HK1091027B - A heat spreader and the method for applying the same to a heat source such as a plasma display panel - Google Patents

Description

Heat sink and method for using the same for plasma display panel

Description of the invention

Known to be me, the U.S. citizen Julian Norley residing in charrin Falls, 17635 Plum Creek Trail (44023), ohio; martin David Smallc, a citizen of America, resident in Parma, 5608 Elyvista Drive (44129); joseph Payl Capp, a citizen of the United states of America, residing in Strongsville, 10094 Juniper Coirt (44136); and Timothy Clevelsk, Inc., residing in North Omsted, 5250 Columbia avenue Apt.407(44070), Ohio invented a novel and useful "heat sink for plasma display panels".

Technical Field

The present invention relates to a heat sink for a plasma display panel, and a method of applying the heat sink of the present invention to a plasma display panel.

Background

A plasma display panel is a display device including a plurality of discharge cells, and displays an image by causing desired discharge cells to emit light by applying a voltage across electrode discharge cells. A panel member, which is a main part of the plasma display panel, is manufactured by bonding two glass substrates together so as to sandwich a plurality of discharge cells therebetween.

In the plasma display panel, the discharge cells that cause light emission to be used for image formation generate heat, thereby constituting a heat source, causing the temperature of the plasma display panel to rise as a whole. The heat generated in the discharge cells is transferred to the glass forming the substrate, but it is difficult to conduct the heat in a direction parallel to the panel surface due to the nature of the glass substrate material.

In addition, the temperature of the discharge cells excited to emit light is significantly increased, while the temperature of the discharge cells not excited is not increased so much. Thus, the panel surface temperature of the plasma display panel locally rises in the area where the image is generated, accelerating the thermal degradation of the affected discharge cells unless some heat dissipation measures are taken.

In addition, since the temperature difference between the excited and non-excited discharge cells is high, and in fact, the temperature difference between the discharge cells generating white light and the discharge cells generating dark color light is also high, stress is applied to the panel member, resulting in the conventional plasma display panel being easily broken and damaged.

When the voltage applied to the electrodes of the discharge cells increases, the brightness of the discharge cells increases, but the amount of heat generated in the cells also increases. Therefore, those cells having a large voltage for excitation are more prone to thermal degradation and exacerbate the cracking problem of the plasma display panel components.

For example, Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji, and Kawashima in U.S. patent No.5,831,374 propose the use of graphite films or sheets as thermal contact materials for plasma display panels. In addition, the ability of the flakes of compressed particles of exfoliated graphite to dissipate heat is also recognized. Indeed, it is commercially available from advanced energy Technology, Lakewood, OhioA material of the like is used as such a material.

Graphites are made up of layered planes of hexagonal arrays or networks of carbon atoms. These layered planes of hexagonally arranged carbon atoms are generally planar and are oriented or ordered to be generally parallel and equidistant to each other. Generally flat, parallel equidistant sheets or layers of carbon atoms, often referred to as graphene layers or basal planes, are bonded or bonded together, with the groups arranged into grains. Highly ordered graphites consist of crystallites of considerable size, the crystallites being arranged or oriented relative to one another and having well-ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred grain orientation. It should be noted that graphite has an anisotropic structure and thus exhibits or has many properties that are highly directional, such as thermal and electrical conductivity.

In short, graphites can be characterized as carbon-layered structures, i.e., structures consisting of laminated layers or laminae of carbon atoms bonded together by weak van der waals forces. When considering graphite structures, two axes or directions are often noted, namely the "c" axis or direction and the "a" axis or direction. For simplicity, the "c" axis or direction may be assumed to be the direction perpendicular to the carbon layers. The "a" axis or direction is assumed to be the direction parallel to the carbon layers or the direction perpendicular to the "c" direction. The graphites suitable for making flexible graphite sheets possess a high degree of orientation.

As mentioned above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der waals forces. Natural graphites can be treated so that the spacing between the laminated carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction, and thus form an expanded graphite structure in which the laminar character of the carbon layers is substantially retained.

Greatly expanded or particularly significantly expanded graphite sheets, such as webs, papers, strips, tapes, foils, mats or the like (commonly referred to as "flexible graphite"), having a final thickness or "c" direction dimension which is about 80 times or more the original "c" direction dimension, can be formed without the use of a binder in the bonding or integration of the expanded graphite sheets. Because of the mechanical interlocking or agglomeration achieved between the plurality of expanded graphite particles, it is believed that the graphite particles expanded to a final thickness or "c" direction dimension that is about 80 times or greater than the original "c" direction dimension can be formed into integrated flexible sheets by compression without the use of a binding material.

In addition to flexibility, as noted above, it has also been found that the orientation of the expanded graphite particles due to strong compression and the graphite layers being substantially parallel to the opposed surfaces of the sheet results in a high degree of anisotropy in thermal conduction, making the sheet particularly useful in heat dissipation applications. The sheet produced has excellent flexibility, good strength and high orientation.

Briefly, a method of making a flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or extruding particles of expanded graphite having a "c" direction dimension which is as much as about 80 times that of the original particles under a predetermined load and in the absence of a binder, thereby forming a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles, which are typically worm-like or vermiform in shape, once compressed, will remain compressively deformed and aligned with the opposite major surfaces of the sheet. The density and thickness of the sheet material are varied by controlling the degree of compression. The density of the sheet may be in the range of from about 0.04g/cc to about 2.0 g/c.

The flexible graphite sheet exhibits significant anisotropy due to the orientation of graphite particles parallel to the opposed, parallel major surfaces of the sheet, with the degree of anisotropy increasing as the sheet compresses to increase orientation. In compressed anisotropic sheets, the thickness, i.e., the direction perpendicular to the opposed parallel sheet surfaces, comprises the "c" direction and the "a" directions along the length and width directions, i.e., the directions along or parallel to the opposed major surfaces, the thermal or electrical properties of the sheets are very different, by orders of magnitude, for the "c" and "a" directions.

One disadvantage of using graphite foil as a heat sink for a plasma display panel is the plasma display panel manufacturing process. In particular, the plasma display panel is manufactured in large quantities, and the process of applying the graphite heat sink to the plasma display panel is required so that a bottleneck is not generated in the manufacturing process. In addition, the method of bonding the graphite heat sink to the display panel must prevent the graphite heat sink from falling off during the manufacturing process and ensure good thermal contact between the graphite heat sink and the plasma display panel without applying a high voltage to the heat sink; however, the attachment method must not significantly adversely affect the thermal properties of the heat sink.

One method of attaching graphite heat sinks to plasma display panels is to use an adhesive applied to the graphite. U.S. patent 6,245,400 to Tzeng, Getz and Weber describes a method of making a pressure sensitive adhesive flexible graphite sheet article with a release liner, wherein the release liner is easily removed from the graphite sheet without delaminating the graphite. Graphite sheets have relatively low cohesive strength and removal of the release liner without delaminating the graphite is a significant challenge. A critical part of the Tzeng et al patent is the application of a primer coating to the graphite sheet prior to application of the pressure sensitive adhesive. A disadvantage of this method is that it requires an additional coating step, which increases manufacturing complexity and cost.

Accordingly, there is a need for a method of making a pressure sensitive adhesive flexible graphite sheet with a release liner for use as a heat sink for a plasma display panel without the use of a primer. In addition, the method enables high speed separation of the release liner from the adhesive coated graphite sheet without delamination of the graphite and without undesirable height reduction of the thermal properties of the graphite heat sink.

Disclosure of Invention

It is, therefore, an object of the present invention to provide a method of applying a heat sink to a plasma display panel in a mass manufacturing process.

Another object of the present invention is to provide a heat sink material that can be used in a large-scale plasma display panel manufacturing process.

It is a further object of the present invention to provide a method of applying a heat sink material to a heat source, such as a plasma display panel, in a high volume manufacturing process wherein the application of a heat sink does not create a bottleneck in the manufacturing process.

It is a further object of the present invention to provide a heat sink material that can be applied to a heat source or a cluster of heat sources, such as a plasma display panel, and that provides good thermal contact bonding between the heat sink and the plasma display panel without applying high voltage to the heat sink to achieve the desired thermal contact.

Another object of the present invention is to provide a heat dissipating material applicable to a heat source or a group of heat sources such as a plasma display panel, which is adhered to the heat source without falling off during an assembling process.

It is another object of the present invention to provide a method of applying a heat sink to a heat source or group of heat sources, such as a plasma display panel, without significantly affecting the thermal properties of the heat sink.

These and other objects, which will become apparent to those skilled in the art upon a reading of the following description, are achieved by providing a method of applying a heat sink to a heat source, such as a plurality of plasma display panels, the method comprising providing a plurality of heat sink composites, each composite comprising a heat sink material having an adhesive thereon, and a release material positioned such that the adhesive is sandwiched between the heat sink material and the release material; removing separation material from the plurality of composite materials; and applying at least one composite material to each of the plurality of plasma display panels such that the adhesive adheres the heat dissipation material to the plasma display panels.

The release material and adhesive should be selected such that the release material will not cause undesirable damage to the heat dissipating material when the release material is released at a predetermined rate. In addition, the adhesive and the release material should also produce an average release load of no greater than about 40 grams per centimeter (g/cm) at a separation speed of about 1 meter per second (m/s). In practice, the average breakaway load should be no greater than about 20g/cm, preferably no greater than about 10g/cm, at a separation speed of about 1 m/s.

Further, the adhesive preferably achieves at least about 125g/cm²Preferably an average lap shear bond strength of at least about 700g/cm²。

To avoid excessive heat loss, the adhesive should increase the thermal resistance through the thickness of the adhesive/heat dissipating material combination by no more than about 100%, preferably no more than about 35%, as compared to the heat dissipating material itself. To meet the requirements for liner separation speed, bond strength and thermal resistance, the adhesive should be no greater than about 0.5 mils thick, with a preferred thickness of between about 0.1 mils and about 0.25 mils.

The heat sink material preferably comprises graphite, and in particular at least one sheet of compressed particles of exfoliated graphite, and may be a laminated layer comprising a plurality of sheets of compressed particles of exfoliated graphite.

Other and further objects, features and advantages of the present invention will become apparent to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings.

Drawings

FIG. 1 is a high speed separation test of example 1.

FIG. 2 shows the high speed separation test of example 1 during the test.

Detailed Description

Graphite is a crystalline form of carbon comprising atoms covalently bonded between planes with weaker bonds to form flat layered planes. In obtaining the raw materials for the flexible graphite sheets described above, particles of graphite, such as natural graphite flakes, are typically treated with an intercalant, such as a solution of sulfuric and nitric acid, wherein the crystal structure of the graphite reacts to form a composite of the graphite and the intercalant. The treated graphite particles are hereinafter referred to as "intercalated graphite particles". Upon exposure to high temperatures, the intercalant within the graphite decomposes and volatilizes, causing the particles of intercalated graphite to expand in size in an accordion-like manner to about 80 or more times its original volume in the "c" direction, i.e., in the direction perpendicular to the crystalline planes of the graphite. The expanded (otherwise known as exfoliated) graphite particles are vermiform in appearance and are therefore often referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes to form small transverse openings by deforming mechanical impact.

The graphite starting material for the flexible sheets suitable for use in the present invention comprises a highly graphitic carbonaceous material capable of intercalating organic and inorganic acids as well as halogens and expanding when exposed to heat. These highly graphitic carbonaceous materials preferably have a degree of graphitization of about 1.0. As used in this specification, the term "degree of graphitization" means a value g according to the following formula:

where d (002) is the spacing between carbon graphite layers in the crystal structure measured in angstroms. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The diffraction peak positions corresponding to the (002), (004) and (006) Miller Indices (Miller Indices) were measured and the spacing that minimizes the total error of all these peaks was derived using standard least squares techniques. Examples of highly graphitic carbonaceous materials include natural graphites from a variety of sources, as well as other carbonaceous materials such as graphites prepared by chemical vapor deposition, pyrolysis of polymers, or crystallization from molten metal solutions. Natural graphite is most preferred.

The graphite starting material of the flexible sheet used in the present invention may contain non-graphite components as long as the crystal structure of the starting material maintains the required degree of graphitization and can be exfoliated. Generally, any carbon-containing material that has the required degree of graphitization in its crystalline structure and can be exfoliated is suitable for use with the present invention. Such graphite preferably has an ash content of less than twenty percent. Preferably, the graphite employed in the present invention has a purity of at least about 94%. In the most preferred embodiment, the graphite employed is at least about 98% pure.

One conventional method for manufacturing graphite sheets is described by Shane et al in U.S. patent No.3,404,061, the contents of which are incorporated herein by reference. In a representative embodiment of the Shane et al method, natural graphite flakes are subjected to an intercalation reaction by dispersing the flakes in a solution containing, for example, a mixture of nitric and sulfuric acids, preferably at a concentration of about 20 to about 300 parts by weight of intercalation solution per 100 parts by weight of graphite flakes (pph). The intercalation solution contains oxidizing and other intercalating agents known in the art. For example, substances containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like; or mixtures, such as concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid; or a mixture of a strong organic acid, such as trifluoroacetic acid, and a strong oxidizing agent dissolved in the organic acid. Alternatively, an electrical potential may be used to oxidize the graphite. Chemical species that can be introduced into the graphite crystal by electrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of sulfuric acid, or a mixture of sulfuric acid and phosphoric acid, and an oxidizing agent, such as nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like. Although less preferred, the intercalation solution may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or halides such as bromine, a solution of bromine and sulfuric acid, or bromine in an organic solvent.

The amount of intercalation solution may range from about 20 to about 350pph, preferably about 40 to about 160 pph. After the flakes were intercalated, excess solution was drained from the flakes and the flakes were rinsed with water.

Alternatively, the amount of intercalation solution may be limited to between about 10 and about 40pph, in which case a rinsing step may not be required as described in U.S. Pat. No.4,895,713, the disclosure of which is also incorporated herein by reference.

The particles of graphite flake treated by intercalation solution, optionally contacted with an organic reducing agent, e.g. by blending, may be selected from alcohols, sugars, aldehydes and esters which react with the surface film of the oxidative intercalation solution at temperatures in the range 25 ℃ to 125 ℃. Suitable specific organic agents include cetyl alcohol, stearyl alcohol, 1-octanol, 2-octanol, decyl alcohol, 1, 10-decanediol, decanal, 1-propanol, 1, 3-propanediol, 1, 2-ethanediol, polypropylene glycol, glucose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl carnosite, diethyl carnosite, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfonate. The amount of organic reducing agent is suitably from about 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid before, during or immediately after intercalation can also bring about an improved effect. These improvements are a reduction in the delamination temperature and an increase in the expanded volume (also referred to as "worm volume"). The expansion aid herein is preferably an organic material that is sufficiently soluble in the intercalation solution to improve expansion. Further limiting, such organic materials containing carbon, hydrogen, and oxygen may be preferably, but not exclusively, employed. Carboxylic acids have been found to be particularly effective. Suitable carboxylic acids for the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic, straight or branched chain, saturated or unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids having at least one carbon atom, preferably up to about 15 carbon atoms, which are dissolved in the intercalation solution in an amount sufficient to produce a measurable improvement in one or more aspects of exfoliation. Suitable organic solvents may be employed to enhance the solubility of the organic expansion aid in the intercalation solution.

Representative examples of saturated aliphatic carboxylic acids are those having the formula H (CH)₂)_nAcids of COOH, wherein n is a number from 0 to about 5, include formic, acetic, propionic, butyric, valeric, caproic, and the like. Instead of carboxylic acids, anhydrides or reactive carboxylic acid derivatives such as alkyl esters may also be used. Representative of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalants are capable of decomposing formic acid, ultimately to water and carbon dioxide. For this reason, it is advantageous to contact formic acid and other sensitive expansion aids with the graphite flake prior to immersion of the flake in the aqueous intercalant. Representative dicarboxylic acids are aliphatic dicarboxylic acids having 2 to 12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1, 5-glutaric acid, 1, 6-suberic acid, 1, 10-sebacic acid, cyclohexane-1-4-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid or terephthalic acid. Representative alkyl esters are dimethyl and diethyl hertoferrinate. Representative cycloaliphatic acids are cyclohexane carboxylic acid and representative aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m-and p-methylbenzoic acid, methoxy and ethoxybenzoic acid, acetoacetamidobenzoic acid, acetamidobenzoic acid, phenylacetic acid and naphthoic acid. Representative hydroxyaromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. Among the polycarboxylic acids, citric acid is predominant.

The intercalation solution will contain water and preferably will contain an amount of expansion aid of from about 1% to about 10%, this amount being effective to enhance exfoliation. In embodiments in which the expansion aid is contacted with the graphite flake prior to or after immersing in the aqueous intercalation solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite.

After intercalating the graphite flake, the intercalated graphite flake is mixed with an organic reducing agent and the mixture is then placed at a temperature in the range of 25 c to 125 c to promote reaction of the reducing agent with the intercalated graphite flake. The heating period is up to about 20 hours, and for higher temperatures in the above range, the heating period is shortened, for example, at least about 10 minutes. At higher temperatures, times of 1 half hour or less, for example on the order of 10 to 25 minutes, may be employed.

The above described process of intercalating and exfoliating graphite flake is facilitated by pretreating the graphite flake at graphitization temperatures, i.e., temperatures in the range of about 3000 c and above, and by including a lubricant in the intercalant.

Pretreatment or annealing of the graphite flake results in a significant increase in the amount of expansion (i.e., increase in expansion volume to 300% or greater) when the flake is subsequently intercalated and delaminated. In fact, it is desirable that the expansion be increased by at least 50% as compared to the same treatment without the annealing step. The temperature used for the annealing step should not be significantly below 3000 c, since even a 100c reduction in temperature results in a significant reduction in expansion.

The annealing of the present invention is performed for a time sufficient to cause the degree of expansion of the sheet upon intercalation and subsequent delamination to increase. Generally, the time required is 1 hour or more, preferably 1 to 3 hours, and more preferably, the treatment is carried out in an inert atmosphere. To achieve the greatest beneficial effect, the annealed graphite flake will also be subjected to other treatments known in the art for increasing the degree of expansion, namely intercalation in the presence of an organic reducing agent, an intercalation aid such as an organic acid, and a surfactant rinse following intercalation. Furthermore, the embedding step 3 can be most repeated for the most advantageous effect.

The annealing step of the present invention is carried out in an induction furnace or other such device known and recognized in the graphitization art; temperatures in the range of 3000 c are used here at the high end of the range encountered during graphitization.

Because it has been observed that worms produced using pre-intercalation annealed graphite sometimes "clump" together, negatively impacting face weight uniformity, additives that contribute to the ability to form "free-flowing" worms are highly desirable. The addition of a lubricious additive to the intercalation solution facilitates a more uniform distribution of the worms on the compression device machine, such as the machine of a calendering station conventionally used to compress (or "calender") the graphite worms into flexible graphite sheets. The resulting flakes have high surface weight uniformity and greater tensile strength even when the starting graphite particles are smaller than those conventionally used. The lubricious additive is preferably a long chain hydrocarbon. Organic compounds having long chain hydrocarbon groups, even in the presence of other functional groups, may also be employed.

More preferably, the lubricious additive is an oil, most preferably a mineral oil, especially considering that mineral oils are less prone to rancidity and odor, which is an important consideration for long-term use. It is noted that certain of the expansion aids described above also meet the definition of lubricious additive. When these materials are used as expansion aids, it is not necessary to include a separate lubricious additive in the intercalant.

The lubricious additive is present in the intercalant in an amount of at least about 1.4pph, preferably at least about 1.8 pph. Although the upper limit of the amount of lubricious additive is not as critical as the lower limit, it appears that the inclusion of the lubricious additive in excess of about 4pph does not provide significant additional advantages.

The graphite particles thus treated are sometimes referred to as "intercalated graphite particles". Upon exposure to elevated temperatures, e.g., at least about 160 c, and particularly from about 700 c to 1000 c and higher, the intercalated graphite particles expand in an accordion-like manner to about 80 to 1000 or more times their original volume in the c-direction, i.e., in the direction perpendicular to the crystalline planes that make up the graphite particles. Expanded, i.e., exfoliated, graphite particles are vermiform in appearance and are therefore commonly referred to as worms. The worms may be molded together into flexible sheets having small transverse openings, as opposed to the original graphite flakes, which can be formed and cut into various shapes, as described below.

Alternatively, the flexible graphite sheets of the present invention may utilize reground flexible graphite sheets rather than freshly expanded worms. The sheets may be newly formed sheets, recycled sheets, scrap sheets, or any other suitable source.

And the process of the present invention may use a mixture of fresh material and recycled material.

The raw material for the recycled material may be a sheet or a trimmed portion of a sheet that has been subjected to the above-described die pressing, or a sheet that has been compressed by a pre-calendering roller but has not been impregnated with resin. Further, the raw material may be a sheet or a trimmed portion of a sheet that has been injected with resin but has not yet been cured, or a sheet or a trimmed portion of a sheet that has been injected with resin and cured. The starting material may also be a recycled flexible graphite PEM fuel cell component such as a flow field plate or electrode. Each of the plurality of graphite starting materials may be used alone or in admixture with natural graphite flakes.

Once the feedstock of flexible graphite sheets is obtained, it can be comminuted by known methods or devices, such as jet milling, air milling, and the like, to form particles. Preferably, the majority of the particles have a diameter that passes through 20U.S. mesh; more preferably, the major portion (greater than about 20%, more preferably greater than about 50%) does not pass 80u.s. mesh. Most preferably, the particles have a particle size of no greater than about 20 mesh. It is also desirable to cool the flexible graphite sheet as it is being impregnated with resin during comminution to avoid thermal damage to the resin system during comminution.

The size of the milled particles is selected so that the machinability and formability of the graphite article are balanced with the desired thermal properties. Smaller particles produce graphite articles that are easier to process and/or shape, while larger particles will produce graphite articles with higher anisotropy, and thus higher in-plane electrical and thermal conductivity.

If the raw material has been impregnated with resin, it is preferred to remove the resin from the particles. Details of the resin removal are described further below.

Once the raw material is ground, any resin is removed and then re-expanded. The re-expansion may be performed using the intercalation and delamination methods described above and in U.S. Pat. No.3,404,061 to Shane et al and U.S. Pat. No.4,895,713 to Greinke et al.

Typically, delamination occurs after intercalation by heating the intercalated particles in an oven. During this delamination step, the intercalated natural graphite flakes may be added to the regenerated intercalated particles. Preferably, the particles expand to a specific volume in the range of at least about 100cc/g to about 350cc/g or more during the re-expansion step. Finally, after the re-expansion step, the re-expanded particles are compressed into flexible sheets, as described above.

If the raw material has been impregnated with resin, the resin is preferably at least partially removed from the particles. This removal step should occur between the grinding step and the re-expansion step.

In one embodiment, the removing step comprises heating the regrind particles containing resin, such as over an open flame. Specifically, the injected resin is heated to a temperature of at least about 250 ℃ to effect resin removal. During this heating step, care should be taken to avoid flash points of resin decomposition products; this can be achieved by careful heating in air or heating in an inert atmosphere. Preferably, heating should be in the range of about 400 ℃ to about 800 ℃ for at least about 10 minutes up to about 150 minutes or more.

Further, the resin removal step may increase the tensile strength of the resulting article produced by the molding process as compared to the same method without removing the resin. The resin removal step has the advantage that, due to the expansion step (i.e. intercalation and delamination), toxic by-products are generated in some cases when the resin is mixed with the intercalation chemicals.

Thus, by removing the resin prior to the expansion step, superior products are obtained, such as the enhancement of strength properties discussed above. The enhanced strength properties are due in part to the increased expansion. The expansion may be limited due to the presence of resin in the particles.

In addition to strength properties and environmental considerations, the resin may be removed prior to intercalation in view of the potential for the resin to react with the acid in an uncontrolled exothermic manner.

In view of the above, it is preferable to remove most of the resin. More preferably, more than about 75% of the resin is removed. Most preferably over 99% of the resin is removed.

In a preferred embodiment, the flexible graphite sheet, once ground, is formed into the desired shape (i.e., a sheet) and then cured (as the resin is injected). Alternatively, the flakes may be cured prior to grinding, although it is preferred that they be cured after grinding.

Flexible graphite sheets and foils are agglomerated to have good processing strength and are suitable for compression, such as by compression molding, to a thickness of about 0.025mm to 3.75mm, typically to a density of about 0.1 to 1.5 grams per cubic centimeter (g/cc). As described in U.S. patent No.5,902,762 (incorporated herein by reference), approximately 1.5-30% by weight of a ceramic additive may be blended with the intercalated graphite flakes to enhance resin infusion in the final flexible graphite product. The additive includes ceramic fiber particles having a length of about 0.15 to 1.5 millimeters. The width of the particles is suitably from about 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive and non-adherent to graphite and are stable up to about 1100 c, preferably about 1400 c or higher. Suitable ceramic fiber particles are formed from macerated quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide fibers and the like.

In some cases, the flexible graphite sheet may also be treated with a resin after curing, and the absorbed resin, after curing, enhances the moisture resistance and processing strength, i.e., stiffness, of the flexible graphite sheet and "fixes" the morphology of the sheet. Suitable resin content is preferably at least about 5 wt%, more preferably from about 10 wt% to 35 wt%, suitably up to about 60 wt%. Particularly useful resins found in the practice of the present invention include acrylic, epoxy and phenolic based resin systems, or mixtures thereof. Suitable epoxy resin systems include diglycidyl ether or bisphenol a (dgeba) based resins and other multifunctional resin systems; useful phenolic resins include resole and novolak phenols.

Although the present application is written with respect to a heat sink for a plasma display panel, it will be appreciated that the method and heat sink of the present invention are equally applicable to other heat sources, or groups of heat sources, particularly heat generated in large scale processing.

Plasma display panel fabrication sizes of 1 meter or more (angle to angle measurement) are now in existence. Thus, the heat sink for cooling and improving the hot spot effect on such display panels also needs to be quite large, on the order of about 270 mm by about 500 mm, or on the order of about 800 mm by 500 mm or more. In the plasma display panel, as described above, there are hundreds to thousands of cells each containing plasma gas. When a voltage is applied to each cell, the plasma gas reacts with the phosphor in each cell, producing colored light. Plasma displays can be very hot due to the considerable energy required to ionize a gas to produce a plasma. In addition, depending on the color in a particular area of the display panel, hot spots can be generated on the screen, which can lead to premature phosphor failure, shortened display life, and thermal stress on the display panel itself. Thus, heat sinks are needed to reduce the effects of these hot spots.

The sheets of compressed particles of exfoliated graphite, and in particular the laminated layers of sheets of compressed particles of exfoliated graphite, have been found to be particularly useful as heat sinks for plasma displays. In practice, this requires a layer of adhesive on the finished graphite heat sink to adhere the heat sink to the plasma display panel, particularly during plasma display panel assembly. It is thus necessary to cover the adhesive with a release liner so that the adhesive is sandwiched between the release liner and the graphite sheet in order to store and transport the graphite heat sink before being adhered to the plasma display panel.

The use of an adhesive coated graphite sheet (or laminate layer of sheets) with a release liner is desirable for meeting certain requirements when used in a large-scale plasma display panel manufacturing process. In particular, the release liner must be capable of being removed from the sheet at high speed without delaminating the graphite. Delamination occurs when the release liner is removed, actually peeling the adhesive and some of the graphite flakes off the sheet, resulting in waste of graphite, damage to the graphite flakes themselves, and loss of adhesive required to adhere the graphite flakes to the plasma display panel, as well as producing an unsightly and undesirable appearance.

Nevertheless, while the adhesive and release liner must be selected so that the release liner can be separated from the adhesive/graphite sheet without delamination of the graphite, the strength of the adhesive must be sufficient to hold the graphite sheet in place on the plasma display panel while the panel assumes any of a variety of orientations and to ensure good thermal contact between the heat sink and the panel.

In addition, the adhesive must not cause a significant reduction in the thermal properties of the heat sink. In other words, the adhesive applied in a layer having a large thickness may affect the thermal properties of the heat sink because the adhesive may affect the conduction of heat from the plasma display panel to the heat sink.

Thus, the adhesive and release liner combination must achieve a balance such that they produce a release load of no greater than about 40g/cm, preferably about 20g/cm, and most preferably about 10g/cm, at a separation speed of about 1m/s, as measured on, for example, a Cheminstruments HSR-1000 high speed separation tester. For example, if removal of the release liner at a rate of about 1m/s is required to comply with large scale manufacturing requirements for plasma display panels, the release liner should have an average release load of no greater than about 40g/cm, preferably about 20g/cm, and more preferably about 10g/cm, so as not to cause graphite delamination when the release liner is removed at that release rate. To accomplish this, the thickness of the adhesive is preferably no more than about 0.3 mils.

Another factor to be balanced is that the adhesive strength of the adhesive must be sufficient to hold the heat sink in place on the plasma display panel during the plasma display panel manufacturing process and to ensure good thermal contact between the heat sink and the plasma display panel. To achieve the desired bond, the adhesive must have at least about 125g/cm as measured on, for example, a ChemiInstruments TT-1000 tensile tester²More preferably an average lap shear bond strength of at least about 700g/cm²。

Throughout the above, the adhesive should not substantially affect the thermal properties of the heat sink. This means that the presence of the adhesive should not cause the thermal resistance of the heat sink in the thickness direction to increase by more than about 100% over the heat dissipating material itself without the adhesive. Thus, the adhesive must meet the requirements of release load and average overlap shear bond strength while being sufficiently thin to avoid an undesirable extreme increase in thermal resistance. To meet these requirements, the adhesive should not exceed about 0.5 mil thick, preferably not exceed 0.25 mil thick.

In order to achieve the above balance required for producing a heat sink sheet applicable to plasma display panels in a high volume manufacturing process wherein the heat sink sheet is a laminated layer of compressed particles or sheets of exfoliated graphite having a thickness of no greater than about 2.0 mils and a density of between about 1.6 and about 1.9 grams per cubic centimeter, the desired results can be achieved by combining an Aroset 3300 pressure sensitive acrylic adhesive of the desired thickness available from Ashland Chemical with a release liner made from silicone-coated Kraft paper, such as the L2 release liner available from the Sil Tech, a distributor of Techicote corporation. Thus, a heat sink composite is formed comprising a laminate of heat sink material, such as a sheet or sheet of compressed particles of exfoliated graphite, having a thickness of adhesive thereon such that the thermal properties of the heat sink material are not substantially compromised, the release layer being provided with adhesive interposed between the heat sink material and the release material. Then, in use, the release material may be removed from the heat sink/adhesive combination and the heat sink material/adhesive combination then applied to the plasma display panel such that the adhesive bonds the heat sink material to the plasma display panel. Further, at least one heat sink/adhesive combination is applied to each of the plurality of plasma display panels when the plurality of plasma display panels are manufactured.

In this way, an excellent heat sink for a plasma display panel is provided in such a manner that mass production of the plasma display panel can be continuously performed while the heat sink is transported and applied to the display panel.

In order to facilitate a more complete understanding of the present invention, a number of examples are set forth below. However, the scope of the invention is not limited to the specific embodiments disclosed in these examples, which are intended for illustrative purposes only. All proportions and amounts mentioned in the following examples are by weight unless otherwise indicated.

Example 1

High speed release liner testing was performed using a chem instruments HSR-1000 high speed release tester. The test conditions were as follows: capstan speed was set at 400,800 feet/minute, separation angle 180 degrees, surface separation speed of 40, 80 inches/second, fin separation speed of 0.5, 0.25 meters/second, and sample size of 2 inches by 8 inches.

Fig. 1 shows a schematic diagram of a high-speed separation test. The release liner 20 was peeled back gently starting from the exposed end of the sample exposing the graphite heat sink 10 and the release liner 20. The exposed end of the graphite 10 is held firmly in place with a clamp 100, while a paper pull 110 is attached to the free end of the release liner 20. The paper trailer 110 is then folded back on itself and loaded between the driving pulley 120 and the driven pulley 125. The test involves driving the capstan 120 at a set speed and compacting the paper trailer 20 supported by the follower 125, as shown in figure 2. The towed article 20 driven by the driving wheel 120 moves at the same speed as the driving wheel 120, peeling the separation pad 20 from the graphite sheet 10.

As the paper trailer 110 is folded back on itself, the gasket 20 is removed from the graphite sheet 10 at a separation angle of about 180 degrees. Causing the adhesion interface to move along the surface of the graphite sample 10 at half the speed of the paper puller 110. As described above, capstan 120 was tested at a speed of 400 to 800 feet per minute, which corresponds to an interface separation speed of 40 to 80 inches per second. These speeds correspond to separation pad separation speeds of 0.5 m/s and 0.25 m/s, respectively.

During each test, the maximum separation force was recorded in grams per 2 inch width of the sample. After the test, each sample was examined for signs of graphite delamination, graphite area elevation, or graphite transfer to the release liner. If any elevation of the graphitic region is observed, or if there is any delamination of the graphite, the sample fails the test. The results for 2 different adhesive compositions are listed in table I.

TABLE I

High speed peel test

eGraf755-L2 release paper

A total of 199 samples of the eagef 755 graphite heat sink material coated with Aroset 3250 binder were tested at a certain separation speed, e.g. 0.5 m/s, as shown in table I. 163 passed the test, while 36 failed the test, the pass rate was 82%. The average maximum separation force measured on samples that failed the test was 154 grams per 2 inches of width, while the average maximum separation force of samples that passed the test was 42 grams per 2 inches of width.

A total of 12 samples coated with Aroset 3300 adhesive were tested, 8 at 0.5 m/s separation speed and 4 at 0.25 m/s separation speed. In both cases all samples passed the test. The average maximum separation force measured when the sample was tested at the slower speed was only 17.4 grams per 2 inches of width, while for the faster speed test the average was only 19.7 grams per 2 inches of width.

Example 2

Overlap shear adhesion testing was performed using a ChemInstructions TT-1000 tensile tester. The test conditions were: the crosshead speed was 0.5 inches/minute and the overlap shear plane size was 1 "by 1". The eGraf sample size was 1 "wide, 4" long. The test substrate material was glass and the test substrate dimensions were 2 "wide and 4" long. A small piece was cut from each sample section after the sample was fixed to the glass substrate and 1,000 grams of gravity was applied to the graphite side of the graphite/glass joint for 20 minutes prior to testing. No other force is applied to the joint.

The sample was placed in a tensile tester with the glass substrate in the upper jaw and the sample in the lower jaw. The test was conducted at a crosshead speed of 0.5 "per minute. The maximum overlap shear for each sample was obtained and is summarized in table II.

TABLE II

Results of overlap shear adhesion test

As shown in table II, 100 samples of the eaget 755 ink fins coated with Aroset 3250 adhesive and 10 samples of the eaget 755 coated with Aroset 3300 adhesive were tested. The average maximum overlap shear strength of the samples coated with Aroset 3250 adhesive was 4129 grams, while the average maximum shear strength of the samples coated with Aroset 3300 adhesive was 3738 grams. The standard deviation for the Aroset 3250 adhesive sample was 1422 grams and the standard deviation for the Aroset 3300 adhesive sample was 1822 grams. Thus, when 3300 adhesive was used, the overlap shear strength decreased by an average of 10%.

Example 3

Probe tack tests (Probe tack test) were performed using a ChemInstructions PT-1000 Probe tack tester. The initial probe viscosity measures the initial "dwell position" of the adhesive on the substrate in the absence of a load. The probe tack loadings for the eaget 755 graphite heat sink samples coated with Aroset 3250 binder and Aroset 3300 binder, respectively, were obtained and the results are summarized in table III.

TABLE III

Initial viscosity test result of probe

As shown in table III, the probe tack tests were performed on 26 samples coated with Aroset 3250 adhesive and 16 samples coated with 3300 adhesive. The Aroset 3250 sample was from 3 heat sinks and the Aroset 3300 sample was from 2 heat sinks. The average probe initial viscosity loading for the samples coated with Aroset 3250 binder was 23 grams, while the average loading for the samples coated with Aroset 3300 binder was 19.1 grams. The standard deviation for the samples coated with Aroset 3250 was 10.5 grams, while the standard deviation for the samples coated with Aroset 3300 adhesive was 9.0 grams, indicating a 17% reduction in probe initial viscosity loading for the 3300 adhesive compared to the Aroset 3250 adhesive, on average.

Example 4

The thickness direction thermal resistance test was performed using a modified ASTM D5470 thermal conductivity test standard. The tests were performed on an eagef graphite heat sink without binder and an eagef 755 heat sink coated on only one side with either an Aroset 3250 binder or an Aroset 3300 binder. Two samples of each material were tested. The sample diameter was 2.0 inches and the test was conducted at a contact pressure of 16psi and a nominal sample temperature of 50 ℃. The results of the tests are summarized in Table IV. As shown in the table, the thermal resistance of the eGraf755 material without the binder was 3.48cm²DEG C/W. The thermal resistance of samples coated with Aroset 3250 was between 4.46 and 4.55cm²The thermal resistance of the samples coated with Aroset 3300 adhesive varied between 3.77 and 3.99cm²The change between C/W indicates that the thermal properties of the samples coated with Aroset 3300 adhesive are significantly better than those of the samples coated with Aroset 3250 adhesive.

TABLE IV

Thermal resistance test in thickness direction

Contact pressure of 16psi

Adhesive type	Number of samples	Thermal resistance (cm)°K/W)
			Is free of	646	3.48
Is free of	653	3.48
			Aroset 3250	649	4.46
Aroset 3250	651	4.55
			Aroset 3300	650	3.77
Aroset 3300	652	3.99

The above examples illustrate the equalization tests required to identify release liners and adhesives that can be used in heat sink materials to achieve the equalization of the heat sink composite of the present invention.

Application example 1

The thermal characteristics of a panasonic plasma tv model TH42PA20, to the back of which an acrylic heat sink was adhered, were analyzed under the following different screen conditions. Black and white patterns were generated on the display and the screen surface temperature was measured using an infrared camera. The background was black in all cases. The composition of the pattern is as follows: 1) three evenly spaced white lines horizontally across the screen (23.9% screen illumination); and 2) a 4 x 3 array of evenly spaced white points (4% screen illumination). After testing the cells with conventional acrylic coolant, the acrylic coolant was removed and replaced with a flexible graphite coolant having a thickness of 1.4mm and an in-plane thermal conductivity of about 260W/m ° K. The plasma display was then tested again under the same conditions as described above, and the results are shown in table 1.

TABLE 1

Pattern(s)	Heat dissipating agent	Tmax	Temperature range of white pattern	Ambient environment
					White line pattern	Acrylic acid	49.3	30	24.1
White line pattern	Flexible graphite	48.6	34.4	23.5
					White dot array pattern	Acrylic acid	51.8	30.4	24.3
White dot array pattern	Flexible graphite	39.3	28.3	23.4

Application example 2

The thermal properties of NEC plasma displays, model Plasmasync 42 "42 XM2 HD, with an aluminum/silicone heat sink adhered to the back of the plasma display panel, were analyzed under the following different screen conditions. Black and white patterns were generated on the display and the screen surface temperature was measured using an infrared camera. The background was black in all cases. The composition of the pattern is as follows: 1) three evenly spaced white lines horizontally across the screen (23.9% screen illumination); and 2) a 4 x 3 array of evenly spaced white points (4% screen illumination). After testing the cells with conventional aluminum/silicone coolant, the aluminum/silicone coolant was removed and replaced with a flexible graphite coolant having a thickness of 1.4mm and a thermal conductivity of about 260W/m ° K. The plasma display was then tested again under the same conditions as described above, and the results are shown in table 2.

TABLE 2

Pattern(s)	Heat dissipating agent	Tmax	Temperature range of white pattern	Ambient environment
					White line pattern	Aluminium/siloxane	61.4	32.9	25.2
White line pattern	Flexible graphite	55.1	33.9	24.9

These examples illustrate the advantages of using flexible graphite coolant over conventional coolant technology, which are observed in both the maximum temperature (Tmax) and temperature range (T-range).

All cited patents and publications mentioned in this application are incorporated herein by reference.

The invention described can obviously be varied in a number of ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims (30)

1. A method of applying a heat sink to a plurality of plasma display panels, comprising (a) providing a plurality of heat sink composites, each comprising a heat sink material having an adhesive thereon and a release material, the release material being arranged such that the adhesive is sandwiched between the heat sink material and the release material; (b) removing separation material from the plurality of composite materials; and (c) applying at least one composite material to each of the plurality of plasma display panels such that the adhesive bonds the heat dissipating material to the plasma display panel, wherein the separating material and the adhesive material are selected such that the adhesive and the separating material do not cause undesirable damage to the heat dissipating material when an average release load of no greater than 40 grams per centimeter is generated by the adhesive and the separating material at a separation rate of one meter per second.

2. The method of claim 1 wherein the average break-away load is no greater than 10 grams per centimeter at a separation speed of one meter per second.

3. The method of claim 1, wherein the adhesive achieves a minimum lap shear bond strength of at least 125 grams per square centimeter.

4. The method of claim 3, wherein the adhesive achieves an average lap shear bond strength of at least 700 grams per square centimeter.

5. The method of claim 3, wherein the adhesive increases the through-thickness thermal resistance of the adhesive/heat spreading material by no more than 35% compared to the heat spreader itself.

6. The method of claim 5, wherein the adhesive has a thickness of no greater than 0.5 mil.

7. The method of claim 6 wherein the adhesive has a thickness of 0.1 to 0.25 mils.

8. The method of claim 1, wherein the heat sink material comprises graphite.

9. The method of claim 8, wherein the heat sink material comprises at least one sheet of compressed particles of exfoliated graphite.

10. The method of claim 9, wherein the heat dissipating material comprises a laminate layer comprising a plurality of sheets of compressed particles of exfoliated graphite.

11. A heat sink suitable for use in the production of a plasma display panel, the heat sink comprising a heat sink composite comprising a heat sink material having an adhesive thereon and a release material disposed such that the adhesive is sandwiched between the heat sink material and the release material, wherein the release material and the adhesive are selected such that the adhesive and the release material do not cause undesirable damage to the heat sink material when subjected to an average release load of no greater than 40 grams per centimeter at a separation speed of one meter per second.

12. The fin of claim 11 wherein the average break-away load is no greater than 10 grams per centimeter at a separation speed of one meter per second.

13. The heat sink of claim 11, wherein the adhesive produces a minimum lap shear bond strength of at least 125 grams per square centimeter.

14. The heat sink of claim 13 wherein the adhesive produces an average lap shear bond strength of at least 700 grams per square centimeter.

15. The heat sink of claim 13 wherein the adhesive increases the thermal resistance through the thickness of the adhesive/heat sink material by no more than 35% as compared to the heat sink material itself.

16. The heat sink of claim 15 wherein the adhesive has a thickness of no greater than 0.5 mil.

17. The heat sink of claim 16 wherein the adhesive has a thickness of 0.1 to 0.25 mils.

18. The heat sink of claim 11, wherein the heat dissipating material comprises graphite.

19. The heat sink of claim 18, wherein the heat sink material comprises at least one sheet of compressed particles of exfoliated graphite.

20. The heat sink of claim 19, wherein the heat dissipating material comprises a laminate layer comprising a plurality of sheets of compressed particles of exfoliated graphite.

21. A method of applying a heat sink to a plasma display panel comprising (a) providing a heat sink composite comprising a heat sink material having an adhesive thereon and a release material disposed such that the adhesive is sandwiched between the heat sink material and the release material; (b) removing the separation material from the composite material; and (c) applying the composite material to the plasma display panel such that the adhesive adheres the heat dissipation material to the plasma display panel,

wherein the release material and the adhesive are selected such that the adhesive and the release material do not cause undesirable damage to the heat dissipating material when the release material produces an average release load of no more than 40 grams per centimeter at a release rate of one meter per second.

22. The method of claim 21 wherein the average break-away load is no more than 10 grams per centimeter at a separation speed of one meter per second.

23. The method of claim 21, wherein the adhesive achieves a minimum lap shear bond strength of at least 125 grams per square centimeter.

24. The method of claim 23 wherein the adhesive produces an average lap shear bond strength of at least 700 grams per square centimeter.

25. The method of claim 23 wherein the adhesive produces an increase in thermal resistance through the thickness of the adhesive/heat spreader material of no more than 35% compared to the heat spreader material itself.

26. The method of claim 25, wherein the adhesive has a thickness of no greater than 0.5 mil.

27. The method of claim 26, wherein the adhesive has a thickness of 0.1 to 0.25 mils.

28. The method of claim 21, wherein the heat sink material comprises graphite.

29. The method of claim 28, wherein the heat sink material comprises at least one sheet of compressed particles of exfoliated graphite.

30. The method of claim 29, wherein the heat dissipating material comprises a laminate layer comprising a plurality of sheets of compressed particles of exfoliated graphite.