Shorter fibres promise automotive benefits

Edited by the Engineeringtalk editorial team Mar 24, 2003

A novel high-performance short-glass-fibre-reinforced polypropylene compound family has the potential to replace long-glass-fibre grades in highly stressed parts for technical automotive applications.

Borealis has developed a high-performance short-glass-fibre-reinforced (HPGF) polypropylene compound family that has the technological and economical potential to replace long-glass-fibre (LGF) grades in highly stressed parts for technical automotive applications.

Borealis has commercialised this new product group under the Xmod brand.

"Glass-fibre-reinforced compounds based on polypropylene (PP) have lead the industry for more than 20 years.

Over time developments have been based on different polypropylene polymers like homo- or copolymers with normal or higher flowability but with no real progress in properties", says Gunther Aumayr, Market Development Leader, Borealis Engineering Applications Business Unit.

"Moreover, advantages from the improvement of all - LGF products are offset by requirements to optimise the whole process chain including screw design in extruders, processing parameters and mould design, thus incurring higher investment and production costs".

By contrast, the processing of HPGF compounds requires no additional investments as it uses standard injection moulding machines.

These compounds are ready to use with low risk and material responsibility at the raw material supplier.

A broader variety of custom-designed polymers becomes possible.

These are more cost effective than pultruded grades and bring the benefits of easier and better coupling of fibre and matrix, and exhibit similar property levels to those obtained with LGF grades.

HPGF compounds suffer lower emissions, lower fogging and lower odour values than LGF grades.

Improved weldability and fatigue behaviour are also achieved.

Additionally, flowline and weldline properties are increased significantly when compared to those of LGF grades.

Xmod G30 grade (containing 30% glass fibres) shows a significant improvement of tensile modulus compared to conventional PP-GF 30 over a range of temperatures tested up to 140C.

Similarly, impact behaviour, measured as notched impact strength or biaxial impact behaviour, also achieves better results.

According to Aumayr: "For the newly developed high performance Xmod family it was possible to increase significantly both tensile strength to 115-120MPa and total penetration energy up to 10J".

Weldlines and flowlines mainly result from the part's design (holes, ribs and multi gating).

For LGF products melt flow values typically ranging from 40 to 150g/10min (or even higher), necessitating the use of a product with a significantly lower molecular mass resulting in lower mechanical performances.

However, the retained weldline strength in the short-glass-fibre grades, Xmod G30 and PP-GF30, is over twice the value for long-glass-fibre-reinforced grades.

Xmod G30 has an achievable absolute burst pressure level more than double that of long-glass-fibre grades over a range of values for welding penetration, amplitude and welding pressure.

Additionally, it offers superior fatigue behaviour, as measured by fatigue crack growth rate, compared to that of standard PP-GF30 or PP-LGF-30, and only at substantially higher stress levels does polyamide PA6-GF30 perform better.

These improved properties of HPGF grades make them eminently suitable for use in the automotive industry, with the potential to replace metal, long glass fibre PP or even glass-reinforced polyamides.

Front-end carriers moulded in HPGF grades could be an economically better solution than those using LGF polymers.

Dashboard carriers require low emission and fogging values, which are achievable with HPGF.

In respect of critical weldline behaviour, HPGF also performs better than LGF in pedal carriers.

Xmod GB305HP has already been tested successfully for air intake manifold applications and projects are underway for serial introduction.

Further potential applications include the manufacture of fan supports and shrouds, drive belt covers, blower wheel covers, bases for air filters, battery supports, engine covers and parts for the cooling system.

Says Aumayr: "This new high performance family of materials achieves mechanical properties close to the LGF but at lower system costs.

And, in some special cases where weldline, flowline and welding performance as well as fatigue behaviour is of importance, these compounds are superior to LGF products".