Vinyl Compound and Phenolic Interpenetrating Polymer Networks

Chapter 24

Vinyl Compound and Phenolic Interpenetrating Polymer Networks Synthesis and Properties K. Yamamoto and A. Takahashi Hitachi Chemical Company, Ltd., Shimodate Research Laboratory, 1500 Ogawa, Shimodate, Ibaraki-ken 308, Japan

Interpenetrating polymer networks (IPNs) composed of vinyl compounds and phenolics were synthesized by combining simultaneous radical polymerization and phenolic curing reactions. To investigate the structure of products, two different types of IPNs were prepared. One, IPN 1, was formed through simultaneous reactions; the other, IPN 2, was formed through sequential reactions. Dynamic mechanical analysis, SEM observations, and tensile tests of these IPNs were performed. IPN 1 showed only one loss maximum, while IPN 2 showed two distinct loss maxima. These results indicate that vinyl polymer chains and cured phenolic chains in IPN 1 were well entangled with each other, while limited intermixing occurred in IPN 2. Compared with common vibration-damping materials, such as chloroprene rubber, the loss maxima of IPNs were at higher temperatures. Judging from the loss tangent values, IPNs are effective damping materials at elevated temperatures. Phenol-formaldehyde resins were f i r s t manufactured commercially i n 1907. (1-2) Even now they are s t i l l widely used i n many f i e l d s because of t h e i r features of temperature resistance and good mechanical and e l e c t r i c a l properties. However, they are b a s i c a l l y b r i t t l e materials, and many types of additives and modifiers have been developed to improve some of t h e i r properties. While working to develop special-purpose phenolic materials, the authors found that the compounds being produced had excellent vibration-damping properties (high loss tangent values), and that these properties were preserved to higher temperatures than elastomers generally used f o r the 0097-6156/90/0424-0431$06.00/0 © 1990 American Chemical Society

432

SOUND AND VIBRATION DAMPING WITH POLYMERS

purpose. I t i s p o s s i b l e t h a t t h e y may be s u i t a b l e f o r s p e c i f i c damping a p p l i c a t i o n s . These compounds have t h e s t r u c t u r e o f an i n t e r p e n e t r a t i n g polymer network (IPN) o f p h e n o l i c r e s i n w i t h m a t e r i a l s h a v i n g e l a s t o m e r i c p r o p e r t i e s . ( 3 - 8 ) Except f o r e a r l y work by A y l s w o r t h i n 1914, t h e r e has been no work on f u l l IPN p h e n o l i c m a t e r i a l s . (9) T h i s paper d e s c r i b e s t h e p r e p a r a t i o n and t e s t i n g o f f u l l IPN p h e n o l i c systems w i t h v i n y l compounds. At f i r s t , m e t h y l m e t h a c r y l a t e (MMA) p o l y m e r i z a t i o n was used as a model r e a c t i o n system. However, p h e n o l i c d e r i v a t i v e s a r e w e l l known as a n t i o x i d a n t s and i n h i b i t o r s f o r r a d i c a l p o l y m e r i z a t i o n ( 1 0 ) , so s u c h a r e a c t i o n would n o t be e x p e c t e d t o occur. However, by p r o p e r c h o i c e o f v i n y l compounds and i n i t i a t o r s , i t was found t h a t such p o l y m e r i z a t i o n i s p o s s i b l e , and t h a t a v a r i e t y o f IPNs c a n be produced. T h i s paper r e p o r t s on r a d i c a l p o l y m e r i z a t i o n o f MMA i n p h e n o l i c r e s o l and c o n f i r m a t i o n o f t h e s t r u c t u r e by measurement o f dynamic m e c h a n i c a l p r o p e r t i e s , s c a n n i n g e l e c t r o n m i c r o s c o p y , and t e n s i l e t e s t s , t h e n t h e damping a b i l i t y o f these v i n y l compound/phenolic IPNs i s e v a l u a t e d . Experimental Model

Methods

Reactions

P h e n o l i c r e s o l was s y n t h e s i z e d by t h e u s u a l p r o c e d u r e (See Scheme 1.); 1 mol o f p h e n o l , 1.2 mol o f formaldehyde, and 0.04 mol o f ammonia were h e a t e d a t 70°C w i t h v i g o r o u s s t i r r i n g f o r 3 h o u r s , t h e n d e h y d r a t i o n was c a r r i e d o u t under vacuum. MMA p o l y m e r i z a t i o n was c o n d u c t e d a t 70°C i n p h e n o l i c r e s o l , as shown i n Scheme 2. The r e a c t i o n p r o d u c t was poured i n t o a l a r g e volume o f methanol t o p r e c i p i t a t e t h e p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA). T h i s was s e p a r a t e d and washed s e v e r a l t i m e s w i t h methanol, t h e n d r i e d a t reduced p r e s s u r e . The p h e n o l i c r e s o l c u r i n g r e a c t i o n and d i m e t h a c r y l a t e p o l y m e r i z a t i o n were c o n d u c t e d a t 170°C f o r 90 m i n u t e s , y i e l d i n g v i n y l compound/phenolic IPN. (See Scheme 3.) The s t r u c t u r e o f t h e polymer o b t a i n e d was d e t e r m i n e d from i t s IR spectrum; t h e e x t e n t o f c o n v e r s i o n by p o l y m e r i z a t i o n and c o n d e n s a t i o n ( p h e n o l i c c u r i n g r e a c t i o n ) were a l s o c a l c u l a t e d from IR s p e c t r a . Synthesis

of Resol

- OH i . o HCHO NHo

(molar r a t i o ) 70°C, 3h •>0

1.2 0.04

P o l y m e r i z a t i o n o f MMA Resol 0.7 -| MMA 0.3 I n i t i a t o r 0.01-

70°C, i . v a c . •> R e s o l Dehydration

(mass r a t i o ) o Me OH > 0 Wash

7

n

Scheme 1

r

•> Polymer

Scheme 2

24. YAMAMOTOANDTAKAHASHI

Vinyl Compound-Phenolic IPNs

Synthesis of IPN (mass r a t i o ) Resol 0.7 170°C, 90min Dimethacrylate 0.3 Initiator 0.01-J Structural

433

Scheme 3

->IPN

Analyses

In order to investigate the structure, v i n y l compound/phenolic IPNs were synthesized as follows (See Schemes 4, 5 . ) : Method 1: Poly (ethylene glycol) dimethacrylate (23G), and phenolic r e s o l (PR) were mixed at 50°C for 10 minutes and dicumyl peroxide (DCP) was added. The polymerization and phenolic curing reactions then took place simultaneously at 170°C for 90 minutes i n 200mm x 10mm x 1.5mm stainless s t e e l mold. This product was c a l l e d IPN 1. Method 2: F i r s t , 23G was polymerized at 70°C for 60 minutes, then the product, poly 23G, was ground f i n e l y . Poly 23G p a r t i c l e size was about 7.9 x 10~^m i n diameter. PR and the finely-powdered poly 23G were mixed at 50°C for 10 minutes and suspended i n phenolic r e s o l , then cured at 170°C for 90 minutes. This product was c a l l e d IPN 2. Synthesis of IPN (mass r a t i o ) Method 1: PR 0.7 23G DCP

0.3

J

50°C, lOmin — Mixing

0.01-

Method 2: 23G 0.7

1 7 0 ° C , 90min *0— >IPN 1 ^Polymerization Curing Reaction :

Scheme 4

-

AIBN

0.01-

PR

0.3

70°C, 60min Polymerization

Grinding

A

50°C, lOmin ^ 1 7 0 ° C , 90min -> 0 -— — '> IPN 2 Mixing Curing iReaction :

Scheme 5

Dynamic mechanical property data were obtained using Du Pont DMA 982 instrument for s t r u c t u r a l analyses and Rheology DVE instrument for measurement of damping a b i l i t y . Scanning electron microscopy was performed on samples etched with strong chromic a c i d . The mechanical properties were measured at 20°C by t e n s i l e test. Damping A b i l i t y IPNs consisting of v i n y l compounds, phenolic novolacs, and epoxies were synthesized for evaluation of damping a b i l i t y . The test specimens were prepared by curing the raw materials to IPN on 0.8mm thick s t e e l sheets. The thickness of these IPNs were about 3.0mm.

434


Damping properties were measured by the impulse hammer technique at resonant frequency, and the logarithmic decrement was calculated by the haIf-power-width method. Results and Discussion Reaction Dynamics Figure 1 shows time-conversion curves of MMA polymerization i n phenolic r e s o l by using various i n i t i a t o r s . When BPO was used, MMA did not polymerize, and a long induction period resulted when DCP was used. However, even i n phenolic r e s o l , MMA polymerization proceeded when c e r t a i n kinds of i n i t i a t o r were used, such as AIBN. Figure 2 shows time-conversion curves of MMA polymerization i n phenolic r e s o l compared with that i n toluene. In phenolic r e s o l , MMA polymerization proceeded more r a p i d l y . However, there was a short induction period. These r e s u l t s suggest that by selecting the appropriate i n i t i a t o r , v i n y l compound/phenolic IPNs can be expected. Next, polymerization and curing reactions were conducted simultaneously. Table I shows the results of the reactions. By using various v i n y l compounds and i n i t i a t o r s , polymerization of over 90% and condensation conversion over 80% was obtained (as measured by IR spectrum of the cured materials). Figure 3 shows t y p i c a l IR spectra of dimethacrylate and phenolic resol IPN. Reaction conversion was calculated from the peak of C-C double bond, methylol, and carbonyl groups. Structural analyses Dynamic mechanical properties of IPN 1, IPN 2, poly 23G and cured phenolic resol (CPR) were examined, i n order to investigate IPN structure. Figure 4 shows l o s s tangent-temperature curves. Each IPN contained 30% of 23G and 70% of phenolic r e s o l . Poly 23G had i t s loss tangent peak i n the narrow region of -60°C to -20°C. In cured phenolic r e s o l , no peak was found, but the l o s s tangent increased gradually at temperatures over 200°C. IPN 1 had a broad peak i n the region of 30°C to 160°C, and a very small one at -40°C, just l i k e poly 23G. On the other hand, two d i s t i n c t peaks were found i n IPN 2, a small one at -50°C, and another at 0°C, and the loss tangent increased gradually at temperatures over 200°C. The curve for IPN 2 indicated a phase separation: the peak i n the lower temperature region was similar to that for poly 23G, and the curve over 200°C was just as for cured phenolic r e s o l . The peak at 0°C was attributed to IPN structure. This peak i s discussed again l a t e r . Compared with IPN 2, the method of making IPN 1 introduced a high l e v e l of compatibility between the components being reacted. Polymerization of 23G proceeds i n a way different from the curing reaction of phenolic r e s o l . A c r o s s - l i n k i n g reaction between 23G and phenolic r e s o l cannot be considered because there


60


120

R e a c t i o n Time

R e a c t i o n Time

—O— AIB — A I B N -A— BPO at 70 °C Figure

1.

Polymerization

— A B O DCP

—A— at o f MMA

90

°C

i n phenolic

resol.

F i g u r e 2. T i m e - c o n v e r s i o n c u r v e o f MMA polymerization. (Reproduced from r e f . 11. C o p y r i g h t 1986 A m e r i c a n Chemical Society.)

435

436


Table I No.

Oligomer

Results of IPN Reactions

Initiator

Temp.

Conversion (%) Polymerization Condensation 81 93 81 84 89 88 82 89 78 90 96 100 95 100 91 91 82 98 76 98

1 2 3 4 5 6 7 8 9 10

Unsaturated Polyester

a) b) c) d)

AIBN 2 , 2 ' - A z o b i s i s o b u t y r o n i t r i l e ACHN l , l * - A z o b i s (1-cyclohexanecarbonitrile) ABO 2,2*-Azobis (2,4,4-trimethylpentane) DCP Dicumyl peroxide

a

AIBN J ACHN ) AB0 ) DCP ) ACHN.ABO ACHN.ABO

170

b

C

d

PEG 23 Dimethacrylate PEG 9 Dimethacrylate Polyurethane A c r y l a t e PEG 23 Dimethacrylate

«

AIBN.ACHN AIBN.ABO

ti



437

are no active points where reaction can occur. These results indicate that when 23G polymerization and the phenolic resol curing reaction take place simultaneously, the polymer chains are well entangled with each other and have a strong i n t e r a c t i o n . On the other hand, when each reaction proceeds sequentially, the polymer chains have poor entanglement, and that permits a phase separation. Figure 5 shows the influence of IPN component r a t i o on Tg. Each sample was of IPN 1 type, prepared by the simultaneous method. Increasing the amount of 23G lowered the Tg of the IPN. This result also suggests that these IPNs show a good a b i l i t y to mix. The Tg of IPN containing 70% of 23G was shown to be about 0°C, just l i k e IPN 2 synthesized by the sequential method. The structure of 70%-23G-IPN can be considered to be cured phenolic resol i n a continuous poly 23G phase. So, the l o s s tangent peak of IPN 2 at 0°C was attributed to the structure of 23G r i c h IPN such as 70%-23G-IPN. Scanning electron microscopy of IPN 1 and IPN 2 was conducted next. Figure 6 shows micrographs of the surface etched with strong chromic a c i d , which s e l e c t i v e l y dissolves the 23G phase. IPN 1 had a smooth surface, indicating that the 23G p a r t i c l e s were very small and evenly distributed, while these of IPN 2 were rough, indicating that the 23G p a r t i c l e s were much larger and l e s s well d i s t r i b u t e d , leaving a larger p i t when removed. IPN 1 showed a high l e v e l of compatibility between the two components, much more than that of IPN 2. F i n a l l y , mechanical properties were measured at 20°C by a t e n s i l e t e s t . The t e n s i l e strength, elongation and modulus are shown i n Table I I .

Table II Sample

IPN 1 IPN 2 Poly 23G CPR

Tensile Strength (MPa) 34 8 1 11

Mechanical Properties Elongation (%) 1.5 0.6 11.0 0.3

Tensile Modulus (MPa) 2650 1270 10 4310

Experiments were carried out at 20°C.

Cured phenolic r e s o l showed low strength, small elongation, and high modulus. This i s a t y p i c a l b r i t t l e material. Poly 23G showed low strength, large elongation and very low modulus. This material i s very weak i n mechanical strength. IPN 2 showed low strength, elongation and modulus. This material i s also weak. However, IPN 1 showed high strength, r e l a t i v e l y large elongation, and moderately high modulus. This i s a tough material compared with the others. These r e s u l t s also suggest that poly 23G chains

438




439

and cured phenolic r e s o l chains i n IPN 1 are well entangled with each other, and that t h i s entanglement makes i t tough. Damping A b i l i t y From the results of the previous section, i t i s considered that IPN 1 has properties of both poly 23G and cured phenolic r e s o l . If a rubber-like polymer i s used as the v i n y l polymer, t h i s IPN w i l l show good damping properties at elevated temperatures. So, butyl acrylate, ethylene g l y c o l dimethacrylate, phenolic novolac, and bisphenol A type epoxies were used as IPN components. The dynamic mechanical properties of these IPNs were examined f i r s t , because the loss tangent i s very important to damping properties. Then the damping properties of IPN and commercial chloroprene rubber were measured at various temperatures. Table I I I shows the composition of IPNs prepared f o r dynamic mechanical analysis.

Table III

No.

Aerylates BA EGD (g) (g)

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

19 28.5 38 47.5 28 23 17 45 45 45 47.5 47.5

1 1.5 2 2.5 2 7 13 5 5 5 2.5 2.5

Phenolics EP PN (g) (g) 30 26 22.5 19 26 26 26 19 19 19 27.5 22.5

Composition of IPNs

Initiator (g)

50 44 37.5 31 44 44 44 31 31 31 22.5 27.5

BA Butyl Acrylate, EGD PN Phenolic Novolac, EP 0.5 wt% of curing accelerator was

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 1.0 0.1 0.1

EGD EP BA PN Acrylates (molar (equiv. (wt%) ratio) ratio) 1.0 20 0.04 0.04 1.0 30 1.0 0.04 40 1.0 50 0.04 1.0 0.08 30 1.0 0.20 30 1.0 30 0.50 1.0 30 0.08 1.0 0.08 50 1.0 50 0.08 0.5 50 0.04 0.75 50 0.04

Ethylene Glycol Dimethacrylate, Epoxy, used.

The relationship between the loss tangent or Tg and acrylate content, the molar r a t i o of dimethacrylate to acrylate, the i n i t i a t o r concentration, or the equivalent r a t i o of epoxy to phenolic are examined. The l o s s tangent was not affected by the acrylate content, but Tg was s l i g h t l y lowered with an increase of acrylate content. (See Figure 7.) Figure 8 shows the influence of the molar r a t i o of dimethacrylate to acrylate, but t h i s parameter had no effect on the loss tangent or Tg. The loss tangent was also not affected by the i n i t i a t o r concentration, but Tg was raised as i t increased. (See Figure 9.) Figure 10 shows the

440


IPN Figure

6.

IPN 2

1

SEM m i c r o g r a p h

o f etched surface

o f IPN 1 and IPN 2.

- 150

10

20

Acrylate Figure

7.

Influence

30

40

50

Content ( w t % )

of acrylate

c o n t e n t on l o s s

tangent and T . g

24.

YAMAMOTOANDTAKAHASHI


-

150

1.0 —

-

c

O

_

0)

100

c

n

o

-

-

_

_

u o

60 H

50

0

i 0.1

i 0.2

i 0.3

i 0.4

Dimethacrylate/Acrylate Figure and T

8.

Influence

i 0.5 (molar

ratio)

o f d i m e t h a c r y l a t e / a c r y l a t e on l o s s

tangent

150 1.0

c

100

0) 60

§

0.5

o-

"O

H

to

60 H

50 J

0

Influence

i

i

0.5

Initiator F i g u r e 9. and T„.

I

L

1.0

Concentration

of i n i t i a t o r

i

(wt%)

concentration

on l o s s

tangent

441

442

SOUND AND VIBRATION DAMPING WITH

-

POLYMERS

150

1.0

c Q) C

0.5

Hcn

cn O 50

- tf 0

0.5

0.75

Epoxy/Phenolic Figure

10.

Influence

1.0 (equiv.

ratio)

o f e p o x y / p h e n o l i c on l o s s

t a n g e n t and

T,

influence of the equivalent r a t i o of epoxy to phenolic. The loss tangent was decreased, and Tg was raised with an increase i n the equivalent r a t i o of epoxy to phenolic (which controlled the density of c r o s s - l i n k i n g i n the cured phenolic). An IPN with an epoxy to phenolic r a t i o of 0.5 had a r e l a t i v e l y large loss tangent, about 0.9 at 50°C. These r e s u l t s lead to the conclusion that the parameter most e f f e c t i v e i n c o n t r o l l i n g the loss tangent and Tg i s the equivalent r a t i o of epoxy to phenolic. In t h i s system, phenolic components mainly determine the dynamic mechanical properties rather than acrylates. An IPN with a r a t i o of epoxy to phenolic of 0.5 had a r e l a t i v e l y large loss tangent, so i t was expected to show good damping properties. Figure 11 shows the loss tangent and the logarithmic decrement of t h i s IPN, along with chloroprene rubber used as a commercial damping material. In the case of IPN, the maximum logarithmic decrement was at 40°C, which was near the Tg of t h i s material. On the other hand, chloroprene rubber, which has Tg of -30°C, showed no damping e f f e c t above room temperature. However, the logarithmic decrement increased with decrease of temperature below -5°C. These r e s u l t s support the proposal that the l o s s tangent i s an index of damping properties. Judging from the loss tangent, IPNs are useful damping materials at elevated temperatures. Figure 12 shows the temperature dependence of the logarithmic decrement of neat IPN and f i l l e d IPN. The logarithmic decrement of neat IPN showed r e l a t i v e high attenuation, while f i l l e d IPN prepared by adding p l a t e l e t f i l l e r s showed an even higher attenuation over a wide temperature range.


F i g u r e 11. I n f l u e n c e and l o s s t a n g e n t .


o f t e m p e r a t u r e on l o g a r i t h m i c

443

decrement

u

o.oi

L_j

i 0

U

50

100

Temperature — • — •

(°C)

F i l l e d IPN Neat IPN

F i g u r e 12. T e m p e r a t u r e dependence of IPN damping m a t e r i a l .

on l o g a r i t h m i c

decrement

Conclusions P r o p e r t i e s o f v i n y l compound/phenolic IPN were d i s c u s s e d and t h e f o l l o w i n g c o n c l u s i o n s drawn. MMA r a d i c a l p o l y m e r i z a t i o n proceeded r a p i d l y i n the presence of phenolic r e s o l . Poly (ethylene g l y c o l ) d i m e t h a c r y l a t e , 23G, and p h e n o l i c r e s o l IPNs were s y n t h e s i z e d by s i m u l t a n e o u s r a d i c a l p o l y m e r i z a t i o n and p h e n o l i c r e s o l c u r i n g reaction. These IPNs had a s t r u c t u r e o f p o l y 23G c h a i n s and c u r e d p h e n o l i c r e s o l c h a i n s so w e l l e n t a n g l e d w i t h e a c h o t h e r t h a t t h e whole a c t e d as a s i n g l e phase. T h i s t y p e o f IPN i s c o n s i d e r e d t o

444


have p r o p e r t i e s o f b o t h v i n y l polymers and p h e n o l i c s . By c h o o s i n g t h e IPN component m a t e r i a l s and t h e i r r a t i o , t h e Tg o f IPNs c a n be d e s i g n e d t o be i n t h e r e g i o n o f 50°C t o 150°C. J u d g i n g from t h e r e s u l t s o f dynamic m e c h a n i c a l a n a l y s e s , IPNs showed more e f f e c t i v e damping p r o p e r t i e s t h a n commercial c h l o r o p r e n e rubber a t e l e v a t e d t e m p e r a t u r e s . I n a d d i t i o n , f i l l e d IPNs prepared by a d d i n g p l a t e l e t f i l l e r s showed even h i g h e r a t t e n u a t i o n ( l o g a r i t h m i c decrement)•

Literature Cited 1. Bakeland, L. H. U.S. Patent 939 966, 1909. 2. Bakeland, L. H. U.S. Patent 942 852, 1909. 3. Azlak, R. G.; Joesten, B. L.; Hale, W. F. Polym. S c i . Technol. 1975, 9A, 233. 4. Demmer, C. G.; Garnish, E. W.; Massy, D. J. R. Br. Polym. J . 1983, 15, 76. 5. Bachman, A.; Müller, K. Plaste Kautshuk 1977, 24, 158. 6. Kosfeld, R.; Borowitz, J. Prog. Colloid Polym. S c i . 1979, 66, 253. 7. C a h i l l , R. W. Polym. Eng. S c i . 1981, 21, 1228. 8. Borowits, J.; Kosfeld, R. Angew. Makromol. Chem. 1981, 100, 23. 9. Aylsworth, J. W. U.S. Patent 1 111 284, 1914. 10. Bailey, A. E. In Encyclopedia of Chemical Technology; Kirk, R. E.; Othmer, D. F . , Ed.; The Interscience Encyclopedia, Inc. New York, 1948; Vol. 2, p.71. 11. Yamamoto, K . ; Kumakura, T.; Yoshimura, Y. Polymer Preprints, Japan, 1986, Vol. 35, p 802. RECEIVED January 24, 1990

Vinyl Compound and Phenolic Interpenetrating Polymer Networks

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