GB2134122A - Process for preparing improved impact resistant propylene copolymers - Google Patents

Abstract

High impact strength propylene copolymer compositions of controlled rheology are produced by subjecting a propylene ethylene block copolymer component blended with from 1 to 30 wt% of an ethylene-propylene elastomer component to a free radical treatment at a temperature of from 400 DEG to 550 DEG F (204 DEG to 288 DEG C).

Description

SPECIFICATION Process for preparing improved impact propylene copolymers A traditional problem experienced with high impact propylene copolymers is that there is typically a trade-off in impact strength vs. melt flow, i.e. the impact strength decreases as the melt flow is increased. As a result, high impact copolymers having low melt flows are difficult to injection mold in thin parts, e.g. battery cases.

The prior art discloses various methods for improving the melt flow/impact strength relationship by oxidative degradation techniques. For instance, Japanese Publication No. 21731/73 discloses improved results obtained by heat treating an ethylene-propylene block copolymer in the presence of a peroxide. Also, U.S. Patents Nos. 3,940,379 and 4,061,694 disclose processes for the oxidative degradation of various substantially crystalline propylene polymers. A necessary feature of the latter two processes is that the oxidative degradation must be carried out in the presence of oxygen and a peroxide.

Although a substantial improvement in melt flow rates has been obtained by these prior art methods, they have not been abie to produce resins which are suitable for injection molding into rigid thin-walled articles having high impact and tensile strengths, especially at low temperatures.

In accordance with the invention there is provided a process for preparing a high impact strength polypropylene copolymer composition of controlled rheology which comprises: I blending (a) from 70 to 99 wt % of a block copolymer containing from 70 to 90 wt % of a propylene polymer portion and from 30 to 10 wt% of an ethylene-propylene copolymer block portion, and (b) from 30 to 1 wt % of any ethylene-propylene elastomer component, wherein the total content of ethylene-derived units of the blend is between 5 and 20 wt %; and II treating the blend in the absence of added oxygen under controlled oxidative degradation conditions with a free radical initiator at a temperature from 4000F to 5500F (204 to 2880C).

It is believed that in the process the polypropylene segments of both the block copolymer (a) and the elastomer (b) undergo chain scission to a higher melt flow, and the ethylene polymer segments of these components crosslink forming higher molecular weight tougher segments. Also, with the simultaneous formation of propylene polymer radicals and ethylene polymer radicals, some of these will interact forming impact modifying grafts. The predominant reaction will be chain scission resulting in increased melt flow, while loss of impact strength will be minimized by formation of more "rubber" and high molecular weight "rubber" from the grafting and crosslinking.

The process results in resin products which have substantially increased melt flows while largely retaining the physical properties of the original lower melt flow resin.

The propylene polymer portion representing 70 to 90 wt % of the block copolymer component (a), is either propylene homopolymer or a random copolymer of propylene and small amounts of ethylene.

Up to 8 wt % of ethylene derived units can be present in said propylene portion. The amount of ethylene-derived units in the total block copolymer is preferably 4 to 1 5 wt %.

Block copolymers of this type can be produced by well known catalytic polymerization techniques and are commercially available. Although for the purpose of this'invention the block copolymer is defined as a single compound, it should be understood that the block copolymerization processes actually result in mixtures of block copolymers with minor amounts of homo and random copolymer of ethylene and propylene.

The ethylene-propylene elastomer component (b) can also be any of the well known and commercially available EP rubbers, which are copolymers of ethylene and propylene containing typically between 30 and 70 wt % polymerized ethylene. A third monomer can, if desired, be incorporated in the ethylene-propylene rubber in order to increase the ability of the rubber to crosslink. The third monomer suitably is a nonconjugated diene containing typically from 6 to 8 carbon atoms, and having one terminal and one non-terminal double bond, e.g. 1 ,4-hexadiene or 1,5-heptadiene. The resulting terpolymers are known in the art as EPDM rubbers and are available commercially. In order to facilitate processing and blending, the elastomer is sometimes compounded with 30 to 50 wt % polypropylene or polyethylene based on the total weight.

The amount of the elastomer component whether it is EP rubber, EPDM rubber or a mixture of one of these with either polypropylene or polyethylene, should not exceed 30 wt % based on the total weight of the resin components (a) and (b) of the composition produced by the process of this invention, and preferably between 3 wt % and 1 5 wt %.

The resin blend to be treated by the oxidative degradation in step II usually has a melt flow rate from 0.1 to 5 g/1 0 min. ( 2300C and about 2.16 kg plunger load.

The free radical initiator used in the controlled oxidative degradation treatment of the resin blend should be one that decomposes normally, i.e. have at least 3 to 6 but preferably no more than about 1 5 half lives during typical extrusion conditions at 400O to 5500F (20pro to 2880C).

Many organic peroxides are useful for this purpose either singly or in combination, e.g. dicumyl peroxide; di-t-butyl peroxide, t-butyl hydroperoxide; t-butyl perbenzoate; 2,5-dimethyl-2,5-di-t-butyl peroxyhexane and 2,5-dimethyl-2,5-di-t-peroxyhexyne. The initiator concentration should broadly be from 100 to 1500 ppm and preferably between 300 and 1000 ppm, depending upon the initial melt flow and the final melt flow desired.

The controlled oxidative degradation suitably is carried out in an extruder or similar equipment which converts the resin to pellets.

An extruder can be operated under normal pelletizing conditions, between 4000F and 5500F (2040 to 2880C). Typical melt residence times in an extruder range from 0.5 minutes to 3 minutes. The extrusion operation is conducted in an inert atmosphere, i.e. oxygen or air is prevented from entering the extruder during the oxidative degradation treatment. The extrudate can be pelletized with normal commercial equipment.

The process is readily controlled and yields products of high melt flow rates, e.g. from 10 to 75 g/l 0 min. at 2300C, and even higher, and are especially suitable for injection molding of thin-walled articles of desired stiffness and having good impact strengths at room and lower temperatures. The oxidatively degraded resin blend is completely homogeneous and is particularly suitable for use in the production of battery cases, gun cases, luggage and fender liners.

The following examples further illustrate the invention.

EXAMPLE 1 Runs were made at several peroxide levels ranging from 0 to 1000 ppm using a pelletized resin blend consisting of 5 parts by weight of Nordel TMNDG 4167, an EPDM elastomer composition(l) available from E.l. du Pont de Nemours and 95 parts of a block copolymer consisting of 86 wt% propylene homopolymer and 14 wt % ethylene-propylene post block, (the ethylene content in the block amounted to about 40%). In each of the runs, the resin blend was further blended with the amount of peroxide indicated in Table 1 and subsequently extrusion pelletized in a 1 cm (2.5 inch) Prodex extruder at 5000F (26000). The results of the runs are summarized in the Table 1.

As shown from the data there was little or no loss in properties up to a melt flow of 26 and even at a melt flow of 66 there was a slight loss only in impact strength.

(1) 33 wt % of a high density polyethylene and 67 wt % of a terpolymer of ethylene and propylene, in about equal portions, and additionally small quantities of 1,4-hexadiene.

TABLE 1

di-t-butyl peroxide. ppm 0 300 400 500 1000 Melt Flow-gms/10 min, 230 C 4 11 15 26 66 Tensile Strength, MPa (psi) at Yield 23.86(3460) 22.06(3200) 21.72(3150) 22.6(3200) 21.37(3100) at Failure 17.65(2560) 16.89(2450) 17.50(2538) 17.72(2570) 15.41(2235) Elongation, % 400 > 575 140* > 670 > 575 Tensile Modulus, GPa (10s psi) 1.00(1.45) 1.01(1.46) 1.04(1.51) 1.08(1.56) 0.993(1.44) Flexural Modulus, GPa (10s psi) 0.972(1.41) 0.855(1.24) 0.855(1.24) 0.855(1.24) 0.793(1.15) Deflection Temp.

C ( F) at 0.46 MPa (66 psi) 36(96) 38(101) 39(102) 41(106) 42(107) Izod Impact J/cm (ft-lb/in) 1.8(3.3) 1.4(2.7) 1.5(2.8) 1.2(2.3) 1.2(2.3) Gardner Impact m/kg (in-lb) at -17 C (0 F) > 17.9( > 320) 17.9(320) 11.6-14.3 8.73-14.3 9.69(173) (208-256) (156-256) at -40 C (-40 F) 3.2(57) 5.0(90) 4.1(73) 3.1(56) 1.7(30) *2 of 5 samples elongated differently.

EXAMPLE 2 A pelletized resin blend of components similar to those of Example 1 but blended in a ratio of 90:10 block copolymer/elastomer was subjected to controlled degradation in the presence of 500 parts per million of di-t-butyl peroxide at 5000F (2600C) in a 1 cm (2.5 inch) Prodex extruder. Comparative data are shown in Table 2.

TABLE 2

Gardner Izod Impact Melt Impact @ -400F flow J/cm (-40 C) Example Peroxide gum/10 min (ft-lb/in) m/kg No. ppm @ 230C @ 230C (in-lb) Control 0 2.0 3.4 (6.3) 14.5 (259) 2 500 11.4 2.3 (4.4) 13.8 (246) The controlled degradation resulted in a resin having a desired high melt flow rate and a very high Gardner Impact value, which did not vary significantly from that of the original low melt flow resin.

Claims (15)

1. A process for preparing a high impact strength propylene copolymer composition which comprises: blending (a) from 70 to 99 wt % of a block copolymer containing from 70 to 90 wt % of a propylene polymer portion and from 30 to 10 wt % of an ethylene-propylene copolymer block portion, and (b) from 30 to 1 wt % of an ethylene-propylene elastomer component, wherein the total content of ethylene-derived units of the blend is between 5 and 20 wt%; and II treating the blend in theabsence of added oxygen under controlled oxidative degradation conditions with a free radical initiator at å temperature from 4000F to 5500F (204 to 2880C).

2. A process according to claim 1 , wherein the propylene polymer portion of component (a) is a propylene homopolymer.

3. A process according to claim 1, wherein the propylene polymer portion of component (a) is a random copolymer of propylene and no more than 8 wt % ethylene.

4. A process according to claim 1,2 or 3, wherein the copolymer block portion of component (a) contains from 4 to 1 5 wt % of polymerized ethylene units.

5. A process according to any one of the preceding claims, wherein the elastomer component (b) comprises an ethylene-propylene rubber containing between 30 and 70 wt % polymerized ethylene.

6. A process according to claim 5, wherein the ethylene-propylene rubber is a terpolymer of ethylene, propylene and a nonconjugated diene having from 6 to 8 carbon atoms and only one terminal double bond.

7. A process according to claim 5, wherein the elastomer component (b) is a mixture of the ethylene-propylene rubber and polyethylene or polypropylene.

8. A process according to any one of the preceding claims wherein the elastomer component (b) represents between 3 and 1 5 wt % of the blend in Step I.

9. A process according to any one of the preceding claims wherein the melt flow of the blend in Step I is between 0.1 and 5 g/

1 0 min. at 23000.

1 0. A process according to any one of the preceding claims wherein the free radical initiator is selected from dicumyl peroxide, di-t-butyl peroxide, t-butyl hydroperoxide, t-butyl perbenzoate, 2,5- dimethyl-2,5-di-t-butyl peroxyhexane and 2,5-dimethyl-2,5-di-t-butylperoxyhexyne.

11. A process according to any one of the preceding claims wherein the free radical initiator concentration is between 100 ppm and 1 500 ppm.

1 2. A process according to claim 11, wherein said concentration is maintained between 300 and 1000 ppm.

1 3. A process according to any one of the preceding claims wherein the melt flow rate of the blend after treatment is between 10 and 75 g/1 0 min. at 2300C.

1 4. A process according to claim 1 substantially as described with reference to Example 1 or 2.

15. Shaped articles of a propylene copolymer composition prepared in accordance with a process as claimed in any one of the preceding claims.

1 6. Shaped articles according to claim 1 5 of thin wall structure obtained by injection molding.