CA2352130A1 - Electrostatic dissipative plastics adapted particularly for use at elevated temperatures - Google Patents
Electrostatic dissipative plastics adapted particularly for use at elevated temperatures Download PDFInfo
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- CA2352130A1 CA2352130A1 CA002352130A CA2352130A CA2352130A1 CA 2352130 A1 CA2352130 A1 CA 2352130A1 CA 002352130 A CA002352130 A CA 002352130A CA 2352130 A CA2352130 A CA 2352130A CA 2352130 A1 CA2352130 A1 CA 2352130A1
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- fibers
- compression molding
- range
- carbon fibers
- resins
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
- C08K7/00—Use of ingredients characterised by shape
- C08K7/02—Fibres or whiskers
- C08K7/04—Fibres or whiskers inorganic
- C08K7/06—Elements
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/20—Conductive material dispersed in non-conductive organic material
- H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
Abstract
Resins suitable for use in compression molding processes are prepared by mixing between about 10 and 30 weight % of chopped partially carbonized fibers, i.e. fibers having a carbon content of between about 70 to 95 % by weight and an average size of about 100 .mu.m, with compression grade moldin g resins. Typically the compression grade molding resins are about 100 .mu.m. When chopped carbon fibers of reduced conductivity are used the curve obtain ed by plotting the resistivity against the concentration of carbon fibers is le ss steep in the critical regions of interest (i.e., between about 1010 and 1012 and between about 106 and 109) than the curve is obtained when high conductivity fibers are used. It has been observed that even if some dielectric breakdown occurs, the connected fibers are not as conductive and the effect has been found not to be as significant as is the case with the usual carbon fibers which have maximized conductivity.
Description
PATENT APPLICATION
For ELECTROSTATIC DISSIPATIVE PLASTICS ADAPTED
P.~1RTICULARL1' FOR USE AT ELEVATED TEMPERATURES
~storv of the App~tion This application is based upon and claims priority from United States Provisional Patent Application Serial Number 60/109.657 filed on November 24. 1998.
Technical Field This invention relates to electrostatic dissipative plastics and, more particularly, to electrostatic dissipative plastics that retain useful physical, chemical and electrical properties at elevated temperatures to at least 160° C. The invention has particular utility in the manufacture and use of electronic components and electronic devices though it is to be understood that it is not intended that the invention be so limited.
a ' 'o is For purposes of this specification and the appended claims, the following terms are 2 0 defined as follows:
a. Antistatic A ent. A material that can increase the conductn~ity of a plastic. It may be incorporated into a plastic or it may be applied as a surface coating or treatment.
b. esis ' 'ri~. Resistivity, the inverse of electrical conductivity, is a 2 5 measure of the resistance to the flow of an electric current and is measured either as a surface or a volume phenomenon. Surface resisti~.~iri~ is expressed in ohms per square (S?Jo) and is measured at the surface of a material, usually at room temperature. Information detailing methods for quantifying surface resistivity is given in ASTM D257 and EOSIESD S 11.11.
c. ( onductor. :A material that has a surface resisti~.iW less than about 10' O2,'c.
d. Insulator. :=~ material that has a surface resisti~.~ih- greater than about lU'3 S~: e.
e. Electrostatic Dissipative R~Iaterial (ESD). ,~ material that has a surface _ resistiv.~iy bet«:een that of a conductor and an insulator, usually defined as between about 1 U6 and 1 Ul' ~~'~.
Backwound :Art E~%ey major plastic resin is, in its natural state. an electric insulator and many have a significant tendency to accumulate static electric charges. The ability of plastics to generate - _ static electt7c charges and accumulate them variously due to the relative movement of one piece to another. separation of surfaces and the turbulent flo~~~ of contaminates in the air is well recognized. ,As an elample, it is the presence of static electrical charges on sheets of thermoplastic fihn. such as the familiar food wraps, that cause the sheets to stick to each other.
The increased compleairi~ and sensiti~.~ity~ of microelectronic dev.lces makes the control of _ _ sttztic discharge of particular concern in the electronic industy. There are many instances in the manufacture and use of electronic components and electronic dev.~ices in W
Itich an excessive accumulation of static charges can range from being a General nuisance to being a disabling or destructive force. Even a loc~~ voltage discharge ma~~ cause damage. to sensitive dev.~ices. The need to control StatlC Chai'a2 buildup by the controlled dissipation may require the total assembly environment to be constructed of electrostatic dissipative materials. It may also require that materials used in storing. handling and shipping electronic dev.~ices be made from electrostatic dissipative materials. Tote bores. strapping tape and sltipping containers are examples.
The prevention of the buildup of static electrical charges wTltich accumulate on plastics oar be controlled tlwough the use of se~~eral kinds of antistatic agents (antistats) ~~~hich variously are incorporated into the plastic as fillers or applied to the plastic as a surface coating.
?~~Iost conunonly. the electrostatic dissipative teclutologies include those that make use of h~~y oscopic agents. those. that make use of conductive particulates. those that make use of conductive fibers. and those that make use of locy molecular weight additives and conductive polymers. While the use of gnu of these techniques can be helpful in alle~.~ating electrostatic build-up. they are all subject to inherent limitations..
Sunace antistat agents are not ah~~ay~s reliable and can be inconsistent in operation.
Some of the surface antistats do not adhere to a surface W ith sufficient tenacih- to avoid being wiped a4yay over a period of use. Several surface anti5tats are essentially hygroscopic materials that cause a conductive film of water to form over a substrate. In this latter case. it is of course obvious that the resistiv.~iy of a given part «~ill be dependent upon and change with the ambient humiditl~.
i iThe other principal approach to the manufacture of ESD materials has been to fill a plastic with conductive materials like carbon black, metal fibers. carbon fibers, etc. This approach ad~.~antageously provides high conductiut~~. rapid static dissipation. reliability and peunanence . Its disadvantages lie in the fact that the mechanical properties of the plastic materials may he adversely affected at the required loading levels. Some materials such as 1 ~: carbon black are of such black intensiy.~ that ii is difficult to color the composite material.- Also it should be noted. the reduction in strength attributable to the filler may cause portions of the surface to Slough off: Tlus is anathema to clean rooms and othem~~ise presents a surface that is never clean. C arhon blacks and particulate graplutic fillers are especially prone to sloughing.
Conductive fillem are also sometunes difficult to disperse uniformly throughout a plastic .. matnl. Lack of unifoimit~~ can create "hot spots" iyithin a plastic matrix W here arcing or damaging static discharge can occur. Some effective fillers like carbon fibers are relatively expensive cvlule metal fibers cause excessive «-ear and abrasion to the flights of the screcys in mixing extruders.
~.~lore recently. attention has been focused upon the use of synthetic.
organic materials as antistatic agents. These antistatic agents can range from lox- molecular weight compounds to comparatively high molecular «~eight compounds and polymeric materials.
For ELECTROSTATIC DISSIPATIVE PLASTICS ADAPTED
P.~1RTICULARL1' FOR USE AT ELEVATED TEMPERATURES
~storv of the App~tion This application is based upon and claims priority from United States Provisional Patent Application Serial Number 60/109.657 filed on November 24. 1998.
Technical Field This invention relates to electrostatic dissipative plastics and, more particularly, to electrostatic dissipative plastics that retain useful physical, chemical and electrical properties at elevated temperatures to at least 160° C. The invention has particular utility in the manufacture and use of electronic components and electronic devices though it is to be understood that it is not intended that the invention be so limited.
a ' 'o is For purposes of this specification and the appended claims, the following terms are 2 0 defined as follows:
a. Antistatic A ent. A material that can increase the conductn~ity of a plastic. It may be incorporated into a plastic or it may be applied as a surface coating or treatment.
b. esis ' 'ri~. Resistivity, the inverse of electrical conductivity, is a 2 5 measure of the resistance to the flow of an electric current and is measured either as a surface or a volume phenomenon. Surface resisti~.~iri~ is expressed in ohms per square (S?Jo) and is measured at the surface of a material, usually at room temperature. Information detailing methods for quantifying surface resistivity is given in ASTM D257 and EOSIESD S 11.11.
c. ( onductor. :A material that has a surface resisti~.iW less than about 10' O2,'c.
d. Insulator. :=~ material that has a surface resisti~.~ih- greater than about lU'3 S~: e.
e. Electrostatic Dissipative R~Iaterial (ESD). ,~ material that has a surface _ resistiv.~iy bet«:een that of a conductor and an insulator, usually defined as between about 1 U6 and 1 Ul' ~~'~.
Backwound :Art E~%ey major plastic resin is, in its natural state. an electric insulator and many have a significant tendency to accumulate static electric charges. The ability of plastics to generate - _ static electt7c charges and accumulate them variously due to the relative movement of one piece to another. separation of surfaces and the turbulent flo~~~ of contaminates in the air is well recognized. ,As an elample, it is the presence of static electrical charges on sheets of thermoplastic fihn. such as the familiar food wraps, that cause the sheets to stick to each other.
The increased compleairi~ and sensiti~.~ity~ of microelectronic dev.lces makes the control of _ _ sttztic discharge of particular concern in the electronic industy. There are many instances in the manufacture and use of electronic components and electronic dev.~ices in W
Itich an excessive accumulation of static charges can range from being a General nuisance to being a disabling or destructive force. Even a loc~~ voltage discharge ma~~ cause damage. to sensitive dev.~ices. The need to control StatlC Chai'a2 buildup by the controlled dissipation may require the total assembly environment to be constructed of electrostatic dissipative materials. It may also require that materials used in storing. handling and shipping electronic dev.~ices be made from electrostatic dissipative materials. Tote bores. strapping tape and sltipping containers are examples.
The prevention of the buildup of static electrical charges wTltich accumulate on plastics oar be controlled tlwough the use of se~~eral kinds of antistatic agents (antistats) ~~~hich variously are incorporated into the plastic as fillers or applied to the plastic as a surface coating.
?~~Iost conunonly. the electrostatic dissipative teclutologies include those that make use of h~~y oscopic agents. those. that make use of conductive particulates. those that make use of conductive fibers. and those that make use of locy molecular weight additives and conductive polymers. While the use of gnu of these techniques can be helpful in alle~.~ating electrostatic build-up. they are all subject to inherent limitations..
Sunace antistat agents are not ah~~ay~s reliable and can be inconsistent in operation.
Some of the surface antistats do not adhere to a surface W ith sufficient tenacih- to avoid being wiped a4yay over a period of use. Several surface anti5tats are essentially hygroscopic materials that cause a conductive film of water to form over a substrate. In this latter case. it is of course obvious that the resistiv.~iy of a given part «~ill be dependent upon and change with the ambient humiditl~.
i iThe other principal approach to the manufacture of ESD materials has been to fill a plastic with conductive materials like carbon black, metal fibers. carbon fibers, etc. This approach ad~.~antageously provides high conductiut~~. rapid static dissipation. reliability and peunanence . Its disadvantages lie in the fact that the mechanical properties of the plastic materials may he adversely affected at the required loading levels. Some materials such as 1 ~: carbon black are of such black intensiy.~ that ii is difficult to color the composite material.- Also it should be noted. the reduction in strength attributable to the filler may cause portions of the surface to Slough off: Tlus is anathema to clean rooms and othem~~ise presents a surface that is never clean. C arhon blacks and particulate graplutic fillers are especially prone to sloughing.
Conductive fillem are also sometunes difficult to disperse uniformly throughout a plastic .. matnl. Lack of unifoimit~~ can create "hot spots" iyithin a plastic matrix W here arcing or damaging static discharge can occur. Some effective fillers like carbon fibers are relatively expensive cvlule metal fibers cause excessive «-ear and abrasion to the flights of the screcys in mixing extruders.
~.~lore recently. attention has been focused upon the use of synthetic.
organic materials as antistatic agents. These antistatic agents can range from lox- molecular weight compounds to comparatively high molecular «~eight compounds and polymeric materials.
,antistatic agents in the form of comparatively low molecular weight organic materials can be blended «-ith the plastic by melt miring. Typical examples would include quaternary arrunoruum salts. fatty acid esters and etho~~~lated amines.. T:ot uncommonly.
however. low molecular «~eight electrostatic agents suffer from poor heat stabiliW and.
depending on the melting point of the base resin. they may not survive melt processing temperatures. Low molecular weight antistats. even when successfully incorporated into a molding powder can cause problems. Low molecular cweight organic antistatic agents can migrate (bloom) to the sut~ace of the molded article. Surface migration may impair the appearance and tactile properties of an article. Furthermore, most of these additives function by absorbing a layer of _ ._ moisture on the sunace of the article and so requu~e a certain threshold of ambient humidit<-, beloc~- which they are ineffective.
It is knocwn that some organic antistatic agents are thermally unstable or chemically incompatible w°ith a palyrner at processing temperatures and cause unacceptable deyadation to or enter into umt~anted side reactions with the host polymer. This is especially so with more =._ complex host resins such as polyesters and polyamides.
Higher molecular weight organic and poly~rneric antistatic agents are available but here the miscibilih- of the antistatic agent w-ith the base resins often becomes a problem. In addition to physical incompatibilit~~ (poor miscibiliW), themnal instability and chemical incompatibiliy may also cause problems.
There are even more difficult reduv~ements imposed upon antistats in meeting special needs for a high temperature electrostatic dissipative material for use in applications such as «~afer back-end testing in «rluch chips ai-e pressed against a fixture (nests.
contactors and sockets) under elevated pressures and relatively high operating temperatures for plastics. such as 160° C . Flzctrostatic Cha1'geS resultlllg from the movement of the equipment. the «~afer itself or _. .. minute particles in the air surrounding the wafer can discharge suddenly and have significant and negative effects on the «~afer.
As discussed generalh~ above the requirements for an electrostatic dissipative plastics material fall in a specific. range of resistiv~iy. \fhen the sunace resisti~'itl, is less than about 106 S2'=. a composition has yew little insulating ability and is generally considered a conductor.
Such compositions are generally poor electrostatic dissipating polymeric materials because the ._ speed of bleed ofd is too rapid and sparking or arcing can occur. A
substrate W ith high conductiv.'iy does not offer protection from a destructive discharge as can result «.°hen the de~~ice is in the prolimin' of or contacts an electrically conductive element. In some applications the leads of a semiconductor deuce are in direct contact ~~~ith the ESD test fixture during the test cycle. It follows that the surface resistiv.~it~.- must be high enough to avoid um~~anted current flo«' bern~een leads. but lozi- enough to bleed off electrostatic charges in a controlled manner.
Summarizing, the sunace resistivih~ of an electrostatic dissipative plastic should lie in a range of from about 106 and 101'- and, when used. a conductive additive should have permanence. it should be not be affected by changes in the humidiW and it should not slougli off conductive particles. also in the case of contact with deuces having closely spaced leads. the = _ electrostatic dlSSlpati\'e plastics should not be shorted out by a cuwent being can-ied beriween rc~~o nearby' leads (cross tall:).
Earlu attempts to devzlop a product fitting these requirements (lugh compressive strength to at least 160° C and the surface conductiv.'iy to bleed off the static.) focused on the conventional approach of adding carbon black and organic antistats. It c~'as found that carbon black sloughs and most commercial organic. antistats are hy-oscopic in thev~ mode of action.
Thus the latter are ineffective in to«~ humidity en~rironments. Fec~'. if any. commercial organic antistats exhibit the thermal stabilih' required for incorporation into and subsequent use of the lugh temperature plastics (dimensional stability at about 160° C' and above). Elcellent thermal stability is especially necessaw for processes such as stock shape extrusion or compression molding in - _ ~shich the blend may be held in the molten state for one hour or more.
Other attempts «'ere made to four electrostatic dissipative plastics by adding carbon fibers at various concentrations. It «~as Ieamed that the cun:e in ~z~hich the r~sistiv~ih' is plotted vs. the concentration of the carbon fibers (sometunes refewed to in the art as the percolation curve) is vey steep in the reGion of desired surface resisti~.iW. This means that a small change in the overall or local concentrations of carbon fibers can cause the surface resistiv~iri~ to vam° from too resistive to too conductive. Poor dispersion and variations in fiber orientation further contribute to the inabilit<- of obtaining consistent and reproducible values of resistiv.7W. It has also been observed that even if a consistent surface resisti~,ny is achieved by yen- careful blending of the carbon fibers in certain substrate. such as polyetheretherketone, the resistance drops quickly and irreversibly if the testing is can led out at voltages exceeding about 100 volts.
It is thought that this happens because there is a dielectric breakdown of the thin sheath of ., polymer separating adjacent carbon fibers and also because carbonaceous lnaterial may be formed that provides conductive pathways.
It is also noted that many applications require that the resisti~.~ity- of the ESD material is limited to a predefined one or 1'vo decade range «~ithin the overall 106 -10'' ESD range. This «~as not found to be possible using additives such as standard or high performance carbon fibers.
1 ~~ again due primarily to the steepness of the response curve. Therefore.
pt~or to this invention, a material for making components W ith controlled resistiW ~ (i.e. in a one or t«~o decade range) for higher temperature (to 160°C j applications has remained an unfulfilled need of the semiconductor industry.
The patent art has recognized that conductive carbonaceous materials in fibrous form can be used advantageously to adjust the surface resistiuxtl- of plastics.
Reference is made. for example, to US patent number x.068,061 which. infer alia, makes use of elongated non-linear non-flarrunabhe conductive carbonaceous fibers havznG reversible deflection ratios greater than 1.'?:l and aspect ratios Greater than 10:1 to control the sunace resisti~-xh-of plastics.
L1S patent x.820.788 is of interest since it teaches the utilitZ- of using chopped linear .. . fibers of about 6 mils in length «~hich have been partially carbonized to a carbon content of betsz-een about 70-8~°o b~° weight. The disclosed invention is adapted for the use in injection moldinG processes in ~z~hich the filled resins are fend to the feed hopper in the form of chips appro~imateln 1-4 inch. The patent is also of interest for its discussion of other- related prior art processes in which carbonaceous conductive materials are used to alter the conducti~~it<- of resins.
Disclosure of the Invention ': <4ccordincly it is an oUject of this invention to prov.~ide plastic materials that have surface resistiv~ities in a range of from about 106 to about 10'' S2~W.
Another oUject of tlus invention is to prov.7de plastic materials that have sul~ace resistivties in the range of from about 10'° to about 10'' S2-''r.
Another object of this invention is to pro~.~ide plastic materials that have surface resistivfties in the range of from about 106 to about 109 52~~~, A further object of this invention to provide molding resins for use in compression molding processes plastic maiel~ials chat can produce shaped articles W luch have surface resisti~.rities that fall in a range of from about one to about t<vo decades in the overall range of from about 106 to about 10'' ~2~ ~~.
<~ further object of the invention is to pra~.~id~ ESD materials made by compression molding processes that retain useful physical. chemical and electrical properties at temperatures of about 160°C an d up«~ard.
<~nother ot~ject of this invention is to provride effective methods means for predictably and consistently providing electrostatic drsslpatrve plastics made by compression molding ' processes that have surface resistiv.~ities beriveen that of a conductor and that of a insulator.
A related object of this invention is to provide electrostatic dissipative plastics with high temperature capabilities that have stable sul~aces and do not Slough off particles.
And vet a further ohiect of this invention is to provide electrostatic dissipative plastics ~~hose surface resistiv.~y does not appreciably chance with changes in temperature or humidity.
_ ._ A still further oUject of this invention is to prov.~idz electrostatic dissipative plastics that substantially retain their initial surface resitiv~itz- after exposure to applied voltages in excess of about 1 OQ volts.
_7_ Best means of Practicing the Invention These and other objects of the invention are achieved by mixing between about 10 and 30 ~~~eight °'o of chopped partially carbonized fibers . i.e. fibers having a carbon content of betvween about 70 to 95°..~o by weight and an average size of about 1()Oum, with compression grade molding resins. Typically the compression grade molding resins are about 100 Eim in size.
~~Men chopped carbon fibers of reduced conductiv~ih.~ are used the cun%e obtained by plotting the resistivity~ against the concentration of carbon fibers is less steep in the critical regions of interest (i.e.. bet<veen about 101° and 1U1'- and beriveen about 106 and 109 ) than is the curve obtained when high conductivity fibers are used. It has been obsen~ed that even if some dielectric breakdown i i:~ occurs. the connected fibers are not as conductive and the effect has been found not to be as significant as is the case with the usual carbon fibers which have maximized conductiv~it~.~.
Since the invention is directed toW and products made by compression processes, the chopped fibers of about 6 mil in length as taught by the prior art in preparing injection molding resins are of little or no utilihy since it is phyysically difficult to mix the fibers with the comparatively small compression molding resins of this invention. It has been found that When it is attempted to mix fibers in the range of about G mils with approximately 100um compression molding resins. the carbonaceous fbers tend to "ball up'' and are difficult to impossible to disperse uniformly tlwoughout the fibers W ith the compression molding resins.
t% anous blends Were therefore made W ith P,~I (polvamidimides), glass fibers (GF) and carbon fibers. Glass fibers were used to help reduce the coefficient of thermal expansion to provide better dimensional st~lbiliy and mechanical properties of the components.
tldditional tests W ere made by including reduced conductii.~ity carbon fibers in other plastics including poly~ethervnides and polyethersulfones. By changing from the use of standard carbon fibers to reduced conducti~.~ih~ chopped fibers, it tvas possible to reproducibly establish materials with surface resistivities in the desired range. It is believed that these results are novel and have not been aclieved by any other means for compression molding high temperature engineering thermoplastics.
_g_ Carbon fibers «luch v~ere tested had carbonization of less than 1 UO
°.o and pailicularly carbon fibers in ~;Much the carbonization has proczeded from 8~ to 9~°.~o proved to be particularlv_ desv~able.
_g_
however. low molecular «~eight electrostatic agents suffer from poor heat stabiliW and.
depending on the melting point of the base resin. they may not survive melt processing temperatures. Low molecular weight antistats. even when successfully incorporated into a molding powder can cause problems. Low molecular cweight organic antistatic agents can migrate (bloom) to the sut~ace of the molded article. Surface migration may impair the appearance and tactile properties of an article. Furthermore, most of these additives function by absorbing a layer of _ ._ moisture on the sunace of the article and so requu~e a certain threshold of ambient humidit<-, beloc~- which they are ineffective.
It is knocwn that some organic antistatic agents are thermally unstable or chemically incompatible w°ith a palyrner at processing temperatures and cause unacceptable deyadation to or enter into umt~anted side reactions with the host polymer. This is especially so with more =._ complex host resins such as polyesters and polyamides.
Higher molecular weight organic and poly~rneric antistatic agents are available but here the miscibilih- of the antistatic agent w-ith the base resins often becomes a problem. In addition to physical incompatibilit~~ (poor miscibiliW), themnal instability and chemical incompatibiliy may also cause problems.
There are even more difficult reduv~ements imposed upon antistats in meeting special needs for a high temperature electrostatic dissipative material for use in applications such as «~afer back-end testing in «rluch chips ai-e pressed against a fixture (nests.
contactors and sockets) under elevated pressures and relatively high operating temperatures for plastics. such as 160° C . Flzctrostatic Cha1'geS resultlllg from the movement of the equipment. the «~afer itself or _. .. minute particles in the air surrounding the wafer can discharge suddenly and have significant and negative effects on the «~afer.
As discussed generalh~ above the requirements for an electrostatic dissipative plastics material fall in a specific. range of resistiv~iy. \fhen the sunace resisti~'itl, is less than about 106 S2'=. a composition has yew little insulating ability and is generally considered a conductor.
Such compositions are generally poor electrostatic dissipating polymeric materials because the ._ speed of bleed ofd is too rapid and sparking or arcing can occur. A
substrate W ith high conductiv.'iy does not offer protection from a destructive discharge as can result «.°hen the de~~ice is in the prolimin' of or contacts an electrically conductive element. In some applications the leads of a semiconductor deuce are in direct contact ~~~ith the ESD test fixture during the test cycle. It follows that the surface resistiv.~it~.- must be high enough to avoid um~~anted current flo«' bern~een leads. but lozi- enough to bleed off electrostatic charges in a controlled manner.
Summarizing, the sunace resistivih~ of an electrostatic dissipative plastic should lie in a range of from about 106 and 101'- and, when used. a conductive additive should have permanence. it should be not be affected by changes in the humidiW and it should not slougli off conductive particles. also in the case of contact with deuces having closely spaced leads. the = _ electrostatic dlSSlpati\'e plastics should not be shorted out by a cuwent being can-ied beriween rc~~o nearby' leads (cross tall:).
Earlu attempts to devzlop a product fitting these requirements (lugh compressive strength to at least 160° C and the surface conductiv.'iy to bleed off the static.) focused on the conventional approach of adding carbon black and organic antistats. It c~'as found that carbon black sloughs and most commercial organic. antistats are hy-oscopic in thev~ mode of action.
Thus the latter are ineffective in to«~ humidity en~rironments. Fec~'. if any. commercial organic antistats exhibit the thermal stabilih' required for incorporation into and subsequent use of the lugh temperature plastics (dimensional stability at about 160° C' and above). Elcellent thermal stability is especially necessaw for processes such as stock shape extrusion or compression molding in - _ ~shich the blend may be held in the molten state for one hour or more.
Other attempts «'ere made to four electrostatic dissipative plastics by adding carbon fibers at various concentrations. It «~as Ieamed that the cun:e in ~z~hich the r~sistiv~ih' is plotted vs. the concentration of the carbon fibers (sometunes refewed to in the art as the percolation curve) is vey steep in the reGion of desired surface resisti~.iW. This means that a small change in the overall or local concentrations of carbon fibers can cause the surface resistiv~iri~ to vam° from too resistive to too conductive. Poor dispersion and variations in fiber orientation further contribute to the inabilit<- of obtaining consistent and reproducible values of resistiv.7W. It has also been observed that even if a consistent surface resisti~,ny is achieved by yen- careful blending of the carbon fibers in certain substrate. such as polyetheretherketone, the resistance drops quickly and irreversibly if the testing is can led out at voltages exceeding about 100 volts.
It is thought that this happens because there is a dielectric breakdown of the thin sheath of ., polymer separating adjacent carbon fibers and also because carbonaceous lnaterial may be formed that provides conductive pathways.
It is also noted that many applications require that the resisti~.~ity- of the ESD material is limited to a predefined one or 1'vo decade range «~ithin the overall 106 -10'' ESD range. This «~as not found to be possible using additives such as standard or high performance carbon fibers.
1 ~~ again due primarily to the steepness of the response curve. Therefore.
pt~or to this invention, a material for making components W ith controlled resistiW ~ (i.e. in a one or t«~o decade range) for higher temperature (to 160°C j applications has remained an unfulfilled need of the semiconductor industry.
The patent art has recognized that conductive carbonaceous materials in fibrous form can be used advantageously to adjust the surface resistiuxtl- of plastics.
Reference is made. for example, to US patent number x.068,061 which. infer alia, makes use of elongated non-linear non-flarrunabhe conductive carbonaceous fibers havznG reversible deflection ratios greater than 1.'?:l and aspect ratios Greater than 10:1 to control the sunace resisti~-xh-of plastics.
L1S patent x.820.788 is of interest since it teaches the utilitZ- of using chopped linear .. . fibers of about 6 mils in length «~hich have been partially carbonized to a carbon content of betsz-een about 70-8~°o b~° weight. The disclosed invention is adapted for the use in injection moldinG processes in ~z~hich the filled resins are fend to the feed hopper in the form of chips appro~imateln 1-4 inch. The patent is also of interest for its discussion of other- related prior art processes in which carbonaceous conductive materials are used to alter the conducti~~it<- of resins.
Disclosure of the Invention ': <4ccordincly it is an oUject of this invention to prov.~ide plastic materials that have surface resistiv~ities in a range of from about 106 to about 10'' S2~W.
Another oUject of tlus invention is to prov.7de plastic materials that have sul~ace resistivties in the range of from about 10'° to about 10'' S2-''r.
Another object of this invention is to pro~.~ide plastic materials that have surface resistivfties in the range of from about 106 to about 109 52~~~, A further object of this invention to provide molding resins for use in compression molding processes plastic maiel~ials chat can produce shaped articles W luch have surface resisti~.rities that fall in a range of from about one to about t<vo decades in the overall range of from about 106 to about 10'' ~2~ ~~.
<~ further object of the invention is to pra~.~id~ ESD materials made by compression molding processes that retain useful physical. chemical and electrical properties at temperatures of about 160°C an d up«~ard.
<~nother ot~ject of this invention is to provride effective methods means for predictably and consistently providing electrostatic drsslpatrve plastics made by compression molding ' processes that have surface resistiv.~ities beriveen that of a conductor and that of a insulator.
A related object of this invention is to provide electrostatic dissipative plastics with high temperature capabilities that have stable sul~aces and do not Slough off particles.
And vet a further ohiect of this invention is to provide electrostatic dissipative plastics ~~hose surface resistiv.~y does not appreciably chance with changes in temperature or humidity.
_ ._ A still further oUject of this invention is to prov.~idz electrostatic dissipative plastics that substantially retain their initial surface resitiv~itz- after exposure to applied voltages in excess of about 1 OQ volts.
_7_ Best means of Practicing the Invention These and other objects of the invention are achieved by mixing between about 10 and 30 ~~~eight °'o of chopped partially carbonized fibers . i.e. fibers having a carbon content of betvween about 70 to 95°..~o by weight and an average size of about 1()Oum, with compression grade molding resins. Typically the compression grade molding resins are about 100 Eim in size.
~~Men chopped carbon fibers of reduced conductiv~ih.~ are used the cun%e obtained by plotting the resistivity~ against the concentration of carbon fibers is less steep in the critical regions of interest (i.e.. bet<veen about 101° and 1U1'- and beriveen about 106 and 109 ) than is the curve obtained when high conductivity fibers are used. It has been obsen~ed that even if some dielectric breakdown i i:~ occurs. the connected fibers are not as conductive and the effect has been found not to be as significant as is the case with the usual carbon fibers which have maximized conductiv~it~.~.
Since the invention is directed toW and products made by compression processes, the chopped fibers of about 6 mil in length as taught by the prior art in preparing injection molding resins are of little or no utilihy since it is phyysically difficult to mix the fibers with the comparatively small compression molding resins of this invention. It has been found that When it is attempted to mix fibers in the range of about G mils with approximately 100um compression molding resins. the carbonaceous fbers tend to "ball up'' and are difficult to impossible to disperse uniformly tlwoughout the fibers W ith the compression molding resins.
t% anous blends Were therefore made W ith P,~I (polvamidimides), glass fibers (GF) and carbon fibers. Glass fibers were used to help reduce the coefficient of thermal expansion to provide better dimensional st~lbiliy and mechanical properties of the components.
tldditional tests W ere made by including reduced conductii.~ity carbon fibers in other plastics including poly~ethervnides and polyethersulfones. By changing from the use of standard carbon fibers to reduced conducti~.~ih~ chopped fibers, it tvas possible to reproducibly establish materials with surface resistivities in the desired range. It is believed that these results are novel and have not been aclieved by any other means for compression molding high temperature engineering thermoplastics.
_g_ Carbon fibers «luch v~ere tested had carbonization of less than 1 UO
°.o and pailicularly carbon fibers in ~;Much the carbonization has proczeded from 8~ to 9~°.~o proved to be particularlv_ desv~able.
_g_
Claims (9)
1. A compression molding resin comprised of a base resin and chopped partially carbonized fibers.
2. A compression molding resin according to Claim 1 wherein the partially carbonized fibers are comprised of from about 70 to 95 wt% carbon.
3. A compression molding resin according to Claim 1 wherein the partially carbonized fibers are present in an amount of from about 10 to 30 weight %.
4. An article made by the compression molding a resin according to Claim 1 wherein the surface resistivity is in a range of from about 10 6 to about 10 12 .OMEGA./~.
5. An article made by the compression molding a resin according to Claim 1 wherein the surface resistivity is in the range of from about 10 10 to about 10 12 .OMEGA./~.
6. An article made by the compression molding a resin according to Claim 1 having a surface resistivity in the range of from about 10 6 to about 10 9 .OMEGA./~.
7. An article made by the compression molding a resin according to Claim 1 having a surface resistivity in a range of from about one to about two decades in an overall range of from about 10 6 to about 10 12 .OMEGA./~.
8. An article made by compression molding a resin according to Claim 1 that retains useful physical, chemical and electrical properties at temperatures of about 160°C an d upward.
9. A compression molding resin comprised of a base resin and chopped partially carbonized fibers in which the base resin is selected from the group consisting of polyphenylenesulphide, polyamidimide, polyetheretherketone, polytetrafluoroethylene.
polyetherimides orpolyethersulfones
polyetherimides orpolyethersulfones
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US10965798P | 1998-11-24 | 1998-11-24 | |
| US60/109,657 | 1998-11-24 | ||
| PCT/US1999/028109 WO2000032682A1 (en) | 1998-11-24 | 1999-11-24 | Electrostatic dissipative plastics adapted particularly for use at elevated temperatures |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2352130A1 true CA2352130A1 (en) | 2000-06-08 |
Family
ID=22328857
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002352130A Abandoned CA2352130A1 (en) | 1998-11-24 | 1999-11-24 | Electrostatic dissipative plastics adapted particularly for use at elevated temperatures |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP1133773A4 (en) |
| JP (1) | JP2003513109A (en) |
| CA (1) | CA2352130A1 (en) |
| WO (1) | WO2000032682A1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9125310B2 (en) * | 2011-12-16 | 2015-09-01 | Sabic Global Technologies B.V. | Electrostatic dissipative composites |
Family Cites Families (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4837076A (en) * | 1985-04-18 | 1989-06-06 | The Dow Chemical Company | Carbonaceous fibers with spring-like reversible deflection and method of manufacture |
| GB8614784D0 (en) * | 1986-06-18 | 1986-07-23 | Griffiths J A | Surface coating |
| US5593776A (en) * | 1994-02-08 | 1997-01-14 | Osaka Gas Company, Limited | Fluororesin composites |
| US5723186A (en) * | 1994-09-09 | 1998-03-03 | Precision Fabrics Group, Inc. | Conductive fabric and process for making same |
| JP3525525B2 (en) * | 1994-11-28 | 2004-05-10 | 出光石油化学株式会社 | Polyarylene sulfide resin composition |
| TW318857B (en) * | 1995-01-20 | 1997-11-01 | Sumitomo Chemical Co | |
| US5599615A (en) * | 1995-11-09 | 1997-02-04 | Xerox Corporation | High performance electric contacts |
| JP3642873B2 (en) * | 1996-03-26 | 2005-04-27 | 日本ジーイープラスチックス株式会社 | Antistatic resin composition |
| JPH107898A (en) * | 1996-06-28 | 1998-01-13 | Sumitomo Chem Co Ltd | Polyether-ketone resin composition and carrier for processing and treating semiconductor wafer |
| US5695197A (en) * | 1996-12-06 | 1997-12-09 | Farley; Michael L. | Seal ring method of sealing and molding composition comprising blend of PTFE copolymer, polyamide and carbon fiber therefor |
| US5820788A (en) * | 1997-01-29 | 1998-10-13 | Sgl Technic Ltd. | Electroconductive antistatic polymers containing carbonaceous fibers |
-
1999
- 1999-11-24 CA CA002352130A patent/CA2352130A1/en not_active Abandoned
- 1999-11-24 EP EP99965054A patent/EP1133773A4/en not_active Withdrawn
- 1999-11-24 JP JP2000585319A patent/JP2003513109A/en active Pending
- 1999-11-24 WO PCT/US1999/028109 patent/WO2000032682A1/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| EP1133773A4 (en) | 2002-06-05 |
| EP1133773A1 (en) | 2001-09-19 |
| JP2003513109A (en) | 2003-04-08 |
| WO2000032682A1 (en) | 2000-06-08 |
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