Modification of mesoporous silica with phosphotungstic acid and its effects on the combustion and thermal behavior of polylactic acid composites

https://doi.org/10.1016/j.polymdegradstab.2018.12.004Get rights and content

Highlights

  • Highly ordered mesoporous silica loaded with phosphotungstic acid was successfully prepared by vacuum impregnation.

  • The combination of intumescent flame retardant and PWA-SiO2 can synergistically enhance the fire resistance of PLA.

  • The improvement of flame retardant properties is mainly due to the catalytic effects of PWA-SiO2.

Abstract

Highly ordered mesoporous silica (SiO2) modified with phosphotungstic acid (PWA), was successfully prepared by vacuum impregnation. The chemical structure of as-designed PWA-mesoporous silica (PWA-SiO2) was characterized by transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectra and small-angle X-ray scattering. PWA-SiO2, in association with intumescent flame retardant (IFR), was introduced into polylactic acid (PLA) by melt blending to obtain flame resistant composites. The flammability evaluation indicated that the presence of 19.0 wt% IFR and 1.0 wt% PWA-SiO2 in PLA achieved the maximal LOI value of 47.2%, passed the UL-94 V-0 rating, and significantly decreased the peak heat release rate from 622.4 kW m−2 of neat PLA to 182.3 kW m−2. The introduction of PWA-SiO2 could also slightly improve the mechanical properties of PLA composites. It is proposed that PWA-SiO2, which exhibits excellent catalytic effect, can play roles together with IFR in both condensed phase through promoting the formation of continuous dense char layer and gas phase through releasing non-flammable gas.

Introduction

Polylactic acid (PLA) has received more and more attention in recent years due to its biodegradable properties which can provide significant environment and financial benefits. PLA is a green polymer obtained from lactic acid, which can be derived from renewable plant resources (such as corn, starch, and cassava, etc.) [1,2]. PLA shows great potential for commercial large-scale production due to its excellent mechanical properties, good processability, and high degree of transparency [3,4]. In recent years, PLA has been gradually used in electronics, aerospace, automotive components and other fields [[5], [6], [7]]. However, because of its poor flammability and heavy melt dripping during combustion, PLA without any further treatment cannot be recommended for applications where excellent flame retardant properties are required [8,9]. Therefore, it will be of great significance to reinforce the fire resistance of PLA.

The incorporation of flame retardants (FR) is the most widely used method to improve the fire resistance of polymer matrix for its ease of fabrication and relatively ideal effects. Many efforts have been undertaken to improve the flame resistant properties of PLA by the incorporating of flame retardants, such as phosphorus/nitrogen compounds, aluminum hydroxide, expanded graphite, nano-additives as well as intumescent flame retardants (IFR) [[10], [11], [12], [13]]. Due to the environmentally friendly advantages (including halogen free, low toxicity and lack of corrosive gas generation), IFR has gained extensively application [14,15]. IFR can effectively insulate the transfer of heat and oxygen and prevent the dripping by forming intumescent char layer. In recent years, many researchers have devoted themselves to develop synergistic flame retardants to improve the efficiency of IFR. For example, Li et al. [16] investigated the effects of organically modified montmorillonite on the flame retardancy and thermal stability of intumescent flame retardant PLA composites, and found that IFR and montmorillonite (MMT) could improve UL-94 grade to V-0 rating and increase the limiting oxygen index (LOI) value to 27.5%. Similar results were also obtained in Liu's work [17], where combined IFR with organo-modified α-zirconium phosphate.

SiO2 is usually used to enhance the mechanical properties in thermoplastics polymers [18]. Mesoporous silica is the mesoporous form of silica, and has applied in catalysis, drug delivery as well as fire resistance. During the past years, mesoporous silica has also been used as flame retardant in polymer composites such as polypropylene, polystyrene and epoxy resins [[19], [20], [21]]. Phosphotungstic acid (PWA) is a kind of typical polyoxometalates with superior catalytic performance in numerous processes in industrial fields. Our previous work demonstrated that PWA can synergistically promote the char formation and improve the fire resistance of PLA/IFR composite [2], however, the addition of PWA intercalated layered double hydroxides (LDH) and IFR decreases the mechanical properties sharply, and cannot fully eliminate the melt dripping phenomenon.

In this study, PWA was loaded into the microvoids of mesoporous SiO2 to prepare PWA-mesoporous silica (PWA-SiO2). It was expected that both SiO2 and phosphotungstic acid can act as char forming catalysts, and the combination of IFR and PWA-SiO2 can significantly enhance the fire resistance, thermal stability and mechanical properties of PLA composites. The combustion behavior of PLA/IFR/PWA-SiO2 composite was evaluated by LOI, UL-94 and cone calorimeter tests. The thermal properties and morphology of residual char were also characterized in order to understand the thermal degradation pathway and the flame retardant mechanism.

Section snippets

Materials

PLA with a specific gravity of 1.24 g cm−3 (3052D, Mn ≈ 12 × 104) was supplied by Nature Works Company, America. Phosphotungstic acid (PWA, AR) was purchased from Fuchen Chemicals, Tianjin, China. Cetyltrimethylammonium bromide (CTAB, AR) was purchased from Guangfu Fine Chemical Research Institute, Tianjin, China. Tetraethoxysilane (TEOS, AR) was provided by Xilong Scientific Co., Ltd, Shantou, China. Hydrochloric acid (HCl, AR) and ammonium hydroxide (NH3·H2O, AR) were purchased from Beijing

Characterization of PWA- SiO2

Fig. 2 shows the FTIR analysis of PWA, SiO2 and PWA-SiO2. The characteristic diffraction peaks of the Keggin structure in phosphotungstic acid at 1085 cm−1, 982 cm−1, 895 cm−1 and 801 cm−1 in the spectrum of PWA are corresponding to Psingle bondO, W=O, Wsingle bondO and Wsingle bondO’ bonds, respectively. In PWA-SiO2 spectrum, the corresponding bands can also be observed at 1079, 984, 898 and 802 cm−1, indicating that the Keggin structure of phosphotungstic acid has not been broken. The above results demonstrate that

Conclusion

Mesoporous silica modified with phosphotungstic acid, PWA-SiO2, has been successfully prepared, and was then introduced into PLA matrix in association with IFR to prepare flame retardant PLA composites. The combination of intumescent flame retardant and SiO2/PWA-SiO2 could synergistically enhance the flame resistance of PLA. IFR/PWA-SiO2 combination was more effective than IFR or IFR/SiO2 on improving the flame retardancy and eliminating the dripping phenomenon. The introduction of PWA-SiO2,

Notes

The authors declare no conflicts of interest.

Acknowledgements

This work was financially supported by Beijing Natural Science Foundation (Grant No.2174083), National Natural Science Foundation of China (Grant No. 21674008) and the Fundamental Research Funds for the Central Universities (JD1818).

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