GB2200357A - Epoxy foams - Google Patents

Abstract

Rigid foams are produced by reacting at an ambient temperature within the range 50 DEG C to 80 DEG C a mixture which includes the following constituents:- (a) a liquid epoxy compound derived from a diglycidyl ether of bisphenol A or from N-glycidylamine, the epoxy equivalent weight of said compound lying between 100 and 300; (b) a carboxylic acid anhydride curing agent selected from methyl nadic anhydride, nadic anhydride, camphoric anhydride, phthalic anhydride and polyhydrophthalic anhydrides, (c) a foaming agent, (d) a tertiary amine, and (e) an aliphatic polymercaptan hydroxy compound. The foams remain stable and rigid at temperatures from 100 DEG C to 140 DEG C, are non-friable and non-brittle, and will withstand severe conditions of compression or impact.

Description

EPOXY FOAMS This invention relates to epoxy foams and, particularly to rigid epoxy foams which retain good mechanical and electrical properties when subjected to high ambient temperatures.

A range of epoxy foams which are suitable for encapsulating fragile electronic components and may also be moulded to form rigid articles is described in our pending European Patent Application No. 85307669.3. These foams, which are based on epoxy resin formulations using amine or mercaptan curing agents in association with tertiary amine catalysts may be conveniently prepared by mixing two liquids and allowing them to react at normal room temperature. While such foams provide excellent mechanical and insulating properties when used in most environments, we have found that these properties degrade rapidly when exposed to temperatures of 700C and above, particularly when used under load bearing conditions.

Temperature resistant foams have hitherto been provided based on such foam forming systems as polyurethanes, silicones or foamed thermoplastics. Such systems generally suffer from certain disadvantages, however; polyurethanes involve the handling of highly toxic isocyanates, silicones and phenolics have comparitively low strength, while polyimides and thermoplastics generally require high processing temperatures.

Foam-forming compositions based on epoxy resins and various carboxylic acid anhydride curing agents in association with a catalyst and a foaming agent are known to provide temperature resistant foams. British Patent Specification No. GB 207111A, British Patent Specification No. 1374616, and U.S. Patent Specification 4090986, for example, disclose compositions in which the catalyst comprises a tertiary amine and foaming takes place at elevated temperature, typically in excess of 1000C.

We have prepared a range of such prior art anhydride cured foams in which various carboxylic axid anhydrides were used as curing agents in association with various tertiary amine catalyst. It was generally observed that, while such foams were rigid and temperature resistant, they were brittle and friable, with a tendency to shatter under conditions of impact or compression, thereby being unsuitable for many applications such as the encapsulation of fragile electronic devices or the provision of moulded structural components.

It is an object of the present invention to provide a rigid temperature resistant epoxy foam which does not suffer from the above mentioned disadvantages.

According to one aspect of the invention there is provided a rigid, temperature resistant foam produced by reacting at an ambient temperature within the range 500C to 800C a mixture which includes the following constituents: (a) a liquid epoxy compound derived from a diglycidyl ether of bisphenol A or from N-glycidylamine, the epoxy equivalent weight of said compound lying between 100 and 300; (b) a carboxylic acid anhydride curing agent selected from methyl nadic anhydride, nadic anhydride, camphoric anhydride, phthalic anhydride and polyhydrophthalic anhydrides, (c) a foaming agent, (d) a tertiary amine, and (e) an aliphatic polymercaptan hydroxy compound.

The mixture may optionally include a surfactant, suitably a silicone glycol co-polymer.

The foaming agent is preferably a halogenated hydrocarbon, suitably trichlorotrifluoroethane.

The tertiary amine may suitably be selected from tris (dimethylaminomethyl) phenol, hereinafter abbreviated to TDMP, benzenedimethylamine, hereinafter abbreviated to BDMA, and tetramethylphenylene di amine, hereinafter abbreviated to TMPD.

The aliphatic polymercaptan hydroxy compound suitably has a long chain structure with molecular weight of substantially 300 and preferably includes an oxypropylene linkage which serves to reduce brittleness and friability in the cured foam. A suitable such compound is obtainable from Diamond Shamrock Chemicals Company under the name Capcure 3800.

Conveniently, the mixture may be in the form of first and second liquid compositions, the first liquid composition including the epoxy compound, the acid anhydride, the foaming agent and, when present, the surfactant, and the second liquid composition including the tertiary amine and the polymercaptan hydroxy compound. Any reaction between the epoxy compound and the anhydride curing agent in the absence of a catalyst is very slow at low temperatures and the first liquid composition may be conveniently prepared in advance and stored in a refrigerated (5 to 100C) sealed container until required, the shelf life of bisphenol A derived compositions being typically three months and of N-glycidylamine compositions, which are more reactive, typically one month.The second liquid composition, which comprises a catalytic system, may also be prepared in advance and similarly stored with a shelf life of typically three months.

The two liquid compositions may readily be mixed to form a mobile liquid which is capable of reacting and foaming within the ambient temperature range 500C to 800C, typically 60 C, to yield a rigid epoxy foam which, in the case of bisphenol A derived compositions remains stable and rigid at temperatures up to 1000C and, in the case of N-glycidylamine compositions, remains stable and rigid at temperatures up to 1400C. Furthermore, foams produced according to the invention are substantially non-friable and non-brittle, and will withstand severe conditions of compression or impact without shattering.

The mobile liquid may be applied to an article required for encapsulation or, if required, may be poured into a mould to form a moulded body. The article or the mould are then warmed in an oven set to the required ambient foaming temperature, typically 60 C, for typically two hours during which time foaming and curing of the mixture takes place.

Pre-warming of the liquid compositions to typically 40 C, which is below the ambient foaming temperature, is generally advantageous to facilitate rapid blending, and when a mould of high heat capacity and high thermal conductivity material such as aluminium is used, pre-warming of the mould is advisable.

The anhydride curing agent may be provided as a liquid solution. A suitable methyl nadic anhydride solution is obtainable from Shell under the name Epikure NMA. The anhydride may alternatively be provided in powder form which may be mixed into the liquid epoxy compound. In the latter case, a storable first liquid composition would be a suspension requiring further mixing shortly before use to ensure an even dispersion.

The relative reactivities of the constituents are chosen so that a uniform foam is produced at the required ambient foaming temperature and so that the speed of reaction throughout the foam is reasonably constant, thereby avoiding the production of large voids or of charring and avoiding undesirable stress or shrinkage effects.

The ambient foaming temperature of 500C to 800C is particularly convenient in two respects: (a) It is sufficiently low for moulds of many plastics materials such as polypropylene to be used, or for the encapsulation of devices which may be damaged if exposed to higher temperatures, and (b) It is sufficiently high for the anhydride curing agent to be stored with the epoxy compound in the first liquid composition at a temperature between 50C and 100C without significant reaction taking place. It should be noted that the prior art practice of including the curing agent as a constituent of the second liquid composition with the catalytic system, as is disclosed in European Patent Application No. 85307669.3, would here be unsatisfactory since the mercaptan compound and the anhydride would react adversely during storage.

It will be appreciated that first and second liquid compositions need not necessarily be prepared in advance and, if required, all the constituents may be mixed to form a single mobile liquid at the time of foam preparation.

Foams according to the invention are substantially non-brittle and non-friable and will remain rigid on exposure to temperatures of at least 1000C. The foams preferably have the following mechanical, thermal and electrical properties: (1) A compressive strength, as defined in British Standard No.4370, at 230C of at least 400 kPa and at 700C of at least 300 kPa, and, in the case of N-glycidylamine based compositions, at 1200C of at least 300 kPa associated with a dimensional stability, measured as described in British Standard No.4370, not exceeding 0.5% dimensional change following heat treatment at 120 C for 100 hours.

(2) Density within the range 70 to 100 Kgm 3 (3) Compressive strength at 230C divided by density is at least 5.0 kPa kgè1 3 least 5.0 kPa kg-1 m .

(4) Compressive strength at 7O0C divided by density is at least 4.5 kPa kg-1 3 least 4.5 kPa kg m3.

(5) Compressive strength at 120 C divided by density in the case of N-glycidylamine based compositions, is at least 3.5 kPa kg m .

(6) Thermal conductivity not exceeding 5 x 10 Wm K (7) Dielectric constant at 10GHz not exceeding 1.3, loss tangent at 10GHz not exceeding 0.005 and an electrical resistivity exceeding 10 ohm metre.

The invention will now be described, but is in no way limited, by the following Examples: EXAMPLE 1 (a) A liquid epoxy resin based on a diglycidyl ether of bis-phenol A, commercially available from Shell under the name Epikote 828 (190 parts by weight) was blended at normal room temperature with a foaming agent comprising trichlorotrifluoroethane (28 parts by weight) , with a surfactant comprising a silicone glycol copolymer available from Dow Corning under the name DC193 (3.4 parts by weight) and a curing agent comprising a liquid solution of nadic methyl anhydride obtainable from Shell under the name Epikure NMA (134.9 parts by weight). This blend represents the "first liquid composition' as hereinbefore described and measurements of the viscosity of a number of samples of the blend at 230C lay within the range 0.8 to 5 Pas.The first liquid composition may be stored in an airtight refrigerated (5 to 1O0C) container until required for the next stage in the process with a shelf life of substantially three months.

(b) An aliphatic trimercaptan compound of molar equivalent weight 300 commercially available from Diamond Shamrock under the name Capcure 3800 (47.5 parts by weight) was blended at normal room temperature with a catylst comprising tris-(dimethylaminomethyl)-phenol (TDMP) available from Ciba-Geigy under the name Araldite HY960 (19 parts by weight), this blend representing the "second liquid composition" as hereinbefore described and measurements of the viscosity of a number of samples of this blend at 230C lay within the range 10 to 20 Pas.

The second liquid composition may be stored in an airtight container at normal room temperature with a shelf life of substantially three months.

(c) In order to prepare a foam, the first and second liquid compositions were warmed to substantially 400C and blended together using a high speed, high shear mixer, e.g.

that produced under the name Silverson Type L2R, for 45 to 60 seconds. The resultant mix, which was a mobile liquid, was poured into a polypropylene mould and warmed in an oven at substantially 60 cm for 2 hours. Foaming commenced within 30 minutes and the resultant foam was easily handleable after removal from the oven and cooling to room temperature.

Physical properties of a number of samples of the foam were as follows: Density Substantially 80 kgm-3 Compressive strength at 23 0C 425 to 525 kPa Compressive strength at 700C 350 to 430 kPa Volume Resistivity 3 to it x 1012 ohm m Thermal Conductivity 3 to 4 x 10-2 Wm1 K-1 Dielectric Constant at 10GHz 1.05 to 1.20 Loss tangent at 10GHz 0.001 to 0.002 The foam was observed to be rigid, substantially non-brittle and non-friable at temperatures up to 1000C.

EXAMPLE 2 (a) The procedure of Part (a) in Example 1 was followed except that 171 parts by weight of nadic methyl anhydride solution and 30 parts by weight of trichlorotrifluoroethane were used in the blend which resulted in the first liquid composition, the viscosity of which was 0.2 to 2.4 Pas at 23 C.

(b) The procedure of Part (b) of Example 1 was followed.

(c) The first and second liquid compositions were blended and the resultant mix foamed, as described in Part (c) of Example 1.

The foam produced had the following properties: Density Substantially 87 kgm 3 Compressive strength at 23 0C 450 to 550 kPa Compressive strength at 700C 450 to 550 kPa Volume Resistivity 1.8 to 2.4 x 1012 ohm m Thermal Conductivity 3.5 to 4.5 x 10 2-Wm 1 K Dielectric Constant at 10GHz 1.08 to 1.12 Loss tangent at 10GHz 0.0014 to 0.002 The foam was observed to be rigid, substantially non-brittle and non-friable at temperatures up to 1000C.

EXAMPLE 3 (a) An epoxy resin based on an N-glycidylamine commer ci ally available from Ciba Geigy under the name of Araldite MY720 (190 parts by weight) was blended at normal room temperature with trichlorotrifluoroethane (47 parts by weight), silicone glycol co-polymer DC193 (5.8 parts by weight) and nadic methyl anhydride solution (260 parts by weight) and the resultant blend may be stored in an air tight, refrigerated (5-100C) container until required for the next stage of the process with a shelf life of substantially one month. This blend represents the first liquid composition, and has a viscosity in the range 0.8 to 7.0 Pas at 230C.

(b) The procedure of Part (b) in Example 1 was followed.

(c) The first and second compositions were blended and the resultant mix foamed, as described in Part (c) of Example 1.

The foam produced had the following properties: Density Substantially 90 Compressive strength at 23 0C 575 to 675 kPa Compressive strength at 70 0C 500 to 600 kPa Compressive strength at 1200C 400 to 500 kPa Dimensional Stability at 1200C (100 hrs) 0.5% maximum change 12 Volume Resistivity 1.5 to 2.0 x 10 ohm m Thermal Conductivity 3.2 to 4.0 x 10 Wm K-l Dielectric Constant at 10GHz 1.05 to 1.15 Loss tangent at 1OGHz 0.001 to 0.004 The foam was observed to be rigid, substantially non-brittle and non-friable at temperatures up to 140 C.

EXAMPLES 4 to 11 These examples were derived from Epikote 828 epoxy resin and their constituents included the anhydrides, mercaptan compound and tertiary amines listed in Table I below: TABLE I Example No. Acid Anhydride Mercaptan Tertiary Compound Amine 4 Epikure NMA Capcure 3800 TDMP 5 Epikure NMA Capcure 3800 BDMA 6 Epikure NMA Capcure 3800 TMPD 7 Nadic Anhydride Capcure 3800 TDMP 8 Phthalic anhydride Capcure 3800 TDMP 9 Tetrahydrophthalic Capcure 3800 TDMP anydri de 10 Rexahydrophthalic Capcure 3800 TDMP anhydri de 11 Camphoric anhydride Capcure 3800 TDMP The acid anhydride in Examples 4,5 and 6 was in the form of a liquid solution, while in Examples 7 to 11, the acid anhydride was in the form of a fine powder.

In each case, 100 parts by weight of Epikote 828 epoxy resin were blended with 71 parts by weight of acid anhydride until an even dispersion was obtained. 4 parts by weight of DC 193 surfactant and 18 parts by weight of trichiorotrifluorethane foaming agent were stirred into the mix which was then pre-heated to 40 C. 25 parts by weight of Capcure 3800 and 10 parts by weight of tertiary amine were blended into the mix using a high shear mixer operating at 5000 r.p.m. for 45 seconds. The mix was poured into a polypropylene mould and placed in an oven at 60 C for two hours. In all cases, foaming commenced with 5 to 45 minutes and the resultant foam was easily handlable after removal from the oven and cooling to room temperature.

Foams prepared according to Examples 4 to 11 were rigid, substantially non-friable and non-brittle, and remained rigid on exposure to temperatures of 1000C.

EXAMPLES OF DENSITY AND COMPRESSIVE STRENGTH RANGES It will be appreciated by those skilled in the art that, by varying the proportion of foaming agent in a mix, foams of various densities can be produced and, in general, foams of high density will have high compressive strength and vice-versa.

While many applications, such as moulded structural components, require the compressive strength to be high, other applications such as encapsulents for electronic components may require a lower density at the expense of some reduction in compressive strength. The parameter (compressive strength/density) provides a useful guideline as to the achievable compromise between compressive strength and density for a family of epoxy foams.

A family of diglycidyl ether of bis-phenol A derived epoxy foams was prepared according to the procedure of Example 1, excepting that the proportion of foaming agent was varied from sample to sample to provide densities within the range 70 to 100 kgm 3. Compressive strengths at 230C and 70 0C and density were measured for each sample and results are summarised in Table II below.

TABLE II Density Compressive (Compressive Foam Range Temperature Strength Strength/Density) Basis kgm-3 OC Range kPa Range kPa kg1m3 70 to 75 360 to 430 5.1 to 5.7 Based on 75 to 80 410 to 475 5.5 to 5.9 Example 1, 80 to 85 23 455 to 535 5.7 to 6.3 Bis-phenol A 85 to 90 515 to 605 6.1 to 6.7 derived 90 to 95 585 to 710 6.5 to 7.4 95 to 100 680 to 850 7.2 to 8.5 70 to 75 335 to 395 4.8 to 5.3 75 to 80 380 to 435 5.1 to 5.4 80 to 85 70 415 to 490 5.2 to 5.8 85 to 90 470 to 560 5.5 to 6.2 90 to 95 540 to 620 6.0 to 6.5 95 to 100 610 to 735 6.4 to 7.3 A similar exercise was carried out in respect of N-glycidylamine derived epoxy foams, based on the procedure of Example 3 excepting that the proportion of foaming agent was varied from sample to sample, and compressive strength was additionally measured at 120"0. The results are summarised in Table III below.

TABLE III Density Compressive (Compressive Foam Range Temperature Strength Strength/Density) Basis kgm-3 oC Range kPa Range kPa kg-1m3 70 to 75 405 to 485 5.8 to 6.5 Based on 75 to 80 460 to 535 6.1 to 6.7 Example 3, 80 to 85 23 52into 595 6.5 to 7.0 N-glycidylamine 85 to 90 570 to 655 6.7 to 7.3 derived 90 to 95 635 to 700 7.1 to 7.4 95 to 100 670 to 750 7.0 to 7.5 70 to 75 345 to 415 4.9 to 5.5 75 to 80 390 to 460 5.2 to 5.7 80 to 85 70 415 to 490 5.2 to 5.8 85 to 90 490 to 560 5.8 to 6.2 90 to 95 540 to 620 6.0 to 6.5 70 to (5 260 to 320 3.7 to 4.3 75 to 80 300 to 365 4.0 to 4.6 80 to 85 120 345 to 415 4.3 to 4.9 85 to 90 395 to 490 4.6 to 5.4 90 to 95 470 to 535 5.2 to 5.6 The above Examples show many of the advantages common to epoxy foams in general, namely high structural strength, with good dimensional stability, low dielectric constant and loss tangent, very high resistivity, and low thermal conductivity.

Furthermore, unlike polyurethanes, the formulations are non-corrosive and non toxic, with low moisture absorption, and the foams may be readily softened in simple solvents such as acetone or methylene chloride to facilitate depotting of encapsulated articles. We have provided examples of non-friable and non-brittle epoxy foams which will withstand in-service temperatures varying from 1000C to 1400C and for which the ambient temperature for foaming and curing may be typically only 60 C.

Alternative embodiments of the invention will be apparent to those skilled in the art.

Claims (23)

1. A rigid, temperature resitant foam produced by reacting at an ambient temperature within the range 500C to 800C a mixture which includes the following constituents: (a) a liquid epoxy compound derived from a diglycidyl ether of bisphenol A or from N-glycidylamine, the epoxy equivalent weight of said compound lying between 100 and 300; (b) a carboxylic acid anhydride curing agent selected from methyl nadic anhydride, nadic anhydride, camphoric annydride, phthalic anhydride and polyhydrophthalic anhydrides, (c) a foaming agent, (d) a tertiary amine, and (e) an aliphatic polymercaptan hydroxy compound

2. A foam according to Claim 1 wherein said mixture includes a surfactant.

3. A foam according to Claim 2 wherein said surfactant comprises a silicone glycol polymer.

4. A foam according to any preceding claim wherein said mixture is provided by mixing first and second liquid compositions, the first liquid composition including the epoxy compound, the anhydri de, the foaming agent and, when present, the surfactant, and the second liquid composition including the tertiary amine and the polymercaptan hydroxy compound.

5. A foam according to any preceding claim wherein the polymercaptan compound has a long chain structure with a molecular weight of substantially 300.

6. A foam according to any preceding claim wherein the polymercaptan hydroxy compound includes an oxypropylene linkage.

7. A foam according to any preceding claim wherein the tertiary amine is selected from tris (dimethylaminomethyl) phenol, benzenedimethylamine and tetramethylphenylenediamine.

8. A foam according to any preceding claim wherein the anhydride is in a liquid solution.

9. A foam according to any of claims 1 to 7 wherein the anhydride is in the form of a powder.

10. A foam according to any preceding claim wherein the foaming agent comprises a halogenated hydrocarbon.

11. A foam according to Claim 10 wherein the halogenated hydrocarbon comprises tri chi orotri fluoroethane.

12. A foam according to any preceding claim which is substantially non-brittle and non-friable and remains rigid at temperatures up to 1000C.

13. A foam according to Claim 12 wherein the liquid epoxy compound is derived from N-glycidylamine, said foam remaining rigid at temperatures up to 1400C.

14. A foam according to any preceding claim having a compressive strength at 23 C of at least 400KPa and a compressive strength at 700C of at least 300KPa.

15. A foam according to Claim 13 having a compressive strength at 1200C of at least 300K Pa.

16. A foam according to Claim 15 having a dimensional stability not exceeding 0.5S dimensional change following heat treatment at 1200C for 100 hours.

17. A foam according to any preceding claim having a density within the range 70 to 100 kg m

18. A foam according to claim 17 wherein the compressive strength at 230C divided by the density is at least 5.0 kPa -1 m3.

19. A foam according to claim 18 wherein the compressive strength at 700C divided by the density is at least 4.5 kPa kg1 3

20. A foam according to claim 19 wherein the liquid epoxy compound is derived from N-glycidylamine and wherein the compressive strength at 1200C divided by the density is at least 3.5 kPa kg m3.

21. A foam according to any preceding claim having a thermal conductivity not exceeding 5 x 10 2 Wm 1 R 1.

22. A foam according to any preceding claim having a dielectric constant at 10 MGz not exceeding 1.3, a loss tangent at 10 GHz not exceeding 0.005 and an electrical resistivity exceeding 1012 ohm metre.

23. An epoxy foam substantially as described herein with reference to the Examples.