Uwe Rietz 1*, Dietmar Lerche 1, Stefan Hielscher 2 and Uwe Beck 2

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1 293 ( 80 ) (Accepted for publication : April 17, 2015) Uwe Rietz 1*, Dietmar Lerche 1, Stefan Hielscher 2 and Uwe Beck 2 1 LUM GmbH, Justus-von-Liebig-Str. 3, Berlin, Germany 2 BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, Berlin, Germany Tel: , Fax: , info@lum-gmbh.de Abstract A new technology for strength determination of adhesive-bonded joints based on analytical centrifugation is introduced. Tensile as well as shear strength can be measured for multiple test specimens simultaneously within a very short time. The experimental procedure is described from sample preparation, application of adhesives, sample mounting within the centrifuge to testing parameters and sequences. All requirements of DIN EN ) are fulfilled and, in addition, a higher throughput and a better reproducibility can be obtained. The experimental results are discussed regarding bonding strength development over time, comparison of tensile and shear strength and various testing conditions. Furthermore, an additional application field of centrifuge technology is described with respect to the determination of the adhesive strength of coatings. The novel centrifuge based technology provides the determination of bonding, adhesive and shear strength on a statistical basis under identical testing conditions for up to eight samples. Moreover, sophisticated two-sided sample clamping, necessary for a tensile testing machine, is replaced by a simple plug-in procedure. Key words : adhesion/tensile strength/shear strength/centrifuge/multiple sample testing (Received : December 23, 2014) 1. Introduction Bonding and coating as well as the use of composite materials are important new fields of processing and application. From lightweight construction e. g. in aircraft and automotive industry, to innovative multilayered optical materials 2) (e.g. sun glasses or mirrors) as well as substrate coating to improve chemical inertness, scratch-resistance or appearance, to name a few. Product designers have to choose among a great diversity of materials, of adhesive and coating dispersions, treatments of substrate surfaces and processing conditions. Despite the multitude of application specific, qualitative or quantitative tests available only the pulloff test gives results in N/mm 2 (DIN EN ) ). Its application is restricted to just one by one sample analysis and geometrical constrains of sample geometry. The multiple-sample centrifuge technology offers many advantages in comparison to the pull-off test, e.g. the simultaneous and fast analysis of up to eight samples under identical testing conditions, easy sample preparation and mounting, utilization as desktop plug-and-test system both for QC and R&D. 2. Theoretical/Experimental 2.1 Measurement principle The principle of the new CATT (Centrifugal Adhesion Testing Technology) is based on the physical law of inertia of a body. Due to rotation the centrifugal force F C is applied to the test stamp (TST) which is bonded onto the surface of the sample prior to testing 3). The load increase is programmed by variation of rotor

2 Special Issue on WCARP Ⅴ Vol. 51 No. S1(2015) ( 81 ) 294 Fig. 1 Principle of CATT (Centrifugal Adhesion Testing Technology) for the determination of adhesive strength of coatings to substrates. In case of adhesive testing (bonding strength) the test stamp is directly bonded to the substrate. rpm. If the applied load exceeds the tensile strength of the sample-assembly, the TST changes its position within a guiding sleeve. The detachment of the TST (at the moment of rupture) is automatically detected and a position-coded IR-signal is send out of the turning rotor transmitting the current rotor speed as well as the rupture time. The force at rupture and the strength is calculated and displayed on-line by measurement software SEPView. The measurement principle is demonstrated in Fig. 1. For calculation of breaking force (F C ) test stamp mass (m), its distance to rotational axis (r) and rotor speed at rupture (ù) were considered. Dividing breaking force (F C ) by bonded area (A) tensile strength (ó) is received. 2.2 Sample preparation The test specimen assembly in general consists of the sample itself - it may be a bulk material or a coated substrate - the test stamp with guiding sleeve and the adhesive. Prior to bonding the adherent surfaces of test stamp and sample have to be pre-treated. When testing the bonding strength of a joint both adherents (sample and test stamp side) are mechanically roughened for example by grinding. This is followed by cleaning using a suitable detergent e.g. acetone or isopropanol. For coated samples usually cleaning is conducted to retain the coating properties. After the pre-treatment procedure sample and test stamp are bonded together by means of the adhesive application kit, in particular a sleeve guiding the test stamp and supporting the sample. This kit is used afterwards also for testing within the centrifuge. Test specimen preparation process is illustrated in Fig. 2. At first adhesive is applied to the test stamp adherent (a) followed by positioning of guiding sleeve on the sample surface (b). Afterwards the test stamp is inserted into the guiding sleeve (c) and moved down to the sample (d) until full contact is established (e). For the application of a defined and reproducible amount of adhesive a direct displacing pipette with a variable volume is used (advantageous because of high viscosities of some adhesives). The amount of adhesive mainly depends on the size of adherent area, required adhesive layer thickness and its shrinkage during curing. Bonding pressure, curing and storage (temperature, time) may be used to optimize adhesive and cohesive characteristics. 2.3 Experimental After curing and storage the test specimen are inserted into the detection modules and afterwards the loaded modules into the drum rotor by an easy plug-in procedure. Prior to testing, the operator has to specify the selected test stamps and to assign the sample identification to the rotor position. Finally, the determination of the appropriate testing sequence has to be considered, i.e. shape and rate of load increase. SEPView software makes use of standard operating procedures (SOPs), i.e. all parameters describing the test sequence can be saved and recalled again. This offers the advantage of easy use, full comprehensibility and the ability to implement measurements at pre- Fig. 2 Preparation of test specimen

3 295 ( 82 ) defined conditions. 3. Results 3.1 Curing behavior of a two-component polyurethane adhesive At the beginning of the measurement series (t 0 ) 40 test specimens have been prepared. Both adherents consist of stainless steel , a two-component polyurethane adhesive SikaForce 7888 L10 was used for bonding. Curing took place at room temperature. Every 30 minutes four test specimens have been tested, starting with the first prepared ones. In Fig. 3 the adhesive strength as a function of curing/setting time of the Sika adhesive is illustrated. For each time interval 4 specimens have been tested (columns). The dashed lines indicate the mean values, dot ted lines show standard deviation. As expected, strength increases with time progression. Within the first hour the mean value remained below 1 MPa, after 3 hours it exceeds 2 MPa. 3.2 Comparison of tensile and shear strength Objective of the measurements is the comparison of tensile and shear strength on the one hand and the investigation of different test stamp masses influencing on strength on the other. In total 16 test specimens have been prepared, 8 for tensile and 8 for shear strength determination. The same materials and adhe- Fig. 3 Strength development of SikaForce 7888 L10 Fig. 4 Comparison of tensile and shear strength for SikaForce 7888 L10

4 Special Issue on WCARP Ⅴ Vol. 51 No. S1(2015) ( 83 ) 296 sive mentioned in section 3.1 have been used but curing time was extended to 80 hours at room temperature. The results obtained are illustrated in Fig. 4. Shear as well as tensile strength determined correlate with values specified in technical data sheet (~20 MPa), whereas shear strength measured is a little lower than tensile strength. For both, tensile and shear strength testing (design of sample assembly for tensile stress 4 (a) and shear stress 4 (b) are shown in front of the columns), two different test stamp types (mass, radius) were used. TSU-M utilizes a medium mass body (about 20 g), TSU-L a large mass body (about 35 g) for the test stamp. Comparing the different test stamp types for each within tensile and shear stress, equal strength values have been determined. Detachment occurs rather for the large and heavy test stamps (at lower rpm) than for the medium and lighter test stamps but load/ strength are similar. 3.3 Influence of substrate thickness on strength and failure pattern for PU-coated metals The samples investigated are PU-coated metals for corrosion protection used in building industry. The identical coatings (~20μm thick) have been deposited on metal substrates with four different thicknesses ranging from 500μm to 1500μm. For evaluating the influence of substrate thickness on strength half the samples of each group have been supported by bonding 3 mm thick stainless steel plates to the backsides (with support), the remaining samples were left in original state (without support). For sample preparation test stamps with aluminum adherents (A=78.5 mm 2 ) and a two-component epoxy adhesive were used, which was cured at 70 for one hour. Afterwards test specimens have been stored for two days at 23. All tests have been carried out with a linear load increase of 5 N/s. Fig. 5 illustrates the mean values of tensile strength and standard deviation, every column represents the mean of four test specimen. It is obvious that high strength (20 MPa to 22 MPa) have been determined for all supported samples. Unsupported samples show lower tensile strength, which increases with substrate thickness. To the right of Fig. 5 failure patterns are presented, on top an unsupported and below a supported sample. The results illustrate the influence of deformation/bending of the samples under stress. The thinner samples start to deform at lower loads. As a consequence the adhesive is stretched at the outer sections especially. Failure starts from these highly stressed zones and leads to total breakdown of the joint. In the upper right section of Fig. 5 coating delamination in the middle of the former adherent can be seen which confirms the effect of substrate deformation under load. It could be shown that the substrate thickness has a significant influence on strength and the appearance of failure pattern. From these results can be concluded that a minimum substrate thickness is required to obtain reliable and reproducible results. Furthermore it has to be stated that strength is a characteristic of the overall sample system which can be influenced by its Fig. 5 Influence of substrate thickness on tensile strength and failure appearance

5 297 ( 84 ) unique components, in this case substrate thickness. 4. Discussion & Conclusions The new Centrifugal Adhesion Testing Technology has the potential to become a strong and powerful tool for mechanical property testing. Because of the structural design of the testing device there is no need for expensive two-sided clamping of the test specimen assembly. Moreover, sophisticated two-sided clamping is replaced by a simple plug-in procedure. Another major advantage is the possibility to test up to eight samples simultaneously under identical testing conditions. This way, the overall testing duration is decreased. Hence, costs are reduced and comparability is increased. Programmable SOPs (Standard Operational Procedures) allow the realisation of almost any testing sequence among them standardised short-term, customized and long-term tests. Applications already range from the evaluation of the tensile and shear strength of adhesive bonded joints to the adhesive strength of coatings. It could be shown that the centrifuge technology has been successfully applied for very different kinds of applications. References 1) DIN EN 15870: : Adhesives - Determination of tensile strength of butt joints 2) U. Beck, G. Reiners, M. Weise, D. Lerche, U. Rietz, H. Niederwald; Quantitative Adhesion Testing of PVD- Coatings by means of Centrifuge Technology, Surface & Coatings Technology, 205, 182 (2011). 3) U. Rietz, S. Hielscher, U. Beck, D. Lerche; Centrifuge technology revolutionises adhesion testing. Dispersion Letters, 4, 1 (2013).