Adhesion and cohesion, Liquid penetration CHEM-E2135 - Converting of Web-Based Products Spring 2019 Jouni Paltakari, Eero Hiltunen 1 Why glue is gluing? • Adhesion: – ”how well glue is attached to the substate surface” – Work needed to separate glue layer from the substrate surface • Cohesion: – ”Internal strenght of glue” – Work needed to split the glue layer 2 Adhesion • ”State in which dissimilar bodies are held together by intimate interfacial contact so that mechanical force can be transferred across the interface”. • Force/work required to separate the bodies. 3 Work of adhesion e.g. glue Substrate B Dupré equation where Is surfacepintaenergia energy Work of adhesion, Wa, is needed to separate dissimilar bodies along their interface, to form two separate material-air interfaces. 4 Work of cohesion e.g. glue two glue ”piece” ”pieces” where Is surface energy Work of cohesion, Wc, is needed to form two air-body interfaces from one body. 5 Cohesion and Adhesion: case liquid state • Molecules in the liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. • For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension. 6 H2O is a polar molecule H2O is a polar molecule, with an electrical dipole moment The oxygen atom has a higher electronegativity than hydrogen atoms By User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia to Commons by sevela.p. Translated to english by by Michal Maňas (User:snek01). Vectorized by Magasjukur2 - File:3D model hydrogen bonds in water.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/ind 7 ex.php?curid=14929959 Cohesion and Adhesion: case liquid state • When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces which leads to an upward turning meniscus at the walls of the vessel and contribute to capillary action. • The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds. 8 Adhesion-Cohesion-Surface tension • Cohesion causes water to form droplets • Surface tension causes them to be nearly spherical • Adhesion keeps the droplets in place 9 Adhesion is a critical factor in several papermaking and converting processes • Fiber-fiber bonding • Retention of size and fillers to paper structure • Side gluing of sacks and boxes • Combining liner and fluting by gluing in corrugated board manufacturing • Combining plastic film and paper in extrusion process • Attachment of coating material to the substrate in dispersion-, pigment-, or hot-melt coating • Interaction between label and release paper and sticking of label to final surface • Gluability of wall paper to the wall surface material 10 Theories of adhesion Mechanical interlocking ”Specific theory” Thermodynamic Electrostatic Chemical Weak boundary Diffusion Polarisation adsorption theory adhesion theory Induced Hydrogen Dipole dipole bonding 11 Theories of adhesion Understanding of wetting, adhesion and surface phenomena has increased a lot in recent years BUT There is still no single theory explaining adhesion completely! 12 Mechanical interlocking • Main mechanism in adhesion is the penetration of one component into irregularities in the other surface. • Phenomenon with porous substrates; e.g. wood, paper, textiles. Example: Penetration of size/glue into the paper structure and adhesion when glue is hardening. 13 Peel strength vs. PE-LD coating weight on three papers with different surface smoothness (calendering treatment). Filled symbols indicate paper matrix failure. 14 Diffusion theory • Compatible materials with equal solubility parameters can form a transition zone where interdiffusion of macromolecules or molecule segments can occur. • Close molecular contact by wetting is necessary for interdiffusion. • Adhesion due to diffusion only occurs with identical or compatible polymers. Examples are adhesion between different layers in coextrusion or autohesion (layers of the same polymer) • Segmental interdiffusion and an interfacial layer of 10-1000 Å forms between two incompatible polymers. • Other important cases where diffusion occurs are the heat sealing of thermoplastics and the film formation of latexes Not present, when adhesion to metal or glass surface Probably not a main mechanism for fibre surfaces Possible fibre examples: * Diffusion of size with small molecules to fiber cell wall in paper * Strenght between board layers (diffusion of hemicellulose) 15 Wetting • The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces • Wetting is important in the bonding or adherence of two materials • As the tendency of a drop to spread out over a flat, solid surface increases, the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability • A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable • For water, a wettable surface may also be termed hydrophilic and a nonwettable surface hydrophobic Water beads on a fabric that has been made nonwetting (hydrophobic) by chemical treatment16 Wikipedia – Creative Commons Diffusion theory – example: adhesives on PET Figure 2. Peel strength and solubility parameter difference for various adhesives on poly (ethylene terephthalate).17 Electrostatic theory • Idea of an electrical double layer forming when different materials come into contact. • Adhesive and substrate act as a capacitor, and adhesion is an attractive force across the double layer. • Has a role in polymer-metal adhesion bonds No big meaning for fibre surfaces 18 Thermodynamic adsorption (or Wetting theory) • Wetting is a precondition for adhesion in this instance • Interdiffusion is not necessary to achieve good adhesion • Close contact and molecular and physical interactions between materials are essential to adsorption theory Has a big meaning for fibre surface 19 a) Covalent b) Hydrogen bond c) Van der Waals nm 1Å = 0.1 nm Figure. Potential energy curves for different types of interaction/interatomic forces. 20 Contact angle (q), Surface energy (g), Work of adhesion (Wa) Vapour gLV gSV = gSL + gLVcosq Liquid q gSV g – g Solid g SV SL SL cosq = gLV Wa = gLV (1+ cosq) 21 DEFINITIONS: WETTING: Wetting refers to the intermolecular interactions when forming an interface (liquid-solid). Wetting occurs if change in free energy <0. A system always tries to reach a lower energy state. The amount of wetting depends on the energies (or surface tensions) of the interfaces involved such that the total energy is minimized. The criteria for wetting, formation of a contact angle; work of adhesion is greater than zero . DGw = Gafter –Gbefore = gSL-(gS+gL) = -Wa<0 => Wa>0. Contact angle with water q >0 (0< q <90°). Precondition for wetting: Work of adhesion > work of cohesion, if liquid is wetting the surface but not spreading over it 22 DEFINITIONS: SPREADING: Spreading refers to the replacement of the solid surface by a solid with liquid interface and a liquid surface after the contact. DGS = Gafter –Gbefore = (gSL+gL) -gS <0 gSLis often small compared to gL à spreading occurs if liquid surface energy is lower than that of a solid. Spreading occurs when the contact angle is zero (q =0) and also indicates the movement of liquid regardless of the contact angle. 23 Hysteresis and metastable state • Hysteresis is the difference between advancing and receding angles. • If the size of a liquid drop on a solid increases or decreases, the same contact angle does not occur. • Common reasons for hysteresis are – roughness – Morphological and chemical heterogeneity of the surface, – interactions between the solid and liquid – variation of areas with low and high free energy on a surface. 24 Figure 5. A drop of liquid in two metastable configurations. 25 Chemical adhesion • In addition to van der Waals forces, stronger metallic bonds exist: – ionic – covalent • Work of adhesion can be 25-40 times larger – metallic bonds 26 Bond energy Bond length Bond (kJ/mole) 8* (Å) 2 chemical covalent 63-710 1-2 ionic 590-1050 metallic 113-347 van der Waals 3.6 dispersion 0.08-42 dipole-dipole 4-21 dipole-ind. dipole max. 2 hydrogen 2.7 involv. fluorine max. 42 excluding fluorine 10-26 * Bond energies originally from Pauling and Good according to Kinloch 8. Table. Various bonds, their energies, and lengths. 27 Examples • Chemical bonding: Interfaces between a polymer and metall. • Covalent and ionic bonding: formation of an ester from an anhydride in a tie resin with the hydroxyl group in ethylene vinyl alcohol during coextrusion. • Example of covalent bonding is between oxygen plasma treated paper and untreated polyethylene due to the hydroperoxides formed on the paper surface. 28 Weak boundary layer theory • Adhesion based on molecular interaction forces: – Dispersion forces – Hydrogen bonds – Induction forces Theory states that a failure within a WBL near the interface causes poor adhesion. WBL theory describes differences in bonding Has a big meaning for paper products 29 Weak boundary layer theory • The surface of paper can have a WBL due to fewer or less bonded fibers. • Practical examples of the WBL theory: – linting – adhesive bonding – paper coating – In pigment coating, polyvinylacetate as a binder forms WBL 30 Testing of adhesion and cohesion in practise 31 ADHESION-COHESION TESTING PEEL TEST Figure 8. Peel test configurations. 32 Value Criteria of evaluation 0 layers do not adhere 1 layers peel off each other 2 layers peel off each other, some fibers are removed 3 fiber tear < 50% of surface area 4 fiber tear > 50% of surface area 5 total fiber tear Table. Criteria of evaluation in hand test. 33 Interaction of liquids with porous materials (paper, board) • Essential event in, e.g. – printing – surface sizing – coating – impregnation Driving force • Dynamic event: Rate = Resisting force • Lucas-Washburn equation & Hagen-Poiseuille eq.