Antimicrobial additives for safer plastic applications

Antimicrobial additives are not one-size-fits-all. Different additives are needed in particular applications and to target specific microbes. Find out the different categories of antimicrobial additives in the following sections¹.

 

Organic antimicrobial additives
 

Organic antimicrobial additives have value in a wide range of applications and products. However, their incorporation into molded plastic products has limited effectiveness. Organic antimicrobials are relatively small molecules that are incompatible with the polymer matrix. As a result, the molecules leach to the surface where they interact with the microorganism preventing its reproduction. This is often referred to as a biostatic effect.

Leaching limits the useful life of the additive, especially on regularly cleaned surfaces, which leads to its use in consumable and disposable products. It also causes discoloration of the product and an unpleasant taste in applications with food and water contact. Organic antimicrobials are sensitive to high processing temperatures. This makes them challenging to successfully incorporate in thermoplastic materials.

 

Examples of organic antimicrobial additives
 

Organic antimicrobials include but are not limited to:
 

• phenolic compounds,
• isothiazolinone treatments,
• thiabendazole,
• zinc pyrithione,
• iodopropynyl butylcarbamate,
• quaternary ammonium compounds, and
• 10,10'-oxybisphenox-arsine (OBPA)
   

The arsenic-based OBPA biocide was preferred due to its low cost and antimicrobial effectiveness. Today, OBPA cannot be used in Europe since it is not BPR-listed. In addition, OBPA is currently under review by the US Environmental Protection Agency. As a result, a generation of newer, greener, and equally effective antimicrobial options has been developed to replace OBPA.

 

Inorganic antimicrobial additives
 

Inorganic antimicrobials are more likely to be incorporated into molded plastic products. Many of these additives are based on metal ions that are unreactive until released in association with another agent, like moisture. These antimicrobials remain stored in the polymer. They are released gradually to the surface, providing continual, long-lasting protection. These additives offer both biocidal and biostatic effects.

 

Examples of inorganic antimicrobial additives

Examples of inorganic antimicrobials are as follows:
 

• Zinc and silver have strong antibacterial activity at low concentrations. However, high levels are needed to achieve adequate antifungal properties.
• Zeolites are porous ceramic-based materials, containing a variety of antimicrobial ions such as zinc. These materials are good for short-term use (5-10 years), which is useful for disposable/limited-use products.
• Synergistic combinations of different actives can lower overall anti-microbial use levels, offer cost savings, and deliver superior antimicrobial performance.

Once you know the types of additives, the next step is learning how to incorporate them effectively into plastics.

Antimicrobial treatments can be applied to polymer products by coating the products or incorporating them while manufacturing the products. There are two primary methods for incorporating antimicrobial additives into plastics.

 

Masterbatch

A masterbatch is a concentrated mixture of the antimicrobial additive and a carrier resin. This can be added to the base polymer during the manufacturing process.

 

Processing methods

Antimicrobial additives can also be directly added to the polymer during processing methods like extrusion or injection molding. This involves mixing the additive with the molten polymer before shaping the plastic product. For example, by adding antimicrobial additives in polymers during the manufacturing process, the vinyl becomes resistant to microbial growth (Figure 1). This maintains aesthetic appeal and tensile strength while increasing the end-use life of the PVC product.
 

Figure 1: Untreated PVC (Left) and Antimicrobial-treated PVC (Right)
Tested using the ASTM G21²

Coatings

Coatings are typically applied on surfaces to prevent the growth and spread of harmful pathogens. There are various types of antimicrobial coatings available, each with its unique properties and applications.
 

Metal-based coatings

These coatings contain antimicrobial metals, such as silver, copper, or zinc. These metals have inherent antimicrobial properties and can be incorporated into coatings through various techniques.

Polymer-based coatings

These coatings are made from polymers (such as acrylics, epoxies, or silicones) that are infused with antimicrobial agents. These agents can be organic or inorganic.

To appreciate their benefits, it’s important to understand the mechanisms that make antimicrobial plastics effective.

Mechanisms of antimicrobial action
 

Antimicrobial additives can act against bacteria, fungi, and algae in a similar fashion. Some general modes of action occurring are summarized below³.
 

Reactive oxygen species (ROS) generation 

ROS are produced after exposure to materials such as metal oxides. Examples of ROS include superoxide anions, hydroxyl radicals, and hydrogen peroxide. These ROS can induce the peroxidation of the polyunsaturated phospholipids in the bacterial cells to damage DNA. This subsequently leads to cell death.

Physical damage

Cell wall membranes of microorganisms can be damaged when interacting with sharp edges of the nanostructured material. Antimicrobial additives work their magic by disrupting the cellular functions of microorganisms.

Binding

Binding materials on the bacterial cell wall can cause loss of cell membrane integrity and efflux of cytoplasmic materials.

Release of metal ions

Metal ions released from the nanomaterials into culture media can inhibit ATP production and DNA replication to destroy the cells.

Figure 2: Schematic Representation of Antimicrobial Mechanisms of Metal Ions³

Specific mode of action against bacteria

A summary of the likely bactericidal mechanism of antimicrobial additives, especially those of metal ions is schematically represented in Figure 2. For example, silver ions can bind to microbial DNA, preventing replication and leading to cell death. This makes silver-based additives highly effective in a variety of applications. For example, medical devices and consumer products.

Based on the mode of action of antibacterial additives they are of two types:
 

• Bactericidal: Bactericidal technologies contribute to irreversible cell death. They are often chosen for materials used in healthcare facilities where killing is targeted to maintain aseptic environments.

• Bacteriostatic: Bacteriostatic treatments differ from bactericidal versions in that they inhibit the growth and multiplications of bacterial cells, rather than directly kill them.

Antibacterial additives can disrupt the cellular functions of bacteria. A study was conducted to check the effectiveness of methicillin-resistant Staphylococcus aureus, also known as "MRSA" on plastic samples. The same amounts of bacteria were added to the sample with and without antibacterial additives. They were then incubated at 37°C (body temperature) for 24 hours.

As seen in Figure 3, large amounts of bacteria were recovered from the untreated plastic (Figure 3, left). In contrast, very few bacteria were recovered from the antibacterial plastic (Figure 3, right).

 

Figure 3: MRSA Recovered from Untreated Plastic (Left); MRSA Recovered from Antibacterial Plastic (Right)
Tested According to ISO 20743 Standard²

Hence, understanding the function of these additives at the molecular level is crucial for appreciating their effectiveness.
 

The effectiveness of antimicrobial additives in polymers depends on several key factors. This is measured according to the ISO 22196 standard⁵.

 

Type of microorganisms
 

Different antimicrobial agents exhibit varying levels of efficacy against different microorganisms. For example:
 

1. Bactericides: Agents may be particularly effective against bacteria
2. Fungicides: Agents effective against fungi
3. Algaecides: Agents effective against algae

Polymer type
 

The compatibility between the antimicrobial agent and the polymer matrix is crucial for optimal performance. Some polymers may be more receptive to certain antimicrobial agents than others. This influences the agent's distribution and activity within the material.

Select antimicrobial additives compatible with various polymers from our Master Catalog, and access technical data or request samples as needed:
 

Concentration of additive

The amount of antimicrobial agent incorporated into the polymer affects its antimicrobial activity. A higher concentration generally leads to greater antimicrobial protection. However, excessive amounts can compromise other properties of the polymer.

 

Processing conditions

The manufacturing process can influence the distribution and activity of the antimicrobial agent within the polymer. Several factors can affect the agent's dispersion and effectiveness in inhibiting microbial growth. For example, temperature, pressure, and mixing time.

 

Durability

Antimicrobial additives must be durable to maintain their effectiveness over time. They should not degrade or lose their antimicrobial properties under normal use conditions.

 

Regulatory compliance

Antimicrobial additives must comply with relevant regulatory standards, especially in industries like healthcare and food processing. These standards may vary depending on the specific application and the intended use of the antimicrobial polymer.
 

With a clear understanding of types, application, and performance, we can now explore where antimicrobial plastics are most beneficial.