Keywords

1 Introduction

Due to substantial improvements in attributes like mechanical and thermal properties in polymer and as a result of its versatile use in multiple industries, polymer-based composite materials have garnered a lot of interest. These polymer composites usually have one or more reinforcing agents (particles, fibers, etc.) and a polymer matrix. Out of thermoplastic and thermosets, scientists commonly prefer thermoplastic polymer matrices because of lower cost of processing, low production cycle and higher reversibility [1]. Among all, Polypropylene (PP) is the most considerable thermoplastic polymers because of its great chemical resistance and mechanical properties. Composite preparation by polypropylene matrix reinforcement with varying fillers load percentage is witnessed [2]. Depending on the nature, filler materials are divided into two types: fibers and particles. Fibers are defined as elongated, thread-like structures that have a high aspect ratio and are capable of being spun into filaments, yarns and strings. The fibers like cellulose [3], wood fibers [4], jute [5], etc. are being used for reinforcing polymers. Similarly, particulate reinforcing agents such as reduced graphene oxide (rGO), graphene oxide (GO) [6, 7], aluminium [8], TiO2 [9], noble metal nanoparticles like Ag and Au [10], Cu [11,12,13] and other metal and non-metal particles are being used by many researchers. The selection of reinforcing agent is based upon the desired applications. As an example, carbon-based filler materials are conditioned to enhance the tensile strength, toughness, and thermal stability of materials [2, 14, 15]. The effect of the mixing of the Copper into polypropylene matrix enhances electrical and thermal conductivity, antimicrobial property [11,12,13]. Aluminium based fillers showed greater thermal stability and lesser tensile strength and strain at break [8]. Zühtü Onur Pehlivanl et al. used boric acid granules as reinforcement material in PP matrix and the composite showed improvement in insulation property or in other words, it decreases the thermal conductivity of the composite [16]. Similarly, several scientists have been using various other materials for reinforcement in polymers to tailor desired functionalities.

The methodologies for preparing these composites can be approached in-situ and ex-situ. In-situ methodology involves synthesizing reinforcing phase within the matrix of the polymer during manufacturing process; this can be achieved by chemical reactions, or polymerization of the co-constituents. For example, in one of the methods, the desired reinforcing agent can be synthesized by decomposition or reduction of the precursor material within the polymer matrix [10]. Another method involves addition of filler material to monomer along with a block copolymer and during the polymerization process, the filler becomes embedded within the matrix material [17]. Whereas in the case of the ex-situ approach, the reinforcing material is synthesized separately and then blended with a pre-polymerized matrix to fabricate the composite. The synthesized composites possess various properties and these are categorized into Thermal and Mechanical properties. These characteristics can be analyzed using various characterization techniques and tests.

2 Different Types of Reinforcement Materials

The researchers select appropriate filler materials and synthetic routes that align with their end goal. A few of the reinforcement materials and methodologies are discussed in Table 1.

Table 1 Methodologies for preparing polymer composites using particulate fillers
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Selection of methodology and processing condition can significantly affect the characteristics of polymer composite. It may have an effect on filler dispersion as well as distribution in polymer matrix. The effect of different methodology on solid loading are focused in the following section.

2.1 Melt Blending

It’s a common process for producing polymer-based composites. The matrix material and reinforcing agent, along with any other additives, are blended together in a molten state. So, in this technique, the matrix material is heated to a temperature slightly above its melting point. The mixture is subsequently subjected to shear forces, which facilitate the dispersion of the reinforcing materials throughout the matrix. It is reported that, melt blending technique produces composites with high porosity, thermal stability and intercalation [28]. This method has the advantage of being easy and inexpensive. However, it could be difficult to achieve consistent filler dispersion across the matrix. Nukala et al. [29] prepared and characterized composites based on wood polymer from recycled polypropylene and sawdust as the filler. The composite was prepared by melt mixing as shown in Fig. 1, followed by compression moulding. The SEM images revealed that SD was evenly distributed throughout the recycled polypropylene matrix. Thermal characteristics revealed that adding SD to the rPP matrix enhanced the thermal stability of recycled polypropylene-sawdust (rPPSD) composites. Moreover, as there was an increase in filler material, so did the tensile and flexural strength and water absorption capacity.

Fig. 1
A flowchart of r P P S D composite preparation includes cleaned recycled polypropylene r P P, cleaned sawdust S D, mixing as per desired weight ratio, melt mixing of 190 degrees Celsius and 8 revolutions per minute, drying and crushing of obtained composite, hot pressing, and r P P S D composite.

Flowsheet for preparation of rPPSD composite [29]

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Osman et al. [8] synthesized Al-PP composites using flaky and spherical shape filler particle and varying loadings (0–55% v/v). The composite was synthesized by melt blending, followed by compression molding. Increases in filler loading resulted in a decrease in strain at break and tensile strength. However, when the filler content increased, so did the voids, modulus, density, and thermal stability. The percolation concentrations for flaky and spherical particles were between 15–30% and 30–45% respectively.

2.2 Solution Mixing

Using the method of solution mixing, fillers are dispersed in a polymer solution by vigorous agitation, followed by cautious solvent evaporation and casting of the polymer composite. It can be implemented to achieve a high degree of reinforcing material distribution. It is, however, more expensive and time consuming than melt mixing, and it involves solution handling and rigorous dispersion methods such as ultrasonication. As a result, it is less desired among industries. Zanrosso et al. [30] fabricated polyvinylidene fluoride (PVDF)/ZnO composite using solution mixing technique. Initially, a solution of ZnO and N–N-dimethylacetamide (DMAc) was prepared. Subsequently, pore forming agents Polyvinylidene fluoride (PVDF) and Polyvinylpirrolidone [PVP] were introduced to the solution, the resulting mixture was subjected to centrifugation, ultrasonication and finally casting was performed to prepare PVDF/ZnO composite films. The findings indicated that the incorporation of ZnO improved the photocatalytic activity of the composite films.

Kuila et al. [31] synthesized efficient graphene-polyethylene (linear low-density) based composites, using solution mixing methodology. To synthesize dodecyl amine moderated graphene (DA-G)/LLDPE composite, predetermined quantities of DA-G and LLDPE were provided separately to xylene, and the solutions were subjected to ultrasonication and constant stirring respectively. The procedure of preparation is shown in Fig. 2. Afterwards, dispersed DA-G was added to LLDPE solution, followed by heating, stirring and casting to obtain the composite. The electrical conductivity, tensile strength and thermal stability of the composite are enhanced after adding DA-G.

Fig. 2
A flowchart of D A-G or L L D P E preparation includes D A-G + xylene, L L D P E + xylene, ultrasonication, constant stirring, vigorous stirring, casting and drying, cast and dried at room temperature, injection molded for 10 minutes at 150 degrees Celsius, and D A-G or L L D P E composite.

Flowsheet for preparation of DA-G/LLDPE [31]

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2.3 Extrusion

This procedure involves applying pressure and heat to melt and blend the polymer matrix with filler materials before driving the mixture through a die to generate a specified shape. Long et al. [7] prepared a composite by dispersing functionalized graphene oxide (modified using phosphorous acid) in the PP matrix. The composite was prepared by extrusion followed by injection molding. Compared to graphene oxide (GO), functionalized graphene oxide (FGO) showed better dispersion in PP. Unlike GO/PP composite, incorporation of FGO to PP matrix exhibited greater increases in thermal stability, mechanical properties and flame retardancy. Palaniyappan et al. [32] synthesized an Acrylonitrile Butadiene Styrene/Silicon (ABS/Silicon) composite by using single screw extrusion, followed by the Fused Deposition Modelling (FDM) procedure as shown in flowchart (Fig. 3). Silicon reinforcement enhanced mechanical attributes like flexural, compressive and tensile strength of the composite.

Fig. 3
A flowchart of silicon or A B S composite preparation includes A B S granules + fine silicon particles, ball milling for 3 hours, single-screw extrusion at 235 degrees Celsius and 20 revolutions per minute, a water bath, composite specimen preparation by fused deposition modeling, and silicon or A B S polymer composite.

Flowsheet for preparation of silicon/ABS composite [32]

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2.4 Reactor Granule Technology

This is a type of in-situ technique in which the precursor of reinforcing material is impregnated with the polymer, followed by melt mixing is carried out to prepare the composite. Precursor material is chemically transformed to the desired filler material during the melt mixing process. Maira et al. [33] prepared PP nanocomposites by integrating oxide nanoparticles. To synthesize PP/Al2O3 nanocomposites, PP reactor granules were impregnated with precursor of Al2O3 i.e., aluminumisopropoxide (Al(OiPr)3). The granule was melt-mixed, followed by hot pressing, to prepare the nanocomposite samples after impregnation and solvent removal in vacuo. In comparison to traditional nanocomposites, the Al2O3 nanoparticles’ more effective dispersion resulted in appreciable increases in the tensile properties, dielectric constants, and thermal conductivity.

Maira et al. [34] fabricated polypropylene based nanocomposites by using an innovative reactor granule technology (additive-free). First, PP powder was impregnated with a solution of magnesium ethoxide. Afterwards, solvent drying was done. Subsequently, pre-hydrolysis and melt mixing of the powder was done to obtain magnesium hydroxide (Mg(OH)2). This process is shown in Fig. 4. TEM images confirmed that the synthesized PP/Mg(OH)2 composite exhibited homogeneous distribution of Mg(OH)2 nano range particles in the matrix at higher filler loadings.

Fig. 4
A flowchart to formulate P P or M g O H 2 composites includes P P reactor granule + magnesium ethoxide, impregnated in methanol or toluene at under N 2 for 50 degrees Celsius for 12 hours, solvent removal in vacuo, melt mixing at 180 degrees Celsius, hot pressed at 230 degrees Celsius, and P P or M g (O H) 2 composites.

Flowsheet to formulate PP/Mg(OH)2 composite [34]

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3 Conclusion

Present days, polymer composites have received huge interest because of wide range of applications. After observing various results, the subsequent inferences can be drawn.

The final characteristics of the composite are predominantly influenced by the selection of fillers and the synthetic routes employed. By strategically choosing appropriate fillers and tailoring the synthesis process, researchers can achieve a high degree of control over the properties of the final material. The synthetic route can influence factors like the interfacial bonding between the filler and the matrix (the surrounding material), ultimately impacting the overall performance of the composite. Particulate fillers are playing a major role because of wide range of applications. The particulate reinforcements like GO, rGO, Al2O3, TiO2, Al etc. are enhancing the thermal stability and mechanical properties including tensile strength, Young’s modulus. The polymer composites with Au, Ag, Cu exhibit strong antimicrobial property and these are having potential use in medical applications.

For preparing the particulate fillers-based polymer composite, the commonly used techniques are melt blending and extrusion as these are economic processes. However, the homogeneity or uniform distribution of fillers is difficult to achieve using these two methodologies. Many morphological studies also revealed the presence of pores and accumulation of fillers in polymer matrices during melt mixing and extrusion.

The solution mixing and in-situ methodologies like reactive extrusion and reactor granule technology are relatively more expensive and time consuming. On the other hand, these techniques are implemented to achieve effective dispersion of fillers. The pores and accumulations of fillers can be reduced to greater extend by employing these preparation techniques.

Out of these techniques, researchers need to choose the most effective pathways to synthesise composite materials as per the desired output.