Metering and Mixing of Components

Metering. The components of multiple-part adhesive or sealant systems must be measured out carefully. The concentration ratios have a significant effect on the quality of the joint. Strength differences caused by varying the curing agent concentration are most noticeable when the joints are tested at elevated temperatures or after exposure to water or solvents. 

Exact proportions of resin and hardener must be weighed out on an accurate balance or in a measuring container for best adhesive quality and reproducibility. Possible problems that can occur by not adhering to the proper mixing proportions can include incomplete polymerization (too little catalyst), brittleness (excessive catalyst), and corrosion of metallic adherends (excessive catalyst). 

In mixing, cleanliness is almost as important as accuracy. Scoops, weighing pans, mixing spatulas, and other accessories should be kept clean. Organic solvents are recommended for cleaning. Adhesives that appear to be similar may be ruined if one is contaminated with the other. Water will spoil many adhesives and sealants. 

For a given type of mixing equipment, the larger the mass, the longer will be the mixing time. This reduces the time available for application of the adhesive. Alternatively, a system of batch replenishment may be employed, but this is sometimes difficult because of the exotherm of the resident batch causing acceleration of the additive batch. 

For mechanical mixing often a variable-speed mixer, preferably air-driven, and a container are all that is required. An air-driven mixer is safer, less costly, and more easily regulated than an electrically driven mixer. For the most reliable mixing operation, the temperature of the compound in the mixing vessel should be uniform. For adhesives or sealants that are stored at cold temperature, adequate mixing viscosity can be reached quickly by the use of a hot liquid jacket around the vessel. 

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