Macromolecular coupling agents for flame retardant materials

https://doi.org/10.1016/j.eurpolymj.2005.10.017Get rights and content

Abstract

Polypropylene (PP) is a large-consumed polymer employed in many applications. For some uses, good flame resistance is desirable and this can be achieved by the addition of metallic hydroxides. However, high loads of metallic hydroxides are needed causing marked deterioration of the physical properties. Addition of interfacial agents is a useful way of minimizing these effects. In this study, PP was modified with vinyltriethoxysilane (VTES) and maleic anhydride (MA) and the products were used as coupling agents for PP/aluminum hydroxide (ATH) composites. The composites were characterized by TGA, SEM, tensile and flammability tests. It was observed that both coupling agents were efficient but PP modified with VTES showed better effect on the mechanical properties. Two types of ATH were used for comparison.

Introduction

PP and PP composites have been largely used in many applications mainly due to their low cost and good properties [1]. As their use grows, flammability and smoke emission problems acquire importance, mainly for applications that show fire risks as is the case of automotive components and electric or electronic devices. Addition of flame retardant compounds is a possible way to reduce this problem. Nowadays, halogenated compounds are the flame retardant additives that show the best cost/benefit relation for PP [2]. However, these compounds have been considered not friendly to the environment since they produce harmful gases during burn.

Aluminum hydroxide (ATH) is a well-known flame retardant for polymers free from halogens. It is an easily handled and relatively non-toxic material used for elastomers [3], [4], termorigid resins [5] and thermoplastics [6], [7], [8]. The thermal decomposition of ATH releases water, subtracting energy from substrate, diluting the combustible present in the gas phase and thus retarding the thermal degradation of the polymer [6], [9]. Moreover, its thermal degradation produces Al2O3, a refractory oxide that acts as a protective layer avoiding oxygen from feeding the fire [6]. However, high ATH concentrations are necessary for adequate flammability levels, in general higher than 50% [6], [7]. As it is common under these circumstances important physical properties losses are found [9].

Good interaction between the phases is important for improving the properties of the composites. This can be achieved through several ways as, for example, by coating the filler with selected materials. The use of coupling agents (organosilanes, for example) tends to increase the tensile strength of the composites [10], [11], [12], fatty acids and their salts increase impact strength in spite of decreasing tensile strength [10]. Addition of polymeric coupling agents can also improve adhesion between filler particles and matrix thus leading to better properties [10], [13], [14], [15], [16]. In many cases, hydrophobic polyolefins are grafted or functionalized with polar molecules becoming more hydrophilic and able to interact with the polar functional groups of mineral fillers. Hornsby and Watson [10] observed that employing 10 wt.% or more of an acrylic acid functionalized PP improved the flexural strength of PP composites with magnesium hydroxide (MTH). Mai, Li and Zeng [13], [14], [16] also worked with PP grafted with acrylic acid as macromolecular coupling agent for PP/ATH and PP/MTH composites and observed improvements in the tensile and flexural strengths and an increase in the melt flow index. The authors suggested that the functionalized PP reacted with the metallic hydroxides by means of acid–base reactions between the carboxylic groups of the modified polymer and the hydroxyl groups of the filler surface. Polyethylenes modified with butyl acrylate, maleic anhydride, acrylic acid and epoxide molecules were employed by Seppälä and coworkers [15] as polymeric coupling agents for PE/ATH and PE/MTH composites. The authors compared the properties of such materials with PE/ATH composites containing estearic acid coated ATH. Observation of the morphology patterns showed that the polymeric coupling agents showed the best effect on improving adhesion. This interfacial action was particularly visible in the mechanical properties of the materials. Yang and coworkers [17] used a commercial silane-grafted HDPE (6 phr) as surface modifier for ATH in HDPE composites and observed a substantial increase in yield strength. Wang et al. [18] suggested that the crosslinking induced by the presence of the silane groups was responsible for the improvements in tensile strength for PE/ATH composites prepared with silane-grafted PE. In fact, the use of organosilanes as functionalizing molecules for PE is a common commercial practice searching for crosslinkable materials. However, similar trends are not found for PP that is mainly modified with maleic anhydride, acrylic acid and epoxide moieties in commercial products.

The effects of particle size and degree of agglomeration on the properties of polyolefin composites with mineral fillers have been studied by some authors [3], [19], [20], [21], [22]. Lee and coworkers observed that tensile strength strongly diminished with ATH particle sizes in EPDM composites [3]. According to these authors, bigger particles show less uniform distribution producing lower tensile strength. Miyata and coworkers [19] filled PP with four different types of MHT, having observed that the tensile strength was strongly dependent on the crystallite size of the filler, more than on the average particle size. They proposed that bigger crystallites show better dispersion in the resin thus improving its effective volume and so the mechanical properties. Cook and Harper [21] compared two magnesium hydroxide fillers of different morphology incorporated into PP. They verified that a platelet filler tended to orient parallel to the flow direction thus causing nucleation, whereas a pseudospherical filler resided isotropically within the matrix.

On the basis of previous studies performed in our laboratory dealing with silane functionalization of polyolefins [23], [24] we studied in this article the usefulness of well-characterized silane-functionalized PP as coupling agents for PP/ATH composites. In this sense, results were compared to those obtained for composites coupled with the more common maleic anhydride-functionalized PP. The properties of composites prepared with two kinds of ATH were determined viewing to relate the morphology of the filler with the interfacial action of the coupling agents.

Section snippets

Experimental

Polypropylene (highly isotactic PP, Braskem SA), vinyltriethoxysilane (VTES, Silan GF56—Wacker Chemie), Maleic Anhydride (MA 98%—Produtos Quimicos Elekeiros SA), Aluminum trihydroxide (ATH V—high purity Vetec and ATH H—Hydrogard Alcoa Aluminio SA), Dicumyl peroxide (DCP—Aldrich Chemical Company) and Luperox 101 (Atofina Brasil Quimica Ltda) were employed as received. ATH particle sizes were determined with a Particle Size Analyser Cilas Laser 1180.

ATH characterization

The curves of particle size distribution show similar patterns for both ATH (Fig. 1). However, the statistical analysis reveals that the mean diameter of ATH H particles is bigger than that of ATH V ones (Table 1).

The differences in the morphology can be seen in the scanning electron micrographs of the two powders in Fig. 2a and b. ATH V (Fig. 2a) is porous and roughly spherical with wide size distribution whereas ATH H (Fig. 2b) exhibits more smooth, well-defined faces even tough remaining the

Conclusions

The addition of ATH to PP imparts some good properties to the polymer, such as increasing decomposition temperature under oxidative atmosphere, increasing flame resistance and increasing limit oxygen concentration for ignition. However, the mechanical properties and the processability of the materials are lowered.

The morphology of the dispersed phase is important for the properties of the composites. It was verified that ATH with bigger particles showed greater deleterious effect on the tensile

Acknowledgements

The authors are grateful to Fapergs and CNPq for financial support, to Braskem S.A. and Alcoa Aluminio SA for raw materials supplying.

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