The invention is directed to a process for electroplating a non-conducting surface such as through-hole walls within a printed circuit board substrate. The process comprises formation of a conductive oxide coating over a substrate, preferably by immersion of said substrate in an aqueous oxidative desmear solution for a time sufficient to form a coating containing conductive dielectric oxidation residue and then, in the absence of a step of forming an additional conductive coating over the residue coating or removing the coating, electroplating metal onto said surface by immersion of the substrate having the coating in an electroplating solution.

1. Introduction
This invention relates to new metal plating processes for a variety of uses including fabrication of printed circuit boards. In one embodiment, the invention relates to direct electroplating of copper over a substrate while avoiding the use of an electroleess metal plating solution or the use of an extraneous conductive layer. The process is selective to non-conducting portions of a printed wiring board base material and therefore, copper clad surfaces of a circuit board substrate remain essentially unaffected by the procedure.
2. Description of the Prior Art
For purposes of discussion only, the major portion of the description that follows is directed to the fabrication of printed circuit boards (PCBs) though the process is suitable for formation of any metal coated substrate. Accordingly, the process is useful for decorative plating, formation of mirrors, formation of circuits inclusive of multilayer circuits, double sided circuits and flex circuits.
A conventional process for fabricating a multilayer circuit board starts with a copper clad epoxy innerlayer substrate. A circuit is formed on the innerlayer, multiple innerlayer circuits are stacked, and the stack laminated using heat and pressure. To provide electrical connections between circuit layers, conductive pathways are formed within and through the stack at desired locations. A first step in the process of forming conductive pathways is to drill an array of holes into the stack at desired locations.
Following the drilling of holes, a pretreatment sequence is required prior to metallizing the hole walls. The process involves subjecting the hole walls to a step of smear removal. Smear refers to the drill smear on the walls of holes formed by frictional heating and melting of epoxy during the drilling step. The elimination of smear is practiced in all processes for circuit fabrication to achieve metal adhesion to the hole wall and full electrical continuity between layers of the board through plated holes or vias. Etchback is a term used in connection with smear removal. The term refers to smear removal and is used to remove epoxy to a level deeper than the thickness of the smear, typically to expose about 0.5 mils on the top and bottom surfaces of the innerlayer copper circuitry. Desirably, innerlayer copper protrudes into the drill hole to provide three point connection for copper bonding as required for some military spec boards.
Four methods are used for smear removal and etchback. All use oxidation, neutralization-reduction and glass etching. For many years, sulfuric acid has been used. Major disadvantages include lack of control which leads to hole-wall pullaway and rough holes, and the corrosive nature of sulfuric acid which makes it hazardous to use and difficult to waste treat. Chromic acid provides more control and a longer bath life but problems are encountered with copper voids due to Cr+6 poisoning, waste water pollution and contamination of plating processes. Plasma etching is a dry-chemical method to achieve dielectric oxidation. In this process, boards are exposed to oxygen and fluorocarbon gases. Persistent problems with the process are non-uniform treatment of holes and high cost of equipment. Moreover, the process is a batch process and not subject to automation, controls must be provided to prevent air pollution and the cost of the process has limited its use. Permanganate is the current method of choice due to improved copper adhesion to the hole wall resulting in less pullaway, smoother plated through-holes and better control. Permanganate desmear is described by C. A. Deckert, E. C. Couble and W. F. Bonetti, xe2x80x9cImproved Post-Desmear Process for the Multilayer Boardsxe2x80x9d, IPC Technical Review, January 1985, pp. 12-19 incorporated herein by reference. Moreover, permanganate is non-toxic and easily waste treated.
It is known that permanganate desmear leaves a residue on the hole wall that is a mixture of oxides of manganese, unreacted permanganate and organo manganese compounds. The art has long considered this residue to be undesirable and therefore, has adopted procedures to remove this residue.
A conventional permanganate desmear and etchback process involves the following steps:
Solvent pretreatment softens the surfaces of the epoxy. The permanganate solution activates the surface of the epoxy by oxidation and etching. The epoxy substrate is composed of organic micelles. Contact with the alkaline permanganate solution is believed to etch or oxidize micelles from the surface of the epoxy resulting in a roughened porous surface into which a subsequently applied metal deposits. This mechanically bonds the metal deposit to the surface of the epoxy. Contact of the permanganate solution with the hole wall reduces permanganate leaving the above described dielectric oxidation/permanganate residues on the surface of the epoxy. Neutralizer (acid solution) is then used to dissolve the dielectric oxidation residues from the surface of the non-conductor. This is believed to be a critical part of the process as the presence of permanganate residues is thought to interfere with bond strength between the substrate and a subsequently applied metallic coating. Finally, a glass etchant is used to remove glass fibers protruding into the hole due to removal of the epoxy surface during the permanganate etch step.
Metallization is a series of chemical steps following desmear. The process is used to make panel side-to-side and innerlayer connections by plating non-conducting epoxy surfaces with copper. Metallization may be conducted using either electroless techniques or by direct electroplating of copper, a process the industry has termed the direct metallization technique (DMT).
The electroless process includes the above described steps of solvent pretreatment, desmear, treatment with neutralizer, racking the PCB substrates, copper microetching, hole and surface catalyzing with a palladium colloid and electroless copper deposition. A typical sequence following desmear and omitting rinse steps, follows:
Step 1: Cleaner-Conditioner. Alkaline cleaning to remove soil and condition holes.
Step 2: Microetch. Acid etching to remove copper surface contaminants.
Step 3: Sulfuric Acid. Used to remove microetch residues.
Step 4: Pre-dip. Used to maintain chemical balance for the next treatment step.
Step 5: Catalysis. Use of an acid solution of palladium and tin to deposit a thin layer of surface active palladium on the holes walls.
Step 6: Acceleration. Used to remove colloidal tin from the palladium colloid.
Step 7: Electroless Copper. Alkaline chelated copper reducing solution that deposits a thin copper deposit on the surfaces of the holes and other surfaces.
Difficulties are encountered using electroless copper solutions. For example, it is difficult to obtain uniform deposits because the solution composition changes with use. Voids may occur within the holes due to improper hole wall preparation. Plated copper on the hole-walls may pull away and form blisters. Electroless copper solutions may decompose resulting in a rapid plate out of the copper onto the circuit board substrates and onto all surfaces exposed to the copper solution such as tanks and racks. Copper to copper bond failure often occurs if copper cladding is inadequately prepared since electroless copper is deposited onto the copper cladding of the printed circuit board. Waste treatment of electroless copper solutions is costly since the spent solutions contain complexing agents and copper ions, both of which must be removed prior to discharge to the environment. The reducing agent used in electroless copper solutions is conventionally formaldehyde, a suspected carcinogen.
The industry is currently adopting direct metallization techniques (DMT) as an alternative to electroless plating. The basic concept of using a palladium system for direct metallization is disclosed in U.S. Pat. No. 3,099,608 incorporated herein by reference. In this patent, a method using colloidal palladium to metallize through-holes for PCB fabrication is disclosed. Defects in the process of the patent prevented early commercialization. The original concept of using carbon and graphite as a conductive layer dates back to the early days of eyelet boards when graphite was used to convert single-sided PCB""s into double-sided boards. Notwithstanding the early direct plate procedures, it was only in the early 1990s that there has been commercial interest in DMT processes for PCB fabrication.
Most current DMT processes fall into three broad categories. They are palladium based systems; carbon and graphite systems; and conductive polymer systems. The first commercially significant palladium based system, known as the EE-1 process, uses a colloidal palladium-tin activator followed by flash electroplating. This process is disclosed in U.S. Pat. No. 4,683,036 incorporated herein by reference. The flash plating bath contains a surfactant to inhibit deposition of copper on foil surfaces without inhibiting deposition on palladium sites. Deposition occurs by propagation from the copper foil and growth epitaxially along the activated surface of a hole during flash plating. The flash plate is then built to full thickness using any electroplating bath.
The DPS system uses a palladium/tin activator with vanillin as an additive in the palladium-tin formulation. This system is disclosed in U.S. Pat. No. 4,933,010 incorporated herein by reference. The system is similar to the EE-1 process but vanillin is said to enhance the rate of epitaxial growth of copper along the catalyzed surface.
The next advance in the use of the palladium/tin activator and the most commercially significant of the palladium-tin activator DMT systems involves conversion of palladium to palladium sulfide as disclosed in U.S. Pat. No. 4,810,333 incorporated herein by reference. This system uses the palladium-tin colloid and an additional step believed to convert adsorbed palladium to palladium sulfide by immersion in an aqueous solution of a sulfide. The sulfide increases conductivity of the coating contributing to lower resistance to electrolytic copper plating. The sulfide form of the palladium colloid has been found to be more chemically resistant to imaging technology.
The second major DMT process uses carbon. One such system is identified as xe2x80x9cBlack Hole(trademark)xe2x80x9d. The process is disclosed in U.S. Pat. No. 4,724,005 incorporated herein by reference. Black hole uses a carbon suspension to form a conductive coating. Non-conductive surfaces are first conditioned with a polyelectrolyte to facilitate absorption of carbon onto the hole walls. Following formation of the carbon layer, the entire board is heated to fix the carbon to the board. To ensure sufficient conductivity, the carbon treatment is performed twice. Carbon residues must be removed from copper foil surfaces by a microetching step.
Black hole was followed by the use of graphite as disclosed in U.S. Pat. No. 5,389,270 incorporated herein by reference. The process sequence is similar to the carbon sequence. The hole-wall is treated with a polyelectrolyte, graphite is adsorbed onto all surfaces inclusive of hole-walls from a graphite suspension and the coating is then fixed to the surfaces by heating. At least one supplier of such chemicals further uses an ionic solution to enhance conductivity.
A conductive polymer system is disclosed in European Patent No. 0 489 759. This process utilizes conductive polymer to form a conductive layer for direct metallization with electrodeposited copper. After the step of microetching copper foil and solvent pretreatment of hole-walls, an alkaline potassium permanganate conditioning solution is used to form a dielectric oxidation coating containing manganese dioxide and other decomposition residues on the walls of the holes. This manganese dioxide portion of the coating functions as a catalytic oxidizing agent for subsequent processing steps used to form the conductive polymer layer. In the catalyzing step, a solution of a thiophene monomer is used to wet manganese dioxide surfaces. The hole-walls are then treated with a sulfuric acid solution to spontaneously oxidize the thiophene monomer to form a black conductive polymer film on the non-conductive areas of the PCB base material. A similar process disclosed in U.S. Pat. No. 5,183,552 combines the catalyzing and fixation steps in a single step using an acid permanganate treatment solution and pyrrole as the monomer to form a polypyrrole conductive film.
All DMTs described above have common characteristics. Following desmear and etchback, each process requires an additional series of chemical treatment steps to form a layer of material (hereafter the xe2x80x9cextraneous layerxe2x80x9d) possessing adequate conductivity to permit electrolytic copper deposition. The additional sequence of steps required to form the extraneous layer has impact on total process costs, total processing time, equipment size and waste treatment requirements. The impact of this extraneous layer on the overall cost and efficiency can be summarized as follows.
DMTs using palladium colloids require the steps of adsorption of palladium colloid onto a substrate, acceleration to activate the palladium and either a flash plate of copper or treatment with a sulfur compound. Each chemical composition must be controlled during use, accommodated in the number of processing steps by the plating equipment; and each additional chemical composition must be waste treated when spent. The processes are not selective and palladium adsorbs onto copper foil as well as onto the hole walls. Therefore, an aggressive etching step and chemical etching solution are required to remove the palladium from the copper foil prior to electroplating. The etchant must be accommodated by the plating equipment; and spent etchant, containing dissolved copper, must be carefully waste treated before discharge to the environment. Each additional step requires additional rinsing with a concomitant requirement for waste treatment and space within the treatment equipment. It has been found that the process is not suitable for the fabrication of multilayer boards.
Carbon and graphite DMT systems have disadvantages similar to the palladium processes. To electroplate copper onto the surface, it is necessary to form the carbon or graphite extraneous conductive coating over the surface. Once formed, the coating must be fixed by a drying and heating step, steps involving significant energy consumption. Because the process is non-selective, the coating must be removed from copper foil without disruption of the coating on hole walls. It is difficult to selectively remove the carbon or graphite from the foil without causing interconnect defects in a finished board. The carbon or graphite solution is a source of contamination to the entire PCB facility. Spent carbon and graphite solutions must be waste treated, but these solutions are difficult to waste treat because they contain substantial quantities of electrolytes and surfactants to maintain relatively large carbon and graphite particles stable in suspension. Copper etchants and rinse waters used to remove the coating from the copper foil must be carefully waste treated because they contain carbon or graphite in addition to dissolved copper.
Conductive polymer DMT systems possess certain advantages over both carbon and palladium systems because formation of the conductive polymer is selective to non-conducting surfaces whereby polymer does not have to be removed from the copper foil. Though the process is selective, there are problems associated with the use of these materials. The conductive polymer is formed by contact of monomer with dielectric oxidant residues formed during the step of conditioning with permanganate. The polymerization reaction used to form the conductive polymer is a condition sensitive in-situ reaction initiated by contact with oxidant. The reaction is difficult to monitor or control within a hole wall. The polymer film formed by this reaction may be unevenly formed over the non-conducting surfaces. Consequently, there are voids in the polymer film. These voids lead to interconnect defects.
The subject invention provides a DMT process that eliminates the need for an extraneous conductive layer such as palladium, carbon, graphite, or conductive polymer. Instead, the process uses dielectric oxidation residue formed from conventional or modified desmear oxidizing chemicals as an initial conductive layer, preferably dielectric oxidation residue formed by a permanganate desmear treatment.
The process arises from the observation of several properties of the oxidation residue following desmear. First, it is known that dielectric oxidation residues formed by oxidative desmear remain on the dielectric surface of a part following treatment, but not on metallic surfaces. Using permanganate desmear as an example, the residues contain unreacted permanganate, organo-manganese compounds and manganese dioxide in varied concentrations dependent upon reaction conditions. Second, dielectric oxidation residue left on the surface of a part, such as manganese dioxide, is a weak electrical conductor. In this respect, manganese dioxide has been used as a conductive electrode material in battery fabrication and for other purposes where a conductive coating is required. Third, it is known from the battery literature that enhanced oxidation of metallic oxide conductors by treatment with oxo compounds such as a peroxydisulfate or hydrogen peroxide enhances the conductivity of such coatings. For example, it is known that conductive manganese dioxide coatings are formed by treatment with peroxydisulfate solutions.
Accordingly, the process described herein involves a desmear step with an oxidant to form a dielectric oxidation residue coating on the dielectric surfaces of the part to be plated, optionally, treatment of the dielectric oxidation residue coating with an oxidizing agent, preferably an oxo compound to enhance the conductivity of the coating, and direct electroplating of the dielectric oxidation coating with a desired metal. The total process of the subject invention desirably includes a solvent pretreatment step. The oxo compound selected is desirably a mild etchant for copper. Since desmear materials selectively react with the dielectric of the PCB base material and do not react with copper foil, mild etchants such as the contemplated oxo compounds are satisfactory etchants for the copper. In this way, it is possible that the steps of conductivity enhancement and copper etching may be combined into a single step.
The following chart compares the steps of the subject invention with the DMT processes currently in use.
The above table illustrates that the process of the invention substantially reduces required chemical materials and processing steps. Since contact with the oxidant is selective to non-conducting surfaces, copper surfaces would not require a severe etch step. This results in a reduction in equipment size and power requirements. Waste treatment requirements are minimized. Extraneous thick intermediate layers between the PCB base material and a copper plate are eliminated avoiding a cause of interconnect defects. The overall process results in a substantially reduced cost to produce a metallized surfaces such as a high resolution circuit board. The circuit board so produced is of superior quality.
For purposes of illustration, the description that follows is directed to the manufacture of printed circuit boards, but it should be understood that the invention is not limited to this application and is suitable for any application involving the coating of non-conducting surfaces with metal.
The first step in the process preferably comprises treatment of hole walls in a circuit board base material with a solvent, for example, an oxygenated epoxy solvent for epoxy circuit board substrates. Solvents sold for this purpose include acetone, methyl ethyl ketone, methyl cellosolve, butyl carbitol, butyl carbitol acetate, butyl cellosolve and butyl cellosolve acetate. Typically these solutions are used in the form of aqueous alkaline solutions to increase attack on the surface of the epoxy. The solvent is diluted with water to form a 15 to 40% by volume water solution.
Following contact of the PCB base material with the solvent and water rinsing, the substrate is desmeared by contact with an oxidant solution, preferably an acid or alkaline permanganate solution. Chromic acid solutions are also suitable, but lesser preferred. Alkaline permanganate solutions are well known in the art and disclosed and discussed in U.S. Pat. Nos. 4,083,740; 4,425,380; 4,515,829; and 4,601,784; by Deckert et al, Improved Post-Desmar Process for Multilayer Boards, Printed Circuit World Convention III, Vol. 13, May 25, 1984; Deckert, Advances in MLB Technology, Circuit World, Vol 13, No. 3, 1987; and Deckert, Adhesion Solderability and Productivity: Next Generation PTH, Printed Circuit Fabrication, Vol. 11, No. 8, August 1988, each incorporated herein by reference. The permanganate solution is typically one having a pH of from 11 to in excess of 13.5. The combination of a high concentration of permanganate and an elevated temperature provides an increased etch rate on hole wall surfaces. Typical permanganate solution consists of sodium and potassium permanganate in a concentration of between 20 and 50 grams per liter. Sodium permanganate is more soluble and therefore preferred, but more costly than potassium permanganate. Because of cost considerations, potassium permanganate is more commonly used by the industry. The pH of the solution is maintained by addition of a base such as potassium hydroxide.
Alkaline potassium permanganate solutions undergo spontaneous reduction. During use, the rate of reduction is increased when the solution comes into contact with the epoxy resin which is oxidized. The reduction of permanganate can be represented by the following equation.
MnO4xe2x88x92+2H2O+3exe2x88x92xe2x86x92MnO2+4OHxe2x88x92
The reaction with epoxy resin is believed to be direct oxidation with the formation of carbon dioxide as shown by the following equation.
xe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94+8KMnO2xe2x86x928MnO2+4CO2+8KOH
Complete oxidation yields only manganese dioxide. Incomplete oxidation produces a surface that is believe to be a mixture of manganese dioxide, unreacted permanganate and organic residue permanganate byproducts.
The permanganate solution contains the permanganate salt in an amount of from 0.5 grams per liter to saturation and preferably, in an amount of from 10 to 60 grams per liter. The pH of the solution may range from about 1 to in excess of 14 though alkaline permanganate solutions are preferred, and solutions having a pH in excess of 12 are most preferred The desired pH is achieved by use of a hydroxide, preferably an alkali metal hydroxide, in an amount required to adjust pH to the desired value. The amount of hydroxide used may vary between about 1 and 70 grams per liter and more preferably, between about 20 and 50 grams per liter.
Though permanganate desmear solutions find predominant use in the industry, other desmear solutions are used to form dielectric oxidation residue coatings on the surface of a dielectric to be plated. Exemplary additional materials are disclosed in U.S. Pat. No. 5,194,313 incorporated herein by reference. A preferred material disclosed in said patent is a cerium solution such as a cerium sulfate solution. Such as solution would be a mixture of the cerium salt, an acid such as hydrochloric acid, and desirably a surfactant. The acid is added in an amount to provide a pH less than 1. The cerium salt is used in about the same concentration as permanganate. The dielectric oxidation coating comprises cerium oxide, unreacted cerium salts and organometallic cerium compounds. Additional oxidants solutions that may be used include sources of ferric ion, rubidium, lithium, chromic acid and periodate.
The above oxidants may be used alone or where compatible, in admixture with each other or further in admixture with one or more conductivity enhancers. As used herein, the term xe2x80x9cconductivity enhancerxe2x80x9d means either a material that increases electrical conductivity of the oxidation residue or the conductive coating formed from the oxidation residue following a step of reduction as discussed below. Cerium salts, as described above, are known conductivity enhancers and fall into the category of materials that increase the conductivity of the oxidation residue. Therefore, one preferred embodiment of the invention comprises a desmear solution that is a mixture of permanganate salt and a cerium salt, the latter used as both an oxidant and a conductivity enhancer. Suitable cerium salts include sulfates and nitrates. A preferred concentration of the cerium salt in the permanganate solution would be from 0.1 percent by weight to saturation, and more preferably, from 1 to 30 percent by weight of solution. Other conductivity enhancers may also be added to the permanganate solution. Examples of materials that increase the conductivity of the coating formed by reduction include easily reducible metal salts such as copper, cobalt and nickel salts that become entrapped in the oxidation residue coating during its formation and are readily reduced to metal during the reduction step described below. Such salts include sulfates, oxides and nitrates and may be added in an amount of from 0.5 to 25 percent by weight of the solution. In solution, they are likely converted to hydroxides, but will still codeposit with the oxidation residue. In addition, a reducible metal ion may be used as the cation of the permanganate salt used to form the permanganate solution. In this respect, alkali and alkaline earth metal permanganates are conventionally used to form permanganate desmear solutions. In accordance with a preferred embodiment of this invention, the permanganate salt used to form the desmear solution is selected from the group of cadmium permanganate, cesium permanganate, silver permanganate for non circuit applications, cobalt permanganate and nickel permanganate. Conductivity enhancers can be used that increase the conductivity of both the oxidation residue and the coating following reduction. Such materials include other conductive oxides such as indium tin oxide, ruthenium tin oxide, tin oxide, zinc oxide, tungsten oxide and zirconium oxide. It should be noted that such materials are not normally conductive, but can be made so by the addition of an impurity and appropriate treatment thereafter. The formation of such materials as conductive particles is disclosed in U.S. Pat. Nos. 4,545,928; 4,457,973; 4,904,526; and 5,742,424, each incorporated herein by reference. These conductive particles, though insoluble in the permanganate solution, may be suspended in solution. They can be added in amounts of from 0.5 to 25 percent by weight of the solution. These materials would be occluded in the oxidation residue to enhance conductivity of the residue and are readily reduced in accordance with procedures to be described below.
Many of the conductivity enhancers discussed above can also be used alone or as a separate treatment solution following formation of the oxidation residue and before the step of reduction. For example, following formation of the oxidation residue, the part could be immersed in a solution of an easily reducible metal salt and then the coating containing adsorbed metal ions could be reduced to a mixture of the manganese and the metal of the metal salt by a step of reduction. Metal salt solutions could be weakly acid to basic solutions of any of the metals described above and can be used in concentrations of from about 0.5 percent by weight to saturation. Alternatively, the conductive metal oxide particles disclosed in the aforementioned patents can be dispersed in a suitable liquid suspending agent, especially an aqueous suspending agent, most preferably water, applied to the surface of the dielectric by immersion of the dielectric into the suspension, and the adsorbed conductive particles reduced to metallic form using the reduction techniques disclosed herein. The particles disclosed in the above referenced U.S. Pat. No. 4,457,973 would be especially suitable for this approach. In this instance, the suspension of the conductive particles may be used with or without the coating of the oxidation residue.
In the most preferred embodiment of the subject invention, conventional permanganate pretreatment chemicals are used modified to enhance conductivity of either the oxide residue coating, the metallic coating following reduction, or both, as described above in combination with a simple oxo treatment step, all as described above.
The above solutions used for the purpose of desmear of hole walls will be referred to herein generically as xe2x80x9cdesmear solutionsxe2x80x9d, a term well known in the art. As used, the term will refer to the formation of coatings suitable for direct electroplating. Coatings formed using desmear solutions will be referred to as xe2x80x9cdielectric oxidation residue coatingsxe2x80x9d.
If desired, a source of fluoride ions may be added to the desmear solution to combine the step of oxidation with a step of dissolution of glass fibers extending into the hole wall. Sodium and potassium fluorides are desirable sources of fluoride ions and may be added to solution in an amount of from 0.1 to 5 percent by weight.
The conditions for treatment of a substrate with the desmear solution are not critical. Typically, the solution is used at a temperature varying from room temperature to slightly below the boiling point of the solution and more preferably, from about 20 to 45xc2x0 C. Contact time with a substrate may vary from about 1 to 15 minutes and preferably from about 2 to 10 minutes.
In the conventional desmear process, following treatment with the desmear solution, the dielectric oxidation residue coating would be stripped from the surface of the hole walls using a neutralizer solution. This solution generally comprises an acid solution of a reducing agent to dissolve residues from the hole walls. This step is desirably omitted from the process of the subject invention. However, if the oxidation residue is excessive, the neutralizer may be used to remove only the excess from the surface such as the hole wall, but not all of the residue coating. In this respect, to be useful for fabrication of PCBs, the dielectric oxidation coating residue has to be firmly bonded to the surface to be metallized and has to be a continuous coating that fully covers the surface. The desmear reaction with the epoxy resin is an oxidizing reaction that when optimized, will form a thin chemisorbed dielectric oxidation on the surface to be plated that is locked within pores formed by the oxidizing reaction. Excessive attack of the oxidant with the epoxy could form a coating that is excessively thick and frangible. This could cause copper blistering during use of the board. Under such circumstances, it is desirable to strip a portion of the coating from the surface, such as the hole wall, using a neutralizer solution, but its use must be carefully controlled so as not to strip the entire residue from the surface. Sufficient dielectric oxidation residue must be left on the hole wall to function as a conductive layer for metallization. In this respect, the process of the invention is characterized by the absence of a step that would remove the dielectric oxidation coating residue from the surface of the substrate. Conditions required to remove excess residue may be determined by routine experimentation and is governed by the overall thickness of the residue.
The residue formed from treatment with the desmear solution will contain conductive metal oxide and some unreacted oxidant in varying ratios dependent upon the reaction conditions. Using permanganate as an exemplary desmear solution, manganese dioxide residue would be left on the surface oxidized. Manganese dioxide is commonly employed as a cathode material in batteries such as heavy duty, alkaline and lithium cells. The use of manganese dioxide as a conductor is shown in U.S. Pat. Nos. 4,402,931; 4,541,172; 4,549,943; 4,604,336; 4,662,065; 4,863,817; 5,156,934; 5,277,890; 5,346,783; 5,348,726; 5,391,365; 5,401,477; 5,419,986; 5,482,796; 5,489,493; 5,505,200; 5,523,073 and 5,532,084, each incorporated herein by reference.
Battery grade manganese dioxide has been derived from naturally occurring manganese dioxide and synthetically produced manganese dioxide. Synthetic manganese dioxide may be in the form of electrolytic manganese dioxide (ED) and chemical manganese dioxide. In the past, the electrolytic manganese dioxide had been formed by electrolysis of a bath of manganese sulfate and sulfuric acid with ED depositing onto an anode. It was believed that chemical manganese dioxide was unsuitable for electrode use. Wang et al. recently discovered a process for formation of chemical manganese dioxide suitable for use as an electrode material thereby substantially increasing the availability of manganese dioxide for such use and lowering its cost. The Wang et al process is disclosed in the above referenced U.S. Pat. Nos. 5,348,726; 5,391,365; 5,482,796 and 5,532,084. Wang et al discovered that battery grade manganese dioxide is formed by reacting an aqueous solution of manganese sulfate with a peroxodisulfate to form a gamma crystal structure of manganese dioxide as a precipitate. The manganese dioxide precipitate is crystalline and in the form of spherical particles with filament like protrusions emanating from its surface uniformly distributed over the particle surface. It is possible that these protrusions serve as bridges between particles contributing to the conductivity of the coating.
In accordance with the subject invention, treatment of dielectric oxidation coating residue on the surface of a non-conductive surface with an oxo compound such as a peroxodisulfate or hydrogen peroxide following the procedures of Wang et al. is believed to convert manganese oxide residues to a more conductive form increasing the direct electroplating capability of the dielectric oxidation coating. The oxidant is preferably applied as an aqueous solution. A preferred solution comprises from about 1 to 70 grams per liter of an alkali or alkaline earth metal peroxodisulfate solution, preferably a solution of from about 20 to 55 grams per liter. The solution desirably contains an acid such as sulfuric acid to assist in the solubility of the salt in solution. From about 1 to 50 ml of sulfuric acid per liter of solution is desirable. Treatment time may vary from about 0.5 to 10 minutes at temperatures varying between about room temperature and 95xc2x0 C. This solution is preferred because it is an oxidant for the manganese residues and a mild etchant for copper. In this way, the step of oxidation and copper microetching may be combined into a single step. Alternatively, a 10 to 25 percent hydrogen peroxide solution may also be used.
The next step in the process comprises direct electroplating of metal over the dielectric oxidation residue coating. Two approaches are possible. One involves proceeding directly to the electroplating solution. The other involves an intermediate step of reduction. Each approach has advantages and disadvantages.
Caution must be exercised when using the dielectric oxidation residue coating as the conductive layer for the electroplating step. Certain of these coatings are soluble in acid solution. Therefore, in certain instances, acid copper electroplating baths conventionally used in PCB fabrication processes may not be suitable for the direct plating processes of the invention. Under such circumstances, an alkaline copper electroplating bath may be used. Such baths are known in the art as illustrated in U.S. Pat. Nos. 4,356,067; 4,417,956; 4,462,874; 4,469,569; 4,521,282; and 4,933,051, each incorporated herein by reference. These alkaline plating baths are cyanide free and therefore easier to waste treat and safer to use. Typically, these baths contain a source of cupric ions, a minor amount of zinc ions, a chelating agent such as an organophosponate, a buffering and stabilizing agent such as an alkaline metal carbonate, a grain refining agent, hydroxy ions provide desired pH and a wetting agent. Cupric ions concentration is in the range of from about 3 to 50 grams per liter. The solutions are used at pH varying between about 7.5 and 10.5 though recommended pH varies between about 9.5 and 10. The baths are known to electrodeposit a fine grained, ductile, adherent copper deposit. The bath is typically operated at a temperature of from about 80 to 170xc2x0 F. though operate best within a temperature range of from 130 to 150xc2x0 F. They may be operated at cathode current density of from about 0.1 to about 250 amp per square foot with a cathode to anode ratio of from about 1:2 to about 1:6.
Though alkaline copper plating baths are known in the art, they do not possess the plating efficiency of the acid copper plating bath. For this reason, for some applications, it may be desirable to use the alkaline plating bath as a strike bath followed by an acid plating bath to plate to full thickness. The acid plating bath is any of those acid electroplating baths known to the art.
Conditions for electroplating conductive metallic surfaces differ somewhat from the conditions used to plate lower conductivity materials such as the dielectric oxidation residue coating contemplated herein. For this reason, milder plating conditions should be used. The plating bath is operated at temperatures and pH recommended by the bath supplier but current density should be maintained at the low end of the recommended range to avoid burn-off of manganese dioxide, at least during the initial plating stage. Current density may be increased until a maximum level is reached as an initial coating of copper is formed over the manganese dioxide.
As an alternative to the use of an oxide residue as the base for electroplating, the oxide may be reduced to metal and the metal used as the conductive layer for the electroplating step. There are at least two methods available to reduce the oxide to metal, electrolytic reduction and chemical reduction.
Electrolytic processes for reduction of oxides to metal are known in the art. A preferred method is disclosed in IBM Technical Disclosure Bulletin 74C 00934, pages 3463 to 3465, March., 1974; and in unexamined Japanese Patent Application Number 10-240365 filed Aug. 26, 1998, Publication Number 11-165217 published Jun. 22, 1999, each incorporated herein by reference. In general, the process involves connecting the substrate to be plated to the negative terminal of a power source and connecting a second conducing electrode, such as a platinum electrode, to the positive terminal. With the electrodes immersed in an electrolyte, passage of current between the electrodes will reduce the oxide to metal. The electrolyte may be acid, basic or neutral though it is desirable to avoid the use of strongly acid electrolytes as this could dissolve acid soluble oxides. A weak basic electrolyte such as 2 normal potassium hydroxide would be suitable. With a voltage drop of between about 5 and 25 volts across the electrodes, approximately 20 to 40 mA flows through the electrolyte, and within a couple of minutes, the oxide is reduced to metal. The temperature of the electrolyte is not critical but elevated temperatures increases the rate of reduction. The reduced metal is then readily electroplated. In another embodiment of the invention, as will be described in greater detail below, the reduced metal may be used as the current carrying metal itself.
In an alternative lesser preferred embodiment of the invention, chemical reduction may be employed to reduce the oxide to the metal. A strong reducing agent is desired. Preferred reducing agents include amine boranes and borohydrides. Dimethyl amine borane is the preferred reducing agent. Reduction is accomplished by immersion of the part to be plated in a hot alkaline solution of the reducing agent. Temperature is desirably in excess of 150 degrees F. and the pH of the solution is desirably in excess of 9.
In a further embodiment of the invention, reduction may take place by a combination of the methods described above. In this embodiment, electrolytic reduction would be used with an electrolyte containing a reducing agent. A reducing agent compatible with the electrolyte would be added to the electrolyte to assist reduction. The reducing agents described above would be suitable for this embodiment. They would be added to the electrolyte in an amount ranging from about 1 to 50 grams per liter and preferably in an amount of from 5 to 25 grams per liter.
The process described herein substantially reduces the number of processing steps required for plating such as in a conventional PCB manufacturing line because the process line is shorter and the process is selective to those areas where metal deposition is wanted. In particular, the process of the invention is characterized by the elimination of the need to form an additional coating of a conductive material over the manganese dioxide containing coating. A reduction in processing steps translates to a reduced number of proprietary chemicals required for PCB manufacture; smaller, more energy efficient equipment; reduced waste treatment; elimination of aggressive copper etching; and a reduced number of waste water treatment steps. The boards are superior in performance to the boards manufactured by current methods because elimination of a thick extraneous layer of conductive material between the board material and a metal plate would eliminate a cause of interconnect defects.
In the preferred embodiment of the invention, the process for fabrication of a circuit board could be fully additive with no etching of copper cladding required. In this process, the fabricator would start with an unclad PWB board base material. The entire board material would be treated with the permanganate to form the conductive oxide coating. A mask of a circuit pattern would be formed over the surface of the board using a photoresist, preferably a high resolution photoresist. Suitable photoresists are known to the art and exemplary materials are disclosed in U.S. Pat. Nos. 4,839,261; 4,830,953; and 4,820,549; each incorporated herein by reference. For some embodiments of the invention, the photoresist is desirably a material have a high dielectric constant that may be left as a permanent coating on the board. Because the oxide coating is conductive, there is electrical continuity over the full surface of the board. Therefore, openings within the mask may be electroplated to form conductors and metallized hole walls. This is accomplished by either using the electroplating solution directly, preferably the alkaline electroplating solution, or by electrolytically reducing the oxide coating to metal and then electroplating, each as described above. For high resolution boards with narrow spacing between lines, the mask would be removed and the underlying conductive material removed. For extremely high resolution, with very fine lines, the reduction product without build up by electroplating would be suitable to function as the conductor lines. The removal of the mask and underlying coating would not be required for low resolution boards because the spacing between lines would be adequate to prevent shorting through the poorly conductive oxide coating. For example, spacing of at least 5 mils between lines should be sufficient to prevent shorting through the oxide layer. Spaces between circuit lines would then desirably be filled with a permanent dielectric. Such a process would be a dramatic improvement over those processes now in use.
The above process has been described in connection with the formation of a single or double sided circuit board. The process would also be suitable for the formation of multilayer boards including those multilayer boards formed by a sequential build process such as that described in U.S. Pat. No. 5,246,817 incorporated herein by reference. The process is also suitable for the formation of through holes in circuit boards using subtractive techniques.
It should be understood that the process described above can be used to form patterned metal layers for purposes other than the formation of printed circuit boards. For example, alternative uses could comprise formation of nameplates, color filters, liquid crystal displays, transistsors, etc. In addition, it should be understood that the above procedure for forming patterned metal on a surface can be accomplished using poorly conductive materials other than the manganese oxide residues.


A paper composition comprising: at least one cationic thermosetting resin comprising the reaction product of (i) at least one polyamide having secondary amine groups made from reactants comprising at least one polyalkylene polyamine, an acid component comprising succinic acid and optionally at least one dicarboxylic acid (other than succinic acid), and (ii) epihalohydrin. The paper is capable of being repulped substantially faster than paper that is substantially the same except that it contains conventional wet strngth resin.

1. Field of the Invention
This invention relates to repulpable wet strength paper and to a process for fmaking the paper.
2. Description of the Prior Art
Wet strength resins are often added to paper including paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%-50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
As stated in Handbook for Pulp and Paper Technologists, Gary A. Smook, Angus Wilde Publications, 1992: xe2x80x9cPaper has traditionally been defined as a felted sheet formed on a fine screen from a water suspension of fibers. Current paper products generally conform to this definition except that most products also contain non-fibrous additives. Dry forming methods are now utilized for the manufacture of a few specialty paper products. Pulp is the fibrous raw material for papermaking. Pulp fibers are usually of vegetable origin, but animal, mineral, or synthetic fibers may be used for special applications. The distinction between paper and paperboard is based on product thickness. Nominally, all sheets above 0.3 mm thickness are classed as paperboard; but enough exceptions are applied to make the distinction somewhat hazy.xe2x80x9d
Because of increased commercial emphasis on developing paper products based on recovered cellulose, there is growing interest in developing paper which is readily repulpable. Paper and paperboard waste materials are difficult to repulp in aqueous systems without special chemical treatment when they contain wet strength resins.
Improving the repulpability of paper containing wet strength resins has generally been achieved by modifying the repulping conditions.
In U.S. Pat. No. 2,872,313, House et al. teach the use of hypochlorite salts to repulp paper containing polyaminoamide/epichlorohydrin wet strength resins. Miller, in U.S. Pat. No. 3,427,217, teaches the use of oxidizing salts such as sodium hypochlorite, ammonium persulfate and others, to repulp wet strength paper containing resins such as polyaminoamide/epichlohydrin resins, urea-formaldehyde and melamine-formaldehyde resins, etc. Although the hypochlorite salts effectively repulp wet strength paper, they are also chlorinating agents under some conditions and can form environmentally undesirable chlorine-containing degradation products in the process effluents.
Schmalz, in TAPPI, 44, no. 4, pp 275-280, April 1961, teaches the repulping of polyaminoamide/epichlorohydrin resin-containing paper with hypochlorite, or with strong alkali. Although the alkali method will not chlorinate organic byproducts, it is a relatively slow process.
Espy, in European Patent Application Publication No. 585,955-A, discloses a composition for repulping paper containing polyaminoamide/epichlorohydrin wet strength resins in an aqueous slurry, which comprises treatment with a non-chlorinated oxidizing agent and an alkali that is a water-soluble buffering salt capable of maintaining a pH of 7-12 in the aqueous reaction mixture.
Caropreso et al., in PCT International Publication No. WO 94/20682 disclose a composition containing a persulfate and a carbonate, bicarbonate or sesquicarbonate, which composition is suitable for oxidizing wet strength resin used in wet strength paper. The combination is stated to decrease the time required to repulp wet strength paper.
In U.S. Pat. No. 5,330,619, Henry et al. teach a method for treating fibrous sheet materials, such as paper or paperboard containing wet strength resin, which comprises reacting the fibrous materials with an enzyme to hydrolyze the resin and thereby improve repulping of the fibrous materials.
Copending application Ser. No. 08/657,242, filed Jun. 3, 1996 discloses a process for repulping paper by providing paper comprising at least one coacervate complex made from at least one cationic polymer and at least one anionic polymer and repulping the paper to obtain recycled pulp fibers.
Since many of the repulping processes used for wet strength paper result in formation of environmentally undesirable chlorine-containing degradation products, involve strong oxidizing agents, or proceed slowly, there is a need for improved methods for making wet strength paper that will be readily repulpable without significantly lowering the wet and dry strength properties of the paper.
According to the present invention there is provided a paper composition comprising: at least one cationic thermosetting resin comprising the reaction product of (i) at least one polyamide having secondary amine groups, made from reactants comprising polyalkylene polyamine, an acid component comprising at least 50 mole % of succinic acid or its ester or anhydride, and optionally at least one dicarboxylic acid (other than succinic acid) or its ester or anhydride, and (ii) epihalohydrin, said paper being repulpable substantially faster than paper that is substantially the same except that it contains conventional wet strength resin.
Further according to the present invention there is provided a process for making repulpable wet strength paper of comprising:
(a) providing an aqueous suspension of pulp fibers;
(b) adding to the aqueous suspension at least one cationic thermosetting resin comprising the reaction product of (i) polyamide having secondary amine groups made from reactants comprising polyalkylene polyamine, an acid component comprising at least 50 mole % succinic acid or its ester or anhydride and optionally at least one dicarboxylic acid (other than succinic acid) or its ester or anhydride, and (ii) epihalohydrin; and
(c) sheeting and drying the aqueous suspension to obtain wet strength paper that is repulpable substantially faster than paper that is substantially the same except that it contains conventional wet strength resin.
Cationic thermosetting resins suitable for use in the present invention comprise the reaction product of polyamides having secondary amine groups and epihalohydrin. The polyamides are made from polyalkylene polyamines, succinic acid or its ester or anhydride and optionally dicarboxylic acid (other than succinic acid) or its ester or anhydride.
The dicarboxylic acids contemplated for use in the present invention are saturated aliphatic dicarboxylic acids, preferably containing from 3 to 8 carbon atoms, such as: malonic, glutaric, adipic, and so on. Of these, the saturated aliphatic acids having from 4 to 6 carbon atoms in the molecule, such as glutaric and adipic are most preferred. Blends of two or more of these dicarboxylic acids may also be used, as well as blends of one or more of these with higher saturated aliphatic dicarboxylic acids, such as azelaic, sebacic, as long as the resulting long chain polyamide is water-soluble.
A variety of polyalkylene polyamines including polyethylene polyamines, polypropylene polyamines, polybutylene polyamines and so on may be employed herein of which the polyethylene polyamines represent an economically preferred class. More specifically, the polyalkylene polyamines of this invention are polyamines containing two primary amine groups and at least one secondary amine group in which the nitrogen atoms are linked together by groups of the formula xe2x80x94CnH2nxe2x80x94 where n is a small integer greater than unity and the number of such groups in the molecule ranges from two up to about eight and preferably up to about four. The nitrogen atoms may be attached to adjacent carbon atoms in the group xe2x80x94CnH2xe2x80x94 or to carbon atoms farther apart, but not to the same carbon atom. This invention contemplates not only the use of such polyamines as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, and the like, which can be obtained in reasonably pure form, but also mixtures and various crude polyamine materials. For example, the mixture of polyethylene polyamines obtained by the reaction of ammonia and ethylene dichloride, refined only to the extent of removal of chlorides, water, excess ammonia, and ethylenediamine, is a very satisfactory starting material. Most preferred are the polyethylene polyamines containing from two to four ethylene groups, two primary amine groups, and from one to three secondary amine groups, e.g. diethylenetriamine (DETA).
The term xe2x80x9cpolyalkylene polyaminexe2x80x9d employed in the claims, therefore, refers to and includes any of the polyalkylene polyamines referred to above or to a mixture of such polyalkylene polyamines and derivatives thereof.
It is desirable, in some cases, to increase the spacing of secondary amine groups on the polyamide molecule in order to change the reactivity of the polyamide-epichlorohydrin complex. This can be accomplished by substituting an aliphatic diamine such as ethylenediamine, propylenediamine, hexamethylenediamine or a heterocyclic diamine such as piperazine or the like for a portion of the polyalkylene polyamine. For this purpose up to about 60% of the polyalkylene polyamine may be replaced by a molecularly equivalent amount of the diamine. Usually a replacement of about 30% or less will serve the purpose.
The temperatures employed for carrying out the reaction between the dicarboxylic acid and the polyalkylene polyamine may vary from about 110xc2x0 C. to about 250xc2x0 C. or higher at atmospheric pressure. For most purposes however temperatures between about 160xc2x0 C. to about 210xc2x0 C. have been found satisfactory and are preferred. Where reduced pressures are employed somewhat lower temperatures may be utilized. The time of reaction depends on the temperatures and pressures utilized and will ordinarily vary from about xc2xd to 2 hours, although shorter or longer reaction times may be utilized depending on reaction conditions. In any event, the reaction is desirably continued to substantial completion for best results.
In carrying out the reaction, it is preferred to use an amount of dicarboxylic acid sufficient to react substantially completely with the primary amine groups of the polyalkylene polyamine but insufficient to react with the secondary amine groups to any substantial extent. This will usually require a mole ratio of polyalkylene polyamine (including aliphatic diamine if present) to dicarboxylic acid of at least about 0.9:1 more preferably at least about 0.92:1. However, mole ratio of at least about 0.8:1 may be used with quite satisfactory results. The mole ratio of polyalkylene polyamine to dicarboxylic acid can be up to about 1.4:1, preferably up to about 1.14:1 and more preferably up to about 1.2:1. Mole ratios outside of these ranges are generally unsatisfactory. Thus, mole ratios below about 0.8:1 result in a gelled product or one having a pronounced tendency to gel while mole ratios above 1.4:1 result in low molecular weight polyamides. Such products do not produce efficient wet-strength resins when reacted with epichlorohydrin.
Of the dicarboxcyclic acids employed in making the polyamide generally at least about 50 mole %, preferably at least about 80 mole % and most preferably about 100 mole % of succinic acid is used. The amount of the other dicarboxylic acid can be up to about 50 mole % and preferably up to about 20 mole %.
The reduced specific viscosity (RSV), which is an indication of molecular weight, for polyamide prepolymer derived from succinic acid and diethylenetriamine can be at least about 0.080, preferably 0.10 and most preferably 0.13 dL/g. The RSV for this prepolymer can be up to about 0.20, preferably 0.175 and most preferably 0.155 dL/g.
The preferred epihalohydrin for use in the present invention is epichlorohydrin.
In converting the polyamide, formed as above described to a cationic thermosetting resin, it is reacted with epichlorohydrin at a temperature from about 45xc2x0 C. to about 100xc2x0 C. and preferably between about 45xc2x0 and 70xc2x0 C. until the viscosity of a 20% solids solution at 25xc2x0 C. has reached about xe2x80x9cCxe2x80x9d or higher on the Gardner-Holdt scale. This reaction is preferably carried out in aqueous solution to moderate the reaction. pH adjustment is usually not necessary. However, since the pH decreases during the polymerization phase of the reaction it may be desirable, in some cases, to add alkali to combine with at least some of the acid formed.
When the desired viscosity is reached, sufficient water is then added to adjust the solids content of the resin solution to about 15% or less, the product cooled to about 25xc2x0 C. and then stabilized by adding sufficient acid to reduce the pH at least to about 6 and preferably to about 5. Any suitable acid such as hydrochloric, sulfuric, nitric, formic, phosphoric and acetic acid may be used to stabilize the product.
In the polyamide-epichlorohyrin reaction, it is preferred to use sufficient epichlorohydrin to convert all secondary amine groups to tertiary amine groups and/or quaternary ammonium groups including cyclic structures. However, more or less may be added to moderate or increase reaction rates. In general, one can use at least about 0.5 mole of epichlorohydrin per mole of polyamide secondary amine. It is preferred to use at least about 0.9 mole and most preferably at least about 1.0 mole of epichlorohydrin per mole of polyamide secondary amine group. Generally, up to about 1.8 moles, preferably up to about 1.5 moles and most preferably up to about 1.3 moles of epichlorohydrin per mole of polyamide secondary amine is used.
The process for manufacturing repulpable wet strength paper according to the present invention comprises a number of steps. One step comprises forming an aqueous suspension of pulp fiber which can be performed by conventional means, i.e., known mechanical, chemical and semi-chemical, etc., pulping processes. Another step comprises adding to the aqueous suspension of pulp fibers the cationic thermosetting resin described above. This can be done at any point, before sheet formation or it can also be applied after sheet formation from a tub size or at a size press or from showers to the dried or partially dried sheet. Yet another step comprises sheeting and drying the aqueous suspension of fine pulp fibers containing the cationic thermosetting resin. This can be done by conventional means.
The amount of cationic wet-strength resin can be as low as about 0.05 weight % on a dry basis, based on the weight of the dried paper, preferably about 0.1 weight % and most preferably about 0.2 weight %. The maximum amount of cationic thermosetting resin is generally about 2 weight % on a dry basis, based on the weight of the dried paper, preferably about 1 weight %, and most preferably about 0.5 weight %.
The process of repulping the paper to obtain recycled pulp fibers can be carried out by any mechanical action that disperses dry pulp fibers into an aqueous pulp fiber suspension. Conditions for repulping, as well as equipment commercially used, are discussed in xe2x80x9cHandbook for Pulp and Paper Technologists, Second Editionxe2x80x9d by G. A. Smook, Angus Wilde Publications, 1992, pp 194-195 and 211-212, which reference is incorporated herein by reference in its entirety.
A reproducible, quantative laboratory method to determine repulpability of wet strengthened paper uses a temperature controlled disintegrator described in TAPPI method T 205 OM-88, (1988), at 3000 rpm which is incorporated herein by reference in its entirety. Fiber yield (repulpability) is measured with a Somerville fractionating screen. Conditions during repulping are 1.5% consistency at 50xc2x0 C. and pH of 7 for 5 minutes. Using this test it was found that paper prepared by the process of this invention can be repulped in substantially less time than is required to repulp the same paper at about the same level of wet-strength, but containing conventional wet strength resin, e.g. Kymene(copyright) 557H aminopolyamide-epi wet strength resin.
Other wet-strength resin types that could be used in combination with the resins of the present invention as long as repulpability is not significantly adversely affected are: aminopolyamide-epi resins (e.g. Kymene(copyright) 557H-resin); polyamine-epi resins (e.g. Kymene(copyright) 736 resin), epoxide resins (e.g. Kymene(copyright) 450 and Kymene(copyright) 2064 resins); polyethylenimine, ureaformaldehyde resins; melamine-formaldehyde resins; glyoxalated polyacrylamides (e.g. Hercobond(copyright) 1000 resin, Parez 631NC); polyisocyanates; and reactive starches (oxidized starch, dialdehyde starch, blocked reactive group starch).
Other chemical additives that can be used in conjunction with wet-strength resins are: rosin size, reactive size (alkenyl succinic anhydride or alkyl ketene dimer), surface size, starch, retention aids, drainage aids, formation aids, flocculants, creping aids (adhesives and release agents), dry strength resins (cationic starch, guar gums, polyacrylamides), defoamers, scavengers for anionic trash and stickies control, fillers (clay, calcium carbonate, titanium dioxide), optical brightening aids and dyes.
The recycled pulp fibers prepared by the repulping process of the present invention can be used to make paper by conventional paper making processes, which comprise providing an aqueous suspension of the recycled pulp fibers and then sheeting and drying the aqueous suspension to obtain paper.
The wet strength resins of the present invention are suitable for use in the following areas: paper towels; napkins; facial tissue; liquid packaging board (milk carton, juice carton); poultry boxes; produce boxes; carrierboard; butchers wrap; bleached bag; poster board; table cloth; wallboard tape; currency paper; map paper; tea bag; corrugating medium; paper plates; molded products (egg cartons); laminating grades; flooring felt; coffee filter; bread wrap; multiwall bag; shingle wrap, etc.


A heat exchanger for swimming pools, formed of an essentially cylindrical housing through which a first medium flows axially, while a second medium flows through a conduit that is installed in the housing and that is fashioned as a coil. The coil-shaped conduit is formed of a corrugated hose, and has separate connections for the second medium, or for the corrugated hose. The corrugated hose is connected at its end segments to separate connections for the second medium via a plug connection. The connections extend from the outside to the inside of the housing, passing through the housing.

The invention relates to a heat exchanger, in particular for swimming pools, formed of an essentially cylindrical housing through which a first medium flows, essentially axially, the housing having, at its axial end faces, connections for connection with adjacent conduit segments for the first medium, while a second medium flows, via two separate connections that extend radially to the housing, through a conduit that is installed in the housing and is formed as a coil, having an axis that extends parallel to or co-incident with the housing axis. The coil-shaped conduit for the second medium is made of a corrugated hose having end segments that run in the radial direction and that are connected to the separate connections for the second medium and are fixed there.
Swimming pool heat exchangers of this sort are used to heat the swimming water, as a first medium, with the aid of a heating water as a second medium, the ratio of the volumes of the streams of swimming water and heating water being on the order of magnitude of approximately 5 to 1. Accordingly, the swimming water flows through the heat exchanger in the axial direction, and thereby flows around the heating water conduit, which is installed in the heat exchanger housing in the shape of a coil in order to enlarge the heat exchange surfaces. In order to obtain a reliable charging of the coil-shaped conduit, a cylindrical displacement element is usually situated inside the coil, and diverts the swimming water stream around the heating water conduit. Such heat exchanger designs are also known for other areas of application, for example also for fuel cooling units, as is disclosed in DE-A 34 40 060. In this prior art, a significant problem in the assembly of the heat exchanger is that the coil-shaped tubular conduit must be welded from the insidexe2x80x94i.e., under very narrow conditionsxe2x80x94against a housing opening so as to line up with the separate connections that extend radially on the outside.
As a rule, for the cited swimming pool water applications, the coil-shaped heating water conduit is made of stainless steel, as is the housing. For applications that are particularly susceptible to corrosion, such as for example mineral water or seawater pools, the components are also manufactured from titanium. In general, the design having a coil-shaped conduit and having a displacement element additionally situated mainly inside the coil necessarily results in a relatively large construction volume, with correspondingly heavy weight, and this large construction volume also results in correspondingly high manufacturing costs, due to the use throughout of high-quality materials.
In view of this prior art, the present invention is based on the object of providing a heat exchanger, in particular for swimming pools, that is distinguished by simplified installation and improved manufacturability.
According to the present invention, this object is achieved in that the separate connections for the second medium extend from the outside to the inside of the housing, passing through the housing, and that the end segments, running in the radial direction, of the corrugated hose in the interior of the housing are connected to the cited separate connections for the second medium, and are fixed there, via a plug connection.
The essential advantage of the inward installation of the connection between the corrugated hose coil and the separate connections for the second medium is that the corrugated hose can be fixed to these inward-protruding separate connections by simple plugging, without requiring a welded connection or a similarly complicated connection method, which would be very difficult in the narrow installation space available in the interior of the housing, or could be achieved only through an additional division of the housing.
The advantages resulting from the present invention are due not only to the plug connection between the corrugated hose and the separate connection, but also to the separate connection that protrudes into the housing, whereby the corrugated hose and the separate connection can overlap over a certain area, which can be exploited on the one hand for mutual sealing, and on the other hand for mutual fixing. The result is a heat exchanger having a considerably reduced assembly and manufacturing expense.
Advantageously, the end segments of the corrugated hose are plugged into the separate connections for the second medium, and are fixed there for example by pins that engage in positively locking fashion in troughs of the corrugation of the corrugated hose. However, the fixing of the corrugated hose in the separate connections is also possible via other fixing means, such as clamps that apply pressure to the corrugated hose in positively locking fashion, locking or gripping elements, and the like, as long as they satisfy the demands and requirements found for example in the present case of application of a swimming pool heat exchanger.
For the plug connection between the corrugated hose and the separate connections, it is particularly recommended that the separate connections for the second medium be fashioned separately from the housing and inserted through openings in the housing, so that during the assembly of the heat exchanger first the corrugated hose coil is brought into the housing, and the separate connections are only then inserted from the outside into the radial housing openings, and finally the plug connection is made between the corrugated hose and the separate connections during the insertion of the separate connections into the housing. As soon as the separate connections are located in the correct end position in relation to the housing, the corrugated hose can be fixed to the separate connections using the previously noted fixing pins.
Various specific embodiments are conceivable for the connection between the separate connections and the housing, such as for example a screw connection. However, because as a rule the separate connections likewise have at their free end, situated at the external side of the heat exchanger, a screw connection for connecting a medium-conducting conduit that is to be affixed thereto, this would entail the risk that the initial tightening moment in the assembly of the medium-conducting conduit to the separate connection would be transmitted not only to the threading involved here, but also to the second threading via which the separate connection is fixed to the heat exchanger housing. For this reason, it is recommended that in the area of connection between the separate connection and the heat exchanger housing there be provided a plug connection, which however should be formed in positively locking, rotationally fixed fashion, in order to receive torsional forces and thus to absorb the mentioned initial torque. This plug assembly can be secured against withdrawal of the separate connections from the housing (similar to the case of the corrugated hose plug connection) by fixing means in the form of fixing pins that apply pressure to the separate connections on the inside of the housing and fix these connections on the housing wall in positively locking fashion, in that the fixing pins abut on the housing inner wall.
In order to secure the mentioned fixing pins for fixing the corrugated hose in the separate connection on the one hand and for the fixing of the separate connection in the housing on the other hand, which pins can usefully be connected with one another in order to simplify the assembly for each separate connection, to prevent falling out during the operation of the heat exchanger, these pins can be fixed in their installed position by screw-in sleeves. The screw-in sleeves can be formed most simply by the connections for the first medium, situated on the front side of the housing.
The cited and required securing against twisting of the separate connections in relation to the housing can be easily achieved by a hexagonal shape, through a knurling or a polygonal shape. The separate connection can have a shoulder or a collar in the noted polygonal shape, while the housing opening should have a collar that abuts thereon in positively locking fashion, at least in some areas.
In addition to the fixing pin for fixing the corrugated hose in the separate connection, it is recommended, as already mentioned, that a sealing element be provided in the overlap area between the corrugated hose and the separate connection, which most simply can consist of at least one O-ring that is plugged onto the associated corrugated hose end segment and that engages in positively locking fashion in at least one corrugation trough, and whose external side, which applies pressure to the radial segment of the separate connections for the second medium, abuts on the inner side of the radial segment. For this purpose, the cited radial segments of the separate connections for the second medium are preferably of a smooth cylindrical construction, in order to form the required tight connection with the sealing element (i.e., preferably, the O-ring).
Moreover, and especially given larger pressure differences between the first and second medium, in order to strengthen the axial fixing of the corrugated hose in the separate connection at least one of the corrugated hose end segments can work together with another fixing means besides the fixing pin. This fixing means can for example create a locking connection through a design having barbs or the like. An additional fixing means can be formed by plugging it onto the corrugated hose in addition to the sealing element, and plugging it into the separate connection together with this sealing element; here the diameter of the fixing means should have, in relation to the diameter of the separate connection, a dimension such that under initial loading it abuts on the inner side thereof, and in addition the fixing means should likewise engage in positively locking fashion in the corrugated hose.
Preferably, the corrugated hose end segment is formed of a corrugated hose section formed by simple cutting to length. However, it is particularly advantageous if the corrugated hose end segment is calibrated through stretching and is reduced somewhat in flexibility, so that it does not give way in the axial direction by compression during insertion into the separate connection.
In addition, however, the end segment of the corrugated hose can also be formed by a smooth-walled tube that is attached to the corrugated hose, for example by welding, which tube then enters into the plug connection with the separate connection. For this purpose, on its external side the tube can have groove-shaped recesses into which the O-rings are placed in order to seal the plug connection. According to the present invention, this specific embodiment should be expressly regarded as a variant, falling under the present main claim, of the corrugated hose end segment.
In order to promote the complete surrounding of the corrugated hose coil by the flow, it is especially advantageous if spacing elements are provided between the corrugated hose coil and the inner surface of the housing jacket, via which the first medium, i.e., the swimming pool water in particular, can also flow through this gap area between the housing and the corrugated hose. The spacing elements should be fashioned so as to promote the previously mentioned plugging assembly.
The mentioned spacing elements also have the further advantage that they support the corrugated hose, in order to enable the prevention of flow-induced noise emissions. In connection with this, it should also be noted that it can be worthwhile, not only in order to improve the flow around the coil but also in order to prevent noise resulting from flow, to install one or more flow-past elements, in the form of baffle plates, in the housing and in the corrugated hose coil.
A particular advantage of the simplified installation of the heat exchanger according to the present invention, and also of the circumstance that the separate connections are not connected in one piece with the housing, but rather are fashioned separately from the housing, is that only the surfaces that are responsible for the heat exchange, i.e. the corrugated hose, must be manufactured from special steel, while the housing can be made of corrosion-resistant plastic. Thereby the costs of the heat exchanger according to the present invention can additionally be drastically reduced. In contrast, in the prior art it was necessary to weld the coil-shaped conduit to the housing in the area of the radial openings and to bring it into connection with the radial connections, so that the housing also required a weldable, corrosion-resistant material, i.e. in particular special steel. In contrast, in the heat exchanger according to the present invention the connection of the corrugated hose and the separate connection can take place through plugging assembly and without welding; and, if it were to be desired likewise to manufacture the separate connection from special steel, the entire housing could still be made of plastic, thus still enabling a significant cost reduction. As a plastic material, in particular PA, PP, PE, PVC-C, and similar materials are possible.
Accordingly, in this case as a rule the first medium is formed by the swimming pool water, while the second medium is formed by the heating medium, or the heating water. Likewise, the present heat exchanger can however also be used for the cooling of fluids; here the second medium must then have a temperature that is reduced in relation to the fluid temperature. In addition, it is of course also possible to use the heat exchanger for other combinations of media, for example in order to heat water for industrial use in gas water heaters, fuel cells, and the like, for the recovery of waste heat, and in general for a large number of industrial applications in the automotive field, etc.xe2x80x94even in areas where heat exchangers having a design with a cooling or heating coil have customarily been used.
In comparison with heat exchangers from the prior art (not all of which enjoy prior publication), the subject matter of the present invention has the advantage that, in a heat exchanger that due to the required volume throughput can have connections for only one medium at its end face, the second medium is conducted into the heat exchanger via radial connections, and there can be transported via a corrugated hose coil having a maximum length, and thus a maximum surface responsible for the heat exchange. That is, no transition areas such as bends or the like are required that create the connection between the corrugated hose and the connections situated on the outside of the housing, and that usually cause a large pressure loss due to abrupt changes in the direction of flow, and also cause reduced exchange efficiency due to their separate length that is not to be used for the heat exchange. In contrast to this, in the subject matter of the present invention, in which the corrugated hose goes over directly into the separate connections for the second medium, there results a significantly reduced pressure loss, or an improved heat exchange efficiency, in comparison with heat exchanger designs having comparable dimensions. In addition, the omission of the welded connection of the metal components enables the use of a greatly simplified connecting technique with regard to the connection of the corrugated hose to the separate connections, and, with regard to the connection between the heat exchanger housing and the separate connections, allows the two objects to be manufactured from different materials, so that the housing can then be made of inexpensive plastic.

Heat exchanger, in particular for swimming pools


In a process for the continuous preparation of glutaraldehyde by reaction of an alkoxydihydropyran of the formula I

The present invention relates to a process for the continuous preparation of glutaraldehyde by reaction of alkoxydihydropyrans of the formula I
where R is a straight-chain or branched C1-C20-alkyl, preferably C1-C8-alkyl, in particular a methyl group, with water at from 0xc2x0 C. to 200xc2x0 C., preferably from 40 to 150xc2x0 C., and a pressure in the range from 0.01 bar to 16 bar, preferably from 0.1 to 5 bar.
Glutaraldehyde is used, for example, as a microbiocide or in tanning.
U.S. Pat. No. 4,244,876 and the associated U.S. Pat. No. 4,448,977 describe storage-stable compositions comprising glutaraldehyde acetals and an organic acid as catalyst from which an aqueous solution of glutaraldehyde can be prepared quickly when required by addition of water. This solution is used, in particular, as biocide for the control of sulfur bacteria in oil wells. For this purpose, separation of the reaction mixture into its pure components is not necessary and the unpurified hydrolysis product is used. A 2-alkoxy-3,4-dihydropyran is reacted with an alcohol or glycol in the presence of an acid catalyst, e.g. a strong acid ion exchanger, to give a mixture of 2,6-dialkoxytetrahydropyran, dialkoxypentanal and 1,1,5,5-tetraalkoxypentane. After the ion exchanger has been separated off, an organic acid is added to the mixture. The resulting mixture is storage-stable. At the point of use, e.g. a drilling site, it can be converted into a glutaraldehyde-containing aqueous solution by addition of water.
The European patent application EP 0 066 224 A1 describes a process for preparing an essentially water-free glutaraldehyde precursor from which glutaraldehyde can be liberated within a short time by addition of water. To prepare the storage-stable precursor, a 2-alkoxy-3,4-dihydropyran is reacted with water in a molar ratio of from 1:1 to 3:1 at from 30 to about 100xc2x0 C. in the presence of an acid catalyst, e.g. an acid ion exchanger. This gives a mixture comprising 2-hydroxy-6-methoxytetrahydropyran as main component together with traces of 2,6-dimethoxytetrahydropyran and glutaraldehyde. After the acid catalyst has been separated off, e.g. by filtration, the mixture is storage-stable. Addition of water results in hydrolysis to form glutaraldehyde. No provision is made for purifying the glutaraldehyde.
JP 72 26 488 discloses a process for the continuous preparation of glutaraldehyde by hydrolysis of a 2-alkoxy-3,4-dihydropyran in the presence of an acid catalyst. For this purpose, a mixture of the 2-alkoxy-3,4-dihydropyran and water in a molar ratio of from 1:2 to 1:100 is heated to from 50 to 200xc2x0 C. The process is carried out so that a concentration of from 0.5 to 10% by weight of the 2-alkoxy-3,4-dihydropyran is maintained in the system. No provision is made for further purification. The reaction product obtained is used as tanning agent for leather.
U.S. Pat. No. 2,546,018 describes the synthesis of glutaraldehyde and C-substituted glutaraldehydes starting from dihydropyrans. The hydrolysis of the dihydropyran can be carried out in the presence or absence of acid catalysts, with an uncatalyzed reaction being preferred because of the simpler work-up of the mixture after the reaction. The reaction can be carried out batchwise, and it is also proposed that the alcohol formed be removed continuously from the reaction mixture during the reaction. It is also proposed that the reaction be carried out continuously. For this purpose, the reaction mixture is said to be passed through a tubular reactor which is heated to an appropriate temperature. The flow rate is selected so that the desired conversion has taken place at the outlet of the reactor. After the reaction mixture has left the reactor, it can either be collected first and then distilled batchwise or, as an alternative, a continuous distillation with the desired fractions being separated off is proposed. In each of the examples in this patent, the hydrolysis of the dihydropyran is carried out in a first step and a fractional distillation is subsequently carried out in a second step. A disadvantage of this process is that an elaborate apparatus is necessary, with the hydrolysis being carried out in a first stage using a first apparatus and the desired product being separated off by fractional distillation in a second stage using a second apparatus. This is cumbersome and incurs costs.
It is an object of the present invention to provide a process by means of which glutaraldehyde or C-substituted glutaraldehydes can be prepared continuously in increased purity in a simple manner, with the outlay for apparatus being kept as small as possible in order to keep production costs low.
We have found that this object is achieved by a process for the continuous preparation of glutaraldehyde by reaction of an alkoxydihydropropan of the formula I with water at from 0xc2x0 C. to 200xc2x0 C. and a pressure in the range from 0.01 bar to 16 bar to form glutaraldehyde and the alcohol corresponding to the alkoxy group, wherein water and alkoxydihydropyran are fed continuously to a reaction column, preferably at a point between the top and the bottom outlet of the reaction column, and a distillate enriched in the alcohol corresponding to the alkoxy group is taken off at the top of the column and a product enriched in glutaraldehyde is taken off at the bottom.
The apparatus used for the reaction can be very compact since the hydrolysis of the alkoxydihydropyran is carried out in the column and it is therefore not necessary to provide a separate vessel for carrying out the hydrolysis reaction. The continuous reaction mode significantly reduces the dimensions of the plant for the same throughput per unit time compared to a batchwise reaction with the alcohol formed and the glutaraldehyde being distilled off separately. Selection of appropriate reaction parameters or appropriate dimensions of the plant makes it possible to achieve virtually complete conversion of the alkoxydihydropyran, so that an increased yield can be achieved and the glutaraldehyde formed has a higher purity than that obtained in processes known hitherto.
The reaction rate can be increased by carrying out the reaction in the presence of an acid catalyst. Suitable acids are organic acids such as saturated and unsaturated carboxylic acids having from 1 to 10 carbon atoms. It is also possible to use polyfunctional acids such as maleic acid. Preference is given to using inorganic acids such as phosphoric acid, boric acid, nitric acid, sulfuric acid or acid salts, e.g. NaH2PO4. If acids which are not readily volatile under the operating conditions of the reaction column are used, they are preferably added in the upper part of the reaction column. When using acids which are volatile under the operating conditions, they can also be added in the lower or middle part of the reaction column. The amount of acid should be such that an acid concentration in the range from 0.0001% by weight to 10% by weight, in particular from 0.01% by weight to 3% by weight, is obtained in the stream leaving the bottom of the column. The catalysts are preferably used as solutions. Suitable solvents are water, alcohol, the alkoxydihydropyran or glutaraldehyde/water mixtures.
In place of liquid acids, catalytically active packing comprising solid acids can also be installed in the reaction column. Such solid acids are, for example, ion exchangers such as Amberlyst(copyright) 15 or the bleaching earth catalysts mentioned in DE 44 29 262. The catalysts are generally arranged in the column so that intimate contact of the reaction solution with the catalyst is possible but the solid catalyst is kept in place in the column by means of screens or filters. An example of such an arrangement is KATAPAK(copyright) from Sulzer. Further solid catalysts which can be used are thin layer catalysts in the case of which a catalytically active composition has been applied directly to internals, ordered packing or random packing in the column, e.g. by impregnation and subsequent drying.
The reaction column preferably has at least 2, in particular at least 3, theoretical plates. In general, the number of theoretical plates is then in the range from 2 to 200, preferably from 3 to 100, in particular from 4 to 50.
The separation of the alcohol is further improved if the reaction column is operated under reflux and the reflux ratio is chosen so as to be in the range from 0.2 to 80, preferably from 0.4 to 40.
If the alcohol is distilled off at the top of the reaction column as an azeotrope with water, it is advantageous for the top of the reaction column to be connected to a phase separation vessel and for an aqueous phase which separates out to be returned to the top of the reaction column. In this case, additional runback to the column may be able to be dispensed with.
The heating power at the bottom of the column is preferably chosen so that the stream leaving the bottom of the column contains less than 10% by weight, preferably less than 1% by weight, of the alcohol liberated in the hydrolysis reaction.
The concentration of glutaraldehyde in the stream leaving the bottom of the column is from 5 to 75% by weight, preferably from 25 to 65% by weight.
Water and alkoxydihydropyran can be fed to the reaction column either separately or as a mixture. If they are fed in separately, the water is preferably introduced in the upper part of the reaction column. The alkoxydihydropyran is preferably introduced in the lower part of the reaction column. Between the point of introduction of the alkoxydihydropyran and the bottom of the reaction column and between this point and the top of the column there should in each case be sufficient theoretical plates so that the alkoxydihydropyran content of the bottom product and the top product, respectively, is within specification. This number of theoretical plates which are necessary depends on the reflux ratio and can easily be determined by preliminary tests. When water and alkoxydihydropyran are fed in as a mixture, they are generally introduced in the middle region of the reaction column.
The amount of alkoxydihydropyran introduced depends on the volume of the reaction column used, on the amount and type of catalyst and on the temperature in the reaction column and can easily be determined by preliminary tests. In general, from 1000 to 5000 kg/h of alkoxydihydropyran can be introduced per 1 m3 of empty volume of the column.
The amount of water should be such that glutaraldehyde is obtained in the desired concentration at the bottom of the reaction column. In general, the molar ratio of water to alkoxydihydropyran is xe2x89xa73, in particular xe2x89xa74.
The reaction column can have various forms of packing or internals. Examples of such internals are random packing elements (e.g. Pall rings), structured packing made of sheetmetal (e.g. Sulzer Mellapak 250Y) or metal mesh (e.g. Sulzer BX or CY) or preferably trays (e.g. bubble cap trays, valve trays, tunnel trays) or else combinations of the internals mentioned. If appropriate, special residence trays having a particularly high liquid level or special constructions of the downflow shafts having a particularly large liquid content can also be used. Furthermore, packing comprising solid acids such as ion exchangers or bleaching earth catalysts can also be installed in the reaction column. Such packing is constructed so that intimate contact of the reaction solution with the catalyst is possible but the solid catalyst is kept in place in the column by means of screens or filters. An example of such an arrangement is KATAPAK from Sulzer.
The dimensions of the reaction column can be reduced if a side offtake for taking off a gaseous or preferably liquid product comprising water, alkoxydihydropyran and possibly the catalyst is provided in the middle section of the reaction column and the stream taken off at this point is conveyed to an associated reaction space, and the stream flowing out of the reaction space is recirculated to the reaction column, preferably in the middle region of the column and below the side offtake. The associated reaction space can be configured as a stirred vessel, a loop reactor or a tube reactor.
The amount of material taken off at the side offtake can vary within a wide range. It preferably corresponds to from 0.1 to 80 times, preferably from 0.2 to 30 times, the amount of alkoxydihydropyran fed to the reaction column.
Preference is given to feeding the water, the alkoxydihydropyran and, if appropriate, the catalyst into the associated reaction space.

Method for the continuous production of glutaraldehyde

Abstract: where R is C1-C20-alkyl, with water at from 0xc2x0 C. to 200xc2x0 C. and a pressure in the range from 0.01 bar to 16 bar to form glutaraldehyde and the alcohol corresponding to the alkoxy group, water and alkoxydihydropyran are fed continuously to a reaction column and a distillate enriched in the alcohol corresponding to the alkoxy group is taken off at the top of the column and a product enriched in glutaraldehyde is taken off at the bottom.

Abstract: This process makes it possible to prepare glutaraldehyde or C-substituted glutaraldehydes continuously in high purity in a simple manner with a low outlay in terms of apparatus.