Note: Descriptions are shown in the official language in which they were submitted.
3~
This invention relates to the dryiny of coatings, fil~s
and the like. By the invention there is provi~ed an improved
process (and resultan~ product) whereb~ said dr~ing is carried
out more efficaciously than be~ore.
Two component systems with their infinitely variable
formulation possibilities and above average dried film
characteristics, have served the purpose wèll of introducing
the VAPOCURE (TM) Process to the industrial coating arena.
Unfortunately their main shortcoming, that of a finite pot
life, makes them unsuitable for use in those applications where
large volumes of paint are consumed on a daily basis. In these
instances there is no time available for the mixing, reduction
and stabilisation of a two component system as a paint kitchen
is invariably employed in which a large amount of paint (which
is usually supplied at application viscosity) is recirculated
continuously between itself and the point of application.
This paint, by definition, has to be a one component type
whose viscosity and other rheological properties remain stable
indefinitely while being used in conjunction with a pressurised
recirculation system. The paint has to cope with being left in
the feed lines during extended stoppages, such as breakdowns or
holiday periods, and flow at the push of a button when again
required. The two component system with its finite pot life,
its constantly varying viscosity characteristics and its
sensitivity to pressure, is clearly unsuitable under these
circumstances.
~ requirement exists for both water and a catalyst to be
present within the drying chamber if moisture curing, one
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~26364~i~
component paint systems are to polymerise to dryness~ The
amount of water per gram of applied paint necessary to couple
with the prepolymer molecules can be calculated accurately and
the empirically established requirement for V~POCURE (TM) two
component systems of one catalyst molecule for every six
reaction sites, holds also for one component systems, which
means an optimum ratio of water to catalyst can be calculated
for the formulation.
When all these considerations are taken into account and
the drying conditions maximised, the one component paint system
will rapidly poly~erise to give dried films of initial hardness
and chemical resistance which overshadow the established bench
marks achieved with the two component formulations.
This is possible because the average molecular weight of
the reacting species in a one component system is much lower,
allowing high solids at the same application viscosity while a
higher NCO percentage results in a higher crosslinking density
which aids in the expulsion of catalyst and solvent as drying
proceeds.
Whereas a two component system has all the necessary
ingredients for polymer formation available in the applied
film, a one component system needs a finite amount of water to
be present if complete polymerisation is to take place. The
water enters the film in a gaseous state from the air
surrounding the painted article, as does the catalyst. A
concentration gradient needs to exist for this water and also
for the catalyst.
It is an object of the present invention to provide a
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hydrated cataly~t cornplex and process or using
same -to dry one component coatings and substantially
overcome or ameliorate the difficultics associated
wi-th drying such coatings.
In one broad aspect, the invention provides
a process for forming a dried coating upon a suitable
substrate, the coating being a one component vehicle
containing free isocyanate groups. The process
is characterized in that the vehicle is dried
by being subjected to treatment by a hydrated
catalyst complex which transports water molecules
into the coating.
In another broad aspect, the coating
is undried and is subjected to treatment with
a hydrated catalyst complex in an atmosphere at
relative humidity of 40 to 80 percent.
The inventio~ finds application in the
drying of paints, lacquers, varnishes, printing
vehicles and printing inks, liquid adhesives,
surface coatings, caulking compounds and the
like.
Definitions
1. The word "coating", when used as a noun, is,
for the purposes of this invention, to be understood
as synonymous with "film" (or the like).
2. The word "drying" (i) includes within its
ambit "curing" and (ii) indicates that the coating
is either free from "tack", insoluble in solvent,
possessed of an advanced degree of integrity,
or able to withstand reasonable abrasion or pressure
without damage.
3. The word "substrate" includes any surface
to which the vehicle can be adheringly applied,
and upon which it will be retained while treatment
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with the agent is being effected.
4. The word "vehicle" includes all paints, lacquers
and the like which contain free isocyanate groups.
5. The expression "drying agent" means the agent
which
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~31~
efects the drying of the coated vehicle and is multi-component
comprising a first component, water, together with at least one
further component selected from an amine, or any other
hydratable compound - such as an organo rnetal or inoryanic
metal salt - which, in association with the ~ater, will
accelerate the desired reaction pathway.
Although no structural analysis has yet been made it is
believed that the water and the further component(s) of the
agent interact in vapour phase to form a hydrated catalyst
complex, which unexpectedly and dramatically accelerates the
rate of drying of the vehicle. However, it is to be understood
that the specification is not to be construed as bound to any
particular theory of drying operation.
6. The term"free isocyanate groups" includes any compound
having potentially free such groups, ie. the pre-polymer has
isocyanate groups which are releasable, or available, for
reaction with water molecules (for the purposes of polymer
propagation and/or film formation); and includes not only
polyisocyanates with urethane and urea structure but also those
with polyisocyanurate, biuret, and allophanate structure.
7~ The term "amine" includes tertiary amines and
alkanolamines and these can either be; a) Polyfunctional, b)
Aromatic, C) Aliphatic or Cycloaliphatic in nature. Specific
examples are triethylamine and dimethylethanolamine (DMEA), and
ditertiary amines such as N,N,N',N'-tetramethylethylenediamine
(TMEDA) and N,N,N',N',2-pentamethyl-1,2-propanediamine (PMT) -
and, indeed, any combination of such amines, proportioned as
required, whereby advantage may be taken of the synergistic
-- 5 --
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effect of such a combination.
8. The word "atmosphere" relates to the gaseous environment
in the drying chamber.
Examples of organo metals are dibutyl tin dilaurate, lead
tetraethyl, titanium acetyl acetonate, dimethyl tin dichloride,
and stannous and zinc octoates.
Examples of inorganic metal salts are bismuth nitrate and
ferric chloride.
The drying agent preferably effects its treatment in the
vapour-phase. The term "vapour-phase" denotes that the agent
is in gaseous, vapour, or other entrained air-borne form (e.g.
dispersion, fog or aerosol) in which it is available for
reaction. Attainment of this phase is achieved by the
atomisation of predetermined quantities of water and the
selected further component. The concentration levels (of water
and further component(s)) may be varied in accordance with
situational requirements. However, there appears to be a
relationship between the extent of hydrated complex formation
and the acceleration of drying.
Usually a substrate, coated with the vehicle, is subjected
to ~reatment in an atmosphere containing water atomised to
provide a Relative humidity level within the range 40%-80~
dependant upon the existing temperature which can lie within
the range 10C - 40C.
The further component(s) is usually present at a "parts
per million" level, and varies with the selected component.
For example fir DMEA, the "atmosphere" can contain 500 to 5000
parts per million, for TMEDA 250 to 2500 parts per million and
-- 6 --
PMT~200 to 2000 parts per million.
The present invention is particularly suitable for the
drying of commercially available one-cornponent vehicles. It is
well known that one-component systems traditiorlall~ require
extended periods to reach a full crosslinked state of dryness
under ambient (temperature and humidity) conditions as the
moisture necessary for curing must permeate into an environment
that is essentially hydrophobic~ Acceleration of the cure by
increasing the temperature has been found to be
counterproductive as this has the effect of minimizing
available water at reaction sites. These factors previously
have made one-component moisture curing systems unviable on a
commercial scale.
However, the present invention does not merely facilitate
the introduction of moisture; rather, it so accelerates the
crosslinking reaction that fully dried films can be produced
within, for example, 3-4 minutes. For instance, the exposure
of a standard paint test panel, coated with a one-component
paint with a thickness up to 1~0 microns, to an atmosphere of
the drying agent of the present invention can result in
transition from liquid to solid in shortened time periods as
indicated above.
This has led to the inevitable conclusion that both water
and catalyst molecules would have to be present at the reaction
site simultaneously. The best theory that explains this
phenomena is that the catalyst molecule undergoes complexing
with available water molecules and this hydrated catalyst
complex permeates the wet paint ~ilm proportionately to its
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vapour pressure concentration gradient. The existance of this
hydrated catalyst complex is suppGrted by the following
experimental data wherein specialised apparatus used in the
determination of relative heat of combustion effects was used
with various chemical entities.
The combustimeter is an apparatus which utilises a change
in the resistivity between a reference and an active filarnent
as generated by the heat of combus tion of any oryanic compound
in the vapour phase and converts it to an OUtpllt signal
measured in millivolts. The millivolts generated is then
proportonal to the heat of combustion of the compound being
tested and its concentration. The following tables and graphs
indicate the results obtained when specific quantities of five
different chemical substances were tested at constant
concentrations and varying Relative humidity levels. Initia~ly
the output voltage obtained with the apparatus was determined
to be zero and independent of Relative humidity from 0 to 100~.
The concentration of each material in the vapour phase was
chosen so that it coincides with optimum output voltages within
the range under investigation.
As evidence ~f the existence of a hydrated ~atalyst
complex five compounds DMEA, TMEDA, PMT, SOLVENT NAPHTHA 100
and ETHANOL were tested to ascertain whether their heat of
combustion (as measured by output voltage) altered with changes
in Relative humidity. The results are set out in Table I.
-- 8 --
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In the drawings which lllustate the invention,
FIGURE 1 is a graph showing heat of combus-
tion alterations wi-th changes in relative humidity for
five compounds;
F~GURE 2 is a graph showing the effect of
temperature on the cure rating of DMEA;
FIGURE 3 is a graph showing the effect of
relative humidity on the cure rating of DMEA; and
FIGURE 4 is a graph showing the effect of
impingement velocity on the cure rating of DMEA.
,
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TABLE I
CATALYST - DMEA
Temperature
Conditions - 25C Concentration = 2055 ppm.
R.H.% Millivolts
0 23.94
22.80
14 21.04
14.25
10.26
7.98
81 5 70
________________ ____________~______________ ___________________
CATALYST = TMEDA
Temperature - 25C Concentration = 776 ppm
~ o ~ Millivolts
1~ 11 . O
o 18.25
17.10
14.25
62 12.83
88 10.26
___ _______ ____ ______ ____ ______________________________ ___
~: _ g _
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CATAI.YST = PMT Concentration = 775 ppm
Temperature Condition = 25C
R.H.% Millivolts
4 17.10
16 17.10
14.25
62 12.54
11.40
________________________________._______________________~________
SOLVE~T NAPHTHA 100 Concentration = 678 ppm
Temperature Condition = 25C
R.H.% Millivolts
14.25
16 14.25
14.25
14.25
14.25
___________________________________
_____________________________
ETHANOL Concentration = 1770 ppm
Temperature Condition = 25C
R.H.~ Millivolts
16 14.25
24 14.82
14.~2
14.82
__________________________________
__.____________________________
The results of Table I are summarised on proceeding graph
I, wherein the vertical scale indicates the millivolt
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reading, whilst the horizontal scale indicates the Relative
humidity as a percentage, at 25C.
The following is a legend for the graphed information.
DMEA is represented by: line A
TMEDA is represented by: line B
PMT is represented by: line C
Solvent Naptha 100 is represented by: line D
Ethanol is represented by: line E
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The results were plotted as output voltaye versus Relative
humidity. Three compounds were found to be affected by
variations in Relative humidity while two were not. The three
affected compounds are all tertiary amines with the
alkanolamine (DMEA) displaying particularly strong deviations.
The other two compounds displayed no dependancy on Relative
humidity giving a steady 14 to 14.5 M.V. output voltaye across
the 0-100 R.H. range. One of these non-dependant compounds is
ethanol which normally would form quite strong hydrogen bonding
with water in the liquid phase. The fact that no deviation was
experienced with ethanol ~urther reinforces the belief that the
complexing with water only occurs at the tertiary amine end of
the DMEA molecule which also contains a hydroxyl group in its
structure.
The graph for DMEA displays a linear relationship with
Relative humidity suggesting that complexing with water
molecules is proportional to the cncentration of the two
compounds and that the complex formed has in fact an altered
heat of combustion to that of anhydrous DMEA itself.
As can be seen from the foregoing data it is important
with one component systems that certain parameters be
maintained above minimum levels to ensure the correct cure
response is achieved within the VAPOCURE chamber. Since
applied film weight, catalyst concentration and impingement
velocities are all either fixed or controllable under existing
equipment design conditions, it is left then to ensure that
humidification and temperature control are available to
complete the requirements.
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The invention will now be described with specific
reference to the following numerical examples. It will again
be appreciate~ that such ensuing description is intended to be
merely illustrative of the invention.
Com~arative Exam~le 1
___ ____________ ____
Two standard paint test panels of rectangular shape (the
substrate) were sprayed with a one component white paint, which
paint had been previously mixed with the quantity of water
calculated as being necessary to effect complete crosslinking.
The first panel was used as a control panel and allowed to dry
in air, while the second panel was used as a test panel and
treated as follows:
The test panel was placed in a sealed drying chamber
wherein was generated, by injection of carefully metered
quantities of DMEA (dimethylethanolamine), an atmosphere of
this material having a concentration measured as 1~50 parts per
million. The temperature was maintained at 25C and the
Relative humidity measured at 40~. This environment was
recirculated at 1.5 metres per second for a period of two
minutes after which a three minute purge cycle commenced.
After the purge cycle had evacuated the chamber of the DMEA and
replaced it with fresh air, the test panel could be safely
retrieved.
The test panel displayed signs of surface skinning while
the underlying regions remained quite wet. It was three - four
hours before the test panel had a cure rate of 3-4 (see page
14. The control panel took some 8-10 hours to reach a similar
cure rate.
~3~
Comparative_Exam~e 2
In this experiment the two panels were sprayed with a one
component white paint, which had been previously mixed with
0.5% w/w DMF~ to catalyse the curing reaction. Again one panel
was left as an air drying control while the second test panel
was treated as follows:
The test panel was placed in a sealed chamber wherein was
generated, by injection of carefully metered quantities of
water to create an atmosphere with a Relative humidity level of
65~ at a temperature of 25C. This environment was
recirculated at 1.5 m/sec for a period of 2 minutes after which
a 3 minute purge cylce restored normal conditions and the test
panel could be retrieved.
The test panel had experienced a slight increase in tack
but it was 4 hours later before it had a cure rate of 3-4 (see
page 14). The control panel took 6 hours to reach a similar
cure rate and considerably longer to achieve resistance to
solvent rub.
Although the transport of the DMEA molecule into the paint
film by means of the concentration gradient, aided by
impingement velocity, is readily demonstrated and understood,
the transport of water rnolecules is obviously less clear. The
very sustainable hypothesis however is that the catalyst, being
extremely hydrophilic, complexes with available water molecules
and thus allows transport into the wet paint film. In the
vicinity of an NCO group the water molecules undergo rapid
reaction, perhaps allowing egress of stripped catalyst
molecules from the film.
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The reliance of ambient conditions to s~pply the necessary
water in the curing tunnel all year round can prove risky,
particularly in areas that experience low Relative humidity at
certain times of the year. For this reason it is necessary to
humidify the curing zone o~ any VAPOCURE enclosure if VAPOCURE
one component systems are to be utilised.
Table I demonstrates the interaction between water and
catalyst as gravimetric additions to a one pot VAPOCURE paint
in bulk conditions not in vapour phase.
TABLE II
__ _ ___ _ _
EFFECTS OF MOISTURE/CATALYST ON VAPOCURE
____________________,___________________
l-POT PAINT SYSTEM
__________________
_________________________________________________ ___
2~ W/W TIME TO TIME TO
ADDITION THICKEN GEL
H20 2 hrs 12 hrs
H2O:DMEA 32 mins 5 mins
1 : 1
_____________________________________________________
DMEA 25 mins 60 mins
Nil - indef1nite
stability in
sealed
containers.
_____________ _______________________________________
Workin~ Exam~le 1
______ ___ _ ____
In this experiment the two standard paint panels were
coated with the one component white, as supplied in its
unmodified stable form. One panel was again left to air dry as
a control while the second was tested as follows:
The test panel was placed in a sealed drying chamber
wherein was generated, by injection simultaneously of carefully
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~L263~
metered quantities of DMEA and water, an atmosphere containing
1250 parts per million of DM~A at a Relative humidity level of
65~ at 25C. This environment was then recirculated for 2
minutes after which a 3 minute purge cycle restored normal
conditions and the test panel was removed. The test panel was
found to have a cure rate of 1-2 (see page 14) and minutes
later showed minimal effect when subjected to 20 double rubs of
methyl ethyl ketone (MEK). The control panel was still quite
wet at this point and even after a twenty four hour period
could be dissolved by contact with MEK.
Working Example 2
____________ _ __
In this experiment the two standard paint panels were
coated with the one component white, as supplied in its
unmodified stable form. One panel was left to air dry as a
controlr while the second was tested as follows:
The test panel was placed in a sealed drying chamber,
wherein was generated by injection simultaneously of carefully
metered quantities of both TMEDA and water, an atmosphere
containing 600 parts per million of TMEDA and a Relative
humidity level of 65~ at 25C. This environment was then
recirculated for 2 minutes after which a 3 minute purge cycle
restored normal conditions to the chamber and the test panel
was removed. The test panel was found to be fully cured at
this point, and minutes later developed full resistance to
contact with MEK. The control panel was quite wet and a check
the next day revealed some dissolving with MEK.
Working Exam~ e 3
In this experiment the two standard paint panels were
~Z~i36~2
coated with the one component white, as supplied in its
unmodified stable form. One panel was left to air dry as a
control, while the second was tested as follows:
The test panel was placed in a sealed drying cha~ber,
wherein was generated by injection simultaneously of carefully
metered quantities of both PMT and water, an atmosphere
containing 500 parts per million of PMT and a Relative humidity
level of 65~ at 25C. This environment was then recirculated
for 2 minutes after which a 3 minute purge cycle restored
normal conditions to the chamber and the test panel was removed.
The test panel was again found to be fully cured at this
point and minutes later developed full resistance to contact
with MEK. The control panel was wet, and a check the next day
revealed some dissolving with MEK.
Other variables which act to change the rate of cure with
one pot systems are catalyst concentration, temperature, film
weight and impingement velocity.
Since catalyst concentration and film weight should be
held constant then temperature, Relative humidity and
impingement velocity are the only other variables to be
considered.
Since temperature dictates the air's capacity to carry
water, then it is obviously important that this variable should
not fall below a preset level. Also the rate of any reaction
is temperature dependent so that very low temperature levels
may have the twoEold detrimental effect of minimising
atmospheric moisture and retarding reaction rates.
The impingement velocity of the recirculating air stream
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aids in providing the necessary concentration gradient to the
water and catalyst mixture so that permeation of the wet paint
film can take place. Velocities should be maximised although a
minimum of 1.0 metre per second can be tolerated.
Graphs II, III and IV demonstrate the effect of these
variables on the cure rating of a one component system.
In relation to graphs II, III and IV the vertical scales
indicate the cure rate (see below for explanation of this)
whilst in relation to
Graph II: the horizontal scale indicates the temperature in
C. For this graph, the impingement velocity
was held at 1.4 metres per second, the Relative
humidity was kept at 60% and DMEA was at a
concentration of 1250 parts per million;
Graph III: the horizontal scale indicates the Relative
humidity as a percentage. For this graph, the
impongement velocity was held at 1.4 metres per
second, the temperature was kept at 25C and
DME~ was at a concentration of 1250 parts per
million; and
Graph IV: the horizontal scale indicates the impingement
velocity in metres per second. For this graph the
~elative humidity was kept at 60%, the temperature
was maintained at 25C, whilst the DMEA
concentration was 1250 parts per million.
The cure rate has a direct correlation to the following
hardness scale.
4~
Cure Rate Hardness
1 'H' pencil hardness
2 '3B' pencil hardness or better
3 '6B' pencil hardness or better
4 imprintable
slight tackiness
6 tacky film
7 film mobile
8 thick liquid
9 liquid
unchanged
Table III, which appears directly after Graphs II,
III and IV, serves to demonstrate how the cure rate of
a specifically chosen one component system is effected
by those critical variables previously outlined.
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TAB LE I I I
PANEL NO. P.P.M. R.H. TEMP I.V. CURE RATE CRITICAL FACTOR
DMEA
1 650 61 20 3.5 9 Low Catalyst
Concentrations
____ _______________ ____ ______________________________________
800 6~ ~6 3.5 ~ Low Catalyst
Concentrations
Low temp.
________________________________________________________________
3 1250 30 32 1.4 4 Low R.H.
________________________________________________________________
4 1350 55 31 0.8 3 Low I.V.
________________________________________________________________
1250 66 20 1.~ 1 Correct
Conditions
6 1000 60 25 1.4 1 Correct ~
Condition
____~..___________________________________________________________
I.V. = Implngement velocity (metres/sec)
R.H. = Relative Humidity (%)
The following conditions need to be maintained for the
successful drying of one component paint systems for DMEA.
(A) Dr~ing Chamber
__ __ ________
1. Temperature 25-30C
2. Relative Humidity 60 65% maximum
3. Air Impingement Velocity not lower than 1.0 m/s.
4. Catalyst concentration (1100-1250) PPM
(B) Post-Dry Chamber
1. Temperature 25C (approximately).
2. Air Impingement Velocity not lower than 1.0 m/s.
Further experiments in which concentration levels of water
and catalyst were varied demonstrated that certain
concentration ratios maximized the curing effect and that this
~o ratio is related to the formation in air solution of a specific
hydrated catalyst complex which facilitates the introduction of
aater into the hydrophobic paint film.
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In summary, the invention provides a process for
accelerating the polymerisation of isocyanate terminated
prepolymers by facilitating the introduction of water into
these systems by way of hydrated catalyst complexes in the
vapour phase.
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