Brittle film adhesion

Further work in this area is underway employing polyamic acid systems which are known to produce higher viscosity solutions (e.g. polyimides derived from 4,4 -oxydianiline and either BTDA or pyromellitic dianhydride). This is being carried out in the belief that higher viscosity solutions will give rise to higher quality, less brittle films and will, thereby, enable a broader spectrum of metal systems to be studied regarding the adhesive and electrical conductance properties of metal ion filled polyimides. 

There are several common forms of solid epoxy adhesives. These include film, tape, powder, and preformed shapes. Certain formulations are better suited for specific forms. For example, casting of tape or film adhesive from solvent solutions lends itself to working with multicomponent hybrid systems, where each resin can be solubilized and blended together in a universal solvent. B-staged systems are generally more brittle and better suited for powders or preformed adhesives. 

When considering a ductile substrate coated with a brittle film and subjected to a uniaxial strain during loading and before debond of the film (perfect adhesion), the displacement is assumed to be continuous at the interface. Then, the substrate deformation is entirely transmitted to the film through the interface. When exceeding the critical cracking strain of the film, a network of transverse cracks develops. At the interface, each crack tip will be surrounded by a... 

Hardness often goes with brittleness, and brittle films - particularly those subjected to temperature changes - do not survive long, failing by cracking and then flaking off if adhesion is only mediocre. 

Phenolic based adhesives are outstanding in their wetting characteristics and strength. In the unmodified form, however, they tend to be brittle. To overcome this problem, phenolic based adhesives usually contain a rubber modifier to increase the crack resistance of the adhesive. The first modified phenolic contained polyvinyl butyral and was known as the "Redux" adhesive. Many modern phenolic adhesives contain nitrile rubbers (copolymers of acrylonitrile and butadiene) as the modifiers. An example of a phenolic based film adhesive is AF-30 which is listed in Table 1 along with its physical properties. This film... 

These PVAs are not normally suitable for use as adhesives in their basic unmodified state as they tend to form brittle films and have limited adhesion capabilities. They are, however, compatible with a wide range of compounding ingredients, which are capable of modifying their properties very considerably. Plasticizers and thickeners are two of the materials most commonly used in the formulation of PVA-based adhesives, but many other modifiers may be incorporated as required, resulting in a very wide range of PVA-based adhesives being available to meet the specific demands of many different applications. 

It should be pointed out that such a composition of 50 vol. % solids dried to an extremely brittle state before the last of the water is removed. In considering the behavior of fillers in plastics, for example, more than about 40% by volume gives a hard, relatively brittle mass. Likewise, less than 40% by volume of colloid in silicate should give a less brittle film as the last of the water is removed. In this case, by similar calculations it is found that this corresponds to a SiO-iNajO fatio of 3.5. Perhaps by coincidence this is near the common ratio used in adhesives. 

The percolation threshold was between 34% and 38% content of polyaniline. However, brittle films are obtained at these concentrations. The electrode coating method was also used in the same work, and it improved the adhesion of polyaniline to an optically transparent electrode, facilitating spectroelectrochemi-cal measurements. 

M. D. Drory and J. W. Hutchinson, Measurement of the adhesion of a brittle film on a ductile substrate 1 indentation, Proc. R. Soc. Land.,... 

Rapid drying is one of the most important characteristics of nitrocellulose-based coatings. Nitrocellulose alone produces hard and brittle films, and therefore, nitrocellulose-based coatings generally are formulated with plasticizers to improve flexibility and adhesion as well as compatibility with other resins. They are also blended with other polymers such as hard resins and non-drying alkyd resins, to name a couple. On the limiting side, nitrocellulose-based lacquers have generally poor resistance to chemicals and poor durability. 

Properties Brittle films, similar to - thin-boiling starch Good adhesion, stable viscosity ... 

Phenolic materials go back to the patents of Baekeland and are based upon the reaction of phenol and formaldehyde to yield a lower molecular weight resinous material that can be formulated with other resins and sometimes other curatives to yield a paste adhesive or a film adhesive. When phenol and formaldehyde are reacted in the presence of an acidic catalyst with an excess of phenol versus formaldehyde, they yield what are known as novolac resins. These materials are soluble in organic solvents and do not react further with themselves thus, they need to have a curative added. That curative is most often hexamethylene tetraamine. Another type of phenolic resin can be generated with an excess of formaldehyde and under basic conditions. These materials, known as resole phenolics, will react with themselves to yield a ftilly cured phenolic and thus must be stored frozen in order to limit this reaction. Thus, resole phenolics do not need an external CTOsslinker. Resole phenolics are widely used in the binding of paper products and the bonding of wood. Adhesives based upon these materials were brittle and could only be used to bond wood, where it is still widely used. A more widely usable adhesive was developed during World War II which modified the very brittle phenolic adhesive with poly(vinyl formal) resins. These materials were... 

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