CA1115768A

CA1115768A - Electrically conductive web

Info

Publication number: CA1115768A
Application number: CA308,738A
Authority: CA
Inventors: Walter M. Westberg; Thomas W. Lindsay
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1977-09-06
Filing date: 1978-08-03
Publication date: 1982-01-05
Also published as: AU517815B2; IT1157170B; AU3956478A; IT7850979A0; FR2402378B1; DE2839036A1; BR7805810A; GB2003791B; GB2003791A; JPS6010847Y2; FR2402378A1; JPS5459078U

Abstract

ABSTRACT
An electrically conductive web for safely and quickly discharging static electricity accumulated on per-sonnel is disclosed. The web comprises a semi-conductive polymeric surface layer, which is in electrical contact with a conductive foraminous layer. The relative conduc-tivity of these layers result in desirable electrical dis-charge characteristics. The composite structure may be bonded to a suitable supporting substrate and suitable electrical connection between the conductive foraminous layer is made with an electrical ground. Charged personnel coming into contact with the semi-conductive surface layer have their static charge quickly and safely discharged to ground potential.

Description

913,967 ~ 7 ~ ~

ELECTRICALLY CONDUCTIVE W~B

Thls inventlon relates to an electrlcall~ conduc-tlve web whlch may be fabricated into floor mats, curtains, clothing, and other article~. The web exhlblts controlled electrical properties which permit it to be efrectively used to discharge statically charged ob~ects coming into contact with the semi-conductive surrace layer.
A person may become statically charged to a po-tentlal of up to 30,000 volts. I~ a charged person comes into contact with a hlghly conductlve, well-grounded sur-race, a substantlally lnætantaneous ~park discharge will result. Such ~tatlc discharges can be both uncomrortable and hazardous.
Conventlonal materlals ~or controlllng statlc electrlclty are commerclally avallable and are used for a varlety Or purposes. Typlcal uses include the ellmlnatlon of static shock discom~ort and spark avoldance ln hazard-ous envlronments such as munitlons plans and medlcal oper-ating rooms. These materlals may also be fabrlcated lnto floor mats ror use wlth computer terminals and other statlc sen~ltlve electronlc equlpment.
There are two prlor approa¢hes to conductlve webs used for control Or static electrlclty.
One prlor art approach 18 to provlde a mat whlch is more conductlve than appllcants'. Unlted States Patent No. 3,406,126 lssued October 15, 1968 to I. Lltant 18 an example of a hlghly conductive web. Hlghly conductlve mats are useful ~or preventlng build up o~ statlc charge.
. ~

., ~ .. .. . . . .

.;
.. :. , ~ ., -. :
' '," ~ ..

7~8 However, these webs are usually too conductive to be used for dissipating static charge already accumulated on per-sonnel. A highly conductlve rloor coverlng or web wlll permlt substantially lnstantaneous dlscharge whlch may result ln hazardous spark dlscharge.
The second conventlonal approach to static con-trol lnvolves the use of highly resistive webs. United States Patent No. 3,891~786 issued October 15, 1973 to ;
C. Conklln is a repre~entative example Or a carbon loaded ~;
prior art web. These webs are usually su~ficient to con-trol the discharge Or accumulated static. However, such webs usually exhlblt electrlcal characteristlcs which are dependent on the amblent humldlty. In most appllcatlons, lt is deslrable to have a conductive web wlth electrlcal dlscharge properties whlch are lndependent o~ the molsture level o~ the operatlng envlronment.
Appllcants' web provldes a means ror both pre-venting the build-up o~ static charge as well as the rapld and sare dlscharge Or accumulated static electrlclty.
Appllcants' web can discharge a statlcally charged person ~rom 30,000 volts to ground potentlal wlthin one or two seconds Or contacting the mat. However, the controlled electrlcal characterlstlcs Or the mat structure also lnsure surflclent reslstlvlty to prevent spark dlscharge.
Appllcants' novel web structure dlfrers from prlor art structures, and exhiblts an unusual and deslrable electrlcal discharge property. When appllcants' web ls grounded and a ~tatlc charge ls applled to the web through an lnsula~lve materlal, appllcants' mat exhiblts a faster discharge rate than a grounded metal plate, under simllar conditions. Although the reason for this anomalous be- ;
havior is not known, it ls a desirable property since it lnsures that a ~tatically charged person wearing s~nthetic ~oled shoes will be rapldly discharged ln spite of the in-sulative nature of his shoes.
Applicants' structure comprises a foraminous layer of material, such as a scrim or open cell foam which is rendered electrlcally conductlve by coatlng it wlth latex or other suitable binder resin containing conductive carbon partlcles. The reslstlvlty o~ the coated foraminous layer should range from approxlmately 103 to 107 ohms.
Thls coated foraminous layer is then bonded to a layer of semi-conductive thermoplastic polymeric material. When a scrim is used as the ~oraminous layer the semi-conductlve polymerlc material penetrates and encompasses the conduc-tlve scrlm making good electrlcal and mechanlcal bond wlth lt. Thl~ obviates the need for chemical adheslon between the conductlve foramlnous scrlm and the seml-conductlve polymerlc layer whlle insurlng good electrlcal and mechan-lcal bond.
The polymerlc seml-conductlve layer should have a volume re~l~tlvit~ between 1 x 107 and 1 x 1012 ohm-cm.
Suitable polymerlc resln for thls purpose lnclude thermo-plastlc materials such as plastlclzed vlnyl, especlallyin plastlsol rorm. Approprlate plastlclzers may be com-blned with the polyvlnyl chlorlde to yleld the desired volume reslstivlty. Addltlonal additives may be combined wlth the polymerlc resln to lmpart deslrable physlcal , . ,, , .
.. . . . .. .
.:. . : .
,. , - . . ~:
.

- .. . ' '.

propertles such a8 abra~ion reslstance, toughness, color and flexlbllity. The ~emi-conductive layer should prerer-ably have a thlckne~s between .OlQ and .125 lnches (.025 cm. and .039 cm.). After curlng, a supportlng sub~trate may be bonded to the web material to produce the rinlshed artlcle.
A conductlve connectlon ls made wlth the rlnished web to enable a ground wlre to be connected to the conduc-tlve roramlnous layer. Typically thls connection may be formed as a metallic grommet which perforates the web maklng electrlcal contact with the coated roramlnous layer.
Flgure 1 15 a rragmentary cross-section Or the completed web structure showlng a conductlve scrim as the foramlnous web; and Flgure 2 1~ a fragmentary cross-sectlon Or a second embodlment 30 Or the web structure showlng a conduc-tlve roam as the foraminous web.
In Flgure l, a coated conductive roramlnous layer

2 18 shown partlally embedded ln a seml-conductive polymer-lc materlal 3. Thls composlte structure 1~ then bonded toa suitable support substrate 4. The support ~ubstrate 15 selected to match the mechanlcal characterlstlcs Or the rlnlshed web wlth lts lntended appllcatlon. A reslllent foam layer may be used to provide cushlonlng when the mat 18 used as a rloor covering over hard sur~a¢es and a thln sheet o~ stl~ plastlc or flberboard may be used when the mat or web 18 used under chalrs especlally over carpetlng.
In Flgure 2, a conductlve foam layer 6 rorms a ~oramlnous web whlch 18 bonded to a seml-conductlve poly-.. . ~ ~ ............................. .. . .
!, ,;, , "

', ` :' . ' : ' '': ' "`' "

meric layer 5. The area 7 shows semi-conductive polymeric material penetrating the foam, insuring a good mechanical and electrical connection with the semi-conductive layer.
The foraminous layer may be an open weave textile scrim of sufficient strength to impart toughness and mech-anical strength to the composite structure. Woven cotton scrim with 12-24 yarns per inch has sufficient strength for use in a conductive floor mat. Such a scrim may be rendered electrically conductive by coating the scrim with carbon mixed with a suitable binder such as chloroprene latex.
The foraminous layer may also take the form of an open cell foam such as urethane foam. This type of forami-nous layer may be rendered electrically conductive by im-mersing it in a carbon loaded chloroprene latex, to coat it.
In either embodiment, sufficient carbon must be included in the completed foraminous layer to produce a resistivity of the web between approximately 103 to 107 ohms as measured between two 2.27 kg, 6.35 cm. (dia.) electrodes placed 1.27 cm. apart on the web with a D.C.
potential difference between the electrodes of 500 volts.
Although the exact amount of carbon loading necessary to meet these parameters depends upon the in-herent conductivity of the foraminous layer, textile scrims having approximately .34 grams/m2 to 7.9 g/m of carbon black suspended in latex or chloroprene binder have been found to pro~ide operable resistivity.
The semi-conductive polymeric material must ex-hibit a volume resistivity of 107 to 1012 ohms - cm. The specific resistivity is selected to match the finished web to its intended application. Generally, the greater volume resistivity the longer it takes the web to discharge a given static potential.
It should be noted that the discharge rate de-creases linearly with the increased thickness of the semi-conductive layer. This parameter may be varied to matchthe electrical and mechanical properties of the completed , web to its intended use.
.

" X

Also, for a given polymeric material, the type and amount of plasticizer can be used to control the volume resistivity exhibited by the semi-conductive layer. In general, increasing the amount of plasticizer used decreases the volume resistivity of the material.
The following two examples serve to illustrate the invention. In each example, the dry and liquid ingredients are mixed with a paddle mixer until well dispersed.
Films cast from these resins are cured for five minutes at 170 C. and are then tested for volume resistivity.

Example 1 Parts by `
Weight a) Resin: Copolymer, 95% polyvinyl 185.0 ~-chloride, 5% polyvinyl acetate b) Plasticizer: A linear dialkyl140.0 phthalate having a mixture of C7, Cg, and Cll alkyl groups, which phthalate is marketed under the trade mark "Santicizer 711"
by Monsanto Corporation. This phthalate is a clear oily liquid having a molecular weight of 414, ~ -a density of 8.06 at 25C calculated as pounds/gallon and a boiling point of 252C at 100 mm. HG.
c) Plasticizer: Epoxidized soybean oil 9.6 d) Stabilizer: Barium/cadmium liquid 5.5 e) Fungicide: 1.7 f) Pigment: 7.0 Volume Resistivity 7 x 10 ohm-cm.

" ~;
. .
i', - . : : , ~

Example 2 Parts by -Wel~ht a) Resin: Cellulose acetate butyrate 5.0 b) Pla~ticlzer-AT: Low vlscoslty, butyl-capped, low molecular welght~ethylene o~lde ollgomer such a~ Moba ~ antlstatic plastlclz-AT (Mobay 18 a registered trademark of Mobay Chemlcals, Plttsburg, Pennsylvanla~ U.S.A.) 4.0 ~olume Re~lstlvlty 5.3 x 101 ohm-cm.

The electrlcally conductive web i8 rabrlCated by combinlng elther one o~ the two precedlng exemplar resins with the conductive foramlnous layer. This may be convenlently accompllshed by laylng a thin t.010 - .125 inch) layer of the resin on a patterned release ~abric.
This layer 18 then heated for three mlnutes at 170 C. to "gel" the resln. The conductlve ~oramlnous layer 19 lald on top of the gelled layer and addltlonal resln must be applled to the surface Or the foraminous layer to flll and slightly overcoat the web materlal lr deslred. The com-poglte 18 then cured at 170 C. ror ix mlnutes. I~
desired, the composlte may be bonded to a suitable support substrate wlth an approprlate adhesive. The rinlshed material exhlbit~ good me¢hanical bondlng between the semi-conductive layer and the conductive roramlnous layer~
When the finished material i~ to be used as a rloor ooverlng, it is deslrable to lnsert a metallic grom-~ met through the rlnished web to make electrlcal contact ; with ~oramlnou~ conductive layer and thu~ permit a ground wlre to be connected to the ~loor co~ering.
.

. , ~. ,......... ,. - . .

Samples of a web formed ln accordance with this invention exhibit electrical dlscharge characteristlcs as delineated in Table l.
Table 1 Volume Dlscharge Time Sample Caliper Resistivity (seconds) No. (lnches) (ohm-cm.) 5kv to 1 kv -~
l .057 7 x 109 .2 2 .094 5 x 109 .2

3 .078 8 x 10 .2

4 .o86 6 x 109 .2 .108 9 x 109 .2 A web ~ormed ln accordance with the present ln- r`
vention also exhlblts an unusual and deslrable electrlcal property when the web ls grounded ror use as a floor mat.
In such appllcatlons, lt ls hlghly deslrable to dlscharge a charged per~on very qulc~ly. Leather soles are rela-tlvely good conduotors and permlt rapld dlscharge tlmes.
However, it has been round that a person wearlng shoes wlth synthetlc soles ls dlscharged very 810wly when ln contact wlth conventlonal materlals. It has been round that a person wlth synthetlc soles may be dlscharged more qulckly when ln conta~t wlth a grounded ~loor mat ~ormed ln accord-ance wlth the present inventlon than when ln contact wlth a grounded metal plate. Thls phenomenon was verlfied by ~measurlng the amount of tlme that was requlred to dlscharge an electrode whlch slmulates a person from 5,000 volts to 1,000 volts through an lnsulatlve layer of polymeric materlaI whlch slmulates the synthetic sole of a shoe.
The in~ulatlng layer has a volume reslstlvlty of 5 x 1012 ,:

ohm-cm. The charged electrode with the lnsulatlve layer was placed in contact wlth a grounded conductlve web and the dlscharge time wa~ measured. Then the electrode and lnsulatlve layer was recharged and placed ln contact with a grounded metal plate and dlscharge tlme was agaln meas-ured. The results o~ thls test are summarlzed ln Table 2.
Table 2 Grounded Conductlve Web Grounded Metal Plate Dlscharge Tlme (sec)Discharge Tlme (sec)

5~000 v - 1.000 v5,000 v - 1.000 v 5.0 23.0 4.8 25.0 5.0 31.0

6.o 7.0

7.0 7.0 6.o lo.o 5,0 16.0 g.o 13.0 7.0 13.0 4 0 19.0 5.0 15.0 6.0 8.0 Average Di~¢hargeAverage Dlscharge Tlme ~ 5.8 se¢. Tlme ~ 12 sec.

,. . . . .

Claims

913,967 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A conductive web for discharging a static electric charge impressed thereon comprising:
a layer of semi-conductive thermoplastic poly-meric material with a volume resistivity between 1 x 107 and 1 x 10l2 ohm-cm., in direct physical and electrical connection with, a foraminous layer coated with carbon loaded resinous material to impart a resistivity of between 103 ohms and 107 ohms, and conductive means in electrical contact with said foraminous layer for connecting said foraminous layer to ground potential.

2. A conductive web for discharging a static electric charge impressed thereon comprising:
a layer of semi-coffductive thermoplastic poly-meric material formed from a resin consisting essentially of a copolymer resin of 95 percent polyvinyl chloride and 5 percent polyvinyl acetate plasticized with diisodecyl phthalate and epoxidized soybean oil exhibiting a volume resistivity between 1 x 107 and 1 x 1012 ohm-cm. in direct physical and electrical contact with, a foraminous layer coated with carbon loaded resinous material to impart a resistivity of between 103 and 107 ohms, and conductive means in electrical contact with said foraminous layer for connecting said foraminous layer to ground potential.

3. A conductive web for discharging a static electric charge impressed thereon comprising:
a layer of semi-conductive thermoplastic poly-meric material formed from a resin consisting essentially of cellulose acetate butyrate plasticized with a low vis-cosity monomeric plasticizer characterized as a butyl-capped, ethylene oxide oligmer exhibiting a volume resis-tivity between 1 x 107 ohm-cm. and 1 x 1012 ohm-cm., in direct physical and electrical connection with, a foraminous layer coated with carbon loaded resinous material to impart a resistivity between 1 x 103 and 1 x 107 ohms, and conductive means in electrical contact with said foraminous layer for connecting said foraminous layer to ground potential.

4. The conductive web of claim 1 wherein said foraminous layer is characterized as an open cell poly-meric roam.

5. The conductive web of claim 1 wherein said foraminous layer is characterized as an open weave textile scrim.

6. The conductive web of claim 1 wherein said conductive means in electrical contact with said foraminous layer is characterized as a metallic grommet which perfor-ates the completed conductive web and passes through said foraminous layer.

7. The conductive web of claim 1 further in-cluding a support substrate bonded to said foraminous layer.

8. The conductive web of claim 7 wherein said support substrate is characterized as a flexible polymeric foam.

9. The conductive web of claim 7 wherein said support substrate is characterized as a rigid sheet.