Method for injection molding shaped resin bodies having a scratch resistant surface

Abstract

Injection molded parts having a scratch resistant surface are produced by applying a liquid mixture of at least one monomer susceptible of free radical polymerization and having more than one polymerizable double bond and of at least one free radical forming initiator to the forming surface of an injection mold, polymerizing the mixture at least partially, and then injecting a thermoplastic or duroplastic molding composition into the closed mold.

Description

The present invention relates to a method for injection molding articles of synthetic resin having a scratch resistant coating.

THE PRIOR ART

It is known to provide synthetic resin articles produced by injection molding with a scratch resistant coating after forming by immersing them in a solution of a material suitable for the formation of a scratch resistant film, volatilizing the solvent, and curing the film. In the second method described in German Patent Publication No. 24 55 715, a solution is used for this purpose which contains a compound susceptible of free radical polymerization and having at least three acryl or methacryl groups, and an initiator forming free radicals. However, the dimensional accuracy of the molded article is affected by the subsequent coating. Moreover, the solvent contained in the coating solution may give rise to cracks in the resin part so treated.

According to German Patent No. 20 58 504, a sheet of glass is coated with a liquid mixture which can be cured to form a scratch resistant resinous coating, a sheet of synthetic resin is placed thereon with exclusion of air bubbles, the free surface of the resin sheet is coated with the liquid mixture, a second sheet of glass is placed thereon, the layers of the mixture are cured, and the glass sheets are removed. While this method does permit the production of dimensionally accurate synthetic resin sheets having a scratch resistant surface, it is not suitable for the production of molded articles of any shape, such as injection molded parts.

For the production of coated injection molded parts, European Patent No. 123,374 proposes that after an injection molded part has been produced, the mold be opened slightly, a liquid composition be forced into it for formation of a surface layer, and the mold then be closed. The composition is spread over the surface of the molded part by the rotary motions executed by the mold halves as the mold is being closed and is then cured. Apart from the fact that many injection molds would not permit rotary motions for the spreading of the composition injected after the molding operation, providing the mold with means for rotating it would complicate fabrication of the mold and add to its cost.

In a number of other prior art methods, molded articles having a scratch resistant surface layer are produced by providing the forming surface of an injection mold with a scratch resistant layer and introducing into so-conditioned mold a curable composition for formation of the inner molded article, and then curing it. Thus, in the first method of German Patent Publication 24 55 715, a layer of a coating material is applied to the inner wall of a mold and photochemically cured in an oxygen free atmosphere. A monomer intended for formation of the molded article, for example a methacrylate syrup, is then introduced into the mold and polymerized under predetermined conditions. In the process, the layer applied to the wall of the mold bonds to the polymerized article being formed and on completion of polymerization can be removed therewith from the mold.

In a very similar method according to German Patent Publication No. 21 64 716, the coating applied to the inner wall of the mold is covered with a foil and is cured by means of free radical initiators, which eliminates the need for an oxygen-free protective gas. After the coating has heen cured, the foil is removed and a monomer for the synthetic resin core is charged to the mold and polymerized. A similar procedure is used in the methods according to German Patent Nos. 31 40 316 and 30 28 562 where, after the inner wall of the mold has been coated with a colored material, an unsaturated polyester resin is injected into the mold cavity and cured.

According to German Patent No. 32 03 540, injection molded parts having a scratch resistant surface are produced by placing a length of foil provided with a scratch resistant coating into the injection mold and then injecting

to the foil under the pressure of the molding composition. This method can only be employed to render parts having flat surfaces scratch resistant.

For the production of injection molded parts having a colored surface film, German Patent Publication No. 28 03 144 proposes applying a film of a coating solution to the inside of an injection mold. The mold is then closed and a molding composition is injected.

According to German Patent No. 24 48 477, the inside of a heated mold is electrostatically coated with a powdered thermoplastic resin, which then agglomerates by sintering into a layer. A molding composition containing a blowing agent is then injected. A molded foamed resin article having a surface layer of the thermoplastic material, which has better scratch resistance than the foamed resin article, is so obtained.

THE OBJECT AND THE INVENTION

The invention has as its object to produce, in a method for making molded synthetic resin articles having a scratch resistant surface, a surface having high scratch resistance by coating the forming surface of an injection mold with a material suitable for formation of the scratch resistant coating, injecting a thermoplastic or duroplastic molding composition into the closed mold, and removing the molded article from the mold after cooling it to a temperature below the softening point.

In accordance with the invention, this object is accomplished by applying to the forming surface of the injection mold, for formation of the scratch resistant coating, a liquid mixture of at least one monomer susceptible of free radical polymerization and having more than one polymerizable double bond and of at least one free radical forming initiator. When using thermoplastic molding compounds, the mold is prewarmed to a temperature below that of the molten thermoplastic mass. In this case, the mixture for forming the scratch resistant coating contains an initiator forming free radicals at temperatures between the temperature of the mold and temperature of the thermoplastic mass. If a duroplastic molding compound is used, the mold is prewarmed to a temperature at which the duroplastic mass hardens. An initiator which forms free radicals at the temperature of the mold is used in the liquid mixture which forms the scratch resistant coating. Polymerization of the monomers sets in either at once or after injection of the molding composition and is completed before the injection molded part has cooled to a temperature below the softening point and is removed from the mold. The polymerization can be regarded as completed when a hard scratch resistant coating is obtained, even though it may still be possible to detect unreacted double bonds therein.

ADVANTAGES OF THE INVENTION

With the process of the invention, molded plastic articles are obtained whose dimensions are exactly those of the mold. Injection molded parts having a scratch resistant surface can be produced by the use of commonly employed existing injection molding machines and molds. Conventional injection molding compositions without reactive groups or conventional duroplastic molding compositions can be used to produce the molded parts. These advantages are made possible through process steps which are readily and quickly carried out and do not unduly slow down or add to the cost of the injection molding process.

USE OF THE INVENTION

The method of the invention is suited for the production of all kinds of scratch resistant injection molded parts whose size and shape are limited only by the boundaries of injection molding technology. By its nature, the invention has its broadest application in the manufacture of parts with a high gloss surface, made in particular from crystal clear plastics. Examples of such parts are optical lenses, spectacle lenses, optical prisms, mirrors, instrument panels, and glazing for automobiles, trailers, ships, aircraft, etc.

Suitable thermoplastic injection molding compositions include those of polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Polycarbonates, such as bis-phenol-A-polycarbonate, and especially acrylic glass, meaning homo- and copolymers of methyl methacrylate (with over 80% MMA), are preferred. The thermoplastic molding compounds harden on cooling below their softening point to give solid bodies.

Suitable duroplastic molding compounds are the known thermosetting resins containing heat reactive components or groups and which harden on heating to form crosslinked high-polymeric materials. Their hardening temperature is generally between 40.degree. C. and 230.degree. C., preferably between 40.degree. C. and 150.degree. C. The duroplastic molding compounds are also termed "duromers" and are defined in DIN 7724. They may be liquid or pasty coating resins or powdered or granular substances suitable for compression molding. Typical known representative duroplastic molding compounds are the phenoplasts, aminoplasts, epoxy resins, thermosetting acrylic resins, alkyd resins, unsaturated polyester resins, and isocyanate resins.

PRACTICE OF THE INVENTION

The monomers used for formation of the scratch resistant coating have at least two and rarely more than six polymerizable double bonds. These may be present in acryloyl, methacryloyl, vinyl, vinylidene, allyl, or methallyl groups. Esters of acrylic or methacrylic acid with polyhydric aliphatic alcohols are preferred. As a rule, these contain from 2 to 10 carbon atoms and from 2 to 6 hydroxyl groups, all or some of which may be esterified.

Illustrative of such monomers are ethylene glycol diacrylate and dimethacrylate, 1,2-propylene glycol diacrylate and dimethacrylate, 1,2- and 1,4-butylene glycol diacrylate and dimethacrylate, glycerol triacrylate and trimethacrylate, pentaerythritol tri- and tetra-acrylate and -methacrylate, trimethylolpropane triacrylate and trimethacrylate, and the acrylic esters of dipentaerythritol. Of the esters mentioned, those of acrylic acid polymerize more rapidly and more completely than those of methacrylic acid. Thus, it will therefore be advantageous to use only acrylic esters or mixtures of acrylic and methacrylic esters in which the former clearly predominate and the latter represent not more than 30 weight percent, and preferably not more than 15 weight percent. Monomers with boiling points above 140.degree. C. are preferred.

Of the polyfunctional monomers, those having three or more carbon-to-carbon double bonds will result in particularly high crosslinking density and consequently good scratch resistance. However, since their viscosity is usually high, they are difficult to process in a pure form at room temperature. Their viscosity can be reduced by including a percentage of difunctional monomers. This can also be accomplished by the addition of monofunctional monomers. While the latter will not contribute to crosslinking, they will enhance the flexibility of the scratch resistant coating. Such monofunctional monomers include, for example, styrene, acrylonitrile, methacrylonitrile, monoalkyl esters of acrylic acid and methacrylic acid having from 1 to 10 carbon atoms in the alkyl group or having substituted alkyl groups, for example those which may carry hydroxyl groups as substituents.

With a view to obtaining good processing viscosity and high flexibility of the scratch resistant coating, the monomer mixture may contain as much as 70 weight percent of mono- and bi-functional monomers, while for the sake of high scratch resistance the proportion of tri- or poly-functional monomers should not be less than 30 weight percent. Surprisingly, the monomer mixture may contain up to 30 weight percent of acrylic acid or methacrylic acid without the scratch resistance being diminished. The viscosity of the liquid monomer mixture is advantageously not greater than 100 mPa/sec (determined at 20.degree. C.).

In addition to or in place of the mono- or di-functional monomers, nonpolymerizable volatile organic liquids may be used to obtain good processing viscosity. Before the mold is closed and the molding composition is injected, these liquids must largely or completely be evaporated from the coating applied to the inner surface of the mold. The unevaporated residue will gradually volatilize after the finished injection molded part has been removed from the mold.

To be able to evaporate, the liquid must have a boiling point below that of the monomers used. The boiling point is advantageously below the temperature of the mold surface. Suitable organic liquids are solvents such as aliphatic esters, ethers, ketones, chlorinated hydrocarbons, and aromatic hydrocarbons, for example. Of the ketones, which are generally preferred, methyl ethyl ketone is particularly suitable.

The function of the initiator forming free radicals is to harden, or cure, by polymerization, the coating of monomer mixture applied to the forming surface of the injection mold. Curing may be completed before the mold is closed and the molding composition is injected. The hardened coating then cannot be dislodged by the molding composition being forced into the mold. However, bonding between the coating and the cooled molding composition is not always fully satisfactory. Better bonding is obtained when polymerization of the coating is completed only after the injection of the molding composition. If polymerization has not progressed sufficiently by the time the molding composition is injected, there is the danger that the coating will be partly dislodged from the mold surface. The operating cycle of the injection molding process therefore is preferably such that the time interval between the application of the monomer mixture to the mold surface and the injection of the thermoplastic or duroplastic molding composition is sufficient for a partial polymerization that will prevent displacement of the coating and allow adequate bonding.

When atmospheric oxygen is admitted to the polymerizing coating on the mold surface, polymerization is inhibited more markedly at the exposed surface of the coating than over the area where it is in contact with the mold surface. As a result, the coating cures more or less completely at the mold surface while the other side remains partially polymerized until air is excluded by the molding composition being injected and polymerization goes to completion with formation of a good bond. If, notwithstanding this desired inhibiting effect of atmospheric oxygen, complete curing of the coating before the injection of the molding composition is desired, it is advisable to close the mold and blow in an inert gas.

Free radical forming initiators with a half-life of less than two minutes at 100.degree. C. are particularly well suited for the process of the invention. (See Ullmanns Enzyklopaedie der technischen Chemie, 3rd Ed., 1970, Supplement, pp. 177-181). Suitable are, in particular, aliphatic peroxydicarbonates, which include:

diethyl peroxydicarbonate;

dichloroethyl peroxydicarbonate;

diisopropyl peroxydicarbonate;

diisobutyl peroxydicarbonate;

di-2-ethylhexyl peroxydicarbonate;

dicyclohexyl peroxydicarhonate;

di(alkylcyclohexyl) peroxydicarbonate;

di(methylcyclohexyl) peroxydicarbonate; and

di(tert.-butylcyclohexyl) peroxydicarbonate.

In this connection, see Swern, Organic Peroxides, John Wiley & Sons, Vol. 1, 1970, pp. 68-73, and Vol. 2, 1971, pp. 863-867.

The initiators, such as the peroxydicarbonates named above, are used in amounts from 0.1 to 10 weight percent, and more particularly from 1 to 7 weight percent, based on the total weight of the monomer mixture. In addition to initiators with a half-life of less than two minutes, initiators having a half-life of over two minutes at 100.degree. C. may be used concurrently in a smaller amount, for example up to one-fifth of that of the first-named initiators. Examples are dilauroyl peroxide, tert.-butyl peroxypivalate, and dibenzoyl peroxide.

Further additives, such as flow control agents, antioxidants, antistatic agents, or UV stabilizers, may be added to the liquid monomer mixture. The customary nonpolymerizable UV absorbers, as listed in Ullmanns Enzyklopaedie der technischen Chemie, 4th Ed., Vol. 15, pp. 253-260, are suitable for use. However, polymerizable UV absorbers such as 3-(2-benzotriazolyl)-2-hydroxy-5-tert.-octylbenzyl methacrylamide are more advantageous.

The monomer mixture consisting of monomers, initiators, optional organic liquids (solvents), and further additives is applied in a coating thickness between 1 and 100 microns, and preferably between 2 and 50 microns, to the forming surface of the mold. By this are meant the areas of the mold which during injection molding come into contact with the molding composition and determine the physical boundaries of the molded part. A lesser coating thickness would not assure scratch resistance, nor would a greater coating thickness enhance the scratch resistance; at most it would reduce the elasticity and the adhesive strength of the coating. The coating is best sprayed on, the nonforming areas of the mold surface and the portions of the forming surface where no scratch resistant coating is to be produced being covered by appropriate templates. Unless the coating is applied by airless spraying, oxygen free gases such as nitrogen are preferably used for spraying in order not to interfere with the polymerization of the coating formed.

In the case of molds of complex design, and especially of multipart molds, it may be advisable to spray the coating into the closed mold by means of one or more spray heads that are integral with the mold so as to apply the coating only to the forming surface areas. If indicated, the mold may then be momentarily opened to vent the solvent vapors.

The mold surface usually has a temperature between 40.degree. C. and 150.degree. C. The applied coating of monomer mixture will assume this temperature unless it is lowered by evaporation of the volatile solvent or raised by the action of a heat lamp. Over that temperature range, the polymerization will proceed rapidly and may be completed within a period ranging from 5 to 600 seconds, especially when atmospheric oxygen is excluded.

The preferred time interval between the application of the coating and the injection of the molding composition is 1 to 20 seconds.

As soon as the coating has cured to the desired extent and the injection mold has been closed, the molding composition can be injected conventionally in the molten state. The final cure takes place during the time that the molded part remains in the closed mold until it has cooled to a temperature below the softening point. Then the mold is opened and the part is ejected.

The quality of the scratch resistant coating on the finished molded part will depend on the composition of the monomer mixture used, and particularly on the crosslinking density, the elasticity, and the bonding to the subjacent molding composition. With judicious selection of the composition, scratch resistance will be as good as or better than that of high quality scratch resistant polysiloxane based coatings. The best coatings that can be produced in accordance with the invention cannot be scratched with No. 00 steel wool even with hard rubbing.

A better understanding of the present invention and of its many advantages will be had from the following examples, given by way of illustration.

EXAMPLES

In the examples which follow, the following starting materials were used:

(A) pentaerythritol tetraacrylate;

(B) hexanediol-1,6-diacrylate;

(C) bis-4-tert.-butylcyclohexyl peroxydicarbonate;

(D) trimethylolpropane triacrylate; and

(E) cyclohexanone.

EXAMPLE 1

9.43 g of a mixture of 56 percent by weight of pentaerythritol tetraacrylate (normal boiling point 450.degree. C.) and 44 percent by weight of hexanediol-1,6-diacrylate (normal boiling point 315.degree. C.) were stirred with 1.6 g of initiator solution in a glass beaker. The initiator solution contained 10.5 g of methylethyl ketone as the solvent and 1.6 g of bis-4-tert.-butylcyclohexyl peroxydicarbonate as an initiator having a half life of 20 seconds at 100.degree. C. The solution was filled into the cup of a micro-injection apparatus and, using nitrogen as a carrier gas at 4-5 bars of injection pressure, was sprayed onto the interior surface of an open injection mold. The coating on the hot mold surface is exposed to the surrounding air until injection of the molding mass. During this time, the surface of the coating remained relatively soft, from which it was recognized that it was only partially polymerized. The mold surface had a temperature of 90.degree. C.

Immediately after spraying on the coating, the injection mold was closed. Four seconds later, a thermoplastic polymethylmethacrylate molding compound (commercially available under the tradename "PLEXIGLAS Y 8 N, clear") was injected at a temperature of 250.degree. C. After a lapse of 50 seconds from injection of the coating, the mold was opened and the shaped piece was removed. The piece was a circular disk having a diameter of 135 mm and a thickness of 3 mm. It had on its surface a scratch resistant coating 5 microns thick. The scratch resistance of the coating was tested using the abrasive wheel method according to DIN 52347E. The increase in scattered light reflected off the surface scratches was measured. The increase in the scattered light after surface scratching amounted to 6.7 percent. On a comparison body not provided with a scratch-resistant layer, an increase in scattered light of 43.7 percent was determined using the same test.

For determining the adhesion of the scratch resistant layer, the cross-cut test according to DIN 53151 was performed. The test showed no loss of the coating.

EXAMPLE 2

5.54 g of a monomer mixture containing 21 percent by weight of pentaerythritol tetraacrylate and 79 percent by weight of trimethylolpropane triacrylate (normal boiling point 315.degree. C.) were combined with 2 percent by weight of initiator solution. The initiator solution comprised 6.6 g of cyclohexanone (normal boiling point 156.degree. C.) and 16 percent by weight of bis-4-tert.-butylcyclohexyl peroxydicarbonate. Further treatment followed as in Example 1.

The scratch resistance of the shaped piece obtained was tested by rubbing with steel wool. The surface proved to be substantially more scratch resistant than the surface of a comparison body not provided with a scratch resistant layer. No loosening was shown on the cross-cut test.

EXAMPLE 3

8 g of a monomer mixture containing 56 percent by weight of pentaerythritol tetraacrylate and 44 percent by weight of hexanediol-1,6-diacrylate were combined with 16 percent by weight of initiator solution. The initiator solution contained 9 percent by weight of the initiator used in Examples 1 and 2 in 23.4 g of cyclohexanone. The spray solution was applied to the mold surface as in Example 1. The mold had a temperature of 100.degree. C. Four seconds after spraying, a polycarbonate molding compound (commercially available as "MACROLON 2800") having a melt temperature of 280.degree. C. was injected. After a period of 50 seconds from the application of the coating, the injection mold was opened and the shaped body removed. It had a scratch resistant coating 3.5 microns thick. Using the steel wool test, the surface proved to be substantially more resistant to scratching, than that of an uncoated comparison body made of the polycarbonate molding compound. No loosening was shown in the cross-cut test.

EXAMPLE 4

A scratch resistant layer as in Example 1 was sprayed onto one-half of a injection mold heated to 85.degree. C. The other half of the mold had a temperature of 60.degree. C. and was uncoated. After coating, the mold was closed and a polystyrene molding compound (commercially available as "PS 180 N") was injected at a temperature of 210.degree. C. Ninety seconds after the application of the coating, the mold was removed. The scratch resistant layer on the shaped piece, on rubbing with steel wool, showed a significantly higher scratch resistance in comparison with an uncoated surface. There was no loosening of the scratch-resistant layer using the cross-cut test.

EXAMPLE 5

One-half of the mold of an injection mold tool was heated to 120.degree. C. and sprayed with a scratch resistant coating as in Example 1. Thereafter, the mold was closed and a duroplastic molding compound was injected. The molding compound comprised 95 percent by weight of an aliphatic diisocyanate, 4 percent by weight of an alkoxy ester, and 1 percent by weight of a catalyst. At a mold temperature of 130.degree. C., the molding compound was hardened within 60 seconds to form a circular disk 123 mm in diameter and 4 mm thick. The disk had thereon a 6 micron thick scratch resistant coating. The scratch resistance was investigated with the abrasive wheel method of DIN 52347E. The increase in scattered light from the surface coated with the scratch resistant coating was 7.5 percent, but was 34.5 percent from an unprotected surface. No loosening was shown in the cross-out test.

Claims

1. A method for making a molded synthetic resin article having a scratch resistant coating thereon, which method comprises the following steps:

(a) heating an injection mold;

(b) applying a polymerizable material suitable for formation of the scratch resistant coating to the forming surface of said injection mold;

(c) partially polymerizing the polymerizable coating material;

(d) injecting a molten thermoplastic polymeric molding composition into the mold when it is closed and while said polymerizable coating material is still only partially polymerized;

(e) removing the molded article from the mold after the coating and the molding composition have hardened;

said mold being heated in step (a) to a temperature range wherein the partial polymerization of step (c) proceeds rapidly, but below the temperature of the molten thermoplastic polymeric molding composition of step (d); the partial polymerization of step (c) proceeding, prior to step (d), for a time sufficient to prevent displacement of the coating material on the mold and to bond the coating material to the thermoplastic polymeric molding composition when injected, said molding composition having a temperature sufficient to cause completion of the polymerization of the coating material; said coating material being a liquid mixture of at least one monomer capable of free radical polymerization and having more than one polymerizable double bond and of at least one initiator forming free radicals at a temperature between the temperature of the injection mold and the temperature of the molten thermoplastic polymeric molding composition.

2. A method as in claim 1 wherein the material for formation of the scratch resistant coating is applied to the forming surface of the mold when said surface is at a temperature between 40.degree. C. and 150.degree. C.

3. A method as in claim 1 wherein said material contains a volatile organic solvent.

4. A method as in claim 3 wherein said solvent has a boiling point below the boiling point of the monomer or monomer mixture.

5. A method as in claim 4 wherein said solvent is free of carbon-to-carbon double bonds susceptible of free radical polymerization.

6. A method as in claim 3 wherein the material containing a solvent is sprayed onto the forming surface of the open injection mold and the latter is closed after the solvent has substantially volatilized, whereupon the thermoplastic molding composition is injected.

7. A method as in claim 1 wherein a compound having more than one acryloyl group is used as said monomer having more than one polymerizable double bond.

8. A method as in claim 7 wherein said material contains at least 70 mole percent of the polymerizable double bonds in acryloyl groups.

9. A method as in claim 7 wherein at least a portion of the acrylic compound used contains three acryloyl groups.

10. A method as in claim 1 wherein polymerizable monomers having a boiling point above 140.degree. C. are used.

11. A method as in claim 1 wherein a free radical forming initiator having a half-life of less than two minutes at 100.degree. C. is used.

12. A method as in claim 10 wherein said initiator is an aliphatic peroxydicarbonate.

13. A method as in claim 1 wherein said thermoplastic molding composition is injected into the closed injection mold at a temperature between 230.degree. C. and 350.degree. C.

14. A method as in claim 1 wherein the material susceptible of free radical polymerization applied to the injection mold is exposed to the action of atmospheric oxygen during its partial polymerization.

15. A method as in claim 1 wherein said thermoplastic molding composition is injected into the injection mold 1 to 20 seconds after said material comprising polymerizable monomers has been applied to its forming surface.