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MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR WOOD COATINGS

MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR WOOD COATINGS. Mari de Meijer - Drywood Coatings Johan Nienhuis - SHR Timber Research. February 2004 - Cost Copenhagen. Content of presentation. Origin of thermal and hygroscopic stress

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MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR WOOD COATINGS

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  1. MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR WOOD COATINGS Mari de Meijer - Drywood Coatings Johan Nienhuis - SHR Timber Research

  2. February 2004 - Cost Copenhagen Content of presentation Origin of thermal and hygroscopic stress Wateruptake of free paint films: swelling, E-modulus and elongation Internal stress measurement method Effect of temperature and R.H. changes on internal stress and stress relaxation Unanswered questions and future work

  3. Temperature E ( VC - VW) S = Moisture VC : volume change coating Vw : volume change wood E : elastic modulus coating  : Poisson ratio coating (0.4) Positive stress: contraction Negative stress: expansion 1 -  VC COATING VW WOOD February 2004 - Cost Copenhagen How internal stress can occur Difference in thermal or hygroscopic expansion between coating and wood

  4. February 2004 - Cost Copenhagen Quantification of IS Calculate maximum stress from difference in expansion or contraction between coating and wood due to temperature or moisture change Measure stress in coating directly, possibility to assess relaxation of IS Determination of E-modulus

  5. February 2004 - Cost Copenhagen Measurement of Internal Stress • Coating is applied on one side of very thin metal strip • Coated strips are exposed to change in T or RH • Expansion or contraction of the coating cause strip to deflect • Deflection is measured as a function of time

  6. deflection February 2004 - Cost Copenhagen Measurement of Internal Stress • Internal stress is calculated as follows: • IS=DEd3/(3L2(d+)(1-)) • IS internal stress • D deflection • E modulus of elasticity of paint • d thickness of substrate (metal strip) • L length of unclamped substrate • Poisson ratio of substrate •  thickness of paint

  7. February 2004 - Cost Copenhagen Measurement of Internal Stress • Application of paint film (80 m dft) on stress free metal strip (155 m) • Drying of paint for 2 weeks @ 20 °C and 65 % R.H. • Place metal strip in clamps • Measure deflection at different temperature and relative humidity • change climate in the morning • measure deflection during the day and overnight

  8. Apparatus for measuring deflection February 2004 - Cost Copenhagen

  9. February 2004 - Cost Copenhagen E-modulus measurement • Tensile test of free paint films (@ Zwick, loadcell 500 N, 50 mm/min, preload 0.1 N) • Free paint films (70 mm length, 15 mm width, 40-60 µm dft) • Additional pretreatments: • immersion in water for 24 h - 28 days • Leaching according to EN 927-5 • Ageing @ 105 °C for 4 h • Volume changes also recorded

  10. February 2004 - Cost Copenhagen Paint systems tested • 4 white pigmented paints: • solventborne alkyd primer / topcoat (SB) • waterborne primer / topcoat (WB) • waterborne primer with good durability(WP1) • waterborne primer with restricted durability (WP2 ) • Various types of unpigmented binders

  11. February 2004 - Cost Copenhagen Change in volume of free paint films

  12. February 2004 - Cost Copenhagen Change in E-modulus of free paint films

  13. February 2004 - Cost Copenhagen Swelling of wood (width) and volume change of free paint films

  14. February 2004 - Cost Copenhagen Results E-modulus / volume change • Volume free coating film increases up to 20 % during immersion in water • Leaching followed by drying and 2nd immersion reduces swelling • Heating reduces volume • E-modulus changes due to moisture, leaching or heating

  15. February 2004 - Cost Copenhagen Internal stress in paint films:changes in R.H. @ constant temperature and changes in T @ constant R.H.

  16. Overview of IS results February 2004 - Cost Copenhagen

  17. Overview of IS results February 2004 - Cost Copenhagen

  18. Increase of RH @ low T February 2004 - Cost Copenhagen 65 % 40 % 99 %

  19. Increase of RH @ low T February 2004 - Cost Copenhagen 65 % 40 % 99 %

  20. Increase in temperature at high RH February 2004 - Cost Copenhagen 40 degrC 1 degrC

  21. Increase in temperature at high RH February 2004 - Cost Copenhagen 40 degrC 1 degrC

  22. Increase in temperature at low RH February 2004 - Cost Copenhagen 50 degrC 20 degrC

  23. Increase in temperature at low RH February 2004 - Cost Copenhagen 20 degrC 50 degrC

  24. February 2004 - Cost Copenhagen Comparison various unpigmented films

  25. Conclusions February 2004 - Cost Copenhagen Swelling due to wateruptake of free paint film (volume) more than coated wood (width) This will cause IS in coating E-modulus changes for immersed, leached or aged coatings: increase in modulus and thereby internal stress: IS=DEd3/(3L2(d+)(1-))

  26. February 2004 - Cost Copenhagen Conclusions Increase of stress at temperature or RH changes Increase of stress (built up of stress) due to changes in climate (R.H. and temperature): no complete stress relaxation

  27. February 2004 - Cost Copenhagen Future work • Tg and E-modulus in relation to stress relaxation • Impact of E-modulus changes on stress • Stress measurements while immersed • Calculation of coating stress on wood • Difference expansion coating-wood • Anisotropy in dimensional changes

  28. February 2004 - Cost Copenhagen Relation IS -durability What levels of stress cause failure? Depending on type of coating both high and low T or RH can be critical (what does this mean for ICP in EN 927-3?) Time between T / RH changes to allow relaxation is also important for durability

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