WO2020230060A1

WO2020230060A1 - Thermally-conductive adhesive films with enhanced through-plane thermal conductivity, and method of making the same

Info

Publication number: WO2020230060A1
Application number: PCT/IB2020/054540
Authority: WO
Inventors: Li Yao; Richard Y. Liu; Jeremy M. Higgins
Original assignee: 3M Innovative Properties Company
Priority date: 2019-05-14
Filing date: 2020-05-13
Publication date: 2020-11-19

Abstract

A thermally-conductive adhesive film comprises an adhesive layer having first and second opposed major surfaces and a thickness therebetween. The adhesive layer comprises thermally-conductive platelets dispersed in a polymer matrix. Each of the thermally-conductive platelets has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially parallel to the thickness of the thermally-conductive adhesive film.

Description

THERMALLY-CONDUCTIVE ADHESIVE FILMS WITH ENHANCED THROUGH-PLANE THERMAL CONDUCTIVITY, AND

METHOD OF MAKING THE SAME

TECHNICAL FIELD

The present disclosure broadly relates to thermally-conductive adhesive fdms and methods for their preparation.

BACKGROUND

The power density of electronic devices has generally increased with reduced device size.

Application areas include, for example, light emitting diodes, electronic packaging, batteries, solar cells, and flexible printed circuits. Thermally-conductive pads have been used to address the problem of thermal management in electronic devices.

More recently, thermally-conductive adhesive fdms have begun to be widely used for such applications. Thermally-conductive adhesive fdms typically have better wettability to component surfaces and are typically much thinner than conventional thermal pads.

SUMMARY

Advantageously, the present disclosure provides improved thermally-conductive adhesive fdms that outperform extruded thermally-conductive adhesive fdms made by conventional processes. The thermally-conductive adhesive fdms have enhanced through-plane thermal conductivity, adhesion, and mechanical properties.

In one aspect, the present disclosure provides a thermally-conductive adhesive fdm comprising: an adhesive layer having first and second opposed major surfaces and a thickness therebetween, the adhesive layer comprising thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally-conductive platelets has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially parallel to the thickness of the thermally- conductive adhesive fdm.

In another aspect, the present disclosure provides a thermally-conductive adhesive fdm

comprising:

an adhesive layer having first and second opposed major surfaces and a thickness therebetween, wherein the adhesive layer has a length and a width, and wherein the adhesive layer comprises thermally- conductive platelets dispersed in a polymer matrix,

wherein thermal conductivity along the thickness of the thermally-conductive adhesive fdm is higher than thermal conductivity along the length and the width of the thermally-conductive adhesive fdm.

In yet another aspect, the present disclosure provides a method of making a thermally-conductive adhesive fdm, the method comprising: a) extruding an adhesive composition to form an extruded adhesive layer, wherein the extruded adhesive layer has first and second opposed major surfaces, wherein the extruded adhesive layer comprises thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally-conductive platelets in the extruded adhesive layer has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially perpendicular to the thickness of the extruded adhesive layer;

b) rolling the first surface of the extruded adhesive layer against the second surface of the extruded adhesive layer to form an elongated convolute adhesive member having first and second opposed ends; and c) slicing a thin film from the first end of the elongated convolute adhesive member to form a convolute adhesive layer, whereby a majority of the respective planes are substantially parallel to the thickness of the convolute adhesive layer.

As used herein:

"convolute" means rolled upon itself as, for example, a jelly roll or a roll of tape;

"polymer" refers to an organic polymer having sufficient molecular weight that addition of a single monomer unit does not result in a significant change in physical properties; and

"substantially parallel" means within ± 30 degrees of parallel.

The term "parallel" as applied to a first plane (e.g., of a platelet) and a line (e.g., a thickness) means that the line is wholly contained within a second plane that is parallel to, or coincident with, the first plane.

Features and advantages of the present disclosure will be further understood upon consideration of the detailed description as well as the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram depicting an exemplary method of making a convolute member 135 according to the present disclosure.

FIG. 1A is an enlarged view of region 1A in FIG. 1.

FIG. 2 is a schematic perspective view of depicting an exemplary method of making a thermally- conductive adhesive film according to the present disclosure from a convolute member.

FIG. 3 is a schematic side view of thermally-conductive adhesive film 300 according to the present disclosure.

FIG. 4 is a schematic perspective view of thermally-conductive adhesive film 300 according to the present disclosure.

FIG. 5 is an enlarged perspective view of a thermally-conductive platelet 150.

FIG. 6 is a cross-sectional scanning electron micrograph of the extruded thermally-conductive layer of Comparative Example C.

FIG. 7 is a cross-sectional scanning electron micrograph of the thermally-conductive film of Example 1. FIG. 8 is a cross-sectional scanning electron micrograph of the extruded thermally-conductive layer of Comparative Example D.

FIG. 9 is a cross-sectional scanning electron micrograph of the thermally-conductive fdm of Example 2.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

Thermally-conductive adhesive film according to the present disclosure can be made by a multi- step process shown in FIG. land described below.

Referring now to FIG. 1, in a first step, an adhesive composition 110 is extruded from extruder 130 to provide extruded adhesive layer 120. Referring now to FIG. 1A, extruded adhesive layer 120 has first and second opposed major surfaces (122,124). Extruded adhesive layer 120 comprises thermally- conductive platelets 150 dispersed in a polymer matrix 160.

Referring now to FIG. 2, each of thermally-conductive platelets 150 in extruded adhesive layer has a respective length LI and width W1 that define a respective plane P, and wherein a majority of the respective planes are substantially perpendicular to the thickness T1 of extruded adhesive layer 120.

Referring to FIGS. 1 and 2, first surface 122 of extruded adhesive layer 120 is rolled against second surface 124 to form convolute member 135, which has first and second opposed ends (132, 134). If the adhesive composition 110 is curable, it may optionally be at least partially cured at this point. In any case, thermally-conductive adhesive film 300 (see FIG. 3) is then formed by slicing it off the first end 132 of convolute member 130 at cut line 138. If the adhesive composition 110 is curable, the thermally- conductive adhesive film 300 may also optionally be at least partially cured at this point in the process.

While the adhesive layer is typically directly wound upon itself (i.e., rolled up) to form the convolute member, it may also, for example, be extruded onto a releasable carrier, then removed from the carrier and wound upon itself to form the convolute member at a later time. Other configurations of the process may also be used.

Referring again to FIG. 3, exemplary thermally-conductive adhesive film 300 comprises a convolute adhesive layer 310 having first and second opposed major surfaces (312,314) and a thickness T1 therebetween. Thermally-conductive adhesive film 300 comprises thermally-conductive platelets 150 dispersed in polymer matrix 160.

Referring to FIG. 5, each of thermally-conductive platelets 150 has a respective length L and width W that define a respective plane P. A majority of the respective planes P are substantially parallel to the thickness T2 of thermally-conductive adhesive film 300. The adhesive composition comprises a polymer matrix containing dispersed (preferably well- dispersed) thermally conductive platelets. In some embodiments, the adhesive composition is extrudable to form an extruded adhesive layer using a conventional hot-melt extruder. In other embodiments, the adhesive composition can be formed into an adhesive layer by other methods such as, for example, molding, embossing, or passing through a nip (e.g., heated or not heated).

The polymer matrix comprises at least one polymer. Preferably, the polymer matrix comprises a hot-melt adhesive, although this is not a requirement. Hot-melt adhesives are typically non-tacky at room temperature but become tacky and capable of bonding adherends at elevated temperatures. These adhesives usually have a glass transition temperature (Tg) or a melting point (T_m) above room temperature. When the temperature is elevated above the Tg or the T_m the storage modulus usually decreases, and the adhesive becomes tacky. Exemplary hot-melt adhesives include those based on ethylene-vinyl acetate copolymers, ethylene-acrylate copolymers, polyolefins, polyamides, polyesters, polyurethanes, styrenic block copolymers (SBCs), polycaprolactone, and polycarbonates. Of these SBCs are preferred in some embodiments.

Some preferred polymer matrixes comprise block copolymers that are styrenic block copolymers (SBCs). SBCs may be hot-melt adhesives and optionally hot-melt pressure -sensitive adhesives. SBCs generally include copolymers of the A-B or A-B-A type or a combination thereof, where A represents a thermoplastic polystyrene block and B represents an elastomeric block, such as polyisoprene,

polybutadiene, poly(ethylene/butylene), poly(ethylene/propylene), or poly(isoprene/butadiene). SBC molecular weights typically range from about 100,000 grams per mole to about 1,500,000 grams per mole.

Examples of useful SBCs include styrene-isoprene block copolymers, styrene -ethylene block copolymers, styrene-propylene block copolymers, styrene-ethylene-propylene block copolymers, styrene- ethylene-butylene block copolymers, styrene -butadiene block copolymers, styrene-isoprene-butadiene- styrene block copolymers, and combinations thereof. In some embodiments, the styrenic block copolymers are diblock, triblock, or higher block copolymers. In some embodiments, the styrenic block copolymer is a styrene-isoprene diblock copolymer, a styrene-isoprene-styrene triblock copolymer, a styrene-isoprene- styrene triblock copolymer, and combinations and mixtures thereof. In some embodiments, functionalized (e.g., maleated) versions of any of the above block copolymers may be used. In some embodiments, the styrene-based block copolymers are styrenic block copolymers comprising styrenic end block and isoprene mid-block. In some embodiments, the styrene-based block copolymers are styrenic block copolymer comprises of diblock of a styrenic block and isoprene block.

SBCs useful in the present disclosure can be in the form of various molecular architectures including linear, branched, radial, star and tapered geometries. Variation of the volume fraction of styrene in the two-phase composition leads to polystyrene domains in the shape of spheroids, cylinders, plates and co-continuous structures. In some embodiments, weight percent of the styrene component in the one or more styrene block copolymers can range from about 5 weight percent styrene to about 50 weight percent styrene, in some embodiments from about 8 weight percent styrene to about 40 weight percent styrene, in some embodiments from about 15 weight percent styrene to 35 weight percent styrene, and some embodiments from about 20 weight percent styrene to about 30 weight percent styrene.

Examples of commercially available SBCs useful in the presently disclosed binder layer or application layer include styrene-isoprene block copolymers such as those commercially available as KRATON D1161, KRATON D1119, and KRATON D1117 from Kraton Corp., Houston, Texas;

VECTOR 4113, and VECTOR 4111A from Dexco Polymers LLP, Taipei, Taiwan; QUINTAC 3620 from Zeon Corp. Tokyo, Japan; and EUROPRENE SOL T 9113 from Versalis, Milan, Italy. Examples of commercially available SBCs useful in the presently disclosed binder layer also include styrene- ethylene/butylene block copolymers such as those commercially available as KRATON G1657 from Kraton Corp.; styrene- ethylene/propylene block copolymers, such as those commercially available as KRATON G1702 from Kraton Corp.; styrene-butadiene block copolymers, such as those commercially available as KRATON D1118X from Kraton Corp.; and styrene-isoprene/butadiene block copolymers, such as those commercially available as KRATON D1117P from Kraton Corp.

In some embodiments, the polymer matrix comprises a pressure-sensitive adhesive which may also be a hot melt adhesive, although this is not a requirement. For example, in some embodiments, SBCs may be modified by the addition of one or more non-polymeric compounds such as tackifiers and/or plasticizing oils to, for example, increase the tack. Any suitable tackifier that is particularly effective in combination with an SBC may be included in the polymer matrix. Amounts of these materials to include will be apparent to those of skill in the art. In some embodiments, the tackifier and the plasticizer may be used alone or in combination with one another. In some embodiments, the tackifier and the plasticizer may be combined with aforementioned tackifier containing non-carbon hetero-atom functionality individually or together.

In some embodiments, a non-styrenic hydrocarbon block copolymer or combination thereof can be used along with a styrenic block copolymer. In some embodiments, the block copolymers may include, for example, isoprene-butadiene block copolymers, ethylene-butylene block copolymers, and ethylene - propylene block copolymers.

In some embodiments, a hydrocarbon block copolymer (e.g., styrenic block copolymer) may include a blend of two or more such copolymers. In some embodiments, the blends of block copolymers include blends of polymers differing solely in terms of overall molecular weight, molecular weight of one or more blocks, degree of branching, chemical makeup of blocks, number of blocks, or molecular weight of block fractions. In some embodiments, the blends of block copolymers have more than one such difference. In some embodiments, a blend of substantially linear triblock copolymer blended with a substantially linear block copolymer may be employed.

The adhesive layer may further include, for example, one or more of tackifier(s), plasticizer(s), pigment(s), stabilizer(s), free-radical initiators (e.g., thermal initiators and/or photoinitiators), and fdlers other than the thermally conductive fdler(s). As used herein, the term "tackifier" (e.g., a tackifying resin) means a material that is part of an adhesive as a rheological modifier to increase glass transition temperature, decrease modulus, increase tack, or a combination of two or more of these. Typically, if a tackifier is included, the adhesive composition will be a pressure-sensitive adhesive (which may also be a hot-melt adhesive or not), although this is not a requirement.

Pressure-sensitive adhesive compositions are known to those of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack at room temperature, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be cleanly removable from the adherend.

In some embodiments, the adhesive composition is curable. Curing may be accomplished by heating, exposure to actinic radiation (e.g., ultraviolet and/or visible light, gamma rays, or an electron beam), and/or chemical agents. Of these exposure to actinic radiation is typically preferred due to ease of implementation. In those embodiments wherein the polymer matrix contains free -radically polymerizable ethylenic unsaturation, the adhesive layer preferably further comprises a photoinitiator (i.e., for free-radical polymerization). If present, the amount of photoinitiator is typically an effective amount that is at least sufficient amount to cause at least partial curing of the adhesive layer upon exposure to sufficient actinic radiation. Typically, effective amounts of photoinitiator comprise less than 10 percent by weight, more typically less than 7 percent by weight, and more typically less than 3 percent by weight of the total adhesive layer. It will be recognized that curing may be complete even though polymerizable

(meth)acrylate groups remain.

Exemplary photoinitiators include a-cleavage photoinitiators such as benzoin and its derivatives such as a-methylbenzoin; a-phenylbenzoin; a-allylbenzoin; a-benzylbenzoin; benzoin ethers such as benzil dimethyl ketal (available as IRGACURE 651 from Ciba Specialty Chemicals, Tarrytown, New York), benzoin methyl ether, benzoin ethyl ether, benzoin n-butyl ether; acetophenone and its derivatives such as 2 -hydroxy-2 -methyl- 1 -phenyl- 1-propanone (available as DAROCUR 1173 from Ciba Specialty

Chemicals) and 1-hydroxycyclohexyl phenyl ketone (available as IRGACURE 184 from Ciba Specialty Chemicals); 2-methyl-l-[4-(methylthio)phenyl]-2-(4-morpholinyl)-l-propanone (available as IRGACURE 907 from Ciba Specialty Chemicals); 2-benzyl-2-(dimethylamino)-l-[4-(4-morpholinyl)phenyl]-l- butanone (available as IRGACURE 369 from Ciba Specialty Chemicals); titanium complexes such as bis(rp-2.4-cyclopentadien- 1 -yl)bis|2.6-difluoro-3-( 1 H-pyrrol- 1 -yl)phenyl |titanium (available as CGI 784 DC from Ciba Specialty Chemicals); and mono- and bis-acylphosphines (available from Ciba Specialty Chemicals as IRGACURE 1700, IRGACURE 1800, IRGACURE 1850, and DAROCUR 4265). One useful photoinitiator, a difimctional alpha hydroxyketone, is available as ESACURE ONE from Lamberti S.p.A, Albizzate, Italy.

Preferably, if an acylphosphine or acylphosphine oxide photoinitiator is utilized, it is combined with a photoinitiator (e.g., 2 -hydroxy-2 -methyl- 1 -phenyl- 1-propanone) having a high extinction coefficient at one or more wavelengths of the actinic radiation. Such combination typically facilitates surface cure while maintaining low levels of costly photoinitiator. Other useful photoinitiators include: anthraquinones (e.g., anthraquinone, 2-ethylanthraquinone, 1- chloroanthraquinone, 1,4-dimethylanthraquinone, 1-methoxyanthraquinone) and benzophenone and its derivatives (e.g., phenoxybenzophenone, phenylbenzophenone).

Useful thermally-conductive fdlers include any thermal fdler that has the form of platelets.

Examples include hexagonal boron nitride platelets (e.g., 3M Boron Nitride Cooling Filler Platelets from 3M Company), graphene (e.g., graphene nanoplatelets), nickel platelets (e.g., Novamet Conductive Nickel Flake from Hart Materials, Staffordshire, United Kingdom), stainless steel platelets (e.g., Novamet Stainless Steel Flake from Hart Materials), graphene oxide platelets, alumina platelets (e.g., as described in U.S. Pat. No. 5,137,959 (Block et al), and combinations thereof. Of these, hexagonal boron nitride platelets are typically preferred.

Thermally-conductive adhesive films according to the present disclosure preferably have in-plane thermal conductivity of from 10 watts per kelvin-meter (W/(K m)) to 10000 W/(K m) and a through-plane thermal conductivity of from 0.1 W/(K m) to 100 W/(K m), although this is not a requirement.

Thermally-conductive fdlers may have any particle size, but preferably have a maximum dimension in the size range of 100 nm to 1 mm. Similarly, thermally-conductive fdlers may have any aspect ratio, but preferably have an aspect ratio of more than 10, more preferably 10 to 100, more preferably, 20 to 70, most preferably 30 to 60.

Single crystal boron nitride can vary in particle size from sub-micron up to a D₅₀ of 50 micrometers (pm) as measured by a laser diffraction particle size analyzer (e.g., a MASTERSIZER available from Malvern Instruments (Worcestershire, UK)). Larger sizes can be used in some

embodiments. Farger particle sizes are generally have relatively higher thermal conductivity while smaller particle sizes generally have lower production costs. In some embodiments, particle agglomerates are used to attain even higher particle sizes.

Single crystal h-BN has a strong anisotropy in thermal conductivity with up to 400 watts per kelvin-meter (W K^-1 m^-1, i.e. W/(K m)) in plane (e.g., the x- and y-axes) and as low as 4 W/(K m) through plane (e.g., the z-axis).

Any amount of thermally conductive fdler may be used in the adhesive layer, but typically the amount is from 10 to 80 percent by weigh, more preferably 20 to 70 percent by weight, and even more preferably 30 to 70 percent by weight.

Referring now to FIG. 4, exemplary thermally-conductive adhesive fdm 400 comprises a convolute adhesive layer 410 having first and second opposed major surfaces (412,414) and a thickness T4 therebetween. Convolute adhesive layer 410 comprises thermally-conductive platelets 150 dispersed in polymer matrix 160. Thermal conductivity along the thickness T4 of the convolute adhesive layer is higher than thermal conductivity along the length F4 and the width F4 of the thermally-conductive adhesive fdm 400. The thermally-conductive adhesive fdm preferably has a thickness of 0.1 to 2.5 millimeters, more preferably 0.5 to 2 millimeters, and more preferably 0.5 to 1.8 millimeters.

Thermally-conductive adhesive fdms according to the present disclosure are useful, for example, for electronic/semiconductor packaging, where an ultra-thin thermally conductive fdm adhesive can replace the currently-used thermal pad for dissipating heat from electronic packaging board into the heat sink. Another use is in conjunction with lithium batteries where heat needs to be efficiently dissipated from the battery to a heat sink in case of a pressure built up inside of the battery. Also in Organic Light Emitting Diode (OLED) displays where heat is generated from the battery, CPU, and chips, a good heat dissipation layer is needed on the backside of the OLED display to dissipate heat and spread the heat evenly to the whole display.

SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a thermally-conductive adhesive film comprising:

an adhesive layer (e.g., a convolute adhesive layer) having first and second opposed major surfaces and a thickness therebetween, the adhesive layer comprising thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally-conductive platelets has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially parallel to the thickness of the thermally-conductive adhesive film.

In a second embodiment, the present disclosure provides a thermally-conductive adhesive film according to the first embodiment, wherein the polymer matrix comprises at least one of a styrene- isoprene-styrene block copolymer or a styrene-butadiene-styrene copolymer.

In a third embodiment, the present disclosure provides a thermally-conductive adhesive film according to the first or second embodiment, wherein the polymer matrix comprises a hot-melt adhesive.

In a fourth embodiment, the present disclosure provides a thermally-conductive adhesive film according to the third embodiment, wherein the hot-melt adhesive is a pressure-sensitive hot-melt adhesive.

In a fifth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the first to fourth embodiments, wherein the polymer matrix contains free-radically polymerizable ethylenic unsaturation, and wherein the adhesive layer further comprises a photoinitiator.

In a sixth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the first to fifth embodiments, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.

In a seventh embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the first to fourth embodiments, wherein the thickness of the thermally-conductive adhesive film is 0.5 millimeter to 1.8 millimeters.

In an eighth embodiment, the present disclosure provides a thermally-conductive adhesive film comprising: an adhesive layer (e.g., a convolute adhesive layer) having first and second opposed major surfaces and a thickness therebetween, wherein the adhesive layer has a length and a width, and wherein the adhesive layer comprises thermally-conductive platelets dispersed in a polymer matrix,

wherein thermal conductivity along the thickness of the adhesive layer is higher than thermal conductivity along the length and the width.

In a ninth embodiment, the present disclosure provides a thermally-conductive adhesive film according to the eighth embodiment, wherein the thermal conductivity along the thickness of the thermally-conductive adhesive film is at least twice the thermal conductivity along the length and the width of the thermally-conductive adhesive film.

In a tenth embodiment, the present disclosure provides a thermally-conductive adhesive film according to the eighth or ninth embodiment, wherein the polymer matrix comprises a hot-melt adhesive.

In eleventh embodiment, the present disclosure provides a thermally-conductive adhesive film according to the tenth embodiment, wherein the hot-melt adhesive is a pressure -sensitive hot-melt adhesive.

In a twelfth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the eighth to eleventh embodiments, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.

In a thirteenth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the eighth to twelfth embodiments, wherein the thickness of the adhesive layer is 0.5 millimeter to 1.8 millimeters.

In a fourteenth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the eighth to thirteenth embodiments, wherein the elongation at break is at least 500 percent.

In a fifteenth embodiment, the present disclosure provides a thermally-conductive adhesive film according to any one of the eighth to fourteenth embodiments, wherein the tensile strength is at least 0.7

N/mm after UV radiation.

In a sixteenth embodiment, the present disclosure provides a method of making a thermally- conductive adhesive film, the method comprising:

a) extruding an adhesive composition to form an extruded adhesive layer, wherein the extruded adhesive layer has first and second opposed major surfaces, wherein the extruded adhesive layer comprises thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally-conductive platelets in the extruded adhesive layer has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially perpendicular to the thickness of the extruded adhesive layer;

In a seventeenth embodiment, the present disclosure provides a method according to the sixteenth embodiment, wherein the polymer matrix comprises a hot-melt adhesive.

In an eighteenth embodiment, the present disclosure provides a method according to the seventeenth embodiment, wherein the hot-melt adhesive is a pressure-sensitive hot-melt adhesive.

In a nineteenth embodiment, the present disclosure provides a method according to any one of the sixteenth to eighteenth embodiments, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

Material abbreviations used in the Examples are reported in Table 1 below.

TABLE 1

Fabrication of Co-extruded Film Adhesive Fabrication

Films used in the Examples (see Tables 2 and 5) were compounded and fabricated using a 30- millimeter (mm) Wemer & Pfleiderer co-rotating twin screw extruder. Components were pre-mixed, then vohimetrically fed into the extruder feed throat and subjected to 300 rotations per minute (rpm) mixing.

The extruder, melt transport, and die temperatures were set to 232 °C for Examples 4 and 5. After compounding, the material was coated directly onto polyester backing at a thickness of 0.004 - 0.014 inches (0.10 - 0.36 mm) film and covered with a polyester release liner.

Fabrication of Vertical Layered Film Adhesives

A hot-melt adhesive composition was hot-melt extruded as a film of 0.15 mm to 0.25 mm thickness that was wound directly upon itself to form a convolute roll. A slice off the end of the convolute roll was made using a cutting blade to convert the in-plane direction into through-plane direction. The slice had a thickness of 0.5 mm to 1.8 mm as illustrated in FIG. 1.

l IV Radiation Procedure

Both sides of the films were exposed by UV radiation with the dosage as listed in Tables 6 and 7 on a HERAEUS NOBLELIGHT FUSION UV CURING SYSTEM (Fusion UV Systems Inc.,

Gaithersburg, Maryland), where they were exposed to UV radiation from a "D" bulb. UV exposure dosage can be adjusted by adjusting the conveyor speed together with running for multiple times and the energy output was recorded from the UVA range (320-390 nanometers (nm)) using a UV POWER PUCK II (EIT LLC, Sterling, Virginia).

Gel Percentage Test Method

Aluminum mesh was wrapped around a known amount of film sample (about 2-3 grams (g)). The wrapped sample was then submerged in tetrahydrofiiran (approximately 100 milliliters (mL)) for 60 hours under constant stirring. The sample was then removed from the solvent and the mass of the remaining sample was recorded. The gel percentage of the sample was calculated as being equal to the ratio of the mass of the remaining film to the mass of original film. Tensile Test Method

Film samples were cut into 1 inch by 5 inches (2.54 centimeters (cm) by 12.7 cm) strips for yield strength, tensile strength, elongation at break tensile tests. Tensile tests were conducted on an Instron single column table top system, Model 5943, with Instron 2712-041 pneumatic action grips (Instron, Norwood, Massachusetts) at the speed of 2 inches/minute (5.08 centimeters/minute).

180° Peel Strength Test Method

Film samples were cut into 1 inch by 3.5 inch (2.54 cm by 8.89 cm) strips and sandwiched between a NANOPLAST PET fdm (1 inch x 5 inches x 0.02 inch (2.54 cm x 12.7 cm x 0.05 cm), 3M Company) and a selected substrate (2 inch x 5 inch x 0.048 inch (5.08 cm x 12.7 cm x 0.12 cm) stainless steel, 304sst (Oakdale Precision, Oakdale, Minnesota); 2 inch ^c 5 inch ^c 3/16 inch (5.08 cm x 12.7 cm x 0.48 cm), CLEARLEXAN Polycarbonate (Aeromat Plastics, Burnsville, Minnesota); or 2 inch x 5 inches x 3/16 inch (5.08 cm x 12.7 cm x 0.48 cm), acrylonitrile butadiene styrene, TP-BLKABS (Aeromat Plastics). The assembled samples were then passed through a 4.5 pound (2.04 kilograms (kg)) weight roller three times to laminate them together. After dwelling at room temperature for 72 hours, the 180° peel strength tests were performed on an Instron single column table top system Model 5943 (1 kN capacity), with Instron 2712-041 pneumatic action grips (1 kN capacity) (Norwood, Massachusetts) at the speed of 12 inches/minute (30.48 centimeters/minute). Samples were inserted into the instrument according to the Instron lab manual and the 180° test was performed. Results are reported in Newtons per millimeter (N/mm).

Static Shear Test Method

Static shear tests were conducted on 1 inch x 1 inch (2.54 cm x 2.54 cm) square film adhesive samples. The square film adhesive samples were laminated between a stainless steel coupon and PET film (2 mil (0.05 mm) thick NANOPLAST PET film, 3M, St. Paul, MN). A 500 g weight was hung from the sample and the sample and hanging weight were placed into a chamber heated to 70 °C. The time until failure (i.e., from the time the weight was hung to the time the weight fell) was recorded.

Thermal Conductivity Test Method

For thermal conductivity measurements, disk-shaped samples were made by pressing a disk-shaped mold into the cured film with a diameter of 12.6 mm and a thickness of 0.10 - 0.36 mm.

Specific heat capacity, c, was measured using a Q2000 Differential Scanning Calorimeter (TA Instruments, Eden Prairie, Minnesota) with sapphire as a method standard.

Sample density was determined using a geometric method. The weight (m) of a film was measured using a standard laboratory balance, the diameter (d) of the disk was measured using calipers, and the thickness (h) of the disk was measured using a Mitatoyo micrometer. The density, p, was calculated as p = m/( ph-(d/2)²).

Thermal conductivity measurements were conducted for both in-plane and through-plane directions. The measurements were made using an LFA 467 HYPERFLASH Light Flash Apparatus (Netzsch Instruments, Burlington, Massachusetts) according to ASTM E1461-13, "Standard Test Method for Thermal Diffusivity by the Flash Method" . Thermal conductivity, k, was calculated from thermal diffusivity, heat capacity, and density measurements according the formula:

k = a· c·p

where k is the thermal conductivity in W/(K m), a is the thermal diffusivity in mm /s, c is the specific heat capacity in J/K-g, and p is the density in g/cm .

Tackiness Measurement Test Method

Tackiness was measured by pressing an ungloved finger on the film adhesive, ranked by high finger tack, medium finger tack, and low finger tack.

COMPARATIVE EXAMPLES A to C

Thermally conductive adhesives were prepared according to the formulations reported in Table 2. Films of the thermally conductive adhesives were prepared according to the Fabrication of Co-extruded Film Adhesive Fabrication procedure. Tackiness measurements were taken according to the Tackiness Measurement Test Method. Process conditions, final film thickness, and tackiness measurements of the thermally conductive adhesives are reported in Table 2. Tensile and static shear properties (obtained by the Tensile Test Method and Static Shear Test Method) for Comparative Examples A to C are reported in Table 3. Results for the 180° peel strength tests (obtained by the 180° Peel Strength Test Method) on stainless steel (SS), polycarbonate (PC), and acrylonitrile-butadiene-styrene copolymer (ABS) substrates are reported in Table 4. Results for thermal conductivity (k), both in-plane and through-plane, were measured according to the Thermal Conductivity Test Method are also reported in Table 4.

TABLE 2

TABLE 3

TABLE 4

COMPARATIVE EXAMPLES D and E

UV-Crosslinkable thermally conductive adhesives were prepared according to the formulations reported in Table 5. Films of the UV-crosslinkable thermally conductive adhesives were prepared according to the Fabrication of Vertical Layered Film Adhesives. Tackiness measurements were taken according to the Tackiness Measurement Test Method described previously. Process conditions, final film thickness, and tackiness measurements of the UV-crosslinkable thermally conductive adhesives are reported in Table 5.

The fdms were then exposed to UV radiation according to the l IV Radiation Procedure. Gel percentages of films exposed to UV radiation were obtained according to the Gel Percentage Test Method and are reported in Table 6. Mechanical properties before and after UV exposure for Examples 1 and 2 were measured according to the Tensile Test Method and are reported in Table 7. Through-plane thermal conductivity (TC) of Examples 1 and 2 were obtained according to the Thermal Conductivity Test Method described previously and are reported in Table 8.

TABLE 5

TABLE 6

TABLE 7

TABLE 8

EXAMPLES 1 and 2

Examples 1 and 2 were sliced fdm adhesives with vertical layers prepared as described previously. Film thickness were ranged from 0.5 mm to 1.8 mm. Results for through-plane thermal conductivity, were measured according to the Thermal Conductivity Test Method and are reported in Table 9. Mechanical properties before and after UV exposure for Example 2 were measured in two directions, perpendicular to the layer interface and along the layer interface, according to the Tensile Test Method described above and are reported in Table 10.

TABLE 9

TABLE 10

All cited references, patents, and patent applications in this application that are incorporated by reference, are incorporated in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in this application shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.

Claims

What is claimed is:

1. A thermally-conductive adhesive fdm comprising:

an adhesive layer having first and second opposed major surfaces and a thickness therebetween, the adhesive layer comprising thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally-conductive platelets has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially parallel to the thickness of the thermally- conductive adhesive film.

2. The thermally-conductive adhesive film of claim 1, wherein the polymer matrix comprises at least one of a styrene-isoprene-styrene block copolymer or a styrene-butadiene-styrene copolymer.

3. The thermally-conductive adhesive film of claim 1, wherein the polymer matrix comprises a hot- melt adhesive.

4. The thermally-conductive adhesive film of claim 3, wherein the hot-melt adhesive is a pressure- sensitive hot-melt adhesive.

5. The thermally-conductive adhesive film of claim 1, wherein the polymer matrix contains free- radically polymerizable ethylenic unsaturation, and wherein the adhesive layer further comprises a photoinitiator.

6. The thermally-conductive adhesive film of claim 1, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.

7. A thermally-conductive adhesive film comprising:

wherein thermal conductivity along the thickness of the thermally-conductive adhesive film is higher than thermal conductivity along the length and the width of the thermally-conductive adhesive film.

8. The thermally-conductive adhesive film of claim 7, wherein the thermal conductivity along the thickness of the adhesive layer is at least twice the thermal conductivity along the length and the width.

9. The thermally-conductive adhesive film of claim 7, wherein the polymer matrix comprises a hot- melt adhesive.

10. The thermally-conductive adhesive film of claim 9, wherein the hot-melt adhesive is a pressure- sensitive hot-melt adhesive.

11. The thermally-conductive adhesive film of claim 7, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.

12. A method of making a thermally-conductive adhesive film, the method comprising:

a) extruding an adhesive composition to form an extruded adhesive layer, wherein the extruded adhesive layer has first and second opposed major surfaces, wherein the extruded adhesive layer comprises thermally-conductive platelets dispersed in a polymer matrix, wherein each of the thermally- conductive platelets in the extruded adhesive layer has a respective length and width that define a respective plane, and wherein a majority of the respective planes are substantially perpendicular to the thickness of the extruded adhesive layer;

b) rolling the first surface of the extruded adhesive layer against the second surface of the extruded adhesive layer to form an elongated convolute adhesive member having first and second opposed ends; and

c) slicing a thin film from the first end of the elongated convolute adhesive member to form a convolute adhesive layer, whereby a majority of the respective planes are substantially parallel to the thickness of the convolute adhesive layer.

13. The method of claim 12, wherein the polymer matrix comprises a hot-melt adhesive.

14. The method of claim 13, wherein the hot-melt adhesive is a pressure-sensitive hot-melt adhesive.

15. The method of claim 12, wherein the thermally-conductive platelets comprise at least one of hexagonal boron nitride, graphene, or graphene oxide.