POLYIMIDE RESTN

Field of the Invention The invention relates generally to cross-linked, heat resistant, thermosetting polyimide resins, used in the manufacture of laminates for the electronics industry. More particularly, it relates to a novel polyimide resin which avoids the aromatic diamines now in common commercial use. Prior Art

Thermosetting resins prepared from bisi ides and aromatic diamines used commercially are disclosed in patents assigned such as those assigned to Rhone-Poulenc, e.g., U.S. 3,562,223, 3,658,764, and 3,878,172. In some instances, such compositions are prepared as prepolymers and combined with epoxy compounds and crosslinked with dicyandiamide or other agents. Examples may be found in

U.S. Patents 4,526,835, 4,525,572, 4,463,147, 4,393,188, and 4,435,560. While resins derived from bisimides and aromatic diamines perform well in electronic laminates, they have several disadvantages. If substitute materials could be found that had improved performance, contained no free aromatic diamines, were less brittle and met UL flammability rating V-O, they should find ready acceptance in the marketplace.

Polymers from cyana ide and bis ides or mixtures of bismides and monoimides are disclosed in U.S. 4,043,986. No mention is made of dicyandiamide or polyphenols as possible components of such polymers and the patent is primarily concerned with molding compounds rather than laminates for the electronics industry.

U.S. Patent 4,482,703 discloses resins made with a polyvalent i ide and dicyanamide compounds described by the formula NCHN-Y-NCHN where Y is a divalent organic group having an aromatic ring. Such compounds are said to produce a prepolymer (which may be further reacted with

epoxy or other compounds) which combines an isomela ine ring and an i ide ring which are linked to other molecules through the Y moiety and the imide nitrogen. By definition, such resins would not include dicyandiamide, which contains no Y group and, therefore, could not provide an isomelamine ring capable of being linked through such a Y group. The '703 patent also suggests ternary resins in which the dicyanamide compound and polyvalent imide are combined with a diamine. In copending and commonly assigned application

Serial No. 850,660 thermosetting resins are produced by reacting bismaleimides (including a prepolymer chain- extended with an aromatic diamine) with styrene-terminated bisphenol (and/or the tetrabromo substituted compound) . It is characteristic of these compositions to be blended together, combined with reinforcing materials, and cured to prepare laminates. Such compositions are not chemically reacted until the laminates are formed. Such resins are shown to have a desirably low dielectric constant, but they exhibit low peel strength and are too brittle and improved resins have been sought.

U.S. Patent 4,038,251 discloses polymers made by reacting bismaleimides with polyhydric phenols in the presence of basic compounds, particularly amines, which are said to be catalysts. Amines are optional components of the polymers. There is no reference to dicyandiamide as a component of such polymers or recognition of its unique qualities in combination with bismaleimides and polyhydric phenols. The present invention relates to a resin which avoids the use of aromatic diamines and yet provides better performance than resins containing such diamines.

Summary of the Invention An improved thermosetting resin suitable for use in laminates for the electronics industry is prepared from (a) a bisimide, preferably a bismaleimide, (b) a polyphenol, preferably a bisphenol, which may be styrene- terminated (i.e. a vinyl benzyl ether of a bisphenol) , and (c) dicyandiamide.

The resin is principally comprised of a bisimide with minor amounts of components (b) and (c) . The weights of (a) , (b) , and (c) preferably will be between 80 to

98%/l to 10%/0.1 to 10%, respectively, most preferably 94 to 98%/l to 2%/1.5 to 3%, respectively, based on the total of (a) , (b) , and (c) .

The thermosetting resin composition may comprise (a) a bisimide having the formula:

where: A = alkylene radicals having 1 to 20 carbon atoms, cycloalkylene radicals having 4 to 40 carbon atoms, heterocyclic radicals containing at least one of 0, S, and N, phenylene or polycyclic aromatic radicals. D = divalent radicals containing a carbon-carbon double bond (b) a polyphenol, particularly a bisphenol; (c) dicyandiamide.

In a preferred embodiment, the bisimide is a bismaleimide having the formula:

CH- CF- where: A = -C ₆ H ₄ -CH ₂ -C ₆ H ₄ -, -C ₆ H ₄ -C-C ₆ H ₄ -, -C ₆ H ₄ -C-C ₆ H ₄ -,

CH, CF,

,

~ ^c _- ^H 4~ ^c _-; ^H it~/ ~ ^C 6__ ^Hn Λ4~' ~ ^c 'e6 ^Hπ 3_5 ^{C( H} 5"f

CF.

-C ₆ H4-0-C ₆ H4-C-C ₆ H ₄ -0-C ₆ H4-, -C ₆ H ₄ -C-C ₆ H ₄ -, CF,

~ ^C 6 ^H 4~ ^CH 2~°~ ^CH 2~ ^C 6 ^H 4~' ~ ^C 6 ^H 4 ^"" ?~ ^C 6 ^H 4 ^"" ' ~ ^C 6 ^H 4 ^" °~ ^C 6 ^H 4 ^" ^

O

~ ^C 6 ^H 4 ^"C"C 6 ^H 4 ^" CH ₃

*^1' *-^2' *^3' -^ ⁼ *■""' ^*^3' ^2*^5' C H ^« --, C _3g, gH-^j. and the bisphenol has the formula

where: Z = H, CH ₂ = CH-C ₆ H ₄ -CH ₂ -

E = CH ₃ -C-CH ₃ , OSO, -0-, HCH, -S-, and a sig a bond

The resin composition may comprise 1-10 mols of (a) , greater than zero to 1 mols of (b) , and greater than zero to 2 mols of (c) .

In preferred embodiments A is -CgH ₄ -CH ₂ -CgH4-; R _lf R ₂ , R ₃ , R ₄ are each H; Z is H or CH ₂ = CH-CgH ₄ -CH ₂ -; E

is CH ₃ -C-CH ₃ ; and B is H or Br.

Description of the Preferred Embodiments The thermosetting resins of the invention can replace those made by reacting a bisimide with an aromatic diamine. Such resins are polymerized and then crosslinked by use of dicyandiamide or related compounds. The present resins, however, introduce dicyandiamide as an intrinsic element of the polymer, thereby producing a superior resin, as will be seen below. Bisimides

Bisimides are compounds having the general formula D(CO) ₂ N-A-N(CO) ₂ D where D is a divalent radical containing a carbon-carbon double bond and A is a linking

group which may be a divalent radical having at least 2 carbon atoms. This nomenclature is adopted from that given for such compounds in U.S. 3,562,223. The compounds disclosed in that patent are generally applicable in the present invention. Particularly preferred are bismaleimide (BHI) compounds where A is an isopropyl radical or an ether linkage between two phenyl radicals which are attached to the nitrogen atoms.

For purposes of the present invention, the bisimide compound will be selected to provide resins having improved toughness, low water absorption and low dielectric constant.

Resins of the invention are usually comprised mainly of the bisimide, preferably from about 80 to 98 weight%, most preferably 94 to 98 weight percent.

Typically, prepolymers of bisimides and aromatic diamines of the type disclosed by Rhone-Poulenc contain smaller amounts of the bisimides than are employed by the present inventors. The Rhone-Poulenc resins are said to be preferably between 1.3:1 and 5:1 bisimide/diamine (U.S. 3,562,223). The bisimide can react with the diamine through the unsaturated carbon-carbon bond of the imide ring, but it may also react with itself in the same manner. In the resins of the invention, a further component of the polymer chain is dicyandiamide, which is more commonly used in other resin systems as a crosslinking agent once the bisimide and diamine have formed a prepolymer. The dicyandiamide here is believed to become part of the backbone of the polymer. Dicyandiamide

This compound has the formula

NH

II NH ₂ CNHC=N

It is capable of reacting with the carbon-carbon double bond of the bismaleimide to extend the polymer chain.

Only relatively small amounts of dicyandiamide are required, broadly about 0.1 to 10 wt% of the resin may be used. Preferably, about 1.5 to 3 wt.% would be reacted to form the resin. Polyphenols

The polyphenols used in the resins of the invention preferably are bisphenols, particularly those within the following formula:

where: Z = H, CH ₂ - CH-C ₆ H ₄ -CH ₂ -

E = CH ₃ -C-CH ₃ , 0S0, -0-, HCH, -S-, and a sigma bond

B = H, Br, CH ₃ Of particular interest are the group consisting of bisphenol A, tetrabromobisphenol A, vinyl benzyl ether of bisphenol A, and vinyl benzyl ether of tetrabromobisphenol A.

The polyphenols are present in the resin in minor amounts, preferably about 1 to 10 wt.%, most preferably 1 to 2 wt.%

Composition of the Resin

A thermosetting resin composition according to the invention may comprise (a) a bisimide having the formula:

where: A = alkylene radicals having 1 to 20 10: carbon atoms, cycloalkylene radicals having

4 to 40 carbon atoms, heterocyclic radicals containing at least one of O, S, and

N, phenylene or polycyclic aromatic radicals D = divalent radicals containing a carbon-carbon 15 double bond

(b) a bisphenol, and

(c) dicyandiamide.

In a preferred embodiment the bisimide is a bismaleimide and the thermosetting resin composition may 2.0 comprise (a) a bismaleimide having the formula:

where: A - -C ₆ H ₄ -CH ₂ -C ₆ H ₄ -, ₆ H ₄ -,

^""C 6 ^H 4 ^""C 6 ^H 4~' ^*"C 6 ^H 4 ^" ' ~ ^C 6 ^H 3 ^CH 3~'

CF,

-C ₆ H ₄ -0-CgH4-C-C ₆ H4-0-CgH4-, -CgH ₄ -C-CgH ₄ -, CF,

^• C6H4-CH ₂ -0-CH ₂ -CgH4-, -CgH ₄ -S-CgH ₄ -, -C ₆ H ₄ -0-C ₆ H ₄ -,

O

~ ^c _f i ^H _ _l ~

R * ^• !i _/ / R ^Λ 2_> i' R ^Λ 3 _"? i' R ^Λ A4 *~ H, CH 3, ,' C2H5c,' C-3-jH"7,' C" ₄ 4H"9Q ,' C5cH11

and the bisphenol has the formula

where: Z = H, CH ₂ = CH-C ₈ H ₄ -CH ₂ -

10

E = CH ₃ -C- ICH ₃ , OSOI, -0-, HC IH, -S-, and a sigma bond

B = H, Br, CH ₃ 155 and (c) dicyandiamide. The term "sigma bond ¹¹ refers to a covalent bond between the aromatic rings and corresponds to a bisphenol in which the aromatic rings are directly joined.

The resin composition may comprise 1-10 mols of 20 (a) , greater than zero to 1 mols of (b) , and greater than zero to 2 mols of (c) .

In preferred embodiments A is -C ₈ H ₄ -CH2-C ₈ H ₄ -; ^R _l ' ^R ₂ ' ^R ₃ ' ^R ₄ ^{are eacn} H ^; Z is H or CH ₂ = CH-Cg^-C^- ^; E

253 I is CH ₃ -C-CH ₃ ; and B is H or Br.

Resin Synthesis

3_0Q The resins of the invention may be prepared by reacting the three components in a solvent at an elevated temperature to form a prepolymer. One convenient procedure is to warm a suitable solvent, such as dimethyl formamide (DMF) , N-methyl pyrrolidone, dimethyl acetamide,

35 acetone, benzene, toluene, and the like to a temperature at which the chosen bismaleimide will be dissolved, say about 90 to 100°C. The bismaleimide is added to the solvent and mixed until dissolved. Then the temperature is increased to the desired reaction temperature, about

40 120 to 140°C, at which time the second and third

components are added and mixed. The three-component mixture in the solvent will be maintained at the reaction temperature for a sufficient period of time to partially complete the reaction. Polymerization is completed during the manufacture of laminates. Use of the Resins

The resins of the invention may be used to prepare laminates for the electronics industry by techniques generally in use in the field. Generally, the resins are diluted with a solvent and then used to impregnate a fabric such as fiberglass and dried at an elevated temperature. The resulting composite may then be laminated with other layers, such as copper foil, and then baked to fully cure the finished laminate. In the following examples, unless otherwise specified, the resins were prepared by this procedure. Dimethyl forma ide (DMF) was added to a glass flask and heated to 100°C. The bismaleimide resin was added to the DMF and the temperature returned to 100°C, when dicyandiamide and other components of the resin were added to the mixture. The flask was heated to 140°C and maintained at that temperature until the desired degree of reaction is reached. This was determined by sampling the mixture and determining the gel time by stroke cure method on a cure plate. When the resin had a gel time of about 6 - 7 minutes at 171°C the reaction was stopped by cooling the flask. The resin was then ready for use.

Exampl i

A BMI resin provided by the Monsanto Company (Skybond 3000) was reacted with dicyandiamide and a bisphenol A (BPA) according to the above procedure. The following summarizes the composition and the conditions of the reaction.

Formula

Monsanto 3000 - 150 gr.

Dicyandiamide - 3.75 gr. Bisphenol A (BPλ) - 2.25 gr.

DMF (solvent) - 100 gr.

Reaction

110 minutes at 135°C

Product

Gel time - 4.15 minutes at 171°C

The resin product was applied to 7628 CS 309 glass cloth provided by the Clark Schwebel Company and then B-staged in an oven for 5.5 minutes at 171°C to yield an impregnated material containing 45.6 % (vt.) of the resin. Eight plies of the B-stage were then pressed together with copper foil on the surface at 176°C for 2 1/2 hours. The copper foil used was Gould JTC polyimide treatment. The laminate was postbaked at 218°C for 32 hours.

The finished laminate was examined and found to have the following physical properties.

P operty Valug

Solder blister at 550°F (288°C) 20 minutes +

Peel strength, condition A 9.6/9.5

Peel strength § E/170°C 8.9/9.2 H 0 absorption D-24/100 0.87%

Tapered pin IR D-24/100 6.9 x lo ⁶

Dielectric breakdown D-24/100 70.0 V

Tg per TMA 268°C

CTE (Z-axis) 62 ppm/°C Fla mability 8.3/14.0

Retained resin 40.8%

Example 2 The above example was repeated with a revised composition and conditions with the results given below: Formula

Monsanto 3000 - 200 gr.

STTBBPA (1) - 6 gr.

Dicyandiamide - 5 gr.

DMF - 132 gr.

(1) Styrene-terminated tetrabro o bisphenol A (a vinyl benzyl ether of tetrabromo bisphenol A) .

ReaqtJPn

205 minutes at 120°C

Pfoduςt gel time - 5 minutes at 171°C

The resin product was applied to 7628 CS 309 glass supplied by Clark Schwebel Company as in Example 1, with the impregnated materials having 40 wt.% resin. The B-stage was cured for 4 -1/4 minutes at 171°C and then pressed at 176°C for 3 hours as described in Exeunple 1. Gould JTC polyimide treatment copper foil was used. The laminate was postbaked for 24 hours at 218°C.

The finished laminate was examined and found to have the following physical properties.

Pro ^p ert ^y Y_ilH≤

Solder blister at 288°C 20 minutes +

Peel strength, condition A 10.3/10.2

Peel strength, § E/170°C 8.7/8.7 H ₂ 0 absorption D-24/100 0.68%

Tapered Pin IR D-24/100 2.8 x 10 ⁶

Dielectric breakdown D-24/100 62.0 V Tg per TMA 256°C

CTE - Z-axis 62 ppm/°C Flammability 0.50/3.0

Retained resin 34.9%

Dielectric constant € 1MHz 4.50

Dissipation factor § 1MHz 0.010

Prepreg volatiles § 300°C 3.29% Prepreg volatiles § 225°C 3.22%

Example 3 A series of compositions were prepared corresponding to Example 2 which varied in the proportions of STTBBPA and dicyandiamide. The formulations and their preparation are summarized below, along with some of the properties of the finished laminates.

Formulation A B C D

M-3000, g 150 150 150 150

STTBBPA, g 2.25 6.75 6.75 2.25

Dicyandiamide, g 2.25 5.63 1.87 5.63

DMF, g 100 100 100 100

Processing

Resin solids 60.7% 61.9% 61.3% 61.2%

Synthesis time 240 min. 110 min. 160 min. 135 min.

Resin Gel

Time @ 171°C 4.24 min. 5.36 min. 4.41 min. 4.04 min.

Viscosity 1068 cip 958 cp 1360 cp 1040 cp

Resins Content 43.1% 45.2% 43.7% 45.5%

Cure Time @ 171°C 5 1/2 min. 6 min. 5 3/4 min. 5 min.

Press Cycle @ 176°C 2 1/2 hrs. 2 1/2 hrs. 2 1/2 hrs. 2 1/2 hrs.

Postbake @ 218°C 24 hrs. 24 hrs. 24 hrs. 24 hrs.

Properties Solder Blister

@ 288°C 185 sec. 36 sec. 326 sec. 40 sec.

Flammability 7.5/ 2.4/ 2.8/ 6.2/

(ave/long) 17.5 sec. 7.0 sec. 6.0 sec. 10.0 sec.

It was concluded that the presence of STTBBPA controls the flammability characteristics of the finished laminate. A UL flammability rating of V-O can be achieved. The solder blister properties are important to the laminate applications. The solder blister improves as dicyandiamide concentration is reduced to 1.87 g.

Example 4 (Comparative) In order to show the importance of the relatively small amount of dicyandiamide as used in the previous formulations, the Monsanto BMI resin of Examples 1 -3 was combined with only 2.91 wt.% STTBBPA in DMF solvent. The resin solids were 60.7%. The synthesis was carried out for 15 hours at 135°C but no gel time could be measured indicating that essentially no resin had formed. This example illustrates that at least some dicyandiamide is essential in order for the reaction to proceed.

Example 5 A formulation similar to those of Examples 1 - 3 was prepared using a bismaleimide in which the linking group A is diphenyl methane, designated XU-292A by Ciba- Geigy, its supplier. A laminate was prepared generally as previously described. Pertinent data are summarized below.

Formulation

XU-292A, g 150 STBPA, g(l) 4.5

Dicyandiamide 3.75

DMF 75

Processing

Resin solids 67.8% Synthesis time 180 min.

Resin gel time § 171°C 5 1/2 min.

Resin content 45.9%

Cure time § 166°C 4 1/2 min.

Press cycle § 176°C 17 hrs. Postbake @ 218°C 32 hrs.

Properties (laminate)

Solder blister § 288°C 120 min.+

Peel strength, condition A 7.2/7.0

Peel strength, § E/170°C 6.6/6.6 H ₂ 0 absorption D-24/100 1.86%

Tapered pin IR D-24/100 7.3 x 10 ⁶

Dielectric breakdown D-24/100 73.0 KV

Tg per TMA 273°c

CTE-Z-axis 45.8 ppm/°C Flammability 2.9/5.4

Retained resin 40.3%

Dielectric constant § 1MHz 4.86

Dissipation factor § 1MHz 0.011

(1) Styrene-terminated bisphenol A (a vinyl benzyl ether of bisphenol A) .

Example 6 The BMI of Examples 4 and 5 was formulated with thiodiphenol and dicyandiamide following the general procedures of the previous experiments. A laminate was produced and tested as in the previous experiments. The pertinent data is summarized below. fprmygation

XU-292A, g 100 Thiodiphenol, g 3.5

Dicyandiamide, g 2.5

DMF, g 65

Processing

Resin solids 62% Synthesis time 75 min.

Resin gel time § 171°C 4.5 min.

Resin content 46.8%

Cure time § 166°C 4.5 min.

Press cycle € 176°C 2 1/2 hrs. Postbake θ 218°C 32 hrs.

Properties (laminate.

Solder blister § 288°C 150 min.+

Peel strength, condition A 7.3/6.7

Peel strength § E/170 6.7/6.7 H ₂ 0 absorption D-24/100 1.24%

Tapered pin IRD-24/100 2.3 X 10 ⁶

Dielectric breakdown D-24/100 69.3 KV

Tg per TMA 345°C

CTE-Z-axis 91.2 ppm/°C Flammability 6.0/12.9

Retained resin 43.2%

Dielectric constant € 1MHz 4.77

Dissipation factor § 1MHz 0.014

Example 7

(Comparative)

The BMI of Examples 5, 6, and 7 was formulated with an epoxy endcapped with dicyandiamide and additional dicyandiamide, following the procedures previously described. A laminate was made with the resin and tested as before. The pertinent data are reported in the following table.

Formulation XU-292A, g 100

Shell 829 epoxy endcapped with dicyandiamide, g 3.3

Dicyandiamide, g 2.5

DMF,g 65 Processing

Resin solids 60%

Synthesis time 80 min.

Resin gel time @ 171°C 3 min.

Resin content 41.9% Cure time θ 166°C 3 min.

Press cycle § 176°C 2 1/2 hrs.

Postbake § 218°C 32 hrs. Properties (laminate.

Solder blister € 288°C 67 min. Peel strength, condition A 6.7/6.8

Peel strength § E/170 5.8/6.0

H ₂ 0 absorption D-24/100 1.89%

Tapered pin IR D-24/100 8.2 x 10 ⁵

Dielectric breakdown D-24/100 40.5 KV Tg per TMA 330°C

CTE-Z-axis 84 ppm/°C

Retained resin 38.7%

Dielectric constant 8 1MHz 4.44

Dissipation factor § 1MHz 0.009 The laminate was found to have lower peel strength than desired, plus relatively high water absorption and thermal expansion.

Example 3 (Comparative) The Ciba-Geigy BMI was formulated with ethylene diamine and dicyandiamide following the procedures previously described. A laminate was made from the resin and tested as before, with the pertinent data summarized below.

Formulation

XU-292A, g 150

Ethylenediamine, g 4.5

Dicyandiamide, g 3.75

DMF, g 75

Processing

Resin solids 67.8%

Synthesis time 113 min.

Resin gel time 8 171°C 4 1/2 min.

Resin content 48.1%

Cure time 8 166°C 3 1/4 min.

Press cycle § 176°C 2 1/2 hrs.

Postbake 8 218°C 32 hrs.

Properties (laminate)

Solder blister 8 288°C 842 sec.

Peel strength, condition A 6.8/6.7

Retained resins 44.9% The laminate had a solder blister test result which was undesirably low.

Example 9 Comparative A laminate was prepared with only dicyandiamide and a bismaleimide and lacking the polyphenol required in resins according to the invention. Forty (40) grams of XU-292A from Ciba-Geigy was added to 66 grams of DMF in a flask and heated to above 70°C. Then 20 grams of dicyandiamide was added and the temperature raised to 130°C and held for 45 minutes. The resin product was applied to 7628 CS309 glass cloth and B-staged in an oven

for 2 minutes at 165°C. A second sample was given a second coating of resin after 1 minute in the oven and then returned to the oven for another minute. A laminate having copper foil on each side was prepared from each sample and pressed at 170°C for 1 hour. The samples were then postbaked for 16 hours at 176°, 204°, and 218°C. All samples showed very weak bonding of the copper foil to the resin. Blistering was noted at the higher postbake temperatures. It was concluded that the bismalei ide- dicyandiamide resin would not produce a satisfactory laminate.