Plastics Fundamentals pdf

1 C C ORE ORE T T ECH ECH S S YSTEM YSTEM plastics.ppt Plastics Fundamentals Plastics Fundamentals This chapter will give you some understanding of the plastic materials in the plastic industry. You will learn : •Characters of plastics •Classification of plastic materials •Applications of plastic materials 2 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Characters of Plastic Materials Characters of Plastic Materials q Lightweight Material ) Density: 0.8-2.2g/cm 3 q Wide Range of Mechanical Properties ) Young’s modulus (1-100,000 N/mm 2 for elastomer- GF/thermosets, for steel: 500,000 N/mm 2 ) q Easy Processability ) processing temperature < 400 o C q Modifiability by Additives ) Fillers, reinforcing materials, color pigments, plasticizers, flame retardants, stabilizers Today, plastics applications are found in almost all areas of life. They are materials with extremely wide spectrum of properties.That’s the reason why their applications are so intensive and extensive. The characters of plastic materials are summarized briefly here. Plastics are light. The density of plastics varies between 0.8 and 2.2 g/cm 3 . In other words, they are lighter than common materials such as steels and ceramics. The weight of plastic-made article is lighter than that of metal-made/ceramic- made products. Plastics exhibit a wide range of mechanical properties. They can be soft and elastic as well as hard and rigid under different processing environment. Some filler-reinforcing plastic material may exhibit a higher value of elastic module than that of metal and can be applied for specific functions. Plastics can be processed easily and economically. Most plastics can be processed below 400 o C .Thus we can produce complex plastic part simply without consuming too much energy. The properties of plastic materials can be modified by compounding. Additives such as fillers, reinforcing materials, plasticizers, stabilizers can be added to modify the processing properties of material or performance of product. 3 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Characters of Plastic Materials Characters of Plastic Materials q Low Thermal and Electrical Conductivity ) k:0.1-1W/m/K,metal:100-1000W/m/K, thermal/electric good insulator q Transparency for Amorphous Plastics ) for lenses, CDs, optial discs q High Chemical Resistance ) not susceptible to corrosion q Permeable ) permit permeation and diffusion q Recyclability and Reusability The thermal and electrical conductivities of plastics are low, these properties make them good thermal and electric insulators. On the other hand, the poor thermal conductivity of plastics will produce some molding difficulties or defects, such as hot spot and burning points. Amorphous plastics such as PS are often transparent and can be colored as desired. This character makes plastic a good candidate for optical article production. Plastics are inert to chemical and are not susceptible to corrosion problem. This character is the reason why plastics have been applied to producing containers for corrosive chemicals. Sometimes the permeability of plastics make cause problem, such as in the applications of IC-packing. Humidity permeation across the plastic packaging may cause corrosion of electric circuits interior the package. However, in some applications, such as application in membrane, this character is essential. Thermoplastics are remeltable and can be recycled and reused. 4 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Introduction to Plastic Materials Introduction to Plastic Materials q A polymer molecule (macromolecule) is a long, large chain composed of many small simple chemical units (structural units) Macromolecule branch Structural Unit A plastic material is composed of polymer chains. Each chain is composed of many repeated simple chemical units (structural units). Macroscopic behavior of plastic material is determined by its microstructure. This difference in microstructure characters between plastic materials and the small molecules (such as that of air or water) causes the difference in their macroscopic behavior. 5 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Example of a Polymer Molecules Example of a Polymer Molecules H H C C H H manomer Ethylene: CH 2 =CH 2 M.W.=28 polymerization LDPE (Low-Density PolyEthylene): -(CH 2 -CH 2 ) n - polymer M.W.=280,000 The basic substances used in producing plastics are called “monomers” (mono=single, meros=part). The macromolecules of plastics are formed by chemical linking manomers. This chemical reaction is called polymerization process because a polymer (poly=many) molecule is produced from monomers. For example, a low density polyethylene (LDPE) molecule is produced from the combination of many thousands of ethylene manomer molecules. The molecular weight of a ethylene manomer is 28 whereas the molecular weight of a LDPE is about 280,000. 6 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Simple Molecules vs. Simple Molecules vs. Macromolecules Macromolecules Simple Molecules e.g.,water liquid near the rotating rod is push outward by the centrifugal force The microstructure of materials may effect their macroscopic behavior. For simple molecules such as water molecules (composed of one oxygen atom and two hydrogen atoms), their response to external force is fast due to the high molecular mobility and high freedom of changing molecule configuration. For example, if we insert a rotating rod into a baker containing water, we see that the liquid near the rotating rod is push outward by the centrifugal force, and a dip in the liquid surface near the center of the baker result.This behavior is typcal for liquids consisting of simple molecules. 7 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Simple Molecules vs. Simple Molecules vs. Macromolecules Macromolecules Polymer Molecules entanglement between polymer chains polymeric liquid moves toward the center of the baker and climbs up the rotating rod Weissenberg Effect (Rod-Climbing Effect) On the other hand, when the rod is rotated, the polymeric liquid will moves in the opposite direction, toward the center of the baker and climbs up the rotating rod. This rod-climbing phenomena is referred to as the Weissenberg effect. The Weissenberg effect is attributed to the normal stress difference arising from the chain-like microstructure of polymeric liquid. The entanglement between polymer chains and flow-induced anisotropic microstructure cause the plastic material to exhibit nonlinear normal stress behavior. Intuitively, you may think Weissenberg effect is a result of polymer chains climb the rotating rod as they are entangled with each other. 8 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Simple Molecules vs. Simple Molecules vs. Macromolecules Macromolecules Simple Molecules e.g.,water shear rate Shear Viscosity lower temperature higher temperature viscosity = f( composition, T,P ) Newtonian Viscosity Viscosity is a measure of the resistance to flow. Low-molecular-weight fluids (composed of simple molecules) exhibit a so-called Newtonian viscosity and they are referred to as Newtonian fluids. The viscosity of a Newtonian fluid is independent of deformation rate and depends only on its composition, temperature and, to a much lesser extent, on pressure. Typically, low-molecular-weight liquids and all gases are Newtonian fluids, that is, they have constant viscosity under constant temper-ature and pressure condition. This type of flow behavior would be expected for small molecules, where the structure, and therefore the resistance to flow, does not change with th intensity of shearing. Note that the Newtonian viscosity is a decreasing function of temperature for the liquids, and in some cases the temperature dependence is very strong. 9 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Simple Molecules vs. Simple Molecules vs. Macromolecules Macromolecules Polymer Molecules shear rate Shear Viscosity lower temperature higher temperature viscosity = f( composition, T,P,shear rate ) Non-Newtonian Viscosity (Shear-Thinning Effect) On the other hand, polymeric liquids with chain-like microstructure are non- Newtonian in that the viscosity changes with changing shear rate ( that is, rate of deformation). Polymer melts and solutions are non-Newtonian and are referred to be pseudoplastic (shear-thinning) fluids because their viscosity (hence resistance to flow) decreases with the intensity of shearing. The shear-thinning behavior of polymeric liquids will be given in next section. In addition, the shear-thinning viscosity also depends on composition, temperature, as well as pressure. The temperature-dependence of polymer viscosity plays a key role in polymer processing operations. 10 plastics.ppt C C OR E OR E T T ECH ECH S S YSTE M YSTE M Shear Rate Viscosity Shear Shear - - Thinning Viscosity of Thinning Viscosity of Polymeric Liquids Polymeric Liquids increasing Temperature The shear-thinning behavior of polymeric liquids can be interpreted in terms of their molecular structure. At low shear rate, the randomizing (disorienting) effect of the thermal fluctuation of the chain segments overcomes any tendency toward molecular alignment in the shear field. The molecules are thus in their most random and highly entangled state, and have their greatest resistance to slippage (flow). This explains why they exhibit higher viscosity under low shear rate condition. As the shear rate is increased, the molecules will begin to untangle and align in the shear field, reducing their resistance to slippage past one another. Thus the viscosity will decrease with increasing shear rate because the increasing orientating behavior of molecular chains. At very high shear rates orientation of molecular chains may be complete and in this range near-Newtonian behavior may be observed. Intense shearing will eventually lead to extensive breakage of main-chain bonds of polymer microstructure and mechanical degradation of material. In injection molding the shear rate has a range of about 100 to 10,000 sec -1 ,most plastics will exhibit a shear-thinning viscosity in this shear-rate change. The shear-rate-dependent viscosity behavior of plastic materials also play a key role in the polymer processing operations. [...]... value of viscosity 21 plastics. ppt Classification of Plastics PLASTICS thermoplastics Thermoplastics Elastomers/Thermosets not crosslinked cross-linked Amorphous (Semi)crystalline Slightly Crosslinked elastomers PC,PS,PVC, PMMA, HDPE,LDPE, PP,PA,POM, PTFE 22 PUR, Polybutadiene, Natural Rubber Strongly Crosslinked thermosets Epoxy,Melamine, Phenolics, Polyester, CORETECH SYSTEM Plastics can be classified... to several criteria If we make the classification according to chemical structure of plastics, two categories can be obtained: thermoplastics and thermosets/ elastomers Thermoplastics are those polymers that have no crosslinking microstructure They are meltable under high temperature and soluble in many solvents Thermoplastics can further be divided into two classics: amorphous and crystalline (or semicrystalline)... or dissolved, but they can be swelled in organic solvents 22 plastics. ppt Thermoplastics q Microstructure: No chemical connection (crossliking) between molecules Held chains together by intermolecular forces and physical entanglement q Properties: Meltable,Moldable Soluble Variety in property at room temperature Next CORETECH SYSTEM Thermoplastics (“thermo”=heat,”plastic”=shapable) are those polymers... structure Molecular chains in a thermoplastics are hold together by intermolecular forces and physical entanglements These rather weak bonding forces can be overcomed upon heating, hence thermoplastics are meltable and moldable That is, they have the ability to flow as the intermolecular forces are broken and the molecules are able to slide past each other with ease Thermoplastics can be melted repeatedly... in comparison to thermoplastics Like the elastomers, they are neither meltable nor soluble They cannot be swelled by solvent due to their dense cross-linking In general, thermosets exhibit excellent mechanical property and resistance to chemicals, thus they have application in electrical outlets and microelectronics packaging 29 plastics. ppt Applications of Crystalline Polymers Plastics Applications... value equals to one Carefully fractioned or anionic addition polymers has a PI value of 1.02 Most commerical plastics have a polydisperse molecular weight distribution and have PI values larger than 2 Values of 3 to 8 are not uncommon, with special purpose polymer having values as large as 30 17 plastics. ppt Macromolecular Structure q Large Number of Internal Degree of Freedom stretch slide entanglement... on many factors such as the type and number of branches or side chains, their mechanical properties vary from a soft condition to a ductile or hard and brittle at room temperature 23 plastics. ppt Classification of Thermoplastics Crystalline or Semicrystalline crystalline regions: orderly arranged tightly packed chains Examples: HDPE,LDPE,PP,PA, POM,PTFE Amorphous slightly branching, linear or very ordered... polymers Plastics with highly branched chains, long side chains, or lagre functional group (side chain chemical group) are hard to arrange molecular chains in an ordered and tightly structure These unstructured polymers are referred to be as amorphous (=have no definite form) polymers Since they are always crystal clear in the unpigment condition, they are also called synthetic or organic glasses 24 plastics. ppt... have been destroyed Amorphous polymers are unstructured and have no significant melting point An increase in temperature causes the molecules to vibrate and cause the plastics to become softer With a further increase in temperature, the plastics exceeds the flow temperature and enters the melt state Amorphous polymers have no distinct melting point They soften and exhibit a variation in specific volume... specific volume within a temperature range In melting state the polymer is able to flow The microstructure is essentially amorphous because the high mobility of molecular chains 25 plastics. ppt General Properties of Thermoplastics in Processing PROPERTY CRYSTALLINE AMORPHOUS Melting or Softing Sharp Softens over a range Density Viscosity Increase with Crystalline Low Lower than crystalline one High . S S YSTEM YSTEM plastics. ppt Plastics Fundamentals Plastics Fundamentals This chapter will give you some understanding of the plastic materials in the plastic industry. You will learn : •Characters of plastics •Classification. and extensive. The characters of plastic materials are summarized briefly here. Plastics are light. The density of plastics varies between 0.8 and 2.2 g/cm 3 . In other words, they are lighter. module than that of metal and can be applied for specific functions. Plastics can be processed easily and economically. Most plastics can be processed below 400 o C .Thus we can produce complex