Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.     

A constitutive model for self-reinforced ductile polymer composites

Self-reinforced polymer composites are gaining increasing interest due to their higher ductility compared to traditional glass and carbon fibre composites. Here we consider a class of PET composites comprising woven PET fibres in a PET matrix. While there is a significant literature on the development of these materials and their mechanical properties, little progress has been reported on constitutive models for these composites. Here we report the development of an anisotropic visco-plastic constitutive model for PET composites that captures the measured anisotropy, tension/compression asymmetry and ductility. This model is implemented in a commercial finite element package and shown to capture the measured response of PET composite plates and beams in different orientations to a high degree of accuracy.     

Preparation and properties of self-reinforced poly(lactic acid) composites based on oriented tapes

The inherent brittleness and poor thermal resistance of poly(lactic acid) (PLA) are two main challenges toward a wider industrial application of this bioplastic. In the present work, through the development of self-reinforced PLA (SR-PLA) or “all-PLA” composites, the high brittleness and low heat deflection temperature (HDT) of PLA have been overcome, while simultaneously improving the tensile strength and modulus of SR-PLA. The obtained composites are fully biobased, recyclable and under the right conditions compostable. For the creation of SR-PLA composites, first a tape extrusion process was optimized to ensure superior mechanical properties. The results show that SR-PLA composites exhibited enhanced moduli (2.5 times) and tensile strengths (2 times) and showed 14 times increase in impact energy compared to neat PLA. Finally, the HDT of SR-PLA was also increased by about 26 °C compared to neat PLA, mainly as a result of an increase in modulus and crystallinity.     

Coefficients of thermal expansion for single crystal piezoelectric fiber composites

Piezoelectric fiber composites were developed to overcome drawbacks of typical monolithic piezoceramic (PZT) actuators. Although piezoelectric fiber composites had many improvements over the monolithic PZT, there are still improvements. Thus, the single crystal piezoelectric fiber composite actuator is proposed. Single crystal piezoelectric materials such as PMN-PT have larger piezoelectric strain constants, higher bandwidth and higher energy density than polycrystalline counterparts. Piezoelectric fiber composites can improve the performance of various structures, and can be subject to wide temperature range where the thermoelastic behavior is important. Therefore, this paper studies the coefficients of thermal expansion (CTE) for single crystal piezoelectric fiber composites. The Macro Fiber Composite (MFC) as the piezoelectric fiber composite is considered. To calculate the effective properties of two orthotropic layers of the MFC, PMN-PT(or PZT)/epoxy and copper/epoxy layers, the rule of mixture is adopted. With the effective properties known for each layers, the two CTE of the MFC actuator are obtained from the classical lamination theory considering thermal effects. The difference of the CTE between the single crystal MFC and the standard MFC is studied.     

In-plane thermal enhancement behaviors of Al matrix composites with oriented graphite flake alignment

Oriented graphite flakes (G_f)/Si/Al composites were fabricated to study their thermal enhancement behaviors. The in-plane thermal conductivity (TC) of the composites increases with the increase of G_f volume fraction. At a given volume fraction, a larger G_f size can achieve a higher in-plane TC of the composites. Microstructural characterization revealed a clean and Al₄C₃-free interface between the side surface of G_f and the Al matrix. Based on the observed microstructures, an analytical model was presented to predict the in-plane TC of the composites with oriented G_f alignment by incorporating interfacial thermal resistance within the framework of effective medium approach (EMA). Comparisons of the present model predictions with the experimental data of the as-fabricated G_f/Si/Al and previously reported G_f/Al and G_f/polymer (polyvinyl butyral, PVB) composites show good agreement. The results indicate that our model can well predict the in-plane thermal enhancement behaviors of the composites at different effective phase contrasts (i.e. the ratio between effective TC of the G_f and TC of the matrix).     

Penetration impact testing of self-reinforced composites

Penetration impact resistance is one of the key advantages of self-reinforced composites. This is typically measured using the same setup as for brittle fibre composites. However, issues with the test configuration for falling weight impact tests are reported. Similar issues have been found in literature for other composites incorporating ductile fibres. If the dimensions of the test samples are too small relative to the clamping device, then the test samples can heavily deform by wrinkling and necking. These unwanted mechanisms should be avoided as they absorb additional energy compared to properly tested samples. Furthermore, these mechanisms are found to occur more easily at lower compaction temperatures due to the lower interlayer bonding. In conclusions, the sample dimensions of ductile fibre composites should be carefully selected for penetration impact testing. If wrinkling or necking is observed, then the sample dimensions need to be increased.     

Failure behaviour of self-reinforced polypropylene at and below room temperature

Self-reinforced polypropylene is a very tough material. It is even thought that its impact resistance increases with decreasing temperature. This was investigated by examining the constituent tapes and matrix. Tensile tests on both drawn polypropylene tapes and self-reinforced polypropylene were similar: the stiffness increased and the failure strain slightly decreased at low temperatures. The matrix, however, embrittled below room temperature due to the glass transition. In contrast with literature data on Izod impact resistance, the penetration impact resistance did not increase at low temperatures. At lower temperatures, the damaged area after non-penetration impact was significantly reduced. This was caused by a change in the damage mode from tape–matrix debonding to matrix cracking, as the matrix went through its glass transition. These conclusions provide the first understanding of the failure behaviour of self-reinforced polypropylene below room temperature, and can be exploited to further optimise the excellent impact resistance of self-reinforced polymers.     

The importance of bonding in intralayer carbon fibre/self-reinforced polypropylene hybrid composites

Polymer composites are usually either stiff or tough, but seldom both. Intralayer hybrids of carbon fibre and self-reinforced polypropylene (PP) do offer the potential to achieve a unique combination of toughness and stiffness. In these hybrids, the bonding between carbon fibre prepregs and PP tapes is a crucial parameter. For a weak bonding, the 20% ultimate tensile failure strain and high penetration impact resistance of self-reinforced PP were maintained. For a strong bonding, the ultimate tensile failure strain was strongly reduced, but the flexural performance was improved. For a homopolymer PP matrix in the prepregs, the weak bonding between fibre and matrix caused the penetration impact resistance to reduce according to a linear rule-of-mixtures. For a maleic anhydride modified PP matrix however, the strong fibre–matrix bonding greatly reduced the penetration impact resistance. These results provide new insights into designing hybrid composites with a unique balance of stiffness and failure strain.     

Three-dimensional carbon nanotube based polymer composites for thermal management

In this study, the porous multiwall carbon nanotube (MWCNT) foams possessing three-dimensional (3D) scaffold structures have been introduced into polydimethylsiloxane (PDMS) for enhancing the overall thermal conductivity (TC). This unique interconnected structure of freeze-dried MWCNT foams can provide thermally conductive pathways leading to higher TC. The TC of 3D MWCNT and PDMS composites can reach 0.82 W/m K, which is about 455% that of pure PDMS, and 300% higher than that of composites prepared from traditional blending process. The obtained polymer composites not only exhibit superior mechanical properties but also dimensional stability. To evaluate the performance of thermal management, the LED modulus incorporated with the 3D MWCNT/PDMS composite as heat sink is also fabricated. The composites display much faster and higher temperature rise than the pristine PDMS matrix, suggestive of its better thermal dissipation. These results imply that the as-developed 3D-MWCNT/PDMS composite can be a good candidate in thermal interface for thermal management of electronic devices.     

Impact characterization of thermoformable fibre metal laminates of 2024-T3 aluminium and AZ31B-H24 magnesium based on self-reinforced polypropylene

In this work, the impact behaviour of an AZ31B-H24 magnesium-based fibre metal laminate (FML) and a 2024-T3 aluminium-based FML with self-reinforced polypropylene (SRPP) is analysed. The study focused on determining the impact properties of the FMLs and explaining the differences between their impact responses. Additionally, in the comparative study, a single SRPP was included as reference base material. Low-velocity impact tests were performed by using a drop-weight machine. The failure energy for cracking of metal sheets, onset of fibre breakage in the composite constituent and total perforation of the laminate were identified by correlating force–time curves measured from impacts with images of impacted specimens obtained by a macroscope. Moreover, the ‘energy profile method’ was used to identify the penetration and perforation thresholds enabling the definition of the no-penetration and the penetration phases. The results revealed that the Al-FML presented more specific resistance and energy dissipation capacity under impact than the Mg-FML.     

Characteristic impact resistance model analysis of cellulose nanofibril-filled polypropylene composites

Notched and unnotched Izod impact strength of cellulose nanofibers (CNFs) and microfibrillated cellulose (MFC)-filled impact modified polypropylene (PP) composites were measured and compared with microcrystalline cellulose (MCC)-filled composites. An Izod impact fracture initiation resistance theory was formulated and a characteristic impact resistance model was developed to evaluate the unique impact characteristics of cellulose nanofibril-filled PP composites. As filler loading increased CNF and MFC-filled composites showed higher characteristic impact resistance than MCC-filled ones. Among the cellulose fillers used in this study, CNF were found to be the most resistant of the three materials tested in terms of characteristic impact resistance. Even though impact resistance in not the only evaluation tool, characteristic impact resistance is an evaluation tool used to determine the material’s unique and hidden impact characteristics. The characteristic impact resistance model is useful for analysis of the impact behavior of any polymer composite material. It was also found that impact modified PP used in this study is more fracture resistant, but more crack sensitive, than conventional PP.     

Statistical continuum theory for the effective conductivity of fiber filled polymer composites: Effect of orientation distribution and aspect ratio

Effective conductivity of polymer composites, filled with conducting fibers such as carbon nanotubes, is studied using statistical continuum theory. The fiber orientation distribution in the matrix plays a very important role on their effective properties. To take into account their orientation, shape and distribution, two-point and three-point probability distribution functions are used. The effect of fibers orientation is illustrated by comparing the effective conductivity of microstructures with oriented and non-oriented fibers. The randomly oriented fibers result in an isotropic effective conductivity. The increased fiber orientation distribution can lead to higher anisotropy in conductivity. The effect of fiber’s aspect ratio on the effective conductivity is studied by comparing microstructures with varying degrees of fiber orientation distribution. Results show that the increase in anisotropy leads to higher conductivity in the maximum fiber orientation distribution direction and lower conductivity in the transverse direction. These results are in agreement with various models from the literature that show the increase of the aspect ratio of fibers improves the electrical and thermal conductivity.     

Coordinate-free derivation and weak formulation of a general imperfect interface model for thermal conduction in composites

In a great number of situations of practical interest, the interfaces between the constituent phases of a composite turn out to be imperfect. In the context of thermal conduction, an interface is said to be imperfect if the requirement that both the temperature and the normal heat flux be continuous across the interface is not satisfied. A powerful method based on mathematical asymptotic analysis has been proposed and developed in the literature by several authors for the derivation of linear imperfect interface models of thermal conduction. This method consists in replacing an interphase of small uniform thickness between two-phases by an imperfect interface of null thickness characterized by the temperature and normal heat flux jump relations deduced by carrying out an appropriate asymptotic analysis. The objective of the present work is threefold. Firstly, it aims to explicitly show and emphasize the key role played by Hadamard’s relation in the method. Secondly, it has the purpose of using a coordinate-free differential geometry theory and Hadamard’s relation to render the method coordinate-free. Thirdly and most importantly, the present work gives a weak formulation for the problem concerning the steady thermal conduction in a composite with the interfaces described by the general temperature and normal heat flux jump relations derived. This weak formulation is a key step toward solving the problem by the extended finite element method (XFEM) presented in a companion paper.     

Synergistic effect of hybrid graphene nanoplatelet and multi-walled carbon nanotube fillers on the thermal conductivity of polymer composites and theoretical modeling of the synergistic effect

We found that the thermal conductivity of polymer composites was synergistically improved by the simultaneous incorporation of graphene nanoplatelet (GNP) and multi-walled carbon nanotube (MWCNT) fillers into the polycarbonate matrix. The bulk thermal conductivity of composites with 20 wt% GNP filler was found to reach a maximum value of 1.13 W/m K and this thermal conductivity was synergistically enhanced to reach a maximum value of 1.39 W/m K as the relative proportion of MWCNT content was increased but the relative proportion of GNP content was decreased. The synergistic effect was theoretically estimated based on a modified micromechanics model where the different shapes of the nanofillers in the composite system could be taken into account. The waviness of the incorporated GNP and MWCNT fillers was found to be one of the most important physical factors determining the thermal conductivity of the composites and must be taken into consideration in theoretical calculations.     

Thermal conductivity of polymer composites based on the length of multi-walled carbon nanotubes

The anisotropic development of thermal conductivity in polymer composites was evaluated by measuring the isotropic, in-plane and through-plane thermal conductivities of composites containing length-adjusted short and long multi-walled CNTs (MWCNTs). The thermal conductivities of the composites were relatively low irrespective of the MWCNT length due to their high contact resistance and high interfacial resistance to polymer resins, considering the high thermal conductivity of MWCNTs. The isotropic and in-plane thermal conductivities of long-MWCNT-based composites were higher than those of short-MWCNT-based ones and the trend can accurately be calculated using the modified Mori-Tanaka theory. The in-plane thermal conductivity of composites with 2 wt% long MWCNTs was increased to 1.27 W/m·K. The length of MWCNTs in polymer composites is an important physical factor in determining the anisotropic thermal conductivity and must be considered for theoretical simulations. The thermal conductivity of MWCNT polymer composites can be effectively controlled in the processing direction by adjusting the length of the MWCNT filler.     

Enhanced thermal and mechanical properties of carbon nanotube composites through the use of functionalized CNT-reactive polymer linkages and three-roll milling

We report enhanced thermal and mechanical properties of carbon nanotube (CNT) composites achieved through the use of functionalized CNTs-reactive polymer linkages and three-roll milling. CNTs were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group resulting in a chemical reaction. To maximize CNT dispersion for practical usage, entangled CNTs are separated and then evenly dispersed within the polymer matrix using three horizontally positioned rotating rolls that apply a strong shear force to the composite. Consequently, accompanying with thermal stability, elastic modulus and storage modulus of such functionalized CNT/polymer composites were increased by 100% and 500% that of the untreated epoxy polymer.     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

Recycling of shredded composites from wind turbine blades in new thermoset polymer composites

As the energy produced from wind increases every year, a concern has raised on the recycling of wind turbine blades made of glass fibre composites. In this context, the present study aims to characterize and understand the mechanical properties of polyester resin composites reinforced with shredded composites (SC), and to assess the potential of such recycling solution. A special manufacturing setup was developed to produce composites with a controlled content of SC. Results show that the SC in the composites was well distributed and impregnated. The composite stiffness was well predicted using an analytical model, and fibre orientation parameters for strength modelling were established. The stress-strain curves revealed composite failure at unusual low strain values, and micrographs of the fracture surface indicated poor adhesion between SC and matrix. To tackle this problem, chemical treatment of SC or use of an alternative resin, to improve bonding should be investigated.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.     

Effect of reinforcing layer on shape fixity and time-dependent deployment in shape-memory polymer textile composites

The purpose of the present study is to model shape fixity and time-dependent deployment in shape-memory polymer composites (SMPCs) and to evaluate the effect of textiles’ tensile and bending moduli on these properties. We constructed an SMPC model by combining SMP layers and a reinforcing layer. We also considered the thermo-viscoelasticity of SMP and the difference in values between the tensile and bending moduli of the reinforcing layer. Employing this model, we simulated deployment under pure bending conditions. Comparison with experimental results confirmed that our proposed model is able to simulate shape fixity and time-dependent deployment in SMPCs. We also confirmed that the bending modulus is an important factor for shape fixity and time-dependent deployment, whereas the tensile modulus has nothing to do with these properties.