Reversible micro/nanostructured adhesives could have high impact applications in robotics, medicine, sport apparel and shoes, automotive, etc. As reversible adhesives, first, gecko foot-hairs inspired elastomer micro-fiber adhesives using intermolecular forces such as van der Walls forces are presented. Optimized mushroom-like microfiber adhesives show strong and tunable adhesion and friction on a wide range of surfaces in dry and wet conditions. Wet and dry contact cleaning and temperature and humidity sensitivity of such microfiber adhesives are demonstrated for real world applications. We also used such adhesives to create new miniature climbing robots and soft robotic manipulators. Next, we show that liquid metal-coated elastomer pillars can have reversible adhesion using dry adhesion of gallium oxide layer of a gallium coating. Such liquid metal layers can reversibly change their phase from solid to liquid or vice versa using minor heating or cooling to adapt and adhere to a given surface roughness/curvature and then release easily. Part manipulation using such reversible and switchable adhesive is demonstrated as an example application.
Innovative dressings can expedite the wound healing processes. artimelt hydrocolloid adhesive wound dressings were developed explicitly for this purpose. They combine unique characteristics that produce an ideal wound climate as needed for optimized healing. On one hand, artimelt hydrocolloids absorb secreted moisture (wound exudate) and on the other, this process creates a gel that reduces pressure and provides a cushioning effect. At the same time, artimelt hydrocolloids establish a barrier against the ingress of microorganisms and promote wound healing by regulating the wound climate.
This presentation characterizes applications and describes the special properties of this adhesive group.
Silicone elastomers have now received a wide acceptance for use in healthcare applications. Soft Skin Adhesive (SSA) gels are particularly used for scar care and wound management, especially for wound dressings. The gel technology secures adhesion to the peripheral skin and enables atraumatic removal by minimizing the stress transfer to the skin during removal. As a result, dressings using soft silicone favorably impact the comfort for the patients and result in overall reduction in cost. Reduced pain upon removal is a desirable property for wound care applications, but a silicone dressing won’t be well accepted if it cannot be removed without residues left on the patient’s skin. In consequence, the design of soft silicone gels and the final performances of the adhesives also depends on the dressing’s construction (substrates, materials…).
Typical advanced wound care dressings are composed of different layers, combining different types of materials: a backing film, an hydrophilic absorbent and a wound contact layer that is designed to guarantee soft adhesion to skin. Soft silicone adhesives combined with polyurethane films are materials of choice for this adhesive layer. However, existing soft silicone adhesives available on the market adhere poorly with polyurethane substrates. To avoid any risk of residue left on the skin, manufacturers generally use a physical or chemical surface treatment but this is a complex and often costly process.
During the present lecture, we would like to draw attention to the design of a new soft skin adhesive gel network and focus on an innovative gel formulation that could adhere directly to polyurethane films and stick softly and efficiently to the skin. Formulation of this adhesive takes into account the different interfaces the product will be in contact with (skin, substrate). Besides good skin adhesion obtained with this new gel, a specific focus will be made on other advantages this new gel brings to the manufacturers of the wound contact layers.
Presently, silicone adhesives are far from having developed all their potential and innovation is under way for the design of innovative products.
The high incidence of soft tissue damage due to trauma or tumor removal asks for new solutions to reconstruct those defects overcoming current limitations. Often the use of adhesives or hydrogel-like sealants able to attach to wet surfaces and bind tissues together, or fill tissue defects seems to be an advantageous alternative to suturing and stapling techniques. As a prerequisite, those hydrogels should not only close or seal defects but also actively promote natural tissue healing mechanisms.
In this context and with regard to the specific constitution and mechanical characteristics of soft tissue, biopolymer-based hydrogels derived from proteins or polysaccharides are promising functional materials in tissue repair. Our own work focuses on biodegradable polysaccharides like dextran, and chitosan, and on the more complex glycosaminoglycans hyaluronan and chondroitin sulfate to prepare cross-linkable, water-soluble macromonomers usable as adhesives or sealants in the wet environment of soft tissue. Polymeric materials with defined molecular weights have been prepared from the mentioned biopolymers by controlled thermal or oxidative degradation processes. Various reactive functionalities including (meth)acrylate, aldehyde, amino and carboxyl groups have been introduced into these biopolymers to increase their cross-linking capability.
Concerning the development of soft tissue defect-filling sealants, the hydrogel formation of dextran or glycosaminoglycan (meth)acrylates was studied in aqueous, buffered solutions using different (photo)initiators. Furthermore, two-component adhesives with proper curing times could be generated by azomethine formation of reactive dextran or hyaluronan dialdehydes with chitosan derivatives. Both types of cross-linked hydrogels showed a gradual degradability and an excellent in vitro and in vivo cytocompatibility. Adhesive systems derived from chitosan represent promising candidates for the repair of injured mucosa tissue in otorhinolaryngology surgery. Further studies are currently ongoing to confirm these in vitro data and provide a basis for the translation of these materials into clinical practise.
In order to protect electronic or mechatronic systems, housings that are manufactured by injection molding are state-of-the-art and extensively used in various industrial sectors, especially in the automotive market. Such protection housings are based on metal lead frames that are coated with thermoplastic polymers like PA 6.6 (see test sample in Fig. 1). The benefits are manifold: low production costs, high volumes per year, low weight, and high design flexibility. One limiting factor in the applicability is the tightness of the metal plastic compound. Along the metal plastic interfaces (see Fig. 2), leakage paths can exist or emerge throughout operation. As a result, vapor, gas, oil, or other highly fluid media can penetrate into the housing during operation and affect the durability of the electronic or mechatronic components inside.
To achieve gas and oil tightness, for example, the interface between the metal inlay and the housing material needs to be remodeled. This approach will avoid subsequent and cost-intensive sealing steps. Moreover, the selective surface treatment of the lead frame can effectively be integrated in the reel to reel stamping process. New and advanced adhesive materials are emerging the market, such as B-stage epoxies, hybrid materials, silane-modified polymer, etc. Especially B-stage epoxies are promising with respect to both the reel to reel production and the injection molding process.
This work will give a comparison of various advanced adhesives (see Table 1). Also selective mechanical treatments are investigated, which were partly combined with the applied adhesives. The test samples are mainly based of PA 6.6 for the housing and pure copper inlays (see Table 2). In order to determine the leakage rates, two setups have been developed. These setups allow a direct comparison with housings that were manufactured without inlays (references). The aim of our work is to achieve leakage rates that are comparable to the references. Thus, this work involves additionally a more in-depth analysis on the metal plastic interface to reveal the mechanism of action for those variations with the lowest leakage rates. From these results design measures as well as options for further optimization will be derived.
E-Motors can be found in a wide field of application. These are, for example, electric razors or tooth brushes, actuators for e.g. valves, motors to adjust car seats or to move car windows, power units for diverse vehicles and many more.
Since 2009, a worldwide applicable regulation for the efficiency of e-motors took effect. The motors were sorted in classes IE1 to IE4 while the lowest class IE1 stands for efficiency higher than 90%. The part of high efficient motors is intended to continuously increase. In 2011, a regulation was introduced in which new motors have to have a minimum of IE2 (efficiency >94%). To reach this high efficiency, adhesive bonding is mandatory for a lot of e-motor designs.
Due to the high numbers, e-motor production must be fast. This means adhesives for e-motor bondings must be able to be implemented in a fast production. Other important trends are improving the capability of static loads and loads under temperature.
Normally, heat curing epoxies fulfill these requirements very well but the high curing temperatures needed for a fast curing can be a limit. Two component epoxy resins can be cured at room temperature or accelerated with lower temperatures but do not have the same strength specifically at a higher temperatures than heat curing systems. New systems have been improved in this behavior and close the big gap between room temperature- and heat- curing epoxy resins.
Metallic bipolar plates (BPP) are typically made through a laser welding process of two half shells. The complex engineering technology, complicated clamping devices, slow production cycles and the input of thermal energy into the material leading to internal material stresses, are the major drawbacks of this joining process. Using newly developed adhesives, the laser welding process can be replaced by a less complex process with the same functionality. The primary requirements for the adhesive purpose are the sealing of the cooling system against the surrounding areas and the electrical contact between the two metal half-shells.
By developing an adhesive-based solution, the production of BPP becomes much easier and faster, which may lead to a cheaper product. The aim is also to improve the electrical conductivity of the system. Particular challenges in the formulation of electrically conductive adhesives can be seen in the selection of suitable electrically conductive fillers. Typical metallic fillers such as gold, nickel or silver either tend to corrosion under the operating conditions of a fuel cell, or are very expensive, which makes the use of these fillers in the fuel cell economically unfeasible. The application of the new conductive adhesives with a suitable roller coater on the patterned substrates of the BPP provides surfaces with a uniform thickness of the adhesive layer and low electrical resistance. Not only epoxy systems that are commonly used for conductive applications, but also polyolefin hot melt adhesives show this good performance. In addition, the completely thermoplastic polyolefin hot melts show the positive side effect that individual cells could be touched up after assembly of the stack, since no irreversible crosslinking of the system takes place.
Polyamide additive technology has become the market reference in order to achieve the best rheological properties in Adhesives & Sealants.
In parallel, Silyl-Terminated-Polymer technology used in Adhesives and Sealants has seen it’s market share increase with high growth rates in the construction sector but also in industrial areas. In addition the demands in High Tack systems are more and more important as Adhesives are replacing mechanical fasteners aswell as being used in a variety of other applications where specific technical properties are required.
For these applications, Crayvallac Polyamide Additives are certainly the products of choice to achieve suitable rheological profiles, combining high early grab and enhanced workability.
This presentation will cover the following items:
A brief introduction of Polyamide technology in terms of chemistry and physical properties will be given. As part of the organogelator family, this technology of additives works by forming a three-dimensional network within the formulation it is incorporated. A reversible mechanism based on weak hydrogen bonds generates a rheological profile with a unique balance between a high viscosity at low shear rates and low viscosity at high shear rates. This shear-thining behaviour has tremendous advantages since it makes it possible to have antisettling properties at rest, as well as anti-slump properties after application, whilst at the same time maintaining ease of application and excellent storage stability.
The paper will then focus on practical examples adapted to STP based High Tack formulations. It will show different methods of characterisation (an analytical one with rheomoter and also an applicative one) and the performance of different additives regarding these properties.
Finally, we will demonstrate the capabilities of the Polyamide additive technology for High Tack STP based Adhesives & Sealants. This will clearly show and prove it’s differentiation when using these technologies in what is a very competitive world.
Academia and industry have extensively studied the utility of blocked isocyanates because blocked isocyanate technology allows 1K polyurethane and 1K/2K epoxy systems to achieve safer processes and higher performance coatings and adhesives. Key improvements include reduced free isocyanates and extended storage stability, specially for one component formulations, by minimizing moisture sensitivity through masking the isocyanate. Commonly used isocyanate blocking agents include Phenol, Nonyl phenol, Methylethylketoxime(MEKO), Alcohols, ɛ-caprolactam, Amides, Imidazoles, and Pyrazoles. The structure of the blocking group has major impact on deblocking temperature and cure rate of the coating or adhesive.
Due to raised health concerns over Phenols and Nonyl phenol (NP), industries have requested for non-toxic alternatives with similar performances and benefits. Cardanols from cashew nutshell liquid have been investigated as non-toxic alternative and have shown lower deblocking temperature than NP and Phenol and excellent stability at room temperature. Cardanol and NP blocked prepolymer systems were tested in epoxy systems cured with Phenalkamines for its bond strength, mechanical properties, and cure speed along with moisture resistance. In this paper we will discuss detailed results and advantages of using Cardanol as a replacement of Phenol and Nonyl phenol.
Even the best and technically most sophisticated product can turn into a real nuisance if presented in the wrong packaging. Looking at this assertion from the point of view of different downstream users the development of user friendly packaging can do very much for the marketability of products.
By showing different examples of tests and developments that we are currently making with flexible packaging based on isocyanates and acrylates, we want to present different possibilities and give ideas when considering packaging as a serious marketing tool for products. Looking at real life application challenges has shown that by including thoughtful details a packaging can be the deciding factor when choosing between different products.
Our current tests with downstream users from the building, aerospace and paint industry show clearly that flexible packaging can do very much for chemical products. The considerable prolongation of shelf life, the allowance of contact free application of chemical substances and the reduction of CO2 footprint are only a few advantages that have been named so far by those users. This very hands-on experience sharing presentation aims to give many practical impulses which can be easily discussed and put into place within a short time.
The efficient implementation of synthesis concepts into robust and reproducible emulsion polymerization processes requires a reliable process control, ideally also with respect to an inline monitoring of polymer nanoparticle growth. If critical process parameters like e.g. emulsification of the monomers, particle formation, colloidal stability during processing, heat flux, and viscosity are monitored inline and continuously, an increased understanding of the polymerization process is achieved, resulting in safe and cost-optimal processes and feedback control strategies. For example, polyvinylacetate based dispersion adhesives (white glues) are usually modified with functionalized comonomers to control glue properties like viscosity, pH, film formation, hydrophobicity, and adhesive strength (via crosslinking). However, the modification will also influence process characteristics like particle nucleation and growth or polymerization rates. Technologies which provide information about these features might improve process control and hence product quality.
To date only a limited number of process analytical technologies, suitable for high concentrations of nanoparticles, exist. The recently developed Photon Density Wave (PDW) spectroscopy allows for the precise and calibration-free characterization of the optical properties of particles and droplets during their processing. It’s fundamental benefit - the quantitative separation of light absorption and light scattering - enables particle sizing also in highly concentrated polymer dispersions (> 40 vol%), in diameter ranges of approx. 50 nm – 500 µm.
The contribution will introduce PDW spectroscopy and discuss its benefits based on the monitoring of polymerization processes like the synthesis of highly concentrated functionalized polyvinylacetate adhesives.
Aqueous PU dispersion adhesives based on high molecular weight, semi-crystalline polyurethane polymers have long demonstrated their outstanding performance and are a well-established bonding technology in several industrial applications. Due to their thermal activation properties and sharp melting area, these adhesives allow for efficient bonding processes with short bond strength build-up times. State of the art is the use of polyester building blocks from fossil raw material resources to synthesize the base polymers.
As sustainability is increasingly a factor in public and customer perception, it is also becoming increasingly important in industrial processes and company strategies. For these reasons, a consortium of industrial and research partners is working to develop bio-based solutions for PU dispersion adhesives using significant amounts (> 50 %) of renewable raw materials. Target applications include the automobile, furniture and shoe industries. The new base polymers are semi-crystalline and exhibit thermal activation behaviors similar to those of their petroleum-based analogues. Results of bonding performance are being presented for 3d furniture lamination as well as automotive interior lamination application, showing similar tack and peel strength values compared to currently used adhesives based on petrochemical raw materials. Using the new biogenic raw materials, the carbon footprint can be reduced by more than 25% versus fossil-based PU dispersions.
This project is supported by the “Fachagentur Nachwachsende Rohstoffe” (FNR) on behalf of the German Federal Ministry of Food and Agriculture (BMEL) by decision of the German Bundestag.
Light weight constructions are key elements for saving power and reducing emissions of cars. The automotive industry is searching for new bonding solutions to join dissimilar materials like CFR plastics and metal. Polyurethane systems are well suited to provide high elongation and shear adhesion but state-of-the art adhesives are 2 C liquids which need to be accelerated by catalysts to build up sufficient handling strength and also primers are requested to provide adhesion to CFRP.
Fast setting 1C PU hot melts based on polyester polyols are well established for high productive assembly and lamination applications. After application of the adhesive at moderate temperatures initial strength is built up by solidification and crystallization, resulting in short cycle times that allow fast downstream processing of the bonded substrates. The subsequent cross-linking by moisture, such as atmospheric moisture, leads to bonds with good final strength and temperature resistance. However, there are limitations in using 1C PU hot melts in case of thick bond-lines as the curing speed is depending on moisture permeability as well as in providing very high cohesive strength for structural adhesives.
Now, innovative hot melt concepts based on new linear and branched polyester polyols were developed. A modular system of tailor-made amorphous, liquid and crystalline polyols enables the formulation of novel two component reactive hot melt adhesives for various (semi-) structural applications. Due to their excellent adhesion to various substrates additional primers are not needed.
2C PU hot melts are a new approach to offer high productive adhesive solutions with very high cohesive strength for industrial applications due to their hot melt character and high cross-link density.
Exploring the potential for 150KG weight reduction of mixed-material bodies with applications of various adhesives.
Showing how high performance adhesives can be used to achieve stiffening and crash resistance, solve delta-alpha and enable multi-material joining.
Evaluating High Strength Bonding as new innovative technology to deliver NVH and crash performance solutions while weight saving.
Reaching an increased level of design flexibility though application of Ultra High Modulus to multi-material bodies.
The use of adhesive bonding in assembly processes has been steadily increasing over the past years. When adhesively bonding structural parts, the surface quality and cleanliness requires a special attention. Before these components can be further processed by adhesive bonding or painting, laborious cleaning is necessary to remove residues of release agents or other contaminants, which otherwise would strongly decrease the stability of an adhesive bond. Often, surfaces are additionally treated (activated) after cleaning, in order to further increase the strength of an adhesive bond.
Here we present a system which determines the wettability of a surface by an aerosol wetting test. Originally patented by Fraunhofer IFAM, we put a strong focus on automated industrial testing applications during industrialization and product development. At this stage our system provides a reliable process for fast surface inspection of large components during the production process.
The surface condition is investigated through the nebulization of a very fine water mist. The contact angle of the water droplets with the surface depends on its bonding characteristics. The distribution of the droplets size along with other parameters is analyzed through image processing. The analysis is reference based and hence can be calibrated, with various “reference samples” in order to achieve a reliable “ready to bond” signal. This technique can be applied to numerous materials and industrial applications (Structural bonding in aircraft or automotive industry, surface monitoring of electronic components…). The use of such a system allows an increase in overall product quality, process reliability and thus enhanced cost effectiveness.
In summary our work including the latest studies show an enhanced ability to detect local contaminations as well as global pre-treatment steps. The latest results even indicate a capability to differentiate activation/contamination levels and concentrations.
The ECHA (European Chemicals Agency) identified diisocyanates as chemicals of concern and started the process of restriction.
A restriction dossier was submitted to the European Chemicals Agency (ECHA) by German REACH competent authorities (BAuA) proposing risk management measures to ensure safe handling of diisocyanates at the workplace.
Three key options are possible:
A) Exempted product/use combination;
b) Training as pre-condition for use and
c) < 0,1% diisocyanate monomer content.
This gives a new opportunity for low monomer isocyanate based pre-polymers and preparations.
As we live in a real world with real challenges and nobody so far find the all-in-one-talent (jack of all trades) to give a 100% toughening to epoxy systems, we combine the best of different toughening worlds to give an optimized solution for the customer.
We like show that a multi modale toughener does make sense as it is an already well balanced toughener combining the best aspects of different toughener worlds in a single new epoxy toughener. The new generation of Polycavit® product range is one result of the implemented tool box (we introduced on the last year in-adhesive 2016) of Struktol® Epoxy Products. Next to the improvement in performance we also like to show that not all combinations make sense.
Another aspect of the Tool Box is the fact to be considered as a living subject where the integration of the customer is a crucial point. That is why we are able to open the toughener range more and more for the application field of 2C epoxy systems which was a drawback of the classic Polydis® product range in the past. The recent months show to us that the epoxy converting industry is not always satisfied with the solution that can be obtained. Because of our integrated Tool Box we are able to give the customer the flexibility back which is most likely lost using a given fixed system.
In the process of curing, gelation and vitrification are important transitions of an epoxy adhesive associated to significant changes of chemical and physical properties. While the cross-over of the loss tangent at different frequencies is indicative for gelation, vitrification is attended by a maximum of the loss tangent. Due to the principles of time-temperature-superposition the maximum of the loss tangent relates to the test frequency. In the liquid state plate to plate rheology is most suitable for investigating viscoelastic properties. Dynamic mechanical thermal analysis (DMTA) is an established method to examine viscoelastic properties of solids. Combining both methods makes it possible to follow the curing process of an epoxy adhesive under isothermal curing conditions from the start of the reaction past gelation and vitrification up to the fully cured high-modulus polymer at different frequencies.
This paper will present data obtained with a plate to plate rheometer in oscillatory multiwave-mode as well as with a high-end Eplexor type of DMTA. The adhesive formulation was based on two epoxy resins and an amine curing agent in a stoichiometric composition. The results were obtained with aluminum adherents under isothermal curing conditions at different temperatures. The results of these experiments provide insight into the frequency dependent vitrification of structural epoxy adhesives at curing conditions below and in the vicinity of the maximum glass transition temperature of the fully cured polymer.
Thermosets, especially epoxy types, offer many desirable properties for adhesives, particularly dimensional stability, heat and chemical resistance, low creep and excellent adhesion to many different types of substrate. For many of the thermoset resins, these properties are partly due to the highly crosslinked nature of the cured resin. However, because these resins are highly crosslinked, they also tend to be somewhat brittle, so formulators often utilize a toughener to overcome the brittle nature of the cured adhesive. In thermoset adhesive formulations, one type of toughener that has found widespread use is Reactive Liquid Polymers (RLP). RLP are low molecular weight, carboxylicacidterminated butadiene or butadieneacrylonitrile copolymers (CTBN). The polymers are added to thermosets and are initially soluble, but as the thermoset begins to cure, the RLP will phase separate and form a sub-micron size rubbery domain within the cured matrix which toughens through a variety of mechanisms, but primarily through enhanced plastic deformation of the matrix ahead of the crack tip. The reactive end group can be derivatized to allow for use in multiple types of thermoset adhesives. The carboxyl group can be reacted with a diepoxide for the epoxy terminated polymer (ETBN) to toughen a one- or two-part epoxy structural adhesive. Diamines can be reacted with the carboxyl group to form an amine-terminated polymer (ATBN) to toughen a 2-part structural epoxy adhesive. The carboxyl group can also be reacted with glycidyl methacrylate to form a vinyl-terminated polymer (VTBN) that can be used in the toughening of acrylic adhesives. In addition to improving the toughness of thermosets, RLP demonstrate additional performance advantages, such as enhanced adhesion to oily substrates and balancing of the A:B ratio in two-part amine cured adhesives. RLP is also increasingly used in emerging thermoset adhesive technologies, such as benzoxazines, for high temperature performance adhesives. All of these properties make RLP extremely versatile tools for toughening thermoset adhesives.
This presentation addresses the fracture mechanical characterisation of industrial adhesives in combination with wooden adherends used for sandwiched composites. Especially, contact zone properties for three different crack separation modes were investigated. Fracture mechanical parameters were determined, which can be used for selecting the suitable adhesive for any bonded composite application. Especially, this can be of great interest for different industries such as aeronautical and defense, medical, automotive and construction.
The ongoing situation shows that material parameters obtained from popular standardised tensile adhesion and peel tests cannot be used to declare fracture mechanical statements. Unfortunately, they merely giving the maximum bonding strength of the adhesively bonded composite. Specifically, since they are pure one-parameter measures assuming continuum mechanics, they are not appropriate for choosing the right adhesive under fracture mechanical aspects. Furthermore, popular fracture mechanical tests such as DCB or ENF do not take account the complex and non-linear nature of industrial adhesives, which leads to distorted and misleading results.
Therefore, alternative methods were developed and used for characterising the cracking behaviour of industrial adhesives applied for bonding wood-adhesive composites. A comparison of common practice standardised tensile strength tests was made in order to show their shortcomings when it comes to fracture mechanical characterisation of adhesive interfaces. This is important, as the interface is the most likely location of an adhesively bonded composite where a crack forms and starts running. In the worst case, this could lead to catastrophic failure of the whole composite component endangering life.
To overcome such limitations, an innovative fracture testing method was newly applied enabling stable crack propagation behaviour after maximum load has been exceeded. This in turn leads to a better understanding of interlace fracture mechanisms behind and therefore fruitful conclusions of how to choose the best adhesive/adherend system can be made.