Phillipe Deligny Edited by
II APPLICATIONS FOR POLYESTER RESINS IN SURFACE COATINGS
1
The first coil productio
used for coating steel shutters (50 mm width and 0.3 mm thickness). Their production rate was 12 m/minute and twelve hours were required in order to produce one ton of lacquered metal. Today, a modem production line can produce 500 tonnes of steel in 12 hours. The current uses of coil coating can be sub-divided as shown in Tabl
TABLE 3-1: CURRENT USES OF COIL COATING Utilisation (in 19
Area Thousand Area Thousand (mil m2) of tons (mil m2) of tons
Buildings 435 2025 125 183
Transport 21 99 23 33
Household appliances 49 227 4 6
Miscellaneous 34 159 4 6
Dealers 56 264 9 14
Various export 49 227 8 12
Total 644 3001 173 254
Applications include:
open ends for food and beverage, drawn cans for food.
However the use of polyester based coatings for coil applications for can coatings is,
r exposure and the very rapid and significant development in the physical properties f some vinyl alkyds. At this time, acrylics rapidly established themselves despite their very
teel whose zinc coating is replaced by a mixture of luminium - zinc. These different substrates are surface treated before being coated. These
od as those om other types of primer. However, their mechanical properties are far superior.
crylics and polyesters cross-linked with melamine formaldehyde resins or blocked
hose unsuitable for acrylic based oatings.
lexible coatings, which are abrasion resistant with good durability, can be formulated from
uch as claddings. The polyesters sed are generally based on isophthalic or terephthalic acid in order to have maximum
Buildings: metallic locks, roller shutters, cladding, roofing, window breasts, siding, awnings, blades for shutters
Internal applications: roof supports, partitions.
Transport industry: caravans, coaches.
Miscellaneous: casing for different appliances such as televisions, radio, camping equipment, switchgear, household appliances.
Food and beverage cans: easy
today, minimal. Epoxy and PVC based systems are the major coatings employed for can coil coatings.
Coil coatings have progressed over the years. At the beginning, all of the resin types present during this period were used. These included vinyl organosols, vinyl derivatives, alkyds, acrylics and epoxies. Failure soon became apparent, with loss of adhesion of vinyl organosols on exterio
o
pungent odour and very dense fumes. A significant improvement was introduced in 1968 with polyesters and silicone based acrylics. Siliconised polyesters soon followed this.
Coil substrates include galvanised steel and aluminium alloys. In 1980, galvalum was introduced which is a galvanized s
a
are, in particular bonded substrates (parker), alodine or even alcomet C which is deposited by a roller to give a homogeneous chemical conversion on the surface of the substrate.
The use of polyesters in coil depends on the use of the final coating. Polyester primers are used quite often in Europe although their anticorrosive properties are not as go
fr A
isocyanates are used in finishes. Acrylics were developed at an earlier stage for interior uses, where they are not exposed to water, chemicals or mechanical stresses. Polyester resins are suitable for a large number of applications, including t
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polyester resins with or without blocked polyisocyanates. The benefit of polyurethane crosslinking improves, once again, the flexibility of the coating.
Silicone modified polyesters give several years exterior resistance, enabling ten or more year guarantees to be offered on some building applications, s
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flexibility and exterior resistance. The use of special diols remains somewhat limited since for this application material prices must be competitive. Molecular weights (Mw) are rather low and the structure rather linear, therefore there are very few triols used. The hydroxyl value will be between 80 to 160 mg KOH/g. The blocked polyisocyanates that are used are the trimers of HDI or 1PDI. In the USA, these polyester systems are starting to become more widespread as they are more tolerant to passivated surfaces than epoxides or water dilutable
Of course, polyester resins can also be found in powder coatings, which are trying to break into the coil coating market. For coil coating there is a slow but regular development of water-dilutable Polyesters which are used when a new production line is put into service.
However, these add an extra constraint in that it is necessary to pre-dry the coating in order to vaporate the water.
. Industrial
eneral the desired properties are sistance towards chemicals, high, gloss and a good appearance. Their functionality is a little hig
Pol t the
system tage that a fatty chain improves pigment wetting,
In two component polyurethane systems, polyesters still have many advantages compared
ew cycloaliphatic: diols enable QUV resistances of 3000 hours without significant loss of a ing. Today, these products remain expensive and have rather igh hydroxyl values, which increases the cost of formulations (due to the higher levels of
ls, acrylics cannot compete and it is ecessary to use reactive diluents that are very expensive and also reduce pot life. Reduced tance also result. Polyesters have a promising future in the field of
oday, in the two-component water based system, the best results are obtained with acrylics
which imparts a large part of the hardness and physical drying characteristics is sed.
owadays, the automotive industry uses a large tonnage of polyester resins that can be found e
2
Polyesters, principally used with melamine formaldehyde resins, are particularly suited for stoving finishes on industrial articles. These are typically the rather standard type of coating being based on phthalic acid or isophthalic acid, as in g
re
her than for those used in coil coatings. Curing temperatures range from, 120°C to 180°C.
yes ers modified with isononanoic acid or Cardura E10 to impart high flexibility to are available. They have the advan
whilst increasing the fatty acid nature of the polyesters.
with resins that are principally used in this sector. Polyesters, either alone or combined with acrylics, impart flexibility which cannot be attained with acrylics alone, for example cold impact resistance. Moreover, the appearance obtained with the latter is also improved.
N
gloss nd practically no yellow h
crosslinking agent required).
New products are being developed, particularly with the objective of obtaining VOC's of ca.
350 mg/I for tints such as orange or red. At such leve n
hardness and chemical resis high solids coatings.
T
but adding polyesters enables their properties to be improved.
Of course, in the majority of polyurethane dispersions, a polyester with varying degrees of branching,
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3. Automotive.
N
in the following types of paints:
• Primers
• Metallised opaque and pearl coats
• Pigmented finishes and varnishes
(i) Car primer surfacer.
For economic reasons and above all for mechanical properties, polyester resins are redominantly used. A primer surfacer for car paint systems is applied on top of a athodically deposited primer and may be covered by a metallised base and finishing varnish.
ensures cohesion of the unit and imparts most of the mechanical roperties. For example, when the birds' droppings resistance test is carried out on car coats, a
is stem has no influence on the result, because this is a mechanical roblem and not a physical attack. Indeed, at high temperatures, the droppings adhere to the
even the electrodeposited coating layer. For grit resistance, the rincipal remains the same, as the primer will absorb the impact of grits and ensure that there
ating. Finally, this primer has a direct action on the appearance of the vehicle as it also serves as a leveller.
is s uirements. Sometimes polyesters are found with extreme flexibility ased upon very long chain diacids or urethane modified polyesters in order to improve the -crosslinking polyesters which impart chemical, adhesion and grit resistance. These products are evolving towards polyesters with higher non-volatile contents, which will impart body, and increasingly flexible products enabling the amount of blocked isocyanate in the formulation to be reduced.
Water-dilutable (water reducible) formulations which are being developed, use polyesters in two ways:
h, due to the use of polyesters as the basic structure, (in combination with the correct diisocyanate) imparts mechanical
olyesters play an important role in these complex formulations. In most cases, these etallic effect. SCA (sagging control agents) olyesters have also become indispensable so as to bring a certain rheology to the system that p
softened coat and during the drying process, lead to the mechanical contraction of the film. If the sealer cannot absorb the strains imposed on the system, the layer of paint will be stripped off down to the primer or
p
is no detachment of the co
The better the appearance of the primer, the better the appearance of the finish which applied afterwards. Thus as can be seen, a primer has an essential role to play and it i common to see up to three different polyesters in a formulation in order to meet different and often contradictory req
b
appearance as well as self
Using water-dilutable polyesters, modified by a fatty chain acid, which assist in obtaining hardness, appearance and ease of application.
Blending polyester with polyurethane dispersions, which are nowadays indispensable in water-dilutable primer formulations and whic
properties.
Polyesters or hybrid polyester/acrylic systems (70/30) are used m automobile primers especially m the USA where more than two million vehicles per year (lorries and cars) are produced.
(ii) Metallised opaque and pearlescent coats.
P
polyesters have a large degree of linear structure in order to impart maximum flexibility to the film and aluminium placement or rather m
p
(iii) Pigm
For one coat pig h ar ite yester resins are
still used but in amounts reviously because of the smaller demand for these system e of bi-layer s is increasi e metallised base plus finish e of the pr n specifications which has improved in
rms of exterior resistance for car paints as well as better tint stability.
polyester resins are used, especially in two component varnishes or they can be used in combination with acrylic resins. Here, they improve film build and especially scratch resistance by softening the film and give a very high level of general mechanical properties.
ed applications. They can be d metal supplies as well as
r
Polyesters for crosslinking with amine compounds such as Primid, PT 9 10, or even the most well known TGIC
nking with an acrylic resin containing epoxide
ith the largest volume. In powder coatings, the polyester-epoxy ratio varies between lyester portion, the better its exterior resistance.
ented finishes and varnishes.
mented finishes, whic e most often wh , red or black, pol significantly smaller than p
s. Instead, the us system ng. They includ ing varnish becaus ogress i
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Acrylics have come into this sector bringing with them improved exterior resistance and a higher gloss although the appearance, build and mechanical properties are not the same, being inferior. All this is available at an equivalent cost. Today, new polyesters are appearing on the market with improved exterior resistance and also increased non-volatile content. It has also been observed that when gas ovens are used, acrylics give coatings with less yellowing in the oven than with polyesters.
For finishing varnishes which are applied to metallised or opaque bases, a large proportion of
In one component varnishes, they are sometimes used in small amounts in order to impart resistance towards scratches and friction due to cleaning brushes (car wash). Their structure must be aromatic-free and relatively linear. This is why their use is limited since they reduce hardness and chemical resistance and remain slightly more expensive than traditional acrylics.
4. Powders.
In this field, polyester resins represent the predominant chemistry (with epoxy resins) and this hemistry is used with different crosslinking agents for many vari
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found in automobiles, aviation, transport material, renovation an for various machines and equipment, coil and m tal containers. e
Today, several types of polyesters for powder coatings can be identified as a function of thei crosslinking mechanism.
High acid value polyester (30 to 60 mg KOH/g) for crosslinking with an epoxide resin with EEW = ca. 700.
Hydroxylated polyester for crosslinking with blocked polyisocyanates.
Carboxylated polyester for crossli groups.
Thermoplastic polyesters.
(i) Acid functional polyesters.
Acid functional polyesters primarily react and crosslink with solid epoxy resins. These are termed hybrid systems or carboxyl systems. With the all-epoxy systems, these are the two
ystems w s
50/50 and 70/30. The greater the po
Acid value is an essential characteristic of the polyester. Its acid value influences its reactivity towards epoxy resins which are most often a type 3 or even 2. The higher the acid value, the greater the reactivity of the polyester.
However, different catalysts with their different chemistries are used in order to regulate this since too high a reactivity of the system reduces the appearance. The coating must flow to form a level film before crosslinking, otherwise the surface is very uneven when cured. These polyester resins use linear structures such as terephthalic or isophthalic acid and some triacids uch as trimellitic anhydride that can be used in two ways in order to utilise its third acidic
position, the flexibility of
e of the polyester can be increased which will further increase,
tructures, which are discussed in the section on side reactions in polycondensation, are the undesirable blooming effect. Today, it is possible to guarantee resins without this effect that detracts from the appearance of the paint.
creasing the T , of the resins, by as much as 50'C, significantly improves stability.
verage paint formulation of a hybrid system comprises 60-62 % of resin
they can be olyisocyanate dimers, tenned uretidiones. Consult Waterborne and Solvent Based Surface
oatings Resins, Volume 3, Chapter 1, for further details.
include polyols that are partially trifunctional in order to increase e excess of hydroxyl functionality. This hydroxyl value varies between 40 to 60 mg KOH/g.
gs are also sistant to scratches. Often trimethylolpropane is used as the triol. Terephthalic acid is often
olyisocyanates dimers have less reactivity m bl o not release volatile agents during curing.
tance towards ageing but still remain rather xpensive compared with other systems. The usual ratios of polyester to isocyanate s
function.
However, with small amounts of TMA present in the monomer com
the polyester can be modified at will depending on the desired application, by replacing for example NPG, which is often used, by hexanediol or adipic acid. By varying the quantity of diacids, the exterior resistanc
the more there is in the formulation.
By reducing the level of branching (tri-dimensional structure) and by reducing the molecular weight, the paint flow can be significantly improved as well as its reactivity. The cyclic s
responsible, in some cases, for
In g
A standard a
(polyester + epoxy), 37-38 % of pigments and fillers, 0.7-1.2 % of surface tension modifier (flow additive) and 1% of other additives such as benzoin.
(ii) Hydroxyl functional polyesters.
Hydroxylated polyesters are cross-linked with polyisocyanates. These isocyanates can be either blocked by a blocking agent that is often caprolactam, or else
p C
The polyester resins used th
In particular applications such as for anti-graffiti paints, polyesters with hydroxyl values of 100 to 300 mg KOH/g are used in order to obtain a very densely cross-linked network which gives protection from chemical attack by inks or aerosol paints. These coatin
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used with flexible diacids such as adipic acid. P than caprolacta ocked isocyanates, but they d The films obtained in general have excellent resis e
crosslinker vary from 70/30 to 85/15 for traditional systems, but for polyesters with high
(iii) TGIC cured systems.
s, there are many varied polyester systems available that enable coating ystems to be formulated for a wide range of requirements:
l systems.
Systems without blooming.
exterior resistance (super durable).
t paints. o
he use of TGIC enables one to achieve good exterior resistance due to its absence of aromatic structures, resistance to UV combined with excellent mechanical performances. It is a product today which tends to be replaced by other polyamines that are not as toxic as TGIC.
Examples are principally Primid (EMS), Araldite PT 910 (CIBA) as alterative for TGIC.
i-crystalline or crystalline powder by infrared radiation to ensure flow,
rosslink modified
crosslinking. There are numerous uses and in terms of mechanical roperties, the significantly more expensive acrylics have difficulty in providing similar roperties.