The present invention relates to a method for acceler¬ ating the hardening of mineral wool impregnated with mela¬ mine binders and mineral wool products which are hardened in a microwave oven with the addition of resorcin to melamine binders as a hardening accelerator.

Mineral wool products are mixed with a binder to im¬ prove their inherent stability. This is generally done by spraying the raw material with a binder solution or disper¬ sion in a fall shaft and then drying it. The mineral wool containing binder is then put into the desired shape, pos¬ sibly surface-polished and then hardened. Hardening can be effected thermally by guiding the product through a kiln. Apart from the expenditure of energy this is problematic because mineral wool is a good insulator so that the thermal energy supplied penetrates into the interior of the product slowly and with difficulty, particularly in the case of large layer thicknesses. One therefore began using a micro¬ wave oven for hardening some time ago, which has proven useful for mineral wool products bound with melamine bind¬ ers. The good penetration characteristics of microwaves in mineral wool products permit much more uniform heating, which furthermore essentially affects binder, and thus at the same time faster hardening.

Despite the advantages of hardening in a microwave oven over conventional thermal hardening, this method is still unsatisfactory in some points. Microwaves also have trouble penetrating into the interior of mineral wool products with large layer thicknesses so that there is also uneven heating in this method. Relatively long exposure times are therefore required to harden the core of thick mineral wool products

su ficiently, although the edge areas have long since hard¬ ened more or less completely.

DE-B-27 56 848 discloses the addition of alkyl resor- cins to improve the water repellence of a melamine binder. This binder is used for insulating materials that are sub¬ jected to thermal hardening. This reference says nothing about this additive accelerating hardening. The expert also knows that hardening accelerators added in thermal methods generally have no such effect on hardening with microwaves.

It is also prior art to use resorcin as a hardening component for binder containing phenol and urea in normal gas-fired drying kilns for chipboard production. The goal is to improve the formaldehyde emission of chipboard.

It is therefore fundamentally desirable to accelerate the hardening of melamine-bound mineral wool or mineral wool products in order to shorten the exposure time of the mi¬ crowaves triggering hardening and thus achieve an improved temperature pattern, in particular in thick mineral wool layers. A shortened holding time in the microwave oven is furthermore desirable in the interests of increased produc¬ tivity and lower energy consumption.

This goal is achieved with a method of the abovemen- tioned type wherein the binder is mixed with a dihydroxy aromatic, preferably resorcin, in an amount of 1 to 15% by weight based on the binder before application to the mineral wool.

Dihydroxy aromatics are in particular dihydroxybenzeneε and substituted dihydroxybenzenes whose substituents do not interfere with the reactions, for example alkyl-substituted benzenes. Preferred dihydroxybenzenes are resorcin and cat- echol, resorcin being particularly preferred and used by way of example in the further description.

The inventive method accelerates the hardening of min¬ eral wool or mineral wool products impregnated with a mela-

mine binder containing resorcin in a microwave oven by up to 20%.

Resorcin is expediently added to the melamine binder in an amount of 2 to 8% by weight, advantageously 2 to 6%, in particular 3 to 5%. The resorcin should be introduced into the binder immediately before application to the mineral wool, preferably via the .sizing line. The reason for this is that resorcin can react with the melamine binder to form insoluble products which impair impregnation of the mineral wool.

The binder containing resorcin is introduced into the mineral wool in the customary way, preferably by spraying in a fall shaft. An aqueous solution or dispersion of the binder has proven to be particularly suitable and ecologi¬ cally acceptable, in particular one with a binder content of 5 to 10% by weight, including the hardening accelerator. The mineral wool is then dried down to a residual water content, expediently 1 to 2% by weight of residual water based on the mineral wool.

The binders used are ones formed in the conventional way from melamine resin, phenolic resin and urea. The phe- nolic resin component expediently contains up to 20% by weight of formaldehyde which is in particular capable of reacting with the urea component. The binder can contain customary additives, for example oil, in an amount of about 0.2% by weight and a bonding agent in an amount of 0.1% by weight, in each case based on the dispersion.

A particularly preferred binder comprises 4 parts by weight of melamine resin, 3 parts by weight of phenolic resin with a formaldehyde content of 17% by weight, and 3 parts of urea, and is processed in 5 to 10% aqueous disper¬ sion. The resorcin is added to this binder in the form of an aqueous solution.

The present invention also relates to mineral wool products of any kind produced using dihydroxy aromatics as a hardening accelerator for melamine binders in microwave hardening. The method is particularly suitable for producing heat-insulating covers as are used particularly for encasing heating tubes and hot-water pipes. These heat-insulating covers can be processed and laminated in the customary way to permit simple mounting on the objects to be insulated.

The invention shall be explained in more detail by the following example.

Example

A binder (MB) was mixed from 4 parts by weight of mel¬ amine resin, 3 parts by weight of phenolic resin with a proportion of 17% by weight of free formaldehyde (R117), and 3 parts by weight of urea and converted into a 5% by weight aqueous dispersion. A 5% by weight resorcin solution (R) was added to this dispersion via the sizing line in an amount so as to yield a hardener content of 3 or 5% by weight based on the binder. The binder dispersion containing resorcin was then immediately applied to freshly produced glass wool in a fall shaft.

The glass wool was then dried to a residual water con¬ tent of 1 to 2% by weight. It could be processed into heat- insulating covers without trouble even after 3 weeks.

The glass wool impregnated by the above method was subjected after drying to a forming process, surface-pol¬ ished and guided through a microwave oven. A perfectly hardened product was obtained that was at least equivalent in all points to a product obtained without a hardening ac¬ celerator, while the hardening time was shortened by 10 to 20%.

The following table compares the band speeds and cycle times in the production of melamine resin-bound heat-insu¬ lating covers made of glass wool with different thicknesses of the insulating layer with and without a hardening accel¬ erator in the binder. In every case there is an appreciable shortening of the cycle time or increase in band speed. The column "Type" states the inside diameters and wall thick¬ nesses of the hardened heat-insulating covers for piping.

MB = Melamine resin binder

R = Resorcin (added as 50% aqueous solution)