From packaging to assembly in countless products made around the globe, it's estimated that the world's hot melt adhesive market will reach a staggering $9.64 billion by the year 2020.
Companies that choose hot melt adhesives can achieve stronger bonds between the aggregates used in their products. These bonds are more durable than other gluing methods because of the adhesive qualities of hot melt. Product or packaging attractiveness is enhanced, and the adhesives are not hazardous, toxic or flammable.
Hot melt adhesives are one of the very few adhesive systems that allow the applicator to control and automate their work process flow. In other words, hot melt adhesives are fantastically adaptable to almost any desired conditions simply by controlling the temperature, process and speed as one applies it.
What Is a Hot Melt Adhesive?
A hot melt adhesive, also referred to as a thermoplastic adhesive, is a substance that is melted to create cohesion when it cools. The most popular and recognizable form of a hot melt adhesive is your basic glue gun.
Like all adhesives, a hot melt adhesive is made up of one or more base polymers with certain additives such as stabilizers and pigments. After the hot melt adhesive has been applied, viscoelastic polymers work to preserve the adhesion even after the cooling process has completed.
Hot melt adhesives are frequently used in trades that assemble, manufacture, seal, package and label products. They are also used in larger-scale production operations, such as RVs and trucks. The ease of use and application of hot melt adhesives can accelerate manufacturing and assembly line operations while limiting the production of pollution and waste.
Hot melt adhesives are always applied in a liquid, molten state.
“Open time” is the term used for the time between applying the liquid hot melt to one substrate and attaching it to another.
Depending on the hot melt, the user must ensure the hot melt does not start to solidify during the open time. The manufacturer’s instructions will provide appropriate data regarding each product’s open time, or how long it takes for it to solidify.
Hot Melt Adhesive Composition
There is a balance of formulated components that make up a hot melt adhesive. The major components are:
- A Base Polymer - The molecular backbone of the adhesive, this gives the adhesive strength and flexibility.
- Thermoplastic resins - These help the polymer become pliable or moldable at a specific temperature, then solidifies upon cooling.
- Plasticizers - These are the processing oils and waxes.
- Tackifiers - Adding tackifiers improves the initial adhesion.
- Antioxidants - An antioxidant provides oxidation resistance and preserves the adhesive at high temperatures.
Hot melt adhesives contain no solvents or water. They solidify when they lose heat, which gives them a very fast setting time. They also have the unique ability to bond non-porous materials, like metal or glass.
Why Do Product Assembly Companies Choose Hot Melt Adhesives?
Product assembly companies find hot melt an excellent choice for several reasons:
- They are biodegradable, which in turn make them recyclable.
- They produce no solvent emissions (VOCs).
- They cause no wastewater problems.
- They emit no airborne contaminants.
- The profit margin is higher in hot melt product assembly because a lighter adhesive coating is needed to create the cohesion.
- They can adhere more product at a faster production speed with better application precision.
- They bring additive value: Hot melt adhesives are compatible with several additives like UV-inhibitors, water repellents, flame-retardants, anti-oxidants and antimicrobials.
An advantage of hot melt adhesives in modern-day product assembly is that they can be applied with various techniques and methods. The manner of applying hot melt will vary depending on the nature of the product, the cost and type of application equipment and hardware, labor expenses and the quality of the hot melt adhesive itself.
One of the more popular methods of hot melt application is using a dispensing nozzle.
Some nozzles are rudimentary, such as on a caulking gun or directly built into the packaging of various products.
Dispensing nozzles can also be much more complex, requiring air or electric actuation. This means the flow of the adhesive being dispensed is blocked and unblocked by a needle-style valve. These nozzles typically do not make contact with the surface of the item they are coating.
Advantages of nozzle application include ease of use in the mechanical equipment and the ability to control precise adhesive amounts.
The only significant disadvantage of nozzle application is that it's not a great method for covering large areas unless you have multiple nozzles working together.
A roll coater for hot melt is like a mechanical or electrical roll coater used for paint. When you have a large area to cover, roll coating is the applicator of choice.
The adhesive sits in a reservoir and is applied to the surface of a coating roller either by being dipped or pumped from the reservoir and brought into contact with the surface of the coating roll. A scraper blade or another roller then removes any excess adhesive, leaving behind a measured layer of hot melt adhesive.
The advantages of roll coaters are speed and efficiency akin to a printing press. Roll coating should only be applied to flat surfaces, however, like large panels for an RV or truck.
Roll coaters work well in conveyor assembly line production.
Transfer printing is similar to roll coating. However, a pattern is placed across the roller or pressed into a pad that picks up the adhesive. The applicator then rolls over the surface of the substrate and transfers the hot melt onto the product.
This method can be used with flat plates instead of rollers. Transfer printing is ideal for applying a very thin layer or an intricate pattern of adhesive and for use in small areas like the flaps and windows of envelopes.
Curtain coating works like a waterfall. The product that needs to be coated is pushed through a curtain of adhesive. A trough is placed underneath to collect the excess hot melt, which is then pumped back up into the waterfall applicator. Curtain coating is best for covering large areas with a moderately heavy coat of adhesive.
Advantages to this method include consistency, speed of application and ease of use in automated production lines. Curtain coating is not ideal for applying thin layers of adhesive, and not all adhesives run smoothly through curtain coating, limiting the types of hot melt eligible for this method.
Screen printing is best suited for applying a controlled layer of adhesive to a specific area on a surface and is sometimes used for spot gluing or laying down a pattern of adhesive. Just like screen printing a T-shirt, the adhesive is pushed through a screen using a squeegee. The size of the openings in the screen will determine the thickness of the coat applied.
This process can be very labor intensive, so automatic screen presses can speed things up and make it a lot easier to work on a large volume of items. Equipment clean-up can be difficult, as screen printing is one of the messier application methods.
Which Application Method Is Best?
The best application method strictly depends on your product and packing or assembly processes.
No matter what application method you use, there are certain things you can do to increase the efficacy of using hot melts:
- Make sure all materials to be bonded are clean, dry and free of grease, oil and dust.
- Apply hot melt adhesives at the correct temperature.
- Make sure the materials you bond together are not heated or cooled to an extreme before application of the glue. If the material is very hot, the adhesive will take too long to set. If the substrates are too cold, the adhesive will set before the materials bond.
- The amount of adhesive applied will affect the bond tremendously. Too little and the adhesive will cool quickly; too much adhesive may prevent a solid bond from forming unless your compression persists for an extended period of time to allow the excess glue to cool and set.
Forms of Hot Melt Adhesives: Chips and Pellets
Most hot melt adhesives that are used in production facilities come in the form of pellets and chips. Workers and machines melt the adhesive to apply it to their product and packaging materials.
Using hot melt chips and pellets results in faster processing rates, saved storage space, reduced waste and improved quality of the end product. The streamlined use of hot melts can enhance the attractiveness of the product, while at the same time remaining hazard-free, toxin-free and non-flammable.
How Hot Melts Are Manufactured
Hot melt chips and pellets are created on conveyors that mix, move and blend the ingredients and resins. Manufacturers of hot melt adhesives use automated processes because cleaning up after coming into contact with the warm, sticky product is difficult.
Variables in Hot Melt Adhesive Application
In order to maximize the bonding strength in a hot melt, there are variables to consider — not only when choosing the right hot melt for your product assembly line but also for each application procedure:
- The temperature at which the adhesive is dispensed should be as high as possible. Nozzle applications typically require higher temperatures due to the heat that is lost when the adhesive leaves the nozzle and travels to the product.
- Pay attention to adhesive heat stability.
- Determine the heat tolerance: What are the maximum and minimum temperatures at which the adhesive will perform optimally?
- Know the limitations of the equipment used to dispense the adhesive.
- Determine the weight of the coat you are applying. A thicker coat will take more time to dry or set.
- Understand whether you need compression to create the right bond.
If the hot melt adhesive is not hot enough in the liquid stage, adhesion may occur initially but fail to stay bonded properly as it cools down further.
In contrast, if the temperature is too high when the adhesive is applied, it will remain a liquid for too long. This may result in the adhesive remaining soft even after cooling.
What Products Work Best with Hot Melt Adhesives?
You will find hot melt adhesives everywhere in your day-to-day life. From the furniture you sit on to the corrugated carton that holds the mailers in your local post office, it's not difficult to come across items bonded using this popular adhesive.
Companies that work with wood, fabric and foam, footwear makers, bookbinders and plastic manufacturers use hot melt almost exclusively for their bonding requirements. Construction workers and contractors will use hot melt adhesive for mounting trim in a new house, laminating a floor or even bonding pieces of concrete together. Hair stylists will use hot melt on hair extensions, and fashion designers will use it to accessorize clothing creations.
Electricians use a hot melt adhesive to seal circuit boards and for wire tacking and encapsulation. Automobile manufacturers use this glue for interior trim and seat assembly in their production lines. Appliance producers use hot melt for trim and gasket application.
Quite possibly the largest consumer of hot melt adhesives is the packaging industry. From the glue strip on an envelope to anything that needs to be coated or printed, a hot melt adhesive simplifies and streamlines the bonding process in a most cost-effective and clean manner.
Hot melts bond well with many substrates including:
- Organic Substrates
Non-porous materials like metal and glass should be bonded with high-tack polyamide hot melt adhesive for best results.
Without a doubt, hot melt adhesives have revolutionized our industrial world. Every single day, business entrepreneurs and regular people come up with new and innovative ways to utilize hot melt applications. The future possibilities are endless and, most likely, beyond our wildest imagination.