No : PCP 001 Date : 02 -01-2014

Adhesive Coating Defects

Introduction

During film formation Adhesive should transform a liquid into a closed, solid film

called coating. In order to give optimum protective and aesthetical properties the

coatings should be smooth and even without any defects. Adhesive Film defects can

ruin protective and aesthetical properties of coatings. A huge amount of time and money

is invested in preventing and resolving defects. A lot of hustle can be prevented by

using available knowledge that is simple and easy. How to resolve a defect turns out to

be logical as soon as the governing phenomena of that defect are known and understood.

A wide variety of coating defects occur and the same are discussed.

Defects , Causes and the Remedies

Adhesive Coating is a simple process

There are various methods available for the same.

In general following methods are used

Gravure

Gravure with Myer Bar

Kiss Coating

Floating Knife

Comma Coating

Or

Reverse Roll coating

We are all aware of these Coating Methods and we know how to use them to the best to

produce an Adhesive Coated Product.

No : PCP 001 Date : 02 -01-2014

Adhesive Oozing / Bleeding

a squeezing out of the adhesive from between layers of tape so that the sides of the roll

are very tacky.

Blistering

A small , dome like bulge in the coated film or a small recess in a coating film. Typical

blisters are caused by trapped moisture , rapid drying and the use

of improper solvents. Small to large broken or unbroken bubbles

Broken edge craters and small blister-like spots can be caused by

oil, grease or contamination on the substrate. Trapped solvents

can also show up as blisters. Blisters will occur as a result of

humidity testing on test panels, indicating moisture penetration

through the film.

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Cratering

Is formation of bubbles (foaming) and resulting small, round concave depressions

(cratering) when bubbles break in a adhesive film, during adhesive application and

drying.

Craters are also called Pitting .Small , uniform

Indentations in coated Film .

This term refers to large blisters, sometimes deep

enough to expose the substrate, appearing like a

volcano. They are caused by oil and grease spots on

the substrate or by unwanted particles of dirt. In this

latter case, a small lump may be visible at the bottom of the crater. Fish eyes are a

special category of crater often caused by silicone

compounds, such as waxes or lubricants. Pinhole

craters are also encountered when solvent is trapped

below the film surface due to insufficient flash-off

or solvents evaporating too quickly.

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Crawling

Crawling, also called retraction or pulling back, is a uniform de-wetting effect: the

surface energy of the solid substrate is too low to let the liquid adhesive , having a

certain surface tension, cover that substrate. The resulting effect is that the liquid

adhesive does not fully wet the substrate but it retracts and forms droplets, leaving part

of the substrate naked.

Chalking Chalking refers to a powdery appearance and a loss of gloss. It is caused by a

disintegration of the film on a molecular level. Poor weather ability of the paint resin,

use of poorly-balanced thinning solvents and insufficient paint

agitation, can cause chalking. Formation of fine powder on the surface of the paint film

during weathering, which can cause color fading.

Cracking

Cracking in conformal coating is where a smooth surface fractures into sections with the

cracks in the coating leaving the area below exposed to potential contaminants.

Contamination Contamination (dirt) can take many forms, particle and fibers all fall into the category of

dirt. However, in some studies of contamination, it was found that more than 50 per cent

of the dirt occurrences were caused by tiny particles. Other sources of

dirt include: Surrounding shop area

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De Lamination

Peeling from undercoat or substrate

Delamination in adhesive coating is where the coating

lifts from the surface , leaving the area below

exposed.

Dirt Pickup: Accumulation of dirt, dust particles ,

fiber and hair , on the adhesive film; may be a major

problem

Fish Eyes

This coating defect is characterized by circular voids or

separation in the coating.

Fish-eyes can be caused by oily spots or silicone particles

and/or by air-borne droplets that are deposited on the coated

surface. This is a typical defect when adhesive is sprayed ,

or coated by pickup rollers.

Separation or pulling apart of wet film to Expose

underlying Finish or substrate. A coating defect

characterized by a small. recessed area in a coating film

with a raised area in the middle , is nothing but Fish eyes.

No : PCP 001 Date : 02 -01-2014

Foam

Foam can already be present in the adhesive before application starts. It can also be

introduced during the application process. Especially during Gravure or Kiss roller

coating , air can be brought into the adhesive . A coating formulator wants to develop a

adhesive that loses its foam as soon as application has finished and film formation

proceeds.

Mottle

A coated defect characterized by a ring like blotch with a

light colored area in the center of the ring. Mottle

typically occurs when metallic flake pigment is used in a

coating.

Mottling is a dull, hazy or blotchy appearance on the

surface of some vinyl films or over laminates.

Orange Peel

Orange Peel is Uneven , rough irregularities in the surface

of a coated film. Orange peel often occurs when a binder

fails to flow uniformly across an object's surface.

Uneven surface formation - much like that of the skin of

an orange - which results from poor coalescence of

atomized paint droplets. Adhesive droplets dry before

they can flow out and level smoothly together.

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An orange peel effect is generally caused by the Adhesive

viscosity that’s too high. However, poor operator

techniques, or too much adhesive coating thickness , will also produce a rough finish.

Pin holing

Pin holing is one of the most common surface issues especially

in sprayed application. This defect is characterized by pore-like

holes of about the size of a pinpoint. It frequently extends

entirely through the coating film.

Tiny deep holes exposing substrate

A small , sunken area that forms on a coating film after a bubble

ruptures. Pinholes are typically larger and less abundant across

a coating's surface than solvent pops.

Peeling

Peeling: Loss of bond due to poor adhesion. Where there is

a primer and top coat of adhesive, peeling may involve

some or all coats.

No : PCP 001 Date : 02 -01-2014

Wrinkling , Rough Surface

Wrinkling: A rough, crinkled adhesive surface,

which occurs when uncured adhesive forms a "skin.”

Defects Related to Substrate Surface Quality

It is well known that defects and imperfections pre-existing on the substrate surface can

be easily duplicated and magnified in the hot-dipping stage, resulting in defective

coatings.

Rough Surface

A rough substrate surface containing iron fines and slivers is prone to the development

of

outbursts in the coating. The resultant coating

is rough with numerous areas of localized

thick coatings. After being flattened in temper rolling, these areas reflect light

differently, appearing as either dark or bright spots depending on the lighting condition.

A typical sample is shown in Figure 1. Studies indicate that the density of this kind of

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defect decreases with increasing coating weight,(4) suggesting that such a defect can be

masked by a thick coating.

The Causes

Adhesive Oozing / Bleeding

Blistering

Possible Causes:

• Applying highly plasticizer loaded or softer adhesive .

• Exposure of the Adhesive film to high temperature , humidity or moisture

shortly after adhesive is coated , especially if there was inadequate exhaust.

No : PCP 001 Date : 02 -01-2014

Cratering

Crawling

Possible Causes:

Use of low quality adhesive or very old adhesive.

Applying (especially rolling) paint too rapidly.

Wetting becomes more critical when the surface energy of the substrate is low and/or

when the surface tension of the liquid Adhesive is high.

A substrate having low surface energy is called

hydrophobic, implying that the surface of the substrate is

not able interacting strongly with a material brought on

top of it. Unmodified plastics, for example, are a group of

materials having low surface energy.

Even when the surface energy of the substrate is

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relatively high, wetting may become a critical factor if highly dynamic application

processes are involved. On fast-running printing presses or roller-coater application, for

example, wetting must be very fast to obtain good results. If the substrate wetting is too

slow, uneven ink-lay, print defects or even poor ink transfer may result.

Under static conditions, the contact angle Q of a liquid drop placed on a given substrate

determines whether wetting is good or poor

Cracking

Cracking in conformal coating is where a smooth surface fractures into sections with

the cracks in the coating leaving the area below exposed to potential contaminants.

Cure temperature too high

Conformal coating heat cured too quickly without allowing enough time for

room temperature (RT) drying

Film thickness too great causing coefficient of thermal expansion (CTE)

mismatch and cracks occurring in the coating

Operating temperature too high or too low causing the conformal coating to flex

too much & crack.

Chalking

• Use of a low-grade, highly pigmented paint.

• Use of an interior paint for an outdoor application.

Contamination

Dirt Pickup:

No : PCP 001 Date : 02 -01-2014

Electrostatic charges develop on surfaces of film webs as they are transported through

various converting processes such as Coating, Laminating , rewinding, slitting. These

charges often cause attraction of Dirt , Dust , fibre and hair on to the web material and

defects in coating uniformity resulting in quality problems and customer dissatisfaction.

Static electricity is the cause of many problems encountered in the transporting and

converting of film webs in today’s modem machinery. As progressing technology

allows faster production speeds, static related problems are also increasing,

hampering production and affecting product quality.

Static charges are generated on the surface of the film as it unwinds from the roll and as

it contacts and separates from surfaces such as idler rolls, nip rolls and printing or

coating rolls.

Charges exist in two polarities: positive and negative. Surfaces charged to the same

polarity will repel each other. Opposite polarities will attract. A charged film of either

polarity will attract to uncharged surfaces of insulators or conductors. These occurrences

become especially evident in converting operations such as sheeting, bag making and

die-cutting where the film is no longer under the control of the mechanical structure of

the web and its transport system.

Film webs tend to develop high charges; 30 kV to 40 kV is common. The strong

electrostatic fields associated with these charges attract dust particles, fibers, bugs and

hair resulting in surface contamination. This causes obvious quality problems

in printing, coating and laminating and cleanliness problems with food, medical and

pharmaceutical packaging films.

Uneven coatings and “wicking” of adhesive are often the result of static charges.

De Lamination

Factors that influence delamination include:

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1. Cleanliness of the surface for adhesion

2. Compatibility between the coating and the surface adhered to due to surface

energies mismatch

3. Permeability to moisture

4. Degree of coating cure

Fish Eyes

"Fish-eyes” are areas in a freshly applied surface coat that tends to thin or open up

on their own accord.

This problem is caused by the high surface tension of the adhesive applied on a

polymer surface, such as a plastic films.

Foam

Two aspects strongly dominate the formation, stabilization and release of foam. First,

foam bubbles can be stabilized by additives having a surfactant (soap) structure. This

problem is experienced most strongly in water based adhesives. The second important

aspect is the rheology of the Adhesive. Despite the viscosity being high the foam

bubbles might be able to move to the surface of the film where they can break.

However, the resulting hole possibly can not level out when the viscosity of the film is

too high,

Mottle

The plasticizers that make films soft, also make them very impressionable. So much so

that when vinyl films and overlaminates are rolled up on a roll, the films can pick the

impression of backside of the release liner.

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Orange Peel

Adhesive might be too viscous ( high viscosity )

Or The coating nip is too tight solvent evaporation too fast

Origin and Potential Causes: Improper gun adjustment and techniques. Too little air

pressure, wide fan patterns or spraying at excessive nozzle distances causes droplets to

become too dry during their travel time to the work surface and they remain as formed

by gun nozzle. Extreme shop floor temperature. When air temperature is too high,

droplets lose more solvent and dry out before they can flow and level properly.

Improper dry.

Factors that can cause orange peel effect include:

1. The improper application of the conformal coating. This manifests in aerosol or

spray gun application where the coating is applied too far away from the substrate

and the coating is too “dry” on the surface to level out.

2. The incorrect thickness of the coating where not enough coating is deposited so

that there is insufficient conformal coating to self level.

3. The incorrect cure profile when using an accelerated curing mechanism. This could

be that the flash off zone before heat curing is inadequate or the oven profile is

incorrect.

Improper flash or recoat time between coats. If first coats of enamel are allowed to

become too dry, solvent in the paint droplets of following coats will be absorbed into the

first coat before proper flow is achieved.

Wrong solvents or reducer. Under-diluted adhesive or adhesive thinned with fast

evaporating solvents or reducers causes the atomized droplets to become too dry before

reaching the surface. Too high viscosity. Low shop floor temperature. Too little solvent

or reducer.

Materials not uniformly mixed. Many finishes are formulated with components that aid

coalescence. If these are not properly mixed, orange peel will result.

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Substrate not treated thoroughly

Peeling

Pin holing

Pin holing is the result of trapped solvents, air or moisture.

Factors that can cause these effects include:

1. When applying the conformal coating the wet film surface can skin over,

entrapping solvents under the surface which can bubble or burst out.

2. If the coating is applied too thick or too viscous any bubbles which are created in

the process of application can become entrapped in the coating before they settle out.

3. Air entrapped under components by the coating process can be forced out during

the drying / curing process and cause bubbles.

4. Pressure pots with conformal coating inside can absorb air which can manifest as

champagne bubbles when applied.

5. Brush coating with too viscous a material or working the coating can cause

bubbles.

6. Incorrect spray equipment or pressures can cause excessive bubbles & foam.

Wrinkling , Rough Surface

Possible Causes:

• Adhesive is applied too thick.

• Adhesive coated during extremely hot weather or cool damp weather, which

causes the adhesive film to dry faster on top than on the bottom.

• Exposure of uncured adhesive to high humidity levels.

• coating of top coat of adhesive to insufficiently cured primer.

• Coating over contaminated surface (e.g., dirt or wax)

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The Remedies

Adhesive Oozing / Bleeding

Blistering

Setting appropriate drying temp is a key to get raid of Blisters.

The drying Air temp and the velocity is critical. Adhesive coated film should never be

exposed to very high temp

in the first zone. As a

matter of fact a temp

gradient setting is essential.

The most ideal temps for

drying zone wise beginning

from First to last zone

should be set. This allows

the entire thickness of the

wet adhesive film warm up

uniformly. Thus the sudden

drying of the top layer , and

entrapment of solvent in the layer below is avoided.

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Cratering

• Use high quality adhesive which will allow good flow and appearance

• coat an appropriate primer before using such Adhesive over the surface.

Crawling

Crawling can be prevented in a number of ways. A pre-condition that has to be met in

order to obtain complete wetting is that the substrate is clean. In some cases the

substrate must be modified in such a way that the surface energy of the substrate is

raised, i.e. the substrate is changed from hydrophobic to hydrophilic. Substrate treatment

is often applied to plastics prior to coating.

Improved Wetting

There are two ways to improve wetting characteristics:

Raising the surface energy of the substrate through cleaning - removal of oils

and/or other contaminants - or through surface treatment (corona pre-treatment,

flaming, acid or caustic wash).

Lowering the surface tension of the coating by using additives made

specifically for this purpose.

Corona treatment helps to a great extent in improving the wettability of the substrate.

Another approach being most often used to prevent crawling is lowering the surface

tension of the liquid adhesive by adding wetting agent. However, the use of wetting

agents can worsen key properties of the system, like hardness, recoat ability and foam

sensitivity.

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Cracking

It is possible to minimise conformal coating cracking effects by

Lowering the initial cure temperature

Allowing an initial drying time at room temperature before exposure to elevated

temperature

Reducing the coating thickness to avoid CTE mismatch issues

Select a coating with a wider temperature range performance.

Select a more flexible coating

Contamination

Dirt Pickup:

NEUTRALIZING CHARGES ON THE FILM WEB

If static charges on the film web can be controlled to a low level, most electrostatic

related problems can be eliminated. A neutral material will have no electric field so

induction charging, particle attraction and most ESD events will not occur.

Unlike conductors, charges on insulative materials cannot be neutralized by simply

grounding. Methods such as modifying the surface of the material to give it some degree

of conductivity or the use of air ionization must be employed.

Humidity

Raising relative humidity is sometimes used to increase surface conductivity of some

normally insulate materials in an effort to reduce static charging.

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Ionization

The use of air ionizers is the most common, cost effective method of controlling static

charges on the film web. Ionizers emit vast quantities of both negative and positive

charged gas molecules called air ions. The ions are made available to the charged web

where the electric field attracts the necessary ions of opposite polarity to its surface, thus

balancing the charge on the web.

The most common ionizers used in film transport systems are static bars and ionized air

blowers. They may be electrically operated, passive, or may use radioactive materials as

the ion generation source.

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De Lamination

Fish Eyes

Reduction of the surface tension is required to avoid fish-eyes. This can be achieved

thanks to surface modifying agents such as surfactants.

Incorporation of surfactants in the formulation will help you to improve the coating

appearance.

Foam

The prevention of foam in films can be arranged in a number of ways. At first, the use

of surfactants should be minimized. Secondly, introduction of air in the adhesive during

production, handling and application should be prevented as much as possible. Finally,

the rheology of the system should be such that the air that was introduced can escape in

a short period of time during the first stage of film formation. Be aware that the use of

defoamer, being an approach that cannot always be prevented, increases the risk of

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cratering , as well as reduction in Peel adhesion values. However addition of around 0.5

to 1.0 % of Defoam / Anti Foam is generally followed .

Mottle

Orange Peel

Adjust the Adhesive viscosity or the nip replace the solvent

Peeling

Pin holing

Surface modifying additives, especially surfactants, will reduce the surface tension, and

help entrapped bubbles to move to the surface.

The use of surfactants in adhesive formulation will avoid pinholes to appear on the dry

paint film.

Wrinkling , Rough Surface

• Scrape or sand substrate to remove wrinkled coating. If using a primer, allow it to

dry completely before applying top coat. Recoat , (avoiding

temperature/humidity extremes) .

Dear Members ,

I have made an attempt to highlight the issue’s related to coating defects in general.

Please feel to write to me for any specific problem you might be facing . It’s a promise

from my side to guide you , in terms of selection of a suitable additive or a process.