Chemical and Physical Properties Chapter 5

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.