joining of composites
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JOININGOFCOMPOSITEMATERIALS
AND TYPESOFADHESIVES
LTFYE ETN2008286011
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http://www.youtube.com/watch?v=HOjy_bRyUh0
http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://www.youtube.com/watch?v=HOjy_bRyUh0http://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
http://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://nzic.org.nz/ChemProcesses/polymers/10H.pdfhttp://nzic.org.nz/ChemProcesses/polymers/10H.pdfhttp://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://ebookbrowse.com/assembly-adhesive-bonding-pdf-d313133029http://nzic.org.nz/ChemProcesses/polymers/10H.pdfhttp://nzic.org.nz/ChemProcesses/polymers/10H.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.alueurope.eu/talat/lectures/4701.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdfhttp://www.cozybuilders.org/Oshkosh_Presentations/AbarisCompositeBondingOshkosh2005.pdf