For two-component adhesives that need to be mixed by hand, mix ratios can be given by weight or volume. The ratios are usually expressed in terms of parts per hundred (PHR), with the correct amounts of each adhesive component calculated by the following formulas:
PHR/100 x Weight of Resin = Weight of Hardener
PHR/100 x Volume of Resin = Volume of Hardener
To get the right mix, the user has to use the correct PHR spec and calculate the appropriate measurements of the resin and hardener. The ingredients are then weighed out on a scale or measured in a graduated cylinder.
Sounds simple enough, but this measurement process can be fraught with errors. Mistakes in the PHR calculations and measurement processes happen all the time.
One of the most common errors involves mixing up the by-weight and by-volume mix ratios, which will differ for a given adhesive product.
If you do confuse the two ratios, which is a common mistake, the adhesive properties will suffer. To illustrate this point, we mixed up two batches of our 20-2180, a popular polyurethane potting and encapsulating product. Batch One has the correct 100:60 ratio by weight. Batch Two had a 100:50 ratio by weight—which is the correct by-volume ratio.
You can see that this seemingly small mix error keeps the adhesive from developing its full hardness at room and elevated temperatures. The drop off in hardness as temperatures increase is also more pronounced with the incorrectly mixed batch.