{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,13]],"date-time":"2026-10-13T00:22:25Z","timestamp":1791850945270,"version":"4.3.4"},"reference-count":23,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2026,5,12]],"date-time":"2026-05-12T00:00:00Z","timestamp":1778544000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"STM Defense Technologies Engineering and Trade Inc."}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Polymers"],"abstract":"<jats:p>In materials science, the increasing use of lightweight and multi-material structures has made improving the interfacial bonding characteristics of polymer-based adhesive systems increasingly important. Accordingly, chemical surface activation and interfacial engineering strategies have attracted considerable attention for enhancing polymer\u2013fiber compatibility and adhesion performance. However, the combined effects of fiber content, surface treatment, and orientation on adhesion behavior remain insufficiently understood. In the present study, natural fibers obtained from the rachis part of the palm tree were chemically modified and incorporated into an epoxy adhesive matrix to investigate the effect of surface functionalization on polymer\u2013fiber interfacial adhesion. In the first stage, the effects of fiber ratios (5\u201320 wt%) and chemical surface treatments (methanol cleaning and methanol +2\u20136% HNO3) on adhesion behavior were evaluated. Tensile tests showed that specimens treated with methanol cleaning followed by 4% HNO3 oxidation and containing 10 wt% fiber exhibited an approximately 48% increase in failure load compared to neat joints. In the second stage, the influence of fiber orientation (0\u201390\u00b0) was examined using the optimized parameters. The results indicate that interfacial load-transfer capability increased as the fiber orientation approached perpendicular alignment, reaching maximum performance at 90\u00b0. Based on SEM observations, nitric acid treatment was found to increase the surface roughness of the fibers and strengthen the polymer\u2013fiber interfacial bond. FTIR, XPS and contact angle measurements suggested the development of oxygen-containing surface functionalities and improved wettability, consistent with enhanced interfacial adhesion. These findings demonstrate that appropriate chemical surface treatment, fiber content, and orientation can effectively enhance the interfacial adhesion and bonding efficiency of epoxy-based adhesive systems, providing practical guidance for the design of high-performance bonded structures.<\/jats:p>","DOI":"10.3390\/polym18101180","type":"journal-article","created":{"date-parts":[[2026,5,12]],"date-time":"2026-05-12T12:33:37Z","timestamp":1778589217000},"page":"1180","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Enhancing Interfacial Strength of Epoxy Adhesive Joints Using Chemically Surface-Modified Palm Fibers: Influence of Fiber Loading, Surface Chemistry and Orientation"],"prefix":"10.3390","volume":"18","author":[{"given":"Iclal Avinc","family":"Akpinar","sequence":"first","affiliation":[{"name":"Office of Occupational Health and Safety, Erzurum Technical University, 25050 Erzurum, Turkey"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Simay","family":"Bayramoglu","sequence":"additional","affiliation":[{"name":"STM Defense Technologies Engineering and Trade Inc., 06530 Ankara, Turkey"},{"name":"Department of Mechanical Engineering, Erzurum Technical University, 25050 Erzurum, Turkey"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3247-991X","authenticated-orcid":false,"given":"Salih","family":"Akpinar","sequence":"additional","affiliation":[{"name":"Department of Mechanical Engineering, Erzurum Technical University, 25050 Erzurum, Turkey"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,5,12]]},"reference":[{"key":"ref_1","unstructured":"Troughton, M.J. (2009). Chapter 17-Adhesive Bonding. Handbook of Plastics Joining, William Andrew Inc.. [2nd ed.]."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"472","DOI":"10.1016\/j.compositesb.2019.02.007","article-title":"Investigation of the effect of use of Nano-Al2O3, Nano-TiO2 and Nano-SiO2 powders on strength of single lap joints bonded with epoxy adhesive","volume":"166","author":"Adin","year":"2019","journal-title":"Compos. Part B Eng."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"896","DOI":"10.1080\/00218464.2016.1187069","article-title":"Mechanical properties of adhesively single lap bonded joints reinforced with multi-walled carbon nanotubes and silica nanoparticles","volume":"93","author":"Ayatollahi","year":"2017","journal-title":"J. Adhes."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"124","DOI":"10.1080\/00218464.2016.1255606","article-title":"An Experimental Study on Composite Adhesives Reinforced with Different Types of Organo-Clays","volume":"94","author":"Gultekin","year":"2018","journal-title":"J. Adhes."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"211","DOI":"10.1007\/s40430-021-02920-x","article-title":"Graphene nanoparticle effect on flexural and shear behaviors of adhesively bonded single lap joints of GFRP composites","volume":"43","author":"Ahmed","year":"2021","journal-title":"J. Braz. Soc. Mech. Sci. Eng."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"469","DOI":"10.1080\/01694243.2024.2411305","article-title":"Nanoparticle integration in adhesive and hybrid single-lap joints: Effect on strength and fatigue life underenvironmental aging","volume":"39","author":"Karimi","year":"2025","journal-title":"J. Adhes. Sci. Technol."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"104199","DOI":"10.1016\/j.ijadhadh.2025.104199","article-title":"Synergistic effects of hybrid h-BN and HNT particles on mechanical performance and fracture characterization of epoxy adhesive","volume":"145","year":"2026","journal-title":"Int. J. Adhes. Adhes."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"135664","DOI":"10.1016\/j.matlet.2023.135664","article-title":"Enhancing the strength of adhesively bonded single-lap composite joints using fiber fabrics","volume":"357","author":"Thirunavukarasu","year":"2024","journal-title":"Mater. Lett."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"802","DOI":"10.1515\/mt-2022-0284","article-title":"Influence of carbon and glass fiber reinforced composite adhesive on the strength of adhesively bonded joints","volume":"65","author":"Akpinar","year":"2023","journal-title":"Mater. Test."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"103954","DOI":"10.1016\/j.ijadhadh.2025.103954","article-title":"The effect of different chemical surface-modified carbon fiber structure and ratio on bond strength in adhesive joints","volume":"138","author":"Akpinar","year":"2025","journal-title":"Int. J. Adhes. Adhes."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"015044","DOI":"10.1063\/5.0183153","article-title":"The Effects of Carbon Fiber Surface Treatment by Oxidation Process for Enhanced Mechanical Properties of Carbon Fiber\/Epoxy Composites for Biomedical Application","volume":"14","author":"Roseno","year":"2024","journal-title":"AIP Adv."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"104020","DOI":"10.1016\/j.ijadhadh.2025.104020","article-title":"A study on the effects of electrochemical oxidation and silane modification applied to fibers in short carbon fiber-reinforced structural adhesives and adhesive joints","volume":"140","year":"2025","journal-title":"Int. J. Adhes. Adhes."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Ilyin, S.O., and Kotomin, S.V. (2023). Effect of Nanoparticles and Their Anisometry on Adhesion and Strength in Hybrid Carbon-Fiber-Reinforced Epoxy Nanocomposites. J. Compos. Sci., 7.","DOI":"10.3390\/jcs7040147"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"6199","DOI":"10.1016\/j.apsusc.2010.03.141","article-title":"Interfacial and Mechanical Properties of Carbon Fibers Modified by Electrochemical Oxidation in (NH4HCO3)\/(NH4)2C2O4\u00b7H2O Aqueous Compound Solution","volume":"256","author":"Liu","year":"2010","journal-title":"Appl. Surf. Sci."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Avinc Akpinar, I., Ko\u00e7yi\u011fit, \u00d6.F., and Atasoy, S. (2025). Effect of Oxidation and Silane Modifications Applied to the Bonded Material and Fibers in Carbon-Fiber-Reinforced Composite Adhesive Joints. Polymers, 17.","DOI":"10.3390\/polym17141893"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"546","DOI":"10.1016\/j.apsusc.2019.04.248","article-title":"A Two-Step Carbon Fiber Surface Treatment and Its Effect on the Interfacial Properties of CF\/EP Composites: The Electrochemical Oxidation Followed by Grafting of Silane Coupling Agent","volume":"486","author":"Wen","year":"2019","journal-title":"Appl. Surf. Sci."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Guo, G., Yang, Z., Cai, M., Wang, S., and Jiang, L. (2024). Surface Activation and Characterization of Basalt Fiber by Plasma Treatment and Its Interfacial Adhesion with Epoxy. Polymers, 16.","DOI":"10.3390\/polym16223181"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"221","DOI":"10.26701\/ems.1702654","article-title":"Investigation of electrochemical oxidation process time and glycidoxypropyl trimethoxy silane coating in glass fiber reinforced composite adhesive bonded joints","volume":"9","year":"2025","journal-title":"Eur. Mech. Sci."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"107874","DOI":"10.1016\/j.compositesb.2020.107874","article-title":"Strength improvement of adhesively bonded single lap joints with date palm fibers: Effect of type, size, treatment method and density of fibers","volume":"188","author":"Delzendehrooy","year":"2020","journal-title":"Compos. Part B Eng."},{"key":"ref_20","unstructured":"(1992). Adhesives-Designation of Main Failure Patterns (Standard No. ISO 10365)."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"283","DOI":"10.1007\/s10019-003-0265-z","article-title":"The mechanism of adhesion and printability of plasma processed PET films","volume":"7","author":"Deshmukh","year":"2003","journal-title":"Mater. Res. Innov."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"95","DOI":"10.1016\/j.ijadhadh.2012.04.007","article-title":"Adhesion phenomena in bonded joints","volume":"38","author":"Baldan","year":"2012","journal-title":"Int. J. Adhes. Adhes."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"1169","DOI":"10.1002\/1097-4628(20010214)79:7<1169::AID-APP30>3.0.CO;2-Q","article-title":"Scanning electron microscopy study of chemically modified coir fibers","volume":"79","author":"Rout","year":"2001","journal-title":"J. Appl. Polym. Sci."}],"container-title":["Polymers"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2073-4360\/18\/10\/1180\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,5,12]],"date-time":"2026-05-12T13:05:01Z","timestamp":1778591101000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2073-4360\/18\/10\/1180"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,5,12]]},"references-count":23,"journal-issue":{"issue":"10","published-online":{"date-parts":[[2026,5]]}},"alternative-id":["polym18101180"],"URL":"https:\/\/doi.org\/10.3390\/polym18101180","relation":{},"ISSN":["2073-4360"],"issn-type":[{"value":"2073-4360","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,5,12]]}}}