Tianjin Key Lab of Pulp and Paper, Tianjin Univeristy of Science & Technology, 29 at the 13th Street, TEDA, Tianjin, China.
Langmuir. 2010 Dec 7;26(23):18331-9. doi: 10.1021/la103675f. Epub 2010 Nov 2.
Surface elemental compositions of model latex clay coatings on an impervious substrate consolidated under various conditions were measured using the XPS technique, in order to clarify when and how colloidal latex particles migrate to the surface during drying. Under similar drying conditions, surface carbon content decreased with the addition of a water-soluble polymer to the coating colors, while remaining virtually unchanged for coatings of different coat weights made with a given color, indicating that surface carbon content variation is mainly caused by migration of latex rather than of water-soluble polymer. The results also showed that for coatings made with a given suspension, surface carbon content decreased with increasing delay time between coating and heating. For coatings frozen during consolidation and dried by sublimation, surface carbon content increased with increasing drying time before freezing. These results suggest that for the model coatings studied, latex migration mainly occurs after coating application before capillary formation during the initial drying stage when coatings are in the liquid phase, contradicting both the conventional capillary transport and boundary wall migration mechanisms. An alternative mechanism which attributes latex migration to surface trapping effect and to higher Brownian mobility of the smaller latex particles compared with pigment appears to provide a systematically consistent explanation to those phenomena. The new particle migration mechanism implies that segregation of colloidal particles is a ubiquitous phenomenon that would occur not only during the drying of paper coatings but also during consolidation of colloidal films containing particles of different sizes. This is of great importance in the control of surface compositions of nanocomposite coatings.
采用 XPS 技术测定了在不同条件下固结的不渗透基底上模型胶乳粘土涂层的表面元素组成，以阐明胶体乳胶颗粒在干燥过程中何时以及如何迁移到表面。在相似的干燥条件下，随着涂层颜色中水溶性聚合物的添加，表面碳含量降低，而对于用给定颜色制成的不同涂层重量的涂层，表面碳含量几乎保持不变，表明表面碳含量的变化主要是由于乳胶的迁移而不是水溶性聚合物的迁移。结果还表明，对于用给定悬浮液制成的涂层，在涂层和加热之间的延迟时间增加时，表面碳含量降低。对于在固结过程中冷冻并通过升华干燥的涂层，在冷冻前的干燥时间增加时，表面碳含量增加。这些结果表明，对于研究的模型涂层，乳胶迁移主要发生在初始干燥阶段涂层处于液相时的涂层应用之后和毛细管形成之前，这与传统的毛细管传输和边界壁迁移机制相矛盾。一种替代机制将乳胶迁移归因于表面捕获效应以及与颜料相比较小乳胶颗粒的更高布朗运动性，似乎为这些现象提供了系统一致的解释。新的颗粒迁移机制意味着胶体颗粒的分离是一种普遍现象，不仅会在纸涂层的干燥过程中发生，而且会在含有不同大小颗粒的胶体膜的固结过程中发生。这对于控制纳米复合涂层的表面成分非常重要。
