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Quality management in the latex laboratory.

Quality management in the latex laboratory

The advent of statistical process control to the manufacturing industry in the U.S. has generated volumes of publications and seminars on its application to the manufacture of solid rubber. However, the use of quality management and process control in the latex laboratory has not been addressed extensively in the literature.

Quality management within a latex laboratory and production facility can be divided into three important areas: supplier quality program, compounding and laboratory process control, and production or manufacturing process control. The interactions between these three areas overshadows the importance of any one area. All three overlap one another and depend on each other for the success of the entire quality program.

Quality management in latex production is tied to the consistency of raw materials and the knowledge of the manufacturing process in which these materials are used. Latex is a complex, natural material with many variables that can affect production. Only with the understanding of these variables can process control be achieved.

Supplier quality program

An effective supplier quality program must begin with investigation and definition of all the requirements for each raw material purchased. These need to be determined within the framework of the performance of the product during and after manufacture. For each material to be purchased, similar questions must be asked:

* What chemical properties are truly significant?

* What size or quantity would make most efficient use of space and cost?

* What packaging and labeling requirements are necessary?

* What particle size or distribution is best?

The answers to such questions can only be found with knowledge of the entire operation, from purchased material to finished product.

Setting up a program is more than organizing files of suppliers and collecting certificates of analysis. As stated by J. Bossert, "When trying to decide how to work with suppliers it is imperative that company directions and goals be taken into account. The bottom line is simple: The main goal of any business is to make money. The way to get there can take many forms. The best way for long-term stability and profitability is to produce the highest quality product at the lowest possible cost. The procurement organization is given the task of developing a supplier base that allows this to happen." (ref. 1).

In the medical industry the majority of our programs are dictated by Good Manufacturing Practices, as spelled in 40 CFR 820. Much of what is stated in these regulations is now becoming "new" quality control principles for many industries: traceability of raw materials, efficient storage and clean, easily read packaging.

General guidelines for supplier approval and performance are necessary for fair and objective evaluation of suppliers. A good set of guidelines offers a basis for:

* consistency between suppliers;

* decisive action when unforeseen circumstances arise;

* laying the groundwork for further programs;

* a sense of mission for the day-to-day work in the area of "critical raw materials." These are the raw materials that become part of our finished product or are an integral part of the formation of the finished product while in production. These require the utmost attention of all the raw materials brought into any production quality and product quality down the line. These are also the materials requiring the most documentation in GMP.

A supplier quality program needs to be directed by the mutual learning and cooperation of both the supplier and the customer to attain the most beneficial relationship for both companies. With each vendor of one of our critical raw materials we have written requirements of packaging, shipping with certificates of analysis, and quality of the individual products. Suppliers need to know their customer's expectations and to know that their product is being fairly evaluated. These needs are established mutually through a three step program:

* Preliminary lab testing - This is the review of product in small quantities, with an initial examination of product performance under laboratory conditions, safety, standard packaging and list pricing.

* Pilot trial or short production run - Once a material is proved to be acceptable in our product, it is next evaluated under pilot trial conditions and a small quantity is obtained for evaluation. This is not only for the evaluation of product quality and consistency, but for the evaluation of packaging, delivery, availability and general operation of the vendor in a purchasing situation. I never pass judgement on any product on one delivery. Pilot trials may be repeated at least twice for new suppliers, preferably three times. If the vendor is already established as an approved supplier for other products, the small quantity purchase may only be done once.

* Production run and/or age testing for final approval - A continuous pilot run or a run in actual production will give an indication of any deleterious effects of new materials on product quality.

Many requirements can be standardized for all raw materials within a group, such as critical raw materials. These requirements can then be used as a basis for maintaining business with a supplier and for evaluation of new suppliers, with all requirements and assumptions from both sides in writing, long term relationships can be formed and maintained to the benefit of both supplier and customer.

Even with the approval of one or two suppliers for every raw material, new sources and new alternatives must be constantly evaluated. Often, the feeling with the tried and true raw materials and suppliers is: "If it ain't broke, don't fix it." However, new products are constantly coming onto the market that could improve processability, improve product performance or reduce costs. In the turbulent global market of today, a compound that has been effective for years may be replaced by your competitors next month. This area is an important function of a vendor quality program as it is of development or quality control.

An organized vendor quality program provides a framework for evaluation of new raw materials and new suppliers. The distinct requirements set out for each product ensure the unbiased evaluations necessary to improve our product. As with the initial examination of materials for the organization of the program, new products would also undergo:

* preliminary lab testing/technical review;

* pilot trial of performance;

* production run for shipping/packaging evaluation.

This entire program only indirectly addresses the problem of raw material cost. With extensive data and records maintained, the purchasing agent can relate total cost to benefits much more readily. Saving a few cents per pound on a material is lost if the supplier is late with shipments, precipitating numerous phone calls and considerable scrambling and wasted time. Cheaper packaging or bulk packaging is not a reduction in total cost when production people spend extra time dealing with more inconvenient or less safe packaging. A supplier with a proven history of quality product and delivery is the most cost-effective over the long term and outweighs any short-term savings of a few cents per pound.

In summary then, a well constructed vendor quality program contains five important elements:

* goals established from company goals;

* written guidelines of approval procedures;

* documented records of performance and quality;

* cooperation of supplier and customer;

* allowance for new materials and suppliers.

Compounding/laboratory process control

Many of us have spent considerable time perfecting ASTM testing methods in our labs for our incoming raw material and production testing. These methods provide a basis for monitoring incoming raw materials and production quality. They also establish the means necessary to correlate various laboratories. ASTM methods should only be considered a starting point. In a good QC operation the lab tests have some identifiable correlation to the process. In other words, ask yourself if all the tests performed are pertinent to the actual use of the raw material or to critical processing characteristics in production. Blackley spent considerable time reviewing latex testing and the true significance of each test. We all are familiar with at least these commonly used tests:

* pH;

* KOH number;

* % alkalinity;

* mechanical stability;

* volatile fatty acids;

* total solids/dry rubber content;

* viscosity.

All of these tests are well described in ASTM D1076. The combination of results of any of these is much more valuable than any one test in determining latex quality.

For example, the interrelationships between the pH, KOH number and % alkalinity tests are an interesting area that is affected by and affects other latex properties. The pH determination tells the presence of the dissociated hydronium ion. The test for alkalinity is the determination of alkali content of a latex. The KOH number test measures the concentration of acid radicals present in natural rubber latex as free acids which, in the presence of the ammonia, exist as the ammonium salts (ref. 2).

The ease of a pH test does not provide the information on the complex nature of the ionic species present. The character of the fatty acid soaps naturally present, along with the relative quantity of alkali present tells much more about the age and processability of the latex than a single pH check. The stabilizing effectiveness of the fatty acid soap system is greater at a pH of 9.5 or higher, but is not well defined. Blackley goes on to explain that the ammonia content has little effect upon the inflection point of the KOH number test, but it does affect the slope of the line, making determination of the inflection point more difficult. All three tests describe an aspect of the ionized contents of the latex, which in turn are related to the mechanical stability.

As another example, the total solids content, dry rubber content, mechanical stability and viscosity tests have such interrelationships also. The total solids test determines the total weight of nonvolatile materials present in the latex. The dry rubber content determines the weight of rubber present. The differences between these give the weight of various proteins, lipids, fatty acids, etc. present. The quantity and character of the species present can be directly correlated to the mechanical stability (ref. 3). The viscosity can be adjusted and controlled with fatty acid or soap additions, or even alkali additions. The viscosity is the most important variable in straight dip operations and can be affected by all of these parameters.

All these standardized tests are dependent on the accuracy and repeatability of the lab testing. So too, their validity is defined by the requirements of the process. For instance, what happens to the latex after it is tested in the lab and it goes to production? If it has been subjected to stand time, heating, or cooling the lab test results may not be relevant to the production process. The mechanical stability test is notorious for being quite subjective. The KOH number test can be difficult to determine accurately in a high ammonia latex. A pH meter used for several of the tests may give erroneous readings if the electrodes have become clogged with proteins from the latex. The testing must be continually monitored for its accuracy. This is as important an aspect of a total quality program as any other. Any SPC program is no better than the accuracy and precision of the testing of the variables. The various tests and their interdependence are a greater assessment of latex quality than any one test. These tests must be carried out with accuracy and repeatability to be valid. Decisions based on SPC work must have a basis in solid numbers.

All of the latex tests are tried and true procedures, but allowances have to be made for new tests, new theories on latex as a fluid and new test methods. For instance, many commonly measured latex variables affect the fluid's surface tension, which is one of the variables controlling coating thickness. This test has only recently become automated enough to be useful.

This complex quantity called natural rubber latex cannot be simply described in one or two test results. This complexity is what makes process control of a latex operation challenging. When this complexity is understood process control is still intricate but not impossible.

Good lab and compounding procedures then contain the following points:

* capability of all standard tests;

* repeatable and accurate results;

* written compounding procedures;

* correlation to production.

Many latex variables can be controlled during compounding. This needs to be done before the production process, where there are many more variables to control than the latex itself.

Production process control

The final quality of a product and the process for producing that product define the controls needed in compounding and raw materials. Statistical process control methods in production are often the first introduction of an SPC program in a company. Improvements in product quality and production methods are the short-term goals of most SPC programs when the program is first instituted. As an SPC program matures, and if it is continually used for the long-term improvement of product quality, lab testing and vendor quality become tied in, even if they were not in the beginning.

In the initial investigations of an SPC program, one way to start is to chart or control as many raw material and compounded latex variables as possible. Then when all possibilities are examined a company will begin to see which variables are affecting quality. For instance, some variables can be maintained within a certain known operating range while others are studied. Or, with advanced knowledge of statistics and with the aid of computer programs available today, designed experiments can be run. The most likely discovery will be that, as with the properties of the latex itself, the interaction of two or more process variables have the biggest effect on quality. In a complex system such as latex this is entirely possible. Charting of individual process variables eventually may be reduced when the complex nature of the latex is understood through the initial set up of SPC.

Production dipping provides plenty of variables to control outside of the latex. Flow rates in tanks, temperatures of tanks and ovens, and air flow in drying and curing have all been proved to be important variables requiring statistical process control.

Viscosity is the controlling factor in coating thickness, and is the variable to control in every straight dip process using nonporous formers. Viscosity is strongly dependent on temperature and therefore temperature control is critical in the production process.

Classic latex dip tank design simply calls for flow that provides surface movement to minimize skinning. This has been described by Blackley and many others. This design needs to be modified for greater efficiency. Surface flow needs to be maximized but not at the expense of shearing the latex and creating coagulum. Volume needs to be minimized to create the fastest latex turnover in the tank, yet an adequate volume must be maintained for production capacity.

In straight dipping or coagulant dipping, the gellation stage of the latex film after the former has been withdrawn from the latex is a complex process stage that cannot be described simply in terms of drying or destabilization. Several different processes of heat transfer, mass transfer, viscosity increase and particle coalescence occur at the same time. Any one of these processes occurring too quickly or too slowly at this stage can be a source of visual defects.

Variability in the cure or drying stage can adversely affect physical strength of the latex film. Even with most of the physical strength of the rubber latex set in compounding and prevulcanization before film formation, excessive or inadequate heat can result in the final product being overcured or undercured respectively.

Everyone in production needs to be educated in the important points of an SPC program and the important points of charting in production. Volumes have been written on the actual SPC charting mechanics. Workers also need to be educated in the basic technology of what they do everyday and why. More importantly, they need to begin to understand the interrelationship of their own everyday work to the work of others on their shift and to the effectiveness of the whole organization. In general, just as natural rubber latex is a complex material requiring greater understanding, so too the skill level of latex production personnel needs to be greater to achieve quality improvement over the long term.

In conclusion, the complicated problems of instituting and maintaining a quality management system in a latex production operation are intensified by the complexity of natural rubber latex. A planned, vigilant approach can make the management of such a system possible though still a big challenge for any company.

The three areas of quality management discussed here give a framework for latex quality management. It is drawn from the multitudes of work done in other industries, with the realization that such a system needs to be customized to the individual goals of the company.


"Correlation of results from curemeters of different designs" is based on a paper given at the Spring 1991 Rubber Division meeting.

"Analysis of N-nitrosamines in compounds" is based on a paper given at the Fall 1991 Rubber Division meeting.

"Quality management in the latex laboratory" is based on a paper given at the Spring 1991 Rubber Division meeting.


[1]"Procurement quality control," ed. J. Bossert, ASQC 1988, p. 7. [2]"High polymer latices," 1966, D.C. Blackley, vol. 2. [3]"Natural rubber science & technology," A.D. Roberts; Oxford Science Publications, Oxford, 1988.
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Author:Butkus, MaryJane S.
Publication:Rubber World
Date:Jan 1, 1992
Previous Article:Analysis of N-nitrosamines in compounds.
Next Article:Correlation of results from curemeters of different designs.

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