Claims
- 1. An electrically conductive polyethylene foam sheet which comprises a sheet having a closed cell foam structure with a smooth closed cell surface and exhibiting a surface resistivity of not more than 10.sup.5 ohms/square; said sheet having been formed by heating an extruded, foamable, non-crosslinked sheet preform containing, per 100 parts by weight of a mixture containing about 100-0% by weight of polyethylene and about 100-0% by weight of an ethylene copolymer, 2 to 15 parts by weight of a chemical blowing agent, 0.5 to 1.5 parts by weight of a crosslinking agent and 12 to 20 parts by weight of conductive carbon black, said polyethylene having a melt index of about 10-40 and the ethylene copolymer having a melt index of about 10-40.
- 2. An electrically conductive polyethylene foam sheet according to claim 1, wherein said mixture contains about 20-80% by weight of polyethylene and about 80-20% by weight of the ethylene copolymer.
- 3. An electrically conductive polyethylene foam sheet according to claim 2, wherein said ethylene copolymer is a copolymer prepared from a monomeric mixture containing at least 60% by weight of ethylene and at least 5% by weight of at least one ethylenically unsaturated monomer copolymerizable with ethylene and capable of providing a copolymer that increases the flexibility of the polyethylene.
- 4. The electrically conductive polyethylene foam sheet according to claim 2, wherein the ethylene copolymer is a copolymer prepared from a monomeric mixture containing at least 60% by weight of ethylene and 5-40% by weight of vinyl acetate.
- 5. The electrically conductive polyethylene foam sheet according to claim 2, wherein the polyethylene has a melt index of from 15 to 30 and the ethylene copolymer has a melt index of from 20 to 25.
- 6. An electrically conductive shaped article formed by molding the electrically conductive polyethylene foam of claim 1.
- 7. An electrically conductive polyethylene foam sheet according to claim 1, wherein said sheet contains conductive carbon black having a particle size not greater than 30 mm and an effective surface area not greater than 900 m.sup.2 /g.
- 8. An electrically conductive polyethylene foam sheet according to claim 1, wherein said sheet has a thickness of from 1/4 to 5/8 of an inch.
- 9. An electrically conductive foamable polyethylene composition capable of being extruded into a profile having a closed cell foam structure and exhibiting a surface resistivity of not more than 10.sup.5 ohms/square, which consists essentially of, per 100 parts by weight of a mixture consisting essentially of about 20-80% by weight of polyethylene and about 80-20% by weight of an ethylene copolymer, 12-20 parts by weight of conductive carbon black, 0.5 to 1.5 parts by weight of a crosslinking agent and 2 to 15 parts by weight of a chemical blowing agent; said polyethylene having a melt index of about 10-40 and the ethylene copolymer having a melt index of about 10-40.
- 10. An electrically conductive, foamable, polyethylene composition according to claim 9, wherein said ethylene copolymer is a copolymer prepared from a monomeric mixture containing at least 60% by weight of ethylene and at least 5% by weight of at least one ethylenically unsaturated monomer copolymerizable with ethylene and capable of providing a copolymer that increases the flexibility of the polyethylene.
- 11. The electrically conductive foamable polyethylene composition according to claim 9, wherein the ethylene copolymer is a copolymer prepared from a monomeric mixture containing at least 60% by weight of ethylene and 5-40% by weight of vinyl acetate.
- 12. The electrically conductive foamable polyethylene composition according to claim 9, wherein the polyethylene has a melt index of about 10 to 40 and the ethylene copolymer has a melt index of about 10 to 40.
- 13. The electrically conductive foamable polyethylene composition according to claim 9, wherein the polyethylene has a melt index of from 15 to 30 and the ethylene copolymer has a melt index of from 20 to 25.
- 14. An electrically conductive foamable polyethylene composition capable of being extruded to form a profile having a closed cell structure and exhibiting a surface resistivity of not more than 10.sup.5 ohms/square, which consists essentially of, per 100 parts by weight of a mixture consisting essentially of about 100-0% by weight of polyethylene and about 100-0% by weight of an ethylene copolymer, 12 to 20 parts by weight of conductive carbon black, 0.1 to 1.5 parts by weight of a crosslinking agent and 2 to 15 parts by weight of a chemical blowing agent; said polyethylene having a melt index of about 10-40 and the ethylene copolymer having a melt index of about 10-40 .
- 15. A composition according to claim 14, wherein said conductive carbon black has a particle size not greater than 30 mm and an effective surface area not greater than 900 m.sup.2 /g.
Parent Case Info
This is a division of application Ser. No. 857,301, filed Apr. 30, 1986, now U.S. Pat. No. 4,719,039 which application is a continuation of application Ser. No. 688,413 filed Jan. 2, 1985 now abandoned.
This invention relates to an extruded, crosslinked polyethylene foam which is electrically conductive, an electrically conductive shaped article formed therefrom, a foamable polyethylene-containing composition, and a method for producing the foam wherein an extruded non-crosslinked preform is prepared by extrusion of a foamable composition comprising a resinous mixture containing carbon black and crosslinking and blowing agents and wherein the preform is thereafter heated to effect crosslinking and foaming of the composition.
For the past several years, the users and manufacturers of communication equipment, photographic processing equipment, electronic devices and the like have become aware of the need to protect their equipment and devices from the adverse effects of static electricity, especially electrostatic discharge (ESD). In order to satisfy this need, a new industry has developed; namely the static control industry, and many different products for controlling static electricity are now commercially available. Among such products are electrically conductive foams of polyurethane and polyolefins such as polyethylene which may be used to provide both physical and electrostatic protection for sensitive electronic parts; mats, films, and sheets of synthetic resins, such as polyesters, containing conductive additives for providing working and walking surfaces that will dissipate static electricity as well as static shielding bags formed from metallic coated synthetic resin, e.g., a polyester, which may be used to provide Faraday Cage Protection for electronic components encased within the bags. Also various carriers such as boxes, trays, parts bins and the like, have been made from conductive plastics and reinforced conductive plastics. Such products require the use of different synthetic resins and often require rather elaborate procedures for producing products having the necessary flexibility, impact resistance, structural rigidity and tensile strength suitable for each intended application.
In order to provide guidelines for establishing and implementing an Electronic Discharge Control Program in accordance with DOD-STD-1686, the Department of Defense of the United States has published a handbook entitled "Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts Assemblies, and Equipment (Excluding Electrically-Initiated Explosive Devices)" which is identified as DOD-HDBK-263, May 2, 1980. On pages 2 and 3 of the handbook the following definitions are given:
Anti-static Material. ESD protective material having a surface resistivity greater than 10.sup.9, but not greater than 10.sup.14 ohms per square.
Conductive Material. ESD protective material having a surface resistivity of 10.sup.5 ohms per square maximum.
Insulative Material. Material having surface resistivities greater than 10.sup.14 ohms per square.
An object of the present invention is to develop an electrically conductive foam which can be used to provide many different static controlling products.
Yet another object of the present invention is to provide a foamable electrically conductive preform by extrusion in the form of a sheet which can be readily fabricated into shaped articles by conventional molding techniques.
Still another object of the invention is to provide a method of producing the crosslinked, electrically conductive polyethylene foam which is continuous and which comprises compounding, extruding, shaping, and cooling to provide a foamable and crosslinkable preform and heating the preform to activate the crosslinking and blowing agents sequentially thereby forming the foam product.
This invention contemplates an electrically conductive, crosslinked extruded polyethylene foam having a density of from 2 to 12 pounds per cubic foot (PCF) and a surface resistivity of not more than 10.sup.5 ohms/square and preferably of not more than 10.sup.4 ohms/square. In addition to its outstanding electrical conductivity, the crosslinked polyethylene foam of this invention exhibits high tensile strength and elongation at break, very low water absorption and excellent flexibility thereby providing a foam material that is readily thermoformable into simple and complex shapes by a variety of conventional fabrication techniques including vacuum forming and compressed air forming, that is thermally weldable, and that is capable of being cemented or laminated with itself or other materials, for example, synthetic resins, paper, metal, etc.
The electrically conductive crosslinked polyethylene foam of this invention is produced from a preform of a foamable composition that contains low-density polyethylene and/or, a copolymer of ethylene and an ethylenically unsaturated monomer copolymerizable with the ethylene; for example, vinyl acetate, propylene, butene, butadiene, acrylic acid, etc., with vinyl acetate being the preferred monomer since the ethylene/vinyl acetate copolymer is particularly effective in reducing brittleness and imparting improved flexibility and moldability to the resulting foam especially a foam prepared from an admixture of polyethylene and an ethylene/vinylacetate copolymer.
In general, the foamable composition comprises 100 parts by weight of a resinous material (hereinafter simply referred to as resin) containing 0 to 100%, preferably about 20-80% by weight and more preferably 40-60% by weight of at least one low-density polyethylene and 100 to 0%, preferably 80-20% by weight, and more preferably 60-40% by weight, of at least one of ethylene copolymer, especially an ethylene/vinyl acetate copolymer, and 12-25 parts by weight of conductive carbon black, 0.5 to 1.5 parts by weight of a crosslinking agent and 2 to 15 parts by weight of a blowing agent per 100 parts by weight of the resinous material. The low-density polyethylene has a specific gravity of from 0.91 to 0.93 (as determined by ASTM D-1505-68) and a melt index of from 10 to 40 and preferably from 15 to 30 (as determined by ASTM D-1238-79). Also mixtures of low-density polyethylenes having different melt indices within the above designated range may be used along or admixed with the copolymer.
The ethylene copolymer used in the foamable composition of the present invention should also exhibit a melt index of from 10 to 40, preferably 20 to 25, and should have an ethylene content of at least 60% by weight. With a copolymer of ethylene and vinyl acetate, the content of vinyl acetate should be from 5 to 40% by weight and preferably from 15 to 30% by weight. The homopolymers and copolymers of ethylene suitable for the present invention are commercially available and are produced by well-known conventional polymerization methods.
The crosslinking agent utilized in the foamable composition are organic peroxides such as dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butyl peroxy) hexane, tert-butyl hydroperoxide, cumyl-tert-butyl peroxide and the like. Among these crosslinking agents, it is necessary to select the crosslinking agent which thermally decomposes at a lower temperature than the decomposition temperature of the blowing agent to be used. Also it will be recognized that the quantity of the crosslinking agent used depends upon the specific type of resinous admixture and the blowing agent and the quantity of the blowing agent. Generally, the crosslinking agent is used in a quantity of from 0.5 to 1.5 parts by weight per 100 parts of the resin and preferably from 0.85 to 1.0 parts by weight of the crosslinking agent per 100 parts by weight of the resin is utilized. Dicumyl peroxide which is the preferred crosslinking agent has a decomposition temperature of about 170%, at which crosslinking commences.
The blowing agent used in the present invention is a chemical blowing agent used to control the density of the foam and has a decomposition temperature higher than the softening point of the resin and also higher than the decomposition temperature of the crosslinking agent in the foam composition. The blowing agent is usually used in an amount of from 2 to 15 parts per 100 parts by weight of the resin and preferably is used in an amount of from 5 to 10 parts by weight per 100 parts of the resin. Suitable chemical blowing agents include azodicarbonamide, dinitrosopentamethylene tetramine, barium azodicarboxylate, hydroazodicarbonamide, p-toluene sulfonyl semi-carbazide, trihydrazine triazine and the like. The preferred chemical blowing agent is azodicarbonamide which has a decomposition temperature above 190.degree. C. that is higher than that of the preferred crosslinking agent.
It will be appreciated that customary additives ordinarily employed in the formation of polyethylene foams, for example, light-protection agents, pigments, fillers, flame retardants, mold release agents, lubricants, or the like, may be added to the composition prior to formation of the non-crosslinked foamable preform.
The electrically conductive polyethylene foam of this invention is produced by a continuous method wherein the resin, that is the ethylene polymers, the conductive carbon black, the crosslinking agent and the chemical blowing agent are compounded together and the resin is plasticized within an extruder unit; the resulting plasticized admixture is extruded from the extruder at a temperature that is below the decomposition temperatures of the crosslinking agent and the blowing agent and that is above the melting point of the resin to provide a non-crosslinked, non-foamed preform or matrix material usually in the form of a sheet; the sheet is sized or calibrated between a pair of rollers to have a uniform thickness; the sheet is cooled by the ambient air and the cooled preform is passed through a multi-zone heating oven to effect crosslinking and foaming. In the first zone of the oven the sheet is heated to a first temperature, e.g. from 175.degree.-200.degree. C., to activate the crosslinking agent (about 70 to 80% of the crosslinking agent is reacted). Then in the second zone the sheet is raised to a second temperature higher than the first temperature to complete the crosslinking and to initiate foaming, e.g. a temperature of from 200.degree. C. to 220.degree. C. Finally, in the third zone of the oven the sheet is heated to a third temperature which is higher than that of the second zone to complete the foaming of the resin, e.g. a temperature of from 220.degree. C. to 240.degree. C. The resulting crosslinked foam product is taken from the oven, cooled with water, for example, water cooled chrome rolls, and then passed to a take-up roll for storage.
The ethylene polymers in the foam product normally are not fully crosslinked and the extent or degree of crosslinking is from 65% to 80%, preferably 70% to 75% in order to provide a foam that is flexible and that can be readily fabricated into shaped articles such as boxes, trays and the like by vacuum-forming.
Advantageously, the non-resinous additives, i.e. conductive carbon black, crosslinking agent and blowing agent each are premixed and formed into small pellets containing resin, e.g. polyethylene and/or ethylene copolymer and the required amount of additive prior to compounding within the extruder. This practice reduces the problems of air pollution at the extruder site, facilitates metering of the foam components or ingredients into the extruder and ensures uniform admixing of the ingredients within the extruder unit.
In the extruder unit the heat pressure, i.e. the pressure at the discharge die, must be controlled in order to prevent premature crosslinking as well as foaming due to an increase in the melt temperature, thereby activating the crosslinking and the blowing agent. Generally the temperature in the extruder unit must be maintained at a range of from 125.degree. to 160.degree. C. In order to maintain this temperature range, the head pressure cannot be allowed to become too high. It has been found that the presence of carbon black in the resin admixture at the level required to impart electrical conductivity to the foam raises the head pressure in the extruder unit. It is also found that the molecular weight of the polyethylene and the ethylene copolymer forming the resin admixture has a direct influence on the head pressure; a higher molecular weight providing a higher melt viscosity which, in turn, raises the head pressure.
Based upon several trial runs using different compounding recipes for producing an electrically conductive foam, it has been determined that a head pressure in excess of 2500 psi at a flow rate of 500 pounds per hour will cause premature crosslinking which is evident by blistering of the foamed product; whereas a head pressure of 1000 psi at the same flow rate is too low since there is incomplete mixing of the components within the extruder unit. Accordingly, a head pressure of from 1300 to 2000 psi at a flow rate of 500 lbs./hr. should be maintained to avoid these problems and to provide an acceptable preform product in the form of a sheet or other desired profile.
It has also been found that when the amount of carbon black is below 10 parts per 100 parts of the resin, the surface resistivity required by the foam product of this invention cannot be achieved. The use of more than 25 parts by weight of carbon black per 100 parts by weight of the resin increases the melt viscosity and is not necessary to achieve the required electrical conductivity. Therefore, the content of conductive carbon black in the foam composition is controlled to be from about 12 to about 25 parts, preferably about 15 to 20 parts, per 100 parts of the resin.
The melt index of the resin in indicative of its molecular weight; the higher the melt index, the lower the molecular weight. In order to control the head pressure in the extruder unit it has been found that the polyethylene as well as the ethylene copolymer must each have a melt index within a selected range of melt indices.
The melt index of the low-density polyethylene is determined, in part, by the content of the conductive carbon black which acts are filler and in part, by the contents of the crosslinking and blowing agents which are temperature sensitive. If the melt index of the polyethylene is too low, e.g. less than 10, then the melt temperature will be raised due to an increase in the melt viscosity and an increase in the head pressure in the extruder unit. In such cases the foam will be blistered by localized crosslinking. If the melt index of the polyethylene is too high, e.g. greater than 40, the foam will collapse during activation of the blowing agent by heat. The expression "melt index of the low-density polyethylene" is used herein is intended to refer to an average melt index of all the polyethylene resin within the resin admixture. For example, a minor amount of a polyethylene with a melt index of 6 or less may be mixed with a major amount of a polyethylene having a melt index of 40 to provide an average melt index of 20, which would be acceptable. Generally, polyethylene resin having a melt index near the middle of the heretofore described range provides the most preferred resin for producing the foam of this invention. The same consideration applies to the selection of the ethylene copolymer.
A number of different conductive carbon blacks are suitable for the invention including channel black, furnace black, acetylene black or thermal black. Particular effective blacks are highly electrically conductive, non-reinforcing furnace-type carbon blacks which have sub-micron particle sizes and which are used for compounding with polymers. These preferred carbon blacks generally have an average particle size not greater than 30 millimicrons and an effective surface area greater than 900 m.sup.2 /g.
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Divisions (1)
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Number |
Date |
Country |
| Parent |
857301 |
Apr 1986 |
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Continuations (1)
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Number |
Date |
Country |
| Parent |
688413 |
Jan 1985 |
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Patent Grant
4800126
