Aqueous gluing of coated paperboard packaging products in North America, Solutions!, Online Exclusives, April 2004

Aqueous gluing of coated paperboard packaging products in North America

By Quintin Parker, BASF Corp

Gluing is a very important process in the converting of coated paperboard packaging products. The main criterion in determining successful gluing of packaging products is almost always 100% fiber tear upon separation of the glue joint. Failure to achieve fiber tear along the glue joint is considered unacceptable by North American converting operations.

Most paper mill manufacturers as well as converting operations have developed qualitative test methods that focus on percent fiber tear or the time to reach fiber tear; however, very few focus on quantitative methods that predict the relative force required to separate the glue joint. Regardless of which approach is currently being used, the glue joint must be strong enough to prevent the loss of product at the consumer level. This paper will focus on the converting process, the mechanism of gluing coated paperboard products, the fundamentals of product design, and test methods used to predict acceptable performance.

Converting of coated paperboard products
The product line typically used for packaging products by coated paperboard manufacturers is called folding carton board (FCB). This product is usually single, double, or triple coated on one side (C1S) using various types of pigmented coating applications. There are specialty applications that require a single coating being applied to the backside for functionality. The next step is usually printing of the coated side using flexography, offset lithography, or rotogravure printing processes. Depending on product specification, a backside treatment application (such as PE extrusion) may occur before printing. The substrate is now ready for the gluing process.

The type of adhesive typically used in gluing coated paperboard substrates is based on polyvinyl acetate (PVAc) chemistry along with water and other additives for functionality. Hot melt adhesives are also used to glue the top and bottom flaps of the cartons upon filling, however this paper will focus more on the use of aqueous adhesives (cold set glues) which are mainly used to connect the side seams of the carton. The glue is applied to the coated side of the substrate using high speed gluing equipment. During the gluing process, a thin strip of glue is applied to the coated side of the carton and then folded over to meet the uncoated side to form the glue joint. The carton is then placed under compression only for a few seconds (depending on machine speed) before being stacked and assembled for shipment to the end user. Quality control checks are performed just before the cartons are palletized for shipment. It is during this period when the converter decides whether an acceptable amount of fiber tear is achieved during separation of the glue joint.

Mechanism of aqueous gluing
To achieve consistent fiber tear when gluing coated/uncoated substrates, the basesheet of either substrate must always be the weakest link upon separation. The mechanism in achieving consistent fiber tear beneath the coated surface is such that glue vehicle (mainly water) must penetrate to the fiber matrix to cause “fiber bond fractures” (weakening) which in turn initiates fiber tear. As this process is occurring, glue particles are being entrapped in the pores of the substrate where they become “set” and form a permanent bond. This is only achievable when “moisture” is able to penetrate either side of the coated substrate. The rate at which this process occurs depends mainly on coated and uncoated sides’ surface pore structure development, which is easily influenced by binder type and level for coated substrates. See Figure 1 below.

Figure 1. Description of glue/glue vehicle penetration into both substrates.

When a barrier effect is created at the surface of either substrate, it slows down the dehydration rate of the glue considerably thereby reducing the tendency to tear fiber upon separation. In laboratory studies conducted at BASF’s Charlotte Technical Center, z-directional (?) penetration of the glue vehicle showed faster glue set times while creating consistent fiber tear than xy-directional (?) spreading. This was observed in Figure 1A when a cross sectional analysis of a glue joint (containing glue tagged with Optical Brightening Agent [OBA]) was taken using a fluorescent lighting SEM technique. The board that exhibited good fiber tear showed glue/glue vehicle penetration down to the fiber matrix. See Figure 1a below.

Two conditions that interfere with the development of good fiber tear are: (1) the application of a surface treatment that reduces penetration of the glue’s vehicle into the fiber network and (2) having a highly bonded, very strong baseboard (high internal bond) exceeding the strength of the coated surface or the glue.

Note that “100% fiber tear” assures the converter a continuous glue joint is present. However, this method does not give a measurement of adhesion strength.
Table 1 highlights the results of a glueability study comparing multiply paperboard at various starch pick up levels. Dennison Wax and Mullen Burst Testing were performed on each condition.

Table 1. Data Table indicating poor fiber tear results as surface sizing treatment increases.

The results of the study indicated the following: First, as the level of starch increased, fiber tear became more difficult to achieve. Secondly, although the adhesion force increased with increasing starch pick up, the point of failure shifted to the coating which is not acceptable to North American converters.

Folding Carton Board (FCB) Product Design for Aqueous Gluing
The binders used in coating formulations for folding carton products have an enormous impact on aqueous gluing performance. Polyvinyl Acetate (PVAc) synthetic polymers have been the choice for many years.

Also, styrene butadiene (SB) synthetic copolymers in conjunction with soy protein (natural co-binder) were another binder system typically used in FCB. However, since the move to brighter and improved print quality in the paperboard market, styrene acrylate (SA) synthetic copolymers in conjunction with high levels of calcium carbonate (CaCO3) have become the new formulations of choice in the SBS board market. These new formulations have been able to maintain and in some cases improve aqueous gluing performance of FCB products. See Table 2 below.

Table 2. Pigment and binder systems typically used in the
various market segments of FCB

When considering reformulation of coating colors for FCB products, the manufacturer must always determine the impact the changes will have on glueability. A product having poor glueability can result in significant claims to the Paperboard producer when the end product is designed for food, beverage, cigarette, cosmetics or soap detergent markets.

As the demand for higher brightness, and improved printability (print snap, reduced mottle) increases, formulating for glueability becomes more challenging due to the adverse requirements of the performance parameters. For example, a PVAc/CaCO3 system may have excellent gluing performance but may negatively impact print quality. The proper selection of raw materials and their levels when designing coating formulations is the key to balancing glueability and print properties.

Test Methods Used to Predict Aqueous Gluing Performance
FCB manufacturers have incorporated various test methods to predict aqueous gluing performance before shipment to the converter. The traditional test is to apply a 4-inch bead of aqueous glue to the coated surface and sandwich the backside surface to it while applying a known weight over a specified period of time. The weight is then removed and the glued substrates set for a known period of time before separated. See Figure 2 below. Upon separation, the observer then determines (qualitatively) the amount of fiber tear (as a percent) achieved from either substrate.

Figure 2. Typical Aqueous Glue Test Method used by FCB Manufacturers

There are other glue tests that focus on the time it takes to reach an acceptable amount (usually 70% or more) of fiber tear. BASF’s Paper Group has incorporated a quantitative method, using the “Mullen Burst Tester”, to predict the force required to detach the glue joint. See Figures 3 & 4 below.

Figure 3. Description of Mullen Burst Tester used by BASF’s Paper Group

Figure 4. Mullen Burst Tester (w/ test sample in place)

Although the Mullen tester provides information that predicts the force (in kPa) required to detach the glue joint, this information alone may not be acceptable to North American converters because they always require a specified level of fiber tear upon separation.

VI. Troubleshooting Glue Problems with BASF Test Methods

When troubleshooting aqueous gluing issues, there are a number of tests that can be used to help determine the cause of failure. See Table 3 below.

Table 3. List of Test Methods used to Troubleshoot Aqueous Gluing Issues

Summary
In summary, aqueous gluing performance of paperboard products is a very important factor to consider when designing coating formulations for FCB manufacturers. Keep in mind that there are a multitude of variables in the papermaking process that could impact the gluing performance. However, one should remember that pore structure development of the coated/uncoated surface(s) is the key to successful dehydration of the glue vehicle down into the substrate(s). Also, FCB manufacturers must be able to quickly assess the gluing performance of their product before shipment to the converter. Incorporating the proper test method that predicts the performance of the product can easily do this.

About the author:
Quintin Parker is a paper coatings specialist at BASF Corp.’s Functional Polymers research facility in Charlotte, North Carolina, USA. He can be reached at 704 588-5280.