Hot Melt Adhesives for Closing Cartons and Boxes

Introduction

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 1960s, 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 "manufacturers joint". The fiberboard thickness, weight, and surface coatings will determine the type of fastening method to be used.

Adhesive bonded manufacturers 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 manufacturers joints are called "folder-gluers". The most common adhesive types used within the packaging industry for closing boxes are INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (1) liquid plasticized polyvinyl acetate (PVAc) emulsion adhesives and INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (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.

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 12