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Lexan 940

December 1 2003

 

Contents

Introduction

Lexan is one of the most popular thermoplastic.  It is one of the toughest and most versatile polymers, and its unique combination of features is used to add high quality and high performance to thousands of products ranging from toys to space helmets. Mechanical properties of polymers, such as elasticity, yield strength, tensile strength, and elongation, are required in many engineering products.  One of the most useful tests to determine multiple mechanical properties is the tensile test.

The LEXAN brand is also a strong signal of quality that can help consumers recognize products that enhance the way we live, work and play. Lexan was created by Dr. Daniel W. Fox of General Electric.  While working on a project to develop wire insulation, Dr. Fox found himself with a gooey substance that hardened to the beaker.  This substance was the birth of Lexan polycarbonate.

The polymer used in this report was Lexan 940 created by Husky. In order to further understand the properties and behaviour of this polymer, a variety of tests were conducted. Tensile tests were conducted in order to determine the level of crystallinity and the yield strength of the polymer. The viscoelastic test was found to determine the effect of strain rate on the polymer. SEM analysis was also conducted on the lexan polymer to determine the chemical composition to determine if any additives were used in the melt.

Chemistry

Molecular Shape and Synthesis

Lexan 940 has an amorphous, polycarbonate structure, and in most cases is a thermoplastic polymer. It is formed of long molecules that are composed of structural entities called monomers. The monomer for lexan is shown in Figure 1. There is an oxygen side atom bonded to the main chain. There are two possible arrangements for this oxygen atom when the monomer unit is repeated in a polymer. The hydrogen can alternate, forming the very common head-to-tail configuration, or it can form on complimentary sides forming the very rare head-to-head configuration. There is also two CH3 side groups on the lexan monomer. Since there are two of these side groups attached to the same backbone atom, there is no alternate configurations for this particular group.  

Figure  1 Lexan monomer [1]

The first step of the synthesis of lexan is shown in Figure 2, where bisphenol A is combined with a phosgene [2][3]. This is an example of making ester linkages (polyesters) through condensation polymerization. Condensation polymerization is also referred to as step growth polymerization. The condensation process occurs because there is a reaction between an acid and an alcohol [4]. The reaction times for condensation are generally longer than for addition polymerization. In order to produce large molecular weights, the reaction time must be sufficiently long in order for the conversion of the monomer reactants to be complete. Condensation polymerization is generally used to create thermosetting polymers and polycarbonates such as lexan. The step-wise process is continued to produce a linear molecule.

Figure  2 First step of lexan condensation polymerization

After the completion of the first step of lexan synthesis, the second step is shown in Figure 3. This final step produces the final product of polycarbonate lexan, called polyphthalate carbonate. Condensation polymerization typically gives off some form of by-product, typically water, however in this case the by-product is HCl [5].

Figure  3 Final step of lexan synthesis

Mer Molecular Weight

The mer contains carbon, hydrogen, and oxygen. The ring strucuture is also included in the monomer structure. In order to calculate the mer molecular weight, the number of molecules in the mer must be determined. Figure 4 shows a ring structure for the molecular weight calculation. The ring structure has six carbon and six hydrogen atoms.

Figure  4 Ring structure[6]

The shape of the polymer chain is influenced by the positioning of the backbone atoms. Polymer chains can typically have a straight chain segment, or a bending and twisting segment. The thermoplastic lexan is amorphous. Amorphous thermoplastics have a sharp melting point and soften gradually. This is because as the temperature is raised, secondary bonding forces are diminished by increasing molecular motion so that the relative movement of adjacent chains is facilitated when a stress is applied. Amorphous thermoplastics have a random chain orientation in both the solid and melt phases, and do not flow as easily as semi crystalline or liquid crystal thermoplastics do [7]. However, the amorphous thermoplastic can be formed into transparent parts.

Scanning Electron Microscope Background

The sample was analysed with a Scanning Electron Microscope (SEM). The SEM has difficulty analyzing polymer samples for multiple reasons. Polymers are not conductors, and in order to be viewed under the electron microscope, the polymer samples must be gold coated. After gold coating, the sample was viewed under the SEM to look at inclusions and other particles, as well as to analyse the composition. The SEM can not detect Nitrogen, Carbon, or Hydrogen. Since Carbon and Hydrogen make up a large part of the compositions lexan, the compositional analysis will instead show what other particles are evident in the samples. This is advantageous since it can help us to understand what elements may be added to the polymer melt to improve performance characteristics. In addition, if the SEM were to stay focused on a part of the sample for too long, or if it were to attempt to zoom in to large magnifications, the sample would be damaged by the electrons. The electrons emitted from the electron gun in the SEM cause the polymer molecules to evaporate. This evaporation is trapped underneath the gold coating and is shown by large bubbles on the polymer. Surface damage on the polymer surface after prolonged exposure at high magnification is shown in Figure 5.

Figure  5 Electron damage and surface scratches

 

Scanning Electron Microscope Analysis

The Lexan sample showed evidence to the presence of elements and is visible in Figure 6. After analyzing one sample area for possible inclusions, various other elements were detected. The elements Silicon (13.2 wt%), Chlorine (8.41 wt%), Titanium (70.26 wt%), and Iron (8.12 wt%) were detected. It is important to remember that these weight percentages do not include Nitrogen, Carbon, and Hydrogen, which are likely in abundance in these samples. However they do show at this particular sample site what weight percentage of additional elements are evident. It is possible that some of these elements are due to contamination. Since these samples were sanded and cleaned up in the materials lab, the Iron may be due to contamination from steel samples. Since Iron has a low weight percent, this furthers this theory. However, samples such as Silicon and Titanium may be purposeful inclusions in the sample. The SEM report for the Lexan sample, including a picture of the area analyzed, is shown in Appendix A. Compositional analysis has shown that most of the spots on the left of the image are not due to electron damage.

Figure 6 Lexan under a Scanning Electron Microscope

 

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