Distillers dried grains with solubles (DDGS) were employed as a bio-based resin/adhesive. DDGS were defatted with hexane, ball ground, and screened prior to use. DDGS flour was mixed dry with Paulownia wood (PW) to make composites using the following conditions: temperature of 150 to 195 °C, PW particle sizes of £ 75 to 1700 mm, pressure of 2.1 to 5.6 MPa, and using DDGS dosages of 10 to 100%. Molded composites were evaluated for their flexural properties. Composites were examined with Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction. The best flexural properties were obtained from composites containing 50% DDGS and 50% PW, using 180 to 250 mm PW particles, pressed at 5.6 MPa, and employing 185 °C. Flexural properties of DDGS-PW composites were similar to composites fabricated using soybean flour (Prolia) as the resin/adhesive. Dimensional stability properties (water absorbance and thickness swelling) of DDGS-PW and Prolia-PW composites were similar. DDGS-PW composite properties satisfied several European Committee Industry Standards for fiberboards in terms of flexural properties but were inferior in terms of thickness swelling properties.
Fiberboard Created Using the Natural Adhesive Properties of Distillers Dried Grains with Solubles
Brent Tisserat,a,* HongSik Hwang,a Steven F. Vaughn,a Mark A. Berhow,a Steven C. Peterson,b Nirmal Joshee,c Brajesh N. Vaidya,c and Rogers Harry-O’Kuru d
Distillers dried grains with solubles (DDGS) were employed as a bio-based resin/adhesive. DDGS were defatted with hexane, ball ground, and screened prior to use. DDGS flour was mixed dry with Paulownia wood (PW) to make composites using the following conditions: temperature of 150 to 195 °C, PW particle sizes of 75 to 1700 m, pressure of 2.1 to 5.6 MPa, and using DDGS dosages of 10 to 100%. Molded composites were evaluated for their flexural properties. Composites were examined with Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction. The best flexural properties were obtained from composites containing 50% DDGS and 50% PW, using 180 to 250 m PW particles, pressed at 5.6 MPa, and employing 185 °C. Flexural properties of DDGS-PW composites were similar to composites fabricated using soybean flour (Prolia) as the resin/adhesive. Dimensional stability properties (water absorbance and thickness swelling) of DDGS-PW and Prolia-PW composites were similar. DDGS-PW composite properties satisfied several European Committee Industry Standards for fiberboards in terms of flexural properties but were inferior in terms of thickness swelling properties.
Keywords: Medium density fiberboard; Particle board; Renewable resources; Paulownia; Flexural properties; Thermal properties; Dimensional stability
Contact Information: a: Functional Foods Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604 USA; b: Plant Polymers Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, Peoria IL 61604 USA; c: Agricultural Research Station, Fort Valley State University, Fort Valley, GA 31030 USA; d: Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, Peoria IL 61604 USA;
* Corresponding author: Brent.Tisserat@ars.usda.gov
It is estimated that by 2030, global consumption of industrial and solid wood will increase by 60% over currently consumed levels, and in addition, there will be a substantial demand for more paper and paperboard products (Elias and Boucher 2014). To satisfy wood needs, engineered wood products are employed such as fiberboard (FB), which includes particleboard (PB), medium density fiberboard (MDF), hardboard (HB), and oriented strand board (OSB) (Hemmilä et al. 2017). These products are composed of various sized cellulosic particles bonded together with synthetic resins or adhesives using heat and pressure. Engineered wood products are expected to grow 25 to 33% by 2020 (Elling 2015). Structural panels made in North America will increase 21% by 2020 (i.e., from 31.5 billion square feet to 38 billion), largely in response to increased housing.
Currently, engineered wood products use petroleum-based thermosetting resins such as urea formaldehyde, phenol formaldehyde, melamine fortified urea formaldehyde, or polymeric diphenylmethane diisocyanate (Hemmilä et al. 2017). Using these resins to create engineered wood products has adverse environmental and health consequences because of the emission of volatile organic compounds (VOCs) such as formaldehyde (Hemmilä et al. 2017). New engineered wood products often employ isocyanate resins since they do not contain formaldehyde and are considered non-volatile when cured. In addition, other additives are incorporated into the FB such as borate compounds (to prevent termites, wood boring beetles, molds, and fungi) and waxes (to increase hydrophobicity) (Hemmilä et al. 2017). Engineered wood products are a major source of formaldehyde off-gassing in US homes and present a serious health problem (CPSC 2013). Formaldehyde is classified as a carcinogen, and its exposure levels are regulated in the USA to avoid health problems (CEPA-ARB 1997; EPA 2010; CPSC 2013; ClassAction.org 2015; Hemmilä et al. 2017). However, wood composite flooring made in China and exported to the US have formaldehyde emission levels that exceed current US safety emission standards, in some cases by as much as 350% (ClassAction.org 2015).
Soy protein-based adhesives derived from soya bean meal (SBM) have been employed to some extent over the last 80 years (Zhong et al. 2001). Soybean proteins are an alternative to petroleum polymers because of their abundance, renewability, biodegradability, and feasibility (Mo et al. 2001, 2003; Mohanty et al. 2002; Lodha and Netravali 2005a, 2005b; Liu and Li 2007; Frihart et al. 2014; Nasir et al. 2014; Xu et al. 2014). Soybean meal is inexpensive, abundant, and easy to handle because it can be cold or hot pressed (Zhong et al. 2001; Amaral-Labat et al. 2008; Jeon et al. 2011; Reddy and Yang, 2011; Gu et al. 2013; Xu et al. 2014). However, SBM was essentially replaced in the 1960s by less-expensive synthetic adhesives. In recent years, interest in their use as adhesives was renewed because they are biodegradable and free of VOCs. For example, Uniboard (Laval, Quebec, Canada) markets a “Nu Green SOYA” particle board utilizing a soy based adhesive (Uniboard Canada Inc. 2016). Soya protein isolate (SPI) has high adhesion strength but costs more than PF and UF (Mo et al. 2001; Zhong et al. 2001; Kumar et al. 2002; Lodha and Netravali 2005a; 2005b; Liu and Li 2007; Vnučec et al.2015). Commercial products such as SPI (Pro-Fam 970) (ADM, Decatur, IL) containing 90% protein (dry basis) and defatted SBM such as Prolia (PRO) (Cargill, Cedar Rapids, IA) containing ≥ 50% protein are available. SBM currently sells for $0.45 to $0.50/lb ($0.99 to $1.10/kg) (Alibaba.com 2016).
Other sources of bio-based adhesives have also been developed (Shukla and Cheryan 2001; Beg et al. 2005; Norström et al. 2014). Of particular interest is the employment of zein, which is derived from corn gluten meal and is highly effective as a resin/adhesive (Shukla and Cheryan 2001). Zein is a prominent storage protein (a prolamine protein) in corn and comprises 35 to 40% of the corn protein. It is extracted from corn gluten meal, which is obtained from the wet milling processing of corn seeds. Unfortunately, the cost of zein is prohibitive to most commercial enterprises such as bio-plastics and bio-composites. Zein sells for $4.54 to $18.20/lb ($10 to $40/kg) (Shukla and Cheryan, 2001). Corn gluten meal itself contains 55 to 70% protein and is considerably less expensive, selling for $0.36 to $0.45/lb ($0.79 to $0.90/kg) (Alibaba, 2016). Corn gluten meal has been employed in a myriad of products including livestock feed, plastic fillers, coatings, and bio-plastics (Beg et al. 2005; Samarasinghe et al. 2007). Nevertheless, there is still a great need to develop even less expensive, abundant bio-based adhesives to be employed in the manufacture of engineered wood products.
The major feedstock used in ethanol production in North America is corn, with over 90% of the 226 operational ethanol plants utilizing corn exclusively (Liu 2011; Ethanolproducer.com 2017). Approximately 30% of the U.S. harvested corn acreage, representing 5,200 million bushels of corn, is used for ethanol production (Wisner 2015). Over 80% of the ethanol produced is derived from the dry-milling process (Kim et al. 2008). Distillers dried grains with solubles (DDGS) are the cereal by-product of the dry-milling process (Shukla and Cheryan 2001). Approximately 38 to 42 million metric tons of DDGSs are produced annually in North America (Clarizio and Tatara 2013). DDGS are a relatively low value by-product, are mainly used as an animal feed, and sell for about $0.03 to $0.05/lb ($0.06 to $0.10/kg) (Irwin 2017). Recently, DDGSs have been employed as a bio-filler blended with thermoplastic resin matrix (e.g. poly(lactic acid) (PLA)), high density polyethylene (HDPE), or thermoplastic starch to fabricate biocomposites (Tatara et al. 2009; Tisserat et al. 2013a; Clarizio and Tatara 2013; Lu et al. 2014a; 2014b; Madbouly et al. 2014; Ju et al. 2016). However, our interest was to utilize the DDGS, not as a filler/reinforcement material, but as the matrix itself in the fabrication of biocomposites. Both zein ( 96% protein) and corn gluten ( 65% protein) have been employed as matrix resin material to fabricate biocomposites (Shukla and Cheryan 2001; Beg et al. 2005; Samarasinghe et al. 2007). DDGS contains 25 to 35% protein, which suggests it could be employed as adhesive/resin to construct engineered wood products as a substitute for zein or corn gluten meal. This study investigated the possibility of developing a DDGS-wood biocomposite product. The filler material chosen in this study was Paulownia wood (PW) derived from trees of Paulownia elongata S.Y. Hu, (Paulowniaceae) grown at the Paulownia Demonstration Plot, Fort Valley State University. This tree is an extremely fast-growing coppicing hardwood that is native to China and cultivated in plantations in China and Japan. PW is highly valued in the construction and furniture industries (Chinese Academy of Forestry Staff 1986; Joshee 2012). Paulownia trees can be established on marginal lands and have deep tap roots, which make them drought tolerant (Joshee 2012). PW is light-weight, insect resistant, pale colored, and heat resistant (Chinese Academy of Forestry Staff 1986; Ashori and Nourbakhsh 2009; Joshee 2012). Paulownia trees offer an inexpensive source of woody biomass for both energy and lumber, and their wood wastes could be employed in the fabrication of engineered wood products (Ashori and Nourbakhsh 2009; Joshee 2012). Juvenile Paulownia trees could be a likely source of woody biomass needed in the future. Hence, this study utilized PW derived from juvenile tree biomass (i.e. 36-month-old).
The influence of processing temperatures, pressures, wood filler particle sizes, and DDGS-filler dosage ratios on the flexural properties of DDGS/PW biocomposites were assessed. The flexural properties of DDGS/PW biocomposites were compared to industry standards for commercial composites (i.e. PB, MDF, and HDF) to assess their potential commercial application. In addition, since SBM flour is commonly employed as a bio-adhesive in engineered wood products, the flexural properties of DDGS-PW composites to PRO-PW composites using various resin and wood dosages were also compared. As part of this study, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) were also employed to assess the bonding nature associated with DDGS and wood particles.
DDGS were obtained as the commercial animal corn feed pellet product (Archers Daniel Midland Co., Decatur, IL, USA). ProliaTM(200/90) (PRO) containing 54% protein and 5% moisture was provided by Cargill Inc., Cedar Rapids, IA, USA and used as provided. Paulownia elongata wood material was obtained from 36-month-old trees grown in Fort Valley, GA. PW shavings were milled successively through 4-, 2-, and then 1-mm screens with a Thomas-Wiley mill grinder (Model 4, Thomas Scientific, Swedesboro, NJ, USA). Particles were then sized through a Ro-TapTm Shaker (Model RX-29, Tyler, Mentor OH, USA) that employed 203 mm diameter stainless steel screen/sieves. The sieve/screens employed were #12, #30, #40, #80, #140, and #200 US Standards (Newark Wire Cloth Company, Clifton, NJ, USA). The shaker was operated for 60 min intervals at 278 rpm to obtain particle separation. Two PW mixtures composed of 50% ≤ 600 µm particle selection, obtained from particles passing through the #30 mesh sieve, and a 600 to 1700 µm particle selection, obtained from particles passing through the #12 mesh sieve and collected on the #30 mesh sieve, were used throughout this study. PW contained 6% moisture. DDGS were ground in a Wiley mill as described. DDGS were defatted with hexane as the solvent using a Soxhlet extractor to obtain a DDGS containing 30% crude protein and 5% moisture. DDGS were ball ground into a flour (< 425 m particles) using a laboratory bench top ball mill (Model 801CVM, U.S. Stoneware, East Palestine, OH, USA) to obtain fine powder. DDGS was ground in Alumina mill jars containing Burundum cylindrical grinding media pellets (13 mm diam, 7.3 g wt.) (U.S. Stoneware) at a speed of 50 rpm for 60 h. DDGS flour was sieved through a #80 mesh to be obtained as 250 µm particles.
To test the influence of mold temperature on DDGS-PW composites, a composite composed of 80 g of DDGS was mixed with 40 g of < 600 m PW particles and 40 g of 600 to 1700 m PW particles. DDGS and PW were mixed in a zip-lock bag and given 15 min of circular agitation using a 0.074 m3 compact dryer (Model MCSDRY1S, Magic Chef, Chicago, IL, USA). An aluminum mold (outer dimensions: 15.2 cm width 30.5 cm length 5 cm depth and mold cavity: 12.7 cm width 28 cm length 5 cm depth) was employed. The mold interior was sprayed thoroughly with mold release (Paintable Dry Spray with Teflon, No. T212-A, IMS, Chagrin Falls, OH, USA). Pressings were conducted using manual hydraulic presses (Model 4126, Carver Press Inc., Wabash, IN, USA). The mold was then transferred to a preheated Carver press at 150, 170, 180, 185, or 190 °C. Initially, molds were given 2.8 MPa pressure for 4 min, and then pressure was released in order to remove internal air build-up within the composite. Then, molds were pressed to 4.2 MPa for 4 min and pressure was released again. Finally, molds were pressed to 5.6 MPa for an additional 4 min. Total heating/compression time was ~12 min. Mold composites were then held at 5.6 MPa pressure while the heating was terminated, and cooling process of the press platens commenced (via cold water). The mold was removed from the Carver press when the mold surface reached 27 °C.
The influence of PW particle size on the mechanical properties of DDGS-PW composites was determined by testing particles obtained from sieve/screens as previously described. Composites composed of 80 g of DDGS were mixed with 80 g of < 1700, < 600, 425 to 600, 180 to 250, 106 to 180, < 74, or 600 to 1700 m PW. DDGS-PW composites were subjected to 185 °C under 5.6 MPa pressure for 12 min. The influence of mold pressure on DDGS-PW composites was tested on composites composed of 80 g DDGS and 40 g of PW consisting of < 600 m particles and 40 g of PW consisting of 600 to 1700 um particles. DDGS-PW composites were fabricated at a temperature of 185°C for 12 min using a pressure of 2.1, 2.8, 4.2, or 5.6 MPa. To compare DDGS to PRO, 15, 25, 50, 75, and 100%, mixtures of DDGS or PRO were mixed with the balance of PW consisting of equal amounts of 74 to 600 m particles and 600 to 1700 um particles. DDGS-PW and PRO-PW composites were subjected to 185 °C temperature under 5.6 MPa pressure for 12 min.
FTIR spectra were measured on an ABB Arid Zone FT-IR spectrometer (ABB, Houston, TX, USA) equipped with a DTGS detector. Test samples were transparent discs that consisted of 1.00 mg solids homogenized with 300 mg of dry spectronic grade KBr, placed in a KBr die, and compressed at 24,000 psi using a Carver press. Absorbance spectra were acquired at 4 cm-1 resolution and signal-averaged over 32 scans. Interferograms were Fourier transformed using cosine apodization for optimum linear response. Spectra were baseline corrected and adjusted for mass differences and normalized to the methylene peak at 2927 cm-1.
DSC and TGA
DSC experiments were performed in duplicate on a TA Instruments DSC, Model Q2000 with refrigerated cooling system (New Castle, DE). Calibration was done without a pan for baseline and with indium for temperature. Samples (9 to 10 mg) were added in Tzero aluminum pans and press sealed (non-hermetic). The DSC was purged with dry nitrogen at 50 mL/min. The sample was equilibrated at -60 °C, then heated to 190 °C at 10 ºC.min-1, and this heating cycle was repeated twice. TGA experiments were conducted in duplicate with a Model Q50 TGA (TA instruments, New Castle, DE, USA) under nitrogen with 60 mL/min flow rate. An approximately 10 mg sample was placed on a platinum sample pan, and the pan was loaded with the autosampler. The sample was heated at 10 °C/min from 25 °C to 800 °C. TA Universal Analysis software was used to analyze the results.
XRD analysis was employed to assess the morphological properties of ingredients and composites. X-ray diffraction spectra analyses were performed using a Bruker D2 Phaser (Bruker AXS Inc., Billerica, MA, USA) X-ray diffractometer. The X-ray source was Cu-Kα radiation at a current of 10 mA and 30 kV, set up using θ/θgeometry. Samples were scanned at 10 to 90°, 2θ, step size 0.01°, 0.2 s/step, and stage rotation of 10 rpm. Initial divergence slit size was 0.6 mm and a 1 mm air scatter screen was used above the sample. A Lynxeye™ detector was used with a 2.5° Soller slit and a Ni-Kβ filter.
Scanning Electron Microscopy (SEM) was conducted at Fort Valley State University, Fort Valley, GA, USA. Samples were mounted on Hitachi aluminum specimen mounts M4, ø15 x 6 mm, (Ted Pella, Inc., Redding, CA, USA) using double sided carbon tape. The samples were then dried at a room temperature (~26 °C) for 48 h to remove excess trapped moisture. After 48 h, the specimens were sputter coated with gold using Denton Vacuum Sputter Coater Desk V (Denton Vacuum, NJ, USA). The vacuum pressure was set at 0.05 tort for 60 sec of sputter coating with 50 Å thickness. A variable pressure SEM (Hitachi 3400 NII, Hitachi Technologies America, Inc., Pleasanton, CA, USA) was used to take pictures at various magnifications. Sample surfaces and edges were examined.
Flexural and Physical Tests
Following pressings, composite panel boards were conditioned at 25 °C and 50% relative humidity (RH) for 72 h. A table saw was employed to cut specimen boards in order to conduct three-point bending tests (EN 310:1993). Specimen board thickness was measured according to the EN 310:1993 test. Specimen board dimensions were 127 mm long, 50 mm wide, and 3.5 to 5.5 mm thick, depending on the treatment. The specimen thickness dictated the free span length. Tests were performed on a universal testing machine [Instron Model 1122 (Instron Corp., Norwood, MA, USA)] using a crosshead speed of 5 mm/min. Densities of the composites were determined using the EN 323:1993 standard. Water absorbance (WA) and thickness swelling (TS) were conducted on 50 x 50 mm squares immersed in water for 24 h according to EN 317:1993 standards utilizing composite formulations of the various DDGS-PW dosages (EN 317:1993).
Five specimens of each formulation were tested. The average values and standard errors were reported. The experimental data obtained were analyzed statistically by analysis of variance for statistical significance, and multiple comparisons of means were accomplished with Duncan’s Multiple Range Test (p 0.05) (Statistix 9, Analytical Software, Tallahassee, FL, USA).
RESULTS AND DISCUSSION
Figure 1 a-d shows the FTIR spectra of ingredients DDGS and PW and composite panels 100DDGS (100% DDGS wt.) and 50DDGS-PW (50% DDGS:50% PW wt.). There was no difference in the absorption bands between the original DDGS and 100DDGS composite panel except for a peak region occurring at 625 cm-1 in the 100DDGS, which was absent in the DDGS ingredient spectra. The 600 to 700 cm-1 region is characteristic of P-S and P=S stretching (Pretsch et al. 2000). That the region shows up in the 100DDGS suggests unfolding of the protein component so that a buried vibration could be observed.
All spectra (Fig. 1a-d) showed characteristic H-bonding and -OH stretching absorption around 3300 to 3500 cm-1 and C-H asymmetric and symmetric stretching of methylene groups around 2800 to 3000 cm-1 (Pretsch et al. 2000; Nagieb et al. 2011; Hemsri et al. 2012; Deka et al. 2015; Li et al. 2016). This region’s band was centered at 3384 cm-1 and is identified as -OH of carbohydrate overlapping the protein -NHs (Pretsch et al. 2000; Deka et al. 2015). A broad alkyl band occurs at 2921 cm-1, which was resolved in the 2nd derivative spectrum into four bands: strong 2958 cm-1 (-CH3 asymmetrical stretching), very strong 2927 cm-1 (-CH2 asymmetrical stretching), 2876 cm-1 (-CH3 symmetrical stretching), and very strong 2858 cm-1(-CH2 symmetrical stretching) (Pretsch et al. 2000; Deka et al. 2015). A residual ester or lignocellulosic carbonyl and protein bands occur at 1738 cm-1 (-C=O, medium carbonyl band either from lignin or uronic esters), and 1656 cm-1 (N-C=O, amide I). The 1543 and 1517 cm-1(H-N-CO) bands are the protein amide IIs (Pretsch et al. 2000; Deka et al. 2015). Bands 1470 cm-1 and 1372 cm-1 are identified as CH2and CH3 deformations, respectively, and are commonly known as the alkyl deformation region of the spectrum (Pretsch et al. 2000). Other prominent bands are located at 1117 cm-1 (O-C=O), 1081 cm-1 (-CHO-), 1050 cm-1 (-CH2O-), and 994 cm-1 (unknown).
Fig. 1. FTIR spectra of ingredients (a) DDGS and (c) PW and bio-composites (b) 100DDGS and (d) 50DDGS-PW
Figure 1b 100DDGS spectrum is similar to Fig. 1a of the DDGS ingredient. As previously noted, a strong band occurs at 625 cm-1representing P-S and P=S stretching of protein. This band was inconspicuous in the spectrum of the folded DDGS protein (Fig. 1a) but now obvious in the unfolded matrix (Fig. 1b).
Figure 1c is the spectrum of PW. Like in many carbohydrates a broad and strong symmetric absorption band is observed at 3420 cm-1 for the (H bonding and -OH stretching) with a moderate intensity alkyl band occurring at 2925 cm-1 (Pretsch et al. 2000; Li et al. 2016). The broad band at 2925 cm-1 is resolved into a sharp 2969 cm-1 (-CH3asymmetrical stretching), 2924 cm-1 (-CH2– asymmetrical stretching), 2876 cm-1 (-CH3 symmetrical vibrations) and 2852 cm-1 (-CH2symmetrical vibrations). The alkyl groups occurring between 2925 cm-1 and 620 cm-1 of the spectrum are easier to identify from its 2ndderivative spectrum because these bands are better defined. A medium carbonyl band either from lignin or uronic esters is also evident at 1739 cm-1 followed by moderate protein absorbances at 1642 to 1505 cm-1 (Pretsch et al. 2000; Li et al. 2016). Alkyl chain deformation bands (-CH2– and -CH3) trending toward the low frequency region ending at 620 cm-1 (see previous paragraphs). The 620 cm-1 band is analogous to that in processed 100DDGS. There is a single carbonyl band at 1738 cm– 1 assignable either to a uronic ester in the wood component or a lignin carbonyl moiety (Pretsch et al. 2000; Li et al. 2016). Next there is a low protein carbonyl absorption at 1656 cm-1, amide I and very intense peaks at 1597 and 1508 cm-1presumably represent the amide II mode. The -CH2– and -CH3deformation bands are shown at 1470 and 1372 cm-1, respectively. There is a sizable O-C=O band at 1246 cm-1 followed by very intense –CHO-, and -CH2O- bands located at 1159, 1128, 1037, and 991 cm-1. Finally an anomalous P-S and P=S band is observed at 670 cm-1(Fig. 1c).
The 50DDGS-PW composite spectra ( Fig. 1d) is characterized by a strong 3410 cm-1 band representing the overlapping -OHs of DDGS and PW together with the -NH stretching bands of the intrinsic protein content. A broad band at 2913 cm-1 is resolved in the 2ndderivative of its FTIR spectrum into four discrete bands: 2959 cm-1 (-CH3 asymmetrical), 2923 cm-1 (-CH2 asymmetrical stretch), 2875 cm-1 (-CH3 symmetrical stretching) and 2854 cm-1 (-CH2symmetrical stretching) absorption. The next absorption series of the spectrum comprise an ester carbonyl band at 1743 cm-1 resulting from the PW, a single amide I absorbance at 1656 cm-1 (probably the α-helix of the DDGS), and prominent amide II peaks at 1596, 1546 and 1517 cm-1. These are followed by the deformation bands, 1470 and 1378 cm-1 of the –CH2– and -CH3 alkyl moieties, respectively. The latter bands are followed by the -O-C=O stretching of the ester at 1246 cm-1 and the –COH- and -CH2O- at 1167, 1130, 1077, 1039 and 992 cm-1 and finally by the P-S and P=S vibration at 621 cm-1.
DSC and TGA Evaluations
Ganesan and Rosentrater (2007) observed glass transition temperatures (Tg) at 20 to 50 °C with unmodified DDGS, defatted DDGS, or de-waxed DDGS using DSC. The defatted DDGS used in this study showed a somewhat higher Tg (54.0 °C) during the first heating cycle, which was shown as a small shoulder in the large water evaporation curve (Fig. 2a). The difference, presumably, resulted from slightly different compositions produced by different processes to obtain the DDGS. The second heating cycle, conducted after the first heating cycle, showed no water evaporation peak and a higher Tg(117.8 °C). The reason for this weak signal was that the DDGS is a mixture of many ingredients and some of them exhibit a “thermoplastic property”. The presence of water lowers the Tg and acts as a plasticizer in a material having glass transition, and thus removing the water increases the Tg (Frascareli et al. 2012). The DSC of PW did not show an obvious Tg during the first or the second heating cycles, indicating that the PW did not have ingredients undergoing glass transition (Fig. 2c). The DSC of 100DDGS showed a slightly lower Tg than that of DDGS in both the first and the second heating cycles, indicating that the materials showing glass transition underwent little change during the process of making this composite (Fig. 2b). In contrast, 50DDGS-PW showed no Tg (Fig. 2d). In the presence of wood, DDGS binds more completely to produce a thermoset composite. This result indicated that in addition to the fact that 50DDGS-PW contained only 50% DDGS, there may have been physical changes in DDGS during the composite-making process.
The physical change such as the interface of DDGS molecules with PW particles may have been one of the possible reasons for the increased strength of 50DDGS-PW over 100DDGS (which will be discussed later), since there were no substantial chemical reactions evident between DDGS and PW as observed in FT-IR spectra.
Fig. 2. DSC of ingredients (a) DDGS and (c) PW and bio-composites (b) 100DDGS and (d) 50DDGS-PW
The TGA analysis confirmed water evaporation from both ingredients and composites was initiated around 100 °C (Fig. 3). DDGS further lost weight starting around 140 °C, while PW did not begin substantial weight loss until the temperature reached 210 °C. Beyond 210 °C, all four samples exhibited remarkable weight loss due to decomposition. This indicated that such process temperatures should have been avoided because of detrimental damage to the composites. However, there should be a certain temperature and enough incubation time for the physical changes required for interfacing of the DDGS molecules with PW particles to occur in order to obtain optimized strength of the composite. Process temperatures that are too high or process times that are too long could cause excessive thermal decomposition of the composite resulting in lower strength. This will be discussed further in a later section.
Fig. 3. TGAs of DDGS, PW, 50DDGS-PW, and 100 DDGS
Figure 4 represents the X-Ray diffractograms of the native DDGS and PW versus the resulting bio-composites (100 DDGS and 50DDGS-PW). Both ingredients (DDGS and PW) are biological, amorphous solids that soften and semi-melt over a temperature range conducted in this study. In addition, it was noted that breaking of the 50DDGS-PW and 100DDGS panels results in curved or irregular faces, which are characteristic of amorphous solids.
The DDGS samples (DDGS, 100DDGS, and 50DDGS-PW) had a doublet peak occurring around 30 degree 2-theta. The occurrence of this doublet has been previously reported (Xu et al. 2008). The PW ingredient had a broad peak occurring around 22 degree 2-theta which is characteristic of amorphous materials such as wood (Devi and Maji 2013). In addition, PW diffractogram showed shoulder peaks occurring before and after the main peak at 22 degree 2-theta.
The 50DDGS-PW diffractogram represented a combination of the DDGS and PW ingredients without any new peaks occurring. The PW peaks of the 50DDGS-PW composite were somewhat masked by the broader DDGS peak. Overall, the composites did not show any additional peaks at higher degree 2-theta values, which would have indicated the occurrence of crystallization (Xu et al. 2008). It was concluded that little or no crystallization occurred in the preparation of the composites from the ingredients employed. This suggested that the interaction between the DDGS and PW is disordered and amorphous in nature.
Fig. 4. X-Ray diffractograms of ingredients (a) DDGS and (c) PW and bio-composites (b) 100DDGS and (d) 50DDGS-PW
Effect of Temperature on Flexural Properties
It should be noted that all composites were found to have smooth tactile feel on their surface, resembling a commercial MDF or even thermoplastic or thermoset resin-wood composite. The temperature employed to hot-press composites was found to have remarkable influence on the flexural properties of the composites. The Fmobtained from specimens using 150, 170, and 180 °C as the hot-press temperatures were similar (Fig. 5). The DDGS-PW composites hot-pressed at 185 °C showed a remarkable increase in bending strength (Fm), while the 190 °C hot-pressed specimens showed a decline. Likewise, the modulus of elasticity (Em) values followed the same pattern (Fig. 5).
Clearly, the mechanical properties of DDGS-PW composites hot-pressed at 185 °C were superior to other hot-press temperatures. This was attributed to the optimum plasticization of the DDGS matrix having occurred at this temperature. Below this temperature (185 °C) some plasticization of DDGS occurred, while above this temperature (190 °C) both plasticization and thermal degradation of the DDGS occurred. DDGS-PW composites hot-pressed at 185 °C had Fm and Em values of 41.4 0.7 and 6073 123 MPa, respectively, while DDGS-PW composites hot-pressed at 150 °C had Fm and Em values of 22.8 2.0 and 3548 157 MPa, respectively. This translated into DDGS-PW composites hot-pressed at 150 °C with Fm and Em values being -45 and -42% less, respectively, than composites hot-pressed at 185 °C.
Fig. 5. Effect of press temperatures on the flexural properties of biocomposites
Effect of Particle Size on Flexural Properties
The size of wood particles plays an important role in the fabrication and overall characteristics of engineered wood (Korosten MDF manufacture 2013; Lias et al. 2014). Particleboard usually consists of large wood particles obtained from inexpensive wood sources, which are hot-pressed with a petroleum based binding agent (i.e.,formaldehyde resins) (Korosten MDF manufacture 2013). MDF in contrast is made of finer wood particles and typically are much denser and thinner than particleboard. MDF may be glued together with lignin and wax (paraffin), and these are therefore more eco-friendly than particleboard (Korosten MDF manufacture 2013; Lias et al. 2014). In this study a variety of wood particles were employed in the DDGS-PW composites (Fig. 6). It was found that the bending strength (Fm) and modulus of elasticity (Em) values of DDGS-PW composites were influenced by the particle size of the wood employed. For example, composites composed of the largest wood particles (600 to 1700 m) had the lowest flexural values when compared to the other composites tested, Fm and Em values were 34.2 1.4 and 5151 244, respectively. The composite that contained 425 to 600 m particles selection also exhibited lower flexural properties. Composites containing 180 to 250 um particles had the highest Fmand Em values that were 45.4 2.3 and 7409 276, respectively. This suggested that this particle size was optimum for producing high quality composites. Composites that contained a mixture of wood particles composed of < 1700 m or < 600 m exhibited higher flexural properties that rivaled the 180 to 250 m composite. Interestingly, composites containing the smallest particle (< 74 m) exhibited lower Fm values but had Em values in par with the other composites. Other researchers have noted the importance of wood particle size on the mechanical properties using thermoplastic resin-wood blends (Stark and Berger 1997; Tisserat et al. 2013b).
Fig. 6. Effect of the Paulownia wood (PW) particle size on the flexural properties of DDGS-PW composites
Fig. 7. SEM micrographs of DDGS-PW composites composed of various particles sizes: (a) < 74 to 1700 m particles-surface, (b) < 74 to 1700 m particles-edge, (c) < 74 to 600 m particles-surface, (d) < 74 to 600 m particles-edge, (e) 425 to 600 m particles-surface, (f) 425 to 600 m particles-edge, (g) 180 to 250 m particles-surface, (h) 180 to 250 m particles-edge, (i) 106 to 180 m particles-surface, (j) 106 to 180 m particles-edge, (k) < 75 m particles-surface, (l) < 75 m particles-edge, (m) 600 to 1700 m particles-surface, (n) 600 to 1700 m particles-edge; scale bar = 200 m
SEM examination of the various composites showed little differences in their surface topography (Fig. 7). However, the cut edges showed the outlines of the particles sizes employed in the composition of the panels (Fig. 7). DDGS coating of the particles often obscured the wood particles (Fig. 7). Deeper fissures occurred in the micrographs of the DDGS-PW composites containing the larger particles sizes compared to that found in the other composites (Fig. 7 b and n). These fissures were often associated with poor interfacial adhesion between ingredients and resulted in lower flexural properties (Fig 6).
The interaction between the DDGS and wood particle size was responsible for flexural properties of the composite. The DDGS matrix somewhat resembled a thermoset matrix, which mimics SBM74 matrix/adhesives. The mode of bio-based adhesion is unclear and complex (Frihart et al. 2010; Frihart 2011; Frihart and Birkeland 2014; Frihart et al. 2014). The adhesive nature of SBM is believed to be attributable to the protein composition (Frihart et al. 2010; Frihart 2011; Frihart and Birkeland 2014; Frihart et al. 2014). Soya seed proteins represent 30 to 50% of the seed mass with storage proteins accounting for 65 to 80% of the total proteins. The main storage proteins in SBM are quaternary globulins, glycinin, and conglycinin (Frihart and Birkeland 2014; Wolf 1970). DDGS is chemically dissimilar from SBM. In addition, yeast contributes 5.3% of the protein content of DDGS (Lim and Yildirim-Aksoy 2008). Corn meal proteins consist of 14% albumin/globulin, 35 to 40% zein (a prolamine protein), and 30% glutelin. It is conjectured that soy protein adhesive properties occur through the denaturation of the quaternary globulins into tertiary structures and crystalline secondary structures, α-helices, and β-sheets (Frihart 2010; Frihart and Birkeland 2014; Frihart et al. 2014). These crystalline secondary protein structures are suspected to provide the optimum adhesive properties that bind protein to wood (Frihart 2010; Frihart and Birkeland 2014). No crystalline structures in the XRD studies with DDGS were observed which suggested a different method of protein adhesion may occur.
Denaturation of proteins can be achieved through a variety of methods such as heat, alkali, or chemical modification (Frihart and Birkeland 2014; Frihart et al. 2014). DDGS powder contained 30% protein and was originally a solid and through the application of heat and pressure undergoes a “phase change or transition” and becomes a “liquid-gel” that binds with wood. These events are recognized as typical in the adhesive process (Adhesives.org 2017). Apparently, the DDGS proteins are denatured under pressure and heat to a state that can then bind to wood. Upon cooling, DDGSs transitions back into a solid, which cannot be melted again.
It should be noted that employing a high moisture DDGS-PW formulation results in explosive stream generation and ultimately an unacceptable composite exhibiting excessive blistering. Therefore, a relatively dry DDGS-PW formulation was employed which was prepared by mixing the ingredients dry and not pre-stirring DDGS in any liquids as commonly reported in the fabrication of SBM biocomposites (Zhong et al. 2001; Amaral-Labat et al. 2008; Jeon et al. 2011; Reddy and Yang, 2011; Gu et al. 2013). In addition, in the preparation method steam was allowed to escape during the molding process by short releases of the molding pressure. Further, it was found that rapid cooling resulted in blistering and internal cracking of the DDGS-PW composites, while slow cooling the composite produced a non-blistered composite (Fig. 8). Blistering is a common problem in thermoset materials (Plenco 2015). Thermoset materials cure as a result of a chemical reaction and are affected by temperature and pressure (Plenco 2015). Blistering is often due to areas of gas trapped beneath the surface. One of the methods commonly employed to address this problem in injection molding is to decrease mold temperature by cooling after molding (Plenco 2015; IDI Composites International 2017). Reddy and Yang (2011) employed prolonged slow cooling to molds to obtain soy-wood composites. Likewise, this problem was recognized and was addressed similarly.
Composites that contained predominately large wood particles formed an interfacial complex with the “melted” DDGS, which formed a biocomposite that was weaker than the DDGS composite matrix that interacted with smaller wood particles. It was speculated that smaller wood particles interacted better with the DDGS matrix because of the ability of the DDGS to obtain a more fluid molten state that could penetrate the wood of smaller particles. Composites consisting of various particle sizes exhibited different flexural properties, suggesting that two types of interactions were occurring: (1) the wood particles were providing a support role based on their size, which contributed to the composite strength, and (2) the DDGS was interacting differently with the wood particles depending on their size. The < 1700 m blend produced a composite that exhibited high flexural properties and was the simplest to prepare when compared to the other composites. This PW composite blend was adopted in all further studies (Fig. 8).
Fig. 8. Comparison of composite panels. Top panel consists of 50:50 mixture of DDGS: PW subjected to fast cooling. Note blistering and internal cracking. Middle panel consists of 50:50 mixtures of DDGS:PW subjected to slow cooling. Note absence of disruptive blemishes. Bottom panel is commercial PB locally purchased
Effect of Pressure on Flexural Properties
The effect of pressure applications on the flexural properties of composites is presented in Fig. 9. Best flexural properties were obtained using the highest pressure tested. The equipment used did not permit higher pressures to be administered. Composites subjected to 5.6 MPa had Fm and Em values of 41.4 0.7 and 6073 123, respectively. Composites subjected to 2.1 MPa had Fm and Em values of 12.6 1.0 and 2354 286, respectively. Fm and Em values of composites subjected to 5.6 MPa were 228% and 158% greater than Fm and Em values of composites subjected to 2.1 MPa, respectively. Similar results have been reported by other investigators (Li et al. 2011). Applied pressure had a profound influence on the flexural properties and affected their physical properties, thickness and density. This phenomenon was attributed to the increased densification of the DDGS-PW composite, which resulted in an enhancement in the interfacial binding between particles and thus improving the flexural properties of the composites. Density of composites treated with 2.1, 2.8, 4.2, or 5.6 MPa were 929, 1,052, 1,212, or 1,275 kg.m3, respectively. Conversely, thickness of the composites decreased when treated with 2.1, 2.8, 4.2, or 5.6 MPa to 0.470, 0.430, 0.361, or 0.358 mm, respectively.
Fig. 9. Effect of various mold pressures on the flexural properties of composites
Influence of PW dosage on the Flexural Properties of Composites
The influence of DDGS-PW and PRO-PW dosages on the flexural properties of composites is presented in Table 1. Neat DDGS (100%) produced a panel that had the highest density and lowest thickness of all DDGS-PW composites tested but exhibited the lowest flexural values of all the DDGS-PW composites tested (Table 1). Increasing PW content increased the flexural values of all the DDGS-PW composites compared to neat DDGS. The highest flexural properties were obtained from composites containing 50:50 DDGS-PW (% wt.). Increasing the concentration of PW in the 10:90, 15:85, and 25:75 DDGS-PW (% wt.) composites resulted in a reduction of flexural properties compared to other DDGS-PW composites (i.e. 50:50 and 75:25 % wt.). DDGS-PW composites had flexural properties comparable to PRO-PW composites for the 50:50 formulations (Table 1). However, the flexural properties of PRO-PW composites to DDGS-PW composites differed depending on the concentrations tested. For example, neat 100% PRO produced a panel that cracked and shattered within 24 h after removal from the mold and therefore could not be tested. Composite formulations of 10PRO-PW, 15PRO-PW, and 25PRO-PW had superior flexural properties compared to 10DDGS-PW, 15DDGS-PW, and 25DDGS-PW. Soy flours (e.g. SBM, Prolia, Prosante, and SPI) have been employed in numerous studies to fabricate fiberboards (Zhong et al. 2001; Mo et al. 2001; Amaral-Labat et al. 2008; Frihart et al. 2010; 2014; United (USB) Soybean 2010; Jeon et al. 2011; Gu et al. 2013; Vnučec et al. 2015). SBM and DDGS have different protein compositions and concentrations. Nevertheless, the adhesive properties of DDGSs were similar to PRO. That observation suggested that DDGS may have merit as a potential substitute for soybean flour as a bio-based adhesive.
Table 1. Comparison of the Flexural Properties of Biocomposites Using Soybean and DDGS Flours*
*Treatment values with different letters in the same column were significant (P 0.05); means and standard errors derived from five different replicates are presented.
Water Absorption and Thickness Swelling of Composites
Water absorption and thickness swelling of DDGS-PW composites were compared to PRO-PW composites (Table 1). Regardless of the adhesive matrix employed, as the concentration of wood flour increased, the amount of water absorbed by the composite increased proportionally. This situation occurred because of the decreasing cohesion that occurs within the composites between the wood and the adhesive (Pan et al. 2006). Higher adhesive contents within the 50DDGS-PW and 75DDGS-PW composites exhibited substantially less water absorption and thickness swelling. TS and WA properties vary with the type of raw materials (i.e. bonding agents, additives, and fillers/reinforcements) employed during manufacturing (Mendes et al. 2012; Melo et al. 2014). Bio-based adhesives have a hygroscopic nature compared to synthetic adhesives derived from petroleum sources which are more hydrophobic. WA and TS for the two different types of adhesive composites were somewhat comparable (Table 1).
Comparison of DDGS-PW Composites Properties with European Committee for Standards
As shown in Table 1, the DDGS-PW composites produced in this study compared well with the nominal flexural requirements for PB, MDF, and HDF established by the European Committee for Standards presented in Table 2 (EN 662-2:1997; EN 622-5:2006; EN 312:2003). However, the non-conventional DDGS-PW composites prepared in this work were often inferior in terms of TS compared to that adopted by the European Committee for Standards (EN 662-2:1997; EN 622-5:2006; EN 312:2003). Clearly, additional research is necessary to improve water resistance properties.
1EN 312:2003, 2EN 622-5:2006, 3EN 622-2:1997.
- Novel DDGS-PW composites were fabricated containing 25 to 75% distillers dried grains with solubles (DDGS): 75 to 25% paulownia wood (PW) through a phase-change process whereby the DDGS flour powder reacts with PW particles under high pressure and temperature to become a “liquid-gel” matrix to interfacially bond to the PW. Upon slow cooling, the DDGS liquid-gel transforms back into a solid to create panel boards exhibiting relatively high flexural properties.
- FTIR analysis failed to show esterification; similarly, thermal analysis using DSC and TGA suggested that plasticization of DDGS occurs around 117 °C. XRD analysis failed to show evidence of crystallization, suggesting the DDGS and PW interaction is associated with the generation of a composite in which the binding material is amorphous in nature.
- Pressure, temperature, PW particle size, and DDGS dosages were found to be important factors in the fabrication of DDSG-PW composite panels in terms of its flexural properties.
- DDGS-PW composite panels were found to have similar flexural properties when compared to PRO-PW panels.
- DDGS-PW composite panels exhibited similar water resistance properties as PRO-PW panels.
- DDGS-PW composite panels were found to be comparable or superior to flexural properties required by Industry Standards (EN 312:2003). However, DDGS-PW composite panels were often found to be inferior in terms of water resistance (i.e.thickness swelling) compared to Industry Standards.
The authors acknowledge Nicholas Montesdeoca and Dr. Louis Reifschneider for technical assistance. Mention of a trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture. USDA is an equal opportunity provider and employer.
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Article submitted: November 16, 2017; Peer review completed: February 2, 2018; Revised version received: February 14, 2018; Accepted: February 16, 2018; Published: February 21, 2018.