The mixture of two or more polymers or copolymers with an ingredient content of more than 2 wt. % is called a polymer blend. Specific and highly compatible polymer blends are called alloys. Currently, polymer blends, alloys, and composites account for over 80 wt % of all plastics, establishing them as an important category in materials science.1
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Polymer blends and alloys offer diverse properties through efficient and inexpensive manufacturing methods, making them applicable in several applications.2 With increasing environmental concerns and a shift towards sustainability, the focus is on developing economical materials with enhanced mechanical properties. These are not intended to substitute steel or wood but rather to recycle and replace conventional polymers.1
Polymer Blends and Alloys: The Basics
Polymer blends can be miscible with a single-phase structure (homogeneous) or immiscible with more than two phases (inhomogeneous). In the former, the mixing of free energy is negative, while in the latter, it is positive. Immiscible polymer blends that show inhomogeneity at a very small scale are referred to as compatible.
Homologous polymer blends contain more than one fraction of a single polymer, each having a different molecular weight. Alternatively, isomorphic polymer blends contain multiple kinds of semi-crystalline polymers that are miscible in both molten and crystalline states.1
Polymer alloys refer to immiscible and compatibilized polymer blends with modified morphology and interface. Although the terms polymer blends and polymer alloys are often used interchangeably, fundamentally, only fully compatible mixtures are considered alloys. Polymer alloys form a specific sub-class of polymer blends that constitute almost all high-performance engineering blends.1
Common polymer blend preparation methods include solution blending, graft copolymerization, melt mixing, latex mixing, and synthesis of interpenetrating polymer networks (IPN).3 The development of solid-state processing methods, like cryogenic mechanical alloying and shear pulverization, has enhanced polymer blending and alloying.1 The choice of processing method largely depends on the materials used, availability of resources, and desired application.
Enhancing Mechanical Performance
Polymer blends and alloys exhibit enhanced mechanical properties, thereby improving the overall performance of products.
During the blending process, brittle polymers become tough, eliminating the requirement of low molecular weight additives, such as the use of plasticizers in flexible polyvinyl chloride (PVC). Blending with a more heat-resistant and rigid resin improves the modulus and dimensional stability of polymers.1 A blend of polyester (PEST) with polycarbonate (PC) demonstrates higher resistance to a chemical solvent. Individually, PC is amorphous, while PEST is semi-crystalline.
The incorporation of non-flammable resins like styrenics or acrylics into flammable PVC improves its fire resistance. It is also possible to obtain permanently anti-static polymer blends using polymers with -SH or -OH functional groups. For instance, blending ethylene oxide-co epichlorohydrin with ABS/PC (acrylonitrile-butadiene-styrene/ polycarbonate).1
A recent study in eXPRESS Polymer Letters demonstrated the enhanced mechanical properties of an innovative blend of recycled polyethylene terephthalate/polyamide 11 (rPET/PA11) by the use of Joncryl® as a compatibilizer. A boost in ductility, Young’s modulus, and flexibility was observed in rPET/PA11 blend synthesized using a twin-screw extruder and injection molding tool.5
Advancing Sustainable Solutions
According to recent estimates, if oceanic plastic pollution remains unchecked, by 2050, the number of plastic bottles in the oceans could surpass the fish population. This highlights the critical need for replacing petrochemical plastics with biodegradable or recyclable alternatives.2 Polymer blends and alloys could play a significant role in this endeavor.
The incorporation of biodegradable resin into polymer blends enhances their sustainability. It also increases the life cycle of polymer alloys by allowing the reconstruction of materials with high molecular weights from partially disintegrated polymers. Hence, high-functional products can be manufactured from plastic waste, providing a way to recycle and reuse industrial plastic.1
Polymer blending simplifies the plastic waste recycling process by eliminating the need to sort mixed plastics through direct processing and further use of compatibilizers to form polymer alloys.3
Bio-based polymers are recognized as sustainable alternatives that remain mechanically stable during the functional stage while still possessing biodegradation capabilities. Key biopolymers are polyamides (PA), polyhydroxyalkanoates (PHA), poly (lactic acid) (PLA), and poly (ε-caprolactone) (PCL). Selecting the appropriate processing and modification methods of these tailored to specific applications in industries or the medical field is crucial.4
The research community is actively working to develop sustainable polymer blends and alloys. A recent study in Materials proposed the preparation of polymer blends from glycol-modified poly(ethylene terephthalate) (PET-G) foils and poly(ethylene 2,5-furanoate) (PEF). The researchers used recycled PET-G, which is commonly used in packaging, disposables, and perishables, and bio-sourced PEF. The blends created were, therefore, environmentally friendly.2
Addressing Blending Challenges
Melt mixing is predominantly used to prepare polymer blends because it leverages well-established constituents and versatile mixing tools. However, this process is highly energy-consuming and unsuitable for chemical modification of the blend ingredients.
Solution blending is preferred at the laboratory scale but requires the identification of a common solvent for all constituents of a blend. Additionally, the recovery of harmful solvents is an important part of the solution blending process.3
Direct recycling of plastic waste to form polymer blends or alloys is an inexpensive process but can lead to compromised product quality. Recycling polymer blends presents additional challenges, as the initial blend could be degraded, notably through photo-oxidation. This degradation can be overcome using an appropriate combination of compatibilizer and blending techniques.3
The adaptability of the blending technology enables the properties of the material to be tailored. These challenges can be overcome by using the most suitable polymer blending method. Selecting the correct compatibilizers for the polymer constituents is crucial for obtaining a homogeneous alloy with the desired properties. This can be achieved by the application of theoretical models and computations in the design stage.3
Future Outlooks
Recent technological advancements in polymer science have improved the mechanical properties of polymer blends and alloys and have increased the scope of their application beyond bottles and foils. They are now being used to fabricate garden tunnels for greenhouse-based plantations, 3D printing filaments, polyester weed control fabrics, as well as yarns and fibers for textiles.2
The initial development stages of a polymer blend or alloy can be expedited at a low cost using reliable models to predict its physical properties. The integration of machine learning and advanced artificial intelligence tools in material science can revolutionize the design and structural analysis of blends.3
While bio-based polymers derived from renewable sources represent a promising way to attain polymer sustainability, a circular economy for plastic materials should be a primary goal of future research in the material sciences.
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References and Further Reading
1. University of Babylon. Introduction to Polymer Blend. [Online] University of Babylon. Available at: https://www.uobabylon.edu.iq/eprints/publication_10_10932_1261.pdf
2. Paszkiewicz, S., Irska, I., Piesowicz, E. (2020). Environmentally Friendly Polymer Blends Based on Post-Consumer Glycol-Modified Poly(Ethylene Terephthalate) (PET-G) Foils and Poly(Ethylene 2,5-Furanoate) (PEF): Preparation and Characterization. Materials. doi.org/10.3390/ma13122673
3. Dorigato, A. (2021). Recycling of polymer blends. Advanced Industrial and Engineering Polymer Research. doi.org/10.1016/j.aiepr.2021.02.005
4. Okolie, O., Kumar, A., Edwards, C., Lawton, L. A., Oke, A., McDonald, S., Vijay Kumar Thakur, Njuguna, J. (2023). Bio-Based Sustainable Polymers and Materials: From Processing to Biodegradation. doi.org/10.3390/jcs7060213
5. Khan, Z. I., Habib, U., Mohamad, Z. B., Raji, A. M. (2021). Enhanced mechanical properties of a novel compatibilized recycled polyethylene terephthalate/polyamide 11 (rPET/PA11) blends. Express Polymer Letters. doi.org/10.3144/expresspolymlett.2021.96
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