Evaluating Antimicrobials

December 3rd, 2009 by manny No comments »

There are several different preservative additives for polymeric materials. The most commonly used are 2-n-octyl-4-isothiazolin-3-one and 10,10’-oxybisphenoxarsine. Because of the severity of the application, there are only a few preservatives that can be used.

Preservatives for polymers are considered pesticides and are registered with the Environmental Protection Agency under the Federal Insecticide, Fungicide, and Rodenticide ACT (FIFRA). All preservatives are currently under review by the EPA.

Performance effectiveness of the preservative should be evaluated in each formulation. Traditionally, there are two ways to evaluate microbiological resistance: controlled laboratory evaluations and actual field trials.

Controlled laboratory studies are used to allow for a rapid determination of the microbiological resistance. By using carefully controlled studies, the susceptibility of the polymer can quickly and easily be examined. By using proper controls, the rate of migration of the preservative, the “natural” resistance to the compound without a preservative, and the effects of various levels of preservative can be examined.

Preliminary tests are useful

Also within the laboratory, artificial environmental conditions from a carbon-arc or xenon-arc Weather-o-meter or Q-Panel can be used before actual microbiological testing. The artificial weathering is generally regarded as appropriate for exterior applications. There are two laboratory tests typically for polymeric materials. ASTM G 21-85, “Determining the Resistance of Synthetic Polymeric Materials to Fungi,” is the standard industry test. It calls for the fungi to get nutrients from the polymeric material. The test looks at the protective nature and not the leachability of the preservative.

The second test is ASTM E 1428-1, “Evaluating the Performance of Antimicrobials in or on Polymeric Solids Against Staining by Streptoverticillium Reticulum (a Pink Stain Organism),” more commonly known as the pink stain test. It relates to a common difficulty with microorganisms where metabolic by-products are produced that diffuse through a polymeric compound and leave a permanent stain. Under test conditions the polymer is in direct contact with the stain-producing organism and must stop the stain diffusion.

There are other tests available for specific applications or environments, such as the soil burial test or tests requiring different combinations of microorganisms. Earlier test methods which relied on leachability of the biocide, a “zone of inhibition”, were previously considered acceptable. It is now generally regarded as not representing the actual resistance of the polymeric compound, particularly with exterior applications. Neither of the test methods outlined above require such an evaluation.

Outdoor exposure is also used to evaluate the performance of an antimicrobial in a practical environment. Most sites for testing of this type are located in Florida and Arizona. Florida is preferred because of its high humidity, which is more conducive to fungal growth. The limiting factors in outdoor exposure tests are the length of the testing required (usually several months) and the unpredictability of the environment.

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Antimicrobials

November 30th, 2009 by manny No comments »

Antimicrobials fall into the category of preservatives and are often referred to as mildewcides, fungicides, bacteriocides, or biocides. No matter the specific moniker used, preservatives serve to prevent microorganisms from attacking polymeric materials. Microbial consumption of the polymer may cause degraded aesthetic appearance, slight to strong mildew odors, brittle products, or a variety of other product defects.

A majority of polymeric materials in their pure form are inherently immune to attack by microbes. However, after various plastic additives are compounded in the master batch, the final formulation may support the growth of microbes that are capable of deteriorating the final product. Plasticizers, lubricants, and even a few heat stabilizers are a few additives that may be prone to microbial growth promotion. Flexible PVC is particularly susceptible to attack by microorganisms because it uses many of these additives in large quantities.

Enhancing Microbial Resistance

There are two major considerations when optimizing polymeric compound’s microbial resistance. First, minimize the use of additives that enable microbial growth. Some level of resistance can be gained through reduction of additives known to affect the susceptibility to microbiological attack. Plasticizers used to provide flexibility to PVC products can dramatically impact the susceptibility of the end product. The selecting a plasticizer with a strong microbial resistance will help to reduce the vulnerability of the compound.

Phthalates, polymeric esters, citrates, and other similar plasticizers have been shown to only slightly increase the susceptibility of flexible PVC. The following plasticizers are considered to be moderately susceptible to microbial attack:

  • Adipates
  • Azelates
  • Pentaerythritol esters
  • Sebacates
  • Epoxidized soybean oil
  • Epoxidized tallates
  • Glycolates

Using a non-migratory plasticizer such as tricresyl phosphate and polyester can also help to reduce the availability of nutrients on the surface of the polymer that may be consumed by any microorganisms.

The second major consideration for maximizing microbial resistance while maintaining desired functionality is the judicious use of a preservative agent. For a preservative to be most effective, it must migrate to the surface of the polymer in sufficient concentrations to stop the growth of surface microbes. However, the preservative should not bloom to the surface too quickly, or leach from the polymer, which could reduce the useful life of the end product.

Cost-effective and convenient liquid solutions are available for a wide range of materials like flexible PVC, acrylics, and silicone-based adhesives. These formulations offer advantages such as enhanced shelf stability, color compatibility, and reduced handling and shipping costs.

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Antifog Application Methods

November 30th, 2009 by manny No comments »

There are two main ways that antifogging agents can be applied. The first method is a topical or coating application. This can be accomplished through dip, spray, or gravure techniques. Topical applications tend to be relatively inexpensive and provide an immediate antifog benefit. However, externally applied antifog coatings may not be long lasting as the coating can be removed easily.

The second application technique is the internal incorporation of the agent by dry blending or extrusion processes. This is the preferred method for obtaining an antifog effect in film. Internally incorporated anti-fog agents are intentionally selected to be incompatible with the polymer carrier. This incompatibility forces the anti-fogger to migrate away from the polymer base. In most cases this migration will be toward the surface of the polymer which is where it will afford the most benefit, a phenomenon known as blooming.

The anti-fogging agent will reach a maximum surface density where no further blooming will take place. In use the agent will can be removed by volatilization or mechanical abrasion. Fortunately, internal antifog agents beneath the surface act like a reservoir in the film to continually replace the additive as it is consumed during its performance. Consequently, internally applied antifog agents can provide long-lasting performance and protection. Additionally, they can provide other benefits such as antistatic, cling, and lubricating properties.

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Anti-fog Agents

November 28th, 2009 by manny No comments »

One major point of consideration for manufacturers of agricultural/horticultural films is the fogging of packaging for products with medium to high moisture content. Water vapor trapped within the packing condenses on the inner surfaces of the film into small droplets which produces a fog-like appearance. This fogging causes light scattering and an undesirable optical barrier. The end results are poor aesthetics and decreased functionally. Anti-fogging agents have been developed to eliminate or reduce the formation of water droplets. These surface-effect chemicals, often added during polymer processing, condense water vapor into a continuous, transparent layer instead of visible and relatively large droplets. The additives work by minimizing the surface tension of the polymer which reduces the contact angle between water and the polymer surface.

  • Sorbitan
  • Ethoxylated sorbitan fatty acid esters
  • Glycerol fatty acid esters
  • Alcohol ethoxylates
  • Proprietary formulations

Quite frequently, different types of antifog agents are combined to get the best anti-fog performance. Typical anti-fog usage levels start at around 0.5% and can go as high as 4.0%.

Polymer processing conditions, film thickness, and additive thermal stability are factors that influence the choice of the most appropriate agent. Polymer processing conditions, compatibility, low-extractability, and physical form of the additive are other important considerations. In food packaging applications in certain cases, local, national, and international government regulations must also be considered.

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