WO2014108382A1

WO2014108382A1 - Polyethylene composition

Info

Publication number: WO2014108382A1
Application number: PCT/EP2014/050118
Authority: WO
Inventors: Carolus Johannes Augustinus ENGELS; Maria Soliman; Dimphna Joahnna Maria VAN BEEK
Original assignee: Saudi Basic Industries Corporation
Priority date: 2013-01-10
Filing date: 2014-01-07
Publication date: 2014-07-17

Abstract

The invention relates to a polyethylene composition comprising (a) multimodal polyethylene (b) vitamin E type stabiliser (c) polyphenolic compound comprising ester groups and (d) phosphite wherein the weight ratio (b): ( c+d) ranges between 1:1 and 1:2. The composition is applied in the production of pipes having improved resistance to chlorine dioxide containing water.

Description

Polyethylene composition

The present invention relates to a multimodal polyethylene composition suitable for pipes for transport of drinking water.

The production processes for bimodal high density polyethylene (HDPE) are summarised at pages 16-20 of "PE 100 Pipe systems" (edited by Bromstrup; second edition, ISBN 3-8027-2728-2).

The production of bimodal high density polyethylene (HDPE) via a low pressure slurry process is described by Alt et al. in "Bimodal polyethylene-Interplay of catalyst and process" (Macromol.Symp. 2001 , 163, 135-143). Bimodal high density

polyethylene may be produced via a low pressure slurry process for the production of comprising a polymerisation stage, a powder drying stage, an extrusion and pellet handling stage, a recycling stage and a wax removal unit. In a two stage cascade process the reactors may be fed continuously with a mixture of monomers, hydrogen, catalyst/co-catalyst and diluent recycled from the process. In the reactors,

polymerisation of ethylene occurs as an exothermic reaction at pressures in the range between for example 0.5 MPa (5 bar) and 1 MPa (10 bar) and at temperatures in the range between for example 75 °C and 85 °C. The heat from the polymerisation reaction is removed by means of cooling water. The characteristics of the polyethylene are determined amongst others by the catalyst system and by the concentrations of catalyst, co monomer and hydrogen. The production of bimodal high density polyethylene (HDPE) via a low pressure slurry process may also be performed via a three stage process.

The concept of the two stage cascade process is elucidated at pages 137-138 by Alt et al. "Bimodal polyethylene-Interplay of catalyst and process" (Macromol. Symp. 2001 ,163). The reactors are set up in cascade with different conditions in each reactor including low hydrogen content in the second reactor. This allows for the production of HDPE with a bimodal molecular mass distribution and defined co monomer content in the polyethylene chains.

Suitable catalysts for the production of bimodal polyethylene include Ziegler Natta catalysts, chromium based catalysts and single site metallocene catalysts. In all potential possible technologies the process and the catalyst have to form a well balanced system. The catalyst is crucial for the polymerisation of bimodal polyethylene polymerisation reaction. By cooperation of process and catalyst a definite polymer structure is produced.

The bimodal polyethylene composition is suitable for pipe applications for the transport of gas, waste water and drinking or potable water. Pipes have a very good resistance to water however their life is shortened when the pipes come into contact with disinfectants which are often added to water for hygienic reasons. The chlorine dioxide used as disinfectant in water degrades most materials including polyethylene (Colin, Aging of polyethylene pipes transporting drinking water disinfected by chlorine dioxide, part I, Chemical aspects; Polymer engineering and Science 49(7); 1429- 1437;July 2009). It is known in the art to apply additives for example antioxidants and stabilizers to prevent said degradation. Several types of additives are proposed to protect polymers during processing and to achieve the desired end-use properties. However, appropriate combinations of stabilizers have to be carefully selected, depending on the desired final properties the polymeric article should have.

It is the object of the present invention to provide a multimodal such as a bimodal polyethylene grade for pipe applications for the transportation of chlorine dioxide containing drinking or potable water with improved service lifetime.

The polyethylene composition according to the invention comprises

(a) multimodal polyethylene

(b) vitamin E type stabiliser

(c) polyphenolic compound comprising ester groups and

(d) phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

The multimodal polyethylene may be for example trimodal or a bimodal polyethylene.

According to a further preferred embodiment of the invention the polyethylene composition according to the invention comprises

(a) bimodal polyethylene

(b) vitamin E type stabiliser

(c) polyphenolic compound comprising ester groups and

(d) phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

The composition according to the invention results in a chlorine dioxide resistant bimodal polyethylene grade. A drinking water pipe based on this grade has an improved protection against chlorine dioxide containing water and consequently a longer life time.

According to a preferred embodiment of the invention the weight ratio (b): (c+d) ranges between 1 :1 and 1 :1.5

More preferably the weight ratio (b): (c+d) ranges between 1 :1.1 and 1 :1.3. Preferably (a), (b) and (c) are added during the granulation step of the bimodal high density polyethylene powder.

According to a preferred embodiment of the invention the components are added to the polyethylene resin while the polyethylene is in a molten state during extrusion.

The components may be added together and may be added separately.

Preferably the components are added in one step.

Preferably the amount of vitamin E in the composition is lower than 1.0 wt%, more preferably of lower than 0.6 wt%. Most preferably this amount ranges between 0.4 and 0.6 wt%. This amount protects the pipe against chlorine dioxide during a long period.

Vitamin E consists of eight naturally occurring vitamers, a family of four tocopherol and four tocotrienol (Drotleff, Eur food Res Technol (1999) 210: 1-8).

The vitamin E type stabiliser may be a tocopherol according to the following formula:

wherein R¹ is an alkyl having 1 to 18 carbon atoms or an alkenyl having 2 to 18 carbon atoms, R² is alkyl having 1 to 5 carbon atoms, R³, R⁴ and R⁵ are the same or different and each is hydrogen atom or alkyl having 1 to 4 carbon atoms and R⁶ is hydrogen atom or alkyl having 1 to 5 carbon atoms.

A preferred vitamin E type stabiliser is tocopherol wherein R¹ is alkyl having 16 carbon atoms and R² is methyl.

The tocopherol may be a- tocopherol, β- tocopherol , γ- tocopherol and/or δ- tocopherol.

A preferred tocopherol is D,L-alfa-tocopherol.

Suitable polyphenolic compounds comprising ester groups include for example tetrakis[methylene-3-(3',5')-di-t-butyl-4-hydroxyphenyl)propionate]methane; bis-[3,3-bis- (4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester; 5-di-t-butyl-4- hydroxyhydrocinnamic acid triester with 1 ,3,5-tris(2-hydroxyethyl)-s-triazine-2,4,6(1 H, 3H, 5H)trione and mixtures. According to a preferred embodiment of the invention the polyphenolic compound is selected from tetrakis[methylene-3-(3',5')-di-t-butyl-4-hydroxyphenyl) propionate] methane and bis-[3,3-bis-(4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester or mixtures thereof .

According to a further embodiment of the invention the phosphite may be a monophosphite, diphosphites and/or polyphosphites.

Suitable examples of phosphite include tris(2,4-di-t-butylphenyl)phosphite, trisnonylphenyl phosphite, trilauryl phosphite, di-isooctylphosphite, triisodecyl phosphite, diisodecylphenylphosphite, diphenyl isodecyl phosphite, triphenyl phosphite and mixtures.

A preferred phosphite is tris(2,4-di-t-butylphenyl) phosphite.

According to a further preferred embodiment of the invention the polyethylene composition comprises

(a) bimodal polyethylene

(b) Vitamin E

(c) tetrakis [methylen- 3-(3 ',5 ')-di-t-butyl-4 '- hydroxyl phenyl)propionate] methane and/or bis-[3,3-bis-(4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester and

(d) tris (2,4-di-t-butylphenyl)phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

More preferably the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :1.5.

More preferably the weight ratio (b): ( c+d) ranges between 1 :1.1 and 1 :1.3. Preferably, the density of the uncoloured bimodal polyethylene base resin is higher than 945 kg/m³

Preferably, the melt flow index of the uncoloured bimodal polyethylene base resin ranges between 0.15 g/10 min and 0.36 g/10 min (load 5 kg).

More preferably the melt flow index ranges between 0.15 and 0.30 g/10 min and even more preferably between 0.15 and 0.27 g/10 min.

The bimodal ethylene polymer may be an ethylene homo- or copolymer. The bimodal ethylene grades to be applied in pipe applications may comprise additives such as for example carbon black , calcium stearate, tetrakis[methylene-3-(3',5')-di-t- butyl-4-hydroxyphenyl) propionate] methane and tris(2,4-di-t-butylphenyl) phosphite.

Preferably the amount of vitamin E in the composition is lower than 1.0 wt%, more preferably of lower than 0.6 wt%. Most preferably this amount ranges between 0.4 and 0.6 wt%. This amount protects the pipe against chlorine dioxide during a long period. Preferably the total amount of other stabilisers in the composition is lower than 15 wt%, more preferably of lower than 10 wt%, still more preferably lower than 8 wt. % based on the total polyethylene composition.

The composition may also comprise other additives such as for example a UV stabilizer for example a sterically hindered amine. The weight amount of this stabiliser may range for example between 0.01 and 1 wt%, more preferred between 0.05 and 0.5 wt%, based on the total polyethylene composition.

The composition may also comprise additives for example acid scavengers for example calcium stearate, fillers, minerals and lubricants. The weight amount of additives may range in an uncoloured pipe for example between 0.01 and 4.0 wt%, based on the total polyethylene composition.

The polyethylene composition according to the present invention may be used in the production of articles. This composition is preferably used in pipes and more preferably in pipes to be applied for the transport of cold water.

EP907676B1 discloses a stabilizer composition comprising a) at least one sterically hindered phenol, b) at least one phosphorus-containing secondary antioxidant, and c) at least one tocopherol compound wherein the weight ratio of component (a) to component (b) is from 2:1 to 1 :4 and the weight ratio of

component (a) to component (c) is from 2:1 to 10:1 for the stabilization of

polyethylene-based thermoplastic polymers against degradation, cross-linking and/or discoloration due to the exposure to heat or light, especially in the presence of oxygen. EP907676B1 does not disclose bimodal polyethylene and EP907676B1 does not refer to pipes for drinking water with improved life time expectation under influence of chlorine dioxide containing water.

EP2014704 discloses a polyolefin composition comprising a polyolefin, a vitamin E-type stabiliser, a phenolic stabilizer and optionally an UV stabilizer. The phenolic stabilizer applied in EP2014704 is free of ester groups.

US 2010/133714 discloses a bimodal polyethylene composition, comprising a high molecular weight component having a weight average molecular weight (Mw) of from 400,000 to 950,000; a low molecular weight component having a weight average molecular weight (Mw) of from 3,000 to 100,000, wherein the high

molecular weight component is present in an amount ranging from 25 wt % to 40 wt % of the bimodal polyethylene composition; and a percent die swell of less than 80%. US 2010/133714 gives a solution for the problem that despite their use in commercial products, high density polyethylene (HDPE) blow molding compositions and their corresponding industrial applications still desire improvements in elasticity and die swell. Consequently, HDPE has not typically been used for particular blow molding applications, such as bottles, where a reduced die swell is desired or required. US 2010/133714 provides for improved HDPE compositions and methods for making the same that exhibit improved blow molding properties including

reduced die swell. In US 2010/133714 Irganox 1010 and Irgafos 168 were added in an amount of 0.005% by weight whereas no vitamin is applied. US 2010/133714 does not disclose, does not suggest and does not provide a bimodal polyethylene grade for pipe applications for the transportation of chlorine dioxide containing drinking water with improved service lifetime.

W097/49758 discloses a stabilizer composition for the stabilization of polyethylene-based thermoplastic polymers comprising a) at least one sterically hindered phenol, b) at least one phosphorus-containing secondary antioxidant, and c) at least one tocopherol compound wherein the weight ratio of component (a) to component (b) is from 2: 1 to 1 :4 and the weight ratio of component (a) to component (c) is from 2:1 to 10:1. W097/49758 does not disclose and does not suggest a polyethylene composition comprising (a) bimodal polyethylene, (b) vitamin E type stabiliser, (c) polyphenolic compound comprising ester groups and (d) phosphite wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2. W097/49758 provides a stabilizer composition for the stabilization of polyethylene-based

thermoplastic polymers such that both processing stability and color of the final products are improved. W097/49758 does not disclose, does not suggest and does not provide a bimodal polyethylene grade for pipe applications for the transportation of chlorine dioxide containing drinking water with improved service lifetime.

The invention will be elucidated by means of the following non-limiting exampl

Examples

The PE 100 grade (A) used as base polymer in all examples was a bimodal high density polyethylene (MFR₅ of 0.18 g/10min, density 958 kg/m³).

Examples l-IV and comparative Examples A-B

The compositions l-IV and Comparative Examples A-B use different additive packages in combination with PE100 to protect the polyethylene from attack by chlorine dioxide (see Table 1 ). Table 1

wherein:

• Irganox 1010 :Tetrakis [methylen- 3-(3 ',5 ')-di-t-butyl-4 '-hydroxyphenyl)

propionate] methane commercially available from Ciba Speciality Chemicals,

• Vitamin E: D,L-alfa-tocopherol ; Irganox E 201 commercially available from BASF;

• Hostanox 03 : bis-[3,3-bis-(4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester ; commercially available from Clariant;

· Tocotrienol ; commercially available from: Parachem;

Compounds were compression molded using IS01872-2 resulting in plaques of size of 50x120x2 mm. Ageing test

The compression molded plaques were aged in a continuous water flow at a temperature of 40 °C with a chlorine dioxide concentration maintained at 1 mg/L and a pH maintained at 7.2. Flow rate regulated at 200 L/h. Water hardness to 20 °F. A constant fresh water flow is added during testing allowing full renewal of the testing water each 4 hrs.

The compression molded samples were aged for 60 days.

IR-microscopy was performed on 50 μιη thick slices, cryo microtomed at -100 °C sample- and knife temperature using a metal knife on the Leica RM 2165 microtome from compression molded plaques. The slices are imaged using a Perkin Elmer Spotlight 400 microscope (IR). All samples are measured in transmission.

The oxidation index (01) of each pixel was calculated by taking the ratio of the area between 1660 and 1800 cm^"1 (oxidation) and the area between 1394 and 1330 cm^"1 (PE reference band). This procedure was performed according to ASTM F2102- 06. The result is given in Table 2.

Table 2

From Table 2 it can be concluded that Examples l-IV demonstrate significantly lower oxidation levels after being exposed to water containing chlorine dioxide than

Comparative Examples A-B. In addition, when comparing Examples I and II it can be concluded that the effect of adding vitamin E had a more profound effect than increasing Irganox 1010 on the oxidation levels as obtained after exposure to water containing chlorine dioxide.

Ageing of pipes.

Pipes produced from composition I and comparative example A were aged in the set- up as described in the previous example. The pipes were tested at 6 bar pressure. As the medium is running through the inside of the pipes, only the inner pipe wall is being exposed to chlorine dioxide. Pipes were exposed until failure. A failure is defined as the time at which water is leaking out of the pipe. The results are reported in Table 3. Table 3

From Table 3 it can be seen that the pipes made from composition I show longer time to failure than the pipes made from composition A At the time of compiling the examples, three samples still had not failed for composition I.

Claims

1. A polyethylene composition comprising

(a) multimodal polyethylene

(b) vitamin E type stabiliser

(c) polyphenolic compound comprising ester groups and

(d) phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

2. A polyethylene composition according to Claim 1 comprising

(a) bimodal polyethylene

(b) vitamin E type stabiliser

(c) polyphenolic compound comprising ester groups and

(d) phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

3. A composition according to any one of Claims 1-2 characterised in that the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :1.5.

4. A composition according to any one of Claims 1 -3 characterised in that

component (c) is selected from tetrakis[methylene-3-(3',5')-di-t-butyl-4- hydroxyphenyl)propionate]methane; bis-[3,3-bis-(4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester; 5-di-t-butyl-4-hydroxyhydrocinnamic acid triester with 1 ,3,5-tris(2-hydroxyethyl)-s-triazine-2,4,6(1 H, 3H, 5H)trione and mixtures.

5. A composition according to Claim 4 characterised in that component (c) is selected from tetrakis [methylen- 3-(3 ', 5 ')-di-t-butyl-4 '-hydroxy phenyl) propionate] methane and/or bis-[3,3-bis-(4'-hydroxy-3'-t-butylphenyl butanoic acid]-glycol ester.

6. A composition according to any one of Claims 1-5 characterised in that

component (d) is selected from tris(2,4-di-t-butylphenyl)phosphite,

trisnonylphenyl phosphite, trilauryl phosphite, di-isooctylphosphite, triisodecyl phosphite, diisodecylphenylphosphite, diphenyl isodecyl phosphite, triphenyl phosphite and mixtures thereof.

7. A composition according to Claim 6 characterised in that component (d) is tris (2,4-di-t-butylphenyl)phosphate.

8. A composition according to any one of Claims 1-7 characterised in that the polyethylene composition comprises

(a) bimodal polyethylene

(b) Vitamin E

(d) tris (2,4-di-t-butylphenyl)phosphite

wherein the weight ratio (b): ( c+d) ranges between 1 :1 and 1 :2.

9. A composition according to any one of Claims 1-8 characterised in that the density of the bimodal polyethylene is higher than 945 kg/m³

10. A composition according to any one of Claims 1-9 characterised in that the melt flow index of the bimodal polyethylene ranges between 0.15 g/10 min and 0.36 g/10 min (load 5 kg).

1 1. A composition according to any one of Claims 1 -10 characterised in that Vitamin E is D,L-alfa-tocopherol.

12. An article prepared using the products obtained with a composition according to any one of Claims 1-1 1.

13. A pipe prepared using the products obtained with a composition according to any one of Claims 1-1 1.