The present invention relates to the field of polymer science, particularly polyurethane polymers for application as electrical and electronic encapsulation with heat dissipation including for high temperature applications
BACKGROUND OF THE INVENTION
Potting compounds are used for protection of electrical & electronic assemblies such as transformers, insulators, capacitors etc. during operation from water ingress, vibration, corrosive agents etc. The resin typically used for encapsulation consists of unsaturated polyesters, vinyl esters, polyurethane, epoxy resins etc. The present invention relates to polyurethane casting resins for production of polyurethane potting compounds obtained from reaction mixture of polyisocyanates, polyols, specialty additives and fillers.
Polyurethane compounds can be employed for encapsulation in applications where temperature rise is well below its glass transition temperature. Thermally stable Polyurethane resins with flame retardant additives can be employed for demanding application and fire safety. Alternatively, heat can be dissipated from assemblies using thermally conductive polyurethane compound, achieving temperature stabilization and ensuring continuous use at high temperatures without any deterioration in performance and degradation of resin. Fillers incorporated to make thermally conductive polyurethane compounds must ensure low processing viscosity in order to fill all interstices without cavities and bubbles, particularly in large systems.
Also, there are few challenges during handling and formulation of polyurethane compound such as effect of moisture on properties of raw material or moisture entrapment rendering compound porous. In prior art, moisture adsorbing agents are employed which adsorb moisture and produce a solid compound after curing. However, adsorbed moisture may release during high temperature application rendering compound to swell and vitiating IP rating for waterproof.

Thus, there is a need to tackle such challenges while formulating and provide a polyurethane encapsulating compound for high temperature applications.
SUMMARY OF THE PRESENT INVENTION
In an aspect of the present invention, there is provided a process for preparing a polyurethane formulation comprising the steps of: (a) reacting at least a polyol with at least a polyisocyanate to synthesize urethane linkage between polyol and polyisocyanate, wherein said linkage takes place in the absence of any catalyst.
In another aspect of the present invention, there is provided a polyurethane formulation comprising at least a polyol, at least a polyisocyanate, at least an additive, and at least a filler, wherein thermal conductivity (ASTM D5930) of the said formulation is about 0.55 W/mK.
DESCRIPTION OF ACCOMPANYING DRAWINGS
The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings. These and other details of the present invention will be described in connection with the accompanying drawings, which are furnished only by way of illustration and not in limitation of the invention, and in which drawings.
Fig. 1 depicts the chemical structure of polyol and polyisocyanate, in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, for the purposes of explanation, various specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, that embodiments of the present invention may be practiced without these specific
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one another or with any combination of other features. An individual feature may not address any of the problems discussed above or might address only one of the problems discussed above. Some of the problems discussed above might not be fully addressed by any of the features described herein. Example embodiments of the present invention are described below, as illustrated in various drawings in which like reference numerals refer to the same parts throughout the different drawings.
The present invention provides a process for preparing a polyurethane formulation comprising the steps of: (a) reacting at least a polyol with at least a polyisocyanate to synthesize urethane linkage between polyol and polyisocyanate, wherein said linkage takes place in the absence of any catalyst.
In an embodiment of the process claimed in the present invention, the at least a polyol is a naturally occurring polyol selected from the group consisting of but not limited to, castor oil, soyabean oil, linseed oil, and rice bran oil. The polyol used in the claimed process is not functionally modified or chemically treated. In a preferred embodiment, the polyol is castor oil. In another preferred embodiment, the polyol is a combination of castor oil, soyabeen oil, linseed oil, and rice bran oil.
In an embodiment of the process claimed in the present invention, the at least a polyisocyanate are aromatic or aliphatic polyisocyanate, selected from the group consisting of, but not limited to TDI, MDI, IDPI, HMDI and the like. In a preferred embodiment, the polyisocyanate is MDI. In another preferred embodiment, the polyisocyanate is a combination of MDI, TDI, IPDI, HMDI and the like. In an embodiment, the NCO content is in the range of 32±2%.
In an embodiment of the process claimed in the present invention, the formulation optionally further comprises at least an additive, wherein said additive can improve at least one of thermal, mechanical, or rheological properties by reducing surface tension or viscosity modification of the said formulation. The said at least additive is selected from the group consisting of

but not limited to acrylates, siloxanes, and combinations thereof. In a preferred embodiment, the additive is modified polysiloxanes
In an embodiment of the process claimed in the present invention, the said formulation comprises 30-80% polyol, 5-25% polyisocyanate, 30-60% fillers, and 0-10% additives. In a preferred embodiment, the said formulation comprises 30-60% polyol, 5-15% polyisocyanate, 40-60% fillers, and 0-5% additives. In a more preferred embodiment, the formulation comprises 34% polyol, 10% polyisocyanate, 55% fillers, and 1% additives.
In an embodiment of the process claimed in the present invention, the reaction mixture is cured at 60-80°C for about 30 minutes. In an alternate embodiment, the reaction mixture is cured at 22-30°C for about 18-24 hours.
In an embodiment of the process claimed in the present invention, the said process does not comprise any foaming agent. In another embodiment, the said process does not comprise any moisture adsorbing agent. Due to absence of any moisture adsorbing agent, under high temperature application and/or high humidity conditions, the resin does not swell and/or exhibit failure of its functionality.
The present invention also provides a polyurethane formulation comprising at least a polyol, at least a polyisocyanate, at least an additive, and at least a filler, wherein thermal conductivity (ASTM D5930) of the said formulation is about 0.55 W/mK.
In an embodiment of the formulation claimed in the present invention, the at least a polyol is a naturally occurring polyol selected from the group consisting of but not limited to, castor oil, soyabean oil, linseed oil, and rice bran oil. The polyol used in the claimed process is not functionally modified or chemically treated. In a preferred embodiment, the polyol is castor oil
In an embodiment of the formulation claimed in the present invention, the at least a polyisocyanate are aromatic or aliphatic polyisocyanate. In a preferred

embodiment, the polyisocyanate is aromatic polyisocyanate like MDI, TDI and combination thereof etc.
In an embodiment of the formulation claimed in the present invention, the said additive can improve at least one of thermal, mechanical, or rheological properties by reducing surface tension or viscosity modification of the said formulation. The said at least additive is selected from the group consisting of but not limited to acrylates, siloxanes, and combinations thereof. In a preferred embodiment, the additive is modified polysiloxanes. The additive improves the homogenization of filler which in turn enhances the thermal conductivity of the formulation.
In an embodiment of the formulation claimed in the present invention, the said filler selected from the group consisting of. Kaolin, dolomite, kyanite, sillimanite, enstatite, diopside, wollastonite, larnite, alumina, alkali feldspar, quartz, clay etc. In a preferred embodiment, the filler is kaolin, sillimanite or combination thereof etc.
In an embodiment of the formulation claimed in the present invention, the said formulation comprises 30-80% polyol, 5-25% polyisocyanate, 30-60% fillers, and 0-10% additives. In a preferred embodiment, the said formulation comprises 30-60% polyol, 5-15% polyisocyanate, 40-60% fillers, and 0-5% additives. In a more preferred embodiment, the formulation comprises 34% polyol, 10% polyisocyanate, 55% fillers, and 1% additives.
In an embodiment, the formulation claimed in the present invention does not comprise any moisture adsorbing agent.
The formulation is capable of high temperature application and high humidity environment as there is no swelling of the resin, thus, the formulation does not exhibit functionality failure in high temperature or high humidity applications.

In an embodiment of the formulation claimed in the present invention, the said formulation is prepared by a process as substantially descried in the present description.
EXAMPLES
Example 1
In a particular exemplification of the present invention, three different polyurethane formulations (trial A, trial B, and trial C) were prepared and evaluated for thermal conductivity.
Process for preparation of formulation of Trial 3
Part A
80 parts of Kaolin is charged into a beaker equipped with stirring blade and containing castor oil. Following proper homogenization, lphr of modified polysiloxane additive is added to increase fluidity. The process is followed with addition of 80phr filler and 1 phr additive with continuous stirring up to 400 rpm in controlled environment.
Part B
1 phr of modified siloxane additive is added to 27 phr of MDI with continuous mixing to prepare homogeneous mixture.
Part B is discharged into Part A and after 5-10 minutes of continuous stirring, the compound is transferred into cavity followed by either 22-35°C curing for 18-24 hours or high temperature curing at 60-80°C for about 30 minutes.
A similar Process is followed for preparing formulation of Trial 1 and Trial 2
Fig. 1 is a representative image of polyol and isocyanate.
The results are provided below in Table 1.

Table 1

Ingredients Trial 1 Trial 2 Trial 3
Physical characteristic Porous (Shore A 35 Solid (Shore A 60) Solid (Shore A 75)
Castor Oil Polyol (grams) 33 (35%) or 100 parts 33 (77%) or 100 parts 33 (34%) or 100 parts
Aromatic Isocyanate MDI (grams) 9(9.5%)or27phr 9 (21%) 27 phr 9(10%) or 27 phr
Modified polysiloxane Additive (grams) 1 (2%) or 3phr l(l%)or3phr
Kaolin Filler (grams) 53 (55.5%) or 160phr 53 (55) or 160 phr
Characteristic
Thermal conductivity W/mK (ASTM D5930) 0.05 0.24 0.55
As seen from Table 1, it can be appreciated that the particular combination of additive and filler (Trial 3) unexpectedly and surprisingly exhibits a synergistic effect in contributing to the thermal conductivity of the polyurethane formulation.
Example 2
Variation in amount of isocyanate with use of different polyol (Linseed Oil)
Table 2

Ingredients Trial 4 Trial 5 Trial 6
Physical Gelly Porous, not Solid but Porous, Solid compound
characteristic useful for low thermal without any
encapsulation and conductivity porosity of thermal
have high conductivity 0.49

Ingredients Trial 4 Trial 5 Trial 6
possibility of water ingress
Linseed oil Polyol (grams) 100 parts 100 parts 100 parts
Aromatic Isocyanate MDI (grams) 5 phr 45 phr 25 phr
Modified
polysiloxane
Additive (grams) 3phr 3 phr 3 phr
Alumina Filler (grams) 130phr 130phr 130 phr
Hence optimal amount of isocyanate is required for good performance and thermal conductivity.
Example 3
Use of different isocyanate (TDI) and additive (Acrylate based) Table 3

Ingredients Trial 7 Trial 8
Physical characteristic Porous, not desirable Solid
Castor Oil Polyol (grams) 100 parts 100 parts
Aromatic Isocyanate TDI(grams) 25 phr 25 phr
Acrylate based Additive (grams) 3phr
Kaolin Filler (grams) 160phr 160phr

Thermal conductivity W/mK (ASTM D5930) 0.53
Example 4
Thermal performance of the polyurethane formulation of the present invention vis synthetic polyol based encapsulating compound
Table 4 illustrates the comparison of performance in terms of temperature stabilization of components of a PCB encapsulated using renewable resource-based polyol polyurethane encapsulating compound against synthetic encapsulating compound with similar thermal conductivity.
Table 4

s.
No. Component Synthetic RM based
potting compound Renewable resource based PL)
compound Synthetic RM based
potting compound Renewable resource based PL)
compound
Test cond" @ 100V Input @300V Input
1 Drum coil 108 111 92 90
2 Diode bridge 116 106 93 91
3 Mosfet 119 118 101 100
4 Transformer winding 112 105 107 101
5 Transformer core 112 110 107 106
6 Diode 102 100 100 99
7 Capacitor 108 94 91 93
Advantages of the present invention:
The polyols used in the present invention is a renewal and sustainable material.

The polyol have no functional moieties modification or post-treatment done.
No catalysts are used in the process.
The polyurethane formulation is customizable to crosslink at room temperature as well as at elevated temperatures.
Additives are added for enhancement of thermal conductivity, heat dissipation, electrical and mechanical properties of the polyurethane.

I/We claim:

1. A process for preparing a polyurethane formulation comprising the step of reacting at least a polyol with at least a polyisocyanate to synthesize urethane linkage between polyol and polyisocyanate, wherein, said linkage takes place in the absence of any catalyst.
2. The process as claimed in claim 1, wherein the polyol is not functionally modified or chemically treated.
3. The process as claimed in claim 1, wherein said polyol is naturally occurring polyol selected from the group consisting of but not limited to, castor oil, soyabean oil, linseed oil, and rice bran oil.
4. The process as claimed in claim 1, wherein the formulation optionally further comprises at least an additive, wherein said additive can improve thermal, mechanical, or rheological properties by reducing surface tension or viscosity modification, wherein said additive is selected from the group consisting of but not limited to acrylates, siloxanes, and combinations thereof.
5. The process as claimed in claim 1, wherein the formulation optionally further comprises at least a filler selected from the group consisting of kaolin, bentonite, dolomite, kyanite, larnite, alumina, sillimantite, ensatite, diopside, wollastonite, alkali feldspar, quartz, clay, and combinations thereof.
6. The process as claimed in claim 1, wherein said formulation comprises 30-80% polyol, 5-25% polyisocyanate, 30-60% fillers, and 0-10% additives.
7. The process as claimed in claim 1, wherein the reaction mixture is cured at 60-80°C for about 30 minutes or at 22-30°C for 18-24 hours.
8. The process as claimed in claim 1, wherein polyisocyanate are aromatic or aliphatic polyisocyanate, such as but not limited to TDI (Toluene

diisocyanate), MDI (Methylene diphenyl diisocyanatej IPDI (Isophorone diisocyanate), HMDI (Hexamethylene diisocyanate) and the like, and wherein the NCO content is in the range of 32±2%.
9. The process as claimed in claim 1, wherein said process does not comprise any foaming agent or moisture adsorbing agent.
10. A polyurethane formulation comprising at least a polyol, at least a polyisocyanate, at least an additive, and at least a filler, wherein thermal conductivity (ASTM D5930) of the said formulation is about 0.55 W/mK.
11. The formulation as claimed in claim 10, wherein said formulation comprises 30-80% polyol, 5-25% polyisocyanate, 30-60% fillers, and 0-10% additives; wherein said additive can improve thermal, mechanical, or rheological properties by reducing surface tension or viscosity modification, and is selected from the group consisting of but not limited to acrylates, siloxanes, and combinations thereof; wherein said filler is selected from the group consisting of (); wherein said polyol is naturally occurring polyol selected from the group consisting of but not limited to, castor oil, soyabean oil, linseed oil, and rice bran oil; and wherein said polyisocyanate are aromatic or aliphatic polyisocyanate.
12. The formulation as claimed in claim 10, wherein said formulation does not comprise any moisture adsorbing agent.
13. The formulation as claimed in claim 8 is prepared by a process as claimed in claim 1.