Polyamide moulding compounds having high impact strength

Abstract

Polymer blends consisting of from 60-99 % by weight of polyamide and of from 1-40 % by weight of a graft product of polyethylene or copolymers of ethylene with grafted units of (meth)acrylic acid and/or (meth)acrylic acid derivatives and/or maleic acid which graft products have been prepared by a special grafting process.

Description

This invention relates to homogeneous polyamide moulding compositions which contain graft polymers of unsaturated acids and/or their derivatives on polyethylenes to improve their impact strength.

The impact strength of moulded articles produced from polyamide moulding compounds depends to a considerable extent on their water content. In dry state they are sensitive to impact and attain their well known high impact strength only by conditioning, i.e. absorption of water. Conditioning of polyamides is, however, a time-consuming procedure which takes several days, particularly since the water must not be concentrated in the surface layers but must be uniformly distributed throughout the moulded article.

It has therefore repeatedly been attempted to mix polyamides with suitable substances which would impat to them in the dry state the impact strength which could otherwise only be produced by conditioning.

The impact strength of polyamide moulding compounds can be improved, for example, by the incorporation of low molecular weight plasticizers, but these plasticizers have the following disadvantages:

1. A part of the plasticizer evaporates, since in most cases they have a high vapour pressure under the conditions of incorporation into the polyamide.

2. The low molecular weight plasticizers tend to be lost by exudation.

3. They fail to plasticize at low temperatures.

4. The improvement of the impact strength is combined with a considerable drip in the flexural strength and elastic modulus.

It has also been attempted to improve the impact strength of polyamides by the addition of polyethylene. Commercial polyethylene, however, is not compatible with polyamides. Test samples of such alloys undergo severe scaling in the buckling test.

According to German Auslegeschrift No. 1,694,802, the compatibility of polyamides with polyethylene can be improved by the addition of copolymers of ethylene and acrylic acid (derivatives). The compatability remains, however, unsatisfactory.

Improvement of the impact strength of polyamide moulding compounds by the addition of copolymers of ethylene and vinyl acetate has been disclosed in German Auslegeschrift No. 1,138,922.

An improvement in the impact strength of polyamide moulding compounds by the addition of copolymers of ethylene and .alpha., .beta.-unsaturated acids or their derivatives has been described in numerous publications, for example in U.S. Pat. No. 3,742,916 and German Pat. Nos. 1,241,606; 1,544,706 and 1,669,702.

All these proposed solutions for improving the impact strength of polyamide moulding compounds have, however, the disadvantage that they result in a relatively severe loss of rigidity and hence of E-modulus and of flexural strength. This is because the copolymers used have a pronounced plasticizing action.

Blends of polyamides with numerous copolymers and graft polymers in which the impact strength is considerably improved have been described in German Offenlegungsschrift No. 2,622,973. High impact strength mixtures of polyamides with graft polymers of acrylic acid and its derivatives on .alpha.-olefin-vinyl ester copolymers have also been disclosed in German Offenlegungsschrift No. 2,454,770, but the graft polymers used for this purpose are prepared by a complicated method of impregnating the copolymer granulates with graft monomers containing the polymerisation initiator in solution. Moreover, not all the important mechanical properties reach the desired level in these blends.

It was therefore an object of the present invention to provide homogeneous high impact resistent polyamide moulding compositions in which the other characteristic polyamide properties would be largely preserved. According to the present invention, this was achieved by blending polyamides with graft products based on polyethylene or ethylene copolymers and grafted units, e.g. of unsaturated carboxylic acids and/or their derivatives, which were prepared according to German patent application No. P 27 34 105.5.

The present invention therefore relates to polyamide blends comprising:

I. from 60 to 99% by weight, preferably from 70 to 95% by weight of at least one polyamide;

II. from 1 to 40% by weight, preferably from 5 to 30% by weight of graft products consisting of:

(a) from 70 to 99% by weight, preferably from 75 to 95% by weight of a graft substrate of polyethylene and/or copolymers of ethylene and a vinyl ester containing up to 50% by weight of the vinyl ester, preferably vinyl acetate, and (b) from 1 to 30% by weight, preferably from 5 to 25% by weight of grafted units of: (.alpha.) from 0 to 100% by weight, preferably from 0 to 50% by weight of (meth) acrylic acid and/or (.beta.) from 0 to 100% by weight, preferably from 0 to 70% by weight of esters of (meth) acrylic acid formed from an alcohol having from 1 to 8, preferably from 2 to 4 carbon atoms and/or (.gamma.) from 0 to 30% by weight, preferably from 0 to 10% by weight of acrylamide and/or (.delta.) from 0 to 30% by weight, preferably from 0 to 20% by weight of maleic acid anhydride, which graft products are prepared by a process of graft polymerisation in which the solvent free melted polymer used as graft substrate is brought into contact with oxygen or oxygen-containing gases under conditions of vigorous mixing at a pressure of from 1 to 150 bar and a temperature of from 80.degree. to 300.degree. C. for a maximum of 10 minutes, the monomers which are to be grafted are added immediately thereafter with vigorous mixing in the absence of oxygen and oxygen-containing gas, and the residual monomers are removed after polymerisation; and

III. from 0 to 20% by weight, preferably from 0 to 10% by weight of polyethylene, wherein the sum of I-II, of (a) to (b) and of (.alpha.) to (.delta.) must in each case be 100% by weight.

As already mentioned above, the graft polymers which are contained in the blends according to this invention are prepared by a very rational process in which the graft substrate need not, as has hitherto been necessary, be used as a solution or as a latex. Nor is it necessary to provide for prolonged swelling of the graft basis by the graft monomers as disclosed in German Offenlegungsschrift No. 2,454,770. Instead, polyethylene or copolymers of ethylene and vinyl acetate containing up to 50% by weight of vinyl acetate are first reacted with oxygen in the molten state under conditions of vigorous mixing, preferably in an extruder, to form peroxide groups as described in German patent application P 27 34 105.5. Grafting of the .alpha., .beta.-unsaturated carboxylic acids and/or their derivatives on the polyethylene then takes place in a second stage of the process, preferably during the same passage through the extruder, the peroxide groups formed in the first stage providing the initiator radicals.

Suitable graft monomers include acrylic acid, methacrylic acid, the methyl, ethyl, n-butyl, tert. butyl and 2-ethylhexyl esters of these acids, (meth) acrylamide and maleic acid anhydride. Acrylic aid, n-butyl acrylate and tert. butylacrylate are particularly suitable.

The quantity of active oxygen (peroxide groups) introduced in the first reaction step is generally from about 1,000 to 10,000 ppm. No significant reduction in the molecular weight of the graft basis occurs during the peroxidization. The activated centres are uniformly distributed over the graft substrate so that, in contrast to the known graft polymerisation processes, a very large number of active centres are available. A large number of relatively short side chains are therefore formed during the subsequent graft polymerisation.

The graft polymers hitherto known, which are prepared by radical transfer to the polymer which forms the backbone, contain 1 or 2 grafting centres per macromolecule. The molecular weights of the side chains are generally from 300,000 to 1,000,000. The graft polymers used for the polyamide moulding compounds according to the present invention have from 10 to 50 graft centres per macromolecule, and the average molecular weight of the side chains is from 5,000 to 40,000.

According to the present invention, aliphatic polyamides having a relative viscosity of from 2.3 to 4.8, preferably from 2.7 to 4.3 (determined on a 1% by weight solution in m-cresol at 20.degree. C.) are preferably used for the preparation of the blends. Polyamide-6 and/or polyamide-6,6 are particularly preferred but also their mixtures and block polymers or copolymers of caprolactam, adipic acid and hexamethylene diamine.

Preparation of the moulding compounds according to the present invention is preferably carried out in commercial two-shaft extruders, but one-shaft extruders and kneaders may also be used.

When graft polymers prepared according to German patent application No. P 27 34 105.5 were mixed with polyamide, the melt viscosity was found to rise. This rise in viscosity was completely unexpected since it had not been observed when polyamide was mixed with the previously known copolymers and graft polymers containing the same quantity of the same comonomers.

When using the graft polymers prepared according to German patent application No. P 27 34 105.5, therefore, homogeneous moulding compounds which have a relatively high viscosity and excellent impact strength even at low temperatures can be obtained from inexpensive polyamides having a low viscosity. Grafting onto polyamides is evidently much improved compared with the polymers previously used.

To prepare high viscosity moulding compositions, it has hitherto been necessary to use high quality, highly viscous polyamides which had to be prepared from low viscosity polyamides by after-condensation.

Moulded articles produced from the moulding compounds according to the present invention show no signs of white fracture where they have been cut, buckled or subjected to pressure, nor do they show any signs of separation into their components. Furthermore, compared with moulded articles produced from known polyamide-polyolefin mixtures, they have very advantageous surface characteristics.

The moulding composition according to the present invention may also contain fillers and reinforcing substances, processing auxiliaries, nucleating agents, pigments and stabilizers in known quantities. The following are examples: chalk, quartz, wollastonite, microvite, talcum, calcium stearate, TiO.sub.2, carbon black, cadmium sulphide and stearically hindered phenols.

The moulding compounds are suitable for processing in extruders and by injection moulding for the production of high impact resistant moulded articles.

EXAMPLES

Preparation of graft polymers I-XIV according to German patent application No. P 27 34 105.5.

Grafting of the monomers listed in Table 1 onto high pressure polyethylene (BAYLON.RTM. 19 N 430 of Bayer AG) was carried out in a counter rotating double shaft laboratory extruder of the point eight type of Welding Inc. having a shaft diameter D of 20 mm and a length of 48 D. The conditions indicated in the Table were used. The melt was compressed by increasing the root diameter of the shafts in the compression zones. The shafts had a continuous pitch of 1 D.

Air was forced into the induction zone under a pressure of from 40 to 60 bar and at a rate of 80 liters per hour and into the oxidation zone at a pressure of from 20 to 40 bar and at a rate of 240 liters per hour. The graft monomers were fed into the polymerisation zone at a pressure of 5 bar. Unreacted monomers were then evaporated off and solidified as a strand and granulated.

The graft products shown in Tables 2-4 were prepared under the conditions indicated there, in a double shaft extruder having both shafts rotating in the same sense and equipped with self cleaning variable screws 32 mm in diameter and 38 D in length. Compression of the melt was achieved by the insertion of a left handed screw thread, sudden changes in the pitch of the thread, and kneading blocks shifted to the left in the double pitch variable shafts. Air was forced into the induction zone at a pressure of from 20 to 40 bar and at the rate of 100 liters per hour and into the oxidation zone at a pressure of from 20 to 30 bar and at the rate of 300 liters per hour. The graft monomers were fed into the polymerisation zone at a pressure of 2 bar. The residual monomers were removed after graft polymerisation, and the graft product was removed in the form of a strand and granulated.

The graft products shown in Table 2 were grafted onto Baylon.RTM. 19 N 430, those in Table 3 onto Baylon.RTM. 10 M 460 (a copolymer of ethylene and 8% by weight of vinyl acetate) and in Table 4 onto Levapren.RTM. 450 (a copolymer of ethylene and 45% by weight of vinyl acetate).

______________________________________Abbreviations used:______________________________________AS acrylic acid Quantities areMAS methacrylic acid always given inEA ethyl acrylate percent by weight.n-BA n-butyl acrylatet-BA tert. butyl acrylateOA ethylhexyl acrylateAA acrylamideMSA maleic acid anhydride______________________________________ TABLE 1__________________________________________________________________________Graft Product I II III IV V VI VII a__________________________________________________________________________Throughput [g/h] 950 950 1900 750 750 750 750Shaft Speed [revs. per min] 75 75 115 50 50 50 50Average dwell time [min.] 7 7 2.5 10 10 10 10Peroxide content afterOxidation [ppm] 1500 1400 700 1000 1000 1000 1000MFI Bed. E after Oxidation[g/10 min] 7.6 7.5 6.8 7.2 7.1 7.2 7.3Monomer feed [g/h] 70 130 300 130 120 130 150Monomer composition AS 15 parts MSA 20 parts AS 10 parts AS n-BA OA 1 part AA 85 parts n-BA 80 parts n-BA 90 parts t-BA 10 parts AS 89 parts n-BAConversion [%] 99 85 95 65 78 72 53Composition of product 7.3% AS 2.0% MSA 3.1% AS 1.1% AS 12.5% n-BA 12.3% OA 0.2% AA 9.8% n-BA 12% n-BA 10.6% t-BA 1.5% AS 9% n-BAResidual peroxidecontent [ppm] 620 580 280 340 380 410 290MFI Bed. E of product[g/10 min] 1.8 0.5 3.7 5.2 4.8 5.0 2.3(conditions)Intake zone 2D 110.degree. 110.degree. 110.degree. 110.degree. 110.degree. 110.degree. 110.degree.Melting zone 5D 180.degree. 180.degree. 180.degree. 180.degree. 180.degree. 180.degree. 180.degree.Compression zone 2D 180.degree. 180.degree. 180.degree. 180.degree. 180.degree. 180.degree. 180.degree.Induction zone 3D 225.degree. 225.degree. 230.degree. 215.degree. 215.degree. 215.degree. 215.degree.Oxidation zone 6D 190.degree. 190.degree. 210.degree. 170.degree. 170.degree. 170.degree. 170.degree.Compression zone 2D 190.degree. 190.degree. 200.degree. 170.degree. 170.degree. 170.degree. 170.degree.Degasification zone 4D 170.degree. 170.degree. 190.degree. 170.degree. 170.degree. 170.degree. 170.degree.Compression zone 2D 160.degree. 150.degree. 190.degree. 170.degree. 150.degree. 150.degree. 170.degree.Polymerisation zone 8D 140.degree. 150.degree. 195.degree. 170.degree. 150.degree. 150.degree. 190.degree.Compression zone 2D 140.degree. 150.degree. 195.degree. 170.degree. 150.degree. 150.degree. 190.degree.Evaporation zone 7D 140.degree. 150.degree. 195.degree. 170.degree. 150.degree. 150.degree. 190.degree.Extrusion zone 5D 140.degree. 150.degree. 195.degree. 170.degree. 150.degree. 150.degree. 190.degree. 48D__________________________________________________________________________ Temperatures are measured in .degree.C. TABLE 2______________________________________Graft Product VII b VIII______________________________________Throughput g/h 3600 3800Shaft speed 30 40Average dwell time 7.5 7.0Peroxide content after Oxidation ppm 1200 1000MFI Bed. E after Oxidation[g/10 min] 4.3 5.4Monomer feed g/h 450 360Monomer composition 1 part AA 20 parts AS t-BA 79 parts n-BAProduct composition 0.2% AA 2.2% AS 7.6% t-BA 8.9% n-BAResidual peroxide content ppm 540 460Conversion % 90 80MFI Bed. E of the product 0.4 1.2(Conditions)Intake zone 2D 100.degree. 100.degree.Melting zone 4D 140.degree. 140.degree.Compression zone 1D 180.degree. 180.degree.Induction zone 2D 230.degree. 235.degree.Oxidation zone 5D 190.degree. 210.degree.Compression zone 1D 180.degree. 200.degree.Degasification zone 2D 170.degree. 190.degree.Compression zone 1D 180.degree. 200.degree.Polymerisation zone 10D 180.degree. 205.degree.Compression zone 1D 180.degree. 205.degree.Evaporation zone 6D 190.degree. 200.degree.Extrusion zone 3D 210.degree. 200.degree.______________________________________ TABLE 3______________________________________Graft Product IX X XI______________________________________Throughput g/h 3600 3600 3600Shaft speed revs.per min 30 30 30Average dwell time(min) 7.5 7.5 7.5Peroxide content afterOxidation ppm 1300 1300 1300MFI Bed. E afterOxidation [g/10 min] 4.6 4.6 4.6Monomer feed g/h 180 450 400Monomer composition 20 parts AS 1 part AA AS 80 parts n-BA 20 parts AS 79 parts n-BAProduct composition 2.4% AS 0.2% AA 4.9% AS 9.4% n-BA 1.6% AS 8.5% n-BAResidual peroxidecontent ppm 510 480 460Conversion % 99 95 96MFI Bed. E of product 1.5 2.1 1.8(Conditions)Intake zone 2D 100.degree. 100.degree. 100.degree.Melting zone 4D 140.degree. 140.degree. 140.degree.Compression zone 1D 180.degree. 180.degree. 180.degree.Induction zone 2D 235.degree. 235.degree. 235.degree.Oxidation zone 5D 210.degree. 210.degree. 210.degree.Compression zone 1D 200.degree. 200.degree. 200.degree.Degasification 2D 190.degree. 190.degree. 190.degree.zoneCompression zone 1D 170.degree. 180.degree. 180.degree.Polymerisation 10D 150.degree. 180.degree. 180.degree.zoneCompression zone 1D 150.degree. 180.degree. 150.degree.Evaporation zone 6D 170.degree. 190.degree. 190.degree.Extrusion zone 3D 170.degree. 200.degree. 200.degree.______________________________________ TABLE 4______________________________________Graft Product XII XIII XIV______________________________________Throughput g/h 2500 2500 2500Shaft speed revs. per min 25 25 25Average dwell time (min) 10.8 10.8 10.8Peroxide content afterOx. ppm 2500 2500 2500Mooney viscosity afterOxidation 13 13 13Monomer feed g/h 120 320 260Monomer composition 20 parts AS AS 80 parts n-BA t-BAProduct composition 4.6% AS 2.3% AS 8.7% t-BA 9.5% n-BAResidual peroxidecontent ppm 820 850 870Conversion % 98 93 83Mooney viscosity ofProduct 32 27 25(Conditions)Intake zone 2D 60.degree. 60.degree. 60.degree.Melting zone 4D 120.degree. 120.degree. 120.degree.Compression zone 1D 120.degree. 120.degree. 120.degree.Induction zone 2D 210.degree. 210.degree. 210.degree.Oxidation zone 5D 180.degree. 180.degree. 180.degree.Compression zone 1D 120.degree. 120.degree. 120.degree.Degasification zone 2D 100.degree. 100.degree. 100.degree.Compression zone 1D 120.degree. 160.degree. 170.degree.Polymerisation zone 10D 150.degree. 180.degree. 190.degree.Compression zone 1D 150.degree. 180.degree. 190.degree.Evaporation zone 6D 170.degree. 180.degree. 190.degree.Extrusion zone 3D 170.degree. 180.degree. 190.degree.______________________________________

EXAMPLES 1-18

Incorporation of graft products I-XIV was carried out in a commercial double shaft extruder ZSK 53 of Werner and Pfleiderer at temperatures of from 265.degree. to 290.degree. C.

The preparation and composition of the alloys and their properties are summarized in Table 5.

The relative solution viscosities (.eta. rel) were determined on 1% solutions of the products in m-cresol at 20.degree. C.

TABLE 5__________________________________________________________________________Production and properties of the blends of polyamide andthe graft product graft mixing .eta.rel impact Polyamide/ polymer/ further Temperature of the strength flexuralExample .eta.rel % by weight additives .degree.C. blend kg/m.sup.2 strength homogeneity__________________________________________________________________________ 1 PA-6/2,86 I/10 -- 265 3,37 22,3 95,3 very good 2 " II/10 -- 265 3,28 21,6 96 " 3 " III/10 -- 265 3,31 23,9 94,2 " 4 " IV/10 -- 265 3,43 22,6 97,1 " 5 " 10% copoly- -- meres of ethy-Comparison lene with 4% 265 3,08 18,8 97 " by weight AS and 7% b.w. t-BA MFI = 6,5 6 PA-6,6/3,06 V/10 -- 285 3,58 25,2 95,3 " 8 " VIIa/10 -- 285 3,62 27,8 93,8 " 9 " VIIb/20 -- 285 3,85 43,9 86,6 " 7 " VI/10 -- 285 3,47 22,8 94,5 "10 " VIII/10 -- 285 3,42 19,1 95 "11 " IX/10 10% Poly- 280 3,35 23,2 96,2 good ethylene12 " X/10 10% Tal- 290 3,57 15,7 93,4 " kum13 mixture of XI/20 -- 280 3,78 41,2 83 very good 50% by weight PA-6 + 50% b.w. PA-6,614 mixture of XII/10 -- 280 3,41 24,5 98,2 very good each 50% b.w. PA-6 u.-6,615 mixture of XIII/10 -- 280 3,45 21,4 95,1 "- each 50% b.w. PA-6 u.-6,616 mixture of XIV/10 -- 280 3,52 23,6 96,9 "- each 50% b.w. PA-6 u.-6,617 PA-6,6/3,98 IV/10 -- 285 4,39 32,8 94,6 "18 " IV/10 -- 285 4,73 68,3 84,7 "__________________________________________________________________________

Claims

1. A polymer blend consisting of

I. from 60 to 99% by weight of a polyamide;

II. from 1 to 40% by weight of a graft product consisting of

(a) from 70 to 99% by weight of a graft substrate of polyethylene and/or a copolymer of ethylene and a vinyl ester containing up to 50% by weight of the vinyl ester, and

(b) from 1 to 30% by weight of grafted units of

(.alpha.) from 0 to 100% by weight, based on the total weight of grafted units, of (meth) acrylic acid,

(.beta.) from 0 to 100% by weight, based on the total weight of grafted units, of an ester of (meth) acrylic acid formed from an alcohol having from 1 to 8 carbon atoms,

(.gamma.) from 0 to 30% by weight, based on the total weight of grafted units, of acrylamide and

(.delta.) from 0 to 30% by weight, based on the total weight of grafted units, of maleic acid anhydride, the sum total of grafted units (.alpha.) and (.delta.) being 100% by weight of the total grafted units of said graft product and said products having been prepared by a process of graft polymerization which comprises bringing a solvent free melted polymer used as a graft substrate into contact with oxygen or an oxygen-containing gas at a pressure of from 1 to 150 bar and at a temperature of from 80.degree. to 300.degree. C. under conditions of vigorous mixing for a maximum of 10 minutes, adding immediately thereafter the monomers which are to be grafted under conditions of vigorous mixing in the absence of oxygen and oxygen-containing gas, and removing the residual monomers after polymerization; and

III. from 0 to 20% by weight of polyethylene.

2. A blend as claimed in claim 1 which contains from 70 to 95% weight of polyamide.

3. A blend as claimed in claim 1 which contains from 5 to 30% by weight of graft product II.

4. A blend as claimed in claim 1 which contains from 0 to 10% by weight of polyethylene.

5. A blend as claimed in claim 1 wherein component II contains from 75 to 95% by weight of the graft substrate, and from 5 to 25% by weight of grafted units.

6. A blend as claimed in claim 1 wherein the ester of (meth) acrylic acid is formed from an alcohol having from 2 to 4 carbon atoms.

7. A blend as claimed in claim 1 wherein the grafted units are derived from acrylic acid, n-butyl acrylate and tert. butylacrylate.

8. A blend as claimed in claim 1 wherein the polyamide is polyamide-6, polyamide-6,6 or mixtures thereof.

9. A blend as claimed in claim 1 wherein the vinyl ester is vinyl acetate.

10. A moulded article when produced from a blend as claimed in claim 1.