Wiley Encyclopedia of Composites. Part 6

385 Pages·100.754 MB·English

by  Nicolais L. et al.

STAINLESS STEEL POLYMER COMPOSITES inherit high effectiveness for ESD control and EMI
shielding. The obvious choice is to use metals such as
FARIDAKHTAR copper and aluminum. But the use of metals in ESD
StockholmUniversity, control and EMI shielding is limited because of heavy
Stockholm,Sweden; weight, cost, and corrosion resistance. The materials of
UniversityofEngineering
choice are then conductive polymer composites. These
andTechnology, Lahore,
composites combine the unique advantages of conductive
Pakistan
material and light-weight polymers. Conductive polymer
composites generally consist of a nonconductive polymer
INTRODUCTION matrixandelectricallyconductivereinforcementorfiller.
In conductive polymer composites, the ESD control and
Staticelectricityreferstotheaccumulationofchargeinan EMI shielding depends on the electrical conductivity,
objectasaresultofslidingorrubbingofamaterial,such permeability, and permittivity. Several parameters
as plastic, which is a good generator of electrostatic volt- control the properties of conductive polymer composites
ages.Thestaticchargeremainslocalizedwithinthearea whichwillbediscussedlaterinthisarticle.
ofcontactforaninsulatingplasticmaterialanddischarges As is well known, plastics are insulating materials
viaanarcorspark,whileitcomesincontactwithanother and show typical surface resistivities of 1014–1017 (cid:2)/sq.
object at a sufficiently different electric potential and Metalsontheotherhandareconductiveandhaveresistiv-
momentaryelectriccurrentflowsthroughtheconducting ities of 10−4–10−10 (cid:2)/sq (Fig. 2). In polymer composites,
path.Ifthemomentarycurrentflow(ESD)happensinan electrical conductivity can be achieved via a conductive
uncontrolledwaythroughanelectronicdevice/equipment network of particles or fibers of electrically conductive
sensitivetoESD,itcandamagethedevice.ESDwasiden- materials.Polymercompositesthatshowdesiredelectrical
tifiedasthemajorsourceofdamageofintegratedcircuits conductivityarecategorizedonthetypeoffillermaterial
during1970s.Thedesignengineersintheelectronicindus- such as metal-based or carbon-based composites. Most
try developed on-chip protection against ESD to ensure common fillers to achieve conductive polymer composites
safe handling and working of ESD-sensitive semiconduc- arestainlesssteelandcarbon.Forelectricallyconductive
tordevices.Since1970s,thecontinuedminiaturizationof polymer composites, three different conductivity ranges
electric equipment and devices and use of sensors in the aredefined:shielding,conductiveorstaticallydissipative,
modernindustryespeciallyaircraftindustryareimposing andantistatic[3].
challengestothedesigners,manufacturers,andmaterial The term reinforcements or filler refers to particulate
scientists to design better and develop new materials for and fiber additives incorporated in polymer matrix. The
ESDcontrol.ThereexistsastrongneedtocontrolESDnot type of particulate additives used to achieve the static
only in electronic industry but also in business machine, dissipationvarieswidely(Table1).Thereareadvantages
automotive, and medical industries. A second damaging and disadvantages of each of these additives (Table 1).
effectoccurringwithESDinelectrical/electronicindustry Dependingonthepropertiesofthefiller,1.5–30wt%filler
istheelectromagneticinterference(EMI)[1]. is required within the polymer composite to achieve con-
The origin of EMI is electrical. EMI can arise ductivity. Incompatibility between matrix and filler can
either by natural phenomenon, including atmospheric prevent formation of a well-dispersed network of disper-
charge/discharge such as lightning and extraterrestrial sant in the host polymer matrix. Also, the matrix and
sources,thatis,radiationfromsun,oritcanbegenerated fillershouldbechemicallyinertinordertoensurethesta-
by artificial sources, such as power lines, electric motors, bleelectricalpropertiesandlowdegradationofproperties
radio transmissions, and computers. For EMI to happen, withtime.
electricalnoise(EMIsource),acouplingpath,andarecep- Carbonisoneofthemostcommonfillersusedtoobtain
torarethecrucialelements.AlthoughEMIisinvisible,its conductivity both in crystalline and amorphous forms.
destructivenatureiseasilydetectedasstaticinterference Thermoplastic materials generally employed as matrix
on amplitude modulation (AM) radio transmission, high include polysulfone, polyester, nylon, polyetheretherke-
error rates on telephone modem communication, as well tone (PEEK), polycarbonate, polystyrene, polypropylene,
asfalsesignalsonsignalanalysisequipment.EMIissues and polyetherimide. Carbon powder polycarbonate com-
have influenced most of the electrical and electronic posites can be typically found in computer housings and
systems.Forinstance,thepowersystemofalaptopcom- business machines where molding to tight tolerances is
puter generates broadband EMI energy and TV antenna critical. Polystyrene and polypropylene carbon powder
catches the energy propagating and causes abnormal composites can be found in office supplies and tote bins
performance. Electromagnetic energy can cause EMI wherealowcostconductivecompositeisrequired.Usually,
issuesatanyfrequencyinthespectrumgiveninFig.1[2]. the strength and impact properties of polymer composite
In electronic, automotive, medical, and aerospace aregenerallylessthanthebaseresin,whencarbonpow-
industries, ESD control is an important consideration derisusedasfiller.Carbonpowdercanmigrateresulting
for design and manufacturing. Traditionally, materials inafinelayerofcarbonpowderonthesurfaceofthemold-
with high dielectric constant and electrical conductivity ing. This phenomenon, known as sloughing, can cause
WileyEncyclopediaofComposites,SecondEdition.EditedbyLuigiNicolaisandAssuntaBorzacchiello.
©2012JohnWiley&Sons,Inc.Published2012byJohnWiley&Sons,Inc.
1
2 STAINLESSSTEELPOLYMERCOMPOSITES
m m m m
n n n n
0 0 0 0
0 0 0 0
4 5 6 7
Visible light
106 m 103 m 1 m 1 mm Wavelength
PowerEMI SRhaideiold winavge sRangeMicrowaves Infrared Ultravoilet Soft X-rays Hard X-rays γ-rays Cosmic rays
Near Far
1 1000 106 109 1012 1015 1018 1021 1024
Figure1. Electromagneticspectrum. Frequency (Hz)
Insulative polymers andwearresistancetothemoldedpart.Thiscombination
1014 ofpropertiesmakescarbonfibercompositesidealforload
1013 carryingapplicationssuchasgearsinbusinessmachines.
110012
110011 Statically dissipating composites Carbon fiber polymer composites offer the advantage of
110010 reduced/negligiblesurfacemigrationoffiller.Partsmolded
109 fromcarbonfiberpolymercompositescanthereforebeused
108
incleanroomenvironmentssuchasintegratedcircuit(IC)
107
106 chipcarrierorinpaperhandlingsystems.Adisadvantage
105 Conductive composites ofcarbonfiberpolymercompositeistheorientationofcar-
104 bonfibersduringmoldinganditcouldresultinanisotropic
103
102 shrinkage. Like carbon powder, composite molded parts
101 areinevitablyblack[2,4,5].
100 Since the discovery of carbon nanotubes (CNTs),
10−1 EMI shielding materials because of their unique mechanical, thermal, and
10−2
10−3 electrical properties, many researchers have fabricated
10−4 Metals CNT-reinforced composites with excellent combination
10−5 of properties of CNTs and host matrices [6–8]. Such as
nanotube-basedpolymercompositesuseCNTsasfillerto
Figure2. Resistivityspectrum.Source:ModifiedfromRef.3.
produceconductivecompositesforelectronics,automotive,
andaerospaceindustry.CNTsareveryeffectivecompared
totraditionalcarbonfillermicroparticles,mainlybecause
concernincleanroomenvironmentsorinpaperhandling oftheirhighaspectratios.Theresultingcompositesshow
applications.Carbonfibersareinherentlyconductiveand very low electrical percolation threshold at 0.0025wt%
are available with a variety of aspect ratio. Loadings CNTs and high conductivity of 2S/m at 1.0wt% CNT
within the composites can vary from 10 to 30wt% to content in epoxy matrices (Fig. 3). A comparison of
achieveresistivitiesanywherebetween103and1012(cid:2)/sq. CNTs and carbon black fillers in Fig. 3 shows that
Carbon fiber polymer composites offer strength stiffness CNTs effectively show varying electrical properties with
Table1. ConductiveFillersandFibersa
Filler Loadingwithinthe ResistivityRangein Advantages Disadvantages
Composite(%) Composite((cid:2)/sq)
Carbonpowder 5–20 103–1012 Costandconductive Lowstrength,
sloughing
Carbonfiber 10–30 103–1012 Conductive,high Anisotropy,shrinkage
strength
Carbonblack 7.5–30 100–104 Costandconductive Lowstrength,
sloughing
Carbonnanotubes 1.5–2.5 100–104 Excellentphysical Extremelyhighcost
properties
Stainlesssteel 5–10 103–105 Colorability,isotropic Cost,conductivity
aDatatakenfromRefs3and4.
STAINLESSSTEELPOLYMERCOMPOSITES 3
(a) (b)
Figure3. (a)SpecificcompositeconductivityversusweightfractionpofCNTs.(b)Comparisonof
carbonblackandCNTsasfillerforepoxymatrix.Source:FromRefs6and7.
small variation in their content. Other advantages of Table2. CompositionofStainlessSteelGrades[6]
using CNTs as filler are good mechanical and thermal
AISIType Composition
properties.ThemainobstacleforthewideuseofCNTsis
theirextremelyhighcost. 410 12.5%Cr,0.15C(Max.)
As mentioned above, metals are good candidates for 420 13.0%Cr,0.13C(Max.)
430 17.0%Cr,0.10C(Max.)
ESD control and EMI shielding. Stainless steel fibers
434 17.0%Cr,1.0%Mo
are very active and require only 5–10wt% (0.75–1.5
302 18.0%Cr,9.0%Ni
vol%) to achieve a conductive composite with a range of
304 19.0%Cr,10.0%Ni
103–105 (cid:2)/sq.Thereareseveraladvantagestousestain-
316 17.0%Cr,12.0%Ni,2.5%Mo
less steel as filler over carbon powder and fibers. Parts
molded from stainless steel composite will not slough; Abbreviation:AISI,AmericanIronandSteelInstitute.
therefore, they can be used in clean room environments
of various types depending on the alloying elements and
and paper handling applications. Low cost and highly
theirmassfraction.Table2summarizesthenominalcom-
conductive composite polymer composites can be molded
positionofstainlesssteelgrades[10].Austeniticstainless
by injection molding and extrusion with large fibers.
steelsarecommonlyusedtodrawstainlesssteelfibers.
Theadvantageofstainlesssteelpolymercompositesover
Fine stainless steel fibers are produced by bundled
carbonfiberpolymercompositeistheisotropicproperties
drawing of stainless steel wires in which a multiple
of parts because of lack of orientation of fibers in the
end bundle of wires is progressively reduced in diame-
moldedpart.Stainlesssteelfibersarelightandhavethe
ter in wire drawing dies. As conductive filler for plastics,
aesthetic appearance of the matrix. Stainless steel fibers
stainless steel fibers are available is various diameters
allow anchoring and prevent pullout after debonding.
(6–200μm)andaspectratio(>1000).Stainlesssteelfiber
The conductive polymer composites with stainless steel
is available in three forms [11]. Tow consists of multi-
fillers provide adequate ESD control and EMI shielding
ple strands of multiple end filaments, typically 1159 and
inadditiontohavebetterstrengthandimpactresistance.
3721fibersperstrand.Towisusuallyunsized.Sizedand
Additionally, stainless steel polymer composites can be
chopped fibers are made by coating strands of tow with
coloredusingavarietyofpigments[2,9].Thisarticlewill
6–30wt% sizing then drying and chopping to a specified
focusontheconductivestainlesssteel-reinforcedpolymer
length. Typical lengths vary from 3 to 8mm. A range
compositesforESDcontrolandEMIshielding.Theeffect
of sizing is available to achieve chemical and rheological
ofthereinforcementsize,shapeofthefiller,content,and
compatibility with the base resin. In this form, the fiber
the polymer on the EDS/EMI properties of the compos-
bundles are nonclumping and can be easily dry blended
ite will be reviewed, discussed, and compared with other
with pellet or granular form resins before compounding.
competitivematerials.
Whenfeedingpurefiberintoprocessequipment,agravi-
metric belt feeder with vibratory prefeed gives the best
STAINLESSSTEELFIBERS results.Thefiberpropertiesdependmainlyonthecompo-
sition of stainless steel and the size of the fiber. Typical
Stainlesssteelinanironalloywithaminimumof11wt% fiber properties of stainless steel 316 fibers are listed in
chromium. Stainless steels exhibit very attractive prop- Table3.
erties compared to carbon steels, for example, corrosion Air laid web is another form of stainless steel fiber
resistance in various environments. Stainless steels are used for conductive plastics. This is nonwoven randomly
4 STAINLESSSTEELPOLYMERCOMPOSITES
Table3. FiberProperties[7] not establish connective pathways for electrical conduc-
tion through the composite, and the composite exhibits
Fiber Electrical Tensile Elongation
high resistivity. As the concentration of conductive filler
Diameter Resistance Strength (%)
(μm) ((cid:2)-cm) (cN) increases, the critical filler content is attained and is
termed as the percolation threshold. At this critical filler
8 170 7–8 1
content, the first connected conductive pathway is estab-
12 80 17–18 1
lished in the composite. On further increasing, the filler
22 30 55–60 1
concentration gives rise to the formation of connected
pathways, and the resistivity of the composite decreases
sharply[13].
In percolation threshold concentration range of filler,
arrayedfiberwebweighingfrom34to406g/m2.Itisused
theconductivityincreasesdrasticallybyseveralordersof
asaconductiveveilforbulkmoldedandsheetproducts.
magnitude (Fig. 4a and b) [12,14]. In percolation theory,
theresistivity(ρ)ispredictedasfollows[13,15]:
PARAMETERSAFFECTINGESDANDEMI (z−2)ρ ρ
ρ = (cid:2) c p (cid:3) , (1)
m A+B (A+B)2+2(z−2)ρ ρ 1/2
Inorderforcurrenttoflowthroughinsultingplastic,con- c p
ducting filler is added. Conducting polymer composites
where
mainly consist of two components: polymer matrix and
conductingfiller.Thefillercanbeintheformofinorganic (cid:4) (cid:5)z(cid:6)(cid:7) f (cid:8)(cid:9)
powder,suchasametaloranalloy,oranorganicmaterial, A=ρc −1+ 1− c (2)
2 f
such as carbon. The polymer composites become conduc-
tive at a critical volume content of the conductive filler. and
At low filler concentration, the average distance between (cid:4) (cid:9)
conductingfillerparticlesislargeandtheconductanceis B=ρ zfc −1 (3)
mainlydependentonthepolymerinsulatingmatrixwith m 2f
high resistivity of the order of 1015 (cid:2)/sq. On increasing
theconductingfillerconcentration,thefillerparticlesget and ρm is the resistivity of polymer-reinforced compos-
closer and can establish linkages at critical filler loading ite, ρc the resistivity of the conductive filler phase, ρp
and result in making conducting path through the com- the resistivity of the matrix polymer phase, z the coor-
posite material[12]. Below thiscritical filler content,the dination number of the conductive filler particles, fc the
compositeiselectricallyinsulative. percolationthreshold,andf thevolumefractionofthecon-
ducting filler phase. The limitation in using the classical
percolationmodeltodescribethecompositesystemisthe
ELECTRICALCONDUCTIVITYOFCONDUCTIVE applicabilityofthemodelnearthethresholdpointatwhich
POLYMERICCOMPOSITE thecompositechangesfrominsulatortoaconductor.
Effective media theory is another theory to predict
Percolation theory describes the mechanism of conduc- theelectricalconductivityofthepolymercompositeswith
tionwithinconductivefiller-reinforcedpolymericcompos- electricallyconductivefillers.Therearetwolimitingthe-
ites.Lowerconcentrationsofconductivefillerparticlesdo ories: symmetric theory models and asymmetric theory
Figure4. (a)Schematicsketchofapolymercompositewithvaryingamountofconductivefiller
volume.(b)Percolationcurveforcarbonfiber-reinforcedthermoplastics[12].
STAINLESSSTEELPOLYMERCOMPOSITES 5
models.Eachlimitingtheoryisonlyapplicabletocertain
GEM equation can be successfully used to predict the
cases of conductive particle concentration in the compos- conductivityofthecompositeonawiderangeofcomposi-
ite. McLachlan proposed a generalized effective media tion of composite [15]. Taya and Ueda [16] proposed the
(GEM) equation by postulating effective media theories followingrelationshipfortheelectricalconductivityofthe
andderivedapercolationequationinmathematicalform. composites:
(cid:4) (cid:9)
(cid:4) (ρ −ρ )[(ρ −ρ )(S +S )+2ρ ]
ρ =ρ 1+ f f m f m 11 33 m . (4)
c m 2(ρ −ρ )2(1−(cid:4) )S S +ρ (ρ −ρ )(2−(cid:4) )(S −S )
f m f 11 33 m f m f 11 33
Forfibercomposites 1015
1014
(cid:10) (cid:7) (cid:8) (cid:11) 1013 Stainless steel fiber
ld2 l l2 1/2 l 1012 Carbon fiber
S11= 2(l2−d2)3/2 d d2 −1 −cos h−1d , (5) cm) 11001110
Ω- 109
y ( 108
where vit 107
sti 106
esi 105
S22=S11, R 110043
S =1−2S , 102
33 22
2 4 6 8 10 12 14 16 18 20 22
Fiber (Wt%)
l is the fiber length, d the fiber diameter, and (cid:4) the
f
packing fraction of the fiber for f number of contacts per Figure5. Percolation curves for stainless steel and carbon-
fiber. reinforcedpolycarbonatecomposites[18].
Another mechanism is the process of charge carrier
transport. In this theory, the motion of charge carriers
through the composite material occurs via hopping, tun-
neling, or metallic conduction [7,17]. Investigators have FIBERLENGTH
hypothesized that the electron flow can occur when the
particles have aseparation of 100A˚ or less via tunneling The percolation threshold reduces drastically with the
fillerswithhighaspectratio(Fig.6a).Carbonfiberswith
mechanism. Hence, the factors that reduce the distance
an aspect ratio of 1000 require 1 vol% content to achieve
between conductive filler particles or fibers or increase
thedesiredconductivity(100(cid:2)-cm),whereascarbonfibers
theircontentwillenhancetheconductivityofthepolymer
with an aspect ratio of 10 require 10 vol% content to
composite.
achieve the similar conductivity of the composite. The
Theimportantfactorsaffectingtheconductivityofthe
fiberlength/aspectratioisanimportantparameterinthe
polymercompositesare
developmentofpolymer-reinforcedcompositescontaining
acontinuousconductingnetwork.
• conductivefillerloading Two different percolation curves are measured for
• conductingfilleraspectratio fibers with different fiber lengths in polyethersulfone
(PES)(Fig.6b).Thethresholdcontentincreasesforshort
• conductingfillerorientation
fibers. The small size of fibers reduces the probability to
• conductivefillerdispersion.
have body-to-body, end-to–body, and end-to-end contact
between the fibers. While using carbon fibers as a filler
material, the fiber length reduces because of breaking of
FIBERCONTENT carbon fibers. A chopped carbon fiber with an original
length of 6350μm may end up at the final length of
The nature of filler material determines the content of 200μm during processing, whereas stainless steel fibers
filler added to achieve low resistivity of the composite. bend rather than break in stainless steel-reinforced
Stainless fibers gives low resistivity to the composites at polymer composites because of their ductility and the
much lower loading content compared to carbon fibers fiber length does not change with processing. CNTs have
(Fig.5)[18].Forexample,inordertoachievearesistivity very high aspect ratio and the composites using CNTs
of 105 (cid:2)-cm of polycarbonate composite, twice as much as conducting filler materials show very low threshold
carbonfibers(10wt%)ofstainlesssteelfibers(5wt%)are concentration of CNTs and exhibit high conductivity at
needed. verylowfillercontent(Fig.3).
6 STAINLESSSTEELPOLYMERCOMPOSITES
104
103 16 Fiber length (µm)
90–140
o 14
ati y 200–250
ect r stivit 12
ber asp 102 ace resi 10
Fi surf 8
101 og
L
6
4
1
0.1 1 10 100 6 8 10 12 14 16 18 20
Volume fraction of conductive filler Milled carbon fiber content (wt%)
(a) (b)
Figure6. (a) Relationship between fiber aspect ratio and minimum concentration required to
produceacompositewitharesistivitybelow100(cid:2)-cm.(b)Percolationcurvesforcarbon-reinforced
PEScompositescontainingfiberswithvaryingfiberdistribution[18].
−0.6
Transversal
Longitudinal
−0.7
m)
c −0.8
Ω-
ρ (
y −0.9
vit
sti
esi −1
R
−1.1
Figure7. Evolution of resistivity as a function of depth −1.2
(from0.2to1.7mm)intransverseandlongitudinaldirections
0 0.5 1 1.5 2
for poly(propylene) (PP)/1.66% v/v long stainless steel fibers
(LSSF)[9]. Penetration depth (mm), φ = 1.66% v/v
The effect of the aspect ratio of stainless steel on the extrusionandinjectionmolding.Hence,duringprocessing,
resistivityofthepolypropylenereinforcedwithlongstain- the fibers can align in the processing direction. For
lesssteelfibershasbeenreportedbyFelleretal.[9].Itwas instance, molded cross section of carbon fiber-reinforced
reportedthat0.12–0.45vol%contentoflongstainlesssteel polymer composites shows two skin layers and one core
fibers was needed to achieve conductive composites. The layerwithdifferentfiberorientation.Inskinlayers,fibers
percolation threshold was different in longitudinal and align in the melt flow direction and show random distri-
transversedirection.Thus,theresistivityofthecomposite butioninthecorelayer.Theskincoremorphologyaffects
was different in longitudinal and transverse direction as themechanicalproperties[19]andelectricalperformance
showninFig.7. [20] of the molded composite part. On the other hand,
Stainlesssteelpolymercompositesdonotshowthelayers
with difference in fiber distribution. The fibers tend to
FIBERDISTRIBUTION form a tangled mat and molded composite parts exhibit
reducedtendenciestoshrinkorwrap.Neitherfiberlength
In fiber-reinforced composites, the fiber orientation nor fiber orientation is a readily controllable parameter
strongly depends on the processing method and in stainless steel composites. The parameters of greater
parameters.Thecommonprocessingmethodsadoptedare importancearefibercontentanddispersion.
STAINLESSSTEELPOLYMERCOMPOSITES 7
Table4. VariationinConductivityforDifferent Various dispersion aids exist for stainless steel fibers.
Molding[18] Imperialchemicalindustries(ICI)hasdevelopedapropri-
etaryformulationtoensureproperdispersionofstainless
Molding SurfaceResistivity((cid:2)/sq)
steelfibers.Conductivityunderavarietyofmoldingcondi-
Plaque 3×103
tionsisreproducible(Table7).Temperaturesfrom234to
1 2×1011 ◦
236 Candinjectionpressuresfrom3.4to10.3MPaallgave
2 2×1011
3 8×1011 parts with resistivities of 103–104 (cid:2)/sq. The processing
4 3×1010 window has been widened significantly by the dispersion
5 8×1010 aid,allowingprocessingtemperaturetobereducedbyas
◦
6 2×109 muchas30 C.Resistivityacrossthepartisalsoconsistent
7 8×1010 (Table8).
INFLUENCEOFPOLYMERMATRIXONTHE
FIBERDISPERSION CONDUCTIVITY
Another important factor to achieve the desired conduc- Themechanicalandthermalpropertiesofpolymermatrix
tivity is the distribution of fibers in the polymer matrix. influence the conductivity of polymer matrix composites.
The difference in the dispersion results in variation in Several parameters affect the properties of conductive
the electrical conductivity of the polymer composite. The polymer composites. These parameters include polymer
dispersion should be adequate to result in a conductive matrix shrinkage, external mechanical actuation, and
composite.Variationsinresistivityfrom103 to1011 (cid:2)-cm thermalexpansion.Thesefactorsplayavitalroleindeter-
were obtained from moldings containing 5wt% stainless miningtheconductivityofconductivepolymercomposites.
steel in polycarbonate matrix (Table 4). It was thought Polymer shrinkage during processing induces high
that variation in dispersion of the stainless steel was internalstressandreducestheinternalparticleresistance
responsible for a wide range of resistivity measured. The and may lead to the formation of cracks and ultimate
effectthatdifferentmoldingconditionshadonresistivity failure of the component. For instance, researchers have
wasthereforestudied.Resistivitiesof7.6×15.2×0.32cm investigatedtheresistivityofepoxyresinwithconductive
plaques were measured as a function of barrel tempera- fillersduringcuring.Ithasbeenfoundthattheresistivity
ture,injectionpressure,backpressure,andboostpressure. decreased between two and seven orders of magni-
Only a narrow processing window allowed the desired tude[14,17,21].Similarly,variationintheresistivitywas
resistivity to be obtained (103–106 (cid:2)/sq) (Table 5). Not observedonapplicationoftheexternalstress.Itinfluences
only did the resistivity vary tremendously from part to the distribution of the conductive filler. The influence
part, it was also shown to vary within a single part of external stress on the variation of the conductivity
(Table 6). Plaques that measured 1013 (cid:2)/sq at the gate strongly depends on the nature and shape of the conduc-
could measure 105 (cid:2)/sq at the opposite end. Since fiber tivefiller[14,18].Thethermalchangeswithtemperature,
orientationandlengtharenotconsideredinthesecompos- for example, structure transitions, thermal expansion,
ites and loading was held at 5wt%, only poor dispersion and contractions, significantly influence the conductivity
canbethecauseofmeasuredwide-rangeresistivities. of the polymer composite [14]. At temperature below the
Table5. EffectofMoldingConditionsonConductivityofPlaquesContaining5%StainlessSteelPolycarbonate[18]
◦
BarrelTemperature( C) InjectionPressure(MPa) BackPressure(MPa) BoostPressure(MPa) VolumeResistivity((cid:2)/sq)
234–236 Couldnotbemolded
250–262 Couldnotbemolded
262–274 3.4 0.69 0 3×105
262–274 3.4 0.69 0.34 6×105
262–274 6.9 0.69 0 >105
262–274 10.3 0.69 0 >105
Table6. VolumeResistivityonStainlessSteelPolycarbonateMoldedPlaques[18]
◦
BarrelTemperature( C) InjectionPressure(MPa) VolumeResistivity((cid:2)/sq)
Gate Middle End
234–236 6.9 2×1015 6×1014 6×1016
250–262 3.4 3×109 2×1013 2×106
262–274 3.4 5×106 2×105 8×104
262–274 6.9 5×1013 2×106 9×105
262–274 10.3 9×1012 3×105 1×106
8 STAINLESSSTEELPOLYMERCOMPOSITES
Table7. EffectofMoldingConditionsonConductivityofStat-KonDS[18]
◦
BarrelTemperature( C) InjectionPressure(MPa) BackPressure(MPa) BoostPressure(MPa) VolumeResistivity((cid:2)-cm)
234–236 3.4 0.69 0.034 4×104
250–262 3.4 0.69 0 5×103
262–274 3.4 0.69 0 2×103
262–274 6.9 0.69 0 2×104
262–274 10.3 0.69 0 2×104
Table8. VolumeResistivityonStat-KonDSMoldedPlaques[18]
◦
BarrelTemperature( C) InjectionPressure(MPa) VolumeResistivity((cid:2)-cm)
Gate Middle End
234–236 6.9 3×105 4×104 6×103
250–262 3.4 2×105 4×104 1×104
262–274 3.4 1×105 4×104 2×104
262–274 6.9 2×105 2×105 5×103
262–274 10.3 7×104 3×104 5×103
melting temperatures of polymer matrix, the conductive NYLON12
particles have closed packing and intimate contact to
neighbors forming conductive pathways to reduce the Nylon12hasmanyapplicationsinfuelhandlingsystems
resistivityofconductivepolymercomposite.Withincrease [17]. One of the desired properties is static dissipation.
in the temperature, the polymer expands more than the Owing to the poor chemical resistance of carbon com-
filler,causesreductioninthecontactpressure,andresults posites, stainless steel is the filler of choice. Various
inmoderateincreaseintheconductivity.Furtherincrease mechanical properties can be obtained by adjusting the
in the temperature near to melting point enhances the molecularweightofnylon12orbyaddingglass(Table10).
effectandhencetheconductivity[22]. RX-S-90417 exhibits high elongation and excellent
impact strength. Its low shrinkage allows parts to be
molded to tight tolerances. RX-S-90463, a glass-filled
system, exhibits high tensile and flexural properties.
This part can be used in applications in which parts not
POLYCARBONATE
only have to dissipate a charge but must bear a load.
RX-S-90545 is an extrusion grade composite. Its surface
In polycarbonate composites, stainless steel filler is not
appearanceandflexibilitymakethisproductanexcellent
reinforcing and the mechanical properties are close to
candidateforuseincableapplications.
thebaseresin.Mechanicalpropertiescanbesignificantly
improvedbytheadditionofglassfibersinadditiontostain-
lesssteelfibers.Infact,strengthandstiffnesscomparable
tothatofpolycarbonate(PC)/carbonfibercompositecanbe POLYETHERSULFONE(PES)
obtainedasshowninTable9[17].Polycarbonatestainless
steel composites have applications in business machine Electrical and mechanical properties have also been
industry, in paper handling, and in applications where obtained for moldings of the stainless steel PES compos-
colorabilityandisotropyarekeyrequirements. ites.PES/stainlesssteelcompositeshavetheadvantageof
Table9. MechanicalPropertiesofStaticallyDissipatingPolycarbonateComposites[17]
Units Polycarbonate Stat-KonDS RX-D-90225 Star-KonDC-1002
Glassfiber % — — 15 —
Carbonfiber % — — — 10
Stainlesssteel % — 5 5 No
TensileStrength MPa 55.2 55.2 96.6 103.5
Tensileelongation % 6.0 5.0 2–3 2–3
Flexuralstrength MPa 89.7 86.2 151.7 148.3
Flexuralmodulus GPa 2.4 2.8 5.5 6.2
NotchedIzod kJ/m 0.64 0.091 0.133 0.064
UnnotchedIzod kJ/m — 1.34 0.69 0.534
Surfaceresistivity (cid:2)/sq 102–106 102–106 102–106
STAINLESSSTEELPOLYMERCOMPOSITES 9
Table10. MechanicalPropertiesofStainlessSteel,Nylon12Composites[17]
Property Units RX-S-90417 RX-S-90463 RX-S-90545
Tensilestrength MPa 46.2 72.4 39.3
Tensileelongation % 110.0 30.0 240.0
Flexuralstrength MPa 55.2 103.5 21.4
Flexuralmodulus GPa 1.34 4.6 0.45
NotchedIzod kJ/m 0.096 0.10 NB
Electricalresistivity10E (cid:2)/sq
Surface 2–6 2–6 2–6
Volume 2–6 2–6 2–6
Table11. ElectricalandMechanicalPropertiesofPESStainlessSteelComposites[17]
StainlessSteel(wt%) SurfaceResistivity((cid:2)/sq) VolumeResistivity((cid:2)/sq) TensileStrength(MPa) NotchedIzod(kJ/m)
5 4×105 5×105 93.1 0.064
10 1×104 1×105 93.1 0.054
10 5×103 3×104 91.0 0.053
Table12. PropertyProfileofPEEK/StainlessSteel/Glass freedombyeliminatingsecondaryprocessesandthepossi-
Composite bilityofleaksduetochippingandscratching.Fordesign,
planar fences shield better against high frequency radia-
Property Units
tionsandspecificdesignofthejointsandseamsdependson
Tensilestrength MPa 146.9 thesourceofemissionandcorrespondingsignalspectrum
Tensileelongation % 2.0
generated[2,23].
Tensilemodulus GPa 12.2
Flexuralstrength MPa 234.5
Flexuralmodulus GPa 11.2
RADIATEDEMIEMISSIONS:NEARFIELD/FARFIELD
NotchedIzod kJ/m 0.08
Electricalresistivity (cid:2)/sq
Surface 3×102 EMI emission starts radiating from an emitting source.
Volume 6×104 TheEMIemissionspropagateviaaradiatingpathtoward
the susceptible receiver. The strength of the EMI radi-
ation depends on the source emitting, the surrounding
media, and the distance from the receiver. Owing to the
◦ expandingsphericalwavefrontoftheexpandingEMIelec-
acontinuoususeatatemperatureof150 Candtheparts
tromagnetic waves, the EMI waves with an electric field
areextremelyisotropic(Table11).
(E)andamagneticfield(H)(perpendiculartoeachother)
travel in all directions around the source. There are two
POLYETHERETHERKETONE(PEEK) aspects of electromagnetic radiation: near field and far
field.Theregionclosetosourceiscallednear,orinduction
Stainless steel has been compounded in PEEK. A 30% field,alsoreferredtoasclosefieldormagneticfield,exists
glass-reinforced stainless steel PEEK composite exhibits withinλ/2π distancefromtheradiatingsource,whereλis
hightensileandflexuralpropertiesandexcellentchemical wavelength. In this near field region, both radiation and
resistanceandprovidesacontinuoususeatatemperature static fields contribute to EMI. In the far field, E/H also
of250◦C(Table12). referred as the impendence Z equals the characteristic
impedance of the medium, that is, E/H=Z =377(cid:2) for
0
air or space and the wave is a plane wave. While in the
SHIELDINGELECTROMAGNETICINTERFERENCE(EMI) nearfield,theE/Hisdeterminedbythecharacteristicsof
thesourceandthedistancefromthesourcetothepointof
Theenclosuredesignofcommercialelectronicequipments observation.Ifthesourcehashighcurrentandlowvoltage
requiresEMIshielding.Thedesignsshouldaccountforthe (E/H<377(cid:2)),thenearfieldispredominantlymagnetic.
structuralcharacteristics,forexample,housingwallthick- Conversely,ifthesourcehaslowcurrentandhighvoltage
ness, gaps between the layers and joints, slot opening on (E/H>377(cid:2)),thenearfieldispredominantlyelectric[2].
theemissions,andthemechanicalreliability.EMIshield- The power at any point in the field is generally propor-
ingrequirementscanbemetbyapplyingconductivepaints tionalto1/r2,whereristhedistancefromthesource.Far
or electroplating part surfaces [23]. Another active solu- field also referred to as E field or transverse electromag-
tionisthemoldingofthepolymerwithaconductivefiller netic wave totally comprises radiated fields. The far-field
to absorb or reflect EMI radiation. The use of conductive wave exists beyond a λ/2π distance from the radiating
fillerstomoldconductivecompositesincreasesthedesign source.Thepowerisgenerallyproportionalto1/r2[18].
10 STAINLESSSTEELPOLYMERCOMPOSITES
1000.0
σ ~ 5.81E5 S/cm
900.0
Cu, 0.051 cm
800.0
700.0
B)
d 600.0
Incident g ( Total
n 500.0
di
el
hi 400.0
S
Reflected 300.0
Absorption
200.0 loss
Transmitted
100.0
Reflected
Reflection loss
0.0
1.0E+05 1.0E+06 1.0E+07 1.0E+08
Frequency (Hz)
(a)
70.00
σ ~ 2 S/cm
60.00 0.2 cm
Total
50.00
B)
g (d 40.00
n Reflection
eldi 30.00 loss
hi
S
20.00
10.00 Absorption loss
0.00
1.0E+051.0E+06 1.0E+07 1.0E+08 1.0E+091.0E+10
Frequency (Hz)
(b)
Figure8. Shieldingeffectivenessahighlyconductivematerialsuchascopper(0.020in)and(b)
performanceofalowerconductivitymaterialwithfourtimesthecopperthickness[23].
Anelectromagneticwavecanbeshieldedbytwomech- Absorption is proportional [2] to thickness (t) and the
anisms: reflectance and adsorption (Fig. 7). Shielding squarerootoffrequency(f),conductivity(G),andperme-
effectiveness(SE)isnormallyexpressedindecibels(dB).It ability(μ):
isdescribedasafunctionofthelogarithmoftheratioofthe
incidentandexitelectric(E),magnetic(H),orplane-wave A=3.3tμfG. (7)
field intensities. The SE therefore can be defined as the
sum of the energy absorbed (A), reflected (R), and a cor- Absorption therefore increases with frequency. This
rection factor for multiple reflections in thin shields (B) equation however assumes that the material is homoge-
[22–24]: neousandelectricallythick[25].Theskindepthδisgiven
asfollows:
SE=A+R+B. (6)
1
δ= . (8)
(π f μG)1/2
Energyisreflectedattheexitinterface,butthisportion
is usually very small and is commonly excluded from The absorption loss is approximately 9dB of one skin
the equation. In cases, B can also be removed from the depth in a shield and skin effect becomes important at
equation for electric fields and plane waves. The effect low frequencies. The skin depth for different metallic
that frequency has on the reflectance and absorption of materialsshowsthatgoodconductorssuchascopperand
shielding materials along with the total SE is shown in aluminumareeffectiveinstoppingcurrentsbuthaverela-
Fig.8[23]. tivepermeabilitysameastheair.Mildsteelwithrelative