WO2015090538A1

WO2015090538A1 - Process for the surface treatment of metal moulds for the injection of plastic materials thereinto and moulds treated therewith

Info

Publication number: WO2015090538A1
Application number: PCT/EP2014/003305
Authority: WO
Inventors: Valentina NICOLINI; Luca MARZANI; Leonardo RETTIGHIERI
Original assignee: Tec Star S.R.L.
Priority date: 2013-12-18
Filing date: 2014-12-10
Publication date: 2015-06-25
Also published as: ITMO20130350A1

Abstract

A process consisting of the treatment of moulds for the production of plastic materials (injection, compression, rotational moulding, etc) characterised by the fact that such process envisages the application in the mould of an alcohol-based solution containing an active agent from the silane family capable of lending the metal surface a property consisting in the non-adhesion to molten polymer, forming an ultra-thin, extremely resistant and long-lasting layer. The invention also relates to the mould for the production of plastic materials to which the treatment claimed herein is applied.

Description

DESCRIPTION

PROCESS FOR THE SURFACE TREATMENT OF METAL MOULDS FOR THE INJECTION OF PLASTIC MATERIALS THEREINTO AND MOULDS TREATED THEREWITH

Technical field

The present invention relates to the field of production of plastic materials, wherever a metal mould is used to give shape to the objects produced (injection moulding, rotational moulding, compression moulding, thermoforming, extrusion, casting, blow-moulding), in particular wherever it is necessary to produce parts with a complex geometry or a very good surface finish (mirror finish).

The complex shapes make the release of the part produced from the mould complicated, as the latter tends to grip the workpiece; it is therefore necessary for the operator to intervene, who manually removes the workpiece, thereby interrupting the machine's production cycle and risking ruining the surface finish of the mould.

The treatment envisaged in this invention reduces the manual intervention times greatly, thus reducing machine downtime and increasing productivity.

Background art

The adhesion of melted polymeric parts to the metal surfaces of the moulds is a problem that has occurred clearly since the use of machines for injection moulding began. In fact, because of the high pressures involved in the moulding process, the moulded part is subjected to high compression, with forces of considerable intensity, which force it into the mould and, in many cases, impair extraction after opening the said mould. In many cases, it has been necessary to use releasing means interposed between the polymer and the mould, in such a way as to limit the adhesion bonds.

For this purpose, conventional technical solutions envisage the use of:

- treatments using wax-based substances, which are usually applied by hand. Drawback of this solution: these treatments coat the mould unevenly and are not suitable for the production of highly aesthetic surface finishes (mirror finishes);

- treatments using polymeric-type substances (varnishes) which coat the mould's surfaces with a layer which can be up to one tenth of a millimetre thick. Drawback of this solution: there are problems which typical occur with varnishes, such as non-standard thicknesses and detachment due to poor adhesion to the metal, which jeopardise the functionality of the treatment and require frequent re- application of said varnishes in the mould;

- treatments which use additives which are added to the polymer (granules) to ob ain release properties. Drawback of this solution: risk of inducing substantial changes within the mechanical properties of the end product;

- use of liquid release agents, which are sprayed on automatically during each production cycle, upon mould opening. Drawback of this solution: considerable material wastage and limited effectiveness over time, as well as the possibility of release agent residues on the surface of the article produced, which can pose problems during subsequent treatments (painting, metallisation, etc.), resulting in costly workpiece cleaning processes.

Disclosure and advantages of the invention

The aim of the present invention is to overcome the drawbacks lamented hereinabove by providing the art with a process for the treatment of the moulds for the production of plastic materials, which lends the metal thus treated a property consisting of the facilitated release of the plastic part produced upon mould opening, or during the steps immediately following the moulding process.

A further aim of the invention is to create a treatment for the application of an ultra-thin layer (i.e. being less than 10 nm thick, given that 1 run corresponds to lO⁹ m) of molecules of an active agent from the silane family, dispersed in an alcohol- based solution; the said active agent/ substance is easily miscible with the solvent indicated.

A preferred embodiment of the application to the surface of the metal mould will now be described, however, such application may be performed using different methods and still fall within the scope of the invention.

More specifically, the application treatment involves the use of a spray dryer powered by compressed gas in order to dampen the mould's surface simply and in a uniform manner, even for complex mould geometries; the treatment proves effective after a few hours' rest at room temperature, or in less time if heat is administered (up to a temperature of about 80-100 ° C).

The scope of protection is further extended to any mould suitable for injection moulding which has been subjected to the treatment described and claimed herein.

The present invention overcomes all the drawbacks lamented and, in particular, allows numerous advantages to be obtained, such as:

- the ultra-thin layer which is formed allows the geometry of the treated surfaces to remain unchanged, in terms of both geometric tolerances and surface finish (roughness); this property of the product allows objects to be moulded with a mirror finish and likewise complex geometries, etc; - the application method is extremely simple and can be adapted for use with moulds which are already fitted and ready for use; this characteristic, combined with the presence of extremely volatile solvents, results in rapid drying (no polymerisation time needed) and a uniform coating of the metal surface;

- the infinitesimal thickness of the layer formed allows the treatment to anchor onto the mould's surface tenaciously, rendering the release effect resistant over time, despite the high pressure and temperature conditions typical of the moulding step; this results in a prolonged anti-adhesion effect even in moulding cycles running continuous over 24 hours and several days (at least 10 working days without the need for new applications);

- the treatment facilitates the distribution of the molten polymer within the mould;

- the effect of the facilitated release from the mould decreases - by approximately 40% - machine downtime caused by workpieces stuck in the mould, regardless of the nature of the polymer;

- since the treatment consists of an ultra-thin layer of material (i.e. a minimum amount of substance applied to the surface) such treatment does not leave residues on the workpieces and does not generate any chemical alteration issues on the surface of the moulded article, thus allowing post-treatment surface without the need to clean the moulded surface;

- If necessary, for periodic maintenance or in the event of loss of effectiveness, the treatment and residues thereof can be removed extremely simply, using common solvents; - the treatment can induce a water repellent (hydrophobic) action in the metal surface, making the mould more resistant to corrosion and oxidation.

The aforesaid aims and advantages are achieved by the process in question in the present invention, for the surface treatment of metal moulds for the production of plastic materials and for the mould produced therewith, which is characterised by the provisions of the claims set out below.

Description of the invention

The aim of the present invention is to provide a process for treating moulds for the production of plastics. Once applied to the mould, such treatment facilitates the release of the plastic part produced upon mould opening and during the steps immediately following the moulding process. The treatment consist of the application of an alcohol-based solution with an active agent from the silane family dispersed therein.

The choice of the type of solvent is important in order to define the properties of the treatment, as the physical properties determine the evaporation kinetics thereof; in order to facilitate the application of the ultra-thin layer of ac ive agent, it is important that the solvent evaporates quickly (so that no stagnation or accumulation of material occurs on the surface, nor sagging, which renders the coating uneven); it is therefore necessary for the boiling temperature thereof to be below 85 ° C. Solvents of this kind may be polar protic or aprotic solvents, such as acetone, benzene, methanol, ethanol, acetonitrile, isopropanol, methyl ethyl ketone, and other similar or apolar solvents, such as hexane, benzene, diethyl ether, chloroform, dichloromethane, ethyl acetate, tetrahydrofuran, chloride methylene and other similar; based on safety criteria concerning users' health, as well as cost- effectiveness and commercial availability criteria, mixes of the aforementioned solvents may also be chosen, such as: ethanol denatured with Lsopropanol (at a rate of 10% by weight with respect to the ethanol).

The active molecule which must lend the surface the anti-adhesion fea ure has been chosen from the silane family, because these rarely pose a health risk and are industrially available. The substance must feature good miscibility with the solvent and a molecular structure that allows both strong adhesion to the surface (chemical bonds) and an organic component, which enables the chemical repulsion of the molten polymeric material.

Members of the silane family which meet these requirements include: trimethoxysilane, vinyltrimethoxysilane, hexadecyl trimethoxysilane, ethyl rimethoxy silane, all yl trimethy 1 silane, chloromethyltrimethoxysilane, hexyltrimethoxysilane, trimethoxymethylsilane, trimethoxypropylsilane, trimethoxyoctylsilane, trimethoxyoctadecylsilane, isobutyltrimethoxysilane and other similar silanes; also in these cases, based on cost-effectiveness and commercial availability criteria, as well as the product's technical properties, specific products maybe chosen, such as: hexadecyl trimethoxysilane (hereinafter referred to for brevity as HDTMS).

This member of the silane family is miscible with the solvent chosen by simply a mechanical stirring action; once the silane is deposited on the surface of the metal, it reacts by forming a chemical bond with therewith.

The chemical process which takes place shall now be described. On the surface of steels, as in the case of other metal materials, there are oxides and hydroxides of different elements (Fe, Cr, Al, Mn, etc.) depending on the type of steel used. The HDTMS molecules form a bond with the substrate thanks to the presence of these oxides and hydroxides. The oxygen on the steel's surface attacks the silicon present in the HDTMS molecule and, simultaneously, one of the three CH3O groups is eliminated from the said molecule due to the fact that such group is a good leaving group (possible SN mechanism). The formation of the bond Met-O-Si (metal - oxygen - silicon) therefore occurs.

Given the molecular structure, one could expect the chain-linked organic part to be branched upwards, thus creating a release effect; however, analyses performed with angle-resolved XPS spectroscopy (X-ray Photoelectron Spectroscopy) - an analytical technique which is extremely sensitive to the surface - have demonstrated the formation and existence of a molecular layer less than 10 ran thick.

The silane concentration in the solvent has proved efficient within the range 0.1 - 15% by weight, and optimal within the range 3-6% by weight.

The solution stated above is applied to the metal mould's surface using a spray dryer powered by compressed gas, which has been optimised as a spray can.

In particular, in relation to this application example and by following the various criteria stated above, the choice of solvent has fallen on a mixture of ethanol denatured with isopropanol (at a rate of 10% by weight with respect to the ethanol), while - as regards the silane - hexadecyl trimethoxysilane (HDTMS) has been chosen.

The purpose of the nebulisation of the solution is to dampen the mould's surface in a uniform manner, without leaving any areas untreated.

After this, the treatment requires 8-10 hours' rest at room temperature - or just one hour at temperatures of approximately 80-100 ⁰ C - before it is effective. The possibility of treating the mould without removing it from the moulding machine is also envisaged, leaving the mould at the normal use temperature for the time stated above.

At the end of the treatment, an ultra-thin layer of product molecules remains on the mould, wherein the term 'thin' refers to a layer with a thickness of less than 10 nm (lnm corresponds to lO⁹ m).

Claims

1. A process consisting of the treatment of moulds for the production of plastic materials (injection, compression, rotational moulding, etc) characterised by the fact that such process envisages the application in the mould of an alcohol-based solution with an active agent from the silane family dispersed therein, which is then left to dry.

2. A process according to Claim 1, characterised by the fact that the said solution is ethanol denatured with isopropanol at a rate of 10% by weight.

3. A process according to Claim 1, characterised by the fact that the said active substance from the silane family is hexadecyl trimethoxysilane.

4. A process according to Claim 1, characterised by the fact that the active agent is mixed with the solvent in a concentration within a range of 3-6% by weight.

5. A process according to Claim 1, characterised by the fact that the active agent is mixed with the solvent in a concentration within a range of 0.1-15% by weight.

6. A process according to Claim 1, characterised by the fact that the active agent can be chosen from:

a. trimethoxysilane,

b. vinyltrimethoxysilane,

c. hexadecyl trimethoxysilane,

d. ethyltrimethoxysilane,

e. allyltrimethylsilane,

f. chloromethyltrimethoxysilane,

g. hexyltrimethoxysilane, h. trimethoxymethylsilane,

i. trimethoxypropylsilane,

j. trimethoxyoctylsilane,

k. trimethoxyoctadecylsilane,

1. isobutyltrimethoxy silane

7. A process according to Claim 1, characterised by the fact that the solution stated above is deposited on the metal mould's surface using a spray dryer powered by compressed gas to dampen the mould's surface in a uniform manner.

8. A process according to Claims 1 and 7, characterised by the fact that following the application of the solution, the drying of the solution is envisaged within a waiting time of 8-10 hours at room temperature, or even one hour at a temperature of approximately 80- 00 ° C.

9. A mould, suitable for the production of polymer parts by injection, comprising a layer of product after at least one treatment consisting in the application of an alcohol-based solution with an active agent from the silane family dispersed therein, as according to Claim 1.

10. A mould according to Claim 9 characterised by the fact that such mould comprises a layer of product which is less than 10 nm thick.