Final Report: Nanocomposite Anchored PlasticizersEPA Contract Number: 68D02060
Title: Nanocomposite Anchored Plasticizers
Investigators: Myers, Andrew
Small Business: TDA Research Inc.
EPA Contact: Manager, SBIR Program
Project Period: June 1, 2002 through June 1, 2004
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2002) Recipients Lists
Research Category: Nanotechnology , Air Quality and Air Toxics , SBIR - Pollution Prevention , Pollution Prevention/Sustainable Development , SBIR - Air Pollution , Small Business Innovation Research (SBIR)
Plasticizers are small, often volatile molecules added to hard, stiff plastics to make them softer and more flexible. Plasticizers are not directly bound to the polymer chain and can escape from the plasticized material. Loss of plasticizers is an undesirable event, leading to brittle and unusable materials and contamination of the surrounding environment. This is particularly a problem for polyvinyl chloride (PVC), which can be highly plasticized and often is used in toys for infants and medical tubing and solution bags. A system that immobilizes the plasticizing agent within a polymer without compromising other necessary physical properties would find a ready commercial market. One solution is to anchor the plasticizing moiety to the surface of a nanoparticle. If properly designed, the plasticizing nanoparticles would show good dispersion without the loss in physical properties observed with larger particle fillers.
TDA Research, Inc. (TDA) has shown that plasticizers anchored on nanoparticles can soften PVC, but cannot escape from the polymer. The plasticizing system consisted of plasticizing groups attached to an inorganic, nanometer-sized (50-100 nm) core. Although the nanoparticles resisted efforts to migrate out of the polymer, the nanocomposite PVC exhibited a lower glass transition temperature, tensile strength, and modulus; indications of forming a softer, more plasticized material. Both rigid and traditionally plasticized PVC formulations showed increased plasticization with the addition of TDA’s nanoparticles.
The additional benefit of increased plasticizer permanence also was discovered with the addition of TDA’s nanoparticles. In PVC formulations plasticized with 50 phr dioctylphthalate (DOP), the addition of small (2-5 percent) amounts of TDA’s nanoparticles significantly decreased the percent of plasticizer lost to air, activated carbon, water, soap solution, cottonseed oil, and hexane. This improved retention feature could increase service lifetimes of PVC materials and would improve consumer acceptance.
Polymer nanocomposites are a combination of a polymeric host matrix and additive particles that are smaller than 100 nm. The nanoparticles have very high surface area to volume ratios. The amount of interfacial area between the polymer and the nanoparticle is significant, and the unusual behavior of the polymer at nanoparticle surface contributes to the overall bulk physical properties of the composite. There is a synergistic effect of combining nanoparticles with polymers that is well beyond the sum of the properties of both phases, and revolutionary improvements in the properties of the resulting composite materials can be achieved.
TDA has developed a nanoparticle technology that shows excellent dispersion and is easily processed using standard industrial melt-mixing methods. TDA’s nanoparticles are small (20-50 nm), flat, surface-modified inorganic particles that can be modified to be compatible in many thermoplastic and thermoset polymers. In contrast to commercially available nanoclay materials, these nanoparticles do not significantly increase the viscosity of the resin, have good dispersing ability within a wide variety of organic materials (including solvents and polymers), and remain dispersed at the nanometer scale. TDA’s nanoparticles are predicted to be inexpensive and effective at low levels (often less than 5 percent), making them attractive as an additive for commodity polymers.
The goal of Phase I was to demonstrate that TDA could prepare a PVC nanocomposite that was softer because of the addition of specially designed nanoparticles. In the case of already plasticized PVC, the retention of traditional phthalate plasticizers was improved by forming a more effective barrier nanocomposite that prevented the plasticizers from migrating to the surface. Nanoparticles were synthesized that had good compatibility with PVC and showed plasticizing properties.
The goal of Phase II was to further develop the nanoparticle anchored plasticizers identified in Phase. TDA’s initial goals were to synthesize nanoparticles with good compatibility and good dispersion with PVC. These requirements narrow both the choices of surface groups for TDA’s nanoparticles and the property changes that can be achieved by using those specific surface groups. Several methods were explored to optimize the mixing of TDA’s nanoparticles into PVC using standard industrial polymer processing methods. Most current PVC compounding is done at temperatures above the melting point of the polymer system. Good melt processing also was a goal of the research project and was evaluated with a Brabender polymer mixer and a two-roll mill. Testing the material to assess the improved properties of the nanocomposite was conducted using current industry standard testing protocols. Appropriate American Society for Testing and Materials methods were followed to measure the plasticizing nature and improved plasticizer permanence of TDA’s PVC nanocomposites. Finally, an engineering and commercialization assessment was conducted to determine possible property changes and the feasibility of incorporating TDA’s nanoparticles in PVC formulations.
Successful nanocomposite formation requires overcoming a number of obstacles. The first (and often largest) challenge is dispersion. Without successful dispersion, the nanoparticles aggregate and resulting composite is no better than those achieved with conventional micron-scale additives. The next obstacle is improving key material properties. In Phase I, TDA successfully overcame both of these hurdles. Nanoparticles were successfully dispersed into PVC formulations, as measured by visual observation of a homogeneous material and appropriate property changes. These nanocomposites exhibit plasticization without the use of external plasticizers, and show an improved permanence of plasticizers when formulated with traditional plasticizers such as DOP. The final properties of both plasticized and nonplasticized PVC varied with the amount and type of nanoparticle used. As such, the addition of nanoparticles offers an additional method of tailoring the final properties of PVC materials. Many properties were similar to, or slightly better than, plastics made without nanoparticle additives, indicating that TDA’s nanoparticles can be incorporated without producing undesirable side effects on other properties.
Careful selection of the organic periphery of TDA’s nanoparticles maximizes compatibility with a given resin. Good compatibility was found with nanoparticles that had been surface modified with organic groups similar to those found in traditional PVC formulations. These included stearic acid (C18) and portions of epoxidized soybean oil (ESO). Stearic acid (C18) is a known lubricant for PVC, and ESO is used as a thermal stabilizer. Polyethylene glycols and caprolactone oligomers have been used as plasticizers and viscosity depressants in PVC and were found to be good compatibilizing groups for TDA’s nanoparticles. Several custom-synthesized molecules were prepared that contained plasticizing groups bound by long flexible chains. Significant scale-up development was carried out, and TDA’s nanoparticles were prepared easily in kilogram quantities using a “bucket chemistry” type process.
During this research project, nanoparticle anchored plasticizers were added to conventionally plasticized and rigid PVC formulations. In the case of PVC plasticized with external plasticizers, DOP was used at levels of approximately 50 phr. Formulations for PVC materials of varying degrees of flexibility were obtained, and one formulation that was fairly standard for a plastic that would be used for a child’s toy or the dashboard of a car was examined. Both of these applications represent risks for plasticizer exposure for children, car interior surfaces being the larger and more significant commercial market. PVC formulations were processed in several melt-mixing methods, using both a Brabender polymer mixer and a two-roll mill. TDA’s nanoparticles showed excellent compatibility with both the host resin as well as the processing methods. Melt viscosity differences with the nanocomposite often were lower than blank formulations because of the plasticizing and lubricating nature of the nanocomposites.
Softer, more plasticized materials display decreased tensile strength and modulus and increased elongation. TDA effected these changes in both rigid and flexible PVC. Most importantly, TDA was able to do so while keeping the nanoparticle anchored plasticizer migration to zero. As an added feature, the permanence of traditional phthalate plasticizers in flexible PVC was improved with the formation of a nanocomposite. Retention of phthalates was improved against loss to volatilization to air, migration to activated carbon, and extraction to aqueous and organic solvents.
TDA’s nanoparticles are produced from inexpensive starting materials and therefore can be economical, value-added additives for the coatings market. TDA’s nanoparticles are based on boehmite, an inexpensive mineral product used in large volumes for catalyst supports, and simple organic compounds that can be selected to provide compatibility between the boehmite and a host thermoplastic. By the nature of the starting materials (a mineral and organic compounds prepared and sold in large volumes), these nanoparticles also are very inexpensive. It is anticipated that most of TDA’s modified boehmite nanoparticles can be produced for approximately $2-3 per pound. Although this cost is larger than the cost of large-volume plasticizers such as phthalates, the low loading levels necessary to make a nanocomposite and improve properties reflects the low addition to the cost of the final compounded resin.
Plasticizers increase the flexibility and softness of a material, are incorporated into many modern high-volume plastics, and are one of the largest segments of the plastics additives market. Plastic modifiers, including plasticizers and impact modifiers, were a $9 billion business in 1997. If successful, the nanoparticle anchored plasticizers could have an extremely large commercial impact and enable the production of safer, longer-lived plastic materials.