Alawode et al. J Wood Sci (2020) 66:12 https://doi.org/10.1186/s10086-020-01860-9 Journal of Wood Science ORIGINAL ARTICLE Open Access Evaluation of Irvingia kernels extract as biobased wood adhesive A. O. Alawode1, P. S. Eselem‑Bungu2, S. O. Amiandamhen1, M. Meincken1 and L. Tyhoda1* Abstract Irvingia tree species have been earmarked for domestication in many countries due to their potential as raw materials for various applications, which include biodiesel, cosmetics, perfume, soap, etc. Presently, there is no information on the utilization of kernel seed extract as a potential source of green wood adhesive. This study is focused on investigat‑ ing the properties of adhesives produced from kernel seeds of two Irvingia wood species i.e. Irvingia gabonensis (IG) and Irvingia wombolu (IW), as well as investigating the improved properties derived from the efect of modifcation using a few selected modifying agents including glutaraldehyde, glyoxal, epichlorohydrin (EPI) and an acid/base type process modifcation. Polyethylene (PE) was used along with the glutaraldehyde, glyoxal and epichlorohydrin modifers in the modifcation process. Fourier transform infrared spectroscopy (FTIR), diferential scanning calorimetry (DSC), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were conducted to study the efect of modifcation on adhesive properties. The glycosidic carbon of the unmodifed extracts and that of the EPI modi‑ fed sample were not sensitive to chain conformations. Principal components (PC) 1 and 2 explained 85.19 and 9.54%, respectively, of the total variability in FTIR spectra among the modifed and unmodifed adhesives. The unmodifed samples for IG and IW exhibited one peak with crystallization temperatures of 18.7 and 14.4 °C, respectively, indicating only one component exhibits some low degree crystallinity. The adhesive properties of the modifed extracts were tested on wood veneers according to ASTM standard. The shear strength of the modifed adhesives ranged from 1.5 to 3.93 MPa and 1.7 to 4.05 MPa for IG and IW, respectively. The modifed samples containing PE showed marked improvement in the shear strength. The highest values were about 63% higher than the shear strength of unmodifed samples with least shear strength. The results indicated that the modifcation of Irvingia‑based adhesives had a great contribution to their performance as natural wood adhesives. Keywords: Irvingia gabonensis, Irvingia kernel extracts, Irvingia wombolu, Natural wood adhesive, Wood adhesive modifcation, Wood composites Introduction harmful to humans and the environment [2]. Te role of It is well established that the global demand for wood com- adhesives cannot be overemphasized for the efcient uti- posites has increased tremendously over the last two dec- lization of wood and other lignocellulosic resources in the ades [1]. With increasing awareness of the impact of the manufacturing of wood products [3]. Products from wood hazardous levels of volatile organic compounds’ emission, composite industries such as particleboard, oriented strand especially formaldehyde from wood composite products board, plywood and laminated timber are essential, and bonded with formaldehyde-based adhesives, it is becom- they are being used for the production of wooden trusses, ing imperative to fnd viable alternatives, which are less kitchen cabinets, wardrobes, bookshelves, etc. Formalde- hyde-based adhesives, which include urea formaldehyde, *Correspondence: [email protected] phenol formaldehyde, melamine formaldehyde and res- 1 Department of Forest and Wood Science, Stellenbosch University, orcinol are the most commonly used adhesives because of Stellenbosch, South Africa their excellent performance, which includes good adhesion Full list of author information is available at the end of the article © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea‑ tivecommons.org/licenses/by/4.0/. Alawode et al. J Wood Sci (2020) 66:12 Page 2 of 13 to diferent lignocellulosic substrates, high water resist- Many studies have been carried out on the modifcation ance (except for urea formaldehyde), low initial viscosity, of numerous potential wood adhesives developed from resistance to environmental degradation, and excellent diferent natural sources such as tannins, lignin, citric thermal stability [4]. Tese formaldehyde-based adhesives acid, carbohydrate, unsaturated oils, liquefed wood, rice are produced from petroleum and natural gas which are bran, and soybean proteins [5, 14–17]. Imam et al. [18] non-renewable resources [5]. Furthermore, the emission successfully developed wood adhesive by blending starch of hazardous compounds from composite products manu- polymers with polyvinyl alcohol (PVOH), hexamine, cit- factured with these adhesives is of great concern because of ric acid and latex. In another report, oxidized starch their efect on health-related issues. As a consequence, they adhesives were modifed to improve their performance were categorised among the carcinogenic materials by the by incorporating sodium dodecyl sulfate, urea, silane World Health Organisation (WHO) in 2004 [5]. Terefore, couplings and olefns. Te authors reported that bond- recent studies have focused on the development of wood ing strength and water resistance of the adhesive were adhesives from renewable sources, which are environmen- signifcantly improved [19]. Umemura et al. [20] investi- tally safe and able to compete with conventional adhesives gated the properties of boards manufactured from citric with regards to performance properties [6]. acid and sucrose adhesive, and reported excellent physical Irvingia species belong to the family Irvingiaceae. Te properties. Particleboards have also been manufactured species include I. gabonensis (IG), I. wombolu (IW), from sweet sorghum bagasse and citric acid adhesive [21]. I. smithii (IS), I. grandifora (IGF), I. robur (IR) and I. Polyethylene (PE) is well known for its high mechani- malayana (IM). I. malayana is usually found in South- cal and impact strength and is widely used in day-to-day east Asia [7]. IG and IW are the most commonly found life applications such as insulation cable and construction in Africa. Tese two species are very similar and difcult materials. PE has chemically inert surfaces with low sur- to distinguish. Te distinction between the two species is face energy. Tis makes them difcult to bond with other that IG has sweet edible fruit pulp, while IW has bitter, substrates. Tese materials are highly chemically robust inedible fruit pulp. In West Africa, they are commonly and exhibit very poor adhesive properties on their own. In known as Dika nut, bush mango or wild mango. Irvingia this study, we provided information on the modifcation of prefers undisturbed low land tropical forest as their habi- IG and IW kernel extract as an adhesive from a renewable tat and the species is well adapted to Ultisol soils in high source (wood) using diferent hardeners as crosslinking rainfall areas. Tese forest fruit trees are widely available agents as well as blending these modifed extracts with PE in western and central African countries. Tey are now to produce wood adhesives of greater mechanical strength. being domesticated in many countries because of their For the frst time, the efective use of modifed kernel seed numerous potential uses [8]. Irvingia trees usually reach extracts/PE blend as potential wood binder was demon- maturity and begin fowering at 10–15 years of age. How- strated. Te extract was modifed chemically with some ever, much earlier fruiting has been reported. Ladipo [9] selected hardeners, which was followed by blending of the described trees that produced fruit at age six. modifed extract with PE. Te chemical and thermal prop- Several studies have shown that various products, such erties of the product were analysed using fourier transform as fuel (bio-diesel), cooking oil, cosmetics, margarine, infrared spectroscopy (FTIR), solid-state nuclear magnetic season cubes, confectionaries and soaps, can be produced resonance (NMR), thermogravimetric analyser (TGA) and from the oil (fat content) from Irvingia kernels [10, 11]. diferential scanning calorimetry (DSC). Te efect of gly- However, limited information is available on the potential oxal/PE, epichlorohydrin/PE, glutaraldehyde/PE, acid/base application of the residues (mucilage). Some researchers modifcation on composition and functional groups of have used mucilage to formulate drug binders in medi- Irvingia wood adhesives was also investigated. cal formulations. Tis mucilage is starch based. Tere was also an attempt by Ikechukwu and Salome [12] to Materials and methods evaluate the possibility of using gum from IW kernels in IG and IW kernels were purchased from a local market