NOVEL HETERO-DIELS-ALDER CROSS-LINKER AND USE THEREOF IN REVERSIBLY CROSSLINKED POLYMER SYSTEMS

Abstract

The invention relates to a novel hetero-Diels-Alder crosslinker, to a process for the production thereof and to the use thereof for reversibly crosslinking polymer systems.

Claims

1. A reversibly crosslinkable formulation, crosslinkable by hetero-Diels-Alder reaction, the formulation comprising a component A having at least two dienophile double bonds, and a component B having at least one diene functionality, wherein: the component A comprises at least one instance of the following structural unit (Z): ##STR00007## and R.sub.1 and R.sub.2 are independently an alkyl or alkylene radical having 1 to 20 carbon atoms, in which the alkylene radical may be bonded to further instances of the structural unit (Z).

2. The formulation according to claim 1, wherein: at least one of the components A or B comprises more than two functionalities; at least one of the components A or B is present in the form of a polymer, and the formulation is crosslinkable at room temperature, and the crosslinking is reversible to an extent of at least 50% at a higher temperature.

3. The formulation according to claim 1, wherein: the component A is a compound having a plurality of the structural unit (Z) and these are bonded to one another with alkylene groups R.sub.1 having between 1 and 5 carbon atoms; and R.sub.2 has between 2 and 10 carbon atoms.

4. The formulation according to claim 3, wherein the component A is at least one of the following compounds: ##STR00008##

5. The formulation according to claim 1, wherein the component B is a polymer.

6. The formulation according to claim 5, wherein the polymer is selected from the group consisting of polyacrylates, polymethacrylates, polystyrenes, mixed polymers made of acrylates, methacrylates and/or styrenes, polyacrylonitrile, polyethers, polyesters, polylactic acids, polyamides, polyesteramides, polyurethanes, polycarbonates, amorphous or semicrystalline poly--olefins, EPDM, EPM, hydrogenated or non-hydrogenated polybutadienes, ABS, SBR, polysiloxanes and block, comb and/or star copolymers of these polymers.

7. The formulation according to claim 1, wherein the component B is one of the following compounds: ##STR00009##

8. The formulation according to claim 1, wherein the component B is a polymer obtained by copolymerizing at least one of the following comonomers: ##STR00010## wherein R3 independently represents hydrogen or an alkyl radical having 1 to 10 carbon atoms.

9. The formulation according to claim 1, wherein the component B is a polyamide, a polyester or a polycarbonate having at least one diene functionality.

10. The formulation according to claim 1, wherein the component A has one of the structural unit (Z), and the component B has one diene group.

11. A process for reversible crosslinking, the process comprising crosslinking the formulation of claim 1 at room temperature by a hetero-Diels-Alder reaction, and at a higher temperature breaking at least 50% of the crosslinks by means a retro-hetero-Diels-Alder reaction.

12. The process according to claim 11, wherein at a temperature above 80 C. at least 90% of the formulation is soluble in a solvent suitable for the formulation before the crosslinking.

13. The process according to claim 11, wherein the crosslinking occurs within 2 min after mixing of the components A and B.

14. The process according to claim 11, wherein the crosslinking occurs within 2 min after mixing of the components A and B with a crosslinking catalyst.

15. A composition, comprising the formulation of claim 1, the composition being selected from the group consisting of adhesives, sealants, moulding materials, foams, varnishes, paints, coatings and inks.

16. A composite, comprising the formulation of claim 1, wherein the composite is adapted to function as a composite in the fields of construction, automotive and aerospace, in the energy industry and in boat- or shipbuilding.

Description

PROBLEM

[0027] The problem addressed by the present invention is that of providing a novel reversible crosslinking method employable in different applications and in a broad formulation spectrum.

[0028] The problem addressed is further that of finding crosslinker structures which have sufficient thermal stability without protecting groups to render the use of cyclopentadiene or similar structures as blocking agents unnecessary.

[0029] Furthermore, the synthesis steps and the yields achieved should be improved to provide a simple and robust method of production to ensure economic production of the crosslinking systems and the reactant costs too should be reduced compared to the systems known from the prior art.

[0030] In addition, the retro-Diels-Alder reaction should take place at temperatures that permit glass transition temperatures of the overall system of greater than 100 C. and simultaneously the processing temperatures of a system that is for example methacrylate-based need not be increased above 240 C.

[0031] Further problems not explicitly mentioned will be apparent from the entirety of the description, claims, and examples which follow.

[0032] Solution

[0033] The problems were solved by developing a novel formulation suitable for performing a reversible crosslinking mechanism which is employable for various polymers irrespective of the formulation constituents such as binders. It was found that, surprisingly, the stated problems can be solved by a novel formulation that is crosslinkable by means of a hetero-Diels-Alder reaction.

[0034] This novel, reversibly crosslinkable formulation which is crosslinkable by means of a hetero-Diels-Alder reaction comprises a component A having at least two dienophile double bonds, wherein component A comprises at least one instance of the following structural unit Z,

##STR00001##

[0035] wherein R.sub.1 is an alkyl or alkylene radical having 1 to 20 carbon atoms, wherein the alkylene radical may be bonded to further instances of the structures shown.

[0036] The formulation further comprises a component B having at least one diene functionality.

[0037] The reversible crosslinking possible with the formulation according to the invention enables a very rapid reaction even at a low first temperature and a breaking-apart of the crosslinks at higher temperatures so that thermoplastic processability is recovered and for example the originally crosslinked layers when employed in the field of individual layers pressed into laminates in composites can be easily separated from one another again or for example the crosslinked individual layers present as prepregs for example can be subjected to forming and pressed into a laminate. A particular aspect is that a plurality of cycles of a crosslinking and a breaking-apart of the crosslinks are possible with the present system.

[0038] The described crosslinker/chain extender molecules in pure form are sufficiently stable to temperature Increases not to require blocking with protecting groups.

[0039] In particular, the formulations according to the invention have the following particular advantages: [0040] no protecting groups/blocking groups are required for the reactive dienophile in the synthesis. [0041] very simple and robust synthesis with cost-effective reactants and high yield [0042] the formulation is temperature-resistant to above 200 C. even without protecting groups [0043] the retro-hetero-Diels-Alder reaction is effected at temperatures permitting melting point/glass transition temperatures of the overall system of greater than 100 C.

[0044] It is preferable when at least one of these two components A or B has more than two functionalities.

[0045] It is similarly preferable when at least one of the components A or B is present in the form of a polymer.

[0046] It is also preferable when the formulation is crosslinkable at room temperature. This crosslinking can be reversed again to an extent of at least 50% at a higher temperature.

[0047] Component A is obtainable by the following general synthetic route for example:

##STR00002##

[0048] For a longer alkylene chain (R.sub.2) it is also possible to employ other diols, for example hexanediol, in place of the ethylene glycol.

[0049] It is particularly preferable when component A is a compound having a plurality of the cited structural units Z. It is especially preferable when the radicals R.sub.1 are alkylene groups having between 1 and 5 carbon atoms by means of which the structural units Z are bonded to one another. R.sub.2 is preferably an alkyl group having 2 to 10 carbon atoms.

[0050] It is very particularly preferable when component A is the compound

##STR00003##

[0051] and/or the compound

##STR00004##

[0052] In a particular embodiment component B is a polymer. Preferred polymers are polyacrylates, polymethacrylates, polystyrenes, mixed polymers made of acrylates, methacrylates and/or styrenes, polyacrylonitrile, polyethers, polyesters, polylactic acids, polyamides, polyesteramides, polyurethanes, polycarbonates, amorphous or semicrystalline poly--olefins, EPDM, EPM, hydrogenated or non-hydrogenated polybutadienes, ABS, SBR, polysiloxanes and/or block, comb and/or star copolymers of these polymers.

[0053] In terms of component B one skilled in the art may choose suitable compounds having diene functions suitable for a hetero-Diels-Alder reaction relatively freely. The following three alternatives have proven particularly suitable:

[0054] In the first alternative component B is one of the following compounds:

##STR00005##

[0055] In a second alternative component B is a polymer obtained by copolymerization of at least one of the following comonomers:

##STR00006##

[0056] The radicals R.sub.3 may be identical or different radicals. R.sub.3 is preferably hydrogen and/or an alkyl radical having 1 to 10 carbon atoms.

[0057] These monomers may be copolymerized with (meth)acrylates and/or styrene for example.

[0058] In the third preferred embodiment component B is a polyamide, a polyester or a polycarbonate having at least one diene functionality.

[0059] In a particular embodiment of the present invention the formulation according to the invention is not crosslinked but rather a chain extension and thus a switching between two different thermoplastic states is effected. In such a formulation component A has precisely one structural unit Z and component B has precisely one diene group.

[0060] The (meth)acrylates notation as used in this text is to be understood as meaning alkyl esters of acrylic acid and/or of methacrylic acid.

[0061] In a further possible embodiment component B is a bifunctional polymer produced by means of atom transfer radical polymerization (ATRP). In this case functionalization with the diene groups is effected via a substitution of terminal halogen atoms that is polymer-analogous or performed during termination. This substitution may be effected by addition of diene-functionalized mercaptans for example.

[0062] A further aspect of the present invention is the process for reversibly crosslinking the formulations according to the invention. When performing this process a formulation composed of at least two different components A and B is crosslinked at room temperature by means of a hetero-Diels-Alder reaction. In a second process step at a higher temperature at least 50%, preferably at least 90% and particularly preferably at least 99%, of the crosslinks are broken apart again by means of a retro-hetero-Diels-Alder reaction.

[0063] When performing this second process step at least 90 wt %, preferably at least 95 wt % and particularly preferably at least 98 wt % of the formulation becomes soluble again in a solvent suitable for the formulation before the crosslinking at a temperature above 80 C. preferably within 5 min, at most within 10 min. The previous crosslinking was so extensive that during a 5-minute washing with the same solvent, not more than 5 wt %, preferably not more than 2 wt % and particularly preferably not more than 1 wt % of the formulation could be dissolved. The term formulation and all percentages associated therewith in this case relate only to components A and B. Further formulation constituents, such as may be added in a coating or adhesive composition for example are not taken into account in this consideration. In the text below, the expression formulation in the context of this specification describes exclusively the components A and B and an optional crosslinking catalyst. The expression composition by contrast comprehends not only the formulation but also additionally added components. These additional components may be additive substances selected specifically for the respective application, for example fillers, pigments, additives, compatibilizers, cobinders, plasticizers, impact modifiers, thickeners, defoamers, dispersing additives, rheology improvers, adhesion promoters, scratch-resistance additives, catalysts or stabilizers.

[0064] Similarly to the previously described formulation, in the process initially components A and B and also optional further additive substances are brought together.

[0065] At room temperature the crosslinking reaction may take place within 10 min, preferably within 5 min, particularly preferably within 2 min and very particularly preferably within one minute. To accelerate the crosslinking a crosslinking catalyst may be added after the mixing of components A and B. These crosslinking catalysts are generally strong acids such as trifluoroacetic acid or sulphuric acid or strong Lewis acids, for example boron trifluoride, zinc dichloride, titanium dichloride diisopropoxide or aluminium trichloride.

[0066] In an alternative embodiment, crosslinking may also be accelerated without a catalyst, for example by thermal means. In this case the activation temperature is below the temperature required for the retro-hetero-Diels-Alder reaction.

[0067] In a further alternative embodiment, independently of the activation of the crosslinking reaction, the formulation comprises a further catalyst which lowers the activation temperature of the retro-hetero-Diels-Alder reaction. These catalysts may be iron or an iron compound for example.

[0068] The formulations and processes of the invention may be employed in a very wide variety of fields of application. The list which follows gives examples of a number of preferred fields of application without limiting the invention in any form whatsoever in this regard. Such preferred fields of application are adhesives, sealants, moulding materials, foams, varnishes, paint, coatings, oil additivesfor example flow improversor inks.

[0069] Examples of moulding materials are for example polyester, polycarbonate, poly(meth)acrylates or polyamide where it is a retro-hetero-Diels-Alder decoupling occurring at elevated temperature combined with the polymer chain linkages reforming at lower temperatures that makes a reduced viscosity/first nascent flowability that is advantageous for plastics injection moulding possible in the first place. The polymer chain linkages which reform at lower temperatures and during cooling improve the mechanical properties of the moulding for example. The moulding compounds may generally be injection-moulding or extrusion moulding materials for example.

[0070] When using the similarly inventive formulations for chain extension said formulations thus at a temperature above the retro-hetero-Diels-Alder temperature, i.e. in the open state, make it possible, through the lowe viscosity of the melt, a) to better reproduce relatively fine structures, b) to produce parts at lower pressures and/or c) having thinner walls which, however, exhibit the properties of higher molecular weight polymers after the relinking.

[0071] When using the formulations according to the invention to achieve crosslinking it is only when said formulations are thus at a temperature above the retro-hetero-Diels-Alder temperature, i.e. in the open state, that they may be employed in the processing methods customary for thermoplastic materials of construction, for example blow moulding, injection moulding or extrusion processes, at all. The properties of the crosslinked polymers achievable after the recrosslinking show a markedly enhanced performance such as is familiar to one skilled in the art also in polymers that have been irreversibly crosslinked by radiative crosslinking.

[0072] These inks are for example compositions that are applied by thermal means and undergo crosslinking on the substrate. Using conductive oligomers or additives for generating conductivity in general affords an electrically conducting ink which may be processed by bubble jet processes for example.

[0073] Examples from the fields of application varnishes, coatings and paint are compositions which are capable of impregnating or wetting for example porous materials particularly readily in the decrosslinked state and as a result of the crosslinking reaction afford highly coherent materials.

[0074] Similar characteristics are important for adhesives which should have a high cohesion but are nevertheless intended to easily wet the surfaces of the materials to be adhesive-bonded. A further application in the field of adhesive bonding is for example a joint which is needed only temporarily and is later to be broken apart such as may occur in various production processes, for example in automotive engineering or in mechanical engineering.

[0075] Another conceivable application is the adhesive bonding of components which, viewed over the lifetime of the product as a whole, are highly likely to be replaced, and which therefore ought to be removable again as easily as possible and without residue. One example of such an application is the adhesive bonding of car windscreens.

[0076] One particular example for adhesives or sealants is use in food packagings which open or can be broken apart automatically during heating, such as in a microwave, for example.

[0077] One example of applications in the field of rapid prototyping for the crosslinking and decrosslinking materials described here can be found in the field of FDM (fused deposition modelling), SLS (selective laser sintering) or in 3D printing by ink-jet methods with low-viscosity melts.

[0078] The application of the formulations according to the invention in the field of composites is particularly preferred.

[0079] Thus the formulations according to the invention may be employed for example as a dispersion for the impregnation of fibre material, for example carbon fibres, glass fibres or polymer fibres. The fibres impregnated in this way may then in turn be used for producing prepregs by known processes.

[0080] The invention thus also relates to emulsion polymers for example which are intraparticulately crosslinked with the inventive crosslinker molecules of the formulation by means of the hetero-Diels-Alder mechanism. The crosslinked polymers can then be wholly or partly decrosslinked by thermal processing, for example in the form of a composite matrix, via a retro-hetero-Diels-Alder reaction and interparticulately recrosslinked on cooling. This provides a second route to storage-stable prepregs for composites. But it is also possible to thus realize other materials that have thermoset properties at use temperature but thermoplastic processing properties at a higher temperature.

[0081] Use as a matrix material for endless-fibre-reinforced plastics thus affords semi-finished composites having improved processing properties compared to the prior art, which are usable for the production of high-performance composites for a wide variety of different applications in the fields of construction, automotive and aerospace, in the energy industry (for example in wind power plants) and in boat- and shipbuilding. The reactive compositions usable according to the invention are eco-friendly, inexpensive, have good mechanical properties, are simple to process and are characterized by good weather resistance and also by a balanced ratio between hardness and flexibility. In the context of this invention, the term semi-finished composite is used synonymously with the terms prepreg and organic sheet. A prepreg is generally a precursor of thermoset composite components. An organic sheet is normally a corresponding precursor of thermoplastic composite components.