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When you mention plastics in the medical industry, most think of injection molded plastics or disposable plastics (i.e., items meant for one use and then discarded). However, extruded stock shape thermoplastics have earned their place within the industry maintaining a highly visible position in one distinct area of this field, the operating room. The number of surgeries, whether invasive or non-invasive, is increasing every year. This is particularly true for orthopedics where the growth potential is excellent and market stability looks promising for at least the next decade.
Extruded thermoplastics, including polypropylene, got their start in the operating room as early as the 1950s, but most of the products that are predominant today have only been in use since the early 1980s. The products using thermoplastics are collectively known as medical devices, and originate from the engineering type resins. This is due to their enhanced capabilities and physical properties.
One of the most important aspects of plastic for medical devices is the ability to withstand repeated sterilization, the most common being steam in autoclaving. Cold/plasma, ethylene oxide (EtO) gas, dry heat, and radiation are other methods of sterilization. Devices can be sterilized and used several times in one day. Resistance to the sterilization process will often determine the life of the device in the field, and material compatibility is critical in this respect.
One area polypropylene has found a niche in the medical market is in containers. Containers, often referred to as surgical instrument or sterilization trays, are designed to hold a wide variety of instruments or an entire system of instruments. A typical design for a container is an outer box with a lid held by latches, an inner tray or multiple layers of inner trays to hold the instruments, and a set of handles incorporated into the lid. It wasn’t until the early 1980s that plastic was considered as an alternative material to metal cases. The primary reason being the possibility of post-operative infections being caused by incomplete sterilization of instruments and tools housed in the cases or trays. Thermoplastics were also chosen because they were lightweight and generally had a lower cost.
PTFE and polyoxymethylene (acetal) materials have been used, but their resistance to repeated autoclaving was limited. Recently, a heat-stabilized polypropylene has been developed to replace most of these materials. It can withstand hundreds, if not thousands, of autoclave cycles without losing dimensional stability due to extremely low water absorption and high heat deflection properties. It is also lightweight, able to be laser marked, and chemically resistant, making it a great material for this application.
This article was written by Westlake Plastics Company.
|In This Issue:
Polypropylene in the operating room
Material meets need in medical industry.
Tank fabricators depend on polymers for durability
Application of extruded and compression molded PP sheets
Test your knowledge
Plastics Education in Philadelphia
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The alarm sounds at the fire station, the firefighters jump on/in the truck and off they go, racing to prevent the destruction of a building or to save a life. They arrive at the site of the fire — a rural area — only to discover that the water in the truck is nearly gone due to a leak in a cracked tank — the result of road vibration.
If that tank had been fabricated from co-polymer polypropylene, the chance of this scenario becoming a reality would be dramatically reduced. Why? Co-polymer polypropylene has excellent impact properties, particularly at low temperatures that reduces the possibility of cracking during transport and installation. In addition, it doesn’t absorb moisture and won’t corrode, maintaining structural integrity.
There is no question that polymers have been widely accepted and specified as the material of choice for tank fabrication. The most commonly used are homopolymer and co-polymer polypropylene, with co-polymer as the top choice of tank fabricators due to key properties that include:
In fact, polypropylene has become the preferred material for the majority of electroplating tank applications because it is resistant to attack from most plating solutions such as brass, cadmium, copper, lead, nickel, silver, tin and zinc. Plating barrels and drums, dip baskets, tank fixtures, baffles, splashguards and secondary containment structures used in electroplating can all be fabricated from polypropylene. In addition, polypropylene tanks are a common sight in the demanding environment of steel processing plants. Its durability and corrosion resistance make it a popular and economical choice for etching solutions, prep and picking tanks.
However, the property that really makes polypropylene the material of choice with tank fabricators is the fact that it can be easily fabricated and welded using commercially available hot air, extrusion and fusion welding equipment. Tank fabricators can quickly meet the stringent requirements of customers in terms of providing a product that won’t deteriorate in an extremely corrosive environment and one that can be easily fabricated in non-standard sizes and shapes utilizing a variety of machining techniques such as sawing, milling and welding.
This article was written by Ken Thomas, Quadrant EPP USA.
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Within the area of industrial application, there are two methods commonly used to produce thermoplastic sheets and slabs: compression molding and extrusion. The two methods differ in terms of the possible thickness of the slabs and productivity levels, as well as the properties of the manufactured slabs. Nowadays the extrusion of sheets up to 70 mm thickness and up to 4,000 mm width is a particularly economical approach, thus replacing the compression molding method in those cases in which it is deemed commercially viable. The more cost-intensive compression molding method is used only when special properties and thicknesses over 70 mm up to 200 mm are of interest. This article provides a concise overview of the properties and applications of polypropylene (PP) sheets, depending on the production method used.
The compression molding method
The key advantages of compression molded sheets are the very low shrinkage and isotropic properties that can be achieved when using optimized parameters.
The molding compound normally falls onto the screw by gravitation and by means of the rotating screw it moves from the filling station to the nozzle. During this movement, high shear forces and friction occur so that the material is heated up and homogenized. The molding compound is pressed through the extrusion die (sheet die) and is flattened on a three-roll mill with decreasing heating, removed by means of rubber cylinders over a roller cooling grate and cut to slabs with gate shears or by saws after cutting the edges.
In addition to internal stresses, which are independent of the production method, and possible thermal stresses arising during application, so-called orientation stresses occur as a result of the extrusion process. Due to the extrusion process, the orientation level of extruded sheets is greater in the direction of extrusion than perpendicularly to it.
Properties of polypropylene
Due to the processing method, the properties of extruded and compression molded sheets also differ substantially when it comes to shrinkage behavior and orientation behavior. Furthermore, the mechanical properties are quite different.
The reason for these differences is to be found in the melt flow characteristic of the molding compound during extrusion. The molecules are stretched in one direction, and depending on the cooling rate, different forms of crystallization can be observed.
Only compression molded PP sheets have approximate isotropic properties as a result of the lack of melt flow during processing. Difficulties usually only occur when the cooling rate is not uniform, e.g., in the case of thick sheets. Therefore, the compression molded sheets are cooled down in the mold under pressure to avoid anisotropic cooling and void formation.
Owing to the isotropy and low shrinkage behavior, compression molded slabs always deliver key benefits when uniform multidirectional properties are required. These uniform properties are essential for mechanically machined sheets where the requirement of dimensional stability has to be fulfilled.
On the other hand, in thermoforming applications a higher shrinkage behavior is necessary and therefore extruded sheets are used, where the shrinkage behavior can be controlled by the processing parameters. Generally, the shrinkage in the extrusion direction is higher than perpendicularly to it. Therefore, the extrusion direction is marked for the thermoforming application to avoid faults in the thermoformed part.
This article was written by John Ploskonka, SIMONA AMERICA Inc.
Polypropylene (PP), a polyolefin, is a thermoplastic made by the polymerization of propylene gas, a by-product of petroleum refining. Various techniques can vary the physical, chemical or mechanical properties of polypropylene.
Polypropylene offers a good balance of thermal, chemical and electrical properties with moderate strength. It possesses a good strength to weight ratio. Because it has such a hard, high gloss surface, polypropylene is ideally suited to environments where there is concern for bacteria build up that can interfere with flow. Like most polyolefins, polypropylene is approved for direct food contact.
Polypropylene has good insulating properties. Its excellent arc resistance and dielectric strength allows polypropylene to be used extensively in electrical applications. It can also be modified to be conductive or antistatic.
As a homopolymer, polypropylene can be used at temperatures ranging from 30°F to 210°F (0 to 100°C) depending on specific chemistry, while copolymers can operate successfully from -20°F to 180°F (-30 to 80°C).
Resistance to chemicals and solvents is very good. It has good resistance to aqueous, salts, acids or alkaline solution. It also has very low permeability to water vapor and gases. Because of its excellent resistance to solvents, it finds extensive use in the semi-conductor industry. It performs well in the presence of deionized water. The resins are attacked, however, by halogens, fuming nitric acid and other active oxidizing agents, and by aromatic and chlorinated hydrocarbons at high temperatures. (See the property table for polypropylene on the IAPD web site at www.iapd.org/new/bookstore/free_resources.html.)
For more information on polypropylene and other plastic materials, IAPD’s Introduction to Plastics is an invaluable training manual. Details about it and other IAPD educational resources are available online at www.iapd.org.
What do you know about polypropylene? Answers are at www.iapd.org/popquiz.html.
1. While polypropylene has good chemical properties, which chemical family would not be compatible with the material?
2. What is the specific gravity (D792) of standard homopolymer polypropylene?
Your IAPD Distributor is your choice in finding the right material for your application. Go to www.iapd.org to find a distributor in your area. You can search by company name, location or product category.
The IAPD Magazine web site at www.theiapdmagazine.com allows you to search by material, trade name and fabrication process. You can also search for fabrication capabilities.
Join IAPD in Philadelphia September 20 for a special one-day educational package specifically for engineers and specifiers to learn about innovations in both plastic materials and applications.
IAPD experts will first present the popular IAPD Plastics Applications Seminar, which offers a look at a wide variety of plastics applications. The seminar introduces and reintroduces materials in a variety of eye-opening applications. After the seminar, be sure to attend the IAPD Plastics Exhibition, a great opportunity to experience first-hand the best of the plastics stock shapes and pipe, valves and fittings distribution industry. The show is the only one of its kind in North America.
After the day of education, finish the evening at Hospitality Suite Night, a unique opportunity to network with IAPD manufacturer members in a social setting. You'll enjoy food and beverages while minging with other delegates!
Information is available online at www.iapd.org/new/pdf/events/ac/one_day.pdf.
Designing with Plastics is published by the International Association of Plastics Distribution. While every effort has been made to ensure accuracy, IAPD encourages you to verify information with a plastics distributor to ensure you select the correct plastic products to meet your needs.