Today’s blog post is about the properties, history, and applications of PMMA as plastic in injection molding. Most people know PMMA better by its proprietary name, Plexiglas®. However, it is also known as acrylic glass.
What is PMMA?
PMMA stands for poly methyl methacrylate, a clear thermoplastic known worldwide as acrylic glass. Produced by chain polymerization, PMMA has an impressive light transmission superior to silicate glass, high impact strength, and lightweight.
PMMA is the acronym for one of the most successful plastics ever made. Of course, polyethylene, propylene, and other plastics used in packaging materials and disposable bottles are produced in large quantities. But PMMA is also something we encounter constantly in our daily lives and it knows how to make a very special impression with its outstanding properties and great quality as a material. PMMA stands for poly methyl methacrylate. As usual, this sounds very complicated and difficult to pronounce for the layman. We will explain later how the name came about. Let’s start by looking at the properties of this amorphous thermoplastic.
PMMA – The All-rounder in Plastic Products
What do the 1972 Olympics, various deep-sea diving records, and the healing of complex leg fractures have to do with each other? In all three cases, acrylic glass played a decisive role. On the gleaming roof of the famous Olympic Stadium, 75,000 square meters were covered with 19,000 acrylic panels of just 4 mm thickness. After a few years, these began to turn yellow. People wanted to sue the manufacturer for false promises because PMMA was advertised as UV-resistant. In fact, it is, just as it is insensitive to many other chemicals. In this case, it is a fire retardant that is added to the plastic during the manufacturing process. For example, in the deep sea, the famous Deep Sea Rover TV submersible is built with observation domes made of acrylic glass. They can withstand an incredible 100 kg of pressure per square centimeter at a depth of 1,000 meters. The dome’s PMMA is only 13.5 cm thick. PMMA is also used in aircraft to safely build windows. Finally, for broken bones, the so-called bone cement is made of PMMA and is used as a bone substitute or also at the dentist for transplants. As a result, this amazing plastic is very stable, easy to mold, and also allows light to pass through more easily than the usual glass.
History of PMMA
Otto Röhm was actually interested in creating an artificial rubber. This was the goal of many researchers around 1900, and Otto Röhm’s personal dream was to produce transparent car tires. This failed, but years later he succeeded in his own company with the technical treatment of enzymes used in detergents and the invention of hard safety glass made of polyacrylate (i.e. polymerized acrylic acid). This earned him enough money to continue his research. Things got even more interesting when acrylic acid was fused with methanol and acetone to form a substance – a monomer.
This MMA, methyl methacrylate, showed such interesting properties that people spent a long time trying to find a way to produce it on a large scale. After all, polymerization is not something else; one of the substances, the monomer, is stimulated by a trigger to cross-link more and more, and more and more of the monomer combines with itself. But this seemed impossible. But chance saved the study. Criminal carelessness led Mr. Bauer of the Otto Röhm team to leave some MMA in a bottle on the windowsill over the weekend. Solar radiation and heat set the polymerization reaction in motion. PMMA, or polymethyl methacrylate, immediately proved its superiority over the glass and caused the bottles to burst. When Otto Röhm saw this, he reportedly exclaimed, “I was baffled,” and invented the brand name Plexiglas® for selling his company’s products.
This new wonder material soon became known because Otto Röhm was first and foremost a businessman. At that time the world did not know that such a transparent and slightly elastic material could be molded at temperatures slightly above 212 °F (100 °C) and retain its shape after cooling, and Otto Röhm made a transparent violin made of PMMA for the 1937 Paris World’s Fair – and the world was amazed. The company grew accordingly until the patent expired in 1952 it was later acquired by BASF. The old name is still protected, which is why polymethyl methacrylate is today often referred to as acrylic glass.
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Polymethyl Methacrylate in Injection Molding
To produce injection molded semi-finished products, the material is heated to 392-482 °F (200-250°C) Celsius. Pellets should be preheated to 158-212 °F (70-100°C) before heating in the cylinder but must be handled with care. Pellets that are heated too early and too strongly will stick together and then must be separated again before entering the injection molding machine. The actual heating inside the cylinder is carefully calculated to avoid sink marks. The same applies to the mold, which must be preheated to a precise level to eliminate stress. The injection rate depends on the thickness of the molded part. Late molding is possible at a temperature of approximately 302 °F (150 °C), and the mold used depends on the degree of bending.
The part should be annealed for a longer period of time at 140-176 °F (60-80 °C) to prevent crack formation. Cooling is done slowly, lasting 3-5 hours, depending on the initial temperature. In post-processing, the plastic can be drilled, (wet) ground, laser, sawed, cut (<1 mm), and scribed. PMMA is easily bonded, although, for some shapes, tempering is best. To guarantee a good optical quality of the product, absolute cleanliness of all tools and machines used should be ensured.
The Properties of PMMA Are a List
If we put these properties in a list, it looks like this.
- Density: 1.18/cm³, so about half the weight of window glass.
- Low scratch sensitivity due to high surface hardness
- High compressive, bending and tensile strength, not strong impact resistance
- Refractive index of 1.492 according to ISO 489
- Resistant to weathering and UV rays
- Temperature range between -40 °F (-40°C) and 158 °F (70°C)
The performance and quality of PMMA can be further improved by using certain additives. It can be made more impact resistant, permeable to UV radiation, and resistant to gamma rays. It can be approved as food safe and stabilized against other chemicals.
Today, polymethyl methacrylate is used in the automotive industry, aerospace, pharmaceuticals, construction, and even in cosmetics and coating technology. Like many plastics, PMMA is available as thermoplastic pellets for injection molding.
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