International Journal of Adhesion & Adhesives 24 (2004) 479–484 A comparative study on the use of photoreagents for the enhancement of adhesion on heavy-duty leather (Salz leather) Catalin Fotea 1 , Claudius D’Silva* Department of Chemistry and Materials, The Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester M1 5GD, UK Accepted 14 August 2003 Abstract This paper aims to expand on work presented by us in previous publications regarding the enhancement of adhesion on leather substrates with photoreagent type primers. Two new reagents were synthesized: 3,5-dihydroxyphenyl-4-azidobenzoate (7) and 4,4 0 - ethylene-1,1 0 -diphenyl azide (8) and used as an intimate mixture with commercial polyurethane adhesive (Solibond PU39), to enhance the adhesion of Salz leather surfaces. The molecular design of the primer molecule was intended to provide 2- and 3-point ﬁxation between the substrate and adhesive. The strength of adhesive joints do not seem to be highly dependent on the molecular structure of the primers investigated in respect to the type of OH groups present (aliphatic or aromatic), both confer high adhesive strengths under dry and wet testing conditions. Cyanuric chloride (CNCl) 3 a commercially used primer when tested under the same conditions for comparison purposes, gave good results but with a high standard deviation. Overall, the azide primers generate good, reliable joint strengths under extreme testing conditions and we believe they present negligible technological problems to industrial scale implementation. r 2004 Elsevier Ltd. All rights reserved. Keywords: A. Coupling agents; B. Surface treatment; C. Destructive testing, Leather 1. Introduction Leather is a natural material whose stabilization occurs after tanning, re-tanning, colouring, fat-liqouring and ﬁnishing. The tanning process produces salt bridges between chromium (Cr 2+ ) ions and the carboxyl (COO  ) groups of collagen protein chains and together with the other stages (of tanning) irremediably affect the quality of the leather surface such as colour, chemical resistance, water repellence and resistance against bacterial attack [1]. Aesthetic requirements (fashion trends) and practical considerations (wear and water resistance) can dictate further applications of coatings and ﬁnishes, therefore producing a leather product with a low surface energy with limited bonding capacity, makes adhesion at the surface a problem. To improve adhesion with leather surfaces, we recently reported a method of bonding based on increasing the degree of molecular contact between the adhesive and the leather surface by using silane primers [2]. This approach created an interpenetrating polymer network (IPN) by polymerization of monomers of the structure X 3 –Si–R–Y to form a cross-linked polysilox- ane network which chemisorbed onto the leather surface via H-bonds, van der Waals and hydrophobic interac- tions [2]. Although the joints formed using an IPN showed increased strength in peel testing, these bonds weakened under wet testing conditions. Therefore a method for covalent bond formation between the adhesive and leather was sought to improve bonding at the surface under extreme conditions. Hydroxyl functionalised aryl azides on photoactivation can insert into the leather surface introducing hydroxyl groups to facilitate adhesion via physical bonds (H-bonds) and covalent bond formation with the adhesive (see Fig. 1). Aryl azides investigated were only found to function effectively as a primer if the hydroxyl groups were not directly conjugated to the azide ring and the aryl group ARTICLE IN PRESS *Corresponding author. E-mail addresses: c.fotea@surrey.ac.uk (C. Fotea), c.dsilva@mmu.ac.uk (C. D’Silva). URL: http://www.chem-mats.mmu.ac.uk/stafﬂist.html. 1 Polymer Research Centre, University of Surrey, Guildford. 0143-7496/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.08.003