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Adhesives – high performers

The following research report documents performance testing carried out by Rohm and Haas in their development of acrylic/urethane hybrid adhesive systems for the flexible food packaging industry in a bid to improve the performance of water-based acrylics by introducing urethane. While this technology is rapidly growing in North America its applications in Europe are still relatively new says Dr Mai Chen, the scientist who recently received the Otto Haas award for scientific accomplishment on this topic

While this technology is rapidly growing in North America its applications in Europe are still relatively new says Dr Mai Chen, the scientist who recently received the Otto Haas award for scientific accomplishment on this topic

Overview

Solvent based polyurethane and polyester adhesives have been used in dry bond laminating adhesives for decades, demonstrating good adhesion on various substrates such as aluminum foil, polypropylene (PP), polyethelene (PE), PET and nylon.

In the flexible food packaging area, solvent-based urethane adhesives have always provided good product resistance to moisture, fatty foods and oil to meet the requirements for different food packaging.

In response to industry trends and demands, extensive research has been focused on developing water-based dry bond laminating adhesives that provide comparable performance to solvent-based adhesives. Both acrylic latex and polyurethane dispersion (PUD) adhesives are currently available in the market.

Water-borne acrylic adhesives typically are used in bottle label and snack food packaging applications.

Polyurethane dispersions have been used to improve the adhesion and product resistance of water-based adhesives.

However, the pot life of two-part PUDs usually is short and the raw material cost of PUDs can be undesirable. The purpose of this work was to improve the performance of water-based acrylics by introducing urethane into the acrylic system to make an acrylic/urethane hybrid adhesive.

Hydrogen bonding and good cross-link density are the two key factors needed to achieve good laminating adhesion.

Due to the nature of their chemistry, water-based acrylic adhesives do not have the strong hydrogen bond provided by urethane adhesives. Also, there is poor cross-linking with most acrylic latexes.

One part acrylic latexes have advantages over other water-based systems such as urethane dispersions. They usually have very high molecular weight, high shear strength, good machine runability and pot life. In order to improve the adhesion of water-based acrylics while still maintaining their good properties, a new two-part acrylic/urethane hybrid water-based adhesive has been developed.

Due to an interpenetrating network (IPN) formed in the urethane/acrylic hybrid system and the cross-linking from the urethane reaction, the urethane/acrylic compound exhibited superior cohesion strength, peel strength, humidity resistance, heat resistance and product resistance compared to one part acrylic adhesives.

A urethane dispersion was also included in the test. The end result of these comparisons was the finding that the acrylic/urethane hybrid has equivalent or better performance than the urethane dispersion system and is vastly superior to basic acrylic systems.

Cohesive strength test

Cohesive strength was tested by a drop shear test. A 1000g-weight cell was loaded on the one end of the primary web of a 0.5x1in laminate and the secondary web was suspended at the top. The time until the weight cell dropped was recorded.

Table 1 shows the drop shear results of the three adhesives. The two-part acrylic/urethane hybrid system showed improved cohesive strength over the single acrylic latex by more than 100 times.

The cohesive strength of the acrylic/urethane hybrid system was more than 10 times higher than the cohesive strength of the urethane dispersion adhesive.

Peel strength test

The three adhesives were applied to various substrates to test the T-peel strength. The acrylic/urethane hybrid system showed improved adhesion as compared to the acrylic latex.

The acrylic/urethane hybrid had similar and sometimes better adhesion when compared to the urethane dispersion adhesive. Figure 1 lists the adhesion comparison between the three adhesives on various substrates.

Humidity resistance test

Another advantage of the acrylic/urethane hybrid system was the improvement of moisture, heat and product resistance when compared to both the single acrylic latex and the urethane dispersion adhesive.

The humidity resistance test was conducted by placing 1in strips of each laminate in a 90% relative humidity oven at 100° F for 100 hours. The peel adhesion was checked before and after the test to compare the loss of adhesion.

The one part acrylic latex adhesive lost its adhesion after the test while the acrylic/urethane hybrid maintained its good adhesion after the test. Figure 2 lists the outcome of the acrylic latex and acrylic/urethane hybrid comparison.

‘Boil-in-bag’ test

A ‘boil-in-bag’ test was also conducted to check the heat and product resistance of the laminates made with the adhesives on various substrates.

Different products including water, corn oil and a mixture of vinegar, corn oil and ketchup were packed in pouches made with laminates. The pouches were boiled in water for one hour. The peel adhesion before and after the test was measured.

The one part acrylic latex adhesive had the worst heat and product resistance. Pouches made with the acrylic/urethane hybrid performed well after the test with no tunnelling or delamination observed. These test results showed the significant improvement in product resistance of the two-part acrylic/urethane hybrid system versus the acrylic latex adhesive.

Mechanical stability

Unlike typical two part polyurethane dispersion, the two-part acrylic/urethane hybrid system has a long pot life.

The viscosity of the acrylic/urethane hybrid mixture stayed about the same 24 hours after mixing.

The two-part acrylic/urethane hybrid system had a clear and transparent appearance.

It can be run at high machine speeds with no foaming or grit formation. In addition, it wets well on various substrates.

Conclusions

The newly developed acrylic/urethane hybrid water-based adhesive has many benefits compared to one part acrylic latex adhesives and urethane dispersions.

For example:

* It has improved the adhesion, moisture resistance and product resistance of one part acrylic latexes as dry bond laminating adhesives.

* The performance of acrylic/urethane hybrid adhesive was equal or better than the polyurethane dispersion adhesive tested.

* It demonstrated extended pot life and superior mechanical properties during application.

* It adheres to a wide variety of substrates with good clarity, good product resistance and minimal slip interaction.

The exceptional performance of the acrylic/urethane hybrid makes it ideal for more challenging flexible food packaging applications.

It is also better suited for advancements in flexible packaging technologies than traditional acrylic latex and urethane dispersion adhesives. More research is currently being conducted in order to assess additional applications for this acrylic/urethane hybrid adhesive system.