GB2241179A

GB2241179A - Coated glass microbeads and pigment for synthetic polymeric material

Info

Publication number: GB2241179A
Application number: GB9103587A
Authority: GB
Inventors: Pierre Laroche
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1990-02-21
Filing date: 1991-02-20
Publication date: 1991-08-28
Also published as: IT1244630B; PT96806B; DK30291A; CA2035692A1; FR2658502B1; FR2658502A1; PT96806A; SE9100504D0; SE9100504L; ITTO910068A1; JPH04216835A; BE1005673A5; NL9100306A; ITTO910068A0; ES2024377A6; LU87682A1; DE4105319A1; GB9103587D0; DK30291D0; ZA911240B

Abstract

The invention relates to glass microbeads having a coating comprising particles of metal compound of refractive index greater than 2 distributed in a binding polymer which provides the microbeads with a coloured external surface and to a synthetic polymeric material incorporating such microbeads; a pigment for synthetic polymeric material comprising such microbeads, and to microbeads for retroreflective paint.

Description

1 1 Coated glass microbeads and 12ig=nt for synthetic polymeric material

The present invention relates to coated glass microbeads, to a synthetic polymeric material incorporating such coated microbeads and to a pigment for synthetic polymeric material. The invention also extends to coated microbeads for retroreflective paint.

It is well known to incorporate various solids into a synthetic polymeric material for different purposes. For example, glass beads can be used as filler for modifying the mechanical properties of the filled polymer, such as its flexibility or its resistance to traction, or to facilitate its application. This incorporation is usually carried out by mixing the beads with a liquid which is polymerizable or contains the polymeric material. It is also known to use coated glass microbeads, the coating being selected so as to adapt the surface properties of the microbeads, such as for example their chemical bonding properties or their surface tension properties with respect to the liquid.

Furthermore, the colouration of polymeric materials is frequently obtained with the aid of organic or inorganic pigments. Among the different types of possible pigments, metal compounds are often used, being chosen because of their stability to heat and light and their chemical stability. These pigments can be incorporated in the form of an insoluble powder consisting of particles of metal compound which is mixed into the material which is polymerizable or contains the polymer, while it is in the liquid state. Certain difficulties are encountered in distributing these particles as regularly as is necessary. To obtain a uniform colour clearly necessitates a regular and homogeneous distribution of the pigment particles in the material. Such a distribution is not easy to obtain, it depends notably on the viscosity characteristics and dispersant power of the liquid material at the time of incorporating the pigment. It is necessary to avoid agglomerating the insoluble particles and obtaining different levels of colouration across the mass of 4083 2 polymer. Furthermore, since the pigments are quite expensive, it is preferable to use small amounts.

A pre-dispersion of the pigment particles in a resin can also be carried out, so as to form a "colouring concentrate% which can then be distributed in the liquid material. However, the operation of distribution remains a difficult operation and this pre-dispersion implies an additional manufacturing step.

It is also common to incorporate glass beads in marking paints to provide them with retroreflective properties for visible light. Such paints, used for example for road markings, can be seen easily at night because they reflect light from the headlights of a vehicle back towards vehicle's driver. To obtain a retroreflection effect requires that at least a part of the beads are exposed at the surface of the paint, and that the glass and the paint have refractive indices sufficiently different for the light to be reflected at the interface between these two media. Conventionally, the glass beads are incorporated in a liquid material consisting of the fresh paint. The degree of sinking into the paint can easily be controlled by using microbeads bearing a coating which adjusts their characteristics of wettability by the solvent of the paint.

The reflection of light at the glass-paint interface is obtained by incorporating pigments in the paint. These pigments, generally formed of particles of a metal compound of high refractive index, must again be properly distributed in the paint.

Certain problems analogous to those described in the context of synthetic polymeric materials can appear for retroreflective paints: the pigments used are quite expensive and it is not always easy to distribute the insoluble particles in the paint as regularly as one would wish.

One of the objects of the present invention is to provide coated microbeads which contribute to the colouration of the material in which they are incorporated, which facilitate good pigment distribution in the material and which enable the quantity of pigment required to colour this material to be reduced to a certain degree.

1 i __I 1 i j 1 4083 3 The present invention relates to coated glass microbeads, characterized in that the coating comprises particles of metal compound of refractive index greater than 2 distributed in a binding polymer and provides the coated microbeads with a coloured external surface.

Because of their shape and size, it is generally much easier to distribute the coated microbeads in a liquid or pasty material than to distribute the very fine particles of pigment, which tend to agglomerate and are difficult to distribute regularly in such a material. Since, according to the present invention, the external surface of the coated microbeads is coloured by the particles of metal compound, a good distribution of the microbeads in a liquid or pasty material leads to a good distribution of these particles in this material and preserves it while this material hardens and solidifies. The colouration is therefore carried out more easily and more regularly in the solid material.

Moreover, it does not require any separate operation of pigment incorporation, and this enables the number of steps necessary to obtain a coloured article filled with microbeads to be reduced.

The invention also enables the quantity of pigment incorporated in the material and the colour obtained to be more easily controlled.

The quantities of pigment associated with the coated microbeads can easily be adapted by modifying the proportion of pigment distributed in the binding polymer. It is consequently easy to adjust the colouration of the material by adapting the quantities of coated microbeads incorporated and the proportion of particles in their coating.

The characteristics of the coated microbeads according to the present invention enable the quantifies of pigment to be limited, and it is even possible to use smaller quantities of pigments than are necessary when microbeads and pigment particles are added separately.

We have discovered that such coated microbeads can be incorporated in a marking paint and bring about an effect of retroreflection of visible light. This is quite surprising since the coated microbeads alone have a dull and opaque appearance. Nevertheless, 4083 4 these coated microbeads incorporated in the paint produce during use a very effective retroreflective effect.

As compared with a synthetic polymeric material, where one attempts generally to distribute a pigment as uniformly as possible in the polymeric mass, in the case of retroreflective paints, it is necessary above all to obtain pigmented particles in sufficient quantity at the microbeads-paint interfaces to permit good retroreflection. The present invention enables these particles to be very well placed, since they are present at the very surface of the retroreflective microbeads. Moreover this characteristic also enables the quantity of pigment necessary to be reduced.

Quite surprisingly, despite the high density of a metal compound in relation to that of a binding polymer, inasmuch as the metal compound is in the form of particles whose size enables the overall sphericity of the coated microbeads to be preserved, the particles of metal compound distributed in the binding polymer appear to cause little or no modification of the wettability characteristics of the coated microbeads, by comparison with coated microbeads which are otherwise similar but do not contain such particles. This characteristic is particularly advantageous since, provided that the binding polymer is judiciously chosen, coated microbeads can be obtained which will sink more or less strongly in the material in which they are incorporated.

For example, coated microbeads can be obtained which, when introduced into a fresh paint, float at the surface of the paint and will therefore be well exposed to cause retroreflection to appear.

It has been found that a certain number of metal compounds can be used to colour the external surface of the coated microbeads. Among these, particles of zinc oxide, for example, can be used to obtain a white colouration, particles of lead chromate or of cadmium sulphide to obtain a yellow colouration, or again particles of chromium (III) oxide to obtain a green colouration. It is especially preferred to colour the external surface by particles of titanium dioxide. Titanium 11 A i i i 1 i 1 i i 4083 dioxide, which provides an excellent whiteness, has a refractive index of 2.9. It opacifies the synthetic polymeric materials very well and permits good retroreflection in marking paints.

In the preferred ernbodiments of the invention the coating comprises a coupling agent.. Coupling agents are useful for binding organic materials to inorganic materials. A coupling agent, by fom-ling a bond between the glass and the binding polymer, enables the binding polymer to be attached easily to the glass beads and favours good adhesion of the coating to the glass beads. This better cohesion facilitates storagle, of the coated microbeads, and their handling and subsequent incorporation in a synthetic polymeric material or in a paint without risk of damaging them. A coupling agent can also couple the particles of the metal compound and the binding polymer; a coupling agent can be chosen which increases the wettability of the particles of the metal compound by the binding polymer and reinforces the adhesion between the particles and the binding polymer.

Organometal compounds, such as silanes or organic titanates are particularly effective as coupling agents. Such coupling agents which can be quoted as examples are aminopropylsilanes, methacryloxysilanes, and isopropyltitanates.

Preferably the coating comprises a surfactant. A surfactant enables the dispersion of the particles of metal compound in the material forming or containing the binding polymer to be facilitated: the particles are dispersed more easily and more regularly. The distribution of the particles in the binding polymer can thus be improved.

Among the different types of surfactants, the titanates, for example, can be used, but a silanated surfactant will preferably be chosen. It is possible to select a silane which acts at the same time both as coupling agent and as surfactant. The number of constituents present in the coating is thus reduced, while simultaneously ensuring two different functions.

6 The coating preferably comprises a thixotropic agent. In this way the flow (during agitation) of the liquid material containing the particles of metal compound is favoured, which permits the easy application of this material to the glass beads without risk of premature hardening.

Thixotropic agents such as ethylcellulose or organophilic clays, for example, can be used.

It is advantageous to use particles of the metal compound with a mean diameter less than 1 gm, and their diameter is preferably between 0.1 and 0.7 gm. Particles of small size enable the overall sphericity of the coated microbeads to be well preserved. Moreover, by comparison with particles of greater size, which can tend to sink in the material forming or containing the binding polymer during their dispersion, particles of small size disperse easily and regularly. They enable a coating of very homogeneous composition to be obtained and 15 an approximately uniformly coloured external surface to be provided on the coated microbeads. On the other hand, it is preferable to avoid using particles whose size is small in relation to the wavelengths of visible light because they can produce a harmful diffusing effect.

Preferably the mean diameter of the glass beads will lie between 100 gm and 2 mm. In general, the size of the glass beads influences the properties of the material in which they are incorporated, and their mean diameter will be chosen as a function of the type of article to be produced and of its field of use. For example, microbeads incorporated as fillers in synthetic polymeric materials are generally smaller than the microbeads incorporated in a retroreflective paint.

Furthermore, the size of the glass beads influences the properties of the coated microbeads according to the present invention, and their mean diameter will be chosen as a function of various criteria.

Since the specific surface area of beads of small diameter is proportionately greater than that of glass beads of greater diameter, weight for weight, a proportionately greater amount of particles of metal compound will become attached to the smaller particles when 1 i i i j 1 i i i i j 1 1 i 1 1 4083 7 applying a coating of given composition to a given thickness. The intensity of the colouration imparted to the beads will depend on the amount of metal compound applied, but since the particles of metal compound are generally fairly expensive, it is usually desired to use them in amounts which are as small as possible. The mean diameter of the glass beads may be chosen according to the desired intensity of colouration, having regard to the cost of the metal compound.

The particle size distribution of the glass beads also influences the properties of the material in which they are incorporated.

It is preferred in general that the glass beads have a quite narrow particle size distribution so as easily to obtain a good distribution of them in the polymeric material or paint: coated microbeads well distributed in a synthetic polymeric material enable a colouration uniformly distributed through the mass to be ensured and, incorporated in a marking paint, they will show a similar degree of sinkage, enabling a high intensity of retroreflection to be obtained if they are exposed at the surface of the paint.

The thickness of the coating is preferably between 0.5 and 4 gm. Coatings whose thickness lies between these values are sufficiently thick to permit a good distribution of the particles of the metal compound in the binding polymer. On the other hand they permit small quantities of metal compound to be used and the size and shape of the glass beads to be preserved.

We have found that many polymers can play the part of the binding polymer, provided that the material forming or containing this binding polymer is presented in the form of a hardenable liquid. Thus the particles of metal compound can be dispersed in liquid material, the material applied to the glass beads, and coated microbeads with coloured external surface formed by hardening the material containing the particles of metal compound.

It is especially convenient to use a thermo-setting resin.

Thermo-setting resin binder coatings tend to be more mechanically 4083 8 resistant than other types of coating in view, and the coated beads are thus more able to withstand handling prior to their incorporation into a polymeric matrix, and they also tend to be better able to withstand the temperatures to which they will be subjected during formation or 5 moulding of such a matrix.

Preferably the binding polymer is selected from silicones, polyurethanes, polyesters, acrylic polymers or epoxy resins. Thbse polymers meet the condition described above: they permit a good distribution of the particles of metal compound, harden easily, for example by polymerization with the aid -of a catalyst, by solvent evaporation or by addition of a hardening agent, and adhere well to the glass.

The binding polymer can be chosen to obtain the surface tension which it is desired to give the coated microbeads. For example a silicone binder enables coated microbeads to be obtained which float in a fresh paint and after drying produce coated microbeads which, while adhering suitably to the paint, are well exposed to provide the retroreflection. One can also introduce to the paint at the same time other coated microbeads, whose surface comprises a binding polymer which confers different surface properties, for example if it is desired that the coated microbeads distribute themselves through the whole thickness of the paint.

Advantageously, the particles of metal compound are distributed in the binding polymer in such a way that the external surface of the coated microbeads comprises particles of metal compound of refractive index greater than 2, surrounded by binding polymer. The particles of metal compound thus distributed can provide a well coloured external surface without for all that using large quantities of particles of metal compound. Moreover, such a distribution enables a very good bonding of the coated microbeads to the synthetic polymeric materials or to the paints to be obtained: by comparison with coated microbeads whose external surface would be entirely covered with particles, a surface t 1 i i 4083 9 containing binding polymer will bond better to a synthetic polymeric material or a paint and will enable filled articles having a high cohesion to be formed.

The bonding of the coated microbeads can be adjusted by selecting the binding polymer as a function of the material in which the microbeads are incorporated. The binding polymer will preferably be of the same nature as this material, in order to favour optimum compatibility. However, a binding polymer different from the polymer incorporating the microbeads can be chosen. For example, a polymer can be chosen which, while being well compatible with the synthetic polymeric material or with the paint as the case may be, provides the coated microbeads with hydrophobic properties and/or modifies the wettability characteristics of the microbeads.

Preferably the coated microbeads are hydrophobic. For this reason they tend not to absorb atmospheric moisture which could otherwise cause the agglomeration of the coated microbeads and the loss of their fluidity properties, notably during the storage prior to their incorporation or during their handling. Moreover, the accumulation of moisture at the surface of the coated microbeads can harm the cohesion between the microbeads and the material in which they are incorporated.

If it is desired to manufacture an article of low density, hollow beads can be used. However, it is generally preferred to use solid beads because of their greater mechanical strength.

It is believed that a synthetic polymeric material incorporating coated microbeads possessing one or more of the characteristics mentioned above has new features, and the invention consequently extends to a synthetic polymeric material incorporating coated microbeads, characterized in that some at least of the microbeads are coated glass microbeads having one or more of the characteristics described above.

The present invention also includes a pigment for synthetic polymeric material characterized in that it comprises coated glass 4083 microbeads whose coating comprises particles of metal compound of refractive index greater than 2 distributed in a binding polymer and which provides the microbeads with a coloured external surface.

Such a pigment is very advantageous because it is generally much easier to distribute in a liquid or pasty material than a pigment consisting of particles of metal compound and it enables a uniformly coloured article to be obtained very easily after hardening the liquid or pasty material. Moreover this pigment enables the quantities of particles of the metal compound to be limited and even, in certain cases, smaller quantities of particles to be used than those required when using a pigment in the form of particles of metal compound, while obtaining a similar colouration.

Preferably the pigment comprises coated microbeads presenting one or more of the characteristics described above.

The invention extends also to microbeads for retroreflective paint, characterized in that they comprise coated glass microbeads whose coating comprises particles of metal compound of refractive index greater than 2 distributed in a binding polymer and which provides the coated microbeads with a coloured external surface.

Quite surprisingly in view of their dull and opaque appearance, these coated microbeads are very suitable for providing retroreflective properties to paints in which they are incorporated.

Localization of the pigment particles on the surface of the retroreflective microbeads also presents the additional advantage of enabling smaller quantities of pigment than before to be used. Now, by the conventional method, the pigment particles must be distributed as regularly as possible through the whole of the paint so as to ensure that the glass beads incorporated afterwards should be sufficiently in contact with them to permit retroreflection; whereas, according to the present invention, the coated microbeads possess directly on their external surface the particles of metal compound required for retroreflection.

The particles of metal compound have quite a high density yet, i 1 1 k 4083 i 11 surprisingly, they do not appear appreciably to modify the wettability characteristics of the coated microbeads by comparison with otherwise similar coated microbeads which do not contain these particles. The binding polymer can consequently be chosen as a function of the desired sinkage of the microbeads in the paint. This enables the production of coated microbeads which remain on the surface of a paint or which sink into the latter, or again the use of a mixture of coated microbeads of different wettabilities so as to distribute the microbeads through the thickness of the paint.

Preferably the microbeads for retroreflective paint comprise coated microbeads presenting one or more of the characteristics described above.

Various preferred embodiments will now be described by way of example.

EXAMPLE 1

Glass microbeads intended for introduction into a road marking paint are manufactured. The glass beads are solid and have a diameter between 250 and 650 gm.

Particles of titanium oxide of mean diameter 0.5 gm are mixed little by little with a solution of ethyl silicate VP 2262 (from Wacker) in a proportion of 200 g of titanium dioxide to 100 ML of solution. A small amount of toluene is added to adjust the viscosity. 40 mL of this mixture are removed in order to treat 1 kg of glass beads. The mixture is poured onto the glass beads. The whole is mixed with agitation, and 25 then dried at WC. The ethyl silicate hardens by hydrolysis and condensation and microbeads with a siliconized coating are obtained. The coated microbeads obtained have an external surface of a fairly dull white. They are hydrophobic and the thickness of their coating is about 1 Km. 30 The coated microbeads are incorporated in a marking paint of the epoxy resin type, and remain well exposed in the surface. They produce a very satisfactory retroreflection.

4083 12 As a variant of this example, the whiteness of the coated microbeads obtained is increased by adding a small quantity of cobalt stannate in the mixture containing titanium dioxide. These coated microbeads are incorporated in a vinyl paint.

EXAMPLE 2

Coated glass microbeads with yellow external surface, intended to colour and fill a synthetic polymeric material, are made.

250 g of lead chromate particles with diameters of about 0.7 gm are slowly mixed into 100 mL of polyester resin of the Palatal P51 type (of B.A.S.F.), to which 0.5 weight % of methacryloxypropyltriethoxy- silane have been added in advance. After homogenization, tert-butyl peroxide is added as polymerization catalyst. The pasty mixture obtained is dispersed by means of a fast agitator on glass microbeads whose diameter is between 20 and 60 gm. The whole is brought to 14WC over several minutes to harden the coating which is about 2 gm thick.

The coated microbeads formed are sieved and then incorporated in polypropylene at a concentration of 40 weight %. The polypropylene thus filled presents a yellow colour.

EXAMPLE 3

2 weight % of N-[2-(vinylbenzylaniino)-ethyll-3-aminopropyltrimethoxysilane are dissolved in 100 mL of toluene. Then 100 g of titanium dioxide particles with diameters between 0.1 gm and 0.7 gm are added with agitation. When dispersion is assured, 500 g of Plexilith 402 acrylic resin (of R6hrn) and 1 weight % of Plexilith 492 are added.

mL of this mixture are used to treat 1 kg of beads of mean diameter 400 gm and form coatings thereon about 3 gm in thickness. At the end of the operation, a polymerization catalyst such as benzoyl peroxide is added. The polymerization lasts for about 30 minutes. The coated microbeads are subsequently dried and sieved.

The whiteness of these coated microbeads is evaluated by j! 1 1 i i i 1 i 1 1 1 4083 d 1 1 13 measuring their luminance (L). The luminance value is 83 for the coated microbeads according to the example, whereas it is 90 for titanium oxide alone and 63.4 for untreated glass beads.

E2 4 (úx M.) An epoxy resin Araldite/GY260, having an initial viscosity of 12000 to 16000 mPa.s, is diluted with toluene in the proportion of 50 mL toluene to 100 g resin, so as to reduce the viscosity to between 1500 and 1600 mPa.s. Ihen an organophilic clay is added to make the solution thixotropic. After homogenization, 150 g of cadmium sulphide particles with diameters of about 0.5 gm are incorporated with vigorous agitation, then 0.5 weight % of isopropyltriisostearoyl titanate, and then a polyamine hardener.

g of this solution are removed to treat 1 kg of glass beads of mean diameter about 60 ptm. The solvent is evaporated under vacuum and the resin hardened rapidly by heating at 1OWC to form a coating about 1 gm in thickness.

The coated microbeads have a yellow external surface and can be incorporated in a paint or in a synthetic polymeric material.

In a variant, coated microbeads of similar appearance are obtained by using a polyurethane as binding polymer.

4M3 1 14

Claims

1. Coated glass microbeads, characterized in that the coating comprises particles of metal compound with refractive index greater than 2 distributed in a binding polymer and provides the coated microbeads with a coloured external surface.

2. Coated glass microbeads according to claim 1, wherein the particles of metal compound are particles of titanium dioxide.

3. Coated glass microbeads according to claim 1 or 2, wherein the coating comprises a coupling agent.

4. Coated glass microbeads according to one of claims 1 to 3, 10 wherein the coating comprises a surfactant.

1 gm.

5. Coated glass microbeads according to one of claims 1 to 4, wherein the coating comprises a thixotropic agent.

6. Coated glass microbeads according to one of claims 1 to 5, wherein the particles of metal compound have a mean diameter below

7. Coated glass microbeads according to one of claims 1 to 6, wherein the particles of metal compound have a diameter between 0.1 and 0.7 pm.

8. Coated glass microbeads according to one of claims 1 to 7, wherein the mean diameter of the glass microbeads lies between gm and 2 mm.

9. Coated glass microbeads according to one of claims 1 to 8, wherein the thickness of the coating lies between 0.5 and 4 gm.

10. Coated glass microbeads according to one of claims 1 to 9, wherein the binding polymer is selected from silicones, polyurethanes, polyesters, acrylic polymers or epoxy resins.

11. Coated glass microbeads according to one of claims 1 to 10, wherein their external surface comprises particles of metal compound of refractive index greater than 2 surrounded by binding polymer.

12. Coated glass microbeads according to one of claims 1 to 11, wherein they are hydrophobic.

i i 1 j 4+100D

13. Coated glass microbeads according to one of claims 1 to 12, wherein the glass microbeads are solid.

14. Synthetic polymeric material incorporating microbeads, wherein some at least of the microbeads are coated glass microbeads 5 according to one of claims 1 to 13.

15. Pigment for synthetic polymeric material characterized in that it comprises coated glass microbeads whose coating comprises particles of metal compound of refractive index greater than 2 distributed in a binding polymer and which provides the coated microbeads with a coloured external surface.

16. Pigment for synthetic polymeric material according to claim 15, wherein it comprises coated glass microbeads according to one of claims 2 to 13.

17. Microbeads for retrorefiective paint, characterized in that they comprise coated glass microbeads whose coating comprises particles of metal compound of refractive index greater than 2 distributed in a binding polymer and which provides the coated microbeads with a coloured external surface.

18. Microbeads for retroreflective paint according to claim 17, wherein they comprise coated glass microbeads according to one of claims 2 to 13.

Published 3991 at The Patent Office. Concept House. Cardifr Road. Newport. Gwent NP9 1RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. CAmfelinfach. Cross Keys. Newport. NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.