Utilization of Metal Oxides Nanoparticles in Modulating Polyvinyl Chloride Films to Resist Ultraviolet Light

Abstract

Modified poly(vinyl chloride) (PVC) films with organic groups (amino group from ethylene di-amine (en) and a suitable aromatic aldehyde from benzaldehyde (BEN)) were synthesized by casting using tetrahydrofuran (THF) solvent. The films were doped with four metal oxides nanoparticles (NPs), namely: CuO, Cr₂O₃, TiO₂, and Co₂O₃, to improve the anti-photodegradation property. The films were irradiated with ultraviolet light and the resulting damage was assessed using different analytical and morphological techniques. These techniques included FTIR, ¹H-NMR, and ¹³C-NMR spectroscopies that were used to examine the chemical structure, while another set of devices, namely optical microscope, scanning electronic microscopy (SEM), and atomic force microscope (AFM) were used to examine the morphology. In order to confirm that modified PVC acts as PVC photostabilizers, the roughness factor (Rq) was measured for the irradiated PVC films. The average Rq for irradiated blank PVC, modified PVC, modified PVC/CuO NPs, modified PVC/TiO₂ NPs, modified PVC/Co₂O₃ NPs, and modified PVC/Cr₂O₃ NPs films were 368.3, 76.1, 62.6, 53.2, 45.8, and 33.8, respectively. Infrared spectroscopy and weight loss determination indicated that the films incorporated with additives showed less damage and fewer surface changes compared to the blank film. All mentioned additives acted as UV screeners against the UV light. The modified PVC/Cr₂O₃ NPs film showed the highest ability to resist the photo-degradation process based on the results data of FTIR spectra, weight loss, and surface morphology. In addition, after 300 h of irradiation, the weight percentage of modified PVC/Cr₂O₃ NPs film was 0.911 in contrast to the blank PVC, 2.896. Among the tested films, modified PVC/Cr₂O₃ NPs film showed the best results.

1. Introduction

Since the beginning of the commercial production of plastics, UV rays have always had a substantially harmful effect on polymers’ outdoor applications. Harsh conditions, e.g., high temperature, long time exposure to sunlight, and moisture, in the presence of oxygen cause plastic to suffer from photo-oxidation observed by cracks, discoloration, and loss of mechanical and physical properties [1,2]. In the past decades, there has been a massive increase in the commercial production of plastics due to high demand [3]. The production volume of polyvinyl chloride (PVC) has increased since 1965 from 3 million tons to an outstanding 40 million tons in 2018; the production is forecasted to grow to 60 million tons in 2025 [4]. Thus, PVC is known as one of the most commercially manufactured plastics [5]. The high chlorine content grants PVC its hardness and toughness. As a result, it is utilized in a variety of applications, such as packaging, healthcare appliances, toys, building materials, electrical wire insulation, clothing, and furniture [4,6]. The conventional membrane has limited application in the separation of organic dyes, biomacromolecules (proteins, polysaccharides, etc.), and microbial cells with small molecule size [7,8]. The environmental degradation mechanisms of plastics can be classified as either (i) physical, referring to changes in the bulk structure, such as cracking, embrittlement, and flaking, or (ii) chemical, referring to changes at the molecular level such as bond cleavage or oxidation of long polymer chains to create shorter molecules. The potential environmental hazards associated with the soluble chemical byproducts of plastics degradation must be considered [9]. In recent years, exceptional efforts have been devoted by researchers to improving PVC optical stability for outdoor applications, one of the deployed methods is by doping suitable additives to prevent photolysis. The de-chlorination and the formation of unsaturated C=C bonds within the backbone structure led to its photolysis, when fragments and polyene residues are produced [10]. Within the backbone of polymers, the production of radicals might be induced by the impurities or distortions presence, which usually results in C-H bonds cleavage [11,12]. Moreover, peroxy-reactive radicals are produced during the replication step when oxygen is present. Additionally, hydroperoxides might be produced, which may lead to the polymer’s oxidation [13]. Recently, the photochemical stability of such polymers has gained a lot of attention in a way to find an effective method to prevent photochemical decomposition. Among the enhancement methods, the use of additives to stabilize polymers and improve their physical, mechanical, and thermal properties showed remarkable outcomes [14]. Here, additives could act as plasticizers, stabilizers, crosslinking agents, fillers, blowing agents, softeners, lubricants, flame retardants, colorants, and UV absorbents. These UV stabilizers can minimize the rate of photo-oxidation of polymeric materials [15]. Additive choice is ruled by different factors, such as the desired stability, color, volatility compatibility, and cost. Furthermore, they must have the ability to absorb or reflect UV rays and energy as heat or rays at a relatively harmless rate. Ultimately, the goal is to have the PVC plastics utilizing plasticizers to obtain a resistive polymer for outdoor use [16]. The most common additives for plastics include, but are not limited to, colorants, plasticizers, flame retardants, and stabilizers [17]. Additives should be easy and inexpensively produced, efficient at a very low concentration, pose no danger to the environment, and not lead to undesirable changes in the physical properties of plastics (e.g., alteration of color). In addition, additives should be chemically stable, involatile, and blend homogenously with polymers; they can be used as powders, beads, spheres, and flakes. Doping these materials aimed to reduce the photodecomposition and photooxidation of plastics, where they can act as absorbers for light, quenchers for energy, decomposers for radicals, and antioxidants [18]. However, the dispersion of additives in PVC and the compatibility between them cannot be ignored, and often affect the overall performance of PVC materials [19]. In our group, PVC films were modified by Cr₂O₃ NPs to enhance the optical properties of the polymer. The modified polymers had a smaller energy gap and became more conductive; as a result, the modified polymers had a higher photostability [20]. In addition, the PVC was modified by Tin(IV) complexes to delay the photodecomposition rate and improve the surface properties. These metallic additives showed a remarkable improvement, where different components showed a difference in performance [21]. In addition, the chains of polymer PVC modified by a Schiff base were grafted to gain a pliable PVC and give a homogenous polymer. This is generated by combining a copper chloride (Cu II) with the PVC compound to give PVC-L-Cu(II). The PVC film was exposed to 300 h irradiation to examine its photo stabilization through the environmental effect [22]. As a final example, PVC films were modified by a Schiff base inserted with nickel chloride to obtain a homopolymer. This homopolymer has a photostability toward the incident wavelength throughout the film. The variables upon the homopolymer surface have produced a porosity due to the change in atom volume of the molecule to promote the performance toward the incident light [23]. In this work, ethylene di-amine, benzaldehyde, and four types of metal oxide nanoparticles (CuO, Cr₂O₃, TiO₂, and Co₂O₃) were used to modify the PVC films and enhance its performance in presence of 313 nm UV light. The physicochemical properties and morphology of these modified films were investigated by different techniques, where all proved the applicability of these materials as stabilizers for PVC.

2. Experimental Part

2.1. Materials and Apparatuses

Ethylene di-amine (98%), benzaldehyde (98%), metal oxides nanoparticles CuO (58.43 nm), Cr₂O₃ (18.51 nm), TiO₂ (48.82 nm), Co₂O₃ (15.14 nm) (98%) [24], and THF solvent were ordered from (Gillingham, UK). Polyvinyl chloride (DOP = 3000) was purchased from Petkim Petrokimya (Istanbul, Turkey). Accelerated weather-meter QUV tester supplied with UV-B 313 lamps was obtained from Philips (Saarbücken, Germany). FT-IR spectra of the modified PVC films were detected via an FT-IR instrument of Model no. 8400 Shimadzu Spectrophotometer (Japan) and frequency range of 400–4000 cm⁻¹. The ¹H-NMR Proton nuclear magnetic resonance and ¹³C-NMR carbon nuclear magnetic resonance spectra of PVC films were obtained by a Bruker DRX300NMR spectrophotometer (Bruker, Zürich, Switzerland). The surface of irradiated and modified PVC films was tested by the scanning electron microscopy (SEM) technique, while energy dispersive X-ray (EDX) mapping was performed using a SIGMA 500 VP microscope (ZEISS Microscopy, Jena, Germany). Further examination of films’ morphology was carried on by atomic force microscopy (AFM) device using a Veeco instrument, and a MEIJI TECHNO microscope was also used for this purpose. For the AFM test, the samples were set in nitrogen to prevent deformation; a tapping mode was used to obtain 3D images at a scanning rate of 0.8 Hz. As this work is one of a series of studies in our group, the detailed examination procedures are illustrated elsewhere [20,21,22].

2.2. Preparation of Modified PVC Films

2.2.1. Preparation of PVC (Blank) Film

The homogeneous PVC films were prepared by the solvent-casting method. The blank PVC films were prepared by dissolving 5 gm of PVC in 100 mL of tetrahydrofuran (THF) for three hours at a stable stirring rate.

2.2.2. Preparation of Modified PVC Film

The PVC films modified with ethylene di-amine (en) were prepared by first dissolving 5 g of PVC and 30 mg of en in the solvent, then 30 mg of benzaldehyde (BEN) was added as shown in Scheme 1. Later, seven drops of acetic acid were added and the volume of THF was completed to 100 mL; the mixture was left to stir for three hours.

2.2.3. Preparation of Modified PVC Film Doped with Different Nanoparticles Metals

The nanoparticles metal oxides (CuO, Cr₂O₃, TiO₂, and Co₂O₃)-doped PVC sheets were prepared by sonicating 5 g of PVC, 30 mg of en and BEN, and 0.35 mg of metal oxide nanoparticles in 100 mL of THF at room temperature for one hour. Then, the blend was heated for three hours with continuous stirring.

2.3. Films Irradiation by UV Radiation

The blank and modified PVC films have been irradiated to UV light (λ = 313 nm with a light source of intensity 1.052 × 10⁻⁸ ein.dm⁻³∙s⁻¹ at 25 °C for 300 h. This test aimed to examine the durability of films against photodegradation and identify the improvement gained after doping the metal oxides NPs.

3. Results and Discussion

3.1. Characterization of PVC Films by NMR Spectrophotometer

The blank PVC and modified PVC films were characterized by ¹H-NMR and ¹³C-NMR in DMSO-d₆ solvent. In ¹H- NMR spectra of a blank PVC, a signal of chemical shift between 2.504–2.512 ppm is observed, which may be attributed to H-C-H. However, the chemical shift at 3.428 ppm corresponds to H-C-Cl. In the ¹³C-NMR spectrum for blank PVC, a signal between 39.27 and 40.52 ppm is detected that corresponded to C-C.

Similarly, in the ¹H- NMR spectrum of modified PVC film, the signals of chemical shift 1.039–1.074 ppm corresponded to N-H, while the peak at 2.509 ppm is attributed to H-C-H. Yet, the shifting at 3.398–3.878 ppm is attributed to H-C-Cl, while the chemical shift at 7.257–8.344 ppm belongs to Ar-H. The ¹³C-NMR spectrum of modified PVC film shows signals of chemical shift at 19.02, 39.31–40.56, 56.49, and 129.12 ppm that belong to C-Cl, C-C, C-N, and C=C aromatic moieties, respectively. The main signals of chemical shift for blank PVC and modified PVC films are tabulated in

Table 1. ¹H-NMR spectral data for PVC films.

1H NMR (500 MHz: DMSO-d6, δ)
Film	N-H	H-C-H	H-C-Cl	Ar-H
PVC	-	2.504–2.512	3.428	-
Modified PVC	1.039–1.074	2.509	3.398–3.878	7.257–8.344

and

Table 2. ¹³C-NMR spectral data for PVC films.

13C NMR (δ, ppm)
Film	C-Cl	C-C	C-N	C=C
PVC	-	39.27–40.52	-	-
Modified PVC	19.02	39.31–40.56	56.49	129.12

for the ¹H-NMR and ¹³C-NMR, respectively.

3.2. Photodegradation Rate Observation by FTIR Spectrophotometer

The ability of complexes to reduce the photo-degradation rate of PVC films was studied using FTIR spectrophotometer. Here, PVC films underwent oxidative photo-degradation at a wavelength of 313 nm for 300 h. The formation of undesirable compounds that contain destructive hydroxyl group and small polymeric fragments that contain carbonyl (C=O; carboxyl and ketone) was obtained. In addition, the polyene (C=C; carbon–carbon double bond residues) groups were observed [25]. Figure 1 (a and b, where (a) is for the (a) blank and (b) modified PVC, and (b) is for the NPs-doped films) shows an illustration depending on the peak intensity. Peaks of O-H (3500 cm^–1), C=O (1722 cm^–1), and C=C (1602 cm^–1) were remarkably higher in the irradiated blank PVC film compared with the original one, while the modified PVC doped with NPs (especially Cr₂O₃) film generated peaks (of the main functional groups) that had significantly lower intensity.

The comparison between blank and modified PVC films was made by comparing the calculated carbonyl index (I_C=O) and polyene index (I_C=C). The assessment focused on comparing the absorbance of main peaks, where the reference peak was at 1328 cm⁻¹. According to Equation (1), the I_s is the functional group index, while A_s and A_r are the absorbance of functional group and reference peak, respectively [26]:

$$I_s=\frac{A_s}{A_r}$$

(1)

Values of I_OH, I_C=O and I_C=C were plotted against the irradiation time of 50 h intervals (Figure 2, Figure 3 and Figure 4).

Clearly, NPs stabilized the PVC after 300 h of irradiation, where I_O-H values were 0.663, 0.439, 0.403, 0.380, 0.318, and 0.268 of the blank PVC, modified PVC, modified PVC/CuO NPs, modified PVC/TiO₂ NPs, modified PVC/Co₂O₃ NPs, and modified PVC/Cr₂O₃ NPs films, respectively. Similarly, the I_C=O values after 300 h irradiation were 0.718, 0.588, 0.553, 0.487, 0.454, and 0.357 of the blank PVC, modified PVC, modified PVC/CuO NPs, modified PVC/TiO₂ NPs, modified PVC/Co₂O₃ NPs, and modified PVC/Cr₂O₃ NPs films, respectively. In the same way, the I_C=C for blank PVC was 0.699 after 300 h irradiation compared to 0.589, 0.520, 0.489, 0.427, and 0.368 of the modified PVC, modified PVC/CuO NPs, modified PVC/TiO₂ NPs, modified PVC/Co₂O₃ NPs, and modified PVC/Cr₂O₃ NPs films, respectively. The growth rate of hydroxyl, carbonyl, and polyene groups increased by increasing the irradiation time and decreased via the addition of NPs.

3.3. Photodegradation Rate Assessment by Weight Loss

The weight lost during irradiation was calculated using Equation (2), where blank and modified PVC films were weighed after irradiation at different time intervals. In the equation, W₀ and W are the weight of films before and after irradiation, respectively [27].

$$\mathrm{Weight}\mathrm{loss}\%=\frac{{\mathrm{W}}_0-\mathrm{W}}{{\mathrm{W}}_0}\times 100$$

(2)

The photo-oxidation of PVC causes dehydrochlorination (elimination of HCl) and reveals residues of volatile organics that lead to weight loss [28]. Here, irradiation was conducted at intervals of 50 h, and the weight loss was tracked using Equation (2). Figure 5 shows the effect of UV light on the blank and modified PVC films, where the weight loss of the modified PVC doped with Cr₂O₃ film was the lowest compared to other PVC films. The weight loss after 300 h irradiation of modified PVC/Cr₂O₃ film was 0.911 compared with 2.896 of the blank PVC film.

3.4. Surface Analysis

3.4.1. Optical Microscope

A light microscope was utilized to study the photo-degradation of the modified PVC films by capturing 400× magnification images for the morphology [29]. Figure 6 shows the polymeric films before and after irradiation, where a smoother surface was obtained before irradiation. By comparing the dark spots, changes in color and roughness appearance of the PVC films’ morphology were detected after irradiation for 300 h. This variation is attributed to the photo-degradation process that generates bonds breakage and HCl elimination at the surface. Hence, the modified PVC polymer decelerated the photochemical process and reduced the HCl elimination [29].

3.4.2. Atomic Force Microscope (AFM)

Moreover, morphologies of films were further examined by a three-dimensional atomic force microscope (AFM) that provides a relatively useful information regarding the features and roughness of the surface [29]. Irradiating films for relatively long periods leads to bond breakage and surface roughness. Figure 7 shows the AFM images of polymeric films’ surface prior to and after 300 h irradiation. The surface of the irradiated modified PVC films was considerably smoother than that of the blank PVC film. According to the Rq values, the modified PVC/Cr₂O₃ film had less roughness compared to the blank and other modified PVC films. The Rq values for the polymeric films were listed in

Table 3. Roughness average of PVC films after irradiation.

PVC Films	Rq (Roughness Average) (nm)
PVC	368.3 ± 41.6
Modified PVC	76.1 ± 11.7
Modified PVC/CuO NPs	62.6 ± 8.3
Modified PVC/TiO2 NPs	53.2 ± 7.8
Modified PVC/Co2O3 NPs	45.8 ± 6.1
Modified PVC/Cr2O3 NPs	33.8 ± 4.8

.

3.4.3. Scanning Electron Microscopy (SEM)

To capture clear images of polymeric film surfaces with high resolution, scanning electron microscopy (SEM) was used [19]. Figure 8 shows the PVC surface film SEM images (a) before and (b) after irradiation. Previous reports indicated that non-irradiated polymers usually have a smoother and more homogeneous surface comparing to the radiated ones [29].

Figure 9, Figure 10, Figure 11 and Figure 12 display the SEM images after irradiation of the modified PVC films at three magnification powers. The surface of the modified PVC films was much smoother and cleaner when compared to that for the non-irradiated PVC film. The particles (may appear as pores) within the modified PVC films introduced new morphological surfaces. The CuO, TiO₂, Co₂O₃, and Cr₂O₃ NPs in the modified PVC films’ morphology looked like bubbles, shine crystals, cement wall, and honeycomb, respectively.

As a way of proving the elemental composition of PVC films, energy dispersive X-ray (EDX) mapping was coupled with the SEM [27]. The EDX graph of blank and modified PVC films are shown in Figure 13. From Figure 14, it can be revealed that the chlorine percentage was 32.0% prior to irradiation and 13.7% after 300 h irradiation. These results indicate the significant impact of dehydrochlorination. A reduction in chlorine content of the modified PVC films after irradiation was observed compared to the blank PVC film. However, the chlorine content was relatively high (19.4%) in the case of the irradiated modified PVC doped with Cr₂O₃ film. Given that, high chloride percentage means lower bonds breakage and photo-degradation.

Nanoparticles have a high recombination rate of photogenerated electrons and holes, so rutile could be used as a UV light protective material [30]. Namely, UV protective behavior occurs primarily due to strong UV light absorption, a high recombination rate of photogenerated electrons and holes, and high light scattering capability provided by nanoparticles. Such absorbed light nanoparticles reemit at a less harmful wavelength, mainly as heat. Here, ethylene di-amine and benzaldehyde behave as UV light absorbers due to the presence of heteroatoms and aromatic rings. Another suggested mechanism is improving the NPs’ role to retard the PVC photogeneration due to the acidity of metal oxides. The metal oxides eliminate the chloride ion of hydrogen chloride generated from PVC chains degradation upon irradiation; a stable substituted metal chloride is produced. The Cr₂O₃ NPs were the most sufficient additive to decrease the rate of PVC photogeneration process because of its binuclear unit and high binding sites capacity. Thus, the Cr₂O₃ oxide had strong interactions with chlorides, hydroperoxide, and peroxide radicals compare to other NPs. Corundum-type Cr₂O₃ nanoparticles are also one of the leading metal oxide materials utilized for a wide range of applications such as optical, catalytic, sensors systems, antimicrobial agents, photovoltaic cells, etc. To the best of our knowledge, the structural stability of Cr₂O₃ NPs against the exposure of dynamic shock waves is not yet reported in the literature. Cr₂O₃ NPs have better stability than the other three metal oxide NPs because of its outstanding structural stability against the impact of shock waves [31].

4. Conclusions

The benzaldehyde and metal oxide nanoparticles (CuO, Cr₂O₃, TiO₂, and Co₂O₃) were blended with polyvinyl chloride to produce modified PVC films in order to increase their stability against UV light. The films were irradiated at wavelength 313 nm for up to 300 h at room temperature. The efficiency of the films was evaluated by FTIR, weight loss, microscope, AFM, and SEM. The FTIR spectrum of PVC films after irradiation revealed absorption bands of hydroxyl group (O-H) at 3500 cm⁻¹, carbonyl group (C=O) around 1720 cm⁻¹, and polyene group at 1604 cm⁻¹. Hydroxyl, carbonyl, and polyene groups’ growth rate increases with increasing time of irradiation. The modified PVC doped with Cr₂O₃ film after irradiation appear dramatically decreased in weight loss values in contrast to blank PVC and other modified films. The microscope, AFM, and SEM images reveal that the modified PVC doped with Cr₂O₃ film showed less roughness and crack.