Latex foam is a well known material. The latex is in the form of an emulsion when delivered to an end user. In certain applications, the emulsion is employed in the manufacture of flooring, wall covering, shoe lining and non-woven materials. The end user may add fillers to enhance desired properties prior to coating a given substrate with a foam layer made from the emulsion. Since products may be stored for extended periods of time, stable emulsions would be highly desirable. Previous attempts to provide stable emulsions required use of curing pastes, gelling agents, accelerators or stabilizers. Latex emulsions that are stable and that do not require such curing pastes, gelling agents, accelerators or stabilizers would be highly desirable. Especially desirable are latex emulsions that will cross-link in the backing process to ensure final end properties have sufficient strength.

The present invention provides a solution to one or more of the disadvantages and deficiencies described above.

In one broad respect, this invention is an emulsion comprising a latex and an epoxy compound. The latex is preferably a carboxylated styrene-butadiene polymer with a bimodal particle size. The composition may also contain stabilizing surfactants as the cross- linking agent to improve physical properties of the resulting foam. The composition may employ a dual catalyst system. Furthermore, the composition may include performance enhancing additives, such as paraffin wax and silicone detackifier. Advantageously, the latex emulsion may be supplied to the point of manufacture where inorganic or organic filler can be added to enhance desired properties. More advantageously, no additional curing pastes, gelling agents, accelerators or stabilizers are required in the practice of this invention. The emulsion, in one non-limiting embodiment, is stable for twelve months at ambient temperatures. During processing, the resulting foam will cross-link in the backing process to improve final end properties such that the product has sufficient strength.

Beneficially, this invention provides a simplified manufacturing process and need not employ any heavy metals, sulfur or nitrosamine releasing accelerators which are conventionally used for making such foams.

In another broad aspect, this invention is a process useful for forming an article of manufacture comprising applying a foam to a substrate wherein the foam is formed from a composition comprising a bimodal latex and an epoxy emulsion. In yet another broad sense, this invention is a process useful for forming a composition useful for preparing a latex foam, comprising: combining a bimodal latex and an epoxy emulsion.

The bimodal latexes used in this invention may be characterized as having two separate and distinct particle size distributions have high solids content, good high shear rheology and good low shear viscosity. The large size polymer particles of the bimodal latex have a heterogeneous character.

The bimodal latex used in this invention may comprise a proportion of large size latex particles and a proportion of small size latex particles. It is desirable to employ large size particles whose diameter is in the range of from 2.5 to 10, most preferably from 3 to 4, times that diameter of the small size particles. It is also desirable that the weight percentage of large size particles in the latex formulation exceed the weight percentage of the small size particles. For example, a latex composition comprised substantially of styrene/butadiene comprising from 50 to 98, preferably from 60 to 80, weight percent large size particles and from 2 to 50, preferably from 20 to 40, weight percent small size particles can be used. It is understood that the proportion of large size particles and the proportion of small size particles, the size distribution of particles, and the amount of solids in the formulation employed can depend on the particular latex which is employed and/or the particular coating device which is employed.

"The large size latex particles can vary in size from 1500 A (0.15 micrometers) to 10,000 A (1 micrometer), more preferably from 1800 A (0.18 micrometers) to 3,000 A (0.3 micrometers) in diameter. The small size latex particle can vary in size from 500 A (0.05 micrometers) to 1000 A (0.1 micrometers), more preferably from 600 A (0.06 micrometers) to 800 A (0.08 micrometers) in diameter." Heterogeneous polymer particles may be employed in the practice of this invention to provide the large size polymer particles of the bimodal latex. Of particular interest are the types of polymer particles disclosed in U. S. Pat. No. 4,134,872. That is, the heterogeneous polymer particles are characterized as having a hard resinous polymer of interpolymer forming a core or core-type region, and a soft preferably interpolymer shell or shell-type region. Also useful herein are the coalescence capable heterogeneous polymer particles, which particles have hard core or core-type regions and soft shell or shell-type regions.

Broadly speaking, the large size heterogeneous polymer particles have a relatively soft polymer domain and a relatively hard polymer domain. It is believed that the hard polymer domain provides a desirable gloss characteristic to the coating formulation; while the soft, deformable polymer domain provides a desirable binding characteristic to the coating formulation.

The heterogeneous polymer particles typically comprise from 10 to 90, preferably 40 to 75 weight percent of a hard polymer domain, and 10 to 90, preferablv 25 to 60 weight percent of a soft polymer domain. Generally, the hard polymer domain comprises from 80 to 100 weight percent types of monomers (for example, monovinylidene aromatic monomers) which form a hard component of the hard polymer domain when polymerized; from 0 to 20 weight percent, preferably from 10 to 20 weight percent monomers such as open chain aliphatic conjugated diene monomers or other such monomers which when polymerized provide a softening character to the hard domain; and from 0 to 10, preferably 0.5 to 5 weight percent of a hydrophilic, hydrolyzable or ionizable monomer such as acrylic acid. Generally, the soft polymer domain comprises from 30 to 70, preferably 40 to 60 weight percent of a monoethylenically unsaturated monomer which (for example, a monomer which can form a hard component of the polymer domain such as a monovinylidene aromatic monomer, or a monomer which can form a soft component of the soft polymer domain such as an acrylate monomer, or a combination thereof) ; from 70 to 30, preferably from 60 to 40 weight percent of a soft monomer such as a conjugated open chain diene; and from 0.1 to 10, preferably 2 to 6 weight percent of a hydrophilic, hydrolyzable or ionizable monomer. Typically, the minimum film formation temperature of the latex composition is less than about 30°C. Preferred heterogeneous polymer particles comprise carboxylated monovinylidene/conjugated diene containing polymer particles. For example, carboxylated styrene/butadiene containing polymer particles having a heterogeneous character are particularly useful.

The small size polymer particles of this invention are prepared from combinations of monomers such that the resulting particles have sufficient adhesive properties for foam coating applications, such as for application to a backing material to provide for example a resilient foam backing layer which is adhered to a second structure, the second structure being the subst-rate for the foam layer. Virtually any latex that can be used as a foam coating and can be prepared for use in a bimodal composition can be employed. It is also desirable that the latex be carboxylated in order to increase colloidal stability and, hence, the degree of binding efficiency to the paper and pigments. Examples of suitable monomers for providing a carboxylate character include acrylic acid, methacrylic acid, itaconic acid and fumaric acid. Typically, the minimum film formation temperature of the latex composition is less than about 25 C. Representative monomers useful in preparing the latexes of this invention and methods for preparing the individual separate particles are described in U. S. Pat. Nos. 3,404,116 and 3,399,080. Examples of monomers suitable for preparing the latexes of this invention can include the olefins such as ethylene and propylene, vinyl acetate, alkyl acrylates, hydroxyalkyl acrylates, alkyl methacrylates, hydroxyalkyl methacrylates, acrylamide, n-methyloylacrylamides, as well as monomers such as vinyl chloride and vinylidene chloride. Especially preferred latexes include modified styrene/butadiene latexes such as, for example, styrene/butadiene/acrylic acid, styrene/butadiene/acrylic acid/itaconic acid, styrene/butadiene/vinylidene chloride, styrene/butadiene/beta-hydroxyethyl acrylate, styrene/butadiene/beta- hydroxyethylacrylate/acrylic acid, styrene/n-butylacrylate/acrylic acid, methyl methacrylate/n-butylacrylate/acrylic acid, vinyl acetate/acrylic acid, vinyl acetate/n- buty! acrylate/acrylic acid, and/or styrene/n-butyl acrylate/butadiene/acrylic acid. Mixtures of carboxylic acids can be employed in the aforementioned latexes.

In the preparation of the small particle size polymer latexes, it is desirable to use a relatively small polymer particle (for example, a"seed"latex) in initiating particle formation. The latexes having separate and distinct particle sizes are then blended together to yield a bimodal latex composition. Alternatively, bimodal latexes can be prepared by intermediate addition of a seed latex during the heterogeneous particle emulsion polymerization process. For example, the core domain of the large size particle can be prepared, and either simultaneously to or after the shell domain of the large size particle is formed, the seed latex can be added in order to provide large size heterogeneous polymer particles having a hard core domain and a soft shell domain, and small size polymer particles having a soft character which is similar to that shell character of the large size particles. Any of the latex formulations can be concentrated, if desired.

In the practice of this invention, it is preferred to employ carboxylated latex comprised of a copolymer of a vinyl aromatic monomer and an unsaturated carboxylic acid monomer. In a preferred form, the copolymer may further comprise a diene monomer.

The vinyl aromatic monomer may be selected from styrene, alpha-methylstyrene, a, r- methylstyrene, a, r-ethylstyrene, alpha-a, r-dimethylstyrene, a, r, a, r-dimethylstyrene, a, r-t- butylstyrene, vinylnaphthalene, methoxystyrene, cyanostyrene, acetylstyrene, monochlorostyrene, dichlorostyrene, and other halostyrenes, and mixtures thereof. The vinyl aromatic monomer may be present in any effective amount. The vinyl aromatic monomer may be present in amounts of from approximately 0 to 75 percent by weight, based on the total weight of the polymer resin. Preferably the vinyl aromatic monomer is present in amounts of from approximately 35 to 70 percent by weight.

The ethylenically unsaturated carboxylic acid may be a monocarboxylic acid, or a dicarboxylic acid or a polycarboxylic acid, such as, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, derivatives thereof and mixtures thereof.

The ethylenically unsaturated carboxylic acid monomer may be present in amounts of from approximately 0.5 to 25 percent by weight, based on the total weight of the polymeric resin. Preferably, the ethylenically unsaturated acid monomer is present in amounts of from approximately 1 to 5 percent by weight and, more preferably, from 3 to 5 percent by weight, based on the total weight of the copolymer.

The diene monomer, when present, may be selected from butadiene, isoprene, divinylbenzene, derivatives thereof and mixtures thereof. The 1,3-butadiene monomer is preferred. The diene monomer may be present in amounts of from approximately 0 to 85 percent by weight, preferably from approximately 30 to 65 percent by weight, based on the total weight of the polymer resin.

The latex may comprise an additional ethylenically unsaturated monomeric component or components. Specific examples of such ethylenically unsaturated compounds include methyl methacrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, lauryl methacrylate, phenyl acrylate, acrylonitrile, methacrylonitrile, ethyl- chloroacrylate, diethyl maleate, polyglycol maleate, vinyl chloride, vinyl bromide, vinylidene chloride, vinylidene bromide, vinyl methyl ketone, methyl isopropenyl ketone and vinyl ethylester. Derivatives thereof or mixtures thereof may be included.

The latex may comprise a styrene/butadiene/-acrylic acid copolymer or a styrene/butadiene/hydroxy-ethylacrylate/itaconic acid copolymer. The latex may also include a mixture of copolymers. A mixture of styrene/butadiene/acrylic acid and styrene/butadiene/-hydroxyethylacrylate/itaconic acid polymers in approximately equal amounts by weight may be used.

Such monomers are copolymerized in an aqueous emulsion containing surfactants and modifiers under conditions of time, temperature, pressure and agitation in accordance with well known principles of emulsion polymerization.

In the practice of this invention, use of an epoxy emulsion is employed in combination with the bimodal latex.

The epoxy resin component is suitably any compound which possesses more than one 1, 2-epoxy group. In general, the epoxy resin component is saturated or unsaturated aliphatic or cycloaliphatic, aromatic or heterocyclic and can be substituted or unsubstituted.

The epoxy resins may be selected from the polyglycidyl ethers of bisphenol compounds, the polyglycidyl ethers of a novolac resin, and the polyglycidyl ethers of a polyglycol. Mixtures of two or more epoxy resins may also be used.

The preferred epoxy resins are the polyglycidyl ethers of bisphenol compounds. The polyglycidyl ethers of bisphenol A or bisphenol F have been found to be suitable. The epoxy resins may be formed as the reaction products of epichlorohydrin and bisphenol A or bisphenol F or derivatives thereof.

The epoxy resin component of the curable latex composition may further include an emulsifier or surfactant. An anionic or a nonionic surfactant may be used. A nonionic surfactant is preferred. An ethoxylated nonionic surfactant is more preferred. An ethoxylated nonionic surfactant having an HLB of approximately 16 to 20 is most preferred.

The non-ionic surfactant sold under the trade designation"Capcure 65"and available from Diamond Shamrock Corporation has been found to be suitable. The emulsifying agent or surfactant may be present in amounts of from approximately 5 to 10 percent by weight, based on the weight of the epoxy resin. Preferably, the emulsifying agent or surfactant is present in amounts of from approximately at least 8 percent by weight. It has been found that where a nonionic surfactant or emulsifying agent is included, the epoxy resin emulsion so formed provides a relatively reduced particle size. The reduced particle size provides an improvement in the stability of the epoxy resin and in turn in the curable latex composition.

Desirably, in the preparation of the epoxy resin emulsion, the epoxy resin and surfactant or emulsifier are homogenized by means of a suitable high shear blender. The particle size of the epoxy resin emulsion thus produced may be approximately two to five times that of tile latex (for example approximately three times that of the latex (for example, less than 1000 nm)). High shear homogenization may continue during phase inversion in order to assist in achieving small particle size.

The level of epoxy resin employed will vary over a wide range depending upon the properties of the final product required, as well as the types of epoxy resin and carboxylic acid used.

Low viscosity resins are preferred as it is easier to produce a stable emulsion from them. Representative commercial epoxy resins include those sold under the trade designations D. E. R. oxo 351-A and D. E. R. (V 330 available from The Dow Chemical Company.

The epoxy resin emulsion component as described above comprises an organo- soluble or organo-miscible catalyst. Suitable organo-soluble or organo-miscible catalysts include the phosphonium salts, such as, for example, ethyltriphenyl phosphonium acetate and ethyltriphenyl phosphonium phosphate and the quaternary ammonium salts, such as, for example, alkylbenzyl dimethyl ammonium chloride, benzyltrimethyl ammonium chloride, methyltrioctyl ammonium chloride, tetraethyl ammonium bromide, N-dodecyl pyridinium chloride and tetraethyl ammonium iodide. The preferred organo-soluble or organo-miscible catalysts are ethyltriphenyl phosphonium acid acetate, ethyltriphenyl phosphonium phosphate and methyltrioctyl ammonium chloride. Ethyltriphenyl phosphonium phosphate is not readily available but it can be manufactured from ethyltriphenyl phosphonium acetate by reaction with phosphoric acid.

The organo-soluble or organo-miscible catalyst may be present in an amount of from approximately 0.1 to approximately 10.0 percent, preferably from 0.3 to 2.0 percent, by weight, based on the weight of the epoxy resin.

The water-soluble catalytic curing agent may be present in an amount of from approximately 0.1 to approximately 15 percent by weight, based on the weight of the copolymer. Suitable catalytic curing agents include tridimethyl aminomethyl phenol, dimethyl aminomethyl phenol, dicyandiamide, polyamines such as, for example, ethylenediamine, diethylenetriamine, triethylene tetramine, tetraethylene pentamine and isophorone diamine.

The curable latex composition according to the present invention may further include standard compounding ingredients such as, for example, fillers, thickening agents, antioxidants, dispersants, pH modifiers and flame retarding agents.

An adjustment of the pH of the mixture of the reactive latex and the coreactive material may be made, if desired, by the addition of usual acidifying or alkalizing agents such as, for example, acetic acid, citric acid, dilute mineral acids, ammonium hydroxide and dilute aqueous solutions of alkali metal hydroxides.

The shelf life of the blend of latex and epoxy resin emulsion may be improved by selecting the pH of the blend such that a substantial proportion of the carboxyl groups on the latex copolymer are protonized. It has been found that if the pH is maintained in the range of approximately 6 to 6.5, extended shelf life may be achieved. The pH may be adjusted in any suitable manner. Addition of an approximate amount of ammonia has been found to be suitable for pH adjustment.

In addition to the bimodal latex, the epoxy emulsion may comprise additional components and additives. Such additional components may include, but not limited to, 0.1 to 10 parts per 100 dry parts of a bimodal latex, preferably about 1 to 4 parts of a paraffin wax emulsion to improve cell tack and water resistance. The composition may include from 0.1 to 5 parts, preferably about 1 part, of a cell detackifier, such as a silicone based cell detackifier, used to prevent the cell walls of the foam from sticking together. The composition may include from 0.1 to 5 parts, preferably about 3 parts, of a froth stabilizer such as a suspension of disodium N-cetostearyl sulphosuccinimate. The composition may include from 0.1 to 5 parts, preferably about 1 part, of a froth booster. The composition may include from 0.1 to about 2, preferably about 0.4 part, of a dispersant for any filler, if present, such as a polyphosphate dispersant added to improve dispersion of a inorganic (mineral) filler. The composition may include 0.1 to 5 parts, preferably about 1.5 parts, of a catalyst or dual catalyst which improves curing time during processing, such as a water based mixture composed of 2, 4,6-tri (dimethylaminomethyl) phenol and ethtriphenylphosphonium acid ester. The composition may include 0.1 to 2 parts, preferably 0.5 part of ammonium sulfate to reduce viscosity. The composition may include 0.1 to 10 parts, preferably about 4 parts of the epoxy emulsion, which serves as a cross-linker. The composition may include a compound to improve resilience, such as 0.1 to 5 parts, preferably 1.5 parts, of ammonium oleate. The composition may include 0.1 to 5 parts of one or more antioxidants, such as about 1.2 parts of a blend of a polymeric hindered phenol and ditridecyl di thio diproprionate.

The compositions of this invention may additionally comprise one or more mineral fillers. Examples of mineral fillers include those known in the art such as clay, titanium dioxide, carbon, silicates, zinc oxide, calcium carbonate, zinc sulfide, potassium titanate and titanate whiskers, glass flakes, clays, kaolin and glass fibers. The amount of filler which is employed can vary, depending upon the density of the filler and the coating properties desired. Each of the aforementioned components is mixed in an aqueous medium to yield a formulation which is about 10 to about 90 percent solids by weight.

It will be understood that the various components of the curable latex compositions of the present invention may be maintained separately until shortly before use because of their ambient temperature curing properties. In some instances, two or more components which do not react with each other can be premixed. For example, the latex and the water- soluble catalytic curing agent may be provided as one component and the epoxy emulsion containing the organo-soluble or organo-miscible catalyst as the other component. The latex may also be combined with the epoxy emulsion containing the organo-soluble or organo- miscible catalyst and then the water-soluble catalytic curing agent may be added separately immediately prior to use. Once combined, the composition may be used directly or may be further diluted with water depending on the solids level desired for the particular method of application to be employed. Advantageously, however, the composition of this invention containing a bimodal latex and an epoxy emulsion may be admixed and stored for long periods of time, for example, up to a year.

The curing temperature may be any suitable temperature above ambient temperature.

Indeed, some curing may occur at ambient temperature, but since the reaction time is extremely slow, such a temperature is impractical.

The preferred temperature range is from approximately 120°C to 180°C. The residence time is variable. Factors influencing residence time include temperature, film thickness, water content and the components of the curable coating composition. With temperatures in that range, a total residence time of approximately five to ten minutes has been found to be suitable.

The generality of the invention should not in any way be restricted by theory based on the results of our experiments; however, it can be postulated that the water-soluble catalytic curing agent will to some extent be transferred into the epoxy resin phase upon drying, and promote polymerization of the epoxy resin, as well as carboxyl-epoxy reaction.

It also makes the latex more miscible with the epoxy resin. The organic soluble catalyst had shown reasonable activity for the acid-epoxy reaction only, hence resin emulsions pre- catalyzed with the organo-soluble catalyst yield a long shelf life. The organic soluble catalyst also makes the carboxylated latex polymer more miscible with the epoxy resin. It is therefore reasonable to assume, that the latex particles are crosslinked with a built-in network of homopolymerized epoxy resin.

The foaming step may be undertaken in any suitable manner conventional manner.

A foam or froth may be generated by methods well known in the art, for example by releasing a noncoagulating gas such as nitrogen, or by causing the decomposition of a gas- liberating material to chemically react with an ingredient in the mixture with the liberation of a non-coagulable gas as a reaction product. The mixture of the reactive latex and the coreactive material is also foamed by whipping or by use of apparatus having commercially available foam heads. Known foaming aids, such as sodium lauryl sulfate, or foam stabilizers, such as potassium oleate, may be added if desired. Preferably, such added materials should be non-reactive with the reactive group in the latex polymer or in the coreactive material and thus the preference may vary with the composition of the mixture.

Other soaps, emulsifiers, wetting agents, and surfactants, however, may be used, even though they may be reactive to a limited extent.

The frothed mixture may be poured into molds, spread on a flat tray or belt, or coated onto substrates. For the purpose of this specification, the term"substrate"is defined as any material such as cloth, fabric, leather, wood, glass or metal or any form of backing to which the frothed mixture will adhere when applied and after it is cured.

In a preferred embodiment in which the foam is used as a textile backing, the foam may be applied to the textile prior to drying and curing. A typical froth formed from the continuous foam will have a density in the range of from approximately 200 to 400 grams per liter in its wet state, preferably approximately 350 grams per liter. The foam may be applied to the substrate utilizing a doctor blade.

Once formed, the foam may be dried and cured at a temperature of approximately 110°C to 150°C. The drying and curing may be undertaken in a forced air circulation oven.

The internal temperature of the oven should be maintained preferably at or above approximately 120°C.

The following examples are illustrative of this invention and are not intended to be limit the scope of the invention or claims hereto. Unless otherwise denoted all percentages are by weight.

Example In a stainless steel reactor equipped with a nitrogen inlet, mechanical stirrer and condenser, 20.75 parts of bisphenol A and 68.48 parts of D. E. R. 330 epoxy resin were heated to 130°C with stirring. 0.05 parts of a 70 percent solution of Al catalyst in methanol was added and the temperature was raised to 150°C to start the exotherm. Under adiabatic conditions a peak exotherm occurring at around 180°C was observed. 30 minutes after the peak exotherm, 0.026 parts of methyl-para-toluene sulfonic ester were added while cooling down the resin to 120°C. 4.3 parts of Aerosol 108 (surfactant from ICI), 5.1 parts of Disponil TA 430 (Henkel) and 1.4 parts of Aerosol TO 75 were added and mixing was allowed for 30 more minutes while the temperature was further decreased to 90°C. An end epoxy equivalent weight (EEW) value of 500 plus/minus 20 was obtained. The resin product was dispersed in a centrifugal pump to yield an epoxy emulsion having volume average particle size below 0.6 micron; an EEW value of 780-910 and a solid content of 56- 61 percent.

An emulsion in accordance with this invention was prepared by admixing the following components: 100 dry parts of a bimodal carboxylated styrene-butadiene latex, 4 parts of a paraffin wax emulsion to improve cell tack and water resistance (IMPERMAX T940, from Govi), 1 part of a silicone based cell detackifier used to prevent the cell walls of the foam from sticking together (PROSIL E70, from Stephenson Brothers), 3 parts of a froth stabilizer (EMPINIM MKB, a suspension of disodium N-cetostearyl sulphosuccinimate, from Alright and Wilson Surfactants Group), 1 part of a froth booster (SLS), 0.4 part of a dispersant for filler (CALGON PT, a polyphosphate dispersant added to improve dispersion of a inorganic filler), 1.5 parts of a dual catalyst which improves curing time during processing (ETPPAAc/Ancamine K54, a water based mixture composed of 2,4,6- tri (dimethylaminomethyl) phenol and ethtriphenylphosphonium acid ester, from Morton Performance Chemicals Europe), 0.5 part of ammonium sulfate to reduce viscosity, 4 parts of the epoxy emulsion which serves as a cross-linker, 1.5 parts of ammonium oleate to improve resilience and 1.2 parts of a blend of antioxidants (EMULSION L, composed of a polymeric hindered phenol (WINGSTAY L) and a secondary antioxidant, ditridecyl di tiho diproprionate (DTDTDP) from Great Lakes in Austria).

The emulsion was employed in a foam backing process by combining 100 parts by dry weight of the emulsion of the preceding paragraph and 160 parts by dry weight of calcium carbonate filler to provide a product having a solids content of 78 percent and a viscosity of 3,500 cps (Brookfield spindle 4 at 20 rpm). The filler was employed to improve resilience and strength of the final foam. Calcium carbonate was used as an extender in the foam manufacturing process. The calcium carbonate was preferably selected to be readily dispersible, exhibit low froth viscosity and stability and provide good final foam properties at high loadings of up to 200 parts per 100 parts dry latex. A representative calcium carbonate filler may currently be obtained from Omya as BL 200. The product was mechanically foamed using air to reduce density, spread onto the substrate and the dried in an oven at approximately 140°C to remove the water from the system.

Further modifications and alternative embodiments of this invention will be apparent to those skilled in the art in view of this description. Accordingly, this description was to be construed as illustrative only and was for the purpose of teaching those skilled in the art the manner of carrying out the invention. It was to be understood that the forms of the invention herein shown and described were to be taken as illustrative embodiments. Equivalent elements or materials may be substituted for those illustrated and described herein, and certain features of the invention may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the invention.