Application of Biobased Phenol Formaldehyde Novolac Resin Derived

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Application of bio-based phenol formaldehyde (PF) novolac resin derived from beetle infested lodgepole pine barks for thermal molding of wood composites Ning Yan, Boya Zhang, Yong Zhao, Ramin R. Farnood, and Junyou Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Industrial & Engineering Chemistry Research

Application of bio-based phenol formaldehyde (PF) novolac resin derived from beetle infested lodgepole pine barks for thermal molding of wood composites Ning Yanab*, Boya Zhangc , Yong Zhaob, Ramin R Farnoodc, Junyou Shia* a: College of Forestry, Beihua University, Binjiang East Road, Jilin City, Jilin Province, P. R. China; b: Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON., Canada, M5S 3B3; c: Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON., Canada, M5S 3E5;

*Corresponding author. Tel: +14169468070; Fax: +14169783834 Email address:[email protected]

Abstract In this study, wood particles were thermally molded into composites using a novolac resin derived from beetle infested lodgepole pine barks with three different resin to wood filler weight ratios (3:7, 5:5, and 7:3). Control composites were made using a lab synthesized novolac resin without bark for comparison. Results showed that mechanical properties of the composites varied with the resin to filler ratios. Bark-derived resin improved the tensile strength of the composites at resin to filler weight ratio of 5:5. Meanwhile, at all three resin to filler weight ratios, the composites made using the barkderived resin showed an improved water resistance than the control composites. However, the composites made with the bark-derived resin exhibited a slightly lower thermal stability than the control composites. This study demonstrated that bark-derived novolac resins have great potential for application in thermal molding of wood composites to improve water resistance compared with novolac resins without bark components.

1. Introduction

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Phenol formaldehyde (PF) novolac resins are produced from the addition and condensation reactions between phenol and formaldehyde under acidic condition with formaldehyde to phenol molar ratio of less than one. Novolac resin is a linear thermoplastic polymer with a partially cross-linked structure. The addition of a curing agent, such as hexamethylenetetramine (HMTA), is needed to cross-link the resin. Novolac resins have been extensively used as molding compounds, wood adhesives and frictional materials due to their outstanding mechanical properties, bonding performance, water and heat resistance and durability.

1

In a thermal molding, a mixture of novolac

resin, curing agent (typically HMTA), reinforcement filler, and additives, such as pigments, accelerating agent and lubricating agent, is used to produce different moldable thermoset materials. The novolac-based molding products are known to exhibit outstanding resistance against heat, flame, and chemical with desired hardness. 2, 3

Previous studies indicated that the intradomain reaction and interdomain reaction existed in the novolac resin gelation process, the growth of the inhomogeneity and gelation mechanism were dependent on the amount of cross-linker and initial formaldehyde to phenol molar ratio.

4, 5

In addition, the curing of novolac resin with HMAT was

dominated by two different polymer-growth mechanisms including power-law and subsequent deviation related to the chain extension and intermolecular reactions between larger molecules.

6, 7

It was found that cured phenolic resins had an inhomogeneity

associated with internal fractal interfaces between voids and phenolic resins. The curing reaction of PF novolac resin occurred by following both autocatalytic and n-order kinetic mechanism.

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The molecular weight and polydispersity index of phenol formaldehyde

novolac resins were found to have a significant impact on the rheological and mechanical properties of the resulting molding composites. 9

In addition to the conventional phenol formaldehyde novolac resin, 4,4′-(1,3-dipropyltetramethyldisiloxane) bis-2-methoxyphenol(SIE) was used to modify phenolic novolac resin through co-polymerization. The modified phenolic novolac resin and surface-treated chopped sisal fiber were used to produce silicon modified phenolic molding composites.

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The resulting composites showed improved mechanical and thermal properties compared to unmodified composites.

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An organic molding composite, that was made by using

silicone-modified phenolic resin (PR) as the matrix, vitreous silica fiber as the reinforced material, nano-aluminum oxide powder as the filler, and low melting point glass powder as the forming additive, demonstrated excellent thermal stability and ablation property. 11 The sisal fiber reinforced biobased silicone modified phenolic composites containing 4.5 wt % of layered double hydroxide (LDH) showed 60% reduction in total heat release than the composite without LDH. The biobased silicone modified phenolic resins had a reduced water absorption rate, increased volume electrical resistance, and improved interfacial interaction with sisal fiber than the control phenolic resins.

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Meanwhile, a

new novolac-type phenol-hydroxymethylfurfural (PHMF) resin was developed and used for making Fiberglass-reinforced composites with hexamethylenetetramine (NMTA) as the cross-linker.13 5-hydroxymethyl furfural (HMF) was in-situ generated from glucose at 120 °C with CrCl2/CrCl3 and tetraethylammonium chloride (TEAC) catalysts.

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It was

found that PHMF resin had a similar structure to phenol–formaldehyde (PF) novolac resin. The molding composites made from PHMF resin and fiberglass demonstrated similar or better tensile strength than that made from conventional PF resins. 14

Phenol, a key raw material for the novolac resin production, is derived from fossil fuel resources, such as petroleum and coal. Due to the increased environmental concerns, there is a strong interest in using renewable resources as alternative feedstock to replace petroleum-derived phenol in the phenolic resin production. 15 Bark, a renewable non-food based biomass material with more phenolic compounds (lignin and tannin) than wood, is largely available as waste residues from forest mills. Bark has shown good promises to partially substitute phenol in synthesizing bio-based PF resins due to its high phenolic content. 15-19

Previous studies showed that molding composites made from novolac resins containing phenol liquefied wood, 20-25 bark, 26 waste paper, 27 lignin, 28-30 and tannin 31 demonstrated good mechanical properties. Lin et al. prepared molding composites using a phosphoric acid catalyzed liquefied wood novolac resin.32 The molding composites showed

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comparable mechanical strength, improved flexural strength and decreased water resistance compared to the molding composites prepared using the conventional novolac resin without wood components. Pan et al. synthesized PF novolac resins using oxalic acid catalyzed phenol-liquefied wood and made wood composites. It was found that the wood composites prepared from the liquefied wood-based novolac resins had poorer dimensional stabilities in water.

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Alma investigated the tensile properties of molding

composites made from phenol liquefied barks catalyzed using sulfuric acid for various tree species (Pinus brutia Ten, Cedrus libani, Eucalyptus camaldulensis, Robina pseudoacacia, Castanea sativa, and Quercus cerris).

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The molding composites made

from the liquefied pine bark had the best tensile properties (tensile strength, Young’s modulus and tensile elongation). Moreover, it was found that the liquefied bark resin composite had comparable tensile properties and better compatibility with the wood filler to the composites prepared using the control novolac resin without bark components. However, there were limited studies on the effects of molding compositions on the properties of the resulting molding composites.

In recent decades, severe outbreaks of mountain pine beetle (MPB; Dendroctonus ponderosae) infestation affected western Canadian forests significantly.

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Infested trees

were harvested in order to slow the spread of the beetle outbreak, resulting in a large amount of infested lodgepole pine biomass available for utilization.

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Zhao et al. have

successfully synthesized PF resol resins using the liquefied infested lodgepole pine barks under alkaline conditions. It showed that the lap-shear specimen glued using the barkderived resol resins had high dry and wet bonding strengths.

35-38

Our recent study has

shown novolac resins can be formulated using either sulfuric acid- or hydrochloric acidcatalyzed liquefied infested lodgepole pine barks as the partial substitute for phenol. The novolac resins containing liquefied barks were found to have higher molecular weights, higher curing activation energies, faster curing rates and similar post-cured thermal stability in comparison with the lab-made control novolac resins without liquefied bark. 39

The liquefied beetle infested pine bark was found to have highly phenolated structures,

which was intrinsically more hydrophobic with desirable phenolic nature to react with formaldehyde. Meanwhile, bark contains tannin compounds and the catechol moiety from

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the condensed tannin can form strong hydrogen bonds with the hydroxyl groups of wood filler that could result in the water-insoluble three-dimensional network to enhance the water-resistance in the resulting composite.

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In addition, bark and bark components

were used as bio-based alternative to polyols and bisphenol-A and successfully incorporated into polyurethane and epoxy as well as melamine-formaldehyde resin. 40-47

However, no previous study has applied liquefied MPB infested lodgepole pine bark derived novolac resins in thermal molding application. Limited efforts in the literature investigated the effect of resin and wood particle compositions on the properties of the resulting wood composites. Moreover, few studies have looked into dimensional stabilities of the molding composites prepared using the liquefied bark novolac resins in water.

In this study, novolac PF resins made from hydrochloric acid-catalyzed liquefied bark were used for preparing molding composites. Maple wood particles were used as the wood fillers. Hexamine (HMTA), calcium hydroxide and zinc stearate were added as hardener, accelerating agent and lubricating agent, respectively. Three molding compositions with 3:7, 5:5 and 7:3 resin/filler weight ratios were prepared. Tensile properties, water resistance, and thermal stability of the resulting composites were measured. Control molding composites were made using lab-made novolac PF resin without bark following the same composition and procedure for comparison. 2. Experimental Section

2.1 Materials The laboratory-made novolac PF resins without bark and hydrochloric (HCl) acidcatalyzed liquefied bark novolac PF resins (denoted as Lab-PF 0.6 and LBPF-C 0.6, respectively) made following the procedure used in our previous study

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for making

molding composite. The composition of the mountain pine beetle (MPB) infested lodgepole pine bark (Pinus contorta, provided by FPInnovations, Vancouver, Canada)

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was found to contain 46.7 wt.% of holocellulose, 20.5 wt.% of α-celluose, 42.6 wt.% of lignin, 17.7 wt.% of ethanol-toluene extractives, 7.5 wt.% of hot water extractives, and 4 wt.% of ash content as measured according to ASTM D1105-96 and ASTM D1106-96.

The liquefied MPB infested lodgepole pine bark was obtained through HCl catalyzed phenol liquefaction at 150°C for 120min with the phenol to bark weight ratio of 3:1, and 3% catalyst loading based on the phenol weight. The liquefied bark was separated from the solid residues and reacted with formaldehyde under acidic conditions to obtain liquefied bark novolac PF resins. Liquefaction reaction was carried out according to the following steps. The liquefied bark and HCl were mixed in a three-neck flask and was heated up to 85°C, formaldehyde solution (37%) was then gradually added into the flask within 60min. After the formaldehyde addition, the reactants were kept at 85°C for another 60min. The resulting solid-state resins were used for making molding composites subsequently. In order to make molding composites, the novolac resins were first dried and ground into 60 mesh powders prior to mixing with the wood particles. The maple wood particles (Acer saccharum, provide by FPInnovations, Vancouver, Canada, containing 60.7 wt.% of holocellulose, 35 wt.% of α-celluose, 29.6 wt.% of lignin, 2.4 wt.% of ethanol-toluene extractives, 3.5 wt.% of hot water extractives, 2.1 wt.% of ash content measured according to ASTM D1105-96 and ASTM D1106-96) were sieved to be smaller than 60 mesh in size prior to be used as wood fillers. Hexamine (HMTA), calcium hydroxide, and zinc stearate were added in the mixture prior to molding as hardener, accelerating agent and lubricating agent, respectively. Laboratory-made novolac resins without bark components were also synthesized and used to prepare control composites for comparison. 2.2 Composite Preparation The composites were prepared using three different resin to wood filler loading ratios. The compositions with regarding to the ingredients and weight ratio of each molding compound are listed in Table 1. The wood powders were dried in the oven at 60°C overnight to have their moisture content to be within 1-2wt.% before mixing with other ingredients. Acetone was used to dissolve the Lab-novolac and LBPF-novolac resin

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powders to make resin solutions first (at 40% solids content). Wood powders, hexamine, calcium hydroxide, and zinc stearate were then mixed uniformly with the resin solutions. The mixture was dried in an oven at 40°C under vacuum overnight for acetone removal. The dried mixture of the molding compounds was ground into 60 mesh fine powders in a mortar and ready to be used for thermal molding.

Approximately 15g of molding compound mixture were molded into test specimens at 190°C for 15min with a set thickness of 3mm in a Laboratory Molding Press. The compression-molded test specimens had a dimension of 3 mm × 19 mm × 165 mm (thickness × width × length). (Figure 1)

Table 1. Compositions of the molding composites Molding Compounds Recipes (weight ratio) Types A B LBPF-novolac resin 7 5 Maple wood particles 3 5 Hexamine (HMTA) 0.1 0.1 Calcium hydroxide 0.05 0.05 Zinc stearate 0.05 0.05

T-thickness W0-overall width L0-overall length D-distance between grips L-length of narrow section W-width of narrow section

3mm 19mm 165mm 115mm 57mm 13mm

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C 3 7 0.1 0.05 0.05


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Figure 1. Specimen dimensions according to the ASTM D638-10 standard

2.3 Composite Characterization 2.3.1 Mechanical Test Tensile strength, tensile strain, and Young’s modulus (MOE) of the composites were tested according to the ASTM D638-10 standard using a Zwick universal test machine (Zwick/Z100, Zwick Roell Group, Germany) with a crosshead speed of 3 mm/min. The average value of the tensile strength, tensile strain and Young’s modulus (MOE) is reported based on a minimum of 10 replicates.

2.3.2 Water Soaking Test For each molding composition, at least 10 composite samples were used for the watersoaking test (according to ASTM D570-98). The initial thickness and weight of each composite specimen were measured before the test. All the test composite specimens were then immersed in the distilled water bath set at 23°C±1°C. The test specimens were regularly removed from the water bath every 24 hours and dried with paper towels to remove excess water on the surface of the specimen. The thickness and weight of each specimen was then measured. The thickness swelling and water absorption after 14 days (total 336 hours) were determined. 2.3.3 Scanning Electron Microscopy (SEM) SEM images of the fractured composites were obtained using a JEOL model JSM-6610 LV type of scanning electron microscope (JEOL USA Inc., MA, USA). The fractured composite samples were attached to metal stubs and coated with a thin layer of gold using a SC7620 model sputter coater manufactured by Qoroum Technology (Quorum Technologies Ltd, Lewes, UK). 2.3.4 Thermal Stability

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Thermal Gravimetric Analyzer (TGA-Q500, TA Instruments, USA) was used for characterizing the thermal stability of the composites. An approximate 10 mg of grounded composites were loaded in a platinum pan and was heated from room temperature to 800°C at the rate of 10°C/min under the nitrogen atmosphere at 20psi. The weight percentage as a function of temperature was analyzed and figures with weight percentage versus temperature were plotted. 3. Results and Discussion 3.1 Mechanical Properties Testing results for composite tensile strength, strain and Young’s modulus (MOE) are presented in Figures 2, 3, and 4 respectively.

Figure 2. Tensile strength of composites with different resin contents. （Columns having different letters are statistically significantly different by ANOVA and Tukey’s test (α = 0.05.))

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Figure 3. Tensile strain of the novolac molding composites. （Columns having different letters are statistically significantly different by ANOVA and Tukey’s test (α = 0.05.))

Figure 4. Young’s modulus (MOE) of novolac molding composites. （Columns having different letters are statistically significantly different by ANOVA and Tukey’s test (α = 0.05.))

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Figure 2 shows the tensile strengths of the molding composites with control and barkderived novolac resin. When the molding composites had the resin-to-filler ratio equal to 7:3, the Lab-PF-0.6 molding composite had a higher average tensile strength than the LBPF-C-0.6 molding composite. When the resin-to-filler ratio decreased to 5:5, the composite molded with LBPF-C-0.6 novolac resin had a higher average tensile strength than the composite molded with Lab-PF-0.6 novolac control resin. For the molding composites made with 3:7 resin-to-filler ratio, i.e. a higher wood filler content, both LabPF-0.6 and LBPF-C-0.6 resin-molding specimens had relatively low tensile strength. In general, the tensile strength obtained from novolac composites with bark component was lower than that of the control novolac composite without bark components except at 5:5 resin to filler ratio. And higher resin content in the composite gave a higher tensile strength. However, based on ANOVA analysis, it was found that the liquefied bark novolac resin molding composite with equivalent parts of resin and filler (5:5) had a higher tensile strength (25% higher) than the control lab-made molding composites. The LBPC-C-0.6 molding composite sample with 5:5 resin-to-filler ratio also had a greater tensile strength than the LBPC-C-0.6 molding composite sample with higher resin content (7:3). It was obvious that the tensile strength of the novolac resin composite was formula-dependent. In comparison to literature studies with similar molding conditions, 26, 48, 49

the tensile strengths of novolac resin composites molded using LBPF-C-0.6 are

within the reported data range.

48, 49

The liquefied bark component in the resin could

enhance mechanical properties of the composite depending on the resin to filler ratio.

Figures 3 & 4 show the tensile strain and Young’s modulus (MOE) of the novolac molding composites. Both tensile strain and MOE of the molding composites followed the same trend as what was observed for the tensile strength. Molding composites with a higher resin content generally had higher tensile strain and Young’s modulus. For the composite molded with liquefied bark PF novolac resin (LBPF-C-0.6), the sample with 5:5 resin-to-filler ratio showed the highest modulus among all LBPF-C-0.6 molding composites. The composites molded with liquefied bark PF novolac resin exhibited a higher tensile strain and modulus than the composites molded with the control Lab PF novolac resin when the resin-to-filler ratio was 5:5. It indicated that the addition of

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liquefied bark component into resin synthesis did not jeopardize the mechanical properties of the molding composites at this resin to filler ratio. The liquefied bark components could improve the composite’s tensile properties depending on the molding composition.

3.2 Water Soaking Test The dimensional stability and water absorption of the molding composites are shown in Figure 5.

Figure 5. Thickness swelling of novolac molding composites. The dimensional change of novolac resin molding composites in 336 hours (14 days) as measured by thickness swelling is shown in Figure 5. Overall, the thickness of all the composite samples had the most visible change within the first 24 hours. The liquefied bark PF (LBPF-C-0.6) novolac composite with resin-to-filler ratio of 7:3 showed the highest dimensional stability after being immersed in water with an overall thickness change of 0.48% at equilibrium. The thickness swelling of LBPF-C-0.6 molding composites with resin-to-filler ratios of 7:3 and 5:5 was lower than the rest of molding

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composite after 24 hours of soaking. For the molding composites made using Lab-PF-0.6 with resin-to-filler ratio of 7:3 and 5:5, the thickness of composite samples tended to stay constant after 48 hours; however, a noticeable decrease in thickness was observed after 144 hours of soaking and then reached the equilibrium. The molding composition with 3:7 resin-to-filler ratio had the least dimensional stability for both LBPF-C-0.6 and LabPF-0.6 molding composites. The descending trend in thickness change with fluctuated value indicated that the PF novolac molding composites with high content of wood filler were not dimensional stable after immersed in water for a long period of time. In general, LBPF-C-0.6 molding composite showed better resistance to dimensional changes than the Lab-PF-0.6 molding composites after soaking in water. However, bio-composites made with liquefied wood PF resin reported by Pan et al.

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showed poor water-

resistance, which was believed to be due to the hydrophilic liquefied wood component in the LWPF resin. In comparison to Pan’s results, our molding composite exhibited a lower thickness swelling, therefore a better water resistance. This may be attributed to the fact that bark component in the resin enhanced resin’s water-resistance. Bark contained a higher tannin and lignin content than wood of the same species.16 Lignin is an intrinsically hydrophobic polymeric material with desirable phenolic nature that can partially replace phenol to react with formaldehyde during the resin synthesis. The high lignin content in the LBPF-C-0.6 resin might contribute to the good water resistance of the resin. Besides, the liquefied bark PF novolac resin contained higher molecular weight components from liquefied bark (Mw and Mn of Lab-PF-0.6 were 1048.06Da and 524.24Da, respectively; Mw and Mn of LBPF-C-0.6 were 1617.74Da and 590.31Da, respectively).

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The higher molecular weight and more complex three-dimensional

structure of the liquefied bark could also improve the water resistance of the resulting LBPF resin and molding composites. Moreover, bark contain tannin, ,

17

the catechol

moiety from the condensed tannin in bark can form strong hydrogen bond with the hydroxyl groups of wood filler that could result in forming water-insoluble threedimensional network in the LBPF resins and therefore enhance the water-resistance in the resulting composite. 1, 35, 40

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In general, composite sample with higher resin contents exhibited better water resistance and dimensional stability since there were less wood powders in their compositions. The wood fillers in the composite are hydrophilic materials and will absorb water and cause the change in sample weight and dimension. It is well known,50 cellulose in wood is intrinsically hydrophilic. The high value of water absorption and change in thickness of the composite with higher filler content are expected, and the weak interfacial adhesion between the resin and the filler could also result in a higher water absorption and thickness swell of the molding composites. 3.3 Thermal Stability of the Molding Composites Thermal stability of the molding composites was tested using the thermal gravimetric analyzer (TGA). The thermal degradation curves of all tested composite samples molded with LBPF-C-0.6 resins and Lab-PF-0.6 resins at different compositions are shown in Figure 6. In general, both LBPF-C-0.6 composites and Lab-PF-0.6 composites molded with resin-to-filler ratios of 7:3 and 5:5 exhibited similar thermal stability below 400°C, while those molded with 3:7 resin-to-filler ratio had a significant weight loss starting at around 250°C and reached approximately 45 wt.% loss of the total weight below 400°C.

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Figure 6. Thermal degradation of molding composites molded with HCl acid-catalyzed liquefied novolac PF resins (LBPF-C) or lab-made control novolac PF resins (Lab-PF) at different resin-to-filler ratios (7:3, 5:5, and 3:7).

The comparison of thermal stability between LBPF-C-0.6 molding composites and control Lab-PF-0.6 molding composites at the same resin-to-filler ratios are presented in Figure 7. The molding composites produced from the Lab-PF-0.6 novolac resins exhibited a better thermal stability than those made with LBPF-C-0.6 novolac resins at all resin-to-filler ratios. For the molding composites formulated with resin-to-filler ratio of 7:3, major weight loss was observed at around 300°C for both LBPF-C and Lab-PF composites. The retained weight at 400°C were 70% and 75%, respectively. The molding composites with resin-to-filler ratio of 5:5 exhibited very similar thermal degradation curve for both LBPF-C and Lab-PF composites. Both LBPF-C and Lab-PF composites made with 3:7 resin-to-filler ratios had poor resistance against high temperature, where a significant weight loss happened at around 250°C and almost half of the total weight was lost below 400°C. In the synthesis of the lab-made control novolac PF resin, phenol reacted with formaldehyde under acidic condition and the resulting Lab PF resins acquired a regular linear structure with repeating units. The HCl catalyzed liquefied bark was utilized partially as the phenol substitute in the formulation of LBPF-C resins. Phenolated liquefied bark contains more complex functional groups (i.e. methylol groups) due to the higher lignin content in bark. The resulting LBPF-C resins might contain more complex phenolated structures due to the liquefied bark components in the resin formulation and these components might have less resistance against high temperature. Therefore, it is understandable that Lab-PF-0.6 molding composites showed a slightly better thermal stability than the LBPF-C-0.6 molding composites with a higher filler content (at the resin-to-fillers ratio of 7:3). However, thermal stability of LBPF-C resin molding composites was comparable to that of Lab PF resin composites at resin-tofillers ratios of 5:5 and 3:7.

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Figure 7. Thermal degradation of molding composites molded with HCl acid-catalyzed liquefied novolac PF resins (LBPF-C) or lab-made control novolac PF resins (Lab-PF) at different resin-to-filler ratios. Thermal stability of LBPF-C-0.6 molding composites with different molding compositions is shown in Figure 8. It was found that composites formulated with higher resin-to-filler ratios exhibited a better thermal stability. Same trend was observed for the conrol Lab-PF-0.6 molding composites. According to the literature,

51

wood filler starts

to degrade at a relatively low temperature, approximately 200°C. However, both labmade control novolac PF resins and liquefied bark novolac PF resins had good thermal stability. Therefore, it is understandable that composites molded with a higher resin amount exhibited a better thermal stability.

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Figure 8. Thermal degradation of molding composites molded with HCl acid-catalyzed liquefied novolac PF resins (LBPF-C) and lab-made control novolac PF resins (Lab-PF) at different resin-to-filler ratios (7:3, 5:5, and 3:7). 3.4 Scanning Electron Microscopy (SEM) images The fractured surfaces of the molding composites have been investigated using SEM. The interactions between resin matrix and fibers were examined. The SEM micrograph of the fractured surfaces of the composite molded using Lab-PF-0.6 and LBPF-C-0.6 at 7:3 resin-to-filler ratio is shown in Figure 9. It was evident that fibers were broken instead of being pulled out (circled region). There were obvious remaining resin matrix attached to the fiber surfaces indicating there was a strong interaction between the resin and the wood fillers. For the composite molded using LBPF-C-0.6 at the resin-to-filler ratio of 7:3, although few broken fibers were exposed on the fracture surface, several pulling channels were also observed revealing that several fibers were pulled out during the tensile test, which indicated a weaker interaction between the fiber and resin matrix. The results agreed with our mechanical test results where the Lab-PF-0.6 molding composite had a higher tensile strength at the resin-to-filler ratio of 7:3. It was attributed to the stronger interaction between the resin matrix and the wood fillers in the Lab-PF-0.6

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molding

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composite.

Figure 9. SEM micrographs of the fractured surfaces of the LBPF-C and Lab PF resin molding composite at the resin-to-filler ratio of 7:3. (Regions indicated by the red circles showing the broken fibers)

The SEM micrograph of the fractured surfaces of the composite molded using Lab-PF0.6 and LBPF-C-0.6 with 5:5 resin-to-filler ratio are shown in Figure 10. Large amount of visible long tunnels were apparent in the SEM micrograph of the composite molded using Lab-PF-0.6, suggesting a number of fibers have been entirely pulled out during the tensile test (circled region). It can be found that fibers were smoothly drawn out, indicating that fibers were weakly attached to the resin matrix in the molding composite. There were several channels and some broken fibers appeared on the SEM micrograph of the composite molded using LBPF-C-0.6. It is found that fibers were not smoothly pulled out from the matrix since there were broken fiber remaining attached along the pullout channels at the fracture surface. This implied a relatively stronger interaction between the resin matrix and fibers in the composites molded using LBPF-C-0.6 at the resin-to-filler ratio of 5:5. As mentioned before, wood fillers were added in the molding composite for reinforcement purpose. If the fiber contents were too high in the composite, cluster of fibers might form during the molding process and fibers could not be evenly dispersed in the resin matrix. If the fiber contents were low, the amount of fibers might be insufficient to be evenly distributed in the matrix and fibers might not play an effective role in reinforcing the molding composites. The uneven dispersion of fibers in the molding

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composite would lead to poor mechanical properties. Therefore, the mechanical strengths of the molding composites were formula dependent. The result also agreed with the mechanical test results, where the molding composites made with LBPF-C-0.6 exhibited higher tensile strength than Lab PF resin molding composites at the resin-to-filler ratio of 5:5. At this resin to filler ratio, the composites demonstrated the best tensile property among all molding composites.

Figure 10. SEM micrographs of the fractured surfaces of the Lab PF and LBPF-C resin molding composites. (resin-to-filler weight ratio : 5:5). (Regions indicated by the red circles showing the pulled out fibers)

The SEM micrographs of the fractured surfaces of the Lab-PF-0.6 molding composite and LBPF-C-0.6 molding composite at the resin-to-filler ratio of 3:7 are present in Figure 11. The fractured surfaces of both molding composites were very porous and chaotic. It was evident that a large amount of pores existed on the fracture surface, indicating that most of the fibers were pulled out during the mechanical test. Furthermore, due to the low resin amount in the molding formulation, the wood filler surface might not be fully covered by resin and that would result in a weak interaction between the resin matrix and wood filler. The results were consistent with the mechanical test results, where the molding composites at the resin-to-filler ratio of 3:7 had the lowest tensile strength for both types of resins.

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Figure 11. SEM micrographs of the fractured surfaces of the Lab PF and LBPF-C resin molding composites. (resin-to-filler weight ratio : 3:7) (Regions indicated by red circle showing pores existed on the fracture surface)

4. Conclusions Wood composites were successfully molded using hydrochloric acid-catalyzed liquefied bark-derived novolac PF resins. In general, a higher resin content in the composite gave a higher composite tensile strength. In comparison to the lab-made control novolac molding composites, the liquefied bark novolac molding composites were found to have a higher tensile strength at 5:5 resin to filler weight ratio. In addition, the liquefied bark components in the novolac resin enhanced water resistance of the resulting molding composites for all resin to filler ratios studied. Moreover, molding composites of the liquefied bark novolac resins showed only a slightly lower thermal stability than the control composites. The SEM micrographs of the fracture surfaces of the molding composites revealed fracture mechanisms associated with the filler resin interactions that were consistent with the results obtained from the mechanical test. The bio-based novolac PF resins derived from liquefied MPB infested lodgepole pine bark showed good potential for application in thermal molding of wood composites.

Acknowledgement

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Application of Biobased Phenol Formaldehyde Novolac Resin Derived

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