Service Life of Exterior Wood Coatings VTT Virtual project by alicejenny


									Service Life of Exterior
Wood Coatings:
A review of measurement and performance factors

                          Jon Graystone

                    The Paint Research
                     Association, UK

 •CTBA Paris 2004 26-27 April
Relevance of Service Life Issues to the
Exterior Wood Coatings Industry - Supply
Chain Participants
•   Architects & Specifiers   •   Retailers (Trade & Retail)
•   Building Constructors     •   Paint Manufacturers
•   Building Owners           •   Raw Material Suppliers
•   Maintenance Officers      •   Wood Providers and
•   Painting Contractors          Converters
•   Trainers and Training     •   Wood Fabricators and
    Organisations             •   Research and
•   Specialist Contractors        Technological
•   Householders                  Organisations
                              •   Standards Organisations
                              •   Industrial Applicators
Relevance of Service Life Issues to the
Exterior Wood Coatings Industry - Benefits
• Shorten Development Cycle
    – reduce risk
• Enhanced Performance
    – Warranty
•   Increase Customer Satisfaction and Confidence
•   Aid New Technology Development
•   Increase Confidence in New Product Launch
•   Test new technology or chemistry
    – VOC and SHE issues
• Understand performance in different climates
    – relative rankings
• Gain Competitive Advantage
Service Life in Context


               Service Life

     Quality                  Durability
Service Life Definitions
•   Durability
     – The period of exterior exposure during which the coating will retain an
        acceptable appearance, protect the substrate and not require any
        maintenance other than cleaning.
     – The degree to which paints withstand the destructive effects of
        weather (BSI)
     – The capability to reliably perform an intended function over a long
        service life under reasonable conditions of use.
•   Statistical Definitions
     – Reliability is the probability that an item will perform its intended
        function for a specified time interval under stated conditions
•   User focussed
     – A reliable product is one that does what the user wants it to do, when
        the user wants it so do so.
•   Quality
     – “on-target with minimum variance”
          • Capability Ratios
          • Statistical Process Control
          • Assumed link with performance
     Service Life Practical Usage of Terminology
•   Service life might be defined as a time period before replacement
    or repair is required.
     – Hence for practical purposes
         • Durability = Service Life
             – qualitative vs quantitative
•   Quality control is associated with production                        Service Life

•   Reliability is associated with design or development
                                                               Quality                  Durability

•   Service Life Prediction and Durability Studies focus on
     – measuring and predicting failure and failure rates
         • probabilistic or deterministic
     – methods to accommodate failure
         • maintenance, tolerances, redundancy, warranty
     – preventing failure
         • improved service life
               – deterministic causes of failure analysis
Difficulties related to the Service Life Prediction of
Wood Coatings
•   Wood as a natural material is a difficult substrate,
     – species, grain orientation, heartwood sapwood ratio etc.
•   Many other causal factors can influence performance
     – design, the nature of previous coatings and the exposure history.
•   An overarching problem is that of the weather itself.
     – As an uncontrolled variable, climatic influences are difficult to
       separate from other sources of experimental variance.
         • Poor test capability, in terms of both repeatability and reproducibility.
         • An unresolved problem that has bedevilled the development of European
           performance standards.
•   Coatings are supplied as an Intermediate
     – Upstream and downstream influences on performance
         • Factory vs Maintenance
         • New work vs restoration
•   Fault Tree Analysis is useful for showing relationships
Fault Tree Analysis

Coated Window Failure          Predictive Tests
                               - Mechanical Properties
                               Phenomenological Tests
                               - Simple panel tests
      Coating Formulation      - Modified panels
                               - Jointed test pieces
                               - Constructions (such as window -
        Design Elements        frames)
                               - Test houses
       Wooden Substrate        Visual Inspection
                               - Real buildings

    Environmental Conditions

       Application Factors

        Damage Factors
                                         Fault Tree - Expanded

                                                                                                                                                                                   C oa te d Wi nd ow Fa ilu re
                                                                                                                                                                                         (Multiple causes)

                    Design Aspects                                                                          Coat ing Syst em                                                                                         Wood        Dist res s fact ors                                                                           Applic at ion                          Climate and
                                                                                                             Formulation                                                                                           Subst rat e      (Damage)                                                                                  Cons ideration                           Weather

Joint s                               Profil es                                            Permeability                                      Mechanical Properties                         Wood Species                                       Ex ternal                                                                Indust ri al
                                     And A ngles                                         Liqui d & Vapour                                    St rength and m odulus                     (Or deriv ed produc ts)                               Fact ors                                                                Applic at ion                       Ex ternal                 Int ernal
                                                                                                                                                                                                                                                                                                                                                          Climate                   Climate

     Glazing                                                                                                                    Poly mer ty pe                   Pigm ent at ion                     Heartwood                                         Moisture                                                  Fact ory
   And seal ant s                                                                                                                 (Binder)                       Phase volume                         Sapwood                                          Movem ent                                                Condit ions                         UV                                    Hum idty
                                                                                                                                                                                                                                                                                                                                                  Radiat ion                            Condensation
   Weathers eal                                                                                                                   Cure                                                               Early and                                         Mechanical                                                       Weather
    And DPC                                                                                                                      Mwt & Tg                                                            Latewood                                           Dam age                                                        (Ex ternal)                 Free                                    Therm al
                                                                                                                                                                                                                                                                                                                                                  Radicals                                Conduct ivit y

     End Grain                                                                                                              Physical Form                                                         Grai n Orientat ion                                   Therm al                                               Temperat ure
      Sealing                                                                                                            Soluti on or Dispersion                                                 Knots and Defect s                                    Movem ent                                                 & Wind                          Temperat ure                           Temperat ure
                                                                                                                                                                                                                                                                                                                                                    Cycle                                 Cycling

       Quality                     Ex pos ure                        Adhesion                                                                      Penetration                                        Porosit y                                                       Biologic al                                Hum idity
     Work manship                And Orientation             Int ercoat and subst rat e                                                             And Fl ow                                         Dens ity                                                          At tac k                                  & Rain                           Freeze

                                                                                                                               Surface Tension                        Rheology                     Ex tract ives                                                                                                      Quality
                                                   Surface Energy                      Col loi dal Forces                          Wett ing                           Visc osity                 Decay resist anc e                              Blue s tain                        Decay Fungi                   Film thicknes s                Precipit aion
                                                                                      Molecular Forces                                                                                                                                         Surface mould                                                                                   And condensat ion

                                                                                                                                                                                                      Im pregnat iom
                                                                                Chem ical                                 Water repellency                                                                                                                                                                           Levelling                        Hai l
                                                                               Resistance                                                                                                                                                                                      Nut rient s                                                      Im pac t dam age

                                                                                                                                                                                               Preservat ive
                                                                                                                                                                                               Treat ments                                                                                                                                      Tim e of Wet nes s
                                                                    Funct ionalit y                                                                                                                                                                                                     Spore dist ributi on         Aeration
                                                                                                                                                                                                                                                                                                                     & pores
                                                                                                                                                                                             Chem ical Type
                                                                                                                                                                                                                                                                                                                                                   Pollut ion
                                                                                      Crosslink densit y                                                                                                                                                            Insect                                                                         Ac id rain
                                                                                      Net work st ructure                                                                                                                                                           At tac k
                                                                                            Opacit y                             Col our                                                                                                       Int ernal                                                                                        Carbon dioxide
                                                                                         & UV absorbtion                        & Gloss                                                                                                        Fact ors
                                                                                                                                                                                               Or Polymers
                                                                                       Fungal Res ist ance                                                                                                                                           Coat ing
                                                                                                                                                                                                                                                 Shrinkage St res s
                                                                                                                                                                                                    And cutt ing

                                                                                                                                                                                               Surface compression

                                                                                                                                                                                                        Ex pos ure

                                                                                                                                                                                                 Nat ural

                                                                                                                                                                                            Chem ical/ Phys ical
                                                                                                                                                                                       Plas ma. Flame, Ac ety lation

                                                                                                                                                                                                         Coat ings

                                                                                                                                                                                              Maintenanc e
                                                                                                                                                                                              & Preparation

                                                                                                                                                                                           Ancilliary P roduct s
                                                                                                                                                                                            Fillers, S ealers

                                                                                                                                                                                           Surface Treat ments
                                                                                                                                                                                               Sil icone etc
                                                       Fault Tree Analysis

                                                      Coated Window Failure

                                                             Coating Formulation

                                                              Design Elements
                                 Coating System
                                                             Wooden Substrate
                                                          Environmental Conditions

                                                             Application Factors
                     Permeability                                                            Mechanical Properties
                   Liquid & Vapour                            Damage Factors                 Strength and modulus

                                                               Polymer type                                Pigmentation
                                                                 (Binder)                                  Phase volume

                                                   Cure                             Physical Form
                                                  Mwt & Tg                      Solution or dispersion

             Adhesion                                     Chemical
      Intercoat and substrate                            Resistance

Surface Energy       Colloidal Forces      Functionality               Crosslink density
                     Molecular Forces                                  Network structure

            Penetration                     Water repellency
              & Flow

Surface tension           Rheology
    Wetting               Viscosity

                       Opacity                     Colour
                   & UV absorbtion                 & Gloss

                  Fungal Resistance
                  Biocidal Properties

                Wood Species                                                                         Impregnation
            (Or derived products)

Heartwood                               Early and                              Preservative                          Monomers
Sapwood                                 Latewood                                Treatment                           Or polymers

                         Grain Orientation                          Chemical               Penetration
                         Knots and Defects                            Type

 Porosity                      Extractives
 Density                     Decay resistance

                                          Machining                                            Previous
                                          And cutting                                          Coatings

                      Surface compression                                      Maintenance                Surface treatments
                                                                               & Preparation                 Silicone etc

                Sanding                                       Ancilliary Products
               & finishing                                     Fillers, sealants


        Natural                     Chemical/Physical
       Weathering               Plasma, Flame, Acetylation
Fault Tree - Design Aspects

                                     Design Aspects

                       Joints                    Profiles
                                                And angles

          Glazing               Weatherseal
        And sealants             And DPC

         End Grain

                             Quality            Exposure
                           Workmanship        And Orientation
Fault Tree - Distress Factors
                                                          Distress factors

            External                                                              Internal
            Factors                                                               Factors

 Moisture              Mechanical                                       Coating
 Movement               Damage                                      Shrinkage Stress

  Thermal              Biological
 Movement                Attack

                         Blue stain
                       Surface mould


                                       Decay Fungi

                           Nutrients          Spore distribution
Fault Tree- Application


                   Industrial              Weather
                  Application             (External)

          Factory                Temperature           Humidity
         Conditions                & Wind               & Rain

                Film thickness


    & pores
           Fault Tree - Climate & Weather

                                                                    Climate and

                                    External                                                          Internal
                                    Climate                                                           Climate

                         UV                                Temperature                   Humidity                Temperature
                       Radiation                              Cycle                    Condensation                 Cycle

             Free                              Freeze                         Thermal
            Radicals                            Thaw                         Conductivity

          Precipitaion                         Pollution
        And condensation                       Acid rain

     Hail          Time of Wetness
Impact damage

                   Carbon dioxide
Durability & Service Life Alternative
• To quantify Service Life
• To Predict Service Life
• To Investigate Mechanistic Aspects of Service Life
   – The different objective will define the type of
      • Probabilistic
      • Deterministic
     Reliability Concepts
Probabilistic Reliability              Deterministic Reliability
Studies overall failure rates          Studies individual failure
independently of causes                mechanisms as a function of time.

Used to predict failure for e.g.       Investigates failure at component, or
maintenance scheduling                 causal level.
Studies system reliability, e.g.       Tries to eliminate failure at the
through redundancy, fault              causal level
elimination etc
Concerned with multiple failure rate   Concerned with single failure
distributions                          distributions and the impact of
                                       random events.

  Probabilistic reliability methods are widely used in military,
  aerospace and telecommunication specification.
  In deterministic studies it is essential that there is a
  conceptual model around which the failure mode can be
Types of Reliability model
• STRESS-STRENGTH: An item fails if the applied stress
  exceeds its strength. Below this level there is no damage
  and failure may be ascribed to random events. The failure
  distribution is exponential. (Hail Damage)
• DAMAGE-ENDURANCE: This relates to accumulation of
  damage and also covers fatigue. Failure is sometime
  described as an endurance limit. In coatings the build up
  of residual stress is known to lead to failure under some
  circumstances. (Moisture Movement)
• CHALLENGE-RESPONSE: Here a defect remains dormant
  until a specific event, or combination of events ‘challenge’
  the coating and cause failure. (Adhesion Failure)
• TOLERANCE-REQUIREMENT: This term is used to
  describe degraded performance that does not immediately
  lead to failure, such that the user accepts inferior
  performance rather than incurring correction costs.
    Physical Ageing and Degradation
•   Physical ageing affects many substances including
    polymers, and is described as the reversible change in a
    property as a function of storage time under no influence
    from external conditions
     – Non -equilibrium (T<Tg> ambient)
        • may lead to contraction, change in creep etc.
•   In contrast with physical ageing, polymers may also
    undergo degradation and in some cases further cross-
    linking in response to external conditions. (Irreversible
    chemical ageing).
•    The effects of UV, water and temperature have all been
    widely studied in the context of durability
•   A useful approach in coating failure studies is to consider
    the chain of events which ultimately leads to failure.
Accelerating Feed Back
• Artificial Weathering
• Intensified Natural Weathering
   – angle of exposure
   – ‘Emmaqua’
• Extrapolation from Early Results
   – Time series
   – Analytical Techniques
• Mechanistic Studies
   – photodegradation studies
    Accelerated Testing
•    “About the only non-controversial thing that can be said
     about accelerated testing is that it can be controversial”
         • L W Condra
•    Accelerated testing (weathering) is based on the premise
     that test panels will exhibit the same behavior after a short
     period at high stresses, as they would exhibit after a longer
     time at lower stresses.
     – Reciprocity Issues
         • Much work amounts to a quest for a ‘black box’ that will give a
           good correlation with natural weathering exposure.
•    Many regard the Coating Industry as behind other
     industries in predicting the service life
     – ”Attempts at refining the current durability methodology have
       been made over the last 80 years without any significant
       improvement in its predictive capability”
         • FCST Mongraph - Methodologies for Predicting Service Life (1996)
             – Martin, Saunders, Floyd, Wineburg [ref.6]
    Reciprocity Issues
•   There is an underlying assumption in much accelerated testing
    that raising the level of a ‘stress’ such as UV radiation will simply
    increase the rate at which changes occur such that the damage
    depends on the total absorbed energy, and is therefore
    independent of radiant intensity and exposure time.
•    This is known as a reciprocity law.
•   A recent review [ref.8] outlines the many exceptions to the simple
    assumption. At various radiant flux thresholds different
    reciprocity relationships have been observed, indicating that
    alternative, system dependent degradation paths have been
•    A model based on band theory is been proposed to explain this
    behaviour, to complement photochemical mechanisms.
    Service Life Prediction Issues (SLP)
•   Practical coating failures invariably show a wide, and often
    non-normal distribution.
•   Incidents of early failure may have more practical
    significance than average service life.
•   Outdoor exposure conditions are not reproducible.
•   Quantification of relevant exposure environments (natural
    or artificial) is poor.
•   Durability data is multivariate.
•   There is no a priori reason to expect correlation between
    different coating chemistries under different service
    conditions whether natural or artificial.
    – A dosage based damage model is required
        • this represents a fundamental change in tactics for many coating
          manufacturers !
Quantification of Exposure Conditions
•   UV Dosage
    – Integrate
        •   spectral irradiance
        •   spectral absorption coefficient
        •   spectral efficiency
        •   duration
•   Temperature
    – Energy balance equations
        • absorptivity
        • emissivity
•   Water
    – time of wetness
    – relative humidity
    – precipitation
    Some Mathematical Aspects of Service Life
    (“Mathematics is a subject in which we do not know what we
    are talking about, nor care whether what we say is true” -
    Betrand Russel )
• Statistics and Experimental Design
      – variance and test capability
•    Regression
•    Non parametric statistics
•    Time Series Forecasting
•    Reliability Methodologies
•    Failure Modeling
•    Bayesian Methods
      – meta-analysis and the use of prior knowledge
Descriptive Statistics
Data Analysis Tools
•   Distribution Functions
     – Measures of location - mean, mode
     – Measures of spread - standard deviation, range
•   Correlations
     – linear association
         • practical implications of magnitude
•   Multivariate data analysis
     – do not depend on a formal probability model
     – information rich summaries to generate hypotheses
         •   principal component analysis
         •   multidimensional scaling
         •   cluster analysis
         •   correspondence analysis
•   Significance Tests
Common Distributions
( “It is a capital mistake to theorize before one has data - Conan Doyle” )
A random variable may be discrete or continuous. Discrete distributions are
described by a probability function, while continuous distributions have a probability
density function. Each is applicable to particular problems.

• Discrete                                   • Continuous
  Distributions                                Distributions
     – Point probability                          – Cumulative or
       functions                                    probability
          •   Bernoulli
          •   Binomial                                  •   Uniform
          •   Poisson                                   •   Normal
          •   Geometric                                 •   Exponential
                                                        •   Gamma
                                                        •   Weibull
The Normal Distribution

                         LAW OF ERROR
                        STANDS OUT IN
                      MANKIND’S EXPERIENCE
                    AS ONE OF THE BROADEST
                 PHILOSOPHY ~ IT SERVES AS THE

                                                          W J Youden
Important Lifetime Distributions
The Four most important distributions used in lifetime studies are:-

• The Normal Distribution
      – Very important in Statistics and Quality control,
         • slightly less common in reliability work
• The Lognormal Distribution
      – very common in reliability work, used particularly for
        failure distributions due to accumulated damage
• The Exponential Distribution
      – applicable to failures which are totally random
        (independent of past history) e.g. electrical fuse
• The Weibull Distribution
      – Widely used in reliability work and failure modeling due
        to its versatility
           • has a shaping factor allowing it to be used for all parts of
             the bathtub curve, or as an exponential distribution.
    Impact of Variance on Measurment

•   It has been common in the past to describe variance as ‘error’,
    and to divide this into systematic and random errors. This is
    generally derogated, and the term ‘common cause’, and ‘special
    cause’ variation are much to be preferred in order to make a
    distinction between ‘mistakes’, and the variance that is often part
    of the normal process of measurement.
•   Common cause variance influences the accuracy of results, while
    special cause errors will affect the precision of the results
•   Accuracy is the difference between the average measurement of a
    sample and its ‘true’ value, while precision is the variation in
    measurements observed with a test method while measuring the
    same sample multiple times on the same instrument by the same
Impact of distribution on Test Capability
           Test Capability


              control limits
Reliability Based Methodologies
Lifetime Distributions
Especially useful where observed times to failure have a high variability

 •   Specimens are exposed until they reach an absolute, or arbitrary relative
     failure point
      – non -failures are said to be ‘censored’
 •   Lifetime Representations
      –   Failure Density (Probability density function)
            •   number of failures/unit time as % of original
                  – decreasing - old is best (infant mortality)
                  – constant - old is as good as new (youth and middle life)
                  – increasing -new is best (wear out, old age)
      –   Cumulative Distribution Function
            •   cumulative number of failures up to a given time
      –   Survival Function
            •   cumulative number of survivors t a given time
                  – taken as a measure of reliability
      –   Failure Rate
            •   number of failures per unit time expressed as fraction of the number of survivors
      –   Characteristic Life
      –   Failures per unit time
      –   Mean time between failure

 Cracking Score



                      10   20   30   40     50    60      70   80   90   100
                                          Time in weeks

Differences between traditional and reliability methodologies for a
hypothetical example. In the traditional approach (a) the cracking
scores are averaged at two years. In the reliability approach (b), the
times to reach a pre-denominated failure level (here 3) are recorded.
Typically less replicates are used in the traditional approach.
    Failure Modeling - Time Transformation Functions

•   Assumes that time to failure distributions are a common set for
    which a function exists to relate distribution parameters to the intensity of
    intensity of weathering
    f weathering
        d mechansims
    se Power Law
        al to stress
         most common
        ss modelling
        re dependent
    ndau, Ferry)
    Some Relatively Recent Research Observations (1)
    e.g. AIR project, Arwood, DOE Multiclient Studies, PRA Multiclient

•   “The type of paint has no effect on the average moisture content in the
    window frames in contrast to the type of wood”
     –   Centrum voor Onderzoek (20 Paint ystems in 13 housing complexes)
•   “In general coating performance on panels and L joints did not correlate
    with that on window components. It appears from this trial therefore that
    these test specimens cannot be used to reliably predici performance on
     –   AIR Project CT94/2463 - Task 1
•   “The results show a multitude of factors with numerous interactions
    influencing the wood moisture content. In absolute terms the most
    important factor was seasonal climatic fluctuations. The coating system
    played only a minor role compared to the influence of the sample design”
     –   AIR Project CT94/2463 - Task 2
•   “The rank order of performance (integrity) correlates with the initial water
    absorption value of the coating”
     –   DOE Multiclient
•   “Some coatings give rise to high localised moisture concentraion at the
    wood coating interface”
     –   PRA DTI Project
    Some Relatively Recent Research Observations (2)
    e.g. AIR project, Arwood, DOE Multiclient Studies, PRA Multiclient
•     “It has been shown that the type of wood has no effect on failures in the
      paint system,”
        –   Centrum voor Onderzoek (20 Paint ystems in 13 housing complexes)
•     “Confirmation that the substrate can affect the performance of all coatings
      is one of the major findings from this trial”
        –   DOE Multiclient
•     “After 12 months exposure it was clear that for many coatings this had
      been insufficient to unequicocally discriminate their performance
•     “…analysis confirmed there was no correlation between performance after
      12 months and after 24 months”
        –   -DOE Multoclient
•     “…correlations of the assessment results between 12 & 24 months were
      good, and this gives a basis on the suggested duration of one year for an
      exposure time in EN 927-3
        –   AIR Project CT94/2463 - Task 1
•     “’opaque’ films can vary significantly in their opacity to light”
        –   PRA DTI Project
•     “Performance (integrity) after 12 months correlates with dry film thickness
        –   DOE Multiclient
Service Life Studies - Current Trends [1]
•   Damage Indices
     – photohydrolytic mechanisims
        • UV, Temperature , Time of Wetness, Humidity
        • Spectral Power Distribution Functions (SPD)
     – effect of colour
•   Dosage models and verification
     – integrated sphere
•   Degradation Rate models
•   Damage precursor studies
•   Non traditional metrics
     – fracture energy, UVA spectra, HALS analysis, photo-oxidation
       analysis, DMA, DSC
•   Newer Techniques
     – FTIR, photoacoustic TOF-SIMS, Atomic Force Microscopy
•   Arrhenius-Weibull Models
     – influence of Tg
•   Time Series Analysis
•   Fundamental Molecular Orbital Predictions
    Recent (2003) NIST Publications
    - all fully available as PDF on web pages
•   Advanced Techniques for Nanocharacterization of Polymeric Coating Surfaces.
•   Effects of Spectral UV on Degradation of Acrylic-Urethane Coatings.
•   Enhancing Sensitivity of Atomic Force Microscopy for Characterisation of Surface
    Chemical Heterogeneity.
•   Integrating Sphere Sources for UV Exposure: A Novel Approach to the Artificial UV
    Weathering of Coatings, Plastics and Composites.
•   Laboratory Apparatus and Cumulative Damage Model for Linking Field and
    Laboratory Exposure Results.
•   Nanoscale Characterisation of Coatings Surface Degradation With Tapping Atomic
    Force Microscopy.
•   Reciprocity Law Experiments in Polymeric Photodegradation: A Critical Review.
•   Relating Laboratory and Outdoor Exposure of Coatings. Part IV: Mode and
    Mechanism for Hydrolytic Degradation of Acrylic-Melamine Coatings Exposed to
    Water Vapour in the Absence of UV Light.
•   Repeatability and Reproducibility of Field Exposure Results.
•   Use of Laser Scanning Confocal Microscopy for Quantitative Characterisation of
    Physical Changes of Polymer Coatings After UV Exposure.
Temperature Controlled Weathering Rack
(PRA - Teddinngton)
     Service Life Prediction Methodology
                                                                        Quantification of Exposure
Environmental        Natural Weathering                                  Environment.
Exposure             -distributed sites.                                - ambient and test panel
                     Analysis of existing                               - UV, temperature, wetness etc.

                     Laboratury Exposures                               Quantification and systematic
                     - 'artificial weathering                           variation of environment
                        - static and dynamic
                                                           Dosage Metrics
  Response          Techniques
  Monitoring        -SEM, AFM
                                                                                       Initial Data Analysis
                    - FTIR
                                                                                       - distribution statistics
                    - Thermal Analysis
                                                                                       - time series
                    - Mechanical Analysis
                                                            Degradation                - reliability methods
                    - Gloss, Colour, Chalking etc.
                    - Biological                            Mechanisms

Delivered             Model Application              Model Validation          Time transformation Model
Objectives            - correlation studies          Data = Fit + Residual     - Arrhenius
Durable Long-life
                                                                               - Power Law
                                                                               - WLF etc.
State of the Art - General Conclusions (1)
Scope for further work

•   Despite the vast amount of published work there remain many
    unanswered questions and contradictions relating to the
    performance of coated wood.
•   Investigating all the many interactions (fault tree) would be hugely
•   There is a need to focus on basic issues which define a framework
    within which specific issues might be prioritised
     – of clear value if answers could be found
         • however funding has proved almost impossible to obtain
•   Further critical meta-analysis of past work could yield valuable
    clues (Knowledge Management Issues)
Scope for further work - some examples

•   Influence of exposure conditions on rank order of performance
     – TC139/WG2 - Round Robin
•   Optimum mechanical properties for long term performance
•   Moisture permeability limits
     – influence on performance
         • Next Presentation
•   Effect of parameters which influence both performance and
    Test Capability
     – Colour, Film Thickness, Wood Species and microstructue
•   Dosage based methods for service life prediction
     – lessons from other disciplines
         • reliability theories
    Summary and Conclusions for Service Life Studies
•    Development of long lived coating systems requires improved
     methods to measure and validate Durability or Service Life
•    There are a wide variety of mathematical and statistical tools that
     aid the prediction of durability and service life.
•    Choice of the right method requires a clear understanding of the
     objectives and the limitations and advantages of each technique.
•    Traditional Methods have failed to deliver the confidence found in
     other industries.
•    Reliability Methods are more appropriate to some types of
     progressive failure and in multi-component situations.
•    Dosage models are needed to interpret Natural Weathering, and
     the bridge the gap between Natural and Artificial Weathering.
•    Service life can in principle be predicted from damage precursors
     before they become or cause catastrophic failure.
•    Time Series Analysis should be considered for sequential
     observations that are not independent.

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