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Almanac: The importance of molecular weight

Maintaining material properties in the molded part depends heavily on this key variable.

In 1920, a German doctoral candidate, Hermann Staudinger, hypothesized correctly that the unusual properties of polymeric materials were attributable to the chain-like shape of the molecules and their unusual length. In other words, the functional properties of polymers were directly related to an unusually high molecular weight.

Like most ideas that change an industry, Staudinger’s pronouncement was not met with universal approval. Even his thesis advisor rejected the idea and recommended that he not pursue it any further. But pioneers in polymer synthesis like Wallace Carothers at DuPont, the creator of both polyester and nylon, used Staudinger’s theory as the basis for their research, and before too long the idea had taken root.

Almost 85 years later, approximately 90% of all plastic product failures exhibit some problem relating to low molecular weight. In an industrial setting, we do not typically make direct measurements of the molecular weight of a polymer, but we have developed convenient and reasonably accurate measurements such as melt-flow rate, melt viscosity, and intrinsic viscosity that enable us to assess the relative average molecular weight of a material. These tests allow us to evaluate different grades of material and to compare raw material to molded parts.

Check Your Material Sources

When the molecular weight of a polymer proves too low to provide the required performance, there are two primary causes—a low initial molecular weight or polymer degradation during processing. In the first case, the material does not have the required molecular weight to begin with. Most materials are differentiated within a product family according to the way they flow, and melt-flow rate is the most common way of measuring these differences.

Low melt-flow rates are associated with higher molecular weight while higher melt-flow rates indicate a lower average molecular weight. Processors prefer higher-flow materials because they facilitate the filling of more intricate parts. They also allow for the use of lower melt temperatures, which translates to shorter cycle times.

However, these higher-flow materials often come with a hidden price: reduced performance. The property that declines most rapidly with reduced molecular weight is impact resistance. This can be difficult to detect in a standard property sheet because sometimes the problem is not apparent at room temperature, where almost all properties are measured.

But even when the data sheets show no differences between, for example, a 5-melt-flow and a 25-melt-flow polycarbonate, they will still be evident in properties such as the ductile-to-brittle transition temperature, creep and fatigue performance, and resistance to stress cracking.

Material suppliers are constantly trying to find ways to reduce the performance penalty associated with lower molecular weight, but the improvements announced in the media regarding these efforts far exceed the actual advances.

An additional issue with low initial molecular weight comes from secondary supply sources. Many primary material suppliers no longer do their own compounding. Compounding involves the incorporation of fillers, colorants, and other additive packages into an existing base resin. A thorough understanding of the chemistry of a resin is required in order to maintain the properties of the polymer. A company that manufactures the polymer will have this understanding.

Compounding represents a melt processing step just like injection molding. So when a material is compounded it has already seen a heat history before it reaches the final processor. If a primary supplier of a material publishes a melt-flow rate or viscosity specification for a particular grade of material, it will ensure that this specification is maintained after compounding. A secondary supplier may not.

Even greater problems may be encountered if recycled or wide-spec feedstocks are used. These products have been manufactured outside the original specification range or are collected as post-industrial or post-consumer scrap and are sold at a discount to secondary suppliers. These materials make up much of the low-cost market that has put such extreme cost pressures on primary polymer manufacturers. Some lots of these materials have properties comparable to virgin resin, yet others represent a significant departure from the intended performance profile. Blending of different feedstocks is sometimes employed to attenuate the property differences inherent in a wide range of initial molecular weights. But this changes the molecular weight distribution. While this is a complicated subject beyond the scope of this article, it is sufficient here to state that a 12-melt-flow-rate material produced directly from the polymerization process has better properties than a 12-melt-flow material made by blending two or three materials of widely varying melt-flow rates.

Trust Only Your Own Measurements

The second cause of reduced molecular weight is degradation of the polymer during processing. This degradation can occur in any material if during melt processing the combination of temperature and time exceeds the thermal stability limits of the material. Materials like high-density polyethylene are very thermally stable and can withstand a broad range of time-temperature exposures in the melt. Other polymers like PVC have an almost legendary sensitivity to elevated temperatures.

Measuring actual melt temperatures rather than relying on barrel setpoints is an important part of process documentation and control. Residence time calculations determine how long the material is exposed to the melt temperature. While material suppliers provide recommended melt temperature ranges in their literature, it is important to remember that a temperature appropriate for 3 minutes may cause catastrophic damage in 10.

It is also critical to note that most calculations of residence time take into account only the material that can be held in front of the fully retracted screw. This ignores the material contained in the screw flights, which can account for 60% to 70% of the material in the melt state at any given moment.

In some polymers such as polycarbonate or polyesters, there is the additional problem of hydrolysis. Excess moisture present in the polymer during melt processing will cause degradation when the polymer chemically reacts with the water. Since few processors actually measure pellet moisture content with an instrument that is moisture specific, detecting problems with elevated moisture levels is often a matter of guesswork and visual inspection.

In most polymers, a comparison of the viscosity of the raw material to that of the molded parts can provide a determination of polymer integrity after processing. Some reduction in viscosity is normal during melt processing, but if it exceeds certain limits, the chances for product failure increase significantly. Even when short-term properties appear to be unaffected, long-term performance may still be compromised.

In an era when new processing techniques are being introduced to provide competitive advantage and allow for the manufacture of new products, it is important to remember that molecular weight retention remains at the foundation of good product performance.

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