Handbook of Modern Coating Technologies

Film formation of polymer latexes

Polymer particles are microscopic solids mostly at 0.1 — 1 pm and observed as suspended in water, which have been the subject of many theoretical and experimental studies. There are plenty of applications for latexes, ranging from adhesives to inks, paints to coatings, films to cosmetics to drug deliveries, many of which require thin polymer films on a substrate surface formed by latexes. Therefore, it has become widely attractive to form a latex film mainly in coating industries. Latex film formation involves a multistep process as a complicated phe¬nomenon, in which it is critical for the particles to have suitable properties [1]. The drying temperature is critical for glass transition temperature (Tg) of the particles that the aqueous or nonaqueous colloidal latex dispersions are called as "hard or high-Tg” at higher levels and the aqueous as "soft or low-Tg” at lower levels. When heating above Tg, the soft latexes are compressed and deformed by the accompanying forces of evaporation and typically trans¬formed into a "latex film” that is transparent and continuous [1,2]. Then, the evaporation of medium contributes to polyhedron shape and close-packed array of the soft latexes. On the other hand, there occurs no deformation for the particles remaining hard. The latex film is required to become homogenous and voidless and its transformation is fulfilled by annealing that is necessary to stimulate diffusion among the soft particles. Nevertheless, if hard latex structure is annealed, the deformation initially closes the voids [3,4] and the diffusion starts to enable the mechanics of hard latex films to gradually develop following the complete
+ Deceased. This paper is dedicated to the memory of late Saziye Ugur. She will remain in our hearts forever.
Handbook of Modern Coating Technologies. DOI: https://doi.org/10.1016/B978-0-444-63239-5.00006-8
© 2021 Elsevier B.V. All rights reserved.
evaporation of solvents and the disappearance of all the voids. The process of coalescence [4] is a key characteristic and worth of study for latex coating. The interdiffusion of polymer chains is the latex film formation through annealing of hard particles after the voids are totally eliminated [5,6] before the polymer-to-polymer interface is healed. This term refers to "interdiffusion” used in polymer science to indicate the molecular process of mixing, inter-mingling, and homogenization. In this issue, there are many studies including several experi¬mental methods to prepare, dry, and analyze a wide variety of latex dispersions under different conditions so as to gain a rich relevant literature, including reviews [6—13] and books [14,15] published from different perspectives.
Table 6-1 Summary of the research results on latex film formation.
Latex system Latex size (nm) Film preparation method Analytical
method Parameters studied References
PS and PMMA — Freeze-drying FFTEM and SEM Packing effect [7]

PBMA 117, 337 Annealing FFTEM Latex size [8]

PBMA 60 Drying SANS Temper time and temperature [9]

PBMA 337 Annealing AFM Annealing time [10]

PiBMA — Spreading-drying AFM Water/air, polymer/air, polymer/water interfacial tensions [11]

Acrylic latex 258 Drying AFM Drying temperature [12]

(S12)
PnBMA — Drying SEM Degree of carboxylation-copolymer composition [9]

nPBMA Drying SANS Molecular mass, crosslinking, incompatibility of matrix material [13]

PS PS 38 Molding hot pressing SANS Annealing time [14,15]

PS — — SANS and DET End group-branch length [16b]

PMMA 1000 Annealing DET Annealing time—temperature [17]

PMMA 100 Melt-pressing DET Annealing time—temperature [18]

PBMA 110 Annealing DET Annealing time—temperature [19]

PBMA, P(MAA- 125, 132 Annealing DET Water amount [20]

co-BMA)
PMMA 1000—3000 Annealing SSF Annealing temperature—time [21]

MMA-BA-co- 133—170 Drying FTIR Surfactant [22]

polymer
latexes
PS/PMM-co-PBA Drying CSEM Drying rate, volume, and mass ratio of latexes [23]

AFM, Atomic force microscopy; CSEM, cryogenic scanning electron microscopy; DET, direct energy transfer; FFTEM, freeze-fracture transmission electron microscopy; FTIR, Fourier transform infrared; nPBMA, poly(n-butyl methacrylate); PBMA, poly(buthyl methacrylate); PiBMA, poly(isobutyl methacrylate); PS, polystyrene; PMMA, poly(methyl methacrylate); PnBMA, poly(n-butyl methacrylate); SEM, scanning electron microscopy; SANS, small-angle neutron scattering; SSF, steady state fluorescence.

The investigative studies have experienced many experimental approaches for distinc¬tive features of film formation process using latex particles (Table 6—1). The conventional technique for morphological analysis of polymer latex films is use of scanning electron microscopy (SEM) [7] and transmission electron microscopy (TEM) [24,25] to identify the array of latex particles in the high solid dispersions. Typically highly ordered films show hexagon pattern similarly as packed face centered cubic [8,26] other than different orders appearing in specific conditions [7]. When observing the films completely annealed, the structure may disappear like in extensive polymer interdiffusion [27]. Several techniques are highly supportive including freeze-fracture TEM (FFTEM) [8,9] and environmental SEM [28—30] for film morphology and atomic force microscopy (AFM) for film formation [10—12]. The performance of latter process may be evaluated based on the indicator of cor-rugation height while for alterations in morphology it is sufficient to determine the perme-ability of the latex film [12,27,31,32] and the effectiveness of the coalescing agent in its formation [33]. A molecular observation of latex film formation called small-angle neutron scattering (SANS) has been utilized for the recent two decades [9,13]. Hahn et al. and then Klein and Sperling et al. [5,14—16,34] investigated interdiffusion in polyfilms of butyl meth-acrylate and polystyrene (PS), respectively. In another paper, Mazur [35] comprehensively reviewed the coalescence of polymer particles along with a brief discussion about the neck growth process with the help of the geometrical approximations prior to the polymer chain interdiffusion. In a study by Winnik et al., the latex film formation has been studied using dye-labeled poly(methyl methacrylate) (PMMA) [17,18] and poly(buthyl methacrylate) (PBMA) [19,20,36,37] latex systems through a combination of fluorescence [transient fluo-rescence technique (TRF)] and direct energy transfer (DET) techniques while that of the steady state fluorescence (SSF) and DET helped to analyze the healing and interdiffusion stages in the PMMA ones [21,38—40]. Here, healing refers to the process in which the junc-tion surface of two polymer samples becomes indistinguishable from any other place located the polymeric material. Besides, for monitoring the transmitted light intensity, Itr Photon transmission technique was appropriate to PMMA and PS particles, as well [41,42]. To examine surfactant—latex molecular level interactions [43] and the surfactant distribu¬tion in the final latex film [22], the attenuated total reflectance Fourier transform infrared spectroscopy has been previously utilized. There is an evidence for the benefits of dynamic mechanical analysis in polymer interdiffusion [44]. The microstructural progress of latex coatings, monodisperse, and bimodal are followed via high-resolution cryogenic scanning electron microscopy (CSEM) on the stage of dehumidification [23]. In recent studies, the drying latex film turbidity for particle deformation has been measured by Pohl and cowor-kers [45] using an alternative detection channel in a standard TCSPC equipment as well as its Forster resonance energy transfer (FRET) efficiency for latex interdiffusion, to describe their correlation in between. In a study by Ludwig et al. [46] the water concentration in thin acrylic latex films was quantified and monitored throughout the drying process in either vertical or horizontal direction and the authors learned the dry film structure in combination with AFM before they searched and checked different characteristics of the film formation.