Sulfopolyester Resins for Water-Based Ink Formulations

Printing Inks and Packaging The packaging printing market continues to grow rapidly, driven by increased demand for unique and customized products, which require brand owners to use a variety of printing inks to effectively distinguish themselves from competition and market to consumers.  New ink formulations are thus constantly being developed for brand owners, who apply them to substrates such as metal foils and plastic films used in flexible packaging, as well as paperboard used in carton and box packaging.

Sustainability, safety, and environmental concerns have made water-based ink formulations increasingly preferred to solvent-based inks.  Water-based ink formulations rely on resins for several key functionalities, which include successfully binding pigments and other formulation additives together, forming visually appealing films, and providing good adhesion onto substrates.  Therefore, selecting the right resin for an ink system is important. 

Eastman AQ™ Polyesters and Eastek™ Polymer Dispersions

Eastman's AQ™ Polyesters are thermoplastic, amorphous, and aromatic sulfopolyester resins that contain anionic sulfonate groups that allow them to be easily dispersed into water without the need for emulsifiers, co-solvents, inorganic bases, amines, or other dispersing additives.  

When dispersed in water, negatively charged sulfopolyester polymer aggregates are suspended and stabilized through electrostatic repulsion.  Compared to water-soluble polymers that form latexes, the AQ™ dispersions form very small aggregates, as shown in the image below.

 
 

 

These resins are available in two formats.  The Eastman AQ™ Polyesters are solid, water-dispersible sulfopolyester resins.  Eastek™ Polymer Dispersions are the same sulfopolyester resins, pre-dispersed in water.  ​

 

Advantages of Using Eastman AQ™ Polymers and Eastek™ Dispersions in Water-Based Ink Formulations

There are two main areas where Eastman AQ™ and Eastek™ provide formulation benefits.   One is the ink’s formulation design and the other is its application and handling.  The specific benefits that Eastman AQ™ and Eastek™ sulfopolyesters provide are detailed below.

Formulation Design Benefits

 

Efficient Film Formation, Adhesion, and Chemical Resistance

While one of the main functionalities of resins for water-based inks is to provide film formation on substrates, it is also important that these films are formed quickly and easily.  Conventional polymer latexes like acrylic latexes typically form films at a temperature that is approximately equal to the glass-transition temperature (Tg) of the polymer.  For polymers used in water-based inks with a Tg above ambient temperature, either heat must be used or a coalescing agent must be incorporated to lower the Tg and complete the cure of the printing ink.   

 

In comparison, Eastman AQ™ and Eastek™ sulfopolyester resins are plasticized by water, which lends to easier coalescence and a minimum film formation temperature around 20°C lower than the listed Tg.  In most cases for these resins, coalescing agents in water-based inks are not necessary. 

Drying times of films made with Eastman AQ™ and Eastek™ sulfopolyester resins are also fast, even allowing for gravure line speeds in the range of 1,900–2,000 ft/min, which are comparable to, or exceed that of which water-based acrylics are capable.  It is thought that such fast-drying speeds are achievable due to the sulfopolyester polymer aggregates being very small within these dispersions.  The figure below illustrates the film-formation mechanism of the Eastman AQ™ and Eastek™ sulfopolyester resins.

 

These films also are very dense, smooth, and have excellent resistance to alcohols, alkali, and acidic water.  In addition, they have good adhesion to a variety of plastic films, paper, and foil, as well as a consistently low coefficient of friction.  Films formed also possess good anti-blocking properties and maintain overprinting ability.  All of these benefits make Eastman AQ™ and Eastek™ sulfopolyester resins preferred for inks used in packaging applications such as flexible packaging.
 

PIgment Dispersion

Pigments are a key component of an ink formulation, which must be successfully dispersed and stabilized within the ink to achieve proper visual qualities.  The Eastek™ 1100 polymer dispersion and Eastman AQ™ 48 Ultra and AQ™ 55S polymers provide dispersion and emulsification capabilities for both organic and inorganic solid pigments in water-based ink.  Pigments may also be treated with these sulfopolyesters to provide water dispersibility.  Below are images that compare a water-based paste ink prepared with the Eastek™ 1100 resin dispersion versus an acrylic resin dispersion.

It is apparent that the water-based ink prepared using Eastek™ 1100 has well-dispersed pigment, providing good flow and viscosity properties, making it ideal for use in gravure printing systems.  On the other hand, the ink prepared with acrylic resin suffers in its consistency, deriving from poor pigment dispersion and agglomeration, where adding a separate pigment dispersion aid would be necessary.    

 

Gloss

As mentioned previously, dispersions of the AQ™ and Eastek™ contain small-sized aggregates of sulfopolyester polymer.  Due to the relationship between specular reflectivity and the refractive index difference at a smooth interface such as a cured ink, the surface gloss is determined by the ink polymer's refractive index.  Sulfopolyesters have a higher refractive index compared to other polymers used in ink formulations, such as vinyl or acrylic polymers.  Therefore, films formed using AQ™ and Eastek™ resins are very smooth and glossy.  If a matte appearance is preferred conventional matting agents such as silicas or clays work well in these systems and may be added.  

Application and Handling Benefits

 

Low Foaming

For many printing processes, including gravure and flexographic printing, the foaming of inks poses a challenge to quality and efficiency.  Moreover, faster line and cylinder operating speeds exacerbate foaming issues.  Eastman Eastek™ sulfopolyesters have been demonstrated to generate less foam and defoam quicker than other water-based ink dispersions.  Below is a comparison of an Eastek™ 1100 solution, acrylic emulsion, and polyurethane dispersion exposed to the same foam generating conditions.  All samples shown did not contain defoamer or co-solvents and had the same viscosity and solids content.

 

Defoaming after 5 minutes

Defoaming after 30 minutes

As seen, the Eastek™ 1100 solution has less propensity to foam and defoams to a greater extent over the same period compared to the acrylic emulsion and polyurethane dispersion solutions. 

 

Shear-Thinning Rheology

Eastman AQ™ polymers and Eastek™ dispersions also have inherent shear-thinning rheology that makes them useful for acheiving the necessary efficiency and print definition for offset and screen-printed inks. The chart below shows the shear-thinning properties of Eastek™ 1100 at various concentrations.
 

 

Odor-Free

Earlier, it was mentioned that Eastman AQ™ polymers disperse in water without the need for dispersing additives like amines.  As AQ™ polymers have very low odor, the exclusion of amines allows dispersions of AQ™ polymers and Eastek™ dispersions to be odor-free.  This makes them especially useful in inks for food packaging applications.  

Eastman AQ™ Polyesters and Eastek™ Polymer Grades

The selection of an appropriate AQ™ Polymer or Eastek™ Dispersion grade to use in a water-based ink formulation will depend on the desired Tg and calculated charge density.   Below is a table of grades recommended for water-based printing inks:  

 

Product Name	Approximate Tg (°C)	Calculated Charge Density
Eastek™ 1400	29	0.45
Eastek™ 1300	35	0.47
Eastman AQ™ 38S / Eastek™ 1000	38	0.43
Eastman AQ™ 55S / Eastek™ 1100	55	0.66
Eastman AQ™ 65S / Eastek™ 1200	63	0.33

 

Sulfopolyesters with higher charge densities may be more easily dispersed in water, have smaller dispersed aggregate sized, and be better at dispersing other additives like pigments.  On the other hand, lower charge density grades tend to produce films with better water and humidity resistance.  Generally, water and humidity resistance can be modified, if necessary, by using hydrophobic or hydrophilic plasticizers.  

Summary

Eastman AQ™ Sulfopolyesters and Eastek™ Sulfopolyester Dispersions provide many more benefits to water-based printing ink formulations than other resin dispersions, such as acrylic or polyurethane resin dispersions.  Such benefits include:

 

• Elimination of emulsifiers, co-solvents, amines, and other additives needed to form polymer dispersions
• Reduction or elimination of pigment-dispersing agents
• Fast and easy formation of chemically resistant films on a variety of substrates
• Production of naturally high-gloss films, with compatibility for matting agents, if needed 
• Reduced need for defoaming agents
• Shear-thinning rheology for application via gravure techniques
• Formation of odor-free films

 

These benefits allow ink formulations to be made higher quality and less costly by requiring fewer additives.  Thus, Eastman AQ™ Polyester Resins and Eastek™ Resin Dispersions are an excellent choice for use in water-based printing ink formulation applications, especially for packaging formats such as flexible packaging.  Contact us below to request a quote or sample of the Eastman AQ™ Polyester Resins or Eastek™ Resin Dispersions for your water-based ink formulation.