Aqueous Film Coating: Get It Right the First Time

Coating’s an unforgiving process, but a clear understanding of key parameters, and straight shooting, can help ensure success.

By Fred Rowley, Solid Dosage Training, Inc.

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Film coating is an unforgiving process. It demands consistent tablet hardness, as well as controlled spray rates, temperature and drying air flow rates. Although automation is improving, the process cannot be fully controlled by a computer.

Expensive mistakes (see Troubleshooting Aqueous Coating below) can occur in seconds, but can easily be avoided when operators (see Are Your Operators in Focus? below) clearly understand the following critical operating parameters:
  • Spray gun geometry and calibration
  • Atomizing and pattern air
  • Pan pressure
  • Pan speed
  • Spray rate
  • Inlet/outlet air temperature
  • Total air volume
  • Adhesion of particles to the gun surface.
This article will concisely present those parameters, review a few of the nuts and bolts of film coating, and offer troubleshooting tips.

The suspension preparation process

Some coatings are not suspensions — for example, a clear coating or a controlled-release membrane. However, most coatings are formulated during the suspension process. Typically, a coating formula will consist, roughly, of 7 to 18% film-forming polymer, 0.5 to 2% plasticizer, and between 2.5 and 7% pigment or colorant. Table 1 summarizes the materials typically used.



The equipment should mix the suspension thoroughly. Operators must check for even distribution of powders. Lumps and fish eyes should not be allowed, and the suspension should be suspended through an 80-mesh screen.

It is important to mix the suspension during preparation, but to stir during actual use. These terms must be written correctly on the batch record, since FDA is very sensitive to their improper usage.

Equipment

The coating equipment and its setup are critical to coating success, and that equipment includes not only what the operator sees, but also what he or she does not see.

Pumping systems, coupled with magnetic flow meters, are typically used to deliver coatings. Choices include gear pumps, used for non-suspension coatings, and peristaltic and rotary lobe pumps. Each has its pros and cons, which are summarized in Table 2.



Spray gun geometry and calibration

Placement of the spray gun (photo, below right) relative to the bed, and boom placement in particular, is very important, yet it is typically overlooked during scaleup or product transfer. The ideal placement will vary between equipment manufacturers and pan models.

Good gun placement includes a fixed gun-to-bed distance and a consistent gun-to-gun setting.

Operators should first look straight down the boom to check that the guns have not become misaligned, or “cocked,” which can easily occur as material is moved into and out of the pan. Cocking can also result when mechanical equipment is adjusted, when fittings loosen or when equipment is abused. The direction in which the guns are cocked is important:
  • Side to side will generally result in overwetting.

  • Up or down will cause solution to stick to the pans.
Gun-to-bed distance

The gun-to-bed distance is fixed, never a range, and the proper value is usually included in the product specification. Usually, for aqueous coatings, settings are 8 or 10 in. from the bed, depending on the spray rate used.

However, the gun-to-gun distance, which is usually assumed to be fixed, may change during maintenance or gun change-out. Therefore, this dimension must be verified prior to each new coating campaign. This value isn’t usually stated as a specification or set point on the worksheet, but the setting is typically 5.5 or 6.0 in. from gun tip to gun tip between guns.

Distance between gun and side of the pan

This setting is widely understood but not always recognized as a variable that should be checked. In order to optimize the distance between the gun and the side of the pan, the operator should measure the tip of the last gun on either side of the boom, either to the front side of the pan or to the back of the drum. If this distance is set incorrectly, there will be solution on the window, solution on the side of the pan, or on both the side and the back of the pan.

The setting can be checked with placebo tablets or a large sheet of paper before the gun is fixed, and should be checked regularly thereafter. A flashlight helps in looking for problems.

Gun-to-bed height

Tablet beds may differ based on pan charge, batch yields and the size and shape of the tablet. Guns should be set at the bottom of the waterfall in the upper third of the bed. This setting should be rechecked at the start of each campaign, or any time pan charge changes.

Calibration

Calibration is also critical. Variation in solution rates between guns is a common problem. This value should not vary by more than +/- 10 ml between guns. If the variation exceeds the recommended value, the needle stroke can be adjusted from the back of the gun.

Atomizing and pattern air

Both atomizing and pattern air levels must be carefully set. Too much pattern air flattens the cone and can result in overspray, wetting and “picking.” Too little concentrates the cone in too small an area within the tablet bed, resulting in tablet erosion or “picking.”

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