Surface decoration of Halloysite nanotubes with POSS for fire-safe thermoplastic polyurethane nanocomposites

doi:10.1016/j.jmst.2021.05.060

Abstract

Abstract:

Halloysite nanotubes (HNTs) have been considered as a promising flame retardant fillers for polymers. In this work, the polyhedral oligomericsilsesquioxane (POSS) containing amino group was covalently grafted on the surface of HNTs with 3-(2,3-epoxypropoxy)propytrimethoxysilane as a chemical bridge. The POSS modified HNTs (HNTs-POSS) dispersed uniformly in the thermoplastic polyurethane (TPU) matrix and endowed TPU nanocomposites with enhanced tensile properties and fire safety. Cone calorimeter tests revealed that the introduction of 2 wt% HNTs-POSS to TPU matrix remarkably reduced the peak of heat release rate (PHRR) and total heat release (THR) by 60.0% and 18.3%, respectively. In addition, the peak CO production rate and total smoke release (TSR) could be significantly suppressed by the addition of HNTs-POSS. The well dispersed HNTs in combination with the ceramified silicon network from the thermal decomposition of POSS contributed to the formation of a continuous and compact char layer, exhibiting a tortuous effect by inhibiting heat diffusion and evaporation of volatile gaseous. In addition, the released crystal water from HNTs could dilute the combustible volatiles and then decline the combustion intensity. The tensile tests demonstrated that introduction of 2 wt% HNTs-POSS would enhance the maximum stress of TPU nanocomposite with a slight decrease of elongation at break. The combination of HNTs and POSS through the construction of effective interfacial interactions provides a feasible way to effectively enhance the fire safety of TPU nanocomposites without scarifying ductility.

Key words: Thermoplastic polyurethane, Halloysite nanotube, POSS, Flame retardancy, Mechanical property

Wei Wu, Wanjing Zhao, Xianjing Gong, Qijun Sun, Xianwu Cao, Yujun Su, Bin Yu, Robert K.Y. Li, Roy A.L. Vellaisamy. Surface decoration of Halloysite nanotubes with POSS for fire-safe thermoplastic polyurethane nanocomposites[J]. J. Mater. Sci. Technol., 2022, 101: 107-117.

Figures/Tables 16

Scheme 1. Preparation route of TPU/HNTs-POSS nanocomposites.

Table 1 The formulations of TPU nanocomposites.

Sample	TPU (wt%)	Pristine HNTs (wt%)	HNTs-POSS (wt%)
TPU	100	-	-
HNTs-0.5	99.5	0.5	-
HNTs-2	98	2	-
HNTs-POSS-0.5	99.5	-	0.5
HNTs-POSS-2	98	-	2

Fig. 1. (a) SEM image of pristine HNTs, (b) TEM image, (c) STEM image, and (d) EDX elemental mapping of N, C, O, Al, and Si of HNTs-POSS.

Fig. 2. (a) FT-IR spectra, (b) XPS survey spectrum, insert is high resolution spectrum of N 1s of HNTs-POSS (c) XRD patterns, and (d) TGA curves of pristine HNTs and HNTs-POSS.

Fig. 3. Digital images of (a) pristine HNTs, and (b) HNTs-POSS dispersed in different solvents (1 mg mL-1. The digital images were taken after 24 h storage).

Fig. 4. SEM images of fracture surfaces of pure TPU and its nanocomposites. (a) TPU, (b) HNTs-0.5, (c) HNTs-2, (d) HNTs-POSS-0.5, and (e) HNTs-POSS-2.

Fig. 5. (a) TGA and (b) DTG curves of TPU and its composites under nitrogen atmosphere.

Table 2 TGA data and LOI values of TPU nanocomposites.

Sample	T₅ (°C)	T_p1 (°C)	T_p2 (°C)	Char yields at 700 °C (wt%)	LOI (%)
TPU	302.5	357.1	415.2	1.97	21.7
HNTs-0.5	307.8	351.7	417.3	2.37	22.3
HNTs-2	300.2	352.8	417.6	4.72	24.7
HNTs-POSS-0.5	308.6	356.6	417.0	2.88	24.0
HNTs-POSS-2	304.3	355.8	417.9	5.41	26.3

Fig. 6. Cone calorimeter curves of TPU composites: (a) HRR, (b) THR, (c) CO production rate, (d) CO2 production rate as a function of time.

Table 3 Combustion data of TPU nanocomposites obtained from cone calorimeter measurements at 35 kW/m2.

Sample	TTI (s)	PHRR (kW/m²)	THR (MJ/m²)	PCOPR (g/s)	PCO₂PR (g/s)	Total smoke (m²/m²)	Char residues (wt%)
TPU	54±1	788.5±20	57.3±0.4	0.0094±0.0003	0.437±0.010	671.6±20.3	4.2±1.0
HNTs-0.5	62±3	645.3±15	56.2±0.4	0.0073±0.0007	0.383±0.007	622.8±18.5	4.6±1.2
HNTs-2	64±2	426.7±30	53.3±0.7	0.0041±0.0004	0.249±0.013	545.3±30.1	5.7±0.9
HNTs-POSS-0.5	63±3	495.6±24	48.5±0.3	0.0052±0.0004	0.304±0.006	602.2±24.3	5.3±0.8
HNTs-POSS-2	62±1	315.1±35	46.8±0.5	0.0043±0.0005	0.201±0.009	484.7±26.5	8.4±1.0

Fig. 7. Digital photos of char residues of (a) pure TPU, (b) HNTs-0.5, (c) HNTs-2, (d) HNTs-POSS-0.5, (e) HNTs-POSS-2; after cone calorimeter test.

Fig. 8. SEM images of char residues of (a, d) pure TPU, (b, e) HNTs-2, and (c, f) HNTs-POSS-2; Raman spectra of (g) pure TPU, (h) HNTs-2, and (i) HNTs-POSS-2 after cone calorimeter test.

Fig. 9. XRD patterns of the char residues for pure TPU, HNTs-2, and HNTs-POSS-2.

Fig. 10. 3D TG-FTIR spectra of (a) TPU, (b) HNTs-2, and (c) HNTs-POSS-2. Absorbance of pyrolysis products for TPU, HNTs-2, and HNTs-POSS-2 as a function of time: (d) Gram-Schmidt, (e) at maximum mass loss rate, (f) CO, (g) hydrocarbons, (h) carbonyl compounds, and (i) alcohols.

Fig. 11. Schematic diagram of flame retardant mechanism of TPU/HNTs-POSS composites.

Fig. 12. (a) Stress versus strain curves, and (b) tensile properties of TPU and its composites.

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