Modifying Adhesives for Superior Performance

Linear saturated copolyester resins are at the foundation of many high-performance, solvent-based laminating adhesives. Widely used to bond laminates constructed of film, foam, or foil substrates, these liquid adhesive systems are highly durable. They meet or exceed the bond specifications of industrial laminations used in cable wrap and flexible circuitry, and they can withstand the rigors of flexible, film-to-foil laminated packaging for medical, industrial, and food products. These high-performance adhesives are even versatile enough to create vinyl/foam laminates used in automotive interior trim applications.

All of these liquid applications use a high-molecular-weight copolyester resin that provides an initial green tack and high bond strength immediately upon lamination.

The actual copolyester resin used to formulate a specific adhesive, however, varies to provide optimum adhesion and physical characteristics for a particular application. To produce physical traits well beyond those inherent only to the resin, certain adhesives must be modified by an appropriate curative system.

Linear Saturated Copolyester Resins
Linear saturated copolyester resins have the structure of a long, straight molecular chain capped at each end with a hydroxyl group, or an organic acid group.

Without modification, the linear chain resin can be used for adhesive purposes — essentially as a thermoplastic system. But, the bond performance for laminates is often suspect. That is because the long structure of the resin allows the molecular chains to slip by each other, contributing to the substrates' gradual separation.

As a countermeasure, more robust adhesives can be formulated by using a cross-linking curative agent. Options are isocyanate systems, aziridines, epoxies, and other chemicals that link the polymer chains together forming a tight-knit, thermoset matrix.

The most common curative agents are based on isocyanate systems. These agents react to the polymer resin's end groups, as well as with trace moisture in the system.

Methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), and isophorone diisocyanate (IPDI) all can be used as reactive isocyanates with polyester resin. They are considered difunctional, because they have two reactive groups per molecule.

Used as is, these curatives would chain extend the polymer structure to create a system with a higher molecular weight but with little if any cross-linking capability. A curative with functionality greater than two reactive groups is necessary to provide a true cross-linking effect.

The effect of cross-link functionality on cross-link density: A high-molecular-weight polymer with low reactivity, which is reacted with a low-functionality cross-linking agent such as MDI, will yield a weak matrix.

By switching to a curative with higher functionality — such as a trifunctional adduct of MDI or a trifunctional adduct of TDI — a polymer with a tighter matrix will result. Ultimate strength is attained by increasing the functionality of the polymer itself. This produces a dense, cross-linked system.

In addition to reacting with the polymer's end groups, isocyanate curatives exposed to the system's trace moisture also can generate various reaction products. With high-molecular-weight resins, these so-called side reactions are at times more critical to the finished adhesive's performance than the primary reaction between the curative and the polymer.

Because these side reactions are instrumental in creating a highly cross-linked system, an excess of isocyanate generally is added to the adhesive mix.

The interaction of isocyanate with water and polymer can yield a variety of reaction products.

The first two reactions show the chain extension that occurs as a diisocyanate reacts with the polymer to increase the resin's molecular weight. This can continue for some time. Among the reaction products that result, urea linkages occur that may contribute more to the toughness of the cured polymer than the chain extension itself.

Effect of Moisture on Cure Properties
All of these reactions should be controlled to produce a finished product with uniform physical properties. To examine the effect of moisture on an isocyanate-cured system, our chemists took a high-molecular-weight copolyester resin and solvated it in toluene.

The resin had a glass transition temperature (Tg) of — 15 deg C. The hydroxyl number, indicating the functionality of the polymer, was 3-6. Chemists reacted the polyester resin-based adhesive with a trifunctional isocyanate system based on a TDI compound. A ratio of isocyanate to resin that would result in a six-fold excess of isocyanate to hydroxyl was established.

As a test experiment, the adhesive was used in a production application to bond vinyl to foam. The line speed was relatively slow, and the adhesive was exposed to ambient air after exiting the drying ovens.

Modulus development, solvent resistance, peel strength, and a number of other physical properties were evaluated in monitoring the cure. Changes to the adhesive's glass transition temperature also were examined over time. The Tg measurement helped assess the polymer's cure development independent of other adhesion characteristics, such as substrate wetting, reactivity, and polarity.

To complete the experiment, an exact amount of water was added to five adhesive samples. The adhesives were cast into films and then exposed to varying levels of humidity.

Reviewing the data, it is evident that the adhesive system is affected minimally by the addition of water. However, exposing the adhesive films to moisture after drying prompts a much greater effect.

In the first set, the samples with 0.1% moisture added showed an increase in Tg from 20 to 32 deg C after 96 hr. With the maximum level of moisture, a slight increase occurred in the Tg at 24 hr but without any real change at 96 hr.

When exposing the same samples to 70% relative humidity (RH), the maximum cure was achieved between 48 and 96 hr. This was true even for the samples with the least amount of water.

In another series of tests, chemists studied the softening point of a sample adhesive resin used to bond vinyl to foam for automotive interior systems. The rapid development of heat resistance was critical for subsequent processing of the automotive parts. Results in this instance followed the previous pattern.

Samples exposed to higher levels of ambient humidity showed a more rapid increase in softening temperature.

What does this mean to the adhesive user?

The key way to achieve rapid development of a high-modulus adhesive is to control moisture exposure levels after drying and before laminating. With spray applications or slow-speed lamination, this can be accomplished by humidifying the area between the coating and lamination stations.

For high-speed applications, it is more difficult to draw moisture into the lamination. Laboratory attempts to affect the moisture level in the nip by spiking the adhesive with water met with limited results. Most of the water boiled off in the oven, leaving little residual moisture to react with the isocyanate.

One possible alternative for introducing more moisture into the lamination is to use a faster curing system such as a trifunctional adduct of MDI. A second option is to add a species that mimics the increased modulus, increased Tg, and improved chemical resistance properties of a humidified adhesive.

Mimicking the Effect of Moisture on Isocyanate Cure
In high-speed laminations, adequate amounts of moisture cannot be brought into the nip to control the modulus development. While there is likely to be some moisture after lamination of a permeable substrate, its impact is negligible at best. Moreover, two impervious substrates will not permit any increase in moisture. A potential remedy rests with a form of copolyester resin that helps reduce the moisture sensitivity of the isocyanate cure system.

These copolyester resins are branched, low-molecular-weight systems with hydroxyl end groups. These particular resins can be highly effective in increasing the rate of cure, since their low-molecular-weight copolyester structure allows their solubility in many of the same solvents used in laminating adhesives.

To demonstrate its effectiveness, chemists measured the softening point after curing a copolyester resin. The adhesive resin under study had a Tg of — 18 deg C. The hydroxyl number, relating to the functionality of the polymer, was 3-6. The polyester resin-based adhesive was reacted with a trifunctional isocyanate system based on a trifunctional adduct of MDI. A ratio of isocyanate to resin was chosen so as to result in a 3.5-fold excess of isocyanate to hydroxyl. This adhesive system was used to bond polyester film to polyester film. The line speed was fast, and the rapid development of high strength was critical.

Melt point measurements of this trifunctional adduct of MDI-cured adhesive showed a softening point by Dynamic Mechanical Analysis (DMA) of 55 deg C. By adding 5% of the copolyester resin and raising the isocyanate level commensurate with the increase in hydroxyl number of the blended system, the softening point increased to 60 deg C. The addition of 10% of the copolyester resin placed the softening point at 90 deg C.

The same effect comes about when modifying coatings with the same low-molecular-weight copolyester resin. A coating resin with a Tg of 50 deg C showed a dramatic increase in softening point after the addition of 5%-10% more copolyester resin. The presence of the specialty copolyester resin greatly increased the cross-link density of the system. The result is a high-functionality polymer cross-linked with a high-functionality curative.

The one significant drawback of using such a system is its effect on modulus. In a system where high extendibility is critical, a dropoff in adhesion can occur with increasing levels of modifier. This is due to a rather excessive modulus level in the finished adhesive, which results in “zippy” bonds.

In conclusion, to control cross-link density and development, either the moisture content of the applied adhesive or the functionality of the polymer system needs to be regulated. By adding 5%-10%, by weight, of a low-molecular-weight multifunctional polymer, the functionality of the polymer systems can be dramatically enhanced. The benefits of both measures are an increase in the system's Tg, improved cross-link density, and a lessened reliance on atmospheric moisture to achieve ultimate cure strength.

The authors are team members in the Analytical Services and Flexible Laminating Adhesives Groups at Bostik Inc., Middleton, MA. For more information contact them at 978/750-7245; 888/571-8558, ext. 7337

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s)./p>

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