Introduction

Vitrolite™ lowers polymer melt viscosity in stress and strain-rate regimes typical of melt compounding and molding of finished articles.  The lower viscosity of polymer melts with Vitrolite™ allows for compounding and molding process adjustments that can increase productivity rates, often with less energy required per unit produced.  Vitrolite™ may increase the efficacy of functional additives because of improved dispersion and distribution, allowing compounders to reduce formulation costs.  Vitrolite™ is unlike any other processing aid on the market because finished polymers with Vitrolite™ have physical, mechanical and appearance properties equal to or greater than the same polymer without Vitrolite™.

The primary foci of this technical bulletin are to demonstrate:

• Quantitative reduction of melt viscosity by Vitrolite™
• Absence of adverse interaction of Vitrolite™ and nylon
• Correlation of lower melt viscosity with improvements in melt compounding and molding

Effect of Vitrolite™ on Nylon Melt Viscosity

Oscillating-plate rheometry is very effective in quantifying the effect of Vitrolite™ on melt properties in a dynamic environment as a function of temperature, shear rate and stress.  Oscillating plate rheometry is particularly suitable because it focuses on internal melt properties as opposed to external melt slip.  The effect of Vitrolite™ is well known to reduce the melt viscosity internally. 

Oscillating plate rheometric evaluation of a toughened nylon 6,6 is illustrative of the effect of Vitrolite™ on nylon melt viscosity.  The effect of shear-rate on nylon melt viscosity is isolated by varying the oscillation frequency (proxy for shear rate in compounding and molding equipment) from 0.1-100 sec-1 at strain amplitudes of 2.5% or less.  By limiting the strain amplitude, melt properties are representative of the linear regime of stress-strain relations.  Although stress-strain regimes in most parts of compounding and molding equipment are non-linear, the effects of Vitrolite™ in the linear regime are relevant to some melt compounding and molding environments and provide a basis for understanding the melt properties in the non-linear regime.

As generally expected for viscoelastic polymer melts, the OSV of nylon 6,6 decreases with increasing oscillation frequency (Figure 1).  At high oscillation frequencies (shear rates) that are more representative of environments in compounding and molding equipment, Vitrolite™ lowers the melt viscosity.  At low oscillation frequency, Vitrolite™ increases the melt viscosity.  This is not considered to have an adverse effect on molding of nylons because:  (i) low shear rate-low stress environments are not dominant in typical equipment; stresses typically are higher and melt properties in these environments are better described by stress amplitude results below and (ii) shear-rate dependent thickening at low oscillation frequencies is unlike any other polymer studied and may be peculiar to the specific toughened nylon.

The behavior of the melt in the non-linear stress-strain regime more typical of compounding and molding environments is readily evaluated by varying the stress applied to the polymer melt while holding the oscillation frequency constant.  For example, the OSV of toughened nylon is relatively constant at stresses below the critical stress value of about 20,000 Pa at an oscillation frequency of 1 sec-1 (Figure 2, left).

Below this critical stress value, the melt stress-strain relation is dominantly linear.  As the critical stress value of about 20,000 Pa is exceeded, the melt behavior becomes strongly non-linear as evidenced by a precipitous drop in melt viscosity.  The presence of Vitrolite™ results in an even greater melt viscosity decrease from 16-67% depending on Vitrolite™ concentration.  At higher oscillation frequency of 10 sec-1 (i.e., higher shear rate), the rheometer is not capable of achieving the critical stress amplitude; therefore, an anticipated strongly non-linear viscosity drop is not detected.  However, nylon melt with Vitrolite™ has a consistently lower viscosity (Figure 2, right) as predicted by the frequency sweep at low stress amplitude in the linear regime (Figure 1). 

Oscillating-plate rheometry evaluation of toughened nylon 6,6 clearly demonstrates that Vitrolite™ lowers the melt viscosity.  It is entirely reasonable, particularly considering compounding and molding studies on non-toughened nylon presented below, that Vitrolite™, in general, lowers the melt viscosity of all nylon melts.

Interaction of Vitrolite™ with Nylon

Detailed studies of numerous polymers other than nylon have yet to identify instances of polymer compositional changes or chemical interaction between the polymer and Vitrolite™ (see Technical Bulletin:  How Does Vitrolite™ Work?).  Rather, the effect of Vitrolite™ on polymer melts is almost entirely a physical interaction between the solid Vitrolite™ and polymer molecules.  Physical and chemical evaluations of nylon 6,6 compounded with Vitrolite™ and without Vitrolite™ likewise indicate that the interaction of Vitrolite™ and nylon molecules is dominantly physical nature:

• The peak crystallization and melt temperature of nylon with 0.5-1.0 wt% Vitrolite™ does not consistently differ from NEAT nylon as determined by Differential Scanning Calorimetry (DSC) at 5-20oC/min heating/cooling rates under N2.
• Vitrolite™ is not a strong nucleating agent.  The enthalpy of recrystallization is slightly higher with Vitrolite™ than without Vitrolite™; however, Vitrolite™ may slow crystallization rates slightly since the recrystallization enthalpy was lower with 1.0 wt% Vitrolite™ and at the fastest 20oC/min cooling rates.
• Vitrolite™ does not accelerate nylon degradation.  Thermogravimetric analysis of weight loss over a 3-hr time interval at 270oC was identical for NEAT nylon and nylon with 0-5-1.0 wt% Vitrolite™.  Thermal degradation rates at 300oC were nearly identical for both NEAT nylon and nylon with 0.5-1.0 wt% Vitrolite™ for up to 1 hour.
• Vitrolite™ does not promote nylon degradation.  The on-set temperature of thermal degradation with 0.5 wt% Vitrolite™ is within 1oC of the 398oC on-set temperature for NEAT.  Nylon with 1 wt% Vitrolite™ had a slightly lower on-set temperature of about 393oC.

Compounding with Vitrolite™

Although the effect of Vitrolite™ on nylon melt viscosity differs in detail from other thermoplastic melts, the effect is broadly similar:  the melt viscosity is lower under conditions relevant to compounding and molding.  Of particular relevance to nylon, the effect of Vitrolite™ is the same or even exaggerated in highly filled formulations as compared to unfilled formulations.   Glass-filled nylon formulations are prized for their physical and mechanical properties, but are often difficult to compound and mold.

Vitrolite™ improves compounding of glass filled nylon by lowering of the melt viscosity.  For example a large compounder ran an initial compounding trial in a 92-mm twin screw compounding extruder to determine the effect of Vitrolite™ on a 30% glass filled nylon 6.  With almost no process adjustments, the compounder noted about 20% increase in output and more than 10% decrease in amp draw and motor torque.

Physical and mechanical testing of the glass-filled nylon 6 from the initial run indicated little change in properties of the nylon with Vitrolite™ as compared to nylon without Vitrolite™ (Table 1).

TABLE 1.  MECHANICAL PROPERTIES OF 30% GLASS-FILLED NYLON 6.

The compounder was highly encouraged by results of the initial compounding not only because of the obvious benefits to them but also because the Vitrolite™ formulation solved a problem that their downstream molder was having in filling a complex part.  The compounder subsequently perfected compounding methods, made minor adjustments to correct the loss in unnotched Charpy impact strength (*) and is in commercial production.  The compounder is accruing benefits in compounding while at the same time providing the downstream molder a product with excellent performance.  

Other benefits may also be available to the compounder.  Vitrolite™ is known to increase the dispersion and distribution of functional additives and pigments particularly those that tend to agglomerate in the polymer matrix.   By improving the dispersion and distribution, concentrations of those additives may be decreased without affecting their performance, thus reducing formulation costs.   

Effect of Vitrolite™ on Nylon Molding

To date, Vitrolite™ has been used in injection molding of nylons as shown in examples below.  The potential benefits of using Vitrolite™ in injection molding are well known; however, their relative importance varies widely because of variation in nylon formulations, molding equipment, mold and runner design, melt temperature and other variables.  Generally, addition of Vitrolite™ results in some or all of four principal improvements: higher productivity due to a shorter cycle time, lower processing temperature, more consistent mold filling and reduced energy consumption per unit produced.  In many of the examples below, objectives were narrowly defined so some of the improvements were not sought.  Other improvements are more subtle and may include improved surface finish of the article, reduced machine wear and tear due to lower operating pressure and less build up of contaminants due to mild purging action of Vitrolite™. 

In the following examples, processing conditions were optimized for NEAT polymer and became the baseline for comparison.  Polymer with Vitrolite™ was fed to the molding equipment and the process was allowed to stabilize.  The process was adjusted to meet the specific objective or fully optimized to maximize the benefits of Vitrolite™.

Unfilled Nylon

Nylon 6,6:  The objective was to determine if Vitrolite™ would allow a reduction in cycle time.  An Arburg 88-ton press was optimized for molding of a small plaque using the standard nylon 6,6.  Nylon 6,6 compounded with 0.5 wt% Vitrolite™ was introduced and the process was adjusted to produce an acceptable part in the least amount of time without adjusting operating parameters.  Relevant process parameters:

The results demonstrate two attributes of processing with Vitrolite™: 

• Cycle time can be reduced even though melt temperature was not reduced.  Even though the part temperature is slightly higher at take-off, Vitrolite™ appears to reduce warp thus allowing earlier take-off. 
• The melt viscosity is lower as indicated by the reduced pressure at transfer.  Lower pressure at transfer has been correlated with lower pressure drops (improved flow) between barrel, runner and gates.  Delivery of higher plastic pressure in the mold results in more consistent mold fill and less part to part weight or dimensional variation.

Nylon 6,6 with molybdenum disulfide (Nylatron GS):  the objective was to reduce the cycle time for molding of a small computer case closure by optimizing all operating parameters. Relevant process parameters:

The total cycle time was reduced by optimizing several process parameters.  Of particular significance: 

• Vitrolite™ allows for lowering of the melt temperature without greatly increasing the melt viscosity.  In spite of a lower melt temperature, injection pressure did not increase and fill time decreased with Vitrolite™, both of which indicate a lower melt viscosity. 

Filled Nylon

25% glass filled Nylon 6:  The objective was to reduce the cycle time for molding of a 90-gram electrical fitting by optimizing process parameters.  Relevant processing parameters:

The cycle time was reduced thus increasing productivity by about 13%.  Of particular significance:

• Vitrolite™ apparently lowered the melt viscosity.  The lower injection pressures and faster fill time for the glass-filled nylon 6 indicate a lower melt viscosity even at the lower set temperatures.

30% glass filled nylon 6,6:  The objectives were to optimize the cycle time necessary to mold closures in a 6-cavity mold and to determine the standard deviation of weight for 32 sets of closures molded at the optimized processing conditions.  Relevant processing parameters:

The cycle time was reduced by 2.9 sec without altering the processing temperature.  Of particular significance:

• Although injection pressure increased in the barrel, addition of Vitrolite™ to the 30% glass filled nylon 6,6 significantly improved mold fill and reduced the weight variation by 35%. 

33% glass-filled nylon 6:  The objective was to increase productivity of a 1.59 kg automotive fan on a 900 ton press.   The primary process adjustments from the optimized NEAT process were to screw RPM, cure time and dosage.  Relevant processing parameters:

The increased productivity objective was readily met primarily by reduction of cure time, even without lowering the melt temperature.  Of particular significance:

• Vitrolite™ apparently lowered the melt viscosity as indicated by the lower pressure at transfer (95 to 81 bars).
• The lower melt viscosity resulted in a more consistent filling of the fan blades, eliminating short shots and greatly reducing the need to balance the fans.

40% glass- and mineral-filled nylon 6:  The objective was to increase productivity of a 154-gm steering column handle on a 150-ton press.  The processing parameters had been intensively optimized over a long production run, so no initial optimization was required.  Relevant processing parameters were:

The cycle time was reduced by slightly over 8 seconds while still producing a high-quality part.  Of particular significance:

• Even though the melt temperature was lowered by 14oC for nylon with Vitrolite™, the injection pressure did not increase (injection speed was constant).   This indicates that Vitrolite™ lowered the melt viscosity of the 40% filled nylon.

Summary of Vitrolite™ effects in nylon molding: 

• Consistent increase in productivity
• Effective in unfilled or filled with up to 40% glass fiber nylon 6 and 6,6
• May allow for lower processing temperatures combined with increased productivity
• More complete fill of complex molds and less part-to-part weight variation