The present work was focused on studying the effects of different carbon black (CB) loadings on rheological, thermal, tensile, dynamic mechanical, and electrical properties of polyvinylidene fluoride (PVDF) and FKM composites. To these ends, dynamic mechanical analysis (DMA) was conducted, and the CB grade and method of compound preparation which was CB(N330) by melt mixing with different shear effects were examined. The composites were melting blended with CB at 190°C in an internal mixer. After that the properties of filled and unfilled composites were compared. DMA showed the glass transition temperatures of the composites. The analysis also revealed that the area under the loss tangent (tanδ) peak decreased; moreover the tanδ temperature of the rubber phase increased by CB loading. The presence of CB improved the mechanical properties, such as Young’s modulus and tensile strength, of the composites and increased their thermal stability due to high thermal stability of CB and the interaction between CB particles and polymer matrix. The increase in the electrical conductivities of the composites under different CB loadings was also examined with different shear effects owing to different dispersion states of CB. The percolation threshold of conductive thermoplastic vulcanizate composite was observed based on conductive CB and the experimental data were well fitted to the general equation model(GEM).

Keywords: composites, poly(vinylidene fluoride), fluoroelastomer, carbon black

In recent years, polymer composites and polymer blends have been used extensively in various applications. Addition of filler into polymeric matrix and blending of polymers are wellestablished ways to improve the properties which cannot be achieved from the individual component. It is an effective cost and economic approach to produce a new material with desired properties.^1-3 However, the difference in molecular structure and affinity of blend component results in the immiscible blend. This has been recognized to cause both inferior mechanical properties such as poor tensile strength, elongation at break and compression set.^4,5 The blending of polymer has been studied extensively over the past decades in order to achieve a set of desired properties and high performances for specific applications.^6,7 The thermoplastic elastomeric combination is an interesting and new class of material, so-called “Thermoplastic Elastomer” (TPE). The TPE consists of a rigid thermoplastic phase and a soft elastomer phase, resulting in a combination of the excellent elastic properties of rubber with melt process ability of thermoplastic. The properties of TPE based on plastic-rubber blend depend on phase morphology and miscibility between two phases, mainly due to phase separation of the blend. There are two types of blending technique to prepare TPE: simple blend and dynamic vulcanization. The simple blend (SB) means blending of plastic and rubber without cross-linker.^8,9 The dynamic vulcanization is a blend with cross-linker to vulcanize rubber phase during melt mixing. The thermoplastic elastomer prepared by dynamic vulcanization is usually called thermoplastic vulcanizate (TPV).^10,11 Thermoplastic vulcanizates (TPVs) are prepared by a dynamic vulcanization technique by adding curative during a mixing operation. The TPVs consist of dispersion of vulcanized rubber domains in thermoplastic matrix, which differs from the simple blends. The viscosity plays a significant role on formation of TPV morphology. When the degree of vulcanization is high, the rubber particles may be broken into micron size of elastomeric particles. The dynamic vulcanization of rubber phase in the plastic matrix leads to formation of materials with improved properties of high elasticity, while thermoplastic phase provides the melt processing. The varieties of TPVs have already found commercial applications, especially in the automotive sector.^12,13
Poly(vinylidene fluoride), or poly(1,1-difluoroethylene) is known by its acronym PVDF. Polymerization procedures, temperatures, pressure, recipe ingredients, monomer feeding strategy, and post polymerization are variables influencing product characteristics and quality.¹⁴ In general, fluoropolymers are thermally more stable than hydrocarbon polymers. The high electronegativity of fluorine atoms on the chain and the high bond dissociation energy of the C–F bond provide the high stability of the fluoropolymers. PVDF has been observed to experience degradation during high-temperature operations. The outstanding heat stability and excellent oil resistance of these materials are due to the high ratio of fluorine to hydrogen, the strength of the carbon-fluorine bond, and the absence of unsaturation. PVDF generally possesses distinction chemical stability against most of the chemicals, including a wide range of harsh chemicals, inorganic acids and solvents.^15,16
Fluoroelastomers (FKM) are a class of synthetic rubber which provide extraordinary levels of resistance to heat, oil and chemicals, while providing useful service life above 200℃. FKM are a family of fluoropolymer rubbers, not a single entity. FKM can be classified by their fluorine content, 66%, 68%, and 70%, respectively. FKM having higher fluorine content have increasing fluids resistance derived from increasing fluorine levels.^17,18 One of these FKM, Viton A, is a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) developed by DuPont and was made available commercially in 1955. Many other reports have focused on chemical degradation of FKM rubbers in specific aggressive aqueous chemical environments. Many reports have been published on thermal degradation properties of FKMs, where attempts have been made to establish degradation mechanisms.^19,20
Carbon black (CB) is a reinforcing filler and also widely used as conductive filler to improve mechanical and electrical properties of high-performance rubber materials. Different types of CB offer differences in mechanical and electrical properties which mainly depend on the specific surface area and/or structure of the primary aggregates.²¹ An important characteristic of CBs is the diameter of the primary particles which is extremely small (typically less than 300 nm), in particular CBs, may even be up to 500 nm. The other characteristic feature is the CB structure. CBs are variously described as “low structure” and “high structure” that correlate with their spatial extent, the former having larger dimensions than the latter. The CB characterized by primary aggregates composed of many prime particles, with considerable branching and chaining, is referred to as a high-structure CB. If the primary aggregates consist of relatively few prime particles, the CB is referred to as a low-structure CB. The reinforcing potential in rubber is mainly attributed to two effects: (i) the formation of a physically bonded flexible filler network and (ii) strong polymer filler couplings. Both of these effects refer to a high surface activity and specific surface of the filler particles. ¹⁹ The fillers covered by the immobilized rubber interface can be considered as physical cross-links. This provides network chains for the rubber matrix in the proximity of CB particles.^22-24
PVDF and FKM have partially similar physical properties. As one of the special functional fluoroplastic, PVDF has been widely studied owing to its remarkable mechanical properties, thermal stability, piezoelectric properties, in combination with good resistance to high temperatures, UV irradiation and aggressive chemicals.^25,26 Incorporation of other types of fillers into PVDF matrix has been studied with the objective to improve its properties, such as mechanical and thermal properties. Incorporation of graphite in PVDF might act as a nucleating agent and accelerated the overall non-isothermal crystallization process of PVDF. The storage modulus and the dielectric constant of the composites increased linearly with graphite concentration.²⁷
The mobility of the polymer chains is restricted to some extent due to filler-polymer interaction, depending on a type of the polymers and fillers, and fillers distance which is mainly controlled by the specific surface area and filler loading.²⁸ The amount of immobilized rubber increases with the increasing content of CB. Stronger polymer-filler interaction would result in a thicker rubber shell for small particle-sized CB which is in combination with a larger interfacial area in the unit volume compound at the same loading. This gives more immobilized rubber shell in comparison with large particle sized CB. It is well established that the immobilized rubber content of higher structure CB is significantly higher than its low-structure counterpart. This is probably due to high structure CB is certainly related to the increased polymer-filler interfacial area and greater adsorption ability of the freshly built surface during mixing.²⁹
In the rubber manufacturing industry, fillers have been used to reinforce polymer matrices to achieve required mechanical properties. As an active filler, CB is typically adopted to reinforce FKM, such as Viton A. FKM products are exposed to intense conditions, i.e. high-temperature and chemical environments, during their lifetime.³⁰ Accordingly, this work investigated and elucidated the effects of different CB loadings on the rheological, thermal, tensile, dynamic mechanical, and electrical properties of PVDF and FKM composites.