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e-Polymers 2012, no. 082

http://www.e-polymers.org

ISSN 1618-7229

Effects of sterically hindered N-alkoxyamines on photo-oxidative stability of reinforced polypropylene Daniela Enescu,* Alberto Frache *Dipartimento di Scienza dei Materiali e Ingegneria Chimica – Politecnico di Torino, sede di Alessandria, Viale Teresa Michel, 5, 15121 Alessandria, Italy; tel/fax: +39 0131 229343/+39 0131 229331; e-mail: daniela.enescu@polito.it (Received: 10 March, 2011; published: 06 September, 2012)

Abstract: In the present paper the influence of Flamestab

® NOR

TM 116 (abbrev.

FNOR), a sterically hindered N-alkoxyamine stabilizer, on the photo-oxidative stability of calcium carbonate reinforced polypropylene (abbrev. CCPP) used for production of plastic collars has been investigated. The samples, prepared by melt compounding, were exposed to artificial accelerated photo-ageing carried out at λ>300 nm and 45

oC in air. FT-IR and UV-VIS analysis revealed that the

deterioration of neat CCPP takes place very rapidly in comparison with the composites containing FNOR. Hydroperoxides and various carbonyl species are formed as the main degradation products during the photo-oxidation process. Moreover, the intensity of the absorption bands grows with increasing of UV exposure time indicating that the material suffers a photo-oxidative degradation process. Indeed, addition of only 0.1 wt.-% FNOR did not significantly modify the length of the induction period of neat CCPP, but a strong efficiency was obtained by increasing the amount of FNOR (0.5 wt.-%) which shows an increase of eight times more as compared with neat CCPP. Therefore, it may be supposed that OIT is linked to the level of concentration of the stabilizer. This remarkable increase in composite stability demonstrated by a lengthening of OIT is typical for sterically hindered N-alkoxyamine stabilizers since they are very efficient at scavenging free-radicals formed as the polymer degrades. In other words, these UV stabilizers are converted to nitroxide radicals (see Scheme 1) which readily trap the polymer degradation species to produce alkoxyamines. Then, the formed alkoxyamines do subsequently react with peroxy radicals to regenerate the active nitroxide radical, thereby this process (called Denisov cycle) proceed in a regenerative cycle, thus retarding oxidative polymer degradation. The increase of absorbance in the carbonyl region as a function of irradiation time was used to determinate the photooxidation rate. It was found out that the photooxidation rate measured from the slope of linear part of the curves is significantly decreased for the CCPP/FNOR0.5 composite compared to neat CCPP. In the light of the above mentioned findings, FNOR could be taken into consideration as an UV stabilizer for CCPP. Keywords: photo-oxidation; induction period; reinforced polypropylene; sterically hindered N-alkoxyamine stabilizer

Introduction

The lifetime extension of the articles made of polymeric materials, under exposure to solar UV radiation, present a great importance for the outdoor applications. For instance, the photo-oxidative degradation of polyolefin homopolymers has been extensively studied and reported in numerous publications [1-9]. In order to extend their lifetime against photo-oxidative degradation, among sterically hindered amine

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light stabilizers (abbrev. HALS; the originally commercialized HALS had structures of secondary (>NH) and tertiary (>NCH3) cyclic amines) [10-18], the sterically hindered N-alkoxyamine stabilizers (abbrev. NOR HAS) have opened new possibilities due to their resistance to the acid environment [19-21]. NOR HAS are of lower basicity having pKa values ~4 [22], whereas >NH and >NCH3 HALS having pKa values between 7.5 and 9.7 are deactivated in the presence of acid impurities [13]. However, similar publications on photo-oxidative degradation of polyolefins composites are extremely limited.

The use of Flamestab® NORTM 116 as NOR HAS has a special interest due to its chemical properties (combine light stability and flame retardancy in one molecule). It has been shown that it can lead to a remarkable improvement of fire retardancy at a concentration of only 0.5 wt.-% passing the NFPA 701 fibre test [23] on polypropylene knitted socks [24]. Later, Kaprinidis and co-workers [25] have reported that Flamestab® NORTM 116 can replace antimony trioxide as synergist for halogen and non-halogen containing flame retardants. Low flame retardant concentrations and no antimony trioxide result in polymers with improved processability, better mechanical properties and reduced smoke density. For example, the authors have shown that it is possible to be achieved UL 94 V-0 and V-2 ratings, respectively in PP injection moulded plaques at concentrations as low as 3.5 wt.-% of formulations containing Flamestab® NORTM 116.

With regard to its usage as UV stabilizer, as far as we know, no paper has been published reporting its influence on the durability behavior for instance on reinforced polypropylene.

In the present paper the influence of Flamestab® NORTM 116 on the photo-oxidative degradation of calcium carbonate reinforced polypropylene used for production of plastic collars was evaluated. Results and discussion

The photo-oxidative degradation mechanism of polypropylene has been investigated in detail in the past years. Most of the photoproducts which appear during irradiation, as a consequence of the presence of chromophoric impurities and/or catalytic residues [26], consisting in the formation of hydroperoxy/hydroxyl species and various carbonyl species (carboxylic acid, esters, peresters, lactones in the region 1800-1600 cm-1) were identified [1,4,9].

FT-IR spectra of neat CCPP (Figure 1a) subjected to accelerated photo-ageing in the SEPAP device shows in addition to the characteristic absorption bands of polypropylene and calcium carbonate (stretching vibration of –CH3 and CH2 groups between 3000 and 2716 cm-1, asymmetric bending vibrations of –CH3 and –CH2

groups at 1460 cm-1, symmetric –CH3 bending vibrations at 1375 cm-1, and of the carbonate anions at 2514, 1798, 1430, 876, and 713 cm-1, respectively; Figure 1a, 0 h) new broad absorption bands at 1711 cm-1 as well as at 3422 cm-1 after 120 h of exposure. These new broad absorption bands in the carbonyl and hydroxyl region belong to the formation of photo-oxidation products such as carboxyl acid, hydroperoxide and alcohols as it have been identified previously [4,9].

Furthermore, Figures 1b and 1c, respectively show the evolution of the FT-IR spectra of neat CCPP in the carbonyl and hydroxyl region upon UV irradiation. It can be seen that the intensity of both bands grows with increasing of UV exposure time. This behavior indicates that the material suffers a photo-oxidative degradation process

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and because neat CCPP films have broken after 320 h of UV exposure, comparison after longer exposure was not carried out.

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

Wavenumber (cm-1)

Abs

orba

nce

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40h80h

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2000.0 1950 1900 1850 1800 1750 1700 1650 1600 1550.0

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orba

nce

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C=O O-H

b)

a)

4

4000.0 3900 3800 3700 3600 3500 3400 3300 3200 3100.0

Wavenumber (cm-1)

Abs

orba

nce

Fig. 1. a) FT-IR spectra of neat CCPP at different UV irradiation times. Evolution of FT-IR spectra of neat CCPP at different UV irradiation times in the (b) carbonyl and (c) hydroxyl domain.

N

POO .

R"

Subst.

N

R"

Subst.

POOH

N

O .

Subst.

nitroxide radical

N

O

Subst.

P .

alkoxyamine

P

POH + P = O

Path A

Path B

P = Polymer

R" = H, CH3, COCH3, CH2-C6H5, O-Alkyl

(NOR)-HAS

O

POO .

Scheme 1. Proposed stabilization mechanism of NOR HAS.

O-H

320h

240h

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120h

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0h

c)

5

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orba

nce

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C=O O-H

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a)

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Wavenumber (cm-1)

Abs

orba

nce

Fig. 2. a) FT-IR spectra of CCPP/FNOR0.5 composite at different UV irradiation times. Evolution of FT-IR spectra of neat CCPP at different UV irradiation times in the (b) carbonyl and (c) hydroxyl domain. In contrast, the CCPP/FNOR0.5 composite (Figure 2a) shows an increase of oxidation induction time (abbrev. OIT - defined as the time prior to the onset of the oxidation) up to 980 h of irradiation. After this induction period it can be observed a progressive increase of photo-oxidative process which can be attributed to the consumption of the stabilizer. Figures 2b and 2c, respectively show the evolution of the infrared absorption spectra of the CCPP/FNOR0.5 composite in the carbonyl and hydroxyl region upon UV irradiation. In comparison with neat CCPP the intensity of carbonyl and hydroxyl bands grows slower. Furthermore, after 1680 h of irradiation no cracks were observed on the surface of the specimen. The long induction period of the photo-oxidation of the composite is in agreement with the reported literature

[27-30] which has proposed the hypotheses that within polymer matrix, NOR HAS are readily converted to nitroxide radicals which scavenge alkyl radicals. Then, the formed alkoxyamines do subsequently react with peroxy radicals to regenerate the active nitroxide radical. Thus, this process (called Denisov cycle) proceeds in a regenerative cycle that accounts for the high stabilizing effect of NOR HAS (Scheme 1). Extensive description of the regeneration mechanism based on revision of the so called Denisov cycle was published recently [31].

In order to compare the rate of photo-oxidation of the CCPP/FNOR0.5 composite with that of neat CCPP, the increase of absorbance in the carbonyl region as a function of irradiation time can be considered (Figure 3, Table 1). It has been found that the oxidation kinetics is modified by the presence of the stabilizer: the induction period increased eight times more as compared with neat CCPP, and the rate of oxidation measured from the slope of linear part of the curves is decreased. This

O-H

1680h

1580h

1460h

1340h

980h

600h

300h

80h

0h

c)

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phenomenon indicates that the presence of the stabilizer leads to an improvement of photo-oxidative degradation process of neat CCPP. Tab. 1. Oxidation induction time (OIT) and the rate of oxidation (slope) obtained from normalized absorption at 1711 cm-1 of neat CCPP and the CCPP/FNOR0.5 composite.

Sample OIT (h)

Slope (10-2 h-1)

CCPP 120 0.2 CCPP/FNOR0.5 980 0.1

Fig. 3. Evolution of absorbance at 1711 cm-1 (normalized absorption corresponding to oxidized products) as a function of the irradiation time of neat CCPP and the CCPP/FNOR0.5 composite. It was investigated also the influence of stabilizer concentration by decreasing the concentration of it at 0.1 wt.-%. The kinetic curves of oxidation of the CCPP/FNOR0.1 composite show a decrease of induction period (data no given) of six times less compared to the CCPP/FNOR0.5 composite. Therefore, it may be supposed that the OIT is linked to the level of concentration of the stabilizer.

Changes in UV-Visible spectra on the irradiated neat CCPP and CCPP/FNOR0.5 films are shown in Figure 4.

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0.00

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Wavelength (nm)

Abs

orba

nce

0h140h360h880h

980h1040h1680h

Fig. 4. Evolution of the UV-Visible spectra at different UV irradiation times of: (a) neat CCPP and (b) CCPP/FNOR0.5 (UV irradiation times are indicated on the curves).

200.0 250 300 350 400.0 0.00

1.0

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Absorb

ance

e 0h

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Experimental Materials

Polypropylene filled with 40 wt.-% calcium carbonate (abbrev. CCPP), density 1.24 g/cm3, melt index (230 oC/2,16 kg) 2 g/10 min, a commercial grade material, supplied by Campia Imballaggi S.R.L (Italy). Flamestab® NORTM 116 (trade name) (Schema 2) a monomeric N-alkoxy hindered amine stabilizer (abbrev. FNOR) supplied by Ciba Specialty Chemicals Inc (Basel, Switzerland).

HN NN NH

R

H

R

R

N

N

N

NC4H9

NC4H9

N N

O O

R=

Scheme 2. Chemical structure of Flamestab® NORTM 116. Preparation of CCPP composites

The composites were prepared via direct melt compounding of CCPP with 0.1 and 0.5 wt.-%, respectively Flamestab® NORTM 116 (abbrev. CCPP/FNOR0.1 and CCPP/FNOR0.5) using a Leistritz ZSE 18 HP co-rotating twin screw extruder (screw diameter (D) = 18 mm, L/D = 40) (Figure 5 shows the detailed screw profile).

Fig. 5. Screw profile: GFA or GFF are conveying type elements and are designed for example, GFA 2-30-60 where G=co-rotating, F= conveying, A= conveyor of the molten material (“self-cleaning”, refer to the fact that the rotating screws continuously wipe the molten material from each other and the barrel walls, so that the material build-up and its degradation over time can be prevent), F= conveyor of the solid material (not “self-cleaning”), 2= number of threads, 30 = length of pitch (mm), 60 =

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length of screw element (mm). Elements indicated by KB4 are kneader block type mixing elements and are designated as follows: for example KB4 2-15-30 where KB= kneading block, 4= number of kneading segments, 2 = number of threads, 15= length of kneading block, 30 = stagger angle between the kneading discs (degrees). The operating temperature of the extruder was maintained at 225-220-217-215-210-210-210-205 oC from hopper to die, respectively. The screw speed was maintained at 150 rpm and the throughput was 3.5 kg/h. The molten material was quenched in water and then palletized. The pellets obtained were further dried under vacuum oven at 80 oC for 6 h and another 6 h only under vacuum, then hot-presses to films (thickness ~60 µm measured using a micrometer) by using a hydraulic press equipped with heating plates (SPECAC®) at 190 oC for 1 min. Photo-oxidation test Ultraviolet (UV) exposure conditions

The samples in form of thin films were exposed to artificial accelerated photo-ageing carried out in SEPAP 12/24 device (ATLAS®) equipped with 4 medium-pressure mercury lamps (400 W) with borosilicate envelops which allows irradiation at wavelengths above 300 nm (see the full description of the device reported previously by Philippart et al. [32]) simulating in this way the condition of natural outdoor weathering [33]. Photo-ageing experiments were performed in air and the working temperature was maintained at 45 oC. For each composition, a total of three specimens were tested in order to ensure significant and reproducible data. The photo-ageing samples were collected at different period of exposure and subjected to chemical analysis. Characterization of the photo-ageing samples

The evolution of the irradiated films was monitored by Fourier Transform Infrared Spectroscopy (Perkin Elmer GX FT-IR spectrometer in a wavenumber range of 4000-400 cm-1, 16 scans at resolution of 4 cm-1) and by Ultraviolet-Visible Spectroscopy (Perkin Elmer Lambda 40 UV-VIS spectrophotometer in the wavelength of 200-800 nm).

The photo-oxidation rate was obtained from the slope of the linear part of the carbonyl absorption curve by plotting the ratio of the absorbance at 1711 cm-1 to that at 2722 cm-1 as a function of irradiation time. The carbonyl absorption band, measured at 1711 cm-1 from FT-IR spectra at different irradiation time, was normalized in order to avoid the differences due to the film thickness by dividing it to the FT-IR absorption band at 2722 cm-1 (characteristic vibration stretching band of polypropylene) whose intensity depend of the thickness of the film. Acknowledgements

This work was financially supported by PLASTICOLLAR, Piedmont Region Industrial Research call 2008. The authors acknowledge the useful comments and suggestions of the reviewers.

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