|Title of Invention||
"AN IMPROVED AQUEOUS PROTECTIVE COATING COMPOSITION"
|Abstract||An queous protective coating composition having a viscosity which allows its application by industrial methods, containing an associative thickener wherein the basic polymer of the associative thickener is hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethyleneoxide, ethylhydroxylethyl cellulose, carboxymethyl cellulose, guar gum, starch, starch ethers, particularly hydroxyethyl starch, locust bean gum, pectin, xanthan gum, methylhydroxyethyl cellulose derivatives and mixtures thereof and a binder system as herein described wherein the binder system has a particle size of 50nm to 200nm and the said composition is preferably 60wt.% latex (200mn particle size); 20% pigment (1000 nm particle size) and 20% additional water; characterized in that the associative thickener is present in a concentration which is below the critical concentration C*, defined as the thickener concentration at which the coils of the thickener polymer start to overlap or entangle, calculated according to the Mark Houwink equation as herein described.|
|Full Text||The invention relates to the use of specific aqueous protective coating compositions containing an associative thickener for industrial coatings and to some specific aqueous protective coating compositions.
Associative thickeners and aqueous coating compositions containing such associative thickeners are we11 known in the prior art. In. this respect, it can be referred to a number of documents e.g. EP-A S66 911, EP-A 339 712, E9-A. 138 614, EP-A 426 086, EP-A 465 992, EP-A 471 866, CA-20 23 C58, DE-A 30 04 161, EP-A 376 196, EP-A 384 167, US-A 4,902,733, EP-A 444 791, DE-A 42 24 617, EP-A 396 576, WO 94/06840 and EP-A 350 414. However, while moat of these documents refer to the industrial applicability of the disclosed protective coatings, this "industrial applicability" is restricted to applying tha coatings by rollers or brushes mainly in the architectural field for buildings and indoor uses etc.. Contrary thereto, the term "industrial coatings" as used herein refers to coatings which are applied by industrial methods as defined below.
It is extensively discussed in the prior art, e.g. in WO 94/06840, that traditional thickeners such ae cellulose derivatives and polyacrylate provide insufficient levelling in
water based paints. So, it was believed that many of the water soluble polymers like carbohydrates and synthetic associative thickeners disclosed in the prior art cannot be practically used for the application of water based paints by industrial methods such as spraying techniques like conventional air spray, high volume low pressure techniques and airless spray techniques. By applying the prior art coa.tings by such industrial mesthods, many associative thickeners have a negative influence on sprayability, film formation of the binder, smoothness of the coating film, water resistance and film gloss, which do not occur when applying these coatings by brushes, rollers etc..
Application by industrial methods becomes even more problematic due to the development of new binder systems with very fine particle sizes of less than about 200 nm down to about 50 nm.
The prior art. tried to overcome this problem by the development of polyurethane thickeners as described e.g. in WO 94/06840 and in practice all known aqueous protective coating compositions comprising binder systems with very small particle sizes (which are used for application by industrial methods) are based on polyurethane type thickeners.
Therefore, there is a problem in the prior art that, with the exception of polyurethane type thickeners, there is no reliable way of providing aqueous coating compositions which can be used for the application by industrial methods. Prior to the present invention, it was believed that an aqueous protective coating composition containing conventional thickeners cannot provide satisfactory results when applied by industrial methods such as conventional air spray techniques, high volume low pressure techniques and airless spray techniques, even if this composition showed good results when applied by brushes or rollers.
Unexpectedly, it was found in the present invention that this problem can be solved by using an aqueous protective coating composition for application by industrial methods containing a binder system and an associative thickener, which is
characterized in that the associative thickener is not a polyurethane thickener and is selected so that its concentration required by the specific application method (e,g, required to spray viscosity) is below the critical
concentration C*, defined as the thickener concentration at which the coils of the thickener polymers start to overlap or entangle, calculated, according to the Mark Houwink equation
wherein [i\] is the limiting viscosity number
Φ and K are proportionality constants
h is the hydrodynamic equivalent mean square end to end
For determining the optimum composition and Concentration of the associative thickener, we need to know the so called Mark Houwink equation for the basic polymers used to make the associative thickener. The Mark Houwink equation as well as the relationship between the radius of gyration, the hydrodynamic equivalent mean square end to end distance of the polymer molecule and the viscosity average molecular weight are
disclosed in various standard text books e.g. in D,W, van Krevelen, Properties of Polymers, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York, 1976, H. Elias, Macromolecules 1 & 2, John Wiley & Sons, S. Sun, Physical Chemistry of Macromolecules, John Wiley & Sons or Polymer Handbook, In the prior art, the Mark Houwink equation is generally usisd to determine the viscosity averaged molecular weight of a polymer from the (measured) limiting viscosity number. It is assumed that the Mark Houwink equation does not change dramatically due to the modification of the basic polymer with hydrophobia groups, as is disclosed e.g. in Robert A. Gelman and Howard G. Barth, "Viscosity Studies of Hydrophobically Modified (Hydroxyethyl) Cellulose" in ACS Conference book on Water Soluble Polymers, 1986, Chapter 6, pages 101 to 110. The hydrophobia groups usually represent less than 5 wt I of the total thickener and it can be assumed that the coil volume of the non-modified and modified polymer are similar.
Prom the following equations
which are disclosed e.g. in the above reference books, for a given associative thickener polymer the maximum possible viscosity average molecular weight can be dete:mined by using the maximum value of the radius of gyration calculated as discussed above. Values of Φ, K and a for various polymers are disclosed e.g. in the Polymer Handbook for synthetic polymers and in R. Lapasin and S. Pricl, Rheology of Industrial Polysacharides - Theory and Applications, Blaekie Academic & Professional, Chapman and Hall for carbohydrate based polymers. Other standard books disclosing these constants are known to a skilled person.
Knowing the viscosity average molecular weight, the limiting viscosity number [n] can be calculated and, thus, the reciprocal value of the limiting viscosity number is correlated to the thickener concentration at which the coils start to overlap or entangle. By selecting the thickenur concentration in the continuous phase below this critical concentration, it is ensured that the- continuous phase will behave as a liquid while elastic properties' will start to show up beyond this critical polymer concentration. Therefore, the use level of the thickener molecule should remain below this level to make sure that the system will flow well.
The associative thickeners which can be used according to the invention can be carbohydrate based thickeners (natural thickeners) or synthetic thickeners. Preferred are the carbohydrate based thickeners such as modified cellulose ethers. Preferred are associative thickeners comprising as a basic polymer hydroxyethyl cellulose, hydroxypjropyl cellulose, methyl cellulose, polyethyleneoxide, . et.Tylhydroxylethyl cellulose, carboxymethyl cellulose, guar gum, starch, starch ethers, particularly hydroxyethyl starch, locust bean gum, pectin, xanthan gum, methylhydroxyethyl cellulose, polyvinylpyrolidone, polyvinyl alcohol, methylhydroxypropyl cellulose, mixed ethers of the above cellulose derivatives and mixtures thereof. Especially preferred are hydrophobically modified hydroxyethyl cellulose, hydrophobically modified methylhydroxyethyl cellulose, hydrophobicully modified hydroxypropyl cellulose, hydrophobically modified polyethyleneglycols, particularly hydrophobe end-capped polyethyleneglycols. Preferred are dodecyl- ami cetyl-modified polymers, e.g. polyethyleneoxides. Preferred thickeners are disclosed e.g. in EP-A 566 911 and US-A 5 574 127. Furthermore, starch and its derivatives are associative thiclceners which can advantageously be used according to the invention.
The present invention is particularly useful for coating compositions comprising new binder systems with very fine particle aizes down to 50 nm, preferably between 200 nm and 50 run. AS la evident from the calculations shown below, the critical concentration for compositions having large particles
of well above 200 nm is quite high and, thus, such compositions are not so likely to flocculate when applied by industrial methods. Particularly preferred are coating compositions with very fine particle sizes of below 100 nm arid those having particle sizes of about 50 nm or above. The particle size of the latexes referred to in this specification in as measured by a Disk Centrifuge of the type Joyce Loebl,
In the following, the invention will be explained in some detail for two typical coating compositions, The invention is not restricted thereto.
As typical coating system 1 a latex can be mentioned with an average particle size of 50 nm and a solids content of 50 wt I. A further typical coating system is system 2 which is a coating system represented by 60 wt % latex (200 nm particle site), 20% pigment (1000 nm particle size) and 201 additional water. The latex has a solids content of 50% w/w, which makes as a solids content of the paint 50 wt % as in the system 1 defined above.
The layer thickness of water around the binder particles can then be calculated for system 1 to be 10 ran and for system 2 to be 60 nm. The calculation of the layer thickness for system 1 is now explained in detail. However, how such a calculation can be done is well within the knowledge of a skilled person.
in system 1 there is a solids content of 50% and the particle size is 50 ma. Per 100 g of the coating therefore 0.050 kg of solid latex pariclea are present. The specific gravity of the latex particle* is known and for the purpose of this
calculation it can be assumed to be p " 1100 kg/m3 , Since the particles are assumed to be spherical, each particle has a volume of V = (4/3) π R3. The surface area of each latex particle is 0 = 4 it R2. The weight of the particle is pV. The number of particles thus follows from the total weight of the solid latex particles and the weight of one part.icle and is N = 0.050 / (pV). The total surface area of the latex particle calculates to (number of particles x surface area of one particle = N' x 0). 50 g of water which are present in the
system equal 50 ml or 50 x 10-6 m3. The layer thickness of the liquid on the latex particles can then be calculated to be L= 50 x 10" / (N x 0). Using the concrete numbers for system 1, it follows:
N = 0.050 /11OO x (4/3) n (25 x 10-9)3 •• 6.945 x 1017 particles,
0 * 4 it (25 : L s 9.17 x 10-9 m corresponding to approximately 10 nm as disclosed above.
Following the calculations as explained above, the layer thickness for any system can be easily calculated,
In order to make sure that the thickener molecule will not flocculate the binder or pigment particles, the hydrodynamic diameter of the coil of the water' soluble polymer ( = thickener/rheology modifier)' should not excoed the diameter of the layer thickness of this continuous phase. Therefore, the radius of gyration of these thickener coils should not exceed 5 nm for system 1 and not exceed 30 nm for system 2. The hydrodynamic volume (Vh) of the corresponding thickener coils in solution can then be calculated for system 1 to be Vh = 1.25 x 10-19 cm3 and for system 2 to be Vh * 2.76 x 10-17 cm3.
Introducing the values for the radius of gyration into the Mark Houwink equation, the viscosity average molecular weight can be calculated and knowing the viscosity average molecular weight, the limiting viscosity number [n] and the critical concentration range can be determined. In particular, by taking the reciprocal value of the limiting viscos:.ty number, the thickener concentration can be determined at which the coils start to overlap or entangle. The Mark Houwirk equation, the viscosity average molecular weight range calculated for systems
1 and 2 and the preferred critical concentration C*p range
calculated are summarized in the following table for a broad
range of basic water soluble polymers.
In order to make sure that the continuous phase containing the thickener molecules will behave as a viscous liquid, the thickener content should remain below the critical polymer concentration calculated. This means for example for
hydrophobic-ally modified hydroxyethylcellulosis in the more
critical system 1: below c*p=l.07 wt % and for the
hydrophobically modified Xanthan Gum in the mors forgiving
system 2: below C*p*1.85 wt %.
The hydrophobe content and type of modification of the associative thickener can be adjusted as required 'to reduce the, concentration of the associative thickener in the aqueous protective coating composition below C* or preferably C*p while nevertheless the associative thickener provides a viscosity which -is sufficiently high to meat the requirements of the envisaged industrial applicability-, e.g. the viucosity which is required to meet the' 40 sec. DIN Cup 4. The hydrophobe types suitable for. this purpose are aikyl and aikyl aryl groups ranging from C8-C24 carbon atoms in the aikyl group. The hydrophobe modification can 'range from 0,4 wt % uj> to 5 .wt I or until material becomes insoluble in water, The amount of hydrophobe needed depends on the strength of the interaction between the binder and the thickener and needs ':o be checked in the final coating system, In practice, the required range of hydrophobe contents is estimated to be between 0,5 wt % and 4.0 wt %. It is known that increasing the hydrophobd content yields an increased thickening efficiency, e,g, G. Kroon, "Associative Behaviour of Hydrophobically Modified Hydrpxyethyl Cellulose (HMHEC's) in Waterborne Coatings" in Progress in Organic Coatings, 22, 1993, Elsevier Sequoia pages 245 to 260 or EP-A 566 911.The composition of the present invention is neither a product of chemical reaction nor a mere admixture resulting in the aggregation of the properties of the components thereof and is in fact a' synergistic mixture having improved and unexpected properties.
Accordingly, there is provided an aqueous protective coating composition having a viscosity which allows its application by industrial methods, containing an associative thickener wherein the basic polymer of the associative thickener is hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethyleneoxide, ethylhydroxylethyl cellulose, carboxymethyl cellulose, guar gum, starch, starch ethers, particularly hydroxyethyl starch, locust bean gum, pectin, xanthan gum, methylhydroxyethyl cellulose derivatives and mixtures thereof and a binder system as herein described wherein the binder system has a particle size of 50nm to 200nm and the said composition is preferably 60wt.% latex (200mn particle size) ; 20% pigment (1000 nm particle size) and 20% additional water; characterized in that the associative thickener is present in a concentration which is below the critical concentration C*, defined as the thickener concentration at which the coils of the thickener polymer start to overlap or entangle, calculated according to the Mark Houwink equation as herein described.
The following examples further described the invention.
In the following examples, in table 2a the value of C*p refers to a polymer solution, whereas in table 2b and in the further tables of the examples the concentrations indicated refer to the total paint which contains only a certain percentage of solids. If nothing else is stated, the solids content of the tested paints is 50%. Therefore, to decide whether or not any of the disclosed compositions falls within the scope of the present invention, the concentration has to be adapted to a polymer solution. For example, the concentrations given in table 2b (50% solids content) have to be multiplied by 2 and
then compared to the value of C*p indicated in table 2a. A corresponding calculation has to be carried out with respect to the other tables.
The concentration of the associative thickeners in the tested aqueous protective coating compositions was adjusted to meet the viscosity according to 40 sec. DIN 4 as discussed in Ernest w. flick, "Water-based Paint Formulations Vol. 3", Noyes Publications, Pack Ridge, New Jersey, USA, 1994 This viscosity of the coating compositions was necessary to apply the compositions with the spray guns used in the exiunples.
The first example shows the performance oil a number of hydrophobically modified hydroxyethyl celluloses with variable molecular weight and hydrophobe type and -content in an anti-corrosion paint based on a styrene acrylic latex (formulation! see Table 1),
Anti-corrosion primer formulation for air- and HVLP-spray application
Solids content at spraying is 50 wt %, The particle size ia approximately 100 run (the exact value is 82 run)
The various HMHECs are described and their performances in the anti-corrosion primer are summarized in Table 2a and bs
Composition of some HMHECs to be evaluated in the anti-corrosion primer
Performance of various HMHECs in an anti-corrosion primer baaed on a styrene acrylic latex and an epoxyeatttr emulsion.
It is obvious! to conclude from the results in Table 2b that the HMHEC samples with a molecular weight within the MV-range and present in the composition at thickener concentrations below C*p, as outlined above, can be applied by industrial methods, while compositions not having a thickener MV and concentration according to the invention cannot. In particular, in formulations 3, 4, 5, 7 and 8 the concentration of the thickener waa below C*p and the formulations could be applied by spraying giving excellent anti-corrosion primers, while in formulations 1, 2, 6 and 9 the concentration of the thickener was above c*p and, therefore, either the sprayability of the formulation was only moderate or the resulting film showed shortcomings (please note, that the cone, [wt %] for 40 s DIN 4 in table 2b has to be multiplied by 2 to allow a comparison with C*p in table 2a).
In the same anti-corrosion primer of example 1 a hydrophobically modified methylhydroxethyl cellulose (HMMHEC 1) and a hydrophobically modified hydroxypropyl cellulose (HMHPC 1) were evaluated. The HMMHEC 1 sample has a molecular weight of 25000 and a dodecyl-substitution of 1.3 wt %. The HMHPC sample had a molecular weight of 66000 and a HP -MS of 2.5 and dodecyl-substitution of 1.9 wt %. The test results are summarized in Table 3. The products were compared with their non-modified precursors MHEC 1 and KPC 1 respectively.
Evaluation of hydrophobically modified MHEC! and HPC in an anti-corrosion primer (formulations Table 1),
The sprayability of the coatings with the hydrophobically modified materials is good while the film appearance of these samples is good as well. This can be expected on the basis of method described before. The molecular weight of HMMHEC 1 and HMHPC 1 is within the range indicated, while the critical polymer concentration is not exceeded of these materials:
C* HMMHECi 0,57 wt % C* HMHPC: 1.90 wt %
Example 1 and 2 were related to carbohydrate based Associative thickeners and more specifically cellulose derivatives, Example 3 shows that the described selection method is also applicable to synthetic thickeners. Dodecyl- and cetyl-modified polyethyleneoxides with variable compositions were synthesized and evaluated in the anti-corrosion primer of Table 1.
The compositions of the hydrophobe end-capped polyethyleneglycols are given in Table 4, whoreas the paint evaluation results are summarized in Table 5.
Composition of hydrophobe end-capped PEG samples.
Performance of hydrophobe end-capped PEGs in an anti-corrosion primer (formulation: see Table 1),
The 3.8 wt % of PEG 100000 required to generate) the 40 seconds DIN cup 4 exceeds the critical polymer concentration at the Mv of 100000 (C* - 1.82 wt: %). Therefore, sprayability and film appearance are poor. The tendency to flocculate the binder particles is illustrated by its poor water resistance and corrosion resistance.
Now it has been shown that the invention is both applicable to carbohydrate-based and synthetic associative thickeners. In order to demonstrate that the invention is also applicable to a wide variety of coating systems two associative thickeners are evaluated in other coating systems according to the invention. The products selected are HKHEC 4 and HMPEG 1. As representative of a product outside the desired composition range HMHEC 2 was selected as reference together with a commercial product Natrosol Pius grade 331 with indicative composition of hydroxyethyl cellulose with Mv, - 300000 and cetyl substitution of approximately 0,6 wt %.
Example 4 shows the performance of the different rheology modifiers in a top coat based on a styrene acryi.ic latex. Table 6 exhibits the formulation of the top coat and table 7 summarizes the paint properties after spraying the coating on cold rolled bondar steel which was protected by an anti-corrosion primer based on the formulation of Table 1 using HMHEC 4 as a thickener.
Formulation of a high gloss top coat based on a small particle aize styrene acrylic latex (82 nm)
The solids content of the formulation is 50 wt
Performance of various thickeners in a top coat based on a styrene acrylic latex.
In this example it is shown that the selected materials HMKEC 4 and HMPEG 1 demonstrate good performance in coatings for wood. In Table 8 and Table 9 formulations are given of a parquet lacquer based on a urethane-acrylic dispersion (Table 8) and of a universal wood coating based on a combination of acrylic latexes (Table 9). The paint properties obtained with the different thickeners are then summarized in Table 10 and 11.
Formulation of a parquet lacquer.
The solids content of this formulation is 34 wt %. The particle size is approximately 100 run.
Formulation of a universal wood coating.
The solids content of this formulation is 38 wt I. The particle size is approximately 100 nm.
The acrylic latex 1 and the acrylic latex 2 are as discussed in the description.
Paint properties of the parquet lacquer (Table 8) using different, rheology modifiers.
HHHEC 4 and HMPEG 1 provide the coating again with good film properties (high gloss and smooth films) and the flow of these products is good. The moderate rating with HMPEG 1 in the hot pan teat is not due to flocculation as can be seen from its good gloss value. It is more a result of its low softening temperature (- 45 C) .
Paint properties of the universal wood ccating using the different thickeners.
In example 6 a number of coating formulations are given using HMHEC 4 or HMPEG 1 as a rheology modifier.
Formulation of airless sprayable pigmented t:op coat,
Formulation of a metal primer based on an epoxyester emulsion.
1. An aqueous protective coating composition having a viscosity
which allows its application by industrial methods, containing an
associative thickener wherein the basic polymer of the associative
thickener is hydroxyethyl cellulose, hydroxypropyl cellulose, methyl
cellulose, polyethyleneoxide, ethylhydroxylethyl cellulose, carboxymethyl
cellulose, guar gum, starch, starch ethers, particularly hydroxyethyl
starch, locust bean gum, pectin, xanthan gum, methylhydroxyethyl
cellulose derivatives and mixtures thereof and a binder system as herein
described wherein the binder system has a particle size of 50nm to
200nm and the said composition is preferably 60wt.% latex (200mn
particle size); 20% pigment (1000 nm particle size) and 20% additional
water; characterized in that the associative thickener is present in a
concentration which is below the critical concentration C*, denned as the
thickener concentration at which the coils of the thickener polymer start
to overlap or entangle, calculated according to the Mark Houwink
equation as herein described.
2. An aqueous protective coating composition as claimed in claim 1,
wherein the associative thickener is a carbohydrate based thickener.
3. An aqueous protective coating composition as claimed in claim 1,
wherein the associative thickener is a synthetic thickener as herein
4. An aqueous protective coating composition as claimed in claim 2,
wherein the carbohydrate based thickener is a cellulose based thickener.
5. An aqueous protective coating composition as claimed in any of the
claims 1 to 4, wherein the hydrophobe content of the thickener is 0.5 wt
% to 4 wt %.
6. An aqueous protective coating composition substantially as herein
described with reference to the foregoing examples.
|Indian Patent Application Number||787/DEL/1998|
|PG Journal Number||12/2008|
|Date of Filing||26-Mar-1998|
|Name of Patentee||HERCULES INCOPORATED|
|Applicant Address||1313 N. MARKET STREET,HERCULES PLAZA,WILMINGTON DELAWARE,19894-0001 UNITED STATES OF AMERICA|
|PCT International Classification Number||C09D 7/12|
|PCT International Application Number||N/A|
|PCT International Filing date|