AU2001100017A4 - Method for producing coloured polymeric articles - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION INNOVATION PATENT Application Number: Lodged: Invention Title: METHOD FOR PRODUCING COLOURED POLYMERIC ARTICLES The following statement is a full description of this invention, including the best method of performing it known to METHOD FOR PRODUCING COLOURED POLYMERIC ARTICLES Field of the invention The present invention relates to a method for producing coloured polymeric articles. In particular the invention relates to a method of producing such articles by photopolymerisation of compositions comprising zwitterion generating photochromic compounds and a suitable monomer.

Background Photochromism is a property shown by various materials whereby the absorption spectrum which they exhibit varies upon irradiation with specific wavelengths of light. In other words these materials change colour when irradiated with light of a suitable energy. This effect is produced by electronic and or structural changes which occur as a result of absorption of light at those specified wavelengths. Of particular interest in relation to the present invention are photochromic compounds which possess the ability to switch between neutral (ie. uncharged) and zwitterionic forms upon irradiation. Such zwitterion generating photochromic compounds are discussed in further detail below.

The present invention obviates the need for separate dye (or pigment) components added to plastics or polymer blends during the manufacture of coloured articles according to conventional processes. This advantage of the present invention is achieved by incorporating the polymer initiating moiety and the colour producing moiety (or pigment) into a single component.

Brief description of the invention The present invention provides a method for producing coloured polymeric articles comprising the steps of preparing a mixture of a zwitterion generating photochromic compound and a monomer and irradiating the mixture with electromagnetic radiation of a wavelength suitable to convert the photochromic compound into its zwitterionic form.

The method of the invention is particularly advantageous in that the photochromic compound can be selected to as to impart a specific colour on the end product.

Furthermore, by appropriate choice of photochromic compound the process can be operated using light in the visible spectrum rather than higher energy (lower wavelength) radiation.

The present invention further provides a photopolymerisable composition capable of polymerisation upon irradiation with electromagnetic radiation comprising a monomer and a zwitterion generating photochromic compound.

Preferably the zwitterion generating photochromic compound is a spiropyran such as 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]naphth[2,1-b][1,4][oxazine] (hereinafter "Photochrome or 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'- [3H]phenanthr[2,1 -b][1,4][oxazine] (hereinafter "Photochrome III").

Preferably the monomer is selected for the group consisting of cationically or anionically polymerisable monomers. Suitable anionically polymerisable monomers are a-cyanoacylates, such as methyl-cyanoacylate, ethylcyanoacylate, butyl-cyanoacylate and hexyl-cyanoacylate.

Detailed description of the invention The process of the present invention is predicated on the finding that the zwitterionic form of a photochromic compound can initiate polymerisation in suitable monomers. Since zwitterionic molecules, by definition, bear both positive and negative charges the term "suitable monomers" refers to monomers which are susceptible to polymerisation by either cationic or anionic initiating species.

As discussed above photochromic compounds change colour when irradiated with light of a suitable wavelength. Generally this colour change is reversible with the compounds possessing the ability to switch between two different coloured configurations or forms. For the purposes of the present invention the neutral (uncharged) form of a zwitterion producing photochromic compound is referred to as the "closed" form, while the zwitterionic form is referred to as the "open" form (see Scheme I below).

Without wishing to be limited to any specific theory the basis for the present invention is believed to be as follows. In the absence of any other interaction it is possible for a molecule to switch back and forth between open and closed forms, as depicted schematically in Scheme I below (the wavy lines represent bonds to additional groups which may be present, see for example formulae and (11) below). However, if the anionic moiety (for example) of an open form interacts with a monomer molecule the switch back to the closed form is prevented. Such an interaction has two consequences. Firstly the photochromic compound is frozen into the open form, having the characteristic open form colour. Secondly the polymerisation process is initiated. Thus the photochromic compound acts as a photoinitiator producing the polymerisation initiating species upon irradiation. It has been found that polymers formed under such circumstances exhibit the colour of the open form of the photochromic compound.

SCHEMEI

H

3 C OH 3 H 3 C CH 3 -N hv -N Shv 2 No 1 I

OH

3 OH 3 Closed form Open form The photochromic compound acts as both the initiator (of the polymerisation reaction) and the colour producing agent (or pigment) for the polymer produced.

Thus by incorporating the polymerisation initiator and the pigment into a single molecule the present invention obviates the need for separate pigment and initiator components in a method for producing coloured polymeric articles.

By modifying the structure of the photochromic compound the colour of the open form (and thus the end product) can be manipulated, and the wavelength of light to which the initiator responses can be tuned. The second of these features is particularly advantageous in that it facilitates the use of visible wavelengths in the manufacturing processes, with significant energy saving and safety benefits. The choice of photochromic compound therefore depends on the desired colour of the end product and the wavelength of the radiation to be used.

Suitable photochromic compounds include Photochrome I which changes from being transparent (in the closed form) to pink (in the open form) and Photochrome III which changes from violet (in the closed form to green (in the open form).

H

3 C CH 3

-N

I(I)

CH

3 Photochrome I

H

3 C CH 3

-N

C NO (11)

I

CH

3 Photochrome III Any wavelength may be used provided that it is of sufficient energy to transform the photochromic compound from the closed form to the open form. Wavelengths in the visible spectrum, such as light characterised by a wavelength of greater than about 420 nm, are preferred.

Any monomer which readily undergoes cationic or anionic polymerisation may be used in the practice of the present invention. The particular monomer chosen should take into account other features of the overall system such as the choice and amount of photochromic compound used, the irradiation conditions used (ie.

wavelength, intensity and duration) and any other components which may (or may not) be present. For example, for 'living' polymeric systems such as those based on the anionic polymerisation of cyanoacrylates it is known in the art that, once initiated, polymerisation will continue in the absence of any chain terminating agents until all of the monomer has been consumed. Therefore under such circumstances the production of only a small concentration of initiating species will lead to complete consumption of the monomer. This has advantages in terms of the amount of photoinitiator and energy (light) required by the process.

The coloured polymeric article can be produced in any desirable shape by using a suitable mold, provided that the mold is transparent to the wavelength of light to be employed or incorporates an aperture or window which facilitates irradiation of at least a portion of the photopolymerisable composition.

It has been found that the introduction of Photochrome I into neat ethylcyanoacrylate monomer to produce a transparent solution has no effect when the solution is keep in the dark. However, upon irradiation with light of X 420 nm for a period of tens minutes, followed by storage in the dark at 4°C, a solid polymeric form is produced. The resultant polymer exhibits the pink colour characteristic of the open form of Photochrome I. A similar effect may be observed when Photochrome III is used as the photoinitiator. In this case the colour change is from violet (closed form in solution) to green (open form in the polymer).

The dark stability of the photochromic compounds in neat monomer and the photoinitiated polymerisation of cyanoacrylate monomers upon irradiation of the mixture with light in the visible spectrum are thus illustrated. The fact that the polymers formed exhibit the absorption characteristics (or colour) of the open spiropyran forms is indicative of initiation of polymerisation by the zwitterionic forms of the photoinitiator.

The term "comprises comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more features, integers, steps, components or groups thereof.