Chemie des Klebens Chemistry of Adhesives Dr. Jochen Stock, Laboratory Manager CRL Germany: Neuss, November 27 th, 2013 Innovative Technologies 1
Overview Chemie des Klebens Chemistry of Adhesives Introduction Surfaces Physical Curing PSA s Structural Adhesives Fundamentals of Adhesives Primers (Silanes) Polymer Solutions Fundamentals of PSA s Crosslinking of Polymers Adhesion (Surfaces) Surface Treatment (Corona, Plasma) Poly(acrylates) Isocyanates Cohesion (Polymers) Block Copolymers Epoxides UV-Crosslinking 2
The Origin of Adhesive Science Physics Adhesives Science is a highly interdisciplinary science in the center of: - mechanics - physics and - chemistry Chemistry Adhesives Science Polymers Mechanics Focus of this presentation on surfaces and polymers 3
Basics of Adhesives Science Adherent Adhesive Adherent adhesion The stability of a joint bond requires: Strong connection to the surface of the adherent (Adhesion) Surfaces High internal strength of the bond itself (Cohesion) Polymers 4
Importance of Wetting adhesive adhesive Good wetting adhesive fills all irregularities of substrate surface good adhesion substrate substrate Poor wetting adhesive can t fill all irregularities of substrate, e. g. due to formation of voids poor adhesion Adhesion may be improved by adjusting the adhesive or changing the substrate surface 5
Primers and Surface Treatment hydrated surface OH OH OH OH metal oxide (MeO) 3 Si-OR Silanes metal metallic surfaces are normally oxidized and hydrated Reaction of silanes with hydroxy groups results in an re-functionalization of the surface (coupling reaction) Depends strongly on the substrate 6
Corona- or Plasma-Treatment δ+ δ- δ- Corona- or Plasma-Treatment of surfaces is a method to change properties of non-reactive & low surface energy surfaces The creation of highly polar chemical groups on the surface creates a negative electric poential (Zeta-Potential) on the surface and results in hydrophilisation δ- 7
Why is Water not an Adhesive? δ- δ+ δ+ Water is a dipol and forms strong, attractive bonds between the single molecules two glass plates will stick to each other This so-called hydrogen bonding is not permanent, the molecules move relative to each other glass plates can be repositioned in x,y-direction Mobility of water molecules leads to evaporation the glue disappears over time Si - O O - O - - O Si Si - O O - O - - O Si Si - O O - O - - O Si Si - O O - O - - O Si Glass 8
Why are Polymers Adhesives? good cohesion between surfaces of adherents due to intermingling and/or crosslinking of polymeric adhesives good adhesion to surface of adherent, due to multiple contact points per molecule 9
Crosslinking (Curing) of Polymers Crosslinking of polymeric adhesives controls many essential materials properties: cohesion of adhesive mechanical properties in general Crosslinking may be of physical or chemical nature: intermingling of polymer chains (physical) attractive forces of electrical origin (physical) electric charges, dipoles, van der Waals forces crosslinking by branching of growing polymer chains (chemical) crosslinking or curing of (pre-)polymer by special chemical reactions (chemical) 10
Physically Curing Adhesives relative content of solvent Model for polymer solutions (aqueous or in solvent) and for polymer dispersions evaporation of solvent (film formation) evaporation (diffusion of solvent out of the film) During the drying process the solvent evaporates and the polymer coils get closer and closer until the hydrodynamic volumes touch each other Further drying will force the polymers to intermingle and form a solid film Intermingling of polymer chains is an important method of physical crosslinking dry film time 11
Physically Curing Adhesives - Glues Methyl-Cellulose Water soluble due to modification of cellulose (methylation) Strong adhesion to HSE Surfaces (like e. g. paper) by hydrogen bond formation Intermingling of polymer molecules in (aqueous) solution Solidification ( crosslinking ) by evaporation of water Application: wallpaper 12
Pressure Sensitive Adhesives (PSA s) Characteristics of PSA s Permanently tacky at room temperature Adhere firmly to a variety of surfaces just upon finger pressure (no curing mechanism or activation required) Require deformability to adapt to rough surfaces Are applied very often on a carrier (e. g. as a tape) Very often based on poly(acrylates), due to their broad chemical variability Liner Adhesive 13
PSA s Poly(acrylates) Why Poly(acrylates)? Chemical composition and corresponding material properties can by varied in a broad range by copolymerization - glass transition, internal softening Molecular weights and its distribution (correlated with mechanical properties) can be adjusted by conditions of synthesis Introduction of functional monomers enable chemical crosslinking and may improve surface wetting Glass Transition [T g ] 105 C 22 C -8 C -43 C -50 C 14
Radical Polymerization of Acrylates 15
Formation of Copolymers 16
Pressure Sensitive Adhesives (PSA s) isoprene block = elastic block styrene block = hard block S-I-S Block Copolymer formation of physical networks 17
Structural Adhesives Withstand high loading bearing conditions Resistant to environmental exposure, like e. g.: temperature, moisture, UV and chemicals They may consist of one or two parts Amongst adhesive the structural adhesives are the Tough Stuff Extreme requirements ask for extreme strength, which can be only achieved by chemical crosslinking 18
Structural Adhesives Chemistry of Isocyanates Hardening with Water 19
Structural Adhesives Chemistry of Isocyanates Hardening with Water schematic presentation of di-isocyanate reactions......... and subsequent network formation... 20
Structural Adhesives Chemistry of Epoxides - Basics Epoxides react with many different nucleophiles Amines are very fast reaction partners, others (like e. g. alcohols) may need catalysts oder high temperatures Resins based on poly(epoxide)s may form complex adhesives 21
Structural Adhesives Chemistry of Epoxides Curing Reaction + Bisphenol A and ist Diglycidylether form only linear Polymers Introduction of multifunctional components allow branching of the polymer chains 22
Curing by Branching Linear polymer chains can be crosslinked while being synthesized by branching of growing polymer chains 23
Crosslinking of Pre-Polymers Synthesis of (non-crosslinked) pre-polymers Curing and network formation of the polymers by a second component 24
UV-Curing Crosslinking with UV-Radiation - Basics 25
UV-Crosslinking in Poly(acrylate) Benzophenone moieties can be incorporated into polymer chains UV-triggered reaction results in chemical crosslinking 26
Questions? 27