COMPOSITE SYSTEM COMPRISING A POLYMER MATRIX AND CORE-SHELL NANOPARTICLES, PROCESS FOR PREPARING IT AND USE THEREOF

Abstract

A polymer matrix/nanoparticle composite (PMNC) comprises core-shell nanoparticles, where the core is made of a material that is different from the polymer matrix and at least part of the shell is made of the same monomer or polymer that is used for said polymer matrix, or is made of a monomer or polymer compatible with said matrix. The core of the nanoparticles has a refractive index that is different from the refractive index of the polymer used for the matrix, at least the matrix is made of transparent materials that do not absorb light.

Claims

1. (canceled)

2. A light diffuser comprising a composite system comprising a polymer matrix that contains a plurality of nanoparticles, wherein a material used for preparing the polymer matrix is a material that is transparent; wherein the nanoparticles are core-shell nanoparticles, and a core of the nanoparticles: is made of a material that is different from the polymer matrix, that does not absorb light, and has a refractive index that is different from the refractive index of the polymer of the matrix, to thereby provide a scattering of white visible light transmitted through the diffuser so as to chromatically separate white visible light into at least two chromatic components: one in which the blue component is dominant and one in which the blue component is low; wherein the nanoparticles are uniformly and randomly distributed within the polymer matrix and the minimum distance between the surfaces of the cores of any two neighbouring nanoparticles is at least 10 nm and at least 0.2 times the nanoparticle size; and wherein the light diffuser is a Rayleigh-like diffuser in that the composite system or at least a portion of the composite system produces a haze, as defined in the ASTM Designation E284-09a, which is at least 1.5 times larger for an impinging light in the spectral interval 400-450 nm than in the interval 600-650 nm.

3. The light diffuser of claim 2, wherein the material of the core is selected from polymers and inorganic compounds.

4. The light diffuser of claim 2, wherein a shell of the nanoparticles is cross-linked.

5. The light diffuser of claim 2, wherein at least part of the shell of the nanoparticles is obtained from the same monomer or polymer that is used for the polymer matrix, or is obtained from a monomer or polymer compatible with the matrix.

6. The light diffuser of claim 2, wherein the concentration of nanoparticles is in the range of 0.001% to 20% by weight.

7. The light diffuser of claim 2, wherein the number of nanoparticles within a volume element delimited by a portion of the composite system having an area of 1 m.sup.2, is N, wherein N≥N.sub.min, and wherein: $N_{\min }=\upsilon \frac{10^{-29}}{D^6}.Math.{.Math.\frac{m^2+2}{m^2-1}.Math.}^2$ where u is a dimensional constant equal to 1 meter.sup.6, N.sub.min is expressed as a number/meter.sup.2, the effective diameter D, which is given by the nanoparticle diameter times the matrix refractive index, is expressed in meters and wherein m is equal to the ratio of the refractive index of the nanoparticle core to the refractive index of the matrix material.

8. The light diffuser of claim 2, wherein the core is made of cross-linked polystyrene, the matrix and the shell layers are made of polymethylmethacrylate (PMMA).

9. The light diffuser of claim 1, wherein the core is made of one or more of: TiO.sub.2, SiO.sub.2, ZnO, ZrO.sub.2, Fe.sub.2O.sub.3, Al.sub.2O.sub.3, Sb.sub.2SnO.sub.5, Bi.sub.2O.sub.3, and CeO.sub.2.

10. The light diffuser of claim 1, wherein the core is made of an inorganic material.

11. The light diffuser of claim 1, wherein the minimum distance between the cores of neighboring nanoparticles is at least 30 nm.

12. The light diffuser of claim 1, wherein the average dimension of the cores of the nanoparticles is in the range of 10 nanometers (nm) to 240 nm.

13. The light diffuser of claim 1, wherein the core is a linear polymer, a cross-linked polymer, or an inorganic material.

14. The light diffuser of claim 1, wherein the polymer material does not absorb light.

15. A process for preparing a light diffuser comprising a composite system configured to provide a scattering of white visible light transmitted through the composite system so as to chromatically separate white visible light into a least two chromatic components, one in which the blue component is dominant and one in which the blue component is low; the composite system comprising a polymer matrix in which a plurality of nanoparticles is dispersed, the polymer matrix being prepared from a polymer material that is transparent and is made of a material that does not absorb light, the process comprising: preparing a plurality of nanoparticle cores of a material B; providing the nanoparticle cores with at least one shell obtained starting from a monomer or a polymer to obtain a first shell comprising a polymer A thereby giving a core-shell nanoparticle, wherein the material B of the core of the nanoparticle is different from the monomer or polymer used for obtaining the matrix and has a refractive index that is different from the refractive index of the matrix; cross-linking the polymer of the first shell; providing the core-shell nanoparticle with an additional shell, the additional shell being obtained starting from a monomer or polymer to comprise a polymer that is not cross-linked; dispersing the core-shell nanoparticles into the monomer or polymer from which the matrix will be obtained, the monomer or polymer used to obtain the matrix being the same as or being compatible with the monomer or polymer used to obtain the additional shell, so as to obtain a dispersion of the nanoparticles in the matrix before polymerization; and polymerizing the matrix; wherein the light diffuser is a Rayleigh-like diffuser in that the composite system or at least a portion of the composite system produces a haze, as defined in the ASTM Designation E284-09a, which is at least 1.5 times larger for an impinging light in the spectral interval 400-450 nm than in the interval 600-650 nm.

16. The process of claim 15, further comprising cross-linking the matrix.

17. The process of claim 15, wherein the matrix is a paint comprising polymer resins and at least a solvent, and wherein the scattering is provided by the composite system after the solvent has evaporated from a layer of paint.

18. The process of claim 15, further comprising cross-linking the core of the nanoparticles.

19. The process of claim 15, further comprising: preparing a core comprising at least one inorganic nanoparticle and a monomer; and polymerizing the monomer to provide a core comprising inorganic nanoparticles and a polymer.

20. The process of claim 15, further comprising: preparing core-free nanoparticles from the monomer or polymer that is used to prepare polymer A, wherein the nanoparticles do not comprise the core material B; and mixing the core-free nanoparticles with an amount of nanoparticles having a core, whereby the core-free nanoparticles provide a starting material for the matrix.

21. The process of claim 15, wherein the nanoparticles have average dimensions in the range of 10 nanometers (nm) to 240 nm.

Description

EXAMPLE 1—PREPARATION OF PTFE-CORE NANOPARTICLES

[0065] Polymerization was carried out in a glass reactor equipped with a reflux condenser, magnetic stirrer (200-300 rpm), nitrogen inlet, and a water jacket for temperature control. The formulation is given in Table 1. In particular, the initial reactor charge was purged with nitrogen to remove dissolved oxygen while heated, followed by addition of a part of the initiator solution. The monomer mixture and the remaining initiator solution were then fed to the reactor over a prescribed period of time (e.g., 3.5 hours) by a pump, respectively.

[0066] The reactor temperature was kept at 80±2° C. during the polymerization. At the end of the monomer feeding, the reaction system was maintained at 85° C. for 1 hour to complete the monomer conversion. Then, the system was cooled down to 40° C., and its pH was adjusted to 7. The final latex was filtered with a filter of 25 micron openings to remove any possible coagulum formed during the polymerization.

TABLE-US-00001 TABLE 1 Typical recipe for emulsion polymerization of PTFE-MMA/BA particles for sky-sun diffusers. Chemicals Weight (g) Note Monomer feed MMA 50-100 The MMA/BA BA 0-50 ratio depends Initial reactor H.sub.2O 125 on Tg charge NaHCO.sub.3 0.3 Surfactant (K30) 0.5 PTFE latex  30 (ϕ.sub.wt = 33%) Initiator solution KPS 0.35 H.sub.2O 24 Total 275 (ϕ.sub.wt = 40%)

EXAMPLE 2—PREPARATION OF POLYSTYRENE-CORE NANOPARTICLES

[0067] Cross linked polystyrene latex with nanoparticle size of 65 nm (diameter measured by dynamic light scattering) was used for seeded emulsion polymerization. 1650 g polystyrene latex, 800 g water, 6 g SDS (surfactant, sodium dodecyl sulfate) and 1.6 g KPS, (potassium persulfate, initiator) were charged in the reactor under mechanical stirring. Then 285 g of MMA mixed with 15 g of cross linker (5% w/w of cross linker DTTA, di-trimethylolpropane-tetraacrylate) was slowly added to the reactor within 3 hours at 80° C. The system was kept at 80° C. for another 2.5 hours to finish the first shell.

[0068] Then, 300 g MMA and 1.4 g KPS were added and fed at 80° C. during two hours; the mixture was heated for another 1 hour to finish the reaction and was then cooled to room temperature. The average size of the nanoparticles was found to be 90 nm, with narrow distribution, and the solid content was about 28% by weight. After polymerization, the latex was mixed with an ion exchange resin and stirred for 2 hours to remove the surfactant.

EXAMPLE 3—PREPARATION OF INORGANIC CORES FOR CORE-SHELL NANOPARTICLES. THE FOLLOWING STEPS WERE CARRIED OUT

[0069] 1. Synthesizing TiO.sub.2 particles in organic solvent under relatively low temperature. In a typical process, precursor TiCl.sub.4 is dropwise added into ethanol. After the heat production is released completely, the mixture is poured into pre-heated benzyl alcohol. The system is maintained by stirring and heating for more than 8.5 hours. When the hydrolysis process finishes, TiO.sub.2 nanocrystallized particles are thoroughly precipitated by adding ether, followed by centrifugation and re-dispersion in ethanol.

[0070] 2. Hydrophobic modification of TiO.sub.2.

[0071] TiO.sub.2 primary particles form nanoclusters in ethanol. To make them compatible with organic solvents, silane coupling agent is added and chemically attaches to the surface of the clusters. Excess silane is removed by centrifuging. The treated nanoclusters are then used to prepare core-shell nanoparticles.

[0072] 3. Preparation of Core-Double-Shell Structure

[0073] The modified TiO.sub.2 nanoclusters are well dispersed in a mixture of monomer (e.g., methyl methacrylate or styrene) and cross linker (e.g., di-trimethylolpropane tetraacrylate or divinylbenzene). The dispersion is dropwise added, under stirring and N.sub.2, to an aqueous solution of the steric surfactant, forming a homogeneous mixture with the assistance of sonication or mechanical separation. After nitrogen purging and mechanical stirring, an aqueous solution of the initiator (potassium persulfate) is introduced into the system and the first emulsion polymerization is carried out, while the N.sub.2 bubbling and stirring are still maintained.

[0074] After the first layer has been polymerized and cross-linked, the second linear shell is provided on the external surface of the nanoparticles. For this second linear shell, a monomer is fed continuously at a low feeding rate, and corresponding amount of initiator is added, without using a cross-linking agent. After the second polymerization process is completed core-shell nanoparticles of the invention are obtained from aqueous phase by freezing.

[0075] There are two main processes for post-treating the nanoparticles obtained according to the invention: direct drying and melting for preparing a film, and coagulation and drying for preparing a PMNC system by redispersion in a monomer.

[0076] Technique A: Direct Drying and Melting

[0077] If the PMNCs are required in the form of films, the typical techniques for making films from polymer latexes can be used. In particular, the obtained core-shell nanoparticles latex can be dried directly to eliminate water, and then the temperature is increased to above the Tg (glass transition temperature) of the polymer constituting the shell, leading to the formation of the PMNCs in the form of films.

[0078] Technique B: Powder Moulding

[0079] This technique includes two steps:

[0080] Step 1: Coagulation of the Core-Shell Particles

[0081] Coagulation or aggregation methods are used to separate the core-shell nanoparticles from the disperse medium. A coagulation process leads the core-shell nanoparticles to form clusters or aggregates with sizes from at least a few tens of microns to hundreds of microns or even to millimetres, thus easy to be separated from the disperse medium by any standard techniques such as filtration, floatation, sedimentation, centrifugation, etc.

[0082] Due to the advantage of the core-shell structure, within the dried clusters or aggregates or powders, the minimum distance among the nanoparticles is maintained by the designed thickness of the shell. To ensure the uniform and random distribution of the nanoparticles within the clusters, coagulation under shear is preferred, since it forms compact clusters with randomly distributed particles. In particular, three types of coagulation are preferred:

[0083] a) Coagulation of latexes in mechanically stirred tanks with addition of a proper amount of electrolytes. The electrolytes can be chosen amongst any salts or base or acid. The use of the electrolytes is required to partially or completely eliminate the electrostatic repulsive interactions among the particles so as to ease the coagulation.

[0084] b) Coagulation of latexes in intense shear flow without making use of electrolytes. In this case, the energy generated by the intense shear flow should be high enough, capable of forcing the particles to overcome the interaction barrier, leading to aggregation. Typical processes that are able to generate so high energies are, for example, forcing the latexes to pass through a microchannel, as described in the open literature (Wu H, Zaccone A, Tsoutsoura A, Lattuada M, Morbidelli M. High shear-induced gelation of charge-stabilized colloids in a microchannel without adding electrolytes. Langmuir. 2009; 25:4715-23).

[0085] Further, if there are specific requirements for the nanoparticle concentrations within the PMNCs, which cannot be satisfied only by the produced thickness of the shell, then, particles of the same materials as the shell (i.e., the same as the polymer matrix), but without the nanoparticle core, are produced using the same emulsion polymerization technique. The obtained latex where the particles do not contain the nanoparticle core will be mixed with the latex where the particles contain the nanoparticle core, in proper ratios based on the requirements in the nanoparticle concentration. The obtained latex mixtures are then coagulated using the techniques described above, so as to produce the dried powders.

[0086] c) Coagulation by freezing.

[0087] The latex of example 2 was stored in a freezer at −18° C. After defreezing the latex, the mixture was centrifuged. The solid wet powder was recovered and dried.

[0088] Step 2: Preparation of the composite.

[0089] The nanoparticle powder thus obtained is then used for preparing the required matrix composite. After having obtained the dried powders where the nanoparticles are distributed, with the required minimum distance among the nanoparticles core being ensured by the presence of the shell, various standard techniques can be used to easily produce the desired, different forms of the PMNCs.

[0090] A preferred technique is bulk polymerization, in which the nanoparticles powder is weighed and dissolved in the monomer of the matrix, before crosslinking it. Re-dispersion is carried out so as to obtain a uniform dispersion of the nanoparticles, that is ensured thanks to the presence of one or, preferably, two layers of the shell; proper agitation and/or use of ultrasonic energy may be advantageous.

[0091] For example, the powder obtained after freezing and drying the nanoparticles of example 2 was weighed and dissolved in MMA. The transparent dispersion was sonicated for 2 hours to get the nanoparticles well dispersed in MMA. A standard bulk polymerization technique was used to convert the monomers into the polymer matrix, in which the nanoparticles are homogeneously distributed, leading to the required PMNCs.

[0092] Other techniques may be used, such as injection moulding, reaction injection moulding, compression moulding, transfer moulding, extrusion moulding, rotomoulding, blow moulding, calendering, knife coating, etc. This possibility of having a starting material that can undergo many different treatments is one of the key advantages of the present invention.

[0093] In another embodiment, nanoparticles having a core with the required refractive index, with or without a polymer, and a non cross-linkedshell, are dispersed in a matrix and sonicated until they reach the required uniform dispersion. The matrix is then polymerized and optionally cross-linked.