hollow glass beads

Journal Pre-proof Influence of interfacial interactions on the mechanical behavior of hybrid composites of polypropylene / short glass fibers / hollow glass beads Gustavo B. Carvalho, Sebastião V. Canevarolo, Jr., José Alexandrino Sousa PII:

S0142-9418(19)31732-5

DOI:

https://doi.org/10.1016/j.polymertesting.2020.106418

Reference:

POTE 106418

To appear in:

Polymer Testing

Received Date: 26 September 2019 Revised Date:

16 January 2020

Accepted Date: 8 February 2020

Please cite this article as: G.B. Carvalho, Sebastiã.V. Canevarolo Jr., José.Alexandrino. Sousa, Influence of interfacial interactions on the mechanical behavior of hybrid composites of polypropylene / short glass fibers / hollow glass beads, Polymer Testing (2020), doi: https://doi.org/10.1016/ j.polymertesting.2020.106418. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

1 INFLUENCE OF INTERFACIAL INTERACTIONS ON THE MECHANICAL BEHAVIOR OF HYBRID COMPOSITES OF POLYPROPYLENE / SHORT GLASS FIBERS / HOLLOW GLASS BEADS Gustavo B. Carvalho, Sebastião V. Canevarolo Jr. & José Alexandrino Sousa* Materials Engineering Department, Universidade Federal de São Carlos, Rod. Washington Luiz Km 235, São Carlos, SP, Brazil - 13565-905. ABSTRACT Ternary composites of Polypropylene (PP)/Short Glass fibers (GF)/Hollow Glass Beads (HGB), with varying total and relative GF/HGB contents and using untreated and aminosilane-treated HGB compatibilized with maleated-PP, were prepared by direct injection molding of pre-extrusion compounded GF and HGB concentrates. The mechanical strength properties (tensile, flexural and Izod impact) were correlated with theoretical model predictions for hybrid composites, which identified synergistic gains over the rule of hybrid mixtures, depending upon the degree of interfacial interactions between the components of the hybrid composite. SEM analysis of cryofractured composites surfaces revealed that presence of untreated HGB particles induces fiber-polymer interfacial decoupling under mechanical loading of the hybrid composites at much lower stress levels than in the presence of treated HGB particles. Higher storage modulus (E') and lower mechanical damping (tan δ) from DMTA established the importance of strong polymer-hybrid reinforcement interfacial interactions in the development of lightweight/high strength PP syntactic foams.

Keywords PP syntactic foams; Hybrid composites; Interfacial interactions; Mechanical properties; DMTA.

1. Introduction Polypropylene (PP) composites reinforced with short glass fibers (GF) are materials widely explored in the scientific and industrial fields, because of the interest associated to their matrix polymer’s characteristics of low cost and weight, ease of processing and, when adequately compatibilized with GF, are considered as cost-effective materials for complex-shaped injection molded parts appropriate for engineering applications. However, as in most fiber-filled thermoplastic composites, the mechanical properties improvements achieved in injection molded parts are highly anisotropic, due to the preferential fiber orientation in the mold fill direction with increased GF content. This effect also contributes to several other problems associated to high differential molding shrinkage, warpage and reduced weld-line

2 resistance [1-13]. Thus, hybridization of high aspect ratio fibers with low aspect ratio particulate mineral fillers is a consolidated industrial technique to minimize such anisotropic effects. Examples of low aspect ratio particulate fillers commonly used to modify GF-reinforced thermoplastics are calcium carbonate, talc, mica and glass beads [2, 4, 14-20]. In the latter case, there are now commercially available options of high strength hollow glass beads (HGB) which, in addition to its potential to mitigate anisotropic effects with respect to fiber-only reinforced thermoplastic composites, also offer several other benefits of syntactic foams, such as weight reduction [21], reduced thermal [22-27] and electrical conductivity [22, 28] and increased sound damping [29] properties, which all put together, constitute important gains for engineering applications in lightweight and high-strength materials for the petrochemical, automotive and electronic home appliances industries. The main points to be taken into consideration when proposing the hybridization of fibrous reinforcements with particulate fillers, so as to assure enhanced mechanical performance in hybrid ternary polymer composites, are related to the influence of the following main aspects: (i) the total volume content of the hybrid reinforcement in the composite with respect to its maximum volume packing fraction delimiting value, (ii) the relative content of high (fibrous) and low (particulate) aspect ratio fillers at a given total content and (iii) the degree of interfacial interactions of both components of the hybrid reinforcement with the polymeric matrix (interfacial adhesion) and also of direct interactions between the reinforcement components, as the total and relative content of the hybrid reinforcement increases. The published literature data on mechanical performance of several ternary thermoplastic composites with different hybrid fibrous-particulate reinforcements, [6-11, 30, 31] indicate that the elastic modulus properties (tensile or flexural) are improved as the particulate filler concentration increases at a given constant content of fibrous reinforcement, usually according to a simple rule of hybrid mixtures. On the other hand, under the same equivalent hybrid reinforcement concentration, the mechanical strength properties (tensile, flexural and impact) are usually reduced to a higher or lower extent with increasing particulate filler content, depending on the particulate filler aspect ratio [2, 4, 14-18, 20, 24, 32-35]. Nevertheless, only a few reports were available [24, 32, 36-39] in a literature survey carried out on specific ternary hybrid thermoplastic composites, where fibers were hybridized with low density hollow microspheres (HGB) for use in lightweight and enhanced strength applications by addition of fibers in small content in plastic syntactic foams. In these composite systems, with increasing HGB content at any given constant fiber concentration, the resultant elastic modulus (tensile and flexural) of

3 the ternary hybrid composites assumes values lower than those predicted by the rule of hybrid mixtures. Also, only limited data on the strength properties (tensile, flexural and impact) of these systems are available, which indicate decreasing strength properties with increasing HGB content. However, to the best of our knowledge, no clear understanding is yet reported regarding how the interfacial interactions of the hybrid reinforcements with the polymer matrix influence both the mechanical elastic modulus and strength properties of these ternary composites with increasing total and relative hybrid-filler content. In this case, as the interparticle distance between the fibrous and particulate fillers decreases substantially so that their stress fields superimpose under mechanical loading, it is expected that the weaker low aspect ratio particulate filler particles may damage the stress transfer efficiency at the fiber-polymer interface to a higher or lower extent, depending on the degree of interfacial adhesion of the particulate filler with the polymer matrix. Thus, this negative effect may contribute to reduced mechanical strength of the hybrid composite in comparison to its reference single fiber-filled binary composite. Taking these interfacial interaction factors into account for the development of lightweight and high strength thermoplastic composites, this work analyses the short-term mechanical properties of ternary PP composites reinforced with several hybrid compositions of PP compatible short glass fiber (GF) and aminosilane surface-treated and untreated hollow glass beads (HGB). Thus, density, tensile, flexural, impact and dynamic-mechanical (DMTA) properties of “reference” binary composites (PP/GF and PP/HGB) and ternary hybrid composites (PP/GF/HGB) were investigated, as the total and relative volume contents of single and hybrid reinforcements are increased. The polymer-hybrid reinforcement interfacial interactions were evaluated through scanning electron microscopy (SEM) of pre-stressed and then cryofractured composite surfaces and through monitoring of the dynamic storage modulus (E') and the mechanical damping properties (tan δ) in DMTA analysis.

2. Experimental 2.1. Materials The polymeric matrix used was an isotactic polypropylene homopolymer obtained by mixing two PP grades from Quattor Polyolefins / Braskem S.A., in the weight ratio of 3:1 of PP HP648S (low molecular weight and MI = 40 g/10 min, at 230°C / 2.16 kg) and PP HP550K (high molecular weight and MI = 3.5 g/10 min, at 230°C / 2.16 kg), respectively. The chopped roving short glass fiber (GF) used was an EC 13 4.5 968 grade, from Vetrotex do Brasil (currently CPIC Ltda.), of 13 µm nominal fiber

4 diameter, 4.5 mm length chopped roving and density of 2.55 g/cm3; "968" being a PP-compatible sizing with supposedly aminosilane coupling agent. The hollow glass beads (HGB) used were an iM30k type Glass Bubbles made of soda-lime borosilicate glass from 3M Company. It consists of a high crush strength HGB (nominal 28.000 psi) for use in injection molding applications, with nominal density = 0.60 g/cm3, average diameter = 16 µm (size distribution: 9 - 29 µm) and an average nominal wall thickness of 0.70 µm. Two types of iM30k beads, as supplied by 3M, were employed: untreated (HGB) and surface treated with aminosilane coupling agent (HGBamino). An interfacial compatibilizer of maleic anhydride grafted PP copolymer (PP-g-MAH) was used to enhance the interfacial compatibilization of both HGB and GF reinforcements in all binary and ternary PP composites. The grade chosen was Polybond 3200, acquired from Addivant, with nominal MAH content = 1.0 wt.% and MFI = 115 g/10 min, at 190°C / 2.16 kg. Finally, a combination of thermal stabilizers and antioxidants (Irganox 1010 and Irgafos 168 in a ratio of 1:2, supplied by BASF) were added at 0.5 wt.% with respect to the base PP matrix.

2.2. Extrusion Compounding In this work, a different compounding procedure was adopted in the preparation of the final binary and ternary PP composites, in order to avoid any severe HGB and GF breakage, especially at higher reinforcement concentrations. Thus, all the final binary and ternary PP composites were obtained by mixing adequate amounts of concentrates of PP/GF and PP/HGB with the base polymer directly in the injection molding process, so as to obtain composites with the desired varying total and relative GF and HGB hybrid filler contents. These concentrates were compounded in a Werner-Pfleiderer ZSK-30 corotating twin-screw extruder (TSE) of 30 mm screw diameter, L/D ratio of 35, 11-barrel sections and equipped with K-Tron gravimetric feeders. The extrusion compounding conditions used a barrel temperature profile of 210 / 225 / 235 / 230 / 230 / 230°C and screw rotation of 150 rpm. Concentrates of PP/GF were extrusion compounded with nominal 45.0 wt.% of GF and concentrates of PP/HGB with untreated and aminosilane-treated HGB were similarly produced with 30.0 wt.% of HGB. It is worth noting that different screw profiles were used for compounding the PP/GF and PP/HGB concentrates. Although the HGB used has high crush strength and, consequently, suitable for extrusion compounding, the screw profile and HGB feeding arrangement were carefully selected to avoid excessive HGB breakage during processing. This was achieved by feeding HGB downstream into the fully molten polymer (barrel section #8) and without use of any aggressive kneading and distributive

5 mixing screw elements, as recommended in the specific literature for compounding HGB-filled thermoplastic composites [40, 41]. In the case of GF concentrates, the fiber was also fed downstream at the same barrel section, and a standard mild kneading and distributive mixing screw elements was used to assure minimum fiber breakage. In all above mentioned concentrates, the actual PP-g-MAH content was defined based on the optimum concentration per unit surface area of the reinforcements, as determined by Lopes et al [1]. Thus, considering that the HGB filler (ρ = 0.60 g/cm3) has a much higher specific surface area/unit mass than that of GF (ρ = 2.55 g/cm3), the maleated PP content in the GF concentrate was 2.5 wt.% and 8.0 wt.% for the HGB concentrate. It should also be noted that the reference PP matrix was obtained by extrusion compounding of the two previously mentioned grades, in the ratio 3:1, along with the thermal stabilizers. This reference PP was used to dilute the concentrates in the desired proportions during the injection molding process. Finally, to analyze the influence of the compatibilizer content on the mechanical properties of the reference PP, a separate base PP matrix with 3.5 wt.% of PP-g-MAH, corresponding to the maximum content of the compatibilizer in any PP composite systems, was also similarly prepared for comparison purposes by dynamic mechanical thermal analysis.

2.3. Injection Molding Tensile (ASTM D638 - type I), flexural (ASTM D790) and Izod impact (ASTM D256) test specimens, composed of several binary and ternary PP composite formulations with different total and relative hybrid GF/HGB content, were injection molded in an Arburg Allrounder 270V/300-120 machine. The injection molding conditions comprised: (i) barrel temperature profile: 200 / 210 / 220 / 225 / 230°C; (ii) mold temperature: 50°C; (iii) injection pressure/speed: 580 bar / 5,0 cm3/s. The screw rotation and back pressure were adequately chosen to optimize good filler dispersion and reduced GF and HGB breakage. Thus, the hybrid composites preparation procedure adopted by direct dilution of the preextruded PP concentrates during the injection molding of test specimens should ensure that all binary and ternary PP composites will present minimum variation in GF length and HGB breakage and, as the total and relative hybrid filler content is varied, are expected to contribute with minimum mechanical properties variations in all binary and ternary PP composites.

6 Injection molding protocol was based on the solid-state mixture of adequate amounts of the PP/GF and PP/HGB concentrates diluted with the reference PP polymer matrix, obtained by simple plastic bag “tumble blending of pellets”, so as to prepare the desired compositions. Table 1 presents the composition of all PP composite formulations, along with the reference PP matrix and their respective formulation code. The filler content in the binary PP/GF and PP/HGB with and without aminosilane surface treatment was restricted up to 14 vol.% in the composites, so as to minimize both GF and HGB breakage. The formulation of the ternary PP/GF/HGB composites was carefully chosen in such a way to evaluate the effect of increasing particulate HGB filler content on the mechanical behavior of the reference binary PP/GF composites at constant fiber content (15, 20, 25 and 30 wt.%). Thus, as the total and relative hybrid reinforcement volume content is increased and the interparticle distance between GF and HGB is reduced so that their stress fields superimpose under mechanical loading, it should be possible to investigate the influence of the interfacial interactions between the high and low aspect ratio reinforcements on the mechanical behavior of ternary PP/GF/HGB composites in the presence of untreated and aminosilane-treated HGB filler.

2.4. Characterization and Testing The real weight concentrations of the composites, indicated in Table 1, were determined through pyrolysis of the PP composite molded test specimens in a microwave oven at 620°C for 30 min. The total volume concentration (ϕ) of filler was calculated using Equation 1, valid for both binary and ternary PP composites. In Equation 1, W and ρ represent the weight fraction and density of the composite components, while the suffixes PP, GF, and HGB are related to the PP matrix, glass fiber and hollow glass beads, respectively. ϕ

=

W ρ

+

W ρ

W ρ

W ρ 1 − (W +

+

ρ

+W

)

(1)

The specific density measurements were carried out with a gas pycnometer equipment from Micromeritics, model AccuPyc II 1340, on both molded composite test specimens and on the pyrolysis residues of each formulation. The latter were used, together with the initial measured densities of the reinforcements as received from the suppliers, to calculate the percentage volume breakage of HGB during processing, according to Equation 2 [41, 44]:

7

% volume loss =

ρ

1

−

1 ρ ρ

1

− −

ρ

1

1 ρ

−

1 ρ

x 100 (2)

where ρHGBi is the initial density of the HGB, ρHGBm is the measured density of HGB from pyrolysis residue and ρsg is the density of solid glass (2.55 g/cm3) [41]. It should be noted that the percentage of HGB breakage in the ternary hybrid composites is calculated assuming the nominal relative content of each type of filler. As the mechanical strength properties of GF reinforced polymer composites are significantly influenced by the average GF length, optical microscopy readings of at least 700 individual glass fibers, obtained from ignition loss residues of injection molded tensile test specimens, were used for weight average GF length (Lw) measurements in both binary PP/GF and ternary PP/GF/HGB composites. These Lw readings were obtained on a LEICA microscope, equipped with an image analysis software (ImageProPlus). The maximum volume filler particles packing fraction data of the individual reinforcements (GF and HGB) and of the hybrid systems of GF/HGB in the relative volume ratios of 100/25, 50/50 and 25/100 were obtained through “spatula rub-out oil absorption” measurements based on ASTM D281-12 standard test method, using Equation 3: V$,

&'

100 ) ρ$ = (3) O. A. 100 ( )+( ) ρ- . ρ$ (

where, Vf,max is the maximum volume packing fraction, O.A. is the oil absorption in grams per 100g of filler, ρf is the filler density (GF or HGB individually or GF/HGB hybrid densities) and ρoil is the oil density. The oil used was DOP (dioctyl phthalate), with nominal density of 0.983 g/cm3. The O.A. tests were carried out using the pyrolysis residue samples from ignition-loss tests performed on the PP/GF(45%) and PP/HGB(30%) extrusion compounded composites concentrates, which were manually mixed in the above-mentioned volume ratios of GF/HGB. These filler oil-absorption tests were carried out to obtain estimates of the maximum content of hybrid GF/HGB filler that can be incorporated in the polymer matrix, as its critical Vf,max value is approached and above which there is no sufficient matrix polymer to wet all the filler particles surface and to fill in the interstitial particles voids at the highest

8 packing configuration. Thus, as the total content of GF/HGB, at any given relative GF/HGB concentration, approaches its delimiting Vf,max value, it is expected that problems may arise with respect to the quality of the hybrid filler particles uniform dispersion in the matrix polymer, leading to the presence of voids and segregation of hybrid filler components in the composite, as reported in literature on these hybrid composites systems [37, 38]. The tensile and flexural tests were carried out with an Instron universal testing machine, model 5569, according to ASTM D638 and ASTM D790 (Procedure A) standard test methods, respectively. At least five test specimens were used for testing each formulation. The tensile tests used a crosshead displacement rate of 5 mm.min-1. The flexural test, in turn, was performed with the support span-to-depth ratio = 16, support span (L) = 51.0 mm, rate of strain (Z) = 0.01 mm/mm/min, rate of crosshead motion (R) = 1.3635 mm/min and maximum strain = 5.0%. Notched Izod impact tests were carried out in a CEAST pendulum impact test equipment, according to the standard test method ASTM D256, with a pendulum of 1.0 J, and using eight test specimens per composite formulation. Dynamic mechanical thermal analysis (DMTA) was carried out in a TA Instruments Q800 equipment. The analysis was performed under 3-point bending method, with a constant strain amplitude of 60 µm (< 1%, in the linear viscoelastic regime), frequency of 1 Hz, temperature range of -30°C to 100°C and heating rate of 3°C/min. All storage modulus (E’) and mechanical damping factor (tan δ) results are presented as an average from triplicate DMTA runs for each composite formulation. Scanning electron microscopy (SEM) observations, using a FEI Company microscope - model Inspect S50, were carried out over the cross-section of cryo-fractured surfaces of tensile test specimens, previously submitted to a strain corresponding to 70% of the strain at maximum tensile strength of each composite. This procedure was adopted to enhance visualization of the interfacial adhesion at the polymer-hybrid reinforcement’s components interfaces.

3. Results and discussion 3.1. Filler Content, Composite Density and GF Length Table 1 indicates the total nominal and real filler concentrations, percentage of HGB breakage and GF attrition (Lw) of all binary and ternary PP composites, as determined from the previously mentioned experimental procedures. The real total volume filler content was calculated by discounting the percentage of HGB breakage determined by Equation 2. Analyzing this percentage of HGB breakage

9 data in the binary PP/HGB and ternary PP/GF/HGB composites, as shown in Table 1, one can observe that these values varied between 8 - 12% and, as a general trend, increased with HGB concentration in both binary and ternary composites.

Table 1

In Figure 1, the experimentally measured densities of both binary and ternary PP composites, as a function of the real total volume content of single and hybrid GF/HGB reinforcements (discounting the percentage volume of HGB breakage), are compared with the corresponding theoretical density values calculated based on the simple rule of hybrid mixtures expressed by Equation 4:

W ρ0 = 1 ρ

+

W ρ

+

W ρ

2

34

(4)

where ρc is the theoretical predicted density of the composite, WPP, WGF and WHGB are the real weight fractions (from ignition loss tests), and ρPP, ρGF and ρHGB are the measured densities of the PP matrix, GF and HGB, respectively. However, it should be noted that in the hybrid composites, the weight fractions of each component (WGF and WHGB) are based on their nominal relative contents. Thus, as observed in Figure 1, the measured density values of both binary PP/HGB and ternary PP/GF/HGB composites are reduced linearly with increasing HGB volume content.

Figure 1

Also, considering that the experimentally measured density values of the binary PP/HGB and ternary PP/GF/HGB composites indicate a good fit over the theoretically calculated values (dotted straight lines shown in Figure 1) based on Equation 4, it is possible to infer that the effective total and relative contents of the hybrid reinforcements are in good agreement with the proposed concentrations in these composites. Nevertheless, the measured density data of the ternary composites of PP/GF(30)/HGB, with both untreated and aminosilane-treated HGB, indicates systematically values slightly higher than their corresponding theoretical values. However, analyzing the evolution of the measured density data with increasing HGB content of this set of ternary composites with the same evolution in the reference

10 binary composites of PP/HGB, as shown in Table 1, one can observe that increasing HGB filler content up to 10 wt.% (approx. 14 vol.%) in the binary composites of PP/HGB contributes to a density reduction of reference PP from 0.906 g/cm3 to 0.870 g/cm3 for the untreated HGB and 0.869 g/cm3 for the aminosilane-treated HGB-filled composites, which correspond to density reductions of 4.0% and 4.1%, respectively. In the case of the ternary hybrid composites at the highest total filler content (26.8 vol.%), i.e. in the PP/GF(30)/HGBamino(10) composite, the measured density value is 1,090 g/cm3, which results in nearly the same density reduction of 4.2% when compared to its reference binary composite of PP/GF(30.0) of density = 1.138 g/cm3. Considering that proportionally comparable density reduction results were also verified for the other ternary PP composites, it can be inferred that the percentage density reduction in the ternary hybrid composites, at any given GF content, is approximately equivalent to the density reduction verified in the respective reference binary PP/HGB composites at the same HGB volume content. This means that the addition of HGB filler in the ternary composites of PP/GF(30)/HGB does not alter the density values of its matrix polymer. Thus, the previously verified small deviations between the measured and theoretical density values of the specific ternary composites of PP/GF(30)/HGB can only be related to the fact that at higher hybrid reinforcement concentrations the actual measured HGB breakage values verified during the processing of this set of ternary composites do not account for the real level of HGB breakage in these composites. Similar deviations in the correlation of measured and theoretical density data of binary and ternary PP composites with HGB fillers were also reported in published literature [36,74] and attributed to difficulties in the measured density values of the hybrid reinforcement’ components or in the HGB breakage values. Finally, analyzing the weight average GF length (Lw) of all GF-filled composites, shown in Table 1, it is possible to observe that the Lw values of the binary PP/GF composites are in the range of 522 – 525 µm, independent of GF content. Similarly, in the ternary PP/GF/HGB composites, the average Lw values for all compositions are centered at 523 ± 15 µm. Considering that all the GF-filled composites were obtained by direct dilutions in the injection molding process of PP/GF concentrates with Lw = 577 µm, it is expected that the obtained Lw values of GF of all composites should vary little and, therefore, will not influence significantly the mechanical strength properties analyzed in this work.

3.2. Filler Oil Absorption (Maximum Volume Packing Fraction)

11 The oil absorption (O.A.) and maximum filler volume packing fraction (Vf,max) results of five (5) different relative volume ratio combinations of GF and HGB fillers, extracted as pyrolysis residues from binary composites concentrates of PP/GF(45%) and PP/HGB(30%), are indicated in Table 2. As expected, the relatively low Vf,max value (0.13) verified for the short GF, in comparison to high Vf,max value (0.68) for HGB, clearly indicate that short microfibers tend to entangle and pack poorly in comparison to microspheres packing characteristics. The relationship between the obtained Vf,max values and the relative volume content of HGB is illustrated in Figure 2 with the polynomial trendline dotted curve fitted to the experimental data, which clearly indicates that introduction of microspheres into short GF contributes towards higher values of Vf max. in the hybrid GF/HGB filler in ternary PP composites. This hybrid filler’s improved volumetric packing characteristics is based upon the simplified expression (1 – Vf/Vf max)-1 used for characterization of the melt flow behavior of highly filled polymer systems, so that increasing Vf max value at any given volume content (Vf) of single or hybrid filler will increase the volume fraction of polymer matrix available for flow in the molten state and decrease the melt viscosity of the polymer composite, as detailed in the published literature [1, 2, 16, 19]. This same effect also implies that in the solid-state mechanical properties, such as the elastic modulus of polymer composites, increasing Vf max. values of the hybrid filler with increasing relative volume fraction of HGB filler in the total hybrid HGB/GF reinforcement, will contribute to increased volume fraction of matrix polymer available to deform and, thereby, reduce the elastic modulus of the hybrid composite for the same reason specified for melt viscosity.

Table 2

Figure 2

In Table 3, similar filler oil absorption data measured directly on the pyrolysis residues of specific injection molded test specimen of PP/GF(30), PP/GF(30)/HGBamino(3.5) and PP/GF(30)/HGBamino(7.5) composites are detailed and their respective Vf,max values are also shown in Figure 2. The good experimental fit of these measured hybrid filler Vf,max values over the trendline dotted curve of the graph in Figure 2 permits use of this trendline curve as a valid reference for the delimiting

12 value of the maximum filler volume content for all ternary formulations of PP/GF/HGB composites investigated in this study, as indicated in the data presented in Figure 3.

Table 3

Figure 3

The data shown in Figure 3 indicates that the filler content in all the ternary hybrid composite systems containing 25 wt.% or less of GF are all well below their estimated trendline maximum filler volume packing fraction values (Vf,max), whereas their content in the ternary composites of PP/GF(30)/HGB and PP/GF(30)/HGBamino is either quite close to or slightly above their Vf,max values. In the latter case, under such critical processing conditions during the injection molding of this set of composites test specimens, as discussed earlier, it is expected that problems may arise with respect to the quality of the hybrid filler particles dispersion and uniform distribution in the matrix polymer, which may lead to the presence of voids and segregation of hybrid filler components in the composite, as reported in literature on these hybrid composites systems [37, 38]. Also, as the interparticle distance between both fibrous and particulate fillers is greatly reduced in this case, then the degree of interfacial interactions between the hybrid filler´s components are expected to significantly influence the mechanical strength properties of this set of hybrid ternary composites, depending upon the degree of interfacial adhesion of the matrix polymer with the weaker low aspect ratio HGB particles.

3.3. Scanning Electron Microscopy The morphology of the binary and hybrid composites was analyzed by scanning electron microscopy (SEM), primarily aiming to visualize the interfacial adhesion at the polymer-reinforcements interfaces. Figures 4 and 5 illustrate the morphology of the cryo-fractured surfaces of pre-strained tensile test specimens (at 70% of their respective tensile strength values) of binary PP/HGB(10) and ternary PP/GF(30)/HGB(5) composites, respectively, using both aminosilane-treated and untreated HGB filler. The SEM micrographs of the PP/HGB(10) and PP/HGBamino(10) binary composites, presented in Figures 4(a) and 4(b) respectively, indicate significant differences in relation to the interfacial adhesion of the glass beads to the thermoplastic PP matrix. The fracture surface of the untreated HGB in the

13 composite PP/HGB(10.0) appears very smooth with no matrix polymer adhered and with profuse microspheres debonding from the matrix polymer, as a clear sign of their poor interfacial adhesion to the PP matrix. The binary composite of PP/HGBamino(10), on the other hand, reveals HGB particles that are pretty well adhered to the PP matrix and with matrix polymer adhered on the surface of few debonded microspheres, as a clear indication of strong polymer-filler interfacial adhesion. Considering that the binary composites of PP with both untreated and aminosilane-treated HGB filler contain a sizable and equal amount of maleated PP (PP-g-MAH) interfacial compatibilizer (8 wt.% w.r.t. HGB filler content in PP/HGB concentrates), the differences in interfacial adhesion between PPHGB filler particles, as verified in the micrographs of Figure 4 (a and b), can be attributed to the distinct interfacial compatibilization mechanisms promoted in the presence or absence of the aminosilane coupling agent treatment of HGB particles. As well documented in the specific literature on interfacial compatibilization of PP/glass filler composites with use of maleated PP compatibilizer [42-48], as it occurs in the composites of this study, the maleic anhydride (-MAH) functional groups of PP-g-MAH may chemically react with the naturally occurring hydroxyl (-OH) groups present on the untreated borosilicate glass HGB particles through ring opening mechanism of the MAH or through physical secondary hydrogen bonding interactions [43, 44]. Thus, in the case of PP composites with untreated HGB filler, only weak secondary hydrogen bonds are most likely to occur, given the low reactivity between the hydroxyl (-OH) and maleic anhydride (-MAH) functional groups, as reported by Orr et al [43]. However, in the case of the PP composites containing aminosilane-treated HGB, the interfacial compatibilization reaction occurs through strong amide and imide covalent bonds between amine (-NH2) and maleic anhydride (-MAH) co-reactive functional groups, which is considered to be of higher reactivity according to Orr et al [42-44]. Thus, the chemical interfacial compatibilization between the maleic anhydride functionalized PP compatibilizer present in PP matrix and the aminosilane-treated HGB has greater efficiency in promoting the verified strong PP polymer-HGBamino filler interfacial adhesion.

Figure 4

Now, analyzing the SEM micrographs of the ternary composites of PP/GF(30)/HGB(5), shown in Figures 5(a) and 5(b) for the untreated and aminosilane-treated HGB filler respectively, a closer examination indicates a much better polymer matrix wetting of glass fibers in the composite with

14 aminosilane-treated HGB filler than compared to that observed in the composite with untreated HGB filler. Considering that both ternary composites contain nearly the same total and relative hybrid GF/HGB content and also that the SEM micrographs in Figure 5 (a and b) correspond to the cryofractured surfaces of pre-strained tensile test specimens at 70% of their respective tensile strength values (72 MPa and 78 MPa for the untreated and aminosilane-treated HGB respectively, as shown in Table 5), it can be deduced that the presence of aminosilane-treated HGB filler contributes to a much lower GF debonding than that observed with the untreated HGB filler in these ternary composites. This fact indicates that untreated HGB filler particles in the vicinity of the GF surface induce fiber-polymer interfacial decoupling at much lower stress levels under mechanical loading than in the presence of treated HGBamino particles, as detailed later in the item 3.5.3 on tensile strength properties of the hybrid PP composites.

Figure 5

3.4. Dynamic Mechanical Thermal Analysis (DMTA) The dynamic mechanical thermal analysis (DMTA) consists of a technique that correlates the macroscopic properties of polymeric materials with their molecular characteristics such as polymer molecular relaxations associated with conformational changes and deformations generated by the molecular rearrangements [42, 49-51]. In this study, the technique was used with the main objective of evaluating the characteristics of the interfacial interactions of PP matrix polymer with GF or GF/HGB reinforcements in the composites [27, 42, 49, 50, 53-56].

3.4.1. Storage Modulus (E') Figure 6 shows the mean spectral curves of storage modulus (E') and mechanical damping factor (tan δ), as a function of temperature (range of -30 to 100°C), for the reference PP matrix (indicated as “PP ref.”) and an additional modified base PP matrix with 3.5 wt.% of PP-g-MAH, corresponding to the maximum content of the compatibilizer in any of the analyzed PP composite systems. Firstly, one can observe an equivalence between the storage modulus (E') and tan δ curves for both PP samples, considering that the curves of each property are practically superimposed. This similarity can also be observed from DMTA data presented in Table 4. By verifying that these two PP samples are equivalent in their dynamic spectral response, it is possible to attest that all previous and further considerations about

15 distinct degrees of polymer-filler interfacial adhesion throughout the work are certainly due to specific differences in the interfacial compatibilities between the MAH-modified PP matrix with the individual and hybrid GF/HGB reinforcements (especially with untreated and aminosilane-treated HGB), and do not arise from the modification of the polymer matrix itself.

Figure 6

Table 4

Now, additionally analyzing the storage modulus (E') spectra, as a function of increasing temperature for the reference binary PP/GF(30) and selected ternary PP/GF(30)/HGB composites (Table 4 and Figure 7), decreasing E' value curves are verified for all materials, as expected with increasing temperature. This decay of E' curves is more pronounced at temperatures around 10°C, which is close to the glass transition temperature (Tg) of the amorphous phase of the i-PP homopolymer [57].

Figure 7

Figure 8

A further important point to emphasize when comparing Figures 6 and 7 is the marked effect of the GF and HGB reinforcements over the storage modulus (E') of neat PP matrix. At room temperature, the E' values of the binary PP/GF and ternary PP/GF(30)/HGB composites are almost four times higher than the modulus of the reference PP matrix, demonstrating the mechanical reinforcement efficiency derived mainly from the GF in these composites (see also Table 4). It should also be noted from Figure 7 that these E' curves spread out significantly at temperatures above 60°C. In order to clarify this behavior, Figure 8 highlights the E' curves of Figure 7 at temperatures higher than 60°C. Such deviations of the elastic storage modulus (E') curves of the analyzed PP composites at room temperature and temperatures higher than 60°C can be related to the influence of residual polymer-filler interfacial thermal stresses in PP composites, derived from the differential of thermal contraction coefficients between the polymer matrix and the filler particles. During the cooling phase of the injection molding process, distinct thermal

16 contractions occur between the polymer matrix and the reinforcements, which leads to a mechanical interlocking of the reinforcement particles by the matrix, even when there are no other physical or chemical interactions at the reinforcement-matrix interface. Thus, at room temperature (23°C) and very low strain levels (< 0.25%) used in DMTA, there are sufficient residual interfacial thermal stresses present so as to ensure an effective stress transfer from the polymer matrix to the reinforcement, regardless of whether there is or not a strong polymer-filler interfacial adhesion [50, 58-62]. However, when the E' storage modulus is determined at higher temperatures, when the effects of interfacial thermal stresses are nullified (above around 60°C in the case of PP), it is possible to clearly verify the contribution of the polymer-filler interfacial adhesion to the measured property [50, 53, 59, 62]. As shown in Figure 8, the storage modulus curve of the reference binary PP/GF(30.0) composite assumes intermediate values between the hybrid PP/GF(30.0)/HGB composites with and without aminosilane-treated HGB filler. The E' values are lower in the case of the untreated HGB-containing ternary composites and assume inferior values with increasing untreated HGB volume content in the ternary composites. On the other hand, a completely opposite behavior is observed for the ternary PP/GF(30.0)/HGB composites with the aminosilane-treated HGB. In this case, the E' values are enhanced, with respect to the reference binary PP/GF(30), proportional to the volume content of HGBamino. This behavior is an important indicator of the influence of the polymer-particulate HGB filler interfacial adhesion on the mechanical properties of the hybrid PP composites, when residual interfacial thermal stresses are nullified with temperature increase. Thus, when the interfacial adhesion is good, as in the case of ternary composites with the aminosilanetreated HGB, the reinforcing effect is positive and proportional to the total volume content of the hybrid reinforcement. When the polymer-particulate filler (HGB) interfacial adhesion is poor, increasing HGB content in the GF-containing composite has a deleterious effect on the storage modulus (E') property; i.e., E' decreases in relation to the binary PP/GF composite system. This particular behavior has also been observed in previous studies with other types of hybrid PP composite systems [2, 18].

3.4.2. Mechanical Damping Factor (Tan δ) The mechanical damping (tan δ) spectra, obtained from the DMTA temperature scan, is a material characteristic related to its molecular relaxation, as the polymer undergoes changes in its physical state related to its primary and secondary thermal transitions: the primary Tm and secondary Tg and sub- and supra-Tg transitions being of importance in the solid state behavior of semicrystalline

17 thermoplastics. Thus, tan δ values indicate how far the polymeric material’s solid-state mechanical behavior deviates from the ideal elastic condition [42, 50, 58, 63]. In polymeric composites, the interfacial interactions between the polymer matrix and the reinforcement component are concentrated at or in the near vicinity of the reinforcement’s rigid surface, which may be denominated as the interface or interphase region. This interface or interphase has properties deferring from those of the polymer matrix, depending upon the type of physical and chemical interfacial interactions existing in the specific composite [12, 13, 50, 53, 56, 58, 63-65]. In a simplified arrangement, the mechanical damping factor of a polymer composite (tan δc) can be defined by Equation 5, as originally defined by Kubát et al. [50]:

tan δ0 = ϕ: . tan δ: + ϕ . tan δ + ϕ . tan δ (5)

where the indices "c", "r", "i" and "m" correspond to the composite, reinforcement, interface or interphase and polymer matrix, respectively, while "φ" corresponds to the volume fraction of each component [50]. Although this equation does not provide a complete estimate of tan δ values for the composite, it is quite useful to evaluate the degree of polymer-filler interfacial adhesion. According to Kubát et al. [50], it is possible to consider that the reinforcement components are perfectly elastic in the temperature range of polymer DMTA characterization and have, therefore, no mechanical damping, so that their contribution to Equation 5 is null, i.e. tan δr = 0. In addition, the volume fraction of the interface/interphase is comparatively negligible with respect to the main components of the composite and, therefore, φi ≈ 0. It is known that strong polymer-filler interfacial interactions tend to reduce macromolecular mobility in the vicinity of the surface of the reinforcement particles, compared to the mobility present in the bulk matrix [50, 53, 58]. This reduces tan δc values proportional to the increasing reinforcement volume fraction. However, in the condition of low polymerreinforcement interfacial adhesion, the contribution of the interface term (φi.tan δi) in Equation 5 increases as a result of a large dissipation of heat related to the interfacial friction of the loose polymer chains at the reinforcement’s rigid surface [50]. This effect is particularly emphasized at temperatures higher than 60ºC for PP composites, when the residual interfacial thermal stresses are nullified [42, 50, 53, 59-61]. Based upon the above described principle, the mechanical damping factor (tan δ) curves of the binary and ternary hybrid PP composites along with the glass-transition temperatures Tg values of the matrix PP polymer are presented in Figure 9 and Table 4. Firstly, analyzing the data on Tg values of the

18 reference binary PP/GF(30) composite (Tg = 10.9ºC) and also of the ternary PP/GF(30)/HGBamino composites (Tg = 9.9ºC), in comparison with that of maleated-PP matrix (Tg = 7.3ºC), there is a clear indication that an efficient interfacial compatibilization of single GF or hybrid GF/HGBamino reinforcements contributes to a slight increase in the matrix polymer Tg values, which can be attributed to the polymer chains immobilized at the rigid reinforcement’s surface. On the other hand, the use of untreated HGB in the ternary PP/GF(30)/HGB composites, contributes to reduced Tg values of 9.4 ºC and 4.1ºC, respectively for 3.5 wt.% and 7.5 wt.% of HGB filler.

Figure 9

Figure 10

In Figure 10, the mean tan δ curves of the same set of binary PP/GF(30) and specific ternary PP/GF(30)/HGB composites shown in Figure 9 are now highlighted in the temperature range between 60ºC to 100°C, when the residual interfacial thermal stresses are nullified. Under such conditions, a substantial difference in intensities of the tan δc curves of the ternary PP/GF(30)/HGB composites with nearly identical content of 3.5 wt.% and 7.5 wt% of untreated and aminosilane-treated HGB fillers can be observed, with the tan δ curve of the reference binary PP/GF(30) assuming an intermediate position between the two set of totally and partially compatibilized ternary hybrid composites. Considering that these two sets of ternary composites have nearly identical total and relative volume contents of hybrid GF/HGB reinforcement (total volume = 17.3 and 21.5 vol.% for untreated HGB composites and 17.1 and 22.9 vol.% for aminosilane-treated HGB composites, as shown in Table 1), it was expected that their tan δ curves should both be positioned below the tan δ curve of their reference binary PP/GF(30) composite with much lower GF content (13.6 vol.%), as prescribed by the previously shown Equation 5. The fact that the tan δ curves of ternary composites of PP/GF(30.0)/HGB(7.5) and PP/GF(30.0)/HGBamino(7.5) are positioned distinctly apart (tan δ intensities at 80ºC of 0.066 and 0.057, respectively for the untreated and aminosilane-treated HGB filler, as shown in Table 4) with respect to the position of their reference binary PP/GF(30) composite at the same temperature (tan δ = 0.061), indicates much higher interfacial heat dissipation in the ternary composites with untreated HGB filler. This fact clearly demonstrates the important role of polymer-reinforcement interfacial interactions in defining the mechanical performance

19 of these ternary hybrid composites of PP with distinct interfacial compatibilities formulated for this specific hybrid fibrous-particulate GF/HGB reinforcement.

3.5. Mechanical Properties 3.5.1. Tensile and Flexural Moduli The graph in Figure 11 illustrates the relationship between the tensile elastic modulus (Ec) and the total volume content of reinforcement of the binary composites of PP/GF, PP/HGB and PP/HGBamino and the ternary hybrid composites of PP/GF/HGB and PP/GF/HGBamino. The detailed results are also indicated in Table 5. Firstly, comparing the reference binary composites at equivalent reinforcement volume content, the addition of GF to PP matrix has a much higher reinforcement effect on the tensile modulus property, as compared to the modulus increment in the HGB-filled composites, which is proportional to the aspect ratio of each type of reinforcement, its effective solid volume content (much lower for HGB filler) and the preferential orientation of glass fibers along the mold fill direction.

Figure 11

Table 5

As expected in the binary PP/GF composites, the tensile Ec values increased linearly from 1.2 GPa , as GF content increased from 0 wt.% to 30 wt.%, respectively. On the other hand, at nearly equivalent filler volume concentrations in the binary PP/HGB composite systems (filler aspect ratio = 1), the Ec values were upgraded to a nearly constant value of approximately 2.0 GPa; apparently independent of HGB content or degree of polymer-filler interfacial adhesion (HGB with and without aminosilane treatment). This effect is indicated in Figure 11 by the low positive slope (0.063) of the straight line fit over the Ec data of these two distinct PP/HGB binary composites. The inability of the HGB to efficiently increase the polymer matrix stiffness with increasing filler content is related to its microstructure being composed of three phases: polymer matrix, thin glass outer shell and the air void inside the HGB. Thus, in the case of hollow microspheres like the HGB, the effective elastic modulus of such plastic syntactic foams is limited to the small solid volume content of the thin outer shell of the hollow glass microspheres, which in turn depends on the wall thickness to the diameter ratio of these particles, as reported in the

20 specific literature [66-68]. Thus, the resultant elastic modulus of such binary PP/HGB composite is much lower than the Ec values of polymer composites with solid glass microspheres. In the case of the ternary hybrid composites of PP/GF/HGBamino with constant GF content up to 25 wt.% (≅ 10.5 vol.%), the tensile modulus is significantly increased with HGB addition with respect to its equivalent reference binary PP/GF composite of same GF content. As shown in Figure 11, this modulus enhancement is more pronounced at low HGB content in the ternary composites of PP/GF(20)/HGBamino(2) and PP/GF(25)/HGBamino(2) and then tends to reduce with further HGB addition. The influence of this effect shall be detailed furthermore in the item 3.5.4 on model predictions of tensile properties. On the other hand, with increasing HGB content in 30 wt.% GF-filled ternary composites (≅ 14 vol.% GF), the tensile moduli values remain practically unaltered with respect to that of their reference binary PP/GF(30) composite, apparently, irrespective of the content or degree of interfacial adhesion of both untreated and aminosilane-sized HGB filler with the polymer matrix. Similar behavior was also reported by other authors [24, 32, 36] for different fiber/HGB-filled hybrid thermoplastic composite systems. This limiting elastic moduli values at increasing total hybrid reinforcement volume content above 15%, when the critical Vf,max values for GF/HGB filler are approached or even exceeded (as shown in Figure 3), may be attributed to the presence of voids and segregation of the hybrid reinforcement components in these composites, as reported earlier in the literature data [37, 38]. Thus, the combined effect of both fillers (GF and HGB) towards increasing tensile modulus is impaired when the total volume hybrid filler content is close to or above the critical Vf,max value of the hybrid filler and, as a consequence, the tensile Ec values of the ternary hybrid composite systems containing 25 wt.% and 30 wt.% of GF become practically merged, as verified in Figure 11. Another important observation to be noted in Figure 11 for the tensile moduli values of the specific ternary composites of PP/GF(30)/HGB is related to the similarity of the slope of the straight line fit over the experimental Ec values (0.061) with increasing HGB filler content, in comparison to that of its binary reference composites of PP/HGB (0.063). This fact implies that the tensile modulus of this set of ternary composites with increasing hybrid GF/HGB filler content and close to their critical Vf,max value is essentially governed by the simple “Rule of Hybrid Mixtures”, which assumes that the tensile modulus of the hybrid composite is derived from simple additive weighted volume combination of the properties of their reference binary composites, as shall be detailed further on in item 3.5.4 on model prediction of tensile modulus. Finally, the fact that the tensile moduli values of both binary reference composites of

21 PP/HGB and ternary composites of PP/GF(30)/HGB are not significantly influenced by the degree of interfacial polymer-HGB filler adhesion verified with untreated and aminosilane-treated microspheres is attributed to presence of sufficient residual interfacial thermal stresses to assure stress transfer at the polymer-HGB filler interface, as reported earlier in detail in the item 3.4.1 on the behavior of storage E' modulus curves with increase in temperature.

Figure 12

In Figure 12 and Table 5 are detailed the flexural elastic modulus data of specific binary PP/GF and PP/HGB composites and ternary composites of PP/GF(30)/HGB (with both untreated and aminosilane-treated HGB filler), as a function of the total volume content of reinforcement. As evidenced earlier for the tensile elastic modulus behavior of the very same set of ternary composites (Figure 11), the flexural moduli values remain practically unaltered with respect to that of their reference binary PP/GF(30) composite; apparently, irrespective of the content of untreated HGB filler and increase slightly with aminosilane-sized HGB content. This slight increase in the flexural modulus of the ternary hybrid composites of PP/GF(30)/HGBamino with increase in aminosilane treated HGB content is attributed to the well-known “skin-core” orientation profile of short glass fibers of injection molded test specimens [1, 2]. Thus, with increase in HGB filler content in the ternary composite, the degree of fiber orientation in the main flow direction of the skin layers of the test specimen increases and, consequently, contributes to the verified increase in FS values, as also reported elsewhere in published literature for PP composites with different hybrid GF/mineral filler reinforcements [16-18].

3.5.2. Tensile Strain at Break In order to analyze, in a simplistic manner, the contribution of the stress concentration factor of both glass fibers and HGB microspheres on the fracture behavior of the ternary PP/GF/HGB composites of this study, the data on the tensile strain at break (εb) of the reference binary and ternary composites is presented in Table 5. Firstly, a comparative analysis of the reference binary PP/GF and PP/HGB with and without aminosilane-treatment, clearly indicates distinct values of strain at rupture at equivalent volume content of GF or HGB fillers. The relatively low values of percentage strain at break of PP/GF composites (εb = 3.2 – 5.2%), in comparison to much higher values registered for both types of PP/HGB

22 composites, can be attributed to the higher stress concentration factor of glass fibers in comparison to that of HGB microspheres, due to their high aspect ratio (L/d ≅ 40 for GF against unity value for HGB in this study) and also to their much stronger interfacial interactions with the polymer matrix. These interfacial interactions for GF-reinforced PP composites are much higher in both their intensity (degree of interfacial adhesion) and extension (interfacial surface area) in comparison to the composites with untreated HGB (εb = 85% up to >300%) and at least in the degree of interfacial extension in comparison to the aminosilane-treated HGB (εb = 15 – 48%). The substantial differences in tensile strains at break of the PP/HGB composites with and without aminosilane-treated HGB filler, of essentially same average diameter and size distribution, is consistent with the estimates for strain at break of particulate-filled polymer composites defined by the well-established Lewis-Nielsen model equation, with emphasis on the degree of polymer-filler interfacial adhesion [63]. Now, analyzing the tensile strain at break data of the ternary PP/GF/HGB composites with untreated and aminosilane-treated microspheres, especially at the highest GF content (30 wt.%) as presented in Table 5, it is quite clear that the previously mentioned influence of the higher stress concentration factor of glass fibers predominates on the strain at break values of this set of ternary composites. These values are quite close to and slightly lower than those of their equivalent reference binary PP/GF(30) composite, irrespective of the volume content or the degree of interfacial adhesion of HGB filler particles. Similar behavior of tensile strain at break of other ternary PP composites with hybrid fibrous-particulate mineral fillers were also registered in the literature [2, 16, 18].

3.5.3. Tensile, Flexural and Impact Strength Properties

Figure 13

The tensile strength (TS) data of the binary reference and ternary hybrid PP composites, as a function of their total volume content of reinforcement, is presented in both Figure 13 and Table 5. In the binary reference composites, one can observe that the TS value of neat PP increases from 33.5 MPa to 84.3 MPa with incorporation of approximately 14.0 vol.% (around 30.0 wt.%) of GF. This indicates the existence of good interfacial adhesion of the GF reinforcement with the MAH-modified PP matrix and, consequently, resulting in an efficient reinforcement mechanism. On the other hand, in the binary

23 PP/HGB composites with untreated and aminosilane-treated HGB, the TS data presents two distinct behaviors, depending on the degree of the polymer-HGB filler interfacial adhesion. In the case of untreated-HGB binary composites, the TS values decreases monotonically (slope = 0.63, as shown in Figure 13) with increasing particulate filler volume content, as also reported in several previous published literature [32, 45-51, 69]. However, the use of aminosilane-treated HGB retains unaltered the TS values and practically equivalent to that of the reference PP matrix polymer, independently of HGBamino content. This behavior of both binary PP/HGB composites is corroborated with the previously presented evidence in the SEM micrographs of Figure 4 (a and b) of the PP/HGB(10.0) and PP/HGBamino(10.0) binary composites, respectively, which indicate significant differences in relation to the interfacial adhesion of aminosilane-treated and untreated HGB microspheres to the matrix PP polymer. In the case of all ternary hybrid PP/GF/HGB composites, at any given constant GF concentration, increasing volume content of aminosilane-treated HGB retains the tensile strength values practically unaltered with respect to those of their corresponding reference binary PP/GF composites of equivalent GF content, as shown in Figure 13. On the other hand, use of untreated HGB in the ternary hybrid PP/GF(30.0)/HGB composites results in a nearly linear decay in the TS values (slope = 1.07) with increasing HGB content. Thus, it appears that the TS properties of the ternary hybrid PP/GF/HGB composites are strongly influenced by the strength properties of their respective “pseudo-matrix” PP/HGB binary composites, which in turn is effectively dependent on the degree of interfacial adhesion at the HGB-PP matrix polymer interface. To elucidate this distinct mechanical strength behavior, it is necessary to consider the interfacial interactions between the components of the ternary PP/GF/HGB composites. The tensile strength in shortfiber reinforced composites depends strongly on the stress transfer efficiency from the thermoplastic matrix to the reinforcement. When such reinforcement has a high aspect ratio (L/d), as in the case of GF, the stress transfer is determined by the “effective average fiber length” in the composite. In other words, at a given constant GF content in the composite, the average GF length (Lf) should be much greater than a minimum critical fiber length (Lcrit) required for efficient interfacial stress transfer, as given by the KellyTyson’s Equation 6 [12, 13]:

L$ d$

0: >

=

σ$ (6) 2. τ A>

24 where σf is the ultimate tensile strength of the fiber; τint is the polymer-fiber interfacial shear strength, and Lf and df are the fiber length and diameter, respectively. Thus, an efficient fiber-reinforcement effect in a polymer composite depends on: (i) the fiber-matrix interfacial shear bond strength, which is associated to the degree of polymer-GF interfacial adhesion, limited at its maximum value by (ii) the shear strength of the thermoplastic matrix [12, 13, 49, 57, 63-65, 71]. On the other hand, when a HGB-filled composite is subjected to an external stress, the interface between the matrix and the particulate filler can transfer only a minor part of the stress due to its low aspect ratio (=1) [13, 19, 63]. Additionally, in HGB-filled composites containing untreated microspheres of nearly same average diameter and, consequently, equivalent surface area, the previously mentioned low stress transfer capacity is further reduced by the poor polymer-HGB interfacial adhesion, which results in more pronounced debonding of the untreated HGB with increasing stress [32, 45-48, 69], as evidenced in the SEM micrographs of Figure 4(a), in comparison to the reduced debonding verified with aminosilane-treated HGB filler particles shown in Figure 4(b). Now, considering that the glass fibers used in all ternary PP composites of this study are strongly bonded to PP matrix through previous maleated-PP compatibilization during the extrusion compounding of PP/GF concentrate and also considering that their weight average GF length (Lw) does not significantly vary in the binary and ternary composites, as previously indicated in Table 1, consequently, the significant decrease in the tensile strength property registered for the PP/GF(30)/HGB composites containing untreated HGB, at constant 30 wt.% of GF, can only be attributed to an apparent increase of the critical fiber length (Lcrit) of such composite system with increasing untreated HGB content. Thus, with increasing total volume content of the hybrid reinforcement and, consequently, reduced interparticle distance between glass fibers and HGB particle to the extent that their stress fields superimpose under mechanical loading, the local shear stress fields around the high aspect ratio GF surface can easily induce, in their vicinity, decoupling of untreated HGB particles from the matrix polymer. This extensive decoupling nucleates microcracks and coalescence of adjacent microcracks leading to bigger cracks, which will effectively impair the interfacial shear stress transfer at the fiber-polymer interface and, thereby, contribute to an increase in the effective Lcrit of GF. Consequently, as Lcrit increases with increase in the untreated HGB content, a given fraction of glass fibers lose their ability to efficiently reinforce the thermoplastic matrix. On the other hand, when aminosilane-treated HGB microspheres are added to the binary reference PP/GF(30) composite, the observed tensile strength values of the ternary

25 PP/GF(30.0)/HGBamino remain unaltered at nearly constant values with increasing HGBamino filler content. Apparently in this case, although the interface between the maleated polymer matrix and the aminosilane treated HGB must shear at its limit interfacial shear bond strength, extensive microspheres debonding from the matrix polymer does not occur and, thereby, microcracks development and their coalescence into bigger cracks is restricted so that the fiber-polymer interfacial debonding is much reduced. The above described behavior is corroborated by the SEM micrographs previously presented on cryofractured surfaces of ternary PP/GF(30.0)/HGB(5.0) composites shown in Figure 5 (a and b), which clearly reveal that untreated HGB particles induce fiber-polymer interfacial decoupling at much lower stress levels under mechanical loading than in the presence of treated HGBamino particles. This same behavior of the TS properties of the ternary hybrid composites of PP/GF(30)/HGB with respect to the influence of interfacial adhesion at the polymer-HGB interface when using untreated and aminosilanetreated HGB filler is also corroborated with the mechanical damping Tan δ values presented earlier on DMTA analysis of Figure 10. In this case, the reduced Tan δ intensity values for the aminosilane-treated HGB and increased Tan δ intensity values for the untreated HGB filler, in comparison to the Tan δ intensity value of their reference binary composite of PP/GF(30), clearly confirm the important role of strong interfacial interactions between the polymer matrix with the hybrid filler components for the definition of improved mechanical strength properties of the investigated ternary composites of PP. Another important point to be noted in the data presented in Figure 13 is related to the higher decay rate (slope = 1.07) of TS in the ternary composites of PP/GF(30)/HGB with increasing untreated HGB content, in comparison with the much lower decay rate (slope = 0.64) of its reference binary composites of PP/HGB. This fact is attributed to the previously discussed disruptive effect of untreated microspheres on the stress transfer efficiency at the polymer-GF interface. On the other hand, considering that the TS values of the binary reference composites of PP/HGBamino and that of all ternary composites of PP/GF/HGBamino exhibit nearly the same asymptotic slopes with increase in aminosilane-treated HGB filler content, as evidenced in Figure 13, this behavior indicates that the reference binary composite of PP/HGBamino acts as the pseudo-matrix of its corresponding ternary composite of PP/GF/HGBamino for the determination of the tensile strength properties of the specified ternary hybrid PP composites.

Figure 14

26 Figure 14 illustrates the flexural strength (FS) data of specific binary PP/GF and PP/HGB composites and selected ternary composites of PP/GF(30)/HGB with both untreated and aminosilanetreated HGB filler, all as a function of the total volume content of reinforcement. Analyzing first the FS property of the binary reference composites in Figure 14, one can observe that the FS value of reference PP matrix increases from 33.7 MPa to 125.8 MPa by the incorporation of approximately 14.0 vol.% (around 30.0 wt.%) of GF, as a consequence of the good interfacial adhesion of the GF reinforcement with the MAH-modified PP matrix, as previously observed for the tensile strength property. On the other hand, at the same equivalent 14%volume content (around 10 wt.%) of HGB filler, the binary composites of PP/HGB exhibit low FS values (39.8 MPa and 49.2 MPa for untreated and aminosilane-treated HGB respectively), which clearly indicate the low reinforcement efficiency of the hollow HGB microspheres due to their low aspect ratio (= 1). Similarly, as evidenced earlier for the tensile strength behavior of this very same set of ternary composites of PP/GF(30)/HGB, with increasing volume content of aminosilane-treated HGB filler, the flexural strength values remain practically unaltered with respect to that of their reference binary composite of PP/GF(30). On the other hand, use of increasing content of untreated HGB in the ternary composites of PP/GF(30)/HGB results in a nearly linear decay in the FS values.

Figure 15

Figure 15 illustrates the notched Izod impact strength (IIS) data of specific reference binary composites of PP/GF and PP/HGB and selected ternary hybrid composites of PP/GF(30)/HGB (with and without aminosilane-treatment), as a function of the total volume content of reinforcement. In the case of the binary PP/GF composites, an increase in IIS value of reference PP from 26 J/m to 98 J/m is observed with addition of around 14 vol.% of GF (≅ 30 wt.%), as also reported in Table 5. This positive behavior is associated to the energy dissipated during fibers decoupling and pull-out work of fracture. On the other hand, when considering the binary PP/HGB composites, with untreated and aminosilane-treated HGB, increasing volume contents of the particulate filler results in the deterioration of the IIS values of these composites, being slightly more pronounced in the case of the composite with untreated HGB. This behavior is consistent with the characteristics of the particle size and low aspect ratio filler-reinforced

27 composites, together with poor filler-matrix interfacial adhesion, as per demonstrated by other authors [44, 49, 52, 60]. Distinct notched IIS performances are also observed in the ternary hybrid PP/GF/HGB composites containing 30 wt.% of GF, when comparing increasing volume content of untreated and aminosilane-treated HGB. Whilst in the treated HGBamino-filled composite system, the notched IIS remains constant with increasing hybrid filler volume fraction, it decreases monotonically with untreated HGB content. It should be also noted that increasing volume content of untreated HGB filler reduces the IIS in the ternary composites in such a way that the difference in the IIS property values of the ternary composites containing treated and untreated HGB is significantly higher than that verified for the binary PP/HGB composite systems. Namely, as reported in Table 5, while the difference of IIS values between the PP/HGBamino(10) and PP/HGB(10) binary composites is 3.3 J/m (19.7 J/m to 16.4 J/m, respectively), in the hybrid ternary PP/GF(30)/HGB(10) composites containing treated and untreated HGB the property difference reaches up to 12.7 J/m at the maximum hybrid GF/HGB reinforcement volume content. This same impact strength behavior was also verified in the tensile and flexural strength properties of the selected ternary composites of hybrid PP/GF(30)/HGB. Thus, it appears that the similarities verified in the behavior of the tensile, flexural and impact strengths properties of the ternary hybrid PP/GF(30)/HGB composites are strongly influenced by the strength properties of their respective “pseudo-matrix” of PP/HGB binary composites, which in turn is effectively dependent on the degree of interfacial adhesion at the HGB-PP matrix polymer interface. Consequently, as the GF-HGB interparticle distance is reduced in this set of ternary composites with increasing total volume content of hybrid reinforcement, the interfacial interactions between the high aspect ratio GF and low aspect ratio HGB filler particles and also between the matrix polymer with both components of the hybrid reinforcement play an important role in maximizing the mechanical strength behavior of PP composites reinforced with hybrid fibrous-particulate reinforcement.

3.5.4. Model Predictions of Tensile Properties The “Rule of Hybrid Mixtures” (RHM) is the simplest and most used model for prediction of mechanical modulus and strength properties of ternary hybrid polymer composites with fibrousparticulate reinforcement. This RHM model assumes that the mechanical properties of the hybrid composite are derived from simple additive weighted volume combination of the properties of their

28 reference binary composites. Thus, no interfacial interactions between the fibers and filler particles are considered in this model mechanical properties predictions. However, experimental mechanical properties data of several hybrid composites indicate significant positive deviations from the RHM predictions, which have led several authors to simulate, predict and validate with experimental results the mechanical properties of hybrid polymer composites with fibrous-particulate reinforcement based on several different approaches to the composite reinforcement mechanism [37, 38, 71-73]. One such model used for prediction of elastic modulus of hybrid particle/short fiber-filled polymer composites is based on the “Laminate Analogy Approach” (LAA) model, which considers that interactions between the short fibers and rigid filler particles uniformly and stochastically distributed in the hybrid composite’s matrix will influence the elastic modulus property of its matrix polymer, based on the premise that the rigid particlesfilled binary composite will act as the “pseudo-matrix” for the reinforcing short fibers. Thus, with increasing particulate filler, the expected higher shear modulus of this pseudo-matrix will influence positively on the stress transfer at the polymer-fiber interface and, thereby, contributing towards improved modulus and strength properties of the hybrid composite, as detailed in the published literature [37, 72, 73]. In the present study, the experimental results of tensile elastic modulus and tensile strength properties of the investigated ternary hybrid composites of PP/GF/HGB were used to establish correlations with the corresponding predicted theoretical values based on the RHM model. Any possible positive deviations of the experimental modulus and strength values from the RHM model, were then analyzed considering the previously mentioned LAA prediction model for hybrid polymer composites. The “Rule of Hybrid Mixtures” for the tensile elastic modulus and tensile strength properties is established according to Equations 6 and 7 respectively [72, 73]:

FGH

FGH

FGH

FGH

FGH

FGH

CDE = CIJ . KIJ + CLIM . (1 − KIJ )

(6)

NDE = NIJ . KIJ + NLIM . (1 − KIJ ) (7)

FGH

FGH

Where CDE and NDE are, respectively, the tensile modulus and the tensile strength of the hybrid FGH

FGH

composite at a certain total volume content of hybrid reinforcement (??). CIJ and CLIM are the tensile modulus of the binary composites of PP/GF and PP/HGB (or PP/HGBamino) respectively, at the same

29 total volume content of reinforcement (??). KIJ is the relative volume fraction of GF in the hybrid composites, so that ?? = KIJ + (1 − KIJ ). Similarly, identical indexes in Equation 7 are used for the FGH

FGH

determination of the tensile strength property of the hybrid composite. The CIJ and CLIM values for the FGH

FGH

tensile modulus and the NIJ and NLIM values for the tensile strength values were obtained from the straight lines fit over the experimental data of the reference binary composites of PP/GF and PP/HGB (or PP/HGBamino) respectively, as shown in the graphs of Figures 11 and 13.

Table 6

Table 6 presents the experimental tensile modulus and tensile strength data for all ternary hybrid composites of PP/GF/HGB of this study in comparison to their predicted RHM values based on Equations 6 and 7. Analyzing first the tensile elastic modulus data of these ternary composites, the correlation between the experimental values and the theoretical predictions is expressed in terms of their relative values of (Ec)Exp./(Ec)RHM. Unity values of this relation represents close fit of experimental data to the RHM model, whereas deviations from unity values indicate positive or negative effects on the modulus property. Thus, as the total and relative GF/HGB hybrid reinforcement contents increase in the ternary composites, as shown in Table 6, the relative (Ec)Exp./(Ec)RHM values increase with addition of aminosilane-treated HGB filler. With increasing GF content, this tensile modulus enhancement is more pronounced at low HGB content, as registered in the ternary composites of PP/GF(20)/HGBamino(2) and PP/GF(25)/HGBamino(2 and 3.5) and then tends to reduce with further HGB addition. In these ternary composites, the experimental tensile modulus registers significant synergistic gains of 24-25% over the predicted values based on RHM model. However, at still higher GF content in the hybrid reinforcement, as evidenced for the ternary composites of PP/GF(30)/HGB with both untreated and aminosilane-treated HGB, this synergistic effect is nullified and the experimental values of tensile modulus are essentially determined by the RHM expressed by equation 6, as the relative values of (Ec)Exp./(Ec)RHM are all rigorously close to or equal to unity, as also evidenced in Table 6. This loss in reinforcement efficiency of the hybrid GF/HGB filler, as its total content approaches its limiting maximum volume packing fraction value (Vf,max) as previously shown by the data in Figure 3, is attributed to the previously mentioned lack of additional matrix polymer available to wet all the surface of the hybrid filler components and, consequently, depreciating the elastic modulus property of this set of ternary composites. On the other

30 hand, the synergistic gains in tensile modulus verified in the ternary composites with GF content up to 25%, are considered to be in line with the above mentioned “Laminate Analogy Approach” model (LAA) proposed by S. Y. Fu, Y. W. Mai, et al. for hybrid composites [72]. In this case, at total volume content of the hybrid filler well below its limiting value of Vf,max and at high relative hybrid GF/HGB volume fraction (i.e. at low HGB content), the increase in the elastic shear modulus of the binary pseudo-matrix of the PP/HGB, as stipulated by the LAA model theory, will contribute with higher stress transfer to the reinforcing short glass fibers. This hybrid effect contributes to the verified synergistic increase in the tensile modulus behavior of these ternary composites, over and above the value of the tensile modulus of its equivalent reference binary PP/GF composite of same GF content, as discussed earlier on the tensile modulus results presented in Figure 11. However, further increase in HGB content in these ternary composites, at a given constant GF content (ie. at lower relative hybrid GF/HGB volume fraction), will contribute towards improved hybrid filler’s packing characteristics, as shown in the data discussed earlier in Figure 3 of item 3.2. Thus, this improved hybrid filler’s packing effect, based upon the previously discussed expression (1 – Vf/Vf max)-1 used to characterize the melt flow behavior of highly filled polymer systems, will increase the volume fraction of polymer matrix available to deform in the solid state mechanical properties of the ternary composite and, thereby, contribute to reduced modulus enhancement with further increase in HGB filler in these ternary hybrid composites. Now, analysis of the correlation between the experimental tensile strength values of the ternary hybrid composites of PP/GF/HGB and their predicted values based on the RHM model, as shown in Table 6, indicates that the relative (σc)Exp./(σc)RHM values increase slightly with addition of aminosilanetreated HGB filler, as the total and relative GF/HGB hybrid reinforcement contents increase in the ternary composites. Similarly as observed for the tensile modulus data with increasing GF content, the tensile strength enhancement is also more pronounced at low HGB content, as registered for the same ternary composites of PP/GF(20)/HGBamino(2) and PP/GF(25)/HGBamino(2 and 3.5). In this case, the tensile strength properties exhibit minor synergistic gains of 9-13% over the predicted values based on RHM model. However, at higher GF content, as registered for the ternary composites of PP/GF/HGB with untreated and aminosilane-treated HGB filler, two distinct behaviors can be evidenced: the relative (σc)Exp./(σc)RHM values indicate synergistic gains with addition of aminosilane-treated HGB filler, whereas with untreated HGB filler, the relative (σc)Exp./(σc)RHM values are quite close to unity and are, therefore, determined by the RHM model. Thus, differently from the behavior verified for the elastic modulus

31 correlation, interfacial interactions between the components of the hybrid composite play an important role in defining the tensile strength behavior of the ternary composites of PP/GF(30)/HGB. Summarizing the observations verified with this correlation exercise established between the experimental results and theoretical predictions of the tensile modulus and strength properties of the ternary composites of PP/GF/HGB, it is possible to conclude that interfacial interactions between the components of the hybrid composite play an important role in defining these mechanical properties. In the case of the tensile modulus property, at lower total hybrid reinforcement content (< Vf max) but high relative GF/HGB content, significant synergistic gains in the modulus value were verified, well above their RHM predictions. These modulus gains are defined by the volumetric packing characteristics of the hybrid fibrous-particulate reinforcement. However, at much higher total hybrid reinforcement content, close to the limiting (Vf max) value of the hybrid filler, these synergistic gains are nullified and the tensile modulus is determined by the RHM model, irrespective of the use of untreated or aminosilano-treated HGB filler. This effect arises from the fact that the elastic modulus property, measured at room temperature and very low stress-strain levels, does not depend strongly on the polymer-HGB filler interfacial adhesion due to sufficient residual interfacial thermal stresses for stress transfer from the matrix polymer to the components of the hybrid reinforcement, as previously evidenced by the storage modulus data of DMTA analysis previously discussed in item 3.4. On the other hand, the tensile strength property, measured at much higher stress-strain levels, are strongly dependent on the degree of interfacial adhesion of the matrix polymer with the components of the hybrid reinforcement. Minor synergistic gains in tensile strength values over the RHM predictions were registered at lower total hybrid reinforcement content (< Vf max) with addition of aminosilano-treated HGB filler. On the other hand, as the total hybrid reinforcement content approaches its limiting Vf max value and, consequently, interfacial interactions between the glass fibers and the HGB particles are intensified, use of untreated HGB filler eliminates any gains and the tensile strength property is determined by the RHM predictions. However, increasing content of aminosilano-treated HGB filler in these same ternary composites contributes with gains in the experimental tensile strength property over their RHM predictions, clearly indicating the important role of strong interfacial adhesion between the matrix polymer and the weaker HGB filler particles in order to achieve improved mechanical properties of thermoplastic ternary composites with fibrous-particulate hybrid reinforcements.

32 Finally, the authors understand that lightweight and high mechanical strength PP syntactic foam composites can be adequately formulated by hybridization of glass fibers and hollow glass beads (HGB) and with tailored mechanical properties obtained depending upon the total and relative content of the hybrid GF/HGB reinforcement, based upon a better understanding of the prevailing interfacial interactions between the components of the hybrid composite of PP.

4. Conclusions Binary reference composites of polypropylene/short glass fiber (PP/GF) and of PP/hollow glass beads (PP/HGB) and ternary hybrid composites of PP/GF/HGB with both untreated and aminosilanetreated microspheres were successfully prepared by the direct dilution of previously twin-screw extrusion compounded PP/GF and PP/HGB concentrates during injection molding process. The influence of the interfacial interactions between the components of the hybrid composites was then analyzed as the total and relative contents of the hybrid GF/HGB reinforcement were increased. The mechanical performance of the ternary hybrid composites of PP/GF/HGB with aminosilanetreated HGB filler, as characterized by the short-term tensile tests, indicated that both the tensile modulus and strength properties were significantly improved with respect to the elastic moduli verified in the reference binary composites of PP/GF of equivalent GF content. Significant synergistic gains of up to 25% and 13% were verified in the modulus and strength properties, respectively, when compared with their corresponding theoretical predictions based on the “Rule of Hybrid Mixtures” model (RHM). These tensile properties enhancement was more pronounced in the hybrid composites of PP/GF(20)/HGBamino(2) and PP/GF(25)/HGBamino(2 and 3.5) at low HGB content (ie. higher relative volume fraction of GF/HGB) and at total hybrid filler concentrations below their limiting volumetric packing fraction (Vf max) value. However, at higher total hybrid filler content and close to its (Vf max) value, as observed for the ternary composites of PP/GF(30)/HGB with both untreated and aminosilane-treated HGB filler, the synergistic gain in the tensile modulus is nullified and the modulus is essentially determined by the RHM model. On the other hand, the tensile strength property of this same set of ternary composites indicate minor synergistic gains with increase in aminosilane-treated HGB filler, whereas use of untreated HGB filler results in severe loss in tensile strength with respect to that of its reference binary composite of PP/GF(30). In the latter case, the decrease in the tensile strength property with increasing untreated HGB filler content follows the RHM model.

33 The above mentioned behavior of the tensile modulus and strength properties of the hybrid composites of PP/GF(30)/HGB, with and without aminosilane treated HGB filler, was similarly reproduced by the flexural modulus and flexural strength and Izod impact strength properties of the very same set of hybrid composites also investigated in this study. Thus, at higher total hybrid filler content and close to its (Vf max) value, when interfacial interaction are more intense, the mechanical strength properties are strongly influenced by the degree of interfacial adhesion of the polymer matrix with the components of the hybrid GF/HGB reinforcement. These mechanical strength properties of the ternary composites were corroborated by the SEM analysis of cryofractured composites surfaces, which revealed that untreated HGB particles induce fiberpolymer interfacial decoupling at much lower stress levels under mechanical loading than in the presence of aminosilane-treated HGB particles. Dynamic-mechanical thermal analysis (DMTA) was also used to corroborate the mechanical performance of the analyzed ternary composites. Higher storage modulus (E') and lower mechanical damping (tan δ) values, obtained at temperatures above 60°C when residual interfacial thermal stresses are nullified, were precisely used to evidence the important role played by the interfacial interactions between the components of the hybrid composite in order to attain improved mechanical performance of ternary hybrid PP/GF/HGB composites when completely compatibilized hybrid GF/HGB reinforcement is used.

Acknowledgements The authors gratefully acknowledge the support provided by the companies 3M Brazil Ltda., Braskem S.A., CPIC Ltda., Addivant-SI Group and BASF in providing the raw materials used in this work. This study was partially financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES - Finance Code 001) and is part of the doctoral thesis of the first cited author approved by the Postgraduate Program in Materials Science and Engineering (PPG-CEM) of Universidade Federal de São Carlos, Brazil.

Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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38 Table Captions

Table 1 - Nominal and real GF/HGB content in binary and ternary PP composites, with their respective measured composite density, HGB breakage and average GF length (Lw) data. Table 2 – Filler oil absorption data of five volume ratios of GF/HGB(a) physical mixtures and their respective calculated maximum filler volume packing fractions (Vf,max). Table 3 – Oil absorption results measured on the pyrolysis residues of the molded test specimen of specific composite formulations.

Table 4 - Storage modulus (E′) and tan δ data for neat PP, maleated-PP matrix, reference binary PP/GF(30.0) and ternary PP/GF(30.0)/HGB composites.

Table 5 – Tensile, flexural and impact mechanical properties of all binary and ternary PP composites.

Table 6 – Experimental and predicted (RHM) data for tensile modulus and strength properties of the hybrid PP/GF/HGB composites.

Figure Captions

Figure 1 – Measured and theoretical density values of the binary and ternary hybrid PP composites, as a function of the total volume content of reinforcement.

Figure 2 – Maximum filler volume packing fraction values ( ) as a function of the relative volume content of HGB in the hybrid physical mixture combinations of GF/HGB. (inlaid experimental Vf,max values of GF/HGB filler ash content extracted from (∆) PP/GF(30.0), (O) PP/GF(30.0)/HGBamino(3.5) and (◊) PP/GF(30.0)/HGBamino(7.5) composites shown in Table 3). Figure 3 – Total hybrid GF/HGB filler volume fraction values, as a function of the relative volume content of HGB of all ternary hybrid PP/GF/HGB composites.

Figure 4 - SEM micrographs of the binary composites of (a) PP/HGB(10.0) and (b) PP/HGBamino(10.0), at equivalent HGB volume content (13.6% and 13.7%, respectively).

Figure 5 – SEM micrographs of the ternary hybrid composites: (a) PP/GF(30.0)/HGB(5.0) and (b) PP/GF(30.0)/HGBamino(5.0), at nearly equivalent HGB volume content (19.4% and 19.8%, respectively). Figure 6 - Mean storage modulus (E') and tan δ curves, as a function of temperature, for the reference PP matrix (PPref.) and the modified base polymer PPref.+3.5% PP-g-MAH. Figure 7 - Mean storage modulus (E') curves, as a function of temperature, for the reference binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites.

39 Figure 8 - Mean storage modulus (E') curves, as a function of temperature, for the reference binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites, highlighted from Figure 12 in the temperature range of 60 - 100°C.

Figure 9 - Mean mechanical damping (tan δ) curves, as a function of temperature, for the binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites.

Figure 10 - Mean mechanical damping (tan δ) curves, as a function of temperature, for the binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites, highlighted from Figure 14 in the temperature range of 60 - 100°C.

Figure 11 – Tensile elastic modulus of the binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

Figure 12 – Flexural elastic modulus of specific binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

Figure 13 – Tensile strength data of binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

Figure 14 – Flexural strength data of specific binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

Figure 15 – Notched Izod impact strength of specific binary and hybrid PP composites as a function of the total volume content of reinforcement.

40 Tables Table 1 - Nominal and real GF/HGB content in binary and ternary PP composites, with their respective measured composite density, HGB breakage and average GF length (Lw) data. Formulation code

PP ref. PP/GF(15.0) PP/GF(20.0) PP/GF(25.0) PP/GF(30.0) PP/HGB(4.0) PP/HGB(5.6) PP/HGB(7.2) PP/HGB(10.0) PP/HGBamino(4.0) PP/HGBamino(5.6) PP/HGBamino(7.2) PP/HGBamino(10.0) PP/GF(15.0)/HGBamino(2.0) PP/GF(15.0)/HGBamino(3.5) PP/GF(15.0)/HGBamino(5.0) PP/GF(20.0)/HGBamino(2.0) PP/GF(20.0)/HGBamino(3.5) PP/GF(20.0)/HGBamino(5.0) PP/GF(25.0)/HGBamino(2.0) PP/GF(25.0)/HGBamino(3.5) PP/GF(25.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(2.0) PP/GF(30.0)/HGBamino(3.5) PP/GF(30.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(7.5) PP/GF(30.0)/HGBamino(10.0) PP/GF(30.0)/HGB(2.0) PP/GF(30.0)/HGB(3.5) PP/GF(30.0)/HGB(5.0) PP/GF(30.0)/HGB(7.5)

Nominal content Total Total filler filler (wt%) (vol%) --------15.0 5.9 20.0 8.1 25.0 10.6 30.0 13.2 4.0 5.9 5.6 8.2 7.2 10.5 10.0 14.4 4.0 5.9 5.6 8.2 7.2 10.5 10.0 14.4 17.0 9.1 18.5 11.5 20.0 13.9 22.0 11.5 23.5 13.9 25.0 16.3 27.0 14.0 28.5 16.5 30.0 19.0 32.0 16.7 33.5 19.3 35.0 21.9 37.5 26.0 40.0 30.0 32.0 16.7 33.5 19.3 35.0 21.9 37.5 26.0

Real content Total filler (wt%)(a) ----14.6 ± 0.2 19.9 ± 0.4 25.1 ± 0.2 31.1 ± 0.0 4.6 ± 0.5 6.2 ± 0.3 8.0 ± 0.3 10.5 ± 0.1 4.4 ± 0.1 5.8 ± 0.2 7.6 ± 0.2 10.7 ± 0.1 17.0 ± 0.3 19.3 ± 0.1 20.2 ± 0.3 21.8 ± 0.1 23.8 ± 0.1 25.3 ± 0.4 26.4 ± 0.5 27.8 ± 0.1 30.4 ± 0.3 32.7 ± 0.1 32.9 ± 0.1 35.2 ± 0.0 37.2 ± 0.0 40.1 ± 0.1 31.6 ± 0.1 33.5 ± 0.1 34.8 ± 0.2 35.5 ± 0.1

Total filler (vol%)(b) ----5.6 ± 0.1 7.8 ± 0.2 10.4 ± 0.1 13.6 ± 0.0 6.0 ± 0.7 7.9 ± 0.5 10.2 ± 0.4 13.6 ± 0.1 5.8 ± 0.2 7.6 ± 0.2 9.7 ± 0.3 13.7 ± 0.2 8.2 ± 0.2 10.8 ± 0.0 12.3 ± 0.2 10.1 ± 0.1 12.4 ± 0.0 14.7 ± 0.3 12.1 ± 0.3 14.0 ± 0.0 16.8 ± 0.2 15.5 ± 0.0 17.1 ± 0.0 19.8 ± 0.0 22.9 ± 0.0 26.8 ± 0.0 14.9 ± 0.1 17.3 ± 0.1 19.4 ± 0.2 21.5 ± 0.1

Measured density (g/cm3) 0.906 ± 0.000 1.000 ± 0.001 1.038 ± 0.001 1.087 ± 0.001 1.138 ± 0.001 0.891 ± 0.001 0.886 ± 0.000 0.879 ± 0.001 0.870 ± 0.000 0.890 ± 0.000 0.885 ± 0.001 0.880 ± 0.000 0.869 ± 0.001 0.992 ± 0.000 0.987 ± 0.001 0.974 ± 0.001 1.036 ± 0.000 1.029 ± 0.001 1.012 ± 0.002 1.071 ± 0.000 1.062 ± 0.001 1.059 ± 0.001 1.147 ± 0.001 1.127 ± 0.001 1.124 ± 0.000 1.112 ± 0.001 1.090 ± 0.001 1.144 ± 0.001 1.136 ± 0.001 1.114 ± 0.002 1.098 ± 0.001

HGB breakage (%) --------------------9.5 11.6 11.1 9.4 10.0 10.2 11.6 10.6 8.3 9.3 11.7 10.0 11.2 10.7 10.5 11.7 12.2 8.5 8.7 9.1 10.2 10.3 8.2 9.1 9.7 10.6

GF length – Lw (µm) ----522.8 525.7 520.8 525.5 --------------------------------532.7 529.9 541.1 557.3 521.5 527.7 526.2 536.1 505.1 515.3 510.7 494.4 532.9 529.0 513.0 514.0 514.8 510.9

(a)

Determined through pyrolysis of the molded test specimens; Calculated by discounting the percentage of HGB breakage in the composites; ( ) * Measured densities of the reinforcements as received: GF=2.594, HGB=0.610, HGBamino=0.604 g/cm3. (b)

Table 2 – Filler oil absorption data of five volume ratios of GF/HGB(a) physical mixtures and their respective calculated maximum filler volume packing fractions (Vf,max). Filler Relative GF/HGB ρGF+HGB(a) (g/cm3) O.A. (%) Vf,max GF 100 / 0 2.594 254.2 ± 28.9 0.131 ± 0.013 GF/HGB 75 / 25 2.450 199.9 ± 10.6 0.167 ± 0.007 50 / 50 2.224 143.3 ± 10.1 0.236 ± 0.013 25 / 75 1.787 89.7 ± 21.1 0.386 ± 0.061 HGB 0 / 100 0.634 76.0 ± 30.3 0.679 ± 0.090 (a) Obtained from the pyrolysis residues of concentrates of PP/GF(45%) and PP/HGB(30%).

Table 3 – Oil absorption results measured on the pyrolysis residues of the molded test specimen of specific composite formulations. Formulation Relative GF/HGB ρGF+HGB (g/cm3) O.A.(a) (%) Vf,max PP/GF(30) 100 / 0 2.594 247.6 ± 39.4 0.135 ± 0.017 PP/GF(30)/HGBamino(3.5) 73 / 27 2.098 215.8 ± 16.5 0.179 ± 0.011 1.724 205.3 ± 19.5 0.218 ± 0.016 PP/GF(30)/HGBamino(7.5) 54 / 46 (a) Average value of 5 samples used per composite formulation.

41

Table 4 - Storage modulus (E′) and tan δ data for neat PP, maleated-PP matrix, reference binary PP/GF(30.0) and ternary PP/GF(30.0)/HGB composites. Formulation Code Sample Description PP ref. PP ref. + 3.5% PP-g-MAH PP/GF(30) PP/GF(30)/HGBamino(3.5) PP/GF(30)/HGBamino(7.5) PP/GF(30)/HGB(3.5) PP/GF(30)/HGB(7.5)

Storage modulus – E' (GPa) 23°C 40°C 60°C 80°C 1.4 1.0 0.6 0.4 1.4 1.0 0.6 0.4 5.4 4.8 4.1 3.4 5.3 4.7 4.1 3.6 6.2 5.5 4.8 4.0 5.2 4.6 3.8 3.2 5.1 4.5 3.8 3.2

Tg* (°C) 7.2 7.3 10.9 9.9 9.9 9.4 4.1

@ Tg 0.0877 0.0876 0.0528 0.0535 0.0487 0.0455 0.0508

Tan δ Intensity 40°C 60°C 0.0858 0.1124 0.0880 0.1126 0.0421 0.0507 0.0425 0.0488 0.0395 0.0469 0.0385 0.0501 0.0445 0.0573

80°C 0.1212 0.1212 0.0609 0.0580 0.0570 0.0595 0.0655

*Tg identified from tan δ peak curves of Figures 11 and 14.

Table 5 – Tensile, flexural and impact mechanical properties of all binary and ternary PP composites. Formulation code

PP ref. PP/GF(15.0) PP/GF(20.0) PP/GF(25.0) PP/GF(30.0) PP/HGB(4.0) PP/HGB(5.6) PP/HGB(7.2) PP/HGB(10.0) PP/HGBamino(4.0) PP/HGBamino(5.6) PP/HGBamino(7.2) PP/HGBamino(10.0) PP/GF(15.0)/HGBamino(2.0) PP/GF(15.0)/HGBamino(3.5) PP/GF(15.0)/HGBamino(5.0) PP/GF(20.0)/HGBamino(2.0) PP/GF(20.0)/HGBamino(3.5) PP/GF(20.0)/HGBamino(5.0) PP/GF(25.0)/HGBamino(2.0) PP/GF(25.0)/HGBamino(3.5) PP/GF(25.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(2.0) PP/GF(30.0)/HGBamino(3.5) PP/GF(30.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(7.5) PP/GF(30.0)/HGBamino(10.0) PP/GF(30.0)/HGB(2.0) PP/GF(30.0)/HGB(3.5) PP/GF(30.0)/HGB(5.0) PP/GF(30.0)/HGB(7.5)

Tensile Modulus (GPa) 1.2 ± 0.0 3.7 ± 0.2 4.5 ± 0.2 5.5 ± 0.2 7.1 ± 0.5 2.0 ± 0.4 2.0 ± 0.4 2.1 ± 0.3 2.1 ± 0.1 1.8 ± 0.0 1.9 ± 0.0 2.0 ± 0.1 2.2 ± 0.1 3.8 ± 0.1 4.1 ± 0.1 4.4 ± 0.3 5.6 ± 0.4 5.6 ± 0.2 5.7 ± 0.1 6.6 ± 0.2 6.7 ± 0.3 6.6 ± 0.5 6.6 ± 0.5 6.7 ± 0.5 6.6 ± 0.4 7.0 ± 0.6 7.3 ± 0.2 6.5 ± 0.2 6.6 ± 0.2 6.7 ± 0.2 6.5 ± 0.2

Tensile Strength (MPa) 30.4 ± 0.2 54.6 ± 0.5 63.5 ± 1.7 72.5 ± 1.9 84.3 ± 1.4 26.8 ± 0.3 25.5 ± 0.2 24.4 ± 0.1 21.5 ± 0.7 30.7 ± 0.2 30.7 ± 0.1 30.7 ± 0.2 30.3 ± 0.2 54.3 ± 1.1 55.9 ± 0.7 54.9 ± 0.7 67.0 ± 0.3 64.6 ± 1.0 65.2 ± 0.6 74.0 ± 0.5 74.3 ± 1.5 74.1 ± 1.2 81.7 ± 2.2 80.7 ± 1.4 77.6 ± 2.8 82.1 ± 1.3 82.3 ± 0.7 75.4 ± 1.5 72.7 ± 2.4 71.7 ± 0.4 68.0 ± 1.8

Elongation at break (%) > 300.0 5.2 ± 0.2 4.5 ± 0.3 3.8 ± 0.2 3.2 ± 0.2 > 300.0 > 300.0 > 300.0 81.2 ± 30.0 48.0 ± 3.1 26.3 ± 1.6 17.3 ± 2.0 15.0 ± 0.4 4.8 ± 0.1 4.8 ± 0.1 4.7 ± 0.2 3.9 ± 0.2 3.9 ± 0.2 3.8 ± 0.2 3.7 ± 0.1 3.6 ± 0.1 3.5 ± 0.1 3.2 ± 0.1 3.1 ± 0.2 3.1 ± 0.1 3.0 ± 0.1 3.0 ± 0.1 3.4 ± 0.2 3.2 ± 0.5 3.4 ± 0.1 3.3 ± 0.1

Flexural Modulus (GPa) 1.2 ± 0.0 ------------5.2 ± 0.4 ------------1.9 ± 0.1 ------------2.1 ± 0.2 ------------------------------------5.5 ± 0.3 5.4 ± 0.2 5.4 ± 0.2 5.8 ± 0.1 5.9 ± 0.3 5.0 ± 0.2 4.8 ± 0.2 5.2 ± 0.3 5.1 ± 0.2

Flexural Strength (MPa) 33.7 ± 0.3 ------------125.8 ± 2.1 ------------39.8 ± 0.3 ------------49.2 ± 0.3 ------------------------------------127.8 ± 4.5 124.7 ± 1.8 122.0 ± 4.8 126.3 ± 2.7 124.2 ± 2.5 114.0 ± 1.2 108.5 ± 2.0 107.1 ± 1.9 103.8 ± 2.3

Notched Izod Imp. Strength (J/m) 25.9 ± 2.5 ----63.7 ± 2.9 ----97.7 ± 2.4 ----18.4 ± 1.3 ----16.4 ± 0.5 ----20.3 ± 2.7 ----19.7 ± 1.2 ------------------------------------96.8 ± 2.4 96.3 ± 3.1 96.4 ± 3.6 97.0 ± 3.7 96.4 ± 2.9 89.8 ± 4.8 86.3 ± 2.7 85.4 ± 3.0 83.7 ± 2.6

42 Table 6 – Experimental and predicted (RHM) data for tensile modulus and strength properties of the hybrid PP/GF/HGB composites. Formulation code

PP/GF(15.0)/HGBamino(2.0) PP/GF(15.0)/HGBamino(3.5) PP/GF(15.0)/HGBamino(5.0) PP/GF(20.0)/HGBamino(2.0) PP/GF(20.0)/HGBamino(3.5) PP/GF(20.0)/HGBamino(5.0) PP/GF(25.0)/HGBamino(2.0) PP/GF(25.0)/HGBamino(3.5) PP/GF(25.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(2.0) PP/GF(30.0)/HGBamino(3.5) PP/GF(30.0)/HGBamino(5.0) PP/GF(30.0)/HGBamino(7.5) PP/GF(30.0)/HGBamino(10.0) PP/GF(30.0)/HGB(2.0) PP/GF(30.0)/HGB(3.5) PP/GF(30.0)/HGB(5.0) PP/GF(30.0)/HGB(7.5) (a) (b)

Total filler (vol%)(a) 8.2 ± 0.2 10.8 ± 0.0 12.3 ± 0.2 10.1 ± 0.1 12.4 ± 0.0 14.7 ± 0.3 12.1 ± 0.3 14.0 ± 0.0 16.8 ± 0.2 15.5 ± 0.0 17.1 ± 0.0 19.8 ± 0.0 22.9 ± 0.0 26.8 ± 0.0 14.9 ± 0.1 17.3 ± 0.1 19.4 ± 0.2 21.5 ± 0.1

Relative Filler content (vol%) GF HGB 64 36 50 50 41 59 70 30 57 43 48 52 74 26 62 38 54 46 78 22 67 33 58 42 48 52 41 59 78 22 67 33 59 41 48 52

Exp. 3.8 ± 0.1 4.1 ± 0.1 4.4 ± 0.3 5.6 ± 0.4 5.6 ± 0.2 5.7 ± 0.1 6.6 ± 0.2 6.7 ± 0.3 6.6 ± 0.5 6.6 ± 0.5 6.7 ± 0.5 6.6 ± 0.4 7.0 ± 0.6 7.3 ± 0.2 6.5 ± 0.2 6.6 ± 0.2 6.7 ± 0.2 6.5 ± 0.2

Tensile Modulus (GPa) RHM(b) Exp./ RHM 3.7 1.03 4.0 1.03 3.9 1.13 4.5 1.24 4.7 1.19 4.8 1.19 5.3 1.25 5.4 1.24 5.7 1.16 6.6 1.00 6.5 1.03 6.8 0.97 6.8 1.03 7.1 1.03 6.4 1.02 6.6 1.00 6.7 1.00 6.5 1.00

Tensile Strength (MPa) Exp. RHM(c) Exp./RHM 54.3 ± 1.1 51.9 1.05 55.9 ± 0.7 52.4 1.07 54.9 ± 0.7 50.9 1.08 67.0 ± 0.3 59.1 1.13 64.6 ± 1.0 59.1 1.09 65.2 ± 0.6 59.0 1.11 74.0 ± 0.5 66.8 1.11 74.3 ± 1.5 65.8 1.13 74.1 ± 1.2 66.8 1.11 81.7 ± 2.2 79.1 1.03 80.7 ± 1.4 76.3 1.06 77.6 ± 2.8 76.8 1.01 82.1 ± 1.3 74.8 1.10 82.3 ± 0.7 74.6 1.10 75.4 ± 1.5 75.2 1.00 72.7 ± 2.4 73.4 0.99 71.7 ± 0.4 70.9 1.01 68.0 ± 1.8 65.2 1.04

Calculated by discounting the percentage of HGB breakage in the composites; Tensile Modulus and Tensile Strength(c) values predicted by the Rule of Hybrid Mixtures (RHM).

1

1.2 1.2

Density (g/cm3)

1.1 1.1 PP ref. PP/GF PP/HGB PP/HGB amino PP/GF(30)/HGB PP/GF(15)/HGB amino PP/GF(20)/HGB amino PP/GF(25)/HGB amino PP/GF(30)/HGB amino Theoretical

1.0 1.0 0.9 0.9 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 1 – Measured and theoretical density values of the binary and ternary hybrid PP composites, as a function of the total volume content of reinforcement.

Maximum volume packing fraction

0.90 0.80 y = 0.5927x3 - 0.2112x2 + 0.1667x + 0.1304 R² = 1

0.70 0.60 0.50

PP/GF(30)/HGBamino(7.5) exp.

0.40

PP/GF(30)/HGBamino(3.5) exp.

0.30

PP/GF(30) exp.

0.20 0.10 0.00 0

0.25

0.5

0.75

1

Relative volume content of HGB Figure 2 – Maximum filler volume packing fraction values ( ) as a function of the relative volume content of HGB in the hybrid physical mixture combinations of GF/HGB. (inlaid experimental Vf,max values of GF/HGB filler ash content extracted from (∆) PP/GF(30.0), (O) PP/GF(30.0)/HGBamino(3.5) and (◊) PP/GF(30.0)/HGBamino(7.5) composites shown in Table 3).

2 PP/GF(30)/HGB PP/GF(15)/HGB amino PP/GF(20)/HGB amino PP/GF(25)/HGB amino PP/GF(30)/HGB amino

Total GF/HGB filler volume fraction

0.30

0.25

0.20 Vf,max trendline curve 0.15

0.10

0.05 0

0.1

0.2

0.3

0.4

0.5

0.6

Relative volume content of HGB Figure 3 – Total hybrid GF/HGB filler volume fraction values, as a function of the relative volume content of HGB of all ternary hybrid PP/GF/HGB composites.

Figure 4 - SEM micrographs of the binary composites of (a) PP/HGB(10.0) and (b) PP/HGBamino(10.0), at equivalent HGB volume content (13.6% and 13.7%, respectively).

3

Figure 5 – SEM micrographs of the ternary hybrid composites: (a) PP/GF(30.0)/HGB(5.0) and (b) PP/GF(30.0)/HGBamino(5.0), at nearly equivalent HGB volume content (19.4% and 19.8%, respectively).

(E') PP ref. (E') PP ref. + 3.5 PP-g-MAH (tan δ) PP ref. (tan δ) PP ref. + 3.5 PP-g-MAH

4.0

0.120

3.0

0.100

2.5 0.080 2.0 0.060

Tan delta

Storage Modulus (GPa)

3.5

0.140

1.5 0.040

1.0

0.020

0.5 0.0

0.000 -40 -30 -20 -10

0

10

20

30

40

50

60

70

80

90 100

Temperature (°C) Figure 6 - Mean storage modulus (E') and tan δ curves, as a function of temperature, for the reference PP matrix (PPref.) and the modified base polymer PPref.+3.5% PP-g-MAH.

4

9.0 PP/GF(30) PP/GF(30)/HGB amino(3.5) PP/GF(30)/HGB amino(7.5) PP/GF(30)/HGB(3.5) PP/GF(30)/HGB(7.5)

Storage Modulus (GPa)

8.0 7.0 6.0 5.0 4.0 3.0 2.0 -40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

Temperature (°C) Figure 7 - Mean storage modulus (E') curves, as a function of temperature, for the reference binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites.

5.0 PP/GF(30) PP/GF(30)/HGB amino(3.5) PP/GF(30)/HGB amino(7.5) PP/GF(30)/HGB(3.5) PP/GF(30)/HGB(7.5)

Storage Modulus (GPa)

4.5

4.0

3.5

3.0

2.5

2.0 60

70

80

90

100

Temperature (°C) Figure 8 - Mean storage modulus (E') curves, as a function of temperature, for the reference binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites, highlighted from Figure 12 in the temperature range of 60 - 100°C.

5

0.080 0.070 0.060

Tan Delta

0.050 0.040 PP/GF(30)

0.030

PP/GF(30)/HGB amino(3.5)

0.020

PP/GF(30)/HGB amino(7.5) PP/GF(30)/HGB(3.5)

0.010

PP/GF(30)/HGB(7.5)

0.000 -40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

Temperature (°C) Figure 9 - Mean mechanical damping (tan δ) curves, as a function of temperature, for the binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites.

0.070 0.065

Tan Delta

0.060 0.055 PP/GF(30)

0.050

PP/GF(30)/HGB amino(3.5) PP/GF(30)/HGB amino(7.5) PP/GF(30)/HGB(3.5)

0.045

PP/GF(30)/HGB(7.5)

0.040 60

70

80

90

100

Temperature (°C)

Figure 10 - Mean mechanical damping (tan δ) curves, as a function of temperature, for the binary PP/GF(30.0) and specific ternary PP/GF(30.0)/HGB composites, highlighted from Figure 14 in the temperature range of 60 - 100°C.

6

8.0

Tensile modulus (GPa)

7.0 6.0 y = 0.0608x + 5.5401 5.0 PP ref. PP/GF PP/HGB PP/HGB amino PP/GF(30)/HGB PP/GF(15)/HGB amino PP/GF(20)/HGB amino PP/GF(25)/HGB amino PP/GF(30)/HGB amino

4.0 3.0 2.0 y = 0.0628x + 1.4166

1.0 0.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 11 – Tensile elastic modulus of the binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

7.0

Flexural modulus (GPa)

6.0 5.0 4.0 PP ref.

3.0

PP/GF PP/HGB

2.0

PP/HGB amino PP/GF(30)/HGB

1.0

PP/GF(30)/HGB amino

0.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 12 – Flexural elastic modulus of specific binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

7

90.0 80.0

Tensile strength (MPa)

70.0 60.0 y = -1.067x + 91.467 50.0

PP ref. PP/GF PP/HGB PP/HGB amino PP/GF(30)/HGB PP/GF(15)/HGB amino PP/GF(20)/HGB amino PP/GF(25)/HGB amino PP/GF(30)/HGB amino

40.0 30.0 20.0 10.0

y = -0.6361x + 30.514

0.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 13 – Tensile strength data of binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

140.0

Flexural strength (MPa)

120.0 100.0 80.0 PP ref.

60.0

PP/GF PP/HGB

40.0

PP/HGB amino PP/GF(30)/HGB

20.0

PP/GF(30)/HGB amino

0.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 14 – Flexural strength data of specific binary and ternary hybrid PP composites as a function of the total volume content of reinforcement.

8

Notched Izod impact strength (J/m)

120.0

100.0

80.0

60.0

PP ref. PP/GF

40.0

PP/HGB PP/HGB amino

20.0

PP/GF(30)/HGB PP/GF(30)/HGB amino

0.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

Total volume content of reinforcement (%) Figure 15 – Notched Izod impact strength of specific binary and hybrid PP composites as a function of the total volume content of reinforcement.

Highlights:

- Novel lightweight and high mechanical strength polypropylene (PP) syntactic foam composites were developed; - Tailored mechanical modulus and strength properties were obtained by hybridization of short glass fibers (GF) and hollow glass beads (HGB) which identified synergistic gains over the rule of hybrid mixtures; - Improved mechanical strength performance is attributed to the strong PP matrix-hybrid filler interfacial adhesion, achieved through adequate interfacial compatibilization with maleated-PP compatibilizer; - SEM analysis revealed that untreated HGB particles induced fiber-polymer interfacial decoupling at much lower stress levels under mechanical loading than in the presence of treated HGB particles; - DMTA analyses contributed towards a better understanding of the prevailing interfacial interactions of the polymer matrix with the high and low aspect ratio components of the hybrid reinforcement.