Effects of Tackifiers

Fig. 15. Effect of tackifying resin on storage modulus of addition-cured silicone PSA.

Figure 6 displays the effect of tackifier on abrasive resistance of mbber compound as the tread mbber of tire. The mbber compound was vulcanized at 170°C for 12 min and then was then evaluated as tread mbber in tire under the conditions of loading of 2.5 kg, a slip ratio of 40%, a temperature of 20°C, and a measuring time for 2 min using a Lamboum abrasion tester manufactured by Iwamoto Sei-sakusho Co., Ltd. The abrasive wear of the tire tread of passenger cars and tracks... 

Shih H.H., Hamed G.R., Peel adhesion and viscoelasticity of poly(ethylene-co-vinyl acetate)-based hot melt adhesives. 1. The effect of tackifier compatibility, 7. Appl. Polym. Sci, 63(3), 1997, 323-331. 

Figure 11. Effect of tackifier resin on the storage modulus of an elastomer I209J... 

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]. 

These substances determine the heat and moisture resistance of the adhesives, but are also responsible for the effectiveness of the tackifying agents. Pure acrylates are among the best PSAs. 

Fig. 22.5 Master curves showing the effect of the addition of 60 wt.% of Escorez 5380 tackifying resin on C (continuous iines) and C" (broken iines) of the SiS biock copoiymer. Reference temperature 20°C.

To illustrate the effect of sample thickness, we can compare adhesives to sealants, which in many cases can be viewed as thick adhesives. As the sample thickness is increased, the benefit of the BTZ is clearly demonstrated. In Table 3 a SEBS/hydrogenated hydrocarbon tackifier sealant formulation was prepared as a hot melt and poured into shallow petri dishes. Although both of these polymers have good inherent stability, sealant applications may require extended exposure to UV radiation. The discoloration data show that the BTZ prevents yellowing of the sealant. However, examination of the sealant surface shows surface crazing and cracking when not protected by incorporation of the HALS. The combination of the two classes of light stabilizers provides the best overall performance. 

The prolonged effects due to thermal oxidation of a tackifier during storage correlate directly with the level of discoloration and viscosity changes in the HMA formulation. An effectively stabilized tackifier will produce a HMA with good color and controlled viscosity. When used in a HMA formulation, an unstabilized tackifier will result in a... 

The effects of storage time of an unstabilized and a stabilized rosin ester tackifier on the properties of an EVA HMA are illustrated in Figs. 15-17. Significant effects on the initial color of the EVA HMA (Fig. 15) are observed when using an unstabilized tackifier. An increased level of hydroperoxides is also noted. In this situation, the addition of an antioxidant to the HMA will not correct the problem. However, the addition of an antioxidant to the HMA may reduce further discoloration during compounding or end-use applications. 

Figure 16 illustrates the effects of an unstabilized tackifier on color formation as a result of high-temperature aging of the EVA HMA formulation. In this scenario the tackifier was aged for 18 days at 50° C and then combined with the other components at 177°C (350°F). The final HMA formulation was then aged at 170°C (338°F). Use of the unstabilized tackifier results in a darker initial color and a more rapid rate of discoloration than that of HMA using the stabilized tackifier. 

TABLE 4 Effect of incorporation of 2.5phr of tackifier on tread rubber... 

There have been very few studies reported on the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. In 1973, M. Sherriff and co-workers (1) reported on the effect of adding poly (j3-pinene) resin to natural rubber. Based on a G master curve, they showed that the resin shifted the entry to the transition zone to a lower frequency and reduced the modulus in the rubbery plateau. G. Kraus and K.W. Rollman (2) reported in 1977 on their study of resins blended with styrene-isoprene-styrene block copolymers. They showed that the addition of a resin increased the glass transition temperature of the rubbery mid-block and decreased the plateau modulus. Accordingly, a satisfactory tackifying resin should produce these changes. 

A summary is presented of topics discussed at a conference on adhesive tapes organised in Athens by AFERA. These included the use of iodopropynyl butyl carbamate for the protection of water-based adhesives from biological attack, the formulation of NR latex based adhesives with tackifiers in aqueous dispersion, the effects of silicon contamination on adhesion to plastics film supports treated in different ways, possible harmful effects... 

The difference between plasticizers and tackifiers is somewhat arbitrary and the effect of this difference is described below in the section on Physical Properties. Plasticizers are typically lower in molecular weight and melting or softening point than are tackifiers. Hydrocarbon oils including aromatic, naphthenic, and paraffinic are commonly used with rubber-based adhesives. Phthalates, such as dioctyl phthalate or dibutyl phthalate, are more typically used in small amounts with acrylics. Since plasticizers are usually low molecular weight compounds, there is often more latitude in the chemical types that will be compatible with the base pol5mier because of the contribution from entropy of mixing. 

In Fignre 5, the storage modulus of a typical cross-linked rubber is compared to that of a PSA made from a mixture of the rubber and tackifier. The rubber has a low which is why it is soft and mbbery at room (or use) temperature, but its modulus is too high for it to be a PSA. Addition of tackifier decreases the modulus below the Dahlquist criterion and allows for the mixture to be a PSA. The effect of dilution of the rubber network with a tackifier can be predicted by rnbber elasticity theory nsing the equation below (31,32). The effect of fillers can also be predicted. 

As described above, the distinction between plasticizers and tackifiers is somewhat arbitrary, but the important difference between the two is that addition of a plasticizer will not affect or will lower the Tg of the PSA formulation. The effect of lowering the modulus is the same as for a tackifier. 

In the final paper, Azrak, Joesten and Hale illustrate the effect of reformulation of acrylic contact adhesives from solvent-based to water-based systems. Requirements on the tackifier and polymer latex are discussed which lead to an adhesive with the proper balance of open time and green strength. 

Tg, in the range of 30-35°C. Such hard resins might not be expected to tackify an already soft acrylic. However, except for heat activated samples, contact tack is measured when there is residual carrier. Therefore, the effect of organic solvents or water on the Tg must be considered. 

It is obvious that relatively small variations in the chemical structure of tackifying resins can exert appreciable effects on its physical properties and these in turn effect the adhesive efficiency of the resin in adhesive applications. Our studies focused on the chemistry of pol3nnerization, have aided us in defining the resin structure and should aid us in the future in designing resins to meet the needs of the adhesives industry. 

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