New Developments of Solvent-Based Polyurethane Resins for Printing Inks.
Solvent-based inks still dominate in this market. However, with the ever increasing variety of substrates, better and faster printing technology and the demands of the customer for more package performance and eye appeal require the formulator to search as well for advanced resin technologies that will help in meeting these demands.
This paper discusses the new generation of solvent-borne polyurethane resins that the formulator can obtain to aid in solving the complexities of not just film lamination printing but those encountered in other printing and coating applications as well.
The flexible packaging industry today is very dynamic. Abroad variety of substrates are introduced into the market every day with different surface properties.
Better and faster printing techniques are available today which are designed to give superior print quality with less down time, allowing the converter to run more and differentiated jobs in a shorter period of time.
Package designs are not only meant to please the eye with its glossy photo-like finish but also to preserve its contents.
For example, potato chip bags, coffee pouches etc. are designed not only to advertise the products and attract the consumers but also to preserve the freshness of the food in the package by eliminating or limiting the passage of moisture, oxygen and flavors.
Similarly the food packages for microwave cooking not only need to protect the food during storage but also need to have good heat resistance. The beverage packages require resistance to the environment found in refrigerator storage and handling. And finally, bakery packaging requires not only good grease resistance but also must be able to release the product for ease of consumption.
To meet the demands of the packaging industry, the ink manufacture today needs to formulate inks that can be used for multiple applications. Some of the application requirements are shown in Table 1.
The various performance requirements of an ink often demand that the ink maker utilizes two or more resins in a formulation to be successful. Nitrocellulose/polyurethane, nitrocellulose/ acrylic, PVC derivatives, PVB and polyamides are simple generic examples of the resin systems required to meet the demands of the converter.
However, it becomes highly inconvenient and confusing to both the ink maker as well as the converter to maintain the large inventory of various resins and inks to solve the myriad of customer demands.
Moreover, there exists the problem that due to incompatibilities between the different resins and ink systems significant press down time results between print jobs due to the need for thorough cleaning of press equipment to prevent contamination of one ink system by another.
In addition, PVC and PVB based inks, used in lamination, are being notoriously difficult to clean up. There exist other concerns such as the chlorine given off during the incineration of old packaging printed with PVC based inks.
For PVB inks, odor and print behavior on the press can be frustrating to the converter.
Bearing these facts in mind, it is obvious that an ink system capable of covering most of these requirements, should be well received in the industry.
Background: Polyurethane Chemistry
In recent years, urethane polymers have received intense attention in the ink, coating and adhesives industries, resulting in the synthesis of many specialized forms.
We can now synthesize polymers containing not only the urethane linkages (Fig 1.) but many other functional groups as integral units in or on the polymer chains, which lead to the versatility of polyurethane products.
Basically polyurethanes are addition reaction products of a (polyether or/and polyester) with a diisocyanate or polyfunctional isocyanate materials (Fig. 2).
Typically a polyurethane has three basic building blocks: a polyol, diisocyanate and the chain extender. The properties of the polyurethane are determined by these basic segments, like the segment flexibility, chain entanglement and inter-chain forces (Fig. 3).
Properties of Urethane Polymers
Urethanes can be synthesized to range from very hard to soft to tacky. Most of the mechanical properties change with changes in the molecular weight of the polyurethane.
Intermolecular forces also influence these properties. Intermolecular forces are the result of hydrogen bonding, dipole moments and polarizability. These intermolecular attractive forces tend to hold polymer chains together in a manner similar to that of primary chemical bonds, but are much weaker and more readily affected by increase in temperature or stress.
Besides the intermolecular forces, obviously also the nature of the basic building blocks are determining the final urethane polymer properties.
Polyesters provide good weathering, good abrasion resistance, chemical resistance and toughness. Polyethers on the other hand provide good flexibility, elasticity and in-can stability.
The type of isocyanate used is also important Aliphatic isocyanates improves hydrolytic stability and resists UV degradation and do not yellow. Aromatic isocyanates on the other hand provide chemical resistance and toughness but yellow on exposure to UV light.
When formulating with polyurethanes it helps to have an understanding of the relationship between the properties we seek for our application and the corresponding polymer structure.
While solvent-borne reactive polyurethane chemistry enjoy wide use in the industrial coatings and/or adhesives markets the reactive sites, curing requirements and solvents often limit their usage in many applications of the graphic arts market.
However, these disadvantages are overcome by polyurethanes which can be considered as "non-reactive," "plasticizing" or "capped."
Plasticizing resins based on polyurethane chemistry have been introduced to the market as a whole during the early 1970s and are now well established in the global printing ink industry.
However, for the most part very little has ever been written about their use. Basically they are reaction products of a polyol (polyether or/and polyester)with a di-isocyanate typically MDI or TDI, where the excess NCO is optionally reacted with a reaction terminating agent which can typically be an alcohol.
This capping of the isocyanate group prevents the polymer from any additional cross linking and only extreme changes in conditions can remove this cap.
As a result of elimination of the free isocyanate groups, the urethane is now compatible with the solvents and resins typically used in the flexo and gravure packaging industry. The ink formulator makes use of the urethane's physical properties rather than its reactivity to enhance the cohesive strength, flexibility and adhesion of ink formulations.
Non-reactive polyurethanes have been used primarily as co-resins for flexibility and to improve adhesion properties of nitrocellulose based flexo- and gravure inks for surface and reverse printing on polypropylene, polyethylene, foil and other film type substrates.
The range goes from very soft and flexible types with maximum plasticizing properties to less flexible types but with improved adhesion properties with a higher polyurethane content. The more flexible types of lower polyurethane content have generally a high alcohol tolerance, while high urethane content grades are generally less alcohol tolerant.
For surface printing formulations Zeneca Resins offers a range of NeoRez products designed to fulfill each desired performance objective.
Typical examples of our range of polyurethane resins are can be found in Table 2.
Another important feature of polyurethanes is its low thermoplasticity, providing good release properties to inks that are printed in the heat sealing area of flexible packages and therefore unlike other plasticizers impart good heat seal resistance.
Adhesion promoters, like titanium chelates, are often used in these ink systems to optimize adhesion properties by means of a complex cross linking mechanism involving the hydroxyl groups present in the binder system of the ink and the corona discharge treated surface of the substrate.
They also maximize heat seal resistance properties up to 180[degrees]C often required when packaging is done on high speed lines.
Inks based on these type of polyurethane resins are used successfully for flexo and gravure inks for surface and reverse printing on polypropylene and polyethylene substrates; heat resistant gravure inks for printing on aluminum foil and also to some extent in flexo-gravure inks for surface printing on polyester and polyamide substrates.
The disadvantages using these conventional types of solvent-borne polyurethane resins are that they do not form films and therefore need to be combined with other resins like nitrocellulose for heat resistance, and anti-blocking.
That is to say, these urethane polymers have to be used as modifiers in formulations containing much harder co-resins.
New Generation Elastomeric Polyurethanes
Unlike the conventional polyurethane binders described above, Zeneca Resins developed a new generation polyurethane binders having an optimized molecular weight balance giving a block free film with elastomeric properties.
This new generation of polyurethane binders are specifically suitable for adhesive and extrusion lamination enabling the ink formulator to achieve a close to an universal ink system; some of these new products can be used as sole binders or, as with conventional urethanes, can be used in combination with another resin like nitrocellulose. These resins when formulated into an ink system provide properties like excellent adhesion to a wide variety of films, excellent bond strengths in laminates, no blocking and good heat seal jaw release.
Table 3 exposes the most important differences between conventional and new generation film forming elastomeric polyurethanes. Zeneca Resins has developed two elastomeric polyurethane binders that meet the needs of the global graphic arts market.
These urethanes (NeoRez D and E) are designed for adhesives and extrusion lamination printing and differ in solvent tolerance. NeoRez E can be formulated into a mono-solvent system, that is ideal for gravure applications with solvent recovery. NeoRez D has broader solvent compatibility and as such can be used in both gravure and flexo applications.
Both resins offer improved adhesion and bond strength over conventional urethanes on multiple substrates (PET, OPP, PE, and Nylon). Moreover NeoRez D&E can be used as pigment grinding medium for white pigments. These urethane also offer good flexibility, nitrocellulose compatibility, ink stability and block resistance. These vehicles offer increase versatility as they can be used for both lamination and surface print.
This new generation of urethane polymer products for printing inks will offer advantages to the formulator that standard urethane chemistry could not accomplish. These higher molecular weight urethanes with elastomeric properties offer faster rate of drying, more cohesive strength for better lamination bonds and broader adhesion latitude.
The benefits offered over conventional systems extend beyond physical properties and include:
* Low solvent retention for reduced odor in final packaging
* Single ink system for both flexo and gravure applications
* Single ink system for multiple structures
* Good print properties with easy clean up
These elastomeric urethanes offer the ink formulator a versatile vehicle that can be formulated into a single resin and/or mono-solvent ink system to meet the flexible packaging converters ever changing needs.
Shaivalini Purohit has a Ph.D. in organic chemistry from J.N.V University, India. She started her career with Council for Scientific and Industrial Research, India and later joined Color Converting Industries as an R&D chemist. She joined Zeneca in 1996 as applications chemist and is currently working with graphic arts group as industry technical manager.
Sjaak Griffioen (1962) graduated in 1988 from Vrije Universiteit Amsterdam in organo-metallic chemistry. Sjaak started his career within Sigma Coatings in sealants and later as R&D manager decorative paints. He joined Zeneca Resins in 1995 as industry manager "adhesives and sealants" and is now, as senior industry manager, responsible for the global product development for film coatings and graphic arts.
The authors would like to thank Joan Tiana, Jim Gistis and Hennie Passier for useful discussions.
* Adhesion to a wide variety of substrates
* Good bond strengths with laminates of different compositions
* Resistance to boiling and retorting treatments
* Deep freeze resistance
* Heat seal resistance
* Specific chemical or product resistance
* Low solvent retention in laminating inks
* Good print properties
NeoRez Range Urethane Adhesion Heatseal Alcohol Flexibility content resistance tolerance A High Best B Medium [up arrow] [down arrow] [down arrow] [down arrow] C Low Best Best Best NeoRez Range Grease resistance A Best B [up arrow] C Properties Conventional Polyurethane Film formation No, remains sticky Co-binder demand in formulation High, to avoid blocking and to optimize heat seal resistance Suitable in gravure and flexo ink Depends on grade systems Adhesion latitude Critical on PET substrates Suitability for different laminating Not suitable for PET and PA applications laminates Solvent release Fair Anti-blocking Needs modification with e.g. NC Heat seal resistance Needs modification with e.g. NC Flexibility Depends on grade Suitable for pasteurization and Critical, yellowing because of high sterilization NC content of the ink Deep freeze resistance Excellent Properties Elastomeric polyurethanes Film formation Forms elastomeric film Co-binder demand in formulation None or very small amounts Suitable in gravure and flexo ink Depends on grade systems Adhesion latitude Excellent Suitability for different laminating Suitable applications Solvent release Fast Anti-blocking Superior Heat seal resistance Superior Flexibility Excellent because of low NC demand Suitable for pasteurization and Suitable sterilization Deep freeze resistance Excellent Properties NeoRez [D.sup.*]) NeoRez [E.sup.*]) Suitable as sole binder Yes Yes Application Flexo Yes No Gravure Yes Yes Suitable for monosolvent ink No Yes systems Multi-Substrate Adhesion Excellent Excellent Lamination: Adhesive Excellent Excellent Extrusion Excellent Poor Ink Stability Excellent Excellent Block Resistance Very Good Excellent Flexibility Good Very Good Nitrocellulose Compatibility Excellent Excellent