GB2318794A

GB2318794A - Free flowing polymer blend dispersions and their preparation

Info

Publication number: GB2318794A
Application number: GB9722941A
Authority: GB
Inventors: Navjot Singh; Joseph Michael Anostario
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1996-11-01
Filing date: 1997-10-30
Publication date: 1998-05-06
Anticipated expiration: 2017-10-30
Also published as: GB2318794B; FR2755442B1; JPH10182845A; FR2755442A1; GB9722941D0; DE19746991A1

Abstract

A method of preparing a blend of a polymer A with a Tg, Tg a with a polymer B with a Tg, Tg b or a melting temp Tm b , where Tg a < Tg b and Tg a < Tm b , comprises mixing the two polymers at a temp between Tg a and the lower of Tg b and Tm b for a time and under shear conditions sufficient to convert polymer A to dispersed free-flowing particles coated with polymer B. Examples of polymer A are polysiloxanes, EP rubber, polybutadiene, polyisoprene, neoprene, and acrylic rubbers. Examples of polymer B are thermoplastics such as polyethylene, polypropylene, PVC, linear polyesters, polyvinyl aromatics such as polystyrene, styrene/acrylonitrile polymers, polyphenylene ethers, polyimides, polyethersulphones, polyetherketones and polyarylene sulphides. In the examples the polymers are mixed in a Waring blender or a Henschel mixer.

Description

2318794 SOLID POLYMER DISPERSIONS AND METHOD FOR THEIR PREPARATION

Background of the Invention

This invention relates to polymer dispersions in solid form and a method for their preparation. More particularly, it relates to the preparation of polymer blends in solid form.

The use of elastomeric (i.e., rubbery) polymers as additives in blends comprising other polymers is known. Various rubbery polymers are useful as impact modifiers, flame retardants and additives conferring other properties on blends in which they are incorporated. Among the polymers which confer advantageous properties are polyorganosiloxanes, particularly those having very high molecular weight and existing in the form of gums.

It is difficult, however, to prepare homogeneous blends of rubbery polymers with other resins, owing to the relative intractabilities of said rubbery polymers and the slow progress of dispersion of said polymer in the blend. Blending usually requires equipment capable of producing extremely high shear, and even then the production of a homogeneous blend containing more than about 20% rubbery polymer is difficult or impossible. Thus, it is problematic at best to prepare masterbatches or blends in which the rubbery polymer is a major or even a significant constituent.

Various patents, including US Patents 3,824,208, 5,153,238, 5,391,594 and 5,412,014, describe the incorporation of fillers such as silica in rubbery polymers to form compositions which exist as free-flowing particles. However, it is sometimes highly desirable to exclude inorganic materials such as silica from polymer blends. It is further desirable to produce such blends in solid, free flowing form capable of extrusion and pelletization.

Summary of the Invention

The present invention facilitates the formation of polymer blends as described hereinabove. In particular, it makes it possible to prepare blends which are solid and free-flowing, said blends comprising high and often major proportions of such rubbery materials such as polyorganosiloxanes and synthetic elastomers, said blends also containing another resinous constituent. Among the blends that can be produced are those useful as products in their own right and those useful as masterbatches suitable for incorporation as additives in other polymer compositions.

In one of its aspects, the invention is a method for preparing a blend of: polymer A having glass transition temperature Tga and polymer B having glass transition temperature Tgb or melting temperature Tmb, where Tga<Tgb and Tga<Tmb, which comprises intimately mixing said polymers at a temperature between Tga and the lower of Tgb and Tmb, for a time and under shear conditions sufficient to convert polymer A to dispersed particles coated with polymer B and produce a free-flowing blend.

Another aspect is compositions comprising blends of polymers A and B as described above, said blends being free-flowing at temperatures between Tga and Tgb or Tmb and comprising particles of polymer A coated with polymer B. Detailed Description; Preferred Embodiments

Polymer A employed according to the present invention is one which has a relatively low glass transition temperature Tga. The value of Tga is generally below about WC and may be below OC. For example, polydiorganosiloxane gums useful in the invention may have Tg values down to about -12TC. Polymer A also typically has a high viscosity, most often in the range of about 500,0005,000,000 centipoise at a shear rate on the order of 10 sec-1.

Polyorganosiloxanes, especially polydialkylsiloxanes such as polydimethylsiloxane and their fluorinated derivatives such as poly(trifluoropropyimethyisiloxane), are usually preferred as polymer A. However, other rubbery polymers including ethylenepropylene rubbers, polybutadiene, polyisoprene, neoprene (polychloroprene) and acrylic rubbers, such as poly(ethyl acrylate), poly(isobutyl acrylate) and poly(n-butyl acrylate) may also be employed.

Polymers useful as polymer B may be amorphous or crystalline. When amorphous, they are characterized by their Tg value; when crystalline, the crystalline melting temperature (Tm) may be more significant. In any event, Tgb or TMb is greater than Tga. Thus, there is a temperature span which is above the glass transition temperature of polymer A and below the glass transition temperature or crystalline melting temperature of polymer B. Illustrative polymers useful as polymer B include olefin polymers such as polyethylene and polypropylene, polycarbonates, poly(vinyl chloride), linear polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate), vinylaromatic polymers including polystyrene and styrene-acrylonitrile copolymers, polyphenylene ethers, polyimides (including po)yethe rim ides), polyethersulfones, polyetherketones and polyarylene sulfides. Especially preferred are those polymers having glass transition temperatures above about 15TC. Most preferred in. many instances are polyphenylene ethers, especially poly(2,6-dimethy]-1,4phenylene ether).

In the method of the invention, polymers A and B are mixed under high shear conditions, at a temperature higher than Tga and lower than the lower of Tgb and Tmb. Mixing is generally conducted in one or more discrete steps rather than continuously as in an extruder, and under high shear conditions sufficient to produce a composition of the type described hereinafter. High shear mixers of this type are known in the art and include Waring blenders, Henschel mixers, Drais mixers and mixer-granulators of the type manufactured by Littleford Bros., Florence, KY.

in general, both polymers are charged in their entirety before mixing begins. It is within the scope of the invention, however, to add polymer A incrementally to polymer B, so as to maintain conditions under which a dispersion of polymer A in solid polymer B is formed.

It has been shown that initially, a dispersion of gum (polymer A) in solid (polymer B) is formed. During the high shear mixing process, a progressive breakdown of the particle size of polymer A occurs. Simultaneously, the particles of polymer B coat those of polymer A to form a solid, particulate blend which is a solid dispersion of polymer A in polymer B and which is free-flowing at temperatures below Tgb and TMb.

The proportions of polymers A and B, as well as the mixing time and conditions, are chosen to ensure that all particles of polymer A are dispersed and coated. If the mixing time is too long, polymer A will form particles so small that the quantity of polymer B will be inadequate to fully coat them, whereupon reagglomeration will take place immediately or upon storage.

Thus, suitable proportions and mixing conditions can be determined by simple experimentation. In general, weight ratios of polymer B to polymer A in the range of about 1-5:1 are preferred. In the case of a rotary blade mixer, blade tip velocities in the range of about 1,500-15,000 cn-dsec are generally adequate to produce the required high shear mixing.

The blending temperature is not critical, so long as it is between Tga and Tgb or Tmb. In the preferred situation where Tga is below about TC and Tgb or TMb is above 150Q blending at moderate temperatures in the range of about 20-75Q and especially at ambient temperature of about 25Q is satisfactory.

However, suitable blending temperatures can be chosen even in less preferred situations. For example, polyethylene with a Tg of about 11 OOC may be employed as polymer A with a polyphenylene ether having a Tg of 21 OOC as polymer B, if blending is at a temperature typically around 1500C.

Following the blending operation of the present invention, it is usually preferred to extrude and to pelletize the polymer blend of the invention to form a storable material. Depending on the constituents employed, this storable material may itself be a useful polymer composition or may be a masterbatch or an additive for incorporation of other polymer compositions.

The invention is illustrated by the following examples.

All parts are by weight.

Example 1

A mixture of 25 parts of a vinyl-terminated polydimethylsiloxane gum having a viscosity of about 3.9 million centipoise at a shear rate of about 10. 14 sec-1 and 100 parts of a po ly (2-6-d i methyl- 1 4-phe nylene ether) having an intrinsic viscosity of 0.4 dilg (in chloroform at 25C) was mixed in a Waring blender at high speed for 10 minutes. The desired blend was obtained a free-flowing powder, with 2.36 parts of unblended silicone remaining. The blend was capable of being molded, as shown by a compression molding operation at 300C.

Example 2

The procedure of Example 1 was repeated, except that mixing was conducted in a Henschel mixer. The product was a free flowing powder capable of extrusion and molding.

Example 3

The procedure of Example 2 was employed to prepare a free-flowing blend of 4 parts of polyethylene powder and 1 part of methyl-stopped polydimethylsiloxane gum having a viscosity of about 3,900,000 centipoise at 10.14 sec-1. The blend was capable of extrusion and molding.

Example 4

The procedure of Example 3 was repeated, substituting 4 parts of polystyrene powder for the polyethylene powder. A similar product was obtained.

Example 5

The procedure of Example 4 was repeated, substituting 4 parts of bisphenol A polycarbonate powder for the polyethylene powder. A similar product was obtained.

Example 6

The procedure of Example 1 was repeated, using a blend of 1 part each of the polyphenylene ether and an ethylenepropylene rubber. A well dispersed, free-flowing powder with a shelf life of at least one month was obtained.

Claims

CLAIMS:

1. A method for preparing a blend of: polymer A having glass transition temperature Tga and polymer B having glass transition temperature Tgb or melting temperature TMb, where Tga<Tgb and Tga<Tmb, which comprises intimately mixing said polymers at a temperature between Tga and the lower of Tgb and Tmb, for a time and under shear conditions sufficient to convert polymer A to dispersed particles coated with polymer B and produce a free-flowing blend.

2. A method according to claim 1 wherein Tga is below about 25%.

3. A method according to claim 2 wherein Tga is below -120C.

4. A method according to claim 2 wherein polymer B is crystalline.

5. A method according to claim 2 wherein polymer B is amorphous and Tgb is above about 15TC.

6. A method according to claim 1 wherein polymer A is a polyorganosiloxane, an ethylene-propylene rubber, polybutadiene, polyisoprene, neoprene or an acrylic rubber.

7. A method according to claim 6 wherein polymer A is a polydiorganosiloxane.

8. A method according to claim 6 wherein polymer B is an olefin polymer, a polycarbonate, poly(vinyl chloride), a linear polyester, a vinylaromatic polymer, a polyphenylene ether, a polyimide, a polyethersulfone, a polyetherketone or a polyarylene 5 suffide.

9. A method according to claim 8 wherein polymer B is a poly(2,6-dimethy]1,4-phenylene ether).

10. A method according to claim 1 wherein said polymers are mixed in a rotary blade mixer at a blade tip velocity in the range of about 1,500-15, 000 cm/sec.

11. A composition comprising a blend of:

polymer A having glass transition temperature Tga and polymer B having glass transition temperature Tgb or melting temperature TMb, where Tga<Tgb and Tga<TMb, said blend being free-flowing at temperatures between Tga and Tgb or TMb and comprising particles of polymer A coated with polymer B.

12. A composition according to claim 11 wherein Tga is below about 2TC.

13. A composition according to claim 12 wherein Tga is down to about -120C.

14. A composition according to claim 12 wherein polymer B is crystalline.

15. A composition according to claim 12 wherein polymer B is amorphous and Tgb is above about 1 STC.

16. A composition according to claim 11 wherein polymer A is a. polyorganositoxane, an ethylene-propylene rubber, polybutadiene, polyisoprene, neoprene or an acrylic rubber.

17. A composition according to claim 16 wherein polymer A is a polydiorganosiloxane.

18. A composition according to claim 17 wherein polymer A is a polydimethylsiloxane.

19. A composition according to claim 16 wherein polymer B is an olefin polymer, a polycarbonate, poly(vinyl chloride), a linear polyester, a vinylaromatic polymer, a polyphenylene ether, a polyimide, a polyethersulfone, a polyetherketone or a polyarylene 5 sulfide.

20. A composition according to claim 19 wherein polymer B is a poly(2,6dimethyi-1,4-phenylene ether).