IR Spectroscopy and Scanning Electron Microscopy Studies of Natural Ageing of Polypropylene

IR Spectroscopy and Scanning Electron Microscopy Studies of Natural Ageing of Polypropylene

Aicha. Mahir

Physics Department, Physics of surfaces and interfaces, Ibn Tofail University, kenitra, Morocco,

Mourad. Hamouya Physics Department, Physics of surfaces and interfaces, Ibn Tofail

University, kenitra, Morocco,

Mohamed. Chafik EL Idrissi Physics Department, Physics of surfaces and interfaces, Ibn Tofail University, kenitra, Morocco,

Abstract- In the present work we are concerned in the study by IR Spectroscopy and Scanning Electron Microscopy of the effect of solar radiation in two different regions on the natural ageing of the polypropylene(PP).The first region of Mohammedia and the second region of Talsint-Figug. The analysis showed a marked degradation of the surface, which is mainly caused by the contamination of the superficial structure by oxygen. This study was conducted with the aid of three samples of PP: the first was new, the second sample (M) old was exposed about four months the region of Mohammedia; and third, sample (t) old was exposed four about months the region of Talsint-Figug. Spectral analysis indicates that, for new sample, oxygen establishes single bonds with carbon. However, through the ageing process, spectral changes

essentially occur in the regions of -3400, -1712, and 1170 cm- 1which correspond to hydroxyl and/or hydrogen peroxide groups, a carbonyl group, and CO. The examin of the bands corresponding to a carbonyl group reveals the presence of a complicated mixture of oxidation products: aldehydes, ketones, esters, acids, peresters, and peracids. Spectral analysis also shows that the most favorable site for oxidation is at the methylene group. This result is a consequence of the increase of

the reactivity of the reactions of oxidation which brings a breaking of the macromolecular skeleton of the formation of peroxide of under products which might act as catalysts of degradation.

Key words: Polymer, Polypropylene, Ageing, Oxidation, FTIR, SEM

1. INTRODUCTION

   The ageing of a polymer is shown by a slow, often irreversible deterioration of the properties of the material as a result of its own instability under the effect of the environment [1]. The polymer used in this study is the polypropylene (PP) in the form of low-thickness films (0.06mm). Polypropylene (PP) is a linear polyolefin having the formula (C3H6)n, obtained by Ziegler-Natta polymerization or by polymerization catalyzed by a metallocene. There are a wide variety of properties and behavior of the PP depending on the chemical nature of its formulation and shaping conditions[2]. This polymer has been used in various applications for human life because of its low density, excellent thermal and mechanical properties and good process ability with its appropriate price[3]. Unfortunately, polymers (polyolefins) are subject to natural (outdoor) weathering; the latter giving rise to

   degradation effects. Weathering is particularly severe for organic materials, because it combines the photophysical and photochemical effects of ultraviolet (UV) radiation, with the oxidative effects of the atmospheric oxygen and hydrolytic effects of water [4]. Weathering is commonly defined as the undesirable change produced by outdoor exposure [4]. In this paper, we have studied and compared the effect of aging on three samples of PP, including a new taken as reference; and other naturally aged in two different regions. In this regard, we have followed the evolution of the composition of the surface of PP samples under natural aging (in particular UV light of the sun) using ATR-FTIR spectroscopy. As photo-degradation occurs mainly in thin polymer samples of surface layers, reasonable experimental methods are commonly used to study changes in the surface structure (decomposition and formation of chemical bonds), such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), which is one of the most widely used non-destructive techniques in the polymers for the characterization of polymer industry.

   In the present article, we present results obtained by means of IR Spectroscopy and Scanning Electron Microscopy. The study was undertaken on three samples: first, a new sample (N) taken as reference; the second sample (M) old was exposed about four months the region of Mohammedia; and third, sample (t) old was exposed about four months the region of Talsint-Figug.

2. EXPERMENTAL

   1. Materials

      The study focused on three films of PP, thin (0.06) intended to be used as pockets and whose use is commonly answered. This is the PP, manufactured in France and marketed by Cambridge, numéro208873 reference. The samples were analyzed by ATR-FTIR Spectroscopy and Scanning Electron Microscopy.

   2. Natural ageing exposure

      The two kinds of samples (PP (M) and PP (T)) were exposed to natural weathering (days and nights) for four months during the period from July to October. Natural ageing of these samples was carried out at the roof of a building in Mohammedia city and in Talsint Figug city,

      Morocco. The support sample is inclined 45° from the horizontal and oriented towards the south in accordance with the norm (NF T51-165).

   3. Spectral analyses

      1. IR Infrared Spectroscopy

         ATR-FTIR spectra were recorded on citya Magna-IR 560 using ZnSe crystal. The spectra were recorded in the 400-4000 cm-1 range, with a resolution of 4 cm-1 and scan number of 400. All spectra were taken in logarithmic absorbance mode. Experiments are performed applying the ATR equipment with the FTIR spectrometer. The sample surface exposed to photo-oxidative conditions has been in contact with crystal. Indeed, ATR-FTIR spectroscopy is well known as a surface characterization and depth profiling technique. The advantage of ATR is the possibility of obtaining the spectra directly from the sheet without any further sample preparation. ATR is a very sensitive technique and is especially suitable for surface analysis. The utmost care should be taken in sample manipulation to avoid contamination.

         The spectral bands of the investigated functional groups of the polymer were normalized with respect to a reference band and internal standard. Obviously, the reference band was present in three samples of polypropylene and its intensity and wavelength did not change upon the ageing time. The band characteristic of methylene groups (1378 cm-1) was accepted as the reference band [5- 6- 7].

      2. Scanning Electron Microscopy

         Scanning Electron Microscopy is a technique of electron microscope capable of producing high resolution images of the surface of a sample by using the principle of electron-matter interactions.

         The observation of the samples in cutting or in surface was realized on a device of method ESEM with a gas pressure in the room being able to go until 26 m bar. It is also the only allowing microscope to work in three methods of different emptiness: high vacuum (10-6 m bar), low vacuum (0,1 1,33 m bar), and environmental (1,33 26 m bar). And for each of these methods a detector of the secondary electrons that furnishes non-drivers without any preparation, it allows also to observe in a manner dynamic of the materials under a given environment (pressure, temperature, gas composition). It remains topographic on the sample. The method ESEM eliminates totally the effects of loads and allows the observation of samples non- drivers without any preparation, it allows also to observe in a manner dynamic of the materials under a given environment (pressure, temperature, gas composition). Equippe with a complete system of microanalyses X (detector edx-edax) and of a detector of the electrons retro diffused, it allows giving the chemical composition of his sample with a detection limit up to Bore [8].

3. RESULTS AND DISCUSSION

1. IR Infrared Spectroscopy

   The IR Spectrum of the new PP sample is shown in Fig.1. It consists of the characteristic bands of PP [9]: a Broad and intense band That extends from 2860 to 3000 cm-1 is attributed to the valence vibrations of the CH bonds, a bands located at 1450 cm-1is characteristic of methylene CH2 Group, and a band at around 1377 cm-1 is attributed to the methylene CH3 group. There are two other types of bands present: CC vibrations are found in the lower frequency region (from 7OO to 12OO cm-1), and another band of intensity comparable to the former, which characterizes dialkyl ether bonds, COC [10], is found at 1168 cm-1. Note also the emergence of a very low intensity band at 1646 cm-1 that corresponds to C=C bonds [12], as well as the existence of another band, also of very weak intensity, in the region of 3114 to 3255 cm-1 that is attributed to the valence vibrations of an alkane CH bond. The absence of a band at 1700 cm-1 relating to the carbonyl, shows that the oxygen in the reference sample sets only single bonds with carbon.

   Fig.1 IR survey spectra of the new polypropylene sample

   Fig.2 shows the spectrum of the old sample (M), about four months the region Mohammedia. In addition to the characteristic bands next to the bands corresponding to the PP bonds(CH, CH2, CH3) [11], the Spectrum reveals the existence of a new band centered at 1712 cm-1that corresponds to a carbonyl bond C=O [12-13]. Also, another new band between 3100 and 3500 cm-1is attributed to a hydroxyl bond and/or a hydro peroxide bond [14-15]. The third band in very low intensity to 1646 cm-1 corresponding to the C = C bond [16] which relates to groups located at the end of the polymer. Note the significantly increased intensity of the band located at 1168 cm-1, which characterizes the C-O bond, the decreased intensity of the band, corresponding to methylene CH2 groups.

   Fig .2 IR survey spectra of the polypropylene sample (M) aged exposed about four months the region of Mohammadia

   Fig.3 is the IR Spectrum of the old sample (T), about four months the region Talsint-Figug It shows an important increase of the bands attributed to C=O, C-O, and OH bands.

   The changes that affect the three spectra of the corresponding PP samples essentially refer to these regions: 3400, 1712, and 1170 cm-1 .These regions respectively show the contributions from the alcohol and/or hydro peroxide groups, the carbonyl group(C=O), and OH bonds and a remarkable increase in the band at 1646 cm-1 corresponding to the C = C bond [14]. Note also the increased intensity of the band at 1166 cm-1, characteristic of the ether bond CO [14].

   Fig. 3 IR survey spectra of the polypropylene sample aged exposed about four months the region of Talsint-Figug

   The spectral changes between the three samples of PP occur mainly in the 3400 cm-1, 1700 cm-1, 1646 cm-1 and 1166 cm-1 respectively assigned to alcohol and / or hydroperoxide, carbonyl, ether and the alkene function. When studying the aging of PP, usually two regions of the IR spectra are examined: the hydroxyl region and / or hydroperoxides (POOH) and regions carbonyl (C = O) [15- 16], we will focus especially in the second region, designated in the literature by the index of aging [16].

   The superposition of the region located between 1900 and 1600cm-1 for the spectra of all There samples (Fig.5) shows in particular that the carbonyl group is absent from the spectrum for the new PP sample while the second sample offers a very significant C=O feature, which is even more remarkable for the third sample (aged about four months the region Talsint-Figug).

   Whereas for the sample (M) and sample (T), the convolution shows the presence of a broad band centered at 1712 cm-1 with two shoulders at 1735 and 1780 cm-1 band to 1712 cm-1 corresponds to an acid carboxylic acid in dimer form [14] or to a ketone. In a similar study [16], the comparison with the XPS spectra indicated that this band can be attributed largely to ketones [14].The first shoulder at 1935 cm-1 corresponds to a free acid and / or ester the second shoulder at 1780 cm-1 corresponds to a peracid function and / or function perester or even -perlactone.

   Fig. 4 IR spectra from CO groups characteristic region for (a) a non treated polypropylene sample, (b) aged exposed about four months the region of Mouhammadia, (c) aged exposed about four months the region of Talsint-Figug

   In addition to the characteristic bands corresponding to the main groups found in the material (CH, CH2,CH3), the IR spectrum of the new PP sample also contains a band related to an ethers CO bond.

   The method proposed by Luongo[12].was used to determine the nature of the configuration of the chains forming the samples. This method is based on the characteristic absorption bands and those specific to the isotactic and atactic phases, its vantagead is that it is independent of the physical nature of the samples and the molecular mass distribution and it is also unaffected by minor constituents such as stabilizers. For mor information on the spectral differences characterizing the PP samples called 100% isotactic and 100 atactic, the reader may refer to Luongo[13].The IR spectrum of the new sample(Fig.1) closely resemble that of the 100% isotactic, in particular, the two bands at 974 and 975cm-1 have almost the same intensity(although these two bands show up in our instrument at 972 and 997cm-1, they are essentially the same) and the band at 1168cm-1 with a shoulder at 1554cm-

   1is also similar to the bands found on the 100% isotactic spectrum.

   of oxidation (Fig.4). However, the closeness of the absorption bands in the complicated mixture of aldehydes, ketones, esters, acids, peresters, and peracids, along with the difficulties of differentiating between hydroperoxides and alcohols, rendered the qualitative analyses more difficult.

   1. Degree of crystallinity

      The infrared spectrums permitted us to calculate, in a quantitative manner, rate of crystallinity XC of different samples starting from two specific picks of 997 and 970 cm-1.The pick 917 cm-1 was taken as reference [15-16].

      = 109 × (997 1 917 1 ) 31.4 × 100

      972 1 917 1

      Fig.5 Determining the configuration of the PP by the method of Luongo

      The fixation of oxygen on the polymer is caused by photooxidation reactions [14]. PP is susceptible to oxidation by servel methods: heating, radiation, UV, and so forth [15-16]. Because the formation of carbonyl group is always significant, certain authors follow the evolution of the carbonyl group as a function of exposure time [17- 18].In this study the interest was on the nature of carbonyl- containing compounds. It looks as though the disappearance of the component located at 1700cm-1 is caused by one of the following there conjugation between the ketone group and the C=C, especially because this latter band is observed at 1637 cm-1 for the old sample (M), about four months the region Mohammedia and is seen at 1640cm-1 on the Spectrum of the old sample (T), about four months the region Talsint-Figug. Second, the loss of the carboxylic acid existing as a dimer [17]. Displaces ketones [13]. By 10-20cm-1and consequently would force the component appearing at 1700cm-1 to add to the component found at 1710cm-1. The concentration of the numerous oxidation products, other than for hydroperoxides and alcohols, are ordered according to the following decreasing order: carboxylic acids > ketones > esters > peracids, peresters > alkenes. These results are in agreement with those obtained by other authors who used chemical reactions to attain this goal [14-15-16].

      The intensity of the band centered at 842cm-1, which is attribued to the CC vibration, is merely affected by the aging phenomenon and hence it can be used as a reference band to normalize the carbonyl absorption band and to compare the aged sample [14]. On the contrary, the intensity of the methylene group shows an interesting alteration during oxidation. Actually, as the intensity of the methylene decreases, that of the carbonyl increases, thus confirming that the oxidative attack preferentially occurs on the methylene group.

      Henceforth, IR spectroscopy allowed us to clearly determine the site of oxidative attack, as corroborated by our XPS results [12]. It permitted the detection of oxygen in the new sample, even though it was present in trace amounts. It also aided in visualizing the increasing degree

   2. Density

      To calculate the density in function of the rate of crystallinity, we often use the following equation.

      = 100 × /( )

      We have the density of the samples a and c which are respectively the densities of PP of amorphous and crystalline phases. Different values of a are stated in the literature [14-16]. We took = 0.850 g / cm3 [14] and

      = 0.936 g / cm3 [14-15] We have taken the following.

      Table-1 presents the density and rate of the crystallinity of three samples (N, M and T)

      Samples	Xc	d (g/cm-3)
      Sample (N)	45,27	0,8889
      Sample (M)	56,67	0,8987
      Sample (T)	71,74	0,9116

      According to the following picture observe us an increase of the rate of crystallinity and densities of the samples age this is hard to the replacement of the liaisons CH by the liaisons C=O in the shapeless region [17].

2. Scanning Electron Microscopy

The results obtained by SEM micrograph have shown a difference between the morphology of its samples.

We remark that the micrograph of the sample (N), the surface is observed without cracks and with some white stains due to the existence of some vacuums from the liberation of gas during the treatment. But for the micrograph of the sample (M), the white stains were bugger with some equivalent cracks with appearance of oxygen in chemical composition of the sample which indicates the formation of some products of oxidation. The something was observed in the micrograph of the sample (T).

The surface showed some deeper and more numerous cracks than those observed in the sample (M), equivalency to the increase of the percentage of oxygen in their chemical composition, This result is a of the increase of the reactivity of the reactions of oxidation which brings a breaking of the macromolecular skeleton of the formation of peroxide of other products which might act as catalysts of degradation. We also observe the appearance of some metals like Mg, Al, Si, and K with different percentage. This is explained by the

existence of dust, polluted air because of the existence of cement producing factory in that region (Talsint-Figug). These results sho accordingly w that the sample(M) and sample(T) undergo a remarkable degradation regarding the climate factors of each region (O, UV, humidity.) which confirms the results of infrared [18].

Fig.6 Scanning electron microscopy micrographs of polypropylene sample

(N) new

Fig.7 Scanning electron microscopy micrographs of polypropylene sample

(M) aged exposed about four the region of Mouhammadia

Fig.8 Scanning electron microscopy micrographs of polypropylene sample

(T) aged exposed about four months the region of Talsint-Figug.

CONCLUSION

IR Spectroscopy and Scanning Electron Microscopy (SEM) were used to study aging natural PP.

The data obtained via IR Spectroscopy and Scanning Electron Microscopy was useful in determining the structural modifications brought about by aging of commercial PP samples. Indeed, three types of PP samples were analyzed: the first sample was absolutely new, the second had been exposed about four months the region of Mohammedia, while the third was exposed about four months the region of Talsint-Figug. The IR spectrum and Scanning Electron Microscopy of the new sample (N) (Fig.1, Mic.6) reveals the presence of oxygen, which is incorporated into the polymeric chain via single chemical bonds, and indicates that the commercial sample is isotactic. The IR spectrum and Scanning Electron Microscopy of the second sample (M) old was exposed about four months the region of Mohammedia, (fig. 2, Mic.7), the spectrum shows the presence of a carbonyl group. The examining of this peak indicates the presence of a complicated mixture of oxidation products: conjugated ketenes, aldehydes, esters, acids (carboxylic acids can exist as stable dimmers, which absorb at 1710cm-1, or as free acids [18], which absorb at 1755cm- 1), peresters, and peracids. The decreased intensity of methylene confirms that it is the favourable oxidation site. The IR spectrum and Scanning Electron Microscopy of third sample (T) aged exposed about four months the region of Talsint-Figug (Fig3, Mic.8). However, the intensity of the

carbonyl group is greater for the third sample, and the

examining of this band shows the absence of the conjugated ketene. Also, the intensity of the numerous oxidation products is increased. These results sho accordingly w that the sample (M) and sample (T) undergo a remarkable degradation regarding the climate factors of each region (O, UV, humidity.).

REFERENCES

[1]. Kaloustin, J., Antonetti, P.; Berrada, A.; Claire, Y.; Perichaud, A. J Thermal Anal 1998, 52, 327.

[2]. E. Niki, C. Decker and F.R. Mayo, J Polym Sci Polym Chem Ed 11 (1973), p. 2813

[3]. Mckellar JF. Allen. NS. Photochemistry of man-made polymers.

Essex, England: Applied Science Publishers Ltd.; 1979. P.31.

[4]. Svetlana I. Kuzina, Alfa I. Mikksilou. Europ. Polym J. Vol.

37(2001) 2319-2325.

[5]. J.K. Pandey, K.R. Reddy, A.P. Kumar and R.P. Singh, Polymer Degradation and Stability 88 (2005), p. 234.

[6]. L. Kupper, J.V. Gulmine, P.R. Janissek, H. M. Heise, Vibrational Spectroscopy, 34 (2004) 63-72

[7]. Laleh Moghaddama, James P. Blincob, John M. Colwellc, Peter J. Halleyd, Steven E. Bottlea, b, Peter M. Fredericksa and Graeme A. Georgea, b, c, Available online 26 January 2011.

[8]. J.D. Peterson, S. Vyazovkin, C.A. Wight, M. J. Condensed Matter 202 (6) (2001).

[9]. Lacoste, J., Vaillant, D.; Carlsson, D. J. J. Polym Sci 1993, 31, 715.

[10]. Lunongo, T. P. J Polym Sci 1960, 9, 302.

[11]. Lascomb, J.; Haurie, M.; Jonsien, M. L. J Chim Phys 1962, 59, 1233.

[12]. Hoekstra, H. D., Spooraker, J. L.; Breen, J., Au douin, L. Verdu, L. Polym degrade Stabil 1995, 49, 251.

[13]. D.J. Carlsson, R. Brousseau, C. Zhang, D. M. Wiles, Am. Chem.

Soc. Symp. Ser., 364, 376 (1998).

[14]. A. Rjeb, L. Tajounte, M.C. El Iddrissi, S. Letarte, A. Adnot and D. Roy et al., IR spectroscopy study of polypropylene natural aging, J Appl Polym Sci 77 (2000), pp.

[15]. J. Lacoste ; D. Vaillant, D. J. Carlsson, J Polym. Sci. Polym. Chem., 31, 715-722 (1993).

[16]. J. P..Mercier, E. Maréchal, Traité des matériaux 13, chimie des polymères, Synthèses, Réactions, Dégradations, press Polytechniques et Universitaires Romandes.

[17]. R. G. Hausermon, M. Johnson. J. Appl. Polym. Sci Vol 20(1967) 2533-2539.

[18]. J. Luston, V. Polusakoua, F. Vass. J. Appl. Polym. Sci vol 47(1993) 555-562.