Thermocatalytic degradation of low density polyethylene films by responding to the actuation of heat

Abstract

A novel thermal catalyst in response to heat excitation was first synthesized and then added into low density polyethylene (LDPE) matrix to prepare corresponding films. The thermocatalytic degradation of LDPE/thermal catalyst (LEPDET) film was investigated under artificial ageing conditions. The changes in the films were evaluated by measuring the surface morphology, average molecular weight, crystallization capacity, chemical group, contact angle, and mechanical property. The results obtained show that the surface of as-prepared film was seriously destroyed because of the thermocatalytic reactions at 40 °C in darkness. A rapid decreasing tendency of the average molecular weight was clearly monitored, simultaneously, the crystallization capacity decreased significantly. The presence of carbonyl groups was observed during degradation process, and the above-mentioned groups were also detected mediately in contact angle view. Additionally, on the basis of the above-mentioned evidences, a possible mechanism for the thermoctalytic degradation was also established in detail. Thus, the effectiveness of thermocatalytic route has been successfully demonstrated by heat excitation at near room-temperature. It is hoped that our study may develop an alternative method for the disposal of polymeric film in the future.

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1. Introduction

Polyolefins are broadly used in daily life because of their low cost, good mechanical properties, light weight and durability. Especially, polyethylene (PE) is frequently used for agricultural films such as greenhouse, mulching, and tunnel film.¹ Polyethylene polymer is known for its remarkabe resistant to degradation. Its chemical and biological inertness has also fostered its application in various products from plastic bags and piping, to the construction of fuel storage tanks.² Therefore, considering several reasons, including raw material cost and availability, and outstanding properties, has incited the widespread use of these materials.³

However, the intense use of polymeric materials has created disposal problems.^4–6 Traditional disposal methods include recycling, incineration and burying in landfills, but every method has its limitations and disadvantages.⁷ It is well-known that polyolefins are bio-inert, that is, they are highly resistant to assimilation by microorganism such as fungi, bacteria. The surfaces of the materials made from polyolefins are hydrophobic, which inhibit the growth of microorganism on them. Therefore, there are very few molecules ends accessible on or near the surfaces of the materials made from these resins.⁸

Any polymeric material degrades gradually during the period of its application under an ambient or specific environment. In the presence of oxygen, the degradation should proceed with the oxidation of the material, which is caused by various factors such as thermal ageing, ultraviolet and ionizing radiation exposure.⁹ On the basis of the above-discussion, the photocatalytic degradation of the plastic films is one of the most available routes. It is reported in many literature that polyethylene could be degraded indeed by photocatalysts.^10,11 However, there are still several practical shortcomings that remain unanswered because of the lack of truly effective ultraviolet (UV) exposure; therefore, photodegradation cannot be completely applied to crack the polymeric films by this catalytic degradation.

As mentioned in our previous study, the abatement of organic pollutant was verified by thermocatalytic route,¹² in this study we were devoted to construct a similar method to degrade LDPE-based film, and then we demonstrated the feasibility of thermocatalytic degradation in the decomposition of the polymeric materials. In this regard, the thermocatalytic degradation of LDPE-based film was investigated by artificial ageing treatment for a particular time. In order to strongly confirm the degradation triggered by heat excitation, we performed various tests, including surface morphology observation, average molecular weight, crystallization capacity, chemical group, contact angle, and mechanical property. To the best of our knowledge, to date, no information on the thermocatalytic degradation of LDPE-based film at near room-temperature is reported in opening literatures. In addition, it is noted that the abovementioned polymeric film could be widely used in agricultural environmentally friendly mulching films, and simultaneously it may also be applied for the disposal of other categorized films after their use life. Hence, we believe that the findings in this work will considerably enrich our fundamental understanding of degradable polymeric film via thermocatalytic route and also bring some new insights into their corresponding applications.

2. Experimental

2.1. Materials

Low density polyethylene (LDPE) was purchased from Sinopec Group. Mn₃O₄ (AR), CuO (AR), NiO (AR), and RuO₂ (AR) were all purchased from Aladdin Industries, Inc.

2.2. Catalyst preparation

The gathering of SiO₂/15–20 wt%, CaO/15–20 wt%, Mn₃O₄/25–45 wt%, CuO/10–25 wt%, and NiO/0–15 wt% was viewed as original system, and then the system was physically mixed for a moderate time. Subsequently, it was placed in muffle furnace, and to increase the temperature to 950 °C by program, it was maintained at 1050 °C for 2 h. After that, the above-obtained mixture was adjusted with RuO₂/2–4%. Detailed results are presented in Table 1. Considering the catalyst composition chosen, elements selected could be excited solely or with each other using heat stimulus. Namely, the activation energy E_a is comparatively low because these elements are oxides, which are intrinsically susceptible to heat excitation. It could be easy to stimulate or to produce certain electronic layers, such as holes, electrons, and other active free radicals, which are the active species for the degradation of polymeric films. Another equally important aspect is that the as-synthesized catalyst acts as semiconductor in nature, which has a special function in responding to heat excitation at near room-temperature.

Table 1 Data obtained by XRF analysis

SiO2/%	CaO/%	MnO/%	NiO/%	CuO/%	RuO2
15.44	15.53	36.32	13.32	14.89	2.26

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2.3. Film preparation

A blend of neat LDPE and thermal catalyst was prepared and then blown into films using a single-screw extruder (HAAKE Polylab OS 600, Ø 30, L/D 25). Wherein the mixing condition was 10 rpm at barrel temperatures ranging from 180 to 195 °C. In order to achieve better loading and the desired dispersion of catalysts (containing 4 wt% of thermal catalyst, and named as LDPET) in neat LDPE matrix. The abovementioned blending system should be further blended using a high speed mixer for an appropriate duration.

2.4. Toxicity analysis

Our study aims at preparing environmentally friendly mulching film in response to heat excitation, as stated in the introduction, which is applied in practical soil fields. In light of the above applications, the toxicity of the as-synthesized catalyst need be discussed, and its corresponding interpretation is described as follows: first, the content of the catalyst added to the prepared film is very few; second, in general, only heavy metal ions can be toxic. However, the as-synthesized catalyst here exist in the form of oxides. Finally, the heavy metal derived from the as-synthesized catalyst itself also existed in the soil, which would be absorbed by various plants. Thus, according to the abovementioned reasons, the polymeric films with the as-synthesized catalyst should be harmless to other creatures in nature.

2.5. Artificial ageing tests

The thermocatalytic degradation experiments were performed at 40 °C in absence of light for 0, 10, 20, and 30 days in an artificial ageing box. The corresponding tests regarding degradation properties, such as surface morphology, roughness value, crystallization, chemical group, contact angle, and mechanical test, were implemented every 10 days or other intervals under complete degradation process.

2.6. Mechanical tests

Type V dumb-bell shape specimens were cut using a CEAST die cutting machine according to ASTM D638-10. The speed of testing was 100 mm min⁻¹. The tensile properties were evaluated in the blown direction before and after the degradation process. Five specimens were tested for each sample type to gain an average value.

2.7. Characterizations

The detected elements of the catalyst were obtained by an X-ray fluorescence spectrometer (XRF, Rigaku Primini, Japan) equipped with a 50 W end window, which was used with Rh anode X-ray tube, operated at a maximum voltage of 40 kV and current of 1.25 mA, control temperature of 36.5 ± 0.5 °C, and gas flow rate of 5–7 ml min⁻¹. A small amount of catalyst powder was dispersed in ethanol to form a suspension, and then a drop of this was subsequently dropped onto a carbon-coated Cu grid for specimen research using transmission electron microscopy (TEM, Zeiss Libra 200 FE) with an accelerating voltage of 200 kV. The surface morphologies of the specimens were characterized using scanning electron microscopy (SEM, Ultra 55, Zeiss Corporation, Germany), and the above samples were coated with gold by sputtering for 5 min before the tests and the voltage was 15.00 kV in SEM. The surface morphologies of the corresponding films were observed using an atomic force microscopy (AFM, SPA300HV, Seiko Instruments, Japan), operating in tapping mode under air atmosphere, and the standard silicon probe (Spring constant: 3 N m⁻¹. Tip geometry: circular cone, the nominal radius of 50 nm) was used for tapping mode imaging (frequency: 65 Hz. Scan amplitude: 1 V. Scan speed: 1 Hz). The surface roughness of the samples was estimated from the region chosen per sample, with a specific scan size for the general agreement of interval tests (for example in Fig. 1). The average molecular weights and other parameters (Triple detector, Eluent: TCB stabilized with 0.0125% BHT) were examined by a high temperature gel permeation chromatograph (HT-GPC, Model PL-GPC 220, UK), and the GPCV calibration method was adopted using the prototype of the narrow distribution of polystyrene. Differential scanning calorimetry (DSC, Q200, TA Instruments, USA) analyses was carried out at a heating rate of 10 °C min⁻¹ and under nitrogen atmosphere. Crystal textures were examined using polarizing microscope (PLM) (Leica 2000, Germany). Fourier transform infrared (FT-IR) spectra were obtained at 400–4000 cm⁻¹ by an infrared spectrophotometer (Nicolet 6700, Nicolet Instruments Corporation, USA). Water contact angles of the sample surfaces were measured with a Kruss DSA 30 (Kruss GmbH, Hamburg, Germany) for hydrophobic properties. The pixel error of the tensiometer is reported to be around 0.01 degrees.

Fig. 1 Schematic diagram of the sample region selected for examining various properties.

3. Results and discussion

3.1. Characterizations of as-synthesized catalyst

3.1.1. SEM analysis. Owing to the importance of catalyst morphologies, the microstructure cases of the as-synthesized catalyst were studied by SEM analysis. A large quantities of macropores with different sizes and various shapes were observed as shown in Fig. 2a, and these macropores were interconnected, existing in the form of many so-called ‘aggregates’. In order to better understand its microstructures, SEM micrograph in Fig. 2a was enlarged and it is seen that a few small particles were attached on the surfaces of the above-mentioned ‘aggregates’from Fig. 2b. All this suggested that the degradation reactions may benefit from the microstructures of the catalyst.

Fig. 2 SEM images of the as-prepared catalyst at different magnifications (a and b). TEM with SAED patterns (inset) (c) and HRTEM image (d).

3.1.2. TEM analysis. The morphologies and crystallization properties of the samples were further investigated by TEM, shown in Fig. 2c and d, and the selected area electron diffraction (SAED) is also shown in the inset of Fig. 2c. It is seen that the sample possess a highly ordered mesopores of around 20 nm in diameter. Fig. 2d represents the presence of polycrystalline particles, and the lattice fringes corresponding to the (211) (d₂₁₁ = 0.322 nm) and (110) (d₁₁₀ = 0.351 nm) crystallographic planes of cubic spinel were most frequently monitored. In addition, multiple electron diffraction rings, which are clearly seen in the SAED pattern of Fig. 2c indicated that the synthesized catalyst was polycrystalline.

3.2. Surface morphology analysis for LDPET film

3.2.1. Surface monitoring of SEM. The surface morphologies of corresponding films are shown in Fig. 3 and 4 wherein Fig. 3 is the phase images of the neat LDPE and LDPET films, respectively. It indicates that neat LDPE and the as-synthesized catalyst were blended uniformly after processing. It is apparent that the surface of the untreated film (Fig. 4a) is smooth compared to the subsequent film surfaces. Namely, the degree of roughness increased significantly with treated time and ultimately, the film transformed into several small fragments. These phenomena may mainly be attributed to the attacks from the active species, responding to the heat of excitation 40 °C. In brief, the above observations indicate the evident signs of degradation.

Fig. 3 Phase images of pure LDPE film (a) and prepared LDPET film (b).

Fig. 4 SEM images of the prepared LDPET film under artificial ageing conditions for 0 (a), 10 (b), 20 (c), and 30 (d) days, respectively.

Additionally, some possible cavities around the catalyst in the film are also observed in Fig. 4. Probably, the forming of these cavities may be induced by the escape of volatile by-products from the LDPE matrix in the early stages. Moreover, this phenomenon was responsible for a number of active points from the as-synthesized catalyst used. It elucidated that the deterioration of the LDPE matrix initiated from the interfaces of the LDPE matrix and catalyst and then resulted in the appearances of the above-mentioned cavities around the catalyst.^13–15 In other words, the degradation of the prepared film first preferred to appear in the LDPE matrix and catalyst interface rather than around catalyst aggregates.

3.2.2. Surface roughness of AFM. In addition to the abovementioned observations, Fig. 5 (3-D view) shows that the surface of the untreated film (0 days) was relatively smooth, however, as expected, the huge changes (more and more rough) were observed in the surfaces of the treated films (10, 20, 30 days treated) as thermal ageing time prolonged; these results were consistent with the surface morphologies derived from SEM analysis. Hence, the increase in surface roughness with increase in time is not strikingly due to the degraded interaction. In fact, the R_a values of LDPET changed from 3.8, 12.1, 30.2, and 39.3 nm (Table 2) respectively, under complete process. It is found that the average roughness of the LDPET film increased significantly. Besides, Fig. 5 (2-D view), reflects a clear view of the increasing roughness of the polymeric film during the degradation process. In general, the lightness of the 2-D AFM images was increasingly lighter according to the reading bands of information; it showed that the film surface under thermocatalytic degradation changed and gradually deteriorated. Therefore, the destructions of the film are further demonstrated under artificial ageing treatment.

Fig. 5 AFM images of the prepared LDPET film under artificial ageing conditions for 0 (A-3D, a-2D), 10 (B-3D, b-2D), 20 (C-3D, c-2D), and 30 (D-3D, d-2D) days, respectively.

Table 2 Experimental results of the roughness values of the film surfaces calculated by AFM analysis

	Ra (nm)
Days	0 d	10 d	20 d	30 d
LDPET	3.8	12.1	30.2	39.3

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3.2.3. Changes of LDPET film around room-temperature. Fig. 6a–c show that the surface changes of LDPET films were observed at different treatment times and temperatures. They reveal that the surfaces of films at 20 and 30 °C looked similar to the original surface, which illustrated that the polymeric material was not destroyed at room-temperature or near it. This is ascribed to the appropriate gap of the as-synthesized catalyst with respect to the critical value from valence band to conduction band. In this case, thermocatalytic degradation does not play an important role in destroying LDPET film. To further show the negligible degradation performance, we tested a model pollutant (methylene blue, MB) in aqueous solution at different temperatures. The results obtained exhibited that their degradation rates were unchanged even at a temperature of 20 or 30 °C in Fig. 6d and e; namely the as-synthesized catalyst almost not work at the above temperatures. In other words, the rates of degradation reactions were both rather slow in spite of degradation for polymeric film or for MB. Briefly, the polymeric film would be substantially stable around room-temperature.

Fig. 6 SEM images of the LDPET films at room-temperature (a, original, untreated), 20 °C (b, 10 days treated), and 30 °C (c, 10 days treated), respectively. Including the degradation curves of the model pollutant (methylene blue, MB) after 10 days treatment at 20 (d) and 30 °C (e), respectively. Details on degrading MB process are described as follows: 50 mg of the as-synthesized catalyst was suspended in all solutions, and this was added to a glass vessels containing 50 ml dye solution of different concentrations (including 10, 20 30, 40 and 50 mg L⁻¹), and the temperatures of the reaction processes were varied at 20 and 30 °C, respectively. Note that, the physical adsorption amounts have been removed in all above tests, and the tests were performed in darkness under air atmosphere.

3.3. M_w monitoring for LDPET film

As shown in Fig. 7, the molecular weight distributions for the tested films are plotted. It is clear that the average molecular weight and its distribution both changed before and after degradation. Generally, the tendency to the lower average molecular weight was found clearly after degradation.^16,17 This is because of the chemical reactions taking place in the degradation process. Specifically, the gradual destructions from the part of lower molecular weight to that of higher one were carried out in the catalytic degradation process. The initial average molecular weight (M_w: 120 058; M_n: 21 670) has declined substantially to 57 691 (M_w) and 12 972 (M_n) in Table 3. In conclusion, the polymeric film could be truly degraded through thermocatalytic degradation. On the other hand, some additional tests were also performed for finding the relation between M_w and the roughness value (R_a); surprisingly, it is very true that there is a clear trend between the molecular and macroscopic scale (presented as R_a). As shown in Fig. 8, the relation between M_w and R_a was close to the linear fitting in a way, such that it is predicted that the degradation property may be controlled by adjusting the component of the LDPET film in the future.

Fig. 7 Data derived from high temperature gel permeation chromatography (HT-GPC) before and after degradation.

Table 3 Data information of the average molecular weight before and after degradation

Sample	Mw	Mn	PD
LDPET(0)	120 058	21 670	4.9811
LDPET(10)	118 071	20 098	4.8701
LDPET(20)	85 621	16 728	4.9028
LDPET(30)	57 691	12 972	4.8218

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Fig. 8 The fitting relation between the average molecular weight (M_w) and roughness value (R_a) (Sampling at 0, 10, 20, and 30 days, respectively).

3.4. Crystalline and thermal properties for LDPET film

3.4.1. DSC analysis. As displayed in Table 4 and Fig. 9, in the case of LDPET(0), the onset crystallization temperature and its crystallization peak temperature both are in lower temperature region as compared to LDPET(30). The initial crystallization temperature of 119.79 °C and crystallization peak temperature of 112.33 °C for LDPET(0) changed to 127.15 °C and 121.91 °C for LDPET(30), respectively. Namely, the crystallization temperature approached the higher temperature area, which showed that the crystallization of LDPET(30) was easier than that of LDPET(0) (before degradation).¹⁸ Additionally, the striking declining trend for the heat of crystallization was noted at the same time in the cooling processes. It indicates that the crystallization regions of polymeric film were destroyed in a comparative less time, which effectively facilitated the degradation of the LDPE matrix in subsequent deterioration.

Table 4 Crystallization data using DSC measurements

Sample	Initial temperature of crystallization (°C)	Crystallization peak temperature (°C)	Heat of crystallization (J g^−1)
LDPET(0)	119.79	112.33	92.97
LDPET(10)	121.93	114.07	90.06
LDPET(20)	124.32	117.82	86.42
LDPET(30)	127.15	121.91	83.62

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Fig. 9 DSC records of the LDPET film before (0 days treated) and after degradation (30 days treated).

3.4.2. PLM analysis. The morphological evolution of the LDPET film has been characterized by texture changes in PLM, as shown in Fig. 10. There were a lot of white spots on the surface of film, which represented the corresponding crystal regions. Apparently, the gradual decreasing trend of the crystal points was found from the initial degradation to the end in a whole view. However, in Table 5, the crystal size increased from the initial value of 4.00 × 10⁻⁶, 4.94 × 10⁻⁶ mm² (10 days treated), 16.00 × 10⁻⁶ (20 days treated) and to 44.44 × 10⁻⁶ mm² (30 days treated). It showed that the crystal performance was weakened instantly, because the total crystal size decreased continuously during degradation. Interestingly, the size of single crystal regions evolved to the bigger ones compared with the previous stages. It was possibly ascribed to the collapses of the crystal structures and the combination of the crystal phase occurring more conveniently because of the action of the parallel substances. Furthermore, this was partly because it was attributed to more short chain segments, which also flexibly moved in cutting chains from degradation. In conclusion, the changes in the crystal size could be also reflected in a great measure by the results measured with GPC, as well as with the variation in film roughness, both of which referred to the length and movement of molecular chains.

Fig. 10 Representative PLM textures of LDPET film under artificial ageing conditions for 0 (a), 10 (b), 20 (c), and 30 (d) days, respectively.

Table 5 The changes in crystal size in complete degradation process

Days	0 d	10 d	20 d	30 d
Square (10^−6 × mm^2)	4.00	4.94	16.00	44.44

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3.4.3. M_w and thermal properties. The average molecular weight gained a huge drop, and it changed from 120 058, to 118 071, to 85 621, and to 57 691 in Table 3. Moreover, the initial temperature of the crystallization was correspondingly increasing from 119.79, to 121.93, to 124.32, and to 127.15 °C in Table 4. This may be because of the polymeric structures that were destroyed with the continuing thermocatalysis, such that chain segments became apparently shorter and shorter. In this regard, there is every reason to believe that M_w has a great impact on the thermal properties of the LDPET film; for instance, it reflected on the initial temperature of crystallization, and additionally, it is easier to form crystallization because of the smaller molecules.

3.5. Chemical groups analysis

Fig. 11 compares FT-IR spectra of the LDPET films before and after thermocatalytic process. The obvious peaks at 719 and 1472 cm⁻¹ were assigned to the long alkyl chain of polyethylene, which were uniform in pure neat LDPE film. While, one new peak at 1715 cm⁻¹ was clearly observed in the LDPET film after catalytic degradation, this could correspond to C=O stretching vibrations. The presence of the carbonyl groups existing in the neat LDPE matrix suggests that the oxidative reaction has taken place and the LDPE matrix easily deteriorates further.^19–23 The peak corresponding to around 3000 cm⁻¹ was seen as shown in the inset of Fig. 11, and therefore, the formation of –COOH was verified completely when combining with the peak corresponding to 1715 cm⁻¹. As for the formation of the carbonyl groups, undergoing the continuous impact of heat excitation, they could form a large number of electron/hole pairs generated in the conduction and valence bands from the catalyst, and then they could migrate to the surfaces of the particles and react with oxygen or water molecules; subsequently, a series of chemical reactions were performed in the degradation process (presented also in Fig. 14 Mechanism).

Fig. 11 FT-IR spectra of the LDPET film before (film untreated) and after (30 days film treated) degradation.

3.6. Contact angle measurements

The presence of the carbonyl groups in the degraded film surface was verified by their strong interaction with water. From the observations, in Fig. 12, the huge change in equilibrium contact angles, which were at 95.61° and 52.55° for before and after degradation in water medium, was clearly observed. In other words, because the carboxyl posed a characteristic of absorbing water molecules, indirectly, the carbonyl groups were formed indeed during the degradation process, respectively. In this case, it was in agreement with the results from FT-IR analysis. Hence, the comparative bad ability to the hydrophobicity of the degraded film appeared after degradation reactions.

Fig. 12 Water contact angles of initial film (a) and degraded film (b) and their corresponding schematic illustrations (c and d).

3.7. Mechanical tests

In Fig. 13a, it is apparent that tensile strength of the LDPET film decreased dramatically with the degradation time. The loss was comparatively slow from 0 to 10 days treatment at the initial stage, but the loss rate gradually increased with the subsequent stages; this may be because of the greater accumulation of heat energy that strongly stimulated the catalyst to form more active species in the attacking polymeric material. In Fig. 13b, even at this less time thermocatalytic level, we found a 34% decrease in strength and a 32% decrease in modulus. These above results may result from the longer chains that were broken down during the degradation, and eventually leading to substantial stress loss.

Fig. 13 The monitoring values for tensile strength of the LDPET films in the degradation process (a). Representative stress-strain curves of the LDPET films before and after degradation (b).

3.8. Degradation mechanism

As schematically illustrated in Fig. 14, when the as-synthesized catalyst is excited by accumulating heat resource, it accordingly gives rise to electrons and holes outside the orbits of the transition metal atoms, which further result in the formation of the free radicals reacted with oxides and water molecules. These free radicals have the ability to attack the C–H groups originating from the LDPE matrix and then produce polyethylene macro radicals, eventually, leading to a series of competing hydroperoxides. After that, it could decompose various oxygen-containing compounds, such as carbonyl groups, as intermediates. The carbonyl groups are excited to the singlet and triplet states, which may further break down via Norrish type I or Norrish type II reactions as shown in the scheme from Fig. 14.^24–26 Hence the LDPET film was very brittle and directly lost its mechanical strength largely during degradation. Simultaneously, it also accelerates the formation of small-size segments making them favorable for biodegradation as larger molecules cannot pass through the cell wall of the microbes.²⁷ That is, chain segments changed from the long ones to short ones after experiencing a set of thermocatalytic reactions. In brief, the proposed mechanism is in good agreement with the results obtained from previous tests, such as GPC, FT-IR, contact angle, and mechanical tests.

Fig. 14 Schematic illustration of the degradation mechanism regarding polymeric film with the catalyst under a thermocatalytic course.

4. Conclusions

In summary, LDPE-based film with the as-synthesized catalyst was investigated at artificial ageing time of 0, 10, 20, and 30 days, respectively. The samples obtained were characterized by techniques, such as SEM, AFM, GPC, DSC, PLM, contact angle, FT-IR, and mechanical test. The results indicated that the surface of the film was intensively deteriorated via thermocatalytic reactions at an artificial temperature of 40 °C; the average molecular weight and crystallization capacity both declined obviously, because of the thermal catalytic degradation. Additionally, the carboxyl groups were monitored after film degradation, in order to further make chemical groups favorable for the carboxyl, contact angle tests were performed to confirm the existence of abovementioned groups from another perspective. Moreover, the plausible mechanism was also proposed systemically and in accord with the results measured. To conclude, the degradation pattern responding to heat excitation at near room-temperature is completely practicable. Hence the development of this kind of catalytic route may lead to an environmentally-friendly disposal in regard to polymeric wastes in the future.

Acknowledgements

This work was supported by the National Key Technology R & D Program, Minister of Science and Technology, People's Republic of China (Grant no. 2007BAE42B04), and Postgraduate Innovation Fund Project by Southwest University of Science and Technology (Grant no. 13ycjj02).

References

1. M. F. Mina, S. Seema, R. Matin, M. J. Rahaman, R. B. Sarker and M. A. Gafur, et al., Preparation of degradable polypropylene by an addition of poly(ethylene oxide) microcapsule containing TiO₂, Polym. Degrad. Stab., 2009, 94, 183–188.
2. J. M. Restrepo-Flórez, A. Bassi and M. R. Thompson, Microbial degradation and deterioration of polyethylene e A review, Int. Biodeterior. Biodegrad., 2013, 88, 83–90.
3. C. K. Williams and M. A. Hillmyer, Polymers from renewable resources: a perspective for a special issue of polymer reviews, Polym. Rev., 2008, 48, 1–10.
4. G. L. Liu, S. J. Liao, D. W. Zhu, Y. M. Hua and W. B. Zhou, Innovative photocatalytic degradation of polyethylene film with boron-doped cryptomelane under UV and visible light irradiation, Chem. Eng. J., 2012, 213, 286–294.
5. J. A. Brydson, Plastic materials, Butterworth-Heinemann Press, Oxford, 7th edn, 1999.
6. B. Singh and N. Sharma, Mechanistic implications of plastic degradation, Polym. Degrad. Stab., 2008, 93, 561–584.
7. C. J. Yang, C. Q. Gong, T. Y. Peng, K. J. Deng and L. Zan, High photocatalytic degradation activity of the polyvinyl chloride (PVC)–vitamin C (VC)-TiO₂ nano-composite film, J. Hazard. Mater., 2010, 178, 152–156.
8. D. M. Wiles and G. Scott, Polyolefins with controlled environmental degradability, Polym. Degrad. Stab., 2006, 91, 1581–1592.
9. M. Sugimoto, A. Shimada, H. Kudoh, K. Tamura and T. Seguchi, Product analysis for polyethylene degradation by radiation and thermal ageing, Radiat. Phys. Chem., 2013, 82, 69–73.
10. X. Zhao, Z. W. Li, Y. Chen, L. Y. Shi and Y. F. Zhu, Solid-phase photocatalytic degradation of polyethylene plastic under UV and solar light irradiation, J. Mol. Catal. A: Chem., 2007, 268, 101–106.
11. W. J. Fa, L. Zan, C. Gong, J. C. Zhong and K. J. Deng, Solid-phase photocatalytic degradation of polystyrene with TiO₂ modified by iron (II) phthalocyanine, Appl. Catal., B, 2008, 79, 216–223.
12. X. G. Luo, S. Z. Zhang and X. Y. Lin, New insights on degradation of methylene blue using thermocatalytic reactions catalyzed by low-temperature excitation, J. Hazard. Mater., 2013, 260, 112–121.
13. N. Guermazi, K. Elleuch and H. F. Ayedi, Investigations on hygrothermal aging of thermoplastic polyurethane material, Mater. Des., 2009, 30, 2006–2010.
14. A. Boubakri, N. Haddar, K. Elleuch and Y. Bienvenu, Impact of aging conditions on mechanical properties of thermoplastic polyurethane, Mater. Des., 2010, 31, 4194–4201.
15. M. Obadal, R. Cermak, M. Raab and V. Verney, Structure evolution of α-and β-polypropylenes upon UV irradiation: A multiscale comparison, Polym. Degrad. Stab., 2005, 88, 532–539.
16. I. Jakubowicz, Evaluation of degradability of biodegradable polyethylene (PE), Polym. Degrad. Stab., 2003, 80, 39–43.
17. A. Ammala, S. Bateman, K. Dean, E. Petinakis, P. Sangwan and S. Wong, et al., An overview of degradable and biodegradable polyolefins, Prog. Polym. Sci., 2011, 36, 1015–1049.
18. M. Atiqullah, M. M. Hossain, M. S. Kamal, M. A. Al-Harthi, M. J. Khan, A. Hossaen and I. Hussain, Thermal behavior of PE-b-PMMA block copolymer: Effect of multiple heating and cooling rates versus mathematical artifact, AIChE J., 2013, 59, 200–212.
19. W. Y. Liang, Y. Luo, S. S. Song, X. M. Dong and X. Y. Yu, High photocatalytic degradation activity of polyethylene containing polyacrylamide grafted TiO₂, Polym. Degrad. Stab., 2013, 98, 1754–1761.
20. S. P. Vijayalakshmi and G. Madras, Photocatalytic degradation of poly (ethylene oxide) and polyacrylamide, J. Appl. Polym. Sci., 2006, 100, 3997–4003.
21. A. Corti, S. Muniyasamy, M. Vitali, S. H. Imam and E. Chiellini, Oxidation and biodegradation of the polyethylene films containing pro-oxidant additives: Synergistic effects of sunlight exposure, thermal aging and fungal biodegradation, Polym. Degrad. Stab., 2010, 95, 1106–1114.
22. A. L. Linsebigler, G. Q. Liu and J. T. Yates, Photocatalytic degradation of the dye sulforhodamine-B: A comparative study of different light sources, Chem. Rev., 1995, 95, 735–758.
23. P. Chen, L. Y. Zhu, S. H. Fang, C. Y. Wang and G. Q. Shan, Photocatalytic degradation efficiency and mechanism of microcystin-RR by mesoporous Bi2WO6 under near ultraviolet light, Environ. Sci. Technol., 2012, 46, 2345–2351.
24. M. Koutny, J. Lemaire and A. Delort, Biodegradation of polyethylene films with prooxidant additives, Chemosphere, 2006, 64, 1243.
25. L. Zan, W. Fa and S. Wang, Novel photodegradable low-density polyethylene-TiO₂ nanocomposite film, Environ. Sci. Technol., 2006, 40, 1681.
26. R. T. Thomas, V. Nair and N. Sandhyarani, TiO₂ nanoparticle assisted solid phase photocatalytic degradation of polythene film: A mechanistic investigation, Colloids Surf., A, 2013, 422, 1.
27. R. T. Thomas and N. Sandhyarani, Enhancement in the photocatalytic degradation of low density polyethylene-TiO₂ nanocomposite films under solar irradiation, RSC Adv., 2013, 3, 14080–14087.

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Footnote

† They equally contributed to this paper.

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