Piston styles include (left to right): solid O-ring, solid lip seal, lip seal with center bleed and air-free (AF) lip seal.
When choosing cartridge system components for a particular formulation, it’s important to convey all of the details of the application to the cartridge system manufacturer. Many adhesive formulators already know which cartridge materials are best suited for - and chemically compatible with - their different formulations. This article is for those who need a primer on selecting the right components.
Most cartridge manufacturers provide cartridge components made of different materials. For example, cartridge bodies are typically molded in either polypropylene or nylon. However, there are other options, such as polyester and fluorinated HDPE (high-density polyethylene). Why are these options necessary? The simple answer is chemical compatibility, which leads to an explanation of adhesive shelf-life stability.
Most materials are fine in polypropylene cartridges, which are the least expensive option. Common examples of these materials include epoxies and urethanes. However, most acrylics - specifically those that contain a methyl methacrylate (MMA) monomer - have shelf-life issues in polypropylene cartridges. Over time, the MMA monomer will permeate through the walls of the polypropylene cartridge, leaving a dried-out, “cakey” component that doesn’t flow or mix as well with the other component, resulting in a compromised final product. Adhesive manufacturers should always evaluate a new package before selling their product in it.
During shelf-life stability tests, which are often accelerated aging tests to simulate longer-term shelf-life in a much shorter period of time, this loss of MMA monomer will be detected by the cartridge’s loss of weight over time, which can be easily measured. This weight loss can then be compared to the specification, which might require that the filled package not lose more than 1% of its weight after one year on the shelf, for example.
There are three different options for these acrylics, or methyl methacrylate materials, that cannot be contained in polypropylene: nylon, polyester and fluorinated HDPE.
Nylon tends to be the most popular choice; it’s easier to mold and less expensive than polyester. In addition, it doesn’t require a secondary operation, like the fluorinated HDPE (cartridges usually have to be sent out to a third-party vendor for fluorination). It is important to note that non-fluorinated HDPE is not much better than polypropylene at containing the MMA monomer; fluorination provides a better barrier.
All of these options are more expensive than standard polypropylene and work to different degrees. It’s been argued that polyester is the best option, but nylon is so close in performance that it has become the traditional choice. Fluorinated HDPE does not perform as well as either polyester or nylon, but it’s the most cost-effective of the three options.
Since polypropylene is a more cost-effective resin, most adhesive manufacturers prefer to use it if possible. For example, if a manufacturer made several different adhesives (e.g., epoxies, urethanes and acrylics), they would typically want to use polypropylene with the epoxies and urethanes, and use nylon for the acrylics (only because they have to).
Some might argue that having one cartridge for all of their materials simplifies ordering and inventory. Because of this, they would consider using the more expensive nylon across the board, figuring the simplification outweighs (or cancels out) the increase in material cost. However, it’s important to note that nylon is not a good choice for urethanes or any other moisture-sensitive adhesives. Urethanes are extremely moisture-sensitive - any contact with moisture causes the isocyanate (ISO) component to crystallize or become a crusty solid, making it difficult to dispense and compromising the final mixed product. Since nylon absorbs moisture almost like a sponge, it’s easy to see that urethanes and nylon are not a good match.
Once the cartridge body material has been chosen, pistons must be considered. Many pistons are made of polyethylene, but instead of the HDPE version mentioned above for cartridge bodies, they tend to be LDPE (low-density polyethylene), a softer material and better for sealing purposes, especially with lip-seal pistons. If nylon or polyester is chosen for the cartridge, it probably means a nylon piston, as there aren’t many pistons molded in polyester.
Keep in mind that the cartridge body is the most important component of the system when it comes to compatibility with, for example, the MMA monomer. This is because the cartridge body has the largest surface area for the MMA monomer to permeate through compared to the pistons and the outlet plug. However, the larger the cartridge size, the larger the piston diameter, making the pistons a significant factor as well.
Cartridge bodies are typically molded in polypropylene (left) or nylon (right).
With pistons, choosing the right material of construction is only part of the final decision, as there are a number of design options to consider. For example, TAH manufactures four different styles of 50 mL 1:1 pistons: solid O-ring, solid lip seal, lip seal with pre-staged center bleed plug, and air-free lip seal.
If the solid O-ring is chosen, there is the issue of O-ring compatibility to consider. For common materials like epoxies and urethanes, however, the standard EPDM (ethylene propylene diene monomer) is the default choice. In contrast, some adhesives are not compatible with these O-rings, causing them to either swell and roll out of the O-ring groove or shrink and flatten out, thereby losing their seal with the inside wall of the cartridge. In both cases, discovery happens during dispensing, where “blow-by” (material gets past the piston seal and gunks up the dispenser) can occur. The aforementioned MMA monomer can also permeate through certain O-rings. If any of these problems occur, either an alternative O-ring (such as silicone, nitrile or Viton®) can be tried, or one of the non-O-ring piston options. Some of the more expensive O-ring materials, such as Teflon®, work well but can be cost-prohibitive.
With the above 50-mL solid O-ring piston example, the O-ring is the primary and only seal, and it is a very reliable one as long as the adhesive is compatible. However, there are other solid pistons that feature a lip seal as the primary seal but may also have an O-ring groove in case an O-ring needs to be added for a secondary seal. This approach might be used with an extremely low-viscosity material that will try to wick its way past the lip seal for a “belt and suspenders” approach. Note: for any solid piston option, a shim must be used to bleed the air out of the cartridge. During the insertion process, there is always the risk of the shim damaging the lip seal, creating a leak path. An O-ring piston is more forgiving in these cases, since the O-ring is less likely to be damaged by the shim. However, in either situation (lip seal only or with O-ring), you will want to use a smooth plastic shim (such as a Teflon strip or oval fishing line) to minimize the risk of damaging the lip and/or O-ring.
To avoid shims altogether, one of the other “self-bleeding” pistons mentioned above can be used. With the center bleed option, air escapes through a tiny hole in the center of the piston. Once the piston hits the material, the pre-staged center bleed plug can be closed. For years, this has been an extremely effective piston. However, it was recently discovered that some viscous materials allow a conical shape to form immediately after filling, the point of which can clog the center bleed hole before the piston is completely inserted. This results in air being trapped between the material and the piston. When this occurs, there is an opportunity for air to eventually work its way into the material, especially during dispensing, thus creating soft spots in the resulting mix and compromising bond strength at that point.
Recent innovations like the air-free (AF) piston combat the conical shape problem by bleeding the air 360° around the outside perimeter of the piston. The piston actually pushes the point of the cone down and, at the same time, the air is exhausted around the outside perimeter until the pressure of the material gets up into the bleed paths and causes the much larger center plug to be closed without a secondary operation.
Air entrapment is a very serious issue and can also occur when inserting solid pistons if the shim doesn’t get all the air out. It’s important to have the right piston-insertion equipment, the right piston and the right expertise. A good filling person can probably make any of these piston options work by using various tricks they’ve learned to get all (or almost all) of the air out. It might be the angle in which they have the cartridge, the side on which they put the shim or how they knock down the cone shape. This is an often-overlooked aspect of the packaging process, but products like the AF piston can provide easier solutions to these problems.
Conclusion
Whether you’re looking to put your adhesive into a cartridge system for the first time or changing to a different cartridge system, some research is essential. Even with some idea of the components that you want or are required to have for a particular material, a thorough evaluation is always recommended.For more information on choosing equipment, contact Craig Blum, TAH, phone (609) 259-9222 or e-mail cblum@tah.com.
Viton and Teflon are registered trademarks of E.I. Dupont.
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