Radiant-Cured 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 radiant-cured adhesives include: 1) performance requirements, including bond strength, durability, and process adaptability; 2) application issues; and 3) curing technology. It should be noted that both process adaptability and application techniques involve 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. Radiant-cured adhesives do not lose thickness after curing. Solvent-based adhesives, on the other hand, lose 50%-70% of their applied thickness while drying and curing and require larger quantities to be applied to achieve the desired coating thickness. (Ref. 13)

Application Issues

Adhesives are applied to substrates by a variety of methodologies. If a considerable amount of application equipment is already in place, a manufacturer may favor an adhesive technology that can be applied by existing equipment. This will avoid the costs of new application equipment, downtime for new maintenance procedures and training, and unfamiliarity with new equipment. With retrofitting, both ultraviolet (UV) light and electron beam(EB)-cured adhesives can be applied by the same equipment used for solvent-based adhesives, although EB systems can be retrofitted onto an existing solvent-based system more easily. The only added cost is the EB-curing mechanism. (Refs. 2, 6)

There are differences between UV and EB systems. As noted previously, UV systems require a photoinitiator, while EB systems do not. Also, UV light does not penetrate thick, dark, or colored coatings or backings very well. (Ref. 6) EB curing allows for curing of a specific area, as well as of a specific depth. EB systems have higher equipment and maintenance costs than UV systems, and are therefore more appropriate for high-production runs.

Drying and Curing

Radiant curing takes far less time than curing with conventional drying ovens, since the process occurs through a high speed, photochemical chain reaction. The most important factor for assuring a proper radiant cure is a consistent and reliable UV light or EB source.

KEY FACTORS LEADING TO SUCCESSFUL APPLICATION OF RADIANT-CURED ADHESIVES

Proper settings of the UV light or EB equipment is very important for assuring a correct cure and includes the following considerations: the use of a radiometer to measure the UV lamps output or EB generating equipment intensities; the height and angle positions of lamps or curing equipment; and the need for dual-cured systems to reach shadowed areas not contacted by the first energy source. The use of a radiometer to measure light output consistency is critical to ensure proper curing conditions, especially in UV systems, and equivalent to the monitoring of curing temperature for solvent-based systems. (Ref. 13)

Other issues to consider when implementing UV/EB-cured adhesive technologies include the light absorbing or reflective properties of the substrates and how they might adversely affect the bond, as well as any post-curing effects that might alter the performance characteristic of the adhesive bonds. For example, if the substrate is exposed to UV light after curing, the radiant-cured adhesive may continue to absorb UV light and weaken the bond.

Case Studies

The literature review of the use of radiant-cured adhesives did not identify EB-cured adhesives in wide use in the United States. However, UV light cured adhesives are in use in a number of different industries. Some examples:

• Automotive Door Shields. A large Midwest plastics film supplier that needed to increase production of door shields determined that using UV-cured adhesive technologies provided the most feasible solution. This decision was based upon considerations such as manufacturing floor space limitations, product specifications, substrate selection and properties, application properties, and the need to eliminate VOC emissions. The company also found that the technology provided fast curing time, which helped speed production, reduce cleaning requirements, and bring about better adhesive performance than the solvent-based adhesive that was replaced. (Ref. 13)

• Laminated Glass Plates. An applicator that laminates pre-coated film onto glass plates selected the UV light cured adhesive technology in order to reduce the frequency with which abrasive belts became coated with adhesives during plate edge grinding. Another objective was reducing adhesive waste. The advantages included fast cure, smooth lay-down, good adhesion, reduced floor space requirements, and easy handling and cleanup. (Ref. 13)

• Nameplate Manufacturer. A nameplate manufacturer found that a UV-cured adhesive gave superior performance over a waterbased adhesive. Since the waterbased adhesive occasionally failed, causing nameplates to become detached, implementation of UV-cured adhesive technology reduced the number of customer complaints. The system also was added to the existing line with little or no down time and minimal additional space requirements. (Ref. 13)

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

Radiant-cured adhesive technologies have numerous advantages over solvent-based adhesives. The primary advantages include removal of HAPs and VOCs from the process; elimination of explosion risk (if all solvent-based adhesive lines are replaced); no need for air pollution control devices; and the removal of dryers in the production line.

There are also some limitations for radiant-cured adhesives, including the initial expense of the equipment required for radiant-cured adhesives and possible need for a secondary post cure step to achieve full strength. (Ref. 2) Table 3 summarizes the advantages and limitations of radiant-cured adhesive technologies as compared to solvent-based adhesives. EB and UV adhesives are specifically noted where they have distinct characteristics.

Advantages	Disadvantages
Operational Costs
Up to 50% fewer employees per line than solvent-based systems	EB: Maintenance costs may be higher (Ref. 2)
Possibility for lower insurance costs – removal of solvents reduces risks and eliminates explosion hazards
EB: Overall capital equipment costs is comparable to solvent-based system, when pollution control equipment is also considered (Ref. 15)
EB: Higher production rates (Ref. 2)
EB: Does not contain photo-initiator, reducing raw material costs compared to UV adhesives (Ref. 2)
UV: Capital equipment costs 25% less than solvent-based systems (Ref. 6) (Note: this does not include the cost of emission control equipment, so there could be even greater cost reductions.)
Performance
Able to bond dissimilar substrates (Ref. 11)	May require a secondary post cure step to achieve full strength (Ref. 2)
Cures in seconds	May continue to absorb UV light after cure, resulting in degradation and poor performance
Excellent chemical and moisture resistance (Ref. 11)	UV: Bond strength may decrease at temperatures above 140 °F (Ref. 13)
Recycled paper can be used in the process (Ref. 6)	UV light does not penetrate thick, dark, or colored coatings (Ref. 6)
EB: does not have to be exposed to the surface in order to cure (Ref. 2)	UV: Parts with complex shapes possibly difficult to cure
100% solids: no VOCs or HAPs, removing both environmental and worker exposure issues (Ref. 6)	Workers must be adequately protected from radiant energy sources (Ref. 2) Shields must be constructed to avoid eye and skin contact with curing lamps
Generates less waste and requires less cleanup (Ref. 2)	Resins have caused skin irritation and skin sensitization in a limited number of exposed workers (Ref. 2)
No hazardous waste is generated during the process (Ref. 6)
Spills and overflows can be cleaned up without solvents (Ref. 6)
Elimination of explosion risk
Environmental/Safety
100% solids: no VOCs or HAPs, removing both environmental and worker exposure issues (Ref. 6)	Workers must be adequately protected from radiant energy sources (Ref. 2) Shields must be constructed to avoid eye and skin contact with curing lamps
Generates less waste and requires less cleanup (Ref. 2)	Resins have caused skin irritation and skin sensitization in a limited number of exposed workers (Ref. 2)
No hazardous waste is generated during the process (Ref. 6)
Spills and overflows can be cleaned up without solvents (Ref. 6)
Elimination of explosion risk
Energy
Do not require ovens or after burners for curing (Ref. 13)	None noted from literature
Do not require pollution control systems, such as solvent recovery systems
EB: Low energy requirements (Ref. 2)
Production
Can be applied by conventional solvent-based application equipment with retrofitting (Ref. 6)	EB: May require inert gas blanketing to achieve an oxygen-free environment (Ref. 2)
Minimal floor space required (Ref. 13)	EB: requires more nitrogen inerting gas than UV (Ref. 2)
Indefinite work life (Ref. 11) and longer shelf life (Ref. 2)	EB: Ratio of energy to raw materials must be properly determined, to avoid damaging backing (Ref. 6)
EB: Can cure adhesives in obscure and hard-to-reach spots (Ref. 2)	EB: Operators need to be better skilled (Ref. 2)
EB: Can be precisely controlled (Ref. 2)

Characteristics specific solely to Electron Beam or Ultraviolet adhesives are noted as EB and UV, respectively.

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