Fluoropolymer dispersions

Details Fluoropolymer dispersions Fluoropolymer dispersions

2001-06-18

2002-03-19

Pezzuto, Helen L. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S220000, C524S316000, C524S317000, C524S375000, C524S545000, C524S546000, C526S243000, C526S247000, C526S248000, C526S253000

Reexamination Certificate

active

06359044

ABSTRACT:

The present invention relates to fluoropolymer dispersions suitable for the cloths impregnation and the obtainment of cast films.
More particularly it relates to fluoropolymer dispersions having improved substratum wetting, i.e. with reduced presence of cissings, and minimum viscosity variation with temperature and critical thickness maintenance, necessary condition to obtain coatings of high thickness and free from cracks with few passages. Moreover the formulations obtainable from the fluoropolymer dispersions of the present invention are such to minimize the discoloration problems of the manufactured article.
It is known that the fluoropolymer dispersions and of PTFE in particular, are used to impregnate cloths and to coat supports able to thermally withstand to the sintering treatment that the fluoropolymer must undergo to form a continuous film. A particular case is formed by PTFE cast films, obtained by successive deposition of PTFE films brought to sintering and then removed from the support.
The use of fluoropolymer dispersions for the cloth coating (glass fiber, KEVLAR, etc.) and for obtaining films has been known for a long time.
One of the problems arising during the use of the fluoropolymer aqueous dispersions for the above purposes consists in assuring a very good and homogeneous covering of the support to be coated with the fluoropolymer. The greatest problems arise when a first layer of fluoropolymer has been deposited on the support and one has to continue to deposit fluoropolymer aqueous dispersion on the first fluorinated layer having low surface energy (for instance for the PTFE &ggr; is about 17 dyne/cm).
Another problem is to avoid that the fluoropolymer dispersion viscosity has a great dependence on the temperature in the range 10°-50° C., preferably 20-35° C. Indeed the temperature of the bath where the dispersion is placed can vary in this range during the deposition process, with negative consequences for the control of the impregnation process of the cloths or of the cast film formation. In general it is preferable that the viscosity absolute value of the formulated dispersion in this temperature range is not higher than 150-200 cPoise, preferably not higher than 100 cPoise, still more preferably lower than 60 cPoise. Industrial processes generally operate continuously and the drying and sintering conditions are fixed at the beginning. A variation of the dispersion viscosity during the time, involving a pick-up variation (amount of deposited dispersion) would oblige to a continuous adjustment of the initial drying and sintering conditions. This way to operate is not suitable from an industrial point of view. On the other hand if these continuous adjustements are not carried out, the quality of the final coating is very poor and besides it does not maintain constant during the time.
A further problem is to have a high critical thickness of the formulated dispersion to allow to deposit high dispersion thicknesses without the formation, during the sintering, of cracks and pinholes which lower the sealing and protection capability of the coating. A further charateristic which it is desirable not to be meaningfully modified is the colour of the final manufactured article, in particular of the cast films obtainable from the dispersions.
To solve these technical problems it is known in the art to use dispersions containing surfactants.
However the fluoropolymer dispersions usually commercialized, are not able of contemporaneously solving all the mentioned problems.
For instance polytetrafluoroethylene (PTFE) dispersions containing about 60% by weight of PTFE and containing as surfactant TRITON® X100, alkylphenoxyethoxylated surfactant commonly used, in an amount of 3% by weight, are not able to give coatings on cloths usable from the industrial point of view since the cloths contain cissings, i.e. zones not homogeneously coated.
Therefore the use of these cloths for instance in the foodsector is not possible since in these zones they do not show the desired antisticking characteristics.
Other used PTFE-based dispersions, which contain besides TRITON® X100 also other alkylphenoxyethoxylates, give a wetting increase but they introduce discoloration problems and viscosity increase.
The Applicant has unexpectedly and surprisingly found that it is possible to prepare fluoropolymer dispersions capable to overcome all the above technical problems.
It is an object of the present invention fluoropolymers aqueous dispersions comprising:
a) 1 to 10% of an hydrogenated nonionic surfactant;
b) 0.1 to 3% by weight of a fluorinated nonionic surfactant selected from the following classes:
R
f
—L—(OCH(R
1
)—CH(R
2
))
n
—O—A  (I)
 wherein:
n is an integer in the range 4-60, preferably 8-30;
L is selected from:
—(CF(Y)—CO—O)
p
R′—; —(CF(Y)—CO—NH)
p
R′—;
—(CF(Y)—SO
2
—NH)
p
R′—;
wherein: p is 0 or 1; Y can be F or CF
3
; R′ is an alkylic radical C
1
-C
5
, linear or branched when possible;
R
1
, R
2
can preferably be both H or the former H and the latter CH
3
,
R
f
can be a radical of (per)fluoroalkane type from 4 to 20 C atoms or of (per)fluoropolyether type comprising repeating units randomly distributed along the polymer chain selected from:
—CF(X)CF
2
O— or —CFXO—, wherein X is equal to F or —CF
3
;
—CF
2
(CF
2
)
z
O— wherein z is an integer equal to 2 or 3;
—CF
2
CF(OR
f
″)O— or —CF(OR
f
″)O— wherein R
f
″ can be —CF
3
, —C
2
F
5
, or —C
3
F
7
;
A is selected from the following ones:
—H;—CH, —CH
2
OH; —CH
2
OCH
2
CH
2
OH; —CH
2
(OCH
2
CH
2
)
n′″
OR′ wherein n′″ is an integer in the range 2-15 and R′ is H, CH
3
, COCH
3
; —CONHCH
2
CH
2
OH;
c) 25 to 75% by weight of water.
The R
f
radical generally has a number average molecular weight from 250 to 1500, preferably from 400 to 1000.
The perfluoropolyether radicals R
f
comprise a T end group selected from —CF
3
, —C
2
F
5
, —C
3
F
7
, ClCF
2
CF(CF
3
)—, CF
3
CFClCF
2
—, ClCF
2
CF
2
— and ClCF
2
—.
In particular the following prfluoropolyether radicals R
f
can be mentioned as preferred:
(a) T—O (CF
2
CF(CF
3
)O)
a
(CFXO)
b
—
wherein: X is F or CF
3
; a and b are integers such that the molecular weight is in the above range; a/b is in the range 10-100 when b is different from 0, and T is one of the above mentioned end groups;
(b) T—O(CF
2
CF
2
O)
c
(CF
2
O)
d
(CF
2
(CF
2
)
z
CF
2
O)
h
—
wherein: c, d and h are integers such that the molecular weight is in the above range; c/d is in the range 0.1-10; h/(c+d) is in the range 0-0.05, z is an integer equal to 2 or 3, and T is one of the above mentioned end groups;
(c) T—O(CF
2
CF(CF
3
)O)
e
(CF
2
CF
2
O)
f
(CFXO)
g
—
wherein: X is F or CF
3
; e, f, g are integers such that the molecular weight is in the above range; e/(f+g) is in the range 0.1-10, f/g is in the range 2-10, T is one of the above mentioned end groups;
(d) T—O(CF
2
O)
j
(CF
2
CF(OR
f
′)O)
k
(CF(OR
f
′)O)
1
—
wherein: R
f
″ is —CF
3
, —C
2
F
5
, —C
3
F
7
; j,k,l are integers such that the molecular weight is in the above range; k+l and j+k+l are at least equal to 2, k/(j+l) is in the range 0.01-1000, l/j is in the range 0.01-100; T is one of the above mentioned end groups;
(e) T—O—(CF
2
(CF
2
)
z
CF
2
O)
s
—
wherein s is an intger such as to give the molecular weight above indicated, z has the above meaning and T is one of the above mentioned end groups;
(f) T—O (CR
4
R
5
CF
2
CF
2
O)
j
, —
wherein R
4
and R
5
are equal to or different from each other and selected from H, Cl or perfluoroalkyl, for instance having 1-4 C atoms, j′ being an integer such that the molecular weight is that above indicated;
(g) T—O(CF(CF
3
)CF
2
O)
j″
—
j″ being an integer such as to give the moelcular weight above indicated.
These compounds and the methods for their preparation are described in GB 1,104,482, U.S. Pat Nos. 3,242,218, 3,665,041, 3,715,378 and 3,665,041, EP 148,482 and U.S. Pat. Nos. 4,523,039, 5,144,092.
The preferred nonionic surfactants of formula (I) are those

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