A large orange cloud overshadows the little town. Ooze bubbles along the sidewalk. A gaggle of news crews and a delightful assortment of inquisitive regulators converge on the manufacturing plant. Manufacturing and assembling product involves using the power of chemical reactivity. Fortunately, most chemical reactions do not result in a news-making scenario and most manufacturers take steps to minimize health and safety risks. Even less exciting chemistry, like curing epoxies, impacts product reliability, customer satisfaction, and your manufacturing process.
We ask you to consider a revolutionary concept for working with epoxies:
FOLLOW THE MANUFACTURER’S DIRECTIONS FOR CURING.
While short cuts may not lead to incendiary, flamboyant events, the wrong curing process can impact structural integrity, surface quality, and even introduction of contaminants. Even making candy involves chemical reactivity; you have to follow the recipe – the chemicals and the processes – to produce the right product.
Rock Candy and Caramel
The process determines the characteristics of the product. Picture a very simple solution with two chemicals: sucrose (refined cane sugar or refined beet sugar) and water. The proportions of water and sugar and the temperature profile determine the outcome. You can create the large sugar crystals popularly known as rock candy by adding a high proportion of sugar relative to water, heating the solution so the sugar dissolves, providing a surface, e.g. a string or a wooden stick, on which crystals can form, and then letting the solution cool down slowly, over a period of days (1).
On the other hand, applying high heat to sugar crystals (sometimes dissolved in water, sometimes alone) can result in aromatic, yummy, gooey caramel. That is – if you do it correctly. The chemistry of caramel formation is complex and not completely understood. In addition to the texture and taste, one clue to the complexity of the chemical reactions is that caramel is aromatic while table sugar has no detectable odor. Caramel is a mixture of chemical compounds. Using high resolution mass spectroscopy in tandem with liquid chromatography, one group (2) reported detecting thousands of molecular components of caramel.
Curing
Epoxies, potting compounds, and conformal coatings are all examples of materials used during product assembly that have to be cured. Curing may involve U.V. light; or it may involve time and/or temperature. Epoxies start out as a liquid or a paste, or sometimes as a two-part system, and then solidify. Therefore, there is a tendency to equate curing with drying or simple evaporation. However, curing is a process; a recipe is involved.
Cook sugar/water incorrectly, and you end up with a burnt saucepan – and that’s not a good thing! Curing is not the same thing as drying; curing is more analogous to making caramel; both instances involve chemical reactions. The difference is that with epoxy resins, you are making a plastic. Epoxy resins have more than one glycidyl or epoxy group. The “Ep” in epoxy is related to “epi”, which means “on the outside of” and refers to the oxygen molecule sticking up outside of the carbon chain (3).
Epoxy group
To form the plastic, the epoxy resin is mixed with one or more chemicals that serve as hardeners. Curing involves providing conditions that allow the mixed epoxy resin/hardener combination to form the crosslinked polymer of the plastic. Among the curing parameters are temperature and time.
Process and Contamination
As throughput increases, there is a temptation to cut down on the curing time or to increase the curing temperature. While the epoxy may appear to be set or hardened, if the epoxy is not completely cured, reactions are still taking place. This means the epoxy is still changing with time. If such a material is incorporated into the product, important functional aspects of the surface or structure may be compromised. In addition, the incompletely-cured epoxy may release contaminants that alight on other parts of the product.
Increasing the temperature can change the way the reaction proceeds and can also result in product contamination in an indirect manner. Julia Markardt of the Performance Review Institute, Staff Engineer-Electronics, MedAccred/Nadcap, reminds us that efforts to speed up curing of silicone-containing materials by baking in an oven may have the unintended consequence of depositing persistent, silicone-based contaminants in the oven. These contaminants can then be transferred onto other products. While silicones are often cited as a source of contamination, we should point out that the problem is not restricted to silicone and that not all silicone-based materials are equally noisome in terms of contamination.
Improperly-cured epoxies are a common problem in product manufacturing. The problem is so common that in tracking down the source of surface residue, particularly where there is an inconsistent, puzzling assortment of residues, we look to see if epoxies are part of the product build. One fairly quick “clue” is the presence of unexpected peaks or an unusual profile using FTIR (Fourier Transform Infrared Spectroscopy). We also check the process directions and observe people conducting the actual steps in the assembly process.
Follow the recipe, know the process
It is important to follow the recipe, but the recipe directions need to be adequate. Just as with candy preparation, this includes not only the curing but also the mixing technique. A colleague related a situation where, in an effort to be precise and to avoid over-mixing, a company called out directions for a two-part epoxy to be blended by mixing a specified number of times clockwise followed by a specified number of times counterclockwise. The assembly process involved bonding the epoxy to the component, then curing. The company even included witness samples that could be used for analysis and destructive testing.
Unfortunately, while the witness samples looked visually acceptable, the product popped out of the epoxy, an event that would have lead to unpleasant consequences had it happened in-use. Our colleague reviewed the written procedure and then actually observed the process. The directions were precise; the assemblers followed those directions carefully; but the directions weren’t correct. The epoxy mixture showed visible striations. The colleague used ATR (Attenuated Total Reflection) Infrared Spectroscopy to find a peak associated with cross-linking. The company was then able to combine analytical and visual observations to develop the right recipe for blending. This involved having the operators develop a trained eye for mixing.
We began by suggesting following manufacturers directions for curing. That’s a great start, but this anecdote illustrates that precision is not enough; you also have to provide the correct processing techniques. It also illustrates that controls or witness samples have to emulate the product.
References
1. T. Husband, “The Sweet Science of Candymaking,” ChemMatters Online, ACS, October, 2014.
http://www.acs.org/content/acs/en/education/resources/highschool/chemmatters/past-issues/archive-2014-2015/candymaking.html
2. Agnieszka Golon and Nikolai Kuhnert, “Unraveling the Chemical Composition of Caramel”, J. Agric. Food Chem., 2012, 60 (12), pp 3266–327
3. http://www.nilsmalmgren.com/epoxy-chemistry/epoxy-plastics-general-chemical-and-physical-properties/ (Accessed 16 November, 2015).
