TGA Fiber Content

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TGA Fiber Content
TA UserCom
Mettler-Toledo GmbH Anlytical
Determination of the fiber content of composite
materials by thermogravimetry
Dr. B. Benzler, Applications laboratory, METTLER TOLEDO, Giessen, Germany
Plastics are frequently filled or reinforced
with very different types of material in order to improve their mechanical and thermal properties. Organic fillers and reinforcing materials (e.g. wood flour) increase the toughness of a plastic. The addition of fibers can result in a major increase in stiffness and structural strength.
Besides natural organic fibers such as jute
and sisal, synthetic inorganic fibers (e.g.
glass and carbon fibers) and organic fibers
such as aramid are widely used for reinforcement purposes. Aramid fibers consist
of poly-p-phenylene terephthalamide and
are remarkable because of their high tensile strength and their relatively high decomposition temperature of about 550 °C.
Fiberglass-reinforced thermoplastics are
being increasingly used for technical products: they can be processed by injection
molding or extrusion and exhibit excellent
mechanical properties. This allows them to
be used in very diverse fields (car manufacturing, precision machines, electrical
engineering etc.).
Some examples of products reinforced with
aramid fibers are high pressure flexible
hoses, belts and bulletproof jackets.
The quality assurance of such composite
materials consists primarily of checking
the desired fiber content. This is very easily
done using thermogravimetry, as will be
shown in the following two examples.
Thermogravimetry (TG) or
thermogravimetric analysis (TGA) measures the mass of a sample that is heated
at a constant rate (usually linear) in a
controlled atmosphere. The examples de-
Mettler-Toledo GmbH
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scribed here deal with the loss of mass resulting from the pyrolytic decomposition
of the plastics under nitrogen.
Example 1: Fiberglass-reinforced
polyamide PA 6
Samples of about 11 mg each of PA 6
(without fiberglass) and fiberglass-reinforced PA 6 were examined by TGA under
the following experimental conditions:
• Instrument: TGA/SDTA 851e STARe System thermobalance
• Heating rate: 10 K/min
• Temperature
25 °C to 800 °C
• Atmosphere: Nitrogen, 50 ml/min.
The resulting measurement curves and
their evaluations are shown in Figure 1.
The unfilled PA 6 sample loses about 2.5%
of its mass at about 200 °C. This effect is
due to the loss of moisture and is well
known for polyamides. Between 400 °C
and 500 °C the sample decomposes almost
completely, i.e. it undergoes almost 100%
As expected, the fiberglass-reinforced
sample behaves similarly except that the
effects are reduced in proportion to the
filler content. The residue of 24.5% that
remains can be assigned to the fiberglass
content of the sample.
A quantitative content determination is
therefore a simple matter because the filler
(fiberglass) does not decompose under the
experimental conditions used, but remains
behind as a residue while the plastic undergoes complete degradation.
Fig. 1. Thermogravimetric analysis of Polyamide 6 with and without glass fibers
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Fig. 2. TGA of an epoxy resin aramid fiber composite (middle): above: pure aramid fibers, below: pure
Example 2: Aramid fiber-reinforced
synthetic resin
The experimental conditions for the
thermogravimetric analysis of the individual components, i.e. the resin and aramid fibers, and the composite material
were similar to those used in Example 1:
DB in the last column means, “dry basis”,
i.e. with respect to the dry content at
300 °C.
The resin content of the composite dry
substance is therefore:
• Instrument: TGA/SDTA 851e STARe System thermobalance
• Heating rate: 10 K/min
• Temperature
25 °C to 800 °C
• Atmosphere: Nitrogen, 200 ml/min.
Recalculated with the actual moisture
content of the composite:
Moisture: 2.1%, resin: 49.8% and as the
difference to 100 %, aramid fiber: 48.1%.
The aramid content could also be calculated from the step between 520 °C and
640 °C, or from the residue at 640 °C. The
result is however not so accurate; the pyrolysis of the aramid is probably different
in the presence of the epoxy resin.
Aramid undergoes decomposition in contrast to the glass fiber used in Example 1.
The TGA curves in Figure 2 can be interpreted as follows:
The pure resin first loses moisture and
then pyrolyzes in a single step process between 300 °C and 520 °C.
The aramid fibers initially lose about 3.1%
moisture. Between 300 °C and 520 °C the
weight loss is only 1.3%. Decomposition
takes place above 520 °C.
The composite material first suffers a loss
of about 2% due to moisture and then, as
expected, decomposes in two steps.
Aramid fiber
Pure resin
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These examples show that
thermogravimetry is an excellent method
for the rapid and accurate determination
of the resin and fiber content in composite
materials. The decomposition temperatures provide additional qualitative information concerning the identity of the
Thermogravimetric steps in %
RT...300 °C
300...520 °C
300...520 °C, DB
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