GB2224738A

GB2224738A - Forming process for moldable thermosetting composition

Info

Publication number: GB2224738A
Application number: GB8826649A
Authority: GB
Inventors: Vincent R Landi; J Mark Mersereau; Walter A Robbins
Original assignee: Rogers Corp
Current assignee: Rogers Corp
Priority date: 1988-11-15
Filing date: 1988-11-15
Publication date: 1990-05-16
Anticipated expiration: 2008-11-15
Also published as: FR2639048B1; FR2639048A1; GB2224738B; GB8826649D0; DE3838717A1

Description

2224738 FORMING PROCESS FOR MOLDABLE THERMOSETTING COMPOSITION is This

invention relates to moldable thermosetting compositions.

Themosetting 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. Thermoplastic materials, on the other hand, become molten and remain so (rather than crosslink) when heated, allowing them to be processed into a variety of shapes at elevated temperatures. Thermoplastic elastomers are block copolymers having thermoplastic blocks that act as physical cross links for the material at ambient temperatures but flow when heated, thereby permitting the material to be molded as a thermoplastic.

In our previous British patent application no. 8811227.1 we describe a thermosetting molding composition that can be formed 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 thermoplastic elastomer. The thermoplastic elastomer includes the following:

(a) between 10 and 100% by weight, inclusive, of a first block copolymer having the formula Xm(YX)n (linear block copolymer) or (graft copolymer) where Y is a polybutadiene or polyisoprene block having at least 50% by weight 1,2 addition, X is a thermoplastic blo,-:-k, and m and n represent the average 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, of a second block copolymer having the formula Wp(ZW)q (linear block copolymer) or:zr I k_A 1 (graft copolymer) where Z is a polyethylene or ethylene-propylene copolymer block, W is a thermoplastic block, and p and q represent the average block numbers in the copolymer, p being 0 or I and q being at least I.

A preferred process for the production of hard, molded, shaped articles for such a composition includes a high temperature cure at a temperature above about 2500C, but below the decomposition temperature of the molding composition. We have now found that this process may also be advantageously used with compositions in which the first block copolymer of the thermoplastic elastomer contains a polybutadiene or polyisoprene block having less than 50% by weight 1,2 addition.

Thus, in accordance with the present invention there is provided a forming process for producing a hard shaped molded article comprising the steps of (a) providing a moldable thermosetting composition comprising a polybutadiene or polyisoprene liquid resin having a molecular weight less than 5,000 and at least 50% by weight 1,2 addition; and a thermoplastic elastomer comprising a first block copolymer having the formula Xm(YX)n or Y where Y is a polybutadiene or polyisoprene 1 k having less than 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; (b) forming said composition into a shape, said thermoplastic elastomer enabling said composition to maintain said shape during said forming process, said liquid resin enabling the viscosity of said composition to be sufficiently low that said shape is readily formed; and (c) curing said composition to produce said article including subjecting said composition to a high temperature cure condition at a temperature greater than about 2500C and less than the decomposition temperature of said composition.

In a preferred embodiment the molding composition employed in the process of the present invention includes a dielectric filler (i.e. a material having a dielectric constant greater than 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 expansion, do not vary more than about 5% throughout the article.

In other preferred embodiments, the molding composition further includes a crosslinking agent capable of co-curing (i.e. forming covalent bonds) with the resin, thermoplastic elastomer, or both. Examples of preferred crosslinking agents include triallylcyanurate, diallylphthalate, divinyl benzene, a multifunctional acrylate, or combinations of these agents. The volume % of the crosslinking agent as a percentage of the combined volume of the resin, thermoplastic elastomer, and crosslinking agent is preferably less than or equal to 20.

The volume to volume ratio of the resin to the thermoplastic elastomer preferably is between 1:9 and 9:1, inclusive.

Preferred thermoplastic blocks for the first 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 copolymer is styrene-(ethylene-propylene)-styrene triblock copolymer (p = q = 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 upon the combined volume of resin, thermoplastic elastomer, crosslinking agent (if any) and filler) is between 5 and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres); corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g. Kevlar), fibreglass, Ba2Ti9O2Or glass spheres, quartz, boron nitride, aluminium nitride, silicon carbide, beryllia, BaO-PbO-Nd2O3-TiO2 or magnesia. They may be used alone or in combination.

Useful articles are prepared according to the process of the invention by forming the composition 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 can readily be molded into a wide variety of shaped articles having favourable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of a particular material e.g. ceramic materials, including gallium arsenide, alumina, and silica through the choice of one or more of the above described fillers. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications, e.g. as specialised substrates for high speed digital and microwave circuits.

Examples of microwave circuits including microstrip circuits, microstrip antennas, and stripline circuits. 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 also 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 environmental 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 applied e.g. bonded to a metal, e.g. for use in a circuit board, the coefficient of the thermal expansion of the cured thermoset materials may be modified by the inclusion of appropriate dielectric fillers such that it 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 preferred embodiments thereof, and from the claims.

We now describe preferred embodiments of the invention. Structure and Preparation The thermosetting molding compositions include 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 composition at a manageable level during processing to facilitate handling. It also crosslinks during cure. Polybutadiene and polyisoprene resins having at least 90% 1,2 addition by weight are preferred because they exhibit the greatest crosslink density upon cure owing to the large number of pendent vinyl groups available for crosslinking. High 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 available 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 reduces cracking during molding. Furthermore, it participates in crosslinking during cure.

As described above, the thermoplastic elastomer portion includes a linear or graft-type block copolymer that has a polybutadiene or polyisoprene block with less than 50% by weight 1,2 addition and a thermoplastic block that preferably is is styrene or a-methyl styrene. The higher the proportion of 1,2 addition in the polyisoprene or polybutadiene block the higher the crosslink densities after the curing step, as is the case with the polybutad--iene or polyisoprene resin described above. A preferred copolymer is a styrene-butadiene-styrene triblock copolymer, e.g.

Kraton DX1300 (commercially available form Shell Chemical Corp.). Other copolymers which may be employed in the process of the present invention include, for example, D1102 and D1184 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 ethylene-propylene copolymer (in the case of polyisoprene). When used in conjunction with the first copolymer, materials with particularly low coefficients of thermal expansion cn 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 2 is added to the thermosetting composition to increase crosslink density upon cure. Examples of preferred crosslinking agents include triallyl cyanurate, diallyl phthalate, divinyl benzene, and multifunctional acrylate monomers (e.g. Sartomer resins available from Arco Speciality Chemicals Co.), all of which are commercially available.

Examples of preferred fillers have been recited 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 dissipation factor to be achieved in the final cured product 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 preferably used.

A curing agent is preferably added to the composition to accelerate the curing reaction. When the composition is heated, the curing agent decomposes to form free radicals, which then initiate crosslinkig of the polymeric chains. Preferred curing agents are organic peroxides, 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 resin.

The homogenised 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, injection, or transfer mold, or an extruder, and the material is molded into the desired shape. The shaped 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 is 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 article is then removed and cooled.

The high temperature cure imparts an unusually high degree of crosslinking to the final article. Molding compositions in which the thermoplastic elastomer has a polybutadiene or polyisoprene block having less than 50% by weight 1,2 addition are suitable. The higher the percentage of 1,2 content, the greater the number of pendent vinyl groups which can participate during the during reaction. Thus, the degree of crosslinking in the final product following the high temperature cure can be adjusted through the choice of the thermoplastic elastomer.

The thermosetting compositions of Examples 1 and 2 below were prepared, molded, and cured by the process of the invention. All amounts are given in volume percent. The cured products are hard plastics having relatively low impact strengths.

ExamDle 1 B3000 resin 21.5 D1102 (a low vinyl content SBS resin available from Shell Chemical Corp.) 15.3 Ti02 41.4 Si02 18.1 Kevlar 2.0 Luperox peroxide curative 0.5 t-butyl perbenzoate curative 0.2 A180 Silane coupling agent 1.0 (Union Carbide Corp.) The final product exhibited the following properties:

Specific Gravity 2.50 Shrink (in/in) 0.009 DK split waveguide 13.08 DF split waveguide 0.003 Flex Strength (psi) 13,700 Modulus X 105 (psi) 1.65 % Water Absorption D48/50 0.19 Impact Strength (ft. lb./in.) 0.40 Coefficient of Thermal Expansion (X) 24-28 OC-1 (Z) 22-25 Copper Adhesion (pli) 1.4 Example 2 is identical to Example 1 except that D1184, another low vinyl SBS resin available from Shell, was substituted for the D1102 resin. The final product had the following properties:

ExamiDle 2 Specific Gravity 2.40 Shrink (in/in) 0.010 DK split waveguide 11.50 DF split waveguide 0.002 Flex Strength (psi) 13,900 Modulus x 105 (psi) 1.49 % Water Absorption D48/50 0.13 Impact Strength (ft. lb./in.) 0.45 Coefficient of Thermal Expansion (X) 27-31 OC-1 (z) 28-29 Copper Adhesion (pli) 1.6

Claims

1. A forming process for producing a hard shaped molded article comprising the steps of (a) providing the moldable thermosetting composition comprising a polybutadiene or polyisoprene liquid resin having a molecular weight less than 5,000 and at least 50% by weight 1,2 addition; and a thermoplastic elastomer comprising a first block copolymer having the formula Xm(IX)n or where Y is a polybutadiene or polyisoprene block having less than 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, provided that said first block copolymer is not Kraton DX 1300; (b) forming said composition into a shape, said thermoplastic elastomer enabling said composition to maintain said shape during said forming process, said liquid resin enabling the viscosity of said composition to be sufficiently low that said shape is readily formed; and (c) curing said composition to produce said article including subjecting said composition to a high temperature cure condition at a temperature greater than about 2500C and less than the decomposition temperature of said composition.

2. A forming process as claimed in claim 1 wherein said polybutadiene or polyisoprene block has a large number of pendent vinyl groups available for crosslinking.

3. A forming process as claimed in claim 1 or claim 2 wherein said liquid resin is polybutadiene and said first block copolymer is styrene-butadiene-styrene triblock copolymer.

4. A forming process as claimed in any one of claims 1 to 3 wherein said composition further includes a second block copolymer having the formula Wp (ZW). or "0 " where Z is a polyethylene 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 I and q being at least 1.

5. A forming process as claimed in claim 4 wherein said liquid resin is polybutadiene, said first block copolymer is styrene-butadiene-styrene triblock copolymer, and said second block copolymer is styrene-ethylenepropylene-styrene triblock copolymer.

6. A forming process as claimed in any one of claims 1 to 5 adapted to produce a dielectric article comprising the further steps of adding to said composition provided in (a) a quantity of dielectric filler to provide to said article a given dielectric constant, said thermoplastic elastomer maintaining said filler dispersed substantially uniformly throughout said composition.

7. A forming process as claimed in claim 6 wherein a plurality of said fillers having different dielectric constants are employed, the respective quantities of said fillers being selected to provide to said article a given dielectric constant.

8. A forming process as claimed in claim 6 or 7 further comprising applying to said cured article produced by (c) a metal layer having a given coefficient of thermal expansion and, in the process 1 1 of forming said article, selecting said filler and the quantity thereof to provide to said article a coefficient of thermal expansion substantially matched to that of said metal layer.

9. A forming process as claimed in any one of claims 1 to 8 further comprising adding a crosslinking agent to said thermosetting composition.

10. A forming process as claimed in claim 9 wherein said crosslinking agent comprises triallyl cyanurate, diallyl phthalate, divinylbenzene, a multifunctional acrylate monomer, or a combination thereof.

11. A forming process as claimed in any one of claims 1 to 10 wherein said curing step further includes subjecting said composition, prior to said high temperature cure, to a catalysed cure at a - temperature less than the temperature of said high temperature cure.

12. A process substantially as described herein with reference to Example 1 or Example 2.

13. An article prepared according to the process of any one of claims 1 to 12.

14. A microwave lens, microwave window, filled wave guide, substrate for a microwave circuit, rod antenna, substrate for a high speed digital circuit, or chip carrier comprising an article prepared by the process of any one of claims I to 12.

Published 1990 at The Patent Office. State House. 66 71 HighHolborn. London WC1R4TP- Further copies maybe obtained from The PateentOffice Sales Branch. St Ma--y Cray. Orpington. Kent BR 3RD Printed by MultIplex techniques Itd. St Mary Cray. Kent. Con. 187