CA1286466C - Method for the manufacture of a dyed, molded, plastic element - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A flat sheet of plastic is coated on at least one side with a plastic foil that is extensible when heated and contains a transfer dye, in such a manner that the foil detachably adheres to the sheet, is heated to the molding temperature of the sheet and molded, whereby dye from the foil is transferred from the foil onto the surface of the sheet. The foil can also be left on the molded element as a protective foil until further processing or until it is placed in use and can be removed according to need.

Description

TITLE OF THE INVENTION

MET~OD FOR THE MANUFACTURE OF A DYED, MOLDED, PLASTIC ELEMENT

BACKGROUND OF THE INVENTION
.. .. . .... . _ _ _ Field of the Invention The invention relates to a method for the manufacture of a dyed, molded plastic element by heating a smooth sheet of plastic, which is coated on at least one side with a detachably adhering plastic foil that is stretchable when heated to the molding temperature of the sheet, molding the sheet while simultaneously molding the foil adhering thereto, and cooling the molded plastic element to a temperature below the molding temperaturer Descriptlon of _he Related Art A related moldlng process .is taught in DE 27 18 510.
In this method a cast sheet oE polymethylmethacrylate is detachably coated with a polyethylene foil, heated to the molding temperature, molded with the Eoil adhering thereto and cooled. Thereafter, the foil can be pulled off or left in place on the molded element to protect the surface until further processing or until use.

With this method, even dyed or patterned sheets of polymethylmethacrylate can be worked. In this case, the polyethylene foil cannot be applied until after the sheet has been provided with the desired dyes or patterns. The transfer printing method according to DE-OS 32 44 355, for example, can be used for this purpose. Uncolored or unpatterned sheets of polymethylmethacrylate are provided ~ith a protective lamination immediately after manuEacture, which protective lamination is removed before the sheet is dyed or patterned dur1ng the transfer printing process or in another manner.
Removal of the protective coating prior to dyeing or patterning and re-coating with a protective lamination after this operational step is not very economical. Canadian Patent Application No. 520,135 describes transfer printing onto foil-coated plastic sheets, whereby the dye transfer takes place from a flat dye vehicle through the foil and onto the surface of the plastic sheet.
DE-A 33 24 709 discloses a method in which a thermally heated plastic sheet is simultaneously molded and dyed by trans~er printing. In this method, the sheet is molded by means o~ a tool which is overlaid with a flat dye transeer vehicle. This method does not permit the manufacture of spherically molded, dyed ~r~6~

elements, or only p-~its it with the limitation that the flat dye vehicl- ~ust first be brought into the corresponding spher_:al shape.
In spite o~ th~bove-described known methods, there remains a nee- Eor new, more economical and simpler methods for ~roducing dyed, molded plastic elements.

SU~ Y OF THE INVENTION
Accordingly, i- is an object of the present invention to provic~ an improved method of manufacturing dyed, ~olded plastic elements in which a separate operating -ep of dyeing or patterning is avoided and the ne~- for energy and related materials is reduced.
This and othe- ~bjects have been achieved by the discovery of a man ~cturing method wherein duriny the molding process a pistic sheet is employed which is coated with a detac::lbly adhering eoil containing a transEer dye, and crin~ the heating of the sheet to the molding temper--lre and/or during the molding, dye is transeerred fro~ :he foil to the surEace of the sheet. As a rule, ~c dye transeer will take place primarily during th ~eating Oe the plastic sheet to the molding temperz- re, but can also continue during the molding proceSc ~self. If the foil is applied to the plastic sheet t-:~ugh the use of heat, a portion of ~ ~J~

the dye can even be transferred then. Since there is no shifting of the foil relative to the sheet dye to adherence, the method step at which the primary quantity of the dye is transferred is not critical to the success of the process. Even the extensions that occur during molding do not lead to blurred pattern transfers, because the adhering foil is extended together with the sheet.
In accordance with the present invention, there is thus provided a method for manufacturing a dyed, molded plastic element, which comprises the steps of:
a) manufacturing a plastic sheet;
b) detachably applying to at least one surface of the plastic sheet, during or immediately after its manufacture, a plastic foil which is extensible when heated and which contains therein a transfer dye;
c) heating the plastic sheet coated with the plastic foil to the molding temperature of the sheeti and d) molding the sheet while simultaneously molding the fo:il adhering thereto to produce a coated, dyed, molded plastic element, wherein during the heating or molding, the dye is transferred from the foil to the surface of the sheet.
The method of the invention leads from an undyed, flat plastic material to a dyed, three-dimensional molded plastlc elemen-t in a single i,.~.~ ~, .

6~
- 4a -opera-tional step. According to the previously used method, the plastic sheet had to be heated both for the transfer printing method as well as for the molding; according to -the invention, it is heated only once. Furthermore, the dye vehicle in the -transfer printing step simultaneously takes over the function of the protective foil, so that auxiliary material for this purpose is unnecessary. Par-ticularly advantageous is the fact that the foil containing the transfer dye is applied elther during the manufacture of the plastic sheet or immediately thereafter, so that the protective effect of the foil is maintained from the time of manufacture of the plastic sheet until the molded and dyed plastic element is put into use, and no further auxiliary materials with planar surfaces are required~
The method of the invention is suitable for the manufacture of dyed or patterned, inscribed or otherwise decorated molded plastic elements which are manufactured by molding flat sheets. sy this method, equal-area cylindrical moldings may be performed with the same ease as spherical moldings with partial surface extension. For example, illuminatable advertising signs, traffic signs, directional indicators, device housings, fashion articles such as hair clips or clasps, packaging containers or closures, light globe halves, sportin~ articles such as surf boards or skateboards, plastic lu~gage, items oE
vehicle equipment, among others can be manufactured.
Undyed plastic sheet5 are those sheets which have not been subjected to dye transfer by the present method, even if the plastic sheets already have a different coloration or patterning. The expression "dyed"
includes any coloration produced through transfer printing, from uniform or continuous colors to single or multiple color patterns or inscriptions.
Suitable for the invention are solid plastic sheets, provided that they can be molded cylindrically or spherically when heated and that they will permanently retain the shape produced during heating ~r~

after cooling to room tempera-ture. It is preferred that -they retain their shape up to at least 50C and do not have an E-module of less than 1,500 Nmm 2. Mold-ability in the thermoelastic or thermoplastic condition should be possible at temperatures of from 50 to 250C. At the temperatures necessary for the dye transfer, the plastic generally should not be decom-posed in any appreciable manner.
As examples of suitable plastics, there may be mentioned polymethylmethacryla-te,polyvinylchloride, polystyrene, polycarbonates, polysulphone plastics, polyester, cellulose ester as well as bonded material sheets made from different layers of such plastics.
By polyester, polycarbonate and polysulfone are meant polymeric materials as described in Kirk-Othmer's Encyclopedia of Chemical Technology (1982), Volume 18, pages 479-494, 549-594, and pages 832-848, respect-ively. By cellulose ester is meant organic esters of cellulose such as those described in Kirk-Othmer's Encyclopedia of Chemical Technology (1982), Volume 5, pages 118-143. The particular plastics chosen are oE
secondary importance relative to the method oE manu-fac-ture disclosed herein. One oE ordinary skill in the art could, without undue experimentation, determine ~J3~

which plastics could be used to success~ully carry out the method of this invention.
Preferably the sheets are colored homogeneously white, because any desired coloration is possible with such sheets. By way of example, the sheets can be from 1 to 15 mm in thickness and may have any desired dimensions, from a few square centimeters to several square meters~
The plastic foil serving as the dye vehicle for the transfer dye must be capable of being molded together with the plastic sheet while at the same temperature as the plastic sheet. It does not create a problem if the foil is softer than the plastic sheet at the molding temperature, as long as it does not become detached from the sheet under the mechanical forces that occur. Preerably the polyester of thç foil has a glass temperature which is not higher than the glass temperature of the polyester sheet. Detachability ater cooling generally remains assured if the plastic of the foil has a low afEinity to that of the sheet.
This is usually the case when one of the two plastics is rich in polar groups and poor in nonpolar groups and the other plastic is more or less free of polar groups. PolyoleEins have adequate minimal afinity relative to the exemplary plastics listed above.
Examples of suitable polyolefins are polyethylene and 6~

polypropyleneO The polyolefins may be homopolymers or copolymers, and may include plasticizers such as those described in Kirk-Othmer's Encyclopedia of Chemical Technology9 Vol~ 18, pages 111-183. A preferred polyolefin ~or purposes of the present method is polyethylene. Furthermore, polyester foils are often well suited. Polyesters include polyethylene terephthalate and others well-known to those of skill in the art. Preferred pairs of plastics for the plastic sheet and the plastic foil are:

polymethylmethacrylate/polyethylene;
polycarbonate/polyethylene;
polysulfone/polypropylene; and acrylonitrile butadiene-styrol co~olymer/polyethylene.

The affinity should, if possible, be so low that after cooling, the foil can be removed with a small application of force without ripping. On the other hand, the affinity matf not be so low that inadequate adhesion is achieved. The adhesion must be great enough tha~ the foil does not shiet relative to the sheet durin~ molding. Xf necessar~f the adhesion can be increased through a corona treatment of the foil surface to be laid on the sheet. It is preferred that the foil be coated on this side with transfer printing dye.

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The particular dyes which could be used in the method of the present invention are not limited, and may be selected from any of the dyes which are known to those of ordinary skill in the art of plastic dyeing.
The following non-limiting examples may be mentioned:

Blue Dyestuffs:

- OX X = rl ' ~r C~i3 O NTI
(~ so20~

,:~

O N~T-~-OCT13 Yellow Dye.stufs~

NC ~ -N-N-C

~15~:2~ ~ N--)~C

C-CH-13- ~C2H40~

Co~C~f ~ N
N ,c--clI-cl~=c~ ~

~3 3CO -C--~--C-OC2~4--1~ _~ CH C~ CN

Orange Dyestuff:

-I~=N--~ -N-N-~ -OI~

Red Dyestuff:

O ~H

[~ ~3 \ CEI;~--( C~I2 ~
O OH

Violet Dyestuff:

~2 ~$~ C2H4--~C~3'?

1~}~2 The amount of the transfer printing dye which may be applied to the carrier foil is preferably from about 1 to about 50 g/m2, relative to the dye-carrying areas.
The dye layer is applied according to known methods of foil printing, for example throucgh offset, screen printing or by roller application. The dye, particularly the binding agent contained therein, can change the adhesive properties of the oil, particularly when the foil is entirely or largely printed upon. It is advantageous if the binding agent has a polarity as low as the foil itself. If the printing dye should result in an undesirably high 6~; `

adhesion, the unprinted side of the foil can be placed on the plastic sheet, since the dyes are capable of di~fusing throu~h the foil. Any of the dyes well known to those of skill in the art of plastic dyeing and molding may be used in the method of this invention.
The foil, which is usually 40 to 150 ~m thick, is best applied by the application of heat, for example by means of light pressure from a roller heated to between 60 and 100C. The foil can also be adhesively applied to the sheet if the sheet has an increased surface temperature of from 60 to 80C. In any case, care should be taken that as the foil is applied no bubbles are formed therein.
A foil coated with pressure sensitive glue can also be used, which can be applied to the sheet without the application o~ heat. The glue layer does not hinder dye transfer.
Although it is possible to efEect dye transfer onto the sheet immediately aEter the placement o~ the foil by additional application of heat at a temperature that is adequate for the dye transfer, for economic reasons it is preferred that no additional application of heat be employed with the goal of dye transfer after the placement of the foil but rather that the coated sheet be allowed to cool, so that only that amount of dye is transferred that results automatically frorn the conditions of the foil coating process.

For thermal molding, the coated sheet is heated again. For spherical moldings, the molding temperature must be reached through the entire thickness of the sheet at least in the area to be molded, while for cylindrical moldings it is sometimes sufficient if the molding temperature is reached at least to the center of the sheet thickness. The heating effectively takes place through heat radiation or hot air. It is generally in this stage of the method that the primary quantity of dye is transferred from the foil to the sheet. Therefore, at least the foil must attain a temperature sufficient for dye transfer, and this temperature must be maintained for the duration of the transfer process.
Many plastic sheets suited for the method of the invention are capable of being molded well in the temperature range of from 120 to 250C. The temperature range in which the plastic primarily has thermoelastic characteristics is preferred. For cast polymethylmethacrylate this temperature lies approximately between 130 and 200C., for extruded polyrnethylmethacrylate it lies ~rom 120 to 180C. In order to achieve these temperatures quickly, the surface of the sheet is generally heated to temperatures above 200C., thereby ~acilitating dye transfer very well. The largest portion of the usable transer dye material can best be transferred in the temperature range from 150 to 250C. The usual heating periods of from 1 to 15 minutes, depending on the thickness of the sheet to be printed upon, are usually sufficiently for the dye transfer.
The molding of the heated sheet while simultaneously molding the foil therewith takes place in a known manner by means of molding tools acting on one or both sides, by one-sided excess or negative pressure, by foldable clamping frames and the like.
After molding, the molded element is allowed to cool to a temperature below the softening temperature. After the molded element is unclamped, the foil is left on its surface and is not removed until the molded element is processed further or placed into use.
The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

Claims (9)

1. A method for manufacturing a dyed, molded plastic element, which comprises the steps of:
a) manufacturing a plastic sheet;
b) detachably applying to at least one surface of said plastic sheet, during or immediately after its manufacture, a plastic foil which is extensible when heated and which contains therein a transfer dye;
c) heating said plastic sheet coated with said plastic foil to the molding temperature of said sheet; and d) molding the sheet while simultaneously molding the foil adhering thereto to produce a coated, dyed, molded plastic element, wherein during said heating or molding, said dye is transferred from said foil to the surface of said sheet.

2. A method according to Claim 1, wherein said sheet is molded cylindrically or spherically during said molding.

3. A method according to Claim 1, wherein said molding is carried out at a temperature of from 50 to 250°C.

4. A method according to Claim 1, wherein said plastic sheet is colored homogeneously white.

5. A method according to Claim 1, wherein one of said plastic sheet and said plastic foil is rich in polar groups and poor in non-polar groups, and the other is substantially free of polar groups.

6. A method according to Claim 5, wherein said plastic which is substantially free of polar groups is a polyolefin.

7. A method according to Claim 6, wherein said polyolefin is polyethylene or polypropylene.

8. A method according to Claim 5, wherein said plastic which is rich in polar groups and poor in non-polar groups is polymethylmethacrylate, polycarbonate, polysulfone or acrylonitrile-butadiene-styryl copolymer.

9. A method according to Claim 1, wherein the amount of said dye in said plastic foil is from about 1 to 50 g/m2.