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:
- Ester plasticizers
- Cellulosic rheology modifiers, and
- 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:
- 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.
- 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:
- PVOH stabilized PVA
- PVOH stabilized VAE
- Cellulose stabilized VASE
- Rubber emulsions (e.g., SBR, natural, and polyurethane)
- Surfactant stabilized VAE or acrylic
- Styrene acrylic
- 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?
How do Biocides Work?
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 buildu-p 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:
- Prevent a potential problem
- 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
Classification of Preservatives
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.
Chemical Family
|
Characteristics
|
Formaldehyde/formalin and formaldehyde donors (e.g. Triazines)
|
Environmental concerns but still used due to low cost
|
Heavy metals (silver, mercury, etc.)
|
Some restricted due to toxicity and environmental concerns
|
Organosulfur: Isothiazolinone-based
|
Combination and separate use of 5-chloro-2-methyl-4-isothiazolin-3-one (CIT) and 2-methyl-4-isothiazolin-3-one (MIT) and 1,2 benzisothiazolin-3-one (BIT)
|
Organosulfur: Pyridine derivatives
|
Zinc pynithiones have low solubility in water
|
Others
|
Organic acids and their salts, nitrogen and phenolics compounds, glutaraldehyde, cyanobutane
|
Blended formulations of the above
|
Allows custom formulation for a specific product, processing range, and microorganism
|
Various Selected Chemical Types of Biocides
Optimal Preservative Selection
Optimal Preservative Selection
The choice and concentration of biocide depends on:
- The type of microorganisms encountered
- The chemical makeup of the formulation
- The adhesive or sealant manufacturing processes, and
- Their use in the service environment
The table below lists the principal features considered important in the choice of preservative.
Feature
|
Characteristics
|
Efficacy
|
- Economical
- Broad-spectrum antimicrobial activity
- Unaffected by manufacturing processes or the physical condition of the product
- Effective over the product shelf life and/or service life
|
Safety, Health, and Environment
|
- Safe at practical use levels and handling
- Appropriate regulatory approvals
- Acceptable environmental profile
|
Compatibility
|
- Compatible and stable with a wide range of ingredients
- Water-soluble or dispersible in the product composition
- Effective over a wide pH range
|
Features Considered Important in Choosing a Biocide
Factors Influencing the Choice of Biocide
Factors Influencing the Choice of Biocide
Although biocides have the lowest incorporation level of any additive in adhesives or sealants, they must be cost-effective. And, any biocide additive must be easy
& safe to use, and acceptable to the environment. This final requirement is a source of consternation in certain regions where strict environmental regulations are limiting the use of what were once common biocide agents.
Other desirable features of a biocide include:
- Rapid biocide activity
- Lack of a strong or offensive odor
- Water solubility
- Heat stability, and
- Microbial tolerance issues
Formulation and processing factors that are important in selecting a biocide and their influence on product performance are described in the following table.
Factor
|
Influence
|
Chemical composition of formulation ingredients
|
Most susceptible: Fillers such as china clay and calcium carbonate; natural materials such as starch and dextrin.
Susceptible: Cellulose derivatives, polyvinyl alcohol solutions.
Less susceptible: Polymer emulsions are generally attacked through their stabilizing system components rather than the polymer itself.
Least susceptible: Concentrated or solid surfactants and defoamers.
|
Compatibility
|
Acid/base interactions can cause compatibility problems. This is generally seen as coagulation or gel formation.
|
pH
|
Preservatives are active over specific pH ranges. Polymer emulsion-based adhesives are prone to yeast and mold growth at pH of 3-7 and bacteria at pH of 6-8. Over pH of 9 and under a pH of 2, emulsion adhesives are less prone to attack. |
Solids
|
Very low solids products have shown a particular sensitivity to attack. |
Climate
|
Polymer emulsion adhesives designed for export to very warm countries are particularly susceptible to microbial attack. This is due to condensation of dilute films onto the product’s surface which are readily attacked.
|
Processing
|
High-temperature processes require the preservative to be heat stable or to be added after heat processing. |
Redox state
|
Excess oxidizing agents can have deleterious effects on certain preservatives. This is also true for reducing agents. |
Nucleophiles
|
Heterocyclic preservatives can be decomposed by nucleophilic attack. Primary, secondary and tertiary amines can deactivate some isothiazolinone preservatives. Sulfur nucleophiles like mercaptans shown the same effect. |
Physical form
|
Liquid preservatives are easier to incorporate than powders and reduce exposure of factory personnel.
|
Solubility
|
The preservatives used should be soluble in the aqueous phase of the product.
|
Versatility
|
A broad spectrum of antimicrobial activity is required. |
Cost
|
The high cost of some preservatives can be offset by their greater efficiency.
|
Factors Influencing the Choice of Biocide[1]
Common Biocides for Polymer Dispersions
Common Biocides for Polymer Dispersions
Some of the common preservatives currently used in polymer dispersions and characteristics important in their selection (e.g., target organisms, effect of oxidizing and reducing agents, and effect of pH and temperature) are shown in the table below.
Biocide
|
Target
Organisms
|
Affected by
Oxidizing Agents
|
Affected by
Reducing Agents
|
Effective
pH Range
|
Effective
Temperature
(°C)
|
1,3-Benzisothiasolin-2-one (BIT) |
Bacterial & Fungal
|
Yes
|
Yes
|
2-14
|
<100
|
Methychloroisothiazolinone (CMIT)/Methylisothiazolinone (MIT)
|
Bacterial & Fungal
|
No
|
Yes
|
3-8
|
<40
|
1,2-Benzisothiaxolin-3-one (BIT)/Methylisothiazolinone (MIT)
|
Bacteria
|
Yes
|
Yes
|
3-10
|
<60
|
Methylisothiazolinone (MIT)
|
Bacteria
|
No
|
Yes
|
Up to 10
|
<45
|
Formaldehyde releasing biocides
|
Bacteria
|
Ammonia
|
Casein
|
*
|
**
|
Note: *pH and temperature will determine the amount of formaldehyde release
**Increased temperature increases formaldehyde release
|
Characteristics of Selected Common Biocides Used in Polymer Dispersions
Concentration of Biocide in an Emulsion
Concentration of Biocide in an Emulsion
Antimicrobial agents are used in amounts ranging from ppm to 1% by weight. Additives or certain processing conditions can degrade the biocide originally present in the polymer emulsion. Therefore, the raw materials may contain little or no biocide by the time they are ready to use by the formulator, and they could be highly susceptible to biological contamination.
In these cases, additional biocide must be added to further preserve the product.
Since the biocide will become depleted with time, the level of biocide present in a formulation must be well above the minimum inhibitory concentration necessary to prevent biological growth.
Factors that can decrease the concentration of a biocide in an emulsion and affect its shelf life are mentioned below:
- Dilution of the emulsion
- Changes in pH
- Use of high temperatures during formulation
- Addition of reducing agents such as sulfites or bisulfites
- Addition of oxidizing agents such as peroxides, persulfates or bleach
- Addition of good nucleophiles such as ammonia, amines, or mercaptans
- Addition of certain metal salts
- Addition of biologically contaminated formulating ingredients
- Addition of starches, modified celluloses, polyvinyl alcohol, sugars, and other naturally occurring polymers
A worrisome event that often plagues formulators is that the microorganism can mutate to become resistant to the selected biocide. This has resulted in the practice of biocide rotation.
Many suppliers will switch biocides every 6 months to 2 years even if there have been no outbreaks. The belief is that by switching products the microorganism is not allowed to become resistant to any specific biocide.
After you have selected a biocide/preservative for your formulation, its recommended to test if it would perform well.
Regulations Driving Future Trends
Regulations Driving Future Trends
Environmental and regulatory concerns dominate the current state of biocide development. Compliance with VOC limits in the United States, the Biocidal Products Directive (BPD) and REACH legislation in Europe are driving much of the product development occurring today.
The proliferation of regulations around the world will continue to dramatically limit the availability of chemistry choices. As a result, many suppliers have reduced their R&D efforts regarding the development of new biocides and are focusing on acquisitions of other companies and chemical variations and blends of current products.
Controlled release of biocides at the right place and at the right time is the ultimate objective of technology to control microorganisms.
To this end, research has begun in biocide microencapsulation technology. A timely and targeted release improves the effectiveness of the biocide, broadens their application range, and ensures optimal dosage. Although this new technology offers great potential to the adhesive and sealant industries, little commercial development has been carried out to date in this field.
Adhesives Professionals - Stay Alert!
Update your knowledge on preservatives for adhesives and sealants by taking live course: Biocides in Waterborne Adhesives: How to Select in Today's Regulatory Environment? by Edward Petrie.
Testing Microbial Resistance of Formulations
Commercially Available Biocides for Adhesives and Sealants
Commercially Available Biocides for Adhesives and Sealants
Many of the biocides used in adhesive and coating systems contain sulfur. Isothiazoline-based biocides are the most popular and this class contains multiple chemistries. Many commercial biocides are made of blends of two or more biocides to improve range and efficiency.
Products in use:
- 3:1 ratio of 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT) and 2-methyl-4-isothiazolin-3-one (MIT) is frequently used. CMIT/MIT has broad spectrum efficacy versus bacteria, algae, and fungi.
- 1,2-benzisothiazolin-3-one (BIT) products have been used in a limited range of industrial applications requiring long-term preservation for bacterial control.
- Recently, a new microemulsion technology was introduced using 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT).
Several commercial biocide agents that are used for in-can preservation and dry-film protection are indicated in
the table that follows. This list is only an example of the commercial biocides in use today. It is far from complete.
Company
|
Active Ingredient
|
Trade Name
|
In-Can Biocide Agents:
|
Lonza
|
1,2-Benzisothiazolin-3-one in dipropylene glycol
|
Proxel™ GXL
|
Lanxess
|
Benzisothiazolone
|
Veriguard 19
|
Lanxess
|
Aqueous dispersion of approx. 25 % 1.2-dibromo-2.4-dicyanobutane.
|
Tekatamer 38 A.D.
|
Lanxess
|
1, 2-dibromo-2.4-dicyanobutane (24%) and isothiazolinones (0.08%)
|
BIOCHEK® 430
|
Lanxess
|
1, 2-dibromo-2.4-dicyanobutane (19%) and 1, 2-benzisothiazolin-3-one (6%)
|
BIOCHEK® 410
|
Lanxess
|
Tetrahydro-3,5-dimethyl-2H-1,3,5-thidiazine-2-thione
|
N-521 Dispersion
N-521 PAC 24
N-521 P
|
Dow
|
1-(3-Chloroallyl)-3,5,7-triaza-1-azoniaadamantane chloride combined with a stabilizer (sodium bicarbonate).
|
DOWCIL™ 75
|
Lanxess
|
o-phenylphenol, tetrahydrate sodium salt
|
P1
|
Dow
|
o-phenylphenol
|
Dowcide 1
|
Lanxess
|
Blend of 15% o-phenylphenol and 5% dazomet.
|
Veriguard OD
|
Troy
|
4,4-dimethyloxazolidine (DMO)
|
Nuosept™ 101
|
Dow
|
5-chloro-2-methyl-isothiazolone and 2-methyl-4-isothiazolinone
|
Kathon LX 1.5%
|
Dry-Film Biocide Agents:
|
Lonza
|
Zinc pyrithione
|
Zinc Omadine®
|
Lanxess
|
2-(4-thizolyl)benzimidazole
|
METASOL® TK-100
|
Dow
|
Diiodomethyl-p-tolysolfone
|
AMICAL™ 48
|
10,10’oxybisphenozyarsine (OBPA)
|
Durotec 7603
|
Troy
|
3-iodo-2-propynylbutyl carbamate (IPBC)
|
Troysan Polyphase AF-1
|
Lanxess
|
o-phenylphenol (OPP) + dodecylguanidine HCL
|
Veriguard 13
|
Tetrachloroisophthalonitrile + p-[(diiodomethyl) sulfonyl] toluol
|
Veriguard 67
|
Biocide Agents for Adhesives and Sealants
By combining different types of biocides (e.g., a fast acting biocide with a long-term preservative) the biocide supplier can target a broader antimicrobial spectrum and offer optimal protection.
Also, certain combinations of active biocide ingredients have been shown to provide synergy – providing performance at lower concentrations than a higher concentration of either active ingredient alone.
Biocides Used in Adhesives and Sealants
References
- Cresswell, M.A. and Holland, K., “Preservation of Aqueous-Based Synthetic Polymer Emulsion and Adhesive Formulations”, Chapter 9 in Preservation of Surfactant Formulations, Morpeth, F.F., ed., Blackie Academic, Glasgow, 1995.