The mechanical and optical qualities of polymers are depleted as they are recycled. For food packaging applications, optical properties are critical. In a previous study, semi-crystalline isotactic polypropylene (iPP), a commercial polypropylene grade, was investigated to ensure a suitable opacity upon recycling [14]. Monoclinic, trigonal, and orthorhombic forms are among the crystal modifications found in semi-crystalline iPP structures [15]. Despite the fact that polypropylene is one of the most widely used plastic packaging components, only around 1% of it is reprocessed, meaning the majority of it ends up in landfills [16] These take 20–30 years to disintegrate. Apart from the harmful chemicals in PP like lead and cadmium, this poses serious environmental concerns. Dioxins and vinyl chloride, both harmful, may be released during incineration.
Companies have done "life cycle" studies to determine how recyclable polypropylene is, which examine at the plastic from production stages to the last phase of waste disposal to determine the materials long-term viability. According to the findings of these investigations, PP has a lot of potential as a long-term product[16].
First, other plastic polymers must be removed from the polypropylene. This is achieved using a process known as "sink-float" separation, which is focused on the peculiar relative density of Polpropylene (0.93–0.95 gram/cm3), which enables it to hover when other polymers, including PET relative density 1.43–1.45 gram/cm3 sink. Despite the fact that major enterprises recycle roughly 30% of polypropylene, a significant portion is still discarded in landfills. Recycling PP is currently not as cost-effective as recycling other polymers, particularly HDPE, LDPE, and PET. With developments in recycling technology, it is believed that this will alter in the near future. [16]
3.1 Polypropylene fibre utilization in construction industry
Concrete has been the most expensive material in the construction sector in recent years. The cost of concrete ingredients is extremely exorbitant and out of reach for most people. In today's world, there is also a scarcity of material. The cost of steel, for example, is extremely high, which drives up the whole building cost [17]. As a result, the researchers should be cautious about the materials utilized in building.
One of the most difficult tasks facing researchers right now is to establish a balance between the needs of the construction sector, environmental preservation, and human health. Thermoplastic materials such as polyethylene and polypropylene can be used to reach this equilibrium. Polyethylene is popular due of its strong tensile strength and compressive strength, low toxicity, good electrical properties, and light weight; polypropylene, on the other hand, has excellent dielectric properties and acid and alkali resistance. Their collection, management, and safe disposal are given special care due to their massive production and consumption [8].
Polypropylene fibres have become the most widely used commercial commodity in reinforced concrete in recent decades, owing to their ease of manufacture and high tensile strength. They have also been utilised to prevent early age cracking caused by concrete shrinkage. Cracking, interfaces, shear behaviour dynamic behaviour, impact behaviour, high performance concrete lightweight concrete, fly ash concrete, and shrinkage have all been studied extensively in the mechanical performance of polypropylene fibre reinforced concrete. When fibres are torn out of cementitious matrix, stress fluctuations arise. The authors of this study concentrate on stress variations during flexure since correct evaluation of these variations could aid in monitoring the performance of fibre reinforced concrete under various service loads [18].
As the fibre content of the FRSCLC samples was increased, the compressive strength did not improve much. Because to nonhomogeneous components, several FRSCLC specimens showed a minor drop in compressive strength. The addition of polypropylene fibre to the FRSCLC specimens increased the modulus of elasticity slightly, although the extent of that gain is unclear, with hybrid fibre Mix showing the greatest improvement in modulus of elasticity. As a result, it can be inferred that the addition of polypropylene fibre to the elastic modulus has a minor effect, with the hybrid fibre samples showing the greatest increases. The addition of polypropylene fibre to the samples results in a little modification in splitting tensile strength, although the difference is not discernible. While some samples show an increase, others show a drop, with the hybrid fibre samples of Mix showing the greatest rises [19].
Polypropylene fibres are used in pavement as they not just to help to improve fracture resistance, but they also have superior chemical resilience and are less expensive than other fibres.. Polypropylene, unlike steel and polyvinyl alcohol fibres, is unreactive, viscous, and has a weak surface free energy, resulting in a low bond between the Polypropylene fibres and the cement matrix. Because of the hydrophobic surfaces of PP fibre, there is a lot of porosity between the fibre and the hydration product, which reduces binding strength. Furthermore, chemically stable PP in an alkaline environment is unlikely to interact with hydration products. To address this issue, several solutions for increasing interfacial characteristics have really been developed. One of the ways was to increase the strength of the cement matrix. However, the cement matrix's mechanical properties, such as young's modulus and hardness, were greater than those of the PP fiber/cement interfacial transition zone, implying that the influence of further cement matrix strengthening on improving interfacial behavior could be limited. As a result, polypropylene fibre surface changes were examined in order to attain this goal [20].
Using physical methods such as mechanical indentation production, improve the surface roughness of Polypropylene fibres and enhance the interfacial binding strength of fibre content or cement. Chemical treatments such as detergent, polyvinyl acetate, and acid–dichromate increased the surface roughness and hydrophilicity of PP fibres, resulting in greater beam bonding and flexural strength. Even after coating PP fibres with nano-SiO2, no chemical bond were created between the altered PP fibre and the cement matrix. It's possible that the coating was not applied thickly enough, or that nano silica escaped during the concrete mixing process [20].
Polypropylene fibres have traditionally been utilised as a supplementary reinforcement in concrete. Polypropylene fibre volume fractions for this application ranged from 0.1–0.5% in most situations. They were discovered to be effective in minimising shrinkage cracking and enhancing concrete impact resistance. When mixing or making concrete with polypropylene fibres, no additional precautions are usually required. Some studies, on the other hand, advocated for lowering aggregate concentration to improve the efficiency of polypropylene fibres in concrete. Concrete is strengthened by the addition of silica fume, which reduces permeability and improves strength. However, due to the limited permeability to water and chloride ions, the presence of silica fume increases a variety of properties, including bonding (concrete steel), drying shrinkage, modulus of elasticity, and resistance to reinforcing steel corrosion and sodium sulphate attack. [18]
Furthermore, over the last decades, considerable effort has been undertaken to produce novel forms of polypropylene fibres capable of imparting substantial toughness to concrete. As a result, polypropylene fibres are widely available.
A recently invented macro-synthetic fibre, made by processing of artificial polymers centered on polypropylene, was used in this competition. To begin, a mix design was studied that was built specifically for generating members in Polypropylene Fibre Reinforced Concrete (PFRC) [21]. In practice, a substantial volume proportion of PP fibres is required for structural applications, and so workability must be carefully ensured. The effectiveness of this fibre type was then tested on a specific beam type, such as Wide-Shallow Beams and fourteen WSBs in reinforced concrete with no shear reinforcement or PFRC were tested at the University of Brescia's structural laboratory with the goal of determining the viability and effectiveness of using non-metallic Polypropylene fibres to improve beam shear behavior. [21]