Polypropylene (PP), one of the most widely used general-purpose plastics, was first synthesized by Prof. Natta in 1954 and subsequently industrialized, thus opening a new chapter in polymer science and engineering (Zheng et al., 2017). PP is a thermoplastic that is obtained by polymerizing propylene gas with a suitable catalyst. Due to the asymmetry of the monomer, the steric insertion of the methyl group (–CH3) in the chain can occur in a number of ways, resulting in PP (Gon Öalves and Margherita, 2015):
•what is polypropylene
Isotactic polypropylene (i-PP), with methyl groups arranged on the same side of the carbon chain, is a semi-crystalline polymer; it accounts for more than 90% of commercially produced polypropylene,
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Syndiotactic polypropylene (s-PP) has methyl groups that alternate along the carbon chain; industrial production of this isomer began less than 15 years ago.
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Atactic polypropylene (a-PP) has methyl groups randomly arranged along the carbon chain; it is an amorphous material with little commercial interest.
The characteristics of PP include: semi-rigidity, toughness, good fatigue resistance, integral hingeability, good heat resistance. PP is highly resistant to chemical agents and solvents, does not suffer from stress cracking issues, and has excellent electrical and chemical resistance at higher temperatures. PP and PP copolymers are widely used in textile, construction, packaging, automobile, medical and other industrial fields due to their light weight, good stability, easy processing, low manufacturing cost, and wide range of uses. The main use of PP in the construction industry is for pipes/ducts (Zheng et al., 2017; Gonçalves and Margarido, 2015; British Plastics Federation).
Table 9.1 provides data on selected attributes of PP.
Depending on the application and the chosen processing method, different PP types (based on i-PP) are available. These include in particular polypropylene random copolymers (PPR), glass fiber reinforced polypropylene (PPGF) and expanded polypropylene (EPP) (Zheng et al., 2017; British Plastics Federation; Yu et al., 2013):
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PPR - Contains comonomer units arranged randomly (as opposed to discrete blocks) along the long polypropylene chain molecule. Such polymers usually contain 1-7% by mass of ethylene. PPR is widely used as base components for films, pipes, furniture, etc. due to its balance of rigidity and toughness relative to polypropylene homopolymer (PPH),
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PPGF - 30% glass fiber reinforcement increases strength, stiffness and heat deflection temperature, but significantly reduces impact resistance,
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EPP - is one of the most produced foam polymers in the world. EPP has high strength and stiffness, good impact strength, and uniform closed-cell structure. EPP is ideal for the production of sports and rescue equipment, used in the automotive and construction industries, as well as in the production of food packaging.
In Europe, PP ranked first in plastics demand with 19.1% of polymer types in 2015, followed by low-density polyethylene (LDPE) with 17.3% (PlasticsEurope, 2016). In the United States, the output of PP is second only to LDPE, ranking second. In contrast, PP has a very low recycling rate of around 0.6%, making it one of the least recycled post-consumer plastics (Ozbakkaloglu et al., 2017). After recycling, PP is used in the production of office supplies, flower pots and also in the production of components for the electrical and automotive industries. Since the use of some other types of plastics has been explored in the construction industry as an alternative to natural aggregates (NA), the use of PP in the production of eco-efficient sustainable concrete seems to be a suitable solution. However, few papers have investigated PP as an aggregate, as so far there has been little research on the possible application of PP in lightweight concrete.