Mixing Technologies for the Production of Low- to High-Viscosity Adhesives
Adhesives are formulated from different chemistries depending on their end use and desired performance. The type and amount of resin, curing agent, fillers, and additives are selected to optimize bonding to a particular substrate in a particular environment. In a similar way, the mixing technologies utilized in the production of adhesives also vary from one formulation to another. Mixer selection is based on a number of factors but primarily viscosity profile and shear input.
The ideas discussed in this paper are recommendations based on Ross’ collective experience as a provider of specialty mixing equipment to the adhesive industries. Mixer testing and simulation trials are recommended to confirm the suitability of a specific mixing strategy.
The preparation of almost all adhesives begins and ends with adequate mixing. From the homogenization of adhesive emulsions, to the dissolution of polymers into solvents, or mastication of rubber and let-down of master batches, the type of mixing equipment and method hugely dictate overall processing efficiency and end-product quality. This paper seeks to provide an overview of effective and updated mixing technologies being implemented across many of today’s competitive adhesives manufacturing plants, as well as new equipment designs increasingly being recognized by the industry as potential solutions to prevailing mixing challenges.
Early equipment used to dissolve polymers into solvent was based on low-speed propeller, turbine, or rake-type agitators in vessels (known as churns). These devices relied heavily on the solvent’s softening action on the polymers and predictably yielded very long cycle times. Mixing in a churn for as long 12-24 hours was typical. The operator would load the vessel with raw materials, turn on the mixer in the morning and shut it off in the evening or the next day. This problem was exacerbated when the resin was supplied in pellet or slab form, making it difficult to dissolve. Even with the introduction of saw-tooth type high-speed dispersers, batch times could take up to several hours just to dissolve the resin.
To hasten the solvation process, a High Shear Mixer is recommended. Composed of a four-blade rotor that turns at high speeds within a stationary stator, this mixing device will mechanically shear large particles and reduce their size. Materials are drawn from below the mix head and expelled at high velocity through the openings of the stator, creating intense hydraulic and mechanical shear. As fast as material is expelled, more is drawn into the rotor/stator generator. Polymer particles are thus broken down into smaller and smaller pieces, which get easier and easier to dissolve. Fillers too such as fumed silica are dispersed faster with a High Shear Mixer compared to lower-energy devices.
Source: Adhesives Magazine and Charles Ross & Son Co.