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Aako BV
P.O. Box 205
3830 AE Leusden
The Netherlands
KvK 31009627

Reversacol

Application
Paints, Eyewear, Inks, Plastics, Brand protection, Ophthalmic lenses
Product type
Photochromic colorants

Vivimed Reversacol™ Photochromic Dyes

Vivimed is a world leader in the development of innovative photochromic dyes. Under the Reversacol trade name, we manufacture and market the most comprehensive range of patented, high performance dyes available in the world.

We have 30 vibrant colours in our dye portfolio, including a unique selection of single-molecule, neutral grey photochromic dyes. For these dyes, we were awarded the prestigious Queen's Award for Innovation in 2006, one of the UK Industry's highest accolades.

The company's long heritage in speciality colour chemistry reaches back over 100 years to the birth of the textile dye chemicals industry in the north of England. We were at the forefront of innovation then, and we remain there today. Our research and development expertise ensures that we bring cutting edge chemistry to all our customers.

Reversacol dyes are sold separately as fine crystalline powders, allowing the user to formulate bespoke recipes according to their own matrix and performance criteria. Our expert team of technical service chemists are available to assist with formulation support for a wide variety of uses.

Applications

Lenses: The main application for Reversacol Photochromic dyes is in prescription ophthalmic lenses that darken in bright sunlight. However, Reversacol dyes can also be used to provide a photochromic effect for sunglasses, ski goggles, and visors for motorcycle helmets.

Plastics: Incorporation into plastics for extrusion and film laminates, allows the use of Reversacol dyes in a wide range of fabricated items, including packaging, plastic toys and novelty items.

Inks & Coatings: Reversacol Photochromic dyes can be incorporated into solvent-based ink and overprint varnishes as well as coating systems for use in gravure and flexo printing, screen print for T-shirts, and nail varnish.

Authentication: As part of a covert or overt security marking or brand-protection system, Reversacol Photochromic dyes can allow a colour change effect when irradiated by a specific wavelength UV.

How do Reversacol™ Photochromic Dyes work?

Under the influence of UV light, a photochromic molecule will change shape, opening up from a twisted figure-8 type structure into a brightly coloured planar form; the open form is a very effective absorber of visible light. This colour change is a reversible equilibrium, when the source of radiation is removed the molecule will revert back to its unactivated or ‘resting’ state.

The rate at which the dyes fade back to their colourless form is dependant on ambient temperature and the chemical structure of the dye.

Vivimed Reversacol™ dyes are generally based on 2 major families of photochromic molecules; Spiroxazines & Naphthopyrans, which allow for a wide variety of innovative derivatives to be prepared, leading to a myriad of dye colours, from vivid purples and oranges for plastics through to neutrally toned greys and browns for ophthalmic lens applications. Reversacol dyes represent the widest palette of commercial dye colours currently available in the market.

Incorporation of different functional groups onto the backbone of the molecules allow different characteristics of activation and fade speeds (we refer to this as dye kinetics) which can allow dyes to be ‘tuned’ for specific performance applications.

Because the photochromic effect relies on a physical twisting of the molecule’s ring structures, dyes only become fully coloured if incorporated into a suitably flexible matrix. The dyes will not exhibit any photochromic effect when in their normal, crystalline, powder state as they will be unable to move.

In this absorbance graph for Reversacol Flame, it can be seen that the unactivated absorption profile (yellow line) is quite flat and low, which equates to its unactivated resting state. The high absorption at 380nm in the UV will ensure strong activation of the dye. Once activated, the red line, maximum absorption occurs at around 475nm in the blue region of the spectrum. Removal of the blue fraction of the spectrum by the dye results in the molecule taking on an overall red-orange colour.

This absorption profile of Reversacol Graphite shows a very different response. The activated form, the upper grey line, shows absorption spread across the spectrum from 430 to 680nm. All visible wavelengths are extracted by the dye's absorption, hence the molecule has no particular colour tone, and appears neutral grey when activated. The company was awarded the UK's coveted Queen's Award for invention of Reversacol Graphite and other similar molecules exhibiting this unique effect.

Some photochromic dyes will not change colour behind automotive windows because the glass absorbs the UV light necessary to produce a colour change. However, some Reversacol dyes, such as Palatinate Purple, Sea Green, Plum and Aqua Green, can be activated by light in the visible light spectrum alone, i.e. around 420nm, allowing the coloured state to develop behind glass or with interior artificial lighting. Of course this can also lead to some residual colouration, typically a light pastel shade in the resting state. Other dyes such as Reversacol Rush Yellow, and Oxford Blue will require higher energy, lower wavelength UV light 360 - 380nm to activate fully and will not convert to their coloured form in the presence of visible or artificial light alone.

The Reversacol dye range provides the most comprehensive selection of photochromic dyes, chosen to present a wide span of activation performance, strength and tonal properties for different application areas.

Lens Applications

Reversacol dyes are typically incorporated ‘in-mass’ within the matrix of an optical monomer, usually a methyl methacrylate resin blend, and then thermally or UV-cured. Photochromics can also be incorporated into a sandwich layer of a lens or a coating via dipping or spin coating. Such methods allow manufacture of photochromic lenses if very rigid plastic is required for the application.

Choosing The Best Reversacol Dye

Vivimed have developed a series of patented ‘dualpeak’ photochromic dyes that activate across a wide range of the visible spectrum. They form an ideal ‘neutral’ base colour for the lens formulator. Toners can then also be added in small quantities (0.5 – 20%) to complement and balance the overall lens colour according to the customers’ requirements.

Matching the various dye kinetics i.e. the speed at which they change from their unactivated to coloured state and back again, needs careful consideration in a successful lens formulation.

This simplified graph shows the difference in activation rates between two dyes. When combined in a matrix, the green dye will activate to its coloured state before the blue dye, as it has a faster response time to UV light. Ideally for lens applications, a toneneutral shade is preferable during both the activation and the fading process, thus toner dyes for lenses function best when their kinetics match those of the neutral base dye in the formulation.

The Reversacol dye technology allows us to fine-tune the characteristics of the dye and its performance. Our wealth of experience and advice helps customers avoid the so-called ‘rainbow effect’ as a lens is exposed to and then removed from the UV source.

Finally, our experience has shown that addition of a small amount (0.5 – 1%) of a strong dye to a lens formulation, such as Reversacol Palatinate Purple, Sea Green or Graphite, can help improve overall activated lens colour depth, without adding to the residual (non-activated) colour, or affecting the overall tone.

Influence of Matrix

With ‘in-mass’ systems, the dye is dissolved in a suitable optical monomer at between 250 and 1000 ppm dye loading, then cast into a lens. The rigidity of the cured matrix system is a key factor in the performance of the photochromic dye, and additives such as siloxanes and other lower functional resins can be incorporated into the matrix to modify cross-linking density and thus improve the photochromic dye performance. The in-mass lens systems have excellent longevity and darkness because they allow good dye concentration in the matrix and can accept further additives and stabilisers to optimise light-fastness.

In a dipped or surface-coated photochromic lens a thin coating resin is applied to the lens at only 5 to 20 micron thickness. The Reversacol dye has to be included in the resin at a sufficiently high concentration to allow a photochromic effect to be observed.

After the photochromic resin is applied, the lenses are often finished with hard coating resin, anti-fog and antireflective coatings to provide additional scratch resistance and enhance the optical properties of the lens. Sometimes these coatings can filter out some of the UV which can lead to reduction in dye performance in the lens.

Plastics

Applications for Reversacol photochromic dyes in plastics include:

  • Toys 
  • Novelties 
  • Packaging 
  • Security markers 
  • Films/laminates 
  • Clothes Cosmetics such as nail varnish

Reversacol Dyes are readily incorporated into plastics extrusion processes due to their high temperature resistance. The flexural modulus of the matrix is an important consideration for the dye performance. Softer, low flexural modulus polymers such as LDPE, HDPE and polypropylene are all excellent media for exhibiting Reversacol photochromic dye activation. Engineering plastics such as polycarbonate, polymethyl methacrylate, PET, styrenes and ABS, tend to have higher flexural modulus which can lead to reduced photochromic response; the matrix can be physically too stiff to allow the photochromic molecules to twist into their fully activated forms.

Reversacol photochromic dyes can be readily incorporated alongside other special effects dyes and pigments such as thermochromic dyes, metallic or pearlescent pigments to provide an effective and visually stimulating variety of colour enhancements for many different substrates.

Inclusion Levels
Standard inclusion level for plastics extrusion process is 0.01 - 0.05% by weight of polymer. Some of the more powerful dyes such as Reversacol Sea Green, Storm Purple and Palatinate Purple activate very easily even with indoor ambient lighting, and this can lead to some residual colouration of the plastic. This can be countered by decreasing the dye content to 0.01% or even as low as 0.005%, depending on the desired visual properties of the device.

It is important to ensure the dye powder is thoroughly dispersed within the plastic granules before loading into the extrusion hopper. Larger commercial quantities are best processed into a suitable masterbatch prior to injection moulding. If you require further information about masterbatch process and availability please contact our sales office or technical team.

Inks & Coatings

Reversacol Photochromic dyes are incorporated into ink and coating systems to enable: 

  • Speciality printing effects, e.g. photochromic overprint lacquer 
  • Decorative dynamic colour prints 
  • Security marking systems
  • Brand enhancement and authentication devices

For use in coatings or inks, please note that soft resin/carrier systems work best. For example, soft acrylics, polyurethanes, polyvinyl butyrals (PVB) and PVC (with acidity controlled), particularly if there is a plasticiser present to increase the flexibility. The key is the flexural modulus of the resin.

UV-cured ink and coatings systems need careful processing as the very high strength of the UV-cure lamps used can cause degradation of the dye molecule.

Reversacol dyes are supplied in powder form and need to be dissolved typically in solvents, vinyls, resins etc., before use. The most effective solvent system for Reversacol dyes is toluene. Our recommendations on solubility are as follows:
Toluene > THF > xylene > ethyl acetate > acetone > ethanol (or any other alcohols) > hydrocarbons e.g. petroleum ether.

Inclusion levels
Wherever possible trials should be conducted in coatings greater than 5 microns. Typically the thinner the layer, the higher the concentration required. For an average 5-micron thickness, we advise around 3% dye on a weight-for-weight basis. In a 10-micron thickness this addition level may be reduced to 1%. Inclusion levels are based on weight for weight, i.e. the total weight of the whole system including any solvent in it.

For incorporation into aqueous resin carriers such as PVA or acrylate dispersion polymer Reversacol dyes can be pre-dissolved in a suitable solvent such as toluene or ethyl acetate, then the solvent solution stirred into the aqueous medium.

General considerations when using Reversacol Photochromic Dyes

Choice of Reversacol Photochromic Dye
Choosing the best photochromic dye for your application often requires settling for the best balance of desired performance characteristics. For example, a dye that fades very quickly is likely to be weaker in colour than a slow fading one, which may sometimes exhibit a stronger residual colour. Palatinate Purple is very fast to activate, very strong colour and very fast to fade but shows more residual colour.

The graphs above represent the four main performance characteristics. By plotting each of the properties on a different axis, one can visualise the difference in properties of the various photochromic molecules in order to aid selection of the correct dye for a particular application.

For example, Reversacol Oxford Blue has good activation speed, weaker strength but very low residual colour.

Reversacol Graphite will be very strong, with slower activation and slow fade.

Reversacol Palatinate Purple will be extremely strong and fast to activate, very fast to fade but have stronger residual (unactivated) colour.

Our technical service team are happy to offer support in choosing the best Reversacol dye to meet the requirements of your end application.

Influence of temperature

Ambient temperature affects the final intensity (optical density) of the colour observed from a photochromic dye as the graph above illustrates. Equilibrium exists between the clear and the coloured form, and as the temperature increases, the rate of back-reaction to the unactivated state increases. Therefore in hot climates the final activated colour strength of the dye will be less than that typically observed in strong sun and cool temperature climate, such as on the slopes of a mountain ski resort. Some Reversacol dyes do have kinetics that are more suited to warmer temperature activation, and our technical service team have the experience needed to guide you in your optimum choice of dye.

Influence of matrix

The absorption wavelength of a dye determines its colour shade, and the matrix in which the dye is incorporated can influence this. Sometimes as much as 20nm shift can be observed in the activated colour when the dye is put into different systems, or if the additive package is changed.

For example, as shown in the diagram above, Reversacol Palatinate Purple is a strong deep purple colour in extruded LDPE plastic, and a blue colour within an optical lens monomer. Our applications team always recommend testing dye samples within your own formulation before final selection of the desired colour.

Application advice

  • Due to the high purity profile, only very small inclusion levels are recommended. For best effect, typically from 0.01 to 2.0% is sufficient, depending on application. At higher loadings (depending on the matrix) the dyes saturate out and the photochromic response is reduced.
  • Reversacol dyes are temperature resistant up to a min of 260°C, with some dyes being stable as high as 300°C.
  • Reversacol dye powders are most soluble in organic polymers and non-polar aromatic solvents such as toluene, THF, xylene. They are sparingly soluble in acetone and ethyl acetate.
  • All photochromic dyes are unstable in aqueous systems, and are generally intolerant to free radical attack which causes degradation. Low pH, acidic media should also be avoided. 
  • Depending on the matrix, photochromic dyes can fatigue when subjected to extended periods of strong UV-exposure. Lifetimes can be greatly extended by the correct use of an optimised matrix together with stabiliser packages, e.g. HALS-type, anti-oxidants, UV-absorbers. 
  • Photochromic dyes are unlike conventional permanent dyes (e.g. textiles dyes) in that they do not 'fix' onto a substrate. They need to be held in place within a suitable matrix for them to function correctly.

For further information on the use or handling of any of our Reversacol dyes, please contact the Vivimed sales office.

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Aako BV
P.O. Box 205
3830 AE Leusden
The Netherlands
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