Preparation of polyethylene waxes

Details Preparation of polyethylene waxes Preparation of polyethylene waxes

2001-03-16

2003-11-18

Killos, Paul J. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S205000

Reexamination Certificate

active

06649788

ABSTRACT:

The present invention relates to a process for preparing polyethylene waxes at from 200 to 350° C. and pressures in the range from 500 to 4 000 bar using molar mass regulators, wherein a peroxide mixture comprising from 5 to 95% by weight of at least one cyclic peroxide of the formula I,
where the radicals R are identical or different and are selected from among alkyl groups and aryl groups, is used as free radical initiator and a molar H
2
/ethylene ratio of from 1:2 000 to 1:40 000 is employed.
The preparation of homopolymers and copolymers of ethylene by high-pressure processes is carried out industrially on a large scale. In these processes, pressures above 500 bar and temperatures of 150° C. and above are used. The process is generally carried out in high-pressure autoclaves or in tube reactors. High-pressure autoclaves are known in squat or elongated embodiments. The known tube reactors (
Ullmanns Encyclopädie der technischen Chemie
, Volume 19, p. 169 and p. 173 ff, (1980), Verlag Chemie Weinheim, Deerfield Beach, Basle, and
Ullmann's Encyclopädie der technischen Chemie,
4th Edition, keywords: waxes, Vol. 24, p. 36 ff., Thieme Verlag Stuttgart, 1977) are easy to handle and have low maintenance requirements and are advantageous compared to stirred autoclaves. However, the conversions which can be achieved in the abovementioned apparatuses are limited and generally do not exceed 30%.
To increase the capacity of available apparatuses, attempts are made to achieve very high conversions. However, limitations are imposed by polymerization temperature and polymerization pressure which, depending on the product type, have a specific upper limit. For LDPE waxes, this upper limit is about 330° C.; above this, spontaneous ethylene decomposition can occur. Furthermore, efforts are made to improve heat removal by means of a very low wall temperature. However, below a temperature of 150° C., heat removal problems can occur as a result of the formation of laminar polyethylene layers which can act as insulator. Furthermore, the pressure drop which occurs is a limiting factor; this pressure drop increases with decreasing temperature.
The conversion can be increased within certain limits by appropriate choice of free radical initiator. Free radical initiators which decompose quickly but can nevertheless be handled safely are desirable. A good method of testing the decomposition rate of a free radical initiator in the high-pressure process is to record the temperature profile. For this purpose, the temperature profile is recorded over the length of the reactor in a polymerization in a high-pressure tube reactor. Immediately after the first introduction of the initiator, the temperature rises due to the polymerization reaction enthalpy liberated and then drops again. At the temperature minimum T
min
, initiator is again introduced and the temperature once more rises steeply and then drops again. At the next temperature minimum, initiator is again metered in. The greater the temperature difference between temperature maximum and minimum, the higher the conversion. A critical indication of the complete reaction of a peroxide is the cooling curve which is steeper when complete decomposition occurs than in cases in which part of the peroxide remains in the reaction mixture even after the actual reaction zone.
In general, a plurality of peroxides of which at least one decomposes at a comparatively low temperature are initially introduced at the starting point, i.e. at the beginning of the reactor.
For various reasons it would be desirable to introduce initiator at a large number of points; however, owing to the high cost of the pumps which are necessary at each introduction point, the number of introduction points is limited by econimic and engineering considerations.
EP-B 0 813 550 discloses that cyclic peroxo compounds of the formulae P
1
to p
3
can be used for polymerizing ethylene in the high-pressure process.
However, it has been found that the conversion is still too low when using the most important conventional free radical initiators. The most important conventional free radical initiators are dibenzoyl peroxide, di-tert-butyl peroxide, tert-butyl perpivalate (“TBPP”) and tert-butyl perisononanoate (“TBPIN”). If the conversion is too low, the economics of the high-pressure process are adversely affected. The conversions when using the peroxides of the formulae P
1
to p
3
are also too low.
The molecular weight of the product in the high-pressure process can be influenced by regulators such as aldehydes, ketones or hydrogen; however, no influence on the covnersion has been found when using conventional peroxides (GB 1,058,967).
It is an object of the present invention to provide a process by means of which the conversion in the high-pressure polymerization of ethylene is increased further.
We have found that this object is achieved by using mixtures of conventional peroxides comprising from 5 to 95% by weight of commercial cyclic peroxo compounds of the formula I and employing a molar H
2
/ethylene ratio of from 1:2 000 to 1:40 000 to increase the conversion further in the preparation of polyethylene waxes by the high-pressure process, thus making it possible to achieve conversions higher than those hitherto customary.
In this formula, the radicals R are identical or different and are selected from among
C
1
-C
8
-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, n-hexyl, isohexyl, sec-hexyl, n-heptyl, n-octyl; preferably linear C
1
-C
6
-alkyl such as methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, particularly preferably linear C
1
-C
4
-alkyl such as methyl, ethyl, n-propyl and n-butyl, very particularly preferably ethyl;
C
6
-C
14
-aryl such as phenyl, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl, 4-phenanthryl and 9-phenanthryl, preferably phenyl, 1-naphthyl and 2-naphthyl, particularly preferably phenyl.
The preparation of such trimeric ketone peroxides can be achieved by condensation of the corresponding ketones with hydrogen peroxide in the presence of strong mineral acids and is described in the literature (for example R. Criegee, in
Methoden der Organischen Chemie
(Houben-Weyl), Vol. 8, p. 46, Georg-Thieme-Verlag, Stuttgart 1952 or EP-A 0 813 550).
The mixtures of the peroxides are made up so that they comprise at least one peroxide decomposing at high temperature, i.e. it does not decompose until a relatively high temperature is reached, and also at least one peroxide decomposing at intermediate temperature.
The distinction between peroxides decomposing at high temperature and peroxides decomposing at intermediate temperature is made by means of the temperatures at which the half lives t
½
. for the decomposition are 10, 1 or 0.1 hours; it is most usual to report the temperature at which the half life is 0.1 hour.
Peroxides decomposing at intermediate temperature have a half life of 0.1 hour at temperatures of from 100 to 140° C.
Peroxides decomposing at high temperature have a half life of 0.1 hour at temperatures above 140° C.
There is a wide choice of commercially available peroxides, for example the Trigonox® or Perkadox® products from Akzo Nobel. Examples of commercially available peroxides decomposing at intermediate temperature are:
didecanoyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoyl-peroxy)hexane, tert-amyl peroxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butyl peroxydiethylisobutyrate, 1,4-di(tert-butylperoxycarbo)cyclohexane as isomer mixture, tert-butyl perisononanoate, 1,1-di(tert-butylperoxy)-3,3,5-tri-methylcyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, methyl isobutyl ketone peroxide, tert-butylperoxy isopropyl carbonate, 2,2-di(tert-butylperoxy)butane and tert-butyl peroxyacetate.
Examples of conventional commercially available peroxides decomposing at high temperature are:
tert-butyl peroxybenzoate, di-tert-amyl peroxide, dicu

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