GB2135675A - Making spread elastomeric foam by the no-gel method - Google Patents

Abstract

A spread foam is made by the no-gel method. In the process, a conventional no-gel foam latex composition, comprising the latex of an elastomeric polymer, is frothed and a thickener latex is added and mixed with the latex composition at the time of frothing; the thickened composition is then spread, for example, on a textile substrate, carpet back or as an unsupported foam, dried and, if desired, cross-linked. The injection of the thickener latex enables a high density foam of good quality to be produced from compositions which could otherwise only produce foams of low and medium densities.

Description

SPECIFICATION Process for the preparations of spread elastomeric foam by the no-gel method This invention relates to a process for the preparation of a spread foam by the no-gel method from a frothed aqueous composition comprising an aqueous latex of an elastomeric polymer.

Throughout the specification the following terms are to be accorded the following meanings:-- a "latex" is a natural rubber latex or an aqueous emulsion or dispersion of a synthetic polymer which has been prepared by any procedure and includes emulsions and dispersions prepared by emulsion polymerisation; a "polymer" is a macromolecular substance and the term includes within the scope "homopolymers" "copolymers", "terpolymers", "graft polymers" and the like; "elastomer" has the meaning given on pages 446 to 640 of Kirk-Othmer "Encyclopedia of Chemical Technology" Vol. 8, 3rd Edition, (John Wiley 8 Sons 1979) and includes rubber and rubber-like polymers; "cross-linking" includes curing and vulcanisation; a "froth" is a composition which comprises an aqueous latex which has been mixed with air while the composition is still in a substantially wet condition; a "foam" is a solid substantially dry product obtained by drying a froth; and "gel", "no-gel" and "gelation" carry the meanings derived from the documents cited below as examples of related art.

A recent brief review of the art of coating the backs of tufted carpets with a layer of spread foam may be found under the heating "Foam Backing" on pages 80 to 91 of the book entitled "Polymer Latices and their Applications" edited by K. O, Calvert, published by Applied Science Publishers, 1982.

The relative advantages of the gel and no-gel methods of foam backing are discussed therein.

Attention is drawn to the concluding comment, reading as follows: "The domestic market (the author is referring to the United Kingdom) is served by all systems but this is where the major advantage of nongel foam technology comes into its own, particularly for carpet qualities at the low to middle end of the market".

It may be deduced from this comment that a need exists for an improvement in no-gel foam technology which would facilitate production of no-gel foam backings having a quality better than that available at the present day.

The process of the present invention is particularly suitable for the preparation of so-cailed high density foam backings for tufted carpets, such as those which are specfied for contract use, as distinct from the lower quality backings intended for domestic use, to which the statement quoted above refers. A special example of such high quality foam backings are the very high density foam backings which are applied to a thickness of 3 mm. or less and which are often known as "unitary backings".

An impression of the range of foam qualities manufactured can be obtained from "Specification for Foam Rubber Backings for Carpets-BCMA 1076", issued by the British Carpet Manufacturers Association, 1976, which states that the minimum thickness, minimum mass/unit area and minimum density of the foams shall be as follows: Heavy Medium Light Duty Duty Duty Minimum thickness mm. 3.2 3.2 3.2 Minimum mass/unit area g./m2. 1040 815 545 Minimum density g./cm3. 0.270 0.215 0.145 Although the process of the present invention may be used for the preparation of medium and light duty foam backings for carpets having properties which conform with those listed in the above table, it is particularly suitable for the preparation of heavy duty foam backings.It is not intended to suggest, however, that all the other specifications of BCMA 1076 for heavy duty foam backings will necessarily be met as a result of making use of the process of the invention, nor to suggest that the usefulness of the process is limited to carpet backing. The process may be applied in the preparation of any product for which a no-gel process yields satisfactory foam properties such as apparel textiles and unsupported foam sheets from which the backing support has been removed after drying, such as underlays for carpets and for other floor coverings.

The range of densities of the foam which is the product of the process of the present invention will be defined by and depend upon the range of densities of the intermediate froth. A suitable range of froth densities for carpet backing is 400 to 1 ,200 g./l., a preferred range being 550 to 900 g./l. For the preparation of high density foam backings for carpets such as those discussed above, a suitable thickness of the spread wet froth is in the range 0.1 to 6 mm., a preferred range being 0.5 to 3 mm.

The preparation of polymeric foams by the no-gel method embodying the step of rapid drying in a current of air at high temperature was first described in British Patent Specification No. 1 ,1 05,538 when it was disclosed that the no-gel method was applicable to these dispersions of polymers which are film-forming at temperatures below 300C F (1 490C.), such as the aqueous latices of such elastomeric polymers as carboxylated and non-carboxylated butadiene polymers, polychloroprene latex and natural rubber latex.

The preparation of foams by the no-gel method was also disclosed in British Patent Specification No. 1 267,458, which described the advantages of using a froth stabilisation agent comprising a mixture of a non-ionic surfactant and a fatty alcohol or amide. The Examples describe the preparation of foams from the latices of an elastomeric acrylic polymer, a carboxylated butadiene polymer and centrifuged natural rubber latex. The latices of such elastomeric polymers are typical of those to which the process of the present invention may be applied.

A review of the prior published art on the subject of the preparation of spread foams from the latices of elastomeric polymers by the no-gel method suggests the existence of certain difficulties and limitations, which may hinder the applicability of the method under a wide range of operating conditions such as may be encountered under practical manufacturing conditions. Thus, for example according to British Patent Specification No. 1 285,631, which prescribes the use of certain amphoteric emulsifiers as froth stabilisers, the highest dry foam density recorded corresponds to a value of 0.34 g./cm3. (Table IV, second sample listed). The highest wet froth density shown in the Examples is 390 gel., yielding a foam density corresponding to 0.274 g./cm3.. (Table II, column 6).

The process described in the latter Specification is said to be suitable for natural rubber latex and for the latices of synthetic elastomers such as styrene-butadiene copolymers. According to British Patent Specification No. 1 ,285,706, however, published on the same day, the no-gel method performs satisfactorily with natural latex but not with the latex of a synthetic styrene-butadiene copolymer. In the latter case difficulties arise if the latex composition is stored before frothing. This difficulty is overcome by inclusion in the formulation of an alkali metal polyphosphate. The preferred solids content of the latex composition is not less than 70 wt.%, the example showing the use of a composition having a solids content of 78 wt.%. The wet froth density corresponded to 1 68 g./I. and the foam density to 0.202 g./cm3..

According to British Patent Specification No.1,558,162, a latex foam product can be prepared from a latex composition having a solids content of at least 82 wt.%, the wet froth density being restricted to the range 100 to 400 g./l., equivalent to a dry foam density of 0.09 to 0.3 g./cm3.

This review of some of the publications of the prior art shows that there is a need for an improved process for the preparation of no-gel foam. Such a process should be more versatile, in that it should be capable of yielding foams of good quality over a wide range of operating conditions, and should be capable of yielding foams of different qualities as desired, preferably from the same latex composition by means of convenient changes to the operating conditions.

In accordance with the present invention there is provided a process for preparation of spread foam by the no-gel method, wherein a conventional no-gel foam latex composition, comprising the latex of at least one elastomeric polymer, is frothed, and a thickener latex is added and mixed with the latex composition at the time of frothing whereafter the resulting thickened composition is spread, dried in hot air, and cross-linked if so desired. Preferably, the mixing of the thickener latex with the nogel foam latex is carried out in the same equipment as that which is used for frothing.

Although it has been proposed to inject a thickener latex into a frothed latex composition, it has not previously been proposed to use the process of thickener injection for the preparation of spread foam, either by the gel or non-gel method.

We have found that the process of the present invention is capable of providing high density spread foam products of excellent quality and to this end the thickened composition may be spread on a textile, on the back of a carpet or on a temporary support to provide an unsupported foam. Moreover such products can be prepared from latex compositions which are readily and economically prepared and handled, and which if need be can also be used to prepared foams of medium and low density without change of formulation. Thus the present process not only renders possible the preparation of no-gel foam of excellent quality, but also enables this result to be achieved by means of a versatile and convenient process of wide applicabi!ity.

British Patent Specification No. 2,038,237, which derives its priority from the same application as United States Patent Specification No.4,205,112, describes a process for producing textile laminates using a frothed latex composition, wherein a low viscosity latex composition is mixed with air and with a polymeric thickener latex in a high speed mixing zone. The frothed composition is then continuously spread on a textile substrate, a second textile layer being applied to the coated material in order to form a laminate. In such a process the purpose of using a frothed, rather than an unfrothed, latex composition is to secure better control of the rate of spread, the degree of wetting of the fibres, the degree of penetration into the interstices between the fibres etc. One of the results is an improvement in the bond between the layers in the laminate. At the moment of lamination the froth collapses, and it was an unexpected and valuable discovery that the process of thickener latex injection can create an outstanding improvement to the process of preparation of a no-gel spread foam.

The choice of thickener latex to be employed in the present process will be determined by various considerations including the properties of the latex composition which is to be frothed. The thickener latex is preferably one which exhibits a relatively low viscosity at a pH value less than about 7, and which even at a low concentration in water will increase rapidly in viscosity when the pH is raised to a value over about 7, forming a viscous solution or dispersion of a colloidal nature. Such an effect is generally due to the presence of combined carboxylic groups.Suitable thickener latices may be prepared by emulsion polymerisation of one or more ethylenically unsaturated carboxylic acids with other copolymerisable monomers, and include those referred to or described in British Patent Specification No. 2,038,237 and in United States Patent Specifications Nos. 4,205,112, 3,366,584 and 2,657,175. A preferred class of thickener latices are those formed by copolymerisation of one or more neutral lower alkyl acrylate or methacrylate esters and acrylic and/or methacrylic acid, such as copolymers derived predominantly from ethyl acrylate and acrylic and/or methacrylic acid.

The pH of the latex composition to be frothed preferably lies within the range of 8.5 to 12.0 before frothing. Its viscosity before frothing preferably lies within the range of 100 mPas to about 10,000 mPas, and is most preferably about 2,000 mPas, measured on a Brookfield viscometer model LVT with spindle 4 at speed 60. The solids content is preferably from 60 wt.% to 85 wt.%, and is preferably about 72 wt.%.

When the thickener latex is rapidly mixed with the latex composition, the viscosity of the mixture rapidly increases. As was stated previously, it is preferred that the thickener latex should be added to the latex composition in the frothing apparatus, such as through an injection nozzle. If necessary, a basic substance may also be injected at the same time at a rate sufficient to adjust the pH to the desired value for thickening to occur. It will be appreciated in any case that this method of operation is highly convenient for commercial scale manufacture, in that a relatively fluid latex composition may be prepared as a batch, and then continuously delivered to the frothing apparatus, from which the thickened and frothed composition is delivered to the equipment in which spreading is carried out as a continuous operation.

As was indicated previously, the elastomeric polymer latex to be used in the present process may be derived from one or more unsaturated hydrocarbons. The polymer may be that of a butadiene polymer containing copolymerised carboxylic acid groups, such as the products sold commercially as "Revinex" 34D10 and "Revinex" 90W10 (Revinex is a registered Trade Mark); or that of a butadiene/styrene polymer, such as the products sold commercially as "Intex" 131 ("Intex" is a registered Trade Mark) and "Polysar" 341 ("Polysar" is a registered Trade Mark); or an evaporated natural rubber latex, such as the product sold commercially as LCS "Revertex" ("Revertex" is a registered Trade Mark); or centrifuged natural rubber latex; or the latex of an elastomeric acrylate polymer.Other types of latices of elastomeric polymers are also suitable, those designated above being known to be in commercial use for the preparation of no-gel foam. Preferably the elastomeric polymer has been prepared by emulsion polymerisation.

A preferred specific example of a commercially available thickener latex is "Revertex" Al 91, which contains a copolymer of the acrylic class containing combined carboxylic acid groups, and which has a solids content of about 20 wt.%. The rate of addition of "Revertex" A191 is generally approximately 1 to 2 parts by weight per 100 parts by weight of the elastomeric polymer composition which is to be frothed, this rate of addition being suitable for metering when the thickener latex is injected into the frothing apparatus. It will be appreciated that the rate of addition of other thickener latices may be greater or smaller, depending on such considerations as their solids contents and the chemical composition of the thickener latex polymer.For example, the rate of addition may range up to 10 parts by weight per 1 00 parts by weight of the elastomeric polymer composition, and the solids content of the thickener latex may vary from 5 to 50 wt.% at the time of addition.

The constituents of the no-gel foam latex composition are conventional, and may include mineral fillers, thickening agents, reinforcing polymers in latex form, frothing agents and froth stabilisers, dispersants, antioxidants, reodorants, crosslinking agents, and so on.

The proportion of mineral filler in the latex composition is that conventionally used for the preparation of no-gel foam i.e. from 50 to 300 parts by weight of ground calcium carbonate per 100 parts by weight elastomeric polymer, a preferred range being from 140 to 200 parts by weight.

The frothing agents used are those appropriate to the elastomeric polymer latex. For example, a mixture of selected anionic and nonionic surfactants may be used with carboxylated butadiene polymer latices, sodium higher alkyl sulphosuccinamates may be used with butadiene/styrene polymer latices and with natural rubber latices, and so on.

It is generally desirable to crosslink the polymer during the step of drying the froth. Where the elastomer is a butadiene polymer, sulphur crosslinking agents may be present, but melamine/formaldehyde resins may be used if the elastomeric polymer contains coreactive groups. A further possibility is to employ the latex of a so-called self-crosslinking elastomeric polymer, which will crosslink on heating in the absence of external crosslinking agents.

As was indicated previously, the density of the wet froth preferably lies within the range of from 400 to 1200 g./l. this range being suitable for high density foam. A preferred range of density is 550 to 900 g./I. The thickness of the spread wet froth is preferably within the range of 0.1 to 6 mm. more preferably 0.5 to 3 mm.

The invention will now be illustrated but not limited by the following Examples: EXAMPLE 1 A latex composition was prepared according to the following formulation: Parts by Weight Including Volatiles Dry "Revinex" 90W10 (carboxylated latex) 177 100 Tetrasodium pyrophosphate 1.0 Carbon Black dispersion, 35 wt.% 4 1.4 Sulphur 2 Zinc dimethyldithiocarbamate 2 Zinc mercaptobenzthiazole 0.5 "Flectol" H 0.75 Zinc oxide 1 "Calmote" AD (ground calcium carbonate) 170 "Mobilcer" A (wax dispersion) 4 2 Sodium lauryl sulphate 3 1 "Empilan" KM50 (non-ionic surfactant) 1 "Celacol" HPM5000 (Methyl cellulose) 12 0.3 22% aqueous solution Water 16.8 "Revinex", "Flectol", "Calmote", "Mobilcer", "Empilan" and "Celacol" are all Registered Trade Marks.

The solids content of this composition was 72 wt.% and the viscosity 2000 mPas, measured on a Brookfield viscometer model LVT spindle 4 speed 60.

The composition was frothed using a conventional frothing machine, i.e. one manufactured by Cowie and Riding, with a 1 4 inch twin rotor. The wet density of the froth was 750 g./l. Using a metering pump a thickener latex, "Revertex" Al 91, was injected continuously into the froth mixing head via the inlet port provided. The thickener latex had a solids content of 20 wt.%, and the rate of addition was estimated at from 1 to 1.5 parts by weight per 1 00 parts by weight of the composition.

The froth was spread on the back of a tufted carpet to which had previously been applied a latex precoat, and the carpet was passed under infra-red lamps into an oven for drying and crosslinking of the polymer. An excellent foam backing was obtained, having a high density, smooth surface with no cracks or pinholes, and a fine-structured foam.

EXAMPLE 2 (Comparative) In a comparative trial, in which all the conditions were identical with those set out in Example 1, except that the injection of thickener latex was omitted, a layer of foam resulted having a very poor quality, and exhibiting extensive cracking of the surface.

EXAMPLE 3 (Comparative) In a further comparative trial, the same composition as in Example 2 was frothed to a lower density, namely 200 g./l., and applied to the back of a tufted carpet without thickener injection.

An excellent low density foam backing was obtained. Using the same formulation and varying the froth density between 200 and 400 g./l., it was possible to obtain foams having a low to medium density without injection of thickener latex into the frothing head. These foams possessed very good quality.

EXAMPLE 4 A latex composition was prepared according to the following composition: Parts by Weight Including Volatiles Dry "Intex" 131 (high solids uncarboxylated SBRlatex) 153.8 100 "Empimin" MKK (sulphosuccinamate) 13.0 4.55 Sodium hexametaphosphate 1.25 1.25 Sulphur 2.5 2.5 Zinc oxide 1.5 1.5 Zinc dimethyldithiocarbamate 1.0 1.0 Mercaptobenzthiazole 0.4 0.4 "Flectol" H 1.0 1.0 Carbon black dispersion 4.0 1.4 "Calmote" AD (ground calcium carbonate) 170.0 170.0 "Mobilcer" A (wax dispersion) 4.0 2.0 Sodium lauryl sulphate 3.0 1.0 Water 12.0 0.0.

"Empimin" and "Calmote" are Registered Trade Marks.

The solids content of this composition was 78% and the viscosity 4000 m Pas measured by a Brookfield viscometer model LVT spindle 3, speed 12 rpm.

The composition was frothed as in Example 1 to a wet density of 750 g/litre and a thickener latex was injected into the mixing head. As in Example 1 the froth was spread on the back of a tufted carpet to which had previously been applied a latex precoat and the carpet was passed under infra-red lamps into an oven for drying and crosslinking of the polymer. An excellent high-density foam backing was obtained having a smooth surface with no cracks or pinholes and a fine-structured foam.

EXAMPLE 5 (Comparative) A comparative trial was carried out in which all the conditions were identical with those set out in Example 4, except that the injection of thickener latex was omitted and the finished foam layer exhibited very poor quality with extensive cracking of the surface.

EXAMPLE 6 (Comparative) In a further comparative trial, the same composition as in Example 4, but omitting the thickener latex, was frothed to a lower density of from 200 to 400 g./l. and foam backings were obtained within the low to medium density range, possessing very good quality with no surface cracks.

These Examples show that high density foam backings can be prepared readily using the same latex composition which can also be employed to prepare low and medium density foams. Since all these foams can be of excellent quality, application of the process of the invention to the preparation of high density foams represents a valuable extension of the usefulness of the method of no-gel production of foamed products.

The tufted carpet backed with the high density no-gel foam prepared according to Example 1, was adhered to a floor using an aqueous adhesive. After drying it was found that the carpet could readily be removed from the floor by application of sufficient force, in such a way that the foam backing remained intact i.e. the floor was left clean. Since contract laid carpeting is required to be removed every few years for replacement, this particular property of a high density no-gel foam backing is desirable and its attainment represents an advance in the art.

Claims (24)

1. A process for the preparation of spread foam by the no-gel method, wherein a conventional nogel foam latex composition, comprising the latex of at least one elastomeric polymer, is frothed, and a thickener latex is added and mixed with the latex composition at the time of frothing, whereafter the resulting thickened composition is spread, dried and cross-linked, if so desired.

2. A process as claimed in Claim 1, wherein the thickener latex is mixed with the no-gel foam latex composition in the same equipment as that which is used for frothing the composition.

3. A process as claimed in Claim 1 or 2, wherein the wet froth has a density of 400 to 1 200 g./l.

4. A process as claimed in any one of Claims 1 to 3, wherein the wet froth has a density of 550 to 900 g./l.

5. A process as claimed in any one of Claims 1 to 4, wherein the thickness of the spread froth is 0.1 to 6 mm.

6. A process as claimed in any one of Claims 1 to 4, wherein the thickness of the spread froth is 0.5 to 3 mm.

7. A process as claimed in any one of Claims 1 to 6, wherein the elastomeric polymer is derived from one or more unsaturated hydrocarbons.

8. A process as claimed in any one of Claims 1 to 7, wherein the elastomeric polymer is a butadiene polymer.

9. A process as claimed in Claim 8, wherein the elastomeric polymer contains copolymerised carboxylic acid groups.

10. A process as claimed in any one of Claims 1 to 7, wherein the elastomeric polymer is a butadiene/styrene polymer.

11. A process as claimed in any one of Claims 1 to 6, wherein the elastomeric polymer is an elastomeric acrylate polymer.

1 2. A process as claimed in any one of Claims 8 to 11, wherein the latex has been prepared by emulsion polymerisation.

13. A process as claimed in any one of Claims 1 to 7, wherein the latex is a natural rubber latex.

14. A process as claimed in any one of Claims 1 to 13, wherein approximately 1 to 2 parts by weight of thickener latex is added per 100 parts by weight latex composition.

1 5. A process as claimed in any one of Claims 1 to 14, wherein the thickener latex is one which exhibits a relatively low viscosity at pH value less than about 7 and which increases rapidly in viscosity when the pH is raised to a value above about 7.

16. A process as claimed in any one of Claims 1 to 15, wherein the thickener latex has been prepared by emulsion polymerisation of one or more ethylenically unsaturated carboxylic acids with other copolymerisable monomers.

1 7. A process as claimed in Claim 16, wherein the thickener latex has been formed by copolymerisation of one or more neutral alkyl acrylate or methacrylate esters and of acrylic and/or methacrylic acid.

1 8. A process as claimed in any one of Claims 1 to 17, wherein the pH of the composition to be frothed is from 8.5 to 12.0 before frothing.

1 9. A process as claimed in Claims 1 5 to 18, wherein a basic substance is added to the composition during the frothing.

20. A process as claimed in any one of Claims 1 to 1 9, wherein the thickened composition is cross-linked by means of sulphur cross-linking agents after spreading.

21. A process as claimed in any one of Claims 1 to 20, wherein the thickened composition is spread on a textile.

22. A process as claimed in any one of Claims 1 to 21 , wherein the thickened composition is spread on the back of a carpet.

23. A process for the preparation of spread foam by the no-gel method substantially as hereinbefore described in Example 1 to 4 of the foregoing Examples.

24. Spread foam prepared by the process claimed in any preceding claim.