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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.