CLIAMS:1. A process for the preparation of high conductivity and high heat capacity ultrahigh molecular weight polyethylene (UHMWPE) products; said process comprising feeding UHMWPE having molecular weight distribution (MWD) ranging from 6 to 25 at the nip of at least one pair of heated polished counter rotating calendaring rollers having a pre-determined roller speed and roller speed ratio with the roller temperature being kept below the melt temperature of the polymer followed by hot stretching to obtain the UHMWPE products.

2. The process as claimed in claim 1, wherein said UHMWPE is at least one selected from the group consisting of UHMWPE powder and compressed UHMWPE preform.

3. The process as claimed in claim 1, wherein said UHMWPE is disentangled.

4. The process as claimed in claim 1, further includes incorporating at least one additive in the UHMWPE polymer fed at the nip of at least one pair of heated polished counter rotating calendaring rollers.

5. High conductivity and high heat capacity UHMWPE products prepared by the process as claimed in claim 1, characterized in that the axial thermal conductivity being greater than 130W/mK, transverse direction thermal conductivity ranging from 0.022 to 0.045W/mK and heat capacity being greater than 24 MJ/m3K. ,TagSPECI:FIELD:
The present disclosure relates to high thermal conductivity ultrahigh molecular weight polyethylene products and their preparation.

BACKGROUND:
Polymeric materials typically have a low thermal conductivity and polymeric substances such as foams prepared from amorphous polymers are widely used for thermal insulation. Materials for heat exchangers and thermal management, however, require high thermal conductivity which is commonly associated with conductors known in the art such as copper, aluminum, titanium, and the like.

Recent reports of high thermal conductivity in polyethylene nano fibers and natural biopolymers have renewed the researchers’ interest in thermally conductive polymers. It is known that polymers with high crystallinity and chain alignment tend to have larger thermal conductivities. Heat in the polymer is conducted in the direction of the covalently bonded molecular chains and in case of oriented products, it depends on the crystallinity, orientation, crystal size, length of molecular chains, chemical bridge points, crystal or amorphous boundary, defects, ends and entanglements of the molecular chains and morphologies composed of crystal and amorphous. The randomly oriented crystal region composed of folded ultrahigh molecular weight polyethylene (UHMWPE) chains changes to highly oriented crystal region composed of extended chains. Heat conduction of the extended chains in the direction of the covalent-bonded chain axis in crystal regions contributes to the high thermal conductivity.

Preparation of fibers and special devices to prepare tapes or sheets of UHMWPE involves the process of gel spinning where large quantities of suitable solvents are used to disentangle the polymer chains so as to achieve highly oriented crystal region composed of extended chains. Furthermore, the processes reported so far are highly energy intensive and cumbersome, thus limiting their use in commercial applications.

The present disclosure therefore envisages a process for the preparation of highly conductive UHMWPE that mitigates the drawbacks associated with the conventional processes.

OBJECTS:
Some of the objects of the present disclosure are discussed herein below:
It is an object of the present disclosure to provide a process for the preparation of UHMWPE products having high axial thermal conductivity and high heat capacity.
It is another object of the present disclosure to provide a process for the preparation of UHMWPE products having high axial thermal conductivity and high heat capacity, which is simple, commercially viable and environment friendly.
It is still another object of the present disclosure to provide UHMWPE products having high axial thermal conductivity and high heat capacity.
It is still further an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Other objects and advantages of the present disclosure will be more apparent from the following description.

SUMMARY
The present disclosure relates to a process for the preparation of high conductivity and high heat capacity ultrahigh molecular weight polyethylene (UHMWPE) products. The process includes feeding UHMWPE at the nip of at least one pair of heated polished counter rotating calendaring rollers having a pre-determined roller speed and roller speed ratio with the roller temperature being kept below the melt temperature of the polymer followed by hot stretching to obtain the UHMWPE products. The UHMWPE has molecular weight distribution ranging from 6 to 25. The high conductivity and high heat capacity UHMWPE products prepared by the afore-stated process is characterized in that the axial thermal conductivity being greater than 130W/mK, transverse direction thermal conductivity ranging from 0.022 to 0.045W/mK and heat capacity being greater than 24 MJ/m3K.

DESCRIPTION:
In accordance with one aspect of the present disclosure, there is provided a process for the preparation of high conductivity and high heat capacity UHMWPE products. The process includes but is not limited to the steps presented herein below.

UHMWPE powder or a compression molded UHMWPE preform is fed at the nip of at least one pair of heated polished counter rotating calendaring rollers having a pre-determined roller speed and roller speed ratio to obtain pre-laminate(s). In one embodiment, the thickness of the compression molded UHMWPE preform is 1 - 3 mm.
Also, in the instance where more than one pair of rollers is used, the rollers may be arranged serially or parallely.

Typically, the roller temperature is kept below the melt temperature of the polymer. In one embodiment, the temperature maintained in this process is 125 oC, the pre-determined roller speed ranges from 47 cm/min to 63 cm/min and the pre-determined roller speed ratio is 1.35.

The width of the pre-laminate is adjusted by setting the required gap parallel to the rollers. In one embodiment, the width and thickness of the pre-laminate is maintained at 52 mm and 0.15+0.01 mm, respectively. The width of the pre-laminate is further adjusted by modifying the gap between the spacers and as per the requirement the pre-laminate is slit to obtain tapes/strips of required width.

The pre-laminate(s) thus prepared is then hot stretched at a pre-determined temperature, at a pre-determined speed to obtain laminates. Stretching at a specific temperature and speed causes the polymer chains to align in the direction of stretching; thereby inducing anisotropy in the resultant product and making it highly conductive. In one embodiment, the pre-determined temperature is a temperature ranging from 130 – 155 oC and the pre-determined speed of stretching is maintained to range from 50 – 60 mm/min. Laminates having thin cross sections are obtained with different stretch ratios.

Conventionally, it is believed that the capacity of UHMWPE to get hot-stretched and therefore to attain conductivity is limited. This is because as the process of stretching progresses, beyond a certain threshold, the chain alignment/orientation morphology encounters a limitation which results in lower thermal conductivity of the polymer. This is a phenomenon, observed commonly for entangled as well as disentangled UHMWPE. However, the inventors of the present disclosure have found that when the molecular weight distribution (MWD) of disentangled UHMWPE is maintained in a specific range and the step of hot stretching is carried out, the above limitation does not hold true. The inventors of the present disclosure have found that when disentangled UHMWPE, having MWD ranging from 6 to 25, is hot stretched at the afore-stated temperature and speed conditions, the thermal conductivity of the polymer continues to increase with stretching, even beyond the threshold, and further demonstrates an increase in elastic modulus. It is therefore evident that the process of the present disclosure uses disentangled UHMWPE having MWD ranging from 6 to 25.

The UHMWPE used in the present process also has certain other pre-determined properties. The UHMWPE is substantially disentangled and has molecular weight ranging from 0.3 to 20 million, being prepared by using suitable catalyst(s) as described in the documents WO2013076733, PCT/IN2013/000016 and 1440/MUM/2013. The powder is also characterized for various parameters such as Reduced Specific Viscosity (RSV), measured by means of ASTM-D 4020-1a; molecular weight measured by the Mark-Houwink method using the equation: M = K[ƞ]α, where K and α are constants, K = 53700, α = 1.37 and η – intrinsic viscosity; bulk density (BD) measured by ASTM D-1895; molecular weight distribution by melt rheometry using Rheometrics RDA-III from T A Instruments employing Orchestrator software; density by ASTM D 792 using measuring unit of Mettler Toledo; melting temperatures (Tm) and heat of fusion during melting (ΔHTm) by differential scanning calorimeter. The molecular weight distribution of the polymer powder is tailored by manipulating the process conditions including but not limiting to temperature and pressure along with other properties like the RSV>17 dl/g, ΔH > 200 J/g and the bulk density <0.3 g/cc. The UHMWPE powder formed as a result of the afore-stated process is highly crystalline, richly disentangled and has a high molecular weight distribution.

The process of the present disclosure further includes the step of incorporation of additives including but not limiting to carbon nanotubes, graphene, carbon black, aluminum powder and boron nitride. Such additives when blended with the polymer can further enhance the thermal conductivity of the resultant products and extend the limit of thermal conductivity as achieved by pure UHMWPE.

In accordance with another aspect of the present disclosure, there is provided high conductivity and high heat capacity UHMWPE laminates or products prepared by the afore-stated process. In one embodiment, the products thus prepared are characterized by axial thermal conductivity greater than 130W/mK, transverse direction thermal conductivity ranging from 0.022 to 0.045W/mK and heat capacity greater than 24 MJ/m3K. The axial thermal conductivity of the product is dependent on the stretch ratio of the sheet. The products prepared in accordance with the process of the present disclosure have high thermal conductivity as well as high heat capacity. This differentiates the present process from the conventional processes. Furthermore, the product prepared according to the present process has see-through clarity.

The present disclosure provides a simple, eco-friendly solvent free process where highly crystalline UHMWPE polymer with high degree of disentangled macromolecular chain preferably having a pre-determined, broad MWD, is processed to obtain products having highest degree thermal conductivity. The resultant products, therefore, find plenty applications in the field such as heat exchanger/sinks in printed circuit boards and electronic devices, computers, printers, automotive interior and exteriors, appliances, batteries, superconducting coils, refrigeration systems, building constructions, interior temperature controls of housings, chemical engineering devices, thermal solar devices and the like. The products of the present disclosure include but are not limited to tape, strip, fiber and film. The thermally conductive articles from UHMWPE film/tape/stripe/fiber can be in the form of composites of single or multiple layers (with or without additive) and based on the alignment of film/tape/strip/fiber, the path of conduction can be defined.

The present disclosure will now be explained in the light of the following non-limiting examples.

Example 1:
Two polymer samples of the following properties were used to prepare thin films by maintaining different stretch ratios. While the RSV and molecular weight (Mw) of the two samples were in a close range, the molecular weight distribution of Sample 2 was higher than Sample 1.

Table 1
Polymer sample DPE
RSV MW MWD BD
1 27.33 5 12.3 0.063
2 28 5.16 19.8 0.074

The conditions of the stretched film sample preparation and their tensile and thermal properties are presented in Table 2. The highest stretch ratio is the limit beyond which the thin film reaches the break point.

The axial thermal conductivity of the stretched film so prepared was found to increase with the increase in tensile modulus which in turn is related to the stretch ratio as seen in sample 1 (table 2). It is found that the polymer sample 2 could be stretched to 128.6 times with thermal conductivity of 128.2 W/mK whereas the tensile modulus is lower than stretched film of lower stretch ratio of polymer sample 1. This may be possibly due to superior alignment of the polymer chains in sample 2 with high stretchability as the molecular weight distribution (MWD) of sample 2 is broader than polymer sample 1. Higher axial thermal conductivity is, therefore, attainable with the increase of stretch ratio instead of increase on tensile modulus with broader MWD.

The axial thermal conductivity of the film prepared from polymer sample 2 was found as high as 130.1 W/mKat 49.4oC which is highest reported value in the literature (Table 5). The thermal conductivity of the polymer film was not found to depend on the temperature range studied i.e. -21.5 oC to about 50 oC (Table 3-5).

The thermal conductivity of the polymer film in the transverse direction was found in the range of 0.022 to 0.045 W/mK.

The heat capacity (Cp) also increased with the stretch ratio of the film from about 7.0 MJ/m3K to as high as 24 MJ/m3K (in the temperature range of about -20 oC to about 50 oC) with the increase in stretch ratio 31 and 85 of sample 1 and about 128 of sample 2 (Tables 3 – 5). This made the solid state thermal properties of the stretched polymer film unique, having a combination of very high thermal conductivity as well as Cp (this is unlike metals which have a combination of high thermal conductivity and very low Cp).

The processing speed of the sheet can be controlled by suitably adjusting the speed and temperature of the two roll mill. Further, the rate of stretching of the sheet was found to depend on the set temperature of the stretching unit.

It was also determined that the composite form of the uniaxial thermally conductive film/strip/tape/fiber of UHMWPE can have multi-axial controlled thermal conductivity depending on the direction of placing their unit forms in the composite layers and achieving the product by applying compressive force to compact them.
Table 2:
Polymer Sample Film Sample Sheet making Hot stretching of sheet, oC Stretch ratio* Tensile modulus of film (hot stretched sheet), GPa Axial Thermal Conductivity (W/mK) @ temp. (oC)
Machine Temp oC Step 1/ Cross head speed Step 2/
Cross head speed Step 3/ Cross head speed
1 5PE31 Two roll mill 125 oC 130/50 145/60 150/60 31.29 75.35 55.0@25.0
5PE53 125 oC 130/50 145/60 150/60 53.8 94 58.6 @ 30.0
5PE77 125 oC 130/50 145/60 152/60 77.11 121.4 71.6 @ 30.0
5PE85 125 oC 130/50 145/60 150/60 85.5 124.4 115.8@25.4
2 5PE128 125 oC 130/50 145/60 152/60 128.6 92.18 128.2@ 25.6

Stretch ratio is ratio of hot stretched film and calendar rolled sheet based on their unit weight and volume.

Table 3 Film Sample 5 PE 31
Temperature, oC Thermal conductivity, W/mK Cp, MJ/m3 K
22.7 57.4 7.20
11.5 56.1 7.08
2.0 55.6 7.16
-11.9 56.9 6.55
-21.1 57.3 6.25
25 55.0 7.64
48.7 56.5 7.89
Table 4 Film Sample 5 PE 85

Temperature, oC Thermal conductivity, W/mK Cp, MJ/m3 K
24.2 112.3 15.38
11.5 112.9 15.06
1.7 113.4 14.76
-12.2 114.5 13.39
-21.3 113.4 13.40
25.4 115.8 15.33
48.7 117.3 17.69

Table 5 Film Sample 5 PE 128

Temperature, oC Thermal conductivity, W/mK Cp, MJ/m3 K
23.5 123.5 21.22
11.4 123.5 19.75
1.9 130.5 19.03
-12.1 124.3 18.82
-21.4 124.6 18.27
25.6 128.2 20.85
49.4 130.1 24.19

TECHNICAL ADVANTAGES AND ECONOMIC SIGNIFICANCE:

- The process of the present disclosure is simple, free from solvent and requires easily available commercial processing tools.
- The UHMWPE products of the present disclosure demonstrate uni-axial as well as multi-axial thermal conductivity.
- The high uni-axial thermal conductivity of the UHMWPE film/tape/stripe/fiber provides flexibility of designing for applications such as heat exchanger/sinks in printed circuit boards and electronic devices, computers, printers, automotive interior and exteriors, appliances, batteries, superconducting coils, refrigeration systems, building constructions, interior temperature controls of housings, chemical engineering devices, thermal solar devices and the like.
- As the products have a unique combination of high axial thermal conductivity and high electrical insulation, they can find use in developing efficient electrical products which include cables, electrical junctions and the like.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.