Mechanical pressure-sensitive adhesive

In the earlier art, there was some consideration that partial incompatibility of the tackifier resin with the rubber was responsible for the appearance of tack, but this no longer is seriously held in light of continuing studies by many investigators. Aubrey [38] has addressed this in his review of the mechanism of tackification and the viscoelastic nature of pressure sensitive adhesives. Chu [39] uses the extent of modulus depression with added tackifier as a measure of compatibility. Thus in a plot of modulus vs. tackifier concentration, the resin that produces the deepest minimum is the most compatible. On this basis, Chu rates the following resins in order of compatibility for natural rubber rosin ester > C-5 resin > a-pinene resin > p-pinene resin > aromatic resin. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

Filled Graft Rubber as the Disperse Phase. Rubber-modified polystyrene is generally obtained by polymerization grafting of a rubber in the presence of styrene monomer. The polymerization is carried out totally or partially in mass with the aid of shearing agitation, as patented by Amos et al. (1). The study on the initial stage of this type of polymerization was first published by Bender (5), and phase inversion similar to that discovered for the two-phase pressure-sensitive adhesives was observed. The mechanism of particle formation has also been reviewed (47). 

The main use of adhesives in labelling applications is in the form of pressure-sensitives, i.e., sticky labels attached either directly or indirectly (behind a potential barrier layer) to a foodstuff. Pressure-sensitive adhesives are a distinct category of adhesives that in dry form are permanently tacky at room temperature. These adhesives will adhere to a variety of substrates when applied with pressure they do not require activation by water, heat or solvents and they have sufficient cohesive strength to be handled with the fingers or by mechanical means in labelling stations. 

The primary mode of bonding for a pressure-sensitive adhesive is not chemical or mechanical but rather a polar attraction to the substrate. This always requires pressure to achieve sufficient wet-out onto the surface thereby providing adequate adhesion. The four main varieties of pressure-sensitive adhesives are derived from rubber-based, acrylic, modified acrylic and silicone formulations. Release liners are used to carry the sticky label and enable it to be printed. The release liners are normally paper, treated with a very thin silicone coating to allow the label to be peeled away easily without tearing. Some transfer of the silicone into the adhesive is inevitable. 

Use Tire carcasses and linings, especially for tractors and other outsize vehicles electric wire insulation encapsulating compounds steam hose and other mechanical rubber goods pond and reservoir sealant. Latex is used for paper coating, textile and leather finishing, adhesive formulations, air bags, tire vulcanization, self-curing cements, pressure-sensitive adhesives, tire-cord dips, sealants. 

Fillers provide films with conductive properties, influence their surface properties, affect their permeability, mechanical and optical properties, and affect their durability against environmental exposure. Various technologies are used to produce conductive films. These include lamination to metal foils (in-plant, using pressure sensitive adhesives), surface coating, and addition of conductive materials. Conductive films are widely used in packaging to limit static electricity. 

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

Bon, Keddy, and coworkers [109] demonstrated that soft armored polymer latex made via Pickering miniemulsion polymerization [i.e., poly(lauryl acrylate) armored with Laponite clay discs] could be used as a nanocomposite additive in standard poly(butyl acrylate-co-acrylic acid) waterborne pressure-sensitive adhesives (PSAs), leading to marked mechanical property enhancements (see Fig. 13). 

Not only new techniques deserve attention, but also any new developments in old techniques, such as scanning electron microscopy (28). Scanning electron microscopy, for example, can now enhance the examination of the adhesive interface in greater detail. Two other old techniques have also found new applications in adhesive chemistry. One is dynamic mechanical analysis (29,30), which has been accepted for the study of pressure-sensitive adhesives and the curing mechanism of epoxy resins (31,32). The other is the use of a fluorescence probe to examine the curing mechanism (33). 

B. C. Copley, "Dynamic Mechanical Properties of Silicone Pressure Sensitive Adhesives," in Ref. 14. 

The fourth and fifth papers have to do with properties of pressure-sensitive adhesives. In particular, the matter of how the materials composing pressure-sensitive adhesives (rubbers and resins) interact and phase separate to produce the phenomenon of tack or pressure-sensitivity is addressed. Both studies use dynamic mechanical measurements to uncover phasing - one in a silicone and the other in natural and styrene-butadiene rubber systems tackified with various resins. 

The paper by Copley presents a study of two types of silicone pressure-sensitive adhesives, of known composition, by dynamic mechanical and thermal analysis. The materials were prepared from a siloxane resin and two silicone gums poly(dimethylsiloxane) and poly(dimethyl-co-phenylsiloxane). 

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