DE1025139B - Process to reduce the tendency of polyolefins to degrade and erode - Google Patents

Description

Process for reducing the tendency to break down and become brittle Polyolefins Is polyethylene in the solid state z. B. after processing into fittings, like plates or pipes, or in solution for a long time in the presence of air at higher levels Temperature, e.g. B. heated to over 100 ° C, it takes place in the course of a few days a strong breakdown of the molecular chains. The rate of degradation increases with increasing Temperature very strongly. The course of degradation over time can be measured by measuring the melt viscosity, the specific viscosity of the solution of polyethylene or track the molecular weight. With the decrease in the size of the molecule, there is deterioration the r application-technical properties linked; above all, the tendency to embrittlement is increasing with increasing degree of degradation. This increases the utility value of the polyethylene reduced.

It is known to be sensitive to oxidation due to the addition of certain substances Substances, called stabilizers for short, e.g. B. of mercapto compounds or of basic organic compounds. The effect of such Stabilizing agent only lasts a relatively short time, and d especially for the use of polyethylene at higher temperatures is the stabilizing effect with the stabilizers proposed so far for permanent use too low. There is therefore still a need for more effective stabilizers to fin. the.

It has now been found that the tendency to breakdown and embrittlement can be reduced of polyolefins, especially Polyethylene, can be reduced by the polyolefin is a mixture of aliphatic straight. ettigen or branched Mercaptans and basic organic compounds whose basic effect is so strong is that its watery. Solutions phenolphthalein alkaline react, add and the overall mixture thus obtained is briefly heated to at least 120 ° C. The stabilizing effect is only present if both components, i.e. the mercaptans and the basic, are. Substances are present at the same time. One of the two components alone still works not or only slightly stabilize d.

Particularly advantageous. ft can be used in the process according to the invention Use polyethylene.

Since the processing of polyethylene to moldings in general in open devices and above its melting point, that for high. pressure polyethylene at about 115 ° C and for those according to patent applications Z 3799 IV c / 39 c, Z 3862 IV c / 39c, Z3882 IV c / 39c, Z 3941 IV c / 39c, Z 3942 IV c / 39c (see also Angewandte Chemie, Vol. 67, 1955, p. 541) produced low-pressure polyolefins is at 125 to 130 ° C, occurs, the boiling point of the mercaptans and the basic collisions should be avoided from ver steam ext. u. should be correspondingly high, e.g. B. higher than 150 ° C.

In the. above-mentioned older, not belonging to the state of the art Proposals is described that one under comparatively mild pressure and ethylene Temperature conditions with mixtures of organometallic compounds and salts. of the elements Ti, Zr, Hf, V, Nb, Cr. Mo, Wo, Th and U can polymerize. Such Polymers are referred to as low-pressure polyethylene.

The main mercaptans that come into consideration are: heptyl, nonyl, Dodecyl or hexadecyl mercaptan; as basic substances are z. B. to name: Aminoethanol, Triethanolamine, trihexylamine and tribenzylamine.

If lower-boiling substances are used, in closed vessels, and can be worked under pressure.

Mixtures of different mercaptans or au. s different. basic substances are used. This possibility comes especially then into consideration when it comes to the use of technically occurring mixtures from mercaptans or from basic substances. Also the presence of the representation of any starting products or by-products still present is not included from the dilution effect-without substantial.

Influence on the stabilizing effect. This can cause the separation the technical mixtures z. B. avoided by distillation.

The stabilization z. B. of polyethylene required amounts of substance depend on the sensitivity of the polyethylene to be stabilized to oxidation. Relatively stable products only need about 0.1% Mercaptan, strongly unstable, on the other hand, up to 5 "/ e. The amount of basic additive can. are almost always kept smaller than that of the mercaptan. Will be expedient a ten. tel to half of the mercaptan, expressed. back in parts by weight, added. In special cases, e.g. B. when incorporating particularly sta, associated bases, like triethanolamine in polyethylene, an excess of basic substance can also be used versus the amount of mercaptan used, so at each point of the total mixture made of polyethylene, mercaptan and basic compound. despite the strong association, the minimum amount of basic required to achieve the stabilizing effect Fabric is present. Higher polyolefins, e.g. B. polypropylene generally require a higher amount of stabilizer than polyethylene.

The incorporation of stabilizers into the polyolefins can run in different ways. It is best to go from the powdery one Product from. For the purpose of even distribution of the stabilizing agents in the Powder is the mixture for some time in one of the usual mixing devices, e.g. B. in a ball mill, in a kneader, od on a roller. Like., Intimately mixed.

If the mixture is then above the melting point of the polyolefin sent through a heatable extrusion press, an even better uniform one is produced Distribution achieved; at the same time, the polyolene falls as being ready-to-use. ized granules. You can, but also the substances to be added in one common solvent or solvent mixture, e.g. B. in a hydrocarbon, an ester, alcohol, ketone, or chlorinated hydrocarbon, or any Mix these solvents, and dissolve the polyolefin powder with this solution mix. After the uniform distribution, the removal of the solvent follows by evaporation. To quickly remove the solvent, its boiling point must be lie as deep as possible; in addition, when using a low-boiling solvent avoided that the stabilizers in the evaporation of the solvent in noticeable Vaporizing amount. There are therefore particularly low-boiling gasoline fractions, Benzene, methyl and ethyl acetate, methanol, ethanol.

Acetone, methylene chloride or carbon tetrachloride as solvents into consideration.

Since it is often difficult to find a solvent that does this at the same time one can dissolve the mercaptan and the basic component in sufficient quantity one or both constituents of the pair of stabilizers, also with addition emulsify an emulsifier in water and z. B. the polyethylene with this aqueous Soak the slurry. Such a system is e.g. B. in the emulsion of dodecyl mercaptan in na. tronalka. soapy water. For good wetting of the polyethylene if the emulsion is expediently a water-soluble wetting agent for polyethylene, z. B. ethanol added. After intimate mixing, the water is and optionally remove the wetting agent by evaporation.

The most intimate distribution of the three components polyols, mercaptan and basic substance is obtained when using a polyolefin solvent or at least strongly swelling substance. This is where the Polvolefin come in particular or aliphatic and aromatic hydrocarbons which dissolve at elevated temperatures in question. For example you can get high-pressure polyethylene in a normal truck in boiling Dissolve benzene; because low-pressure polyethylene only takes place at a temperature that is 20 to 30 ° C higher is soluble, the solution must either be pressurized, e.g. B. at 3 to 5 atm when using of benzene, or it must be a higher-boiling solvent such. B. xylene, be used. The stabilizing agents must be in the solvent for polyethylene not necessarily be soluble. It is only necessary to use the stabilizing agent by very intensive stirring, kneading or the like. As homogeneously as possible in the solution of the Polyethylene or in the paste from the starl swollen polyethylene and the source agent to distribute. After mixing, the solvent must be removed again, z. B. by heating the mixture in vacuo.

In principle, granules can also be stabilized in a similar way to powder, only here in any case the mixture of polyethylene granulate is passed twice and steelizing agent by a granulating machine expediently, so that an intimate Distribution of the stabilizing agent is achieved.

Regardless of the way in which the stabilizing agents are incorporated z. B. in the polyethylene, the mixture must always be a few minutes Temperature of at least 120 ° C.

Only then do the stabilizers become effective and retain their own However, it is also effective after cooling down to a lower temperature. One such Heating always takes place when the polyethylene is above its melting point is processed into fittings; this heating process is sufficient that To make stabilizers effective. On the other hand, there is an unheated one Mixture of z. B. Polväthylenpulver and the pair of stabilizers still so unstable like the original polyethylene.

A disadvantage of the above-mentioned mercaptans is that they are unreasonably stronger Odor. Even the molded pieces still smell of mercaptan for some time, until the excess of mercaptan has evaporated.

By heating the granulate or the shaped pieces in an air stream or in a vacuum on z. B. 100 ° C, the samples are odorless faster. For many purposes z. B. for the use of Polväthylens for the production of sewer pipes or for cable sheathing, the disadvantages of the unpleasant odor present are insignificant compared to the much more important necessity here, degradation and embrittlement of the material as much as possible.

The experiments to determine the degradation behavior of polyethylene samples were mostly carried out at 120 ° C. This temperature is already very high close to the temperature at which the polyethylene grades with the highest melting point (125 to 130 ° C), and therefore rarely comes in practical use into consideration. The test temperature was deliberately chosen so high that the Investigate degradation under extreme conditions while simultaneously reducing the trial period abbreviate significantly. The polyethylene sensors used were mostly low-pressure polyethylene used because its melting point is correspondingly high. The usual high pressure polyethylene show the same degradation phenomenon, but these cannot be below the same level Temperatures are worked, since the melting point is generally below 120 °.

Molecular degradation was determined by measuring the reduced viscosity #red = # spec / c (in with 0.1% phenylß -naphthylamine stabilized tetrahydronaphthalene as a solvent at 120 ° C for the concentration c = 0.5 g of polyethylene in 100 cm3 solution).

To determine the embrittlement associated with the degradation, the polyethylene samples by pressing powder or granulate under 300 atmospheres at 150 ° C 0.5 mm thick plates produced. The plates were heated to 120 ° C and daily for their embrittlement after the hand bending test (bending of the plate by about 30 to 40 ° by hand).

Very brittle specimens break apart immediately, while not brittle samples e.g. B. can be bent up to 180 ° without breaking.

Example 1 a) 20 g of polyethylene and 20 cm3 of p-xylene are 10 minutes heated to 140 ° C; then the p-xylor is heated up for 6 hours 100 ° C evaporated in a stream of nitrogen. The reduced viscosity #red of this sample is 2.65. After 9 days of heating at 100 ° C, 3red only takes 2.20 After heating to 120 ° C for 2 days, #red sogra has dropped to 1.81. A sample of the the same material already becomes brittle under the same conditions. b) Will the in example 1, a) mentioned mixture 1% dodecyl mercaptan, based on polyethylene, added, the mixture homogenized by stirring and finally dried, so The viscosity of the sample pretreated in this way does not initially drop as much, z. B. to 2.21 after 2 days of heating to 120 ° C, but again with further heating stronger. During the brittleness attempt, the plate breaks after being heated for 4 days. c) To the mixture of Example 1, a) 1% dodecyl mercaptan and 0.2% ethanolamine, each based on polyethylene, according to the procedure described in Example 1, b) added. After 9 days of storage at 100 ° C there is none at all and after 50 days Store at 120 ° C a decrease in viscosity of only 6%. d) The one in the example 1, c) mixture is heated to 115 ° C instead of 140 ° C. The stability is only marginally better than that of the starting polyethylene.

Example 2 a) The granules of a polyethylene sample that are somewhat more sensitive to oxidation than those in Examples 1, a) to 1, d) is related, is heated to 120 ° C. After 24 hours, #red has dropped by 40%. b) In a solution of 2 g of tribenzylamine In 200 cm3 of a gasoline fraction with a boiling point of 60 to 80 ° C, 300 g of polyethylene are produced of the same variety as in Example 2, a) stirred in. The petrol is at 20 ° C in the Vacuum evaporated and the amine-containing polyethylene at 170 ° C on an extruder processed into granules. The granules behave just like that when heated Starting product of the previous example 2, a). c) Are those mentioned in Example 2, b) 3 g of dodecyl mercaptan were added to the solution of tribenzylamine in the gasoline fraction and otherwise worked as indicated in Example 2, b), 'q' falls after Heating to 120 ° C for only 10 ° / o.

Example 3 a) The reduced viscosity r of the granules of another Polyethylene sample sinks after 3 days of heating to 120 ° C from 2.97 to 1.53 Sample has become brittle after storage for 4 days at 120 ° C. b) Become the same Polyäthvlen, as used in Example 3, a), but in the one mentioned in Example 2, c) Way 0.6% hexadecyl mercaptan and 0.2 "/ o triethanolamine, both dissolved in the 100 times the amount of acetone added, the reduced viscosity decreases accordingly produced granules after 40 days at 120 ° C by 5 ouzo, and the sample is after this time still flexible.

Claims (1)

PATENT CLAIM: Process for reducing the tendency to break down and become brittle of polyolefins by mixing the polyolefins with either organic mercapto compounds or with basic organic compounds, characterized in that the Polyolefin mixture of aliphatic straight-chain or branched mercaptans and from organic basic substances, the aqueous solutions of which are phenolphthalein-alkaline react, adds and thereby the total mixture to a temperature of at least Heated to 120 ° C. Documents considered: French patent specification No. 956,489; British Patent No. 609 177.