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

2. P OLYMERS  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 (10 4 —10 7 ).

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 Thermosettin g →T hermē (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 → U nsaturated 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 APPLICATION S  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. P OLYMER M ATRIX C OMPOSITE (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 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: THE FIBER-MATRIX INTERFACE

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