Selecting preservatives and testing microbial resistance

Microorganisms can breakdown adhesive or sealant products before their service life is complete or, in some cases, even before their service life has even begun. These microorganisms include bacteria, fungi, yeast, and mold. 

Microorganisms exist everywhere, especially where water and appropriate nutrients are available for their growth and survival. They thrive primarily at 20 - 30°C and high humidity. 

Given their relatively simple needs for life, microorganisms can attach and grow in many adhesive and sealant products that contain water or naturally-based ingredients. 

Some good sources of food include:
 

• Starch
• Dextrin
• Cellulose
• Animal fats
• Vegetable oils
• Polymers that contain aliphatic hydroxyl and ester groups
  • Polycaprolactone
  • Polyester-based urethanes, and
  • Biopolymers

Synthetic polymer water emulsions that are especially susceptible to microbial contamination include polyvinyl acetate, polyvinyl alcohol, and ethylene/vinyl acetate. 

Even RTV silicone sealants that do not inherently support microbal growth are subject to microbial degradation. External chemicals commonly found near construction sealants can migrate into the sealant, and these chemicals may support microbial growth.

The base polymer in an adhesive formulation is not the only component that the formulator needs to worry about when fighting microbial growth. Formulation additives are also often an excellent nutrition source and become a primary focus of biological attack. These include:
 

1. Ester plasticizers
2. Cellulosic rheology modifiers, and
3. Epoxy ester stabilizers among others

Microbial contamination can manifest itself in a number of ways. There are primarily two stages in which microbial infection can become dominant:

1. When the adhesive or sealant is in liquid form. The microbes can feed off the moist environment and nutrients supplied by additives and raw materials.
2. After the adhesive or sealant is applied and cured. The microbial attack can occur on the finished surfaces of the polymer film.

Microbial growth in polymer emulsions

Microbial growth on polymer emulsions could lead to costly customer quality issues and down time for factory decontamination. Bacterial growth can also contribute to a decrease in indoor air quality and lead to human health problems. 

The effects of microbial growth in polymer emulsions are listed in the table below.

 

The adhesive and sealant emulsions that are most susceptible to microbial growth are listed below in order of susceptibility:
 

1. PVOH stabilized PVA
2. PVOH stabilized VAE
3. Cellulose stabilized VASE
4. Rubber emulsions (e.g., SBR, natural, and polyurethane)
5. Surfactant stabilized VAE or acrylic
6. Styrene acrylic
7. Acrylamides, N-methyl acrylamide, N-butyl methyl acrylamide

Most of the microorganisms encountered in industrial practice are in the range of 4-9 pH.
 

• Fungal organisms are more prominent at acidic pH, and
• Bacterial organisms are more prominent at neutral to slightly alkaline pH.

Polymer emulsions generally fall in the ideal pH range for microbial growth (table below).
 

Now, let's explore how to prevent microbial degradation and the properties of an ideal preservative for adhesive and sealants.
 

Antimicrobial agents, collectively known as preservatives, biocides/ or fungicides, are added to certain adhesive and sealant formulations in order to inhibit the growth of microorganisms either during shipping and storage (in-can preservation) or after the product is applied (dry-film preservation). Each antimicrobial agent has a specific spectrum of activity depending on the microbial agent encountered and the susceptible material.

 

Biocides have mostly low molecular weight molecules that kill or suppress the growth of microorganisms. Molecular aspects of the action of these agents are outside can be found in the literature.^[1]

 

They generally work by entering the microorganism's cell membrane to damage and inhibit the cell's genetic activity or to interact directly with the cell membrane to destroy its integrity.

 

Mechanism of antimicrobial agents

 

 

Preventing microbial degradation

Biocides are particularly effective when used proactively in a formulation, however, they can also be used for clean-up of contaminated water or equipment. Proper factory maintenance strategies can prevent microbial infection from the source and reduce the need for a biocide.

The various plant hygiene preservation strategies are summarized below. 
 

• Keep incoming tank loading lines clean and dry or full of product protected with biocide. Avoid piping configurations that allow for the buildup of stagnant areas of emulsion.
• Avoid leaving residues in storage tanks prior to filling with additional product, whenever possible.
• Agitate storage tanks, if possible, and/or use materials in storage tanks on a first-in, first-out basis.
• Avoid stagnant water. Avoid water in loading and packaging lines. Purge or drain all lines prior to use.
• Check for contamination of process water and other key raw materials, including emulsions. This can be done by streaking samples of raw material onto appropriate growth media and incubating the growth media to observe the level of microbial growth (streak testing).
• Clean and sanitize tanks, lines, hoses, and any surface that may come in contact with polymer emulsions. Cap hoses if possible. Inspect the cleaned areas visually after cleaning and sanitizing is completed, if possible.
• Keep the manufacturing area clean and dry.
• Conduct sterility testing in the manufacturing area on a monthly basis. Include visual check of the top of the storage tank each month.
• Conduct streak testing on the outgoing product to confirm that they are free from microbial contamination.

There are various preservation strategies for the formulator to use to protect his or her formulation. These include:
 

• Checking and treating the water supply
• Checking raw materials
• Improving plant design and hygiene
• Using a broad spectrum biocide

 

 

 

Properties of an ideal preservative for polymer emulsions

In an adhesive or sealant formulation, the main function of a biocide is to kill or inhibit the growth of microorganisms. In this respect they are used either to:
 

1. Prevent a potential problem
2. Correct a problem that already exists

In either case, the biocide must meet certain requirements to be an effective product. Some of the basic properties required for all biocides that are used in polymer emulsions are mentioned below.
 

• Broad-spectrum activity against bacteria, molds, yeasts
• Stability over a wide pH range
• Stability at high temperature, not volatile
• Resistance to redox agents
• Water-soluble at low concentrations with correct partition characteristics
• Compatibility with the polymer emulsion type and formulation components
• No effect on viscosity
• Low toxicity/ecotoxicology (generally free from heavy metals, formaldehyde, chlorophenols, etc.)
• Relative regulatory approvals
• Cost-effectiveness

 

 

How to improve fungus resistance in adhesives & sealants? Find out how »

 

Classification of preservatives

 

Biocides can be classified in many different ways. The most practical method of classification is by how they work. The classes of biocides that will be discussed mainly are In-can and Dry-film preservatives.
 

In-can preservatives
 

• Biocides work either with the product during manufacture and storage to increase shelf life or with the product after application to a substrate to prevent premature failure.
• In-can preservatives inhibit microbial growth in water-based products during the manufacturing process and product storage. 

Dry-film preservatives
 

• Dry-film fungicides inhibit mildew and/or algae growth in an applied adhesive or sealant. In dry film, the biocide additives are somewhat different than they are for in-can preservatives.
• Fungicides and mildewcides are used in both aqueous and solvent-borne adhesives and sealants to inhibit fungal and algae growth in the dry adhesive film.
• The primary requirement for a dry-film biocide is low water solubility for it is necessary that the active ingredient does not migrate out of the adhesive or sealant with time. Due to the requirement for fungicidal and algaecide activity, combinations of biocides are generally used.
• Dry film preservatives are used in both aqueous and solvent-based systems.

Biocide chemical families

Biocides are complex chemicals, typically with long chemical names. Formulators know many biocides by their general chemical classification or trade name.

A variety of chemicals can be used to stabilize polymers against biological attack. A general chemical classification scheme is listed in the table below.