Indian Journal of Engineering & Materials Sciences Vol. 9, June 2002, pp. 197-202

Conducting polymeric products from metal powder-filled castor oil-based polyurethanes

Anil Srivastava", R C Chauhan" & Paramjit Singhb

'Department of Chemical Technology, Sant Longowal Institute of Engineering & Technology, Longowal, Sangrur 148 106, India

bpanjab University, Chandigarh 160 014, India

Received 9 May 200/; accepted 4 March 2002

In the present studies, the polyurethanes were prepared by reaction between low cost vegetable oil, i.e., castor oil and its polyols and toluene 2,4-di-isocyanate. The fine metal powders, i.e., aluminum and graphite of 200 mesh size were dispersed in the castor oil and its based polyols before preparation of polyurethane pre-polymer. The pre-polymers of polyurethanes achieved by these methods were reacted by vinyl monomer followed by in-situ polymerization technique. Powder filled composites were prepared by compression molding technique. The composite specimens prepared by these methods were characterized for volume resistivity and surface resistivity to find out electrical conductivity. The products are expected to find suitable application as floor-tiles in the areas, where electrostatic charges are of great concern; conducting coatings, adhesives for electrical industries and junction boxes, panels particularly in the areas of EMI and RFI concerned.

The recent development in the information technology has put forward the need of finer, portable, cleaner, faster and dense devices. Polymeric materials, which are known to be easy to process and modify even in the atomic/nano scale are regarded as a promising candidate for many such applications l

.

Polymers are known for their excellent insulating resistance2 but the electrical conductivity in the polymers is extremely important to develop these devices for (i) lowering the dissipation of electrostatic charges from plastics parts, (ii) minimizing the effects of electromagnetic waves interferences due to the shielding of plastics boxes and (iii) restricting the dust collection on the surface of plastics products2

.5

•

Conducting polymeric products have interesting applications as display panels, sensors, optical discs, rechargeable batteries, electrodes and solar cells including dissipation of electrostatic charges and miniature electric/electronic circuitry. Conjugated polymers are also in news, which provide a class of processible film forming semi-conductors and metals. Polymer-polymer heterogeneous can be exhibited both in LED and photovoltaic cells. Polymers semi­conductors can be used in field-effect transistor, for which control of order is important6

.7

. Conducting characteristics can be introduced in polymers by a number of different routes and techniques. Metal­filling in polymer system is one of the techniques to

introduce conductivity in commercially available polymers at the desired level. These filled polymers have a number of advantages in terms of (i) absorbing the specific radiation, (ii) increasing the density, (iii) displaying permanent magnetism, (iv) improving thermal stability, (v) eliminating the dissipation of electrostatic decay, (vi) enhancing the thermal and electrical conductivity, (vii) imparting strength and (viii) reducing the cost factor and processing ease due to secondary operation to achieve conductivity g. lo.

Plastics composites at low concentration of metal filler remain an effective insulator, but there is a sharp rise in the conductivity level and improvement in tensile strength as well with the filling of conducting fillers up to the critical volume at which the composite can conduct electricity . There is a smaller effect after the critical concentration5

. Frenkel I I proposed the electron tunneling theory to describe the contact resistance between two metallic bodies. Many authors have reported that tunneling effected by thermal fluctuation of the electrical potential is dominant mechanism under conditions. Kusy and Turner l2 and Malliaris and Turner l3 have reported the arrangement of particles filled in the polymer matrix or embedded on the glass fiber Material. Nielsen 14 has proposed the following equation, which relates the increase in thermal conductivity in the multiple order of two with the filler loading of 20-30%.

198 INDIAN J. ENG. MATER. SeL,JUNE 2002

k I + ABe -----km I-Bce

A=Ice-I,

The same formula can be applied to calculate the electrical conductivity by replacing k and km by E and Em respectively. Here ke is the Einstein coefficient, k is the composite conductivity, Vr is the volume frac­tion, 8 is the packing fraction.

Chakledar and Bhattacharya 15,16 supported the general equation that thermal conductivity increases with increasing fill er content, temperature and with particle size. Gurl ;amd 17

, Kusy and Turner l2, and Milliaris l3 have also supported the view of increase of conductivity with the filler loading. Inorganic particles improve Young's modulus also and, thus, unfortunately lead to the reduction of fracture strain. Nielsen 14 analyzed the fracture of polymer with a model in which the composites were described by a cubic array of particles. Kelly and Tyson 18 proposed a model to find the total modulus of composites as shown below:

where Me is the modulus of composites, Mr is the modulus of fi bre, Mrn is the modulus of resin , Cr is the fractional volume of glass fibre, K=I for parallel arrangement, <1 for random arrangement, 0.5 for two directionally oriented fibers. Bazhenov l9 proposed that an increase in the filler content leads to a transition in deformation mechanism. Bonini et al. 20

have supported the increase in modulus in the presence of fillers . Plasticised PVC formulated with 40/h of zinc adipate ester possesses the highest conductivity and poor mechanical properties making it of great interest in fabrication of electronic and microwave devices and battery electrode appl ications21 . The volume resistIvity of the polyethylene film decreases with the carbon fiber content as expected, as long as carbon fibers are relatively well distributed in separated filament ,

which is effected by the physical and chemical state of carbon fiber surface. The tensile strength is found to be quite scattered with the carbon fiber content, implying that the reinforcing effect of carbon fibers is negative. It may be due to poor adhesion of carbon fiber surface to the polyethylene matrix and trapped void inside the film22,23. The use of multiphase materials brings interesting solutions, particularly in decreasing the percolation threshold, carbon black content, improving the electrical characteristics and reproducibilit/4,25. Materials, which show a dramatic resistivity increase within a temperature transition region, are of interest for thermo-electric switching devices. Maximum values of dielectric loss occur in the polymer carbon black composite is found at percolation concentration. Electrically conductive materials have shown self-heating characteristics. Carbon black distributions in mUlti-component have shown as a useful technique for achievement of low resistivity at low carbon black content26. The blends of polyaniline and polystyrene can exhibit insulating character at low shear level; however above a certain level, smooth surface filament are generated, which dramatically increases the conductivities27. The behaviour of composites made with an insulator polymer particularly in polymeric blends such as polyurethane IPNs and inorganic conductive polymer/ fiber fillers/metallic or non-metallic fine powder, viz., aluminum and graphite powder is qui te interesting also because of the incomparable properties of the resulting composites from the originating material. The induction of IPN route has opened the avenues to modify the mechanical properties of thermoset brittle plastics by adding vinyl type monomer at the pre­polymer stage followed by in-situ polymerization, thereby yielding the controlled morphology for achieving improved mechanical properties28. Polyurethane based on castor oil and IPDI has shown the conductivity level in the order of 10,13 Ohm' l-cm' l and its polyurethane IPNs have attained the conductivity in the order of 10,15 Ohm' l-cm' l as reported29.3o. Metal-filled Castor oil and its polyols -based polyurethane IPNs have exhibited the improved

h . I . 31 mec anlca properties .

Experimental Procedure The swollen unfilled pre-polymer of polyurethane

have been prepared by the reaction of toluene 2,4-di­isocyanate with a renewable resource. viz., castor oil and its polyols of hydroxyl values 160, 225, 354 mg KOHlg. The untreated fine metai powder of

SRfV ASTA V A el al.: METAL POWDER-FILLED CASTOR OIL-BASED POLYURETHANES 199

aluminum and graphite were dispersed in the polyols in the weight fraction of 0 .071 , 0 .133, 0.187 and 0.235 to prepare the filled polyurethane. The vinyl rnonorner32

, i.e, styrene as a 30% by weight was mixed with the pre-polymer to prepare the polyurethane IPNs. The mixture was stirred at room temperature for about four hours and as the mass started becoming viscous, it was then poured into the polished mild steel mold. The mold was kept under hydraulic press ~or 24 h under the pressure of 10 kN. The polyurethane: composites sheets/panels prepared by the above method were cut as per ASTM-257 by power press for evaluation of electrical conductivity. Electrical conductivity was measured by high voltage source unit model 237. Measurement of electrical conducti vity is performed by mounting the standard size sample between the top and bottom electrodes by opening the thumb-screw fasteners holding the top electrode of the model 237 high voltage source measure unit. Before mounting the sample, it is cleared by ethanol to remove the dirt etc. and dried for an hour to apply a coating of sil ver paste. A high conducting material suppli ed by Eltecks Corporation, Bangalore is coated on its surface to provide conducting path in the insulating material. The current is also measured by applying 1000 V for 60 s. The volume and surface res istivity of the sample are calculated as 33.3.\.

(i) . . . 22.9 V

Volume resIstIvity (Vr)= Ohm-cm X ",

(i i) S f . .. (5) 53.4 Oh ur ace res lstl vlty r =-- m 1

Electrical conductivity (Ec) = (volume res istivity)"', where V is in vo lt, I is in ampere and Xw (thickness) is III cm.

Results and Dir.cussion In the present study, the effects of (i) fi lI er

concentrati on, (ii) hydroxy l value and (iii) shape factor are discussed on the surface and volume res istivi ty. The effects on surface and volume resistivity of I e tal powder filling in the composite filler have been shown in Figs I and 2. They exhibit that the filler concentrati on has decreased the surface and volume resistivity in polyurethane, polyurethane IPN and g lass fiber reinfo rced polyurethane lPNs, but the rate of decrease is higher in case of graphite powder than the aluminum powder filled composite. The value of vo lume and surface res istivity are found

13.0

12.5

12.0

11.5

] 11 .0 o

:~ 10.5

"! 10.0

~ Vl

o 9.5 '" :3

~

~ *=-- ' --.....:.:::_,--..--- ~ ~

.

...... --......... "'-

~ " " ~ -........ ""'>~ "" ~~'"'' " ~

~~

,~ ~~

9.0 • Aluminum powder fill ed pu "~, 8.5 • Graphite powder filled pu ,~ . ___ ~~ ' ..........

.l Aluminum powder fill ed Pu IPN composite '-~.

... Graphite powder filled Pu IPN composite 'fit ' ........... B.O ~ Aluminum powder fill ed glass '" -.

fi be r rei nforced pu IPN "'.-

+ Graphite powder filled glass "'" libe r reinforced pu IPN •

7.5

7.0 +--.--.--,--..----.-..--,---.-,-----r-,---I 0.00 0.05 0.10 0.15 0.20 0.25

Weight Fraction

Fi g. l--Effeci of fill er concentration on surface resistivity of fill ed PU, PU IPN and glass fiber reinforced PU IPN

12.5 ;-

12.0 + •

11 .5 +". 11 .0 't '.j '

• r\ lw,nllllUnll"' .... lio;' f,lItt.! ru E • Graphit(' puwdn fill ed p"

~ 10.5 ... Alunllnum J'IO",'d(r filled IOU II 'N Com~l tc ~ + 0 ... C;r.ptIlIC ~.t.kr filll:tl ru IP'" o:nfltpo."Kllc

~. Aiumlilurn powder fillrd "las. fih<,

§. 10.0 f(: ln too;cd pu IPN composites. ,,~ + vraphllc po ..... der tilled glan fibeT

'G rt"infun:nl J'IU ",rN ... on\rmile~.

~ 9.5 ~ "0 ~

0

'" .3 9.0 I ,,'

B.S ~I

B.O • • 7.5

7.0 "I

0.00 0.05 0.10 0.15 0.20 0.25 We ight Fra(:liofl

Fi g. 2--Effect of fill er concenlralion on volume resisti vity of fill ed PU, PU IPN and glass fiber reinforced PU rPN

200 INDIAN J. ENG. MATER. SeI.,JUNE 2002

more with the glass fiber reinforced polyurethane IPNs than homopolyurethane and polyurethane IPNs. The value of volume resistivity is lower than the surface resistivity in all composites. The difference between the value obtained from graphite and aluminum powder filled composite is more in surface resistivity and less in volume resistivity . The volume resistivity of graphite filled composite is very close to aluminum filled composite, but in case of surface resistivity the same is not found. The surface resistivity of both the fillers are different. It appears that the fillers are randomly embedded on the surface of the composite and their conductivity may be hindering while measuring the surface resistivity of the composite. The unfilled glass fiber composite has shown the higher volume and surface resistivity due to non-conducting behaviour of glass fibers The little deviation to higher side in surface and volume resistivity have also been noted due to increase of hydroxyl value as shown in Table I. The shape factor has not shown significant changes in conductivity level with the metal powder, but the glass fiber has increased the surface and volume resistivity as shown in Table 2. The presence of un-reacted styrene and polystyrene has imparted the decrease in conductivity level as shown in Figs 1 and 2. The lowering of conductivity in polyurethane IPN from the original homo-polymer is due to microscopic swelling of the polystyrene inside the polyurethane matrix . These swelling might have created voids and pockets in the layers of rubbery and plastics phases and hollow space between the core fine metal powder and polymer particle surface, resulting to decrease In

Table I- Effect o f hydroxy l value on surface and volume resisti vity PU and PU IPN s

Hydroxyl value Log o f vo lume Log o f surface (mg KOH/g) res isti vity res isti vity

(Ohm-c m) (Ohm)

160 (PU) 9 .5 129 9 .91 2 1 160 (PU IPN) 11.75 11 .89 225 (PU IPN ) 12.3060 12.621 2 354 (PU IPN ) 13. 1090 13.378 1

conductivity level. The other reason of lowering down the conductivity level in the case of polyurethane IPNs may be due to non-conducting coating of polystyrene on the metal fine powder and its arrangement in the polymer matrix . Unfilled polyurethane IPNs have shown the lower side conductivity level than homo-polyurethane because of phase change in morphology arises from the brittle thermoplastics and pre-polymer of polyurethane blends. The hydroxyl value of polyols have also played role in giving a slight increase in volume and surface resistivity . The present conductivity of the castor oi l based polyurethane is in the order of 10-10

(Ohm-cmr l, which is ten times smaller than the

conducti vity reported 12 of the polyurethane foam and rubber from the petrochemical resources and hundred times higher than the result reported29 for the polyurethane from castor oil and IPDI. This is perhaps due to the presence of bulky group in form of monoesters , di-esters tri-esters and tetra-esters in polyols chain, which may be supportive to resistivity. The other factors are voids in the polymer surface, fine particle arrangement in the polymer matrix , adhesion properties of the fillers and glass fibers with the resins. The better conducting result shown by graphite powder with respect to electrical conductivity is because of the better interaction and coupling action with the (i) double bond of styrene and (ii) the presence of un-reacted castor oil in the system, resulting to increase of cross-link density and spherullite formation. The rod like cluster and sliding nature of graphite powder surface might have supported the tunneling effect resulting to decrease in the volume and surface resi stivity of the filled

. 12·1 8 27 32·36 compos ite .. .

Lastl y, the ai m of the present studies was to develop the conducting composite from castor oil based polyurethane IPNs and to investigate the effects of various factors in deve loping the conducting polyurethane, polyurethane IPNs through meta l filling route . Many researchers28

.29 have reported the

conductivity is in the range of 10- 13_10- 15 (Ohm-cm)"

Table 2- Effect o f shape factor o n surface and volume resisti vity in PU IPN s

Name o f the filler

Aluminum powder

Graphite powder

Glass fiber

Shape factor

1.5

3.0

30 .0

Weight fraction

0 .235

0. 235

0 .235

Log o f volume resistivity Ohm-cm (H;tdrox;t l value, mg KOH/g)

160 225 354

9 .9 10.3364 11 .3388

9.838

12.529 13. 124 1 13.3496

Log o f surface resistivity , Ohm (H;tdrox;t l value, mg KOH/g)

160 225 354

10.334 10.5751 11.6875

10.01

13.00 13.5225 13.7894

SRIVASTAVA et al.: METAL POWDER-FILLED CASTOR OIL-BASED POLYURETHANES 201

of castor oil based polyurethane-PMMA, Polyurethane-isoprene and polyurethane-polystyrene IPNs. The conductivity of Si02/CaC03 filled castor oil based polyurethane IPN has been reported30 in the range of 10- 13 (Ohm-cmf'. These results are very much comparable with the present studies and support that non-conducting filler cannot impart conductivity in the composite. The un-treated aluminum and graphite powder filling have raised the conductivity level of the castor oil and its polyols based polyurethane IPNs . The trend found to raise the conductivity in polyurethane IPNs is encouraging. Potentially improved metal-filled composites in terms of tensile modulus of castor oil based polyurethanes have already been reported31

. The term composites have been applied due to heterophase materials when the dimensions involved approach the macroscopic particle/fiber. In the present studies, a material system is composed of a mixture or a combination of two or more macroscopic constituents that differ in form or material composition and are essentially insoluble to each other35

. The present conductivity of the filled castor oil based polyurethane and its IPNs are in the reach of the results reported by Okazaki36

, who have achieved the conductivity of carbon/aluminum silicate filled polyurethane materials in the range of 10-5_10-8

(Ohm-cmf' and has recommended for coating material to automobile door handle. Therefore, the metal filled polyurethane IPNs based on castor oil could be accepted as a conducting composite. The present products could be usefu l for conducting gasket, ceiling plates, flooring tiles, junction boxes etc. The efforts are made first time to develop the conducting composite from castor oil based polyurethane IPNs and further efforts could be made to develop the conducting composites from castor oil based polyurethanes IPNs for the suitable applications.

Conclusions From the above findings, it is concluded that (i)

filler concentration imparts the conductivity in composites made of castor oil and its polyols based polyurethane and polyurethane lPNs, (ii) shape factor of filler particle does not have the significant effects on the conductivity part of the composite, (iii) the bulky groups of polyols playa significant role in decreasi ng the conducting character of composites, (iv) un-reacted styrene and polystyrene dilutes also the conductivity at the surface level, (v) cylindrical and spherical particles both improve the strength of

particle filled composites, (vi) graphite powder increases the metallic character of polyurethane IPN composites resulting to increase in conducting character of the composite, (vii) glass fiber reinforcement improve the mechanical properties in terms of modulus31 but lowers the conductivity level of the composite, (viii) graphite filled castor oil-based polyurethane IPN system could be a good combination for low cost conducting composites, (ix) the level of conductivity is irrelevant for the selection of conducting filler and (x) the phase morphology has a greater role in deciding the conductivity of the composite which depends on the mixing process, characteristics of the plastic or rubbery domain in the LPN and the pressure applied during the curing process.

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