med

med

Medvance 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.



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.


How do Reversacol™ Photochromic Dyes work?

med

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 dependent on ambient temperature and the chemical structure of the dye

Spiroxazines & Naphthopyrans

Medvance Reversacol™ dyes are generally based on two 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.

Dye Kinetics

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.

Light Absorption

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.

Neutral Grey

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.

Activated Energy

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.


Reversacol™ Applications

med

Reversacol photochromic dyes find many applications in plastics, films, coatings and inks as colour-enhancing effects or functional UV-indicators. The widest area of application is in prescription photochromic lenses where dyes can be incorporated in-mass or as a surface coating onto the lens.

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.


med

Plastic

Applications for Reversacol photochromic dyes in plastics include; Toys, Novelties, Packaging, Security Markers, Films/Laminates, Clothes and Cosmetics such as Nail Varnish.


med

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.

Inks and 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.

Usage Advice

med

There are several important factors to consider when using Reversacol photochromic dyes, in order to maximise the effectivity of the dyes in a particular application. This section deals with the recommended inclusion levels, suitable solvents, processing temperatures and various situations to avoid which may affect performance and lifetime of the dye.


Our 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.

Reversacol™ Product Range Table

med

Please refer to our product list and for further enquiries and Custom requirements, kindly send us an email on info@medvancesolutions.com