Hot melt adhesives are important in the carton and corrugated box sectors of the packaging industry primarily because of their faster production speed, excellent adhesion to a broad variety of new substrates, and reproducible adhesion quality. By the 1960's, when speed became a limiting factor for waterborne adhesives, hot melts began to be used commercially. Today, hot melt adhesives are required to bond to difficult boards and coatings at high speeds in sometimes difficult environments.
This article will review the requirements and hot melt adhesive systems that are commonly used for closing cartons and boxes. The factors that result in a successful application will be reviewed as well as common test methods employed by the industry. The process and equipment required for high speed production operations will also be described.
Box Closing Substrates and Adhesives
Due to a combination of properties such as high strength-to-weight ratio, low cost, and recyclability, corrugated packaging is the leading choice for transporting goods. Corrugated cardboard is a material consisting of fluted sheets and at least one flat liner. The materials of construction and manufacturing process have been previously described in a SpecialChem article.1 Boxes and cartons made from corrugated fiberboard are made into boxes by creasing or scoring to provide controlled bending of the board and then slotted to provide flaps for closure. The flaps are then closed and secured with an adhesive.
Folding cartons are available in a wide variety of designs and with different finished surfaces and coatings. The Regular Slotted Container (Figure 1) is the most common style of corrugated box in the industry.
Figure 1: Corrugated box blank and assembly into a container
(SpecialChem Fig. Ref.: Bondability index of elastomers)
The ends of the box blank are joined together with pressure sensitive tape, staples or adhesives. These joints are known in the industry as the "manufacturer's joint". The fiberboard thickness, weight, and surface coatings will determine the type of fastening method to be used.
Adhesive bonded manufacturer's joints have become popular because they provide higher strength and productivity. They also are considered when rough handling, scratchable contents, or box printing is contemplated. The types of equipment used to apply the adhesive and assemble the manufacturer's joints are called "folder-gluers". The most common adhesive types used within the packaging industry for closing boxes are (1) liquid plasticized polyvinyl acetate (PVAc) emulsion adhesives and (2) hot melt adhesives.
Liquid PVAc adhesives are applied to one of the surfaces to be joined, the substrates are mated, and then they are held under compression while the water in the adhesive is absorbed by the fiberboard. As the water is removed from the adhesive, it sets and a strong bond develops. Hot melt adhesives are applied in a molten state and the two substrates are held under compression until the adhesive gels due to cooling. Hot melt adhesives provide bond mainly to the surface of the fiberboard and offer less penetration than do liquid adhesives. Table 1 summarizes some of the advantages and disadvantages of each type of adhesive.
Liquid Polyvinyl Acetate Emulsion Adhesives
Penetration into substrate
High bond strength
Easy clean-up with water
Wettability and flow over entire joint areas
Moisture sensitive once set
Lack of freeze resistance during storage
Viscosity is temperature dependent
Low bond strength to coated fiberboard
Hot Melt Adhesives
Fast set times
Relatively good flexibility
Good moisture resistance
Good bond strength to coated fiberboard
Waste can be reused
Capable of rebonding with heat
Relatively high cost
Fast open times
Relatively low penetration into substrate
Poor elevated temperature resistance
Table 1: Advantages and Disadvantages of the Main Types of Adhesives Used to Manufacture Boxes and Containers
Because of the slower setting times required for liquid waterborne adhesive systems, hot melt adhesives are attracting much attention in the industry and they are gradually becoming the preferred adhesive. However, hot melts have a number of shortcomings. They generally cost about two times more than PVAc emulsion adhesives, and the application equipment is several times the cost of waterborne application equipment. Hot melt adhesives are also relatively viscous and they set rapidly. This limits their penetration into the porous substrate and also results in difficulty in spreading into a larger bond area to provide a strong adhesive bond.
Modern-day hot melt adhesive systems have negated many of these problems, and these adhesives will be the focus of this discussion. The selection of the type of adhesive (liquid emulsion or hot melt) will depend on many factors including the length of the pressure belt used and the quality of the fiberboard. On modern, fast running equipment production speeds of up to 120 closings per minute are possible and hot melt adhesives are preferable because of their fast set.
Hot Melt Adhesive Requirements
Initial Joint Performance
Usually the cohesive strength of the adhesive is sufficient for the packaging application as the fibers in the fiberboard provide a relatively weak matrix. However, it is important that the adhesive grab onto these fibers by penetrating into the fiberboard. Thus, the major characteristics of the adhesive that will affect joint performance are related to wetting and the speed of set. Other adhesive characteristics affecting joint performance are summarized in Table 2.
Wet out capability
Surface tension or specific adhesion
Adhesion to low energy (polymeric) coatings or synthetic fiberboard
Set time and open time
Window of processing time
Ability to hold substrates together before adhesive sets
Fiber tear strength
Measure (qualitative) of adhesion to fiberboard
Strength of the adhesive in bulk form
Strength of the adhesive joint (adhesive and substrate); usually measured in tension or peel
SAFT (shear adhesion failure temperature)
Time to joint failure under constant shear load at rising temperatures
PAFT (peel adhesion failure temperature)
Time to joint failure under constant peel load at rising temperatures
Figure 2 illustrates good and poor wetting of an adhesive applied to a fiberboard box substrate. Wetting is the degree to which an adhesive is able to flow and achieve intimate contact with the substrate on a microscopic level. Wetting is primarily related to the viscosity and surface tension of the adhesive. A lower viscosity and / or longer time in the liquid state obviously provides greater bond area and mechanical interlocking. A low adhesive surface tension will encourage penetration and also allow good bond strength to boards that have a low surface energy (e.g., coated boards or boards made with synthetic materials).
Figure 2: Illustration of good and poor wetting
(SpecialChem Fig. Ref.: Bondability index of elastomers)
With high speed box manufacture, it is necessary to control the complete adhesive-application process in order to assure the quality of the joint and the required productivity. This process includes the application of the adhesive on the substrate, the wetting of the substrate by the adhesive, the pressure and time of pressure applied on the mated joint, and the setting speed of the adhesive. The most important parameters to control these elements are:
Open time - the time from application of the adhesive on one surface until the other surface is mated with it.
Pressure time - the time during which the adhesive joint is under pressure.
Pressure on the joint - the pressure that is applied on the adhesive joint in the folder gluer.
Amount of applied adhesive - the amount of adhesive that is applied on the surface of the initial substrate.
Temperature of the applied adhesive - the temperature of the adhesive as it exits the application extruder or nozzle.
Set time - the time between application of the adhesive until the time in which the adhesive hardens sufficiently to provide fiber tearing properties.
Of these the most important to the box converter may be open time and set speed as these will affect both productivity and joint strength. Figure 3 illustrates the relationship of open time and set time to the properties of the adhesive joint.
Figure 3: Relationship of open and set times to the properties of the adhesive2 (SpecialChem Fig. Ref.: HMA Time Relationship)
Demands on the adhesive will also result from the service environment to which the joint will be exposed once the box is manufactured and filled with its contents. These demands can include mechanical stresses such as the weight of stacked boxes and dynamic forces due to transportation. Climate factors must also be considered, and these include factors such as temperature, relative humidity, and exposure to moisture or oil. The adhesive formulation must be chosen to provide both the necessary initial joint strength as well as the durability throughout box loading, transportation, storage, and retail use.
Hot Melt Adhesives
A typical hot melt adhesive has three main components as indicated in Table 2. The base polymer is often either a vinyl ethylene copolymer or a polyolefin. The base polymer gives the adhesive its cohesive strength, the resin or tackifying agent is used to improve wetting of the substrate, and the wax regulates viscosity and adhesion or tack. Other minor components, such as antioxidants, are also required in hot melt adhesives.
Hot melt adhesives, of course, have been used for some time as an environmentally acceptable alternative to solvent-borne pressure sensitive adhesives for tape and labels. They are also preferred in certain applications to waterborne adhesives because of lower energy costs during their manufacture and significant improvement in moisture resistance.
A current trend in hot melts for the packaging industry is the development of low-temperature hot melts. These hot melts can be processed at considerably lower temperatures (between 120°C and 130°C) than traditional products (between 160°C and 180°C). As the hot melt is exposed to lower temperatures this leads to improved viscosity stability and a color. Due to the lower application temperature the equipment is less stressed and so its service life expectancy is extended, less maintenance is needed and less energy is consumed. Also low temperature hot melts can be applied to heat sensitive substrates. Because of the small difference between processing and room temperature these products show a faster setting/curing, which makes it possible to shorten the production time.
An even newer trend is the use of reactive hot melt adhesives. These include polyurethane and silicone chemistries. These adhesives are applied and initially bond like a low-temperature hot melt, but they then crosslink with time on exposure to ambient humidity. As a result reactive hot melt adhesives have superior heat and creep resistance when compared to conventional hot melts.
Application processes and hot melt formulation have been optimized to the extent that it is possible to seal as well as close folding boxes. With this process is it possible to do without the interior pouches used in the past, which is economically as well as ecologically advantageous because less packaging material is needed and less has to be disposed.
In addition to formulation improvements a number of improvements have also been made in the form of the adhesive system. For example, hot melt adhesives are now being used on some packaging tapes as an alternative to pressure sensitive tape. The advantages of hot melt in these applications include strong tack and bonding power, moisture insensitivity (unlike water borne pressure sensitive adhesives), and quiet and easy unwind. This type of tape can be applied at temperatures of about 50°C.
A similar product is a hot melt adhesive impregnated with polyester fiber. Known sometimes as "packstring", this type of high strength tape is used with ethylene vinyl acetate type hot melt. As a tape for cartons and boxes it provides a low profile with extremely high strength that is resistant to corner tearing. In corrugated board, packstring can sometimes be placed inside the board where added strength is needed.
Another innovation in the packaging industry is the reduction of adhesive cost through the use of foaming agents and formulations that provide excellent adhesion with low coating weights. Formulations consisting of the use of bio-based waxes and tackifying agents also promise to further reduce cost via the use of non-petroleum based, agricultural feed stock.
Another driver in the development of hot melt adhesives is biodegradability and compatibility with paper pulping processes. The paper stock is generally biodegradable (depending on any coatings that may be used) and it would be preferable that the tapes, labels and adhesives used for their attachment also be biodegradable. To this extent, several new biodegradable tape and label products have come on the market where both the face stock as well as the adhesive is biodegradable and / or compatible to re-pulping processes.
Testing of Adhesive Joints and Boxes
To determine the adhesive joint quality, generally peel tests are employed. Traditionally the tests are performed by manually peeling one substrate from another and then studying and grading the result. Generally adhesive fracture with greater than 50% fiber tear is required. The Y-peel test (Figure 4) is often preferred in the industry since it is easy to use and the set-up is relatively well defined. Moreover, by evaluation of the energy consumption (dissipative energy) during the Y-peel test, it is possible to obtain not only a qualitative but also a quantifiable assessment of the adhesive joint. 3
There are also several regulations related to corrugated products, such as those set by carriers (rail and truck). Defined specifications which can be considered as industry standards for corrugated materials are provided by the Fiber Box Association (FBA) or the Association of Independent Corrugators (AICC). Equipment and machinery guidelines and standards can be obtained from the Packaging Machinery Manufacturers Institute (PMMI).
The carrier rules provide the following guidelines for the manufacturers' joints. For example, boxes must have manufacturer's joints not less than 3.18 cm overlap and gluing the entire contact area with a water resistant adhesive. Most corrugated box material and corrugated package testing procedures are provided by the Technical Association of the Pulp and Paper Industry (TAPPI) and the American Society of Testing and Materials (ASTM). For example, TAPPI Standard T813 provides a tensile test for the manufacturer's joint of fiberboard shipping containers (applicable to taped, stitched, or glued joints).
Please share your thoughts using the "Rate and react" tools below.