Modifying thermoplastic polyolefin surfaces by gas-phase methods – relevance to adhesion for

waterborne coatings in automotive applications

K. Grundke1, K. Estel1, A. Marschner1, C. Bellmann1, B. Joos-Müller2 and A. Stoll3 1 Leibniz-Institut für Polymerfoschung Dresden e.V.

2 Fraunhofer-Institut für Produktionstechnik und Automatisierung, IPA 3 Forschungsinstitut für Leder und Kunststoffbahnen Freiberg, FILK

Thermoplastic polyolefins (TPO) are widely used in the automotive industry for the manufacturing of bumpers. For esthetic and protective reasons, the decorative painting of bumpers is desirable.

The low surface free energy and the lack of polar functional groups of these materials are the reasons for their poor paintability. Effective surface modification techniques are, therefore, needed

to improve the adhesion properties of TPO.

It was the aim of this work to compare the modification effects of three gas-phase methods (flame, plasma jet treatment, gas phase fluorination), especially with regard to a better understanding of

the interplay between chemical composition, surface energetic properties and surface morphology of the pretreated TPO on the one hand and the adhesion of waterborne coating systems on the

other hand.

Methods and Materials

Comparison of modification effects

Impact on paint adhesion

Acknowledgements

This project (no. 387 ZBG) was funded via AiF by the German Federal Ministry

for Economic Affairs and Energy within the governmental R&D support „Industrial

cooperative research“.

The authors want to be grateful to the companies of the project board especially

BMW, Daimler, Plasmatreat and Wörwag for making available the materials as

well as their support in carrying out the experiments.

Conclusions

Flame treatment and gas phase fluorination had only a minor influence on surface roughness and heterogeneity of TPO materials.

A dramatic increase in roughness and heterogeneity was observed after plasma jet treatments. But, by optimizing the treatment

conditions surface roughness could be kept unaltered.

Depending on the treatment conditions oxygen and nitrogen containing functional surface groups are introduced. In this way,

it is possible to adjust the acid-base surface characteristics (influence on paint adhesion is expected).

Higher amounts of additives in the surface region of TPO materials caused insufficient paint adhesion.

A relation between the lowest and highest oxygen content in the surface region and paint adhesion was found.

From XPS analysis of the fracture surfaces, it can be concluded that interfacial failure predominated when the modification effect

was too small. Cohesive failure in the TPO material was observed when the surface was over-treated.

0.0

0.5

1.0

1.5

2.0

Ra

[µm

]

flame

treatedgas phase fluorination plasma jet

static nozzle

plasma jet

rotating nozzle

untr

eate

d

wx

Materials TPO: commercially available from Basell, Total, Borealis, Sabic;

blends: polypropylene (PP), ethylene propylene diene monomer rubber

(EPDM), additives, talc (10%, 20%, 30%)

Automotive coating system: 2K hydroprimer (15-25 µm), metallic base

coat (10-14 µm), 2K clear coat (25-35 µm) provided by Wörwag

Effect of different flame procedures

Plasma jet treatment: surface roughness and

heterogeneity were dramatically increased

depending on the treatment parameters High fluctuations in the oxygen content of the outermost surface region

Relation between oxygen content and adhesion ?

Flame treatment air/ propane (22:1); variation of

exposure time, burner capacity, and

flame distance

additionally: flame treatments under

industrial conditions in a paint line

for plastic parts

Plasma jet treatment static and rotating nozzles;

variation of nozzle distance and

exposure time

Gas phase fluorination tunnel machine using a gas mixture

of F2/ N2/ O2 (oxyfluorination); batch

process using a mixture of F2/ N2;

variation of fluorine concentration

and time of exposure

Complementary surface characterization techniques

0 2 4 6 8 10 12 14 16

-20

-10

0

10

20

pHIEP

pHIEP

pHIEP

basic groups

amphoteric groups

acidic groups

pH

[mV]

Zeta potential XPS AFM, confocal light microscopy Contact angle

Estimation of paint adhesion Vapor jet test (DIN 55662): after conditioning (7 day at

23°C and 50% rel. humidity) and after TWT test (3 cycles

each with 15h, 105°C, 30 min RT, 8h -40°C, 30 min RT);

coated test panels contained a cross-cut, adhesion level

was assessed by a number between 0 and 5

TPO surfaces treated in a paint line contained higher amounts of nitrogen

resulting in a change of the acid-base properties

Effect of different gas-phase methods

Adhesion of a waterborne paint system on pretreated materials

TPO materials A and C: destruction of

the lacquer layer

For the lowest and the highest oxygen

content in the surface region of TPO,

adhesion was insufficient

XPS analysis of the fracture surfaces

-500

0

500

1000

F E D

C B

A

Atomic Force Microscopy - Section (50 µm)

0 50 100 150

-500

0

500

1000

he

igh

t [n

m]

x-position [µm]

Section analysis of different TPO-materials XPS high-resolution spectra of the C1s peak

after plasma jet pretreatment

BFa

01

BFa

02

BFa

03

GFa

02

GFa

03

PLa

08

RPLa

16

0

5

10

15

20

25

715 15

4

10

3

10

15

5

qu

anti

ty

characteristic value 3-5

characteristic value 2

characteristic value 0-1

10

A B C D E F0

5

10

15

20

25

3

3

15

1010

14

1

1213q

uan

tity

material

characteristic value 3-5

characteristic value 2

characteristic value 0-1

17

A B C D E F0

10

20

30

40

37

131411

33

mg/

m2

16

material

Acid-base characteristics Surface free energy

0 50 100 150 2000

20

40

60

80

100

120

SV

= 39 mJ/m²

untreated, cleaned surface

treated surface (gas phase fluorination)

R

wa

ter

co

nta

ct

an

gle

[°]

time [s]

A

SV

= 23 mJ/m²

0,0

0,1

0,2

0,3

0,4

0,5

wx [

O]:[C

]

flame

treatedgas phase fluorination plasma jet

static nozzle

plasma jet

rotating nozzle

un

trea

ted

BFa01 PLa08

0

1

2

3

4

underside

of the paint

substrate substrate

fracture surfaces

after painting

TPO before

painting

ato

mic

co

nce

ntr

ati

on

[%

]

Mg

Si

N

PLa08

0

1

2

3

4BFa01

0

4

8

Primer

Insufficient adhesion of the paint correlates with

higher amounts of soluble additives in the surface

region of the different types of TPO materials

Flame treatment

– small modification effect –

Plasma jet treatment

– overtreatment –

Motivation

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0-40

-30

-20

-10

0

10

20

Zeta

pote

ntia

l [m

V]

pH

A-wx

B-wx

C-wx

E-wx

A-02

B-02

C-02

E-02

X-wx industrial flame treatment

X-02 flame treatment #2