Thermo101 Alphabet
A TECHNICAL ARTICLE 2004 VOLUME 23, #2
Alphabet Soup?
It seems that plastics people never
tire of their alphabet soup ABS,
PTFE, PVC, PUR, and on and on. The
soup continues when we consider
evaluating the characteristics of plastics.
In this short series, we consider a
few of the letters in this soup.
Tg, Tm and DSC
Weve already discussed the first
two. Tg is glass transition temperature,
or the temperature above which
polymers become rubbery rather than
glassy. Tgs for polystyrene and acrylic
are around 210°F (100°C). Tg for
rigid PVC is around 185°F (85°C) but
can be as low as -25°F (-30°C) when
highly plasticized. The glass transition
temperature for nylon 6 is only
122°F (50°C). The Tg for polyethylene
is around -125°F (-90°C) and that for
homopolymer PP is 15°F (-10°C).
Youll recall that Tm is the melting
temperature for crystalline polymers
such as polyethylene, polypropylene
and nylon. The melting temperature
for HDPE is around 275°F (135°C). The
melting temperature for homopolymers
PP is 330°F (165°C), and that for
nylon 6 is 430°F (220°C).
One popular method for measuring
Tg and Tm is with DSC. So, what is
DSC? Differential scanning calorimetry.
Consider heating a substance from
room temperature, say, to a specific
processing temperature. Lets use water
as an example. It takes exactly one
calorie of energy to heat one gram of
water one degree Centigrade. In British
units, it takes one British Thermal
Unit of energy to heat one pound of
water one degree Fahrenheit. This rule
works until water reaches its boiling
point of 212°F (100°C). At the boiling
point, the temperature remains constant
even though substantial energy
is inputted to the water. If we were
to compare the energy uptake of a
substance that did not boil with that
of water, we would see that the temperature
of the non-boiling substance
would continue to climb while that of
water would remain constant until all
the water had evaporated.
Conversely, if we cool water from
room temperature to 32°F (0°C), it
freezes. At the freezing point, the
temperature remains constant even
though substantial energy is removed
from the water.
Physical changes that take up
energy with little or no temperature
change, such as boiling or melting, are
called endothermic changes. Physical
changes that give off energy with
little or no temperature change, such
as freezing or crystallizing, are called
exothermic changes.
We can build a device that compensates
for these temperature differentials.
The device uses a well-characterized
substance as its reference. The
substance to be tested is then heated
at the same rate as the reference substance.
This is done by carefully controlling
the energy ratio between the
reference substance and the test substance.
Since the device is measuring
calories or units of energy, it is called a
calorimeter. Since the device measures
the temperature difference between
two substances as they heat, it is a
scanning device. And since the device
is looking at the difference between
the two substances, it is a differential
device. It we put this all together we
see that the device is a differential
scanning calorimeter, or DSC!
What Can We Learn From
DSC?
First, we must realize that the DSC
can be used either in a heating mode or
in a cooling mode. Samples are usually
heated beginning at room temperature
and they are usually heated at a fixed
temperature rate such as 10°C/minute.
The temperature range and energy
requirements of transitions are the
primary information gathered from
heating DSCs. The most common
transitions are the glass transition
temperature and the melting temperature,
if any.
Between transitions, the DSC provides
relative energy uptake by the
test substance. This is directly related
to that for water, as specific heat or
heat capacity. As we saw above, the
amount of energy absorbed by water
is 1 cal/gm°C or 1 Btu/lb°F. So its heat
capacity is 1.0. It takes 100 cal/gm°C or
180 Btu/lb°F to heat water from 32°F
to 212°F. We find that polystyrene has
about 55% of the heat capacity of water
and that for PVC has about 37% of that
of water. PP has about 85% of the heat
capacity of water and that for LDPE
is about the same as that of water.
Remember, now that these values are
between transitions.
DSC is important when trying to
determine the extent of crystallization
of a polymer. Consider the case for a
100% crystalline polymer that requires
100 cal/g to melt. If that polymer is
cooled from the melt and DSC determines
that only 50 cal/g was liberated
during recrystallization, it is safe to say
that the polymer at room temperature
is only 50% crystalline.
In the last lesson, we learned that
coPP melts around 155°C but recrystallizes
at around 100°C. How did we
know that? From DSC, of course.
The DSC can teach us another aspect
to polymer characterization. As
we increase the cooling rate for some
crystalline polymers, we retard the
temperature at which recrystallization
begins. And we reduce the final
level of crystallinity. How do we know
this? Consider PET. It has a melting
temperature of 510°F (265°C). If we
cool PET very slowly, we find that it
recrystallizes at around 250°C to about
40-45%. If we cool PET very rapidly,
we find that there is no recrystallization
region. PET remains amorphous
at room temperature and beyond. DSC
is therefore a tool for determining how
rapidly a plastic crystallizes. ¦
Keywords: glass transition, melting,
recrystallization, calorimeter,
endothermic, exothermic
THERMOFORMING
101
A TECHNICAL ARTICLE 2004 VOLUME 23, #3
ABCs of Alphabet Soup?
In our last lesson, we learned about
Tg, Tm, and DSC. These are important
letters in our alphabet soup. In this
lesson, we look at some new letters.
IR and FTIR
IR means infrared and FTIR means
Fourier-transform infrared. We usually
heat our plastic sheet with radiant
heaters. These heaters emit infrared
energy or IR. Infrared energy is part
of the electromagnetic spectrum of energy.
The spectrum is usually defined
in terms of the length of the emitting
rays. And the length is usually given
in microns. The length is identified
by the symbol .m. Radio waves are
long-length waves, in the range of 107
to 1010 .m. They reside near one end of
the electromagnetic spectrum. Gamma
rays are short-length waves, in the
range of 10-4 to 10-7 .m. They reside
near the other end of the electromagnetic
spectrum.
In contrast, visible light has the
wavelength range of 0.4 to 0.7 .m. It
is about in the middle of the electromagnetic
spectrum. Energy of shorter
wavelengths, between 0.4 .m and
about 10-2 .m, is ultraviolet energy. Energy
of longer wavelengths, between
0.7 .m and about 103 .m, is called
infrared energy. The infrared energy
wavelength range is usually separated
into near infrared energy, having wavelengths
between 0.7 .m and about 2.5
.m, and far infrared energy, having
wavelengths between about 2.5 .m
and 103 .m.
Our thermoforming radiant heaters
typically operate in the wavelength
range of about 3.5 .m to about 20 .m or
so. Or in the far infrared energy wavelength
range. As you might expect, as
the temperature of the radiating body
increases, the peak wavelength shifts
to short and shorter wavelengths. And
as the radiating body temperature increases,
the amount of energy emitted
increases as well.
For example, if your heater temperature
is 400°F (~200°C), the peak wavelength
is 6.06 .m, and the maximum
amount of energy emitted at this wavelength
is 2.94 units1. If you raise the
heater temperature to 600°F (~315°C),
the peak wavelength is 4.92 .m, and
the maximum amount of energy emitted
is 6.78 units. If you raise the heater
temperature to, say, 900°F (~480°C), the
peak wavelength is 3.83 .m, and the
maximum amount of energy emitted
is 17.3 units.
A couple of reference points, please!
Certainly! The sun radiates at 12,000°F
(~6500°C), its peak wavelength is 0.43
.m, and the maximum amount of energy
it emits is nearly 120,000 units2!
What about us! Because radiation is
electromagnetic energy interchange,
we radiate back to the sun at 98.6°F
(37°C). Our peak wavelength is 9.35
.m, and we emit about 0.52 units.
Were pretty feeble radiators!
Okay, what about the sheet were
trying to heat? Well, as the sheet
heats, the amount of energy it radiates
increases. Suppose the heater temperature
is 600°F. If the sheet reaches 400°F,
it radiates a maximum of 2.94/6.78
= 43% of the energy it gets from the
heater back to the heater! Really! Isnt
infrared energy fun?
Okay, what is FTIR? Keep in mind
that heaters emit and sheet absorbs
far infrared energy. However, plastics
do not absorb energy uniformly.
The amount of energy absorbed at
any given wavelength depends on
the type of plastic being heated. Very
few plastics uniformly absorb radiant
energy only on their surfaces. When
the radiant energy is not entirely absorbed
in the surface layer, some of it
is transmitted into the plastic. If the
sheet is thick enough, all the radiant
energy that impinges the sheet surface
is absorbed in the polymer. Plastics
such as polyethylene are notoriously
poor at absorbing energy on the sheet
surfaces. Others, such as PVC, absorb
a substantial portion of the incoming
energy on the sheet surface.
But how can we tell whether a sheet
of plastic is absorbing most of its energy
on its surface or inside the sheet?
Thats where FTIR comes in. For the
moment, ignore the FT part of the
alphabet soup. I take a thin film of my
plastic, place it in an infrared or IR
scanner, and pass a monochromatic3
infrared beam through it. I measure the
decrease in energy transmitted through
the plastic film at that wavelength. I
change the wavelength of the infrared
beam and again measure the transmitted
energy. If I scan the film with an
infrared beam of wavelength range of
2 .m, say, to 20 .m, I will have covered
the majority of the wavelength range
of our heaters.
Now, I double the film thickness and
repeat the scan. This tells me how deep
the infrared energy penetrates into my
film. I continue doubling the film thickness
until none of the infrared energy
is transmitted through the film.
The IR scanner is mostly used in an
analytical polymer laboratory, where
polymer chemists determine the general
composition of the polymer and
its additive packages. The various
peaks that are generated at specific
wavelengths are directly related to the
molecular confirmation of the polymer.
For example, the carbon-hydrogen
bond is stretched at a frequency of
about 3.5 .m. As a result, all polymers
containing C-H bonds absorb 100% of
the 3.5 .m infrared energy. PP does.
PE does. PTFE doesnt because it has
no C-H bonds.
What about the FT portion of
FTIR? The polymer chemist obtains
infrared scans of various recipes in order
to compare the ingredients. These
scans are then mathematically encoded
so that the polymer chemist (or his
computer software) can arithmetically
subtract the infrared spectrum of the
polymer, say, from the spectrum of the
recipe. Whats left must be the additive
package. By subtracting the infrared
spectrum from a known additive package,
for example, the polymer chemist
can determine the composition of any
unknown in the recipe. The mathematical
encoding is called Fourier Transformation
or FT. And weve just put the
FT back into FTIR.
Do we care what the recipe of our
plastic is? Not really. We just need to
know how much infrared energy our
plastic absorbs. So we just piggyback
on the polymer chemists FTIR
device. ¦
Keywords: Infrared, far infrared,
electromagnetic energy, wavelength,
Fourier Transform
THERMOFORMING
101
1 The units are kW/m2 – .m.
2 Think Clearwater Beach, Florida at 1400
hours on July Fourth!
3 Monochromatic: Of or composed of radiation
of only one wavelength.
A TECHNICAL ARTICLE 2004 VOLUME 23, #4
XYZs of Alphabet Soup?
This is the third in a series on plas-
tics alphabet soup. So far, weve
tackled Tg and Tm. And we spent time
with DSC and IR and FTIR. Are there
more acronyms1 that we should know
about? Sure. Lots. In this discourse, we
look at four more HDT, DTA, DMA,
and DTMA.
What is HDT?
HDT stands for heat deflection temperature2.
It is an ASTM test (D648) and
an ISO test (75-1, 75-2)3. The test focuses
on the three-point deflection of a
plastic bar of very specific dimensions4.
The bar is placed in an oil bath. A dead
weight is placed in the center5. The oil
bath is heated at a very specific rate6.
As the plastic increases in temperature,
it softens. HDT is the temperature at
which it sags a fixed amount7. This
test, without ASTM or ISO numbers,
is more than 60 years old.
HDT is often used to sort or rank
plastics. And it is often used for
quality control. In thermoforming, it
provides a crude, early estimate of
the lower temperature for forming.
Unfortunately, it is not a very good
test. For example, it has no value for
plastics that are relatively soft at room
temperature, such as some plasticized
PVCs and TPOs. The test yields a
single data point that should not be
used to predict long-term behavior.
The test is often tricked by residual
stress in the test specimen. And it is
tricked if the polymer has very low
thermal conductivity or if the oil bath
is not vigorously stirred.
Here is another way the data can
mislead. Consider HDT values for
polycarbonate and nylon 6 for two
loads:
Plastic HDT @ HDT @
66 psi 264 psi
Polycarbonate 290°F 275°F
Nylon 6 370°F 150°F
What is the real HDT value for
nylon 6?
And one more reason to avoid HDT
values. Consider glass-reinforced polycarbonate
and nylon 6 HDT values:
Plastic Unrein- Reinforced
forced @ HDT @
264 psi 264 psi
Polycarbonate 275°F 295°F
Nylon 150°F 420°F
Again, what is the HDT of nylon 6?
Nuff, said!
DTA, DMA, and DTMA.
Are These the Same?
First, DTA. DTA stands for differential
thermal analysis. Remember our
discussion on DSC, differential scanning
calorimetry? Well, DTA uses the
same equipment and the same analysis
as that for DSC. The difference is that
the data are interpreted differently for
DTA. For example, DTA yields specific
heat, or the amount of heat absorbed
by the plastic as a function of temperature.
Time-dependent changes such
as rate of crystallization are obtained
by running the DSC/DTA at different
heating (or cooling) rates.
DMA is the acronym for differential
mechanical analysis and DTMA is the
acronym for differential thermal mechanical
analysis. The earliest device is
described in ASTM D 2236. A review of
various mechanical testing techniques
is described in ASTM D 4065. In general,
a solid plastic test bar is subjected
to torsional or flexural oscillation.
Usually, if the device is operated at
a fixed temperature and the frequency
of oscillation is varied, the test is
called DMA. If the device is operated
at a fixed oscillation frequency and
the temperature is varied, the test is
called DTMA. There is an imperfect
correlation between these two testing
procedures.
What is DMA/DTMA used for?
If the plastic is elastic, its resistance
matches that of the oscillating device.
If the plastic is fluid or viscous, its
resistance is out-of-phase with that of
the oscillating device. As the temperature
or frequency is changed, device
detects shifts from elastic to viscous
or viscous to elastic character in the
polymer. As a result, the data will yield
the glass transition temperature of the
polymer. More importantly, however,
the test will show plastic resistance to
applied load over a temperature range
up to its melting temperature. This is
important for thermoformers, since
we are stretching the plastic while it
is primarily in its rubbery solid state.
For example, we can quickly assess
the forming temperature range of the
plastic. And within a given polymer
family, we can determine which polymer
recipe yields the broadest forming
window. DMA/DTMA will probably
be the subject for a TF 101 lesson.
DMA/DTMA data obviate single-
point values such as those obtained
with HDT/DTUL devices. ¦
Keywords: mechanical analysis,
heat deflection, heat distortion,
oscillation frequency
THERMOFORMING
101
1 Acronym: a word formed from the initial
letters of a multi-word name.
2 HDT was originally called heat distortion
temperature.
3 Another acronym used for the ASTM test
is DTUL, meaning deflection temperature
under load.
4 5 inches long by 1/2-inch thick by a width
not to exceed 1/2-inch. ISO specifications
are similar but in metric units.
5 The weight is equivalent to either 66 or
264 lb/in2 fiber stress. ISO specifications
are similar but in metric units.
6 2 +/- 0.2°C/min.
7 0.010 inches. ISO specifications are
similar but in metric units.