Medium density ethylene polymers, a process to prepare these...

Details Medium density ethylene polymers, a process to prepare these... Medium density ethylene polymers, a process to prepare these...

2000-11-20

2002-06-18

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S352000, C526S315000, C526S316000, C526S219600, C526S064000, C524S272000, C524S274000

Reexamination Certificate

active

06407191

ABSTRACT:

The present invention relates to medium density ethylene homo and copolymers, and more in particular to medium density LDPE-type (low density polyethylene) resins. In addition, the present invention relates to a high pressure ethylene homo or copolymerization process and to the use of carbonyl group containing chain transfer agents such as ketones or aldehydes, and especially methyl ethyl ketone (MEK) or propionaldehyde, in the polymerization process.
Medium density ethylene homo and copolymers, which polymers have a density of between 0.925 and 0.935 g/cm
3
, are well known in the art. These known polymers can, for instance, be prepared in high pressure radical initiated polymerization processes, wherein a wide variety of different chain transfer agents can be used.
Chain transfer agents or telogens are used to control the melt flow index in a polymerization process. Chain transfer involves the termination of growing polymer chains, thus limiting the ultimate molecular weight of the polymer material. Chain transfer agents are typically hydrogen atom donors that will react with a growing polymer chain and stop the polymerization reaction of said chain. These agents can be of many different types, from saturated hydrocarbons or unsaturated hydrocarbons to aldehydes, ketones or alcohols. By controlling the concentration of the selected chain transfer agent, one can control the length of polymer chains, and, hence, the weight average molecular weight, M
w
. The melt flow index (MFI or I
2
) of a polymer, which is related to M
w
, is controlled in the same way.
After the donation of a hydrogen atom, the chain transfer agent forms a radical which can react with the monomers, or with already formed oligomers or polymers, to start a new polymer chain. This means that any functional groups present in chain transfer agents, for instance carbonyl groups of aldehydes and ketones, will be introduced in the polymer chains.
A large number of chain transfer agents, for example propylene and 1-butene which have an olefinically unsaturated bond, can also be incorporated in the polymer chain, themselves, via a copolymerization reaction. This generally leads to the formation of short chain branching of respectively methyl and ethyl groups, which lowers the density of the polymers obtained.
In many processes, two types of chain transfer agents are used in order to control both the M
w
(and MFI) and the density of the polymers prepared.
The melt flow index of the product polymer can be controlled by varying the amount of chain transfer agent present during the polymerization, usually by mixing varying amounts of transfer agent with the monomer or the mixture of monomers prior to polymerization. Polymers produced in the presence of chain transfer agents are modified in a number of physical properties such as processability, optical properties such as haze and clarity, density, stiffness, yield point, film draw and tear strength.
The use of carbonyl group containing compounds, and especially ketones or aldehydes, such as MEK and propionaldehyde, as chain transfer agent and molecular weight regulator in high pressure polyethylene (co)polymerization processes using autoclave and tubular reactors, is well known for at least 30 years.
DE-OS-19 08 964 teaches an ethylene homopolymer preparation process using two tubular reactors in series, and using organic peroxides as radical initiators. The reaction temperatures and pressures are within the ranges of 250-340° C. and 1500-4000 kg/cm
2
. As an example of a suitable polymerization controller MEK is mentioned. The polymer product is said to have a narrow molecular weight distribution (MWD), a good transparency and a good gloss.
In an article in the Journal of Polymer Science: Part A-1; vol 4, 881-900 (1966), Mortimer describes the use of, a.o., aldehydes such as propionaldehyde, and ketones such as MEK, as chain transfer agents in a high-pressure free-radical polymerization process.
U.S. Pat. No. 3,129,212 teaches that chemical modifiers such as propylene and MEK can be used to prepare polyethylenes having a narrow MWD and a high density.
Also, U.S. Pat. No. 3,334,081 teaches that chain transfer agents can be used to increase the density of solid polyethylenes. Among various chain transfer agents, MEK and aldehydes are mentioned.
In U.S. Pat. No. 3,317,504, a high pressure ethylene polymerization process is described using a tubular reactor and, a.o., MEK as transfer agent. The polymers obtained were said to have a density of up to 0.940 g/cm
3
.
East German patent 108,546 teaches a high-pressure ethylene homo-, co- or terpolymerization using free radical forming initiators. As polymerization regulators, among other, MEK and propylene are mentioned, as well as a combination thereof. This document teaches the addition of chain regulators in two tubular reactor zones to reduce molecular weight fluctuations, which leads to better workability and better film forming properties.
U.S. Pat. No. 3,293,233 describes that polymers of ethylene can be obtained when certain chain transfer agents are employed in a high-pressure polymerization process. Among the huge amount of chain transfer agents, MEK is explicitly referred to.
In U.S. Pat. No. 3,691,145 a high pressure polyethylene preparation process is described using polymerization regulators. The polymerization regulators used may be “those usual in the art, such as alkanes, alkenes of more than two carbon atoms, alcohols, ethers, aldehydes, ketones or mixtures of such substances”. Reference is made to the above-mentioned article of Mortimer.
U.S. Pat. No. 3,917,577 describes a continuous process for the production of ethylene homopolymers in a tubular reactor having at least two reaction zones. Ethylene, initiator and regulator are continuously introduced at the beginning of each reaction zone. It is the object of this patent to provide a multistage process giving a polyethylene having a narrower MWD. In order to achieve this, one has to use the polymerization regulators having a high C value as described by Mortimer. MEK and propionaldehyde are mentioned among the preferred regulators.
U.S. Pat. No. 4,076,919 teaches to use conventional regulators such as propylene and MEK in a high pressure two-zone tubular reactor ethylene polymerization or co polymerization process.
U.S. Pat. No. 4,085,266 teaches the same chain transfer agents in a two-zone autoclave ethylene copolymerization process. In the top zone of the autoclave reactor ethylene is polymerized at a pressure of between 1,000 and 1,800 kg/cm
2
and at a relatively low temperature of between 130 and 200° C., followed by a reaction in the bottom zone at a pressure in the same range and at a high temperature of between 220 and 280° C.
In U.S. Pat. No. 4,123,600, a high pressure LDPE preparation process is described using a battery of two or more autoclave reactors, which are operated in the same way as the autoclave reactor described in the previous paragraph.
In the high-pressure polymerization process described in U.S. Pat. No. 4,168,355, the melt index of ethylene homo and copolymers is taught to be affected in a conventional way by the addition of a chain transfer agent.
U.S. Pat. No. 4,988,781 teaches the production of an improved homogeneous interpolymer of ethylene and an &agr;-olefinically unsaturated carboxylic acid or ester in a stirred autoclave. The polymer product is said to have a substantially narrow MWD, appreciably reduced levels of long-chain branching, substantially improved extrusion stability, and appreciably improved draw-down. The improvements are obtained by using a telogenic modifier. MEK is referred to as an especially preferred telogen.
When in these known processes using carbonyl group containing compounds, such as MEK or propionaldehyde, as chain transfer agent (CTA) polymer products having a medium density are prepared, the obtained polymer products usually have a narrow molecular weight distribution of less than 3.0 and contain reduced amounts of high molecular weight fractions. This is due to the high reactivity of the carb

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