Application of Polymer Materials in Automotive Industries

Because they are lightweight and due to their property tailorability, design flexibility, and processability, polymers and polymer composites have been widely used in automotive industry to replace some heavy metallic materials. Table 6 (Szeteiová) and Fig. 22 (Polymers in the automotive industry polymotive 2005) show the typical polymers used in a car.
The cars with more components made from polymer composites are lighter. Figure 23 (APME 1999) shows the typical weight saving that can be made in various car parts when using plastics to substitute conventional materials.
Cars with more polymers are lighter; in turn, chassis, drive trains, and transmission  parts can all be made lighter as a result of having to support a lower overall car weight. The benefits from the weight reduction are the improved efficacy of fuel and less greenhouse gases release. It has been estimated that for every 10 % reduction in a vehicle’s total weight, fuel consumption reduces by 7 %. This also means that for every kilogram of weight reduced in a vehicle, there is about 20 kg of carbon dioxide reduction (Frost & Sullivan 2005). It is estimated that the polymers will account for 18 % of average vehicle weight by 2020, up from 14 % in 2000 as shown in Fig. 24 (A.T. Kearney Inc 2012).

The plastic components have been used from simple interior in the early stage to interior, exterior, powertrain, chassis, engines, electrical systems, and fuel systems in the present. The enormous growth of polymer components in automotives accompanies the demand for high-performance polymer materials to meet the requirements of the components; one of the advanced materials developed to apply in automotive sectors is polymer nanocomposites, which was firstly used by Toyota Motor Co. in 1991 to produce timing belt covers as a part of the engine for their Toyota Camry cars (Polymer nanocomposites drive opportunities in the automotive sector).
Polymer nanocomposites, which are among the most widely watched technology areas in the plastics arena, are a new class of materials containing nanoparticles or nanofillers dispersed in the polymer matrix. The commonly used nanofillers for polymer reinforcement include nanoclays, carbon nanotubes, carbon nanofibers, nanosilica, nano-oxides, and polyhedral oligomeric silsesquioxanes; they usually can improve a wide range of properties of polymers at low filler fraction owing to their size and shape.

Approaches of Fabricating Polymer Nanocomposites

There are several ways to fabricate polymer nanocomposites; the commonly used are in situ polymerization, solution mixing, and melt compounding.
In situ polymerization of polymer nanocomposites includes emulsion, emulsifier-free emulsion, miniemulsion, and dispersion polymerization; nanofillers are usually added directly to the liquid monomer, and a polymerization then starts either thermally or chemically. Significant property improvement can be achieved due to good interaction between the nanofiller and polymer matrix. Figure 25 (Liang et al. 2008) shows the organic-modified nanoclay-filled nylon 6 nanocomposites formed by in situ polymerization, Fig. 26 (Liang et al. 2008) shows the barrier property, and Table 7 (Liang et al. 2008) tabulates the mechanical properties and heat distortion temperature (HDT) of the nanocomposites as a function of the filler fraction.

The solution mixing to prepare the polymer nanocomposites may consist of several steps: (a) dissolving polymer matrix into an appropriate solvent to make a solution, (b) dispersing the nanofiller into the solution to make a suspension, and (c) casting the new mixture to evaporate the solvent to produce final nanocomposites. A method through solution blending and then compression molding is also usually used by obtaining the mixture powders after procedure (b) and compression molding into the mixture powders into the desired panel or prototype. One of the advantages for the solution mixing is the molecular level of mixing. Figure 27 (Bhattacharya and Chaudhari 2013) shows the mechanical properties of nanosilica-filled polyamide composites prepared by formic acid mixing.

The composite film exhibits an increased tensile strength with an increase in silica content. However, composite film containing 1.0 wt% nanosilica exhibits much lower tensile properties as compared to the neat polymer due to poor particle distribution.
In the melt compounding process, nanofillers are mixed with the polymer matrix at the molten state of polymers in the absence of any solvents. The dispersion of the nanofillers depends largely on the thermodynamic interaction between the polymer chains and the nanofillers. Comparing to other techniques, melt compounding is simple, versatile, and suitable for mass production; the resultant nanocomposites usually have high purity as the process is essentially free of contaminations. Figures 28 and 29 show the mechanical and thermal properties of the polypropylene (PP) nanocomposites as a function of graphene fraction (Pingan et al. 2011). The graphene used in the study was firstly coated with polypropylene latex and then melt-blended with PP matrix.

The graphene sheets were well dispersed in the PP matrix and considerable enhancement of the mechanical and electrical properties of PP was achieved by incorporating very low loading of graphene. By addition of only 0.42 vol% of grapheme, about 75 % increase in yield strength and 74 % increase in the Young’s modulus of PP were achieved.

Toughened Polymer Nanocomposites Prepared by Melt Compounding

Tensile strength and impact strength of materials are among the most important properties for the materials applied in automotives. There are a few reports that show nanoclays improve both tensile strength and impact strength for the nanocomposites prepared by melt compounding (Liu and Wu 2002). However, the increase of the impact strength is usually only achieved at low nanoclay fraction; the further increase in the nanoclay fraction results in decrease in impact strength as shown in Fig. 30 (Kelnar et al. 2005).

Nanoclays usually can increase the tensile strength and modulus of the polymers due to their rigid inorganic nature, nanoscale dimension, and huge adequate interfacial contact area between the nanoclay and the polymer matrix. The increment of impact strength at low nanoclay fraction is perhaps caused by formation of submicron voids within the intra-gallery of clay layers under impact loading, which prevents crack propagation. On the other hand, nanoclays, particularly exfoliated nanoclays, actually act as stiff fillers which hinder the mobility of the surrounding chains of polymers and thus reduce the impact strength of polymers. Moreover, the size range of individual clay layers is in nanometer scale, which is perhaps too small to provide toughening via mechanisms like crack bridging.
The common approach to improve the impact strength of polymers is to add elastomers with long molecular chains. The cavitation of elastomer particles followed by plastic deformation of the matrix is usually the main toughening mechanism in the polymer composites. Nevertheless, the soft nature of elastomers generally decreases the tensile strength of polymers. In order to obtain polymer composites with improved impact strength without sacrificing mechanical strength, ternary polymer nanocomposites with the presence of nanofillers such as nanoclay and elastomer have been designed. It has been found that the mixing sequence of the polymer, elastomer, and nanofiller have great effect on the properties of the nanocomposites, particularly the impact strength of the composites. It has been found that for polyamide 66 or nylon 66 ternary nanocomposite filled with nanoclay of Cloisite® 30B and elastomer of styrene–ethylene/butylene–styrene tri-block copolymer grafted with 1.84 wt% of maleic anhydride (SEBS-g-MA) (Dasari et al. 2005), the location of the nanoclay in the nanocomposites is different with different mixing sequences based on microstructure study. The two-step mixing–blending nylon 66 and nanoclay initially and later mixing with SEBS-g-MA is the preferred blending sequence to maximize the notched impact strength due to the maximum amount of the exfoliated nanoclay in the nylon 66 matrix. The presence of nanoclay in SEBS-g-MA elastomer phase reduces the cavitation ability of SEBS-g-MA particles. For the polyamide 6 nanocomposites filled with nanoclay of Cloisite® 93A and elastomer of maleic anhydride-grafted-poly(ethylene–octene) (POE-g-MA), it has been found that the one-step compounding of PA 6 with the nanoclay and the elastomer shows the synergetic effect of the two types of the fillers in improving the tensile modulus and impact strength (Yu 2012; Fig. 31).

As compared to PA 6, the impact strength of the nanocomposite is remarkably increased by 96.3 % in one-step mixing, while the impact strength remains the same in two-step mixing–blending of PA 6 with elastomer first and then blending with nanoclay. The nanoclay used in the study has a higher affinity to the PA 6 than to the POE-g-MA; the nanoclays disperse mainly in the PA 6 phase to enhance the tensile property in the one-step process.
On the basis of the study on the extrusion sequence, the properties of the ternary composites are further optimized by investigating the filler fraction. Figure 32 (Yu 2012) shows the tensile and impact properties for PA 6 composites with optimized filler fractions. The results are obtained with drying of the test pieces in the oven before testing to eliminate the effect of moisture absorbed on the mechanical properties of the composites.
With the optimization in concentration of the fillers, PA 6 composites with much improved impact strength have been obtained; the tensile strength and modulus, at the same time, are not sacrificed. With 7.5 % of nanoclay and 10 % of toughening agent, the impact strength is increased by 110.0 %, modulus is increased by 9.2 %, and tensile strength is similar to that of neat PA 6.

Green Composites for Automotive Applications

With the drive of lightweight materials with super performance, improved fuel efficiency, and less CO2 emissions in automotive industry, the usage of polymer nanocomposites in a car will continue to grow. Recently, there is increasing interest about green composites with growing environmental awareness. Green composites are composites that are designed to reduce environmental burden through their life cycles. The examples of green composites are natural fiber-reinforced polymer composites and biopolymer composites. Biopolymer composites are composites in which the polymer matrix is bio-based or biodegradable or bio-based and biodegradable. The research on application of green composites in automotives is as early as 1990s, the first car component made from natural fiber composites is believed to be a door quarter panels made of a LoPreFin PP/PET/natural fiber composite appeared on the 1999 Saab 9S. Figure 12 shows use of natural fibers for composites in the German automotive industry from 1999 to 2005 (Karus et al. 2006; Fig. 33).

Both natural fiber and biopolymers generally are hydrophilic, easily absorb moisture, and have relatively poor processability and low mechanical properties. These disadvantages restrict their application, particularly as exterior automotive components. Research and development are required to overcome these obstacles to allow more green composites which are eco-friendly, lightweight, and costeffective to be applied in automotive sector. One of the approaches to modify the properties of natural fiber-reinforced polymers and biopolymers is incorporated with nanofillers. Chieng et al. (2012) reported that with the addition of only 0.3 wt% of graphene nanoplatelet, the tensile strength and elongation at break of poly(lactic acid)/epoxidized palm oil blend increased by 26.5 % and 60.6 %, respectively. Nemati et al. (2013)studied mechanical properties of wood plastic composites made from wood flour, recycled polystyrene, and nanoclay. The obtained results indicated that the tensile strength and flexural strength were increased by raising nanoclay content in the composites as shown in Fig. 34.

Guigo et al. (2009) prepared lignin and natural fiber nanocomposites filled with sepiolite or organically modified nanoclay by extrusion. It was found that the incorporation of 2 % or 5 % w/w of sepiolite does not influence the mechanical and thermal behavior compared to the reference lignin/natural fibers composite, while nanoclay-based nanocomposites have shown improved properties. Shi et al. (2006) investigated the effect of single-walled carbon nanotubes (SWNTs) and functionalized SWNTs (F-SWNTs) on electrical and mechanical properties of  unsaturated, biodegradable polymer poly(propylene fumarate) (PPF). It was found that nanocomposites with 0.1 wt% F-SWNTs loading resulted in a threefold increase in both compressive modulus and flexural modulus and a twofold increase in both compressive offset yield strength and flexural strength when compared to pure PPF networks, whereas the use of 0.1 wt% SWNTs gained less than 37 % mechanical reinforcement. The SWNT also increased significantly the electrical conductivity of the PPF polymer matrix as shown in Fig. 35.

It has been proven that that the addition of nanofillers is an effective way to improve properties of neat polymers; the green polymer nanocomposites are considered to be the next-generation materials for automotive and other industries.