Chinese Lacquer: Precursor of present coatings

The Chinese lacquer has been known for 1,000 years, we observed that resin (organic secretion produced by some plants) of trees, applied on a surface hardened with time, protecting the wood and other objects, it was the first natural polymer that was discovered.

Natural resin

This natural rubber had a high adhesive strength and a high gloss finish, making it very useful to preserve and beautify wood.

The natural resin extracted from different trees, treated, filtered and mixed with colouring (iron oxides) and applied to the wood, producing a layer that preserved from scratches and bumps.

These secretions, out of Asian subtropical plants such as Rhus Verniciflua with CAS 225234-38-8 and IUPAC name for chemicals Verniciflua Anacardiaceae Rhus.

Chineese LacquerThis polymer is a colloidal system consisting of water emulsion with an oil whose main component is the urushiol.

By oxidation of an enzyme, the hydrocarbons polymerize in the presence of moisture, obtaining the polymer.

The chemical formula of Urushiol is a mixture of phenols with different R in Para position. The chain has a length between 15 to17 carbons.


lacachina2 Urushiol
R = (CH2)14CH3
R = (CH2)7CH=CH(CH2)5CH3
R = (CH2)7CH=CHCH2CH=CH(CH2)2CH3
R = (CH2)7CH=CHCH2CH=CHCH=CHCH3
R = (CH2)7CH=CHCH2CH=CHCH2CH=CH2
Urushiol hydrocarbon has different R saturated and unsaturated chains (some conjugates). For this reason are polymerized by Diels Alder reactions.

Drying and later curing is by evaporation, obtaining the characteristic appearance of the lacquer: hard and shiny.

This resin would be the precursor natural of the current synthetic.

Main features of natural resin

  1. It is very adhesive
  2. It is moulded by pressure or temperature (type: thermoplastic at present).
  3. It is an excellent sealing and protecting wood.
  4. it has a high hardness.
  5. It is very tough at time.
  6. It can have brightness or not.

Chineese LacquerWith this natural resin developed a technique to embellish and highlight of the wood: The
Chinese Lacquer, we can get a protective coating and decoration, through a special product that gives transparency, texture and color to the wood.

In the eighteenth century, this natural resin is introduced in Europe by Filippo Bonanni, who wrote a treatise on the techniques of
Chinese Lacquer.

Polymerization mechanism

Looking at the current oils used in manufacturing coatings or modified resins, they are composed mainly of the followings fatty acids: oleic acid (one unsaturation), linoleic acid (twice unsaturated) and linolenic acid (three times unsaturated). The proportion composition depends on initial oil: Linseed oil, Castor, Soybean Rapeseed, Tung, etc

By heat, the present double bonds isolated double in the fatty acid chains start to move through the carbon chain, occurring the conjugation, if we add the oxidation enzyme is produced a Diels-Alder reaction, forming cyclohexene as link between two fatty acid chains.

Example of synthesis of Diels-Alder reaction between two fatty acids
dietaldereg 

This reaction is carried out in all natural oils with fatty acids: oleic, linoleic and linolenic

Depending on the length of the unsaturated carbon chains and the number double bonds, the polymerization process is conducted at faster or slower, depending on time and the speed of reaction, we can obtain a specific viscosity, very important for drying.
The number of intermolecular bonds determines the viscosity of the finished product.

Comparing the composition of natural oils with unsaturated fatty acids and Urushiol base resin (lacquer). We can say that Urushiol polymerizes by Diels Alder reaction, as unsaturated fatty acids.

Formulas of unsaturated for fatty acids Total carbon 
Oleic acid: HCOO-(CH2)7-CH=CH-(CH2)7-CH3 18
Linoleic acid: HCOO-(CH2)7-CH=CH-CH2-CH=CH-(CH2)4-CH3 18
Linolenic acid: HCOO-(CH2)7-CH=CH-CH2-CH=CH-CH2-CH=CH-CH2- CH3 18
 

 
Formulas of the remains in the position to the phenol Total carbon
R - (CH2)14CH3 15
R2 - (CH2)7-CH=CH-(CH2)5-CH3 15
R3 - (CH2)7-CH=CH-CH2-CH=CH-(CH2)2-CH3 15
R4 - (CH2)7-CH=CH-CH2-CH=CH-CH=CH- CH3 15
R5 - (CH2)7-CH=CH-CH2-CH=CH--CH2-CH=CH2 15
 
We can see that there are many similarities between the formulas of the fatty acid composition of the radicals R of phenol.

Developments in the field of coating
s

Chinese lacquer is an expensive product, due to its origin, at the beginning, it was used mainly to decorate furniture and ceramics.

Years later, the wood was protected by oils, as Linseed oil by its color, due to high content of linolenic acid.

With the arrival of teak furniture, it was development, one blend for this application: teak oil, it is a 50% mixture between tung oil containing elaeosteárico acid (C18: 3 conjugate) of 82% and Linseed oil containing linolenic acid (C18: 3) of 65%.

In this case the polymerization process is easier, due to the large amount of fatty acids with three conjugated bonds, which reduces the curing time and increases the hardness of the polymerization and the dark brown mixture, it was used for  for outdoor wood preservation.

The descendants of
Chinese Lacquer

Over the years, new findings of products, got to give high resistant to wood. In 1845, Christian Schönbein discovered the nitrocellulose used as protective coating cars in 1920. In 1937, Otto Bayer synthesized polyurerthanes used as coatings woods, metals. Later modified resins and the lasures in the 60's.

 
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