WO1991007465A1

WO1991007465A1 - Polyacetal compositions

Info

Publication number: WO1991007465A1
Application number: PCT/GB1990/001719
Authority: WO
Inventors: Silvestro Cartasegna; Jacques Paul Eugène Joseph HORRION
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1989-11-09
Filing date: 1990-11-08
Publication date: 1991-05-30
Also published as: JPH05503724A; GB8925341D0

Abstract

The impact strength of polyacetal resins can be improved by including a terpolymer comprising an alpha-olefin, a polyunsaturated ester monomer, and a monomer which confers on the terpolymer an ability to form an ion-dipole interaction between the terpolymer and the polyacetal.

Description

POLYACETAL COMPOSITIONS

The present invention relates to blends of

polyacetal and a polymeric impact modifier. The invention also relates to shaped articles formed from such blends.

Polyacetals, also known as polyoxymethylenes, have particularly good heat resistant and mechanical properties, and in particular the mechanical properties vary little within a broad temperature range. The potential applications for polyacetals are therefore extensive. However, they have the disadvantage of a low impact strength. Polyacetals may be moulded to form a variety of shaped articles, including parts for machines, but the low impact strength of the articles renders them weak, especially at surface

imperfections.

Various impact improvers have been proposed for polyacetals. For example, GB-1202454 describes a blend of a polyacetal and a copolymer of an alpha-olefin, and an

alpha-beta unsaturated carboxylic acid, the carboxyl groups of which are at least partially neutralised with metal ions, preferably sodium ions.

However, as discussed in the introduction of US Patent 4556690 this impact improver is not sufficiently effective. US Patent 4556690 itself describes an impact improver comprising a modified alpha-olefin polymer being a graft-copolymer of an alpha-olefin polymer and an unsaturated carboxylic acid or its derivative.

The present inventors have surprisingly found that an impact modifier which comprises an alpha olefin monomer, a polar unsaturated ester monomer and a monomer which confers on the impact modifier an ability to interact with the

polyacetal is particularly effective at improving the impact strength of polyacetal.

The present invention provides a blend of polyacetal and a completely or partially neutralized

terpolymer derived from monomers comprising (i) an

alpha-olefin of 2 to 5 carbon atoms, (ii) a polar unsaturated ester monomer and (iii) a neutralizable monomer which confers on the neutralized terpolymer an ability to form an

ion-dipole interaction between the terpolymer and the

polyacetal.

The forming of the ion-dipole interaction is strong enough to provide sufficient adhesion between the polyacetal and the impact modifier to ensure a homogenous dispersion, provided the ion concentration in the terpolymer is

sufficiently high.

Polyacetals which may be used in the present invention are homopolymers of formaldehyde, trioxane or tetraoxane, copolymers of two or more of these monomers, or copolymers of one of these monomers with one or more of cyclic ethers, cyclic esters, or vinyl compounds.

Examples of cyclic ethers which may be used include ethylene oxide, propylene oxide, oxacyclobutene, styrene oxide, 1,3-dioxalate and epichlorohydrin.

Examples of cyclic esters include betapropi olactone. Examples of vinyl compounds include

isobutylene, styrene, vinyl acetate, ethyl acetate, methyl methacrylate and ethyl methacrylate.

Suitably the polyacetal comprises 90 weight % or more of oxymethylene units, and 10 weight % or less of the co-component (s). The melt index (MI) of the polyacetal is preferably from 0A to 30, more preferably from 2 to 20, as measured at 190°C/2.16 kg in accordance with ASTM 1238E.

Suitable polyacetals which are currently commercially

available include ULTRAFORM H2320 (BASF) which has an MI (otherwise termed melt flow rate, MFR) at 190°C/2.16 kg of 2 .5.

The olefin of the terpolymer may be any olefin of 2 to 5 carbon atoms eg. ethene, propene, butene-1, 2 methyl prop-1-ene or pentene-1. Ethene or propene, particularly ethene, are preferred.

Examples of poly-unsaturated ester monomer which may be used in the terpolymer include vinyl acetate and acrylates or methacrylates such as methyl (meth) acrylate and butyl (meth.) acrylate.

The third monomer of the terpolymer is one which confers on the terpolymer an ability to form an ion-dipole interaction with the polyacetal.

Typical monomer proportions of the terpolymer are 50 to 90 preferably 60 to 85 weight % of alpha-olefin, 5 to 45 preferably 10 to 35 weight % of polyunsaturated ester and 1.0 to 20 preferably 2 to 15 by weight % of the neutralizable monomer (iii).

The polyacetal blend preferably comprises 50% or more of the polyacetal and up to 50% by weight of the

neutralized terpolymer. More preferably the blend comprises 70 to 95% by weight of polyacetal and 5 to 30% by weight of neutralized terpolymer.

The form of interaction between the impact modifier (ie. the neutralized terpolymer) and the polyacetal is an ion-dipole interaction. A dipole is produced between the oxygen atoms and the carbon atoms of the polar polyacetal. In the blend the impact-modifying terpolymer is neutralized, preferably with ions and will therefore form an ion-dipole interaction with the polyacetal.

Neutralization of the terpolymer may be, for example, by means of an amine group, but preferably a metal ion is employed, for example derived from a metal salt or hydroxide. Most preferably the metal is a transition metal. As used herein the term "transition metal" means those metals contained in groups 3 to 12 (new notation) of the Periodic Table as published in Handbook of Chemistry and Physics, 66th edition (1985-86); however the term also includes the metal zinc, since zinc has d-orbitals in its electronic

configuration, albeit filled. Preferably monomer (iii) is an unsaturated monomer containing at least one carboxylic acid group, for example acrylic acid or methacrylic acid. Alternatively monomer (iii) may be, for example, an unsaturated monomer containing at least one sulphonic acid or phosphonic acid group.

A suitable way of neutralizing the terpolymer to form the neutralised impact modifier which is effective with polyacetal is to use, for example, acrylic acid or a

derivative thereof, such as methacrylic acid, maleic acid or itaconic acid as the third comonomer with the alpha olefin and the polar unsaturated ester, and to neutralise the acid in the resulting terpolymer with a metal eg. transition metal salt such as acetate to form an ionomer.

The acid may be wholly or partially neutralised. The neutralisation should be sufficient for the resulting ionomer to adhere to the polyacetal via the ion-dipole interaction. The amount of neutralization will of course depend on the concentration of acid groups in the

un-neutralized terpolymer. Thus in order to achieve an effective level of ion-dipole interaction to give an impact modifying effect to the blend, a terpolymer having a low concentration of acid groups will have to be neutralized to a greater extent than a terpolymer containing a high

concentration of acid groups. Neutralisation may be, for example, from 10 to 90 %, more preferably from 15 to 35%.

Preferred transition metals include zinc, nickel, and cobalt, particularly zinc. A particularly preferred ionomer is a terpolymer of ethylene, methyl acrylate and acrylic acid, which terpolymer is wholly or partly neutralised with zinc. Preferably the terpolymer comprises 60 to 80 % by weight of polyolefin, 15 to 30 % by weight of the polar unsaturated ester monomer (polyunsaturated ester) and 3 to 10 % by weight of acrylic acid or derivative thereof.

The ionomer typically has an MFR at 190°C/2.16 kg (ASTM 1238 E) of 0.5 to 15, generally 1-8, for example about 2. The ionomer may be made using techniques known in the art, eg. copolymer ising the alpha-olefin, the polar

unsaturated ester and monomer (iii) eg. acrylic acid, to form a terpolymer, and reacting this with a neutralizing agent such as metal acetate to form the ionomer. Such ionomers are commercially available.

The ionomer impact modifier is generally incorporated in an amount of up to 50 % by weight with the polyacetal, with the precise amount being adjusted depending on the desired impact properties of the blends.

The impact modifiers of the present invention may be blended with polyacetals to produce fine dispersions of the modifier (for example of particle size in the range 1 to 10 micrometres) within the polyacetal, the dispersed

particles of modifier adhering well to the polyacetal phase.

The terpolymer, prior to neutralization, may be prepared by copolymer ization techniques well known in the art .

The blend of polyacetal and modifier may be made by known methods eg. mixing the two components together in a mixer to obtain a homogenous mixture. Typical mixers which may be used are Brabender, an extruder or an internal mixer.

The blends may be moulded eg. by compression moulding, extrusion moulding or injection moulding to produce shaped articles.

The following examples illustrate the invention. Comparative examples are prefixed by the letter (C).

EXAMPLE 1

The polymers were mixed together in a laboratory

Brabender mixer at 200ºC and 40 rpm until the torque was stabilized. The mixing was then continued for a further 3 minutes to obtain a homogenous mixture. After removal from the mixing chamber the blends were each compression moulded at 190ºC and under 10 tonnes/square metre pressure to room temperature. Specimens for testing were cut from the

compression moulded articles. The modifiers of the invention were ionomers, which formed an ion-dipole interaction between the terpolymer and the polyacetal. Table I gives details of the blends used and their properties after moulding. Impact Strength was measured according to ISO R 180, and flexural modulus according to ASTM D790 MI.

The notched izod impact of blends of the invention, such as blends 3 and 7, compare favourably with blends containing an equivalent amount of a non-ionomeric modifier, such as blend C3. In blends of the invention, notched Izod impact strength increases, with increasing ionomer content (see blends 1 to 4 and blends 5 to 7). In contrast the blends which contain terpolymer which has not been even partially neutralized (C1 to C4) show a lower impact strength and no trend with increasing terpolymer concentration. This is believed to be because such blends do not have the

improved adhesion between modifier and matrix polyacetal which is present in blends of the invention. Indeed it may be seen that C1 to C4 have impact strengths which are only very little different from straight polyacetal (blend C5). In contrast the blends of the invention in all cases show an improvement over straight polyacetal. The degree of impact strength improvement can be controlled by adjusting the concentration of neutralised terpolymer, and by adjusting the chain length in the polar ester monomer (ii).

Claims

1. A blend of polyacetal and a completely or partially neutralized terpolymer derived from monomers comprising (i) an alpha-olefin of 2 to 5 carbon atoms, (ii) a polar unsaturated ester monomer and (iii) a neutralizable monomer which confers on the neutralized terpolymer an ability to form an ion-dipole interaction between the

terpolymer and the polyacetal.

2. A blend as claimed in claim 1 in which (i) is ethylene.

3. A blend as claimed in claim 1 or 2 in which (ii) is vinyl acetate, methyl acrylate, butyl methacrylate or ethyl acrylate.

4. A blend as claimed in any one of the preceding claims in which (iii) is an unsaturated compound containing at least one sulphonic or phosphonic acid group.

5. A blend as claimed in any of claims 1 to 3 in which (iii) is an unsaturated compound containing at least one carboxylic acid group.

6. A blend as claimed in claim 5 in which (iii) is acrylic or methacrylic acid.

7. A blend as claimed in any one of the preceding claims wherein the terpolymer, after neutralisation, contains metal ions.

8. A blend as claimed in claim 7 in which the metal is a transition metal.

9. A blend as claimed in claim 8 in which the metal is zinc, nickel or cobalt.

10. A blend as claimed in claim 1 in which the terpolymer is derived from the monomers (i) ethylene, (ii) methyl acrylate and (iii) acrylic acid and is wholly or partially neutralized with zinc.

11. A blend as claimed in any preceding claim in which the neutralised terpolymer is derived from monomers incorporated in the proportions by weight of from 60 to 85% (preferably 60 to 80%) monomer (i); from 10 to 35%

(preferably 15 to 30%) monomer (ii); and from 2 to 15%

(preferably 3 to 10%) monomer (iii), based on the total of (i), (ii) and (iii).

12. A blend as claimed in any preceding claim in which the terpolymer is from 15 to 35% neutralised.

13. A blend as claimed in any preceding claim which comprises at least 50 weight % polyacetal.

14. A blend as claimed in any preceding claim which comprises from 70 to 95 weight % polyacetal and from 5 to 30 weight % partially or wholly neutralized terpolymer.

15. A shaped article formed from a blend as claimed in any one of the preceding claims.