3.1 Introduction
Aliphatic polyamides have got
very good applications as engineering plastic as well as fiber material. During
its normal use it is exposed to sunlight, which causes extensive degradation to
the polymer. Degradation is a very crucial factor and deteriorate the useful
properties of polymeric materials. Nylon 66 degradation is studied widely
during the past several decades by several authors1-4. It causes severe changes in its chemical,
physical and mechanical properties upon natural/artificial weathering. The
degree of changes depends on the wavelength of the UV radiation and the
atmospheric conditions. Various techniques have been used to characterize the
photodegradation in nylon 66. Moore2 compared the photodegradation
products obtained from the model amides and polyamides and characterized them
by several techniques like loss in tenacity, change in intrinsic viscosity, UV
spectroscopy and end group analysis. The hydroperoxide analysis and second
derivative of UV spectroscopy are also important and have been used to
characterize photodegradation in polyamides 5-8. Allen et al.9-12 have studied the photochemistry of nylon
polymers in details. Kinetics of photoaging has been widely studied2,4,10,12
for nylon 66. It was observed that wavelength of light has significant
influence on photoproduct formation during photodegradation of nylon polymers6,7.
Chromophore defects and impurities initiate the hydroperoxidation when nylon
polymers are exposed to light of higher wavelength (l
> 340 nm). Whereas at lower
wavelength (l = 254 nm) direct photoscission
occurs, which is independent of the length of carbon chain.
Presence of water was the
another important factor plays ver crucial role during the photo-oxidation of
aliphatic polyamides.
Chain
scission upon photo-irradiation is also widely studied in nylon polymers. Do et
al.13 studied the photodegradation and photo-oxidation in nylon
films at shorter wavelength (l = 254 nm) by FT-IR
spectroscopy. They also studied the evolution of the gases and changes in
crystallinity with UV exposure. They observed that photo-irradiation of nylon
polymers in nitrogen atmosphere gives extensive chain cleavage.
While photo-oxidative
degradation gives the following photoproducts.
Although
a few workers have studied photo-oxidation14-16 of nylon 66 but the photo-oxidative
kinetics is not well defined. To improve all aspects of the stability of
polymers, a fundamental understanding of degradation process involved is
essential, therefore in present study we have made an attempt to study the
chemical and physical changes in nylon 66 with polychromatic irradiation (l ³ 290 nm)
3.2 Experimental
3.2.1 Materials
Commercial samples of nylon 66 (Zytel 101 L NC 10) were received
from M/s E.I. du Pont de Nemours, USA and used without further purification.
3.2.2 Sample preparation
Thin films (thickness ~50 mm) of commercial nylon 66 were prepared by
pressing the polymers in preheated Carver press at 270°C under ~14 kg/cm2 pressure for
two minutes. The films were quench cooled rapidly in the press.
3.2.3 Photo-irradiation
All samples were irradiated in
a SEPAP 12/24 (an accelerated photoirradiation chamber l ³ 290 nm) at 60°C
for different time intervals. The
details of this equipment are given elsewhere17.
3.3 Analysis
3.3.1 FT-IR analysis
Photo-oxidative products and
kinetics of photodegradation were studied by FT-IR (Fourier Transform Infrared
16 PC spectrometer).
3.3.2 UV absorption analysis
The ultra-violet absorption
measurement for thin films (~ 50 mm) were carried out on Hewlett
Packard 8452A Diode Array Spectrophotometer.
3.4 Results and Discussion
3.4.1. Change in hydroxyl regions
FT-IR spectroscopic technique was used to characterize
the photo-oxidation in the polymer films. Changes in hydroxyl region (3700-3200
cm-1) during the
Figure
3.1 Change in hydroxyl region of
IR spectra of photo-irradiated nylon 66 sample.
photo-irradiation are quite
different than what usually observed in polyolefines and styrenic polymers18,19.
Figure 3.1 shows that already formed hydroperoxides, which might have
been generated during the melt processing of the polymer and are decomposed upon
photo-irradiation. The rate of decomposition of hydroperoxide/hydroxyl groups
is much faster than that of generation during the initial period of
photo-oxidation, which leads to a decrease in the absorbance range 3700-3200 cm-1
in the initial 100 h. irradiation but after that there is no consistency in the
changes as shown in Figure 3.2. Here the peak intensity in case of
hydroxyl/hydroperoxy absorption is an integrated partial area from 3700 to 3350
cm-1 calculated using base line from 3700 to 2200 cm-1. In case of carbonyl absorption, the peak
intensity is also an integrated partial area from 1820 to 1690 cm-1
calculated using baseline from 1800 to 972 cm-1. Decomposition of hydroperoxide/hydroxyl
species can form imide or aldehyde respectively20.
Existence of hydroperoxy/hydroxyl species in unexposed
sample is due to the
Figure 3.2. Change in
hydroxyl and carbonyl peak absorption of nylon 66 with exposure time.
formation of these groups
during melt processing (while preparing the films) of nylon 66. Moreover, these species are stable at room
temperature and easily detected in unexposed samples. We did not observe the
presence of these species in solution-cast films.
In a separate study we tried to
examine the decomposition behavior of hydroperoxy/hydroxyl species during the
initial period of photo-irradiation. Pre-existing hydroperoxy/hydroxyl species
started getting decomposed with increasing exposure time (Figure 3.3) up
to 25 h. During that period there was a rapid increase in carbonyl population.
After 25 h, the hydroperoxy/hydroxyl species started increasing while carbonyl
population remained rather constant. After 90 h again
Figure
3.2. Change in hydroxyl and
carbonyl peak absorption of nylon 66 with photo-irradiation during initial
period of exposure.
hydroperoxy/hydroxyl species
started getting decomposed and the carbonyl population increased sharply. Thus,
decomposition of hydroperoxy/hydroxyl species resulted in the formation of
carbonyl species.
3.4.2 Change in carbonyl regions
The IR spectral changes in the
1800-1690 cm-1 region with exposure time are shown in Figure 3.4.
The increase in the absorbance with irradiation time in this region indicates
an increase in the imide group population. From Figure 3.2 also, it is
clear that there is a continuous increase in carbonyl absorption, moreover,
this increase is more rapid during initial period of exposure (up to 100 h)
after that it continues to increase but with slower rate. This observation,
again, gives perfect evidence about the decomposition of hydroperoxide/hydroxyl
groups in the carbonyl groups. Straight increase in carbonyl group
concentration with irradiation time is an indication for the prevalence of
single photo-oxidation mechanism throughout the photo-oxidation process.
Figure
3.4 Change in carbonyl region of
IR spectra of photo-irradiated nylon 66 sample.
3.4.3 Change in methylene
regions
As per mechanism suggested2,19,20
for photo-oxidation of model polyamides,
it was observed that the primary free radical formed upon UV irradiation
is:
which suggests that there
should be continuous decay of methylene group adjacent to ¾NH¾
group [¾CH2¾(NH)¾]
population. We have also tried to study the kinetics of ¾CH2¾(NH)¾ decomposition with exposure time. The
absorption band at 1180 cm-1 is due to ¾CH2¾(NH)¾
group24 and we found that population of methylene group, vicinal to ¾NH¾
group, decreases linearly with irradiation time (Figure 3.5). This is an
additional evidence to the mechanism suggested by Do et al.13
that decomposition of hydroperoxides leads to the formation of carbonyl groups:
Absorption of methylene groups
linked both the side with carbon atom show absorption at 2900 cm-1.
We did not observe any significant change in this particular band intensity.
Figure
3.5 Change (%) in the methylene
group, adjacent to the ¾NH¾
group, population with photo-irradiation time.
3.4.4 Change in amorphous and crystalline
regions
Many semicrystalline polymers
show infrared absorption bands corresponding to motions in the crystalline and
amorphous phase. In case of nylon 66 crystalline and amorphous bands appear at
936 and 1140 cm-1, respectively21. The origin of this
bands has been assigned21a to a CO twist (1140 cm-1)
and ¾C¾ CO stretch (936 cm-1).
Figure 3.6 shows that intensity of this peak decreases with exposure
time.
In Figure 3.7 amorphous
and crystalline peak intensity is plotted against the exposure time and from
this figure it is very much clear that the amorphous region is the only site
for photo-oxidation, which decomposes continuously with exposure time. Somewhat
similar results were observed by Do et al.13 during exposure
of nylon 66 by short wavelength irradiation (l =
254 nm). The reason for decomposition of amorphous region, exclusively, can be
justified by the lower oxygen diffusivity and permeability for nylon 66. This
phenomenon might have limited photo-oxidation to amorphous region only where
oxygen diffusion would be sufficient to oxidize the polymer.
Aliphatic
polyamides are reported to have very less oxygen diffusivity as compared to
polyolefines. Importance of oxygen diffusion in determining the rate
Figure
3.6 Changes in amorphous region
of IR spectra with photo-irradiation.
Figure
3.7 Change in crystalline and
amorphous peak intensity with exposure time.
of thermal and photo-oxidation
of polymers has been discussed in details22-26. Carlsson and Wiles
observed26 very high surface oxidation of during the
photo-irradiation of polyolefines and indicated that oxygen availability at the
surface of polymer causes very high degree of oxidation. Furneaux et al.27
studied the natural and accelerated artificial weathering of low density
polyethylene. The natural and artificial samples showed very similar reaction
profiles. They found that there was a significant drop in photo-oxidation
reaction at the center of the sample. They too concluded that this particular
behavior was due to the limited supply of oxygen at the core of sample. It is
fact that gas diffusion in any semicrystalline polymer is limited to the
amorphous phase of the polymer. As crystallinity increases the gas diffusivity
is declined. Prevalence of
photo-oxidative processes in amorphous region supports the fact that oxygen
diffusivity is one of the important factor that can control the rate of
oxidative degradation in any polymer.
Do et al. also reported the emergence of
absorption band at 1620 cm-1 during photo-oxidation of nylon 66,
according to them this band is due to absorption of certain species bearing
double bonds. However, in our study we did not observe such kind of absorption
at ~ 1620 cm-1.
3.4.5 Change in
UV absorption
Exposure of
nylon 66 films to polychromatic radiation led to the development of UV absorption
band at ~244 nm. The spectra shown in Figure 3.8 are in the form of
spectral subtraction of unexposed sample spectra from each of the spectra of
the photo-oxidized sample at different time duration.
Figure
3.9 shows that the peak height (l =
244 nm) is increasing continuously with exposure time. Increase in peak height
with exposure time is due to the formation of imide groups as it is already
reported28
that open chain imides show a peak in the region of 250 nm and 230 nm with a
tail extended out to 280 nm. Also the nature of kinetic curves in Figure 3.4
and Figure 3.9 are similar, which again confirms the formation of,
mainly, imide species as a result of photo-oxidation in nylon 66. Allen
reported29 the UV absorption bands at ~230 nm and 294 nm in
photo-oxidized nylon 66 (due to formation of a, b-unsaturated
carbonyl species) but we did not observe the same in our studies.
Figure 3.8 Change
in UV absorption nylon 66 photo-irradiated at different time intervals.
Figure 3.9
Increase in UV absorption at 244 nm with exposure time
3.5 Conclusions
The presence of
hydroxy/hydroperoxide groups in melt processed film was found in quite a higher
concentration and according to us that is the main driving force for the higher
rate of photo-oxidation during the initial period of exposure. The continuous
increase in carbonyl group concentration concludes the existence of a single
photo-oxidation mechanism throughout the oxidation process. Primary
photo-oxidation process in polyamide is the oxidation of methylene group
adjacent to the ¾NH¾
group. Physical changes in polymer matrix like disappearance of amorphous phase
with exposure time is the main evidence for effectiveness of photo-oxidation in
amorphous phase. Finally, the UV absorption study suggests that the increase in
UV absorption at ~244 nm is due to the imide formation.
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