Cold-box foundry binder systems

Details Cold-box foundry binder systems Cold-box foundry binder systems

2001-09-27

2004-02-03

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C164S016000, C164S526000, C164S529000, C523S145000, C523S427000, C523S436000, C523S438000, C523S466000

Reexamination Certificate

active

06686402

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to foundry binder systems, which will cure in the presence of sulfur dioxide and a free radical initiator, comprising (a) an epoxy novolac resin; (b) preferably a bisphenol F; (c) an acrylate; and (d) an effective amount of a free radical initiator. The foundry binder systems are used for making foundry mixes. The foundry mixes are used to make foundry shapes (such as cores and molds) which are used to make metal castings.
(2) Description of the Related Art
Foundry binder systems, which cure with vaporous sulfur dioxide, are known in the art. For instance, U.S. Pat. No. 3,879,339 discloses that certain synthetic resins can be cured in the presence of a free radical initiator and sulfur dioxide. Examples of such resins are furan, urea formaldehyde, and phenol formaldehyde resins. On the other hand, U.S. Pat. No. 4,526,219 discloses a cold-box process for making foundry shapes
1
, whereby certain ethylenically unsaturated materials are be cured by a free radical mechanism in the presence of a free radical initiator and vaporous sulfur dioxide.
1
Typical foundry shapes are cores and molds.
U.S. Pat. No. 4,518,723 discloses a cold-box process for making foundry shapes with foundry binders comprising an epoxy resin. Although the patent broadly covers binder systems based upon epoxy resins alone, it is known that bisphenol A epoxy resins and bisphenol F epoxy resins, cured with SO
2
in the presence of a free radical initiator, do not work effectively when used alone in a commercial setting, where high productivity is required. In order for the epoxy resins to be useful in these situations, the epoxy resin must be used in conjunction with an acrylic monomer or polymer, typically trimethyolpropane triacrylate (TMPTA). These binders have excellent tensile strengths and can be used in typical high production core-making facilities.
Typically, these binders are packaged in two parts. One part (Part I) is a mixture of a bisphenol-A epoxy resin (bisphenol-F epoxy resin is also used, but not as commonly) and cumene hydroperoxide (free radical initiator). The other part (Part II) is a mixture of a bisphenol-A epoxy resin, a multifunctional acrylate, and optional components. The multifunctional acrylate is typically trimethyolpropane triacrylate and is typically used in amount of about 15 weight percent to about 20 weight percent based on the amount of epoxy resin, but in some cases is used in an amount of 25 weight percent.
The Part I and Part II of the binder are mixed with a foundry aggregate, typically sand, to form a foundry mix. The total amount of binder used to form the foundry mix is typically from about 0.5 to 2 weight percent based on sand (bos). The foundry mixed is blown or compacted into a pattern where it is gassed with SO
2
to produce a cured core or mold. Foundry mixes made with these binders have extended benchlife and foundry shapes made with the binder have excellent physical properties.
Although foundry shapes made with these binders have good tensile strengths, it is often necessary to coat the foundry shapes with a refractory coating prior to use in order to minimize the erosion of the foundry shape during casting. The cured core or mold is immersed into the water-based refractory coating to improve the quality of castings made with the foundry shapes. Because of the moisture in the coating, it is necessary to dry the foundry shapes in an oven to evaporate the water in the refractory coating.
If a conventional gas fired convection oven is used, the coated foundry shapes are typically heated for about 20 minutes at a temperature of about 175° C. to 200° C. Then they are extracted from the oven and allowed to cool, so they can be handled without breaking. If the cool-down time is inadequate, the foundry shapes may crack, sag, or distort when handled. This results in waste and inefficiency because defective foundry shapes cannot be used to cast metal articles.
Recently, there is a growing interest in using microwave ovens to dry coated foundry shapes because drying times can be reduced to 5 minutes or less and post-curing in a conventional oven can be eliminated. The disadvantage of using a microwave oven to dry the coated foundry shapes is that this process degrades the binder (even though the tensile strength of the coated foundry shape is good), and heating is uneven. Because the heating is uneven, the surface temperature of the coated foundry shape depends on the local concentration of water (due to the water in the core wash), and may vary as much as 50° C. from one location to another over the surface of the coated foundry shape. This phenomenon does not occur when coated foundry shapes are dried in conventional ovens. In conventional ovens, the surface temperature of the coated foundry shape varies only by few degrees from one place to another.
An even greater problem with using a microwave oven to dry foundry shapes is the high concentration of water vapor in the oven atmosphere during the drying operation, which is a problem because of the poor air circulation. In current industrial microwave design, the airflow through the microwave oven is only about 5000 cubic feet per minute (cfm), compared to 40,000 cfm in a typical conventional oven. Because of this, the atmosphere in the microwave oven is saturated with moisture, and cores and moulds emerging from the oven are not completely dry. In addition, in the microwave process, steam from the evaporating water is driven through the core, rather than evaporating from the surface as in a conventional oven. This entrained hot moisture degrades the strength of the organic binder. As a result, the coated foundry cores do not survive the microwave process without extensive degradation or warpage, and thus are unacceptable for use.
In current practice, about 90% of the binders cured by the cold-box process using SO
2
are based on bisphenol-A epoxy resins exclusively. Coated foundry shapes made with these binders typically cannot be handled when they emerge from the oven. When handled, the foundry shapes often sag, crack, or collapse. The larger the core or mould, the more pronounced this effect. Typically, cooling times of 45-60 minutes are required before the foundry shapes can be handled, which is an unacceptable condition in most foundries.
In a small percentage of cases, binders, cured by the cold-box process using SO
2
, are based on bisphenol-F epoxy resins exclusively. Bisphenol-F epoxy resin is the diglycidyl ether of bis (hydroxyphenyl)methane, prepared by the condensation of phenol and formaldehyde, and has a functionality of approximately 2.05. Although binders based on bis F epoxy resin show some advantages over bisphenol-A epoxy resins in microwave applications, the foundry shapes emerging from the oven are still soft and subject to distortion or cracking, if stressed before a cool down time of 20 minutes or so, particularly when the foundry shapes are coated with a refractory coating.
In view of the problems associated with drying foundry shapes in conventional ovens and microwave ovens, there is an interest in modifying the binders to reduce cracking of the foundry shapes and reduce drying times.
Examples 16-17 of U.S. Pat. No. 4,518,723 (hereinafter the '723 patent) teach that an epoxy novolac resin EPN-1139, manufactured by Ciba-Geigy Corp, can be used to prepare cores. EPN 1139 is an epoxy novolac resin having an average functionality of about 2.3, an epoxide equivalent weight of about 180, and a viscosity of approximately 50,000 centipoise at 25° C. However, it is noteworthy that in both of these examples, the EPN-1139 is blended with Epon 828 (a bisphenol A epoxy resin) to obtain satisfactory cores. It is also noteworthy that the binder of Example 17 does not contain TMPTA, while the binder of Example 16 only contains 7 weight percent

Affiliated with

Also associated with

Rating

Cold-box foundry binder systems does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Cold-box foundry binder systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cold-box foundry binder systems will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3325104