**2.1 Polymeric blends**

Polymer blends are mixtures of two or more polymers in concentration greater than 2%wt. The blends can be miscible or immiscible, a parameter that depends on the thermodynamics of the system and molecular structure, weight, and polymer concentration. More information on the complicated thermodynamics that govern polymer blend miscibility can be found in the Polymer Blends Handbook [18, 19]. Miscible polymer blends are also known as homogeneous blends and are monophasic while immiscible blends with morphologies that differ such as, spheres, cylinders, fibers, or sheets (**Figure 4**) [12].

Subject to polymer compatibility, polymer blends can exhibit synergistic, antagonistic, or additive behavior. A common method used to assuage the immiscibility of polymers blends is the inclusion of compatibilizers—a polymeric surface tension reduction agent that promotes interfacial adherence—in the blend. The three most common types of compatibilizers are reactive functionalized polymers, nonreactive polymers containing polar groups, and block or graft polymers [12, 19, 20].

The difficulty during polymer blend recycling lies in the different properties presented by its component parts such as melting points and processing temperatures between polymers [12]. Most recycling efforts are concentrated on the procedure of pyrolysis to extract energy through the oils, wax, char, and gasses produced. Furthermore, research in recent years has focused on the use of various, different catalysts in order to lower the energy consumption of the whole process and increase the exploitable yield. Along with those some novel methods of polymer blend recycling will be explored.

#### **Figure 4.**

*(a) and (b) are a visual representation of the differences between miscible and immiscible Polymer Blends. Images (c), (d), and (e) show the spherical, fibrous, and cylindrical morphologies of immiscible Polymer Blends, respectively. Image inspired by Ragaert et al. [12].*

#### *2.1.1 Production of composite materials*

Polymer composites are made up of two or more elements resulting in a multiphase, multicomponent system that exhibits superior properties compared with the constituent materials due to a synergistic effect. It comprises two parts:


One way that polymer blend can be recycled is by acting as the matrix for secondary elements creating composite materials. In this way it is possible to unite the two components in a form that reinforces the secondary materials and reuses the polymer blends. This method can be adapted to use natural fillers or fibers as the reinforcing fillers. Those can be added along with a coupling agent to optimize the interaction of the fillers with the matrix further and have the positive side effect of making the whole process environmentally friendly. It is important, however, that these fillers have the capacity to be chemically treated.

In a research conducted by Choudory et al., [22], Low-density polyethylene (LDPE)/Linear low-density polyethylene (LLDPE) blend extracted from milk pouches was used as a matrix for coir fibers. The result was composites with properties only slightly lacking from the virgin material ones. In case a maleated styrene pretreatment was applied, the mechanical properties and thermooxidative stability were drastically increased [23].

In another research conducted by Lou et al., [24], PET/PP blend and bamboo charcoal were used to create extruded or injection-molded composite materials. A great increase in mechanical properties was observed in the injection-molded composites, which maintained their mechanical properties even after three rounds of processing. The percentage of total mass of PET in the blend plays a particularly significant role in the product's final behavior [23].

#### *2.1.2 Pyrolysis*

Pyrolysis is a promising choice as regards the recycling of polymer blends. With pyrolysis, high levels of conversion of the polymer blend into oil and gas with high calorific values can be attained. These can be used afterward to either fuel the process, or they can be utilized elsewhere [25]. This can be an invaluable asset to the petrochemical industry and a green way for the recycling of plastic waste [26].

Another advantage of pyrolysis is that a sorting process is not needed in contrast to other recycling methods that are extremely susceptible to contamination. This can of course save money and time when recycling polymer blends. Lastly, with the use of the pyrolysis procedure, waste management becomes easier as it is a cheap and environmentally friendly method. In the meanwhile, it allows for minimization of landfill capacity—a serious contemporary difficulty [5]. As the combination of polymers that make up polymer blends is wide, with every blend presenting different properties and pyrolysis behavior, it would be impractical to analyze each one of them. Instead, this chapter will focus on the pyrolysis route taken for the most common polymer blends by examining the research conducted by scientists in the field.

In general, the pyrolysis process can be either thermal or catalytic. In practice, however, the latter is widely preferred by the industry as it demands lower

#### *Current Topics in Plastic Recycling DOI: http://dx.doi.org/10.5772/intechopen.101575*

operating temperatures—and thus cost is minimized—that produce a more satisfactory yield of pyrolytic oils, if the correct catalyst has been elected [5].

In a study conducted by Vasile et al., [26], a blend with a composition similar to that originating in municipal waste—24%wt high-density polyethylene (HDPE), 39%wt LDPE, 21.5%wt isotactic polypropylene (IPP), 10%wt PS, 4%wt ABS, and 1.5%wt PET—was investigated. The blend underwent the process of catalytic pyrolysis two separate times each with a different catalyst—HZSM-5 in the first batch and PZSM-5 zeolite catalyst in the second batch, in order to find which catalyst led to better results. It was concluded that the PZSM zeolitic catalyst was characterized by higher selectivity and stability. The optimal temperature for the pyrolysis was found to be 450–480°C, and the gas produced increased sixfold in comparison to the non-catalytic process. Furthermore, the liquid products were found to contain high concentrations of aromatic hydrocarbons. As such, both the liquid and the gas phase can be utilized by the petrochemical industry. Lastly, the pyrolysis oil could be useful as petrochemical feedstock [26].

A novel research conducted by Bober et al. [27] proposed a way to produce hydrogen gas from the catalytic pyrolysis of different consistency HDPE/ poly(methyl methacrylate) PMMA polymer blends. After trial and error, the optimal temperature for maximum hydrogen production was found to be 815°C, a temperature where the catalyst used, Ni/Co, operated the best for hydrogen production. It was also found that, the higher the HDPE content in the blend, the bigger the hydrogen output. In contrast, when PMMA was the dominant polymer in the blend, CO was produced at a greater rate than the previous procedure. The research team proposed that the best ratio for HDPE/PMMA in the blend is 4:1 [27].

It must also be noted that concerning the production of hydrogen from pyrolysis of polymer blends, a popular option is the co-pyrolysis of the polymer blends with biomass [28].

A largely untapped potential of Polymer Blends is their recycling as feedstock for the chemical industry. A study presented by Plastics Europe [29], displays that only 2–3% of the collected plastic waste in Europe is utilized as feedstock (**Figure 5**).

**Figure 5.** *The fate of the European collected plastic waste. Image inspired by Donaj et al. [30].*

A possible procedure for the creation of feedstock from pyrolysis of Polymer Blends on the group of polyolefins was suggested by Donaj et al., [30]. For the purposes of the process, the researchers used a blend of polyolefins—46% LDPE, 30% HDPE, 24% PP- taken from MSW/plastic waste. The collected material was firstly reduced in size to about 3 mm pieces and then pyrolysis ensued under temperatures of 600–700°C in a fluidized bed reactor and with the use of steam and a catalyst if that was deemed feasible as the latter materials increase the yield of olefines. To optimize the procedure, Ziegler-Natta catalyst was used.

The research noted that after the procedure's conclusion, plastic pyrolysis had directly yielded 15–30% gaseous olefins that can then be channeled directly into a polymerization plant. The residue produced consists of a naphtha-like consistency. To be used, this residue must undergo reformation via petrochemical technologies to be upgraded into olefins. Also, as in the previous cases of pyrolysis, the products of the process can be used to fuel the procedure itself. However, work still needs to be done on this field as the process described is not as cost-effective as desired [30].

#### *2.1.3 Melt processing*

A last noteworthy method for the utilization of immiscible Polymer Blends is their direct melting processing into fibers with good mechanical properties proposed by Shi et al. [31]. The blend used in this research was PS/PP while fibers were chosen due to two distinct reasons: (a) The fiber spinning technique is known to endow improved properties to polymer blends. (b) Fibers from polymer blends may display new properties in comparison to pure polymers. This method is widely cost-effective for preparing strong fibers for the industry, and it is expected to see great development in the coming years [31].

#### **2.2 Multilayer packaging**

In this age of climate change and overall pollution, it has been the priority of policymakers to ensure the viable and sustainable future of human development. An example of this is the EU with the European Plastic Strategy dictating that all packaging used should be reusable or recyclable by 2030 [32].

A prime example of the challenges the industry faces to reach this standard is Tetra Pak, a multilayer packaging used mostly in the food, medicine, chemical, and commodities industry. Tetra Pak most usually consists of three elements: paper cardboard, aluminum, and LDPE.

As stated by the Tetra Pak company, its composition is as follows: (a) 71% paperboard, (b) 24% plastics, and (c) 5% aluminum foil (**Figures 6** and **7**).

These three make up the six layers that combined make Tetra Pak. Each layer has a particular use elaborated on below:

However, this is not an absolute rule. For example, certain products with a short shelf life have no need for the protection given by the aluminum layer. On the other hand, when the aforementioned shelf life needs to be extended, the LDPE layers can be substituted by PP providing a chance for further heat treatment of the product. HDPE, PET, and PA are also possible options for replacing the LDPE layers. Lastly, polyurethanes and EMAA are often utilized as adhesives between layers [34] while the Tetra Pak carton may also contain various chemical additives such as plasticizers, stabilizers, lubricants, fillers, foaming agents, colorants, flame retardants, and antistatic agents [35].

As Tetra Pak cartons are composed of mainly paper, the removal and recycling of the carboard layer are of much significance. As such there are two main processing routes: recycling without hydropulping and recycling with hydropulping. The initial *Current Topics in Plastic Recycling DOI: http://dx.doi.org/10.5772/intechopen.101575*

#### **Figure 6.**

*Raw materials used to produce Tetra Pak. Image inspired by the Tetra Pak site information.*


#### **Figure 7.**

*The layers of Tetra Pak. Image inspired by Georgiopoulou et al. [33].*

procedure processes the cartons as a whole, while the latter uses the technique of hydropulping to first separate the cellulosic fibers from the Al-LDPE laminate.

#### *2.2.1 Recycling without hydropulping*

The main aim of those following this route is energy recovery or downcycling. Energy recovery is attained in combination with solid municipal waste through

means of pyrolysis, gasification, or incineration. However, this method comes with many downsides. Paper—the main ingredient of Tetra Pak cartons—has a low heat combustion (16 MJ/Kg), high moisture content, and a significantly high ash value. This makes the entire process inefficient, and thus it is in general not widely used [34].

### *2.2.2 Recycling with hydropulping*

Before proceeding with the options in this category, it would be useful to briefly go over the hydropulping process. When the soon-to-be recycled material first arrives into the recycle unit, the hydropulper breaks apart the paper with rotating blades that use high pressure water and a slurry of fibers is produced. Further processing ensues in centrifugal cleaners that remove heavy materials such as sand, adhesives, staples, etc. [36]. The end result of this procedure is a pulp of cellulosic fibers and can be used as a substitute for wood pulp, in the production of brown paper and pulp board [37]. What remains after the process is the external LDPE layer and the Al-LDPE laminates. However, residual cellulosic fibers can account for up to 5% of the finished products (**Figure 8**).

### *2.2.3 Pyrolysis*

The appeal of this method lies in its simplicity and cost-effectiveness. The pyrolysis procedure has two steps: (a) the degradation of paper (200–400°C) and (b) the devolatilization of LDPE (420–515°C) [38–40]. It should be noted that the temperature plays an important role in the composition of the final products. For example, the production of char is minimized with higher temperatures, and the opposite is true for wax.

The solid products that follow the process are aluminum, char caused by paper degradation, wax from LDPE degradation and tar. A great deal of gaseous products are also formed that mainly consist of CO2, CO, H2, CH4, C2–6 hydrocarbons, and volatile matter. Lastly, there is an aqueous phase consisting of water and phenols.

Many uses have been proposed for those pyrolytic products. The produced gases could be used to sustain the pyrolysis procedure itself or used elsewhere entirely, the char and tar can be exploited as a solid and oil fuel, respectively, while char can also act as a primal resource for the production of carbon-based materials. Lastly, the wax and aqueous phase can readily be utilized as a raw material for the chemical industry [39, 40].

**Figure 8.** *The main recycling routes.*

#### *Current Topics in Plastic Recycling DOI: http://dx.doi.org/10.5772/intechopen.101575*

A novel approach has been taken by researchers in Mexico and Spain who have used the char and the aluminum from the pyrolysis to have them act as absorbents of mercury in aqueous solutions. By means of trial and error and using thermodynamical analyses, they did conclude that char obtained from pyrolysis at 600°C at a 3 h procedure demonstrated the most promising mercury adsorption capacity at 21.0 mg/g. The field of char absorbents is still expanding with hopes of Tetra Pak pyrolysis chars acting as major absorbents for industry in the future [41].
