Manufacturing deadlines loom. You get impatient with the product cleaning process. Can’t you make it run faster?
Is it getting hot in here?
Most critical and industrial cleaning processes run at elevated temperatures, and for good reason. A soil’s reactivity to the cleaning agent can be enhanced with heat. Chemists learn that—as a rule of thumb—the rate of reaction doubles with every 10°C increase.
Particularly with spray systems, aqueous cleaning agents may need to be brought up to a certain temperature just for the additive package to behave properly. For some near-neutral-pH aqueous cleaning agents, a higher temperature may be needed to avoid bacterial growth. And heat is necessary to liquefy soils like wax and metalworking fluid.
Because the drying step can be the most time-consuming, rinsing at high temperatures is helpful for getting heavy parts to flash dry. However, operating at a higher temperature can have some drawbacks. At high temperatures, greater reactivity may be offset by undesirable soil modifications. One example is caramelization; if you’ve ever burned a cake or overheated a custard, you know what that means.
At higher temperatures, soils can polymerize, that is, form larger, long-chain molecules. Polymers can be very difficult to remove; think about how difficult it can be to remove cured epoxies.
Soils can also react with the substrate at high temps; recall that the substrate is the object or part you are trying to clean. When this happens, cleaning can become nearly impossible.
With immersion cleaning, flash drying can occur between the wash and rinse step or between rinse steps. Soils or cleaning agent residues that dry on the part surface are likely to be more difficult to remove. Baked-on dirt is more adherent. Sometimes, adding a spray or mist system between process baths can help prevent premature drying.
Excessive heat can cause material compatibility problems. The substrate can itself be modified by heat, or by the combination of heat, cleaning agent and the physical force of the process. Occasionally, you may want to achieve such surface modification. For example, solvents in the vapor phase are sometimes used to achieve surface smoothing of polymeric materials. Most of the time however, surface modification is undesirable.
With ultrasonic cleaning, higher temperatures are not necessarily better. Cavitation (a major cleaning vector in ultrasonic cleaning) becomes much less effective near its boiling point. This is because the bubbles (actually the tears in the liquid) become so full of vapor that they do not collapse effectively, making for wimpy cavitation and eliminating the benefits of the process.
We sometimes see representatives of ultrasonic equipment move the temperature up to near the boiling point , arguing that it helps with cleaning. To us, this basically means that temperature—not cavitation—is removing the soil. Test before you purchase ultrasonic cleaning equipment. If heat alone does the job, revise your capital equipment purchase plans.
Both solvents and aqueous cleaning agents may pose environmental risks associated with air emissions, such as VOC or air toxins releases at high temperatures. There are also employee safety problems due to direct exposure to the hot process bath and to inhalation exposure.
You also have to consider flammability issues. Before changing processes by adding heat, check with your safety and environmental professionals.
Flash drying at higher temperatures can also cause safety risks. Drying times can sometimes be shortened by using a high-temperature drier or rinse water, but the parts themselves take longer to cool this way. Employees can be put at risk of burning themselves, and you certainly don’t want anyone to drop a heavy component on his foot! Some of our clients have found that drying can be accomplished at lower temperatures, and employees find it easier to handle the parts.
In terms of resources, heating water or solvents takes energy, and energy costs money. The hotter the cleaning agent, the greater the evaporative losses. This means more frequent replenishment or replacement of cleaning agent and—you guessed it—more money. Of course, you do have to expend some sort of energy to achieve high-quality critical cleaning. Using higher temperatures can improve cleaning, but don’t assume. Test for the optimal temperature, or temperature range. It’s the Goldilocks approach: not too hot, not too cold—just right!
Barbara Kanegsberg and Ed Kanegsberg Ph.D. are industrial product cleaning consultants with BFK Solutions LLC, and industry leaders in critical/precision and industrial product cleaning. For questions or to receive their newsletter, contact them at 310-349-3614 or firstname.lastname@example.org.