Category Archives: General Knowledge
Touring some of the local hobby shows and speaking with some of our younger customers, I’m finding a growing interest in robot building.
In routing around the Net I happened upon John Palmisano’s site, TheSocietyOfRobots.com
John offered me these thoughts on getting started and what’s available on his site:
Robots today are no longer only made out of traditional tin and steal.
Instead, an ever increasing number of robots consists of large
quantities of plastics – not just for aesthetic casings but also as
main structural components. Societyofrobots.com covers all aspects on
building your own robot, including materials and structural
* * *
So, Plasticguy says check’m out!
Fairfax folds on plastic bag ban
Bowing to threats of a lawsuit by the plastics industry, Fairfax officials have decided to make the town’s ban on plastic grocery bags voluntary. “Basically, we’ve gotten legal advice that we are not likely to prevail if we fight it,” said Councilman Lew Tremaine. “We don’t want to waste a bunch of people’s money, so we’re going to alter the ordinance so that it’s voluntary.”
In July, the council voted unanimously to bar grocery stores, restaurants and retail shops from using plastic bags. The ordinance, which did not require businesses to comply until Feb. 10, 2008, allowed the use of recyclable paper, compostable bags or reusable containers. Violators would have been fined $100 for a first offense, $200 for a second and $500 for any subsequent offenses. The town approved the ban over threats from the plastics industry, which argued that the ordinance amounted to an endorsement of paper bags, made from trees that consume greenhouse gases. An attorney for Emerald Packaging Co. and Fresh Pak Corp. accused Fairfax of ignoring the California Environmental Quality Act’s requirements for a full environmental assessment before enacting the ban.
“A big part of me thinks the whole concept that we have to do an EIR is patently absurd, and I’d love to see that dragon slain in a court of law,” Tremaine said. “But reality is reality.” Emerald Packaging representatives said the lawsuit would not move forward if the town follows through with plans to change the ordinance’s wording to discourage the use of plastic bags, but referred further questions to Donne Dempsey, managing director of the Progressive Bag Alliance, who said an EIR is in order.
“The increased use of paper would contribute to deforestation,” Dempsey said.
Fairfax faced similar opposition to its long-standing ban on polystyrene take-out containers. When the plastics industry threatened a lawsuit, environmental activists turned to Fairfax voters, who made the issue a ballot initiative. Tremaine said something similar is likely to happen with the bag ban.
“I think what’s going to happen is environmental activists are going to put it on the ballot. We’ve been down this road before,” Tremaine said. “The group I was working with at the time – the Fairfax and San Anselmo greens – did a petition drive and an initiative, and it passed by a ridiculously large margin.”
Andy Peri agreed.
“My understanding at this point is that we’ll be going to voters to collect the signatures necessary to get this on the ballot, probably in November 2008,” said Peri, a member of Green Sangha, the organization that initially pushed for the Fairfax ban. “Once it goes to the ballot, the plastic industry can’t play its shenanigans, making us do an assessment under CEQA law, when this ban is clearly in the interest of the environment, and in the interest of supporting clean air, clean water and wildlife.”
Because so many businesses have supported the ban, Peri said he doesn’t believe life in Fairfax will change once the council modifies its ordinance – which Tremaine said is likely to happen at its Nov. 7 meeting.
Hobby greenhouse helps gardeners stay busy year-round
The weather is clearly changing as nighttime low temperatures dip to the 40s and the tree leaves start to change colors and get ready for their autumn dance to earth. You can smell the cooler fall weather, and meteorologists are talking about wind chill temperatures again. The first frost and then the first freeze normally arrive in the next two or three weeks on the calendar.It’s time to start planning for any tropical or annual plants you want to save and bring inside for winter. Some plants may be too large to save, but you can take stem cuttings to root and carry over for spring. Most tropicals need to be kept near windows in good light if you are going to try to winter them in the house.
Many people look at the killing fall freeze as the end of a special season or memory and a chance for a clean palette to start their garden anew next spring. Others can’t bear to lose beloved plants, large porch or patio gardens, or special plant collections.
You can save a few plants in your house, but space and access to light often limit the number of plants you can bring in the house. This makes a hobby greenhouse the best option.
A hobby greenhouse can also be a fun way to produce vegetables through the winter, to grow seeds and cuttings, to start transplants for next spring or to start a collection of orchids, begonias, bonsai or the special plants of your choice.
You can buy a kit greenhouse at local stores or via the Internet, or you can build a greenhouse frame and cover it with a number of greenhouse skins or glazings. Decent kit greenhouses start at about $1,000 and go as high as $20,000. Most of the inexpensive kits are made of lightweight galvanized metal or polycarbonate extrusions and are covered with plastic film or single-wall polycarbonate panels. The frames go up in cost as the metal frame gets heavier or you switch to aluminum frames or painted frames. A few kits use redwood or cedar wood as the frame, but more than 85 percent of the kits sold use metal frames, which probably offer the most strength for the money.
The earliest types of greenhouses that used more glazed glass are still available but not used as often because of the glazing cost and because it takes more structure to support the glass. Glass is available in 1/8-inch-thick tempered glass and in energy-saving insulated glass panels. The most popular glazings these days are single-wall corrugated clear polycarbonate panels or the energy-saving twin-wall polycarbonate in 6- or 8-millimeter-thick panels.
Some kits still use corrugated fiberglass, and newer kits use the more expensive corrugated or twin-wall acrylic panels that will stay clear longer. Greenhouse-grade ultraviolet-resistant polycarbonate panels offer the most strength, durability and light transmission for the money.
Twin-wall panels save almost 30 percent to 35 percent in energy. Greenhouse copolymer plastic film is the least expensive glazing. When you install two layers and blow air between the layers, greenhouse plastic film is one of the most energy-efficient choices.
If you build your own wood or metal frame or convert a garage, shop or other building, you can buy good greenhouse glazing to put on your frame. You can use 4-foot-wide corrugated polycarbonate panels that install by overlapping the panels and screwing them to the frame.
You will need heat in your greenhouse. A unit heater is usually the best choice. Natural gas is generally the cheapest commercial fuel, followed by propane. Electric heat is the cheapest to install but costs the most to operate.
Even if you only use the greenhouse in fall, winter and spring, you will need ventilation, as all greenhouse owners very quickly appreciate the power of solar energy. You can provide winter cooling with side and roof vents or with a motorized shutter on one end and an exhaust fan on the opposite end.
It is usually best to automate the ventilation on a thermostat, because we often need cooling near the middle of the day even in the winter. It is not unusual to have the fan come on to ventilate on a 40-degree or 45-degree day. If it is clear and sunny outside, it is possible for the greenhouse temperature to rise to more than 100 degrees because of solar radiation if you are not ventilating.
You will probably need to add shade cloth and an evaporative cooler if you want to use the greenhouse in summer.
Although you can buy hobby greenhouses from mail-order catalogs or over the Internet, I would encourage you to buy this specialized equipment locally for the best advice on frames, glazing materials, sizing, heating and cooling equipment and shade percentages for this area. There are several good suppliers in Oklahoma City and Tulsa that would be familiar with our area’s conditions to help you select the right hobby greenhouse for your application and crops.
Now is the time to plant pansies, viola, ornamental kale and cabbage and to select and plant spring flowering bulbs such as tulips, crocus and daffodils. This is also a great time to mulch your more tender hardy plants to help insulate their roots and protect them for the winter ahead.
Rodd Moesel serves on the Oklahoma Horticulture Industrial Council and the Oklahoma State University agriculture dean’s advisory committee. He is a former president of the Oklahoma Greenhouse Growers Association. E-mail garden and landscape questions to firstname.lastname@example.org.
A reader left a comment yesterday about careers in polymer engineering. Not really sure of what a polymer engineer is I did some digging and found the following at Penn State’s Department Of Chemical Engineering. Perhaps someone in the field might offer an article more specific to the job opportunities in the field.
WHAT DO POLYMER ENGINEERS DO?
Polymers are the most rapidly growing sector of the materials industry. As long ago as 1981, polymer production exceeded 24 million metric tons per year. No wonder–polymers are found in everything from compact discs to high-tech aerospace applications. As polymer production has grown, so has the number of people who work in this field. Today, it’s estimated that 50 percent of the chemical engineers and chemists in the United States work in the polymer industry.
Polymer engineers need to apply the traditional skills of chemical engineers, such as plant design, process design, thermodynamics, and transport phenomena, to various problems involving the production and use of polymers. Many of the engineers currently working in industry trying to solve thes problems have background in either chemical engineering or polymers. The Polymer Engineering Option in Chemical Engineering is designed to produce engineers with both sets of skills.
More than half the chemical engineers in the world work with polymers in one form or another. As a graduate of the Polymer Engineering option, your knowledge of polymers can give you an advantage when competing for chemical engineering jobs including:
- Process engineering in polymer-producing chemical companies
- Process engineering in polymer processing or fabrication
- Scale-up of new synthetic chemistry from laboratory development to pilot plant and large-scale production
- Research and product development in polymer synthesis (current hot topics include biodegradable polymers and compatibilizers for recycling polymers)
- Research and process development in polymer processing
- Learn the fundamentals of polymer technology from the basics of polymer chemistry, structure, synthesis, and processing to more advanced topics, including structure-property relationships and end-use polymer design
- Learn about polymer mechanical properties, the applications of polymeric materials, and how to choose the correct polymer for a particular application
- Learn about polymer rheology and processing–where transport phenomena enter into the polymer field
- Prepare for internships with polymer companies that have strong ties to the Polymer Science and Engineering program and the Department of Chemical Engineering
- Receive highly personalized instruction in courses with low student to faculty ratios
- Participate in the activities of the Polymer Science Club
42% of all plastics destined for extrusion
Slumping residential building and maturing applications pose a threat to growth, but demand for extruded plastic in the U.S. is still projected to expand by 2.8% annually to nearly 40 billion lb in 2011, according to a new study entitled Extruded Plastics from The Freedonia Group (Cleveland, OH). Total market value in terms of final product cost is projected to reach $81.3 billion according to the report, driven in part by expected advances in extrusion’s cost efficiency, processing easy, and machinery improvements. Extrusion as a process will remain the largest consumer of plastics, converting 42% of resin in 2011.
Freedonia forecasts that polyvinyl chloride (PVC) and low-density polyethylene (LDPE) will account for the lion’s share of the business, taking up 60%. PVC is expected to grow 2.4% annually, driving by high-volume construction applications in pipe, siding, and flooring, with faster growth expected in LDPE on the basis of its use in food packaging. Polypropylene will offer the strongest growth, driven by the sheet and film markets. High-density PE is expected by grow 3.4% annually, but polystyrene will contract, giving ground to polyolefins and polyesters in food packaging. That market, combined with construction, accounted for 78% of all extruded plastics demand in 2006. For more information, go to www.freedoniagroup.com.—email@example.com
Through years of R&D, Reynolds Polymer Technology has developed an ability to chemically bond acrylic, creating a nearly invisible seam, while maintaining over 90% of the parent materials strength.
For larger projects, “on-site bonding” has become a way of creating massive acrylic structures on-site as shipping the finished product would be impossible. In some cases months are spent in clean room environments in order to create that exacting bonding atmosphere as would be at home.
Sudbury Neutrino Observatory Project
Sudbury, Ontario, CanadaThe Sudbury Neutrino Observatory in Ontario, Canada, is a scientific masterpiece. Designed for astrophysics research, this forty-foot (12.2m) sphere is made of 2″ – 4″ (5cm – 10cm) thick acrylic, was created with over 1,550 feet (470m) of bonds, and was constructed 1.25 miles (2km) beneath the surface of the earth in a clean room environment.
The Sudbury Neutrino Observatory was a collaborative effort sponsored by the United States, Canada and the United Kingdom to increase the scientific understanding of particle physics and astrophysics. Approximately 74,000 pounds (33,566kg) of acrylic and over 1,550 feet (470m) of bonds were used to create this 40’ (12.2m) cast acrylic sphere.
The forty-foot (12.2m) diameter seamless acrylic sphere at the Sudbury Neutrino Observatory Project is used for astrophysics research. Reynolds Polymer Technology provided the scientific research and design, engineering, manufacturing and on-site installation of the Sudbury Neutrino Observatory (SNO) project.
Copyright ©2007 R-Cast™ is a registered trademark of Reynolds Polymer Technology, Inc. All rights reserved.
Description: “Wet Benches” are stations for wet etching and cleaning of wafers and devices. (“Litho Benches” in contrast, are used for resist processing.) The various wet benches differ in the specific process modules available and the materials allowed at each station. For information about some of the various wet processes available, click <here>. The available Wet Benches are:
Wet Bench Fire Safety: Update
By: Lise Laurin and Marcia Tate
Where there’s smoke there’s fire…but where there’s fire there doesn’t have to be smoke, corrosive fumes, or catastrophic damage, because cooperation between semiconductor and plastics manufacturers, industrial insurance providers, and wet bench manufacturers has resulted in safer, more efficient wet benches.
Today’s wet benches, made from specially engineered plastics, have come a long way from their beginnings in the 1970s, when they were simply modified work benches made of painted steel. By the 1980s, specialized manufacturers were making wet benches from plastics such as polypropylene. In the late 1980s, attempts to produce safer wet benches led to the development of fire retardant polypropylene (FRPP); subsequent research, however, has shown that although the FRPP formula lengthens ignition time, the corrosive smoke from burning FRPP actually causes more damage than was caused by the earlier generations of polypropylene. All plastics burn, but not all burn in the same way.
Readily available, easily fabricated, and able to resist corrosive chemicals without deteriorating or leaching out into the bath, specialized plastics are the material of choice for wet bench construction. Yet in the semiconductor industry, the very nature of the wet bench process—an electronically controlled process usually containing corrosive chemicals and sometimes incorporating electrically heated baths—means there will always be a potential source of ignition nearby. While fire isn’t the only safety issue associated with wet benches, it’s certainly the one that attracts the most attention because of the astronomical amount of damage one fire and its resulting smoke and corrosive fumes can cause.
With loss control as motivation, industrial insurance pro-viders joined plastics manufacturers and wet bench manufacturers to find plastics that don’t propagate fire, release little smoke, and don’t produce corrosive fumes as they burn.
Factory Mutual Research, a division of FM Global, began the move toward safer wet benches by establishing an index against which to measure cleanroom materials: Test Standard FM4910, Cleanroom Materials Flammability Test Protocol (1997). The first and most important feature of the FM4910 test standard is the Fire Propagation Index (FPI), which indicates tendency of a material to propagate fire. The test also indicates the comparative amounts of smoke generated according to the Smoke Development Index (SDI). Initially, the test also measured corrosive fire byproducts (CDI), but that measurement was later dropped.
In order for a material to be considered “fire-safe,” it must have an FPI equal to or less than the established acceptable level of 6. Regular polypropylene, FRPP, and polyvinyl chloride (PVC)—the conventional wet bench materials and the first materials tested—do not meet this minimum requirement.
With this measure as a guide, plastics manufacturers began formulating plastics to meet the specifications and submitting them for testing. Semiconductor manufacturers became concerned that while these new materials might be fire-safe, they might not be compatible with their processes. To address these fears, International SEMATECH, a research consortium of semiconductor manufacturers, established testing procedures to determine the process compatibility of materials destined for use in wet bench construction. The aim was to establish a set of parameters that would allow anyone to test the compatibility of any plastic material.
The Price of Progress
International SEMATECH’s tests of new plastics show that many concerns about the need to compromise purity to gain fire safety are unwarranted. The new standards have not only given plastics manufacturers an incentive to develop new materials, but also provided guidelines as to what criteria the new products must meet. As a result, many of the materials submitted for testing are proving to be superior to conventional wet bench materials.
While most new plastics are more expensive than conventional ones, the materials usually represent only a small percentage of the cost of a wet bench; in most cases the added cost of new materials is less than the cost of the built-in fire suppression system.
Complicating the conversion to new materials, however, is the lack of “off-the-shelf” parts made of fire-safe materials. To maximize safety, wet bench manufacturers must custom-build parts, and this adds to costs. Todd Thomas, president of Amerimade Technology, a wet bench manufacturer, states that at present his company is choosing its new plastics from the poly-propylene family. Since many of the smaller parts, such as fittings and latches, are not yet available in the new materials, keeping materials’ properties as similar as possible allows for easier construction and welding. Although new materials may meet criteria for construction of the wet bench structure, no test results are yet available to qualify these materials for use in process baths. To achieve a totally fire-safe system, the process baths must be upgraded to Teflon or PVDF, materials that are inherently fire-safe.
As more plastics measure up to the FM4910 and SEMA-TECH standards, and as wet bench manufacturers become more comfortable with the characteristics of the new plastics, the costs associated with their use are decreasing. Customers, too, are becoming aware of and educated about the new materials. At first, semiconductor manufacturers showed some reluctance to switch from the materials with which they were familiar; there was uncertainty about whether the change was necessary and concern about potential adverse effects on production. Increased research results, several catastrophic fab fires, and the potential of reduced insurance coverage, however, helped overcome these obstacles. According to Thomas, as recently as one year ago, wet benches constructed of the new fire-safe plastics accounted for only 10-15% of his company’s sales; that figure is now up to 50%. Informed customers are now requesting specific materials.
As plastics manufacturers rushed to submit materials for testing, it became apparent that the limited availability of the Factory Mutual Research test equipment and subsequent high cost and long lead time of the tests were hindering the process. Acceptable alternative materials were not becoming available soon enough to meet demand. Many materials that passed the fire propagation and smoke tests (20% of those submitted for testing) failed the corrosion tests. Specifically, most of the semitransparent plastics suitable for windows in minienvironments were found to produce corrosive fumes as they burned.
Concerned engineers proposed eliminating the corrosion index, allowing the use of materials that do not comply to FM4910 as long as they are not close to an ignition source, and moving to qualify more labs to run the 4910 tests. In response to industry pressure and after a review board concluded that the CDI results were not adequately repeatable, Factory Mutual Research removed the corrosion damage index from the FM4910 test standard. Factory Research Mutual researchers and others involved felt that if a material passes the stringent fire propagation and smoke development tests, it would likely cause minimal corrosion damage.
Still, there was pressure to speed up the qualification process and to make it more economical. HSB Industrial Risk Insurers chartered Underwriters Laboratory (UL) to formulate a less expensive test that could produce results as reliable as those of FM4910. While Factory Mutual Research’s tests require equipment with limited availability, the UL test uses a standard cone calorimeter. No one involved disputed Factory Mutual Research’s contributions, but input from another source was welcome. This “second opinion” validated Factory Mutual Research’s effort and research.
Working with samples donated by plastics distributors, UL and Factory Mutual Research have completed a round-robin of testing to verify the equivalency and repeatability of their tests. Representatives from UL, FM Global, HSB, and several plastics manufacturers met on November 17, 1999, to discuss the results of UL’s test. UL disclosed the results of the tests using the standard cone calorimeter. Valid test results from this less costly, more readily available equipment (installed in more than 100 laboratories worldwide), would enable quicker, less expensive testing and result in more options and lower prices. It would also allow testing in intermediate steps during R&D.
The goal of the round-robin was to formulate a single set of “fire-safety” testing standards to present to the National Fire Protection Association (NFPA) at its annual spring meeting. This would align all parties involved (insurers, materials manufacturers, semiconductor manufacturers, and process equipment manufacturers) striving for the same result, which would ultimately result in better products and safer cleanrooms.
Factory Mutual Research retested eight of the nine plastics used in the UL tests; the results of this round of testing were consistent with Factory Mutual Research’s previous findings but not identical to UL’s findings.
Two types of tests include a “full-scale” test (Photo 1) and a “small-scale” test (Photo 2). Results of the full-scale (“Parallel Panel”) test are obtained by setting up two 2 x 8 ft. panels of the same plastic and starting a fire between the panels. Researchers monitor the panels to discover how quickly they ignite and what happens to them as they burn.
In the small-scale test, small samples of material are burned in a piece of laboratory equipment, where they are monitored to measure the time it takes the material to ignite, how much heat it releases, and the amount of smoke it generates. Since a much smaller sample of plastics is used in the small-scale test, results must be compared to the Parallel Panel test results. UL correlated the cone calorimeter tests (ASTM E1354) to the Parallel Panel tests.
UL and Factory Mutual Research achieved very similar results from the panel tests, but slightly less clear-cut results from the small-scale testing. Factory Mutual Research’s apparatus and the standard cone calorimeter yielded slightly different data. Factory Mutual Research felt that further testing on a wider selection of plastics was advisable; only nine plastics were thoroughly tested using the UL procedure as opposed to the hundreds that have been tested using FM4910. While there was total agreement on the properties of the plastics at the extremes of the test parameters, there were gray areas in the middle ranges.
After reviewing the data, the NFPA committee responsible for presenting recommendations for updating the NFPA 318, the standard of protection of cleanrooms, to the NFPA annual spring meeting discussed adopting FM4910 and UL2360, the new UL subject test standard, as reference standards. A note was included indicating that the subject test standard UL2360 was under development, since UL had not completed its internal standards review process. The recommendation, which will be submitted for final approval at the international annual conference in May 2000, is that in order for a material to be considered fire-safe or noncombustible, it must pass a reference standard test conducted by a recognized testing laboratory and be “listed” by that laboratory. Robert Pearce from HSB stated that both UL and Factory Mutual Research are acceptable testing laboratories.
FM Global and its insureds are already working under a version of the fire-safe standard. Since most wet benches are custom fabricated, Factory Mutual Research must inspect each of its clients’ wet benches, both at the manufacturer’s site and on-site in its final location, to determine whether or not the bench can be considered fire-safe without the addition of a fire suppression system. Once manufacturers have a full complement of fire-safe materials, along with a set of clear-cut standards to follow, this process will flow more smoothly.
Factory Mutual Research has submitted the FM4910 to the NFPA Fire Test Committee, ASTM, and ANSI, which effectively puts it in the public domain. The test would become NFPA Standard 287 upon final review and acceptance in 2000. The test is undergoing final review and comment from ASTM, and adoption is also expected in 2000.
Looking beyond wet benches, other major applications for fire-safe materials in semiconductor applications include wafer carriers, parts cleaners, wall panels, and insulation materials that can represent significant combustible loads. FM Global’s Paul Higgins states that his company is planning to partner with several wafer transport equipment manufacturers to move away from ordinary polypropylene and polycarbonate carriers. New 300 mm wafer carrier storage systems, known as stockers, could present fire protection challenges. Sprinkler systems would not prevent smoke and water damage to their contents, and the larger enclosures required could be very difficult and expensive to protect with fire suppression systems.
So far, the push for fire safety has been a win-win process for all involved. The reduced risk of fire will simplify business for the risk managers and insurers. Semiconductor manufacturers will benefit from more stable insurance coverage—from the knowledge that their facilities and workers are more secure and from knowing that wet benches made from new materials are more process-compatible than their predecessors. Plastics manufacturers have created new markets for superior materials, tailored specifically to their clients’ needs.
From the very beginning of the research to the presentation of the joint findings to the NFPA, there has been a spirit of cooperation among all parties involved. Everyone stands to gain from a clear definition of “fire-safe” and the adoption of standards to ensure uniform requirements throughout the industry.
Thanks to Todd Thomas, Amerimade Technology (Pleasanton, CA); Paul Higgins, FM Global (Johnston, RI): Robert Pearce, HSB Industrial Risk Insurers, (San Francisco, CA).
Surface Modification of Plastics for MEDICAL DEVICE APPLICATIONS -
By: June-Jim Schepple,Jeff Dykhouse
Plastics, polymers, and resins have become widely accepted for in vivo and in vitro medical applications. Many of these materials have properties that lend themselves well to the manufacture of medical appliances or devices; they are relatively inexpensive and easily molded or formed into complex shapes, and bulk physical properties may be selected from a wide range of parameters such as rigidity and temperature stability. Unfortunately, fabrication procedures that require bonding are difficult to achieve, and biological interface reactions within the body or in the laboratory can limit their in vivo and in vitro performance.
Gas plasma technology offers a technique for easing these limitations by modifying the surfaces of these polymers. By altering just the first few atomic layers, the surfaces of most medical polymers can be rendered wettable so that adhesive bonding can be achieved to troublesome materials such as polyolefins, silicones, and fluoropolymers. In a similar fashion, more exotic processes such as plasma grafting and polymerization can produce totally new custom surfaces without loss of the desirable characteristics of the bulk material.
Plasma is a low pressure, gaseous “glow discharge” process that has been used in the aerospace, semiconductor, and electronics industries for more than 30 years for cleaning, etching, and surface treatment of various materials. Plasma treatment does not affect the bulk of the materials and plasma treated parts are generally visually and physically indistinguishable from untreated parts.
Plasma is now routinely used for controlling the wettability of test tubes and lab vessels, for pre-bonding preparation of angioplasty balloons and catheters, for treating blood filtration membranes, and to manipulate surface conditions of in vitro structures to enhance or prohibit culture cell growth.
Given enough energy, any gas can be excited into a “plasma,” which is a mixture of ions, electronics, excited species, and free radicals. There are many temperature and pressure conditions where this phenomenon will occur, but for practical considerations, radio frequency or microwave energy is commonly used, enabling these processes to take place at low temperatures (25-100¡C) and low pressure (0.1-1 torr), where surface reactions with polymers are feasible without bulk interactions.(1,2)
Plasma “treatment” usually refers to a plasma reaction that either results in modification of the molecular structure of the surface or atomic substitution. Even with be-nign gases such as oxygen or nitrogen, plasma treatment can create highly reactive species at low temperatures. High energy ultraviolet light is emitted in the process, which along with the high energy ions and electrons provide the energy necessary to fracture polymer bonds and initiate chemical reactions at the surface. Only a few atomic layers on the surface are involved in the process, so the bulk properties of the polymer remain unaltered by the chemistry, while the low process temperature eliminates concerns about thermal modification or distortion of the bulk. Unique reactions can be promoted by appropriate choice of reactant gases, and unusual polymer byproducts and structures can be formed.
In many instances, plasma cleaning with benign gases such as oxygen or nitrogen provides adequate surface activation for enhanced wetting and adhesive bonding. With other targeted end results or substrate materials, it may be necessary to utilize reactants which result in “grafting” or surface chemistry modification to achieve the desired results.
It is frequently possible to select reactants that form volatile byproducts upon reaction of the plasma with the substrate material. These, upon desorbtion from the surface of the treated material, are removed by the vacuum pump, resulting in etching of the surface without the necessity for further scrubbing or neutralizing.
Oxidizing species such as air, oxygen, water vapor, or nitrous oxide are often used to remove organics, leaving functional oxygen-containing groups on the surface. These groups greatly enhance wetting, improve adhesive bonding, and, in some instances, create acidic surfaces.(3,4) In at least one instance, sterilization of components has been reported with the use of strong oxidizers such as ozone or hydrogen peroxide vapors.
Reducing gas species such as hydrogen or methane-often diluted with argon, helium, or nitrogen-may be used to remove organics from surfaces sensitive to oxidation. This chemistry may also be used to partially substitute hydrogen atoms for fluorine or oxygen in polymer surfaces. The noble gas species, such as argon or helium, are chemically inert, so they do not combine or become part of the surface chemistry. Instead, they transport energy to break chemical bonds in polymer chains. Broken polymer chains result in “dangling bonds,” which recombine with other reactive sites, resulting in significant molecular restructuring and cross-linking. The creation of dangling bonds allows for chemical “grafting” reactions to occur. This process is involved in several of the biomedical applications.(5)
Active gases, such as ammonia, are used to create amino groups on the surface. This type of functionality has an influence on important surface properties such as pH and Lewis basicity.
Fluorinated species such as Tetrofluoromethane (CF4), Sulfur Hexafluourine (SF6), and perfluorohydrocarbons may be used to induce substitution of fluorine atoms for hydrogen atoms in the surface structure. Teflon-like structures may be created, resulting in a very hydrophobic, chemically inert surface with significant chemical stability.
Polymerization, or deposition, processes may include reactions utilizing a wide variety of gases, including some of the organic or organo-metallic compounds, which deposit nonvolatile polymer films.6 In many instances these reactant gases maybe toxic, corrosive, or otherwise hazardous and require special handling such as heated gas transfer plumbing and measurement instrumentation, reactor exhaust scrubbing, and trapping of reaction byproducts. Polymerization processes will generally necessitate frequent cleaning of the reaction chamber, since all surfaces ex-posed to the plasma will be coated.
BIOMEDICAL DEVICE APPLICATIONS
Surface Wetting. Plasma treatment of polyethylene or polypropylene disposable Petri or Assay dishes greatly enhances wetting. Contact angles as low as 22û have been demonstrated on these materials after only a few minutes of oxygen plasma exposure (see Table 1). When these parts are properly packaged after treatment, the contact angle has been seen to be stable for several years.
Conversely, many medical polymers can be made extremely hydrophobic. Teflon-like films and other similar surface treatments can be easily accomplished on most polymers using fluorinated gases. For example, small diameter tubes can be treated so that when immersed in aqueous solutions they do not draw fluid by capillary action.
One of the simplest techniques used to evaluate plasma surface treatment is a wetting angle test using a contact goniometer. Surface roughness and substrate cleanliness need to be tightly controlled to obtain quantitative data. Standard wetting solutions are often used to obtain accurate surface energy values.
Most untreated polymers are only poorly wettable. Initial contact angles may vary from 60-100û. Table 1 shows a sample of some typical contact angle measurements:
Typical wetting angles,
before and after
Oxygen plasma treatment.(7)
After 5 min. of oxygen plasma treatment, the contact angle decreases significantly.
Adhesive Bonding. Many intravascular devices, such as balloon catheters, are assembled by adhesive bonding of polyethylene components. Chemical surface activation or mechanical surface roughening techniques provide only modest bonding performance, with bond failures noted after as few as eight repetitive inflations. With plasma treatment, up to 40 repetitions are achievable. Typical bond strength data are shown in Table 2.(7)
Typical lap-shear bond
strengths (psi), without and
with plasma treatment.
An oxygen plasma not only removes organic residues but also chemically reacts with the surface to form strong covalent carbon-oxygen bonds, which are much more polar and more reactive than the initial carbon-hydrogen bonds. The increased polarity of the surface accounts for the substantial increases in wettability and adds a degree of covalent bonding to the surface-adhesive interface. (Note that other gases may be used to attain similar results in instances where oxidizing species may be harmful to components of the assembly.)
The bond strength ultimately realized will certainly be affected by:
1. Initial cleanliness of the surface(s).
2. Wetting of the surface by the adhesive.
3. Cross-linking effects.
4. Chemical interaction of the adhesive with the surface.
Any mold release compounds, unpolymerized monomers, plasticizers, or additives that may have migrated to the surface must be removed either by plasma cleaning or washing before surface modification is attempted. Immediate assembly is usually advised after the surface has been prepared. Once the surface has been optimized and bonded, the bond is permanent and does not degrade over time.
In-vivo and in-vitro applications. To increase biocompatibility in vivo, the issue of thrombogenesis (the propensity of a surface to form or initiate clotting) must be addressed. Many unmodified materials encourage protein binding and thus initiate the process of clot formation. To combat this process, antithrombonin (anticlotting) coatings are often applied to the surface, but when dealing with polymers these antithrombonin coatings often fail to effectively bond to the target surface.
Using an active gas plasma, surfaces may be modified by heparinizing or by grafting of antithrombotic functional groups, which achieve effective chemical bonding to previously inert material surfaces. Process variables are dependent upon a range of factors including selection of the base materials, composition of the antithrombotic, and the expected product lifetime.
Animal test results of the procedure on surface modified and heparin coated polyure-thane catheters revealed no protein attachment after a 30-day indwelling. Simultaneous testing on surface modified but uncoated polyurethane catheters disclosed only slight protein attachment, while unmodified and uncoated catheters show severe thrombus formation.
In tests performed on plasma modified blood filters, results showed substantial reduction in platelet retention compared to untreated materials.
Attachment of in vitro cultures. It is sometimes necessary to manipulate surface conditions of in vitro structures so as to encourage or enhance culture cell growth. In specific cases where cell attachment is necessary to ensure proliferation, plasma modified in vitro culture cell containers yielded dramatic improvement over untreated containers. Testing has confirmed that, by using gas plasma surface modification procedures, materials such as PET, polyethylene and K-Resin can yield substantially higher performance than in the untreated state.
1 J.R. Hollohan and A.T. Bell, Techniques and Applications of Plasma Chemistry, John Wiley & Sons, New York, NY, 1974
2 Chapman, Glow Discharge Processes, John Wiley & Sons, New York, NY, 1980
3 T.J. Hook, J.A. Gardella, and L. Salvati, Journal of Materials Research, 2, 132, 1987
4 D.S. Everhart and C.N. Reilly, Annals of Chemistry, 53, 665, 1981
5 M. Szycher and W.J. Robinson, Editors, Synthetic Biomedical Polymers, Technomic, Westport, CT, 1980
6 R. d’Agostino, Editor, Plasma Deposition, Treatment, and Etching of Polymers, Academic Press Inc., San Diego, CA, 1990
7 E.M. Liston, Journal of Adhesion, 30, 198, 1989
New Water-Based Acrylic Pressure-Sensitive
Adhesive Grabs Higher Performance Applications
PHILADELPHIA, Oct. 10 /PRNewswire-FirstCall/ — Rohm and Haas announced today that it has developed a new water-based acrylic adhesive, ROBOND(TM) Prohesion, designed for pressure sensitive tapes and other demanding applications traditionally served by solvent-based adhesives. The product exhibits characteristics and stamina never before seen or thought possible in an aqueous acrylic adhesive.
“Water-borne acrylic PSAs may have traveled a long way, but until now, solvent-based choices have continued to dominate most higher-performance adhesive requirements,” says Chris Urheim, Rohm and Haas North American region marketing manager, pressure sensitive adhesives. “With ROBOND(TM) Prohesion, we have taken another significant leap forward in delivering a cost effective acrylic adhesive that doesn’t compromise performance. Feedback from our customers so far has been amazing.”
Shear adhesion and resistance properties of this new technology are said to be groundbreaking. No other emulsion PSA has ever come close to the product’s heat resistance capabilities while retaining its level of adhesion. Tests indicate that ROBOND Prohesion withstands punishing hot shear tests at 150 degrees F (62 degrees C) for more than 50 hours, exceeding values of some solvent-based choices. In addition, the product’s humidity resistance is outstanding, eliminating a traditional shortcoming of aqueous systems. ROBOND Prohesion retains more than 80 percent of its peel adhesion after prolonged exposure to moisture under severe conditions of 90 percent humidity at 95 degrees F (35 degrees C).
ROBOND Prohesion offers an environmentally advanced formulation that holds up to performance demands of the automotive, construction and general industrial markets, all of which are requesting that suppliers lower volatile organic compounds in their products. The adhesive adheres aggressively and exhibits superior anchorage to substrates ranging from high surface energy applications, like stainless steel for example, to equally challenging low surface energy materials like high-density polyolefin foams.
To learn more about ROBOND(TM) Prohesion, visit us at, http://www.rohmhaas.com/wcm/information/robond_prohesion/index.page
About Rohm and Haas Company
Leading the way since 1909, Rohm and Haas is a global pioneer in the creation and development of innovative technologies and solutions for the specialty materials industry. The company’s technologies are found in a wide range of industries including: Building and Construction, Electronics and Electronic Devices, Household Goods and Personal Care, Packaging and Paper, Transportation, Pharmaceutical and Medical, Water, Food and Food Related, and Industrial Process. Innovative Rohm and Haas technologies and solutions help to improve life every day, around the world. Based in Philadelphia, PA, the company generated annual sales of approximately $8.2 billion in 2006. Visit http://www.rohmhaas.com for more information. imagine the possibilities(TM)