Polyurethane Varnishes or Uralkyds (Single Pack) (Technical Note: ST 17)

General Types Properties Compatibility Solubility Recommended Driers Mode of Supply Specifications Comparative Film Properties Superiority of Oil Modified Polyurethanes Over Other P.U. Systems Applications Cost Advantages Typical Formulations Technical Service

1.0 General

Uralkyds such as Silac U210, Silac U220 and Silac U230 more often known as Urethane Oil media or simply as ‘U’ varnishes, represent the most modern technology in the field of Surface Coatings. They are polymers built up by Urethane linkages containing air-drying polyunsatured ester groups as substituents. In effect the modern Urethane Oils are a type of Alkyd resin varnishes in which the dibasic acid is replaced by a diisocynate.

The lacquers based on ‘U’ varnishes are high-tech film formers very widely used in abroad and are unequalled in properties such as toughness, hardness, durability and rapid drying.

1.1 Though our ‘U’ varnishes are as convenient to use as ordinary oil modified Alkyds, being single pack systems, and require the common solvents such as MTO and conventional driers, yet yield surface coatings which are far superior to those possible with Alkyds or other known media. The free isocynate content is also nil and hence can be used even by an unskilled worker.

The two pack Urethane Resins systems are soluble only in special costly solvents, require selective pigments, have limited pot life and may contain free isocynate.

2.0 Types

we manufacture ‘U’ varnishes in 3 types, the broad differences between them are as follows:

Silac U 210: Very rapid drying, flexible film but film yellows slightly with time.

Silac U 220: Rapid drying, flexible film which is almost non-yellowing.

Silac U 230: Modified with Alkyd, medium-drying time, and better non-yellowing properties.

3.0 Properties

The films of ‘U’ varnishes exhibit the following properties, which are superior to those of conventional media.

3.1 Almost colourless, hence suitable for all shades and colours including pure whites.

3.2 Very rapid rate of initial drying even at low ambient temperature.

3.3 Thorough drying.

3.4 No special solvents required, conventional solvent eg: MTO is adequate.

3.5 Can be processed in the manner similar to those for Alkyds with which all manufacturers are familiar.

3.6 Inspite of (3.5) they have the advantages associated with Polyurethane compounds.

3.7 Excellent abrasion resistance and scratch hardness.

3.8 Excellent resistance to chemicals and water.

3.9 Better resistance to most of the solvents.

3.10 Presence of free NCO is negligible and is completely non hazardous and absolutely safe for handling even by unskilled workers.

3.11 Can withstand temperature from +150°C to -25°C

3.12 Very tough film.

3.13 Dust repellent and antifungal

3.14 Excellent salt spray resistance

3.15 Extremely glossy finish.

3.16 Ability to cure fast at very low oven temperatures such as 2 mts at 140°C, leading to higher rate of production and low energy consumption.

4.0 Compatibility

‘U’ Varnishes (Silac U 210, U 220 and U 230) are compatible with all drying oils, long and medium oil alkyds, epoxies, estergums, phenolics and maleics in all proportions and only to a limited extent in chlorinated resins, short oil alkyds and urea resins.

5.0 Solubility

Silac U 210, U 220 and U 230 are soluble in Mineral Turpentine Oil (MTO) in all proportions. They are also soluble in most of the common paint solvents like Xylene, Napthas, Esters and Ketones. Wherever the ‘U’ varnishes have to be used with other resins of limited solubility in MTO like Epoxies & Vinyls etc., other suitable solvents like Toluene, Xylene or Ethyl methyl ketone should be used.

6.0 Recommended Driers

Since ‘U’ varnishes are very rapid drying, comparatively smaller quantity of Cobalt drier i.e., 0.03% to 0.05% of Cobalt as metal on the solid resin content will be adequate. However, if faster drying is required driers upto 0.4%, Zirconium and 0.1% Cobalt as metals on the resin contents can be added. After addition of driers ageing for 24 hours in airtight containers is recommended.

6.1 The oil modified urethane coatings dry (or cure) by the usual oxidation process as applicable to drying Alkyds and unsaturated oils and hence requires conventional driers.

While the other types of urethanes such as moisture cured, blocked type or the two pack systems cure by the reaction of Isocyanate groups with Hydrogen and thus require special type of drier chemicals.

7.0 Mode of Supply

The ‘U’ varnishes are supplied in 200 or 20 Ltrs packings as 45% solids.

8.0 Specifications

Product Colour Specific Gravity Viscosity* Seconds in Ford (B) 4 Drying Time** Scratch Hardness after 48 hrs. (gms) Acid Value Resistance Surface Dry Hard Dry At 140oC Water 5% Sulphuric Acid 2% Sodium hydroxide Solvent Silac U-210 Very light Pale 0.8 ± 0.02 120 to 159 5 mts. 15 mts. 2 mts. > 1500 <4 Excellent Excellent Excellent Excellent Silac U-220 Water White 0.8 ± 0.02 75 to 100 10 mts. 25 mts. 2 mts. > 1500 <4 Excellent Excellent Excellent Excellent Silac U-230 Water White 0.08 ± 0.02 200 to 250 20 mts. 1½ hrs 5 mts. > 1200 <4 Very good Very good Very good Excellent

9.0 Comparative Film Properties

‘U’ Varnishes represent the latest high technology development in the surface coatings field. They have made possible the manufacture of superior grade enamels, which have made them very popular abroad both for decorative and industrial applications.

Their performance is usually compared with Alkyds, which are widely used by the Paint Industry.

The following table presents the comparative advantages and disadvantages of ‘U’ Varnishes with Alkyd Resin Varnishes, Cardphenol based styrenated phenolic resin is also included as a reference point being our very popular resin. The comparison is made between the films of varnishes of the above with 30% solids and MTO as solvent.

Properties Alkyd Silex 55 (65%) Silac U 210 Silac U 220 Silac U 230 Drying Properties Fair Good Excellent Excellent Very good Scratch Hardness Poor Good Excellent Excellent Very good Abrasion resistance Poor Good Excellent Excellent Very good Outdoor durability Very good Poor Fair Fair Good Water resistance Poor Very good Excellent Excellent Very good Chemical resistance Poor Very good Excellent Excellent Very good Solvent resistance Poor Very good Excellent Excellent Very good Colour retention Excellent Poor Fair Good Very good Colour Water White Golden yellow Very light pale colour Water white Water white Brushing property Excellent Very good Excellent Excellent Excellent

Excellent > Very good > Good > Fair > Poor

10.0 Superiority of Oil Modified Polyurethanes Over Other PU Systems

It can be seen from the following table that all other P.U. Systems require special costly thinners, special techniques for pigmentation, costly driers and specially trained labour for handling paints based on them as they contain free N.C.O. They also have limited pot life. The only advantage which they have over oil modified P.U. Systems is better hardness of the film and better chemical resistance in some cases.

Comparison of properties of different types of polyurethanes

 

S.No. Type Curling Process Free NCO Potlife Pigmentation Solvent Drying time Scratch hardness gms. Resistance to water Resistance to chemicals Resistance to weather Cost 1 Single pack system: a) Oil Modified, eg. Silac U 210 + U 220 In air Nil Unlimited Conventional Mineral Turpentine Fast drying 15 to 20 min. 1500 Excellent Excellent Fair Moderate b) Moisture cured Moisture Yes Limited Special techniques Special thinners Slower 2000 Excellent Excellent Fair Costly c) Blocked Heat Blocked Limited Special techniques Special thinners On heating 2500 Excellent Excellent - Costly 2 Two pack system: 1) Pre polymer/catalyst Catalyst Yes Limited Special techniques Special thinners Fast 2500 Excellent Excellent Good Very costly 2) PolyIsocyanate/ polyol NCo/OH/chemical/reaction Yes Limited Special techniques Special thinners Fast/slow 2500 Excellent Excellent Good Very costly

11.0 Applications

The unique and excellent film forming properties of ‘U’ Varnishes have made possible their use in a very wide range of applications. Some of the areas where they have shown considerable promise are:

Water Proofing of Cement structures *** Fast dry super enamels *** Floor varnishes ** Fast baking enamels *** Traffic Paints ** Marine industry * Furniture finishes *** Under water paints and bath room wall paints ** Leather industry *** Printing inks ** Rubber and FRP *** Primers and undercoats ** Maintenance Coats ** Insulating varnishes ** Chemical process Industry *** Medium for aluminum paints ** Wooden Bobbins ***   *** Very Highly Suitable ** Highly Suitable ** Suitable

12.0 Cost Advantages

The initial comparatively higher cost of the ‘U’ Varnishes is offset many times by their performance advantages. The better protection which they provide against corrosion and abrasion etc., reduces the maintenance effort. Moreover the faster drying time and lower curing temperature reduces the application costs and increases the production rate. All these factors mean economy and lower costs.

The ‘U’ Varnishes have proven their worth the world over and using them for coating costly equipment such as aeroplanes, railway coaches, chemical tankers, concrete structures, ships etc., has become a standard practice.

13.0 Typical Formulations

The following compositions can be regarded as suggestive only to illustrate the application of these very versatile and high tech ‘U’ Varnishes.

1. Anti corrosive primer for under water structures: (Parts by weight)

 

Basic Lead Sulphate 200.0 Aluminium Powder (66%) 100.0 Iron Oxide 100.0 Barytes 100.0 PU Medium (45%) 655.0 6% Cobalt Octoate 2.0 24% Lead Octoate 2.5

2. Zinc Chromate-Iron Oxide Primer: (IS-2074-1962) (Parts by weight)

 

Zinc Chromate 85.0 Talc 45.0 Iron Oxide red 300.0 PU Medium (45%) Solids 250.0 6% Cobalt Octoate 1.25 24% Lead Octoate 2.0

3. High gloss Enamels-Super White (Parts by weight)

 

Titanium dioxide 325.0 Barytes (Super White) 25.0 PU Medium (45% Solids) 660.0 Violet toner 1.0 24% Lead Octoate 4.0 6% Cobalt Octoate 1.8 Anti skinning agent 0.7

Procedure: Charge the Ball mill with the above ingredients except the antiskinning agent and grind them together for 12 hours to get a fineness of 8 on Hegman gauge (0.002^H). Discharge the material. Wash the mill with 80 Parts MTO, add the antiskinning agent and finally adjust the viscosity in Ford cup No.4 for 60 seconds with mineral turpentine.

4. One pack Aluminum paint: (Parts by weight)

Aluminium paste (66% leafing) 180.0 PU Medium (45% Solids) 600.0 Solvent Naptha 100.0 6% Cobalt Octoate 1.5

Procedure: Disperse the Aluminium paste in PU Medium using a high speed disperser. Add the driers and finally adjust the viscosity with thinner to 60 seconds in ford cup No.4

5. High Gloss Enamel-Dark smoke grey (Parts by weight)

 

Titanium Dioxide (Rutile) 476.0 Talc 47.0 Carbon black 8.0 Ultramarine blue 12.0 Middle chrome 9.0 PU Medium (45% Solids) 168.0 6% Cobalt octoate 4.5 24% Lead octoate 6.0 Antiskinning agent 2.0 Procedure as in 1,2 and 3

6. Wood furniture varnish: (Parts of weight)

 

PU clear lacquer 100 gms ½ Sec. Nitro cellulose (30%)   10 gms Toluene     5 gms Butyl acetate (or) MIBK     5 gms

14.0 Technical Service

The Service of our well equipped Application Laboratory and technically qualified staff are at your disposal further technical information and suggestions in regard to the applications of these varnishes.

Caution: Because we can not anticipate the conditions under which our product may be used, we do not guarantee the applicability or the accuracy of the above information. Users should make their own tests to determine the suitability of our products for their particular purpose. The products are sold without any such warranty either express or implied. Moreover the above information is not intended as recommendation to use our product in the infringement of any patent.