AU8583582A - Molding compound - Google Patents

Description

MOLDING COMPOUND

Background of the Invention

There are a variety of commercially available solid epoxy molding compounds. The compounds have been employed for many applications including electric or electric part encapsulation. Those molding compounds generally contain catalysts to provide a rapid cure rate at molding temperature. Due to the catalysts, however, the compounds suffer a corresponding loss of shelf-life. Even under refrigeration, they may harden and so lose their usefulness before they can be used.

U. S. Patent No. 3,812,214 of Mark Markowitz describes certain hardenable epoxy resin compositions. Those compositions generally contain: a) an epoxy resin having more than one 1,2-epoxy group per molecule; b) a metal acetylacetonate having solely metal to oxygen bonds; and, if desired, c) a phenolic accelerator.

The compositions are said to possess outstanding characteristics including rapid cure rates and storage stability. The patent does not, however, describe the use of these compositions as molding compounds.

Introduction to the Invention The present invention is directed to improved epoxy molding compounds. These molding compounds comprise an admixture of: a) from about 5 to 90% by weight of epoxy resin having more than one 1,2-epoxy group per molecule; b) from 0.001 to 5% by weight of metal acetylacetonate having solely metal to oxygen bonds; c) from 0.025 to 30% by weight of phenolic accelerator; and d) from about 10 to 95% by weight of filler, based on the total weight of the composition. These compositions, which may be in either solid or liquid form, not only exhibit rapid cure rates and storage stability but also superior strength, shrinkage and allied physical properties important to their use as molding compositions.

Description of the Invention The epoxy resins employed in this invention can be any 1,2-epoxy resin having more than 1 epoxy group per molecule. They include cycloaliphatic epoxy resins such as 3 ,4-epoxycyclohexylmethyl-3(3,4-epoxy)cyclohexane carboxylate (sold under the trademarks ERL 4221 by Union Carbide Plastics Company or Araldite CY 179 by Ciba Products Company); bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate (sold under the trademarks ERL 4289 by Union Carbide Plastics Company or Araldite CY 178 by Ciba Products Company); vinylcyclohexane dioxide (EEL 4206 made by Union Carbide Plastics Company); bis (2,3-epoxycyclopentyl) ether resins (sold under the trademark ERL 4205 by Union Carbide Plastics Company; 2-(3,4-epoxy) cyclohexyl-5,5 spiro (3,4-epoxy) cyclohexane-m-dioxane (sold under the trademark ERL 4234 by Union Carbide Plastics Company or Araldite CY 175 by Ciba Products Company); glycidyl ethers of polyphenol epoxy resins, such as liquid or solid bisphenol A diglycidyl ether epoxy resins (such as those sold under trademarks as Epon 826, Epon 828, Epon 830, Epon 1001; Epon 1002,

Epon 1004, etc. by Sheel Chemical Company); phenol-formaldehyde novolac polyglycidyl ether epoxy resins (such as those sold under the trademarks DEN 431, DEN 438, and DEN 439 by Dow Chemical Company); epoxy cresol novolacs (such as those sold under the trademarks ECN 1235, ECN 1273, ECN 1280 and ECN 1299 by Ciba Products Company); resorcinol glycidyl ether (such as ERE 1359 made by Ciba Products Company); tetraglycidoxy tetraphenylethane (Epon 1031 made by Shell Chemical Company); glycidyl ester epoxy resins such as diglycidyl phthalate (ED-5661 by Celanese Resins Company); diglycidyl tetrahydrophtha late (Aradite CY 182 by Ciba Products Company) and diglycidyl hexahydrophthalate (Araldite CY 183 made by Ciba Products Comapny or ED-5662 made by Celanese Resins Company) and flame retardant epoxy resins such as halogen containing bisphenol A diglycidyl ether epoxy resins (e.g. DER 542 and DER 511 which have bromine contents of 44-48 and 18 - 20% respectively, and are made by Dow Chemical Company).

The foregoing epoxy resins are well known in the art and are set forth for example, in many patents including U.S. Patent Nos . 2 , 324 , 483 ; 2 , 444 , 333 ; 2 , 494 ,295 and 2 ,511 , 913. Moreover, it often is advantageous to employ mixtures of these epoxy resins, e.g., a glycidyl ether epoxy resin such as Epon 828 with a cycloaliphatic epoxy resin such as ERL 4221 to control the cure rate of the thermosetting resin.

The present compositions ordinarily contain from about 5 to 90%, more preferably from about 5 to 50%, of epoxy resin by total weight. This amount generally provided optimum properties for the present molding compounds.

The metal acetylacetonates of the present invention can be characterized by the following structural formula:

wherein M is a metal ion and n is 1 to 4 corresponding to the valence number of the metal ion. They are characterized by the presence of solely metal to oxygen bonds. Included within the scope of the invention are metal acetylacetonates in which one or more hydrogen atoms of the methyl or methylene groups are substituted by a halogen atom or by an alkyl, aryl, or an alkaryl substituent. An example of a halogen-substituted metal acetylacetonate is a metal hexafluoroacetylacetonate or trifluoracetylacetonate. An example of an alkyl-substituted acetylacetonate is dipIvaloylethane in which the three hydrogen atoms on each of the methyl groups are substituted with a methyl group. The acetylacetonate hardeners of the present invention should not be confused with similar compositions containing a labile halogen atom. In the present compositions, the halogens, if present, are attached directly to a carbon atom of the methylene or methyl groups and are therefore extremely stable. Labile halogen atoms in epoxy resin curing agents normally form halogen acids, and the presence of such an ionic constituent in the cured resin would raise many problems, including poor electrical properties and corrosive action on metals. Metal acetylacetonates in which the metal is aluminum, titanium, zinc or zirconium are a particularly preferred class of metal acetylacetonates within the scope of the invention. However, essentially any metallic acetylacetonate may be used, including those aluminum, barium, beryllium, cadmium, calcium, cerous, chromic, cobaltic, cobaltous, cupric, ferric, ferrous, gallium, hafnium, indium, lead, lithium, magnesium, manganic, manganous, molybdenum, molybdenyl, nickel, palladium, platium, potassium, rhodium, rubidium, ruthenium, sodium, strontium, thallium, thorium, titanium, tungstyl, uranyl, vanadium, vanadyl, zinc, and zirconium. Acetylacetonates of the rare earth elements, scandium, cerium, yttrium, lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium are known and can reasonably be expected to be useful in the practice of the present invention.

The metal acetylacetonates are used in small catalytic quantities of from 0.001 to 5.0 percent, based upon the weight of the epoxy resin. Optimum results have been achieved with from 0.05 to 3.0 percent. It is important to note that the acetylacetonates of the invention are catalytic hardeners, which do not in a significant way become a part of the hardened epoxy molecule as do curing agents added in much larger or near stoichiometric amounts.

Among the phenolic accelerators which can be effectively used in this invention are bisphenol A (i.e., 2,2-bis(4 hydrophenyl-propane)), pyrogallol, dihydroxydiphenyls as well as ortho-, meta- and para- hydroxybenzaldehydes, such as salcylaldehyde, catechol , resorcinol , hydroquinone , and phenol-formaldehyde and resorcinol-formaldehyde condensates. Examples of other phenolic accelerators suitably employed in this invention also include halogenated phenols such as ortho-, meta- and para chloro phenols or bromophenols and ortho-, meta-, and para nitrophenols. The phenolic accelerator may be present in concentrations ranging from 0.025 to 30 percent, with optimum cure rates being produced with phenolic accelerator concentrations between 0.5 and 10 percent by weight of the epoxy resin. As in the case of the acetylacetonate, the phenolics are added in relatively small amounts because they are accelerators or catalysts rather than curing agents of the stoichiometric type which form a significant reaction with the epoxy resin and become a significant part of the epoxy molecule.

The fillers of the present invention are generally chemically inert. They ordinarily will not react with any of the epoxy resins, metal acetylacetonate or accelerator. They instead function by stabilizing the physical properties of the molding compound, particularly during and after resin cure. Any conventional filler may be utilized in the present compositions. Representative fillers include: clays, like kaolin and calcined clays; silica, like novaculities, ground sand and amorphous glass; mica; talc; carbon black; alumina; and wollastonite. Alternatively, or in addition, a structural filler may be employed. These fillers include such fibrous materials as glass fiber, mineral wool and the like which may provide enhanced product strength. The present molding compounds should contain from about 10 to 95% filler by total weight. More preferably, from about 50 to 95% filler is utilized. This optimizes the advantages of the present invention.

In preparing the present compositions, the various ingredients may simply be admixed, usually at ambient temperatures. Solid molding compounds in particulate form are, for example, generally prepared in this manner. For liquid form compositions, however, it is preferred to divide the liquid resin Into two parts. These parts may be separately admixed with metal acetylacetonate and accelerator under elevated temperatures to facilitate dissolution. After cooling, they may then be combined. In this two-part process, filler may be added at any stage of the procedure. In utilizing the present compositions, conventional techniques may be employed. For example, a solid composition may be compression, transfer, or injection molded while a liquid one is normally injection or pultrusion molded. While under molding pressure, the composition should be heated, generally to from about

150 to 200°C. Under these conditions, curing occurs in minutes. Molded articles having virtually any configuration or size may be formed.

In preferred embodiments of the present invention, the molding compound may contain one or more auxiliary components: For example, up to about 10%, desirably from 1 to 5%, by weight of a flame retardant may be incorporated. Most commercial retardants, including antimony oxide or halogenated hydrocarbon may be utilized. Mold release agents such as wax, ordinarily in an amount of from 0.2 to 4% by weight, are also highly desirable. These and other components may simply be admixed with the composition prior to molding to obtain the benefits for which they are already known. The following examples are given by way of illustration only and are not intended as a limitation on the scope of this invention. Many variations are possible without departing from its spirit and scope. Unless otherwise specified herein, all proportions are provided on a weight basis.

EXAMPLE I Comparative sample molding compounds differing in filler composition are made and tested. In each instance, they are prepared by dissolving the accelerator in one half of the epoxy resin at 130°C under mechanical agitation; the metal acetylacetonate is dissolved in the other half at 110°C under similar agitation. The two halves are then cooled to room temperature and mixed with filler, as indicated, under vacuum in a Ross planetary mixer.

The compositions and properties, before and after molding at 175°C for 3 minutes, are shown in the table below: Cycloaliphatic Epoxy Resin (ERL 4221) 95 95 95 Phenolic Resin Accelerator 5 5 5 Aluminum Acetylacetonate 1 1 1 Silica Filler - 0 - 250 1500

Rigidity (mils at 175°C) 62 165 55

Linear Shrinkage (mils/inch) 34 32 18 Flex Strength (psi x 10³) 6.3 10 .2 11.4

Flex Modulus (psi x 10⁶) 0.25 0 .69 0 .86

Comparative Sample A is totally unsatisfactory is molding compound. In addition to the difference in properties shown, cracking and charring attributed to a higher curing exotherm are apparent. For the other two samples, the improvements in properties are dramatic and are directly related to increasing filler composition.

The respective gel times of the samples show a slight increase in speed of reaction as filler concentration increases. The differences in rigidity for the samples

(measured as the deflection induced by weight on an edge suspended disc 30 seconds after it is demolded) show the improvement in resistance to deformation upon de-molding afforded by filler. Shrinkage difference (measured against the cooled mold) indicate the increased stability and uniformity of highly filled molding compounds. Finally, both flex properties for the molded samples also increase with higher filler concentrations.

EXAMPLE II Comparative sample molding compounds are prepared and tested as described in Example I. Their compositions and results are as follows: v

Contrasting samples I and J, which, lack filler and metal acetylacetonate, with the other samples shows the advantages of the present invention. Samples D through H exhibit superior stability and curing/molded properties. EXAMPLE 3 A molding composition is made by the method of Example I consisting of 95 parts of EPON 828 epoxy resin, 5 parts of EPON 871 epoxy resin, 2 parts zirconium acetylacetonate catalyst, 7.5 parts of catechol accelerator, 125 parts crystalline silicon filler, 25 parts of OCF 405 BB 1/8 bundled glass fiber. The composition was compression molded Into excellent parts. The molded parts have a flex strength of 10,600 psi, a flex modulus of 1.2 x 10⁶ psi and a notched izod impact of 1.1 ft.1b/ in.

EXAMPLE 4 5 parts of EPON 1004 epoxy resin and 7.5 parts plastic resin are dissolved in 42.5 parts of ERL 4221 at 120°C while separately, 1 part of aluminum acetylacetonate is dissolved in 42.5 parts of ERL 4221. After cooling, these resins at mixed for minutes in a planetary mixer under vacuum with 108 grams of crystalline silica, 54 parts of milled glass and 0.33 parts of candelilla wax powder as a mold release agent. This composition is compression molded into a 1/8" X 4" disc at 350°F for 3 minutes and gives excellent parts which release from the mold readily. The parts have an average hot rigidity of 96 mils.

EXAMPLE 5 Three sample molding compounds are prepared from solid epoxy resins. In each case all ingredients are ground to a fine powder and dryblended in a paint shaker for 5 minutes. They are then melted on a two-roll mill for 2-3 minutes at 150-155°F. The melted sheets are regreund and transfer molded into 4" diameter parts at 175°C for 3 minutes. The compositions and test results are as follows :

TABLE

1

EXAMPLE 6 Sample molding composition with additive flame retardants, are prepared and tested as in Example 1. The compositions and results are as follows:

This data shows that fire retardance can be imparted without any loss of other desirable physical properties for the present molding compounds. The above mentioned patents and/or publications are incorporated herein by reference. Obviously, other modifications and variations of the present invention are possible In light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention described which are within the full intended scope of the invention as defined by the appended claims.

Claims (11)

1. CLAIMS 1. A molding compound comprising an admixture of: a) from about 5 to 90% by weight of epoxy resin having more than one 1,2-epoxy group per molecule; b) from 0.001 to 5% by weight of metal acetyl acetonate having solely metal to oxygen bonds; c) from 0.025 to 30% by weight of phenolic accelerator; and d) from about 10 to 95% by weight of filler, based on the total weight of the composition.
2. 2. The molding compound of Claim 1 wherein the filler comprises a fibrous material.
3. 3. The molding compound of Claim 1 wherein the admixture is in solid, particulate form.
4. 4. The molding compound of Claim 1 wherein the admixture is in liquid form.
5. 5. The molding compound of Claim 1 wherein the filler comprises from about 50 to 95% by total weight.
6. 6. The molding compound of Claim 5 wherein the epoxy resin comprises from about 5 to 50% by total weight.
7. 7. The molding compound of Claim 6 wherein the metal acetylacetonate comprises from 0.5 to 3% by total weight.
8. 8. The molding compound of Claim 7 wherein the phenolic accelerator comprises from 0.5 to 10% by total weight.
9. 9. The molding compound of Claim 1 wherein the metal of the acetylacetonate is selected from the group consisting of aluminum, titanium, zinc and zirconium.
10. 10. The molding compound of Claim 1 which additionally contains a fire retardant.
11. 11. The molding compound of Claim 1 which additionally contains a mold release agent.