Addition of Halloysite Nanotubes Prevents Cracking in Drying Latex

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Addition of Halloysite Nanotubes Prevents Cracking in Drying Latex Films Junqiang Qiao,†,‡ Jörg Adams,‡ and Diethelm Johannsmann*,‡ †

Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China ‡ Institute of Physical Chemistry, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany

ABSTRACT: Investigating the process of film drying from aqueous dispersions containing a polymer latex as well as halloysite nanotubes (HNTs), we found that composite films could be formed without cracking under conditions where films of the pure polymer would always crack. Scanning electron micrographs showed that the HNTs were well dispersed and, further, that the distribution of fiber orientations was close to isotropic. The pendulum hardness of films formed from acrylate dispersions strongly increased upon addition of the inorganic phase. The pencil hardness, on the other hand, was poor, which presumably goes back to insufficient coupling between the organic and the inorganic phase. All films were white in appearance. For fiber concentrations higher than 10 vol %, the final films were porous.

I. INTRODUCTION The occurrence of cracks in drying films is an everyday experience and, also, a pertinent problem in the coatings industry.1,2 Cracking has recently attracted a considerable amount of research interest from the theoretical side. It is a pattern formation process with intriguing complexity.3 The patterns may be regular or fractal,4 they may or may not have preferred directions,5,6 the cracks may be straight or wavy,7 there may be a hierarchy of crack sizes,5 and there may or may not be the possibility of crack healing.8 All these types of behavior depend on the properties of the material, on the one hand, and on the drying conditions (drying speed, film thickness, drying directionality), on the other. On a fundamental level, films crack when the tensile stress inside the layer exceeds some critical level.9 Tensile stresses are difficult to avoid because drying necessarily entails a shrinkage in volume. The substrate constrains this shrinkage, and unless the material is very soft, it responds with mechanical tension. Mechanical stress, stress relaxation, and material transport induced by stress gradients are of much importance in film formation even if the film does not eventually crack. As first pointed out by Griffith in 1921,10 cracking is a nucleated process. Griffith balanced the elastic energy released upon crack propagation against the energy needed to newly form the inside surface of the crack. Since the elastic energy © 2012 American Chemical Society

increases with crack depth more strongly than the surface energy, only cracks with a certain minimum depth grow.11 According to Griffith, the critical depth for crack nucleation can be estimated as

a=

2EGc 1 π σF 2

(1)

with E the Youngs modulus, Gc the energy needed to create the surface, and σF the critical stress at fracture. E, Gc, and σF are materials parameters. In the context of coatings, the size dependence established by Griffith implies that thick films of any given material crack more easily than thin ones. This thickness dependence follows from the fact that a crack in a film can never be deeper than the film itself. One can define a “critical cracking thickness” (CCT)12,13 as an indicator of a material’s tendency to crack, which allows one to compare different materials and drying conditions. Below, we used the CCT to assess the effectiveness, with which halloysite nanotubes (HNTs) suppress crack formation. Received: March 19, 2012 Revised: April 20, 2012 Published: May 22, 2012 8674

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respectively. At moderate pH, the outside surface is negatively charged.27 The point of zero charge is at pH 2.7.28 The use of HNTs as an additive to polymer materials has been reported before. For instance, Rooj et al. reinforced a rubber by the addition of halloysite nanotubes.29 Interestingly, the inorganic filler not only increased the mechanical moduli but the decomposition temperature as well. Barrientos-Ramirez et al. have studied means to functionalize the fiber surface.30 This is important for the application as a reinforcing agent for plastics because it may allow for a covalent linkage between the nanotubes and the matrix. The authors of ref 30 employed aminosilanes. They used these HNTs as a solid support for a heterogeneous atom transfer radical polymerization (ATRP). Films containing HNTs were also produced electrophoretically31 and with the layer-by-layer approach.32 In the former case, poly(acrylic acid) was used as the dispersing agent. The approach was shown to be very flexible with regard to the choice of the inorganic phase. Materials deposited included multiwalled CNTs, HNTs, silica, and titania. In ref 32, the binder was poly(ethylenimine). The authors advertize the resulting structure as a potential scaffold for enzyme immobilization. Cavallaro et al. blended HNTs with biopolymers dissolved in water.33 Film formation occurred by simply drying. These authors were mostly interested in the structural aspects. When using a cellulose derivative, the HNTs and the polymer formed a sandwich structure. No such structure was found with citrus pektin. In principle, HNTs are hollow and can therefore be used as a container for active substances.34−36 This might include anticorrosion agents36 or catalysts inducing selfhealing.37 This aspect is of minor importance here. While halloysite consists of tubes, in principle, we only make use of it as structural elements; that is, we employ it as an assembly of nanorods.

The strategies to prevent cracking in drying colloids can be roughly grouped as follows: (a) Employ materials, which are soft at the time of drying and become harder later on. In soft materials, the gain in elastic energy upon crack propagation is small. (b) Ensure that the material has good internal cohesion at the time of maximum stress. With good cohesion, the energy needed to create a new surface, Gc, is high, which shifts the critical stress upward. (c) Slow down the drying process, thereby giving the material more time to relax. (d) Employ a heterogeneous material, which deflects the cracks, thereby increasing the surface energy and, also, distributing the stress at the crack tip more evenly.14 (e) If cracking is mostly caused by capillary pressure, one may attempt to lower the capillary pressure. At the same time, one of course also lowers the driving force for compaction. Today, the first option is the dominant approach in the coatings industry. There are numerous ways to harden films, most of which involve cross-linking after film formation.15 The good mechanical performance of two-component (2K) polyurethane coatings to some extent builds on this principle.16 Good cohesion (b) mainly amounts to an early onset of polymer diffusion across the interparticle boundaries.17 This topic has earned little attention in the published literature so far, although it may have implicitly been addressed in empirical studies. Slow drying (c) is always a possibility but often is in conflict with other constraints of the coatings process. Internal heterogeneity as a means to prevent cracking (d) is well-known from structural materials.11 This principle is, for instance, applied in the design of high impact polystyrene18 and also (in a wider sense) in the field of fiber reinforced plastics.19,20 Fiber reinforcement on the nanoscale has been thoroughly studied employing carbon nanotubes (CNTs).21,22 The success is mixed,23 where major problems are control of the morphology and insufficient adhesion between the fiber and the matrix.24 Heterogeneity was implicitly addressed in the study by Murray and co-workers.25 These authors investigated how pigments affected cracking and report a correlation between the CCT and the specific surface area (SSA) of the dispersion. Films containing large pigment particles (and having a correspondingly low SSA) cracked less than films with smaller particles. Halloysite nanotubes as used here can be viewed as (anisotropic) pigments, and we therefore believe that there is a correspondence between our findings and ref 25. The working hypothesis when starting this study was that nanofibers might prevent cracking based on crack deflection and crack blunting. Cracking in drying latex films is indeed much reduced by blending inorganic nanofibers into the latex dispersion, but more than one single mechanism is at work. We come back to this question in the last paragraph of the discussion section. This study focused on drying-induced cracking as opposed to the hardness of the final coating. Actually, the results show that nanofibers are probably not a promising additive, if the toughness of a coating is the main engineering target. Note that the term “film formation” often is understood in the sense that the final coating is compact. The coatings produced when adding halloysite fibers at a concentration larger than 10 vol % were not compact. The nanofibers employed here were the naturally occurring aluminosilicate halloysite.26 Halloysite nanotubes have a co mposition similar to kaolinit e (sum fo rm ula: Al2Si2O5(OH)4·2H2O). The silica surface and the alumina surface are on the outside and the inside of the tube,

II. MATERIALS AND EXPERIMENTS Polymer latexes were prepared by miniemulsion polymerization. The monomers used were methyl methacrylate (MMA, Aldrich, >99%), butyl acrylate (BA, Aldrich, 99%), and acrylic acid (AA, Fluka, 99%). Hexadecane (HD, Aldrich, 99%) was added as the costabilizer and azobis(isobutyronitrile) (AIBN, recrystallized from ethanol) as the initiator for polymerization. The MMA/BA ratio determines the softness of the polymer. When quoting glass temperatures below, these were calculated from the Tg’s of the pure components (105 and −54 °C for PMMA and PBA, respectively38) with the Fox equation.39 We employed MMA/BA ratios of 30/70, 40/60, 50/50, 60/40, and 70/30 (all numbers are percentages by weight). These correspond to glass temperatures of −22, −10, 4, 20, and 34 °C (cf. Figure 7). AA, HD, and AIBN were added in amounts of 1.5, 4, and 1.5 wt % (based on monomer), respectively. The emulsifier was the nonionic surfactant Lutensol AT50 (12.5 wt %, BASF). The organic phase and aqueous phase were mixed with a magnetic stirrer and sonicated (Branson Sonifier 450, output 70%) for 4 min. The resulting miniemulsions were polymerized for 20 h at 70 °C. The final solids content was 20 wt %. Dispersion of Halloysite in Water. Halloysite nanotubes were purchased from Sigma-Aldrich. They are delivered as a powder. HNT is classified as cancerogenic to humans (group 1) by the Internatial Agency for Research on Cancer (IARC). The length of the tubes varies between 1 and 3 μm. The outer diameter is about 50 nm. Further material properties are a refractive index of 1.54, a specific surface area of 64 m2/g, a density of 2.53 g/cm3, and a cation exchange capacity of 8.0 mequiv/g.40 The powder has a slight gray coloration, which goes back to contaminants. In a first step, the HNTs were dispersed in water at a concentration of 15 wt % (1.5 g HNTs/10 mL water). This mixture was magnetically stirred and sonicated. Under these conditions, the halloysite particles readily sediment. Poly(ethylene 8675

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Figure 1. Optical micrographs of films prepared from a halloysite/PMMA dispersion (composition 37/63 by volume). The dry thickness is 45 μm. The shaking times were 0, 0.5, 1, 2, 4, and 8 h for panels A−F, respectively. With prolonged shaking, the films become brighter in appearance. At the same time the number of cracks (dashed circles) decreases. The image in panel G was taken on the same sample as the image in panel B. It shows the cracks in more detail. glycol) (PEG) was added for stabilization of the halloysite dispersion. PEG (MERCK-Schuchardt) with a molecular mass of 1500 g/mol was dissolved in water at a concentration of 15 wt % (0.3 g PEG/2 mL water). Subsequently, these two liquids were mixed at a ratio of 1/5 (PEG/halloysite). After sonication, a viscoplastic fluid results. Presumably, the gelation is caused by bridging flocculation.41 The halloysite content in this dispersion is 5.3 vol % (12.5 wt %). In most cases, the dispersions were mechanically agitated for a few hours prior to further use. We elaborate on the shaking process in the Results section. The halloysite dispersion and the latex dispersion were mixed with a magnetic stirrer. Reference experiments, where PEG alone (without halloysite) was added to a PMMA latex, showed that PEG alone has no discernible influence on cracking. Films were prepared by manually spreading the dispersion onto a glass slide. Film thicknesses quoted below are dry thicknesses as calculated from the applied volume of the dispersion, the spreading area, and the solids content. Drying occurred at room temperature. Scanning Electron Microscopy (SEM). SEM images were acquired with a Helios Nanolab 600 instrument (FEI, Eindhoven, NL). These samples were spread on a metal substrate and evacuated overnight prior to imaging. A conductive 2 nm carbon layer was deposited on the sample surface prior to imaging in order to avoid charging. The energy of primary electrons was 3 keV. Hardness. The pendulum hardness (cf. Figure 7) was determined with the pendulum hardness tester supplied by BYK-Gardner (Geretsried, Germany). The procedure follows ISO 1522. Briefly, a sphere connected to a pendulum on its upper side rests on the film surface and performs a reciprocating rolling motion. The instrument counts the number of oscillations until the pendulum comes to rest. Good pendulum hardness corresponds to a large number of oscillations. PMMA surfaces have a pendulum hardness of around 100. Pendulum hardness is mostly related to rolling friction and tackiness. It is to be distinguished from scratch resistance, as, for instance, quantified by pencil hardness.42 The pencil hardness was measured with a Wolf-Wilburn Pencil Hardness tester (BYK-Gardner, Geretsried, Germany) following ISO 15184. The pencil hardness scale extends from 9H (good) to 9B (poor).

Critical Cracking Thickness. The critical cracking thickness (CCT) was determined by forming numerous samples with different nominal thicknesses (that is, thickness calculated from the applied amount and the solids content). The lower end of the error bar always denotes a film which is completely free of cracks. The upper end denotes films, which show a continuous pattern of cracks. In-between those two thicknesses, one finds a rather broad range with small microcracks which do not span the entire sample (cf. Figure 1). Drying always occurred at room temperature.

III. RESULTS Refinement of the Dispersions by Shaking. The properties of the films formed from halloysite/latex blends could be much improved by overnight shaking prior to film formation. We employed a standard laboratory shaker (IKA Vibrax Orbital Shaker, Model VXR Basic). Note again that the sample is a weak gel because of the bridges formed by the PEG. The most important consequence of continued shaking is that one eventually finds a film of colored contaminants adsorbed to the wall of the container. Presumably, the acrylate phase selectively coagulates the contaminants and, over time, induces adsorption of the coagulates to the walls of the container. Figure 1 shows a sequence of optical micrographs taken from halloysite/PMMA films (composition 37/63 by weight) with a nominal dry film thickness of 45 μm and varied time of shaking. Films formed from material having undergone shaking refinement had an improved brightness. Also, they showed less cracking (dashed circles in panels A−C). We interpret this finding in the sense that shaking amounts to continued shear melting of the gel. The larger aggregates thereby are allowed to sediment. Contaminants are allowed to reach the wall of the container, where they adsorb. Interestingly, a related mechanism has recently been proposed to fractionate non-Brownian rodlike particles.43 Madani et al. suspended glass fibers in a 8676

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viscoplastic fluid and adjusted the yield strength of the fluid such that the heavier particles sediment to the bottom, while the lighter ones stay in suspension. Since the fibers employed here are much smaller than the fibers in ref 43, gravity alone does not induce the fractionation. By mechanical agitation, the bridges formed by the PEG are repeatedly broken and reformed. It is the combination of this dynamic equilibrium with gravity which eventually leads to the removal of the larger aggregates and the contaminants. Halloysite/PMMA Composite Films. By adding halloysite nanofibers to a latex dispersion, one can easily form composite films with polymers, which do not dry crack-free under the same conditions. Among these is PMMA, which has a glass temperature around 105 °C. PMMA remains turbid when dried as a film in the usual way because the particles do not deform. Also, these films show cracks. However, one can form a film by mixing PMMA with halloysite. The weight fraction of the inorganic phase has to be higher than 50 wt %. Figure 1 shows a series of images from such films. Panels A−F correspond to variable shaking times as noted in the figure caption. One can still see a few cracks in panels A−C. These do not span the entire sample. Also, they are curved. Figure 2 displays the

Figure 4 shows the kinetics of water uptake from humid air (85% RH). The weight increase never exceeded 12%, which is

Figure 4. Water uptake versus square root of time for halloysite/ PMMA composite films exposed to humid air (85% RH). The halloysite contents were 28, 37, 48, 61, 78, and 88 vol % (50, 60, 70, 80, 90, and 95 wt %), where the arrow indicates increasing halloysite content.

less than what is expected from the porosity as evidenced in the SEM images. Only part of the void volume is filled with water, which can be explained if one assumes that the inside of the film is poorly wetted by water. The kinetics of water uptake was plotted versus the square root of time in order to compare it to the sorption kinetics expected from Fickian diffusion.45 Although the water uptake is linear in t1/2 at small times, it does not make sense to explain this behavior with Fickian diffusion. A more likely mechanism is capillary condensation.46 The speed of uptake is highest for the lowest halloysite content. This can be rationalized by assuming that the pore size increases with halloysite content and that capillary condensation is fastest for the smallest pores. The data as such do not allow to distinguish between water entering the cores of the tubes and water condensing in the open space outside the tubes. However, the amount of water uptake is too large to be explained with the core volume only. Halloysite/Acrylate Composites with Variable Tg of the Polymer Phase. In order to improve the mechanical stability of the films, we lowered the glass temperature of the polymer phase by adding BA to the recipe. Generally speaking, the films looked similar to what was observed with pure MMA. Figure 5 shows SEM images for a MMA/BA ratio of 45/55 and varied halloysite content. These films had been annealed at 150 °C for 2 h after film formation in order to improve the stability. Clearly, the material is porous if the halloysite content is 28 vol % (50 wt %) and larger. At 9 vol % (20 wt %), a continuous film is formed. This limit is in fair agreement with predictions

Figure 2. Critical cracking thickness (CCT) of halloysite/PMMA composite films as a function of halloysite content. Drying had occurred at room temperature.

critical cracking thickness (CCT) as a function of halloysite content. For halloysite contents below 28 vol % (50 wt %), film formation was impossible. Above 28 wt %, the CCT increased with halloysite content up to a plateau, which is reached at about 60 vol % (80 wt %). Even pure halloysite forms films, but the mechanical properties of these films are poor. We come back to this issue in the context of the pendulum hardness. Figure 3 displays SEM images of halloysite/PMMA composite films with a halloysite content of 78 vol % (90 wt %). These films consist of a loose arrangement of fibers and spheres. The fibers are well dispersed and there seems to be no preferred orientation. Given this structure, mechanical strength is not expected. Potential applications presumably are in fields where a highly porous medium is required. For instance, such films might be used as scaffolds for biological assemblies.44

Figure 3. SEM micrographs taken on a sample containing PMMA and HNTs (78 vol %, 90 wt %). The scale bars correspond to 2, 1, and 0.5 μm on the left, the center, and the right, respectively. 8677

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Figure 5. Scanning electron micrographs taken on annealed halloysite/acrylic composite films. The MMA/BA ratio was 45/55. The samples had been heated for 2 h at 150 °C. (A, B) 9 vol % (20 wt %) halloysite; (C, D, E) 28 vol % (50 wt %) halloysite; (F, G, H) 61 vol % (80 wt %) halloysite; (I, J, K) 78 vol % (90 wt %) halloysite. The scale bars correspond to 2 μm on the left, to 1 μm in the center, and to 0.5 μm on the right.

The scratch resistance of the composite films was inferior to films formed from the pure latex dispersions, as it is to be expected given the porous nature. Pencil hardness tests yielded a hardness in the range of 8B and below. The lower end of the pencil hardness scale is 9B. Probably, the hardness can be improved by silanization of the halloysite material,30 thereby improving the adhesion between the filler and the polymer. Compatibilization is outside the scope of this work. Interestingly, the pendulum hardness did increase upon addition of HNTs to the recipe. The pendulum hardness measures the energy dissipated by a sphere rolling on the film surface. As Figure 7 shows, halloysite nanotubes very significantly reduced the rolling friction. The highest pendulum hardness was found for the highest Tg and at 28 vol % (50 wt %) HNT content. Films formed from pure halloysite have a reduced pendulum hardness compared to those values. As the examples discussed above show, cracking is much reduced in the composite films compared to films produced from the polymer latexes only. However, this is not necessarily a consequence of mechanical heterogeneity only. Since the composite films are porous, the driving force for cracking (which is capillary pressure in most latex films2) is reduced. The capillary force scales as the curvature of the menisci in the interstitial voids. The radius of curvature is comparable to the pore size, and larger poresas produced by the open structure in the compositesreduce the capillary pressure. Of course,

of the maximum achievable packing density for elongated rods in ref 47. Figure 6 shows the CCTs for films with a MMA/BA ratio of 70/30. This material has a Tg of 37 °C. Films formed from the

Figure 6. Critical cracking thickness of halloysite/acrylate composite films. The glass temperature of the pure polymer is 37 °C. Drying had occurred at room temperature.

pure polymer turn clear, but they crack. As with pure PMMA, cracking is much reduced in the presence of halloysite. Interestingly, the CCT for the softer material is about the same as the CCT of PMMA/halloysite composites. We have no simple explanation for this finding. More extensive investigations would be needed to determine whether or not this coincidence has a deeper reason. 8678

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REFERENCES

(1) Lee, W. P.; Routh, A. F. Why Do Drying Films Crack? Langmuir 2004, 20, 9885. (2) Tirumkudulu, M. S.; Russel, W. B. Cracking in Drying Latex Films. Langmuir 2005, 21, 4938. (3) Lazarus, V.; Pauchard, L. From Craquelures to Spiral Crack Patterns: Influence of Layer Thickness on the Crack Patterns Induced by Desiccation. Soft Matter 2011, 7, 2552. (4) Borodich, F. M. Some Fractal Models of Fracture. J. Mech. Phys. Solids 1997, 45, 239. (5) Allain, C.; Limat, L. Regular Patterns of Cracks Formed by Directional Drying of a Colloidal Suspension. Phys. Rev. Lett. 1995, 74, 2981. (6) Jagla, E. A. Stable Propagation of an Ordered Array of Cracks during Directional Drying. Phys. Rev. E 2002, 65, 7. (7) Goehring, L.; Clegg, W. J.; Routh, A. F. Wavy Cracks in Drying Colloidal Films. Soft Matter 2011, 7, 7984. (8) Cho, S. H.; White, S. R.; Braun, P. V. Self-Healing Polymer Coatings. Adv. Mater. 2009, 21, 645. (9) Jagota, A.; Hui, C. Y. Mechanics of Sintering Thin-Films 2. Cracking Due to Self-Stress. Mech. Mater. 1991, 11, 221. (10) Griffith, A. A. The Phenomena of Rupture and Flow in solids. Philos. Trans. R. Soc. London 1921, A 221. (11) Lawn, B. Fracture in Brittle Solids; Cambridge University Press: New York, 1993. (12) Chiu, R.; Garino, T.; Cima, M. Drying of Granular Ceramic Films 1. Effect of Processing Variables on Cracking Behavior. J. Am. Ceram. Soc. 1993, 76, 2257. (13) Chiu, R.; Cima, M. Drying of Granular Ceramic Films 2. Drying Stress and Saturation Uniformity. J. Am. Ceram. Soc. 1993, 76, 2769. (14) Padture, N. P.; Lawn, B. R. Toughness Properties of a SiliconCarbide with an in-Situ Induced Heterogeneous Grain-Structure. J. Am. Ceram. Soc. 1994, 77, 2518. (15) Taylor, J. W.; Winnik, M. A. Functional Latex and Thermoset Latex Films. JCT Res. 2004, 1, 163. (16) Melchiors, M.; Sonntag, M.; Kobusch, C.; Jurgens, E. Recent Developments in Aqueous Two-Component Polyurethane (2K-PUR) Coatings. Prog. Org. Coat. 2000, 40, 99. (17) Voyutskii, S. S.; Ustinova, Z. M. Role of Autohesion during Film Formation from Latex. J. Adhes. 1977, 9, 39. (18) Donald, A. M.; Kramer, E. J. Craze Initiation and Growth in High-Impact Polystyrene. J. Appl. Polym. Sci. 1982, 27, 3729. (19) Jones, R. F.; Jones, M. R.; Rosato, D. V. Guide to Short Fiber Reinforced Plastics; Hanser Gardner Publ.: Cincinnati, OH, 1998. (20) Bakis, C. E.; Bank, L. C.; Brown, V. L.; Cosenza, E.; Davalos, J. F.; Lesko, J. J.; Machida, A.; Rizkalla, S. H.; Triantafillou, T. C. FiberReinforced Polymer Composites for Construction-State-of-the-Art Review. J. Compos. Constr. 2002, 6, 73. (21) Breuer, O.; Sundararaj, U. Big Returns from Small Fibers: A Review of Polymer/Carbon Nanotube Composites. Polym. Compos. 2004, 25, 630. (22) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Small but Strong: A Review of the Mechanical Properties of Carbon NanotubePolymer Composites. Carbon 2006, 44, 1624. (23) Wang, T.; Lei, C. H.; Liu, D.; Manea, M.; Asua, J. M.; Creton, C.; Dalton, A. B.; Keddie, J. L. A Molecular Mechanism for Toughening and Strengthening Waterborne Nanocomposites. Adv. Mater. 2008, 20, 90. (24) Wang, T.; Keddie, J. L. Design and Fabrication of Colloidal Polymer Nanocomposites. Adv. Colloid Interface Sci. 2009, 147−48, 319. (25) Murray, M. Proceedings of the Meeting of the Society of the Chemical Industry, 20 April 2009, http://www.soci.org/News/ ∼/media/Files/Conference%20Downloads/ Rideal%20Lectures%20Apr%2009/Murray.ashx, downloaded on 3/6/ 2012. (26) Joussein, E.; Petit, S.; Churchman, J.; Theng, B.; Righi, D.; Delvaux, B. Halloysite Clay Minerals - A Review. Clay Miner. 2005, 40, 383.

Figure 7. Pendulum hardness of films with a thickness of 50 μm with varied halloysite content and varied MMA/BA ratio. PMMA surfaces have a pendulum hardness in the range of 100. The numbers in the legend are the glass temperatures as calculated with the Fox equation.39

the dried films are not compact for the same reason. Also, the drying time for the composite films typically was hours, whereas the drying time for the corresponding polymer films was less than an hour. This is not meant to imply that the elastic network formed by the HNTs was of no importance at all. The fact that the films can be annealed at high temperatures (cf. the samples shown in Figure 5) shows that the network is able to contract. It does yield to some extent without brittle fracture. However, the reduced capillary pressure also contributes to the integrity of the final film.

IV. CONCLUSIONS Employing halloysite nanotubes, crack-free films have been formed from composite dispersions where the other phase was a polymer dispersion which would not dry crack-free under the same conditions. All films with a halloysite content of about 10 vol % (∼20 wt %) and more were porous. The critical cracking thickness and the brightness improved, if the starting dispersion had been shaken overnight before application. This process removes large aggregates and contaminants by a combination of sedimentation and continued shear melting. The nanofibers were well dispersed and distributed isotropically. For low-Tg acrylate dispersions, the pendulum hardness increased upon addition of halloysite. The pencil hardness, on the other hand, was poor. Presumably, the mechanism by which HNTs prevent cracking is a combination of various factors, where porosity plays a role.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Frederick Meyer, Astrid Peschel, Irina Nikiforow, Anne Finger, Arne Langhoff, and Reinhard Göhrke for technical help with the experiments and Katja Pohl for critical reading of the manuscript. This work was funded by the National Science Foundation of China (Grant No. 61176005), the China Scholarship Council, and the Deutsche Forschungsgemeinschaft (DFG, Grant No. Jo278/18-1). 8679

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dx.doi.org/10.1021/la3011597 | Langmuir 2012, 28, 8674−8680