adhesion enhancement of chromium tanned heavy

7/31/2019 Adhesion Enhancement of Chromium Tanned Heavy

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Adhesionenhancement of

chromiumtannedheavy-dutyleather

(Salzleather) underextremeconditions usingphotoreagents as surfaceprimers

Catalin Foteaa, , , Claudius DSilvab, , a LBSA, School of Pharmacy, University of Nottingham b Department of Chemistry and Materials, The Manchester Metropolitan University,

John Dalton Building, Chester Street, Manchester M1 5GD, UK

Accepted 18 December 2002. Available online 23 March 2005. http://dx.doi.org/10.1016/j.ijadhadh.2002.12.001,How to Cite or Link Using DOI Cited by in Scopus (2) Permissions & Reprints

Abstract

The work described investigates the development of photoreagents as surfaceprimers for the

enhancement of adhesion with heavy-dutyleathersurfaces (Salz). Several reagents were

investigated which included 4-azidophenol (1), 3-azidophenol (2), 4-(2,2-iminodiethanol)-3-

nitrophenylazide (4) and 4-azido-(2,2-iminodiethanol)benzamide (6) and used as an intimate

mixture with commercial polyurethane adhesive (Solibond PU39) to enhance the adhesion of

Salzleathersurfaces. The photoreagents upon UV irradiation covalently attach to the

leathersurface enriching it with new functionalities to provide sites for bonding with the

adhesive layer. The adhesion strength of photo primer treated leather samples were tested

using the T-peel test against abraded leathersurfacesunder dry and wet conditions.

Compounds 4 and 6 on T-peel testing were found to enhance adhesion strength by 99% under

dry conditions and 163% and 157%, respectively, under wet conditions. The observed failure

type was cohesive within the substrate when photoreagents were used, suggesting covalent

bonds formation at the interface in addition to the usual van der Waals forces and hydrogen

bonds affecting adhesion.

Keywords

Coupling agents; Surface treatment; Destructive testing; Chromiumtannedleather
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1. Introduction

Leather is a naturally occurring material containing collagen as its major protein constituent.

The characteristic properties of leather result after re-tanning, colouring, fat-liqouring and

finishing[1]. The above treatments irremediably alter leather's chemistry (surface and bulk),

with many groups on the peptide backbone being appreciably affected by the tanning process,

making adhesion a problem. In an attempt to improve adhesion, we recently reported a

method of bonding based on increasing the degree of molecular contact between adhesive

and the leathersurface by enhancing adsorption[2]. In this approach an IPN (Interpenetrating

Polymer Network) was created by polymerisation of monomers of the structure X3SiRY

to form a cross-linked polysiloxane network which chemisorbed onto the leathersurface via

H-bonds or van der Waals interactions whilst the Y group reacted with an isocyanate reagent

to increase the overall hydrophobicity and adherence of the resultant IPN[2].

Although the joints formed using an IPN based on 3-(2-aminoethylamino)

propyltriethoxysilane (AEAPES) showed an increased total joint strength, of 96.9 N under

dry and 51.2 N under wet conditions[2], further improvements in the joint under wet

conditions were considered necessary to meet the stringent requirements for use of this

technology to heavy-dutyleather boots. Chemisorbed bonds weaken under wet conditions

therefore an ideal solution would involve the formation of a covalent bond between the

adhesive and leather. The tanning process together with the waterproof treatments applied on

the skin side make it chemically inert. Therefore surface enrichment with active chemical

groups was sought to enhance adhesion and to produce strong, durable joint assemblies.

1.1. Surfaceprimers

Aryl azides belong to a group of compounds capable of generating reactive electron deficient

species (nitrenes) on irradiation with a UV light. Nitrenes are reagents of electrophilic

character sharing chemical similarities with other electrophilic nitrogen reagent[3]and[4],

but posseses four non-bonded electrons which can accept electrons from electron rich species

[5], to form covalent bonds (seeFig. 1).
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Fig. 1. Decomposition pathways of aryl azides with the amino acid residues of

proteins. Inset: 1, 3-Dipolar nature of azides.

The thermal stability of azides are critically dependent on the nitrogen substituent, whilst aryl

azides have high thermostability, alkyl or acyl structures are unstable and can decompose

violently on exposure to heat, mechanical shocks or aggressive chemical reagents.

Aryl azides have been used to investigate the binding site of macromolecules and to bond

polymers to glass[6]. In the latter approach a photoreactive siloxane primer of the structure

N3RSi(OR)3 was used to react with functional groups (OH) present on the glass surface

via the siloxane function. On photoactivation of the aryl azide group a nitrene species is

generated which bonds covalently to the backbone of the polymer film. In the approach

proposed to functionalise leather we considered the preparation of a variety of hydroxyl

functionalised aryl azides which on photoactivation could covalently bond to the

leathersurface and introduce hydroxyl groups to facilitate covalent bond formation with the

adhesive (seeFig. 2). In this paper we report on the synthesis of two hydroxyl and two new

dihydroxyl photoprimers (seeFig. 3) and their evaluation in the enhancement of adhesion to

leathersurfacesunder dry and wet conditions.

Fig. 2. Schematic representation for the use of aryl azides as coupling agents.
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Fig. 3. Structures of hydroxy and dihydroxy aryl azides evaluated as surfaceprimers

for leather.

2. Experimentation

2.1. Chemicals

Phloroglucinol.2H2O, 4-hydroxylaniline and 2,4-dinitrobenzene were from Avocado, UK;

diethanolamine (2,2-iminodiethanol), sodium azide and 4-aminobenzoic acid were from

Aldrich Chemical Co. Ltd. (UK). Preparative silica gel plates Analtech (1212 cm; 2 mm)

were obtained from Alltech. Chem. Co. The adhesive used was Solibond PU39 (Crispin-

Adhesives Ltd) an opaque one component polyurethane polymer solution which hardened on

solvent release. Manufacturer's specification: d 0.860.88 g cm3

, drying time at 20 C: 10 to

30 min. Solvents: acetone, EtOAc. Melting points were uncorrected. ATR and FT-IR spectra

were analysed with Spectrum Lite software (PerkinElmer).1H and

13C NMR spectra were

recorded at 270.05 and 67.80 MHz, respectively, using TMS as an internal standard. MS

were recorded at 70 eV and ESIMS at 3.5 or 3.0 kV, 70 C, in a H2O/MeOH (1:1) matrix.

2.2. Aryl azide synthesis

2.2.1. 4-Azidophenol (1)

To a stirred ice cooled solution of 4-hydroxylaniline (1.0 g; 0.01 M) dissolved in a (1:1)

mixture of AcOH/ H2O (50 cm3) was added NaNO2 (2.1 g; 0.03 M) slowly. After 5 min were

added sodium azide (2 g; 0.03 M) and diethyl ether (50 cm3). The diethyl ether layer was

separated, washed with water, dried on MgSO4, evaporated and the resultant black oil

triturated with hexane (2x) and decanted. The residue was applied to a prep plate (1212 cm)eluted with CHCl3 and the upper band isolated as a reddish black oil on evaporation in vacuo

(0.63 g: 65%). H1

NMR (CDCl3) 6.8 (s, 4x ArH); C13

NMR (CDCl3) 116, 120, 132 (COH),

152.5 (CN3); HREIMS (high resolution electroionisation mass spec) calc for C6H5N3O (M+)

135.0433, found 135.0433; IR () 2112 cm1

(N3), 3330 cm1

(OH); UV (THF) max 258 (max

1123), 290 nm (max 277 m2

mol1

).

2.2.2. 3-Azidophenol (2)

3-Azidophenol (2) was obtained using the same procedure as for (1) but with 3-

hydroxylaniline as the starting material. After 20 min of reaction the solution was filtered to

remove black insoluble polymeric residues, extracted with CHCl3, separated, washed withwater, dried MgSO4, and evaporated in vacuo to give a yellow/brown oil. The oil was applied
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to a prep plate (1212 cm) eluted with CHCl3 and the product on evaporation in vacuo

isolated as a dark brown oil which on standing solidified to give a brown semi-solid (0.14 g;

11.3%); H1

NMR (CDCl3) 7.15 (t, , ArH), 6.6 (d, , 2x ArH), 6.5 (s,

ArH); C13

NMR (CDCl3) 106, 111, 112, 113, 130 (COH), 157 (CN3); EIMS m/z

135.1(M+), 29), 107.0 (M

+N2), 100); HREIMS calc for C6H5N3O (M

+) 135.0433, found

135.0432; IR (thin film) () 2115 cm1

(N3), 3300 cm1

(OH); UV (THF) max 210 (max2036), 250 (max 719), 282 (max 451), 324 (max 219), 380 nm (max 207 m

2mol1) (seeFig. 4).

Fig. 4. Schematic for the synthesis of 4-azidophenol (1) and 3-azidophenol (2).

2.2.3. 4-Fluoro-3-nitrophenylazide (3)

To a stirred ice-cold solution of 4-fluoro-3-nitroaniline (3.9 g; 0.025 M) dissolved in 1.2 M

HCl (50 cm3; 0.06 M) was added dropwise a solution (20 cm

3) of sodium nitrite (2.07 g;

0.03 M). After 5 min sodium azide (3.25 g; 0.05 M) dissolved in water (30 cm3) was added

(seeFig. 5). The reaction mixture was stirred for 0.5 h at RT and then extracted with EtOAc,

washed in water, dried (MgSO4) and evaporated to yield 3 (2 g; 75%), re-crystallized

(EtOAc); mp 5455 C (lit. 5152 C[7]); IR () 2132 cm1

(N3). H1

NMR (D6MSO) 8.3 (dd,

Ar-6H), 7.8 (s, Ar-2H), 6.95 (d, Ar-5H).

Fig. 5. Schematic for the synthesis of 4-(2,2-iminodiethanol)-3-nitrophenylazide (4).

2.2.4. 4-(2,2-Iminodiethanol)-3-nitrophenylazide (4)

A mixture of 4-fluoro-3-nitrophenylazide (3) (1.18 g; 0.0065 M), diethanolamine (2,2-

iminodiethanol) (0.68 g; 0.0065 M) and EtOAc (20 cm3

) was heated overnight on an oil bathwith stirring, at 50 C. The reaction mixture was washed with water (320 cm

3), dried

(MgSO4) and evaporated in vacuo to give the product as a red solid, re-crystallized from

EtOAc to give 3 (38%). H1NMR (CDCl3) 7.41 (d, J=8.8, ArH), 7.35 (d, J=2.6, ArH), 7.17

(dd, J=2.6, 8.8, ArH), 3.7 (t, J=5.1, 2xCH2OH), 3.3 (t, J=5.1, 2x(CH2)N), 2.3 (bs, 2x

OH); HRESIMS (high resolution electrospray ionisation mass spec) calc for C10H14N5O4

(M+H) 268.1045, found 268.1042; IR () 2110 cm1

(N3).

2.2.5. 4-Azidobenzoic acid (5)

To a stirred ice-cold solution of 4-aminobenzoic acid (4.2 g; 0.03 M) dissolved in 1.2 M HCl

(30 cm3; 0.06 M) was added dropwise a solution (60 cm3) of sodium nitrite (6.2 g; 0.09 M).After 90 min, diethyl ether (45 cm

3) was added followed by sodium azide (9.75 g; 0.15 M)
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Compounds 1 and 2 were dark oils indicating a high degree of internal and through space

(charge transfer) conjugation, between the OH and the azide group, respectively, which is

supported by the values ofmax 258 nm (max 1123) for 1 and 250 nm (max 719) and 380 nm

(max 207) for 2.

Photolysis of1 (0.65104

mol l1

) in THF was monitored by following the decrease in theabsorbance peak at 258 nm and the formation of a pronounced long wavelength product, max

300 nm. The photolysis of1 showed two isobestic points one tightly centred at 275 nm and

the other at 230 nm (seeFig. 7), with an apparent first-order rate constant of 6.9103

s1

(

). The prominent product formed at 300 nm (seeFig. 7) may be a

quinoneimine formed as a result of internal conjugation of the electron deficient nitrene with

the hydroxyl-activated aromatic ring rather than ortho addition to give an azanorcaradiene

intermediate, which undergoes electrocyclic rearrangement to an azepine (seeFig. 1).

Fig. 7. UV decomposition of 4-azidophenol (1). Spectra recorded at 0, 0.5, 1, 1.5, 2, 3,

4, 5, 6, 8 and 10 min intervals.

The photolysis of2 (0.82104

mol l1

) under the same conditions as 1 was monitored by

following the decrease in the peak at 250 nm. The photolysis showed a tight isobestic point at

270 nm with an apparent first-order rate of decomposition of 2.88103 s1 ( )

which was slower than 1. The photolysis of4 (5104

mol l1

) under similar conditions

showed an isobestic at 315 nm but was biphasic in character indicative of a complex

photolytic process (Fig. 8). Nitro-substituted aryl azides generate reactive nitrenes on

irradiation but their chemical properties are profoundly affected by the photolytic properties

of the nitro-substituent. The conclusion for 3-nitrophenylazides (4) from several reviews[10]and[11]is that they obey higher-order kinetics.
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Fig. 8. UV decomposition of 4-(2,2-iminodiethanol)-3-nitrophenylazide (4). Inset UV

decomposition.

Photolysis of6 (0.77104 mol l1) resulted in the decrease in the peak at 264 nm. The

photolysis showed a tight isobestic centred at 284 nm (Fig. 9) with an apparent first-order rate

constant of 8.9103

s1

( ) similar in magnitude to 1 with the formation of a

product with a broad adsorption around 290 nm.

Fig. 9. UV decomposition of 4-azido-(2,2-iminodiethanol)benzamide (6). Spectra

recorded at 0, 0.5, 1, 1.5, 2, 3, 4 and 5 min intervals.

3.2. Surface treatment and testing

The surface treatment and testing for adhesion was reproduced according to our initial studies

on the subject[2]. Leather panels cut to size (25.530.5 cm) were washed with a liquid soap

solution (1:1 v/v Mardsen Chem Ltd in warm tap water, on the skin side). The samples were

rinsed with cold tap water and stored open to the atmosphere in dust proof boxes to dry. After

drying they were superficially abraded with abrasive paper (Buehler P60, both skin and inner

flesh side).
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The adhesive (Solibond PU39, Crispin-Adhesives Ltd) was spread with an adhesive spreader

(B&Q, UK), only on the skin side of some test panel. The adhesive treated panels were left in

the open at RT for solvent flash-off. The total flash-off time was 14 min (determined

experimentally elsewhere[2]) which was divided into two parts a 9 min allowance for solvent

evaporation in the open atmosphere followed by a 5 min photo-irradiation at a distance of

30 cm using a convection cooled 75 W Xenon arc lamp (Ushio) vertically mounted on an A-1010 lamp unit. The lamp was fitted with a pyrex window and f/4.5 reflector (Photon

Technology International-PTI) and was powered by a direct current power supply (LPS-220)

with igniter (PTI). The spectral characteristics of the xenon lamp were continuous in the

visible range, extending far into the UV and NIR. After flash-off and irradiation the panels

were pressed together (ca. 0.4 kg cm2

), the adhesive coated skin side to the superficially

roughened flesh side of an uncoated panel, for a minimum of 5 min and allowed to set

overnight at RT (similar to[2]).

Quantities of compounds 1, 2, 4 and 6 dissolved in EtOAc (8 cm3) were uniformly mixed

with adhesive (80 cm3) and the resultant mixture applied to leather panels for T-peel testing.

The washings and roughening procedures applied to leather panel before adhesion increase its

surface area, make it very porous and introduce the necessary hooks and grooves to effect

adhesion via mechanical interlocking mechanisms in the first instance. An azide dissolved in

an organic solvent (acetone or EtOAc) if applied to this surface would penetrate the

superficial layer and enter the corium. It was for this reason that mixtures of azide and

adhesive solution were used, to ensure the localization of the primer at the interface and not

in the corium.

The molar concentrations of photoreagents4 and 6 used and the average peel strength

recorded are presented inFig. 10,Fig. 11andFig. 12, respectively.

Fig. 10. Determination of optimum concentration of compound 4 on T-peel strength.
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Fig. 11. Determination of optimum concentration of compound 6 on T-peel strength.

Fig. 12. T-peel test values under wet conditions. Adry and Awet from[2].

T-peel tests used for the testing of various surface treatments and assessment of adhesion

strength were undertaken using ASTM D1876-72 as a reference. The test consisted of peeling

apart two lap bonded pieces of leather (25.4 mm wide, 10 test samples cut from the largepanels described before) fastened in the roll-over jaws of a tensiometer (Hounsfield with

H.T.M. series software). Common practice requires first the initiation of a crack with the first

and the last centimetre of the joint being ignored when recording the peel force. The speed of

peeling is kept constant during the joint fracture, 254 mm min1 and the average force (N)

required to separate the parts recorded over a fixed distance of 127 mm.

To test the quality of joints underconditions of water saturation, bonded leather assemblies

were soaked in a water basin for 24 h at RT, prior to removal and testing (whilst wet,Fig.

12). Compounds 1 and 2 showed limited adhesion strength on testing under dry conditions.

Samples failed at small loads and they could be easily peeled off by hand and therefore they

were not investigated further on the Hounsfield. This correlates with the UV decompositions

studies namely the possibility of internal conjugation in the case of1 and azepine formation

in the case of2 reducing the reactivity of the nitrene to external addition so the bridges

between surface and the adhesive layer did not develop, hence the easy failure of the joints.

Compound 4 showed the highest values of joint strength at the corresponding concentrations

inFig. 11. The maximum recorded value was attained at a concentration of 4.2 mM of

photoreagent in the adhesive mixture. Unlike compounds 1 and 2 the presence of a nitro-

group in this compound deactivates the aromatic ring to addition by the nitrene and so is

capable of forming bridges with the surface hence the higher joint strength.
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Compound 6 gave a similar maximum value for the peel strength but at a slightly higher

concentration, 5.5 mM (adhesive solution) seeFig. 12. In this case the presence of a carbonyl

group in the aromatic ring deactivates the nitrene to addition with the ring and therefore it

forms productive bridges with the surface so the greater peel strength of joints.

The observed failure type in both cases was of the cohesive type within the leather substrate.

Wet tests on 4 and 6 were performed at the optimum concentrations determined on dry

samples and gave the results shown inFig. 12. The observed failure type for wet samples was

of the adhesive type at the substrateadhesive interface.

For comparison purposesFig. 12shows values obtained for the standard procedure of surface

treatment: liquid soap washings followed by superficial abrasion with a 10% dilution of the

adhesive solution and 14 min solvent flash-off time (referred to as Aabraded samples, from

[2]).

4. Discussion

Several factors influence surfaceadhesion: the nature of the surface, chemical impurities, the

rate of adsorption of the adhesive into the surface and its solvent evaporation. To achieve

optimum adhesion and reproducibility these factors have to be addressed in the preparation of

samples prior to testing. Regarding adhesion to the leathersurface one of the conclusions of

our previous work on the subject[2]was the requirement of aggressive surface treatments to

achieve adhesion. In this respect we found that in addition to superficial roughening the use

of washing with liquid soap and organic solvents (CHCl3) removed most of the weak

boundary layers and significantly improved the joint strength. In all subsequent testing we

used the standard procedure of liquid soap washing (Mardsen Chem.) and superficialabrasion (Buehler P60) with the respective joint strengths reported (T-peel test, 55.2 Ndry

and 33.7 Nwet for A samples). These values were used as a reference for comparison

against the testing of photoreagents.

Both compounds 4 and 6 gave a total increase in peel strength of 99% compared to the

standard procedure. In our previous studies, AEAPES gave an increase in peel strength of

75% dry and 53% for wet samples[2]. Adhesive bonds are believed to be accomplished via

mechanical interlocking, the further increase in strength using azides suggests the formation

of covalent bonds across the surface. These observations hold even for testing under wet

conditions as both compounds showed much higher strength values than the wet standard (A)

and wet AEAPES samples.

Water is the most common hazard encountered by bonded joints exposed to the elements.

Under water attack joints would fail due to diffusion through the adherent and/or adhesive or

chemical attack of the adhesive layer (swelling, wicking). In this respect water would attack

the most vulnerable position on an assembly: the interface region. The high values obtained

for 4 & 6 suggest a better stress transfer across the surface and stronger bonds at the interface.

The overall increase in peel strength for wet tests was 163% for 4 and 157% for 6.

Looking at the chemical structures of compounds 1, 2, 4 and 6 it would appear that molecules

like 1 and 2 with an electron donating hydroxyl group directly attached to the nitrene ring are

not suitable as photo primers due to problems of direct or indirect conjugation, respectively,
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reducing the reactivity of the nitrene species. Compounds 4 & 6 with the hydroxyl groups not

directly conjugated to the nitrene ring and containing electron withdrawing groups (nitro,

carbonyl) produce nitrenes capable of forming covalent bonds with the surface, due to

deactivation of the aromatic ring discouraging the formation of an azepine ring by

intermolecular addition. The presence of two hydroxyl ethylene groups give twice as many

strong hydrogen bonds with the adhesive layer (physical bonds are additive). This explainsthe higher values for peel strength recorded with this type of reagent.

5. Conclusion

The excellent behaviour of photoreagents as surfaceprimers makes them a prime candidate

for industrial applications. The technology is easy to implement and presents limited health

and safety hazards. The irradiation stage uses a powerful UV lamp and it can be contained in

a confined space for continuous batch process[12]. However even the use of this type of

coupling agents does not exclude the necessary requirement of a abrasion stage which

removes most of the weak boundary layer but at the same time affects the water proof layers.In this respect, the abrasion areas must be kept to a minimum and restricted to the regions to

be bonded only.

Future work aims at the design and use of new types of primers/photoprimers or

combinations of photoreagents to produce strong joints under adverse conditions that would

replace the need for stitches in boot manufacture.

Acknowledgements

The authors thank the Defence Logistics OrganisationResearch and Project Support (DLO-

RPS) for a grant (Molecular Adhesives for Leather Seam Sealing) and the Overseas Research

Award Scheme (ORS) for financial support. Thanks is expressed to D.E. Games of the

EPSRC mass spectrometry Service Centre, Swansea, United Kingdom for EIMS, CIMS and

HREIMS measurements.
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adhesion enhancement of chromium tanned heavy

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