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Industrial is a supplier of High
Performance Plastic Shapes, Self-Lubricating
Plane Bearings, composites and custom
machined
components. |
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| Disclaimer: The
following on-line seminar is presented in good
faith; the information supplied herein is
gathered from publicly available sources as
well as practical field experience. Poly-Tech
Industrial is not responsible for accuracy or
completeness. |
Introduction
"High
performance" is a relative term often used to
describe materials and/or attributes of
materials. Often over-used and misapplied by
zealous marketing types, consumers are often
misled by the term and usually are given no
point of reference from which to begin material
comparisons. So let's start at the beginning
and define High Performance as it pertains to
plastics.
|
Polymers
|
The two main
sub-Groups are: Thermosets /
Thermoplastics
|
To better
understand the family tree of plastics; it is
helpful to review their history. (See "The History
of Plastics") However, for this discussion, and as
a point of reference, let's divide the entire
family of polymers into two main groups;
"Thermosets" and "Thermoplastics". (We'll further
divide them into sub groups
later.)
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"Thermosets are like
eggs"
|
Thermosets are those materials
which when polymerized take an irreversible set.
You can liken thermosets to concrete (or an egg if
you prefer) where once subjected to a catalyst
and/or elevated temperature undertake changes as
the molecular level. After this change, they cannot
be returned to their original raw state. Some
examples of thermosets would be materials like
epoxies, the composites family of Poly-Texx HPV,
and Canvas or Linen phenolics, (better known as
Bakelite and Micarta). Because they cannot be
easily reformed or melted back down, these
materials do not lend themselves easily to
recycling, scraps and off-cuts are usually
discarded to land fills.
|
"Thermoplastics are like
ice"
|
Thermoplastics are those materials that even
after an initial polymerization can be
reconstituted to their original state usually
through heat and/or a chopping process. At that
point they can be reprocessed and reused (Commonly
known as recycling). Thermoplastics can be thought
of like ice, they can be melted, down and
re-polymerized so that they can be used again.
Examples of thermoplastics are PETG (2-liter soda
bottles), polyethylene and polypropylene
(Squeezable ketchup bottles). It should be noted
however that once a plastic has been recycled, some
of its original properties are diminished. This is
primarily due to degradation caused by subsequent
heating cycles, as well as the introduction of
debris.
|
"Thermoplastics
Sub-Groups"
|
Though it
could be argued that there are some thermosets that
could be described as high performance, we will
concentrate on the thermoplastic side of the field.
For this discussion, it is convenient to divide
thermoplastics into three main sub-groups:
standard, engineering, and high performance
plastics. We will spend our time today on the last
group, but let's first define the standard and
engineering materials.
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"Standard Plastics"
|
| Standard
plastics are generally those materials that have a
maximum operating temperature below 180 degrees F.
Low electrical properties, and/or chemical
resistance. They are usually low in cost and easily
available from a large number of producers. These
materials are by far the most widely used on a
volume-only basis. Essentially, they are useful due
to one or two main attributes that specifically
match the needs of an application. As a result many
standard plastics find their way into consumer
products. |
| |
- ABS
(Computer
housings)
- NYLON
(Power tool
casings)
- ACRYLIC
(Point of purchase
displays)
- POLYSTYRENE
(Credit
cards)
- POLYETHYLENE
(Milk
containers)
- POLYPROPYLENE
(Food
containers)
- POLYVINYLCHLORIDE
(PVC / Water
pipe)
|
|
| From an
engineering standpoint, the most useful member of
this group may be Ultra High Molecular Weight
Polyethylene (UHMW-PE). A member of the polyolefin
family, It is a highly abrasion resistant, tough,
low cost plastic. Commonly used for bearing and
wear components such as chain guides, rollers and
pulleys. |
"Engineering Plastics"
|
Engineering
plastics form the largest group of plastics in
regard to the number of materials from which to
choose. They generally handle temperatures up to
350 oF. The physical properties are good while
chemical and electrical properties vary. The costs
are moderate to high.
|
| |
- ACETAL(HOMOPOLYMER)
-
Delrin
- ACETAL(COPOLYMER)
- Ensital, Tecaform,
Celcon
- FILLED
NYLONS - Vekton,
Nylatron
- PHENOLICS -
Micarta,
Bakelite
- PTFE -
Teflon, Halon,
Hostaflon
- PVDF -
Ensikem, Kynar &
Chemfluor
- ETFE -
Tefzel,
Hostaflon
- POLYETHYLENE
TEREPHTHALATE -
Ensitep BT,
(PET)
- POLYBUTYLENE
TEREPHTHATE - Hydex
4101(PBT)
- PPO -
Noryl
- POLYCARBONATE
- Ensicar, Lexan,
Tuffack
|
|
Acetal
|
The key to
choosing the right engineering grade plastic for an
application is to first understand the application
as completely as you can. Everything from loading,
stresses, wear properties, etc. Then the trick is
to match the materials capabilities to the
application, thereby minimizing the shortcomings of
the plastic. For example, the Acetal family
(Delrin, Celcon, and Tecaform) is a crystalline
material with good physical properties, good
chemical resistance and is easily machined. At one
time, acetal was only available in a homopolymer
(Single resin system). Unfortunately homopolymer
acetal extruded shapes such as sheet and rod
exhibit a porosity line created when bubbles form
and converge during cooling. This line is actually
an area, which is more porous than the rest of the
cross-section. Though homopolymer acetal is still
commonly used, resin suppliers have since corrected
this problem with the introduction of the copolymer
version (Multi resin system), which minimizes
porosity, and eliminates the centerline porosity
and discoloration.
|
Nylon
|
The Nylon
family is also highly useful for many applications
including pulleys, bearings, bearing and wear
components. It is strong, tough, resistant to
abrasion, and has a low coefficient of friction.
Nylon's weak point is moisture absorption. Some
types of Nylon can absorb up to 8% moisture at
saturation. The result is not only swelling, but at
saturation, water acts to lower its glass
transition temperature to 120 degrees F. This means
that fully saturated Nylon will tend to soften as
temperatures rise over 100 oF.
Engineering plastics are used for many close
tolerance machined parts. They are available in
similar materials that are injection moldable, and
therefore used as prototypes before injection
molding is considered. Thus, engineering plastics,
while among the most useful and common, each have
attributes and detrimental qualities that require
that we understand the application as completely as
possible before deciding upon a
material. |
"High Performance
Plastics"
|
Now that we have defined
"Standard" and Engineering" plastics and
understand that these classes of materials
are limited by a particular attribute like
temperature, chemical resistance or moisture
absorption. Then, by definition, "High
Performance Plastics" are those materials
that maintain their physical properties under
thermal, chemical or electrical stress. These
materials are relatively high in cost, and in
some cases only available from a single
source. The unique characteristics of each of
these often allow them to solve problems not
possible with other materials and therefore
have earned the right to be called "High
Performance".
For this discussion we will review nine materials.
We will subdivide them into three groups;
amorphous, crystalline, and imidized. |
