GB2166976A - Process for curing/drying coatings - Google Patents

Abstract

Hydrated catalyst complexes and a process for accelerating the polymerisation of isocyanate terminated prepolymers by facilitating the introduction of water into these systems by way of the hydrated catalyst complexes in the vapour phase.

Description

SPECIFICATION Hydrated Catalyst Complex and Process This invention relates to the drying of coatings, films and the like. By the invention there is provided an improved process (and resultant product) whereby said drying is carried out more efficaciously than before.

Two component systems with their infinitely variable formulation possibilities and above average dried film characteristics, have served the purpose well 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 sensitivityto pressure, is clearly unsuitabie under these circumstances.

A requirement exists for both water and a catalyst to be present within the drying chamber if moisture curing, one 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 Vapocure (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 polymerise 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 molecularweightofthe 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 hydrated catalyst complex and process for using same to dry one component coatings and substantially overcome or ameliorate the difficulties associated with drying such coatings.

In one broad aspect, the invention provides a process for forming a dried coating upon a suitable substrate comprising applying a vehicle, such as a one-component vehicle, upon a substrate and subjecting the vehicle to treatment with a drying agent as hereinafter defined.

The invention 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 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 effects 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 metal or inorganic metal salt-which, in association with the water, 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 vehicie. 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, i.e.

the pre-polymer has isocyanate groups which are releasable, or available, for reaction with water molecules (for the purposes of polymer propagation and/orfilm 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 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 treatment in an atmosphere containing water atomised to provide a Relative humidity level within the range 40%-80% dependent upon the existing temperature which can lie within the range 1 O"C--40"C.

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 PMT 200 to 2000 parts per million.

The present invention is particularly suitable for the drying of commercially available one-component vehicles. It is well known that one-component systems traditionally 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 100 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 film proportionately to its vapour pressure concentration gradient. The existence of this hydrated catalyst complex is supported 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 filament as generated by the heat of combustion of any organic compound in the vapour phase and converts it to an output signal measured in millivolts. The millivolts generated is then proportional 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. Initially 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 of the existence of a hydrated catalyst 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.

TABLE I Catalyst=DMEA Temperature Conditions=25 C Concentration=2055 ppm R.H.% Millivolts 0 23.94 10 22.80 14 21.04 45 14.25 60 10.26 75 7.98 81 5.70 Catalyst=TMEDA Temperature=25"C Concentration=776 ppm R.H.% Millivolts 0 18.25 15 17.10 45 14.25 62 12.83 88 10.26 Catalyst=PMT Temperature Condition=25 C Concentration=775 ppm R.H.% Millivolts 4 17.10 16 17.10 45 14.25 62 12.54 80 11.40 Solvent Naphtha 100 Temperature Condition=250C Concentration=878 ppm R.H.% Millivolts 0 14.25 16 14.25 45 14.25 65 14.25 85 14.25 Ethanol Temperature Condition=25 Concentration=1770 ppm R.H.% Millivolts 16 14.25 24 14.82 45 14.82 80 14.82 The results of Table I are summarised on the graph shown in Figure 1, in which the vertical scale indicates the millivolt reading, whilst the horizontal scale indicates the Relative humidity as a percentage, at 25"C.

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 Naphtha 100 is represented by: line D Ethanol is represented by: line E.

The results were plotted as output voltage 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 dependency on Relative humidity giving a steady 14 to 14.5 M.V. output voltage across the 0--100 R.H. range. One of these non-dependent 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 further 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 concentration 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.

The invention will now be described with specific reference to the following numerical examples. It will again be appreciated that such ensuing description is intended to be merely illustrative of the invention.

COMPARATIVE EXAMPLE 1 Two standard painttest 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 1250 parts per million. The temperature was maintained at 25"C 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 underiying regions remained quite wet. It was three--four hours before the test panel had a cure rate of 3--4 (see page 18. The control panel took some 8--10 hours to reach a similar cure rate.

COMPARATIVE EXAMPLE 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 DMEA 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 25"C. This environment was recirculated at 1.5 m/sec for a period of 2 minutes after which a 3 minute purge cycle 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 34 (see page 18). 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 molecules 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.

The reliance of ambient conditions to supply 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 of 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 1-Pot Paint System 2% w/w Addition Time to Thicken Time to Gel H20 2 hrs 12 hrs H2O:DMEA 3- mins 5 mins 1:1 DMEA 25 mins 60 mins Nil indefinite stability in sealed containers WORKING EXAMPLE 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 metered quantities of DMEA and water, an atmosphere containing 1250 parts per million of DMEA at a Relative humidity level of 65% at 25"C.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 18) 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 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 metered quantities of both TMEDA and water, an atmosphere containing 600 parts per million of TMEDA and a Relative humidity level of 65% at 25 C. 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 EXAMPLE 3 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 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 metered quantities of both PMT and water, an atmosphere containing 500 parts per million of PMT and a Relative humidity level of 65% at 25 C. 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 twofold detrimental effect of minimising atmospheric moisture and retarding reaction rates.

The impingement velocity of the recirculating air stream 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.

The graphs shown in Figures Il, Ill and IV demonstrate the effect of these variables on the cure rating of a one component system. In the graphs of Figures II, Ill and IV, the vertical scales indicate the cure rate (see below for explanation of this) whilst in relation to the respective graph of: Figure 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; Figure III: the horizontal scale indicates the Relative humidity as a percentage, For this graph, the impingement velocity was held at 1.4 metres per second, the temperature was kept at 25"C and DMEA was at a concentration of 1250 parts per million; and Figure IV: the horizontal scale indicates the impingement velocity in metres per second. For this graph the Relative humidity was kept at 60%, the temperature was maintained at 25"C, whilst the DMEA concentration was 1250 parts per million.

The cure rate has a direct correlation to the following hardness scale.

Cure Rate Hardness 1 'H' pencil hardness 2 '3B' pencil hardness or better 3 '6B' pencil hardness or better 4 imprintable 5 slight tackiness 6 tacky film 7 film mobile 8 thick liquid 9 liquid 10 unchanged Table lil serves to demonstrate how the cure rate of a specifically chosen one component system is affected by those critical variables previously outlined.

TABLE Ill Panel No. P.P.M. DMEA R.H. Temp I.V. Cure Rate Critical Factor 1 650 61 20 3.5 9 Low catalyst concentrations 2 800 64 16 3.5 4 Low catalyst concentrations low temp.

3 1250 30 32 1.4 4 Low R.H.

4 1350 55 31 0.8 3 Low IV.

5 1250 66 20 1.4 1 Correct conditions 6 1000 60 25 1.4 1 Correct conditions l.V.=lmpingement 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) Drying Chamber 1. Temperature 2--30"C 2. Relative humidity 6065% maximum 3. Air Impingement Velocity not lower than 1.0 m/s 4. Catalyst concentration (1100--1250) PPM (B) Post-Dry Chamber 1. Temperature 25"C (approximately) 2. Air Impingement Velocity not lowerthan 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 ratio is related to the formation in air solution of a specific hydrated catalyst complex which facilitates the introduction of water into the hydrophobic paint film.

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.

Claims (15)

1. A process for forming a dried coating upon a substrate comprising applying a vehicle upon the substrate and subjecting the vehicle to treatment with a drying agent which is a hydrated complex catalyst.

2. A process as claimed in Claim 1, wherein said vehicle is a one component vehicle containing free isocyanate groups.

3. A process as claimed in Claim 2, wherein said vehicle has a polyisocyanurate structure.

4. A process as claimed in Claim 2, wherein said vehicle has a urethane or urea structure.

5. A process as claimed in Claim 2, wherein said vehicle has a biuret structure.

6. A process as claimed in Claim 2, wherein said vehicle has a allophonate structure.

7. A process as claimed in any one of Claims 1 to 6, wherein said drying agent is exposed to said vehicle in an atmosphere at a relative humidity of 40% to 80%.

8. A process as claimed in any one of Claims 1 to 7, wherein said drying agent is exposed to said vehicle at a temperature in the range of 100 to 40"C.

9. A process as claimed in any one of Claims 1 to 8, wherein said catalyst is exposed to said vehicle at a temperature of approximately 25"C at a relative humidity of approximately 65%.

10. A hydrated catalyst complex for drying one component vehicles (as herein defined), said catalyst being comprised of water and either an amine, tertiary amine, alkanolamine, organo metal or inorganic metal salt.

11. A hydrated catalyst complex as claimed in Claim 10, wherein said catalyst is formed in an atmosphere having a temperature in the range of 1 0 C to 40"C.

12. A hydrated catalyst as claimed in Claim 10 or 11, said complex being formed in an atmosphere having a relative humidity in the range of 40% to 80%.

13. A hydrated catalyst complex as claimed in any one of Claims 10 to 12, said catalyst being formed at a temperature of 25"C at 65% relative humidity.

14. A method of rapidly drying a one component vehicle, said method being substantially herein described with reference to any one of Working Examples 1 to 3.

15. A hydrated catalyst complex substantially as herein described with reference to any one of Working Examples 1 to 3.