The packaging industry changes rapidly as the needs of the various sectors fluctuate. However, one fact remains true nearly 80 years after the invention of the first adhesive product designed to combat the challenges of temperature and humidity—sectors of all types rely on hot-melt adhesives. At the time of its invention, this versatile adhesive solved multiple problems posed by traditional glues. It quickly became the adhesive of choice for the construction, manufacturing, furniture, automotive, packaging, and bookbinding industries, among others.
Today’s hot-melt adhesives are available in a wide variety of forms, including glue sticks, pellets, blocks, pillows, and more. As time has progressed, hot-melt adhesives have become more versatile than ever and continue to feature several advantages over other adhesives. Advantages of hot-melt adhesives include:
- Rapid set time
- Long-lasting bond
- Bonding to almost any substrate
- Resistant to temperature changes and moisture
- Simple, versatile application methods
- Easily automated dispensing process
- Long shelf life
- Minimal waste
- Relatively inexpensive compared to other adhesive options
THE IMPORTANCE OF A SUCCESSFUL BOND
While each of the above advantages plays a crucial role in the decision to utilize hot-melt adhesives, it is important to note that the advantages only exist if a successful bond is achieved during the application process. It is crucial to ensure the right factors are in place to promote an ideal bond between the substrates and adhesive. To understand why these factors are so important, one must first understand the nature of hot-melt adhesives.
Composed primarily of polymers, resins, and a wax or oil, hot-melt adhesives are a thermoplastic that becomes malleable at and above the melting point of the polymer. Once liquid, the adhesive seeps into any pores in the substrate materials it is applied to; then, the second substrate is applied to the adhesive. As the adhesive cools and fills in the two substrates’ pores and cavities, it hardens and adheres to each, providing a strong bond between the two.
FACTORS THAT AFFECT BONDING STRENGTH
Hot-melt adhesives are manufactured in several formulations, each constructed specifically to provide the unique viscosity, melting point, and adhesion strengths necessary to make it ideal for a certain application. Depending on the strength of the bond needed, the ideal set time, the preferred viscosity, and the type of substrate that will be used, manufacturers can alter certain adhesive properties to fit it for almost any use. Hot-melt adhesive factors that impact bonding include:
- Type of polymer. The type of polymer used primarily affects the adhesive melting point and can impact the final bond’s strength. In addition, different polymers can add flexibility, increase or reduce adhesion, and add chemical resistance. Finally, the polymer chosen can also affect the final application process.
- Adhesive temperature. The temperature of the hot-melt adhesive used affects the bond strength and appearance of the final product. Within the ideal temperature range, the adhesive becomes liquid and can adequately seep into the substrate’s pores. Outside the ideal range, the adhesive can remain too stiff to bond with the substrate, have improper surface tension, fail to wet the surface, or char in the pot and become less effective.
- Environmental temperature. While adhesive temperature is of primary concern while in the application machinery, ambient temperature can affect bonding. If the substrate or surrounding air temperature is too cool, adhesive can set too quickly, preventing bonding. If it is too warm, set times may be longer than is ideal for your production process.
- Amount of adhesive product. A larger surface area of the adhesive product allows more of the hot-melt adhesive to seep into the substrate’s pores and provide a strong bond. The amount of product necessary can depend on the adhesive strength, the substrate used, and the nature of the particular application.
- Porosity of the substrate. The surface characteristics of the substrate used can impact bond strength. Absorbent or rough materials may consume too much adhesive, while glossy or non-porous materials may not allow the proper bonding between substrates and adhesive.
- Compression. Adding pressure to the substrate and hot-melt adhesive can increase the available surface area for bonding. In addition, it can aid the adhesive’s movement into substrate pores and further strengthen the bond.
- Open time and set time. Open and set times are defined as the amount of time between adhesive application and placement of the substrate and between the application of the substrate and bonding completion, respectively. Ensuring proper open times and set times ensures the other factors like viscosity, temperature, and so on are ideal for successful bonding.
STEPS TO A SUCCESSFUL BOND
After careful consideration of all the above properties, choose the hot-melt adhesive best for your purposes. Then, follow these steps to ensure a successful bond:
- Exercise temperature control. Ensure that the hot-melt temperature is within the ideal limits to promote proper melting, prevent charring, and provide proper surface tension and wetting. Also, ensure the ambient temperature is within the prescribed range to keep the adhesive in its liquid state for the ideal amount of time.
- Ensure substrate and hot-melt are contaminant free. A dusty or oily substrate can cause a poor bond. Similarly, contaminants within the hot-melt adhesive can compromise adhesive performance and appearance. Ensure all metal parts and substrates are properly cleaned using a cleaner like Melt-O-Clean.
- Apply the proper amount of adhesive. Depending on the end application, differing amounts of adhesive may be necessary for successful bonding. Apply hot-melt adhesive to reach the necessary surface area required.
- Pay attention to open and set times. Different hot-melt adhesives have different ideal open times and set times. Ensure you pay strict attention to each to ensure the ideal bond.
- Apply pressure. If necessary for the type of hot-melt adhesive and application, apply pressure to the substrate to ensure an adequate bond. However, many hot-melt adhesives, like Technomelt Supra 106M and Technomelt 250A, do not require pressure to form a strong bond.
NEED MORE INFORMATION?
Achieving a great adhesive bond requires careful consideration of the right hot-melt adhesive for your needs and attention to each step of the application process. If you’d like advanced technical specifications regarding each of our hot-melt adhesive products, or information regarding heating units, replacement parts, and more, contact APPLIED Adhesives. Reach out online or call (800) 274-9801.