WO2002020657A1

WO2002020657A1 - Compounds for manufacturing cross-linked products with great softness and high thermal dimensional stability

Info

Publication number: WO2002020657A1
Application number: PCT/EP2000/008667
Authority: WO
Inventors: Sergio Brunetti
Original assignee: Api Applicazioni Plastiche Industriali Spa
Priority date: 2000-09-05
Filing date: 2000-09-05
Publication date: 2002-03-14
Also published as: AU2000275149A1

Abstract

The invention realizes a compound for manufacturing cross-linked products with great softness and thermal stability made by injection moulding, consisting of a mixture comprising: master polymers such as ethylene vinyl acetates (EVA), or polyethylene (PE); cross-linking agents, such as peroxides or similar. The compound presents in addition styrene-ethylene-propilene-styrene (SEPS) or styrene-ethylene-butylene-stryrene (SEBS) block polymers.

Description

COMPOUNDS FOR MANUFACTURING CROSS-LINKED PRODUCTS WITH GREAT SOFTNESS AND HIGH THERMAL DIMENSIONAL STABILITY The invention concerns a compound for manufacturing foamed cross-linked products especially suited for producing very soft items with thermal dimensional stability.

Cross-linking polyethylenes (PE) and ethylene vinyl acetates (EVA) through the use of peroxides is a known technology for manufacturing injection moulded products such as, for example engineering articles, footwear, wheels, etc. The use of cross-linked products allows to obtain, against thermoplastic materials, polymers with greater physical and mechanical properties together with high heat resistance.

Cross-linked products can be basically divided into two types, compact and foamed, which can reach very low minimum densities of around 100 Kg/m3. It is mainly in the foamed version that these products demonstrate the positive contribution of cross-linking in their mechanical properties and thermal resistance.

It is these properties that have made cross-linked, foamed EVA products so successful in the manufacture of soft products, while cross-linked PE products have proven their success in the manufacture of more rigid products.

The manufacturing process of any of the products can basically be divided into two stages.

In STAGE ONE the mixture or so-called compound is prepared with the components required to obtain, in the final transformation process, a product that complies with the desired mechanical properties.

The compositions of known compounds generally comprise:

- Master polymers such as EVA and PE mixtures, or Ethylene-propylene rubbers (EPR), otherwise low and high density polyethylenes, having various Melt Flow Index (MFI) levels, or ethylene vinyl acetates with different MFI's and different vinyl acetate contents;

- Peroxides, which are cross-linking agents that initiate activity at a given temperature, such as dicumyl peroxide, Bis (tert-butylperoxypropyl) benzene, 2.5 - Bis (tert-butylperoxy)- 2.5- dimethylhexane;

- Coagents. which block the secondary reactions of peroxides optimising cross-linking, such as triallylcyanurates, triallylisocyanurates, ethylene glycol dimetacrilate and trimethylolpropane trimetacrilate;

- Foaming a^gents such as various forms of azodicarbonammides, which generate gas through their thermal destruction such as the types known industrially as Porofor® and Genitron®; - Lubricant agents, which assist ejection of products from the mould and limit friction in the injection machinery, such as stearin and stearates;

- Anti-degradation agents, which protect the polymer from degrading during the manufacturing process, such as 6 - tert -butyl-m-cresol;

- Kickers, which control the degradation temperature of the foaming agents, such as metal oxides for example Zinc or glycols;

- Various forms of filler, such as calcium carbonates or silicas, are implemented both as nucleating agents to favour uniform dispersion of the components and gases, and to reduce costs in the mixture;

- Pigments, which are used to die the mixture; - Oils that plastify the compound and/or reduce volatile dust in the mixture being processed.

One known method for manufacturing the compound prescribes that, after these ingredients have been adequately batched, they are mixed in special mixers until a uniform mixture is obtained, which is then fed into extruders that melt and blend the components in order to obtain a mixture that has all the additives finely dispersed in the master polymer/s.

Special care is taken with the extruders, which have to be kept at a temperature that does not initiate peroxide activity and does not decompose the foaming agents. So at this stage no reaction is initiated, which makes this cycle exactly the same as the manufacturing and transformation process for thermoplastic composites.

The resultant final product is to all effects a thermoplastic ready to cross-link and foam if taken to the right temperature. Another known method of manufacturing differs from the former in that the preliminary stage does not prescribe the preparation of a uniform blend. This method uses an extruder equipped with weight batchers that first supply the extruder with polymer that is melted and then the various additives, which are amalgamated in the mass of molten polymer thereby creating the uniform compound described previously. Another method of manufacturing the compound, normally used in rubber manufacturing, prescribes the hot preparation of a preliminary mixture in Banbury internal mixers, this mixture is feed onto roll mills in order to obtain a laminate of specific thickness that, when cut in a pelletiser, produces a cube- shaped granule.

The end result of all three of the above methods is that a compound in granule form is obtained, ready for transforming by the end user in extrusion or injection moulding processes. STAGE TWO involves the transformation of the product from a thermoplastic into a final thermoset polymer.

To do this the compound is fed in an injection press for plastics comprising a punching screw injector consisting of a cylinder and a screw suited to feeding the granules into the cylinder which, when heated to the required temperature, melts or plastifies the compound. By appropriately controlling the cylinder's temperature so that it does not reach initiation temperature, that is normally around 110 - 130°C, of both the cross- linking agent and the foaming agent, the cycle can be performed within the material's thermoplastic range. The now molten compound can then be injected into the mould which, if kept at a low temperature (i.e. 20 - 60°C), does not initiate the cross-linking and foaming processes while, if heated to a high temperature of around 150 - 190°C for sufficient time, it heats the polymer initiating peroxide activity, thereby starting up the cross-linking process in the polymer and transformation of the foaming agents into gas. Even though the gas develops a high pressure, it is confined within the mould by the clamping force of the press, thereby remaining finely dispersed throughout the polymer.

The cross-linking process then progressively proceeds according to the established rheological graph in the following stages: - decomposition of the peroxide with formation of free radicals;

- extraction of hydrogen atoms by the free radicals from the polymer chains that gives rise to products of decomposition of the stable peroxides and a polymer with free radicals;

- combination of the radicals linked to the polymer and formation of cross- linking. As is known, both quantity and type of peroxide added will control the degree of cross-linking.

In particular, if the stoichiometric quantity of peroxide added to the mixture is sufficient total cross-linking of the polymer can be achieved otherwise cross- linking levels will be less than 100%.

By selecting the right temperature and polymer curing time in the mould, the cross-linking reaction is completed only when all the peroxide is consumed thereby preventing a subsequent, further increase in viscosity, so that partial or total cross-linking is achieved. On conclusion of the cross-linking process the mould is opened allowing the gases to suddenly expand the product, making it eject automatically from the mould with a kind of small explosion.

The quantity of foaming agent in the compound, and consequently the gas volume generated, together with the resilience and the resistance to the gas expansion pressure offered by the polymer determine the amount of volume increase in the product.

To be more precise the volume of the final cross-linked product is directly proportional to the gas volume generated, the physical and mechanical properties of the cross-linked polymer and the level of cross-linking reached by the polymer.

The foamed product is characterised by an expanded core and a very smooth, compact outer surface due to the greater level of cross-linking and therefore resilience that the part of surface polymer in contact with the mould reaches first. The product appears with a compact surface, brilliant appearance and coloured depending on both the types and quantity of pigments in the compound.

Its shape traces the mould's form even though it is proportionally larger in size, depending on the foaming agents in the compound less shrinkage due to thermal contraction created by the difference in temperature between mould and environment.

A fundamental characteristic of the cross-linked products described above is their softness which, together with their specific weight, enable extremely lightweight materials to be obtained, capable of substituting more expensive products such as cork, rubber, etc. Known compounds have a formulation given by weight for the mixture described in the table below:

The mixture blended in a Ribbon blender and extruded with a twin-screw extruder at a temperature of 100 - 110 °C, produces cylindrical granules approx. 3 mm in diameter and 2.5 mm in length which, moulded in a punching screw injection press fitted with the mould of a plate approx. 6 mm thick, 90 mm in width and 120 mm in length, heated to 180 °C with a cylinder temperature of 95°C and a cross-link curing time of 4 minutes, produce a parallelepiped cross-linked, foamed product.

Some experimental measurements made on the foamed plate are given below that, against known parameters, will allow to evaluate certain mechanical properties of the final product:

In particular the hardness of a product obtained from the compound described earlier is 72 Asker C at 23°C, a value which provides a material that is of little interest for applications where material softness is an critical factor, such as for instance in manufacturing soles for shoes or slippers.

According to the description above, for the final product to be made softer it is sufficient to vary the type of EVA, in particular using a compound having the following formulation:

On the same conditions the following experimental measurements are obtained:

As can be seen, the hardness expressed in Asker C at 23°C goes from 72 to 62 making the material sufficiently soft, however this is obtained at the expense of the plate's thermal stability which, comparing the experimental data goes from 1.7% at 70°C in 24 h in the first example, to 5.4%, and from 0.6% at 50°C in 24 h to 1.3%.

This enables us to highlight a great limitation of these cross-linked, foamed products arising from their thermal dimensional instability. In fact these products can, whether accidentally or for reasons of application, be exposed to greater temperatures than environmental temperature. As a consequence these materials can incur permanent and irreversible shrinkage. Therefore they cannot be used in applications where thermal dimensional stability is an essential requirement, as mentioned earlier in their use for producing slipper or shoe soles.

This creates the need for materials with a thermal stability, intended as conservation of their original dimensions, falling within acceptable tolerances and without compromising their application. In the production of for instance soles, together with thermal dimensional stability, it is important to create products that are also sufficiently soft, with shrinkage remaining below pre-set values.

In fact a sole made with cross-linked material is normally glued or stitched to the upper; a poor thermal dimensional stability of the sole may lead, for instance, during transport where the footwear may undergo temperatures of 50 - 70°C, to a deformed upper and as a result deformed footwear, unacceptable in appearance and with its size non corresponding to the declared size. Therefore it seems even clearer how important it is to have a very low thermal stability, possibly below 2-2.5% while maintaining adequate softness. Unfortunately, it has been seen that the softness obtained with the aforesaid polyolefins is tied to the type of polyolefin used while in polyethylenes it is tied to the grade used.

High grades, in other words having a high MFI, with high shrinkage at 70°C - 24 h, in particular, as highlighted by the experimental measurements, EVA's with a high vinyl acetate content and high MFI, make soft products with shrinkage above 2.5 %, which is unacceptable for any commercially viable product.

As a final consequence it can be concluded that there is a limitation when producing very soft cross-linked, foamed EVA's using high vinylacetate content EVA's or high grade polyethylenes. In particular, cross-linked, foamed EVA's have reached interesting degrees of use regardless of the delicacy of their manufacturing process, primarily because of their light weight. Moreover the softness of products that can be obtained with acceptable thermal dimensional stability limits their possibilities of development. The scope of this invention is to overcome the limitations illustrated above. In fact the intention is to create a compound formulation for producing foamed and compact cross-linked polyolefins that allow to manufacture soft products with high thermal dimensional stability.

Said scopes are achieved by a compound for manufacturing cross-linked products obtained by injection moulding that in accordance with the main claim consists of a mixture comprising at least:

- master polymers, such as ethylene vinyl acetates (EVA), polyethylene (PE) and their mixtures, rubber, ethylene-propylene (EPR);

- cross-linking agents, such as peroxides or similar; - various forms of foaming agents such as azodicarbonammides or similar, said compounds characterised in that they have in addition styrene-ethylene-propilene-styrene

(SEPS) block polymers or Styrene-Ethylene-Butylene-Styrene (SEBS) block polymers. Said scopes and advantages will be better illustrated during the description of a preferred form of execution of the invention given as a guideline but not a limitation.

It has been seen how the addition of Styrene-Ethylene-Propylene-Styrene

(SEPS) or Styrene-Ethylene-Butylene-Styrene (SEBS) type block copolymers, allows to obtain compounds producing products with lower hardness without altering thermal dimensional stability of the initial polyolefinic polymer.

Similar effects are found when the compound contains both of the two copolymers SEPS and SEBS. These thermoplastic elastomers (SEPS) are obtained by hydrogenation of known Styrene-lsoprene-Styrene (SIS) based thermoplastic rubbers, as with

SEBS, which are obtained by hydrogenation of SBS (Styrene-Butadiene-

Styrene).

The thermoplasticity of these elastomers is tied to the styrenic group, or rather polystyrenic, which if heated melts and allows the polymer to be transformed by traditional thermoplastic processes.

So SEPS and SEBS have a thermoplastic nature and because of the hydrogenation process they have a saturated structure and their resistance to ageing and oxidation is therefore commonly accepted. Their heat resistance is directly linked to their molecular weight and to the ratio of blocks.

Compatibility of SEBS and SEPS with polyolefins is recognised.

Their glass transition temperature (Tg) is decisively lower than 0°C and to be more precise, below -50°C. If these products, in other words SEPS and/or SEBS, are added to cross- linking polyolefin compounds, in quantities ranging from 1 to 100 by weight and preferably from 3 to 40 by weight, using conventional manufacturing technology as described earlier, softer products can be obtained having a lower hardness with the same density with excellent thermal dimensional stability and very good resistance at low temperatures.

Their function is to elastomerise and soften the compound without taking part in the cross-linking unlike other known polyolefin modifying processes using EPR (i.e. Ethylene-Propilene Rubber) or with EPDM (Ethylene-Propylene- Diene Rubber) . The final result therefore obtains foamed or compact cross-linked polyolefin composites that are soft and thermally stable.

Since SEPS and SEBS show excellent behaviour at low temperatures, their related compounds also react better at low temperatures. In fact it is known that cross-linked, foamed polyolefins have increased hardness if taken to low temperatures; this behaviour is tied to the carbon- carbon interlinks generated by radical cross-linking. EXAMPLE 1

According to a preferred form of execution of the invention the compound, comprising the Styrene-Ethylene-Propylene-Styrene (SEPS) type block copolymer, has a formulation given by weight for the mixture described in the table below.

The SEPS block copolymer has the following characteristics:

The mixture blended in a Ribbon blender and extruded with a twin-screw extruder at a temperature of 100 - 110 °C has given cylindrical granules approx. 3 mm in diameter and 2.5 mm in length which, when moulded in a punching screw injection press fitted with the mould of a plate approx. 6 mm in thickness, 90 mm in width and 120 mm in length, heated to 180°C using a 95°C cylinder temperature and with a cross-link curing time of 4 minutes, have produced a parallelepiped product.

The table below gives some mechanical properties obtained by experimental measurements made on the cross-linked foamed plate produced as above.

EXAMPLE 2

According to a preferred form of execution of the invention the compound, comprising a Styrene-Ethylene-Butylene-Styrene (SEBS) type block copolymer, has a formulation given by weight for the mixture described in the table below.

The SEBS block copolymer has the following characteristics:

The normal procedure gives the following characteristics:

EXAMPLE 3

The same composition used in example 1 has been repeated adding a greater quantity of the SEPS block copolymer in example 2.

The normal procedure gives the following characteristics:

Moreover the plates obtained from the above compounds are chilled to -20°C and on comparison their rigidity is proportional to their SEPS content.

Claims

1) Compound for manufacturing cross-linked products with great softness and thermal stability made by injection moulding, consisting of a mixture comprising at least: - master polymers such as ethylene vinyl acetates (EVA), or polyethylene (PE) and their mixtures, otherwise ethylene-propylene rubber (EPR);

- cross-linking agents, such as peroxides or similar;

- foaming agents, such as azodicarbonammides or similar characterised in that it has in addition styrene-ethylene-propilene-styrene (SEPS) or styrene-ethylene-butylene-styrene (SEBS) block polymers.

2) Compound according to claim 1) characterised in that it has in addition styrene-ethylene-propilene-styrene (SEPS) and styrene-ethylene- butylene-styrene (SEBS) block polymers.

3) Compound according to claim 1) characterised in that it contains said block polymers (SEPS) in quantities by weight ranging from 1 to 100 and preferably between 3 and 40 by weight.

4) Compound according to claim 1) characterised in that it contains said block polymers SEBS in quantities by weight ranging from 1 to 100 and preferably between 3 and 40 by weight. 5) Compound according to claim 1) characterised in that it has the following formulation:

EVA 2MFI 18 % VA 100

Silica amorphous powder 5 Bis (tert-butylperoxypropyl) benzene 2 - Master 40% in EVA

Azodicarbonammide 50 % in ZnO 3.5

Zn Stearate 0.5

Stearin 1

SEPS 8

Master carbon black 50% in EVA 2

6) Compound according to claim 1) characterised in that it has the following formulation:

EVA 2MFI 18 % VA 100

Azodicarbonammide 50 % in ZnO 3.6

Zn Stearate 0.5

Stearin 1

SEPS 18

Master carbon black 50% in EVA 2

7) Compound according to claim 1 ) characterised in that it has the following formulation:

EVA 2MFM8 % VA 100

Silica amorphous powder 5

Bis (tert-butylperoxypropyl) benzene 2

- Master 40% in EVA

Azodicarbonammide 50 % in ZnO 3.5

Zn Stearate 0.5

Stearin 1

SEBS 8

Master carbon black in EVA 2

8) Compound according to claim 1 ) or 2) or 3) or 5) or 6) characterised in that said SEPS type block copolymer has the following properties: Styrene content 13 %

Hardness 38 Shore A

Tensile strength 11.8 Mpa

Elongation at break 1110 %

Physical form Granules

9) Compound according to claim 1) or 2) or 4) or 7) characterised in that said SEBS type block copolymer has the following properties:

Styrene content 12 %

Hardness 46 Shore A

Tensile strength 2.7 Mpa

Elongation at break 980 %

Physical form Granules