North American CFS papers (#1, #2, #3) have seen a large increase in brightness over the last five years. This increase is in part a response to increased market demand for brighter and whiter papers,…
North American CFS papers (#1, #2, #3) have seen a large increase in brightness over the last five years. This increase is in part a response to increased market demand for brighter and whiter papers, accelerated by the imported papers from Europe and the availability of new coating raw materials. To achieve this increase in brightness, many paper makers now use high levels of carbonate, requiring a different approach to maintain printing strength. Also, as the carbonate level is increased, the need to maintain opacity, gloss and stiffness has led to the increased use of narrow particle size distribution (NPSD) pigments. However, with the move to NPSD, the coated papermaker risks coating colour water holding, machine runnability and coated sheet strength. We also know that stiffness can be delivered through a variety of mechanisms to the coated paper, such as the fibre matrix, double-coating or through the coating ingredients. As the market moves in these new directions, will the current latex system provide the strength and print properties for which latex is known? Can an appropriate latex design improve the wet coating behaviour associated with NPSD-rich formulations? Two latex technologies will be examined for their impact on coated paper. In the first portion of this article, precipitated calcium carbonate (PCC) will be used as a representative NPSD pigment and to compare latex performance in a PCC rich environment. The second portion will demonstrate a high strength latex offering. The technology covers a broad range of other performance profiles, including high stiffness.
Latex impact on NPSD performance
Experimental
The size, shape and amount of NPSD pigments used in a coating formulation will be dependent on the requirements for the particular grade of paper. For the purposes of this work a 0.4( average particle size acicular aragonite precipitated calcium carbonate (PCC) is used to represent a NPSD pigment. The particle size distribution of the PCC is compared to that of a 0.7( average particle size ground calcium carbonate (GCC), a conventional CFS pigment.
Formulations
All formulations were carbonate rich. Three different latex families were evaluated and the latex amount was maintained at 12 parts. All formulations were kept simple to highlight the impact of the latex design on coating performance.
MaxFoS Latex technology
The role and importance of latex and its chemistry on the rheology of the coating colour has been documented in several papers. The chemistry of the latex has the advantage that it can be designed to fulfil a desirable product profile. In this work, three latex chemistries were investigated in the PCC-rich coating formulation. The Standard SB latex is a carboxylated styrene butadiene (SB) latex, which is widely used in conventional coating formulations. The Std High Strength latex is a more highly modified SB latex that is used commercially today in arenas where higher strength is needed. The MaxFoS DMX 1 and DMX 2 latexes are SB products that were designed to broaden the overall particle size distribution of the coating colour and to lower the interactions between the latex and the pigment by controlling the surface chemistry. These modifications reduce the thickness of the hydration layer around the latex particle resulting in a significantly reduced hydrodynamic diameter of the latex particle.
Test Methods
All testing was completed in the Dow Paper Testing Laboratories, using standard operating procedures.
Results and discussion
NPSD pigments produce a different packing structure than traditional coating pigments. The structuring of the coating produced by these pigments leads to significant increases in optical properties compared to either traditional clays or GCC. PCC pigments offer coated paper producers the opportunity to produce high brightness grades without suffering the low gloss usually encountered with GCC. Thus, PCC formulations are claimed to provide the glossability of clay and brightness of carbonate. With the benefits provided with PCC, there are obstacles to overcome in order to run high levels successfully. PCC is known to have problems with water holding, machine runnability and coated printing strength. We have found that the degree of surface modification of the latex can eliminate or reduce the problems associated with high levels of PCC in a CFS formulation. This work compares the performance of a standard styrene butadiene (SB) and a standard high strength SB to the new MaxFoS technology latexes.
The performance gaps identified in comparing PCC to GCC can be minimized or overcome by optimizing the latex properties to perform in this NPSD coating system. The surface modification of a latex can further impact the other performance parameters. Several latexes are discussed here. While the MaxFoS latex technology in general improves the wet packing characteristics of the formulation, some specific grades additionally contain high degrees of surface modification, which results in improved strength and printability.
Water Holding
Narrow particle size distribution pigments increase the packing volume of the pigment particles, leading to a faster dewatering and, consequently, a lower immobilization solids and higher high shear viscosity. Therefore, water holding is a key concern for coated paper manufacturers as they move to PCC pigments. The nature and size of the PCC pigments generally offer less water retention than do the GCC pigments. Because of the lower immobilization solids, PCC tends to be run at lower solids than GCC and therefore the binder must be designed to achieve a lower high shear viscosity.
The dynamic water retention of the coating is an indication of the coating colour dewatering tendencies under the blade or under the shear of the coating operation. A coating colour that shows higher dynamic water retention will hold water longer under shear than one with a shorter time interval. The control of water holding can be achieved by latex design. For example, the increased particle packing with the DMX 2 latex increases water retention.
Machine Runnability
Machine runnability can be measured by the ACAV Ultra High Shear Viscometer. The coating colour viscosity at 1,000,000/sec is representative of the viscosity at the shear rates exhibited by a blade application. A higher viscosity could be an indication of problems such as scratching or dry whiskering at the blade. Using high levels of PCC in coatings can lead to runnability problems if not addressed by other coating ingredients. Machine runnability can also be positively impacted by the latex choice in a PCC rich formulation. The MaxFoS technology latex design has a large impact on ultra high shear capillary rheology. At 1,000,000/sec the viscosity drop with the DMX 1 and DMX 2 latexes is about 30 mPaS from that of the Standard SB and Std High Strength products.
Coated Sheet Strength
Another deficiency often observed with NPSD systems is the higher binder demand required to provide sufficient print strength. The DMX 1and DMX 2 latexes offer higher passes to fail than the Standard SB and the Std High Strength latexes due to low slope. The higher degree of surface modification in the DMX 2 latex improves slope beyond that previously available for PCC rich formulations.
Prufbau Wet Pick
This method is a relative measure of the force required to pick coating from a wetted coating surface. It compares the ability of coated surfaces to withstand film split forces of a printing ink after the sheets have been moistened with a fountain solution, which the paper would experience in an offset printing application. The impact of latex choice on effectiveness of a PCC-rich coating is evident in the dramatic strength difference seen between the latexes shown through this measurement.
High Strength T
echnology
Now we will introduce latexes for improved coated sheet strength. Latex glass transition temperature (Tg) and dynamic mechanical properties, along with application parameters such as finishing variables, can be optimized to improve coated paper stiffness. Latex film formation becomes more difficult to achieve with increasing Tg and drying temperature must increase, which typically results in lower strength. The objective of this work is to demonstrate the development of the Dow ProForte latex technology, which offers a high strength latex that maintains stiffness.
EXPERIMENTAL
Formulations
All formulations were carbonate-rich. Three different latex families were evaluated. The properties measured were for latex at 11 parts and with a 15% binder reduction at 9.35 parts. All formulations were kept simple to highlight the impact of the latex design on coating performance.
Latex Properties
Latex glass transition temperature (Tg) and dynamic mechanical properties, along with application parameters such as finishing variables, can be optimized to improve coated paper stiffness.
Coating Application
The coatings were applied onto both sides of a 60# wood-free basestock using the Dow Laboratory Web Coater in the blade-metering configuration. Each coating was coated at 10 lbs/3300 ft2 on each side. The paper produced was lab calendered using 4 nips at 150F and 800 PLI.
RESULTS AND DISCUSSION
In some environments, the need to maintain printing strength and sheet stiffness with the high levels of carbonate is acute and can be based on basestock, coating ingredients, or printing requirements. The DPF technology offers a higher strength latex that can be used in environments where increased print strength is needed.
Gurley Stiffness
The Dow ProForte (DPF) technology offers improved stiffness beyond the Standard SB at both binder levels. As expected, stiffness increases with increasing Tg. Stiffness was not impacted by coat weight variations, as the average and standard deviation in coat weights of the samples were 20.5 0.3 lb/3300 ft2.
Deltack Passes to Fail and Slope
At both the 11 and 9.4 parts of binder, the DPF latexes offer equivalent, to increased passes to fail over the Standard SB. Similarly, slope data shows the DPF latexes to offer lower slopes from the Standard SB and the Mod Stiff SB.
Conclusions
This work demonstrated that the performance of NPSD pigment coatings is highly dependent on the design of the latex used. In the case of NPSD pigments, as represented by PCC, the MaxFoS latex technology brings improvements in water holding, machine runnabiltiy and coated sheet strength.
The Dow ProForte technology shows high utility in formulations where strength increase is needed. The technology can be used to maintain strength at lower binder levels (relative to the standard SB) and maintain coated sheet strength while providing stiffness equivalent to known high stiffness latexes.
Acknowledgements
The authors would like to thank the many at The Dow Chemical Company who contributed to this paper. To learn more about MaxFoS and ProForte technologies and the rest of Dow Latex’s offerings, visit our website at www.dowpaperlatex.com.
For complete graphs and references, please contact the authors through Jennifer Holzinger at JLHolzinger@ Dow.com
Excerpt from an article by Holly Dunnill, Linda Kim-Habermehl, Femi Kotoye, John Oates, Don Ventresca, Greg Welsch
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