Tackification

Class and Chu [34] have studied the tackification of natural rubber and SBR over a wide range of resin concentrations for several tackifiers. From their graphical data it can be estimated that 1 1 tackification (by weight) with a poly(/-butyl styrene) resin, MW 850 and Tg = 59°C, gives a PSA with Tg about — 13°C, and storage modulus, G about 8.8 x 10 Pa, well within the PSA window. 

The net effect is that tackifiers raise the 7g of the blend, but because they are very low molecular weight, their only contribution to the modulus is to dilute the elastic network, thereby reducing the modulus. It is worth noting that if the rheological modifier had a 7g less than the elastomer (as for example, an added compatible oil), the blend would be plasticized, i.e. while the modulus would be reduced due to network dilution, the T also would be reduced and a PSA would not result. This general effect of tackification of an elastomer is shown in the modulus-temperature plot in Fig. 4, after the manner of Class and Chu. Chu [10] points out that the first step in formulating a PSA would be to use Eqs. 1 and 2 to formulate to a 7g/modulus window that approximates the desired PSA characteristics. Windows of 7g/modulus for a variety of PSA applications have been put forward by Carper [35]. 

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. 

Besides the higher volume pressure sensitive adhesives discussed above, the industry also uses other synthetic elastomers as the base component for PSA formulation. Most of these elastomers require some form of tackification to make the materials tacky. However, a few materials are low enough in Tg and sufficiently compliant to be useful without requiring compounding with tackifiers. 

A review of the traditional use of resins as tackif iers for elastomers in solvent-applied adhesives is presented. Following is a discussion of how this technology is extended to water-based systems. Data is discussed on the use of various backbone polymer latexes, modified with the proper resin emulsions to achieve pressure—sensitive adhesive properties. Recommendations are also given for the proper choice of resin type to achieve optimum properties with each specific class of elastomer. 

Table 9. Tackification of Acrylic Latexes with Emulsions of the Glycerol Ester of Highly Hydrogenated Rosin or the Phthalate Ester of Hydroabietyl Alcohol...

The basic resin and dispersion technology has been developed to meet the tackification needs of the industry as it moves forward to water-based products no matter what backbone polymer is chosen. 

Acrylic adhesives tend to be more stable than rubber-based adhesives since they have a saturated backbone. They are also generally designed so that they function as PSAs without the need for any tackification or plasticization, although tackifiers can be added to modify adhesion or reduce cost. Both of these features help to differentiate them from rubber-based PSAs. The ability to avoid use of tackifiers and antioxidants makes acrylics a good choice for medical applications where minimal skin irritation is desired. 

Kumar KD, Tsou AH, Bhowmick AK (2010) Unique tackification behavior of needle-like sepiolite nanoclay in brominated isobutylene-co-p-methylstyrene (BIMS) rubber. Macromolecules 43 4184—4193... 

Ethylene-propylene and ethylene propylene-diene rubber compounds have poor building tack, so, when such a property is required (e.g., for assembling operations), the use of tackifers (cumarone resins, xylene formaldehyde resins, etc.) is indispensable. 

Plasticizers and softeners include the phthal-ates such as dioctyl phthalate and diisobutyl phthalate, natural oils such as lanolin, and paraffin, naphthenic and aromatic oils obtained from petroleum refining. Liquid resins from rosin or petroleum feedstocks can serve the dual purpose of tackification and plasticization. 

The tackification of the triblock copolymers differs from that of other elastomers in that two tackifiers, of quite different solubility parame-... 

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