Text: Assistant professor Essi Sarlin and Researcher Vsevolod Matrenichev, Research group of Plastics and Elastomer Technology, Tampere University
Photo: Essi Sarlin

New application areas and technological development create new business opportunities in the plastic and polymer composite industry. Simultaneously, it generates new challenges for the materials development and research. New materials and structures designed for soft robotics, energy harvesting or self-healing systems are one of the main topics in the conferences. Of course, more incremental improvements in materials performance, processing and simulation including the environmental aspects form the other established half of the discussion. In this column, we want to highlight some of the popular topics discussed in the plastic and polymer conferences this year.

Sensing and actuation

To sense, think and actuate is the basis for numerous active and autonomous systems. Materials science can contribute especially in the sensing and actuation functionalities, while in computer science novel tools, such as neural networks, are developed to predict the behaviour and to control the materials.

There are three main approaches to use integrated sensors in polymeric structures: In the R&D phase of new materials and structures, sensors can be embedded into the test coupons and prototypes to provide detailed information of their performance. In the processing phase, the sensors can be used to monitor e.g. the state of cure and validate the quality of the final component and provide on-line feedback about the processing parameters. In addition, the sensors can be used to monitor the structural health and possible damages during storing, transportation and use of the component. As the size of the sensors is small, they can be used e.g. in composite structures commingled into the reinforcing fabric without any effect on the local or global material properties. Also, the sensors can survive such processing methods as resin transfer moulding (RTM).

For actuation, there is a variety of strategies to achieve the movement and the materials selections should be done according to the selected operational principle. Shape memory polymers, such as polyurethane-based thermoplastics, are one recently discussedgroup of materials for polymeric actuators. In these copolymers alternating soft amorphous and hard crystallized segments cause phase separation and further the shape memory effect. Liquid crystalline elastomers are another novel possibility for polymeric actuators. These soft, anisotropic materials exhibit large shape transformations and can be controlled with various stimuli. The aforementioned materials can be 3D printed. In addition to the research focused on the materials, the movement can be caused by anisotropy of the macroscopic structure, which is also a very popular approach in robotics.

Novel processing methods

Faster processing cycles would contribute in the cost efficiency of polymeric structures. In plastics, new curing systems, such as the frontal polymerization by the ring-opening metathesis polymerization, are recently excessively studied. In frontal polymerization, external heat (or UV light) is required only to initiate the exothermic curing reaction. After that the reaction is self propagating. Thus, frontal polymerization is a combined system of curing reaction and thermal diffusion. Depending on the positioning of the initial heat source, the polymerization front proceeds either though the component length or thickness. The reaction itself is very fast, similar to the infusion process, which reduces the energy requirements and cure times significantly compared with conventional oven or autoclave curing. The achievable mechanical properties of these materials are comparable to basic bisphenol A based epoxies.

There are a lot of incremental attempts to improve the quality of the automated tape layering process. An example is the tool-less approach to prepare composite parts. In this system, there are two 6-axis robots: the first one is the conventional tape placement robot heating the prepreg while the second one forms a solid frame that provides the boundaries of the structure being fabricated and cools down the prepreg. The movements of the robot heads are  synchronized in the 3D space being able togenerate contoured and complex shapes. Although new processing methods like this are primarily in the interest of the space industry they might give an insight into the possibilities of the commercial processing methods of the future.

Sustainable materials and recycling Environmental aspects keep their position as one of the main drivers of research. From materials science point of view, there are two main focus areas in the shift towards more sustainable materials: the origin of the raw material and the end-of-life behaviour.

New bio-based and waste-based monomers respond in the need to reduce the use of virgin petroleum-based materials. The resulting polymers have very similar or even identical chemistry to the conventional ones but also new monomers that result in polymers with novel property combinations are developed. For example, acrylated vegetable oils and glycerol can be used to generate highly tunable block copolymers for high-performance formaldehydefree adhesives for wood composite materials. Alternatively, some of the research is focused to develop styrene free resins in order to meet the current and future restrictions of styrene compounds.

In the field natural fibre composites, the research is very active in Europe: the use of different plants, extraction methods as well as treatments for improved adhesion and better resistivity to humidity and heat are very popular themes. Also, introduction of new thermoplastic matrices in the field of natural fibre composites is a widely research topic as the available selection is still somewhat restricted to low temperature thermoplastics such as polypropylene. Alternative sustainable choice for reinforcing fibres are recycled carbon and glass fibres. Although recycled carbon fibres are already available on the market, a lot is to be done to discover and establish the most suitable ways to treat, process and use them. For example, there are interesting new approaches to use nonwoven recycled carbon fibres mats oversaturated with a B-stage resin in a resin transfer pressing process. This method allows the storing of the preforms at room temperature while the preparation of thefinal part can be done by compression moulding. As often concluded in the composite forums, the use of recycled fibres especially in stiffness critical applications has a lot of potential, while for strength critical components they introduce often too much uncertainty.