GB2265374A

GB2265374A - Improved curable resin systems

Info

Publication number: GB2265374A
Application number: GB9304274A
Authority: GB
Inventors: Achim Hansen; Peter Adolphs; Stephan Schroeter
Original assignee: Ruetgerswerke AG
Current assignee: Ruetgers Germany GmbH
Priority date: 1992-03-03
Filing date: 1993-03-03
Publication date: 1993-09-29
Anticipated expiration: 2013-03-03
Also published as: GB9304274D0; ITRM930048A1; GB2265374B; FR2688794A1; ITRM930048A0; IT1261171B

Abstract

A particulate resin mixture for the manufacture of grinding wheels, clutch and brake linings, moulding compositions, as well as for use as reactive hot-melting adhesive for connection points which are subjected to high thermal and mechanical stress, comprise a particulate resin wherein at least a proportion of the particles comprise a cure accelerator for the system. The resin may be a phenolic novolac, especially an epoxy phenolic novolac, and the accelerator may be an imidazole.

Description

IMPROVED CURABLE RESIN SYSTEMS The present invention relates to resin preparations useful in such applications as the manufacture of grinding wheels, clutch and brake linings, moulding compositions and reactive hot-melting adhesive for connection points subject to high thermal and mechanical stress, the resin preparations comprising a mixture of a phenolic resin and an epoxy resin.

Given that products having good compressive strength, shear strength, heat stability and chemical resistance can be produced from phenolic resins, such resins have become well established for numerous applications over the course of time.

Phenolic resin systems frequently used in the production of highly stressed articles include novolacs and resols, or mixtures thereof, with hexamethylenetetramine or p-formaldehyde as a cure accelerator.

High-grade moulding compositions which yield products of high strength, good bondability, resistance to oxidation and excellent dielectric properties can be obtained using such systems.

However, since these resin mixtures can be moulded and hardened only at elevated temperatures, generally in the region of 120 to 2000C, problems arise in the form of, inter alia, odour-intensive dissociation by-products, such as released amines and ammonia, and water produced during the condensation reaction. These problems have repercussions both on the process and on product quality. Not least of the problems is that odiferous amines, ammonia, formaldehyde and other dissociation products require elaborate ventilation and vent air purification.

Gaseous dissociation products can cause bubbles and inclusions to form in the resin as it hardens, as a result of which product quality can be markedly diminished. Condensation water liberated during the manufacturing process and remaining in the moulding after the pressing operation is a particular impediment to the work cycle, and requires additional ventilation as well as the application of a temperature closely matched to the corresponding ventilation cycles. These complicated ventilation cycles have to be adhered to in order to remove dissociation products during the manufacture of mouldings, especially thick-walled products, and such cycles are both costly to implement and time-consuming.

It is known that odour-intensive emissions and the formation of condensation water during the preparation of high-grade products having especially high thermal, mechanical and chemical resistance and strength can be avoided, if pulverised phenolic resins and pulverised epoxy resins are hardened in the presence of an imidazole at anelevated temperature.

However, end product quality diminishes if the resin mixtures are subjected to prolonged storage before processing; after only 1 week, drastic changes in the characteristic values, such as flow and gelation time, take place. The resulting hardened material then has substantially reduced hardness and a relatively low tensile strength. Investigations have shown that these changes are attributable to a reaction which takes place between the resin types used, even at storage temperatures.

It has surprisingly been found that high strengths and undiminished degrees of hardness, even after prolonged storage, are obtainable if the cure accelerator is first mixed with the phenol resin and this mixture is mixed with pulverised epoxy resin, particularly where imidazole is used as cure accelerator and is first mixed with molten phenolic novolac and, after cooling and grinding, is mixed with the pulverised epoxy resin.

Thus, in a first aspect, the present invention provides a particulate resin for use as part of a curable phenol/epoxy resin system, wherein at least a proportion of the particles comprise a cure accelerator for the system.

It is particularly preferred that the particulate resin is in powder, or pulver, form. The terms 'powder' and 'pulver', and related terms, are used interchangeably herein.

The term 'curable phenol/epoxy resin system' is used herein with regard to a system of a phenolic resin and an epoxy resin which, when blended in the presence of a cure accelerator, generally at an elevated temperature to melt the mix, is curable to yield a hardened product, such as might be used in the manufacture of grinding wheels, clutch and brake linings, moulding compositions, or as reactive hot-melting adhesive for connection points which are subjected to high thermal and mechanical stress.

The preferred phenolic resin is a phenolic novolac, while the preferred epoxy resin is an epoxy phenolic novolac.

While the invention envisages incorporating the cure accelerator in either of the resins, the conditions employed to achieve incorporation may lead to undesired side-effects if the epoxy resin is selected as the carrier, such as congealing. Accordingly, it is very much preferred for the novolac to be used as the carrier.

Any suitable method may be used to incorporate the cure accelerator in the resin (referred to, for convenience, as novolac hereon in), but the most convenient is to melt the novolac and to blend in the accelerator. The mix, which will often be a solution of the accelerator in the novolac, is then cooled, typically to form a solid solution, and ground to an appropriate size.

Appropriate sizes will be readily apparent to those skilled in the art, and may range in size from granules down to fine dust. For most purposes, pulverised resin is most useful, as it provides ready mixability, for example.

The cure accelerator is preferably an imidazole, as exemplified hereinbelow, and may be solid or liquid at room temperature.

The cure accelerator may be evenly distributed throughout all of the granules of the resin, or it may be incorporated in a portion of the granules. This latter approach can be useful with accelerators which are liquid at room temperature to avoid excess contact between the ingredients of the system in advance of requirements.

The only requirement of the resin incorporatin the accelerator is that the combination of resin and accelerator be solid at the storage temperature.

Resin not containing accelerator may optionally be fluid at storage temperatures. If the systems are stored in the ready-mixed form, then this may result in the system being in liquid form for storage.

It is a particular advantage of the present invention that the system can be stored in a ready-mixed condition without loss of flowability. Surprisingly, the characteristic values of the binder thus obtained change very little, if at all, even during storage of 3 months or more.

Thus, the present invention further provides a phenol/epoxy resin system comprising a particulate resin as defined herein.

If desired, the systems of the invention may comprise more than one resin incorporating accelerator, although there is not generally any advantage in so doing.

The present invention is advantageous in that it provides a resin mixture having a low content of monomers and low-molecular compounds which is useful for the manufacture of grinding wheels, clutch and brake linings, moulding compositions, or indeed for use as reactive hot-melting adhesive for connection points subjected to high thermal and mechanical stress.

Thus, systems of the present invention are useful to manufacture products having comparable properties to those of the art using phenolic resins used; are simply prepared; are stable in storage; and, during hardening, neither release undesirable dissociation products nor tend to form condensation water.

In an alternative aspect, the present invention provides resin mixtures, characterised in that a pulverised "solid solution" comprising a cure accelerator and a phenol-formaldehyde novolac is mixed with an epoxy resin.

Suitable novolacs for use in the present invention are those acid-condensed phenol-formaldehyde novolacs having the general formula

wherein n is a number between 2 and 15, inclusive, R', R", R, R2, R3 and R4 may each be the same or different, and R' = H, OH, C19 -alkyl, p-phenyl, R" = H, OH, C1 g-alkyl, R1 = H, CH3-, C2H5 -, isopropyl, n-propyl R2= H, CH3-, C2H5 -, isopropyl, n-propyl, R = H, CH3-, C2H5-, isopropyl, n-propyl, and R = H, CH3-, C2H5-, isopropyl, n-propyl.

Starting compounds for the preparation of the phenolic novolacs include, for example: phenols; o-, mand p-cresols; longer-chain alkylphenols; resorcinol; and nonylphenols, as well as p-phenylphenol or bisphenols, such as bisphenol-A, bisphenol-F or bisphenol-S. These starting compounds are condensed in an acid medium with formaldehyde, or with a compound which generates formaldehyde under condensation conditions, the molar ratio of phenol : formaldehyde being within the range 1 : 0.6 to 1 : 0.9.

Another acceptable process for the preparation of resins of the present invention involves heat-curing resins obtained by polycondensing phenols with aralkyl halides or aralkyl ethers, such as with 3,5-dimethoxy-p-xylene.

Thus, within the broadest sense, all novolacs which carry free, cross-linkable phenolic OH groups and can be ground at room temperature are utilisable. Those phenolic novolacs which melt at temperatures of approximately 70 to 1150C are preferably used.

When molten, these resins may be mixed with a liquid or solid cure accelerator, so that the latter is encapsulated in the resin to a greater or lesser extent after cooling and grinding.

Suitable substances which may be used as cure accelerators include pure imidazole and substituted derivatives thereof, such as: 1-methylimidazole, 2-methylimidazole, 4-methylimidazole, 2-ethyl-4-methylimidazole, 2-isopropylimidazole, 1-vinylimidazole, 1- (3-aminopropyl) imidazole, 4-nitroimidazole, 2-methyl-4-nitroimidazole, 2-isopropyl-4-nitroimidazole, 1-aminoethyl-2-methyl- imidazole, 2-methyl-4-ethylimidazole and 4-methyl-2 phenylimidazole.

Imidazoles which are solid at room temperature are preferred.

As a result of mixing the molten phenolic novolac with the cure accelerator, a "solid solution" is generally obtained on cooling, which may be ground and mixed with a liquid or pulverulent epoxy resin and stored for a protracted period, without any appreciable change in the mixture.

Epoxy resins suitable for use in accordance with the present invention are especially the so-called "epoxidised" novolacs, which may be obtained by simple reaction of the aforementioned phenol-formaldehyde novolacs with epichlorhydrin, under reaction conditions identical to those for the preparation of bisphenol-Adiglycidyl ethers. The molar quantity of epichlorhydrin to be reacted is dependent on the number of free phenolic OH groups of the novolac to be modified. In the ideal case all phenolic hydroxyl groups are epoxidised by the end of the reaction.The epoxidised novolacs obtained in this manner may be represented by the following general formula:

wherein n is a number between 2 and 15, inclusive, R', R", R1, R2, R3 and R4 may each be the same or different, and R' = H, OH, C~g-alkyl, p-phenyl, R = H, OH, C1 -alkyl, 1-9 R1 = H, CH3-, C2H5 -, isopropyl, n-propyl, R2 = H, CH3 -, C2H5-, isopropyl, n-propyl, R3 = H, CH3-, C2H5-, isopropyl, n-propyl, and R4= H, CH3-, C2H5-, isopropyl, n-propyl.

Depending on the average molecular weight of the phenol-formaldehyde novolacs utilised, epoxidisation yields resins which have a liquid, semi-solid or solid consistency, all of which may be used, irrespective of their appearance, for the preparation of resin mixtures according to the present invention.

Solid or crystalline epoxy resins will, in general, be finely ground and intensively mixed with the pulverised "solid solution" of phenol-formaldehyde novolac and cure accelerator. Semi-solid epoxidised novolacs may be heated to a temperature at which they are homogeneous and liquid, and mixed with the pulverised "solid solution" at a temperature whereat the "solid solution" remains solid. After cooling of the mixture, either a semi-solid paste or a pourable powder is obtained. Liquid epoxidised novolacs may be mixed with the pulverised "solid solution" at room temperature.

Resins which can be ground, obtained by melting, homogenisation and subsequent cooling, are also useful.

These are ground together with the "solid solution".

In general, the epoxidised novolac is mixed with the "solid solution" as far as possible in a stoichiometric ratio 1 : 1, based on the epoxy groups contained in the resin and the free phenolic hydroxyl groups contained in the phenol-formaldehyde novolac, to prepare the resin mixtures according to the present invention.

The ratio of the epoxy groups to hydroxyl groups may be varied according to the properties of the hardened resin desired, so that the properties of the epoxy resins are emphasised in the hardened resin if an epoxy group excess is used, or the properties of the phenolic novolacs, if a phenolic hydroxyl group excess is used.

Depending on the purpose of use, good properties are obtained if the novolac is mixed with the epoxidised novolac in a ratio of hydroxyl groups to epoxy groups of anywhere from about 1 : 2.5 to about 2.5 : 1.

The cure accelerator can be either uniformly melted in all the phenol-formaldehyde novolac which is to be utilised in a quantity of from 0.01 to 10.0 parts by weight, calculated on the total mixture, or it may alternatively be contained in only a part thereof, to prepare the whole mixtures. In the latter case, the smaller surface area considerably reduces the contact potential between the epoxy resin and the cure accelerator. In this manner the reactivity of the resins at room temperature is additionally diminished.

It has therefore been found to be advantageous to melt the cure accelerator in approximately 5 to 20 wt-W of the phenol-formaldehyde novolac to be utilised in the mixture, to grind the "solid solution thus prepared finely, and to mix the powder obtained intensively with the remaining novolac powder and the epoxidised novolac.

Depending on the starting resins and the mixing ratio of the novolac to the epoxidised novolac, the resin mixture thus obtained is usable for the manufacture of grinding wheels, moulding compositions, especially clutch and brake linings, as well as for the manufacture of adhesive compounds which are subjected to high thermal and mechanical stress.

The resin mixture may be mixed with fine-particle fillers, such as aluminium hydroxides and magnesium hydroxides, red phosphorus, dolomite, chalk, quartz flour, vitreous silica flour, talc, mica, alumina or other substances, such as fibrous substances, for example, asbestos, glass fibres, aramid fibres, carbon fibres, mineral fibres, metals, or substances such as abrasive grains or cryolite, in a quantity of from 0.5 to 95 wt-W, calculated on the total weight, and hardened, in order to manufacture moulded products.

Virtually no undesirable emissions are observed during hardening of the resin mixtures according to the present invention during the manufacture of plastics products.

When the binders according to the present invention are used, clearly improved properties, such as increased elongation at break, clearly increased impact strength and tear resistance, or increased abrasion resistance are obtained, compared to when conventional phenolic resins are used.

The following Examples serve further to elucidate the present invention, without being restrictive thereon.

EXAMPLE 1 (Comparative Example) 630 g (3.44 molar equivalents) of an epoxidised novolac (epoxy equivalent 183), prepared by reacting a phenolic novolac with epichlorhydrin, having a softening point of approximately 750C, and 370 g (3.44 molar equivalents) of a phenolic novolac prepared by reacting phenol and formaldehyde, are ground very finely with 5 g of 2 -methylimidazole.

The powder resin is then stored at room temperature. The product proves to be unstable in storage. In the following Table, and in the remainder of the Examples, the term "fresh" indicates that the product was tested just after preparation.

Storage time Fresh 1 day 2 days 7 days 4 weeks Flow 53 mm 49 mm 42 mm 24 mm no flow (acc. ISO 8619) Pourability + + + EXAMPLE 2 (Comparative Example) 665 g (3.18 molar equivalents) of an epoxidised novolac (epoxy equivalent 210), prepared by reacting an ortho-cresol novolac with epichlorhydrin, having a softening point of approximately 810C, and 335 g (3.18 molar equivalents) of a phenolic novolac, prepared by reacting phenol and formaldehyde, are ground very finely with 7.5 g of 2-phenylimidazole.

The powder resin is then stored at room temperature. The product proves to be unstable in storage.

Storage time Fresh 1 day 2 days 7 days 4 weeks Flow 41 mm 37 mm 34 mm 16 mm no flow (acc. ISO 8619) Pourability + + + EXAMPLE 3 10 g of 2-methylimidazole and 90 g of a phenolic novolac having a softening point of approximately 98"C are melted together, homogenised and cooled. 45 g (containing 0.38 molar equivalents of phenolic novolac) of this product are then ground very finely with 630 g (3.44 molar equivalents) of an epoxidised novolac (epoxy equivalent 183), prepared by reacting a phenolic novolac with epichlorhydrin, having a softening point of approximately 750C, and 325 g (3.06 molar equivalents) of a phenolic novolac (softening point approximately 980C) prepared by reacting phenol and formaldehyde.

The powder resin was then stored at room temperature. The product proved to be stable in storage.

Storage time Fresh 1 day 7 days 14 days 12 weeks Flow 56 mm 56 mm 56 mm 55 mm 54 mm (acc. ISO 8619) Pourability + + + + + EXAMPLE 4 15 g of 2-phenylimidazole and 90 g of a phenolic novolac having a softening point of approximately 980C are melted together. After cooling, the product (I) is ground finely.

665 g of an epoxidised novolac (epoxy equivalent 183), prepared by reacting an ortho-cresol novolac with epichlorhydrin, having a softening point of approximately 81"C, and 290 g of a phenolic novolac, prepared by reacting phenol and formaldehyde, are ground very finely and then mixed intensively with 52.5 g of product (I).

Storage time Fresh 1 day 7 days 14 days 8 weeks Flow 41 mm 41 mm 41 mm 41 mm 40 mm (acc. ISO 8619) Pourability + + + + + EXAMPLE 5 Example 3 was repeated with the exception that 686 g of an epoxidised bisphenol-A novolac, prepared by reacting bisphenol-A, formaldehyde and epichlorhydrin, was utilised in place of the epoxidised novolac.

The powder resin was stored at room temperature after grinding. The product proved to be stable in storage.

Storage time Fresh 1 day 7 days 14 days 12 weeks Flow 26 mm 26 mm 25 mm 25 mm 24 mm (acc. ISO 8619) Pourability + + + + + EXAMPLE 6 Example 3 was repeated. The phenolic novolac used had a softening point of 1060C.

Storage time Fresh 1 day 7 days 14 days 24 weeks Flow 39 mm 38 mm 37 mm 37 mm 34 mm (acc. ISO 8619) Pourability + + + + + EXAMPLE 7 Example 4 was repeated. The quantity of product (I) was increased from 52.5 g to 105 g.

Storage time Fresh 1 day 7 days 14 days 8 weeks Flow 25 mm 25 mm 25 mm 25 mm 23 mm (acc. ISO 8619) Pourability + + + + + EXAMPLE 8 15 g of 2-phenylimidazole and 90 g of a phenolic novolac having a softening point of approximately 980C are melted together. The product (I) is ground finely after cooling.

805 g of an epoxidised novolac, prepared by reacting an o-cresol novolac with epichlorhydrin, having a softening point of approximately 810C, and 290 g of a phenolic novolac, prepared by reacting phenol and formaldehyde, are ground very finely and then mixed intensively with 52.5 g of product (I).

EXAMPLE 9 Example 8 was repeated. The quantity of epoxidised novolac was changed from 805 g to 550 g.

EXAMPLE 10 Example 8 was repeated. 290 g of phenolic novolac were replaced by 330 g of an o-cresol novolac prepared by reacting o-cresol with formaldehyde.

EXAMPLE 11 Example 3 was repeated. 1-methylimidazole was used in place of 2-methylimidazole.

The end product was stored at room temperature after grinding. The powder resin proved to be stable in storage.

EXAMPLE 12 Example 3 was repeated. 895 g of a commercially available so-called Advancement resin, Rütapox (m) 0194 (epoxy equivalent 900), prepared by reacting bisphenol-A with bisphenol-A-epoxy resin (epoxy equivalent 186) 'was utilised in place of the epoxidised novolac. The quantity of phenolic novolac was changed from 325 g to 105 g.

Storage time Fresh 1 day 7 days 14 days 12 weeks Flow 96 mm 96 mm 95 mm 93 mm 91 mm (acc. ISO 8619) Pourability + + + + +

Claims

Claims 1. A particulate resin for use as part of a curable phenol/epoxy resin system, wherein at least a proportion of the particles comprise a cure accelerator for the system.
2. A resin according to claim 1, wherein the particulate resin is in pulver form.
3. A resin according to claim 1 or 2, wherein the particulate resin is a phenolic resin.
4. A resin according to according to any preceding claim, wherein the phenolic resin is a phenolic novolac.
5. A resin according to according to any preceding claim, wherein the epoxy resin is an epoxy phenolic novolac.
6. A resin according to any preceding claim, wherein the cure accelerator is incorporated into the resin by melting the resin and blending in the accelerator, followed by cooling and grinding to an appropriate size.
7. A resin according to any preceding claim, wherein the cure accelerator is an imidazole.
8. A phenol/epoxy system comprising a phenolic resin according to any preceding claim in combination with an epoxy resin.
9. A resin mixture, characterised in that a pulverised "solid solution" comprising a cure accelerator and a phenol-formaldehyde novolac is mixed with an epoxy resin.
10. A system or mixture according to claim 9 or 10, characterised in that the cure accelerator is selected from; imidazole, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole, 2-ethyl-4-methylimidazole, 2 - isopropylimidazole, 1-vinylimidazole, 1- (3-aminopropyl) imidazole, 4-nitroimidazole, 2-methyl-4-nitroimidazole, 2-isopropyl-4-nitroimidazole, 1-aminoethyl-2-methylimidazole, 2-methyl-4-ethylimidazole and 4-methyl-2 -phenylimidazole.
11. A system or mixture according to claim 9, 10 or 11, characterised in that the phenolic resin is a novolac selected from an acid-condensed product of phenols, o-, m-, p-cresols, longer-chain alkylphenols, resorcinol, nonylphenols, p-phenylphenols, bisphenols, and formaldehyde, and compounds which dissociate formaldehyde under condensation conditions, wherein the molar ratio of phenol : formaldehyde is from 1 : 0.6 to 1 : 0.9.
12. A system or mixture according to claim 11, wherein the bisphenols are selected from bisphenol-A, bisphenol-F and bisphenol-S.
13. A system or mixture according to any of claims 9 to 12, characterised in that the epoxy resin is an epoxidised novolac, obtained by reacting epichlorhydrin with a phenol-formaldehyde novolac, and wherein the phenol-formaldehyde novolac is an acid-condensed product of phenols, o-, m-, p-cresols, longer-chain alkylphenols, resorcinol, nonylphenols, p-phenylphenols, bisphenols, and formaldehyde, and compounds which dissociate formaldehyde under condensation conditions.
14. A system or mixture according to claim 13, wherein the bisphenols are selected from bisphenol-A, bisphenol-F and bisphenol-S.
15. A system or mixture according to any of claims 9 to 14, characterised in that the resins are in a stoichiometric mixture ratio of from about 1 : 2.5 to about 2.5 : 1, calculated on the ratio of the epoxy groups and phenolic hydroxyl groups contained in the mixture.
16. A system or mixture according to claim 15, wherein the ratio is about 1 : 1.
17. A system or mixture according to any of claims 9 to 16, wherein the cure accelerator is present in the phenolic resin in a quantity of from 0.01 to 10 wt-W, calculated on the resin mixture.
18. A system or mixture according to any of claims 9 to 17, wherein the cure accelerator is present in from 5 to 20 wt-k of the phenolic resin contained in the resin mixture.
19. Use of a system or mixture according to any of claims 9 to 18, for the manufacture of grinding wheels, moulding compositions, clutch and brake linings, and of adhesive compounds which are subjected to high thermal and mechanical stress.
20. A resin mixture comprising a phenolic resin, an epoxy resin and a cure accelerator, substantially as defined herein.