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Predicting Polyamide Powder Performance
In combination with FTIR analyses of the photochemical
degradation, specific accelerated UV weathering tests are
able to simulate natural outdoor ageing of powder coatings
based on aliphatic polyamide 11, and provide a good
understanding of the complex phenomena occurring in the
coating.
Serge Gaumet, Patrick Delprat, Marc Audenaert.
Thermoplastic powders based on semi-crystalline aliphatic
polyamide 11 (known under the trade name "Rilsan") are
used to coat, protect and provide several high performance
properties to metallic surfaces. These coatings show an
unusual combination of high performance properties. These
include low friction coefficient, good abrasion resistance,
outstanding cavitation resistance, high impact strength, high
hardness, good flexibility, good electrical insulation, good
chemical resistance and good thermal insulation. All these
properties are retained over a wide range of working
temperatures.
The powder coatings may easily be processed by ordinary
workshop processes, allowing them to be used in a wide
variety of engineering applications. It is often critical for the
engineer, architect or designer to construct with low cost
materials, such as steel, and use the coating to impart
"Nylon" properties to their designs, whilst retaining the
structural rigidity, choice of shapes and low cost that these
systems allow.
Pigmented polyamide 11 fine powders are used in the
automotive industry, where their outstanding chemical
resistance and abrasion resistance meets the very high
specifications of specific applications (coating of sliding door
rails, break tubes, spline shafts...). Many fine powder grades
have also got food contact approval. Therefore, they are
used in the drinking water industry to protect steel from
corrosion, abrasion and cavitation. They are used to coat
dishwasher baskets whenever high performance is
recommended, i.e. when the coating has to protect the
metallic basket during the entire machine lifetime. Other
specific outdoor coating applications require outstanding
weathering resistance, e.g. shopping trolleys, outdoor
furniture and stadium seats.
Durability studies
These latter applications have benefited from a good
understanding of the durability of the coatings and the
photochemical mechanisms responsible for degradation of
their mechanical properties. However, performing relevant
accelerated weathering tests is even more important for
such high performance coatings because their lifetime is
considerably longer.
Degradation criteria used so far to estimate coatings
evolution have been mainly based on changes in aspect.
These criteria are not very informative, sometimes
ambiguous and moreover not transferable from laboratory to
natural outdoor exposure conditions. For pigmented
coatings, changes in aspect can be attributed to two
different natural phenomena:
- Direct degradation of the binder by UV irradiation, which
affects both pigmented and clear coats (photochemical
degradation).
- Degradation of the binder by radicals generated through
the photo-activity of pigments and additives.
These two phenomena may occur together and even
interact. For instance, pigments partially absorb UV light
leading to a decrease of the photochemical degradation.
However, although the two mechanisms can be coupled,
each phenomenon can lead to quite different chemical
changes. It is possible to describe these changes as follows:
Oxidative changes
Most of the time, the macromolecular chains undergo
oxidation. This yields scissions and cross-linking, causing
micro cracks and deterioration of mechanical properties.
The products of such changes generally accumulate in the
matrix as inert products.
Yellowing
Yellowing generally appears when new chemical groups are
formed during degradation that are able to absorb visible
light. Such products alone, however, which are generally
unsaturated, cannot be used for an accurate measurement
of the process of polymer matrix photo oxidation and the
evolution of physical properties. The formation of such
products can only indicate which type of chemical group is
being degraded. Measurement of the colour change in a
coating is therefore quite difficult to interpret.
Another aspect of the durability of a high performance
protecting coating is the metal corrosion protection.
However, corrosion protection will not be developed in this
paper as salt spray tests and other specific tests are
available to address the anticorrosion durability issue.
Oxidative changes and yellowing depend to a great extent
on experimental conditions. It is consequently very difficult
to simulate natural outdoor conditions in accelerated
weathering tests in a reasonable time frame. Specific
accelerated tests have been developed, however, for a good
prediction of the durability of a given coating, accelerated
weathering tests are needed, which are able to simulate
natural ageing, and also provide a good understanding of
the complex phenomena occurring in the coating. The
following study shows that, if these conditions are fulfilled,
laboratory ageing can yield very relevant results.
Spectroscopy for a molecular approach towards ageing
The concentration of chemical species that form on a
macromolecular chain during degradation is obviously low. It
is generally accepted that the oxidation of only 1% of the
monomer units is more than enough to lead to a complete
change of most of the polymer matrix's initial physical
properties. The concentration of oxidation products is still
much higher than that of yellowing products. However,
resonance spectroscopy using Fourier transform has
reached such a high level of sensitivity that the
concentration of the inert final products that accumulate
after oxidation can be monitored. One might then expect a
relationship between the concentration of these chemical
groups, i.e. the level of photochemical degradation, and the
change in physical properties.
Resonance techniques can also be used to detect
intermediate products, which form during the chain
transformation of the macromolecules. It is thus possible to
follow the conversion reaction leading to the final products.
Among all the possibilities of chemical changes, one can
then determine, which one - in laboratory tests and in
natural weathering - explains the physical degradation of the
coating: Typically, the degradation leads to the accumulation
in the matrix of one critical group. The concentration of this
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critical group can then be correlated with the change in
physical properties, which determine the coating's lifetime.
An acceleration factor can be calculated as the ratio
between the initial rate of formation of critical groups under
accelerated conditions and the same rate at the beginning of
the natural ageing. This factor allows to predict the coating
lifetime under natural conditions from accelerated tests. That
is to say, such an acceleration factor provides a link
between the laboratory time scale and the natural time
scale.
This value has been named the "acceleration factor for
physical properties evolution" (AFPPE).
Yellowing changes and shade evolution do not always have
the same origin (at the molecular level) as the physical
properties evolution. However, the same approach can be
used for another acceleration factor, the "acceleration factor
for discoloration" (AFD).
Comparison of accelerated ageing and outdoor ageing
The discussion will be focused first on natural polyamide 11
(PA 11), i.e. on un-pigmented clear coats.
After outdoor ageing in Florida, a significant change of the
FTIR spectra was observed compared to the spectra
obtained before ageing. This FTIR spectral change can
mainly be seen in the carbonyl absorption domain (1900 1500 cm-1) (Figure 1).
A widening of the initial amide carbonyl band was observed.
In addition, an absorption band at 1715 cm-1 and two
shoulders at 1735 and 1690 cm-1 appeared in the spectra.
This was attributed to the formation of specific carbonyl
products. The intensity of these peaks is directly related to
the degree of photo oxidation of the polyamide matrix. The
FTIR spectra of the laboratory-aged samples (Figure 2)
showed exactly the same spectral changes.
FTIR (as well as NMR and UV-visible) spectroscopy showed
that the same chemical evolution occurs under natural or
accelerated ageing. In other words, the stoichiometry of the
various degradation products is the same and therefore the
photochemical ageing mechanism is the same [3]. This
leads to the conclusion that, under well-controlled
parameters, laboratory ageing can simulate outdoor
weathering.
It has been shown that this is not true when, for example,
QUV-B is used, because the lower wavelengths used with
this technique cannot be found, at ground level, in the
outdoor exposure solar spectrum.
Critical photoproducts and physical properties
On the basis of previous studies, a complete photo oxidation
mechanism of PA 11 has been proposed [2, 4, 5, 6]. Based
on this mechanism, terminal acids were identified as the
major "critical" photoproducts, absorbing at 1715 cm-1(∪ -CH
2-COOH) in the FTIR spectrum. Thus, this photoproduct is a
good indicator of the photo oxidation level. Its formation can
be correlated with the evolution of the physical properties.
For example, the evolution of the Taber abrasion (ISO
standard 9352 measurement) as a function of the acid
concentration measured via the FTIR optical density (OD) at
1715 cm-1 is shown in Figure 3.
Acceleration Factor for Physical Properties Evolution
(AFPPE)
By knowing the degradation mechanism and using an
accelerated ageing capable of simulating outdoor aging, it is
now possible to calculate an Acceleration Factor defined as
the ratio between laboratory and outdoor ageing rates [7].
Specifically, the photo oxidation rates, OD(1715 cm-1) =
f(exposure time), for a sample exposed in natural conditions
and another in accelerated conditions can be compared.
Acceleration factors have been calculated for the dark blue
coating (Figure 4a) and for the grey coating (Figure 4b).
By combining data from Figure 3 and Figure 4, acceleration
factors for abrasion resistance can be calculated. The
results obtained for the un-pigmented and pigmented
coatings are shown in Table 1.
It is worth noticing that AFPPE appear to be dramatically
formulation dependent. Thus, for a specific chosen physical
property, such as the abrasion resistance, and when using
the conditions of this study, 1 hour of accelerated aging was
found to be equal to 18 hours of Florida exposure for the
un-pigmented coating, while, for the pigmented coatings, the
equivalent Florida exposure time can be as low as 3 hours.
Acceleration Factor for Discolouration (AFD)
The same analysis can be made for the colour shade
evolution. It is known that the discolouration mechanism is
similar for outdoor and accelerated ageing. Discolouration
versus irradiation time was plotted for natural and
accelerated ageing (Figure 5).
Just as for the previous abrasion data, the acceleration
factors for discolouration (AFD) were calculated for the
different pigmented coatings (Table 1). As the table shows,
AFD values are also formulation dependent. Moreover, they
are very different from the AFPPE factors. These differences
are due to the fact that the photo oxidation mechanism of
the polyamide 11 matrix and the mechanism leading to the
discolouration of the pigmented coatings are chemically
different.
Acceleration factors depend on formulation and
polymer type
In conclusion, in can be said that significant accelerated
ageing of powder coatings based on aliphatic polyamide 11
is possible under specific conditions.
Laboratory experiments allow a complete photo oxidation
mechanism to be determined [2, 4, 5, 6]. With this
knowledge and a suitable accelerated ageing method, it is
possible to define an Acceleration Factor (AF) as the ratio
between laboratory and outdoor ageing times. Specifically,
acceleration factors characterising the evolution of the
physical properties (AFPPE, correlated with macromolecular
chain scission and photo oxidation) and characterising the
discolouration (AFD, depending on chemical changes in the
matrix, and on the use of different pigments, have been
defined. These two acceleration factors are generally
different, because of the different underlying chemical
mechanisms.
It must be emphasized that the acceleration factors depend
on the powder formulation. Knowing this, specific UV
resistant polyamide 11 coatings can be designed. Moreover,
predicting outdoor ageing of different polymer materials, by
using laboratory accelerated ageing rates in order to
compare their performances, is only possible if the
acceleration factors of each polymer material are known.
References
[1] R.M. Dittmar, R. A. Palmer, R.O. Carter III, Applied
Spectro. Reviews, 29 (2), (1994), 171
[2] O. Vasseur, Thèse d'Université, Université Blaise Pascal,
Clermont-Ferrand II, (1999)
[3] J. Lemaire, Pure and Appl. Chem., 54, (1982), 1667
[4] D. Fromageot, J. Lemaire, D. Sallet, Euro. Polymer J.,
26, (1990), 1931
[5] A. Roger, . J. Lemaire, D. Sallet, Macromolecules, 19,
(1986), 579
[6] M. R. Krejka, K. Udipi, J. L . Middeleton,
Macromolecules, 30, (1997), 4695
[7] S. Gaumet, ACS Meeting PMSE Presentation August
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20-24, 2000, Washington DC
Experimental
Sample Preparation
Polyamide 11 (PA 11) based powder coatings were applied
on primed metallic grit blasted plates using a fluid bed
dipping method. A hot metallic article was dropped in a tank
of cold aerated powder, whereby the finely divided plastic
particles melt in contact with the surface of the hot metal.
With this process, the coating thickness was usually in the
order of 200 - 400 µm.
The powders used in this work were:
- Natural powders (without pigments, fillers and additives),
yielding clear coats.
- Grey powders (containing pigments and additives).
- Dark Blue powders (containing pigments and additives).
Pigments were different from those used in the formulation
of the Grey powder.
- Light Blue powders (containing pigments and additives).
Pigments were different from those used in the formulation
of the Grey and Dark Blue powders.
Exposure conditions
Natural aging
Samples were exposed to natural conditions in Florida
(South Florida Testing Service condition, 24 to 36 months)
and in Normandy (France, 5 years).
ageing.
- Terminal acids were identified as the major "critical"
photoproducts, absorbing at 1715 cm-1 (∪ -CH2-COOH) in
the FTIR spectrum. The formation of this final photoproduct
can be correlated with the evolution of the physical
properties.
- By knowing the degradation mechanism and using an
accelerated ageing capable of simulating outdoor aging, it is
possible to define an Acceleration Factor defined as the
ratio between laboratory and outdoor ageing times.
- The acceleration factors for physical properties evolution
(AFPPE) and for discolouration (AFD) appear to be
dramatically formulation dependent and they also differ very
much from each other.
The authors:
> Dr Marc Audenaert joined Atofina in 1985, working on the
characterization of polymers at the Cerdato Research
Center. He is now manager of the Coatings Department.
> Dr Patrick Delprat joined Atofina in 1994 as head of a
laboratory involved in accelerated weathering of polymers.
In 2002, he joined the development team of Atoglas for
acrylic resins.
> Dr Serge Gaumet joined Atofina in 1998 and, until 2001,
worked on the formulation of fine powder coatings at the
Cerdato Research Center. He is now manager of the
Polymer Application Laboratory in Lyon, France.
Accelerated aging
All samples were exposed in the laboratory using a Xenotest
1200 device according to the ISO 4892 standard and
Renault D 27 1380 exposure conditions at 63 W/m2 between
300 and 400 nm.
Analytical Techniques
A variety of analytical techniques were used in the
determination of PA 11 photooxidative degradation. It was
shown that Fourier transform infrared spectroscopy (FTIR) is
a powerful tool for the analysis of the photo oxidation
products. FTIR spectroscopy in the acoustic mode (PAS)
was also used [1]. This technique is especially well suited to
analyzing organic coatings because there is no need to
remove the coatings from their metallic substrate. The
spectra in Figures 1 and 2 were obtained using a Nicolet
"Magna 860 FTIR" spectrometer.
Analysis of the compositional changes occurring in the
polymer during the photon irradiation were carried out using
various techniques, such as NMR- and UV/Visible
spectroscopy [2].
Colour changes were monitored using a "ColorQuest"
spectrometer from Hunterlab (CIELab* 76 reference).
Surface aspect modifications were examined using an
Olympus AX70 optical microscope.
Taber abrasion resistance was measured using a Taber
"Teledyne 505".
Results at a glance
- After outdoor weathering in Florida of powder coatings
based on aliphatic polyamide 11, a widening of the initial
amide carbonyl band and absorption bands at 1715 cm-1
and two shoulders at 1735 and 1690 cm-1 appeared in the
FTIR spectrum. This was attributed to the formation of
specific carbonyl products. The intensity of these peaks is
directly related to the degree of photo oxidation of the
polyamide matrix. Laboratory-aged samples showed exactly
the same spectral changes, indicating that the same
chemical evolution occurs under natural or accelerated
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Figure 1: The FTIR spectra before and after Florida ageing show significant changes in
the carbonyl absorption domain (1900 - 1500 cm -1).
Figure 2: The laboratory-aged samples show exactly the same spectral changes as the
samples aged in Florida.
Figure 3: Evolution of the Taber abrasion resistance (ISO 9352) as a function of the
acid concentration, as measured via the FTIR optical density at 1715 cm -1.
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Figure 4 a: Determination of AFPPE (Acceleration Factor for Physical Properties
Evolution): For the dark blue coating..
Figure 4 b: Determination of AFPPE (Acceleration Factor for Physical Properties
Evolution): For the grey coating..
Figure 5 a: Discolouration vs. irradiation time for natural and accelerated ageing, and
determination of AFD (Acceleration Factor for Discolouration): For the dark blue
coating..
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Figure 5 b: Discolouration vs. irradiation time for natural and accelerated ageing, and
determination of AFD (Acceleration Factor for Discolouration): For the grey coating..
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.
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