150 years

Challenges in Achieving Class A Finish on Carbon Fiber Composites

Donald H. Campbell

April 2015

150 years

Lightweighting efforts have increased interest for carbon fiber composites as

exterior body panels.

These panels are expected to exhibit a class A surface finish

Continuous Fiber panels

Resin transfer molding (RTM) – shorter cycle times, carbon fiber weaves

Prepreg with autoclave – unidirectional carbon fiber layers or weaves

Both unidirectional and weaves cause varying degrees of texture in the panel

surface

This texture is currently reduced by multiple coats with sanding between coats

Chopped Fiber panels

Likely choice for high volume exterior panel applications

Fiber protrusion and porosity cause pop-like defects – similar to SMC issues

Challenges in Achieving Class A Finish on Carbon Fiber Composites

24/20/2015

150 years

Cross-section of RTM CF Composite

Large CF bundles (48k tow) – 1.5-2.0 mm x 0.25 mm

Resin rich domains – 0.25 mm x 0.25 mm

150 years

Multiple surfacers were screened for coverage of surface texture

Topcoating was done with low bake (100°C) 2K clear and water basecoat

Low bake products were generally better

Unsanded surfacer showed 0.25 mm and wider 2 mm surface structure

Even when surfacer was sanded smooth, 0.25 mm structure was seen after

topcoating!

Multiple coats of primer plus sanding did not improve this

Panels were reheated to 100°C and evaluated while still hot

No surface structure was seen in unsanded areas!!

Sanded areas had reverse of structure

Optical profilometry study planned to map coated and uncoated areas at 25°C and

100°C

Filling of Texture in ContinuousCF Substrate (RTM)Results of Technology Screening

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150 years

Unsanded primer + Topcoatwide depressions at 25°C, almost gone at 100°C

25°C

100°C

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Uncoated RTM CF Compositedeeper depressions at 25°C

25°C

100°C

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Uncoated RTM CF CompositeDifference between 25°C and 100°C

-4

-3

-2

-1

0

1

2

3

4

0 2 4 6 8 10

dis

pla

cem

en

t (m

icro

ns)

position (mm)

Overlay of 25C and 100C (uncoated)

25C

100C

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 2 4 6 8 10

dis

pla

cem

en

t (m

icro

ns)

position (mm)

Difference between 25C and 100C (uncoated)

Unsanded primer + Topcoat at 25C

Results and Conclusions The difference in the substrate

(25°C vs 100°C) is similar to the texture that is seen in the unsanded panel at 25°C.

Change in substrate surface during cooling is causing appearance defects

Changes are larger than epoxy surface domain

150 years

Sanded Primer + Topcoatsmall depressions at 25°C, reversed at 100°C

Small depressions 0.25 mm x 0.25 micron = epoxy surface domains

Depressions are reversed at high temperature

25°C

100°C

150 years

Small Deformations (0.25 mm x 0.5 microns)

Comparison with Domain Sizes and CTE

• Carbon fiber CTE (transverse) is 5-10ppm/C • Resin CTE measured at 55ppm/C, 45ppm differential CTE to CF domains• 0.34% differential thermal expansion between 25°C and 100°C• 0.8 micron of surface expansion above these 250µm domains

Table 1: CTEs for some common

electronic materials

material CTE (ppm/°C)

silicon 3.2

alumina 6–7

copper 16.7

tin-lead solder 27

E-glass 54

S-glass 16

epoxy resins 15–100

silicone resins 30–300

150 years

Why Sanding is ineffective

substrate

primer

Primer sanding

clearcoat

baked primer on CF substrate

clearcoat at room temperature system at bake temperature

Clearcoat baking

system after cooling

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Summary

• Surface texture on continuous CF composites can be related to non-

homogenous CTE within the substrate

• Glass fiber (GF) composites can be designed with matched CTE between fiber

and matrix

• Potential means to counteract this issue include:

• Prepreg weave has smaller resin domains than RTM samples

• Unidirectional prepreg have smallest resin domains

• Low CTE epoxy resins

4/20/201511

150 years

150 years