Plywood has been manufactured using foaming for many years.

Foaming is not a new concept; plywood has been manufactured using this technology for many years. The first attempt to bring foaming to the converting market occurred in the 1980s. This failed due to resistance from adhesive and converting equipment manufacturers. Today's end users are more informed about the benefits associated with foaming. In addition to substantial adhesive cost savings, adhesive end users understand that products manufactured with foamed adhesive have less warp, resulting in another cost savings: less waste. This has presented adhesive manufacturers with puzzling challenges, but has left the adhesive end user with the double benefit of cost savings plus better product.

The mixing head incorporates the air into the adhesive, or “foams” the adhesive.

The foamed adhesive is supplied to the application point and used in exactly the same manner as a non-foamed adhesive.

The two main types of adhesive foaming systems are high-shear and low-shear. The critical difference is in the design of the mixing head. High-shear mixers are designed with concentric rows of mixing teeth covering a front stator, rear stator and rotor. These mixing heads have slim mixing-head profiles and low residence time. Low-shear mixers have "pins" instead of teeth inside an elongated mixing chamber. To compensate for the lower shear "pins," the mixing head is elongated for longer residence time. This raises the temperature of the foamed adhesive as it passes through the head, resulting in poor product stability and significantly lower cost savings for the end user. For the application of foaming adhesive, it is best to use a high-shear mixing head and a system that will consistently produce uniform micro-cellular foam.

It is important to size the foaming equipment to an application and to ensure that the foamer can adjust to meet customer needs and applications. One size does not fit all when it comes to foaming a water-based adhesive.

BENEFITS OF FOAMING

• Better lay flat: less glue means less water
• Better bond: more adhesive at the interface of the substrates
• Better hold out on absorbent substrates
• Improved open times
• Faster setting
• Increased wet tack
• Improved filming characteristics
• Reduced adhesive usage, up to 50%

APPLICATIONS FOR FOAMING

Following are the only limiting factors to foaming.

• The ability of the adhesive companies to formulate a foam-friendly version of their adhesive brand
• A low coat weight limit, usually 1 mil.
• An application method not suitable for foaming, e.g., recirculating adhesive system, spray, etc.

CONVERTING:

Bulk-box laminating

Tube winding

Paper-to-paper laminating

Artist mat board

Ceiling tiles

Foam-to-paper and -board

Multi-wall bags

PRODUCT ASSEMBLY:

Door manufacturing

Paper and vinyl to particle board, MDF, Lauan and gypsum

Drawer wrap

FSK to OSB

Panel manufacturing

Veneering

FORMULATION CONSIDERATIONS

When formulating a foam-able adhesive for an application, these basic fundamentals need to be kept in mind.

• Let the foamer do the foaming. In order to achieve the desired density reduction and to keep the adhesive stable, the foam needs to be produced by the foamer only. If the adhesive continues to foam upon introduction to the application point, the adhesive will become "overfoamed" and will not perform. The addition of foaming agents to a formulation is usually not needed.
• The application method will determine formulation. Considerations must include the type of roller being used, the amount of shear the foamed adhesive will be subjected to on the roller, and whether or not it is an extrusion application.
• The adhesive must perform both foamed and un-foamed. An adhesive must not be formulated in such a way so it will only work for a customer when it is foamed.
• If a customer could use a filled product, they would be using it now. Foaming will not change the basic bonding characteristics of an adhesive. Trying to recover lost margin dollars by filling a product with clay or calcium carbonate will only result in adhesive failures.
• In almost all cases, the only defoamer that can be in the adhesive is the defoamer that is in the raw polymer itself. Even a very small amount of defoamer added to an adhesive to aid in the adhesive manufacturing process will dramatically decrease the foamability of the adhesive and its effectiveness for an application.
• Highly plasticized PVAs (10%+) will not foam as well. The addition of a surfactant may be needed.
• Foaming increases viscosity. Foamable adhesives are usually lower in viscosity. Determine the lowest possible viscosity for an application and then formulate the adhesive.
• Foaming changes the rheology of an adhesive. The flow characteristic of an adhesive will change as the amount of air increases. This change allows a foamed adhesive to act as a void filler and to stay where it is applied, keeping more of the adhesive at the substrate interface.
• Upper limit to density reduction. When an adhesive is foamed, the density is reduced by the amount of air that is incorporated into the adhesive, usually in the 10-45% range. It is usually not recommended to reduce the density by more than 50%. At this point, the foamed adhesive begins to act like a solid rather than a liquid, and problems can occur with the application.

Foamed to 35% Air

MARKETING STATEGIES

Unfoamed Adhesive

One possible strategy would be to develop primarily foamable adhesives regardless of whether the customer plans to foam or not. Remember, the adhesive needs to work both foamed and unfoamed. By using this strategy, a manufacturer can avoid a dual product line and keep costs down. The advantage to a single product line would be that if a customer is using a foamable product but not foaming and then wishes to foam their adhesive, they can use the same product, eliminating down time with prolonged testing procedures and any necessary approvals or certifications.

Producing foamable adhesive may result in adhesive manufacturing issues that need to be addressed. Static in-line mixers can be used in the manufacturing process to eliminate air incorporation and to allow cup weight specifications to be met. Large adhesive companies that polymerize both PVA and VAE can formulate products right out of the reactor and thereby eliminate some of the issues presented to a formulator of adhesives.

CONCLUSION