UV + PVD Vacuum Coating for Plastic Products: A Comprehensive Guide to Treatment Agent Selection, UV Paint Composition, and Spraying Process

In industries such as consumer electronics, home appliances, and automotive interiors, the demand for metallized decoration of plastic products is growing increasingly. The combined process of UV coating and PVD (Physical Vapor Deposition) vacuum coating has become the mainstream solution for plastic surface metallization due to its advantages of environmental friendliness, high efficiency, and strong metallic texture. The core of this process lies in selecting treatment agents based on the characteristics of plastic materials, precisely regulating the composition of UV metallic paint, and strictly following standardized spraying procedures. Only through the synergy of these three elements can strong adhesion between the coating and the substrate, a perfect metallic appearance, and excellent durability be achieved.

I. Treatment Agents and Their Chemical Names for Different Plastic Materials

Differences in surface polarity and crystallinity of plastic materials directly affect the adhesion of PVD coatings. Targeted pretreatment agents are required to improve surface properties. Based on the difficulty of adhesion, plastics can be divided into two categories: easily adherent substrates and difficult-to-adhere substrates. The selection of treatment agents and their chemical compositions are as follows:

(I) Easily Adherent Substrates: No Special Treatment Agents Required

ABS, PC, and ABS+PC alloys are the most commonly used easily adherent plastics for PVD coating. These materials have moderate surface polarity, and the polar groups in their molecular structures (such as nitrile groups in ABS and carbonate groups in PC) can form good bonding with conventional solvent-based coatings. In actual production, only ethanol or isopropanol is needed for surface cleaning to remove oil, dust, and no additional special treatment agents are required. Regular UV primer formulations can achieve stable adhesion.

(II) Difficult-to-Adhere Substrates: Special Treatment Agents and Their Chemical Compositions

Materials such as PP, PET, PA (nylon), and glass fiber-reinforced PC require surface modification with special treatment agents (Primers) to ensure PVD coating adhesion due to their low surface tension, high crystallinity, or inert groups.

1. PP (Polypropylene): As a typical non-polar plastic with extremely low surface polarity, it requires the use of special chlorinated PP polymer treatment agent (chemical name: Chlorinated Polypropylene Resin Solution). Its chemical composition takes chlorinated polypropylene as the core film-forming substance, supplemented by mixed solvents such as ethyl acetate and toluene. It improves the polarity and roughness of the PP surface to build a bonding bridge between the primer and the substrate. This treatment agent appears as a clear, transparent, slightly yellow liquid with a density of 0.87g/cm³ and a flash point of approximately 6.3℃, suitable for vacuum plating pretreatment of home appliance casings, IT product plastic parts, etc.

2. PET (Polyethylene Terephthalate): With a smooth surface and high crystallinity, it requires polyurethane-based adhesion promoter (chemical name: Polyester-Modified Polyurethane Resin Treatment Agent). Its main component is a polyester-modified polyurethane prepolymer, combined with ketone solvents (such as acetone), which can form a reactive film on the PET surface to enhance the adhesion strength of the subsequent UV primer.

3. PA (Nylon): Due to the presence of amide bonds in its molecular structure, it is prone to water absorption, leading to decreased coating adhesion. It requires epoxy-modified polyamide treatment agent (chemical name: Epoxy-Terminated Polyamide Resin Solution). The epoxy groups in this treatment agent can chemically react with the amino groups on the PA surface to form chemical bonding, while also having a moisture-proof effect to ensure coating stability.

4. Glass Fiber-Reinforced PC: The addition of glass fiber reduces the surface adhesion of the material, requiring silane coupling agent-modified acrylic treatment agent (chemical name: γ-Aminopropyltriethoxysilane-Modified Acrylic Resin Treatment Agent). The alkoxy groups of the silane coupling agent hydrolyze and bond with the hydroxyl groups on the glass fiber surface, while the acrylic resin is compatible with the UV primer, forming a stable "substrate-treatment agent-primer" bonding system.

II. Core Composition of UV Paint for Different Metallic Colors

The core of UV metallic paint is to achieve a metallic texture through the oriented arrangement of metallic pigments, combined with a UV-curable resin system to ensure the mechanical properties and weather resistance of the coating. The difference in composition between different metallic colors mainly lies in the type and proportion of metallic pigments, while the resin system is mainly based on acrylates. The specific compositions are as follows:

(I) Silver UV Paint

Silver UV paint is the most widely used metallic coating, with core components of flaky aluminum powder (pigment) and acrylate oligomers/monomers (binder). The aluminum powder has a particle size of 10-30μm and a flaky structure, accounting for 8.6%-12% of the solid content. It forms a silver metallic luster by reflecting light through oriented arrangement. The resin system includes polyurethane acrylate oligomers and trimethylolpropane triacrylate (TMPTA) monomers, combined with acylphosphine oxide (TPO)-based photoinitiators (to solve the shielding effect of metallic pigments on short-wavelength UV light), supplemented by additives such as leveling agents and antioxidants. The solvent is a high-volatility ketone or ester solvent, with a construction solid content of approximately 38% and a viscosity of 15mPa.s (25℃).

(II) Gold UV Paint

The pigment of gold UV paint is copper-zinc alloy powder (commonly known as gold powder), and the binder is similar to that of silver UV paint. The color of the gold powder is determined by the copper-zinc ratio: a zinc content of 8%-12% results in a reddish gold color, 20%-30% zinc content results in a greenish gold color, and the intermediate ratio gives a reddish-green gold color. In practical formulations, the gold powder has a particle size of 800 mesh and is surface-coated with a surfactant to improve compatibility with the resin, accounting for 10%-15% of the solid content. The resin system uses high-transparency acrylic/polyurethane resin to ensure that the metallic luster of the gold powder is not obscured, and UV absorbers are added to improve weather resistance and avoid discoloration after long-term use.

(III) Gunmetal UV Paint

Gunmetal (dark gray metallic color) UV paint uses composite pigments of nickel powder and graphite powder, with a chemical composition of nickel powder (particle size 20-40μm) and graphite powder (particle size 5-10μm) at a mass ratio of approximately 3:1, with a total content of 12%-18% of the solid content. The resin system selects epoxy acrylate oligomers, which have both hardness and adhesion. The photoinitiator is a composite system of acetophenone and TPO to ensure deep curing of the coating. This formulation forms a calm gunmetal effect through the metallic reflection of nickel powder and the light absorption of graphite powder, suitable for high-end decorative parts.

III. UV Spraying Process for UV + PVD Vacuum Coating

UV spraying is a key link connecting plastic pretreatment and PVD coating. Strict control of construction parameters and process steps is required to ensure coating quality and subsequent coating effects. The complete process includes four stages: pretreatment, spraying, leveling, and curing, as detailed below:

(I) Pretreatment Stage

1. Substrate Cleaning: An ultrasonic cleaner is used with ethanol or isopropanol as the cleaning medium to remove oil, dust, and mold release agent residues from the plastic surface. The cleaning time is 3-5 minutes, and the temperature is controlled at around 40℃.

2. Treatment Agent Coating: For difficult-to-adhere substrates, a special treatment agent is sprayed using an electrostatic spray gun with a coating amount of 8-10g/㎡ and a film thickness of 3-5μm. After spraying, infrared drying is performed at 50-60℃ for 3-5 minutes to ensure complete solvent evaporation.

(II) UV Primer Spraying

1. Coating Parameters: A special PVD UV primer is selected with a solid content of 40%-55%, a viscosity of 10-12s (NK-2# cup, 25℃), and solvents mainly composed of ketones and esters.

2. Spraying Operation: A reciprocating spray gun is used with a spraying pressure of 0.3-0.5MPa and a distance of 15-20cm from the workpiece. Uniformly apply 1-2 coats with a coating amount of 40-50g/㎡ and a target film thickness of 15-25μm.

3. Leveling Treatment: After spraying, place the workpiece in a leveling room for 5-8 minutes at room temperature, or infrared leveling at 60℃ for 3 minutes to eliminate spraying marks and ensure a smooth coating.

4. UV Curing: A mercury lamp curing machine is used with a curing energy of 400-800mj/cm². After curing, the coating hardness reaches above 2H without sticking.

(III) UV Mid-Coat Spraying (Optional)

For products requiring high surface roughness, an additional UV mid-coat process is needed. The mid-coat has a solid content of 15%-30% and uses high-volatility solvents. After spraying, the film thickness is 5-8μm. After infrared drying at 50-70℃ for 3-5 minutes, UV curing is performed at an energy of 300-500mj/cm² to fill small defects and improve surface flatness.

(IV) UV Metallic Paint Spraying

1. Coating Preparation: Fully stir the UV metallic paint before use to ensure uniform dispersion of metallic pigments. A small amount of anti-settling agent can be added to prevent pigment sedimentation.

2. Spraying Parameters: Low-pressure spraying is adopted with a pressure of 0.2-0.3MPa and a coating amount of 15-20g/㎡. Apply 2-3 thin coats to avoid sagging.

3. Leveling and Curing: After spraying, level for 3-5 minutes, then perform UV curing with an energy of 500-800mj/cm² to ensure complete curing of the coating and good oriented arrangement of metallic pigments.

(V) UV Top-Coat Spraying (Protective Coating)

As the final protective coating, the UV top-coat must have wear resistance, chemical resistance, and glossiness. A high-transparency UV clear coat is selected with a solid content of 45%-60% and a film thickness of 15-40μm. After spraying, infrared leveling is performed at 60℃ for 5-8 minutes, followed by UV curing at an energy of 600-1200mj/cm². The cured top-coat can pass the RCA wear test for more than 300 cycles, and meet the requirements of the boiling water resistance test (80℃, 60 minutes) and humidity resistance test (80℃, 96 hours, 95% relative humidity).