Thermoplastic polymer heat sinks are coming to market fast as a replacement of metal heat sinks. Although it is obvious that metals have much more thermal conductivity than polymers, but the advantage of using the latter is case specific. If a system is conduction limited then metal heat sinks are better but if the system is convection limited, then polymer heat sinks are claimed to be more efficient.

Check out these links:

http://www.ptonline.com/articles/plastics-that-conduct-heat

http://www.rtpcompany.com/rtp-company-introduces-thermally-conductive-compounds-for-enhanced-led-luminaires/

http://www.ledinside.com/interview/2013/9/replacing_aluminum_with_thermoplastic_in_heat_sinks_cuts_cost_by_20_30

As far as LED is concerned the mode of heat transfer in convection and radiation. Then as you siad Thermal plastics are a better option for heat sink and fixture right!!

This  material as a heat sink is relatively new and it is not well proven in the field. So, one has to be cautious with their use in the LED luminaries. This is beause the  LED array is expensive and it is wanted to last for long time for the pay back.

So, there is no cash answer for your question. One has to prove that this thermal conductive thermoplastics is really qualified for such applications by theoretical analysis and experimental investigations. Accelerated failure tests must be performed to judge with certainty. 

So, proceed in your research work.

wish you success.

Thank you for your response sir. Is there any theoretical analysis available to prove the thermal conductive thermoplastics? 

Dear Lilli,

In analyzing the thermal performance of your luminare, you can use the thermal model which is an electrical analog for heat conduction. Every layer of material is represented by a parallel combination of a thermal conductance and a thermal capacitance. The heat flux produced by electrical power consumption in the diode is represented by a current source. You build this model and solve it as you solve electrical circuits.

For more details please revert to the links: lrt.sagepub.com/content/early/2013/07/10/1477153513495689.full.pdf

and www.ledsmagazine.com/.../mentor-graphics-addresses-thermal-modeling..

and https://www.researchgate.net/publication/256346392_The_bipolar_transistors_theory_and_basic_applications

wish you success. 

Book The bipolar transistors, theory and basic applications