17 Performance of Conductive Carbon Blacks in a Typical Plastics System J.
H.
SMUCKLER
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Conductive Polymer Corp., Marblehead, Mass. 01945 P.
M.
FINNERTY
Cabot Corp., Billerica, Mass. 01821
Carbon black, used primarily as a reinforcingfillerin rubber, exhibits properties of electrical conductivity and physical form which render it the preferredfillerfor imparting con ductivity to normally insulative elastomeric and thermo plastic polymers. The properties of carbon black important in imparting conductivity to polymer systems are discussed. The effects of three different carbon black types at varying concentrations on the electrical, physical, and rheological properties of poly(ethylene vinyl acetate) (EVA) in both thermoplastic and thermoset form are also illustrated. Reference is also made to the phenomenon of significant resistivity variations with increasing temperature of carbon black-loaded plastics systems.
/Conductive c a r b o n b l a c k is t h e p r e f e r r e d filler f o r i m p a r t i n g a n t i - s t a t i c ^
a n d c o n d u c t i v e p r o p e r t i e s to p o l y m e r systems.
C o n d u c t i v e plastics
compounds containing carbon black are w i d e l y used i n the wire a n d c a b l e i n d u s t r y as m e t a l c o n d u c t o r s t r a n d a n d p r i m a r y i n s u l a t o r s h i e l d i n g i n h i g h v o l t a g e cables. S i m i l a r plastics c o m p o u n d s a r e u s e d t o p r o d u c e anti-static s h e e t i n g , b e l t i n g , hose, a n d m o l d e d goods w h i c h m i n i m i z e static b u i l d u p i n h a z a r d o u s e n v i r o n m e n t s s u c h as m i n e s a n d o t h e r areas w h e r e explosive v a p o r s m a y collect. T h i s p a p e r r e v i e w s b r i e f l y t h e reasons c a r b o n b l a c k is classified e l e c t r i c a l l y as a " s e m i - c o n d u c t i v e " m a t e r i a l a n d h o w these
conductive
p r o p e r t i e s a r e i m p a r t e d to p l a s t i c systems. P r i m a r i l y , h o w e v e r , w e i l l u s trate t h e effects of three c a r b o n b l a c k types at v a r i o u s concentrations o n 171 Deanin and Schott; Fillers and Reinforcements for Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
172 the
FILLERS
AND
REINFORCEMENTS
electrical, physical, a n d rheological properties
acetate
copolymer,
in both
of
FOR
PLASTICS
ethylene—vinyl
t h e r m o p l a s t i c a n d thermoset
(chemically
crosslinked ) form. Carbon Black Properties Which Affect
Conductivity
F i g u r e 1 shows the r e l a t i o n s h i p b e t w e e n the c o n d u c t i v i t y of c a r b o n b l a c k , p u r e p o l y m e r s , c a r b o n b l a c k - f i l l e d p o l y m e r s , a n d metals.
Semi-
c o n d u c t i v e m a t e r i a l s , as o p p o s e d to metals, e x h i b i t a n e g a t i v e t e m p e r a t u r e coefficient of resistance, s h o w i n g a n increase i n c o n d u c t i v i t y w i t h
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increasing temperature.
F u r n a c e b l a c k s , s u c h as the grades
here, h a v e v e r y s m a l l n e g a t i v e t e m p e r a t u r e coefficients
discussed
a n d thus v a r y
only slightly i n d r y resistivity w i t h temperature variation. Log Volume Resistivity, ohm—cm
£
=
2
Figure 1. Volume resistivity of polymer pounds and reference materials T h e s e c a r b o n b l a c k s of l o w v o l a t i l e content
com-
(chemisorbed
c o m p l e x e s ) e x h i b i t d r y resistivities i n the r a n g e of 0.2 to 1.0 m e a s u r e d at a b u l k d e n s i t y of
