GB2218925A - Use of migratory catalysts to increase cure rate - Google Patents

Description

2 2218925 USE OF MIGRATORY CATALYSTS TO INCREASE CURE RATE The present

invention relates to the use of migratory catalysts in the curing reaction of curable compounds, and particularly to the use of migratory catalysts for accelerating the cure of the binders used in magnetic media.

A wide variety of polymeric coated substrates are made by coating a curable liquid onto a substrate and effecting a chemical cure of the coating. Such polymeric coatings often include a pigment, such as a magnetic pigment, a reflective pigment, or an abrasive pigment.

is Magnetic pigments dispersed in a cured polymeric binder provided on a support are used as magnetic recording media. The various types of magnetic media include video tape, audio tape, computer tape, diskettes and many other forms of magnetic pigment bound to a non-magnetic support.

The production of polymeric coated substrates in general is similar to the typical production procedure of magnetic media outlined below. First, a non-magnetic support or substrate, for example, a thin polyester or polyolefin film, is fed into a coating apparatus where it is coated with a thin layer of a liquid dispersion of a magnetic pigment in a chemically curable binder composi tion. The solvent is evaporated, allowing the binder to cure. The non-magnetic support with a coating of almost uncured binder then continues through the coating apparatus where it encounters various rollers, guides, pins and the like. In its uncured or partially cured state, the binder is extremely susceptible to marring, scratching, contamination by dust and dirt, and related problems.

Problems with the magnetic coating can go unnoticed and uncorrected until long after the coating process.is completed. Typically, a roll of coated media is often kept in a temperature controlled environment for several days in order to obtain a higher degree of cure.

1 1 After this curing period testing is conducted to determine whether the coating is azceptable. This lag time makes it difficult to correct deficiencies by adjusting the parameters of the coater and the coating process while the material is being coated. Entire batches must be scrapped when it is discovered, after the cure period, that the coating is, for example, scratched, marred, rough, or has an incorrect-caliper. insufficient cure can also result in blocking of the media in the roll form. Blocking is the tendency of media to adhere to itself when in, for example, a roll or a stack of diskettes.

An-:electron-beam initiated, free-radical cure is one method employed to effect a faster cure than that typically found in an uncatalyzed chemical cure. E-beam cured polymeric systems may be undesirable for certain end uses, and chemical cures continue to be widely used despite the processing difficulties caused by the relatively slow rate of cure.

In an attempt to avoid the above problems encountered by the partially cured polymeric coating during processing, catalysts are sometimes added to the bulk curable liquid to increase the rate of cure. (See, for example, U.S. Patent Nos. 4,557,813; 4,560,456 and 4,567,096.) A catalyst added to the bulk liquid before coating increases the rate of the curing or cross-linking reaction in the bulk dispersion, thereby causing the viscosity of the dispersion to gradually increase. As the composition becomes increasingly cured it becomes difficult to obtain a smooth coating surface. Thus, the pot life, or the time the curable composition can stay in the bulk form before becoming too viscous to successfully coat, is decreased as more catalyst is added. Therefore, it is impractical to add enough catalyst to the bulk curable coating to avoid the above problems encountered as the partially cured coating passes through the coater.

1 j 8 No conventional chemically cured coating system has successfully overcome these problems. It is therefore desirable to provide a method for coating a substrate with a chemically curable composition which cures rapidly throughout the thickness of the coating. It is particularly desirable to provide a method for producing a magnetic media in which the binder components, after coating, cure rapidly-throughout the thickness of the binder layer, thereby avoiding damage during processing.

The method should also provide a chemically curable coating, for example, a dispersion of binder and magnetic particles, which is easily coatable and which has an acceptably long pot-life.

The present invention discloses a method for providing rapid curing of a chemically curable coating throughout the thickness of 'Che coating after applying the coating to a substrate. The method of the present invention comprises applying a polymeric composite, which includes a catalyst layer and a chemically curable coating layer, to a substrate. The catalyst layer contains a migratory catalyst composition which will migrate into the curable layer to accelerate the cure of the layer through- out its thickness. The catalyst layer is preferably applied to the substrate first with the curable coating layer applied on top of the catalyst layer.

A preferred embodiment includes a pigment dispi--rsed in the curable coating layer, such as, a magnetic, reflective, abrasive or conductive pigment. preferred embodiment, the polymeric composite coated substrate, before substantial migration, comprises a substrate, a catalyst layer carried by the substrate and, overlying the catalyst layer, a curable coating layer comprising a liquid dispersion of pigment in a chemically -1 4 In a curable binder resin composition. The catalyst layer, when coated, contains a migratory catalyst composition in an amount effective to migrate into the curable coating layer and accelerate curing of the layer.

The curable coating layer, as originally coated, comprises a homogenous liquid dispersion of:

a) a chemically curable coating resin; b) an effective amount of a curing agent capable of reacting with the coating resin to effect a substantial cure of the layer; and c) preferably, a pigment.

The coating layer may optionally include a low level of a resident catalyst which does not substantially affect the pot life of the bulk coating, but which has an effecl- on the rate of cure of the coating when combined with the migratory catalyst composition which is greater than an additive effect.

Preferably, the catalyst layer is coated onto the -irst and the dispersion of pigment in the substrate If curable coating layer is coated thereon. The migratory catalyst composition migrates into the curable layer and catalyzes the curing reaction between the chemically curable coating resin and the curing agent, preferably an isocyanate curable resin and an isocyanate compound, 25- respectively.

An effective amount of the migratory catalyst composition is defined as an amount of a catalyst or catalysts which provides sufficient concentrations of catalyst to migrate into the coating layer and provide a substantial increase in the cure rate throughout the thickness of the layer as compared to the absense of such catalyst. An effective amount of the curing agent is defined-as an amount sufficient to effect a selected amount of cure, which is typically a complete cure.

CI The present invention also contemplates dissolving the migratory catalyst composition in a carrier polymer and coating the mixture onto the substrate. After drying, the carrier polymer holds the migratory catalyst in place during 2rocessing until the coating layer is coated thereon. Tae migratory catalyst composition then migrates from the carrier polymer into the coating layer to catalyze the cure reaction.

-iary.amine and metal It has been found that tert salt catalysts are examples of suitable migratory catalysts for the present invention, particularly tertiary amines -which contain at least one bridgehead nitrogen. A bridgehead nitrogen is defined as a nitrogen which has all three of its bonds as part of a non-invertible structure, thereby providing a highly reactive nitrogen atom.

The present invention provides an improved method of manufacturing a substrate coated with a chemically cured polymeric composite, for example, a magnetic medium, by rapidly curing the coating during processing, thereby avoiding damage or contamination thereto.

There are three basic components to the system of the present invention: the substrate, the catalyst layer and the curable coating layer.

Substrates Substrates upon which polymeric coatings of the present invention are typically applied are thin flexible polymeric materials. Examples of typical substrates include polyesters, such as polyethylene terephthalate or polyethylene-2-6-naphthalate; polyolefins, such as polyethylene or polypropylene; cellulose derivatives, such as cellulose triacetate; plastics, such as polycarbonate, polyamides and polyimides; and other flexible polymeric materials well known in the art. The thickness of the substrate can vary widely depending on the end use. In the magnetic media area, substrates are generally between about 5 and 100 micrometers, and preferably, between about 8 and 75 micrometers.

Catalyst Laver An effective amount of a migratory catalyst comprising one or more catalysts is coated, preferably between the support or substrate and the curable coating layer. General considerations in choosing a suitable migratory catalyst composition include: 1) the catalyst's effectiveness in catalyzing the reaction between the curable coating resin and the curing agent, such as between an isocyanate and an isocyanate-curable resin and 2) the catalyst's ability to migrate into the curable coating so as to be available to catalyze the reaction. Tertiary amine migratory catalysts, particularly containing a bridgehead nitrogen, have been found to be suitable. It is preferred that the catalyst also be multi-functional, i.e., contain two or more reactive nitrogen groups, for higher rates of cure. Examples of preferred migratory catalysts include triethylene diamine, dimethylethanol amine, quinuclidine and mixtures thereof.

It is also preferred that a small amount of a resident catalyst be added to the bulk curable coating mixture. The resident catalyst should preferably have the following characteristics: 1) efficient at low concentrations, 2) have a greater than additive effect in combina- tion with the migratory catalyst, and 3) not detrimentally affect the pot- life of the coating composition at low concentrations. Examples of suitable resident catalysts include-metal salts such as dibutyltindilaurate. The concentration of the resident catalyst should be low enough so as to not substantially affect the pot-life or viscosity h of the bulk coating. A suitable concentration range of the resident catalyst has been found to be between about 0.2 and about 0.8% by weight based on coating solids.

In a preferred embodiment of the present invention the migratory catalyst composition is dissolved in a carrier polymer and the mixture is then coated onto the substrate. The substrate coated with the carrier polymer/migrato-Y catalyst layer can either be formed into a roll for storage and subsequent coating with the curable coating layer or the entire process can be done in a single, on-line procedure. If a carrier polymer is not used to support the migratory catalyst composition, the composition may be applied directly as a liquid under proper processing conditions.

A suitable carrier polymer is preferably a higher molecular weight polymer which is coated by dissolving, in a suitable solvent, and is thus not cured. The carrier polymer preferably should have the following characteristics: 1) the migratory catalyst should be very soluble in the carrier polymer film; 2) the migratory catalystcarrier polymer film should be clear, smooth and abrasionresistant, and 3) the carrier polymer film should swell when the curable coating layer is coated thereon to release the migratory catalyst and allow it to migrate into the coating layer.

Examples of suitable carrier polymers include polyesters, vinyls, phenoxies and mixtures thereof. In magnetic media, the carrier polymer layer having the migratory catalyst dispersed therein is typically coated at a thickness of between 0.02 micrometers and 1.0 micro meters, and preferably is coated in a thickness of between about 0.1 and 0.5 micrometers. The concentration of the catalyst composition is typically in the range of between about 1 x 10-5 g/cm2 and 6 x 10-5 g/cm2' and preferably between about 3 x 10-5 g/cM2 and 5 x 10-5 g/CM2 and 4 1 f -a- S X 1C)-5 g/clu2 for carrier polymer thicknesses of between about 0.1 micrometers and 0.5 micrometers.

- The carrier polymer/migratory catalyst composition is preferably coated between about 10 and about 14 percent solids. Catalyst layers having percent solids of greater than 14 solids have shown lower gloss, increased roughness and, hence, decreased electrical performance of the fInal magnetic media. The concentration of the migratory catalyst composition on the carrier polymer is typically between about 15% and about 40%, and preferably between about 20 and about 30%.

Coatincr Laver The chemically curable coating layer is comprised of a liquid mixture of a chemically curable coating resin and an effective amount of a curing agent capable of curing the coating resin. The coating layer may include added pigments such as magnetic, reflective, abrasive or conductive pigments, along with other additives necessary for processing or end use properties. The coating layer when applied must be sufficiently liquid to obtain a sufficiently smooth coating surface.

A preferred curable coating resin/curing agent system, particularly in the magnetic media area, is an isocyanate and isocyanate curable resin system. Isocyanate curable resins suitable for use in the present invention include: partially hydrolyzed copolymers of vinyl chloride and vinyl acetate, a terpolymer of vinyl chloride, vinyl acetate and vinyl alcohol, a terpolymer of vinyl chloride, vinyl acetate and maleic acid, a partially hydrolized copolymer of vinyl chloride and vinylidene chloride, mitrocullulose, phenoxy(bisphenol- epoxy), polyester urethanes, and other hydroxy containing polymers, polyols such as glycerol and trimethylolpropane, and phenoxies, such as PIMH commercially available from Union Carbide.

i -g- Isocyanate compounds suitable for use as curing agents in the present invention are preferably aromatic. In addition the isocyanates are at least tri-functional. Examples of aromatic isocyanate compounds which are at least tri-functional and suitable for use in the present invention include Modur HC, Mondur MRS, and CB-60 commercially available from Mobay Chemical Corp. and Desmodur IL, commercially available from FarberfabrIken Bayer AG.

CB-60 is believed to be tris-N,N1,N" (3-isocyanato-4-methyl-phenyl)-trimethylol propane carbamate; Desmodur IL is believed to be bis-1,3-di(3isocyanato-4-methyl-phenyl)-isocyanurato-4methyl benzene; Mondur HC is believed to be n-1,6-(di-(3-methyl-4- is isocyanatorhenyl)isoc,.ranurato)-isocyanatohexamethylene-3methyl-4isocyanatophenyl-isocyanurato-hexane; and Mondur MRS is believed to be poly(isocyanato-l-phenyl-3-methylene) with 2 to 3 repeating units.

The chemically curable coating layer is preferably coated on top of the catalyst layer. The catalyst layer is a dried polymeric layer with one or more migratory catalysts dispersed therein. The polymeric layer is preferably comprised of higher molecular weight polymers coated in a suitable solvent, but which is not cured. Presumably curing prevents migrazion of the catalyst composition.

After the curable layer is coated onto the catalyst layer the catalyst composition begins to migrate into the curable layer to catalyze the curing reaction.

Depending on such parameters as the compositions of the catalyst layer and the curable layer, the solvents, and the degree of compatibility of each layer, it is believed that varying amounts of mixing or loss of integrity between the layers may occur.

k The types of magnetic pigments which are suitable for use in the present invention include gamma-iron (111) oxide, finely divided magnetite, undoped or doped ferromagnetic chromium dioxide, cobalt-doped gamma-iron (111) oxide, ferromagnetic alloy fine powders, barium ferrite, and many other magnetic or magnetizable particles or pigments which are well known in the art.

Additives In addition to magnetic or magnetizable pigments or particles, magnetic media typically require other additives necessary for processing or end use characteristics. For example, lubricants, dispersing agents, antioxidants, and non-magnetizable pigments may be added as needed.

Lubricants such as saturated or unsaturated higher fatty acids, fatty acid esters, higher fatty acid amides, higher alcohols, silicone oils, mineral oils, edible oils, fluorinated compounds, molybdenum disulfide and others known in the art may be added to the magnetic pigment dispersion as needed for the desired end use physical properties.

Dispersing agents may be added to chelate the magnetic pigment such that they are individually suspended within the uncured binder to provide optimum magnetic performance in the magnetic coating after curing. Effective dispersants include carboxylated polyesters, phosphate esters, lecithin, fatty acid amides, titanate coupling agents and the like.

Examples of antioxidants which may be used in the magnetic media of the presentinvention include salicylanilide, tin oxide, mercury phenyloloate, copper naphthenate, zinc naphthenate, trichlorophenol, pdinitrophenol, sorbic acid, butyl p-oxybenzoate, dihydro- acetoacetic acid and the like.

t, f.

1 Non-magnetic pigments which may be added as a head cleaning agent, or for other purposes, include silicon dioxide, titanium dioxide, aluminumoxide, chromium dioxide, calcium carbonate, zinc oxide, Fe203. talc, kaolin, silicon carbide, carbon black, and the like.

The invention is further described by the following non-limiting examples wherein all parts are by weight.

ExamiDle 1 and ComDarative ExamDle 1 Magnetic media of the present invention were made, using a reverse gravure technique, by coating a catalyst layer consisting of 26% by weight triethylenediamine, commercially available as DABCO from Air Products, Inc., and 74% by weight of a partially hydrolyzed vinyl chloride/vinyl acetate copolymer in a solution of methyl ethyl ketone (MEK) at 12% solids, onto a 30 cm wide, 75 micrometer thick polyester backing. At a separate coating station, a dispersion of binder and magnetic pigment comprising: gamma iron oxide, carbon, alumina pigments, present as 75% of the solids, bisphenolepoxy and polyester urethane binders, present at 11% of solids, Mondur CB isocyanate curative, present as 6% of the solids, and dispersants and lubricants present as 8% of the solids, was coated, oven-dried at 160 degrees F and calendered.

The coating was found to be 40% cured as it exited from the coater and exhibited excellent green strength. Diskettes were prepared from the above coated roll. The durability of the diskettes of Example 1 was greatly improved over the diskettes of Comparative Example A. The ExamDle 1 diskettes survived more than 15 million passes without visual scratches, while the Comparative Example A diskettes survived only 6.7 million passeV.

t Examples 2-6

Magnetic media of the present invention were made according to the compos itions and procedures of Example 1, except that instead of coating the catalyst layer at 12% solids, the catalystlayer solution was coated at 2, 4, 6, 8 and 10 solids, Examples 2-6 respectively. There was no significant difference in the percent cure, green strength or overall coating toughness between Examples 2-6 and Comparative Example A.

ExamRles 7-9 Magnetic media of the present invention were prepared by preparing a catalyst layer of 25% triethylenediamine and polyester binder such as Bostik 7975. This mixture was applied onto a 30 cm wide, 75 micrometer thick polyester support. After the catalyst layer was coated onto the support and was oven dried, the coated support was wound up in a roll for later use. Three such rolls were made using 10, 11, and 12% solids solutions (Examples 7, 8, 9, respectively). After three days a magnetic dispersion very similar to the one described in Example 1 was coated onto the three supports having cured catalyst layers, was oven dried at 160 degrees F and was calendered. The Example prepared using a 12% solids solution (Example 9), was sufficiently cured so as to give no coating transfer when a 4 inch piece of SCOTCH brand transparent tape was pressed onto the magnetic coating immediately after coating and peeled off with force. At 11 solids (Example 8) there was appreciable transfer of binder, and at 10% solids (Example 7) the transfer was significant and could not be distinguished from the uncatalyzed control (Comparative Example A).

The amount of triethylenediamine present in a known area (18.75 cm2) of the catalyst/polyester carrier polymer coating was determined spectrophoto.metrically using h the ultraviolet absorption of a methylene chloride extract. The catalyst concentration of Example 9 was thus found to be about 4 X 10-5 5/CM2.

ExamDles 10-12 Magnetic media of the present invention were prepared in a manner similar to the one described in Example 1. A phenoxy (bisphenolepoxy) binder was used as the carrier polymer for triethylenediamine catalyst, at 26% catalyst concentration and 10, 11 and 12% solids in the solutions, (Examples 10, 11 and 12, respectively). The magnetic coatings, applied in a separate pass were significantly cured as indicated by their reluctance to disintegrate when immersed in dimethyl sulfoxide (DMSO). The uncatalyzed coating (Comparative Example A) completely disintegrated in a matter of seconds when immersed in DMSO immediately after coating. As in Example 1 (vinyl carrier) and Example 3 (polyester carrier), at 26% triethylenediamine concentration, the preferred percent solids of the catalyst/polymer solution appeared to be about 12%.

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Claims (12)

WHAT IS CLAIMED IS:

1. A method of-making a magnetic medium, said method comprising the steps of (a) coating at least one side of a nonmagnetic support with a catalystlayer, including an effective amount of a migratory catalyst composition; (b) coating a liquid dispersion of magnetic pigment in a chemically curable binder resin onto each of said at least one catalyst-layers, said dispersion comprising i) an isocyanate curable binder resin; ii) an effective amount of an isocyanate compound capable of reacting with said binder resin; and iii) magnetic pigment; wherein said binder resin is cured upon migration of said catalyst composition into said dispersion of magnetic pigment to catalyze the curing reaction between said isocyanate compound and said isocyanate curable binder resin.

2. The method of claim 1 wherein said catalyst layer also includes a carrier polymer having said effective amount of a migratory catalyst composition dissolved therein.

3. The method of claim 2 wherein said catalyst composition has a concentration in said carrier polymer of about 15 - 40% by weight.

4. A magnetic recording medium comprising a non- magnetic support and at least one magnetic layer, sLid magnetic layer comprising a cured composite derived from (a) a catalyst-layer comprising an effective P p 1.

amount of a migratory catalyst composition said layer coated on said nonmagnetic support; and (b) a liquid dispersion of magnetic pigment in a curable binder said dispersion coated onto said catalyst layer; said curable binder comprising a chemically curable binder resin. and an effective amount of a curing agent capable of reacting with said binder resin.

5. The magnetic medium of claim 4 wherein said z chemically curable binder resin is an isocyanate curable binder resin and said curing agent is an isocyanate compound.

6. The magnetic medium of claim 4 wherein said catalyst-layer also includes a carrier polymer in which said effective amount of a migratory catalyst composition is dispersed.

7. The magnetic medium of claim 4 wherein said migratory catalyst composition includes a catalyst which has at least one bridgehead nitrogen.

8. The magnetic medium of claim 4 wherein said migratory catalyst composition is selected from the group consisti ng of triethylenediamine, dimethyl ethanolamine, quinuclidene, and mixtures thereof.

9. The magnetic medium of claim 6 wherein said catalyst layer has a thickness of about 0.1 - 0.5 micrometers.

1

10. A method of making a polymeric composite coated substrate comprising the step of providing a composite on a substrate said composite comprising a) a catalyst layer having an effective amount of a migratory catalyst.

16 composition; and b) a chemically curable layer comprising i) a chemically curable coating resin; and ii) an effective amount of a curing agent capable of curing said coating resin; and wherein said curable layer is cured upon migration of said catalyst composition into said curable layer and the catalyzing of the curing reaction between said curable coating resin and said curing agent.

11.. A method of making magnetic medium as claimed in Claim 1 substantially as herein described with reference to any one of the Examples.

12. A magnetic recording medium as claimed in Claim 4 substantially as herein described with reference to any one of the Examples.

a Published 1989 atThePatentOfnee,State House,66/71 HighHolbomLondon WClR4TP.Further copies maybe obtainedfrornThePatentomce. Sales Branch, St Mary Cray, Orpington, Rent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87 4