Technology Notebook: Pultruding polyurethane composite profilesTechnology Notebook: Pultruding polyurethane composite profiles
November 9, 2005
Guidelines for injection box design, component metering equipment, and processing.
Unsaturated polyesters and vinyl esters have been the workhorse resins in the pultrusion industry for a quarter century or more. In efforts to expand their markets and differentiate their products, however, pultruders are continually seeking out new materials and processes that can mitigate some of the limitations of traditional resins such as brittleness or slow reactivity.
During the past three years, polyurethane resins, which exhibit superior strength and damage tolerance as compared to traditional polyester and vinyl ester pultrusion resins, have been successfully commercialized. Such advantages do not come without new challenges as polyurethane resins require distinctive material handling and processing methods. A schematic of a typical polyurethane pultrusion setup is shown in the diagram above.
Cost Analysis
The outstanding material performance of polyurethane profiles comes at a cost premium as compared to common isophthalic unsaturated polyester resins, considering resin cost alone. In the current climate of severe price volatility in resin costs, PU resins are typically 40-60% more expensive than UPE resins and are similarly priced to many vinyl esters (VE). While the cost of PU resin is typically greater than UPE resin, the toughness, damage tolerance, and strength of polyurethane resins bring potential to simplify and reduce the cost of the reinforcement layup in pultrusion profiles by replacing mat with rovings. Further, converting mat to rovings in the design increases profile stiffness and has the potential to reduce the overall composite geometry. These two design elements together can actually offer a cost reduction for PU versus UPE.
Equipment Design
Polyurethane resins are two-component systems comprised of a polyol and an isocyanate, typically based on a modified polymeric MDI (diphenylmethane diisocyanate). The polyol is a fully formulated blend of base resin, catalyst, internal mold release (IMR), and other additives. This blend does not usually include filler, colorant, or UV stabilizers, for example.
To process this system, a two-component metering unit is required because of the limited potlife of the mixed resin (15-20 min depending on ambient temperature and mix quality). The metering unit dispenses mixed resin into a closed injection box or injection die. However, limited openbath runs are possible even for relatively large parts provided the bath volume is kept to a minimum, and fresh resin is regularly added.
The required metering unit is comprised of several subsystems that are commercially available including: metering pumps or cylinders, resin tanks, a mixhead and transfer hoses, mixing elements, and a solvent flush system. Complete metering systems with a broad range of technical sophistication can be purchased from several commercial suppliers. It is feasible to operate such turnkey systems directly from the pultrusion machine controller, allowing realtime component metering control and almost completely automated resin processing.
Pumps/cylinders?Components may be metered either by air-driven cylinders or by electrically driven pumps. Single-acting cylinders can be used, but double-acting cylinders are preferred as they provide minimal flow fluctuations when the cylinder reverses direction. Cylinder pumps are rugged, reliable, and can be either fixed or in a more versatile variable ratio setup that allows for processing with and without fillers. An air-pressure regulator must be installed to control the flow rate from the pump.
Electrically driven gear or progressive cavity pumps can also be used, providing a smooth, continuous resin flow with little pressure fluctuation. Electric motor(s) control either both pumps (fixed ratio) or each component individually (variable ratio). Component ratio and flow rate are controlled by the motor speed, which is critical to maintaining optimum mixing. Gear pumps are not suitable for use with abrasive fillers (generally in the polyol side). Progressive cavity pumps can be used with fillers but are generally more expensive than gear pumps. A typical arrangement would use a gear pump for the isocyanate and progressive cavity pump for the polyol.
Pumps must be sized according to the largest product to be run and linespeed requirements. The volume of resin per length of product times the line speed determines the lowest total resin volume output for which the pumps should be sized. However, very large pumps may be difficult to control for very small resin throughput.
Resin tanks?One tank each is required for the isocyanate and polyol to provide resin supply during pump operation. The tanks are usually installed to provide gravity or pressure feed for the pumps and should be sized to minimize the need for refilling during operation. Both tanks should be sealed to minimize exposure to humid atmosphere and have a desiccant filter installed to provide dry makeup air. An agitator is required in the polyol tank to ensure the IMR and filler, if any, remain homogenized. The hose from the isocyanate tank to the pump should be polyethylene lined for maximum life. An inline strainer on the isocyanate feed is also suggested to prevent debris, if any, from entering the pump. For the highest level of process control, both the polyol and isocyanate tanks may be jacketed to maintain constant component temperature, but this stringent action is not required.
Mixhead?The mixhead controls the on/off flow of resin, combining the polyol and isocyanate streams to begin the mixing process. In simplest form, the mixhead consists of two ball valves with one handle and a body in which the streams combine. More sophisticated mixheads can be pneumatically or electrically operated. Check valves at the entrance to the mixhead valves will prevent backflow into the resin hoses in the case of pressure differentials between the two components. It is also feasible, but somewhat costly, to set up a mixhead with a third stream to add colorant or other liquid additives directly online. This approach would prevent contamination of the polyol tank and allow for rapid changeover of profile color. Component mix ratios must be adjusted accordingly to compensate for any additive.
Mixing elements?Typically, static mixtubes are connected to the outlet of the mixhead to thoroughly mix the two components before introduction into the injection box. The size of the mixtube and number of elements should be sized to provide good mixing without creating high backpressure. Sizing parameters are dependent on flow-rate requirements, isocyanate to polyol ratio, component viscosity, and component compatibility. Three .375-inch OD mixtubes attached in series with 24 mix elements in each tube provide good mixing for most resin combinations.
Solvent flush system?It is necessary to flush the mixhead, mixtubes, and injection box during shutdown of a pultrusion run to prevent resin from curing in place. The flush solvent is typically introduced into the mixhead through a threeway ball valve. Resin is initially purged out of the mixhead and mixtubes with air followed by thorough flushing with solvent. Typically a 2-gal paint pot pressurized with air works well as a solvent container. The solvent should have a high flashpoint to minimize the hazard of passing it through a hot die. Propylene carbonate is a satisfactory flush solvent to remove uncured polyurethane resin. Pressure regulators should be installed to control air pressure in the solvent pot and for flushing and the mixhead.
Injection box? Injection processing for pultrusion has been practiced in development and production for more than a decade. The approach reported by Gauchel & Lehman using a teardrop injection die is probably the best known. Resin injection can be performed internally within the die, typically by modifying it with a weir surrounding the profile, or within an injection box added on to the die in place of a typical wetbath. Each has its advantages. Development work is best suited for an injection box approach as it can be fabricated rapidly and inexpensively from high-density polyethylene (HDPE). Steel or anodized aluminum is more robust for the heavy use of a production environment. The configuration of the wet-out chamber for closed injection processes is considered to be highly proprietary to pultruders. However, there are several rules of thumb that are known in the public domain.
The most critical design parameter is limiting dead spots in the injection zone, especially for fast-reacting resins such as polyurethanes, where the resin can accumulate and possibly cure. The entire volume of the injection box or die must be replenished 35 times before the gel time to ensure long runs can be made without gelation. There are no special die design requirements for pultruding polyurethane resins compared to styrene-based resins.
Recommended Process Conditions
There is some perception within the pultrusion industry that polyurethane resins are difficult to process and are prone to die lock. With a well formulated PU resin system and appropriate process conditions, polyurethanes exhibit quite robust and reproducible processing. High-quality PU pultrusion profiles have been run continuously for over 48 hours at 60-inches/min using a standard resin having a 20-min gel time. This run wasn?t terminated due to gelation in the injection box or die lock, but simply because sufficient profile was produced.
Using a well-designed closed injection box and dosing resin with a metering pump, runs of indefinite time are feasible. Short runs of 3060 minutes are also possible in an open bath with minimized dead spots and good resin mixing. In contrast to die lock issues found in epoxy pultrusion, line purges up to 5-min long have been performed without restart problems during PU runs when required to address saw or reinforcement concerns.
The polyol component should be mixed vigorously before use and continually during use. Care should be taken to limit exposure to air as the polyol is somewhat hygroscopic. Since CO2 is generated when MDI reacts with H2O, the absorbed moisture can lead to resin foaming upon mixing with the isocyanate. Similarly, any additive such as CaCO3 filler or colorants should contain minimal moisture. However, it is not required to dry the additives or reinforcements since a limited amount of moisture can be tolerated without degrading processing or profile quality.
The MDI should remain in enclosed containers except during transfer operations. With prolonged exposure, MDI reacts with H2O in air foaming the PU resin, solidifying some of the MDI in the tank, and perhaps clogging the equipment filters. Reaction with moisture in the air is not an issue for short-term exposure (up to several hours time) for typical material transfer operations. Small amounts of unused MDI can be decontaminated with a mixture of water, surfactant, and ammonia according to the MSDS.
Alternately, the MDI can be reacted with residual polyol in a well-ventilated area. Care should be taken to limit resin volumes to prevent excessive exotherm generation.
Editor?s note: This article was adapted by permission from an original article written by Michael Connolly, product manager?urethane composites for Huntsman Corp. in Auburn Hills, MI, with the assistance of John King, senior development engineer; Trent Shidaker, development manager for polyurethane composite and rigid foam development; and Aaron Duncan, development engineer. The original, full-length article was presented in September at Composites 2005, Columbus, OH.
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