CHAPTER-1 INTRODUCTION

Transcription

1 CHAPTER-1 ITRDUCTI This chapter deals with the introduction of thermosetting plastics, the need for toughening, methods of improving properties and different toughening mechanisms. Bismaleimide resin and its properties are discussed along with its advantages and drawbacks and scope for improvement.modification methods of bismaleimides, bismaleimide based composites and their applications are also discussed. bjectives of the research work and thesis outline are given at the end of the chapter. 1.1 Method of improving Properties of Thermosetting Polymers The inertness, low specific weight, low production cost, variability of mechanical properties, corrosion resistance, fluidity and other unique properties of thermosetting polymers and their composites make them more attractive for industrial, automotive, aerospace and other electronics applications [1]. The drawbacks, such as, low fatigue resistance and non-bio-degradability made them limited usage in severe environment [2-3]. Polymer finds application in many different forms ranging from structural composites in the construction industry to the high technology composites of the aerospace and space satellite industries [3-5]. Due to lower thermal stability compared to metals, active research is being done to improve the thermal stability of the polymers and polymer composites. Researchers use alternate methods for strengthening the polymer which includes cold working [6], hot working [7], fiber-reinforced polymer composites [8], and blends of the polymer materials [9]. The thermosetting polymers generally exhibit brittleness due to their high crosslinking density within the structure. [10] To overcome the above cited limitation / drawbacks of the thermosetting polymers can be modified by adding additives such as, anti oxidant, surfactant, and cupling agents etc. The additives that are usually added to thermoset resin systems to improve the flameretardant features and toughness of network polymers are known as flame retardants and toughening agents, respectively [11]. University of Mysore 1

2 Thermoset resins are susceptible to degradation through oxidation reactions during processing at a high temperature, during thermal treatment, and outdoor exposure. This problem can be addressed by adding antioxidants like B-carotene and A-tocophenol are examples of natural antioxidants.[12] The incompatibility of filled polymer systems, which generates weak interfaces, leading to a drastic reduction in mechanical properties can be addressed by using coupling agents such as, trichlorovinyl silane, triethoxyvinyl silane, and G- glycidoxypropyl-trimethoxy silane which are examples of coupling agents for thermosetting resins [13]. Surfactants such as, sodium dodecyl sulfate (SDS) and sodium salt of dodecyl benzene sulfonic acid (addbsa) are added to a thermoset resin system to promote the dispersion of fillers in the resin matrix. The design of such additives to make a high-performance resin has been investigated extensively in recent years by researchers [14-15]. 1.2 Toughening Requirements and Toughening Mechanism Due to high crosslink density and a rigid structure, thermosetting polymers exihibit brittleness in nature which leads to pre-mature failure during real applications[16]. Therefore, enhancing toughness of polymer is a key factor for present context. Recent years, many advances have taken place in the theoretical understanding of the toughening mechanisms [17] for thermosetting resin systems. Some important aspects of these toughening mechanisms are summarized below: 1.2.1General toughening mechanisms for thermosetting polymers Crack-pinning mechanism This theory states that as the crack begins to propagate through the resin, the crack front bows out between the filler particles but remains pinned at the particles. This mechanism is based on the function of small particles as toughening agents. Because this mechanism operates mainly with inorganic fillers that resist fracture during failure of the matrix resin, it is generally less important in ductile matrix materials [18]. University of Mysore 2

3 Microcracking mechanism Microcracks due to reinforcing particles cause tensile yielding and, thus, a large tensile deformation. Debonding or microcracking effectively lowers the modulus in the frontal process zone around the crack tip, and thus effectively reduces the stress intensity there. But this theory could not explain many phenomena such as, stress-whitening, large amount of plastic deformations, higher fracture toughness at a higher temperature, and the fact that nonreactive rigid thermoplastic particles also may toughen some systems [19] Localized shear yielding (or Shear banding) mechanism The mechanism that involves dilatational deformation of the matrix and cavitation of the rubber particles in response to the triaxial stresses near the crack tip is combined with shear yielding between the holes formed by the cavitated rubber particles. The stresswhitening was attributed to light scattering by these holes, and the major energy absorption mechanism was suggested to be the plastic deformation of the matrix. Plastic deformation blunts the crack tip, which reduces the local stress concentration and allows the material to support higher loads before failure occurs [20] Particle bridging (Rigid particles) mechanism In this toughening mechanism, it is proposed that a rigid or ductile particle plays two roles: (1) It acts as a bridging particle that applies compressive traction in the crack wake. (2) The ductile particle deforms plastically in the material surrounding the crack tip, which provides additional crack shielding. Sigl et al also pointed out that the shielding which resulted from yielded particles is negligible, and that the particle bridging provides most of the improvement in toughness [21] Crack-Path deflection mechanism The crack-path deflection may explain the increase in toughness by a stress intensity approach. There are both mode I and mode II characters of the crack opening. Most materials are more resistant to mode II crack opening. The deflection of the crack path decreases the mode I crack opening, but increases the mode II crack opening and hence the materials exhibit a higher apparent toughness [22]. The toughening mechanism of thermosetting resins may be a combination of the above two or several more mechanisms [23-26]. University of Mysore 3

4 Even though there are various toughening mechanisms proposed by different researchers, it seems that a single theory cannot explain every experimental result and phenomenon of toughening. ne reason may be the discrepancy of the raw materials chosen by different researchers, because the initial properties of raw materials have significant influence on the final fracture properties of thermoset materials. However Crack-Path deflection mechanism is the most feasible method adopter according to the literature. 1.3 Bismaleimides: Properties and Advantages Bismaleimides (BMIs) are a relatively new class of polyimides which have gained acceptance for wide industrial application. They offer improved high temperature properties compared to the epoxy systems. In polyimides, BMIs (Fig.1.1) still dominate the high temperature polymer scenario due to their thermal and oxidative stability, flame retardence, low propensity for moisture absorption, ease of synthesis and cost effectiveness. A large variety of addition polyimides, with good processability and thermal stability has been successfully used as matrix resins in high performance and high temperature resistant composite applications. The double bond of the maleimide is quite reactive. Grund Schober first reported their homo polymerization by simply heating the monomer to a temperature between o C. Crosslinked BMI polymers are insoluble in all solvents, infusible, rigid and brittle. They have relatively high densities ( gm/cc) and exhibit Tgs generally in the range of o C. The strain to failure is typically below 2%. The moisture absorption levels in BMI resins tend to be much the same as in Epoxy resins (4-5%by weight) but saturation occurs faster than in epoxy resins [27-30]. University of Mysore 4

5 R Bismaleimide CF 3 C CF 3 CH 2 CH 2 C n Fig.1.1. Structures of condensation polyimide and addition polyimide 1.4 Drawback of Bismaleimides and Scope for Improvement Due to high crosslink density and a rigid polymer structure, bismaleimides exhibit high Tg (> 400 C) and low fracture toughness (with Gic << 50 J/m 2 ). Therefore, enhancing toughness of bismaleimides is a key factor for the advancement of these materials. The major drawbacks of BMIs include, (i) brittleness due to their high crosslinking densities after curing, (ii) poor solubility in ordinary solvents leading to poor processibility, (iii) high crystalline melting temperatures of the monomers and (iv) narrow temperature window for processing (the temperature difference between the melting point of bismaleimide monomer and its onset point of curing reaction) which necessitates only solution processing method [31-33]. University of Mysore 5

6 The scope for improving the toughness of BMIs lies in the outstanding property of - substituted maleimide is the reactivity of the maleimide double bond. This high reactivity is from two adjacent electron-withdrawing carbonyl groups, which make the maleimide double bond very electron-deficient. When heated above their melting points, BMIs undergo a free radical polymerization reaction without the need for a catalyst. In addition to homopolymerization and copolymerization, there are many other chemical reactions involved in BMI chemistry. For example, BMI can undergo Diels-Alder reaction with a suitable diene; Ene reaction with allyl compounds; Michael addition with primary and secondary amines; and other addition reactions with cyanates, isocyanates, azomethines and epoxies. Although BMI has a brittle nature that limits their applications, the versatile reaction capability of maleimides provides the great possibilities to modify BMIs with reactive components such as reactive diluents, additives, comonomers and viscosity modifiers so that the BMI systems with desired properties can be developed [34-35]. 1.5 Chemical Toughening/ Modification of Bismaleimides The reaction capability of the maleimide groups with the active double bond is utilized to form different, modified of high performance polymeric systems. Maleimides undergo Michael addition reaction with aromatic diamines such as, primary and secondary amines, phenols, thiols, etc to form chain extended polymers. BMI being a bisdienophile, can undergo Diels-Alder reactions with dienes. Allylphenol reacts with BMI via an 'ene' reaction. Vinyl and allyl type ethylenic double bonds with maleimides are also reported. ther approaches for improving the toughness include modification with high performance thermosets and incorporation of engineering thermoplastics. Some of the methods for the chemical toughening of Bismaleimides are described below [36-38] Bismaleimide / Allyl Phenol Copolymers The rigid aromatic units tightly held by the short methylene linkages make the BMI matrix brittle. Allyl phenyl and allyl phenol compounds have been proved to be good comonomers for BMI resins. The reaction between the two components proceeds via the Ene reaction at relatively low temperature at first. The unsaturated Ene adduct University of Mysore 6

7 intermediate easily undergoes further Diels-Alder type reaction with excess BMI to give the bis- and tris adducts (Fig.1.2). The intermediate step (Diels-Alder) is sometimes referred to as Wagner Jauregg reaction too. The total reaction sequence is also referred to as Alder-ene reaction [39]. Fig.1.2: Chemical structure of BMPM/DABA adducts Bismaleimide / Michael-Addition Copolymers BMI can react with bis-nucleophilic species to form the crosslinking structure via Michael-Addition reaction (Fig.1.3). Dithiols and diamines are the favored bisnucleophilic because they have high basicity. ne of the most important developments to thermosetting polyimides is the non-stoichiometric Michael-Addition reaction between BMI and aromatic diamine. A typical adduct structure of prepolymer from BMPM and 4,4 -diamino diphenyl methane is shown is Scheme 1.3. The fracture and Tg were reported as the function of diamine content [40]. HAc H 2 Ar H Ar Ar Ar Ar n Where Ar = Where Ar = H 2 CH 2 H 2 MDA MDA Fig.1.3: Chemical structure of BMPM / 4, 4 -diamino diphenyl methane adduct. University of Mysore 7

8 1.5.3 Bismaleimide / Epoxy Copolymers Many efforts have been made to blend epoxy and BMI resins to achieve both high temperature performance and processing ease. Introduction of epoxy backbone between the maleimide ends could improve the toughness of BMIs and have less sacrifice in thermo-oxidative stability. The BMIs with epoxy and silicon linkages or phosphorus groups exhibited good organosolubility, low melting points, and excellent processability. The cured polymers were found having high glass transition temperatures above 210 C and good thermal stability over 350 C[41] Bismaleimide / Cyanate Copolymers Copolymers of aromatic cyanate esters and bismaleimide have been realized with systems having good physicochemical properties, i.e. the thermal characteristics of bismaleimide and the toughness of cyanate esters. This is one of the best modification approaches reported for the bismaleimide systems and active research is underway in this direction [42] Modification with Thermoplastics Modification of thermosets by engineering thermoplastics has been an area of research for long time. The important objective of the modification is to improve the toughness of the brittle matrices by the incorporation of thermoplastics. In the case of BMIs also, by using good heat resistant engineering thermoplastics, the toughness can be greatly improved. Currently, the most widely used thermoplastics are: Acrylonitrilebutadiene styrene (ABS), Polybutylene Terephthalate (PBT), polyethersulfone (PES), and polyetherimide (PEI). The toughening effect depends on the thermoplastics chain structure, molecular weight, particle size, and end groups. ne of the disadvantages of thermoplastic modification is the enhancement of the viscosity after the loading of thermoplastics, which inturn results in the increase of the overall viscosity of the resin, which further leads to the processing difficulty. Again, the thermal stability of the thermoplastic modifier determines the overall temperature resistance of the matrix [43-45]. University of Mysore 8

9 1.6 Bismaleimide based Composites and their Application Recent innovations in science and technology has created a necessity for higher and higher performance based materials in aerospace, military, defense and engineering applications[46-49]. As a result, the conventional structural materials are being replaced by different types of composite materials, the properties of which can be tailored to meet any specific requirement by proper selection and modification of the matrix, reinforcement, interface and processing technique [50-52]. Day by day world is witnessing a spectacular growth in the application of bismaleimide based composites in every possible use and which has led to the innovation of cost effective material solutions for replacing metals, alloys and ceramics for a wide spectrum of applications. These have been developed primarily for aerospace industry in which the demand for strong and stiff lightweight structures overcomes the prohibitive costs of early composite materials [53-54]. Extensive use of fiber reinforced bismaleimide polymer matrix composites has been identified as a viable means of reducing weight and thus increasing the fuel efficiency and payload in aerospace applications. In the development of reusable space transport systems, one of the major challenges is the development of materials and structures to meet the flight requirements, and withstand the loads and adverse conditions like high temperature and aggressive plasma environment. Polymer matrix composites are widely applied in aerospace areas for their high specific stiffness and strength, low coefficient of thermal expansion and good fatigue resistance. High performance BMI resin is one of the most favorable matrix materials of all heat resistant polymers [55-60]. As a class of high temperature thermosetting resin, BMI resin has superior performance to traditional epoxy resin on account of its durability under exposure to harsh service environment in space. In addition, BMI resin-based composites possess good processing performance in reducing the structure assembly time and overall costs as compared to other thermoplastic composites. Thus, carbon/bmi composites have gradually taken place of epoxy resin based composites as ideal candidates for construction materials in aerospace applications [60-66]. University of Mysore 9

10 1.7 utline of the Thesis The thesis has been divided in to six chapters. Chapter one deals with the introduction to high performance thermosets and its composites. The main focus of the first chapter is the introduction to bismaleimides. Chapter two deals with the recent literature review on the developments in modification field of BMIs and its composites with respect to thermoplastic modification, olifinic compounds, cyanate esters, epoxy resins etc. Chapter three gives a brief account of the various materials used in the present study for the processing of polymer blends and composites along with the methods and processing techniques employed for their production. A brief account of the characterization techniques adopted for the performance / quality evaluation of the chemicals, materials, polymer blends and composites are given in this chapter along with the details of the instruments used for the same. Chapter four deals with the modification of various BMIs with allyl compounds. The first part deals with BMIP/DABA system and the second part deals with BMPM/ Allyl ovolac system. The synthesis, characterization, curing studies and finally the composite preparation and characterization is also described in the chapter. Chapter five of the thesis deals with synthesis of BMI oligomers having both Bismaleimide and allyl group in the same back bone. The synthesis, characterization, curing studies and finally the composite preparation and characterization is also detailed in the chapter. Chapter six of this dissertation deals with the modification studies of the BMIs with thermoplastic Acrylonitrile-butadiene-styrene (ABS). This chapter also describes the characterization of the modified resin and the comparison of the modified resin with unmodified one. University of Mysore 10

11 Chapter seven summarizes the overall conclusions from the different investigations carried out for the present study and highlights the scope for further quality improvement for new modification and blending. University of Mysore 11