Surface Morphology of Prevulcanized Natural Rubber Latex Films by

6208

Langmuir 1999, 15, 6208-6219

Surface Morphology of Prevulcanized Natural Rubber Latex Films by Atomic Force Microscopy: New Insight into the Prevulcanization Mechanism C. C. Ho* and M. C. Khew Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Received November 16, 1998. In Final Form: May 17, 1999 Prevulcanization of natural rubber latex was investigated as a function of maturation duration. The morphology of the film formed from these latexes was monitored using atomic force microscopy (AFM) as the film aged. The morphology is correlated with the cross-link density of the rubber molecules. Film formed from prevulcanized latex was further postvulcanized, and their film morphologies were compared. Inhomogeneous latex particles cross-linked on the surface with an unvulcanized core were obtained during prevulcanization. This gives rise to the characteristic indentation structure when these hard shell-soft core latex particles coalesce to form film. Basically the prevulcanization mechanism is controlled by the relative rates of the diffusion of vulcanizing reagents and the cross-linking reaction within the latex particles. It is clear that the partially vulcanized particles have a profound influence on the film-forming property of the prevulcanized latex.

Introduction Commercial natural rubber (NR) latex concentrate is used almost exclusively in the production of various dipped goods, the bulk of which is in the form of examination gloves. Technically the dipping process using NR latex involves the following basic steps: (i) latex compounding by mixing the latex with dispersions of vulcanizing reagents (sulfur, zinc oxide, and organic accelerator such as zinc diethyldithiocarbamate (ZDEC)) and additives such as surfactant, antioxidant, pigment, filler, and so forth; (ii) agitating the mixture during a period of maturation at 20-70 °C; (iii) forming the articles by dipping so as to deposit a thin layer of the compounded latex on a former (A former is a solid surface onto which a thin layer of latex can be coated by dipping it in the latex and then withdrawing it. The thin latex film formed after drying can be stripped off. The former, made of ceramic, takes the shape of a hand in the case of glove manufacturing.); (iv) leaching followed by heating to dry and vulcanize the wet gel; and (v) stripping the film from the former. Step (ii), known as “maturation” or “prevulcanization”, is the period for which the compounded latex is stored after mixing, prior to use in the production line. During maturation cross-linking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex. Prevulcanization offers a means of control of the extent of cross-linking needed in the final vulcanized film. Many hypotheses have been proposed on the transport of the vulcanizing reagents through the aqueous phase. Early studies1,2 suggested, and others3 have just assumed, that vulcanization takes place upon direct contact between the latex particles and the vulcanizing reagents, since the latter (sulfur, zinc oxide, and ZDEC) are insoluble in water. Yet others4-8 preferred the premise that the * Correspondence to this author. (1) Geller, T. I.; Sandomirskii, D. M.; Ustinova, Z. M.; Fodiman, N. M.; Dogadkin, B. A. Kolloid Zh. 1963, 25, 291. (2) Ustinova, Z. M.; Fodiman, N. M.; Geller, T. I.; Sandomirskii, D. M.; Dogadkin, B. A. Kolloid Zh. 1965, 27, 773. (3) Gorton, A. D. T. International Polymer Latex Conference, London, 1978, 11/1.

reactants must first dissolve in the aqueous phase before diffusing into the latex particles. Recently, Porter et al.9 showed that both the accelerator and sulfur dissolve in the aqueous phase of the latex before migrating to the rubber phase. It had also been suggested4 that sulfur and ZDEC first react in the aqueous phase to form some intermediate species before they are transported to the surface of the latex particles into which they diffuse and then cross-link the rubber molecules therein. However, the sequence of events following the arrival of the vulcanizing reagents at the surface of the latex particles is of paramount importance in influencing the morphology of the latex particles. The structure of the prevulcanized latex particles is expected to be dependent on the relative rates of diffusion of the vulcanizing reagents in the rubber phase and of their reaction with the rubber molecules to form cross-links. When diffusion is much faster than the vulcanization reaction, unimpeded diffusion of these reactants into the interior of the latex particles occurs followed by cross-linking. This would lead to the formation of homogeneously cross-linked latex particles. However, when vulcanization is much faster than diffusion, cross-linking takes place on the surface of the latex particles before the reactants can diffuse into the core. This will lead to an unvulcanized core surrounded by a highly cross-linked shell of rubber molecules. Hu and coworkers10 claim, without providing evidence, that the rate of cross-linking is much greater than the rate of diffusion. Hence cross-linking occurs rapidly as the vulcanizing reagents enter the surface of the latex particles, and the cross-links formed hinder further diffusion of the reactants into the interior of the particles. In view of the difficulty (4) Blackley, D. C. Proc. Int. Rubber Technol. Conf. Penang, Malaysia, 1988, 3. (5) Van Dalfsen, J. W. Rubber Chem. Technol. 1943, 16, 318. (6) Blackley, D. C. High Polymer Latices; Applied Science: London, 1966; Vol. 1, 835. (7) Van Gils, G. E. Rubber Chem. Technol. 1977, 50, 141. (8) Loh, A. C. P. Ph.D. Thesis, Council for National Academic Awards, England, 1982. (9) Porter, M.; Rawi, R.; Rahim, S. A. J. Nat. Rubber Res. 1992, 7 (2), 85. (10) Hu, Y. M.; Chou, Y. F.; Chen. W. T. Proc. Int. Rubber Conf. Moscow, 1984, A27.

10.1021/la981601v CCC: $18.00 © 1999 American Chemical Society Published on Web 07/28/1999

Prevulcanized Natural Rubber Latex Films

of separating the effect of diffusion from cross-linking, the actual mechanism of latex prevulcanization has never really been elucidated experimentally. The structure of the prevulcanized latex particles was never visualized or confirmed. In contrast, several studies on the effect of cross-linking on particle coalescence in latex films have been carried out using different synthetic latexes.11-18 For example, the diffusion coefficient of the interdiffusion process of polymer chains of neighboring poly(butyl methacrylate) (PBMA) latex particles was found to decrease with increasing molar mass of the matrix material (PBMA) of the film whereas highly cross-linked latex particles in the film showed an abnormal increase in diffusion coefficient.11 This was explained by invoking an inhomogeneous morphology of the cross-linked latex particles, the deformation of which into polyhedrons in the film was incomplete, and entanglement or interdiffusion of polymer chains across the particle-particle interface is no longer possible. On the other hand, the cohesive strength of a styrene-butadiene (SB) copolymer latex film was strongly dependent on the cross-linking between latex particles capable of undergoing H-bonding or ionic dipolar interactions at the particle surface.12 In another investigation, the effects of SB composition and cross-linking on the diffusion coefficient of polymer chain segments and the further coalescence process of the partially cross-linked SB copolymer latex film were studied.13 The “cellular latex film” model, by Joanicot et al.,19 of a hydrophobic particle core surrounded by a hydrophilic shell of a different polymer material is another example that bears some resemblance to our prevulcanized NR latex film, except that the NR latex particles of cross-linked and un-crosslinked rubber chains are of the same polymer type and the particles are surrounded by an adsorbed layer of charged long-chain fatty acid soaps, proteins, and polypeptides.20 It was also shown that annealing of the film at temperatures that caused fragmentation of the cell walls produced interdiffusion of high-molar-mass polymers.19 Of the various synthetic latex systems studied, the one that bears the closest resemblance to the prevulcanized NR latex is perhaps that by El-Aasser and co-workers.16-17 Using an amino-telechelic polybutadiene (PBD-NH2) latex as a cross-linking agent for the latex of a copolymer of poly(methyl methacrylate/n-butyl acrylate) containing dimethyl meta-isopropenyl benzyl isocyanate (PMBT), they found that PBD-NH2 was located primarily at the surface of the PMBT particle, clearly indicating the diffusion of the PBD-NH2 into the PMBT particles and cross-linking therein.17 They also demonstrated that it is important that the rate of cross-linking be so controlled that cross-linking occurred primarily among the interdiffused polymer chains.16 (11) Hahn, K.; Ley, G.; Oberthur, R. Colloid Polym. Sci. 1988, 266, 631. (12) Richard, J.; Maquet, J. Polymer 1992, 33, 4164. (13) Richard, J.; Wong, K. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1395. (14) Cohen-Added, J. P.; Bogonuck, C.; Granier, V. Macromolecules 1994, 27, 2111. (15) Cohen-Added, J. P.; Bogonuck, C.; Granier, V. Macromolecules 1994, 27, 5023. (16) He, Y.; Daniels, E. S.; Klein, A.; El-Aasser, M. S. J. Appl. Polym. Sci. 1997, 64, 1143. (17) Xu, J.; Dimonie, V. L.; Sudol, E. D.; Shaffer, O. L.; El-Aasser, M. S. J. Appl. Polym. Sci. 1998, 69, 977. (18) Zosel, A.; Ley, G. Macromolecules 1993, 26, 2222. (19) Joanicot, M.; Wong, K.; Cabane, B. Macromolecules 1996, 29, 4976. (20) Ho, C. C.; Kondo, T.; Muramatsu, N.; Ohshima, H. Colloid Polym. Sci. 1996, 178, 442.

Langmuir, Vol. 15, No. 19, 1999 6209

With the advent of sophisticated instrumentation, various experimental techniques have now become available for the study of latex film formation and film morphology. These range from nonradiative energytransfer techniques21,22 to small angle neutron scattering,23,24 ellipsometry, environmental scanning electron microscopy25 (ESEM), freeze-fracture transmission electron microscopy, and atomic force microscopy (AFM).26 AFM is particularly powerful in the study of latex film morphology because it can provide high-resolution threedimensional images of the film surface without any sample pretreatment. It can be performed in an essentially nondestructive mode and thus is extremely useful for aging studies where samples can be re-examined many times as a function of time. Recently,27 we have examined, using AFM, the film morphology of monodisperse PBMA latex in the presence of post-added alkali-soluble resin containing carboxylated random copolymers (ASRs). We were able to distinguish a hard shell-soft core type of structure formed by the higher Tg ASR (hard) adsorbed on the soft PBMA particle surface. This type of particle structure gives rise to a unique morphology of indentations and cave-ins, observed for the first time, of the latex particles in the film during their gradual coalescence. The occurrence of these characteristic indentations on the latex particles in the film was attributed to the difference in the extent of fusion of the two phases during film formation. The aim of this work is to study the surface morphology of prevulcanized NR latex film from the nascent stage of its film formation using AFM. The structures of the vulcanized particles and the film formed from them can be elucidated from the change in film morphology and cross-link density with maturation duration and from the difference in morphology after postvulcanization. Hence, the mechanism of prevulcanization could be deduced. The extent of cross-linking has a profound effect on the structure of the vulcanized particles and hence the surface morphology of the film. There was striking similarity in the particle structure and film morphology of these vulcanized films with those formed from the hard shellsoft core particles of the PBMA-ASR latex mixtures mentioned ealier.27 Experimental Section Latex Compounding. Commercial high-ammonia (HA) NR latex was used. The average particle size of this latex was 1053 ( 259 nm, but it also contained a small number of smaller size particles (mostly in the range of 300 nm) and thus appeared to be slightly bimodal. All compounding ingredients were commercial chemicals, used without further purification. Aqueous dispersions of sulfur, zinc oxide, and ZDEC were prepared by ball milling under standard conditions. Latex was compounded according to the formulation shown in Table 1. After compounding, the mixture was stirred slowly for 30 min and allowed to mature at room temperature (ca. 28-30 °C) for 2, 4, 6, and 8 days. The viscosity and pH of this prevulcanized latex were determined using a Brookfield viscometer and a UNICAM pH meter, respectively. (21) Kim, H. B.; Winnik, M. A. Macromolecules 1995, 28, 2033. (22) Kim, H. B.; Winnik, M. A. Macromolecules 1994, 27, 1007. (23) Hahn, K.; Ley, G.; Oberthur, R. Colloid Polym. Sci. 1988, 266, 631. (24) Kim, J. D.; Sperling, L. H.; Klein, A. Macromolecules 1994, 27, 6841. (25) Keddie, J. L.; Meredith, P.; Jones, R. A. L.; Donald, A. M. Macromolecules 1995, 28, 2673. (26) Wang, Y. K.; Kats, A.; Juhue, D.; Winnik, M. A. Langmuir 1992, 8, 1435. (27) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1998, 14, 5419.

6210 Langmuir, Vol. 15, No. 19, 1999

Ho and Khew

Table 1. Formulation of Prevulcanized Latex ingredients

parts per 100 rubber (phr)

60% (w/w) HA latex 50% (w/w) sulfur dispersion 50% (w/w) zinc oxide dispersion 50% (w/w) ZDEC dispersion

100 1.0 1.0 0.5

Surface Morphology by AFM. After the required maturation period, the prevulcanized latex was deposited onto a freshly cleaved mica surface. The latex film morphology was monitored from the nascent stage of film formation (30 min after latex was deposited) and during aging at room temperature up to 3 months using an AFM. The effect of post-vulcanization on surface morphology was studied by heating the prevulcanized latex film for 30 min at 110 °C prior to imaging. All films were imaged in air at 25 °C using a Nanoscope III atomic force microscope (Digital Instruments, Inc., Santa Barbara, CA) operating in the tapping mode. The AFM-E piezoelectric scanner can scan a maximum surface area of 10 × 10 µm2. The spring constant of the silicon cantilever was 50 N m-1. In the tapping mode, the cantilever on which the tip is mounted is oscillated at a frequency of approximately 250 kHz. The oscillation is driven by a constant driving force, and the amplitude of its oscillation is monitored. The tip is brought toward the sample surface until it begins to touch the surface, which reduces the oscillation. The feedback control loop of the system then maintains this new amplitude constant as the oscillating, or tapping, tip traverses the surface. This is done by the z component of the piezoelectric scanner, changing the tip height to exactly adjust for the surface height variations as the tip scans across the sample surface. Since the tip moves across the surface at a relatively slow rate (1 s/scan line), while tapping at a high rate (200 kHz), there is no longer any lateral shear or frictional force exerted across the surface to tear it or to push items around. Equilibrium Swelling Volume Measurements. Approximately 0.5 mm thick prevulcanized NR latex films were prepared by casting on level glass plates after the respective maturation duration. The latex films were dried at room temperature for 3 days before the swelling tests. Postvulcanized NR latex films were prepared by heating these latex films at 110 °C for 30 min. Accurately weighed samples (ca. 0.2 g) cut from these films were immersed in toluene at 40 °C and allowed to swell for 48 h (with one change of solvent after 24 h). At the end of this period, surface solvent was removed from the sample with filter paper and the sample was weighed immediately in a closed weighing bottle. The imbibed toluene was removed by heating at 60 °C in an air oven (to constant weight). Values of vr, that is, the volume fraction of rubber in the swollen gel, were calculated.28 The value of the physical degree of cross-linking (nphys) was calculated using the Flory-Rehner equation.29

-ln(1 - vr) - vr - χvr2 ) 2Fvonphysvr1/3 where F is the density of the rubber network (F for vulcanized rubber ) 0.9203 g/cm3), v0 is the molar volume of toluene, and χ, the rubber-toluene interaction parameter, is assumed to have a value of 0.39.

Results and Discussion Surface Morphology of Prevulcanized Latex Films. No change in the pH of the latex was observed during maturation. It remained constant at pH 10.6. The values of the viscosity, the volume fraction of rubber (vr), and the physical degree of cross-linking (nphys) for the prevulcanized latex films are shown in Table 2. A slight increase in viscosity was observed after a gradual thickening in ammonia-preserved NR latex, under normal storage conditions, as a result of the slow liberation of zinc ions from the accelerator zinc dialkyl dithiocarbamates present. The physical degree of cross-linking increases (28) Porter, M.; Wong, W. S. Polym. Latex III Int. Conf., London, 1989, 25/1. (29) Flory, P. J.; Rehner, J., Jr. J. Chem. Phys. 1943, 11, 521.

Table 2. Effect of Maturation on the Viscosity, the Volume Fraction of Rubber (vr), and the Physical Degree of Crosslinking (nphys) of the Prevulcanized Latex maturation (day)

pH

viscosity (cP s)

vr

nphys (mol/g RH)a

0 2 4 6 8

10.7 10.6 10.6 10.6 10.6

17.5 17.5 19.0 19.0 19.0

0.0176 0.0184 0.0304 0.0462 0.0599

7.10 × 10-7 7.66 × 10-7 1.82 × 10-6 3.84 × 10-6 6.14 × 10-6

a

RH ) rubber hydrocarbon.

Figure 1. Effect of maturation duration of prevulcanization on the physical degree of cross-linking of latex films: (2) films from prevulcanized latex dried at room temperature; (b) films from prevulcanized latex were postvulcanized at 110 °C for 30 min.

with prevulcanization maturation duration, indicating an increase in vulcanization with maturation. Thus, it is clear that prevulcanization duration should be controlled to achieve the desirable mechanical properties. The low degree of cross-linking observed for prevulcanized films confirms that intraparticle cross-linking occurs during maturation. Interparticle cross-linking can only take place when the particles come into close contact in the latex film and after postvulcanization. This is reflected in the almost 2 orders of magnitude increase in the degree of cross-linking after postvulcanization, as shown in Figure 1. Surface morphologies of the prevulcanized latex films after 0, 2, 4, 6, and 8 days of maturation, respectively, were monitored using AFM. AFM images were recorded at the nascent stage of film formation and after aging at room temperature. Figure 2 shows the AFM images of prevulcanized latex films 30 min after film formation. Distinct spherical contours of the NR latex particles were clearly seen for the control NR (Figure 2a) and prevulcanized NR latex films matured for 0, 2, 4, and 6 days (Figure 2b-e). However, for prevulcanized latex film at 8 days of maturation, no spherical contours of the NR latex particles were seen. Instead, many indentations were observed on this film surface (Figure 2f). The appearance of these indented structures first became evident after the films were aged for 2 h for prevulcanized latex at 2 and 4 days of maturation (Figures 3b and 4a) and after 1 h aging time for prevulcanized latex at 6 days of maturation (Figure 5a). The occurrence of these indenta-


Langmuir, Vol. 15, No. 19, 1999 6211

Figure 2. Surface morphology of the control (NR latex film) (a) and prevulcanized latex films matured for (b) 0 days (without maturation), (c) 2 days, (d) 4 days, (e) 6 days, and (f) 8 days at room temperature. Films formed from these latexes were observed 30 min after deposition on a freshly cleaved mica surface.

tion structures and their depths were dependent on both the maturation period and the aging time of the films. Both the number of indented particles and the indentation depth increased with maturation duration and film age. The width of each indentation varies; typically it ranges from 0.5 to 1.5 µm. The indented structure was believed to be due to the collapse of the prevulcanized latex particles during film formation, leaving as “indented” the top of individual latex particles within the top surface layer of the film.

Film obtained from latex after 2 days of maturation and with nphys ) 7.7 × 10-7 mol/g RH (see Table 2) shows only a few indentations compared with the highly indented surface of film from highly cross-linked particles (nphys ) 6.1 × 10-5 mol/g RH) obtained from latex after 8 days of maturation (Figures 3b and 6a). Typical section profiles for these film surfaces are shown in Figure 7. Since the degree of cross-linking increases with prevulcanization duration, this indicates that the number of latex particles collapsed increases with maturation duration, as shown

6212 Langmuir, Vol. 15, No. 19, 1999

Ho and Khew

Figure 3. Effect of aging at room temperature on the surface morphology of prevulcanized latex films matured for 2 days.

in Table 2. Figure 8 shows that the depth of indentation of the prevulcanized latex films increases rapidly with film aging time until a maximum and then decreases. All the prevulcanized latex films show the same behavior: the maximum depth occurred 6-8 h after film formation. The maximum depth attained increases from approximately 60 nm after 2 days of maturation to approximately 570 nm after 8 days maturation of the latex. It would appear that film prepared from highly cross-linked particles is more prone to indentation formation compared with that of the lightly cross-linked particles. However, the depth of indentation of the prevulcanized latex film

decreases with prolonged aging time (Figure 8). This behavior suggests that the rubber molecules of these particles flow at room temperature despite being surrounded by a hard shell of cross-linked rubber initially. The depth of indentation for these latex films after 3 months of aging at room temperature is dependent on the maturation duration of the latex. Film formed from latex after 8 days of maturation shows the biggest depth of indentation, among the various films studied. The effect of aging at room temperature is influenced by the gradual coalescence of the latex particles and diffusion of the polymer chains during film formation.


Langmuir, Vol. 15, No. 19, 1999 6213


Those rubber molecules at the surface, once they are crosslinked, would have their mobility restricted whereas the mobility of the unvulcanized molecules in the particle core remain unaffected. For film from lightly cross-linked particles, that is, prevulcanized latex after 2 days of maturation, the particles coalesced reasonably well and formed film with only a few surface indentations. Thus the surface mean roughness (Ra) value for this film was similar to that obtained from raw NR latex film (Figure 9). However, the spherical contours and indentations of the film surface persisted up to 8 weeks after film

formation (Figure 3f) compared to only 1 day for raw NR latex film.30 The low Tg (-64 °C) of un-cross-linked NR enables a rapid gradual coalescence of the latex particles in the film to be achieved. It is evident that cross-linking in the prevulcanized latex particles occurs preferentially at the particle surface, and the hard cross-linked molecules retard the disappearance of the particle contours even at low cross-linking density due to the cross-linked network (30) Ho, C. C.; Khew, M. C. Proc. Int. Workshop Green Polymers, Bandung, Indonesia, 1997, 102.

6214 Langmuir, Vol. 15, No. 19, 1999

Ho and Khew


present at the particle-particle interfaces. Hence, the spherical contours persist throughout the aging period. For film formed from highly cross-linked particles, that is, prevulcanized latex after 8 days of maturation, indented structures were seen once the particles came into close contact at the very early stage of film formation. It seems the particles collapsed immediately upon coming into contact (Figure 2f). At 4 h after film formation (Figure 6b), these indentations were transformed into voids due to complete cave-ins and flowing of the rubber molecules in the bulk underneath the top layer after coalescence. The width and depth of these voids increase initially due

to apparent merging among themselves (Figure 6). However, as the film ages, they become smaller and shallower following slow diffusion of the cross-linked polymer chains. Void structures (Figure 6f) were still clearly visible on these films even after aging for 9 weeks at room temperature, as reflected by their highest Ra values among those of the prevulcanized latex films (Figure 9). For films at a moderate degree of cross-linking, for example, from prevulcanized latex at 4 and 6 days of maturation, morphologies with intermediate features between the two extremes were observed (Figures 4 and


Langmuir, Vol. 15, No. 19, 1999 6215


5). This indicates that a certain degree of cross-linking must be achieved before collapse of the surface-vulcanized particles occurs. It is clear from these results that, during maturation of the prevulcanized latex, cross-linking occurs primarily at the latex particle surface, resulting in an inhomogeneous particle. Even after maturation for 8 days, prevulcanized latex does not form homogeneously cross-linked latex particles. When the sulfur and accelerator or their active intermediates arrive at the latex particle surface, cross-

linking takes place on the surface before the reactants can diffuse into the interior. This cross-linked shell acts as a recipient surface for cross-linking reagents, arriving later at the particle, as well as a barrier for diffusion of these materials into the particle core. The cross-linked surface is harder, denser, and thus heavier compared to the unvulcanized core. During gradual coalescence, as the rubber molecules in the core flow and the particles deform, the heavier cross-linked shell cannot support itself and collapses inward. This basically is due to the inherent

6216 Langmuir, Vol. 15, No. 19, 1999

Ho and Khew

Figure 7. Section profiles of (a) lightly cross-linked (after 2 days of maturation) and (b) highly cross-linked (after 8 days of maturation) prevulcanized latex films. Figure 9. Surface mean roughness Ra of prevulcanized latex films as a function of aging time of the films at room temperature. Prevulcanized latex films were matured for various durations. Table 3. Effect of Postvulcanization on the Volume Fraction of Rubber (vr), the Physical Degree of Cross-linking (nphys), and the Depth of Indentation of the Latex Film Prepared from Prevulcanized Latex Films maturation (day)

vr

nphys (mol/g RH)a

depth of indentation (nm)

0 2 4 6 8

0.1275 0.1322 0.1482 0.1565 0.1635

2.60 × 10-5 2.79 × 10-5 3.52 × 10-5 3.93 × 10-5 4.30 × 10-5

70.6 ( 31.8 51.7 ( 9.3 237 ( 86 438 ( 85

a

Figure 8. Effect of aging at room temperaure on the depth of indentation of particles in prevulcanized latex films matured for various durations.

difference in the fusion rates of the cross-linked shell and the unvulcanized core. The reticulated surface of fine structure, as revealed by SEM on unleached prevulcanized NR latex film reported earlier,31 can easily be explained now by the above observations. Similar indentation structure was observed recently27 in the film formed from soft PBMA latex particles surrounded by a hard ASR shell. Analogous reasoning has been proposed for the phenomenon. Surface Morphology of Postvulcanized Latex Films. Postvulcanization of the prevulcanized films at 110 °C for 30 min was carried out to ensure as complete a cross-linking of the rubber molecules as possible was achieved and to exhaust the remaining vulcanizing reagents. The surface morphology of these postvulcanized latex films is expected to be influenced, at this high vulcanization temperature, by (i) further cross-linking within the particles and (ii) greater mobilities and mixing of polymer chains across the particle-particle interface. Both these effects would lead to an increase in the degree (31) Robert, A. D., Ed. Natural Rubber Science and Technology; Oxford University Press: New York, 1988; p 83.

RH ) rubber hydrocarbon.

of cross-linking, but the latter, in addition, would promote gradual coalescence between particles and lead to better film-forming properties. In this respect, it is interesting to note that the degree of cross-linking of a purely post-vulcanized film without any prevulcanization (i.e. zero maturation duration) at 2.6 × 10-5 mol/g RH was much higher than that of the prevulcanized latex matured for 8 days at 6.1 × 10-6 mol/g RH (see Table 3). Also the degree of cross-linking of the prevulcanized latex after different maturation duration increases further on postvulcanization, reaching a level of 4.3 × 10-5 mol/g RH after 8 days of maturation. This means that further cross-linking occurs during postvulcanization of the prevulcanized latex (even after maturation for 8 days) when the excess cross-linking reagents are consumed. This also confirms the advantage and usefulness of employing prevulcanization to control the degree of cross-linking, in addition to using it to achieve a higher degree of cross-linking compared with that of a purely postvulcanized latex film. Figure 10 shows the characteristic morphology with indentation structure of these postvulcanized films prepared from prevulcanized latexes after different maturation duration. More indentation structures were noted in these postvulcanized films than in the prevulcanized latex films at the same aging time, but the overall surface roughness of these films was comparatively low (Figure 10c and d). However, deep indentations and voids were found on postvulcanized films prepared from prevulcanized latexes matured for 6 and 8 days respectively


Langmuir, Vol. 15, No. 19, 1999 6217

Figure 10. Effect of postvulcanization (at 110 °C for 30 min) on the surface morphology of the control (a) and prevulcanized latex films matured for (b) 0 days (without maturation), (c) 2 days, (d) 4 days, (e) 6 days, and (f) 8 days at room temperature.

(Figure 10e and f). It would appear that coalescence of pre-cross-linked latex particles produces film with a different morphology compared to that of film post-crosslinked with the same vulcanizing system after the particles have coalesced. A lightly cross-linked particle by prevulcanization would be able to undergo greater fusion via interparticle entanglement and cross-linking of its molecules during postvulcanization of the prevulcanized latex films, resulting in a comparatively well-coalesced film surface (Figure 10c and d). On the other hand, a further but slight increase in the degree of cross-linking during

postvulcanization was noted for an already highly crosslinked particle at the surface through prevulcanization (Figure 10f). The extremely rough surface and large number of voids and indentations of this film surface attest to this. However, the present technique does not distinguish how much of this increase is due to interparticle cross-linking made possible by the higher annealing temperature during postvulcanization. It is reasonable to assume that perhaps some interparticle cross-linking did occur, leading to some improvement in the gradual coalescence of the particles in the film. The morphology

6218 Langmuir, Vol. 15, No. 19, 1999

Ho and Khew

Figure 11. Proposed mechanism of prevulcanization of latex and its film formation.

of the postvulcanized film tends to indicate the rate of cross-linking is faster than the flow of polymer chains at the postvulcanization temperature. And a highly crosslinked shell could still retard the flow of polymer chains across the particle-particle interface even at the postvulcanization temperature. It is obvious that the latex particles remained inhomogeneous even after postvulcanization. Part of the core of the particles remained soft and unvulcanized. The free flow of rubber molecules between the particles during the gradual coalescence stage of film formation is impeded, giving rise to the observed morphology. Nonetheless, the continuous and elastic film produced from the postvulcanization of the prevulcanized latex, under optimum conditions, is a clear proof that sufficient interparticle diffusion of polymer chains and interfacial cross-linking have occurred during the gradual coalescence stage of the film formation of the pre-crosslinked particles to allow cohesive and mechanical strength to fully develop. Scanning through the literature on film formation of synthetic latices,11-19 it is perhaps not too unreasonable to drawn a parallel on some similar behavior of these prevulcanized NR latexes with their synthetic counterparts. For example, the study on the influence of crosslinking on the structure of PBMA latex films by Zosel and Ley18 demonstrates that interparticle diffusion and entanglements of molecular chains between particles were not possible in latex films formed from highly cross-linked particles. These films remain brittle even after annealing at elevated temperature. The same latex particles, but un-cross-linked, form a continuous latex film, with gradual coalescence of the particles occurring via interdiffusion between particles, during which the final mechanical strength of the film develops. Of course, the prevulcanized NR latex system is different from these two limiting situations: the Tg of NR at -64 °C is very much lower

than that of PBMA at 29 °C. So there should be relatively greater mobility of the rubber molecules and interparticle fusion of the particles in the NR film, both prevulcanized and postvulcanized, even at room temperature, compared with the PBMA system. The excellent mechanical strength of NR latex gloves is a good testimony of this. This shows that gradual coalescence of the partially cross-linked NR (at the surface) latex particles in the film is retarded but not completely prevented, as in the case of the highly crosslinked PBMA latex.18 Our AFM results are in agreement with the conclusions of El-Aasser et al.16,17 on PMBT latex with respect to the cross-linking/film-forming relationship deduced mainly from ultramicrotome electron microscopy and mechanical strength analysis. The cellular model of Joanicot et al.19 provides further confirmation of our conclusion based on AFM data. However, since the same NR material is involved in the prevulcanized latex system, it is envisaged that the demarcation between the dense vulcanized shell and the soft partially unvulcanized core would be gradual and diffuse, unlike that for their core/ shell structured synthetic counterpart,16,17 where different core/shell materials are likely to give rise to a distinct interface between core and shell within the same latex particle. Mechanism of Prevulcanization. The present study on the surface morphology of the prevulcanized latex film provides, for the first time, concrete new evidence of the latex prevulcanization mechanism. Figure 11 shows a complete schematic representation of the prevulcanization process, based on our AFM results, from the transport of cross-linking reagents across the aqueous phase (by other authors), through to their absorption, diffusion, and reaction within the latex particles. The film formation process of the prevulcanized latex is also presented. It is clear that the rate of the cross-linking reaction is much faster than the diffusion rate of the reagents in the rubber


phase during prevulcanization using the present vulcanizing system. Hence, a highly cross-linked shell is formed surrounding an as yet unvulcanized core, resulting in inhomogeneous latex particles. This cross-linked shell acts as the recipient surface for the arrival of more crosslinking reagents and becomes a hindrance to further diffusion of these materials into the core. The difference in fusion rates of the two phases during the gradual coalescence of the film results in the collapse of the denser shell of the particle. The unusual film morphology of the prevulcanized latex clearly attests to this. The hardened shell of the partially vulcanized particles also retards the diffusion of the rubber molecules and thus hinders the further gradual coalescence of the latex particles in the film. Hence, the film surface remains rough and uneven for the highly cross-linked latex films. Conclusions Latex prevulcanization has been known for many years and is a common practice in the manufacturing of rubber dipped goods. However, many aspects of it are still not fully understood, especially the mechanism of prevulcanization. While the actual cross-linking of the rubber molecules within the latex particles (whether pre- or postvulcanized) is likely to correspond to known bulk rubber vulcanization reactions, the process of diffusion of the active vulcanising reagents within the particle prior to cross-linking is largely unresolved and remains speculative. Some work on the transport and dissolution

Langmuir, Vol. 15, No. 19, 1999 6219

behavior of the vulcanizing reagents in the serum phase has been reported,7-9 but what actually happens within the particles during prevulcanization has been less amenable to study experimentally until now. By following the film formation process of the prevulcanized latex and noting the changes in film morphology with time using AFM, we are able to elucidate for the first time the structure of the prevulcanized latex particles and deduce that vulcanization occurs on the latex particle surface. The highly cross-linked shell of the partially vulcanized particles retards gradual coalescence of the film. A homogeneously vulcanized particle is not achieved by prevulcanization. Complete vulcanization could not be achieved even after postvulcanization: the particle core remained somewhat unvulcanized, and the film morphology remained rough and uneven. Hence, an optimally matured prevulcanized latex is one that can strike a balance between cross-link density and film-forming property so that full cohesive and mechanical strength development can ensue smoothly. These findings would be of significant technological importance to the rubber dipped goods manufacturing industry in providing a means for effecting better product quality control and improvement. Acknowledgment. We acknowledge the financial support given by MPKSN under IRPA project no. 03-02-03-0226. LA981601V

Surface Morphology of Prevulcanized Natural Rubber Latex Films by

Recommend Documents