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AUTOMOTIVE SUNROOF SYSTEMS AND FRAMES IN XIRAN®

SMA/ABS

Marcie Kurcz, Henri-Paul Benichou

Polyscope Polymers B.V., The Netherlands

Abstract Automotive sunroof systems, which have become a must-have for the added comfort and

styling to today s cars, increasingly rely on engineering plastics functionalities to replace metals. Structural and semi-structural sunroof module components, sunroof frames in particular, typically need to meet a wide range of technical requirements, with a clear focus on integration of functions, safety, cost and weight reduction.

The glass-reinforced materials, thermoplastics and thermosets, currently used for sunroof frames are mostly based on PBT/ASA, PBT, PA, PP and unsaturated polyester SMC. These products are not a perfect match for the application needs of today and the future. Glass-reinforced SMA/ABS on the other hand offers an ideal, unique combination of properties required in sunroof frames and systems. SMA/ABS-GF compounds such as Polyscope s Xiran®

SG grades have clear technical and commercial benefits:

High dimensional stability and precision

Very low warpage, compliance to mold cavity shape

Good performance at low wall thicknesses

High creep resistance

Excellent adhesion without surface treatment

Low density, high economic value

Good chemical resistance

Easy recyclability with efficient waste streams

Introduction to SMA/ABS-GF SMA/ABS-GF is an impact-modified compound of styrene maleic anhydride (SMA)

copolymer, ABS and glass fibers. SMA/ABS-GF, such as Polyscope Xiran SG grades, is mainly intended for high performance automotive interior components and use proprietary technology developed by DSM and Polyscope.

All SMA/ABS-GF compounds can be molded utilizing typical hot-runner, valve-gated and conventionally designed mold systems. Extra-rigid SMA/ABS-GF grades have a mold shrinkage of 0.20-0.35%, are easily recyclable and have an excellent shot-to-shot reproducibility. Thanks to its flexible production process, Polyscope is able to supply standard or tailor-made SMA/ABS-GF grades in natural or coloured pellets.

SMA/ABS-GF have glass fiber reinforcement levels ranging from 10 to 30% by weight. Interaction between the polar SMA co-polymer and the glass fibers yields an unmatched property profile. Vicat B50 heat resistance ranges from 125° to 150°C. The extra-rigid SMA/ABS-GF grades are well suited to automotive applications such as soft instrument panel carriers, floor consoles, center stacks and structural brackets, sunroof frames, over-head console racks, door panel substrates, painted mirror housings, windscreen demisters and air intake grilles.

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Sunroof Frame Case Study

Technical Requirements

With few exceptions, the main technical requirements of sunroof frames are similar at most OEMs and system suppliers. A high priority is put on a low distortion of the material and on good bonding with adjacent components.

Furthermore, the material should meet the following requirements: minimize the impact of end-of life vehicles on the environment by being fully recyclable and containing no prohibited substances; fulfill sun simulation and high-low temperature tests with no surface degradation, no visible deformation and no loss of properties; have limited emissions of volatile organic compounds (VOCs), meet relevant weather resistance and fogging standards and exhibit resistance to a wide range of chemical products, including cutting and drawing oils, rust inhibitors, cleaning compounds and screw oils.

Properties

Sun roof materials must typically meet the following requirements:

Tensile modulus: MPa 5000-9000;

Stress at break: MPa 60-100;

Elongation at break: % 2.0-3.0;

Charpy notched impact strength @23°C:

kJ/m2 8-11; @-30°C: kJ/m2 7-10.

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Table 1: SMA/ABS-GF compounds properties

Typical data

SMA/ABS-GF

Glass Content % 15% 20% 30% Mechanical properties Units SI Value Value Value Charpy notched impact (ISO 179/1eA) at +23 °C kJ/m² 9 10 10 at -40 °C kJ/m² 8 9 9 Charpy unnotched impact (ISO 1791eU) at +23 °C kJ/m² 28 30 30 at -40 °C kJ/m² 32 35 30 Flexural test (ISO 178) flexural strength N/mm² 120 132 142 flexural modulus N/mm² 5400 6000 8500 Specific properties

Density (ISO 1183) g/cm³ 1,17 1,20 1,23 Mould shrinkage % 0.35 0,25 0,20

Material Benchmark

SMA/ABS-GF also fulfills additional needs of sunroof designers and manufacturers. They exhibit: limited warpage; constant dimensional stability over a wide temperature range; excellent compatibility with polyurethane adhesives; good paintability; high creep resistance under load; low coefficient of linear thermal expansion (CLTE), closer to that of metal than many other polymers; recyclability: good strength per unit weight; competitive system cost; consistent property performance over temperature.

Warpage & Shrinkage

Warpage leads to fit and finish issues in the assembly and performance of sunroof modules. These issues increase costs by requiring the OEM or system supplier to add plastics and metal elements or adjust toolings to secure the assembly. Therefore, a material such as SMA/ABS-GF, that offers excellent dimensional stability, improves the benefits from standpoints of cost, performance and manufacturing.

Components molded with amorphous SMA/ABS-GF exhibit much less warpage than with semi-crystalline materials (PP-LGF, PBT/ASA-GF, PBT-GF, PA-GF).

Issues affecting semi-crystalline materials may include significant deformation, long set-up times and mold cavity shape adjustment. These materials have a higher shrinkage (Fig 1) than SMA/ABS-GF, while the long glass fibers in LGF compounds create different levels of shrinkage in the flow and cross-flow directions. The warpage is difficult to predict with CAE in order to guarantee the correct part shape. More injection gates are also needed.

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Fig 1: Crystalline polymers have a higher shrinkage than amorphous polymers

Warpage of SMA/ABS-GF vs. PP-GF

Warpage depends on many factors, such as processing conditions, process variations, uneven cooling, gating and design. These factors can create stresses in the part and influence warpage and shrinkage values.

The following warpage study (Table 2) of a 2.5 mm thick molded part demonstrates the better dimensional performance of SMA/ABS-GF15 compared to several PP types:

Table 2: Molded part deflection is lower in SMA/ABS-GF than in PP-GF or PP-talc

Polypropylene Deflection increase compared to SMA/ABS-GF (15% glass fiber reinforced)

10% GF 200 %

30% GF 170 %

10% Talc 178 %

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Another warpage study (Fig 2) with SMA/ABS-GF30 vs. PP-GF30 shows the excellent dimensional stability of the former. IAC Group, in partnership with Polyscope, has achieved a much lower molding warpage of SMA/ABS-GF vs. PP-GF on a M.A.N. truck sunroof component.

SMA/ABS - 30% GF No Warpage

Polypropylene - 30% GF High warpage

Fig 2: Molded part warpage is lower in SMA/ABS-GF than in PP-GF. (IAC Group warpage comparison on an M.A.N. truck sunroof component)

Adhesion to PUR adhesive or foam

High polarity SMA/ABS-GF has an excellent adhesion to PU adhesives used to bond sunroof frames to metal sheet roofs. SMA/ABS exhibits: high surface energy - strong chemical adhesion without primer or pre treatment, thanks to the chemical reactivity of the maleic anhydride with polyurethane - excellent durability over service life.

Polypropylene and PBT/ASA blends have low polarity and hence no or limited adhesion to PUR adhesives.

Flame or plasma treatment or primer is therefore needed

Reliability is weak, the system is not foolproof (poka yoke)

Additional costs are involved in primer pretreatment step / investment / plant floor space

Source: IAC Group

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Table 3: Adhesion strength as a function of surface roughness and foam types of SMA/ABS-GF vs.other polymers

Adhesion properties of SMA/ABS-GF vs. other polymers

Substrate materials

1 2 3 4 5 PP GF30 PC / ABS PPO HIPS GF16

SMA GF20 a SMA GF20 b

Source: > SAE Paper 950860 > 2007 Journal of applied Polymer science DOI 10.1002 / app (Vol. 104, 479-488 )

In the Table 4 hereunder, bonding tests results with SMP adhesive (PUR alternative) show the excellent adhesion on SMA/ABS-GF15 without pre-treatment, initially and after ageing. (test results show a 100% cohesive failure, ie. the material fails before the adhesive, indicating the best achievable bonding.)

Table 4: SMA/ABS-GF has an outstanding adhesion to SMP or PUR adhesives

Test method : Quick-knife test (peel test) Application method: Adhesive applied with a triangular bead 9x9mm and pressed down to. 2-3 mm thickness Steps: 1) Curing for 7 days at 23°C and 50%RH 2) followed by 7 days water immersion at room temperature 3) followed by 7 days at 70°C 4) followed by 7 days at 70°C/100%RH + 16hours at 25°C

1. Best 5. Worst

Source: Bostik

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Creep

When a stress is applied over a long period of time, all plastic materials tend to creep. The

very low creep of SMA/ABS-GF ensures tight mechanical fixations and screw torque retention over the part lifetime. SMA/ABS-GF parts subjected to loads over temperatures ranging from -40°C to +120°C retain their dimensional stability in the long term.

Most sunroof frame designs are screwed and/or glued with polyurethane adhesives to the sheet metal roof. No gaps or delamination are permitted. Therefore, creep resistance, together with high polarity and chemical reactivity for excellent adhesion, low warpage and dimensional stability, are critical factors in material selection.

SMA/ABS-GF creep (Fig 3) is significantly less than PP-LGF or PBT/ASA-GF and so ensures better screw torque retention. Fewer metal inserts (or in some cases none at all) are needed for a SMA/ABS-GF sunroof frame.

Creep curves Xiran vs PP LGF

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 1 10 100 1000

Time [Hours]

Cree

p [%

]

XIRAN 15 GF PP 20 LGF

Creep curves Xiran vs PP LGF

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0,2

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0,1 1 10 100 1000

Time [Hours]

Cree

p [%

]

XIRAN 15 GF PP 20 LGF

Fig 3: SMA/ABS-GF creep is significantly less than PP-LGF

Behaviour in temperature

Acceptable sunroof frame performance requires a material capable of maintaining its properties over the vehicle service temperature. The following graphs (Fig 4 & 5) reflects the high property retention of SMA/ABS-GF under thermal variation.

The consistency of properties over a wide range of temperatures allows an optimal, cost effective design.

SMA/ABS-GF shear modulus remains more stable in temperature variations than semi-crystalline materials such as PP-LGF. Dynamic deformation of SMA/ABS-GF molded parts is therefore lower at high temperature.

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SMA/ABS-GF13 (XIRAN® SG226) & GF15 (XIRAN® SG223) vs. PP-LGF23 & 30

Fig 4: Dynamic deformation of SMA/ABS-GF molded parts is lower than in PP-LGF

SMA/ABS-GF15 (XIRAN® SG230) & GF30 (XIRAN® SG260)

Flexural Modulus vs Temperature

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1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-40 23 50 80 110

Temperature °C

N/m

m2

XIRAN SG230 XIRAN SG260

Fig 5: The rigidity of SMA/ABS-GF remains very stable over temperature variations

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SMA/ABS-GF15: Serial Applications in Sunroof Systems

Chrysler WD/WK Platform Chrysler Grand Cherokee (1,5 kg, 115 x 97 cm )

Chrysler WD/WK Platform Dodge Durango (1,5 kg, 65 x 90 cm )

Chrysler RT/RM Platform / Dodge Caravan, Chrysler Town & Country, VW Routan (1 kg, 65 x 85 cm )

Recycling SMA/ABS-GF is currently recycled from rejected instrument panel carriers and die-cut trims.

Up to 20% of reclaimed SMA/ABS-GF is blended with 80% virgin SMA/ABS-GF (Table 5) to mold sunroof frames or other components, and still complies with virgin material specifications. Finished parts are shredded, the different layers (substrate, foam, adhesive, skin) are mechanically separated (Fig 6) , further steps remove paint, coatings, etc. 100% reclaimed SMA/ABS-GF can be used in less critical applications (airducts, center consoles, retainers, etc.)

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Table 5: Up to 20% of reclaimed SMA/ABS-GF is blended with 80% virgin SMA/ABS-GF

Excellent Recycling Properties of SMA/ABS-GF13

Recycling Process Flow Diagram of SMA/ABS-GF

MoldingResin supplier

Foam/Skin Assy OEM

Recycling - Recompounding End User

Fig 6: Finished parts are shredded, the different layers (substrate, foam, adhesive, skin) are mechanically separated

PP-LGF recycling is more critical due to the breakdown of long glass fibers. A significant loss of properties occurs from the second recycling loop onwards.

Cost Analysis Cost analysis made on serial sunroof frames and sunroof components demonstrate the

better cost effectiveness of SMA/ABS-GF over PBT/ASA-GF (Table 6), thanks to a lower density and an excellent bonding to polyurethane adhesives without primer or surface pre-treatment.

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Table 6: Better cost effectiveness of SMA/ABS-GF over PBT/ASA-GF

Typical cost benchmark PBT/ASA-GF vs. SMA/ABS-GF

PBT/ASA GF30 SMA/ABS-GF30 SMA/ABS-GF15

Density 1.47 Weight 2500 g Part.cost = 100 Primer cost = 10 Total cost = 110

Density 1.27 Weight 2160 g Part.cost = 86 Primer cost = 0 Total cost = 86 Saving 21%

Density 1.17 Weight 1990 g Part.cost = 80 Primer cost = 0 Total cost = 80 Saving 27%

Conclusion SMA/ABS-GF compounds such as Polyscope s XIRAN® SG grades are the logical choice

for current and future sunroof frames applications, thanks to an outstanding combination of benefits, in particular a proven dimensional stability, a negligible warpage or distortion after molding, a high adhesion to PUR adhesive without prior surface treatment and a low density. These key advantages compared to conventional engineering plastics make SMA/ABS-GF the ideal solution for improving weight and cost effectiveness of sunroof frames.