A Study of Physico-Mechanical Properties of Hollow Glass Bubble, Jute Fibre and Rubber Powder Reinforced Polypropylene Compounds with and without MuCell® Technology for Lightweight Applications

Lightweighting is one of the key solutions to reduce the carbon footprint of vehicles. Nowadays, it is still challenging to achieve this target because there is a conflict between the cost and final material performance, as well as the fact that many lightweight solutions are restricted to laboratory or small-scale production. In this work, a commercially feasible strategy was adopted to fabricate materials for lightweight applications. Hollow glass bubbles, jute fibres, and rubber powder were used as fillers with polypropylene as the base polymer. Various samples were fabricated using conventional and MuCell® injection moulding. Their performance was then characterised by their density and morphological, mechanical, and rheological properties. A comparison among hybrid fillers/polypropylene compounds with and without MuCell® technology was investigated. The filler hybridisation resulted in not only a density reduction of up to approximately 10%, but also improved tensile/flexural modulus and strength. The use of MuCell® led to a further reduction in density of roughly 10%. Meanwhile, although some compounds fabricated by MuCell® exhibited some deterioration in their tensile yield strength, tensile modulus, and impact strength, they maintained acceptable mechanical properties for automotive applications.


Introduction
Vehicle lightweighting has become one of the essential tasks in order to reduce fuel consumption, and thus to reduce CO 2 emissions in view of EU legislation establishing a lower CO 2 emission target for vehicles manufactured after 2020 [1,2]. In the automotive industry, injection moulding is a widely employed technology for mass production of car components due to its short cycle time. However, the main limitations of conventional injection moulding are the demanding clamping pressure and the relatively heavy weight exhibited by the resultant plastic components. In order to address this issue, thermoplastic manufacturing processes such as microcellular injection moulding (MuCell ® ) [3] have been developed to produce microcellular foam structures [4]. MuCell ® not only has the advantages of reduced material usage/cost and shorter cycle times, but also allows for more tool design freedom due Conventional injection moulding was carried out on a Boy ® 22A Pro machine (Dr. Boy GmbH & Co. KG, Neustadt, Germany). The injection speed was 55 mm/s. MuCell ® injection moulding was performed on a 300-ton Engel Victory machine (ENGEL Austria GmbH, Schwertberg, Austria) equipped with MuCell ® technology (Trexel, Inc., Woburn, MA, USA). This machine had a screw diameter of 60 mm. The injection speed was 250 mm/s and the supercritical fluid nitrogen (SCF-N 2 ) content was 0.7 wt%. This content is recommended in [28], where a maximum cell density was obtained at an SCF-N 2 level of 0.7 wt% for a carbon fibre/polypropylene composite foam.

Morphology and Filler Dispersion
Scanning electron microscopy was carried out using a FEI Quanta 200 (Thermo Fisher Scientific Inc., Waltham, MA, USA). The samples were immersed in liquid nitrogen for five minutes prior to fracture, and then were coated with 6 nm Au/Pt (40 wt%/60 wt%) using a Quorum Q150T ES sputter coater (Quorum, Laughton, UK) before imaging. Both secondary electron (SE) and backscattered electron (BSE) signals were used to determine surface morphology and filler phase distribution [29].

Density Measurement
Density measurements were carried out using a Weightron Bilanciai Sartorius machine (Weightron Bilanciai Ltd, Chesterfield, UK), which utilises Archimedes' principle. Plastic pellets were weighed both in air (m air ) and water (m water ) and the density of the test sample (ρ sample ) was calculated as follows [6]: ρ sample = ρ water m air m air − m water (1) where ρ water is the density of water at 23 • C and atmospheric pressure. For each of the mechanical tests mentioned above, a minimum of five specimens per sample were used.

Rheology
The rotational rheological analysis was performed on an AR-G2 rheometer (TA Instrument, New Castle, DE, USA). The lower plate was fixed on the machine, whilst the upper parallel plate was rotating. The distance between the plates was set as 1500 µm and the temperature as 200 • C (equivalent to the injection temperature). The test samples were 25 mm in diameter with a thickness of 1.54 mm and three tests were conducted for each material. The viscosity was recorded for increasing values of shear rate up to 50 s-1 ; beyond this value the results were not reliable due to the effect of the centrifugal force on the measuring system.

Melt Flow Index (MFI) Measurement
Melt flow index tests, which are often used for controlling the fluidity of the melt [30], were carried out on a ZwickRoell Mflow machine (ZwickRoell, Leominster, UK) according to ISO 1133. A normal load of 2.16 kg was applied onto the molten polymer in a heated cylinder at a constant temperature of 230 • C.

Interfacial Morphology and Filler Dispersion of Materials
All of the materials listed in Table 2 were fabricated using both conventional and MuCell ® injection moulding. However, there were processing difficulties (short shot issues and occasional injection sprue blockage issues) when using MuCell ® to fabricate GB3 and JUTE3, which contained the largest percentage of hollow glass bubbles and jute fibres. Therefore, GB3 and JUTE3 do not have MuCell ® counterparts. The middle column in Figure 1 shows the filler phase dispersion within the matrix using the BSE signal, while those in the left and right columns reveal the surface morphology using the SE signal. It can be seen that the HGBs are evenly dispersed in the matrix without any visible agglomeration. The inserts for GB1 and GB2 show a good interface between the HGBs and polypropylene matrix. In the case of jute compounds, the fibres were well distributed in the matrix for JUTE1 whilst, in the case of JUTE2, some jute fibre bundles were observed. In terms of foamability in the MuCell ® process, PP Current has the best foamability among all of the compounds, indicated by more homogeneous cellular structures with an average pore diameter of 250 µm. In contrast, pores appear to be more elliptical in the compounded materials. Although previous research has shown that the presence of a small amount of fillers facilitates cell nucleation [13], the presence of high amounts of HGBs and jute fibres could interfere with cell growth, leading to cell collapse and coalescence. The reduced foamability is associated with their weak melt strength during foaming, where the cell walls are not able to retain the gas during the foaming/expansion process [31,32]. In addition, in the Polymers 2020, 12, 2664 5 of 13 case of HGB compounds, the glass bubbles act as a lubricant, leading to easier sliding between polymer chains under extensional deformation and, hence, a lower melt strength [33].
Polymers 2020, 12, x FOR PEER REVIEW 3 of 4 addition, in the case of HGB compounds, the glass bubbles act as a lubricant, leading to easier sliding between polymer chains under extensional deformation and, hence, a lower melt strength [33].   Figure 2 presents the normalised density values of various materials using conventional and MuCell ® injection moulding, and Table 3 summarises the normalised density changes as a result of a change in formulation and switching from conventional to MuCell ® injection moulding. Normalised density was calculated as the density of a compound divided by the density of PP Neat prepared by conventional injection moulding. The addition of jute fibres led to a slight increase in density. However, the increase was approximately 11% lower than that for materials reinforced with the same amount of glass fibres reported in the literature, which can be attributed to the difference in densities between jute and glass fibres [34]. Compared to PP Neat, the introduction of HGBs had a significant impact on the density, e.g., there was a 9.6% reduction in the case of GB3. Since GB3 also consists of a certain amount of jute for toughness and damage tolerance considerations, the reduction in density in response to compounding PP with only HGBs would be even greater. Figure 2 presents the normalised density values of various materials using conventional and MuCell ® injection moulding, and Table 3 summarises the normalised density changes as a result of a change in formulation and switching from conventional to MuCell ® injection moulding. Normalised density was calculated as the density of a compound divided by the density of PP Neat prepared by conventional injection moulding. The addition of jute fibres led to a slight increase in density. However, the increase was approximately 11% lower than that for materials reinforced with the same amount of glass fibres reported in the literature, which can be attributed to the difference in densities between jute and glass fibres [34]. Compared to PP Neat, the introduction of HGBs had a significant impact on the density, e.g., there was a 9.6% reduction in the case of GB3. Since GB3 also consists of a certain amount of jute for toughness and damage tolerance considerations, the reduction in density in response to compounding PP with only HGBs would be even greater.

Density
Using MuCell ® , the bulk density of the materials was further reduced by approximately 9%-10% when compared to conventional injection moulding, with the exception of GB2, where the density decreased by only 5%. The level of reduction in bulk density was related to the porosity of the materials, which depends on their foamability. As discussed in Section 3.1, due to the large amount of fillers GB2 possesses (25wt% in total), these fillers interfere with cell growth, leading to cell collapse and coalescence. The presence of a large amount of HGBs acts as a lubricant, promoting the sliding of polymer chains and leading to reduced melt strength of GB2, resulting in reduced foamability and porosity.
Using MuCell ® , the bulk density of the materials was further reduced by approximately 9-10% when compared to conventional injection moulding, with the exception of GB2, where the density decreased by only 5%. The level of reduction in bulk density was related to the porosity of the materials, which depends on their foamability. As discussed in Section 3.1, due to the large amount of fillers GB2 possesses (25wt% in total), these fillers interfere with cell growth, leading to cell collapse and coalescence. The presence of a large amount of HGBs acts as a lubricant, promoting the sliding of polymer chains and leading to reduced melt strength of GB2, resulting in reduced foamability and porosity. Figure 3 shows the effect of shear rate on the shear viscosity at a temperature of 200 • C for various compounds. It can be seen that a higher loading of fillers increases the viscosity of both jute and HGB compounds in the low shear rate regime, a trend that is similar to that expressed by the Maron-Pierce relationship [35]. This is also reflected in the MFI values of the materials, as shown in Figure 4. With increasing shear rate up to 50 s −1 , the viscosity of filled polymers drops dramatically as the mechanism of polymer chain orientation and disentanglement prevails at a higher shear rate [36]. Moreover, highly filled compounds (JUTE2 and JUTE3) experience an earlier drop in viscosity, probably due to the breakdown of networks formed by the high aspect ratio of fibrous fillers [37]. Note: (−) and (+) denote a reduction and increase in density values, respectively; bulk density loss = (density using conventional process -density using MuCell ® process) / density using conventional process × 100%. Figure 3 shows the effect of shear rate on the shear viscosity at a temperature of 200 °C for various compounds. It can be seen that a higher loading of fillers increases the viscosity of both jute and HGB compounds in the low shear rate regime, a trend that is similar to that expressed by the Maron-Pierce relationship [35]. This is also reflected in the MFI values of the materials, as shown in Figure 4. With increasing shear rate up to 50 s −1 , the viscosity of filled polymers drops dramatically as the mechanism of polymer chain orientation and disentanglement prevails at a higher shear rate [36]. Moreover, highly filled compounds (JUTE2 and JUTE3) experience an earlier drop in viscosity, probably due to the breakdown of networks formed by the high aspect ratio of fibrous fillers [37].

Mechanical Properties
To understand the reinforcing effect of hybrid fillers, the tensile, flexural, and impact properties were evaluated, and the results are summarised in Figures 5-7. All of the data were normalised with Note: (−) and (+) denote a reduction and increase in density values, respectively; bulk density loss = (density using conventional process -density using MuCell ® process) / density using conventional process × 100%. Figure 3 shows the effect of shear rate on the shear viscosity at a temperature of 200 °C for various compounds. It can be seen that a higher loading of fillers increases the viscosity of both jute and HGB compounds in the low shear rate regime, a trend that is similar to that expressed by the Maron-Pierce relationship [35]. This is also reflected in the MFI values of the materials, as shown in Figure 4. With increasing shear rate up to 50 s −1 , the viscosity of filled polymers drops dramatically as the mechanism of polymer chain orientation and disentanglement prevails at a higher shear rate [36]. Moreover, highly filled compounds (JUTE2 and JUTE3) experience an earlier drop in viscosity, probably due to the breakdown of networks formed by the high aspect ratio of fibrous fillers [37].

Mechanical Properties
To understand the reinforcing effect of hybrid fillers, the tensile, flexural, and impact properties were evaluated, and the results are summarised in Figures 5-7. All of the data were normalised with

Mechanical Properties
To understand the reinforcing effect of hybrid fillers, the tensile, flexural, and impact properties were evaluated, and the results are summarised in Figures 5-7. All of the data were normalised with respect to the corresponding data obtained from conventional moulded PP Neat, which is shown in Table S1 (see Supplementary Materials). When using conventional injection moulding, comparing the jute compounds with PP Neat, Figure 5a,b show that both the tensile yield strength and tensile modulus increase with an increasing amount of jute fibres, thus confirming the reinforcing effect of jute. This can be explained by the fact that the external load was effectively transferred from the PP matrix to the jute fibres [38]. Comparing JUTE1 with GB1, there is a slight reduction in the yield strength with the addition of HGB. Among the HGB compounds, the tensile modulus increases with an increase in HGB content whilst the tensile yield strength experiences a slight decrease. Nevertheless, the yield strength of the HGB compounds is still greater than that of PP Neat, as shown in Figure 5a,b. The incorporation of glass bubbles lowers the yield strength (weakening effect), but it improves the modulus of the material (stiffening effect). By making use of the hybridisation effect of jute and HGB fillers, a relatively strong material can be obtained without compromising the material's density. At the same time, the yield strain and breaking strain are gradually reduced with increasing filler contents for both jute and HGB compounds, see Figure 5c,d. This can be attributed to the reduced polymer chain mobility in the presence of fillers [39].
The MuCell ® moulded materials experienced a reduction in their tensile strength (25-42%), tensile modulus (20-56%), yield strain (2-10%) and breaking strain (2-9%) when compared to their conventional counterparts. This reduction is due to the non-uniform porous structure of the materials, as revealed by the SEM images (Figure 1), especially in highly filled polymer compounds. Because of the existence of fillers and the weak melt strength of materials, the resulting porous structures are less uniform. In spite of these reductions, it is pertinent to note that a lower value in certain properties does not render it unsuitable. These reductions are acceptable for an application as long as its strength and modulus properties are comparable with those of the MuCell ® counterpart of PP Current, the benchmark material.
Polymers 2020, 12, x FOR PEER REVIEW 3 of 4 respect to the corresponding data obtained from conventional moulded PP Neat, which is shown in Table S1 (see Supplementary Materials). When using conventional injection moulding, comparing the jute compounds with PP Neat, Figure 5a,b show that both the tensile yield strength and tensile modulus increase with an increasing amount of jute fibres, thus confirming the reinforcing effect of jute. This can be explained by the fact that the external load was effectively transferred from the PP matrix to the jute fibres [38]. Comparing JUTE1 with GB1, there is a slight reduction in the yield strength with the addition of HGB. Among the HGB compounds, the tensile modulus increases with an increase in HGB content whilst the tensile yield strength experiences a slight decrease. Nevertheless, the yield strength of the HGB compounds is still greater than that of PP Neat, as shown in Figure 5a,b. The incorporation of glass bubbles lowers the yield strength (weakening effect), but it improves the modulus of the material (stiffening effect). By making use of the hybridisation effect of jute and HGB fillers, a relatively strong material can be obtained without compromising the material's density. At the same time, the yield strain and breaking strain are gradually reduced with increasing filler contents for both jute and HGB compounds, see Figure 5c,d. This can be attributed to the reduced polymer chain mobility in the presence of fillers [39]. The MuCell ® moulded materials experienced a reduction in their tensile strength (25-42%), tensile modulus (20-56%), yield strain (2-10%) and breaking strain (2-9%) when compared to their conventional counterparts. This reduction is due to the non-uniform porous structure of the materials, as revealed by the SEM images (Figure 1), especially in highly filled polymer compounds. Because of the existence of fillers and the weak melt strength of materials, the resulting porous structures are less uniform. In spite of these reductions, it is pertinent to note that a lower value in certain properties does not render it unsuitable. These reductions are acceptable for an application as long as its strength and modulus properties are comparable with those of the MuCell ® counterpart of PP Current, the benchmark material.  impossible. In contrast, the flexural strength and modulus of the MuCell ® samples did not see a significant deterioration in spite of the porosity introduced in the materials, and in certain cases (for example, in the case of the flexural modulus of GB2), there was even a slight improvement. This is due to the fact that the bending strength and modulus are strongly influenced by the compact skin layer (the thickness of the skin layer is around 0.4 mm) where the maximum stress occurs [40]. Fillers are pushed towards the skin layer in the case of MuCell ® samples during foaming.  Figure 7 shows the normalised impact strength of various materials. Although previous research shows that adding short fibres into a matrix material tends to improve its toughness [20], this research indicates that, when comparing jute compounds with PP Neat using conventional injection The flexural properties for conventional moulded samples (black column) show a similar trend in the tensile properties, that is, a reduction in strain at flexural strength and an increase in modulus with increasing filler loading ( Figure 6). The normalised breaking strain information is not shown as PP Neat did not fracture during the experiment, making normalisation of this series of data impossible. In contrast, the flexural strength and modulus of the MuCell ® samples did not see a significant deterioration in spite of the porosity introduced in the materials, and in certain cases (for example, in the case of the flexural modulus of GB2), there was even a slight improvement. This is due to the fact that the bending strength and modulus are strongly influenced by the compact skin layer (the thickness of the skin layer is around 0.4 mm) where the maximum stress occurs [40]. Fillers are pushed towards the skin layer in the case of MuCell ® samples during foaming.
incorporation of jute had a significant negative effect on the impact strength of the compound, which was worsened by an increase in the amount of jute. The impact strength was further worsened by adding HGBs, and the extent of this reduction was proportional to the amount of HGBs added to the compound. Compared to conventional moulding, the MuCell ® counterparts exhibited a lower impact strength, reduced by 4%-8%, which can be attributed to the presence of a non-uniform cell structure, as shown in Figure 1.

Conclusions
In this study, both hybrid fillers and MuCell ® technology were utilised for the fabrication of lightweight materials. A comparison between materials fabricated using conventional and MuCell ® injection moulding was made, and the following conclusions can be drawn: • The combination of filler hybridisation and MuCell ® technology can lead to a total weight reduction of up to 20% when compared to the current plastic materials available on the market. For instance, replacing PP Current with GB3 using conventional moulding or with GB2 using MuCell ® moulding can lead to a weight reduction of 18.7% and 16.4%, respectively.

•
Owing to the incorporation of high-stiffness glass bubbles and reinforcing jute fibres, the newly developed PP compounds manufactured by conventional moulding exhibited increased tensile and flexural modulus and strength compared to PP Current, which is currently used for automotive components. • Materials fabricated by MuCell ® exhibited some deterioration in their tensile modulus, tensile breaking strain, and impact strength, but they still possess acceptable mechanical properties for automotive applications.

•
Investigating the influence of shear rate on the viscosity of compounded materials revealed that highly filled compounds (JUTE2 and JUTE3) experience an earlier drop in viscosity with increasing shear rate, probably due to the breakdown of the networks formed by the high aspect ratio fibrous fillers.  Figure 7 shows the normalised impact strength of various materials. Although previous research shows that adding short fibres into a matrix material tends to improve its toughness [20], this research indicates that, when comparing jute compounds with PP Neat using conventional injection moulding, the addition of jute fibres reduces the impact strength. This may be caused by the presence of fibre bundles leading to premature failure. Research has shown that micro rubber powders tend to improve the toughness of materials [25,26], but in this study, it was observed that the addition of micro rubber powders did not counteract the detrimental effect caused by the jute fibres. This effect might be compensated by incorporating a sufficient amount of rubber powder in a future study. The incorporation of jute had a significant negative effect on the impact strength of the compound, which was worsened by an increase in the amount of jute. The impact strength was further worsened by adding HGBs, and the extent of this reduction was proportional to the amount of HGBs added to the compound. Compared to conventional moulding, the MuCell ® counterparts exhibited a lower impact strength, reduced by 4-8%, which can be attributed to the presence of a non-uniform cell structure, as shown in Figure 1.

Conclusions
In this study, both hybrid fillers and MuCell ® technology were utilised for the fabrication of lightweight materials. A comparison between materials fabricated using conventional and MuCell ® injection moulding was made, and the following conclusions can be drawn:

•
The combination of filler hybridisation and MuCell ® technology can lead to a total weight reduction of up to 20% when compared to the current plastic materials available on the market. For instance, replacing PP Current with GB3 using conventional moulding or with GB2 using MuCell ® moulding can lead to a weight reduction of 18.7% and 16.4%, respectively.

•
Owing to the incorporation of high-stiffness glass bubbles and reinforcing jute fibres, the newly developed PP compounds manufactured by conventional moulding exhibited increased tensile and flexural modulus and strength compared to PP Current, which is currently used for automotive components.

•
Materials fabricated by MuCell ® exhibited some deterioration in their tensile modulus, tensile breaking strain, and impact strength, but they still possess acceptable mechanical properties for automotive applications.

•
Investigating the influence of shear rate on the viscosity of compounded materials revealed that highly filled compounds (JUTE2 and JUTE3) experience an earlier drop in viscosity with increasing shear rate, probably due to the breakdown of the networks formed by the high aspect ratio fibrous fillers.
As a result of the combination of reduced weight and acceptable mechanical properties, the newly developed materials are promising candidates for the large-scale industrial manufacturing of lightweight components.