Sepiolite

Chapter 25

Downloaded by NORTH CAROLINA STATE UNIV on December 20, 2012 | http://pubs.acs.org Publication Date (Web): December 18, 2012 | doi: 10.1021/bk-2012-1118.ch025

Thermal Stability and Fire Retardancy of Polypropylene/Sepiolite Composite W. C. Cao,a L. J. Wang,*,a X. L. Xie,b and C. A. Wilkiec aCollege

of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, People’s Republic of China bCollege of Chemical and Biological Engineering, Guilin University of Technology, Guilin 541004, People’s Republic of China cDepartment of Chemistry and Fire Retardant Research Facility, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201 *E-mail: [email protected]

Polypropylene/sepiolite and PP/organo-sepiolite composites were prepared via melt extrusion, sepiolite was pre-modified with cetyltrimethyl ammonium bromide (CTAB). X-ray diffraction, transmission electron microscopy were used to characterize the morpohlogy of composites. Thermal stability and flame retardancy of the composites were evaluated by thermogravimetric analysis(TGA), cone calorimetric analysis, limiting oxygen index(LOI) and the UL94 protocol. The peak heat release rate of PP/sepiolite and PP/organo-sepiolite composites, with 10% sepiolite loading, decreased by 54% and 66%, respectively. The other fire retardancy also improved by addition of sepiolite, fire performance index (FPI) of PP/sepiolite composite and PP/sepiolite composite increased by 41.7% from 0.014sm2/kW of PP to 0.024 sm2/kW, and that of PP/organo-sepiolite composite increased by 150% reached 0.035 sm2/kW. The smoke production rate (SPR) of PP/organo-sepiolite composite decreased by 40% , from 0.117m2/s of PP to 0.07m2/s of composite, while mass loss rate (MLR) decreased by 41%, from 0.45g/s of PP to 0.20g/s of composite. The limiting oxygen index of composite increased © 2012 American Chemical Society In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

by 3, from 17.1(in PP) to 20.1. All of the composites could obtain an HB in the UL-94 horizontal burning test (UL-94HB) grade but were not classified in the vertical burning test (UL-94VB). Keywords: retardancy

Sepiolite;

polypropylene;

composite;

fire


Introduction Flammability is a major concern for polymeric materials. Because the important industrial applications of polymeric materials are restricted due to inherent high flammability, reducing the flammability of polymeric materials is a major and challenging task. Polypropylene (PP) is an important commodity plastic used extensively in many fields, such as housing, transportation, and electrical engineering materials, but its usage is often limited because of its poor flame retardancy (the limiting oxygen index (LOI) is often lower than 18%). Therefore, studies on flame-retardant PP have attracted considerable interest during the last decades (1–3). Many flame retardant have been developed for PP in the last three decades. Early fire retardants were halogen-based, such as polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), which are very efficient and easy to use. However, interference with flame reactions inhibiting hydrocarbon oxidation and the conversion of CO to CO2 results in very smoky, highly toxic fire effluents. In addition, halogen flame retardants have been shown to leach out of polymers into the natural environment, where their presence is now ubiquitous, and some are proven endocrine disruptors (4). European Union (EU) directive on the “Reduction of certain hazardous substances in electrical and electronic equipment” (RoHS, 2002/95/EC) prohibited the use of PBB and PBDE. These problems have driven the search for alternative “halogen free” fire retardants, which include metal hydroxide, carbonate fillers, phosphorus compounds and clays (5–7). In general, halogen-free fire retardants are much more polymer-specific and less efficient than halogen based flame retardants, requiring higher loadings (up to 70% by weight) in order to meet required flammability standards. Flame retardants with very small particle size have been of great interest. Nanometer size fire retardants have been thought to have essential advantages over flame retardants with larger particle size in terms of efficiency. In these systems, concurrent improvements across multiple properties are typically achieved, such as improved barrier, flammability, and biodegradability behavior, compared to the unfilled polymer (8). Polymer/clay nanocomposites have attracted wide interest in recent years for exhibiting improved mechanical, thermal, barrier, optical properties and fire retardancy. A great fraction of published data on polymer/clay nanocomposites focused on lamellar layered silicates, especially on the intercalation and 392 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


exfoliation of montmorillonite and layered double hydroxides (9, 10), whereas polymer/ sepiolite (nano)composites have not been studied to great extent. Sepiolite, which belongs to the structural family of phyllosilicates, is a hydrated magnesium silicate clay with Si12Mg8O30(OH,-F)4(OH2)4·8H2O as the theoretical half-unit-cell formula. This clay mineral has a nanometer-size tunnel structure and exhibits microfibrous morphology. The particular structure provides a high specific surface area (> 300 m2g-1) and porous volume (~0.4 cm3 g-1). Due to its specific structure, sepiolite can be easily modified. The modification can enhance the interaction between the polymer matrix and therefore sepiolite can improve the properties of polymers (11, 12). The objective of this paper is to prepare polypropylene/sepiolite composites via melt extrusion with CTAB modified sepiolite as an additive, and study the fire retardancy of sepiolite/PP (nano)composites using thermogravimetric analysis, limiting oxygen index, UL-94 and cone calorimetrcy.

Materials and Experimental Materials

Polypropylene (PP, homopolymer, HD 120 MO) was supplied by China National Petroleum Co., Ltd., Maleic anhydride grafted polypropylene (MAH-g-PP, Exxelor PO1020) was supplied by Huadu Technology Co., Ltd. Nanjing. Sepiolite was provided by Xixia Wanli sepiolite Factory (China). Cetyltrimethyl ammonium bromide (CTAB) was purchased from Sinopharm Chemical Reagent (China).

Modification of Sepiolite

To improve the dispersion of sepiolites in PP, it is nessary to enhance the compatibility of sepiolites with PP. According to the structural characteristics of polypropylene, CTAB is an appropriate material used to modify sepiolite to enhance the compatibility of the interface between inorganic additive and organic matrix. The typical method is as follows: 25g sepiolite and 240mL deionized water were mixed in a 500mL three-neck flask. 150mL of 1.2mol/L HCl was added dropwise into the flask with vigorous stirring for 6h at 75 °C and then centrifuged. The material was washed several times with deionized water. The resulting slurry was dried at 105 °C overnight and sieved to obtain a 200-mesh acidic-sepiolite. 12g acidic-sepiolite was mixed with 180mL deionized water in one-neck flask, 5.7mL of 10 wt% CTAB solution was added to the suspension. The mixture was vigorous stirred for 12h at 75 °C, then centrifuged and washed several times with deionized water, then dried overnight in a vacuum oven. 393 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Preparation of PP/Sepiolite Composite


The (nano)composites were prepared by extrusion molding in a tightly intermeshing co-rotating twin screw extruder (TS-20,Nanjing rubber & plastic machinery co., Ltd) with screw diameter of 27mm and L/D ratio of 36. A masterbatch was fabricated by melting maleic anhydride grafted polypropylene (MAH-g-PP) and sepiolite. Then the masterbatch was melt mixed with pure PP. The temperature in the extruder barrel was 200 °C at the feed region and 225 °C at the die section; the screw speed was 12 rev/min; the feed rate was 5 kg/h. The formulations are shown in Table 1.

Table 1. Formulation of composites Samples

Pure PP (g)

MAH-g-PP ( g)

Sepiolite (g)

Organosepiolite (g)

PP

97

3

0

0

PP/3% sepiolite

94

3

3

0

PP/5% sepiolite

92

3

5

0

PP/10% sepiolite

87

3

10

0

PP/3% organo-sepiolite

94

3

0

3


92

3

0

5


87

3

0

10

Characterization XRD experiments were performed on a PANalytical X’pert PRO powder diffractometer with Cu Kα generator (l5.5405Å ) at 40 kV and 40 mA. Scans were taken at 2θ=5°to 80°at 2°/min. Polymer composite samples were pressed into 20×15×1mm3 thickness plaques by compression molding. Every sample was tested five times at different positions. FT-IR spectroscopic analyses were carried out on a NICOLET 470 Fourier transform infrared spectrometer using the KBr method. Spectra were recorded between 400 and 4000 cm-1 at a resolution of 4 cm-1. The TEM imaging was carried out using a transmission electron microscope JEM-2100 with an accelerating voltage of 75 kV. The samples were ultramicrotomed with a glass knife on an AO-E microtome at room temperature to give sections of ~90 nm in thickness. The sections were transferred from the knife-edge to Cu grids. Bending test was carried out on an AG-201 machine at room temperature according to Chinese National Standard GB/T 1040-1992. The specimens were 100×4×1.5mm3 in size and the crosshead speed was set at 50 mm/min. A minimum of five specimens were tested for each data point and the averages were reported. Thermogravimetric analysis data were obtained on a NETZSCH STA-449 thermal 394 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


analysis instrument at 12-16mg scale under N2 atmosphere; all TGA samples were run in duplicate and the average values were reported. The thermal degradation measurements were carried out at a heating rate of 10 °C /min at a temperature ranging from room temperature to 600 °C. The UL94 protocol was performed on a DMS Horizontal-Vertial flame chamber testing instrument. Sample size is length of 130±5mm, width of 13±0.5mm, thickness of 1.0±0.1mm, angular radius

Sepiolite

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