SURFACE AND INTERFACE CHARACTERIZATION OF POLYMERS October 31

SURFACE AND INTERFACE CHARACTERIZATION OF POLYMERS October 31, 2018

AGENDA • Introduction • Brief Review of the Principles of XPS and TOF-SIMS • Case Study #1 – Defect Analysis • Case Study #2 – Failure Analysis • Buried Layers and Interfaces • Summary

INTRODUCTION • Polymeric Films – Impacted by quality of the interface of the layers of the films along with the surface characteristics. – These impact many properties such as adhesion, barrier performance, printability, wettability, strength and appearance. • Metallizing Polymers – The value of the converted polymer product revolves around the surface modification of the polymeric material and the coating(s) utilized.

PRINCIPLE OF XPS • X-Ray Photoelectron Spectroscopy (XPS) – Also known as Electron Spectroscopy for Chemical Analysis (ESCA). – A photon ionizes an atom which results in the ejection of a core electron. – The kinetic energy (KE) of these photoelectrons is related to the x-ray source energy (hν) by the photoelectric effect. • KE = hν – BE (BE = binding energy of the core electron) – Each element on the Periodic Table of Elements has a different electronic configuration (does not detect hydrogen). – This is how XPS can be used to identify the elements in a sample.

XPS (Continued) Sample Carbon Nitrogen Oxygen Treated 79. 0 3. 0 18. 0 Untreated 99. 0 0. 0 1. 0

XPS (Continued) Four Main Attributes of XPS – Surface sensitivity (1 -10 nm sampling depth). – Elemental and chemical state identification. – Quantitative without the use of standards. – Ability to examine highly insulating samples.

PRINCIPLE OF TOF-SIMS Time of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS) Extraction Field TOF-SIMS Requires • Pulsed Ion Source • Extraction Field • Flight Tube • Detector Capable of Detecting the Individual Ions

TOF-SIMS (Continued) Positive Mass Spectrum of PDMS contaminated PET. PDMS has been shown in blue for clarity: Si+, Si. CH 3+, Si. C 3 H 9+, Si 2 C 5 H 15 O+

TOF-SIMS (Continued) Four Main Attributes of TOF-SIMS – Surface sensitivity (<0. 5 nm sampling depth). – Elemental and molecular identification. – Ability to chemically map elements/molecules on the surface of a material with <1µm image resolution. Mg distribution on surface Organic molecule – TOF-SIMS has the ability to examine highly insulated materials.

CASE STUDY #1: IDENTIFICATION OF DEFECTS ON METALLIZED POLYMER-PSA LAMINATE • Laminate consisting of an acrylic-based pressure sensitive adhesive (PSA) on a polyester film bonded to a metallized PET with a silicone release layer – experienced isolated defects 50 -200 µm in size. • Defects – undesirable transfer of the metallization to the PSA surface.

CASE STUDY #1 (Continued) • In adhesion failures, it is desirable to study the mating sides as the contaminant can preferentially transfer one side or the other. • This investigation can also help confirm the locus of failure as it is not always straightforward in multilayer laminates. • Small area XPS was performed on the mating side of the 200µm defect. Mating Side (i. e. Void Defect Side) PSA Side of the Failure (i. e. underside of the Al Flake)

CASE STUDY #1 (Continued) • Mating Side: CHx, C-O, O-C=O and silicones – consistent with PET covered in 1 -2 monolayers of silicone. – Concern silicone had migrated to the defect post surface. • PSA Side: Al, Al 2 O 3, CHx, C-O, O-C=O and silicones. Mating Side (i. e. Void Defect Side) PSA Side of the Failure (i. e. underside of the Al Flake)

CASE STUDY #1 (Continued) • Quite clear locus of failure was at the polyester-Al interface. Area Carbon Oxygen Aluminum R-Si Si. O 2 Metal Side of Defect 67. 3 27. 9 0. 0 4. 7 0. 0 PSA Side of Defect 43. 2 35. 4 17. 0 4. 5 0. 0 Release Layer 44. 1 30. 6 0. 0 18. 7 6. 6 • Close examination at the carbon spectrum from the Al flake underside revealed C-O and O-C=O at equal intensities. – This is indicative of an ester – only organic species should have been from CH 3 and silicone and any airborne organic materials.

CASE STUDY #1 (Continued) • PET to metal wasconsistent also considered. The high Conclusion: Suchtransfer shifts are with the presence of an resolution XPS image gave subtle indications that the ester aliphatic ester present on the surface of the PET prior to was not from an aromatic source (i. e. PET). metallization. This was the ultimate cause of film failure. PET Reference – Top Defect Spectrum – Bottom. See shifts in the binding energies of the C-O and O-C=O. There is no evidence of the weak aromatic band around 292 e. V on the defect.

CASE STUDY #2: FAILED HEAT SEAL • Sterile package from a medical device that experienced an adhesion failure at the polyethylene-ethylene acrylic acid (copolymer) heat seal to the polyethylene interface. • EAA (ethylene acrylic acid) – added at 3% to improve adhesion and lower % crystallinity. • Good and bad samples were compared using both XPS and TOF-SIMS.

CASE STUDY #2 • XPS identified only carbon and oxygen on the surface. Confirmation of the acrylic acid species.

CASE STUDY #2 (Continued) • Making the assumption the oxygen detected comes from the EAA, the following conclusions were reached: – Good heat seal contains the expected 3% EAA. – Bad heat seal contains almost 5% EAA. Sample Carbon Oxygen Good PE-EAA Surface 98. 6 1. 4 Failed PE-EAA Surface 97. 7 2. 3 3% EAA-PE, Theoretical 98. 8 1. 2

CASE STUDY #2 (Continued) • TOF-SIMS was then run on the surfaces to confirm the EAA concentrations. • Both surfaces contained ions that were indicative of PE (C 2 H 3, C 3 H 5, C 4 H 7, etc. ) and EAA (CH 3 O, C 2 H 5 O, etc. ). • However, the bad heat seal contained peaks for a hydroxyhydrocinnamate (HHC) compound (aka antioxidants known under the brand name of Irganox® ). Irganox ® 1010

CASE STUDY #2 (Continued) • Weak HHC peaks were observed on the Good heat seal at roughly 1/3 the intensity as seen on the Bad heat seal. • XPS could only identify the alkyl carbon and a weak O-C=O band. – Because both samples contained the HHC component, it was impossible for XPS to identify the contaminant. – XPS did detect the higher level of oxygen in the Bad seal. • TOF-SIMS was able to determine the excess oxygen was not due to EAA but rather from excessive antioxidants levels on the surface. • TOF-SIMS allowed for the determination of the root cause of failure.

BURIED LAYERS AND INTERFACES • The ability to access exposed surfaces and directly or through delamination sample them is straightforward with XPS and TOF-SIMS. • There is interest though in looking at layers that may be buried many microns below the top of the surface. • Historically, the main choices for analysis were techniques such as FTIR, Raman, SEM-EDS or TOF-SIMS. – These are generally done on soft materials using a microtome. – This gives lateral resolution of the sample and the technique limits the thickness of the buried layer/interface that can be examined.

BURIED LAYERS AND INTERFACE (Continued) • Limitations – SEM-EDS, Raman and TOF-SIMS – resolution is ~1µm. – FTIR – resolution is ~15µm. – The poor lateral resolution of XPS (20 -30µm) limits the use of XPS for these types of applications. • SEM image of a crosssectioned paint sample (substrate not shown). – Layers of interest are >5µm , can use TOF-SIMS. Clearcoat Basecoat Primer

BURIED LAYERS AND INTERFACE (Continued) Green = From Primer Corrosion Red = Al & Ti from Basecoats Blue = Cl from Clearcoat Basecoat Primer

BURIED LAYERS AND INTERFACE (Continued) • What about the layers <1µm? – Gas Cluster Ion Beam (GCIB) – can sputter into organic systems and maintain chemical state information. Using TOF-SIMS and GCIB, the molecular signal from two different Irganox molecules were obtained as a function of depth.

SUMMARY • XPS is a surface sensitive analytical tool for probing chemical phenomenon on polymers. – Standardless quantitation. – Ability to identify not only the elements on the surface, but also the chemical state. • TOF-SIMS offers two complementary attributes to XPS: 1. Determination of specific organic compounds present on the surface (rather than simply the functional groups). 2. Chemical mapping with ~1µm resolution. • Combined, these two tools provide the most complete chemical analysis of polymer surfaces and interfaces. • Recent advances open up these techniques and this field of analysis to new characterization opportunities and multilayer polymeric systems.

THANK YOU FOR YOUR ATTENTION. QUESTIONS?