Hydrophobic silica

Abstract

Methods of making hydrophobic, pyrogenically produced silica having a tamped density, of 55 to 200 g/l. Said silica is produced by hydrophobizing pyrogenically produced silica by reaction with a halogen free silane and then compacting it with means consisting of a roller compactor or a belt filter press. The silica may be used for the production of dispersions.

Description

INTRODUCTION AND BACKGROUND

This invention relates to a hydrophobic, pyrogenically produced silica, to a process for the production thereof and to the use thereof.

It is known to compact hydrophilic, pyrogenically produced silica (EP0 280 851 B1). Disadvantageously, as tamped or bulk density increases, thickening action declines in a linear manner. Dispersibility also falls as density increases. This results in unwanted speckling. Thus, once compacted, a hydrophilic, pyrogenically produced silica may only be used for a limited number of applications.

It is therefore an object of the present invention to avoid the problems of compacted, hydrophobic, pyrogenically produced silica of the past.

SUMMARY OF THE INVENTION

The above and other objects of the present invention can be achieved by developing a hydrophobic, pyrogenically produced silica having a tamped density of 55 to 200 g/l.

The tamped density is preferably from 60 to 200 g/l.

A feature of the present invention is a process for the production of the hydrophobic, pyrogenically produced silica having a bulk density of 55 to 200 g/l, which process is characterised in that pyrogenically produced silica is hydrophobized using known methods and then compacted.

Hydrophobing can preferably be performed by means of halogen-free silanes. The chloride content of the silica can be less than or equal to 100 ppm, preferably from 10 to 100 ppm.

Compaction can be performed by means of a roller compactor. Compaction can preferably be performed by means of a belt filter press according to EP 0 280 851 B1, which reference is relied on and incorporated by reference.

The hydrophobic, pyrogenically produced silica used for purposes of the present invention can be, for example, the silicas known as:

• Aerosil R 812 or Aerosil R 8128, having the group —0—Si (CH₃)₃
• Aerosil R 202, Aerosil MS 202, Aerosil MS 202, Aerosil R 106 or Aerosil R 104 having the group

• Aerosil R 805 having the group

These are commercially available products from Degussa Hüls AG.

The hydrophobic, pyrogenic silica according to the invention having a tamped density of 55 to 200 g/l exhibits the following advantages:

Transport costs are distinctly lower as a result of the higher tamped density.

Once dispersed, the silica according to the invention is in the form of relatively small aggregates. In other words, the dispersions are more finely divided because the silica according to the invention is more readily dispersible.

The dispersions produced using the silica according to the invention exhibit a lower Grindometer value.

Both UV transmission transparency and visual transparency of the dispersions are distinctly improved by using the silica according to the invention.

Dispersions containing the silicas according to the invention exhibit distinctly increased stability because the tendency towards settling is distinctly lower.

Another advantage of the silica according to the invention is reduced dusting during incorporation and the distinctly reduced incorporation or wetting time in, for example, liquid systems.

In comparison with hydrophobic, pyrogenic silica of a lower bulk density, the hydrophobicity of the silica according to the invention is unchanged. Thickening action is also unchanged.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be further understood with reference to the following detailed embodiments thereof.

EXAMPLE 1

Various hydrophobic, pyrogenically produced silicas are investigated, wherein different compaction states are compared.

The following definitions apply:

bulk =  pulverulent, unmodified silica
CF =    silica compacted with a Carter filter
VV 60 = silica compacted to a tamped density of approx. 60 g/l
VV 90 = silica compacted to a tamped density of approx. 90 g/l

Aerosil grades R 202, US 202, US 204, R 812, R 812S and R 805 are investigated. The results are shown in Table 1.

As evaluated by the Corning Glass methanol wettability method, the degree of compaction has virtually no appreciable influence on hydrophobicity. Viscosity also exhibits no clear systematic dependency upon tamped density. Especially for R 812, dispersibility improves with increasing density. R 812S, which contains more SiOH groups than R 812, exhibits the above phenomenon less markedly.

US 202 and US 204 have very comparable theological properties and are inferior to AEROSIL R 202.

Surprisingly, the compacted variants, in particular of R 812, R 202 and US 202/4, exhibit an incorporation time reduced by up to half. The compacted silicas moreover exhibit reduced dusting.

		444701	444702	444703	444704	444705	444706	444707	444708
		AER	AER	AER	AER	AER	AER	AER	AER	444709	444710	444711
		202	202	202	202	805	805	805	805	AER812	AER812	AER812
PA	Test method	CF	CF	VV60	VV90	bulk	CF	VV60	VV90	bulk	CF	VV60
0330	Viscosity, epoxy before cure	459	456	382	430	190	181	185	178
0335	Viscosity, epoxy after cure	54.4	54.7	49.4	52.8	42	41.7	39	43
0340	Thickening action									11.7	13.3	11.5
0410	Grind motor value									127	102	92
0420	Methanol wetability
0701	Tamped density	45	50	51	75	44	62	55	68	45	44	50
0920	Agglemerate strength		11	20	18		15	15	20
0930	Hand: |sic|sieve oversize	0	24	4	27	0	36	7	19	0	0
0955	Effectiveness	258	274	203	266	235	260	236	258	166	185	169
0965	Effectiveness(UT)	280	290	226	295	271	281	270	288	197	213	209
0975	Settling(effectiveness)	15	15	15	8	10	15	10	5	13	15	8
		444712	444713	444714	444715	444716
		AER	AER	AER	AER	AER	444717	444718	444719	444720	444721	444722	444723
		812	812S	812S	812S	812S	US202	US202	US202	US204	US204	US204	US204
PA	Test method	VV90	bulk	CF	VV60	VV90	bulk	CF	VV60	bulk	CF	VV60	VV90
0330	Viscosity, epoxy						350.4	377.6	380.8	379.2	350.4	358.4	368
	before cure
0335	Viscosity, epoxy after cure						50.7	45.9	45.3	49.9	47	52.6	50.7
0340	Thickening action	11.1	17.3	17.3	18.2	17
0410	Grind motor value	77	93	110	110	100
0420	Methanol wetability
0701	Tamped density	73	49	50	58	75	39	50	67	44	45	57	71
0920	Agglemerate strength	22				28		10	15			16	23
0930	Hand: |sic|sieve oversize	12	0	0	0	4	0	27	36	0	0	3	20
0955	Effectiveness	159	168	169	187	209	320	304	320	186	193	192	201
0965	Effectiveness(UT)	225	201	200	216	235	336	327	346	223	225	225	230
0975	Settling(effectiveness)	5	8	8	3	0	15	10	3	10	10	10	10

EXAMPLE 2

Investigation of the Influence of Higher Compaction on Applicational Properties

		AE R 812,	AE R 812,
	AE R 812,	compacted	compacted
	uncom-	RHE	RHE
	pacted	UB 3847-2	UB 3847-3
	UB 3847-1	(4)	(5)	AE R 812
	10 kg	15 kg	20 kg	RHE
	sack	sack	sack	specific.
Tamped density (DIN ISO	g/l	50	87	106	approx.
787/11)					50
Effectiveness, ethanol		184	214	209	216 1)
(0955)
Effectiveness (UT),		218	260	290	236 1)
ethanol (0965)
Settling	vol. %	10	1	1	1)
(effectiveness, high-
speed mixer)

1. Determined on standard sample (UB 3391)

RHE in the above table indicates the Rheinfelden plant located in Germany.

Rheological Testing:

• Polymer: Araldit M (biphenol-1-expoxy resin by Ciba-Geogy, in the form of clear yellow liquid).
• Thixotroping agent: R 202 and R 812 Additive:—

Sample production date: 24 Feb. 1994 Spindle: 5
	Storage time	5 rpm	50 rpm
	in days	[mPa*s]	(mPa*s)	T.I.
	Sample A R 812 10 kg 2-10123
	0	16600	4460	3.72
		80-85 μ
	Sample A R 812 15 kg 1.0/8 min
	0	15100	4060	3.72
		50-60 μ
	Sample AR 812 20 kg 0.6/14 min
	0	15100	4020	3.73
		50-60 μ

Compaction may amount to a type of predispersion. Accordingly, effectiveness values rise with tamped density, i.e. the particles effectively present in the ethanol dispersion become smaller and the compacted samples exhibit distinctly less settling. Any suitable organic solvent can be used to form the dispersion.

The compacted samples accordingly have a more favourable Grindometer value in Araldit. However, since the larger particles have a decisive influence on thickening action, the property declines slightly on compaction.

It may be seen from the effectiveness values that, while the highly compacted AEROSIL R 812 sample may indeed still be broken up with the Ultra-Turax® mixer(0965), it can no longer be broken up with the high speed mixer (0955). Due to the smaller surface area of AEROSIL R 202 (and to the consequently theoretically improved dispersibility), this phenomenon hardly occurs with AEROSIL R 202. The parenthetical values (0965) and (0955) are the PA numbers in the tables on pages 5 and 6.

As compaction rises, the particles effectively present in an ethanol dispersion thus become smaller and 90° angle scattering rises due to Rayleigh scattering. Total scattering (over all angles), however, falls and the samples become distinctly more transparent on visual inspection, as is also substantiated by the UV transmission spectra.

Compaction has no influence on hydrophobicity, which in each case substantially corresponds to that of the standard sample.

EXAMPLE 3

Investigation of the Influence of Higher Compaction on Applicational Properties.

		AE R 202,	AE R 202,
	AE R 812,	compacted	compacted
	uncompacted	RHE	RHE
	UB 3848-1	UB 3848-2	UB 3848-3
	2-02024	2-01024-	2-01024-	AE R 202
	10 kg	(2)	(3)	RHE
	sack	15 kg sack	20 kg sack	specific.
Tamped density (DIN ISO 787/11)	g/l	51	93	119	approx. 60 3)
Effectiveness, ethanol (0955)		319	334	336	334 1)
Effectiveness (UT), ethanol (0965)		346	365	373	339 1)
Settling	vol. %	10	5	1
(effectiveness, high-speed mixer)
1) Determined on standard sample (UB 3391)
3) Guide value

The compacted AEROSIL R 202 samples behave in a similar manner to the compacted AEROSIL R 812 samples.

Reference is thus made to Example 2 with regard to the discussion.

The parameter of “effectiveness” reported in the tables herein relates to the high degree of fineness of the particle. This is therefore an indicator of high transparency and good stability of the resulting dispersions.

Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.

German priority application filed Dec. 22, 2000 199 61 933.6 is relied on and incorporated herein by reference.

Claims

1. A process for the production of the finely divided, readily dispersible hydrophobic, pyrogenically produced silica in the form of aggregates having a tamped density of 55 to 200 g/l comprising hydrophobizing pyrogenically produced silica by reaction with a halogen-free silane and then compacting said silica with means consisting of a roller compactor or a belt filter press.