Volume 4 Issue 1

What’s Happening at Missouri S&T:

Basic Composition of Coatings is next week!!!
**There are a few last minute open spots, so register today for next week's Basic Composition course!!**

This spring we will be offering “Basic Composition of Coatings¿? March 12-16, 2007 and. The Basic Composition course is intended for new personnel in the coatings profession. It targets the components of coatings (resin, pigments, extenders, solvents and additives), testing and specifications, general formulation and manufacturing methods. Basic Composition is primarily a lecture course with several laboratory demonstrations. For more information see our web site at http://coatings.mst.edu and to register contact Michael Van De Mark at coatings@mst.edu or call 573-341-4419. **This course is held on the Rolla campus**

Introduction to Paint Formulation
**Register Today!**

This spring we will be offering “Introduction to Paint Formulation¿? May 14-18, 2007 . This course is intended to give the person a fundamental knowledge of how to approach a starting formulation and troubleshoot it. This course involves both lecture and laboratory work. For more information see our web site at http://coatings.mst.edu and to register contact Michael Van De Mark at coatings@mst.edu or call 573-341-4419. **This course is held on the Rolla campus**

Summer 2007 Short Course Dates

Next summer we will be offering "Introduction to Coatings Composition and Specifications" July 16-18, 2007 , in St. Louis Missouri.

Technical Insights on Coatings Science

The Influence of Temperature, Time, and Air Quality Factors on Ambient Cure Coatings
By Dr. Michael Van De Mark, Director, Missouri S&T Coatings Institute

Relative humidity, temperature, defects and time

The term ambient air cure, or ambient air dry, coatings implies that the normal conditions of the application area allow proper drying or curing of the coating. Nothing could be farther from the truth.

The air temperature and relative humidity are obviously important. When coatings dry the solvent must leave the film and with it goes the heat of vaporization which can cause condensation if the surface temperature drops below the dew point. In water borne coatings, if the relative humidity is very high the coating will either dry very slowly or remain wet. Water loss is inversely proportional to the relative humidity and thus as the relative humidity goes up, the dry time increases.

The temperature is not always what we think it is. We generally measure the air temperature near where we are coating a part. However, the most important temperature is that of the surface of the part. If the surface of the part is cold, the coating may be more sensitive to dew point issues but if the surface is very warm, it may flash off the solvent and yield more orange peal due to a lack of flow. If the air is warm or cold it will accelerate or retard the evaporation of the volatiles respectively.

Another problem with air is that we can have particulates in the air which can land on the part and create surface defects. This is critical for high gloss or Class A finishes. The creation of various forms of dust from exterior sources, surface finishing processes, clothing, hair, sweeping, or unseen general dust can all create surface defects. In addition, aerosol cleaning solutions, personal care sprays, and even diesel exhaust can put micron size droplets of liquid material on the part or the coating which can cause defects until the coating is fully cured.

We often think of dry time as the time needed to reach tack free or hard dry. This, however, is not the true final dry time. It may require 4-30 days for all the solvent to leave a coating. In addition, reaction cure coatings such as epoxies or alkyds may require 30 days to fully react or even longer as can be seen for most alkyds. The properties of a coating will change for a considerable length of time and will continue to be dependent upon its environment. For example, an alkyd exposed to direct sun light will heat up considerably more than one in the shade and will therefore cure more rapidly.

Air movement, humidity and dust control

Since both air movement and humidity can play an important role, their control is critical to a reproducible coating quality. Air can be purified by cooling the air to remove water and then raising its temperature to the desired point. The correct relative humidity and temperature are therefore easy to control but not inexpensively.

The dust content is somewhat harder to control in an absolute sense. We can install a multi-stage air filter with the final filter being a HEPA filter if needed. The instruction of no sweeping, grinding or generation of any particulate matter in the vicinity of the paint line can further reduce dust. Proper industrial hygiene can also keep dust down. Aerosols must be legislated by management to be kept out of the area with severe penalties for violations. In exterior operations the air quality may not be controllable.

The coating's operation will be subject to the environment. This does not mean that we can ignore the above problems. It does mean that we need to monitor the temperature of the substrate and the relative humidity as well as watch the weather to insure that the climate will be appropriate for painting. Remember that the temperature must be within a good range until the coating has fully cured. Ideally, temperature and humidity for the surroundings and the surface should be logged with a computer so that the information can be tracked and easily checked. Some systems, such as alkyds, are more forgiving of low temperatures, but water-borne coatings are usually the least forgiving of high humidity and low temperatures.

What tests would be affected by these factors?

Before the coating is fully cured, the above variables can adversely affect tests such as adhesion, hardness, abrasion, corrosion, flex, impact, elongation and tensile strength. Control over all the variables is necessary to produce reliable test data. Engineers who set specifications on the coatings systems must understand that these variables have limits and their control is needed for the end product to meet their needs. If the technology will not perform at the engineer's specified thickness or application condition, the formulator will need to work with the customer to educate them on the limits and optimum performance criteria of the technology.

Some tests may indicate that the environment is not under control when the coating is applied, such as gloss, permeability to oxygen or water, and hardness if the coating is not fully cross-linked. Also, if the coating is not fully cured, the tensile strength is reduced which will affect the hardness, flexibility, and impact strength. If dust is incorporated into the coating, the distinctness of image (DOI) will be lowered. If the dew point is reached and the coating is not fully cured, moisture can infiltrate the coating causing the color to be distorted. All these plus more can be altered if the environment is out of control. The coating selected for a given application must take into consideration the environment in which it is to be applied.

Is there a topic you would like discussed? Contact us by e-mail at coatings@mst.edu.