Fluoropolymer coating composition

Aqueous fluoropolymer formulations containing in finely divided form from 15 to 75% by weight of a fluoropolymer which preferably has at least 40% by weight of tetrafluoroethylene units, from 0.5 to 10% by weight of an organic compound which has at least two, preferably 2-4, isocyanate groups and, optionally, up to 20% by weight of auxiliary or additive substances are prepared and used for coating yarn, in particular sewing yarn, and textile sheet materials.

DESCRIPTION 
The present invention relates to an aqueous fluoropolymer formulation and 
to the preparation thereof, this fluoropolymer formulation making it 
possible to produce strongly adherent fluoropolymer coatings on textile 
materials made of organic synthetic fibers. 
European Patent Application 0,224,262 disclosed laminating textile sheet 
materials on both sides with a polyvinyl fluoride coating. In this known 
process, a solution of the polymer in an organic solvent is cast on a 
conveyor belt into a thin layer which is made to gel by heating. The gel 
layer is then pressed onto the surface of the textile material to be 
laminated. In this process, the fluoropolymer coating is fixed to the 
textile material only at the surface and virtually no impregnation of the 
individual filaments with the fluoropolymer takes place. 
German Offenlegungsschrift 3,301,270 discloses a process for sheathing 
fiber or filament yarns with a fluorine-containing polymer. In this 
resource-intensive process, the filament to be sheathed is pulled through 
a central hole in an annular spinneret while at the same time a tube of 
fluoropolymer is extruded from the annular spinneret. The extruded tube 
therefore surrounds the filament drawn out of the central hole as a 
loose-fitting sheath. No firm bonding takes place between the 
fluoropolymer tube and the filament sheathed therewith. 
It is also known for the manufacture of awnings, air-houses, flexible 
containers and similar products to coat textile sheet structures, usually 
woven fabrics, preferably those made of synthetic organic fibers or 
filaments, with polymer materials, usually with polyvinyl chloride (PVC). 
This coating is effected by impregnating the textile materials in 
suspensions of polyvinyl chloride in organic liquids. In the course of 
this coating, even the individual filaments of the textile material become 
enveloped by the polyvinyl chloride coating. To obtain sufficient adhesion 
between the polyvinyl chloride coating and the synthetic fibers, the 
coating is carried out in two stages. First a basecoat is applied 
comprising a mixture of a PVC paste or suspension with an adhesion 
promotor; this is followed by the application of a top-coat comprising a 
pure PVC formulation. The adhesion promotors suitable for this purpose are 
known. It is usual to use two-component adhesives comprising an organic 
substance having a plurality of hydroxyl groups, preferably a 
hydroxyl-containing polyester, and an organic substance having a plurality 
of isocyanate groups. 
It is also already known to coat materials such as threads or sheetlike 
structures made of organic synthetic fibers with fluoropolymers in order 
to confer particularly advantageous properties, for example a low 
co-efficient of friction, a high chemical resistance and a soil-repellent 
effect, on their surfaces. To this end, the synthetic fiber materials are 
impregnated or coated with commercially available aqueous dispersions of 
fluoropolymers and the resulting polymeric coating is fixed by means of a 
heat treatment. 
However, in order to make composite materials based on 
fluoropolymer-treated synthetic fibers suitable for a wide range of 
applications, for example the manufacture of membranes for textile 
construction, flexible containers, conveyor belts, fabric tubes and the 
like, it is absolutely necessary that the fluoropolymer should show 
adequate adhesive strength to the synthetic fiber. Adhesive strength is 
here to be understood as meaning the resistance to separation of base 
material and coating for a 5 cm wide strip as determined in line with 
German Standard Specification DIN 53 530. Adequate performance capability 
of the composite is ensured when, depending on the intended use, adhesion 
values of from 10 to 15 daN/5 cm are obtained. 
However, the production of strongly adherent fluoropolymer coatings of 
synthetic fiber materials presents even greater difficulties than the 
production of polyvinyl chloride coatings. This is because it is found 
that fluoropolymers are far more inert with respect to synthetic fibers, 
for example polyester fibers, polyamide fibers or aramid fibers, than 
polyvinyl chloride; that is, they show great reluctance to enter permanent 
physical or chemical bonds with synthetic fiber surfaces. Moreover, 
fluoropolymers which, on the basis of the physical data, might be thought 
suitable for use as coating agents for synthetic fibers are in general 
commercially available in the form of aqueous dispersions or pastes. It is 
therefore not possible using these known fluoropolymer dispersions or 
pastes to produce coatings on synthetic fiber materials showing adequate 
adhesive strength for all the abovementioned industrial uses. 
Nor is it possible to obtain a significant improvement in the adhesion of 
fluoropolymer coatings by using the one-or two-component adhesion 
promotors used successively in the production of PVC coatings. 
It has now been found, surprisingly, that it is possible to produce very 
strongly adherent fluoropolymer coatings on synthetic fibers by applying 
at least as the first impregnation or as the first coat (basecoat) a 
fluoropolymer formulation which, in place of conventional adhesion 
promotors, merely contains an organic compound having a plurality of 
isocyanate groups. 
The present invention thus provides an aqueous fluoropolymer formulation 
containing in finely divided form from 15 to 75% by weight, preferably 
from 25 to 60% by weight, of a fluoropolymer, from 0.5 to 10, preferably 
from 1 to 5, % by weight of an organic compound which has at least two, 
preferably 2 to 4, isocyanate groups and, optionally, up to 20% by weight, 
preferably up to 10% by weight, of auxiliary or additive substances. 
The fluoropolymers of the formulation according to the invention contain to 
an extent of at least 40% tetrafluoroethylene units and to an extent of up 
to 60% units derived from other unsaturated fluorine-containing monomers 
copolymerizable with tetrafluoroethylene. Advantageously, the amounts and 
identities of the fluorine-containing monomers copolymerized with the 
tetrafluoroethylene are chosen in such a way that the copolymer has the 
desired mechanical properties, such as strength, flexibility, elasticity, 
low slip friction and the like, and also good chemical resistance. More 
particularly, the composition should be chosen in such a way that the 
fluoropolymer is readily dispersible in water and also gives dispersions 
of good long term stability. Preference here is given to those 
fluoropolymers whose melting points are below the melting point of the 
synthetic fibers to be finished with the formulation. More particularly, 
fluoropolymers of advantage there are those whose melting points are from 
5.degree. to 50.degree., preferably from 20.degree. to 40.degree. C., 
below the melting point of the synthetic fibers to be finished. 
High suitability within the meaning of the above specifications is 
possessed by tetrafluoroethylene copolymers containing to an extent of up 
to 60% units derived from hexafluoropropylene and vinylidene fluoride. A 
fluoropolymer particularly suitable for preparing the fluoropolymer 
formulation according to the invention contains from 40 to 60% by weight 
of tetrafluoroethylene units, from 10 to 30% by weight of 
hexafluoropropylene units and from 20 to 40% by weight of vinylidene 
fluoride units. 
The preparation of such fluoropolymers is known, and numerous 
representatives of this class of compounds are commercially available. 
Particularly high suitability for the preparation of the fluoropolymer 
formulation according to the invention and for producing strongly adherent 
fluoropolymer coatings on synthetic fibers is possessed by the range of 
.RTM.Hostaflon TFB products available from Hoechst AG. 
Organic compounds which have a plurality of isocyanate groups and which can 
be incorporated as adhesion promotors into the fluoropolymer formulations 
according to the invention are known in large numbers, and numerous 
representatives of this class of compounds are commercially available. A 
number of industrially important tri- and polyisocyanates which are also 
suitable for the use according to the invention may be found for example 
in Ullmann's Enzyklopadie der technischen Chemie, 4th edition, Volume 13, 
page 355, Volume 15, page 634 and Volume 19, pages 303 and 304. It is of 
course also possible to use other di- and polyisocyanates which are 
dispersible and sufficiently stable in aqueous systems in the 
fluoropolymer formulations according to the invention. Preferred di- and 
polyisocyanates are for example the isomeric 2,4-diisocyanatotoluenes and 
mixtures thereof, 1,5-diisocyanatonaphthalene, diisocyanatodiphenylmethane 
and its technical-grade isomer mixtures, dimerized and trimerized 
diisocyanatotoluene, and adducts of diisocyanatotoluene with 
trimethylolpropane and tris[isocyanatohexyl]biuret. Particular preference 
for use in the fluoropolymer formulations according to the invention is 
given to the aforementioned derivatives of diisocyanatotoluene, in 
particular its dimerization product, which is available from Bayer AG 
under the name .RTM.Desmodur TT. 
As additives there may be present in the fluoropolymer formulations 
according to the invention various auxiliary and additive substances. 
These include in particular surface-active agents (surfactants) which may 
serve as wetting agents in the preparation of formulations and in their 
application and dispersants which facilitate and/or make possible the 
dispersion of fluoropolymers and cyanato-containing compounds in the 
aqueous phase and increase dispersion stability. Again such surface-active 
agents are described in the literature in large numbers. 
We cite for example Ullmann's Enzyklopadie der technischen Chemie, 4th 
edition, Volume 22, pages 455 et seq., in particular pages 466 and 488 and 
also Volume 23, page 120. It is possible to use for example anionic, 
cationic or amphoteric surfactants, but preferably nonionic surfactants 
derived from ethoxylates, terminally blocked ethoxylates, fatty acid 
esters and polyhydroxy compounds. The possibilities also include 
silicone-based surfactants or fluorosurfactants. If a novel fluoropolymer 
formulation having a relatively low fluoropolymer concentration and a 
correspondingly low viscosity is to be used, it can be advantageous also 
to add as an auxiliary and additive substance a thickening or 
antimigration agent, for example agents from the group of the 
carboxymethylated polysaccharides, xanthans or alginates, of the starch or 
cellulose derivatives, of the acrylate copolymers or of polyvinyl alcohol. 
Even inorganic thickening agents such as pyrogenic silica or bentonites 
may be incorporated into the fluoropolymer formulation according to the 
invention in low concentration. 
For specific applications, in particular for the topcoat, it is also 
possible to use mixtures of formulations according to the invention with 
blend components specific to the intended use. Such blend components are 
for example pigments, fillers, flame-retardant agents and modifying agents 
which are capable of modifying the properties of the fluoropolymer 
coating, for example plasticizers, lubricants or agents which modify the 
surface properties. These can be incorporated into the formulations 
according to the invention in a total amount of up to 25% by weight, 
preferably up to 15% by weight. However, it is advantageous not to use 
this second group of additives in formulations according to the invention 
which are to be used in the first coat (the basecoat). 
It is of course advantageous and particularly preferable to choose the 
amount of the auxiliary or additive substances in the fluoropolymer 
formulations according to the invention in such a way that the effect 
intended with these additives is obtained to an optimum degree. 
The fluoropolymer formulation according to the invention is prepared in a 
conventional manner by homogenizing, if 100 parts by weight of the 
formulation are to be prepared, from 15 to 75 parts by weight of 
fluoropolymer, from 0.5 to 10 parts by weight of the organic compound 
having at least two isocyanate groups and, optionally, up to 20 parts by 
weight of further auxiliary or additive substances in finely divided form 
with sufficient water to make up to 100 parts by weight, in a suitable 
mixing apparatus. If the fluoropolymers used here are solid, they must 
first be converted into a very finely divided form and then by vigorous 
stirring or kneading into a uniform dispersion or paste in the requisite 
amount of water, if necessary with the addition of a sufficient amount of 
dispersing auxiliaries and/or wetting agents. At the same time or 
thereafter the requisite amount of the selected organic compound 
containing a plurality of cyanate groups is added. 
The mixing operation is advantageously carried out at room temperature or 
at an only moderately elevated temperature. Further additive or auxiliary 
substances may be added to the blend batches from the start, or they may 
be incorporated once a stable fluoropolymer dispersion or paste has been 
obtained. 
It is advantageous to prepare the fluoropolymer formulations according to 
the invention by incorporating into an aqueous fluoropolymer suspension or 
fluoropolymer paste which may contain further auxiliary or additive 
substances from 0.5 to 10% by weight, based on the final weight of the 
mixture, a finely divided form of the organic substance which contains at 
least two isocyanate groups, and homogenizing it. Aqueous fluoropolymer 
suspensions or pastes which are highly suitable for producing the 
formulations according to the invention are for example the commercially 
available .RTM.Hostaflon TFB range mentioned above. 
Textile materials for the purposes of this invention can be one- or 
two-dimensional; that is, they are threads or else settlelike structures, 
for example woven fabrics, knitted fabrics, laid fabrics or nonwoven 
fabrics of various thicknesses, or even laminates of identical or 
different sheetlike structures of this type, possibly combined with other 
raw materials. 
The filaments or fibers of the textile materials to be coated can by dyed 
or undyed, smooth or textured. There is no evidence that commercial 
textile dyes migrate out of the synthetic fibers into the fluoropolymer 
coating. 
In principle, the fluoropolymer formulation according to the invention can 
be applied in one operation to the synthetic fiber material to be coated. 
To produce thicker fluoropolymer coats, the fluoropolymer formulation 
according to the invention is advantageously applied in a plurality of 
operations, in which case, depending on the chosen composition of the 
formulation and the associated consistency, an impregnation by dipping or 
padding or alternatively a paste application, for example by knife-coating 
or roller-coating, may be carried out. In this advantageous application of 
the formulation according to the invention it is particularly advantageous 
to carry out the first application, i.e. the production of the basecoat, 
with a fluoropolymer formulation according to the invention which, aside 
from the organic compound having a plurality of isocyanate groups, 
contains no or only little other additives, in particular no solid 
additives, for example pigments or flame-retardant additives. 
It is also possible to prepare fluoropolymer coatings on organic synthetic 
fibers by using the fluoropolymer formulation according to the invention 
only to prepare the basecoat and to use for the basecoat or basecoats a 
normal aqueous fluoropolymer dispersion or paste which is free of 
isocyanato-containing organic compounds but which, of course, may contain 
further additives, for example dispersants, wetting agents, pigments, 
flame-proofing agents or other filling and auxiliary substances. 
As stated above, it is also possible to carry out the fluoropolymer finish 
using the fluoropolymer formulation according to the invention in a single 
step, for example by impregnating, this being advantageous in particular 
in the application to threads made of synthetic filaments or fibers. In 
the application of the fluoropolymer formulation according to the 
invention, each application by impregnation or coating is in general 
followed by a heat treatment of the material to dry the application and to 
fix the freshly applied fluoropolymer to the substrate. 
The fluoropolymer coatings applied to the above-specified application 
methods have a high adhesive strength of at least 10 daN/5 cm, on use of 
preferred formulations according to the invention in general more than 20 
daN/5 cm. The adhesive strength of the fluoropolymer to the synthetic 
fiber can be controlled by the amount which is added of the organic 
compound containing two or more isocyanate groups. Furthermore, the 
adhesive strength can be influenced by the temperature and duration of the 
heat treatment carried out following application of the formulation 
according to the invention. 
The present invention also provides the textile sheet materials, for 
example woven fabrics, knitted fabrics, laid fabrics or nonwoven fabrics 
made of synthetic fibers, preferably polyesters, polyamide or aramid 
fibers, in particular polyester fibers, which are coated with the 
fluoropolymeric formulation according to the invention. Textile sheet 
materials for the purposes of the present invention also include laminates 
of identical or different sheetlike structures, including any mixtures 
with other raw materials. The coated sheetlike structures are notable for 
having, at least in the fluoropolymer layer directly adjoining the fiber 
surface, one or more adhesion-promoting constituents derived from an 
organic compound having at least two isocyanate groups. Furthermore, the 
fluoropolymer-coated synthetic fiber textile materials according to the 
invention are notable for the fact that the coating has an adhesive 
strength, as measured in line with German Standard Specification DIN 53 
530, of at least 10 daN/5 cm. Owing to the high mechanical strength 
properties of polyester, polyamide or aramid fibers, the materials which 
have been fluoropolymer-coated according to the invention also have 
excellent mechanical strength values which very much widen their range of 
possible industrial uses. Moreover, the coated materials have low 
coefficients of friction, show high chemical resistance and are 
soil-repellent. 
The present invention further provides the synthetic fiber threads coated 
with the fluoropolymer formulation according to the invention. The term 
threads is here to be understood as meaning monofilaments, multifilament 
yarns or even staple fiber yarns. Suitable fiber material here too 
comprises in particular polyester, polyamide or aramid synthetic fibers, 
preferably polyester fibers. The fluoropolymer-coated yarns thus obtained 
have not only the high mechanical strength values characteristic of 
synthetic fibers but also a particularly high-slip, chemically resistant 
and soil-repellent surface. 
The fluoropolymer formulations according to the invention prove 
particularly useful for the bonding of sewing yarn. This is to be 
understood as meaning that the fluoropolymer coating brings about a 
flexible bond between the individual filaments of the yarn. The bonded 
sewing yarns thus produced have a particularly high strength and excellent 
sewing properties. A particular advantage here is that the bonding action 
of the extremely inert fluoropolymer coating does not disappear on dyeing 
or on application of sewing finishes. 
The threads coated according to the invention and the bonded sewing yarn 
are also notable for the fact that they have, at least in the 
fluoropolymer layer immediately adjoining to the fiber surface, one or 
more adhesion-promoting constituents which are derived from an organic 
compound having at least two isocyanate groups and that the fluoropolymer 
coating has such adhesive strength that it does not become detached due to 
mechanical stress such as is experienced in the further processing of the 
threads or in the course of the intended use of the sewing yarn, for 
example in the course of winding, weaving or knitting or in the course of 
sewing.

The working examples that follow illustrate the preparation of the 
fluoropolymer formulation according to the invention and its application 
to yarn and textile sheet materials. 
EXAMPLE 1 
500 g of a commercially available 50% strength by weight aqueous paste of a 
fluorocopolymer of tetrafluoroethylene, hexafluoropropylene and vinyl 
difluoride (.RTM.Hostaflon TFB X 7900 from Hoechst AG) containing 2% by 
weight of a thickener based on an acrylate polymer and 15 g of a finely 
pulverulent 
1,3-bis-(4-methyl-3-isocyanatophenyl)-1,3-diazacyclobutane-2,4-dione (for 
example Desmodur TT from Bayer AG) are kneaded for 15 minutes in a kneader 
at 20.degree.-25.degree. C. and packaged. This gives a pasty fluoropolymer 
formulation which is highly suitable for coating textile materials (yarn 
or textile sheet material) made of synthetic fibers. It contains 48.5% by 
weight of fluoropolymer, 2.9% by weight of the diisocyanato compound and 
1.9% by weight of thickener. The formulation can also be used to good 
effect as a primer coating in the coating of textile materials onto which 
firmly adherent fluoropolymer topcoats can be applied by means of 
commercially available fluoropolymer pastes or dispersions. 
EXAMPLE 2 
10 g of wetting and dispersing agent based on an ethoxylated alkylphenol 
are dissolved with vigorous stirring in 315 ml of demineralized water at 
40.degree. C. 175 g of a finely ground fluorocopolymer of 55% by weight of 
tetrafluoroethylene, 15% by weight of hexafluoropropylene and 30% by 
weight of vinylidene fluoride are then slowly introduced with stirring. 
Stirring is continued until the dispersion is completely homogenized, and 
the dispersion is then packaged. The 35% strength by weight aqueous 
fluoropolymer formulation obtained is highly suitable for the dip 
impregnation and the coating of textile sheet materials and in particular 
yarns made of synthetic fibers. 
EXAMPLE 3 
Coating of Woven Polyester Fabric 
A 220 gm/m.sup.2 woven polyester fabric with 9 ends and 9 picks of 1100 
dtex per cm is initially provided with the following basecoat: 
A fluoropolymer paste according to Example 1 is applied to both sides of 
the fabric, in each case at a rate of 60-80 g/m.sup.2, by spread coating 
by means of an air knife or a rubber blanket doctor blade. 
The material is then sintered at 210.degree. C. for 2 minutes. Topcoating 
is effected with a 50% strength .RTM.Hostaflon TFB X 7900 paste (aqueous 
fluoropolymer pate) which, if desired, may additionally be pigmented or 
flameproofed (for example by the addition of 5% of TiO.sub.2 and/or 10% of 
Sb.sub.2 O.sub.3, based on solid fluoropolymer). This formulation is 
applied by spread coating with a steel roll doctor or rubber blanket 
doctor blade in a plurality of coats (for example 6 coats on the front, 4 
coats on the back), each coat being followed by an intermediate drying at 
210.degree. C. for 2 minutes. 
The total amount applied to the front of the fabric is 300 g/m.sup.2 and to 
the back of the fabric 160 g/m.sup.2. 
After the last coat has been applied, the fabric is sintered at 210.degree. 
C. in the course of a residence time of 10 minutes. Thereafter the surface 
of the front is embossed by means of a calender. The adhesive strength 
obtained is 20.7 daN/5 cm. 
If this example is repeated exactly as described above with a fluoropolymer 
paste which contains no dicyanato compound but which otherwise has the 
composition of the paste prepared in Example 1, an adhesive strength of 
9.1 daN/5 cm is found on the front of the coated fabric and an adhesive 
strength of 6.9 daN/5 cm on the back. 
EXAMPLE 4 
Fabric: as in Example 3. 
Basecoat: as in Example No. 3, the basecoat being sintered at 210.degree. 
C. in the course of a residence time of 10 minutes. 
Topcoating: as in Example 3. 
Adhesive strength obtained: 30.2 daN/5 cm. 
EXAMPLE 5 
Fabric: as in Example 3. 
Basecoat: as in Example No. 4. 
Topcoating: 
Formulation: Hostaflon TFB X 7100 Melt granules+5% of TiO.sub.b +10 % of 
Sb.sub.2 O.sub.3 +1% of Hoechst Wax OP. 
Application method: roll melt unit. 
Amount replied: 
front 300 g/m.sup.2 
back 160 g/m.sup.2. 
Processing conditions: 
Roll temperatures: 205.degree. C., 
Preheat roll: 150.degree. C., IR radiator: 90%, 
Machine speed: 3 m/minute, 
Rubber roll pressure: 7 bar, 
Film weight: 
front 300 g/m.sup.2, 
back 160 g/m.sup.2. 
Adhesive strength obtained: 21.5 daN/5 cm. 
EXAMPLE 6 
Bonding of a Sewing Thread 
A black 266-dtex 64-filament 3-ply polyethylene terephthalate yarn is 
impregnated with 15-20% by weight (dry add-on based on weight of fiber) of 
a formulation as described in Example 2 by dipping. 
The impregnated thread is then heat-treated in a hot oven at 220.degree. C. 
for at least 60 seconds. The sewing thread obtained does not show a change 
in hue, nor any migration of the dye into the bonding. It produces very 
good sewing results even under high stress, for example in industrial 
sewing machines. No abrasion takes place either in winding or in sewing, 
nor, following prolonged storage in bobbin form, is there any adhesive 
coalescing of yarn layers. 
In the same way, the above-described dip impregnation can also be effected 
with a commercially available 35% strength fluoropolymer dispersion 
(.RTM.Hostaflon TFB X 7100 from Hoechst AG) into which 1.5% by weight of 
1,3-bis-(4-methyl-3-isocyanatophenyl)-1,3-diazacyclobutane-2,4-dione (for 
example .RTM.Desmodur TT from Bayer AG) has been homogeneously 
incorporated beforehand. 
To test the quality of the sewing thread produced, it is subjected to a 
tough sewing test such as that described in DE Offenlegungsschrift 
3,431,834. 
In this test, the thread is used in sewing under the following conditions: 
industrial sewing machine Pfaff 363, stitch length 1 mm, zigzag seam 8 mm 
width, 3,250 cycles/minute, 200 cN pre-tension, 4 layers of cotton twill. 
The thread bonded according to the invention permits in this test on 
average over 4,000 stitches without thread breakage, while an unbonded 
thread permits on average only about 300 stitches.