Fluorescent whitening agents, also known as optical brighteners, may be added to coating formulations to increase sheet brightness. Polyvinyl alcohol (PVOH) is recognized worldwide as the most effective “carrier” for optical brighteners. The precise mechanism is not fully understood. One theory proposes an interaction similar to hydrogen bonding which links the optical brightener to the PVOH near the surface of the coating. Another theory proposes that PVOH locks the optical brightener into its active planar or transposition which absorbs UV light and readmits it in the blue-white range of the visible spectrum.
Regardless of the mechanism, by utilizing PVOH as the carrier, it is possible to boost brightness an additional 4+ units. Further, PVOH also provides superior pigment binding strength as well as good water retention properties enabling the coating formula to be optimized for reductions in latex and hydrocolloid levels.
When PVOH is added to a coating formula containing optical brightener, a reduction in the low shear Brookfield viscosity is observed. This may allow coatings to be run at higher solids resulting in a quicker immobilization point with reduced binder migration and better coating uniformity.
The following is an excerpt from our brochure on Selvol PVOH as an optical brightener for paper and paperboard coatings.
The trend in bleached board coatings over the past several years is to replace a portion of the fine particle size clay utilized in the topcoat with fine particle size calcium carbonate (CaC03). This has resulted in a brightness increase from 79-81 units with the all clay topcoats to 84-86 units with the clay/CaC03 blends. With FDA approval of certain tetrasulfonated optical brighteners*, brightness can be increased even further. However, the FDA clearance limits addition to approximately 0.75 wet parts of optical brightener per 100 dry parts of pigment which allows for only modest brightness gains. PVOH, which has a synergistic effect with optical brighteners, can be used to further boost and maximize brightness.
To illustrate the effect of PVOH on optical brightener response in typical board coatings, the following study was performed. A series of topcoats with PVOH varying from 0-3 pph was formulated with total binder level kept constant. The topcoats, listed in Table 5, were applied to a 120#/3000 sq. ft. (10 pt) SBS base-sheet on a CLC-6000 coater equipped with a blade metering head. A common basecoat (also shown in Table 5) was utilized with a weight target of 6.0 +/- 0.5 #/3000 sq. ft. The experimental topcoats were applied to the base-coated board also at an approximate coat weight of 6.0 +/- 0.5 #73000 sq. ft.
Coated sheets were TAPPI conditioned, gloss calendered (175 °F/ 300 pli) and then reconditioned per TAPPI procedures prior to testing. Brightness and fluorescence of the coatings were measured and are shown in Figures 5 and 6, respectively. As can be seen: Brightness in all pigment compositions followed the same trend:
Fluorescence results show the same trends as brightness results for all pigment compositions:
For specific information on formulating with Selvol PVOH, please contact our Technology Center.
* Benzenesulfonic acid, 2,2’-(1-2-ethenediyl) bis[5-[[4-[bis(2- hydroxyethyl) amino]-6-[(4-sulfophenyl) amino] -1,3,5-triazin-2-yl] amino]-, tetrasodium salt (CAS Reg. No.16470-24-9); manufactured by Bayer Corporation, Ciba Specialty Chemicals Corporation and Clariant Corporation.