1. Introduction
The automotive industry is paying great attention to lightweighting to meet the requirements against carbon dioxide emissions. Foam injection molding is a practical method for manufacturing plastic trim parts in an ecofriendly manner. The amount of plastic trim is supposed to be approximately 13% of the total weight of a vehicle [
1]. Unfilled heterophasic polypropylene copolymer (co-PP) is a type of cost-efficient, impact-modified copolymer that is widely used in exterior unaesthetic trim parts. Engine covers, trims, and luggage components are applications with co-PP, and their main functions are as coverings. Thus, flexural and impact behavior are more important mechanical properties of these materials than the tensile property is due to the main functionality of this application. Additionally, sound insulation is a desired function.
However, co-PP foams exhibit a nonuniform cell size and distribution, resulting in deteriorated mechanical properties [
2]. This behavior occurs because of the low melt strength, poor gas solubility, and two-phased morphological structure of the material. Thus, the ultimate weight reduction attained in practice is below 8%, considering processing problems at the industrial scale such as short shot or postblowing [
3]. Although numerous studies have been reported in the literature on the foaming of polypropylene materials, few focused on the relations among material structure, process parameters, and part performance. Investigations were mostly carried out on lab-scale batches or semicontinuous processes throughout the standard test bars or plates, and consequently provide limited insights. However, most automotive trim parts have a complex design consisting of elements such as bosses, ribs, and holes. Moreover, the high shear rate plays a dominant role in industrial-scale processing, which has a strong influence on foam formation. Frictional forces, loss on the pressure and melt temperature become much more significant through the long flow path from the gate to the endpoint in the mold of complex structures. Industrial foam samples have a macrocellular cell structure in the core layer regarding the orientation of additives governed by fountain flow and in efficient cooling [
4,
5]. To address this problem, there are many studies examining the utilization of nanoparticles from many perspectives [
6,
7,
8,
9,
10], and graphene nanoplatelets (xGnP) represent the most promising option among nanoparticles thanks to their scalable production and affordability.
The preparation of xGnP-co-PP nanocomposites is challenging due to agglomeration caused by particle–particle interactions [
10]. Moreover, the enhancement of the exfoliation degree of nanoplatelets is a requisite condition to fulfill the utmost improvement of mechanical and other properties. Many attempts to enhance the dispersion and exfoliation of the xGnP in the polymer matrix were reported in the literature, and the utilization of supercritical fluid (SCF) is a promising approach to address this challenge relating to its strong solvent and intercalation role [
11,
12]. SCF-assisted injection molding studies showed that the properties of thermal and electrical conductivity electromagnetic interference shielding can be improved by exfoliation of platelet structures [
13,
14,
15,
16]. In the literature, it was proven that the pretreatment of layered structured nanoparticles with SCF gases under batch conditions resulted in an increase in interlayer gallery spacing [
17,
18,
19]. However, the processing time during which a nanoparticle is subjected to SCF is not sufficient, and shear forces are predominant in continuous systems. There are few studies in the literature concentrating on the foam injection molding process. The cocontinuous concept of foam injection molding, first involving the pretreatment of clay nanoparticles with SCF during twin screw extrusion with low-density polyethylene, and subsequently foam injection molding of a derived nanocomposite as a second step, was investigated. Although ultimate weight reduction levels of about 15% and a decrease in average cell size were attained, the concept has drawbacks. The shelf-life limitations of the first step eventuated in an additional storage unit of semiproducts, and the aforementioned steps are not collocational operations in one single manufacturing site [
20]. The dispersion and exfoliation states of graphene nanoplatelets were investigated in a homopolymer polypropylene matrix by using subcritical gas-assisted processing during melt blending [
21]. The expansion of bubbles provides equibiaxial flow during cell growth. A dominant extensional flow driven by bubbles had a dominant effect on agglomerates accompanied by shear forces. As with xGnP, the effect reported on clay with SCF provides insights into this phenomenon. The mechanism of the enhancement of the dispersion and exfoliation of xGnP powder treated with SCF before foam injection molding was also reported [
22]. SCF gas molecules penetrate through the layers of clays thanks to their high diffusivity. Separation between interlayers occurs due to cell growth during the foaming steps, especially in the case of the subjection of SCF gas in the prefoaming stage. Consequently, exfoliation and dispersion are accompanied by a uniform foam morphology. In another study, xGnP with high-density polyethylene showed similar properties, in agreement with these findings [
23].
In this study, a new and facile approach to prepare co-PP foams with enhanced cell morphology by means of the utilization of dispersed and exfoliated xGnP is studied. We hypothesize that providing a practical study for designers and engineers in the automotive industry, choosing a representative exterior trim part design, and performing the experimental studies via industrial-scale equipment fill the gap in the literature from the industrial perspective. Therefore, we aimed to investigate (i) the enhancement of the dispersion and exfoliation degree of xGnP particles driven by the high shear forces of industrial-scale equipment; and (ii) the interaction between the SCF gas and xGnP platelets, and its effect on nanocomposite properties and structure formation during cell growth with the arrangement of xGnP particles through cell growth. Through a series of experiments, we evaluated the incorporation of xGnP into injection molding grade with commercial co-PP. Predispersed xGnP nanoplatelets in masterbatch form dilution were tested via foam injection molding in a one-step process, using 1 and 2 wt % xGnP. Specifically, these experiments are important for evaluating the two levels of SCF at 0.25 and 0.35 wt % with regard to performance to understand the effect on cell formation and dispersion state to compare the findings with the traditional nanocomposite foam preparation approach [
3]. Therefore, the foam samples were prepared in a two-step process. First, 1 and 2 wt % xGnP were melt-mixed with co-PP via twin-screw extrusion. Second, nanocomposites were subjected to foam injection molding. To ensure sufficient xGnP–co-PP interaction, maleic anhydride grafted polypropylene was used as a coupling agent. Lastly, we characterized the premixed samples in terms of physical, mechanical, and thermal properties, and evaluated all foam samples through morphological, mechanical, rheological, and structural analyses, and sound transmission loss measurement.
4. Conclusions
This study proposed a practical one-step approach via diluting a graphene nanoplatelet (xGnP) masterbatch as a process aid to enhance the weight reduction and mechanical properties of copolymer polypropylene (co-PP)-based foam automotive trim parts. According to our results, the incorporation of xGnP enhances the flowability, mechanical and thermal properties, and foaming behavior of co-PP. The presence of xGnP reduces the nonuniformity and polydispersity of the material. It contributes to the increase in the skin-foam thickness ratio of the foam and the elimination of cell coalescence. Increased cell size and decreased cell density were achieved in all samples comprising 2 wt % xGnP. However, the presence of a coupling agent and possible chain scission in twin-screw extrusion steps exhibited grainier cell sizes compared to those of the proposed method in this study. Flexural properties were improved with the addition of 2 wt % xGnP for both methods. However, the impact strength of the samples needs optimization. Increased supercritical fluid (SCF) levels improved flexural behavior in Method 1 regarding the orientation of particles and the exfoliation of layers. The sound transmission loss behavior of the nanocomposites was improved by up to 40 dB regarding the enhanced modulus for frequencies at which the bulk modulus plays a more important role than that of thickness. The foam of the 2 wt % xGnP nanocomposite prepared via Method 1 with high SCF enables improved sound transmission loss in lower frequencies compared to neat foams.
Consequently, the proposed method provided postulated enhancement on the cell formation, and mechanical and sound insulation performance of the foams by the addition of xGnP with 2 wt %. Method 1 with a high level of SCF was attained as the optimal process for the nanocomposites in this study. Additionally, processing the 12% weight-reduced part was enabled without any shot-shot or postblowing problems. The flow-oriented nanoplatelet structured morphology provides an opportunity for applications relying on enhanced thermal and electrical conductivity. Therefore, the proposed method has potential for electromagnetic interference shielding and the elimination of metal thermal dissipater subparts of engine cover parts. The approach seems feasible for manufacturing plastic parts such as luggage trims and engine undertrims in automotive applications.