Properties and characteristics of novel formaldehyde-free wood adhesives prepared from Irvingia gabonensis and Irvingia wombolu seed kernel extracts

International Journal of Adhesion and Adhesives 95 (2019) 102423 Contents lists available at ScienceDirect International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh Properties and characteristics of novel formaldehyde-free wood adhesives prepared from Irvingia gabonensis and Irvingia wombolu seed kernel extracts A.O. Alawodea, P.S. Eselem Bungub, S.O. Amiandamhena, M. Meinckena, L. Tyhodaa, a b T ⁎ Department of Forest and Wood Science, Stellenbosch University, Stellenbosch, South Africa Department of Chemistry and Polymer Science, Stellenbosch University, Stellenbosch, South Africa A R T I C LE I N FO A B S T R A C T Keywords: Formaldehyde-free Irvingia gabonensis Irvingia wombolu Natural adhesive Wood composites There is renewed interest in the domestication of Irvingia tree species due to the potential use of various parts of the tree as raw materials for a wide range of applications such as biodiesel production, cosmetics, perfumes, soap, weight-loss supplement etc. The current study investigates the properties of extracts from the seed kernels of two Irvingia species – Irvingia gabonensis (IG) and Irvingia wombolu (IW) as natural wood adhesives. Three extraction methods using various solvent/solute media were compared in terms of yield, composition and mechanical properties. Statistically, the analysis revealed significant differences between the different extraction methods. The adhesion properties of the extracts were tested on wood veneers according to the American Society for Testing and Materials standard (ASTM D – 906-64). The shear strength of the extracts ranged from 0.55 to 1.5 MPa and 0.86–1.7 MPa for IG and IW, respectively. The initial decomposition temperature of all Irvingia Kernel extract ranges from 138.3 to 149.11 °C for IG and 129.5–145.3 °C for IW. As a result, the hot melt temperature for the adhesive experiments was set around 150 °C. The results indicate that Irvingia kernel extract is a more promising source of non-formaldehyde based adhesives in wood composite production. 1. Introduction Wood adhesives play vital role in the production of various wood composites; and increase in the production of reconstituted wood panels has resulted in an increase in the consumption of adhesives, which are typically formaldehyde and petroleum based [1]. Zhang et al. [2] and Khosravi et al. [3] listed formaldehyde based adhesives such as urea formaldehyde (UF), phenol formaldehyde (PF) and melamine-urea formaldehyde (MUF) to be the most commonly used in the wood processing industries because of their superior adhesion properties. In addition, they exhibit several advantages, which includes; good adhesion to different lignocellulosic substrates, high water resistance (except UF), low initial viscosity, resistance to environmental degradation, and excellent thermal stability [1,4,5]. However, Langenberg et al. [6] reported that despite the advantages of these synthetic adhesives, they have some limitations such as emission of formaldehyde which has been categorised to be carcinogenic by the Environmental Protection Agency (EPA) since 2008 [6]. Furthermore, their prices depend largely on the oil market with its usual result price instability. The depletion of fossil fuel reserves is a big concern, which makes availability of these synthetic adhesives uncertain in the future. Therefore, there is need for the development of wood adhesives from renewable sources, which will ⁎ Corresponding author. E-mail address: ltyhoda@sun.ac.za (L. Tyhoda). https://doi.org/10.1016/j.ijadhadh.2019.102423 Received 31 May 2019; Accepted 26 July 2019 Available online 08 August 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved. be formaldehyde-free and possess comparable strength properties to commonly used synthetic adhesives. Renewable material based adhesives are made from plant or animal sources and have various advantages over synthetic adhesives. They are developed from materials, which are abundant, sustainable, environmentally friendly and low cost. However, they have some shortcomings such as low gluing strength and bio-degradation resistance. Renewable materials have been used to develop wood adhesives for the production of wood composite products such as plywood, particleboard, and oriented strand board. Zhang et al. [2] reported that several efforts have been made by various researchers using different biological materials such as; lignin, tannin, furfural, or soybean either to reduce the formaldehyde content in adhesives formulation or to even develop adhesives completely from natural materials. Khosravi et al. [3] showed that both, soy protein isolate (SPI) and wheat gluten (WG) can be used as binders for particleboards. Moubarik et al. [7] discovered that particleboard bonded with formaldehyde-free corn starch-tannin adhesive showed comparable mechanical properties to the panels made with the commercial UF resin. Wang et al. [8] developed a modified starchbased wood adhesive that exhibits good adhesive property at room temperature by grafting vinyl acetate monomer onto waxy corn starch backbone. However, there has been limited research on utilization of International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Bondtite 345 was supplied by Bondtite® (Pty) Limited, South Africa. wood adhesives based on natural seed gums. Norstrom et al. [9] reported high molar mass and viscosity for gums from the seed of locust bean, guar, xanthan, and tamarind, with locust bean gum showing the best bonding performance. This study seeks to investigate the potentials of Irvingia gabonensis and Irvingia wombolu as a raw material for the development of environmentally-friendly wood adhesives. Several works have been reported regarding the description and multi-potentials of Irvingia species. In one report, Ainge and Brown [10] described the species as underutilized, highly valued fruit trees, native to Africa and Southeast Asia, belonging to the family Irvingiaceae and that their genus description was carried out in 1860. Singh [11] reported that there are different species of Irvingia which include, Irvingia smithii (IS), I.malayana, I.gabonensis, I.wombolu, I. grandiflora, I.robur, and I.malayana in Southeast Asia. In Africa, two species are common: I. gabonensis and I. wombolu, and these forest fruit trees are widely available in Western and Central Africa. Ikechukwu and Salome [12] reported that Irvingia species are commonly known as African mango, Dika nut, bush mango or wild mango. Etebu and Tungbulu [13] made a distinction between the two, noting I. gabonensis has a sweet edible pulp while I. wombolu has a bitter inedible pulp. Ainge and Brown [10] enumerated various potential industrial applications of Irvingia kernels as reported by several researchers in Africa and Asia. These include cooking oil, margarine, perfume, soap, and pharmaceuticals binders. Bello et al. [14] investigated possibility of using the oil extract as possible alternative fuel in diesel engines and deduced that African bush mango oil is a suitable alternative fuel for diesel engines. There has been a number of attempts to investigate the potential of Irvingia kernels extracts as binder in pharmaceutical applications. Ikechukwu and Salome [12] discovered that I. wombolu gum contains alkaloids, flavonoids, saponin, tannins and glycosides. They concluded that natural gum from I. wombolu has good potential to be used in formulating normal tramadol capsules [12]. In a similar study, Eraga et al. [15] inferred from the results of their investigation that the suspension prepared with the co-precipitate of I. gabonensis gum and gelatin as suspending agent appears to be superior to those prepared with the gum or gelatin alone. A major drawback in mass processing of Irvingia kernels is fungal infestation and spoilage. Ikhatua et al. [16] reported that the fungus (Aspergillus fumigatus) is the predominant isolate and the pathogen of spoilage of Irvingia seeds. However, Awono et al. [17] revealed that the proximate analysis of the nutritive composition of healthy and spoilt kernels of I. gabonensis and I. wombolu obtained from retailers showed no significant differences in some nutritive components. In a related study, Ikhatua and Falodun [18] reported that there were no significant differences in chemical compositions of both healthy and spoilt kernel of Irvingia species. As a result of this, rejected seeds may be utilized for the gum formulation. This is another way of waste utilization, since the chemical composition remains unaltered irrespective of their physical appearances. Irvingia kernels extracts are biomaterials with the potential to be used as renewable adhesives and can serve as green and cost-effective wood adhesive with desirable bonding properties. According to Bello et al. [14], extracts of Irvingia species can be obtained as by-products after the kernel fat content has been used for manufacture of various consumer products. To the best of our knowledge, this is the first approach of evaluating the extracts from these species as wood adhesives. This study additionally describes the feasibility of extracting and formulating adhesives from Irvingia kernels for wood composite applications. 2.1. Chemical analysis The kernels of both species were air dried for 14 days. Thereafter, they were milled using a hammer mill fitted with a 1 mm sieve. The obtained particles were analysed for protein, fat, ash and carbohydrate content using the Association of Official Analytical Chemists Methods [19]. 2.2. Extraction Several methods for extracting the gum from Irvingia kernels have been reported. Some of the methods employed in this study using different solvents are described below and compared in terms of efficiency, yield, and properties of the Kernel extract. 2.2.1. Sodium chloride extraction This extraction followed the procedure described by Ogaji et al. [20]. 100 g of milled I.gabonensis was transferred to a 2 L beaker containing 2 L of 1% w/v sodium chloride pre-heated to about 78 °C. The mixture was heated on a hot plate to about 78–85 °C. The mixture was mixed gently for an hour with the aid of a magnetic stirrer. The mixture was then left to stand at room temperature (~21 °C) for 24 h. Solidification of the lipids component took place at the top and the bottom of the mixture and the lipids were subsequently removed by filtration through a 105 μm cloth sieve. The lipids were air dried and stored in an airtight container until ready for use. The remaining sample containing the gum was centrifuged (Allegra 6R centrifuge, Beckman Coulter™, USA) at 3440 rpm for 10 min to remove other impurities. The resultant clear supernatant mucilage (kernel extract) was dried in a lyophilizer and stored in an airtight container until required for further analysis. 2.2.2. Water extraction Water extraction was carried out according to the method reported by Eraga et al. [15] with slight modification. According to the method, 100 g of the powdered cotyledon of I.gabonensis and I.wombolu was dispersed in 2 L of distilled water in a plastic container. The mixture was mixed gently for 1 h with the aid of magnetic stirrer. The dispersion was homogenized for about 1 h and left for 24 h. The mucilage formed was then filtered through a clean muslin cloth to obtain a viscous filtrate (gum). The filtrate was centrifuged (Allegra 6R centrifuge, Beckman Coulter™, USA) at 3440 rpm for 10 min to separate the fat. The supernatant mucilage (kernel extract) gum was then dried in a lyophilizer. The resulting powder was kept in an airtight container until required for further analysis. 2.2.3. Sodium metabisulphite (Meta) extraction This extraction was carried out according to the methodology developed by Ikechukwu and Salome [12] with slight modification. 100 g of the powdered cotyledon of I.gabonensis and I.wombolu kernels were soaked in 2L of distilled water containing 1% sodium metabisulphite for about 12 h. Thereafter, it was filtered and the gum was centrifuged (Allegra 6R centrifuge, Beckman Coulter™, USA) at 3000 rpm for 10 min to separate the fat. The supernatant mucilage (kernel extract) was then dried in a lyophilizer. The resulting powder (extract) was kept in an airtight container until required for further analysis. 2. Materials and methods 2.3. Extract yield Irvingia gabonensis and Irvingia wombolu kernels were purchased from a local market in Nigeria. Sodium metabisulphite and sodium chloride were purchased from Associated Chemical Enterprises, South Africa. Hexamine was purchased from United Scientific, South Africa. The yield from different extraction methods for the two species was determined by weighing the dried Irvingia extract and calculated on a wet basis. 2 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. 10min. Ten replicates of each sample were prepared. The test specimens were placed in a conditioning room at 65% relative humidity (RH) and 20 °C for 96 h prior to shear strength testing analysis. 2.4. Physical properties 2.4.1. Specific gravity The specific gravity (S) of the Irvingia extracts was determined by dividing the weight of the resin by the weight of an equal volume of water. Specific gravity = Resin weight (g) weight of an equal volume of water (g) 2.7. Tensile strength test This was conducted according to the method given by Dongre et al. [4] and Tappi method T 494-OM-01. Whatman GF/C glass microfiber filter material was used for the experiment. It was cut into 102 × 25.4 mm strips and oven dried at 105 °C and weighed. 5 g of dried Irvingia extract was dissolved in 37 ml of water. The strips were immersed completely in the formulated adhesives for 5 min, removed and left to dry overnight. This procedure was repeated with a commercial urea based resin Bondtite 345 incorporated with hexamine as hardener for comparison. The dried strips were placed on smoothed aluminium foil and covered by another layer of smoothed aluminium foil, to prevent sticking to the caul plates during the hot pressing stage. The dried strips were then pressed in an electric hydraulic press at a temperature of 150 °C and pressure of 2 MPa in order to gain strength upon solidification and crystallization. Solidification and crystallization is necessary in order to further enhance the adhesive-adhesive and the adhesive-substrate (glass fibre) cross-linking reaction [2]. The pressed glass fibres were oven dried again at 105 °C and weighed to determine the amount of adhesive absorbed by the samples. The strips were conditioned at 20 °C ± 2 °C and 65% relative humidity prior to testing. The tensile strength of the strips was measured using an Instron Universal machine (Model 3322), with a crosshead speed of 2 mm per min and operated at 5 KN. The inert glass fibres do not absorb water and any change in its properties can be attributed to the adhesive rather than the fibre substrate. Ten replicates of each sample were also prepared for this test. The tensile strength of the adhesive was calculated using the formula: [1] 2.4.2. pH The pH of all the Irvingia adhesives was measured with a digital pH meter (Mettler Toledo S220). 2.5. Adhesive characterization 2.5.1. Thermogravimetric analysis (TGA) The thermal stability of the Irvingia extract powdered samples was analysed using a TGA Q50 thermogravimetric apparatus. Approximately 5 mg of each powdered sample was placed on a balance located in the furnace and heat was applied over the temperature range from room temperature to 800 °C at a heating rate of 20 °C/min in a nitrogen atmosphere. The derivatives of weight loss vs. temperature thermograms were obtained to show the different decomposition processes. 2.5.2. Differential scanning calorimetry (DSC) analysis The melting temperature (Tm) of each Irvingia extract was determined using a TA Instrument Q100 calorimeter. The instrument was calibrated with an indium metal standard according to standard procedures. About 4 mg of the powdered samples were put into an aluminium pan, with the usage of the empty pan as reference. The samples were heated up at a rate of 10 °C/min within the temperature range of −15 and 150 °C under inert nitrogen atmosphere. A constant ramp rate of 10 °C/min was used for both heating and cooling cycles. T= TR − TB M [2] where T = Tensile strength per gram (N/m.g), TR = tensile strength of reinforced fibre (N/m), TB = tensile strength of the blank fibre (N/m), M = mass of adhesive absorbed by the glass fibre. 2.5.3. Fourier transform infrared (FTIR) spectroscopy analysis The Irvingia extract was analysed using fourier transform infrared (FTIR) spectroscopy operating in Attenuated Total Reflectance (ATR) mode. The powdered extract samples were pressed against the diamond crystal surface with a spring-loaded anvil of a Thermo Nicolet, Nexus™ model 470/670/870 FT-IR spectrometer equipped with ZnSe lenses. Spectra were collected in ATR mode at a resolution of 4 cm−1 and 32 scans per sample within the absorption bands in the region of 4000–650 cm. Data collection and further processing were carried out in the Thermo Scientific OMNIC software. To distinguish between the spectra pattern exhibited by the different Irvingia adhesives samples, principal component analysis (PCA) was performed using Statistica software (Statsoft V12). 2.8. Statistical analysis The statistical analysis was performed using STATISTICA (version 13). The data were analysed through a one-way ANOVA (analysis of variance) at 5% significance, with a post-hoc Fisher LSD test at 5% for pair comparison to evaluate if there were statistically significant differences among extracts. 3. Results and discussion 3.1. Chemical composition of kernels 2.6. Plywood preparation and shear test The chemical composition analysis data of Irvingia wombolu and Irvingia gabonensis kernels are presented in Table 1. The results agree well with those obtained by Ikhatua 2010 [16], although the values The shear strength properties of all the Irvingia extracts were determined according to the ASTM D-906-64 test method. This standard measures adhesive shear strength in a plywood construction by tension loading (Instron Universal testing machine). A three-layered plywood of 9 mm thickness was prepared from sweet birch (Betula lenta) veneer with a thickness of 3 mm. Each veneer was coated with 250 g/m2 of extract and exposed to air for about 15 min to evaporate excessive moisture. This procedure was repeated with a commercial resin, Bondtite 345 for comparison. Bondtite 345 is a tannin-based resin. Hexamine was added to the Bondtite 345 as a hardener. 5 g of Bondtite 345 was dissolved in 10 mL of distilled water and 1 g of hexamine was added to the solution to produce the reference binder. Glued veneers were hot pressed at 150 °C temperature with a pressure of 1.6 MPa for Table 1 Compositional analysis of Irvingia wombolu and Irvingia gabonensis kernels. Species Carbohydrate (%) Protein (%) Fat (%) Ash (%) Moisture Content (%) IW 21.2 (0.82) 16.83 (0.82) 6.5 (0.36) 7.97 (0.51) 67.5 (1.69) 70.5 (1.92) 2.3 (0.05) 2.4 (0.08) 2.5 (0.05) 2.3 (0.08) IG a Values represent mean of three replicates with standard deviation in parenthesis. 3 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. 3.3.2. Chemical composition of Irvingia wombolu and Irvingia gabonensis extract The results of the chemical characteristics of Irvingia extract are presented in Table 3. The ash content of extract produced using NaCl and metabisulphite extraction methods were higher than those produced from water extraction. The extract produced through metabisulphite extraction method had a lower carbohydrate content. This may be due to a shorter extraction time of 12 h. The protein contents of all the extract are in a close range between 4.8 and 7.9. This suggests that the extraction method did not have any effect on the protein yield. The extract produced from water extraction have, however, higher carbohydrate and fat contents. Table 2 Physical properties of Irvingia extract as determined by DSC, TGA, pH meter and SG (resin weight/water weight). The pH was determined at ambient temperature [25.3 -27.0 °C]. Extraction Method Phys. Qty I.wombolu (IW) I. gabanensis (IG) water pH SG Tm [°C] Tc [°C] Td [°C] ∆Hm [J/g] pH SG Tm [°C] Tc [°C] Td [°C] ∆Hm [J/g] pH SG Tm [°C] Tc [°C] Td [°C] ∆Hm [J/g] 5.6 0.98 36.0 13.6 393 53.9 5.1 0.99 35.3 13.6 35.3 8.3 5.8 1.01 36.1 13.6 359 28.4 5.6 0.99 37.6 16.1 400 70.2 5.5 0.98 37.4 16.8 37.4 14.8 5.6 1.01 38.5 21.7 360 47.5 NaCl Na2S2O5 3.4. Thermogravimetric analysis (TGA) TGA analysis was used to determine thermal stability and decomposition temperature of the adhesives (extract). It is important to understand the thermal properties of these adhesives since heat is required during adhesion. Fig. 2 a-f shows the individual TGA and DTG curves. In accordance with the TGA curves, approximately 2–4% weight loss is observed with the extract at a temperature ranging between 0 and 120 °C, with the highest weight loss of 4 wt % reported for the water extract of IW. The changes are associated with the loss of water and volatiles. The maximum decomposition temperature of 405 and 365 °C, was recorded for the water and sodium metabisulphite extracts, respectively, whilst 305 and 325 °C was recorded for NaCl extract of IG and IW, respectively. This clearly indicates that the extraction method has a marked influence on the thermal property of the final extract. The different extraction methods provide extract with different composition. In the case of the water extraction method, three distinct weight loss of 8, 18 and 58% (IG) and 6, 20 and 54% (IW) were recorded at decomposition temperatures of 200, 285 and 405 °C, respectively as shown in Fig. 2 (a and b). The weight loss of 58 and 54% is associated with degradation of the starch macromolecules as indicated by Rudnik et al. [21]. On the other hand, the associated peaks at 200 and 285 °C are due to protein and fatty acid degradations [22,23], respectively. Plots of extracts from the sodium chloride extraction method display two major decomposition peaks as illustrated in Fig. 2 (c and d). Approximately, 32 and 36% weight loss is reported at 200–400 °C, with peak decomposition temperatures, at 303 and 326 °C, for IG and IW, respectively. This observation suggests that the starch molecules are hydrolysed in the presence of NaCl, resulting in a reduced thermal resistance as indicated by the low melting enthalpy (See Table 3).The hydrolysis of starch is confirmed by the appearance of a new peak at 540 °C, which is typical of the degradation temperature of glucose [24]. The 65 and 51 % wt. loss at 900 °C is associated with the decomposition of NaCl residue, indicating a higher amount of starch. This result correlates well with the result presented in Table 3. TGA and DTG curves of the sodium metabisulphite extracts are presented in Fig. 2 (e and f). As indicated earlier, the effect of starch hydrolysis is observed largely as indicated by the higher weight loss of glucose (19% IG and 11% IW) between 400 and 600 °C. The peak at 285/265 °C and 165/184 °C in IG/ IW are due to fatty acid and protein macromolecules respectively. may vary slightly due to differences in the geographical location of the seed origin and species. 3.2. Physical properties Table 2 shows the results of the physical properties of the extracted Irvingia adhesives. These properties have a major effect on the adhesive performance and determine the penetration and interaction of the adhesive with the substrate. The pH results show that Irvingia adhesives are mildly acidic with a pH range between 5.09 and 5.76. PH values, although most adhesives reported in literature are in the alkaline region. Dongre et al. [4] investigated the effect of pH on the mechanical properties of the wood adhesive and the three pH conditions tested were in the acidic medium. The authors concluded that acidic wood adhesives resulted in a high cross-linking reaction rate and enhances adhesive modification. The specific gravity values ranged from 0.99 to 1.01. All Irvingia extract obtained had lower pH and specific gravity compared to those reported from other sources such as wheat protein, lupin flour, soy protein and lignin-based. 3.3. Effect of extraction on the adhesive properties 3.3.1. Irvingia extract yield The extract yield varies with species and extraction methods as presented in Fig. 1. For I.wombolu species, Meta extraction gave the highest yield and water extraction resulted in the lowest yield, but for I.gabonensis species, NaCl extraction produced the highest yield while Meta extraction gave the least. The analysis of variance showed that there was a significant difference (p < 0.05) in the effect of species and extraction methods on the adhesive yield. 3.5. Differential scanning calorimetry (DSC) The extracted products were analysed with DSC to observe their crystallization and melting behaviours. All Irvingia extracts display a single melting and crystallization peak. The peaks are associated with the fatty acid component [25]. While the IW extracts display peaks crystallization temperatures at 13.6 °C, those of IG display peak crystallization temperature at 16.1, 16.8 and 21.7 °C for the water, NaCl and Na2S2O5, respectively, indicating fatty acid with a longer backbone in IG as indicated in the thermograms in Fig. 3a. Interestingly, a crystallization temperature change of 4.7 °C was observed between the NaCl and Na2S2O5 extracts of IG, but not in the case of IW. This difference Fig. 1. Extract yield from Irvingia species using different extraction methods. 4 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Table 3 Compositional analysis of Irvingia extracts; percentage composition of each component after the extraction process. Samples Extraction method Notation Carbohydrate (%) Protein (%) Fat (%) Ash (%) Moisture content (%) IW IG IW IG IW IG NaCl NaCl Water Water Metabisulphite Metabisulphite IWN IGN IWW IGW IWM IGM 23.8(0.47) 18.0(0.15) 43.5(0.16) 39.8(0.31) 17.1(0.46) 13.1(0.13) 6.95(0.16) 5.78(0.11) 4.77(0.28) 7.92(0.76) 5.30(0.75) 5.11(0.14) 17.5 20.2 40.2 41.5 34.8 36.7 47.8 (0.57) 52.4 (0.46) 6.1 (0.12) 6.6 (0.16) 31.4 (0.12) 33.7 (0.67) 4.2 (0.15) 3.6 (0.06) 5.4 (0.25) 4.1 (0.25) 11.4 (0.38) 11.4 (0.17) (2.4) (2.4) (5.6) (0.7) (3.0) (2.8) Fig. 2. TGA curves of Irvingia extracts a) Water extracted I.gabonensis, b) Water extracted I.wombolu, c) NaCl extracted I.gabonensis, d) NaCl extracted I.wombolu, e) Metabisulphite extracted I.gabonensis, and f) Metabisulphite extracted I.wombolu. presents the highest peak melting temperature. See Table 2 for more themo-physical differences. may be associated with differences in the fatty acid types since the same extraction method was applied. Presented in Fig. 3b, are the individual melting endotherm. In this case, no major differences in the peak melting temperature was observed, although NaS2O5 extract still 5 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Fig. 3. DSC thermograms of water extracted Irvingia wombolu extract. glycosidic (C–O–C) and peptide linkages of starch and protein backbones, respectively [28,29]. All these different functional groups were observed for all the extraction products. Interestingly, an additional absorption band was observed at 1102 cm−1 and 960 cm−1 for the extract obtained with the sodium metabisulphite method. These two peaks are due to the vibration of the inorganic molecules, which is indicative of starch content in the final extract [30]. In the case of sodium chloride extract, no additional peak was reported since sodium chloride is transparent at the reported IR region. This result correlates well with the result reported for the compositional text in Table 3. Spectra of the Irvingia adhesives extract are very similar irrespective of the extraction methods. Interestingly, the absorption bands of the protein molecules (amide I, amide II and the peptide) is significantly lower for the sodium metabisulphite extract when compared to the other counterparts. This could be associated with the loss of protein content during the extraction process as sodium metabisulphite, readily for sodium bisulfite in water and is notorious for increasing the solubility of protein [31]. This reflects well with the result depicted in Table 3 since the lowest protein content was reported for this extract. 3.6. Fourier transformed infrared spectroscopy (FTIR) analysis FTIR spectroscopy was used to identify specific functional groups pertaining to the different components present in the adhesive extracts. The analysis helps to identify any structural modification that may have occurred during the extraction processes. Presented in Fig. 4 (a and b) are the individual FTIR spectra for the extract of IG and IW, respectively. According to the FTIR findings, the observed broadband between 3014 and 3669 cm−1 is associated with N–H and O–H stretching vibration of the protein and starch backbones respectively. The vibration band at 2960 cm−1is due to CH3 stretches, while the vibration band at 2917 and 2851 cm−1are due to symmetric and asymmetric CH2 vibration of the fatty acid backbone. The C]O stretching vibration band at 1736 cm−1 is attributed to the carboxylic acid functionality of the fatty moiety. The absorption associated with amide I functionality of the protein backbone accounts for the C]O vibration at 1654, while the absorption at 1547 cm−1 is attributed to the N–H vibration band of amide II functionality [26,27]. The absorption band at 1464 is due to bending vibration of the CH2 group of the fatty acid backbone. In addition to all the different functional groups, the absorption band at 1179 and 1048 are associated to the C–O and C–N stretches of the Fig. 4. ATR-FTIR spectra of Irvingia adhesives. 6 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Fig. 5. Shear strength of the adhesives. Fig. 6. Tensile strength of the extracts. presence of hexamine as a hardener in its formulation, because Bondtite 345 as specified by the manufacturer cannot be used as a resin without a hardener. Panels bonded with extracts produced from water extraction had highest shear strength among panels bonded with Irvingiabased extracts, with values of 1.7 and 1.5 MPa for IWW and IGW, respectively. This can be attributed to the carbohydrate and ash content in the samples, as explained above. According to the results, panels made with IWW, IWN, IWM, IGW and IGN were not significantly different from each other but were significantly different from IGM. The lowest shear strength for both species were observed in the panels bonded with extracts produced from metabisulphite extraction: IWM 3.7. Shear strength of the extracts (plywood) The shear strength of the Irvingia samples and Bondtite 345 were determined according to ASTM D 906-64 test method. This standard measures adhesive shear strength in a plywood construction by tension loading (Instron Universal testing machine). Fig. 5 shows the mean values of shear strength for the panels made from Irvingia-based extracts and a commercial resin, Bondtite 345 for comparison. Panels produced with Bondtite 345 had the highest shear strength of 4.71 MPa. There was a significant difference between Bondtite 345 bonded panel and panels bonded with Irvingia-based extracts. This may be due to the 7 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Fig. 7. Elastic modulus of the extracts. they were from different species and also produced with different extraction methods. Analysis of the elastic modulus results revealed that there were significant differences among the panels produced. The effect of carbohydrate concentration on the adhesive strength performance was also determined. In all cases, it was discovered that an increased carbohydrate concentration resulted in enhanced strength properties. A high amount of simple sugars indicates better adhesion. This can be observed in the adhesives produced using water extraction process, which exhibited higher strength properties due to the higher carbohydrate quantity compared to adhesives extracted through other processes. This may also be explained by the absence of organic salt in the composition. had 0.86 and IGM had 0.55 MPa. This can also be correlated to the chemical composition as explained above, as samples with higher ash and lower carbohydrate contents had lower tensile strength and elastic modulus. According to EN 16352 standard, the minimum requirement for the characteristics shear strength for panel production for cross layer bond lines is 1 MPa [21]. Therefore panels bonded with Bondtite 345, IWM, IWN and IGW extracts met the minimum requirements. 3.8. Extracts strength performance (glass fibers) Tensile strength is one of the fundamental properties of polymers and natural fibre-reinforced composites. Tensile strength is the resistance of a material to breaking under tension. The elastic modulus is a measure of elasticity as ratio of stress to strain produced. As a complimentary method to determine the strength properties of all the Irvingia extracts, the tensile test of the extracts was conducted using glass fibres impregnated with the adhesives. Figs. 6 and 7 show the tensile strength and elastic modulus of the extracts produced using the three methods. A comparison was made with the tensile strength of the commercial tannin-based adhesive Bondtite 345. The tensile strength and elastic modulus of Bondtite 345 was 24.6 N/m-g and 56.7 GPa respectively which were 40.2% and 17.6% higher than the highest tensile strength and elastic modulus respectively, of the Irvingia-based extracts. There were significant differences in the tensile strength and elastic modulus results of panels bonded with Bondtite 345 and Irvingia extracts. Extracts produced with water extraction had the highest tensile strength and elastic modulus for both species, with values of 14.7 and 13.6 KN/m-g and 46.7 and 44.1 GPA, respectively. This can be attributed to the carbohydrate and ash content in the samples. It was also observed that the adhesives produced from metabisulphite extraction exhibited the lowest tensile strength and elastic modulus for both species (7.4 and 5.0 GPA respectively, for the tensile strength; 31.3 and 17.9 KN/m-g respectively, for the elastic modulus). This can also be correlated to the chemical composition, as samples with higher ash and lower carbohydrate contents had lower tensile strength and elastic modulus. The tensile strength results showed that there was no significant difference among panels bonded with IWM and IGN, although 4. Conclusions This study investigated the possibility of developing wood adhesives from Irvingia gabonensis and Irvingia wombolu kernels extract. The results show that all Irvingia extract is mildly acidic. The chemical characterization showed that extract produced from water extraction had higher carbohydrate (43.5 and 39.8%), and fat content (40.2 and 41.5%) compared to the extract obtained from other extraction processes. Based on the strength comparison with a commercially available tannin-based resin Bondtite 345, water extracted Irvingia adhesives, though significantly lower than Bondtite 345, met the minimum requirement for the characteristics shear strength for panel production for cross layer bond lines which is 1 MPa according to EN 16352 standard. This is imperative in view of developing alternative formaldehyde-free adhesives for wood-based applications. Therefore, the study concluded that extracts developed from Irvingia kernel can be used for wood composites manufacturing with the added advantage of environmental sustainability. In order to improve the strength properties of the adhesives, non-toxic cross linkers and hardeners are recommended for further study. Conflicts of interest The authors declare that there is no conflict of interest. 8 International Journal of Adhesion and Adhesives 95 (2019) 102423 A.O. Alawode, et al. Acknowledgements engines. 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