Readily programmable materials: Shape memory plastics
After it undergoes an injection molding or extrusion process, an innovative shape memory plastic (SMP) from Covestro can be stabilized in a different form and then restored to its original shape on command. The secret to this smart material lies in its network structure, which is already improving countless applications.
From the playful bounciness of “flying rubber” (or flubber) to the menacing liquid metal robots in Terminator 2, popular culture has held a long-running fascination with shapeshifting materials. But the first materials with a certain “awareness” of their own form were developed in real life as early as the 1930s. Known as shape memory alloys (SMAs), these substances are also referred to as smart metals or even muscle wires. They include alloys such as nickel titanium (“nitinol”). Superelastic SMAs are used as lightweight actuators and pneumatic valves in the automotive industry, robotics and other applications.
With the discovery of polyurethane and advances in polymer research, smart materials have widened their scope to include various polymers. Sometimes called the “plastic with a brain”, shape memory plastics (SMPs) offer several properties that make them more attractive than SMAs. They offer an extremely high capacity for elastic deformation, a much lower cost, low density, and a wide range of tunable application temperatures. These benefits are joined by easy processing, as well as the potential for biocompatibility and the inherent biodegradability of certain polyurethane grades and other components.
Though they differ in their specific properties, SMAs and SMPs both rely on a similar principle that defines their structure: two different materials are chemically bonded yet “phase-segregated”, much like oil and water. While they do not segregate on a visible level, they do pull apart enough to form a structured network of hard and soft segments (also called a block copolymer). The job of the softer segments is to enable temporary deformations, while the harder segments support the recovery of the material’s original shape.
Drawing on this basic principle along with specific polymer expertise, Covestro scientists have come out with a smart new thermoplastic polyurethane (TPU) with plenty of potential for inspiring applications.
Desmopan® 2795A SMP is a shape memory plastic (SMP) synthesized by a patented process. The polyester TPU is injection molded or extruded to fabricate components that can then be stabilized temporarily in another shape by means of thermo-mechanical treatment. Yet despite this deformation, the original, permanent shape remains stored in the material. It can be restored almost completely by heating the TPU above a “switching temperature” (or the more scientifically correct “phase transition temperature of the polyol”) of approximately 40°C. The polymer thus “remembers” its original shape.
The TPU was developed in close cooperation with BAM Federal Institute for Materials Research and Testing in Berlin, where the mechanical properties, aging behavior and “shape memory performance” in long-term use were investigated. The material has an average hardness of approx. 92 Shore A at room temperature and 72 Shore A at 60 °C, thus exhibiting a significantly greater difference in hardness at these temperatures than conventional TPU. Otherwise it offers all the same advantages of TPU: abrasion resistance, elasticity, and flexibility over a wide range of temperatures. It is also resistant to many chemicals. Desmopan® 2795A SMP is even usable in food contact applications.
The new grade of Desmopan is suitable for reuse. It can be stably re-melted without degradation and reprocessed in further applications.
We can imagine applications in areas ranging from mechanical engineering and the automotive, textile, sports and leisure industries to toy manufacturing and aerospace engineering.
Two concrete applications for the new SMP have already been developed in collaboration with BAM: self-erecting garden tunnels and tamper-proof QR code labels.
The first potential application involves the integration of Desmopan® 2795A SMP in garden tunnels that are used to extend the growing season of flowers and vegetables. These portable structures consist of supporting arches and a cover for which many gardeners rely on lightweight plastic pipe and film. The set-up process for these mini-greenhouses is relatively labor-intensive. But when the arches are injection molded and extruded with Desmopan® 2795A SMP, they can be temporarily stabilized in a flat shape and attached to transparent films. These films are then rolled out over the bed and when they are heated, the arches recover their permanent shape. The result: self-erecting garden tunnels for an efficient setup at virtually any scale.
A second application in the making is the use of shape memory TPU for QR code labels. After dyeing and engraving the TPU label with a QR code, the label is temporarily deformed to render it unreadable. A scanning device (e.g. smartphone) can only be used to read the label after it is heated and it recovers its original form. These smart TPU labels are ideal for tamper-proof labeling and the identification of goods on demand. A slightly different process can even be used to render symbols, logos, etc. in TPU tags temporarily invisible for increased protection.
Currently, a vast range of opportunities are conceivable for this emerging technology. The outlook is promising. Jürgen Hättig, Head of Application Development for TPU at Covestro, explains “We can imagine applications in areas ranging from mechanical engineering and the automotive, textile, sports and leisure industries to toy manufacturing and aerospace engineering.” Shape memory plastics have a wide-ranging potential for use in artificial muscles, hinges, solar sails, self-opening and -closing packaging, reprintable braille signs, or textiles that iron out their own wrinkles. In the future, new material combinations and highly tunable polymers are sure to open up even more options.
Would you like to learn more about Desmopan® 2795A SMP or request application support or a sample? We look forward to your query.
- Excellent elasticity
- Easy processing
- Resistant to chemicals and abrasion
- Suitable for reuse / recycling