Study of water-based resins for graphic arts applications

This study correlates the most important physical-chemical polymer properties such as polymer composition, glass transition temperature (Tg), acidity, and molecular weight to the most important application properties pertaining to specific end markets such as gloss, adhesion, dry & wet crinkle resistance on difficult substrates, water, grease, chemical resistance, and heat resistance.

Introduction

Most waterborne graphic arts polymers are either low molecular weight resins made in a solution polymerization process, or emulsions, which incorporate low molecular weight resins as raw materials. In either case, the chain of manufacturing processes leading to most waterborne inks begins with a process called solution polymerization. In solution polymerization, a solvent is required to produce the “solution”. After polymerization, most of the residual solvent is removed.

However, because 100% removal is impossible, all solution polymers contain some level of residual solvent. In terms of converting and packaging fields, most low molecular weight graphic arts polymers are polymerized in a glycol-ether solvent, and therefore contain residual glycol ether. Analytical testing shows these products, as shipped to the ink maker, generally contain glycol ethers at levels between 2% and 7% of the dry weight of the actual polymer.

Studies proved that some glycol ethers may cause DNA lesions that could explain the spontaneous abortions, the malformations and the testicular cancers observed in exposed individuals to this type of substances. Some glycol ethers are also highly odorous and may migrate from printed packaging inks, affecting food flavor.

During polymerization, monomers are reacted to form a polymer. However, because chemical reactions are not 100% efficient, some monomers are not converted into polymers. These unconverted materials, known as residuals, are organic materials (VOCs) which can evaporate. These residual monomers can then migrate into packaged products, tainting aroma and taste. For these reasons, glycol ether and residual monomers are proving unacceptable to many packaged–goods marketers.

The Food and Drug Administration (FDA) sets limits on materials that come in contact with foods and administers regulations. When print on packaging can come in contact with food, or the package contains a printed insert, the ink’s components must be recognized as safe, either as a direct food additive or as an indirect additive, introduced by contact with the food. Therefore FDA compliance is a must for packaging inks polymers.

Chemically referred to as “environmental estrogens”, Alkylphenol ethoxylates (APEs) are suspected of causing birth defects in both humans and wildlife through disruption of the endocrine system. Given the reported concern factors of APE products, there is a growing trend towards the use of APE-free polymers in printing inks.

This presentation emphasizes a sample of 3Dresyns emulsion polymerisation consulting offer to resin manufacturers interested in producing emulsions, dispersions and resin solutions for water based Graphic Arts flexo inks & OPV´s: a portfolio of high performance products which are glycol ether free, alkyl phenol ethoxylate (APE) free, have very low residuals, and broad FDA compliance.

3Dresyns products covered in our synthetic consulting services are designed to cover the specific needs of a large number of water based graphic arts end application markets. Among those, there are certain segments that are almost exclusively water-based, including folding cartons, multi-wall bags, and corrugated products, both pre-printed and post-printed. Another sector where water-based is replacing solvent-based systems is printing on flexible substrates (e.g. films; polyethylene, polypropylene, polyester, PVC, etc) in applications such as shopping bags, grocery sacks and food packaging. 3Dresyns glycol ether free polymers offer packagers the advantage of using human and environmentally safe, stable and low flammability products.

Another packaging application sector where solvent based systems are used traditionally and water based usage is witnessing a considerable growth, is heat resistant and sealable inks and coatings applications. Our polymers offer the heat resistance and sealability that is required in markets like pouches, lidding, paper plate, and bakery over wraps.

For the chemical resistance required in certain markets, such as wall covering, flooring, household products, tissues and wipes, 3Dresyns offers products with improved chemical resistance complying with DIN646 (standard for resistance to specific chemicals). Additionally, 3Dresyns offers products with superior oil and grease resistance for the food wrapping applications.

A growing trend in the folding carton market is the requirement for an increase in the gloss levels of water based systems to compete with the UV technology. This represents a technical challenge due to the limitations related to the lower solids of WB systems in comparison to UV. Nevertheless, the further increase of gloss that 3Dresyns offers provides converters with the opportunity to comply with the market needs. On the other hand, the corrugated inks market leans towards economical systems. This market requires low resin solids systems (cost) balancing up with good press runnability (transfer and resolubility during printing) as well as good pigment binding of the printed ink.

Table 1 compares the performance properties of the three main water based polymer types. Typically, styrene acrylic emulsions are used for GA applications. Generally, they exhibit low acid numbers and high molecular weights (Mw). Emulsions are organized into two basic categories: rheology controlled RC and non-rheology controlled emulsions, depending on the type of surfactant (emulsifier) used to stabilize the emulsions.

Solution resins, which normally refer to solutions of solid resins in water at basic pH, are characterized by having excellent pigment grinding performance, transfer, resolubility, leveling, press cleaning, etc. Their compositions are mainly based on high acidic low Mw styrene acrylic monomer compositions. Mainly, the high acidity and low Mw are responsible for the performance properties stated previously.

Colloidal dispersions have acid numbers and molecular weights intermediate to emulsions and solution resins. Indeed, their performance characteristics are also between both product types. Upon neutralization dispersions become clear solutions due to their hydrophilic nature and their water solubility at basic pH.

Experimental

HEUCOSPERSE™ -BS 9590 (water based pigment blue 15:3 paste) was used as pigment paste for making the starting point water based flexographic ink formulations. The emulsions (letdowns) were added to the pigment pastes at 65/25 ratio for making the water flexo inks. The inks were formulated and diluted to application viscosity of c.20 seconds in Ford cup #4 at 25ºC and applied on several absorbent (coated paper) and non absorbent substrates (films and aluminum).

The inks were applied with a K bar #2, at approximately 12 microns wet, on oriented polypropylene (OPP), low density polyethylene (LDPE), polyethylene terephthalate (PET), aluminum (Al) and on a standard coated paper. The ink applications were dried at 70ºC for 20 seconds. The tape adhesion was measured with a tape immediately and after 24 hours on different substrates.

The wet and dry crinkle resistance was measured after 24 hours by crinkling the films between the fingers (under tap water for the wet crinkle). The scratch or mar resistance and gloss of the inks on coated paper were measured with the nail. The chemical resistance to different chemical was measured with the drop test for 60 seconds.

Results & discussion

Table 2 describes the general product properties, such as solids, Tg, acid number (AN), pH, minimum film forming temperature (MFFT), viscosity, neutralizer type, etc., as well as several general previously tested characteristics like transfer, resolubility and press cleaning.

As commented previously, most products are alcohol-, glycol ether- and APE-free. Additionally, they exhibit broad FDA clearance, being suitable for food contact applications.

The final ink application results are shown in Table 3 and were ranked as follows: 3 Excellent, 2 Good, 1 Poor.

In general, high Tg polymers offer higher scratch resistance, but lower flexibility and lower adhesion. In Table 3, it can be observed that high Tg products show low wet crinkle resistance and low adhesion on more difficult substrates like aluminum. This can be solved by either increasing the adhesion by softening the polymers with lower Tg, or by increasing the polarity (AN). Nevertheless, increasing the polarity (AN) normally decreases water resistance due to the hydrophilic nature of the polar acidic groups.

On the other hand, low Tg products tend to show lower scratch resistance, although they show improved flexibility and therefore better adhesion and wet crinkle resistance. For example, Resyncryl GA-E-25 (Tg=-25ºC) and Resyncryl GA-E-15 (Tg=-15ºC) exhibited the best wet crinkle resistance, as well as good adhesion.

Table 3 shows that the best products in terms of gloss were Resyncryl GA-E-25 and E-30. Gloss is related to the flow and leveling of inks, which is improved in products having medium Tg values, which is related to a balance between flexibility and surface hardening.

Oil resistance and water resistance are typically better for high Mw emulsions and lower for neutralized colloidal dispersions. This is related to the intrinsic hydrophobicity of emulsions compared to colloidal dispersions and solution resins.

The medium-high Tg Rheology Controlled RC emulsions, such as the blend of Resyncryl GA-E+100/GA-E+20 at 6/4 ratio, had worse adhesion properties due to the higher rigidity of the macromolecular backbone. Low Tg emulsions had good overall adhesion properties.

The neutralized colloidal dispersion, Resyncryl GA-CD1, had also good adhesion properties, but reduced wet crinkle resistance under tap water. This is due to its inherent poor water resistance.

Wet crinkle resistance properties are related to a combination of design variables, including adhesion, water resistance, flexibility of the macromolecular backbone, which is related to the softening and to the glass transition temperature “Tg”, etc.

Conclusions

3Dresyns water-based resins portfolio complies with most advanced environmental, health and safety regulations while exhibiting excellent performance properties for a broad variety of GA end applications, including food and tobacco flexographic packaging, where high purity products complying with the most demanding health and safety legislations are required.

Contact us to consult about our 2D emulsion and colloidal dispersion synthesis / formulation technology-transfer (consulting) services and your specific performance goals at: info@3dresyns.com