Foams
Polymer foams have unique properties thanks to their cellular structure. Their low weight and mechanical damping, coupled with excellent thermal and acoustic insulation, make foams attractive for many areas of application. These include applications in thermal insulation, packaging, footwear technology, structural lightweight construction, and many more.
We research the targeted adjustment of the structure and properties of polymer foams. To this end, we modify polymers, use existing manufacturing processes, develop new processes, and investigate the resulting properties in detail.
Selected areas of application for our research
- Transport (lightweight construction, energy absorption, insulation)
- Energy technology (lightweight construction, thermal insulation, function integration)
- Sports technology (shoes)
- Electric & electronics (printed circuit boards)
- Packaging (thermal insulation, energy absorption)
Foam extrusion is a continuous process for the production of foam board and foil. The polymer is impregnated in the melt with one or more blowing agents under high pressure. Additives such as nucleating agents or fillers can also be used. The rapid pressure drop at the nozzle creates a thermodynamic imbalance in the polymer melt, which leads to foaming of the polymer. In order to separate the process steps of plasticising / impregnating and cooling / temperature conditioning of the melt, two extruders are often used in tandem.
We have two such tandem extrusion lines available for research. This enables us to cover production at the laboratory and pilot plant scale. Our experience includes the entire spectrum of polymers, from standard plastics, to engineering thermoplastics, and high-temperature thermoplastics.
We are also versed in bio-based and biodegradable polymers such as polylactic acid (PLA). PLA, for example, has comparatively good mechanical characteristics and a low CO2 footprint. These properties make it particularly interesting for the packaging industry. We specifically influence the low melt strength of PLA and its resulting poor foamability by using additives. By using chain extenders, we achieve lower density, smaller cell size, and higher compressive strength.
Just one way we wish to contribute to sustainability is with foams made from recycled “bottle PET” (polyethylene terephthalate, PET). Using reactive foam extrusion, we can produce foams with good solvent resistance, high stiffness, and temperature stability. These properties are important, for example, for the core materials of rotor blades in wind turbines. Furthermore, the foam can be made fire-resistant by using flame retardants.
We optimise the foamability of another polyester, polybutylene terephthalate (PBT), by using supramolecular additives. These additives are soluble in the polymer melt and form supramolecular nano-structures during cooling. The use of these selected additives in the extrusion process makes possible foams with a significantly more homogeneous cell structure and small average cell size. Furthermore, printing properties can be significantly improved.
In the field of high-temperature thermoplastics for transportation and electronics, we are developing a thermoplastic circuit board based on polyetherimide (PEI) and lightweight, inherently flame-resistant foams for aviation and rail transport in application-oriented projects.
Particle foams such as expandable polystyrene (EPS), expanded polypropylene (EPP), and expanded polyethylene (EPE) play an important role in the field of packaging and insulating materials. The advantages of particle foams are their low and adjustable density (down to less than 10 g/l), their favourable price, and the simple production of foamed moulded parts. For this purpose, foam beads are welded together, for example by steam injection, to form complex geometries in a mould. However, the thermal and mechanical properties of EPS, EPP, and especially EPE are limited.
In material development, we deal with the modification of existing and the development of new particle foams, for example, designed for higher temperature resistance or toughness. For particle production, we have autoclave and extrusion processes at our disposal. Furthermore, we investigate the processability and energy consumption of new particle foams in the moulding machine.
For instance, we are currently working on PLA particle foams that are bio-based as well as biodegradable. PLA also offers itself as a sustainable alternative to established particle foams because of its low carbon footprint. A central component of our research here is the improvement of foamability through additives, and the investigation of welding mechanisms during moulded part production.
In cooperation with Neue Materialien Bayreuth GmbH, we are also researching particle foams made of engineering thermoplastics. Compared to EPS and EPP, particle foams made of PBT have higher continuous operating temperatures and are thus suitable for certain processes (e.g. sandwich consolidation or oven drying after painting) or applications (e.g. in the engine compartment). Here, too, special attention is paid to targeted chemical modification in order to improve melt strength, and thus particle production or welding. Especially at elevated temperatures, E-PBT has a higher compressive strength than standard particle foams.
Batch foaming is a discontinuous process conducted with an autoclave. Basically, this process can be divided into pressure-induced and temperature-induced batch foaming. In the first case, a rapid pressure drop is generated in the autoclave by opening the outlet valve, which supersaturates the polymer. As a result, cell nucleation and growth take place and the material foams. In temperature-induced batch foaming, the polymer is first saturated at low temperature under high pressure. By abruptly increasing the temperature of the saturated polymer, for example, by immersing it in a hot medium, the system becomes thermodynamically unstable, which causes the polymer to foam.
Due to the small amount of material and the easily controllable process conditions, batch foaming is suitable for the systematic investigation of foamability. However, the process also has commercial significance and is used by some companies for foam production.
We use the method, for example, to investigate the foaming behaviour of a wide variety of amorphous and semi-crystalline polymers. Here we analyse the influence of viscosity, crystallisation behaviour, and process conditions on foam formation. In particular, the rate of pressure drop has a significant influence on morphology and the density of the foam.
In recent years, polymer research has increasingly focused on bioplastics. However, the term bioplastics is not clearly defined, and includes materials that can be produced from renewable resources and/or are biodegradable. However, it also includes plastics such as polyethylene, which can be produced as drop-ins from natural raw materials but do not degrade biologically.
One focus of our research is on polylactide (PLA) – a polymer whose raw material can be obtained, for example, from corn starch via fermentation. Another area of research is wood plastic composites (WPC). These are a combination of thermoplastics and wood fibres that are recyclable, lightweight, and have good mechanical properties.
The great potential of PLA lies in the fact that it is both bio-based and biodegradable, has comparatively good mechanical characteristics, and boasts a low carbon footprint. In future, processing the material by foam extrusion will be especially interesting for the packaging industry, and offers great opportunities in terms of ease of disposal. A major challenge in foaming PLA is its low melt strength, which is why we are investigating various modifications to improve the foamability of PLA and the effect on foam morphology.
WPC offers an environmentally friendly and sustainable alternative to previous polymer composites, while displaying greater weather resistance. To this end, we are currently researching WPC made of wood fibre in combination with bio-based polymers in cooperation with SKZ Würzburg.
The established types of particle foam, EPS and EPP, have comparatively low continuous use temperatures and are therefore unsuitable for certain processes (e.g. sandwich consolidation or oven drying after painting) and applications (e.g. in the engine compartment). Consequently, we are researching alternatives based on engineering thermoplastics in cooperation with Neue Materialien Bayreuth GmbH. The polyester polybutylene terephthalate (PBT) is suitable for use at elevated temperatures (approx. 200 °C), but has a low melt strength, which makes foaming difficult. Through targeted chemical modification, however, we have managed to produce particle foams and to weld them into moulded parts. The moulded parts made of E-PBT have a higher compressive strength under the influence of increased temperatures than standard particle foams.
We are also working on the use of supermolecular additives to control the morphology of PBT foams. These additives are soluble in the polymer melt and form supramolecular nano-structures during cooling. The use of these selected additives in the foam extrusion process leads to foams with a significantly more homogeneous cell structure and small average cell size. Furthermore, printing properties can be significantly improved. In addition to PBT, we also have experience in foaming other engineering and high-temperature thermoplastics.
Since their introduction, synthetic foams have been used in many areas. In most applications, mechanical properties play an important role. The mechanical properties of polymer foams depend on the base polymer, foam density, cell size, and other structural details of the foam cells. Furthermore, external parameters such as temperature and loading rate have a major influence.
An important area of research for us is therefore the mechanical characterisation of polymer foams – from long-term creep behaviour to quasi-static loading conditions and dynamic fatigue properties. We have various methods for investigating stress under tension, compression, shear, bending, and torsion at our disposal. For a deeper insight, we couple advanced methods such as Digital Image Correlation (DIC) with mechanical tests.
Duromer foams are porous, cellular-structured thermosets. They offer all the advantages of compact epoxy with the addition of lower density and improved insulating properties. The cellular structure can be adjusted to open or closed cell by choosing different physical or chemical blowing agents. The use of templates such as hollow spheres results in closed-cell structures or so-called syntactic foams. In the production of thermoset foams, the curing and foaming mechanisms run in parallel. In order to obtain an optimal morphology and the best possible properties, optimal process conditions are necessary for the respective system.
Due to our many years of expertise in epoxy resins, we have focussed on this class of materials. Epoxy resins also have excellent properties in terms of Tg, chemical stability, and thermomechanical properties. The current trend is to use CO2 as a blowing agent in order to establish an environmentally friendly process for foaming epoxy resins. The CO2 can first be added to an amine hardener in an upstream synthesis step to form a carbamate. This can subsequently be introduced into the resin as an additive with a dual function (as a blowing agent and hardener after its decomposition). CO2 can also be used directly in gaseous form as a blowing agent for a pre-cured epoxy system using the solid-state process.
For efficient production of polymer foams, a fundamental understanding of the rheological properties of blowing agent-laden polymer melts is essential. Even relatively low blowing agent concentrations can significantly reduce the viscosity of the polymer melt. The experimental investigation of the rheology of blowing agent-laden polymer melts requires special methods, as the devices used must be able to withstand high pressures and be absolutely gas-tight.
We have two methods available for this – a pressure cell for a rotary rheometer for low shear rates, and a specially developed nozzle for our foam extrusion plant (in-line rheometer) for high shear rates, which are typical for foam extrusion. Both methods can be used to generate a deep understanding of the influence of dissolved blowing agents on the flow properties of polymers.
Furthermore, we investigate extensional rheological properties to determine melt strength and strain hardening. Two extensional rheometers are available for this purpose.