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By: Betsy Kearns, Ron Kelley, Mark Mulready, Eugene Sommerfeld, and Tom Warren, Bostik.
High-molecular-weight, linear saturated copolyester resins offer a great deal of versatility when formulating adhesives and coatings for flexible laminating and industrial converting. Applied through a solution or an extrusion coating method, these resins give copolyester and vinyl films many of their high-performance features.
Copolyester resin-based coatings anchor printing inks to polyethylene terephthalate (PET) and polyvinyl chloride (PVC) films. They also create a plasticizer barrier on vinyl films; and are used as heat-seal coatings on PET and PVC films.
As laminating adhesives, copolyester resins are combined with a trifunctional isocyanate curative, and used to bond various film-to-film and film-to-foil laminates.
For all their natural capabilities, the properties of copolyester resins are often impacted by a seemingly benign outside influence — storage.
Storage conditions affect the stability of resins and their solutions. They can alter the molecular weight of a resin. And, with regard to the aesthetics of a coating or an adhesive, storage can even distort the appearance of a resin in solution.
The Role of Moisture and Temperature
Moisture is a critical condition affecting the storage stability of polyester and copolyester resins. Polyester resins hydrolyze in the presence of moisture. The higher the temperature, the faster hydrolysis occurs.
Consequently, resins used in an extrusion process must be dried just prior to melting. A moisture content of less than 0.05% is usually sufficient and can be achieved with a hopper-dryer device.
Glass-transition temperature (Tg) is another important factor regarding the stability of copolyester resins. A resin stored above its Tg may hydrolyze on standing.
Bostik’s technical specialists have measured the intrinsic viscosity (IV) of copolyester resins at various storage temperatures. These data, as plotted in Figure 1, show that the higher the storage temperature, the more rapid the decrease in molecular weight. While this is now a recognized effect in the industry, it was not addressed until recently.
To lessen the likelihood of a drop in molecular weight, Bostik stores its low-Tg resins in refrigerated warehouses. This ensures that customers receive a much more consistent product.
Factors Affecting Resin Stability
The effect of long-term storage on copolyester resins may not be the same in every situation. Outcomes vary depending on whether the resins are used to formulate heat-seal coatings or laminating adhesives. As an example, Bostik chemists have measured the heat-seal profile of Vitel® 1902B at various molecular weights. The findings, as seen in Figure 2, show that gains in peel strength correspond to increases in molecular weight as measured by intrinsic viscosity (IV). Conversely, heat-seal strength is likely to lessen as a resin’s molecular weight drops due to hydrolysis. By contrast, the decline in molecular weight of a resin used in a laminating adhesive may not have as critical an effect on the finished properties of the laminate. Here’s why. Companies that formulate laminating adhesives generally use an excess amount of isocyanate compared to the hydroxyl portion of the resin. Even though hydrolysis generates more hydroxyl groups, enough isocyanate exists to fully cure the system. The lower molecular weight that results may have some effect on the green strength. But, unless the resin is particularly old, there should be no loss of adhesive performance. A change in the glass-transition temperature of a polymer is another storage effect that can influence resin performance. For example, if a polymer is stored below its Tg, it will not hydrolyze. But, an amorphous polymer with very low Tg will tend to block and cold flow. This can be seen with Vitel 3550, an amorphous copolyester with a Tg of -15°C. If a slab of this resin — at room temperature — is left hanging off the edge of a table, it will very slowly cold flow to the floor. Vitel 2700, an amorphous copolyester with a much higher Tg of +47°C, presents an entirely different scenario. This polymer remains a free-flowing granulated resin until it is stored in a hot area of the warehouse or factory. Once Vitel 2700 is exposed to temperatures above +47°C, the free-flowing resin will become tacky. Crystalline polymers react differently to storage conditions once they are crystallized. Prior to crystallizing, these resins demonstrate characteristics much like their “amorphous cousins.” A crystalline polymer stored above its glass-transition temperature will be tacky and block together. Once crystallized, it remains a free-flowing resin in environments well above its Tg. Crystalline copolyester resin Vitel 1902B has a Tg of -9°C. Once crystallized, it maintains a free-flowing state until it approaches its 130°C melt point, then becomes tacky. It should be noted that some crystalline polymers take time to crystallize. When initially extruded or cast from solution, Vitel 1902 will be amorphous. It will take a while to fully crystallize. Other resins like Vitel 1050, which is generally used as an extrusion resin, will crystallize almost immediately.
Stability of Resins in Solution
High-molecular-weight copolyester resins solvated in urethane-grade methyl ethyl ketone (MEK) and toluene are quite stable, generally. The resin will not hydrolyze in either of these solvents, and, as a result, the intrinsic viscosity is more stable over time.
Certain polyester resins, however, do exhibit one negative effect in solution. They sometimes turn cloudy.
Polymers based on ethylene glycol and terephthalate will form low-molecular-weight cyclic oligomers during polymerization. The oligomers are finely dispersed when the resin is first being solvated. Over time, these invisible particles begin to amass or agglomerate.
These clusters appear as a very fine haze before becoming much more noticeable later on. The agglomerated particles ultimately settle out and can be filtered. However, if some random particles escape the filtering process, they will once again cluster and re-form the haze.
Cyclic oligomers are found to form more rapidly in high-solids solutions. Solutions that have undergone several freeze/thaw cycles experience a faster generation of these particles.
Interestingly, different lots of the same resin may show significant differences in oligomer production. Generally, though, the oligomers do not hinder resin performance. They instead draw attention to aesthetics. To limit the chances of oligomer formation in critical applications, Bostik specialists recommend that the solutions be stored at low-solids levels.
Storage conditions of linear saturated copolyester resins and their solutions may influence their performance. When practical, resins should be stored at, or below, their glass-transition temperatures. Low storage temperatures often result in a longer product shelf life.
If dry solvents are used, the solutions made from these resins are usually stable enough to withstand shifts in molecular weight.
Finally, when clarity is critical — and whenever it’s practical — the lowest level of solids should be used to store the solution.