Discussion :: Building Materials - Section 1 (Q.No.42)
Varnish is a transparent or semi-transparent solution of resinuous substances in
Answer: Option D
No answer description available for this question.
|Anna said: (Apr 2, 2018)|
|What is the difference between varnish and paint?|
|Tdentaa said: (Sep 4, 2018)|
|The Right one is C.|
|Pranita said: (Mar 6, 2019)|
|Varnish is a preparation consisting of resinous matter, as copal or lac, dissolved in an oil (oil varnish) or in alcohol (spirit varnish) or other volatile liquid. When applied to the surface of the wood, metal, etc. It dries and leaves a hard, more or less glossy, usually transparent coating.|
|Azim said: (Nov 17, 2019)|
Main article: Drying oil.
There are many different types of drying oils, including linseed oil, tung oil, and walnut oil. These contain high levels of polyunsaturated fatty acids. Drying oils cure through an exothermic reaction between the polyunsaturated portion of the oil and oxygen from the air. Originally, the term "varnish" referred to finishes that were made entirely of resin dissolved in suitable solvents, either ethanol (alcohol) or turpentine. The advantage to finishes in previous centuries was that resin varnishes had a very rapid cure rate compared to oils; in most cases they are cured practically as soon as the solvent has fully evaporated. By contrast, untreated or "raw" oils may take weeks or months to cure, depending on ambient temperature and other environmental factors. In modern terms, "boiled" or partially polymerized drying oils with added siccatives or dryers (chemical catalysts) have cure times of less than 24 hours. However, certain non-toxic by-products of the curing process are emitted from the oil film even after it is dry to the touch and over a considerable period of time. It has long been a tradition to combine drying oils with resins to obtain favourable features of both substances.
Many different kinds of resins may be used to create a varnish. Natural resins used for varnish include amber, kauri gum, dammar, copal, rosin (pine resin) , sandarac, balsam, elemi, mastic, and shellac. Varnish may also be created from synthetic resins such as acrylic, alkyd, or polyurethane. A varnish formula might not contain any added resins at all since drying oils can produce a varnish effect by themselves.
Originally, turpentine or alcohol was used to dissolve the resin and thin the drying oils. The invention of petroleum distillates has led to turpentine substitutes such as white spirit, paint thinner, and mineral spirit. Modern synthetic varnishes may be formulated with water instead of hydrocarbon solvents.
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