Polymer adhesives initial problems

The primary challenge facing adhesive bonding of metals is to obtain sufficient durability of a bonded structure. Initial bond strength in metal-polymer adhesive joints is almost invariably excellent. Challenging the application of adhesives in polymer-polymer joining, however, is the problem of obtaining a joint that is... 

Undesirable adhesion to metal surfaces may cause a problem in injection molding, melt spinning, and thermoforming. In the melt spinning of PAS a variety of adhesion-related problems have been observed. These problems include the build up of polymer on spinneret surfaces resulting from both initial filament extrusion and lick-back from filament breaks. These adhesion-related problems lead to prematme spin pack failure and equipment down-time to replace spent spinneret packs, and can limit the use of PAS in certain melt spinning operations. 

In numerous applications of polymeric materials multilayers of films are used. This practice is found in microelectronic, aeronautical, and biomedical applications to name a few. Developing good adhesion between these layers requires interdiffusion of the molecules at the interfaces between the layers over size scales comparable to the molecular diameter (tens of nm). In addition, these interfaces are buried within the specimen. Aside from this practical aspect, interdififlision over short distances holds the key for critically evaluating current theories of polymer difllision. Theories of polymer interdiffusion predict specific shapes for the concentration profile of segments across the interface as a function of time. Interdiffiision studies on bilayered specimen comprised of a layer of polystyrene (PS) on a layer of perdeuterated (PS) d-PS, can be used as a model system that will capture the fundamental physics of the problem. Initially, the bilayer will have a sharp interface, which upon annealing will broaden with time. 

The problem of adhesion between a polymer and a metal is strongly dependent on the specific type of polymer and metal involved, as well as on the deposition process under which the interface between the two is formed. In order to improve adhesion, different pretreatment methods can be used, but the development of such techniques requires detailed information about metal-polymer interfaces. Particularly, in the case of thin metal films deposited by physical vapor deposition (PVD) in ultra high vaccum (UHV), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used to obtain chemical information about initial film growth modes,... 

One of the more common difficulties in bonding pine veneers and chips is adhesive dryout. Dry-out is associated with the high liquid absorbancy of pine sapwood and it appears especially during long assembly times. This problem can be overcome by using resins modified through reaction with alkylated phenols, especially 3,4-xylenol [52]. Another technique used to achieve similar results is the manipulation of synthesis procedures used in preparing a standard PF resin [52]. The dry-out resistance imparted by alkylated phenols is due to an initial semithermoplastic character in the resin. This is derived from their monomer bifunctionality and the linear polymer that is consequently formed. 

Problems with electrically conducting adhesives provide another possible opportunity for conducting polymer-dopant compositions. Today s adhesives, such as used to replace soldered connections, conduct because of metal or carbon fillers. While high loadings are necessary to obtain high conductivities, these loadings dilute the mechanical properties of the adhesive and the conductive particles serve as possible nucleation sites for crack initiation. 

Giniewicz [17] has studied the effects of mixing procedure on filler distribution for the manufacture of 0-3 composites. The filler was added to the polymer phase and mixed by hand with a laboratory spatula until it appeared to be well mixed. When the composites were formed, a careful examination showed several defects, most of which were introduced during the initial mixing and dispersion stages of the compounding process. The key problems were inadequate distribution of the polymer, poor adhesion between component surfaces and air entrapment. 

---