Car manufactures, like their clients, are under pressure. Reducing CO₂ emissions is turning into a commercial argument, but despite efforts to promote public transport, sustainable urban mobility will not be able to disregard the car completely. Migration towards towns and cities requires a greater ability to adapt to changing priorities which go beyond just shared transport. Even if it is just a start, some towns and cities are looking to develop electric car hire services, in addition to public transport. The growth in the number of vehicles on the road worldwide is unavoidable.

The electric car demonstrates that the reduced weight of a “city car” will be the key to its success. In Europe, regulation EC No. 443/2009 sets emission performance for new passenger cars as part of the European Community’s integrated approach to reduce CO₂ emissions from light-duty vehicles.

Development of a Concept Car Using a Rotomolded Chassis

Figure 1 shows a concept car developed by Total. This two-seat concept car is designed for urban mobility; the overall weight is around 500 kg, the maximum speed 120 km/hr, and the range between charges around 120 km. This concept car does not foreshadow a “Total” production car. That’s not what it’s about. The aim is to serve as a technological showcase for the group and its various subsidiaries.

This electric vehicle is produced with a skin-foam-skin chassis. Figure 2 illustrates the main part of the chassis, and Figure 3 the front part of the car. This chassis was produced by rotomolding with a three-layer structure of skin-foam-skin. (The foaming process is also described in the references.^1,2,3) The production of plastic foam involves the dispersion of gases within a polymer matrix. The gaseous phase is generated from either a physical blowing agent or a chemical blowing agent. In very low pressure foaming systems, such as rotational molding, chemical blowing agents have been the most common method to produce cellular structure. (Chemical blowing agents are solid or liquid materials that decompose under certain conditions to produce gas.)

A foam structure in the rotational molding process requires the polymer matrix to form a continuous melt phase; this is to minimize losses of the liberated blowing gas to the atmosphere and reduce the adverse effect of incoherent polymer melt on the bubble morphology. Polymer powder particles tend to decrease their total surface area by fusing together when in contact with each other at elevated temperatures—this is called coalescence or sintering. The driving force for sintering is surface tension, and the main factor opposing this mechanism is the resistance to flow, expressed by viscosity.

In the rotational foam molding process, the sintering step should be completed before the onset of bubble nucleation and growth, to produce foams with higher bubble density. A blowing agent should be dispersed in the matrix and when in solid form, wetted by the molten polymer. The gases released in the polymer melt above the decomposition temperature of the blowing agent initiate the foaming expansion process, which generally comprises fundamental steps: bubble nucleation, bubble growth, bubble coalescence, and stabilization.

The bubble nucleation step occurs simultaneously with the decomposition of the blowing agent particles, and it begins at an initiation site within the polymer melt that has been supersaturated with the blowing gas. Owing to the continual supply of gas, the pressure in the nucleated bubbles increases, whereas the pressure of the polymer matrix is maintained at atmospheric pressure during the entire process. This pressure difference is the main driving force for the growth of the bubble nuclei.⁴

The interest in foaming in rotomolding is driven by the possibility to produce thick layers of a very low density: from 150 to 300 kg/m³. This allows the production of thick parts that will give much more strength with a good weight balance. The rotomolding technology used to produce this car is referred to as TP-Seal® technology from Total Refining and Chemicals.

Materials

The resins used are described in Table 1. The shot weights of the different materials were set to give the following wall thicknesses:

• Outer skin: 4.5 mm
• Foam: 10 mm
• Inner skin: 1.5 mm

The rotomolding cycle is around 1½ hours. The cycle, materials and thicknesses are the exact same for the front part. Tomography analyses were performed to characterize the foam structure (bubble size and dispersion). The overall weight of the two rotomolded parts is 85 kg.

For compression tests, some parts were produced with a single layer of PE1. The part was produced with an overall weight the same as the skin-foam-skin structure; the part produced has a wall thickness of 7.0 mm. (It was estimated that the same chassis produced in metal should weigh between 200 and 300 kg.)

Results

The rotomolded parts produced have been characterized for mechanical performance:

Impact performance (using crash simulation): The front part was tested at a speed of 20 km/hr. No cracks were noticed, and the deformation remained in elastic behavior.

Mechanical strength in a compression test: Some parts were cut off the main chassis of the skin-foam-skin structure and compared to a monolayer structure. A compression test was performed, as presented in Figure 4. We can see on this graph the huge effect of the part thickness while having the same weight. For a 2% global deformation, the skin-foam-skin structure gives five times more strength than the monolayer part of equivalent weight. For a 10% global deformation, the skin-foam-skin structure gives eleven times more strength than the monolayer part of equivalent weight (Figure 4).

Recycling

Twenty rotomolded skin-foam-skin chassis, newly produced, have since been ground up mechanically and then compounded again on a twin-screw extruder. Tensile properties of the obtained resin show that the compound still has very good mechanical performance and good ductility. This material can be reused in the rotomolding process.

Conclusions

The use of a skin-foam-skin rotomolded structure can be considered to produce a small electric car chassis with a very good weight/mechanical-strength balance, with a possibility to recycle the parts.

References

1. M. Emami, E. Takacs, and J. Vlachopoulos, “Study of Foaming Mechanisms in Rotational Molding,” ANTEC 2010.
2. M. Emami, M. R Thompson, E. Takacs, and J. Vlachopoulos, “Rheological Effects on Foam Processing in Rotational Molding,” ANTEC 2011.
3. M. Emami, M. R Thompson, E. Takacs, and J. Vlachopoulos, “Rheological Aspects of Rotational Foam Molding,” PPS 2011.
4. M. Emami, E. Takacs, M.R Thompson, J. Vlachopoulos, and E. Maziers, “Visual Studies of Model Foam Development for Rotational Molding Processes,” Advances in Polymer Technology, Vol. 32, No. S1, E809-E821 (March 2013).