Varnishes. Components, Classification, Applications and Literature

Index

1 Varnishes: An Introduction

2 Components ofVarnishes

3 Classification of Varnishes

4 Literature About Varnishes

5 Applications ofVarnishes

6 Research Work About Varnishes

Varnishes -An Introduction

Wood is valuable for structural purpose and decorative purpose also. Wood has plant origin. The wood used for building construction is known as Timber. Forests produce a huge quantity of timber. Cellulose, Hemicellulose, Lignin and other substances are the constituents of wood. Aliphatic compounds, phenols, fats, waxes, terpenes, terpenoids etc. are found in woods. Stilbenes, Tannins, Flavonoids and Lignanas are phenolic compounds available in woods.

Woods are of different types:

(1) Hardwood
(2) Softwood
(3) Heartwood
(4) Sapwood
(5) Springwood
(6) Summerwood.

Coating of varnishes is useful for wood, metal and their derivatives. It is also useful for plastic and masonry.

Varnish is a formulation of resinous matter, as copal or lac, dissolved in oilor in alcoholor any other liquid. When a varnish is applied to the surface of wood or metal, it dries and leaves a hard and generally transparent coating. Itis a shiny coating which is applied on a floor or furniture. It dries clear and luminous. It gives a hard, lustrous and transparent finish to the surface.

It gives furniture, wooden materials, artwork and other objects a glossy look. It also protects the surface. Usually varnish is prepared from resin, solvent and oil. The Medieval Latin root word for varnish is ‘ vernix’ that means odorous or fragrant resin.

Some varnishes harden immediately as the solvent evaporates. So, it produces a film. The other varnishes harden slowly over a period of time. The process of hardening involves oxidation and polymerization.

Shellac, Resin and Lacquer dry immediately. Acrylic and some of the water based varnishes evaporate the water or solvent and dry over a period of time. It is also known as curing process of varnishes. Oil based, polyurethane and epoxy varnishes also dry slowly to hard finish.

Generally, the factors like following affect curing process of a varnish:

- Temperature
- Humidity of atmosphere
- Components of varnish.

Wood kept outside is normally exposed to big variations of temperatures and weather. It is also exposed to Ultra Violet(UV) rays in the atmosphere. Changes in humidity also affect the wood which results in contraction or expansion of wood. These all factors damage the wood or wooden articles. Hence, varnishes can be used to protect wood and wooden articles.

Generally, a varnish contains Linseed Oil or Tung Oil, resins like phenolic, polyurethane or alkyd and solvents like naphtha, mineral spirit or thinner. In preparation of a varnish, the amount of oil with respect to resin used affects the applicability of the varnish. Less amount of oil but a large amount of a resin in a varnish may result in a hard and brittle finish. Besides this, a drying agent and additives also serve as components of a varnish.

Paints and Varnishes

Paints and varnishes - both are used to protect and decorate the surfaces. Both give aesthetic look to life. But, there are also obvious differences between paints and varnishes. A paint normally contains a pigment while a varnish usually contains a resin. In a paint, pigment is dispersed in a vehicle while in a varnish, a resin is dispersed in oil or thinner. In most of cases, the film produced by a paint is opaque whereas varnish produces a transparent film.

Characteristics of a Good Varnish

A good varnish should create uniform appearance on a surface. When a varnish dries, it should create a hard film. The process of drying should be rapid. The coating generated by a varnish should be long-lasting. A good varnish should not crack after drying process. Various factors like moisture content of wooden surface, ambient temperature, radiation exposure, etc. affect the stability of varnishes. Abdullah Sonmez et al. worked on finding the effect of wood moisture on adhesion process of varnish coatings. In their work, they prepared samples from Eastern beech (Fagus orientalis L.), Scots pine (Pinus sylvestris L.) and oak (Quercus petraea L.). The samples were coated with cellulosic(nitrocellulose), two-part polyurethane (urethane alkyd) and waterborne(polyurethane) varnishes. They found that the moisture content had significant effect on the adhesion of varnishes, applied on wooden surfaces[1].

General Uses of Varnish

Varnishes can be used to protect wooden surfaces like windows, doors, floors and roof trusses from atmosphere. Varnishes may be applied as a topcoat on worn finishes. Varnishes can be used to brighten the look of natural grains in wood.

Newly Developed Varnishes

Some newly developed varnishes are also reported. Their properties are compared with traditional varnishes and tried to find out the better option. B. Kaygin and E. Akgun reported comparison of cellulosic, polyurethane, polyester, synthetic and nanolacke ultraviolet varnishes on beech(Fagus orientalis L.) and oak(Quercus robur L.) wood samples[2].

A study about the preparation of oleo-resinous wood varnishes by utilizing safflower(Carthamus tinctorius) seed oil is published by M. Rafiquzzaman et al. The varnish reported was water resistant and could be applied on wood surface as tack free coating[3].

New varnishes are developed with following properties:

- Abrasion resistance

- Scratch resistance
- Moisture resistance
- Chemical resistance
- Hardness.

Nanotechnology is also applied in manufacturing new varnishes. Besides wooden surfaces, varnishes have been developed by researchers and various companies for paper, plastic and metal surfaces.

References

[1] Abdullah Sonmez, Mehmet Budakci, Mehmet Bayram, Effect of Wood Moisture Content on Adhesion of Varnish Coatings, Scientific Research and Essay, December 2009, 4(12), 1432-1437.

[2] B. Kaygin, E. Akgun, A Nano-Technological Product: An Innovative Varnish Type for Wooden Surfaces, Scientific Research and Essay, January 2009, 4(1), 1-7.

[3] M. Rafiquzzaman, M. S. Rahman, M. A. Hossain, A.J.M. Moynul Hasan, Studies on the Preparation of Oleo-Resinous Wood Varnishes by Utilizing Safflower(Carthamus Tinctorius) Seed Oil, Bangladesh Journal of Scientific and Industrial Research, 2006, 41(1-2), 77-82.

2. Components of Varnishes

Components present in a varnish obviously affect the properties like brightness, toughness and fastness(surface sticking resistance) of the varnish. Murat Ozalp studied the effect of pentahydrate added into a varnish[1]. In that study, cellulose bright varnish was applied to upper surfaces of Austrian pine and beech wood to assess toughness, brightness and surface sticking resistance of the varnish. Determining the resistance of sticking of the varnish, the following formula was used:

Abbildung in dieser Leseprobe nicht enthalten

where, X = Sticking resistance,

F = Splitting force,

d = radius of a sample cylinder(mm).

The major components of a varnish are as follows:

- Resins

- Solvent and Thinner

- Drying oil

- Metallic dryers.

Resins

A resin is considered as a major component in many varnishes. Resins are natural or manmade(synthetic) compounds.

It may be a viscous liquid or non-crystalline substance.

Natural Resins

Natural resins are obtained from plant secretions. They can be obtained from trees of pine and firs. They are discharged from these kinds of trees. Plants produce resins either naturally or in reaction to certain injuries.

Copal, shellac, mastic, gum, dammar, amber, sandarac, balsam, etc. are resins.

Rosin is a natural resin. It is derived from long leaf pine trees.

Amber is a hard natural resin. It is a fossil resin. It is also known as Resinite. It is obtained from coniferous and the other trees.

Copal may also be found as sub fossil resin. It becomes tacky if a drop of chloroform(CHCl3) or acetone(CH3COCH3) is added on it[2].

Mastic is also a natural resin obtained from mastic tree. It is produced in droplets. So, it is known as ‘ tears of Chios’ in Greece(Chios are Greek islands).

Natural resins are normally insoluble in water. They become soluble in organic solvents like alcohol and ether. They have typical odour. They have hydrocarbon material. They can be classified into following classes:

(1) Gum Resins

Resin of Boswelia serrate and Gukkulu of Commiphora mukul are gum resins.

(2) Hard Resins

Hard resins do not have typical odour. Copal, Dammar, and Shellac are hard resins. Shellac is obtained from Butea monosperma. Plants of Dipterocarpaceae and Bursaceae family give Dammar. Plants of Caesalpinaceae family give copal.

(3) Oleo Resins

These are soft resins. Balsam, Benzoin and Turpentine are oleo resins.

Synthetic Resins

Synthetic resins have properties similar or different from natural resins. They are of two types:

(1) Thermoplastic Resins

These resins are also called thermosoftening resins which remain plastic after heating. Thermoplasticresinis apolymerwhich becomes soft or fluid when heated and then gains its original solid phase when it is cooled.

(2) Thermosetting Resins

These resins become infusible after heating. Itis aprepolymer existingin a soft solid or viscous phase which varies irretrievably into an infusible polymer by curing process.

Solvent and Thinner

Linseed oil, walnut oil, turpentine, wood naphtha, methylated spirit of wine, denatured alcohol, Methyl ethyl ketone, etc. are used as solvents in varnishes. Linseed oil, walnut oil and tung oil contain polyunsaturated fatty acids. These solvents can thin a varnish, so, they also play a role of thinners.

Turpentine

It has very strong odour.

It is useful in paints and varnish applications.

Earlier the mid-20th century, it was widely used as a solvent for paints and varnishes. It is also used as a thinner. It is a distillate of pine tree sap. The petroleum solvents having cheaper and less odour properties have replaced turpentine.

Many substitutes of turpentine are available in the industry. Terpenoids, mineral spirit etc. are good substitutes for it.

Petroleum Distillates

Petroleum distillates include substances like mineral spirits naphtha, toluene and xylene.

They are used for varnishes such as polyurethane varnishes. They are also used for cleaning brushes.

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