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Low Glass Transition Temperature (Tg) Rubber Latex Film Formation Studied by Atomic Force Microscopy C. C. Ho* and M. C. Khew Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Received February 22, 1999. In Final Form: October 25, 1999 Latex with very low glass transition temperature (Tg) polymers forms a continuous film on drying. The physical and mechanical properties of the film are dependent on the extent the latex particles are able to coalesce and fuse into each other. Any hindrance to the film formation process would result in a poorly formed film and a drop in performance. The film formation process of natural rubber (Tg ∼ -65 °C) latexes and synthetic latexes with low Tg are monitored as a function of time using atomic force microscopy (AFM). The influence of the leaching method of the film, the presence of additives (some added after preparation) and nonrubber materials [specific for natural rubber (NR) latex only], and gel content on film morphology and flattening of the particles in the film is studied. The influence of the leaching procedure on the effectiveness of nonrubber removal from NR latex films and their effect on film formation is highlighted. The effects of nonrubbers and high gel content of NR latex in slowing down the NR film formation is discussed and contrasted with the synthetic polyisoprene and chloroprene latexes. The change of the surface mean roughness, Ra, with time provides a convenient means of comparing the rate of flattening of the polydisperse particles in these films.
Introduction Polymer latex films are formed by spreading waterbased latex dispersion onto a substrate and allowing the water to evaporate until the particles come into contact and fuse together. The fundamental process involves the transformation of the particles in a stable latex dispersion into a continuous film. Early work on the mechanism of latex film formation was carried out by Dillon et al.,1 Brown,2 Voyutskii,3 Bradford and Vanderhoff,4 and Mason.5 Over the years, various improvements to these early models have appeared,6-10 and the general picture that emerged recognizes, conceptually for an idealized model, the film formation process as being divided into three distinct stages. The first stage involves the linear cumulative water loss with time of the concentrated latex dispersion, with increasing restricted Brownian motion of the particles until they come into contact. The packing of the latex particles with interstices water depends on the polydispersity of the particle size and the ionic strength of the original latex dispersion. Further, slower evaporation of water leads to deformation and coalescence of the soft deformable particles in the second stage. Deformation is driven primarily by capillary and osmotic forces and resisted by electrostatic/steric repulsion and viscous and elastic deformation of the polymeric particles. At the end of this stage, the film is dry but particle contours are still discernible, the particles having deformed into a polyhedral structure. At the final stage, interdiffusion of (1) Dillon, W. E.; Matheson, D. A.; Bradford, E. B. J. Colloid Sci. 1951, 6, 109. (2) Brown, G. L. J. Polym. Sci. 1956, 22, 423. (3) Voyutskii, S. S. J. Polym. Sci. 1958, 32, 528. (4) Bradford, E. B.; Vanderhoff, J. W. J. Macromol. Chem. 1956, 1, 335. (5) Mason, G. Br. Polym. J. 1973, 5, 101. (6) Vanderhoff, J. W. Br. Polym. J. 1970, 2, 161. (7) Bradford, E. B.; Vanderhoff, J. W. J. Macromol. Sci., Phys. 1972, 6, 671. (8) Eckersley, S. T.; Rudin, A. J. Coating Technol. 1990, 780, 89. (9) Dobler, F.; Pith, T.; Lambla, M.; Holl, Y. J. Colloid Interface Sci. 1992, 152, 1. (10) Dobler, F.; Pith, T.; Lambla, M.; Holl, Y. J. Colloid Interface Sci. 1992, 152, 12.
Table 1. Physical Properties of Latexes Used latex type
Tg (°C)
HA
-63.6
DPNR
-64.4
ONR
-65.0
IR
-60.6
CR
-40.6
particle size (nm)
molecular weight
gel content (wt %)
292 ( 81 1053 ( 259 314 ( 106 971 ( 146 275 ( 73 1097 ( 284 331 ( 76 1564 ( 395 217 ( 45
1.4 × 106
27.0
8.7 × 105
53.3
1.5 × 106
25.8
1.6 × 106
0.7
polymer chains across the particle-particle interface occurs (termed further gradual coalescence or autoadhesion), if the film is at a temperature above the glass transition temperature (Tg) of the latex particles, resulting in a mechanically continuous homogeneous film. Any residual water left in the film would escape by diffusion through capillary channels between the deformed particles or through the polymer itself. Latex film formation is a very important aspect of synthetic latex technology, especially for the paint, paper, adhesive, and coating industries. Much effort has been directed toward understanding the relationship between film structure and mechanical properties and ultimately the mechanism of film formation. Numerous reports on the experimental aspect of film formation can be found in the literature.11-13 Most of these studies look into the molecular aspects of interparticle diffusion of polymer chains 12,14,15 and latex particles coalescence under different conditions.11,16-19 One of the parameters closely monitored (11) Joanicot, M.; Wong, K.; Cabane, B. Macromolecules 1996, 29, 4976. (12) Zhao, C. L.; Wang, Y. C.; Hruska Z.; Winnik, M. A. Macromolecules 1990, 23, 4082. (13) Rharbi, Y.; Boue, F.; Joanicot, M.; Cabane, B. Macromolecules 1996, 29, 4346. (14) Feng, J. R.; Pham, H.; Stoeva, V.; Winnik, M. A. J. Polym. Sci. B: Polym. Phys. 1998, 36, 1129. (15) Wang, Y. C.; Zhao, C. L.; Winnik, M. A. J. Chem. Phys. 1991, 95, 2143. (16) Winnik, M. A.; Wang, Y. C.; Haley, F. J. Coating Technol. 1992, 811, 51.
10.1021/la990192f CCC: $19.00 © 2000 American Chemical Society Published on Web 02/19/2000
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Table 2. Leaching Procedures for the Various Latex Films procedure (leaching type)
1 (no leaching)
HA DPNR ONR IR CR
x x x x x
2 (post leached with water)
3 (post leached with acetone)
4 (gel leached)
5 (gel- and post-leached)
x x
x x x
x
x
x
Figure 1. Surface morphologies of HA latex films (HA1) as a function of aging time at room temperature: (a) 2 h; (b) 1 day; (c) 4 weeks, and (d) 12 weeks.
in film formation studies is the morphology of the latex film surface. For example, electron microscopy studies of film morphology and structure20 have contributed much to the knowledge of the film forming process. More recently, atomic force microscopy (AFM) has become a very powerful tool in the study of latex film morphology because it can provide high-resolution three-dimensional (3-D) images of the film surface without any sample pretreatment. It can be operated in an essentially nondestructive mode and is extremely useful for film aging studies where samples can be reexamined many times as
a function of time.21-23 Some latex film can be imaged within a few minutes after preparation, thus enabling the nascent stage of film formation to be monitored.24 When a latex film ages above the Tg of the polymer, flattening of the latex particles in the film occurs concurrently with interparticle diffusion of polymer chains.22,25 However, because the AFM technique monitors the change in surface morphology as the tops of the latex particles deform with time, it measures essentially the rate of flattening of the latex film surface and not the interparticle chain migration.
(17) Xu, J.; Dimonie, V. L.; Sudol, E. D.; Shaffer, O. L. El-Aasser, M. S. J. Appl. Polym. Sci. 1998, 69, 977. (18) Kim, H. B.; Winnik, M. A. Macromolecules 1995, 28, 2033. (19) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992, 270, 806. (20) Eckersley, S. T.; Rudin, A. J. Appl. Polym. Sci. 1994, 53, 1139. (21) Lin, F.; Meier, D. J. Langmuir 1995, 11, 2726.
(22) Goh, M. C.; Juhue, D.; Leung, O. M.; Wang, Y.; Winnik, M. A. Langmuir 1993, 9, 1319. (23) Goudy, A.; Gee, M. L.; Biggs, S.; Underwood, S. Langmuir 1995, 11, 4454. (24) Butt, H. J.; Kuropka, R.; Christensen, B. Colloid Polym. Sci. 1994, 272, 1218. (25) Perez, E.; Lang, J. Macromolecules 1999, 32, 1626.
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Table 3. Surface Mean Roughness, Ra (nm), for NR Latex Film Surfaces 1 Day after Film Preparation latex type
1 (no leaching)
HA DPNR ONR
29.3 ( 3.1 25.0 ( 2.7 51.3 ( 28.1
2 (post leached with water)
3 (post leached with acetone)
63.3 ( 1.3 76.4 ( 5.5
93.2 ( 7.8 66.2 ( 17.3 86.8 ( 24.3
4 (gel leached)
5 (gel- and post- leached)
30.1 ( 1.0
30.8 ( 5.3
Figure 2. Surface morphologies of post-leached (by distilled water) HA latex films (HA2) as a function of aging time at room temperature: (a) 1 day; (b) 1 week; (c) 4 weeks, and (d) 8 weeks.
In contrast, there is very little work done on the film formation of natural rubber (NR) latex, even though NR latex is used extensively in the manufacture of gloves, prophylactics, and other dipped-goods that also involve film formation of the latex particles. In fact, the necessary condition of such applications is the formation of a continuous film with the appropriate mechanical strength. Apart from the much larger particle size and wider size distribution, one obvious difference between synthetic and NR latex is that NR latex also contains a host of nonrubber materials in small amounts. These nonrubbers (mainly carbohydrates, lipids, and proteins) are either soluble in the aqueous phase or adsorbed on the latex particle surface.26,27 The latex particles in commercial latex concentrate are stabilized by an adsorbed layer of mainly (26) Ho, C. C. Colloid Polym. Sci. 1989, 267, 643. (27) Ho, C. C.; Kondo, T.; Muramatsu, N.; Ohshima, H. J. Colloid Interface Sci. 1996, 178, 442.
long-chain fatty acid soaps (the hydrolysis products of phospholipids), polypeptides, and proteins.27 NR latex forms a continuous, almost transparent film when allowed to dry as a thin spread on a substrate. Once initial contact between the latex particles is achieved by gelation following the uniform destabilization of the latex particles, dehydration of the aqueous phase retained in the interstices of the 3-D gel network of latex aggregates follows. In addition, it is a common industrial practice to leach the latex film in water at slightly elevated temperature to remove the nonrubbers and other processing additives that are retained in the wet film. In this study, the images from AFM of various NR latex films, such as commercial high ammonia (HA) latex film, low protein natural rubber (DPNR) latex film, and oilextended natural rubber (ONR) latex film were captured. The aim was to obtain a better understanding of the overall film forming process of NR latex, a low Tg polymer (-63.6
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Figure 3. Surface morphologies of post-leached (by acetone) HA latex films (HA3) as a function of aging time at room temperature: (a) 1 day; (b) 2 weeks; (c) 4 weeks; and (d) 8 weeks.
°C, see Table 1). The influence of nonrubber materials and additives and their behavior at the latex film surface during film formation were investigated. A systematic study of the removal of these materials from the film surface by leaching was included. The film can be imaged within 30 min after preparation. Thus, the effect of aging of the latex film in air at room temperature was monitored by imaging the surface at regular intervals. A comparison with low Tg commercial synthetic latexes, such as polyisoprene latex (IR) and chloroprene rubber latex (CR), was made. Experimental Section Materials. NR latex film was prepared from commercial HA latex concentrates. DPNR was prepared from commercial HA latex by incubation with proteolytic enzyme and surfactant(s) followed by washing (centrifugation).28 ONR was prepared by adding 15 parts per hundred rubber (phr) palm oil in the form of an emulsion to HA latex followed by incubation at 50 °C.29 Typically, the protein contents of HA latex is 0.21 wt % (in terms of nitrogen content),30 whereas that for DPNR is
