Acrylic polymer nanocomposite resins for water borne coating
applications
Proefschrift
Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de rector Magnificus prof. dr.ir. J.T. Fokkema voorzitter van het College van Promoties,
in het openbaar te verdedigen op dinsdag 27 februari 2007 om 12.30 uur door
Marie Louise NOBEL scheikundig HBO ingenieur
Prof. dr. S. J. Picken
Samenstelling promotiecommissie:
Rector Magnificus, prof. dr.ir. J.T. Fokkema, voorzitter Prof. dr. S.J. Picken, Technische Universiteit Delft, promotor Prof. dr. J.A Moulijn, Technische Universiteit Delft
Prof. dr. R.A.T.M. van Benthem, Technische Universiteit Eindhoven Prof. dr. A.M.van Herk, Technische Universiteit Eindhoven
dr. ing. G.J.M. Koper, Technische Universiteit Delft Dr.ir. F.G.H. van Wijk, Nuplex resins
Dr. A.D. Gotsis, Technische Universiteit van Kreta
Prof. dr. J. Schoonman, Technische Universiteit Delft (reservelid) This project is supported with a grant of the Dutch Programme EET (Economy, Ecology,
Technology) a joint initiative of the Ministries of Economic Affairs, Education, Culture and Sciences and of Housing, Spatial Planning and the Environment. The programme is run by the EET
Programme Office, a partnership of Senter and Novem.
Copyright © 2007: M.L. Nobel All rights reserved
No part of the material protected by this copyright notice may be reproduced or utilizes in any form or by any means, electronic or mechanical, including photocopying, recoding or by any information storage retrieval system, without permission from the author.
ISBN
Acrylic polymer nanocomposite resins for
water borne coating applications
Onderzoek naar de bereiding en eigenschappen van acrylaat polymeer nanocomposieten voor toepassing in watergedragen coatings
Chapter 1 General introduction 3
Chapter 2 Preparation of polymer nanocomposite resin systems for coating applications 25
Part I
A closer look on the morphology vs. performance of Organic Layered Silicate containing polymer nanocomposites based on organosolvent containing resins
Chapter 3 Morphological study of Montmorillonite containing acrylic resin formulations: TEM and X-ray diffraction 59
Chapter 4 Dynamic mechanical properties of Montmorillonite containing acrylic
resin formulations: DMTA and rheology 71
Part II
A closer look on the morphology vs. performance of water borne polymer
nanocomposite emulsions prepared from organosolvent containing resins
Chapter 5 Morphological study of acrylic resin emulsions containing
Montmorillonite-, Boehmite- or Laponite nanoparticles:TEM and X-ray diffraction 93
Chapter 6 Dynamic mechanical properties of acrylic resin emulsions containing Montmorillonite-, Boehmite- or Laponite nanoparticles: DMTA,
rheology and particle size analysis 107
Part III
A closer look on the morphology vs. performance of water borne polymer nanocomposites based on commercial acrylic resin dispersions
Chapter 7 Morphological study of Laponite containing Setalux 6768 dispersion:
Chapter 9 Conclusions and Recommendations 149
Summary 153
Samenvatting 155
Dankwoord 157
1K 1 component
2K 2 component
AAS atomic absorption spectroscopy
AFM atomic force microscopy
AR aspect ratio
C30B Cloisite 30B
cryo-TEM cryo-transmission microscopy
DMTA dynamic mechanical thermal analysis
HT Halpin Tsai
L/d length/thickness
MMT Montmorillonite
nm nanometer
nvc non volatile components
OEM original equipment manufacturing
SAXS small angle X-ray scattering
SB solvent borne
SB PNC solvent borne polymer nanocomposite
TEM transmission electron microscopy
TGA thermogravimetric analysis
um micrometer
VFL volume fraction Laponite
VFr volume fraction resin
VOC volatile organic components
WAXS wide angle X-ray spectroscopy
WB water borne
WBAA water borne all acrylic
WB PNC water borne polymer nanocomposite
1
Chapter 1
General introduction
1.1. History of coatings- and nanoparticles technology
1.1.1. An overview of the history of paint technology
that VOC’s and other paint components are released. VOC´s refers to a class of chemicals that evaporate readily at room temperature. They are present in all oil-based paints as solvents. Many latex paints (which use water as the main solvent or carrier) also contain VOC’s as part of their paint formulation. When these VOC’s off-gas, they may cause a variety of health and environmental problems. Research in paint manufacturing is exploring ‘how low can you go’ in the field of VOC reduction as well as investigating other coating system alternatives like high solid paints and powder coatings.[2]
1.1.2. A first acquaintance with nanotechnology
A way to describe nanotechnology is the ability to do things (measure, see, predict and make) on the scale of atoms and molecules and exploit the novel properties found at that scale. That’s not an easy job because a nanometer (nm) is pretty small: one must move a Post-It note a distance equivalent to half-way around the earth for it to appear 3 nanometer in size.[3]
After the findings on the Lycurgus chalice further development of nanotechnology can be followed in the middle of the nineteenth century.
Figure 1.1: the Lycurgus Chalice
Nobel laureate in 1965. Finally, in 1974 the word ‘nanotechnology’ was first used. Norio Taniguchi of the University of Tokyo used the word to refer to ‘production technology to get the extra high accuracy and ultra fine dimensions, i.e. the preciseness and fineness on the order of 1 nm’. [5] Nowadays, research to nanoscale technology is extensively funded. When defined as anything with a size between 1 and 100 nanometers with novel properties, this broad definition can encompass cutting-edge single molecule semiconductor research, several topics in supramolecular chemistry, and indeed advances in materials such as nanocomposites and coating formulation. The known and imagined possible hazards of nanotechnology are similarly divers, as by the broadness of the subject. From molecular manufacturing -the development of molecular machines that control the chemical manufacture of complex products (as described by Drexler starting in 1981)- to the production of nanoparticles, it is all under debate.
In 1992 Drexler published Nanosystems, a technical work outlining methods to manufacture high-performance nanomachines from a molecular carbon lattice ("diamondoid"). Meanwhile, he was also actively engaged in raising public awareness of the implications of the technology. The Drexler version of advanced nanotechnology has also been the subject of public concern, largely centered on the notion that nanotechnology could spiral out of control and convert all life on Earth into "gray goo." Drexler, who originally introduced this apocalyptic prospect in “Engines of Creation”, has since repeatedly distanced himself from it, but grey goo combined with other unknown effects of using nanotechnology retained its grip on the public imagination.[6, 7]
expressed fears about the potential ecological and health consequences of mainstream nanotechnology, and have called for increased research into safety of nanoparticles.[6] Nanotechnology in the field of coatings is mostly materials science. Material science nanotechnology is an interesting field, with some impressive possibilities for improving our lives with better materials and tools.
1.1.3. Future expectations of nanotechnology used in coatings
Various potential uses of nanotechnology for coating applications have been considered over the lost couple of years. Since 2001, passive nanostructures in coatings, nanostructured metals, polymers and ceramics have been developed. For example, a clearcoat-containing nano-silica is the first automotive clearcoat to use nanoparticle technology. The clearcoat possess a combined resistance to scratches, mars and acid etch. The patented nanoparticle technology creates a highly cross-linked silica network at the surface of the coating providing superior resistance to damage caused by day-to-day use, car washes and environmental factors like acid rain and tree sap.[8]
Another example are new spray-on, nanotech coatings that could keep window screens from scratching, make paper products waterproof and perform other minor miracles.[9] Other potential improvements are extreme flexibility, improved adhesion and resistance to corrosion and microbial growth. Whatever the final application may be, nanotech coating systems share a similar goal: they are aimed to be cheaper, easier to apply and more environmentally friendly than the substances currently in use.
1.2. From solvent borne to water borne coating systems
1.2.1. General alternatives for solvent borne resin systems
the organic solvent evaporates, allowing the coating to cure to its final strength. The main advantage of these coating systems is the rapidly evaporating solvent, allowing the coatings to dry rather quickly. Some solvent borne coatings contain large quantities, as much as 70-75% of VOC’s that are released into the atmosphere as they dry, VOC’s are known to contribute to low level ozone which can have detrimental health effects and an impact on the environment e.g. smog. Equivalent water borne coatings generally have a VOC content of around 10%, which clearly has a lower environmental impact. With the trend towards environmental awareness, this is an increasingly important issue. Alternatives for solvent borne coatings are available in the form of high solid coatings, powder coatings and radiation cured coatings. As the name suggests, high solids coatings have higher solids content and, therefore, a lower solvent content than conventional solvent borne coatings. The application of high solids coatings produces lower emissions than the application of conventional solvent borne coatings. Powder coatings contain no organic solvent. These coatings are either thermoplastic or thermosetting powders. Thermoplastic powder coatings melt and flow when heat is applied, but continue to have the same chemical composition upon cooling and solidifying. Thermosetting powder coatings melt when exposed to heat, flow into a uniform thin layer, and chemically cross ink with themselves or with other reactive components to form a higher-molecular weight reaction product. Radiation cured coating involves photocuring mixtures of low-molecular weight polymers or oligomers dissolved in low-molecular weight acrylic monomers. These formulations contain no carrier solvent and can be cured using either electron-beam or ultraviolet light sources.
lower solvent content of water borne coatings, significant emission reductions over conventional solvent borne coatings applications can be achieved. When using water as the main solvent, the compatibility of the resins and additives that make up the solids portion of the formulation also has to be considered. Most resin systems are insoluble in water and must be incorporated into the paint as emulsions. These emulsions are essentially resin particles suspended in water. These resin particles stay in suspension, but as the water evaporates, they coalesce to form a continuous film. Whether a coating uses water or an organic solvent as the carrier, the composition and function of the solids portion of the paint are roughly similar. Resins, additives and pigments form the dried film. The appearance and performance of this dried film should be similar, whether it is a water-borne coating or a conventional solvent coating.[10]
1.2.2. The journey to water borne resin systems for automotive lacquers
Since the beginning of the automobile industry at the turn of the twentieth century, coatings have been used on automobiles. The original coatings were the same as those used on carriages at the time. They were oleoresinous based and were produced by grinding pigments in a varnish made from natural resins and oils[11]. These early varnishes were very slow to dry and required many coats to be brushed on for coverage. Therefore, they needed large storage bays for the automobiles while they dried, with the entire process taking up to six to seven weeks. Furthermore, the finishes had poor durability and would only last for several months of exposure before chalking. Consequently, they needed to be polished often to keep their gloss.[12] These coatings did have advantages - they were easy to repair, readily available to the owner and could be easily brushed on.[13]
could be achieved with several coats by spraying or brushing. The lacquer topcoat had to be hand buffed, but within six hours of forced drying, a hard and glossy finish could be attained. By 1925, the entire finishing process dropped to about 50 hours. This breakthrough allowed for the production line manufacture of automobiles.[12] Nitrocellulose lacquers remained a major component of automobile finishes for more than 30 years.[14] Limited colours were available in the very early finishes. In fact, one automobile supplier, in order to maximize manufacturing efficiencies, offered only black vehicles. Customers’ demand for more colours, and a willingness to pay for them, spurred the development of new pigments during the 1920’s.
The next significant development occurred in the mid to late 1930’s with the advance of alkyd resins. Coatings formulated from these resins, which were originally based on glyceryl phthalate and drying oil fatty acids, were higher solids than nitrocellulose lacquers and did not need to be rubbed to achieve high gloss. Alkyds coatings form films by crosslinking via the chemical process of oxidation. In North America, automobile manufacturers such as Chrysler, Ford, and others adopted alkyd resin coatings for topcoats. Meanwhile General Motors embraced the use of nitrocellulose coatings.[14] In addition to topcoats, undercoats were also formulated around these two technologies. The original equipment manufacturing (OEM) coating process consisted essentially of three steps – primer, surfacer, and topcoat. With some exceptions in the use of combination primersurfacers, it remained that way until the advent of colour coat/clear coat technology in the 1970’s.
crosslinkers. During the 1950’s also the advent of acrylic lacquer topcoats was found. With improved gloss and weathering, acrylic lacquers replaced nitrocellulose lacquers. In the 1960’s, acrylic enamels, which were crosslinked with melamine formaldehyde resin, provided improved weathering over alkyd enamels. These were widely adopted by the North American industry and were used into the 1990’s. In the 1960’s and 1970’s, improved weathering alkyds were developed. These melamine cure alkyds gained acceptance in Europe.[15] With the introduction of colour coat/clear coat technology in Europe in the 1970’s, a major leap forward in finish appearance was available for the customer. The advantage of the two-stage process was improved durability and the ability to incorporate new, more colourful pigments as well as metallic and other effects pigments. The use of colour coat/clear coat finishes has increased -where today virtually all automobiles are coated in this manner.
Acrylic melamine clear coats were popular along with 1K blocked isocyanate systems. To make more acid etch resistant clear coats, 2K urethane and new technologies such as carbamates are being utilized. For colour coats, both solvent borne and water borne technology is employed. A “wet-on-wet” process is used whereby the colour coat is applied, followed by the clear coat and then both layers are baked. When the colour coat is water borne, it goes through a flash-off tunnel to remove water before the clear coat is applied.
On the undercoat side of finishes, a significant improvement over nitrocellulose and alkyd enamel primers and surfacers was made with the introduction of epoxy esters in the 1950’s. The fatty acid epoxy ester was cured with urea formaldehyde resin to make a baking primer with improved adhesion, corrosion resistance and chip resistance. This technology was used into the mid 1980’s.[14]
electrodeposition. This coating used bisphenol A epoxy modified with pendant amine groups and a blocked isocyanate crosslinker. This approach yielded coatings with much improved corrosion resistance compared to the anodic process.[14] Cathodic electrodeposition was revolutionary and is in use in almost every automobile plant today. As stated before, regulatory pressure began in the late 1960’s to reduce the VOC’s in coatings. This was, and is, a major driver in coatings development and has led to the use of water borne, powder and higher solids solvent borne finishes. There is a high possibility that water borne paints are going to be the future of automotive painting as the technology and non-toxic, environmentally friendly properties are just too beneficial to be ignored. Paint manufactures are currently devoting a lot of their R&D into water-based paints. As environmental concerns develop, solvent borne will give way to water borne paints. The future development of water borne coatings will focus on the top clear-coat material and use of water borne systems in high production environments.
1.2.3. Film formation of water borne resin systems
During the film formation of water borne dispersions, the dispersion changes from a heterogeneous polymer in water dispersion into a transparent homogenous film. The film formation process can be grouped into 6 stages, which can be grouped into three events.
During water evaporation and flocculation (see stages 1 and 2 in figure 1.2) external factors like air flow, relative humidity, and temperature of the environment influence the drying. There is a balance between the kinetics of drying and the kinetics of fusion of the particles (coalescence). Therefore, water evaporation followed by consolidation and fusion of the particles (stages 1-2 and 3-4 in figure 1.2) can happen at the same time, where the balance between attraction and repulsion forces determines the time scale. Particles in the nm range have huge attraction forces, and repulsion must be obtained through electrostatic interactions due to charges on the particle surface. These charges are compensated by counter ions in solution and an electrical double layer is formed. During drying the electrolyte concentration increases, causing the double layer to shrink and the dispersion becomes less stable. For the best hexagonal packing during coalescence it is important that the particles stay stable for as long as possible. During the deformation stage (=consolidation), particle boundaries are still present. The presence of water is crucial for coalescence because during the last stage of the film formation the particles have to loose their identity and the polymer chains from adjacent particles become entangled. During this last stage the film obtains its final strength, which can take several weeks.[16]
1.2.4. Performance objectives of water-borne coating systems
By definition, a coating for paint applications is a polymer-based substance that covers an object for decorative and/or protective purposes. In the field of automotive applications, protection against corrosion of the substrate, protection against abrasion and chemical attack and stone chip resistance of the top coating layer is important. Gloss, colour and flop (the metallic appearance) are important factors for decorative purposes.
even "upside down." Thus, the viscosity of the paint must be low enough to permit flow (levelling), but high enough so that the paint doesn't sag.
The curing process of the coating is important. The time required for the solvent or water to dry, and the time frame for the crosslinking reaction are important parameters for application. On the other hand, the paint should not cure in the storage container. One way to prevent this is to make a two-component system where the two species that react to make the polymer binder, are stored separate.
The pot life of paint is the period of time you still can work with a paint system once it has been readied for use and is normally on the order of hours for a 2K system. Some of the parameters above are expected to be affected positively or negatively by the presence of nanoparticles.
1.3. Size, morphology and composition of nanoparticles and nanocomposites
1.3.1. Polymers for coating formulations and composite preparation
In the case of exterior automotive purposes, both thermoplastic and thermoset polymers can be used for topcoats. Thermoplastic polymers do not branch, but make use of their high molecular weight to guarantee good mechanical properties. Thermoset polymers make use of branching and network formation to achieve good properties.
A conventional automotive lacquer is a thermoplastic polymer dissolved in an organic solvent. High molecular weight polymers are necessary, and since the polymer molecules are dissolved, the amount of polymer dissolved in the solvent is rather low. Acrylic lacquers are often used, they are good for brilliant metallic colors, have excellent exterior durability and gloss retention. In case of thermoplasticity they remain soluble, which makes repair jobs easier.
crosslinking is not necessary for good mechanical properties. Of course also thermosets with an internal crosslinker or two component systems can be prepared.
1.3.2. Nanoparticles: importance of size, shape and consistence
Nanoparticles are available in various sizes shapes (see figure 1.3) and compositions. In this thesis the reinforcing group of nanoparticles can be distinguished by dividing them into three categories: inorganic particles, clay particles and carbon nanotubes. The common factor is that at least one of the dimensions of the particle should be in between 0.1 and 100 nm.
Figure 1.3: an image of spherical, needle and platelet shaped nanoparticles. The nanometer range is denoted by the arrows
Due to their small size and larger surface area nanoparticles can be beneficial for a wide range of properties. Inorganic nanoparticles like zinc oxide and cerium oxide are known for their UV absorbance, catalytic activity and conductivity while silica particles are beneficial for scratch and chemical resistance.
Clay particles are special due to their unique dimensions. The types of clay most often used are derived from the smectite family, a naturally occurring mineral structure of three layers. Alumina atoms lay between two layers of silicon atoms in a 2:1 structure as shown in figure 1.4.
The industry has developed many synthetic counterparts of the natural smectite family over the last few years. Advantages of synthetic clay particles are a well-defined size distribution and less pollution of unwanted minerals like iron, which give rise to discoloration of the composite. Since individual clay particles are exceptionally thin (in the order of 1nm) they are excellent for obtaining transparent composites. Their enormous aspect ratio (L/d (= length over thickness) can be up to 1000) gives excellent barrier properties and can control the rheology. The use of clay particles for nanocomposite manufacturing has also a big disadvantage. Clay sheets are very hard to separate and therefore completely exfoliated composites are rare. Synthetic layered silicates could be supplied as individual particles, which may solve this problem. Carbon nanotubes are a completely different class of nanoparticles and will not be used in this thesis. They are up to 100 times the strength of steel and have a conductivity as high as copper, which provides opportunities for strong, light-weight conductive composites.
1.3.3. Montmorillonite, Boehmite and Laponite
In this thesis Montmorillonite (MMT), Laponite and Boehmite nanoparticles are used for the preparation of water borne polymer nanocomposite (WB PNC) resins. Montmorillonite, a type of smectite, has two layers of silicon atoms and one layer of alumina in 2:1 structure (see figure 1.5). It is an expanding clay mineral where bonding of divalent cations and water with the basal oxygen atoms of the tetrahedral sheets holds the different layers together. The formula for Montmorillonite is (Si7.8Al0.2)4(Al3.4Mg0.6)6O20(OH)4. The formula indicates that there is substitution of Si4+ by Al3+ in the tetrahedral layer and of Al3+ by Mg2+ in octahedral layer.
The Montmorillonite platelets and edges can be surface modified with organic compounds to allow complete dispersion and exfoliation and to provide miscibility with the polymer matrix.
technical ceramics, thin solid films and catalysis. [18] Preparation from Alumina precursors via hydrothermal synthesis can yield Boehmite particles as colloidal rod-like species with a high anisotropy.
Figure 1.5: Montmorillonite structure
Their aqueous dispersions exhibit flow birefringence, thixotropy and elasticity. [19] Like many of the colloidal rod-like systems, they are capable of forming a nematic phase above a certain particle concentration. In aqueous Boehmite dispersions, the particles are stabilized by electrical double-layer repulsion. [20] Addition of an organic solvent destroys the repulsive interaction and results in flocculation of the particles. In order to use the conventional composite preparation techniques (meltblending/ extrusion, solution-blending or in situ polymerization), it is essential to apply surface modification to the particles. [21]
organo-modification via ion exchange processes possible. Laponite suspensions of individual Laponite discs can be easily prepared under low shear conditions.
Figure 1.6: ideal Laponite structure (left) and representation of a single Laponite crystal (right) [22]
1.4. Scope of this thesis
1.4.1. Research objectives
Due to environmental and safety regulations the use of VOC´s containing lacquers for exterior automotive purposes is under growing pressure. The emission of VOC’s during and after application of coating systems is considered a health, safety and environmental threat. As a consequence there is a demand for more environmentally friendly alternatives like water borne coatings, high solid coatings, powder coatings and radiation cured coatings (see paragraph 1.2.1).
In general, water borne resin systems show qualitatively poor behaviour when it comes to decorative and protective properties (see paragraph 1.4.1). The development of water borne resin systems with a superior mix of material and application properties might be realized by the addition of high aspect ratio, reinforcing inorganic nanoparticles (see paragraph 1.1.3 and 1.3.2).
The research for this project performed at Delft University of Technology, section NanoStructured Materials is focused on the incorporation of nanoparticles in (model)resin systems. The appraisals made to realise the incorporation of reinforcing nanoparticles in the polymeric matrix are discussed and potential nanoparticle-resin partners are investigated (see paragraph 2.2).
The effect of the incorporation of nanoparticles on water borne coating systems is expected to vary with the formulation and synthesis of the PNC systems. To study this kind of effects, three routes for preparation of nanocomposite resin systems are followed (see paragraph 2.3 to 2.5). By varying the formulation and synthesis of the PNC systems it is also tried to achieve variations in nanoparticle dispersion-and nanocomposite morphology of the resin films.
The addition of nanoparticles might hinder the coalescing process of resin dispersion droplets. Hindrance of the coalescence process of nanocomposite resin dispersions will effect the performance of the resin film. Therefore the mechanical properties of the nanoparticle containing cured coating films is studied (see chapters 4, 6 and 8).
The morphology of the nanoparticle containing resin oligomers, dispersions and emulsions (eg. dispersion of nanoparticles in the resin phase vs. nanoparticle dispersion in the water phase) is probably also of influence on the rheological properties of the resin dispersion. In addition to the mechanical properties, rheological parameters are investigated too.
Summarized: this thesis describes routes to prepare water borne polymer nanocomposite resins via post-emulsification of a model resin system and by simple mixing of the nanoparticles with commercially available water borne resin dispersion. The experimental techniques as mentioned above are used to investigate the morphology and resulting properties of WB PNC resins.
1.5. Outline of the thesis
References
1. History of paints and coatings, National Paint & Coatings Association. 2006 2. Tracton, A; Coatings Technology handbook. 3 ed. Boca raton. 2006
3. http://www.nsf.gov/news/speeches/bordogna/rochester/sld006.htm, Sept. 2005 4. Reinhart, K; Latest news on colloidal gold. 2005
5. Haraguchi, Y; and Sasaki A; Philosophical transactions of the Royal Society, 147(1857): p. 145. 1980
6. Keiper, A; Molecular manufactoring.
http://www.nanotechnow.com/Press_Kit/nanotechnology-history.htm, Nov. 2002
7. Ajayan, M; Schadler L; and Braun P; Nanocomposite Science and Technology. Weinheim. 230. 2003
9. Gardner, J; http://www.wired.com/news/technology/. March 2006 10. Polymer Materials Handbook. Surface Coatings. Wiley New York. 2003 11. Weed, F.G; The Finishing of Automotive Equipment and Other Metal
Surfaces, in Protective and Decorative Coatings. Wiley and Sons: New York. chapter 14. 1943
12. Armour, A.G; Automotive Paints and Coatings, ed. Fettis G; Weinheim: VCH. 1995
13. PPG, Automotive coatings....A brief history. 2001
14. Dickerson, J; 50 years of Epon resins, in Paint and Coatings industry. 2002 15. Poth, U; Topcoats for automotive industry. Automotive Paints and Coatings,
ed. Fettis. G; Chapter 5. Weinheim: VCH. 1995
16. Binder Chemistry and Film formation, in Training Coating technology module 2 PTN. 2002
17. Okada, K; et al., J Colloid Interf Sci, 253: p. 308. 2002 18. Music, S; et al., Mat Sci Eng B, 52: p. 145. 1998
19. Buining P.A; Lekkerkerker H.N.W; Langmuir, 10: p. 2106. 1994
20. Buining, P.A; Preparation and properties of dispersions of colloidal Boehmite Rods. University of Utrecht: PhD Thesis. 1992
2
Chapter 2
Preparation of polymer nanocomposite resin
systems for coating applications
Abstract
2.1. Introduction
The quantity of nanoparticles available for nanocomposite preparation seems almost unlimited. The strength of all nanoparticles is that they are effectively a bridge between bulk materials and atomic or molecular structures. Where bulk material should have constant physical properties regardless of size, at the nano-scale this is often not the case. The properties of materials change as their size approaches the nanoscale. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered as super hard materials that do not exhibit the same malleability and ductility as bulk copper. The interesting and sometimes unexpected properties of nanoparticles are partly due to the effect of the surface of the material dominating the properties. The percentage of atoms at the surface of a material becomes significant as the size of that material approaches the nanoscale. For bulk materials larger than 1 micrometer (um) the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent together with Brownian motion is strong enough to overcome differences in density. Nanoparticles often have unexpected optical properties like transparency or color changes because they are small enought to scatter visible light rather than absorb it, also the electronic properties of small particles are different compared to the bulk material.
nanoparticle-chemical complex using an organic or semi-organic nanoparticle-chemical capable of bonding to the surface often containing an ammonium or phosphonium functional group. The groups modify a nanoparticle surface by ionically bonding to it, converting the surface from a hydrophilic to an organophilic species. Ion dipole interactions are a type of chemical bond formed between a charged ion and a molecule that contains a dipole moment. A classic example is water of hydration in many ionic compounds. A complex with a nanoparticle may have a specific ratio of organic or polymer material depending on the charge on the nanoparticle. The chemical structure of many nanoparticles by nature favours the attraction of water or materials, which are soluble in water. After compatibilization the nanoparticles are dispersible in organophilic environments. When using nanoclays, compatibilization is essential to achieve exfoliation. During the exfoliation process the stacks of nanoclay platelets are separated from one another in the polymer matrix. Platelets in the outermost region of each clay packet cleave off, exposing more platelets for exfoliation.
In this chapter the selection of nanoparticles and matrix for the preparation of the nanocomposite resins is motivated. Subsequently, the routes of preparation used to prepare a variety of nanocomposite resin dispersions are described.
2.2. Selection of nanoparticles and resins, a matter of compatibility versus performance
Nanoparticle sample
Organophilic modification Size of individual particle Additional comments Nanomer series (MMT based) Nanomer I24 TL
During production carboxyl end groups are available for reaction (e.g. with amino groups after caprolactam ring
opening) Diameter: up to 1000nm Thickness: 1nm Swellable in e-caprolactam, exfoliation via emulsion polymerisation. Nanomer I28 E
Onium ion surface modified Montmorillonite* Which one unknown
”
Designed for anhydride cured epoxy resins. Not suitable for amine cured resins Nanomer I30 TC N+ C H3 (C18) CH3 CH3 ”
Intended for extrusion compounding with nylon-6
Nanomer I34 TCN
Onium ion surface modified Montmorillonite* Which one unknown
Nanofil serie (organomod. Bentonite) Nanofil 15 distearyldimethylammonium- chloride Diameter: up to 500nm Thickness: 1nm
Suitable for polyolefins
Nanofil 784 See nanofil 15, probably
variation of chain length ”
idem
Nanofil 948 See nanofil 15, probably
variation of chain length ”
Cloisite serie (organo MMT) Na+ MMT none Diameter: up to 1000nm Thickness:1 nm Cloisite 30B Diameter: up to 1000nm Thickness:1nm Boehmite serie Boehmite powder ind.grade: Disperal
-None: aqueous dispersible -Acidic groups -Organic groups Available in various particle shapes and sizes. Colloidal Boehmite alumina powders, to be dispersed in appropriate solvent Boehmite needles
-None: aqueous dispersible -Titanate coupling agent -Dodecylbenzene sulphonic acid Tunable aspect ratio. Length: 250 to 700 nm Thickness: 10 to 25nm
Lab scale prepared aq. suspension of needle shaped AlOOH
Boehmite powder
-None: aqueous dispersible
Laponite serie (smectite) Laponite RD/JS RD: None. -Phospholan PNP9 -Ethoquad C12 JS: TSPP (tetrasodium- pyrophosphate) Diameter: 25nm Thickness: 1nm Laponite JS is a sol forming grade by the aid of TSPP
Table 2.1: known properties of nanoparticles tested for nanocomposite resin preparation
The nanoparticles summarized in table 2.1 were selected by their expected performance as nanofillers in resins based on their physical properties and/or ease of preparation. The series Cloisite (Southern clay chemicals), Nanomer (Nanocor) and Nanofil (Sud- Chemie) are organo-modified natural layered silicates. With single clay sheet dimensions of a diameter up to 1 um and a thickness of 1 nm the large aspect ratio is the most significant reason for selection of these types of nanoparticles. The ease of dispersing these clay nanoparticles as single clay sheets into a resin matrix depends on the compatibility between the resin in solvent system and the organo-modified clay in solvent dispersion.
The Somasif series is a family of synthetically prepared clays with a mica type structure. The dimensions and dispersibility are similar to the Cloisite, Nanomer and Nanofil serie. The additional reason for selection of these nanoparticles is the expectation of less discoloration of the nanocomposite end product as caused by the purified structure of synthetic clay particles compared to natural clays.
The advantage of these particles is their tuneable aspect ratio, combined with the opportunity to produce a sol of single particles without the need of pre-dispersing the particles before nanocomposite resin preparation. Dried powder agglomerates of these Boehmite sols unfortunately are not re-dispersible in solvent.
The Laponite series (Rockwood additives) consist of two commercially available types of synthetic smectite. The two grades “JS” and “RD” differ in organo-modification. Both Laponite grades consist of disc shaped single particles, supplied as powder agglomerates. The powder agglomerates are easily dispersible in water upon applying moderate to high shear. Additional organo-modification can be performed subsequently.
From this list of nanoparticles, three types were selected for nanocomposite resin preparation. The selection parameters chosen were size and shape, (non)polar nature and ease of obtaining individual nanoparticles. Boehmite needles and Laponite discs are good candidates because of their unique dimensions and easy preparation of a solution of individual nanoparticles. Concerning the third selection parameter, Boehmite and Laponite particles are stable in aqueous environments and can be transferred easily to organic solvent via organo-modification of the surface.
The last type of nanoparticle that was selected for nanocomposite resin preparation was selected from the series of clays that was summarized in table 2.1. Individual nanoclay sheets with a large aspect ratio complement the needle shaped Boehmite and disc shaped Laponite particles.
The resin used for nanocomposite preparation contains beside acrylic oligomers the solvents n-methylpyrrolidone (NMP) and ethylethoxypropionate (EEP). The resin/solvent mixture mixes well with o-xylene, which allows the removal of excess solvent as it causes a lower boiling point of the total mixture.
Cloisite 30B clay, which is disposed into 90 millilitres of o-xylene, in separate 100 millilitres graduated cylinders. The clay is added at such a rate that the particles are wetted evenly and clumping does not occur. The samples are allowed to equilibrate after all the clay has been added in approximately 30 minutes. The volume occupied by the clay is then recorded in tenths of a millilitre; this number is called the swelling volume (see table 2.2) since it is related to the amount of solvent able to penetrate in between the clay sheets. The notation of the swelling volume in table 2.2 is the amount of swelling in respect to the added volume.
The mixture is than vigorously shaken 50 times, 10 times horizontally, 40 times vertically, and allowed to stand overnight. The volume occupied by the clay is again recorded in tenths of a millilitre; this value is called the settling volume.
The swelling volume gives an indication of the compatibility of the organic portion of the organo-modified clay with the xylene; the settling volume gives a rough indication of the ease of dispersion of the clay in xylene under low shear conditions. Because of variances in the rate of sifting of the clay into the xylene and the vigour with which the sample is shaken, the numbers are not absolute. Small differences in the volumes are not considered significant. The values are intended for comparison only. The results of the xylene compatibility test are shown in table 2.2.
sample nr
clay Swelling
volume (x times the added volume) Settling volume (ml) Appearance 01 Nanomer I24 TL 4 7 - 02 Nanomer I.28 E 1.8 2 +- 03 Nanomer I 30 TC 2.5 2.7 +- 04 Nanomer I 34 TC 5 2 + 05 Nanofil 15 2 5 -- 06 Nanofil 784 2.2 4.9 -- 07 Nanofil 948 3 5 - 08 Somasif MEE 3 2.5 + 09 Somasif MAE 3.5 3 + 10 Cloisite 30B 5.2 0.6 ++
++ good dispersion, gel formation + good dispersion,
+/- suspension
- highly swollen, but flocculated -- precipitation
Table 2.2: compatibility of organo-modified layered silicates with xylene in terms of swelling, settling and visual appearance
As a result of the xylene compatibility test was C30B chosen as a nanofiller to manufacture nanocomposite resins (see paragraph 2.3) or with the nanofillers located in the resin phase of water borne composite resin dispersions (see paragraph 2.4). As written previously, Boehmite needle shaped nanoparticles and Laponite disc shaped nanoparticles are used to manufacture nanocomposite resins with the nanofillers located in the resin phase of water borne dispersions (see paragraph 2.4). Finally, Laponite particles are also used to prepare nanocomposite resins with the nanoparticles located in the aqueous phase of the resin dispersions (see paragraph 2.5).
2.3. Preparation of the organic layered silicate containing polymer nanocomposites based on organosolvent containing resins used in part I of this thesis
2.3.1. Material
The organic layered silicate (OLS) used for the preparation of the polyacrylate nanocomposite coatings was supplied by Southern Clay Products Inc. (Gonzales, TX) under the trade name Cloisite 30B. It is a natural Montmorillonite treated with an organic modifier. Montmorillonite is a clay with a low content of alumina, sodium, iron and magnesium. It belongs to the smectite family and is found in the shape of thin sheets consisting of three layers. These layers consist of octahedral and tetrahedral planes in which certain atoms can be replaced by others. Thus, the silicon of tetrahedral layers can be replaced by alumina, whereas in the octahedral layers, alumina atoms can leave the network and can be replaced by various elements, such as iron or magnesium. These substitution mechanisms, give rise to a deficit of electronic charge that can be compensated by ions or other interfoliated layers.
Figure 2.1 and 2.2 give the structural information, weight loss upon heating and the X-ray diffraction pattern of the organoclay. The material is available in the form of a fine powder. The clay was dried for 48 hours at 80°C, hereby removing 2% of internal moisture before processing.
Figure 2.1: TGA curves of organoclay Cloisite 30B. The inset shows structural information of the organomodification (see section 3.3.1 for TGA)
A resin formulation was selected as a model system to study the influence of organic layered silicates. BASF patent US 5,670,600 relates to the invention of a 2 component aqueous polyurethane coating composition comprising a water dilutable polyacrylate resin and a polyisocyanate component as a crosslinking agent.[1] The aqueous polyacrylate resin is obtainable via a 2-step solution polymerization. Based on examples stated in this patent, Nuplex Resins synthesized an oligomer solution in ethoxyethyl propionate. After composite preparation these can be emulsified in a next stage via addition of water and removal of the organic solvent. Table 2.3 provides the composition and characteristics of the synthesized solvent-based polyacrylate resin used for composite preparation.
Composition Explanation
Method 2-stage
SV 30 Mixture of butyl acetate and ethoxyethyl propionate Cardura E10 10 Glycidylester of versatic acid
n-BMA 20 n-butyl methacrylate
MMA 16 Methyl methacrylate
Styrene 15.4
EHA 10 Ethylhexyl acetate
HEMA 22 Hydroxyethyl methacrylate
AA 6.6 Acrylic acid
TBPEH 6.0 t-butylperethyl hexanoaat
Characteristics
Non-volatile (%) 73.7 Solids content AN (mgKOH/g) 34.8 Acid number
Viscosity (Pa.s) ntb Viscosity in Pa.s determined by Physica Appearence clear Color 68 APAH determined by SBM 008F
Table 2.3: Composition (in parts) and characteristics of the solvent based polyacrylate resin prepared according to US patent No. 5,670,600
As a crosslinking agent for this system Nuplex Resins provides US138BB-70, a solution of non-plasticized melamine resin with a very high reactivity, under the brand name Setamine.
2.3.2. Compounding and preparation of the polyacrylate nanocomposites
The polyacrylate nanocomposites are prepared as shown in scheme 2.1. Cloisite 30B powder is swollen in xylene under high shear (6000rpm). While the mixture is still under high shear is after 20 minutes 1,2-propylene carbonate (PC) added (C30B: PC = 1:1). This polar activator migrates to the surface of the plates and therewith weakens the Van der Waals forces that keep the platelets together. As a consequence within 15 minutes, a large viscosity increase is observed due to the increased spacing between the platelets and the occurrence of OH-bridging via the platelet edges. To promote exfoliation at this stage 15 minutes of simultaneous ultrasonication is applied. Dispersion by ultrasound is a consequence of microturbulence caused by fluctuations of pressure and by cavitation. Cavitation bubbles expand and subsequently implode within a very short time. The implosions locally result in very high pressures and temperatures.
To block the platelet edges from OH-bridging, 1,1,1,3,3,3-hexamethyldisilazane (HMDS) is added at a fixed proportion with the PC/clay addition: C30B: PC: HMDS = 1:1:1. HMDS is a weak methylsilyl donor, often used in silylating mixtures where the active hydrogen is replaced by an alkyl silylgroup. HMDS can be used for silylating acids, alcohols, amines and phenols. The reaction is shown below:
The reaction is viewed as a nucleophilic attac upon the silicon atom of the silicon donor, producing a bimolecular transition state. The silicon leaving group possesses low basicity in order to stabilize a negative charge in the transition state and to complete the reaction. The ease of derivation of functional groups is in the order of alcohol>phenol>carboxylic acid>amine>amide. The ease of reactivity for alcohols follows the order primary>secondary>tertiary.
2.4. Preparation of the water borne polymer nanocomposite dispersions used in part II of this thesis
2.4.1. Material
WB PNC resin emulsions containing Montmorillonite sheets, Laponite discs or Boehmite needles interacting with the resin phase are prepared.
For characteristics of the C30B particles and preparation of a nanosized suspension of C30B particles in organic solvent we refer to paragraph 2.3 and Part I of this thesis: ‘morphology vs. performance of OLS containing PNC’s based on organosolvent containing resins’.
Rockwood Additives LTD supplied synthetic disc shaped Laponite under the trade name Laponite JS. In paragraph 2.5 the characteristics and the process to disperse Laponite in aqueous environment are described which are used in part III of this thesis: ‘morphology vs. performance of WB PNC’s based on acrylic resin dispersions’.
The use of Boehmite particles is only for part II of the thesis: ‘morphology vs. performance of WB PNC emulsions prepared from organosolvent containing resins’. The preparation as well as the surface modification of the particles is extensively described below. Boehmite needles were manufactured by the Akzo Nobel Innovation Unit according to the processes described in patent WO2005/051845 and the method of Buining et al. [2, 3]
The patent describes hydrothermal preparation of quasi-crystalline Boehmites. The term hydrothermal processing encompasses all processes in which a hydrocarbon feed is reacted with water at elevated temperatures and under elevated pressure. Quasi-crystalline Boehmite or pseudo Boehmite have higher surface areas and smaller crystal sizes than micro-crystalline Boehmite and are dispersible in water and acids.
destroys the repulsive interaction and results in flocculation of the particles. In order to use composite preparation techniques, it is essential to apply surface modification to the particles.
The process that is followed to obtain Boehmite needles is a combination of the two methods and comprises the following steps:
- Preparation of an aqueous precursor mixture comprising a water-soluble alumina source. A mixture of alumina tri sec butoxide (Fluka) and alumina iso propoxide (Janssen) was dissolved in demineralised water and acidified with hydrochloric acid to decrease the pH of the precursor mixture from for example 4 to 2. Subsequently, the pH of the mixture was increased from for example 2 to 4. The resulting solution was stirred for one week
- Aging of the mixture under hydrothermal conditions to form quasi crystalline Boehmite. The mixture was crystallized into Boehmite in an autoclave at 150°C for 22 hours. In order to remove alcoholic by-products, the colloidal dispersions were dialyzed against demineralised water for 1 week.
Figure 2.3: TEM image of the Boehmite needles after evaporation of the aqueous environment
The synthesis yielded Boehmite needles in stable aqueous dispersions at a pH of approximately 4.3. Boehmite concentration in aqueous dispersions used in the preparation of WB PNC resins was 0.91% (w/w).
As for the dimensions of Boehmite needles, TEM characterization revealed an average length of 470 nm and an average width of 14 nm (see figure 2.3). Size estimation of the needles was performed by use of software analysis tools.
2.4.2. Surface modification of nanoparticles
Before preparation of polymer nanocomposite resins is possible, it is necessary to organomodify the surfaces of the nanoparticles. The Boehmite-, MMT- and Laponite nanoparticles are covered with organic groups to stabilize the colloidal dispersion of individual nanoparticles when transferred to organic solvent. The stability of colloids depends on a delicate balance between inter-particle forces, which are mainly electrical double-layer repulsion, sterical hindrance and Van der Waals attraction. [4] If the electrical double-layer repulsion in the system is not sufficiently strong, attraction
prevails and agglomeration of the particles occurs. As the inter-particle forces strongly depend on the surface charges and pH/ionic strength of the medium, these parameters should be controlled throughout the surface treatment procedure.
oppositely charged with respect to the surface attach to the particles by means of ionic attractions. Colloidal stability is obtained by covering the surface with surfactant molecules, which, due to their long alkyl parts, provide a steric barrier against agglomeration of the particles. The main disadvantage is the lack of covalent bonds between the surfactant molecule and the colloidal species, which makes the system less stable against changes in the chemical environment or application of high shear forces, ultrasonication, etc. Another alternative is the use of coupling agents.
Coupling agents can covalently bond to the surface to be modified and act as compatibiliser between two dissimilar systems, such as organic polymers and inorganic surfaces. This is an important feature since upon proper tuning of the alkyl tails it can ensure interaction of the inorganic particles with the resin matrix. In our research, both surfactants and coupling agents are used. The processes of organomodification and the characteristics of the organomodifiers used are explained below:
Organomodification of the Boehmite needles
Three main types of coupling agents are distinguished: zirconates/titanates, silanes and zircoaluminates.[9] In literature it is mentioned that Titanate coupling agents seem to have major advantages. Monte et al. have studied this type of coupling agents extensively and have provided a major contribution to the field.[10]
The general structure of Titanate coupling agents is as follows:
(RO-)
n-Ti-(-OXR'Y)
4-n(1) (2) (3)(4)(5)(6)
butyl, octyl, aromatic benzyl etc.; (5) Y: thermoset functional group such as methacryl, mercapto, amino, etc.; (6) 4-n: organofunctionality (mono-, di- or tri-).
For the organomodification of the Boehmite needles, the titanate-type coupling agent: titanium IV, tris[2-[(2-aminoethyl)amino]ethanolato-O],2-propanolato was used. This coupling agent is known commercially under the brand name KR-44 and is provided by Kenrich Petro Chemicals.[10]
The modification proceeded as followed (see figure 2.4):
- n-propanol (Acros organics) was added drop wise into the Boehmite aqueous dispersion (0.91w/w%) under ultrasonification and moderate stirring.
- Next a azeotropic distillation with further addition of n-propanol to maintain a constant volume was performed. This leads to a stable dispersion of Boehmite in propanol
The change of solvent was required as the first step, because of insolubility of the Ti-coupling agent in water and the incompatibility of the resin oligomers in organic solvent with water, before the neutralization step is performed.
- Finally, the coupling agent (0.25g/g of Boehmite) dissolved in a small amount of propanol was introduced in a drop wise manner into the dispersion, still accompanied by ultrasonification and moderate stirring.
Figure 2.4: impression of organomodification of the surface of Boehmite inorganic needles with Titanate coupling agent.
The dispersions were transparent and stable over time. Observations of dispersions that were kept for several months revealed no agglomeration or sedimentation. In order to get a quantitative measure of the Titanate agent present on the Boehmite surface, atomic absorption spectroscopy (AAS) was performed.
For this purpose, identical amounts of Boehmite dispersions were treated with varying amounts of the Titanate agent. After equilibration, the Boehmite needles could be precipitated by centrifugation. Supernatant parts were analysed for their Ti-content using AAS. By this method, the amount of Titanate agent retained on the Boehmite surface could be calculated.
Figure 2.5: TEM image of Titanate modified Boehmite needles after evaporation of 1-propanol
Figure 2.6: added vs adsorbed amount of Titanate coupling agent per gram of Boehmite
However, above the addition of 0.1g added Titanate agent/g Boehmite the increase of adsorbed Ti-agent/g Boehmite values start to level off. The overall trend shows that the amount of adsorbed Titanate agent at the point of 0.25g added Titanate agent/g Boehmite is approximately 5% of the Boehmite weight.
Organomodification of the Montmorillonite sheets
As stated in paragraph 2.3 is the bare surface of sodium Montmorillonite covered with ammonium cations via an ion exchange process and distributed by Southern Clay Chemicals. The with methyl tallow bis -2-hydroxyethyl quaternary ammonium ions covered product is available under the trade name Cloisite 30B. Figure 2.7 gives an impression of the organomodification.
Organomodification of the Laponite discs
A small amount (3% on particle weight) of phosphonic ester (Phospholan PNP9, Akzo Nobel surfactant, see table 2.5)) is used to make the Laponite discs more compatible with the acrylic resin matrix. Figure 2.8 shows an impression of this anionic phosphate ester when attached to the edges of the Laponite discs. The dispersion process of the Laponite discs is further elaborated in paragraph 2.5. The exact composition of the Phospholan PNP9 compound is unknown. However, for practical reasons as will be described in paragraph 2.5 this surfactant is chosen. Phospholan PNP9 is an anionic phosphate ester with a maximum of 10% of nonionic material. It is fully soluble in water and therefore is expected to have a large hydrophilic part (high HLB) which favours oil in water emulsification. Phospholan PNP9 has an acid number between 135 and 143 mg kOH/g resulting in a pH of 2 upon 5% dissolved material in water. Upon addition of the Phospholan PNP9 to the Laponite in water dispersion the hydrolyzed phosphate ester is able to attach to the positively charged edges of the Laponite. The free acidic groups are expected to cause bridging between the Laponite discs and the resin particles (see scheme 2.3) The nonylphenol/ethanol groups of the phosphate ester might cause a slight increase of the hydrophobicity of the discs, depending on the length of the PEG part. This could cause interactions with the surfactant of the resin particles. However, this is uncertain because of the poor amount of information available.
After the Laponite powder particles are dispersed as individual discs in water the temperature of the nanoparticle dispersion is raised to 85°C. A small amount of Phospholan PNP9 dissolved in water is added under moderate stirring. After 4 hours 1-propanol is added to the dispersion while the water is removed during azeotropic distillation. After organomodification the dispersion of individual Laponite particles is maintained. Figure 2.9 shows the Laponite discs before and after organomodification.
Figure 2.9: TEM images of Laponite discs before (left) and after (right) organomodification with Phospholan PNP9
To investigate the amount of Phospholan PNP9 adsorbed on the Laponite surface TGA analysis was performed. Figure 2.10 reveals that approximately 2.8% of organomodification was present on the surface of the Laponite discs.
2.4.3. Compounding and preparation of the polyacrylate nanocomposites
The general route of preparation includes a two-stage process. First solvent borne (SB) resin oligomers are mixed with a dispersion of organomodified Boehmite, -Laponite or -MMT nanoparticles in a matching carrier solvent like xylene or 1-propanol. The SB resin oligomers consist of a polyacrylic resin dissolved in organic solvent. The characteristics are given in part I of this thesis where the performance of this intermediate solvent borne polymer nanocomposite (SB PNC) resins containing MMT has also been analysed.
Next, the resin dispersion process is performed. The mixture of nanoparticles and resin is stirred at 110°C. This nanoparticle containing resin in solvent is added under vigorous stirring to a water/dimethylethanolamine (DMEA) mixture at 50°C. The quantity of DMEA is such that the polyacrylate resin is neutralized to a degree of 80%. Organic solvent is removed from this dispersion by azeotropic distillation in vacuum until GC can detect no more than 15% organic solvent. While the organic solvent is separated off, the water is returned to the reactor. Finally, the dispersion is brought to a solids content of 40% by weight.
For the preparation of polymer films Nuplex Resins provides a crosslinking agent for this system under the brand name Cymel 328 resin. This is a partially methylolated, highly alkylated, high solids, high imino melamine resin, which is added in a 80:20 resin:crosslinker ratio. Films are formed during a curing process of 30 minutes at 80°C followed by 140°C during 30 minutes.
2.5. Preparation of water borne polymer nanocomposite dispersions used in part III of this thesis
2.5.1. Material
water layers. Each layer comprises three sheets, two outer tetrahedral silica sheets and a central octahedral magnesium sheet. Part of the magnesium in the central sheet is replaced by lithium, resulting in a net negative charge of the layer. This is balanced by sodium ions located between adjacent layers in a stack. The empirical formula of the Laponite structure is Na+
0.7 [(Si8 Mg 5.5 Li 0.3) O20(OH)4]- 0.7.[11]
The process of dispersing Laponite from powder to individual particles is shown in scheme 2.2.
Scheme 2.2: scheme of Laponite dispersion in water [11]
The powder is solvated in aqueous environment under moderate shearing using a high-speed disperser (2000 rpm). The aggregated particle stacks are dispersed into primary particle stacks when the speed is raised to 6000 rpm. This rotation speed is maintained for 1 hour.
A 1K, self-crosslinkable dispersion formulation was selected in which the addition of Laponite particles to the water phase could be studied. Nuplex Resins (former Akzo Nobel Resins) provided Setalux 6768, a specialty acrylic dispersion (see Table 2.4). Setalux 6768 acrylic dispersion is stabilized by electrostatic repulsion of the anionic (negatively charged) resin particles. It was originally developed for topcoats in the general metal market and is at the moment finding many applications on other substrates such as wood and plastic. Upon drying the crosslinking mechanism is triggered resulting in a fully cured coating.
Acrylic polymer stabilized anionic acrylate dispersion in demineralised water with a core shell structure consisting of an hydrophobic core an a hydrophilic shell
MFFT 7°C Non-vol (%) 39-41
PH 7-9 Tg core (°C) +- 125
Density 1.04kg/dm3 Tg shell(°C) +- 40 Appearance clear
Table 2.4: characteristics of the all-acrylic dispersion formulation Setalux 6768
2.5.2. Compounding and preparation of the polyacrylate nanocomposites
The polyacrylate nanocomposite dispersions and films are prepared as described in scheme 2.3.
Laponite is dispersed to 4% w/w in demineralised water according to scheme 2.2
At this point, the Laponite particles are separated into individual platelets. By trial and error during the preparation, the empirical observation is that Phospholan PNP9 (see table 2.5) modified Laponite discs disperse fully in the aqueous emulsion. Nonmodified Laponite discs tend to phase separate from the resin emulsion because of aggregation of the Laponite discs. A small amount (2.8% on particle weight) of a phosphonic ester (Phospholan PNP9, Akzo Nobel surfactant) is added (see paragraph 2.4.2). RPO32-X 2+ 4h, 65°C Application with doctors blade,
dry at RT Resin particle boundary Substrate interface
with enclosed Laponite particles Accumulation of Laponite particles depended of Laponite concentration Air interface
0.35%≤φv≤0.48% 0.60%≤φv≤2%
Laponite dispersion
S6768/ butylglycol Slow Stirring
Scheme 2.3: scheme of WBPNC preparation
Phospholan PNP9 Hydrophobic part: Nonylphenol, EO X+=H+, K+, N+H(C 2H5OH)3 R-O-(CH2CH2O)n R=Cn, R-O-(CH2CH2O)n P-O-X+ O n>0 Cn,
Table 2.5: general structure phosphate ester Phospholan PNP9
The relationship between the weight percentage of the Laponite and volume fraction Laponite(VfL) can be calculated using equation 2.1:
(
)
1 1 11
L L fL L L L sw
V
w
w
ρ
ρ
ρ
− − −=
+ −
2.1 with:ρ
L=
2.65 /
g cm
3 and 30.998 /
sg cm
ρ
=
For example, 4% w/w Laponite upon non volatile component (nvc) content of the dispersion contains 0.0161 VfL of Laponite discs in the cured coating film.
The relationship between the weight percentage of solid resin in the dispersion and the volume fraction of solid resin in the dispersion can be calculated using equation 2.2
(
)
1 resin resin
1 1
water resin resin
1
resin fRw
V
w
w
ρ
ρ
ρ
− − −=
+ −
2.2with:
ρ
resin=
1.04 /
g cm
3 and 3 water0.998 /
g cm
ρ
=
2.6. Summary and Conclusions
Material properties of the nanoparticles play an important role in the final properties of the cured resin films. Nanoparticles in general are expected to improve the mechanical properties of the resin matrix while maintaining optical properties like transparency and gloss. Synthetic nanoparticles are in particular interesting for optically high demanding applications. The crystal structure can be modified to contain no ions that can give rise to discoloration in the final coating film. However, the major advantage favouring of the use of synthetic nanoparticles instead of nanoparticles made by nature is the ease of dispersing the nanoparticles as individual particles in (organic) solvent. While processing natural (clay) nanoparticles a lot of energy is needed for acquiring a solution of individual nanoparticles to mix with the matrix or in dispersing the nanoparticles as individual particles directly in the matrix. By using (preferably homemade) synthetic nanoparticles, individual particles can be obtained during nanoparticle manufacturing.
It is possible to prepare nanocomposite resins containing alumina oxide particles (Boehmite), synthetic Hectorite particles (Laponite) or natural Montmorillonite particles (Cloisite 30B) dispersed on a nanoscale. After drying or curing of the nanocomposite resins films with varying morphologies depending on the location of the nanoparticles are expected. Compatibilization agents are used to promote either nanoparticle exfoliation (in the case of Montmorillonite clay) or nanoparticle stabilization during dispersion of the particles in the matrix (Boehmite, Laponite and Montmorillonite). Compatibilization agents play also a role in the interaction between the resin matrix and the nanoparticle which may influence the nanocomposite morphology of the dried coating layers.
References
1. Nienhaus; et al., United States patent number 5,670,600. Assignee: BASF. Sept. 23,1997
3. Laheij, E; et al., Hydrothermal process for the preparation of quasi crystalline Boehmite, Albemarle Netherlands BV Akzo Nobel NV Petroleo Brasileo: Netherlands, Brazil. patent. EP2004/013226. 2005
4. Buining, P.A; Prepraration and properties of dispersions of colloidal PhD thesis, Utrecht University. 1992
5. Buining, P.A; et al., Langmuir, 10: p. 2106. 1994 6. Philipse, A.P; et al., Langmuir. 10: p. 4451. 1994 7. Buining, P.A; et al., Langmuir. 12: p. 4851. 1996 8. Bruggen, M.P.B.v; Langmuir. 14: p. 2245. 1998
9. Levering, A.W; Interphases in zirconium silicate filled high density polyethylene and polypropylene. PhD thesis, Delft University of Technology. 1995
10. Monte, S.J; Polym. Compos. 10 1. 2002
11. Technical data sheets August available on:. www.laponite.com. 2003
Part I
A closer look on the morphology vs. performance of
Organic Layered Silicate containing polymer
nanocomposites based on organosolvent containing
resins
3
Morphological study of Montmorillonite containing
acrylic resin formulations: TEM and X-ray diffraction
Abstract
Several acrylic resin based polymer nanocomposites containing Montmorillonite type layered silicate plates were analysed by transmission microscopy (TEM) and wide angle X-ray diffraction (WAXS) to characterise the nanoscale dispersion of the layered silicate. The polymer nanocomposites investigated consist of acrylic oligomers in organic solvent with a dispersed phase of Cloisite 30B. The results of this study reveal that the overall nanoscale dispersion as shown by a combination of XRD and TEM is a mixed morphology (regions of both exfoliated and intercalated nanostructures), depending on the liquid suspension vs. cured film state of the polymer nanocomposites and the concentration of the added Montmorillonite phase.
3.1. Introduction
