Coating quality fundamentals based on resin substrates, mold release agents, solvent resistance, and drying temperature

Introduction

The following questions often arise in plastic coating consultations.

Do you have a paint that can coat plastic?
We are looking for a paint that adheres to resin parts.
Our current paint peels off; is there one with better adhesion?
Please suggest a primer for plastic.

Immediately proposing a paint name in response is risky.

Plastic is a broad term, and coatability varies greatly with the material type.

Is it ABS?
PP?
PC?
PS?
PA?
PBT?
FRP?
A molding compound such as SMC or BMC?

A change of material changes paint adhesion, solvent resistance, the upper drying-temperature limit, surface preparation methods, and suitability for electrostatic coating.

In plastic coating, identifying the material before selecting paint is therefore important.

This article organizes the basic initial checks for plastic coating from the perspectives of materials, molding, release agents, solvents, and drying conditions.

1. The word plastic alone is not enough to define coating conditions

In metal coating, differences between steel, stainless steel, aluminum, and other materials are often recognized.
Resin parts, however, may all be grouped under the name plastic.

There are many types of plastic.

Even representative materials include the following.

• ABS
• PP
• PE
• PC
• PS
• PA
• PBT
• PVC
• FRP
• SMC
• BMC

They may look similar, but their coatability is not the same.

Some materials are easy for paint to adhere to, while others are very difficult.
Some are relatively solvent resistant, while others crack, swell, or whiten under solvent exposure.
Some withstand high-temperature drying, while others deform at low temperatures.

The broad category of paint for plastics is therefore not enough to achieve stable coating quality.

Identifying the material is the starting point for plastic coating.

2. What can happen when the material is unknown?

Selecting paint without knowing the material can cause various problems.

Typical problems include the following.

• Poor adhesion
• Coating film delamination
• Cissing
• Blistering
• Whitening
• Solvent cracking
• Substrate swelling
• Deformation during drying
• Uneven gloss
• Contamination adhesion
• Film thickness variation during electrostatic coating

For example, paint containing a highly solvating thinner can attack the surface of a solvent-sensitive resin.
This can cause fine cracks, whitening, swelling, and surface roughness.

Materials such as PP and PE have low surface energy and may be difficult for paint to adhere to.
Ordinary paints can peel off easily in these cases.

Proposing coating without identifying the material is like prescribing medicine without a diagnosis.
Material identification is the first step toward stable plastic coating quality.

3. Different resin materials require different checks

The points to check in plastic coating vary by material.

Examples of the differences follow.

ABS

ABS is often used for automotive parts, appliance parts, and exterior parts.
It may be relatively easy to coat, but solvent effects and internal molding stresses require attention.

Checks:

• Does solvent cause cracking?
• Do molding stresses remain?
• Does cracking occur near gates or edges?
• Is drying temperature appropriate?

PP and PE

PP and PE are typical materials to which paint adheres poorly.
Surface treatment or dedicated primers may be required.

Checks:

• Is surface treatment needed?
• Is a primer needed?
• Are flame, corona, or plasma treatments applicable?
• Adhesion test results
• Will the coating resist peeling in its service environment?

PC

PC may be used for parts requiring transparency and strength.
However, solvent-induced cracking requires attention.

Checks:

• Are the solvents in the paint too aggressive?
• Does solvent cracking occur?
• Does the drying temperature cause deformation?
• For transparent parts, do appearance defects occur?

PS

PS is also readily affected by solvents.
Strong solvents may roughen or crack the surface.

Checks:

• Does solvent attack the substrate?
• Does whitening or surface roughness occur?
• Can low-temperature drying be used?

PA

PA absorbs water.
Its moisture condition may affect dimensions and coatability.

Checks:

• Water absorption condition
• Predrying treatment
• Adhesion
• Effects of moisture in the service environment

FRP, SMC, and BMC

FRP, SMC, and BMC are molding materials containing fibers or fillers in a resin.
Fillers, thickeners, release agents, and other constituents may influence the interior and surface of the substrate.

Checks:

• Molding conditions
• Type of mold release agent
• Surface voids and pinholes
• Effects of fillers
• Need for putty or primer
• Occurrence of foaming or blistering

Knowing the material therefore reveals which risks need to be checked.

4. Residual mold release agents cause poor adhesion and cissing

Mold release agents may be used in plastic molding to make molded parts easier to remove from the mold.

They are useful in molding.
However, they can create major problems during coating.

A release agent remaining on the surface makes paint adhesion difficult.
It can also repel the paint and cause appearance defects.

Even a visually clean surface may retain a release agent as a thin oily film.

Check the following about release agents.

• Is a mold release agent used?
• What type is it?
• Is it silicone-based?
• Is it wax-based?
• Is it stearate-based?
• Is it removed?
• Which cleaner is used to remove it?
• Is adhesion checked after cleaning?

When adhesion problems occur in plastic coating, check for release agents before suspecting the paint.

5. Plasticizers can migrate into the coating film

Some plastics contain plasticizers to make them flexible.

A plasticizer is a component that softens resin.
Plasticizers can be problematic, particularly in flexible PVC.

Migration of plasticizers into the coating may cause the following problems.

• Drying defects
• Surface tackiness
• Blocking
• Coating film softening
• Gloss changes
• Poor adhesion

Blocking is a phenomenon in which coated surfaces stick to each other when coated products are stacked.

Preventing plasticizer migration requires checking compatibility between the substrate and paint and, as needed, selecting a barrier primer or an appropriate coating system.

In plastic coating, constituents within the substrate may affect the film after application.
This is a major difference from metal coating.

6. Excessively aggressive solvents damage the substrate

Thinner and solvents in paint require attention in plastic coating.

In metal coating, solvents mainly dilute paint and facilitate application.
In plastic coating, however, solvents can attack the substrate.

Solvents unsuitable for the substrate cause the following problems.

• Solvent cracking
• Whitening
• Swelling
• Surface roughness
• Cracking
• Uneven gloss
• Poor adhesion
• Deformation

Solvent cracking is cracking of the resin surface caused by solvent exposure.
It is particularly likely in areas retaining internal molding stresses.

The following areas require attention.

• Near gates
• Edges
• Bent sections
• Areas with different wall thicknesses
• Areas retaining molding stresses

In plastic coating, the idea of cleaning thoroughly with a strong solvent or applying with an aggressive thinner is risky.
A balance is needed between removing dirt and avoiding substrate damage.

7. Drying temperature can cause deformation

Some plastics are sensitive to heat.
Drying-temperature settings are therefore very important.

A baking temperature acceptable for metal parts can cause deformation, warping, dimensional changes, or cracking in resin parts.

Check the following.

• Substrate heat-deflection temperature
• Drying temperature
• Drying time
• Actual product temperature
• Whether fixtures prevent deformation
• Deformation risk from wall-thickness differences
• Deformation during handling
• Dimensional changes after drying

Plastic coating must achieve adequate film drying without deforming the substrate.

Raising the drying temperature may accelerate drying.
However, a deformed substrate makes the product unusable.

It may be necessary to consider low-temperature-drying paints and review drying time, fixtures, and handling methods.

8. Static electricity attracts contamination

Plastics readily develop static charge.
They therefore tend to attract dust and dirt before coating.

Even after apparently thorough cleaning, static electricity can attract airborne dust, which appears as dirt nibs or contaminant defects after coating.

Check the following.

• Is static charge removed before coating?
• Is air blowing appropriate?
• Does the air contain oil or moisture?
• Dust control inside the booth
• Fibers from operator clothing
• Contamination during handling
• Time between surface preparation and coating

For contamination prevention in plastic coating, static elimination is important as well as cleaning.

9. Check conductivity when using electrostatic coating

Electrostatic coating is relatively easy to apply to metal parts.
Metals conduct electricity, allowing paint to be attracted electrically.

Plastics, however, generally conduct electricity poorly.
Electrostatic coating may therefore have limited effect without further treatment.

The following checks are needed when applying electrostatic coating to plastics.

• Is a conductive primer used?
• Is a conductive treatment liquid used?
• Is grounding provided through the fixture?
• Is conductivity at contact areas sufficient?
• Is surface resistance controlled?
• Are humidity or elapsed time after treatment having an effect?

Using electrostatic coating equipment does not necessarily increase transfer efficiency.
Workpiece conductivity and grounding design are important.

10. Recyclability also affects the coating specification

Higher adhesion is not always better in plastic coating.

The coating must not peel during use.
A coating that peels easily is a quality defect.

From a recycling perspective, however, excessively strong adhesion can sometimes be problematic.
If the coating cannot be separated during recycling, reusing the substrate may be difficult.

Plastic coating therefore requires the following balance.

• No peeling during use
• Required appearance and durability are achieved
• Separability is considered where recycling requirements apply

Recyclability may become increasingly important for automotive and appliance parts in the future.

Plastic coating needs an overall assessment of adhesion, appearance, durability, and recyclability.

11. Basic checks for plastic coating

The following checks are important when considering plastic coating.

Material

• What is the resin material?
• Is it ABS, PP, PC, PS, PA, FRP, or another material?
• Is it a thermoplastic or thermosetting resin?
• Does it contain fillers or additives?
• Does it contain plasticizers?
• Is there variation between material lots?

Molding

• What is the molding method?
• Is a mold release agent used?
• What type of release agent is used?
• Where are the gates?
• Does the shape tend to retain internal stress?
• Are there wall-thickness differences?

Surface preparation

• Is the release agent removed?
• Which solvent is used for wiping?
• Does that solvent attack the substrate?
• Is static charge removed?
• Is a primer needed?
• Is surface modification treatment needed?

Coating and drying

• Does the paint suit the substrate?
• Does the thinner attack the substrate?
• Does drying temperature suit the substrate?
• Does deformation or warping occur?
• Is the coating too thick?
• Are adhesion tests performed?

Use and recycling

• Is the service environment indoors or outdoors?
• Is there contact with water, oil, chemicals, or heat?
• Is abrasion resistance needed?
• Are there recycling requirements?
• Does coating-film separability need to be considered?

12. Points to note at factories in Vietnam

The following points require particular attention when coating plastics at factories in Vietnam.

12-1. Information exchange between molding and coating companies

When molding and coating are performed by different companies or departments, the coating side may not know the material or release-agent information.
This lack of information causes poor adhesion and cissing.

12-2. A hot, humid environment

High humidity can affect material storage, surface preparation, drying, static electricity, and contamination.
Water absorption must also be considered for some materials.

12-3. Variation in wiping between operators

Wiping methods greatly affect plastic coating quality.
Wiping too hard, using too much solvent, coating before the surface has dried, or using dirty cloths.
Such variation causes defects.

12-4. Inadequate low-temperature drying conditions

Using metal-coating drying conditions unchanged can deform resin parts.
Drying conditions for resin substrates need to be defined.

12-5. Inadequate static elimination and contamination control

Plastics readily attract dirt through static electricity, making static elimination before coating and booth environmental control important.

13. Questions for customer visits

For the factory manager

• Have you received complaints about poor adhesion or cissing in plastic coatings?
• Are the coated products automotive parts, appliance parts, electronic parts, or another application?
• Is there much touch-up or recoating after application?
• Are there recycling or environmental requirements?

For production engineering

• What is the resin material?
• Is a release agent used during molding?
• Do you know the type of release agent?
• How is surface preparation performed?
• Does the solvent in use affect the substrate?
• Does the drying temperature deform the substrate?

For quality assurance

• Are adhesion tests performed?
• Where do defects occur?
• Are there cracks near gates or edges?
• Is the relationship between molding lots and coating defects checked?
• Is coatability evaluated when release agents change?

For purchasing

• Is resin material information shared with the coating process?
• Are coatability effects checked when molding suppliers change?
• Is information about release agents and additives checked?
• Are surface preparation or primers omitted solely for cost reasons?

For the coating shop-floor supervisor

• Is coating carried out without identifying the material?
• Does wiping solvent cause whitening, cracking, or swelling of the substrate?
• Is static charge removed before coating?
• Does deformation occur in the oven?
• When adhesion is poor, are substrate and molding conditions checked as well as paint?

14. How CoatureVietnam can help

CoatureVietnam can support plastic coating through material identification, surface preparation, drying conditions, and adhesion evaluation, as well as paint selection.

Specific support can include the following.

• Organizing resin material information
• Checking molding conditions
• Checking release-agent information
• Checking surface preparation methods
• Selecting primers for resin substrates
• Checking thinner effects
• Checking drying temperature
• Organizing solvent-cracking causes
• Designing adhesion test conditions
• Checking conductivity treatments for electrostatic coating
• Creating surface preparation SOPs for operators
• Preparing customer improvement proposal materials

Paint alone does not reveal the cause in plastic coating.
Materials, molding, surface preparation, paint, and drying need to be considered together.

Summary

Plastic coating starts with identifying the material.

Even resin parts that look similar differ greatly in coatability between ABS, PP, PC, PS, PA, FRP, SMC, and BMC.

Selecting paint without identifying the material may cause poor adhesion, cissing, solvent cracking, whitening, deformation, and drying defects.

Check the following for plastic coating.

• Material identity
• Molding method
• Mold release agents
• Plasticizers
• Solvent resistance
• Heat-deflection temperature
• Surface preparation method
• Need for a primer
• Suitability for electrostatic coating
• Recycling requirements

CoatureVietnam’s principle of “Coating the Future with Science and Care” means carefully examining these material differences and translating them into shop-floor conditions that reduce the likelihood of failure.

Science means considering plastic coating in terms of materials, molding, release agents, solvents, and drying conditions.
Care means clarifying the initial checks so that operators and customers do not face uncertainty.

Do you have a paint that can coat plastic?
The first answer to that question is not a paint name.

First, identify the material.
That is where stable plastic coating quality begins.