2 сентября, 2015 
Malyar The use of CAB in these systems improves flow and levelling and reduces the tendency for ‘fish-eyes’. There are many different grades and viscosities available. All have a hydroxyl value and will, therefore, react with the isocyanate to reduce the solvent sensitivity of the CAB. Suppliers include Eastman Chemicals and Bayer.
Silicones can be used to improve flow, slip and reduce foaming, but care must be taken in selection and addition levels as the wrong silicone or level can give rise to ‘craters’ or ‘foaming’. Suppliers include Byk, Bayer, Th. Goldschmidt, Efka, and Dow Coming.
Silica matting agents are normally used for effect finishes. However for low sheen finishes it may be necessary to use blends of silicas with different properties to obtain the best appearance as problems of unevenness or patchiness can occur. Suppliers include Degussa, Grace, Croxton & Garry, and Crossfield.
Waxes are not normally necessary, but could be used to improve feel. Саге must be taken to avoid air entrapment caused by the waxes which can lead to surface defects such as pinholes. Suppliers include Alfa chemicals, Capricorn chemicals, Elem Chemicals, and Hoechst.
Accelerators are available, if required, to shorten cure times. They can lead to shorter pot lives, poorer physical properties and cause yellowing. Suppliers include Bayer, and Akros.
Some starting formulations for wood finishings are given below:
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FORMULATION 5-5: WHITE PU PRIMER
(l) 50% solids, OH content 1% |
The first five components are high speed dispersed or bead milled. The 1/2 Sec Nitrocellulose (80/20 Plasticiser) is predissolved in the solvents (toluene, acetone and ethyl acetate) before it is added to the above. The nitrocellulose could be replaced with a suitable grade of CAB. Mixed viscosity is 40-45 secs B4 at 20°C with a specific gravity (S. G.) of 1.16.
Solvents used are fast evaporating for shorter handling time. Desmodur L is a TDI based isocyanate which is cheaper and reacts faster than aliphatic versions. It is acceptable in primers where extra protection comes from the topcoat. The nitrocellulose reduces touch dry time.
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FORMULATION 5-6: SATIN WHITE TWO PACK PU
(1) (2) (3) (4) Eastman Chemicals Dow Coming Byk Additives 50% solids, OH content 1.8% |
The titanium dioxide and hydroxy acrylic resin are high sheer dispersed or bead milled. The CAB is predissolved in the solvents, before the resin and pigment are added, with stirring in the silicone. Mixed viscosity is 35-40 secs B4 at 20°C. S. G. is 1.02. The silicone improves mar resistance. Desmodur N75 is used where colour retention and durability are important. Curing is slower but this means longer pot life.
FORMULATION 5-7: SATIN CLEAR PU
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CAB 381-0.3(1) |
8.00 |
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Xylene |
13.00 |
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Methyl Ethyl Ketone |
25.00 |
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Methyl isobutyl ketone |
11.00 |
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Ethyl Acetate |
7.00 |
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n-Butyl Acetate |
14.00 |
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Silica Matting Agent |
2.00 |
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Hydroxy Acrylic Resin ® |
20.00 |
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Desmodur L 75% In Ethyl Acetate |
5.00 |
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Total |
100.00 |
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^ Eastman Chemicals (2> 50% Solids, OH content 1.8% |
The CAB 381-03 is predissolved in the solvents before the hydroxy acrylic resin and the matting agents are added to give a viscosity of 30-35 secs B4 at 20°C, sheen 28-32% and
S. G. of 0.90.
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FORMULATION 5-8: MATT LACQUER FOR WOOD
Synocure 869X, 52% in xylene, 1.55% hydroxyl |
Viscosity (20°C, centipoise) 200
Vehicle solids 25.4
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TABLE 5-6: PHYSICAL PROPERTIES OF FORMULATION 5-8
KEY: 0 Substantially affected 5 Unaffected Mix parts I and II, and spray apply. |
Lancowax TF1778 (Capricorn Chemicals) is a micronised wax. Aerosil TS100 (Degussa) is a silica matting agent.
The commercially available resins recommended for wood finishing include:
Macrynal SM548 Synocure 213 Synocure 869SX Setalux C-1184SS51 Desmophen A450
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