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Article

Physical Foam Injection Molding of Cellulose Fiber Reinforced Polypropylene by Using CO2: Parameter Variation and Comparison to Chemical Foam Injection Molding

by
Claudia Pretschuh
1,
Matthias Mihalic
1,
Christian Sponner
2,
Thomas Lummerstorfer
3,
Andreas Steurer
2 and
Christoph Unterweger
1,*
1
Wood K Plus—Kompetenzzentrum Holz GmbH, Altenberger Strasse 69, 4040 Linz, Austria
2
ENGEL Austria GmbH, Ludwig-Engel-Strasse 1, 4311 Schwertberg, Austria
3
Borealis Polyolefine GmbH, St. Peter Strasse 25, 4021 Linz, Austria
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(1), 50; https://doi.org/10.3390/jcs9010050
Submission received: 29 October 2024 / Revised: 18 December 2024 / Accepted: 9 January 2025 / Published: 20 January 2025
(This article belongs to the Special Issue Feature Papers in Journal of Composites Science in 2024)
Browse Figures
Figure 1
<p>Research design and aim of the present study.</p> "> Figure 2
<p>Reduction in weight of several PP–cellulose plates prepared with MuCell<sup>®</sup> FIM with CO<sub>2</sub> and N<sub>2</sub>, and with chemical FIM for the complete plates (y-axis) and for the middle part of the plate (x-axis). The line represents equal values for weight reduction in the middle and complete plates.</p> "> Figure 3
<p>Flexural modulus (<b>a</b>) and flexural strength (<b>b</b>) of the PP–cellulose plates prepared using FIM.</p> "> Figure 4
<p>Specific flexural modulus (<b>a</b>) and specific flexural strength (<b>b</b>) of the PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent the 95% confidence interval.</p> "> Figure 5
<p>Charpy impact strength, notched (<b>a</b>) and unnotched (<b>b</b>) of PP–cellulose plates, prepared using FIM.</p> "> Figure 6
<p>Charpy impact strength, specific, notched (<b>a</b>) and unnotched (<b>b</b>), of PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent a 95% confidence interval.</p> "> Figure 7
<p>Microscopy images of foamed PP–cellulose. MuCell<sup>®</sup> with CO<sub>2</sub>: Sample 1 (<b>a</b>), Sample 5 (<b>b</b>), Sample 6 (<b>c</b>) and Sample 7 (<b>d</b>); MuCell<sup>®</sup> with N<sub>2</sub>: Sample 10 (<b>e</b>) and Sample 16, derived using chemical FIM (<b>f</b>).</p> ">
Versions Notes

Abstract

:
The use of cellulose fiber-filled polypropylene (PP) composites in combination with foam injection molding has enabled the lightweight design of injection-molded parts. The study provides achievements for the physical foam injection molding (MuCell®) process of PP–cellulose fiber compounds by using CO2 as the direct foaming agent, including a comparison of MuCell® foaming with N2 and a comparison to a chemical foaming process. Weight and density reductions, foam structure and specific mechanical properties are highly dependent on the applied processing parameters. The maximum weight reduction reached values of up to 16%, and density reduction even reached 33% in relation to the compact plates. The extent of weight and density reduction could be adjusted, among other factors, by a reduction in the shot volume. Setting the density reduction to 22% allowed for simultaneously decreasing weight while sustaining the specific flexural properties and limiting the loss of specific impact strength. By using optimized FIM parameters, the mechanical performance could be improved, with specific modulus values even outperforming the compact reference sample. This presents a significant benefit for the preparation of lightweight products and sets the basis for further optimization and modeling studies.

1. Introduction

Lightweight design is a key focus in the automotive industry’s efforts to improve vehicle efficiency; the energy required to move a vehicle is directly proportional to its mass, and thus a reduction in weight results directly in a reduction in fuel consumption [1]. In order to promote weight reduction in polymer composites, it stands to reason to use lightweight fillers in the material. Natural or cellulose fibers as fillers support lightweight design via a reduced composite density compared to glass fiber or talcum-filled PP composites [2]. The selection of different available lightweight and natural filler types—fibrous cellulose powder, fine wood powder, larger wood particles and other natural fibers—helps to adjust and optimize the composite properties, and further combinations of different fillers can be used [3]. Furthermore, natural and cellulose fibers as fillers in polypropylene (PP) composites are well-known for a potential reduction in primary energy use and greenhouse gas emissions; the corresponding life cycle assessment (LCA) literature was reviewed by Civancik-Uslu et al. [4], while LCA of Kraft pulp fiber composites was carried out by Hesser [5], and LCA for man-made cellulose fibers is described by Shen and Patel [6].
Besides the use of lightweight materials, the weight of polymer composite parts can effectively be reduced by microcellular foam injection molding (FIM). In 2023, Feng et al. [7] investigated the resource and environmental impacts of plant-fiber-reinforced PP automotive components and microcellular foam molding processes using supercritical N2. As one conclusion, widespread use of FIM should be promoted in the automotive sector as an effective approach for lightweight vehicles and green development.
FIM can be applied for a reduction in weight and thereby for the development of injection-molded ultra-lightweight thermoplastic parts. It offers a number of advantages, such as low material consumption, shorter cycle time and lower injection pressures [8], as well as lower clamping forces [9].
Two injection molding methods are known to produce ultra-lightweight plastics: the application of a chemical foaming agent (e.g., sodium hydrogencarbonate; “chemical FIM”) on the one hand, and direct gas injection (“physical FIM”) on the other hand. Such lightweight product design positively impacts CO2 footprints through saving materials, as well as through CO2 reductions during transport to customers and final use [10]. Regarding physical foaming, in most cases a supercritical fluid—usually N2 or CO2—is injected into the plasticization zone of the injection molding machine; the MuCell® process is the most widespread example using this technology and has become an industrial standard method [11]. In addition to physical foaming by supercritical gas injection, several foaming methods using low-pressure gas injection have been proposed in recent years, such as RIC-FIM [12,13,14] or Ku-Fizz® [1], which are claimed to be competitive with supercritical gas injection methods while at the same time saving costs and improving safety due to the lack of high-pressure equipment.
Fine fillers can improve the foam cell nucleation of a polyolefin in such a foam injection molding process due to heterogeneous nucleation at the interfaces between solid filler and the polymer melt [15]; some recent examples include studies on the effect of mineral nanofillers such as CaCO3 [16,17,18,19], montmorillonite (MMT) [18,19], talc nanoparticles [18] and carbon nanotubes (CNT) [20], as well as PTFE nanofibers [20,21,22]. Moreover, the presence of a dispersed polymer phase can have the same effect as a solid filler, with the polymer interfaces acting as heterogeneous cell nucleation sites [12,16,21,23]. Gosselin et al. [24] used chemical foaming with azodicarbonamide for birchwood fiber composites. They showed that foam cell size distributions and the thickness of the unfoamed skin are a function of the wood content. However, the specific complex moduli in flexion and torsion were reduced by foaming. Using the same chemical foaming agent, Cai et al. [17] found that, with increasing CaCO3 content in ternary PP/rice-husk fiber/CaCO3 nanoparticle composites, initially the average cell diameter and cell aspect ratio decreased, whereas the cell density and cell orientation increased, followed by the opposite trends at CaCO3 contents higher than 2 wt%. Faruk et al. [15] concluded that the specific tensile strengths of hardwood fiber–PP microcellular composites show only small differences between different types of chemical foaming agents. Furthermore, the type of PP and a variation of injection parameters on mold temperature, front flow speed and filling quantity, as well as the wood fiber type and length, had a great influence on the final properties of such microcellular wood fiber–PP composites. Finer wood fibers were correlated with a finer microcellular structure. This indicates the suitability of very fine cellulose fibers for foaming applications.
In general, there are only a few studies known on the foam injection molding of wood-fiber thermoplastic composites using physical blowing agents; many of the studies were carried out in earlier years. Wood-fiber high-density polyethylene (HDPE) composite foams were derived via N2 as the physical blowing agent in a MuCell® system [25]. The study suggested that wood–polymer composite foams exhibiting a 20% weight reduction are promising materials in terms of their physical properties. Kuboki [26] also used N2 as a physical blowing agent and investigated the effects of cellulose content on injection-molded PP composite foams. The void fraction was defined by the shot size. In this study, the specific flexural properties could be maintained in the foamed parts. Notched Izod impact strength was not decreased at void fractions below 15%.
Mihalic et al. [27] used the core-back method (mold opening) on PP–cellulose fiber and PP–wood compounds. This process enabled an almost linear variation of density and, furthermore, a roughly linear correlation between the tensile or flexural properties and the density. The results demonstrated the considerable weight-saving potential of FIM. In this study, the chemical FIM process resulted in improved mechanical properties compared to the MuCell® process. However, different machinery sizes and molds were applied for the different foaming techniques. In a recent study by Zhao et al. [28], physical FIM of PP–tea stem fiber composites showed that, with filler contents increasing up to 30 wt%, the cell structure and mechanical properties were improved, whereas a further increase in the filler content led to the formation of agglomerations that had detrimental effects.
Mold opening FIM was also used for the preparation of PP–glass fiber (GF) composite foams, as studied by Yang et al. [29] via N2 dosage and under various mold-opening distances. The addition of GF led to the improved foaming behavior of PP and a rather uniform cellular structure. Li et al. [22] investigated the properties of PP/PTFE nanofiber composites foamed by the MuCell® process with mold opening and concluded that larger mold opening distances resulted in larger average foam cell size, lower cell density, larger cell aspect ratio and higher orientation, and that the highest impact and tensile strengths were achieved at the smallest mold opening gaps. Mendoza-Cedeno et al. [30] investigated the influence of the mold opening gap on the behavior of different PP grades in chemical foaming, showing that low-expansion applications favored polymers with low molar mass and high-expansion applications favored polymers with high molar mass and broad molar mass distribution.
By using the core-back foam injection molding technique, PP–cellulose nanofiber composite foams were also achieved. N2 was applied at the small level of 0.2% [31]. It was found that the used cellulose nanofibers were well dispersed and aligned along the cell wall in the core-back direction. Influences of cellulose fiber addition on the crystallization behavior were presented by Amash and Zugenmaier [32], who showed an increase in the crystallization temperature and crystallinity of PP attributed to the nucleation effects. Borja et al. [33] determined the crystallinity of lyocell fiber-reinforced PP: cellulose I promoted the crystallization; the use of a coupling agent reduced this effect in cellulose I but induced crystallization when cellulose II (lyocell) was used.
Li et al. [34] studied the effects of different gas loading in the MuCell® FIM of PP with 0.2 wt% of decorative Al flakes. They found that, with N2 content increasing up to 0.6 wt%, the foam cell density increased and the tensile and flexural properties improved (to such an extent that the flexural modulus and strength even exceeded the respective values of the solid material), but deteriorated again at gas loadings higher than 0.6%. The average cell diameter increased steadily with increasing N2 content.
Effects of different chemical foaming agents on composite properties were studied previously [35]. In this earlier study, the highest density reduction was achieved by using sodium bicarbonate. This type of foaming agent also resulted in the highest specific notched impact strength; however, it reduced the flexural strength.
In long-chain branching PP, well-dispersed cellulose nanofibers were also used to prepare composite foams by short-shot FIM [36]. Broad molecular weight distribution and long-chain branched PP were studied without fillers only for mold-opening FIM by utilizing chemical blowing agents [37]. This PP type showed increased flexural modulus at the cost of decreased toughness. However, its high molecular weight contributed to improved foam morphologies. Wu et al. [38] prepared injection-molded β-PP foams with high ductility and improved cell structure. Inferior mechanical performances occurring after using FIM technology remain a challenge for successfully preparing lightweight plastic foams.
Using CO2 in a supercritical condition in the injection molding of pure PP has been studied previously. The crystallization behavior of PP has been investigated [39]: the presence of supercritical CO2 affected the size of α and β crystals. Investigations using a commercial PP copolymer in combination with CO2 and backpressure optimization [40] yielded an optimized backpressure of 120 bar at a gas concentration of 3.5%.
It has been demonstrated previously that natural fiber-reinforced PP composites can be competitive with their mineral-filled counterparts [41]. When considering biobased fillers, man-made cellulose fibers offer the advantage of being available in constant quality and thus allow for process development unaffected by fluctuations in raw material properties. While the literature on the properties and processing of cellulose-reinforced PP is plentiful, to the authors’ awareness no study has been published yet that focusses on physical foaming of this material type using CO2 as the blowing agent, and the influence of different processing conditions and achievable properties in comparison with the more widely used N2-based physical FIM are not very well known yet. Therefore, in the present work, for the first time, a comparison between physical FIM (MuCell®), under varied gas types, and chemical FIM was carried out on one PP–cellulose fiber compound type. By using the same machinery and the same screw and mold type during all three tested foaming processes, influences on the residence and cooling time could be prevented, thus presenting comparable results for different FIM techniques on PP–cellulose fiber compounds for the first time. Injection molding parameters have been varied to study their impact on the density and weight reduction potential of the PP–cellulose compound. Gas type and gas concentration, injection volume and injection speed have been varied, and mold opening was applied. Their impacts on the local foam structure, on the achievable density and overall weight reductions and on the mechanical properties of foamed injection-molded plates are discussed.

2. Materials and Methods

2.1. PP–Cellulose Compound

Fibrous cellulose powder (FCP) with an average diameter of 10 µm and an average length of 300 µm (Lenzing AG, Lenzing, Austria) was used as a lightweight and reinforcing filler for PP. FCP was used in an amount of 20 wt%, which corresponds to 13.2 vol%. This filler content was chosen, as higher amounts can lead to high viscosity of the melt, resulting in reduced flow properties during the injection molding process. Lower amounts are often not so suitable for reasonable improvements in the mechanical performance.
PP BJ400HP (Borealis Polyolefine GmbH, Linz, Austria), a special low-viscosity type, with a density of 908 kg/m3 and a melt flow rate (230 °C/2.16 kg) of 100 g/10 min, was used as the matrix polymer. As the coupling agent, 2 wt% of a maleic anhydride-grafted PP with a 1.4% grafting level, Scona TPPP 8112 FA (Byk Altana, Wesel, Germany), was added.
A parallel, co-rotating twin-screw extruder, Brabender DSE20 (Brabender GmbH, Duisburg, Germany), comprising a screw diameter (d) of 20 mm and a length of 40 d, was used for compounding. An extruder screw speed of 375 rpm and a throughput of 12 kg/h were chosen. A temperature profile decreasing from 210 °C in the melting zone to 180 °C at the die was used. The maximum melt temperature, measured at the screw tip, was 180 °C, which is suitable for avoiding the thermal degradation of the fibers. All components were fed via gravimetric dosing scales (Motan-Colortronic GmbH, Friedrichsdorf, Germany). Two vertical ports were used for venting. A twin-screw side feeder (d = 20 mm) was employed for feeding FCP at position 11d, i.e., after the plastification zone, in order to reduce the mechanical degradation of the fiber structure. For granulation, an ECON EUP50 underwater pelletizer (Econ GmbH, Weisskirchen, Austria) was used. The granules were dried at 80 °C until the moisture content was below 0.08%.

2.2. Foam Injection Molding

Injection molding (IM) was performed on an ENGEL e-victory 220 (ENGEL Austria GmbH, Schwertberg, Austria). Plates were produced via a 200 × 150 × 3 mm3 plate tool equipped with one film gate. The used temperature profile started at 180 °C for the heating zones and ended with 200 °C at the nozzle. The tool temperature was set to 40 °C, unless otherwise mentioned. The used processing parameters for compact injection molding, the MuCell® FIM and the chemical FIM are summarized in Table 1. For compact reference plates, the applied packing pressure was 505 bar for 10 s.
In the MuCell® FIM process, packing pressure was reduced to a time of 0.2 s. Cooling and cycle times were employed similar to compact molding. CO2 and N2 were used in this physical FIM process. The content (weight percent) of CO2 dosing was varied; gas content of N2 was fixed at 0.6 wt%. Shot volume was reduced from 114 to 108, then to 100 and 95 cm3. Injection speed was varied, and a mold-opening step (core-back process) was additionally applied for selected trials. For this core-back process, the cooling time was reduced to 22 s. The cycle time without mold opening was 37 s, while for the core-back process the cycle time was extended to 45.5 s, unless otherwise mentioned.
For the chemical FIM process, the chemical foaming agent (CFA) Hydrocerol ITP 845 (Clariant International, Muttenz, Switzerland) was used. This is an endothermic foaming and nucleating agent masterbatch with 40% effective components; its decomposition starts at 150 °C. Back pressure was set between 155 and 160 bar; one trial was performed at 83 bar. Mold openings of 0.3 to 0.5 mm, according to the core-back process, were applied. Two temperature profiles were used, one from 180 to 200 °C and the other from 150 to 185 °C, with the higher temperature at the nozzle.
Weight reduction was determined from the rectangular plates by weighing up. Density and weight from the middle part of the plate were determined as mean values from 3 measurements using a buoyancy method, employing an analytical balance (density determination kit, Sartorius AG, Goettingen, Germany), according to ISO 1183-1. Flexural properties were determined on a Messphysik BETA50 universal testing machine (Messphysik Materials Testing GmbH, Fürstenfeld, Austria) according to EN ISO 187. From each sample, six specimens were tested. Charpy impact strength, notched (1eA) and unnotched (1eU), were tested according to ISO 179 using an Instron CEAST 9050 impact pendulum (ITW Test and Measurement Italia S.r.l., INSTRON CEAST division, Pianezza, Italy). Ten specimens were used for each test. For the preparation of microscopy images, cross-sections were cut from the middle part of the plates, embedded and polished. Pictures were taken using an Olympus BX-RLA2 reflected light microscope in the bright-field mode (Olympus Corporation, Tokyo, Japan).
An overview of the research design and the aim of the present study is shown in Figure 1.

3. Results and Discussion

3.1. Processing Parameters and Achieved Weight Reductions

In Table 2, the reductions in the material densities and weight of the plates derived from the MuCell® process are presented. While weight reduction is more relevant for industrial application, we included density reduction in order to consider the observed variation in plate thickness as well. Regarding the reduction in weight, values from 3% to 19% were reached in relation to the compact plates. Despite generally larger variation in density values, similar trends could be observed for density and weight reductions.
In general, mainly by decreasing the shot volume, density and weight reductions could be increased. No significant differences in density or weight reductions were observed by variation of the CO2 gas content, which is an indication that the gas was sufficiently dissolved in the polymer matrix. By applying mold opening at a reduced shot volume during this process, a small impact on the density in the middle part of the plates was achieved. However, the less accurate weight values were not significantly changed. A higher injection speed of 250 cm3/s caused a lower density and weight reduction. This can be explained by the reduced time, which is then available for the expanding of the foam in the mold, and in addition by the higher pressure that exists in the mold. Reduction in the mold temperature to 30 °C, as well as the variation of the screw speed, showed no significant influences on the weight. Therefore, they were not further studied.
In chemical FIM trials, which were used as a reference for the physical MuCell® process, narrower ranges of density and weight reductions could be covered by the studied process parameter variation. Table 3 shows that for chemical FIM, weight reductions of the complete plates between 11 and 15% and corresponding reductions in the material density between 22 and 26% were obtained. In comparison to CO2 as a physical blowing agent, the use of N2 and the use of chemical FIM resulted in almost identical maximum weight reductions of approximately 15% for the entire plates. However, a higher density reduction of up to 33% was achieved by N2 physical FIM.
According to the literature on PP–talc-foamed specimens [42], a higher range of density reduction up to 29% could be achieved by the variation of chemical FIM processing parameters. For the use of exothermic chemical foaming agents on wood fiber PP, density reductions up to 30% are also mentioned in the literature [43]. Despite the variation of several parameters, only minor changes in weight and densities could be achieved in our study. An increase in the mold opening gap (Sample 13) showed no significant influence on the density reduction or on the weight of the plates. Raising the foaming agent content to 4 wt% could not reduce the weight either. Lowering the screw speed (Sample 14) rather decreased the weight reduction in the plate, probably caused by an insufficient dispersion of the foaming agent in the melt. A decrease in the melt temperature was carried out (Sample 15); thereby, the chemical decomposition reaction of the foaming agent was decelerated, and this can result in a more homogenous foaming. The weight reduction could be further increased by softening the shear process through reduction in the back pressure and by lowering the dwell time during the plasticization (Sample 16).
By applying foam injection molding, high weight reductions in the prepared plates have been achieved. The MuCell® FIM of PP–cellulose with CO2 injection resulted in high weight reductions of up to 16% for an entire plate. However, the degree of foaming can differ between the middle of the plate and the edges—most samples show significant deviations from the straight line, representing equal values for weight reduction in the middle and the complete plates, as presented in Figure 2. This discrepancy is dependent on the overall degree of foaming. At low foaming degrees, the weight reduction in the middle is smaller compared to the overall reduction, whereas at higher foaming degrees, where the overall weight reduction is limited to 16%, weight reductions of up to 19% can occur in the middle of the plates. Interestingly, all chemically foamed samples showed smaller weight reductions in the middle compared to the overall reduction in the full plates.

3.2. Flexural Properties

In Figure 3, the results from flexural tests are presented in relation to the density reduction at the middle part, as the specimens for testing had been prepared from spots close to the middle of the plates. The stress–strain curves of selected samples can be found in the supplementary data file (Figures S1–S6). The flexural modulus decreased for the MuCell® process with CO2 if the density reduction was adjusted to more than 7%. At the lowest achievable densities for each type of FIM, quite similar flexural properties for the PP–cellulose plates were obtained. However, for chemical FIM, the achieved density range was much narrower than for the MuCell® process with CO2. The samples, which allowed a moderate density reduction below 15%, were derived only by the MuCell® process with CO2 injection and not via chemical FIM. In particular, by using CO2 as the physical foaming agent, it was possible to obtain PP–cellulose composite plates with only slightly reduced flexural properties. For PP–talc composites, Yetgin et al. [42] presented comparable linear reductions in the measured mechanical properties. Yang et al. [29] presented, exclusively for a foamed PP sample with 20 wt% GF and with an expansion ratio of about 1.25, an even higher tensile strength and tensile modulus than its solid sample. The outcomes from the literature indicate that the resulting mechanical properties are highly dependent on the processing parameters. In the present study, the flexural properties show almost linear reductions with increasing density reductions (see Figure 3) but with no clear influence of the processing parameters, i.e., how these density reductions have been achieved seems to have no obvious impact.
In order to better estimate the performance obtained, the mechanical properties are presented as relative specific values (absolute values divided by sample density in relation to a compact reference sample) in Figure 4. The properties are presented depending on their sample number according to Table 2 and Table 3. In contrast to the absolute properties, differences among the 17 samples are much smaller for the specific values. For MuCell® CO2 samples, a higher specific flexural modulus was obtained if the mold opening process was not used (sample numbers 1, 2, 3, 5 and 6). According to the literature for foamed PP–carbon fiber composites [44], the mold opening process can result in a lower fiber orientation in the flow direction, causing a decreased tensile modulus in this direction. This corresponds well to the presented modulus results in Figure 4. MuCell® N2 and chemically foamed samples all have specific modulus values slightly above those of the compact reference but do not reach the values of the best MuCell® CO2 samples.
Two CO2-foamed samples, 1 and 6, maintained the specific flexural strength of the compact reference, in addition to slightly improved specific flexural modulus values. These samples were produced both with equal gas content and with no mold opening, but different shot volumes and injection speeds were applied. Similar results on maintaining the specific flexural properties have been reported by Kuboki [26] for PP–cellulose compounds, albeit with N2 as the blowing agent.
The lowest specific flexural strength among the CO2-foamed materials was achieved by the samples that were prepared with mold opening (4, 7, 8 and 9). This is consistent with the observations made for the specific flexural modulus (see above). However, the size of the mold opening gap does not appear to have an influence on the outcome. Increasing the gas content of the MuCell® samples seems to result in a lower specific flexural strength, as exhibited by Samples 3 and 5.
The application of a higher injection speed at otherwise equal process parameters (Samples 7 and 9) had no noticeable effect on the specific flexural properties. This is consistent with the findings by Kuboki [26].
The samples prepared using N2 as the blowing agent (10 and 11) exhibited slightly better specific flexural properties than their CO2-foamed counterparts prepared under otherwise equal conditions (Samples 7 and 8). This may be a consequence of the more effective density reduction in N2 in the middle of the plates, where the test specimens were taken from.
In contrast to previous reports in the literature [27,45,46,47], no obvious trend can be observed regarding the comparison of physical versus chemical foaming in terms of the overall mechanical properties. The results indicate that influence from the type of foaming process seems to be lower compared to the influences obtained from the individual degrees of foaming, depending, e.g., on the shot volume and the mold opening.

3.3. Impact Strength

Charpy impact strength decreased for every sample after applying FIM, as presented in Figure 5; the notched specimens lost a minimum of 15% of their value compared to the compact plates, and the unnotched ones lost nearly 40%. The foaming cells can act as crack initiation sites, which could explain the drop of the unnotched impact strength for FIM specimens. In addition, the introduction of several gas cavities probably prevents crack propagations and thereby energy absorption. If the achieved density reduction was above 20%, the reduction in the impact strength was even higher. In these cases, more or bigger foam cells were formed.
Specific impact strength values are presented in Figure 6. Of all MuCell®-foamed samples, the highest specific impact strength value was obtained by the process with the highest applied injection speed (Sample 5). However, in comparison with the compact samples, the specific impact strength was always decreased due to the foaming process. The specific notched impact strength could be nearly maintained for the CO2-foamed Sample 5 and Sample 6 via processing at a reduced shot volume, but without using the mold-opening process. In addition, one chemically foamed sample resulted in nearly maintaining the specific notched impact strength; however, at a low unnotched impact strength. According to the literature [48], the specific Charpy impact strength was higher for samples if they were foamed by exothermic chemical foaming agents. However, the foamed composites could exhibit a higher damping index. Only Kuboki [26] was able to present the maintenance of the notched Izod impact strength, but this was the case exclusively for samples with void fractions below 15%. For special long-chain branched PP–cellulose nanofiber composite foams [36], specific Charpy impact strengths could also be higher than those of the solid samples.

3.4. Structure of the Foams

The microscope images of MuCell®-foamed PP–cellulose present, on the one hand, the foam cells as more or less circular, usually dark phases (in Figure 7f also white phases due to the embedding resin not completely filling all pores); on the other hand, dispersed cellulose fibers appear as a dark phase, but in an apparently elongated and very thin form. Several differences in the foam structures are clearly visible in the presented samples: Sample 1, Figure 7a, shows the fewest foam cells of all samples, as it was the sample with the lowest foaming degree with an achieved density reduction of 7%. In Sample 5, Figure 7b, the sample derived at a density reduction of 15%, several more foam cells are observable. Sample 6 and Sample 7, with density reductions of 22% and 28%, respectively, corresponding to Figure 7c,d, present significantly bigger individual foam cells in the middle layer.
Foaming with N2, as shown in Figure 7e, resulted in a significantly different foam structure compared to the MuCell® FIM process with CO2. A very high amount of larger circular foam cells is visible in Figure 7e, especially in the middle layer. As N2 is a less reactive gas type than CO2, such arrangements of the foam cell structure in different layers can occur. Figure 7f shows a foam structure from chemically foamed plates: in comparison to the other FIM methods, a significantly higher amount of larger circular foam cells was derived.
The highest absolute and specific Charpy impact strength among the foamed samples was achieved in Sample 5. This can be partly explained by its foam structure via microscope images; see Figure 7. The finer cell morphology of Sample 5, Figure 7b, compared to Sample 6, Figure 7c, is likely a result of the higher injection speed, as there is less time for the gas cells to expand if the injection into the mold happens faster. It must also be noted that despite having the highest gas loading, Sample 5 exhibits the lowest amount of density reduction of all samples prepared with the same shot volume. In addition, it has been suggested that a higher injection speed causes a higher amount of shear-induced heating near the skin layer, resulting in a relatively larger core [49,50]. However, no such observation can be made here. Sample 1, Figure 7a, shows a fine foam structure as well, but with lower specific flexural and impact strengths; regardless, this sample was prepared at a higher shot volume and with a lower degree of foaming. A correlation of better mechanical properties with a finer cell structure has also been reported by Gong et al. [51].
A study by Hou et al. [52] on CO2 physically foamed PP–talc composites identified the packing time in the core-back foaming process as an important factor in the development of the foam structure. This may be an interesting aspect that should be considered in future research on this topic.

4. Conclusions

The results of this study confirm the very good foamability of PP–cellulose compounds using different FIM techniques: MuCell® FIM by CO2 injection was equally as implementable as by N2 injection and chemical FIM. High reductions in the weight of the plates of up to 16% and density reductions of up to 33% could be achieved. It was possible to almost maintain stiffness and flexural strength, with various samples outperforming the compact reference in terms of the specific modulus. However, unnotched impact strength was reduced for all foamed plates. The homogeneity, appearance, size and number of the foam cells depended highly on the used FIM process itself and on the adjusted processing parameters.
Regarding the mechanical properties of the foamed samples, the following observations can be made:
  • Mold opening during foaming results in reduced mechanical performance, whereas the size of the mold-opening gap appears to have almost negligible influence.
  • Higher injection speed during the MuCell® process led to a finer foaming cell morphology, resulting in the highest absolute and specific impact strengths of all MuCell®-foamed samples.
  • With all other processing conditions being equal, N2-based MuCell® foaming tends to produce slightly better-performing materials than CO2-based MuCell® foaming.
  • In terms of other processing parameters, such as shot volume, gas content and injection speed, the influence of the individual parameters on the overall mechanical performance is less clear; instead, the main factor affecting the mechanics seems to be the density reduction, regardless of the manner in which it was achieved.
Despite not all correlations being clear yet, it can be concluded that processing parameters must be optimized for each material type and each FIM technique. The study showed that, even for new materials, it can be possible to simultaneously decrease weight while sustaining or even increasing the flexural modulus and strength. Nevertheless, further optimization in order to improve the impact properties is still required. This work offers a database and initial guideline for the preparation of injection-molded foams with desirable mechanical properties for applications that require ultra-lightweight materials.
This study could be a first step for future modelling and simulation studies, where selected output data like foam cell size or mechanical properties may be directly adjusted using correlated processing parameters, e.g., injection speed, which was identified as a significant impact factor in this study. The combination of already light materials, like cellulose-filled PP, with processing methods for the production of ultra-light foamed parts is very promising for various applications, e.g., in the transporting sector, with the target of saving materials and energy.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs9010050/s1, Figures S1–S6: Stress-strain curves from flexural testing of trial numbers 0, 4, 5, 6, 11 and 15.

Author Contributions

Conceptualization, C.P. and C.S.; methodology, C.P., C.S. and M.M.; investigation, C.P., C.S., M.M. and A.S.; resources, T.L., C.P. and M.M.; writing—original draft preparation, C.P. and M.M.; writing—review and editing, T.L., C.S. and C.U.; validation, T.L., C.S., A.S. and C.U.; visualization, C.P. and C.U.; supervision, A.S.; project administration, C.P.; funding acquisition, C.P., T.L. and A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research study was funded within the scope of the COMET program. Kompetenzzentrum Holz (Wood K plus) is a COMET Centre within the COMET (Competence Centers for Excellent Technologies) program and funded by the Austrian ministries BMK, BMDW and the federal states of Upper Austria, Lower Austria and Carinthia. COMET is managed by the Austrian Research Promotion Agency FFG. The writing, review and editing was funded additionally by the SSbD4CheM project within the use case automotive. SSbD4CheM is a European Union Horizon Europe research and innovation program funded under grant agreement number 101138475.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

Christian Sponner and Andreas Steurer were employed by the company ENGEL Austria GmbH. Thomas Lummerstorfer was employed by the company Borealis Polyolefine GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Jcs 09 00050 g001
Figure 1. Research design and aim of the present study.
Figure 1. Research design and aim of the present study.
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Figure 2. Reduction in weight of several PP–cellulose plates prepared with MuCell® FIM with CO2 and N2, and with chemical FIM for the complete plates (y-axis) and for the middle part of the plate (x-axis). The line represents equal values for weight reduction in the middle and complete plates.
Figure 2. Reduction in weight of several PP–cellulose plates prepared with MuCell® FIM with CO2 and N2, and with chemical FIM for the complete plates (y-axis) and for the middle part of the plate (x-axis). The line represents equal values for weight reduction in the middle and complete plates.
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Figure 3. Flexural modulus (a) and flexural strength (b) of the PP–cellulose plates prepared using FIM.
Figure 3. Flexural modulus (a) and flexural strength (b) of the PP–cellulose plates prepared using FIM.
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Figure 4. Specific flexural modulus (a) and specific flexural strength (b) of the PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent the 95% confidence interval.
Figure 4. Specific flexural modulus (a) and specific flexural strength (b) of the PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent the 95% confidence interval.
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Figure 5. Charpy impact strength, notched (a) and unnotched (b) of PP–cellulose plates, prepared using FIM.
Figure 5. Charpy impact strength, notched (a) and unnotched (b) of PP–cellulose plates, prepared using FIM.
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Figure 6. Charpy impact strength, specific, notched (a) and unnotched (b), of PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent a 95% confidence interval.
Figure 6. Charpy impact strength, specific, notched (a) and unnotched (b), of PP–cellulose plates, prepared using FIM in relation to a compact reference. Error bars represent a 95% confidence interval.
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Figure 7. Microscopy images of foamed PP–cellulose. MuCell® with CO2: Sample 1 (a), Sample 5 (b), Sample 6 (c) and Sample 7 (d); MuCell® with N2: Sample 10 (e) and Sample 16, derived using chemical FIM (f).
Figure 7. Microscopy images of foamed PP–cellulose. MuCell® with CO2: Sample 1 (a), Sample 5 (b), Sample 6 (c) and Sample 7 (d); MuCell® with N2: Sample 10 (e) and Sample 16, derived using chemical FIM (f).
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Table 1. Processing parameters used for PP–cellulose prepared by compact IM (Comp.), MuCell® FIM (by using CO2 and N2) and chemical FIM (Chem.).
Table 1. Processing parameters used for PP–cellulose prepared by compact IM (Comp.), MuCell® FIM (by using CO2 and N2) and chemical FIM (Chem.).
Shot
Volume [cm3]		Gas/CFA
Content
[wt%]		Mold
Opening [mm]		Injection Speed
[cm3/s]		Screw Speed
[m/s]		Back
Pressure [bar]		Cooling Time
[s]		Cycle
Time
[s]
Comp.114001140.3702542
CO295–1081.0–1.60–0.6110–2500.3–0.5150–17522–3037.5–45.5
N2950.60.3–0.62000.51602245.5
Chem. 96.53–40.3–0.51930.3–0.583–1602231–45.5
Table 2. Density and weight reductions for PP–cellulose prepared by MuCell® FIM in relation to the studied processing parameters.
Table 2. Density and weight reductions for PP–cellulose prepared by MuCell® FIM in relation to the studied processing parameters.
Type Nr.Shot
Volume [cm3]		Gas Content [wt%]Mold
Opening [mm]		Injection Speed
[cm3/s]		Density
Reduction,
Middle [%]		Weight
Reduction, Middle [%]		Weight
Reduction, Plate [%]
Compact011400110000
CO211081.201106.83.45.4
21001.0020010.29.110.0
31001.6017010.79.69.7
41001.20.620025.98.510.8
5951.6025014.813.512.8
6951.2020022.219.315.2
7951.20.320027.816.416.0
8951.20.520030.716.515.7
9 *951.20.324026.815.915.4
N210950.60.320028.617.015.4
11950.60.520033.517.115.5
* Nr. 9: processed at 30 °C mold temperature.
Table 3. Density and weight reductions for PP–cellulose prepared using chemical FIM related to the studied processing parameters.
Table 3. Density and weight reductions for PP–cellulose prepared using chemical FIM related to the studied processing parameters.
Nr.CFA Content [wt%]Mold
Opening
[mm]		Temperature
Nozzle
[°C]		Screw
Speed
[m/s]		Density
Reduction, Middle [%]		Weight
Reduction,
Middle [%]		Weight
Reduction,
Plate [%]
1230.32000.524.513.514.7
1330.52000.526.613.214.6
1440.52000.326.09.211.2
1540.51850.322.47.811.8
16 *40.51850.325.49.213.3
* Nr. 16: processed at lower back pressure and a cycle time of 30 s.
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MDPI and ACS Style

Pretschuh, C.; Mihalic, M.; Sponner, C.; Lummerstorfer, T.; Steurer, A.; Unterweger, C. Physical Foam Injection Molding of Cellulose Fiber Reinforced Polypropylene by Using CO2: Parameter Variation and Comparison to Chemical Foam Injection Molding. J. Compos. Sci. 2025, 9, 50. https://doi.org/10.3390/jcs9010050

AMA Style

Pretschuh C, Mihalic M, Sponner C, Lummerstorfer T, Steurer A, Unterweger C. Physical Foam Injection Molding of Cellulose Fiber Reinforced Polypropylene by Using CO2: Parameter Variation and Comparison to Chemical Foam Injection Molding. Journal of Composites Science. 2025; 9(1):50. https://doi.org/10.3390/jcs9010050

Chicago/Turabian Style

Pretschuh, Claudia, Matthias Mihalic, Christian Sponner, Thomas Lummerstorfer, Andreas Steurer, and Christoph Unterweger. 2025. "Physical Foam Injection Molding of Cellulose Fiber Reinforced Polypropylene by Using CO2: Parameter Variation and Comparison to Chemical Foam Injection Molding" Journal of Composites Science 9, no. 1: 50. https://doi.org/10.3390/jcs9010050

APA Style

Pretschuh, C., Mihalic, M., Sponner, C., Lummerstorfer, T., Steurer, A., & Unterweger, C. (2025). Physical Foam Injection Molding of Cellulose Fiber Reinforced Polypropylene by Using CO2: Parameter Variation and Comparison to Chemical Foam Injection Molding. Journal of Composites Science, 9(1), 50. https://doi.org/10.3390/jcs9010050

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