FIELD OF THE INVENTION

The present invention relates to a method for preparing an oxidized polyethylene wax, an oxidized polyethylene wax obtainable this way, the use thereof and a wax emulsion comprising the oxidized polyethylene wax.

BACKGROUND OF THE INVENTION

An oxidized wax is a polar form of a polyethylene wax which is widely used in emulsions for coating agents and/or a lubricant agents. The oxidized polyethylene wax can be produced by oxidizing polyethylene wax. Depending on the oxidation process parameters and polyethylene wax feature, various properties of the oxidized polyethylene wax can be obtained. If the process conditions are not appropriate, the oxidation may cause cross-linked by-product which leads to an increased viscosity of the mixture during the process and formation of a gel-like product, resulting in difficulties to control the product specification. This also causes poor emulsifiability and poor clarity of the wax emulsion.

There were some attempts to overcome this problem. For example, US6211303B1 discloses a process for preparing polar wax products by oxidation of nonpolar polyethylene waxes wherein the oxidation of the polyethylene wax is carried out with addition of inorganic or organic acids to the reaction mixture before commencement or in the early stage of the oxidation,

CN104277229A discloses a method for preparing a high-density polyethylene wax emulsion by adding 20-50% fatty alcohol ethoxylates as a dispersant to thermal cracked high-density polyethylene before oxidation process to prevent cross-linking reaction.

US2952649A discloses a new emulsifiable self-polishing wax composition that can be used as a carnauba replacement by adding 5-30% of a very low molecular weight species of paraffin to the polyethylene prior to oxidation. However, the resulting wax shows an acid number in the range of 3-6 which is more difficult to emulsify. Thus, it is needed to add some alkali carbonates or amines to stabilize to give a faster reaction and obtain the wax with acid number of 1-15. Besides, the very low melting point of paraffin may lead to colorant in the final product after process at high temperature. Therefore, it is an object of the present invention to overcome drawbacks of the prior art, in particular the disadvantage of crosslinMng during oxidation. Particularly, it is an object of the invention to provide the method for preparing an oxidized polyethylene wax suitable for reducing cross-linking curing preparation of the oxidized polyethylene wax.

DETAILED DESCRIPTION

The above object is achieved by a method for preparing an oxidized polyethylene wax comprising oxidizing a mixture of a polyethylene wax having a number average molecular weight of more than 1000 to 4000 g/mol, preferably 1000 to 3000 g/mol, even more preferred 1200 to 2500 g/mol according to gel permeation chromatography and a low molecular weight polyethylene wax having a number average molecular weight of 100 to 1000 g/mol, preferably 100 to 900 g/mol, even more preferred 200 to 800 g/mol according to gel permeation chromatography .

Preferably, the polyethylene wax has a weight average molecular weight of 4000 to 20000 g/mol, preferably 4000 to 18000 g/mol, even more preferred 4500 to 16000 g/mol according to gel permeation chromatography and/or the low molecular weight polyethylene wax has a weight average molecular weight of 100 to 30000 g/mol, preferably 1000 to 25000 g/mol, even more preferred 1500 to 20000 g/mol according to gel permeation chromatography. By mixing the low molecular weight polyethylene wax and the polyethylene wax prior to the oxidation, the viscosity of the mixture can be controlled and crosslinking between the polymer chains can be reduced. Thus, the satisfactory properties of the oxidized polyethylene wax can be controlled. Furthermore, the oxidized polyethylene wax has a suitable acid number which is easy to emulsify to a wax emulsion with good clarity.

In the present invention, the polyethylene wax may have a density in the range of 0.92-0.97 g/cm ³ and an acid number of zero.

In one embodiment, the polyethylene wax can be obtained from thermal cracking of polyethylene. Thermal cracking may be initiated by heat or light and takes place at C-C bond to generate the shorter chain hydrocarbon. The degree of cracking may be controlled by the reaction time and temperature. Aguado et al., Energy & Fuels, 2002, 16, 1429-1437, discloses a respective wax formation using pyrolysis of polyolefins. Also, polyethylene wax can be obtained from polyethylene polymerization which occurred via metallocene-catalyzed polymerization or traditional Ziegler/Natta-polymerization or from by-product of polyethylene polymerization. US 5,023,388 A and WO 2013/027958 Al disclose the preparation of polylethylene wax in the presence of a metallocene catalyst. A process for preparing polyethylene waxes using Ziegler-Natta catalyst systems is disclosed in US 2010/0050900 Al.

In another embodiment, the low molecular weight polyethylene wax may have a weight average molecular weight of 100 to 30,000 g/mol, preferably 1,000 to 25,000 g/mol, evenmore preferred 1 ,500 to 20,000 g/mol according to Gel Permeation Chromatography, a density in the range of 0.92-0.96 g/cm ³ , a viscosity of 20-7000 cP, and an acid number of zero. Preferably, it has a viscosity of 30-1000 cP, more preferably, has a viscosity of 40-200 cP.

The low molecular weight polyethylene wax can be obtained from polyethylene polymerization or as a by-product of polyethylene polymerization either via meta!locene- catalyzed polymerization or traditional Ziegler/Natta-poIymerizati on.

In a preferred embodiment, the mixture of polyethylene wax and low molecular weight polyethylene wax comprises 50-97 wt% of polyethylene wax and 3-50 wt% of low molecular weight polyethylene, more preferably, 75-90 wt% of polyethylene wax and 10-25 wt% of low molecular weight polyethylene.

In a further preferred embodiment, the oxidized polyethylene wax has an acid number of 10 to 40, preferably 15 to 35, more preferred 15 to 30 according to ASTM D1386.

In a preferred embodiment, the ratio Mw/Mn with respect to the polyethylene wax is from 3 to 10 and/or the ratio Mw/Mn with respect to the low molecular weight polyethylene wax is from 3 to 60.

Preferably, "comprising" in terms of the invention may be "consisting of.

The oxidizing (oxidation) may be carried out in a temperature range of 120-190°C and a pressure range of 2-10 bar using pure oxygen, air, ozone or an oxygen containing gas, optionally in the presence of an oxidation catalyst e.g. di-tert-butylperoxide, other organic peroxides or hydroperoxides. More preferably, in the temperature range is of 150-170°C and the pressure range of is 4-7 bar.

Stabilizing agents, such as alkali carbonates or amines, may be used in the oxidation to accelerate the reaction by stabilizing the oxidation intermediates. The oxygen feed rate may be regulated to control the oxidation rate which might affect the viscosity of the reaction and also the acid number of the oxidized polyethylene wax product. In the present invention, air may be fed at 1000-10000 L/hr/kg; preferably, at 2000-5000 L/hr/kg.

The oxidizing process may further comprise stirring to make a homogeneous mixture and oxidation may occur throughout the homogeneous mixture. The stir speed rate may be from 300 to 1000 rpm. However, the speed rate may also depend on the size of the reaction vessel and the viscosity of the mixture.

The oxidation time may be about 1-10 hours depending on the oxidation temperature, molecular weight of starting polyethylene wax, feed rate of oxygen, the presence of the catalyst and/or stabilizing agent and the desired acid number of the oxidized wax.

An acid number in a range of 15-30 is preferred for use in a wax emulsification. Wax with lower acid number is more difficult to emulsify.

The object is further achieved by an oxidized polyethylene wax obtainable by the method according to the present invention.

The object is further achieved by a use of the inventive oxidized polyethylene wax in a wax emulsion, or as a coating agent and/or a lubricant.

Finally, the object is achieved by a wax emulsion comprising a) the inventive oxidized polyethylene wax; b) a non-ionic surfactant; c) potassium hydroxide and/or sodium hydroxide; d) sodium metabisulfite; and e) water.

In one embodiment, the amount of the oxidized polyethylene wax in the wax emulsion is from 5 to 35 wt% with respect to the total weight of the wax emulsion, preferably, 10 to 30 wt% with respect to the total weight of the wax emulsion.

In this regard, it may be preferred that the non-ionic surfactant is comprised in the inventive wax emulsion in an amount from 2 to 12 wt.%, preferably 3 to 10 wt.%. The potassium hydroxide and/or sodium hydroxide may be comprised in the inventive emulsion in a total amount from 0.5 to 1.5 wt.%, preferably 0.5 to 1.2 wt.%. The sodium, metabisulfite may be comprised in the inventive wax emulsion in an amount from 0.05 to 0.5 wt.%, preferably 0.1 to 0.4 wt.%. The amount of water may be from 50 to 90 wt.% and is selected to balance the composition to a total amount of 100 wt.%.

In another embodiment, the oxidized polyethylene wax has a density of 0.93-0,99 g/cm ³ . More preferably, has density of 0.93-0.97 g/cm ³ .

In the preferred embodiment, the oxidized polyethylene wax was obtainable by oxidizing the mixture comprising 50-97 wt% of polyethylene wax and 3-50 wt% of low molecular weight polyethylene with respect to the total weight of the mixture. More preferably, 75-90 wt% of polyethylene wax and 10-25 wt% of the low molecular weight polyethylene with respect to the total weight of the mixture.

Measurement methods

Molecular weights, weight average molecular weight (Mw) and number average molecular weight (Mn), were measured by Gel Permeation Chromatography (GPC). Around 24 mg of sample was dissolved in 8 ml of 1 ,2-dichlorobenzene at 150°C for 60 min. Then the sample solution, 200 μΐ, was injected into the high temperature GPC with IR4 detectors (Polymer Char, Spain) with flow rate of 1 ml/min at 140°C in column zone and 150°C in detector zone. The data was processed by GPC One® software, Polymer Char, Spain.

Molecular weight distribution is described by the ratio of Mw to Mn.

%Transmission was measured by UV-visible spectrophotometer at 550 nm.

EXAMPLES Example

Polyethylene wax having weight average (Mw) molecular weight of 6575 g/mol (number average molecular weight (Mn)=1363 g/mol, Mw/Mn=4.82) ₅ a density of 0,92 g/cm and a viscosity of 170 cP was mixed with low a molecular weight polyethylene wax having weight average molecular weight of 3565 g/mol (Mo=435 g/mol, Mw/Mn=8.19) and viscosity of 50 cP at various ratio as shown in. Table 1. The mixture was then oxidized. The temperature and pressure of the reaction was maintained at 150-170°C and 4-7 bar. The properti.es of the resulting oxidized wax are shown in Table 1 ,

Table 1 The properties of the oxidized wax obtained from various ratio of polyethylene wax and low molecular weight wax

When comparing the example 1-7, it was found that the oxidized wax from polyethylene wax without addition of low molecular weight polyethylene wax shows a much higher viscosity while by mixing some amounts of low molecular weight polyethylene wax with polyethylene wax prior to the oxidation, the viscosity of the oxidized wax is reduced. Comparative example_8

Polyethylene wax having weight average molecular weight of 6575 g/mol (number average molecular weight (Mn)=1363 g/mol, Mw/Mn=4.82), density of 0.92 g/cm ³ and viscosity of 170 cP. Low molecular weight wax having weight average molecular weight of 3565 g/mol (Mn=435 g/mol, Mw/Mn ^~ 8.19) and viscosity of 50 cP. Each wax was oxidized separately. The temperature and pressure of the reaction was maintained at 150-170°C and 4-7 bar. The oxidized polyethylene wax and the oxidized low molecular weight wax were then (after being oxidized separate from each other) mixed at the ratio of 84:16. The oxidized wax mixture shows the properties as in table 2.

Table 2 The properties of the oxidized wax obtained from separately oxidation, of polyethylene wax and low molecular weight wax

The oxidized polyethylene wax without addition of low molecular weight polyethylene wax shows an. increased viscosity of 400 cP while the oxidized low molecular weight polyethylene wax shows the viscosity of 10 cP. The oxidized polyethylene wax and the oxidized low molecular weight wax were mixed at the ratio of 84:16. The viscosity of the mixture decreases to 350 cP; however, viscosity is still higher compared to that of the inventive examples 2-6. The result shows the inventive of addition of low molecular weight polyethylene prior to the oxidation.

Preparation of polyetylene wax emulsion

The oxidized wax obtained previously (examples 1 and 3) was emulsified by mixing 18 wt% of the oxidized wax with 7 wt% of polyethylene glycol having ethylene oxide content of 9 and hydrophilic-lipophilic balance of 13, 0.5 wt% of potassium hydroxide 85%, 0.18 wt% of sodium metabisulfite and water. The mixture was then stirred rigorously. The properties of wax emulsion are shown in Table 3. Table 3 The properties of the wax emulsion obtained from the oxidized wax prepared by the present invention.

The result shows that the oxidized wax from mixing of polyethylene wax and low molecular weight polyethylene prior to the oxidation (example 3) have higher clarity of wax emulsion than, the oxidized wax from 100% polyethylene wax (example 1 ).

The features disclosed in the foregoing description and in the claims may, both separate or in any combination, be material for realizing the invention in diverse forms thereof.