WO2003033571A1

WO2003033571A1 - Colouring mixture for neutral products to be crosslinked and expanded by injection molding

Info

Publication number: WO2003033571A1
Application number: PCT/EP2001/014964
Authority: WO
Inventors: Sergio Brunetti
Original assignee: API Applicazioni Plastiche Industriali SPA
Current assignee: API Applicazioni Plastiche Industriali SPA
Priority date: 2001-10-11
Filing date: 2001-12-18
Publication date: 2003-04-24
Anticipated expiration: 2004-04-11
Also published as: ITVI20010210A1

Abstract

A colouring mixture for neutral products to be crosslinked and expanded by injection moldings disclosed, comprising a carrier consisting of a basic polymer and a pigment comprising crosslinking agents.

Description

COLORING MIXTURE FOR NEUTRAL PRODUCTS TO BE CROSSLINKED

AND EXPANDED BY INJECTION MOLDING

The present invention relates to a coloring mixture for neutral products to be crosslinked and expanded by injection molding. The method of crosslinking polyethylenes (PE) and ethylene vinyl acetate

(EVA) by using peroxides for making injection molded manufactured articles such as technical articles, footwear, wheels and so forth is well known.

Use of crosslinked products allows to obtain in comparison with thermoplastic materials, foamed polymers of considerable lightness and with valid physical and mechanical characteristics.

The process of making any manufactured article of this kind may be generally divided into two stages.

In the first stage one proceeds to preparation of the coloured mixture called compound, containing the components required to obtain in the final transformation cycle, a manufactured product meeting the desired mechanical features.

The known compound compositions generally comprise:

- basic polymers such as EVA and PE mixtures or ethylene propylene rubbers (EPR) or low and high density polyethylene, having different degrees of melt flow index (MFI) or ethylene vinyl acetate with various

(MFI) and different vinyl acetate contents;

- peroxides being crosslinking agents which are activated at a predetermined temperature, such as decoumyl peroxide, bis-tert-butylperoxy propyl benzene, 2,5 bis-tert-butyl-peroxy 2,5 dimethyl exane; - coaqents. blocking the secondary reactions of peroxides optimizing the crosslinking yield such as triallylcyanurates, triallylisocyanurates, ethylene glycol dimethacrylate and trimethylolpropane-trimethacrylate;

- foaming agents such as azodicarbonamides of various nature which generate gas through their thermal distruction such as the industrial known types like Porofor® and Genitron®;

- lubricants enhancing the release of manufactured articles from the molds and limiting onset of friction in the transformation machinery such as stearin, stearates;

- anti-degradation agents protecting the polymer from degradation during the manufacturing process such as 6-tert-butyl-m-cresol; - kickers regulating degradation temperature of the foaming agents such as metal oxides or salts like for instant zinc or glycols;

- fillers of various types such as calcium carbonates or silica or talk which are used as nucleating agents to enhands a homogeneous dispersion of components and gases as well as to diminish the mixture cost;

- pigments allowing to colour the mixture;

- ojjs allowing to plasticize the compound and/or diminish dustyness of the mixture to be worked.

A predetermined coloration as a function of the used pigments is associated in this stage to the compound.

These ingredients properly dosed are mixed in proper blenders until a coloured and homogeneous blend is obtained. The coloured blend is then introduced into extruders melting and homogenizing the components so as to obtain a product in which the dyes and all the additives are finally dispersed in the basic polymer or polymers.

The so obtained final product is in all respects a coloured thermoplastic product adapted to be crosslinked and foamed when taken to adequate temperatures.

The second stage provides for transformation of the product from a coloured thermoplastic one to a final thermoset polymer. In order to obtain this the coloured compound is introduced into a press for injection of plastic material. By properly controlling temperature of the injection press cylinder, the working cycle is carried out in the range of material thermoplastic condition (normally temperatures between 85 and 120°C). The molten coloured compound is injected into the mold which being heated to high temperature in the range of 150- 190°C for an adequate time, heats the polymer triggering the peroxide action, so as to start the polymer crosslinking process and transformation of the foaming agents into gas. At the end of the crosslinking process the mold is opened allowing the gases to expand suddenly the article causing its automatic ejection from the mold with a kind of little explosion.

The quantity of foaming agent in the compound and consequently the volume of generated gas, together with the toughness and resistance that the crosslinked polymer opposes to the gas expansion pressure, defines the blowing amount of the article. More particularly the generated volume of the crosslinked article is directly proportional to the volume of the generated gases, the physical mechanical characteristics of the crosslinked polymer and the degree of crosslinking reached by the polymer. The foamed article is characterized by a foamed core and a very fine and compact coloured outer surface.

The shape of the article follows the configuration of the mold that made said article, but has proportionally greater size as a function of the foaming agents present in the compound less the shrinkage due to the thermal contraction generated during cooling by the difference of temperature between mold and ambient.

The manufactured article has a compact surface with bright look and colour function of the kind and quantity of pigments present in the compound.

A first drawback of said method consists in that the article coloration is decided when preparing said compound. Indeed as above mentioned, the colouring agents or pigments are added when making the mixture constituting the compound. Said pigments give to the compound a fixed coloration that cannot be changed thereafter.

Another drawback linked to the preceding one consists in that the article manufacturers must order the supply of the raw material, that is the compound, specifying not only quantity, density and the desired mechanical characteristics, but also the colour.

This causes a considerable increase of complex handling the raw materials and an inevitable increase of costs due to leftover stock. Indeed the market such as the footwear market, always requires products with different colours linked to the fashion trend.

This causes considerable trouble also for the compound producers. As a matter of fact these producers in order to meet quickly the supply orders of the customers, are also obliged to keep adequate stock of materials with clear stockage and handling costs.

In the worst case this leads to have unused leftover stock that not only weighs upon the production costs and selling prices, but they must be properly disposed as well.

In order to remove said drawbacks the prior art tried to develop methods allowing to give to the article the desired coloration at the molding operation, that is in the second stage of the production process. The known method generally consists in producing a colourless basic compound called neutral compound, and adding a granular colouring mixture containing the pigments diluted in a suitable carrier compatible with the neutral compound at the molding operation.

A first drawback of said method consists in that the used colouring mixtures alter both the mechanical characteristics and mainly the compound foaming and crosslinking features.

More particularly it was noted that the neutral compound being equal, different manufactured articles are obtained by combining colouring mixtures of different coloration and composition.

More particularly neither repeatability nor dimensional consistency of the manufactured articles is warranted.

Conducted studies highlighted that the different colouring mixtures change the composition of the compound in such a way that both expansion and thermal shrinkage of the article after the molding operation are unpredictable.

Said drawbacks make such a known method totally useless. As a matter of fact in order to apply such a method, that is to warrant repeatability and the same mechanical characteristics and expansion and crosslinking features of the article when changing the colouring mixtures, it is necessary to make different neutral compounds each to be associated with a predetermined colouring mixture.

The object of the present invention is to overcome the foregoing drawbacks.

More particularly a first object of the invention is to provide a colouring mixture for making foamed crosslinked articles allowing pigmentation of the article at the molding operation starting from a neutral compound.

Another object is to provide a colouring mixture that does not alter expansion and thermal shrinkage of the article when added to a neutral compound if its pigmentation features are changed. More particularly the object is to obtain that expansion and thermal shrinkage of the article are a function only of the features of the neutral compound.

The foregoing objects are attained by a colouring mixture for neutral products to be crosslinked and foamed by injection molding, that according to the main claim comprises: - at least a carrier consisting of at least a basic polymer; - at least a pigment; characterized by comprising also crosslinking agents or foaming agents or foaming and crosslinking agents.

Said objects and advantages will be better understood by reading the following description of a preferred embodiment of the invention given as an illustrative but non limiting example.

The colouring mixture for producing crosslinked articles made by injection molding according to the present invention comprises:

- a carrier consisting of one or more basic polymers mixed together; and - a pigment.

The invention provides that the mixture comprises either crosslinking agents or crosslinking agents and foaming agents as well.

According to a preferred formulation the crosslinking agents consist of peroxide or analogs while the foaming agents consist of azodicarbonamides or analogs. As to the carrier, it consists of basic polymers such as ethylene vinyl acetates (EVA), or polyethylene (PE) and their mixtures or ethylene propylene rubber (EPR) or ethylene propylene diene terpolymer (EPDM) or a thermoplastic elastomer.

The pigment may be either an inorganic type or an organic type. In some cases the pigment may also be fluorescent or metalizing or pearlscent.

EXAMPLE No. 1

According to a preferred embodiment of the invention, the colouring mixture comprises:

- polymeric carrier EVA (ethylene vinyl acetate) or polyethylene or mixtures thereof;

- organic pigments: 2% to 30% or inorganic pigments: 5% to 60% or their mixtures;

- peroxide 1 % to 3% of bis tert-butylperoxypropyl benzene at 40% of active portion. The mixture may also contain: various additives:

- process oils (paraffin oil),

- lubricants (stearin, zinc stearate, polyethylene waxes and so forth),

- dispersants (surfactants of various kind), - heat stabilizers (antioxidants), - fillers (calcium carbonates, silica and so forth).

The latter should not be necessarily present in all the formulations, when included, the percentage used will be in the range of 0.1% and 10% on the total amount of the formulation components. The percentage of colouring mixture on the neutral compound varies from 5 to 15% and is preferably in the range between 6 and 10%. In operation the colouring mixture in the form of granules according to the foregoing composition and the neutral compound are prepared separately. Such a neutral compound of a kind known per se and obtained according to the preceding disclosure indicated as first stage, may have the following formulation: - EVA 18% V.S. 2MRI 100

- Amorphous silica 5

- Bis (tert-butylperoxy-propyl benzene at 40% in EVA 2 - ADCM at 40% / 50% ZnO 3.6

- Zn stearate 0.5

- Stearin 1

Once the neutral compound and the colouring mixture are prepared, the second stage of molding the article is started. More particularly the second stage consists as already mentioned, in transforming the compound from a thermoplastic material to a thermoset final polymer. The granules of neutral compound and colouring mixture properly mixed intimately, are dry introduced into a conventional injection press for plastic material. More particularly such a press comprises a punching screw injector consisting of a cylinder and a screw adapted to move the granules inside the cylinder which is heated to the desired temperature allowing the plasticization of the compound.

By properly controlling the cylinder temperature so as not to reach the triggering temperature which is generally in the range of 85-120°C of both the foaming agent and the crosslinking agent, it is possible to carry out the working cycle in the material thermoplastic area.

The molten compound is finally mixed with the pigments of the colouring mixture thus obtaining a product with homogeneous colour that can then be injected into the mold. When the mold is heated to a high temperature in the range of 150-190°C for an adequate time, the polymer is heated triggering the action of peroxide thus starting the polymer crosslinking process and transformation of the foaming agents into gas.

Although said gases develop a high pressure, they are bound inside the mold by the press closing action so as to remain finally dispersed in the polymer. The crosslinking then proceeds according to the known rheologic curve with the following phases:

- decomposition of peroxide with formation of free radicals;

- extraction of hydrogen atoms by the free radicals from the polymeric chains leading to stable peroxide decomposition products and polymer with free radicals;

- combination of the free radicals with the polymer and crosslinking formation.

It is known that either quantity or type of introduced peroxide allow to adjust the crosslinking degree.

More particularly if the stoichiometric quantity of peroxide introduced into the mixture is sufficient, it is possible to reach the total polymer crosslinking, otherwise crosslinking levels lower than 100% are obtained. By selecting properly temperature and residence time of polymer in the mold, the crosslinkling reaction is completed until all the peroxide is consumed so as to prevent a further subsequent increase of viscosity, thus obtaining partial or total crosslinking.

At the end of the crosslinking process the mold is opened thus allowing the gases to expand suddenly the article, causing its automatic ejection from the mold with a kind of little explosion.

The amount of foaming agent present in the neutral compound and possibly in the colouring mixture and consequently the volume of generated gas together with toughness and resistance of the polymer opposing the gas expansion pressure, define the article blowing amount. More particularly the volume of the generated crosslinked article is directly proportional to the volume of generated gases, the physical mechanical characteristics of the crosslinked polymer and the reached degree of polymer crosslinking. In any case irrespective of the size, crosslinking speed and therefore the time of the molding and crosslinking cycle are exactly those of the neutral compound.

Claims

1 ) A colouring mixture for neutral products to be crosslinked and expanded by injection molding, comprising:

- at least a carrier consisting of at least a basic polymer; - at least a pigment; characterized by comprising also crosslinking agents.

2) The mixture according to claim 1 ) characterized by comprising also foaming agents.

3) The mixture according to claim 1 ) characterized in that said crosslinking agents are peroxides or analogs.

4) The mixture according to claim 2) characterized in that said foaming agents are as dicarbonamides.

5) The mixture according to any of the preceding claims characterized in that said at least a basic polymer is ethylene vinyl acetates (EVA). 6) The mixture according to claims 1 ) to 4) characterized in that said at least a basic polymer is polyethylene (PE).

7) The mixture according to claims 1 ) to 4) characterized in that said at least a basic polymer consists of a mixture comprising ethylene vinyl acetates

(EVA) and polyethylene (PE) polymers. 8) The mixture according to claims 1 ) to 4) characterized in that said at least a basic polymer is ethylene propilene (EPR).

9) The mixture according to claims 1 ) to 4) characterized in that said at least a basic polymer is EPDM.

10) The mixture according to any of claims 1 ) to 4) characterized in that said at least a basic polymer consists of a mixture comprising two or more basic polymers including:

- ethylene vinyl acetates (EVA);

- polyethylene (PE);

- ethylene propylene(EPR); - EPDM.

1 1 ) The mixture according to claims 1 ) to 4) characterized in that said at least a basic polymer is a termoplastic elastomer.

12) The mixture according to any of the preceding claims characterized in that said at least a pigment is of the inorganic type. 13) The mixture according to any of claims 1 ) to 1 1 ) characterized in that said at least a pigment is of the organic type.

14) The mixture according to any of the preceding claims 1 ) to 11 ) characterized in that said at least a pigment consists of a mixture comprising organic and inorganic pigments. 15) The mixture according to any of claims 1 ) to 14) characterized in that said pigment is of the fluorescent type.

16) The mixture according to any of claims 1 ) to 14) characterized in that said pigment is of the metalicized effect type.

17) The mixture according to any of claims 1 ) to 14) characterized in that said pigment is of the pearl effect type.