Summing Up: Where We Are Now; Next Steps to Take - ACS

Chapter 24

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Summing Up: Where We Are Now; Next Steps to Take F. Louis Floyd PRA Laboratories, Inc., 430 West Forest Avenue, Ypsilanti,MI48197

The proceedings of the first conference were published as ACS Symposium Series #722, "Service Life Prediction of Organic Coatings: A Systems Approach," Bauer and Martin, editors, 1999 (ACS), Oxford University Press. (ISBN 0-8412-3597-X). Unfortunately, the end-of-conference summary and planning session chapter was omitted from that proceeding. It is included here to complete the record. The first conference focused on setting the stage for the service life prediction debate. It defined the need for better laboratory testing, for better quantification of the environment ("natural" weather), for the use of different (reliability-type) protocols, and for construction and use of verified materials databases in the coatings industry. Attendees. There were 70 participants from the U.S. and 9 foreign countries. The top four employers of participants were government labs, coatings companies, raw materials suppliers, and universities. • Government labs: USA--National Institute for Standards and Technology (NIST), National Renewable Energy Lab (NREL), Wright Patterson Air Force Base (WPAFB), Federal Hwy Adm, and Forest Products Labs (FPL); foreign -Swedish National Testing and Research Institute, NILU (Norway), Fraunhofer Institute for Solar Energy Systems (Germany), Singapore Productivity & Standards Codes, and Japan Atomic Energy Research Institute. • Companies: DuPont, Ford, Shell, Cook Composites & Polymers, Duron, Reichhold, Rohm & Haas, 3M, Standard Products, GE, Owens-Corning, Monsanto, Elf Atochem, Cytec, Americhem, Atlas Weathering Services, Courtaulds, Akzo-Nobel, BHP Coated Steel, SC Johnson, Dow, and Sherwin-Williams. • Universities: Washington State, Iowa State, Univ of Belgium, Univ of Colorado, Univ of Cincinnati, Pascal Univ (France). The last day of the conference was given over to afree-formdiscussion that attempted to capture a consensus of the conferees understanding of the subject matter, and where they felt we as a coatings community should go next. Following is a summary

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© 2002 American Chemical Society

Martin and Bauer; Service Life Prediction ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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of that discussion, arranged in three broad subject areas: what did you learn, what techniques did you feel were being underutilized in SLP studies, and what is needed in order to improve our chances of succeeding in our attempts to predict service life. A. Based on your previous knowledge, the formal presentations that you've heard this week, and the informal discussions that you've had with other symposium attendees, what broad conclusions have you reached concerning the current status and future direction of SLP methodology? 1. Time is the currency of today. Accelerated testing must be utilized to make commercial decisions. We no longer have the luxury of waiting for outdoor exposures. This makes SLP methodologies essential, if we are to avoid major blunders. 2. Time, however, is not the proper abscissa for service life comparisons. Cumulative-damage-events is the proper basis for comparison. This circumvents the problem resulting from varying damage event frequency over time. 3. Biggest single driving force today in product development programs: risk. Business is driving hard to do more in shorter times. R&D needs to determine how to accomplish this without incurring unacceptable risk levels. Unfortunately, risk is not being given the forum for discussion it deserves in corporate venues. 4. "Failure" is a defined event, and may be either loss-of-protection or loss-ofappearance. Failure may evolve from either continuous or discontinuous processes. Failure definitions will vary based on the end-use market served. 5. There are actually two different kinds of failure: infant mortality (at beginning of life - frequently from errors in application), and true product capability (late in life). The former is the source of most product liability claims, while the latter is the focus of product development efforts. Much more attention needs to be paid to the early failures - in particular to diagnosing their causes and eliminating them. 6. Our current condition of replication-deficit prevents us from understanding our current systems, and knowing with reliability whether new systems are better. 7. We are heavily focused on chemical changes during weathering, and overlook the role of physical stresses and changes. 8. Degradation occurring during accelerated aging tests must occur by the same mechanism(s) responsible for natural aging degradation. Any acceleration that produces chemistry that is different from natural aging is inherently flawed, and potentially misleading. 9. It is essential that the end-use environment be completely characterized. "Micro-environment" differences are real, and quite significant for product performance. 10. Useful models do not necessarily require perfect understanding. 11. We can, and should, learn from other industries already using reliability methodology. 12. We can improve data credibility with image analysis techniques.



478 Β. What are some of the currently available techniques that you think are underutilized? 1. Cyclic experiments; measuring cyclic stresses 2. Utilizing fracture energy measurements to predict cracking behavior 3. Adequate replication / population descriptions 4. Acoustic emission for sub-visible mechanical damage measurement 5. Image analysis for quantification of results 6. Thermal analysis (DSC, DMA, TGA, etc.) 7. Electrochemical measurement of corrosion 8. Chemiluminescence for early detection of oxidative processes C. What capabilities and/or knowledge are needed in order to improve our ability to make more reliable service life predictions? 1. Spectral UV data (dose vs. wavelength) on light sources (natural and artificial) 2. Spectral UV responses of materials. 3. Venue to discuss hardware, software, and best practices in this field at future conferences. 4. Some kind of lab / testing certification process for SLP protocols. 5. Investigation of "induction periods." This is currently viewed more as an artifact than a behavior worthy of study. 6. Internet discussion group on Reliability Theory, where we can exchange ideas and results between now and future SLP workshops and symposia 7. We need to be able to get to point where we truly trust our test data as an industry. Until that time, we'll continue wasting substantial time confirming each other. This means that we need to carefully develop the field of "meta data": complete disclosure of experimental details under which data are collected. We also need to develop "meta data" standards for use in peer review and publication. This will lead to industry standardization. [Sounds like a natural role for NIST] 8. Workshop / short courses / tutorials needed: reliability theory, statistical methods, experimental design, image processing, meta data, techniques for utilizing "old" data. Should definitely be part of agenda for future conferences. 9. R&D must educate general management regarding reliability methodology. 10. We need to vastly speed up the approval process for new standards.

Summary of the Second Conference (1999) The second conference focused heavily on the photochemical aspects of weathering, with special emphasis on automotive clear topcoats, since that is the coatings industry's biggest single product liability event in recent memory. While there was some recognition that cyclic exposures were important, there were few papers treating this subject.



479 There were 65 attendees at the second conference from the U.S. and 6 foreign countries: • Suppliers: DuPont, Millennium, Kerr-McGee, Rohm & Haas, DSM, Dow Chemical, Dow-Corning, Elf Atochem • Academics: North Dakota State, U Missouri® KC, Iowa State, Colorado School of Mines • Agencies & independent labs: U.S. Fed Highway Admin, U.S. National Renewable Energy Lab, U.S. National Institute for Standards & Technology, Aspen Research Corp, PRA Labs (Ypsilanti, MI), USDA Forest Products Lab (Madison, WI), Japan Atomic Energy Research Institute, Swedish Institute for Wood Technology, EMPA - Switzerland, Singapore Productivity & Standards Board, KTH Center for Building Environment - Sweden, Inst of Macromolecular Chemistry - Czechoslovakia, Federal Inst for Materials Research - Germany, Fraunhofer Inst, for Solar Energy Systems - Germany • End-users: Ford, Daimler-Chrysler, 3M, GE, Northrop-Grumman, PPG • Paint manufacturers: Sherwin-Williams, Carboline, BASF, Visteon, AkzoNobel • Testing & equipment: Atlas (exposure services; accelerated testing equipment) • Software: Galactic of Salem, NH • Consultants: several retired folksfromvarious companies, still keeping their hand in The last day of the conference was given over to an open discussion that had as its goal the capturing of the attendees perspectives regarding where we are now, and where we should go next on the issue of service life prediction. Following is a summary of the discussion and conclusions drawn by the attendees during that session. A. What did you learn at this conference that was of particular significance to you? 1. the state of the art world-wide in weather monitoring is much more extensive than realized before the conference; 2. substantial variations occur in our weather over even short time intervals; 3. photo degradation behavior of materials as a function of depth into polymer films and as a function of UV wavelength; 4. UV absorber performance and permanence over time; 5. the commonalities we have with other industries; 6. the need to embrace a wider range of variables in order to adequately describe "weathering" (e.g. acid rain,fractureenergy, cyclic processes, temperature, humidity, time-of-wetness, corrosion, adhesion, and of course UV dosage); 7. the surprising possibility that today's coatings materials are so much better in UV resistance than their predecessors that photochemical resistance may no longer be the most important component in "weathering" resistance; the possibility that hydrolysis may now be as important as UV resistance in determining "weathering" resistance, [presumption: the first property to



480 "fail" is considered to be the mode of failure. UV degradation was that first property to fail in the past, but may no longer be.] 8. the possibility that in the future, we will be spending a lot more time and resource on feeding verified data into data bases, and then mining the data bases instead of running new experiments [while this is certainly hard to accept in the coatings world today, other industries have already made this conversion - materials science, electronics, aerospace]; 9. reliability theory offers a route to shorter R&D cycle times without increasing risk - but it requires very different mind sets and techniques to be successful.

B. "Weathering" is a complex issue. What sticks-in-the-sand can we place today regarding what we think we know about the weathering process, and our ability to predict the effects of weathering on coatings? [These can be used to measure our progress during future conferences] 1.

These appear to be the principal components of weathering, in the form of [cause (manifestation)]: ο UV resistance (gloss loss, color change, chalking) ο Water resistance (blistering, wrinkling) ο Adhesion - particularly under wet conditions (blistering, peeling, flaking) ο Hydrolysis resistance (erosion, color change, chalking, gloss loss - even under mild pH conditions) ο Fatigue failures (primarily cracking)fromcyclic stresses: wet/dry, hot/cold,freeze/thaw,light/dark (implicit hot/cold and wet/dry in diurnal cycle). Fracture energy, fatigue resistance, hysteresis (in stress-strain tests), and permeability probably all play a role in fatigue failures. ο Corrosion resistance (blistering, rusting, delamination, peeling) ο Surface fouling (dirt pick-up, mildew) ο Staining bleed-through from substrate (tannin, stains)

2.

UV resistance is strongly related to chemical bond energy (higher is more resistant). The bonds (or functional groups) susceptible to UV degradation are the same ones susceptible to free radical grafting reactions, and in turn are inversely related to published chemical bond energies in handbooks. This can be compensated for by using screening agents (UV absorbers) which prevent the UV light form reaching the susceptible polymer, or by free radical traps (HALS) which function to intercept degradation reactions at early stages, thereby preventing the chain of events which lead to physical property changes.

3.

UVA and HALS effectiveness is very sensitive to its (micro) environment. They appear to be far more useful in clear coats than monocoats, perhaps due to the considerably thicker path length for clear


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coats (mils) relative to monocoats (tenths of a mil for clear glossy surface layer). Certain systems do not benefit from the presence of HALS (e.g. pMMA). Acid rain and acidic components in the coating can de-activate HALS, which may explain why waterborne systems tend to have less success in utilizing HALS than solvent-borne and powder systems. Hydrolysis resistance probably follows the rules set down by Turpin, et al. for water-reducible polymer systems: hydrolysis is reduced by increases in polymer hydrophobicity, increases in steric hindrance, and reduction in anchimeric (neighboring group) effects. Hydrolytic degradation may be as significant as UV-induced degradation in determining the overall appearance durability of a coating (gloss loss, chalking, color change). Cracking of paintfilmsare of two kinds: drying (or curing)-related, and cyclic stress (fatigue) related. The drying/curing-related types are known as ο mud-cracking (related to solids and wet film thicknesses which are excessive ~ characterized by cracks in surface skin which do not usually reach all the way through a film), ο film-formation cracking (too high T or too low coalescent level characterized by "star" appearance with cracks radiating from central flaw), ο checking, alligatoring, (related to shrinkage caused by continuing reaction of residual reactive functionality ~ may start as starcracking, but takes on a cellular structure appearance once cracking is well-advanced), The cyclic stress fatigue related types are usually known as ο grain-cracking (related to film permeability (initiation) and fracture energy (propagation) - appearance is one of long linear cracks, which mimic the grain pattern of the substrate), ο cold-checking (typical of furniture lacquers exposed to hot/cold cycles), and ο blister-cracking (related to blistering/wrinkling of water-sensitive films - cracks mimic wrinkling patterns when film dries out). g

Surface fouling is probably related most strongly to surface hardness and surface energy (wetting). Surface hardness probably has a thermal (softening) component and water (softening) component. There is also a wetting/adhesion (surface energy) component, but that is far less well studied. Fouling probably starts with environmental "dirt" becoming embedded in the surface, followed by active growth of whatever species is present. In the case of marine paints, an additional component is organisms that seek and adhere to surfaces, such as barnacles. A laboratory test for dirt resistance that correlates well with exterior results utilizes independent exposure to both water and heat.



482 C. What issues were not adequately treated during this conference? 1. Attendees are not sure that all the necessary tools are yet in place to accomplish SLP. Perceived gaps: characterization of environmental causative agents; deficiencies in accelerated testing devices, particularly in light source filtration. As a result, the coatings community is not comfortable today with any leaps to prediction. 2. Not enough on wavelength-dependence of materials degradation. 3. Want more on actual prediction techniques (case studies; specific examples of how to do it) 4. Statistics (how to deal with probability distributions, apart from discrete values) 5. Cyclic processes and mechanical integrity issues were not well treated ~ may be at least as important as photochemistry 6. Is there an ISO standard on SLP? Need references and linkage. Building materials - NIST should have references. 7. Want more on effect of water on degradation processes. 8. Need better ways to cross-communicate on SLP issues. Website? Who, how, etc? [NIST may well take the lead on this] 9. Train of variability through the whole SLP process. Critical step in accomplishing SLP. Need to quantify each step. 10. Cumulative distribution vs. differential distribution;frameworkfor choosing which to deal with. 11. What is the role of matrix in weathering chemistry? ("matrix" here is used to describe the local micro-environment that a species sees, which may be substantially different from any overall composition) 12. Do we know enough about the light to get it right? Many think that existing accelerated testing light sources may not be accurate enough. But it is unclear whether betterfiltrationis the answer, primarily because the experiments haven't been run as yet. 13. What is the role of cycles in getting it right? 14. Lots of other issues in "weathering": cycles, acid deposition, hydrolysis, mechanical failure, corrosion. 15. Need simple models that work. How good is good enough? D. What steps should we take to improve the next conference? 1. Hold the next conference only when enough work has been completed to warrant it - perhaps 3 years' hence (2002). ο Site the conference on the east coast of the U.S., to facilitate foreign travelers, who largely comefromEurope, ο Use the "unanswered questions" from die following section to guide the content and organization of the next conference, ο Need to broaden foreign participation. How does work in US stack up with the rest of the world? ο Consider publishing extended abstracts in advance of next meeting. 2. Extend subject matter to include: ο plastics;


483 ο sealants; ο automated real-time measurements of performance; ο influence of lifetime costs on assessment of service life; ο beta testing as a means of reducing risk; ο other modes of failure, such as corrosion, cracking, blistering; ο role of pigments other than Ti0 ; ο risk: measurement of, tolerance of, definition(s) of. 3. Include more papers on role of: ο water, ο cycles, ο mechanical integrity, ο corrosion, ο fouling (dirt, mildew, etc), ο acid rain, ο micro-structural characteristics. 4. Need an actively managed web site for communication between conferences [NIST will set this up during 2000] 5. Use many more case studies. They are an excellent means of communicating issues. Helps clarify suitability of competing approaches. 6. To facilitate the measurement of progress on such a complex issue, place some sticks in the sand today, representing what we think we know, and then compare future work/findings/learning to them to measure our progress? [see section Β of this discussion] 7. Clearly define (and publicize) the scope of the conference in advance in order to achieve a better match between attendees' expectations and actual conference content. 8. Add sessions on: ο effect of quality of artificial light source on reliability of results vs. natural weathering. Press providers of accelerated weathering devices to develop and report on UV filters for their existing artificial weathering devices that "exactly" match the spectrum of sunlight. ο effect of cyclic stresses on coating performance: hot/cold, light/dark, wet/dry, freeze/thaw. ο fracture mechanics of polymeric materials, and how such measurements aid in predicting lifetimes. ο effect of residual (unreacted) cure functionality on survivability of a coating exposed to substantial swings in temperature during its lifetime, like clearcoats on automobiles (summer days cycle between 60° F and 180° F) 9. Form a study group to explore the implications to reliability theory for the coatings industry. Ask them to develop a list of suggested roles for various players in the food chain. Ask them to prepare a paper for the next conference, describing what they've found or concluded. 10. Invite appropriate software companies to participate via a poster session, with hands-on, live-time demonstrations of their wares. This includes


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statistics, design-of-experiments, database, and artificial intelligence software. //. Put out a callfor papers now in the key topical areas noted above, to encourage research on these topics on atime-scaleusefulfor the next conference. 12. Provide some sort of tutorial process to bring newcomers up to speed. Possibilities: plenary lectures that review key areas; encourage attendees to procure and read prior proceedings.

E. What important questions remain largely unanswered? What needs haven't been adequately met? 1. Chaos theory suggests (among many other things) that such things as weather may be deterministic in nature, but that it has multiple causes, those causes interact in unknown fashion, and initial conditions can never be completely specified. The result is a lack of predictability, or chaos, or randomness, depending on one's background. How much of our variability in service life (and resultant lack of predictability) is simply due to such things as our inability to • adequately measure initial conditions? • truly reproduce materials for "replicate" tests? • adequately measure all causative factors during test? 2. How can we detect the onset of unexpected (different) failure modes when pushing a particular mode of acceleration? What about changing modes of failure over time? 3. What role should each level of the "food chain" (e.g. raw material suppliers, manufacturers, users of paint) play in reliability testing? What should suppliers be obliged to do before approaching coatings companies? What information can/will be believed between levels in the chain? Goal: stop duplicating effort. 4. Given that time is not the correct X-axis for comparisons (cumulative damage events is the correct one), how can we translate SLP information back into a time-scale, which the commercial world both needs and demands? 5. If accelerated weathering device manufacturers were to develop a "perfect" filter for their light source (i.e. one that reproduces natural sunlight "exactly"), would that result in an improvement in our predictive capability? Would that be sufficient to develop a simple model that works well enough? 6. Is there a relatively simple combination of short-term lab tests today that together yield an adequate (for commercial purposes) prediction of service life? For example: • fracture mechanics for mechanical integrity • cycling of physical conditions (wet/dry, hot/cold, freeze/thaw) • photochemical degradation • permeability (water, oxygen) for substrate protection


7.

8.


9. 10. 11.

12. 13.

14. 15. 16.

485 • fouling resistance (dirt, mildew, etc; hardness, bio-resistance) • adhesion (probably with a wetting/penetration component) What is the environment really like (how variable is it) on the micro scale? To what extent does this variation contribute to question #1 above? [Even today, monitoring of environment is broad and crude, compared to typical monitoring of lab conditions.] Are there "healing" processes that occur and are significant during the weathering process? Is the ultimate arbiter of performance simply one of comparison to some "known" system? What is the rate-limiting step for various failure modes? What are some good commercially-available software packages for DOE/AOE? Criteria: user-friendly, simple, easy, minimal learning curve, readily understandable (and of course competent). [One liked ANOVA T M from ASD. A few like Stat-Ease.] What are the minimum number of properties that need to be specified to guarantee the "quality" of a coating? What are the minimum number of environmental conditions that need to be specified/monitored in order to account adequately for "weathering" processes? What is the role of residual unreacted cure capability on the performance of coatings (particularly clearcoats)? How much risk are we willing to accept, and how do we determine that level? [relates to acceptable failure rate vs. time]. Approach the polymeric binder community with the need for more robust adhesion-promoting technology. Current technology is strikingly deficient in robustness: too easily poisoned by other ingredients; too-easily defeated by environmental condition swings; no significant penetration (binding) of porous (unsound) substrates.


Summing Up: Where We Are Now; Next Steps to Take - ACS

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