The Network Structure of Epoxy Systems and Its Relationship to

Chapter 11

The Network Structure of Epoxy Systems and Its Relationship to Toughness and Toughenability 1

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Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 7, 2013 | http://pubs.acs.org Publication Date: August 8, 2000 | doi: 10.1021/bk-2000-0759.ch011

H.-J. Sue , P. M. Puckett , J. L. Bertram , and L. L. Walker 1

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123 Epoxy Products Department, The Dow Chemical Company, Freeport, TX 77541 2

Beginning in about 1980, epoxy system networks were developed via an Edisonian approach that formed tough high T polymers (e.g., CET, crosslinkable epoxy thermoplastic resins). The driving force for the innovation was the need for inherently tough resins that could provide a base for formulation of aerospace prepregs and adhesives. Classical approaches to toughening epoxies either involved addition of toughening agents (rubber or thermoplastic) or reducing network crosslink density. Both approaches had significant drawbacks in their effects on polymeric properties and/or processability. This inherently tough CET chemistry was modified and extended in about 1990, to make it more suitable for liquid molding applications (e.g. resin transfer molding, pultrusion, etc.). Even though hundreds of CET formulations were investigated, no systematic investigation of the structure property relationships for this resin type was undertaken until recently. Model CET epoxy networks, with variations in crosslink density and monomer rigidity, were prepared to study how the network structure affects modulus, T , and toughness (toughenability). Diglycidyl ethers of bisphenol-A, tetrabromobisphenol A, and tetramethylbisphenol-A, along with the corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties of epoxy polymers. The present study indicates that, as expected, the backbone rigidity of the epoxy network, not the crosslink density alone, will strongly influence the modulus and T of epoxy resins. Upon rubber toughening, it is g

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© 2000 American Chemical Society

In Toughening of Plastics; Pearson, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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found that the rigidity of the epoxy backbone and/or the nature of crosslinking agent utilized are critical to the toughenability of the polymer formed. That is, the well-known correlation between toughenability and the average molecular weight between crosslinks (M ), which usually corresponds to the ductility of the epoxy resin, does not necessarily hold true when the nature of epoxy backbone molecular mobility is altered. The potential significance of the present findings for a better design of toughened thermosets for structural applications is discussed. Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 7, 2013 | http://pubs.acs.org Publication Date: August 8, 2000 | doi: 10.1021/bk-2000-0759.ch011

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Introduction A very high percentage of the materials used in fabrication of aerospace composites and adhesives have been made from epoxy resins. Formulators are constantly pushing to extend the limits of epoxy technology to generate higher T , tougher polymers, with improved modulus, and lower moisture absorption, while retaining all the processability of conventional epoxy systems. Development of inherently tough epoxy resins that could either be toughened further by formulation with rubbers, elastomers, or thermoplastics, or could be used in liquid molding applications without addition of second phase toughening agents was believed to be particularly desirable. In the last 20 to 30 years, it has been demonstrated that low molecular weight epoxy resins, which usually exhibit good processability, when cured with multifunctional curing agents, are brittle and cannot be effectively toughened [1-5]. The use of these materials for many structural and electronic applications is limited by this lack of ductility. On the other hand, when epoxy monomer molecular weights are high, although they become much more toughenable, their processability, rigidity, and use temperature can be greatly reduced. As a result, they begin to lose their suitability for composites, adhesives, and coatings applications. Resin formulation choice for these applications becomes a continuing series of trade-offs between viscosity, modulus, cure time, T , moisture resistance, and toughness. The common approach to generating a toughened epoxy resin has been addition of a rubber or thermoplastic. As early as the late 1960s, rubber modification was reported as an effective approach for toughening brittle epoxy resins [1]. Since then, many workers have developed an understanding of how rubber particles toughen epoxy systems. The size and type of rubber particle in the epoxy network [2-4], the curing agent and crosslink density of the epoxy polymer [5-7], and the effect of the curing schedule on rubber particle phase separation [6,7], all influence the extent of matrix toughening achieved in epoxies and other highly crosslinked thermoset systems. While progress was made in understanding the physics of rubber toughening and the mechanics offractureand crack propagation, compromises brought about by rubber toughening were often found to be too big a compromise for many applications [2-7]. This was especially true as compromises that enhanced properties began to decrease the processability of resin formulations. This type of compromise ultimately effects the cost to manufacture, a trade-off that industry is very reluctant to make. g

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To avoid the drawbacks associated with rubber toughening, thermoplastic modifiers were utilized to replace rubber particles in toughening [8-12]. However, it is found that the toughening effect becomes significant only when phase inversion takes place or when the thermoplastic composition reaches approximately 30% or more by weight [10,12]. Dispersion of thermoplastic particles in epoxy is typically a difficult art. In addition, the processability of these systems in comparison to unmodified resins is normally more complex and the other physical and mechanical properties of the modified resin are compromised for the sake of matrix toughness. Even though thermoplastic modified epoxies are commercially available as prepreg and adhesive formulations, there is still room for significant progress in this area. Even with all the limitations of thermoplastics and rubbers as toughening agents for epoxy, state-of-the-art prepreg materials (e.g. Toray T800/3900-2, Hexcel IM7/8551-7 or 8552, and IM-7/977-2) have been developed which provide exceptionally good impact resistant composite systems when properly processed [13-18]. These damage tolerant composites typically possess a resin rich interlaminar region, which has been toughened by the addition of relatively large elastomeric or thermoplastic particles. This type of tough composite micro-architecture based on second phase toughening agents cannot be used in applications when a liquid molding process forms the composite. The use of large particulate toughening agents is unworkable primarily because the fibers act as afilter,trapping particles as the resin flows through them and making a controlled interlaminar thickness impossible. Controlling the thickness of the resin rich interlaminar region is a large contributor to the overall composite toughness [19]. Furthermore, second phase toughening tends to increase resin viscosity to a level that makesfiberwet out and saturation through thick fibrous bundles prohibitively difficult. An approach to generating impact resistant composites that does not use toughened resin formulations is the use of a three dimensional (3-D) preform [20-22]. The presence of fibers in a Z-axis orientation physically ties together the lamina and that suppresses failure via a delamination mode. Composite fabrication via liquid molding techniques (e.g. resin transfer molding, RTM) using 3-D preforms and even resins known to form a very brittle polymeric matrix have generated some remarkably damage tolerant composite systems as measured by CAI [23-27]. However, the triaxial stress states that are induced by resin shrinkage and coefficient of thermal expansion mismatch during elevated temperature curing in the interstitial resin rich regions of a X-Y-Z reinforced composite are almost always relieved by microcrack formation. Use of a ductile resin that will not microcrack in this type of stressfieldis the preferred solution for RTM. Development of ductile epoxy systems that are inherently tough and can serve as the basis for high viscosity prepreg and adhesive formulations as well as much lower viscosity liquid molding formulations was begun in the early to mid 1980s. This chemistry does not require second phase toughening to produce a ductile crosslinked polymer with high fracture toughness, but the addition of rubber and/or elastomers as would be expected further increases toughness.


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Crosslinkable Epoxy Thermoplastic Resins The crosslinkable epoxy thermoplastic (CET) chemistry, developed by Bertram, et. al. [28], has made it possible to produce commercially viable high T , high modulus, and low viscosity epoxies for high performance structural applications. The chemical design was based on two fundamental concepts: (1) the utilization of low molecular weights epoxy monomers with stiff backbones or bulky side groups to achieve high modulus and high T polymers [28-31] and (2) the promotion of in-situ chain extension of the epoxy backbone during cure, which then leads to a lower crosslink density epoxy network with greater processability and excellent inherent toughness and toughenability. The measured toughness for many of these formulations was often comparable to that of engineering thermoplastics, and they were referred to as CET resins. A driving force in the creation and evolution of the CET materials was the desire to produce a formulated epoxy that was exceptionally tough, but was more processable than a typical two phase toughened epoxy system. Second phase toughening can be problematic to fabrication processes for a variety of reasons including, high viscosity, poor wet out of fibers or fillers, and final matrix toughness that is dependent on the thermal history of the cure. Because the CET chemistry begins with low molecular weight monomers, it has none of these problems. The rate of cure in the system can be adjusted by altering the cure temperature without any significant effect on the properties of the finished matrix. The first commercial CET resin (designated Prepreg Resin 1) introduced required the addition of a curing agent and was targeted at the prepreg and adhesive formulators market. This CET chemistry evolved over the next several years to become a fully formulated resin system that could be used directly by molders that were fabricating by either pultrusion or RTM. A representative material of this class is shown as RTM Resin 2. While the starting materials used to make these two representative CET resins are different, the properties are remarkable not only for their similarity, but also for their excellent overall performance. Table 1 compares the properties of unreinforced polymers of Prepreg Resin 1 (cured with stoichiometric DDS, diaminodiphenylsulfone) and RTM Resin 2. The ductility of the CET resin translates well, forming a very tough composite structure without the need for addition of rubber or thermoplastic. Properties of unreinforced clear castings are provided for only two resins, but they are representative of properties observed for dozens of CET systems that were made and characterized. When RTM Resin 2 and similar formulations were converted to composites (55% fiber volume) using AS4-6K 5HS fabric excellent properties were obtained. Typical ambient properties were SB S of 70 MPa, Compression Strength of 725 MPa and, CAI Strength of 262 MPa. With the low equilibrium moisture absorption of these materials, typically about 1.5%, there is excellent retention of properties up to temperatures of 121°C. The examination of processing characteristics for CET resins also showed some unexpected results. A typical epoxy system on cure will release 300-400 joules/gram of energy. However, a typical cure exotherm for a CET resin is only 150-180 joules/gram. With significantly less energy being liberated during cure, there is less residual thermal stress found in a composite part. Also, typical cure shrinkage g


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Table I. Unreinforced Mechanical Properties of Prepreg Resin 1 and RTM Resin 2.*

Test

Measurement

Prepreg Resin 1

RTM Resin 2

Tensile

Strength (MPa) Modulus (GPa) Elongation (%)

90 3.07 5.5

83 3.03 5

Flexural

Strength (MPa) Modulus(GPa)

131 3.10

145 3.28

Moisture Absorption

Weight (%)

1.4

Density

Resin (g/cc)

1.5

1.3

Thermal --DSC -DMA[dry/wet] -DMA[dry/wet]

Tg (°C) Tg (Tan δ, °C) Tg(E" , °C)

168 168/160/--

167 174/167 165/155

CLTE

below Tg, ppm/°C

59

61

Cure Shrinkage

density change (%)

0

Viscosity

The Network Structure of Epoxy Systems and Its Relationship to

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