Thermoplastic Resins Composition and Selection
Thermoplastic Resins Composition and Selection
The market share for thermoplastic resins is expanding within a growing
composites market. For example, the automotive market is seeking new solutions for
lighter vehicles and high volume production. Some applications of thermoplastic
composites in the automotive industry include:
- Seat frames
- Battery trays
- Bumper beams
- Load floors
- Front ends
- Valve covers
- Rocker panels, and
- Under engine covers
In many cases, thermoset resins, such as epoxies, are not satisfactory for these
applications and, thus, there is an increased interest in the use of thermoplastic
materials.
Advantages of Thermoplastics over Thermosets in Composites
Advantages of Thermoplastics over Thermosets in Composites
Compared to thermoset resins, there are two major advantages of thermoplastics
in composites. The first is that many thermoplastic resins have an increased impact
resistance
compared to comparable thermoset materials. In some instances, the
difference is as high as 10 times the impact resistance.
The other major advantage of thermoplastic composites is the ability to reform.
When heat and pressure impregnate a reinforcing fiber with a thermoplastic resin, a
physical rather than a chemical change occurs. This allows the thermoplastic
composites to be reformed and reshaped. Thus, the thermoplastic composite can be
recycled at the end of its life.
Need to Select Right Polymers for Thermoplastics Composites
Need to Select Right Polymers for Thermoplastics Composites
One of the key factors that affects the performance of a thermoplastic composite
is the polymer that is used as the basis for the composite material. There are many
factors that need to be considered when selecting a matrix polymer for use in
thermoplastic composites.
Making the selection of the appropriate matrix material is complex, because, in general, the choices do not excel in every category.
For example, one
material might be inexpensive and have good corrosion resistance but has a very low
glass transition temperature and is susceptible to creep. Another material might offer a
very high glass transition temperature but be expensive and have relatively poor
hydrolytic stability or solvent resistance.
Thermoplastics that have high mechanical
properties and that can operate in more varied and aggressive environments tend to
cost more. It is necessary to select the best candidate for the specific needs of the
particular demanding application.
Thermoplastics Resins Used in Composites
Thermoplastics Resins Used in Composites
Thermoplastic are commonly used in composites but are not limited to the following:
Each of these resins provide certain advantages and disadvantages to the resulting final
thermoplastic composite. In that regard, the final
application is extremely important in defining the resin requirement for the matrix
polymer.
Before discussing the features of these type of resins, it needs to be pointed out
that the thermoplastic polymers that are used in thermoplastic composites can be
divided into two general classes:
The classification is based mainly on the maximum service temperature
of the polymers, which in turn is based on the glass transition temperature or Tg. This
is the temperature at which the amorphous portion of the polymer changes from a
glassy to a rubbery phase on heating. Semi-crystalline
thermoplastic polymers may
carry a load above the Tg, as only the amorphous phase of the polymer has become
rubbery. The crystalline portion of the polymer remains solid until the melt temperature,
or Tm, is reached.
The following Table summarizes the glass transition temperatures, Tg, melt
temperatures, and processing temperatures, process of the thermoplastic polymers
that are commonly used in thermoplastic composites.
| Polymer |
Morphology |
Tg (°C) |
Tm (°C) |
Process (°C) |
Cost
(Relative)
|
| PBT |
Semi-crystalline |
56 |
223 |
250 |
$$ |
| PA6 |
Semi-crystalline |
48 |
219 |
245 |
$ |
| PA12 |
Semi-crystalline |
52 |
176 |
224 |
$ |
| PP |
Semi-crystalline |
-20 |
176 |
190 |
$ |
| PEEK |
Semi-crystalline |
143 |
343 |
390 |
$$$ |
| PEI |
Amorphous |
217 |
-- |
330 |
$$ |
| PPS |
Semi-crystalline |
89 |
307 |
325 |
$ |
| PEKK |
Semi-crystalline |
156 |
306 |
340 |
$$ |
Common Thermoplastics Used in Composites
Based upon the definitions previously provided, PEEK, PEI and PEKK can be
classified as high temperature thermoplastics while the other polymers can be
categorized as engineering thermoplastics. In general, as can be seen from the
information in the Table above, high temperature thermoplastics tend to have a higher
relative cost than do the engineering thermoplastics. This is an important consideration
for the selection of resins in certain applications.
Factors Influencing Thermoplastic Resin Selection
Factors Influencing Thermoplastic Resin Selection
In general, there are three categories that are regarded as
the most important criteria:
- Cost
- Operating environment and the associated environmental
resistance of the matrix, and
- Mechanical properties
Factors Influencing Thermoplastic Resin Selection
For the operating
environment, concerns are generally items like operating temperature and exposure to
sun, water and chemicals. For mechanical properties, items like strength, stiffness, and
toughness are important features. It does need to be noted that the mechanical
properties of the thermoplastic composite are influenced not only by the choice of the
matrix polymer but also by the reinforcing fiber and the interface between the matrix and
the reinforcement.
The use of such
resins is a requirement to make thermoplastic composites economically feasible in such
applications.
Cost
For high volume applications, such as automotive uses, the cost of the
thermoplastic resin is a key consideration. Thus, resins that have a low relative cost,
such as polypropylene and polyamide are often the resin choice for these type of
applications. However, the environmental considerations are also an important factor in
making the decision as to which of the engineering thermoplastics should be used in a
particular high volume application.
For example, polyamides are sensitive to moisture
in the environment and can undergo degradation and a subsequent loss in mechanical
properties if there is a large quantity of water present. Thus, if the use conditions
present a situation in which moisture is an issue, polyamides would not be a good
choice for the thermoplastic polymer to utilize. Instead, in that case, polypropylene,
which is not sensitive to the presence of water, would be a better choice for the polymer
in the thermoplastic composite.
Summarizing the information provided in this guide, then, the major driving force
for the selection of a particular thermoplastic resin in thermoplastic composites is the
desired application for which the final composite will be utilized.
Operating Environment
Even within the class of high volume applications, the operating
environment for the thermoplastic composite is a significant factor in the choice of the
the appropriate matrix resin. Thus, if moisture or solvent resistance is required in the
desired application, the resin features in that regard must be considered. For example,
polyamide resins are sensitive to moisture in the operating environment and the
performance of such resins will be dependent on the surrounding conditions. For high volume applications that require a resin that is not sensitive to moisture effects,
polyolefins, such as polypropylene and polyethylene are reasonable choices.
Related Read: Advancing polyolefin composites for new applications »
Temperature
Another factor that will determine the choice of the correct thermoplastic resin to
use in the thermoplastic composite is the intended use temperature for the finished part.
Thus, while resins such as polypropylene and poly(butylene terephthalate) offer low
cost, their temperature performance is quite low. Therefore, they would not be the resin
of the choice for applications that involve temperatures above about 100°C.
On the
other hand, the high temperature thermoplastics, such as PEEK and PEI, can be used
for applications that require high temperature performance. The potential negative of
the use of such resins, however, is their relatively high cost compared to resins such as
polypropylene and polyamides. The high cost of these resins will limit their use to more
specialty type
applications such as aerospace uses.
As has
been shown in this guide, different resins are useful over different temperatures. The
selection of the appropriate resin for the desired use temperature will help guarantee
the success of the product in its application.
The following Table summarizes some of the key current composite applications
for several of the resins:
Composite Applications for Various Resins
The information contained in this Table shows that the lower cost resins, such as PP,
are utilized in high volume automotive applications while the higher cost, high
temperature thermoplastics find specialty applications in the aerospace and medical applications.
As additional applications are developed for thermoplastic composites, it will
continue to be necessary to define the correct resin for utilization as the matrix material.
The rules that have been provided here can be used as the basis for the
selection criteria. In that way, there will be a greater likelihood of the successful
implementation of the composite material in its desired utilization.
Reach Faster Higher Heat Resistance with your Thermoplastic Composite Materials
Talk to Mark DeMeuse where he will share how to reach higher heat resistance with your thermoplastic composite materials by learning to improve your filler/polymer interface properties. He will also help you tailor surface properties (fiber sizing, use of coupling agents, reactive functionalization…) for high-temperature applications (automotive, aircraft, electronics…) thereby improving HDT, strength & fracture toughness.
