**4. Conclusion**

This review was devoted to summarizing the main literature studies about TC of polypropylene and the recent developments of heat transport ability in PP based compounds.

The TC of polypropylene has been measured in the range approximately between 0.1 and 0.2 W/mK, but different parameters as polymer cristallinity, chain structure and orientation, processing conditions and methods, temperature and pressure have played a positive or negative role on its thermal transport behaviour. In details, for the examined semi-crystalline polymer, starting from room temperature and going up to melting point, an opposite trend of TC have been reported. The TC remained almost the same as the temperature increase until it decreased during melting due to a possible breakup of crystalline regions. On the contrary, opposite results showed a strong increase of TC really at melting point. Finally, below 100 K, a growing tendency of TC against temperature has been verified. The effect of acting pressure on TC of polypropylene seemed to be always positive, probably for the induced stress in a longitudinal direction that led to anisotropy of the tested feature. For the same reason, processes like extrusion, injection molding and foaming, by causing an orientation of the polymeric molecular chains, could determine an increase of TC in the same direction of the inferred one. In the molten state, the TC of polypropylene appeared to be a complex function of molecular weight and chain branching; it continued to increase by pressure but resulted almost unaffected by temperature.

The devices, adopted for measuring the TC of PP and of its composites have been prevalently the Guarded Hot Plate Method and the Heat Flow Meter Method, based on a steady state approach, or the Flash Method and the Transient Hot Wire Method, based on transient approach.

**51**

**Author details**

Antonella Patti1

Naples-Federico II, Naples, Italy

provided the original work is properly cited.

\* and Domenico Acierno2

2 Centro Regionale di Competenza Tecnologie, Naples, Italy

\*Address all correspondence to: antonlla.patti@unina.it

*Thermal Conductivity of Polypropylene-Based Materials DOI: http://dx.doi.org/10.5772/intechopen.84477*

among themselves (lower contact resistance).

Different efforts have been spent in literature in the improvement the heat conduction of PP, by the addition of inorganic fillers (metallics, carbon-based, ceramics and minerals) in micro- or nano-size, one-, two- or three-dimensional, with a higher thermal transmission compared to the pristine resin. By increasing filler loading, positive but not always satisfactory increases of TC in the respective compounds have been achieved. The size and shape of particles, their orientation and distribution in the polymer, the interfacial interaction between filler and matrix and between filler and filler, have been identified as crucial aspects in the optimization of final heat transport in the polymeric composites. All these factors contributed to realize effective thermally conductive pathways in the composites, actualized among particles with an advanced dispersion, good interfacial adhesion with the pristine material (lower interfacial resistance) and the proper contact

The filler functionalization (i.e., the introduction of functional group on filler surface) and the addition of compatibilizer in polymer/particles mixtures, have been considered a useful approach for developing the compatibility between the two phases, and consequently for improving the dispersion and the interfacial interaction. Another approach has been the combination of two or more fillers, having different size and shape, to optimize the filler packing and their distribution in the matrix, so to realize and support an effective thermal conductive network. By comparing data on PP-based compounds, despite the difference in filler loading, the greater efficiency in improving the TC of the matrix seemed to be realized in the case of combined micro- and nano-sized carbonaceous particles in the resin.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Department of Chemical, Materials and Industrial Engineering, University of

### *Thermal Conductivity of Polypropylene-Based Materials DOI: http://dx.doi.org/10.5772/intechopen.84477*

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

conductive bridge by dispersing between BN and PP.

with larger ones (CaCO3 and Talc).

almost unaffected by temperature.

Method, based on transient approach.

**4. Conclusion**

compounds.

to the filler type, its size and added content.

combined carbonaceous nano- and micro-particles.

to the pristine PP) was recorded. Cheewawuttipong et al. [70] added BN and vaporgrown carbon fiber (VGCF) into the PP resin. They found that by increasing the content, the distance between fillers could be reduced and the development of conductive structure was attained. VGCF/BN hybrids possessed a better heat transport behaviour than of composites incorporating BN single size, since VGCF contributed to generate a

Finally, the development of carbon-based thermally conductive composites with low electrical conductivity was actualized by PP-based ternary formulations, combining CNTs (a thermal and electrical conductive filler) with additional thermally conductive, but electrically insulating, particles (ZnO,CaCO3, BN, and Talc) having different sizes and shapes [71]. Results showed that, in ternary formulations, an increase of TC was always verified for all kinds of secondary particles, in particular through the union of CNTs with BN. Significant reduction of electrical conductivity was achieved, despite the presence of CNTs, with the addition of smaller secondary species (BN and ZnO), while a further increment of the same feature was obtained

**Table 3** shows some values of TC reached in the PP-based composites, according

An effective comparison of reported TC values has not been possible due to differences in the filler loadings in each study. Yet, higher TC measurements were verified in the presence of microparticles, in particular with layered shape (talc); the same results have never been reached by adding nanoparticles. The highest improvement of heat transport in PP was recorded with the introduction of two

This review was devoted to summarizing the main literature studies about TC of polypropylene and the recent developments of heat transport ability in PP based

The TC of polypropylene has been measured in the range approximately between 0.1 and 0.2 W/mK, but different parameters as polymer cristallinity, chain structure and orientation, processing conditions and methods, temperature and pressure have played a positive or negative role on its thermal transport behaviour. In details, for the examined semi-crystalline polymer, starting from room temperature and going up to melting point, an opposite trend of TC have been reported. The TC remained almost the same as the temperature increase until it decreased during melting due to a possible breakup of crystalline regions. On the contrary, opposite results showed a strong increase of TC really at melting point. Finally, below 100 K, a growing tendency of TC against temperature has been verified. The effect of acting pressure on TC of polypropylene seemed to be always positive, probably for the induced stress in a longitudinal direction that led to anisotropy of the tested feature. For the same reason, processes like extrusion, injection molding and foaming, by causing an orientation of the polymeric molecular chains, could determine an increase of TC in the same direction of the inferred one. In the molten state, the TC of polypropylene appeared to be a complex function of molecular weight and chain branching; it continued to increase by pressure but resulted

The devices, adopted for measuring the TC of PP and of its composites have been prevalently the Guarded Hot Plate Method and the Heat Flow Meter Method, based on a steady state approach, or the Flash Method and the Transient Hot Wire

**50**

Different efforts have been spent in literature in the improvement the heat conduction of PP, by the addition of inorganic fillers (metallics, carbon-based, ceramics and minerals) in micro- or nano-size, one-, two- or three-dimensional, with a higher thermal transmission compared to the pristine resin. By increasing filler loading, positive but not always satisfactory increases of TC in the respective compounds have been achieved. The size and shape of particles, their orientation and distribution in the polymer, the interfacial interaction between filler and matrix and between filler and filler, have been identified as crucial aspects in the optimization of final heat transport in the polymeric composites. All these factors contributed to realize effective thermally conductive pathways in the composites, actualized among particles with an advanced dispersion, good interfacial adhesion with the pristine material (lower interfacial resistance) and the proper contact among themselves (lower contact resistance).

The filler functionalization (i.e., the introduction of functional group on filler surface) and the addition of compatibilizer in polymer/particles mixtures, have been considered a useful approach for developing the compatibility between the two phases, and consequently for improving the dispersion and the interfacial interaction. Another approach has been the combination of two or more fillers, having different size and shape, to optimize the filler packing and their distribution in the matrix, so to realize and support an effective thermal conductive network.

By comparing data on PP-based compounds, despite the difference in filler loading, the greater efficiency in improving the TC of the matrix seemed to be realized in the case of combined micro- and nano-sized carbonaceous particles in the resin.
