**1. Introduction**

Reduce, reuse, recycle – this is the current strategy to prevent the world from being flooded by plastic waste. There are many initiatives led by governments, regulators, and also companies and entrepreneurs to follow these guidelines: the so-called "Single Use Plastics" Directive (SUP) [1]. The Circular Plastics Alliance, and The Alliance to End Plastic Waste – to name a few. Incineration and energy recovery of plastics seem to no longer be the promoted solution due to increasing carbon dioxide emissions and causing the loss of the potential hidden within polymers. Although landfilling of plastics practically does not emit CO2, it can lead to microplastic release into the environment [2].

Chemical recycling is now more recognised as a potential solution to recycling or ending the life of plastic, as it can potentially treat the waste that cannot be mechanically recycled, keeping "carbon" in the industry loop. But there is no one official understanding of the term "chemical recycling" worldwide. European

regulation defines recycling only at a very general level as any operation that takes waste and makes products, materials, and substances, except fuel components [3]. ISO 15270 is even more precise, limiting chemical (feedstock) recycling to cracking, gasification and depolymerisation processes to convert plastic waste into monomer of new raw materials, excluding energy recovery and incineration [4]. On the other hand, American Chemistry Council (ACC) defines chemical (advanced) recycling as any technology that converts post-use plastics into monomers, specialty polymers, feedstocks and fuels [5]. What is more, the process of waste to value-added chemicals is also known as upcycling [6].

Recycling itself, both mechanical or chemical, can produce two types of products. Suppose the properties of the recycled material are not considerably different from those of the virgin material and can be used in the same application. In that case, the recycling process is called "closed-loop recycling". This approach is difficult for the mechanical recycling of some polymers for specific applications (like food packaging or specialty applications) as, in many cases, the processing of plastic waste causes partial degradation of the polymer structure and a change to its mechanical properties. When recycled material has different properties and is used in different applications to the original one, the recycling process is called "open-loop".

In the current chapter, both types of chemical recycling technologies of polyolefins (polyethylene and polypropylene) – closed-loop and open-loop, will be described.

### **2. Structure of polyolefins**

Polyethylene (PE) and polypropylene (PP) are polymers called polyolefins. These are the two largest plastic resins based on production worldwide. One of the reasons for this is the variety of applications where these plastics are used: mainly in packaging but also in toys, piping, cable covers, automotive parts, ship ropes or even bulletproof vests. Polyolefins are inert, have low thermal conductivity (are good insulators) and are not subject to attack by most chemicals. As thermoplastic resins, most polyolefins can be mechanically recycled; however, thanks to the chemical and hydrocarbon structure, they are also proper materials for chemical recycling via cracking (pyrolysis) and gasification.

Polyethylene is a product of ethylene polymerisation. Depending on the production process and consequent chemical structure and properties, many types of PE are produced. The major ones are:


LDPE is a type of polyethylene with the most branched structure and a density lower than that of HDPE. HDPE is a polymer with a linear structure with a low degree of branching. LLDPE is produced by the polymerisation of ethylene with other olefins with longer hydrocarbon chains, like 1-butene, 1-hexene or 1-octene. The result, a linear backbone with short and uniform branches. Finally, PEX is a polymer obtained by crosslinking the process of polyethylene. This crosslinking

*Chemical Recycling of Polyolefins (PE, PP): Modern Technologies and Products DOI: http://dx.doi.org/10.5772/intechopen.99084*

**Figure 1.** *Structures of polyethylene and polypropylene.*

changes the properties of polyethylene significantly – increased temperature, pressure, or corrosion resistance. Thermoplastic polyethylene becomes thermoset which limits the possibility of mechanical recycling of this material significantly.

Polypropylene is a product of the polymerisation of propylene. It means that every monomer has a methyl side group. Due to the presence of asymmetric carbon atoms in the chain, stereoisomerism is observed for this polymer.

Differences in the structures are presented in **Figure 1**. These differences influence the polymer's mechanical properties and the cracking or gasification process conditions, and the composition of products from chemical recycling processes.

It should be noted that many different additives are used to change the properties of the polymer during the production of plastic items. For example, pigments and dyes are added to change the colour; glass fibres can be added to alter the mechanical properties of the polymer, and talk is sometimes added to reduce the price of the final goods. Stabilisers (like UV stabilisers), flame retardants, lubricants and plasticisers are other types of additives commonly used in the plastic industry. The quantity of these additives may be vast, from parts of a percent up to 60–70%. What is more, in many cases, polyolefins are used together with other polymers. For example, in multilayer films used for packaging, PE is used together with poly(ethylene terephthalate) (PET). What is more, flexible packaging is often highly printed. The final yield and composition of chemical recycling products will depend on the type and quantity of all of the impurities.
