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Need for Preservatives

Need for Preservatives

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

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.

Property Change Due to Microbial Infection
Impact
Viscosity change
Polymer dispersions can become thinner or thicker depending on the effect of increased concentration of acidic by-products. Phase separation can also occur. Viscosity increase and microbial infection can also restrict the flow within the factory equipment piping, filters, etc.
pH change
The metabolic by-products often are acidic in nature. The reduced pH will cause destabilization of the polymer dispersion and promote a corrosive environment both in the factory (surface of plant equipment) and once in service (corrosion of substrates).
Odor production
Bacteria are often sulfur-reducing. Other microbes have the ability to produce odors based on their biochemical reactions.
Gas production
Bacteria can produce hydrogen sulfide gas which leads to odor and gas production problems.
Color change
Microbes can change the color of the product before or after application. Sulfur-reducing bacteria generally blacken the polymer dispersion or the finished product.
Visible surface growth
Microbes lead to color and viscosity change (see above).
Corrosion
Corrosion of plant equipment and of substrates can occur from metabolic byproducts and acid production.
Change in properties (due mainly to reduction in molecular weight)
Breakdown of the polymer molecular weight and/or change of dispersion property characteristics can affect the end-use properties of the adhesive or sealant.
Effect of Microbial Growth in Polymer Emulsions


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).

Types of Polymer
pH
Ethylene vinyl acetate
Polyvinyl acetate
PVA/acrylic
PVA/Versatate and PVA/Acrylic
Acidic (pH 3.5-6.5)
Styrene acrylic
Polyolefins
Alkaline (pH 7.0-9.5)

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


How do Biocides Work?

Classification of Preservatives

Optimal Preservative Selection

Factors Influencing the Choice of Biocide

Common Biocides for Polymer Dispersions

Concentration of Biocide in an Emulsion

Regulations Driving Future Trends

Testing Microbial Resistance of Formulations

Commercially Available Biocides for Adhesives and Sealants

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