Polyvinyl
alcohol is an excellent water-based adhesive property. Generally
speaking, the product is in the form of flakes, granules or flocs.
It is not easily dissolved in cold water, when it is dissolved,
heating is generally required. This property greatly limits its
application range. However, if subjecting it to fine pulverization
and surface treatment, it will have a good cold water solubility.
But due to its thermoplasticity, it is difficult to be in the
pulverization
process.
The vinyl acetate is generated by the
polymerization of vinyl acetate and then the polyvinyl alcohol with
different degree of alcoholysis can be gained through the polyvinyl
acetate in different alkali amount. The cold-soluble polyvinyl
alcohol powder a water-soluble environment-friendly white solid
powder prepared by deep-cooling milling of a partially
alcoho-decomposing polyvinyl
alcohol.
It has good film forming properties and
adhesion, solvent resistance, abrasion resistance and tensile
strength.
The
particles are fine, the amount of addition is small, and it is easy
to disperse. It can be dissolved in cold water. Providing good
bonding properties and increasing mortar strength. Prevention
cracking and detachment of mortar, increasing the adhesion strength
and
smoothness.This
product is non-toxic, tasteless and environmental
friendly.
PVA is
used in a variety of medical applications because of its
biocompatibility, low tendency for protein adhesion, and low
toxicity. Specific uses include cartilage
replacements, contact
lenses, and eye
drops.[5] Polyvinyl
alcohol is used as an aid in suspension polymerizations. Its
largest application in China is its use as a protective colloid to
make polyvinyl
acetate dispersions. In Japan its major use is the
production of Vinylon fiber.This fiber is
also manufactured in North Korea for
self-sufficiency reasons, because no oil is required to produce
it. Another application is photographic film.
Nowadays PVA-based polymers are being
used widely in additive manufacturing. For example, 3D printed oral
dosage forms demonstrate great potential in the pharmaceutical
industry. It is possible to create drug-loaded tablets with
modified drug-release characteristics where PVA is used as a binder
substance. Medically,
it may also used as the embolic agent in a Uterine Fibroid
Embolectomy (UFE)
Polyvinyl acetals are prepared by
treating PVA with aldehydes. Butyraldehyde and formaldehyde afford polyvinyl
butyral (PVB) and polyvinyl formal (PVF),
respectively. Preparation of polyvinyl butyral is the largest use
for polyvinyl alcohol in the U.S. and Western
Europe. Preparation
Unlike
most vinyl
polymers, PVA is not prepared by polymerization of the
corresponding monomer as the
monomer, vinyl
alcohol, is thermodynamically unstable with respect to
its tautomer acetaldehyde. Instead, PVA is
prepared by hydrolysis of polyvinyl acetate,[3] or
sometimes other vinyl ester-derived polymers with formate or
chloroacetate groups instead of acetate. The conversion of the
polyvinyl esters is usually conducted by
base-catalysed transesterification with
ethanol:
[CH2CH(OAc)]n +
C2H5OH
→ [CH2CH(OH)]n +
C2H5OAcThe
properties of the polymer are affected by the degree of
transesterification. Worldwide consumption of polyvinyl alcohol was
over one million metric tons in ***6.Large producers
include Kuraray (Japan, Europe, and
US) and Sekisui Specialty Chemicals (US), while mainland China has
installed a number of very large production facilities in the past
decade and currently accounts for *5% of world
capacity.
Structure and
properties
PVA is
an atactic material that
exhibits crystallinity. In terms of
microstructure, it is composed mainly of 1,*-diol linkages
[-CH2-CH(OH)-CH2-CH(OH)-]
but a few percent of 1,*-diols [-CH2-CH(OH)-CH(OH)-CH2-]
occur, depending on the conditions for the polymerization of the
vinyl ester precursor.
Polyvinyl alcohol has excellent film
forming, emulsifying and adhesive properties.
It is also resistant to oil, grease and solvents. It has high tensile
strength and flexibility, as well as high oxygen and aroma barrier
properties. However, these properties are dependent
on humidity:
water absorbed at higher humidity levels acts as a plasticiser,
which reduces the polymer's tensile strength, but increases its
elongation and tear strength.
Safety
and environmental
considerations
PVA is
widely used, thus its toxicity and biodegradation are of interest.
Solutions containing up to 5% PVA are nontoxic to fish. It
biodegrades
slowly。