Select Zinc Silicate Primers MJM&MS

8/8/2019 Select Zinc Silicate Primers MJM&MS

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How to Selec t Z inc S il i cate Pr imers

As Published in the Protective Coatings Europe Journal

Mike Mitchell & Mark SummersInternational Protective Coatings

Akzo Nobel

www.international-pc.com

There is often uncertainty and confusion amongst users of zinc silicate primers regardingparameters such as volume solids and spreading rates which cause few difficulties withconventional paints. The following note is intended to clarify these issues and to give somesimple guidelines in selection of zinc silicates.

It is accepted that this could be argued to be an oversimplified view, but it is believed thatthis is worthwhile in the quest for clarity. Formulation can allow products to fall outside theparameters discussed but, in this instance, benefits should be demonstrable by performancetesting.

Zinc silicates are amongst the most widely used primers in the Protective Coatings Industry,and it is necessary to ask why use zinc silicate primers?

The answer is because of:

Corrosion resistance : Damage resistance on handling.

: Underfilm corrosion creep on damage.

: General anti-corrosive performance.

For zinc silicates the anti-corrosive and mechanical properties are highly dependent uponthe level of zinc dust present. Twenty-five years ago it was not unusual to use zinc silicatescontaining 90% or more zinc dust in the dry film, with the only other components being theethyl silicate and anti-sag agents. Cost pressures and practicality has driven this down toaround 86% zinc dust, with other components often being reinforcing extender pigmentspresent to give improved film properties (i.e. reduce mudcracking in thick areas). Theselevels give zinc -zinc steel contact and thus potentially allow cathodic protection to takeplace.

Figure (1) shows the corrosion potential against time for an 85% zinc dust silicate. Untilaround 80 days cathodic protection of the steel substrate is definitely occurring, andsubsequently up to 140 days partial cathodic protection and partial barrier effect. After thisprotection is primarily by barrier effect.

Most long term current track record is based on zincs with around 85% zinc dust (higherthan in most standards such as SSPC 20 or ISO 12944), in most instances the zinc beingtopcoated.


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How to Selec t Z inc S il i cate Pr imers

There are also many instances of exceptional performance of high zinc containing zincsilicates being used as single coat systems, generally in more arid environments such asdesert regions, but also on bridges and other infrastructures. Often water based alkalisilicates are used as binders in this instance but all of the comments in this note applyequally to these as to the more common organic ethyl silicate (tetra ethyl ortho silicate)based materials.


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Page No. 3

Figure 1

It is necessary to consider the composition and the typical fi lm of a zinc silicate to fullyunderstand what is being purchased.

Zinc silicates are unusual coatings and, along with some flat wall and masonry paints, areone of the few coatings which are designed so that all of the solid pigment particles are notcoated with polymer and all of the gaps between particles are not filled with polymer, i.e.they are designed to be porous films.

Idealised coating showing zinc dust, joined by silicate with many air gaps

Corrosion Potential vs Time for Zinc Rich Silicate (85% Zinc Dust)

-1200

-1000

-800

-600

-400

-200

0

0 50 100 150 200 250

Time/Days

Ecorr/mVvsSCE

Zinc Silicate

AirGaps

60-70m

SilicateBinder

ZincDust

Particle Size6-9m


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Page No. 4

Consider the 90% w/w zinc dust zinc silicat e described earlier.

In the dry film composition is roughly as follows:-

Weight S.G. Volume

Zinc Dust 90.0 7.1 12.68

Silicate Binder 9.0 2.5 3.60

Anti-sag Agents etc 1.0 2.0 0.50

PVC 78.5*

* PVC is pigment volume concentration and is used to give an indication of pigment loadingin a paint film. Gloss finishes typically have values of 10-20% and conventional primers40-50%.

In the more widely used 86% w/w zinc dust system: -

Weight S.G. Volume

Zinc Dust 86.0 7.1 12.11

Silicate Binder 10.0 2.5 4.00

Mica 3.0 2.8 1.07

Anti-sag 1.0 2.0 0.50

PVC 77.4

It can be seen from these pigment content figures, which are considerably over a simplisticcubic close packing scenario for the zinc particles of between 62 and 66%, that a veryconsiderable portion of the film is void, i.e. the film is porous (this is not a problem as withweathering it fills with salts and becomes a barrier but it does cause the well knownpinholing problem seen with topcoats applied over fresh zinc silicate). The fact that there is

not sufficient polymer to fully wet and coat the pigment (zinc) particles present ensureselectrical contact and Cathodic protection.

85% Zinc Dust 85% Zinc Dust(Alternative Sample)

60% Zinc Dust

SEM of Cross-sections(86% Zinc Dust & 60% Zinc Dust)


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Page No. 5

Theoretical calculations

If a purely theoretical calculation is undertaken for a zinc silicate, assuming resin, zincand extender all film form to give a totally void free coherent film, then typically volumesolids figures of around 35-45% are achieved for commercial zinc silicates. This type offigure if clearly not representative of the wet to dry film thickness ratio obtained inpractice, nor of the spreading rates normally achieved.

The difference between the practical figures and calculated fig ures is a feature whichcauses many problems in the assessments of the commercial value of various zincsilicates and can lead to poor system performance and poor value for users.

Volume Solids Determinations

The industry norm for volume solids quotations (necessary both to determine spreadingrate and in practice to give a method of accurate commercial assessment betweenvarious suppliers coatings) is to use methods of accurately determining the wet to dryfilm thickness ratio of the coating.

The actual methods normally used are those described in ISO 3233 or ASTM D2697(generally modified to allow for ambient curing). One key aspect of both of thesemethods is accurate determination of film thickness by measuring the volume of the dryfilm, utilising Archimedes principle and weighing in air and water. This works well for allnormal organic coatings which are formulated with the aim of achieving void free films,and values obtained correlate well with practice.

However, with zinc silicates using water, volume solids figures reflect the theoreticalvolume solids calculated, not that which is observed in practice.

This is due to water penetrating into the pores and voids of the coating and the volumemeasurement, thus not reflecting the actual position of the surface.

Therefore, to use this type of method, an alternative approach is needed, e.g. NACEItem No. 54165, which is a modification of ASTM D2697. In this instance, the water isreplaced by mercury, which has a surface tension such that it does not pe netrate the

voids in the silicate film and in order to sink in the mercury application must be totungsten which is of sufficiently high S.G. not to float.

Alternative methods have been described, for example in OCCA Monograph No. 4,which utilise a Profilometer to determine thickness.

Both of these methods give sensible values for volume solids, which relate well to practice.It must be pointed out that generally it is found that some practice by the experimenter isrequired before consistent realistic results can be obtained by either of the methods, which Ibelieve may be a contributing factor to some of the erroneous values which can be quoted.In the past there have been a number of other approaches, for example, spinning down thesolids using a centrifuge which generally gives unrealistically high figures. Other attemptsinclude trying to measure wet film thickness with a wet gauge and dry with a typical d.f.t.gauge, again giving a highly inaccurate assessment. Unfortunately, because of this diffic ulty

in practically measuring volume solids, in this instance some manufacturers basically guessvalues, assuming their products are in line with competitors.


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Page No. 6

It may be questioned as to the real relevance of this volume solids determination. Basically,it gives a measure of how much zinc the purchaser is obtaining on each square metre ofsurface, which is presumably why they are buying a relatively expensive primer in the firstplace i.e. in the belief that the presence of the correct level of zinc is the key to obtaininglong term corrosion protection.

Clearly it is beyond the scope of most purchasers of zinc silicates to check or becomeinvolved in volume solids determinations, and the considerations above are given todemonstrate the complexity of the situation and to illustrate to the prospective purchasers ofthis type of product that they can be misled by erroneous data sheet information, especiallyin the area of volume solids.

Zinc content generally is not so difficult, as is often stated, or the fact that the productcomplies with a certain standard with regard to zinc content is mentioned (but there havebeen instances of zinc dust simply being removed from the powder component of zincsilicates simply to reduce costs).

How best can a specifier/ purchaser then select which zinc silicate to use. Much informationcan actually be obtained from the data, or requested from the suppliers, as figures whichtend to be correct can easily be checked.

(1) Quoted Volume Solids

(In some instances this is not quoted and instead a spreading rate is given this is ineffect the same data), e.g., theoretical spreading rate to give 75 microns d.f.t. is8.0 m2/l and corresponds to an assumed volume solids of 60%

(2) S.G.

The higher the S.G., the more zinc present, and the volume solids will also be higher.

(3) Packing Weights of Binder and Zinc Powder

When the weight of zinc powder is available a calculation can easily be made to givethe weight of zinc per square metre and thus an evaluation of potential effectiveness.For units between 10-15 litres it is suggested 1 kg is allowed for packing weights.

(4) Quoted VOC

Relates directly to the volume solids, the higher the VOC, the lower the volume solids(in general). Figure needs to be treated with some care as ethanol is evolved duringthe curing reaction and some ignore this and calculate from the formula thus giving aslightly low figure.

The data sheets for the zinc ethyl silicates supplied by a number of major suppliers aroundthe world were analysed in this way in order to determine comparability of potentialperformance relative to zinc level on the steel.

It is interesting to note that a minority of companies actually give this weight of zinc per unitarea as part of their standard data sheet information, and if this was given as a standard itwould take much confusion out of the comparison of these coatings.


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Page No. 7

These results are shown in Table 1, and then separately they have been split out intogroupings of basically:-

85-86% Zinc Dust Type Table 2

High Solids Table 3

Reduced Zinc 78-80% Zinc Dust Table 4

Viewed in this way it becomes quite easy to identify materials which seem false in spreadingrate (or volume solids), by comparing S.G., which is an accurate figure easily measured, andweight of zinc metre2, the total amount in any pack is unquestionable, what can be varied isthe spreading rate.

This enables:-

Product 4 to be questioned in 85-86% group

Product 5 in high solids group

Products 2 & 6 in reduced zinc group

It also enables the basic following parameters to be set for the various types of zinc ethylsilicate:-

(1) 85% Zinc Dust Products >60% volume solids

>2.4 S.G.

>200 g/m2 zinc

(2) High Solids >75% volume solids

>3.2 S.G.

>250 g/m2 zinc

(3) Reduced Zinc >60% volume solids

2.2 S.G.

>160 g/m2 zinc

Perfectly good products can be formulated below these suggested parameters which willgive excellent performance, but the lower level of zinc per metre 2 should be reflected in theproduct selling price.


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Page No. 8

Table 1 Summary of Extracted Data Sheet Information on Zinc Silicates

Product

QuotedVolume

Solids(%)

SpreadRate for

75m DFT(m

2/l)

Wt of ZincDust in Dry

Film (%Total)

S.G.PackingWeights

(Pack/Zinc)

VOC(g/l)

Wt of Zincper m

2

(g)

PackSize

A (61) 8.2 86 (3.17) 49kg/33.2kg 0 (268) 4 gal kit

B (61) 8.2 >85 (2.64) 51kg/33.2kg 479 (215) 5 gal kit

C (75) 10.0 84 (3.38) 47kg/33.2kg 288 (243) 3.6 gal kit

D (75) 10.0 85 (3.28) 47kg/33.2kg 389 (237) 3.7 gal kit

E (75) 10.0 75 (2.60) 46.7kg/33.2kg 384 (189) 4.65 gal kit

F 64 8.5 (85) 2.65 Wt Mix 515 (211)

G 62 8.2 (76) 2.4 Wt Mix 515 (171)

H 70 9.3 (78) Vol Mix N/A (157)

I 60 8.0 Vol Mix N/A (217)

J 66 8.8 2.46 Wt Mix 521 (183)

K 62 8.2 2.38 Wt Mix 510 (190)

L 79 10.5 3.36 Wt Mix 293 (249)

M 58 7.7 2.13 18kg 558 8L Mix Unit

N 65 8 .7 2.13 18kg 558 8L Mix Unit

O 62 7.9 85 2.51 462 220

P 76 9.7 83 3.22 312 248

Q 68 8.7 90 2.95 0 251

R 63 8.4 85 2.50 11.7kg/25.8kg 480 (203) 14L Units

S 60 8.0 80 2.02 540 (145)

T 62 8.2 80 2.4 13.5kg/24.8kg 458 (185) 15L Units

U 55 7.35 60 1.95 519 (119)

V 65 8.5 79 (2.28) 474 (168)

W 65 8.7 76 2.3 Vol Mix 525 (155)

X 58 7.7 (85) 2.62 Vol Mix 470 (266)

Y

Z

( ) Indicates calculated from data given

0 VOC corresponds to water based alkali silicates


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Page No. 9

Table 2 Standard 85% Zinc Products

Product B F I K O R X

Volume Solids 61 64 60 62 62 63 58

Zinc (%) >85 85 85 85 85

S.G. 2.64 2.65 2.38 2.51 2.50 2.62

Wt. (g/m2) 215 211 217 190 220 203 266

Table 3 High Solids Products

Product C D L P E

Volume Solids 75 75 79 76 75

Zinc (%) 84 85 83 >75

S.G. 3.34 3.28 3.36 3.22 2.57

Wt. (g/m2) 243 237 249 248 189

Table 4 Reduced Zinc Products

Product H S T V W U

Volume Solids 70 60 62 65 65 55

Zinc (%) 80 80 80 79 60

S.G. 2.02 2.4 2.28 2.3 1.95

Wt. (g/m2) 157 146 185 168 155 119

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Select Zinc Silicate Primers MJM&MS