Chemical and Physical Properties
Chapter 5
Professor Joe Greene
CSU, CHICO
1
MFGT 041
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
2
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
3
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
Melt
Temp
• 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
Tm
Rubbery
Tg
Glassy
Polymer
Form
4
Glass Transition Temperature, Tg
• Glass Transition Temperature, Tg: The temperature
by which:
– Below the temperature the material is in an immobile
(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 5
characteristic.
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)
Vol.
Tg
-50C 50C 100C 150C 200C 250C
Temperature
Tg
Tg
-50C 50C 100C 150C 200C 250C
Temperature
6
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
Char
• Amorphous Materials
– Melt, rubbery, stiff
– Have a reported Tg
Melt
Vol.
• Crystalline materials
– Melt, stiff
– Have a reported Tm, Tg is
not usually used
• Themoset Materials
Hard,
Stiff
Tg
Hard,
Stiff
Tm
-50C 50C 100C 150C 200C 250C
Temperature
Tchar
7
Crystalline Polymers Tg
• 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
-50C
50C
100C
150C
200C
250C
Temperature
8
Thermal Properties
• Table 3.2 Thermal Properties of Selected Plastics
9
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
10
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
11
gas), phosphorous compounds form char barriers
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, Cazinc, Mg-Zinc, etc..
• Impact Modifiers
– Elastomers added to polymers
– PVC is toughened with ABS, CPE, EVA, etc.
12
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
13
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-2ethylhexyl phthalate (carcinogenic in animals)
14
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
15
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-phenylbenzotrizoles
• Most developments involve hindered amine light stabilizers
(HALS)
• HALS often contain reactive groups, which chemically bond
onto the backbone of polymer molecules. This reduces 16
migration and volatility.
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
17
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.
18
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
19
the surface of a specimen. (Fig 10-5)
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
20
• Permeability
Testing
– 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 with21
multilayer films.