Tackifying effect

Inorganic whiskers with a tiny size can be used in coatings as tackifiers to improve coating viscosity Increase thixotropy  

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Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

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]. 

Class and Chu demonstrated that if a tackifier is chosen that is largely incompatible with the elastomer, a modulus increase due to the filler effect is observed and little change in Ta results, and once again a PSA would not be obtained. This was observed for mixtures of low molecular weight polystyrene resin and natural rubber. The same polystyrene resin did tackify SBR, a more polar elastomer that is compatible with the resin. Hydrogenating the polystyrene to the cycloaliphatic polyvinylcyclohexane changed the resin to one now compatible with the less polar natural rubber and no longer compatible with SBR. These authors also provide... 

The presence of these low molecular weight tackifiers and plasticizers may also have other negative effects on the PSA performance. For example, the reduced entanglement of the polymer typically reduces the cohesive strength of the PSA, although crosslinking may be used to compensate for this loss in property. Plasticizers and tackifiers may also be susceptible to migration and/or oxidation, both... 

Similar to the tackifiers discussed earlier, plasticizers have a very dramatic softening effect on the rubbery plateau modulus of the PSA. For this reason, high levels of plasticizers have to be avoided to maintain good cohesive strength in the adhesive, especially at elevated temperatures. Indeed, if high cohesive strength is desired, the amount of plasticizer used in a PSA is typically kept to a minimum, if used at all. 

Continued speculation exists about the exact interaction of the MQ tackifier with the polysiloxane gum. Dynamic mechanical thermal analysis of a typical silicone PSA commonly shows two major transitions a Tg at low temperatures close to that of the pure gum, and a second T at higher temperature. Increasing tackifier loadings have little effect on the first transition but, as shown in Fig. 15, they shift the second transition to increasingly higher temperature [111]. By using... 

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

A more quantitative estimation of compatibility can be obtained with the solvent cloud point test. The solvent cloud point is based on the idea that resins will be compatible with elastomers of similar chemical nature. Thus aliphatic resins will be effective tackifiers for aliphatic elastomers, such as natural rubber, while aromatic solvents are needed for aromatic elastomers, such as SBR. Solvent cloud point tests are carried out in three solvent systems which represent aliphatic, aromatic, or polar systems [16j ... 

In most adhesives, tackifier is the ingredient present in the highest proportion. Tackifying resins are primarily used to reduce adhesive viscosity and adjust the 7g of the adhesive s amorphous matrix phase. Through their effects on the other ingredients and the overall system they can also dramatically affect wet out, hot tack, open time, set speed, and heat resistance. 

Common plasticizers are used to reduce viscosity and to aid adhesion. Most plasticizers commonly utilized in PVC are also used in urethanes. One of the most common plasticizers is diisodecyl phthalate, though many others are used equally effectively. In some cases tackifiers, such as certain esters or terpine phenolics, are utilized to obtain specific adhesion characteristics. 

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. 

The pure diblock-triblock blends previously described cannot develop sufficient tack onto surfaces, in particular because the modulus level in the terminal zone is too high (Dahlquist criterion [3]). In order to improve tack properties, tackify-ing resins are added to the copolymer blend. They have in fact two important effects [4, 5] ... 

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... 

HMA with high degree of discoloration and an unstable viscosity. Stabilizing a HMA formulation, however, will not correct for the addition of an unstabilized or preoxidized tackifier. The best performance can be achieved with the addition of an effective stabilizer to both the tackifier and the HMA. 

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. 

Borax (sodium tetraborate) in the presence of small amounts of sodium hydroxide is the most widely used additive to starch-based adhesives. It is commonly used in dextrin adhesives, where it increases the viscosity and acts as a tackifier and viscosity stabilizer. These effects are particularly important in machine application of adhesive to substrate. When used in adhesives, borax is often added in amounts up to 10% based on dry starch before the starch is cooked. Enough sodium hydroxide is added to convert the borax to sodium metaborate, which is the active boron species in thickening. The metaborate is able to hook two starch molecules together, forming a complex (Fig. 5) [10]. If additional sodium hydroxide is added, the complex will dissociate the viscosity of the suspension will begin to decrease with increasing sodium hydroxide [11]. 

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... 

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

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