white paper: lighter weight automotive...

PolyOne Corporation • 33587 Walker Road • Avon Lake, Ohio USA 44012

A new natural fiber reinforced thermoplastic material can reduce part weight by 5% or more with no performance tradeoffs.

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LIGHTER WEIGHT AUTOMOTIVE PARTS, NATURALLY

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Weight reduction is one of the pillars of automotive original equipment manufacturers’ (OEMs) efforts to improve their vehicles’ fuel efficiency, as required by global regulations. Not only do lower-weight vehicles use less fuel, but they also generate fewer carbon dioxide (CO2) emis-sions. It is estimated that every kilogram of weight saved in a car reduces its CO2 emissions by 75 to 100 mg per kilometer traveled.

The penalty for not reducing the average CO2 emissions is significant. For carmakers marketing their vehicles in Europe, failure to reach in-cremental CO2 limits imposed by the European Commission (average emission/vehicle for the entire fleet at 130g/km in 2015, and then 95g/km in 2021 – equivalent to a fuel efficiency standard of 57.4 miles per gallon) would force OEMs to pay a significant fee up to 95 ¤ per g CO2

over the limit and for each new car sold.

Put another way, the potential cost for an OEM having an average fleet positioned 10g CO2/km above the legal limit and selling one million cars per year in Europe represents ¤ 710 million. If an OEM would decide to absorb these costs, it would have a significant impact on profitability. If the OEM would decide to pass these costs on to con-sumers in the form of higher-priced cars, it would risk a loss of com-petitiveness and market share. Since neither scenario is palatable for automotive OEMs, they instead are leaving no stone unturned in their efforts to find materials that offer the optimum performance-to-weight for each application.

LIghTWEIghTINg PrIOrITIES: FOCuS ON ThE FrONT aNd rEduCE dENSITy by aT LEaST 5%

There essentially are two strategies for lightweighting versus incum-bent material solutions. One is to increase performance while keeping a part’s density at its current level, and the second is to maintain the current performance at a lower density. The latter is typically the more feasible approach.

IMPOrTaNT TO KNOW abOuT LIghTWEIghTINg

• Weightsavedatthefront of a vehicle has a greater effect on fuel savings

• Newregulations-andthe potential costs of not complying- make lightweighting a priority at automotive OEMs

• 1kgofweightsavedreduces a car’s CO2 emissions by circa 75 - 100 mg/km traveled

• OEMswantweightsavings of at least 5% to justify switching to a new material

PolyOne Corporation • 33587 Walker Road • Avon Lake, Ohio USA 44012 2

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but where should lightweighting efforts be focused? On most cur-rent passenger cars, the weight of a vehicle is generally 60% at the front and 40% at the rear of the car. This balance, plus a low center of gravity of the car, is crucial for proper vehicle dynamics. Therefore the most critical areas of focus for lightweighting are primarily located in the front of the car – for example in under-the-hood applications and front components such as lighting systems. Instrument panel carri-ers and other semi-structural parts also are of interest as these weigh enough to justify closer examination for reduced weight.

The incumbent material for many of the applications in the front sec-tion of a car is injection molded short glass fiber reinforced polypro-pylene (PP-SgF). It offers a relatively low density along with good me-chanical properties, and is readily available around the world. If not for the new Eu regulations, it is unlikely that automotive engineers would be seeking alternatives to the proven PP-SgF.

Instead, new regulations are forcing a re-think about even the most established incumbent materials. In general, though, automotive OEMs have said that a new material solution has to offer at least a 5% reduc-tion in density, at comparable performance, to be seriously considered as a replacement.

NaTuraL FIbErS, PaST aNd PrESENT

Perhaps a new reinforcing material should be considered. Natural fibers have been used for decades to reinforce plastics used in auto-mobiles, well before the use of glass fibers. Suppliers of thermoplastic compounds have developed materials reinforced with many different types of natural fibers. Some of these materials have established them-selves in applications that do not require high strength or mechanical performance.

however, natural fiber reinforced thermoplastics (NFr-TP) have seen limited use in vehicle applications requiring superior mechanical per-formance. recently, though, a new NFr-TP solution has been devel-oped that offers automobile manufacturers and their suppliers the opportunity to choose natural fibers and still realize high-level mechanical performance.

uPhILL baTTLE FOr MOST NaTuraL FIbErS

E-glass fibers are the reinforcement most commonly used in thermo-plastic compounds in the automotive industry. These fibers perform well and are cost efficient. however, tests have shown that alterna-tive reinforcement materials to E-glass could provide a better perfor-mance-to-weight ratio. These alternatives include carbon, basalt and S-glass fibers.


Natural fiber reinforced plastics are not new, but they have seen limited use in automotive applications that require high strength and stiffness.

hemp fibers

rice hulls

Wood flour

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In addition to these synthetic fibers, some natural fibers have a specific strength or modulus that in some cases can outperform E-glass. Some of them are agro-based, others are wood derived; very few of them are post-industrial.

Several factors can influence the performance of agro-based natural fibers, including genetics, growing conditions, seasonality and weather. From hemp, flax, ramie, and jute to kenaf, miscanthus, and bamboo, performance is often a function of factors that cannot be controlled. In addition, fibers are not available on a global basis. until now, these challenges have limited the broader adoption of natural fiber rein-forced solutions in many end-use applications and markets, despite the best efforts of many leading plastics suppliers and OEMs. While broader commercial success is lacking, enough programs have been attempted to confirm that parts with low densities can be manufac-tured when reinforced with agro-based natural fibers. Mechanical per-formance has been low, though, and not competitive with glass fiber reinforced composites.

Wood-derived fibers also are an option. These typically have been by-products of the wood industry and have been used as fillers in several types of polymers for Wood Plastics Composite (WPC) applications such as fencing and decking. Their reinforcement effect is more limited than agro-based fibers, but the production of these fibers is closer to an industrial process.

Simply mixing natural fibers, whatever their origin, into a petro-based polymer matrix would result in poor homogeneity and low mechanical performance, since there is a limited chemical compatibility between the two components. The challenge consists in making a non-polar component (the polymer), and a polar component (the fiber) as com-patible as possible. Steps taken to try to achieve this compatibility in-clude fiber pre-treatment (similar to sizing of glass fiber), the addition of maleic anhydride polypropylene (MaPP), or other steps.

In short, the first two key challenges before natural fibers can be suc-cessfully introduced in automotive parts are:

• Identifyanaturalfibermanufacturedtodeliverahighlevelof consistency in shape and performance, with global availability, and with a chemical pre-treatment for compatibility purposes, and

• Identifyatechnologythatwouldcouplethetwocomponents (the fiber and the matrix) to create strong bonding forces, and therefore deliver high mechanical performance in finished parts.


WhaT PrEVENTEd NFr-TPS FrOM brOadEr SuCCESS?

• Lackof(global) availability

• Limitedmechanicalstrength

• Concernsabout consistent performance

• Concernsabout odor and mold

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dEVELOPMENT aNd CharaCTErIzaTION OF POTENTIaL SOLuTIONS

The optimal target property profile for a material for semi-structural automotive applications is defined in the following table:

This target property profile is representative of a typical short glass fiber reinforced PP used in automotive, but with at least a 5% lower density.

When it comes to reinforcing polypropylene with natural fiber, many routes, variables, and choices can have a critical influence on the final solution:

• typeofPP(homo-orcopolymer)• meltindex• naturalfibertype• couplingtechnology• manufacturingprocess,etc.

using the statistical method of a design of experiment (dOE), these input variables could be reduced down to three: the type of fiber, cou-pling technology, and manufacturing process (compound extrusion). a PP homopolymer was determined to be the optimal matrix material.


Property Conditions Standard Target

Specific gravity (g/cm3)

23°C ISO-1183 <1.08

Tensile Strength at break (MPa)

23°C50 mm/min

ISO-527

75

Tensile Modulus (MPa)

23°C1 mm/min

5500

Flexural Strength at break (MPa) 23°C

2 mm/min ISO-178

100

Flexural Modulus (MPa)

4500

Charpy unnotched impact strength (kJ/m2)

23°C ISO-179/1eu 20

heat deflection Temperature (°C)

1,8 MPa ISO 75/a 140

0,45 MPa ISO 75/b 155

underhood applications remain a target for light-weighting projects.

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EVaLuaTION OF VarIabLES rEVEaLS SIgNIFICaNT dIFFErENCES

Tests were conducted on two types of fibers: fiber 1 is an engineered natural fiber manufactured by an industrial process, while fiber 2 is an agro-based fiber. They have been processed in the same conditions (standard compound extrusion), and the below spider graph gives a direct mechanical performance comparison between the two fibers. It appears that there is a clear advantage for fiber 1 vs. fiber 2.

Performance comparison between fibers 1&2


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Fiber performance was then compared in terms of two coupling tech-nologies; the coupling technology strengthens the bond between fiber and the thermoplastic matrix material. Coupling technology 1 is an advanced technology while the second one is a more commonly used coupling technology. The superior performance of technology 1 in comparison to technology 2 was confirmed on both fibers.

Performance comparison between the two coupling technologies on fiber 1

The tests revealed that not only does fiber 1 offer better mechanical performance than fiber 2, but its response to the coupling technology 1 also is stronger than fiber 2.


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Now that the fiber and the coupling technology have been selected, it was necessary to investigate whether the compounding process (limited to two different technologies) had an influence on the property profile at various reinforcing fiber amounts (30% and 40% in weight, fiber 1).

Performance comparison between the two processing methods on fiber 1 at

40%

In the end, process 1 has proven to show the best overall performance, for both reinforcement levels, and the combination of fiber 1 together with coupling 1 and process 1 offers the highest property profile that could be achieved. These lab tests were confirmed on a commercial scale.


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With successful tests completed, the resulting solution was named reSound™ NF natural fiber reinforced solution. reSound is the brand name used by PolyOne to signify a solution contains at least 30% re-newable resourced materials.

The properties of reSound NF solutions obtained at industrial scale with the combination of the different technologies (fiber, coupling, compounding) are very similar to the original target, and realize a den-sity reduction versus short glass fiber alternatives of at least 5%.

Comparison of reSound NF 40% natural fiber reinforced solution vs. ultimate

target profile


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PrOCESSINg STudIES PrOVE POTENTIaL

Injection molding of tensile bars proved the mechanical performance and easy processability of reSound NF compounds. Like most natu-ral fiber reinforced thermoplastics, reSound NF formulations should be processed at temperatures below 200°C in order to maintain the integrity of the fibers. using low temperatures also offers potential energy savings and short cooling times, for efficient yield at manufac-turers.

Experiments also were conducted on molding of parts molded from reSound NF solutions on machines outfitted with the MuCell® foam-ing process from the company Trexel. The MuCell foaming process is a well-known foaming technology for lightweighting parts via the con-trolled injection of supercritical fluids during injection molding. This technology is largely used for automotive components, especially with homopolymer PP-SgF (PPh-SgF) materials in order to reduce the ap-parent density up to 20%. The process affects material properties.

Trials have been performed in collaboration with Trexel and the univer-sity of Lüdenscheid in germany. ISO bars have first been molded with PPh-SgF and reSound NF solutions, without MuCell foaming technolo-gy. Then, MuCell foaming technology has been used in order to obtain a weight reduction up to 20% with both materials.

Finally, mechanical properties (tensile and flexural) have been mea-sured and plotted versus the weight reduction obtained in comparison with standard glass fiber reinforced grades of PP. These results confirm that combining PPh-SgF with MuCell foaming technology in order to reach a 5% to 10% lower part’s weight results in similar or lower me-chanical performance than using a reSound NF formulation without foaming, for the same final part’s low weight. This finding opens new ways for automotive Tier Suppliers not using MuCell foaming technol-ogy to manufacture lightweight parts while optimizing their current equipment and system costs

Further testing revealed that reSound NF compounds are compatible with the MuCell foaming technology and show robust property reten-tion. The retention is as robust as PPh-SgF for tensile properties, is even more robust for flexural properties, and clearly more stable for impact properties. The combination of both reSound NF solutions and MuCell foaming technologies opens additional possibilities for OEMs and Tiers to further reduce parts’ weights while keeping high level of performance.


bENEFITS OF ThE NEW SOLuTION

• 5%-10%lowerdensitythan comparable gFr-PP formulations, at equivalent performance

• Greaterdensityreduc-tion is possible when processed using MuCell foaming technology

• >20%improvementin tensile and flexural properties versus other NFr formulations

• 10°Cto20°Chigherheat deflection tempe-rature than other NFr formulations

• >50%improvementinimpact strength versus other NFr solutions

• Processonstandardmachinery and tooling

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EVaLuaTION OF ON-LINE dILuTION OF a CONCENTraTE

Some customers value the flexibility of using a concentrate and dilut-ing it with a neat resin, in order to adjust final reinforcement levels vs. application needs, or go below a 30% reinforcement level. Therefore trials have been conducted in order to evaluate the applicability of diluting a 50% reinforced reSound NF compound. This concentrate has been mixed with neat PPh on an injection molding machine, in order to respectively obtain a 40% and a 30% reinforced material. results obtained confirm a robust and compatible behavior of parts formed with the diluted solution

ON-LINE INTrOduCTION OF rEgrOuNd MaTErIaL

The behavior and the robustness of both reSound NF formulations and PPh-SgF technologies have been evaluated in regards to on-line use of reground material. Tensile bars have been molded, then ground, and finally molded again with the regrind material only. This was repeated three times.

These results show a more stable performance of the reSound NF solutions with reground material in contrast to PPh-SgF with the same level regrind content. The explanation for this difference of behavior is linked to the L/d

(length over diameter) ratio of the natural fibers in reSound NF formulations. a decrease of this L/d

ratio generally results in lower mechanical performance.

While a glass fiber is physically cut during the injection and regrinding processes, therefore decreasing the L/d

ratio, a natural fiber tends to be cut as well but defibrillated at the same time. In this case, L and d both decrease, keeping the L/d

ratio constant, and so also maintaining the mechanical properties. as a result it seems the limitation in the on-line re-introduction of reground PPh-SgF into the manufacturing process does not apply to reSound NF solutions, meaning this new solution of-fers a potential “no scrap” process for users.


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LargE ParT MOLdINg TrIaLS SuCCEEd

PolyOne and Fraunhofer ICT in germany selected two representa-tive tools in order to perform processing and mechanical tests. One of these injection molds is the “Pegasus” tool, a semi-structural flat tripod, represented here below.

Pegasus shape and main dimensions

This part has been molded in Fraunhofer ICT’s technical center with four types of materials:

Materials molded and their respective densities


600 mm

35

0 m

m25

mm

Material density (g/cm3)

PPh-SgF 30 1,14

reSound rS5200-5003 NF 1,06

PPh-SgF 20 1,05

reSound rS5200-5005 NF 1,02

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The two PPh-SgF have been molded at 240°C and 190°C, while the reSound NF grades were molded at 190°C. The tool temperature was set at 50°C, and injection molding parameters were recorded. The injection molding pressure at 190°C shows a similar injection molding pressure of around 850 bar between a PPh-SgF30 and its equivalent reSound NF lightweight alternative. The process is confirmed to be very stable for all materials. This same comparative processing ap-proach was completed at injection molding machine supplier arburg in germany on a mold for the Fraunhofer ICT beam geometry, a 390mm long, open-ribbed beam, with similar conclusions.

both Pegasus and ICT beam parts were tested at Fraunhofer ICT in tensile and flexural configuration. For Pegasus parts, tensile tests were performed at 23°C and 110°C, while flexural tests were performed at 23°C only.

Flexural testing set-up for ICT beam

The tests confirm that reSound NF formulations have a similar modu-lus as the comparable PPh-SgF grades. Strength was measured based on force at break, and here again the resound NF parts performed as well as the PPh-SgF ones. as a reminder, reSound NF solutions have a 5 to 10% lower density compared to equivalent SgF materials. Similar conclusions were made based on tensile testing, especially at 110°C, a demanding temperature for polyolefins.


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CaE simulations have been carried out in PolyOne’s Innovation Cen-ter, in order to reproduce test conditions and failure modes with the Pegasus geometry. The results of the simulations are consistent with real laboratory tests.

deflection and Von Mises stress

rEady FOr a ChaNgE?Following a rigorous innovation process, advanced material and manu-facturing technologies have been combined to create a solution that offers the required property profile with a low specific gravity. The result, reSound NF natural fiber reinforced formulations, are highly en-gineered, strong and sustainable alternatives to glass fiber reinforced formulations, with densities 5-10% less dense than comparable glass fiber formulations.

Compared to other natural fiber reinforced solutions, reSound NF solutions offer mechanical property improvements of more than 20% for tensile and flexural properties, 10°C to 20°C higher heat deflection temperature, and more than 50% in impact strength.

Customers can process reSound NF compounds on existing machin-ery and tooling, MuCell foaming technology included, at low injection molding temperatures, resulting in short cycle times. These reSound NF solutions are an excellent choice for technical applications in many industries. These new natural fiber reinforced polymer formula-tions enable automotive OEMs and their suppliers to meet goals for lightweighting, sustainability, production efficiency and performance, opening new weight-out opportunities in critical automotive segments including under-the-hood components, lighting systems, semi-struc-tural applications, and many more. PolyOne has identified more than 15 potential applications in passenger vehicles for these formulations.


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Key automotive OEMs are going through their own internal evaluation processes to qualify reSound NF formulations, combining lab tests and physical tests on real parts. The material has made its way onto the ap-proved material list for a critical application in one case.

Customers from non-automotive industries that value lightweighting and sustainable solutions in technical applications can benefit from reSound NF solutions too.

Parts molded with reSound NF natural fiber reinforced solutions

To investigate the potential of reSound NF natural fiber reinforced solutions in your application, contact PolyOne at [email protected].


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