Polymers
Division
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NIST
Combinatorial Methods Center Focused Project
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| High Throughput Methods for the Evaluation of Adhesive Performance |
| The NIST Combinatorial Methods Center is pleased to announce initiation of a two year focused project to develop an integrated measurement approach for adhesives properties characterization. The focus project will adapt existing MCAT (Multi-lens Combinatorial Adhesion Test), peel and wedge test methods to measure adhesion properties as a function of important process variables characterizing pressure sensitive adhesives and thermally curing epoxies. We are inviting NCMC member organizations and others to join the project. The members of the focused project will help the NCMC select model systems, prioritize the characterization required and specify processing variables of interest. Research results will be available to focused project members throughout the project period and member organizations will be able to participate directly with NIST researchers in development of the measurement systems. |
| Statement of Work |
| High-throughput MCAT, peel and wedge methods will be developed to measure bulk and surface properties of model adhesives formulations, specifically pressure sensitive adhesives (PSA) and thermal curing epoxy resin systems. In PSA’s we will characterize composition, relative tack and plasticizer content, blend morphology and crystallinity. In epoxy systems, the composition and thermal rate of curing will be mapped. Measurables include PSA adhesion against gradient surface energy and rough surfaces, and epoxy adhesion strength against silica (SiO2) and polyimide (PI) material surfaces. Characterization methods include microscopy, florescence and IR imaging. Measurements will be performed as a function of important process parameters that include sample gradient temperature and temperature cycling, sample aging and sample quench conditions. |
| Contact: Christopher Stafford |
| Phone: (301) 975-4368 |
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E-mail |
| Introduction: |
| A wide variety of PSA’s and epoxy resins are used in industrial and commercial applications. PSA performance is strongly dependent on the nature of the adhesive base, key formulation components, environmental parameters, substrate parameters and other specific factors. Likewise, a thermally curing epoxy resins’s performance is affected by composition (epoxy resin: curing agent), pre and post curing temperature and rate, substrate surface characteristics, and other specific factors. This makes the development of a high throughput approach for the evaluation of PSA's and epoxies a challenging task. This project is directed towards developing a testing framework that can be used to evaluate adhesive and epoxy resin performance as a function of key manufacturing and environmental variables. This focused project aims to develop a high-throughput test method that will quickly characterize and evaluate adhesive and epoxy resin performance, correlate performance to measured adhesive and epoxy characteristics and provide sufficient experimental information to facilitate selective in-depth study of these systems. |
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Background: |
| Peel and tack tests are often used to measure the performance of an adhesive. The former is a more industrially relevant qualitative measure of pressure sensitive adhesive performance [1] and the latter has recently emerged as a potential method to better characterize and visualize the mechanisms occurring during the debonding of a soft adhesive layer from either a flat or spherical probe [2,3]. Each test represents a slightly different approach. The peel test is a qualitative method that measures the maximum force required to separate an adhesive strip from a substrate. It allows one to quickly compare the performance between different adhesives and is a valuable tool for industrial testing. Tack tests not only measure the force exerted by the adhesive on the probe during debonding, but also the adhesive-probe contact area and the displacement required to completely separate the probe from the adhesive. A semi-quantitative test that quickly determines relative or absolute adhesive performance, but also permits further investigation of adhesive debonding mechanisms would be a unique and powerful tool for adhesive investigations. Wedge tests are useful to probe interfacial debonding at glassy interfaces (e.g. the fully cured epoxy interface). A sharp wedge such as a razor blade is driven into the interface at a known velocity and the crack propagation front is imaged to determine the work of adhesion and interfacial energy, given the modulus of the two materials is known. |
| Combinatorial methods utilize high-throughput measurement techniques to investigate a multi-variant parameter space more efficiently. The NIST combinatorial methods center (NCMC) has been actively developing combinatorial approaches to probe polymer adhesion. One such approach builds on the contact mechanics proposed by Johnson, Kendall, and Roberts (JKR), where the adhesion of a spherical indenter to a substrate is ascertained by following the contact area of the indenter during a loading and unloading cycle. The multi-lens combinatorial adhesion test (MCAT) is a high-throughput adhesion test bed currently under development at the NCMC. This high-throughput adhesion test employs an array of hemispherical lenses attached to a vertical actuator to perform from several hundred to several thousand probe tack tests at one time; see Figure 1. When these lenses are brought into contact with a gradient library, adhesion is measured over a large parameter space. The MCAT geometry is extremely flexible, allowing investigation of several different types of materials. The lens array may be fabricated from a material as rigid as glass, fully or partially cured epoxy or even an elastomer such as polydimethylsiloxane. The surface of the array may be chemically modified with a monolayer or coated with a thin film of polymer [4]. The MCAT measurable variables from each lens include the lens displacement (d), the contact area (a). Potentially the load (P) applied to each lens, within the lens array, is measurable, although not in the present MCAT set-up. To first order, from this information, relevant measures of adhesive performance can be evaluated knowing the maximum load (Pmax) achieved during unloading, and the total displacement (dtotal) to detach each lens on the array from the adhesive. The freedom to change MCAT geometry ensures the applicability of this technique to a large number of adhesive-substrate systems. |
| The current MCAT geometry is well suited for the measurement of adhesive forces across partial to fully cured epoxy resins, but is unsuited for investigation of soft materials such as PSA’s and uncured epoxy films. In order to better understand the debonding process for the soft materials, the current lens array will require modification from an array of hemispherical lenses to an array of posts. The post array ensures constant contact area between the probe and adhesive layer, which simplifies the stress-strain analysis required to quantify the debonding process. The array of posts may be created using developed soft lithography techniques and modified using techniques perfected on the current hemispherical lens arrays in use with the MCAT technique. |
| Technical Approach: |
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This focused project will allow integration of MCAT for comparative adhesion tests on model non-proprietary adhesive and epoxy systems (prioritized by the participating members) involving important variables associated with library characterization and mapping intrinsic physical and chemical characteristics. High-throughput adaptations to traditional peel, tack and wedge tests will provide results for cross-checking against standard industrial adhesion test methods, while MCAT will exist as a platform for quickly evaluating adhesive performance and semi-quantitatively investigating debonding process. At this point, the project encompasses four differing, but parallel thrusts using 1 to 2 selected model adhesive and epoxy systems in consultation with focused project members: |
| 1. Establish techniques for library generation of identified parameters Currently the NCMC has a suite of techniques available to create gradient libraries. We will develop and adapt gradient methods for MCAT, peel and wedge tests, as appropriate, using select model PSA and epoxy systems in consultation with the focused project members. This includes creating the post array that will be used in subsequent experiments. In MCAT we expect Pmax and dtotal to change as a function of adhesive variables – film thickness, composition, crystallinity, plasticizer, tackifier, copolymers and other formulation components. Therefore, the ability to create and test gradients of these variables is important to the performance of PSA and likely the adhesive and viscoelastic behavior of curing epoxies (under slowly curing or quenched conditions). |
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2. Characterize
libraries and map properties of interest Properties of interest in the model PSA include tack, bulk vs. surface composition, morphology and viscoelasticity, while the rate and degree of cure, interfacial-bonding strength, and cured physical property values are important for epoxy systems. These properties will be measured as a function of temperature under different conditions (constant, gradient, cycling, quench). Surface and bulk compositions of libraries will be mapped by IR microscopy (and SIMS, if needed). MCAT arrays would probe a unique combination (e.g. tackifier/temperature) allowing for development of a semi-quantitative comparative adhesion map across a library. Film morphology is imaged during debonding, with MCAT at each debonding location during the study. Similar to the peel test, the maximum load achieved during unloading is indicative of adhesive performance [5], but additional useful information is obtained from the entire tack curve as shown in Figure 2. In this regard, the MCAT combinatorial approach out performs traditional measurement techniques by affording better control of the loading/unloading cycle. The ability to measure load does not currently exist for MCAT and tests are required to determine its feasibility. Consequently, using load sensor arrays to map load for an individual or a local set of microlenses is a goal for MCAT (see Figure 2). Finally, library design and measurements will be integrated using the NCMC informatics system. |
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3. Correlate properties
with parameters to develop understanding of cause-effect
relationships The third thrust correlates cause-effect relationships between PSA parameters such as film thickness, composition, crystallinity, tackifier, copolymer and additives and applied temperature, time, quench and aging to the measured tack and viscoelastic properties. Likewise, the relationship between composition of epoxy, curing temperature and time, filler (if any) and interfacial adhesion energy will be developed. These require physical insight into mechanisms involved during the debonding process to understand cause-effect relationships. For instance, in MCAT, the contact area between individual lenses and the adhesive film is imaged throughout the debonding process. The onset of fingering instabilities, cavitation, geometry of the instability, and the area in contact with each lens of the array during debonding will provide details relative to the viscoelastic behavior of the adhesive and adhesive performance. Varying the lens velocity during contact and retraction and correlating load changes with the onset of fingering or cavitation instabilities may provide insights into the viscoelastic nature of the adhesive. |
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4. Calibration against
standard test methods We envision working jointly with the focused project members to have measurements on identical or similar library samples conducted in their industrial laboratory settings in order to cross-calibrate the high-throughput MCAT, peel and wedge-test against traditional and industrial test methods. |
| References: |
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1. Satas D. Handbook of
pressure-sensitive adhesive technology, Van Nostrand Reinhold,
New York, 1982. |
| Preliminary
Results: |
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| Figure 1: A schematic of the MCAT test bed, current geometry. The adhesion across the sample film is measured at many points with the lens array. The contact area is visualized through the sample film with an inverted microscope. |
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| Figure 2: A sample film on a flat substrate whose composition (φ) is varied orthogonal to the applied temperature (T) gradient. Below the film is a schematic of proposed force sensor arrays to measure P(δ) "locally" as function of (T,φ). Right hand plot shows the type of quantative information that could be obtained with this enhanced capability, and correlated with optical image sequence at each δ. |
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| Figure 3: Schematic approach to PSA adhesion and Epoxy curing for high-throughput characterization and properties measurements. |
| Project
Meetings, Website, Reports and Lab visits: |
| A discussion meeting will be arranged six weeks after the formal launch of the project as well as at six-month intervals for the duration of the project. NCMC will facilitate dissemination and communication among members of the focus project. Quarterly reports will be sent to the members, with updates more frequently via conference calls. In order to facilitate the collaboration, specifications for methods, instruments, programs, data analysis, and other aspects of this work will be available to members during the course of the project. A summary report will be provided within two months of the end of the project. The NCMC labs will be open to prearranged visits from member scientists interested in hands-on participation in method development. |
| As with base level membership in the NCMC, all of the research carried out in the Focused Project is non-proprietary and is intended for publication in the public domain. No proprietary information or materials will be solicited or accepted by NIST from member organizations. The scope of the work by NIST included in this focused project is limited as described in Appendix A of the focused project agreement. |
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Project Milestones: |
| First year |
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§ Select suitable model adhesive-tackifier blend and an epoxy system and establish techniques for library generation of identified parameters. § Conduct preliminary tests on at least one selected model PSA and one selected epoxy system. |
| Second year |
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§ Develop a systematic process for library generation, characterization and testing of adhesive and epoxy films and integration with the (NCMC) informatics database. § Correlate properties with parameters to develop understanding of cause-effect relationships for the model PSA and epoxy system (s) studied and draft summary report on each. § Assist focused project members investigation of a suitable non-proprietary commercial blend system to determine cause-effect relationships between parameters and performance and provide feedback for optimization. |
| Membership
Fee: |
| The membership fee payable to NIST is $ 20,000 per project year. |
| Send
a completed Project Agreement Form and fee to: |
|
NIST |
| The
project is scheduled to begin on September 1, 2003.
NIST reserves the right to cancel the focused project and
refund the membership fees in the case of insufficient member
participation. |
| NIST Materials Science & Engineering Laboratory - Polymers Division |
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| Contact Information: | |
| Cher H. Davis | |
| Technical Coordinator | |
| combi@nist.gov | |
| (301) 975 6488 | |
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