Rubber-Modified Thermoset Resin - American Chemical Society

8 Rubber-Toughened Polyimides

Downloaded by COLUMBIA UNIV on March 20, 2013 | http://pubs.acs.org Publication Date: December 5, 1984 | doi: 10.1021/ba-1984-0208.ch008

A. J. K I N L O C H , S. J. SHAW, and D. A. TOD Ministry of Defence (PE), Propellants, Explosives and Rocket Motor Establishment, Waltham Abbey, Essex, United Kingdom The effects of adding up to 100 parts per hundred parts resin (phr) of a rubber to a bismaleimide resin on such properties as fracture energy, modulus, flexural strength, glass transition temperature, and decomposition temperature are described. The two-phase morphology resulting from this rubber addition is discussed in terms of data obtained from both scanning electron microscopy and dynamic mechanical analysis. The results obtained suggest that up to 50 phr may be incorporated to produce a 20-fold increase in fracture energy without major sacrifices in other important properties.

P O L Y I M I D E S A R E O R G A N I C P O L Y M E R S that i n general possess very high thermal and thermo-oxidative stabilities. They are used i n a number of industrial applications, especially as adhesives and as matrices for fiber-composite materials. Most polyimides may be classified either as condensation-type or addition-type polymers according to the type of polymerization reaction used to produce them from their constituent(s) (I). C o n d e n sation-type polyimides are generally prepared from dianhydrides and diamines via the formation of a soluble polyamic acid precursor. T h e formation of polyimides from the polyamic acid involves the evolution of volatiles, w h i c h can cause voids in the final product. Addition-type polyimide prepolymers are cured by an addition reaction that over comes the problem caused by evolution of volatiles. Addition-type polyimides are prepared by several routes. O n e of the most common, and the one employed i n this work, involves using the activated maleimide double bond. However, one disadvantage of addition-type polyimides based upon bismaleimides is that they depend to a large degree upon a highly cross-linked structure for their high-temperature capability. This structure results in them being extremely brittle. Some of the more brittle polybismaleimides have been toughened by chain ex tension of the imide prepolymer molecules. The resulting polyimides

0065-2393/84/0208-101/$06.00/0 Published 1984 American Chemical Society

In Rubber-Modified Thermoset Resins; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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R U B B E R - M O D I F I E D T H E R M O S E T RESINS

have more open, flexible molecular structures than their unmodified, brittle counterparts. However, the increase i n toughness is usually offset b y a major reduction i n other desirable properties of the poly imides, such as glass transition temperature (T ), thermal stability, and mechanical strength, because chain extension of the prepolymer reduces the density of intermolecular cross-links i n the polyimide structure. W e describe some i n i t i a l exploratory studies c o n c e r n e d w i t h achieving a two-phase microstructure consisting of a dispersed rub bery phase embedded i n and bonded to a matrix of polyimide. I n brittle, cross-linked epoxy resins this microstructure increases the toughness w i t h o u t g r e a t l y r e d u c i n g o t h e r i m p o r t a n t p r o p e r t i e s (2-8).


g

Experimental Materials and Preparation. The basic bismaleimide resin employed was a commercially available material reported (1) to consist of three bismale imide species (I) that form a eutectic mixture with a melting point of approxi mately 70 °C. The rubber used was a carboxyl-terminated, random copolymer of butadiene and acrylonitrile that can be represented by H O O C [(CHaCHCHCH^CH^HCNy^OOH.

The carboxyl content was 2.37 wt%, the acryonitrile content was 18 wt%, and the molar mass of the rubber was 3500 g/mol. Rubber modification was carried out by adding the liquid rubber to the molten bismaleimide at 120 °C. A prereaction period of 24 h at 120 °C was allowed to develop a copolymer from the constituent materials. Curing of the bismaleimide systems was achieved by heating for 2 h at 170 °C followed by 5 h at 210 °C. A total of five bismaleimide-rubber formulations were studied. Formula tions 1-5 contain 0, 10, 30, 50, and 100 parts per hundred parts resin (phr) of rubber, respectively. Thermal Properties. The T values of the cured polymers were deter mined by using two techniques: tnermomechanical analysis (TMA) and differ ential scanning calorimetry (DSC). The former was conducted with a thermomechanical analyzer (Stanton Redcroft Model 790) at a heating rate of 5 °C/min in static air. DSC was conducted with a differential calorimeter (Perkin-Elmer) at a heating rate of 20 °C/min in a nitrogen atmosphere. The thermal stabilities of the cured formulations were determined by using two techniques. First, thermogravimetric analysis (TGA) was employed with a thermobalance (Linseis LSI) in an atmosphere of helium. A heating rate of 5 °C/min was employed with sample weights of approximately 15 mg. Differ ential thermogravimetric analysis (DTA) was conducted simultaneously. Second, isothermal weight loss measurements were performed on formulations 1, 3, and 4 with a thermogravimetric analyzer (Perkin-Elmer). Samples were examined under an oxygen environment, at a gas flow rate of 40 mL/min. The temperature program consisted of an initial temperature of 250 °C, followed by heating at 200 °C/min to 330 °C, which was then maintained for 100 min. Mechanical Properties. Sheets of cured material, 6 mm thick, were cast from the five formulations. The neat bismaleimide, or prereacted rubber-bismaleimide mixture, was degassed at 120 °C and then poured into a heated casting mold. The bismaleimide systems were cured by heating for 2 h at 170 °C followed by 5 h at 210 °C.


8.

103

Rubber-Toughened Polyimides

KINLOCH ET AL.

mp. 1 5 4 ~ 1 5 6 ° C


di ( 4 - M A L E I M l DO P H E N Y L \ M E T H A N E

mp.

172-174°C

2 , 4-BISMALEIMl DO-TOLUENE

0

0 CHo

[ T \ it

I N

CH

2

II

r~

CH*j

I

C-—CH

2

mp. 70-130°C

CH — C H

2

CH

2

N

II

CH.

1,6-

BlSMALE!MID0-2,2,4-TRiMETHYLHEXANE

I D M A experiments were conducted on rectangular strips measuring 85 X 10 x 6 mm by using a mechanical spectrometer (Rheometrics). The specimens were subjected to torsional sinusoidal oscillations at a frequency of 1 Hz. Values of storage shear modulus, G , and loss shear modulus, G", were obtained, and values of the loss tangent, tan 8, were calculated from Equation 1. Measure ments were taken at approximately 10 °C intervals between —160 and 340 °C, with a heating rate between test temperatures of approximately 5 °C/min. f

tan 8 = -^r

(1)

Flexural experiments were conducted on rectangular strips measuring 110 x 20 x 6 mm by using a three-point-bend assembly attached to a mechanical testing machine (Instron). The experiments were conducted at a crosshead speed of 1 mm/min according to ASTM standard method D790-1971. Values of flexural modulus, E, and flexural strength and strain at failure were determined from the resultant load-displacement curves. Fracture properties of the cured formulations were determined by using compact-tension specimens (3) (Figure 1), which were machined from the cast sheets. Prior to testing, a saw cut was made in the position indicated in Figure


104

R U B B E R - M O D I F I E D T H E R M O S E T RESINS


72mm

Figure 1. Compact tension specimen used for determining the stress-intensity factor, K , and fracture energy, ) - I85.5(a/wf + 655.7(a/t# - I0l7(a/w) + 638.9(a/ w) ]; P = load at crack initiation; B = thickness of specimen; w = width of specimen, as defined in Figure 1; and a = crack length. The K values were converted to fracture energy,

Rubber-Modified Thermoset Resin - American Chemical Society

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