Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
MP0866-US1 165/110
aACKGROUND OF THE INVENTION
The present invention relates to temperature
indicating compositions for use on recoverable articles so
that heating to produce recovery or to activate a heat-
activatable sealant can be monitored.
Temperature indicating compositions have foundwide-spread use where warning of temperature excursion is
required or where a heating process is to be monitored.
They are of special use where the desired maximum temperature
would not otherwise be apparent to the operator; an example
of this is in the use of heat-recoverable materials where a
certain temperature is required for proper recovery but
where a higher temperature could cause damage. Such recover-
able materials are commonly used for sealing, the effective-
l~ ness of which is enhanced by provision of a heat-activatable
sealant, such as a hot-melt adhesive, a reactive epoxy
adhesive or a softenable mastic, on a surface of the recover-
able article which will contact the substrate to be sealed.
This sealant will not be visible to the installer, who
therefore needs some indication that the correct bond-line
temperature has been reached. The problem has been overcome
by applying to a visible surface of the article to be heated
a composition, the color of which changes when an internal
surface, for example, has reached some desired temperature.
2~ Such temperature indication is also applicable to articles
which are recovered by means other than heat, but which
require heat solely for activation of an adhesive; or which
mav experience heat during use. 3uch recoveraDle articles
may be temporarily maintained in their dimensionally unstable
~3 configuration by a hold-out member which is removed by
dissolving the adhesive bond between the hold-out and the
recoverable article or by other means.
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A temperature indicating composition which is
suitable for use on a recoverable material will contain
components that are common to any paint or ink, but they
will be chosen and combined with other components to insure
suitability for a substrate that will be subjected to a
range of temperatures, that changes si~e or shape, and that
should remain environmentally sealing.
The basic components are a colorant, a binder,
and often a solvent. The colorant of course gives the
composition the desired color and in most paints and inks
has the characteristic of permanence. The binder, or binder
together with solvent where used, wets the pigment and gives
the composition tack and the rheological properties appropriate
to the method by which the composition is to be applied to
l~ its substrate. Once the composition is on the substrate it
is dried, for example by evaporation of solvent, if present.
After application, the binder will be responsible for the
final film properties of the paint or ink. In general, the
colorant must function as a pigment rather than as a dye for
its intended application, by which is meant that it must be
opaque, rather than transparent. In order to do this it
must have hiding power, and the physical properties which
endows a colorant with hiding power are its refractive
index and insolubility. Where the colorant alone has not a
2~ sufficiently high hiding power, a further component, called
an opacifier and generally white, should be added.
The formulation of a paint or ink is related not
on'v to the method by which it is to be applied and to the
properties that the final film must possess~ but also to the
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drying method to be used. Many methods of drying are
available. A brief review will be made of some of the
generally applicable methods available for drying inks and
paints.
As used herein, the term "drying" refers to all
processes by which a temperature indicating composition
changes from (i) the form in which it exists before it is
applied to a substrate to (ii) the form in which it exists
after it is fixed on the substrate. The term "drying"
includes all the processes described below, including curing
by UV rsdiation.
Further, as used herein the term "binder" refers
to that component of the composition which imparts film
forming characteristics to the composition. The binder may
comprise a polymeric or pre-polymeric material ~hich cures
or polymerizes, or it may comprise such a polymeric or
pre-polymeric substance together with a solvent. The term
binder is-used for the film forming component at all states.
Thus the binder can be in liquid form as it is before
2Q it is applied to the substrate or in solid form as it is
after it has been applied to the substrate and dried.
Drying can occur by penetration: the entire
composition is absorbed into the substrate. This is of more
use for dyes than pigments since a dye is entirely dissolved
2~ in its vehicle. lt has not been used on heat-shrink products
which are generally non-absorbent.
A second method for drying is hardening of a
~inder by oxidation. This has not been used on heat-shrink
products.
MP0866-US1 ~ 78~ 165/110
An important method of drying is by solvent
evaporation, and this is the technique used for paints that
have been used on recoverable products. The paint comprises
a pigment suspended in a resin-solvent system, and after
application the solvent is removed leaving behind the solid
resin. This system is applicable both to paints and to the
three major printing techniques: typographic, planographic,
and intaglio. The rate of drying can easily be controlled:
it is increased by heat, and with heat can be very rapid.
~0 Many resin-solvent systems have been developed which satisfy
all the requirements as regards, odor, toxicity, color,
solvent power, rate of evaporation, boiling range, combusti-
bility and viscosity. The system is highly versatile
allowing application by a variety of methods to a variety of
substrates.
Polymerization of a binder can also cause the
desired change from liquid to solid. A common example of
this is styrenated polyester resins rontaining an appropriate
catalyst which promotes curing of the resin after application
Z0 to the substrate. Newer techniques involve initiating a
polymerization reaction by means of infrared radiation,
microwaves, dielectric9 electron beams or ultra-violet
radiation.
Another technique for drying is by means of preci
pitation. The vehicle in which the colorant is carried is a
solvent plus binder, and a further substance is added
selectively to precipitate the binder.
,
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Drying can also be obtained by using a temperature
indicating composition that includes a UV curable binder and an
initiator which on exposure to UV radiation initiates a polymeriza-
tion reaction in the binder.
Some of the systems that have been used for temperature
indicating compositions will now be reviewed.
In United States Patents Nos. 4,142,416, 4,108,001 and
3,995,489 to Smith, compositions are used to detect overheating in
electrical apparatus. A current of air is passed through the appa-
ratus to cool it, and parts of the apparatus that are cooled bythe current of air are coated with an organic composition which
decomposes and thermoparticulates. If overheating occurs, part-
icles of the decomposed composition will be detected downstream oE
the apparatus. The specification is not concerned with thermo-
- chromism, but it mentions that the organic compounds produced char
when heated.
United States Patent No. 3,816,335 discloses incor~ora-
tion of an inorganic thermochromic colorant throughout the body of
a heat-recoverable plastic article. A problem that can occur with
some inorganic systems is a tendency for the base polymer to be~
come degraded under the action of the colorant and heatO This pro-
blem was solved in United States Patent No. 4,105,583 where it was
proposed that zinc borate and zinc sulfide should be added to some
inorganic systems. Other active ingredients were also mentioned.
In U.K. Patent Publication No~ 2,038,478, a move was made away from
inorganic colorants to the use of organic ma-terials that melt and
decompose at the desired temperature, thus causing a color change.
--7--
In this case the thermochromic ingrectient is applied in an aqueous
or other vehicle, or as part of a solid formulation such as a wax.
This use of organic ~aterials on recoverable articles
can be improved by selection of the binder so that decomposition
products of the thermochromic coloran-t were trapped thereby insur-
ing irreversibility. This is disclosed in U.K. Patent Publication
No. 2,077,919.
The majority of these compositions comprise a pigment,
an opacifier, a binder and a solvent. They thus rely, like many
paints and inks, on evaporation of a solvent.
U.S. Patent ~pplication Serial No. 403,451, filed on
July 30, 1982, by Andrew B. Brown, Tamar Gen, and Gary R. Weihe,
discloses W curable temperature indicating compositions.
The following are desirable features of temperature
indicating compositions:
1. No toxicity, either before or after being heated.
2. No toxic fumes when heated.
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3. No adverse affects on the substrate to which
the composition is applied.
4. An easily visible color change: preferably a
color change from a bright color vlsible even in dark
manholes and above ground when viewed silhouetted against
the sky, to another easily visible color such as black.
5. A color change that is irreversible.
6. Good hiding properties, i.e. opaque, partic-
ularly when the substrate is black, both before and after
the color change.
7. A sharp color change at the conversion
temperature.
- Great difficulty has been experienced in finding
suitable thermochromic indicating compositions for many heat
recoverable substrates, such as substrates made of materials
that recover at temperatures below 150C.
For example, difficulty has been experienced in
finding a suitable tempersture indicating composition for
heat recoverable polyethylene, which recovers at about
135C. The thermochromic colorant often used for polyethylene
is allantoin. Allantoin has poor hiding power, and thus
must be applied in thick layers. Therefore, it is generally
unsatisfactory for UV curable systems which are applied
in thin layers. Further, when allantoin is applied in thick
la~ers, only the outer layer may change color. In use, the
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MP0866-U51 165/110
outer layer can rub off or the inner layer can bleed through
to the outer layer, thereby hiding the color change. Thus,
when the part is inspected at a ]ater time, it is impossible
to tell whether the part was adequately heated.
In view of this problem~ it is apparent that there
is a need for temperature indicating compositions that
satisfy the above-mentioned requirements, and also can be
used over a wide range of temperatures, including temperatures
below 150C.
SUMMARY
The present invention is directed to temperature
indicating compositions that satisfy this need, and also
heat recoverable articles having the dried temperature
indicating composition on a surface thereof.
What has been discovered is a technique for
adjusting over a very broad range the temperature at which
temperature indicating compositions change temperature. The
technique is particularly useful for adjusting over a very
broad range the temperature at which temperature indicating
compositions containing folic acid change color. The
technique also has the effect of increasing the blackness of
the final converted temperature indicating composition
containing folic acid.
According to the present invention, a liquid
temperature indicating composition comprises:
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a) a binder;
b) a thermochromic colorant that changes color
at a temperature T1; and
c) an effective amount of activator that the
colorant changes color irreversibly at a temperature T2, T2
being at least 2ûC less than T1, the color change being
substantially complete over a 10C temperature range.
A preferred composition comprises as the activator
a crystallinP organic acid, such as tartaric acid, that has
a pK of less than 4.2 and that is solid in the composition
at room temperature, both before and after the composition
dries. Such an activator is particularly suitable when the
colorant is folic acid.
While not bound by theory, it is believed that
what occurs with an acid activator is that the activator
melts at about T2, thereby activating the thermochromic
coIorant so that the color change occurs. With folic acid,
color changes at temperatures as low as room temperature
have been effected, while a temp0rature indicating compositions
containing folic acid without the activator change temperature
at a~out 240 to 250C.
In another version of the invention, the activator
is an acid precursor that when exposed to UV light or heat
forms an acid that has a pK of less than 4.2. For example,
the precursor can form an acid when exposed to UV light7
~here the acid is solid in the dried composition and melts
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at about T2. Alternatively, the precursor can form an acid
when heated to about T2, where the acid is soluble in the
composition. Alternatively, the precursor can form an acid
when exposed to heat at a temperature less than T2, where
the acid does not dissolve in the composition until the
composition is heated to about T2.
A temperature indicating composition particularly
suitable for heat recoverable polyethylene has folic acid as
the colorant and tartaric acid as the activator. Preferably
the composition comprises at least about 5,Do by weight folic
acid based on the weight of the composition so that an
easily visible color change occurs, and less than about 80Do,
by weight folic acid. For a composition having a UV-curable
binder, preferably the composition contains less than about
400D by weight folic acid. For effective activator activity,
the composition contains at least 5~0 by weight activator
based on the weight of folic acid. Preferably the composition
contains less than about 100o~ and more preFerably less than
about 40~D~ by weight activator based on the weight of the
folic acid because at higher levels, no beneficial effect
results and it can lead to an over-filled composition.
These and other features 3 aspects, and advantages
of the present invention will become better understood with
reference to the following description and appended claims.
DESCRIPTION
The present invention provides a liquid temperature
indicating composition suitable for application such as by
coating to a heat recoverable substrate. The composition
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MP0~66-US1 165/110
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comprises a binder, an organic thermochromic colorant that
changes cclor when heated, and an activator that causes the
thermochromic colorant to change color at a temperature at
least 20C less than the temperature at which the colorant
would change color in the absence of the activator. The
color change is sharp, the color change being substantially
complete over a narrow temperature range of about 10C.
Preferably the temperature indicating composition
is UV curable for reasons that will be presented below.
Thus, the present invention will be described principally
with regard to UV curable systems. However, it is to be
realized that this invention is usable with other temperature
indicating compositions, including those that dry by penetra-
tion, by oxidation, solvent evaporation, and precipitation.
l~ Further, the composition need not be a liquid, but can also
be a paste or a crayon.
The recoverable article may be a recoverable
polymeric material or a memory metal such as a beta brass or
a nickel-titanium alloy. In particular, the recoverable
article may be a wrap-around or tubular sleeve, or other
hollow article, having a heat activatable adhesive on an
inner surface.
A UV-curable temperature indicating composition
comprises:
2~ (a) a UV curable binder which on curing is
capable o~ adhering to a recoverable substrate;
(b) a thermochromic colorant which produces in
the composition a substantial color change
when the composition is heated to a temperature T1;
MP0866-US1 -13- 165/110
(c) an initiator which on exposure to UV radiation
initiates a polymerization reaction in the
binder; and
(d) an effective amount of an activator for
causing the thermochromic compound to change
color at a temperature T2, T2 being at least
20C less than T1.
The binder ordinarily will contain a reactive
oligomer which polymerizes when initiated by the action of
UY radiation on the initiator or by other means. The
reactive oligomer is the primary, and may be the sole,
- component of the binder and is thus responsible for the bulk
of the performance properties of the binder. The binder
may, however, also contain a reactive diluent which will
l~ in general comprise one or more monofunctional and multifunc~
tional monomers; the function of this component is to act as
a non-evaporating solvent and to provide further cross-linking
sites.
Of all the drying techniques available, the use of
UY curing gives to the composition oarticular advantages.
A correct choice of binder~ colorant, initiator, catalyst,
and optional additives allows a composition to be produced
which is suitable on articles such as polyolefins and other
plastics.
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MP0866-US1 165/110
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The following description will examine the several
requirements of such a composition, outline the chemistry of
the major components and finally give examples of preferred
formulations. In the light of this the chemist skilled in
the arts of thermochromism, UV curing, and printing will be
able to select a formulation which fulfills his requirements.
The first quality of paint or ink is its color, and
in the case of temperature indicating paint or ink, color above
and below the transition temperature must be considered. The
color must be chosen bearing in mind the color of the substrate
to which it is to be applied, and the conditions under which it
is to be viewed. Recoverable articles for encapsulation are
often black and in dark manholes or above ground and viewed
silhouetted against the sky ~here they may be difficult to see~
In either case a bright colored composition is desirable. Where
the substrate is black the composition must have good hiding
power, either before or after the color change, and any defi-
ciency here can be made good by the addition of an opacifier
such as titanium dioxide, the higher refractive index allotrope,
rutile, being preferred. If the thermochromic colorant has
ideal properties except for its color, this can be modified by
adding a tinting agent.
The color change is preferably quick and irrevers-
ible. Where the color change is irreversible the composition
will act as a permanent record of the temperature to which
the substrate was subjected so that inspection can be made
sometime after the heating occured.
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The composition must adhere well to the substrate,
so the chemical nature of the substrate and the adequacy
of any surface cleaning being undertaken must be considered.
In the context of recoverable plastics, adherence is regarded
as satisfactory if the composition is not removed by adhesive
tape pulled from a surface that has previously been scored
with a blade. Adhesion is a tougher requirement in the case
of a recoverable material than a dimensionally stable
material since the conformational change may lead to cracking
io or peeling. Furthermore, it is desirable that the coating
can retain its integrity under those conditions which the
recoverable material itself is expected to be able to
endure. The recoverable material may be tested for elongation,
pressure or temperature cycling and flexing, etc., and
proper adhesion of the composition is preferably maintained
throuohout such tests. In addition to remaining adhered, it
is clearly necessary that the composition does not impair
the performance of the substrate to such an extent that it
no longer passes the tests by which it will be judged.
The technique used and the conditions under which
the composition is applied to the substrate will also affect
the choice of components of the composition. The following
can be regarded as desirable features of the composition
before coating:
1. Ready availability
2. Viscosity suitable for printing
3. Long shelf life and no activation by normal
incandescent lighting
4. Low volatility and flammability
3~ 5. Good hiding power for very thin layers
6. Fast cure rate
7. No ancillary component to be recycled
8. Heat not required for satisfactory cure rate
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MP0~66-US1 -16- 165/110
The UV system of curing allows a composition to be
produced which is particularly valuable in each of these
respects. In particular, very high cure rates can be obtained
which, together with a printing technique, allows material to
5 be coated with the composition extremely rapidly. The coating
can be applied very thin, if printed, and formulations can be
prepared which give sufficient hiding power at a thickness of 5
microns or less. Where the colorant itself lacks sufficient
hiding power and it is undesirable to add normal opacifying
agents because of their modifying influence on the final
temperature converted color, it has been discovered that the
components of the binder itself can be chosen to act as an
opacifier on curing. Thin coatings have the advantages of a
satisfying appearance9 prevention of cratering when flame
treated, and a reduction in the quantity of composition that
has to be manufactured, stored, processed, and used. A UV
curable system need not, and generally will not, involve any
solvent; this avoids the energy required for vaporization and
avoids any costly re-cycling apparatus. The fact that no
thermal energy is required is of particular use in conjunction
with an irreversible thermochromic colorant and with an essen-
tially irreversible recoverable substrate. If excessive heat
had to be applied to adhere the composition to the substrate
the resulting product would of course be quite useless.
The following remarks on the chemistry of the
components of the composition show how these objectives
can be achieved. The binder will be discussed first.
The basis of the binder is a reactive oligomer
or prepolymer which polymerizes when subjected to UV radiation
in the presence of a suitable initiator. A second component
of the binder is a reactive diluent which modifies the cure
rate and, for example, the viscosity of the uncured composition.
MP0866-US1 -17~ 165/110
The principal properties of the final cured resin, however,
are generally imparted by the reactive oligomer component.
The reactive oligomer preferably constitutes about 30-80o by
weight of the composition. The binder must be capable of
adhering to the substrate on curing, but it may of course
also wet or adhere to the substrate before curing.
Reactive oligomers or prepolymers are customarily
used in conjunction with free monomeric and polyfunctional
compounds. The functions of added monomers are to modify
the properties of the final cured resin and to reduce the
viScosity of the oligomer which ordinarily would be too high
for conventional application techniques without these
"reactive diluents." Even when reactive diluents are used,
it is often difficult with filled systems, such as compositions
containing thermochromic pigments, to achieve the low
viscosity or rheological properties needed for the chosen
printing or other coating technique. The "reactive diluent"
may comprise from 5-~0, preferably from ~5-65,~c, by weight of
the UV curable binder~
Four broadly defined classes of binder oligomers
will be mentioned: epoxides; unsaturated polyesters; acrylated
resins such as acrylated epoxides, acrylated polyesters,
acrylated polyethers, acrylated polyurethanes; and polyene/
thiols.
2~ An example of the first group is the cycloaliphatic
diepoxide, ERL 429~ available from Union Carbide. This
product will give excellent adhesion to polyethylerle and
other plastics substrates. The chain length between the two
I~P0866-US1 165/110
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functional groups can be altered to vary the flexibility of
the final film, for example, by selecting alternative
diepoxides. If need be, flexibility can be further improved
by incorporating an extender such as polyethylene glycol, a
molecular weight of about 400 being recommended. An alterna-
tive epoxide is that manufactured by Ciba Geigy under the
designation CY179. They can be produced by alkaline conden-
sation of epichlorohydrin and a dihydric phenol.
The preferred group of binders is the acrylated
resins, which can give high cure rates. Polymerization can
be carried out in a solvent and the resin/solvent system
applied to the substrate, followed by evaporation of the
solvent. What is preferred, however, is to initiate polymer-
ization after the composition is applied, thereby dispensing
with the solvent. Since these resins contain vinyl groups,
polymerization is initiated by free radicals. Some free
radical polymerizations are inhibited by oxygen, and may
thus require provision of an inert atmosphere. Initiators
required can easily be formulated free of other undesirable
elements, such as fluorine~ which is present in many cationic
initiators and which is considered undesirable in some
environments.
Acrylated oligomers sometimes are low molecular
weight acrylic polymers which contain residual unsaturated
acrylate or methacrylate functionality. Usually, however,
the term "acrylated oligomer" applies to low MW polymers of
several sorts which are terminated by unsaturated acrylic or
methacrylic ester groups. These base polymers commonly are
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MP0866-US1 165/110
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in the class of polyester, ether, urethanes, and epoxides,
or combinations of these, but for ease of discussion they
may merely be referred to as reactive or unsaturated acrylates,
polyesters, urethanes, epoxides, etc. It should be understood
S that the only active functionality in these resins is that
of the terminal acrylate or methacrylate groups in most
cases. Seven broad classes are listed below, together with
some examples.
~ ~ ~ 1. Acrylated acrylics Celrad~ 7ûO
(Celanese Corp.)
2. Acrylated esters
3. Acrylated urethanes Uvithane 893
also Uvithanes 782, 783
and 788 (Thiokol Corp)
4. Acrylated epoxides Epocryl 370 (Shell
Chemical Co.)
Celrad 3200 (Celanese Corp.)
5. AcryIated ether- Purelast 186
urethanes (Polymer Systems Corp.)
6. Acrylated amide
7. Acrylated carbonate
Mono-functional as well as these multi-functional
products can be produced. A multi-functional product, such
as a difunctional product, will be used to produce a cross-
linked resin.
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In addition to the series of acrylates, a corre-
sponding series of methacrylates exist. In general, the
methacrylates are harder than the corresponding acrylates
due to a more rigid molecular structure, and this may result
in a reduction in flexibility. Acrylates will in general
cure faster than the corresponding methacrylates.
Acrylated urethanes can be produced by reacting
hydroxy-containing acrylates with isocyanates. Diisocyanates
would produce urethane diacrylates. When these compounds
are used as the oligomeric or prepolymeric sole component of
the binder, they usually have too high a viscosity. As
mentioned above, this can be overcome by adding a reactive
diluent. Acrylated urethanes, for example, will be chosen
where good flexibility and abrasion resistance is desired.
Where the molecular chain is thus extended the acrylated
urethane behaves more like a urethane and less like an
acrylate.
Acrylated epoxides will be similar to acrylated
esters, but they are classified in this way due to the
method by which they have been prepared. They are usually
derived from epoxy ring opening by acrylic acid.
The binder may also comprise a mercaptan/olefin
system which reacts by free radical addition of mercaptan
to an olefin. Olefins have a tendency to homo polymerize
so the conditions must be chosen to insure that the rate of
hydrogen transfer from mercaptan is faster than the homopoly
merization rate.
The reactive diluent preferably used in conjunction
with the reactive oligomer may be either monofunctional or
33 multifunctional monomers or combinations thereof. Both are
used for viscosity adjustment and act as nonvolatile solvents.
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Where reduced viscosity is the primary consideration,
monofunctional diluting monomers may be used, and where high
curing rates and a highly cross-linked product are desired,
diluting monomers of higher functionality should be added.
Materials suitable as the reactive monomers include monoacry-
lates, diacrylates, triacrylates, tetraacrylates, and other
polyacrylates, corresponding methacrylates, vinyl ethers,
vinyl esters, vinyl acrylates and unsaturated acids and
their corresponding anhydrides. Some examples are given
below:
isobornyl acrylate
ethylhexyl acrylate
trimethylpropyl triacrylate
butyl acrylate
tetrahydrofurfuryl acrylate
trihydroxypropyl methacrylate
ethoxyethoxyethyl acrylate
hydroxypropyl methacrylate
hexanediol diacrylate
polyethylene glycol diacrylate
polyethylene glycol dimethacrylate
propylene glycol acrylate
triethylene glycol diacrylate
ethoxyethyl acrylate
pentaerythritol acrylate
phenoxyethyl acrylate
hydroxypropyl acrylate
pentaerythritol triacrylate
pentaerythritol tetraacrylate
vinyl ester of versatic 10 acid
N-vinyl pyrrolidone
styrene
vinyl toluene
itaconic acid
maleic anhydride
MP0~66-US1 165/110
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The initiator required in the temperature indicating
composition can be any of many well known compounds and it
preferably is present in an amount equal to about 2-7DD by
weight of the binder. The initiator will be chosen according
to whether ionic (almost invariably cationic) or free
radical polymerization is appropriate. Free radical polymeri-
zation is generally preferred since a greater variety of
reactive materials are available, cure rates are generally
faster, use of a wider range of pigments is possible, and
cure whitening, to be discussed below, can occur. A disadvan-
tage however is oxygen inhibition. The types of initiator
available can be classified in terms of mechanism or in
terms of chemical type (especially acyloins, aromatic
carbonyls and polyhalogenated compounds). For free radical
l~h~ polymerization, benzoin alkyl ethers, which work by homolytic
fission, are preferred Examples include Darocur~ 173 (EM
Chemicals) and Irgacure~184 (Ciba-Geigy), which are 2-hydroxy-
2-methyl-1-phenyl propanone and 1-hydroxy-cyclohexyl phenyl
ketone, respectively. An example of an aromatic carbonyl
which works by hydrogen abstraction or electron transfer is
benzophenone.
The preferred initiator for cationic polymeriza~
tion is a 3M product designated FC508, which is a proprietary
product believed to be a triarylsulfonium hexafluorophosph~te
salt.
In either case the preferred amount of initiator
is from 1-10DD, more preferably from about 2-8Do by weight,
based on the total weight of the composition.
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The thermochromic colorant is at the heart of the
system, and all the other components are present in order
that the colorant may be applied to a substrate quickly,
cheaply and permanently.
The first consideration when choosing a colorant
is the temperature of the color change and whether or not
the change is reversible. Secondly, it must be able in the
composition to provide the desired hiding power. Thirdly,
it must be compatible with a binder system already chosen,
or it must not put undue restraints on the choice of a
binder system. Fourthly, it must not adversely affect the
substrate to which it is applied.
The colorant can have an effect on the efficiency
of the UV cure~ on rheological properties, on storage
stability and on the surface or gloss of the final film. If
the pigment absorbs or scatters radiation of the wavelength
that activates the initiator, then efficiency of the initiator
will be reduced. If the colorant becomes involved chemically
in the initiator reaction it may thereby deactivate the
initiator.
Both organic and inorganic compounds can be used
as the thermochromic colorant, and attention is directed to
the prior art patents referred to above which discuss
suitable compounds.
The color change of an inorganic thermochromic
r;aterial can result from decomposition of the molecule,
which is preferred because it is irreversible, from a cnange
in crystalline phase, ligand geometry or number oF molecules
o~ solvent in a co-ordination sphere. Also, colorant change
31 can result from an alternation in the equilibrium between
~arious complexes in solution.
78~
-24-
With an organie thermoehromie eolorant, a ehange in
equilibrium between the following speeies ean .be responsible for
the eolor change:
Aeid - base
Keto - enol
Lactone - lactam
Stereoisomers
Crystal struetures
For further information on the chemistry of color change,
the reader is referred to J.H. Day, Chem Reviews, 1968 pp. 649-657
and 1963 pp. 65-80.
Since hiding power is of importance, especially where
the substrate to be coated is black, the colorant will generally
be a solid material until decomposed. In very general terms, the
eolor ehanges that ean be expeeted are:
White to blaek or brown
Yellow to black or brown
Blue t~ blaek
Green to blaek or brown
Red to blaek
Orange to Red
Yellow to Orange
Green to yellow to orange.
,~
7~
POB66-US1 165-/110
-25-
Many thermochromic colorants, although often
described as pigments, have poor hidino power and must be
used in hioh concentration, often with an opacifier such as
titanium dioxide, applied in thick layers. Thick layers
have to be applied by brush, spray or by dipping which can
involve a lot of waste. A large amount oF an opacifer is
undesirable, not only because it leads to higher viscosity,
but also because it lessens the color change. By employing
one of the UV curable binders of this invention it is possible
to produce a temperature indicating composition havino
sufficient hidino power at a thickr,ess of 1 micron or less,
even when pigments of poor hiding power are used, rather
than the more usual 25 to lOO microns. The reason that this
can be done by the present invention is that certain of the
UV curable binders that we have discovered can themselves
act as an opacifier if desired, giving the enhanced opacity
necessary for thin film coatings having good hiding power.
Althouoh we do not wish to be limited by any theory, the
way in which the binder achieves this is believed to be by
forming, on curing, at least two micro-dispersed substantial
homopolymeric phases. They are able to do this in preferred
embodiments due to the presence of two polymerizable components:
the reactive oligomer, which preferably has acrylic ester end
group functionality, and the diluting monomer which preferably
is a vinyl ester. The reactivity between acrylic esters and
vinyl esters during free radical initiated polymerizations is
unfavorable. The unfavorable reactivity ratio (a well defined
term between these two polymerizable constituents in the science
of copolymerization kinetics) leads to the formation of two
incompatible homopolymer systems and subsequently to microphase
separation, which is manifested by a whitened appearance. The
resulting cured product has hiaher opacity and can thus aive
MP0866-US1 165/110
-26-
the colorant the desired extra hiding power. In designing a
formulation, therefore, one can choose the colorant for its
temperature change and color change, decide what is the
maximum permissible thickness of final film, and then choose
a combination of binder components with reference to reactivity
ratios to give the correct hiding power. Some colorants do
not, of course, require this additional hiding power, in
which case one has more freedom in the choice of binder.
The preferred colorant is folic acid. It has been
found, as demonstrated by the examples below, to be particularly
sensitive to adjustment of the temperature at which the
color change occurs. Folic acid, without adjustment,
changes color at about 240 to 250DC. Compositions containing
folic acid with suitably chosen catalysts change color at
temperatures less than 230C, and even less than 200~C and
150~C. Another advantage obtained with folic acid is that
folic acid normally changes color from yellow to a brown/black.
Compositions according to the present invention containing
folic acid change color to a dark black, thereby providing a
more visible color change.
Allantoin has been found to be an unsatisfactory
colorant in this invention. Attempts to reduce the conversion
temperature of allantoin with a strong organic acid have
been unsuccessful in that although the conversion temperature
was reduced~ conversion occurred over a broad temperature
range. A narrow temperature range of conversion on the
order of 1 o5c is required for heat-recoverable articles.
:"
I~P0866-U51 165/110
-27-
In one version of the invention, the activator is
an organic acid. Only relatively strong acids have been found
to be effective. Thus, the acid requires a pK of less than
about 4.2. Of the acids tested, only one acid having a pK
substantially less than 4.2 failed to reduce the conversion
temperature of a composition containing folic acid. That acid
is o-toluic acid which has a pK of 3.91 according to the CRC
~andbook of Chemistry and Physics.
It is necessary that the activator not adversely
affect the properties of the other components of the composi-
tion. It must not interfere with the UV curing process, it
must not cause premature and uncontrollable degradation of
the temperature indicating composition, and it must cause
color change over a narrow temperature range. Preferably
the activator is solid at room temperature and is insoluble
in the binder system both before and after the binder system
is cured. Acid activators which dissolve in the binder
system have been found to prematurely and uncontrollably
degrade the thermochromic colorant when it has been folic
acid. Also, volatile strong acids as hydrochloric acid and
trifluoroacetic acid have not been found useful, most likely
because they are lost by evaporation.
The activator can also be an acid precursor that
when heated or subjected to UV light forms a strong acid
2~ having a pK less than about 4.2. Such an acid precursor can
be a compound that when heated as the recoverable article on
which it is coated is heated degrades Dr decomposes to yield
a strong acid at T2. Another example of an acid precursor
is a material that generates an acid when it is exposed to
U\' lisht, where the acid melts or dissolves in the composition
al about T2.
-2~-
Not to be bound by theory, it is believed that this
adjustment of the temperature at which the temperature indicating
composition chan~es color is a result of acti.vation o:E -the thermal
decomposition of the colorant. It has been found that soluble
acids such as sulfuric acid when added to a coating composition
containing folic acid cause decomposition of the folic acid and
color change at room temperature. Crystalline, insoluble acids
when added to a folic acid composition, have been ~ound to promote
the decomposition and color change of folic acid at a temperature
g~nerally related to the melting point of the added acid. Conver-
sion occurs at a temperature 10-40C below the nominal melting
point of the acid.
The temperature at which color change occurs can vary
depending on the physical form of the catalyst, i.e., powder,
crystals, etc., and the method of incorporating the catalyst into
the composition, i.e. grinding, milling, or simple mixing.
Exemplary of materials which serve as acid precursors
and which have been found to be effective are triarylsulfonlum
salts which generate strong acids when exposed to UV light or heat.
These salts are available from 3M under catalog numbers EC508 and
509. These salts are described in U.S. Patent Nos. 4,058,401;
4,138,255; and 4,161,478.
The composition of this invention contains from about 5
to about 80% by weight colorant. Preferably the composition con-
tains at least 5% by weight activator, based on the weight
.~
8~
MP0866-US1 165/110
-29-
of colorant~ for effective activity. Preferably the composi-
tion contains up to about 100~o weight of the activator,
based on the weight of the colorant. Greater quantities can
be used, but there is generally no beneficial effect and it
can lead to an over-filled composition.
All percentages herein are based on the weight of
the composition unless stated otherwise.
The major optional components of the composition
are as follows:
1. Surfactants such as FC-171, a fluorocarbon
surfactant (3M)
r~
~is 2. Thickening agent such as Aerosil~972, a
~' hydrophobic silicon
3. Color tinting agents such as phthalocyanine
green 85
4. Opacifiers, such as titanium dioxide
5. Sensitizers to improve activity of the initiator,
such as ~C-510 (3M)
6. Stabilizers or deactivators, such as zinc
sulfide or zinc borate.
MP0866-US1 165/110
-~0-
Where these components are used, the following
quantities are preferred:
Surfactant û.5 - 1.5Do wt.
Thickening agent û - 10o wt.
Oolor tint 0 - 5c wt.
ûpacifier 0 _ 3Do wt.
Sensitizer 0 - 8DG
Deactivator 0 - 10o
Depending on the opaqueness of the composition and
the color of the substrate, the composition is applied to a
substrate to produce a coating from about 5 micrometers to
about 20 micrometers thick. The more opaque the coating,
the less thick need be the coating to obtain adequate
hiding.
EXAMPLEs
.The invention is now further illustrated by the
following examples. The percentages given are percentages
by weight.
MP0866-US1 165/110
-31-
Example 1, Control Sample
A typical UV curable TI temperature indicating
composition employing folic acid as the thermal indicating
substance is composed as follows:
~D b~ weight
VV10 (vinylester of versatic acid, 39
Shell Chemical Co.)
Irgacure 184 (1-hydroxycyclohexyl phenyl 4.5
ketone, Ciba-Geigy)
lO Celrad 3200 (acrylated epoxy oligomer,27
Celanese)
Folic Acid (USP, ground powder) 25
Igepal C0-430 [nonylphenoxypsly (ethyleneoxy~ 3.5
ethanol, GAF]
15 Thixatrol ST (castor oil derivative thickening l.0
agent, NL Industries)
The above composition was first mixed in a Waring
Blendor and then ground on a three roll paint mill. This
composition when coated onto a polyolefin sheet substrate
at a thickness of 10-15 microns and UV cured was bright
yellow in color with good hiding power. Upon heating this
sample to a temperature in the range of 240 to 250C, the
color changed sharply from yellow to dark brown-black. The
color change was irreversible.
33788
MP0866-US1 165/110
-32-
Examples 2-27, Acid Containinq Samples
The composition described in Example 1 was modified
by the addition of the acids listed below in table 2. Test
specimens for observing and measuring the thermal conversion
temperatures ~ere prepared as in Example 1. The initial
appearance of the coated and cured samples was identical to
that of Example 1, the unmodified composition. The results
ere summarized in the following table. Color changes were
irreversible, and were substantially complete over a 10C
temperature range.
Effect of Catalysts on the Thermal Conversion
of Folic Acid Containing, UV Cured Coatinqs
Conversion Final
Example # Activator M.P.,C pK Temp. DC Appearance
2 Maleic Acid 131 1.83 120 Black
3 meso-Tartaric Acid 146 3.22 120-130 "
4 Citric Acid 153 3.14 130 "
d-Tartaric Acid170 2.89 13û-140 "
6 Malonic Acid136 2.83 135 "
7 Succinic Acid188 4.16 135-145 "
8 Oxalic Acid 187 1.23 145 "
9 Glycolic Acid79 3.83 150 "
Ascorbic Acid193 4.10 150-160 "
11 d,l-Tartaric Acid 205 2.98 155-165 "
2~ 12 Salicylic Acid160 2~97 160 "
,,~
!'IPO866-US1 ~997~ ` 165/110
Conversion Final
Exam~le # Catalyst M.P.,C pK Temp.C Appearance
13 cis-1, 2-cyclohex- 195 3.45 180 "
nedicarboxylic acid
14 Fumaric Acid 287 3.03 210 "
Tannic Acid 200 19û-205 "
16 2-Naphthoic Acid 185 4.17 210 Black-
Brown
17 o-Toluic Acid 103 3.91 * " "
18 Benzoic Acid 122 4.19 * " "
19 Phenylacetic Acid 77 4028 * " "
Adipic Acid 153 4.43 * " "
21 p-Anisic Acid 185 4.47 * " "
22 Suberic Acid 140-4 4.52 * " "
23 Crotonic Acid 71 4.64
24 Neoheptanoic Acid 25 4.89 *
Stearic Acid 70 5.00 *
26 Anthranilic Acid 146-7 6.97 * " "
27 p-Toluenesulfonic 104-5 0.7 90 Black
Acid
* The conversion temperature was essentially the same as
that of the control sample, Example 1.
9~
.. ,
~lP0866-US1 165/110
-34-
Examo~ 8
A preferred UV curable composition is as follows:
Component O
V~'10 Vinyl ester of versatic acid,
Shell Chemical Co. 35.1 DC
Celrad 3200 acrylated aromatic/aliphatic
epoxy blend, Celanese Corp.28.7,o
Irgacure 184 free radical photoinitiator
Ciba Geigy 4.6Do
lû Folic acid thermochromic colorant25.0,o
Phthalocyanine Green BS
E~ASF~ û .1 ,o
Ganex~V-216 a poly(vinylpyrrolidone)
surfactant GAF Corp. 1.0o
d,l-Tartaric Acid 5,0,o
Thixatrol ST Colloidal Thixotropic
agent, National Lead 0.5~o
This composition exhibits cure whitening and has
excellent hiding power and contrast at coatings as thin as
20 5 microns. The coatings produced were tough and flexible
and adhered well to polyolefins, including heat recoverable
polyethylene.
The composition changeo color from yellow-green to
a dark black over a temperature range of 160-170C.
~9~
~P0866-US1 165/110
It was applied to a polyethylene substrate with
a aravure printer.
xample 29
The following prospective formula show the use
of the present invention with a temperature indicating
composition that dries by solvent evaporation.
Component
Acryloid B66, methyl 4.2
methacrylate polymer, Rohm ~ Haas
Perchlorethylene l9.2
.1 ~1 ! 1- Trichloroethane 57.3
Folic Acid 13.D3
Monastral green, DuPont .07
Irganox 1U10, anti-oxidant .27
Ciba Gegy
Plastanox 1212, anti-oxidant .27
American Cyanamid
Dioctylphthalate 55
Thixatrol ST .ll
d,l-Tartaric Acid 5.û
~9~
MP0866-U51 165/110
-36-
EXAMPLE 30
Using the binder of Example 17 the folic acid
was replaced with Congo Red and d-tartaric acid was used as
an activator. When the composition was placed on a slide,
it changed color about 130 - 140DC. When the d-tartaric
acid was left out of the composition7 it changed color at
about 200C. The color changes were irreversible and were
substantially complete over a 10C temperature range.
EXAMPLE 31
_
Using the composition of claim 1, the folic acid
was replaced with sugar. In one test, tartaric acid was
added as an activator, and the composition chanoed color at
about 130 - 140C. In another test, toluene sulfonic acid
was added as an activator, and the uncured composition
changed color at about 9û~C. The color changes were irrever~
sible and were substantially complete over a lûC temperature
range.
~XAMPLE 32
__
Using the composition of Example 1, the folic
acid was replaced with Methyl Orange and tartaric acid was
added 3S an activator. The composition changed color at
about 130 - 14ûC. The color change was irreversible and
was substantially complete over a 10C temperature range.
~lP0866-US1 165/110
-37-
Although the present invention has been described
in considerable detail with reference to certain preferred
versions thereof, other versions are possible. Therefore
the spirit and scope of the appended claims should not
necessarily be limited to the description of the preferred
versions contained herein.
