Characteristics of medical polyurethanes.
Polyurethanes are cast, molded, and extruded into a variety of products for the medical device industry. These applications include pacemaker leads, peripheral and central catheters, feeding tubes, balloons, and condoms. The characteristics of polyurethanes are defined by the class and grade of material, and by the process of fabricating the component. In the casting of thin components, molding of integral components, and extrusion of tubing, particular variables must be controlled to make an acceptable component. The physical properties of a material can be greatly reduced by improper processing.
Classes of Polyurethanes
The class of a polyurethane(1) designated by the diisocyanate, macrodiol (polyol), and short-chain diol (extender) used to synthesize the urethane. The grade of polyurethane is defined by the ratios of the raw materials. The ratio of polyol (the soft segment) to extender and diisocyanate (the hard segment) determines the hardness of the material. Solution-grade polyurethanes are usually made with a slight excess of hydroxyl functionality to increase solubility. Injection molding and extrusion-grade polyurethanes are usually made with an excess of isocyanate to maximize physical properties. In the medical industry, two common diisocyanates are used: the aromatic methylene 4,4'-diphenyl diisocyanate (MDI) and the aliphatic methylene biscyclohexyl diisocyanate (HMDI). Polytetramethylene ether glycol (PTMEG) is the dominant macrodiol. Polyesterdiols are also used in polyurethanes, and recently, much attention has been directed to the polyurethanes made with poly-carbonatediols. The most common extender is butanediol (BDO). See Table 1 for a description of medical polyurethanes.
Melt Processing - General
Injection molding and extrusion are the methods of choice in processing polyurethanes for a wide variety of products. Two important variables are common to both processes: the temperature profile and dryness of the resin. Resin suppliers recommend a temperature range for each class and grade of polurethane. Generally, soft materials process at a lower temperature than hard materials; aromatic polyurethanes melt at higher temperatures than aliphatics. Polyurethanes are hygroscopic materials and can absorb 1% or more of water at ambient conditions. Melt processing a dry resin is very important - most polyurethane resin manufacturers recommend drying polyurethanes before melt processing the materials. Table 2 shows the drying conditions recommended by several resin manufacturers. Moisture contents above 0.10% result in poor surfaces and, at higher levels, complete degradation of the polymer. The dry resin should not be exposed to ambient moisture before processing the material. Short contact with moist air may result in significant water gain.
Melt Processing - Injection
Molding Standard injection molding machines successfully mold polyurethanes. The size of the machine depends on the desired component and runner system. Clamping pressure should be more than 3000 psi/inch (3200 Kpa/[cm.sup.2]) of surface area. Below this pressure, flashing is very difficult to avoid. Polyurethanes degrade when exposed to high temperatures. If a polyurethane remains in the barrel for long periods, the material's tack increases, turns yellow, and loses physical properties. The time required for significant degradation is temperature-dependent. Polyurethane degradation is initiated at temperatures as low as 150 [degrees] C, and significant degradation occurs at 230 [degrees] C to 250 [degrees] C. Aliphatic resins that mold at a melt temperature of 170 [degrees] C may remain in the barrel for 30 min without changes in appearance or physical properties. However, aromatics that mold at 220 [degrees] C may start to turn yellow within 5 min. Dwell time of the resin should not exceed five cycles.
Die designs must include a generous runner system and a substantial gate. Polyurethanes have a tendency to thicken when forced. The hard segment of polyurethanes is responsible for their thixotropic character Degree of hard segment organization determines the extent of thickening. Runner diameters should be 6.3 mm or larger The gate depends on the component size. For small components (less than 3 grams), a 1.0- to 1.5-mm gate is sufficient. For larger components (3 grams or more) and components with thin walls, a 1.5-mm or larger gate will be required. Thin-walled components (0.13 to 0.25 mm) can be achieved with proper gating, venting, and temperature profile. Injector pin placement and number of pins are dependent on material. For hard materials (those with Shore durometer of 60D or higher), standard pin placement is sufficient. Softer materials, especially below 90A, require stripper plates and stripping sleeves to facilitate removal of the component from cavities and cores. Surface-impregnated mold releases are recommended for long production runs with all polyurethanes.
Each resin has a preferred temperature profile. Machine settings such as injection pressure, cycle times, and injection properties depend upon the grade. A Shore 85A durometer aliphatic polyetherurethane will mold with similar machine settings and mold configurations as an 85A aromatic polyetherurethane and an 85A polycarbonateurethane or polyesterurethane. Shrinkage characteristics are dependent on the shape of the component, gating, cycle settings, class and grade of material, temperature profile, [TABULAR DATA FOR TABLE 1 OMITTED] and moisture level.
Melt Processing - Extrusion
Extruder requirements depend on the size of the tube and take-up equipment. An extruder motor should run at 25% to 90% of capacity. Take-up equipment can generally handle 130 m/min. Above 130 m/min, the operator has difficulties initiating the collection process. Above 200 m/min, collection of material is virtually impossible to initiate; if initiated, it is difficult to maintain.
The preferred extruder has a minimum of three barrel heat zones with air or water cooling, and a heated die. Cooling the screw for polyurethanes, as for other materials, is not necessary or feasible because of the small size of extruders and medical tubing. The screw should have an L/D ratio of 24:1, minimum; 30:1 is preferred. The melting and mixing portions of the screw should be 30% to 40% and 40% to 50% of the screw length, respectively. The preferred wall clearance is 0.08 mm. Recommended screw designs are the standard, Maddox, and, in low-output applications, a Barrier-type. The preferred drawdown ratio for a die is between 1.8:1 and 2.5:1. Drawdown ratios as low as 1:1, and as high as 10:1, are possible. However, gauge control may be difficult at these extremes.
Several variables must be considered when polyurethane resins are being extruded into tubing. Given the same extrusion system, the resulting physical properties of the tube may be altered. In our experience, moisture is the biggest reason for poor tubing. Excessive moisture causes the extrudate to foam. The combination of heat and water results in chain scission and, possibly, formation of amines. The gases that cause the foam are steam, carbon dioxide, and even - under extreme conditions - small molecular organic compounds. With moisture slightly above the recommended level, the extruded material has poor physical properties, rough surfaces, or what appear as irregular die lines. To achieve optimal material properties, the preferred temperature profile may change with the moisture content of the material. A slight amount of water (0.00% to 0.05%) appears to plasticize the melt without affecting final properties of the material. At a moisture level between 0.05% and 0.10%, die head pressure decreases, as if plasticized. Under extremely dry ambient conditions, and if ideal drying equipment is used, a resin may be dried to a point that the lack of moisture affects the extrusion parameters. The actual moisture level for this phenomenon is well below common measuring capabilities. Although rare, a very dry resin will extrude with a high die-head pressure. The resulting extrudate may have difficulties maintaining the dimensions, and physical properties may appear as if the material were extruded cold. Good tubing can be made by increasing the extrusion temperature profile above the recommended values.
The temperature profile that is used to extrude the resin will determine the properties of the tube. Extrusion that uses a hot temperature profile results in a tube with greater elongation, less strength, and an unusually high creep for a polyurethane. With a cold temperature profile, the resulting tubing has low elongation and a different mechanism of failure. These changes in mechanical properties were obtained by extruding at extreme temperature profiles [ILLUSTRATION FOR FIGURE OMITTED].
Shrinkage is very difficult to predict. Controllable variables are the temperature profile, moisture level, drawdown ratio, die-to-water spacing, screw speed, puller speed, and water bath temperature. Extrusion of a single lot of resin at the same conditions results in a tubing of consistent shrinkage. Most lot changes result in little change in shrinkage characteristics. Occasionally, a single lot of material that is within all the supplier's specifications results in much larger or smaller shrinkages than other lots. Shrinkage variations seen in different lots are not understood. Currently, no methods exist to accurately predict shrinkage values from measurable properties of polyurethanes. Development of a reliable formula to predict shrinkage characteristics, although beneficial, would be very time consuming for resin and tubing manufacturers. Therefore, tubing manufacturers may prefer to run a prototype extrusion on new lots of materials where tight tolerances are required. Possible causes for the shrinkage inconsistencies are molecular weight distribution, distribution and organization of the hard and soft segments, or a variation in monomer ratios at synthesis.
Consistent thickness, strength, and appearance are among the desirable characteristics of cast films. To achieve a consistent film, a solution-grade polyurethane and an appropriate solvent must be chosen. A polymer with the desired physical and mechanical properties should be selected for the application. Common solvents are dimethylacetamide (DMAC), tetrahydrofuran (THF), and methylene chloride. Other solvents - such as dimethylformamide, N-methylpyrrolidone, cyclopentanone, cyclohexanone, dioxane, and chloroform - also dissolve polyurethanes. THF dissolves most polyurethanes and is easily removed from the material. However, THF is flammable and can form peroxides, which can be explosive if the solvent is reduced to dryness. Methylene chloride dissolves some polyurethanes, is easy to remove, and is not flammable. However, it is an ozone-depleting solvent and very volatile. DMAC is commonly used for polyurethanes that are difficult to dissolve. Casting from DMAC produces clear, strong films in most cases, but DMAC can be difficult to remove from polyurethanes in some processes.
The appearance of the film depends mainly on the solvent system. Good, dear, bubble-free films require a solvent that completely dissolves the polymer and makes a clear solution and proper evaporation rate. In most cases, casting from a cloudy solution will result in a cloudy film. The evaporation rate becomes important in the manufacture of thick films. For example, if a 0.08-mm film is being cast from 15% solids THF solution, bubbles will form in the film. Addition of DMAC to the THF eliminates the formation of bubbles. Moisture can also produce a cloudy film. It is important to note once again that polyurethanes - and all the solvents mentioned - are hygroscopic. The amount of water affects the viscosity of the solution. Small amounts of water may cause a decrease in viscosity in some systems. With increasing water concentration, the polyurethane will start to precipitate and viscosity will increase.
Consistent film thickness is achieved by controlling the solution viscosity, solution solids concentration, and the dipping process. With a constant dipping process, viscosity of the solution determines the viscosity of a wet coat. The solids concentration will determine the film thickness when dried. Lot-to-lot variations in materials should be considered in the design of a product. If, when a solution is being prepared from the polyurethanes to a set viscosity or solids level, a significant deviation from the desired coating thickness is obtained, a solids:viscosity relationship must be determined. For example, producing standard solution viscosity of a high-viscosity material will require a lower-solids solution. The wet film thickness of the solution would be the same, but the dry thickness would be less. Therefore, the viscosity of the solution must be slightly more than that of the standard solution [TABULAR DATA FOR TABLE 2 OMITTED] to increase the wet film thickness, which will dry to the desired thickness. The converse can be used for polymers with viscosities considerably lower than the standard.
The solution casting process is simple in theory. However, reducing the theory to practice requires control of several variables. First, all surfaces must be free of oil and dirt. The solution must be free of foreign matter, including undissolved polymer, which will lead to defects such as pin holes or bumps. The coating thickness is controlled by using a controlled dip rate or by knifing an exact thickness. Casting a uniform thickness requires a counteracting of gravity. Dipping on a mandrel (casting die) requires inversion of the mandrel as the solvent is removed. Flat films require a level surface. A balanced evaporation rate must be achieved to maximize output versus producing a bubble-free film. Heating a component on a mandrel removes the last traces of solvent and improves the physical properties of the material. Multiple coatings require the proper solvent(s). Aggressive solvents result in favorable binding between layers; however, the aggressive solvent may distort the base material. Control of the solvent contact time and use of a mandrel can greatly reduce distortions.
Polyurethanes make excellent products. They have excellent strength and favorable biocompatibility, and are available in many grades. The materials are shaped into a variety of components by many processes. The development of a process must adjust for slight changes in materials. Quality components are achieved by controlling the variables, such as temperature profile, moisture level in the injection molding and extrusion processes, or viscosity and solids in casting.
1. M.D. Lelah and S.L. Cooper, Polyurethanes in Medicine, CRC Press, Boca Raton, Fla. (1986).
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|Author:||Walder, Anthony J.|
|Date:||Apr 1, 1998|
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