Interpenetrational crosslinked networks

Abstract

A method of manufacturing an interpenetrational crosslinked polymer network is disclosed in which first and second polymers are crosslinked together. The method comprises the steps of crosslinking a f

Description

Interpenetrational Crosslinked Networks. Within the broad field of polymer chemistry exist two very well-known arts which involve thermal polymerisation or crosslinking. In one particular art, developed for matters of dentistry, for casting of transparent objects, for sculpting of acrylic nails and similar, there is a technique which involves the mixing of a high molecular weight polymer powder containing a polymerisation catalyst with a solvating monomer, which in turn softens and partly dissolves the polymer powder the solvating monomer then being polymerised by virtue of the catalyst contained therein (e.g. US2,234,993 A). This art includes one form of the binder jetting technology marketed by Voxeljet and similar technologies for 3D printing. It is relevant to note that in order to achieve any thermoset element to a Voxeljet printed part, the company has had to recourse to materials which are self-reactive and / or require the presence of a solvent to achieve a thermosetting matrix. A completely separate art is the formulation of two pack crosslinking systems, for example in powder coating where reactive components and a catalyst are melted together and react to form a thermoset network. This kind of approach is also embodied within two pack photo-imageable solder mask and similar two-pack curing reactions (e.g. US 4,358,477 A, US 4,943,516 A). The limitation of this latter art is that reaction between the two components is only achieved if the reactive groups on one component are kinetically able to access the reactive groups on the other component. Thus as the molecular weight of both components increases the capability of each reactive moiety to interact with its counterpart decreases. That art is therefore limited to reactions between materials of relatively low molecular weight, and to achieve the required physical properties of the finished crosslinked matrix, a relatively high degree of crosslinking is required to generate that theoretically infinite molecular weight crosslinked network. These prior arts then are perceived as separate, and practitioners of powder / liquid technologies are resigned to the fact that their high molecular weight resins cannot be directly crosslinked with functional groups on another discrete polymer backbone. Conversely, the practitioners of the two-pack crosslinking art are resigned to the use of low molecular weight polymers and resins in order to achieve the required degree of reactivity. According to the present invention, there is a method of overcoming the limitations of each of the preceding arts and achieving a crosslinking system which is both high in molecular weight and will undergo efficient crosslinking. By formulating the liquid monomer system used for a powder liquid system to contain a monomer with the appropriate reactive group thereon, and using this liquid in conjunction with a complementary polymer powder which contains the second reactive moiety, it is possible to polymerise the liquid component to a high molecular weight polymer which interpenetrates the dissolved / softened powder component and therefore permits subsequent crosslinking. The required monomer blend still needs to fulfil the original function of dissolving / penetrating the polymer bead, and any catalysis used to encourage the crosslinking reaction must by definition be slower than the liquid polymerisation reaction at the temperature at which that polymerisation reaction is carried out. In one aspect of the invention, a copolymer of a non-reactive (meth)acrylate with a (meth)acrylate bearing an appropriate crosslinkable moiety is prepared by suspension polymerisation in a manner so as to leave adequate residual polymerisation catalyst therein. This may then be blended with a mixture of non-reactive (meth)acrylate monomer in combination with the appropriate complimentary monomer and a crosslinking catalyst. Of course the selection of the two complimentary groups is limited to those that will crosslink without the elimination of any small molecule during the reaction as this would be trapped within the crosslinking matrix. The resultant mixture may then be introduced to a casting mould, and cured under pressure in an autoclave at a relatively low temperature (e.g. 50-100C) to polymerise the monomers. Subsequently, the solid cast piece can be removed from the mould (and if necessary finished, machined, polished etc.) and then placed in a hotter oven (e.g. 120-180C) to promote the crosslinking reaction. Due to the high molecular weights of the polymers which are being crosslinked, the degree of crosslinking required to add solvent, heat, water etc. resistant to the final matrix is lower than that required for a more traditional crosslinking mixture. This lower crosslink density brings other surprising advantages. With a catalyst capable of reversibly crosslinking or exchanging crosslinks, then vitrimeric behaviour is possible whereby at a sufficiently high temperature, the thermoset matrix may become temporarily thermoplastic, thus enabling forming operations on a crosslinked system. In another aspect of the invention, the crosslinking catalyst can be a photo-activated material. Again employment of a base suspension copolymer where the crosslinkable functional group is carried pendant to an appropriate backbone and is blended with a solvating monomer mixture with the correct complimentary monomer blend is required. Once this system is polymerised, the crosslinks remain inert until the catalyst is photo-activated. In this particular aspect the polymer systems need to be sufficiently transparent for the activation of the catalyst. In a similar aspect to the above, the photocatalyst may be replaced by a latent thermally activated catalyst, provided that the activation of said catalyst does not generate outgassing volatiles. In both of the preceding aspects the invention is capable of preparing a thermoplastic material which can, on demand, be crosslinked to form a thermoset. The advantages of such a material for production of laminated composites is clear. One preferred embodiment of the invention is to use methyl methacrylate as the principal monomer for both the powder and the liquid, formulated in conjunction with the appropriate crosslinkable complimentary co-monomers. In this preferred embodiment, the transparency inherent to PMMA polymers can dominate the final properties and (with correct formulation and processing) can produce a thermoset material of remarkable transparency. Example 1. 600g of a suspension polymerised copolymer of MMA with glycidyl methacrylate, having an epoxy value of 955g / eq and a median particle size of 82.7 microns was taken and blended with 400g of a liquid comprised of 74.7% Methyl Methacrylate, 22.5% Methacrylic acid, 2.5% ethyl methacrylate and 0.3% triphenyl phosphine. The resulting mixture was stirred carefully (to prevent inclusion of air bubbles until it was beginning to thicken and was then poured into a lined mould (i.e. not metal) to produce a bar shaped casting. The mould and the mixture therein were left for a few hours at room temperature until the surface was solid to the touch and then autoclave treated at 55°C for 12 hours @ 6 Bar in an inert atmosphere. At the end of the autoclave cycle, the free-radically polymerised casting was hardened and ready for machining. The machining operation involved both lathe and mill processing. Once machining to required dimensions was complete, the casting (without any further mould) was “heat treated” in a fan assisted oven for one hour at 150°C to effect the crosslinking. The timing of the hour was from the point where the casting had achieved equilibrium with the temperature of the oven. The finished casting was cooled to below 120°C before removal from the oven. Further annealing was not necessary as the temperature of crosslinking removes the initial polymerisation stress from the acrylic and prevents the development of further stress due to differential recrystallisation. Final polishing was then carried out using a sequence of acrylic polishing agents. A finished cast and polished bar was reheated to 150°C and could be gently bent without damaging the surface, and with retention of the new shape upon cooling. This process could be reversed upon reheating. The bar was tested for solvent resistance using a range of solvent including: cyclohexane, toluene, benzyl alcohol, dichloromethane, dichloroethylene, THF, diethyl ether and ethyl acetate. No swelling or damage to the part was observed. The bar was also subjected to steam sterilisation which also had no effect. Example 2. 600g of a suspension polymerised copolymer of MMA with glycidyl methacrylate, having an epoxy value of 955g / eq and a median particle size of 82.7 microns was taken and blended with 400g of a liquid comprised of 73% Methyl Methacrylate, 24% Hydroxy ethyl methacrylate (HEMA), 2.5% Speedcure 938 (Lambson) and 0.5% isopropyl thioxanthone. The resulting mixture was processed as per Example 1 until after the machining step. At this point the casting was subjected to UV irradiation (2000mJ, UV LED — 385nm, rotating chamber) before being subjected to a post-exposure bake at 150°C for 90 minutes. The resulting bar was also found to be resistant to solvent and steam. Example 3. 600g of a suspension polymerised copolymer of MMA with HEMA, having a hydroxy value of 910g / eq and a median particle size of 87 microns was taken and blended with 400g of a liquid comprised of 73% Methyl Methacrylate, 24% Glycidyl methacrylate, 2.5% Speedcure 938 (Lambson) and 0.5% isopropyl thioxanthone. The resulting mixture was processed as per Example 2. The resulting bar was also found to be resistant to solvent and steam. Example 4. 600g of a suspension polymerised copolymer of MMA with glycidyl methacrylate, having an epoxy value of 280g / eq and a median particle size of 103.4 microns was taken and blended with 400g of a liquid comprised of 59.7% Methyl Methacrylate, 40% Methacrylic acid, and 0.3% triphenyl phosphine. The resulting mixture was processed as per Example 1. Final testing demonstrated complete resistance to all the solvents listed, but the flexibility of the finished casting at 150°C was substantially reduced.

Claims

1. A method of manufacturing a crosslinked polymer matrix, the method comprising the steps of crosslinking a first solid polymer and a second non-identical high molecular weight polymer, the first polymer being of greater than 100K Daltons molecular weight, the second polymer being formed in situ by the polymerisation of one or more monomer species capable of acting as an effective solvent / swelling agent for the first, polymer, whereby the first polymer is supplied in the form of a bead.

2. A method according to claim 1 where the first and second polymers are copolymers with complimentary reactive groups on their respective chains.

3. A method according to claim 2, wherein the chains of the first and second polymers are functionalised to a similar molar level in terms of crosslinkable groups.

4. A method according to claim 2, wherein one of the first and second polymers is present in excess.

5. A method according to any preceding claim, wherein the first polymer is formed by suspension polymerisation.

6. A method according to any preceding claim, where the rate of polymerisation of the monomers forming the second polymer is substantially faster than the crosslinking reaction at a given temperature.

7. A method according to claim 6, wherein the crosslinking reaction is latent- or photoinitiated.

8. A method according to any preceding claim, where the polymerisation step forming the second polymer is a free radical polymerisation and the crosslinking reaction is driven by a different mechanism.

9. A method according to any preceding claim, where the first polymer is a random copolymer of methyl methacrylate with one or more comonomers possessing a secondary functionality, the secondary functionality being selected from, carboxylic acid, hydroxyl, carbonate, oxirane, oxetane, aceto-acetyl, maleic, tetrahydrophthalic or cyclic ketal.

10. A method according to claim 9, wherein the secondary functionality is selected from oxirane, hydroxyl, carbonate or maleic.

11. A method according to any preceding claim, where the one or more monomer species comprises a mixture of a methyl methacrylate having a complementary functionality monomer to that in the first polymer.

12. A method according to claims 5 - 7, where polymerisation to form the second polymer is catalysed by a catalyst either incorporated within the solid polymer bead, or is deposited on the surface of the bead.

13. A method according to claim 12, wherein the polymerisation catalyst is incorporated within the bead.

14. A method according to any preceding claim, where the free-radical polymerisation catalyst is one which decomposes without the emission of gaseous materials.

15. A method according to claim 14, wherein the free radical polymerisation catalyst is a peroxide.

16. A method according to claim 15 wherein the free radical polymerisation catalyst is benzoyl peroxide.

17. A method according to any preceding claim, where a crosslinking catalyst suitable for catalysing the reaction of the complimentary crosslinking moieties (at a rate slower than the polymerisation of the in situ monomers) is incorporated into either the first polymer or the monomer used to form the second polymer.

18. A method according to claim 17, wherein the crosslinking catalyst is incorporated into the first polymer.

19. A method according to any preceding claim, where the polymerisation of monomer to form the second polymer is carried out in an autoclave at a temperature from 50 to 80°C.

20. A method according to claim 19, wherein the temperature is from 60 to 70°C.

21. A method according to claim 19 or claim 20, wherein polymerisation takes from 6 to 12 hours22. A method according to any preceding claim, where the polymerised interpenetrating first and second polymers are crosslinked at a temperature of 130-180°C.

23. A method according to claim 22, wherein the temperature is from 150-170°C.

24. A method according to claim 22 or claim 23, wherein crosslinking takes at least one hour.