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

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Presentation on theme: "MEASUREMENT OF HYGROTHERMAL STRESS IN EXTERIOR WOOD COATINGS Mari de Meijer - Drywood Coatings Johan Nienhuis - SHR Timber Research."— Presentation transcript:

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

2 February 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 February Cost Copenhagen How internal stress can occur Difference in thermal or hygroscopic expansion between coating and wood Temperature Moisture VWVW VCVC COATING WOOD S = E (  V C -  V W ) 1 - V C : volume change coating V w : volume change wood E : elastic modulus coating : Poisson ratio coating (0.4) Positive stress: contraction Negative stress: expansion

4 February 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 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 February Cost Copenhagen Measurement of Internal Stress Internal stress is calculated as follows: IS=DEd 3 /(3  L 2 (d+  )(1- )) IS internal stress Ddeflection Emodulus of elasticity of paint dthickness of substrate (metal strip) Llength of unclamped substrate Poisson ratio of substrate  thickness of paint deflection

7 February 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 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 February Cost Copenhagen Apparatus for measuring deflection

9 February 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, µm dft) Additional pretreatments: immersion in water for 24 h - 28 days Leaching according to EN °C for 4 h Volume changes also recorded

10 February 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 Cost Copenhagen Change in volume of free paint films

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

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

14 February 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 Cost Copenhagen Internal stress in paint films: changes in constant temperature and changes in constant R.H.

16 February Cost Copenhagen Overview of IS results

17 February Cost Copenhagen Overview of IS results

18 February Cost Copenhagen Increase of low T 40 % 65 % 99 %

19 February Cost Copenhagen Increase of low T 99 % 65 % 40 %

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

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

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

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

24 February Cost Copenhagen Comparison various unpigmented films

25 February Cost Copenhagen Conclusions 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=DEd 3 /(3  L 2 (d+  )(1- ))

26 February 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 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 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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