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Lexan vs Acrylic

December 1 2003

 

Contents

Summary

The failure of plastic parts can often be traced to mistakes in design, production methods, or material choice. Selecting the material for a particular application involves careful consideration of the end-use environmental conditions and functional requirements of the part. Basing the selection on incorrect criteria can lead to over-designing the part or specifying a material with properties that exceed the demands of the application, often at the expense of scratch or chemical resistance, clarity, or other desirable qualities.

Although Acrylic (Polymethyl methacrylate) is beneficial in a variety of applications, it is not suitable for applications that demand very high impact resistance such as the ice-hockey face shield where protection against the fast moving puck or hockey stick is very important. Hence, Lexan (Polycarbonate: PC) is the material of choice, not only because of its high impact strength but also because of its high values of elastic modulus, tensile strength and percent elongation. It is for these properties that make the material the preferred choice for visual sports safety equipment such as the ice-hockey face-shield.

From research into ice-hockey safety equipment at Sport Mart, one of the largest distributors of sports safety equipment in North America, it was found that Lexan is in fact the material of choice for many brands of face-shields including I-Tech, Nike, Speedo and Bauer.

High impact strength is very important in sports safety equipment like ice-hockey face shields. Even though the standard grade of Acrylic has lower impact strength as compared to Lexan, impact-modified grades of Acrylic that are softer and less rigid than standard formulations are specified for sports safety equipment applications that typically require increased toughness. Since Lexan is prone to problems related to moulded-in stress, such as crazing and cracking, it is recommended that an immiscible polymer blend be used such as High Impact Polystyrene (HIPS) such as a combination of polystyrene and Acrylic which has high impact strength and does not undergo too many problems in mould stress. A PC/ABS immiscible blend is also an option because it has good combination Izod impact strength of 550-560 J/m and is relatively less expensive than ABS.

1.0   Introduction

 

Polymers have been with us since the beginning of time. Natural polymers include tar and shellac, tortoise shell and horns, and tree saps that produce amber and latex. These polymers were processed with heat and pressure into useful articles like hair ornaments and jewelry. Natural polymers began to be chemically modified during the 1800s to produce many materials. The most famous of these were vulcanized rubber, gun cotton, and celluloid. The first truly synthetic polymer produced was Bakelite in 1909 and was soon followed by the first synthetic fiber rayon, which was developed in 1911. [1]

This report details the task of investigating and studying the mechanical behaviour of two polymers: Acrylic and Lexan, and comparing them for mechanical applications focusing on sports safety equipment, in particular ice-hockey face shields. This comparative study is based on data recorded from tensile tests using the Instron machine as well as from research into their mer structures, impact properties, processability, bonding systems, cost, and availability.

Properties of Lexan are discussed in detail in comparison with Acrylic to determine the optimal materials based on the application presented.

Finally, recommendations are made as to which polymer is best suited for applications in sports safety equipment using the recorded tensile test data and performing an in-depth study into the molecular structure of Acrylic and Lexan.

2.0   Short background of Polymers

 

A polymer is composed of chains of many units. Each link of the chain is the "mer" or basic unit that is usually made of hydrocarbons. To make the chain, many links or "mers" are joined or polymerized together.

 

2.1    The Structure of Polymers

Many common classes of polymers are composed of hydrocarbons. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the backbone. Below is a diagram of polyethylene, the simplest polymer structure.

 

 

Figure 1: Polymer structure of polyethylene [4]

 

There are polymers that contain only carbon and hydrogen. Polypropylene, polybutylene, polystyrene, and polymethylpentene are examples of these. [2]

 

Although the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chorine, fluorine, nitrogen, silicon, phosphorous, and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine; nylon contains nitrogen; and Teflon contains fluorine. Polyester and polycarbonates contain oxygen. There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone. These are considered inorganic polymers. [2]

 

2.2    Characteristics of Polymers

Polymers are divided into two distinct groups: thermoplastics and thermosets. The majority of polymers are thermoplastic, meaning that once the polymer is formed it can be heated and formed repeatedly. This property allows for easy processing and facilitates recycling. Thermosets on the other hand, cannot be remelted. Once these polymers are formed, reheating will cause the material to scorch. [4]

 

Every polymer has very distinct characteristics, but most polymers have the following general attributes:

 

  1. Polymers can be very resistant to chemicals.

  2. Polymers can be thermal and electrical insulators.

  3. Generally, polymers are very light in weight with varying degree of strength.

  4. Polymers can be processed in various ways to produce thin fibres or very intricate parts.

 

Although plastics deteriorate and never decompose completely, neither does glass, paper, and aluminium. Plastics make up 9.5 percent of yearly waste by weight compared to paper, which constitutes 38.9 percent. Glass and metals make up 13.9 percent of waste. [7]

 

3.0   Background

3.1    Background on Acrylic

Acrylic polymer, derived from the monomer methyl methacrylate (MMA), a thermoplastic, was first developed more than 60 years ago. Since then, formulations have extended the material's performance range, resulting in varying levels of melt flow, impact resistance, colourability, gamma recovery, and other controlled characteristics. General-purpose Acrylic grades contain a comonomer, added during the polymerization process to facilitate flow during injection moulding and extrusion. Specialty grades are formulated for applications requiring high impact strength and heat resistance. UV light-transmitting formulations are also available, and are specified for certain critical diagnostic equipment in which even minor UV absorption or variation in material flow could be detrimental.

Acrylic has been used in medical and health-care applications since its discovery. One of the first uses of Acrylic sheet was for incubators. The first intraocular Acrylic prosthesis was implanted in 1955, and ever since Acrylic has been used in contact with human tissue. Its biocompatibility led to the adoption of Acrylic for aircraft canopies during World War II. From this, pilots suffered fewer infections from shards of Acrylic than what they had from glass. [6]

The leading applications of Acrylic in the medical industry today are for cuvettes and tubing connectors, but it is also used to produce test kits, syringes, luers, blood filters, drainage wands, flow meters, blood-pump housings, fluid silos, surgical-blade dispensers, incubators, and surgical trays. Acrylic polymers are resistant to many biological and chemical agents. Medical grades of Acrylic have passed USP Class VI biological testing procedures and comply with FDA regulation 21 CFR 177.1010.  [4]

Acrylic is also used as fibrefill, lenses, light covers, glazing (particularly in aircraft), light pipes, meter covers, bathroom fittings, outdoor signs, skylights, baths, and toys.  Acrylic film is laminated over ABS sheet to provide UV protection. 

3.2    Background on Lexan

Lexan is renowned across the world as for high impact strength applications. One of the most distinctive attributes of Lexan is its high impact strength. Few other engineering applications can match its ability to resist high impact and be subjected to harsh weather conditions.

Amongst its high impact strength, Lexan is also regarded as possessing high durability, high heat and flame resistance, UV stability, and sufficient processability. Lexan’s processability is very significant as industries today are faced with constraints as to manufacture materials by the most efficient techniques.

Lexan is a proprietary term given to polycarbonate by General Electric. Lexan is an amorphous polymer belonging to the thermoplastic family. It was first developed by Dr. Daniel W. Fox after he conducted several experiments on an insulation material. He came across a porous material that was essentially “unbreakable” upon hardening. Despite his efforts, Dr. Fox could not find any way to “break” the material which labelled the material as “indestructible”. It was this discovery that revolutionized society’s living and play habits. The use of Lexan became popular when NASA began using the material for visor and pressure helmet assemblies. Due to the growing concern of safety and weight in automobiles, many auto manufacturers also began to implement the material. This resulted in cars with greater defensive properties and light weight for improved performance. [4]

After Lexan’s light weight and impact strength were utilized in space and automotive applications, other industries began to find the material considerably advantageous. Compact disc technology took notice of the light weight and the unbelievable sound quality it produced. Safety also became a growing concern in optical and protection equipment. This led to refinement in optical lenses and sports helmet visors. [1]

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