Mutual protection against UV aging of EVA composites using highly active optical conversion additives

Abstract

The mutual ultraviolet (UV) degradation protection and optical conversion mechanism of ethylene-vinyl acetate copolymer (EVA) rare earth organic complex (REOC) composite were investigated. The uniform Sm(TTA)₃Phen (MSTP) particles were successfully synthesized via microwave ultrasonic technique, and were uniformly embedded into the EVA long chain cross-linked structure with excellent dispersion. The MSTP doped EVA film provides an excellent active optical conversion and highly visible transmission performance, the properties of which are strongly dependent on the size and concentration of the doped particle, and its absorption and emission properties. MSTP was effective as a UV light resistant agent and was a highly active optical conversion additive for EVA rare earth organic complex composites with perfect network structure and excellent adhesion properties, which through the destruction of the structure can transform the heat energy into the form of light energy release via hydrogen bonding.

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Introduction

Owing to outstanding sunlight transmission, flexibility, adhesion to organic/inorganic materials,¹ ethylene-vinyl acetate copolymer (EVA) is widely applied in many fields, such as solar photovoltaic cell,^2–5 food packaging,⁶ mortar and concrete modifier,⁷ shoes and so on. However, the long-term exposure to ultraviolet (UV) irradiation in sunlight will accelerate the aging rate of EVA. The vinyl acetate (VAc) units of EVA are more vulnerable to heat, oxygen, and UV light radiation, which can easily form reactive radical or unstable hydroperoxides, which facilitate further irreversible chemical damage.² In order to ensure optical transparency, many traditional solutions are added to UV resistant agent, most of which release energy in the form of heat leading to thermal aging. One way to achieve improved light utilization and decreased UV degradation rate is through composite rare earth organic complex (REOC). The REOC species absorbs the UV wavelength photons and re-emits them at visible wavelength before they reach the solar cells.⁸

The conventional synthetic methods for REOC are co-precipitation and hydrothermal method, which have large size distribution and amorphous particles. Nowadays, particles with uniform size and controlled morphology are synthesized successfully by a novel microwave ultrasonic technique, which focuses on inorganic particles.^9–12 The researchers found that the microwave ultrasonic synergy method is also applicable to REOC in obtaining particles of uniform size and controlled morphology.

Based on our long term investigation in the morphology of REOC, the objective of this work was to make further detailed studies of REOC and EVA compounds. Through dual protection mechanism, EVA long chain and Sm(TTA)₃Phen particle slow down the rate of UV degradation. Meanwhile, Sm(TTA)₃Phen particle can convert UV energy to available red-orange light energy. Photoluminescence (PL) and infrared spectroscopy (FT-IR) are used to discuss the degradation mechanism, the intensity of light conversion and the relationship between Sm(TTA)₃Phen and EVA chain. Island microstructural morphology was observed by field emission scanning electron microscopy (FSEM).

Experiment

Materials

The raw materials of complex Sm(TTA)₃Phen (MSTP) were a-thenoyltrifluoroacetone (HTTA, 99.00%), 1,10-phenanthroline (Phen, 99.00%), NaOH (99.00%), ethanol (99.70%), benzoyl peroxide (BPO, 98.0%), tetrahydrofuran (THF, 99.0%) obtained from Sinopharm Chemical Reagent Company Ltd. (China). The ethylene vinyl acetate (EVA) copolymer was provided using Mitsui EVA 150 (33 wt%, Vac, Japan). SmCl₃·6H₂O (99.99%) were purchased from Funing Rare Earth Industrial Company Ltd. (China). All chemicals were of analytical grade and used as received without further purification.

Synthesis of Sm(TTA)₃Phen powder (MSTP)

Uniform and small rectangular MSTP powder was prepared by microwave ultrasonic method. As for the synthesis of MSTP, SmCl₃·6H₂O, HTTA, and Phen with a molar ratio of 1 : 3 : 1 were thoroughly dissolved in a minimum amount of ethanol to give three ethanolic solutions. The HTTA solution was firstly added to the microwave ultrasonic reactor at 60 °C under the microwave power of 500 W and ultrasonic power of 500 W into four flat bottom flasks. Then, the SmCl₃·6H₂O and Phen solutions were slowly dropped into the reactor by a peristaltic pump. The pH value of the mixture was adjusted to between 5 and 6 by sodium hydroxide solution (1 mol L⁻¹). After 4 h, the samples were washed with ethanol and water three times and their powder dried at 60 °C for 48 h.

Synthesis of MSTP doped EVA film (EMSTP)

The MSTP complex and ethylene vinyl acetate (EVA) were mixed at different weight ratios (0, 0.2, 0.4, 0.6, 0.8% MSTP doped) and dissolved in THF (5 wt% in concentration). The solution was dispersed under ultrasonic power at 500 W in 30 min. Then, 1% benzoyl peroxide (BPO) was added to the mixed solution and stirred for 2 h. The solution was distilled at 60 °C under vacuum for approximately 24 h to remove the solvent, which gave the films. After evaporating of the solvents on EVA films, the samples were heated to 145 °C for 10 min at 10 MPa by a flat-panel vulcanizer, in which EVA films with the thickness in the range of 150 ± 10 μm were sandwiched between two PET films, then taken out and cooled in air.

Characterization

Photoluminescence (PL) excitation and emission spectra were recorded on a Horiba Jobinyvon FL3-221 fluorescence spectrophotometer equipped with a 450 W Xe light source with an excitation wavelength of 380 nm. Field emission scanning electron microscopy (FESEM) was performed with an S-4800 scanning electron analyzer with an accelerating voltage of 15 kV. Absorption and transmission spectra were measured on a Shimadzu UV3101PC spectrophotometer (Shimadzu Corporation, Japan). FT-IR was performed under ATR technique on a PerkinElmer model Frontier FT-IR, USA. Spectra were collected over 32 scans with a 4 cm⁻¹ resolution from 400 to 4000 cm⁻¹. The artificial accelerated UV aging was applied using a Q-SUN1000 xenon lamp test chamber (Q-Panel, USA). The distance between the light source and the samples was kept at a constant 20 cm. The irradiation intensity was maintained at 0.51 W m⁻² @ λ = 340 nm, with a constant temperature of 65 °C (ISO 4892-2:2006). The aging time ranged from 0 h to 800 h. The heat effect of the EVA films were recorded with a professional infrared thermal imaging camera (FLIR A615) and the light source was provided by ShangHai GuCun Optic Instrument Factory (model ZF-7A hand UL ultraviolet ray examining lamp, 16 W 365/254 nm).

Results and discussion

FT-IR

Fig. 1 shows the FT-IR spectra for the EVA films doped with different concentrations of MSTP. It is observed that an increase in MSTP concentrations increases the intensity of the C–H bending bands in –CH₃ and –(CH₂)_n groups, i.e. the peaks at 1466 and 1372 cm⁻¹, and the C–H stretching bands at 2800–3000 cm⁻¹.¹³ The appearance of peaks at 1736 cm⁻¹ correspond to the stretching vibration of C=O. The bending vibration of C–H at 719 cm⁻¹ belongs to the –CH₂ groups of EVA as shown in the expanded region on the right in Fig. 1. As the MSTP doping concentrations increases from 0 wt% to 0.8 wt%, the new bending vibration of C–H at 730 cm⁻¹ appears and increases in intensity. The bending vibration of C–H splits into two peaks, owing to the incorporation of the MSTP molecules into the EVA cross-linking network structure. The more doped MSTP there is in EVA, the more intense the peak at 730 cm⁻¹. During the MSTP particle restriction, the C–H bond crystalline regions rearrange. This indicates that the MSTP molecules were embedded into the cross-linking network, which affected the extent of the degree of crystallinity which protects against UV damage.

Fig. 1 FTIR spectra of MSTP powder and MSTP doped EVA films of concentrations from 0–0.8 wt% (left: 400–4000 cm⁻¹ full spectrum, right: 650–800 cm⁻¹ expansion).

PL

The absorbance and emission of EMSTP films characteristics are shown in Fig. 2. It can be seen in the Fig. 2(a) that the pure EMSTP-0 only can weakly absorb UV under 270 nm. However, the MSTP doped EVA films have a broad and strong UV absorption from 250 nm to 400 nm, because the organics in the MSTP powder has a strong absorption for UV light. As the doped concentrations increases from 0.2 wt% to 0.8 wt%, the absorption for UV light becomes higher. Above the doped concentration of 0.2 wt%, the growth of absorption rate started to slow down because of optical transmission impact on the films. Owing to the effect of residual solvent tetrahydrofuran (THF), the pure EVA (EMSTP-0) has a strong absorption peak at 270 nm. As the MSTP doping concentrations increases from 0.2 wt% to 0.8 wt%, the 345 nm UV absorption peak gets wider and more intense. The emission spectra (Fig. 2(b)) are similar to the excitation spectra. The luminescence spectra are similar in which there are three main peaks: 564, 600 and 645 nm, corresponding to ⁴G_5/2 → ⁴H_j/2 (j = 5/7/9) transition of the Sm³⁺ ion. With the increase in doping concentrations, the emission peak intensity is also enhanced. The rate of increase levels off with increasing concentrations.

Fig. 2 Room-temperature (25 °C) absorbance (a) and emission (b) spectra of MSTP doped EVA films at concentrations from 0–0.8 wt%, all samples were excited at 360 nm.

FESEM

The FESEM images of MSTP powder synthesised using microwave ultrasonic assisted synthesis is shown in Fig. 3(a). The size of small rectangular particle is about 3–5 μm, which is more uniform than using the ones synthesized by co-precipitation (Fig. S1, ESI†). This reveals that the ultrasonic microwave synergistic effect is able to change the surface properties of the particles of powder to achieve control over the particle size of the REOC. Fig. 3(b) exhibits the SEM images of 0.8 wt% MSTP particle distribution in EVA film. The MSTP particle can dissolve in THF because of the growth mechanism. The Fig. 3(b) in the EVA film particle is slightly smaller than the size of the powder (Fig. 3(a)). Other MSTP in EVA films distribution is similar, except that the concentration and aggregation of the dispersion is different. The MSTP particles have excellent dispersibility in EVA film that can ensure transparency for sunlight and improved optical conversion efficiency. The MSTP particles are connected to the long-chain of EVA polymer, which can reduce MSTP UV degradation.

Fig. 3 FESEM images of MSTP powder via microwave ultrasonic method (a), the surface of a representative 0.8 wt% film of MSTP on EVA (b).

Transmission

Transmission measurements were performed through several points of each of the MSTP doped EVA film at different concentrations in order to confirm that no additional scattering of light was introduced by the molecules. The results are plotted in Fig. 4(a). It is found that the transmittance of EMSTP decreases rapidly from wavelength 400 nm and that of pure EVA film only decreases from 300 nm, which shows the MSTP doped EVA films have a strong absorption in UV light. The gradual increase in concentrations from 0.2 wt% to 0.8 wt%, the visible and ultraviolet of transmission has a slight decrease, which means the ultraviolet absorption capacity has increased. Especially, the 0.6 wt% transmission is higher than other MSTP doped EVA samples because of the antireflection effect. Fig. 4(b) photograph is the physical map transmission of EVA films doped with MSTP at concentrations from 0–0.8 wt%. The EVA film appears slightly yellow with increasing MSTP content.

Fig. 4 Transmission (a) UV-vis spectra and optical images (b) of MSTP doped EVA films at concentrations from 0–0.8 wt%.

Aging ATR-FTIR

Influences from photo-oxidation on the surface of pure EVA (EMSTP-0) and 0.8 wt% MSTP doped EVA films (EMSTP-0.8) were first characterized though ATR-FTIR. As presented in the spectra in Fig. S2,† notable formation of functional groups in EMSTP-0 and EMSTP-0.8 appear during UV irradiation. The carbonyl index (CI) values (CI = A_IR/A₂₈₅₀)^14,15 of specific functional groups are listed in Table 1. The EMSTP-0 and EMSTP-0.8 correspond to the degradation of infrared peaks. The new bond carbonyl ν_C=O stretching in the slow form that is close to zero in ketone structure, shows a growth at the absorption shoulder at 1718 cm⁻¹ and another broad band with an absorption maximum at 1176 cm⁻¹. It could have originated from the acetaldehyde evolution process in the Norrish III photolysis reaction. The absorption band around 1735 cm⁻¹ and the emergence of new carbonyl vibration at 1778 cm⁻¹ are mostly due to the lactone formation reported by Allen. Variations in A₁₇₁₈/A₂₈₅₀, A₁₁₇₆/A₂₈₅₀ and A₁₇₇₈/A₂₈₅₀ all indicate that ketone forms more intensively during the first 400 h exposure than the latter irradiation.

Table 1 CI of pure EVA and MSTP doped EVA films in 0.8 wt% concentration irradiated for various time from ATR-FTIR spectra

Sample	Aging time (h)	νcrystalC–C	νO–C=O	νC=O		νlactoneC=O	νO–H
		A730/A2850	A1735/A2850	A1718/A2850	A1176 × 10^−4/A2850	A1778 × 10^−4/A2850	A3500/A2850
EMSTP-0	0	0.017	2.111	—	0.532	1.349	0.338
	100	0.021	2.063	—	0.525	0.206	0.215
	400	0.009	1.865	—	—	0.196	0.145
	800	0.008	1.777	—	—	0.782	0.209
EMSTP-0.8	0	0.039	2.423	0.009	1.759	1.593	0.358
	100	0.014	2.202	0.003	1.114	0.645	0.153
	400	0.007	2.015	—	0.471	0.851	0.167
	800	0.008	1.913	—	0.737	0.885	0.181

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Combining the obvious O⋯H bands at 3600–3200 cm⁻¹ (Fig. S2(f), ESI†) is 1.06 times higher than pure EVA (Fig. S2(f), ESI†) without irradiation. This data infers that hydrogen bonds^16–19 form between MSTP and the EVA chain. Then, the A₃₅₀₀/A₂₈₅₀ had a decreasing tendency at irradiation, where the hydrogen bonds were broken because of the temperature increase under irradiation.

As the elongation of time, A₇₃₀/A₂₈₅₀ refers to the methylene crystallization peak, the peak tendency is first growing and then dropping. The EMSTP-0.8 methylene crystal first at 0 h is bigger than that of EMSTP-0, which indicates that MSTP embedded into the EVA cross-linking network causes the methylene crystalline region to be more closely aligned. Then, the thermal motions of methylene crystalline regions increased, the degree of orderly arrangement reduced along with the increase in UV irradiation time. Finally, owing to the heat energy, the methylene group activities improve to increase the ability of rearrangement of the crystals. The data analysis reveals that MSTP added in cross-linking structure enhances EVA resistance to UV light radiation.

Aging PL

Comparing with the previous the data published of SiO₂ coated and uncoated MSTP samples.²⁰ the aging behavior of fluorescence highest intensity of EVA films doped with MSTP in 0.8 wt% (EMSTP-0.8) at 400 h UV radiation is shown in Fig. 5. The calculation of decay percentage is characterized by photoluminescence at room temperature (25 °C). The intensity of curve of EMSTP-0.8 is higher and flatter than others, especially before 100 h of irradiation. After exposure for 400 h, the decay percentage of EMATP-0.8 was 1.79 times (75%) of that in SiO₂ coated (42%) and 3.41 times of that in uncoated MSTP (22%). The phenomenon might be explained by the MSTP particles are bound by a cross-linked structure that restrict bond movement and reduce the non-radiative energy loss. Thus, fluorescence decay intensity slowly decreased under UV light radiation. The EVA long chain and MSTP particle decreased the rate of ultraviolet degradation through mutual protection mechanism.

Fig. 5 Aging behavior of fluorescence highest intensity of MSTP doped EVA films at 0.8 wt% (EMSTP-0.8) and previous data published on SiO₂ coated and uncoated MSTP samples (Ni et al.²⁰ ©2013 J. Mater. Sci.) at room temperature (25 °C).

FLTR

The temperature of pure EVA (EMSTP-0) (left) and MSTP doped EVA films at 0.8 wt% (EMSTP-0.8) (right) at 365 nm and 254 nm UV irradiation (16 W) for 120 h via thermal imaging and visual imaging are shown in Fig. 6. First, the temperature of EMSTP-0 is close to EMSTP-0.8 without irradiation. Then, as UV irradiation continued, the EMSTP-0.8 was at a lower temperature than EMSTP-0, and the temperature difference increased.

Fig. 6 Thermographic images of pure EVA (0) (left) and MSTP doped EVA films at 0.8 wt% (0.8) (right) under UV irradiation (16 W @ 365/254 nm) for various time, (a) simulate test picture, (b) optical image, (c) UV irradiation at 0 min, (d) UV irradiation for 30 min, (e) UV irradiation for 90 min, (f) UV irradiation for 120 min.

Because of the UV light was irradiated from top to bottom, the heat temperature transported heat from top down. Due to the MSTP high efficiency of the down-shift conversion luminescence, which convert UV light to red-orange light, the MSTP successfully achieved transformation of the high-energy ultraviolet heat to light energy. As shown, the MSTP particle can absorb UV light and convert it into visible energy, which can transfer the heat energy to light energy. Hence, the MSTP doped EVA film has a lower temperature than pure EVA film and simultaneously reduce the UV degradation.

Mechanism

The proposed mechanisms of molecular chain structure and three-dimensional network structure for MSTP doped EVA polymer are shown in Fig. 7. When the EVA cross-linking process between some long-chain, MSTP particles are embedded in the cross-linking structure. EVA cross-linking structure could absorb UV energy and transfer that energy to MSTP through hydrogen bonds (Fig. 7(b) (right)), which can continuously convert UV energy to visible light. The proposed mechanism suggests that the heat energy for destroying the structure timely dissipate in the form of light energy. The EVA stability network structure is not susceptible to be broken in UV degradation and the MSTP particle can long-term transform light energy.

Fig. 7 Proposed mechanism for MSTP doped EVA polymer and energy transfer. (a) molecular chain structure of hydrogen bonds, (b) three-dimensional network structure (left) and energy levels (right).

Conclusion

The work presented in this paper has shown that EVA rare earth organic complex composites can provide effective mutual protection from both UV degradation and light conversion properties. The proposal mechanism might be that the MSTP particles are bound by a cross-linked structure via hydrogen bonding that restricts bond movement and reduces the non-radiative energy loss. Then, the EVA chain transfers energy to MSTP, which is converted to light energy, which can obviously decrease heat energy. The adjustable and uniform REOC MSTP materials have been successfully synthesized via the microwave ultrasonic technique. The absorption of UV light by doped EVA decreases rapidly from wavelength 400 nm and that of pure EVA film only decreases from 300 nm and the visible transmission has decreased slightly, where the MSTP doped EVA film at 0.6 wt% exhibits the best visible transmission. The higher the MSTP doping concentrations, the higher the excitation, emission and absorption intensity in light. After exposure to UV radiation for 400 h, the emission intensity decay percentage of EMATP-0.8 (75%) is the best compared to data from previous research (22%, 42%). The work demonstrates that EVA rare earth organic complex composites with perfect network structure and hydrogen bonding are effective at mutual protection for avoiding ultraviolet degradation.

Acknowledgements

This work was supported by the Key University Science Research Project of Jiangsu Province (Grant no. 10KJA430016), Jiangsu Province Postdoctoral Fund (Grant no. 1302096C) and a Project Funded by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03748k

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