Main
Work
Play
Portfolio
Mail me



Material Degradation
Copyright © 1998-2005 by Pichai Rusmee
Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Objectives

This lab will expose students to polymeric type materials. This includes a brief introduction to polymer morphology. In addition, they will be made aware of synergistic effects that can arise when a material is subjected to more than one kind of action, environment, etc. The effect of synergism will be demonstrated using the polymer subjected to a load and a corrosive environment.
Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Lecture

What is Polymer?
Polymer is a synthetic or natural material consisting of molecules stringing together to form a long chain. The properties of a given polymer depends on the basic molecules (a "mer" unit, repeating unit) making up the polymer, the length of the chain, and the arrangement of the polymeric chains. Polyethlylene is the most simple commodity polymer available. It's repeathing unit consists of two carbons backbone each with two hydrogen atoms attached to it. The number of repeating units strings together determine whether this polymer is a gas, liquid, or solid. In solid form, the polymeric chains may pack neatly or in random fashion.

There are three basic type of polymer packing: chain, branch, and network as shown in Figure 1.1. A chain type polymer is a polymer consisting primarily of long chains of polymer without any irregularity in the chain. When a polymer chain exhibits some irregularity, i.e., some side atoms or molecules are replaced with chains of the same plymer, then it is said that this particular polymer has a branch structure. If the side branches joined up with other polymer chains, they are forming a network, hence network type structure. Sometimes, polymers chain can arrange themselves in regular, close-pack manner forming polymer crystals. Usually, only chain type polymers is capable of forming polymer crystal lattices.


Figure 1.1 Polymer structures.

Homopolymer is a polymer consist of one and only one type of monomer unit. Copolymer incorporates two or more type of monomers into its chains. Polymer blend is a polymer resulting from physically blending different kind of polymers without incorporating the different kind of polymers into the chain. In a given chain, the polymer could be either homopolymer or copolymer of some kind.

Class of polymer
There are roughly three classifications of polymer:
  • elastomer
  • thermoset
  • thermoplastic
    • crystalline
    • amorphous
Common polymers by rough classification
Thermoset plastics are usually the network type of polymer where all the polymer chains are tightly knit together. The common example of this type of polymer is the epoxy type adhesive. Epoxy adhesive is not a true glue that is water or solvent base and harden by the evaporation of solvent. Epoxy hardens by polymerization. Uncured epoxy usually consists of uncured polymer or partially cured polymer. Once the polymer is exposed to the condition "favorable" to polymerization, it will start crosslinking and formed a crosslinked network that is directly related to the strength of the adhesive. Depending on the type of adhesive, favorable condition may include air, heat, catalyst, initiator, moisture, etc.

Elastomer plastics are usually the chain, branch, or even network type. The polymer chains are very loosely intertwined. As a result, the elastomers are pliable and stretchy. Another name for elastomer is rubber. An example of this type of polymer would be silicon rubber.

Thermoplastics are usually the polymers of the chain, branch, or loosely networked type with some capable of forming crystal structure. These polymers are what commonly called plastics. They are usually meltable and formable through heating. There are different classes of thermoplastics such as polyolefins and polyamides. Some of them are: polyethlene, polypropylene, polytetrafluoroethylene (Teflon®), polycarbonate, etc.

For an abbreviate list of common polymers with their letter designations see Standardized polymer abbreviations.

Phase and properties
Polymers are highly rate and temperature dependent (Figure 1.2). They are mainly useful in the glassy region when they are solid. Passing the transition point, the solid polymer will turn rubbery and, depending on the type, will either burn or melt. This is the general trend for the amorphous type polymers. For the crystalline type polymers the transition temperature is not so obvious since they would have a crystalline structure instead of glassy structure.


Figure 1.2 Rate and temperature dependency of material property, modulus, in polymer.

Synergism
One of the effect being studied in the laboratory is the effect of synergism. That is when two or more agents acting together having a larger effect than each one of them acting separately. The effect may be positively reinforced or negatively reinforced. This laboratory will demonstrate the negatively reinforced synergism effect where the material degrades much more severely than each agent acting separately.

Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Lab Work

There are three sets of data that will be presented. The first set will be the failure load vs. preload time for some fiber specimens. The second set will be the failure load vs. the time exposed to corrosive agent. The third will be the failure load vs. the preload concurrently to exposure time.

This is a dry lab due to hazardous nature of the experiment. The students only have to plot the data given in the handout and discuss them.

Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Data Reduction

Figure 1.3 Failure load vs. exposure time load-environment combination of Nylon 66 fiber.


See Miscellaneous for comments on the graph.

Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Handout

Material Degradation
Materials are continually changing, both on a molecular and macroscopic scale. Molecularly, atomic structures are inherently unstable and are continually changing to a more stable state; however, for many materials this rate is a rather slow process. Molecular changes can also occur as a result of "other" energies subjected onto (surface or boundary) and in (volume) the material properties.

Macroscopic changes can occur as a result of energy input to the material system. This can result in morphological changes that can drastically alter material behavior. For example, ultra-violet and/or gamma irradiation of polyethylene changes the macroscopic and molecular structure, such that the ultimate strength is increased and the fracture toughness is decreased. There also comes a point after a "great deal" of radiation that the polyethylene has so many cracks in it that it is rendered useless. To help us become more familiar with materials other than metals, a general overview of polymer morphology will be discussed in the lecture.

Unlike all the other labs in this course, this lab will not be a "hands-on" lab due to the dangerous nature of the experiment and laboratory time constraints. The experiment will be explained in the lecture and if opportunity arises an experiment in progress will be shown. This experiment investigates the effect of different environments on Nylon 66 fibers. Actual data obtained from the Nylon 66 degradation studies are provided for use in the report write-up. The data can be interpreted as follows:
  1. This data was determined first. Samples were preloaded at 80% of their ultimate load (predetermined value) for the amount of time shown. The samples were then placed in the Instron loading frame and pulled to failure. The fracture loads are shown with the corresponding preload time.
  2. This data was determined second. Samples were exposed to a gas environment of 1% NOx for the amount of time shown. The samples were then removed from the gas environment and pulled to failure in the Instron load frame. The fracture strengths are shown with the corresponding exposure time.
  3. This data was determined last. Samples were exposed to 80% of their ultimate strength and 1% NOx concurrently for the amount of time shown. The samples were then placed in the Instron loading frame and pulled to failure. The fracture strengths are shown with the corresponding preload/exposure time.
Tasks
Your introduction should include some background on material degradation and how it applies to different materials.
  1. Define synergism.
  2. Give an example of synergism not related to this study.
  3. Graph all of the Nylon 66 data points. Choose the graph format that will allow you to compare the three sets of data.
  4. Relate the phenomenon of synergism to this study. What can you say about the combined effect of the load and the NOx environment?
  5. Prepare a one page data sheet about your favorite engineering polymer (homopolymer or copolymer). As a minimum, include the chemical structure, material properties, and commercial uses for this polymer. Other topics to consider are manufacturing techniques, type of failure, cost, etc. You will probably have to visit the library so plan ahead. Make it similar to the one supplied for PMMA (polymethyl methacrylate). You may write on any engineering polymer you choose except PMMA. Below is a list of common polymers in case you haven't picked a favorite. You are not limited to a choice from this list.
Polyethylene, polypropylene, polyisobutylene, polyisoprene, polybutadiene, polystyrene, polyvinyl chloride, polyvinyl fluoride, polyvinylidene chloride, polytetrafluoroethylene (Teflon®), polychloroprene, polyacrylonitrile,polyvinyl alcohol, polyvinyl acetate, polyethylene oxide, polyoxymethylene, polyamide (Nylon), polyethylene terephthalate, polycarbonate, polydimethyl siloxane, polyetheretherketone, polyphenylene sulfide, polyamide-imide, polyether sulfone, polyether-imide, polysulfone, polyimide, phenol-formaldehyde (phenolic), epoxy, acrylonitrile butadiene styrene, high impact polystyrene, etc.

Raw Data

80% of Fracture Load
Lab Air
Zero Load
1% NOx
80% of Fracture Load
1% NOx
Fracture Strength (lbs) Time Held (min) Fracture Strength (lbs) Time Held (min) Fracture Strength (lbs) Time Held (min)

193   0   187   20   187   20  
195   0   193   20   193   20  
189   0   191   20   186   20  
196   400   191   120   185   50  
199   400   185   120   181   50  
201   400   189   120   184   50  
198   650   187   320   175   120  
195   650   190   320   176   120  
205   650   184   320   173   120  
    181   450   150   250  
    183   450   148   250  
    178   450   155   250  
        120   320  
        141   320  
        0   320  
        0   450  
        0   450  
        0   450  


Polymethyl Methacrylate (Acrylic)


Properties
Tensile Modulus     0.40E6 psi
Flexural Modulus     0.44E6 psi
Tensile Strength     9.5E3 psi
Yield Strength     8.0E3 psi
Impact Strength     0.23 ft-lbf/in
Specific Gravity     1.18
Water Absorption     10.3 %
Melting Temperature     120-160 C
Glass Transition Temperature     3 C

Applications
Drinking tumblers; faucet knobs; camera, projection, and viewer lenses; signal light devices, nameplates, automotive instrument panels, aircraft windows

Competitive Materials
SAN, PC, PS, clear ABS

Miscellaneous
Impact modified grades: Acrylic hard phase and an acrylic modifier (all acrylic). Impact modified grades: Clear styrene-butadiene modified (not as free from haze). Impact Strength: 1-2 ft . lbf/in.

Failure
Absorb water, surface left in tension after drying, leads to crazing.

General
Sheet PMMA is manufactured by placing initiated monomer between polished glass surfaces and allowing it to polymerize. Pellets are also produced for molding and extrusion fabrication.

(Ref. Engineered Materials Handbook: Engineering Plastics, 1988)

Objective
Lecture
Lab work
Data Reduction
Handout
Miscellaneous
Miscellaneous

Not all type of graphs is appropriate for the presentation of the given data. The data contain multiple values of load for the same exposure time. A category plot where each time data is taken to be a separate name category is inappropriate. The data should be plotted as a scatter plot.

Connecting each data point together with lines is also inappropriate. Doing so implies that the order that the data is presented is significant which is not the case. Use distinct symbols for group of data in the graph instead.

Plotting a simple average of the data hides statistical trend within a time set and may skew the overall trend of the data. Use curvefitting line to indicate trends in the data. Synergism means that the sum is greater than its parts taken separately. This must be shown in the graph and clearly stated in the report.

 
Last Modified
Sep 2005




__ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __ __