When an adhesive is being compounded and applied either in solution or at 100% solids, it is simply a matter of mixing to bring the various components of the system into the homogenious state required for use. Today, however, the use of adhesives based on water dispersions of polymers is an indispensable part of the industry. When the base polymer for the adhesive is dispersed in water, the tackifiers and other components of the system must either disperse readily in the latex or be supplied in dispersed form for easy combination with the base latex.
Early resin dispersions were used to liquefy hard resins to permit their dispersion. This solvent cut technique worked well, but produced a material with a significant content of volatile organic solvent in the system. With the increasing demand to protect the environment, solvent cut-based dispersions are no longer acceptable and solvent free resin dispersions are now required.
The production of resin dispersion is usually done by emulsifying the resin at high temperature (the resin becomes liquid) into the aqueous phase. Once the system has cooled down, the resin becomes hard again and remains finely dispersed in the aqueous phase. For some specialized applications, room temperature liquid resins are dispersed as well. The resin dispersion can be mixed into the polymer dispersion easily. The adhesive film formed upon drying is an intimate mixture of elastomer and tackifier resin.
In combination with a given polymer, the type and concentration of dispersed resin drives the properties of the final formulation. In order to be useful, the resin dispersion must not cause undue changes such as coagulation or grit formation in the elastomer dispersion. The solid resin must also be compatible with the dry elastomer. The amount and type of emulsifier used to form the dispersion are the main factors determining the storage stability, intercompatibility between the resin dispersion and the elastomer latex, and processability on coating equipment. Because many variables must be considered, a broad range of resin dispersions targeted for specific applications are available:
Rosin acid based dispersions - The dispersed phase is based on aging resistant stabilized rosin acids. They provide good compatibility with all polymer dispersions commonly used for the production of pressure sensitive adhesives. Good adhesion to many substrates can be expected. The low molecular weight of the base resin may be a disadvantage due to the reduced cohesion that results from their use.
Rosin ester based dispersions - The dispersed phase is based on aging resistant stabilized rosin esters. Rosin esters have a higher molecular weight than rosin acids and therefore provide good cohesion. Ester type resin dispersions provide good compatibility and improved adhesion in all polymer dispersions commonly used for the production of pressure sensitive adhesives.
- Hydrocarbon resin-based dispersions - While not compatible with as wide a range of elastomers as the rosin resin-based dispersions, they can impart specific adhesion properties to polyolefin substrates that are superior to rosin resin. Hydrocarbon resin dispersions generally have a cost advantage over other types.
- Resin dispersions based on hybrid feedstock - The dispersed phase is based on a combination of a hydrocarbon resin and stabilized rosin acids or esters. Compared to the ester- and acid-based types, these resin dispersions provide similar or superior properties in the final formulation and generally have a better cost structure. Apart from the acrylic latex polymers, traditionally well evaluated in pressure sensitive label formulations, many polymer dispersions are available. Sometimes, the compatibility of the hybrid resin dispersions may be limited. Proper testing and evaluation of each resin dispersion is recommended.
Eastman Chemical Company produces resin dispersions in North America and Europe. Detailed information on the resin dispersions produced in North America can be found in publication WA-85, Storage and Handling of Eastman Water-Based Resin Dispersions (see page 6). The publication contains a summary of the physical properties of each of the products, links to their information pages, a description of the applications, and types of polymer compatibility expected for each product.
The transportation of resin dispersions is difficult due to their limited shelf life, water content, and low freeze-thaw resistance. Eastman resin dispersions are not available in all regions of the world.