Mix Ratio Related Problems

Symptoms

The mix ratios for Epoxies and Polyurethanes are usually specified as ratios by weight and/or volume. These ratios are determined by product chemistry. The reactive parts are the resin and hardener, being the liquid components of the formulation. The fillers, if present, do not take part in the reaction.

The mix ratio specified for filled systems takes into consideration the amount of fillers present in a given volume of the homogeneous material. If the fillers settled to the bottom of the container and the material is not mixed thoroughly, product taken from the top of the container will be "liquids rich". In this case, applying the specified mix ratio will result in an off ratio product that will not cure or if it cures, will be deficient in cured properties.

Do the following to quickly confirm that material (resin and hardener) is good:

1. Take a clean plastic container.
2. Place the container on a gram scale and tare.
3. Mix the resin in its container thoroughly and pour 200grms into the empty container on the scale.
4. Thoroughly mix the hardener in its container.
5. Weigh the appropriate amount of hardener, as indicated on the data sheet, into the plastic container on top of the resin.
6. Thoroughly mix the resin/hardener together, scraping the sides of the container to make certain that all the hardener is thoroughly and evenly dispersed in the resin.
7. Pour any excess material out into another container, leaving only the amount of product indicated on the data sheet (next to the heading "Gel time").
8. Gel the mixture according to the cure instructions on the data sheet. Check the sample after the specified gel time.

Results:

If, after the specified gel time, the sample is solid and firm throughout, the product is reacting as it should and the problem is elsewhere.

If, after the specified gel time, the sample is soft throughout, re-check the mix ratio that was used to prepare the sample. If the correct mix ratio was used and the sample was gelled at the specified temperature, there is something wrong with the product and the supplier should be contacted.

Soft spots evident within the gelled sample indicate that the resin and hardener were not mixed thoroughly enough and the test should be repeated.

Some handy conversion formulas:

Calculate the Sg (specific gravity) of the mixed material

Weight of Part "A" + Weight of Part "B" (according to stated mix ratio)

Weight of "A" + Weight of "B" = Sg Of Mixed Material

Sg of Part "A" Sg of Part "B"

To convert Mix Ratios from "Parts by Weight" to "Parts by Volume"

PBW of Part "A" X Sg of Part "B" = Part By Volume of Part "A" to 1 Part by Volume of Part "B"

PBW of Part "B" Sg of Part "A"

^{* PBW = Parts by Weight (from the stated mix ratio on Technical Bulletin) Sg = Specific Gravity (as specified on the Technical Bulletin)}