Chemistry

Adhesive

Adhesive



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Area of ​​Expertise - Materials science, macromolecular chemistry

Adhesives connect workpieces made of materials such as wood, textiles, paper, glass, metal or plastic due to their surface adhesion and internal strength (cohesion). The joined bodies are held together by a macromolecular adhesive film. The connection between the different bodies should be stronger than the materials to be connected themselves. In the event of a mechanical load, the break should not occur on the adhesive surface or in the adhesive film, but in the bulk of the bonded body.

Learning units in which the term is dealt with

Polyacrylates40 min.

ChemistryMacromolecular ChemistryPolymers

The structure, properties, production and applications of the polyacrylates are described. Since they are mainly used as paints and adhesives, the required properties of these materials will be briefly discussed.


How do tape and glue stick?

Adhesives are characterized by two basic properties: on the one hand, they must have a strong bond with the parts to be bonded (adhesion), on the other hand they must have strong internal strength so that the bond holds together well (cohesion).

Adhesives

In order to be able to wet the surfaces to be glued as well as possible & # 8211 so that the best possible adhesion is achieved & # 8211, the solid adhesive substances in most adhesives, such as the classic "UHU all-purpose adhesive", are dissolved in a solvent. That can also be water.

Only when the adhesive is distributed as evenly as possible on the parts to be bonded does the solvent escape and the adhesive begins to "work". As the solvent escapes, the cohesion of the adhesive substances increases and a firm bond is created.

"Reaction adhesives", such as 2-component or superglues, work a little differently. In these, the internal strength is not created by the evaporation of a solvent, but by a chemical reaction between two components. In the case of superglues, the second component that triggers the hardening process is, by the way, normal air humidity!

Adhesive tapes

In addition to its obvious advantages, the permanent tack property also has its pitfalls. Who does not know the towel hook in the bathroom that suddenly comes off after several weeks or months? This is due to the rather weak cohesive forces in pressure-sensitive adhesives. A heavy wet towel slowly but steadily pulls the connection apart.


Posts tagged & # 8216Physik & # 8217

With further different surface pretreatments, a significantly improved adhesion of the adhesive to the material can be achieved. In the case of "normal" metals, the cleaning described above is usually sufficient. Further possibilities are described below in order to achieve optimal bonding results.

Mechanical surface pretreatments:
This is one of the mechanical surface pretreatments Sanding, brushing and blasting. They serve to roughen the surface and must be carried out after degreasing in order not to work any grease residues deep into the material of the component to be joined. These surface pretreatments enlarge the surface and the adhesive can also anchor itself positively in the surface. Blasting is not suitable for thin parts to be joined, as it compacts the surface. Tensions arise in the workpiece, which deform it. There may still be oil residues in the compressed air used for blasting. Therefore, degreasing must be carried out again afterwards.

Chemical surface pretreatment:
That Pickling belongs to the chemical surface pretreatment processes. The non-purely metallic layers are removed from the surface of the parts to be joined with diluted acids. This process is subject to strict safety regulations, so that it is very complex and is only carried out when there are high demands on the bond, such as in aircraft construction.

Physical surface treatment processes:
These procedures include the Corona processes, plasma processes and flaming. These processes are mostly used for plastics, but metallic materials can also be treated. Plastics have the property of being difficult to wet, but this is a prerequisite for good bonding. In the physical process, oxygen atoms are built into the surface & # 8222 & # 8220, which improve the wetting properties and the adhesive properties.

At the Corona procedure a shower of sparks is applied to the surface to be treated by means of an electrode. This creates ozone, which is stored in the surface in the electric field. Extruded aluminum profiles for foils, chains for moldings and fine needle points for small areas are used as electrodes.

At the Plasma process Oxygen atoms are built into the surface, thereby increasing the surface tension. This results in better wettability of the adhesive. A distinction is made between two procedures:
Low pressure plasma process: here the plasma is applied to the workpiece in a chamber in an almost vacuum. As a result, this method can only be used for relatively small workpieces (size of the chamber).
Plasma process at atmospheric pressure: Here, plasma is applied to the surface of the material to be treated from a plasma nozzle with the aid of air. The size of the workpieces is arbitrary. Large surfaces are treated either using a nozzle or a series of nozzles (foils, plates). There are surface tensions of
60 N / m reached.

At the Flame the surface is briefly heated with a gas flame that has an excess of oxygen without melting the plastic.
(UHU-Vertrieb GmbH)


From quantum glue to electron slingshot

The world only exists because its atomic components are held together by the quantum mechanical properties of matter. This effect prevents electrons in a metal, for example, from simply making their way out of the dust. The “quantum glue” holds the electrons in the metal even when it is highly negatively charged. With classic behavior, the electrons would fly away due to the repulsive forces. In a new work, the transition from quantum mechanical attraction to classical repulsion behavior on individual nanoparticles was followed with a previously unattainable degree of accuracy.

Even in physics class, schoolchildren learn that two charges of the same name repel each other. If you go to the nanometer scale and gradually charge a metallic particle only a few atomic diameter with electrons, an ever higher, energetic coulomb wall arises around the particle, similar to a medieval castle fortification. How long can electrons stay in the particle and when do they manage to penetrate the wall in order to escape the quantum forces?
To investigate these questions, metal clusters, consisting of a precisely set amount of silver atoms, were created as free-flying particles in a vacuum and charged with a known number of electrons. With the help of the photo effect, it was possible to distinguish emitted electrons based on their energy, which either passed over the Coulomb wall or tunneled through it. By successively increasing the photon energy of the laser, the wall could be “climbed”, ie scanned, which expressed itself in a changing electron emission. It was shown, also using model calculations, that the electrons inside are to be understood as part of a complex quantum system. Obviously there is an energy trough in the clusters in which electrons are trapped, much like electrons in orbitals of atoms. With the number of additional electrons, the height and width of the barrier at the edge of the bowl can be adjusted in a targeted manner. With this a special system has been found, on which fundamental questions of physics can be studied. It is still unclear how much time electrons need to “tunnel through” an energy barrier that would be insurmountable in a classical world. This question may also become important technologically in the future, because the functioning of increasingly miniaturized electrical circuits is increasingly determined by quantum effects.


Visco-elastic behavior of wood-adhesive connections under cyclic tension-compression loads

Correspondence: B.Sc. David Zerbst, University for Sustainable Development Eberswalde, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 EberswaldeSearch for more papers by this author

Eberswalde University for Sustainable Development, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 Eberswalde

Eberswalde University for Sustainable Development, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 Eberswalde

Bern University of Applied Sciences for Architecture, Wood and Construction, Institute for Materials and Wood Technology, Solothurnstrasse 102, Postfach 6069, CH-2500 Biel / Bienne 6

Eberswalde University for Sustainable Development, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 Eberswalde

Correspondence: B.Sc. David Zerbst, University for Sustainable Development Eberswalde, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 EberswaldeSearch for more papers by this author

Eberswalde University for Sustainable Development, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 Eberswalde

Eberswalde University for Sustainable Development, Working Group Chemistry and Physics of Wood, Schicklerstrasse 5, 16225 Eberswalde

Bern University of Applied Sciences for Architecture, Wood and Construction, Institute for Materials and Wood Technology, Solothurnstrasse 102, Postfach 6069, CH-2500 Biel / Bienne 6

Abstract

In this work, a test method was used that simulates the static, cyclic loading of adhesive joints as a result of swelling and shrinking of the wood and shows plastic deformations in the low load range. The elastic behavior of tensile shear test specimens made of beech wood glued with three different types of wood adhesive (MUF, PRF, PUR) was examined under cyclic tensile and compressive loads and the loss and storage energy were evaluated. All adhesives tested showed viscous proportions even at a very low load level of 3 MPa, which increased with higher load. A more elastic behavior was observed for the PRF bond at the load level of 7 MPa than for the other bonds. The increasing loss of energy recorded for the PUR bonding indicates a softening of the adhesive joint.

Abstract

Visco-elastic behavior of bonded wood under cyclic tensile and compression loading

In the present contribution, a test method is used to simulate the static, cyclic loading of adhesive joints due to swelling and shrinking of the wood and to demonstrate the plastic deformation in the low load range. For tensile shear specimens prepared from beech wood and bonded with three different adhesives (MUF, PRF, PUR), the elastic behavior under cyclic tensile and compression loading was investigated and the loss and storage of energy was determined. All tested adhesives showed viscose parts even at a very low load level of 3 MPa. At a load level of 7 MPa, the PRF joints revealed a more elastic behavior than the other. The increased loss energy determined for the PUR bonding indicates a softening of the adhesive joint.


This new generation of adhesives and adhesive sealants was developed on the basis of alternative raw materials. In contrast to conventional PUR construction and assembly adhesives, whose ingredients are subject to labeling due to the Hazardous Substances Ordinance, with COSMO HD adhesives we offer alternative systems for consistently environmentally conscious processors.


History, application

Cyanoacrylate was first discovered by the American chemist Harry Coover, who worked at Eastman Kodak in New York, during World War II. The substance found stuck to all the equipment that was supposed to work with it. But this property was soon marketed profitably: Cyanoacrylate was patented in 1956 and included Eastman 910 The first superglue on this basis came onto the market in 1958. Cyanoacrylates are now used as superglues in handicrafts and model making and are available in a variety of viscosities and properties.

In 1964, the Eastman company applied for medicine to the FDA, the US Food and Drug Safety Authority, to be allowed to use cyanoacrylate adhesives to bond human tissue and wounds. The special polymerization reaction (see above) helps in the event of accidents or seamless surgical interventions. Thanks to its ability to stop massive bleeding, cyanoacrylate quickly became an important tool for surgeons and has saved numerous lives to date. In the Vietnam War, cyanoacrylate sprays were used as a quick wound dressing, but because of possible skin irritation, these adhesives were not approved for civil use. Not until 1998 the variant 2-octyl cyanoacrylate was developed, the spray bandage was also able to spread in civil health care.

In forensic science, cyanoacrylate is used to make fingerprints visible. To do this, the liquid is heated and the vapors formed are reflected on fingerprints, which, however, must still have a certain amount of residual moisture. The fingerprint will then be visible as a white pattern.


1 answer

The epoxy resin adhesive is usually a two-component adhesive consisting of a low-viscosity solution of short-chain epoxy resins and the so-called crosslinking component (usually smells a bit like amines / fishy) If you put the two components together, a highly crosslinked, hard epoxy resin is created

A laminating resin already has high molecular weight polyepoxides, which makes it very viscous / tough from the outset. The viscosity can be reduced by adding the thixotropic agent - but no cross-linking reaction will occur, as with the epoxy adhesive. Therefore, the laminating resin is not suitable as an adhesive.

quote: A laminating resin already contains high molecular weight polyepoxides, which makes it very viscous / tough from the start. The viscosity can be reduced by adding the thixotropic agent.

Are you sure here? The laminating resins I used were always less viscous (thinner) than the adhesives (e.g. UHU plus). With the thixotropic agent, however, I can make the laminating resin more viscous (higher viscosity). Guess you have committed yourself to that.

What can I imagine by a networking reaction? This term doesn't mean anything to me. - I'm more of an electrical engineer. LG

Cross-linking reaction - in other words, form longer molecular chains? The laminating resin also needs a hardener. - There is also a cross-linking reaction here, otherwise the stuff wouldn't harden. Are the molecular chains of the adhesive "stronger" networked after curing?

You're right. Thixotropy is also a stiffening property. depending on the shear rate.

Most polymers (PVC, polyester etc.) are long-chain molecules. If these are chemically connected to one another at several points, a so-called polymer network is obtained. In fact, the whole thing can then become one giant molecule.

Yes essential. The 2-component resins are usually mixed in a ratio of 1: 1.

UHU consists of high molecular weight polymers that are dissolved in a volatile organic solvent. When the solvent evaporates, the polymer layer remains. but these can always be removed with solvent. This is no longer possible with a networked system - they are insoluble.

OK thanks. UHU plus is an epoxy resin system. - But now you are talking about the conventional "everything glue". I'm definitely smarter than before. thanks


Glue seals blood vessels

Blood vessels are lined on the inside with a single layer of cells. These cells have special adhesive proteins on their surface, which they use to weld together tightly. Normally this ensures a perfect seal of the blood vessels.

The most important adhesive protein is the so-called VE-cadherin. It can be destabilized in various pathological conditions & # 8211, for example in the case of sepsis, when bacteria have worked their way into the bloodstream and flood the whole body. This infection triggers inflammatory processes, and this opens up gaps in the sealing of the blood vessels. Blood fluid escapes, life-threatening organ swelling and bleeding in the tissue can be the consequences.

So far there is no means of sealing excessively permeable blood vessels. That would be very helpful, for example when treating patients with water in the lungs or with organ swelling caused by allergies.

Small peptides ensure cohesion

Researchers from the Institute for Anatomy and Cell Biology at the University of Würzburg have taken a step in this direction: They have developed small peptide molecules that strengthen the bond between the vital VE-cadherin adhesive proteins. This stabilizes the sealing of blood vessels against inflammatory stimuli.

How do the peptide molecules work? Like glue: They bridge the adhesive proteins together because they are modeled on the structure with which the VE cadherins weld together closely. They develop their cross-linking effect as tandem peptides arranged one behind the other & # 8211 similar to a plaster with two sticky ends.

Use on humans even at a distance

& # 8222These results open up new approaches for the treatment of the pathologically increased permeability of blood vessels & # 8220, says Professor Detlev Drenckhahn. But there is still a long way to go before it can be used on humans. Because the molecules in their current form are not suitable for this.

According to Drenckhahn, administering peptides to a person is always difficult & # 8211 because unexpected immune reactions can be expected. The next step for the Würzburg researchers is to find other molecules that are similar in structure and effect to the peptides.

Publication in the Journal of Cell Science

The leading Würzburg scientists Wolfgang-Moritz Heupel, Jens Waschke and Detlev Drenckhahn describe their new approach in the current issue of the Journal of Cell Science. They collaborated with structural biologist Thomas Müller from the Biozentrum, who designed the peptide molecules on the computer. The peptide molecules were tested in various systems with the chemist Athina Hübner, the physician Nicolas Schlegel and other employees of the Anatomical Institute.

The researchers were able to show the effectiveness of the novel molecules by means of atomic force microscopy on isolated VE-cadherin adhesive proteins and also in the living organism: If mice are injected with the protective & # 8222 adhesive & # 8220 into the blood vessels, their seal no longer breaks when an experimentally generated inflammatory stimulus is triggered .

& # 8222 Endothelial barrier stabilization by a cyclic tandem peptide targeting VE-cadherin transinteraction in vitro and in vivo & # 8220, Wolfgang-Moritz Heupel, Athina Efthymiadis, Nicolas Schlegel, Thomas Müller, Yvonne Baumer, Werner Baumgartner, Detlev Drenckhahn, Jens Waschke J Cell Sci. 2009 May 15122 (Pt 10): 1616-1625, doi: 10.1242 / jcs.040212


Adhesive - chemistry and physics

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