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Organization of hydrogen energy technologies Organization of hydrogen energy technologies

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No. ESF/2004/2.5.0-K01-045No. ESF/2004/2.5.0-K01-045

Main organization - Lithuanian Energy Institute

Partner - Vytautas Magnus University

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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

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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

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The Theory of Adhesion: Contact angle equilibrium

Young’s equation :

γsa = γlv cos θ + γsl

Harkin’s and Livingston’s solid-“air-plus-vapor” tensor and modified Young’s equation:

γsv = γsa – πe → γsa = γlv cos θ + γsl + πe

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The Theory of Adhesion: Work of Adhesion

The work of adhesion (Wa) 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:

Wa = γlv (1 + cos θ) + πe

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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

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The Theory of Adhesion: Types of Adhesion failure

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Adhesion Testing: Cross-Cut test

A grid pattern of cuts trough the coating is made. A standard tape is applied, then quickly stripped

classification 0 1 2 3 4

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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

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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

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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

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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

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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

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Experimental results: Cross-Cut test results (classification)

Air O2 N2

1 2 3 3

5 1 1 1

10 2 1 2

15 0 1 2

20 1 1 2

Time, minGases

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Experimental results: Optical microscopy photos (air 1 min)

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Experimental results: Optical microscopy photos (air 5 min)

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Experimental results: Optical microscopy photos (air 10 min)

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Experimental results: Optical microscopy photos (air 15 min)

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Experimental results: Optical microscopy photos (air 20 min)

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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