Autoadhesion

Environmental conditions under which solvent release from the adhesive on the substrate is produced must be carefully controlled. Humidity is critical because loss of heat due to solvent evaporation may allow attainment of the dew point (the evaporation of the solvent is an endothermic process), and then condensation of water on the adhesive can result. This phenomenon is often called moisture blooming. The presence of water on the adhesive film causes a detrimental effect because the autoadhesion of rubber chains is greatly inhibited. Therefore, humidity must be controlled and avoided by increasing the temperature during solvent evaporation. 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

Voyutsky, S. S. Adhesion and Autoadhesion of Polymers (1960), Engl. Trans. Interscience Publ. (1963)... 

Barsberg, S. and Hassingboe, J. (2003). Laccase catalysed enhancement of the autoadhesion of TMP fibres. In Proceedings of the First European Conference on Wood Modification, Ghent, Belgium, Van Acker, J. and Hill, C.A.S. (Eds.), pp. 381-388. 

D 1995 Test Method for Multi-Modal Strength of Autoadhesives (Contact Adhesives)... 

Further, Yamawaki et al. [24] hot-pressed surface cyanoethylated wood fibers at 240°C to make a board by autoadhesion of the thermally melted fibers. When the cyanoethylated fibers were chlorinated, the pressing temperature was reduced to 130 C. The thermal flow temperature was decreased... 

Podczeck F, Newton JM. James MB. Assessment of adhesion and autoadhesion forces between particles and surfaces. I. The investigation of autoadhesion phenomena of salme-terol xinafoate and lactose monohydrate particles using compacted powder surfaces. J Adhesion Sci Techn 1994 8 1459-1472. 

Podczeck, F., J. M. Newton, and M. B. James. 1995b. Adhesion and autoadhesion measurements of mi-cronized particles of pharmaceutical powders to compacted powder surfaces. Chem. Pharm. Bull. 43 1953-1957. 

The strength of fibrils and microfibrils is determined by different structural elements. With microfibrils, the intrafibrillar tie molecules are the main source of strength. With fibrils, however, the few interfibrillar tie molecules are of minor importance but the enormous surface of the long (10//,), thin (100-200 A) microfibrils produces autoadhesive forces so great that the deformation of fibrils by axial displacement of microfibrils is very difficult to achieve. 

This produces high bending moments and hence a relatively greater acceleration of the fibre after junction failure. The opportunity for the secondary junction formation is then reduced since the kinetic frictional force will decrease rapidly with velocity. There is some analogy to the dwell time effect in autoadhesion. 

After removing the protective mylar film, the Tel-Tak measured the maximum force in psi required to separate a inch wide rubber test specimen from another identical rubber specimen after these test pieces are touched together under a given load for a given dwell time that is defined as between 0,1 and 6,0 minutes and a. selected contact pressure that is defined a.s between 16 and 32 psi. The rate of separation is 1 in. min. The tack (autoadhesion) is reported as the force required to separate the two identical rubber specimens. This test can be performed again where a stainless steel strip replaces the lower rubber specimen in order to measure the stickiness" of the upper rubber specimen on contact with this polished stainless steel surface. Lastly the so called true tack is calculated by subtracting the stickiness value (rubber-to-metal) from the original rubber-to-rubber tack value. 

In the EjE type, the segments, and to some extent also the macromolecules of the adherent and the adhesive themselves, are able to move. Therefore, they can diffuse into one another. If adherent and adhesive are chemically equal, then self-diffusion is observed for this type. Self-diffusion leads to self-adhesion (autoadhesion) which is responsible for the tack of freshly cut natural rubber. 

Voyutskii, S.S., Autoadhesion and Adhesion of High Polymers. Wiley-Interscience. New York, 1963. 

Discharge treatment of low-density polyethylene (LDPE) has been studied by many workers, particularly with reference to the autoadhesion enhancement effect. LDPE autoadheres when two surfaces are contacted under pressure... 

The autohesion effect is especially good, therefore, when weak crystallization occurs on applying pressure or during annealing, as, for example, with natural rubber or with l,5-trans-poly(pentenamers) (physical cross-linking). On the other hand, if the crystallization is too strong, the deformability of the adhesive is too small (see Section 7.4.2). If adherent and adhesive are chemically different, then in the EfE type this leads to interdiffusion and thence to heteroadhesion. Of course, marked interdiffusion is only possible when the different macromolecules are compatible with one another, and the strength of the autoadhesion or heteroadhesion depends on both diffusion and adsorption. 

Tackifiers. Pine tar, coumarone-indene resins, zylol-formaldehyde, and other resins are used to increase the tack of rubber compounds. Tack, here, means stickiness of the un-cmed rubber stock to itself, rather than to other things, such as metal surfaces. Tack has also been called autoadhesion. It is extremely important for building up structures such as tires. Natural rubber inherently has good natural tack, but most synthetic rubbers do not. 

The open time is the time by which the coalescence between two identical adhesive films drops to an unacceptable level, that is, the time after which autoadhesion and diffusion cannot be adequately produced. During the open time, several changes in the adhesive films are produced because of the loss of residual solvent, which affects the viscoelastic properties of the film (loss of plastification) and the crystalUzation of the rubber chains. 

Autoadhesion (tackiness) n. The ability of two continguous surfaces of the same material, when pressed together, to form a strong bond that prevents their separation at the place of contact. 

Sheet-molding compound (SMC) n. A fiberglass-reinforced thermosetting compound in sheet form, usually rolled into coils interleaved with plastic film to prevent autoadhesion. This term was chosen to replace the term prepr, which was deemed to be confusing and insufficiently definitive. SMC can be molded into complex shapes with little scrap, and is low in cost. 

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