Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-04-20
2001-07-10
Baxter, Janet (Department: 1752)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S556000, C524S524000, C524S321000, C524S322000
Reexamination Certificate
active
06258888
ABSTRACT:
The present invention relates to an aqueous emulsion of a certain organic polymeric material, and to the polymeric material derived from said emulsion, particularly in the form of a coating.
The use of aqueous emulsions of polymers, commonly known as aqueous polymer latices, is well known in the art for numerous applications, and in particular for the provision of the binder material in coating formulations.
In many coating applications, and in particular in waterborne printing ink and overprint lacquer formulations, it is desirable for the aqueous polymer emulsion to possess, or provide in the resulting coating, a combination of various features and properties:
1) The capability of allowing a very high polymer solids content in the emulsion if desired, since this leads to fast drying to a polymeric film coating and high gloss in the resulting coating.
2) Good film formation (low minimum film forming temperature MFFT, and a low moisture vapour transmission MVRR in the applied coating).
3) The property of reversibility in the polymer of the emulsion. Reversibility (sometimes called redispersability or resolubility) is a property, well known to the printing industry, whereby dry polymer obtained from an aqueous polymer emulsion is redispersible or redissolvable in that same emulsion when the latter is applied thereto. This is of great importance in the process of printing which generally involves applying the waterborne ink-formulation by various cylinders (smooth, engraved or flexo diches); these can become blocked with polymer by evaporation of the water and other volatile organic compounds (VOC's) and/or the ink formulation can dry on the roller surface (e.g. during a short stoppage of the process for one reason or another) and this would obviously create problems when the process is restarted if the polymer were not reversible.
4) The emulsion should have acceptably low viscosity for ease of application when using standard coating techniques.
The problem to be overcome therefore is to simultaneously achieve such properties or features in an aqueous polymer emulsion.
We have now invented aqueous polymer emulsions which can provide the property of reversibility with regard to the polymeric material thereof, may have very high solids contents if desired, have acceptably low viscosity concomitant with both high solids content and reversibility, exhibit improved drying rates and good film formation.
According to the present invention there is provided an aqueous emulsion of an organic polymeric material, said organic polymeric material comprising
1) a hydrophobic polymer part which has a polymodal particle size distribution, and
2) an oligomer part bearing acid groups, which oligomer part imparts, or is capable of imparting, reversibility to the organic polymeric material of the emulsion.
There is further provided according to the invention the use of an aqueous polymer emulsion as defined above in coating applications, and in particular in graphic arts applications such as printing inks and overprint lacquers.
There is yet further provided according to the invention a polymeric material derived from an aqueous polymer emulsion as defined above, particularly in the form of an applied coating, and more particularly in the form of an ink coating or overprint coating.
Thus, conventional polymer emulsions having a monoroodal particle size distribution generally have a solids content of ≦45 wt %. Above 45% solids, the viscosity tends to increase sharply to an unacceptably high value. It is known to achieve high solids content in a polymer emulsion concomitant with acceptably low viscosity by arranging for the polymer to have a polymodal particle size distribution (PSD), i.e. a distribution wherein the particles are of varying size with (or grouped around) two or more distinct maxima in the PSD curve (wt % or intensity—ordinate or y axis; size-abscissa or x axis). For the purposes of this invention a dispersion having a very broad PSD without any discernible maaima (broad gaussian distribution curve) is also considered as having a polymodal PSD. Such polymodal polymer latices can be prepared using known techniques, either by blending preformed monomodal latices of different particle size or by various types of in-situ preparation using integrated synthetic procedures where the polymodal PSD may be derived from essentially different polymers of differing size or essentially the same polymer in which the polyrerisation has been tailored to result in a polymodal PSD. Examples of in-situ preparation of polymodal polymer emulsions are described in EP 81083, EP 567811, U.S. Pat. No. 4,254,004, U.S. Pat. No. 4,539,361, DD 274229 and U.S. Pat. No. 4,780,503. For example in EP 81083, two seed latices are used vwith different particle size which are reacted further vwith monomer. In U.S. Pat. No. 4,254,004 a method is described based on a change in feed rate. In U.S. Pat. No. 4,539,361 and EP 567811 a small sized seed latex is added during a conventional emulsion polymerisation, in DD 274229 a nonionic surfactant is used and in U.S. Pat. No. 4,780,503 an extra shot of anionic surfactant is added during a conventional emulsion polymerisation. However, none of these disclosures address the problem of also achieving reversibility in the polymer of the aqueous emulsion.
On the other hand, it is known to prepare aqueous polymer emulsions comprising an alkali-soluble or alkali-swellable polymer or oligomer and a monomodal hydrophobic polymer, either by blending the preformed latices or by an in-situ preparation to achieve separate and/or composite particles of the two polymer types. In order to achieve reversibility in such polymer emulsions it is necessary to render the emulsion alkaline (if it is not already so) to dissolve the alkali-soluble material. Unfortunately, this dissolution in itself results in general in an unacceptably large increase in emulsion viscosity at solids content >50 weight % for many applications. We have found that this drawback, combined with the essentially monomodal nature of the hydrophobic polymer which also (as mentioned above) tends to incur very high emulsion viscosity at high solids contents, again (in our experience) allows only low solids contents be achieved in most cases—again usually ≦50 weight % more usually ≦45 weight %. Accordingly, one would not expect the use of polymodality to overcome the combined effect of monomodality and oligomer dissolution in yielding high emulsion viscosity at high solids content.
The polymodal nature of the invention polymer emulsion composition thus surprisingly allows one to achieve high solids with acceptably low viscosity in reversible systems, thereby allowing a fast drying rate.
Further still, we have discovered that the use of a polymodal PSD in a reversible polymer emulsion allows one to achieve better film formation (lower MFFT and lower MVRR) in comparison to a corresponding reversible polymer emulsion having a monomodal PSD, and this holds both at low emulsion solids content in so far as film formation is concerned as well as at high emulsion solids content—which is surprising. The very much faster drying rate of an aqueous polymodal reversible emulsion is particularly significant, especially in printing ink applications, where the slow drying rate of conventional water-based systems is a serious drawback in comparison to solvent-based systems, especially on non-porous substrates like those derived from polyolefines. Current waterborne systems often involve overlong drying times which of course reduces production speed in the particular coating application being employed. Furthermore, some heat-sensitive substrates cannot be coated with conventional waterborne systems because they cannot resist the high drying temperatures which are necessary to obtain the required short drying times.
All these drawbacks of waterborne reversible emulsions discussed above are overcome by the polymodal emulsion systems of the present invention—particularly in the realm of waterborne printing ink applications.
By an aqueou
Overbeek Gerardus Cornelis
Peters Antonius Carolus Ida Adrianus
Baxter Janet
Lee Sin J.
Pillsbury & Winthrop LLP
Zeneca Resins BV
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