CA1038577A - Rotational molding of chemically cross-linked polyethylene foam - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to a process for the manufacture of foamed, crosslinked polyethylene products by rotational molding. The process comprises the blending of polyethylene with a blowing agent in a non-acidic medium and heating at a temperature between 190°C and 300°C, the feature of the invention being a simultaneous blowing and crosslinking of polyethylene. The character-istic feature of the invention is the use of an organic peroxide as crosslinking agent having a half life-time in the range of 3 to 25 minutes, measured at 160°
in benzene, and a vapor pressure not higher than 5mm Hg at 20°C. The preferred crosslinking agents are 2,5-dimethyl-2,5-di(tert-butyl-peroxy) hexyne-3 and 2,5-dimethyl-2,5-di(tert-butyl-peroxy) hexane in their liquid state.

Description

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The present invention relates to a process for rotational molding of foamed, chemically crosslinked polyethylene. The proc~ss enables hollow objects to be achieved having a cellular wall structure rom solid polyethylene.
Foamed polyethylene structure offers certain advantages over solid polyethylene without excessive sacrifice o ~he excellent ~ -~
inherent properties of polyethylene such as chelmical and moisture resistance, flexibility and toughness. The chief advantages arle reduction of density, thus allowing a saving in weight of polyethylene, -and improvement in the insulating and isolating properties.
A ew years ago, a U.K. Patent, 1,038.810, was issued on the ;~
rotational molding of polyolefin foams rom a finely divided powder having a par~icle size of 800 microns using a blowing agent.
Optionally, a crosslinking agent selected from a polysuloneazide, a polyazide or an azidoformate is suggested to be added in order to obtain a "finer and more uniform cell structure". The disadvantage of this method is perhaps the relatively high cost of the unco~mon crosslinking agent claimed. The patent does not specify the extent of crosslinking ob~ained in the polyolefin but it seems that no specific requirement is imposed on this feature, since tbe presence of the crosslinking agent is also only optional.
J The crosslinked polyethylene compounds have a number of important advantages over the original ~hermoplastic olefin, these advantages baing directly correlated ~o the extent o crosslinking.
Thus, for example, highly crosslinked polyethylenes are not embrittled even when high loadings of fillers are used; they are not subjected to environmental stress cracking, exhibit excellent ~ -weathering properties and have ou~standing resistance to chemical ~ ~ -solvents.
' ' J' - :~0385~77 It is the object of the present invention to provide a process for rotational molding of foamed, chemically crosslinked polyethylene.
According to the invention there is provided a process for the rotational molding of rela~ively high, chemically crosslinked, foamed polyethylene in which solid polyethylene of the high, medium o~ low density type having a melt flow index above 1.8 is blended with a chemical blowing agent, a crosslinking agent and an activator for regulating the decomposition temperature of the blowing agent in a non-acidic medium and the blend is heated in hollow mold at a temperature between 190C and 300C while simultaneously rotating the mold, wherein the process is characterized by simultaneous blowing and crosslinking of the polyethylene to produce a foamed, crosslinked polyethylene having a gel content o~ above 50%, the crosslinking agent being an organic peroxide having a halflife in the range of 3 to 25 minutes and a vapor pressure not higher than 5 mm Hg at 20C, said organic peroxide being present in an amount of O.l - 2% by weig~1t of the ;
polyethylene.
The half lifetime figures of 3 to 25 minutes are measured at 160C in benzene. The applicants have tried out a large number of other crosslinking agents and even organic peroxides which have a vapor pressure higher than 5 mm Hg at 20C but only poor crosslinking, degree of expansion or appearance of the objects obtained by the rotational mold m g of the solid polyethylene ;~
resulted. Examples of organic perosides which are suitable for the present invention are 2,5-dimethyl-2,5-di(tert-butyl-peroxy) hexane, 2,5-dimethyl-2,5-di(tert-butyl-peroxy~ hexyne-3, characterized by their half life-time in the range of 3_25 minuSes (zzasur~d at 160CentigeJde in bznzene) and vzpor ~ .

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pressure not higher than 5 mm Hg at 2nc. Their use in the liquid state is preferred.
After a thorough investigation of the prob]Lem in order to find an explanation for the phenomenon, it was found that the two parameters of the c~osslinking agents - half life-time and vapor pressure - are very critical for the present invention. As known, ~;~
the ter~ 'half life' is commonly used to express the rate of decomposition at a particular temperature, being defined as the time required for one half of the organic peroxide originally present to `
decompose. The inventors have found that, in order to obtain products ~-with relatively high crosslinking and good appearance, the foaming and crosslinking processes should occur simul~aneously. This is achieved when the crosslinking agents are characterized by the two above-mentioned parameters. Poor results are obtained when the crosslinking agents possess only one of the parameters. Thus, for example, when di-tert-butyl peroxide which has a high life-time even ~ ;
higher than that of 2,5-dimethyl-2,5-di~tert-butyl-peroxy) hexyne-3 t20.4 minutes Yersus 18 minutes, both measured at 160C in benzene) was used at the same concentration, poor crosslinking and foam collapse in the products were observed. This is due to the relatively high volatility - 19.5 mm Hg at 20C - o~ the di-ter-butyl peroxide which causes poor crosslinking and instability of the fosm, especially at the lower range of foam density. ;
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The same poor results were obtained when di-t-butyl peroxy benzoate was used; this organic peroxide is a well-known catalyst for high-temperature molding, being a non-vo~tile liquid, but its half life-time is only 1.5 minutes ~measured at 160C in benzene).
The product obtained by rotational molding had an extre~ely rough internal surace and a density of 0.57 g/c~3, which indicatas t~
30 poor foam formation.

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i Other reagents used in the rotational molding of the crosslinked ~
foamed polyethylene are temperature regulators and mold release ;
agents.
The temperature regulators are very import~t components of the ~ -present invention dus to their role in decreasing the decomposition temperature of the blowing agent. Examples of t:he temperature ~ re regulators~various alkaline substances such as magnesium oxide, calcium carbonate, zinc oxide or stearats, titanium oxide, etc.
These reagents are used in amounts in the range of 0.1 to 1 parts by weight of the polyethylene.
Examples of the mold release agents found su.itable for the rotationai molding of foamed, crosslinked polyethylene are: calcium stearate, silicone spray, Te1On etc.
The foamed polyethylene products obtained according to the process of the present invention are characterized by a relati~ely high degree of cross-linking, i.e. j above 50%, which gives rise to ; ::
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improved properties of the products obtained.
It is known that organic peroxides which form free radicals upon decomposition are considered chemically reactive and chemically capable o forming crosslinking. However, rom the above-mentioned U.K. Patent 1,038.810, it would appear that organic peroxides are 1 not suitable or the crosslinking o foamed polyethyljne in a I rotational molding in view of the danger of an excessive degradation of the polymer.
The amount o crosslinhing agent used has to be in the range ¦ of 0.1-2% by weight of the polyethylene and preferably between 0.3-1%.
, Amounts o crosslinking agent less 0.1% by weight of the polyethylene i do not crosslink the polyethylene sufficiently to impart the necessary ` strength to withstand the force of the gas produced by the decomposed blowing agent, especially in the lower range of foam density.
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On the other hand, amounts of crosslinking agents in excess of 2%
by weight may crosslink the polyethylene excessively and inhibit foaming. The extent to which the polyethylene is crosslinked, i.e.
the extent to which its structure is transformed into a three-dimensional structure, is determined by the gel content. Specifically, `
the crosslinked polyethylene, upon immersion in a boiling liquid hydrocarbon, leaves an insoluble residue, known as gel, which is due to the crosslinked structure. This amount of gel is indicative of ~
the degree to which the polyethylene is crosslinked. ~;
The gel was determined by refluxing a weighed sample of the foamed polymer in boiling toluene; after drying, the insoluble portion of the sample was weighed to calculate the percentage of gel as follows:

gel = wt insoluble sam~le 100total wt of sample : ~ :
~This method is described in ~abelitem No. 128, Union Carbide ~1964) ;~
"Cure Testing of Vulcanized Polyethylene").
A non-acidic system is required to prevail since any acidic ~ -component will inhibit the crosslinking reaction.
It was found that, to a certain extent, the presence o oxygen disturbs the crosslinking of the polyethylene and that absorption of oxygen from air by the polyethylene during the heating accompanying the crosslinking react;on was responsible for the great variability in cell size and ruptured cells. Thus, by carrying out the heating in the substantial absence of oxygen, a foam is produced wherein the cell size throughout the foam is reasonably uniformJ and a homogeneous ~, crosslinking is achieved through all the thickness of the wall, as expressed by the constant percentage of gel content. However, when exygen was present, it was ~ound that a lower gel content exists in the thin inner layer, the gel per cent beîng higher at the outer .;

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wall in contact with the mold than at the inner wall. Tharefore, it is possible to obtain either a product with a constant, high crosslinking content throughout the thickness of ~he wall when oxygen ; is absent in the system or a product with a lower gel content at the ; ~;~
inner surface, when oxygen is present in the system. ~A.'person skilled in the art will select the proper conditions according to the specific requir~ments of the desired products. `~
The process according to the presen~ invent:ion can be carried - ~ -out by rotating the mold around two axes at a right angle to each other, at a rotational speed in the range of 2-40 rpm. Another application of , the process according to the present invention is the production of i articles by the centrifugal molding, fo r example, in the production of.cast tubes by a process which comprises rotating a cylindrical mold around its longitudinal axis while said axis is horizontal.
The rota~ional speed or centrifugal molding may be varied over a broad range, generally being between 200 and 2000 rpm.
The polyethylene suitable for the present invention includes low, medium and high density homopolymers, provided that their melt flow 7 index is above 1.8. The use of a high quality polyethylane powder is essential in rotational molding if good end products are to be I obtained.
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Preferred types of polyethylene are the high density polyethylene, I which has a density of about 0.96 and a melt flow index of about 5, and the low density polyethylene which has a density of about 0.92 ;
! and a melt flow index of about 5.
The blowing agents to be used in the process according to the ~ ~
present invention are the common blowing agents known for this art, ~ ;
such as: 4,4' oxybis (benzene sulfonyl hydrazide3, dinitroso pentamethylene tetraamine 80%, 4,4' oxybis (benzene sulfonyl semicarbazide); 1-1 azobisformamide (azodicarbonamide). The :Last one is the preferred blowing agent for the present invention.

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All these blowing agents act by producing N2, COJ C02, etc.
The gases expand the resin and control tha final density of the foam.
The amount of the blowing agent utilized in the preparation of rotationally molded produc~s can be varied over a wide rango and will obviously depend upon the degree of blowing desired. Usually, this amount is in the range of 0.3-5~ and preferably in the range of 0.5-2%, based on the total weight of polyethylene. Amounts of blowing agents outside this range generally do not produce desirable `;
foams. All the above-mentioned blowing agents have the property that they decompose over a definite and short range of temperatures, the gas being uniformly dispersed through the polymer. 'A~cording to the present invention, it is possible ko obtain a process which is ,~3~.
largely self-regulating by the proper selection of the amounts o~ ;
blowing and crosslinking agents. This comes out because the cell walls become sufficiently strong to resist further expansion by the blowing ;~
agent. Accordingly, within reasonable limits, it is possible to adjust the foam density of the finished products by adjusting the mutual proportions of the crosslinking and blowing agents and other `
components of the system. A person skilled in the art will certainly find the proper amounts within the range mentioned in the present `
specification in order to obtain products with the desired properties.
In accordance with another embodiment of the present invention, ;1 it is also possible to obtain composites of solid layers of a thermoplastic resin and foamed layers of polyethylene. This is achieved by a two-stage process: in the first one, a solid outer layer is produced by rotational molding of a thermoplastic resin and~ in the second stage, solid polyethylene, including the o~her ingredients required according to the present invention, is added into the rotating mold, thus obtaining an inner layer o foamed, crosslinked 1~
polyethylene. The outer layer may also be crosslinked, non-expanded .;

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',' polyethylene, in which case, the article produced by rotational molding, although having two separate structural layers, will be .. :
based entirely on polyethylene. ~or certain purposes, this will be of great advantage.
Fillers may also be used, either to reduce the cost of the products or to impart specially desired properties to the foamed, crosslinked polyethylene. The factors which have to be considered ;~
when adding fîllers are the viscosity of the mixture and the required mechanical properties of the products.
It is absolutely requisite that a homogeneous mixture result$
from the mixing of the filler with the polyethylene, since otherwise -~ -the products obtained will have a different appearance and feel on ~ ~-their surfaces. Typical classes of fillers which can be used are carbon black, cellulosic and other wood-derived fillers, inorganic salts and oxides, inorganic flakes and fibers, etc. Examples of `~
the various fillers which can be used in the rotational molding of crosslinked, foamed polyethylene are carbon black, calcium silicate, aluminium silicate, silica gel, barium carbonate, glass beads, etc.
The proportion and particle size of the filler used depend upon ~;;
the rigidity and surface quality required in the product. Where the proportion of filler is relatively low, the molding may have a continuous surface skin of the polyethylene. High proportions of filler, e.g. proportions of the order of 50% by weight, give products having a rougher finished surface suitable for such applications as insulating board or acoustic tiles.
The process according to this invention may further be used for producing colored polyethylene molded items; to this end, plgmen$s may be distribu~ed through the mixture entering into the rotational :
mold.

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The process can be employed to obtain unusual shapes of large articles which would be difficult, if not impossible, to achieve by conventional moldings. Shaped7 hollow articles o any size or shape ; can readily be prepared by rotational molding according to thepresent invention. For example, molds can be used for fabricating .
luggage, boat hulls, fuel tanks, aircraft stOTage tanks, etc.
The molds used are made of any s~eel or even aluminium. Aluminium has been successfully used and found desirable from the standpoint of temperature uniformity throughout the mold.
In order further to illustrate the nature of this invention and the manner of practicing the same fully, the following examples are presented for clearness of understanding therefrom, without being limited thereto, as modifications will be obvious to those skilled in the art. The physical properties of the foamed products given in the examples were determined according to the test procedure described in ASTM D-638.
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~ Example 1 ¦ 100 parts of high density polyethylene ~melt ~low index 5.0, density 0.95) were mixed with 0.7 parts of azodicarbonamide, 1 part zinc oxide, 0.2 parts calcium stearate and 0.6 parts liquid 2,5-dimethyl-2,5-di ~tert-butyl~peroxy) hexyne-3. All the ingredients were homogenized for 2 minutes, introduced into a rotating cubic mold of aluminium ~100 x 100 x 100 mm) and inserted into a cold oven; the mold was then rotated at a speed of 20 rpm. The oYen , was heated to 275C for 25 minutes while the mold continued to -`
i rotate. The oven was then opened and the rotating mold cooled by air ;~
i followed by water spray. A perfect cube of foamed, crosslinked`,~ polyethylene was obtained, having an average gel content of 70%, density of 0.45 g/cm , tensile strength of 908 lb/sq in, and 40 per -;
' 30 cen~ elongation at failure.
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Example 2 :
100 parts of low density polyethylene (melt flow index 6.5, density 0.918) were mixed with 0.5 parts of azodicarbonamide, 0.25 -~
parts zinc~ oxide, 0.1 parts calcium stearate and 0.75 parts liquid

2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexyne-3. All the ingredients were homogenized for 2 minutes, introduced into a `
rotating cubic mold of aluminium (100 x 100 x 100 mm) and inserted into a cold oven; the mold was rotated at a speed of 20 rpm.
The oven was heated to 250C for 20 minutes while the mold continued to rotate. The oven was then opened and the rotating mold cooled by air, followed by water spray. A perfect cube of foamed, crosslinked ~` !
polyethylene was obtained, having an average gel content of 70%, density 0.45 g/cm3, tensile strength 250 lbtsq in, and 95 per cent elongation at failure. ``

Example 3 The same experiment as in the previous example ~2) was repeated except that 0.5 parts of NaHC03 as blowing agent wereused instead of ; ;
azodicarbonamide and 1 part zinc stearate instead of zinc oxide.
The other reagents used and amounts were the same as in the previous example. The cube o foamed, crosslinked polyethylene had a gelcontent of 65% and a density of 0.395 g/cm3.

Example 4 -The same procedure as that in experiment 1 was used in this example, with the same high density polyethylene and other ingredients -~
as mentioned there, except that, in this experiment, 2,5 parts of powdered 2,5-dimethyl-2,5-di (tsrt-butyl-peroxy) hexane (50% acti~e) were used. The cube of foamed, crosslinked polyethylene had a gel content of 52%, density 0.42, tensile strength 725 lb/sq in, 34.4 per cent elongation at failure and 26% elongation at yield.

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Example 5 The same procedure at that in experiment 1 was carried out in this example, using high density polyethylene (melt flow index 4.5 and density 0.945). The amounts of the ingredients were the same as in experiment 1, the only difference being the use of 0.7 parts ~ ;
of tryhydrozinotria~ine (Trademark Genitron ~HT~ as blowing agent instead of azodicarbonamide. In this experiment, the oven was `
heated to 290C. A perfect cube of a foamed, crosslinked polyethylene was obtained which ha~ a gel con~ent of about 70% and density of 0.39 g~cm3.

i Example 6 50 parts of high density polyethylene (melt flow index 5.0, density 0.95) were introduced lnto a rotating cubic mold o~ aluminium (100 x 100 x 100 mm), the mold put into a cold oven and heated to 275C for 20 minutes while the mold continued to rotate. l`he oven was then opened and the rotating mold cooled by air followed by water -~
spray. The mold was opened, the cube taken out and a hole of 1 cm ``
diameter drilled, through which a homogeneous mixture consisting of the following ingredients was introduced: 100 parts of high density polyethylene (melt flow index 5.0, densit~ 0.95), 0.7 parts ;
, azodicarbonamide, 1 part zinc oxide and 0.6 parts liquid 2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexyne-3. The cube was again inserted into the mold and rotated at a speed of 20 rpm for 25 minutes at a temperature of 275C. The oven was then opened and the rotating mold cooled by air followed by water spray. A perfect - : .:
cube of a double layer of polyethylene was obtained, the outer layer being solid high density polyethylene and the inner one being foamed, crosslinked polyethylene of 68% gel content and 0.43 g/cm3 density.
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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the rotational molding of relatively high, chemically cross-linked, foamed polyethylene in which solid polyethylene of the high, medium or low density type having a melt flow index above 1.8 is blended with a chemical blowing agent, a cross-linking agent and an activator for regulating the decom-position temperature of the blowing agent in a non-acidic medium and the blend is heated in a hollow mold at a temperature between 190°C and 300°C while simultaneously rotating the mold, wherein the process is characterized by simultaneous blowing and cross-linking of the polyethylene to produce a foamed, cross-linked polyethylene having a gel content of above 50%, the cross-linking agent being an organic peroxide having a halflife in the range of 3 to 25 minutes and a vapor pressure not higher than 5 mm Hg at 20°C, said organic peroxide being present in an amount of 0.1 -2 % by weight of the polyethylene.

2. A process according to claim 1 wherein the organic peroxide is selected from 2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexyne-3 and 2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexane.

3. A process according to claim 2 wherein the organic peroxide is used in the liquid state.

4. A process according to claim 1 wherein the said activator for regulating the decomposition temperature of the blowing agent is selected from zinc oxide, zinc stearate and titanium oxide.

5. A process as claimed in claim 1 carried out in the substantial absence of atmospheric oxygen.

6. A process according to claim 1 in which the blowing agent is selected from azobisformamide, sodium bi-carbonate, 4,4' oxybis (benzene-sulfonyl-hydrazide), dinitro-zopentamethylene tetramine and 4,4' oxybis (benzenesulfonyl-semicarbazide).

7. A process according to claim 1 wherein a shell of solid polyethylene is first formed in said mold and said blend of solid polyethylene, chemical blowing agent and cross-linking agent is added into said shell to form said relatively high, chemically cross-linked, foamed polyethylene within said solid polyethylene shell.