Foaming Affects Rheology, Performance of Adhesives for Paper Laminating

October 5, 2000

The paper-laminating industry is always looking for ways to decrease warpage of its board. Looking at other industries, Reichhold discovered that foamed adhesives could provide solutions to this problem. The benefits of foaming come from having greater control of the adhesive rheology and adhesive density.

Reichhold developed the TRIAD™ adhesive-modification system initially for the single-face laminating industry. It is currently used in other paper-related markets as well. It provides three main benefits, including the ability to control rheology through foaming, monitoring of operations and reduced costs.

When employing Reichhold’s TRIAD System, the adhesive must be foamed in a proper manner in order to provide benefits. This foam is made up of small bubbles, which collapse only when compressed between the substrates. The resulting film has no gaps or voids. Adhesives may be formulated to not collapse, if desired.

Foam quality is also very important. High-quality foam has small, uniformly sized bubbles. This produces the longest foam life. It also produces an even consistency throughout the foam.

Equipment selection is key to the success of a foaming system. The TRIAD™ System equipment is installed at the customer’s facility near its adhesive applicator. Normally, the machine can be integrated into the adhesive feed line from the adhesive storage to the applicator.

The equipment Reichhold has chosen produces a consistent foam at all outputs without the use of a surge tank. (See photo above.) It does this by constantly monitoring adhesive demand and adjusting its mix ratio accordingly. Injecting air and adhesive at a calculated ratio into a specially designed mix head produces the foam. Performance additives can also be mixed inline as needed for special runs.

Advances in computer technology have allowed the integration of computers into the foaming equipment. TRIAD’s computer monitors the consistency of the foam 300 times per minute and instructs the equipment to make necessary changes to produce uniform foam. The computer simultaneously records data such as line speed, adhesive consumption or ambient conditions.

One common piece of data is adhesive usage per 1,000 ft² of board produced. Reichhold can provide a program that allows customers to measure progress and performance of adhesive used to produce a set amount of product. This equipment can be remotely accessed, and data can be downloaded for monthly monitoring and reporting.

We have been mainly concerned with foaming water-based emulsion adhesives, although other adhesive types can also be foamed. Reichhold’s TRILOK™ adhesive line is specially formulated to produce uniform foam with TRIAD units. Reichhold’s Packaging Systems team initially developed the TRIAD system for its packaging customers and is now introducing its benefits to other industries.

Single-Face Lamination First

Reichhold chose to initiate TRIAD’s development with the single-face laminating industry because of the solutions the system can bring. Some of the problems this industry experiences include warpage of sheets and an inability to use thinner paper. Most of these problems involve water; so introducing less water allows single-face laminators to produce a better product.

Less Adhesive Used = Less Water, Stronger Board and Less Ribbing

Most single-face-lamination applicator roll systems are set up to apply a set volume of adhesive. To decrease the amount of water introduced, high-solids products have historically been used. But, with foamed adhesive, the air helps decrease solids based on volume by acting almost as a secondary carrier. Therefore, the same volume of foamed adhesive will introduce less water into the board. This also leads to reduced adhesive usage, which decreases costs as well. Applying less water helps the paper retain its strength. This allows the use of lighter weight papers while maintaining sufficient board strength.

Ribbing or strike-through of the flutes into the facing sheet also is reduced when less water is applied. Reichhold has not experienced any decrease in bond strength when a foamed adhesive is used properly. In fact, we believe bond strengths may actually be increased for certain applications. This increase is due to controlling the rheology of the adhesive.

Thixotropic Flow

As air is introduced into the adhesive, the rheology changes. As shown in Figure 1, the low- and high-shear Brookfield viscosities rise according to the amount of air introduced. However, the high-shear viscosities do not rise as fast as the low-shear viscosities. This indicates that adding air to the adhesive gives it thixotropic flow.

Thixotropy occurs when structures between particles are built when the system is at rest. This causes the viscosity to rise. Under stress, the structures break down at a rate proportional to the shear stress applied. Thixotropic materials are characterized by a decrease in viscosity under shear. The high speeds and small metering gap of the adhesive applicator apply shear to the adhesive.

A foamed adhesive will demonstrate a lower viscosity when applied with a metered roll applicator due to the shear applied. But after it is applied, the adhesive begins to recover its structure because there is very little shear stress acting on it. This causes a fast rise in viscosity. The higher viscosity prevents the adhesive from penetrating too quickly into the substrate. This property, called “hold-out,” can decrease adhesive usage for paper laminating without affecting bond strength. This is because the adhesive remains at the surface of the paper where it is needed most.

The level of thixotropy can be stated by the thixotropic index – a ratio of the low-shear viscosity to high-shear viscosity. For our purposes, the thixotropic index is the Brookfield viscosity at 5 rpm divided by the viscosity at 50 rpm. Figure 2 shows how the thixotropic index changes as more air is incorporated into the system.

Putting Adhesive Where It’s Needed

Thixotropy also provides another benefit for single-face laminating. In single-face laminating, the adhesive is transferred from a metering roll to the tips of the paperboard flutes. The facing sheet is then mated onto the adhesive line.

Most of the adhesive, which is not immediately between the contact points, does not contribute much to the bond strength. Most nonfoamed adhesives run on these types of applicators have a viscosity of approximately 1,000 cps, run at 20 rpm on a RVF Brookfield. At this viscosity without thixotropic properties, some of the adhesive runs down the sides of the flute, as shown in the photo above and in Figure 3.

This improperly placed adhesive is wasted and also contributes to warp because excess adhesive is being applied. The TRIAD™ system causes the adhesive to apply like a low-viscosity adhesive but build very quickly to a much higher viscosity. The low-shear viscosity of the material prevents it from running down the sides of the flute and being wasted. Since adhesive is not wasted when compared to conventional means, much less adhesive is required to provide a comparable bond.

Better End Products With Less Adhesive

Many adhesive applicators have benefited from the foamed adhesive system. They have achieved a better finished product. In addition, their control of the adhesive rheology has reduced the amount of adhesive required.

New TRILOK specialty adhesive grades are also under development to provide applicators with even more control of their processes. This system allows Reichhold to provide even more solutions to customer needs. The TRIAD System truly is proving to be “A Smarter Way to Apply Adhesives™”.