Substrates coated with neutralized carboxylated polymers

A composite comprising: a substrate; and a polymeric coating adhered to at least one surface of said substrate, said polymer coating having a thickness of about 1 to about 100 micrometers, wherein said polymer coating comprises a neutralized carboxylated polymer having a carboxylate content of about 5 to about 300 meq. per 100 grams of said neutralized carboxylated polymer.

FIELD OF THE INVENTION 
The present invention relates to polymeric coatings having improved barrier 
properties wherein the polymeric coating is formed from an organic 
solution of a neutralized carboxylated polymer. 
DESCRIPTION OF PRIOR ART 
Solids (e.g., substrates, pipes, slabs, sheets, etc.) can be protected from 
the external environment with the use of barrier or protective coating 
materials. For protection from water or moisture, polymer or organic 
materials are widely used. For cost effectiveness, however, these 
materials are generally applied as thin films. The thickness of the film 
depends upon the desired degree of water protection. The thicker the film 
the more likely that water penetration would be slowed down. In practice, 
applying an effective thin coating is difficult because of the various 
stresses tending to make the film discontinuous (e.g., film-rupture, pin 
holes). Films will rupture when a threshold stress is exceeded. The 
lateral stress tending to rupture a film is inversely proportional to an 
expotential power of the film thickness. The thinner the film, the more 
easily it will rupture. To provide film strength current practice requires 
the establishment of crosslinks in the coating by curing. Crosslinking 
(curing) can also improve the coating's resistance to some chemicals. Thin 
films which consist of molecules in relatively random configurations with 
a high degree of entanglements are superior to films containing molecules 
in relatively coiled states with few molecular entanglements. Accordingly, 
polymers containing associating ionic groups (ionomers) which have a high 
degree of molecular interactions should make excellent protective or 
barrier films. 
There are many applications for thickened or gelled soltuions of polymers 
in organic liquids which are quite diverse. There are also a number of 
physical and chemical techniques for preparing such systems. The present 
invention concerns a process for forming a polymer coating having improved 
barrier properties. 
Coatings which can be protective, decorative or special purpose are usually 
applied at thicknesses of as high as 50 micrometers or thicker in order to 
provide the desired properties required of such coatings. Such high 
thicknesses are required in order to compensate for coating defects or for 
poor coating material properties. 
Coatings with improved properties may be applied as thin films having a 
thickness range of 1-100 micrometers, with a preferred range of 2-20 
micrometers. In order for such coatings to be functional, they have to 
meet one or more of the following criteria: the coating material should 
show improved barrier properties; the applied thin coating should be a 
continuous film with few or no defects; and there should be a proper 
adhesion between the coated material and the coating. 
The material used in the thin film coating should have an optimized balance 
of properties, such as elasticity, toughness, hardness, abrasion 
resistance, etc., for durability under adverse conditions. For special 
coatings, surface properties, such as surface tension or tribological 
properties, may need to be met. 
The discovery of the film forming properties of ionomers has made possible 
the extension of their use to coating applications, including controlled 
release products in agriculture (e.g., controlled release fertilizer). In 
controlled release fertilizer applications coatings of ionomers will act 
as barriers to water soluble constituents of the fertilizer, shielding 
them from premature release in aqueous environments for periods ranging 
from several days to several months. Because of their unique barrier 
properties ionomers can potentially be used to make cost effective 
controlled release fertilizers. The benefits obtained by the use of these 
coatings can include labor savings, increased crop yield, increased 
nitrogen utilization efficiency and time savings. The amount of premium is 
proportional to the cost of coating used on the controlled release 
product. Therefore, it is of economic importance to use as little coating 
material as possible to make a desirable agricultural product. The amount 
of coating which should be applied on the controlled release product, 
however, is not only dictated by economic considerations, but also by the 
required performance. In most cases the performance requirements include 
the control of the release or dissolving property of the agricultural 
material, achievable with the application of coatings free of fine 
pinholes or defects. Herein lies the major problem in controlled release 
fertilizer, particularly with existing conventional coatings, because the 
thinner the coating or the less coating material is applied the less 
likely that defect free coatings can be made. Thus, commercially available 
controlled release fertilizer products are with thick (&gt;40 microns) 
coatings to yield acceptable performance (e.g., &lt;20% release of water 
soluble nutrient in seven days in water at 20.degree. C.). As a 
consequence, these products are expensive and have found limited uses. 
With the discovery of ionomer coatings, however, the application of thin 
(&lt;20 microns), defect-free films on controlled release fertilizer can now 
be achieved; thus, its use presents a potential route for making 
affordable controlled release fertilizer. 
The instant invention teaches that a solution of a neutralized carboxylated 
polymer can meet many of the requirements for an improved thin film 
coating. 
SUMMARY OF THE INVENTION 
The present invention relates to a process for forming a polymeric coating 
having improved barrier properties from an organic solution of an organic 
liquid and a neutralized carboxylated polymer. 
GENERAL DESCRIPTION 
The component materials for forming the polymeric coatings of the instant 
invention generally include a water insoluble neutralized carboxylated 
polymer dissolved in an organic solvent system to form a solution with a 
concentration level of 0.1 to 20 weight percent of the neutralized 
carboxylated polymer. The solvent system comprises an organic solvent with 
or without a cosolvent. The solvent can be an organic liquid which is 
capable of dissolving the polymeric backbone. A cosolvent may be needed to 
break up associated domains resulting from aggregation of ionic species. 
The water insoluble carboxylated polymers of the instant invention will 
comprise from about 1 to about 500 milliquivalents of pendant carboxylate 
groups per 100 grams of polymer, more preferably from 5 to 300 meq. 
pendant carboxylated groups. The carboxylate groups are neutralized with 
counterions selected from, but not limited to, Groups IA, IB, IIA, and IIB 
of the Periodic Table of Elements, as well as lead, tin, zinc and 
antimony, or ammonium and amine counterions. 
The degree of neutralization of the carboxylate groups of the neutralized 
carboxylated polymers may vary from 0 (free acid form) to 100 mole 
percent, preferably 50 to 100 mole percent. With the utilization of 
neutralized carboxylated polymers in this instant invention, it is 
preferred that the degree of neutralization be substantially complete, 
that is, with no substantial free acid present and without substantial 
excess of the base, other than that needed to ensure neutralization. The 
neutralized carboxylates possess greater thermal stability and better 
mechanical properties (such as toughness) compared to their acid form. 
Thus, it is clear that the polymers which are normally utilized in the 
instant invention comprise substantially neutralized carboxylated groups 
and, in fact, an excess of the neutralizing material may be utilized 
without defeating the objects of the instant invention. 
The neutralized carboxylate polymers of the instant invention may vary in 
number average molecular weight from 1,000 to 10,000,000, preferably 5,000 
to 1,000,000, most preferably from 10,000 to 600,000. These polymers may 
be prepared by methods known in the art, such as a copolymerization where 
one of the monomers is a carboxylate containing monomer. 
Neutralized carboxylated polymers used in the instant invention are 
characterized by the formula: 
##STR1## 
wherein Y is about 0.1 to about 30 mole percent, more preferably about 0.5 
to about 20, and most preferably about 1 to about 15; R is hydrogen, an 
ethyl or a methyl group; wherein M.sup.+ is selected from the group 
consisting of ammonium counterions, amine counterions and metal 
counterions selected from, but not limited to, the group consisting of 
lead, antimony, zinc, tin and Groups IA, IB, IIA and IIB of the Periodic 
Table of Elements. 
The concentration of the neutralized carboxylated polymer in the solution 
of the neutralized carboxylated polymer and the organic solvent, and 
optionally the cosolvent, is about 0.1 to about 20 weight percent, more 
preferably about 0.5 to about 10, and most preferably about 0.5 to about 
6.0. 
The organic solvent is selected from the groups consisting of aromatic 
solvents, oxygen-containing solvents, such as esters, ketones, ethers, 
aldehydes and carboxylic acids, and amines, amides, alcohols and mixtures 
thereof. Preferred organic solvents are tetrahydrofuran, acetic acid, 
xylene and toluene. 
In order to reduce the viscosity of an organic solution of the neutralized 
carboxylated polymer so as to be able to employ the organic solution in a 
casting process, a polar cosolvent may be added to the organic solution of 
the neutralized carboxylated polymer to solubilize the pendant carboxylate 
groups. The polar cosolvent will have a solubility parameter of at least 
10.0, more preferably at least 11.0, and may comprise from 0.01 to 15.0 
weight percent, preferably 0.1 to 5.0 weight percent, of the total mixture 
of organic liquid, water insoluble neutralized carboxylated polymer and 
polar cosolvent. 
Normally, the polar cosolvent will be a liquid at room temperature, 
however, this is not a requirement. It is preferred, but not required, 
that the polar cosolvent be soluble or miscible with the organic liquid at 
the levels employed in this invention. The polar cosolvent is selected 
from the group consisting essentially of alcohols, amines, amides, 
acetamides, phosphates, or lactones and mixtures thereof. Especially 
preferred polar cosolvents are aliphatic alcohols, such as methanol, 
ethanol, n-propanol, isopropanol, 1,2-propane diol, monoethyl ether of 
ethylene glycol and n-ethylformamide. 
The elastomeric coatings of the instant invention are formed by applying 
the organic solution of the neutralized carboxylated polymer over the 
substrate at an ambient temperature or at 5.degree.-80.degree. C., by 
either dip-coating or spray-coating or with the use of other techniques 
for thin spreading (such as brushing). The organic solvent system is then 
permitted to evaporate with or without the aidn of forced drying gas, such 
as air or nitrogen gas. This step is called the drying process. The drying 
gas temperature can be from ambient temperature up to the boiling point of 
the organic solvent system. Preferably the temperature of the drying gas 
is between 20.degree. C. to 100.degree. C. The most preferred temperature 
of the drying gas should be about 70.degree. C. for fast evaporation of 
the organic solvent system. After drying the thickness of the applied 
coating should be about 1 micrometer to about 100 micrometers. Most 
preferred, the coating thickness should be about 2 to about 20 micrometers 
for both performance of the applied coating, the solution concentration of 
the neutralized carboxylated polymer is applied at 0.5 to 10 weight 
percent. Most preferably, the concentration should be about 2 to 5 weight 
percent. The coating solution of the neutralized carboxylated polymer can 
be applied in single or multiple layers, depending on the desired coating 
thickness. In any instance, the organic solvent system is evaporated after 
each layer application. The neutralized carboxylated polymer coating can 
be applied over the substrate of interest or over a previous coating. In 
the latter case, such practice can modify or improve the performance of 
the coated system. 
A variety of substrates which are discrete particulate solids may be 
encapsulated to form advantageous products. In some applications 
substrates are required to be released in a slow or controlled manner in 
given environments. Examples include: fertilizers, micronutrients, coated 
seeds, synthetic reagents or catalysts, pharmaceutical and drugs. 
Substrates can also be modified by encapsulation in cases where their 
solid surfaces need to be more compatible when they are added to other 
materials. Examples are engineering plastics, adhesives or rubbers with 
incorporated filler particles, such as ground lime or titanium dioxide. 
The neutralized carboxylated polymeric coating can be used as a barrier or 
controlled release coating for applications such as fertilizer, 
micronutrients or other solid materials. 
Urea or other water soluble fertilizer granules can be coated to maximize 
the plant uptake up the applied fertilizer through the minimization of 
losses, including vaporization, nitrogen fixation and leaching. The 
coating of urea can be achieved by spraying a solution of a carboxylated 
ionomer, such as zinc salt of ethylene-methacrylic acid copolymer onto a 
cascading stream of urea granules through an appropriate technique, such 
as fluidized bed coating. Examples of fluidized bed coating processes are: 
conventional spray coating wherein the solid particulates are coated by 
spraying the coating solution above or below the bed; a Wurster 
configuration; or a fluidized bed with a rotating bed support plate. It is 
envisioned that coated urea or other fertilizer particulates can be 
utilized in a variety of environmental conditions and yet the release of 
nitrogen or other water soluble nutrients can be controlled in such a way 
that they are available when the target plant (e.g., cereal) needs them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following Examples will demonstrate the performance of a neutralized 
carboxylated polymer as a barrier coating. 
EXAMPLE 1 
Improved Barrier Properties of an Ionomer Coating 
Two different grades of zinc-ethylene/methacrylic acid ionomers (Surlyn 
9910 and Surlyn 9970 made by DuPont Co.) were dissolved in boiling 
tetrahydrofuran (THF). The polymer concentration of each solution was 2 
weight percent. These solutions were used for dip coating of the ionomer 
over solid, dry urea samples in order to determine the barrier properties 
of the encapsulated urea to water extraction. 
To determine barrier properties of films formed from solution, urea slides 
were coated for immersion tests. The procedures for preparing coated 
samples of urea slides and conducting immersion tests are described as 
follows: 
Urea samples were prepared by depositing reagent grade urea (Fisher 
Scientific) over microscope glass slides. This was done by dipping glass 
slides into molten urea at a temperature of about 135.degree.-145.degree. 
C., followed by cooling and solidification of the urea layer. The urea 
layer was built up to about 7 mm by four to five successive dipping and 
cooling cycles. These urea samples were then coated by a polymeric film 
using a second dipping procedure. Urea slides were repeatedly dipped into 
polymer solutions, such as those described above, followed by drying in a 
vacuum oven at 70.degree. C. for about 3 hours. The dipping and drying 
cycles were repeated until the film thicknesses shown in Table I were 
obtained. The carboxylated ionomer solutions in THF were kept at an 
elevated temperature of 40.degree.-60.degree. C. during the dipping 
process to avoid polymer precipitation. 
The barrier properties of the various polymeric films were determined by 
immersion of each coated urea slide in about 100 g of deionized water at 
room temperature. The amount of urea released into the water was 
determined by recovering the urea after evaporating the water. Each sample 
was initially immersed for 1 day, followed by immersion in fresh water for 
3 days and for weekly intervals thereafter. 
Table I shows the permeabilities of urea solution extracted from the coated 
slides which were immersed in water at room temperature. The 
permeabilities of the coating materials were determined by applying Fick's 
law of diffusion at steady state. Fick's law states that: 
##EQU1## 
where J.sub.m =mass flux (loss) through the film or membrane, A=transport 
area, .DELTA.C=concentration gradient, .delta.=film or membrane thickness 
and D=membrane diffusivity constant which is equal to the ratio of 
permeability (P) over the solubility ratio (K) of urea in the membrane and 
in water. 
The performance of the ionomer coatings was compared with that of two 
commercially used coating materials. The first commerial coating solution 
was a tung oil solution made by Formby of Mississippi at 30 weight percent 
solids in petroleum distillate. The second commercial coating solution was 
linseed oil modified polyurethane Type I made by Minwax Paint Co. of NJ at 
45% solids in petroleum distillate. The two commercial coatings were cured 
at 70.degree. C. for 48 hours after coating. 
The permeability of urea solution through the ionomer films was found to be 
about 2 orders of magnitude lower than either that of tung oil or that of 
polyurethane. Tung oil and polyurethane were disclosed as release control 
coatings for water soluble fertilizers in U.S. Pat. Nos. 3,321,298 and 
3,223,518. 
The reason for scatter in the permeability data for ionomer coatings shown 
in Table I is believed to be a result of the coating quality. Existence of 
pin holes will increase the apparent permeability as calculated above. One 
should, therefore, assume that the lowest number corresponds to a more 
perfect coating. Permeabilities for the other polymers in Table I do, on 
the other hand, agree with literature data for perfect coatings with these 
polymers. 
This Example shows that encapsulated urea having an ionomer coating is much 
more resistant to extraction by water than is the urea encapsulated by 
commercially used coatings. One can, therefore, apply a thinner coating of 
the ionomer for equivalent results to obtain a cost advantage or the 
ionomer coatings can be useful for a slower release. 
TABLE I 
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Permeability of Urea Solution from Coated 
Urea Slides in Water at Room Temperature 
Film Permeability 
Sample Coating Thickness (P = DK) 
No. Material Microns cm.sup.2 /sec 
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141-3 Tung Oil 75 4.3 .times. 10.sup.-9 
141-6 Tung Oil 125 7.6 .times. 10.sup.-9 
158-4 Polyurethane 
100 1.3 .times. 10.sup.-9 
158-5 Polyurethane 
40 2.1 .times. 10.sup.-9 
S-9910 Ionomer 70 4.2 .times. 10.sup.-9 
S-9970-A Ionomer 70 .sup. 2.7 .times. 10.sup.-11 
S-9970-B Ionomer 70 .sup. 2.8 .times. 10.sup.-10 
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