Influence of liquefied wood polyol on the physical-mechanical and thermal properties of epoxy based polymer

Influence of liquefied wood polyol on the physical-mechanical and thermal properties of epoxy based polymer

Polymer Testing 64 (2017) 207–216 Contents lists available at ScienceDirect Polymer Testing journal homepage: Influ...

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Polymer Testing 64 (2017) 207–216

Contents lists available at ScienceDirect

Polymer Testing journal homepage:

Influence of liquefied wood polyol on the physical-mechanical and thermal properties of epoxy based polymer


Anuj Kumara,b,∗, Tomáš Vlachb, Pavla Ryparovàb, Andrijana Sever Škapinc, Janez Kovačd, Stergios Adamopoulosa, Petr Hajekb, Marko Petriče a

Department of Forestry and Wood Technology, Faculty of Technology, Linnaeus University, Lückligs Plats 1, 35195 Växjö, Sweden Czech Technical University in Prague, Faculty of Civil Engineering, Department of Building Structures, Thákurova 7, 166 29 Prague 6, Czech Republic c Slovenian National Building and Civil Engineering Institute, Dimičeva 12, SI-1000 Ljubljana, Slovenia d Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia e University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Jamnikarjeva, 101, 1000 Ljubljana, Slovenia b



Keywords: Liquefied wood Epoxy based polymer Tensile properties Thermo-mechanical properties

Epoxy resins are mostly produced from petroleum-based bisphenol A and epicholorhydrin. Bisphenol A is synthesized from non-renewable petroleum-based phenol and acetone. Biomass derived epoxy-based polymers (EBPs) are becoming the most promising alternative for petroleum-based counterparts, but still these biomassbased EBPs have inferior properties. In the present work, two types of epoxy resins were prepared with different weight percentages of resin (bisphenol A) and hardener. They were then modified with different weight percentages of liquefied wood from spruce sawdust. The derived EBPs were analysed in terms of tensile strength and tensile modulus, fractured surface morphology, thermal stability, long-term water adsorption and resistance to brown-rot fungus decay. The results revealed that the percentages of hardener and liquefied wood significantly influenced the overall properties of the EBPs.

1. Introduction Epoxy resins have broad applications in adhesives, coatings, electrical engineering, constructions and electronics due to their wideranging properties [1,2]. Epoxy resins are mostly produced (approximately 90%) from bisphenol A and epichlorohydrin. Bisphenol A is an untenable petroleum product. Also, epichlorohydrin is carcinogenic, and bisphenol A can cause estrogen-like long-lasting effects on living organisms [3]. The availability of petroleum is uncertain and the global political and institutional tendencies urging the sustainable development in the chemical industries. So, the use of renewable resources in order to synthesize bio-based chemicals and products is developing fast [3]. The use of renewable resources for epoxy monomer synthesis results in reduction of negative environmental impacts. In the recent years, renewable bio-based epoxy resins have been produced using various biomasses such as lignin derived vanillyl alcohol [4,5], bark extractives [6] and tannin derivatives [7]. Lignocellulosic wood and agricultural crop residues are considered to be the most abundant categories of renewable biomass. Lignocellulose biomass mainly consist of cellulose (30–35%), hemicelluloses (15–35%) and lignin (20–35%) [8]. All of these polymers are

highly functionalized materials, rich in hydroxyl groups making them a promising feedstock to produce bio-based polyols. Liquefaction of woody biomass is usually conducted at elevated temperatures (150–250 °C) under atmospheric pressure, using polyethylene glycol (PEG) and glycerol or other polyalcohols as liquefaction solvents, through an acid or base catalysed reaction [8–11]. Kobayashi et al. [12,13] investigated the preparation of liquefied wood/epoxy resins and they found that liquefied wood can form a cross-linked copolymer network. In their work they used triethylene tetramine (TETA) as a curing agent and the liquefied wood/epoxy resin system was cured at 150 °C. Further, they analysed the viscoelastic properties of the prepared liquefied wood/epoxy resins using dynamic mechanical analysis. Asano et al. [14] first treated wood with ozone and then liquefied it and blended up to 53% with neat epoxy resin to produce wood adhesive. They used citric acid and TETA as curing agents and analysed the viscoelastic properties of liquefied wood/epoxy resins. Kishi et al. [15] prepared wood-based epoxy resins that were synthesized from resorcinol-liquefied wood. Wood was first liquefied in the presence of resorcinol with or without a sulfuric acid catalyst at high temperature. Epoxy functionality was introduced to the liquefied wood by glycidyl etherification. Wu and Lee et al. [16,17] evaluated the preparation of

Corresponding author. Department of Forestry and Wood Technology, Faculty of Technology, Linnaeus University, Lückligs Plats 1, 35195 Växjö, Sweden. E-mail addresses: [email protected], [email protected] (A. Kumar). Received 30 August 2017; Received in revised form 7 October 2017; Accepted 8 October 2017 Available online 12 October 2017 0142-9418/ © 2017 Elsevier Ltd. All rights reserved.

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epoxy resins from phenol liquefied wood and PEG/glycerol liquefied bamboo. The synthesis started with the reaction of epichlorohydrin and bisphenol A (molar ratio 5/1) at 110 °C for 2 h under stirring and dripping of aqueous NaOH. The resulting epoxy resin had similar curing behaviour to the petroleum-based epoxy resin. The thermal degradation temperature of liquefied bamboo epoxy resin was lower than that of the petroleum-based epoxy resins. Wastes of epoxy resins and their compounds can cause various environmental problems because they are difficult to dispose from their network structures [18]. Landfilling is an option for disposing epoxy resins, but it is not a sustainable approach and harms the environment. On the other hand, chemical recycling of epoxy resins require very corrosive chemicals such as nitric acid [19]. Addition of liquefied wood into epoxy-based polymers (EBPs) might improve the biodegradability of resins, and this approach has not been explored in the existing literature. Increased biodegradability could decrease the problems of disposing waste epoxy resins, and therefore we considered it as an interesting topic to investigate. In the present work, liquefied wood was prepared from spruce wood and used as biopolymer reinforcement for epoxy resins. Two types of epoxy resins were prepared with two different resin to hardener ratios. Further, two weight percentages of liquefied wood i.e. 2.5% and 5% were added into both types of epoxy resins. The physical, mechanical and thermo-mechanical properties of liquefied wood/epoxy resins were evaluated. Their biodegradation was judged by using standard brownrot fungus decay test.

Fig. 1. Sample shape and dimensions (a) for tensile testing, and (b) E3:1 and (c) E3:1LW5% samples during the test.

2. Methodology 2.1. Chemicals The 2-part solvent free epoxy (Sikafloor®-156) was supplied by Sika, UK. Part A epoxy resin was composed of bisphenol-A diglycidyl ether (> 60%), bisphenol F epoxy (1–10%) and glycidylether of C12-C14 alcohols (1–10%). The part B, the hardener, is composed of benzyl alcohol (30–60%), m-xylene a,a’-diamine (10–30%), cyclohexanemethanamine (10–30%) and tetraethylene pentamine (1–10%). 2.2. Preparation of liquefied wood Liquefaction of spruce wood was conducted in a 1-L reactor using a mixture of polyethylene glycol #400 (PEG) at a 9:1 mass ratio as the reactive solvent and sulphuric acid as the catalyst. The wood-to-solvent mass ratio was 1:3, and the catalyst-to-solvent mass ratio was 3:100. Glycerol, PEG #400, sulphuric acid and wood sawdust were charged into the reactor and refluxed under continuous mechanical stirring for 90 min at 180 °C. After this time, the liquefied mixture was cooled down and diluted with a 1, 4-dioxane/water mixture (4:1, v/v). The residue from liquefaction was removed by filtration under vacuum through a filter disk (Sartorius 388 grade, 12–15 μm particles retention). Liquefied wood (LW) was obtained after evaporation of dioxane and water.

Fig. 2. FTIR spectra of epoxy resins and EBPs.

different weight percentages of LW i.e. 2.5% and 5% were mixed mechanically at room temperature into to the E3:1 and E2:1 to obtain epoxy-based polymers (EBPs). The sample names were respectively E3:1LW2.5%, E3:1LW5%, and E2:1LW2.5%, E2:1LW5%. Special type of silicone moulds were used to prepare the dog-bone epoxy samples with desired dimensions as shown in Fig. 1, as per ASTM D638 [23] standards. All types of resins were poured into these moulds and cured at room temperature for 2 weeks before further analysis.

2.3. Determination of the hydroxyl number and acid number of LW The hydroxyl (OH) and acid numbers of the LW were determined according to ASTM D4274-05 [20] and D974 [21] standards respectively, as reported earlier by Hrastnik et al. [22]. The calculated values of hydroxyl number and acid number for LW were 333.57 and 11.16, respectively.

2.5. Biodegradation test Epoxy (E3:1) and EBP blocks measuring 20 × 10 × 3 mm3 were exposed to Trametes versicolor (L.) Lloyd in petri dishes containing 3% malt extract agar, pre-inoculated 1 week prior to the test. The samples were incubated for 7 weeks on sterile glass sticks; the samples were inoculated by fungal segments (cutouts) on agar at a constant temperature of 25 °C. After 7 weeks, the samples were removed. After

2.4. Preparation of epoxy and LW/epoxy resins Two different types of epoxy resin were prepared in this work; 1) epoxy resin with 3 parts of resin and 1 part of hardener (E3:1) and 2) epoxy resin with 2 parts of resin and 1 part of hardener (E2:1). Two 208

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Fig. 3. XPS spectra of epoxy resins and EBPs.

that the relative error of calculated concentrations is about 15% of reported values.

removing carefully the mycelia, the surface morphology of the degraded resins was studied using stereoscopic observation and scanning electron microscopy.

2.6.3. Scanning electron microscopy (SEM) The surface morphology of the cured epoxy, EBPs and biodegraded resins was studied by scanning electron microscopy (SEM, JEOL JSM5500LV, Japan). All surfaces of samples were cleaned with alcohol, in order to remove dust, and then coated with a thin evaporated layer of gold to improve the conductivity prior to analyses.

2.6. Characterizations 2.6.1. FT-IR analyses FT-IR analyses of cured resins (epoxies and EBPs) were performed with a PerkinElmer (USA) Spectrum One FT-IR spectrometer using the horizontal attenuated total reflection (HATR) technique (with a HATR ZnSe 45° flat plate). All spectra were recorded at a 4 cm−1 resolution, and each was the average of 32 scans. The samples were oven dried at 80 °C for 2 h before the measurements to remove the moisture.

2.6.4. Tensile testing The tensile testing of epoxies and EBPs were performed with a FPZ 100 testing machine, coupled with Dewetron 500 data acquisition system to measure the strains from strain gauges installed on the samples as shown in Fig. 1. The data acquisition system combines stress values measured by the test equipment with strain values measured by the strain gauges. The speed of tensile loading was approximately 0.01 mm/s. The tensile modulus was evaluated using the measured strain by the installed strain gauges, and the tensile strength was evaluated using the cross-section of samples as per ASTM D638 standards [23].

2.6.2. XPS analyses The X-ray photoelectron spectroscopy (XPS or ESCA) analyses of cured resins (epoxies and EBPs) were carried out with a PHI-TFA XPS spectrometer (Physical Electronics Inc). The samples were cut into thin slices by a clean cutter. Slices were then inserted into the XPS spectrometer at two places 3 mm apart and mounted on the metallic sample holder. The vacuum during the XPS analyses was in the range of 10−9 mbar. The analysed area was 0.4 mm in diameter and the analysed depth was about 3–5 nm. Sample surfaces were excited by X-ray radiation from a monochromatic Al source at photon energy of 1486.6 eV. The survey wide-energy spectra were taken over an energy range of 0–600 eV with pass energy of analyser of 187 eV in order to identify and quantify present elements on the surface. The high-energy resolution spectra were acquired with an energy analyser operating at resolution of about 0.6 eV and pass energy of 29 eV. During data processing, the spectra were aligned by setting the C 1s peak at 285.0 eV, characteristic for C-C/C-H bonds. The accuracy of binding energies was about ± 0.3 eV. Quantification of surface composition was performed from XPS peak intensities taking into account relative sensitivity factors provided by instrument's manufacturer (Ref. Moulder). We estimate

2.6.5. Water absorption The water absorption test was conducted by dipping epoxy and EBP cube samples of 18 mm3 into water at different duration up to 6 days. It was measured the weight change of samples with the dipping time. 2.6.6. Thermogravimetric (TGA) and differential thermal analyses (DTA) The thermo-gravimetric (TG) measurements were performed with a Netzsch STA 409 Instrument from room temperature up to 800 °C with a heating rate of 15°C min−1. The samples with an initial mass of around 10–30 mg were placed into Al2O3 crucibles. During the measurement, the furnace was purged with an air flow with a rate of 20 mL min−1. The baseline was automatically subtracted from the 209

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Fig. 4. High resolution XPS of C 1s peak analysis of epoxy resins and EBPs. Fig. 5. SEM images of fractured surfaces of epoxy and EBP samples (a) E3:1; (b) E3:1LW2.5%; and (c) E3:1LW 5%.


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Fig. 6. SEM images of fractured surfaces of epoxy and EBP samples (a) E2:1; (b) E2:1LW2.5%; and (c) E2:1LW5%.

Fig. 7. Tensile strength and tensile modulus of E3:1 and EBPs (a); (b) stress-strain curves.

Fig. 2. All the samples exhibited similar FTIR spectra and were comprised of a broad absorption band at 3400 cm−1 of aromatic and aliphatic -OH groups. The C-H methylene stretching bands distinctly appeared at 2920 cm−1 and 2870 cm−1 for CH2 and CH3 stretching vibrations in epoxy as well as in LW/epoxy (EBPs) samples. The characteristic epoxy linkage CO-C was formed in between 820 and 920 cm−1 [24], which confirmed the compatibility between the epoxy and LW. Some other notable characteristic absorption bands of the epoxy polymer were present, such as at 1248 cm−1 (aromatic ether band) and at ∼ 1600 cm−1 (aromatic ring of phenyl group). Similarly, the XPS results did not confirm differences in the chemical bonding of epoxy resins after addition of LW at different weight percentages (Fig. 3). The C 1s spectra with a binding peak near ∼284.5 eV was characteristic of C-H bonds [24,25] and the binding peaks of O 1s for C-O were found at ∼ 532 eV [25]. In all samples a narrow peak (FWHM∼1.5 eV) at 399.6 ± 0.3 eV of nitrogen was

measured TGA curve. Differential thermal analysis (DTA) measurements were performed simultaneously with the same instrument. 2.6.7. Dynamic mechanical analysis (DMA) The DMA of epoxy resins and EBPs was performed using a Q800 DMA (TA Instruments). With samples measuring 35 × 7.5 × 2.5 mm3 (length × width × thickness). Dual cantilever bending mode was employed with a single frequency of strain. A ramping rate of 2 °C/min was set to reach the maximum temperature of 160 °C from 20 °C to evaluate the storage modulus and tan delta of the samples. 3. Results and discussion 3.1. Chemical properties FTIR absorption spectra of cured epoxy and EBPs are shown in 211

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Fig. 8. Tensile strength and modulus of E2:1 and EBPs (a); (b) stress-strain curves.

Fig. 10. DMA results of E2:1 epoxy resin and EBPs: (a) storage modulus and (b) tan delta. Fig. 9. DMA results of E3:1 epoxy resin and EBPs: (a) storage modulus and (b) tan delta.

distributed into chemical bonding associated to C-C/C-H and C-O/C-N at ∼284.9 eV and 286.4 eV binding energy, respectively. The C 1s peak in E3:1 sample covered 70.57% area by C-C/C-H and 29.43% area by CO/C-N bonding. After the addition of 5% LW (E3:1LW5%) the area of C-

detected and it can be related to the C-N type of characteristic bonds for organic matrix. Furthermore, high resolution of C 1s peak was carried out and the C 1s disintegration is shown in Fig. 4. The C 1s peak mainly 212

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Fig. 11. Thermo-gravimetric analysis: (a) TG curves; (b) DTA curves of E3:1 and EBPs.

Fig. 12. Thermo-gravimetric analyses: (a) TG curves; (b) DTA curves of E3:1 and EBPs.

Table 1 Thermal degradation temperatures at 10% (T10%), 25% (T25%), 50% (T50%) and 75% (T75%) mass loss, and maximum mass loss derivative temperature (TDTA max) for epoxy and LW/epoxy resins as obtained from TGA measurements under N2. Sample

TG10% (°C)

TG25% (°C)

TG50% (°C)

TG75% (°C)


E3:1 E3:1LW2.5% E3:1LW5% E2:1 E2:1LW2.5% E2:1LW5%

201 213 197 180 196 196

326 326 301 291 299 299

372 380 368 367 364 364

487 436 436 440 431 433

374, 382, 390, 386, 386, 392,


557 560 561 549 561 561

C/C-H bonding increased to 73.08% and C-O/C-N decreased to 26.92%. In the case of E2:1 sample, the area of C-C/C-H peak was 63.51%, which is lower than at the E3:1 sample and C-O/C-N peak was 36.49% higher than in the case of E3:1 sample. After the addition of 5% LW (E 2:1LW5%) into E2:1 the peak area of C-C/C-H increased considerably to 70.88% and C-O/C-N decreased considerably to 29.12%; which is very similar to the E3:1 sample. So, it can be postulated that the addition of LW into the epoxy resin hindered the bonding or affected negatively the curing properties for the E3:1 sample with high percentage of resin (part A) and low percentage (part B) of curing agent; but had a positive effect in the case of E2:1 sample having low resin content and high

Fig. 13. Water absorption by epoxy resins and EBPs.

curing agent percentage. This might be explained by the presence of free hydroxyl (-OH) groups in LW, which were able to replace the missing -OH quantity in the sample E2:1 and increase the number of –OH groups into E3:1, while the quantity of curing agent was sufficient 213

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for curing the system properly. 3.2. SEM analysis SEM provided details of the fractured surface morphology of the fully cured epoxy resins and EBPs (Figs. 5–6). The sample E3:1 showed a smooth morphology of the fractured surface (Fig. 5a). After the addition of LW into E3:1LW2.5% and E3:1LW5% resins, the morphology of fractured resin was not smooth anymore, like in the case of E 3:1 (Fig. 5b and c). The fractured surface was very distinct, like layer-bylayer breakage during the force loading. SEM micrographs also revealed the uniform distribution of LW as no LW particles appeared (see Fig. 5b–c). The cured E3:1 sample was partially transparent and after the addition of LW, the epoxy resin became blackish in colour and the transparency was reduced (see Fig. 1b and c). The fractured surface of E2:1 is shown in Fig. 6a. The morphology appeared very scattered and non-uniform compared to that ofE3:1, because of lower content of resin and excessive hardener percentage (Part B). The fractured surface morphology of E2:1 became smoother and denser with addition of LW as shown in Fig. 6b for E2:1LW2.5% and Fig. 6c for E2:1Lw5%, respectively. The excessive Part B in E2:1 was reacted with LW and improved the curing density of the resin system. 3.3. Tensile properties The tensile strength of E3:1 was 40 MPa and the value of the tensile modulus was 1900 MPa as shown in Fig. 7a. After the addition of 2.5% LW, the tensile strength decreased to 28 MPa and the tensile modulus value to 1300 MPa. Further increasing of the weight percentage of LW to 5% in the E3:1 resin had a negative effect on tensile strength and modulus, as shown in Fig. 7a. The linear reduction of tensile properties of E3:1 with the increasing content of LW, might be due to a lower cross-linking of epoxy resin. Fig. 7b shows the stress-strain curves of the epoxy and EBP samples: the addition of LW in E3:1 increased the displacement (strain) and lowered the stress values. The addition of LW into E3:1 reduced the cross-linking between resin (part A) and hardener (part B), and the LW was behaved like a polyol, which possibly increased the reactive sites in epoxy resin while the current hardener

Fig. 14. Photographs and stereoscopic micrographs of epoxy (E3:1) resins and EBPs after 8 weeks of exposure to the brown-rot fungus.

Fig. 15. SEM images of biodegraded epoxy and EBP samples (a) E3:1; (b) E 3:1LW2.5%; and (c) E3:1LW5%.


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the E3:1 resin absorbed 3.5% water after 6 days and the E2:1 resin absorbed almost double quantity of water. After the addition of LW into the E3:1 system, the WA changed considerably, as shown in Fig. 13. This is most likely due to improper curing of the newly formed EBP. The E2:1 system behaved differently with the addition of LW. The addition of 2.5% LW did not affect the WA substantially, but with the addition of 5% LW, the WA improved considerably. The water sorption behavior of epoxy was related to the free volume or voids, which depend on the networking of polymer during the curing process [30–33].

percentage was not enough to cross-link the system uniformly. The tensile properties of E3:1 were slightly lower than the values reported in previous studies [26,27] but similar to some other results shown in literature [28]. Asano et al. [14] prepared an epoxy/LW polymer and reported 29 MPa and 700 MPa tensile strength and tensile modulus, which is very similar to the results of the present study. The ductile behavior of E3:1 sample was increased with the addition of LW into epoxy system, but the tensile strength was decreased. E2:1 sample showed considerably lower tensile properties than the E3:1 sample. In detail, the tensile strength was 5.8 MPa and the value of the tensile modulus was 200 MPa. However, the addition of 5% LW into E2:1 had a positive influence on tensile properties as the values of the tensile strength and modulus were increased considerably to 10 MPa and 600 MPa, respectively (Fig. 8a). The LW addition to E2:1 also improved the stiffness of E2:1 sample significantly (see Fig. 8b). We believe that LW acted as a resin (part A) and improved the crosslinking of E2:1, which have higher cross-linker (part B) percentage. The ductile behavior was considerably decreased and tensile stress was improved significantly with the increasing loading of LW into E2:1.

3.6. Biodegradation of the cured epoxy resin and EBPs The infected samples of epoxy resins and EBPs by the brown-rot fungus Trametes versicolor are shown in Fig. 14. The growth of fungus on the E3:1 surface is shown in Fig. 14a and the stereoscopic image reveals details on the attachment of fungal hyphae on sample surfaces. However, the hyphae did not penetrate through the epoxy surface, as shown on the SEM micrograph (Fig. 15a). LW addition accelerated the biodegradation of epoxy resin: Fig. 14b demonstrates the fungal hyphae growth on the E 3:1LW2.5% sample. Fig. 14b shows the penetration of hyphae into epoxy and Fig. 15b (SEM image) demonstrates the epoxy surface degradation. Similarly, the sample with 5% LW further facilitated the fungal degradation of the epoxy sample. The voids created in the epoxy sample after the addition of LW and the biological origin of LW might be the reasons for the enhanced biological degradation of LW containing samples (EBPs).

3.4. Thermal and thermo-mechanical properties The storage modulus temperature profiles of E3:1 and LW loaded samples are shown in Fig. 9a. The storage modulus in both glassy and rubbery regions, decreased with increasing the LW loading. Glass transition temperature (Tg) was more clearly identified as a maximum in the curves of the tan delta, as shown in Fig. 9b. The incorporation of LW into the epoxy resin (E3:1) caused a decrease of the Tg value, as compared to the Tg of the sole epoxy. The reduction in Tg was attributable to improper curing of E3:1 samples with the addition of LW polyols. The E3:1/5% LW sample showed dual Tg peaks, one was around 35 °C and the second one was at 62 °C, confirming the improper curing of epoxy resin with the addition of LW, as shown in Fig. 9b. Kishi et al. [15] reported on the viscoelastic properties of an epoxy resin, which was synthesized from resorcinol-LW, and the number of reactive sites (-OH groups) significantly altered the Tg as well as the rubbery plateau of the epoxy resin. The storage modulus and tan delta curves of the resin E2:1 are shown in Fig. 10. The glassy and rubbery regions of E2:1 were considerably lowered in comparison to the ones of E3:1 (Fig. 10a), but the addition of LW in E2:1 totally reversed the effect on the viscoelastic properties in comparison to the E3:1 sample. With the addition of LW, the storage modulus in both rubbery and glassy regions considerably improved. The Tg point in tan delta also shifted to a higher temperature, from 34 °C to 56 °C with 5% loading of LW. TGA measurements were carried out to obtain information on the effect of LW addition on the thermal stability of the epoxy resin. Fig. 11a shows the TGA curves of the E3:1 resin with the added LW. It can be seen that the thermal stability of the epoxy/LW composites did not change. In addition, the DTA peaks did not reveal any differences in thermal degradation process of epoxy and EBPs, as shown in Fig. 11b. The degradation mechanism of an epoxy resin can be broadly understood in terms of a two-step process, starting with dehydration and followed by chain scission [29]. Table 1 summarizes the thermal degradation temperatures at 10% (T10%), 25% (T25%), 50% (T50%) and 75% (T75%) mass losses, and the maximum mass loss derivative temperature (TDTA max). Similar behavior was noticed at the E2:1 and LW loaded E2:1 in TGA and DTA analysis (see Fig. 12). The E3:1 sample showed the best thermal stability among all the samples in two sets of experiments, as shown in Table 1, and no notable changes occurred with the addition of LW.

4. Conclusions In the present work, cured epoxy and EBPs were prepared with formulations of two different ratios between the resin and the hardener. LW polyol from spruce sawdust was added into the prepared epoxies at two different weight percentages, i.e. 2.5% and 5%. The addition of LW into the E3:1 sample had a negative influence on the polymer's physical and mechanical properties. The tensile properties reduced significantly with the increase in the share of LW. On the other hand, the addition of LW had a positive effect in the case of the E2:1 set of experiments: all studied physical and mechanical properties of E2:1 improved considerably. With the addition of LW the Tg of the epoxy resin decreased in the case of E3:1 and increased at E2:1. The FTIR and XPS results did not reveal any changes in the epoxy formation after the addition of LW. The SEM and DMA results revealed an improper curing of the epoxy resin with the addition of LW. In conclusion, this work brings new ideas for partial or complete replacement of the synthetic bisphenol-A diglycidyl ether in epoxy resin formulations with sustainable bio-based polyols. Acknowledgements AK and PH wish to acknowledge the Technology Agency of the Czech Republic (TA ČR) funded project no. TH02020512 Subtle concrete furniture and small structures for railway station. MP and ASS acknowledges the financial support of the Slovenian Research Agency through the research programme P4-0015 “Wood and lignocellulose composites” and P2-0273 acknowledge by ASS. AK and SA acknowledge the funding provided by the Formas project 942-2016-2 titled “Utilisation of renewable biomass and waste materials for production of environmental-friendly, bio-based composite”. References [1] H. Lee, K. Neville, Handbook of Epoxy Resins, McGraw-Hill, New York, 1967. [2] C. May (Ed.), Epoxy Resins: Chemistry and Technology, CRC press, 1987. [3] R. Auvergne, S. Caillol, G. David, B. Boutevin, J.P. Pascault, Biobased thermosetting epoxy: present and future, Chem. Rev. 114 (2) (2013) 1082–1115. [4] E.D. Hernandez, A.W. Bassett, J.M. Sadler, J.J. La Scala, J.F. Stanzione III, Synthesis and characterization of bio-based epoxy resins derived from vanillyl alcohol, ACS

3.5. Water absorption (WA) properties The addition of LW into epoxy also influenced the WA properties of the epoxy systems. Fig. 13 shows the WA behavior of epoxy and EBPs; 215

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561–570. [19] W. Dang, M. Kubouchi, S. Yamamoto, H. Sembokuya, K. Tsuda, An approach to chemical recycling of epoxy resin cured with amine using nitric acid, Polymer 43 (10) (2002) 2953–2958. [20] American Society for Testing and Materials (ASTM), D4274. Standard Test Method for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols, ASTM, Pennsylvania, 2005. [21] American Society for Testing and Materials (ASTM), D974. Standard Test Method for Acid and Base Number by ColorIndicator Titration, ASTM, Pennsylvania, 2008. [22] D. Hrastnik, M. Humar, B. Kričej, M. Pavlič, P. Pori, C. Yona, M. Arnaud, M. Petrič, Polyurethane coatings from liquefied wood containing remains of a copper-, chromium-, and boron-based wood preservative, J. Appl. Polym. Sci. 131 (19) (2014). [23] American Society for Testing and Materials (ASTM), D638. Standard Test Method for Tensile Properties of Plastics, ASTM, Pennsylvania, 2014. [24] S.G. Hong, The thermal-oxidative degradation of an epoxy adhesive on metal substrates: XPS and RAIR analyses, Polym. Degrad. Stab. 48 (2) (1995) 211–218. [25] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, Wiley, 1992. [26] Y.J. Wan, L.X. Gong, L.C. Tang, L.B. Wu, J.X. Jiang, Mechanical properties of epoxy composites filled with silane-functionalized graphene oxide, Compos. Part A Appl. Sci. Manuf. 64 (2014) 79–89. [27] Y.J. Wan, L.C. Tang, L.X. Gong, D. Yan, Y.B. Li, L.B. Wu, J.X. Jiang, G.Q. Lai, Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties, Carbon 69 (2014) 467–480. [28] A. Chira, A. Kumar, T. Vlach, L. Laiblová, A.S. Škapin, P. Hájek, Property improvements of alkali resistant glass fibres/epoxy composite with nanosilica for textile reinforced concrete applications, Mater. Des. 89 (2016) 146–155. [29] V. Bellenger, E. Fontaine, A. Fleishmann, J. Saporito, J. Verdu, Thermogravimetric study of amine cross-linked epoxies, Polym. Degrad. Stab. 9 (4) (1984) 195–208. [30] O. Becker, R.J. Varley, G.P. Simon, Thermal stability and water uptake of high performance epoxy layered silicate nanocomposites, Eur. Polym. J. 40 (1) (2004) 187–195. [31] C. Maggana, P. Pissis, Water sorption and diffusion studies in an epoxy resin system, J. Polym. Sci. Part B Polym. Phys. 37 (11) (1999) 1165–1182. [32] A. Kumar, T. Vlach, A. Chira, L. Laiblova, A.S. Škapin, J. Tywoniak, P. Hajek, Nanocoating on alkali-resistant glass fibers by octadecyltrichlorosilane to improve the mechanical strength of fibers and fibers/epoxy composites, Text. Res. J. (2017) p.0040517517693977. [33] A. Tcharkhtchi, P.Y. Bronnec, J. Verdu, Water absorption characteristics of diglycidylether of butane diol–3, 5-diethyl-2, 4-diaminotoluene networks, Polymer 41 (15) (2000) 5777–5785.

Sustain. Chem. Eng. 4 (8) (2016) 4328–4339. [5] A. Maiorana, S. Spinella, R.A. Gross, Bio-based alternative to the diglycidyl ether of bisphenol A with controlled materials properties, Biomacromolecules 16 (3) (2015) 1021–1031. [6] P.Y. Kuo, L. de Assis Barros, M. Sain, J.S. Tjong, N. Yan, Effects of reaction parameters on the glycidyl etherification of bark extractives during bioepoxy resin synthesis, ACS Sustain. Chem. Eng. 4 (3) (2016) 1016–1024. [7] C. Aouf, J. Lecomte, P. Villeneuve, E. Dubreucq, H. Fulcrand, Chemo-enzymatic functionalization of gallic and vanillic acids: synthesis of bio-based epoxy resins prepolymers, Green Chem. 14 (8) (2012) 2328–2336. [8] A. Kumar, M. Petrič, B. Kričej, J. Žigon, J. Tywoniak, P. Hajek, A.S. Škapin, M. Pavlič, Liquefied-wood-based polyurethane–nanosilica hybrid coatings and hydrophobization by self-assembled monolayers of orthotrichlorosilane (OTS), ACS Sustain. Chem. Eng. 3 (10) (2015) 2533–2541. [9] A.M. Cheumani-Yona, F. Budija, D. Hrastnik, A. Kutnar, M. Pavlič, P. Pori, Č. Tavzes, M. Petrič, Preparation of two-component polyurethane coatings from bleached liquefied wood, BioResources 10 (2) (2015) 3347–3363. [10] A.M.C. Yona, F. Budija, B. Kričej, A. Kutnar, M. Pavlič, P. Pori, Č. Tavzes, M. Petrič, Production of biomaterials from cork: liquefaction in polyhydric alcohols at moderate temperatures, Industrial Crops Prod. 54 (2014) 296–301. [11] F. Budija, Č. Tavzes, L. Zupančič-Kralj, M. Petrič, Self-crosslinking and film formation ability of liquefied black poplar, Bioresour. Technol. 100 (13) (2009) 3316–3323. [12] M. Kobayashi, K. Tukamoto, B. Tomita, Application of liquefied wood to a new resin system—synthesis and properties of liquefied wood/epoxy resins, Holzforschung 54 (1) (2000) 93–97. [13] M. Kobayashi, Y. Hatano, B. Tomita, Viscoelastic properties of liquefied wood/ epoxy resin and its bond strength, Holzforschung 55 (6) (2001) 667–671. [14] T. Asano, M. Kobayashi, B. Tomita, M. Kajiyama, Syntheses and properties of liquefied products of ozone treated wood/epoxy resins having high wood contents, Holzforschung 61 (1) (2007) 14–18. [15] H. Kishi, A. Fujita, H. Miyazaki, S. Matsuda, A. Murakami, Synthesis of wood-based epoxy resins and their mechanical and adhesive properties, J. Appl. Polym. Sci. 102 (3) (2006) 2285–2292. [16] C.C. Wu, W.J. Lee, Synthesis and properties of copolymer epoxy resins prepared from copolymerization of bisphenol A, epichlorohydrin, and liquefied Dendrocalamus latiflorus, J. Appl. Polym. Sci. 116 (4) (2010) 2065–2073. [17] C.C. Wu, W.J. Lee, Curing and thermal properties of copolymer epoxy resins prepared by copolymerized bisphenol-A and epichlorohydrin with liquefied Dendrocalamus latiflorus, Polym. J. 42 (9) (2010) 711–715. [18] J. Economy, A.G. Andreopoulos, A new concept for recycling of thermosetting resins I: the case of crosslinkable copolyesters, Polym. Adv. Technol. 7 (7) (1996)