Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-06-01
2002-10-15
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S142000
Reexamination Certificate
active
06465542
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
In the production of moulds and cores, polyurethane-based binder systems are used in large amounts, in particular for mould and core production for the cold-box or polyurethane no-bake process.
Polyurethane-based binder systems for the cold-box and for the polyurethane no-bake process typically comprise two essential binder components, namely:
(1) a polyol component which comprises a binder having at least two OH groups per molecule and
(2) a polyisocyanate component which comprises a binder having at least two isocyanate groups per molecule.
These components are optionally solvent-containing and are usually packed and sold in separate containers.
Usually, the polyol component (first component) comprises a phenol resin having at least two OH groups per molecule. Of these, phenol resins of the benzyl ether resin type have become particularly important. These are the condensates of a phenol of the general formula I
in which A, B and C are hydrogen, alkyl groups or alkoxy groups, with aldehydes of the general formula R′CHO, in which R′ is a hydrogen atom or an alkyl group having 1-8 carbon atoms. The reaction of phenols of the stated general formula with aldehydes of the general formula R′CHO is carried out in the liquid phase, typically at a temperature below 130° C. Catalytic amounts of ortho-directing divalent metal ions, such as Zn
2+
, are added to the reaction mixture.
Preferred benzyl ether resins correspond to the following general formula II:
Here, R is hydrogen or a phenolic substituent in the ortho, meta or para position relative to the phenolic hydroxyl group; the sum of m and n is at least 2 and the ratio m
is at least 1; X is hydrogen or CH
2
OH, the ratio of hydrogen to CH
2
OH being at least 1.
For use in a two-component binder system, phenol resins, in particular benzyl ether resins, are usually used as a solution in an organic solvent. The solvent is required for reducing the viscosity of the phenol resin for mixing with a moulding material and reacting with the polyisocyanate component.
The isocyanate component (second component) of the two-component binder system for the cold-box or polyurethane no-bake process usually comprises an aliphatic, cycloaliphatic or aromatic polyisocyanate having preferably between two and five isocyanate groups; mixtures of such polyisocyanates may also be used. Particularly suitable polyisocyanates among the aliphatic polyisocyanates are, for example, hexamethylene diisocyanate, particularly suitable ones among the alicyclic polyisocyanates are, for example, 4,4′-dicyclohexylmethane diisocyanate and particularly suitable ones among the aromatic polyisocyanates are, for example, 2,4′- and 2,6′-toluene diisocyanate, diphenylmethane diisocyanate and their dimethyl derivatives. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylene diisocyanate and their methyl derivatives, polymethylenepolyphenyl isocyanates (polymeric MDI), etc. Although all polyisocyanates react with the phenol resin with formation of a crosslinked polymer structure, the aromatic polyisocyanates are preferred in practice. Diphenylmethane diisocyanate (MDI), triphenylmethane triisocyanate, polymethylene polyphenyl isocyanates (polymeric MDI) and mixtures thereof are particularly preferred.
The polyisocyanate is used in concentrations which are sufficient to effect curing of the phenol resin. In general, 10-500% by weight, preferably 20-300% by weight, based on the mass of (undiluted) phenol resin used, of polyisocyanate are employed. The polyisocyanate is used in liquid form; liquid polyisocyanate can be used in undiluted form, and solid or viscous polyisocyanates are used in the form of a solution in an organic solvent, it being possible for the solvent to account for up to 80% by weight of the polyisocyanate solution.
In choosing the solvents for the phenol resin component and optionally for the polyisocyanate component, it should be noted that although these do not participate in a relevant manner in the reaction between the isocyanate and the phenol resin in the presence of a catalyst, they may very well influence this reaction. One problem arises in particular from the situation that the two binder components phenol resin and polyisocyanate have substantially different polarities. This polarity difference between the polyisocyanate and the phenol resin limits the number of solvents which may be used to those which are compatible with both binder components. Such compatibility is necessary in order to achieve complete reaction and curing of a binder system. Although polar solvents of the protic and aprotic type are usually good solvents for the phenol resin, they are not very suitable for the polyisocyanate. Aromatic solvents in turn are compatible with polyisocyanates but are not very suitable for phenol resins.
In practice, mixtures of polar and nonpolar, aromatic-containing solvents which are tailored to the respective binder system (phenol resin and polyisocyanate) are therefore usually used. Moreover, the individual components of the solvent mixture should not have too low a boiling range, so that the solvent cannot become ineffective too rapidly as the result of evaporation.
Nonpolar, aromatic-containing solvents used to date are preferably mixtures of high-boiling aromatic hydrocarbons, i.e. mixtures of aromatic hydrocarbons having a boiling range above 150° C. at atmospheric pressure. Polar solvents which have been used are, inter alia, specific sufficiently high-boiling esters, such as, for example, the “symmetrical” esters which are described in German Patent 27 59 262 and in which both the acid radical and the alcohol radical have a relatively large number of C atoms (about 6-13 C atoms) in the same range.
With all advantages of the polyurethane binder for casting technology, it is still felt to be a disadvantage that excessively high benzene emissions occur during pouring of a casting in a mould which comprises a binder based on a polyurethane. These benzene emissions during pouring, but also evaporation and devolatilization prior to the pouring, constitute considerable workplace pollution which generally cannot be trapped by protective measures, such as extractor hoods or the like.
SUMMARY OF THE INVENTION
It was therefore the object of the present invention to provide a polyurethane-based binder system for the cold-box and for the polyurethane no-bake process, which binder releases only small amounts of aromatic compounds during foundry operation.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, this object is achieved by providing a two-component binder system consisting of a phenol resin component and a polyisocyanate component, the phenol resin component comprising a phenol resin having at least two OH groups per molecule and the polyisocyanate component comprising a polyisocyanate having at least two isocyanate groups per molecule, and at least the phenol resin component containing a solvent, wherein
the solvent for the phenol resin comprises a substance which is selected from the group which comprises alkyl silicates, alkyl silicate oligomers and mixtures thereof (i.e. mixtures of different alkyl silicates, mixtures of different oligomers and mixtures of alkyl silicate(s) with oligomer(s) and/or
the polyisocyanate component contains a solvent which comprises such a substance, i.e. a substance which is selected from the group which comprises alkyl silicates, alkyl silicate oligomers and mixtures thereof.
The amount of alkyl silicates, alkyl silicate oligomers and mixtures thereof in the phenol resin component is advantageously in the range between 1 and 40% by weight.
The amount of alkyl silicates, alkyl silicate oligomers or mixtures thereof in the polyisocyanate component (if this requires a solvent) is advantageously likewise in the range between 1 and 40% by weight.
The invention is based on the surprising discovery that alkyl silicates, i.e. alkyl esters of silicic
Bartsch Dietmar
Ladégourdie Gérard P. M.
Seeger Klaus
Torbus Marek
Cain Edward J.
Hüttenes-Albertus Chemische Werke GmbH
Lee Kataryna W.
Pendorf & Cutliff
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