Polyurethane resin-based adhesives - curing reaction and

Polyurethane resin-based adhesives - curing reaction and

(Parte 1 de 2)

Polyurethane resin-based adhesives: curing reaction and properties of cured systems

Giulio Malucellia, Aldo Priolaa,*, Franco Ferreroa, Andrea Quagliaa, Mariaenrica Frigioneb, Cosimo Carfagnac aPolitecnico di Torino, Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, C.so Duca degli Abruzzi 24, Torino 10129, Italy bUniversit"a di Lecce, Dipartimento di Ingegneria dell’Innovazione, Strada per Monteroni, Complesso Stecca, Lecce 73100, Italy cUniversit"a di Napoli, Dipartimento Ingegneria dei Materiali e della Produzione, Piazzale Tecchio 80, Napoli 80125, Italy

Accepted 24 April 2004

Available online 17 June 2004

Abstract

A polyether, moisture curable, polyurethane resin was used as an adhesive on plastic and aluminium substrates, in order to investigate the influence of the surface properties on adhesion. The curing kinetics of the adhesive, in controlled temperature and humidity conditions, was evaluated and an elastomeric product was obtained after curing. The surface properties of the cured resin and of the different substrates (PPO/PA6 blend, polypropylene and aluminium) were evaluated and compared by using a contact angle technique. Adhesion measurements were performed using single-lap joints; the strength values obtained were correlated to the structure of the adhesive and to the surface treatments performed on the substrates. r 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Polyurethane; B. Plastics; Aluminium; C. Lap joints; contact angles

1. Introduction

Polyurethane (PU) resins are widely used in the field of coatings and adhesives due to their high reactivity, high flexibility in formulation and application technologies, mechanical and adhesion properties and weathering resistance [1,2]. In particular PU are used in the automotive industry for joining components since the obtained joints are resistant to petrol, oils, greases and water [2].

We investigated the behaviour of typical PU adhesives coated on different substrates, in order to evaluate their performance and the influence of the surface properties of the substrates on adhesion. In order to achieve the best adhesion properties, it is very important to induce strong interactions between the two phases of the joint and to optimize the interface by means of suitable surface treatments [3,4]. Three substrates were chosen for evaluating adhesion with the PU resin, namely a blend of polyphenylene oxide with polyamide 6 (Noryls), a polypropylene (P) substrate and aluminium sheets as a model of a metallic substrate. They were subjected to oxidation and acid treatments.

The curing kinetics of the adhesive were studied through FT-IR analysis, and the conditions for obtaining the completeness of the curing reaction were established. The thermal, mechanical and surface properties of the cured adhesive were evaluated. Standard single-lap joints, according to ASTM methods were prepared in order to measure the adhesion properties.

2. Experimental 2.1. Materials

An isocyanate-urethane prepolymer (SaBesto 0890 by

Wurth, Bolzano, Italy) was used as starting material; it contains polyether chains (polypropylene oxide chains)

*Corresponding author. Tel.: +39-1-5644-656; fax: +39-1-5644- 699. E-mail address: aldo.priola@polito.it (A. Priola).

0143-7496/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2004.04.003 and isocyanate groups curable by means of atmospheric humidity. The resin contains mineral fillers; the residue after thermal treatment at 650 C was 29.5% w/w, mainly constituted by CaCO3 (60%) and TiO2. Elemental analysis of the resin shows an N value of 2.14% corresponding to a total amount of toluene diisocyanate (TDI) equal to 1.4%.

The following substrates, as 2mm thick sheets, were used:

* noryls GTX (General Electric, USA): blend of polyphenylenoxide (PPO, 53% w/w), polyamide (PA6, 45% w/w) and carbon black (2% w/w),

* P (Moplen HF500N, Basell) as sheets of pure P;

P+Talc (20% w/w)+carbon black (2% w/w); P+Talc (40% w/w)+carbon black (2% w/w),

* Al rolled sheets of 1mm thickness purchased from M.H. Hall & Sons Ltd (UK).

As adhesion promoters, two products supplied by Wurth were employed, namely a polyisocyanate for the P sheets and a mixture of polyvinylbutyralphenolic resin for the aluminium substrate.

2.2. Procedures

The curing kinetics were measured by coating the adhesive on a KBr disk which was maintained in air at 25 C and 50% R.H. The decrease of the isocyanate IR band at 2270cm 1 was monitored as a function of time.

The substrates were firstly washed with acetone. The

P sheets were oxidized by sulfochromic treatment for 1h at room temperature. They were washed with distilled water and dried in an oven at 40 C for 2h according to ASTM D2093-97.

The Al sheets, after acetone washing, were put in a 5% w/v hydrochloric acid solution for 1h at room temperature. They were then washed with distilled water and dried in an oven at 40 C for 2h.

The treated P and Al sheets were coated by the adhesion promoters and dried for 30min in the air.

Standard single-lap joints, according to ASTM D1002 method, were prepared; the overlapping area of the adherends was 0.5in2; the thickness was 250mm. At least five measurements were performed on each type of joint and the values averaged.

2.3. Analyses and characterization

The FT-IR analyses were performed with a Mattson

Genesis I Instrument (USA) on KBr disks. All the spectra were collected in air at 25 C and 50% R.H.

DSC measurements were performed using a Mettler

DSC30 (Switzerland) Instrument, equipped with a lowtemperature probe, ranging from 150 C up to +70 C. The heating rate was 10 C/min. All the temperature scans were performed under nitrogen atmosphere (flux: 20ml/min).

Dynamic mechanical thermal analyses (DMTA) were performed with a Rheometric Scientific MKIII (UK) apparatus, at a frequency of 1Hz in the tensile configuration.

TGA analyses were carried out with a LECO

TGA601 Instr. (USA) in air between 25 and 850 C with a heating rate of 2 C/min.

The surface properties were performed by dynamic contact angle measurements by means of a Kruss DSA10 Instrument (Germany), equipped with a video camera. The analyses were made at room temperature by means of the static sessile drop technique. Three to five measurements were performed on every sample and the values averaged. The measuring liquid was bidistilled water (surface tension g¼ 72:1m N=m).

The roughness measurements were performed with a

Mitutoyo Instrument (Japan) and evaluated as the average peak-to-valley distance.

The mechanical properties and the adhesion values were measured with a Sintech 10D Dynamometer (USA).

3. Results and discussion 3.1. Curing kinetics

In Fig. 1 the FT-IR spectrum of the PU resin is reported. The presence of the band at 2270cm 1 attributable to the unreacted isocyanate groups was evident besides other peaks, at 3320cm 1 and in the region 1600–1720cm 1, which were attributable to urethane groups; in the region 1000–1400cm 1, peaks were attributable to ether groups.

In Fig. 2 the kinetic curing curve of the resin, in the presence of 50% R.H., at 25 C, is reported. It has been obtained by following the decrease of the IR isocyanate

Wavenumbers % Transmittance

Fig. 1. FT-IR spectrum of the PU resin.

G. Malucelli et al. / International Journal of Adhesion & Adhesives 25 (2005) 87–9188 peak as a function of time. It shows an asymptotic trend indicating that, in the adopted conditions, the reaction completion is reached after about 100h.

3.2. Properties of the cured resin

The cured adhesive is a rubbery product; its DSC thermogram is reported in Fig. 3. The DMTA spectrum of the cured product is reported in Fig. 4 showing the E0 modulus and tan d curves as a function of temperature.

A Tg value of about 40 C is obtained considering the maximum of tan d curve. The higher value determined via DMTA with respect to DSC technique

(Parte 1 de 2)

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