Hot Melt 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 hot melt adhesives include: 1) performance requirements, including bond strength, durability, and process adaptability; 2) application issues; 3) curing technology; and 4) recyclability. 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. Hot melt adhesives, for example, do not lose thickness after curing. On the other hand, solvent-based adhesives lose 50% to 70% of their applied thickness while drying and curing. (Ref. 14) Consequently, different adhesives will require different amounts to be applied, in order to achieve the desired coating thickness.

Application Issues

Hot melt adhesives require special application equipment that is not compatible with solvent-based application equipment. The equipment used for hot melt adhesives is designed to operate in the temperature range needed to apply the hot melt adhesives. Unlike solvent-based adhesives, which can be applied with room-temperature equipment, hot melt adhesives must be kept at elevated temperatures, often above 300°F for proper application (Ref. 14).

Drying and Curing

Drying and curing of hot melt adhesives takes far less time than curing with conventional drying ovens. There are no drying issues with hot melts, and the hot melt adhesive cures as it cools. As a result, the curing and drying time necessary for solvent-based adhesives that must be cured in an oven is eliminated with hot melts.

Compatibility with Recycling

Many adhesives vary in their compatibility with substrate recycling, particularly with regard to paper recycling, an issue that is important since adhesives are used extensively in the manufacture of cardboard packaging and paper consumer products. Although not all hot melt adhesives allow easy paper recycling, PURs are easily separated and removed from paper fibers during the recycling process, because of their rigidity after curing. (Ref. 12)

KEY FACTORS LEADING TO SUCCESSFUL APPLICATION OF HOT MELT ADHESIVES

One of the most critical factors for successfully applying hot melt technology is using reliable heat sources capable of keeping the adhesive material at the constant temperatures necessary for the hot melt adhesive to be properly applied. Reductions in temperature may cause the adhesive material to harden or cure prematurely. To prevent this, equipment in a hot melt system must be kept at a suitable and stable temperature with thermostatically controlled electrical wires, a requirement known as "heat tracing."

Case Studies

Hot melt adhesives have been in use since the 1950’s. (Ref. 11) PUR adhesives, introduced in the U.S. in the early 1980’s (Ref. 12), have become increasingly accepted for hot melt applications. Several industries where PURs are in use instead of other hot melt adhesives include:

• Picture Frame Manufacturer. A manufacturer began using a PUR adhesive to withstand the temperature extremes the frames experience during warehousing and transport. The PUR adhesive increased picture frame quality and substantially reduced the reject rate and customer returns. (Ref. 12)

• Footwear Fabrication. In many footwear products, especially athletic shoes, soles are no longer sewn on, so adhesive performance is critical. PURs’ performance has been satisfactory in bonding the different materials found in soles and uppers. Other advantages of PURs in footware products include less handling of shoe components during manufacturing, reduced cure time, and stronger bonds. (Ref. 12)

• Kitchen Countertops. Post forming is a process in the kitchen countertop fabrication industry in which a piece of laminate forming the top layer of a countertop is heated, bent around the front edge of the countertop, and glued to the countertop to form a seamless surface. Solvent-based contact cements have worked satisfactorily for initial bonding, but countertops can delaminate over time when subjected to moisture and heat, as would be typical above a dishwasher installation. Since PUR hot melts are heat and moisture-resistant, they perform well in these environments. (Ref. 12)

ADVANTAGES AND LIMITATIONS OF HOT MELT ADHESIVE TECHNOLOGY, AS COMPARED AGAINST SOLVENT-BASED ADHESIVES

Hot melt adhesive technologies have numerous advantages over solvent-based adhesives. The primary advantages include the reduced cost of the adhesives, on a per pound basis; 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. A corollary advantage of reduced risk is increased potential for lower insurance costs. (Ref. 6)

Limitations to hot melt adhesives include the initial expense of the equipment required to apply hot melt adhesives, the high temperature necessary for adhesive application, and, for PURs specifically, the short shelf life of the adhesives. Table 3 summarizes the advantages and limitations of hot melt adhesive technologies as compared to solvent-based adhesives. PUR adhesives are specifically noted where they have characteristics that are different from other hot melt adhesives.

Advantages	Disadvantages
Operational Costs
Capital equipment costs 50%-70% less than solvent-based systems; includes pollution control equipment savings (Ref. 6)	Heat tracing equipment necessary (Ref. 13)
Raw adhesive costs are 80% less expensive per dry pound than solvent-based adhesives (Ref. 6)	Specialized application equipment required (Ref. 14)
Requires much less floor space than solvent-based systems (Ref. 6)
Up to 50% fewer employees per line than solvent-based systems (Ref. 6)
Lower freight costs due to reduced volume of adhesives shipped (Ref. 6)
Lower inventory cost due to shorter cure times, allowing product to be shipped much sooner (Ref. 6)
Possibility for lower insurance costs – removal of solvents reduces risks and eliminates explosion hazards (Ref. 6)
Performance
Cures in seconds	Lower shear strength at elevated temperatures (Ref. 6)
Very thick films possible (Ref. 14)	Need constant, reliable heat source to melt adhesive (Ref. 15)
Unaffected by water, moisture, or humidity (Refs. 2, 3)	Gelling may occur if not kept at correct temperature (Ref. 15)
PUR: Able to bond dissimilar or temperature sensitive substrates (Ref. 12) [note: other hot melts cannot be used on heat-sensitive materials (Ref. 2)]	Low solvent resistance (Ref. 6)
PUR: Bond points precisely pre-located to function like screws (Ref. 13)	Temperature resistance can be limited (Ref. 14)
PUR: Bonds invulnerable to UV cured technologies (Ref. 13)	PURs: Easily contaminated by air. (Ref. 13)
PUR: Works well in high temperature and humid environments (Ref. 12)	PURs: The material can be difficult to handle (Ref. 13)
Environmental/Safety
100% solids: no VOCs or HAPs, eliminating both environmental and worker exposure issues (Ref. 14)	The adhesive must be applied at elevated temperatures, up to 300 °F (Ref. 14)
Reduced generation of hazardous waste	Potential for worker burns (Ref. 15)
Elimination of explosion risk	If adhesive contains resin acids, heating could produce worker exposures which cause or exacerbate asthma (Ref. 16)
Energy
Do not require ovens, increasing production rates (Refs. 6, 12)	Extensive heat tracing systems may consume constantly high amounts of electricity
Do not require pollution control devices (Ref. 6)
Production
Pre-mixed formulations, in both liquid form and as solid pellets (Ref. 6)	Cleanup must be done immediately after use; if not, equipment could be lost (Ref. 15)
Rapid set up, no curing required (Ref. 14)	PURs: Short shelf life (Ref. 13)
Floor space requirements are less than one-fourth that necessary for solvent-based line (Ref. 6)	PURs: Prone to cross-linking (Ref. 13)
PURs: End-product parts can be pre-finished and quickly assembled (Ref. 13)	PURs: Adhesive must be formulated to each specific substrate (Ref. 13)

Note: Characteristics specific to PURs (as a type of hot melt) are noted as PUR

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