IE42332B1 - Method of making wax-like polyethylenes from residues of polyethylene production - Google Patents

Description

This invention relates to a process for treating residues which are mixtures of low molecular weight polyethylenes and high molecular weight polyethylenes so that only the high molecular weight polyethylenes, and not the low molecular weight fractions are degraded and a wax-like polyethylene (paraffin) is obtained.

Throughout this specification percentages are by weight, unless otherwise stated.

It is known to produce low molecular weight polyethylenes, i.e. wax-like polyethylenes, by thermal degradation of high molecular weight polyethylenes, i.e. polyethylenes having a molecular weight above 12,000. The high molecular weight polyethylenes are decomposed to low molecular weight polyethylenes having an average molecular weight of about 3,000 to 9,000. High molecular weight polyethylene itself shows a special toughness and good heat insulation properties. Therefore very fast running extruders must usually be used for the degradation, which by their mechanical energy impart the necessary heat to the polyethylene.

It is also known to prepare wax-like, low molecular weight polyethylenes by controlled polymerisation of ethylene, whereby a broad range of wax-like polyethylenes of different molecular weight can be produced. For the desired molecular range of about 1000, high temperatures and / or hydrogen concentrations are necessary. These known processes have the disadvantage that in addition to formation of the desired wax-like low molecular weight poly25 ethylenes, paraffin oils and even gaseous hydrocarbons having molecular weights up to that of ethane are formed in considerable quantities. These by-products have only unimportant uses and are therefore without particular value.

In a further known process the residues which are formed in a polyethylene production plant which uses a Ziegler process at temperatures of 400°C are changed into liquid low molecular weight distillates. In this process paraffin oils are formed as by-products.

These by-products are of unimportant value.

The main object of the present invention is to provide a process for the production from polyethylene residues of wax-like polyethylenes, in which process the aforesaid disadvantages are minimised or eliminated.

Accordingly the present invention provides a process for the preparation of wax-like low molecular weight polyethylenes from the polyethylene residues from a Ziegler polymerisation process which residues contain low molecular weight and high molecular weight polyethylenes, which process comprises heating the polyethylene residues at a temperature of 350 to 385°C, preferably 360 to 380°C for a time of | to 3 hours whereby the high molecular weight polyethylenes present in the residues are degraded to low molecular weight polyethylenes, while the low molecular weight polyethylenes present in the residues remain substantially unaffected.

In general the high molecular weight polyethylenes in the residue have a molecular weight of about 40,000 to 250,000 and the low molecular weight polyethylenes have a molecular weight of about 250 to 1,000. The latter can be straight chained or branched and hard.

The mixtures of low and high molecular weight polyethylenes contain in general 5 to 20 wt.% of paraffin oil which can be distilled off under vacuum before cracking. The removal of the paraffin oil can also be carried out after cracking and for special purposes it is also possible to leave the paraffin oil in the product.

By using the above mentioned temperature range only the high molecular weight polyethylenes in the residue are changed to solid low molecular weight polyethylenes which are wax-like at room temperature. The low molecular weight polyethylenes already present in the residue are not affected. 42333 By this process it is possible to avoid an inconvenient waste and to produce a valuable product.

In the degradation there is a loss of oxygen or of gas mixtures containing free oxygen. Therefore the resulting wax-like polyethylenes contain no acid or ester groups. No catalyst is used in the present process. For the degradation any suitable apparatus can be used. The process can be continuous or discontinuous/ For discontinuous processes a stirring reactor can be used. A continuous process which is preferred is one in which the molten mixture of polyethylene residues is pumped through a coiled tube which is heated to 350° - 385°C.

The cracking mixture usually contains high molecular weight polyethylenes in an amount of 5 to 15%, 10 to 20% of paraffin oils and the remainder is low molecular weight poly15 ethylenes.

The average molecular weight of the resultant wax-like polyethylene, comprising low molecular weight polyethylenes derived from decomposed high molecular weight polyethylenes and undecomposed low molecular weight polyethylenes reaches substantially the average molecular weight of the undecomposed low molecular weight polyethylene. There is no additional loss of paraffin oil.

The original high molecular weight fraction is decomposed by heat treatment to linear highly crystalline, wax-like, low molecular weight polyethylenes. The relative amounts of high and low molecular weight fractions can be determined by extraction in benzene, the low molecular weight fraction being taken up by benzene and the high molecular weight fraction not being extracted in benzene. It is generally found that a small decrease only of the penetration hardness (measured according to Richardson) results from the degradation, depending on the reaction conditions.

By this means it has been shown to be possible to obtain economically a highly valuable material from the residual byproducts of polyethylene manufacture by Ziegler processes.

The product of the present process shows the characteristics of a micro-crystalline wax or a paraffin wax.

The polyethylenes formed from the high molecular weight polyethylenes form together with the low molecular weight poly5 ethylenes already present, a valuable wax-like polyethylene of more uniformly low molecular weight.

The physical properties of the product can be varied to some extent by performing the degradation at different temperatures within the given range. Thus, by working at the upper temperature limit relatively softer polyethylenes are obtained and by working at the lower temperature limit relatively harder wax-like polyethylenes are formed.

The products of the present process can be used for example in floor polish, in the leather industry and as additives for printing colours.

The invention will now be further described with reference to the following Examples.

The molecular weights in these Examples were determined by solution viscosity according to DIN 53 728. 0 EXAMPLE 1.

The residual by-products of a Ziegler polyethylene synthetic process was placed in a reactor and heated in a metal bath at 375°C while stirring under hydrogen for 3 hours. The resultant oily fraction (16 wt.%) was removed by distillation under vacuum 5 up to 200°C/l Torr.

A sample of the original by-product was distilled under vacuum in the same way. After the vacuum distillation, the remaining product had a penetration number (according to Richardson) of 5.6 . By a benzene extraction at 90°C the substance was separated into a fraction soluble in benzene of 87 wt.% (M== 800) and an insoluble fraction of 13 wt.% (M= 50,000). The product formed upon degradation of the original residual by-product was a wax42332 - 5 like polyethylene which had substantially the same penetration hardness (according to Richardson) as the original residual by-product. An extraction with benzene of the degraded product (as described above) showed that while the ratio of the soluble to the insoluble fractions was practically unchanged, the soluble fraction had an average molecular weight of— 750, while the insoluble fraction had a molecular weight of 1700 only.

EXAMPLE 2.

The Ziegler residual by-products A and B mentioned in the following io table were partly decomposed in a heated tube reactor having an interior diameter of 9 mm and a length of 13 m. The residual by-products are substances obtained directly as residues from a Ziegler polymerisation of ethylene, the residues not being subjected to vacuum distillation to remove paraffin oils. The results and degradation conditions are also shown in the table.

The wax-like polyethylene was dissolved under hydrogen in a container, led over a metering pump into the tube coil and collected in a distillation container. Afterwards the oily fractions were distilled off. All operations were carried out strictly with -° the exclusion of air and of free oxygen containing gases. - Ί 42332 EXAMPLE 3.

The residual by-products from a Ziegler process which contained 9 wt.% oily fractions (as measured by distillation at 200° C/! Torr) and which had 3 wt.'Z residue with a molecular weight of ' 200,000 (as measured by exhaustive extraction with benzene at 95°C), was heated in the apparatus described in Example 2 at 380°C for 3 hours. After this treatment and distillation of the paraffin oil, a wax-like polyethylene was obtained which had at a penetration of 7, a residue of—3 wt.% fraction which 0 was insoluble in benzene. The molecular weight of the insoluble fraction was ~ 700. ro ro -5 Οχ x ro ω rt- rf· ω to “J T -‘•I α» ο» Cl ρ η π c cl rf rf ro cu -i. c+ O 4=» Oi =3 Oi -h O rf 3 ~s ro ”5 Ό -S co o ro I I co co o ΓΟ —< o tn o cn co cn ο o ο ο ο σ ro ro -s ro χ x ro *->· rf rf V> ro -5 -S -*.

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Claims (5)

1. A process for the preparation of wax-like low molecular weight polyethylenes from a polyethylene residual by-product obtained from a Ziegler polymerisation process which by-product contains 5 low molecular weight and high molecular weight polyethylenes, the process comprising heating the polyethylene residual by-product at a temperature of 350 to 385°C for J to 3 hours whereby the high molecular weight polyethylenes present in the residual by-product are degraded to low molecular weight polyethylenes, while the low 10 molecular weight polyethylenes present in the residual by-product remain substantially unaffected.

2. A process according to Claim 1, wherein the polyethylene residual by-product is heated at 360 to 380°C.

3. A process according to Claim I or 2, wherein the polyethylene is residual by-product contains 5 to 15% by weight of high molecular weight polyethylene, 10 to 20% by weight paraffin oils and the remainder is low molecular weight polyethylenes.

4. A process of preparing wax-like low molecular weight polyethylenes according to Claim 1 substantially as described in any 20 one of the foregoing Examples 1 to 3.

5. Wax-like low molecular weight polyethylenes when prepared by a process as claimed in any one of the preceding claims.