The background of the invention can be described most clearly, and hence the invention can be taught most effectively, by subdividing this section in three subsections. The first subsection will provide some general background regarding the role of crosslinked (and especially stiff and strong thermoset) particles in the field of the invention. The second subsection will describe the prior art that has been taught in the patent literature. The third subsection will provide additional relevant background information selected from the vast scientific literature on polymer and composite materials science and chemistry, to further facilitate the teaching of the invention.
A. General Background
Crosslinked polymer (and especially stiff and strong thermoset) particles are used in many applications requiring high stiffness, high mechanical strength, high temperature resistance, and/or high resistance to aggressive environments. Crosslinked polymer particles can be prepared by reacting monomers or oligomers possessing three or more reactive chemical functionalities, as well as by reacting mixtures of monomers and/or oligomers at least one ingredient of which possesses three or more reactive chemical functionalities.
The intrinsic advantages of crosslinked polymer particles over polymer particles lacking a network consisting of covalent chemical bonds in such applications become especially obvious if an acceptable level of performance must be maintained for a prolonged period (such as many years, or in some applications even several decades) under the combined effects of mechanical deformation, heat, and/or severe environmental insults. For example, many high-performance thermoplastic polymers, which have excellent mechanical properties and which are hence used successfully under a variety of conditions, are unsuitable for applications where they must maintain their good mechanical properties for many years in the presence of heat and/or chemicals, because they consist of assemblies of individual polymer chains. Over time, the deformation of such assemblies of individual polymer chains at an elevated temperature can cause unacceptable amounts of creep, and furthermore solvents and/or aggressive chemicals present in the environment can gradually diffuse into them and degrade their performance severely (and in some cases even dissolve them). By contrast, the presence of a well-formed continuous network of covalent bonds restrains the molecules, thus helping retain an acceptable level of performance under severe use conditions over a much longer time period.
Oil and natural gas well construction activities, including drilling, completion and stimulation applications (such as proppants, gravel pack components, ball bearings, solid lubricants, drilling mud constituents, and/or cement additives), require the use of particulate materials, in most instances preferably of as nearly spherical a shape as possible. These (preferably substantially spherical) particles must generally be made from materials that have excellent mechanical properties. The mechanical properties of greatest interest in most such applications are stiffness (resistance to deformation) and strength under compressive loads, combined with sufficient “toughness” to avoid the brittle fracture of the particles into small pieces commonly known as “fines”. In addition, the particles must have excellent heat resistance in order to be able to withstand the combination of high compressive load and high temperature that normally becomes increasingly more severe as one drills deeper. In other words, particles that are intended for use deeper in a well must be able to withstand not only the higher overburden load resulting from the greater depth, but also the higher temperature that accompanies that higher overburden load as a result of the nature of geothermal gradients. Finally, these materials must be able to withstand the effects of the severe environmental insults (resulting from the presence of a variety of hydrocarbon and possibly solvent molecules as well as water, at simultaneously elevated temperatures and compressive loads) that the particles will encounter deep in an oil or natural gas well. The need for relatively lightweight high performance materials for use in these particulate components in applications related to the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells thus becomes obvious. Consequently, while such uses constitute only a small fraction of the applications of stiff and strong materials, they provide fertile territory for the development of new or improved materials and manufacturing processes for the fabrication of such materials.
We will focus much of the remaining discussion of the background of the invention on the use of particulate materials as proppants. One key measure of end use performance of proppants is the retention of high conductivity of liquids and gases through packings of the particles in aggressive environments under high compressive loads at elevated temperatures.
The use of stiff and strong solid proppants has a long history in the oil and natural gas industry. Throughout most of this history, particles made from polymeric materials (including crosslinked polymers) have been considered to be unsuitable for use by themselves as proppants. The reason for this prejudice is the perception that polymers are too deformable, as well as lacking in the ability to withstand the combination of elevated compressive loads, temperatures and aggressive environments that are commonly encountered in oil and natural gas wells. Consequently, work on proppant material development has focused mainly on sands, on ceramics, and on sands and ceramics coated by crosslinked polymers to improve some aspects of their performance. This situation has prevailed despite the fact that most polymers have densities that are much closer to that of water so that in particulate form they can be transported much more readily into a fracture by low-density fracturing or carrier fluids such as unviscosified water.
Nonetheless, the obvious practical advantages [see a review by Edgeman (2004)] of developing the ability to use lightweight particles that possess almost neutral buoyancy relative to water have stimulated a considerable amount of work over the years. However, as will be seen from the review of the prior art provided below, progress in this field of invention has been very slow as a result of the many technical challenges that exist to the successful development of cost-effective lightweight particles that possess sufficient stiffness, strength and heat resistance.
B. Prior Art
The prior art can be described most clearly, and hence the invention can be placed in the proper context most effectively, by subdividing this section into four subsections. The first subsection will describe prior art related to the development of “as-polymerized” thermoset polymer particles. The second subsection will describe prior art related to the development of thermoset polymer particles that are subjected to post-polymerization heat treatment. The third subsection will describe prior art related to the development of thermoset polymer composite particles where the particles are reinforced by conventional fillers. The fourth subsection will describe prior art related to the development of ceramic nanocomposite particles where a ceramic matrix is reinforced by nanofillers.
1. “As-Polymerized” Thermoset Polymer Particles
As discussed above, particles made from polymeric materials have historically been considered to be unsuitable for use by themselves as proppants. Consequently, their past uses in proppant materials have focused mainly on their placement as coatings on sands and ceramics, in order to improve some aspects of the performance of the sand and ceramic proppants.
Significant progress was made in the use of crosslinked polymeric particles themselves as constituents of proppant formulations in prior art taught by Rickards, et al. (U.S. Pat. No. 6,059,034; U.S. Pat. No. 6,330,916). However, these inventors still did not consider or describe the polymeric particles as proppants. Their invention only related to the use of the polymer particles in blends with particles of more conventional proppants such as sands or ceramics. They taught that the sand or ceramic particles are the proppant particles, and that the “deformable particulate material” consisting of polymer particles mainly serves to improve the fracture conductivity, reduce the generation of fines and/or reduce proppant flowback relative to the unblended sand or ceramic proppants. Thus while their invention differs significantly from the prior art in the sense that the polymer is used in particulate form rather than being used as a coating, it shares with the prior art the limitation that the polymer still serves merely as a modifier improving the performance of a sand or ceramic proppant rather than being considered for use as a proppant in its own right.
Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress towards the development of lightweight proppants consisting of high-strength crosslinked polymeric particles for use in hydraulic fracturing applications. However, embodiments of this prior art, based on the use of styrene-divinylbenzene (S-DVB) copolymer beads manufactured by using conventional fabrication technology and purchased from a commercial supplier, failed to provide an acceptable balance of performance and price. They cost far more than the test standard (Jordan sand) while being outperformed by Jordan sand in terms of the liquid conductivity and liquid permeability characteristics of their packings measured according to the industry-standard API RP 61 testing procedure. [This procedure is described by the American Petroleum Institute in its publication titled “Recommended Practices for Evaluating Short Term Proppant Pack Conductivity” (first edition, Oct. 1, 1989).] The need to use a very large amount of an expensive crosslinker (50 to 80% by weight of DVB) in order to obtain reasonable performance (not too inferior to that of Jordan Sand) was a key factor in the higher cost that accompanied the lower performance.
The most advanced prior art in stiff and strong crosslinked polymer particle technologies for use in applications in oil and natural gas drilling was developed by Albright (U.S. Pat. No. 6,248,838) who taught the concept of a “rigid chain entanglement crosslinked polymer”. In summary, the reactive formulation and the processing conditions were modified to achieve “rapid rate polymerization”. While not improving the extent of covalent crosslinking relative to conventional isothermal polymerization, rapid rate polymerization results in the “trapping” of an unusually large number of physical entanglements in the polymer. These additional entanglements can result in a major improvement of many properties. For example, the liquid conductivities of packings of S-DVB copolymer beads with wDVB=0.2 synthesized via rapid rate polymerization are comparable to those that were found by Bienvenu (U.S. Pat. No. 5,531,274) for packings of conventionally produced S-DVB beads at the much higher DVB level of wDVB=0.5. Albright (U.S. Pat. No. 6,248,838) thus provided the key technical breakthrough that enabled the development of the first generation of crosslinked polymer beads possessing sufficiently attractive combinations of performance and price characteristics to result in their commercial use in their own right as solid polymeric proppants.
2. Heat-Treated Thermoset Polymer Particles
There is no prior art that relates to the development of heat-treated thermoset polymer particles for use in oil and natural gas well construction applications. One needs to look into another field of technology to find prior art of some relevance. Nishimori, et. al. (JP1992-22230) focused on the development of particles for use in liquid crystal display panels. They taught the use of post-polymerization heat treatment to increase the compressive elastic modulus of S-DVB particles at room temperature. They only claimed compositions polymerized from reactive monomer mixtures containing 20% or more by weight of DVB or other crosslinkable monomer(s) prior to the heat treatment. They stated explicitly that improvements obtained with lower weight fractions of the crosslinkable monomer(s) were insufficient and that hence such compositions were excluded from the scope of their patent.
3. Thermoset Polymer Composite Particles
This subsection will be easier to understand if it is further subdivided into two subsections. As was discussed above, the prior art on the use of polymers as components of proppant particles has focused mainly on the development of thermoset polymer coatings for rigid inorganic materials such as sand or ceramic particles. These types of heterogeneous (composite) particles will be discussed in the first subsection. Composite particles where the thermoset polymer plays a role that goes beyond that of a coating will be discussed in the second subsection.
a. Thermoset Polymers as Coatings
The prior art discussed in this subsection is mainly of interest for historical reasons, as examples of the evolution of the use of thermoset polymers as components in composite proppant particles.
Underdown, et al. (U.S. Pat. No. 4,443,347) and of Glaze, et al. (U.S. Pat. No. 4,664,819) taught the coating of particles such as silica sand or glass beads with a thermoset polymer (such as a phenol-formaldehyde resin) that is cured fully (in their terminology, “pre-cured”) prior to the injection of a proppant charge consisting of such particles into a well.
An interesting alternative coating technology was taught by Graham, et al. (U.S. Pat. No. 4,585,064) who developed resin-coated particles comprising a particulate substrate, a substantially cured inner resin coating, and a heat-curable outer resin coating. According to their teaching, the outer resin coating should cure, and should thus enable the particles to form a coherent mass possessing the desired level of liquid conductivity, under the temperatures and compressive loads found in subterranean formations. However, it is not difficult to anticipate the many technical difficulties that can arise in attempting to reduce such an approach reliably and consistently to practice.
b. Thermoset Polymers as Matrix Phase Containing Dispersed Finely Divided Filler Material
McDaniel, et al. (U.S. Pat. No. 6,632,527) describes composite particles made of a binder and filler; for use in subterranean formations (for example, as proppants and as gravel pack components), in water filtration, and in artificial turf for sports fields. The filler consists of finely divided mineral particles that can be of any available composition. Fibers are also used in some embodiments as optional fillers. The sizes of the filler particles are required to fall within the range of 0.5 microns to 60 microns. The proportion of filler in the composite particle is very large (60% to 90% by volume). The binder formulation is required to include at least one member of the group consisting of inorganic binder, epoxy resin, novolac resin, resole resin, polyurethane resin, alkaline phenolic resole curable with ester, melamine resin, urea-aldehyde resin, urea-phenol-aldehyde resin, furans, synthetic rubber, and/or polyester resin. The final thermoset polymer composite particles of the required size and shape are obtained by a succession of process steps such as the mixing of a binder stream with a filler particle stream, agglomerative granulation, and the curing of granulated material streams.
4. Ceramic Nanocomposite Particles
Nguyen, et al. (U.S. 20050016726) taught the development of ceramic nanocomposite particles comprising a base material (present at roughly 50% to 90% by weight) and at least one nanoparticle material (present at roughly 0.1% to 30% by weight). Optionally, a polymeric binder, an organosilane coupling agent, and/or hollow microspheres, can also be included. The base material comprises clay, bauxite, alumina, silica, or mixtures thereof. It is stated that a suitable method for forming the composite particulates from the dry ingredients is to sinter by heating at a temperature of between roughly 1000° C. and 2000° C., which is a ceramic fabrication process. Given the types of formulation ingredients used as base materials by Nguyen, et al. (U.S. 20050016726), and furthermore the fact that even if they were to incorporate a polymeric binder in an embodiment of their invention said polymeric binder would not retain its normal chemical composition and polymer chain structure when a particulate is sintered by heating it at a temperature of between 1000° C. and about 2000° C., their composite particulates consist of the nanofiller(s) dispersed in a ceramic matrix.
C. Scientific Literature
The development of thermoset polymer nanocomposites requires the consideration of a vast and multidisciplinary range of polymer and composite materials science and chemistry challenges. It is essential to convey these challenges in the context of the fundamental scientific literature.
Bicerano (2002) provides a broad overview of polymer and composite materials science that can be used as a general reference for most aspects of the following discussion. Many additional references will also be provided below, to other publications which treat specific issues in greater detail than what could be accommodated in Bicerano (2002).
1. Selected Fundamental Aspects of the Curing of Crosslinked Polymers
It is essential, first, to review some fundamental aspects of the curing of crosslinked polymers, which are applicable to such polymers regardless of their form (particulate, coating, or bulk).
The properties of crosslinked polymers prepared by standard manufacturing processes are often limited by the fact that such processes typically result in incomplete curing. For example, in an isothermal polymerization process, as the glass transition temperature (Tg) of the growing polymer network increases, it may reach the polymerization temperature while the reaction is still in progress. If this happens, then the molecular motions slow down significantly so that further curing also slows down significantly. Incomplete curing yields a polymer network that is less densely crosslinked than the theoretical limit expected from the functionalities and relative amounts of the starting reactants. For example, a mixture of monomers might contain 80% DVB by weight as a crosslinker but the final extent of crosslinking that is attained may not be much greater than what was attained with a much smaller percentage of DVB. This situation results in lower stiffness, lower strength, lower heat resistance, and lower environmental resistance than the thermoset is capable of manifesting when it is fully cured and thus maximally crosslinked.
When the results of the first scan and the second scan of S-DVB beads containing various weight fractions of DVB (wDVB), obtained by Differential Scanning Calorimetry (DSC), as reported by Bicerano, et al. (1996) (see FIG. 1) are compared, it becomes clear that the low performance and high cost of the “as purchased” S-DVB beads utilized by Bienvenu (U.S. Pat. No. 5,531,274) are related to incomplete curing. This incomplete curing results in the ineffective utilization of DVB as a crosslinker and thus in the incomplete development of the crosslinked network. In summary, Bicerano, et al. (1996), showed that the Tg of typical “as-polymerized” S-DVB copolymers, as measured by the first DSC scan, increased only slowly with increasing wDVB, and furthermore that the rate of further increase of Tg slowed down drastically for wDVB>0.08. By contrast, in the second DSC scan (performed on S-DVB specimens whose curing had been driven much closer to completion as a result of the temperature ramp that had been applied during the first scan), Tg grew much more rapidly with wDVB over the entire range of up to wDVB=0.2458 that was studied. The more extensively cured samples resulting from the thermal history imposed by the first DSC scan can, thus, be considered to provide much closer approximations to the ideal theoretical limit of a “fully cured” polymer network.
2. Effects of Heat Treatment on Key Properties of Thermoset Polymers
a. Maximum Possible Use Temperature
As was illustrated by Bicerano, et al. (1996) for S-DVB copolymers with wDVB of up to 0.2458, enhancing the state of cure of a thermoset polymer network can increase Tg very significantly relative to the Tg of the “as-polymerized” material. In practice, the heat distortion temperature (HDT) is used most often as a practical indicator of the softening temperature of a polymer under load. As was shown by Takemori (1979), a systematic understanding of the HDT is possible through its direct correlation with the temperature dependences of the tensile (or equivalently, compressive) and shear elastic moduli. For amorphous polymers, the precipitous decrease of these elastic moduli as Tg is approached from below renders the HDT well-defined, reproducible, and predictable. HDT is thus closely related to (and usually slightly lower than) Tg for amorphous polymers, so that it can be increased significantly by increasing Tg significantly.
The HDT decreases gradually with increasing magnitude of the load used in its measurement. For example, for general-purpose polystyrene (which has Tg=100° C.), HDT=95° C. under a load of 0.46 MPa and HDT=85° C. under a load of 1.82 MPa are typical values. However, the compressive loads deep in an oil well or natural gas well are normally far higher than the standard loads (0.46 MPa and 1.82 MPa) used in measuring the HDT. Consequently, amorphous thermoset polymer particles can be expected to begin to deform significantly at a lower temperature than the HDT of the polymer measured under the standard high load of 1.82 MPa. This deformation will cause a decrease in the conductivities of liquids and gases through the propped fracture, and hence in the loss of effectiveness as a proppant, at a somewhat lower temperature than the HDT value of the polymer measured under the standard load of 1.82 MPa.
b. Mechanical Properties
As was discussed earlier, Nishimori, et. al. (JP1992-22230) used heat treatment to increase the compressive elastic modulus of their S-DVB particles (intended for use in liquid crystal display panels) significantly at room temperature (and hence far below Tg). Deformability under a compressive load is inversely proportional to the compressive elastic modulus. It is, therefore, important to consider whether one may also anticipate major benefits from heat treatment in terms of the reduction of the deformability of thermoset polymer particles intended for oil and natural gas drilling applications, when these particles are used in subterranean environments where the temperature is far below the Tg of the particles. As explained below, the enhancement of curing via post-polymerization heat treatment is generally expected to have a smaller effect on the compressive elastic modulus (and hence on the proppant performance) of thermoset polymer particles when used in oil and natural gas drilling applications at temperatures far below their Tg.
Nishimori, et. al. (JP1992-22230) used very large amounts of DVB (wDVB>>0.2). By contrast, much smaller amounts of DVB (wDVB≦0.2) must be used for economic reasons in the “lower value” oil and natural gas drilling applications. The elastic moduli of a polymer at temperatures far below Tg are determined primarily by deformations that are of a rather local nature and hence on a short length scale. Some enhancement of the crosslink density via further curing (when the network junctions created by the crosslinks are far away from each other to begin with) will hence not normally have nearly as large an effect on the elastic moduli as when the network junctions are very close to each other to begin with and then are brought even closer by the enhancement of curing via heat treatment. Consequently, while the compressive elastic modulus can be expected to increase significantly upon heat treatment when wDVB is very large, any such effect will normally be less pronounced at low values of wDVB. In summary, it can thus generally be expected that the enhancement of the compressive elastic modulus at temperatures far below Tg will probably be small for the types of formulations that are most likely to be used in the synthesis of thermoset polymer particles for oil and natural gas drilling applications.
3. Effects of Nanoparticle Incorporation on Key Properties of Thermoset Polymers
a. Maximum Possible Use Temperature
As was pointed out by Takemori (1979), the addition of rigid fillers has a negligible effect on the HDT of amorphous polymers. However, nanocomposite materials and technologies had not yet been developed in 1979. It is, hence, important to consider, based on the data that have been gathered and the insights that have been obtained more recently, whether nanofillers may be expected to behave in a qualitatively different manner because of their geometric characteristics.
A review article by Aharoni (1998) considered this question and showed that three criteria must be considered. Here are the most relevant excerpts from his article: “When a combination of the following three conditions is fulfilled, then the glass transition temperature . . . may be increased relative to that of the same polymer in the absence of these three conditions . . . . First, very large surface area of a rigid heterogeneous material in close contact with the amorphous phase of the polymer. Such large surface areas may be obtained by having a rigid additive material extremely finely ground, preferably to nanometer length scale. Second, strong attractive interactions should exist between the heterogeneous surfaces and the polymer. In the absence of strong attractive interactions with the heterogeneous rigid surfaces, the chain segments in the boundary layer are capable of relaxing to a state approximating the bulk polymer and the Tg will be identical or very slightly higher than that of the pure bulk polymer. Third, measure of motional cooperation must exist between interchain and intrachain fragments. Unlike the effects of high modulus heterogeneous additives on the averaged modulus of the system in which they are present, the elevation of Tg of the polymer matrix was repeatedly shown to require not only that the polymer itself will be a high molecular weight substance, but that the additive will be finely comminuted to generate very large polymer-heterophase interfacial surface area, and, especially important, that strong attractive interactions will exist between the polymer and the foreign additive. These interactions are generally of an ionic, hydrogen bonding, or dipolar nature and, as a rule, require that the foreign additive will have surface energy higher than or at least equal to, but never lower than, that of the amorphous polymer in which it is being incorporated.”
Almost by definition, Aharoni's first condition will be satisfied for any nanofiller that has been dispersed well in the polymer matrix. Furthermore, since a thermoset polymer contains a covalently bonded three-dimensional network structure, his third condition will also be satisfied if any thermoset polymer is used as the matrix material. However, in most systems, there will not be strong attractive interactions “generally of an ionic, hydrogen bonding, or dipolar nature” between the polymer and the nanofiller, so that the second criterion will not be satisfied. It can, therefore, be concluded that, for most combinations of polymer and nanofiller, Tg will not increase significantly upon incorporation of the nanofiller so that the maximum possible use temperature will not increase significantly either. There will, however, be exceptions to this general rule. Combinations of polymer and nanofiller that manifest strong attractive interactions can be found, and for such combinations both Tg and the maximum possible use temperature can increase significantly upon nanofiller incorporation.
b. Mechanical Properties
It is well-established that the incorporation of rigid fillers into a polymer matrix can produce a composite material which has significantly greater stiffness (elastic modulus) and strength (stress required to induce failure) than the base polymer. It is also well-established that rigid nanofillers can generally stiffen and strengthen a polymer matrix more effectively than conventional rigid fillers of similar composition since their geometries allow them to span (or “percolate through”) a polymer specimen at much lower volume fractions than conventional fillers. This particular advantage of nanofillers over conventional fillers is well-established and a major driving force for the vast research and development effort worldwide to develop new nanocomposite products.
FIG. 2 provides an idealized schematic illustration of the effectiveness of nanofillers in terms of their ability to “percolate through” a polymer specimen even when they are present at a low volume fraction. It is important to emphasize that FIG. 2 is of a completely generic nature. It is presented merely to facilitate the understanding of nanofiller percolation, without implying that it provides an accurate depiction of the expected behavior of any particular nanofiller in any particular polymer matrix. In practice, the techniques of electron microscopy are generally used to observe the morphologies of actual embodiments of the nanocomposite concept. Specific examples of the ability of nanofillers such as carbon black and fumed silica to “percolate” at extremely low volume fractions when dispersed in polymers are provided by Zhang, et al (2001). The vast literature and trends on the dependences of percolation thresholds and packing fractions on particle shape, aggregation, and other factors, are reviewed by Bicerano, et al. (1999).
As has also been studied extensively [for example, see Okamoto, et al. (1999)] but is less widely recognized by workers in the field, the incorporation of rigid fillers of appropriate types and dimensions in the right amount (often just a very small volume fraction) can toughen a polymer in addition to stiffening it and strengthening it. “Toughening” implies a reduction in the tendency to undergo brittle fracture. If and when it is realized for proppant particles, it is an important additional benefit since it reduces the risk of the generation of “fines” during use.
4. Technical Challenges to Nanoparticle Incorporation in Thermoset Polymers
It is important to also review the many serious technical challenges that exist to the successful incorporation of nanoparticles in thermoset polymers. Appreciation of these obstacles can help workers in the field of the invention gain a better understanding of the invention. There are three major types of potential obstacles. In general, each potential obstacle will tend to become more serious with increasing nanofiller volume fraction, so that it is usually easier to incorporate a small volume fraction of a nanofiller into a polymer than it is to incorporate a larger volume fraction. This subsection is subdivided further into the following three subsections where each type of major potential obstacle will be discussed in turn.
a. Difficulty of Dispersing Nanofiller
The most common difficulty that is encountered in preparing polymer nanocomposites involves the need to disperse the nanofiller. The specific details of the source and severity of the difficulty, and of the methods that may help overcome the difficulty, differ between types of nanofillers, polymers, and fabrication processes (for example, the “in situ” synthesis of the polymer in an aqueous or organic medium containing the nanofiller, versus the addition of the nanofiller into a molten polymer). However, some important common aspects can be identified.
Most importantly, nanofiller particles of the same kind often have strong attractive interactions with each other. As a result, they tend to “clump together”; for example, preferably into agglomerates (if the nanofiller is particulate), bundles (if the nanofiller is fibrous), or stacks (if the nanofiller is discoidal). In most systems, their attractive interactions with each other are stronger than their interactions with the molecules constituting the dispersing medium, so that their dispersion is thermodynamically disfavored and hence extremely difficult.
Even in systems where the dispersion of the nanofillers is thermodynamically favored, it is often still very difficult to achieve because of the large kinetic barriers (activation energies) that must be surmounted. Consequently, nanofillers are very rarely easy to disperse in a polymer.
b. High Dispersion Viscosity
Another difficulty with the fabrication of nanocomposites is the fact that, once the nanofiller is dispersed in the appropriate medium (for example, an aqueous or organic medium containing the nanofiller for the “in situ” synthesis of the polymer, or a molten polymer into which nanofiller is added), the viscosity of the resulting dispersion may (and often does) become very high. When this happens, it can impede the successful execution of the fabrication process steps that must follow the dispersion of the nanofiller to complete the preparation of the nanocomposite.
Dispersion rheology is a vast area of both fundamental and applied research. It dates back to the 19th century, so that there is a vast collection of data and a good fundamental understanding of the factors controlling the viscosities of dispersions. Nonetheless, it is still at the frontiers of materials science, so that major new experimental and theoretical progress is continuing to be made. In fact, the advent of nanotechnology, and the frequent emergence of high dispersion viscosity as an obstacle to the fabrication of polymer nanocomposites, have been instrumental in advancing the state of the art in this field. Bicerano, et al. (1999) have provided a comprehensive overview which can serve as a resource for workers interested in learning more about this topic.
c. Interference with Polymerization and Network Formation
An additional potential difficulty may be encountered in systems where chemical reactions are taking place in a medium containing a nanofiller. This is the possibility that the nanofiller may have an adverse effect on the chemical reactions. As can reasonably be expected, any such adverse effects can be far more severe in systems where polymerization and network formation take place simultaneously in the presence of a nanofiller than they can in systems where preformed polymer chains are crosslinked in the presence of a nanofiller. The preparation of an S-DVB nanocomposite via suspension polymerization in a medium containing a nanofiller is an example of a process where polymerization and network formation both take place in the presence of a nanofiller. On the other hand, the vulcanization of a nanofilled rubber is a process where preformed polymer chains are crosslinked in the presence of a nanofiller.
The combined consideration of the work of Lipatov, et al. (1966,1968), Popov, et al. (1982), and Bryk, et al. (1985, 1986, 1988) helps in providing a broad perspective into the nature of the difficulties that may arise. To summarize, the presence of a filler with a high specific surface area can disrupt both polymerization and network formation in a process such as the suspension polymerization of an S-DVB copolymer nanocomposite. These outcomes can arise from the combined effects of the adsorption of initiators on the surfaces of the nanofiller particles and the interactions of the growing polymer chains with the nanofiller surfaces. Adsorption on the nanofiller surface can affect the rate of thermal decomposition of the initiator. Interactions of the growing polymer chains with the nanofiller surfaces can result both in the reduction of the mobility of growing polymer chains and in their breakage. Very strong attractions between the initiator and the nanofiller surfaces (for example, the grafting of the initiators on the nanofiller surfaces) can potentially augment all of these detrimental effects.
Taguchi, et al. (1999) provided a fascinating example of how drastically the formulation can affect the particle morphology. They described the results obtained by adding hydrophilic fine powders [nickel (Ni) of mean particle size 0.3 microns, indium oxide (In2O3) of mean particle size 0.03 microns, and magnetite (Fe3O4) of mean particle size 0.1, 0.3 or 0.9 microns] to the aqueous phase during the suspension polymerization of S-DVB. These particles had such a strong affinity to the aqueous phase that they did not even go inside the S-DVB beads. Instead, they remained entirely outside the beads. Consequently, the composite particles consisted of S-DVB beads whose surfaces were uniformly covered by a coating of inorganic powder. Furthermore, these S-DVB beads rapidly became smaller with increasing amount of powder at a fixed powder particle diameter, as well as with decreasing powder particle diameter (and hence increasing number concentration of powder particles) at a given powder weight fraction.
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