International Journal of Adhesion & Adhesives 24 (2004) 1–7 . The use of silane reagents as primers to enhance the adhesion of chromium tanned heavy-duty leather (Salz leather) C. Fotea, C. D’Silva* Department of Chemistry and Materials, The Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester M1 5GD, UK Accepted 20 June 2002 Abstract This paper investigates the use of silane reagents in a non-traditional role as a primer for modifying the chemical characteristics of chromium (Cr) tanned heavy-duty leather (Salz leather). The silanes investigated were 3-aminopropyltriethoxysilane (APES) and 3- (2-aminoethylamino)propyltriethoxysilane (AEAPES), applied as a polymer solution to leather surfaces treated with an isocyanate monomer to create an interpenetrating polymer network (IPN), prior to bonding with the polyurethane adhesive. The assembled strengths of APES and AEAPES samples were measured using the T-peel test against 2-hydroxyethyl methacrylate samples and conventionally treated samples (abrasion, chloroform and surfactant washing), under dry and wet testing conditions. The physical structure of leather surfaces was investigated using scanning electron microscopy (SEM) and the chemical composition by electron dispersive X-ray spectroscopy (EDX) for Cr, Si and Al impurities. The results indicate that APES and AEAPES treatments significantly enhance surface adhesion with respect to abrasion alone and in the latter case by 179% (dry) and 52% under wet conditions. The adhesive joints tested showed cohesive failure indicating that the improvements in adhesion were due to the generation of a consistent network across the interface due to increased physical adsorption of the silane polymer with the leather surface and interaction with the polyurethane adhesive via van der Waals forces and/or hydrogen bonding. r 2003 Elsevier Ltd. All rights reserved. Keywords: A. Coupling agents; B. Surface treatment; C. Destructive testing; Cr tanned leather 1. Introduction Silanes have proved effective coupling reagents with hydroxyl (OH) functionalised surfaces such as glass and silica [1–2] and the oxides of aluminium and silver [3]. They have also been investigated in bonding to the non-oxide surfaces such as gold being activated by the oxidative introduction of OH functional groups [4]. Leather is a natural occurring material containing collagen as its major protein constituent. Chrome tanning is now the principal tanning process and in almost all cases is supplemented by other methods. Chromium tanned leather is lightly blue in colour with the fibres stabilized against microbial attack but does not have the feel of leather. If the leather were dried at this point only a stiff unattractive product will result. Its characteristic properties result after re-tanning, colour- ing, fat-liqouring and finishing [5]. The above treatments irreversibly alter leather’s chemistry (surface and bulk), with the groups on the peptide backbone being chemically altered by the tanning process, making covalent adhesion a problem. One of the primary objectives of this study was to improve the adhesion of leather by an understanding of its nature and tanning treatments. Leather is a complex natural material whose properties of thermal resistance, durability to cracking and flexing is a result of the tanning process. Chrome tanned leather is a stable material the result of attachments of chromium salts via unipoint or multipoint fixation with the carboxyl residues of glutamic and aspartic acid groups of collagen [1–3]. The chemical limitations resulting from the tanning process prevent the use of silane coupling agents due to the lack of reactive OH groups on the leather surface [6]. In an attempt to improve molecular contact between adhesive and protein we pursued a classical approach to ARTICLE IN PRESS *Corresponding author. E-mail address: c.dsilva@mmu.ac.uk (C. D’Silva). URL: http://www.chem-mats.mmu.ac.uk/stafflist.html. 0143-7496/02/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. PII:S0143-7496(02)00023-4