The purpose of Module 4 is to provide a framework for understanding the basic molecular structures, general classifications and  properties, and a brief overview of plastic processes.  One could literally spend years on this subject; however, the purpose of this course is to look at polymers from a material properties and processing point of view and not an in-depth undertaking.

OBJECTIVES:

After completing this module you should be able to do the following:

   Identify common elements used in polymers
   Distinguish between addition and condensation polymerization
   Differentiate between thermoplastic and thermoset
   Identify common characteristic of thermoplastics and thermosets
   Explain the difference between crystalline and amporphic structures
   Explain what determines the properties of plastics
   Identify common processes used for thermoplastics and thermosets

TERMS:

     Mer
     Monomer
     Polymer
     Polymerization
     Addition
     Condensation
     Amorphous
     Crystalline
     Plasticizers
     Catalysts
     Stabilizers
     Reinforcers
     Fillers
     Expanders
     Thermosets
     Thermoplastics
      Cross-linking
      Network

INTRODUCTION

     It is not difficult to see the impact of plastics and the way they have changed our society.  We use plastics daily, anything from plastic garbage bags to compact discs to the cars we drive incorporate some type of polymer.  Generally speaking, polymers refer to the intermediate stage before the final plastic product is produced.  The term polymer implies many "mers" or the building blocks....similar to the unit cell in metals.  A polymer is a chemical compound or mixture of compounds formed by a process called polymerization, a chemical reaction in which tow or more molecules combine to form larger molecules.  We will look at the basic "monomers", some of  the elements that are used in the polymerization, and some common polymer processes.

BASIC BUILDING BLOCKS

     Plastics are derived from organic materials and are in abundance.  The raw materials commonly used in the production of polymers are coal, air, water, wood, petroleum, limestone, and salt.  The most common material used is petroleum.  These materials contain the basic elements that are used in forming polymers...carbon, hydrogen, oxygen, nitrogen, chlorine, and fluorine.

     Carbon is considered to be the backbone of polymerization.  It is important to look at the basic structure of the elements to gain a better understanding of how polymers are formed.   The general "model" is show below:

     If different elements are substituted in for "X" in the model show above, and heat and pressure is applied, then the double carbon bond is broken and polymerization occurs producing multiple "mers".   Depending on what "X" is, the resulting polymer will be different.   For example,  if  "X" is hydrogen, then the monomer would be ethylene, a colorless, orderless gas.  This monomer is used to produce polyeythelene.    On the other hand if chlorine is substituted for "X" then the monomer is transformed into poly-vinyl chloride through polymerization.  Some of the more common thermoplastic monomers are shown below.

Note:  This is quite overly simplified and the chemistry involved during the polymerization process goes beyond the intentions of this course.  Literally thousands of formulas are used to change the desired polymer and its properties.

POLYMERIZATION

     So how does a monomer become a polymer?  Plastic polymers are called macromolecules because smaller molecules are joined together to form polymer chains.  This process is called polymerization.   Those organic molecules that are suitable for polymerization are the monomers we just discussed.  In order for a monomer to polymerize, it must be at least capable of forming two covalent bonds,  one on the front, one on the back.  The result is a polymer chain.

     Some of these monomers are poly functional, meaning that three or more bonds can be formed, resulting in a network.  This network structure determines the properties of the material.  There are two classifications of polymerization called addition and condensation, with addition being the most common.  During the addition polymerization process, each double bond is broken, and the atoms add onto one another to form the polymer chains.  No atomic change takes place....meaning no atoms are added ...but rather simply connect to the "arms" of their neighbor.  Structural branching can be achieved by causing a macromolecule to grow at several locations rather than just at the ends.  This results in a non-linear macromolecule called a branched polymer.  These polymers will not  pack together as tightly as linear polymers.  An example of a branched polymer is low density polyethylene; however high-density polyethylene remains essentially linear.

      A polymer that is formed through this process is called a thermoplastic material.  Some of the properties of thermoplastics in general are that they become soft when heated and harder when cooled.  These materials have no strong bonds between individual molecules and can be softened by heat and remolded.  Thermoplastics are recyclable materials.
 
 
 

       Thermoplastics may have varying degrees of crosslinking from none to heavy.

The diagram below shows the affect of increased crosslinking on a thermoplastic polymer with respect to increased rigidity (higher modulus of elastiscity).

     The second polymerization method is condensation.   While it is similar to addition, a basic but major difference occurs.  The monomer is changed chemically.  In a simplified description,  some atoms are removed from the monomer which allows atoms from one molecular chain to be bonded to adjacent chains.  Since some atoms are removed, a by product must be formed.  This is often in the form of water.   This chemical bonding of long chain molecules across chains is called cross linking.

Thermosets are completely cross-linked into chemically bonded network.  These materials are harder and more brittle and is not lost lost upon cooling.  This is typicay of network molecular structures which are formed through step growth.    Chemical reactions occur in steps that are accelerated by higher temperatures and are not reversible..

ADDITIVES FOR PLASTICS

     Catalysts are used to accelerated the reaction velocity between the reaction of substances.  This along with the application of heat and pressure is really what causes polymerization.  This is a very simplified description of the process.  In reality, the formulation and control of polymerization in chemical labs is quite complex.
Other agents are added to achieve desired properties of the material.  Some of those include plasticizers, fillers, reinforcing agents, and stabilizers.  Below is a brief description of the general purpose of these additives.

     Plasticizers are added to reduce the viscosity and improve flow characteristics.  The lower something called the glass temperature.  Glass is quite brittle and thus the glass temperature is very important for changing the properties of some materials.  For example, PVC is brittle at room temperature...but if di-iso-octyl is added as a plasticizer, it becomes quite rubbery.  In this state the PVC can be used for applications like electrical tape.

     Fillers are simply low cost materials that are added to reduce the cost of producing the plastic.  Often these additives are inert and do not react ot the polymer, but may, in fact, increase hardness and impact strength.

     Reinforcing agents are added to improve mechanical properties and increase strength, impact resistance stiffness, and hardness.   Various types of fibers are used as reinforcing agents.  These fibers may include glass fibers, carbon fibers, or even metal fibers.

     Stabilizers are added to maintain the integrity of plastic during forming and service.  These agents may be as simple as carbon black to prevent deredation of strength through reaction to ultraviolet light.  Sometimes, lead compounds are added to increase or stabilize the material against weathering if it is to be used in an outdoor product.

     Extenders are simply organic materials such as oils or waxes.  Extenders are added to reduce the volume of plastic material needed per unit area required.

MOLECULAR ARRANGEMENT AND STRUCTURAL PROPERTIES

Molecular arrangement determines the size and weight of the plastic molecule. As the molecular size and weight increases, it affects several mechanical properties.  Among those most affected are strength, stiffness, and hardness.  Molecular arrangement also affects the viscosity of the plastic.  This property is very important for processes that involve molding.

     There are two general types of structure that also relate to properties of polymers.  These include crystallinity and amorphous structure.   The modulus of elasticity as temperature increases is much higher with crystalline thermoplastics as compared to amorphous as shown in the diagram below.

 Amorphous and crystalline structures also  have differences in other properties.  Generally speaking, crystalline plastics tend to be more elastic while amorphous plastics flow more readily and are transparent.  These structures can be modified to influence the desired properties.  Thermoplastics, which are normally crystalline can be forced into some amorphous regions and become semi crystalline.  Some generalized characteristics of these structures are shown below.

          Crystalline Structure:

                 1.  Causes cloudiness in polymers due to light refraction.
                 2.   Highly crystalline polymers have a more defined melting point
                 3.   Shrinkage rates are high during cooling
                 4.   Can be used at higher service temperatures than amorphous materials
                 5.   High crystallinity correlates to high density

                 Examples:  Nylon, polypropylene, tetrafluorethylene, acetal, polyethlenes.

          Amorphous Structure:

                 1.  Higher clarity;
                 2.  less shrinkage upon cooling;
                 3.  Easier to process;

                 Examples:  Polystyrene, acrylic, polyvinyl chloride.

GENERAL CHARACTERISTICS AND PROPERTIES OF PLASTICS

      Electrical resistance.  This is due to covalent bonding being the primary type of bond present and locks the electrons into relatively fixed positions.  However, electrical conductivity increases with the hydroscopic materials.  That is, some plastics absorb and how water more easily. In this case ionic conduction may be present.
     Thermal Conductivity.  Plastics have very low thermal properties and are relatively good insulators against heat and cold.  Since electrons are closely bound by covalent bonds, and there are larger spaces between atoms, heat transfer is inhibited.  Further, voids are often present and create insulating  "gas" pockets.
     Density.  Plastics exhibit very low densities unless heavier reinforcing materials are added.  Typically, commercial plastics range from about .3 to .75 pounds per cubic foot.  As a reference, water weighs 62.4 pounds per cubic foot.
     Corrosion Resistance.  Polymers have an excellent tolerance to corrosion.  The large size of plastic molecules prevent transitions into solution.  However, plastics are subject o a special type of corrosion called swelling.  This happens due to the large molecules allowing solvent agents to penetrate.  For example compounds like grease and oil may penetrate and cause swelling in certain polymers.
 
 
 

     The basic building blocks of polymers is called a "mer"  This mer if repeated becomes a monomer.  Depending on what element is added to the carbon backbone, different materials can be produced.  The process of creating multiple "mers" is called polymerization.  There are two types of polymerization:  1) Addition (or chain like combinations where the compounds are linked end to end, similar to railroad cars.  Thermoplastics are produced through addition.  2) Condensation polymerization (also called step-reaction) where multiple molecules are linked in at least two dimensions (directions).  Chemical reactions produce by-products such as water.  Thermosets are produced through condensation.   Agents are added to polymers to change the characteristics of the material including catalysts, fillers, plasticizers, reinforcers, stabilizers, and expanders and colorants.
     The molecular structure,  arrangement and density affect the properties of polymers.  The structure may be crystalline or amorphic.  Above the glass temperature, all plastics are amorphous.  As the temperature increases the state will change and affect the properties of strength and flowability.  Polymers can be strengthened by orienting the direction for the molecules and by crystallization.
     Processing of plastics involves several varied shaping or forming processes depending on the material and desired product.  The more common processes include injection molding, compression molding, transfer molding, rotational, molding, blow molding, extrusion, blown film molding, thermoforming, laminating, calendaring, and fibering.   These processes will be covered in Module 5.

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