**1. Introduction**

The circular economy promotes sustainability by combining the hierarchy of reduction, reuse and recycling, considering economic and environmental issues [1]. The use of post-consumer materials to manufacture new ones for a new production cycle to minimize the waste of natural resources is the goal of the circular economy [2–6]. The same can be said of plastic, an important class of materials that meets many of society's needs.

Plastics are present in our daily life under different forms and applications, such as office supplies, toys, footwear, civil construction, electrical and electronic components, aerospace, food, the medical and textile industries, packaging, paint and varnish, as well as the automotive industry, among others. This array of applications is due to their desirable properties in a range of sectors [7, 8].

However, not all plastics return to the production cycle after use. In 2015, only 9% of plastics produced worldwide were recycled, 12% incinerated and the rest buried in landfills [9]. Recycling causes less environmental impact, as reported by Bernardo et al. [10], who assessed recycling in terms of global warming and total energy use, concluding that plastic materials generally display environmental and economic advantages over conventional materials throughout their life cycle, from raw material extraction to synthesis, transformation, transport, use, recovery and destination. Duval and Maclean [11] also found a decline in greenhouse gas emissions and energy required during recycling [12]. Mechanical recycling involves the addition of virgin or recycled material to maintain properties [13].

Plastics can be recycled in different ways, including mechanically, chemically and energetically. Chemical recycling involves physical processes, such as remolding [14–16], and the final product is a monomer or oligomer that can be used in the synthesis of other products. In energy recycling, the energy released from the burning of waste material is reused [14].

 Closed-loop recycling occurs when the recycled material replaces the virgin material in the same production cycle as the original product [17, 18]. Open-loop recycling is when the recycled product is used in a different production cycle, that is, the product to be recycled is used to manufacture a product different from the original [17, 19, 20].

 A crucial point to stimulate recycling is the search for a different market for the recycled material and more environmentally sustainable processes. Traditionally, recycled products compete with virgin material, which may hinder their market entry. Scientific studies that focus on recycling should also seek to obtain more economically feasible and technically useful recycled products.

 The most widely used and manufactured plastics are high-density polyethylene (HDPE), polypropylene (PP), low-density polyethylene (LDPE), poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC) and polystyrene (PS) [9]. In the automotive industry, PP is used to manufacture the following items: car trunk lids; battery trays and boxes; heater boxes; tool boxes; seat belt buckling boxes; rear view mirror boxes; electric junction boxes; hubcaps; carpets; battery guards (protection against short circuit); steering wheel covers; shock absorber covers; vacuum hoses; air hoses; consoles; bumpers; glove boxes, among several other uses [21–24].

To comply with the main technical demands of automobile manufacturers, PP compounds must exhibit a suitable balance between stiffness and tenacity, with good thermal resistance, as well as fewer imported raw materials, thereby achieving more competitive prices. In addition to these properties, PP shows good processability [24–26].

 An important supplier of materials to the automotive industry is the industrial sector responsible for manufacturing laminated and tempered glass used in motor vehicle windows (laminated glass for windshields and tempered for the other windows). However, in the tempering and laminating processes an industrial residue consisting of glass powder is generated and disposed of in landfills, with no specific use for this material [27]. In addition to the origin of glass powder in laminating and tempering processes [28], the windows that are removed from automobiles are also discarded when they cannot be reused. In such cases, these parts can be collected and recovered, then submitted to separation processes (polymer protection film) and grinding. The glass powder produced can be incorporated into polymer materials, resulting in composites with different properties [29].

*Study of the Technical Feasibility of the Use of Polypropylene Residue in Composites… DOI: http://dx.doi.org/10.5772/intechopen.81147* 

Incorporating mineral loads into PP has been the object of studies on the production of materials with different properties [30, 31]. Improving the properties of the final product depends on the type of load, particle size of the mineral load being used and degree of dispersion of these particles in the polymer matrix. The most widely used commercial mineral loads are talcum and calcium carbonate [32–34].

This study describes the addition of glass powder to a PP matrix in order to obtain reinforcement properties and compare them with those of conventional composites. The aim is to acquire different properties in the polypropylene composites and reuse a residue (in this case, glass powder). We also assessed the effect of adding recycled polypropylene on the final properties of composites in order to reuse both industrial (glass powder) and urban residue (PP recycled from packaging).

### **2. The use of polypropylene in a composite or mixture**

Polymers have been increasingly used in a number of applications as a substitute for traditional materials such as metal and ceramic, as homopolymers; formulated with additives, in the form of mixtures and polymer composites; or simply for their different properties, such as lightness, low transformation cost, resistance to corrosion, optimal thermal and electric insulation and easy conformation into complex shapes [33].

In general, the mechanical properties of polymers are not suitable in a number of applications owing to their lower resistance compared to metals and ceramics. However, the thermoplastic industry is growing due to ecological issues, in addition to the promising potential of these materials as mixtures or a composite matrix [35].

Compound systems formed by the combination of two polymer materials (mixtures) or a polymer material and a load (composites) are of significant technological interest due to the cost–benefit ratio. In both cases, the material consists of a continuous (matrix) and disperse phase, whose properties depend on good interaction between them [35].

The properties of interest for the automotive industry can be modified with studies on improving the polymer matrix, load, and polymer-load interface, among others. The interface is a link between the surface of the load and the matrix, and since the matrix receives the reinforcement, there is close contact between them, and there may or may not be adhesion. For a same combination of materials, different adhesion mechanisms can occur, such as mechanical, chemical, and electrostatic adhesion and by interdiffusion. The degree of reinforcement or improvement in mechanical behavior depends on a strong matrix-particle interface bond [36, 37].

The stress–strain behavior of many reinforced polymers or plastics can be changed by adhesion promoters and interfacial coupling agents (such as maleic anhydride) that alter adhesion and the nature of the matrix-load interface [38].

Polypropylene (PP) is a recyclable thermoplastic, that is, it melts when heated and hardens again when cooled, in a reversible process. Moreover, PP is easily mixed, primarily with organic reinforcing loads such as natural or inorganic fibers, including calcium carbonate, clay and talcum, and is widely used in structural applications [39–41].

The use of modified PP, especially for applications in the automotive industry, requires a suitable balance between stiffness and tenacity. In this scenario, the process of incorporating elastomeric materials, as well as mineral loads such as talcum and calcium carbonate (CaCO3), into the PP matrix has been widely used to achieve different properties [42, 43].

Nanofillers, such as silica and calcium carbonate nanoparticles, have been added to improve the final properties of the PP matrix [44, 45].

#### **2.1 Use of glass as an additive to the composite**

The use of glass in a polypropylene matrix has been extensively studied and employed its glass fiber form in materials in which mechanical properties such as tensile strength and resistance to impact are important [46, 47].

There are several groups of glass, including silica, oxynitride and phosphate, but the first is the most important raw material used in composites [48]. Short E-glass fibers, obtained from a mixture of Si, Al, B, Ca and Mg oxides, are normally used as reinforcement for thermoplastics due to their low cost when compared to aramid and carbon [49], in addition to better impact strength and stiffness [50].

 The interfacial interaction of glass composites with a thermoplastic matrix is often very weak. Particularly with polyolefin polymers such as polypropylene, there is little or no chemical reaction between the glass and the matrix. The interest in polypropylene for applications as a matrix in composites has been growing and the adhesion of this nonpolar polymer to the glass surface, which is also nonpolar, is a daunting challenge [51, 52].

In addition to the use of glass fiber, there are also glass microsphere applications [53]; however, residual glass powder remains a poorly explored load as reinforcement.

### **2.2 Environmental justification for polypropylene and window glass, materials contained in automobiles**

Initiatives to develop more sustainable technological innovations and ecologically responsible management programs have been driven by a growth in environmental awareness and increasingly rigid legislation. The accumulation of plastic waste caused by the increase in per capital consumption of thermoplastic resins has prompted enormous research and efforts to substitute traditional thermoplastics [54].

To improve the production process, it is necessary to diagnose the flowchart of the process and manage inputs (water, energy, raw materials, etc.) and outputs (products, residues, effluents, atmospheric emissions, etc.). In general, inputs are natural resources that often cause environmental impact, such as ecosystem destruction, atmospheric pollution, etc. Outputs are environmental liabilities created by activities and residual materials (solid, liquid or gas) that, if not suitably managed, may cause permanent environmental impacts [55].

A sustainable production process contains a circular flow, where outputs are reintegrated into the process, which reduces impacts and costs in the generation of inputs and the destination and treatment of outputs. Recycling is an example of this type of sustainable production strategy and is therefore an attempt to reuse the material, natural resources and entropy expenditures in the production of a solid residue, reintroducing it into a new production process, thereby transforming the output of a process into the input of the same or another process [55].

The automotive industry is attempting to transform the car into a more sustainable and efficient product, not only in terms of the environment, but also from the consumer's financial standpoint. As such, the automotive industry has been working within the so-called DFE (Design for the Environment), that is, designing for the environment and introducing environmental variables in all the production strategies of the factory, such as product design (automobiles and parts), the process (manufacture of parts and assembly) and associated technologies (material treatment, painting, etc.) [28].

It is important to underscore that all participants in the life cycle of a product have shared responsibility. Thus, manufacturers, importers, distributors,

*Study of the Technical Feasibility of the Use of Polypropylene Residue in Composites… DOI: http://dx.doi.org/10.5772/intechopen.81147* 

merchants, consumers and public cleaning concessionaires should promote the reuse of solid residues, transfer them to the production chain, reduce residue generation and encourage the development of products derived from recycled materials. The automotive industry, like all companies, is responsible for the entire process, from acquiring raw materials to discarding components, such as bumpers. Moreover, polypropylene is present in many automobile components.
