GB2204588A

GB2204588A - Moldable thermosetting composition

Info

Publication number: GB2204588A
Application number: GB08811227A
Authority: GB
Inventors: Vincent R Landi; J Mark Mersereau; Walter A Robbins
Original assignee: Rogers Corp
Current assignee: Rogers Corp
Priority date: 1987-05-14
Filing date: 1988-05-12
Publication date: 1988-11-16
Anticipated expiration: 2008-05-12
Also published as: DE3816443A1; JPS64135A; FR2615196B1; DE3816443C2; GB2204588B; JP2758899B2; GB8811227D0; JPH02145624A; FR2615196A1; JP2758907B2

Description

11 1 P1210 4 5 8 8 T I NG ON MOLDABLE, THERMOSE & COMPOS111T L 1

Backcround of the Invention This invention relates to moldable thermosetting compositions.

Thermosetting materials are materials that are initially formable but which covalently crosslink when exposed to heat or radiation, thereby forming a three-dimensional network that is infusible and insoluble. ThermoiDlastic materials, on the other had, link) when become molten and remain so (rather than cross. heated, allowing them to be processed into a variety of shapes at elevated temperatures. ThermoiDlastic elastomers are block copolymers having thermoplastic blocks that act as physical crosslinks for the material at ambient temperatures but flow when heated, thereby permitting the material to be molded as a thermoplastic. Summary of the Invention

In a first aspect, the inventign features a -ting molding composition that can be formed thermoset into a shape and then cured to a shaped thermoset article. The composition includes a polybutadiene or polyisoprene resin having a molecular weight < 5,000 and at least 50% by weight 1,2 addition; and a therr-noplastic -orner includes the elastomer. The thermoplastic elas1W following:

(a) between 10 and 100% by weight, inclusive, of a first block copolymer having the formula X (YX) (linear block copolymer) or (graft M n 1 4 TX, 3Q copolymer) where Y is a polybutadiene or pAyisoprene block having at least 50% by weight 1,2 addition, X is a 1 thermoplastic block, and m and n represent the average 1 block numbers in the copolymer, m being 0 or 1 and n being at least 1; and (b) between 90 and 0% by weight, inclusive, -he formula of a second block copolymer having t W (ZW) (linear block copolymer) or _) P q W (graft copolymer) where Z is a polyethylene or I.

ethyl ene-propyl ene copolymer block, W is a thermoplastic block, and p and a represent the average block nun.bers 4n the copolymer, p being 0 or 1 and q being at least 1.

In a second aspec:, the invention features the above-described molding COM-DOSItion with a dielectric filler (i.e., a material having a dielectric constant greater than about 1.2 at microwave frequencies) homogeneously dispersed throughout the composition to the extent that when. the composition is cured the properties of the cured article, e.g., dielectric constant and coefficient of thermal exiDansion, do not vary more than about 5 % throughout the article. In preferred embodiments, the molding composition further includes a crosslinking agent caDable of co-curing (i. e. forming covalent bonds) with the resin, thermoplastic elastomer, or both. Examples of preferred crosslinking agents include triallylcyanura-, e, dial lylphthalate, divinyl benzene, a multifunctional acrylate, or combinations of these agents. The volume % of the cross! iinking agent as a percentage of the combined volume of the resin, thermoplastic elastomer, and crosslinking agent is preferably less than or equal to 20.

In other preferred embodiments, the resin and the polybutadiene or polyisoprene block of the first block copolymer making up the thermoplastic elastomer have at least 90% by weight 1,2 addition. The volume to - 3 volume ratio of the resin to the thermoplastic elastom.er preferably is between 1:9 and 9:1, inclusive.

Preferred thermopl astic blocks f or the f irst or second block copolymer, or both, of the thermoplastic elastomer are styrene and a-methyl styrene. Particularly preferred compositions are those in which the resin is polybutadiene, the first block copolymer is styrene-butadiene-styrene triblock copolymer (m = n 1), and the second block coDolymer is styrene(ethyl ene-p-r opyl ene) -s tyrene triblock copclymer ( p = c = 1), the ethylene-propylene block being the hydrogenated form of an isoprene block.

When the molding composition includes a dielectric filler, the volume % of the filler (based 4n, upon the combined volume of res. thermoplastic elastomer, crosslinking agent Cif any) and filler) is between 5 and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium Ititanate, silica (particles and hollow spheres); corundum, wollasto.nite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba 2 Ti 9 0 20' glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, or magnesia. They may be used alone or in combination.

Useful articles are prepared from the molding compos 41. t ions of the invention by forming the compos-LICion into the desired shape (the viscosity of the composition being sufficiently low as a result of the liquid resin such that the shape is readily formed); and curing the composition to a shaped thermoset article (the thermoplastic elastomer maintaining the shape during the cure step). A curing agent (preferably a peroxide) is used to accelerate the cure.

The compositions of the invention can readily be molded into a wide variety of shaped articles having favorable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of ceramic materials, including gallium arsenide, alumina, and silica. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications, e.g., as specialized substrates for high speed digital and microwave circuits. Examples of microwave circuits include microstrip circuits, microstrip antennas, and stripline ciruits. Examples of shaped articles which may be formed include microwave lenses, microwave windows and filled wave guide cavities. The cured products are also useful as rod antennas and chip carriers.

The compositions have several processing advantages. First, they are easy to handle because the polybutadiene or polyisoprene resin maintains the composition's viscosity at a manageable level. The sizes and shapes that can be prepared are limited only by the mold used. Processing is ilso economical, especially compared with ceramic processing.

The thermoplastic elastomer portion of the composition prevents the dielectric filler from separating from the resin during processing, thereby preventing the formation of "filler-rich" and "filler-poor" regions. Thus, the thermal and dielectric properties of the cured article are substantially uniform throughout the article. The thermoplastic elastomer also reduces the tendency of the composition to crack during molding operations.

The cured articles exhibit good envronmental resistance, e.g., to water, high temperature, acid, alkali, and high pressure. Thus, the compositions are useful as encapsulating resins for articles expected to be exposed to such conditions. Furthermore, where the cured composition is to be bonded to a metal, e."., for use in a circuit board, 9 the low and isotropic coefficient of the thermal expansion of the cured thermoset materials matches that of many metals. Thus, debonding during thermal cycling due to differential thermal expansion of the metal substrate is prevented.

Other features and advantages of the invention will be apparent from the following description of the 'oreferred embodimenzs thereof, and from the claims.

Description of the Preferred Embodiments

We now describe preferred erbodiments Of the invent'-con.

Structure and Prep-aration The thermosetting molding compositions of the invention inc-lude a polybutadiene or polyisoprene resin portion (molecular weight less than about 5,000, preferably between 1,000 - 3,000) and a thermoplastic elastomer portion. The resin portion, which is a liquid at room temperature, maintains the viscosity of the compos. tion at a manageable levell during processing to fac'.!.'L-.ate han--17----g. It also crossilnks durinc cure.

Polybutadiene and polyisoprene resins having at least 90% 1,2 addition by weight are preferred because they exhibit the greatest crosslink density upon curre owing to the lar=e num-Ler of pend=-- nyl arouzs avEilab.e -,, I v for crosslink-ing. Hich crosslink densities are desirable because the products exhibit superior high temperature properties. A preferred resin is B3000 resin, a low molecular weight polybutadiene liquid resin having greater than 90 wt. % 1,2 addition. B3000 resin is commercially availlable from Nippon Soda, Ltd.

The thermoplastic elastomer portion maintains the shape into which the composition is formed during molding. It also prevents the filler from separating from the resin and redices cracking during molding. Furthermore, it participates in crosslinking during cure.

:ks described in the Surrinary of the Invention above, the thermoplastic elastomer portion includes a linear or graft-type block copolymer that preferably has a polybutadiene or polyisoprene block with at least 90% by weight 1,2 addition and a thermoplastic block that preferably is styrene or cx-methyl stlyrene. The high proportion of 1,2 addition in the polyiso-orene or polybutadiene block leads to high crosslink densities after the curing step, as is the case with the polybutadiene or polylsoprene resin described above. A preferred copolymer is a styrene-butadiene-styrene triblock copolymer, e.g., Kraton DX1300 (commercially available from Shell Chemical Corp.).

The thermoplastic elastomer may also contain a second block copolymer similar to the first except that the polybutadiene or polyisoprene block is hydrogenated, thereby forming a polyethylene block (in the case of polybutadiene) or an ethyl ene-pr opyl ene copolymer (in the case of Dolyisoprene). When used in conjunction with the first copolymer, materials with particularly low coefficients of thermal expansion can be produced. Where it is desired to use this second block copolymer, a preferred material is Kraton GX1855 (commercially available from Shell Chemical Corp.) which is believed to be a mixture of styrene-high 1,2 butadiene-styrene block copolymer and styrene-(ethylene-propylene)-styrene block copolymer.

A crosslinking agent having a functionality.1 2 is added to the thermosetting composition, to increase crosslink density upon cure. Examples of preferred crosslinking agents include triallyl cyanurate, diallyl phthalate, diviny! benzene, and multifunctional acrylate monomers (e.g., Sartomer resins available from Arco Specialty Chemicals Co.), all of which are commercially available.

Examples of preferred fillers are recited in the Summary of the Invention, above. Particularly preferred fillers are rutile titanium dioxide and amorphous silica because these fillers have a high and a low dielectric constant, respectively, thereby permitting a broad range of dielectric constants combined with a low dissip.ation factor to be achieved in the final cured r)roduct by adjusting the respective amounts of the two fillers in the composition. To improve adhesion between the fillers and resin, coupling agents, e. g., silanes, are pref erably used.

A curing agent is preferably added to the composition to accelerate the curing reaction. When the composition is heated, the curing agent decompos es to form free radicals, which then initiate crosslinking of the polymeric chains. Preferred curin4 agents are orcranic iDeroxides, e. g., Luperox, dicumyl peroxide, and t-butylperbenzoate, all of which are commercially available.

In general, the thermosetting compositions are processed as follows. First, all the ingredients (polybutadiene or polyisoprene resin, thermoplastic elastomer, fillers, coupling agent) are thoroughly mixed in conventional mixing equipment along with a peroxide curing agent. The mixing temperature is regulated to avoid substantial decomposition of the curing agent (and thus premature cure). Mixing continues until the filler is uniformly dispersed throughout the res.n.

The homogenized mixture is then removed, cooled, and ground into particles for molding. Next, the particles are poured, pressed, or injected into a mold, e.g., a compression, injecItion, or transfer mold, or an extruder, and the material is molded into the desired shat)e. The sha-oed article is then cured in a two-step cure to a crosslinked thermoset article. First, the article is cured in a conventional peroxide cure Step; typical cure temperatures are between 150 and 2000C. Next, the peroxide-cured article is subjected to a high temperature cure step to increase crosslink density. The temperature is greater than about 2500C but less than the decomposition temperature of the resin (typically about 4000C). The ar:ticle is- then removed and cooled.

The following thermosetting compositions were prepared, molded, and cured. -All amounts are given in weight percent, The cured products are hard plastics having relatively low impact strengths.

ExamiDle 1 B3000 resin 7.6 Kraton DX1300 5.1 TiO 2 (rutille) 71.2 Sio 2 (amorphous) 14.5 Keviar polyaramid fibers 1.1 LuPerox peroxide curative 0.2 t-butyl perbenzoate curative 0.1 A189 Silane coupling agent 0.3 (Union Carbide Corp.) claims.

- 9 1 ExamT)le 2 B3000 10.4 Kraton DX1300 7.3 Sio 2 (amorphous) 79,9 E glass fibers 1.5 LuiDerox 0.4 A189 Silane 0.3 A172 Silane (Union Carbide) 0.3 Example 3

B3000 10.4 Kraton GX1855 7.3 Sio 2- (amo.-phous) 81.3 Luperox 0.4 A189 Silane 0.3 A172 Silane 0.3 Other embodiments are within the following 1

Claims

1. A thermosetting molding composition capable of being formed into a shalpe and cured to a shaped ±hermoset article comprising a nolybutad-.e.ne or polyisoprene resin having a molecular weight less than 5,000 and at least 50% bv weight 1,2 addition; and a thermoplastic elastomer comprisinzI (a) between 10 and 100% by weight, inclusive, 10 of a first block copolymer having the formula X m CY.7) n or n where Y is a polybutadiene or X po'Lyis.-urene block having a: least 50% by weight 1,2 addition, X is a thermoplastic block, and m and n represent the average block numbers in said copolymer, m 15 being 0 or 1 and n being at least 1; and (b) between 90 and 0% by weight, inclusive, of a second block copolymer having the 10oimula W D (ZW) a or where Z is a polyethylene or ezhylene-proDyl.ene W&)e copolymer block, W is a thermoplastic block, and p and a 20 represent the average block numbers in said copolymer, :71 - p being 0 or 1 and q being at least 1.

2. A hcmoceneo-usly filled thermosetting mold-ing ccr-,,positJ.on capable of being formed into a shape and cured to a shaped, homogeneously filled, thermoset article comprising a polybutadiene or polyisoprene resin having a molecular weight less than 5,000 and at least 50% by weight 1,2 addition; a thermoDlastic elastomer comprising (a) between 10 and 100% by weight, inclusive, of a first block copolymer having the formula X (YX) or where Y is a polybutadiene or m n Q 1 - 11 polyisoprene block having at least 50% by weight 1,2 addition, X is a thermoplastic block, and m and n represent the average block numbers in said copolymer, m being 0 or 1 and n being at least 1, and (b) between 90 and 0% by weight, inclusive, of a second block copolymer having the formula Wp(ZW)q or f where Z is a polyethylene 0 Cr or ethylene-propylene copolymer block, W is a thermoplastic block, and p and q represent the average block numbers in said copolymer, p being 0 or 1 and q being at least 1; and a filler homogeneously dispersed throughout said composition.

3. A molding composition as claimed in claim 2 wherein said composition comprises between 5 and 80% by volume, inclusive, of said filler.

4. A molding composition as claimed in claim 2 or 3 wherein said filler comprises titanium dioxide, barium titanate, strontium titanate, silica, corundum, wollastonite, polytetrafluoroethylene, fiberglass, aramide fibers, Ba2Ti9O2Of glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, magnesia, or a mixture of two or more of these.

5. A molding composition as claimed in any one of claims 1 to 4 further comprising a crosslinking agent.

6. A molding composition as claimed in claim 5 wherein said crosslinking agent comprises triallyl cyanurate, diallyl phthalate, divinyl benzene, a multifunctional acrylate monomer, or a mixture of two or more of these.

7. A molding composition as claimed in any one of claims 1 to 8 wherein said thermoplastic block of said first or second block copolymer, or both, comprises styrene or a- methyl styrene.

8. A molding composition as claimed in any one of claims I to 7 wherein said resin is polybutadiene and said first block copolymer is styrenebutadiene-styrene triblock copolymer.

9. A molding composition as claimed in any one of claims 1 to 8 wherein said resin is polybutadiene, said first block copolymer is styrenebutadiene-styrene triblock copolymer, and said second block copolymer is styrene-ethylene-propylene-styrene triblock copolymer.

10. A molding composition as claimed in any one of claims 1 to 9 wherein said resin has at least 90% by weight 1,2 addition.

11. A molding composition as claimed in any one of claims I to 10 wherein said polybutadiene or polyisoprene block of said first block copolymer has at least 90% by weight 1,2 addition.

12. A molding composition as claimed in any one of claims 1 to 11 wherein the volume to volume ratio of said resin to said thermoplastic elastomer is between 1:9 and 9:1, inclusive.

13. A molding composition as,&laimed in any one of claims 1 to 12 wherein the volume % of said crosslinking agent as a percentag(i of the combined volume of said liquid resin, said thermoplastic elastomer, and said crosslinking agent is less than or equal to 20.

14. A molding composition as claimed in claim 1 or claim 2 substantially as described herein with reference to any one of the Examples.

15. A shaped thermoset article formed from a molding composition as claimed in any one of claims 1 to 14.

16. A forming process for a curable thermosetting molding composition comprising a polybutadiene or polyisoprene liquid c resin having a molecular weight less than 5,000 and at least 50% by weight,1,2 addition; and a thermoplastic elastomer comprising (a) between 10 and 100% by weight, inclusive, of a first block copolymer having the formula Xm(7X)n or( Y' where Y is a X Yin polybutadiene or polyisoprene block having at least 50% by weight 1,2 addition, X is a thermoplastic block, and m and n represent the average block numbers in said copolymer, m being 0 or 1 and n being at least 1; and (b) between 0 and 90% by weight, inclusive, of a second block copolymer having the formula W where Z is a polyethylene P(ZW)q or or ethylene-propylene copolymer block, W is a thermoplastic block, and p and q represent the average block numbers in said copolymer, m being 0 or 1 and n being at least 1, said process comprising the steps of 20 forming said composition into a shape, the viscosity of said composition being sufficiently low such that said shape is readily formed; and curing said composition to a shaped thermoset article, said,thermoplastic elastomer maintaining said shape during said cure step.

17. A process as claimed in claim 16 wherein said composition further comprises a dielectric filler dispersed substantially uniformly throughout said composition.

18. A shaped thermoset article prepared according to the process of claim 16 or 18.

19. An article as claimed in claim 15 or 18 wherein said article comprises a microwave lens.

20. An article as claimed in claim 15 or 18 wherein said article comprises a mi crowave window.

21. An article as claimed in claim 15 or 18 wherein said article comprises a filled wave guide cavity.

22. An article as claimed in claim 15 or 18 wherein said article comprises the substrate for a microwave circuit.

23. An article as claimed in claim 22 wherein said microwave circuit comprises a microstrip circuit.

24. An article as claimed in claim 22 wherein said microwave circuit comprises a microstrip antenna.

25. An article as claimed in claim 22 wherein said microwave circuit comprises a stripline circuit.

26. An article as claimed in claim 15 or 18 wherein said article comprises a rod antenna.

27. An article as claimed in claim 15 or 18 wherein said article comprises a substrate for a high speed digital circuit.

28. An article as claimed in claim 15 or 18 wherein said article comprises a chip carrier.

29. An article as claimed in any one of claims 15 and 18 to 28 wherein the dielectric constant of said article is substantially the same as that of gallium arsenide.

I$ Published 1988 at The Patent Office, State House, 66.71 High Holborn, London WClR 47P. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Cam 1/87.

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