GB2488367A

GB2488367A - Ultra-violet absorbing material

Info

Publication number: GB2488367A
Application number: GB1103359.4A
Authority: GB
Inventors: Yingqian Xu
Original assignee: Energenics Europe Ltd
Current assignee: Energenics Europe Ltd
Priority date: 2011-02-28
Filing date: 2011-02-28
Publication date: 2012-08-29
Also published as: WO2012117238A1; GB201103359D0

Abstract

A UV absorbing material is disclosed which comprises particles of cerium oxide mixed with at least one other metal oxide, the other metal oxide having a band gap less than that of cerium oxide. This increases the range of UV wavelengths over which the material is an effective UV absorber. Examples of such metal oxides include iron, copper, manganese, cobalt, cadmium, bismuth, indium, vanadium, tungsten, silver, lead and palladium. The material may be used in coating formulations for protecting wood or in cosmetic products.

Description

Ultra-violet Absorbing Material

TECHNICAL FIELD

This invention relates to material for absorbing ultra-violet radiation.

BACKGROUND

Many materials, such as polymers, coatings and wood, are adversely affected by light, in particular ultra-violet (UV) light. Such materials can discolour, crack, or disintegrate under prolonged exposure to UV light. This is known as UV degradation.

One way to mitigate UV degradation is to use a UV absorber to filter incident UV light.

This may be applied as part of a coating, or in the bulk of a product. One use of UV absorbers is in coatings for wood. A coating for wood typically includes organic material, and includes one or more organic UV absorbers. However, organic UV absorbers are not always particularly effective, and degrade over time under UV exposure. A coating for wood containing one or more organic UV absorber will therefore only provide protection for a limited amount of time.

In recent years, there has been growing interest in the use of inorganic UV absorbers, particularly in the field of coatings for wood. This is because inorganic UV absorbers do not degrade over time, unlike organic UV absorbers, and so will provide longer term protection against UV radiation. Inorganic UV absorbers that have been considered include metal oxides, including ZnO, Ti02, and CeO2. These materials are semiconductors with a band gap of around 3.1 to 3.4 eV. The absorbance mechanism is that under UV illumination, a photon with a higher energy than the band gap is absorbed to create an electron hole pair.

A problem with TiO2 and ZnO is that they are highly photoactive. This causes them to generate free radicals under UV exposure, which can cause oxidation and degradation of other organic molecules in the coating. While the TiO2 particles may reduce UV degradation, the generation of free radicals still has a deleterious effect on the coating.

It is possible to coat TiO2 particles prior to using them as a UV absorber in a coating, but this increases the cost.

Furthermore, Ti02 particles have a high refractive index, which means that they are opaque. This affects the colour of a coating in which Ti02 particles are used, and so Ti02 particles are not suitable for use as a UV absorber in clear coatings.

CeO2 particles absorb UV radiation without being photoactive. This is thought to be because charge carriers recombine before they reach the surface of the particle and form free radicals. CeO2 particles therefore do not contribute to degradation of a coating, and also absorb incident UV radiation. Furthermore, a dispersion of nano-sized CeO2 particles has a higher transparency than that of Ti02 in the visible spectrum, and so CeO2 UV absorbers do not affect the colour (or transparency) of a coating.

As CeO2 is less photoactive than Ti02 and ZnO, and more transparent, it is a more suitable UV absorber for applications such as clear coatings. Some studies have attempted to improve the absorbance of CeO2 by using Ca or Zn as dopants. Figure 1 is a graph showing the absorbance against radiation wavelength of CeO2, Zn-doped CeO2 and Ca-doped CeO2. The graph was obtained using particles having a size around 20 nm in a 0.05 wt% concentration in hydrocarbon solvent.

It can be seen that all of the samples show a UV absorbance cut-off threshold at around 360 -370 nm. However, UV degradation still occurs in the wavelengths between around 360 to 400 nm, and it would be desirable to eliminate this.

Furthermore, the use of Zn and Ca as dopants reduces the effectiveness of the CeO2 at higher wavelengths.

SUMMARY

It is an object of the invention to improve the UV absorbance characteristics of inorganic particles, and in particular to improve the UV absorbance characteristics of inorganic particles at wavelengths up to 400 nm. The work leading to this invention has received funding from the European Union Seventh Framework Programme FP7/2007-201 3 under grant agreement number 246434.

According to a first aspect, there is provided a UV absorbing material comprising particles of cerium oxide mixed with at least one other metal oxide, the other metal oxide having a band gap lower than that of cerium oxide.

As an option, the other metal oxide has a band gap of between 0.1 and 3.1 eV. As a further option, the other metal oxide has a band gap between 1.0 and 3.0 eV. As a further option, the other metal oxide has a band gap between 2.0 and 3.0 eV.

The cerium oxide may be mixed with the other metal oxide in any of several ways. For example, at least some of the cerium oxide particles may be doped with the other metal oxide. At least some of the cerium oxide particles may coated with the other metal oxide. The particles may comprise at least some discrete particles of cerium oxide and discrete particles of the other metal oxide As an option, the particles have an average size in the range of 1 to 200 nm, and as a further option, the particles may have an average size in the range of 5 to 100 nm. The size of the particles can affect the UV absorbance properties.

As an option, the other metal oxide is present in the range of 0.5 to 60 mole%. As a further option, the other metal oxide is present in the range of 10 to 40 mole%.

According to a second aspect, there is provided a UV absorbing material comprising particles of cerium oxide mixed with at least one other metal oxide selected from an oxide of iron, copper, manganese, cobalt, cadmium, bismuth, indium, vanadium, tungsten, silver, lead and palladium.

As an option, at least some of the cerium oxide particles may be doped with the other metal oxide. At least some of the cerium oxide particles may be coated with the other metal oxide. The particles may include at least some discrete particles of cerium oxide and some discrete particles of the other metal oxide Optionally, the particles have an average size in the range of I to 200 nm. As a further option, the particles have an average size in the range of 5 to 100 nm.

The other metal oxide is optionally present in the range of 0.5 to 60 mole%, and may be present in the range of 10 to 40 mole%.

According to a third aspect, there is provided a UV absorbing material comprising a dispersion of the UV absorbing particles as described above in either of the first or second aspects.

The UV absorbing particles may be dispersed in a liquid.

As an option, the UV absorbing material is intended to be used in a coating formulation.

As a further option, the UV absorbing material is intended to be used in a protective coating formulation for wood. As a further option, the UV absorbing material is intended to be used in a protective, substantially clear coating formulation for wood As an option, the UV absorbing material is intended to be used in a preservative for As an alternative option, the UV absorbing particles are dispersed in a polymer.

In a further alternative option, the UV absorbing particles are dispersed in a cosmetic product such as a sunscreen.

Note that in addition to absorbing UV radiation, the particles may further protect other components of the UV absorbing material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph of UV absorbance of cerium oxide, cerium oxide mixed with calcium oxide and cerium oxide mixed with zinc oxide; Figure 2 is a graph of UV absorbance of cerium oxide, and two samples of cerium oxide mixed with iron oxide; Figure 3 is a graph of UV absorbance of cerium oxide, and cerium oxide with different concentrations of iron oxide; Figure 4 is a graph of UV absorbance of cerium oxide, cerium oxide mixed with copper oxide, cerium oxide mixed with europium oxide, cerium oxide mixed with manganese oxide and cerium oxide mixed with chromium oxide; and Figure 5 is a graph of UV absorbance of cerium oxide, cerium oxide mixed with manganese oxide and cerium oxide mixed with iron oxide.

DETAILED DESCRIPTION

Different additives to cerium oxide particles have been tested to determine their effect on UV absorbance of particles of cerium oxide. The following description refers to metal oxides being mixed with cerium oxide. "Mixed with" in this context is used to mean any of doping, coating and mixing. Doping refers to incorporation of another metal oxide in the cerium oxide crystal lattice. The doping may be interstitial or may be a direct replacement of a cerium atom in the crystal lattice with another type of metal atom. Coating refers to a type of mixing in which a cerium oxide particle has a surface coating of another type of metal oxide. The coating need not cover the entire particle.

Mixing refers to a mixture of particles containing discrete cerium oxide particles and discrete particles of the other metal oxide. In practice, particles of cerium oxide mixed with another metal oxide may contain doped particles, coated particles and discrete particles.

It has been found that some metal oxides, when mixed with cerium oxide, provide an improvement to the UV absorbance characteristics of cerium oxide. Some metal oxides can extend the UV absorbance of cerium oxide up to 400 nm, and some can extend the UV absorbance of cerium oxide without compromising the transparency of the particles, making them suitable for transparent coatings that are required to give UV protection.

In order for the particles to be effective UV absorbers, a particle size of between around I and 200 nm is required. There are many ways in which such particles can be prepared. These include precipitation, co-precipitation, homogeneous precipitation, hydrothermal methods, solvothermal methods, mechanical milling, high-energy milling, mechanochemical processing (MCP), spray pyrolysis, sol-gel methods including preparation of xerogels and aerogels, physical vapour deposition (PVD), chemical vapour deposition (CVD), and other well-known methods of manufacturing particles.

The particles of mixed cerium oxide and other metal oxide can then be used as a UV absorber. Typically they will be dispersed in another medium. For example, where used in a coating for wood they will be mixed with other substances such as a resin, a thinner and a solvent. The particles may be dispersed in a monomer, which is subsequently polymerized to form a polymer, or may be dispersed directly in the polymer itself. In other words, when making a coating, the particles may be dispersed in a monomer that is subsequently polymerized to manufacture the coating, or may be dispersed in a coating formulation after polymerization. Similarly, when adding the particles to another type of UV absorbing material, such as a bulk polymer used for packaging, the particles may be dispersed in a monomer prior to polymerization of the monomer. The particles may be dispersed in any suitable precursor during manufacture of a bulk polymer, coating, varnish and so on, or at the end of a manufacturing process when manufacturing a liquid UV absorbing material such as a coating or varnish.

In order to test the UV absorbance characteristics of the particles, they were dispersed in a hydrocarbon solvent such as Isopar L, Isopar M, Isopar P, Exssol D40, D60, D80, Solvesso 100, 150, 200 with a dispersion agent added to reduce agglomeration of the particles. The metal oxide mixtures in all cases were added to the solvent at 2.0 wt%.

Note that the particles could also be dispersed in water for testing.

While there are many ways to prepare suitable particles, the following is provided by

way of example only:

1. In order to prepare a mixture of cerium oxide and iron oxide, 0.005 mol of FeCl36H2O was dissolved in 20 ml of de-ionized water, and added to CeO2 sol containing 0.045 mol of CeO2, and stirred for 30 minutes.

2. Ammonia solution was slowly added to the de-ionized water until the mixture pH reached 9.5. The mixture was then stirred for another 30 minutes.

3. The resulting precipitation was separated and washed with water three times, and then dried to obtain a metal oxide mixture (in this example, Ceo 9Fe0 102).

4. The metal oxide mixture was then dispersed in a hydrocarbon solvent with a dispersion agent at a concentration of 0.05 wt%.

5. UV absorbance characteristics were obtained at a scan speed of 600 nm/mm and a path length of 10 mm.

Metal oxide additives with two or more common oxidation states were investigated.

These include iron, copper, europium, manganese and chromium. Iron occurs most commonly in the 2 and 3 oxidation states, copper occurs most commonly in the V and 2 oxidation states, europium occurs most commonly in the 2 and 3 oxidation states, manganese occurs most commonly in the 2+ oxidation states but can also occur in the 3, 4, 6 and P states, and chromium occurs most commonly in the 3 oxidation state but can also occur in the 6+ oxidation state as well as 1, 2, 4 and s Figure 2 shows the UV absorbance characteristics of mixtures of iron oxide with cerium oxide at a level of Ce03Fe0202. One sample was prepared using an iron (II) salt, the other with an iron (Ill) salt to determine whether the oxidation state of the precursor had any effect on the absorbance characteristics. As described above, the tested particles may contain any of cerium oxide particles doped with iron oxide, discrete particles of cerium oxide and iron oxide, cerium particles coated with iron oxide and/or iron oxide particles coated with cerium oxide. A sample of cerium oxide is also shown by way of comparison.

It can be clearly seen that the cerium oxide particles show a sharp decrease in UV absorbance after around 360 nm, and by 400 nm provide almost no UV absorbance at all. Both iron oxide treated samples, on the other hand, show a decrease in UV absorbance at a slightly higher wavelength, and still provide significant absorbance over the UV spectrum up to 400 nm. Furthermore, the presence of iron oxide did not affect the transparency of the particles in dispersion, making the particles suitable for use as a UV absorber in a transparent or translucent coating. The oxidation state of the iron salt used as a precursor did not appear to affect the absorbance characteristics of the samples. This is thought to be because the iron oxide state of both samples remains the same. This may be iron (II) oxide, iron (III) oxide or the presence of both oxidation states.

In order to assess the effect of the amount of iron oxide present, a further sample of cerium oxide mixed with iron oxide was prepared at an iron level of Ce09Fe0.102.

Figure 3 is a graph showing the absorbance of CeO2, Ce0 9Fe0 102 and Ce03Fe02O2. It can be seen that although the presence of 10 mole% iron oxide gives a great improvement over cerium oxide without any other metal oxides, the effect on UV absorbance is not as great as that observed with 20 mole% iron oxide.

Turning now to Figure 4, the UV absorbance of several other metal oxides mixed with cerium oxide is shown. The metal oxides shown here are copper, europium, manganese and chromium. For each sample, the level of metal oxide used was Ce0 9M0102, where M is selected from any of Cu, Eu, Mn and Cr.

Cerium oxide mixed with copper oxide produced a dark green powder that showed a much higher absorbance than that of cerium oxide alone, and provided good absorbance up to 400 nm. The improvement in UV absorbance was comparable to that of Ce09Fe01O2. However, Ce09Cu0102 also showed absorbance in the visible light range, making these particles unsuitable for applications where transparency or translucency is required. An advantage of using copper oxide is that it is also a fungicide. Of course, there are many applications where transparency or translucency is not required, for example where the particles might be dispersed in a bulk polymer.

Neither Ce09Eu0102 nor Ce0.9Cr0.102 showed any discernible benefits for UV absorbance.

Cerium oxide mixed with manganese oxide produced a very dark/black powder that showed a much higher absorbance than that of cerium oxide alone, and provided good absorbance up to 400 nm. The improvement in UV absorbance was better than that of Ce09Fe0102. However, Ce09Mno.102 also showed absorbance in the visible light range, making these particles unsuitable for applications where transparency or translucency is required.

Figure 5 shows a comparison of Ce09Fe0102 and Ce09Mn0102. It can be seen that the absorbance of Ce09Mn0102 is better than that of Ce09Fe0102 over the spectrum measured, but the absorbance characteristics of Ce09Mn0.102 in the visible light range make it unsuitable for use as a UV absorber in a transparent or translucent coating.

Compositions of metal oxides mixed with cerium oxide can be varied between 0.5 mole% and 60 mole% in order to achieve improvements in UV absorbance, and a range of 10 mole% to 40 mole% has been found to be most suitable. As mentioned above, a particle size of between I and 200 nm is suitable, and a range of between 5 and 100 nm has been found to be most suitable.

Although it is not intended that the invention be bound by any specific theory, it is suggested that the mechanism for the improvement in the absorbance characteristics of CeO2 mixed with other metal oxides is related to the band gap of CeO2 and the band gap of the other metal oxide. The following theory could account for the observed improvement in UV absorbance up to 400 nm: UV radiation occurs over the wavelength range of around 200 nm to 400 nm. UV radiation at higher wavelengths has a lower energy than UV at lower wavelengths.

This energy range corresponds with the energy range required to cause excitation of an electron above the band gap. The band gap is an energy range in which no electron states exist, and is typically observed in materials such as dielectrics and semiconductors. The band gap refers to the energy difference between the valence band and the conduction band. An incident UV photon can excite an electron and promote it from the valence band to the conduction band. In this way the energy of the UV photon is absorbed. In order for the UV photon to excite the electron, it must possess energy that is at least equal to the band gap.

CeO2 has a band gap of around 3.2 eV, which ensures that it absorbs UV radiation up to around 360 nm. It is believed that if CeO2 is mixed with another metal oxide that has a lower band gap, the resultant particles will be able to absorb lower energy (and hence higher wavelength) radiation, up to 400 nm.

The amount of other metal oxide affects how much the absorption is increased at higher wavelengths, as does the selection of the other metal oxide.

By way of support, it has been found that Fe, Mn and Cu all improve the absorbance of UV at higher wavelengths. The band gap of iron oxide is between 2 and 3 eV, the band gap of copper oxide is either 1.2 or 1.7 eV, depending on the oxidation state, and the band gap of manganese (IV) oxide is around 0.25 eV. All of these other metal oxides have a band gap lower than that of CeO2, and all gave rise to an improvement in the absorbance of UV by CeO2 at wavelengths up to 400 nm. Note that the band gap of each metal oxide varies according to its oxidation state.

Conversely, Ca, Zn and Cr all gave no improvement or were detrimental to absorbance of UV at wavelengths up to 400 nm. The band gap of zinc oxide is around 3.4 eV, the band gap of chromium oxide is between 4.7 and 5 eV, and the band gap of calcium oxide is around 7.1 eV. All of these other metal oxides have a band gap higher than that of CeO2, and all gave no improvement or were detrimental to the absorbance of UV by CeO2 at wavelengths up to 400 nm.

Metal oxides that have a band gap below that of CeO2 include oxides of iron, copper, manganese, cobalt, cadmium, bismuth, indium, vanadium, tungsten, silver, lead and palladium. Other suitable oxides include mercury and thallium. However, these oxides may be too costly or toxic to be used in practice.

For applications where the transparency of the resultant particles is of importance, the absorbance of photons can be carefully manipulated by selecting a metal oxide with a lower band gap than that of CeO2, and selecting the quantity of metal oxide to add to CeO2, to ensure that absorbance of UV photons is maximized whereas absorbance of visible light photons is minimized. In this case, it is important not to add too much of the additional metal oxide with a band gap lower than that of CeO2, as this could increase the absorbance at higher wavelengths, compromising the transparency of the particles. It has been found that iron oxide is particularly suitable for use as a metal oxide to mix with CeO2 where transparency is required while extending the range of wavelengths over which the particles are an effective UV absorber.

Note that where the particles are used in a dispersion in another medium, the particles can provide UV protection to the other medium. For example, where the particles are used in a clear coating for wood, the particles provide not only UV protection to the wood to which the coating is applied, but also to other components in the coating that might be susceptible to UV degradation.

Note also that the presence of cerium oxide mixed with another metal oxide in a dispersion containing organic polymers can reduce damage to the organic polymers owing to its properties as a free radical trap. The presence of free radicals can degrade organic polymers, including those which are commonly used as coatings, paints and varnishes. Cerium oxide mixed with another metal oxide has been found to act as a free radical trap, and so in addition to protecting organic components of a coating (or other medium in which the particles are dispersed) by absorbing UV radiation, the presence of the particles can also reduce degradation of organic coatings caused by free radicals. Furthermore, the UV absorbing properties of the cerium oxide mixed with another metal oxide can protect other components of a UV absorbing material in which the particles are dispersed simply by absorbing incident UV radiation before it can cause UV degradation of the other components. For example, the particles may be dispersed in a polymer packaging for medicines, in which case the particles not only protect the contents of the packaging from UV degradation, but also the packaging itself.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention as defined in the appended claims. For example, it will be appreciated that cerium oxide mixed with another metal oxide need not be used on their own as UV absorbers, but can be used with other UV absorbers or free radical traps.

Claims

CLAIMS: 1. A UV absorbing material comprising particles of cerium oxide mixed with at least one other metal oxide, the other metal oxide having a band gap lower than that of cerium oxide.
2. The UV absorbing material according to claim 1, wherein the other metal oxide has a band gap of between 0.1 and 3.1 eV.
3. The UV absorbing material according to claim 2, wherein the other metal oxide has a band gap between 1.0 and 3.0 eV.
4. The UV absorbing material according to claim 2, wherein the other metal oxide has a band gap between 2.0 and 3.0 eV.
5. The UV absorbing material according to any of claims I to 4, wherein at least some of the cerium oxide particles are doped with the other metal oxide.
6. The UV absorbing material according to any of claims I to 5, wherein at least some of the cerium oxide particles are coated with the other metal oxide.
7. The UV absorbing material according to any of claims I to 6, further comprising discrete particles of cerium oxide and discrete particles of the other metal oxide
8. The UV absorbing material according to any of claims 1 to 7, wherein the particles have an average size in the range of I to 200 nm.
9. The UV absorbing material according to claim 8, wherein the particles have an average size in the range of 5 to 100 nm.
10. The UV absorbing material according to any of claims I to 9, wherein the other metal oxide is present in the range of 0.5 to 60 mole%.
11. The UV absorbing material according to claim 10, wherein the other metal oxide is present in the range of 10 to 40 mole%.
12. A UV absorbing material comprising particles of cerium oxide mixed with at least one other metal oxide selected from an oxide of iron, copper, manganese, cobalt, cadmium, bismuth, indium, vanadium, tungsten, silver, lead and palladium.
13. The UV absorbing material according to claim 12, wherein at least some of the cerium oxide particles are doped with the other metal oxide.
14. The UV absorbing material according to claim 12 or 13, wherein at least some of the cerium oxide particles are coated with the other metal oxide.
15. The UV absorbing material according to claim 12, 13 or 14, further comprising discrete particles of cerium oxide and discrete particles of the other metal oxide
16. The UV absorbing material according to any of claims 12 to 15, wherein the particles have an average size in the range of I to 200 nm.
17. The UV absorbing material according to claim 16, wherein the particles have an average size in the range of 5 to 100 nm.
18. The UV absorbing material according to any of claims 12 to 17, wherein the other metal oxide is present in the range of 0.5 to 60 mole%.
19. The UV absorbing material according to claim 18, wherein the other metal oxide is present in the range of 10 to 40 mole%.
20. A UV absorbing material comprising a dispersion of the UV absorbing particles as claimed in any one of claims I to 19.
21. The UV absorbing material according to claim 20, wherein the UV absorbing particles are dispersed in a liquid.
22. The UV absorbing material according to claim 20 or 21, where the UV absorbing material is intended to be used in a coating formulation.
23. The UV absorbing material according to claim 22, wherein the UV absorbing material is intended to be used as a protective coating formulation for wood.
24. The UV absorbing material according to claim 22, wherein the UV absorbing material is intended to be used as a protective, substantially clear coating formulation for wood.
25. The UV absorbing material according to claim 20, wherein the UV absorbing material is intended to be used as a preservative for wood.
26. The UV absorbing material according to claim 20, wherein the UV absorbing particles are dispersed in a polymer.
27. The UV absorbing material according to claim 20, wherein the UV absorbing particles are dispersed in a cosmetic product.
28. The UV absorbing material according to any of claims 20 to 27, wherein the UV absorbing particles further protect other components of the UV absorbing material.