I. Introduction
H.W. Sands' photochromics reversibly alter their color upon
exposure to ultraviolet sources. These chameleon like dyes
respond to natural solar irradiation as well as artificial
sources such as 365 nanometer "black light." When
sunlight or ultraviolet (UV) radiation is applied, the dye
becomes excited and the molecular structure is changed allowing
a color to appear. When the stimulus (sunlight/UV) is removed,
the dye will return to a state of rest, which is its colorless
form. Photochromic molecules are not reactive in their crystalline
state and need to be dissolved in a solvent or polymer to
function.
II. Color
Colors |
Photochromic
Dye |
Photochromic
Pigment |
Yellow |
MSA6553 |
MSA6547 |
Orange |
MSA6554 |
MSA6548 |
Red |
MSA6556 |
MSA6549 |
Blue |
MSA6557 |
MSA6550 |
Royal Blue |
MSA6558 |
MSA6551 |
Violet |
MSA6559 |
MSA6552 |
III. Available Products
A. Photochromic Dye - optically grade material
compatible with solvent based flexographic, UV, screen, offset,
gravure and epoxy ink formulations.
B. Photochromic Pigment - microencapsulated
photochromic dye for high temperature processes.
IV. Solvent Compatibility
Compatible Solvents
Toluene |
Xylene |
Methyl Ethyl Ketone |
Ethyl Acetate |
Most Organic Solvents |
Non-Compatible Solvents
*While the photochromics are insoluble in water, they can
be treated like other water insoluble materials and "tricked"
into water based systems.
V. Recommended Concentrations for Ink, Paint and Plastic
Formulations
| |
Ink/Paint
(Solvent) |
Ink/Paint
(Aqueous) |
Plastic
Injection/Extrusion |
| Dye |
0.02% - 0.1% |
N/A |
0.01% - 0.1% |
| Pigment |
3.0% - 30.0% |
3.0% - 30.0% |
0.2% - 5.0% |
VI. Dispersing/Mixing
The Photochromic dye/pigments are compatible with
standard mixing procedures. Be careful not to grind or damage
the pigment version through aggressive dispersion techniques
(e.g. Ball Mills). This will destroy the structure of the
microcapsules and color changing properties.
VII. Printing Recommendations
A. General Recommendations
Photochromic inks must be printed and dried in a clean environment.
Always be sure that the screens, squeegees, knives, spatulas,
or any other equipment that comes into contact with the photochromic
inks are clean and dry, completely free of all solvents or
other matter. In addition, the sensitivity of the photochromic
inks to UV light and high temperatures and specific chemicals
should always be considered.
B. Screen Inks
Photochromic screen inks can work on hand or automatic
(rotary or flat-bed), sheet or web-fed screen printing equipment.
A US 80-150 mesh polyester monofilament screen or lower is
recommended. It is preferable to use a medium or medium-hard
(65 durometer) rounded edge squeegee.
VIII. Opacity
The opacity of the photochromic pigment is directly related
to the color, concentration and print thickness. Their color
intensity is lower than a typical color pigment. Dark colors
such as black can be used to hide an image whereas lighter
colors such as yellow can not. Achieving this concealing effect
requires higher concentrations and multiple print passes.
IX. Heat
H.W. Sands’ photochromics can be imbibed, extruded,
injection molded, cast or mixed in plastisol. H.W. Sands’
photochromics can withstand processing temperatures of 180°C
to 240°C (350°F - 460°F) for short periods without
degradation. Thermal degradation is a function of the combination
of time and temperature. Therefore, avoid prolonged exposure
to high processing temperatures.
X. Miscellaneous and FAQ’s
Q.
How long-lasting is the photochromic feature?
A. Certain photochromics will not last as long in materials
that form acids when exposed to sunlight. In plastisol, half
lives of greater than 40 hours in a QUV panel have been achieved.
(The half life is the time it takes to reach one half the
initial change in color intensity.) Products made from solvent
cast urethane have shown a half life greater than 2,000 hours.
Q.
From what does the photochromic need to be protected?
A. The photochromic molecule needs to be protected
from free radicals such as singlet oxygen, oxidizers such
as peroxides, acids and high energy ultraviolet, UVB. While
it is robust in the closed or colorless form, it is most susceptible
to attack in its open or colored state.
Q.
Some days the photochromic is darker than on other
days. Why?
A. The photochromic feature is affected by the following:
• The amount of activating UV present (the UVA spectrum
changes during the day)
• Temperature (high temperature promotes conversion
of the colored to the colorless form)
Q.
Sometimes there is a residual color when it should
be clear. Why?
A. There is some residual color with all photochromics that
can be controlled by:
• A reduction in the photochromic loading
• A reduction in the processing temperature
XI. Storage
Store at ambient temperatures (20-25°C) out of direct
sunlight and preferably in the dark. Storage longer than 12
months is not recommended for the powders.
All applications using the photochromic products should
be thoroughly tested prior to approval for production.
The information herein is believed
to be reliable and is to assist customers in determining whether
our products are suitable for their applications. However,
no warranty, express or implied, is made as to its accuracy
or completeness and none is made as to fitness of this material
for any purpose. Our products are intended for sale to industrial
and commercial customers. We request that customers inspect
and test our products before use and satisfy themselves as
to contents and suitability. Nothing herein shall constitute
any other warranty, express or implied, including any warranty
of merchantability or fitness, nor of protection from any
law or patent to be inferred. All patent rights are reserved.
The exclusive remedy for all proven claims is replacement
of our materials and in no event shall we be liable for special,
incidental, or consequential damages. We shall not be liable
for damages to person or property resulting from its use.
Consult the Material Safety Data Sheet for additional information
