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FIELD OF THE INVENTION

The present invention relates to a hot-melt resin for dissipating heat and which at the same time is electrically insulating or better has no electrical conductivity.

BACKGROUND ART

Most electronic components are low voltage and produce negligible amounts of thermal energy.

However, there are also many devices - such as CPUs, power diodes, power transistors and so on - which produce significant amounts of heat and it is important that this heat is removed quickly and efficiently from the device to ensure reliable component performance and avoid premature failure.

The use of increasingly powerful devices, for example electronic circuits, batteries, gear mechanisms, results in an increasingly high and dangerous heat development for the devices themselves.

In the automotive field, for example, the growing attention for a reduction of polluting emissions is giving a strong development to the spread of electric cars and therefore of electric batteries increasingly capable of ensuring a high autonomy combined with a higher charging speed.

Even in the telephony field, smartphones are increasingly larger, more powerful and therefore capable of developing more heat.

To date, all devices such as computers, laptops, monitors and many more are designed with increasingly sophisticated processors but, at the same time, the latter develop high loads of thermal energy/heat which must be dissipated in order not to alter the electronic features and their performance, in addition to their durability.

In physics, thermal resistance is defined as the difficulty of heat in passing through a solid, liquid or gaseous medium. Imagine a process of heat exchange between two points a and b maintained at constant temperatures different from each other, Ta and Tb, with Ta>Tb (think for example of the internal and external points of a perimeter wall of a house heated during the winter): the heat will pass spontaneously from point a to point b, and the speed of the process, once the temperatures Ta and Tb are fixed and maintained, depends precisely on the thermal resistance of the medium.

In stationary conditions, i.e. leaving aside the transitional period of the beginning of the heat exchange and considering only after this period (in the previous example, imagine that the house is already heated and one only has to keep the internal temperature constant), the radiant flux W is defined as the amount of heat exchanged in the unit of time and is measured in Joules per second [J/s], i.e. watts [W] The ratio of the temperature difference (Ta - Tb) to the radiant flux W that it causes in a medium is the thermal resistance of the medium, and is measured in Kelvin per watt [K/W], or equivalently in degrees Celsius on watts [°C/W]

In the international system, the unit of measurement for thermal resistance is given by kelvin per watt (K/W), or equivalently in degrees Celsius (°C) per watt (°C/W) (in the equations used only the difference of temperature between two points appears, which has the same value if measured in degrees Celsius or kelvin: 1 K = 1 °C; what changes is only the reference: 0 °C corresponds to 273.15 K).

The thermal resistance of materials is of great interest in electronic engineering because most electronic components generate heat which must be dispersed. The electronic components are sensitive to the operating temperature: the maximum die temperature (i.e. the thin sheet of semiconductor material on which the electronic circuit of the integrated circuit has been made, also called the chip) is specified in the datasheets, for which correct operation is ensured and reliability is linked exponentially to this parameter (the lower its temperature the lower the probability of failure). Therefore, in the design phase, it is important to calculate and check the temperatures of the components in the most stressful operating conditions.

Heat dissipation means are known to be coupled to electrical devices comprising metal bodies, for example aluminum and its alloys, having dissipative fins. Rolled aluminum has a very high thermal conductivity, 290 W/(m*K).

Disadvantageously, a metal coating is heavy, expensive and not applicable to electronic components present in micro circuits designed for devices in general.

The use of plastics as a coating is also known: the low cost, however, collides with the low thermal conductivity, less than 0.5 W/(m*K).

CN-103320076A (Suzhou Howbond New Material Co. Ltd.) describes a solvent-free anisotropic conductive hot melt adhesive which is composed of 75% to 97% by weight of hot melt glue and 3% to 25% by weight of conductive particles, in which the hot melt glue is selected from polyester resin, polyamide resin or polyurethane resin whose softening point is greater than 90 °C. The constituent conductive particles are selected from: silver, nickel or golden metal particles; ball-shaped or random platinum particles which have been coated with silver, nickel, gold or platinum; or monodisperse polyacrylate microspheres.

EP-2466811A (BASF SE) describes a thermoplastic molding composition which comprises, based on the weight of the composition: (a) at least one polyamide (PA), copolyamide or a polymeric mixture comprising polyamide; (b) 0.1% to 10% by weight of carbon nanotubes and/or graphenes; and (c) 0.1% to 3% by weight of ionic liquids. The thermoplastic molding composition does not comprise polyamide-12 units.

JP-2010006998A (Mitsubishi Materials Corp.) describes an electroconductive composition which comprises a polyamide resin in which an electroconductive charge and an electroconductive auxiliary agent selected from nitrogen-based dyes are incorporated. In a preferred embodiment, the resin composition comprises 0.01% to 5% by weight of nigrosine as an electroconductive auxiliary agent. In a further embodiment, the composition comprises 0.1% to 10% by weight of carbon nanofibers as an electroconductive charge.

WO-2003035739 (Hitachi Chemical Co. Ltd) discloses an electroconductive resin composition which comprises: a silver-plated copper powder (al) having an aspect ratio of 1 to 20 or a silver-plated copper powder (a2) having cooper part of the surface thereof; an electrically conductive powder containing a silver powder (Al); a thermoplastic resin (Bl) having one or more functional groups selected from the group consisting of an amide group, an ester group, an imide group and an ether group; and an organic solvent (C). In an alternative embodiment, an electro-conductive resin composition is described which comprises: said electro-conductive silver powder (Al), a thermoplastic resin (B2) selected from the group consisting of a polyamide silicone resin, a polyamide silicone resin and a polyimide silicone resin; and an organic solvent (C).

US-20140374032A1 (Heucher et al.) describes a hot-melt adhesive containing: 20% to 90% by weight of at least one polyamide having a molecular weight (Mw) from 10,000 g/mol to 250,000 g/mol; 1% to 25% by weight of at least one organic or inorganic salt; 0% to 60% by weight of further additives, where the adhesive has a softening point from 100 °C to 220 °C. Further described is a process for the reversible bonding of substrates, where the adhesive bond is released under tension after the application of an electric voltage.

WO-2017182621A1 describes an electrically conductive hot melt molding or adhesive composition field. Some formulations have been identified which, in order to ensure thermal conductivity and therefore thermal dissipation, use glass spheres. Disadvantageously, this type of filler, as already formulated for the electronics field, of polyamide-based hot-melt injection entails an innumerable series of problems in terms of application, in which the application tools used (pumps, fusers and dispensers) are abraded even after only 4 hours of use of the product.

The prior art mentioned above has not provided a solution to the problem of incorporating fillers having consistent heat dissipation but which at the same time are not electrically conductive in polyamide-based molding compositions which:

It is the object of the present invention to provide a solid or semi-solid product to be applied to electrical and mechanical devices which is capable of dissipating heat and which does not act as an electrical conductor since the electrical conductivity would cause a short circuit.

A further object of the present invention is that said product is simple to be manufactured and coupled to the device.

A still further object of the present invention is that said product is not polluting, since it is recyclable.

SUMMARY OF THE INVENTION

According to the invention, said and further objects are achieved by a hot-melt resin for dissipating heat produced by devices to which the resin is applied, characterized in that it comprises an adhesive loaded with boron nitride.

Advantageously, the resin is in a solid state, is free of solvents, is not reactive, does not conduct electric current, is not abrasive, but above all has a thermal conductivity and therefore dissipative, clearly greater than the adhesive.

The resin according to the present invention is applicable by mechanical means without wearing them, for manufacturing electronic and electrical components, for example cables and connectors.

These and other features of the present invention will appear more clearly from the following detailed description of a practical embodiment thereof, made by way of a non-limiting example.

DETAILED DESCRIPTION OF THE INVENTION A hot-melt resin comprises an adhesive raw material (glue) loaded with boron nitride powders (filler).

The raw material is normally a polyamide, but it may also be polyolefin, APAO (amorphous-poly-alpha-olefms) or EVA (ethylene- vinyl-acetate), or a mixture thereof.

The raw material has a solid or semi-solid form.

By resin (artificial or synthetic), it is meant a viscous material capable of hardening when cold or hot. It is generally a large class of different and complex polymers.

A hot-melt resin is a thermoplastic adhesive (glue) usually sold in the solid state which can be applied hot by means of suitable means, for example melters connected to injectors or heated nozzles which by injection, casting or other specific instrument, transfer the hot-melt resin where the product requires it.

Said raw material has a limited cost and is easily producible but has a low thermal conductivity: polyamides for example have a thermal conductivity lower than 0.3 W/(m*K), which is clearly lower than an aluminum alloy whose conductivity is close to 300 W/(m*K).

In an embodiment example tested by the Department of Energy of the Politecnico di Milano, the hot-melt resin of the invention is made up of 67% by weight of polyamide and 33% by weight of boron nitride.

A resin specimen was obtained by hot-dissolving the polyamide resin in a vacuum container (at about 200 °C) and then slowly adding boron nitride in the form of a powder. The vacuum is necessary to preserve the polyamide from -OH groups present in the environment in the form of humidity. The latter humidity would alter the morphological features of the final product as it has oxidizing power.

The hot mixture of polyamide and boron nitride was then pushed by a pump into an extruder from which the final product emerged which, upon exiting the extruder, solidified rapidly (about 3/6 seconds). The final product was in the form of balls, but it may take another shape according to the type of cut to be obtained and used (for example cubes).

Preferably, the hot-melt resin according to the present invention is in the solid state.

Preferably, the hot-melt resin according to the present invention is solvent-free.

Preferably, the hot-melt resin according to the present invention is non reactive.

Preferably, the hot-melt resin according to the present invention does not conduct electric current, i.e. it is insulating, but above all it has a higher thermal conductivity than the adhesive raw material.

Preferably, the hot-melt resin according to the present invention is non abrasive.

Preferably, the hot-melt resin according to the present invention does not contain any type of metallic filler.

In particularly preferred embodiments, the hot-melt resin according to the present invention is insulating and solvent-free. More preferably, this hot- melt resin is in the solid state and has a higher thermal conductivity than the adhesive raw material.

The final product described above has been tested according to three known conductivity measurement methods:

- MTPS (Modified Transient Plane Source) method: the method consists of applying a thermal stress to the surface of the specimen and measuring the surface temperature thereof during the resulting transient, using a special instrument as described in ASTM D7984- 16; the test was performed with the C-Therm Technologies TCi instrument, with an estimated accuracy of 5% of the measurement, with an initial specimen and sensor temperature equal to the ambient temperature;

- HFM (Heat Flow Meter Apparatus) method: this method, unlike the previous one, consists in keeping the two faces of the specimen at two different temperature values, and in measuring the resulting thermal flow, as described in the ASTM C518 and E1530 standards, and by the ISO 8301; the test was performed with TA Instruments Fox 50 instrument, with an estimated accuracy of 4% of the measurement; the temperature difference between the two faces is 10 °C, while the test was carried out for average temperature values (average T) equal to 25 °C and 85 °C.

The thermal conductivity results k (W/m*K), where k is the coefficient of thermal conductivity, were the following:

C-Therm = 1.9;

- Fox 50 (average T = 25 °C) = 1.78;

Fox 50 (average T = 85 °C) = 1.56.

The result is an average value of almost 1.75 k (W/m*K), which is much greater than the thermal conductivity of the raw material (polyamide).

The resin of the present invention preferably has one or more of the following features:

Brookfield viscosity (at 200 °C - Th.27 at 10 rpm, ASTM D 3236 method) between 6000 mPa*s (milli Pascal per second) and 12000 mPa*s, preferably between 9000 mPa*s and 11000 mPa*s, even more preferably between 9800 mPa*s and 10200 mPa*s;

softening point (R&B, ASTM D 3461 method) between 150 °C and 180 °C, preferably between 160 °C and 170 °C, even more preferably between 162 °C and 168 °C;

- water content less than 0.2% by weight;

density less than 1.3 cm ³ , preferably less than 1.15 cm ³ (density gradient column, ISO 1183);

hardness (at 23 °C, ISO 868 method) between 65 and 75 shore D, preferably equal to 70 shore D;

- water absorption (by immersion at 23 °C, ISO 62 method) comprised between 1% and 1.5% by weight, preferably equal to 1.2% by weight;

neutral/whitish color;

self-extinguishing features: reference Flammability UL94: V.0; strength at break (50 mm/min, at 23 °C, ISO 527 method) between

4 and 4.5 MPa*s, preferably equal to 4.2 MPa*s;

yield strength (50 mm/min, at 23 °C, ISO 527 method) of 5.3

MPa*s.

Said boron nitride is preferably in the form of powders having the following composition:

- boron nitride > 98.5%

- total oxygen < 0.5%

- boron oxide < 0.2%

Preferably, the specific surface area (BET) of such powders is comprised between 5 m ² /g and 10 m ² /g, preferably equal to 7 m ² /g. The specific surface area is a measure of the exposed surface of a solid sample on a molecular scale. The BET theory is the most used model for determining the area, which can be measured using instruments such as TriStar and 3 -Flex (Micromeritics).

Preferably the average particle size (D50) of these powders is between 10 pm and 15 pm, preferably equal to 12 pm. This measurement is carried out by laser diffraction, e.g. using an instrument such as a Mastersizer 3000 granulometer (Malvern Panalytical).

The features described above allow applying the resin according to the present invention by mechanical means without wearing them: the applicator, for example a heated gun, preferably has an operating temperature between 195 °C and 230 °C.

The final product (resin) can be used in the construction of electronic and electrical components, for example cables and connectors.

It is also possible to coat electrical circuits by inserting the component in a mold suitable for containing the circuit itself: the operating temperature in the mold is preferably between 20 °C and 60 °C.

The melting of the raw material preferably takes place between 180 °C and 220 °C, even more preferably at 200 °C as in the test described above.

Preferably, the hot-melt resin according to the present invention comprises 62% to 72% by weight of adhesive raw material, on the weight of the resin.

Preferably, the hot-melt resin according to the present invention comprises 28% to 38% by weight of boron nitride, on the weight of the resin.

The percentage by weight of raw material and boron nitride can vary with respect to the optimal one of the test: the content of raw material ranges from 62% to 72% by weight, the content of boron nitride therefore ranges from 28% to 38% by weight, on the weight of the resin.

An increase in boron nitride improves thermal dissipation but involves a decrease in raw material which disadvantageously increases the viscosity too, generating undesired injection problems.

Conversely, an increase in the percentage of raw material improves viscosity but penalizes heat dissipation.

The choice of the optimal mixture depends on the application of the product, the test mixture being however to be considered as preferred for most applications.

In some embodiments, the hot-melt resin of the invention essentially consists of an adhesive raw material loaded with boron nitride, as described above. The expression“essentially consists of’ means that these are the only components to be present in the hot-melt resin of the invention, which make the latter non-reactive, insulating, non-abrasive, and with greater thermal conductivity and therefore dissipative, while any further components do not significantly alter these properties of the resin.

In other embodiments, the hot-melt resin of the invention consists of an adhesive raw material loaded with boron nitride, as described above. Advantageously, the resin according to the present invention does not create any problem of abrasion on the application tools such as first of all the gears present in the melters, the extrusion pumps, the dosing units, and the nozzles which dispense the resin into the molds.

Unlike polyamide resin, polyolefin resins, APAO resins and EVA resins formulated with boron nitride, although part of the present invention, cannot be used in the production of microcircuits, such as those for the field of mobile telephony.

If polyolefin-based resins reach a softening point greater than 140 °C, their TG (glass transition temperature) would be too low and the final product would have a high viscosity. Therefore they are not suitable for the production of microcircuits, since the latter require small quantities of adhesive and this is applied at low pressures in the related molds, which in turn are filled through collectors with a diameter of the order of a few millimeters.

Polyolefin resins, APAO resins and EVA resins can however be used for casting in molds which require large quantities of product, such as compartments containing rechargeable batteries for electric vehicles.

Being very similar to traditional polyolefin resins, APAO resins have melting points and softening points which are between 90 °C and 110 °C, therefore not suitable for ensuring resistance to temperatures required in the telephony market. However, they can be used in the electronic field where particular thermal stresses and compound hardness are not required.

The resin of the present invention obtained based on EVA or polyolefins require the addition of tackifier resins type C9 -C5 - C5-C9, rosin, esterified rosin, coumarin resins, so that they can be considered adhesives, a condition without which it could not adhere to electronic components.

Raw materials with high viscosity, little flowability, hardness and low softening point can be used only for the production of electronic material with reduced features compared to polyamide resins.

The present invention essentially relates to the incorporation of charges which are not electrically conductive but with high heat dissipation performances within hot-melt adhesive or molding compositions, which are based, in particular, on polyamides and on the use of said compositions in injection molding techniques including encapsulation, under-ring and over molding of electronic components.

In particular, the resin claimed

1) does not require high pressure injection molding techniques;

2) implements a simple and clean process with operating cycle times;

3) satisfactorily solves problems of settling of the filler and heterogeneity of the filler.

It is to be understood that all possible combinations of the preferred aspects of the components of the resin are described and thus likewise preferred, as indicated above.

It is also to be understood that all the aspects identified as preferred and advantageous for the resin and its components are to be considered likewise preferred and advantageous also for the preparation and the uses of the resin itself.

The following are non-limiting embodiment examples of the present invention.

EXAMPLES

Example 1.

A hot-melt resin according to the present invention was prepared consisting of 67% by weight of polyamide and 33% by weight of boron nitride.

This resin was subjected to the following tests:

1) MTPS (Modified Transient Plane Source) method using C-Therm, at room temperature,

2) HFM (Heat Flow Meter Apparatus) method using Fox 50 (average T = 25 °C),

3) HFM (Heat Flow Meter Apparatus) method using Fox 50 (average T = 85°C),

as described above.

The results of conductivity k (W/m*K) were the following:

The result is an average value of almost 1.75, which is much greater than the thermal conductivity of the raw material (polyamide) as is.

The resin also had the following chemical-physical features:

Brookfield viscosity (at 200 °C - Th. 27 at 10 rpm, ASTM D 3236 method) of 9800-10200 mPa*s;

softening point (R&B, ASTM D 3461 method) of 162-168 °C;

- water content less than 0.2% by weight;

density < 1.15 cm ³ (density gradient column, ISO 1183);

hardness (at 23 °C, ISO 868 method) of 70 shore D;

- water absorption (immersion at 23 °C, ISO 62 method) of 1.2% by weight;

neutral/whitish color;

strength at break (50 mm/min, at 23 °C, ISO 527 method) of 4.2 MPa*s;

yield strength (50 mm/min, at 23 °C, ISO 527 method) of 5.3 MPa*s.

The extinguishing degree according to UL 94 was also measured.

The tests according to the American Underwriters Laboratories are the most used to determine the flammability of a plastic polymer.

The test consists in measuring the time in which the vertically mounted plastic specimen of a specified thickness (3-6 mm) continues to burn after being impinged by a Bunsen flame for 10 sec:

• The material is classified V.O when the flame goes out within 10 seconds. • The material is classified V.l when the flame goes out within 30 seconds without dripping.

• The material is classified V.2 when the flame extinguishes within 30 seconds with dripping.

The resin of the present example has been classified with: V.0, i.e. the flame went out in less than 10 seconds.

Example 2

A hot-melt resin was prepared consisting of 35% by weight of polyamide and 65% by weight of boron nitride.

It was observed that an increase in boron nitride improved thermal dissipation but involved a decrease in the adhesive raw material which disadvantageously increased the viscosity too, generating undesired injection problems.

Example 3

A hot-melt resin was prepared consisting of 95% by weight of polyamide and 5% by weight of boron nitride.

Unlike Example 2, it was observed that an increase in the percentage of adhesive raw material improved viscosity, but disadvantageously penalized heat dissipation.