Interfacial Chemistry and Adhesion Phenomena: How to Analyse and How to Optimise John F Watts Department of Mechanical Engineering Sciences
The Role of Surface Analysis in Adhesion Studies Assessing surface characteristics (cleaning/pretreatment) Forensic analysis of failed joints, coatings etc. Fundamental studies of adhesion o o o Sectioning of real systems XPS and ToF-SIMS of thin films Adsorption isotherms
Surface Analysis
The Problem Faced by the Analyst 10 s m - mm 100 s m - mm d Adhesive or Coating Interface Region Substrate ARXPS d ~10nm X-ray spectroscopies d ~200nm RBS d ~1μm
The Buried Interface Obtaining analytical information from intact interfaces is very difficult. Carrying out in-situ experiments within the spectrometer can be useful but only rarely is the interphase chemistry exposed in this manner J F Watts, Surf Interf Anal, 12, 497-503, (1988)
TEM/PEELS Cross-Sections PAA treated joint abrupt interface PAA + primer joint diffuse interphase A J Kinloch, M Little, J F Watts, Acta Materialia, 48, 4543, (2000)
Epoxy + GPS/Aluminium Joint Fig. 2. Composite image of the cross-section of a sample containing 1% (w/w) of APS showing the aluminium/adhesive interface from several TEM images. Fig. 5. Graph representing the relative oxygen content over the oxide layer for the various APS concentrations, obtained from EELS spectra. At 0.5% APS, the oxygen profile is bi-lobar, whilst at 1% and 2% the oxygen concentration has only one maximum, near the silane and near the Al respectively. Fig. 3. EELS spectrum of the core loss region of the oxygen K edge along the aluminium oxide layer for a sample containing 0.5% (w/w) of APS. J Bertho, V Stolojan, M-L Abel, J F Watts, Micron, 41, 130, (2010)
Oxide Stripping + Depth Profiling Chemical removal of metal substrate, depth profiling of oxide in situ by ion sputtering. Interphase can then be analysed directly J F Watts, J E Castle, J Mat Sci, 18, 2987, (1983)
Interface Chemistry Fe(II) Fe(II) satellite Iron 2p3/2 spectrum showing Fe(II) component at interface. Oxide is entirely Fe(III).
Ultra Low Angle Microtomy Microtome Knife Angled Sectioning Block Polypropylene Block Angle Sectioning Block 12 x 12 x 7 mm 3 + 25 m = 0.03 O + 50 m = 0.07 O + 100 m = 0.15 O + 200 m = 0.33 O
ULAM Geometry Depth resolution attainable depends on the taper angle and the size of the analytical probe. In XPS small area XPS in the range 15-500 m, for ToF-SIMS < 1 m q Small area XPS analysis mode (100 m) Coating Substrate XPS spot size/ m ULAM taper angle/ o 0.03 0.33 2.0 100 60 600 3500 15 13 100 500 Depth Resolution ULAM/nm S J Hinder, C Lowe, J T Maxted, J F Watts, J Mater Sci, 40, 285, (2005)
Atomic % XPS Line Scan Along Taper PVdF Topcoat on Polyurethane Primer 20 X-ray Spot = 15 m 4 18 16 Step Size = 18 m 3.5 14 N: DZ = 27 nm 3 12 2.5 10 2 Fluorine Nitrogen 8 6 13nm F: DZ = 50 nm 1.5 1 4 2 0.5 0 0 0 0.013 0.026 0.039 0.052 0.065 0.078 0.091 0.104 0.117 0.13 0.143 0.156 0.169 PVdF Depth / um Polyurethane
Relative peak intensity ToF-SIMS Line scan Positive ToF-SIMS image, characteristic ion of the primer formulation 1 500 µm - 128 pixels p=128 p=1 pixel intensity 0.8 0.6 0.4 0.2 0 0 1 2 Depth (µm) 500 µm For each of the 128 rows Pixel depth = f(cutting angle) 13
Stitched Images: Two Coat System Large scale overview of the interface and species migration Topcoat Primer 1 mm 1 mm Negative ToF-SIMS images : 8 (500 µm x 500 µm) squares 14
relative peak intensity Line Scan : Species Segregation Characteristic fragment of the topcoat Characteristic fragment of a primer component 2.5 2 1.5 1 0.5 0 Line scan 0 0.5 1 1.5 depth (µm) 500 µm - 128 pixels 15
ToF-SIMS of ULAM Section a) +ve SIMS b) -ve SIMS FoV is 500 m Polyurethane ions (a) m/z = 149: C 8 H 5 O 3 + (b) m./z = 26: CN - (c) m/z = 59: C 3 H 4 F + c) d) (d) m/z = 19: F - 2 3 500 m 250 nm 1 PVdF ions S J Hinder, C Lowe, J T Maxted, J F Watts, Surf Interf Anal, 36, 1575, (2005)
Counts Counts ToF-SIMS Anions from Images a) 3500 3000 2500 25 Point 2: Bulk Polyurethane 2000 66 c) 1500 1000 41-42 49 100 121 2 500 0 0 20 40 60 80 100 120 140 160 180 200 15000 m/z c) 19 13500 3 1 12000 10500 9000 Point 1: Bulk PVdF 7500 6000 4500 39 3000 1500 49 85 0 0 20 40 60 80 100 120 140 160 180 200 m/z
Counts ToF-SIMS at Interface Negative ToF-SIMS b) 1400 1200 19 Point 3: PU and PVdF at Interface 1000 85 c) 800 2 600 31 400 55 71 87 200 141 185 0 0 20 40 60 80 100 120 140 160 180 200 m/z 121 3 1 PCA on Positive Spectra from Interfacial Region to Identify Those that are not Characteristic of PVdF or PU
Counts Minor Additive 3000 2700 31 2400 2100 41 71 1800 85 1500 1200 900 600 55 x10 300 185 0 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 m/z A negative ion ToF-SIMS mass spectra of the pure acrylic co-resin component of the PVdF topcoat formulation in the mass range 30-200m/z.
ToF-SIMS Images of Acrylic Ions a) b) c) d) Negative Ion Mass Selected Images (a) m/z = 31: CH 3 O - (b) m/z = 55: C 3 H 3 O - (c) m/z = 71: C 3 H 3 O - 2 (d) m/z = 85: C 4 H 5 O - 2 e) f) (e) m/z = 87: C 4 H 7 O 2 - (f) m/z = 141: C 9 H 13 O 4 -
Coil Coating: Adhesion Promoter 25 µm Red: Top coat Green: Primer Blue: HDGS 500 µm 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. ULAM 128 pixels 8 x 128 of Recording Relative Intensity (RI) for each specific fragment 500 µm 128 pixels= 16 strips x 8 pixels Convert RoI to Depth RI vs. RoI 8 pixels = 15.6 µm 1.08 µm = 2 RoI: Region of Interest RI vs. Depth
Surface Concentration% XPS Line Scan: ULAM Section 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. 70 60 50 40 30 20 10 0 C Co N O Zn 0 5 10 15 20 25 30 35 40 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.
Correlative Surface Analysis: Surface Concentration% RPI x 10000 XPS and ToF-SIMS 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. 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). 70 60 50 40 30 20 10 0 C N O Zn 0 5 10 15 20 25 30 Depth (µm) C2H6N C3H8N C4H10N C5H10NO2 Molecular ion x 50 Zinc 50 40 30 20 10 0 0 2 4 6 8 10 12 Depth (µm)
The Thin Film Approach 10 s m - mm 100 s m - mm d Adhesive or Coating Interface Region Substrate thin (< 2 nm film on substrate) select sample from the plateau region of the adsorption isotherm
PMMA on Oxidised Metals 0.2 ev S R Leadley, J F Watts, J Adhes, 60, 175, (1997)
Polymer Conformation PMMA on Oxidized Si and Al J F Watts, S R Leadley, J E Castle, C J Blomfield, Langmuir, 16, 2292, (2000).
Organosilane Adhesion Promoters Molecular Dynamics Models of: (a) Epoxy (b) Amino (c) Vinyl
High Resolution ToF-SIMS The intense SiOAl + peak is indicative of a covalent bond between the aluminium oxide and the organosilane adhesion promoter
Specific Interactions HO OH Si O H (CH 2 ) 3 O CH 2 CH CH 2 O O Al H HO OH Si O Al (CH 2 ) 3 O CH 2 CH CH 2 O
Flame Treatment Used for Many Years
Flame Treatment of Automotive Polypropylene
Example XPS Spectra C1s O1s XPS spectra for Sample A untreated (lower) and treated (upper).
As Received PP Samples C1s Sample B Sample C Sample A Sample D O1s
Mono XPS Valence Band characteristic of pendant methyl group Untreated 1 Pass 3 Passes 7 Passes
XPS/Cluster Profiling X-ray photoelectron spectroscopy performed using the Thermo Scientific K-Alpha system Monochromated X-ray source Fully automated acquisition MAGCIS (monatomic and gas cluster ion source) used to produce craters Ar Cluster size up to 2000 atoms Ion energy = 2-8 kev Monatomic Ar ion beam
Atomic percent (%) Depth Profiling 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 Etch Depth (nm) C1s O1s (5x) Analysis on a K-Alpha using large cluster ions to etch (Ar 1000 ) Depth calculation using estimate from Irganox reference.
ToF-SIMS Identification of Oxygenation C 4 H 9 treated C 3 H 5 O untreated
Intensity Intensity Intensity Intensity Intensity Intensity ToF-SIMS of Oxidation Process 4 x10 2.0 C 4 H 5 O C 5 H 9 8 passes 1.0 C 3 HO 2 4 x10 2.0 1.0 6 passes 4 x10 2.0 1.0 3 passes 4 x10 1.5 1.0 0.5 4 x10 2.0 1.0 2 passes 1 pass 4 x10 4.0 2.0 69.00 69.05 69.10 mass / u Untreated
Additives: ToF-SIMS Spectra Additive Chemical structure Characteristic peaks Ethylene Bis-steramide 282, 310 Irganox 1010 57, 203, 219, 259
Ablation of Additives Ethylene Bis-steramide Irganox 1010 treated treated C 20 H 40 NO C 15 H 23 O C 18 H 36 NO untreated untreated C 17 H 23 O
Conclusions Chemical analysis of interfaces responsible for adhesion requires careful sample preparation. In model systems explicit bond formation can be identified. In commercial systems the inter-diffusion and segregation of the individual components in a formulation may be equally (or even more) important. The use of this films allows bonding, configuration and bond density to be established. For extruded products residual processing aids or additives as well as low functionality may compromise adhesion. Pretreatments must remove the former and enhance the latter.