Abstract
Polyurethane (PU) bio-adhesive was synthesized from bio-polyol based on renewable source. The bio-polyol was synthesized from epoxidized soybean oil (ESBO) using citric acid as ring opening agent. Molecular structure of the bio-polyol was modified with tetra ethyl ortho silicate. PU wood adhesive was obtained by reaction of the modified bio-polyol with polymeric 4, 4`-diphenylmethane diisocyanate (pMDI). The performance of the prepared bio-adhesive was improved by using zinc oxide (ZnO) nanoparticles (NPs), triethylene glycol (TEG) and Dibutyltin dilaurate (DBTDL) as additives.
The effect of mentioned additives on the adhesive lap shear strength was investigated and optimized using Taguchi method. The optimum condition was obtained (6.478 MPa) and was approximately in accordance with predicted amount.
The molecular structures of the prepared bio-polyol and adhesives were studied by FTIR, and 1HNMR. Thermogravimetric analysis (TGA), contact angle, SEM and lap shear strength values were also investigated.

Keyword: Epoxidized soybean oil, Bio-based polyol, ZnO NPs, PU bio-adhesive, experimental design.

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1. Introduction
Adhesives as nonmetallic materials are able to hold similar and dissimilar articles together by surface bonding (Ebnesajjad and Landrock, 2014; Gierenz and Karmann, 2008). Indeed, adhesives are substances that are utilized to surfaces of materials to join them constantly by an adhesive bonding process (Ebnesajjad and Landrock, 2014). Adhesives can distribute stress more than mechanical methods and they can also reduce or prevent galvanic corrosion between dissimilar materials (Ebnesajjad and Landrock, 2014). As you know adhesive bonding is a substitute to more common mechanical technique of assembling substances (Pocius, 2012). Therefore, it can be said that adhesives have an important role in our daily lives (Petrie, 2000).
Adhesives have different types such as epoxy, urea-formaldehyde, acrylic, polysulfide, PU adhesive and etc. but among the various types of adhesives, PU type can be mentioned (Ebnesajjad and Landrock, 2014). PU adhesives are one of the most important and versatile class of polymeric materials and they also have unique features consist of superior adhesion, resistance to weathering and etc. These adhesives are widely used in leather, footwear, textile, packaging and other fields (Akram et al., 2013; Jia-Hu et al., 2015; Khoon Poh et al., 2014; Moghadam et al., 2016). PU adhesives are prepared from reaction between a polyol (as soft segment) and a diisocyanate (as hard segment) (Akram et al., 2013; Cz?onka et al., 2018; Khoon Poh et al., 2014; Sheikhy et al., 2013). The polyols are very important components for PU adhesives and they can affect PU final properties (Cui et al., 2017; Moghadam et al., 2016; Petrovi? and Javni, 1989). Currently most of polyols were made from non-renewable petrochemical sources. As you know petroleum-based materials are toxic and cause environmental pollution. On the other hand, petroleum resources are considerably reducing and becoming expensive (Cui et al., 2017; Cz?onka et al., 2018; Khoon Poh et al., 2014; Mishra and Sinha, 2010; Moghadam et al., 2016).
Furthermore, increasing of chemical requirement affects manufacturing techniques. Industry development’s attention concentrate more on optimizing exist processes rather than on extending new ones, although there are some opposite examples. Despite the rapid development of the industry during the early years, there wasn’t any attention to environmental risks caused by production methods. During recent years it has revealed that chemical industries have brought considerable problems for environment. Needs for environmentally friendly methods have driven industry to create greener chemistry (Wenda et al., 2011).
In the case of PU adhesives in modern studies many attempts have been carried out in order to replacement of petroleum-based polyols with polyols synthesized from renewable materials. Renewable sources such as starch, agricultural wastes, sawdust and vegetable oils have been utilized for polyols production. Among these sources the vegetable oils due to their law cost, accessibility, versatilities, technical facilities in polyol synthesis and because of their special Structure, they have some advantages (Aung et al., 2014; Cz?onka et al., 2018; Khoon Poh et al., 2014; Petrovi?, 2008; Santan et al., 2018).
The use of vegetable oils in the production of adhesives, coatings and inks has many advantages, but studies have shown that high viscosity oils have the most efficient result (Dewasthale et al., 2015; Santan et al., 2018). Crosslinking vegetable oil chain structure with silanes is one method to increase the viscosity. Besides this it has been reported that modification of polyurethanes (Pus) structure with silicon amends the thermal stability. Also, PUs with high silicon content show fire resistance. In addition to studies have showed that production of contiguous layer of silica which postpone the oxidation of the char caused thermal stability (Dewasthale et al., 2015; Xia and Larock, 2010). Furthermore, in recent studies different types of silanes were used as adhesion promoter in PU and dental composite (Cao et al., 2012; Matinlinna et al., 2008).
Besides this in order to improve PU adhesive mechanical strength and thermal stability, many researches have been derived recently. Different types of additives, pigments, nanoparticles, chain extenders, and catalysts have been used to enhance PU adhesives properties (Fu et al., 2015; Ismail et al., 2011; Jia-Hu et al., 2015; Sheikhy et al., 2013). Recently researchers have focused on nano filled elastomers, because their usage has considerably enhanced materials’ features such as thermal stability and mechanical strength (Nikje and Garmarudi, 2011). Organic-inorganic nanocomposites have organic polymers and inorganic materials properties simultaneously so they may get some novel properties (Guo et al., 2007). One of the most widely used nanoparticles are ZnO NPs. ZnO NPs not only are cost effective but also are bactericide and biocompatible compounds. Furthermore, the negative charge concentrated on ZnO NPs that cause hydrogen bonding with compounds that have active hydrogen such as N-H and O-H increases and improves mechanical properties of polymer containing active hydrogen (Dorraji et al., 2018). PU adhesives have unique properties including fast curing, toughness and good chemical resistance originate from their dual microphases structure (the thermodynamic incompatibility of hard and soft segment). Cohesive strength of PU adhesives can be improved by controlling the ratio of microphase separation between soft and hard segment. Incorporation of chain extender is an approach that can increase immiscibility between hard segment and soft segment and make it possible highly polar hard segment to enter phase separation into hard domain through hydrogen bonding. Finally, soft segment is reinforced and mechanical and physical properties are improved (Sheikhy et al., 2013; Tan et al., 2017). On the other hand, as you know catalyst has an important role in PU production. Catalyst not only can have effect on the rate of reaction but also it affects adhesive final properties (Frisch and Rumao, 1970; Silva and Bordado, 2004). Dibutyltin dilaurate (DBTDL) catalyst is used in PU production commonly. It can exert a significant effect on isocyanate-hydroxyl reaction. In fact, DBTDL is the main agent for crosslinking in PU. DBTDL is a cost-effective catalyst and can improve the mechanical properties of PU (Dorraji et al., 2018; Luo et al., 1997).
In the present study bio-based polyol from vegetable oil was synthesized by ring opening of ESBO using citric acid.
The molecular structure of prepared bio-polyol was modified with tetraethyl ortho
silicate (as crosslinking agent and adhesion promoter), ZnO NPs and TEG, and PU wood adhesive was prepared through reaction of prepared bio-based polyol and pMDI in presence of DBTDL as catalyst. To scrutinize the optimum conditions effective factors (ZnO NPs, DBTDL and TEG) were investigated by using taguchi’s method.
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2. Methodology
2.1. Materials
ESBO (oxirane oxygen content = 7.35%) was purchased from Persian shimi Co, Iran. Polymeric 4,4?-metylene diphenyl diisocyanate (pMDI) was obtained from Karoon petrochemical Co, Iran. Tetra ethyl ortho silicate was purchased from pishroo mobtaker peyvand Co, Iran. TEG and citric acid were obtained from Merck.
Analytical reagent grade of hydrochloric acid, zinc sulfate and sodium hydroxide were obtained from Merck and consumed as purchased for preparation of ZnO NPs.
2.2. Methods for synthesis and characterization
2.2.1. Measurement methods
Fourier transform infrared (FTIR) spectra, ranging from 400 cm-1to 4000 cm-1, were studied by FTIR spectrophotometer (Hitachi S4 160, AIS2 100) using thin films or KBr discs. 1H NMR spectra were recorded by a Bruker 250 MHz spectrometer. Hydroxyl number was measured according to ASTM D1957-86: sample was first acetylated and then titrated by KOH solution 0.5N. Thermogravimetric analysis (TGA/DTG) was accomplished on SDT Q600. Adhesive was heated from 25 to 620 °C at a heating rate of 10 °C/min under nitrogen. Each prepared sample was tested for lap shear strength by a Santam STM50 according to ASTM D906-82. The crystal structure of NPs was characterized by X-ray diffraction (XRD; Simens D-5000 diffractometer, Germany) with monochromatic Cu K? radiation. Scanning electron microscope (SEM, model: AIS2100, Seron Technology Co.) was used to study the morphologies of the nanoparticles and adhesive. The contact angle values of water on adhesive sample was measured using an optical contact angle meter (CAM200, KSV Instruments, LTD).
2.2.2 Experimental design
One of the most common procedure in the design process is Taguchi method, mainly for design of experiment. Taguchi method is utilized for optimization of the process condition and also to improve the quality of product with decreasing the development and manufacturing cost.
Taguchi’s techniques have main trust that is using of parameter design and focuses on specifying the parameter (factor) settings producing the best levels of quality. For achievement to the best process design it will be necessary to select a strategically designed experiment, which cause the process to be exposed to various levels of design parameters.
The main aim of this work is to find the factors that have considerable effects on attaining the maximum enhancement of lap shear strength of synthesized adhesive and obtaining the desirable condition in a limited number of experiments.
In this work 3 factor in 3 level (L9) were considered, factors and level were shown in Table 1.
2.2.3. Wood specimens preparation
According to ASTM D906 wood pieces were cut into a rectangular piece with size of
100 × 25 × 4 mm3. Wood pieces were polished with sand paper grit no.60 before application.
2.2.4. Preparation of PU adhesives according to experimental design
In order to preparation PU adhesive first modified bio-polyol, TEG, ZnO NPs and DBTDL were mixed according to the Taguchi Table 2 in a plastic sample container and were well stirred for 6 min at room temperature. Then the stirring was stopped and pMDI was transferred to reaction mixture and stirring was continuing for 1 min. The PU adhesives were synthesized by NCO/OH ratio of 1.5. The reaction mixture was used on wood surface by a film applicator and cured at room temperature.
2.2.5. Lap shear strength
Lap shear testing was accomplished according to ASTM D906 method. The adhesive mixture was applied onto wood specimens’ surface using a film applicator to form a film of 0.1 mm thickness with a bonding area of 30*25 mm2, then two pieces of wood were placed together and a load of 2.5 kg was used onto wood specimens for 24 h at room temperature. After the adhesive bonding were cured at 25 °C with humidity of 50% ± 5% for seven days.
Lap shear strength tests for bonded wood joints were accomplished using universal testing machine. Samples were gripped by two screw-type flat-plate grips and pulled with a pulling rate of 3 mm/min.
2.3. Synthesis
2.3.1. Preparation of bio-polyol
The reaction was performed in a 500 ml three-neck round-bottom and the flask was equipped with a thermometer, condenser and magnetic stirrer. ESBO was first heated at a temperature of 50 °C in an oven and then 150 g of heated ESBO was charged into the round-bottom flask. Also, the 35 g of citric acid first dissolved in distilled water and then was charged into flask. The reaction was completed at a temperature of 80 °C with steady stirring for 10 h. Hydroxyl value of the synthesized prepared bio-polyol was determined (see scheme 1).