APPLICATION INFORMATION
ADDITIVES FOR GEL COATS

TS-A 2

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Additives for gel coats

Contents
03 Introduction
04 Air release additives
05 Additives to improve flow/leveling and prevent craters and fisheyes
07 Additive to improve thixotropy
08 Additives to prevent flooding and floating
09 Additives to prevent porosity
10 Anti-tack additives
11 Additives for gel coats – summary

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Introduction
Gel coats are a particularly important and qualitatively demanding areaof application for unsaturated polyester resins, as they represent theouter, visible layer of a finished glass fiber reinforced plastic component.
Depending on the application, the requirements in terms of appearance,mechanics, and resistance to water, chemicals, or UV radiation, for example,
are particularly high.
Gel coats are the first layer to be applied to the surface of a mold in a
thickness of about 400-700 µm. For this reason, the demands on the liquid
gel coat regarding rheology (run-off behavior), deaeration, and flow arealso very high.
This brochure shows how additives help to meet these high demands.

For additional information
on additives and technical
topics, please contact us:
Thermosets.BYK@altana.com

Note
To ensure the best appearance
and full functionality, please
open in Adobe Acrobat.

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Additives for gel coats

Air release additives
Entrapped air bubbles are a common problem inmanufacturing or applying unsaturated polyester gel coats.
They are difficult to remove mainly because most gel coats
are highly thixotropic.
In addition, they are usually applied by airless sprayequipment in a single pass to a wet film thickness of 400
to 500 μm. These same air bubbles ultimately develop intopinholes, which increases porosity and dramatically reduces
the gel coat’s water resistance.
The example (G. 01) shows a spray gel coat with andwithout BYK-A 555. Hot water resistance at 98 °C isdramatically improved in the samples with BYK-A 555.
The following standard additives can be used in almost all
gel coat formulations to improve air release properties
during manufacturing and application.

Impact of air entrapment on hot water resistanceSampleWithout additive

0.5 % BYK-A 555

BYK-A 555 is widely used in all types of gel coats. It is themost efficient air release additive.

Hours
0

BYK-A 515 can also be used in various gel coats and is
especially recommended for vinyl ester based systems.
BYK-A 550 and BYK-081* are highly effective with minimalhaze and are recommended for transparent gel coats.

* For the use in food contact applications, please check the food contact sheet.

150300G. 01

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Additives for gel coats

Additives to improve flow/leveling andprevent craters and fisheyes
Depending on the application method, uniform flow andleveling is required for the applied gel coat. Brushed gelcoats can vary in film thickness because of insufficient flow
and leveling properties.
It is critical for the mold surface to be sufficiently wet by thegel coat! Poor substrate wetting occurs when the gel coathas a higher surface tension than the mold surface (G. 02).

Craters
Substance withlower surface tension

Fisheyes develop when there are surface tension differences
between the mold releasing film, the gel coat film, and dust
particles (G. 02). When a dust particle falls into the gel coatfilm, the gel coat cannot wet the particle because of its highsurface tension.
The surface tension of the gel coat thus needs to be
reduced in order to solve the fisheye problem.

Liquid with higher surface tension

Possible solutions to improve substrate wetting andeliminate fisheyes are:
G. 02

• Silicones in case of large surface tension differences.
• Acrylate leveling additives in case of small surface
tension differences.

Substrate Wetting
Poor mold surface wetting
because of gel coat’s highsurface tension.

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Additives for gel coats / Additives to improve flow/leveling and prevent craters and fisheyes

Silicones

BYK-378 significantly reduces surface tension and provides
low foam stabilization. It improves substrate wetting andprevents cratering.

BYK-330 reduces the surface tension of the gel coat,
improves flow and leveling, and eliminates fisheyes.
Because of its controlled compatibility in the gel coat,
it does not stabilize foam.

BYK-361 N improves the flow and leveling properties of theapplied gel coat film and eliminates craters and fisheyes.
BYK-361 N is more compatible than BYK-S 706 and can beused in clear gel coats.

Acrylates

BYK-310 is a compatible silicone that reduces surface
tension and improves flow and leveling.

BYK-S 706 is widely used in unsaturated polyester gel coats.
It improves the flow and leveling properties of the appliedgel coat film and helps surface deaeration. It is often used incontinuous lamination processes to improve the flow andleveling of the gel coat film and prevent fisheyes. BYK-S 706
will introduce a slight turbidity in clear gel coats.

Effect of BYK-330

Comparison of BYK-370 and BYK-330 in a clear gel coat

BYK-370 improves the flow and leveling of the gel coat.
It is typically compatible with unsaturated polyester resins.
BYK-370 is especially recommended in clear gel coats.

Wax

BYK-370

BYK-330

0.3% BYK-330

ControlSemi-permanentmold release

G. 03

BYK-370 provides good substrate wetting without causing turbidity.

G. 04

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Additives for gel coats

Additive to improve thixotropy
Thixotropy is very important in gel coats but various
problems may occur:

into the gel coat resin. Wetting the fumed silica can bechallenging depending on the base resin type.

• no thixotropy development• thixotropy development is too slow
• thixotropy drift over timeRHEOBYK-R 605 was developed to improve fumed silica
wetting and dispersion and enhance thixotropic properties.
In order to achieve this, RHEOBYK-R 605 should be added
to the resin before the silica. This not only improves silica
dispersion and enhances the thixotropic effect, but also
maintains thixotropy during storage.

Since fumed silica is typically used to introduce thixotropy,
it is very important that the fumed silica be well dispersedVinylester gel coats
Hydrophilic fumed silica is usually not effective in vinylester
resins. Hydrophobic fumed silica can be used to create
thixotropy, but it is very difficult to obtain air release andgood surface appearance. By using RHEOBYK-R 605 incombination with hydrophilic fumed silicas, it is possible tocreate the required thixotropy while retaining good air
release, flow, and leveling properties.

RHEOBYK-R 605 exhibits best results in vinylester gel coats
Thicknessin μm

Gel coat with hydrophilic
fumed silica

Gel coat with hydrophobic
fumed silica

Gel coat with hydrophilic
fumed silica and RHEOBYK-R 605

50100

Vinylester gel coat with RHEOBYK-R 605

150Component

TypeVinylester resin

Resin

BYK-A 515

Air release additive

0.5

Fumed silica

Thixotrope

1.8

RHEOBYK-R 605

Rheology additive

0.6

Pigment

Pigment

10.0

450

Cobalt (1 %)Cobalt accelerator

3.0500DMA (10 %)Amine accelerator

1.0Peroxide

Peroxide

2.0

200
250
300
350
400

G. 05

Amount in g
100.0

T. 01

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Additives for gel coats

Additives to prevent flooding and floating
Pigment flooding and floating can be influenced by many
variables. The most important variables are: the type ofpigments or pigment mixtures, the grinding resin, degreeof dispersion and pigment stabilization, the thixotropy of
the gel coat, and the application method. It is critical thatthe pigments used are properly dispersed in the polyester
resin, and properly stabilized after grinding so thatre-agglomeration and uncontrolled flocculation do notoccur.
There are basically two possible approaches for addressing
flooding and floating problems:

1. Stabilizing the pigmented system via
controlled flocculation of the pigmentsIn this case, the wetting and dispersing additive is able todevelop a loose bridging network between the pigment
particles (G. 06). Typical products are BYK-W 940,
BYK-W 980, and BYK-220 SN.
The advantage of these products is that they do not
dramatically influence the thixotropy of the gel coat.
They should be added to the system before the pigment
grinding stage. In some cases, it is possible to correct
flooding and floating by post-adding BYK-W 940 andBYK-220 SN to the final gel coat.

2. Stabilizing the pigmented system via
deflocculation of pigments
Using high molecular weight block copolymers withmany pigment affinic groups provides excellent stericstabilization, thereby preventing pigment flocculation.
This stabilizes the color strength and hue of pigments
in gel coats and pigment concentrates.
Typical products are DISPERBYK-2163, DISPERBYK-171,and DISPERBYK-192. These additives must be introducedin the pigment grinding stage. They are highly effective,
and mainly used in color pastes for gel coats.
It is important to check for possible detrimental effects on
the thixotropy of the final gel coat. The additive’s wetting
effect on fumed silica may reduce the fumed silica’s
ability to develop thixotropy.

Controlled flocculation

Deflocculation with high molecular weight block copolymers

G. 06

G. 07

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Additives for gel coats

Additives to prevent porosity
Porosity is a well-known phenomenon and one of thehardest to evaluate. The effect is strongly dependent on thesurface tension of the mold caused by the mold releaseagent, the surface tension of the gel coat, and the efficiencyof the air release additive. The type of peroxide used shouldalso be taken into consideration.

Porosity
Without additive

With 0.25 % BYK-A 515 + 0.25 % BYK-A 525

The gel coat’s surface tension has to be adjusted to thesurface tension of the release agent, whereas wax-based
release agents are easier to wet than others. We
recommend surface-active additives such as BYK-A 525 forsmaller adjustments and BYK-330 for larger adjustments.
An air release additive can support the displacement ofair on the mold’s surface. BYK-A 515 or BYK-A 555 canbe used to accomplish this.
Best results have been achieved by combining asurface-active additive with an air release additive (G. 08).
G. 08

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Additives for gel coatsAnti-tack additives
Sometimes it is necessary to step on a gel coat because of
the mold size (boat hulls) or to fix surface defects. If thesurface of the gel coat is sticky, the worker would destroy
the coating by stepping on it. BYK’s range of anti-tackadditives helps overcome this issue by covering the surfaceof the gel coat with a non-sticky film (G. 09).

Step-on test
Gel coat with BYK-S 780

A lab test can easily be performed with a cotton pad. One
hour after application, the pad is placed on the gel coat’s
surface and a 1 kg weight is positioned on the pad. Afterone minute, the weight is removed and the pad is liftedfrom the gel coat by picking it up at the center (G. 10).
Interlaminar adhesion is normally not affected by theanti-tack additives, but should be carefully checked ineach system (G. 11).

0%

0.5 %

0.6 %

0.7 %

0.8 %

0.9 %

1.0 %
G. 09

Dosage of BYK-S 780 based upon gel coat

BYK-S 780, a wax dispersion, is recommended for allsystems, especially for vinylester resins where interlaminaradhesion is not required.
BYK-S 782, a combination of modified waxes, is designed
for applications at higher temperatures (i.e. in the summer).
It can be applied in every system that requires interlaminaradhesion. The additive can be melted before use so that it
is easier to handle.

Cotton pad test

Test of interlaminar adhesion

Without additive

Perfect adhesion even with 1 % BYK-S 782
on gel coatWith BYK-S 780

Laminateafter

G. 10

1 day

4 daysG. 11

11Additives for gel coats

Additives for gel coats – summary
Application

Air release

Flow and leveling

Thixotropy

Flooding and floating

Porosity

Anti-tackPigmented gel coats

BYK-A 515
BYK-A 555
BYK-A 501

BYK-330
BYK-378
BYK-306
BYK-310
BYK-361 N
BYK-370
BYK-S 706

RHEOBYK-R 605*²

BYK-220 SN
BYK-W 940
DISPERBYK-167
DISPERBYK-192
DISPERBYK-2152
DISPERBYK-2163
BYK-W 980
DISPERBYK-171

BYK-A 515
BYK-A 525BYK-330
BYK-A 555

BYK-S 782
BYK-S 780

Transparent gel coats

BYK-081*¹
BYK-A 500
BYK-A 550

BYK-310
BYK-361 N
BYK-370RHEOBYK-R 605*²

First recommendation  

Second recommendation*¹ For the use in food contact applications, please check the food contact sheet.  *² in combination with fumed silica

BYK-370

T. 01

02/2022
TS-A 2BYK-Chemie GmbH
P.O. Box 10 02 4546462 Wesel
GermanyTel +49 281 670-0
Fax +49 281 65735

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info@byk.com
www.byk.com

The information herein is based on our present knowledge and experience. The information merely describes the properties of our products but noguarantee of properties in the legal sense shall be implied. We recommend testing our products as to their suitability for your envisaged purpose
prior to use. No warranties of any kind, either express or implied, including warranties of merchantability or fitness for a particular purpose, are maderegarding any products mentioned herein and data or information set forth, or that such products, data or information may be used without infringing
intellectual property rights of third parties. We reserve the right to make any changes according to technological progress or further developments.
This issue replaces all previous versions.