Engineering Thermoplastics Guide. Chapter 4: Advancements in Thermoplastic Materials Science

This is an excerpt from Chapter 3 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes. 

Introduction

 Since the early days of thermoplastic materials science, continued research and innovation have been driving the industry. Throughout the decades, this has led to the discovery of materials with unprecedented properties. In the 1930s, for example, the discovery of nylon substituted the use of silk in consumer goods and military applications. In the 1960s, continued research in materials science led to the discovery of Kevlar, a material with an outstanding strength five times greater than steel.

Today, research in the field of thermoplastics is focused both on the exploration of new formulations, like thermoplastic elastomers and liquid crystal polymers, and on the development of new processing methods, capable of expanding the technological possibilities of these materials. Smart and responsive materials and composites are also pushing the boundaries of thermoplastic technology.

In this chapter, you will learn about the latest advancements in thermoplastics materials science and processing technology.

New Formulations and Polymer Chemistry

One of the most interesting features of thermoplastics is the wide diversity of their chemical structures and formulations. Today, research in materials science and engineering is uncovering new thermoplastic materials with unique properties for advanced applications.

Thermoplastic elastomers (TPEs) are a versatile family of polymeric materials that exhibit thermoplastic processability while retaining elastomeric characteristics traditionally associated with thermoset rubbers. The composition of TPEs involves the block copolymerization of molecularly dissimilar polymers, resulting in the creation of "hard" and "soft" segments within the polymer chain. The mechanical properties of these materials are dictated by their molecular composition.

Noteworthy TPE types include:

• TPE-S, whose molecular structure consists of repeated styrene and butadiene units;

• TPO, a blend of polypropylene or polyethylene with another elastomer;

• TPE-E, high-performance thermoplastic elastomers consisting of polybutylene terephthalate combined with polyesters.

TPEs offer many advantages over regular elastomers, including reduced energy consumption during production, recyclability, and improved mechanical properties such as strength and creep resistance. Additionally, TPEs are easily processed through standard thermoplastic techniques like injection molding, 3D printing, and blow molding.

Common applications of TPEs span across various industries, including electronics (condenser sheaths, plugs, and sockets), and medical devices, such as breathing tubes, syringe seals, ventilation masks, and catheters. ^[1]

Liquid Crystal Polymers (LCPs) are a distinctive class of materials that can maintain molecular order in both liquid and solid states. LCPs exhibit two distinct phase changes: the transition from solid to liquid crystal and, subsequently, the transition from liquid crystal to full liquid. The intermediate phase is also known as mesophase. LCPs are categorized as either lyotropic, processed by solvent addition, or thermotropic, melt-processable like conventional thermoplastics.

Three of the most common forms of liquid crystal polymers include semi-aromatic copolyesters, copolyamides, and polyester-co-amides. The aromatic amide polymer Kevlar is one the most well-known examples of an LCP. Due to its outstanding strength and toughness, superior to almost any other synthetic fiber, it is used for bulletproof vests, aerospace materials, sporting goods, and other highly demanding applications.

During processing, the rod-like molecules of LCPs align in a specific direction, resulting in anisotropic mechanical properties. In general, these materials possess outstanding strength, excellent temperature, flame, and chemical resistance, and optimal dimensional stability. The remarkable combination of strength, stiffness, low shrinkage, and stability makes LCPs a preferred material in diverse applications. In electronics, LCPs find use in connectors, switches, and displays. Medical applications include dentistry and micro-surgery. ^[1]

Continued innovation and research are pivotal for advancing thermoplastic materials science. One recent groundbreaking development is 2DPA-1, a 2D polyaramide material obtained through an innovative polymerization process by researchers at MIT in 2022. 2DPA-1 exhibits remarkable properties, including an elastic modulus four to six times greater than bulletproof glass and a yield strength twice that of steel, despite being one-sixth as dense. The mechanical properties of this novel material, combined with its low gas and water permeability, make it a promising candidate for advanced applications such as ultrathin coatings in engineering and aerospace. ^[13]

Hybrid processing technology

A notable advancement in thermoplastic processing technology is the use of 3D printing to craft continuous fiber-reinforced composites. While the additive manufacturing of short fiber-reinforced thermoplastics is already well-established, the 3D printing of long fiber-reinforced composites presents additional challenges.

Today, several technologies are being developed for 3D printing continuous fiber-reinforced composites. These hybrid processing approaches hold the potential to significantly improve part quality and performance while reducing production costs. The demand for printers capable of fabricating continuous fiber-reinforced plastics is high, promising substantial cost-savings and increased automation in the field of composites.

This technology holds promise for diverse applications, including the automotive and aerospace sectors, custom products like orthopedic implants, and prosthetic limbs in healthcare. Replacing traditional labor-intensive manufacturing with 3D printing is a transformative shift leading to a new generation of thermoplastic composites. ^[14]

Overmolding with thermoplastic composites represents another significant step forward in composite manufacturing. It allows the production of complex 3D structures with exceptional structural performance and high functional integration.

Overmolding technology is a processing method that allows the combination of two distinct materials into a unique component. Typically, it involves thermoforming a first material (thermoplastic or thermoplastic composite), followed by injection molding of a second material.

The overmolding process can significantly reduce production times and costs, particularly in aerospace and automotive applications. With its short cycle times, it enables the combination of characteristics from two or more materials without the need for mechanical interlocking or adhesive bonding, eliminating assembly steps.

Overmolding can be used to combine composite materials and other thermoplastics, or thermoplastic materials with thermosets, obtaining hybrid and multifunctional components for versatile applications. The main challenge of overmolding is optimizing the adhesion while preventing interdiffusion between the two different material layers. To address this challenge, new adhesive technologies, based on nanomaterials, are currently being developed. ^[15]

Large-scale additive manufacturing (AM) technologies have revolutionized various industries, from aerospace to construction, by unlocking the versatility of 3D printing in unprecedented applications. While traditional 3D printers have limitations in terms of print volumes and deposition rates, there is a growing focus on overcoming these constraints to facilitate the manufacturing of larger components.

The emergence of large-format additive manufacturing (LFAM) promises increased speed, precision, mechanical strength, and customization in the production of complex, lightweight, large-scale geometries. LFAM can be adapted to different additive manufacturing methods including both powder bed fusion and extrusion-based systems.

Pellet-based technologies within fused filament fabrication (FFF) have also emerged as a notable advancement, offering reduced production times and material costs, as well as the flexibility to use a wide array of materials, including composites. These innovations in LFAM overcome the limitations of conventional 3D printing, paving the way for efficient and cost-effective large-scale manufacturing across diverse sectors. ^[16]

Towards “smarter” thermoplastic materials

Smart thermoplastics are a fascinating class of modern materials designed to respond to external stimuli by changing their shape and properties. Among these materials, shape-memory polymers, or SMPs, stand out for their ability to memorize a specific shape and recover their original form upon exposure to an external stimulus, commonly heat.

This “smart” behavior and the advantages of low density, easy processing, and cost-effectiveness provided by thermoplastics make SMPs promising candidates for various applications. Unlike thermosets, thermoplastic SMPs are processed quickly and cheaply using conventional thermoplastic technology.

Notably, some SMPs can be 3D printed with conventional FFF printers. PLA, for example, is a well-known polylactic acid used in additive manufacturing that also possesses shape-memory properties. A combination of SMPs with new design strategies, such as origami and bio-inspired structures, can result in responsive components.

The integration of 3D printing and SMPs has led to the emergence of "4D printing," enabling printed structures to actively change configurations over time in response to environmental stimuli like heat or humidity. This innovative technology holds significant promise for applications such as minimally invasive biomedical devices, such as stents and dilators. ^[17]

Another example of “smart” materials is self-healing thermoplastics. The quest for materials  with self-healing capabilities has gained substantial attention in the past 15 years, particularly for applications in aerospace, electronics, robotics, and sporting goods. The ability of materials to autonomously repair damage holds significant promise for enhancing longevity, reducing replacement costs, and improving safety. Structural polymer materials and composites are prone to damage and degradation, with cracks often forming deep within the structure. Detecting and repairing such cracks pose considerable challenges.

Self-healing thermoplastics offer an alternative solution, allowing for the spontaneous repair of degraded and cracked structures. These materials are broadly categorized as intrinsic or extrinsic self-healing systems, depending on the repair mechanism. Extrinsic self-healing requires the incorporation of additional materials. Intrinsic self-healing thermoplastics, on the other hand, can autonomously heal themselves, triggered by stimuli such as heat, light, or static load.

Among self-healing materials, thermoplastic elastomers and nanocomposites containing graphene and carbon nanotubes have shown great promise in various engineering applications, spanning from electronics to robotics. This technology represents a ground-breaking advancement in materials science, offering the potential to revolutionize how we approach durability and maintenance in diverse industries. ^[18]

Looking towards the future

In conclusion, the evolution of thermoplastic materials is being improved by remarkable new research and industry advancements. The contemporary landscape of thermoplastics is characterized by a rich diversity of formulations, including the emergence of thermoplastic elastomers and liquid crystal polymers, bringing unique properties and applications.

The quest for innovation extends to hybrid processing technologies, such as 3D printing for  continuous fiber-reinforced composites, large-format additive manufacturing, and overmolding. These new technologies further enhance the versatility, applicability, and potential of thermoplastic materials. Recent breakthroughs in thermoplastic research, such as self-healing and shape-memory polymers, are paving the way for new applications of thermoplastics in engineering, medicine, and aerospace.

Looking forward, the future of thermoplastic research holds exciting possibilities. Integration of thermoplastics with nanomaterials, such as carbon nanotubes, graphene, and silica nanoparticles, can exponentially improve the mechanical and functional properties of composites. Additionally, the rise of biodegradable, bio-derived, and recycled plastics is bringing the thermoplastics industry in alignment with global sustainability goals.

You have just been reading an excerpt from Chapter 4 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes.