Stability of the adhesive

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

This increase in polymer thermal stability translates to improved thermal stability of the adhesive, as shown in Fig. 10 for the steel lapshear adhesive strength after thermal aging at 121 °C for 48 h. 

Where color and light stability of the adhesive are important, and cure speed or cost is less important, aliphatic isocyanates are frequently used. Adhesives derived from isophorone diisocyanate (IPDI), hexamethyl-ene diisocyanate (HDI), or 4,4 -dicyclo-hexylmethane diisocyanate (H12MDI) are available. 

Adhesive Nonmetal, liquid, paste-like or even solid material, joining adherends by means of adhesion forces (surface adhesion) and cohesion forces (inner stability of the adhesive layer) (Chapter 6). 

The pot life refers to the stability of the adhesive, and is related to its acceptable residence time in the application machinery. 

Creep tests on structural adhesives can be divided into tests on bulk hardened adhesive specimens and tests on adhesively bonded joints. The former provides information on the mechanical properties of the adhesive rather than the joints made from them. Fig. 2.28 displays the change in creep modulus with time for a range of cold-cure epoxy adhesives(26). These curves were derived from four point bend tests on adhesive prisms loaded in accordance with Fig. 2.16 using extreme fibre stresses ranging from 0.25 to 2.0 N/mm. The curves represent the stability of the adhesive with time under... 

The following chapters are devoted to applications of phosphorus-based materials. Thus Chapter 8 by Mozsner and Catel deals with the use of polymerizable phosphonic acids (PAs) and dihydrogen phosphates (DHPs) for dental applications. Several PAs and DHPs were synthesized to notably improve the shear bond strength to dentin and enamel, the stability of the adhesive formulation, and the chemical adhesion to tooth tissues. Some of these monomers are nowadays included in commercial dental adhesives. 

There are, obviously, significant limitations with this approach. The adhesive matrix has to be solvent free and must have a high enough viscosity at temperatures above the toughening polymer s melting point to effect dispersion. Further, the polymer must not be soluble in any of the formulating ingredients and must melt at a temperature in which the stability of the adhesive is not affected. 

Thanks to the stability of the adhesion to the substrate and their conductive nature, ICPs often constitute the support of choice to anchor at the electrode substrate a number of further components committed to playing more speciflc... 

Initiators, accelerators, and inhibitors of cyanoacrylate polymerization are used to modify the cure speed and storage stability of these adhesives. They can also be used to broaden the range of materials which can be bonded with cyanoacrylates. Initiators are those materials which are capable of polymerizing cyanoacrylate esters upon contact. These are, therefore, applied either to the substrate surface ( surface primers ), or mixed with the adhesive just prior to application. Accelerators are materials which do not cause polymerization on contact with monomer, but which increase the cure rate once the adhesive is applied. These chemicals are most often compounded with the monomer in the adhesive formulation. The distinction between these two classes can be blurred, as some additions will not cause immediate polymerization on contact but will shorten shelf life in the long run. Anionic polymerization inhibitors are Lewis or Bronsted acids which retard or completely inhibit anionic polymerization. Radical inhibitors prevent polymerization by adventitious, radical sources and are used to prolong the storage stability of the adhesive they generally do not affect cure speed. 

Furfuryl alcohol is used as a viscosity reducer for epoxy formulations. The addition of furfuryl alcohol to urea-formaldehyde resins improves the craze resistance and heat stability of the adhesives. Furfuryl alcohol serves as an effective wetting agent and solvent for phenolic resins in the manufacture of phenolic resin abrasive grinding wheels. Other miscellaneous uses of furfuryl alcohol include use in alkaline paint strippers and cleaning formulations, as a solvent and carrier for dyes in textile printing, and as a chemical intermediate. 

The heat stability of the adhesive is a very important property. Running temperatures that are greater than the adhesive degradation temperature result in charring and decreased overall properties. The addition of stabilizers to the adhesive formulation contributes to the stability of the material by hindering the acceleration of degradation due to the presence of oxygen. 

Cyanoacrylate adhesives are extremely sensitive to traces of impurities, and must be manufactured, stored and used under controlled conditions. Basic impurities or contaminants can seriously affect the shelf-life or stability of the adhesives conversely, acidic materials can slow down or completely inhibit curing. Peroxides or free-radical stabilisers in a potential additive can also seriously affect performance. Because of the sensitivity to contaminants, it is not possible to formulate cyanoacrylates with the wide range of thickeners, fillers and other additives available to formulators of other adhesive systems. 

The clinical technique can also affect the performance of dental adhesive systems. Significant reduction in the degree of conversion and mechanical properties of adhesive systems was observed when solvents were not properly evaporated [23-26]. The application of simplified-step adhesive systems to an excessively wet dentin surface may lead to phase separation and a hydrophobic-poor and hydro-philic-rich zone may be formed, lowering the stability of the adhesive interface [13]. Acidic monomers remain active when poorly polymerized resulting in continuous etching of the underlying dentin [27]. 

Application of the adhesive Application must be performed - at the speed of the equipment - as an even coat, with the required coverage - Viscosity - Wetting of materials - Coverage, consumption - Stability of the adhesive in the tanks - Melting temperature of the hot melts... 

Apart from hardener, some adhesives in this category also contain an additional activator (e.g., catalyst such as monurone - also used as cross-linking agent). With that, a faster rate of reaction can be achieved, which effectively shortens the duration needed for the adhesion process or lowers the activation temperature. However, this also leads to reduced storage stability of the adhesive. 

The test program should also subject the coating to environmental stress (time, temperature, chemical, mechanical fatigue, etc.) in order to evaluate the stability of the adhesion in the service environment. 

---