The present invention concerns a cross-linked polyolefin foam which has been foamed with water and cross-linked by hydrolysing and condensing silyl groups on the polyolefin.

Cross-linked polyolefin foam is presently produced by using chemi¬ cal foaming agents, such as azodiacarbonamide, which decompose on being heated and generate gaseous nitrogen. The cross-linking is usually achieved by the aid of a radical former, such as dicumyl- peroxide. The cross-linking reaction is also achieved with the aid of heat. Cross-linked polyethylene foam manufacturing processes have also been developed, but in their case cross-linking is ac¬ complished with the aid of irradiation. The above-mentioned pro¬ ducts have very low densities, for which reason no applications requiring strength and rigidity can be contemplated. When an organ- ic peroxide is used for cross-linking agent, control of the process is difficult because the foaming and cross-linking process, are both temperature-dependent. Endeavours are, as a rule, to proceed so that when being foamed the polyethylene is partly cross- linked because in this way a more uniform foam is obtained. The above-mentioned processes require extra apparatus after the ex¬ truder for cross-linking and foaming.

Polyethylene is also cross-linked by grafting to it an unsaturated alkoxysilane and by hydrolysing and condensing silyl groups by treating the end product with hot water or steam, with the aid of a condensing catalyst. Nowadays, polyethylene that is cross-linkable in this way is available in the form of two mix components, one of them containing grafted alkoxysilane and the other containing con¬ densation catalyst. These granulates are mixed prior to extrusion and the end product is cross-linked with hot water or steam in a separate process step. Also other silane cross-linking processes exist, which differ somewhat from the process just described. In one process, all raw materials are supplied directly into the ex-

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truder at the step in. which the end product is being manufactured. A separate cross-linking step is required in this case too.

It is true that foam cross-linked with silane is being produced to some extent, but all these processes use azodicarbonamide as foam¬ ing agent, and cross-linking takes place by treating the end product with hot water or steam.

The invention concerns foamed polyolefin foam cross-linked with silane for which no cross-linking step performed afterwards is necessary. The silane-cross-linked polyethylene foam of the inven¬ tion is characterized in that it contains 60-99.9% polyolefin, 0.1-10% chemically bound hydrolysed silane, 0-5% condensation catalyst, 0.1-5% water, and 0-20% water carrier substance.

The invention also concerns a new procedure for manufacturing foamed, silane-cross-linked polyolefin foam. The procedure of the invention is characterized in that for polyethylene to be extruded is used polyethylene which contains, as foaming agent, 0.1-5% water I and 0-20% water carrier substance.

The polyolefin according to the invention to be foamed and cross- linked may be any polyolefin (LDPE,LLDPE, HDPE, PP, etc., or their copolymers or mixtures) , and the foaming agent may be any water- containing substance which is miscible with and dispersible in molten polyolefin (a compound containing crystal water, such as CaS0 ₄ ^* H ₂ 0, CaS0 ₄ ' 1/2 __ ₂ 0 and 1 ₂ 0_ ^* 3 H„0, or water-absorbing compound, such as CaCl ₂ and artificial silica, or a water-solving substance miscible with the polyolefin, such as ethylene glycol, propylene glycol, polyethylene glycol and polypropylene glycol) . For silane may be used any unsaturated hydrolysable silane which can be grafted to the polyolefin chain or copolymerised with the polyolefin by a radical reaction (by organic peroxide, electron irradiation or other means). Silyl peroxides may also be used towards this end.

The aim is, independent of the mode of adding foaming agent, silane

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or organic peroxide, that the molten polyolefin emerging from the extruder should contain 0.1-10%, preferably 0.5-3%, CaS0 ₄ ^* 2 __ ₂ 0 or an organic or inorganic additive containing and equivalent quan¬ tity of water, 0-10%, preferably 0.5-3%, vinyltrimethoxysilane (VTMO) or an equivalent quantity of other silyl groups, and 0-5%, preferably 0.02-0.1%, dicumylperoxide (DCP) or an equivalent quan¬ tity of radicals produced in another way. Furthermore there may be added 0-5%, preferably 0.05-0.5%, dibutylstannic dilaurate (DBTL) or of an equivalent hydroxylising and condensing catalyst, such as Z stearate. n

If the addivitives are ready-mixed to the polyolefin as a compound, the contents stated above apply. If however the additives are added in the form of master batches, the CaSO, ^* 2 H_0 content may be up to 70% and dicumylperoxide may be 10%, and dibutylstannic dilaurate may be 10%. A typical two-component system may be: Compound I; LDPE containing 2% vinyltrimethoxysilane; Compound II; LDPE con¬ taining 20% CaSO, ^* 2 KLO + 20% carbon black + 1% dicumylperoxide + 1% dibutylstannic dilaurate.

The cross-linked foam, which in this case is fairly hard and rigid, is appropriate either as such or together with a non-foamed plastic layer, or layers, to be used in cable manufacturing, tube manufac¬ turing, as jacket material (e.g. of an area heating pipe) and in mould blowing (big LDPE canisters e.g. for chemicals), and one achieves in these instances material savings as regards the poly¬ olefin raw material, and properites of the cross-linked polyolefin (heat tolerance, cold tolerance, tolerance of chemicals, and resistance to stress cracking) . The adhesion to metal is also good (owing to the silane) . It is possible to use less carbon black in the foamed product. Film products may be foamed and cross-linked by this procedure in order to obtain a strong, breathing (holes) film with high friction. It is also possible to produce strong and soft film for packaging purposes (soft and tough at low tempera- tures) . The adhesion properties of silane may be utilized in coex- trusion with other types of plastics. In extrusion coating, the friction and adhesion characteristics of foam produced in this

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manner could be utilized. By the die-casting technique, cross-linked products may be manufactured, in which case the degree of foaming and the degree of cross-linking can be regulated by regulating the die pressure and the temperature cycling. Cross-linked foam products may be manufactured by a rotation casting process, in addition to the technique mentioned. These products may also be manufactured by foaming with water and cross-linking using other procedures (organic peroxide, electron radiation, etc.), or they may be left un-cross-linked (omitting the silane). In that case, naturally, the advantageous foaming, machinabllity, adhesion and strength prop¬ erties resulting from silane will not be achieved.

Deviating from procedures known in the art, in the procedure of the invention the water needed for hydrolysis and condensation of silane is added to the polyolefin raw material in connection with extruding the end product, whereby it can be mixed with the granulate or supplied at a later stage directly into the molten polyolefin. The -water may also be ready-mixed in the plastic raw material (compound). When water is added in this manner to plastic, it must as a rule be mixed with another substance which is well miscible with the poly¬ olefin (e.g. propylene glycol), or the water may be physically or chemically bound to such a substance (e.g. in the form of crystal water) . If the water is added in a polyolefin master batch, it has to be prepared at a temperature so low that the compound has not undergone foaming. With water present in this way when the poly¬ olefin is molten, the silane that has been grafted or is present as a comonomer begins to hydrolyse and to condense partly at the extrusion step already. In this way, the foaming starting after the extrusion orifice takes place in a partly cross-linked and elastic polyolefin melt. The advantage is then that the foam is more uniform and the density is lower. Also the molten strength is higher, thus facilitating the shaping of the end product (e.g. in mold blowing). The shrinkage (increase in density) of the foam in connection with the cooling of the end product is also reduced in this way. The most important advantage is however that the hydryolysis and condensation of silane has already been able to start in the extruder in the foaming step, and the cross-linking reaction will then continue

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of itself without any separate cross-linking step. The product need only be stored in normal storage conditions. It is also important that cross-linking starts on the cell walls at an early stage.

All additive components may be added as they are into the extruder together with the polyolefin granulate or powder, or at a later stage into the polyolefin melt. But it is easier to use a ready- made polyolefin compound which contains all, or part of, the above- mentioned components. If all additive components are included in the same compound, it has to be prepared at very low temperature (below the foaming temperature and preferably below the decomposi¬ tion temperature of peroxide) . This is because the aim is that the silane is not yet grafted in connection with the compound prepar- tion but only at the processing step. In this way, the compound has a longer shelf life. As taught by the invention, however, the silane may also be grafted in connection with the compound preparation. Another alternative is that two compounds are used, Compound I, which contains the silane and which may be prepared at high tem¬ perature and with high production rate, and Compound II, which con- tains the foaming agent and the organic peroxide and which must be prepared at lower temperature and, for this reason, at low production speed. The other requisite components, such as carbon black, may be admixed to either component. In this way, one is enabled according to application and to what is needed to regulate the product's degree of foaming and degree of cross-linking. Com¬ pound I and Compound II may, as required, also be diluted with the basic polyolefin.

In conjunction with extrusion of the end product, conditions must be such that the steam and/or the silane cannot escape through the rear. There must be a plug of molten plastic in the extruder, and at the decomposition temperature of the foaming agent the tempera¬ ture must be high enough so that no foaming occurs in the extruder. It must also be possible to regulate the temperatures and delay times so that the silane is completely grafted in the extruder and that hydrolysis and condensation of silane are partly accomplished. The foaming and the condensation proper of the silane take only

place after the extruder. It is then to advantage if the polyolefin melt has high viscosity (low melt index, low temperature, high degree of cross-linking, etc.), whereby the melt strength is high.

At the beginning, cooling must he between the water condensing temperature and the crystallizing temperature of the polyolefin, whereby a lighter foam and more efficient silane condensation are obtained.

The Idea of manufacturing polyolefin foam foamed with water and cross-linked with silane is illustrated by means of the following examples. Strips were run of the following formulations as dry mixes with a Reifenhauser strip extruder (45 mm, 25 L/D) in such manner that the temperature profile was 105°C, 125°C, 180°C, 180°C, T°C, T°C. The orifice temperature (T) was normally 190°C, the screw speed (V), 40 min , and the foamed, cross-linked strip material was air-conditioned during one week at 23 C, at 50% R.H. , before testing.

Example 1

A polyethylene mix was prepared which contained common low density polyethylene (melt index 0.3 g/10 min) and 0-2% by weight gypsum

3 (CaSO, ^* 2 H ₂ 0). The density of the mixture was 0.92 g/cm .

From the mixture, a strip was produced in a strip extruder (45 mm, 25 L/D) in such manner that the temperature profile was 105, 125, 180, 190, 190°C. The screw speed was 40 min ^" . The foamed poly¬ ethylene strip was air-conditioned during one week at 23 C and at 50% R.H. prior to testing. In Table I are stated the product recipes and the characteristics that were found by measurement.

It is evident from Table I that when the quantity of gypsum in¬ creases the density of the polyethylene foam which is produced goes down, while at the same time the elongation and tensile im¬ pact strength deteriorate substantially. If on the other hand the absolute values of the specimens (not standardized with reference

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to cross-section area) are observed, there is no substantial lower¬ ing. - ^•

Example 2

As in Example 1, foamed polyethylene strips were run from mixtures containing 1% by weight vinyltrimethoxysilane, 0.1% by weight dicumylperoxide, and 0-0.1% by weight dibutylstannic dilaurate. The recipes and the characteristics of the product are presented in Table II.

Table II reveals how cross-linking with silane improves the ten¬ sile impact strength values and, furthermore, reduces the density. When too much silane is added (3%) , the density increases again because the silane consumes too much of the water produced by the gypsum. The degree of cross-linking does not increase substantially however (about 30%) .

Example 3

As In Example 1, polyethylene foam strips were produced in a strip extruder from a mixture containing 1.5-3% by weight gypsum as foam¬ ing agent. The cross-linking agent was vinyltrimethoxysilane (2%). In Table III are presented the recipes that were used and the characteristics measured for the products.

Table III reveals that when the silane content is constant (2%) the density first decreases but then increases again. The degree of cross-linking, on the other hand, first increases with decreas- ing gypsum content, to go down once more thereafter. Lowest density and highest degree of cross-linking are obtained when the gypsum content is 1.5% by weight.

Example 4

As in Example 1, polyethylene foams were produced in a strip ex¬ truder. The cross-linking agent was vinyltris(betamethoxy)silane

and vinyltrimethoxysilane. The foaming agent was gypsum, calcium chloride and 1:1 propylene glycol.water solution. The recipes and the characteristics of the products are presented in Table IV.

In Table IV is seen the effect of silane type and foaming agent type on the characteristics of the LDPE foam. VTM0E0 yields higher degrees of cross-linking than VTMO, but this may be due to the lower building point of the latter, and it may escape in greater amount during the extrusion step. Using CaCl ₂ or 1:1 water/propylene glycol solution for foaming agent, dross-linked foam is certainly obtained, but the densities are rather high and the degrees of cross-linking rather low.

Example 5

In this example the effect of melt index, density (HDPE) and car¬ bon black (2.5%) on the properties of the cross-linked polyethyl¬ ene foam were examined, said properties being shown in Table V. It is observable that when high melt index LDPE is used (SI - 7.5 g/10 min) the density of the foam does not go down enough. The melt strength of the foam is too low. The degrees of cross-linking are, however, fairly high. With HDPE, again, relatively low den¬ sities can be achieved, but the degree of cross-linking remains rather low. Regarding polyethylene containing carbon black it may be said that the foam is impaired both as regards density and cross- linking. Carbon black absorbs part of the water, silane, peroxide and condensing catalyst. This must be taken into account when the recipe is optimized.

Example 6

In the example was studied the effect of extrusion conditions on density and degree of cross-linking. In Table VI is also shown the effect of the foam-solidifying technique on the results, in that the foam has been allowed to crystallize in boiling water, whereby the water in the cells has been in vapour form during crystalliza¬ tion and cross-linking of the polyethylene. In this manner, clearly

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lower densities and slightly higher degrees of cross-linking were obtained. In Table VI it is also seen that when a lower melt tem¬ perature is used a lower density is obtained, but also a slightly lower degree of cross-linking. For the delay time an optimum also exists (in the present instance, 40 min ). When the delay time is too short, the silane has not time to become grafted and condensed, and when the delay time is too long, the silane evap¬ orates before grafting. When there is too much condensing catalyst (0.2% DBTL) and reactive components in sufficient quantity (2% gypsum and 2% VTMO), the cross-linking reaction may be so violent that the polyolefin emerges from the extruder in powder form. The degree of cross-linking is then very high (76%). By reducing the delay time in the extruder by increasing the r.p.m. of the screw, the cross-linking reaction can be prevented from running this far.

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TABLE I

TABLE II

TABLE III

TABLE IV

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TABLE VI

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