CA1037197A - Fluoroelastomers in powder form - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Elastomeric fluorinated copolymers in powder form, suitable in extrusion, injection-molding and similar fabri-cating techniques for the formation of shaped articles are obtained by copolymerizing in aqueous emulsion in the first stage of a two-stage polymerization process vinylidene fluoride with pentafluoropropene, hexafluoropropene, chloro-trifluoroethylene, or a perfluorinated alkyl vinyl ether and optionally with tetrafluoroethylene to at least about 90%
monomer conversion to a polymer having a melting point of less than about 60°C.; polymerizing in the same emulsion in the second stage at least one of the same monomers as in the first stage but in such proportions that the second stage polymer if made separately would have a melting point above 120°C.; flocculating the dispersion; and spray-drying at 121-177°C.

Description

1037~
BACKG~OUND OF 'l~E INVE~TION

This invention relates to novel, powdery, polymeric composltions and to a process ror the manufacture thereo~.
Many fluoropolymers are known. Typical are polymers and copolymers of vinylidene fluoride, hexafluoro-propene, tetrafluoroethylene, chlorotrifluoroethylene, and others. Depending on the nature and proportions o~ monomers, those polymeræ may be predominantly plastic or elastomeric.
Blends of rluorop01ymeræ also are known. ~or exampl~, U.S.

2 789 959 discloses blends of ~inylide~e fluoride/chloro-trifluoroetbylene copol~mer with chlorotrifluoroethylene homopolymer.
U.S. 3 494 784 of R. De Coene et al, issued February 10, 1970, discloses a method of preparing dry, non-adhesive elastomeric powders, wherein a small a unt o~ hard polymer is allowed to coagulate in the presence of a coagulate of an elastomer latex.
U.S. 3 745 196 of C.A. Lane et al, issued July 10, 1973 describes a multistage polymerization process in which a first stage elastomeric acrylic polymer is at least partially encapæulated with subsequent elastomeric acrylic polymer.
SUMMARY OF THE INVE~TION
According to the present invention, there are now provided novel fluoroelastomeric compositions in powder form, said compositions being prepared by the followin~ process:
1. CopolymRrizing in the first stage in an aqueous emulsion and in the presence of a free radical catalyst to at least about 90% monomer conversion a mixture of monomers which will result in a copolymer of about (a) 65-100 weight percent of vinylidene ~r ~ 2 1'0371~Y7 ~ uoride (hereinafter VF2) and a second monomer selected from the group pentafluoropropene (PFP), hexafluoropropene (HFP)~ chlorotrifluoroethylene (CTFE), and a perfluorinated C2-C4 alkyl vinyl ether (PFAVE), the weight ratio of VF2 to the second monomer being within the range 0.67:1 to 3:1;
and (b) 0-35 weight percent of tetrafluoro-ethylene (hereinafter TFE), the resulting first-stage copolymer having a crystalline melting point of less than about 60C.;
2. polymerizing in the second stage in the same emulsion at least one of the same monomers as in the first stage but in such proportions that the second stage polymerJ if made separately, would have a crystalline melting point of more than about 120C.;

3. flocculating the resulting dispersion; and

4. spray-drying the dispersion at a temperature of about 121-177C.
The relative proportions of the components in the resulting polymer should be maintained within the range of about 65-go weight percent of the first stage copolymer and 10-35 weight percent of the second stage copolymer~
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagram showing the elastomeric and plastic compositions of PFP, HFP, CTFE, PFAVE, VF2, and TFE.
Fig. 2 illustrates the flow diagram of a cascade reactor system suitable in the process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The relative proportions of monomers in the first 3o and second stages necessary to produce copolymers of the required compositions can be readily calculated by one skilled in the art 1037~Y7 from either known or easily determinable polymerization rat~s of the individual mono~eræ under the appropriate temperature and pressure conditions.
The Pirst two steps Or the process of the present invention, the two polymerization stages J can be carried out in sequence either batchwise in a single reactor or in a con-tinuous, cascade, process in two separate reactors. A cascade reactor æystem substantially similar to that disclosed in Canadian Patent 1 015 898 of S. David Weaver, issued August 16, 1977, would be particularly suitable in the instant process.
In the prac~ical operation of this process, VF2, a second monomer rrom the abo~e-defined group, and, option-ally, TFE in the required ratios, water, and a water-æoluble free radical generator at a concentration of about 0.001-2 welght percent based on total monomers are introduced into the first two reactors equipped with agitating means and main-tained at a temperature Or about 50-130C and a pressure Or about 250-1500 psig at such a flow rate that at least about 90% conversion of monomers i5 obtained during the residence of the monomers in the reactor.
Suitable rirst-stage monomer comb~nations within the scope of this in~ention and suitable polymerization con-ditions are described, for e~ample, in U.S. Patent 3 051 677 Or D.R. ~exrord, issued August 28, 1962; 2 968 649 of J.R. Pailthorp, issued January 17, 1961; 2 738 343 of A.L.
Dittman et al, issued March 13, 1956; 3 136 745 of J.R. Albin et al, issued June 9, 1964; 3 235 537 of J.R. Albin, et al, issued February 15, 1966; 3 331 823 of D. Sianesi et al, issued July 18, 1967 and 3 335 106 of D. Sianesi et al, issued August 8, 1967. The resulting first-~tage copolymers will ha~e composi-tions shown in the dlagram in Fig. 1 within Area 1. As the diagram shows, all these copolymers are elastomeric.
The product stream is withdrawn at the same rate 103719'7 as the materials are introduced and immediately passed to a second reactor equipped with agitating m~ans and maintalned at a tempcrature o~ about 50-130C, and a pressure of about 250-1500 psi~.
In the second polymerization st~ge, additional VF2 and/or TFE are introdueed to the second reactor, their pro-portions being such that the second stage product will have a high degree of crystallinity and/or of ~ti~fness. To achieve this goal, it is nece~sary to produce in the second reactor a polymer containing more than about 80 weight percent VF2 or 50 weight percent of TFE. Referring again to Fig. 1, it can be seen that suitable second ~tage copolymers will be located either in Area 2 or in Area 3 on the diagram, both areas being ln the plastic region. Compositions containing a high proportion of VF2 or TFE will be highly crystalline;
they have melting points o~ more than about 120C.
me second stage polymers are thus ~ormed in the presence of particles of the ~irst-stage polymer, after the original latex has attained at least 90% polymerization.
Since the e~fluent from the first reactor which enters the second reactor usually contains about 10% or less of un-changed starting monomer~ even though no additional make-up monomers other than VF2 and/or TFE are added, there is normally pre~ent in the second reactor a su~ficient pro-; portion o~ PFP, ~FP, CTFE or PFAVE to ~orm a copolymer con-taining that monomer. Suitable PFAVEI~ include perfluori-nated methyl vinyl ether, ethyl vinyl ether, and all isomers o~ propyl and butyl vin~l ethers.
It is to be understood that theoretically it - 30 is possible to have a quantitative conversion in the first 10~371g7 reactor, no unchanged starting monomer from the first stage being present during the second stage polymerization. Such a situation, even though not likely to occur in a commercial process, also is within the cont~mplation of this invention.
The critical requirement is that the polymer formed in the second stage have a melting point higher than about 120C.
The second stage polymer will thus usually be a copolymer but may in an extreme case be a homopolymer.
A chain transfer agent may be added to either stage to modify the molecular weight of the resulting polymer in a manner generally known in the art. However, normally a chain transfer agent would be considered unnecessary in the process of the present invention since it is desired to obtain high molecular weight polymers, especially in the secQn-d st~ge.
The latex effluent from the second reactor is flocculated in a known manner, for example, by additlon of an electrolyte such as potassium alum or a polyethylene poly-amine, for example, triethylenetetramine. The coagulated dispersion is spray-dried in a stream of a hot gas, especially, hot air. It is practical, although not re~uired, to add either before or during the spray-drying operation a small amount of an anti-tack agent such as, for exam~le, silica, calcium silicate, calcium carbonate, etc.
A possible cascade reactor arrangement is shown in Fig. 2. The first and second reactors are designated 1 and 2, respectively. A solution of free radical generator in water is introduced into the first reactor through the feed line 4. YF2, a second monomer selected from PFP, HFP, CTFE, and PFAVE, and optionally TFE, are introduced through the 103719~7 ~eed line 5, usually as compressed gases. me overflow ~rom the first reactor i8 lntroduced at the bottom of the second reactor, Fresh VF2 and/or TFE si introduced through the ~eed line 6. In thi~ particular ~low diagram, the catalyst is introduced only to the first reactor but not to the second reactor, and no transrer agent is used. Additional catalyst and/or chain transfer agent would require additional ~eed lines.
me overflow from the top of reactor 2 is intro-duced through a let-down valve to separator 3, from which the unchanged gaseoùs monomers are reco~ered throueh the vent 7, and the reactlon product is withdrawn through the drain 8.
me residence time of the monomer mixture in the first reactor must be surficient to allow the polymerizing nomers to reach a conversion Or at least about 90% at a practlcal flow rate and the prevailing temperature and presæure.
The residence time ln the second reactor may be different from ~ that in the first reactor, the relative resldence t1mes being ; 20 dependent on the relative sizes of the two reactors. Such parameters as temperatureæ, residence timeæ, polymerization rates and operating pressures can be adJusted for each re-actor independently of the other. The free-radical gener-atlon rate depends, among others, on the catalyst feed rate.
The appropriate calculatlons can be made both for the case ; where catalyst iæ added only to the first reactor and that where catalyst læ added to both reactors using the equat-ionæ given in the above-mentioned Canadian Patent of S0 ~avid Weaver.
me polymerization is initiated by a free radical `~-103719~
generator, which can be any inorganic persul~ate, peroxide, perpho~phate/ perborate, or percarbonate. HoweverJ the preferred initiators are ammonium persulfate, sodium per-sulfate, potassium persulfate, or a mixture of two or more such compounds. me initiator can be used in combination with a reducing agent such as an alkali metal or ammonium sul~ite, biæulfite, metabisulfite, hudrosulrite, thiosulfa~e, phosphite, or hypophosphite; or in combination with a ferrous, cuprous, or silver æalt, or other easily oxidized metal compound. Such initiator systems are well know~ to those skilled in the art o~ polymerization. The preferred initiator is ammonium per-sul~ate, without a reducing agent.
Since this polymerization is carried out in an a~ueous emulsion, the reaction catalysts should be water-soluble. me catalystæ such as persulfates or peroxide~
can be decomposed either by reducing agents ~n a redox system or by therm~l decomposltion. The pH of the reaction medium normally iB quite low, for instance, from about 2 to about 7-1/2. me solids concentration in each reactor w ually is ~rom about 10 to about 30 weight percent, a concentration o~ product Or about 15-25 weight percent being preferred. Nevertheless, the concentration doeæ not have to be the same in both reactors. The maximum practical concentra-tion o~ solids in each reactor i8 about 50 weight percent æince above that concentration the viscoæity of the resulting latex sgstem is too high for effective stirring. In the practical operation o~ this process, both reactors are filled with liquid, the back preæsure being controlled.
The monomers muæt be under suf~iciently high presæure to enter the reactors. The nomer presæure usually is 10;~719~7 maintained at about 600-900 pSi~, but the exact pressure is not critical, as long as it is sufficient to propel the monomers into the reactors. The preferred polymerization temperature is ab~ut 100-130C.
It is often useful to add to the first reactor an ~ emulsion stabilizer such as a surfactant and/or an alkaline ; compound to improve the stability of latex. The alkaline compound can be, for example, sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, sodium phosphate, disodium phosphate, monosodium phosphate, sodium fluoride, and such. Not all surfactants are suitable in the process of this invention since ordinary aliphatic acid soaps, for example, inhibit polymerization. Fluorocarbon acid soaps are preferred. They include, for example, ammonium ~-hydroperfluoroheptanoate and ammonium perfluorooctanoate.
Oth~r dispersing agents include, for example, salts of chlorendic acid. The amount of the surfactant is about 0.0~-0.2~ of the weight of the water used in the reaction.
If alkaline compounds are used, their concentration should be preferably such as to maintain a pH of about 3.5-6 in the reactor. Alternatively, the ratio of the alkaline compounds to the initiator should correspond to about ; O.1-0.25 g. of sodium hydroxide per gram of ammonium persul~ate.
The product recovered from the second reactor could be considered to be a blend of the predominantly elastomeric material formed in the first reactor and of the predominantly plastic material formed in the second reactor.
Yet, it is believed that the final product possesses properties different from those of ordinary intimate blends, _ g and that the second polymer is in fact somehow attached to the first. While theoretically encapsulation of the first polymer by the second is possible, there is at present no evidence of the formation of a continuous coating of the plastic polymer on the elastomeric material. The particles of the second polymer may be partially formed within the particles of the first; or chemical bonding, i.e., grafting, may be taking place.
Flocculation of the latex recovered from the second stage brings the particle size of the polymer to about 0.5-1 micron. Spray-drying produces a product having 4 particle size of up to about 10 microns. The spray-drying temperature range is critical to the success of this step.
The preferred lower temperature limit of incoming air is about 138C., the most suitable temperature being up to about 157C. The air temperature at the exit will be about 66-93C., preferably 71-82C.
Depending on the proportion of the plastic com-ponent? the spray-dried powder may have a tendency to re-agglomerate. The addition of anti-tack agent is, therèfore, ! .
sometimes recommended. The amount of silica or similar agent isiusually up to about 2% by weight, based on the total polymer, especially about 0.5-2% by weight, about 1~ being normally satisfactory.
; The powder product of the present inverltion is useful in extrusion, injection molding and similar fabri-cation techniques for the formation of shaped articles, such as gaskets, tubes, rods, and other articles. These simple fabricatin~ techniques require conventional equipment.
Powder can bc continuously conveyed to fabricating equipment 103719~7 and formed into desired shapes. Precompounding can be accomplished in simple powder blenders, eliminating the necessit~ of heavy precompounding equipment. The novel copoly~ers of the present invention are also suitable in ~ire-coatin~ and in similar coating applications.
This invention is now illustrated by the following representative examples of certain preferred embodiments thereof, where all parts, proportions, and percentages are by weight unless otherwise indicated.
In the following examples, glass transition temperature and crystalline melting point of the products were determined using a Du Pont 99-0 Thermal Analyzer, using a differential scanning calorimeter unit. This analytical technique is reported in B. Carrol, Physical Methods in Macromolecular Chemistry, Marcel Dekker, New York, 1972, page 253. The inherent viscosity was determined at 80C., in a solution containing 0.1 weight percent of polymer in a mixture of 87 weight percent tetrahydrofuran and 13 weight percent di-methylformamide.

Two 2L autoclaves with stirrers were arranged as shown in Fi~. 2 with associated piping, flow meters, pumps, compressors and feed tanks for feeding gaseous monomers and aqueous solutions of initiator and removing the product from the second reactor. The tem~erature of both reactors was controlled at 110C. by the temperature of steam/water in a jacket, and the pressure was maintained at 61.2 a~m. ~y a control valve at the exit of the second reactor. Gaseous monomers were measured, mixed, comprcssed and fed to the first reactor in one stream and a solution of ammonium per-sulfate initiator, sodium hydroxide and water were fed in a second stream. Vinylidene fluoride was fed at a rate of 1100 ~, glhr. and hexafluoropropene was fed at a rate of 900 g/hr.
Ammonium persulfate was fed at a rate of 16.0 g/hr. and sodium hydroxide at 3.0 g/hr. in 8.OL of water per hour.
The nominal residence time in the first reactor was thus 0.25 hr. Under these conditions the effluent of the first reactor contained 1850 g/hr. polymer tl9% solids), 20 }O g/hr. unconverted vinylidene fluoride and 130 g/hr. unconverted hexafluoropropene (93% conversion of total monomer~. This product was fed immediately to the second reactor where addi-tional vinylidene fluoride was fed at a rate of 800 g/hr. The polymerization proceeded because of the presence in the effluent of the first reactor of undecomposed initiator t20% of initial feed). During the 0.25 hr. residence in the second reactor, additional 740 g/hr. polymer was formed to give a total of 2590 g/hr. polymer t24.5% solids) in the effluent from the ; second reactor; lSO g/hr. VF2 and 60 g/hr. HFP remained un-converted. ', The composition of the polymer formed in the first -reac*or was approximately 58~ VF2 and 42% HFP and this constituted 71~ of the total product. The polymer formed in the second reactor contained approximately 91~ VF2 and 9%
HFP and constituted 29~ of the product.
The latex effluent of the second reactor was collected ` over a period of several hours. A solution of 0.5 wt. %
*riethylenetetramine in water was added until the polym~r flocculated, and the flocculated dispersion was fed to a conical bottom spray-drier at a rate of 0.168-0.227 k~./min.

, .
- 12 _ ~037~9 7 throu~h a two-fluid atomizing nozzle along with air preheated to 138-157C. Under these conditions, the water evaporated, and a powder was collected which had an a~erage particle size of about 50 microns and a water content of about 0.5%.
m is composite product contained 68% VF2 and 32% HFP. Its inherent viscosity, ninh, was 0.89. Its glass transition temperature, Tg, was -20C, charac~eristic o~ the elastomeric component, and its cryst~lline melting point, m.p., was 1~8C, characteristic of its plastic com-ponent.
The powder (100 parts) was mixed with 30 parts MT carbon black, 6 parts calcium hydroxide, 3 parts magnesium oxide, 2 p~rts bisphenol AF and 0.7 part benzyl-triphenglphosphonium chloride and compression-molded in a press for 10 mln. at 177C, then oven-cured 24 hours at 232C. The elastomeric vulcanizate had somewhat higher ~odulw and tensile strength than a vulcanizate o~ the first-stage polymer alone, but its elongation and com-pression ~et were comparable. The properties of thevulcanized product were compared with t~ose of physical ~lxtures of ~irst-stage copolymer ~nd commercial polyvinyli-dene ~luoride in weight ratios 70:30 and 75:25, respectively~
The re~ults are shown in Table I, below.

~ 1037197

5; ~
C`~
~4 ~4 o ~ o a~ :t I t~ t~ ~1 ~ ~. C~
o dP ~P
r- ~

~: ~
~C`J C~l 14 ~ O o o o ~
o o o a In c~ a) ~/
>.
,, _, o o dP dP
o o ~`

Q¦ 4 E-~O ~1 a) o o o o 0~; u7 0CD O~
a) cO u~ ,~
'aO ~ ~
~ x a) dP h o 0 .C h bl h ~ o tn h C~
O ~4 :~ 0 0 ^ .~ o ,1 0 0 o O ~' ~ 0 0U~
h dP 0 ~ a o ~: O h o a~ _I ~ o ~037~9q The above data show that the vulcaniæed co-polymers made by the process of the present invention have distinctly different physical properties from those of blends of equivalent composition._ This especially can be seen from modulus, elon~ation, and compression set figures.
EXAMPI,F. 2 The apparatus and procedure of Example 1 were used to produce an elastomer powder from three monomers. Tempera-ture, pressure and residence time were the same. The initiator system was ammonium persulfate (16.4 g/hr) plus sodium bisul-flte (2.73 g/hr) and sodium hydroxide (3.0 g/hr) in 8.0 lJhr of water. Monomer feed to first stage and polymer produced are shown in Table II.
TABLE II
, Unchanged Feed Monomers Polvmer ~ wt. ~g/hr. ~ O

The product latex was fed to the second reactor along with an additional initiator solution of 4.56 g/hr ammonium persulfate and 1.0 g/hr sodium hydroxide in 0.5 1/hr water. Monomer feed to the second stage and polymer produced are shown in Table III.

_ 15 -1037~97 TABLE III

Feed from New Unchanged Second-~ta~e Stage l FeedMonomer~ Polymer g/hr. ~/hr.g/hr. ~/hr. wt.

HFP 65 0 ~20 45 8 Total 590 The overall product from the second reactor thus contained 590 gthr. plastic polymer in a total weight of 2690 g/hr. or 22~, of the total.
The composition of this composite product was 65S VF2, 19% HFP, and 16~ TFE; ninh = 1.03; Tg - -25C.
and a minor m.p. of 45C. (both characteristic of the elasto-meric component); and a major m.p. of 152C., characteristic of the plastic component ~ XAMPI.E 3 Product was made in the same way as in Example 2 except that the monomer feed to the second stage was predomi-20 nantly TFE instead of VF2. Monomer feed to the second stageand products are shown in Table IV.
TABLE IV

Feed from New Unchanged Second-Stage Stage 1 FeedMonomers Polymer ~/hr. ~ r. ~/hr. ~/hr. wt. %

Total 500 The overall product from the second reactor thus contained 500~hr plastic polymer ;n a total weight of 2600 g/hr or 19% of the total.

.

` ` 1037~g7 This composite product contained 53~ VF2, 19%

HFP~ and 28% TFE; ~ inh = 0.78 for the soluble portion of the product (66~ of total product), TG = -25C. and minor m.p. = 45C. (both characteristic o~ the elastomeric com-ponent); and a major m.p. = 198C., characteristic Or the plastic component.

Claims (13)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for the preparation of an elastomeric, fluorinated copolymer in powder form, said process comprising the following steps:
(1) copolymerizing an aqueous emulsion in the first stage of a two-stage polymerization process and in the presence of a free radical catalyst to at least about 90% monomer conversion a mixture of monomers which will re-sult in a copolymer of about (a) 65-100 weight percent VF2 and a second monomer selected from the group consisting of PFP, HFP, CTFE
and a perfluorinated C2-C4 alkyl vinyl ether, the weight ratio of VF2 to the second monomer being within the range 0.67:1 to 3:1, and (b) 0-35 weight percent TFE, the resulting first-stage copolymer having a crystalline melting point of less than about 60°C.;
(2) polymerizing in the second stage in the same emulsion at least one of the same monomers as in the first stage but in such proportions that the second stage polymer, if made separately, would have a crystalline melting point above about 120°C.;
(3) flocculating the resulting dispersion; and (4) spray-drying the dispersion at a temperature of about 121-177°C.;
the relative proportions of the components in the resulting polymer being 65-90 weight percent of the first-stage copolymer and 10-35 weight percent of the second-stage copolymer.

2. The process of Claim 1 wherein the spray-drying step is carried out at 138-157°C.

3. The process of Claim 1 wherein the only fresh monomers added to the emulsion in the second stage are selected from the group consisting of VF2 and TFE.

4. The process of Claim 1 which is conducted batchwise in the same reactor.

5. The process of Claim 3 which is conducted in a continuous, cascade, two-reactor system.

6. The process of Claim 5 which is conducted at a temperature of about 50-130°C. and a pressure of about 250-1500 psig at such a flow rate that at least about 90%
of conversion of monomers is obtained in the first reactor;
the polymer produced in the second reactor containing more than 80 weight percent of VF2 or more than 50 weight percent of TFE; the concentration of the free radical catalyst in the first reactor being about 0.001-2 weight percent based on total monomers; the emulsion being introduced into the second reactor as soon as it is withdrawn from the first reactor.

7. The process of Claim 6 wherein no additional free radical catalyst is added to the second reactor.

8. A fluoroelastomer in powder form prepared by the process of Claim 1.

9. A fluoroelastomer in powder form prepared by the process of Claim 2.

10. A fluoroelastomer in powder form prepared by the process of Claim 6.

11. A vulcanizable fluoroelastomeric composition in powder form consisting essentially of (1) 65-90 weight percent, based on total polymer, of a first-stage copolymer consisting of (a) a copolymer of 65-100 weight percent VF2 and a second monomer selected from PFP, HFP, CTFE, and a C2-C4 PFAVE, the weight ratio of VF2 to the second monomer being within the range 0.67:1 to 3:1; and (b) 0-35 weight percent of TFE; the resulting first-stage copolymer having a crystalline melting point of less than about 60°C.;
(2) 10-35 weight percent, based on the total polymer, of a second-stage polymer of at least one of the monomers of the first-stage copolymer made in the presence of an emulsion of the first-stage copolymer, said second-stage polymer having a crystalline melting point of more than about 120°C.; and (3) about 0-2 weight percent, based on the total polymer, of an anti-tack agent.

12. A composition of Claim 11 wherein the amount of anti-tack agent is about 0.5-2 weight percent.

13. A composition of Claim 12, wherein the anti-tack agent is silica.