Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-03-23
2002-07-30
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S389900, C427S391000, C427S392000, C427S393500
Reexamination Certificate
active
06426121
ABSTRACT:
BACKGROUND OF THE INVENTION
Crosslinking systems for effecting cure of emulsion polymers are used to provide nonwoven articles, particularly cellulosic webs such as paper towels, with some desired property such as water or solvent resistance. Most crosslinking systems for emulsion polymers which are employed today require temperatures in excess of 100° C. to ensure the development of a decently cured system. While high temperature cures may be acceptable for many applications, such temperatures may be unacceptable in other applications because of an unsuitability of certain types of substrates, operational difficulties, and lastly, they may represent economic hardship due to the high cost of energy.
Many ambient crosslinking technologies for nonwoven articles have been investigated and are employed within some application niches. However, none are widely used today perhaps due to cost, inefficient cures or some chemical incompatibilities. These systems include the crosslinking of an acetoacetoxyethyl methacrylate (AAEM) containing polymer with a multi-primary amine functional moiety. This combination has a very short pot-life making it unsuitable for a one-part system without the addition of some blocking agent. Typically the use of blocking agents requires either temperature to activate the reactants or a pH change thereby reducing their applicability for many applications.
Epoxy functional co-monomers such as glycidyl methacrylate and allyl glycidyl ether have been evaluated, however the epoxy group is readily subject to hydrolysis in water to be of practical use in emulsion polymerizations.
The following patents are representative of crosslink chemistries for the crosslinking of polymeric emulsions.
U.S. Pat. No. 5,534,310 discloses a method for improving adhesive durable coatings on weathered substrates. The durable coatings are based upon latex binders formed by the polymerization of acrylic and methacrylic esters, such as methyl methacrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc., along with vinyl monomers and the like. Durability is enhanced by incorporating acetoacetate functionality into the polymer, typically by polymerization of monomers such as acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate (AAEA), allyl acetoacetate, and vinyl acetoacetate. Enamine functionality is incorporated into the polymer for improving adhesion by reaction of the latex containing the acetoacetate functionality with ammonia or an amine.
U.S. Pat. No. 4,645,789 discloses the use of highly crosslinked polyelectrolytes for use in diapers and dressings which are based upon acrylic acid-acrylate copolymers, acrylic acid-acrylamide copolymers, acrylic acid and vinyl acetate copolymers, and so forth. Preferred aziridines include the triaziridines based upon trimethylolpropane tripropionates, tris(1-aziridinyl)phosphine oxide, and tris(1-aziridinyl)-phosphine sulfide.
U.S. Pat. No. 4,605,698 discloses the use of polyfunctional aziridines in crosslinking applications. One type of polyaziridine is based upon the reaction of ethylene imine with acrylates of an alkoxylated trimethylolpropane or other polyol. Vinyl acetate/carboxylated urethanes and styrene/acrylics are shown as being crosslinked with polyfunctional aziridines to produce coatings having a low temperature crosslinking functionality.
U.S. Pat. No. 4,278,578 discloses coating compositions for plastic substrates based upon carboxy functional acrylic copolymers, which are crosslinked with from about 0.2 to 3% of a polyfunctional aziridine. Examples include N-aminoethyl-N-aziridylethylamine with a most preferred aziridine being a trifunctional aziridine having equivalent weight of 156 atomic mass units
U.S. Pat. No. 3,806,498 discloses the use of (1-aziridinyl)alkyl curing agents for acid-terminated polymers. A wide variety of polymers having terminal-free acid groups are described as being crosslinkable through the use of the (1-aziridinyl)alkyl curing agents, and these include those formed by the reaction of esters of carboxylic saturated and unsaturated acids with aziridinyl alcohols.
U.S. Pat. No. 6,117,492 discloses emulsion polymers utilizing a dual crosslinking package which contains a moiety with an active methylene group. This group is reactable with dialdehydes, while the other functionality is a carboxylic acid, which is reactable with the tri-aziridine. The active methylene group was derived form acetoacetate.
BRIEF SUMMARY OF THE INVENTION
The invention relates to improved crosslinking system comprised of crosslinkable polymer and multiple crosslinking agents which are capable of reaching full cure under ambient conditions. More particularly the invention relates to an improved process for forming a nonwoven web bonded with a dual crosslinkable polymeric emulsion wherein a polymeric emulsion is applied to the nonwoven web, the water removed, and the crosslinkable polymer subsequently crosslinked. The improvement comprises:
utilizing a polymeric emulsion wherein the crosslinkable polymer has pendent carboxylic acid functionality and is formed in the presence of poly(vinyl alcohol) stabilizing functionality;
crosslinking the hydroxyl functionality in the crosslinkable polymer by reaction with an effective amount of a polyaldehyde; and,
crosslinking the carboxylic acid functionality by reaction with an effective amount of a polyaziridine compound.
There are numerous advantages of the dual crosslinker system described herein; these advantages include:
an ability to from a polymer that can achieve >90% of total cure in the test conditions, typically either 150° F. for two minutes or 200° F. for 90 seconds;
an ability to achieve a degree of cure sufficient to approach the target performance requirements as currently achieved by a thermally activated system based on aminoplast technology; and,
an ability to provide for a formulation which is eminently workable at the site of use.
DETAILED DESCRIPTION OF THE INVENTION
In practicing the invention for producing nonwoven webs incorporating crosslinkable polymeric systems capable of reaching full cure under ambient conditions, ethylenically unsaturated monomers wherein at least one has pendent carboxyl groups are polymerized in the presence of poly(vinyl alcohol) protective colloid. Thus, the resultant polymers provide for at least two mechanisms for crosslinking. Crosslinking of the hydroxyl functionality sites is effected by reaction with a polyfunctional hydroxyl reactive compound, i.e., a polyfunctional aldehyde and crosslinking of the acid sites is effected by reaction with a polyfunctional aziridine.
The nonwoven web can be a cellulosic web, such as pulp, or a synthetic fiber based web, such as a polyester (e.g., polyethylene terephthalate), a polyolefin (e.g., polypropylene), a polyamide (e.g., nylon), and fiberglass. The nonwoven substrate can also be a blend of synthetic fibers, or a blend of synthetic fibers with non-synthetic fibers, such as cellulosic fibers.
The poly(vinyl alcohols) suited for forming dual crosslinkable emulsion polymers is related to the type of monomers being polymerized. Polymer systems employing vinyl acetate may use poly(vinyl alcohols) having a molar hydrolysis values of about 85% and above. Fully hydrolyzed poly(vinyl alcohols) may be used but may affect viscosity and stability.
The production of all acrylic emulsions presents a different problem. One of the keys to producing a high solids, e.g., greater than 45% by weight all acrylic emulsion without the use of surfactants, solubilizers, and microfluidization techniques resides in the use of a poly(vinyl alcohol) selected from the group consisting of substantially fully hydrolyzed poly(vinyl alcohol) and a partially hydrolyzed poly(vinyl alcohol), >86%, as the stabilizing agent where the number average molecular weight ranges from about 5,000 to 13,000. A preferred type is one having a molar hydrolysis value of at least 96.5%, i.e., 96.5% of the acetate groups in poly(vinyl acetate) are converted to hydroxyl groups. When less than 96.5% of the acetate groups are con
Goldstein Joel Erwin
Pangrazi Ronald Joseph
Air Products Polymers, L.P.
Bongiorno Mary E.
Cameron Erma
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