Tackifier selection

Bal Harbour, FI., 25th.-28th. June 2000, p.97-104 SMART TACKIFIER SELECTION. KEY CONSIDERATIONS BEYOND PERFORMANCE... 

As mentioned earlier, the critical characteristic is the correct tackifier and components ratio. Although no rules exist for tackifier selection, there are certain shortcuts, based on chemical compatibility and melt point. Certain classes of tackifiers work well with specific types of elastomers. For example, aliphatic hydrocarbons generally work better with... 

Terpene resins, because of their low odor and acceptable FDA clearance, are used as tackifiers for the natural and synthetic gum bases used in chewing gum. Selected petroleum resins are also used as gum bases. 

Adhesives. Contact adhesives are blends of mbber, phenoHc resin, and additives suppHed in solvent or aqueous dispersion form they are typically appHed to both surfaces to be joined (80). Evaporation of the solvent leaves an adhesive film that forms a strong, peel-resistant bond. Contact adhesives are used widely in the furniture and constmction industries and also in the automotive and footwear industries. The phenoHc resins promote adhesion and act as tackifiers, usually at a concentration of 20—40%. In solvent-based contact adhesives, neoprene is preferred, whereas nitrile is used in specialty appHcations. The type and grade of phenoHc resin selected control tack time, bond strength, and durabiHty. 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

The selection of tackifiers for use in hot melt adhesives is discussed with reference to the area in which to place the emphasis in seeking the information needed for this... 

The basic resins need some form of modification to achieve (1) suitable application viscosities, (2) flexibility, and (3) reduction in costs if possible. Suitable polyamide resins (those of the more flexible variety) are thus frequently modified by the addition of EVA copolymer (high-viscosity, high-melt-point grade). The amount of EVA that can be added is restricted to a maximum of 25% in most cases because of compatibility problems. The blend is then further modified with selected tackifying resin addition and small quantities of filler, to reach an optimum balance of performance properties. To achieve maximum adhesion, it is common for polyamide hot melts of this type to be used in conjunction with a polyamide resin solution primer system for edging material. The primer is invariably a dilute solution of the base polyamide resin. 

The plasticizer is selected from esters of citric acid or benzoic acid. Further a tackifying resin can be added. The tackifier may be a rosin derivative, a terpene derivative, poly(lactic acid), or poly(hydroxy-valerate butyrate) (1). 

These studies demonstrate that in selecting resins to tackify either natural rubber or styrene-butadiene rubber, consideration must be given to the resin structure. An aliphatic resin is more likely to be compatible with natural rubber, while an aromatic resin would be compatible with styrene-butadiene rubber. 

Shell Oil Co. EB/UV radiation cure of composition comprising a monoalkenyl arene/conjugated diene block copolymer, tackifying resin, and a di-tetra functional acrylate or methacrylate selected from the group consisting of acrylic and methacrylic acid esters of polyols. Improved high-temperature properties and solvent resistance. PSA properties,... 

The relative decline in interest in using rosin as a tackifier in adhesive formulations, clearly reflected by the paucity of publications up to the mid-1990s, is counterbalanced by a dramatic increase in the number of papers published in the last decade in scientific and technical journals (more than 50), as predicted by an editorial in Chemical Week in 1995 [74]. A selection from this large output is given here to summarize some of the most salient features arising from the return to the utilization of this renewable resource as a key adhesive component. 

For optimum adhesion, the wetting capability of the hot-melt system must be good. Wetting is related to viscosity and is again largely influenced by tackifying resin selection and quantity. 

The basic polyamide resins need some modification to obtain suitable application viscosity, correct flexibility and to reduce their high cost, when this is possible. The more flexible types of polyamide resins are then often modified by the addition of up to 25% of higher viscosity and higher melt index EVAs. This type of blend can be further modified by addition of selected tackifying resins and small amounts of fillers, to optimize the balance of properties of the system. To achieve maximum adhesion, it is common for polyamide hot melts to be used in conjunction with a polyamide resin primer for edging material. The primer is generally a dilute solution of the basic polyamide resin. 

Modern-day adhesives are often fairly complex formulations of components that perform specialty functions. Very few polymers are used without the addition of some modifying substance such as a plasticizer, tackifier, or inert filler. The selection of the actual ingredients will depend on the end-properties required, the application and processing requirements, and the overall cost target of the adhesive. 

Another example of the compounding latitude afforded by polymer selection in the polyisobutylene family is tack. Tack is increased with Vistanex LM low molecular weight PIB butyl rubber can be depolymerized to any desired tacky state, or a wide range of compatible tackifiers can be incorporated. 

As has been pointed out previously a wide range of properties, particularly rheological properties, can be obtained within the isobutylene family of polymers. In addition, a broad range of tackifiers and plasticizers can be used to extend the viscosity range and to control the tack and cohesive strength level. A common plasticizer is polybutene.This material is available in a number of molecular weights so the viscosity and volatility can be selected for the application. Other liquid materials used as plasticizers include paraffinic oils, petrolatum, and certain phthalates with long, aliphatic side chains such as ditridecyl phthalate. 

Tackifiers are included in adhesive formulations to increase the adhesion of the polymer to various substrates. This is accomplished by facilitating the wetting of the substrate by reducing the viscosity of the hot melt. Besides the adhesive characteristics provided by the tacki-fier, its selection is also based on several physical properties such as color, heat, UV, and oxidation stability, and cost. The suitability of the resin is also largely dependent on its compatibility with other components of the adhesive blend, particularly the polymer. 

Tackifying and modifying resins are represented by a variety of products of many different chemical types and physical properties. To a new adhesive formulator, the array of resins offered must seem bewildering and the choice of an appropriate resin must seem to be nothing more than mix and test. There is, however, a body of knowledge concerning elastomers and resins which allows for a systematic approach to the selection of the best modifying resin for a particular system. This systematic approach will be discussed in more detail shortly. 

The properties that determine whether a product can be labelled as a pressure sensitive adhesive are tack, peel and creep. Tack is the property related to bond formation. Peel defines the tension or force necessary to remove the adhesive tape. Creep is the property describing the flow characteristics of the PSA. Formulators need to be aware of any factors that can directly affect these three properties such as temperature, aging, film thickness, cure rate and post-cure parameters. There are also several formulation variables - these include oligomer selection, tackifier addition, monomer structure, molecular weight and glass transition - that directly impact tack, peel and creep. 

For styrenic block copolymers, the tackifier is normally chosen to be either mid-block (soft block) or end-block selective. For SIS, SEBS, or SEPS the mid-block selective tackifiers are primarily aliphatic materials. For SBS, higher polarity is needed for the mid-block. Tackifiers composed of up to 30% aromatic monomers are used. Rosin ester can be used with any of these polymers, but in all cases is not entirely mid-block selective. A portion of the rosin ester will be incorporated into the styrenic end-block leading to a decrease in softening point, but an extension of the adhesive s open time. More details can be found in Hu and Paul 2009. The level of mid-block tackifier in PSAs based on styrenic block copolymers is very high, often more than half the formula and twice the amount of rubber. 

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