Adhesion mechanical work

A needled felt, on the other hand, is a fabric composed of natural, synthetic, or a combination of natural and synthetic fibers physically interlocked by the action of a needle loom with or without combination of other textile fabrics and with or without suitable combination of mechanical work, chemical action, moisture, and heat, but without weaving, knitting, stitching, thermal bonding, or adhesives (16). 

A multilayer-type structure probably due to cords in the molten zone between single arc sprayed (0.25 MPa) Ni droplets and steel substrate were found in AES point depth profiles [2.158]. That particular arc spraying condition turned out to yield the best adhesion. Plasma-sprayed AI2O3 layers separated from pre-oxidized Ni Substrate had a micrometer-thick NiO layer on the substrate-sided face and micrometer-deep oxide interdiffusion [2.159]. In this work also, AES point depth profiling substantiated technological assumptions about adhesion mechanisms. 

The most-often cited theoretical underpinning for a relationship between practical adhesion energy and the work of adhesion is the generalized fracture mechanics theory of Gent and coworkers [23-25] and contributed to by Andrews and Kinloch [26-29]. This defines a linear relationship between the mechanical work of separation, kj, , and the thermodynamic work of adhesion ... 

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

Research aimed at imderstanding bacterial pathogenesis has established the importance of bacterial adherence in disease. This research has led to the identification of a number of both bacterial adhesins and potential host cell receptors. By imderstanding the detailed interactions between a bacterial adhesin and host receptor, it is possible to develop new mechanisms to prevent bacterial adhesion, thereby averting disease. Many promising anti-adhesion mechanisms have been developed and studied, but much more work is needed, both in vitro and in vivo, to establish the feasibility of these mechanisms. 

Additional work must be completed before these hydration inhibitor treatments will be widely used. However, it appears that combined FPL/inhibitor pretreatments have the potential of producing water stable aluminum oxides with structures that promote mechanical aspects of adhesion in a relatively simple manner. Since mechanical adhesion mechanisms are not greatly affected by water, these pretreatments show promise as a means of increasing the durability of metal/polymer adhesion systems in wet environments. 

Mechanical equilibrium is established when small reductions in free energy due to formation of adhesive contact just balance small increases in mechanical work of deformation of the vesicle (8,11). Tnis variational statement leads to a direct relation between the free energy potential for adhesion and the suction pressure applied to tne adherent vesicle,... 

Usually j/ is very much larger than Go- This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than work of adhesion or work of cohesion. However, a modest increase in Go may result in a large increase in practical (measured) adhesion as ijr and Go are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

The practical aspect of the iPP surface modification by the corona discharge can be demonstrated on the adhesive properties of modified and unmodified iPP carp>et fabrics treated by rubbing pastes based on butyl acrylate-vinyl acetate copolymer. The results are summarized in Table 1. According to Table 1 the value of mechanical work of adhesion towards PP is rising with the increase of the rubbing paste mass. 

Table 1. Mechanical work of adhesion in the adhesive joint iPP backing fabric-butyl...

Modification is the most important factor, since with 50 g.cm-2 of the rubbing paste on iPP fabric the mechanical work of adhesion was higher than that for unmodified PP fabric with 150 g.cm-2 layer of the rubbing paste. The adhesive properties given as the values of the work of adhesion improved, on average, by 65 % when compared the backing fabric based on modified PP fabrics non-modified material. 

The higher efficiency of the modification for extruded iPP when compared to biaxially oriented iPP was explained by different susceptibility to oxidation due to different crystallinity. Linear dependence of either PC of the surface energy or hydroperoxide concentration of modified iPP on current density was observed. A decrease in the exposition time of iPP foil resulted in lower modification effect. The sensitivity on exposition time was more pronounced for extruded iPP. Aging of modified PP foil leads to a significant drop in surface energy during first 24 hours for extruded iPP, while for biaxially oriented iPP the decrease was small. An increase of the mechanical work of adhesion was observed if temperature of auto-adhesive joints rised. The observed decrease of hydroperoxide concentration after a destruction of the auto-adhesive joints is in accordance with the previous data. 

Fig. 12. Mechanical work of adhesion to polyvinyl acetate for iPP modified by corona discharge during long-term aging a - biaxially oriented iPP, b - extruded iPP...

The results in Fig. 16 show that the adhesion of iPP to more polar polymers as well as to metals (Fig. 17) is low. The difference between a polarity of iPP and other more polar polymer as a consequence of iPP grafting by lA was diminished. The adhesion pwameters in the system iPP grafted by lA - more polar polymer increases with an increase of the lA grafted content. Grafting of iPP by 5 wt.% of lA leads to a growth of mechanical work of adhesion of iPP 6.9 times, 7.2 times and 18.1 times when considering the adhesion of iPP to cellulose acetate, polyamide and aluminum, respectively. 

Fig. 16. Mechanical work of adhesion of iPP grafted by itaconic acid to cellulose acetate (a) and polyamide (b) against itaconic acid grafted concentration...

The mechanical work of adhesion of iPP modified by chromyl chloride and chromo sulfuric acid vs. time of modification is illustrated in Fig. 26. It was foimd that the mechanical work of adhesion is 36 % higher in the case of chromyl chloride compared to the modification with chromo sulfuric add. The value of mechanical work of adhesion increased rapidly, in a short time (2 min.) reached the maximum value and soon afterwards assumes a constant value. 

Fig. 26. Mechanical work of adhesion of iPP modified by chromyl chloride to polyvinyl acetate against time of modification a - chromo sulfuric acid, b - chromyl chloride...

The surface and adhesive properties of iPP modified by UV radiation in the presence of POCI3 have been studied. Modification of iPP by UV/PCXZI3 results in an increase in surface energy of polymer as well as in an improvement of the mechanical work of adhesion of modified iPP to more polar polymers. 

Here, Si, S2, S3 are the interfaces of solid-liquid 1, solid—liquid 2, and liquid 1-liquid 2, and v is the rate of separation. When cementing in water and oil products, the adhesive is liquid 1 and the medium is liquid 2. Equation (5.2) indicates that spontaneous wetting (without mechanical work to the interface) is possible when the surface tensions of the adhesive and the liquid are equal, i.e., for crad Hq = although the maximal value of (Tadliq is desirable because it is directly proportional to the adhesion thermodynamic work. 

Thus, it is impossible to obtain a strong adhesive-bonded joint when cementing in liquid media without supplying mechanical work to the adhesive-substrate interface, even when thermodynamic conditions are appropriate for selective wetting of the substrate by the adhesive. Practically, the mechanical work can be supplied by mutual displacement of the adhesive and the cemented surfaces, for example, by pumping the adhesive into the gap between the surfaces, by pressing the surfaces together so that the adhesive is expelled from the gap, by ultrasonic treatment of the adhesive layer, and so on. 

Application of mechanical work to the adhesive-metal interface (by adhesive flow through the gap between the dollies at various speeds) had greater effect on the adhesion strength when cementing was performed in the liquids than in the air (see Fig. 5.2). This can be explained by the fact that the work done facihtates the selective wetting of the substrate by the adhesive the flowing adhesive picks up drops of liquid from the substance surface and removes them. 

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