Advancing matte finish in plastics using polymer additives

Matte finish has a microscopically textured surface that diffuses light, making it non-reflective. This finish is better at hiding scratches while being more durable and easier to maintain. In contrast, a gloss finish has a smooth surface that reflects light directly, creating a shiny appearance. The glossy surface shows more scratches but offers better aesthetics and color depth.
 

• Gloss finish: Highly reflective and shiny. Often used for applications where a luxurious or high-end appearance is desired.
• Matte finish: Non-reflective and dull. Commonly used for applications where a more understated or practical appearance is preferred.
   

In the following table, you will find a comparison of characteristics between matte finish and gloss finish.

 

Figure 1 shows two identical parts in terms of their mechanical characteristics that differ only in the finish. The left image has a matte finish which reduces reflections as compared to the right image which shows a gloss finish. The matte finish is also a little rougher to touch. The type of finish can thus be chosen based on the desired look for the prototypes or series parts.²

 

Figure 1: Matte Finish vs. Gloss Finish Plastic Surfaces²

Some characteristics typically associated with matte finish include³:
 

• Matte appearance: It provides a non-glossy, velvety surface finish. It is ideal for applications where a sleek, matte appearance is desired.
• Improved surface coating: This allows manufacturers to achieve consistent and uniform surface coating. This is especially important for packaging materials and consumer goods.
• Texture modification: It can alter the tactile feel of plastic products. It enhances grip and reduces slipperiness.
• Cost-effective: It allows surface modification without the need for additional processing steps.

Several polymer additives are involved to get the desired matte finish in plastics. They work primarily through surface modification by migrating to the surface during processing. They disrupt the smooth plastic surface and create a diffuse reflection that appears matte to the eye. Let us understand the role of each polymer additive in detail.

 

Matting agents

Matting agents are a type of polymer processing additives that are added to polymers to reduce their glossiness and achieve a matte finish. They are also known as flatting agents or dulling agents. These additives work by scattering light and preventing it from reflecting off the surface in a concentrated manner¹. Lowering the extrusion die temperature provides a matte finish.

Matting agents can also provide other benefits, like:
 

• Hiding imperfections: They can help to hide surface imperfections, such as scratches, dents, and unevenness.
• Improving adhesion: They can improve the adhesion of paints and coatings to surfaces, especially in high-humidity environments.
• Enhancing durability: They can enhance the durability of paints and coatings by making them more resistant to abrasion and scratching.

Matte masterbatch additives compound is a versatile solution for achieving specific visual and functional effects in plastic products. Several matting agents are available in the market depending on their particle size and chemical composition. The best choice can be made depending on the application.

Some commonly used matting agents for polyethylene (PE) are discussed below:
 

• Calcium carbonate: It is a versatile filler commonly used in polyethylene formulations. CaCO₃ can effectively reduce gloss while also improving other properties like stiffness and heat resistance.
• Talc: This soft mineral has a platy structure that allows it to scatter light, creating a matte finish. Talc also enhances the smoothness and printability of polyethylene products.
• Silica: It is an effective matting agent for PE made from finely ground sand with excellent light-scattering properties. It can also improve the scratch resistance and dimensional stability of the final product. It is effective in reducing gloss and is also chemically inert, making it suitable for a wide range of applications.

Select matting agents compatible with various polymers available in our database:
 

Plasticizers

Plasticizers increase the flexibility and softness of the polymer. Certain plasticizers can also affect gloss. Fast-solvating plasticizers can lower process temperature and reduce gloss. They offer a range of benefits, such as:
 

• Compatibility: Plasticizers compatible with PVC formulations facilitate fusion at lower temperatures, resulting in a matte finish.
• Suitability for plastisol applications and can be easily used in virtually any polymer system requiring increased flexibility and softness.

Dibutyl phthalate (DBP), butyl benzyl phthalate (BBP), and benzoate plasticizers are notable examples of fast-solvating plasticizers⁵.

Nitrile rubber is a prime example of a solid plasticizer, often referred to as a "flexibilizer" or "plastifier." It is commonly used in conjunction with liquid plasticizers to enhance the flexibility and softness of polymers. However, when combined with liquid plasticizers, nitrile rubber can absorb the liquid and coagulate during high-speed mixing. To prevent this, it is recommended to add nitrile rubber to the cooler mixer at approximately 80°C instead of the hot mixer.

Nitrile rubber is often supplied in powder form under the grade of Chemigum® P35 or Chemigum® P83. It not only improves the surface finish, making it matte but also enhances the compression set, rivaling that of thermoplastic elastomers (TPEs)⁵.


 

Minerals

Minerals, such as aluminum oxide and magnesium oxide, can also be used as matting agents. They are often used in high-performance applications where durability and resistance to scratching are important. Functional fillers like diatomite, kaolin, synthetic silica, and titanium dioxide can be used as matting agents.

Matte finish majorly depends on the coating thickness and the loading level of the fillers. Apart from the loading level of the fillers, the level of gloss is strongly affected by:
 

• particle shape,
• particle size, and
• particle size distribution (PSD)
  
   

Waxes

Waxes, such as polyethylene wax and paraffin wax, are effective matting agents. They are often used in conjunction with silica to provide a more durable finish. Waxes are incorporated in plastics in different forms, such as:
 

• micronized
• micro powder
• emulsified, and
• dispersions

Oxidized high molecular weight PE waxes contribute to a matte finish. These low molecular weight PE waxes come in LDPE and HDPE forms. The partially oxidized LDPE and oxidized HDPE variants are particularly effective. Oxidized HDPE waxes, promote fusion while simultaneously reducing gloss.

 

Acrylic-type matting agents
 

They offer a distinct advantage over inorganic gloss control agents. They are non-abrasive, extending the lifespan of processing equipment. This makes them a preferred choice for applications where equipment longevity is crucial⁵.
 

• Crosslinked acrylate terpolymer: Commonly known as SUNIGUM®, it is a versatile additive that enhances engineering thermoplastics and thermoplastic elastomers. Compatible with polar polymers like PVC, SUNIGUM® imparts softness, a matte finish, and improved resistance to weathering, heat, UV radiation, chemicals, oils, and fuels⁵.
• Acrylic heteropolymer: PARALOID™ KF-710 is an acrylic heteropolymer that effectively reduces gloss in plasticized PVC compounds. When incorporated at a level of 2.5 phr, KF-710 can decrease 60° gloss from 42% to 16%, and further to 10% at 5 phr. This performance surpasses that of silica gel, which only reduces gloss to 23% at 2.5 phr and shows no further improvement at 5 phr⁵.

If impact strength is not a concern, ultrafine polymer particles of acrylics or polyamides can be used. For flexible vinyl compounds, 2-5% of acrylic impact modifiers of higher molecular weight are used.

 

PVC selection

Higher K-value resin & cross-linked resin reduces the gloss in flexible PVC.
 

• K-value: PVC resin K-value plays a role in glossiness. Generally, as the K-value increases, the gloss tends to decrease. For plasticized PVC products, opting for a higher K-value resin, such as K 70-72 or higher, can contribute to a matte finish compared to K-67 resin⁵.
• Crosslinking: Partially cross-linked PVC resin can be used as a partial replacement for conventional PVC to achieve a matting effect. These resins, offered by suppliers like Kaneka, inherently provide a matte finish due to their cross-linked structure.

Matte finish is preferred in certain plasticized PVC products to get a rich, sober, and rubber-like look. This is preferred in gaskets and interiors for automobiles, travel goods, cables, etc. Moreover, scratches are not immediately visible on the matte-finished surface. Therefore, there is a need to identify additives and processing conditions responsible for providing a matte finish.

 

Ethylene-vinyl acetate (EVA)
 

EVA, when used with PVC at 65-70% VA content, exhibits good compatibility. At approximately 45% VA, EVA starts to form alloys with PVC, functioning as an impact modifier for rigid PVC. Increasing the VA level to 50% transforms EVA into a plasticizer for PVC⁵.

However, EVA copolymers with a lower VA content (8-28%) serve as effective dulling agents in flexible PVC. These copolymers possess limited compatibility with PVC, disrupting the typically homogeneous glossy surface of flexible PVC and yielding a smooth matte finish⁵. Extrusion process parameters can also influence gloss reduction. Factors that contribute to a reduction in gloss include:
 

• a decrease in die and length,
• an increase in die and roughness,
• a higher extrusion rate, and
• a lower extrudate temperature

One practical approach to achieve this is by employing fast-solvating plasticizers, which can lower processing and die temperatures.

 

Other matting agents
 

• Pigments: They provide color to the polymer and can also affect its glossiness.
• Antioxidants: These prevent the degradation of the polymer due to oxidation. Antioxidants can sometimes influence gloss.
• UV stabilizers: They protect the polymer from the harmful effects of ultraviolet radiation. UV stabilizers can also affect gloss.

There are several ways to achieve a matte finish on plastic. Figure 2 shows the basic categories to get a matte finish/extinction on plastics¹.


 

Figure 2: Basic Categories of Extinction Method

Types of matting agents

Adding matting agents to the polymer during processing is the most common method to achieve a matte finish. 
 

• Inorganic matte agents: Adding these agents at a level of 1-3 phr (parts per hundred resin) can contribute to a matte finish. Their larger particle size helps to protrude particles, effectively scattering light and reducing gloss. Examples include silica or talc.
• Organic matting agents: Includes polymer micro-beads/sphere resins, prepared by dispersion polymerization. Examples include polymethyl urea particles, polystyrene composite microspheres, and melamine formaldehyde micro-sphere resins.

Several chemical and physical properties of matting agents will affect its matting effect. These include pore volume, average particle size, particle size distribution, whether the particle surface is treated or not, etc.
 

• Concentration of the matting agent: The gloss of the system is reduced with an increased concentration of matting agents.
• Particle size: The bigger the particles of the matting agent, the more effective the matting. This is especially true for precipitated silica and silica gels. However, particle size should be taken into consideration concerning paint film thickness while selecting the matting agents. Coarse matting agents can negatively affect other properties in a thin coating film.
• Pore volume (density): The porosity of this highly porous material is expressed by the pore volume, which indicates the number of internal voids (e.g., silica). The high pore volume of a matting agent can influence the rheology of the system.

Mechanical polishing
 

Techniques like sanding, blasting, or flame treatment can create a textured surface that reduces gloss. These are the most economical and popular surface finishes, polished using fine stone powder as shown in table 2. Sometimes called stone finish, it provides good release and helps hide machining marks.

 

Surface etching
 

Using chemicals to etch the surface of the plastic can create a micro-textured surface that scatters light.

 

Chemical extinction
 

Chemical extinction refers to the synthesis of specific polymer resins through certain chemical reactions. This uses the relationship between its structure and properties or the difference of properties between components, without the addition of various matting agents. This helps to achieve the effect of bulk extinction or self-extinction. The degree of extinction differs for different resin structures.

 

• Resins containing light-absorbing groups, such as nitrocellulose, amino resin, and short oil alkyd resin are easy to matte.
• Resins like saturated polyesters and polyurethanes are difficult to matte.

When the hybrid resin contains some extinction groups, it is also beneficial to the formation of the extinction effect. The chemical extinction coatings were first developed as powder-based coatings, and then gradually extended into UV-curing chemical matte coatings and water-based chemical matte coatings (Figure 3).

Compared with the traditional physical extinction method, chemical matte coatings can avoid the coating defects caused by the addition of an extinction agent. At the same time, they can lower the cost of raw materials and streamline the technological processes.

 

Figure 3: Film Formation Process of Traditional Water-based Matte Coatings¹

Matte masterbatch finds applications in various industries, such as packaging, automotive, consumer goods, and textiles. It is commonly used in the production of matte films, laminates, and injection molded parts.
 

• Automotive: Dashboards, door panels, and other interior components.
• Consumer electronics: Housings for smartphones, tablets, and laptops.
• Packaging: Bottles, containers, and films.
• Building and construction: Roofing materials, siding, and flooring.
• Household goods: Appliances, furniture, and kitchenware.

Achieving matte finish in polymers is a multifaceted process that combines the use of specific matting additives and processing techniques. This finish enhances the aesthetic appeal of products by providing a non-reflective surface. In addition, it also offers benefits like improved durability and ease of maintenance. These benefits lead to the increased demand of matte finished polymers in several industries like automotive, consumer goods, packaging, electronics, etc.