Method of reducing the dust generation of silicates

Production of Polyorganosiloxane-coated Wollastonite

1000 kg of wollastonite is comminuted on a counterflow impact grinding unit (Majac). The jet mill operates with 2 boron carbide nozzles at an operating pressure of 7 bars. The classifier used to separate non-comminuted coarse particles is operated at a rotationalispeed of 1800 rpm. Separation of the final product is effected in a filter.

75 kg of wollastonite thus comminuted is coated in a Thyssen-Henschel FM250D fluid mixer using 750 g of a polysiloxane-polyether copolymer (TEGOSTAB B 8427, TH. GOLDSCHMIDT AG, Essen, Germany).

The rotational speed of the mixing tools (bottom scraper, fluidizing blades, horn tools) is 1200 rpm. The.fluid mixer is double-walled and enables adjusting particular temperature patterns independent of the frictional heat by indirect heating with hot water or steam. The temperature pattern adjusted in the wollastonite reaches 80° C.

Thereafter, the coated wollastonite having a material temperature of 80° C. is cooled to room temperature in a doublewalled cooling mixer (Thyssen-Henschel Type 650). The mixing tools are comparable to the mixing tools of the fluid mixer. The rotational speed is 140 rpm.

A low-dust, coated wollastonite is obtained as final product.

FIG. 1 shows an FT-IR spectrum of the obtained product as a pressed pellet. The Figure shows the transmittance in % as a function of the wave number in cm −1 . The vibrational bands are characteristic for the polyorganosiloxane-coated wollastonite system.

FIG. 2 shows another FT-IR spectrum of the obtained product. Assessment is effected as an illustration of the absorbance as a function of the wave number in cm −1 . Only wave numbers of from 2,400 to 3,600 have been covered, and the wollastonite raw material has been subtracted mathematically from the FT-IR spectrum within the scope of the present analysis. As a consequence, FIG. 2 merely shows the vibrational bands of the surface coating, namely, of the polyorganosiloxane on the surface of wollastonite, particularly in the wave number range of from 2,800 to 3,100. In particular, the CH vibrations are typical in this wave number range. Therefore, the illustration thus obtained is a characteristic fingerprint of the surface additive, describing the specific structure of the polyorganosiloxane employed.

FIGS. 3, 4 and 5 characterize the obtained product with respect to needle length (FIG. 3 ), needle diameter (FIG. 4 ), and average L/D ratio (length/diameter), as a function of the needle length (FIG. 5 ).

Dust Measurement

In this Example, the product of the invention produced according to Example 1 was examined for its dust behavior in comparison to prior art products. More specifically, the following products were tested:

a) wollastonite according to Example 1;

b) wollastonite with no additive (prior art);

c) wollastonite with aminosilane as additive (prior art).

The dust behavior was determined using the following method:

1. Dust measurement using the Heubach Dustmeter

2. Determination of the specific surface according to DIN 66132-BET

1. Dust Measurement Using the Heubach Dustmeter

A Heubach Dustmeter, an analytical balance, as well as a filter GF 92 (S+S) are used in this investigation. The measuring instrument consists of a base instrument with air measurement (volume, temperature, driving motor, vacuum pressure pump). It includes brackets having

holders for mounted structures

dust formation vessel

coarse separator

air filtration vessel

Assembly

The suction pipe with tube connector is screwed tightly on the threaded socket, the stabilizer pipe is fitted onto the bolt, fixed with the clamping block, and the foot is secured to the pipe ends. The short stand rods can be fixed at each position of the bracket. The desired array is furnished using the double sleeve and the stand clamp.

b) Function and Operation

The substance to be tested is kept in motion in the rotating dust formation vessel. The dust is taken up by an axially entering stream of air and is, moved past a coarse separator made of glass wherein coarse, non-dispersible particles undergo sedimentation. The dust remaining in the stream of air is deposited on a filter. The air pressure required in testing is generated by a vacuum pressure pump, the air meter measurement being effected on the pressure side.

c) Performing the Measurement

Adjusting the desired air flow:

An air volume of 100 l at a measuring period of 5 min (20 l /min) is recommended.

One unit of the counter corresponds to 0.1 l, i.e., for e.g. 100 l of air, “1000” must be preselected on the setpoint generator of the four-digit counter. The air filtration vessel covered with filter paper is connected to the pump, the needle valve on the flowmeter is fully opened, the power switch is actuated, the selector switch is disengaged (automatic operation), 20 l (“0200”) is preselected on the setpoint generator. After pressing the “START” button, the pump is set in motion, switching off automatically after 20 l. The needle valve on the pump is opened or closed to such a degree that the preselected 20 l will be conveyed precisely within 1 min. Now, all of the following measurements will be effected at a flow rate of 20 l of air per minute.

d) Dust Measurement

The weighed test material is introduced in the dust formation vessel with 25 g, 50 g or 100 g, depending on the bulk density. Up to a bulk density of 0.5 g/ml, the initial weight should be 25 g or 50 g, and above that, 100 g.

Prior to beginning the measurement, the air filtration vessel covered with filter paper is weighed (m 1 ). The desired amount of air is preselected on the four-digit digital setpoint generator. Once all the connections from the dust formation vessel up to the pump are established, the “START” button is pressed, setting both the gear motor and the pump in operation. Once the preselected amount of air is reached, both of them will be stopped automatically. Reweighing of the filtration vessel furnishes m 2 . Prior to weighing, the rubber plug remaining on the filter should be cleansed on its outside using a cleaning paper.

e) Standard Measuring Conditions for QW Products

In any case, air flowing through the measuring instrument is passed through an upstream drying tower in order to eliminate the effect of humidity on dust formation.

Air volume: 20 l/min

Measuring period: 5 min

Initial weight: 50 g

Filter: GF 92, Schleicher & Schüll

f) Assessment

The dust formation number “s” corresponds to the amount of dust in mg formed from 100 g of test material under standardized conditions.

Said number is calculated according to the following equation: s = m 2 - m 1 m 0 × 10 5

Therein:

m 0 represents the initial weight of sample in g;

m 1 represents the mass of the filter casing with inserted filter prior to measurement;

m 2 represents the mass of the filter casing with inserted filter after measurement.

The dust formation number “s” of all of the three samples was determined. Table 1 shows the dust formation number in mg/100 g.

As can be seen from Table 1, the wollastonite according to Example 1 has a dust formation number which is by 50% lower than that of prior art wollastonite coated with aminosilane.

2.Determination of the specific surface area according to DIN 66132-BET

In the following test, the specific surface area of the wollastonite according to Example 1 and of prior art comparative products was determined. The specific surface area is particularly high in materials having a high dust content, because dust has a large specific surface area. For these products, the following data were determined, which are illustrated in Table 2.

As can be seen from Table 2, the specific surface area of the product according to the invention is lower than the specific surface area of wollastonite with aminosilane.

Production of a Plastic (Polyurethane) Molded Article Including Reinforcing Wollastonite Filler

Using the wollastonite product of Example 1 and a comparative product, namely, wollastonite coated with aminosilane, a plastic molded article having the following composition was produced:

Polyol

Polyether 64.4 parts by weight glycerol/85 PO/15 EO, OH number=35

Diethyltoluylenediamine 35 parts by weight

Diazabicyclooctane 0.5 parts by weight

Dibutyltin dilaurate 0.1 parts by weight

Semi-prepolymer

Polyether glycerol/85 PO/15 EO/OH number=35

Diphenylmethane-4, 4 ′-diisocyanate (NCO content 18%)

The mixing ratio was 100 parts by weight of polyol and 107 parts by weight of semi-prepolymer, as well as 20 parts by weight of wollastonite in the elastomer. Using the plastic material thus produced, a plastic molded article was injection-molded and examined for its mechanical properties. As illustrated in Table 4, the following results were obtained:

As can be seen from the Table, the mechanical properties of the plastic material wherein the wollastonite of Example 1 is used are superior over the mechanical properties of conventionally reinforced plastic materials.

Production of a Plastic (Polypropylene) Molded Article Including Reinforcing Wollastonite Filler

A polypropylene copolymer is produced with a level of 20 wt.-% of wollastonite of Example 1 and with a comparative product, namely, wollastonite coated with alkylsilane. Using this material, a plastic molded article plastic is produced and its mechanical properties are determined. As illustrated in Table 5, the following results were obtained:

As can be seen from the Table, the mechanical properties of the plastic material wherein the wollastonite of Example 1 is used are improved over conventionally reinforced polypropylene.