WO2008009395A1

WO2008009395A1 - Expandable cross-linked polyethylene, method for manufacturing it, and apparatus for performing the method

Info

Publication number: WO2008009395A1
Application number: PCT/EP2007/006254
Authority: WO
Inventors: Joseph Cirasa; Paolo Cirasa
Original assignee: Masterplast S.R.L.
Priority date: 2006-07-20
Filing date: 2007-07-13
Publication date: 2008-01-24
Also published as: ITMI20061418A1

Abstract

A new expandable cross-linked polyethylene, a method for manufacturing it and an apparatus for performing the method. The method in particular relates to the possibility to separate in time and, if so wished, physically the steps of cross-linking and expansion. In this manner it is possible to isolate an expandable cross-linked polyethylene which has particular properties with respect to conventional products.

Description

EXPANDABLE CROSS-LINKED POLYETHYLENE, METHOD FOR MANUFACTURING IT, AND APPARATUS FOR PERFORMING THE METHOD

Technical Field The present invention relates to a new expandable cross-linked polyethylene, to a method for its production and to an apparatus for performing the method. Background Art

Expanded materials are products whose relative density is lower than that of the base material of which they are composed. This effect is achieved by creating, by means of gas bubbles within the structure of the base material, voids which increase its volume.

Synthetic foams are produced usually by using plastic materials which have the ability to change shape when they are subjected to certain temperature and pressure conditions; their volume is made to increase by taking advantage of this characteristic. The final volume can be 1.5 to 60 times the initial one; thus, products are obtained which range in general from approximately 15 to approximately 600 kg/m³.

In order to obtain polymeric foams (for example polyolefin foams, synthetic rubber foams, polyurethane foams, et cetera) it is generally necessary to proceed with a step for cross-linking the monomers, creating inside them microcavities (usually by adding a nucleating agent), and then proceed with a step for inflating said microcavities (usually thanks to the addition of a blowing agent). The cross-linking and blowing steps can occur separately or simultaneously.

During the cross-linking and blowing steps, control of process parameters is decisive in order to prevent the expanded material from collapsing prematurely with respect to the complete cooling of the foam.

A polymer is termed "cross-linked" if there are at least two different paths for connecting any two points of its molecule. Otherwise it is termed "linear" or "branched", depending on whether side chains are attached or not to the main chain.

The most widely used polymer cross-linking methods are the chemical one and the physical one. The so-called chemical cross-linking process begins by virtue of the addition of a molecule known as initiator. A typical example of an initiator is dicumyl peroxide. The particularity of initiators is that their breakup gives rise to two fragments of the original molecule, known as initiator fragments, each of which has a pair of unpaired electrons. Since the radicals are unstable, they find a way to pair without generating a new radical, generating a three-dimensional structure which is interesting since one wishes to provide polymers with stability against the deformation caused by temperature.

In physical cross-linking, the physical properties of high-energy beams are instead used to form the first radicals, which by propagating then give rise to the cross-linking process.

The main difference between chemical and physical cross-linking methods is the moment and the temperature at which the cross-linking occurs. Physical cross-linking has an immediate effect, since the breaking of the double bonds with high-energy beams and the consequent "rebalancing joining" are produced immediately and at a relatively low temperature. Chemical cross-linking occurs instead by means of the heat which breaks down the cross-linking agent. The triggering and the kinetics of chemical cross-linking are tied essentially to the following parameters: percentage of initiator, reaction time and process temperature.

Regardless of the cross-linking method, the expansion step is usually performed by taking advantage of the thermal breakdown of a blowing agent and the subsequent formation of gases which fill the microcavities generated during cross-linking. With specific reference to the production of cross-linked expanded polyethylene in continuous rolls, it is known in the field that the process that uses chemical cross-linking ensures distinctly superior mechanical and thermal characteristics of the product with respect to physical cross-linking. The differences are due to the use of the peroxide. The best thermal and mechanical characteristics are ensured by its use, since such molecule is capable of allowing a firmer cohesion between the molecular chains of the ethylene polymer, thus ensuring higher resistance to mechanical stresses such as traction or compression and use at higher temperatures. However, physical cross-linking ensures a product which is characterized by the absence of yellowish coloring of the finished product, which is instead typical of chemical cross-linking and often interferes with additional colorings to be given to the polyethylene. It should further be noted that the physical cross-linking product has a better cell uniformity and size. This aspect is due essentially to the possibility to separate in time the cross- linking and the expansion. Peroxide initiators used in chemical cross-linking in fact react within the same reaction temperature range that is typical of blowing agents and therefore it is impossible to divide the cross-linking and expansion steps. This impossibility leads to a less than ideal cross-linking kinetics and to a worse cell size distribution. In other words, the overlap of the breakdown temperature range of the peroxide and of the blowing agent causes the molecules of blowing agent to begin their breakdown, and therefore the release of gas - an element which is indispensable for expansion - before the cross-linking agent has completed its breakdown (therefore upon creation of the three-dimensional molecular structure), and is capable of ensuring the optimum cell structure of the foam.

Conventionally, these drawbacks are overcome by means of the physical cross-linking, in which, by using the bombardment of the polymer with high-energy beams when cold, the cross-linking reaction is completed before the breakdown reaction of the blowing agent begins. However, this solution is is not devoid of disadvantages, since as mentioned above physical cross-linking causes an overall decrease of the mechanical and thermal characteristics of the product, is distinctly more complex to provide than chemical cross-linking, and accordingly has a very high industrial cost. Primarily, the high cost is due to the need to use a reactor which is capable of generating the high-energy beams required for bombardment of the product.

Likewise, it would be technically unthinkable to cross-link chemically to a mixture of just polyethylene and radical initiator and then add one or more expanding agents after cross-linking. The addition of a peroxide in fact transforms the polyethylene mixture from a thermoplastic material to a thermosetting material.

Therefore, it would be desirable to have an expanded cross-linked polyethylene which combines the currently acknowledged advantages of chemically and physically cross-linked products, but can be obtained by way of a method which can be controlled precisely in its steps, is characterized by high productivity per work unit, has a low technological difficulty, is economically convenient and has broad industrial margins.

Disclosure of the Invention The aim of the present invention is therefore to provide an expandable cross-linked polyethylene which overcomes the drawbacks of the background art.

Within this aim, an object of the present invention is to provide a cross-linked polyethylene which can be expanded easily for example by thermal methods and which, once expanded, is technically valid and suitable for the most disparate uses.

A further object is to provide an expandable cross-linked polyethylene which, once expanded, has excellent properties of mechanical strength and thermal resistance, lacks any coloring and has an optimum size distribution of the cavities contained therein. Another object of the invention is to provide a method for preparing an expandable cross-linked polyethylene as defined above, in particular a continuous method for producing an expandable cross-linked polyethylene in rolls. Another object is to provide a method as defined above which allows fine control over the individual steps, in particular of the cross-linking step and the expansion step, if provided, characterized also by high productivity per work unit, low technological difficulty, economically convenient and with broad industrial margins. Another object is to provide an apparatus for performing a method as defined above.

This aim and these and other objects are achieved by a method for obtaining an expandable cross-linked polyethylene, said method being characterized in that it comprises the steps of: a) cross-linking a mixture which comprises polyethylene, at least one expanding agent and at least one radical initiator, by subjecting said mixture to a given temperature T_a and to a given pressure P₃, where:

- the temperature T₈ is equal to, or higher than, the breakdown temperature to of said radical initiator, and - the pressure P₃ is higher than the pressure p₀ generated by the gaseous products formed by the breakdown of said at least one expanding agent and optionally by said at least one radical initiator, at said given temperature T₃; and after step a), b) cooling the cross-linked mixture obtained in step a) at least to a temperature T_b, which is lower than the breakdown temperature U of said at least one expanding agent, wherein step b) occurs while maintaining the pressure P₃ of step a).

The aim and objects of the invention are also achieved by an expandable cross-linked polyethylene which can be obtained with a method as defined above.

The aim and objects of the invention are also achieved by an apparatus particularly for performing a method as described above, said apparatus being characterized in that it comprises: A) means for heating and compressing a mixture which comprises polyethylene, at least one expanding agent and at least one radical initiator; and

B) means for cooling and compressing the cross-linked mixture in output from the heating and compressing means. It is understood that any characteristic which mentioned with reference to only one of the aspects of the invention but can be referred also to other aspects is to be considered equally valid as regards said other aspects even though it is not repeated explicitly.

Brief description of the drawings Further characteristics and advantages of the invention will become better apparent from the description of the following Figures 1 to 6, given by way of non-limiting example, wherein:

Figure 1 a illustrates a block diagram, which describes a conventional method for producing chemically cross-linked expanded polyethylene, wherein the two steps of expansion and cross-linking are inevitably simultaneous.

Figure Ib illustrates a block diagram, which describes a method for producing chemically cross-linked and expanded polyethylene according to the invention, in which the time separation of the two steps for cross-linking under pressure and expansion is evident;

Figure 2 is a view of a conveyor which is adapted to provide steps a) and b) of the method, in which therefore one can appreciate both the application of an external pressure in both steps (vertical arrows), and the separation between the cross-linking step and the cooling step. In Figure 2, an extruder is also shown upstream of the conveyor. A sheet of matrix (a term which can be used interchangeably with "mixture") of polyethylene mixed with expanding agent and radical initiator is sandwiched and entrained within the conveyor by an advancement system. The sheet of cross-linked matrix is then released at the end of the cooling step and directed toward the subsequent expansion (not shown);

Figure 3 contains a table which relates the variations of the cross- linking reaction times as the temperature varies. The table highlights the times measured when the reaction has reached a torque value of 50% (T 50) and 90% (T 90). The torque value is defined as the variation of the shearing stress (torque, indeed) as a function of time at constant temperature which a double-cone rotor applies to a sample of polyethylene during cross-linking;

Figure 4 is a chart of the relationship between time and pressure applied at 230⁰C by 0.4 g of a 12% mixture by weight of ADC, 0.6% by weight of dicumyl peroxide, 87.4% melted LDPE. The chart highlights that expansion occurs within approximately 40 seconds of the triggering of ADC breakdown and that the maximum pressure applied by the system settles at around 350 kPa;

Figure 5 contains two charts which plot details of the chart of Figure 4, wherein the first chart of Figure 5 magnifies the curve for a time comprised between 0 and 2 minutes while the second chart magnifies the curve for a time comprised between 0 and 1 minute; and

Figure 6 contains a chart which plots the relation between temperature and pressure applied by the products of azodicarbonamide (ADC). The chart highlights in particular that the breakdown temperature of ADC is around 200 ⁰C.

Ways of carrying out the Invention

The expression "expandable cross-linked polyethylene" is used to reference a polyethylene polymer in which there are at least two different paths for linking any two points of its molecule. The degree of polymerization may be any. The term "expandable" refers to the fact that the cross-linked polymer that is obtained already contains the expanding agent internally, uniformly mixed therewith, so that it is possible to achieve uniform and complete expansion of the polyethylene by simple heating above the breakdown temperature of the specific expanding agent used. The polyethylene according to the invention is thus cross-linked chemically, with the advantages that arise from this choice, but at the same time can expand at a moment which is temporally separate from cross-linking, thus also benefiting from the advantages cited above of polyethylenes which are physically cross-linked and then expanded. The term "expanding agent" designates a substance which is capable of breaking down thermally, generating one or more gaseous products. Expanding agents can be selected among azodicarbonamide, oxybis(benzenesulfonyl hydrazides), toluenesulfonyl hydrazides, toluenesulfonyl acetone hydrazone. Preferably, the expanding agent is azodicarbonamide (ADC).

The expanding agent is typically used in the form of a mixture with one or more optional synergistic agents, which are well-known to any person skilled in the field of expanded polymers.

The expanding agent breakdown temperatures are known and are reported by the manufacturers of such agents.

The pressure generated by the gaseous products of the breakdown of the expanding agent are a function of the temperature of the system constituted by the mixture being cross-linked. It should be noted that the breakdown of the expanding agent must not necessarily lead only to gaseous products, but can also lead to the formation of solid products, which however are irrelevant for calculating P₃.

The expression "radical initiator" designates a substance which is used conventionally in the field of polymers to achieve polymerization of a quantity of monomers by forming radical derivatives through thermal breakdown of the initiator itself. Radical initiators can be selected among 2 ,5 -dimethy 1-2 , 5 -di(tert-buty lperoxy)-3 -hexene ; 2 , 5 -dimethy 1-2, 5 -di(tert- butylperoxy)-hexane; di-tert-butyl peroxide; tert-butyl cumyl peroxide, di(tert-butylperoxy isopropyl)benzene; dicumyl peroxide; butyl-4,4-di(tert- butylperoxy); 1 , l-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane; tert- butylperoxy benzoate; di(4-methylbenzoyl)peroxide; dibenzoyl peroxide; di(2,4-dichlorobenzoyl)peroxide. Preferably, the radical initiator is dicumyl peroxide or 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

The invention considers the (less preferred) case in which the radical initiator also can break down to a minimal extent during cross-linking, generating a slight pressure which is added to the pressure potentially generated by the expanding agent. For this reason, the term "higher" referred to P_a in the third item of step a) of the method means that the pressure P₃ to be applied in step a) of the method is at least equal to the resultant of the two pressures and is preferably slightly higher than the resultant of the two pressures. As will be seen hereinafter, by acting with the overpressure P₃ preferably before the actual expansion is triggered, it is possible to achieve the selected result with relatively low pressures, well below those theoretically necessary to contrast the immediate expansion of all, or most, of the expanding agent that is present, when the expansion reaction is triggered.

The term "to compress", in the context of the apparatus, is used to reference the pressurization of the environment in which cross-linking occurs and subsequent cooling.

In a first aspect, the invention relates to a method for obtaining a new expandable cross-linked polyethylene.

In a preferred embodiment of the invention, the radical initiator comprises a peroxide, particularly dicumyl peroxide, and the temperature T₃ ranges from 190 to 260 ⁰C, preferably from 210 to 250 ⁰C.

In a preferred embodiment of the method, the expanding agent is ADC, optionally in a mixture with conventional synergistic agents, the pressure P₃ ranges from 0.5 to 5 bars and the temperature T_b ranges from 100° to 140 ⁰C.

It should be noted that the breakdown reaction is not immediate but starts after approximately 2-3 minutes both when using azodicarbonamide (the expanding agent most commonly used for this application) and when using other types of expanding agents. In this period of time, the pressure to be applied in order to prevent the expanding agent from reacting is much lower than the pressure that would have to be applied if one waited for the reaction to begin. Therefore, in step a) of the method according to the invention, the expression "cross-linking a mixture which comprises polyethylene, at least one expanding agent and at least one radical initiator, by subjecting said mixture to a given temperature T_a and to a given pressure P₃" means:

- bringing the mixture to the pressure P₈ before reaching the temperature T₃, or

- bringing the mixture to the temperature T₃ and simultaneously to the pressure P₃, or

- bringing the mixture to the temperature T₃ and, at the most within approximately 3 minutes, also to the pressure P₃, preferably within approximately 2 minutes, more preferably within approximately 1 minute, where a shorter period of time corresponds to an advantage in terms of pressure P₃ to be applied. Obtaining an efficient process at lower pressures in fact entails lower costs.

In a preferred embodiment, the radical initiator is dicumyl peroxide, the expanding agent is ADC optionally mixed with conventional synergistic agents, the temperature T₈ ranges from 190 to 260 ⁰C, preferably from 210 to 250 ⁰C, the pressure P₈ ranges from 0.5 to 5 bars, and the temperature T_b ranges from 100 to 140 ⁰C.

In the embodiment of the method in which the temperature T₃ ranges from 190 to 260 ⁰C, step a) can have a duration ranging from 10 to 60 seconds, where the higher the temperature, the shorter the duration in seconds.

In a highly preferred embodiment, the method according to the invention also comprises an additional step for expanding the cross-linked polyethylene. In this manner, the method is used to produce a new expanded cross-linked polyethylene. In this embodiment, after step b) as described above, the method therefore comprises a step of: c) expanding the cross-linked and cooled mixture obtained during step b), by subjecting said mixture to a given temperature T_c which is equal to, or higher than, the breakdown temperature t! of said expanding agent.

The breakdown temperatures ti of the expanding agents are known and reported at purchase by the manufacturers.

In a further embodiment, the method according to the invention comprises the additional steps of: al) mixing polyethylene, at least one expanding agent and at least one radical initiator, and a2) extruding the mixture obtained in step al).

Steps al) and a2) are to be performed in the order in which they are presented here and before step a). In other words, in this embodiment the mixture to be cross-linked in step a) is the one obtained and extruded in steps al) and a2).

In another aspect, the invention relates to an expandable cross-linked polyethylene which can be obtained with a method which comprises the steps a) and b) as described above. The expandable cross-linked polyethylene according to the invention is new, since it has the best mechanical and thermal characteristics of a chemically cross-linked product, ensured by firmer cohesion between the molecular chains of the ethylene polymer. At the same time, however, since it can work at high temperatures without the risk of also triggering expansion, it is possible to make the radical initiator react completely, removing even those quantities of unreacted or poorly reacted initiator which caused aesthetic problems in terms of coloring of classic cross-linked polyethylene. Moreover, like a physically cross-linked polyethylene, it has a uniform dispersion of unreacted expanding agent inside it. Accordingly, it can be subjected to an expansion step the result whereof is an expanded cross-linked polyethylene with a performance, in terms of mechanical strength and thermal resistance, which is similar to those of chemically cross-linked polyethylenes, but with aesthetic, cell size and cell uniformity properties comparable to those of physically cross-linked polyethylenes. In another aspect, the present invention relates to an expanded cross- linked polyethylene which can be obtained with a method which comprises steps a), b) and c) as described above. The expanded cross-linked polyethylene according to the invention is new with respect to conventional similar products, since its thermal resistance and mechanical strength performance are similar to chemically cross-linked polyethylenes while having aesthetic, absolute cell size and cell size uniformity properties which can be compared to those of physically cross-linked polyethylenes.

In another aspect, the invention relates to an apparatus for performing a method as defined above in all of its embodiments. In a particularly preferred embodiment, the heating and compression means A) and the cooling and compressing means B) can be constituted by a single element which is capable of performing all the technical functions. Therefore, the expression "in output from the means A)" does not necessarily mean that the means B) are physically separate from the means A), but only that the means B) must act after the means A). For example, said single element is a conveyor such as the one that will be described in detail below by way of example. However, it is understood that in the light of the information provided here, the average person skilled in the art will know how to replace the conveyor with other technically equivalent known elements or, as an alternative, how to selected individual elements to provide the pressurized cross-linking step and pressurized cooling steps so as to maintain the advantages of the present invention.

There are different known technical solutions which are capable of complying with the notion of conveyor as understood in the present invention. In any case, both in the embodiment in which the means A) and the means B) are both comprised within the context of the same apparatus and in the embodiment in which they are comprised within the context of separate apparatuses, they must comply with the aspects of:

- heating, - cooling, and

- compression.

Moreover, it is also preferred that upstream of the means A), the apparatus according to the invention also comprises means Al) which are adapted to obtain a mixture of polyethylene, expanding agent and radical initiator.

Moreover, it is also preferred that the apparatus according to the invention also comprises, upstream of the means A) but downstream of the means Al) if provided, means A2) for extruding the mixture in output from the means Al). For example, the means A2) can be a conventional extruder or a calender.

Moreover, it is also preferred that the apparatus according to the invention also comprises, upstream of the means A) but downstream of the means Al) and A2) if provided, means A3) which are adapted to convey the mixture of polyethylene, expanding agent and radical initiator, optionally extruded, from the mixer/extruder to the conveyor.

Moreover, it is also preferred that downstream of the means B), the apparatus also comprises means C) for heating the cross-linked and cooled mixture of polyethylene and expanding agent in output from the means B), so as to cause the expansion of the expanding agent. In a preferred embodiment, the means C) are an oven which is conventionally used for this purpose.

As regards the means A), it can be seen that the operation for heating the matrix can be performed effectively with different currently existing constructive solutions. The sector of temperature regulation in fact offers different solutions for the transfer of heat by conduction, such as the use of resistors or heating with diathermic oil. Diathermic oil ensures a more uniform heat distribution within the heating elements, but this advantage does not rule out the use of electrical resistors, which are easier to use.

As regards the means B), their choice should be made advantageously by considering the heat to be removed from the matrix depending on the type of production and on the production capacity of the plant. Cooling can be ensured by means of air, refrigerated water or liquid nitrogen. Refrigerated water, for the amount of heat to be removed and the ease of use, is the best solution, although the use of liquid nitrogen would allow to reduce the size of the cooling area.

As regards both the means A) and the means B), it has been seen that in substantially all of the embodiments, the pressure to be applied is relatively low, and the commercially available devices, including pneumatic ones, can ensure effectively that the required parameters are reached. One preferred solution is constituted by conventional compression means, which ensure pressure uniformity throughout the duration of the process for pressurized cross-linking. For example, preference is given to a series of pneumatic pistons so as to regulate the pressure constantly and uniformly.

As regards the means A3) suitable to convey the matrix to the means A) and B), there are already diffrent solutions which can be used to convey the matrix through heat exchangers, systems with metal belts, Teflon belts, and others. One should bear in mind that is it is very advantageous not to mark the surface of the material, since any defect would be amplified in an optional and subsequent expansion step. For this reason, the use of Teflon belts is preferred because it allows to convey the material through the exchangers without marking the surface of the matrix.

The invention is now described in the particularly preferred embodiment of a continuous method for producing expanded cross-linked polyethylene in rolls, wherein the cross-linking agent is dicumyl peroxide and expanding agent is ADC.

The process for continuous chemical cross-linking under pressure has been performed with the aid of a machine, known as conveyor, for which indications related to its structure will be provided hereafter.

In output from an extruder, a dense sheet of the mixture of polyethylene and suitable additives, hereinafter referenced as a matrix, is introduced in a temperature-controlled conveyor.

The temperature regulation of the conveyor has two distinct and opposite steps. In a first step, the matrix must be brought to a temperature ranging from 190 to 260 ⁰C, depending on the formulation of the product and on its thickness.

The conveyor must ensure a constant pressure throughout the cross- linking step and the subsequent cooling step; said pressure ranges from 0.5 to 5 bars. The conveyor must be sized so as to ensure a holding time at the set temperature of from 10 to 60 seconds in order to take into account the different formulations, the thickness and the initial temperature of the matrix. Sizing of the conveyor must be calculated by taking into consideration the linear speed of the matrix within the conveyor. This calculation can be provided advantageously by using the following formula:

Lug = Vc x time, where Lug designates the length of the conveyor of the cross-linking step (step a) of the method, Vc designates the speed of the belt of the conveyor, and "time" is the holding time of the matrix in the cross-linking area.

The cross-linking step is followed by a second step (step b)) for cooling. This step occurs advantageously again within the conveyor, which must maintain an operating pressure which is sufficient to avoid the expansion of the expanding agent. Advantageously, this is the same pressure set for cross-linking, since a pressure drop during cooling would lead to premature expansion of the product. During this step, the temperature- controlled conveyor must dissipate the heat of the matrix to bring it to a temperature which is lower than the breakdown temperature of the expanding agent. To avoid even partial breakdown, it is preferred to keep the matrix temperature from 100 to 140 ⁰C, depending on the formulation of the product. This section of the conveyor also can be advantageously sized so as to ensure the best holding time for bringing the matrix from the cross- linking temperature to the process end temperature.

It is possible but optional to use a calender to even out the thickness of the matrix, where the calender is arranged before or preferably after the cross-linking process continuously under pressure. The main goal of the process of cross-linking under pressure is to make the peroxide react, inhibiting the reaction of the expanding agent. This result is achieved by optimizing the pressure and temperature parameters.

As regards the temperature, the literature provided by peroxide manufacturers indicate a breakdown temperature of 60 ⁰C and a process temperature of 130 ⁰C, while the cross-linking temperature is indicated as approximately 170 ⁰C. The term "process temperature" references the maximum temperature at which it is possible to process the peroxide without undergoing a chain breakdown reaction. However, in addition to the values indicated by the literature, it is advisable to also consider the increase in the reaction speed in relation to the temperature increase. Measurements made on physical cross-linked products show that the cross-linking rate generally does not exceed a value from 38 to 40%, while chemical cross- linking products normally achieve a cross-linking rate from 65 to 68%. This rate is maintained by the present invention. Therefore, it is possible to modulate the holding time under pressure (until it is reduced to 10 seconds) and the temperature (bringing it even below 210 ⁰C) if one wishes to limit the cross-linking rate. Moreover, beyond 240 ⁰C the time used to achieve a 50% torque variation is entirely comparable to the time required to achieve a 90% torque variation. This means that at this temperature the reaction is immediate once it is triggered. On the contrary, at a temperature of 230 ⁰C the same sample took 30 seconds to complete the cross-linking reaction.

To conclude, the optimum process temperature was found to range from 210 ⁰C to 250 ⁰C, with a corresponding time ranging from 10 to 60 seconds. These values were found to be ideal for each embodiment of the method if one uses the pair constituted by dicumyl peroxide ADC as an initiator and expanding agent respectively. However, it is possible to work with higher or lower temperatures by using the appropriate adjustments to the reaction times.

The pressure applied by the conveyor in the cross-linking process, and in the subsequent cooling step, allows to bring the matrix to a high temperature. This allows to start and complete the peroxide breakdown reaction, forcing the molecules into a compact three-dimensional structure for cross-linking and inhibiting expansion. The possibility to complete the cross-linking reaction and to make the peroxide react entirely without thereby being concerned about the beginning of expansion, allows to avoid the coloring phenomena of polyethylene cross-linked chemically with conventional criteria.

The reaction of the blowing agent in practice generates a gas. The result of the reaction is therefore an increase in the volume of the system. By applying Le Chatelier's principle of chemical kinetics, it is possible to deduce that increasing the total pressure of the system limits the effect of the blowing agent.

The minimum theoretical pressure required by the process for cross- linking under pressure must contrast the increase in pressure of the matrix brought to the peroxide reaction temperature. Experimental measurements of the pressure variation produced by the matrix at a temperature of 230 ⁰C, as a function of time, indicate that the matrix expansion reaction generates, at its peak, a pressure of approximately

350 kPa and that expansion occurs in approximately 40 seconds and begins approximately 2 minutes after the beginning of the reaction.

These data are confirmed by the indications of suppliers, which indicate a breakdown of the expanding agent at a temperature which is slightly higher than 200 ⁰C and a time of slightly less than three minutes.

Focusing the analysis of the pressure increase of the matrix on the reaction times defied for cross-linking of the peroxide (from 10 to 60 seconds), it has been noted that the pressure generated by the matrix is not very high. The matrix generates a pressure of 40 kPa after two minutes at

230 ⁰C, while said pressure drops to 15 kPa if one does not exceed 60 seconds (see for example the chart of Figure 6). Experimental measurements of the pressure variation applied by the matrix at a temperature of 250 ⁰C do not show significant variations of the pressures in the indicated times with respect to measurements performed at 230 ⁰C.

To conclude, it has been possible to increase greatly, for a brief period of time, the temperature during the cross-linking step and to inhibit the expanding agent breakdown reaction, subjecting the matrix to a light pressure ranging in particular from 10 to 500 kPa.

After the cross-linking step, it is necessary to return the matrix to its original temperature (preferably to the temperature at the output of the extruder, which generally ranges from 100 to 140 ⁰C). This is step b) of the method.

The process requires the pressure to remain unchanged throughout the cooling step, during which the heat absorbed by the matrix in the preceding step is removed. Once the required temperature has been reached, the matrix is returned to atmospheric pressure for the subsequent step of expansion in the oven (step c) of the method according to the invention. It has also been noted that lowering the temperature of the matrix to approximately 100-140 ⁰C, in addition to bringing the blowing agent embedded in the matrix to a temperature which is lower than its reaction temperature and preventing its breakdown, allows to detach the product more easily from the conveyor.

In summary, the present invention allows the extruded mixture to cross-link, preventing its expansion by using an external pressure. Subsequent cooling of the cross-linked mass then allows the return to a temperature which is lower than the temperature required for expansion. The method thus leads to the production of a foam of expanded polyethylene which is cross-linked by means of the chemical method, adding a step of controlled cross-linking under pressure. This step, advantageously inserted just after the extrusion process, consists in heating the matrix under pressure before it is introduced in an oven where the subsequent expansion step occurs. In this manner, cross-linking of the product is ensured, bringing the material to the peroxide breakdown temperatures, controlling the expansion reaction thereof by using pressure.

Currently, in the process for producing a chemically cross-linked expanded polyethylene foam it is not possible to separate completely the reaction for breakdown of the cross-linking agent from the reaction for breakdown of the expanding agent. The present method instead allows to proceed with the reaction for breakdown of the cross-linking agent under pressure in order to inhibit the reaction of the expanding agent, separating distinctly (in time and, if one wishes, also in terms of equipment) the cross- linking process from the expansion process. The present method instead allows to combine the advantages of the use of peroxide with the better aesthetic and cell characteristic of the physical process, all without having to resort to the use of a reactor capable of generating high-energy beams.

Although only some preferred embodiments of the invention have been described in the text, the person skilled in the art will readily understand that it is in any case possible to obtain other equally advantageous and preferred embodiments.

The disclosures in Italian Patent Application No. MI2006A001418 from which this application claims priority are incorporated herein by reference.

Claims

1. A method for obtaining an expandable cross-linked polyethylene, said method being characterized in that it comprises the steps of: a) cross-linking a mixture which comprises polyethylene, at least one expanding agent and at least one radical initiator, by subjecting said mixture to a given temperature T₈ and to a given pressure P₈, where:

- the temperature T_a is equal to, or higher than, the breakdown temperature t₀ of said radical initiator, and

- the pressure P₃ is higher than the pressure p₀ generated by the gaseous products formed by the breakdown of said at least one expanding agent and optionally by said at least one radical initiator, at said given temperature T_a; and after step a), b) cooling the cross-linked mixture obtained in step a) at least to a temperature T_b, which is lower than the breakdown temperature ti of said at least one expanding agent, wherein step b) occurs while maintaining the pressure P_a of step a).

2. The method according to claim 1, further comprising, after step b), a step of: c) expanding the cross-linked and cooled mixture obtained in step b), subjecting said mixture to a given temperature T_c which is equal to, or higher than, the breakdown temperature t! of said expanding agent.

3. The method according to claims 1 or 2, further comprising the steps of: al) mixing polyethylene, at least one expanding agent and at least one radical initiator and a2) extruding the mixture obtained in step al), wherein said step al) and, if provided, a2) are to be provided in the order in which they are presented here and before step a).

4. The method according to one or more of the preceding claims, wherein the expanding agent is selected from the group that consists of azodicarbonamide, oxybis(benzenesulfonyl hydrazides), toluenesulfonyl hydrazides, toluenesulfonyl acetone hydrazone, preferably azodicarbonamide.

5. The method according to claim 4, wherein the expanding agent is in the form of a mixture with one or more synergistic ingredients of the expanded polymer sector.

6. The method according to one or more of the preceding claims, wherein the radical initiator is selected from the group that consists of:

- 2,5 -dimethy 1-2 , 5 -di(tert-buty lperoxy)-3 -hexene ; - 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane;

- di-tert-butyl peroxide;

- tert-butyl cumyl peroxide; - di(tert-butylperoxy isopropyl)benzene;

- dicumyl peroxide; butyl-4,4-di(tert-butylperoxy); - 1,1 -di(tert-butylperoxy)-3 ,3 ,5-trimethylcyclohexane;

- tert-butylperoxy benzoate;

- di(4-methylbenzoyl)peroxide; - dibenzoyl peroxide;

- di(2,4-dichlorobenzoyl)peroxide, preferably dicumyl peroxide or 2,5-dimethyl-2,5-di(tert- buty lperoxy )hexane .

7. The method according to one or more of the preceding claims, wherein the radical initiator is dicumyl peroxide, the expanding agent is

ADC optionally mixed with synergistic agents of the expanded polymer sector, the temperature T₃ ranges from 190 to 260 ⁰C, preferably from 210 to 250 ⁰C, the pressure P_a ranges from 0.5 to 5 bars, and the temperature Tb ranges from 100 to 140 ⁰C.

8. The method according to one or more of the preceding claims 2 to 7 for continuously producing expanded cross-linked polyethylene in rolls.

9. The expandable cross-linked polyethylene obtainable with a method as defined according to claim 1.

10. The expanded cross-linked polyethylene obtainable with a method as defined according to claim 2.

1 1. An apparatus particularly for performing a method as defined according to one or more of claims 1 to 8, said apparatus being characterized in that it comprises: A) means for heating and compressing a mixture which comprises polyethylene, at least one expanding agent and at least one radical initiator; and

B) means for cooling and compressing the cross-linked mixture in output from the means for heating and compression.

12. The apparatus according to claim 1 1 , further comprising, upstream of the heating and compression means, means adapted to obtain a mixture of polyethylene, expanding agent and radical initiator.

13. The apparatus according to claim 1 1 , further comprising, upstream of the heating and compression means, means for extruding the mixture in output from the means adapted to obtain a polyethylene mixture.

14. The apparatus according to claims 12 or 13, further comprising, upstream of the heating and compression means but downstream of the means adapted to obtain a mixture of polyethylene and means adapted to extrude the mixture, if provided, - means adapted to convey the mixture of polyethylene, expanding agent and radical initiator, optionally extruded, from the means for obtaining a mixture of polyethylene and means adapted to extrude the mixture to the heating and compression means.

15. The apparatus according to one or more of claims 1 1 to 14, further comprising, downstream of the means for cooling and compression, - means for heating the cross-linked and cooled mixture in output from the means for cooling and compression.