WO2018210712A1

WO2018210712A1 - Process for manufacturing polyethylene

Info

Publication number: WO2018210712A1
Application number: PCT/EP2018/062313
Authority: WO
Inventors: Jan Martijn VAN DER SCHUUR; Bart Fischer; Martinus Catharinus Tammer; Maurice Ludovicus Josephina Frijns
Original assignee: Akzo Nobel Chemicals International BV
Current assignee: Nouryon Chemicals International BV
Priority date: 2017-05-16
Filing date: 2018-05-14
Publication date: 2018-11-22
Anticipated expiration: 2019-11-16
Also published as: TW201900686A

Abstract

Process for manufacturing a polyethylene homo-or copolymer by conducting polymerization of ethylene, optionally in combination with one or more co- monomers, at a pressure in the range 500-5000 bar, wherein bis(n-butanoyl) peroxide is used as polymerization initiator.

Description

PROCESS FOR MANUFACTURING POLYETHYLENE

The present invention relates to a process for manufacturing polyethylene by high-pressure polymerization in a tubular reactor.

Low density polyethylene (LDPE) is generally made by high pressure polymerization in either an autoclave reactor (a high pressure continuously stirred tank reactor) or a tubular reactor. The choice of reactor affects the properties of the LDPE. The extent of branching of "autoclave LDPE" is higher than of "tubular LDPE". This is due to the residence time in the reactors. A tubular reactor operates under plug flow conditions, meaning that the entire reaction mixture has the same residence time. In autoclave reactors, back mixing results in a spread of residence times. The result is a higher degree of branching of autoclave LDPE compared to tubular LDPE.

Organic peroxides are generally used to initiate the polymerisation of ethylene, both in autoclave reactors and in tubular reactors. In both reactors, several different organic peroxides are generally used. In autoclave reactors, diacyl peroxides such as di(3,5,5-trimethylhexanoyl)peroxide are at least one of the types to be used; in tubular reactors, peroxyesters like tert-butyl peroxy-2- ethylhexanoate and tert-butyl peroxypivalate are conventionally present.

Ideally, peroxides decompose by unimolecular homolysis of the 0-0 bond. However, various rearrangement and non-radical decomposition reactions reduce the initiator efficiency.

For instance, diacyl peroxides may undergo non-radical decomposition via the so-called carboxy inversion process, yielding acyl carbonates.

Peroxyesters may undergo non-radical decomposition via the Criegee rearrangement, a process analogues to carboxy inversion. It has now surprisingly been found that carboxy inversion can be reduced by using bis(n-butanoyl)peroxide as initiator:

CH₃-CH2-CH2-C(=0)-0-0-C(=0)-CH2-CH2-CH₃

Hence, this peroxide is more efficient than conventionally used peroxides.

In addition, due to its low molecular weight, this peroxide has a high active oxygen content and its decomposition products are volatile and therefore don't end up in the resulting polymer.

The present invention therefore relates to a process for manufacturing a polyethylene homo- or copolymer by conducting polymerization of ethylene, optionally in combination with one or more co-monomers, at a pressure in the range 500-5000 bar, wherein bis(n-butanoyl) peroxide is used as polymerization initiator.

The polymerization is carried out at pressures that are in the range 500-5000 bar, preferably 1000-5000 bar, more preferably 1500-3500 bar, and most preferably 2000-3300 bar.

At lower pressures, the conversion is very low. This is not only uneconomical, but also leads to polyethylenes with a very low molecular weight, so-called polyethylene wax.

The reaction temperature is preferably in the range 100-350°C, more preferably 130-330°C, and most preferably 160-320°C.

The process can be performed in tubular and autoclave (i.e. high pressure stirred tank) reactors, preference is given to the performance in an autoclave reactor. Bis(n-butanoyl)peroxide can be dosed to the reactor 100% pure or, more preferably, as a solution in hydrocarbons, such as odorless mineral spirit, isododecane, chain transfer agents (e.g. butane, propylene, propionaldehyde), or one or more reactive diluents. A reactive diluent is a liquid unsaturated hydrocarbon that can copolymerize with ethylene. Examples of reactive diluents are olefins, more preferably C₆-12 alpha-olefins.

The bis(n-butanoyl)peroxide concentration in such solutions is preferably in the range 5-50 wt%, more preferably 20-40 wt%. According to the present invention, bis(n-butanoyl)peroxide is preferably added to the reactor in amounts of 100 to 1000 ppm (weight parts per million weight parts), more preferably 100-500 ppm, calculated as pure peroxide and based on the weight of monomer. The process of the present invention can be used both for the homo- polymerization of ethylene and for the co-polymerization of ethylene with other monomers, provided that these monomers undergo free-radical polymerization with ethylene under high pressure. Examples of suitable co-polymerizable monomers are α,β-ethylenically unsaturated Ca-Ce-carboxylic acids (e.g. maleic acid, fumaric acid, itaconic acid, citraconic acid, acrylic acid, methacryiic acid, or crotonic acid), α,β-ethylenically unsaturated C₃-Ci₅-carboxylic esters or anhydrides (e.g. methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, or tert-butyl methacrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, 2- ethylhexyl acrylate, tert-butyl acrylate, methacryiic anhydride, maleic anhydride , citraconic anhydride, or itaconic anhydride), a-olefins (e.g. propene, 1 -butene, 1 -pentene, 1 -hexene, 1 -octene, or 1 -decene). In addition, it is possible to use vinyl carboxylates, particularly preferably vinyl acetate, as co-monomers.

The proportion of co-monomers in the reaction mixture is preferably in the range 0-45 wt%, more preferably 3-35 wt%, based on the weight of ethylene monomer. Most preferably, the process is used for the manufacture of ethylene homopolymer, more in particular low density polyethylene homopolymer (LDPE).

The polymer resulting from the process of the present invention preferably has a density in the range 910-940 kg/m³, more preferably 918-926 kg/m³ and most preferably 920-925 kg/m³. The density is mostly controlled by the reactor pressure and temperature profile and can also be influenced by means of the chain regulators and/or co-monomers.

Low density polyethylene (LDPE) is defined as having a density in the range 0.910-0.940 g/cm³.

The melt flow index of the resulting polymer in accordance with DIN 53 735 (190°C/2.16 kg) is preferably less than 50 g/10 min, more preferably less than 10 g/10 min, and most preferably less than 5 g/10 min.

If the process is performed in a tubular reactor, the polymerization initiator(s) is/are preferably introduced into the tubular reactor along the length of the tube at from 1 to 6 inlet points, so that from 1 to 6 reaction zones are obtained in which polymerization is initiated. More preferably 2-6, and most preferably 3-5 initiator inlet points are used and preferably 2-6, and most preferably 3-5 reaction zones are created.

Each of the reaction zones has its own temperature profile.

Bis(n-butanoyl)peroxide is introduced in at least one of the reaction zones. It preferably is introduced in a plurality of reaction zones, alone or in admixture with other peroxides (co-initiators). Most preferably, it is introduced in every reaction zone.

The peroxide or mixture of peroxides that is introduced in each reaction zone can be the same or can differ per zone. An autoclave reactor generally also contains multiple (preferably 1 -6, more preferably 2-4) reaction zones, each zone being isothermal. As a result of this constant temperature per zone, only one type of peroxide is introduced in each zone. Bis(n-butanoyl)peroxide is introduced in one of the reaction zones.

In addition to bis(n-butanoyl)peroxide, one or more co-initiators can be used in the process of the present invention. Such co-initiator may have a higher reactivity (i.e. shorter half-life) or a lower reactivity (i.e. longer half-life) than bis(n-butanoyl)peroxide at a specific temperature.

Co-initiators are preferably selected from the following groups. It is noted that bis(n-butanoyl)peroxide is used at temperatures up to about 200°C.

Group 1 - suitable up to about 160°C: di(2-ethylhexyl)peroxydicarbonate, tert- butyl peroxyneodecanoate, cumyl peroxyneodecanoate, 1 ,1 ,3,3- tertamethylbutyl peroxyneodecanoate, di-sec-butyl peroxydicarbonate, dibuytyl peroxydicarbonate. Di(2-ethylhexyl)peroxydicarbonate and tert-butyl peroxyneodecanoate are the preferred co-initiators of group 1 .

Group 2 - suitable up to about 240°C: tert-butylperoxy 2-ethylhexanoate

Group 3 - suitable in the range 240-280°C: tert-butylperoxy-3,5,5- trimethylhexanoate, tert-butylperoxybenzoate, tert-butyl peroxyacetate, and 2,2- di(tert-butylperoxy)butane. Tert-butylperoxy-3,5,5-thmethylhexanoate and tert- butylperoxybenzoate are the preferred co-initiators of group 3.

Group 4 - suitable above about 280°C: di-tert-butyl peroxide and 3,6,9-triethyl- 3,6,9,-trimethyl-1 ,4,7-triperoxonane.

In tubular reactors, it is conventional to use a mixture of peroxides covering the entire temperature profile. For the production of LDPE, it is preferred to use of a combination of at least four peroxides. One of these peroxides is bis(n- butanoyl)peroxide, the other are chosen from each of groups 2, 3, and 4. For the production of ethylene co-polymers in tubular reactors, it is preferred to use, in addition of bis(n-butanoyl)peroxide, co-initiators from each of groups 1 , 2, and 3. In autoclave reactors, one peroxide is usually added per zone, although it is also possible to add a mixture of peroxides per zone. Each zone is at constant temperature: the temperature may, however, differ per zone. If the autoclave contains more than one zone, the most reactive peroxides - i.e. bis(n- butanoyl)peroxide and optionally a co-initiator of group 1 - is used in the top zone(s), whereas the least reactive co-initiators (groups 3 and/or 4) are used in the bottom zone(s).

In the process of the present invention, the molar mass of the polyethylene to be prepared can be regulated in conventional ways by the addition of molecular weight regulators. Examples of such regulators are aliphatic and olefinic hydrocarbons (e.g. pentane, hexane, cyclohexane, propene, pentene, or hexene), ketones (e.g. acetone, diethyl ketone, or diamyl ketone), aldehydes (e.g. formaldehyde or acetaldehyde), and saturated aliphatic alcohols (e.g. methanol, ethanol, propanol, or butanol). Particular preference is given to using saturated aliphatic aldehydes, in particular propionaldehydes, or a-olefins such as propene or hexene.

After the last introduction of polymerization initiator, the reaction mixture is cooled in order to allow discharge of the product from the reactor. After discharge of the reaction mixture, the polymer is separated from any unreacted monomers by depressurization, after which the monomers can be re-circulated to the reactor. The resulting polyethylene is highly suitable to make high clarity polyethylene films (tubular LDPE), for injection molding applications, wire and cable production, and extrusion coating. EXAMPLE

In this example, the decomposition of bis(n-butanoyl)peroxide and bis(3,5,5- trimethylhexanoyl)peroxide (Trigonox® 36, ex-AkzoNobel) under conditions comparable to those in an ethylene polymerization process was studied.

Solutions of bis(n-butanoyl)peroxide in n-heptane (0.1 M) and bis(3,5,5- trimethylhexanoyl)peroxide in n-octane (0.1 M) were prepared and the peroxides were completely decomposed in a continuous flow reactor at temperatures of 155°C, 175°C, and 195°C, and pressures of 100, 1000, 2000, and 3000 bar. Samples of the totally decomposed peroxides were collected under helium atmosphere and analyzed by gas chromatography. The amounts of decomposition products found were recalculated in moles per mol peroxide. Based on these amounts, mass balances were made. The results are displayed in Tables 1 and 2.

Carboxy inversion products of bis(n-butanoyl)peroxide would be propyl-butanoyi carbonate, propane-propanoic anhydride, and mixed carboxylic carbonic anhydrides. However, no such products were detected.

Carboxy inversion products of bis(3,5,5-trimethylhexanoyl)peroxide are 2,4,4- trimethylpentyl-3,5,5-trimethylhexanoyl carbonate, 2,4,4-trimethylpentane-2,4,4- trimethylpentanoic anhydride, and mixed carboxylic carbonic anhydrides. These products were indeed detected and listed in the Table below as "carboxy inversion products". The results of Tables 1 and 2 show that bis(n-butanoyl)peroxide gives no carboxy inversion products.

Table 1 - Decomposition products and mass balances at 2000 bar and various temperatures. Normalized to 100% mass balance on R. For bis(n-butanoyl)peroxide (Inv), R= propyl and S= heptyl (from solvent). For bis(3,5,5-trimethylhexanoyl)peroxide (Comp), R= trimethylpentyl and S= octyl (from solvent)

Decomp. product 155°C 175°C 195°C (mol/mol peroxide) Inv Comp Inv Comp Inv Comp

1. co₂ 1.346 0.907 1.490 0.912 1.044 0.806

2. R^" (alkane) 0.783 0.657 0.625 0.642 0.754 0.629

3. R⁼ (alkene) 0.088 0.035 0.071 0.037 0.099 0.041

4. R-R 0.325 0.410 0.369 0.422 0.350 0.431

5. R(0)OH 0.088 0.010 0.103 0.008 0.074 0.007

6. R(0)OR 0.129 0.079 0.150 0.073 0.1 16 0.068

7. R-S 0.086 0.135 0.127 0.156 0.1 12 0.172

8. R(0)OS 0.012 0.000 0.012 0.000 0.008 0.000

9. S-S 0.090 0.120 0.122 0.1 14 0.091 0.108

10. S⁼ 0.078 0.000 0.065 0.000 0.059 0.000

1 1. R-OH 0.036 0.043 0.025 0.037 0.022 0.035

12. Carboxy inversion 0.000 0.071 0.000 0.065 0.000 0.059

Mass balance R (%): 100 100 100 100 100 100

Table 2 - Decomposition products and mass balances at 175°C and various pressures. Normalized to 100% mass balance on R. For bis(n-butanoyl)peroxide (Inv), R= propyl and S= heptyl (from solvent). For bis(3,5,5-trimethylhexanoyl)peroxide (Comp), R= trimethylpentyl and S= octyl (from solvent)

Decomp. product 100 bar 1000 bar 2000 bar 3000 bar (mol/mol peroxide) Inv Comp Inv Comp Inv Comp Inv Comp

1. co₂ 1.094 0.826 1.308 0.944 1.490 0.912 1.191 0.945

2. R^" (alkane) 0.649 0.500 0.733 0.606 0.625 0.642 0.772 0.654

3. R⁼ (alkene) 0.099 0.042 0.091 0.039 0.071 0.037 0.083 0.036

4. R-R 0.404 0.516 0.345 0.441 0.369 0.422 0.347 0.419

5. R(0)OH 0.046 0.002 0.068 0.004 0.103 0.008 0.080 0.009

6. R(0)OR 0.090 0.049 0.121 0.066 0.150 0.073 0.124 0.076

7. R-S 0.201 0.234 0.151 0.203 0.127 0.156 0.081 0.1 14

8. R(0)OS 0.003 0.000 0.006 0.000 0.012 0.000 0.015 0.000

9. S-S 0.050 0.063 0.086 0.092 0.122 0.1 14 0.104 0.122

10. S⁼ 0.034 0.000 0.057 0.000 0.065 0.000 0.059 0.000

1 1. R-OH 0.015 0.023 0.020 0.032 0.025 0.037 0.026 0.043

12. Carboxy

0.000 0.035 0.000 0.051 0.000 0.065 0.000 0.077 inversion

Mass balance R (%): 100 100 100 100 100 100 100 100

Claims

1 . Process for manufacturing a polyethylene homo- or copolymer by conducting polymerization of ethylene, optionally in combination with one or more co-monomers, at a pressure in the range 500-5000 bar, wherein bis(n-butanoyl) peroxide is used as polymerization initiator.

2. Process according to claim 1 wherein the polymerization is conducted at a temperature in the range 160-350°C.

3. Process according to any one of the preceding claims wherein the polyethylene is low density polyethylene (LDPE).

4. Process according to any one of the preceding claims wherein the polymerization is conducted in a high-pressure continuously stirred tank reactor.

5. Process according to any one of claims 1 -3 wherein the polymerization is conducted in a tubular reactor.

6. Process according to any one of the preceding claims, wherein one or more co-initiators are used.

7. Process according to claim 6, using, as co-initiator, at least one peroxide selected from the group consisting of di(2-ethylhexyl)peroxydicarbonate, tert-butyl peroxyneodecanoate, tert-butylperoxy 2-ethylhexanoate, tert- butylperoxy-3,5,5-trimethylhexanoate, tert-butylperoxybenzoate, di-tert- butyl peroxide, and 3,6,9-triethyl-3,6,9,-trimethyl-1 ,4,7-triperoxonane.