Photochromic and thermochromic Materials

[fog37]

Hello Forum,

Thermochromic materials , either LC or leuco dyes, change color with a change in temperature. They are commonly available and not expensive. Photochromic materials, instead, change color when illuminated by UV light (clear to color or viceversa?).
Considering that light (visible or UV radiation) and heat (infrared radiation) are two different forms of electromagnetic energy, I feel that photochromic and thermochromic materials are somewhat similar in the sense that they are both reversible (if the room temperature heat is available) and their color change is activated in both case by absorbing EM radiation (only in a different spectral region). Both materials change structure when they absorb either UV or infrared and their reflectance properties change producing a different color...

Is that correct?

Does anybody know if piezochromic materials (additives), which change color under pressure, are also available and not too expensive?thanks.
fog37

 

[Kevin McHugh]

I have worked with both photochromic pigments and thermochromic pigments for plastics and paints. I am not aware of any materials that change color due to pressure.You really can't compress solids or liquids to the point of molecular rearrangement, except under extreme temps and pressures such as geologic sediments.

 

[TeethWhitener]

At least one company claims to sell a variety of piezochromic pigments:

http://olikrom.com/en/olikrom-products/piezochromic-pigments/

I am not aware of any materials that change color due to pressure.You really can't compress solids or liquids to the point of molecular rearrangement,

You can compress certain polymers enough to change either their absorbance or their reflectivity. This is most easily seen with interference effects in thin polymer films ("thin" = on the order of the wavelength of light). By stretching or compressing them, you change their thickness, which leads to changes in reflectance based on interference within the material. One dramatic example I'm more familiar with is reflectin, a protein in the skin of some squids (used for dynamic camouflage) which changes thickness upon exposure to a pH gradient. This thickness change causes a change in the reflectance of the material (because of interference), which leads to a color change. In this case, the mechanism is chemical, but I could imagine it happening mechanically as well.

 

[fog37]

Hello Teethwhitener,

So piezochromic materials are not really dyes but some special polymer types which under compression, as you say, change their absorbance and consequently their reflectivity color. In general an ordinary polymer is made to have a certain color by mixing it with either a dye or a pigment. For example, a red piece of plastic contains a red pigment. What about colored piezochromic polymers? How do they get their color before they are compressed?

thanks!

 

[TeethWhitener]

So piezochromic materials are not really dyes but some special polymer types

That was an example. There very well might be dyes that change color upon application of mechanical force.

change their absorbance and consequently their reflectivity color.

Absorbance or reflectivity. You only need one. Absorbance is when a material absorbs a photon, generally exciting an electron to a higher energy. Reflectivity includes things like interference effects. A polymer film with thickness on the order of the wavelength of incident light will display peaks and troughs in its reflectance spectra because some wavelengths of light will constructively interfere and some will destructively interfere. However, in this example, photons aren't necessarily absorbed.

There are plenty of polymers which are inherently (absorptively) colored without adding a dye. Polyaniline jumps to mind, but most conjugated polymers have at least some absorbance. I don't know how much pressure you'd have to apply to get their absorbance spectra to change. However, polymers like polyethylene (which does not absorb light in the visible range) can still appear colored if the polymer is thin enough because of interference effects of light reflecting off the polymer. Since the reflectance spectrum is a direct function of the thickness of the film, changing that thickness by applying mechanical pressure would lead to a color change (this is closely related to the iridescence of an oil slick or a soap bubble).

I don't know what materials the commercial vendors use, but from the (rather limited) search that I've done just now and the picture on their website, it looks almost like it could be a FRET (Forster resonance energy transfer) effect, which involves a transfer of energy from a molecule in an excited state to a nearby molecule in its ground state. In this case, the energy transfer is strongly dependent on the distance between the two molecules (in fact, the effect completely disappears if the molecules are separated by more than about 5 nm), and even comparatively small distance changes (< 1nm) can lead to big changes in FRET efficiency. The idea here is that an efficient energy transfer tends to kill off any fluorescence that a material might display. So you would have relatively widely separated fluorophore molecules at ambient pressures which exhibit strong fluorescence with inefficient FRET. When pressure is applied, the molecules move closer together, FRET becomes more efficient and the fluorescence is quenched. Just a guess, though.

 

[Kevin McHugh]

At least one company claims to sell a variety of piezochromic pigments:

http://olikrom.com/en/olikrom-products/piezochromic-pigments/
You can compress certain polymers enough to change either their absorbance or their reflectivity. This is most easily seen with interference effects in thin polymer films ("thin" = on the order of the wavelength of light). By stretching or compressing them, you change their thickness, which leads to changes in reflectance based on interference within the material. One dramatic example I'm more familiar with is reflectin, a protein in the skin of some squids (used for dynamic camouflage) which changes thickness upon exposure to a pH gradient. This thickness change causes a change in the reflectance of the material (because of interference), which leads to a color change. In this case, the mechanism is chemical, but I could imagine it happening mechanically as well.

Thank you for that link. I was in the coloring business for 22 years, and I was unaware of these pigments. There are special effects pigments made up of many layers of ultra-thin polyester film. These are quite common in paints and plastics. They change color depending on viewing angle. But, these are interference pattern pigments, not piezo.

 

[fog37]

Hello Kevin McHugh,

Thanks for your inputs. If I may, I have a few questions,since you have 22 yrs of experience in the color business.

a) From a practical standpoint, what is the difference between dyes and pigments? Are dyes generally more soluble than pigments which tend to maintain their particulate identity in the plastic they are mixed with? Leaching of dyes or pigments from a polymer is an issue. Do pigments tend to be locked inside the host polymer better? I have read about solvent dyes in the context of polymers. Are these dyes that have high solubility (affinity) with polymers like nylon or polyester?

b) Are dyes and pigments used in the ink industry the same as the dyes and pigments used in the textile sector?

c) Do the photochromic pigments go from clear to colored or from colored to clear when exposed to UV? For piezochromic pigments go the activating stimulus is UV light. Do photochromic materials get very hot in the sunlight? As far as thermochromic pigments go, do they heat up the material they are in once they absorb heat or do they use the incident thermal energy to change their molecular structure and cause the color change without causing an increase in temperature of the hosting material?

I will look into the interference pattern pigments in the next days...very interesting.

Thanks!

 

[Kevin McHugh]

a. The basic difference between dyes and pigments is solubility. There are several classes of dyes. There are ionic dyes which are water soluble such as basic and acid dyes. There are solvent dyes and disperse dyes. The Color Index is an invaluable source of info on dyes and pigments (very expensive). Pigments are dispersed into the matrix (polymer, ink, paint). The better the reduction in particle size by shear, the greater the color development (more chroma). Dyes are solvated by the matrix. Polar resins such as nylons, acrylics, polycarbonates and polyesters (and styrenics) are all suitable for dyes. But these engineering resins have high processing temps, so the dyes must have excellent thermal stability. Dyes do not well in polyolefins. They will bleed out due to incompatibility.

b. Some, yes. Most textile fibers are polyester, nylon, or polypropylene. Ink dyes do not need high thermal stability, but textile dyes do. Ink and textile dyes must have excellent light stability (they should not fade upon exposure to light). Dyes cannot be used in polypropylene fiber.

c. Photochromic pigments can go from cleared to colored, or change colors based on temp. I am not certain of the thermodynamics of the photochromic pigments, but common sense suggests that absorption of some light energy by the pigment will go to molecular rearrangement, with excess given off as heat. For a reversible reaction, the system must stay close to equilibrium. Color changes in thermochromic pigments are induced by temperature changes in the matrix. I am not certain if the associated molecular rearrangement is exothermic or endothermic, But every thermochromic pigment has a unique temp at which it will change color.

Engelhard was big in the interference pigments business.

 

[fog37]

Hello Kevin McHugh,

Thank you for your answers! Very helpful.

In general, what causes dye or pigment migration out of a polymer? Does heat cause the dye to sublime (solid to gas) and come out of the polymer?
Does contact with water during washing cause leaching?

Have you worked with fluorescent pigments and dyes as well in your career? I am trying to learn about them as well...

 

[Kevin McHugh]

Dyes bleed out due to insolubility in the polymer matrix. No high temp necessary, it will happen in time. Pigments OTH, are dispersed and wetted out by the polymer, so bleed out is not a concern. Most polymer processing does not come into contact with water. Injection molding, blown film, blow molding and rotomolding are all waterless processing. Pelletization will almost always come into contact with water. The dyes used in plastics are not water soluble, so it is not an issue. I've pelletized nylons, polyesters, styrenics and acrylics containing solvent dyes that do not leach into the process water.

Fluorescent pigments are actually fluorescent dyes encapsulated int a polymer matrix and ground to fine particle size. The encapsulation makes them behave like pigments.

 

[fog37]

Dyes bleed out due to insolubility in the polymer matrix. No high temp necessary, it will happen in time. Pigments OTH, are dispersed and wetted out by the polymer, so bleed out is not a concern. Most polymer processing does not come into contact with water. Injection molding, blown film, blow molding and rotomolding are all waterless processing. Pelletization will almost always come into contact with water. The dyes used in plastics are not water soluble, so it is not an issue. I've pelletized nylons, polyesters, styrenics and acrylics containing solvent dyes that do not leach into the process water.

Fluorescent pigments are actually fluorescent dyes encapsulated int a polymer matrix and ground to fine particle size. The encapsulation makes them behave like pigments.

hi Kevin, I have a simple question regarding the use of a spectrometer to measure the "color" of a substance. For instance, let's consider a red apple. If we place the spectrometer (which in my case has a fiber optic cable connected to it) in front of the apple, will the spectrometer show a strong wavelength pick at around 630 nm or so if I keep the room lights on? Should I turn the lights on the room off and illuminate the apple in some particular way? How? Do you have any advice?
clearly, we need to measure only the light that is reflected by the apple without any other spurious contribution.
Maybe just pointing the spectrometer toward the apple with the room light on should show that expected wavelength peak...

Thanks

 

[Kevin McHugh]

Measuring color can be very subjective. The eye perceives reflected light from an object. The perceived color will be a function of the illluminant, the angle of reflection, the surface of the object (matte or glossy), thickness of the object (where undertones become apparent in thin samples), etc. That is why color science specifies these parameters in color spectrophotometers. Most color specs use a sphere coated with barium sulfate so the incident light is diffuse rather than specular. You cannot achieve that with your set up, so measuring color will be ad hoc so to speak. All that being said, you should see a maximum wavelength near the wavelength of the color of the object. So red should be somewhat less than 700 nm.

 

[Laird M]

I am an artist interested in creating drawing inks that can be irreversably erased (like the thermochromic leuco dye used in PILOT's erasable pen) and that are lightfast (like carbon).I've experimented incorporating thermochromic "pigment" into standard ink binders (gum arabic, various starches and acrylic media) and the results have been fairly good in terms of the resulting handling qualities, such as the flow from a dip pen. The chalky gray of the "black" pigment is less than desirable, however. And I have not been able to procure a leuco "pigment" that erases permanently in response to the level of heat generated by the friction of a polyurethane (or somesuch material) bead rubbed on paper, as the PILOT engineers have succeeded in doing.One wonders if a piezochromic dye could flip to a transparent state simply in response to pressures associated with hand erasure… Given the discussion above, it seems highly unlikely.As regards leuco dyes’ instability vis-a-vis UV exposure one could imagine applying a thin coat of a commercially available nano-encapsulated zinc (sometimes marketed as a “pearlescent” pigment) as a prophylactic—that is as a kind of sunscreen for a drawing completed with made with a fugitive leuco-based ink.I’d be very interested in further thoughts on these matters!

 

[HAYAO]

Hello Forum,

Thermochromic materials , either LC or leuco dyes, change color with a change in temperature. They are commonly available and not expensive. Photochromic materials, instead, change color when illuminated by UV light (clear to color or viceversa?).
Considering that light (visible or UV radiation) and heat (infrared radiation) are two different forms of electromagnetic energy, I feel that photochromic and thermochromic materials are somewhat similar in the sense that they are both reversible (if the room temperature heat is available) and their color change is activated in both case by absorbing EM radiation (only in a different spectral region). Both materials change structure when they absorb either UV or infrared and their reflectance properties change producing a different color...

Is that correct?

Does anybody know if piezochromic materials (additives), which change color under pressure, are also available and not too expensive?thanks.
fog37

From a practical standpoint, you are right. They absorb some form of energy to change its form.

Scientifically, they are quite different. Thermochromic materials involve transition between vibrational modes within ground electronic state. Photochromic materials involve transition between electronic states (and vibrational modes of both excited and ground electronic state).

Also, heat does not necessarily mean infrared radiation (that is photon). It can also be in the form of phonon.However, at the very low energy limit, photons and phonons does pretty much the same thing. For example, Eu,Dy:SrAl₂O₄, which is a very common example of long-persisting phosphorescent material (so-called "afterglow"), involves 4fⁿ-4f^n-15d¹ electronic transition of Eu²⁺ by absorption of photon followed by electron that is thermally excited to the conduction band of the host material (SrAl2O4). The excited electron will be trapped by a trapping site, which can return back to the conduction band with either very low energy photon (infrared light) or heat (phonon), because both can excite the electron to conduction band.

If you have any further question, feel free to ask.

 

[Kevin McHugh]

I
As regards leuco dyes’ instability vis-a-vis UV exposure one could imagine applying a thin coat of a commercially available nano-encapsulated zinc (sometimes marketed as a “pearlescent” pigment) as a prophylactic—that is as a kind of sunscreen for a drawing completed with made with a fugitive leuco-based ink.I’d be very interested in further thoughts on these matters!

Zinc oxide is a good UV absorber, but it has drawbacks in terms of transparency. There are plenty of UV absorbers commercially available for this purpose. Oligomeric hindered amines offer the best protection. Pearlescent pigments are finely ground mica, not zinc oxide.

 

[fog37]

Zinc oxide is a good UV absorber, but it has drawbacks in terms of transparency. There are plenty of UV absorbers commercially available for this purpose. Oligomeric hindered amines offer the best protection. Pearlescent pigments are finely ground mica, not zinc oxide.

Hi Kevin, how are you.

I would like to ask you a couple of questions about color measurement if I may:

1) I would like to measure the color of a fabric or a plastic chip and obtain its CIE L*, a*, b* coordinates. Would I have to buy a reasonably priced and portable colorimeter or do I really need a costly spectrophotometer?

2) Let's say I measured the L*a*b* coordinates of color A (my target color). I want to match a different color B with its own different L*a*b* coordinates to color A.
Do I simply need to add/remove the suitable amounts of red or green or blue or yellow to color B until it matches color A? The CIELab color space seems to be be based on the opposite colors red/green and yellow/blue and luminosity (black/grey/white).

Thanks,
Fog37