Silica products

FIELD OF INVENTION
The invention relates to wax coated silica matting agents which are widely used in the surface coatings industry to reduce gloss but also to enhance film properties such as feel, scratch resistance and flexibility.
BACKGROUND TO THE INVENTION
It is well known that micronised waxes will impart these desirable features to the paint or lacquer film. However, when a silica is coated/impregnated with the wax a process usually performed by jointly grinding the silica and wax in a fluid energy mill (microniser), further product benefits are immediately available. The mixed phenomenon of coating and impregnating, resulting from the inherent porosity of the silica, is hereinafter referred to as "coating". The wax can improve compatibility of the silica with the paint or lacquer preventing interaction with other components in the formulation, but another function is to prevent the formation of hard sediment during storage which is not redispersible. Whilst the mechanism by which this protection is afforded is still not fully understood, its benefit to the paint or lacquer manufacturer is widely recognised because, if hard sediments which cannot be redispersed are formed, none of the benefits described above can be realised.
1. Literature
Two types of wax coated silicas are generally disclosed in the literature. GB 798,621 (Grace) discloses a silica matting agent produced by co-milling an intermediate density silica gel with a microcrystalline wax in a fluid energy mill. GB 1,236,775 teaches a silica matting agent can be prepared by treating precipitated silica with an aqueous emulsion or dispersion of a wax, including thermoplastic materials. The properties of the silica matting agents prepared by the so-called "dry" co-micronising route have been further improved by adding fatty acids (GB 1,461,511 - Grace) or synthetic polyethylene waxes of varying molecular weight (U.S. Pat. No. 4,097,302--SCM) to the microcrystalline wax before feeding to the fluid energy mill. The former improves the dispersibility of the wax coated products, whereas the latter shows products having enhanced settlement characteristics compared with commercially available materials can be prepared. The "dry" processing routes described above use temperatures in excess of the melting points of the waxes or wax blends whereas GB 1,538,474 (BASF) discloses a process which can produce satisfactory wax coated silicas at micronising temperatures of below 50.degree. C., where functional waxes such as montan acid ester are employed.
GENERAL DESCRIPTION OF THE INVENTION
The general trend in the paint and lacquer industry to shorter mixing/cycle times by the use of higher shear and/or ultra sonic dispersing equipment makes it imperative that the wax coating on the silica is robust and more effective in preventing the formation of hard sediment on storage. In addition, the change to high solids systems demands the wax coated silica matting agent is more compatible with the active components in the paint or lacquer formulation. It would be desirable, therefore, to provide a wax coated silica for use as a general purpose matting agent which has enhanced capability of preventing the formation of hard sediment on storage and can lower the interaction between itself and the other components in the paint and lacquer formulation.
The invention proposes the use of a ternary blend comprising a hard microcrystalline wax, a plasticising microcrystalline wax, and a synthetic polyethylene wax for coating on to silica resulting in a product with an improved capability for preventing hard settlement in the paint or lacquer and which is more compatible with the other components in the formulation. Surprisingly, the amount of plasticising microcrystalline wax present in the blend plays the dominant role in determining whether the resulting wax coated silica exhibits the enhanced desirable features. Plasticising microcrystalline waxes have melting points in the range 70.degree. to 85.degree. C., a needle penetration (25.degree. C.) of from 1.5 to 3.0 mm, and is exemplified by the grades Ultraflex.RTM., Victory.RTM. and BeSquare.RTM. 175 available from Petrolire. The hard microcrystalline waxes have melting points in the range 90.degree. to 95.degree. C., a needle penetration (25.degree. C.) of from 0.4 to 0.8 mm and are typified by Crown.RTM. 700 from Petrolite, Cerit.RTM. HOT from Cerilite SA, Brazil and Lunacerin.RTM. H193 from H. P. Fuller, Germany. The synthetic polyethylene wax has a melting point in the range 110.degree. to 140.degree. C., a molecular weight of 1000 to 4000, and is exemplified by the grades Polywax.RTM. 1000 and Polywax.RTM. 2000 available from Petrolite. The three wax components may each comprise mixtures of waxes satisfying the requirements of the component.
Accordingly the invention provides for an improved wax coated amorphous silica matting agent, characterised in that the wax coating consists of a ternary mixture comprising from about 5 to about 30% w/w, preferably from about 10 to about 20% w/w of a plasticising microcrystalline wax, from about 5 to about 20%, preferably from about 5 to 15% w/w of a synthetic polyethylene wax, and from about 50 to about 90% w/w, preferably from about 65 to about 85% w/w of hard microcrystalline wax. When the amorphous silica is a xerogel or precipitated silica the ternary wax coating will usually be present in an amount from about 5 to about. 20% w/w, preferably from about 5 to 15% w/w, in order to obtain the optimum effect. In these ranges the lower levels are sufficient to give the effect sought, whilst the upper levels give the desirable features but they are considered less cost effective.
The amorphous silica supporting the ternary wax coating has a surface area (SBET) to nitrogen from about 200 to about 550 m.sup.2 g.sup.-1, and it is either a silica xerogel or a precipitated silica with a preferred pore volume between 0.8 to 2.5 ccg.sup.-1.
The particle size distribution of the ternary wax coated amorphous silica will be commensurate with obtaining the necessary fineness of grind when dispersed in the paint or lacquer system, and the most effective reduction in gloss in the subsequently dried paint film. Consequently, the ternary wax coated amorphous silica has a weight mean particle size of from about 5 to about 9.mu.m, with a 10 percentile of about 2.mu.m to about 4.mu.m and 90 percentile from about 9.mu.m to about 20.mu.m.
An essential feature of the invention is that the waxes are mixed together in the molten state, either spray cooled, or slabbed, and milled to a particle size range similar to the amorphous silica, before being milled jointly in the fluid energy mill. The inlet temperature of the air being supplied to the fluid energy mill must be high enough to ensure the ternary wax mixture melts within the residence time profile of the milling equipment.
TEST PROCEDURES
i. Matting Efficiency
The matting efficiency of silica matting agent is determined by dispersing the silica in an alkyd urea lacquer, adding a catalyst, drawing a film on a black glass plate, curing in an oven and measuring the glossmeter readings using a BYK multi-glossmeter.
______________________________________Formulations Parts by Weight______________________________________a) Alkyd urea lacquer (35% Solids)Butylated urea formaldehyde 40resin; BE660 (53%)Castor oil modified alkyd of short 30to medium oil length BA548 (60%)Xylene 28Isopropyl alcohol 14 112Silicone Oil/Type AL (Wacker) 1.12b) Acid CatalystButyl alcohol 87Concentrated sulphuric acid 11Distilled water 2 100c) Test LacquerAlkyd urea lacquer (35%) 100Acid catalyst 5.2Silica matting agent 1.46 107.05______________________________________
The test lacquer contains 1.37 % w/w of the silica matting agent.
Into a 250 ml beaker weigh 100 g alkyd urea lacquer (formulation(a)). Using a Heidolph stirrer (fitted with a Cowles-type head) stir the lacquer at 1000 rpm and add 5 g of the silica under test. When the silica completely wets out increase the stirrer speed to 3000 rpm (equivalent to a peripheral speed of 4.87 ms.sup.-1) and stir for 10 minutes. Allow the dispersion to de-aerate, then weigh 6.5 g into a clean beaker followed by 15 g of the alkyd urea lacquer (formulation (a)), and then hand stir in 1.1 g of the acid catalyst. Draw down a film onto a black glass plate (12".times.4") using a 100 .mu.m block applicator, cure in an oven at 150.degree. C. for 10 minutes, allow the plates to cool and then measure the glossmeter reading at 60.degree. and 85.degree. using a BYK multi-glossmeter.
ii. Fineness of Grind
The fineness of grind of a dispersed silica matting agent is measured in 35% solids alkyd urea lacquer with 0-50 micron Hegman Gauge. The result indicates the level of dispersion that can be achieved for a given silica matting agent. The same procedure is followed as for matting efficiency but 5 g of silica is dispersed in 100 g of alkyd urea lacquer (35% solids), equating to 4.76% w/w silica. The dispersion is then spread on a Hegrnan Gauge, viewed with the aid of a display cabinet, and the result recorded in microns.
iii. Accelerated Settlement at Temperature
To obtain information on settlement behaviour and the nature of particulate bed formed, matting agents are usually dispersed in a paint or lacquer at the required concentration and then stored in cans at room temperature. Over a period of weeks/months observations on settlement behaviour are recorded, and in particular the time taken for a hard sediment to form is noted. Even after storage for as long as 3 months it is difficult to discriminate performance of silicas coated with different waxes.
As a consequence an accelerated settlement behaviour test has been devised, by which a dispersion in alkyd urea lacquer prepared in exactly the same way as for matting efficiency, but containing 4.76% w/w silica, is transferred into sealed measuring cylinders and the measuring cylinders placed in a water bath at 70.degree. C.
It is interesting to note the varying settlement behaviour of the silica matting agents containing different waxes, and relate this to the nature of the settled bed of particulates. Two extreme cases are observed. In one, the system appears to "flocculate" giving rise to a sediment layer which is thick and fluffy in appearance and the solvent layer is completely clear, whereas in the other flocculation is not apparent and the particulate layer is thinner and denser, and the solvent layer is cloudy immediately above the settled bed, changing to clear towards the surface of the vehicle. The interface between the sedimented particles and liquid is less marked in the latter case and can initially give the formulator a misleading stance as to the stability of the system. The true stabilisation performance is revealed on attempting to redisperse the bed of settled particles. In the "flocculated" case redispersion can be achieved by shaking or simple stirring whereas the sediment in the absence of flocculation is impossible to resuspend.
The time taken for the dispersed silica to settle to a hard sediment which will not resuspend is recorded. As the dispersions are stored at 70.degree. C. the observations can now be made in days compared to weeks and months at room temperature, making it much easier to differentiate between the wax coated silicas in their propensity to prevent the formation of hard sediments.
iv. Weight Mean Particle Size
The weight mean particle size is determined with the aid of a Malvern Particle Size, Model 3600E. This instrument, made by Malvern Instruments, Malvern, Worcestershire uses the principle of Fraunhoffer diffraction utilising a low power He/Ne laser. Before measurement the sample is dispersed ultrasonically in water for a period of 7 minutes to form an aqueous suspension.
v. Surface Area/Pore Volume
Surface area is determined by standard nitrogen adsorption methods of Brunauer, Emmett and Teller (BET) using a multi point method with an ASAP 2400 apparatus supplied by Micromeritics of the USA. The samples are outgassed under vacuum at 270.degree. C. for at least 1 hour before measurement. Surface area is calculated from the adsorption data measured in the P/Po range 0.05 to 0.3, and pore volume is calculated from the volume of nitrogen gas adsorbed at P/Po 0.98.
vi. Ignition Loss at 1000.degree. C. (TVM)
Ignition loss is determined by the loss in weight of a silica when ignited in a furnace at 1000.degree. C. to constant weight.
vii. Oil Absorption
Oil absorption is determined by the ASTM spatula rub-out method (American Society of Test Material Standards D 281).
The test is based on the principle of mixing linseed oil with a silica and rubbing with a spatula on a smooth surface until a stiff putty-like paste is formed which will not break or separate when it is cut with a spatula. From the volume of oil used the oil absorption, usually reported in g/100 g, can be calculated.
viii. Polyurethane Concentrate Gelling Test
To obtain information on the interaction of the wax coated silica products with the active components in a lacquer system it has been noted that the rheology of a polyurethane concentrate containing bentone is particularly sensitive to the "type" of silica matting agent dispersed in the concentrate. When an uncoated silica is used the system gels irreversibly within 4 hours whereas a totally hydrophobic silica has little effect on the concentrate. Thus, it would appear that this concentrate can be used to measure the level of the interaction between the components in the lacquer system and the silica matting agent.
______________________________________Formulation______________________________________Bentone Paste 100 gUralac AY694 .times. 60 250 g (a polyurethane obtainable from D. S. M. Resins Europe)Xylene 200 gSilica 150 g______________________________________
The silica was added to the lacquer in a 1 liter paint can and dispersed using a Cowles dissolver operating at 4000 rpm for 15 minutes. With the mixer operating at 2000 rpm a solvent mixture containing 240 g of xylene and 60 g of isobutylalcohol was added with care. The mixes containing the different coated silicas were allowed to stand at room temperature and examined for "seeding" and for "gelling". The time taken for the system to gel was recorded, and the shorter this time the greater the interaction between the components of the lacquer system and the silica matting agent. Formation of a gel prevents the lacquer being pourable and spreadable over a surface.
ix. Wax content
The wax content of the coated silicas is determined by solvent extraction using the Tecator Soxhlet HT2. A known weight of sample is transferred to the "thimble" on the apparatus and extracted with 70 mls of chloroform at reflux for 20 minutes. The thimble is rinsed for 45 minutes, air blown for 5 minutes, removed from the equipment and dried in an air circulated oven at 105.degree. C. for 15 mins. The wax content (in % w/w) can be calculated from the weight difference to constant weight.

SPECIFIC DESCRIPTION OF THE INVENTION
The invention is further illustrated but not limited by the following examples in which parts and percentages are by weight, unless otherwise indicated.
In all the examples two basic feedstocks have been utilised, one based on a precipitate or stirred vessel route (A), and the other a conventional high porosity gel (B).
FEEDSTOCK A
This silica was prepared using the method disclosed in EP A 0361 622 Example 1 (Run 1; Table 1) but before filtration and washing the slurry was aged at pH 7.0 for 2 hours at 80.degree. C. After washing the aged hydrogel was then flash dried.
FEEDSTOCK B
This material was made utilising the procedures disclosed in EPA 0377 301 but the silica hydrogel, after washing was aged at pH 7.0 for 6 hours at 80.degree. C. The aged hydrogel was then flash dried.
The properties of the two silica feedstocks are given in Table 1.
TABLE 1______________________________________Properties of Feedstocks______________________________________ Weight Surface Pore Oil MeanSillica Area Volume Absorption ParticleFeedstock m.sup.2 g.sup.-1 cm.sup.3 g.sup.-1 g/100 g Size .mu.m______________________________________A 420 (b) 1.7 (b) 220 (b) 60B 320 (b) 1.8 (b) 230 (b) 200/300 (a)______________________________________ (a) Estimated from sieve separation (b) Mean results from 5 samples
EXAMPLE 1
Comparison
A number of potential waxes and wax blends were screened for their ability to provide the desirable features of improved settlement, and better compatibility, when coated onto a suitable silica.
Feedstock (A) or (B) was mixed with the candidate waxes or wax blends (prepared by mixing the melted waxes, allowing them to cool and then milling) so that the resultant silica/wax mixture contained between 8 to 12% w/w of the wax or wax blend.
This blend of silica and wax was then fed to an AFG 200 fluid bed mill (supplied by Alpine AG, Augsburg, Germany), operating at an air inlet temperature of 180.degree. C., the classifier speed and feed rate being set commensurate with obtaining a micronised product with a weight mean particle size in the range 5 to 9 microns.
The wax coated silicas were subjected to the following measurements:
i. Matting efficiency
ii. Accelerated settlement 70.degree. C.
iii. Polyurethane gelling test.
The results of these selection tests are summarised in Table II, together with properties of the micronised products. Clearly none of the prior art materials provide the necessary enhancement in either settlement behaviour or compatibility, though the micronised product containing the wax blend 10 polyethylene (MW 2000): 90 microcrystalline did offer improved performance in accord with the teaching of U.S. Pat. No. 4,097,302.
TABLE II__________________________________________________________________________ MICRONISED PRODUCT PROPERTIES Weight MeanTEST/FEEDSTOCK LITERATURE Particle Size TVM WAXTYPE WAX/WAX BLEND SOURCE .mu.m % w/w % w/w__________________________________________________________________________1/B Synthetic -- 6.5 17.4 9.3 Polyethylene MW 5002/B Microcrystalline GB 798,621 6.5 17.2 9.63/A Microcrystalline GB 798,621 6.0 15.2 9.34/B 25 Polyethylene US 4,097,302 5.8 16.0 9.7 (MW 500) 75 Microcrystalline5/A 25 Polyethylene US 4,097,302 5.5 17.2 9.5 (MW 500) 75 Microcrystalline6/A 10 Polyethylene US 4,097,302 7.0 13.5 8.0 (MW 2000) 90 Microcrystalline7/B Montan acid GB 1,538,474 7.9 18.0 10.6 Ester__________________________________________________________________________MATTING INTERACTION WITHEFFICIENCY NATURE OF SEDIMENT/TIME POLYURETHANE CONCENTRATETEST 60.degree. 85.degree. ALKYD UREA 70.degree. C. ROOM TEMPERATURE__________________________________________________________________________1 22 38 Soft sediment up to 7 days Thickened over 24 hours - Gelled after 3 days2 21 40 Hard sediment 24 hours Irreversible gelling within 6 hours3 22 42 Hard sediment 24 hours Irreversible gelling within 6 hours4 20 42 Hard sediment 24 hours5 19 44 Hard sediment 24 hours6 20 27 Hard sediment 6 days Thickened over 24 hours7 19 32 Hard sediment 2 days Irreversible gelling after 24 hours__________________________________________________________________________ MW = Molecular Weight TVM = total volatile material and includes the wax
The performance of the micronised silica containing the polyethylene wax (MW 500) is interesting, because at room temperature a hard sediment formed at 24 hours, whereas at 70.degree. C. a soft sediment has been observed up to 7 days. All the other materials gave rise to soft sediments when stored in alkyd urea for up to 2 months at room temperature.
EXAMPLE 2
Following the procedure for the preparation of the wax coated silicas described in Example 1, a range of wax blends were vetted as a possible coating for the base silica feedstocks A and B, they included:
Plasticising microcrystalline wax Victory.RTM.;
Hard microcrystalline - Crown.RTM. 700 and Cerit.RTM. HOT.
Synthetic polyethylene wax's (MW 500, and 2000) Polywax.RTM. 500 and Polywax.RTM. 2000
The waxes were blended together in the molten state, cast and then granulated and pin milled. They were then mixed with the appropriate silica feedstock and fluid energy milled. The micronised silicas containing the wax blends made from the above waxes were subjected to the same testing programme as that carried out on the products of Example 1. The results are summarised in Table III together with the properties of the micronisedproducts. Samples I, II, X, XI and XII are comparative examples.
It can be seen that the micronised product containing the ternary blends, consisting of hard microcrystalline wax as the base and differing levels of plasticising microcrysta, line wax and synthetic polyethylene wax (MW 2000), offer improved performance in terms of settlement behaviour and compatibility. All the products gave rise to soft sediments when stored in alkyd urea for up to 1 month at room temperature.
From the above data and from general experience with the compositions the ternary wax blend gives superior results as a matting agent with respect to minimising interference with the lacquer system, when applied to the same silica feedstock either precipitate or gel derived using similar conditions in the comicronising process. The preferred ternary blend contains 75.+-.10% hard microcrystalline wax, 15.+-.5% plasticising microcrystalline wax, and 10.+-.5% synthetic polyethylene wax.
TABLE III__________________________________________________________________________ WAX BLEND COMPOSITION MICRONISED PRODUCT PROPERTIES HARD MICROCRYST. WEIGHT MEANTEST/FEEDSTOCK MICRO. PLASTIC PARTICLE TVM WAXTYPE % W/W WAX % W/W POLYETHYLENE SIZE .mu.m % W/W %__________________________________________________________________________I/A 90 (Cr) 10 -- 6.6 14.7 8.5II/A 80 (Cr) 20 -- 6.9 14.0 8.3III/A 85 (Cr) 5 10 (P2) 6.4 16.4 9.3IV/A 80 (Cr) 10 10 (P2) 8.2 18.4 10.0V/A 70 (Cr) 20 10 (P2) 6.8 15.8 9.0VI/A 60 (Cr) 30 10 (P2) 6.6 14.5 8.5VII/A 80 (Ce) 10 10 (P2) 6.6 14.8 8.5VIII/B 80 (Ce) 10 10 (P2) 6.7 17.3 9.5IX/A 80 (Cr) 10 10 (P5) 7.0 15.5 9.0X/A 90 (Ce) -- 10 (P2) 6.3 15.0 8.7XI/A 90 (Ce) -- 10 (P5) 6.2 17.0 9.3XII/A -- 50 50 (P5) 6.2 13.8 8.0__________________________________________________________________________MATTING INTERACTION WITHEFFICIENCY NATURE OF SEDIMENT/TIME POLYURETHANE CONCENTRATETEST 60.degree. 85.degree. ALKYD UREA 70.degree. C. ROOM TEMPERATURE__________________________________________________________________________I 22 38 Hard sediment 24 hoursII 21 25 Hard sediment 24 hours Thickened slightly no gellingIII 18 33 Soft sediment up to 7 daysIV 23 25 Soft sediment up to 14 Thickened slightly no days gelling up to 3 weeksV 20 37 Soft sediment up to 7 daysVI 17 29 Hard sediment in 7 days Slight thickening no gellingVII 19 33 Soft sediment up to 14 Thickened slightly no days gelling up to 3 weeksVIII 18 35 Soft sediment up to 14 Thickening slightly no days gelling up to 3 weeksIX 18 28 Hard sediment 24 hoursX 18 33 Hard sediment 6 daysXI 19 32 Hard sediment 24 hoursXII 19 33 Hard sediment 24 hours__________________________________________________________________________ Cv = Crown 700 P5 = Polywax 500 Ce = Cerit HOT P2 = Polywax 2000

Number	Name	Date
3816154	Baldyga et al.	Jun 1974
4097302	Cohen et al.	Jun 1978
5034207	Kerner et al.	Jul 1991
5074918	Babler	Dec 1991
5087594	Kato et al.	Feb 1992
5123964	Kerner et al.	Jun 1992

Number	Date	Country
0341383	Feb 1989	EPX
798621	Jul 1958	GBX

Silica products

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