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HPHP- A Component for Enviornmentally Advanced 2 fd-2 r:
DFCoatings
Don Morris
Texas Eastman Division, P.O. Box 7444 Longview, Texas 75607
The coating’s industry is undergoing a dramatic shift from
solvent based paints to technologies that are more
environmentally acceptable. Much of this change is due to
changing regulations on emissions and waste disposal.
Technologies such as water-borne, high-solids, and powder
coatings help the coatings industry meet the regulations
and can offer increased performance over solvent based
coatings. These technologies are often called Compliant
Coatings because they meet or exceed EPA guidlines for
coatings emissions.
Markets for powder coatings are growing because of the
high quality of the resulting coatings and the increased
emphasis on reducing solvent emissions from commercial
paint lines. Powder coatings markets have grown 10-15
percent per year since 1981 and are now used to coat
metal products such as patio furniture, bicycles, lawn
mower housings, many automobile components, and kitchen
and laundry appliances. Most powder coatings systems are
composed of a polyester resin and a crosslinking agent
which react chemically to form the final coating. This
crosslinking occurs at elevated temperatures after the
powder has melted and flowed out into a smooth film.
Compared to solvent borne paint systems, powder
coatings offer reductions in the emissions of organic
solvents to the environment. Typically 25 to 50% of the
weight of a solvent borne paint is solvent which
evaporates once the paint is applied to the substrate.
Powder coatings contain no solvent and produce less than
0.1 pounds volatile organic compounds per gallon (VOC)
during application. This is considerably below the EPA
control VOC of 2.8 pounds per gallon for coating metal
surf aces.
Powder coating resin manufactures must now rely on a
fairly limited number of diols and diacids to improve the
performance of polyester resins. Neopentyl glycol ( N P G
Glycol*) is an important diol in the preparation of many
types resins because its chemical structure provides the
weatherability and chemical resistance needed to make
high performance coatings. NPG is of particular value in
polyester powder coating resins because, in addition to its
excellent performance, it results in resins with the desired
glass transition temperature (Tg) and melt viscosity. The
powder coating resin must have the hardness to be ground
into a powder (friability) but must melt in a fairly narrow
temperature range to provide controlled flow as the
polyester is crosslinked. The Tg of the resin must be high
enough that the resin does not agglomerate during
storage, that is it must remain free flowing so it can be
applied easily. The melt viscosity controls how the
polymer flows as it is melted.
The surface appearance of the coatings is greatly affected
by the melt viscosity and Tg of the polyester resin.
Smoothness, as measured by gloss and distinctness of
image, is closely related to flow of the polymer as it is
cured. Powder coatings can be used in additional
applications if the smoothness is enhanced. A large
potential market in automotive coatings could be served if
the appearance of powder coatings can be made to match
solvent based automotive coatings.
)
.
A glass transiton temperature of between 50-8OoC is
desired for the resin in order that the powder will be
stable during storage and will flow correctly when melted.
The Tg and melt viscosity of a powder resin is dependent
on the monomers used to make the polyester resin. The
resin is the largest component of the powder and provides
the mechanical strength and overall performance for the
resin. The resins are made by condensation of mixtures of
glycols, polyols, diacids and/or polyacids to produce ester
linkages by the elimination of water. Most polyester
powder coating resins use neopentyl glycol as the major
glycol component in combination with terephthalic acid as
the major diacid to provide a tough weatherable coating.
Improvements in the performance and appearance of the
finished powder coating can be attained by using modifying
monomers in the resin formulation during condensation
step. The modifing monomer is used to change the Tg and
melt viscosity of the polyester resin in order to enhance
desired physical properties such as smoothness, UV
stability, hydrolysis resistance, impact strength and
flexibility. Larry Johnson and coworkers of Eastman
Chemicals summarized the effects of several new
polyester intermediates on resin properties in a paper
given at the Eighteenth Water-borne, Higher-solids, and
Powder Coatings Symposium in New Orleans in February,
1991.
Hydroxypivalyl hydroxypivalate (HPHP) is a monomer
closely related to neopentyl glycol that is very useful in
modifying the properties of polyester powder coating
resins. This glycol is essentially two neopentyl glycol
molecules joined through an ester linkage.
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The major change observed when substituting HPHP for
neopentyl glycol is a lowering of the Tg and melt viscosity
as seen in tables 1-3. The resin manufacture substitutes
HPHP for a portion of the neopentyl glycol to control the Tg
and melt viscosity of the powder resin. Adjusting the Tg
and melt viscosity allows the storage stability (resistance
to agglomeration) to be balanced with the flow out of the
resin as it cures. The appearance of coatings containing
HPHP may be enhanced because of improved gloss and
fewer surface defects and the performance of the coating
may be improved becaused of greater flexibility and
resistance to hydrolysis. Other glycols can be used to
lower the Tg and melt viscosity of polyester resins,
however, HPHP results in smoother more attractive and
more durable coatings while maintaining storage stability
of the powder.
Substituting HPHP for neopentyl glycol increases the
aliphatic character of the polymer because of the
increased molecular weight of HPHP as compared to NPG.
This increase in molecular weight means that less
terephthalic acid, for example, is used in each pound of
resin. Decreasing the amount of aromatic character of a
resin can increase the UV stability of the resin.
The decreasing aromatic charcacter can also result in
coatings with increased flexibility because rigid
terephthalic acid-NPG links are replaced with more
flexible HPHP links. The structures of both HPHP and NPG
contain hydroxyl groups that are protected by two methyl
groups on the next carbon atom. The ester linkage in HPHP
is surrounded by four methyl groups. These protected
hydroxyl groups and the protected ester group enhance
the chemical stability of polyesters made with typical
diacids such as terephtalic acid. The hydrolytic stability of
the resulting polyester is increased dramatically versus
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unprotected ester linkages. This makes coatings made
with the resins more detergent and acid rain resistant.
Acid catalyzed cleavage of the ester bonds in the
polyester result in loss of polymer strength and eventual
failure of the coating without the protected ester linkages.
Eastman Chemicals Company is a manufacturer of a number
of diols and diacids that are used in resin synthesis.
Neopentyl glycol and HPHP are produced at the Texas
Eastman Division in Longview, Texas. Neopentyl glycol has
been manufactured since 1962. Eastman was the first
manufacturer in the world of this key coatings
intermediate. HPHP production at Texas Eastman began in
1991. HPHP is synthesized in a multipurpose manufacturing
plant that was designed to provide for very low emissions.
By-products from HPHP manufacture are processed in the
neopentyl glycol plant where they are recovered as
neopentyl glycol and isobutanol. A single high boiling
stream from the HPHP plant is incinerated. The chemistry
of HPHP manufacture is similar to neopentyl glycol
manufacture in that both products are made by the aldol
condensation of isobutyraldehyde and formaldehyde.
Neopentyl glycol is made by hydrogenating this aldol
product and HPHP is made by condensing two molecules of
the aldol product in a Tischenko reaction that produces the
ester linkage. Eastman Chemical Company is committed to
providing the coatings industry with intermediates that
can be utlilized to improve coatings performance and
environmental acceptability.
NPG is a trademark of Eastman Kodak Company
Table 1
The E f f e c t s of S u b s t i t u t i n g HPHP f o r NPGGlycol i n
Hydroxyl-Functional Powder Coating Resins
IC1 Visc.
0
NG
NG/HPHP
HPHP
To C
'
Mn
(71700~1.E
-
54
2426
30
4/1
49
2771
22
-
26
2885
4
Table 2
The E f f e c t s of S u b s t i t u t i n g HPHP f o r NPG-Glycol i n
Carboxyl-Functional Powder Coating R e s i n s
IC1 V i s c .
Ta.OC
Mn
(20OOC).
-
59
1979
12
NG/HPHP
3/2
50
1948
11
NG/HPHP
2/3
43
2005
9
-
36
1907
7
GLycols
NG
HPHP
Ratio
E
Table 3
The E f f e c t s of S u b s t i t u t i n g HPHP f o r NPG-Glycol i n
Carboxyl-Functional Powder Coating Resins
IC1 Visc.
CHDM/NG
4/1
68
1762
>loo
CHDM/HPHP
4/1
56
1599
71