Polymeric Materials for Future Automobiles

2.1 Target for 2050

At the G8 Hokkaido Toyako Summit it was decided that we must halve CO₂ emissions worldwide by 2050, and it was said that the developed countries bear greatest responsibility and must therefore decrease emissions by 80%. With regard to how Japan will respond to this target, the respective positions of the Ministry of the Environment and the Ministry of Economy, Trade and Industry differ and are not reconciled. How is CO₂ emission reduction progressing in Japan, as a developed country? It has been suggested that decreasing CO₂ emissions will adversely affect the economy. Figure 1 shows the CO₂ emissions per GDP since 1990 (plotted by the author using Ministry of Economy, Trade and Industry data on the current status of greenhouse gas emissions (23 January 2015)). In 1990 (the base year of the Kyoto Protocol), Japan's CO₂ emissions per GDP were low, but since then, although Japan's CO₂ emissions have hardly changed, those of Europe and the US have decreased, such that Japan now emits more CO₂ than several of the European countries. The same can be said of Japan's emissions per capita, and so already our position is nothing to be proud of. This graph suggests that in the US, efforts to consistently decrease emissions are continuing despite the change in government attitude to the prevention of global warming since the arrival of the new administration.

OPEN IN VIEWER

Figure 1

Trends in greenhouse gas emissions per GDP in developed countries

Whatever the case, the world has started to work toward this target, and strict targets will also have to be set for automobiles. Toyota, Nissan and the like are announcing that a 90% decrease will be necessary.

Figure 2 shows the relationship between CO₂ emissions per 1 km trip and vehicle weight. Here, the author plotted the emitted CO₂ per 1 L petrol, calculated as 2.322 kg, from the catalogue fuel mileage (JC08) stated by the respective company. The data for ordinary petrol engine cars and the data for hybrid cars are plotted separately, and the graph reveals that there are two ways to decrease CO₂ emissions (improve fuel mileage). One is to lower the amount of energy required to run the car by making the car as light at possible, and the other is to increase energy efficiency by electrification, as with hybrid cars. Fundamentally, hybrid cars use energy effectively because they are motor-driven in areas where the petrol car has its poorest fuel mileage, whereas under ordinary running conditions they operate as a petrol car, and the energy generated during braking is recuperated as electricity.

OPEN IN VIEWER

Figure 2

CO₂ emissions by passenger vehicle weight

3.1 Common issues in the move towards electrification

Next generation automobiles refers to hybrid electric vehicles (HEVs), plug-in hybrid vehicles (PHEVs), electric vehicles (EVs) and fuel cell vehicles (FCVs) (the usual definition of next generation automobiles also includes clean diesel vehicles, but these are not included in this discussion). As all of the abovementioned vehicles are either partly or wholly driven by electricity-powered motors, “next generation automobiles” means electrification. There are three important components that are common to all of these vehicles: secondary batteries for storing electricity, high-performance motors for driving and for generating electricity, and inverters for controlling the electricity. These components are not present in the internal combustion engines used to date. A characteristic problematic feature of electric vehicles is that their driving range is short because they cannot store very much energy, and so they need to be lighter than conventional petrol cars.

The polymer-related challenges associated with electrification are summarised in Table 1 . In all parts of an electric motor, Joule heat is generated when a large current flows. If this Joule heat cannot escape, the temperature will increase uncontrollably, and because organic solvent is used in the battery as electrolyte, this could lead to increased vapour pressure, solvent degradation, reaction at the electrodes, etc., such that the battery becomes a fire hazard. Too high a temperature in the motor can result in demagnetisation of the high-performance neodymium magnets. In order to improve the heat resistance (holding power) of a magnet, the amount of dysprosium or the like must be increased, but this tends to have an adverse effect on magnetic flux density [1]. Moreover, Japan becoming 100% reliant on China for dysprosium would constitute an increased resource security risk. If heat cannot escape from the inverter, it will be difficult to achieve compactness. Provision of an insulator having high thermal conductivity so that generated heat can escape may seem trivial, but it is no exaggeration to say that the fate of the high-performance motor depends on it.

Figure 3 shows the structure of the lithium-ion battery in the Nissan Leaf. A special feature of the Nissan battery is its laminated structure, which allows it to be thin, to facilitate temperature control [2]. The separator and the anode and cathode binders are polymers that play a crucial role in this structure. The separator is usually a microporous PP membrane, which is manufactured to utilise the characteristic properties of the lamellar structure of PP [3]. The role of the separator is to reliably prevent shorting between the anode and cathode, and to enable ions to flow quickly and with little resistance; it therefore has to be a thin membrane. The binder must bind the active materials of the anode and of the cathode onto the respective electrode plates, must not readily swell in the electrolyte, and must be flexible, electrochemically stable, etc., so that it can withstand volume change due to entry and exit of the lithium ions on charging and discharging [4]. The binder is very important, because if it swells too much and the distance between the active substances becomes too great, charging and discharging are adversely affected. At present, PVDF is the main substance used on the anode side, and SBR is the main substance used on the cathode side.

OPEN IN VIEWER

Figure 3

Structure of the lithium-ion laminate battery

Polymeric materials are electrical insulators, but have poor thermal conductivity. Ceramics are electrical insulators and have high thermal conductivity, but unmodified ceramic material cannot be molded into the necessary shape. Development is underway of high thermal conductivity insulators comprising large amounts of ceramic powder of good thermal conductivity mixed into polymeric material of good moldability. Figure 4 shows the predicted improvement in thermal conductivity afforded by packing with filler, according to a prediction formula based on Kanari's empirical formula [5]. It can be seen that packing with high thermal conductivity filler does not greatly improve thermal conductivity, whereas much better results are achieved if the thermal conductivity of the matrix resin can be improved[6]. High hopes are therefore focussed on the development of an epoxy resin having a mesogenic backbone, such as a biphenyl backbone; to what extent these molecules might improve the thermal conductivity of low-thermal conductivity polymeric materials is being investigated [6].

OPEN IN VIEWER

Figure 4

Increase in thermal conductivity when filler is packed into resin

4.1 Weight reduction using plastics

Plastics have not as yet made a major contribution to weight reduction. Figure 5 shows a comparison of the calculated weight reduction achieved when iron is replaced by different materials and rigidity is kept constant. As plastics are not usually used for high-strength parts, it is more appropriate to compare them using isorigidity statistics. Replacing the iron with aluminium halves the weight, and replacing it with magnesium decreases the weight to a third. Plastics are light, but they are soft and so the parts must be thicker; most of the plastics only decrease the weight by 30–40%. These data are obtained using the elastic modulus at room temperature, and so at working temperatures they will be lower. However, the use of CFRP is remarkably effective, affording a 70–80% weight decrease. This is why CFRP is increasingly being used in aircraft. Life cycle assessment (LCA) results can be used to assess to what extent the use of resins is actually lowering weight and decreasing CO₂ emissions in practice.

OPEN IN VIEWER

Figure 5

Comparison of weight reduction performance using isorigidity statistics (compared to steel)

Figure 6 shows an example of the use of resin as the material for the radiator core support, which is the main part in the front end module. The material used was changed from steel to glass fibre (GF) filled PP injection molding, and then, for further weight reduction, it was changed to carbon fibre (CF) filled PP injection molding. The weight was 5.8 kg using steel, 4.1 kg using PP-GF, and 3.1 kg using PP-CF, resulting in a 20% decrease and 24% decrease in life cycle CO₂ emissions, respectively, as shown in Figure 7 . When CF is used, the decrease in CO₂ emissions is not proportional to the weight decrease because there are high CO₂ emissions associated with CF production. At present, therefore, use of PP-CF radiator core supports is limited to high performance vehicles that require weight balance. Another reason for using resins is that they can easily be molded into complex shapes, which not only allows parts to be integrated, resulting in weight reduction, but also simplifies the molding process, thereby saving high-cost materials.

OPEN IN VIEWER

Figure 6

Changing the front end module (radiator core support) material

OPEN IN VIEWER

Figure 7

LCA results for PP radiator core support

A major aim is to decrease the weight by using resin as the base in module construction. The base part has many parts attached to it, and is therefore complex in shape, and plastics can be injection molded to make complex shapes in a single operation, which is advantageous.

Another example of the shift to resins is the introduction of CFRP propeller shafts, as shown in Figure 8 . The propeller shaft rotates at high speed, and resonance must be avoided. It must therefore be light and very rigid. When a single steel shaft is used, there is loss of rigidity and the shaft is very heavy, and so a centre bearing is provided in the middle in order to decrease or prevent resonance. CFRP is light and very rigid, and so it is possible to increase the resonance frequency such that a centre bearing can be omitted. Another important design consideration is where, in the event of impact, might the shaft break and yet not damage the engine. Steel propeller shafts depend on the centre bearing for this, but CFRP shafts are rigid in the direction of rotation, and their strength in the direction of compression can be adjusted by changing the fibre angle, and so they can be made to break anywhere in the shaft body or its coupled parts. This obviates the need for a centre bearing, thereby affording considerable weight reduction. Figure 9 shows the decrease in life cycle CO₂ emissions. Although the effect is considerable in that CO₂ emissions are decreased by at least 30%, CFRP use is limited by cost issues.

OPEN IN VIEWER

Figure 8

Weight reduction by use of CFRP propeller shaft

OPEN IN VIEWER

Figure 9

LCA results for CFRP propeller shaft

Recently, CFRP has been tested and adopted for use as car body material, due to the weight reduction it affords. Obviously, CFRP is an anisotropic material and so some design difficulties remain. The level of reinforcement also depends on whether the fibres will be subjected to tensile stress, compressive stress or shear stress. Cost-reduction, including process innovation, is being investigated, and although CFRP is still expensive and difficult to use in ordinary cars, it is expected to be a trump card in next generation weight reduction. Interestingly, BMW are using a CFRP car body in their electric car “i3”, and keeping CO₂ emissions down by using hydroelectric power as the energy for CF production [7].

Although rubbers have low specific gravity, in most cases carbon black is incorporated, which increases the specific gravity. Thermoplastic elastomers tend not to contain much filler, and so they have low specific gravity. Glass runs are large rubber parts that used to necessitate the use of around 1 kg EPDM per vehicle. Now, however, most are made from TPOs, which affords a 1520% decrease in weight.

Furthermore, many thermoplastic elastomers are hard materials, and so in order to maintain flexibility, they are often used thinly. This also contributes to weight reduction. A disadvantage of the hardness is poor sealability, and so careful design is necessary. For example, Figure 10 shows a constant-velocity joint boot made from thermoplastic polyester elastomer (TPEE). Constant-velocity joint boots used to be made from CR, but due to durability issues such as ozone cracking, most are now made from TPEE. TPEE is not as flexible as CR, and so in order to maintain joint boot flexibility, the TPEE is used thinly and there are more folds in the boot. Using an injection-blow molding for the aperture part improves precision, and seal parts are being designed. Using CR is said to result in a 54% decrease in weight [8].

OPEN IN VIEWER

Figure 10

TPEE CVJ boot

Resin (thermoplastic elastomer) is also being used for hoses. Changing from conventional single-part fuel hoses made from FKM/ECO/polyester fibre/ECO to electroconductive ETFE/PA12 fuel hoses has resulted in a near 90% decrease in weight. A “quick connector” is used, and fluorine rubber and fluorosilicone are used together for the O-ring to ensure the seal. An example of a resin fuel hose is shown in Figure 11 . Changing from rubber to resin lowers the fuel permeability, and, because the resin is more rigid and therefore must be used thinly, it also affords considerable weight reduction.

OPEN IN VIEWER

Figure 11

Example of a resin fuel hose

There are many cases where changing from rubber to TPE has led to weight reduction, but TPEs readily become permanently distorted at some temperatures, and so the use of TPEs for parts often requires careful design.

There are three aspects to the rationale for using resin for weight reduction as described above.

(1)

The moldability of the material affords freedom with regard to the shape. Rigidity can be maintained by using the necessary thickness or by creating a local rib structure. Changing from rubber to TPE means that the parts will have to be thinner in order to maintain flexibility, and this will bring the weight down.

(2)

Good moldability means that parts can be integrated. The main consequence of integration is that parts disappear, rather than decrease in weight. Even losing just one bolt will lower the weight. This technology includes integration and the use of TPEs for plastic bodies and seal parts.

(3)

Composites comprising highly rigid material have the specific characteristics of the respective elements. Also, the part will maintain its shape, and stress will not be borne uniformly. It is therefore possible to achieve effective weight reduction by considering how the load will be borne when multiple materials are used. CFRP can provide high rigidity, plus the high strength of CF.

5. Other Measures to Prevent Warming

5.1 Fuel-saving tyres

When a car is running, up to 15% of the energy losses are attributed to the tyres. Electrification will not allow us to dispense with tyres, and so the proportion of the energy losses attributed to the tyres will increase. Fuel-saving tyres will therefore become even more important. When the car is running, much of the energy lost from the tyres is released as heat, due to deformations in the tread. It would therefore be advantageous to use rubber of low tan δ, except that this tends to adversely affect the wet skid properties. The characteristic frequency when running is equivalent to several tens of Hz, whereas the characteristic frequency when braking is equivalent to around 10,000 Hz, due to unevenness in the road surface. Calculating this difference in frequency using the temperature/time rule reveals that fuel saving is good at around 50°C when tan δ is small, and the braking characteristic reveals that fuel saving is good at around 0°C when tan δ is large. Therefore a recent trend is to use liquid polymer SBR with a controlled microstructure as the rubber material, and to use silica instead of carbon black as the reinforcing material. The effects of changing to silica are shown in Figure 12 [9].

OPEN IN VIEWER

Figure 12

Relationship between tan δ, temperature and frequency (comparison between silica compound and carbon compound)

5.2 Air-conditioning refrigerants and the prevention of global warming

In the 1990s, destruction of the ozone layer prompted CFC-12 to be replaced by HFC-134a as the refrigerant in cars. However, although HFC-134a does not damage the ozone layer, it does have a global warming potential of around 1410 (the GWP of CO₂ is 1), and so the development of a refrigerant with a lower global warming potential would be desirable. Using CO₂ as refrigerant was considered, but in view of efficiency and the fact that this would increase the size of the system, it is currently thought that the next generation refrigerant will be HFO-1234yf. Refrigerant and oil that also acts as lubricant are mixed in the air-conditioning unit, and the development of a hose material that will last sufficiently long under these conditions is also progressing.

6. Conclusions

The trends in the use of polymers for next generation automobiles have been discussed above. The main target is that at least the developed countries must decrease greenhouse gas emissions by 80% or more by 2050. In order to achieve this, we will have to switch to electric cars that do not use petrol (although it may be possible to use biofuel for some vehicles). Whether the main power source is electricity or hydrogen, it will be secondary energy, and so wherever the energy comes from, however it is made, and by whatever route it is delivered into the car, we will have to shift to total energy efficiency. It is not simply a case of saying “it doesn't emit CO₂ so it's okay”. Having experienced the Fukushima nuclear accident we will perhaps hesitate to depend on nuclear power, and ultimately we will have to rely on renewable energy. Of course, it may not be easy to become a technologically advanced society that has sufficient power. In order to achieve this we must think about how to use our precious energy as efficiently as possible.

These are tempestuous times. Immediately after the 2018 cars were launched, California's zero emission vehicle (ZEV) standards were greatly revised. Hybrid cars, which have hitherto been seen as advanced environmental technology, will perhaps come to be seen as ordinary vehicles. Therefore we can count only on EVs or FCVs for our response to these ZEV regulations. In other words, EV development is a matter of urgency.

Fuel-saving regulations are also getting stricter in Europe, where emissions are limited to no more than 95 gCO₂/km. In response, the countries of Europe are investing in electric cars, hybrid cars, and plug-in hybrid cars. Energy sources may change, but we are nevertheless approaching an era in which we will have to use energy efficiently.

Under these circumstances, polymers will play an important role as structural materials and as functional materials. Surely, if we use our ingenuity, this period of transformation could constitute a great business opportunity.