Polyurethane binders are often used in the foundry industry to hold shaped foundry aggregate together as a mold or core. See for example U.S. Pat. Nos. 3,409,579 and 3,676,392. They are also used as adhesives to hold foundry molds and cores together in an assembly. See for example U.S. Pat. Nos. 4,692,479 and 4,724,892 which describe such foundry pastes.
One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry mix which is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
One of the processes used in sand casting for making molds and cores is the cold-box process. In this process a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core.
A polyurethane-forming binder system commonly used in the cold-box process is cured with a gaseous tertiary amine catalyst. The polyurethane-forming binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry mix.
When the two components of the binder system are mixed with the sand to form a foundry mix, they may prematurely react prior to curing with the gaseous catalyst. If this reaction occurs, it will reduce the flowability of the foundry mix when it is used for making molds and cores, and the resulting molds and cores will have reduced strengths.
The bench life of the foundry mix is the time interval between forming the foundry mix and the time when the foundry mix is no longer useful for making acceptable molds and cores. A measure of the usefulness of the foundry mix and the acceptability of the molds and cores prepared with the foundry mix is the tensile strength of the molds and cores. If a foundry mix is used after the bench life has expired, the resulting molds and cores will have unacceptable tensile strengths.
Because it is not always possible to use the foundry mix immediately after mixing, it is desirable to prepare foundry mixes with an extended bench life. Many patents have described compounds which improve the bench life of the foundry mix. Among the compounds useful to extend the bench life of the foundry mix are organic and/or inorganic phosphorus containing compounds.
Examples of organic phosphorus-containing compounds used as benchlife extenders with polyurethane-forming binder systems are disclosed in U.S. Pat. No. 4,436,881 which discloses certain organic phosphorus containing compounds such as dichloroarylphosphine, chlorodiarylphosphine, arylphosphinic dichloride, or diarylphosphinyl chloride, and U.S. Pat. No. 4,683,252 which discloses organohalophosphates such as monophenyldichlorophosphate. Examples of inorganic phosphorus-containing compounds which extend the bench life of polyurethane-forming binder systems are disclosed in U.S. Pat. No. 4,540,724 which discloses inorganic phosphorus halides such as phosphorus oxychloride, phosphorus trichloride, and phosphorus pentachloride, and U.S. Pat. No. 4,602,069 which discloses inorganic phosphorus acids such as orthophosphoric acid, phosphoric acid, hypophosphoric acid, metaphosphoric acid, pyrophosphoric acid, and polyphosphoric acid.
Also see U.S. Pat. No. 4,760,101 which describes the use of certain carboxylic acids, such as citric acid, to extend the benchlife of polyurethane-forming foundry binders.
In order for a compound to be effective as a bench life extender, it first must be compatible with the polyisocyanate component of the urethane forming binder and mix well with sand. Furthermore, in addition to improving the bench life of foundry mixes made with sand having a range of temperatures normally found in foundry environments, such compounds should have low volatility to minimize inhalation by workers in the foundry. Additionally, such compounds should not create unacceptable stress to the environment.