Issue 28/2013 - Research progress in EVOH resin production

Research progress on EVOH resin production

□ Cui Xiaoming

EVOH resin, also known as ethylene vinyl alcohol copolymer abbreviation (Ethylene Vinyl Alcohol, also known as EVAL), is a random copolymer obtained by saponification reaction or partial saponification reaction of ethylene vinyl acetate copolymer, the proportion of which is usually 20% ~ 40% ethylene, 60% ~ 80% vinyl alcohol, can be regarded as PVA (polyvinyl alcohol) modification, its production process is similar
to PVA 。 It is called the three major barrier materials together with polyvinylidene chloride (PVDC) and polyamide (PA), which is a new type of polymer synthetic material
that integrates the good processing performance of ethylene polymer and the high gas barrier performance of vinyl alcohol polymer.
The melt mass flow rate of EVOH resin is 0.
7~20g/10min, the density is 1.
13~1.
21g/cm3, the melting point is 158~189 °C, and the tensile breaking strength is 44 MPa
.
Because EVOH packaging materials have the advantages of high performance, low cost and low pollution, they have a wide range of applications in packaging materials, medical materials, textile materials, structural materials and polyethylene modifiers, and have broad
prospects for development and utilization.

In the 50s of the 20th century, DuPont of the United States obtained EVOH resin
by copolymerization and alcohololysis of ethylene and vinyl acetate.
In 1972, Japan's Kuraray company successfully developed EVOH film on the basis of PVA process technology, thus realizing the first industrial production
of EVOH resin.

Since vinyl alcohol monomer does not exist stably, it is usually prepared by alcohololysis reaction of ethylene-vinyl acetate (EVA) copolymer
.
The preparation of EVOH resin is carried out in two steps, first ethylene-vinyl acetate copolymer is prepared by polymerization, and then ethylene-vinyl alcohol copolymer is prepared by alcohololysis (or saponification) reaction
.

Polymerization EVA can generally be obtained
by solution polymerization, suspension polymerization, bulk polymerization and emulsion polymerization
.
EVA used to manufacture EVOH resin with high barrier properties is usually obtained by solution polymerization because the solution method is relatively easy to control the composition, regularity, branching, polymerization degree and molecular weight distribution
of the copolymer.
The solvent can be an aromatic or aliphatic alcohol solvent
.
tert-Butanol is the best solvent because it reacts quickly and has less solvent chain transfer, allowing high molecular weight EVOH copolymers to be obtained, and does not generate aldehydes or ethers
like methanol.

The alcohols used in saponification saponification
reactions can use primary alcohols, secondary alcohols, tertiary alcohols, and usually methanol
.
Caustic soda
is commonly used in catalysts.
At present, saponification methods often include homogeneous saponification method, water dispersion saponification method, melt saponification method and heterogeneous saponification method
.
The saponification method prepared by EVOH resin with high barrier can be divided into two categories: homogeneous saponification and heterogeneous saponification according to the saponification process of EVA.
According to the operation method, it can be divided into two categories
: continuous saponification and batch saponification.

Homogeneous saponification method The main feature of the homogeneous saponification method is to first dissolve EVA in a reaction medium, which is usually a low-level linear fatty alcohol, the most commonly used is methanol; Then an alcohol solution of alkali as a catalyst is added to the EVA solution and saponified
at a certain temperature.
Generally, in order to achieve a saponification degree of more than 99%, the EVA resin saponified in alcohol still needs to be saponified in an aqueous solution of alkali at a higher temperature for secondary saponification, and finally the saponification product is repeatedly washed to remove impurities
such as sodium acetate and methyl acetate contained therein.
The advantage of homogeneous saponification method is that the reaction process is easy to control and the equipment is simple, but it requires the use of a large number of alcohol solvents, and the final saponification product contains a large amount of impurities such as sodium acetate, which needs to be washed
with a large amount of water.
In addition, because the saponification of EVA in alcohol is an equilibrium reaction, in order to achieve more than 99% saponification, it is generally necessary to saponify the second stage in aqueous solution after the first saponification, and the saponification process is long
.

Heterogeneous saponification method The main feature of the heterogeneous saponification method is that EVA constitutes a suspension system in the form of particles or powders in a non-solvent, or in the form of molten EVA droplets in a non-solvent
.
The advantage of this method is that the particle size of the saponification product EVOH can be controlled, eliminating the granulation process
of EVOH.
However, the heterogeneous reaction is limited by the diffusion process of the reaction reagent to the EVA particles, and the saponification reaction is generally required at a higher pressure and temperature to achieve a high degree of saponification, so this method has high
equipment requirements.

Continuous saponification method This method is characterized by continuous saponification
in a tower reactor.
In this method, the tower as a saponification reactor is generally required to be a reactor with a plate with a long residence time
.
The methanol solution of EVA is added from the upper part of the tower, and the sodium hydroxide methanol solution as a catalyst is added from the middle of the tower, and methanol vapor is continuously blown in at the bottom of the tower, so that the by-product of the EVA saponification reaction in the tower, methyl acetate, etc.
is continuously brought out from the top of the tower to ensure that the saponification reaction is carried out completely and EVOH resin
with a saponification degree of 99.
5% is obtained.
The EVOH resin obtained at the bottom of the tower contains almost no impurities such as sodium acetate, and there is no need to repeatedly wash
the EVOH resin like the batch saponification method.
In this saponification tower, the general working pressure is 0.
05~1.
0MPa, and the temperature in the tower is usually 45~150 °C at the boiling point temperature of methanol, which has the advantage that the consumption quota and cost are relatively economical
.

Conclusion EVOH resin is widely used, the production process is similar to the production process of PVA, and the current
EVOH resin manufacturers in the world are leading PVA manufacturers
.
China has built a number of PVA production equipment, and the process technology is relatively mature, the future production capacity will also be excessive, but the production of EVOH resin is almost blank, therefore, in addition to accelerating the technical development of EVOH resin, and striving to achieve the industrial production of EVOH resin as soon as possible, it should also strengthen its application research to expand its application field and scope
.