D

Journal of Materials Science and Engineering A 3 (2) (2013) 123-131

DAVID

PUBLISHING

Induced Degradation of Polypropylene with an Organic Pro-Degradant Additive Larissa Stieven Montagna, Maria Madalena de Camargo Forte and Ruth Marlene Campomanes Santana Laboratory of Polymeric Materials, Lapol of Federal University of Rio Grande do Sul, 91.501-970 Porto Alegre, Brazil Received: September 20, 2012 / Accepted: October 17, 2012 / Published: February 10, 2013. Abstract: The aim of this study was to evaluate the effect of an organic pro-degradant additive on the degradation rate of polypropylene (PP) under biotic and abiotic conditions. The PP samples were processed by thermal compression molding (TCM). PP specimens in plate form were evaluated after being submitted to natural ageing and to simulated composting during 120 days. The PP samples were analyzed by viscosimetry, differential scanning colorimetry (DSC) and scanning electron microscopy (SEM). The organic additive was effective in the PP degradation as monitored through the decrease in the polymer molecular weight and variation in the crystallinity and morphology of the PP samples submitted to both conditions. The samples exposed to natural ageing showed a higher degree of crystallinity and lower molecular weight than the samples submitted to the simulated soil composting. The SEM analysis revealed a deterioration of the surface of the PP-modified samples after exposure to different environments. Key words: Polypropylene, degradation, pro-degradant organic, natural ageing, composting.

Nomenclature PP: MI: D: Xc: ∆Hm: ∆H°m: Mv: K: c: k and a:

Polypropylene Melt Flow Index Density Degree of crystallinity Melting enthalpy Enthalpy of 100% crystalline PP Molecular weight Huggins coefficient Concentration of polymeric solution Constants for the polymer-solvent system

Greek letters [ƞ]:

Intrinsic viscosity

1. Introduction The polyolefin family is one of the most commonly-used polymeric materials in manufacturing and many of the products produced can be easily Corresponding author: Ruth Marlene Campomanes Santana, Ph.D., professor, research fields: solid waste recycling, biotic and abiotic degradation, processing of thermoplastic polymer composites, functionalization of polymers to promote adhesion; development of ecological PU foams, PU adhesives base among the most important. E-mail: [email protected].

recycled. They are very convenient for a wide range of applications but there are some products that cannot be properly recycled, for example, disposable napkins [1, 2]. Polyolefins are highly stable and take a long time to degrade in the environment, where the degradation process can span many decades. Thus, if the non-recyclable polyolefin products were biodegradable and degraded in a shorter time their environmental impact could be minimized [3]. Biodegradable polyolefins have been produced by mixing them with biodegradable polymers, such as cellulose, starch, and vegetable oil, obtained from renewable resources [3], or with synthetic polymers like polyesters and polyvinyl-alcohol obtained from petrochemical resources, by incorporating special additives to the mixture, which accelerate the degradation process [4]. The pro-degrading additives promote the polymer degradation process by accelerating the polymer oxidative degradation under heat and UV. As a result of the degradation process there is a breakdown of the polymer long chains with a significant production of

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Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

oxidized low molecular weight products. After this biotic degradation process polyolefins can undergo an abiotic degradation process, which means that the polyolefins are biodegradable [5-7]. The most commonly-used additives are stearate (St) complexes of transition metals such as zinc (ZnSt), copper (CuSt), silver (AgSt), cobalt (CoSt), nickel (NiSt), manganese (MnSt), chromium (CrSt) and vanadium (VSt), or alkaline earth metals such as magnesium (MgSt) and calcium (CaSt) [8]. These ion complexes as additives possess remarkable ability to decompose the hydro peroxide produced during the polymer oxidation process [9]. The oxo-degradation process is a two-step process beginning with the oxo-degradable additive that promotes firstly abiotic (photo or thermo) oxidation, and secondly microbial biodegradation. In the initial abiotic stage involving natural weathering, heat (mainly from sunlight, solar radiation including UV-rays and ambient temperature), or artificial UV light, there is an extensive breakdown of the molecule chains producing low molecular products with oxygenated groups increasing in the biodegradation potential [6, 9, 10]. In the second stage, the biotic degradation takes place via the activity of microorganisms growing in the polymeric material. This microbial growth is dependent on the polymer constitution and properties as well as the environmental conditions (humidity, weather and atmospheric pollutants). Polypropylene, for example, is known to be sensitive only to ultraviolet and its photooxidation mechanism is well understood and predominantly influenced by temperature [11, 12]. In this context, in this study the effect of a pro-degradant additive on polypropylene degradation was evaluated under two different environmental conditions, abiotic (natural weathering) and biotic (composting). The polypropylene degradability was verified by monitoring changes in the physical, thermal and morphological proprieties in comparison with the neat PP.

2. Materials and Methods 2.1 Materials Polypropylene homopolymer (Grade: H125) (MI = 38 g/10 min; d = 0.905 g/cm3) used for the manufacturing of non-woven disposable items was kindly donated by Braskem. Benzoin (Sigma-Aldrich, purity > 98%), containing at least one 1, 2-oxo-hydroxy group free of transition metals, and potassium salt were used as a catalyst and a co-catalyst, respectively, and together comprised the pro-degradant additive. Decalin (Vetec) was used to dissolve the PP. 2.2 Polypropylene with Additive Samples of PP were extruded with the pro-degradant additive in a single-screw extruder brand Ciola (L/D = 22) at 210 °C and 45 rpm, and then pelletized. The PP pellets with additive were ground in liquid nitrogen and oven dried at 40 °C for 24 h. The PP samples with and without the additive, in plate form with 5 cm and 2 mm thickness, were obtained using a molding press at 220 °C under 2 tons for 5 min. Samples of neat PP were submitted to the same treatment and used as a reference. 2.3 Degradation Test The degradation testing of the neat and modified PP samples was performed at room temperature both in the case of the biotic environmental and outdoor weathering. The samples were removed after different time periods: 30, 60, 90 and 120 days. 2.3.1 Simulated Soil (Composting) The PP samples were covered with simulated soil consisting of an organic soil, solid organic waste and gardening material, according to the standard method ASTM D5338-98, and kept under this condition for 120 days, from July to November 2011. Samples were removed from the composting conditions after 30, 60, 90 and 120 days and washed carefully in water, dried at 40 °C for 24 hours.

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Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

2.3.2 Outdoor Weathering Test The exposure of the samples to abiotic natural weathering was conducted from July to November of 2011 on platforms built at an angle of 30° to the ground, facing the equator, in Porto Alegre, RS (Brazil), Latitude 30°05’ S; Longitude 51°11’ W, following the standard method ASTM D 1435-05. During the outdoor weathering test, the average UV radiation index, temperature and the rainfall were obtained from CPTED-INPE (Center for Weather and Climate Studies - National Institute for Space Research), and are listed in Fig. 1.

decaline (Vetec) at 135 °C for 30 minutes, under stirring,

at

four

different

concentrations (0.2, 0.4, 0.6 and 0.8 g/dL). Viscosity measurements

were

carried

out

using

an

Ubbelohde-type capillary viscometer at a temperature of 135 °C (±0.01 °C) which was controlled by a circulating silicone bath (SOLAB, model 159 SL), maintaining the viscometer immersed in the bath, and for each concentration three measurements of flow time were performed [14]. Firstly, the specific viscosity (ηsp) was obtained and

the

reduced

specific

viscosity

vs

Temperature (°C) UV radiation index Total rain (mm)

50 40 30 20

was determined from the intrinsic viscosity [n] of the

0

relationship [n] = kMva. The samples were dissolved in

for

60

10

Mark-Houwink-Sakurada

values

concentration of the polymer solutions (ƞsp/c): nsp  [ ]  k  [ ][ ] 2  c (2) c where ηsp is the specific viscosity, ηsp/c is a reduced specific viscosity, [η] is the intrinsic viscosity, k is the Huggins coefficient and c is the concentration of polymeric solution (g/dL). The intrinsic viscosity was determined from the plot of (ηsp/c) vs (c), by extrapolation of the line obtained for the linear regression when c = 0, according to the Huggins equation. The viscosimetric molecular weight average (Mv) of the samples was determined using the Mark-Houwink-Sakurada Eq. (3), which relates the

The viscosimetric average molecular weight (Ṁv) the

analyzed

equation was used, Eq. (2), corresponding to the plot of

The thermal, physical and morphological changes which took place due to biotic and abiotic degradation of the samples were monitored through the determination of the degree of crystallinity (Xc), average molecular weight (Mv) and morphology by scanning electron microscopy (SEM). 2.4.1 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry analysis of the neat and modified PP samples exposed to two different degradation conditions was carried out with a DSC-Q20 calorimeter (TA Instruments. Scans were recorded using approximately 5-6 mg of the samples at a heating and cooling rate of 10 °C min-1 in nitrogen atmosphere. To remove the thermal history, samples were first heated from 25-250 °C, then cooled to 25 °C and again heated to 250 °C. The percentage crystallinity (Xc) was calculated according to the following Eq.(1): % Crystallinity = 100  ∆Hm/∆H°m (1) where ∆Hm is the melting enthalpy, ∆H°m is the enthalpy of 100% crystalline polypropylene, that is, 209 Jg-1, which is taken as the reference [13]. 2.4.2 Molecular Weight

using

then

to estimate the intrinsic viscosity [n] the Huggins

2.4 Methods for Analysis

samples,

and

July

August

September

October

November

Exposure time (months)

Fig. 1 Climatic conditions during the weathering samples exposition from July to November of 2011.

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Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

intrinsic viscosity average to the viscosimetric molecular weight of the polymer as shown in the following Eq. (3): [η] = k Mva (3) where k and a are the constants for the polymer-solvent system, which is dependent on the polymer, solvent and temperature, according to the literature [14], where k = 11  10-5 dL·g-1 and a = 0.80 for polypropylene in a solvent system consisting of decaline at a temperature of 135 °C. 2.4.3 Scanning Electron Microscopy (SEM) The morphology of the PP samples before and after exposure to the abiotic and biotic environmental conditions for 30, 60, 90 and 120 days were examined with a JEOL, JSM-6060 scanning electron microscope operating at 10 kV. The samples were sputter-coated with gold.

3. Results and Discussion The results obtained for the study of the abiotic degradation

(natural

weathering)

and

biotic

degradation (composting) of polypropylene (PP) samples (neat and modified with an organic additive) for 120 days are reported herein. The degradation of the samples was analyzed based on the rheological, thermal and morphological properties. One method of evaluating the degradation of PP is to assess the chain scission and investigate the decrease in molecular weight by the viscometer method, that indicate the level of degradation of PP, which can be influenced by the environment degradation The variation in viscosity is shown in Table 1, and Fig. 2 shows the values for the average viscosimetric molecular weight of the samples before and after natural weathering and composting. The reduction in the average viscosimetric molecular weight of the samples indicated that there was a reduction in the molecular chain size and the chain scission, indicating degradation of PP. In Table 1, decrease in the viscosity values can be observed as a function of the exposure time, this being

Table 1 Intrinsic viscosity of the samples before and after exposed in soil and natural weathering during 120 days. Exposure time (days) 0 30 60 Neat PP 90 120 0 30 PP/Modified 60 90 120 Sample

Simulated Soil Outdoor weathering

[n] (dL/g) 1.110 1.101 1.116 1.099 1.053 1.082 0.835 0.835 0.825 0.777

[n] (dL/g) 1.110 1.082 1.079 1.068 1.052 1.082 0.824 0.817 0.807 0.655

more evident for the modified PP samples. It was noted that the natural weathering affected the decrease in the viscosity more than the degradation in soil, which indicates that the variables involved in weathering (time, solar radiation, acid rain, wind, and other factors) influenced the decrease in the intrinsic viscosity, which is directly proportional to the molecular weight. The neat PP sample showed a reduction in molecular weight over 120 days for both degradation environments (Fig. 2). Regarding the Mv of the modified PP samples, there was a marked decrease with increasing time of exposure to the abiotic and biotic degradation. It has been previously reported that during the degradation processes (abiotic and biotic), in order to reach a significant degradation in a reasonable time period, the average molecular weight of the oxidized polyolefin should be less than 5,000 g/mol for samples exposed to natural weathering and around 6,500 g/mol for samples degraded in soil. The Mv results also indicate that the processes which occur during composting and natural weathering caused a reduction in the molecules of high molecular weight, leading to a greater number of chains with smaller molecular weight, which may be associated with chain cleavage and an increase of the total number of molecules in the polymer [12]. The samples after natural weathering showed a greater reduction in the Mv compared to samples after

127

Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

120

120 Neat PP

PP modified

Neat PP

PP modified

100

80

Mv x 10³ (g/mol)

Mv x 10³ (g/mol)

100

60 40 20 0 0

30

60

90

120

Exposure time in natural weathering (days)

80 60 40 20 0 0

30

60

90

120

Exposure time in simulated soil (days)

(a) (b) Fig. 2 Average viscosimetric molecular weight of the neat PP and PP samples after been exposed to the (a) natural weathering and (b) simulated soil.

composting, this being more evident in the case of modified PP samples. The same behavior was observed in a study reported by Brandalise [15] in which the biotic (composting system) and abiotic (UVB radiation and condensation) degradation processes were examined for samples of post consumer high density polyethylene (HDPEr) containing poly(vinyl alcohol), using maleic anhydride and dicumyl peroxide as the compatibilizing agents, which showed a greater reduction in the Mv in samples exposed to UV radiation. Polypropylene absorbs UV radiation at wavelengths of above ~250 nm, for instance, solar radiation, which reaches the surface a wavelength above 290 nm, and the initiation of the photodegradation of polymers is attributed to the absorption of UV chromosphores. This is in agreement with the explanation offered by Rabello and White [16] regarding the reason why, under the action of ultraviolet radiation, polymeric materials undergo a series of oxidative chemical reactions that can cause premature failure. This degradation process, which basically involves the absorption of ultraviolet radiation and subsequent oxidative reactions, can cause a reduction in molecular weight and change the chemical structure of the polymer. In the case of PP samples exposed to natural weathering, it is probable that this absorbs ultraviolet radiation which causes a homolytic splitting, and the initiation of

photodegradation takes place in impurities (called chromosphores) such as the catalyst residues and hydroperoxides generated during processing, or even the pro-degradant additive used for the modification of the PP samples. Thus, reductions in the Mv of the samples of PP can be modified by the presence of the pro-degradant [16]. However, the Mv values of the samples after the degradation process in the composting system showed a lower reduction in molecular weight when compared to natural weathering process, indicating the effectiveness of UV radiation in the modification of the physical structure of the polymer when compared with biotic degradation. The kinetics of photodegradation (temperature, pH and humidity) involves the interaction between the peroxide macro-radical and the hydrogen atom of the polymer chain, which is a type of charge transfer. The complex formed absorbs UV photons producing the corresponding excited state. In the excited state simultaneous rupture of the carbon-hydrogen bonds and carbon-oxygen and hydrogen-oxygen bond formation occurs [17]. The result of this reaction is the formation of two alkyl macro-radicals and a hydroperoxy radical, where the latter can recombine with one of the macro-radicals, the result being similar to that of a classical hydrogen abstraction reaction or, due to its greater mobility, the

128

Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

hydroperoxy radical may escape from the cell, allowing the recombination of the two macro-radicals formed after one or both have become peroxide macro-radicals, depending on the amount of molecular oxygen available in the middle [18]. In the case of polypropylene, the initial peroxide macro-radical will be mainly tertiary, however, the second macro-radical photoreaction may be tertiary, secondary or primary. Since the photoreaction is not selective the nature of the second radical is determined statistically by the number of hydrogen atoms attached to tertiary, secondary and primary carbons available [19]. Fig. 3 shows the degree of crystallinity of the samples with and without modification, comparing the results before and after exposing the samples to abiotic and biotic conditions for different periods (0, 30, 60, 90 and 120 days). On analyzing the results for the neat and modified PP samples before degradation, it can be observed the crystallinity values for the modified samples are slightly lower than those of the unmodified samples (the difference being within the margin of error). This may be due to the presence of the pro-degradant additive in the modified samples, which influences the increase in the free volume, reducing the crystallinity, due to the insertion of the side and/or functional groups [20].

The results for the samples exposed to natural weathering and soil composting show an increasing in the Xc value. However for the modified samples exposed to the natural abiotic environment, the increase in crystallinity is more pronounced, in comparison with the other samples, this being more evident in the first 30 days. The degradation rate is influenced by changes in the degree of crystallinity and other morphological aspects of the samples [20]. This phenomenon is well known and is called chemi-crystallization, which involves an amorphous arrangement of molecules having a certain limit, and after a certain period the crystallization tends to decrease due to high levels of degradation [21]. The samples subjected to natural weathering show an increase in the Xc value, because ultraviolet radiation breaks chemical bonds with the presence of stressed segments and entangled or knotted molecules, in which mechanical stresses can indicate accelerated photodegradation of polymers, due to a reduction in the energy barrier to the chemical reaction or a lower rate of free radical recombination. According Popov et al. [22] the localized stresses may have effects similar to those of external stresses in terms of the reactivity of the stressed semi-crystalline PP bonds and thus entangled and knotted molecules would be preferentially attacked. The increase in crystal defects occurring with 60

60

Neat PP

PP modified

50

50

40

40

Xc (%)

Xc (%)

Neat PP

30

PP modified

30

20

20

10

10 0

0 0

30

60

90

120

Exposed time in natural weathering (days)

0

30

60

90

120

Exposed time in simulated in soil (days)

(a) (b) Fig. 3 Degree of crystallinity of PP and PP/modified samples after been exposed to the (a) natural weathering and (b) simulated soil.

Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

oxidative degradation, such as oxygenated groups, double bonds, chain ends and branched sites, results in smaller crystals with more imperfections [23]. However, the increase in crystallinity during degradation is usually explained by the dominance of the UV degradation mechanisms in the amorphous phase of the PP and an analogous preference for biodegradation in the amorphous phase when the material is buried in soil. It is known that polymer degradation usually proceeds in a selective manner, with the amorphous regions being preferentially degraded first as compared to crystalline ones [23]. Another factor that may explain the increase in crystallinity is the packing of chains of smaller size and decreasing molecular size [10]. This behavior was observed in the samples of modified PP exposed to natural weathering for 120 days, which showed an increase in the Xc value and consequently a decrease in the molar weight (Table 1). From the results of the morphological analysis performed by scanning electron microscopy it was possible to evaluate the influence of the environmental conditions studied on the modification of the PP surface. Fig. 4 shows micrographs of the neat and modified PP samples at a magnification of 1,000, before and after 30, 60, 90 and 120 days of abiotic or biotic degradation. Changes in the sample surface can be observed for both types of degradation (abiotic and biotic) and for all the periods analyzed. The micrographs of the samples after the longest exposure time (120 days) showed a greater surface roughness and surface erosion as compared to the samples before exposure and after the other exposure periods. Fig. 4 shows the images of the neat modified PP samples before and after composting and natural weathering for 30, 60, 90 and 120 days. For the sample surface of the PP modified with the pro-degradant additive there was a significant change in the surface compared with the neat PP sample before degradation. The samples after exposure showed a region

129

extensively eroded by oxidation in the samples subjected to degradation by natural weathering and several fragments were present on the surface of the sample due to the effect of oxidation, possibly due to UV light, acid rain, high precipitation or high pH, resulting in high pressure and surface attrition. Wind and particulates can also increase the roughness of the samples, which was more evident for the samples after 120 days of exposure to natural aging [19]. In the samples subjected to degradation in soil (Fig. 4), the deterioration of the sample surface can be observed with patterns and ripples over the entire length. The surface after 120 days of exposure to soil showed greater wear than the samples exposed for 30, 60 and 90 days and samples which were not exposed. The SEM analysis revealed the same surface behavior during biodegradation as that observed by Luckachan et al. [23]. These authors studied the degradation in soil of samples of low density polyethylene grafted with polysaccharide (Su-g-LDPE), for four to twelve months, and observed surface erosion with increasing degradation time, which is associated with microbial degradation.

4. Conclusions The results of this study showed the influence of an organic pro-degradant on PP under the two different environmental conditions (abiotic and biotic), investigated through the degree of crystallinity, average molecular weight and changes in the morphological surface. It was verified that samples of modified PP subjected to abiotic and biotic degradation showed a decrease in molecular weight and increase in the degree of crystallinity, these being more evident in samples after degradation in the natural environment for 120 days. Also, changes in the surface morphology were observed with increasing exposure time, this being more pronounced after exposure to natural weathering. The use of an 1, 2-oxo-hydroxy additive seems to be effective in enhancing the PP degradation when exposed to natural weathering and soil conditions.

130

Induced Degradation of Polypropylene with an Organic Pro-degradant Additive

Natural weathering PP

Simulated soil

PP modificated

PP

0 day

30 days

PP modificated 0 day

30 days

0 day

30 days

60 days

60 days

60 days

90days

90 days

90 days

90 days

120 days

120 days

120 days

120 days

Fig. 4 SEM image of neat PP and PP samples before and after been exposed to the natural weathering and to the simulated soil for 30, 60, 90 and 120 days.

Acknowledgments

[2]

The authors are grateful to CNPQ and FAPERGS for the financial support, to Braskem for providing the polymeric material and to CME for the morphological analysis. [3]

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