Co-intercalated layered double hydroxides as thermal and photo-oxidation stabilizers for polypropylene

An elegant and efficient approach consisting in the co-intercalation of stabilizing molecular anions is described here. The thermal stabilizer calcium diethyl bis[[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]phosphonate] (Irganox 1425, MP-Ca) and a photo-oxidation stabilizer (hindered amine light stabilizer, HALS) are co-intercalated into the interlayer regions of layered double hydroxides (LDH) in a one-step coprecipitation. These hybrid organic–inorganic materials are successively dispersed in polypropylene to form HnMn′-Ca2Al/PP composite films (with H = HALS and M = MP) through a solvent casting method. The corresponding crystalline structure, chemical composition, morphology as well as the resistance against thermal aging and photo-oxidation are carefully investigated by various techniques. The results show that the powdered HnMn′-Ca2Al-LDHs hybrid materials have a much higher thermal stability than MP-Ca and HALS before intercalation. In addition, the HnMn′-Ca2Al/PP composites exhibit a higher overall resistance against thermal degradation and photo-oxidation compared to LDHs intercalated with only HALS or MP. This underlines the benefit of the co-intercalation. The co-intercalated LDH materials pave a new way in designing and fabricating high-performance multifunctional additives for polymers.


Introduction
Hindered phenols and hindered amines, containing the functional groups 2,6-di-tert-butylphenol and 2,2,6,6-tetramethylpiperidine, respectively, are widely used as functional additives in polymers to prolong the service life [1][2][3]. Generally, the anti-aging agents effectively inhibit the degradation in two ways: (1) through capturing generated free radicals and stopping autooxidation and (2) through decomposing and eliminating hydroperoxides [4,5]. However, anti-aging agents are often organic chemicals that easily migrate and volatilize from the polymer, reducing the anti-aging efficiency and increasing environmental pollution [6]. Therefore, it is of interest to explore novel multifunctional additives for polymers with high anti-aging performance together with high migration resistance.
Recently, inorganic-organic hybrid functional additives have attracted increasing attention for their wide applications in polymers [7]. Organic anti-aging species have been immobilized onto inorganic supports (e.g., carbon nanotubes, SiO 2 , graphene oxide) to produce inorganic-organic composites with higher migration resistance [8][9][10]. More recently, layered double hydroxides (LDHs), a layered host-guest material, have emerged as promising inorganic nanocontainers for functional organic active species to enhance the thermal and photo-oxidation stability of interleaved organic species as well as to endow the polymer/LDH composites with the desired properties [11][12][13][14]. In our previous work, a series of intercalated antioxidants and photo-oxidation stabilizers with a single active component have been prepared by coprecipitation. In these polymer/LDH compounds, the resistance against aging was significantly improved [15][16][17]. For example, the antioxidant Irganox 1425 (see Figure 1, abbreviated as MP-Ca) was intercalated into Ca 2 Al-LDH through coprecipitation of MP-Ca and Al(NO 3 ) 3 at pH 10, to yield MP-Ca 2 Al-LDH. Here, the MP-Ca was used the source of Ca for the host sheet and that of MP for the guest anions. Polypropylene (PP) protected with the prepared MP-Ca 2 Al-LDH exhibited enhanced thermal stability and anti-migration behavior in comparison with MP-Ca/PP composites. Lately, some studies have demonstrated much better performance of multi-component intercalation compounds compared to the corresponding single-component intercalation compounds as well as to the physical mixtures of the components [18,19]. The benefit of the co-intercalation is attributed to synergistic effects between the different active species associated to a higher dispersion in the composites [20,21].
In this work, we designed and fabricated a series of novel co-intercalated thermal and photo-oxidation stabilizers (H n M n′ -Ca 2 Al) through straightforward co-precipitation of HALS and MP-Ca ( Figure 1) [16,17], and examined the resistance of the H n M n′ -Ca 2 Al/PP composites against thermal degradation and photo-oxidation as a function of the molar ratio between HALS and MP in the interlayer regions.

Results and Discussion
Analysis of H n M n′ -Ca 2 Al-LDHs  004) and (006) reflection peaks at low angles and the weaker (110) peak at a higher angle, corresponding to the layered structure and the intra-layer structure in the host  sheet [22]. The (002) reflection peaks of HALS-Ca 2 Al and MP-Ca 2 Al are located at 11.5° (d 002 = 0.77 nm) and 3.4°( d 002 = 2.52 nm), respectively. Simultaneously, for LDHs co-intercalated with HALS and MP (H 2 M 1 -Ca 2 Al, H 1 M 1 -Ca 2 Al, H 1 M 2 -Ca 2 Al, H 1 M 3 -Ca 2 Al), the (002) reflection peaks appear at ca. 3.4°, corresponding to the d-spacing values of 2.55, 2.68, 2.55, and 2.75 nm, respectively. The enlarged d-spacing of H n M n′ -Ca 2 Al-LDHs suggests that HALS and MP anions were co-intercalated into the LDH, and the different ratios of HALS/MP result in a slightly different arrangement of guest anions leading to minor variations of the d-spacing values. The full width at half maximum values of the (002) reflection of all H n M n′ -Ca 2 Al compounds are smaller than those of HALS-Ca 2 Al and MP-Ca 2 Al, indicating that the number of stacked platelets was decreased due to the co-intercalation. The results show that co-precipitation yields Ca 2 Al-LDHs free of CaCO 3 by-product [23]. Figure 3 shows FTIR spectra of all the H n M n′ -Ca 2 Al-LDHs. One can observe characteristic stretching-vibration bands of LDHs, for example, the broad band at ca. 3445 cm −1 associated to the OH groups of interlayer water molecules and brucite-like    Table 1 summarizes the corresponding data. In our previous work, the decomposition of HALS and Irganox 1425 molecular anions occurred with an exothermic DTA peak at 300 and 295 °C, respectively [16,17]. Here, three major stages of mass loss in the TG curve of H n M n′ -Ca 2 Al-LDH samples can be observed. The first mass loss up to 180 °C is assigned to the release of adsorbed water and crystal water; The second one in the range of 180-250 °C is attributed to the dehydroxylation of the metal-hydroxide layer. The third large mass loss stage corresponding to the decomposition of HALS and MP ions appears at 250-450 °C with endothermic peaks between 300 and 360 °C in the DTA curve. The thermal stability of H n M n′ -Ca 2 Al-LDHs was expressed through the temperatures associated to a certain weight loss (i.e., T 25% is the temperature at which the sample has lost 25 wt %) in Table 1. For intercalated Ca 2 Al-LDHs, the thermal oxidative decomposition occurs at temperatures higher than those of HALS and Irganox 1425. Moreover, the co-intercalated H n M n′ -Ca 2 Al-LDHs exhibit a higher decomposition temperature than HALS-Ca 2 Al and MP-Ca 2 Al, especially H 1 M 2 -Ca 2 Al (356 °C). For the co-intercalated H n M n′ -Ca 2 Al-LDHs, the T 25% values gradually increase from 257 °C for H 2 M 1 -Ca 2 Al to 299 °C for H 1 M 3 -Ca 2 Al with an increasing content of M. The above results illustrate that the thermal stability of HALS and MP anions are enhanced after the co-intercalation of both anions into the interlayer region of LDHs.  Table 2 lists the element analysis data and the calculated chemical compositions of H n M n′ -Ca 2 Al-LDHs analyzed by CHN elemental analysis for the organic moieties and ICP atomic emission spectrometry for metal cations. The content of interlayer water is determined from the mass loss between 100 and 200 °C in the TG curves ( Figure 5a). The fractions of HALS and MP anions are calculated based on the content of Al and C taking into account the charge balance. The molar fractions of the guest anions are close to the feeding ratio, suggesting the ratio between HALS and MP can be adjusted as designed.
These results also suggest that both of HALS and MP anions have been co-intercalated into Ca 2 Al-LDH.
Analysis of H n M n′ -Ca 2 Al-LDHs/PP composites        anions are also observed after addition of H n M n′ -Ca 2 Al-LDHs. Figure 7b demonstrates the visible-light transmittance of H n M n′ -Ca 2 Al-LDH/PP composite films, which is one of crucial properties of the PP products. All the samples show a similar trend demonstrating that there is a good dispersion of Ca 2 Al-LDHs in the PP matrix without affecting its visible-light transmission. Figure 7c, Performance of H n M n′ -Ca 2 Al/PP composites

Conclusion
In this work, we have successfully co-intercalated a hindered amine light stabilizer (HALS) and a hindered phenolic antioxidant (MP) into the interlayer region of Ca 2 Al-LDHs with different molar ratios through coprecipitation. The concomitant intercalation of HALS and MP significantly enhances the thermal stability of the powders due to the host-guest interactions between guest anions and the host LDH. Subsequently a series of H n M n′ -Ca 2 Al/PP composite films was prepared. The results show that the addition of H n M n′ -Ca 2 Al-LDH has no negative effect on the crystallization behavior of PP, while it improves significantly the stability of the composites against thermal degradation and photo-oxidation. Undoubtedly, the co-intercalation method for LDH framework will open a way to design and fabricate multifunctional additives for polymer composites.

Fabrication of HALS
The HALS was synthesized as reported [16]. Typically, succinic anhydride (15 mmol) was dissolved into 10 mL of dioxane at 80 °C under vigorous stirring, and tetramethylpiperidinamine (15 mmol) in 10 mL of dioxane was dropwise added. The solution was kept at 80 °C for 40 min. The product was washed three times using dioxane and ether. Finally, the powdered product HALS was collected after vacuum filtration.

Characterization
Powder X-ray diffraction (XRD) measurements were performed on a Shimadzu XRD-6000 X-ray diffractometer with a wavelength of 0.154 nm at 40 kV and 30 mA in a 2θ range of 3-70° at 10°·min −1 . Fourier-transform infrared (FTIR) spectra were recorded on a Bruker Vector 22 infrared spectrophotometer with KBr pellets (sample/KBr of 1:100 by weight) or thin films. Thermogravimetry and differential thermal analysis (TG-DTA) was performed on a PCT-IA instrument in the range of 25 to 700 °C at 5 °C·min −1 under flowing air. Scanning electron microscopy (SEM) images were taken with a Zeiss scanning electron microscope by dropping dilute ethanol suspension at room temperature. Elemental analysis for metal elements (Ca and Al) was carried out on a Shimadzu ICPS-7500 inductively coupled plasma (ICP) atomic emission spectrometer. About 30 mg of the samples was dissolved in a few drops of concentrated nitric acid (65%) and diluted to 10 mL using water. CHN elemental analysis was carried out on a Vario EL III, Elementar instrument. The content of water in the samples was obtained by thermogravimetry. The UV-vis spectra in the range of 200 to 800 nm were collected by using a Shimadzu UV-2501PC spectrophotometer.
Stability evaluation of H n M n′ -Ca 2 Al/PP composites Here, two methods were employed to evaluate the thermal stability of H n M n′ -Ca 2 Al/PP composites. One way was to examine the composite samples with TG-DTA, for example, ca. 7 mg of the samples was heated from 25 to 600 °C at 10 °C·min −1 in flowing air. The other was to perform an accelerated thermal aging test in an oven [15]. For this, H n M n′ -Ca 2 Al/PP composite films were tailored to a size of 20 × 20 × 0.1 mm and thermally aged at 150 °C. Every 80 min, the composition was monitored by FTIR. For the quantitative analysis of the degradation, the integrated area of peaks in the range of 1810-1660 cm −1 , assigned to carbonyl groups was used.
The photo stability of H n M n′ -Ca 2 Al/PP composites (20 × 20 × 0.1 mm) was examined in an accelerated photoaging instrument with an ultraviolet high-pressure mercury lamp (P = 100 W, λ max = 360 nm) and the degradation degree was monitored every 5 min by FTIR [27]. The data processing method was the same as during the thermal aging.