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Nitrile Gloves, Good Alternative to Latex Product
A Technical Presentation
By William L. Howe, Expert in polymer dipping technology
Mr. William L. Howe is sales manager of DipTech Systems, Inc., Kent, Ohio, USA, a major worldwide Supplier of polymer dipping technology. He was formerly employed by ACC Automation Co. He has been in the dipping industry for 15 years and has been involved in dipping system design, project management, and technical consultation for over 200 dipping plants. He has also held the position of general manager of a latex examination glove plant during his career in the latex dipping industry. He owns one patent for automatic latex glove removal technology. His 20+ year career has taken him through industrial engineering and project management/engineering positions, leading up to his current position at DipTech Systems. He has earned a BSIM from the University of Akron, and MBA from the University of Ashland.
Nitrile_Examination_Gloves

One fact seems certain for companies who manufacture disposable gloves - stay still too long and the market will not only pass you by, but leave a company gasping for its business breath and survival. Such go the trends in this industry.

In 1987, the powdered latex glove explosion bit the market allowing new players to enter a market which tripled in demand in one short year. By 1989, the market demand goes sour and small manufacturers fight for their existence. In 1992, shock waves hit the USA latex products market over heightened concerns of growing cases of latex allergies, more accurately termed as "latex anaphylactic shock".

Modified dipping lines

Once again, glove manufacturers scramble to respond to justifiable Food and Drug Administration (FDA) scrutiny and directive to reduce levels of extractable proteins from the latex product. Glove makers began processing latex more carefully and modified glove dipping lines to include post leaching after the on line curing stage of the process, which assisted the lowering of protein levels in latex goods.

Other firms searched for latex alternative materials, electing to invest in research and development of polymers such as styrene butadiene, silicone, and polyurethane. This research, conducted primarily by large glove and condom multinational firms, led to the market rediscovery of solvent based dipping systems. Sixty years ago, condom manufacturing was accomplished by a process called "cement dipping". New medical -glove polymers such as Tactylon® and Elastyren® grew in interest among glove distributors and users, even in spite of their cost compared to latex.

During the same period, testing revealed that much of the criticism attributed toward skin sensitivity was also caused by glove donning powders. Hence, the growth of "powder free" latex gloves. The term "hypoallergenic" became a term of misery to the FDA, as the majority of latex glove producers took claim to the term, attempting to substantiate the quality of their glove. One major problem existed with this new term - how to quantify and verify that a glove achieved the auspicious acclaim as being "hypoallergenic". Enter the powder free chlorinated latex glove. By 1994, every major glove manufacturer had introduced a chlorinated latex glove, which removes powder (and reduces protein levels), accomplished through a non-complicated process of chlorine washing and rinsing, typically conducted off line, prior to packaging.

Nitrile plunge

Often lost in all of this shuffle during this period, was the growth of a long standing polymer in the dipping industry - nitrile. In today's disposable glove market, dominated in use by the USA marketplace, thin film nitrile is the hot new entity. Described as a synthetic latex, this polymer is water based, an advantage against solvent based latex alternative materials, which require sophisticated solvent recovery units and/or environmental scrubbing systems. Additionally, many of the solvents used in the solvent glove dipping process necessitate that all electrical controls in and around the dipping process, be upgraded to "explosion proof" or "intrinsically safe" technology, adding 15% to 20% to the cost of the basic machinery.

Nitrite polymers have long been used in the glove dipping process, primarily for supported and unsupported industrial gloves. The process for these heavy industrial gloves was typically accomplished on batch dipping lines (indexing pallet systems). These systems are notoriously slow and high on maintenance, and thus seldom considered practical for the high volume, competitive examination glove market.

One firm--- Best Glove Company, a division of Tillotson Rubber in the USA, pioneered the advent of thin film unsupported nitrile gloves. Normal thickness for unsupported nitrile industrial gloves were typically that of 15-mil to 18-mil. Best Glove Company developed a 5-mil unsupported glove prior to 1990, using synthetic nitrile latex from a major Supplier of nitrile materials in the USA. Today that very product has found a new home in the medical, utility, and food handling industries.

The glove quickly created interest in the medical community, plus food handling industry. Unique characteristics of abrasion and cut resistance, and form fitting features, made for an intriguing alternative to natural rubber. Protein levels are zero and the glove is comfortable to wear. As for the manufacturing technology, the thin film product was manufacturable on continuous chain lines, thus meeting the need for competitive cost for a commodity product. A chart summary of important physical characteristics to compare nitrile latex to natural rubber latex is shown in Figure 1.

Figure 1
Physical Characteristic Comparison Chart For Nitrile Gloves

     Tensile strength at break psi
     Elongation %.
     M100, psi
     Tear resistance, lb/in
     Abrasion resistance, gm loss @ 2000 cycles
     Puncture resistance, lb./in 0
     Hexane swell, %
     ASTM #2 Oil @ 70, %
2000 to 4000
500 to 750
200 to 300
200 to 500
0.06 to 0.08
700 to 900
5 to 10%
0
Machinery conversion

The beauty of manufacturing thin film nitrite is that existing latex glove lines can be easily converted to make this product. With proper attention to certain changes and proper technical guidance, a machine conversion package for either rotating form technology or fixed form technology should not intimidate the experienced glove producer.

Before proceeding further it must be mentioned that the writer is limited to providing information which compromise relationships with existing nitrile glove Manufacturers in the USA. DipTech Systems, Inc. is active with many major suppliers of thin film nitrile gloves, and will protect specific process information associated with each of these companies. However, the data following should prove beneficial to companies looking to reduce their conversion on time and investment in manufacturing this product.

Nitrile_Glove_Machne

Process patents

Furthermore, the writer suggests a comprehensive review first be conducted into existing process patents which exist for thin film nitrile. Best Glove Company owns a process patent in the USA for this product. The writer is uncertain if the patent exists for other countries. Additionally, the manufacturing base for this product is dominated by veteran glove companies, such as Best, Ansell, and Safeskin. Therefore, machinery conversion is only one topic of consideration when evaluating the long term business climate for thin film nitrile gloves.

As previously mentioned, nitrile polymer is water based. Therefore, existing electrical components within a typical natural rubber latex line are safe for use. The second attractive feature for considering conversion is that sequentially, the manufacturing process is essentially identical to natural rubber latex. Here is a typical step-by-step process for natural rubber latex dipping;

      1. Form cleaning tanks
      2. Form temperature conditioning (oven)
      4. Coag dip
      5. Coag drying (oven)
      6. Latex dip
      7. Fluid dispersion (camming)
      8. Latex gel and set up (oven)
      9. Water leaching
     10. Ring (bead) rolling
     11. Final cure (oven)
     12. Powder apply (slurry)
     13. Drying (slurry dry oven or ambient space)
     14. Glove removal

A typical chain dipping process for the above will encompass between 30 and 45 minutes for a complete product cycle, dependent upon the latex formulation used and efficiency of glove curing techniques. Chain lines for natural rubber dipping operate at speeds ranging from 3.5 meters per minute, up to high speed lines from DipTech Systems as high as 15 meters per minute. Obviously, the faster the line speed, the longer the chain length and subsequent footprint size for the dipping machine.

The following representation for thin film nitrile manufacturing is that typically recommended by raw material suppliers of nitrile polymers. One can immediately note the similarities to that of natural rubber glove dipping. The step by step process is the same.

Steps to conversion

One must look carefully to identify the one minor difference in the sequential process step between the two products that of the latex gel step. Natural rubber dipping normally necessitates the use of an oven to gel the latex, while for nitrile manufacturing air dry is necessary. Failure to have this step can lead to product "wash out" in the leach tank, whereby some of the latex bleeds away from the former into the leaching bath. However for thin film nitrile, exposure of approximately 45 to 60 seconds to the ambient air, sufficiently gels this synthetic polymer.

Therefore, step one to conversion is to eliminate use of the latex gel oven.

Step two is to conversion consideration is the leach tank. The writer considers adequate leaching for natural rubber to encompass 5 minutes on line, for ultimate quality. There is more good news for those owning machines which severely reduced leaching time exposure. An exposure time of between I to 3 minutes can adequately serve as the online process for nitrile. Many machines manufactured in Taiwan were extremely short in leach tank length, making it nearly impossible to achieve an adequately leached glove.

Step three is to conversion consideration is that of ring (bead) rolling. Many latex lines were designed with pre-leach ring rolling stations, which is not normally advised for nitrile films. The film rolls more nicely when first leached. Therefore, relocation of this station may be necessary. If your previous line contained a slurry tank in advance of the curing oven, as many did, you can relocate your ring (bead) rolling unit in this space.

Another consideration for converting this function from natural rubber to nitrile, is that of roller length and construction. For natural rubber, the choices are rolled I fabric, urethane, and nylon brush construction. For nitrile, a hardened roller is advised, which would roll the bead more gradually and less aggressive with respect to time exposure, when compared with! natural rubber. Rolling the bead stock more gradually and slower will result in a more consistent appearance. The writer is unable to reveal a recommended exposure time due to industry process confidentiality restrictions.

Step four is a key element to conversion, and one which may require deepest consideration towards converting to thin film nitrile glove manufacturing. Final curing profiles are significantly different for nitrile. During curing, the nitrile film "swells as water is being driven from the process. Early zonal exposure to too high a temperature will Cause the glove to blister, leaving cosmetic defects and weak spots within the glove film.

Nitrile_Latex_Examination_Gloves

Curing Zone

Many natural rubber glove machines contain only one large curing zone, often without the benefit of convection air movement. The open gas burner system existing in many machines is not recommended for this product, unless locating far enough away from the glove travel, so as to negate hot" spots which would blister the glove. The writer suggests a minimum of 2 oven temperature zones, with preferable arrangement of 3 to 4 oven zones. The early zones can be effective with temperatures ranging from 80"C to 90"C. After suitable withdrawal of water from the glove, the last zone can be increased significantly even tip to levels of 160" C.

Once again, industry confidentiality restrictions inhibit the writer's ability to chart a suitable temperature curve and time exposure within the curing zone. However, a note of consideration can be offered here for further evaluation. Many dipping lines were designed to productively operate using a prevulcanized compound or using a "hot" formulation whereby curing times for natural rubber have dropped to as low as 6 minutes exposure. Many DipTech Systems lines typically achieve adequate curing with 10 minutes exposure. However, curing a thin nitrile glove in a 10 minute exposure time may prove to be impractical, thus forcing the manufacturer to reduce the chain speed of the existing line, which subsequently reduces overall machine output.

Conversion Step five for manufacturing nitrile exam gloves is to give serious consideration to chlorinating gloves online. By using a coagulant formulation which does not introduce powders into the process for mould release, Plus incorporation of a chlorination dip tank within the dip line, a powder free nitrile latex glove can be produced.

The chlorination tank is located after the cure oven, followed by a neutralization water rinse tank. Inserting this station on line is not as simple as it may seem, in that chlorine gas fumes will destroy even stainless steel components within the dipping machine. The key is to incorporate adequate side draft ventilation at the chlorine tank, to protect the machinery from exposure to these hazardous fumes.

Companies considering a transition to thin film nitrile can now enjoy the benefits of industry experience on the part of these raw material suppliers, plus qualified sources of dipping machinery technology. Inclusion of both of these valuable technical resources in converting natural rubber latex production to nitrile latex, will minimize risk and cost for the manufacturer.