Over the years, resin and vehicle manufacturers tested a product’s solubility using the traditional titration method. This method provides information as to the percent solids at kick out or differences in solubility using various oils or solvents. In vehicles and finished inks, solubility plays an important role, in that this characteristic influences tack, tack stability, yield value, flow, gloss and set speed. With the actual trend for less aromatics in mineral distillates, solubility becomes more important than ever before.
Both methods provide the technician with numerical testing results that can be used for statistical analysis. The titration method gives results in milliliters or grams, while the cloud point method yields results in degrees Centigrade.
Site-to-site and person-to-person variation plays a significant role in trying to determine the correct values for solubility, method repeatability and reproducibility. The titration method has many steps along its process that influence the test outcome:
• Hot plate temperature
• Up-heat and cool down ramps
• Resonance time at top temperature
• Solvent addition rate and method
• Calling the end point
The cloud point method eliminates many of the variables associated with the standard dilution method. The points along the process line that could cause variation are:
• Electric eye
Data showing the differences in variation while performing the two different procedures will be shown later in this article.
Why Cloud Points?
Laboratories change methods for a variety of reasons, many of which are because the labs feel the results give them a better understanding of how their products perform. I believe cloud point testing offers a number of reasons to change:
• Testing time reduced
• Generates more usable data (multiple point /and solvents)
• Less procedural variation
• Little site-to-site variation
• Virtually no clean-up
• Minimal training
Typical uses for cloud point testing may included incoming raw material testing, in-process vehicle testing, new material characterization, vehicle formulation aid and R & D projects.
Interpreting the Results
A typical cloud point test is run at 10% N.V. solids and generates a single point result. This one point method is commonly used for quality control and in-process testing. The test is quick and allows the technician to make on the spot determinations of fitness for use.
When evaluating new materials or comparing one material to another, it is best to run multiple tests at different percent solids. This enables the technician to visually see differences in materials by looking at the slope of the curves and differences in solubility in different solvents (see Fig. 1 and Fig. 3).
By running multiple points, it enables the technician to evaluate not only the solubility but also distinguish between resins that exhibit similar solubility traits (see Figure 3).
The solvents or oils used in the manufacture of paste ink vehicles have a dramatic effect on the solubility of the resin system. Figure 2 shows the same resin tested in three different oils at two different solids levels. The results can help the formulator choose the oil which best fits the required parameters for tack, stability, dry and flow.
Figure 2 results illustrate the same resin tested in several standard solvents. Halterman 6/9 oil has the lowest solubility for this particular resin. The tack and tack rise (stability) will be higher with the Halterman 6/9 oil than the other oils tested here.
Materials may have the same solubility or cloud point when testing only one point. In Figure 3, two different resins are tested in the same solvent. They both have the same cloud point at 10% solids; however, at 20% solids there is a 35-degree difference between these resins in the same solvent. This illustrates that there are methods to evaluate materials that will show separation between like materials or give additional technical information from QC tests.
Vehicle Formulation Aid
Cloud points can be used to identify material or amounts of materials to use in vehicle formulations. Today’s vehicles contain a myriad of materials to either control solubility or rheology. The cloud point method enables the technician to evaluate different solvents, alkyds, oils and solubilization agents to give a more stable finished product or to control solubility in a tighter range. In Figure 4, different materials are evaluated to determine which one solubilizes this resin the best.
As you can see, each material affects the solubility differently. It now becomes the technicians’ job to pick the one that will work best in the formulation. Now the technician has hard data, not past experience or an educated guess.
When evaluating resin mixtures however, the curves are not always a straight-line function. If the resins differ in solubility the results show a curve because the resin with the lowest solubility kicks out of solution first. The cloud point will not be the weight average of the resin mixture (see Figure 5).
Lower Test Variation
In today’s world of SQC, 6 sigma, and reduced incoming material testing, it is imperative that test procedures are reliable, reproducible and have reduced variation in both site-to-site and person-to- person testing.
The cloud point method fits this measure. The following two tables illustrate the site-to-site and method variation.
As mentioned earlier, it is important to remove as much testing variation as possible. If the test variation is large then, there is less room for variation in materials and processing if the product is to stay within the specification limits at a high confidence level.
As shown in Table 2, the variation between the standard titration and cloud point method is considerably lower using the cloud point method.
Also, it has not always been easy to compare dilution results between sites mainly because of the variations stated earlier in this article. Tests were run at three labs in two solvents at two different solids levels. The results conclusively show little if any variation between sites. Suppliers and customers can finally agree on test results without retesting or sending samples back and forth.
The cloud point method is a reliable, simple procedure that reduces testing time while providing less test variation and more usable information.