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Effect of UV radiation and pro-oxidant on PP biodegradability.


Over the years, the volume of domestic and industrial waste generated by modern society has increased exponentially, leading to serious environmental problems, in particular, because the number of available space (land-fills) is limited in the vicinity areas of large cities and industries (1), (2). These environmental problems are even more serious when the waste is hazardous, such as hospital wastes. Plastic materials represent the largest volume of the solid waste due to their relative low density. Plastic wastes can stay undegraded for more than 50 years and solutions to either decrease the volume of plastic waste or to a faster degradation of these materials have been suggested for the last decade.

A possible solution to decrease the volume of plastic waste could be the use of biopolymers. Biopolymers can serve as nutrients for macro/microorganisms and/or enzymes, decreasing substantially the time for the degradation of the polymer in landfills. Biopolymers have an additional advantage over petrochemical polymers of being produced from renewable sources. Unfortunately, the engineering properties of the present biopolymers use still inferior to the common petrochemical polymers, such as poly(propylene) (PP) and PE. Polymers to which additives to enhance photodegradation (prooxidant additives) and further degradation have been developed by different groups (1), (2). Those polymers could be an interesting solution for the problem of decomposition of polyethylene films. Prooxidant additives accelerate photo-and thermo-oxidation and consequently, polymer chain cleavage, rendering the product more susceptible to biodegradation. Hakkarainen and Albertsson (3) enhanced the degradability of polyethylene by blending it with biodegradable additives or photo-initiators or even by copolymerization.

Pro-oxidant additives act as initiators for the oxidation of the polymers leading to a decrease of molecular weight and formation of hydrophilic groups. The molecular weight reduction and oxidative products are then responsible for a possible polymer biodegradation, the carbon-chain backbones being nutrients for the microorganisms once the molecular weight of the polymer is less than ~1000 (4). Hydrocarbon chains are very resistant to biodegradation as the enzymes are not able to break the C-C bonds (5). Therefore, the initial step for biodegradation of many inert polymers depends on a photo-oxidation of those polymers. During this photo-oxidation, hydroperoxides are incorporated through Norrish type I and II mechanisms which generate carboxylic groups within the polymer. Then the microorganisms attack the carboxylic parts of the polymer, releasing two carbon chain fragments that can be further used in either the anabolic or the catabolic cycle during biodegradation.

The only studies in the literature, which evaluated the effect of UV radiation on biodegradability of polymers to which pro-oxidant additives have been added, reported results using polyethylene (6-11). To our knowledge, the effect of UV radiation on biodegradability of PP with pro-oxidant had never been studies before and the aim of the present work was to evaluate if a pre-exposition to UV radiation affects the biodegradation of Pp to which pro-oxidant additives have been added.

PP samples with and without pro-oxidant additive were obtained by extrusion. The specimens were exposed to ultraviolet radiation in the laboratory for periods of up to 480 h. After selected exposure intervals, the extent of photo-chemical degradation was examined using differential scanning calorimetry (DSC), Fourier transformed infrared spectroscopy (FTIR), and size exclusion chromatography (SEC). After UV exposure the samples were submitted to biodegradation being buried in soil compost. After was accessed and the samples were analyzed by DSC, FTIR, and optical microscopy.



Two types of PP one with (PPOx) and one without (PP) pro-oxidant in a concentration of 3% wt/wt were used in this work. The pro-oxidant consists of calcium stearate ([CaS[t.sub.2]]) and [C[o.sub.3]] [O.sub.4] (3, 8%), which is further dispersed in PP matrix. According to the manufacturer, the samples did not contain UV stabilizers. Ribbons of both types of PP were obtained by extrusion and cut in `15 X 15X 1 [m[m.sup.3] plates.

Photo-Oxidation Conditions

The exposure to ultraviolet radiation was done in a Luzchem LZC-ICH2 chamber equipped with a set of medium pressure mercury lamps with maximum intensity of 14.2 W/[m.sup.2] at 365 nm. The exposure was done continuously (i.e., without a dark interval) with air circulating and temperature kept at 50[degrees]C. The specimens were taken out of the camber after 120, 288, 384, and 480 h. The irradiation dose was obtained by a spectroadiometer. The emission spectrum of the lamps is showed in Fig. 1.



The chemical changes caused by photo-oxidation process were investigated by FTIR using a Nicolet Magna IR-560 equipment with a resolution of 2 [cm.sup.-1]. FTIR analyses were also carried out by transmission, giving an indication of the total change throughout the film cross-section (bulk properties). DSC studies were performed using a Shimadzu DSC-50. Approximately 5 mg from the bulk of each sample were used for this experiment. The experiments were carried out in a nitrogen atmosphere. The thermograms were obtained using the following thermal cycle. The sample was heated at a heating rate of 10[degrees]C [min.sup.-1], then cooled at a rate of 10[degrees]C [min.sup.-1] and finally reheated at a rate of 10[degrees]C [min.sup.-1]. The crystallinity was calculated using the following equation.

Crystallinity = [DELTA][H.sub.m]/[DELTA][H.sub.m.sup.0] x 100% (1)

where [DELTA][H.sub.m] and [DELTA][H.sub.m].sup.0] are the melt enthalpy of the sample and of 100% crystalline PP (209 J/g), respectively.

The measurement of biodegradability was done in soil compost, which consisted of 23% loamy silt, 23% organic matter (cow manure), 23% sand, and 31% distilled water (wt/wt). Calcium hydroxide was added to the compost to provide a pH of 11.0. Samples were buried, in triplicate, in that soil compost at room temperature (25[degrees]C). The mass of the samples was measured every 7 days for 8 weeks to evaluate biodegradation. After various times of exposure the samples were analyzed by SEC. SEC analyses were conducted using a Waters chromatographer (model GPCV 2000) equipped with a series of Toso-Hass columns at 140[degrees]C and a refraction index detector. Specimens for SEC were dissolved in 1, 2, 4 tricholorobenzene and the filtered solutions were injected into the equipment was callibrated using narrow molecular weight polystyrene.

The surfaces of the molded exposed to UV radiation and biodegraded samples were observed using an Olympus B201 optical transmission microscope.


After UV Radiation

The crystallinity and melting temperatures ([T.sub.m]) of PP and PPOx samples before and after exposure to UV radiation were reported before (12). It was shown that an increase of radiation dose leads to a decrease of [T.sub.m] and increase of crystallinity for both samples, PP and PPOx. However, the effect of UV radiation was stronger for PPOx than or PP. These results are not surprising, as the pro-oxidant additive acts as an initiator for photo-oxidation of the hydrocarbon polymer chains. The decrease of melting temperature and increase of crystallinity after UV exposure can be easily explained: photodegradation occurs mainly in the amorphous phase, leading to an increase in the surface free energy of the crystals which in turns results in a reduction of melting temperature (13) Also, the chemical reactions that occur during PP photo-degradation lead to chain scissions and carbonyl group formation. These oxidation reactions occur predominantly in the noncrystalline phase and produce small chain segments that crystallize into the pre-existing crystal increasing the crystallinity by "chemi-crystallization" (14).

FTIR spectrums of carbonyl and hydroperoxide regions of PP samples and PPOx are presented in Figs. 2-5. It can be seen that for both samples carbonyl and hydroperoxide absorbances increase with increasing time of UV exposure. As expected, a greater increase of these absorbances can be observed for PPOx samples. For both samples there is a dominating band at 1712 [cm.sup.-1] specific of terminal ketones, which can be explained by chain scission as confirmed by SEC analysis as shown below.





Figures 6 and 7 show the SEC curves for PP and PPOx samples exposed to UV radiation for different durations. It can be seen that the molar mass of PP and PPOx decreased with increasing time of exposure. After 480 h, the absolute value of molecular weight of PPOx reached values between 300 and 100,000 and the one of PP reached values between 1800 and 180,000. Also, after 480 h, the molecular weight curve of PPOx sample changed from monomodal to tri or tetramodal whereas the molecular weight curve of PP kept its shape. The multimodal molar mass distribution curves obtained for PPOx samples after UV exposure can be explained by a possible heterogeneous photodegradation of the samples. During the injection of the samples the additive already incorporated within the polymer may have migrated to the amorphous regions of the sample during cooling and lead to a preferential photodegradation of these regions. This heterogeneous photodegradation process could lead to molecular scission only in the amorphous region resulting in multimodal SEC curves.



After Biodegradation

The cumulative weight loss of PP and PPOx samples measured during 56 days is reported in Fig. 8. No substantial weight loss can be observed for either type of PP. Only the samples exposed for 480 h and buried for 56 days presented a weight loss greater than 5%. Also the exposure to UV did not seem to affect the weight loss. The results seem to indicate that the degradation induced by photodegradation (lower-molar weight and greater amount of carbonyl and hydroperoxide groups) was not enough to turn the samples biodegradable. These results indicate that the molecular weight of PP has to drop to values below 1000 to turn the polymer biodegradable differently to what was observed for PE (1), (15). However, as shown next (Table 1 and Fig. 9) the biodegradation process led to damages into crystalline phase and surface of both samples.
TABLE 1. Crystallinity and [T.sub.m] of samples after 56 days in soil

 Exposure Radiation [T.sub.m]
Sample time (h) dose (J) ([degrees]C) Crystallinity (%)

PP 0.0 0.0 156.2 28.3
 120 1357.2 -- --
 288 3305.2 158.4 35.9
 384 4426.1 -- --
 480 5532.6 153.7 40.6

PPOx 0.0 0.0 161.8 28.7
 120 1357.2 -- --
 288 3305.2 151.6 31.3
 384 4426.1 -- --
 480 5532.6 150.4 35.4



Table 1 shows the crystallinity and melting temperature ([T.sub.m]) of PP and PPOx samples after 56 days in soil compost. Previously reported data (12) and the ones presented Table 1 show that the melting temperature and crystallinity of PP samples did not change significantly after 56 days in soil compost. The same behavior was observed for PPOx samples that were not exposed to UV radiation. However, the PPOx samples which were exposed to 288 and 480 h show a significant reduction of melting temperature and crystallinity. As shown above when analyzing the FTIR results, PPOx degraded samples (288 and 480 h) contain, in their structure, more functional groups than any PP degraded sample. Probably, these groups turned the PPOx molecules accessible to be attacked by microorganisms.

Carbonyl and hydroxyl index of PP and PPOx samples exposed to different durations of UV radiation before and after immersion in soil compost are presented in Tables 2 and 3, respectively. These indexes were calculated as follows: (16).
TABLE 2. Carbonyl Index of samples after UV radiation and after 56 days
in soil compost.

Sample Exposure time (h) Before biodegradation After biodegradation

PP 0.0 0.44 0.43
 120 0.46 0.42
 288 0.57 0.55
 384 0.80 0.71
 480 1.00 -- (a)

PPOx 0.0 0.44 0.55
 120 0.80 1.02
 288 1.20 1.06
 384 1.42 1.09
 480 -- (a) -- (a)

(a) It wasn't possible because the sample is completely

TABLE 3. Hydroxyl index of samples after UV radiation and after 56 days
in soil compost.

Sample Exposure time (h) Before biodegradation After biodegradation

PP 0.0 0.51 0.50
 120 0.50 0.50
 288 0.55 0.56
 384 0.63 0.62
 480 0.73 -- (a)

PPOx 0.0 0.47 0.55
 120 0.62 1.33
 288 0.72 1.38
 384 0.81 1.35
 480 -- (a) -- (a)

(a) It wasn't possible because the sample is completely fractured.

Carbonyl index = Abs 1712 [cm.sup.-1]/Abs 2720 [cm.sup.-1] (2)

Hydroxyl index = Abs 3450 [cm.sup.-1]/Abs 2720 [cm.sup.-1] (3)

The absorbance at 2720 [cm.sup.-1] was used to normalize the data for sample thickness.

It can be seen from Tables 2 and 3 that the chemical structure of PP samples was not affected significantly during the immersion in soil compost. In the case of PPOx samples exposed to UV radiation for durations longer than 100 h, the immersion in soil compost resulted in a decrease of carbonyl index and increase of hydroxyl index.

The results indicate that the increase of carbonyl and hydroxyl groups undergone by both PP and PPOx samples during exposure to UV (for duration up to 388 h) was not large enough to turn the samples biodegradable. However the results presented Fig. 8 indicate that PP and PPOx samples exposed to UV radiation for 480 h underwent some biodegradation. These results indicate that a decrease of molecular weight and not only a change of chemical structure are necessary for the polymer to become biodegradable.

Figure 9 shows the pictures of the PP and PPOx samples after 56 days in soil compost. It can be seen that 480 h of previous UV exposure are necessary for PP sample to become bio-fragmentable whereas only 288 h of previous UV exposure are necessary for PPOx.

All results about photodegradation and biodegradation from both samples, PP and PPOx, were used to build a scheme (Scheme 1) that tries to explain the similarities and differences between the mechanisms of degradation for types of PP. The first part of Scheme 1 shows the photodegradation process for the two samples (17), (18). PP does not absorb UV radiation near 365 nm as it only contains C--H bonds in its structure. Therefore, the initiation step of PP photodegradation is due to light absorption that produces radicals in the presence of air (oxygen). These radical produce hydroperoxides, and the decomposition of these hydroperoxides yields alkoxy radicals responsible for chain scission and formation of a variety of carbonyl products. In the case of sample containing prooxidant additive other chemical reactions accelerate the photo-oxidation process. One of these reactions is due to cobalt, which is a catalyzer for the decomposition of hydroperoxides reactions (18). The other one is due to carbonyl groups present in the CaS[t.sub.2] structure. The carbonyl groups generated by the photo-oxidation absorb light and produce excited states. The most reactive state is the triplet state (T*) in which carbonyl groups exist as a biradical. The biradicals could abstract hydrogen from a polymer molecule (RH) and formation of a kelyl radical (R*) (18). These two new routes for PP photo-oxidation lead to acceleration of the degradation of the samples. Indeed it was shown experimentally, in the present and former work (12), that the quantity of chain scission (n), carbonyl (carb), and hydruxyl (hyd) groups for PPOx samples were higher than for PP.

Samples After 56 days in soil compost without previously UV exposure did not show any level of fragmentation, while samples previously exposed to UV radiation for time [greater than or equal to] 288 h (PPOx) and time [greater than or equal to] 480 h (PP) showed level of fragmentation. The weight loss only was observed in both samples that were exposed to UV radiation for 480 h.



In this work, the influence of previous exposure to UV radiation on the biodegradability of PP samples with (PPOx) and without (PP) pro-oxidants were studied. Weight loss undergone by the samples when buried in soil compost was used to assess the biodegradability of PP and PPOx samples. After 56 days in soil compost, samples previously exposed to UV radiation showed levels of fragmentation, but only samples exposed for 480 h showed slight level of weight loss. These results indicate that the drop of molar mass and chemical modification undergone during the pholodegradation process for both types of samples were not large enough to turn the polymer biodegradable. The results presented in this work indicate that the biodegradability of synthetic polymers to which prooxidants are added depends strongly on the degradation process before degradation.


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Correspondence to: G.J.M. Fechine; e-mail:

Contract grant sponsors: FAPESP, The State of Sao Paulo Research Funding Agency.

DOI 10.1002/pen.21230

Published online in Wiley InterScience (

G.J.M.Fechine, (1) D.S.Rosa, (2) M.E.Rezende, (2) N.R.Demarquette (1)

(1) Metallurgical and Material Engineering Department, University of Sao Paulo, Cidade Universitaria, 05508-900 Sao Paulo, SP, Brazil

(2) Universidade Sao Francisco, Laboratorio de Polimeros Biodegradaveis e Solucoes Ambientais, Rua Alexandre Rodrigues Barbosa, [] 45, Centro, 13251-900, Itatiba, SP, Brazil
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Article Details
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Title Annotation:ultraviolet radiation, polypropylene
Author:Fechine, G.J.M.; Rosa, D.S.; Rezende, M.E.; Demarquette, N.R.
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:3BRAZ
Date:Jan 1, 2009
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