Manufacturers in several different industries have benefited from using hot melt adhesives in the years since its development. Primarily, these manufacturers point to several qualities that contribute to hot melt adhesives becoming a packaging and bookbinding favorite for their respective sectors, including:

• Versatility of use
  

• Strength and durability

• Water and moisture resistance

• Efficiency in application

• Speed of setting, and

• Low cost compared to other adhesives

As a result, it's no surprise the hot melt adhesives market has been projected to rise to an estimated 9.64 billion USD by the end of 2020.

What, exactly, are hot melt adhesives? How do hot melt adhesives work? How can you use hot melt adhesives in your industry? In this guide, we'll answer all these questions and more.

What Are Hot Melt Adhesives?

Hot melt adhesives are 100% thermoplastic polymer adhesives that are solid at room temperature. When heated above their softening point—a relatively low temperature compared to many packaging components—hot melt adhesives liquefy, allowing for their application to several packaging and other applications. It is in this liquid state that the hot melt adhesive activates, allowing the surfaces in question to bind together.

Hot melt adhesives can exist in many forms in their solid state, including pellets, beads, chips, blocks, pillows, and glue sticks. After heating, depending on the application, the hot melt can be sprayed, extruded from a nozzle, or rolled onto surfaces. However, no matter the solid state or method of application, hot melt adhesives are generally made the same way.

How Are Hot Melt Adhesives Made?

As mentioned, hot melt adhesives are thermoplastic polymers. The term thermoplastic denotes that hot melt adhesives become plastic, or malleable, upon heating and harden upon cooling; meanwhile, the term polymer indicates that hot melt adhesives are made up of a large number of components bonded together. Hot melt adhesives contain no water or other liquids.

Hot melt adhesives consist of three major components:

• Polymers give the adhesive its main properties, flexibility and strength. The polymers make the hot melt adhesive durable and capable of maintaining a bond.
  

• Resins make the adhesive feel “wet” and provide the primary bond between the two surfaces.

• Plasticizers alter the viscosity (thickness and fluidity) of the adhesive and ease application.

Also, hot melt adhesives can include waxes to control set speed, viscosity, and flexibility as well as antioxidants to prevent degradation and biocides to prevent bacterial growth. Once all the vital components are in place, the hot melt adhesive hardens into its final form for delivery, ready for use in several industries. Common types of hot melt adhesives are:

• Metallocene
  

• EVA hot melt

• Polyethylene

• Polyurethane

How Are Hot Melt Adhesives Applied?

Hot melt adhesives are heated to become liquid. The liquid is applied to the initial surface by extruding, spraying, or rolling the prescribed application amount and then exposing it to a short “open time” to reach the optimal temperature and “wetness” for binding to the initial surface. After the prescribed open time, the two surfaces are put into contact with one another. At this point, the adhesive infiltrates the second surface and cools, returning to a stable, solid state and bonding the two together. In some cases, compression may be necessary to promote optimal bonding.

Maintaining a proper application temperature and open time are especially crucial for a firm hot melt adhesion. Ideal temperature guidelines for some common industry uses are:

• Packaging hot melt adhesive — for this primarily cardboard and organic substrate market, hot-melt case seal should reach 350 degrees Fahrenheit, include a 3- to 4-second open time to reach ideal bonding temperature, and remain effective at temperatures in a wide range of cold and hot conditions.

• Pressure-sensitive and liquid labeling— after application to the label in question, optimal label adhesion takes place at 40 to 50 degrees Fahrenheit, including a 24- to 48-hour ideal dwell time before consumer handling.

• Graphic arts hot melt adhesive — typically, graphic arts adhesives are applied at 350 degrees Fahrenheit, have variable open times, and remain effective between 20 and 120 degrees Fahrenheit.