chemical and physical properties chapter 5
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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) - PowerPoint PPT PresentationTRANSCRIPT
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Chemical and Physical PropertiesChapter 5
Professor Joe Greene
CSU, CHICO
MFGT 041
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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
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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
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Form of Polymers• 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
Temp
Glassy
Rubbery
Melt
PolymerForm
Increa
sing T
emp
Tm
Tg
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Glass Transition Temperature, Tg• Glass Transition Temperature, Tg: The temperature by
which:– Below the temperature the material is in an immobileimmobile (rigid)
configuration– Above the temperature the material is in a mobilemobile (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.)
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Glass Transition Temperature, Tg• 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
Modulus (Pa) or (psi)
Temperature-50C 50C 100C 150C 200C 250C
Tg
Vol.
Temperature-50C 50C 100C 150C 200C 250C
Amorphous
Cry
stal
line
TgTg
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Thermal Stability Temperature• 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
LeatheryVol.
-50C 50C 100C 150C 200C 250C
Amorphous
Cry
stal
line
TmTg
Hard,Stiff
Melt
Hard,Stiff
Tchar
Char
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Crystalline Polymers Tg• Tg: Affected by Crystallinity level
– High Crystallinity Level = high Tg– Low Crystallinity Level = low Tg
Modulus (Pa) or (psi)
Temperature-50C 50C 100C 150C 200C 250C
Tg
High Crystallinity
Medium Crystallinity
Low Crystallinity
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Thermal Properties
• Table 3.2 Thermal Properties of Selected Plastics
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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
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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
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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.
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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
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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)
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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
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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.
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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
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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.
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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)
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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
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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.