Joining of Composites

7/28/2019 Joining of Composites

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JOININGOFCOMPOSITEMATERIALS

AND TYPESOFADHESIVES

LTFYE ETN2008286011


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http://www.youtube.com/watch?v=HOjy_bRyUh0
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Adhesion is defined as the adhesive force acting betweenthe adhesive and the surface ofthe material. This force is the result of the mechanicalinterlocking between adhesiveand the material surface roughness (mechanical

adhesion) as well as the physical and/orchemical interaction between the adhesive and thematerial (specific adhesion).Cohesion is the strength of the adhesive itself. This is a result

of the mechanicalentangling and interlocking of the adhesive moleculesand their physical and/orchemical affinity for each other


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This theory explains adhesive attraction forces in terms ofelectrostatic effects at an interface.This is based on the phenomenon of a electrical double

layer formed at the junction of twomaterials. At any boundary an electrical double layer isproduced and the consequentcoulombic attraction might account for the adhesionand resistance to separation.


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In this theory adhesion is attributed tointermolecular entanglements at the interface.

This is applied to the union of high polymers. The

fundamental concept is that adhesion

arises through the interdiffusion of theadherend and the adhesive. It is based upon

the chain

nature of the structure with the consequent

flexibility and the ability of the chains to

undergo

Brownian movements on a sub-molecular

scale.


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When the adhesive is applied in solution

(most probably) and if the adherend is

sensibly soluable in the solvent the substrate

molecules will also diffuse to an appreciable

extent into the adhesive layer. Overall the

clean-cut boundary between the adherend

and the adhesive disappears and is replacedby a

layer representing a gradual transition from

one polymer to the other. A major difference

inthis theory is that it implies a three dimensional

volume process rather than a two

dimensional surface process.


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Adhesive joints are composite systems

whose strength depends on both the

geometrical

design and loading type as well as on theschematically illustrated individual strengths

of the components to be joint, the adhesive

and the interface layer.

As in every composite system consisting ofdifferent members, the overall strength is

limited by the weakest member


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The intermolecular bonds (secondary valency

bonds) act between the adhesive

molecules as well as between adhesive and

the surfaces of the joining parts and are thusrelevant for the cohesion and adhesion

strength


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Numerous adhesives contain polar molecule

groups (dipoles) which have a strong

polarising action on the metallic joining parts,

the latter being non-polar in themselves.The dipole forces can operate effectively

only if these molecule groups can approach

to

within about 0.1 mm of the surface of thejoining parts Theabove

is only possible if the adhesive can wet the

solid surfaces optimally


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Hydrogen bonds are a special form of

intermolecular bonds. These are, for example,

responsible for the relatively high cohesion

strengths of PUR and PA adhesives

Hydrogen bonds can also be formed between

adhesives and solid surfaces when thelatter are oxidised or contain adsorbed

hydrogen molecules.


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Adhesives can be classified into three main

types given below. There are of course several

products that are combinations of these threetypes but essentially all adhesives can be

grouped into these categories.


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Basically an adhesive of this type is supplied in a

low molecular weight form and after

application a polymerisation reaction is allowed

to take place. This polymerisation can beachieved by:


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Two component pack:

Supply the produce as a two component pack, i.e.base plus hardener. Examples of this type

are: Epoxy adhesives based on epichlorhydrin bisphenol.

Cured with amines or polyamide. Phenolics adhesives i.e. a novalac type with

hexamethylene tetramine. Unsaturated polyesters using an organic peroxide,

i.e. M.E.K. peroxide and cobalt naphthenate. Polysulfide with lead dioxide or an isocyanate.

Polyurethanes with isocyanate. Silicone polymers utilising a metal salt of' an organic

acid, e.g. lead octoate. Reactive acrylics - rely on peroxides or amines.


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Moisture:

Polymerisation can be achieved by relying onmoisture either on the surface of the adherend

or in the atmosphere to effect a cross-linkingmechanism on some other "natural" component.

In this case the adhesive is supplied as a singlecomponent. Examples are:

Polyurethane containing an isocyanate group. Cyanoacrylates. These are the instant bond

adhesives. Silicones containing an acetyle group. These are

the common R.T.V. silicones which

with moisture releases acetic acid causing a cross-linking of the paste to an elastomer. Anaerobic which rely on absence of oxygen.


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Heat:

The final method of curing a chemical reactive type isby utilising heat to polymerise the

adhesive components. Examples are: Expoxies with the catalyst incorporated in the

adhesive in a latent form, e.g. dicyandiamide which will require a temperature of

175o C to effect a cure.

Phenolics of the resole type. Polyvinyl acetates which are based on polyvinyl

alcohol reacted with an aldehyde. The conversion is normally about 80% and on heating

after application the crosslinking

is completed. Urethanes incorporating a blocked isocyanate. The

free isocyanate groups are all

reacted with a temporary blocking agent such asphenol which is stable up to 150oC.


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Basically the adhesives in this class are thermoplasticin nature which means they are heatedto a sufficient temperature where they will flow andwet the substrates and then set and

develop the bulk strength on cooling. The ideal HotMelt adhesive is a solid up to atemperature of 80o C (as a minimum) but will thenmelt sharply to give a low viscosity fluidthat is easily applied and capable of wetting the

adherend followed by rapid setting uponcooling. They normally contain a base high molecularweight polymer together with tackifying resins andviscosity depressants.


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Examples of polymers that are used as the base for Hot Meltadhesives are:

Ethylene vinyl acetate - a polyethylene chain containing thehighly polar acetate

group. Ethylene-ethyl acrylate which has an ethyl acrylic grouping.

Ionomers - derived from ethylene acrylic acid copolymers butincluding a metal

cation or some of the pendant carboxyl groups. The metal cation isfree to cross-link

with the anionic side groups similar to a thermosetting resin but thereaction is

thermally reversible.

Phenoxies - similar chemical structure to epoxides.

Polyamides of low to intermediate molecular weight based onthe unsaturated dibasic

acids of vegetable origin. Polyesters (saturated).

Vinyl resins such as polyvinyl acetate, polyvinyl butyral andpolyvinyl ethers - used

in various special areas.


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In adhesives of this class the adhesive polymers

is essentially in its final form however,

wetting of the adherend is achieved by

dissolving or dispersing the polymers in asuitable

solvent.


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Solvent Based Systems:

Rubber adhesives - usually based on an elastomerwhich is either natural or synthetic.

The synthetic rubbers that are used in adhesives arepolychloroprene nitrile (a

copolymer of butadiene and acrylonitrile) butyl (acopolymer of isobutylene and

isoprene) and styrene butadiene rubber. Natural rubberis essential isoprene .

Normally resins, usually phenol-fomaldehyde based, areincorporated.

Phenolic or resorcinced formaldehyde resins dissolvedin a solvent.

Vinyl resins such as polyvinyl acetate, polyvinyl

chloride, polyvinyl ether etc. Acrylic resins based on methyl methacrylate, ethyl

acrylate, acrylic acid etc. Miscellaneous resins such as cellulose acetate and

polyamides.


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Water Based Systems:

There are very few polymers of sufficient

molecular weight to be attractive as adhesivesthat

will dissolve in water. However, dispersions oremulsions are very important. Examples are:

Rubber latices - again either natural orsynthetic such as polychloroprene, nitrile or

styrobutadiene rubber.

Vinyl resins, where polyvinyl acetate

emulsions are very widely used. Acrylic resins which offer some advantages

over PVA types such as water and

solvent resistance.


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Thermosetting plastics - The basic molecules cross-linkacross a number of free mainvalencies to a spatial molecule structure. This results inhigh strengths and rigidities.

Thermoplastics have a linear molecular structure (string-like macromolecules). A largenumber of molecule strings are held together byphysical secondary valency bonds.Amorphous thermoplastics have a "cotton wool"

structure. In semi-crystallinethermoplastics, parts of the microstructure depict adefinite structural arrangement sothat the attractive forces in these regions are more

intensive than in the amorphous areas


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An adhesive bonded joint has many good

properties. To make the most

of these properties, it is important to think

about adhesive bonding at

the design stage.


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Various materials can be joined Galvanic corrosion can be avoided Joint is permanent Makes structures stronger and more rigid Distributes load and stress more evenly in the joints Stress concentrations can be avoided. The adhesive seals and bonds and prevents

crevice corrosion

Low costs for finishing Good fatigue characteristics Dampens vibration Reduces weight and number of required

components


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Should not be handled directly after bonding High temperature results in reduced strength A bonded component is difficult to

disassemble for repair and service Pre-treatment of the surface before bonding

is essential for structural bonding and to obtain satisfactory quality in

corrosive environments It is necessary to ensure that the adhesive

completely covers the surface Potential health, environmental, and safety

issues


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1. Influence of time on process properties 2. Pretreatment of joining parts surfaces 3. Limited form stability 4. Process parameters must be held within very narrow range;low tolerance 5. Change of properties of joint with time (ageing of adhesive layer etc.) 6. Complicated control of process 7. Low peeling strength, creep sensitive

8. Low adhesive layer strength must e compensated by large joining area 9. Repair possibilities limited 10.Complicated strength calculation


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http://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdf

http://www.alueurope.eu/talat/lectures/4701.pdf

http://nzic.org.nz/ChemProcesses/polym

ers/10H.pdf

http://ebookbrowse.com/assembly-

adhesive-bonding-pdf-d313133029
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Joining of Composites