Colour By Design – Storyline notes

CD1 – Ways of making colour.

·        Colours are caused by either the scattering/refraction of light (e.g.?) or by the presence coloured compounds.

·        Early coloured compounds were pigments from natural sources - often minerals from the ground, e.g. red ochre which is iron(111) oxide. These were mixed with oil or mud to form a paste to stick to surfaces.

·        Pigments are no good for colouring fabrics – for this you need a dye and again early dyes were natural substances from plants or animals e.g. the blue dye indigo from the woad plant (used for?) and the red dye cochineal from insects (used for ?)

·        Read the green box on page 209 of SL and list as many differences as you can between pigments and dyes………

·        Early dyes and pigments were expensive – only for the rich! – but when chemists learned to make synthetic dyes these were much cheaper.

·        Many synthetic dyes can be made from coal tar an unwanted (and cheap) by-product of the coal gas industry.

·        Chemists were able to copy natural colours and also develop new colours.

·        Modern colour chemists have a vast range of compounds available to them, they need to understand;

·        Why compounds are coloured

·        Which structures lead to particular colours

·        How to bind coloured substances to different fibres/surfaces

·        Once these things are understood it is possible to design a coloured molecule for a particular purpose – colour by design!!

CD4 – Chemistry at the art Gallery.

The Incredulity of St. Thomas.

·        Painted by Cima da Conegliano in 1504 – he used a wide range of pigments which was rare at that time e.g.

Malachite CuCO₃ – green

Haematite Fe₂O₃ – red/brown

Natural ultra-marine – blue

·        Chemists identified the actual blue pigments present by shining EM radiation on the painting and examining the wavelengths reflected. The resulting reflectance spectrum is characteristic of a particular pigment because different pigments absorb different wavelengths of the incident radiation.

·        It took 15 years to restore the painting and analytical chemists played an important role – identifying pigments, binding media and the materials used in the coatings beneath the paint.

·        How did the painting get into such an awful state to need all this work?

·        Read ‘the history of the painting on pages 218-219. This explains how the painting became so damaged and the early work done to restore it. Chemists became involved at the later stage i.e. restoring the actual paintwork, when information was needed about the pigments and binders used by Cima so that anything used on the painting could be kept as close as possible to the original materials.

What Medium did Cima Use.

·        Egg yolk was used as an early binding medium – it is an emulsion of fat globules and protein in water (Read ‘ The Binding Medium’ and ‘Emulsions’ from page 219) The egg yolk dries and hardens as the water evaporates.

·        Later – by the time Cima was painting – oils were used instead.

·        Many drying oils contained triesters based on palmitic acid and stearic acid.

·        Different oils contain different ratios of the two acids, and the ratio can be found using glc (CI 7.6) hence this technique can be used to identify the drying oil used by Cima.

What were the pigments.

·         To help identify a pigment the elements present can be found using Laser microspectral analysis – LMS. Read green box on pg 220.

·         Once the elements are known it is often much easier to identify the pigment.

·         A newer technique called Energy dispersive X-ray fluorescence – EXD is now also used.

·         Once the pigment and medium are identified, substitutes can be made up accurately and used to restore the painting – being careful to ensure that any restoration done can be removed if desired at a later date (why?)

·        Historians can also help identify pigments etc. (more in Act CD4.4)

CD5 – At the Start of the Rainbow.

·        In the 19^th Century chemists learned to imitate the properties of natural substances by developing new compounds.

·        Colourists enjoyed a high regard in industry.

·        As dyes were developed the understanding of organic chemistry improved.

·        Three young entrepreneurs were at the heart of these developments;

·        William Perkin (1838-1907)

·        Ivan Levinstein (1845-1916)

·        Heinrich Caro (1834-1910)

Perkin

·        When 18 years old he devised and synthesised a dyestuff – Perkins Mauve.

·        He was trying to produce quinine (a cure for malaria)

·        He began with coal tar but his experiments failed, so he tried aniline (phenylamine) as a starting material and although he did not produce quinine, he did produce a purple solution – which was brilliant in colour and had good fastness to fibres.

·        He had developed the first synthetic dye.

·        He produced this dye commercially (see fig 29 pg 222) and this led to the development of other synthetic aniline dyes.

·        Process;

Levinstein 

·        Worked on aniline dyes.

·        Manufactured aniline green and aniline red at a young age.

·        Produced dyes in Manchester – near the textile industry!

Caro

·        He saw the economic potential of dye manufacture. He was a great chemist and engineer, and worked out a key step in the synthesis of the red colour found naturally in madder dye.

But Was it Chemistry?

·        It is important to realise that much knowledge we take for granted today was unknown to early colourists. The first synthetic dyes were found by chance and there structures were unknown.

Alizarin

·        Until Perkins work dyes originated from plant or animal material.

·        E.g. Madder root gave red dyes, known today to contain alizarin

·        Alizarin only sticks fast to cloths if they have been impregnated with a metal compound e.g. Aluminium sulphate Al₂(SO₄)₃ – This process is known as Mordanting.

·        It was found that the metal used in the mordanting compound affected the colour produced;         Al – Red

Sn(11) – Pink

Fe(11) – brown

·        Mordanting takes place if pH > 7 . M(OH)_x is deposited on fibres.

·        The M⁺ ion on fibres binds to the dye molecule forming chelate rings. See fig 26 page 224.

·        Alizarin has been used for many years but its structure was unknown. In 1868 it was found to be derived from anthracene, then a race was on to synthesise it.

·        Graebe and Leibermann were first. They devised route 1 of fig28 pg 226. But this was very expensive as bromine was used, and the chemistry of this route was complex (In final stage an isomerisation reaction occurs)

·        An alternative route was found using route 2 of fig28 pg 226. This route was devised simultaneously by Perkin in London and also by Caro, Graebe and Leiberamnn working for BASF.

·        They decided to share the market for alizarin – and their synthetic product devastated the Madder industry!

CD6 – Chemists design colours.

·        Witt related colour to structure and questioned why certain structures led to cmpds being coloured and why small structural changes led to changes in colour. He worked on diazo reactions (see CI 13.10)

The first Azo Dyes.

·        Azo dyes are made from a diazonium salt and a coupling agent see fig 30 page 227. (also CI 13.10)

·        Witt used phenylamine (aminobenzene) to produce yellow dye

              And  triaminobenzene to produce brown dye, so predicted that

                    diaminobenzene would produce an intermediate orange colour.

·        He was right!

·        He produced the orange azo dye chrysoidine.

·        Azo dyes still dominate the yellow orange and red parts of the spectrum.

How does stucture affect colour?

·        A dye molecule contains a group of atoms called a chromophore.

·        The chromophore is responsible for the dye’s colour.

·        A chromophore contains unsaturated groups e.g. –C=O or –N=N- or benzene rings.

·        These unsaturated groups become part of an extended delocalised system.

·        E.g. chrysoidine

chromophore

·        The functional groups around the chromophore;

·        Modify or enhance the colour of the dye

·        Make the dye more soluble in water

·        Alter the dyes ability to attach to fibres or cloths

·        Azo dyes have general formula  X-N=N-Y and chemists have studied extensively the affect of changing X and Y.

·        Different X and Y groups affect the colour and characteristics of the dye.

·        With 50 diazonium salts and 52 coupling agents to choose from a whole rainbow of azo dyes can be produced, though these are mainly yellow, orange or red, with very few blues and greens.

CD7 – Colour for cotton.

·        Wanted dyes which were fast to light, washing and rubbing.

·        Wool and silk are protein based, so have –COOH and

-NH₂ groups which can be ionised and form electrostatic

interactions with groups on the dye molecules.

                                                                                Fig33interactions between

                                                                                        A dye molecule and a protein chain

·        Cotton is a cellulose fibre made of bundles of polymer chains with no readily active parts.

          Fig 34 two ways of depicting a cellulose fibre. In the top diagram the molecule is shown to be a chain of glucose units, in the bottom digram only the reactive –OH groups are shown.

·        Alizarin uses a mordant (e.g. Al3⁺) while indigo is a vat dye – it is oxidised and precipitates within the fibres.

·        Direct dyes are applied in solution and are held to the fibres by hydrogen bonding and Instantaneous – induced dipoles. These are weak bonds and the dye will only be fast if they are long straight molecules which can line up with the fibres and form several hydrogen bonds.

Fig 35 The structure of direct blue (CI 24410)

A Dyemaker’s Dream.

·        If we could have strong covalent bonds between dye and fibre we would have very fast dyes.

·        William Stephen modified azo dyes by adding reactive groups which he hoped would combine with amino groups on wool fibres.

Fig 36 building a reactive dye to react with wool.

Fig 37 The planned reaction of the new dye with amino groups in wool fibres.

·        The reaction didn’t work with wool – thought to need alkali conditions, but these would damage wool fibres.

·        Cotton not damaged by alkaline, so tried new dye with cotton – it worked!!

·        Called fibre reactive dyes. E.g. procion yellow RS and procion red 2BS (see fig 39 page 231)

Paradox and problem.

·        New dyes were fast because of reaction with hydroxyl groups on cotton.

·        Reactivity was destroyed by hydrolysis with water.

·        Most dyes are used commercially by dissolving in water!!

·        Stephen developed a system of buffers to keep the pH within strict limits to control the hydrolysis. (more on buffers in the Oceans topic)

CD8 – High-Tech colour.

·         Colour chemists can now produce colours to order for a particular application.

·         Recent new development has been modifying dyes for high tech applications e.g. high speed ink jet printers and electronic photography.

A Smudgy Problem.

·         An early problem with ink jet printers was smudging.

·         Dye was needed that was soluble in water but insoluble once it reached the paper.

·         Of the ~ 100 known black inks one was selected – Food black 2 (for structure see bottom of page 233) – which was cheap, non toxic and light fast.

·         Its structure was then modified – some of the sulphonic acid groups             (-SO₂OH) were replaced by carboxylic acid groups (-COOH).

·         Most modern paper is acidic – ink is slightly alkaline.

COOH     -             soluble in alkaline environment i.e. ink solution

                        Insoluble in acidic environment i.e. paper.

·         As a final refinement the ammonium salt of the acid dye is used as this ensures that the conversion to the insoluble carboxylic acid is complete (ammonium salts break down on heating).

One in a million.

·         Challenge was to find three dyes with correct properies to print photographs taken by an electronic camera.

·         Need yellow, cyan and magenta dyes.

·         Fig 43 pg 234 explains how the dye diffusion thermal transfer process works.

·         Dyes nedeed had to have;

·         Colours which blended to cover the whole visible range.

·         High thermal stability (up to 400°C)

·         Of the 1 million known dyes none would do! New ones had to be designed.

·         Computer modelling helped modify existing dyes till the structures shown in fig 44 pg 235 were developed.