Special Issue of adhäsion KLEBEN & DICHTEN 3/2004 Photo: Volkswagen AG Nano-modified ambient temperature curing epoxy adhesives Figure 1: Car assembly line Ambient temperature curing structural adhesives have to meet ever increasing demands. By modifying epoxy adhesives with reactive liquid rubbers and nanoparticles they can be raised to new levels of performance. Stephan Sprenger, Christian Eger, Anthony Kinloch, Jin H. Lee, Ambrose Taylor, Dave Egan he use of room temperature curing adhesives is of great interest in many areas, including automotive T applications. In quite a few future applications (figure 1) the two component adhesive has to possess the same performance as the heat curing adhesives currently used, regardless of the other advantages the room temperature system might have to offer. This is a challenge to adhesive formulators, as well as to adhesive raw material manufacturers. Most crash-resistant structural epoxy adhesives currently in use are modified with reactive liquid polymers (HYCAR RLP) /1, 2/. Investigations of heat curing one component epoxy adhesives have shown that the performance of rubber-modified adhesives can be increased significantly by the addition of a small amount of filler particles with an average particle size of approximately 20 nm /3/. Initial tests with room temperature curing systems confirmed the existence of a synergy between "soft" µm-sized rubber domains and "hard" nm-sized filler particles for two part epoxy adhesives /4/. The results of our recent extensive research work will be presented in this paper. Table 1: Formulations of two-component model epoxy adhesives 2C series containing approx. 18 % Hycar ATBN Reactive liquid rubbers in two part adhesives Butadiene acrylonitrile copolymers with amino end groups (Hycar ATBN) are commonly used in the hardener component of two part epoxy adhesives. The desired property profile of the hardener is achieved by further blending with polyamines, cycloaliphatic polyamines, polyamidoamines, imidazoles, accelerators etc.. Although, the partial immiscibility of the reactive liquid rubber with some amines and the higher viscosity have to be recognised in the formulation. During the cure, a partial phase separation occurs and µm-sized rubber domains are formed (usually 1 - 5 µm). The rubber molecules in the domains are crosslinked to the epoxy resin matrix by the amine endgroups, thus improving the toughness of the resin dramatically. A fraction of the rubber is not participating in the domain formation; but these long flexible polymer chains are crosslinked into the resin matrix. As a result of the lower network density, the adhesive elongation is increased significantly; the modulus and glass transition temperature are lowered. It is of interest to note that no domains are formed when using these reactive liquid polymers with methacrylate end-groups (Hycar VTBN) in acrylic adhesives. The reason might be different solubility parameters and compatibilities. Nano particles in epoxy applications Nanoparticles are of great interest as raw materials for adhesives; having several Table 2: Formulations of two-component model epoxy adhesives 2Ca series containing approx. 12.5 % Hycar ATBN Table 3: Formulations of two-component model epoxy adhesives 2Cb series containing approx. 9.5 % Hycar ATBN advantages compared to conventional micron-sized fillers: at the same loading level the number of particles (and their surface area) is orders of magnitude higher. As a consequence the interactions with the resin matrix are much more intensive. Even at high loading levels (40 %) the viscosity is not increased much because there is no formation of agglomerates. Nano-modified resins are transparent due to the small particle size; this is especially important for coatings applications. The particles can penetrate close meshed fabrics and are therefore very suitable for the reinforcement of composites, especially if injection methods are used for manufacturing. Recently masterbatches of surface-modified SiO2-nanoparticles in epoxies and acrylates have become available in technical quantities /5/: Nanopox‚ and Nanocryl‚ respectively. In industrial production, spherical particles with a particle size of 20 nm and a very narrow particle size distribution are formed using a sol gel process /6/. The surface modification prevents agglomeration and enables crosslinking into the resin matrix during cure. The use of these nanoparticles in epoxy resin formulations gives significant improvements in several properties: high toughness and high modulus (unlike the "classic" toughening using elastomers which lowers the modulus). The coefficient of thermal expansion can be improved. Thermal and chemical stabilities are unchanged and can be improved at higher loading levels. Viscosity and rheology of the resin is changed only slightly. By adding more than 20 % of nanoparticles, scratch resistance and wear resistance can be increased significantly. Compression strength can be improved as well. Figure 2: Mode I fracture energy versus addition level of nano-SiO2 Figure 3: Lap shear strength on untreated aluminium versus addition level of nano-SiO2 Synergy between reactive liquid rubber and nanoparticles In an earlier investigation /3/ of heat curing epoxy adhesives it was found that adhesives modified with nanoparticles only could not compete with a toughened, rubber-modified adhesive. Very surprisingly a synergy between the two different modifications was discovered /3, 7/: the performance of a very tough rubbermodified one part epoxy adhesive can be improved significantly by adding small amounts of nano-SiO2. The next step is to investigate the combination of "soft" µm-sized rubber domains Figure 4: Lap shear strength on oil-treated EG steel versus addition level of nano-SiO2 and "hard" nm-sized filler particles in room temperature curing epoxy adhesives. It has to be taken into account that in such systems besides rubber domains and silica particles there are long, flexible elastomer molecules (of the reactive liquid rubber) which are linked within the matrix. This is different compared to heat curing one part epoxy systems. The formulations of the model adhesives investigated are given in tables 1 - 3. All the adhesive formulations are toughened by adding an amine functional butadiene acrylonitrile copolymer (Hycar ATBN) to the polyamidoamine hardener. In the 2C series 33 % of the polyamidoamine hardener is replaced by the reactive liquid rubber; in the 2Ca series this is 25 % and in the 2Cb series 20 %. The Nanopox was added to the resin component in various concentrations. The NBR-content of the formulations varies slightly within a series due to stoichiometry. All adhesive joints were cured for 24 hours at room temperature, followed by a 2 hour postcure at 60 °C; regardless of the substrate used or the test performed. Note that 90 - 95 % of the lap shear strength of the postcured samples can be achieved by simply curing for one week at room temperature. Hence, postcuring at elevated temperatures is therefore not necessarily a "must". The glass transition temperatures, determined by DMTA, were found to be around 70 – 80 °C for all formulations. The addition of nanoparticles has no apparent influence on Tg. The adhesive fracture energy, determined using the tapered double-cantilever beam test (BS 7991) was investigated first. The substrate material was a chromic-acid etched automotive aluminium alloy. The results are shown in figure 2. There is a remarkable increase in toughness for low addition levels of nanoparticles. When lower elastomer levels are used, in series 2Ca and 2Cb, the maximum toughness moves towards lower nano addition levels, and the loss in performance with higher levels of nanoparticles is more drastic. Lap shear tests were performed according to DIN 55-283 using untreated automotive aluminium alloy (grade 6016) and Anticorit RP 4107 S oil pretreated electrogalvanized steel. (0.8 mm thick). The results are given in figures 3 and 4. With untreated aluminium substrates, significant improvements in lap shear strength are found when surface modified Figure 5: Roller peel strength on chromic acid etched aluminium versus addition level of nano-SiO2 Figure 6: Wedge impact performance on degreased steel versus addition level of nano-SiO2 Figure 7: Lap shear strength after VDA ageing versus addition level of nano-SiO2 nanoparticles are added. Again the performance goes through a maximum at low addition levels. However, in contrast to the fracture energy results, the maximum shifts to higher addition levels of nanoparticles with lower rubber contents. Chromic acid etched aluminium substrates show similar behaviour. The lap shear test results with oil-treated steel as substrate are less evident. Series 2Cb shows the typical curve with significantly improved lap shear strengths and a maximum at low concentrations of nanosilica. With 12.5 % and 18 % reactive liquid rubber (series 2Ca and 2C) there seems to be no improvement. For further investigations the model adhesive formulations of series 2C were chosen. Figure 5 shows the results of the roller peel tests according to ASTM D 3167 using a chromic acid etched aluminium alloy (type 2024 T3). Tests were performed at room temperature and at -40 °C. At low temperatures a more distinct maximum and a higher absolute value are found, together with a slight shift of the maximum towards lower levels of nanoparticles. Compared to the toughened rubber-modified adhesive without nanoSiO2 the performance is improved by more than 100 %. For the wedge impact tests according to ISO 11343, performed at 2 m/s, degreased steel (grade ST 05, 0.8 mm thick) was used as the substrate. Cohesive failure was found with all test specimens. The test results are given in figure 6. Again the typical picture: an extraordinary improvement in toughness is found at low levels of Nanopox in the formulation. Further trials at low temperatures are under way. Lap shear strength test specimens using the untreated automotive alloy (grade 6016) were subjected to 10 cycles of ageing according to VDA 621-415. Each cycle consists of 1 day of salt spray, followed by 4 days of condensed water climate changes, followed by two days of room temperature storage. The unaged test specimens were stored for 70 days at ambient temperature. After ageing the typical picture regarding the relationship between lap shear strength and nano filler particle content is still more or less the same. The performance level is still above the automotive industry requirements regarding hem flange adhesives for hang-on parts. When evaluating the lap shear strength after ageing it should be noted that model adhesive formulations have been used. By adding corrosion inhibitors, e. g. zinc phosphates, and other stabilizers the loss in strength after ageing should be reduced significantly. The results of further investigations regarding low temperature performance (e. g. wedge impact) and cataplasma ageing should be available soon. Having conclusively shown the existence of a synergy between reactive liquid rubbers (Hycar RLP) and surface-modified SiO2nanoparticles (Nanopox), still the question regarding the mechanism responsible is not answered yet. In other studies it was confirmed that nanoparticles are extremely effective at reinforcing resins due to their huge surface area causing very intensive interactions with the resin matrix. The molecular network of the room temperature curing systems investigated is relatively wide meshed, due to the crosslinked elastomer molecules which do not participate in the phase separation and rubber domain formation. The polyamidoamine molecules of the hardener exhibit a certain molecular chain length as well. Evidently the two separate mechanisms of reinforcement by filler particles and toughening by elastomer domains complement each other. The clarification of the mechanism of this "µm - nm"-synergy is the subject of intensive research. The results of this investigation clearly show the potential of room temperature curing epoxy adhesives. There is a huge interest not only for automotive applications, but also for adhesive bonding of composites, e. g. wind turbine blades, and aerospace applications. Summary Toughening using reactive liquid rubbers is used in many structural adhesives applications and gives an excellent price/performance ratio. By the synergistic combination of "classic" rubber toughening and the addition of surface-modified SiO2-nanoparticles, room temperature curing epoxy adhesives can be raised to new levels of performance. These new high-performance raw materials open up exciting new applications for two part epoxy adhesives. π Authors & Literature Prof. Dr. A. J. Kinloch, J.H. Lee, Dr. A. C. Taylor: Imperial College London, Mechanical Engineering Dept., Exhibition Road, London SW7 2BX, U.K., www.me.ic.ac.uk/AACgroup/ D. Egan: Noveon Inc., 9911 Brecksville Rd, Cleveland, OH 44141, U.S.A., www.noveoninc.com Dr. C. Eger, Dr. S. Sprenger: hanse chemie AG, Charlottenburger Str. 9, D-21502 Geesthacht, Germany, www.hanse-chemie.com stephan.sprenger@hanse-chemie.com Acknowledgements: The authors wish to thank DaimlerChrysler AG Sindelfingen (Germany) PWT/VWL for performing the VDA ageing tests. /1/ N.N.: "Review: Two-Component Structural Adhesives for Metals, Plastics and other Substrates", Technical Information BFGoodrich Company (1992) /2/ Kinloch, A.J.: "Adhesives in Engineering"; Proc. Inst. Mech. Eng.; 48th T. Hawskley Mem. Lect., London, 11.12.96 (1996) /3/ Sprenger, S.; Eger, C.; Kinloch, A.; Taylor, A.; Lee, J.; Egan, D.: "Nanoadhesives: Toughness and high strength", Adhäsion, Kleben & Dichten, 03/2003, pages 24 – 30 (2003) /4/ Kinloch, A.J., Lee, J.H., Taylor, A.C., Sprenger S., Eger, C., Egan, D.: "Toughening Structural Adhesives via Nano- and Micro-Phase Inclusions", J. Adhesion 79, Number 8 – 9, pages 867 – 873 (2003) /5/ Product profile Nanopox®; hanse chemie AG (2002) /6/ Patent application WO 02/08 37 76 A1 (2002) /7/ Patent pending