Adhesives Wiki – Advice in the Adhesives Industry

Cyanoacrylates (instant adhesives)

Cyanoacylates are popularly known under the term superglue. This term is not 100% applicable, as many cyanoacrylates do not harden within a second, but in a window of (mostly) several seconds.

In terms of their chemical structure, cyanoacrylates are thermoplastics, which, however, often have very little flexibility and, in direct comparison with 2-component reactive adhesives, have very little moisture and heat resistance.

One can assume an approximate temperature resistance, with continuous load, of approx. 70-80 ° C and a peak load of 110 ° C. This is of course different from adhesive to adhesive, and the various manufacturers now have temperature specifications of up to 130 ° C.

These restrictions, which are considerable for some applications, can be reduced by modifying the molecules in the direction of making the molecules more flexible. Since the elasticity of the adhesive layer increases with the length of the alkyl ester, the adhesive can be manipulated in a targeted manner!

In terms of application technology, the resulting products are mostly between the brittle / fast-curing and the thixotropic / slow-curing adhesives, but have improved dynamic strength and higher moisture resistance. This means that these modified adhesives can also be used to bond different materials.

Typical applications come from the area of plastic and / or rubber bonding. Materials such as nitrile rubber, polycarbonate, polymethyl methacrylate (CAUTION! Stress cracking), polystyrene or melamine-formaldehyde resins can be bonded very well.

For materials that have a non-polar property, such as polyethylene or polypropylene, special primers have been developed which now make it possible to simply glue these materials together.

Of course, cyanoacrylates can be used not only for gluing plastic, but also for gluing glass thanks to their mostly clear and transparent adhesive layer.

Mixed bonds such as plastic / metal or glass / metal are also no problem for many cyanoacrylates.

Another area that is growing in size is its use in medicine. Special adhesives have been developed here, for example to close wounds and for so-called spray dressings.

UV and light-curing acrylate-based adhesives

UV and light-curing acrylate-based adhesives contain photoinitiators, which break down into free radicals when exposed to certain wavelengths.

The irradiated wavelength must be matched to the photoinitiator. As long as the UV lamp is switched on, free radicals are released, which generate cured polymers in a chain propagation via the double bonds of the monomers.

The polymerization takes place until all double bonds are saturated. If the lamp is switched off too early, the curing process will be incomplete as the production of radicals will be stopped.

The higher the radiation intensity, the more radicals are delivered – the hardening process is accelerated.

Hg-doped emitters have been used for years to produce the full spectrum of light that carries out the curing process. Photoinitiators were used for this, which absorb a broadband spectrum and react particularly effectively with shorter-wave radiation.

New photoinitiators are used for the use of LED spotlights, which absorb radiation in the longer wavy, narrow band area.

Our long-life UV-LED lamps deliver a narrow-band spectral range, but with extremely high intensity, many times higher compared to Hg-doped lamps – but without the heat and the dangerous short-wave radiation.

Epoxy adhesives

When using epoxy adhesives, special attention must be paid to the mixing ratios. Even if the hardening reaction is triggered, if the ratio of resin to hardener is not 100% correct, there will be parts of non-hardened material in the thermoset that has hardened later, which have a negative effect on the strength of the bond. So the thought: more hardener = faster or harder curing is an absolute fallacy.

Therefore always observe the mixing ratios specified by the manufacturer (by weight)!

The curing of epoxy adhesives can also be subdivided into the area of thermosetting and cold-curing epoxy adhesives.

Cold curing system

These are the typical adhesives that are known in most applications. These epoxy adhesives have shorter pot lives and often lower bond strengths than their thermosetting counterparts. However, they are often easier to use in their application, since no additional heat source (oven) is required. The lower strength is due to the not so strong crosslinking of resin and hardener. This type of crosslinking occurs from a temperature of 65 ° C and can (to a limited extent) also be achieved after curing by tempering.

Thermosetting system

Compared to the cold-curing systems, the warm-curing systems have a very slow reaction process at room temperature and require an additional component for correct and perfect curing, namely the temperature.

Nevertheless, longer curing times result even at curing temperatures between 60-180 ° C (depending on the adhesive). A big advantage is the long pot life and the high forces that can be transferred later. In addition, thermosetting epoxy adhesives have a higher temperature resistance , temperatures of up to 230 ° C can be achieved here.

These adhesives are available as 2K adhesives or as 1K reaction adhesives, to which, however, have been added as a second component, e.g. carboxylic acid anhydrides.

Mixing errors cannot be made with 1K reactive adhesives because the components have already been mixed by the manufacturer.

MS polymers

The special properties of MS polymer are:

• solvent-free – practically odorless
• Surfaces are not dissolved due to the absence of solvents. This means that adhesive residues can be removed mechanically without leaving any residue.
• temperature resistant, typically from -40 to 120 degrees, briefly 180 degrees
• rubber-like, tough-elastic
• elastic adhesive bonds often hold better, especially when exposed to impact, shock and vibration
• Very good adhesion to a wide variety of surfaces, porous or smooth
• polymerized, also in thick layers to fill gaps
• Polymerization is triggered by surface moisture (moisture-curing)
• transparent or colored available
• little risk potential
• can also be used as a sealant
• Supernatants can be wiped off or removed very easily when they are fresh with benzine or ethanol, in contrast to silicone sealants.
• Can be painted over / overpainted (typically wet process, i.e. directly on the adhesive, which is still soft, but which must have already formed a skin, after complete hardening, adhesive primer for plastics may be necessary.)
• does not shrink much when hardening (typical< 3%), can also be processed in thick layers or to fill gaps.
• Hardening typically 2-3 mm per 24 hours at 20 degrees and 50% humidity
• Temperature during processing: typically 5-35 degrees
• can also be used on damp surfaces or under water
• Elongation at break typically 200-400%
• Open time typically 10-30 minutes
• Adhesive that has not set can be easily removed with alcohol or acetone. Sometimes petrol works too. Fully hardened adhesive can only be removed mechanically.
• Typical hardness: Shore A hardness DIN 53505: 40-60 (for comparison: bath silicone typically Shore A: 20)
• quite resistant to numerous solvents (short-term exposure)

Restrictions:

• not as UV-stable as silicone, for example. The transparent varieties in particular are less UV-stable. They can yellow and also decompose over time. This is especially true when used outdoors in direct sunlight.
• Due to its elastic consistency, the adhesive tears when subjected to high loads. Tensile strength is typically 2-5N / mm² (20-50Kg / cm²), epoxy resin typically creates 30-80N / mm².
• Shelf life is typically given as 1 year. In practice, typically 2-4 years can be achieved. Some varieties can no longer be used after a short overlay or only harden very slowly.
• Very low chemical resistance (more detailed information can be found e.g. at Weicon in the “Elastic Adhesives” brochure )
• In the case of textile bonds, the adhesive degrades over time through washing in the washing machine. It then becomes soft and sticky.
• 2 dense materials cannot be bonded over a large area, because no air humidity can reach the adhesive joint, which the adhesive needs to harden. If, for example, a mirror is glued to tiles, several strands are laid parallel to each other with enough space in between. This allows air to circulate on the strands of adhesive during the curing process, and the adhesive cures.
• When it sets, a skin is formed on the surface first. If you wait too long before joining, this skin can prevent proper adhesion. Note the open time of the adhesive.
• Creep behavior: If a thick adhesive joint is permanently stressed, the adhesive joint may creep, i.e. permanently plastically deform, e.g. stretch. This is particularly important for precise bonds. See also Wikipedia: Creep in plastics
• When it cures, small amounts of methanol are produced, which may be significant in the case of large-area bonds.
• Small amounts of organotin compounds (eg DBT) can be included as a catalyst. Check the safety data sheet of the product.
• like most other adhesives, certain plastics cannot be glued (PTFE, PE, PP)
• In some cases, in contact with oiled wooden surfaces, there is white, salt-crystal-like efflorescence or white adhesive edges (blooming, fogging, frosting). Can also occur in contact with synthetic resin lacquers painted surfaces and drag on for weeks. Not all products tend to be. Similar effects are also known from superglues, but here this process stops a few hours after bonding.

Source: Wikipedia.org | Winfried Müller, as of 2014