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ACRYLIC POLYMERS: A CLEAR FOCUS

Medical Plastics and Biomaterials
Magazine
| MPB Article Index

Originally published January 1996

Optical clarity is an important quality in medical devices or diagnostic equipment that rely on visual inspection and therefore require a high level of transparency. Acrylic, the common name for polymethyl methacrylate (PMMA), has long been used in the manufacture of cuvettes, tubing connectors, speculums, and many other medical devices requiring impact strength, chemical resistance, biocompatibility, and clarity. Acrylic polymers thus occupy a prominent place in the market for clear, disposable plastics--only glass transmits light as well. Acrylic also is commonly used in the manufacture of reading glasses, due to its superior optical qualities.

This article addresses the key properties of acrylic materials and compares them with other thermoplastics competing for a share of the medical market. Benefits to both the manufacturer and the end-user are highlighted, as are applications for which acrylic is well suited and those for which it is not recommended. In addition, the paper discusses acrylic's chemical resistance and performance after sterilization, along with disposal, recycling, and other environmental issues. Design and processing guidelines for manufacturers are also included as is a case study of a specific application.

Overview of Acrylic

Acrylic polymer, derived from the monomer methyl methacrylate (MMA), 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, colorability, gamma recovery, and other controlled characteristics. General-purpose acrylic grades contain a comonomer, added during the polymerization process, to facilitate flow during injection molding and extrusion. Specialty grades are formulated to perform in 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 slight UV absorption or variation in material flow could be detrimental.

Easily processed and assembled, acrylic has been used in medical and health-care applications since its introduction. 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: pilots suffered fewer infections from shards of acrylic than they had from glass.

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, and drainage wands as well as flowmeters, 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.

The use of acrylic polymers in the medical industry has been steadily increasing over the past several years. This is especially noticeable in the area of diagnostics, due to the fact that acrylic is an inert material that does not react with the reagents used in testing. For medical devices, special impact-modified grades formulated to resist breaking and cracking are employed more often than standard grades.

Physical Properties

Acrylic offers light transmittance of 92%--theoretically the maximum obtainable--with particular clarity at lower wavelengths of 270 to 350 nm. For example, acrylic is the material of choice for disposable cuvettes, used to contain blood and other fluids, through which a spectrum of UV light is passed for analysis. Although quartz glass can be used for the most demanding applications, since it transmits light as low as 220 nm, it is not cost-effective in an application that increasingly favors disposable plastics.

Other prominent physical properties of acrylic include good mechanical strength and dimensional stability, along with high tensile and flexural strength (see Table I). Medical-grade acrylic can be supplied for intricate, thin-wall applications in which maximum optical clarity is required: it offers excellent injection molding flow properties (13 g/10 min) and a tensile strength of 10,600 psi. Acrylic also provides good surface hardness for scratch resistance, an important quality in medical applications.

Because acrylic is a rigid material, standard grades do not provide high impact resistance. Therefore, impact-modified grades--softer and less rigid than standard formulations--are specified for applications that typically require increased toughness. Acrylic is not recommended for applications that demand very high impact resistance or those that put surfaces under high pressure. Acrylic does perform well in electrical applications, due to its insulating nature; an increase in absorbed moisture makes it more conductive.

As temperatures increase, acrylic becomes more flexible and exhibits less flexural strength. Under sustained loading, strain on the material can induce excessive molecular movement that increases with time under load and higher temperatures and results in the phenomenon known as creep that is common to all thermoplastics.

Chemical Resistance

Acrylic is resistant to a wide range of chemicals including salts, bases, aliphatic hydrocarbons, fats and oils, most common gases and inorganic chemicals, dilute mineral and organic acids, and dilute and concentrated solutions of most alkalis. It is attacked by strong acids, chlorinated and aromatic hydrocarbons, ketones, alcohols, ethers, and esters. Of course, the chemicals and other materials to which a molded part will be exposed should be carefully considered before selecting any thermoplastic.

Isopropyl alcohol tends to promote crazing in acrylic, as it does in many transparent plastics. Some acrylic grades are more alcohol-resistant than others: resistance is typically a function of the molecular weight of the polymer, with higher molecular weight providing better alcohol resistance. Diluted solutions of isopropyl alcohol can be used to wipe down acrylic without adverse reaction. Acrylic copolymers, particularly those with styrene, offer improved chemical resistance but do not transmit light as well as 100% PMMA.

Sterilization Effects

Acceptable sterilization technologies for acrylic are E-beam or gamma irradiation or dry ethylene oxide gas. While gamma sterilization has a tendency to discolor (yellow) most acrylics, this yellowing is temporary and recovery can be complete, with the parts retaining their original integrity. The higher the radiation dosage, the greater the yellowing and the longer the required recovery time. Until recently, when rapid-gamma-recovery formulations were introduced, acrylic took as long as 60 days to recover from the effects of gamma irradiation. This time has been cut to a week for some grades of acrylic. Wet ethylene oxide and steam sterilization methods are not recommended for acrylic.

Material Comparisons

In the area of diagnostics, polystyrene is the closest clear-plastic competitor in optical properties, but its performance at lower light wavelengths does not rival that of acrylic. (Polystyrene begins to absorb light at about 350 nm and, consequently, does not transmit it as well as acrylic.) Polycarbonate, a key material in the medical device area, is normally not used for diagnostic equipment when maximum clarity is a requirement. In nondiagnostic applications, acrylic competes with other clear plastics: these include polystyrene, styrene acrylonitrile (SAN), polyvinyl chloride (PVC), polyethylene terephthalate glycol (PETG), and clear acrylonitrile butadiene styrene (ABS). Each has its own advantages and disadvantages, depending on the application.

Polycarbonate is clearly dominant when strength and toughness are the prime requirements. It tends to be overspecified, especially for large parts whose failure in a medical environment could have severe consequences. Styrene copolymers are starting to gain on polycarbonate because of their toughness, clarity, and moderate pricing; on the downside, they are fairly soft and easily scratched. Acrylic offers better chemical resistance than polycarbonate. In terms of impact resistance and strength, polycarbonate is tougher, although impact-modified acrylic grades can be used in many applications that specify polycarbonate. Acrylic outperforms polycarbonate in clarity, scratch resistance, and UV transmission, and generally offers better processability, since polycarbonate is prone to problems related to molded-in stress, such as crazing and cracking.

Other tough materials that often are chosen in place of acrylic are clear ABS and clear PETG. ABS is less susceptible to residual molding stresses than polycarbonate, while PETG is less susceptible to chemical attack. But neither ABS nor PETG provides the clarity, scratch resistance, or light transmission of acrylic, and PETG is more difficult to process, tending to warp under adverse molding conditions. PVC is often selected when a flexible material is required, although it does not provide the clarity or scratch resistance of acrylic.

Additional acrylic competitors in the medical market are polystyrene and styrene-containing copolymers such as SAN. These styrenic materials exhibit low melt viscosity and are easy to process, but cannot quite match the performance of acrylic in the areas of clarity and UV transmittance. Polystyrene typically is used for complex parts because of its excellent processability, which reduces the molded-in stresses that can affect light transmission in very complex parts.

Material Selection

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 overdesigning 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. Environmental conditions that figure into the selection include chemicals likely to be encountered, sterilization methods, humidity, temperature, and thermal cycling. Other important factors are mold and part design, processing requirements, and assembly methods.

In selecting the best grade of acrylic to use, these same factors must be considered. Higher-molecular-weight grades are more resistant to crazing from chemical exposure and mechanical stress, but have lower melt-flow rates. As the level of impact modifier is increased in impact-grade acrylic, other properties such as clarity, light transmission, and tensile strength often diminish.

Processing requirements can be difficult to predict. Flow patterns and mold-filling problems can result from large part surfaces, deep ribs or flanges, asymmetrical geometries, and unbalanced thick and thin sections. Experimenting with several different grades in the prototype tool is recommended. Molds designed for acrylic, ABS, and polycarbonate parts typically accommodate a shrinkage level of 0.006 in./in., allowing for convenient trial runs of acrylic in molds used for ABS or polycarbonate parts. Direct comparison of the parts molded from different materials can help determine the best one for the application.

Processing Considerations

Compared with other polymers, acrylic is relatively easy to process. It can be molded with little or no residual stress and is available in formulations specifically designed for injection molding or extrusion in a wide range of melt-flow rates. Under normal processing conditions, acrylic produces melts that are typically higher in viscosity than those of many other thermoplastic polymers. The higher-molecular-weight grades are generally recommended for extrusion. Because of acrylic's higher- viscosity flow properties, injection molding runners and sprues used to process it need to have larger diameters than those handling polystyrene or polyethylene. Large injection-molded parts or parts with thin-wall geometries may require a high melt-flow rate.

When acrylic is processed in molds built for polycarbonate or polyester, lower injection pressures should be used, as it is less likely to warp than polyesters and is more forgiving in molds not designed with well-balanced gates. Because acrylic has a higher melt viscosity than polystyrene or styrenic alloys, it requires higher injection pressures when selected in place of those materials. Higher clamping forces may also be required. A hygroscopic material, acrylic absorbs water and must be dried prior to molding: if molded while wet, it exhibits moisture splay, leaving streaks, bubbles, and a rough surface on the part.

Typical wall thicknesses for acrylic parts range from 0.040 to 0.500 in. Thicker or thinner parts can be achieved with special designs or processing methods such as injection/ compression molding. Consistency is the key; any changes in thickness should be gradual, and feature radiused edges. Vertical walls should be the same thickness as the rest of the part to avoid pressure variations on the flow front, which can lead to stressed areas and voids caused by trapped air. Moderate residual stress does not affect part performance, but high levels of stress can reduce impact strength and resistance to chemical or heat crazing, and can undermine the dimensional stability of the molded part. Before the part and mold design are completed, a mold-flow analysis should be performed to help avoid costly mistakes and downtime.

Bonding Systems

Acrylic parts can be fastened by chemical bonding, ultrasonic welding, and heat staking. Two types of agents are commonly used to chemically bond acrylic: solvents and polymerizable adhesives. Solvents such as dichloromethane dissolve the surfaces of two acrylic parts, which harden after the solvent evaporates and bond to one another. Solutions of acrylic polymer dissolved in a solvent or methyl-methacrylate monomer work similarly. Two-part polymerizable adhesives contain a viscous acrylic resin base and a liquid catalyst that when mixed together provide a strong joint.

Ultrasonic welding is an efficient method of fusing two parts made from the same material. Both contact (near-field) welding and transmission (far-field) welding can be used for joining acrylic parts. However, materials with different melting points are not good candidates for ultrasonic welding, since even a few degrees difference can result in one material melting before the other reaches its melting point, preventing a fusion between the parts.

Mechanical fastening, which concentrates loads at fastening points, is not recommended for acrylic parts, as the act of drilling holes or torquing fasteners can introduce potentially damaging stress. Holes should be cored out rather than drilled.

Acrylic Case History

Selecting the wrong grade of acrylic can lead to unsatisfactory results. Recently, a manufacturer of a blood-clot analyzer experienced early production problems with several complex parts molded from a general-purpose grade of acrylic. With the formulation initially chosen, parts developed stress cracks upon ejection from the tools, primarily because of complicated geometries that included sharp corners, edges, and points. Streaking also occurred. It was determined that additional toughness and impact strength were required, in a high-flow grade that would fill the molds.

An impact-modified, gamma- resistant acrylic--which is supplied in injection-grade pellets--was then selected and found to solve some tricky molding problems. High tensile strength and impact resistance were critical; since the parts are used in the assembly of a cassette intended to contain blood, they must be resilient enough to resist cracking if accidentally dropped. Dimensional precision was also important, given the complexity of the processes that occur under pressure inside the cassette during blood analysis. A third requirement was UV transmissability, which was necessary for curing the adhesive used in the cassette assembly.

Molded from the impact-modified grade, the cassette parts have proven to be sufficiently durable, both during fabrication and in use. The acrylic material provides the requisite impact strength as well as a high degree of optical clarity and scratch resistance, and presents a pleasing, glossy surface finish. After use in the blood-clot analyzer, the disposable cassettes are incinerated with other medical waste.

Disposal and Recycling

Acrylic burns extremely clean, providing virtually smokeless combustion with end products of carbon dioxide and water (per ASTM E 662). In addition, the material offers superior recyclability: acrylic can be reground and reused, which results in less material waste during molding.

Another characteristic of acrylic is its ability to be depolymerized back to its monomer, thoroughly purged of impurities, and repolymerized back into PMMA. Commercial processing facilities set up for this process typically use a molten lead bath to vaporize the acrylic material. The vapors are captured and recondensed into MMA, while eliminating any biomedical waste. This is a true recycling process, taking the polymer back to its monomer, whereas most other "recycling" processes involve crushing the material and using it in applications with lower specifications. For environmental reasons, commercial molten lead baths are no longer operated in the United States, but do exist in England, India, and several other countries.

Conclusion

Used in medical applications for many years, acrylic offers a number of advantages compared with other clear thermoplastic materials traditionally specified for diagnostic equipment and medical devices. Benefits include unsurpassed clarity, superior toughness, rapid gamma recovery, excellent UV-light transmittance, biocompatibility, and good chemical and scratch resistance. Acrylic is easily processed and assembled and offers the potential to be recycled back to its monomer and used again, an important property as the pressure to recycle increases.

Acrylic resin can be custom formulated to meet the requirements of a broad range of medical applications. Since it can be processed in molds designed for polycarbonate and other thermoplastics, manufacturers using those materials should consider trial runs with acrylic. Direct comparisons between parts molded with acrylic and with competing thermoplastics may reveal potential cost savings as well as unexpected side benefits such as improved surface finish, transparency, processing, and part performance.

W. A. Whitaker III is marketing manager for the polymer division of ICI Acrylics, Inc. (Memphis, TN). At ICI since 1972, he has held a number of positions within the company, including sales, technical services, market development, and commercial management.

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Do you know of a medical grade solvent to bond two acrylic pieces together? I'm trying to bond two medical grade acrylic parts and still have the assembly meet USP Class VI requirements.