Waterbased Adhesives
Technical Issues

Evaluating a switch to any alternative solvent-free adhesive requires the close scrutiny and effective teamwork of numerous parties to resolve any technical issues. The successful application of new adhesive technology requires the input of multiple decision makers, in particular product development engineers, adhesive suppliers, application equipment vendors, and end product customers.

• Product development engineers determine the adhesive performance specifications such as thickness, required surface finish or substrate preparation, and structural bonding strength.
• Adhesive suppliers can best determine which products will perform to designed performance specifications. They will also know specifics about adhesive-to-substrate compatibility issues that are important to the product development engineers.
• Application equipment vendors know the limitations and capabilities of their equipment. Issues such as waste generation, energy consumption, corrosiveness of adhesives, and routine equipment maintenance can be addressed by these specialists.
• Customers ultimately determine the overall quality required for the product being manufactured.
A few of the most important issues regarding waterbased adhesives include: 1) performance requirements, including bond strength, durability, and process adaptability; 2) application issues; 3) curing technology; and 4) mixing options. It should be noted that both process adaptability and application techniques include the incorporation of manufacturing issues such as equipment and plant layout.

Performance Requirements

Ultimately, the performance of an adhesive depends upon the material’s ability to durably and successfully bond substrates in a manner that is acceptable to the end user. Among the durability issues are:

1. Working load and types of stress on the joint;
2. Service temperature range; and
3. Expected life.

The use of adhesives in the manufacture of goods requires an analysis of the application limitations of the adhesive, in order to ensure that the production line is not adversely affected by the method chosen. The production issues that need to be addressed for adhesives include:

1. Adhesive fixture time and positioning desired;
2. Reaction methodology;
3. Sag and flow properties;
4. Manual or automated assembly; and
5. Maintenance requirements for the system.

An important performance requirement to evaluate is the adhesive coating thickness before and after application. Both solvent-based and waterbased pressure-sensitive adhesives lose 50%-70% of their applied thickness while drying and curing. (Ref. 17) Consequently, different adhesives will require different amounts to be applied to achieve the desired coating thickness. In turn, that could have production and environmental consequences, such as longer drying times, dryer requirements and/or increased air emissions.

Application Issues

If a considerable amount of solvent-based application equipment is already in place, a manufacturer may favor waterbased adhesives over other alternatives because they can be applied by existing equipment. This avoids the costs of new application equipment, downtime for new maintenance procedures and training, and unfamiliarity with new equipment. However, chemical properties of the adhesive and equipment must be compatible. For example, waterbased adhesives may require the use of stainless steel piping and tanks to prevent rusting from occurring.

Drying and Curing

Drying and curing of waterbased adhesives relies on large ovens to evaporate carrier fluids and cure the adhesives. Variables that affect drying and cure time include the oven characteristics: source of heat, temperature, residence time in the oven, circulation and air velocity. Due to the lower volatility characteristic of water carrier fluids, waterbased adhesives may take longer to dry than solvent-based adhesives. However, infrared (IR) heaters may be added to the ovens to reduce the drying time.

Mixing Options

Depending on production needs, dry adhesive can be mixed with water at the manufacturing site or the formulation can be purchased pre-mixed. The former method requires close attention to achieving the proper adhesive formulation. Although it is possible that errors may occur while mixing, and batches of waste adhesives might be generated as a result, it may be more cost-effective to ship smaller quantities of dry adhesive than premixed adhesive formulations. There are also several advantages to purchasing pre-mixed adhesive in reusable totes:

• Less chance of mixing errors
• Supplier is responsible for adhesive performance
• Reusable totes can be easily located throughout the facility and require little hard mounted plumbing
• Additional mixing can occur in the shipping container for adhesive customization
• Totes are returned to the supplier and refilled to reduce clean-up and packaging disposal wastes.

KEY FACTORS LEADING TO SUCCESSFUL APPLICATION OF WATERBASED ADHESIVES

There are a number of factors involved in the successful application of waterbased adhesives and, if mixing onsite, the adhesive formulation and mixing process. Most waterbased adhesives in use are not suspended in the liquid, but are emulsions. As a result, the adhesive particles can settle out of the water carrier fluid. Consequently, whether the adhesive is mixed onsite or not, it is important to keep waterbased adhesives well agitated, so that the adhesive particles do not have an opportunity to settle out. This may lead to another difficulty with waterbased adhesives: foaming. Too much agitation, or excessive pumping rates, can trap air bubbles in the adhesive, identified by foaming and resulting in unevenness or gaps in the adhesive when applied. (Ref. 6)

A factor to consider in the mixing process is the potential for dirt or impurities to contaminate adhesive formulations. Contamination can occur whether the mixing takes place at the manufacturer’s site or at the adhesive supplier’s facility. Foreign substances such as dirt can affect the adhesion characteristics and potentially reduce durability or strength. Filtration is often used for solvent-based systems, and should be considered for waterbased adhesives. Filter cloths can be used to remove impurities above the micron rating of the filter cloth that might have fallen into the formulation during mixing. Filtration can be accomplished either with a system within the piping, or by pouring the adhesive material over a filter. The filters used in a solvent based system cannot be used for waterbased adhesives; waterbased adhesives have larger particle sizes than solvent-based adhesive, requiring a larger mesh size than solvent-based adhesives. (Ref. 6)

Case Studies

Several industries where waterbased adhesives are currently in use include:

• Crystal Cabinet Works, Inc., Princeton, Minnesota. The conversion from a solvent-based spray contact adhesive to a waterbased spray contact adhesive has eliminated use of methylene chloride in two facilities and decreased reportable toxic chemical releases by 32,000 pounds annually. To make the switch, the manufacturer experimented with the alternative for 18 months and evaluated performance data with different application conditions, substrates, and curing scenarios. The statistical data evaluation method that was used was called Design of Experiments. Using this statistical evaluation, the adhesive with the highest bond strength was selected, but it did require an IR curing system. (Ref. 18)

Personnel at the facility noted that the biggest obstacle when implementing waterbased adhesives was the impact of ambient conditions (in particular, temperature and humidity) on dry/open air time. This had a negative impact on production throughout the process. Therefore, IR curing had the dual advantage of increased bond strength and improved production efficiency (decreased dry time). It should be noted that prior to using waterbased adhesives, no ovens were used for drying or curing. (Ref. 19)

• Nashua Corporation, Omaha, Nebraska. Nashua, a masking tape manufacturer, chose to phase out the use of solvent-based adhesives in 1987. The motivating factors were

• to avoid future regulatory costs;
• to become more involved in the developing field of this adhesive technology; and
• the executives’ belief that the move was an environmentally correct decision. (Refs. 6, 20)

Nashua completed its conversion to waterbased adhesives before December 31, 1993, a self-imposed deadline. One product line was discontinued to meet this goal, since a suitable waterbased adhesive formulation could not be found. Nashua personnel noted that this was a relatively small percentage of the plant output, and was expected to be offset with increased sales of the waterbased product. (Ref. 6) Nashua did not need to increase its oven capacity to incorporate waterbased adhesives, but did need to add IR heaters to increase drying/curing speeds. (Ref. 20)

Nashua personnel consider the conversion to waterbased adhesives a success, as indicated by the following: (Ref. 20)

• Waterbased adhesives can compete with solvent-based adhesives for its applications.
• Nashua remained profitable while converting most of its operations.
• Nashua is removing itself from future hazards and costs associated with solvent-based adhesives.
• The belief that Nashua has "moved toward a more environmentally sound means of production, above and beyond monetary considerations."

• FLEXcon Corporation, Spencer, Massachusetts. FLEXcon manufactures six main categories of pressure-sensitive products: graphic films, packaging labels, electronic printing labels, microembossed films, medical films and labels, and custom-performance products, using a batch operation process. (Ref. 21) FLEXcon has approximately 50 adhesive-coating formulations, and uses waterbased adhesives for about ten of those formulations. These ten formulations are not conversions from solvent-based adhesives, but new product lines. (Ref. 6)

Many of FLEXcon’s products are "exposed to relatively extreme temperature and humidity, rain, and ultraviolet radiation," and current waterbased adhesives cannot meet these conditions, nor can they meet the performance requirements of its clients, which are met by solvent-based adhesives. (Ref. 6) Consequently, FLEXcon is not considering any waterbased adhesives as replacements for its solvent-based products, but is searching for new markets in which it could use waterbased adhesives. (Refs. 6, 21)

ADVANTAGES AND LIMITATIONS OF WATERBASED ADHESIVE TECHNOLOGIES, AS COMPARED AGAINST SOLVENT-BASED ADHESIVES

Waterbased adhesives technologies have numerous advantages over solvent-based adhesives. The primary advantages include the removal of HAPs and VOCs from the process; a complete removal of explosion risk (if all solvent-based adhesive lines are replaced); and the reduced generation of hazardous waste. A corollary advantage of reduced risk is increased potential for lower insurance costs. (Ref. 6) There are some limitations to waterbased adhesives, including the need for an oven (and/or possible retrofit of an existing oven), which takes up space and possibly increases energy requirements for the facility, to cure the adhesive; increased generation of wastewater and sludge; stricter production run requirements; and a limitation on complete substitution of waterbased adhesives for solvent-based adhesives. Table 3 summarizes the advantages and limitations of waterbased adhesive technologies.

Advantages	Disadvantages
Operational Costs
Possible to use the same equipment as solvent-based adhesives	Retrofit of equipment somewhat complicated and requires (sometimes costly) water-resistant piping (PVC or stainless steel) (Ref. 6)
Raw adhesive costs can be 15%-50% less than solvent-based raw adhesive (Ref. 6)	Oven capacity may have to be increased, or production lines slowed (Ref. 6)
	Waterbased adhesives have a much higher heat capacity, affecting oven capacity (Ref. 6)
Performance
Withstands wide temperature ranges (Ref. 6)	Can have lower peel and shear strength, backing compatibility, and humidity resistance (Ref. 6)
	Some backings are not suitable (Ref. 6)
	High temperatures of dry/cure ovens can damage substrate (Ref. 6)
Environmental/Safety
High solids, resulting in few or no VOCs or HAPs; eliminates both environmental and worker exposure issues (Ref. 6)	Increased generation of wastewater and sludge material (Ref. 6)
Non-flammable; elimination of explosion risk	For paper backed substrates, more waste is generated before marketable product is produced due to longer equilibration time needed for backing (Ref. 6)
Reduced generation of hazardous waste, perhaps down to conditionally exempt small quantity generator status	May need onsite wastewater treatment (Ref. 6)
Can be removable and reusable (Ref. 6)
Energy
Will not require solvent recovery systems (Ref. 6)	Dry/cure ovens have high energy consumption
May be able to remove air pollution control devices
Production
Widely available (Ref. 6)	Some types may freeze at low temperatures (Ref. 17)
Potential for new markets for adhesives increases (Ref. 6)	Cleaning is more time and energy intensive. Removing dried adhesive is more difficult (Ref. 6)
	More suited for application runs of 2,500 yards or longer; due to profitability (Ref. 6)
	Takes longer to achieve steady-state process performance during production runs; requires stricter process control (Ref. 6)
	Requires more raw materials for startup on the coating line (Ref. 6)
	Possibility of foaming during application
	Some downtime for worker retraining
	May not be able to convert all solvent-based lines to waterbased adhesives (Ref. 6)
	Requires more labor for maintenance (Ref. 6)
	Surface contamination can cause more problems for waterbased adhesives than solvent-based adhesives (Ref. 2)
	When formulating adhesives on site, storage in fiberglass tanks requires mixing blades and continuous viscosity measurements (Ref. 6)
	May have to add IR heaters

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