Core/mold binder systems: an environmental/productivity update.

Binder system technology has responded to foundries' needs for better quality and productivity while reducing environmental impact.

Since the 1970s, tightening regulations in regard to worker health/safety, plant waste and most recently, air emissions, have created a need for environmental improvements in U.S. foundries. In fact, a recent survey of North American foundries revealed that 66% planned to change one or more of their processes in the next three years for regulatory compliance reasons.

In addition to the regulatory changes in the U.S., the need for productivity improvements also challenges the foundry industry worldwide. The objective is to develop or maintain a competitive edge in an industry that is becoming increasingly competitive. As technology changes, foundries competing locally force lower costs and improved quality.

Another contributing factor is the ever-expanding global casting market. Participation in a global economy demands continual improvements in casting cost and quality, and will likely require local improvements in environmental operating conditions. Therefore, it is necessary to improve productivity, while improving casting quality and environmental operating characteristics.

While hazardous air pollutants plaguing foundries can result from various casting operations, the main operation affected by the binders is core/moldmaking. Therefore, the question is: What is being done to meet the requirements for reduced odor, chemical emissions, smoke and waste disposal, while maintaining - if not improving - productivity? More specifically, what is being done about core/mold binders to meet this challenge?

Knowledge of foundry processes and their emissions will allow foundries to make the best decisions regarding compliance. This article highlights some recent developments among the hotbox, shell, phenolic urethane (coldbox and nobake) and ester-cured phenolic binder systems.

Hotbox Process

Hotbox binders are used in many high-production foundries. The major environmental issue in the use of these binders is formaldehyde emissions. Of particular concern is the amount of formaldehyde emitted during curing.

New resin formulations have dramatically reduced the amount of formaldehyde evolved during curing. This 85% reduction in formaldehyde emissions in the last three decades (to less than 0.2% today) has been accomplished without any coreroom productivity or an increase in the nitrogen (N) content of the binders. Time weighted average (TWA) exposure levels of formaldehyde for coreroom machine operators of less than 0.5 ppm have been documented via these low-formaldehyde systems.

Shell Process

The shell (Croning) process is still the most widely used core process in the U.S. because of its ability to make hollow cores, the nearly infinite shelf life for coated sand, and its unique handling ability due to the dry nature of the material.

One of the most effective means of reducing shell process emissions is during the development of low free-phenol flakes. Conventional phenolic shell resins can contain about 5-10% free phenol. Recent developments, however, have provided resins with 60-90% reductions in free phenol, without any sacrifice in performance. This results in a 60-90% reduction in phenol air emissions during sand coating, and produces resin-coated sand with lower free phenol content. This also results in lower phenol emissions during core or moldmaking.

Another significant development in shell sand technology is low-odor resin-coated sands. These products emit fewer odors during coremaking and pouring due to the release of lower levels of ammonia. Tensile performance is equal to or better than conventional shell systems. Cure times can be faster, reducing cycle times and improving productivity, as shown in Fig. 1. In addition, low-odor formulations are lower in N content by 50-80%. This reduction will lessen the tendency of N-related casting defects.

One advantage of the shell process is that it works well on reclaimed sands, potentially reducing waste sand disposal. Tensile strengths obtained with thermally reclaimed shell sand can actually be higher than with new base sand. Even thermally reclaimed green sand can work well with the shell process, whereas other binder systems often show a significant loss of tensile strength. This performance advantage is shown in Fig. 2.

These factors can assist foundries using the shell process to reduce emissions, odor, smoke and waste sand, while improving cycle times and casting quality. Simply changing the binder system, however, may not be sufficient to meet regulatory requirements. When this occurs, emission control equipment can be used effectively. For example, emissions can be controlled by passing the exhaust from the core and moldmaking processes, as well as from the metal pouring area, through acid or biological scrubbers.

Phenolic Urethane Coldbox and Nobake Binders

New developments in urethane binder technology has allowed both environmental and productivity improvements. Some of these developments include the reduction in free formaldehyde and free naphthalene, and the application of new solvents/plasticizers.

Recently, phenolic urethane Part I resins that contain low free formaldehyde (0.1%) have been developed. These resins are typically 80-90% lower in free formaldehyde than conventional systems; thus greatly reducing coreroom odor and foundry emissions. These new resin formulations also contain less than 1% naphthalene and haven't shown any performance deficiencies.

A novel application of a new solvent/plasticizer to urethane core/mold binders has allowed significant performance and environmental improvements. Used in both Part I and Part II resin formulations, it generally improves core/mold release and emissions of volatile organic compounds (VOCs) before pouring. Added benefits may include improved tensile strength and bench life, and better humidity resistance of cores and molds.

A new solvent/plasticizer for urethane coldbox formulations (specifically designed to work on alkaline sand) was tested on a thermally-reclaimed alkaline (9.6 pH) silica sand using a binder level of 1% based on sand (BOS), and a Part I to Part II ratio of 50:50. Results showed that it provided greater tensile strengths, better bench life under high humidity conditions, significantly better release and less corebox buildup than conventional systems. In addition, it eliminated the special alkaline additive needed in the conventional system.

This solvent/plasticizer has also improved the release characteristics of cores produced in urethane nobake systems. Modifying a Part II resin with the new material delivers comparable tensile strengths to conventional systems. Figure 3 illustrates the benefits derived from the use of the new solvent/plasticizer, in terms of improved core release. The load necessary to release the cores should increase with time. This occurs, as the data in Fig. 3 shows. However, for the system using the modified Part II resin, not only is the load required for release less, but the incremental increase in the load over time is reduced, as compared to the unmodified system.

Binders containing less organic solvent and thus reduced VOC emissions are also available. Tests have shown that such systems are comparable in tensile performance, while the new system releases 87% less volatiles during mixing and through 24 hr of core storage.

The use of the low-VOC formulations addresses the issue of lustrous carbon formation that sometimes can be attributed to excess solvent retention during pouring. Solvent retention can be balanced by using a low-VOC formulation that contains a significantly lower volatile fraction to begin with so that the total amount of material avail-able for lustrous carbon formation is reduced.Therefore, it is possible to have a low-VOC system that is a low emitter because of the reduced volatility of the solvents used in the formulation, and a low generator of organic carbon because of the reduced amount of solvents used in the formulation.

Another development in urethane core/mold binder technology is the application of moisture-resistant systems. The move to the environmentally favored water-based coatings created a need for urethane binders that withstand the characteristic degradation that conventional urethane systems undergo upon contact with water. These new binders possess the same attributes of conventional high-performance urethane systems, but are significantly more resistant to water-based coatings.

These systems, which can be formulated as either a phenolic urethane coldbox binder (PUCB) or as a phenolic urethane nobake binder (PUNB), offer a two-fold advantage to foundries using water-based coatings. First, less core and casting scrap may be anticipated because the cores produced via this process are more robust. Second, increased binder levels may not be called for in those situations where the binder level was increased in response to core degradation by a water-based coating.

Figure 4 illustrates the new system's resistance to a water-based coating. In this test, cores were dipped in a conventional water-based refractory coating and dried in an oven at 315F (156C) for 25 min. At 30 min post-baking, the new system developed nearly twice the tensile strength of the conventional system.

Figure 5 shows the productivity effects of incorporating the improved release technology into the new system. As the data illustrates, it is possible to achieve a significant improvement in release characteristics with this new system. The chart shows the reduced rate of buildup on the corebox, resulting in less downtime for corebox cleaning. A direct correlation exists between resin buildup and ease of release.

Ester-Cured Phenolic Binders

Ester-cured phenolic nobake and coldbox binders have grown rapidly since their introduction in the early 1980s. A major reason for this growth has been the environmental advantages this system offers over other organic binders. In particular, the smoke generated during pouring is quite low. While lower comparable tensile strengths have reduced the utility of the process, recent binder advancements have yielded substantial tensile improvement.

The ester-cured phenolic nobake systems have been particularly successful in steel applications where their low N and hot tear potential are valuable. The reclaimed sand levels, which can be rebonded with ester-cured phenolics, has been lower than other nobake systems, however. Improvements in sand reclamation have also been achieved in a number of ways. Improved binders have been developed for use with reclaimed sands that give higher strengths and allow higher levels of reclaimed sand to be used. The flexural strength, after 40 reclamation cycles at an 85% reclaimed sand level, is higher than the older technology binder at a 70% reclaimed sand level.

Table 1. Ester-Cured Phenolic Nobake Thermal Reclamation Tests

Sand Description             Tensile Strength   Tensile Strength
                                (PSI) 1 hr         (PSI) 24 hr

100% mechanical reclaim             23                 39

thermal reclaim 1100F/0.6%
additive (test 1)                   63                128

thermal reclaim 1100F/0.6%
additive (test 2)                   53                129

thermal reclaim 1400F/0.6%
additive                            59                137

new silica sand                     65                150

Another approach to obtain improved performance with mechanically reclaimed sand is the use of special additives. Tensile strength increases of two to three times can be obtained on mechanically reclaimed sand, as compared to using no additives. The use of a third part addition to the sand mix can be avoided by employing recently developed modified hardeners. Modified hardeners provide improved rebonding strengths with ester-cured phenolics. This can result in improved strengths at a given reclaimed/new sand ratio, or provide the opportunity for increasing the percentage of reclaimed sand. An example of the improvements obtained with the modified hardener is shown in Fig. 6.

Thermal reclamation can also be successfully used with ester-cured phenolic binders. Reclaimed sand can be obtained that performs at nearly the same level as new sand, with ester-cured phenolic nobake systems, as illustrated in Table 1. To avoid sand flow problems in the thermal reclamation unit, an additive is mixed with the sand fed into the thermal reclaimer. The additive functions by reducing the tendency of the ester-cured phenolic sand to clump in the thermal reclaimer, particularly at higher processing temperatures. The sand obtained performs particularly well with ester-cured phenolics.