The Rôle of the Adhesion Promoter in a Model Water-Borne Primer Siavash Adhami, Marie-Laure Abel, Chris Lowe, John F. Watts Department of Mechanical Engineering Sciences October 13-18, 213 Cagliari, Sardinia (Italy)
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, 154, (212)
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, 4, 285, (25)
Coatings Formulation XPS Spectra of HDGS Polymeric Top Coat: Polyester-based, cross-linked with melamine + titanium dioxide and phthalocyanine blue pigments. Applied at ca. 2µm 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. Zn2p 2.5% O1s 41.7% Al2p 1.8% Zn2p 51.9% Al2p 3.5% O1s 34.4% C1s 25.5% C1s 1.2% IPA Cleaned HDGS Zn3s Al2s Zn3p Zn3s Zn3p KOH Cleaned HDGS Al2p Zn3s O2s Al2s Al2p
Failure of Water-Borne Systems Level of adhesion promoter has profound effect on durability! 3 µm 1 µm SEM micrograph of MIFS with the lower SEM micrograph of MIFS with the higher concentration of adhesion promoter concentration of adhesion promoter a & b: both sides of the failed interface of the panel subjected to salt spray for 1 hrs c & d: both sides of the failed interface of the panel subjected to humidity for 1 hrs e & f: optical images of both sides of the failed interface of the panels subjected to humidity for 1 hrs due to the mechanical pull off testing
ToF-SIMS of Amine Adhesion Promoter Characteristic Fragment Ions C 3 H 8 N: 58.6u H 3 C N + H 3 C OH 2 C 4 H 1 NO: 88.7u C 8 H 14 NO 2 : 156.1u C 4 H 5 O 2 : 85.2u O - + O H 3 C CH 2 2-dimethylaminoethyl methacrylate Mw = 157.11 gmol -1 C 2 H 6 N: 44.5u C 4 H 1 N: 72.8u C 5 H 1 NO 2 : 116.7u
ToF-SIMS Terminology Analyser Resolution in ToF-SIMS = M/Δm At mass 29 on silicon ca. 1 for TOF.SIMS 5 Mass Accuracy in High Resolution ToF-SIMS = Δ Δ = 2 ppm Δ = (Assigned Experimental Mass Theoretical Mass) x 1 6 Theoretical Mass Mass reported to 4 decimal places e.g. H 1 = 1.78 u Δm Normally expressed in ppm, where, < 1 ppm is acceptable, < 5 ppm very good, to enable the peak assignment to be confirmed. M 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.
Intensity Intensity 4 Intensity 4 Intensity 4 Intensity 3 Intensity 2 Characterising the Adhesion Promoter Positive Positive ToF-SIMS ToF-SIMS spectrum (m/z= (m/z= 1-19 1-19 u ) u of ) the of the adhesion promoter used in in the the primer, employing cryostage primer; 1,2 dimethyl amino-ethyl methacrylate SIMS x1 5 3. C 3 H 8 N C 4 H 1 N 2.5 2. 1.5 C 2 H 6 N C 4 H 1 NO C 6 H 9 O 2 C 8 H 14 NO 2 C 8 H 16 NO 2 1..5 2 4 6 8 1 12 14 16 18 mass / u x1 x1 3.5 x1 x1 x1 2.5 1.4 8. 3. 4. 1.2 2. 7. 2.5 6. 1. 3. 2. 1.5 5..8 1.5 4. 2..6 1. 3. 1..4.5 2. 1..2.5 1. 44.1 44.2 44.3 44.4 44.5 44.6 44.7 44.8 44.9 mass / u 58.2 58.4 58.6 58.8 58.1 58.12 mass / u 72.2 72.4 72.6 72.8 72.1 72.12 72.14 mass / u 88.2 88.4 88.6 88.8 88.1 88.12 88.14 mass / u 115.6 115.8 116. 116.2 116.4 mass / u C 2 H 6 N: 44.5u, M= 17.6 ppm C 3 H 8 N: 58.6u, M= 17.5 ppm C 4 H 1 N: 72.8u, M= 66.8 ppm C 4 H 1 NO: 88.7u, M=2.5 ppm C 5 H 1 NO 2 : 116.7u, M=16.3 ppm.1u
Surface Concentration (atomic %) Surface Concentration (atomic %) Adhesion Promoter Adsorption on HDGS by XPS 6 5 4 3 2 1 C O N Zn Al 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 7 6 5 4 3 2 1 C O N Zn Al 2 1.3.6.9 Concentration (Mole/Litre) IPA Cleaned HDGS 2 1.3.6.9 Concentration (Mole/Litre) KOH Cleaned HDGS.5.1.5.1 Very little adsorption of amine adhesion promoter on solvent cleaned HDGS, virtually none on the KOH treated HDGS, which gives better performance.
Adhesion Promoter Adsorption on HDGS by ToF-SIMS 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? RPI 7 6 5 4 3 2 C2H6N+ C3H8N+ C4H1N+ C4H1NO+ C5H1NO2+ C8H14NO2+ C8H16NO2+ C9H18NO2+ 1 3 25 2 15 1 5.3.6.9 Concntration (Mole/Litre) 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..5.1
Ultra Low Angle Microtomy Microtome Knife Angled Sectioning Block Polypropylene Block Angle Sectioning Block 12 x 12 x 7 mm 3 + 25 mm =.3 O + 5 mm =.7 O + 1 mm =.15 O + 2 mm =.33 O
25 µm Experimental Sample preparation/ XPS & SIMS Line Scans Red: Top coat Green: Primer Blue: HDGS 5 µm ULAM 128 pixels 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. 8 x 128 of Recording Relative Intensity (RI) for each specific fragment 5 µm 128 pixels= 16 strips x 8 pixels 8 pixels = 15.6 µm 1.8 µm = 2 RoI: Region of Interest Convert RoI to Depth RI vs. RoI RI vs. Depth
Surface Concentration% 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. 7 6 5 4 3 2 1 C Co N O Zn 5 1 15 2 25 3 35 4 Depth (µm) ------------Topcoat-----------Primer--------------HDGS------ 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.
RPI x 1 Surface Concentration% Distribution of the Adhesion Promoter Comparing XPS-SIMS Line Scan 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. 7 6 5 4 3 2 1 5 4 C N O Zn 5 1 15 2 25 3 Depth (µm) C2H6N C3H8N C4H1N C5H1NO2 Molecular ion x 5 Zinc 3 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 5). 2 1 2 4 6 8 1 12 Depth (µm)
Surface Concentration 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. 9 8 7 6 5 4 3 2 1 C N O.5 1 1.5 2 Outer surface of the primer---------»hdgs Depth (nm) 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, 154, (212)
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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