Fundamentals for preventing surface preparation problems in plastic coating

Introduction

The following concerns commonly arise when problems occur in plastic coating.

“The paint does not adhere.” “Cissing appears on the coated surface.” “It remains tacky after drying.” “Fine cracks appeared after coating.” “The same paint produces defects in some lots.”

When these phenomena occur, the paint or thinner is often suspected first.

Paint and thinner selection are, of course, important. However, judging the cause from the paint alone is risky in plastic coating.

Plastic coating is strongly affected by residues on the substrate surface before application, components inside the material, residual molding stress, and compatibility with the solvents used.

Three factors are particularly important.

1. Mold release agents
2. Plasticizers
3. Solvent cracking

These are problem factors specific to plastic coating that differ from those in metal coating.

This article sets out operational checks on mold release agents, plasticizers, and solvent cracking that require attention during surface preparation for plastic coating.

1. Surface preparation for plastic coating requires a different approach from metal coating

In metal coating, the main purpose of surface preparation is to remove oil, rust, moisture, acids, alkalis, salts, and other contaminants to create a substrate surface that readily accepts paint.

Plastic coating, however, requires different checking points.

The following factors can cause problems with plastics.

• Mold release agents used during molding
• Plasticizers in the resin
• Additives
• Internal molding stress
• Sensitivity to solvents
• Heat deflection temperature
• Dust contamination caused by static electricity
• Low surface energy
• Variation between molding lots

Surface preparation for plastic coating is therefore more than a cleaning process.

It creates a surface to which paint can adhere without damaging the substrate.

Achieving this balance is difficult.

Aggressive cleaning may remove contamination. However, wiping with a strong solvent can cause surface cracking, whitening, or swelling in the resin.

More aggressive preparation is not necessarily better in plastic coating. Selecting preparation appropriate for the substrate is important.

2. What is a mold release agent?

A mold release agent is a material used to make molded products easier to remove from a mold.

Plastic parts are often made by introducing resin into a mold. If the product sticks to the mold, removal becomes difficult. Mold release agents may therefore be used on the mold or resin surface.

Mold release agents are useful in the molding process.

However, they can cause problems in the coating process.

If a mold release agent remains on the molded product surface, paint has difficulty adhering to the substrate. Paint can also be repelled by the surface, causing appearance defects.

A mold release agent can therefore assist molding while obstructing coating.

3. Residual mold release agents cause poor adhesion and cissing

Mold release agents remaining on the surface can cause the following problems after coating.

• Poor adhesion
• Coating film peeling
• Cissing
• Uneven gloss
• Dirt nibs
• Coating film detachment
• Localized areas that appear uncoated

Cissing is a particularly clear example. The paint does not wet and spread uniformly and appears to be repelled by the surface.

Mold release agents may be invisible. Even a clean-looking molded surface can retain a thin film of release agent.

It is therefore risky to judge a surface as clean by appearance alone.

When poor adhesion or cissing occurs, first check the following points.

• Is a mold release agent used during molding?
• What type of mold release agent is it?
• Is it silicone-based?
• Is it wax-based?
• Is it stearate-based?
• Is there a process to remove the mold release agent?
• Is the wiping solvent appropriate?
• Is adhesion checked after cleaning?

Changing the paint without information about the mold release agent can leave the root cause unresolved.

4. Cleaning agent selection is important for mold release agent removal

Solvent wiping or washing may be used to remove mold release agents.

The important point here is the selection of the cleaning agent.

A solvent that removes contamination effectively is not necessarily suitable. If it damages the resin substrate, it can remove the release agent while creating another surface defect.

For example, wiping with a strong solvent can cause the following problems.

• Surface whitening
• Swelling
• Cracking
• Uneven gloss
• Surface roughness
• Solvent cracking
• Poor adhesion

Wiping operations are also prone to variation.

Operators use different wiping pressures. They use different solvent quantities. Cloth cleanliness differs. Drying time after wiping differs. Repeated wiping with the same cloth spreads contamination instead of removing it.

Such variations in work practices also affect coating quality.

Mold release agent removal requires standardization of the following.

• Cleaning agent used
• Wiping method
• Cloth type
• Cloth replacement frequency
• Number of wipes
• Drying time
• Time before coating
• Operator training
• Adhesion checking method

Establishing SOPs for surface preparation is very important in plastic coating.

5. What is a plasticizer?

A plasticizer is a component added to make plastic softer.

It is used to make rigid resin more flexible or easier to process. Plasticizers play a particularly important role in flexible PVC and similar materials.

From a coating perspective, however, plasticizers can cause problems.

The issue is that plasticizers move toward the coating film over time. This is sometimes called plasticizer migration.

Migration into the film affects its drying, curing, and surface condition.

6. Problems caused by plasticizer migration

Plasticizer migration toward the coating film can cause the following problems.

• Inadequate drying
• Surface tackiness
• Blocking
• Gloss changes
• Coating film softening
• Poor adhesion
• Dirt pickup
• Sticking during stacking

Blocking is the phenomenon in which coating films stick together when coated products are stacked.

Even if the coating looks acceptable immediately after application, its surface may become tacky later or stacked products may stick together.

Simply extending drying time may not solve this problem. If plasticizer migration is the cause, the paint system, primer, substrate selection, and specification design need to be reviewed.

When plasticizer migration is suspected, check the following points.

• Does the substrate contain plasticizers?
• Is it flexible PVC or a similar material?
• Does tackiness develop after coating?
• Does blocking occur during stacking?
• Does gloss or surface condition change over time?
• Is a primer with barrier properties required?
• Is the service environment temperature high?

Plasticizers are a factor within the substrate. Surface wiping alone may not solve the problem.

7. A barrier layer is an important concept in controlling plasticizers

Preventing plasticizer migration requires a coating construction that makes migration toward the film difficult.

A primer or intermediate layer with barrier properties may be used for this purpose.

A barrier layer stops or reduces the movement of components from the substrate.

However, using a barrier layer does not guarantee a solution.

Check the following points.

• Plasticizer type
• Substrate flexibility
• Compatibility with the paint system
• Primer adhesion
• Drying conditions
• Service environment temperature
• Stacking conditions for coated products
• Surface changes during long-term use

Plasticizer control requires evaluation over time as well as short-term adhesion testing.

A coating may be satisfactory immediately after application but develop tackiness or gloss changes after one week, one month, or use. These cases also need to be considered.

Plastic coating requires evaluation of changes over time.

8. What is solvent cracking?

Solvent cracking is cracking of a resin surface caused by solvents in paint or cleaning agents.

The word solvent refers to the dissolving medium. A crack is a fracture.

Solvent cracking therefore means resin cracking caused by solvent.

Some resin materials are sensitive to particular solvents. Contact with paint, thinner, or cleaning agents can attack the resin surface and cause fine cracks.

Cracks are particularly likely in areas with residual internal molding stress.

Typical locations include the following.

• Near the molding gate
• Edges
• Areas with different wall thicknesses
• Areas subjected to bending stress
• Areas with residual molding strain
• Internal corners of the product

Fine whitish cracks on the surface may be cracks in the resin substrate rather than in the coating film.

9. Solvent cracking may be substrate cracking rather than a coating film defect

An important point about solvent cracking is that it can look like cracking in the coating film.

Operators may interpret it in the following ways.

“The paint cracked.” “The coating film is weak.” “The drying conditions are poor.” “A more flexible paint is needed.”

In fact, the cracks may be in the resin substrate itself rather than in the film.

Changing the paint alone will not solve the problem in this case.

Check the following points.

• Material name
• Molding conditions
• Internal molding stress
• Cleaning solvent used
• Thinner used
• Solvent composition of the paint
• Drying temperature
• Crack locations
• Results of magnified observation
• Whether fine cracks were already present before coating

Solvent cracking requires the substrate, molding, and solvents to be checked together with the paint.

10. Annealing can be effective in some cases

Molded resin products can retain internal molding stress. High internal stress makes cracking more likely on contact with solvents.

Annealing is sometimes used to address this.

Annealing treats a material at a specified temperature for a specified time to relieve internal stress. For resins, it is performed within a temperature range that does not deform the substrate, with the aim of reducing molding strain.

However, annealing is not a universal solution.

Check the following points.

• Is the temperature appropriate for the substrate?
• Will heat cause deformation?
• Will dimensions change?
• Is the treatment time appropriate?
• Does it reduce cracking after coating?
• Can it be implemented in mass production?
• Is the benefit appropriate to the cost?

Annealing is one measure against solvent cracking. However, it must be considered after checking the substrate, shape, and suitability for mass production.

11. Assuming that a strong solvent is the answer is risky

Surface preparation for plastic coating must remove dirt and mold release agents. However, assuming that a strong solvent should be used for this purpose is risky.

A strong solvent may remove contamination effectively while also having a strong tendency to damage resin.

The following materials require particular care.

• ABS
• PC
• PS
• Acrylic resins
• Some transparent resins
• Molded products with residual stress

Strong solvents can cause whitening, swelling, surface roughness, and cracking.

Surface preparation must balance the following requirements.

• Ability to remove mold release agents
• Ability to remove oil
• No attack on the substrate
• Safety for operators
• No residue after evaporation
• Reproducibility in mass production

Cleaning agents should be considered part of the coating specification for plastics.

12. Dust contamination from static electricity is also a surface preparation problem

Plastics readily accumulate static electricity. They therefore tend to attract dust and dirt before coating.

In addition to mold release agents, plasticizers, and solvent effects, static electricity can cause preparation problems.

Contamination on the substrate surface before coating causes the following defects after application.

• Dirt nibs
• Foreign matter
• Roughness
• Coating film defects
• Appearance defects
• Increased rework

Examining the paint and spray booth alone is insufficient in these cases. Static elimination, air blowing, transport methods, and the work environment before coating also need to be considered.

Check the following points.

• Is static eliminated before coating?
• Is the air used for blowing clean?
• Are operators’ clothes shedding fibers?
• Is the time between surface preparation and coating too long?
• Does contamination attach during transport?
• Does recontamination occur at the booth entrance?

Surface preparation for plastic coating requires both contamination removal and prevention of renewed contamination.

13. Surface preparation problems likely in Vietnamese factories

The following surface preparation problems are likely when coating plastics in Vietnamese factories.

13-1. Insufficient information between molding and coating

When molding and coating are performed by different companies, the coating operation may lack information about mold release agents or resin materials. This can result in unexplained poor adhesion and cissing.

13-2. Variation in wiping between operators

Without standardized wiping, quality varies between operators. Solvent quantities, the number of wipes, cloth replacement, and drying time need to be standardized.

13-3. Substrate damage from strong solvents

Strong solvents used to remove contamination can damage resin surfaces. ABS, PC, and PS require particular care.

13-4. Effects of hot, humid conditions on drying and storage

High humidity can affect drying and surface condition during storage. Water absorption must also be considered for some substrates.

13-5. Foreign matter deposition caused by static electricity and dust

Resin parts readily attract dust through static electricity, so foreign matter can attach during storage, transport, or waiting before coating after surface preparation.

14. Basic checks for surface preparation in plastic coating

Check the following items during surface preparation for plastic coating.

Substrate information

• Resin material name
• Thermoplastic or thermosetting
• Presence of plasticizers
• Presence of additives
• Molding lot
• Molding conditions

Mold release agent information

• Whether a mold release agent is used
• Mold release agent type
• Mold release agent removal method
• Cleaning agent
• Wiping conditions
• Adhesion check after cleaning

Solvent effects

• Wiping solvent
• Solvents in the paint
• Thinner type
• Presence of solvent cracking
• Presence of whitening, swelling, or surface roughness
• Need for annealing

Surface preparation operations

• Cloth management
• Number of wipes
• Drying time
• Static elimination
• Air blowing
• Time before coating
• Variation between operators

Evaluation

• Adhesion testing
• Cissing check
• Crack check
• Tackiness check
• Blocking check
• Appearance check after elapsed time

15. Questions for customer visits

For factory managers

• Have there been complaints about poor adhesion or cissing in plastic coating?
• Do defects increase for particular products or lots?
• Are molding and coating performed in the same factory or in separate operations?
• Is the burden of rework or recoating significant?

For production engineering

• What is the resin material name?
• Do you use a mold release agent during molding?
• Do you know its type?
• What solvent is used in surface preparation?
• Have you verified that this solvent does not attack the substrate?
• Have you considered annealing?

For quality assurance

• Do you record cissing and poor adhesion separately?
• Do you check the possibility of solvent cracking?
• Does plasticizer migration cause tackiness or blocking?
• Do you link molding lots with coating defects?
• Do you perform adhesion testing when surface preparation changes?

For purchasing

• Do you obtain material names and mold release agent information from the molding manufacturer?
• Are changes to mold release agents or additives shared with the coating operation?
• Are cleaning processes or primers being omitted solely to reduce cost?
• Do you check the effect on coatability when changing substrates?

For coating operation supervisors

• Do wiping methods differ between operators?
• Are wiping cloths kept clean?
• Do you allow sufficient drying after wiping before coating?
• Do you eliminate static before coating?
• When cracks occur, do you check the substrate and solvents as well as the paint?

16. How CoatureVietnam can help

For surface preparation problems in plastic coating, CoatureVietnam offers support that includes substrate information, surface preparation, solvent effects, and adhesion evaluation as well as paint recommendations.

Specifically, we can provide the following support.

• Organizing resin substrate information
• Checking mold release agent information
• Reviewing surface preparation methods
• Evaluating cleaning agents and wiping conditions
• Analyzing solvent cracking causes
• Checking plasticizer migration risks
• Selecting primers
• Designing adhesion test conditions
• Creating surface preparation SOPs for operators
• Preparing improvement proposal materials for customers

Plastic coating requires molding, substrates, surface preparation, and drying to be considered together with the paint.

Summary

Surface preparation problems in plastic coating have a different structure of causes from those in metal coating.

For metals, the main factors are rust, oil, moisture, and residual treatment chemicals. For plastics, mold release agents, plasticizers, solvent effects, internal stress, and static electricity are important.

Residual mold release agents cause poor adhesion and cissing. Plasticizer migration into the film causes tackiness and blocking. Solvents incompatible with the substrate cause solvent cracking, whitening, or swelling.

Before selecting paint for plastic coating, it is therefore necessary to check the material name, molding method, mold release agents, plasticizers, solvents, and drying conditions.

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means analyzing these hard-to-see causes and translating them into reproducible surface preparation conditions.

Science means separating plastic coating defects into substrate, mold release agent, plasticizer, solvent, and internal stress factors. Care means creating clear SOPs for operators and reducing rework and complaints.

Surface preparation for plastic coating is more than removing contamination. It creates a surface that accepts paint adhesion without damaging the substrate. This is fundamental to stable plastic coating quality.