WO1985003080A1

WO1985003080A1 - Procedure for manufacturing shaped articles containing cross-linked polyolefin

Info

Publication number: WO1985003080A1
Application number: PCT/FI1985/000003
Authority: WO
Inventors: Christer BERGSTRÖM; Johan Brenner
Original assignee: Neste Oy
Priority date: 1984-01-06
Filing date: 1985-01-04
Publication date: 1985-07-18
Also published as: DE3590010T1; BE901442A; GB2168056A; BR8602863A; SE8504137L; SE8504137D0; GB8520470D0; NL8520002A; FR2557879A1

Abstract

Procedure and means for manufacturing shaped articles containing cross-linked polyolefin from a mixture which contains 60-99% polyolefin, 0.01-10% chemically bound hydrolyzable silane and 0-5% condensing catalyst. The polyolefin mixture to be shaped contains 0.1-5% water and 0-20% water carrier agent, and that cooling the product subsequent to shaping is carried out under pressure. The means (20) comprises nozzle members (21) for pressing the shaped article of the polyolefin mixture, and members (22) for cooling the shaped article under pressure to a temperature which is below the boiling point of the water present in the mixture. The chamber (22) also includes means (24) for directing microwave radiation on the shaped article for releasing the water present in the form of crystal water in the polyolefin mixture.

Description

Procedure for manufacturing shaped articles containing cross- linked polyolefin

The present invention concerns the manufacturing of cross-linked polyolefin products. In particular, the invention concerns the manufacturing of shaped articles from polyolefin which are to be cross-linked with the aid of silane and water.

Currently, polyolefin is cross-linked mostly with the aid of organic peroxides. The peroxides are decomposed by heat or in another way after extruding the end product. The drawback is then that the extruding has to be performed at rather low temperatures, in order to avoid decomposition of the organic peroxide, and as a consequence the extrusion rate is low. In addition, subsequent to the extrusion step separate cross-linking lines are needed, which are expensive, space-consuming and energy-intensive. Polyolefins as such or in mixture with peroxides may also be cross-linked by irradiating them. Procedures of this kind require major investments, and the wall thickness of the products is limited in them.

A polyolefin cross-linking procedure which has become very popular in recent years is silane cross-linking. The procedure is based on grafting to the polyolefin a hydrolyzable unsaturated silane, for instance vinyltrimethoxysilane, and in the extrusion step also condensing catalyst is added, for instance dibutylstannic dilaurate. Such mixture can be extruded at high speed at rather high temperatures because the cross-linking is accomplished in an after-treatment with the aid of water, after extrusion. The cross-linking is accomplished in a batch process in hot water or in hot steam. It is a time and energy consuming process, in particular if the product has great wall thicknesses, for the reason that the water must penetrate into the polyolefin product, such as a cable sheathing for instance, and condense the silane.

It is also known in prior art to accomplish cross-linking with the aid of water added to the polyolefin raw material or in the extrud ing step, whereby cross-linking already begins in the extruder. If it is desired altogether to avoid the cross-linking carried out afterwards with the aid of water, one has to add so much water to the polyethylene that the product becomes foamed when emerging from the extrusion nozzle. This kind of manufacturing procedure of polyolefin foam cross-linked with the aid of silane, based on the use of water, is disclosed in the Finnish patent application No. 834062. In many instances, however, the cross-linked polyolefin products are of such kind that no foaming at all is desired or it is desirable to reduce it or to concentrate it in the core part of the end product (integral foam) .

The object of the invention is to achieve a procedure for cross-linking polyolefin with the aid of silane and of water added to the polyolefin raw-material, or in the shaping step, in such manner that the foaming of foam can be avoided or limited. The procedure of the invention for manufacturing shaped articles containing crosslinked polyolefin from a mixture which contains 60-99% polyolefin, 0.1-10% chemically bound hydrolyzable silane and 0-5% condensing catalyst is characterized in that the polyolefin mixture to be shaped contains 0.1-5% water and 0-20% water carrier agent, and that the cooling of the product after shaping is carried out under overpressure.

The present invention also concerns a means for cross-linking polyethylene mixtures containing silane and water. The means of the invention is characterized in that the means comprises nozzles for pressing the shaped object of the polyolefin mixture, and members for cooling the shaped article under pressure to a temperature which is below the boiling point of the water in the mixture.

The procedure is applicable in manufacturing shaped articles by various techniques, including e.g. extrusion, die casting, blow moulding, spin casting and deep drawing. In application for instance in extrusion, the procedure is based on cooling the molten polyolefin mixture emerging from the nozzle under pressure. When the pressure is higher than the pressure of water vapour at the temperature of the melt that has emerged from the nozzle, the polyolefin mixture will not foam at all, yet the cross-linking (hydrolyzis of the silane chemically bound in the polyolefin and its condensation with a condensing catalyst) takes place at high speed in the extruder, in association with cooling and calibration, and thereafter, due to the high temperatures and the presence of a large quantity of water. If it is desired to allow some foaming, and fast cross-linking is desired at the same time, lower overpressure may be used, whereby the mixture has a little time to foam before it has cooled down below the boiling point of water at that pressure. On the other hand, integral foam can be produced in the way that the temperature of the melt is higher in the core of the wall than the boiling point of water at the prevailing pressure, and the temperature of the melt on the surface of the wall is lower than the boiling point of water at the prevailing pressure. Also in the instance in which the polyolefin mixture is extruded at a temperature so low that for instance the water present in the form of crystal water will not separate and is instead separated later in other ways, the product must be under pressure. Otherwise, foam cells will appear in it if it is in molten state, or cracks if it is in solid state. The latter, in particular, substantially impair the strength properties of the end product.

The polyolefin to be cross-linked used in the procedure of the invention may be any polyolefin (LDPE, LLDPE, MDPE, HDPE, PP, PB, or their copolymers or mixtures). The water carrier agent may be any substance containing water which is miscible with and dispersible in the molten polyolefin. For water-containing substance can be contemplated, for instance, compunds containing crystal water, such as CaSO₄ · 2H₂O, CaSO₄ · 1/2 H₂O and Al₂O₃ · 3H₂O, or waterabsorbing compounds, such as CaCl₂ and artificial silica, or a watersolving substance miscible with the polyolefin, such as ethylene glycol, propylene glycol, polyethylene glycol and polypropylene glycol. The required water may also be water produced by means of a condensing reaction or by any other reaction.

For silane may be used any unsaturated hydrolyzable silane which can by a radical reaction (by organic peroxide,, electron irradiation or any other means) be grafted to the polyolefin chain or copolymerized with the olefin. Silyl peroxides may equally be used to the purpose .

Independent of the mode in which water, silane or organic peroxide is added, it is sought to achieve that the molten polyolefin merging from the extruder contains 0.1-10%, preferably 0.5-3% CaSO₄ 2H₂O or inorganic or organic additive containing an equivalent quantity of water, 0.1-10%, preferably 0.5-3%, vinyltrimethoxysilane (VTMO) or an equivalent quantity of silyl groups, and 0.01-5%, preferably 0.02-0.1%, dicumylperoxide (DCP) or an equivalent amount of radicals produced in other ways. In addition to those mentioned, the mixture may contain 0-5%, preferably 0.05-0.5%, dibutylstannic dilaurate (DBTL) or equivalent silane hydrolyzing and condensing catalyst, such as Zn stearate.

If the additives are ready-mixed in the polyolefin, the concentrations stated above apply. If the additivies are added in the foam of a separate concentrate, the CaSO₄ content in such concentrate may be up to 70% and the quantity of dicumylperoxide up to 10%, and the quantity of dibutylstannic dilaurate up to 10%. A typical two-component system could be as follows: Mix 1: LDPE containing 2% vinyltrimethoxysilane; Mix 2: LDPE containing 20% CaSO₄ · 2H₂O + 20% carbon black + 1% dicumylperoxide + 1% dibutylstannic dilaurate.

The water required for hydrolyzing and condensing the silane may be added to the polyolefin raw material in conjunction with extruding the end product; it may then be mixed with a granulate or supplied at a later stage directly into the molten polyolefin. The water may also be present ready-mixed in the plastic raw material. When water is present while the polyolefin is in molten state, the grafted silane, or silane present as a comonomer, begins to undergo hydrolysis and in part to condense during the extrusion step already. The melt strength is then higher, thus facilitating the moulding of the end product (for instance in blow moulding), but the most important advantage is that silane hydroly sis and condensation have already come under way in the extruder, and the cross-linking reaction will then continue without any separate cross-linking steps.

All additive components may be added as such in the extruder together with the polyolefin granulate or powder, or at a later stage to the polyolefin melt. A prepared polyolefin compound may also be used which contains all or part of the components mentioned in the foregoing. If all additive components are included in the same compound, it has to be prepared at a very low temperature (below the foaming temperature and preferably below the decomposing temperature of the peroxide). This is because it is aimed at that the silane should not yet become grafted to the polyolefin in the compound phase, but only in the refining phase. Longer shelf life of the raw material is hereby achieved. However, it is also possible in the procedure of the invention to graft the silane in the compound-preparing phase.

Another alternative is that two compounds are used, of which one contains the silane and may be manufactured at high temperatures with high production rate, while the other compound contains the organic peroxide and water with carrying agent and is manufactured at lower temperatures and at a lower production rate. The rest of the components required may be admixed to either component.

The procedure of the invention for manufacturing shaped articles of polyolefins cross-linkable with the aid of silane and water is applicable in cable, pipe and tube, and strip extrusion, in blow moulding and in die casting. In addition, with polyolefin mixture mentioned it is possible to do spin casting, deep drawing and equivalent shaping under pressure and thus to cross-link the product (and, furthermore, to foam the product in controlled manner, if required).

The invention also concerns a means for manufacturing shaped articles containing cross-linked polyolefin of the mixtures described in the foregoing. The means of the invention comprises nozzle elements for extruding the shaped articles of a molten polyolefin mixture, and members for cooling the shaped article under pressure to a temperature which is below the boiling point of the water in the mixture.

The invention is described in detail referring to certain advantageous embodiments of the invention, presented in the figures of the drawing attached, but to which the invention is not meant to be exclusively confined.

Fig. 1 presents an advantageous embodiment of the means of the invention in sectioned elevational view.

Fig. 2 presents another advantageous embodiment of the means of the invention in sectioned elevational view.

In Fig. 1, an embodiment of the means is depicted in which the nozzle members 11 for pressing the shaped article, and the cooling members for cooling the article under pressure, constitute an integral entity 10. Fig. 1 presents schematically a nozzle intended for manufacturing a cross-linked pipe or tube. The nozzle 11 is provided with cooling with such effect that the temperature of the tube when it emerges is below 100ºC. Hereby, a cross-linked tube is produced from the polyolefin containing silane and water, which is not foamed. Higher temperatures cause foaming of varying degrees. The cooling of the nozzle 11 has preferably been arranged so that the temperatures of the individual nozzle segments 11a, 11b, 11e, 11d

(also that of the mould core) can be separately regulated with the aid of cooling fluid flows 12a, 12b, 12c, 12d.

The temperature of the mix in the means of Fig. 1 has to be kept slightly above the melting point of the particular polyolefin in those parts of the nozzle where the substantial shaping process takes place. In the ultimate part of the nozzle 11 , the polyolefin crystallizes and the passages become narrower as the density of the polyolefin changes. The passages should be designed so that the pressure in the mix surpasses the vapour pressure of water in every part of the nozzle. The interior surfaces of the ultimate part of the nozzle 11, where the product slides in crystal form, may be lubricated with water or with another substance, or the inner surfaces may be made of a material with low friction against the product. With this means 10, also other cross-linked products may naturally be manufactured, such as cable sheaths, profiles, strips, fibres and sheets.

In Fig. 2, another embodiment of the invention is shown schematically, in which the cooling members form a separate chamber 22.

We are concerned with an apparatus 20 for manufacturing cross-linked cable insulation, in which the nozzle 21 is a standard nozzle such as is used in cable manufacturing. In the chamber 22 a pressure higher than atmospheric is maintained. If the pressure is higher than the vapour pressure of water at the respective temperatures of the insulating material, an insulating material is produced of the polyolefin containing silane and water, which is not foamed. With the aid of the pressure, the degree of foaming may also be regulated. The packings 23 represent the state of art. In the chamber 22, the insulation is cooled with water or other substance. If the temperature of the insulation is below 100ºC after the packing 23, one achieves cross-linking of an insulation which is not foamed. This technique may, of course, be utilized in the manufacturing of other cross-linked products as well, such as pipes or tubes, and various sections.

In blow moulding it would be possible by regulating the compression, the after-pressure and the cooling, to regulate the degree of foaming of a cross-linked, blow-moulded article. If prevention of foaming is desired, filling of the mould must be rapid, for instance by using wide feed passages. In order to prevent foaming in the filling phase, sufficiently well-sealed moulds should be used in order to produce a counterpressure. To increase the counterpressure in the empty mould, for instance nitrogen supply may also be used. The counterpressure as well as the after-pressure should be higher than the vapour pressure of water at the respective mix temperature. The article should cool down below 100ºC under pressure in the mould. In spin casting, cross-linked, non-foamed products can be manufactured in a pressure-proof mould from polyolefin containing silane and water by performing the casting under pressure. The overpressure in the mould must then surpass the vapour pressure of water at the casting temperatures concerned. The pressure must not go down before the product has cooled below 100°C.

In the attached examples is illustrated, with the aid of laboratory experiments, the silane cross-linking with the aid of water added to the polyolefin raw material, or added in the extrusion step. In Table I are stated the densities and degrees of cross-linking of the polyolefin mix-cross-linked with silane, in the case that the melt emerging from the nozzle of the extruder has been allowed to cool down without overpressure. In Table II are shown the respective characteristics when the cross-linking took place under pressure.

The results in Table I were measured from strips manufactured with a strip extruder (45 mm; 25 L/D; 105°C, 180°C, 180°C, 190°C, 190°C; r = 40 r.p.m.), the strips being conditioned during one week at 20ºC, 50% R.H. , prior to testing. A dry mixture was prepared of all raw materials before extruding. The results reveal that LDPE is rather amply foamed by effect of 1% CaSH₄ · 2H₂O, and when moreover silane (e.g. VTMO) is grafted to the LDPE, the foaming becomes less, the higher the silane content. Part of the water reacts with the silane, and at the same time the degree of cross-linking increases.

When the CaSO₄ · 2H₂O content is increased to 2%, the density no longer goes down, because the foam shrinks again as water condenses in the cells, but the degree of cross-linking does increase substantially. When 3% CaSO₄ · 2H₂O are added, disintegration of cells and reduction of the water vapour quantity ensue. Hereby, the density increases and the degree of cross-linking decreases. Vinyltrimethoxyethoxysilane (VTMOEO) has an effect on cross-linking and density like that of VTMO. Experiments were also made with CaCl₂ in equilibrium with the humidity of air (23°C, 50% R.H.) and with 11 water-propyleneglycol solutions, but fairly low degrees of cross-linking and rather high densities were obtained, obviously owing to imperfect dispersion. With HDPE, similar densities as with LDPE were obtained, but the degrees of cross-linking were clearly lower.

The results presented in Table II have been measured from sheets which were cross-linked in an autoclave under pressure. First, a strip was produced with a 22 mm strip extruder from a dry mixture containing LDPE (melt index = 0.3 g/10 min, density = 0.922 g/cm³),

2% VTMO and 0.1% DCP. The temperature profile was 130°C, 180°C,

180°C, and the screw speed was 30 r.p.m. This LDPE strip grafted with VTMO was comminuted, and 2% CaSO₄ · 2H₂O were added to it, and in Examples 11-17 0.1% DBTL (in Example 18 no DBTL) , and from this mixture once again a strip was run with temperature profile

120°C, 120°C, 120°C and at speed 30 r.p.m. The strip was pressed at 120ºC into a sheet, and the sheet samples were placed in an autoclave which had been preheated to 120ºC. High pressure nitrogen was introduced in the autoclave as shown in Table II, and the temperature was thereafter raised as quickly as possible so that the end temperature and total time were those given in Table II. Thereafter, the autoclave was immersed in cold water until the temperature of the sample was below 100ºC (about 3 min.), whereafter the pressure was released, and measurements were made of the samples' density (with air pyknometer) and degree of cross-linking (6 hrs in boiling xylene) .

Table II reveals that very high degrees of cross-linking have been achieved. This is evidently due to the fact that the cross-linking could take place in molten state, which is different from conventional silane cross-linking processes. In solid state, only the amorphous part becomes cross-linked. Differences due to temperature or time do not occur either. Obviously there was enough water, and the reaction was very fast at these temperatures (below 163°C only 1 1/2 H₂O is set free). In comparison with the results obtained with silane cross-linked foam (Table 1) , the degrees of cross-linking were very high, too. However, in the absence of DBTL the cross- linking reaction was so slow that at 140ºC and in 30 minutes only 13% cross-linking was achieved (laboratory example 18) . When the pressure was lower than the vapour pressure of water, the polymer became foamed (laboratory example 17). In this case, however, the degree of cross-linking was high. In other instances, no foaming of the polymer took place.

A sheet conforming to the basic recipe presented in Table II (containing 0.1% DBTL) was also placed in a microwave oven (kitchen model) and kept there for 30, 60 and 120 seconds. Hereby, the crystal water was set free, but the product was not foamed because the plastic mixture was in solid state. However, small cracks formed because the treatment did not take place under pressure. When this treatment step was long enough, rather high degrees of cross-linking were achieved (Table III).

Claims

1. A procedure for manufacturing shaped articles containing cross-linked polyolefin from a mixture which contains 60-90% polyolefin, 0.1-10% chemically bound hydrolyzable silane and 0-5% condensing catalyst, characterized in that the polyolefin mixture to be shaped contains 0.1-5% water and 0-20% water carrier agent and that cooling of the product after forming is carried out under pressure.

2. Procedure according to claim 1, characterized in that the pressure is generated in an extruder or in another means intended for shaping the product.

3. Procedure according to claim 2, characterized in that in the extruder a nozzle is used in which at least the ultimate end can be cooled.

4. Procedure according to claim 2 or 3, characterized in that for nozzle is used a nozzle composed of segments which are individually coolable.

5. Procedure according to claims 2-4, characterized in that for nozzle is used a nozzle with tapering passage cross-section.

6. Procedure according to claim 1, characterized in that pressure is generated after the extruder or other shaping means, mechanically, hydraulically or pneumatically.

7. Procedure according to claim 6, characterized in that after forming, the polyolefin mixture is cooled in a pressurized chamber.

8. Procedure according to any one of the preceding claims, characterized in that the water carrier agent is a compound containing crystal water or a compound absorbing water or a substance dissolvitig water and miscible with the polyolefin, or a substance producing water through a chemical reaction.

9. Procedure according to claim 1, characterized in that the water present in the form of crystal water in the water carrier agent is set free by microwave oven technique.

10. A means for manufacturing shaped articles containing cross-linked polyolefin from a mixture containing polyolefin, chemically bound hydrolyzable silane, condensing catalyst, water and/or a water carrier agent, by a procedure according to claims 1-9, characterized in that the means (10; 20) comprises nozzle members (11; 21) for pressing the shaped article of the polyolefin mixture, and members (12;22) for cooling the shaped article under pressure to a temperature which Is below the boiling point of the water present in the mixture.

11. Means according to claim 10, characterized in that the nozzle members (11) also comprise cooling members (12).

12. Means according to claim 11, characterized in that the nozzle members (11) consist of segments (11a, lib, etc.) which can be separately cooled.

13. Means according to claim 10, characterized in that the cooling members (22) consitute a chamber which is kept under pressure higher than atmospheric.

14. Means according to claim 13, characterized in that the chamber (22) also comprises means (24) for directing microwave radiation on the shaped article for releasing the water present in the form of crystal water in the polyolefin mixture.