Elsevier

Applied Clay Science

Volume 194, 1 September 2020, 105700
Applied Clay Science

Research Paper
Synergetic light stabilizing effects of reducing agent and UV absorber co-intercalated layered double hydroxides for polypropylene

https://doi.org/10.1016/j.clay.2020.105700Get rights and content

Highlights

  • SA–UVA–LDH was prepared by co-intercalating a reductant (SA) and an UV absorber (UVA) into MgAl-LDH.

  • SA–UVA–LDH has good thermal stability and excellent UV-shielding property.

  • SA–UVA–LDH significantly improved the thermal and light stability of PP.

  • There are synergistic light stabilizing effects between SA and UVA in LDH.

Abstract

Light stabilizers are widely used in the plastic industry to extend the service life of plastics for outdoor use. Herein, a novel organic–inorganic hybrid light stabilizer was prepared by co-intercalating a reducing agent sorbic acid (SA) and an UV absorber 2-hydroxy-4-methoxy-benzophenone-5-sulphonic acid (UVA) into the interlayer galleries of MgAl layered double hydroxides (LDH) by a separate nucleation and aging steps method. The prepared SA–UVA–LDH was characterized by XRD, FT–IR, SEM, TG–DTA and UV–vis, and was added into the matrix of PP as light stabilizer to evaluate its stabilization efficiency. The results showed that SA–UVA–LDH was evenly distributed in PP, and can offer excellent UV–blocking effect for PP. The addition of SA–LDH, UVA–LDH and SA–UVA–LDH can enhance both the thermal and light stability of PP. It was found that the stabilizing mechanism of SA–LDH arising from the reducing ability of SA is different from the UV–blocking mechanism of UVA–LDH. SA–UVA–LDH/PP has better light stability than SA–LDH/PP and UVA–LDH/PP due to the existence of synergistic stabilizing effect between SA and UVA. Therefore, the prepared SA–UVA–LDH can be potentially used as novel hybrid light stabilizer for PP.

Introduction

Polypropylene (PP) is one of the most used polymers with vast outdoor applications in industry, agriculture, construction and civil use due to its low cost, good mechanical properties and excellent chemical stability. However, the mechanical properties of PP can easily deteriorate when exposed to sunlight, heat, and oxygen because there are plentiful tertiary proton in the molecular chain, resulting in cracking and delaminating. Studies reveal that free radicals are readily generated on the tertiary carbon atoms of PP molecular chains under UV irradiation in sunlight. The formed free radicals will automatically react with atmospheric oxygen to produce peroxy radicals or abstract hydrogen atoms from the proximate tertiary carbons to generate new free radicals, leading to the auto−oxidation degradation of PP and loss of mechanical properties which ultimately shorten its service life (Joseph et al., 2002; Mailhot et al., 2003; Zanetti et al., 2004). Therefore, it is very important to prevent or restrain the photodegradation reactions of PP for prolonging its service life in the outdoor use.

In order to enhance the UV resistance of PP, a series of light stabilizers, such as hindered amine light stabilizers, organic UV absorbers and inorganic UV–shielding materials have been successfully developed and widely used as additive in PP (Búcsiová et al., 2000; Basfar et al., 2003; Wang et al., 2013a, Wang et al., 2015). Among these light stabilizers, organic UV absorbers are one of the most attractive UV stabilizers because of their high UV absorption capability. The stabilizing efficiency of organic UV absorbers strongly depends on their physical properties, such as thermal stability and solvent extraction resistance. However, the conventional organic UV absorbers are low−molecular−weight compounds which suffer from the weakness of easy volatilization at high processing temperature and poor extraction resistance toward solvents in practical application. It is thereby desirable to find ways to prepare novel light stabilizers with high thermal stability, good solvent extraction resistance and excellent UV absorption ability.

Recently, inorganic–organic hybrid materials have attracted considerable attention because they may show novel functionalities because there are host–guest interactions among the inorganic and organic parts (Sanchez et al., 2011). Among various inorganic matrices, layered double hydroxides (LDH), which can be expressed as [M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O (where M2+ and M3+ represent various divalent and trivalent metal cations in the LDH sheets, x stands for the ratio of M3+/(M2++M3+) and An− is the anions in the interlayer), received much attention owing to their desirable properties including large versatility in terms of chemical composition, ion−exchange capability, and good thermal stability (Bernardo et al., 2017; Langry et al., 2017; Nagaraju et al., 2017; Naseem et al., 2018). The isomorphous substitution of M2+ by M3+ ions results in positively charged host layers, which are compensated by anions between host layers. The flexible combination of hydroxide layers and functional interlayer anions results in a large number of functional materials which can be used as catalysts (Wang et al., 2017; Ping et al., 2018; Qin et al., 2018; Tokudome et al., 2018), absorbents (Linghu et al., 2017; Menezes dos Santos et al., 2017; Perez et al., 2017; Mu'azu et al., 2018; Liu et al., 2019), drug delivery (Saha et al., 2017; Choi et al., 2018; Deák et al., 2018), supercapacitors (Chen et al., 2014; Bandyopadhyay et al., 2018; Liang et al., 2018), molecular containers (Benicio et al., 2017; Bernardo et al., 2017), fire retardants (Wang et al., 2013a, Wang et al., 2013b; Cai et al., 2016; Edenharter et al., 2016; Li et al., 2018; Xu et al., 2018; Du et al., 2019), and so on (Xu et al., 2018; Du et al., 2019; Li et al., 2020). It has been reported that the addition of LDHs materials into PP can enhance the light stability of PP. Wang et al. prepared fluorescent anions inserted ZnAl–LDH with UV-blocking capability and found the obtained LDH enhanced the light stability of PP (Wang et al., 2015). Gao et al. discovered that the incorporation of 4,4-diaminostilbene-2,2-disulfonic acid intercalated LDH into PP can enhance the photo-stability of PP (Gao et al., 2017). Our group also revealed that UV absorbers intercalated LDH can be used as UV–blocking materials to enhance the photo-stability of PP (Chai et al., 2008; Cui et al., 2010; Zhu et al., 2011).

Generally, only one kind anion was intercalated into the interlayer of a majority of LDH materials, and very few papers reported the co-intercalation of two kinds of anions into the interlayer of LDH. LDH materials with single kind of interlayer anion usually show disadvantage of limited performance due to the sole function of the single kind of anion. Nevertheless, co-intercalating two kinds of different anions into the LDH gallery endows LDH with many advantages: first, different anions can contribute different properties to the LDH, which may result in multifunctional materials; second, the combination of different anions will generate new properties, which differ from those of the single anion intercalated LDH due to the guest−guest interactions; third, the synergistic effect between the two different anions can enhance the performance of the materials. In our previous works, organic dye and UV absorber co-intercalated LDH may be used as both pigment and UV–blocking material (Li et al., 2014); two different organic dyes with yellow and blue color co-intercalated LDH showed different color characteristics from the single dye intercalated LDH (Tang et al., 2014; Chen et al., 2017); two kinds of UV absorbers co-intercalated MgZnAl−LDH showed enhanced UV–shielding ability compared to the single UV absorber intercalated MgZnAl−LDH (Ma et al., 2019a). In this work, we further report a novel hybrid organic−inorganic material by co-intercalating two different functional guests (a reducing agent and an UV absorber) into a MgAl−LDH (SA–UVA–LDH), and demonstrate the synergistic anti−photoaging effect between the two organic guests. Sorbic acid (SA, Scheme 1a) is selected as the reducing agent due to its light intercalation into the interlayer of LDH and moderate reducing capability. The strong UV absorption ability of 2-hydorxy-4-methoxy-benzophenone-5-sulphonic acid (denoted as UVA, Scheme 1b) as well as its easy intercalating property make it an appropriate UV absorber for co-intercalation. SA–UVA–LDH was prepared by separate nucleation and aging steps (SNAS) method developed by our laboratory, and light aging tests demonstrated that SA–UVA–LDH has better light stabilizing effect than UVA–LDH and SA–LDH toward PP. Therefore, this work not only provides a method for preparation of a highly efficient light stabilizer, but also offer a strategy to prepare novel organic−inorganic hybrid materials by taking advantage of synergistic effects between the organic guests with different functions.

Section snippets

Chemicals

Analytical reagents including acetone, ethanol, hexane, xylene, NaOH, Mg(NO3)2·6H2O and Al(NO3)3·9H2O and industrial grade UVA, SA and isotactic PP were used as received. Decarbonized and deionized water was used in the synthesis and washing process.

Preparation of SA and UVA co-intercalated LDH

SA and UVA co-intercalated MgAl−LDH (SA–UVA–LDH) was synthesized by a method involving separate nucleation and aging step (SNAS). Firstly, a salt solution was prepared by dissolving Mg(NO3)2·6H2O (0.04 M) and Al(NO3)3·9H2O (0.02 M) into 80 mL of

Crystal structure, composition, and morphology

Fig. 1 shows the XRD patterns of SA–LDH, UVA–LDH and SA–UVA–LDH. One sees typical characteristic diffraction peaks of (003), (006) and (009) of LDH materials in the XRD patterns of the three samples, suggesting that LDH materials were successfully prepared. The basal spacings (d003) of SA–LDH and UVA–LDH, are about 1.66 nm and 2.18, respectively. The interlayer spacings of SA–LDH and UVA–LDH calculated by subtracting the thickness of LDH layer (0.48 nm) are about 1.18 and 1.70 nm, respectively,

Conclusions

A novel kind of hybrid UV light stabilizer SA–UVA–LDH was prepared by co-intercalating an organic reductant SA and an UV absorber UVA into MgAl−LDH via SNAS method. The analysis results show that there are host−guest interactions between MgAl−LDH layers and UVA and SA anions and guest−guest interactions between UVA anions in SA–UVA–LDH. SA–UVA–LDH inherits the prominent UV absorption capability of UVA and the UV–blocking ability of LDHs host layers, and thus shows excellent UV–blocking

Author contributions

Ruoyu Ma and Tingwei Chen conducted the experiments, collection of data, and data analysis. Yongjun Feng and Dianqing Li contributed to the research concept and design of this manuscript. Pinggui Tang also contributed to the research concept and design, data analysis as well as writing and revising the article.

Declaration of Competing Interest

There are no conflicts of interest to declare.

Acknowledgments

The authors are grateful for the financial supports from the National Natural Science Foundation of China (U1507119, 21627813, 21521005, 21571015), National Key Research and Development Program of China (2016YFB0301600) and the Fundamental Research Funds for the Central Universities (XK1802-6).

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