The Surface Analysis Laboratory

The Rôle of the Adhesion Promoter in a

Model Water-Borne Primer

Siavash Adhami, Marie-Laure Abel, Chris Lowe, John F. Watts

The Surface Analysis Laboratory

Department of Mechanical Engineering Sciences

October 13 - 18, 2013

Cagliari, Sardinia (Italy)

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Introduction

For many years industry has used chromate conversion coatings which

provided:

Outstanding corrosion protection

Improved paint adhesion

Nowadays the industry demands chrome-free processes which benefit from:

Cost effective

Environmentally compatible

large reductions in VOC emissions

Reduction in fire risk

Worker exposure to organic vapours

The performance of a chrome-free processed, organic coated panels,

applying a novel water-based primer system (containing an amine-based

adhesion promoter at two different concentrations), to alkali cleaned HDGS,

was investigated previously†.

‡ S. Adhami, M-L. Abel, C. Lowe, J. F. Watts, Surf. Interface Anal, 44, 1054, (2012)

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Outline

The interfacial regions of a model multilayer coatings system on an alkali cleaned

zinc coated steel substrate has been investigated by time-of-flight secondary ion mass

spectrometry (ToF-SIMS).

Ultra-Low-Angle Microtomy (ULAM)† was employed to expose the interface

between the primer and the metal substrate at the depth of ≈ 25 µm.

The interfacial chemistry of the interface has been revealed by reconstruction of the

spectra from optimised regions of interests (ROIs) across the metal/primer and

primer/top coat interface to obtain the best spatial and chemical resolution.

The findings confirm that a fraction of the adhesion promoter in the formulation

segregate toward topcoat/primer interface where they enhance the adhesive

properties; such phenomenon are not observed in vicinity of metal/primer interface.

‡ S. J. Hinder, C. Lowe, J. T. Maxted, J. F. Watts, J Mater Sci, 40, 285, (2005)

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Al2p 10.8%

C1s 25.5%

Zn2p 20.5% O1s 41.7%

Al2p 3.5%

C1s 10.2%

Zn2p 51.9%

O1s 34.4%

IPA Cleaned

HDGS

KOH Cleaned

HDGS

Coatings Formulation

XPS Spectra of HDGS

Polymeric Top Coat: Polyester-based, cross-linked with melamine + titanium

dioxide and phthalocyanine blue pigments. Applied at ca. 20µm thickness

Water-Based Primer: Novel system based on acrylic chemistry with “amine-based

adhesion promoter”. Applied to ca. 5µm thickness

Substrate: Alkali cleaned hot-dipped galvanised steel.

Al2p

Al2s

Zn3p Zn3s

Al2p

Al2s

Zn3p

Zn3s

Zn3s O2s

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Failure of Water-Borne Systems

“Level of adhesion promoter has profound effect on durability!”

SEM micrograph of MIFS with the higher

concentration of adhesion promoter

SEM micrograph of MIFS with the lower

concentration of adhesion promoter

30 µm 100 µm

a & b: both sides of the failed interface of the panel subjected to salt spray for 1000 hrs

c & d: both sides of the failed interface of the panel subjected to humidity for 1000 hrs

e & f: optical images of both sides of the failed interface of the panels subjected to

humidity for 1000 hrs due to the mechanical pull off testing

The Surface Analysis Laboratory

ToF-SIMS of Amine Adhesion Promoter Characteristic Fragment Ions

C2H6N: 44.05u

C4H10N: 72.08u

C4H10NO: 88.07u

C5H10NO2: 116.07u

C8H14NO2: 156.10u

C4H5O2: 85.02u

Mw = 157.11 gmol-1

C3H8N: 58.06u

+

O-

O

CH3

CH2

N

CH3

CH3OH2

+

2-dimethylaminoethyl methacrylate

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ToF-SIMS Terminology

Δ = 20 ppm

Δm

M

Analyser Resolution in ToF-SIMS = M/Δm

At mass 29 on silicon ca. 10 000 for TOF.SIMS 5

Mass Accuracy in High Resolution ToF-SIMS = Δ

Δ = (Assigned Experimental Mass – Theoretical Mass) x 106

Theoretical Mass

Mass reported to 4 decimal places e.g. H1 = 1.0078 u

Normally expressed in ppm, where, < 100 ppm is

acceptable, < 50 ppm very good, to enable the peak

assignment to be confirmed†‡.

†Reichlmaier S, Hammond JS, Hearn MJ, Briggs D. Surf. Interface Anal. 1994; 21: 739.

‡Briggs D. Surface Analysis of Polymers by XPS and Static SIMS. Cambridge University

Press: Cambridge, 1998.

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Positive ToF-SIMS spectrum (m/z= 10-190 u ) of the adhesion promoter used in the

primer; 1,2 dimethyl amino-ethyl methacrylate

Characterising the Adhesion Promoter

C2H6N

C 3 H 8 N

C 4 H 10 N

C4H10NO

C 6 H 9 O 2

C8H14NO2

C8H16NO2

mass / u

20 40 60 80 100 120 140 160 180

5 x10

0.5

1.0

1.5

2.0

2.5

3.0

Inte

nsity

Positive ToF-SIMS spectrum (m/z= 10-190 u ) of the adhesion promoter used in the primer, employing cryo-

stage SIMS

mass / u 44.01 44.02 44.03 44.04 44.05 44.06 44.07 44.08 44.09

4 x10

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Inte

nsity

mass / u 58.02 58.04 58.06 58.08 58.10 58.12

4 x10

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Inte

nsity

mass / u 72.02 72.04 72.06 72.08 72.10 72.12 72.14

4 x10

0.5

1.0

1.5

2.0

2.5

Inte

nsity

mass / u 88.02 88.04 88.06 88.08 88.10 88.12 88.14

3 x10

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Inte

nsity

mass / u 115.6 115.8 116.0 116.2 116.4

2 x10

1.0

2.0

3.0

4.0

Inte

nsity

C2H6N: 44.05u, ∆M= 17.6 ppm C3H8N: 58.06u, ∆M= 17.5 ppm C4H10N: 72.08u, ∆M= 66.8 ppm C4H10NO: 88.07u, ∆M=20.5 ppm C5H10NO2: 116.07u, ∆M=16.3 ppm

0.1u

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Adhesion Promoter Adsorption on

HDGS by XPS

0

10

20

30

40

50

60

0 0.3 0.6 0.9

C O N Zn Al

Concentration (Mole/Litre)

Su

rfac

e C

on

cen

trat

ion

(at

om

ic %

)

0

10

20

30

40

50

60

70

0 0.3 0.6 0.9

C O N Zn Al

Concentration (Mole/Litre)

Su

rfac

e C

on

cen

trat

ion

(at

om

ic %

)

0

1

2

0 0.005 0.01

0

1

2

0 0.005 0.01

IPA Cleaned HDGS KOH Cleaned HDGS

Very little adsorption of amine adhesion

promoter on solvent cleaned HDGS,

virtually none on the KOH treated HDGS,

which gives better performance.

Adsorption curves (uptake of the

compounds vs. concentration) for both

solvent cleaned and KOH treated

HDGS,

Uptake of the adhesion promoter

molecule on HDGS using surface

concentration obtained by XPS in a

range of solvated adhesion promoter, N

is used as an indicator for the adhesion

promoter molecule

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Adhesion Promoter Adsorption on

HDGS by ToF-SIMS

0

1000

2000

3000

4000

5000

6000

7000

0 0.3 0.6 0.9

C2H6N+ C3H8N+ C4H10N+ C4H10NO+

C5H10NO2+ C8H14NO2+ C8H16NO2+ C9H18NO2+

Concntration (Mole/Litre)

RPI

0

500

1000

1500

2000

2500

3000

0 0.005 0.01

There is no specific interactions

between adhesion promoter and

zinc, very low concentration in

the case of surface aluminium.

Role of adhesion promoter in

enhancing durability?

Adsorption curves (uptake of the compounds VS.

concentration) for solvent cleaned HDGS,

Uptake of the adhesion promoter molecule on HDGS using

all the relative peak intensity (RPI) of fragments associated

to adhesion promoter molecule.

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Ultra Low Angle Microtomy

Angle Sectioning Block

12 x 12 x 7 mm3

+ 25 mm = 0.03O

+ 50 mm = 0.07O

+ 100 mm = 0.15O

+ 200 mm = 0.33O

Microtome Knife

Angled Sectioning Block

Polypropylene Block

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Experimental Sample preparation/ XPS & SIMS Line Scans

500 µm

128 pixels= 16 strips x 8 pixels

128 pixels

RoI: Region of Interest

∑8 x 128 of Recording Relative

Intensity (RI) for each

specific fragment

RI vs. RoI

Convert RoI to Depth

RI vs. Depth √ = 2°

8 pixels = 15.6 µm

1.08 µm

Red: Top coat

Green: Primer

Blue: HDGS

500 µm

25 µm

ULAM

Approximately same

area is used for XPS

line scan.

Depth profile of a various fragments

obtained by reconstruction of SIMS spectra

across the top coat/primer/metal area from

16 regions of interest.

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XPS Line Scan

The region associated with the primer

layer can be determined considering the

change in the elements concentration:

Analysis point at a depth of ~18 µm

where the concentration of carbon starts

to decrease.

Analysis point at a depth of ~25µm where

Zn signal is observed to become more

intense.

Having Characterised all the individual

components present in the formulation,

it can be concluded that nitrogen is

associated with the adhesion promoter

molecule.

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40

Su

rfac

e C

on

cen

trat

ion

%

Depth (µm)

C Co N O Zn

Changes in C, O, Zn, Co and N elemental

concentration traversing a ULAM taper which has

exposed the buried topcoat/primer/metal substrate

interfaces of the sample with higher concentration

of adhesion promoter.

|------------Topcoat-----------Primer--------------HDGS------|

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Distribution of the Adhesion Promoter Comparing XPS-SIMS Line Scan

0

10

20

30

40

50

0 2 4 6 8 10 12

RP

I x 1

00

00

C2H6N C3H8N C4H10N

C5H10NO2 Molecular ion x 50 Zinc

Depth (µm)

Depth profile of the fragments originating from

the adhesion promoter molecule obtained by

reconstruction of SIMS spectra across the

metal/primer/topcoat area from 16 regions of

interest. the intensity of molecular ion reaches the

maximum intensity at the primer/topcoat interface

(the intensity of the molecular ion has been

multiplied by 50).

Adhesion promoter segregates to the primer

surface when added to the primer

formulation at the higher concentration.

Adsorption studies on this molecule proves

that there is no chemical interaction between

the metal substrate and adhesion promoter

molecule.

The concentration of the adhesion promoter

at the higher level is beyond the critical

miscibility limit of the primer and the

adhesion promoter is rejected from the

formulation towards the outer surface.

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30

Surf

ace

Co

nce

ntr

atio

n%

Depth (µm)

C N O Zn

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The Role of Adhesion Promoter

No chemical interaction of the

adhesion promoter molecule with the

metal substrate and therefore

segregation of this molecule toward the

primer/top coat interface due to the

thermal flux result from the curing

process, which have the effect of

improving the durability of the

system†.

The rejected adhesion promoter at the

outer surface is able to interact with

the topcoat when it is applied

enhancing the overall performance of

the systems.

0

10

20

30

40

50

60

70

80

90

0 0.5 1 1.5 2

C N O

Depth (nm)

Su

rfac

e C

on

cen

trat

ion

|Outer surface of the primer---------»HDGS

Changes in elemental concentration with

depth of the primer with high level of

adhesion promoter obtained by AR-XPS.

‡ S. Adhami, M-L. Abel, C. Lowe, J. F. Watts, Surf. Interface Anal, 44, 1054, (2012)

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Conclusions

In these model systems, the adhesion promoter molecules tend to segregate toward

the primer surface instead of segregating to and improving the adhesive properties

of the primer/metal interface; which was unexpected!

The segregation of the adhesion promoter molecules toward the primer surface

improves the performance of the panels as on application of the topcoat, the

primer/topcoat interface becomes critical for durability.

The durability of the system improves as the concentration of the adhesion

promoter is increased.

This highlights the important role to be played by the primer/topcoat interface in a

multi-coat organic coatings system.

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Thanks for Your Attention

Any Questions?