Plastics PMMA

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CAEDSInjectionMouldingMaterials

Poly(methylmethacrylate)(PMMA)

TechnicalUniversityofGabrovoMilenaKoleva

GeneralPoly(methacrylates)arepolymersof theestersofmethacrylicacids. The

mostcommonlyusedamongthemispoly(methylmethacrylate)(PMMA).

Poly(methylmethacrylate)orpoly(methyl2methylpropenoate) is thepolymerof

methylmethacrylate,withchemicalformulaC5H8O2)n.Itisaclear,colourlesspoly

mer available on the market in both pellet and sheet form under the names

Plexiglas, Acrylite, Perspex, Plazcryl, Acrylplast, Altuglas, Lucite etc. It is

commonlycalledacrylicglassorsimplyacrylic.

Another polymer, poly(methyl acrylate) (PMA) is a rubberymaterial, similar to

poly(methylmethacrylate),butsofter than it,because its longpolymerchainsare

thinnerandsmootherandcanmoreeasilyslidepasteachother.

Poly(methylmethacrylate) isproducedby freeradicalpolymerizationofmethyl

methacrylate in mass (when it is in sheet form) or suspension polymerization

accordingtothefollowingchart:

Properties

Polymethacrylates 1

PMMAisalinearthermoplasticpolymer.PMAhasalackofmethylgroupsonthe

backbonecarbonchain itslongpolymerchainsarethinnerandsmootherandcan

slidepasteachothermoreeasily,sothematerialbecomessofter.MainphysicalcharacteristicsofPMMAareshownintable1.

PMMAhashighmechanicalstrength,highYoungsmodulusandlowelongationat

break.Itdoesnotshatteronrupture.Itisoneofthehardestthermoplasticsandis

alsohighlyscratchresistant.Itexhibitslowmoistureandwaterabsorbingcapacity,

duetowhichproductsmadehavegooddimensionalstability.Bothofthesecharac

teristics increase as the temperature rises. Table 2 shows some of mechanical

characteristicsofPMMA.

General

Structure

Physicaland

mechanical

properties


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Polymethacrylates 2

Table1.TypicalphysicalpropertiesofPoly(methylmethacrylate)

Physical

Properties

Value

Density 1.15 1.19g/cm3

Water

Absorption

0.32%

Moisture

Absorption

at

Equilibrium 0.3 0.33%

LinearMouldShrinkage 0.003 0.0065cm/cm

Melt

Flow 0.927g/10min

Itsstrengthpropertiesduringinjectionmouldingdiffersignificantlyinlongitudinal

and transversedirection as a result of the orientation effect.As in the casewith

otherthermoplastics,themechanicalpropertiesofPMMAvaryasthetemperature

changes.Thismaterialtendstocreep.Itisnotsuitableforoperationundermultiple

dynamicloads.

PMMAisoneofthepolymersthatismostresistanttodirectsunshineexposure.Its

strength characteristics exhibit fairly small variations under the effect of UV

radiation,aswellasinthepresenceofozone. ThesepropertiesofPMMAmakeit

suitableforproductsintendedforlongopenairoperation.

Table2.MechanicalcharacteristicsofPolymethylmethacrylate

Mechanical

Properties

Value

Hardness,

Rockwell

M 63 97

TensileStrength,Ultimate 47 79MPa

ElongationatBreak 1 30%

Tensile

Modulus 2.2 3.8GPa

Flexural

Modulus 3 3.5GPa

Izod

Impact,

Notched 1.220kJ/m2

Izod

Impact,

Unnotched 11kJ/m2

TensileCreepModulus,1h 1800 2700MPa

Tensile

Creep

Modulus,

1000

h

12001800

MPa

The lowwaterabsorptioncapacityofPMMAmakes itvery suitable forelectrical

engineeringpurposes. Itsdielectricpropertiesareverygood,butpolystyreneand

LDPE are superior to it. Its resistivity depends on the ambient temperature and

relativehumidity.Thedielectricconstant,aswellas the loss tangent,dependson

thetemperature,therelativehumidityofairandthefrequency.

Electrical

characteristics


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Polymethacrylates 3

Table3. ElectricalpropertiesofPoly(methylmethacrylate)

Electrical

Properties

Value

Electrical

Resistivity 1014 1015.cm

SurfaceResistance 1014 1016

Loss

factor,

20

C,

1000

Hz,

60%

humidity 0.04

DielectricConstant 2.8 4

DielectricConstant,LowFrequency 3 4

Dielectric

Strength 17.7 60kV/mm

Dissipation

Factor

0.03 0.55

Arc

Resistance 100 180s

The thermal stability of standard PMMA is only 65oC.Heatstabilised types can

withstandtemperaturesofupto100oC.PMMAcanwithstandtemperaturesaslow

as70oC.Itsresistancetotemperaturechangesisverygood.

PMMAisacombustiblematerial,whichcontinuesburningevenaftertheflame is

removed theproducts separated in theprocessof thermaldestructionhave an

intoxicatingeffect.

Table4.

Thermal

properties

of

Poly(methyl

methacrylate)

ThermalProperties Value

CTE,

linear

20C 60 130m/m.C

CTE,linear20CTransversetoFlow 70 90m/m.C

Specific

Heat

Capacity 1.46 1.47J/g.C

Thermal

Conductivity 0.19 0.24W/m.K

Maximum

Service

Temperature,

Air

41 103C

MeltingPoint 130C

Vicat

Softening

Point 47 117C

GlassTemperature 100 105C

PMMAexhibitsverygoodopticalpropertiesittransmitsmorelight(upto93%of

visible light) than glass. Combined with its good degree of compatibility with

humantissue,itcanbeusedforreplacementintraocularlensesorforcontactlenses.

Unlikeglass,PMMAdoesnotfilterultravioletlight.IttransmitsUVlightdownto

300nmandallowsinfraredlightofupto2800nmtopass.

Thermal

properties

Optical

properties


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Polymethacrylates 4

Table5.OpticalpropertiesofPoly(methylmethacrylate)

OpticalProperties Value

Haze

1 96%Transmission,Visible 80 93%

Refractive

Index 1.49 1.498

Acrylics are unaffected by aqueous solutions of most laboratory chemicals, by

detergents, cleaners,dilute inorganic acids, alkalis, and aliphatichydrocarbons

however,acrylicsarenot recommended forusewith chlorinatedoraromatichy

drocarbons, esters,or ketones. Itdissolves completely in chloroform,di and tri

chlorethane,which is used for production of glues.The chemical resistancewill

varywithstresslevel,temperature,reagentsanddurationofexposure.

PMMAarephysiologicallyharmless.Duetotheirlowmoistureabsorptioncapacity

theyarenotattackedbymouldsandenzymes.

PMMA is suitable for injection moulding, extrusion, extrusion blow moulding

(impactmodifiedacrylicsonly),thermoforming,casting.Themeltflowindexofthe

materialvariesaccordingtothemethodoftreatment:

lowmeltflowindex(0.82.5)materialsintendedforextrusion;

mediummeltflowindex(2.5 10)generalpurposePMMA;

highmeltflowindex(>10)PMMAforinjectionmouldingofproducts

havingacomplexshape.

For injectionmoulding themould temperature shouldbebetween 40 and 80oC,

dependingonthetype,thematerialtemperatureshouldbebetween200and250oC.

Usuallyhighinjectionpressuresareneededbecauseofpoorflowproperties.Thisis

particularly important foropticalmouldingswherevisibleweld lineswill form if

the correct parameters are not used.During injectionmoulding internal stresses

oftenbuildup,which canbe eliminatedbyheating ata temperatureabout80C

(tempering).

Mouldsnozzle isan important factorwhenmanufacturingPMMAproduct.The

sizeofthenozzlegatevariesdependingontheshapeandthesizeoftheproduct.

Productsmanufactured from PMMA aremainly clear and transparent and it is

important that theydonothaveanyscratches. So that it isrecommend touseatleastdraftangleof1(rather2).BecausePMMAhashighviscosity,thegasremov

ingchannelscanbequitedeep,from0.04to0.08mm.

Misoperationdue tohigher temperature causesdestructionofPMMA, accompa

niedbyreleaseofmethylmethacrylateorotherlowmoleculevolatileproducts.The

processalsoleadstoalossofmass,whichmakesitundesirable,andagoodknowl

edgeofthethermophysicalpropertiesofthepolymerisnecessaryinordertoavoid

it,especiallyinthetemperaturerangeprocessing(220250C).

Chemical

resistance

Processing

methods


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Polymethacrylates 5

Acrylicsareeasilysawed,drilled,milled,engraved,and finishedwithsharpcar

bidetippedtools.Cutsurfacesmaybereadilysandedandpolished.Theyarealso

readilybentorthermoformedatlowtemperatureandsolventbondingofproperly

fitting parts produces a strong, invisiblejoint. PMMA canbeweldedby all the

plasticsweldingprocessessuchashotblade,hotgas,ultrasonicorspinwelding.

Applications

PMMA isan economical,versatilegeneralpurposematerial. It isavailable inex

trudedand/orcastmaterialinsheet,rodandtubeforms,aswellascustomprofiles.

Various types of acrylics are used in awide variety of fields and applications,

including:

Optics:Dustcoversforhifiequipment,sunglasses,watchglasses,lenses,

magnifyingglasses;

Vehicles:Rearlights,indicators,tachometercovers,warningtriangles;

Electrical

engineering:Lampcovers,switchparts,dials,controlbuttons; Officeequipment:Writinganddrawinginstruments,pens;

Medicine:Packagingfortablets,pills,capsules,suppositories,urine

containers,sterilisableequipment;

Others:Leafletdispensers,shatterresistantglazing,showercubicles,

transparentpipelines,illuminatedsigns,toys.

Brands

LuciteL(LuciteInternational,Inc,USA)

Plexiglas(AltuglasInternational)

Acrylite GP(PiedmontPlastics,CYROIndustries,USA)

* The rangesgiven in the tables indicate the minimum and the maximum value of the respective

property,foundexperimentallyandpublishedforthedifferentbrandsandtypesofthepolymer.

References:

1.Harper,CharlesA.,HandbookofPlasticProcesses,JohnWiley&Sons,2005

2.Harper, CharlesA., EdwardM. Petrie. PlasticsMaterials and ProcessesJohn

Wiley&Sons,2003

3. Stevens,Malcolm P. Polymer Chemistry:An Introduction.OxfordUniversity

Press,USA,1998

4.VanKrevelen,D.W.Propertiesofpolymers, ELSEVIER,2003

5.Crawford,R.J.Plasticsengineering.ButterworthHeinemann,1998

6.JrvelP.etal.,Ruiskuvalu,Plastdata,2000.

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