METHOD AND APPARATUS FOR CONTROLLING THE DENSITY OF A FOAM

Abstract: The density of a foam is controlled by measuring the electrical conductivity of the foam and adjusting its density in response to the measured electrical conductivity. Apparatus for carrying out this method comprises means (10) for generating a foam connected by an enclosed pathway (13) to an application outlet (14). Located within the pathway (13) is an electrical conductivity sensor (15) which is electronically connected (16) to a translation means (17). The translation means (17) is connected (18) to a gas glow controlling means (19) whereby, in response to the signal received from the conductivity sensor (15), the translation means (17) provides a signal to the gas glow control means (19) to vary gas supply to the means (10) for generating the foam. The invention provides an accurate method of controlling the density of the foam within a narrow range, thus enabling the application of the minimum amount of foam necessary to achieve the desired physical properties.

Description: This invention relates to a method and apparatus for controlling the density of a foam. Specifically, the invention relates to a method and apparatus for controlling the density of a foam on a carpet structure. In the production of carpets, such as carpets of the tufted type wherein a plurality of loops of yarn are deposited in a primary backing material, the back surface of said carpet structure is generally coated with a foam. The foam coating serves to anchor the loops of yarn in the primary backing material. The foam generally comprises a natural or synthetic latex composition. Examples of synthetic latexes employed to bind the loops of yarn in the primary backing material are styrene/butadiene copolymer latexes. After application of the foam coating, a second woven or nonwoven backing material is generally applied to the carpet structure. An example of such secondary backing materials is a woven mat of jute. The amount of foam applied to the back of the carpet structure is most desirably the minimum amount of foam suitable for achieving a desirable degree of adhesion between the loops of yarn and the primary backing material as well as a desirable degree of adhesion between the primary backing material and the secondary backing material. It has been found desirable to apply the material which binds the loops of yarn in the primary backing material (binding material) as a foam on the back surface of the carpet structure for several reasons. First, it has been found that application of the binder material as a foam tends to inhibit penetration of the binder material onto the carpet facing. Second, the application of binder material in the form of a foam has been found to produce carpet structures having a more uniform distribution of the binder material as well as producing carpet structures with the minimum amount of binder material necessary to achieve the desired properties. It is desirable that the density (varies inversely with the amount of gas incorporated into the foam) of the foam be maintained at a relatively constant level. This is desirable in order to achieve the desired physical properties through application of a minimum amount of foam. Since a relatively constant thickness of foam is applied to the carpet structures, variations in the density of the foam results in a surplus of binder material being applied to the carpet structure when the density of the foam is too great and a dearth of binder material being applied to the carpet structure when the density of the foam is below a desirable level. In the past, the density of the foam has been controlled by visual inspection of the foam as it is applied to the carpet structure. This method of control has been found to be less than desirable. The lack of the ability to carefully control the density of the foam has necessitated the application of more than the minimum amount of foam (binder material) necessary to achieve the desirable physical properties. It would be desirable to find an accurate method of controlling the density of the foam within a narrow range thus enabling the application of the minimum amount of foam necessary to achieve the desirable physical properties. FR-A-2067572 (published 20th August 1971) discloses a method for detecting gas bubbles in liquid, especially blood, by monitoring the electrical conductivity of the liquid. US-A-4154537 (issued 15th May 1979) discloses the control of density of a bakery dough or batter by varying the flow of gas into a mixer in response to signals from a radiation sensitive densitometer. The densitometer is located downstream of the mixer to monitor the density of the dough or batter. Reference is made to application of the method and related apparatus to control the density of other, unspecified, liquid products. US-A-417917 (issued 30th October 1979) discloses the manufacture of a dual-purpose temporary carpet and carpet underlay by applying a latex foam layer to a decorative wear layer coated with a copolymer latex adhesive whereby the adhesive binds together the latex foam and wear layers. No provision is made for monitoring the density of either the adhesive or the latex foam. One aspect of the present invention provides a method of applying a foam coating to a carpet structure characterised in that the density of the foam is maintained at a predetermined level by: (a) measuring the electrical conductivity of the foam during its preparation and prior to deposition of the foam on the carpet structure; and (b) adjusting the density of the foam in response to the measured electrical conductivity. Another aspect of the present invention provides an apparatus for applying a foam coating to a carpet structure comprising: (a) means for generating the foam; (b) an application outlet suitable for applying the foam to the carpet structure; and (c) an enclosed pathway connecting the means for generating the foam and the application outlet, said enclosed pathway being suitable for conveying the foam from the means for generating the foam to the application outlet, characterised in that the apparatus further comprises: (d) an electrical conductivity sensor located in the enclosed pathway, said electrical conductivity sensor being capable of measuring the electrical conductivity of the foam in the enclosed pathway; and (e) means for altering the density of the foam in response to said measured electrical conductivity. The understanding of this invention will be facilitated by referring to the accompanying drawing in which Figure 1 is a schematic representation which illustrates one embodiment of this invention. Referring to the drawings in more detail, Figure 1, which represents a preferred embodiment of the present invention, depicts a means 10 for generating a foam material. Means 10 has an inlet pipe 12, for composition feed, an inlet pipe 11, for gas used in forming the foam and an outlet pipe 13 through which the foam flows to reach application outlet 14. Located along outlet pipe 13 is an electrical conductivity sensor 15 which measures the electrical conductivity of the foam as it passes through outlet pipe 13. Conductivity sensor 15 is connected by lead 16 to translation means 17. Translation means 17 is capable of translating a signal from conductivity sensor 15, which signal relates to the conductivity of the foam coating, into a signal capable of passing through lead 18 to controlling means 19. Based on the signal received from translation means 17 controlling means 19 regulates the amount of gas flowing through gas inlet pipe 11 into means 10, for generating a foam which has a density related to the electrical conductivity thereof. The terms "conductivity" and "electrical conductivity" are used interchangeably throughout the specification. Means 10 are well-known in the art. Such means suitably comprise mechanical means for incorporating a desirable amount of gas into a material to be foamed. In one preferred embodiment of the present invention, the material to be foamed is suitable for binding loops of yarn (hereinafter tufts) to a primary backing material. Exemplary compositions suitable for binding the tufts to the primary backing material are natural and synthetic latexes. Generally, the natural or synthetic latexes have incorporated therein additives such as fillers, surfactants and thickeners. The latexes (with additives) have a solids content of at least 40 weight percent, preferably from 45 to 90 weight percent, based on total weight of latex and additives. Latexes with a solids content below 40 weight percent produce foams which are unstable and possess insufficient binding power when applied at normal levels. Latexes with a solids content above 90 weight percent are difficult to foam. Exemplary synthetic latexes suitable for use in binding tufts to the primary backing is a latex of a polymer of: styrene and butadiene; ethylene and vinylacetate; ethylene and vinyl chloride; vinyl chloride, styrene, and butadiene; styrene, butadiene and an acrylate monomer having from 1-6 carbon atoms; polybutadiene; and vinylidene chloride. Desirably, the latex comprises a copolymer of styrene and butadiene. Preferably the latex comprises a copolymer of from 40 to 75 weight percent styrene and from 25 to 60 weight percent butadiene. The material to be foamed flows into means 10 for generating a foam through composition inlet pipe 12. The gas which means 10 incorporates into the foamable composition enters means 10 through gas inlet pipe 11. Any gas capable of incorporation into the material to be foamed is suitable for use in the practice of the present invention. For reasons of economic operation and safety, it is generally preferred that the gas incorporated into the material to be foamed comprise the mixture of gases present in the ambient atmosphere (air). Generally, the amount of gas incorporated into the material to be foamed is within the range of from about 35 to about 95 percent by volume. The percent of gas incorporated into the material to be foamed is calculated according to the formula: wherein D F is the density of the foamed material and D NF is the density of the material to be foamed prior to foaming. Usually the density of the foam will be varied by the incorporation of a greater or lesser amount of gas into said foam. The foam exits means 10 through an enclosed pathway such as outlet pipe 13. The enclosed pathway leads to application outlet 14. Any enclosed pathway capable of conveying the foam from means 10 to application outlet 14 is suitable for use in the present invention. As depicted in Figure 1, the enclosed pathway is suitable a general circular pipe. Any application outlet capable of directly or indirectly applying foam to its intended use is suitable for use in the present invention. Those skilled in the art are appraised of application outlets suitable for use in the practice of the present invention. In one preferred embodiment wherein the foam is suitable for binding loops of yarn to a primary backing material application outlets suitable for use include a spray nozzle capable of applying the foam directly to a carpet structure, and an application nozzle capable of discharging the foam into a coating pan for indirect application to the carpet structure. Located within the enclosed pathway leading from means 10 for generating a foam to application outlet 14 is a conductivity sensor 15. Conductivity sensors are well-known in the prior art. Conductivity is defined as the ability of a substance to conduct electric current. Any sensor capable of measuring the conductivity of the foam is suitable for use in the present invention. An example of a conductivity sensor suitable for use in the present invention is a contacting electrode conductivity sensor. Such a sensor operates by having two electrodes spaced from one another with the material to be tested located between the electrodes. A charge is generated in one electrode, the amount of charge passing through the material to be tested to the second electrode gives a relative measure of conductivity. In one preferred embodiment of the present invention, the conductivity sensor is an electrodeless conductivity sensor. Electrodeless conductivity sensors are well-known in the art. Generally, electrodeless conductivity sensors measure an induced current in a loop of solution. The electrodeless conductivity sensors contain 2 toroidally wound coils encapsulated in close proximity within the conductivity sensor. The sensor is immersed in the material which is to have its conductivity measured. An alternating current electrical signal is applied to one toroidal coil. Thereby, a current is generated in the material for which the conductivity measurement is sought, which current is directly proportional to the conductance of the material. This current induces a current in the second toroid. The strength of the current induced in the second toroid is directly proportional to the strength of the current induced in the material for which the conductivity measurement is sought. Electrodeless conductivity sensors are preferred because they avoid some problems associated with contacting electrode conductivity sensor. Contacting electrode conductivity sensors can give inaccurate readings due to polarization of the electrodes. Electrodeless conductivity sensors avoid the problem of polarization. Applicants have discovered that the conductivity of foams suitable for use in binding loops of yarn to a primary backing varies in a generally linear relationship with the density of said foams. This relationship is generally linear within the density ranges and temperature ranges desirable for application of suitable foams to carpet structures. When a styrene/butadiene copolymer latex is employed as the material to be foamed, the conductivity of foam formed varies in a generally linear relationship with density within the density range of from 20 to 95 volume percent gas incorporation based on total foam volume. The signal from conductivity sensor 15 travels through lead 16 into translation means 17. Translation means 17 may translate the signal received from conductivity sensor 15 into a number of different readings. For example, translation means 17 can convert the signal into an output reading of conductivity. Based on the output reading of conductivity, the foam density can be altered to achieve a desired degree of conductivity. Alternatively, the signal received by translation means 17 can be displayed directly as an output of voltage or current. In one preferred embodiment of the present invention, based on the signal received from conductivity sensor 15, translation means 17 generates a signal which travels along lead 18 to gas volume control means 19. Based on the signal received from translation means 17, gas volume control means 19 either increases the amount of gas flowing to means 10 thereby lowering the density of the foam or decreases the volume of gas flowing to means 10 thereby increasing the density of the foam. Since the conductivity of the foam varies with the density of the foam, the change in the density of the foam causes a change in the signal received by translation means 17 from conductivity sensor 15. Translation means 17 continues to alter the volume of gas supplied through gas volume control means 19 until receiving the desired signal from conductivity sensor 15. It is understood that control of gas volume control means 19 can be done electronically through translation means 17 or if translation means 17 simply outputs a conductivity reading or an output voltage or current, gas volume control means 19 may be adjusted manually to achieve a desired conductivity reading or output voltage or current reading which is effected by the lowering or raising of the density through the increase or reduction of the volume of gas contained in the foam. Those skilled in the art will recognize a wide variety of means suitable for use as gas volume control means 19. Examples of such means are valves, and research control valves. Another aspect of the invention concerns a method for controlling the density of a foam. The method comprises measuring the conductivity of the foam. The conductivity of the foam is then adjusted by altering the density of the foam until the desired conductivity is achieved, thereby achieving the desired foam density. Practice of the method of the present invention requires the use of foams the conductivity of which varies with the density of the foam. By being able to determine the conductivity of the foam when said foam possesses the most desirable density, one is able, through the practice of the present invention, to control the density of the foam simply and predictably. It is a matter of simple experimentation to determine the most desirable density for the foam and to determine the conductivity thereof. Once these values are determined it is simple, through the practice of the present invention, to directly or indirectly, manually or electronically, control the density of the foam. Moreover, if it is desirable to quickly change the desired foam density it is easy to change the targeted desirable conductivity for the foam to the level of conductivity predetermined to be possessed by a foam having the desired density. The present invention is illustrated in further detail by the following example. The example is meant to be illustrative only and is not meant to limit in any manner, the scope of the invention as set forth in the following claims. All parts and percentages are based on weight unless specifically noted. Example In the following example, the material to be foamed was formed by blending a latex of a copolymer of styrene and butadiene (60/40), a calcium carbonate filler, surfactants, and thickeners. The latex of the copolymer of styrene and butadiene had a solids content of 52 weight percent based on total latex weight. The latex was blended with the calcium carbonate filler, surfactants, and thickeners to produce the material to be foamed which had a solids content of 82 weight percent based on total weight of material to be foamed. The material to be foamed was foamed in a Texicote foam machine. It had varying amounts of air incorporated therein as set forth in Table I. The foamed material exited the foam head of the Texicote foam machine and passed through an enclosed pathway. Present in the enclosed pathway was an electrodeless conductivity sensor manufactured by Foxboro and sold under the trade designation Foxboro® 1210 Electrodeless Conductivity Sensor. The electrodeless conductivity sensor output a signal to a control device commercially available from Taylor under the trade designation Taylor TM Controller. The electrodeless conductivity sensor output a signal related to the conductivity of the foam in the enclosed pathway. The conductivity of the foam was measured in millisiemens. Based on the signal received from the electrodeless conductivity sensor the control device output a signal to a control valve which controlled the amount of air provided to the foam head. The amount of air provided to the foam head was adjusted until the desired conductivity signal was output by the electrodeless conductivity sensor. This resulted in a foam having a desirable density which density was controlled within a narrow range of deviation. After passing past the electrodeless conductivity sensor, the foam passed through the remaining enclosed pathway and was applied to a carpet structure. The amount of air incorporated in the foam was varied and the density of the foam and conductivity of the foam was determined. The results appear in Table I. Table I indicates that as the density and percent air vary so does the conductivity of the foam.

Claims: A method of applying a foam coating to a carpet structure characterised in that the density of the foam is maintained at a predetermined level by: (a) measuring the electrical conductivity of the foam during its preparation and prior to deposition of the foam on the carpet structure; and (b) adjusting the density of the foam in response to the measured electrical conductivity. A method as claimed in Claim 1, wherein the conductivity of the foam is determined by an electrodeless conductivity sensor. A method as claimed in any one of the preceding claims, wherein the foam binds loops of yarn to a primary backing material. A method as claimed in any one of the preceding claims, wherein the foam is formed from a latex comprising a copolymer of styrene and butadiene, said latex having a solids content of from 45 to 90 weight percent based on total latex weight. A method as claimed in Claim 4, wherein the copolymer of styrene and butadiene comprises from 40 to 75 weight percent styrene and from 25 to 60 weight percent butadiene, based on total styrene/butadiene copolymer weight. A method as claimed in any one of the preceding claims, wherein the density of the foam is adjusted by varying the amount of gas incorporated into said foam. An apparatus for applying a foam coating to a carpet structure comprising: (a) means for generating the foam; (b) an application outlet suitable for applying the foam to the carpet structure; and (c) an enclosed pathway connecting the means for generating the foam and the application outlet, said enclosed pathway being suitable for conveying the foam from the means for generating the foam to the application outlet, characterised in that the apparatus further comprises: (d) an electrical conductivity sensor located in the enclosed pathway, said electrical conductivity sensor being capable of measuring the electrical conductivity of the foam in the enclosed pathway; and (e) means for altering the density of the foam in response to said measured electrical conductivity. An apparatus as claimed in Claim 7, wherein the electrical conductivity sensor located in the enclosed pathway is an electrodeless conductivity sensor. An apparatus as claimed in Claim 7 or Claim 8, wherein the means for generating the foam is capable of forming a foam from a latex comprising copolymer of styrene and butadiene, said latex having a solids content of from 45 to 90 weight percent based on total latex weight. An apparatus as claimed in any one of Claims 7 to 9, wherein said means for altering the density of the foam controls the amount of gas incorporated into said foam. An apparatus as claimed in Claim 10, wherein said means for altering the density of the foam is a gas flow control means in a gas inlet pipe to said means for generating foam.

Inventors: KERNSTOCK, JOHN M., KOTOYE, FEMI O.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: G05D11
IPC codes: B01F15, D06M17, D06N3, B05B7, G05D11, B32B5, D06M23, B05D1, B05D5