Neutralising and controlling materials sent to the press.

Recent advances in polymer technology are allowing for out-of-the-box solutions to some of the most persistent problems in the graphic arts area. Two areas that continue to challenge ink makers and press and obtaining low odour. The polymer suppliers are well positioned to assist with these

The main characteristic of today's graphic arts polymers are those of alkaline solubility on press, and water insolubility off press in the finished print. In the past, an adequate balance of these two properties has been achieved through the use of an extremely hydrophobic polymer segment, usually polystyrene, copolymerised with just enough acid functionality to impart water dispersibility. In this way, products are made that have enough alkaline solubility to run well on press, yet exhibit high hydrophobicity after drying and possess optimum performance in the final print.

These polymers are made at high temperatures in a continuous reactor that allows for the production of most styrene polymers, copolymerised with acrylic acid. Acid numbers range from 190-240. The resulting solid resin is relatively low in molecular weight (1000-15,000mw), and more hydrophobic arid readily alkali/water dispersible when compared to polymers made in water via emulsion polymerisation. This resin technology is then used as the polymeric surfactant for a final styrene/acrylic emulsion polymer. Manipulation of the Tg by optimising the blend of styrene and acrylates allows for designing a final latex that meets the specific performance requirements.

The success of this classic approach in penetrating the broad graphic arts market proves its usefulness, but there are also opportunities for improvements. This is where out-of-the-box solutions are proving effective. Because of the high hydrophobicity of the standard polymers, an excess of ammonia is needed to adequately dissolve the polymer in water, resulting in typical pH readings of 8.0-9.5. Because the viscosity of the polymer solution is pH sensitive, small changes in pH can cause large changes in viscosity, especially below 7.8. This fundamental characteristic, however, often leads to problems on press with stability resolubility and odour, especially ammonia odour.

Back to basics

To address these issues Rohm & Haas (Morton International) has developed technology that optimises the balance of alkaline solubility on press and the hydrophobicity of the final product. Because alkaline solubility is obtained without an excess of neutralising agent, stable materials can be produced that are as low as 5.5-6.0 in pH, although those most useful for graphic arts fall in the range 6.5-7.2. Once dried and the neutralising agent is removed, high hydrophobicity is imparted to the print for maximum performance attributes.

Products based on this chemistry exhibit improved stability and rewet/resolubility characteristics on press. Laboratory evaluations in open containers with mild stirring over an eight hour period demonstrated no pH drift, little viscosity drift, and no skinning/congealing of material at the surfaces - problems readily observed in comparable materials from the standard product line. Because the pH starts out neutral, there is little free ammonia to be driven off, resulting in little or no viscosity change because of pH drift. Any increase in viscosity was directly attributable to the increasing solids caused by evaporation of water from the system rather than from decreasing pH. Because of the optimised solubility characteristics in the development products, any surface drying readily redispersed into the coating, and no skinning-over or congealing was observed.

Pressing applications

Evaluations using a laboratory gravure press gave typical rewet times of less than 60 seconds with the development products, regardless of their film forming temperature. Their offsets in the standard product lines were more variable, ranging from two to three minutes for non-forming films and over 15 minutes for soft-film formers, improved characteristics with regard to rewet, resolubility, and open time translate into improved press run-ability and print quality over long runs at high speed.

However, resolubility cannot come at the cost of performance properties. Many products have been introduced into the graphic arts market that exhibit outstanding rewet characteristics that have not been accepted broadly because of poor performance characteristics. That is where the neutral pH products differ from the many earlier attempts. Overall performance properties are equal to, or better than, comparable offset from the standard product line. Ink transfer, gloss, hold-out, aged water resistance, block resistance and printability all consistently equal or excel the standard counterpart products. The neutral pH products have been incorporated successfully into film and foil inks, paper and paperboard inks, and a variety of overprint coatings. In all cases, end use performance criteria were met with these products.

Rohm & Haas offers five products based on this technology. Morez 400, a solid resin, and Morcryl 435LA, a neutral pH solution of Morez 400, are both recommended for use in pigment dispersions and as formula modifiers. Lucidene 4015 is a hard, non-film forming polymer recommended for use in paper inks. Lucidene 4035 is a room temperature, film forming polymer for overprint inks and varnishes. Film and foil inks should use Lucidene 4045, a soft film forming polymer, which can also be used as a film modifier in overprints. Because of their pH characteristics, these products exhibit maximum press and performance properties when used as the sole vehicles or in combination with other neutral pH products, although they are compatible with standard products if their pH is raised above 8.0pH.

While the solubility characteristics of this new line of products are fundamental to their success, being neutral in pH also contributes to odour reduction. The neutral pH virtually eliminates the ammonia odour associated with standard systems. Odour is further minimised by the high degree of monomer conversion that is achieved with this technology, thereby minimising residual monomer levels.

Table 1: resolubility test

                           pH neutral    standard
Test                       product       product

Water resistance(1)        Good          Very good

Resolubility(2)
 Flexo handproofer         Excellent     Good
 Geiger press              < 1 min       < 5 min

pH/viscosity(3)
 7.0                       500cps        Very high
 7.5                       450cps        3400cps
 8.0                       425cps        1700cps
 8.5                       420cps        1200cps
 9.0                       410cps        1050cps

Air contact stability(4)
 Initial viscosity/pH      325cps/9.0    300cps/9.0
 1 hour                    Good/9.0      Good/8.9
 2hours                    No skin/8.8   Slight skinning/8.8
 4hours                    No skin/8.0   Heavy skinning/8.6
 5hours                    No skin/7.7   Very thick; skin
                                         pulled into mix/8.5
 8hours                    800cps/7.5    Gelled; could
                                         not run/8.0
Dry speed                  Slightly
                           slower        --

(1.) Flexo hand proofs were dried at RT for three hours. Water soaked for two minutes and effect was observed after blotting dry.

(2.) Two tests were run. Ink was allowed to run dry on anilox roll both a Geiger press and a flexo handproofer. The Geiger press was started and prints were pulled through every minute until rewet was reached. The flexo handproofer was placed under running water and removal from anilox was observed.

(3.) Latex material was made up at various pH readings and measured for viscosity.

(4.) Each latex was elevated to a pH of 9.0 by the addition of ammonium hydroxide (28%). They were then placed in a wide open mouth container and stirred with a magnetic stirrer. Observations were made every hour. It is important to remember that in addition to the pH going down, the solids contents is going up because of the loss of water vapour from the open container.3