New Mineral interactive latex binders for improved Paper Coating Strength and Print Quality

John Haigh, Dr Ravi Mukkamala, Carmen Serrano - Rohm and Haas Company.

Introduction

The strength of a paper coating depends on the type of latex binder used and, to a large degree, on how well it is able to bind pigments and other minor ingredients in the formulation together and to the substrate. We have recently developed functionalized, pigment interactive latex binders for coating strength improvement.

These novel latex polymers are able to offer superior binding efficiency to mineral pigments and other additives in various paper coatings as evidenced by significant strength improvements up to 20%. These new, experimental latex binders have been tested in multiple pilot coater trials in Europe and North America, and the coated paper was printed in commercial press settings.

Currently, we are partnering with some of the key paper producers to further understand the value of this new class of latex binders. The higher coating strength imparted by this technology also enables maintained print quality at lower binder use levels thus offering significant raw material savings to the paper manufacturer. The technology has been evaluated in both offset and rotogravure papers and binders are presently being commercialized for both printing techniques. 

In this article, we present some of our key findings on the paper coating improvements, the influence of formulations and ingredients on the overall latex performance, and also briefly dwell on possible mechanisms by which these latex binders are able to impart properties that are advantageous to the paper coatings.

Background

An innovative approach is offered by using a newly developed, proprietary latex chemistry containing specialty monomers ("SM"). These specialty monomers contain groups capable of binding to pigment surfaces which significantly advance pigment association, allowing for a more uniform distribution of binder and pigment, therefore enhancing coatings performance properties as well as optimizing binder usage.

The specialty monomers used in this development have also been used in the area of architectural and industrial paints. In these applications they bring improvements in opacity and cleanability for architectural paints and in industrial coatings they provide excellent corrosion resistance due to their adhesion to metals under wet conditions.

The interaction at the pigment surface is observed in figure 1 where we see the latex which contains the specialty monomer clusters more densely on the surface of the pigment. The binder pigment interaction seems to be particularly strong for TiO2 and calcium carbonate pigments making this technology of particular interest to the paper and paint industries. The ability to space TiO2 can be observed in Figure 2 which shows the improved dispersion (smaller pigment clusters) of TiO2 in the presence of a standard latex compared a modified latex. This yields higher opacity in TiO2 rich systems.


Figure 1: FE-SEM microphotograph of GCC and binder.

 
Figure 2 – Spacing of TiO2 via Pigment Interactive Latex

Experimental Design

The synthesis of emulsion polymers was based on standard emulsion polymerization techniques found in the literature. The bulk compositions of the polymers were based on combinations of butyl acrylate, styrene and acrylonitrile monomers along with a carboxylic functional monomer ("CFM") (control composition) or a specialty monomer – ("SM"). 

Latex particle diameters were in the range of about 100 nm to 300 nm with glass transition temperatures ranging from -25°C to + 8°C. The non-volatile content of the polymers was 45 -50% by weight. The principal difference between the polymer compositions tested was in the glass transition temperature, which is adjusted by varying the ratio of butyl acrylate to styrene.

	System A - Offset		System B - Rotogravure
Binder id	I	II	III	IV
Functional Monomer	CFM	SM	CFM	SM
Glass transition temperature	8°C	8°C	-25°C	-25°C
Particle size	150 nm	150 nm	250 nm	250 nm

After laboratory scale testing to confirm the formulation stability, runnability (via capillary rheometry) and improvement in strength in paper coatings (via wet pick testing), the binders were scaled up for pilot coater trials at KCL in Espoo, Finland. The trial conditions and coating formulations used were as follows:

Offset Formulation

GCC (Covercarb 75)	80 parts
Clay (HG-90)	20 parts
Binder	9, 10, 11 and 12 parts
Solids	58.50%
Viscosity	1000 mPas
Thickener	RM-232DE (associative acrylic thickener)
Coater station	Opticoat Jet
Coater speed	1800 m/min
Basepaper	65 gsm (precoated)
Coat weight	7 + 7 gsm

 
The coated paper was calendared on a hot-soft supercalendar to a sheet gloss of approximately 70% at 3.5% humidity.

The calendared paper was tested in the laboratory for standard properties such as sheet gloss, print gloss, smoothness and porosity. To verify whether the strength of the coating was improved by the specialty monomer we ran a simplified wet pick test which allows ranking of the papers pick strengths. Figure 3 shows the reference samples used to assign a ranking from 1 (best) to 5 (worst).

In this test a drop of water is applied to the coated paper on the first station of a Prufbau and then a tacky ink applied on the second station. The whiteness of the resulting sample gives an indication of how much coating has been removed by the high tack ink due to the wetting of the paper coating. The test is also performed with fountain solution instead of water. This test suggested significantly higher wet strength of the specialty monomer based system (Binder II) when compared to its conventional equivalent (Binder I).

The papers were then printed by Heat Set Web Offset printing and Cold Set Sheet Fed printing to evaluate whether improved laboratory wet pick strength correlated to better print performance. The prints were evaluated visually by several observers and an average ranking established for each of the samples (see figures 4, 5 and 6 for wet pick and print quality).

This blind test confirmed that the SM modification produces coatings with improved print performance. As expected pick strength and print quality decreased as the binder level was decreased for both the standard and modified binder. The improved print performance of binder II (SM modified) versus the standard demonstrates that it is possible to reduce binder levels significantly whilst maintaining print quality. This enables significant cost savings for the paper manufacturer as the binder is often the greatest raw material cost contributor to the coating process.


Figure 3 – Prufbau Wet Pick Ranking References

 

Figure 4 – Simplified Prufbau Wet Pick Test - Rankings (1 = best)

 
Figure 5 – Cold Set Sheet Fed Offset Print Quality Ranking (1 = best)
 

Fig ure 6 – Heat Set Web Offset Print Quality Ranking (1 = best)

 
Figure 7 – Dot Enlargement Heat Set Web Offset (Black)

 

Rotogravure formulation

Talc	80 parts
Clay	20 parts
Binder	9, 10, 11 and 12 parts
Thickener	58.50%
Solids	1000 mPas
Viscosity	RM-232DE (associative acrylic thickener)
Coater speed	Opticoat Jet
	1800 m/min
Base paper
Coat weight	65 gsm (precoated)
Coat weight	7 + 7 gsm

The reference binder III was run at 4.7 parts (the typical use level commercially) and the modified binder was run at 4.7, 4.2 and 3.8 parts. The coated samples were calendared at KCL and the basic properties such as sheet gloss, porosity, brightness and opacity were evaluated. No significant property differences were noted so the results are not presented here.

The reels were roto-printed at Printforum in Germany with and without ESA. Figure 8 shows the printers ranking of the prints without ESA (this accentuates differences and increases the frequency of the missing dots). In this case the data suggests that a 10% binder reduction can be achieved without loss in print quality.


Figure 8 – Print Quality Rankings Rotogravure without ESA (1 = best)

Summary

The binders modified with the specialty functional monomer "SM" showed benefits in wet pick strength over the equivalent binders which contained only the standard carboxylated functional monomer. They have been further scaled up and successfully tested at several pilot coaters (CIC Trois Rivieres , KCL in Finland and PTS Vestra in Germany). They have now been commercialized as AvanseTM EXL-8 (Binder II) and AvanseTM EXL-25 (Binder IV). They have been evaluated positively in paper mills in several countries including South Africa and we have confirmed the opportunity to reduce binder levels thus offering significant savings to papermakers.