sustainability

Case Report Compression Molded Thermoplastic Composites Entirely Made of Recycled Materials

Petri Sormunen * and Timo Kärki Fiber Composite Laboratory, Department of Mechanical Engineering, LUT University, P.O. Box 20, 53850 Lappeenranta, Finland; timo.karki@lut.fi * Correspondence: petri.s.sormunen@lut.fi; Tel.: +358-503-412-551



Received: 4 December 2018; Accepted: 23 January 2019; Published: 25 January 2019


Abstract: Recycled post-consumer high-density polyethylene pipe plastic was agglomerated into composite samples with wood, glass fiber, mineral wool, gypsum, and soapstone as recycled particulate fillers. The tensile strength, tensile modulus, impact strength, and hardness were the mechanical properties evaluated. Scanning electron microscopy was performed on the broken surfaces of tensile strength samples to study the interfacial interactions between the composite matrix and the filler materials. Heat build-up, water absorption, and thickness swelling were the physical properties measured from the composites. The addition of particulate fillers demonstrated the weakening of the tensile and impact strength but significantly improved the rigidity of the post-consumer plastic. The composites filled with minerals had mechanical properties comparable to compression molded wood plastic composites but higher resistance to moisture. A lack of hot-melt mixing affected the mechanical properties adversely.

Keywords: composites; recycling; mechanical properties; physical properties

1. Introduction The addition of organic and inorganic fillers to polymers has been an important industrial method in creating new materials with tailored properties for specific applications. The legislative pressure pertaining to waste materials in Europe reinforces the need for identifying new uses for previously discarded wastes as recycled materials. The use of selected recycled particulates from post-consumer and industrial by-product waste currents as property changing fillers for polymers could be a way to reuse such materials in the manufacturing of new products. Properties of polymers such as corrosion resistance, light weight, and ease of processing into a variety of shapes can be combined with the unique properties of fillers to form composites with modified appearance, cost, mechanical strength, thermal and electrical conductivity, thermal stability, magnetic characteristics, flame retardant, electromagnetic shielding, dielectric, and barrier properties [1,2]. Productivity can also be increased with particulate fillers due to decreased specific heat and increased heat conductivity [3,4]. The filler addition affects the composite system also by introducing new interfaces, imperfections, impurities, and flaws, which in turn affect the thermal properties and can interfere the processability [5]. Particulate fillers have traditionally benefitted polymers by decreasing the amount of costs of production, which can best be achieved by using the largest amount of filler possible, while retaining the integrity of the polymer matrix holding the product together. The most common fillers for thermoplastics are calcium carbonate, talc, silica flour, clay and wood fiber. To change the properties of polymers, the use of high filler rates is often required, which will lower most of the mechanical properties of the material, reducing the possible field applications. If the filler content of the recycled particulates was high, the environmental, cost, and property-changing effects would potentially be greater in the composite. The high filling rate has often a negative effect on the impact strength, elongation to break, tensile strength, and flexural

Sustainability 2019, 11, 631; doi:10.3390/su11030631 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 631 2 of 17 strength [2]. Improvements by particulate fillers are most likely to take place in the elastic modulus of the composite. The use of low-cost filler content decreases the costs in thermoplastic composites when the relatively virgin plastic is replaced [6]. It could also be an economical way to change the properties of the recycled thermoplastics. The main aim of this paper is to study mechanical, moisture resistance, and heat build-up properties of compression molded high-density polyethylene (HDPE) composites filled with recycled wood fiber, glass fiber, mineral wool, gypsum, and soapstone particles by empirical testing in order to evaluate the effect of recycled low-cost fillers on the recycled plastic. The analytical methods presented by Halpin-Tsai [7] or Mori-Tanaka [8], which are often used to evaluate the effects of filler geometry, stiffness and orientation were not used in this study due to diverse types of materials used, expected variance in the recycled material and the strong particle–particle interactions caused by the high filler content. For more information about the issues related to analytical method the reader is referred to the work of Fornes & Paul [9]. The comparison of composite properties achieved with different recycled materials could help potential users to rate materials for a variety of open loop recycling thermoplastic based applications such as plastic lumber [10], packaging material, pallets [11], green space and outdoor construction, agriculture and livestock, public construction and traffic barriers [12]. Previously, the compression molding of recycled material composites with a high filler content has been done using kinetic mixing to heat the material [13], a heated mold [14,15], and pressing extruded re-heated composite sheets [16]. In this study, the material is heated in an oven without pressure or mixing.

2. Materials and Methods

2.1. Materials Recycled high-density polyethylene (rHDPE) from a construction waste sewer pipe was supplied by the Destaclean company and used as matrix material with all the samples. The plastic was identified by manual sorting and recycled mechanically, after which it was crushed to smaller fractions using Untha LR 630. Spruce (Picea abies) was prepared into two particle sizes, fine wood flour of 20 mesh (0.85 mm) and rough hammer milled wood flour of over 20 mesh (max 4.00 mm). The specific gravity of wood flour is 1.3–1.4 g/cm3 [17]. The size of the wood was reduced with a chipper combined with a hammer mill after which it was sieved to the designated size. Glass fiber was the by-product reject from the glass fiber mat production of the Ahlstrom Corporation. The glass fiber reject was chopped to a size of 25 mm with the density 2.5 g/cm3, which became even smaller in the agglomeration phase. Recycled rock wool from construction waste was crushed and milled to powder. The Paroc Group provided by-product grinding dust from rock wool manufacturing, which was collected with a 3 vacuum used in the process. Rock wool consists mainly of silicon dioxide (40–52%, SiO2, 2.65 g/cm ), calcium oxide (10–12%, CaO, 3.35 g/cm3), magnesium oxide (8–15%, MgO, 3.58 g/cm3), and aluminum 3 oxide (8–13%, Al2O3, 3.95 g/cm )[18]. Recycled construction and demolition waste gypsum was provided by the material handling facility Etelä-Karjalan Jätehuolto. The gypsum waste was ground to powder, and the powdered gypsum wall board specific gravity is 1.76 g/cm3 [19]. Tulikivi provided the by-product soapstone powder from its stone sawing process. Soapstone consists primarily of 3 3 talc (40–50%, 2.98 g/cm ) Mg3Si4O10(OH)2 and magnesium carbonate (40–50%, 2.96 g/cm ) MgCO3 in almost equal quantities. In addition, there are small proportions of chlorite (2–10%, 2.42 g/cm3) 3 H4(Mg,Fe)2Al2SiO11 and magnetite (0–15%, 5.15 g/cm ) (Fe3O4)[20]. As a coupling agent, 3% of DuPont™ Fusabond® E226 anhydride modified polyethylene was used with the density of 0.93 g/cm3, the melt flow rate (190 ◦C/2.16kg) 1.75 g/10min, and the declared melting point 120 ◦C. Maleic anhydride modified polyethylene has been used to improve the interfacial adhesion between the organic fillers and polymer matrix [21–24]. Processing additive Struktol® TPW 113—a blend of complex modified fatty acid ester—was used with all of the samples, with a density of 1.005 g/cm3 and a dropping point of 70–88 ◦C. Sustainability 2019, 11, 631 3 of 17

2.2. Agglomeration of Composites The filler material, recycled HDPE, coupling agent and processing additive were compounded using a TRL 100/FV/W turbomixer combined with an RFV-200 cooler. The wood materials for NFF and NFR recipes were not pre-dried. The composites were prepared into 14 different formulations. Tables1 and2 present the composition of the prepared composite agglomerates. The density by rule of mixture (RoM) was calculated according to the materials used. After the composites were produced, the density was estimated using the weight and the molded volume of the samples.

Table 1. Composite formulations, part I (the proportion of material by weight).

REF NFR40 NFR60 NFF40 NFF60 GF40 GF60 HDPE, recycled 94% 54% 34% 54% 34% 54% 34% Wood flour, rough recycled 0% 40% 60% 0% 0% 0% 0% Wood flour, fine recycled 0% 0% 0% 40% 60% 0% 0% Glass fiber, by-product 0% 0% 0% 0% 0% 40% 60% Coupling agent 3% 3% 3% 3% 3% 3% 3% Processing additive 3% 3% 3% 3% 3% 3% 3% Density by Rule-of-mixture g/cm3 0.95 1.09 1.16 1.13 1.22 1.57 1.88 Measured density g/cm3 0.95 ± 0.00 1.02 ± 0.01 1.03 ± 0.02 1.02 ± 0.02 1.05 ± 0.01 1.21 ± 0.03 1.24 ± 0.10 Porosity 0.1% 6.5% 11.3% 9.8% 14.0% 23.0% 34.1%

Table 2. Composite formulations, part II (the proportion of material by weight).

MWR40 MWR60 MWB40 MWB60 GYP40 GYP60 SS40 SS60 Mineral wool, recycled 40% 60% 0% 0% 0% 0% 0% 0% Mineral wool, by-product 0% 0% 40% 60% 0% 0% 0% 0% Gypsum, recycled 0% 0% 0% 0% 40% 60% 0% 0% Soap-stone, by-product 0% 0% 0% 0% 0% 0% 40% 60% Coupling agent 3% 3% 3% 3% 3% 3% 3% 3% Processing additive 3% 3% 3% 3% 3% 3% 3% 3% Density by rule-of-mixture 1.62 1.95 1.62 1.95 1.28 1.44 1.76 2.17 g/cm3 Measured density g/cm3 1.28 ± 0.02 1.37 ± 0.01 1.33 ± 0.01 1.57 ± 0.01 1.22 ± 0.01 1.37 ± 0.03 1.32 ± 0.01 1.62 ± 0.02 Porosity 20.8% 29.7% 17.7% 19.4% 4.3% 4.7% 25.1% 25.3%

2.3. Composite Manufacturing A compression molding tool was used to make 110 × 110 × 10 mm square plates for the manufacturing of impact strength and water absorption samples. The composite agglomerate was heated in an oven to 165–200 ◦C, after which it was manually transferred to the compression molding tool see Figure1 for the set-up. A pressure of 24.3 MPa was applied for 60 s to mold the samples. The temperature of the material inside the oven before compression and immediately before demolding was measured with Mastercool Infrared Thermometer OUTPUT <1 mW AT 630~670 nm CLASS II and documented. The tensile strength specimens were prepared with a compression molding tool see Figure2. The tool and the press had no heating or cooling capability. The mold had channels to regulate overfilling caused by the manual transfer of heated material into the mold. The tools were mounted on a hydraulic C-frame press made by Stenhøj A/S coupled with CA-1000 connector accessory with VI Logger version 2.01 made by National Instruments. The processing of 110 × 110 × 10 mm plates was successful with all the materials, but the thinner tensile strength specimens were not possible to process with the GF60 composition as the specimens broke during demolding. Sustainability 2018, 10, x FOR PEER REVIEW 4 of 17 Sustainability 2019, 11, 631 4 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 4 of 17

Figure 1. The setup for the compression molding of samples. Figure 1. The setup for the compression molding of samples. Figure 1. The setup for the compression molding of samples.

Figure 2. AA tool tool constructed constructed for the compression molding of tensile strength samples. Figure 2. A tool constructed for the compression molding of tensile strength samples. 2.4. Mechanical Property Measurements 2.4. Mechanical Property Measurements 2.4. MechanicalThe tensile Property properties Measurements strength and elastic modulus were measured with a Zwick Roell Z020 The tensile properties strength and elastic modulus were measured with a Zwick Roell Z020 testing machine. The measurement of the tensile modulus was performed according to ISO 527-1 [25] testingThe machine. tensile propertiesThe measurement strength of and the tensileelastic modulus waswere performed measured according with a Zwick to ISO Roell 527-1 Z020 [25] with a testing speed of 1 mm/min. The compression mold specimens were of the ISO 527-2/1A/1 type, testingwith a testing machine. speed The of measurement 1 mm/min. The of the compression tensile modulus mold specimens was performed were accordingof the ISO to527-2/1A/1 ISO 527-1 type, [25] and 12 specimens were prepared for each sample see Figure3. The samples for the Charpy impact withand 12 a testing specimens speed were of 1 preparedmm/min. Thefor each compression sample see mold Figure specimens 3. The weresamples of the for ISO the 527-2/1A/1 Charpy impact type, strength were sawn from molded 110 × 110 × 10 mm square plates (see Figure4) into the length andstrength 12 specimens were sawn were from prepared molded for110 each× 110 sample × 10 mm see square Figure plates 3. The (see samples Figure for 4) intothe Charpythe length impact 80 ± 80 ± 1 mm, width 10 ± 1 mm, and thickness 4 ± 1 mm; 12 samples of each material were prepared. strength1 mm, width were 10sawn ± 1 frommm, moandlded thickness 110 × 1104 ± ×1 10mm; mm 12 square samples plates of each (see materialFigure 4) were into theprepared. length 80The ± The Charpy impact strength was measured at 21 ◦C with the Zwick 5102 impact tester. The impact 1Charpy mm, width impact 10 strength± 1 mm, wasand thicknessmeasured 4 at ± 121 mm; °C with12 samples the Zwick of each 5102 material impact were tester. prepared. The impact The velocity of the pendulum was 2.93 m/s, the pendulum length 225 mm, and the angle of deflection Charpyvelocity ofimpact the pendulum strength was 2.93measured m/s, the at pendul 21 °C umwith length the Zwick225 mm, 5102 and impa the anglect tester. of deflection The impact 160 160 degrees. Brinell hardness was tested with Zwick Roell indentation testing equipment. The diameter velocitydegrees. ofBrinell the pendulum hardness waswas 2.93tested m/s, with the Zwick pendul Roeumll length indentation 225 mm, testing and theequipment. angle of deflectionThe diameter 160 for the pressing ball was 10 mm and the pressing force of 1 kN was applied for 25 s. The dent was degrees.for the pressing Brinell hardnessball was 10 was mm tested and thewith pressing Zwick Roeforcell indentationof 1 kN was testingapplied equipment. for 25 seconds. The diameterThe dent measured with a caliber after the minimum wait time of 2 min. The fracture morphology of the broken forwas the measured pressing with ball awas caliber 10 mm after and the the minimum pressing wait force time of 1 of kN 2 minutes.was applied The for fracture 25 seconds. morphology The dent of tensile strength samples was studied with a Hitachi SU3500 scanning electron microscope (SEM). thewas broken measured tensile with strength a caliber samples after the was minimum studied withwait atime Hitachi of 2 SU3500minutes. scanning The fracture electron morphology microscope of the(SEM). broken tensile strength samples was studied with a Hitachi SU3500 scanning electron microscope (SEM). Sustainability 20192018, 1110, 631x FOR PEER REVIEW 5 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 17

Figure 1. Tensile strength samples after molding. Figure 1. 3. TensileTensile strength samples after molding.

Figure 4. Compression molded square plate before cutting. Figure 4. Compression molded square plate before cutting. Figure 4. Compression molded square plate before cutting. 2.5. Moisture Absorption and Thickness Swelling 2.5. Moisture Absorption and Thickness Swelling 2.5. MoistureThe moisture Absorption absorption and Thickness and thickness Swelling swelling of the composites were determined according to EN 317:1993The moisture [26]. Forabsorption each material, and thickness 12 samples swelling were manufacturedof the composites by compressionwere determined molding according see an The moisture absorption and thickness swelling of the composites were determined according toexample EN 317:1993 in Figure [26].5. MoistureFor each material, absorption 12 (MA) samp andles were thickness manufactured swelling (TS) by compression were calculated molding with thesee to EN 317:1993 [26]. For each material, 12 samples were manufactured by compression molding see anfollowing example equations in Figure 5. Moisture absorption (MA) and thickness swelling (TS) were calculated with an example in Figure 5. Moisture absorption (MA) and thickness swelling (TS) were calculated with the following equations mt − mo the following equations MA = × 100% (1) 𝑚 m𝑚o MA 100% (1) 𝑚𝑚𝑚 where mo and mt are the mass of the sampleMA in grams before 100% and after immersion. (1) 𝑚 where m0 and mt are the mass of the sample in grams before and after immersion. Tt − To where m0 and mt are the mass of the sampleTS = in grams× before100% and after immersion. (2) 𝑇 T𝑇 TS o 100% (2) 𝑇 𝑇 𝑇 T T TS 100% (2) where o and t are the thickness of the sample in𝑇 millimeters before and after immersion. where T0 and Tt are the thickness of the sample in millimeters before and after immersion. where T0 and Tt are the thickness of the sample in millimeters before and after immersion. Sustainability 2019, 11, 631 6 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 6 of 17

Figure 5. Moisture absorption samples immersed in water. Figure 5. Moisture absorption samples immersed in water. 2.6. Heat Build-up and Color Measurements 2.6. Heat Build-up and Color Measurements The heat build-up of the composites was tested according to EN 15534-1 [27]. The samples with dimensionsThe heat 100 build-up× 10 × 4of mm the werecomposites tested. was Three tested specimens according for each to EN type 15534-1 of composite [27]. The were samples measured with withdimensions the black 100 control × 10 × 4 specimen. mm were An tested. infrared Three heat spec lampimens with for 250each W type nominal of composite power was were used measured to heat withup the the samples. black control The distance specimen. from An the infrared lowest heat part lamp of the with lamp 250W to the nominal bottom power of the boxwas wasused 375 to heat mm. Theup the method samples. measured The distance the relative from the value lowest of heat part build-up of the lamp compared to the bottom with a of black the controlbox was specimen 375 mm. Theunder method the defined measured conditions the relative and doesvalue notof heat predict build-up the application compared temperaturewith a black incontrol real situations.specimen Theunder used the blackdefined control conditions specimen and was does made not predict of the same the application material as temperature the reference in (REF) real situations. so that the The test usedresults black would control show specimen the possible was changes made of caused the same by the material application as the of reference filler materials. (REF) so that the test resultsA would Minolta show CM-2600d the possible spectrophotometer changes caused was by the used application to determine of filler the materials. colors of the composite samplesA Minolta according CM-2600d to the CIE spectrophotometer (Commission Internationale was used to de determine l’Eclairage) the L*a*b colors color of system. the composite samples according to the CIE (Commission Internationale de l’Eclairage) L*a*b color system. 3. Results and Discussion 3. Results and Discussion 3.1. Mechanical Properties of Composites 3.1. Mechanical Properties of Composites 3.1.1. Impact Strength 3.1.1.The Impact impact Strength resistance of polymers depends on geometry, the mode of loading, the load application rate, the loading environment, and polymer properties such as chain length, packing, chemical The impact resistance of polymers depends on geometry, the mode of loading, the load arrangement units in the polymer chain, alignment, and bonding forces. Therefore, the strength is application rate, the loading environment, and polymer properties such as chain length, packing, the sum of properties that contribute to the dissipation of the forces of impact [28]. The effects of chemical arrangement units in the polymer chain, alignment, and bonding forces. Therefore, the strengtha high fill is rate the onsum the of Charpy properties impact that strengthcontribute of theto the recycled dissipation plastic of were the forces significant of impact (see Figure[28]. The6). Theeffects impact of a high strength’s fill rate decreasing on the Charpy effect withimpact an st increasedrength of particle the recycled content plastic [29] waswere found significant across (see the ± 2 ± 2 Figurestudied 6). materials, The impact with strength’s an exception decreasing in SS40 effect (4.86 with1.31 an kJ/m increased) and SS60particle (5.27 content2.59 [29] kJ/m was), wherefound the mean value with a 60% fill rate was higher. Compared to the REF (72.31 ± 9.94 kJ/m2) material, across the studied materials, with an exception in SS40 (4.86 ± 1.31 kJ/m2) and SS60 (5.27 ± 2.59 kJ/m2), the drop-in impact strength was on average 90%, as Figure6 shows. Glass fiber GF40 had the highest where the mean value with a 60% fill rate was higher. Compared to the REF (72.31 ± 9.94 kJ/m2) ± 2 material,impact strength the drop-in of the impact measured strength composites was on withaverag fillere 90%, (12.71 as Figure4.76 kJ/m6 shows.). According Glass fiber to GF40 previous had studies, the high impact test results with glass fiber thermoplastic composites have attributed to strong the highest impact strength of the measured composites with filler (12.71 ± 4.76 kJ/m2). According to previousinteraction studies, between the the high silanol impact groups test of theresults glass with surface glass and fiber the anhydridethermoplastic group composites of the coupling have attributedagent [30,31 to]. strong The standard interaction deviation between in GF40the sila resultsnol groups was the of highestthe glass of surface all tested and materials, the anhydride which wasgroup likely of the caused coupling by the agent agglomeration [30,31]. The phase standard where deviation the fibers in were GF40 broken results into was random the highest lengths of and all testedclustered materials, heavily which together, was creating likely caused discontinuities by the agglomeration and porosity in phase the thermoplastic where the fibers matrix. were This broken same intoreason random might belengths the cause and ofclustered the higher heavily results together, in GF40, creating as the clustering discontinuities of fibers and also porosity created in zones the wherethermoplastic the tough matrix. thermoplastic This same was reason the dominant might be material. the cause This of seemsthe higher to be results supported in GF40, by the as SEM. the ± 2 Thisclustering effect of was fibers not also as strong created in zones the composite where the GF60tough (5.72 thermoplastic2.45 kJ/m was), the which dominant could material. be related This to seemsthe plastic to be zones supported being by smaller the SEM. with This a higher effect glass was fibernot as content. strong Thein the use composite of rough GF60 or fine (5.72 saw ± flour 2.45 kJ/mdid not2), which seem tocould have be a related significant to the difference plastic zones in the being impact smaller resistance with a propertieshigher glass in fiber thewood content. plastic The ± 2 ± 2 usecomposite of rough samples or fine NFR40saw flour (6.87 did not1.07 seem kJ/ mto )have and a NFF40 significant (6.77 difference1.63 kJ/m in the) with impact a 40% resistance fill rate. properties in the wood plastic composite samples NFR40 (6.87 ± 1.07 kJ/ m2) and NFF40 (6.77 ± 1.63 kJ/m2) with a 40% fill rate. Both of the particle types decreased the impact strength compared to SustainabilitySustainability 20182019,, 1011,, x 631 FOR PEER REVIEW 77 of of 17 17 unfilled REF, which is in line with the previous studies [14,32]. The composite NFF60 (5.61 ± 0.89 kJ/mBoth2) of with the particlefine wood types flour decreased performed the impactbetter than strength the sample compared with to rough unfilled wood REF, flour which NFR60 is in line (4.91 with ± 2 0.65the previouskJ/m2). The studies variance [14,32 in]. the The impact composite strength NFF60 was (5.61 generally± 0.89 kJ/mhigher) in with samples fine wood with flour a 60% performed fill rate. Withoutbetter than the the bonding sample witheffect rough of the wood matrix, flour the NFR60 high (4.91 aggregation± 0.65 kJ/m of 2).fillers The varianceleads to ininsufficient the impact homogeneity,strength was generallyrigidity, and higher low in impact samples strength, with a 60% and fill the rate. aggregated Without theparticles bonding act effectas crack of theinitiation matrix, sitesthe high in impact aggregation [1]. The of fillersmineral leads wool to insufficientcomposites homogeneity,with recycled rigidity, raw material and low MWR40 impact strength,(11.04 ± 2.94 and kJ/mthe aggregated2) and MWR60 particles (6.32 ± act 1.53 as kJ/m crack2) initiationhad higher sites impact in impact strength [1]. than The mineralthe MWB40 wool (7.13 composites ± 1.59 kJ/m with2) 2 2 andrecycled MWB60 raw (4.86 material ± 0.89 MWR40 kJ/m2) (11.04 made± of2.94 by-product kJ/m ) and mineral MWR60 wool, (6.32 which± 1.53 is kJ/msurprising) had higheras the tensile impact propertiesstrength than in MWB the MWB40 cases were (7.13 superior.± 1.59 kJ/m However,2) and MWR40 MWB60 had (4.86 quite± 0.89 a high kJ/m value2) made for ofelongation by-product at breakmineral 2.81%. wool, The which composite is surprising GYP40 performed as the tensile well properties in impact in testing MWB with cases a small were deviation superior. However,in results 8.86MWR40 ± 0.65 had kJ/m quite2, but a high as the value gypsum for elongation content was at break increased, 2.81%. the The results composite dropped GYP40 as performedin other tested well 2 samplesin impact GYP60 testing 6.32 with ± 1.53 a small kJ/m deviation2. in results 8.86 ± 0.65 kJ/m , but as the gypsum content was increased, the results dropped as in other tested samples GYP60 6.32 ± 1.53 kJ/m2.

64.00

32.00

-82% 16.00 -85% -90% -88% -90% -91% -92% -91% -91% -93% 8.00 -92% -93% -93% -93% 4.00 Impact strength (kJ/m2) strength Impact

2.00

1.00 REF SS40 SS60 GF40 GF60 NFF40 NFF60 NFR40 NFR60 GYP40 GYP60 MWR40 MWR60 MWB40 MWB60

Impact strength (kJ/m2) % Difference to REF

Figure 6. Charpy unnotched impact strength. Figure 6. Charpy unnotched impact strength. 3.1.2. Tensile Strength 3.1.2. Tensile Strength The tensile strength decreased with the addition of filler material compared to fully plastic REF (17.31The± 0.48tensile MPa) strength due to decreased weak interphase with the adhesion addition in of the filler filler material interface compared (see Figure to7 fully). The plastic calculated REF (17.31specific ± 0.48 strength MPa) showsdue to thatweak although interphase the adhesion tensile properties in the filler of interface MWB, GYP, (see Figure and SS 7). filled The composites calculated specificwere similar, strength the shows GYP40 that performs although better the when tensile considering properties theof MWB, material GYP, weight. and TheSS filled tensile composites strengths wereof the similar, compression the GYP40 molded performs wood better plastic when composite considering samples the were material significantly weight. The below tensile the previously strengths ofreported the compression values [14 ,molded33]. The wood composite plastic NFF40 composite (11.29 samples± 1.11 MPa) were had significantly 18% higher below tensile the strength previously than reportedthe composite values NFR40 [14,33]. (9.13 The± composite1.47 MPa), NFF40 which (11.29 would ± 1.11 seem MPa) to suggest had 18% some higher influence tensile by strength the fiber than size. theHowever, composite thewood NFR40 plastic (9.13 composite ± 1.47 MPa), samples which with woul a 60%d seem fill rate to suggest demonstrated some theinfluence opposite by affect, the fiber and size.the NFF60 However, (6.90 the± 2.24 wood MPa) plastic had 18%composite smaller samples tensile strength with a 60% compared fill rate with demonstrated NFR60 (8.16 the± 1.81 opposite MPa). affect,This seems and the to NFF60 be in conflict (6.90 ± with2.24 MPa) previous had reports18% smaller where tensile the increased strength compared fiber size and with fiber NFR60 ratio (8.16 has ±improved 1.81 MPa). both This the seems modulus to be in of conflict elasticity with and previous maximum reports strength where although the increased in these fiber studies size and the fiber fiber ratiosize hashas previouslyimproved beenboth significantlythe modulus smallerof elasticity [33,34 and]. Wood maximum plastic strength composite although samples in NFR40, these studies NFR60, theNFF40, fiber and size NFF60 has previously all had relatively been significantly high variance smaller in results. [33,34]. In the Wood NFF60 plastic with 20composite mesh natural samples fiber NFR40,filling of NFR60, 60%, the NFF40, deviation and inNFF60 results all was had over relatively 30% of high the resultsvariance average. in results. The In measured the NFF60 densities with 20 in meshNFR40, natural NFR60, fiber NFF40, filling and of 60%, NFF60 the were deviation close toineach results other was despite over 30% the differentof the results fill rate, average. and while The measuredNFF60 composite densities samples in NFR40, were NFR60, the heaviest, NFF40, NFR60 and NFF60 performed were betterclose bothto each in strength other despite and in the the differentmodulus. fill The rate, agglomerate and while heatingNFF60 composite method and sa relativelymples were low the (24.3 heaviest, MPa) moldNFR60 pressure performed might better also bothhave in affected strength the and results in the of modulus. the tensile The strength agglomerate negatively heating in method composites and relatively with a high low natural (24.3 MPa) fiber moldcontent. pressure The blanket might createdalso have by affected the wood the fibers results in Figureof the 8tensile acted strength as insulation negatively to the thermoplastic,in composites withleading a high to a longernatural heatingfiber content. time and The uneven blanket temperature created by in the the wood plastic fibers matrix. in Figure The mineral 8 acted filled as insulation to the thermoplastic, leading to a longer heating time and uneven temperature in the Sustainability 2018, 10, x FOR PEER REVIEW 8 of 17 Sustainability 2019, 11, 631 8 of 17 plastic matrix. The mineral filled composites performed better as group compared to the fiber filled composites; an exception to this are the samples MWR40 (7.81 ± 0.86 MPa) and MWR60 (5.85 ± 0.73 MPa).composites The difference performed between better asrecycled group comparedand by-produ to thect mineral fiber filled wool composites; composites an MWB40 exception (9.96 to this ± 0.73 are MPa)the samples and MWB60 MWR40 (9.28 (7.81 ± 1.79± 0.86 MPa) MPa) might and be MWR60 in the non-hardened (5.85 ± 0.73 MPa). resin The component difference in between the by-product recycled material.and by-product The composites mineral wool with compositesby-product MWB40 mineral (9.96wool± filling0.73 MPa) tensile and properties MWB60 (9.28are close± 1.79 to MPa the ) previouslymight be inreported the non-hardened 9.00 MPa with resin a componentrecycled HDPE in the matrix by-product and 40% material. rock wool The filling composites [35]. Wood with plasticby-product composites mineral with wool 40% filling fiber tensile fill have properties previously are close had significantly to the previously higher reported tensile 9.00 strengths MPa with [36]. a Similarly,recycled HDPE there matrixwas a significant and 40% rock variance wool filling in the [35 results]. Wood of plasticglass fiber composites composite with GF40 40% fiber(7.81 fill ± have2.84 MPa)previously with 40% had filling. significantly While higherprocessing tensile the strengths GF60 tensile [36]. strength Similarly, and there elastic was modulus a significant samples variance the matrixin the was results not of able glass to fiberhold compositethe material GF40 together (7.81 and± 2.84 they MPa) broke with during 40% demolding. filling. While In this processing study, thethe cut GF60 glass tensile fiber strengthalso reduced and elasticsignificantly modulus in size samples. In a study the matrix conducted was not in the able same to hold laboratory the material [37], thetogether glass fiber and theywas agglomerated broke during demolding. with the same In thismeth study,od, and the the cut size glass of fiberprocessed also reduced glass fibers significantly ranged fromin size. 20 to In 460 a study µm with conducted the mean in the size same of about laboratory 100 µm. [37 Processing], the glass is fiber known was to agglomerated affect the fiber with length the andsame distribution, method, and and the sizelikewise, of processed anisotropic glass fibersparticle ranged fillers from cleave 20 toand 460 µexperiencem with the considerable mean size of delaminationabout 100 µm. in Processing processing is [1]. known In both to affect natural the and fiber glass length fiber and composites, distribution, the and aggregation likewise, anisotropic of fibers createdparticle weak fillers spots cleave in and the experiencematrix leading considerable to crack delaminationpropagation. In in processinggeneral, the [ 1mineral]. In both fillers natural with and a smallglass particle fiber composites, size performed the aggregation better in the of tensile fibers createdstrength weak test, which spots insuggests the matrix better leading dispersion to crack in thepropagation. matrix than In with general, the fiber the fillers. mineral The fillers tensile with strength a small of particle mineral size wool performed composites better MWR in and the tensileMWB rangedstrength between test, which 5.85–9.96 suggests MPa. better The lowest dispersion result in was the matrixwith MWR than60 with and the the fiber highest fillers. with The MWB40. tensile Bothstrength MWB of samples mineral performed wool composites better than MWR the and samples MWB with ranged post-consumer between 5.85–9.96 recycled MPa. mineral The lowestwool. Thisresult could was be with due MWR60 to the andreinforcing the highest effect with of th MWB40.e non-hardened Both MWB resin samples in by-product performed mineral better wool than whichthe samples was activated with post-consumer in the processing recycled of the mineral compos wool.ite. The This materials could be with due the to least the reinforcing variation in effect the mineralof the non-hardened filled composites resin group in by-product where the mineral samples wool with which recycled was gypsum activated GYP40 in the (10.07 processing ± 0.31 MPa) of the andcomposite. GYP60 (10.48 The materials ± 1.04), and with a thegood least dispersion variation into in thethe mineralmatrix is filled visible composites in the SEM group analysis. where The the tensilesamples strength with recycled SS40 (9.74 gypsum ± 0.68 MPa) GYP40 and (10.07 SS60 ±(9.640.31 ± 2.62 MPa) MPa) and were GYP60 similar, (10.48 but± there1.04), was and greater a good variationdispersion in intothe results the matrix of SS60 is visible probably in the caused SEM analysis.by the higher The tensile probability strength of local SS40 concentrations (9.74 ± 0.68 MPa) of soapstoneand SS60 (9.64in the± mixture2.62 MPa) with were a 60% similar, fill rate. but there was greater variation in the results of SS60 probably caused by the higher probability of local concentrations of soapstone in the mixture with a 60% fill rate.

20.0 20.0 18.0 18.0 16.0 16.0 14.0 14.0 12.0 12.0 10.0 10.0 8.0 8.0 6.0 6.0 Tensile strength (Mpa) strength Tensile Elongation at break % break at Elongation 4.0 4.0 No data No data 2.0 2.0 0.0 0.0 REF SS40 SS60 GF40 GF60 NFF40 NFF60 NFR40 NFR60 GYP40 GYP60 MWR40 MWR60 MWB40 MWB60

Tensile strenght (Mpa)

Specific strenght (Tensile Strenght/Density)

Elongation at break %

Figure 7. Tensile strength of HDPE-based composites. Figure 7. Tensile strength of HDPE-based composites. Sustainability 2019, 11, 631 9 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 9 of 17

Figure 8. Top and bottom view of heated wood plastic agglomerate. Figure 8. Top and bottom view of heated wood plastic agglomerate. 3.1.3. Tensile Modulus 3.1.3. Tensile Modulus The addition of mineral and fiber fillers increased the tensile modulus of recycled HDPE in all samplesThe exceptaddition in GYP40of mineral (0.89 and± 0.05fiber GPa) fillers (see increased Figure9 ).the The tensile standard modulus deviation of recycled in samples HDPE NFR60 in all samples(1.78 ± 0.28 except GPa), in GYP40 NFF60 (0.89 (1.58 ±± 0.050.51 GPa) GPa), (see GF40 Figure (1.39 9).± The0.38 standard GPa), and deviation MWR60 in ( 1.37samples± 0.73 NFR60 GPa) (1.78was high ± 0.28 due GPa), to the NFF60 clustering (1.58 ± of 0.51 particulates GPa), GF40 to high (1.39 concentrations ± 0.38 GPa), and in theMWR60 tensile (1.37 specimens. ± 0.73 GPa) With was the high60% recycleddue to the mineral clustering wool of fill part MWR60iculates some to high individual concentrations tests resulted in the tensile in a lower specimens. tensile modulusWith the 60%than recycled in the REF mineral material wool (0.94 fill± MWR600.04 GPa). some Also, individual the tensile tests modulus resulted ofin MWR40 a lower (tensile1.16 ± 0.16modulus GPa) thanwas quitein the closeREF material to the REF(0.94 material.± 0.04 GPa). Mineral Also, the wool tensile by-product modulus MWB40of MWR40 (1.38 (1.16± ±0.09 0.16 GPa)GPa) was and quiteMWB60 close (1.72 to the± 0.07 REF GPa material.) performed Mineral generally wool by-p betterroduct than MWB40 the recycled (1.38 ± 0.09 mineral GPa) wool and probablyMWB60 (1.72 due ±to 0.07 the GPa) activated performed non-hardened generally resin better in than the by-product the recycled component. mineral wool Wood probably plastic due composites to the activated NFR40 non-hardened(1.40 ± 0.17 GPa resin) and in NFF40the by-produ (1.51 ±ct 0.10component. GPa) had Wood smaller plastic variance composites in results NFR40 than (1.40 the ± other 0.17 GPa) fiber andcomposites NFF40 (1.51 due to± the0.10 good GPa) dispersion had smaller of variance wood particles in results into than the plasticthe other matrix fiber and composites the effect due of the to thecompatibilizer. good dispersion Composites of wood filled particles with into by-product the plastic mineral matrix wool and (MWB40the effect and of the MWB60), compatibilizer. gypsum CompositesGYP40 and GYP60filled with (1.65 by-product± 0.11 GPa mineral) and soapstone wool (MWB40 SS40 (1.17 and± MWB60),0.12 GPa) gypsum and SS60 GYP40 (1.86 ±and0.10 GYP60 GPa) (1.65all had ± 0.11 small GPa) variation and soapstone in the results SS40 (1.17 because ± 0.12 of theirGPa) relativelyand SS60 (1.86 good ± dispersion 0.10 GPa) inall thehad matrix. small variationThe particulate in the andresults fiber because filling increasedof their relatively the stiffness good of dispersion the composites in the with matrix. the The exception particulate of GYP40. and fiberA great filling deal increased of variation the stiffness is visible of in the the composit tensile moduluses with the of exception NFR60, NFF60, of GYP40. GF40, A andgreat MWR60. deal of variationA common is factorvisible for in the all oftensile these modulus is the large of amountNFR60, NFF60, of porosity GF40, revealed and MWR60. by the SEMA common analysis factor and thefor allcalculated of these differenceis the large between amount of the porosity supposed revealed and calculated by the SEM density. analysis Both and GF40 the calculated and MWR60 difference exhibit betweenheavy clustering the supposed of small and fibers calculated without density. proper adhesion Both GF40 to theand matrix, MWR60 which exhibi explainst heavy the clustering variation of in smallthe tensile fibers modulus. without proper adhesion to the matrix, which explains the variation in the tensile modulus. Sustainability 2018, 10, x FOR PEER REVIEW 10 of 17 Sustainability 2019, 11, 631 10 of 17

2.5

89% 68% 46% 2.0 98% 48% 83% 76% 61% 49% 47% 1.5 24% 25%

1.0 -5% Tesnile modulus (Gpa) modulus Tesnile 0.5 No data No data

0.0 REF SS40 SS60 GF40 GF60 NFF40 NFF60 NFR40 NFR60 GYP40 GYP60 MWR40 MWR60 MWB40 MWB60

Tensile modulus (Gpa) Specific modulus (E-Modulus/Density) Difference to REF in %

Figure 9. Tensile modulus of HDPE-based composites. Figure 9. Tensile modulus of HDPE-based composites. 3.1.4. Brinell Hardness 3.1.4. BrinellHardness Hardness is the degree of permanent deformation of a material under an applied force. Figure 10 displays the results for the measured Brinell and the hardness in relation to density. There was a small Hardness is the degree of permanent deformation of a material under an applied force. Figure increase in the Brinell hardness in the composites MWR60 (6.0 ± 0.5 HB), GYP60 (6.1 ± 0.3 HB), and 10 displaysSS60 (6.1 the± results0.2 HB) for compared the meas withured the Brinell REF (5.6 and± 0.5 the HB) hardness material. in However,relation to when density. the increase There inwas a smalldensity increase is takenin the intoBrinell consideration, hardness in soapstone the composites is not a veryMWR60 efficient (6.0 filler ± 0.5 for HB), increasing GYP60 the(6.1 surface ± 0.3 HB), and SS60hardness (6.1 of± 0.2 the HB) composite. compared In a previouswith the study, REF the(5.6 addition ± 0.5 HB) of mineralmaterial. wool However, (40 wt %) when to the the recycled increase in densityHDPE is increased taken into the consideration, Brinell hardness soapstone by 37% [35 is]. not Another a very study efficient reported filler that for compositesincreasing withthe surface 20, hardness30, and of the 40% composite. of recycled In mineral a prev hadious 10.5, study, 16.5, the and addition 20.6% lower of mineral Brinell wool hardness, (40 wt respectively, %) to the recycled than HDPEthe increased reference the [18]. Brinell The smaller hardness content by of37% the [35]. filler Another and better study interfacial reported adhesion that ofcomposites particles to with the 20, 30, andpolymer 40% of matrix recycled was mineral used to explain had 10.5, the 16.5, greater an hardnessd 20.6% lower in the lowerBrinell filler hardness, content respectively, [18]. Here, the than ± the referencefilling effect [18]. of The mineral smaller wool content lowered of the the Brinell filler hardness and better in MWR40interfacial (4.7 adhesion0.8 HB) of by particles 16% and into the MWB60 (5.2 ± 0.5 HB) by 7%. However, in the MWB40 (5.7 ± 0.3 HB) and MWR60 samples, the polymer matrix was used to explain the greater hardness in the lower filler content [18]. Here, the hardness increased by 1% and 7%, respectively. It would seem that the influence of mineral wool filling effect of mineral wool lowered the Brinell hardness in MWR40 (4.7 ± 0.8 HB) by 16% and in filling on hardness is not completely predictable with high fill rates. The highest hardness value was MWB60obtained (5.2 ± in 0.5 the HB) SS60 by composite; 7%. However, it had thein leastthe MWB40 amount of(5.7 variation ± 0.3 HB) in results. and MWR60 This is surprising,samples, the hardnessas soapstone increased mainly by 1% consists and 7%, of talc, respectively. of which the It Mohswould hardness seem that is lower the influence than that ofof gypsummineral orwool fillingsilicon on hardness dioxide, is which not completely is one of the predictable main components with high in mineralfill rates. wool The [highest38]. The hardness composite value SS40 was obtained(4.9 ± in0.4 the HB SS60) had composite; a significantly it had lower the hardness least amount than SS60 of andvariation REF. This in results. could be This due tois thesurprising, fill rate as soapstonedecreasing mainly the consists elastic properties of talc, of of which plastic the but Mohs not creating hardness highly is lower packed than zones that of of mineral gypsum material. or silicon dioxide,GYP60 which (6.1 ±is0.3 one HB of) hadthe main the same components hardness value in mineral as SS60 wool and the [38]. result The for composite GYP40 (4.9 SS40± 0.4 (4.9 HB ±) was0.4 HB) had ahigher significantly than for SS40,lower which hardness might than be due SS60 to betterand REF. packing This of could material, be asdue the to density the fill comparison rate decreasing in the elasticTable1 propertiesshows. Composites of plastic that but had naturalnot creating fiber filling highly demonstrated packed zones elastic of behavior mineral and material. spring-back, GYP60 which explains the rather high values for hardness compared with the composites with mineral fillers. (6.1±0.3 HB) had the same hardness value as SS60 and the result for GYP40 (4.9 ± 0.4 HB) was higher Both glass fiber samples GF40 (4.7 ± 0.5 HB) and GF60 (2.6 ± 0.3 HB) had the poorest results for than for SS40, which might be due to better packing of material, as the density comparison in Table hardness due to their soft fiber bundles on the surface of the material that gave way during the 1 shows.indentation Composites test. In that the GF60had samples,natural fiber there filling was an demonstrated approximately 1elastic mm layer behavior of loose and and spring-back, soft fiber whichmaterial. explains Previously, the rather when high glass values fiber has for been hardness added ascompared reinforcement with to the a wood composites plastic composite, with mineral a fillers.small Both increase glass fiber in hardness samples has GF40 been (4.7 reported ± 0.5 [HB)37]. However,and GF60 the (2.6 previous ± 0.3 HB) studies had used the onlypoorest 10% results of for hardnessglass fiber, due which to their dispersed soft fibe morer bundles homogenously on the into surface the matrix of the and material did not createthat gave soft fiberway bundles.during the indentation test. In the GF60 samples, there was an approximately 1 mm layer of loose and soft fiber material. Previously, when glass fiber has been added as reinforcement to a wood plastic composite, a small increase in hardness has been reported [37]. However, the previous studies used only 10% of glass fiber, which dispersed more homogenously into the matrix and did not create soft fiber bundles. The hardness for wood plastic composite samples NFF40 (5.4 ± 0.8 HB), NFF60 (5.2 ± 0.5 HB), NFR40 (5.8 ± 0.9 HB), and NFR60 (5.1 ± 0.6 HB) was lower than in the REF material, probably because of the Sustainability 2019, 11, 631 11 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 11 of 17 The hardness for wood plastic composite samples NFF40 (5.4 ± 0.8 HB), NFF60 (5.2 ± 0.5 HB), NFR40 wood(5.8 fibers± 0.9 close HB), to and the NFR60 surface (5.1 that± 0.6 gave HB) way was un lowerder thanpressure. in the REFWhen material, taking probablyinto consideration because of the increasethe woodin density fibers closeand the to the lowering surface thateffect gave of waythe undertested pressure. fillers, it When can be taking stated into that consideration they are not advantageousthe increase for in hardening density and the the plastic. lowering The effect refe ofrence the tested material fillers, HDPE it can beis statedbetter thatprotected they are from permanentnot advantageous deformation for by hardening its uncompromised the plastic. The elasticity. reference material HDPE is better protected from permanent deformation by its uncompromised elasticity.

8.0

7.0 2% 7% 8% 9% -4% 1% -1% 6.0 -9% -8% -16% -7% -17% -13% 5.0

4.0 -54% 3.0

Brinell hardness (HB) hardness Brinell 2.0

1.0

0.0 REF SS40 SS60 GF40 GF60 NFF40 NFF60 NFR40 NFR60 GYP40 GYP60 MWR40 MWR60 MWB40 MWB60

Brinell hardness (HB) Specific hardness (HB)/Density Difference to REF in %

Figure 10. Brinell hardness of HDPE-based composites. Figure 10. Brinell hardness of HDPE-based composites. 3.2. Morphology of Fractured Surfaces 3.2. MorphologyThe scanning of Fractured electronic Surfaces microscopy can be used to analyze the topography of the broken tensile strength samples to determine fracture mechanisms and the dispersion of filler materials in the matrix. TheFigure scanning 11 displays electronic the SEM microscopy images for can selected be used specimens. to analyze The the broken topography surface of of the the REF broken material tensile strengthshows samples small impurities. to determine It is unlikely fracture that mechanisms these small particlesand the would dispersion have had of afiller major materials effect on thein the matrix.mechanical Figure 11 properties, displays as the they SEM seem images to be just for on se thelected surface specimens. of the material. The broken Small bitssurface of unmelted of the REF materialplastic shows of different small impurities. colors were It visible is unlikely on the that surface these of small processed particles HDPE would samples. have Small had quantitiesa major effect on theof mechanical impurities are properties, often carried as tothey the seem plastic to material be just fromon the the surface recycling of process. the material. The wood Small fibers bits of unmeltedseem toplastic have beenof different clustered colors in some were parts visible of the brokenon the surface;surface theof concentrationsprocessed HDPE of wood samples. without Small quantitiesthe plastic of impurities matrix are are weak often points carried in the to composite. the plastic Signs material of fiber from pullouts the recycling are visible process. in parts The of the wood fiberswood seem plastic to have composite. been clustered Debonding in some and fiber parts pullouts of the arebroken dominant surface; deformation the concentrations processes when of wood adhesion between the fiber and the matrix is poor [1]. The GF40 sample shows a small amount of without the plastic matrix are weak points in the composite. Signs of fiber pullouts are visible in parts plastic material on the glass fiber surface suggesting weak bonding between the matrix and the fiber. of the wood plastic composite. Debonding and fiber pullouts are dominant deformation processes The scanning electron microscopy indicates the heavy clustering of glass fibers, which has probably whenimpeded adhesion the between flow of plastic the fiber during and molding,the matrix likely is poor causing [1]. theThe relatively GF40 sample high variation shows a in small the GF40 amount of plastictensile material strength on results. the glass Surprisingly, fiber surface the impact suggesting strength resultsweak ofbonding GF40 were between the second the highestmatrix afterand the fiber.REF. The The scanning glass fiber electron samples micr withoscopy 40% and indicates 60% had the both heavy relatively cluste highring fill rates,of glass and fibers, it is likely which that has probablywith aimpeded smaller fill the rate, flow the of matrix plastic would during have molding, a more heterogeneous likely causing structure the relatively and better high processing variation in the GF40quality. tensile The fiberstrength surface results. in MWR40 Surprisingly, is clean of plasticthe impact material, strength suggesting results weak of GF40 bonding were of the the filler second highestto theafter thermoplastic REF. The glass matrix. fiber In the samples mineral with wool 40% based and MWB40 60% andhad MWB60, both relatively the broken high surfaces fill rates, of the and it is likelycomposite that sampleswith a showsmaller the fill dislocations rate, the of ma thetrix filler would from thehave matrix. a more The he matrixterogeneous in both MWB40 structure and and betterMWB60 processing shows quality. signs of aThe tear fiber by stretching; surface in there MWR40 are also is grooves clean of which plastic show material, signs of fibersuggesting pull-outs. weak In the GYP60, the gypsum particulates have dispersed well into the material, which is demonstrated bonding of the filler to the thermoplastic matrix. In the mineral wool based MWB40 and MWB60, the also by smaller deviation in the test results for mechanical properties in the samples. The gypsum filler broken surfaces of the composite samples show the dislocations of the filler from the matrix. The in GYP40 and GYP60 displays clean surfaces without plastic to create proper adhesion in the points matrixof in break. both The MWB40 porosity and and MWB60 dislocation shows of thesigns particulates of a tear fromby stretching; the matrix there can be are seen also in thegrooves fracture which show signs of fiber pull-outs. In the GYP60, the gypsum particulates have dispersed well into the material, which is demonstrated also by smaller deviation in the test results for mechanical properties in the samples. The gypsum filler in GYP40 and GYP60 displays clean surfaces without plastic to create proper adhesion in the points of break. The porosity and dislocation of the particulates from the matrix can be seen in the fracture scans of SS40 and SS60, but more in SS60. The large deviation in the tensile strength results of SS60 samples is related to the strong cavity formation around the soapstone particles. Sustainability 2019, 11, 631 12 of 17 scans of SS40 and SS60, but more in SS60. The large deviation in the tensile strength results of SS60 samplesSustainability is 2018 related, 10, x to FOR the PEER strong REVIEW cavity formation around the soapstone particles. 12 of 17

Figure 11. Scanning electron microscopy of broken tensile sample surfaces.

3.3. Effects of Porosity in Mechanical Properties The porosity for the testedtested compositescomposites is presentedpresented inin TablesTables1 1and and2. 2. The The strength strength of of the the particulate filledfilled compositecomposite depend depend strongly strongly on on the the stress stress transfer transfer between between the the filler filler and and the matrixthe matrix [39]. The[39]. voidThe void content content and theirand their shape, shape, size, size, and locationand location effect effect on the on mechanicalthe mechanical properties properties and and to the to mechanismsthe mechanisms leading leading to mechanical to mechanical failure stress/strain failure stress/strain concentration concentration effects [40]. Thiseffects phenomenon [40]. This is visiblephenomenon in the strainis visible curves in ofthe composite strain curves SS60 whichof composite had relatively SS60 which high porosityhad relatively (25.3%) high see Figureporosity 12. Similar(25.3%) see behavior Figure could12. Similar be seen behavior in the could composites be seen with in the lower composites volume with of porositylower volume suggesting of porosity also irregularitysuggesting also in the irregularity dispersion in of fillerthe dispersion material. Theof filler presence material. of both The resin presence and interlaminar of both resin voids and is verifiedinterlaminar in the voids SEM is of verified broken in tensile the SEM samples of broken see Figure tensile 11 .samples The influence see Figure of high 11. The porosity influence is most of clearlyhigh porosity seen in is the most soapstone clearly filledseen compositesin the soapstone SS40 andfilled SS60. composites Both SS40 SS40 and and SS60 SS60. had Both high SS40 variance and inSS60 impact had high and tensilevariance strength in impact properties, and tensile whereas strength GYP40 properties, and GYP60 whereas had littleGYP40 porosity and GYP60 and small had variance.little porosity The lackandof small heating variance. in the moldThe lack probably of heating affected in adverselythe mold theprobably properties affected of the adversely composite the as theproperties trapped of air the had composite not enough as timethe trapped to move air through had not the enough channel time before to themove solidification through the of material.channel before the solidification of material. Sustainability 2019, 11, 631 13 of 17 Sustainability 2018, 10, x FOR PEER REVIEW 13 of 17

600

500

400

300

200

Standard force [N] 100

0 00.511.522.53 Nominal strain [%]

FigureFigure 12. 12.Strain Strain curve curve of SS60 SS60 composite composite samples. samples.

3.4. Moisture3.4. Moisture Absorption Absorption and and Thickness Thickness Swelling Swelling MoistureMoisture absorption absorption and and thickness thickness swellingswelling di dividedvided the the tested tested materials materials into two into distinct two distinct categoriescategories by performance: by performance: fiber fiber and an particled particle filled filled composites composites (see(see Table 33).). The The reference reference material material REF REF did not absorb water during the immersion time of 28 days. The wood plastic composites NFR60 did not absorb water during the immersion time of 28 days. The wood plastic composites NFR60 and and NFF60 absorbed approximately 13 wt % of water during the test period. The difference in NFF60 absorbedperformance approximately between the 40% 13 and wt 60% % of mixture water was during considerable the test period.as the NFR40 The and difference NFF40 absorbed in performance between5 wt the % 40%of water. and The 60% thickness mixture swelling was considerablein wood plastic ascomposites the NFR40 NFR60 and and NFF40 NFF60 was absorbed 4%, while5 wt % of water. Thethe NFR40 thickness and swellingNFF40 had in only wood 1%. plastic The fiber composites mesh size had NFR60 only anda minimal NFF60 effect was on 4%, the while moisture the NFR40 and NFF40absorption, had only as shown 1%. The also fiberin previous mesh sizestudies had [33] only. It is a notable minimal that effect the wood on the plastic moisture composite absorption, as shownsamples also with in previous both mesh studies sizes had [33 significantly]. It is notable smaller that thickness the wood swelling plastic than compositereported previously samples with for compression molded wood plastics [41]. This can be attributed to the used coupling agent maleic both meshanhydride sizes hadmodified significantly polyethylene, smaller which thickness has been sh swellingown to improve than reported the water previously absorption resistance for compression moldedof wood wood plastic plastics composites [41]. This [42]. canThe glass be attributed fiber 60% composite to the used GF60 coupling absorbed approximately agent maleic 10 anhydride wt modified% polyethylene,of water during whichthe immersion, has been performing shown to more improve weakly the than water the absorptionhydrophilic wood resistance plastic of wood plastic compositescomposites NFR40 [42]. Theand NFF40. glass fiber In both 60% of compositethe glass fiber GF60 compositions, absorbed the approximately poor surface quality,10 wt lack % of water during theof plastic immersion, film on the performing surface, clustering more weakly of fibers than and theweak hydrophilic bonding formed wood pathways plastic for composites the water NFR40 to enter into the composite increasing the water absorption to a relatively high level considering the and NFF40. In both of the glass fiber compositions, the poor surface quality, lack of plastic film on hydrophobicity of the materials used. The poor quality of the glass fiber samples was evident also in the surface,the thickness clustering swelling of fibers measurement and weak as the bonding material formedwas actually pathways separated for from the the water samples to enterduring into the compositethe immersion increasing period. the water absorption to a relatively high level considering the hydrophobicity of the materials used. The poor quality of the glass fiber samples was evident also in the thickness swelling measurement as theTable material 3. Moisture was absorption actually and separated thickness fromswelling the after samples immersion during of 28 days. the immersion period. RE NFR NFR NFF4 NFF6 GF4 GF6 MWR MWR MWB MWB GYP GYP SS4 SS6

F 40 60 0 0 0 0 40 60 40 60 40 60 0 0 M Table 3. Moisture absorption and thickness swelling after immersion of 28 days. A 0.4 5.0 13.5 5.0 12.9 2.3 9.7 0.3 0.8 0.3 0.3 0.2 1.1 0.3 0.1 wt REF NFR40 NFR60 NFF40 NFF60 GF40 GF60 MWR40 MWR60 MWB40 MWB60 GYP40 GYP60 SS40 SS60 % MA wt % 0.4 5.0 13.5 5.0 12.9 2.3 9.7 0.3 0.8 0.3 0.3 0.2 1.1 0.3 0.1 TS TS % 0.0 1.2 1.2 4.4 4.4 1.2 1.2 4.34.3 -0.3− 0.3-1.1− 1.1 -0.4− 0.4 1.2 1.2 -0.1 −0.1 -1.2 −1.2 0.0 0.0 1.0 -0.2 1.0 -0.4− 0.2 −0.4 %

The waterThe water absorption absorption and and thickness thickness swelling swelling of of the the particle particle based based composites composites was negligible. was negligible. The highest value for thickness swelling in particle-filled composites was approximately 1% in the The highest value for thickness swelling in particle-filled composites was approximately 1% in the MWR60 and GYP60 composites. A small amount of filler material was separated during the MWR60immersion and GYP60 time, composites. which would A smallseem to amount suggest of an filler incomplete material covering was separated of the particulates duringthe by immersionthe time, whichmatrix would on the surface seem toof the suggest composite an incomplete samples. This covering could be seen of the from particulates the visible mineral by the particles matrix on the surfacein of the the water composite container samples. after drying This the couldtested samples. be seen from the visible mineral particles in the water container after drying the tested samples.

3.5. Evaluation of Heat Build-Up and Color Measurements The rise of temperature in the composite material exposed to thermal radiation is related to the optical properties [43]. Figure 13 presents the measured CIE L*, a*, and b* color values, with specular component included (SCI) and specular component excluded (SCE). The lightness *L represents the Sustainability 2018, 10, x FOR PEER REVIEW 14 of 17

3.5. Evaluation of Heat Build-Up and Color Measurements Sustainability 2019, 11, 631 14 of 17 The rise of temperature in the composite material exposed to thermal radiation is related to the optical properties [43]. Figure 13 presents the measured CIE L*, a*, and b* color values, with specular darkestcomponent black atincluded *L = 0, (SCI) and brightand specular white atcomponent *L = 100. Theexcluded measured (SCE). values The lightness for a* and *L represents b* represent the the truedarkest neutral black grey at value*L = 0, at and a* bright = 0 and white b* = at 0. *L The = 100. green The color measured is represented values for at a* negative and b* represent a* values the and redtrue at positiveneutral grey a* values. value at The a* b*= 0 negative and b* = values0. The green represent color blueis represented and the positive at negative b* values a* values represent and yellowred at color. positive The a* composite values. The CIE-LAB b* negative values values were represent estimated blue using and Colorizer the positive [44 ].b* The values black represent reference sampleyellow REF color. had The an composite *L value ofCIE-LAB 25.20, whereasvalues were the estimated results for using the Colorizer mineral filled [44]. The samples black werereference in the rangesample 25.50–28.26. REF had Thean *L exception value of 25.20, to this whereas was MWR60 the results with *Lfor 32.61.the mineral For the filled wood samples plastic were samples, in the the range 25.50–28.26. The exception to this was MWR60 with *L 32.61. For the wood plastic samples, the *L value ranged between 44.01 and 51.06, resulting in decreased heat build-up. The composites with *L value ranged between 44.01 and 51.06, resulting in decreased heat build-up. The composites with the mineral fill rate 40% increased the heat build-up by 4.5% compared with the reference material REF. the mineral fill rate 40% increased the heat build-up by 4.5% compared with the reference material The samples with a 60% mineral fill rate had no significant change in heat build-up compared with REF. REF. The samples with a 60% mineral fill rate had no significant change in heat build-up compared The addition of wood fibers had on average a 10% decrease in heat build-up. The use of wood particles with REF. The addition of wood fibers had on average a 10% decrease in heat build-up. The use of resultedwood inparticles lighter resulted colors than in lighter the black colors REF than material. the black The REF results material. for wood The plasticresults compositefor wood plastic samples werecomposite close to samples previously were reported close toones previously with recycled reported materials ones with [14 recycled]. Both woodmaterials and [14]. glass Both fiber wood filling hadand the glass greatest fiber impactfilling had on thethe opticalgreatest properties impact on comparedthe optical withproperties the reference compared material. with the The reference mineral filledmaterial. composites The mineral had relatively filled composites low values had for relative reflectancely low values compared for reflectance with wood compared plastic and with glass wood fiber samples.plastic Thisand glass can leadfiber to samples. higher heatThis build-upcan lead to values, higher as heat Figure build-up 14 shows. values, The as Figure carbon 14 black shows. originally The usedcarbon to color black the originally recycled used sewer to pipecolor plasticthe recycled had the sewer greatest pipe plastic influence had on the the greatest optical influence properties on ofthe the composite.optical properties The addition of the of composit minerale. and The fiberaddition fillers, of mineral however, and decreased fiber fillers, the however, reflectance decreased of the plastic, the givingreflectance it more of of the a dull plastic, type giving appearance. it moreThe of a filler dull particulatetype appearance. size could The alsofiller haveparticulate affected size the could results ofalso the colorhave affected measurements, the results as of the the wood color fibersmeasurements, were several as the times wood larger fibers thanwere anyseveral of the times other larger filler typethan particles any of thatthe other were filler milled type into particles dust in that the agglomeratingwere milled into stage. dust in The the importance agglomerating of color stage. depends The onimportance the application of color and depends market on trends; the application therefore, and the effectmarket of trends; filler on therefore, the visual the look effect of of the filler product on shouldthe visual be assessed look of casethe product by case. should be assessed case by case.

Sustainability 2018, 10, x FOR PEER REVIEW 15 of 17 Figure 13. CIE L*, a*, and b* color coordinates of the studied composites. Figure 13. CIE L*, a*, and b* color coordinates of the studied composites. 54 6% 5% 5% 52 4% 2% 3% 50 -1% 0% -2% -3% 48 -8% 46 -10% -10% -10% 44

42

40

38

36 REF SS40 SS60 GF40 GF60 NFF40 NFF60 NFR40 NFR60 GYP40 GYP60 MWB40 MWB60 MWR40 MWR60 Heat build-up ΔT=ΔT(exp) Difference to REF in %

Figure 14. Heat build-up of the studied composites.

Figure 14. Heat build-up of the studied composites.

4. Conclusions In this study, compression-molded composites were made of recycled materials, and their mechanical properties were examined. The morphology of the HDPE/recycled material composites was analyzed by observing the tensile pull-fracture surface. The recycled filler materials were able to significantly improve the tensile modulus of the recycled HDPE but had a decreasing effect on impact strength, tensile strength and elasticity. The Brinell hardness of the plastic did not improve with the addition of mineral fillers even with 60% fill rates, and most of the composites had lower values for hardness with the exception of MWR60 and GYP60. The measured densities of the composites did not follow the rule-of-mixture, which would seem to suggest uneven mixing in the processing phase and porosity in the samples. This was supported by voids and clustered particles found in the SEM analysis and in the variance in the tensile strength and tensile modulus properties especially in composites with fiber filling. Porosity and clustering of filler material were identified as major causes for variance in mechanical properties. The lack of hot-melt mixing during material processing probably had a negative effect on the final composite mechanical properties. The hot-melt mixing would have improved the dispersion of the filler materials into the matrix. The tested mineral particle filled composites could be used in a variety of compression molded products previously made of recycled plastic, especially where a large volume of material is needed. The composites with mineral fillers had better resistance to moisture compared to wood plastic composites, which would be beneficial especially in outdoor applications. The heat built-up in composites with mineral fillers was higher than in the ones with wood, which suggest that natural fibers are still a better option for producing decking products. The recycled glass fiber did not function optimally with the used recycled plastic as it did not mix properly into the matrix, therefore, the combination as such is not advisable due to the risk of fiber separation. The use of filler type specific coupling agents and improved mixing during processing could further improve the properties throughout the tested material range. Future studies on the methodology of designing products with recycled material composites are required. Furthermore, the economics and carbon footprint of such production should be verified to better understand the potential of the recycled materials in composites.

Author Contributions: P.S. designed tooling, produced the samples, tested the materials, performed the analysis and investigation part and prepared the paper; T.K provided supervision for the project and contributed to the methodology selection.

Acknowledgements: This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors. Sustainability 2019, 11, 631 15 of 17

4. Conclusions In this study, compression-molded composites were made of recycled materials, and their mechanical properties were examined. The morphology of the HDPE/recycled material composites was analyzed by observing the tensile pull-fracture surface. The recycled filler materials were able to significantly improve the tensile modulus of the recycled HDPE but had a decreasing effect on impact strength, tensile strength and elasticity. The Brinell hardness of the plastic did not improve with the addition of mineral fillers even with 60% fill rates, and most of the composites had lower values for hardness with the exception of MWR60 and GYP60. The measured densities of the composites did not follow the rule-of-mixture, which would seem to suggest uneven mixing in the processing phase and porosity in the samples. This was supported by voids and clustered particles found in the SEM analysis and in the variance in the tensile strength and tensile modulus properties especially in composites with fiber filling. Porosity and clustering of filler material were identified as major causes for variance in mechanical properties. The lack of hot-melt mixing during material processing probably had a negative effect on the final composite mechanical properties. The hot-melt mixing would have improved the dispersion of the filler materials into the matrix. The tested mineral particle filled composites could be used in a variety of compression molded products previously made of recycled plastic, especially where a large volume of material is needed. The composites with mineral fillers had better resistance to moisture compared to wood plastic composites, which would be beneficial especially in outdoor applications. The heat built-up in composites with mineral fillers was higher than in the ones with wood, which suggest that natural fibers are still a better option for producing decking products. The recycled glass fiber did not function optimally with the used recycled plastic as it did not mix properly into the matrix, therefore, the combination as such is not advisable due to the risk of fiber separation. The use of filler type specific coupling agents and improved mixing during processing could further improve the properties throughout the tested material range. Future studies on the methodology of designing products with recycled material composites are required. Furthermore, the economics and carbon footprint of such production should be verified to better understand the potential of the recycled materials in composites.

Author Contributions: P.S. designed tooling, produced the samples, tested the materials, performed the analysis and investigation part and prepared the paper; T.K. provided supervision for the project and contributed to the methodology selection. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors. Conflicts of Interest: The authors declare that they have no conflict of interest in this work.

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