Tackifier resin

Compounding is quite different for the two systems. The solvent base system is dependent on magnesium oxide and a /-butylphenoHc resin in the formulation to provide specific adhesion, tack, and added strength. Neither of these materials have proven useful in latex adhesive formulations due to colloidal incompatibihty. In addition, 2inc oxide slowly reacts with carboxylated latexes and reduces their tack. Zinc oxide is an acceptable additive to anionic latex, however. Other tackifying resins, such as rosin acids and esters, must be used with anionic latexes to provide sufficient tack and open time. 

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. 

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

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. 

Tackifiers. Resins are generally added to adjust the desired tack. In general, resins must be used with plasticizers to obtain a good balance between tack and cohesive strength. Typical tackifiers are polyterpenes, although hydrocarbon resins and modified rosins and rosin esters can also be used. In some cases, terpene-phenolics or phenol-formaldehyde resins are added to increase adhesion. 

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. 

Case and carton sealing is a high volume application for hot melt adhesives. Typical formulas use 20-30% wax, 25-40% polymer, and 35-55% tackifier resin. The basic requirements for packaging adhesives will be described followed by formulating guidelines for specific market areas. The focus of this discussion will be on EVA and EnBA-based systems, which are the most common. 

Schlademan, J., Tackifier resins. In Satas, D. (Ed.), Handbook of Pressure Sensitive Adhesive Technology, 2nd edn. Van Nostrand Reinhold, New York, 1989, p. 527. 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

The reactive monomer, besides being a diluent, plays a role similar to that of tackifying resin, namely, affecting the deformability of the adhesive. 

Applications and Formulation. Pressure-sensitive adhesives are most widely used in the form of adhesive tapes. The general formula for a pressure-sensitive adhesive includes ail elastomeric polymer, a tackifying resin, any necessary fillers, various antioxidants and stabilizers, if needed, and cross-linking agents. 

Butyl Rubber. Butyl rubber is used in conjunction with other thermoplastic hydrocarbon rubbers (for example, polyisobutylene) to make pressure-sensitive hot-melt adhesives. Such formulations include also tackifying resins and oils. Adhesives of low viscosity are produced, and these may be applied by the hot-melt spray technique (see page 106). 

Styrene Copolymers. The so-called thermoplastic rubbers based on styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers can be used for hot-melt adhesives, particularly when extended with tackifying resins and oils. They can be made into pressure-sensitive adhesives, as melts with low viscosity—being applied from fine spinnerets which are oscillated to make a... 

The two most prominent liquid radiation curable adhesives are free radical polymerization epoxy acrylates and cationic polymerization epoxies. Such adhesives are generally used as polymerizable syrups. A wide range of prepolymers can be acrylated including epoxies, urethanes, polyesters, polyethers, and rubbers. Elastomer-tackifying resin blends are often used in these formulations. 

Figure 21.13 Storage and loss moduli of an SIS triblock copolymer as a function of temperature. Formulation with tackifying resins brings the peak in loss modulus to near ambient temperatures for adhesives...

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