Carbonized Lignin as Sustainable Filler in Biobased Poly(trimethylene

Dec 16, 2015 - market, but the price premium of the renewable feedstock chemicals typically limits ... the specific stiffness and strength, i.e., to c...
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Research Article pubs.acs.org/journal/ascecg

Carbonized Lignin as Sustainable Filler in Biobased Poly(trimethylene terephthalate) Polymer for Injection Molding Applications Petri Myllytie,† Manjusri Misra,‡,† and Amar K. Mohanty*,‡,† †

Bioproducts Development and Discovery Centre (BDDC), Crop Science Building, Department of Plant Agriculture, University of Guelph, 50 Stone Road East, Guelph, N1G 2W1 Ontario, Canada ‡ School of Engineering, Thornbrough Building, University of Guelph, 50 Stone Road East, Guelph, N1G 2W1 Ontario, Canada ABSTRACT: Light weight and sustainability are the key drivers in the development of novel biobased thermoplastic compounds for automotive applications. This paper reports the engineering properties of thermoplastic compound consisting of a novel bioresourced carbon filler in combination with partially biobased poly(trimethylene terephthalate). The bioresourced carbon filler, which was derived from lignin residue of cellulosic ethanol production, has a clear advantage in terms of density compared to glass fiber and other minerals, and shows potential for weight reduction with 7% lower density at 20% filler content. Polymer processing conditions were optimized in terms of thermomechanical properties, and use of a reactive chain extender additive was studied for improving the performance of the compound. At the optimized conditions, good dimensional stability, 89% increase in heat deflection temperature, 60% increase in flexural modulus, and 14% increase in flexural strength was attained in comparison to neat PTT polymer. Theoretical modeling based on a rule-of-mixture approach showed good agreement of the predicted and experimental modulus of the studied composites. When compared to existing mineral filled engineering polyester resin, many properties of the prepared compounds were on a comparable or favorable level, indicating good potential of the bioresourced carbon filler for light weighting and highly sustainable engineering applications. KEYWORDS: Lignin, Biomass valorization, Thermal conversion, Filled plastics, Biopolymers, Biocomposites, Injection molding

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INTRODUCTION

other lightweight materials will remain in the focus of the industry for years to come.2 Most of the engineering plastics originate from nonrenewable petroleum based feedstock, and the sustainability benefits of using engineering plastics include improved fuel economy as well as reusability and recyclability of the plastic parts. Engineering plastics from renewable and sustainable resources are getting more and more of a foothold in the market, but the price premium of the renewable feedstock chemicals typically limits the use to niche applications instead of large volume manufacturing where price is a crucial property, if technical properties are at par level. In some cases, the renewably resourced chemical feedstock is readily competitive with the petroleum based counterpart. One example is poly(trimethylene terephthalate) (PTT) polymer, which is a partly biobased thermoplastic engineering polyester commercialized by DuPont (Wilmington, DE, USA). The biobased ingredient of the PTT polymer is 1,3-propanediol (PDO)

The automotive industry is one of the largest users of engineering plastics. The use of plastics and composites has been steadily increasing in strive for weight reduction, improved passenger comfort, and safety features that typically deploys advanced design and engineering resins to maximize the technical performance while providing economically viable mass production by injection molding. Weight reduction by advanced and innovative uses of engineering plastics is one of the most important attributes for automotive industry to use and develop new applications for engineering plastics.1,2 The global commitment for reduction of greenhouse gases (GHG’s) has placed legislative pressure on the automotive industry to improve the fuel economy and reduce the emissions of new and future vehicles. In the US, targets have been set for the corporate average fuel economy (CAFE) whereas in Europe the targets are for the amount of CO2 emissions per kilometer per average vehicle produced.2,3 Because weight reduction turns directly into savings in fuel economy, and reduced overall weight helps in down gauging other components in a vehicle, a spiral of light weighting is created, and plastics, composites and © 2015 American Chemical Society

Received: August 1, 2015 Revised: September 27, 2015 Published: December 16, 2015 102

DOI: 10.1021/acssuschemeng.5b00796 ACS Sustainable Chem. Eng. 2016, 4, 102−110

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ACS Sustainable Chemistry & Engineering obtained from an industrial scale fermentation process.4 As an engineering plastic, in terms of technical properties, the PTT competes with two other polyesters, PET and PBT, which are widely used for example in automotive applications. In terms of sustainability, the partly biobased PTT with 37% renewable content clearly outperforms the PET and PBT polyesters while providing a comparable level of technical performance.5,6 Injection molded glass fiber reinforced PTT composites have been thoroughly characterized and successfully introduced as sustainable, high performance materials for automotive interior applications.7,8 For optimal end use performance, engineering plastics are often compounded with mineral fillers and glass and/or carbon fibers. Mineral fillers like talc, calcium carbonate, and mica improve stiffness, thermal properties, and economics of the engineering plastics but the high density of 2.4−2.8 g/cm3 of the minerals, and glass fibers as well, adds to the weight of the materials.9 Therefore, when designing new plastic parts for engineering applications and assigning new materials for specified end uses, in some cases it is important to evaluate the specific stiffness and strength, i.e., to calculate the stiffness and strength divided by density. Materials with high specific properties are preferred for light weighting purposes and are widely used in aerospace applications where lightweight is crucial. As the price per performance of the high performance materials gradually reduce with increasing applications and market volumes, the materials will find their way to more cost sensitive high volume applications, like automotive. Because of the high density of minerals, mineral filled engineering plastics may not be the best way to reach high specific properties for the plastic compound. Therefore, it is justified to research and develop lower density fillers that could improve the specific properties of filled plastics. Hollow glass spheres and mineral tubes are good examples of low density fillers for plastics, but the higher cost is likely to limit the applications.10,11 PTT has previously been used in composites with carbon nanotubes and carbon fibers;12,13 however, the present study involving PTT composites with carbonized lignin is the first of this type. Recently, a renewably sourced carbon material was introduced as a sustainable alternative to petroleum based black pigment commonly used in plastics.14 The sustainable alternative for carbon black pigment was derived from the lignin residue of second generation bioethanol production. The sustainable nature of bioresourced carbonaceous materials has been studied in depth, and pyrolyzed biomass is identified as a viable method of carbon sequestration and greenhouse gas reduction.15,16 Current studies have analyzed the use of biocarbon as a soil amendment; the use of biocarbon in composite materials provides another novel application that allows for carbon sequestration. The lignin residue was thermally converted into carbonaceous material with relatively high elemental carbon content and mechanically processed to a size range suitable for compounding with plastics.14,17 The aim of this work was to study the use of the renewably sourced biobased carbon material as low density filler in partly biobased thermoplastic polyester matrix. Laboratory scale extrusion and injection molding was used for preparation of the compounds and molded samples, followed by mechanical, thermal, thermomechanical, and morphological characterization of the prepared materials. Material properties were reported and complemented by comparison to theoretical model of thermoplastic composite materials. Specific attention was paid to the engineering characteristics of the material in order to evaluate

the potential against the established material solutions in the plastics market.

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EXPERIMENTAL SECTION

Materials. The partly biobased thermoplastic engineering polyester poly(trimethylene terephthalate), Sorona PTT, was obtained from DuPont (Wilmington, DE, USA). The polymer was dried overnight in an air circulation oven at 105 °C prior to processing. The lignin residue of bioethanol production was received from Mascoma (Brampton, ON, Canada). The lignin residue was stored frozen and the moisture content of the material was around 47−51%. The lignin material has been characterized earlier and it consisted mainly of lignin and crystalline cellulose.18 Reactive epoxy functional polymeric chain extender (CE) used was Joncryl 4368 manufactured by BASF (BASF, Ludwigshafen, Germany). Thermal Conversion of Lignin. The frozen lignin residue of bioethanol production was dried in an air circulating oven at 105 °C. After drying, the material was pulverized in a Retsch PM100 planetary ball mill (Retsch GmbH, Haan, Germany). The dry lignin residue was pulverized by ball milling with two 40 mm diameter steel balls at 250 rpm for 2 h. The pulverized lignin was transferred into ceramic crucibles and Carbolite (Hope Valley, UK) GHA horizontal tube and Carbolite VST vertical tube furnaces were used for the thermal conversion process. The conditions of the thermal conversion process, which was essentially a slow pyrolysis under nitrogen gas atmosphere, had been optimized in an earlier work.14,17 The heating ramp rate, isothermal temperature, and dwell time at the isothermal temperature for the thermal conversion were 20 °C/min, 900 °C, and 6 h, respectively. Continuous flow of nitrogen was used to purge the tube furnaces during the thermal conversion process including the cooling phase. The product of the thermal conversion process was a porous solid carbonaceous material, which was pulverized by ball milling with two 40 mm diameter steel balls at 250 rpm for 2 h. The pulverized carbonaceous material was then characterized by elemental analysis, functional group analysis, and sieve analysis. The pulverized carbonaceous material was subsequently used without further modification as sustainably sourced low density filler for the partly biobased thermoplastic poly(trimethylene terephthalate) polymer. Characterization of the Carbon Filler. Elemental composition of the sustainably sourced carbon filler was measured by Thermo Scientific (Waltham, MA, USA) Flash 2000 analyzer, capable of determining elemental carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O). The C, H, N, and S elements were measured against known calibration sample of 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT). Fourier transform infrared spectroscopy (FTIR) was performed with Thermo Scientific Nicolet 6700 FT-IR spectrometer in attenuated total reflectance (ATR) mode using diamond ATR crystal. FTIR spectra were recorded with resolution of 4 cm−1 and 64 scans per sample. Particle size fractions of the carbon filler were evaluated by sieving the material through a 20 μm sieve. A Retsch AS 200 air jet sieving machine (Retsch GmbH, Haan, Germany) was used for the sieving, and sample size and sieving time were 2 g and 10 min, respectively. Processing of PTT and Carbon Filled PTT Compounds. Laboratory scale extrusion and injection molding equipment, DSM Xplore from DSM (Heerlen, Netherlands), was used in processing and preparation of tensile, flexural, and impact test specimens. Neat PTT and PTT containing 20% of carbon filler with and without chain extender additive were extruded and injection molded at selected processing conditions. 0.4% of chain extender by weight was incorporated into the corresponding blends. Melt temperature and barrel temperature were set to 250 °C, mixing time in the twin-screw extruder was 2 min and screw speed was 100 rpm. Mold temperature was varied between 30 and 100 °C in order to find balance between mechanical and thermal performance of the PTT polymer. After initial testing, the studied compositions were processed at mold temperature of 80 °C. The injection parameters were adjusted to get properly filled parts without short shots, sink marks, or excessive flash, and the 103

DOI: 10.1021/acssuschemeng.5b00796 ACS Sustainable Chem. Eng. 2016, 4, 102−110

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Figure 1. (a) DSC first heating curves, and (b) TMA thermal expansion curves for neat PTT molded at different mold temperatures.

Figure 2. Notched Izod impact strength and heat deflection temperature of (a) neat PTT at different mold temperatures, and (b) neat PTT with increasing amount of chain extender at constant mold temperature of 80 °C. heating rate of 2 °C/min. Span length of the sample was 50 mm and the applied force was calculated to match the 0.455 MPa stress according to ASTM D648 standard. Heat deflection temperature was taken as the temperature where the strain reached the value specified by the standard. Thermal expansion of the materials was measured with TMA in temperature range from 30 to 130 °C with heating rate of 5 °C/min. Some of the samples exhibited large change of dimensions near the glass transition temperature and thermal expansion values were determined both below and above glass transition temperature. Because of the chosen sample geometry, the coefficient of linear thermal expansion (CLTE) was measured perpendicular to the melt flow direction. All reported CLTE values were determined at the linear regions of 35−50 °C and 80−120 °C, below and above the glass transition of the PTT polymer. Morphology and Density of the Materials. Fracture surfaces of the notched impact test pieces were imaged with an FEI (Hillsboro, OR, USA) Inspect S50 scanning electron microscopy (SEM) instrument using 20 kV acceleration voltage. Prior to imaging, the samples were sputter coated with gold. Density of the materials was measured by displacement method according to ASTM D792 using Alfa Mirage MD300 densimeter (Alfa Mirage Co., Ltd., Osaka, Japan).

parameters varied slightly depending on the composition of the melt and the specimen molded. Mechanical Characterization of PTT and Carbon Filled PTT Compounds. Tensile, flexural, and impact properties of the PTT and carbon filled PTT composites were measured according to standards ASTM D638, D790, and D256, respectively. Tensile and flexural test were performed with Instron 3382 Universal Testing Machine (Instron, High Wycombe, UK). Tensile tests were performed at deformation rate of 5 mm/min and flexural tests were carried out at deformation rate of 1.4 mm/min. A TMI 43-02 Monitor Impact Tester (Testing Machines Inc., New Castle, DE, USA) with 0−5 × 0.05 ft·lbs standard impact hammer was used to measure notched Izod impact strength. Thermal and Thermomechanical Testing. A differential scanning calorimetry (DSC) Q200 from TA-Instruments (New Castle, DE, USA) was used for thermal characterization of the materials. Thermal transitions including glass transition temperature (Tg), melting and crystallization temperatures, and associated changes in enthalpies were measured with DSC. The DSC measurements were performed in heat/cool/heat cycles between 0 and 250 °C using a heating rate of 10 °C/min and cooling rate of 5 °C/min. A dynamic mechanical analyzer (DMA) Q800 and thermomechanical analyzer (TMA) Q400 from TA Instruments were used for thermomechanical characterization of the materials. Heat deflection temperature of the samples was determined with DMA in three point bending mode at

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RESULTS AND DISCUSSION Optimization of Mold Temperature for PTT Processing. Semicrystalline engineering plastics, such as PTT and PBT 104

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Table 1. Tensile and Flexural Properties of Poly(trimethylene terephthalate) PTT and 20% Bioresourced Carbon Filled PTT Composites with and without Chain Extender (CE) Molded at 80 °C Mold Temperature material PTT PTT PTT PTT PTT PTT

+ + + + +

0.1% CE 0.2% CE 0.4% CE 20% carbon filler 20% carbon filler, 0.4% CE

Et, MPaa 2630 2720 2840 2870 4210 4290

(48) (29) (57) (38) (127) (66)

σY, MPa 59.1 60.0 61.7 65.1 62.0 62.0

(0.7) (0.6) (0.7) (0.6) (0.9) (2.5)

εY, % 4.3 3.7 5.2 8.8 2.2 2.2

(0.3) (0.1) (1.0) (0.3) (0.1) (0.4)

Ef, MPa 2669 2695 2800 2884 4327 4273

(20) (36) (27) (32) (63) (23)

σf, MPa 98.2 (1.0) 100.7 (1.5) 103.9 (1.1) 102.6 (0.6) 103.2 (1.4) 112.1 (0.9)

a Symbols: Et, σY, εY, Ef, and σf denote tensile modulus, tensile strength, yield elongation, flexural modulus, and flexural strength, respectively. Standard deviation for each measurement is given in parentheses.

narrow temperature range around the Tg of the polymer. Only at mold temperatures of 80 °C and above does the trend in thermal expansion become fairly linear through the measurement range of 30−130 °C. The CLTE of PTT below Tg reduced with higher mold temperatures whereas above Tg the CLTE slightly increased compared to low mold temperatures. The CLTE values of PTT measured below Tg were in the range of 6.5−7.9 × 10−5 1/°C and above Tg in the range of 1.3−1.7 × 10−4 1/°C. In comparison to previously reported values for PTT (4.1 × 10−5 and 1.4 × 10−4 1/°C below and above Tg, respectively),24 the obtained values above Tg were similar, whereas below Tg the measured CLTE values were somewhat higher. Difference may arise from measurement direction, which was not stated, and from different grade of PTT used in the study.24 The observed large transition in a narrow temperature range in the low mold temperature samples (Figure 1b) can be considered a risk in terms of end-use performance. From a part design and manufacturing perspective, it is essential that the thermomechanical properties, like HDT and CLTE are known and predictable. Therefore, the mold temperature of 80 °C was considered optimal for the purpose of this study. Use of Chain Extender To Improve the Properties of PTT. Polyester thermoplastics are typically sensitive to moisture, and despite of proper drying prior to processing hydrolysis can occur in melt processing causing reduction of molecular weight and mechanical performance. Reactive chain extenders are particularly useful in processing of recycled plastics where degradation occurs due to moisture, contaminants, and increasing number of melt processing cycles.25 Chain extenders can help to maintain the molecular weight and balance the viscosity and mechanical properties of PTT already in the first melt processing cycle.26 Because the objective of this work was to develop compounds as intermediate products for further processing, such as injection molding of technical parts, it was justified to explore the effect of chain extender additive on the PTT properties in order to find the optimum level of addition for the subsequent compounding experiments. Figure 2b presents the impact and thermal resistance and Table 1 comprises the tensile and flexural properties of PTT modified with reactive chain extender additive. The studied addition levels were limited below 0.5% to avoid gel formation by crosslinking as reported in an earlier study about long-chain branching of PTT.27 The addition of chain extender to neat PTT in the first processing cycle caused only minor improvements on the tensile and flexural properties (Table 1) and the largest increase was observed in the thermal resistance, as evidenced by the HDT results (Figure 2b). Addition of fillers in thermoplastics typically decreases the ductility and elongation at yield of the materials. In the case of

and polyamides 6 and 6.6, are typically molded with elevated mold temperatures whereas polyethylene and polypropylene can be molded with cold mold due to faster crystallization.19 Optimum mold temperature depends on the application and the product attributes, like surface quality. For molded technical parts made of polyesters and polyamides, high crystallinity is needed to ensure good mechanical and thermo-mechanical performance, whereas amorphous polymer structure can be useful in film applications. In this work, the aim was to evaluate the applicability of renewably sourced carbon material as low density filler in technical molded parts, and the first step was to find optimum mold temperature for the experiments. Process optimization was conducted based on the lab scale processing equipment used in this study. Figure 1a shows DSC first heating curves, and Figure 1b shows TMA thermal expansion of neat PTT molded at different mold temperatures. At a low mold temperature of 40 °C, the polymer exhibits cold crystallization transition between 50 and 75 °C (Figure 1a). Higher mold temperature promotes the crystallization of the polymer and already at 60 °C mold temperature the cold crystallization transition is considerably decreased. With further increase of the mold temperature to 80 and 100 °C, the cold crystallization was not observed anymore (Figure 1a). The obtained DSC results correlated well with earlier injection molding studies of PTT polymer.20 The neat PTT molded at different mold temperatures was also evaluated by impact and heat resistance and thermomechanical analysis because the properties were known to be sensitive to the mold temperature.21,22 Figure 2a shows the notched Izod impact strength and HDT of the neat PTT at different mold temperatures. The impact strength of neat PTT (Figure 2a) shows a decrease between the mold temperatures of 40 and 60 °C. The reduction in the impact strength can be related to the crystallization of PTT at 60 °C and above, and it correlates with the disappearance of the cold crystallization transition in the DSC results (Figure 1a). The HDT of neat PTT molded with 40 °C mold is low, only about 46 °C, whereas the HDT at mold temperatures of 80 and 100 °C is 99 and 117 °C, respectively. The relatively low HDT of neat PTT molded with cold mold is in accordance with earlier studies.21−23 Because of quick cooling in a cold mold, the PTT polymer remains amorphous to a high extent and there is not much crystalline structure to contribute to the mechanical resistance at temperatures above glass transition. Indeed, a considerable decrease in storage modulus has been observed by DMA in the neat PTT above the glass transition temperature (Tg) in the earlier reports on PTT composites where low mold temperature was applied.21,22 In terms of thermal expansion (Figure 1b), the low mold temperature samples (40 and 60 °C) show large transitions, both expansion and contraction, within a 105

DOI: 10.1021/acssuschemeng.5b00796 ACS Sustainable Chem. Eng. 2016, 4, 102−110

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produced a particle size that is within the range of typical mineral fillers used in plastics compounding.9,29 Mechanical Properties of the Bioresourced Carbon Filled PTT Composites. Figure 4 presents the impact and

neat PTT, the highest level of chain extender (0.4% addition) increased the elongation at yield as much as 105% (Table 1). Therefore, to counterbalance the decrease in yield elongation and ductility by filler addition, it was considered worthwhile to test the 0.4% addition level of the chain extender in the subsequent compounding experiments of PTT with the bioresourced carbon filler. Properties of the Bioresourced Carbon Filler. FTIR analysis was done on the bioresourced carbon filler used in the compounding experiments. Because the material was prepared by thermal conversion at high temperature, 900 °C for 6 h under nitrogen atmosphere, it was expected that very little functionality would be seen in the FTIR spectra, as reported earlier.14 Figure 3 shows FTIR spectra of bioresourced carbon

Figure 4. Notched Izod impact strength and heat deflection temperature of neat PTT and 20% bioresourced carbon filled PTT composites with and without chain extender (CE) at constant mold temperature of 80 °C.

thermal resistance, and Table 1 contains the tensile and flexural properties of the prepared PTT composites filled with 20% of sustainably sourced low density carbon filler in the absence and presence of reactive chain extender. The decrease in impact performance (Figure 4) and the reduction in yield elongation compared to neat PTT (Table 1) points out increased brittleness by incorporation of the carbon filler into the PTT matrix. The less ductile impact failure is also evidenced in the SEM images comparing the crack initiation points of neat PTT and 20% carbon filled PTT composite, presented in Figure 5.

Figure 3. FTIR-ATR spectra of lignin raw material and of the bioresourced carbon fillers prepared in horizontal and vertical tube furnaces.

filler prepared in the horizontal and vertical tube furnaces together with spectra of the lignin raw material. The observed peaks in the carbon filler spectra are relatively small and hard to resolve, indicating considerable reduction in chemical functional groups during the thermal conversion. The FTIR spectra of the bioresourced carbon filler prepared with both horizontal and vertical tube furnaces are similar to very high extent, demonstrating that the carbonization conditions (900 °C for 6 h under nitrogen atmosphere) were comparable between the furnaces. The elemental analysis results of the bioresourced carbon filler, shown in Table 2, showed quite high carbon content of 88%, which is well in agreement with the SEM-EDS results reported earlier.14 The high carbon content was expected based on studies of high temperature pyrolysis of lignocellulose materials under inert nitrogen atmosphere.28 Furthermore, the particle size fractions of the carbon filler were evaluated by air jet sieve analysis. It was found that 99% of the carbon filler particles passed through the 20 μm sieve, confirming that the pulverization by ball milling efficiently

Figure 5. SEM images of impact fracture surfaces of (a) neat PTT and of (b) and (c) 20% bioresourced carbon filled PTT composite.

The appearance and topography of the fracture surface is very different in the 20% carbon filled PTT composite (Figure 5b) in comparison to neat PTT (Figure 5a). The deep grooves and plastic deformation in the fracture surface of the neat PTT (Figure 5a) are not present when the polymer is filled with 20%

Table 2. Elemental Carbon (C), Hydrogen (H), Nitrogen (N), Sulphur (S), and Oxygen (O) Content of Pulverized Bioresourced Carbon Filler Determined by CHNS-O and SEM-EDS Techniques

a

sample

method

C, %

H, %

N, %

S, %

bioresourced carbon filler carbonized lignina

CHNS-O SEM-EDS

87.8 (0.9) 90.2 (1.4)

0.32 (0.01)

0.86 (0.07)

0

O, % 9.8 (1.4)

Literature reference values.14 Standard deviation for each measurement is given in parentheses. 106

DOI: 10.1021/acssuschemeng.5b00796 ACS Sustainable Chem. Eng. 2016, 4, 102−110

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Figure 6. (a) DSC curves at first cooling and (b) DSC curves at 2nd heating of neat PTT and 20% bioresourced carbon filled PTT composites with and without chain extender (CE) at constant mold temperature of 80 °C.

composites. The addition of chain extender caused only a minor increase in the HDT of the carbon filled composites (Figure 4), whereas for the neat PTT, the chain extender clearly increased the HDT with increasing addition level (Figure 2b). The substantial rise in the HDT can be ascribed both to the reinforcing action of the carbon filler and to the increased crystallinity of the PTT matrix due to nucleation effect of the filler particles. Figure 6a shows DSC curves recorded on first cooling, and Figure 6b shows DSC curves recorded on second heating for PTT and 20% carbon filler containing PTT composites in the absence and presence of reactive chain extender. It is clear that the carbon filler particles advance the crystallization of the PTT by acting as nucleating sites for the crystallization. The crystallization peak and onset temperatures are shifted toward higher temperature (Figure 6a). The nucleation effect of the carbon filler is more pronounced in the absence of the chain extender, and the crystallization peak temperature is 201 °C compared to the value of 182 °C of the neat PTT. The combined effect of the carbon filler and the chain extender falls between the two with crystallization peak temperature of 195 °C (Figure 6a). It is plausible that the reactive chain extender suppresses the nucleation effect of carbon particles because of long chain branching of the polymer chains, which reduces chain mobility and creates segments that are not capable to fold into crystallites, as opposed to the linear polymer chains in the absence of the chain extender. The effect of reactive chain extender on the crystallization of thermoplastic polyester poly(lactic acid) has been reported earlier, and reduced crystallinity was observed with the use of a chain extender.30,31 The double melting peaks in the 20% carbon filled PTT composites (Figure 6b) support the observed nucleating effect of the carbon filler (Figure 6a). Likely, the formed crystallites at the polymer−particle interface are different from the bulk crystallites, leading to the formation of the minor peak at lower temperature than the major peak of the neat PTT (Figure 6b). The degree of crystallinity for the three samples was calculated based on the crystallization peak of the first cooling. The crystallinities of neat PTT, PTT with 20% carbon filler, and PTT with 20% carbon filler and 0.4% chain extender were 35.3%, 36.8%, and 35.9%, respectively. The increase of crystallinity in the 20% carbon filled PTT confirms the nucleating effect of the filler particles. Moreover, the

of carbon and the fracture surface become much smoother with less plastic deformation (Figures 5b,c). The observed difference can be related to reduced ductility, and it agrees with the measured impact performance. The impact strength and yield elongation for the 20% carbon filled PTT are practically the same regardless of the addition of the reactive chain extender, which suggests that the chain extender only improved ductility in the absence of the carbon filler (Table 1). According to the elemental and FTIR analysis of the carbon filler (Table 2, Figure 3), the high temperature thermal conversion process was shown to produce high carbon content with little functional groups. Therefore, the carbon filler was considered to be inert without much interaction with the PTT matrix polymer. The only improvement in the mechanical properties of carbon filled PTT composite by the reactive chain extender addition was slight increase in the flexural strength of about 9% compared to the composite without chain extender, and no improvement in yield elongation was observed, in contrast to the effect of chain extender on the neat PTT (Table 1). Addition of 20% of the bioresourced carbon filler considerably increased the tensile and flexural modulus of the composites. Tensile modulus increased from 2.6 GPa of the neat PTT to 4.2−4.3 GPa of the 20% carbon filled PTT composite, and flexural modulus similarly from 2.7 to 4.2−4.3 GPa. It is known that particulate mineral fillers typically increase stiffness and reduce ductility while strength can remain unaffected, especially when the filler has low interaction with the polymer matrix.9 In many cases the fillers are surface treated by, e.g., surfactants and coupling agents in order to improve and modify the compatibility of the filler with the matrix polymer used. By filler modification, it is possible to counteract the reduced ductility and further improve the thermal and mechanical performance of the filled polymer composites.9,29 The bioresourced carbon filler used in this study was considered chemically inert with low interaction or compatibility with the PTT polymer matrix, thus leaving opportunities for improvement of the stiffness-toughness balance by filler−matrix interphase modification. Thermomechanical and Thermal Properties of the Bioresourced Carbon Filled PTT Composites. The most prominent effect of the 20% addition of carbon filler into the PTT matrix was a considerable increase of HDT from 99 °C of the neat PTT to 180−187 °C of the 20% carbon filled PTT 107

DOI: 10.1021/acssuschemeng.5b00796 ACS Sustainable Chem. Eng. 2016, 4, 102−110

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modulus is considered to be rather insensitive to particle size, especially in the absence of strong filler−matrix interactions, whereas particle shape and aspect ratio, and possible orientation during processing will have more influence on the modulus.9 Again, the agreement with the theoretical model is considered promising, while justifying future research work on interfacial engineering of the bioresourced carbon filler to broaden the attainable range of properties for engineering applications. Benchmarking against Commercial Mineral Filled Engineering Thermoplastic. To evaluate the potential of the compound containing 20% renewably sourced carbon filler in the PTT polymer matrix, it is useful to compare the material properties against existing and established engineering resin, such as 20% mineral filled PBT/PET compound commonly used in automotive applications.33,34 Table 3 compares the

presence of chain extender slightly reduced the crystallinity of the 20% carbon filled PTT, which lends support to reduced chain mobility and formation of branched or cross-linked segments not capable of forming crystallites, as a contrast to the linear polymer chains.30,31 Overall, the measured crystallinity values for PTT were in agreement with previous studies.22,23 Figure 7 presents the thermal expansion measured by TMA for

Table 3. Property Profile of the Bioresourced Carbon Filled Poly(trimethylene terephthalate) PTT in Comparison to Commercial Mineral Filled Engineering Polyester Compound PTT + 20% bioresourced carbon fillera

commercial mineral filled PBT/PET compoundb

g/cm3 % MPa

1.38 20 4210

1.49 20 5880

MPa

62

56

%

2.2

2

MPa

4327

4680

MPa

103

93

J/m

17

27

°C

180

187

MPa/(g/cm3)

3136

3141

MPa/(g/cm3)

75

62

%

50

0

property density filler content tensile modulus tensile strength yield elongation flexural modulus flexural strength Izod impact strength, notched heat deflection temperature specific flexural modulus specific flexural strength biobased content

Figure 7. TMA curves of neat PTT and 20% bioresourced carbon filled PTT composites with and without chain extender (CE) at constant mold temperature of 80 °C.

neat PTT and 20% carbon filled PTT composites with and without chain extender. The CLTE values of 20% carbon filled PTT measured below Tg were in the range of 5.6−5.9 × 10−5 1/°C and above Tg in the range of 1.8−2.1 × 10−4 1/°C, thus the carbon filler decreased CLTE below Tg but increased it slightly above Tg in comparison to neat PTT. As pointed out earlier in the mold temperature optimization, relatively linear thermal expansion for neat PTT was achieved above 60 °C mold temperature (Figure 1b), and the TMA curves presented in Figure 7 confirm similar behavior for the 20% carbon filled composites at the mold temperature of 80 °C. The enhanced crystallization of PTT by the carbon filler could provide further opportunity for optimization of the mold temperature in terms of molding cycle time without adverse effects on thermal and thermomechanical performance of the composites. However, that kind of study will remain outside the scope of this research. Theoretical Consideration of the Bioresourced Carbon as Filler in the PTT Matrix. The increase in modulus by the 20% bioresourced carbon filler can be assessed against theoretical models of composite materials to estimate the efficiency of the filler addition in relation to the highest obtainable modulus predicted by the theoretical models. On the basis of the rule-of-mixture (ROM) approach and using modulus and density values of 2.6 GPa and 1.33 g/cm3 for the matrix PTT and 12 GPa and 1.6 g/cm3 for the carbon filler,32 the ROM predicts modulus of 4.1 GPa for the 20% carbon filled composite, which is in agreement with the experimental modulus of 4.2−4.3 GPa. In the above calculation, modulus of the bioresourced carbon filler was assumed to be similar to anthracite coal for which the modulus is reported.32 Better estimates for the modulus are expected to be obtained as the research and characterization of bioresourced carbon fillers advances in the future. Typically, fillers increase the modulus of thermoplastics linearly until around 20% by volume, and

unit

Experimental data at 80 °C mold temperature. bDatasheet values for the commercial product.33,34

a

mechanical properties between the 20% carbon filled PTT and the commercial 20% mineral filled PBT/PET engineering resin. The property profile of the 20% carbon filled PTT compares well to that of the commercial compound for most of the properties. For example, the tensile and flexural strength of the 20% carbon filled PTT exceeds the strength of the commercial compound. Because of the lower density of the carbon filler, the specific strength is even higher. On the other hand, the impact strength and the tensile modulus of the 20% carbon filled PTT are lower. In terms of flexural strength, the 20% carbon filled PTT compares favorably with the commercial compound, and when considering the specific flexural modulus the materials are nearly equal (Table 3). The lower density of the 20% carbon filled PTT clearly indicates the light weighting potential of the 108

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ACS Sustainable Chemistry & Engineering

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biobased carbon filler. At 20% addition level, the carbon filler reduces the density over 7% compared to the mineral filled counterpart, and the density difference and weight reduction potential increases with higher filler levels. In addition to the very desired light weighting potential, the sustainably sourced biobased carbon filler adds to the biocontent of the partly biobased PTT polymer creating a composite material with 50% biobased content, a level that only a few materials in the class of thermoplastic engineering resins can currently claim, whereas the majority of the materials originates from nonrenewable petrochemical resources. In applications where impact strength is not critical and weight savings and sustainability are considered of prime importance, the low density biobased carbon filler shows good potential. The studied composition of 20% bioresourced carbon filled PTT has, in many terms, comparable or favorable property profile against commercial mineral filled compound, justifying future research and development of sustainable biobased carbon fillers for thermoplastic compounds and engineering applications.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*A. K. Mohanty. E-mail: [email protected]. Tel.:+1-519824-4120, ext. 56664. Fax: +1-519-836-0227. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Natural Sciences and Engineering Research Council (NSERC) Canada for the Discovery Grants (Project # 400322); and the Ontario Ministry of Economic Development and Innovation (MEDI), Ontario Research Fund, Research Excellence Round 4 (ORF-RE04) program (Project # 050231 and 050289) are gratefully acknowledged for financial support. E. I. du Pont de Nemours, Wilmington, DE, USA is acknowledged for providing Sorona PTT sample.

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REFERENCES

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CONCLUSIONS

This study reports the engineering properties of injection molded thermoplastic compound consisting of lignin-based bioresourced carbon filler and biobased poly(trimethylene terephthalate) polymer. The lignocellulose derived biobased carbon filler was characterized by chemical composition and particle size. The carbon content of the filler after thermal conversion of the raw material was approximately 88%, a typical value for high temperature pyrolysis products of lignocellulosic origin. Particle size of the carbon filler was below 20 μm, which is in the size range of common mineral fillers used in plastics. In the first part of the work, processing conditions were optimized with regard to the thermomechanical performance of the PTT polymer, and use of a reactive chain extender was investigated in order to improve further the attainable performance profile of the PTT. Composites containing 20% of the bioresourced carbon filler were prepared in the presence and absence of the reactive chain extender at the optimized process conditions. Addition of 20% carbon filler into the PTT matrix significantly improved the HDT and stiffness of the compound, whereas ductility was decreased as indicated by reduction of impact strength and yield elongation. The addition of reactive chain extender did not have as clear an effect on the properties of the carbon filled compounds, despite it was found to improve the performance of the neat PTT polymer. The carbon filler particles advanced the nucleation of the PTT polymer and increased the crystallinity of the polymer; otherwise, the filler was considered inert without specific interaction with the PTT matrix. In comparison to a mineral filled engineering polyester resin, the studied compound had comparable strength, specific flexural stiffness, and HDT, thus showing promise for engineering end-uses. When considering the potential for light weighting and the sustainability aspects against high density mineral filled, nonrenewably sourced petroleum based engineering resins; the bioresourced carbon filled polyester demonstrated particularly desirable properties. Although the reported material, 20% biobased carbon filled PTT, already showed an interesting property profile versus existing mineral filled engineering resin, future work is required on the stiffness−toughness balance to improve the property profile and to extend the potential application fields. 109

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