Increasing the Impact Toughness of Cellulose Fiber Reinforced Polypropylene Composites—Influence of Different Impact Modifiers and Production Scales

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Article Increasing the Impact Toughness of Cellulose Fiber Reinforced Polypropylene Composites—Influence of Different Impact Modifiers and Production Scales

Matthias Mihalic * , Lukas Sobczak , Claudia Pretschuh and Christoph Unterweger

Division Biobased Composites and Processes, Wood K plus–Kompetenzzentrum Holz GmbH, Altenberger Strasse 69, 4040 Linz, Austria * Correspondence: [email protected]; Tel.: +43-732-2468-6779

Received: 29 June 2019; Accepted: 4 August 2019; Published: 8 August 2019

Abstract: While cellulose fiber reinforced polypropylene (PP) composites typically offer good stiffness and strength in combination with ecological benefits and a high potential for lightweight construction, they often require measures taken to improve their impact performance. In this work, the influence of different types of impact modifier on the mechanical performance of a PP–cellulose composite was systematically investigated, with a particular focus on the improvement of the notched impact strength and the accompanying loss of stiffness. Among the tested impact modifiers, ethylene-octene copolymers appeared to be the most suitable class to achieve a good overall performance. A high modifier viscosity increased its potential to improve the notched impact strength of the composite. Additionally, composite production on a larger scale improved the impact performance without significantly affecting the tensile properties. Several composites from this study surpassed the overall mechanical performance of a benchmark commercial PP–cellulose composite. While the impact strength of commercial high-impact PP–talc composites could not be reached, the considerably lower density of the PP–cellulose composites is worth mentioning.

Keywords: cellulose ﬁber; polypropylene; impact modiﬁcation; mechanical properties; injection molding

1. Introduction Natural-fiber reinforced composites (NFC) based on polypropylene (PP) can be of special interest for the automotive industry as a substitute for conventional composites (e.g., with talc or glass fibers), as they offer comparable stiffness and strength while at the same time exhibiting a higher potential for lightweight construction due to their lower density. In addition, they provide the ecological benefit of partially consisting of renewable raw materials [1,2]. In 2017, the European production of wood polymer composites (WPC) and NFC was 410,000 tons, distributed over three main application fields: Decking, siding, and fencing (49%); automotive (36%); and technical applications, furniture, and consumer goods (15%). The latter sector was the strongest growing application in recent years, with an annual growth rate of roughly 30% between 2012 and 2017. In total, 80,000 t of granules for injection molding or extrusion were produced and traded, with PP being one of the most widely used base polymers (other important polymers for WPC and NFC are biopolymers such as polylactic acid (PLA), polyethylene (PE), thermoplastic elastomers, acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyvinyl chloride (PVC), and poly(methyl methacrylate) (PMMA) [3]. One very interesting type of natural fillers is Kraft pulp fibers, i.e., cellulose fibers obtained from the sulphate process. These offer a good combination of perennial availability, good reinforcing potential, and price, and thus appear particularly suitable for the production of composites for the automotive industry.

J. Compos. Sci. 2019, 3, 82; doi:10.3390/jcs3030082 www.mdpi.com/journal/jcs J. Compos. Sci. 2019, 3, 82 2 of 13

Because of the low intrinsic impact toughness of PP, often a heterophasic copolymer instead of a homopolymer is used as a base polymer for composites. Otherwise, additives are often introduced to improve the impact strength. The most common approach to enhance the mechanical performance of PP-based NFCs is to increase the fiber-matrix interaction [4]; commonly used coupling agents are maleic anhydride (MA) grafted polymers [5–10] or silanes [11]. However, the addition of a coupling agent can adversely affect the notched impact strength, as effective energy dissipation during crack propagation requires debonding and pull-out of the fibers from the matrix [12]. If this is prevented by a very strong fiber-matrix interaction, the fibers will break upon impact; for the materials used in this study, this has been shown to dissipate less energy than the fiber pull-out, which is why no coupling agents were used for the present work. Recently the addition of polymer fibers, such as polyvinyl alcohol (PVA) [13] or polyethylene terephthalate (PET) [14] has been shown to improve the mechanical performance of PP composites considerably. A different approach, especially in the case of PP-based composites, is to add an elastomeric phase to improve the impact performance of the matrix. Next to ethylene-propylene-based rubbers (EPR, EPDM) [15–17], an important class of materials for the impact modification of polymers are thermoplastic elastomers [18]; among those, the most relevant modifiers for PP are styrenic elastomers (SBS, SEBS) [19], which are often used in combination with coupling agents [20–23], and polyolefinic elastomers [24,25]. The use of ethylene vinyl acetate (EVA) as impact modifier for PP has also been reported [26]. While much literature on the mechanical performance of NFCs is available, a systematic investigation of the influence of different impact modifiers is so far lacking. Therefore, the main objective of the present study is to fill this gap. With regard for the possible application of impact-toughened NFCs in the automotive industry, the aim for the mechanical performance was to achieve the highest possible notched impact strength, while at the same time limiting the reduction of the tensile modulus. A target value for the modulus of 1.8 GPa or higher was set, as this was approximately the average of the moduli of the commercial automotive-grade reference materials that were used for comparison (see Section 3.3).

2. Materials and Methods

2.1. Materials The basic formulation for all composites that were investigated in this study consisted of an injection molding grade, metallocene-catalyzed polypropylene (PP) homopolymer with a melt flow rate (MFR) of 140 g/10 min (230 ◦C, 2.16 kg), and bleached Kraft cellulose fibers with an average fiber length of 800 µm (prior to processing) and a thickness of 15 µm. The fiber content in all composites was 20% by weight. The reference formulation consisting of 80 wt% PP and 20 wt% fibers will subsequently be referred to as “PP20F”. The neat PP was characterized as well in order to show the effect of the fibers on the mechanical properties. The experimental work for this study consisted of two main parts: First, a screening of different types of impact modifiers that were added to the basic formulation on a laboratory scale; these included several grades of metallocene-catalyzed polyolefinic elastomers (POE; ethylene-octene (EO) and ethylene-propylene (EP)), styrene-butadiene-based elastomers (SBS, SEBS), and ethylene vinyl acetate (EVA). Second, the preparation of selected formulations from the first part, in an upscaled production in order to emulate the processing conditions of commercial materials more realistically than on a laboratory scale; additionally, for this step, an ethylene-propylene rubber (EPR) and an ethylene-propylene-diene rubber (EPDM) were introduced due to their historic importance as impact modifiers for PP. A schematic of the experimental program is illustrated in Figure1. A complete list of the impact modifiers is shown in Table1. J. Compos. Sci. 2019, 3, 82 3 of 13 J. Compos. Sci. 2019, 3, x 3 of 15

Figure 1. Flow chart to illustrate the experimental program. a FormulationsFormulations with with 30 30 m% m% of of modifier modiﬁer were only prepared with the ethylene-propyleneethylene-propylene (EP)(EP) gradesgrades andand withwith styrenicstyrenic elastomerselastomers (SBS).(SBS).

Table 1. ListList of impact modifiers; modiﬁers; melt melt flow ﬂow rate rate ( (MFR)MFR) data data as as measured measured at at Wood Wood K plus unless otherwise stated, other material data as shown on the technical data sheets.

MFR Additional Material CodeCode DensityDensity (g/cm³) (gHardness/cm3) Hardness Shore A ShoreMFR(2.16 A kg; 190/230 °C)(2.16 (g/10 kg; 190min)/230 ◦AdditionalC) Material Information Information Only laboratory scale production(g/10 min) EP1 0.865 A 71 Only laboratory18.5 scale(190 °C) production/48(230 °C) 13% ethylene content

(190 °C) (230 °C) EP2 EP10.863 0.865A 64 A 718.5 18.5/20(190 ◦C) /48(230 ◦C) 15%13% ethylene ethylene content content

EP3 EP20.862 0.863A 67 A 641.5(190 °C) 8.5/3(190(230 ◦°C)C) /20(230 ◦C) 16%15% ethylene ethylene content content

EP3 0.862 A 67(190 1.5 °C)(190 a ◦C)/3(230 ◦C) 16% ethylene content EO1 0.885 A 84 30 a Mooney viscosity 2 MU EO1 0.885 A 84 30(190 ◦bC) Mooney viscosity 2 MU SBS 0.94 A 71–81 <1(200 °C, 5 kg) [Datasheet] b 31% styrene content SBS 0.94 A 71–81 <1(200 ◦C, 5 kg) [Datasheet] 31% styrene content SEBS 0.90 A 47 1.8(190 °C)/9.4(230 °C) 13% styrene content SEBS 0.90 A 47 1.8(190 ◦C)/9.4(230 ◦C) 13% styrene content EVA 0.952 A 75 2.5(230 °C) c c 28% VA content EVA 0.952 A 75 2.5(230 ◦C) 28% VA content LaboratoryLaboratory scale scale and and upscaled production EO2 0.870 A 70 5(190 °C)/10.8(230 °C) Mooney viscosity 9 MU EO2 0.870 A 70 5(190 ◦C)/10.8(230 ◦C) Mooney viscosity 9 MU EO3 0.857 A 57 1(190 °C)/2.2(230 °C) Mooney viscosity 25 MU EO3 0.857 A 57 1(190 ◦C)/2.2(230 ◦C) Mooney viscosity 25 MU EO4 0.870 A 51 0.5(190 °C)/1.4(230 °C) Mooney viscosity 45 MU EO4 0.870 A 51 0.5(190 ◦C)/1.4(230 ◦C) Mooney viscosity 45 MU OnlyOnly upscaled upscaled production 72% ethylene content EPR n/a n/a n/a d 72% ethylene content EPR n/a n/a n/a d Mooney viscosity 44 MU Mooney viscosity 44 MU 69% ethylene content 69% ethylene content EPDM n/a n/a n/a d 4.4% diene content EPDM n/a n/a n/a d 4.4% diene content MooneyMooney viscosity viscosity 60 60 MU MU aa b b ViscosityViscosity ofof EO1 EO1 at at 230 230◦C was°C was too lowtoo forlow reliable for reliable MFR measurement. MFR measurement.Viscosity ofViscosity SBS was of too SBS high was at either too c hightemperature at either for temperature MFR measurement for MFR with measurement a load of 2.16 kg. withViscosity a load of of ethylene 2.16 kg. vinyl c Viscosity acetate (EVA)of ethylene at 190 ◦ Cvinyl was too high for reliable MFR measurement. d Viscosity of ethylene-propylene-based rubbers (EPR and EPDM) was too acetate (EVA) at 190 °C was too high for reliable MFR measurement. d Viscosity of high at either temperature for reliable MFR measurement. ethylene-propylene-based rubbers (EPR and EPDM) was too high at either temperature for reliable MFR measurement. The amounts of impact modifier in the composites were 10% and 20% by weight, and 30% in selectedThe amounts formulations. of impact Regarding modifier thein the nomenclature, composites were the formulations10% and 20% used by weight, for this and study 30% will in selectedsubsequently formulations. be referred Regarding to according the to the nomenclature, scheme “ModifierCode_wt% the formulations”; e.g., used the designation for this study “EP1_30” will subsequentdescribes thely formulation be referredconsisting to according of 30 to wt% the EP1, scheme 50 wt% “ModifierCode_wt% PP and 20 wt% fibers.”; e.g., For the the designation comparison “ofEP1_30 production” describes scales, the the formulation suffix “_Lab” consisting is used forof 30 the wt% laboratory EP1, 50 scale wt% production, PP and 20 wt% and fibers. “_Up” For is used the comparisonfor the upscaled of production production. scales, the suffix “_Lab” is used for the laboratory scale production, and “_Up”Two is used commercially for the upscaled available production. talc-reinforced polypropylene copolymers that are being used as high-impactTwo commercially grades for theavailable automotive talc-reinforced industry werepolypropylene tested as referencecopolymers materials: that are Hostacom being used TRC as high333N-impact (20%talc grades filled) for andthe Hostacomautomotive EKC industry 330N were (16% tested talc filled), as reference both from materials: LyondellBasell. Hostacom TRC As a 333Nreference, (20% specifically talc filled) forand PP–cellulose Hostacom EKC composites, 330N (16% the talc mechanical filled), both properties from LyondellBasell. of the commercially As a referenceavailable, highspecifically impact for grade PP– Formicellulose EXP composites, 20 (20 wt% the cellulose mechanical filled properties PP) from UPMof the Biocomposites, commercially availableaccording high to the impact technical grade data Formi sheet EXP [27], 20 were (20taken wt% intocellul account.ose filled PP) from UPM Biocomposites, according to the technical data sheet [27], were taken into account.

J. Compos. Sci. 2019, 3, 82 4 of 13

2.2. Sample Preparation For the laboratory-scale production, the compounds were prepared on a Brabender DSE20/40D twin screw extruder (Brabender GmbH & Co KG, Duisburg, Germany) with a 20 mm screw diameter, at a screw speed of 600 rpm and a throughput of 5 kg/h. The upscaled production was performed on a Leistritz ZSE 50 MAXX 48D twin screw extruder (Leistritz Extrusionstechnik GmbH, Nürnberg, Germany) with 50 mm screw diameter, a screw speed of 400 rpm and a throughput of 240 kg/h. For the mechanical characterization, standard shoulder bars according to ISO 527-2 were prepared on a Battenfeld HM1300/350 injection molding machine (Battenfeld Kunststoﬀmaschinen GmbH, Kottingbrunn, Austria). Prior to testing, the specimens were conditioned at 23 ◦C and 50% RH for at least 120 h.

2.3. Characterization Methods Tensile tests according to ISO 527 were carried out on a Messphysik BETA20-10 universal testing machine (Messphysik Materials Testing GmbH, Fürstenfeld, Austria). The tensile modulus E was determined in the range between 0.05% and 0.25% elongation at a rate of 1 mm/min, afterwards the elongation rate was gradually increased to 50 mm/min to determine the tensile strength σM. Charpy unnotched (acU) and notched (acN) impact strength according to DIN EN ISO 179-1eU (unnotched) or -1eA (notched) were tested on an Instron CEAST 9050 impact pendulum (ITW Test and Measurement Italia S.r.l., INSTRON CEAST division, Pianezza, Italy). Depending on the impact toughness of the materials, a pendulum with an energy of 0.5 J, 2 J or 7.5 J was used. The heat deﬂection temperature (HDT-B) according to DIN EN ISO 75 was determined on an Instron CEAST HDT Vicat Series 3 Station Tester, using a load of 0.45 N/mm2. This test was carried out for selected samples. Melt ﬂow rate (MFR) according to ISO 1133 was tested on a Zwick 4105 Extrusion Plastometer, at a temperature of 230 ◦C and using a load of 2.16 kg. The density ρ was measured according to DIN EN ISO 1183-1 using a Sartorius BP 301 S Analytical Balance with a YDK 01 Density Kit. The morphology of the samples was investigated via scanning electron microscopy (SEM), using a Phenom ProX desktop microscope.

3. Results

3.1. Fiber Reinforcement of the Neat PP Compared to the neat PP, the formulation PP20F shows a very strong increase in stiffness and a moderate increase in tensile strength (cf. Table2); these changes are in the expected range for NFC [ 1]. However, the most obvious effect of the addition of the fibers is an increase of the notched impact strength by almost a factor of four, whereas the unnotched impact strength is not significantly affected. The introduction of natural fibers is typically detrimental to the impact performance of PP [1]; however, in the present case the neat PP has a very poor intrinsic impact performance (most likely due to its very low viscosity which indicates a very low molar mass and therefore a high brittleness), which is therefore improved by the addition of the fibers.

3.2. Characterization of the Compounds from the Laboratory-Scale Production Literature comparing different types of impact modifiers for PP-based NFCs is surprisingly scarce. The first experimental stage of this study was therefore to produce and characterize a series of compounds containing different types and amounts of impact modifiers (see the list of materials in Table1) that were introduced into the basic formulation PP20F. The three EP grades were supplied by the same manufacturer and mainly differed in their viscosity, while the differences in ethylene content and Shore hardness were considered negligible in comparison. Therefore, these grades were suitable to investigate the influence of the viscosities of otherwise very similar modifiers on the mechanical properties of the compounds. The details of this investigation are described in Section 3.2.1. A similar investigation might have been possible with the four EO grades; however, due to the fact that EO2 was supplied by a different manufacturer than EO1, EO3, and EO4, J. Compos. Sci. 2019, 3, 82 5 of 13 and due to the large variation in density and Shore hardness, it would not have been feasible to focus solely on the effect of the modifier viscosity.

Table 2. Absolute and speciﬁc mechanical properties of the compounds from the lab-scale production. The properties of the neat polypropylene (PP) are shown for comparison.

Absolute Values Speciﬁc Values (Normalized by Density)

Code ρ MFR HDT-B E σM acU acN E_sp σM_sp acU_sp acN_sp 3 2 2 Unit g/cm g/10 min ◦C GPa MPa kJ/m kJ/m GJ/kg MJ/kg kJ m/kg kJ m/kg PP 0.906 140 a 95.8 1.55 34.7 40.1 1.1 1.71 38.3 44.3 1.2 PP20F 0.981 9.2 134.6 2.76 40.1 41.0 4.5 2.81 40.9 41.8 4.6 EP1_10 0.978 7.3 129.4 2.07 35.6 39.4 5.3 2.12 36.4 40.3 5.4 EP1_20 0.978 6.3 119.1 1.74 30.8 40.1 7.5 1.78 31.5 41.0 7.7 EP1_30 0.973 6.0 104.0 1.32 24.7 38.6 9.7 1.36 25.4 39.7 10.0 EP2_10 0.978 7.1 132.6 2.05 35.3 39.0 5.6 2.10 36.1 39.9 5.7 EP2_20 0.978 4.6 122.9 1.72 30.4 41.0 8.1 1.76 31.1 41.9 8.3 EP2_30 0.974 3.6 108.0 1.33 24.2 36.9 11.1 1.37 24.8 37.9 11.4 EP3_10 0.981 5.1 136.1 2.19 36.6 39.3 6.2 2.23 37.3 40.1 6.3 EP3_20 0.976 2.6 127.7 1.83 31.0 39.1 9.2 1.88 31.8 40.1 9.4 EP3_30 0.972 2.2 114.3 1.33 24.3 36.9 12.8 1.37 25.0 38.0 13.2 EO1_10 0.984 6.4 - 2.30 36.7 44.3 6.7 2.34 37.3 45.0 6.8 EO1_20 0.980 5.2 - 1.83 31.9 44.5 8.4 1.87 32.6 45.4 8.6 EO2_10 0.981 6.7 - 2.36 34.2 42.2 7.6 2.41 34.9 43.0 7.7 EO2_20 0.977 4.7 - 2.03 29.4 38.4 10.2 2.08 30.1 39.3 10.4 EO3_10 0.978 6.2 - 2.44 35.1 35.8 7.2 2.49 35.9 36.6 7.4 EO3_20 0.972 3.5 - 2.09 29.3 34.4 10.1 2.15 30.1 35.4 10.4 EO4_10 0.974 5.5 - 2.30 33.6 37.8 7.1 2.36 34.5 38.8 7.3 EO4_20 0.979 3.0 - 2.03 28.8 36.7 11.6 2.07 29.4 37.5 11.8 SBS_10 0.982 4.3 - 2.29 32.7 41.9 6.8 2.33 33.3 42.7 6.9 SBS_20 0.993 2.4 - 2.12 28.5 40.9 9.6 2.13 28.7 41.2 9.7 SBS_30 0.997 1.5 - 1.72 22.3 37.8 14.2 1.73 22.4 37.9 14.2 SEBS_10 0.982 4.5 - 2.27 34.8 41.3 7.5 2.31 35.4 42.1 7.6 SEBS_20 0.980 3.1 - 1.86 28.6 40.4 11.4 1.90 29.2 41.2 11.6 EVA_10 0.990 5.8 - 2.30 35.1 39.3 6.2 2.32 35.5 39.7 6.3 EVA_20 0.996 5.3 - 1.93 31.3 38.2 7.3 1.94 31.4 38.4 7.3 a MFR of the neat PP was taken from the technical datasheet; the viscosity was too low for a reliable measurement.

The investigation of the influence of different types of modifiers on the mechanical performance is discussed in Section 3.2.2. For that part of the study, the determination of the heat deflection temperature was omitted, as the results from the comparison of the three EP grades showed a close correlation of the HDT-B to the tensile modulus, and thus the measurement of the HDT-B did not seem to give an additional benefit for the purposes of the present study. Table2 shows the measured data, as well as the density-normalized specific values, from the mechanical characterization of all compounds prepared at the laboratory scale.

3.2.1. Influence of the Viscosity of the Impact Modifier For all EP grades, increasing the concentration of the modifiers resulted in a strong increase of the acN and in a reduction of the tensile modulus and strength. Additionally, flowability and HDT were reduced as well. The unnotched impact strength, however, remained largely unaffected by the modifier (cf. Table2). The e ffect of the modifier content on the tensile modulus and the notched impact strength is illustrated in Figure2. The more or less linear reduction of the sti ffness with increasing modifier content is consistent with the literature for impact-toughened PP [15,28] and basically follows the rule of mixtures according to which a larger amount of the soft dispersed phase decreases the volume of the stiff matrix and thus the modulus [17]. J. Compos. Sci. 2019, 3, x 6 of 15

With increasing viscosity of the elastomer, the effect of the impact modification also increased. The highest acN value was thus obtained for EP3_30, which was 184% higher than the acN of the reference NFC material. In comparison, with EP1_30 the acN was increased by 115%. Moreover, a higher viscosity of the modifier slightly reduced the detrimental effect on the HDT. The downside of a higher viscosity of the modifier was obviously an increase in the total viscosity of the compound, as indicated by decreasing MFR values; nonetheless, all compounds could be processed by injection molding without difficulty. Regarding the tensile properties and the unnotched impact strength, the viscosity of the modifier did not appear to have a significant influence. A further illustration of the J.modulus Compos. Sci. and2019 the, 3, 82acN is given in Figure 2, as the focus regarding the mechanical performance6 oflies 13 mostly on these two properties.

3.0 13 2.8 12 2.6 11 2.4 10 2.2 9

2.0 8

E [GPa] E

N [kJ/m²] N c 1.8 7 a 1.6 6 1.4 5 1.2 4 0 5 10 15 20 25 30 Modifier content [wt%] E - EP1 E - EP2 E - EP3 acN - EP1 acN - EP2 acN - EP3

Figure 2. 2. Tensile modulus (ﬁlled(filled symbols; left y-axis)-axis) and Charpy notched impact strength (open symbols; right y-axis)-axis) ofof thethe compoundscompounds containingcontaining thethe EPEP elastomers.elastomers.

WithWhile increasing it has been viscosity shown of in the the elastomer, literature the that eff aect high of the viscosity impact of modification the modifier also is increased. generally Thebeneficial highest for a ctheN valueimpact was toughness thus obtained [29], the for reason EP3_30, for this which cannot was be 184% explained higher in than a straightforward the acN of the referenceway. On the NFC one material. hand, a high In comparison, molar mass withdirectly EP1_30 influences the ac Nthe was impact increased strength by in 115%. a positive Moreover, way. aOn higher the othe viscosityr hand, of thea high modifier viscosity slightly ratio reduced (i.e., the the viscosity detrimental of the eff dispersedect on the HDT.phase, The divided downside by the of aviscosity higher viscosityof the matrix) of the is modifier likely to was increase obviously the particle an increase size [30]. in the Then total again, viscosity the processing of the compound, history asalso indicated has a strong by decreasing influence MFRon the values; morphology nonetheless, [28]. allWhile compounds generally could a small be processedparticle size by improves injection moldingthe impact without behavior diffi [16,31],culty. the Regarding phase compatibility the tensile properties also has a andstrong the influence unnotched on impactboth the strength, impact theperformance viscosity ofas thewell modifier as the morphology did not appear [28]. to have a significant influence. A further illustration of the modulusSEM pictures and the of athecN fracture is given insurfaces Figure of2, asEP1_20, the focus EP2_20 regarding and EP3_20 the mechanical clearly show performance the presence lies mostlyof the cellulose on these twofibers; properties. however, no significant differences in the morphology are visible. In fact, a separationWhile it between has been matrix shown and in the the literature dispersed that phase a high cannot viscosity be of distinguished the modifier is in generally these images beneficial (cf. forFigure the impact 3). Based toughness on the [29 available], the reason data, for a this definite cannot statement be explained on whether in a straightforward the increase way. in impactOn the oneperformance hand, a high with molar increasing mass directly modifier influences viscosity the is impact only strength related to in a the positive molar way. mass On or the is other also hand,influenced a high by viscositythe morphology ratio (i.e., can the therefore viscosity notof be the made, dispersed but given phase, the existing divided knowledge by the viscosity from the of theliteratu matrix)re, a iscombination likely to increase of both the factors particle appears size to [30 be]. the Then most again, likely the explanation. processing history also has a strong influence on the morphology [28]. While generally a small particle size improves the impact behavior [16,31], the phase compatibility also has a strong influence on both the impact performance as well as the morphology [28]. SEM pictures of the fracture surfaces of EP1_20, EP2_20 and EP3_20 clearly show the presence of the cellulose fibers; however, no significant differences in the morphology are visible. In fact, a separation between matrix and the dispersed phase cannot be distinguished in these images (cf. Figure3). Based on the available data, a definite statement on whether the increase in impact performance with increasing modifier viscosity is only related to the molar mass or is also influenced by the morphology can therefore not be made, but given the existing knowledge from the literature, a combination of both factors appears to be the most likely explanation.

3.2.2. Effects of Different Types of Impact Modifiers Because of the poor tensile properties of the previously tested compounds containing 30 wt% EP (1,2,3) (cf. Table2), for most other modifiers except SBS only formulations with 10% and 20% modifier content were prepared, while formulations with 30 wt% modifier content were omitted. An exception was made in case of the SBS because of the comparatively high stiffness of the formulation SBS_20. J. Compos. Sci. Sci. 2019, 3, x 82 77 of of 1315

(a) (b) (c)

Figure 3. SEM images of cryo cryo-fractured-fractured surfaces: ( a) EP1_20, ( b) EP2_20, ( c) EP3_20.

3.2.2.All Effects measured of Different mechanical Types dataof Impact are summarized Modifiers in Table2. The tensile modulus and the a cN of selected formulations are illustrated in Figure4. In order to improve the visibility, only formulations Because of the poor tensile properties of the previously tested compounds containing 30 wt% containing up to 20 wt% of modifier are plotted. In comparison with the EP elastomers, most of the EP (1,2,3) (cf. Table 2), for most other modifiers except SBS only formulations with 10% and 20% other modifiers led to a further improvement of the acN, the notable exceptions being the EO1 and the modifier content were prepared, while formulations with 30 wt% modifier content were omitted. An EVA (cf. Table2). The a cU was slightly improved with EO1, but reduced with the other EO types, exception was made in case of the SBS because of the comparatively high stiffness of the formulation while it was only marginally affected with the other modifiers. The reduction of tensile strength was in SBS_20. aJ. Compos. similar Sci. range 2019, for3, x all modifiers, whereas for the stiffness slightly larger differences were observed,8 of 15 All measured mechanical data are summarized in Table 2. The tensile modulus and the acN of with the best value being reached with SBS. Generally, all observed (positive and negative) effects were selected formulations are illustrated in Figure 4. In order to improve the visibility, only formulations stronger with a higher modifier concentration. containing up to 20 wt% of modifier are plotted. In comparison with the EP elastomers, most of the other modifiers led to a further improvement of the acN, the notable exceptions being the EO1 and 3.0 12 the EVA (cf. Table 2). The acU was slightly improved with EO1, but reduced with the other EO types, while it was only marginally2.8 affected with the other modifiers. The reduction11 of tensile strength was in a similar range for all2.6 modifiers, whereas for the stiffness slightly10 larger differences were observed, with the best2.4 value being reached with SBS. Generally, all9 observed (positive and negative) effects were stronger with a higher modifier concentration.

E [GPa] E 2.2 8

N [kJ/m²] N c 2.0 7 a 1.8 6 1.6 5

1.4 4 0 5 10 15 20 Modifier content [wt%] EP3 EO4 SBS SEBS EVA

FigureFigure 4.4. InfluenceInfluence of of various various impact impact modifiers modifiers on on the the tensile tensile modulus modulus (filled (filled symbols; symbols; left yleft-axis) y-axis) and theand Charpy the Charpy notched notched impact impact strength strength (open (open symbols; symbols; right righty-axis). y-axis).

AmongAmong the POEs, POEs, one one of of the the best best overall overall performances performances was was achieved achieved with with 20 wt% 20 wt%of EO4, of EO4,with with an increase of the acN by 157% and a comparatively moderate loss of stiffness ( 26%). However, an increase of the acN by 157% and a comparatively moderate loss of stiffness (−26%).− However, a areduction reduction of of the the ac aUcU was was observed, observed, which which might might be be related related to to a a rather coarse distribution of the the modifiermodifier in the PP PP matrix, matrix, as as indicated indicated by by the the SEM SEM images images shown shown at atthe the end end of ofthis this subsection subsection.. In Incontrast, contrast, the the best best acU ac U of of all all for formulationsmulations was was achieved achieved with with EO1, EO1, which which may may be be due due to a better better compatibilitycompatibility betweenbetween matrixmatrix andand modifier.modifier. However, thethe aaccN and stistiffnessffness were noticeably reduced comparedcompared toto thethe otherother EOEO grades.grades. AmongAmong the the other other compounds compounds containing containing 20 wt% 20 wt% of impact of impact modifier, modifier, the highest the a highestcN was achieved acN was withachieved SEBS with (+153%), SEBS which (+153%), is very which close is tovery that close of EO4_20 to that but of whichEO4_20 came but atwhich the cost came of a at comparatively the cost of a lowcomparatively stiffness, evenlow stiffness, though iteven was though still above it was the still abovementioned above the abovementioned target value target of 1.8 value GPa of from 1.8 comparableGPa from comparable commercial commercial materials. Themateria formulationsls. The formulations containing SBScontaining showed SBS promising showed mechanical promising properties,mechanical withproperties, the ac Nwith being the slightlyacN being lower slightly than lower that ofthan EO2 that but of theEO2 E-modulus but the E-modulus being slightly being slightly higher. Therefore, an additional formulation with 30 wt% SBS was produced. With this material, by far the best acN of all tested samples was achieved (+213% compared to PP20F). The stiffness was significantly higher compared to the compounds with 30 wt% EP elastomer, despite being slightly below 1.8 GPa. The drawbacks of this formulation were a comparatively poor tensile strength and flowability and, especially, a higher density than most other materials. The samples containing EVA showed comparable tensile properties and acU to the other formulations but a significantly lower acN as well as a higher density. An Ashby plot of the modulus versus the acN is given in Figure 5, which shows that at a concentration of 20 wt%, EO4 gives the best combination of stiffness and notched impact strength, while at 10 wt%, the performance of EO2 and EO3 appears actually better than that of EO4. Among the other impact modifiers, the good mechanical properties of compounds containing SEBS are worth mentioning, but they are surpassed by EO2 and EO3 (at 10 wt%) and by EO4 (at 20 wt%).

J. Compos. Sci. 2019, 3, x 8 of 14

compatibility between matrix and modifier. However, the acN and stiffness were noticeably reduced compared to the other EO grades. Among the other compounds containing 20 wt% of impact modifier, the highest acN was achieved with SEBS (+153%), which is very close to that of EO4_20 but which came at the cost of a J.comparatively Compos. Sci. 2019 ,low3, 82 stiffness, even though it was still above the abovementioned target value of8 of 1.8 13 GPa from comparable commercial materials. The formulations containing SBS showed promising mechanical properties, with the acN being slightly lower than that of EO2 but the E-modulus being higher.slightly Therefore,higher. Therefore, an additional an additional formulation form withulation 30 wt%with SBS30 wt% was produced.SBS was produced. With this With material, this bymaterial, far the by best far ac Nthe of best all testedacN of samples all tested was samples achieved was (+ 213%achieved compared (+213% to compared PP20F). The to stiPP20F).ffness wasThe significantlystiffness was highersignificantly compared higher to thecompared compounds to the with compounds 30 wt% EPwith elastomer, 30 wt% despiteEP elastomer, being slightlydespite belowbeing slightly 1.8 GPa. below The drawbacks1.8 GPa. The of drawbacks this formulation of this were formulation a comparatively were a comparatively poor tensile strength poor tensile and flowabilitystrength and and, flowability especially, and, a higher especially, density a higher than most density other than materials. most other The samples materials. containing The samples EVA showedcontaining comparable EVA showed tensile comparable properties andtensile acU toproperties the other and formulations acU to the but other a significantly formulations lower but ac Na assignificantly well as a higher lower density.acN as well as a higher density. An Ashby plot of the modulus versus the accN is given in Figure 55,, whichwhich showsshows thatthat atat aa concentration of 20 wt%, EO4 gives the best combinationcombination of stiffness stiffness and no notchedtched impact strength, while at 10 wt%,wt%, thethe performanceperformance ofof EO2EO2 andand EO3EO3 appearsappears actuallyactually betterbetter thanthan thatthat ofof EO4.EO4. Among the otherother impactimpact modifiers, modifiers, the the good good mechanical mechanical properties properties of compounds of compounds containing containing SEBS areSEBS worth are mentioning,worth mentioning, but they but are they surpassed are surpassed by EO2 by and EO2 EO3 and (at EO3 10 wt%) (at 10 and wt%) by and EO4 by (at EO4 20 wt%). (at 20 wt%).

3.0 without modifier 2.8

2.6 10 wt% modifier content

2.4 20 wt% modifier content 2.2 E [GPa] 2.0

1.8

1.6 4681012

acN [kJ/m²]

PP20F EP1 EP2 EP3 EO1 EO2 EO3 EO4 SBS SEBS EVA

Figure 5. Ashby plot: Tensile modulus vs. notched impact strength—comparison of all tested impact modiﬁersmodifiers from the laboratory-scalelaboratory-scale productionproduction inin concentrationsconcentrations ofof 0,0, 10,10, andand 2020 wt%.wt%.

Figure6 6shows shows SEM SEM images images of cryo-fractured of cryo-fractured surfaces surfaces of the compoundsof the compounds containing containing 20% modifier. 20% Inmodifier. contrast In to contrast the EP formulation, to the EP formulation, the matrix and the dispersedmatrix and modifier dispersed phase modifier can clearly phase be can distinguished clearly be indistinguished all cases except in all EO1_20. cases except Between EO1_20. EO2, EO3,Between and EO2, EO4, theEO3, morphology and EO4, the appears morphology to become appears slightly to coarserbecome withslightly increasing coarser viscosity with increasing of the modifier. viscosity The of the respective modifier. dispersions The respective of SBS dispersions and EVA are of even SBS coarser;and EVA this are can even probably coarser; be this explained can probably by the high be explained modifier viscosity by the high in combination modifier viscosity with a low in compatibilitycombination with with a the low matrix compatibility in both cases. with Inthe contrast, matrix in the both dispersion cases. In of contrast, the SEBS the appears dispersion to be clearly of the finerSEBS thanappears that to of be the clearly SBS, and finer similar than that to that of the of the SBS, EO2. and similar to that of the EO2.

3.3. Upscaling and Comparison with Commercial Reference Materials For the comparison with commercially available high-impact materials, selected formulations from the present study (EO2_20, EO3_20, and EO4_20 and the base material PP20F as a reference) were produced on a larger scale. The EO grades were chosen because of the good overall mechanical performance of the compounds containing 20 wt% modifier. Despite their high respective acN values, SBS_30 and SEBS_20 were omitted from the up-scaling because of their lower stiffness and, in case of the SBS, also because of its high density. In addition, because of the historic importance of these materials as impact modifiers for PP, a grade of ethylene-propylene rubber (EPR) and a grade of ethylene-propylene-diene rubber (EPDM) were introduced for comparison with the EO grades. As commercial references, the PP–talc J. Compos. Sci. 2019, 3, 82 9 of 13 composites Hostacom TRC 333N (H_TRC; 20% filler content) and Hostacom EKC 330N (H_EKC; 16% filler content) were tested. Due to its very similar composition, the commercial high-impact J. Compos. Sci. 2019, 3, x 10 of 15 PP–cellulose composite Formi EXP 20 (20 wt% filler content) was added as a reference, using mechanical data as given on the technical data sheet [27].

(a) (b) (c) (d)

(e) (f) (g) (h)

Figure 6. SEMSEM images images of of cryo cryo-fractured-fractured surfaces: surfaces: (a) ( aEP3_20,) EP3_20, (b) ( bEO1_20,) EO1_20, (c) (EO2_20,c) EO2_20, (d) (EO3_20,d) EO3_20, (e) (EO4_20,e) EO4_20, (f) SBS_20, (f) SBS_20, (g) (SEBS_20,g) SEBS_20, (h) (EVA_20.h) EVA_20.

3.3. UpscalingAccording and to C theomparison data from with PP20F, Commercial EO2_20, Reference EO3_20, M andaterials EO4_20, the compound production on a larger scale resulted in a notable improvement of the impact properties, especially the unnotched impact strength.For the This comparison may be related with tocommercially a reduced damage available of thehigh polymers-impact materials, and fibers selected during compounding formulations (becausefrom the ofpresent the higher study normalized (EO2_20, EO3_20 throughput, and in EO4_20 the upscaled and the production base material and thusPP20F a reducedas a refere energynce) wereinput), produced and in the on case a larger of the scale. impact modified compounds, also to a better dispersion of the modifier, as illustratedThe EO bygrades a comparison were chosen of the because SEM images of the of good EO4_20_Lab overall mechanical and EO4_20_Up performance (cf. Figure of the7). J. Compos. Sci. 2019, 3, x 11 of 15 compoundsMeanwhile, the containing tensile properties 20 wt% modifier. remained Despitealmost una theirffected high (cf. respective Table3). acN values, SBS_30 and SEBS_20 were omitted from the up-scaling because of their lower stiffness and, in case of the SBS, also because of its high density. In addition, because of the historic importance of these materials as impact modifiers for PP, a grade of ethylene-propylene rubber (EPR) and a grade of ethylene-propylene-diene rubber (EPDM) were introduced for comparison with the EO grades. As commercial references, the PP–talc composites Hostacom TRC 333N (H_TRC; 20% filler content) and Hostacom EKC 330N (H_EKC; 16% filler content) were tested. Due to its very similar composition, the commercial high-impact PP–cellulose composite Formi EXP 20 (20 wt% filler content) was added as a reference, using mechanical data as given on the technical data sheet [27]. According to the data from PP20F, EO2_20, EO3_20, and EO4_20, the compound production on a larger scale resulted in a notable improvement of the impact properties, especially the unnotched impact strength. This may be related(a) to a reduced damage( b of) the polymers and fibers during compounding (because of the higher normalized throughput in the upscaled production and thus a Figure 7. SEM images of cryo cryo-fractured-fractured surfaces: EO4_20_Lab ( a) and EO4 EO4_20_Up_20_Up (b). reduced energy input), and in the case of the impact modified compounds, also to a better dispersion of the modifier, as illustrated by a comparison of the SEM images of EO4_20_Lab and EO4_20_Up ATable comparison 3. Absolute of the and ethylene-octene specific mechanical grades properties EO2, EO3, of and the EO4compounds shows that from the the viscosity upscaled of the (cf.modifiers Figureproduction increases7). Meanwhile, (suffix in the “_Up”), the order tensile comparison EO2 properties< EO3 with< EO4, remained laboratory while thealmost-scale Shore productionunaffected hardness (cf. (suffix decreases Table “_Lab”) 3). in that and order. Likewise,commercial the ac referenceU and ac materialsN values (these of the are, corresponding by definition, “Up”). compounds from the up-scaled production increase in this order, while the tensile strength and (albeit not significantly) the modulus decrease. Absolute Values Specific Values (Normalized by Density)

E M acU can E_sp M_sp acU_sp acN_sp Code [g/cm3 [GPa] [MPa] [kJ/m²] [kJ/m²] [GJ/kg] [MJ/kg] [kJ m/kg] [kJ m/kg] ] PP20F_Lab 0.981 2.76 40.1 41.0 4.5 2.81 40.8 41.8 4.6 PP20F_Up 0.980 2.82 41.0 43.4 5.5 2.88 41.9 44.4 5.6 EO2_20_Lab 0.977 2.03 29.4 38.4 10.2 2.07 30.1 39.3 10.4 EO2_20_Up 0.972 2.03 28.9 39.7 10.7 2.09 29.8 40.9 11.0 EO3_20_Lab 0.972 2.09 29.3 34.4 10.1 2.15 30.1 35.4 10.3 EO3_20_Up 0.973 2.01 28.3 41.8 12.2 2.06 29.1 42.9 12.5 EO4_20_Lab 0.979 2.04 28.8 36.7 11.6 2.08 29.5 37.5 11.9 EO4_20_Up 0.976 1.96 26.8 43.3 13.3 2.01 27.4 44.4 13.6 EPR_20_Up 0.973 2.18 29.4 36.1 8.7 2.24 30.3 37.1 9.0 EPDM_20_Up 0.976 2.13 28.6 35.2 8.5 2.18 29.3 36.0 8.7 H_TRC 1.062 1.90 21.4 >160 a 31.7 1.79 20.2 >151 b 29.9 H_EKC 1.020 1.63 19.4 >160 a 27.3 1.60 19.0 >157 b 26.8 Formi EXP 20 b 0.99 1.80 33 45 8.8 1.82 33.3 45.5 8.9 a With both Hostacom grades, no fracture occurred upon impact with a 7.5 J pendulum. b Data for Formi EXP 20 were obtained from the technical data sheet [27].

A comparison of the ethylene-octene grades EO2, EO3, and EO4 shows that the viscosity of the modifiers increases in the order EO2 < EO3 < EO4, while the Shore hardness decreases in that order. Likewise, the acU and acN values of the corresponding compounds from the up-scaled production increase in this order, while the tensile strength and (albeit not significantly) the modulus decrease. These trends are much clearer in the up-scaled production compared to the lab-scale production, especially between EO2 and EO3, which may again be related to a better dispersion. There was little difference in the mechanical properties between the compounds containing EPR and EPDM. Thus, the diene content of the EPDM did not appear to have a significant influence on the mechanical properties. In both cases a slightly higher stiffness and similar tensile strength, compared to the EO grades, were achieved. However, the impact performance, in particular the acN, was considerably reduced. In comparison with the talc reinforced Hostacom grades, none of the materials tested in this study come close to their impact performance. However, all of the cellulose fiber reinforced PPs have a higher tensile strength and a significantly lower density (cf. Table 3). Especially the latter point is of particular importance for all considerations concerning lightweight construction and may therefore

J. Compos. Sci. 2019, 3, 82 10 of 13

These trends are much clearer in the up-scaled production compared to the lab-scale production, especially between EO2 and EO3, which may again be related to a better dispersion.

Table 3. Absolute and specific mechanical properties of the compounds from the upscaled production (suffix “_Up”), comparison with laboratory-scale production (suffix “_Lab”) and commercial reference materials (these are, by definition, “Up”).

Absolute Values Speciﬁc Values (Normalized by Density) E a U can E_sp _sp a U_sp a N_sp Code [g/cm3] M c M c c [GPa] [MPa] [kJ/m2] [kJ/m2] [GJ/kg] [MJ/kg] [kJ m/kg] [kJ m/kg] PP20F_Lab 0.981 2.76 40.1 41.0 4.5 2.81 40.8 41.8 4.6 PP20F_Up 0.980 2.82 41.0 43.4 5.5 2.88 41.9 44.4 5.6 EO2_20_Lab 0.977 2.03 29.4 38.4 10.2 2.07 30.1 39.3 10.4 EO2_20_Up 0.972 2.03 28.9 39.7 10.7 2.09 29.8 40.9 11.0 EO3_20_Lab 0.972 2.09 29.3 34.4 10.1 2.15 30.1 35.4 10.3 EO3_20_Up 0.973 2.01 28.3 41.8 12.2 2.06 29.1 42.9 12.5 EO4_20_Lab 0.979 2.04 28.8 36.7 11.6 2.08 29.5 37.5 11.9 EO4_20_Up 0.976 1.96 26.8 43.3 13.3 2.01 27.4 44.4 13.6 EPR_20_Up 0.973 2.18 29.4 36.1 8.7 2.24 30.3 37.1 9.0 EPDM_20_Up 0.976 2.13 28.6 35.2 8.5 2.18 29.3 36.0 8.7 H_TRC 1.062 1.90 21.4 >160 a 31.7 1.79 20.2 >151 b 29.9 H_EKC 1.020 1.63 19.4 >160 a 27.3 1.60 19.0 >157 b 26.8 Formi EXP 20 b 0.99 1.80 33 45 8.8 1.82 33.3 45.5 8.9 a With both Hostacom grades, no fracture occurred upon impact with a 7.5 J pendulum. b Data for Formi EXP 20 were obtained from the technical data sheet [27].

There was little difference in the mechanical properties between the compounds containing EPR and EPDM. Thus, the diene content of the EPDM did not appear to have a significant influence on the mechanical properties. In both cases a slightly higher stiffness and similar tensile strength, compared to the EO grades, were achieved. However, the impact performance, in particular the acN, was considerably reduced. In comparison with the talc reinforced Hostacom grades, none of the materials tested in this study come close to their impact performance. However, all of the cellulose fiber reinforced PPs have a higher tensile strength and a significantly lower density (cf. Table3). Especially the latter point is of particular importance for all considerations concerning lightweight construction and may therefore be of great interest for automotive applications [2]. The tensile modulus of most formulations containing up to 20 wt% modifier (except EP2_20, EP3_20) is comparable or better than that of H_TRC. The only formulations of which the stiffness values do not clearly exceed that of H_EKC are those with 30 wt% of any of the EP grades. If the specific mechanical properties are considered, the advantage of the NFCs in terms of stiffness and strength compared to the talc-filled grades becomes even more obvious, while their deficit regarding the impact performance becomes slightly less pronounced (cf. Table3). The cellulose reinforced reference material Formi EXP20 has a higher acU than all other materials prepared for this study; however, the majority of tested formulations containing 20 wt% of modifier, particularly those with the EO grades (except EO1), SBS and SEBS, have a clearly superior acN. Almost all formulations with 20 wt% modifier have a lower tensile strength but a comparable or higher stiffness than Formi EXP20, the exceptions being EP2_20 and EP3_20, which have a lower stiffness. With most modifiers, except SBS and EVA, a lower density than that of Formi EXP20 was achieved (cf. Table3). Figure8 shows an Ashby plot of the modulus versus the a cN of the formulations prepared in the upscaled production and the commercial reference materials. Evidently the ratio between stiffness and notched impact strength, while at 10 wt%, the performance of EO2 and EO3 appears actually better than that of EO4. Among the other impact modifiers, the compounds containing SEBS show the J. Compos. Sci. 2019, 3, x 12 of 15

be of great interest for automotive applications [2]. The tensile modulus of most formulations containing up to 20 wt% modifier (except EP2_20, EP3_20) is comparable or better than that of H_TRC. The only formulations of which the stiffness values do not clearly exceed that of H_EKC are those with 30 wt% of any of the EP grades. If the specific mechanical properties are considered, the advantage of the NFCs in terms of stiffness and strength compared to the talc-filled grades becomes even more obvious, while their deficit regarding the impact performance becomes slightly less pronounced (cf. Table 3). The cellulose reinforced reference material Formi EXP20 has a higher acU than all other materials prepared for this study; however, the majority of tested formulations containing 20 wt% of modifier, particularly those with the EO grades (except EO1), SBS and SEBS, have a clearly superior acN. Almost all formulations with 20 wt% modifier have a lower tensile strength but a comparable or higher stiffness than Formi EXP20, the exceptions being EP2_20 and EP3_20, which have a lower stiffness. With most modifiers, except SBS and EVA, a lower density than that of Formi EXP20 was achieved (cf. Table 3). Figure 8 shows an Ashby plot of the modulus versus the acN of the formulations prepared in J.the Compos. upscaled Sci. 2019 production, 3, 82 and the commercial reference materials. Evidently the ratio between11 of 13 stiffness and notched impact strength, while at 10 wt%, the performance of EO2 and EO3 appears actually better than that of EO4. Among the other impact modifiers, the compounds containing SEBS bestshow overall the best mechanical overall mechanical properties, properties, but they are but surpassed they are bysurpassed EO2 and by EO3 EO2 (at and 10 wt%) EO3 and(at 10 by wt%) EO4 (atand 20 by wt%). EO4 (at 20 wt%).

3.0

2.8 PP20F

2.6

2.4 EPR / EPDM

2.2 EO elastomers commercial PP–talc E [GPa] E 2.0

1.8 commercial 1.6 PP–cellulose 1.4 4 9 14 19 24 29 34

acN [kJ/m²] PP20F_Up EO2_20_Up EO3_20_Up EO4_20_Up EPR_20_Up EPDM_20_Up Hostacom TRC333N Hostacom EKC330N Formi EXP20

Figure 8. 8. AshbyAshby plot: plot: Tensile Tensile modulus modulus vs. vs. notched notched impact impact strength—comparison strength—comparison of compounds of compounds from thefrom up-scaled the up-scaled production production with commercialwith commercial materials. materials.

4. Conclusions By adding a thermoplastic elastomer, thethe notched impact strength of cellulose reinforced PPPP can be considerablyconsiderably improved without significantlysignificantly aaffectingffecting the unnotched impact strength. However, thethe enhanced enhanced impact properties come at the cost of a reduction in stistiffnessffness and tensile strength; depending onon thethe desireddesired application,application, thisthis isis likelylikely oneone ofof thethe mostmost importantimportant limitinglimiting factors.factors. While a high viscosity and a low hardness of the modifiermodifier appear to be beneficialbeneficial for the impact behavior, a a soft soft material material also also leads leads to a to more a more pronounced pronounced loss in loss sti ff inness. stiffness. Among Among the materials the materials tested intested this study,in this ethylene-octene-based study, ethylene-octene POE-based and POE styrene-butadiene-based and styrene-butadiene elastomers-based elastomers led to the best led overall to the mechanicalbest overall performance. mechanical Within performance. the given Within boundary the conditions, given boundary for most conditions, of the modifiers for most a content of the of 20modifiers wt% yielded a content the best of results; 20 wt% in yielded the case the of the best used results; SBS grade in the with case a of content the used of 30 SBS wt%, grade a very with good a notchedcontent ofimpact 30 wt% strength, a very was good achieved notched while impact at the same strength time was retaining achieved acceptable while attensile the same properties time butretaining at the cost acceptable of an increased tensile propertiescomposite density. but at theAn increased cost of an production increased scale composite did not significantlydensity. An affect the tensile properties, but a clear tendency towards an improved impact performance could be observed. Generally, the observed influence of the different types of modifiers on the mechanical performance of the PP–cellulose composite reflects what is known from the impact modification of neat PP—according to the literature, thermoplastic elastomers, in particular ethylene-octene elastomers, appear to be more effective than conventional modifiers such as EPR or EPDM [32,33]. One can therefore conclude that the impact modifiers that were used for this study mainly affect the properties of the PP matrix and that any possible interaction with the fibers plays only a minor role. This conclusion is supported by the SEM images, which do not suggest any adhesion between the fibers and the impact modifiers. Of all the formulations that were tested in the course of this study, the most well-balanced property profile, with an excellent impact performance and also good tensile properties, was achieved with 20 wt% of EO4. Some of the tested materials showed a clearly improved mechanical performance over the commercial cellulose reinforced reference material. While the impact properties of the commercial talc-filled reference materials were out of reach, their tensile properties could be surpassed. In addition, due to the lower density, a cellulose reinforced compound might find opportunities for applications where weight reduction plays a role. J. Compos. Sci. 2019, 3, 82 12 of 13

The mechanical property profiles indicate a high potential for use in automotive interior applications. In that regard, the EO-modified composites are of particular interest, as metallocene-catalyzed polymers offer the additional benefit of a low level of volatile organic compound (VOC) emissions because of their well-defined molar mass distributions [34–36].

Author Contributions: Conceptualization, M.M. and L.S.; Investigation, M.M.; Methodology, M.M. and L.S.; Project administration, L.S., C.P. and C.U.; Supervision, L.S., C.P. and C.U.; Visualization, M.M.; Writing—original draft, M.M.; Writing—review & editing, M.M., L.S., C.P. and C.U. Funding: This work was funded within the scope of the COMET programme of the Austrian Research Promotion Agency (FFG). Part of this work was performed within the project BioCarb K, supported by the European Regional Development Fund (EFRE) and the province of Upper Austria through the program IWB 2014–2020. Conﬂicts of Interest: The authors declare no 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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