WO2000034020A1

WO2000034020A1 - Method and apparatus for improving the stability of a latex froth

Info

Publication number: WO2000034020A1
Application number: PCT/US1998/027450
Authority: WO
Inventors: Nicholas S. Hanlon; David L. Wilkinson
Original assignee: The Dow Chemical Company
Priority date: 1997-09-02
Filing date: 1998-08-31
Publication date: 2000-06-15
Also published as: CA2311046A1; AU2092799A

Abstract

This invention discloses a method for improving the uniformity of a latex compound froth, such as that used on carpet backing, by controlling the temperature of the latex prior to frothing.

Description

Method and Apparatus for Improving the Stability of a Latex Froth

This invention relates to a method for frothing a latex compound.

Frothed latex compound is known to be useful for coating fabric substrates. For instance, in carpet manufacturing, frothed latex compound is used as an adhesive to attach carpet backing to the carpet top layer. The latex compound used to prepare carpet substrate is typically highly filled and comprises a latex and additives, such as fillers, froth aids, and thickeners.

In processes for preparing carpet backing for attachment to the carpet top layer structure, the froth is typically dispensed directly onto the carpet backing substrate as the fabric is conveyed past the dispensing apparatus. Yet it may be difficult to produce a froth which has a relatively uniform consistency, either as it is dispensed or as it cures or dries, since several parameters may affect the froth consistency. In particular, the parameters of froth density, froth viscosity, and froth mechanical stability are key factors for the uniformity of the froth. Froth density may be used to control the weight of the frothed latex compound coating which is applied to the fabric substrate, and froth viscosity may be used to control the placement of the frothed latex compound on the fabric substrate and also assist in controlling the coating weight.

If the froth viscosity is less than desirable, then problems may arise. For instance, when froth viscosity is too low, the froth mechanical stability may be less than desirable. The mechanical stability can be improved by adding more surfactant. Since increasing the amount of surfactant may have the effect of decreasing froth viscosity, then more thickener may need to be added to compensate. As a result, the correlation between froth viscosity and froth density may be less than desirable.

One method that has been used to increase the uniformity of the froth has been to increase the froth viscosity by injecting additional thickeners into the latex compound while it is being frothed as described in U.S. Patent 4,205,112. This method does not, however, desirably improve the correlation between density and viscosity. Additionally, several problems do arise from increasing viscosity by increasing the amount of thickener added to the latex compound. For instance, such thickeners increase the production cost because more raw material is required and because a complex pumping system may also be required. Increasing the amount of thickener may also cause solubility problems since such thickeners may not be completely soluble in the latex compound in the amounts needed to achieve the desired effect. Finally, if too much thickener is added, then the resistance to flow may become so great that the flow of the latex compound will be inadequate for practical production use.

It would be desirable to find a frothing process which controls viscosity but avoids the problems arising from increased amounts of thickeners and has an improved correlation between density and viscosity.

The present invention is a method for frothing a latex compound comprising:

(a) adjusting the temperature of the latex compound; and (b) mixing the latex compound with a gas in a mixing zone under conditions sufficient to provide a froth containing about 30 to 90 volume percent gas;

wherein the temperature of the latex compound is in the range of about 9° to 27°C and is selected to result in the froth having a desired stability.

When using a latex compound froth for coating applications, the ability to control and produce high viscosity in the latex compound adds to the versatility of the coating. This is particularly desirable in the adhesive coating of fabric substrates such as in the primary backing of a carpet substrate for securing a secondary fabric backing to it or to lock tufts in position in the formation of carpet.

It has been discovered that controlling the viscosity of a froth by controlling the temperature of a latex compound provides a means to produce a stable latex compound froth without increasing the surfactant level and without creating the problems associated with increasing the amount of thickener.

Several advantages do arise from controlling froth viscosity by controlling the temperature of the latex compound. First, production costs are decreased, by reducing both material and clean-up costs, since the amounts of thickener and surfactant can be reduced in the latex compound. Raw material costs are reduced because decreased amounts of thickener and surfactant are required. Clean-up costs are also reduced because the stored latex compound has a lower viscosity, which provides easier pumping and flow through process equipment, such as pipes and filters, and reduces clean-up costs of the storage containers for the latex compound. Second, the coating's sensitivity to water is decreased since higher amounts of surfactants and thickeners decrease the water resistance of the final latex compound- containing coating.

Third, the froth produced is more consistent from batch to batch. For example, high temperatures often necessitate increasing amounts of surfactant and thickener to improve uniformity. By varying the amounts of these additives, the froth may be inconsistent from batch to batch, but the process of this invention does not require an increased amount of thickener to increase viscosity or an increased amount of surfactant to increase the volume percent gas. The use of latex compound at high temperatures is not uncommon. For instance, the latex or additives may be at an elevated temperature such as when the filler is used immediately after being ground, or when the latex is freshly made, or when the additives are stored at elevated temperatures. In particular, the latex and filler are often used at elevated temperatures in the summer. Thus, by controlling temperature, seasonal variability and batch-to-batch variability are reduced.

Fourth, consistency from batch to batch also increases because the latex compound has an increased ability to keep filler in suspension during the agitation, or frothing, process, wherein the latex compound and gas are mixed to produce a froth. This reduces the variability in the carpet coating process and minimizes filler phase separation in transfer lines and onto coating equipment. By reducing filler phase separation, the invention allows higher solids content latex compounds to be employed. Finally, by using temperature, instead of chemical additives, to control froth viscosity, density is also controlled by improving the mechanical stability. This improved correlation allows viscosity to be used as an indicator for froth uniformity and stability.

By improving froth uniformity, stability, and batch-to-batch consistency, both the application of the frothed latex compound and the physical characteristics of the coated substrate improve. These and other advantages of the invention will be apparent from the description which follows.

Figure 1 illustrates a preferred process for preparing a latex compound froth.

The process of the invention involves mixing a latex compound with a gas.

Any gas capable of incorporation into the latex may be used. For economy and safety, the preferred gas is air. Other examples of suitable gas include any material which is in the form of a gas at the pressure and temperature at which the process is carried out such as, for example, nitrogen and carbon dioxide.

The term "latex compound" as used herein refers to a composition comprising latex and additives such as, for example, fillers, surfactants, thickeners, and flame retardants. This term is widely used in the carpet industry.

Latexes suitable for use in the process include suspensions of elastomeric and flexible polymers which have good film formation under drying conditions suitable for carpet production, such as at temperatures between 120° and 235°C, and preferably between 150° and 205°C, and which, upon being mixed with a gas, are capable of providing a froth containing from 30 to 90 volume percent gas. Preferably, the latex compound froth contains from 40 to 80 volume percent gas.

Examples of latexes include suspensions of polymers made from one or a combination of the following monomers: styrene; butadiene; unsaturated carboxylic acids, such as acrylic, itaconic, and fumaric acids; vinyl chloride; vinylidene chloride; vinyl acetate; olefins, such as ethylene, propylene, butene, and octene; acrylics; polyurethanes; natural rubber; neoprene and mixtures or combinations of these. Many of these polymers may be made by the emulsion interpolymerization of a conjugated diene, such as butadiene, with an ethylenically unsaturated functional monomer such as an α,β-unsaturated carboxylic acid, unsaturated dicarboxylic acids, mono-esters of such dicarboxylic acids, acrylamides, and N- methylolacrylamines. In addition to the conjugated diene and the functional monomer, the polymerization mixture may also contain a secondary copolymerizable monomer such as styrene, acrylonitrile, methyl methacrylate, or vinylidene chloride. Mechanical or artificial suspensions, dispersions or emulsions of polymers in water may also be employed as the latex of the latex compound.

Examples of latexes suitable as an adhesive for carpet backing include suspensions of copolymers 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; and polymers such as polybutadiene; and poly(vinylidene chloride). Most preferably, the latex comprises a suspension of a copolymer of styrene and butadiene. Preferably the latex comprises a suspension of a copolymer of from 40 to 75 weight percent styrene and from 25 to 60 weight percent butadiene. Most preferably, the latex compound to be mixed with a gas is suitable for use as an adhesive for carpet backing. Such latex compounds are well-known in the art and many of the above- described latexes are commercially available.

Suitable additives for the latex compound include fillers, surfactants, thickeners, and flame retardants. Examples of fillers include calcium carbonate, ground limestone, calcium sulfate, pigments, and fly ash. Fillers may be added in amounts of up to about 1000 parts per hundred parts latex, commonly known in the art as parts per hundred rubber (phr), on a dry basis. These materials provide secondary properties such as opacity, fire retardance, stiffness, and color. Examples of thickeners include polyacrylates, cellulosics, clays, and gums. Examples of surfactants include sulfated or ethoxylated alcohols or fatty acids, sulfosuccinates, and sulfosuccinimates. Examples of flame retardant additives include aluminum trihydrate, calcium sulfate dihydrate, antimony oxide, halogen-containing compounds, magnesium hydroxide, and organophosphates.

The latex compounds typically used have a solids content of at least about 40 weight percent, preferably from 45 to 90 weight percent, based on total weight of latex and additives.

Figure 1 illustrates a preferred process for preparing a latex compound froth comprising a mixture of a latex compound and air. Compound storage tank 10 contains a latex compound suitable for carpet backing. Compound storage tank 10 has feed conduit 11 which is connected to the intake side of circulating pump 12 which pumps the latex through filter 13, through conduit 14, and through a means for adjusting the temperature of the latex, such as heat exchanger 15. Heat exchanger 15 adjusts the temperature of the latex. The latex compound is further pumped through conduit 16 and into mechanical foaming head 19 wherein compressed air is delivered from air compressor 17 through line 18, and the latex compound and air are subjected to high speed agitation. Mechanical foaming head 19 is a mixing zone for generating a frothed mixture of latex and gas. The frothed mixture of latex compound and air exits through enclosed pathway 20 connected to application outlet 21.

To measure froth viscosity, the froth may be collected as it exits application outlet 21. The froth viscosity may be measured by a rotational viscometer. Accordingly, the temperature of the unfrothed latex compound may be controlled by a means for adjusting the temperature, such as, for example, heat exchanger 15, to obtain a predetermined desired froth viscosity. An alternative method may involve monitoring the froth viscosity by an in-line viscometer placed along enclosed pathway 20. There may also be a means for altering the temperature of the heat exchanger in response to the measured viscosity. Automated process control techniques, which are well-known to those skilled in the art, may be employed in this embodiment of the invention.

Means for adjusting temperature include heat transfer equipment which may be designated by type, such as fixed tube sheet or outside packed head, or by function, such as chiller, condenser, heat exchanger, or cooler. Preferably, the means for adjusting temperature is a heat exchanger. More preferably, the heat exchanger is a single pass shell-and-tube type.

The latex compound is pumped into mixing zone 19, which is a means for combining a latex compound with a gas to form a froth, and the latex compound and gas are mixed under conditions sufficient to produce a froth. Mixing zones 19 for generating the frothed mixture are well-known in the art. Such mixing zones suitably comprise mechanical means for incorporating a desirable amount of gas into a to-be-frothed latex compound.

The frothed mixture exits mixing zone 19 through enclosed pathway 20. The enclosed pathway, or conduit, leads to application outlet 21. Any enclosed pathway capable of conveying the mixture from mixing zone 19 to application outlet 21 is suitable for use in the present invention. Any application outlet capable of directly or indirectly applying the mixture to its intended use is suitable for use in the present invention. In one preferred embodiment wherein the mixture is suitable as an adhesive for carpet backing, application outlets suitable for use include a nozzle or manifold capable of applying the mixture directly to a carpet structure and an application nozzle capable of discharging the froth into a coating pan for indirect application to the carpet structure. To measure the viscosity of the frothed mixture of latex compound and gas, a sample of the mixture may be collected as it exits application outlet 21.

The viscosity of the frothed mixture is measured by conventional means, such as by a rotational viscometer. The measured viscosity is the input used to determine whether a temperature adjustment of a latex compound that is subsequently fed into mixing zone 19 is needed.

The temperature of a latex compound subsequently fed into mixing zone 19 is adjusted to provide a frothed mixture of the latex compound and gas with a desired viscosity. The desired viscosity is that which optimizes the desired physical properties of the coated substrate. The viscosity, as measured by a rotational viscometer at 20 rpm with a Brookfield RV #5 or #6 spindle, suitably is between about 8,000 and 30,000 centipoise, preferably is from 12,000 to 24,000 centipoise, and most preferably is from 15,000 to 20,000 centipoise. By adjusting the temperature of the latex compound that is subsequently fed into the mixing zone, the viscosity is modified. For instance, by cooling the latex compound prior to mixing the latex compound with a gas in mixing zone 19, the viscosity of the frothed mixture increases. Conventional means for adjusting the temperature may be used, such as means of heat exchange as described hereinabove. Preferably the temperature of the mixture is adjusted to be between about 9° and 27°C, more preferably from 18° to 24°C.

In addition to measuring viscosity, a mechanical stability test, using density as an indicator, may also be conducted on the froth to gauge its stability. "Mechanical stability" for the purposes of the present invention is measured by the following test. A sample of froth is collected from application outlet 21 , the froth density is measured, and then the froth is tested for mechanical stability within 30 minutes, preferably within 20 minutes, and more preferably within 5 minutes of the froth exiting application outlet 21. The mechanical stability test involves subjecting the froth sample to high shear agitation for one to two minutes in a means for mixing. An example of such means is a Cowles Blade mixer. If the froth is stable, the froth density after mixing remains substantially equivalent to the froth density before mixing. Preferably, the froth density after mixing increases by no more than 10 percent as compared to the froth density before mixing. More preferably, the froth density after mixing increases by no more than 5 percent.

The process of the invention is useful for producing an adhesive coating for securing a secondary fabric backing to a carpet substrate and for locking tufts for carpet into position.

The present invention is illustrated further by the following examples. The examples of the invention are meant to be illustrative only and are not meant to limit in any manner the scope of the invention as set forth in the following claims. All parts and percentages are by weight unless otherwise specified.

Comparative Example 1

A latex compound was prepared from 600 phr carpet grade finely ground white

CaCO₃ filler, LXC 803F NA carpet latex (available from The Dow Chemical Company), 1.9 phr SCT 801 brand surfactant (available from Southern Chemical & Textiles, Inc. of Dalton, GA), and 1.2 phr PARAGUM 265 thickener (available from Para-Chem Southern, Inc. of Dalton, GA). The formulation is given in phr, known in the art as parts per hundred rubber, on a dry basis. The latex compound had a solids content of 84.5 weight percent based on the total weight of the latex compound. The latex compound at 25.6°C (78J°F) was introduced to a mixing head and was mixed with air to produce a latex compound froth, hereafter referred to as froth, which contained about 73 volume percent air and had a temperature of 28.3°C (82.9°F). The froth passed from the mixing head via a conduit and exited through an application outlet.

The froth viscosity was measured with a Brookfield RV viscometer using a #6 spindle at 20 rpm measured at 30 seconds. The viscosity was 1 1 ,000 centipoise, which was less than desirable.

The mechanical stability of the froth was tested within 5 minutes of exiting the application outlet by subjecting the froth to shear agitation. The froth density was measured before agitation and was 0.232 g/cm³, and again after 1 minute of agitation and was 0.290 g/cm³. The change in the froth density indicated that the mechanical stability of the froth of 75 percent (calculated as 0.75 = 1 -(0.290-0.232)/0.232) was less than desirable. The lack of stability was also confirmed by two additional observations. Severe plate-out of the filler in the latex compound was observed, for example, plate out of the filler on the inside of the conduit between the mixing head and the application outlet. Second, the appearance of the froth was non-uniform as evidenced by uneven bubble sizes and collapse of the bubbles in the froth.

Example 2

The procedure of Comparative Example 1 was repeated except that the latex compound was passed through a heat exchanger before it entered the mixing head. The compound temperature prior to the heat exchanger was 25.6°C (78J°F). After exiting the heat exchanger, the latex compound, at a temperature of 20°C (68J°F), entered the mixing head and was mixed with air.

The froth viscosity was 21 ,000 centipoise. The viscosity had improved in comparison to Comparative Example 1. Thus, by decreasing the temperature of the latex compound, the viscosity of the froth increased. The mechanical stability of the froth was tested within 5 minutes of exiting the application outlet by subjecting the froth to high shear agitation. The froth density was measured before agitation and was 0.232g/cm³, and again after 1 minute of agitation and was 0.232g/cm³. The change in the froth density indicated that the mechanical stability of the froth of 100 percent was improved dramatically versus that found in Comparative Example 1. Thus, as the temperature of the latex compound decreased, the froth mechanical stability increased. There was no evidence of filler plate out and the appearance of the froth had uniform bubble structure. The latex compound of this Example was observed to be more uniform than that of Comparative Example 1.

Claims

CLAIMS:

1. A method for improving uniformity of a froth comprising:

(a) adjusting the temperature of a latex compound; and

(b) mixing the latex compound with a gas in a mixing zone under conditions sufficient to provide a froth containing about 30 to 90 volume percent gas;

wherein the temperature of the latex compound measured after it passes through the mixing zone is in the range of from 9° to 27°C and is selected to result in the froth having a predetermined desired mechanical stability.

2. The method of Claim 1 wherein the froth contains from 40 to 80 volume percent gas.

3. A method for improving uniformity of a latex compound froth comprising:

(a) mixing a latex compound with a gas in a mixing zone under conditions sufficient to provide a froth;

(b) measuring the viscosity of the froth; and

(c) comparing the measured viscosity of the froth to a predetermined desired viscosity value;

(d) if the measured viscosity is not a predetermined desired viscosity, adjusting the temperature of the latex compound that is subsequently fed into the mixing zone in response to the measured viscosity, to produce a froth having the predetermined desired viscosity.

4. The method of Claim 3 wherein the froth contains 30 to 90 volume percent gas.

5. The method of Claim 3 wherein the froth comprises from 30 to 90 volume percent gas and has a mechanical stability of at least 90 percent, without changing the composition of the latex compound which is subsequently fed into the mixing zone.

6. The method of Claim 5 wherein the froth has a mechanical stability of at least 95 percent.

7. A method for applying froth to a carpet backing, the method comprising:

(a) adjusting the temperature of a latex compound; and

(b) mixing the latex compound with a gas in a mixing zone under conditions sufficient to provide a froth containing 30 to 90 volume percent gas;

(c) depositing the froth onto carpet backing;

wherein the temperature of the latex compound is in the range of from 18 to 24⁹C and is selected to result in the froth having a predetermined desired mechanical stability.

8. A method for improving the uniformity of a froth of a gas and a latex compound, the method comprising: controlling the mechanical stability of the froth by adjusting the temperature of the latex compound to be mixed with the gas.

9. An apparatus for preparing and dispensing a latex compound froth comprising:

(a) a means for adjusting the temperature of a latex compound;

(b) a means for combining the latex compound with a gas to form a froth, said means being connected to the temperature-adjusting means by a conduit; and

(c) a means for dispensing the froth connected to the means for combining the latex with the gas by an enclosed pathway.

10. The apparatus of Claim 9 wherein the means for adjusting the temperature comprises a heat exchanger.