Ethylene copolymers, process for their manufacture and use thereof.

The present invention relates to binary and ternary copolymers of ethylene. The binary copolymers consist of ethylene and 2,4,4-trimethylpentene (1), the ternary ones additionally contain a vinyl ester, a vinyl ether or an ester of acrylic acid or methacrylic acid. The copolymers have a melt index MFI (l90 / 2, l6) of 0.4 to 20 g / l0 min, a density of 0.9l5 to 0.960 g / cm³ and an impact strength of 850 to 2,600 mJ / mm².

The present invention further relates to a process for the preparation of the copolymers according to the invention by reacting the monomers together under the conditions of the free-radically initiated high-pressure polymerization.

Due to their excellent properties, the new copolymers are particularly suitable for the production of films.

DE-AS 20 l8 7l8 describes a process for the production of modified high-pressure polyethylenes, in which the polymerization of ethylene and optionally other compounds which can be copolymerized with ethylene are carried out in the presence of oligomers of isobutylene which are composed of 4 to 100 isobutylene units.

The polymers obtained by the process of DE-AS 20 l8 7l8 are suitable for the production of films. The addition of oligomers of isobutylene has the effect that the surface stickiness of the films, referred to as blocking, is avoided.

In contrast, if diisobutylene is used, inhomogeneous polymers are obtained. The films produced therefrom are cloudy and have specks (cf. DE-AS 20 l8 7l8, column 2, lines 32 to 38 and column 6, comparative example 2). They are therefore unsuitable for packaging purposes.

Certain requirements are placed on films that serve as packaging material for goods stored in particular at low temperatures. They should have high transparency, rigidity, gloss and good impact strength. These properties must be maintained even at low temperatures.

The present invention has for its object to provide copolymers of ethylene, which provide films with these desired properties, and a method for producing the copolymers.

This object is achieved by copolymers of ethylene which contain 0.2 to 5.0 parts by weight of 2,4,4-trimethylpentene (l) and optionally 2.0 to 20.0 parts by weight of 100 parts by weight of ethylene. Parts contain vinyl esters or vinyl ethers or esters of acrylic acid or methacrylic acid with aliphatic C₁ to C₈ alcohols, a melt index MFI (l90 / 2, l6) of 0.4 to 20 g / l0 min, a density of 0.9l5 to 0.960 g / cm³ and an impact strength of 850 to 2,600 mJ / mm².

The binary copolymers each contain 100 parts by weight of ethylene 0.2 to 5.0, in particular 0.3 to 4.0, preferably 0.35 to 3.5 parts by weight of 2,4,4-trimethylpentene (l ).

The ternary copolymers consist of ethylene, 2,4,4-trimethylpentene (1) and another monomer from the following group of substances: vinyl ester, vinyl ether, acrylic acid ester or methacrylic acid ester. Vinyl esters and the esters of acrylic acid or methacrylic acid are particularly suitable as monomers; acrylic acid esters and in particular vinyl esters are preferred.

Among the vinyl esters, those whose carboxylic acid residues contain 2 to 4 carbon atoms, preferably vinyl acetate, are particularly suitable. The preferred vinyl ethers contain 4 to 8 carbon atoms. An example of these compounds is vinyl ethyl ether. The acrylic acid esters and methacrylic acid esters are derived from aliphatic alcohols with 1 to 8 carbon atoms in the molecule. The alcohols can be straight-chain or branched and carry the OH group on a primary secondary or tertiary carbon atom. Methyl, ethyl and tert-butyl acrylate and methacrylate have proven to be particularly suitable.

The ternary polymers each contain 100 parts by weight of ethylene 0.2 to 5.0, preferably 0.3 to 4.0 and in particular 0.35 to 3.5 parts by weight of 2,4,4-trimethylpentene (1 ) and 2.0 to 20, preferably 2.2 to 15 and in particular 2.5 to 10 parts by weight of the above monomers.

The copolymers according to the invention are characterized by a melt index MFI (l90 / 2, l6) of 0.4 to 20.0, preferably 0.5 to 10.0 and in particular 0.6 to 4.0 g / l0 min. The above-mentioned melt index is determined in accordance with the German standard DIN 53735. Empirically, each melt index measured can be assigned an average molar mass determined by the gel permeation chromatography method. The corresponding values ​​can be seen in the comparison below.<tables id="tabl0001" num="0001"><img file="EP0254022A2_D0001.tif" /></tables>

The density of the copolymers is 0.915 to 0.960, preferably 0.920 to 0.945 and in particular 0.925 to 0.940 g / cm³.

The new products are further described by their impact strength, which is in the range from 850 to 2,600, preferably 870 to 2,500 and in particular 900 to 2,200 mJ / mm². The impact strength is determined in accordance with the German standard DIN 53448.

The claimed binary and ternary copolymers of ethylene can be prepared by copolymerizing the monomers at 50 to 350 MPa and 100 to 350 ° C. in the presence of oxygen or radical-forming compounds. For this purpose, mixtures of 100 parts by weight of ethylene, 0.4 to 10.0 parts by weight of 2,4,4-trimethylpentene (1) and optionally 1.5 to 25.0 parts by weight of vinyl ester or vinyl ether or ester are used polymerized from acrylic acid or methacrylic acid and an aliphatic C₁ to C₈ alcohol.

Since the polymerization rate of the monomers generally differs, the composition of the polymer often differs from the composition of the monomer mixture.

The polymerization is carried out under a pressure of 50 to 350, preferably 100 to 300, in particular of 125 to 275 MPa. The polymerization temperature is 100 to 300, preferably 120 to 325 and in particular 130 to 300 ° C.

Oxygen or radical-forming compounds are used as polymerization initiators. Compounds which form free radicals include organic peroxides, hydroperoxides or azo compounds. Of the organic peroxides, tert-butyl perbenzoate, tert-butyl peroxipivalate and dilauryl peroxide have proven successful. Azo-bis (isobutyronitrile) is successfully used as the azo compound. Tert-butyl peroxipivalate has proven particularly useful. The compounds can be used alone or in a mixture. The polymerization initiators are used in a concentration of 3 to 50, preferably 5 to 40, in particular 10 to 25 ppm by weight, based on ethylene. They are fed to the polymerization directly or as a solution in an organic solvent. Hydrocarbons such as isooctane, benzene, toluene or gasoline fractions are used as solvents.

An essential feature of the novel polymer preparation process is the absence of separate molecular mass regulators (moderators) during the polymerization. Molecular mass regulators are compounds which are added to the monomers in different concentrations in order to influence the growth of the polymeric molecule and thus the molar mass of the polymer. The molecular weight regulators include representatives from a wide variety of substance classes. Examples include: hydrogen, alkanes, unbranched alpha-olefins, alcohols, aldehydes and ketones (cf. Progr. Hochpolym.-Forschg. 7 (3), 386-448).

In this context, it must be pointed out that diisobutylene, ie a mixture consisting mainly of 2,4,4-trimethylpentene (1) and - (2), itself acts as a molecular weight regulator (cf. Progress. High-polymer research 7 (3), 4l7). Accordingly, monomer mixtures with a higher content of 2,4,4-trimethylpentene (l) polymers with a lower molar mass than monomer mixtures in which the 2,4,4-trimethylpentene (l) concentration is lower.

If the polymerization is carried out except in the presence of 2,4,4-trimethylpentene (1) in the presence of further molecular weight regulators, polymers are obtained which are not suitable for the production of films. The ability to pull out is particularly unsatisfactory. In addition, the films have specks and are unusable as packaging materials due to the lack of transparency. It was not foreseeable that the omission of the separate addition of a moderator in addition to the comonomer 2,4,4-trimethylpentene (1), which also acts as a molecular mass regulator, leads to polymers which can be processed excellently into films with excellent properties.

The monomer mixture converted by the process according to the invention always contains 2,4,4-trimethylpentene (1). Its proportion is 0.4 to 10.0, in particular 0.6 to 8.0, preferably 0.7 to 7.0 parts by weight per 100 parts by weight of ethylene used.

2,4,4-Trimethylpenten- (l) can also be fed to the polymerization reaction in the form of a mixture with other C₈-olefins. Diisobutylene is particularly suitable as the mixture containing 2,4,4-trimethylpentene (1). This is understood to mean the mixture consisting essentially of 2,4,4-trimethylpentene (1) and 2,4,4-trimethylpentene (2), which is used in the dimerization of 2-methylpropene (i-butylene) with acidic catalysts (eg ion exchangers) is formed. Depending on the preparation, it contains 50 to 90, preferably 55 to 80 and in particular 60 to 75 parts by weight of 2,4,4-trimethylpentene (I), in each case based on 100 parts by weight of diisobutylene.

In addition to ethylene and 2,4,4-trimethylpentene (1), the monomer mixture polymerized by the process according to the invention may also contain further monomers. These include vinyl esters, vinyl ethers, acrylic acid esters and methacrylic acid esters. Their proportion in the monomer mixture is 1.5 to 25.0, preferably 2.0 to 20.0 and in particular 3.0 to 15.0 parts by weight, in each case based on 100 parts by weight of ethylene used. The residence time of the monomer mixture in the polymerization stage is 30 to 180, preferably 50 to 160 and in particular 60 to 140 seconds.

The process according to the invention can be carried out in the high-pressure reactors known for the polymerization of ethylene and ethylene-containing monomer mixtures. These include stirred autoclaves and tubular reactors (cf. Ullmann's Encyclopedia of Technical Chemistry, 4th edition, Verlag Chemie Weinheim-Basel l980, volume l9, pages l69, l72 to l75). When using a tubular reactor, the entire monomer mixture already containing initiators can be fed to the reactor in one stream. However, it is particularly advantageous to use a tubular reactor with cold gas and initiator replenishment and to divide the monomer mixture into at least two substreams. A partial stream is fed to the reactor inlet, and further partial streams are fed along the reactor, usually in the region of a peak temperature, to the reaction zone.

Films made from the new copolymers have high strength (measured as impact strength) and remarkable transparency. These properties are retained even at low temperatures, such as those that occur during the storage of frozen foods. In addition, the material is characterized by its high pull-out capacity (measured as the smallest film thickness that can be produced when pulling out without tearing or holes); it is 5 to 30, preferably 8 to 25 and in particular 10 to 20 μm.

In the following examples AF, the new polymers and the process for their preparation are explained in more detail. Examples 1-4 relate to comparative tests which show that compliance with the claimed features (material properties and process parameters) are necessary in order to achieve the object on which the invention is based.

In the examples, 2,4,4-trimethylpentene (l) is always used in the form of the mixture of various C₈-hydrocarbons called diisobutylene. It has the following composition:<tables id="tabl0002" num="0002"><img file="EP0254022A2_D0002.tif" /></tables>

It boils at 10 ° C, its density d<img file="EP0254022A2_D0003.tif" /> is 0.7ll g / cm³.

The following methods were used to determine the properties of the substance: Melt index MFI (l90 / 2, l6) g / l0 min: measured according to DIN 53735; Impact strength measured according to DIN 53448; Extractability determined by processing the copolymer on a blown film line. The throughput is 7 kg / h, the nozzle gap has a diameter of 1 mm, the inflation ratio is 2.5: 1; Transparency: visual evaluation of the foils.

The 2,4,4-trimethylpentene (l) content in the polymers is determined from the (gas chromatographically determined) difference in the 2,4,4-trimethylpentene (l) content of the monomer mixture at the reactor inlet and outlet.

The vinyl acetate content in the polymers is determined by pyrolysis and measurement of the released acetic acid using an aqueous iodide / iodate solution.

Examples A - F:

The copolymers are produced in a continuously operated high-pressure reactor (stirred autoclave) which is followed by a high-pressure gas separator and a low-pressure gas separator. The monomer mixture intended for the polymerization is brought to the desired pressure and introduced into the high-pressure reactor. At the same time, the amount of polymerization initiator required for maintaining the polymerization is added (tert-butyl peroxipivalate, dissolved in gasoline). The residence time of the reaction mixture in the high-pressure reactor is about 90 seconds.

Reaction conditions, initiator concentration (based on ethylene used) and composition of the monomer mixture are shown in Table 1, the properties of the copolymers in Table 2.

Examples 1 to 5 (comparative tests):

Examples 1 to 5 are carried out analogously to Examples AF, but in deviation from these, the monomer mixture contains a molecular weight regulator (moderator) separately. The details of the test procedure can be found in Table 1.

The composition and properties of the copolymers obtained according to Examples 1 to 5 are listed in Table 2.

The copolymers of Examples A-F according to the invention have, compared to the copolymers of Examples 1-6 produced for comparison, a considerably increased impact strength, a significantly increased extensibility and a significant improvement in transparency. They are therefore particularly suitable for the production of films.

Examples 6 to 14 (comparative tests):

If the addition of diisobutylene is dispensed with and replaced by conventional molecular weight regulators, products are obtained which are likewise unsuitable for the production of films. The reaction conditions belonging to Examples 6 to 14 are again shown in Table 1, and the properties of the resulting copolymers are shown in Table 2.<tables id="tabl0003" num="0003"><img file="EP0254022A2_D0004.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0254022A2_D0005.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0254022A2_D0006.tif" /></tables><tables id="tabl0006" num="0006"><img file="EP0254022A2_D0007.tif" /></tables>