Siloxan-modified novolacs, their preparation and their use in the manufacturing of friction linings

The invention relates to siloxane-modified novolaks, their production and use for the production of friction linings.

The use of novolaks for the production of friction linings is known (cf. rubber, asbestos, plastics 9, (1978) year 31, pp. 666-670). Such resins serve to glue the numerous fillers and additives and enable the molding compounds to subsequently harden by heating.

Moldings which are resistant to high temperatures, such as friction linings, which are preferably used for the production of clutch and brake linings, should have the highest possible decomposition temperature and a low evaporation rate of thermal decomposition products. The high temperature range is generally understood to be the range of 250-800 ° C. The effectiveness of friction linings and their service life is drastically reduced by a high proportion of volatile cracking products such as mainly benzene, toluene, xylene, phenol, cresol as well as by shape instability (breaking out of friction lining fragments) when exposed to high temperatures.

According to the procedure described in DE-OS 2306463, by mixing phenol-formaldehyde resins and polysiloxanes in solution and then curing the moldings after removal of the solvent, high-temperature-resistant moldings can be obtained, from which friction linings can be produced.

However, the friction linings obtained in this way show an increased breakout of parts of the friction lining molded part at high temperature loads compared to friction linings which were manufactured on the basis of phenolic novolacs, which is apparently caused by an insufficient bond between the low molecular weight components of the novolake and the silicone resin in the friction lining molded part.

The object of the invention is to significantly reduce this decomposition of high-temperature-stressed phenol and / or cresol-formaldehyde resins by a suitable chemical modification thereof and to increase their dimensional stability under high temperature stress by improving the elasticity, tear resistance and tensile strength.

It has been found that phenol and / or cresol-formaldehyde resins of the novolak type with alkoxi-, alkylaryloxy or aryloxy-end group-containing polysiloxanes under acidic catalytic conditions with elimination of alcohol or phenol can be converted to siloxane-modified novolaks which on the one hand are easy to assemble and at the same time resistant to high temperatures Molding compounds with improved elasticity, improved tear propagation and tensile strength result. These resins according to the invention can be processed into friction lining qualities which, when subjected to high temperatures, additionally have significantly improved noise damping and excellent dimensional stability.

The invention thus relates to a polysiloxane of the formula containing novolak<chemistry id="chem0001" num="0001"><img file="EP0013402B1_D0001.tif" /></chemistry>in the<ul id="ul0001" list-style="none"><li>A the same or different a novolak residue, a C₁-C₈-Alkyl-, a C₆-C₁₂Aryl and / or a C₇-C₁₂Represents alkylaryl group, where the novolak residue must be present at least once,</li><li>R the same or different a C₁-C₈-Alkyl-, a C₆-C₁₈-Cycloalkyl group, and / or a C₆-C₁₂ Aryl and / or C₇-C₁₂ Alkylarylgruppe means and</li><li>n represents numbers from 3 to 100.</li></ul>

Another object of the invention is a method for producing the polysiloxane containing a novolak, which is characterized in that a C_i-C_aAlkyloxy, C6-C12 aryloxy and / or C.₇-C₁₂ Polysiloxane containing alkylaryloxy groups with a phenol and / or cresol formaldehyde resin of the novolak type in the presence of an acidic transesterification catalyst at temperatures of 110-180 ° C., the polysiloxane in amounts of 20-120 parts by weight based on 100 parts by weight Novolak is used, and the resulting volatile components are distilled off.

Another object of the invention is the use of the polysiloxanes according to the invention for the production of friction linings.

In the formula given, A and R represent a C.₁-C₈Alkyl radical such as methyl, ethyl, propyl, n-, iso-, tert-butyl, n-, iso-pentyl, n-, iso-hexyl, n-, iso-heptyl and n-, iso-octyl; a C₆-C₁₂Aryl radical such as phenyl, α-naphthyl;<ul id="ul0002" list-style="none"><li>a C₇-C₁₂-Alkylarylrest such as tolyl, p-tert. Butylphenyl, n-, iso-hexylpenyl; and / or a novolak residue.</li></ul>

R can also be a C₆- C₁₈ Represent cycloalkyl group.

As a novolak residue and as a novolak resin which is used in the production process, phenol and / or cresol-formaldehyde resins are suitable, which are obtained by reacting phenols and / or cresols with, for example, aqueous formaldehyde solutions in a molar ratio of 1: 1.05 to 1: 1.4 be prepared in the presence of an acidic catalyst in a known manner (cf. Ullmanns Encyclopadie der techn. Chemie, vol. 13, p. 459, publisher: Urban &amp; Schwarzenberg, Munich-Berlin, 1962 ).

The resin is preferably bound to the polysiloxane via a hydroxymethylene group, from which a hydrogen is split off during the reaction. The phenol groups present in the resin are less reactive and may therefore only react towards the end of the reaction.

A polysiloxane of the following formula is used to produce the poly siloxanes according to the invention:<chemistry id="chem0002" num="0002"><img file="EP0013402B1_D0002.tif" /></chemistry>

Mean in the formula<ul id="ul0003" list-style="none"><li>B the same or different a C₁-C₈-Alkyl, C₆-C₁₂-Aryl or C₇-C₁₂Alkylaryl radical,</li><li>R a C₁-C₈-Alkyl or C₆-C₁₈Cycloalkyl or C₆-C₁₂Aryl and / or C₇-C₁₂Alkylaryl radical,</li><li>n numbers from 3 to 100.</li></ul>

These polysiloxane resins can be dissolved in a solvent such as benzene, toluene and xylene, but are preferably used without solvents. These solvent-free polysiloxanes are liquids and have a viscosity of 40-20,000 (measured at 20 ° C).

Polysiloxane resins of this type can be prepared by processes known from the literature, for example by targeted hydrolysis of silanes of the general structure (R.₂) Si (X)₂, where R is already defined at the outset, and X can represent, for example, chlorine and / or O-alkyl having 1-8 carbon atoms, and the hydrolysis, if appropriate, in the presence of organic solvents such as toluene or xylene and, if appropriate, in the presence of alkali or acid and an appropriate amount of chain terminating C₁-C₈-Hydroxyalkyls or C₆-C₁₂-Hydroxyarylene or C₇-C₁₂-Hydroxy- alkylaryle is carried out (see, for example, BW Noll, "Chemistry and Technology of Silicones", Verlag Chemie GmbH, Weinheim / Bergstrasse 1968, pages 162-171).

With regard to the novolaks used, reference is made to the above explanations.

The amount of the polysiloxane used in the reaction is preferably 30-80 parts by weight based on 100 parts by weight of the novolak.

The reaction is preferably carried out in the presence of 0.0001-0.5% by weight of an acidic transesterification catalyst (based on the total weight of the reaction mixture), such as, for. B. an organic or inorganic acid such as oxalic acid, formic acid, acetic acid, trichloro- or trifluoroacetic acid, benzenesulfonic acid, p-toluenesulfonic acid, hydrochloric acid, sulfuric acid, phosphoric acid or a Lewis acid such as aluminum chloride, iron-III-chloride, zinc chloride, antimony-III-chloride, Germanium dioxide or zinc oxide.

The reaction temperature is preferably 130 170 ° C. The "B residue" split off during the reaction is distilled off under normal pressure, but expediently under a vacuum of 1-500 mbar.

The liquid resin according to the invention is drained from the reaction vessel, expediently worked up on a pasteling belt or a cooling roller and can then be ground.

The resins according to the invention thus obtained generally have a softening point of 60-110 ° C., but preferably 70-100 ° C. (measured according to DIN 53180). They show a higher average molecular weight (measured by osmometry) than the components introduced for the reaction. For example, the reaction of a novolake with a molecular weight of M results_os= 530 with a polysiloxane containing M with methoxy end groups_os= 510 an end product with <o>M</o>_os= 760. This molecular weight analysis enables the chemical conversion of the novolake with the polysiloxane to be checked and verified in a simple manner.

In order to produce friction linings, the resins according to the invention obtained are filled with inorganic and / or organic fillers such as metals and / or metal oxides, fluorspar, asbestos, silica, butyl, nitrile and / or chloroprene rubber in the presence of formaldehyde-releasing substances such as hexamethylenetetramine, trioxi- or tetraoxymethylene mixed intimately on industrial mixing units such as kneaders or internal mixers, preferably at temperatures of 60 to 150 ° C. Formaldehyde-releasing substances are added in amounts by weight of 4-16% by weight, preferably 7-14% by weight, based on 100 parts by weight of the resin according to the invention.

The mixtures obtained in this way are generally pressed at 140 ° C. to 180 ° C., in particular at 140 ° to 160 ° C., optionally under a pressure of 150 to 250 bar, to form friction linings (cf. Vieweg / Becker "Plastics Manual", vol. 10 , Duroplast, pp. 93-95, Carl Hanser Verlag, Munich, 1968).

The thermal properties of the moldings obtained are demonstrated using Table 1, which contains information about the weight loss of 2 resin samples determined by thermogravimetry under nitrogen (heating rate: 20 ° C./min.).<tables id="tabl0001" num="0001"><img file="EP0013402B1_D0003.tif" /></tables>

The invention is illustrated by the examples below.

example 1

660 g of a novolake are used, which was prepared from phenol and formaldehyde in a molar ratio of 1: 1.08 in a known manner in the presence of an acidic catalyst such as oxalic acid and has a molecular weight of <o>M</o>_os= 530 and has an OH number of 535 (cf. Ullmann's Encyclopedia of Technical Chemistry, 13th vol., P. 459, publisher: Urban Schwarzenberg, Munich-Berlin, 1962).

This novolak is reacted together with 330 g of a polydiphenylsiloxane containing methoxy end groups, which has the following characteristics:<ul id="ul0004" list-style="none"><li><o>M</o>_os=510</li><li>Viscosity (DIN 53 015): 110 mm².</li><li>% By weight methoxy groups: 14-16</li></ul>

The reaction is carried out at 160 ° C. in the presence of 0.002% by weight of p-toluenesulfonic acid (based on the total weight of the reaction mixture). The mixture is left to react for 1 hour at 160 ° C. under a nitrogen atmosphere and with vigorous stirring, and then 36.5 g of methanol are distilled off at a pressure of 350-10 mbar. At the same time, 3.0 g of phenol and water are removed by distillation.

The liquid resin is then removed from the reaction vessel and ground after cooling. The product has the following characteristics:<ul id="ul0005" list-style="none"><li>Ep. (According to DIN 53 180): 89 ° C,</li><li><o>M</o>_os=760</li><li>OH number = 330</li><li>Silicon content: 6.2%</li></ul>

The resin produced according to Example 1 was ground, mixed with 12% by weight hexamethylenetetramine and examined by thermogravimetry (cf. Resin 2 in Table 1).

The friction linings produced from the resin according to Example 1 show at temperatures above 300 ° C in addition to the significantly lower evaporation rate of low molecular weight compounds shown in Table 1 as an example, increased elasticity, tear propagation and tensile strength and a higher hardness than the friction linings, which are based on a phenol-formaldehyde resin or which were produced on the basis of a mixture of polysiloxane and phenolic resin (cf. Table 2).

Table 2 contains a property comparison of friction lining mixtures that<ul id="ul0006" list-style="none"><li>A) based on a mixture of acrylonitrile-butadiene copolymer and a phenol formaldehyde novolak with an average molecular weight of M._{os =}530 and an OH number of 535, which</li><li>B) based on a mixture of acrylonitrile-butadiene copolymer and a siloxane-modified novolak according to the resin from Example 1 and the</li><li>C) based on a mixture of acrylonitrile-butadiene copolymer and a phenol novolak silicone resin mixture according to Resin 4 of DE-OS 2 306 463.</li></ul>

This mixture of phenolic novolak and silicone resin according to Resin 4 of DT-OS 2 306 463 is produced as follows:<ul id="ul0007" list-style="none"><li>33 % By weight (based on solids) of a methyl silicone resin with a 3 Mcl% proportion of phenylsiloxane, in the present case as a 51% by weight solution in xylene, which has a viscosity of 40-60 mm², measured at 20 ° C, is mixed with 60 wt .-% of a phenol novolak of molecular weight <o>M</o>_os= 530 and an OH number of 535. - </li></ul>

This mixture is intimately mixed with the abovementioned acrylonitrile-butadiene copolymer and with the other components of the friction lining mixture, the xylene being removed at 100 mbar and 80.degree.

The friction lining mixtures (A), (B) and (C) harden by pressing for 1/2 hours at 150 ° C under a pressure of 200 bar. Sheets of 4 mm thickness were produced, from which standard test bars were made.<tables id="tabl0002" num="0002"><img file="EP0013402B1_D0004.tif" /></tables>

Friction lining qualities, which are manufactured on the basis of the siloxane-modified novolaks according to the invention, are characterized by lower mixture viscosities and thus a more favorable processing behavior in comparison to phenol novolak-containing friction lining qualities (see table 2 values ​​for the Mooney viscosity). Furthermore, the tensile strength (F), the elasticity (E) and the tear resistance (WRF), the dimensions for the dimensional stability of the friction lining are significantly improved compared to a friction lining based on a phenol novolak silicone resin mixture (see mixture (B) and (C) in Table 2.11 The curing effect of the resins according to the invention in friction lining mixtures is also higher than that of a pure phenol novolak or a phenol novolak silicone resin mixture (cf. HD values ​​in Table 2.11 for mixtures (A), (B) and (C)).