THERMOSETS

1. THERMOSETS By AHMAD CHOUDHARY 2009-MS-MME-05

2. Polymers • Term polymer derived from the Greek word poly meaning many & mer meaning part. • “---- A polymer may be defined as; a large molecules built up by repetition of small simple chemical units held together by covalent bonds. ----” • Every polymeric substance has a definite indentifying unit of molecular structure which is called a part or mer. • Polymer are complex and giant molecules (macromolecule) of higher molecular weight (104—107).

5. PLASTICS • Plastikos → Plastic (fit for molding) • “___The term plastics is given to a group of synthetic chemical compound which are highly polymers and which are at some stage plastic and capable of being shaped by heat with or without pressure, to stable form.___” • It must be noted that all plastics are polymers but not all polymers are plastics. • Plastics are available in variety of forms including fiber, coatings, moldings, castings, adhesives , films, etc.

6. PLASTICS as ENGINEERING MATERIALS • Plastics & plastics based composite have become one of the most important classes of advanced engineering materials today. • There is not a single sphere of human and economic activity today, which does not requires plastics. • Outstanding feature which justify their widespread use as engineering material are as under: • Inherently low in density →taking load → energy save • High molecular weight → high specific strength & high specific stiffness • Excellent Resistance against corrosive media (except organic solvents) → frequency of replacement less. • Relative low cost • Design flexibility → molded to any shape • Good thermal & electrical insulating properties (can be made conductive some extent) • Damping properties are good

7. Lower Coefficient of friction • In Some cases refractive index in quite high • Show direct end usability → less finishing • Easily available • Can be shaded into variety of color • Good aesthetic values CLASSIFCATION OF PLASTICS • From engineering prospectus, plastic are divided into two categories • Thermoplastics →× • Thermosetting → √ • Plastics are also classified according to level of performance • High performance plastics • Engineering plastics • Transition plastics • Commodity /general purpose plastics

8. THERMOSETTING PLASTICS Thermosetting →Thermē (heat is required to permanently set the plastic) × “____The plastics formed into a permanent shape and cured or set by a chemical reaction, cannot be remelted and reformed into another shape but degrade or decompose upon being heated at too high temperature, is called thermosets.____” CHEMISTRY OF THERMOSETTS • CROSSLINKING • “____A network polymer is formed as a result of the chemical interaction between linear polymer chains or the build-up from monomeric resinous reactant of a three dimensional fish-net configuration and the process of interaction is called cross linking____” • Cross linking is the main distinguishing element of a thermosetting.

9. The network polymer formed has an “infinite” molecular weight with chemical interconnects restricting long chain macro movement or slippage. • Molecular functionality (i.e., number of reactive moieties per mole of reactant) dictates the potential for a cross linking reaction.

10. Linear chain formation & Crosslinking via addition polymerization Linear chain formation & Crosslinking via condensation polymerization

11. INFLUENCE of TIME, TEMP. & MASS Viscosity vs. time at constant temperature for a liquid thermosetting system. • The abrupt an irreversible transformation from a viscous liquid to an elastic gel or rubber is called the gel point. • The gel point of a chemically cross-linking system can be defined as the instant at which the weight average molecular weight diverges to infinity.

12. Reaction continues beyond the gel point to complete the network formation, where physical properties such as modulus build to levels characteristics of a fully developed network. • Gelation is the incipient formation of a cross- linked network, and it is the most distinguishing characteristic of a thermosets. A thermoset loses its ability to flow and is no longer processable above the gel point, and therefore gelation defines the upper limit of work life. • For example a five minute epoxy

13. Influence of ambient cure temperature on the gel time of thermosets.

14. Influence of mass on the gel time of thermosets.

15. Mechanical property of a thermosetting polymer vs. time

16. HISTORICAL MILSTONES 1839 Goodyear discovered vulcanization of rubber. 1909 Baekeland granted his ‘Heat and Pressure’ patent for phenolic resins. 1926 Alkyd introduced. Aniline-formaldehyde introduced in U.S. 1928 Urea-formaldehyde introduced commercially. 1931 Hyde began research on organo-silicon polymers. 1933 Ellis patented unsaturated polyester resins. 1935 Henkel made melamine-formaldehyde resins. 1937 Automatic compression molding introduced commercially. Polyurethanes first produced. 1938 Melamine introduced commercially. 1939 First patent (in Germany) on epoxy. 1941 Urethane-polyester type-introduced in Germany. 1942 Dow Coming made silicone industrially. 1943 Castan’s patent issued on epoxy. 1946 Polyurethane elastomers introduced. 1947 Epoxy introduced commercially. 1954 Polyurethane introduced in U.S. 1957 Urethane-polyether type-introduced in U.S. 1964 Polyimides introduced as a fabricated product.

17. CLASSIFICATION OF THERMOSETS • Thermosets can be classified as: • Temperature activated →Formaldehyde (FOR), phenoplasts (PF), Amnioplasts (UF), polyester, • vinylester, Alkyd, Allyl, furan, some • epoxies, and Polyimides • Catalyst activated → Unsaturated polyester resin (UPR) • Mixing-activated → Polyurethane • OR • According to broad classification: • General Purpose → Phenolics, aminos, polyesters • Engineering →Epoxy, polyurethane • Specialty →Silicones, allyls, high temperature thermosets,

18. PROPERTIES • Thermosets usually posses • Good dimensional stability • Thermal stability • Chemical resistance • Electrical properties APPLICATIONS • Adhesives • Primary and secondary structural members in aerospace • Countertops and floors for manufacturing facilities and homes • Printed circuit boards, • Conductive polymer elements, • Encapsulation materials for electronic applications; • Dental materials, especially adhesives • Recreational products such as tennis racquets, bicycle frames, golf clubs and fishing rods

19. THERMOSETS IN COMPOSITES

20. Polymer Matrix Composite (PMC) • “____The composite in which polymer is used as a matrix is called polymer matrix composite.____” • The medium of reinforcement is the fibers. Fiber may be in the form of continuous, discontinuous (either aligned or randomly oriented). • PMC is most important type of composite, which is used in great diversity of applications. • Representative properties of PMC is of following • high strength-to-weight ratio • Light weight (low specific gravity) • Significant anisotropy in properties • Low thermal expansion, leading to good dimensional stability • Good fatigue strength • Ease of fabrication (not involve high pressure and high temperature & simpler equipment) • Cost is relatively low

21. Constituents • Resins • Fibers • Fillers & additives • MATRIX – RESIN • Functions of the Matrix • Transmit force between fibers • Arrest cracks from spreading between fibers • Do not carry most of the load • Hold fibers in proper orientation • Protect fibers from environment • Mechanical forces can cause cracks that allow environment to affect fibers • Demands on Matrix • Interlaminar shear strength • Adhesive Properties • Toughness • Moisture/environmental resistance • Temperature properties • Cost

22. PROPERTIES OF THERMOSET RESINS • Thermally stable • Chemical resistance • Stress relaxation • Low viscosity • Common material for fabricators • RESINS SYSTEM • Polyester • Vinyl resin • Epoxy • Phenolic • Polyurethane

23. COMPARISON of RESINS

24. The fiber-matrix interface • The interface between fiber and matrix is crucial to the performance of the composite in particular fracture toughness; corrosion; moisture resistance. • Weak interfaces provide a good energy absorption mechanism - composites have low strength and stiffness, but high fracture toughness. • Strong interface results in a strong and stiff, but brittle composite. • Adhesion between fiber and matrix is due to one (or more) of 5 main mechanisms:

25. Adsorption and Wetting- depending on the surface energies or surface tensions of the two surfaces. Glass and carbon are readily wetted by epoxy and polyester resins, which have lower surface energies. Inter-diffusion - diffusion and entanglement of molecules: Electrostatic attraction - important in the application of coupling agents. Glass fiber surface may be ionic due to oxide composition:

26. Chemical bonding - between chemical group in the matrix and a compatible chemical on the fiber surface: Mechanical adhesion - depending on degree of roughness of fiber surface.

27. THE END