Patent Application
20080034998
The methods and apparatus described herein to make a printing blanket from cast polyurethane layers may utilize two-part polyurethanes, moisture curable polyurethanes, UV or radiation curable polyurethanes, or cure-blocked or delayed-cure polyurethanes.
Suitable polyurethane casting compositions for use in the present invention are described in U.S. Pat. No. 3,211,701, the disclosure of which is hereby incorporated by reference. Such compositions comprise the reaction product of an isocyanate-terminated prepolymer with an organic chain extender or crosslinking agent (which may be a polyamine or a polyhydric alcohol) with a functionality of at least 2 and a molecular weight from 18 to 600. The isocyanate-terminated prepolymer is prepared from a hydroxyl-terminated polyester, polyether, or polybutadiene polyol or mixtures thereof having a molecular weight of 300 to 6000 and a functionality of at least 2 and optionally, a hydroxyl containing chain extending agent with a functionality of at least 2 and a molecular weight of 18 to 600, with an excess of organic diisocyanate.
The polyether polyols useful for the prepolymer are made by polymerization of cyclic ethers such as ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, and the like. Such cyclic ethers can be used individually or as mixtures or in successive fashion when making a polyether.
Suitable polyesters containing hydroxyl groups include, e.g. reaction products of polyhydric (preferably dihydric) alcohols, optionally with the addition of trihydric alcohols, and polybasic (preferably dibasic) carboxylic acids. Instead of free polycarboxylic acids, the corresponding polycarboxylic acid anhydrides or corresponding polycarboxylic acid esters of lower alcohols or mixtures thereof may be used for preparing the polyesters. The polycarboxylic acids may be aliphatic, cycloaliphatic, aromatic, and/or heterocyclic and they may be substituted, e.g. by halogen atoms, and/or may be unsaturated. Exemplary compounds include succinic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, trimellitic acid, phthalic acid anhydride, tetrahydrophthalic acid anhydride, hexahydrophthalic acid anhydride, tetrachlorophthalic acid anhydride, glutaric acid anhydride, maleic acid, maleic acid anhydride, dimeric and trimeric fatty acids such as oleic acid. Exemplary polyhydric alcohols include ethylene glycol, propylene glycol, butylene glycol, hexanediol, octanediol, neopentyl glycol, cyclohexane dimethanol, 2-methyl-1,3-propanediol, glycerol, trimethylolpropane, hexanetriol, butanetriol, trimethylolethane, pentaerythritol, mannitol, sorbitol, methyl glycoside, diethylene glycol, triethylene glycol, dipropylene glycol, polypropylene glycol, dibutylene glycol, polybutylene glycols, and the like. The polyesters may also contain a proportion of carboxyl end groups. Polyesters of lactones may also be used. The polyesters have at least 2 and generally from 2 to 8, preferably 2 or 3, hydroxyl groups.
Suitable polybutadiene polyols are Poly Bd polyols from Sartomer and liquid polybutadiene Krasol polyols from Kaucuk.
Suitable isocyanates for the prepolymers include aromatic or aliphatic diisocyanates and triisocyanates commonly known to those skilled in the art. Examples include 2,2′-, 2,4′-, or 4,4′-methylenediphenylene diisocyanate (MDI), polymeric MDIs, MDI variants, carbodiimide-modified MDIs, modified di- and polyisocyanates (urea-, biuret-, urethane-, isocyanurate-, allophanate-, carbodiimide-, or uretdione-modified, etc.), hydrogenated MDIs, 2,4 or 2,6-toluene diisocyanates or mixtures thereof, p-phenylene diisocyanate, TMXDI, isophorone diisocyanate, adducts of isophorone diisocyanate such as the urea, biuret trimer, dimer and allophanate, 4-diisocyanatobutane, 1,4-cyclohexanediisocyanate, hexamethylene diisocyanate,the adducts of hexamethylene diisocyanate such as biuret, trimer, dimer, allophanate and the like, and mixtures thereof.
Illustrative, but non-limiting examples of hydroxyl containing chain extenders or cross-linkers include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-propanediol, 2-methyl-1,3-propane diol, neopentyl glycol, 1,3- and 2,3-butylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, hydroquinone bis[2-hydroxyethyl ether], and the various bisphenols and their bis[hydroxyalkyl ether] derivatives, glycerin, trimethyol propane and ethoxylated derivatives thereof.
Suitable curing agents for the isocyanate-terminated prepolymers of the present invention include, for example, sterically hindered aromatic polyamines, sterically hindered aromatic diamines, diamines substituted with electron withdrawing groups and mixtures thereof. Examples of aromatic diamines which are rendered less active by electrical effects of ring substituents include 4,4′-methylene-bis(2-chloroaniline) (MOCA or MbOCA) and 4,4-methylene-bis(3-chloro-2,6-diethylaniline) (MCDEA).
These sterically hindered aromatic diamines have molecular weights of less than 500 and include, for example, 1-methyl-3,5-diethyl-2,4-diamino benzene, 1-methyl-3,5-diethyl-2,6-diamino benzene, 3,5-dimethylthio-2,4-toluene diamine, 3,5-dimethylthio-2,6-toluene diamine, 1,3,5-trimethyl-2,4-diamino benzene, 1,3,5-triethyl-2,4-diamino benzene, 3,5,3′,5′-tetraethyl-4,4′-diamino diphenylmethane, 3,5,3′,5′-tetraisopropyl-4,4′-diamino diphenylmethane, 3,5-diethyl-3′,5′-diisopropyl-4,4′-diamino diphenylmethane, 3,5-diethyl-5,5′-diisopropyl-4,4′-diamino diphenyl-methane, 1-methyl-2,6-diamino-3-isopropyl-benzene, trimethylene glycol di-p-amino-benzoate, and mixtures of the above diamines, such as, for example, mixtures of 1-methyl-3,5-diethyl-2,4-diamino benzene and 1-methyl-3,5-diethyl-2,6-diamino benzene in a weight ratio between about 50:50 to 85:15, preferably about 65:35 to 80:20. Some hindered amines are commercially available and sold as Baytec CUR W or Ethacure 100 (a mixture of 3,5-diethyl-2,4-toluenediamine and 3,5-diethyl-2,6-toluenediamine; Bayer Corp. or Albemarle Corporation) and Ethacure 300 from Albemarle Corporation (a mixture of 3,5-dimethylthio-2,4-toluenediamine and 3,5-diethyl-thio-2,6-toluenediamine). The difunctional and polyfunctional aromatic amine compounds may also exclusively or partly contain secondary amino groups such as 4,4′-di-(methylamino)-diphenylmethane, or 1-methyl-2-methylamino-4-amino-benzene.
Suitable prepolymers for the two-part polyurethanes of the present invention are commercially available from Chemtura (formerly Crompton Corp.), Sika Deutschland GmbH, ITWC, Bayer, and Dow.
The cure-blocked and/or delayed-cure polyurethanes are preferably derived from either blocked isocyanates or blocked or delayed action curatives, depending on the casting method employed. Where the polyurethanes are derived from blocked isocyanates, a prepolymer such as those described above for two-component systems is reacted with a blocking group such as methylethyl ketoxime, caprolactam or other active hydrogen-containing compound prior to adding a chain extender or crosslinking agent to the system. Curing is initiated only after the mixture is applied to a substrate and heat is supplied. In the presence of heat, the blocking group is released from the original isocyanate group, thus allowing the isocyanate group to react with other active hydrogen containing entities in the matrix.
Where the polyurethane is derived from blocked or delayed action curatives, such curatives may comprise a complex of methylene dianiline (MDA) and sodium chloride dispersed in dioctyl phthalate. The blocked or delayed action curative is added to a prepolymer such as those described above for a two component system (it replaces the chain extender or cross-linker in the two component cast system). Curing is initiated after the mixture is applied to a substrate and heat is supplied. At room temperature, this complex reacts very slowly with free isocyanate groups, but at elevated temperatures, the salt compound unblocks, releasing MDA which reacts rapidly with the free isocyanate present. Examples of suitable cure-blocked and/or delayed-cure polyurethanes include the MEKO and Caytur type systems from Chemtura.
Suitable moisture-cure polyurethanes for use in the present invention include urethane prepolymers which are isocyanate-capped polyols, such as polyesters, polyethers and polyester/polyols that do not contain any internal cross-linking agent (i.e., water cross-links the polymer and gives the desired physical properties). Typical prepolymers for moisture cured polyurethanes are the same as those described above for two component cast polyurethane systems, but normally the final free NCO content of the prepolymer for a moisture cured systems will be 5% or less while typical prepolymers used in two component cast systems range from greater than 2% up to about 12%. Preferred moisture curable polyurethanes for use are commercially available from Bayer, Futura, Sika and others.
A typical UV or radiation-curable polyurethane system contains an oligomer, which may or may not contain reactive functional groups (such as double bonds), a crosslinking agent, a reactive diluent for viscosity control, and a photosensitizer or photoinitiator. By selecting an oligomer which contains at least two points of reactive unsaturation, or a reactive diluent which contains at least two points of reactive unsaturation, a crosslinking agent may be eliminated. Control over the properties of the cured systems can be exercised via the structure of the oligomer backbone, including such factors as degree of chain-branching, types of functional groups, number and types of unsaturated bonds, molecular weight, etc.; functionality and level of crosslinking agents; nature and level of reactive diluent; kind and level of the sensitizer or photoinitiator; and the like. An exemplary oligomer is an unsaturated urethane oligomer obtained by reacting an isocyanate-functional prepolymer with unsaturated compounds containing an isocyanate-reactive active hydrogen group. The unsaturated urethane oligomers are typically the reaction product of at least one organic isocyanate compound having at least two isocyanate groups; at least one polyether or polyester polyol with a functionality of at least 2 (similar to those described above); and at least one unsaturated addition-polymerizable monomeric compound having a single isocyanate-reactive active hydrogen group such as hydroxyl ethyl(propyl)-(methyl)acrylate. Before any polymerization can occur, free radicals must first be produced via the photoinitiator. The production of free radicals by the photoinitiator is a wave length function of the actinic radiation. Once the radicals are formed, propagation of polymer growth rapidly advances through chain reaction. Suitable UV or radiation curable polyurethanes are available from companies such as Sartomer, Radcure and others.
Referring now to
In an alternative embodiment illustrated in
As described above, after the polyurethane is coated onto the sleeve, curing is initiated at a location downstream from the coating apparatus. For example, curing may be initiated on the side of the cylinder which is opposite the slot die and isolated from the point of coating. After the sleeve is cured, it may then be rotated back to the slot die apparatus for application of further layers.
Referring now to
An example of a suitable electrostatic spraying process is described in U.S. Publication Nos. 2003/0033948 and 2003/0116044, which are incorporated herein by reference. This method may be used in embodiments where solvent-free polyurethane systems are used, and is designed to produce one or more layers of solvated elastomer on a printing blanket or sleeve such that the boundary within one layer of the sleeve comprised of two components is a gradient or such that the boundary between two layers is a gradient.
In this embodiment, the polyurethane preferably comprises a two-part polyurethane or a moisture curable polyurethane. Such polyurethanes are preferred because cure initiating equipment is not required. However, it should be appreciated that UV or radiation curable and cure-blocked or delayed-cure polyurethanes can also be used in such a system where the substrate with the polyurethane coating is transported downstream or rotated away from the spraying apparatus where cure is initiated by exposure to UV light, electron beam, or heat.
As shown in
It should be appreciated that the surface area within the spray nozzle is sufficiently small and the polyurethane is under sufficient pressure such that the polyurethane is nearly completely refreshed along the inner surfaces of the spray nozzle. Accordingly, build-up of cured or partially cured polyurethane is not a significant issue in this method, and prevention of premature exposure to moisture is not as difficult as in prior art methods.
The indexing paper is supplied via rotating rolls 52, 54 and may comprise any paper which has sufficient strength to resist tearing/breaking and which is capable of performing the cleaning function. While indexing paper is preferred for use in the present invention, it should be appreciated that substrates such as plastic films or fabrics may also be used to carry away the partially cured or cured urethane.
Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/836,218, filed Aug. 8, 2006, entitled METHOD OF MAKING A PRINTING BLANKET OR SLEEVE INCLUDING CAST POLYURETHANE LAYERS. The entire contents of said application are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60836218 | Aug 2006 | US |
