A novel phosphorous-containing polymeric compatibilizer: Effective reinforcement and flame retardancy in glass fiber reinforced polyamide 6 composites

Highlights

• A novel phosphorus-containing polymeric compatibilizer (PPC) with multiple anhydride reaction sites was synthesized.

• Both PA6 and silane-coated GF can be grafted onto the PPC backbone simultaneously during processing.

• Adding PPC can simplify the preparation process and improve the mechanical properties and flame-retarded efficiency of GFPA6.

Abstract

Glass fiber (GF) can be deemed as a double-edged sword, which improves the strength of polymers but impairs the flame retardant efficiency and toughness. Herein, a novel phosphorus-containing polymeric compatibilizer (PPC) with multiple anhydride reaction sites was synthesized from radical copolymerization. PPC was directly compounded with 3-aminopropyltriethoxysilane modified GF, polyamide 6 (PA6) to prepare fire retarded glass fiber reinforced polyamide 6 (GFPA6) containing aluminium diethlyphosphinate. The multiple anhydride groups on PPC chain can react simultaneously with the amino groups on modified GF and the terminal amino groups on PA6 chain during processing. The modified GF and PA6 matrix are linked in this way, thereby enhancing the interface adhesion between GF and PA6 matrix. The addition of PPC can effectively heighten the tensile, flexural and Izod notched impact strength of GFPA6 composites. When GFPA6 burns, the addition of PPC leads to more continuous and dense carbonaceous char on GF surface, which weakens the “wick effect” and improves the flame retardant efficiency. This work also opens the door for new generations of inorganic reinforced polyamide, polyester and polyurethane.

Introduction

Glass fiber (GF) with low cost, high tensile strength, excellent corrosion resistance and insulation properties is commonly used as a reinforcing material to effectively improve the mechanical strength of the polymer matrix [[1], [2], [3], [4], [5]]. Specifically, polyamide 6 (PA6), one of the important polyamide materials with excellent comprehensive performance, is usually reinforced with GF to improve the tensile strength and heat distortion temperature [6,7]. However, the performance improvement of glass fiber reinforced PA6 (GFPA6) composites has remained relatively limited on account of the poor interfacial interaction between inorganic GF and organic polymer matrix [8,9]. The surface modification of GF is generally desirable to offer better compatibility between inorganic GF and organic polymer matrix [10,11]. Meanwhile, the incorporation of GF will cause considerable loss of toughness and ductility, which usually requires the addition of appropriate plasticizer [12]. Most importantly, flammability and melt-dripping are the notable shortcoming of GFPA6 composites, which pose serious fire hazards in their application [13]. The melt viscosity of PA6 is low, and the melt will infiltrate the GF surface at high temperature and generate interfacial tension, causing the proverbial “wick effect” [14,15]. Driven by the interfacial tension, the melt tends to wet, spread and flow along the GF surface to the combustion area, which provides fuel supply for further accelerating combustion, making it harder to prepare flame retardant GFPA6 composites [16].

In the face of the increasingly widespread application of GFPA6 composites, it is of great social and economic benefit to develop flame-retardant PA6 products with excellent comprehensive performance. The main mean for preparing flame retardant GFPA6 composites is to add flame retardant by mechanical blending [[17], [18], [19]]. Due to the merits of low toxicity, low smokiness and low corrosiveness, phosphorous flame retardants have emerged as ideal candidates for designing high-performance flame retardant materials in academia and industry, which adhere to the stringent halogen-free requirement for flame retardants [[20], [21], [22], [23], [24], [25]]. The phosphorous flame retardants can act in condensed-phase through facilitating the dehydration and carbonization of the polymer matrix, and the resulting char layers can prevent the permeation of oxygen and heat and protect the underlying substrate from further combustion [26]. In gas-phase, the phosphorous flame retardants generate phosphorous radicals (e.g., PO· or HPO·) upon heating, which can trap active radicals (e.g., H· or ·OH) and terminate the combustion chain reaction [27,28]. In the present researches, phosphorous flame retardants, such as phosphonates and phosphates, have been widely used in GF reinforced polyamide and polyester systems [29]. These flame retardants are uniformly distributed in polymer matrix and work in its positions, also known as a conventional bulk flame retardant model [30]. However, in practice, the combustion rate of the interface areas is more intense than the bulk areas arising from the “wick effect”, which makes the flame retardation of the interfacial region more critical [14]. For the conventional bulk flame retardant model, it is often unsatisfactory to overcome the remarkable “wick effect” unless a high loading of flame retardant is added, which in turn destroys the mechanical performances and fabricability of the composites [31]. It is therefore imperative to increase the flame retardant concentration in the interface areas for reducing the loading of flame retardant and maximizing its efficiency.

So far, a few works on the flame-retardant modification of the GF interface have been reported, in which phosphorous flame retardants are grafted onto GF surface at an early stage, and then blended with polymer matrix without chemical reaction [14,[31], [32], [33]]. Although it can effectively increase the concentration of flame retardant in the interface areas, the preparation process is tedious, which is not conducive to mass production of GF reinforced composites. Moreover, phosphorous flame retardants are highly desired to enhance the compatibility between polymers and GF and lower the interfacial tension, acting as applicable compatibilizers [34]. To develop effective modifiers that can simultaneously realize the improvement of flame resistence and mechanical performances (especially toughness), which is essential to overcome the drawbacks of GF reinforced composites [35].

In this work, a novel phosphorus-containing polymeric compatibilizer (PPC) with multiple anhydride reaction sites on the backbone was synthesized from radical copolymerization of 2-((diphenylphosphoryl)oxy)ethyl acrylate and maleic anhydride. The GF we used is a kind of commercial GF modified by 3-aminopropyltriethoxysilane (KH-550), with amino groups on its surface. The PPC was directly blended with PA6 and GF to prepare flame retardant GFPA6 composites containing aluminium diethlyphosphinate (ADP). The anhydride groups of PPC can be grafted with the amino groups on the GF surface at the processing temperature, so that PPC can be uniformly distributed at the GF interface, which is also called an interface distribution model. Simultaneously, multiple anhydride groups remaining on the PPC molecular chain can also react with the terminal amino groups on the PA6 chain. PPC is used as a double-grafted compatibiliser to increase the interface adhesion between GF and PA6 during processing and application, thus enhancing the mechanical properties of GFPA6 composites. When the GFPA6 burns, it can act as a phosphorous flame retardant to form a successive char layer at the interface to weaken the “wick effect”. Therefore, the addition of PPC can not only simplify the preparation process of flame retardant GFPA6, but also improve the mechanical properties and flame retardant efficiency.