Phenolic Interpenetrating Polymer Networks for Laminate Applications

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11 Phenolic Interpenetrating Polymer Networks for Laminate Applications K. Yamamoto Hitachi Chemical Co., Ltd., Shimodate Research Laboratory,1500Ogawa, Shimodate, Ibaraki-ken 308, Japan

Interpenetrating polymer networks (IPNs) composed of phenolics and vinyl compounds have the properties, such as heat resistance and flexibility, of each component. Phenolic IPN varnishes prepared by dissolving phenolic prepolymers in monomeric acrylates are expected to have a rapid curing property, which will improve productivity in commercial applications. We have applied these IPN varnishes to paper-based phenolic laminates, which are important materials in the electronics industry. The curing time of prepared phenolic IPN varnishes was measured by differential scanning calorimetry. The curing rate was found to depend on the species of radical polymerization initiator and the ratio of the components. Various formulations of varnish were proposed and laminates were prepared on a laboratory scale. The resulting IPN laminate has advantages that include improved productivity, processability, and electrical properties compared with a conventional commercial product.

PHENOLIC-RESIN PAPER-BASED COPPER-CLAD LAMINATES (phenolic lami nates) are important in the electronics industry because they have good electrical, chemical, and physical properties. Phenolic laminates also exhibit good processability and through-holes can be punched easily. In Japan, such laminates are widely used for consumer products such as televisions, air conditioners, audio sets, refrigerators, and tape players. The demand for phenolic laminate is expected to increase continuously at a rate of approxi mately 5 % a year (1, 2). Currently, these materials are used mainly in Asian countries, especially Japan. 0065-2393/94/0239-0233$06.00/0 © 1994 American Chemical Society

In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


234

INTERPENETRATING POLYMER NETWORKS

Although phenolic resin has the previously cited advantages, it does have the problematic properties, also. One problem is the brittleness of the resin, which necessitates modifications for industrial applications (3-10). Tung oil is commonly used as a flexibilizer for the phenolic laminates. However, the supply of tung oil is very unstable because it is a natural product and its growth and yield depend on the weather conditions. Another disadvantage of tung oil is that it lowers the reactivity of modified phenolic resin; that is, the curing time of phenolic resin modified with tung oil is so long that a huge manufacturing plant is necessary to mass-produce the laminate. Therefore, the development of the new flexibilizer for the laminate is greatly desirable. We have previously synthesized phenolic interpenetrating polymer net works (IPNs) composed of phenolic and vinyl compounds to improve the brittleness of phenolic resins (11-15). These IPNs possess useful properties of each component, such as heat resistance and flexibility, that make them of potential interest for phenolic laminates. In a similar manner, by dissolving phenolic prepolymers in monomelic acrylates, phenolic I P N varnishes can be prepared, and these varnishes can be used to impregnated kraft paper to produce the laminates. The preparation of I P N varnishes is simple compared with tung-oil-modified phenolic varnish. I P N varnishes are expected to have rapid curing and have less potential for environmental pollution because they contain no removable solvent; instead monomelic acrylates act as the reactive solvent. These advantages of I P N varnishes offer the potential to improve productivity in commercial applications. In the work described in this chapter, the curing time of various varnishes was measured with a differential scanning calorimeter (DSC). Then, laminates that employed the most promising I P N varnishes were prepared on a laboratory scale and compared with a conventional commercial product.

Experimental Methods Preparation of IPN Varnishes. A fist of the materials used is given in Table I. Phenolic novolac, epoxy, and flame retardants were dissolved in acrylates at 70 °C and radical polymerization initiators and curing catalysts were added to the mixture at 40 °C to obtain clear IPN varnishes (see Figure 1). DSC Measurement. The curing time of the IPN varnishes was measured with a pressure-type D S C (DuPont 910) under 2-MPa nitrogen atmosphere. The temperature of the varnish was raised from 40 to 170 °C at a rate of 30 °C/min and neld for 15 min at 170 °C. Figure 2 is an illustration of the DSC result for a typical I P N varnish. In Figure 2, time A is the time to complete the reaction after reaching 170 °C; time Β is the time after setting the sample in the measurement cell to the first peak, peak 1. Measurement of Gelation Time of IPN Varnishes. The gelation time of IPN varnishes was measured at 160 °C on a heated plate.


11.

YAMAMOTO

Phenolic IPNs for Laminate Applications

235

Table I. Materials and Suppliers Material

Chemical composition


Phenolic novolac HP800N Epoxy R 140P Acrylates BA ESB400A Flame retardants ESB 400 ESB 400A

Supplier

Phenolic novolac Diglycidyl ether of bisphenol A

Hitachi Chemical Mitsui Petrochemical

η-Butyl acrylate Diglycidyl ether of tetrabromobisphenol A acrylic acid adducts

Mitsubishi Rayon Synthesized

Diglycidyl ether of tetrabromobisphenol A Diglycidyl ether of tetrabromobisphenol A acrylic acid adducts Triphenyl phosphate

Sumitomo Chemical

TPP Curing catalysts 2E4MZ 2-Ethyl-4-methylimidazole BDMA N, N-Dimethylbenzylamine Radical polymerization initiators 2,2 -azobis(isobutyroAIBN nitrile) DCP Dicumyl peroxide 3M 1, r-Bis(teri-butylperoxy)3,3,5-trimethylcyclohexane PL Lauroyl peroxide PH Cyclohexanone peroxide t-Butylperoxy (2-ethyl PO hexanoate) ,

Synthesized Daihachi Chemical Shikoku Chemical Wako Pure Chemical Wako Pure Chemical Nippon Oil & Fats Nippon Oil & Fats Nippon Oil & Fats Nippon Oil & Fats Nippon Oil & Fats

Preparation of Laminate. The phenolic-IPN paper-based copper-clad laminate (IPN laminate) was prepared as follows (see Figure 3). Kraft paper was first treated with modified melamine varnish (which also acted as a flame retardant) and then was impregnated with an IPN varnish to obtain a preimpregnate (prepreg) for laminate. Seven prepreg sheets and a copper foil were laminated and then pressed under 2-MPa pressure at 170 °C for 2 min. To complete the curing reactions, the heating was continued for an additional 5 min at 170 °C under atmospheric pressure. Evaluation of Laminate. The general properties of laminate were mea sured according to JIS standard C-6481 or the standard of Hitachi Chemical Co., Ltd. (see Table II).

Results and Discussion Determination of Factors That Affect Curing. To determine the factors that affect curing, D S C measurement of various I P N varnishes was carried out. Generally speaking, the speed of curing is improved by



Curing catalysts

Initiators

Acrylates

Flame retardants

Epoxy

Phenolic novolac

Figure 1. Preparation of IPN varnish.

dissolving cooling dissolving -Ô 0— —0 IPN v a r n a t 40°C a t 40°C a t 70°C a t 70°C

heating



-

Β

*|

5 Time (min)

10

15

Figure 2. DSC thermogram for a typical IPN varnish.

A: Time to complete reaction after reaching 170°C B: Time to peak 1

F—Γ

Heat flow

Temperature L>

20



170°C, 5min

heating

m o d i f i e d melamine t r e a t e d k r a f t paper

foil

Phenolic Figure 3. Preparation of laminate.

Copper

>

with Prepreg

IPN

laminate

pressing, heating . > 2MPa, 170°C, 2min

impregnating IPN v a r n i s h


11.

YAMAMOTO


239

Table II. Measurement of the General Properties of the Laminate Test

Conditioning of Specimen


Water absorption Solvent resistance Inflammability Insulation resistance Dielectric constant Tracking resistance Solder resistance Peel strength Punchability Flexural strength

Immersed in water at 2 3 °C for 24 h Immersed in boiling 1,1,1-trichloroethylene for 1 0 min As received (according to UL94-vertical) As received Immersed in water at 5 0 °C for 48 h As received (according to ÏEC, comparative tracking index) As received (measured at 260 °C) As received As received As received

optimization of the phenolic-epoxy equivalent ratio (P:E), addition of curing catalysts or initiators, and combinations of the two methods. In these systems, acrylate components were cured first; then phenolic and epoxy components were cured. Therefore, the time to complete the reaction (time A ) is the most significant parameter for estimation of the time required to manufacture laminates. Table III presents the results of D S C measurements of I P N varnishes with various kinds of initiators. The influence of initiator on curing speed was thus determined. Runs 3, 5, and 6 show a short time A . These results indicate that the curing time of a varnish depends strongly on the species of initiator and that 3 M is a good initiator for rapid curing of I P N varnishes. The data in Table IV show the influence of phenolic-epoxy equivalent ratio (P:E) on curing speed. The curing rate increased, to a lesser extent, with decreased P : E . P : E = 1:1 and 1:2 I P N varnishes are preferable for rapid curing. The results listed in Table V show the influence of the amount of curing catalyst. To avoid heterogeneity of initiator concentration in I P N varnish, sample varnishes that used P O instead of P L were prepared. This step was taken because P L is only partially soluble in these varnishes. The half-life period of P O is similar to that of P L initiator. The difference between the two initiators is their state at room temperature: P O is liquid, whereas P L is solid. The amount of 2 E 4 M Z curing catalyst had little effect on the curing speed; B D M A catalyst was inferior to 2 E 4 M Z . From the viewpoint of laminate manufacture, the time to peak 1 (time B) is also significant. This time period is the time to the gelation point of the acrylate component. However, in some systems, no time Β was observed, and this time was seen to be insensitive to the initiator, the P : E , and the curing catalyst. Varnishes that contain both P L and 3 M initiators generated some gela tion (run 6 in Table III and runs 13 and 14 in Table V), which made them


240


Table III. Results of DSC Measurement: Influence of Initiator Run No.


Curing Catalyst

AIBN 0.2 DCP 0.8 DCP = 1.0 3M = 1.0 PL 0.2 DCP 0.8 3M 0.2 PH 0.8

1 2 3 4 5

PL 0.2 3M ~ 0.8 None

6 7 a

Initiator (phr)

P:E min) (equivalent Time (' Β Gelation A ratio)

Pot Life

a

0.5

1:1

5.8

4.1

No

Long

0.5 0.5

1:1 1:1

6.5 3.9

— —

No No

Long Long

0.5

1:1

8.9

3.5

No

Long

0.5

1:1

3.9

4.6

No

Long

0.5

1:1

4.4

—

No

Long

0.5

2:1

6.1

—

No

Long

Long means 7 days or more.

Table IV. Results of DSC Measurement: Influence of P:E Time Time Β A Curing Catalyst (min) (min)

Run No.

P:E (equivalent ratio)

Initiator (phr)

8 3 9

2:1 1:1 1:2

3M = 1.0 3M = 1.0 3M = 1.0

0.5 0.5 0.5

5.1 3.9 4.0

10

2:1

3M 0.2 PH ~ 0.8

0.5

5

1:1

11

1:2 2:1

7 a

3M 0.2 P H " 0.8 3M 0.2 PH ~ 0.8 None

Gelation

Pot Life

—

No No No

Long Long Short

6.1

4.2

No

Long

0.5

3.9

4.6

No

Long

0.5 0.5

3.2

4.4

No No

Short Long

6.1

a

Long means 7 days or more; short means less than 3 days.

unsuitable for impregnating kraft paper in preparation for laminate manufac ture. In addition, varnishes that contained a large amount of curing catalyst and low P : E had short pot life (runs 9 and 11 in Table IV and run 15 in Table V), which is undesirable because special preservation equipment is necessary. The foregoing results indicated that the varnishes used in runs 3, 5, and 15 had the desired property for rapid curing. Therefore, further tests on laminates that used these I P N varnishes were conducted.


11. YAMAMOTO

241


Table V. Results of DSC Measurement: Influence of Curing Catalyst

R

u

n


No.

Curing Catalyst 2E4MZ BDMA

Initiator (phr)

p.j? (equivalent ratio)

T i l m

( m i n )

A

Β

Pot Gelation Life

a

5

0.5

0

3M 0.2 PH ~ 0.8

1:1

3.9

4.6

No

Long

12

1.0

0

3M 0.2 P H ~ 0.8

1:1

4.3

—

No

Long

6

0.5

0

PL 0.2 3M ~~ 0.8

1:1

4.4

—

Yes

Long

13

1.0

0

PL 0.2 3M ~ 0.8

1:1

4.0

3.6

Yes

Long

14

0

1.0

PL 3M

1:1

5.8

3.4

Yes

Long

15

1.0

0

1:1

4.0

4.3

No

Long

16

2.0

0

1:1

3.8

4.4

No

Short

7 17

0.5 1.0

0 0

2:1 2:1

6.1 5.1

— —

No No

Long Long

a

0.2 0.8

PO 0.2 3M ~ 0.8 PO 0.2 3M " 0.8 None None

Long means 7 days or more; short means less than 3 days.

Evaluation of Laminates. Table V I lists the gelation times of the selected I P N varnishes and the appearance of the test I P N laminates. The varnish made with laminate I P N 2 showed the shortest gelation time (17 s) at 160 °C. However, blow holes were observed inside the laminate and the appearance of the laminate was not acceptable. The gelation time of I P N 1 was also short enough (34 s), but its appearance was also unacceptable because blow holes were observed at the surface of the laminate. Laminate I P N 3 had a satisfactory appearance and a varnish gelation time of 37 s. Tung-oil-modified phenolic varnish has a long gelation time—more than 5 times that of the I P N varnish (200-300 s). These results indicate that the gelation times of the varnishes of lami nates I P N 1 and 2 are too short for preparation of laminates by the process conducted in this work. Table VII fists the general properties of laminate I P N 3 compared with a conventional commercial product. Laminate I P N 3 showed superior proper ties, including lower water absorption, lower dielectric constant, higher resistance to tracking, and better punchability. This laminate passed the Japanese Industrial Standard (JIS) (PP7F grade), American Society for Test ing and Materials (ASTM) (FR1 grade), and National Electrical Manufactur-


242



Table VI. Gelation Time of Selected IPN Varnishes and Appearance of Laminates Gelation time at 160 °C (s)

Run No.

Curing Catalyst (phr)

Initiator (phr)

IPN 1

4

0.5

3M = 1.0

34

IPN 2

5

0.5

3M 0.2 PH ~ 0.8

17

IPN 3

15

1.0

PO 0.2 3M ~ 0.8

37

Laminate Code No.

a

Tung-oil-modified phenolic varnish a

—

Appearance of Laminate NG (blowholes at surface of laminate) N G (blowholes inside laminates) satisfactory

—

200-300

2-Ethyl-4-methylimidazole.

Table VII. General Properties of Laminates Laminate Properties Physical Water absorption Solvent resistance Inflammability Electrical Insulation resistance Dielectric constant at 1 MHz Tracking resistance CTI Mechanical • Solder resistance Peel strength Punchability at 20 °C Flexural strength at 20 °C Manufacturing time

Unit

IPN3

Conventional

% — s

0.63 Good 2.5

0.78 Good 2.0

X10 Ω — V

17.0 4.27 465

2.7 4.58 310

s kN/m — MPa min

32 1.9 Better 120 10

35 1.8 Good 140 120

12


11.

YAMAMOTO


243

ers Association ( N E M A ) ( X P C F R grade) standards. In addition, less than 1 0 min was required to obtain the laminate from prepregs. From an engineering viewpoint, the short manufacturing time is an obvious advantage when compared with the time for a conventional commercial laminating process (more than 1 2 0 min).


Summary and Conclusions DSC measurements were used to estimate the curing properties of I P N varnishes. Then, formulations of varnish for laminate were determined based on the results, and the properties of the I P N laminates, prepared on a laboratory scale, were compared with a conventional commercial product. The most important determinants of curing speed were the species of radical polymerization initiator, followed by phenolic-epoxy equivalent ratio. The I P N varnish identified as I P N 3 gave good results in both curing time and appearance. The resulting I P N laminate had the advantages of low water absorption, low dielectric constant, high resistance to tracking, and good punchability, and offers the promise of higher speed of laminate manufacture compared with a conventional commercial product.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15.

Itou, K. Electron. Parts Mater. 1991, 30(10), 32-40. Aoki, M . Surf. Mount Technol. 1992, 12(6), 1-11. Takahashi, Y. J. Appl. Polym. Sci. 1961, 5, 468-477. Hsu, E. T.; Pascavage, D. Abstracts of Papers, TAPPI Annual Meetings Proceed ings; Technical Association of the Pulp and Paper Industry: Atlanta, GA, 1980; pp 277-290. Jayabalan, M.; Balakrishnan, T. Angew. Makromol. Chem. 1983, 118, 65-80. Noguchi, K. Matsuda, Y.; Yasui, S.; Saai, S. Japanese Patent 5,970, 1980. Matsumura, M.; Sakamoto, K.; Sakamoto, T. Japanese Patent 43,147, 1980. Nakazato, L; Tsurumi, Y. Japanese Patent 73,721, 1980. Hara H.; Orii, S.; Araki, Y. Japanese Patent 154,153, 1980. Tajima, Y. Yokoo, T. Ema, K.; Ikado, S. Japanese Patent 133,354, 1981. Yamamoto, K.; Kumakura, T.; Yoshimura, Y. Polym. Prepr. Jpn. 1986, 35, 802. Yamamoto, K.; Yasuzawa, K.; Nomoto, M.; Takahashi, Α.; Yoshimura, Y.; Nanaumi, K. Abstracts of Papers, Proceedings of the Thermosetting Plastics Symposium, Japan; Japan Thermosetting Plastics Industry Association: Tokyo, Japan, 1988; pp 23-26. Yamamoto, K.; Takahashi, A. In Sound and Vibration Damping with Polymers; Corsaro, R. D.; Sperling, L. H . , Eds.; ACS Symposium Series 424; American Chemical Society: Washington, DC, 1990; pp 431-444. Yamamoto, K.; Yasuzawa, K.; Nomoto, M.; Kumakura, T. Japanese Patent 225,551, 1987. Yamamoto, K.; Yasuzawa, K.; Nomoto, M.; Kumakura, T. Japanese Patent 225,552, 1987. ;

;

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RECEIVED for review October 9, 1 9 9 1 . ACCEPTED revised manuscript Septem ber 10, 1 9 9 2 .


Phenolic Interpenetrating Polymer Networks for Laminate Applications

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