Sticking to surfaceswood273.sites.olt.ubc.ca/files/2014/08/Lecture-8... · 1. Mechanical...
Transcript of Sticking to surfaceswood273.sites.olt.ubc.ca/files/2014/08/Lecture-8... · 1. Mechanical...
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Sticking to surfaces
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Examples of natural adhesion- temporary to permanent
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Some adhesives are inspired by nature
https://ninithi.wordpress.com/2015/09/11/spiderman-gloves-more-nanotechnology-enables-gecko-pads/
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Other adhesion examples
http://4.bp.blogspot.com/-pEB7Z8jM2ws/UlmeaOVtf6I/AAAAAAAAQCk/CLStC-DkEK0/s1600/DSCN6334+-+Copy.JPG
https://www.e-know.ca/wp-content/uploads/2015/10/Burr-head-dog.jpg
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Adhesion
Liquid Solid
Intimate contact Keeping things together
MonomersMeltsSolutionsSuspensions
EnvironmentApplied stress
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Theories to why things “stick”
1. Mechanical Interlock– Tooth fillings to velcro2. Inter-diffusion– Plastic pipe welding3. Secondary interactions– Intermolecular forces: London dispersion,
dipole-dipole interactions, hydrogen bonds, specific attractions 4. Electrostatic charge model: permanent charge (electrostatically
layered films)5. Covalent linkages: chemical bond formation with the adherend i.e.
coupling agents
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Why things don’t “stick”
All molecules are attracted to each other– sometimes self attraction is much greater than attraction to adherend
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Surface tension (liquid) Surface free energy (solid)Work to create additional surface areaisothermally and reversibly by unit amount
γ0
Water --- 72.8 mN/mBenzene--- 28.9 mN/mHexane --- 18.4 mN/mAcetic acid ---27.6 mN/mAcetone ---23.7 mN/mEthanol --- 22.3 mN/mOctanol --- 27.5 mN/m
http://www.educationquizzes.com/library/KS1-Science/Blowing-Bubbles/Blowing-Bubbles-Main.jpg
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Cohesion and work of adhesion
• Work of cohesion• Work to pull apart a column of water• Work of Cohesion = 2X surface energy of liquid• Wc= 2γL
• Work of adhesion• Spreading (work of adhesion must be greater than the work of cohesion)• Dupre’equation
• Wa=γA+γB -γAB
• Work of Adhesion = γgL x (1 +cos θ) (theta equals contact angle of droplet)
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Probing surface chemistry using the contact angle method
http://www.attension.com/$2/sessiledrop3angles-1.jpg
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Surface issues with adhesion– the good and bad• Roughness
• Macroscale roughness to nanoscale roughness
• Contamination (surface different from bulk)• Arising from “air”
• Aging• Contamination as well as migration of extractives
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Native lumens also can be textured “warty layers” etc.
Engineered Wettability in Tree CapillariesMika M. Kohonen*Langmuir 2006 22 (7), 3148-3153
Singh et al. High variability in the thickness of the S3 layer in pinusradiate tracheids. Holzforschung, 2002 56:111-116.
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All wood surfaces are variable at the micro down to the molecular level
www.branhamcorp.com http://www.oakwoodveneer.com/tips/cuts.html
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Wood surface impacted by processing history
Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002
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Wettability changes as a function of heating temperature
Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002
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Wood surfaces are not static either
From “effects of surface ageing on wettability, surface energy, and adhesion of wood” by M. Gindland co-workers published in Holz RohWerkst (2004) 62:273-280
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Surface chemistry correlates with adhesive performance – “can’t bond a bad surface”
Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002