Transcript Document

MET 470 w/ help from B. Young
Plastics
Plastics
Just cover 4 – 15 thermoset vs.
thermoplastic – rest is review
MET 470 w/ help from B. Young
Plastics
The word POLYMER means many ‘mers’
–
–
–
A ‘mer’ is a unit
Polyethylene means many ‘ethylenes’
The molecular weight of a polymer (length of the chain – number
of ‘mers’) will effect the properties.
• 10-20 ethylenes – greases or oils
• 200-300 waxes
• 20,000 + polyethylene
H H
| |
C=C
| |
H H
Ethylene
H HHHHHHHHHHHHH
| | | | | | | | | | | | | |
[-C-C-C-C-C-C-C-C-C-C-C-C-C-C-]
| | | | | | | | | | | | | |
H HHHHHHHHHHHHH
Polyethylene
Basic Molecular Structure:
Polyethylene = many
molecules of ethylene
Ethylene molecules
attach to each other
through covalent
bond between
carbon atoms
(needed to satisfy
valence
requirements for
carbon)
Many of these ethylene molecules
join together producing polyethylene
–
See physical structure
MET 470 w/ help from B. Young
Plastics
2 basic types of polymer materials
•
•
Thermoset – ‘eggs’ – Undergo a chemical process which crosslinks the polymer
chains. Will not re-melt
Thermoplastic – ‘butter’ – Soften when heated and harden when cooled;
repeatable process

Two basic types of thermoplastic materials
– Semi-crystalline – orderly, dense polymer
molecular chains – packing of molecular chains in
ORDERLY array – not the same as atoms making
“grains” in metals or ceramics!!
– Amorphous – without form, random array of
polymer chain molecules
MET 470 w/ help from B. Young
Plastics
Thermoplastic vs. Thermoset - Properties
–
–
–
–
Chemical Resistance
•
Thermoplastic – Some have good chemical resistance, but most can
be dissolved or weakened by at least some chemical compounds
•
Thermoset – Better chemical resistance than any thermoplastic
Dimensional Stability
•
Thermoplastic – Because they respond to heat, their dimensional
stability is related to their service temperature and their glass
transition temperature.
•
Thermoset – Excellent dimensional stability
Creep (cold flow)
•
Thermoplastic – Although it can be improved by adding fillers and
reinforcements, they are not as good as Thermosets
•
Thermoset – Much better than thermoplastics
Molded-in-stresses (warpage)
•
Thermoplastic – caused by uneven cooling, part design, mold design,
and process parameters
•
Thermoset – relatively low stresses which yield less distortion.
MET 470 w/ help from B. Young
Plastics
Thermoplastic vs. Thermoset - Properties
–
–
–
–
Toughness
•
Thermoplastic – Inherently tough, although some types can be brittle
•
Thermoset – not tough, brittle unless reinforced – fiberglass
Coloration
•
Thermoplastic – Easily colorable and color is maintained
•
Thermoset – Limited options and colors tend to fade or discolor over
time.
Clarity
•
Thermoplastic – Many clear polymers are available
•
Thermoset – Very few options for clarity
Shrinkage
•
•
Thermoplastic – Varies based on type of
polymer (Semi-crystalline/Amorphous)
Thermoset – Varies by process
MET 470 w/ help from B. Young
Plastics
Thermoplastic vs. Thermoset - Properties
–
–
Long term properties
• Thermoplastic – Need to be estimated based on short term
data.
• Thermoset – Can be predicted based on experimental data
Cost
• Thermoplastic – Less expensive and easier to process
• Thermoset – Need more skill and produce more scrap
which cannot be reprocessed.
– Tool Wear
• Thermoplastic – Less tool wear with unreinforced
polymers
• Thermoset – More prevalent mold wear and greater
potential for mold damage. Most Thermosets are
filled.
MET 470 w/ help from B. Young
Plastics
Thermoplastic vs. Thermoset - Properties
–
–
–
Density
•
Thermoplastic – Lightweight, very close to the weight of water
•
Thermoset – Limited in some applications due to their higher weight.
Cycle time
•
Thermoplastic – Generally fast, dependent on wall thickness of part cooling
•
Thermoset – Generally slower, dependent on wall thickness of part –
heating and holding
Flammability
•
Thermoplastic – some burn freely, while others will self-extinguish
•
Thermoset – Inherently non-flammable
MET 470 w/ help from B. Young
Plastics
Entanglement
Plastic material can be envisioned as a plate of spaghetti. You
have very long molecules which get entangled with each other
as they try to move.
–
–
–
–
This entanglement holds the material together
along with secondary forces.
Entanglement is why plastics burn before they
enter a gaseous form
Plastic molecules do not share primary bonds
with the adjacent plastic molecules. If they
did, they would be cross-linked and depending
on the degree of cross linking, would not remelt when re-heated
Side groups and secondary forces can greatly
increase the entanglement forces and thereby
increase the viscosity of the material
MET 470 w/ help from B. Young
Plastics
Non-Newtonian Flow
•
•
A Newtonian fluid is one which you push harder to get it to
flow faster. The relationship between force (viscosity) and
speed (shear rate) is linear.
With plastics, as you push them faster, they flow easier
–
–
The molecules ‘line up’ as the shear rate increases
The faster you go, the more Newtonian the material behaves
MET 470 w/ help from B. Young
Plastics
Hygroscopic
• Some materials will absorb
moisture into their matrix
based on the relative humidity
of the surrounding environment
• These materials need to be dried
prior to use or the moisture in the
matrix will cause rapid degradation
of the material which will reduce the
mechanical and visual properties of
the final parts (splay)
• Nylon is the most moisture sensitive
material
MET 470 w/ help from B. Young
Plastics
Orientation
• When
plastics flow quickly,
the molecules align
themselves in the
direction of flow
• If the molecules are frozen in that
aligned orientation, the part will have
greater mechanical properties in the direction of
orientation, but weaker properties
transverse to the alignment.
• Shrinkage is also affected
• Very prevalent in thin walled parts
• Causes internal stresses
MET 470 w/ help from B. Young
Plastics
Crystallinity
• Some polymers tend to ‘fold up’ and form densely packed regions in at least a
portion of the polymer matrix. (>35% crystallized = Semi-crystalline)
• These materials are referred to as semi-crystalline
• Semi-crystalline materials have a much sharper melt temperature range.
• Semi-crystalline materials require more energy to melt
–
You have to melt the crystals
• Side groups, secondary branching, and cooling rate all affect the degree of
crystallinity of the final product
• Crystalline materials tend to be more chemically resistant
MET 470 w/ help from B. Young
Plastics
• Amorphous (PC, PS, PVC, PMMA, ABS)
Generally
• Higher viscosity than semi-crystalline (s/c) materials
– harder to make flow
• Shrink less than s/c (0.005-0.007 in/in)
• Don’t have a true melt temperature – soften more
above Glass Transition Temperature – Tg
• Less chemically resistant than s/c
• Clearer than s/c (can be translucent/optical quality).
• Better weather resistance vs. s/c
• Better creep vs. s/c
MET 470 w/ help from B. Young
Plastics
•Semi-crystalline (PE, PP, PA, PET, POM)
Generally
• Lower viscosity than amorphous materials – flow easier –
allows them to form crystals
• Wide range of shrinkage values (.008-.050 in/in) –
depends on degree of crystallinity
• Have a clearly defined melting point and a Glass Transition
Temperature – Tg
• Usually translucent to opaque
• More brittle than amorphous
Misc.
•Strength – thermoplastics have no elastic limit. Some have
endurance limit (fatigue limit) but most do not. Various
methods used to calculate strength and E.
•Elongation – can have enormous % elongation. Key: elastomers
elastic strains > 100%. % elongation can be 1,000% for
polymers!
•Properties highly dependent on temperature. Properties also
dependent on aging, strain rate, loading speed, etc..
•Electrical – nearly all plastics good electrical insulators.
•Chemical – most plastics resistant to deterioration by most
chemicals but be careful!
•Thermal conductivity – low thermal conductivity therefore
good heat insulating materials.
MET 470 w/ help from B. Young
Molecular Weight –Plastics
KEY POINTS:
I. Molecular Weight
•Understand what molecular weight means when
dealing with polymers
•Understand the effect of molecular weight on
material properties
•Understand entanglement
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Molecular Weight
When we talk about molecular weight in terms of polymers, we are
really talking about the length of the individual chains.
The polymerization process is subject to variation so there is no
single chain length, there is actually a wide range of lengths, so
when we discuss molecular weight, we
really mean the
average molecular weight of the material. This average is found by
measuring samples of the material as it is produced.
MET 470 w/ help from B. Young
I. Molecular Weight
Plastics
Molecular Weight
There are two different categories of molecular weight average that
are commonly used:
The first is the Number Average Molecular Weight (
The second is the Weight Average Molecular Weight (
)
)
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
Increasing the molecular weight of the material increases many of the
properties of the material by increasing the entanglement of the
molecules.
A higher molecular weight:
•Increases the chemical resistance - to a point
– It takes more damage to the main chains of the molecules before
it will affect the strength of the material
– The big loophole to this is if you have a chemical
that is very similar to the chemical makeup of the
main chain, it will dissolve it much more easily
»Like Dissolves Like
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
A higher molecular weight:
•Increases how far the material can stretch before
rupturing (ductility)
– The higher degree of entanglement allows the material
to be pulled further before the chains break
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
A higher molecular weight:
– Increases ductility: A candle and Polyethylene (PE) have basically the
same molecular structure. The chain length of the candle is just much
shorter than that of the PE. If you bend a bar of PE in half – it will
bend, if you bend a candle in half, it will fracture.
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
A higher molecular weight:
•Increases the impact resistance of the material
– The higher degree of entanglement means that in order
to rupture, more polymer bonds need to be broken, this
means that the polymer can absorb more energy before
failing.
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
A higher molecular weight:
•Increases the weather resistance of the material
– Same type of reasoning behind the increase in chemical
resistance, the chains are longer, so they can withstand
more damage before the mechanical properties will start
to diminish
MET 470 w/ help from B. Young
Properties
Plastics
I. Molecular Weight
A higher molecular weight:
•Increases the viscosity of the material – makes it harder to process the
material using conventional methods
– The longer the chains, the harder it is to get them to flow
» More tangled
MET 470 w/ help from B. Young
Plastics
I. Molecular Weight
Properties
Processors want materials that will flow easily in order to form complex
geometries, but that can affect the properties of material used to create the
product.
Many times it turns out to be a trade-off between the required properties and
processability of the material.
CD’s and DVD’s are made from the same material as most safety glasses,
Polycarbonate.
Safety glasses require a higher molecular weight in order to provide the
necessary property of impact resistance.
CD’s and DVD’s require a lower molecular weight material in order to fill out
the thin walls. CD’s and DVD’s can shatter, safety glasses don’t.
MET 470 w/ help from B. Young
Plastics
Properties
I. Molecular Weight
MET 470 w/ help from B. Young
Plastics
II. Polymerization
Addition Reactions
In addition reactions, the double or triple bonds between the atoms of the molecule are broken
and the chain grows longer when another molecule that has also had its bonds broken
links together with it.
In Polyethylene, the double carbon bond in the ethylene molecule separates and links with
another carbon bond from another ethylene molecule.
MET 470 w/ help from B. Young
Plastics
II. Polymerization
Condensation Reactions
In condensation reactions, a portion of the ‘mer’ molecule reacts with another
‘mer’ molecule to form a new bond and gives off water, carbon dioxide, or
possibly an acid.
The portion of the ‘mer’ that reacts is known as the functional group.
Condensation reactions usually take longer than addition reactions
• In addition reactions any chain end will react with any other chain end and
the molecules grow at different rates depending on what size chains
combine.
• In condensation reactions the chains typically grow at the same rate as the
chemicals that make up the polymer chain are consumed, the reaction rate
slows down.