An introduction to hot melt adhesives, including the properties, fundamental formulation, application, and advantages of this type of adhesive.
What are hot melt adhesives and how do they work?
Hot melt adhesives (HMA) are adhesives which are applied when hot, in a liquid form and once cooled down solidify. HMA have no carrier such as water or solvent for the adhesive, as in effect, heat is the carrier which allows for the adhesive to be applied.
In addition, most HMA do not undergo any chemical reaction such as cross-linking, or removal of a carrier, such as evaporation of solvent. What this means is that 100 % of the formulation is adhesive, and there is no need for a correct ratio of components to be obtained either within the formulation or during application. As well as this, no additional processes such as mixing are required either before or during application.
All hot melts work by “wetting out” the surface of the substrates that are being adhered together and this can easily be achieved when the hot melt is being applied in its liquid form. However, depending on the substrate temperature, the hot melt may quickly cool which can sometimes make adhesion to the second substrate difficult.
There are two main subcategories of hot melt adhesives: Pressure Sensitive Adhesives (PSA) and Non-Pressure Sensitive Adhesives (non-PSA). The difference is that PSA hot melts remain tacky once cooled to room temperature and when bonding non-PSA hot melts, the joining of substrates must be done whilst the HMA is hot to allow for the wetting of adhesive onto both substates to ensure a good bond is achieved. Therefore, the HMA must stay hot for as long as required to wet out both substrates and to achieve this, specialist equipment is available dependent on the adhesive and application requirement.
When using PSA, substrates can be joined when hot, however depending on the adhesive and substrates being used, this may occur when at room temperature as the adhesive will still be tacky. If the substrates are bonded together at room temperature, then the tack level of the PSA must be such that it is able to wet out the second substrate being applied to the HMA.
When cooled to room temperature, regardless of the type of HMA used, it must have enough internal cohesive strength as well as maintain its adhesive properties to keep the two substrates bonded.
Properties of hot melt adhesives
When using HMA there are several properties which need to be considered:
Pot life stability
This is the time the adhesive is stable at elevated temperatures. When applying the hot melt, it is important to ensure that the adhesive doesn’t break down or decompose too quickly when at the elevated temperature.
Open time
This is the time after the adhesive is applied that a suitable bond can be made. This is dependent on the application temperature, as generally the higher the application temperature, the longer the open time.
Tack
This relates to how sticky the surface is and can be referred to as hot tack, or for PSA hot melts – room temperature tack. Room temperature tack is often measured using a rolling ball test – although there are various other tests, including a loop tack test.
Softening point
This is the temperature the hot melt softens at; however, it will lose cohesive strength at a lower temperature especially if under strain. Often softening point is measured using a ring and ball.
Hardness
Hardness of hot melt when at room temperature. Usually measured using Shore A or D.
Melt viscosity
This is the viscosity of hot melt at an elevated temperature. The temperature that the viscosity is measured at needs to be stated as this will decrease with an increase in temperature. This can be measured using a rotational spindle viscometer and heating chamber, or a cone and plate rheometer.
Formulating hot melt adhesives
Hot melts are generally made of three main raw materials which are base polymer, tackifier or resin, and oil or wax with a small quantity of filler, pigment, and antioxidant.
Typical levels of the main raw materials of hot melts are shown in the table below.
Base Polymer
This is the core of a hot melt as it provides the strength, chemical resistance, and many application characteristics of the adhesive. One of the most common polymers in a HMA is ethylene vinyl acetate (EVA) and is used to produce general purpose hot melts used in many different industries.
EVA is very versatile, has good compatibility to many types of waxes and oils and is available in a variety of grades. Other common polymers used in a HMA are styrene-based block co-polymers such as styrene-isoprene-styrene (SIS) or styrene-butadiene-styrene (SBS). These types of polymers are often used for pressure sensitive hot melts, although they can also be used for non-PSA, as they are often softer than PVA based hot melts and have a better low temperature flexibility.
Other common polymers used include polyolefins (PO), amorphous polyolefin (APO), metallocene polyolefins (mPO), polyamides (PA) and polyurethanes (PUR). Each polymer has different characteristics and dependent on the requirement of the final hot melt will determine which polymer is most suitable.
Tackifier / Resin
Generally low molecular weight resins with a glass transition temperature above room temperature. As the name suggests they are part of the formulation to impart tack and improve adhesion when hot onto the final hotmelt. They can also sometimes improve the room temperature tack. Chemicals used as tackifiers include rosin acid derivatives and their esters, aromatic or aliphatic petroleum-based resins and terpenes. One of the most important factors when choosing which tackifier to use is compatibility between the tackifier and other raw materials in the hot melt.
Oil / Wax
Oils or waxes are added to hot melts to help with the mechanical and thermal properties of the final product. They will change the melt viscosity, softening point, adhesion properties and the ease of manufacture of the hot melt. Along with tackifiers they also aid in cost reduction of the hot melt.
The table below shows how each of the three main raw materials can affect some of the properties of a hot melt adhesive. Also included are fillers which can affect several of the main properties as well as change the density, and cost profile of the hot melt.
The other raw material which is regularly found in hot melt formulations is antioxidants. Antioxidants are used to stop excessive oxidation which may result in changes to the hot melts mechanical or physical properties including the change in appearance of the hot melt. It is important that the antioxidant is soluble in or can diffuse through the other raw materials in the hot melt formulation. If this is not possible the antioxidant may migrate to the surface and cause poor adhesion.
Application of hot melt adhesives
Hot melts can be applied in a variety of ways and the most common methods are machine applied or manually applied. Both methods allow for the hot melt to be either bead applied, or spray applied – usually resulting in a swirl pattern rather than atomisation which would occur with water-based or solvent-based adhesives.
There are advantages and disadvantages to machine application / manufacture, and these include:
Manually applied hot melts are more likely to be spray applied, although this does depend on the industry and the whole process is likely to be manual. There are advantages and disadvantages to hand application which include:
The choice of whether to apply by hand or machine will depend on many factors including cost, application, and space. When the adhesive has been applied, regardless of a manual or machine method, if there are issues with the final product, they are usually caused by either incorrect application or incorrect adhesive used.
When considering the type of application method and hot melt to use, several factors should be assessed including:
- Application temperature
- Viscosity required at application temperature
- Bead coverage i.e. how many grams per meter
- Correct amount of adhesive (if sprayed)
- Nozzle size
- Softening point
- Open time
- Beaded or sprayed
- Pressure applied and time of pressure
- Substrates
Advantages of hot melt adhesives
Some of the advantages of using HMA depend upon the industry they are intended for; however, many are universal and include:
- No chemical hazards, therefore, no requirement for extraction. Only PPE relating to heat factors
- Fast ‘curing’ i.e. quick to harden allowing for a fast turnaround
- Bond to a wide range of substrates
- Easy to apply whether machine or hand applied
- Relatively inexpensive – although equipment can be costly if machine applied
Conclusions
Hot melts are versatile, low cost, easy to apply adhesives that can be used in a variety of industries. They are environmentally safer and less hazardous for operators to use than many other adhesives making them a great alternative. However, to get the most out of hot melt adhesives for each usage the correct application parameters and adhesive must be utilized.
Written By: Dr. Keith Berry
Date of Issue: August 2021
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