Effect of flame retardants on mechanical properties, flammability and foamability of PP/wood–fiber composites

Abstract

The mechanical properties, flame retardancy, thermal degradation and foaming properties of wood–fiber/PP composites have been investigated. Ammonium polyphosphate (APP) and silica were used as flame retardants. The limiting oxygen index (LOI), thermal gravimetric analysis (TGA) and cone colorimeter (CONE) were employed for the study of fire retardance. At the same time, wood–fiber/PP composite foams were produced with the batch foaming technique using CO₂ as blowing agent. The effects of APP and silica content, pressure and temperature on the final cell structure were investigated. According to LOI, TGA and cone calorimeter results obtained from the experiments, APP and silica are effective flame retardants for wood–fiber/PP composites, and silica was shown to have a flame retardant synergistic effect with APP in wood–fiber/PP composite. The mechanical properties of the composites decreased with addition of flame retardants, except for the tensile strength of small amount of silica filled wood–fiber/PP composite. The results also revealed that the cellular morphologies of the foamed wood–fiber/PP composites are a strong function of the content of APP and silica as well as foaming conditions.

Introduction

Wood fiber reinforced plastic composites represent an emerging class of materials that combine the favorable performance and cost advantage attributes to both wood and thermoplastics [1]. By comparison with other fillers, the natural wood fiber reinforced polymer composites are more environmentally friendly, and are widely used in transportation, military applications, building and construction industries, packaging, consumer products, etc [2]. Polypropylene (PP) has been widely used for production of natural fiber/polymer composites because of its low density, high water and chemical resistance, good processability, and high cost-performance ratio [3], [4], [5]. Due to the poor compatibility between natural fibers and PP matrix, so a compatibilizer should be added to improve adhesion between matrix and fibers which leads to enhancement of mechanical properties of composites, a prominent method that represents the addition of maleic anhydride polymers as compatibilizers (such as maleic anhydride-grafted poly(propylene) (PP-g-MA) and poly(styrene)-blockpoly (ethene-co-1-butene)-block-poly(styrene) triblock copolymer (SEBS-g-MA)) has been widely used by some research workers [4], [5], [6], [7], [8].

Another drawback of wood–fiber/plastic composites (WPCs) are their high flammability. As organic materials, the polymers and the wood fibers are very sensitive to flame; improvement of flame retardancy of the composite materials have become more and more important in order to comply with the safety requirements of the wood fiber-composite products [9]. There is little research on the flammability of nature fiber and wood fiber composites in the literature. Yap et al. [10] investigated the effects of phosphonates on the flame retarding properties of tropical wood–polymer composites. Anna et al. [11] studied surface treated cellulose fibers in flame retarded PP composites by constituting a high-performance intumescent FR system in the PP matrix, and one of their results showed that the addition of ammonium polyphosphate (APP) to the cellulose fiber containing composite would result in an FR compound. Sain and Kokta [12] investigated the properties of the composites of PP and chemithermo mechanical pulp reactively treated with bismaleimide-modified PP or premodified pulp, the results indicated that in situ addition of sodium borate, boric acid, or phenolic resin during processing of the composite decreased the rate of burning of PP. Li and He [13] investigated the flame retardancy and thermal degradation of linear low-density polyethylene (LLDPE)–WF composites. In their study, APP and the mixtures of APP, melamine phosphate (MP) or pentaerythritol (PER) were used as FRs, and experimental results demonstrated that APP is an effective FR for LLDPE–wood–fiber composites by promoting char formation of the composites, however, the addition of APP (30–40 phr) reduced the Izod impact strength and hardly affected the tensile strength of the composites. Sain et al. [14] found that magnesium hydroxide can effectively reduce the flammability (almost 50%) of natural fiber filled polypropylene composites. No synergetic effect was observed when magnesium hydroxide was used in combination with boric acid and zinc borate, but marginal reduction in the mechanical properties of the composites was found with addition of flame-retardants. Zhao et al. [15] reported the mechanical properties, fire retardancy and smoke suppression of the silane-modified WF/PVC composites filled by modified montmorillonite (OMMT), and observed that the fire flame retardancy and smoke suppression of composites were strongly improved with the addition of OMMT. Guo et al. [16] investigated the effects of nanoclay particles on the flame retarding characteristics of wood–fiber/plastic composites (WPC), the result indicates that using a small amount of nanoclay can significantly improve the flame retarding properties of HDPE/WF nanocomposites.

Ammonium polyphosphate (APP) is an effective intumescent fire retardant for several kinds of polymer-based materials [17], [18], [19]. It is a sort of chain phosphate with high molecular weight. Its efficiency is generally attributed to increase of the char formation through a condensed phase reaction. Silica is usually used as enhancing agent in thermoplastic polymers to increase the mechanical properties, such as tensile strength and toughness. And it has also been recognized as inert diluents and shows some flame retardant effect. Kashiwagi et al. [20] have reported the flame retardant mechanism of silica in polypropylene blends. Fu and Qu [21] reported the synergistic flame retardant mechanism of fumed silica in ethylene–vinyl acetate/magnesium hydroxide blends, the results indicated that the addition of a given amount of fumed silica apparently increased the LOI value and decreased the loading of MH in EVA blends. This study mainly devoted to report the influence of APP and silica on the flammability and thermal decomposition behavior of wood–fiber/PP composite.

Recently, foaming technology has penetrated into the research and development of wood–fiber plastic composite products [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. As a result, their drawbacks such as higher density, lower ductility, and poor impact resistance compared with neat plastics and/or solid wood could be overcomed with the presence of cellular structure within the composites. Foaming of plastic/wood–fiber composites can be produced by utilizing either a chemical or physical blowing agent. A pressure-quench method described by Goel and Beckman was widely used for making microcellular polymers via supercritical carbon dioxide (scCO₂) [33]. They found that the microcellular structure could be achieved by rapid depressurization to allow the cells nucleation and growth as in the batch process after saturating polymers with scCO₂. In this study, the influence of APP and silica on the foamability of wood–fiber/PP composite was also investigated.