23 Polymers & Adhesives 4

8/18/2019 23 Polymers & Adhesives 4

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Polymers


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Polymers:• Defnition

–Engineered materialscharacterized by large moleculesthat are built up by the joining

together o smaller molecules


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Polymers:


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Polymers:

• Advantages: – Lightweight• Specifc gravity 1.11.! compared to "g

1.#$

– Ease o abrication• %an be molded& e'truded& cast& made intothin flms& ( applied as coatings

• %an oten produce products in a singleoperation

• E'cellent asormed suraces )lowfnishing cost*

• Low abrication and tooling cost

• Design versatility


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Polymers: (Advantagescontinued)

– +ood corrosion resistance

– Electrical ( thermal insulator

– ,ntegral color

• %an be transparent

– -ossible ease o assembly• Snap fts and sel tapping asteners


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Polymers:• Limitations

– Low strength )but may be oay instrength to weight ratio*

– Low rigidity

– /ot suitable or elevated temp• $002 is considered upper limit

– -oor wear resistance

– -oor dimensional stability – 3igh thermal e'pansion

– Sensitive to moisture and radiation


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Classifcation o Polymers:

• Polymerization Mechanism: – 4ddition

– %ondensation

• Polymer Structure – Linear& %hain& or 2iber

– /etwor& 2ramewor

• Polymer Behavior

– 5hermoplastic – 5hermosetting

– Elastomer


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Polymerization Mechanisms:

• Addition – "onomers lin together to orm a

polymer with a repeated unit& nownas a mer

– 4ctivators or catalysts initiate andterminate the chain

• 6elative amounts determine the averagelength o the chain or degree o

polymerization )7 o repeated units in thechain*

• Degree o polymerization can range rom#$#$0

–E'ample8 Ethylene

-olyethylene


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Addition mechanism

he unsaturated dou!le !ond in themonomer is !ro"en to #roduce activesites$ %hich then attract additionalre#eat units to either end to #roducea chain


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&nitiation andermination

&nitiation o a#olyethylene chain !ychain'gro%th mayinvolve:

(a) #roducing ree

radicals rominitiators such as!enzoyl #eroide$

(!) attachment o a#olyethylene re#eatunit to one o theinitiator radicals$ and

(c) attachment oadditional re#eatunits to #ro#agatethe chain


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– Co#olymers• 5wo di9erent types o mers are combined

into the same addition chain

• %opolymers oten have more desirable

physical and mechanical properties

• -roperties can be altered by changingproportions o the mers

• E'ample8 :inyl chloride %;3


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Addition erminology:

• 2unctionality8 5he number o siteson a monomer wherepolymerization lins can orm. – 2 = 1 no polymerization

– 2 = ; linear polymers

– 2 = < or more networ polymers

• Degree o -olymerization8 5he

average number o mers in eachpolymer chain – D.-. = "olecular >eight o -olymer

"olecular >eight o "er


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• %ondensation -olymerization – 6eactive molecules combine with one

another to produce a polymer plus a

small byproduct molecule – 3eat& pressure& or catalysts may be

re?uired to drive the reaction

– E'ample8

-henol @ ormaldehyde Aaelite@ 3;B


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CondensationMechanism


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Structure Classifcation:

• Linear8 – %& B& or Si chains with 3& %l& 2 or

benzene ring pendants

– 4ll bonds within the molecule arestrong primary bonds

– ,ntermolecular attraction is by weaer:an der >aals orces

– 4lso mechanical tangling o the fbers

– "echanical model8 the cottonball


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Structure Classifcation:

• 2ramewor – 4ll atoms are connected to one

another by strong covalent bonds toorm a strong


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Classifcation:

(a) Linear. Many van der Waals bonding between the chains hold it together. Examples of linear polymers are

polyethylene, polyvinyl chloride, polystyrene, nylon and the fluorocarbons.

(b) Branched. Sidebranch chains connect to the main ones during synthesis of the polymer. !hese reduces

the pac"ing efficiency, so lower density.

(c) Crosslinked. #d$acent linear chains are actually connected covalently bonding the chains. Many of the rubber

materials consist of polybutadiene crosslin"ed with S atoms, the process is called vulcanisation

(d) Network. Mer units with three active covalent bonds form %& networ"s. e.g. epoxies


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Behavior Classifcation:

• 5hermoplastics – 2or linear polymers& the mechanical

and physical properties are largely

determined by the :an der >aalsorces

– 5hese plastics soten with increasingtemperature and become harder and

stronger when cooled – 5he sotening and hardening can be

repeated indefnitely& and nochemical change occurs


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5hermoplastics )continued*

– Aecause the molecules have di9erentlengths& thermoplastics materials donot have a defnite melting point& butsoten over a range o temperature

– 4s the temperature drops& transitionsoccur

• Li?uidCpour and cast& or injection mold

• 6ubberyClarge amounts o plastic

deormation are possible& can mold ore'trude

• LeatheryCstronger and sti9er

• %rystalline or glassy


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5hermoplastics )%ontinued*

• %ontrolling or modiying properties – "odel with cotton ball

– 4lter properties by interering withslippage between fbers


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5hermoplastics 4ltering -roperties

• ,ncrease the degree opolymerization – Longer chains are stronger

• se nonsymmetrical monomers – Larger side structures )‘bumps”*

• EF8 %l in -:% )polyvinylchloride*

• EF8 Aenzene groups in polystyrene

• %rosslin adjacent chains• -romote crystallization

– 5he parallel alignment o chainsenhances vander>aals bonding


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y#e o monomer


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Crystallization

*) easiest in sim#le monomers+) most li"ely for slo% cooling,) can occur as a result of

deformation


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5hermoplastics 4ltering -roperties

• Aranching – Aranching generally weaens due to

lower density and greater chainseparation

• %old woring – Arings chains into parallel alignment

– Enhances vander>aals interaction

• -roduce %opolymers


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-eormation ohermo#lastics*) Stress initially straightens

out the individual chains+) Continued stress then

causes the chains to slide#ast each other (!rea" .ander /aals !onds)


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Co#olymers


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5hermoplastics )%ontinued*• -rocessing a 5hermoplastic

– Easily molded when hot and sot

– "old must be cooled to induce hardening

– -roduction is slowed by heating and coolingo the mold

• 6ecycling thermoplastics – , the various types o resins and additives

can be separated& many thermoplastics canbe recycled into useul productsCusuallywith some downgrading in use

– EF8 polyethylene )mil bottles* (polypropylene )twoliter soda bottles*


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0orming o hermo#lasticPolymers

y#ical forming #rocesses for thermo#lastic:(a) etrusion$ (!) !lo% molding$ (c) in1ectionmolding$ (d) thermoforming$ (e) calendaring$

and (f) s#inning


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• 5hermosets8 –


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5hermosets )%ontinued*

– Since deormation re?uires breaingo primary bonds• Strong& but brittle

– %ompared to thermoplastics&

thermosets are• Stronger

• Sti9er

• Lower in ductility

• -oorer in impact

– E'amples8 epo'ies& melamine&polyester& phenolics


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5hermosets )%ontinued*

• -rocessing thermosets – "olds operate at constant elevated

temperature& but time is re?uired or the

curing or “setting” reaction – Since the cured material has strength

and rigidity even when hot& productremoval rom the mold is possiblewithout mold cooling


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• Elastomers – 4 special class o linear polymers with

high elastic deormation& such as arubber band

– Elasticity is not the stretching obonds& but the uncoiling o twisted orcurled molecules

– %rosslining can be used to restrict

viscous How and tailor properties• Sot ( He'ible hard ( brittle

• Bnce crosslined& cannot uncrosslin

• E'ample8 :ulcanization o rubber by

sulur crosslins


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2lastomers


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ailoring Pro#erties hroughAdditives:

• 4dditive agents& such as fllers&plasticizers& lubricants& coloring agents&stabilizers& antio'idants& antistaticagents& Hame retardants& and

reinorcement can be incorporated intopolymers to8 – ,mprove properties

– 6educe costCe'tenders

– ,mprove moldability

– ,mpart color

– -revent deterioration in variousenvironments

• Pro#erty com#arison8


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• Pro#erty com#arison8

-olymers versus "etals

Material Condition TS (ksi) E(106psi) Elo

Polyethylene Branched 2 0.025 90-650

Polyethylene Crystallized 4 0.100 50-800

Polyvinyl chloride Cl-sides 8 0.35 2-40

Polystyrene Benzene-sides 0.500 1-3

Bakelite !ra"e#$r% 1.0 1

Aluinu &nnealed 13 10.0 15-30

10!0 steel &nnealed 5 30.0 30


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Structural Adhesives:

-olymers orm the basis o the

structural adhesives and come romall three behavior amilies8

5hermoplastic hotmelt glues

5hermoset twopart epo'ies

Elastomer silicone bathtub caul

Structural Adhesives


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Structural AdhesivesTreendous advances have "een ade recently in the developent# use#

and relia"ility o$ structural (i%e% load&"earin') adhesives%

se has increased rapidly# even in such uality& and dura"ility&conscious

areas as the autootive and aircra$t industries%

Both etals and nonetals can "e "onded# as *ell as the plastics and

coposites%

Structural adhesives span the entire ran'e o$ polyeric types and $ors#includin' theroplastic resins (such as hot elt adhesives)# therosettin'

resins (like the epo+ies)# and arti$icial elastoers (like silicones)%

They can "e applied in a variety o$ $ors# includin' drops# "eads# pellets#

tapes# and coatin's# and are availa"le in the $or o$ liuids# pastes# 'els#

and solids%

Curin' is the process "y *hich the properties o$ the adhesives are

chan'ed# and ay involve condensation reactions# polyeri,ation# or

vulcani,ation%

Curin' can "e "y eans o$ heat# radiation or li'ht# oisture# activators#

catalysts# ultiple&coponent reactions# or co"inations o$ these%

Soe o$ the coon types o$ adhesives include


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1% Epo+ies' )hese "ay *e sin+le c$",$nent resins that

cre #ith heat $r t#$ c$",$nent "itres that tilize s$"e

/$r" $/ crin+ a+ent.

-% Cyanoacrylates' )hese are liid "$n$"ers that

,$ly"erize #hen s,read int$ a thin /il" and are in the

,resence $/ "$istre. )he c$""ercial “s,er +les” are in

this /a"ily. Crin+ is at r$$" te",eratre.

.% Anaero"ics' )hese are $ne c$",$nent "aterials that,$ly"erize #hen sht $// /r$" $y+en in the ,resence $/ a

"etal catalyst. $"e $/ these adhesies can een *$nd

thr$+h $il /il"s.

!% Acrylics' ne ,$ssi*ility here is t$ a,,ly adhesie t$

$ne sr/ace and catalyst t$ an$ther. )hese can *e st$red/$r s$"e ti"e *t #ill *$nd at r$$" te",eratre ,$n

c$ntact.

/% rethanes' er"it lar+e de+ree $/ elastic strain.

Soe o$ the coon types o$ adhesives include

Soe other coon types o$ adhesives include


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6% Silicones' Cre /r$" the "$istre in the air t$ /$r" a

ery /lei*le l$#-stren+th seal.

% 2i'h&teperature adhesives' esi+ned /$r se nder

eleated te",eratre serice c$nditi$ns.

3% 2ot&elt adhesives' )hese are ther"$,lastics #hichs$/ten $r lii/y #hen heated and reharden ,$n c$$lin+.

4% Evaporative' 7ater $r a s$lent ea,$rates t$ leae a

s$lid resin. a",les' c$-ce"ent "$del +les.

10%Pressure sensitive' ch as r**er ce"ent $st-it

$tes etc.

Soe other coon types o$ adhesives include

Advanta'es o$ Adhesives


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'

&dhesies hae a n"*er $/ adanta+es #hen c$",ared t$ the

alternatie "eth$ds $/ :$inin+ sch as #eldin+ *razin+ s$lderin+

"echanical :$ints /asteners s,$t #elds shrin% /its etc.

1. &ll "aterials $r c$"*inati$ns $/ "aterials can *e :$ined.

)hese "aterials can readily acc$""$date di//erences in

ther"al e,ansi$n $r c$ntracti$n.

2. ;$inin+ can *e ,er/$r"ed at l$# te",eratres. )here is n$

heat a//ected z$ne.

3. )hin $r delicate "aterials can *e :$ined.

4. )he entire :$int area can *e *$nded. )his ,r$ides +$$d

stress distri*ti$n $er the entire :$int area. )he l$#er

stren+th $/ the adhesie is c$",ensated *y the increasedarea $/ the *$nd ne,ensie

6. ?i+ht #ei+ht :$inin+


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Advanta'es o$ Adhesives & continued

. )he "aterial is a ther"al and electrical inslat$r.

8. )he adhesie als$ acts as a n$ise sh$c% and i*rati$n

da",er. )his can sere t$ enhance the /ati+e li/e $/ the

asse"*ly.

9. )he adhesie ,r$ides a c$rr$si$n *arrier. Bein+ a

n$nc$ndct$r it *rea%s +alanic cells.

10. )he adhesie als$ seres t$ seal the :$int a+ainst

,enetrati$n *y "$istre +ases and /lids.

11. )here is sally little sr/ace ,re,arati$n reired. >t is

,$ssi*le t$ *$nd directly t$ $ides and r$+h sr/aces areactally *ene/icial since they ,r$ide enhanced +ri,,in+.

12. ?ess ,recisi$n is reired in :$int /it-,. )he adhesie /ills

the aria*le +a,s.


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5iitations o$ Adhesives

$"e $/ the si+ni/icant li"itati$ns are'

1. )he adhesies are +enerally n$t sta*le a*$e 350@!.

)here/$re they sh$ld n$t *e sed /$r c$nditi$ns $/

eleated te",eratre serice.

2. )he :$ints are di//iclt t$ ins,ect /$r ality. !e# $/ the

standard n$ndestrctie tests #$r% /$r the ,$ly"eric-

*ased adhesies.

3. C$nsistent reslts reire c$ntr$l $/ the sr/ace

,re,arati$n adhesie ,re,arati$n a,,licati$n and crin+.

4. )he li/e e,ectancy $/ the :$ints is hard t$ ,redict.


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Documents

23 Polymers & Adhesives 4