Acrylic Coatings
A range of advances in materials, equipment and processes make it easier than ever for formulators to create the right pharmaceutical coating.
Walter Strathy
Director, Scientific Affairs
Emerson Resources, Inc
Veteran formulators cross Generation Gap bridge when they start a conversation with a younger colleague using the phrase “I remember when…”
Well, sometimes it can be nice on the other side of that bridge. Formulation science has come a long way over the years. Gone are the days when our science was referred to as an “art.” Key to the advancement of formulation science are advances in equipment and environmental instrumentation and controls, process reproducibility/reliability expectations (i.e. process validation), training and education in design of experiments (DOE) and improvements and innovations in raw material and excipients.
I remember when (here I go again) coating using acrylic systems was a formulation scientist’s worst nightmare. These projects were usually reserved for the younger/naive scientists. What was the big deal in those days? Coating with acrylic systems was problematic for several reasons. Some of those reasons included relatively low glass transition temperatures, incompatibility with other coating polymers, high tablet to tablet cohesion, high tablet to equipment adhesion, sensitivity to the addition of pigments, extended processing times and difficulty in equipment clean-up
In an effort to reduce adhesion and cohesion issues, talc was traditionally added to the acrylic coating solution/dispersion. I remember when talc was actually used in baby powder (modern baby powder is based on corn starch). As good as it was to keep baby’s bottom dry, it was not a good choice to add to a coating solution, as talc can create myriad processing issues.
Using talc in acrylic coating systems can impede adhesion of the film to the substrate being coated. This “competition” for adhesion results in the need for more polymer and consequently increases processing time, which in turn has a rather significant cost impact. It is very challenging to successfully suspend talc in the coating system. Talc readily settles out of the coating suspension in the mixing vessel, the liquid delivery system and fluid lines. The amount of mixing shear required to keep talc suspended in the coating solution is very high and can result in a negative impact on acrylic dispersions. Typically, at the end of a coating process, one is left with a significant “sludge layer” of talc at the bottom of the mixing vessel. This layer of leftover talc is evidence that the spray delivered during the coating process was not consistent.
Talc used in acrylic coating systems is also problematic due to its relatively hydrophobic characteristic, especially when suspending in an aqueous base coating system. The suspended talc that manages to make it to the spray guns tends to separate out of the coating system. The talc that does not adhere to the coating substrate ends up getting drawn into the dust collector and could impede process air-flow. In addition, the talc that does happen to adhere to the substrate can have a deleterious effect on film flexibility.
Talc Alternatives
Due to the relatively low glass transition temperature of acrylic coating systems, something has to be done to reduce tablet to tablet adhesion and tablet to equipment cohesion during the acrylic coating process. Alternative acrylic coating system additives have been used to replace talc. Two lipophilic compounds, glycerol monostearate1 and acetylated monoglycerides,2 are both widely-accepted alternatives to talc. In order for these lipophilic compounds to be effective in the acrylic systems, they must be emulsified into an oil-in-water system. Moreover, the oil phase needs to have a micelle size of approximately 30 microns.
Preparing an oil-in-water emulsion is the last thing a coating manufacturing department needs to worry about in an effort to eliminate using talc. Value added, emulsified systems of glycerol monostearate and acetylated monoglycerides are available commercially from Emerson Resources, Inc. under the trade names Plasacryl and Plasacryl AC respectively. These systems are sold as stable, ready-to-use emulsions. Typical use levels of Plasacryl are five to 10 percent of the acrylic polymer on a solids basis. Plasacryl functionality in an acrylic coating system is very similar to talc. It is a “de-tackifier,” preventing tablets from sticking together and to the equipment and aiding in the continuous spraying process by decreasing clogging at the spray nozzle. Plasacryl will not settle out of the coating system. The ingredients in Plasacryl work synergistically with the acrylic polymers, yielding a robust and flexible film, and often require lower weight gains to attain the desired release characteristics.
Compatibility Issues
Another potentially troubling issue related to coating with acrylics is compatibility with pigments. I remember when producing a pigmented, acrylic-coated delivery system meant using multiple coating steps. Typically, a formulator had to “overcoat” the acrylic-coated delivery system with a traditional pigmented coating system to achieve a desired color. Unfortunately, over-coating the acrylic-coated delivery system has its own set of issues. Several polymers used in traditional pigmented systems are not compatible with most acrylics. One way to overcome the challenge of incorporating pigments into acrylic coating systems is to add the pigment to the coating in the form of a stabilized, aqueous color concentrate. The resulting system is a one-step process.
Novel Systems
Colorcon, West Point, PA, has an acrylic coating system in its value added line of products called Acryl-EZE.3 Acryl-EZE is an aqueous based acrylic system that yields a delivery system with an enteric release. Acryl-EZE is a dry powder formula that can be dispersed in water at a level up to 20 percent solids. Colorcon promotes the benefits of Acryl-EZE as follows: A one-step pigmented or white, aqueous acrylic system; easy to disperse; uses existing coating equipment and provides for a consistent, reproducible enteric release profile.
Regardless of the coating solution formulation, it is extremely important to maintain tight process controls when coating with acrylic polymer systems. Swings in any process variable can be detrimental to the success of the coating process. A conventional coating system process is generally more forgiving relative to reasonable process variable fluctuations. A two-degree drift around the target temperature set-point is not an issue in coating with cellulosic coating polymers. However, that same (seemingly normal) temperature fluctuation can cause a disaster in an acrylic coating process
Coating equipment manufacturers understand the importance of process control and, as a result, all of the major equipment suppliers offer extremely effective process control. Assuming a robust, scientifically based formulation and process has been developed, and all of the manufacturing equipment has been qualified, the final step prior to commercialization is transferring the technology to the manufacturing department.
Equipment Advances
Acrylic systems have a greater propensity to build up on the spray nozzles. Two recent advances in spray nozzle technology limit this potential buildup. Thomas Engineering, Hoffman Estates, IL, has developed a spray bar.4 The new spray bar is a neat and organized system for delivering the coating solution into a side vented coating pan. Gone are the days of the conventional “trombone” style spray gun racks with their myriad hoses and pipes. The “wingless” nozzles contained on the Thomas spray bar have virtually no surface to which the atomized coating solution/suspension can adhere.
Spray Systems Co., Wheaton, IL, has also developed a nozzle that has reduced the opportunity for buildup. The reduction of coating solution buildup is aided by a unique configuration of the fluid nozzle as it relates to the atomization air orifice.
The technology transfer of an acrylic coated formulation and manufacturing process include larger scale process engineering, process capability studies and process validation. During the process engineering exercise, variables such as coating pan (or column) load and equipment surface area as well as all of the other process parameters must be taken into consideration. Process capability studies should be developed that utilize experimental design. Full-scale process capability studies should be an extension of parameter range studies carried out during the development of the formulation and process. The process capability studies will confirm the variable relationships previously established and provide data that justify parameter ranges. Upon completion of the process engineering and process capability work, process validation is carried out to confirm that the acrylic coating system and process are consistently in control.
I remember when acrylic coating was perceived as a nightmare. Have no fear—new ingredient advances, together with improved equipment controls and a good scientific-based development process have all come together to help make coating with acrylic systems easier than ever.
References
- Degussa website, www.degussa.com, List of excipients for joint processing with EUDRAGIT acrylic polymers.
- Emerson Resources, Inc. website, www.emersonresources. com, Plasacryl AC product bulletin and technical data sheet.
- Colorcon website, www.colorcon.com, Pharmaceutical/modified release/Acryl-EZE
- Thomas Engineering website, www.thomasengineering.com, Spectrum spray bar
