BONDING AND DEBONDING MECHANISM OF PRESSURE SENSITIVE ADHESIVES A thesis submitted for the degree of Doctor of Philosophy By SAMUEL AKOGYERAM Department of Engineering and Design, Brunel University, Uxbridge, Middlesex. June 2010 Brunel University, Uxbridge, Department of Engineering and Design; June 2010. BONDING AND DEBONDING MECHANISM OF PRESSURE SENSITIVE ADHESIVES Samuel AKOGYERAM ABSTRACT Pressure-sensitive adhesives (PSAs) are complex macromolecular-based blend formulations that, in dry form will adhere permanently to diverse surfaces with the application of mere finger pressure. This thesis addresses the bonding and debonding mechanisms of coated films of different commercially available PSAs by systemically investigating the film characteristics on multiple levels. The methods implemented involve a novel procedure in investigating viscoelastic properties with Dynamic Mechanical Analysis, film surface chemistry with Time-of-flight Secondary Ion Mass Spectrometry and film morphology, modulus and bonding characteristics with Atomic Force Microscope. The theoretical aspect invoked rubber elasticity, viscoelasticity and thermodynamic concepts in representation of film morphology with corresponding adhesion nature. The results indicate that the bonding and debonding behaviour of PSA films are of a viscoelastic nature, dictated mainly by two fundamental morphological elements. These elements are; (1) the formation of phase-separated self-assembly of polystyrene-rich- copolymer nano-domains within the adhesive matrix and (2) the inter-linking of the nano- domains by elastically active elastomer segments into a physical crosslinked network system that is highly efficient in dissipating large strain energy. These morphological factors are manifested through a profound contribution to the peel strength of the adhesive films when either coated at high temperatures or annealed. Increasing the content of the polystyrene endblock-tackifier in the adhesive blend formulation increased the PSA’s performance sensitivity to the film coating temperature. Meanwhile increasing the cis-C=C bond concentration in the formulation reduced the film’s performance sensitivity to coating temperature, as polydienes are premised to promote the entropy-elasticity of the film matrix by contributing to the nano-domain interconnections. This thesis generates many qualitative similarities, despite the significantly different adhesive blends investigated and hopefully the results reported here are more universal than one might expect. 1 Table of Contents Abstract 1 ............................................................................................................................................................................. .............. Table of Contents................................................................................................................3 List of Tables.......................................................................................................................8 List of Figures.....................................................................................................................10 Acknowledgements.............................................................................................................19 Chapter 1 Introduction 20 1.1 General Introduction ......................................................................................... 20 1.2 Pressure-sensitive adhesives .............................................................................. 22 1.3 Thesis Structure ................................................................................................. 25 1.3.1 Aims of the Thesis ......................................................................................... 25 1.3.2 Outline of the Thesis ...................................................................................... 25 Chapter 2 Polymers, Block copolymer blends and pressure sensitive adhesive 27 2.1 Introduction ........................................................................................................ 27 2.2 Polymers and copolymers .................................................................................. 27 2.2.1 Polymers ......................................................................................................... 27 2.2.2 Copolymers .................................................................................................... 31 2.3 Polymer mixing and phase separation ............................................................. 32 2.3.1 Entropy of mixing .......................................................................................... 32 2.3.2 Enthalpy of mixing ......................................................................................... 36 2.3.3 Phase separation ............................................................................................. 39 2.4 Block copolymer self-assembling and mesophase ........................................... 42 2.4.1 Block copolymer self-assembly in solution ................................................... 45 2.4.1.1 Diblock versus triblock ........................................................................... 48 2.5 Viscoelasticity of PSAs ....................................................................................... 52 2.5.1 Elasticity of polymer network systems .......................................................... 55 2.6 Pressure-sensitive adhesives .............................................................................. 59 2.6.1 Principles of development of PSAs................................................................ 59 2.6.1.1 Role of Styrene ....................................................................................... 60 2.6.1.2 Role of Tackifiers ................................................................................... 60 2.6.2 Role of mineral oil ......................................................................................... 65 2.6.3 Current understanding about structure of PSA and theory ............................ 66 2 Chapter 3 Materials and characterisation of coated hot-melt PSA films 71 3.1 Materials ............................................................................................................. 71 3.1.1 The PSAs ........................................................................................................ 71 3.1.1.1 The triblock copolymers ......................................................................... 71 3.1.1.2 The tackifying resins ............................................................................... 72 3.1.1.3 Blending oil............................................................................................. 73 3.1.1.4 The formulation process ......................................................................... 74 3.1.2 The non-woven fabrics ................................................................................... 76 3.1.2.1 Cotton fabric ........................................................................................... 76 3.1.2.2 Nylon ...................................................................................................... 76 3.1.2.3 Microfibre ............................................................................................... 77 3.2 Characterisation ................................................................................................. 78 3.2.1 Dynamic mechanical analysis ........................................................................ 78 3.2.1.1 Thermorheological spectrum typical for linear amorphous polymers .... 80 3.2.2 Rotational Rheometer..................................................................................... 81 3.2.3 Peel test .......................................................................................................... 83 3.2.4 Scanning electron microscope........................................................................ 85 3.2.5 Atomic force microscope ............................................................................... 87 3.2.5.1.1 The AFM set-up ................................................................................. 88 3.2.5.2 Modes of imaging ................................................................................... 90 3.2.5.3 Advantages and disadvantages ............................................................... 93 3.2.5.4 Colour contrast assignment of domain regions....................................... 94 3.2.5.5 Morphology investigation with the AFM ............................................... 96 3.2.5.5.1 Micro scale domain size quantification .............................................. 97 3.2.5.6 Nano scale analysis ................................................................................. 99 3.2.6 AFM force-distance measurement ................................................................. 99 3.2.6.1 Adhesion force measurement with the AFM ........................................ 102 3.2.6.2 Elastic modulus measurement with the AFM ....................................... 104 3.2.7 Time –of-flight static secondary ion mass spectrometry ............................. 105 3.2.8 Static limit of SIMS ..................................................................................... 109 3.2.9 The Mass analyzer........................................................................................ 109 3.2.10 Problem with surface charging..................................................................... 109 3.2.11 Film chemical nature characterisation with ToF-SSIMS ............................. 110 3 Chapter 4 Classical investigation of the mechanical properties of PSA and the peel strength to non-woven fabrics 112 4.1 Introduction ...................................................................................................... 112 4.2 Experimental .................................................................................................... 113 4.2.1 Sample preparation and characterization ..................................................... 113 4.2.1.1 Dynamic shear modulus measurement ................................................. 113 4.2.1.2 Tensile testing ....................................................................................... 113 4.2.1.3 Peel Test ................................................................................................ 114 4.3 Results and Discussion ..................................................................................... 114 4.3.1 Dynamic shear moduli of DF645 bulk ......................................................... 114 4.3.2 Tensile test ................................................................................................... 117 4.3.3 Peel test measurements with non-woven fabrics. ........................................ 120 4.3.3.1 Effect of the film coating temperature .................................................. 120 4.3.3.2 Effect of the adhesive bonding rate ...................................................... 121 4.3.3.3 Effect of the substrate surface. .............................................................. 122 4.4 Summary ........................................................................................................... 123 Chapter 5 Thermal Phase Transition and Viscoelastic Behaviour of Styrene-based hot-melt Pressure Sensitive Adhesive Films 125 5.1 Introduction ...................................................................................................... 125 5.2 Experimental .................................................................................................... 126 5.2.1 Materials and sample preparation ................................................................ 126 5.2.2 DMA measurement ...................................................................................... 128 5.3 Results and Discussion ..................................................................................... 129 5.3.1 Thermal dynamic mechanical studies of the constituent block copolymers 129 5.3.2 Thermal dynamical mechanical studies of the constituent middle block and end block tackifiers .................................................................................................... 132 5.3.3 The effect of coating temperature on phase transitions of DF645 coated films. 133 5.3.3.1 Self-assembling of multi-phase structure within coated films of hot-melt PSAs 137 5.3.4 Activation energy involved with T s of DF645 coated films. ..................... 140 g 5.3.5 Phase transitions of three different adhesive blends films and the relationship to peel strength ........................................................................................................... 144 5.3.5.1 The phase transitions of the adhesive films coated at low temperatures 144 4 5.3.5.2 The phase transitions of the adhesive films coated at high temperatures 148 5.4 Summary ........................................................................................................... 150 Chapter 6 Micro- and nanoscale morphology investigation of hot-melt PSA films with atomic force microscopy 152 6.1 Introduction ...................................................................................................... 152 6.2 Experimental .................................................................................................... 153 6.2.1 Sample preparation and characterisation ..................................................... 153 6.2.1.1 Atomic force microscope ...................................................................... 153 6.2.1.1 ToF S-SIMS analysis ............................................................................ 154 6.3 Results and Discussion ..................................................................................... 154 6.3.1 Effect of coating temperature on microscale film morphology ................... 154 6.3.1.1 DF645 adhesive coated films ................................................................ 154 6.3.1.2 7M8 adhesive coated films ................................................................... 159 6.3.1.3 DFC600 adhesive coated films ............................................................. 163 6.3.2 Effect of coating temperature on nanoscale film morphology ..................... 165 6.3.2.1 DF645 adhesive films ........................................................................... 165 6.3.3 7M8 adhesive films ...................................................................................... 167 6.3.3.1 DFC600 adhesive films ........................................................................ 171 6.3.4 Discussion of film morphologies and the impact on adhesive properties .... 174 6.3.5 Effect of annealing on film morphology ...................................................... 182 6.3.5.1 Micro scale film morphology ............................................................... 182 6.3.5.2 Nano scale film morphology................................................................. 183 6.3.6 Chemical group analysis on surfaces of coated films .................................. 187 6.4 Summary ........................................................................................................... 194 Chapter 7 Adhesion force and elastic modulus of coated hot-melt pressure-sensitive adhesive films on a nano-scale level 196 7.1 Introduction ...................................................................................................... 196 7.2 Experimental .................................................................................................... 197 7.3 Results and Discussion ..................................................................................... 197 7.3.1 Effect of film coating temperature on adhesion force .................................. 198 7.3.1.1 7M8 adhesive series .............................................................................. 198 7.3.1.2 DF645 adhesive series .......................................................................... 205 7.3.2 The effect of the film annealing condition on adhesion force ..................... 208 7.3.2.1 7M8 and DF645 annealed samples ....................................................... 208 5 7.3.3 Quantification of adhesion force and thermodynamic work of adhesion .... 212 7.3.1 Studies and discussions of the complex nature of the retraction force curves 217 7.3.1.1 Role of rubbery phase ........................................................................... 217 7.3.1.2 Role of PS domains............................................................................... 223 7.3.1 Elastic modulus determination of thin coated films ..................................... 225 7.4 Summary ........................................................................................................... 228 Chapter 8 Conclusion and future work 231 8.1 Conclusions and Future work ......................................................................... 231 8.1.1 Conclusions .................................................................................................. 231 8.1.2 Future work .................................................................................................. 236 8.1.2.1 Dynamic viscoelastic studies of the network system............................ 236 8.1.2.2 AFM morphology studies of the network system ................................. 237 8.1.2.3 Bonding between PSA and non-microfibre .......................................... 238 6 List of Tables Table 2.1: Solubility parameters of some selected materials at 25oC. ................................ 42 Table 2.2: Diblock versus triblock copolymer gel systems .................................................. 50 Table 3.1: Block copolymer characterisation data ............................................................... 72 Table 3.2: The subcomponents of the various PSA formulations with their total weight percentages (wt-%). ..................................................................................................... 75 Table 3.3: Characteristics of common techniques for imaging and measuring surface morphology271 .............................................................................................................. 94 Table 4.1: Mechanical properties of the investigated annealed adhesive bulk samples by Tensile Testing ............................................................................................................120 Table 4.2: Peel strength of DF645 adhesive films bonded to cotton fabric, at peel rate of 300 mm.min-1. ............................................................................................................121 Table 4.3: Average peel force as function of the adhesive bonding rate ..........................121 Table 5.1: The glass transition temperatures of the subcomponents and the adhesive samples of the DF645 formulation. ...........................................................................137 Table 5.2: The T values of the rubbery matrix at different frequencies ..........................142 g Table 5.3: T values of the PS-rich domains at different frequencies ................................143 g Table 5.4: The main phase transitions of the different PSA films coated at low temperatures. ............................................................................................................146 Table 5.5: Peel strength of films of the three hot-melt blends coated at low temperatures ....................................................................................................................................147 Table 5.6: Major phase transitions of hot-melt PSA films coated at high temperatures. .149 Table 5.7: Peel strength of films of the three hot-melt blends coated at high temperatures ....................................................................................................................................150 Table 6.1: Estimation of PS-rich-copolymer domain population, diameter and inter- domain distance of the adhesive films coated at different temperatures. ...............179 Table 6.2: Estimation of PS-rich-copolymer domain volume fraction and surface area per 1µm x 1µm film surface area, in relation to film coating temperature. ....................182 Table 6.3: Effect of annealing condition on the nano scale PS-rich-copolymer domain characteristics. ...........................................................................................................186 Table 6.4: Estimation of nano-domain volume fraction and surface area per 1 x 1 µm2 annealed film surface area. ........................................................................................187 7 Table 6.5: The surface energies of some of the constituents in the adhesive formulations are listed. ....................................................................................................................191 Table 7.1: The results from the AFM force-distance measurements presenting the quantification of the adhesion force and energy values. ..........................................217 Table 7.2: Quantification of the elastic moduli for DF645 adhesive coated films. ............228 8 List of Figures Figure 1.1: The original temporary dipole of the electron cloud to the left induces temporary dipole with opposite charge to the approaching molecule to the right, generating Van der Waals attractive force between the two atoms11 ....................... 22 Figure 2.1: An illustration depicting a polymer with ordered or crystalline-like regions mixed together with disordered or amorphous regions35. .......................................... 29 Figure 2.2: Classification of polymers34 ............................................................................... 31 Figure 2.3: Microstructure of olefin block copolymer typically used for pressure-sensitive adhesive application .................................................................................................... 32 Figure 2.4: Schematic of solvent-solvent, solvent-polymer and polymer-polymer arrangements in a lattice of N cells, a visual illustration of combinatorial entropy42. 36 Figure 2.5: Schematic diagram of the two types of phase boundaries commonly encountered in polymer solutions. The two-phase regions characterised by the upper critical solution temperature (bottom shaded region), and the lower critical solution temperature (upper shaded region), with a single phase region, lying between the two34. ...................................................................................................... 38 Figure 2.6: Strong segregation limit (SSL) equilibrium morphologies for (A-B) type block n copolymers. The order-disorder transition compositions shown apply to polystyrene- polyisoprene diblock copolymers where f corresponds to the polystyrene volume A fraction87. ..................................................................................................................... 44 Figure 2.7: The additional microstructures of diblock copolymer self-assembly to the classical ones72. ............................................................................................................ 45 Figure 2.8: Possible micelles and aggregates formed by diblock and triblock copolymers in selective solvents. (a) Solvent is selective towards endblock or B block in case of AB and BAB block copolymers. (b) For high loop/bridge ratio, flower-like micelles in case of BAB triblock copolymer where solvent is selective towards A block. (c) A block selective BAB triblock with some free B block extending into solution. (d) A block selective BAB triblocks, branched structure formed where strands of B blocks extending into the core of different micelles102. ......................................................... 47 9