Amine • Phosphite • Thioesters

  11. Additives • Antistatic Agents • Compounded into plastic attract water to surface and thus making it more conductive to dissipate charges • Major types • amines, quarternary ammonium compounds, phosphates, glycol esters • Flame Retardants • Emit a fire-extinguishing gas (halogen) or water when heated, • Swell or foam the plastic and forming an insulating barrier against heat and flame • Based on combinations of bromine, chlorine, antimony, boron, and phosphorous • Major Types • alumina trihydrate (ATH emits water), hologenated materials (emit inert gas), phosphorous compounds form char barriers

  12. Additives • Heat Stabilizers • Retard thermal decomposition for PVC • Based on lead and cadmium in past. 28% Ca pollution came from plastics • New developments based on barium-zinc, Ca-zinc, Mg-Zinc, etc.. • Impact Modifiers • Elastomers added to polymers • PVC is toughened with ABS, CPE, EVA, etc.

  13. Additives • Lubricants • Needed for making plastics. • Reduce friction between resin and equipment • Emulsify other ingredients with lubricant • Mold release for the mold • Causes surface blemishes and poor bonding • Common materials • waxes (montan, carnauba, paraffin, and stearic acid) • metallic soaps (stearates of lead, cadmium, barium, calcium, zinc) Table 7-1

  14. Additives • Plasticizers • Chemical agent added to increase flexibility, reduce melt temperature, and lower viscosity • Neutralize Van der Waals’ forces • Results in leaching for • Food contamination • Reduced impact and reduced flexibility, PVC hoses • Over 500 different plasticizers available • Examples: Dioctyl phtalate (DOP), di-2-ethylhexyl phthalate (carcinogenic in animals)

  15. Additives • Preservatives • Protects plastic (PVC and elastomers) against attacks by insects, rodents, and microorganisms • Examples • Antimicrobials, mildewicides, fungicides, and rodenticides • Processing Aids • Antiblocking agents (waxes) prevents sticking • Emulsifiers lowers surface tension. • Detergents and wetting agents (viscosity) • Solvents for molding, painting, or cleaning

  16. Additives • UV Stabilizers • Plastics susceptible to UV degredation are • Polyolefins, polystyrene, PVC, ABS, polyesters, and polyurethanes, • Polymer absorbs light energy and causes crazing, cracking, chalking, color changes, or loss of mechanical properties • UV stabilizers can be • Carbon black, 2-hydroxy-benzophenones, 2-hydroxy-phenyl-benzotrizoles • Most developments involve hindered amine light stabilizers (HALS) • HALS often contain reactive groups, which chemically bond onto the backbone of polymer molecules. This reduces migration and volatility.

  17. Additives • Heat stabilizers • Retard decomposition of polymer caused by heat , light energy, or oxidation, or mechanical shear. • PVC has poor thermal properties and has used a large amount of stabilizers, mostly cadmium based. (28% of waste Cd from PVC) • Lead and cadmium stabilizers have been replaced with • barium-zinc, calcium-zinc, magnesium-zinc, phosphite formulations

  18. Testing • Electrical Testing • Plastics are good insulators, handles for screw divers etc. • Ability to withstand exposure to electrical current. • Conditioning samples • ASTM D-618: 73F (23C) and RH of 50% for > 40 hours • Dry samples to get consistent results • Dielectric Strength • Amount of voltage required to arc through a specimen of plastic (figure 10-1) • Voltage starting at 0 Volts is applied to one side of specimen and increased until it arcs through.

  19. Testing • Dielectric Constant • The electrical capacitance of a specific plastic cross section as a ratio to that of a similar cross section of air. • Volume Resistivity • Ability of a plastic to resist an electric current through its bulk. (Fig 10-3) Used for electrical insulators. • Surface Resistivity • Ability of a plastic to resist current across its surface. (Fig 10-5) • Arc Resistance • Amount of time required for an electrical arc to carbonize the surface of a specimen. (Fig 10-5)

  20. Testing • Permeability • How easily gases or liquids pass through material • Diffusion Constant, D • Characteristics of material (plastic, metal, or ceramic) • If plastic material is solvent sensitive to a particular gas or liquid then D is large. • High D equals high permeability or low barrier properties • Diffusion variables, Figure 5.5

  21. Testing • Permeability • Barrier Properties of Plastic Materials, Table 5.2 • Packaging materials need to keep foods fresh and away from moisture, oxygen, or keep CO2 in soda or beer. • Barrier properties are due to chemical structure • Polar films let polar gases through but not nonpolar • Non-polar films let non-polar molecules but polar • Example, • ethylene vinyl alcohol (polar due to polar groups along chain) has low permeation rate for O2 (non-polar) but a high permeation rate for water (polar) • Polyethylene is has no polar groups along chain and has low permeation for water but a much higher rate for non-polar oxygen • Barrier properties can be modified with additives, or with multilayer films.

More Related