Lower Gloss Research Articles

The impact of rheological modifiers on water-borne coatings

While low volatile organic components (VOC) architectural coatings like interior wall or exterior façade paints are accepted to a large extent by the end-users, conventional solvent based alkyds are dominating in the area of high performance laquers. The lower acceptance of water-borne coatings corresponds to aesthetic problems like lower gloss, poorer flow and leveling as well as application disadvantages like shorter open-time, lower brush drag and limited weather conditions. To solve these problems it is mandatory to understand dispersions rheology and the different ways to adjust rheological properties of water-borne paints and laquers by special modifiers. In this paper a short review is given about the thickening mechanisms of three main groups of rheological modifiers (cellulose ethers HEC and HMHEC, acrylics ASE and HASE, and urethanes HEUR) in the presence of an acrylic and a polyurethane dispersion. Furthermore measurements on two model coatings are presented. These results allow a discussion about several factors which influence the coatings rheology and performance.

Calendering of coated paper and board in an extended soft nip

Abstract Several coated paper products were calendered at different line loads with an extended soft nip, a conventional soft (polymer/steel) nip and a hard (steel/steel) nip. The central roll of the calender was heated to different temperatures; 140, 180 and 210 oC with the extended soft nip, 140 oC with the conventional soft nip and 140 and 180 "C with the hard nip. Calendering with the extended soft nip resulted in general in a paper product with a more uniform structure than that obtained when the other two types of nips were used. This was reflected in lower gloss variations of the coated surface and in a lower variance in the reflectance of paper samples subjected to a bum-out treatment. The latter may be interpreted in terms of a more uniform pore structure in the coating layer. Offset prints on a coated woodfree paper chiendered with the extended soft nip were less mottled than prints on the same paper calendered with the conventional soft nip. There was apparently a correlation between the variance in the bum-out reflectance of the paper samples and the mottle in the offset print. The effect of the extended soft nip on gloss, print gloss, surface roughness and bending stiffness is also reported.

Approaches for characterizing interactions in paper coating suspensions

The interactions of clay particles with latices were characterized by determining the maximum packing volume fractions in aqueous suspensions and in dry coatings applied on an impervious substrate using a conventional latex binder and a latex containing functional groups designed to create interactions with clay. In addition, the changes in both gloss and reflectance over black during the drying stage of the coating suspensions were monitored and two critical solids volume concentrations were established that were then correlated with the volume fractions of the maximum packing determined above. Analysis of the flow curves confirmed that the addition of latex leads to the formation of bulkier structures when the latex interacts with clay. However, upon centrifugation all the suspensions, independent of their composition, gave similar packing behavior. In the dry coatings the addition of latex that interacts with clay resulted in enhanced void fraction, light scattering, and oil absorption rate, and lower gloss. The effect of the interacting latex on the properties of both the aqueous suspensions and the dry coatings tended to disappear at high latex content. A comparison of the values of concentration of maximum packing showed that the bulky structures formed in the wet suspensions collapse upon drying, the extent of collapse increasing with latex content. These results indicate that the information obtained from measurements made on the coating suspension cannot always be used to predict the properties of the dry coatings.