1 Organization of hydrogen energy technologies training No. ESF/2004/2.5.0-K Main organization - Lithuanian Energy Institute Partner - Vytautas Magnus University

2 Introduction to the topic: In recent years we have witnessed a remarkable growth in the use of synthetic organic polymers in technology, both for high-tech and for consumer-product applications Polymers have been able to replace more traditional engineering materials such as metals, on account of their many desirable physical and chemical characteristics (high strength-to-weight ratio, resistance to corrosion, etc.) and their low cost Fundamental differences between polymers and other engineering solids have also created numerous important technical challenges An important example is the characteristic low surface energy of polymers and their resulting intrinsically poor adhesion

3 Formulation of the problem: Investigate general terms of adhesion theory Review methods of adhesion testing Overlook methods of surface treatment of plastics; advantages of plasma technologies Examine plasma effects on polymer surfaces Review experimentally received results for adhesion changes of polymers affected by plasma

4 The Theory of Adhesion: Contact angle equilibrium Young’s equation : γ sa = γ lv cos θ + γ sl Harkin’s and Livingston’s solid-“air-plus-vapor” tensor and modified Young’s equation: γ sv = γ sa – πe → γ sa = γ lv cos θ + γ sl + πe

5 The Theory of Adhesion: Work of Adhesion The work of adhesion (W a ) is the thermodynamic work necessary to separate the liquid from the solid without performing any additional work When a liquid is separated from a solid, new solid-air and liquid-air interfaces are created, and solid interface is destroyed. This is described by Dupré equation: W a = γ lv (1 + cos θ) + πe

6 The Theory of Adhesion: Theoretical adhesion models The absorption or chemical reaction theory, which states that bond strength is mainly determined by physi- or chemisorption at the interface The electrostatic theory of Deryagin, which is based on contact charging when two dissimilar materials are intimately joined The diffusion theory of Voyutskii, which claims that the bond strength of polymers is governed by diffusion rate across their interface The rheological theory of Bikerman, which states that the performance of bonded system is governed by the mechanical properties of the materials comprising the joint, and by local stresses in the joint The mechanical interlocking or “hooking” theory, based on the micro-geometry of the interface

7 The Theory of Adhesion: Types of Adhesion failure

8 Adhesion Testing: Cross-Cut test A grid pattern of cuts trough the coating is made. A standard tape is applied, then quickly stripped classification

9 Adhesion Testing: Tensile Methods A stud, normally made of steel, is glued with the coating and is subjected to axial or radial tension until detachment of paint film occurs. The adhesion strength is the maximum tensile stress that is possible

10 Adhesion Testing: Indentation debonding A needlelike indenter is perpendicularly pressed into the surface of the coating, most of the deformation will occur within the film, but there also will be a certain debonding effect at the interface

11 Methods of polymer surface treatment: Solvent cleaning Detergent cleaning Mechanical treatment Chemical treatment Primers Flame treatment Exposure to Ultraviolet radiation Drying Glow discharge plasma treatment Corona discharge

12 Advantages of low-pressure plasma surface treatment: Four main effects – cleaning, ablation or etching, crosslinking, and surface chemical modification – occur together in a complex synergy Effects only the surface leaving bulk unchanged Can be applied to objects of wide range of sizes and all possible geometries, also including fine particles such as powders, fibers or flakes Causes low or even no environmental problems

13 Experimental scheme: Working gases: air, oxygen, nitrogen Working pressure 2.0 at thermopolic Scale (about few hundreds Pascal's) Exposure time: 1, 5, 10, 15, 20 min

14 Experimental results: Cross-Cut test results (classification) AirO2O2 N2N Time, min Gases

15 Experimental results: Optical microscopy photos (air 1 min)

16 Experimental results: Optical microscopy photos (air 5 min)

17 Experimental results: Optical microscopy photos (air 10 min)

18 Experimental results: Optical microscopy photos (air 15 min)

19 Experimental results: Optical microscopy photos (air 20 min)

20 Conclusions: Despite the fact that there is quite precise definition for adhesion, the fundamental principles of adhesion phenomena is still not very clear There are a lot of methods for adhesion measurements. The appropriate method can be chosen according to polymer and the film properties Several surface treatment exists all of them having own advantages ad disadvantages Plasma treatment can be very promising method for polymer surface activation and modification Experimentally received results confirm the effectiveness of plasma treatment – fine level of adhesion is achieved after 1 minute of the treatment. It is also seen that the effect of the plasma treatment strongly depends on process time and working gas