US3434867A - Method of etching urea-formaldehyde for making printing elements - Google Patents

Description

United States Patent US. Cl. 117-47 6 Claims ABSTRACT OF THE DISCLOSURE An aqueous solution of formic acid and hydrochloric acid is used to toughen the surface of a urea-formaldehyde resin article which is representative of a series of articles to be etched prior to plating them with a metal coating. The degree of roughness achieved by etching the sample for a given time with a standard solution is determined by the degree of curing of the resin. The roughness achieved is measured and the concentration of acids in the etching solution is adjusted based on this measurement to impart the correct degree of roughening to the remainder of the series of articles.

Disclosure of the invention This invention relates to the roughening of plastics with acid as a pro-treatment prior to metal plating. The invention is primarily concerned with the electroless plating of metal on condensation polymers such as ureaformaldehyde.

Certain metals do not adhere well, either by chemical or physical interaction, when coated on certain substrates. It appears to be generally true that conductive metals have only weak afiinity to nonconductive plastics, such as urea-formaldehyde.

The prior art has found it necessary to roughen nonconductive plastics prior to metal plating. The roughened surface apparently presents minute extensions which extend into a metal coating subsequently applied to mechanically lock the metal coating to the non-conductive plastic.

One common method of toughening for this purpose is grit blasting. Grit blasting is analogous to the commonly known technique of abrading surfaces by sand blasting. Grit particles impact the surface with sufficient kinetic energy and rigidity to deform and thus minutely roughen the surface. Grit blasting is effective, but it requires extensive equipment and constant control to assure a grit spray which will not overroughen the plastic surface. The initial cost of equipment and the constant expense of controlling the potentially excessive grit blasting process thus render grit blasting undesirable for some applications.

It is an object of this invention to provide a roughening process for urea-formaldehyde Which avoids the use of grit blasting equipment.

It is a further object of this invention to provide a roughening process for urea-formaldehyde which requires relatively inexpensive materials and equipment.

It is a still further object of this invention to provide a roughening process for urea-formaldehyde which is capable of use in volume production without critical control and critical monitoring of the parameters of the process, although, of course, reasonable control and monitoring of the process will be necessary.

It is a more specific object of this invention to provide a toughening process for urea-formaldehyde which is defined within specific scientific limits so that the best combination of materials can be selected for each purpose.

3,434,867 Patented Mar. 25, 1969 It is still another more specific object of this invention to provide a roughening process for urea-formaldehyde which is usable within the range of different variations in the characteristics of the urea formaldehyde being roughened.

Acid roughening of plastics for this purpose is known in the prior art. This invention is an acid roughening process, but it is believed that this invention for the first time teaches the proper formulas and utilizations of the formulas for use under various production conditions. In a patent application filed in the United States Patent Officc, by William M. Boggs and George E. Rousselot, Ser. No. 467,748 filed June 28, 1965, in which patent application I am a joint inventor, a more specific formulation of the acid roughening process herein descrbied is claimed.

In accordance with this invention, a. combination of formic acid and hydrochloric acid is used to roughen urea-formaldehyde in the manner specifically taught in my copending patent application. However, further in accordance with this invention tests are made on sample parts of the urea-formaldehyde resin and, in accordance with my scientific findings, an optimum. formic acid-hydrochloric acid combination is selected for use with production quantities of parts having characteristics similar to the urea-formaldehyde part tested.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention.

The part The item to be processed in accordance with the preferred embodiment of this invention is a light, spherical, truncated ball carrying the images of an entire alphabet of characters in the form of raised salients. The ball will be used as the print element of a typewriter or printer. The ball is rotated and tilted by the mechanisms of the printer to select the proper character on the ball. The ball is then impacted into a print station to effect printing. Printers using such a ball are now well known in the art.

The ball is molded under heat and pressure to form a resinous solid essentially comprising urea-formaldehyde resin, alpha-cellulose filler, pigments, and plasticizers. The urea-formaldehyde is a major structural component and the entire ball bound together by the urea-formaldehyde must be acted upon if effective, minute roughening is to be accomplished. Also, excess roughening must be avoided if the image definition of the character salients on the ball is to be preserved.

Molding from powder The molding powder is Plaskon SM, a product of the Allied Chemical Corporation. This mixture is a combination made of materials which coact to form a urea-formaldehyde resin and also containing the alpha-cellulose filler, pigments, catalysts, and plasticizers as significantly important compounds. When the molding compound is submitted to heat and pressure, it goes through a first melting phase in which cavities in a mold containing the mixture are filled, and then it goes through the irreversible condensation and polymerization reactions, common to thermosetting resins, to be converted into a hard, infusihle, and insoluble material. The further the curing is carried out, the more the properties of the thermosetting plastic will be marked, because the degree of polymerization of the material depends on the curing conditions.

Should the molded part be used without further coating, only the physical properties such as hardness, impact resistance, precision mold filling characteristics, and

dimensional stability would have to be considered. As the part is to be surface treated in electrolytic media to thereby achieve a metal coating, however, the chemical properties of the part are of significant importance. The behavior of the part during the surface treatment cycle will be conditioned by the degree of its initial polymerization.

In accordance with this invention definite polymerization conditions are defined for which mechanical, physical, and chemical properties are satisfactory for both the metal plating operation and the subsequent uses ,of the ball as a single element typehead. For these reasons, molding of the plastic ball is not disassociated from plating. It is contemplated in accordance with this invention that once the interdependence of the degree of polymerization of the material and its surface treatment is established, noticeable modifications of the curing cycle which would have repercussions on the plating, will be avoided. This practice additionally leads to the further incidental benefit of achieving a very consistent end product having optium quality level.

Need for surface preparation Whatever be the nature of the substrate, it is well known that the surface preparation prior to electroplating plays a determining part in the achievement of results. This is of special importance for non-metallic materials. Due to their nature, plastic materials have a low chemical activity, and they are hydrophobic and nonconductive. Conventional methods of preparation commonly used for metals therefore can not be employed. A special conditioning specific to this material is required.

To a more or less degree, all plastic materials have the major properties of hydrophobia, chemical passivity, and electrical resistivity; and therefore all plastic materials fall into the group of nonconductors entirely opposite in chemical properties to metallic materials. The purpose of the sequences followed with regard to urea-formaldehyde etch is to transform the non-conductive plastic to a condition in which its surface will receive and retain a metal coating.

Cleaning The cured urea-formaldehyde ball should first be cleaned. The choice of the type of cleaning is governed by the nature of soil to be eliminated and the sensitivity of the material to the chemicals used. Cleaning may be conducted either in a solvent or in an aqueous media. As a general rule, thermoplastics are sensitive to organic solvents which can swell or dissolve them, whereas thermosetting plastics of the urea-formaldehyde type are insensitive to such solvents.

Chlorinated hydrocarbons (trichlorethylene) do not attack urea-formaldehyde resins, but their monovalent cleaning action is restricted to the dissolution of fatty soils (solvency). They have no detergent action, unless they are used with an ultransonic agitation. Electrostatic charges are not eliminated. Cleaning an an aqueous media is much more suitable because it provides a wide range of possibilities by changing the solution composition and operating conditions. In the production process herein contemplated soils are mainly due to handling, whereas solid particles are from deburring and similar factors having to do with the molding operation.

The preferred cleaning solution therefore should be detergent, anti-static, and wetting in order to readily eliminate from the surface any dirt particles which could, if not removed, be entrapped by the metal plate thus creating roughness which would deform the raised type images and be detrimental to printing quality. The cleaning solution should be alkaline enough to neutralize finger marks and soils of a similar nature. Furthermore, the surface-active agents (surfactants) of the cleaning solution having the desired qualities should be materials which are absorbed b the urea-formaldehyde surface, since this tends to regulate the subsequent acid etch.

The following solution and conditions are preferred for the step of cleaning. This solution is easy to use and very economical compared to the chlorinated solvent methods. It does not require any special equipment or particular safety precautions.

. Composition Trisodium phosphate (PO Na l2H O) g./litre 2O Tetrasodium pyrophosphate (P O Na do 10 Celanol 251* (see below) cc./litre l5 Solusol T (see below) do 15 Celon (see below) g./ litre 5 H O, remainder.

pH Colorimetric (adjusted with NaOH)12.5.

Surface tension30 dynes/cm.

Temperature-room temperature (can be heated up to 80 C. if required).

Agitationmechanical.

Immersion time--6il minutes at room temperature;

2:0.5 minutes at 60 C.

In the above formula the action and further description of the components is as follows:

Trisodium phosphate--an agent to bring buffered alkalinity to the solution.

Tetrasodium pyrophosphatea defiocculating agent, it increases the detergency of surfaceactive agents, it is a softening agent.

Celanol 25l*an anionic surface'active agent, it is an oxyethylenated derivative, sulphated (25% of active material).

Solusol T*-a nonionic surface-active agent, it is a fatty alcohol oxyethylenated (50% of active material).

Celon*a sequestering agent, it is ethylenediaminetetracetic acid (EDTA), sodium salt.

Satisfactory solution control of the cleaning solution has been obtained by pH measurements using either electrometric or colorimetric measurement. Free alkalinity was measured by alkalimetric titration. Surface tension was measured by stalagmometer. The solution is not altered by ageing or by reasonable amounts of normal use. In use components may be added periodically to compensate for change in free alkalinity, for materials carried away by the parts cleaned, and for similar changes in the solution.

Acid etch-theory of part structure The chemical etching is important because it creates surface hydrophility and micro-porosity, thus conditioning for the homogenous absorption of the subsequent electroless plating solutions and the adhesion of the final metal deposits. In accordance with this invention the change in the surface condition is accomplished without chemical destruction of the material as might occur with oxidizing agents and without degradation of the geometrical precision of the characters which might occur through improperly controlled grit blasting.

The chemical etching is significantly affected by the interdependence of molding and surface treatment. This basic point requires a review of the more detailed theory of molding. The transformation of urea-formaldehyde is from a urea monomer and a formaldehyde monomer to an intermediate which is dimethylolurea and further to the final three dimensional urea-formaldehyde polymer. The intermediate condensation product, i.e., dimethylolurea, is tetrafunctional, and its molecular weight is low. It is a soluble monomer. The molding powder used in the preferred molding in accordance with this invention contains dimethylolurea, along with the alpha-cellulose fillers, the plasticizers, the pigments, and the catalyzers.

When this molding compound, containing dimethylolurea, is subjected to heat and pressure, it goes through a plastic phase, and the H and the OH groups of a lIltllCHtGS a trademark registered product by Consortium dc lroduits Clunnqucs ct tle Synthcse, Bczons, \"al-dOisc, France.

molecule react with the H and the OH groups of another molecule to form water evolved as water vapor. Ultimately, each molecule of urea is linked to the other urea molecules through methylene (-CH bridges, resulting in a rigid tridimensional lattice structure. The obtained polymer is hard, insoluble and infusible. The alpha-cellu lose used as a filler improves the mechanical properties of the part, and the pigments further influence the characteristic of the product, although the pigments are trapped in the lattice of the urea-formaldehyde and do not participate in any reactions.

In accordance with this invention it is recognized that polymerization in a system such as above described is never complete. By complete polymerization is meant the almost total lack of monomer after curing. In industrial production, it is not feasible to get ideal theoretical conditions due to shapes, wall thicknesses, viscous flows, temperature gradients, and similar environmental and practical factors. The curing cycle established in industrial production is a compromise taking into account the various practical, economical, and technical imperatives or limitations. Under these conditions, for commercially acceptable molding; there will always remain some molecules of non-transformed methylolurea which represent the degree of polymerization.

In accordance with this invention it is recognized that the degree of polymerization directly conditions the mechanical and physical properties of the material. These properties are acceptable within certain limits below which (generally characterized by undercuring) mechanical and chemical resistances are insufficient, and above which (generally characterized by overcuring) material becomes brittle and quite insoluble. Within these limits an optimum polymerization condition exists for obtaining the final qualities desired. For balls to be used as typeheads, this condition has been thoroughly studied by measuring the changes of impact resistance and hardness with increasing curing cycles and also measuring chemical resistance to a given reagent.

By mere visual inspection, without any etching, it is not possible to judge the condition of a mold part after curing. The usual boiling water emergent test shows only the most outstanding cases of undercuring and lack of sensitivity. On the other hand, a suitable acid etching is much more sensitive. It makes it possible to check whether curing was adequate, and, above all, whether all points of the plastic were equally polymerized. Consequently, the distribution of thermal energy within the mold to form the ball, which depends on the part geometry, wall thickness, and similar factors, can be corrected to obtain the required homogeneity.

Acid etch-theory of acid action When the curing conditions specified above are obtained, the material is made up of the three-dimensional rigid lattice structure of the polymer in which a certain quantity of monomer (dimethylolurea), pigment and a filler (alpha-cellulose) are dispersed. If the monomer and pigment can be reached and dissolved without destroying the lattice, michro-porosities will be created at the surface and will allow the absorption of the subsequent solutions and the anchoring of the metal deposit. The etching action must, then, be exerted through a selective dissolution. This function is fairly fulfilled by mixtures of formic and hydrochloric acid within a multitude of adjustments of the amounts of the two acids. Depending on requirements, the activity of the solution is adjusted by changing the ratio of the two acids.

Detailed tests have shown that formic acid, at any concentration, has almost no capacity of dissolution and cannot therefore be used alone. Similar detailed tests have shown that the rate of dissolution of hydrochloric acid increases with concentration, but its action seems ing that dissolution affects only the monomer but not the pigment. Further detailed tests have shown that when both acids are mixed in various proportions the rate of dissolution increases with the hydrochloric acid concentration, and the level of dissolution rate is significantly higher than obtained with hydrochloric acid alone at equivalent concentrations. In all tests, of course, the two acids are contained in aqueous solutions. Significantly, the formic-hydrochloric acid mixtures at any significant ratio are very strongly colored, showing that the dissolution under the mixture affects the pigment as well as the monomer.

These various findings may be explained as follows Hydrochloric acid is a strong acid totally ionized at any concentration for most practical purposes. It is then logical that its activity increases with increased CL- and H+ ion concentrations. On the other hand, formic acid is a weak acid not highly ionized in aqueous solution. Used alone, it will have no action on the material, but its behavior will be quite different when in solution with hydrochloric acid. Presence of the CL* ions exalts the acid function of the formic acid, increasing its ionization and therefore the activity of the solution very noticeably. Even if no real substitution of radicals exists, the increase of activity is important and the mixture behaves like a formal chloride (HCOCl). The same exaltation phenomena occurs with acetic acid and hydrochloric acid mixtures but at a lower degree. Considering for example a mixture of 50% by volume aqueous solution of hydrochloric acid containing HCl within the limits of 36.5% to 38% by Weight, the other 50% being an aqueous solution of formic acid containing 88% by weight formic acid, it was found that 11 milligrams per minute of material was removed from a urea formaldehyde part of the type above described as distinguished from 4.0 milligrams per minute of removal for a similar 50% hydrochloric acid solution in water alone, and 0.3 milligram per minute removal for a similar 50% formic acid solution of formic acid in Water alone. Thus, the exaltation effect increases the rate of dissolution by 3 times.

It was further found that even a relatively small amount of hydrochloric acid produces the exaltation effect. Thus tests indicated that a mixture of grams per litre which is 75 parts by volume of an aqueous solution of formic acid as above described and 25 parts by volume of an aqueous hydrochloric acid solution as above described produces a significantly enhanced effect, and the degree of exaltation appeared to diminish at higher hydrochloric acid concentrations. However, a similar mixture, but with 25 parts by volume formic acid and 75 parts hydrochloric acid functioned with greatly increased etching action and entirely in accordance with the purposes of this invention. Thus, the degree of polymerization of the part is recognized in accordance with this invention as a primary factor. The higher the degree of polymerization, the higher the hydrochloric acid can be to achieve proper roughening. It seems, in fact, that the CL ion, smaller and more mobile, is the etching agent which by penetrating the material, lowers the interface tension, allowing a better material-solution contact and the solvent action of formic acid to take place. A suitable surface active agent helps in this line and makes easier the subsequent operations.

Whatever be the selected etching condition, the weight lost per head can be observed, to thereby define that the etch has been active enough to modify the head to give thorough absorption and good adhesion. Appearance of the part after etching should be dull with an even satin finish without bright spots or evidence of white alphacellulose fillers.

The etching time to achieve these results depends, of course, on the selected composition and the initial polymerization condition of the material. Practically, it varies from one to ten minutes.

One preferred process For molding of urea-formaldehyde balls, a suitable formula is as follows:

The cleaned part is first washed with Water, It is then immersed in the acid etch solution.

Hydrochloric acid (HCl within the limits of 36.5% to 38% by weight in an aqueous solution grams Formic acid (an aqueous solution containing 88% by [Weight formic acid) grams Solusol T (nonionic surface agent, a fatty alcohol oxyethylenated, 50% active material) cc./litre 10 *Solusol T is a trademark registered product by Consortium de Produits Chimiques et de Synthese. Bezons, Val-d-Oise, France.

In using the preferred formula the temperature was room temperature. Surface tension was found to be lower than 35 dynes per centimeter, and the agitation was mechanical. Time of immersion was two to four minutes, depending on the actual solution composition and polymerization. In compounding the solution the acids were titrated by acidimetry, and the surface tension was measured with a stalagmometer.

In actually working with such a solution, water is dragged in by parts and by racks holding the parts. Some solution is lost by evaporation and by the solution being dragged outwith the parts and the racks holding the parts. Consequently, concentration of both acids tends to decrease. Therefore, it is necessary to occasionally check by analysis the composition of the etching bath and to adjust it to its original proportions. The hydrochloric acid concentration should not be lower than 120 grams per litre.

Upon removing the ball and cleaning in any suitable manner, such as washing in water, the urea-formaldehyde is minutely roughened in a form exceptionally suitable for metal plating for subsequent use as a typing element. The ball is not unduly roughened as is true with other acid etch processes examined. By minutely roughened is meant a condition at which strong adhesion of a metal film is attained, but the eye cannot significantly discern roughness in the edges of a character printed from the metal coated ball.

Although in accordance with this invention the degree of curing of the urea-formaldehyde need not be at a preselected level, a control throughout quantity production of the amount of polymerization is, of course, contemplated. Furthermore, overcuring leading to degradation of the polymer must be avoided, as well as undercuring since undercured spots etch away greatly under the acid etch process.

Roughness measurements The desired result obtained and consequently the concentration of the etching bath as related to the particular degree of curing of a part in a production environment is best defined by the degree of roughness obtained by the acid etch. A molded article is etched in accordance with this invention and the surface characteristics obtained are observed with a profilometer. A Micrometrical Manufac turing Company of Ann Arbor, Mich, Type QC Amplifier profilometer has been used to define the proper roughness of a typehead of the kind herein under consideration. The proper roughness has been found to be defined by profilometer reading of 251-5. A profilometer is a test instrument which actually traces the surface of an article with a needle like member and registers the changes on a gage. Steep sides on the minutely roughened surface are also important for the metal plating purpose, but these are inherently provided by the acid etch of this invention.

The gage numbers on the profilometer used are believed to be standard scales found in most profilometers in the United States regardless of manufacturer. A proper, plating bath for use with parts cured within particular limits in accordance with this invention may also be determined by observing the adhesion and smoothness of a metal layer subsequently applied. The metal plated type ball, for example, may be tested by thermal shock. Thus a plated head can be immersed in deionized water at 100 C.+0, -2, for 15 minutes and then quickly dipped into cold running water. Also, immersion in methyl alcohol at C. for 5 minutes with natural warming at room, ambient conditions is another good thermal shock test. No blistering of the metal coating should occur. A blister which does occur can be cut and lifted. If the inner metal and the urea-formaldehyde under it have a dark grey appearance, then an outer layer of the metal coating has severed through poor cohesion. If, however, the plastic side has the reddish color of the pigment component in the plastic, then the failure may be attributed to the roughening step, although other factors are, of course, probable.

Whether the profilometer or other tests are used, the proper concentration of the plating bath should be established by such tests and then the plating bath and the degree of cure of the urea-formaldehyde during quantity production should be controlled.

Metal plating The subsequent metal plating in this preferred embodiment is now known in the art. The roughened plastic is pre-sensitized by emersion in a bath which deposits catalytic amounts of stannous ions. The subsequent immersion in palladium ions deposits 2. continuous layer of palladium into the material. Further electroless and electrolytic plating steps may be conducted to bring the metal layer up to the thickness desired. Another metal, such as preferably nickel, may be plated over the palladium.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim: 1. A process to roughen a plurality of items comprised of urea-formaldehyde comprising the steps of:

immersing a representative sample of said items in a preselected aqueous solution consisting essentially of formic acid and hydrochloric acid for a preselected time, said solution containing a concentration of formic acid per parts by volume of at least 25 parts by volume of an 88% by weight aqueous formic acid solution and a concentration of hydrochloric acid per 100 parts by volume of at least 25 parts by volume of a 36.5% by weight aqueous HCl solution, then comparing said item with criterion indicative of the degree of etch required to achieve minute roughening of said urea-formaldehyde for metal plating, then selecting the proper combination of an aqueous solution consisting essentially of formic acid and hydrochloric acid on the basis of said comparison, said solution containing a concentration of formic acid per 100 parts by volume of at least 25 parts by volume of an 88% by weight aqueous formic acid solution and a concentration of hydrochloric acid per 100 parts by volume of at least 25 parts by volume of a 36.5% by weight aqueous HCl solution,

immersing each of said items with said proper combination acids for a time suflicient to minutely roughen each of said items, and then plating each of said items in a metal plating bath.

2. The process of claim 1 in which said items are printing elements carrying at least one character in the form of a raised salient.

3. A process to roughen a plurality of items comprised of urea-formaldehyde comprising the steps of:

immersing a representative sample of said items in a preselected aqueous solution consisting essentially of formic acid, hydrochloric acid, and a nonionic surface active agent for a preselected time, said solution 9 10 containing a concentration of formic acid per 100 tion of acids for a time sufiicient to minutely roughen parts by volume of at least 25 parts by volume of each of said items, and plating said items in a metal an 88% by weight aqueous formic acid solution and P ting batha concentration of hydrochloric acid per 100 parts PQP of claim} in which said items f by volume of at least 25 parts by volume of a 36.5% 5 Punting elemenfs y at least one character In by weight aqueous HCl solution, then the form a f' comparing said item with criterion indicative of the The Procass of clalmlm whfch Sald mmute rougher! degree of etch required to achieve minute roughening mg defined by profilonieter readmgs of of said urea-formaldehyde for metal plating, then T process of claim 3 wherein sald minute rough selecting the proper combination of an aqueous solul0 enmg 1s defined by Profilometer readings of 25:5

tion consisting essentially of formic acid, hydro- References Cit d ctll1loric acid, and a nonionic surface active agent on UNITED STATES PATENTS t e basis of said comparison, said solution containing a concentration of formic acid per 100' parts by 2,252,143 9/1941 Wood XR volume of at least 25 parts by volume of an 88% 3035944 5/1962 by weight aqueous formic acid solution and a con- 306O55O 10/1962 Smith centration of hydrochloric acid per 100 parts by JACOB H STEINBERG, Pri E i volume of at least 25 parts by volume of a 36.5% by weight aqueous HCl solution, immersing each of said items with said proper combina- 156--2, 3, 14; 101-368, 426