应用文献——采用动态应变流变仪对刚性聚乙烯的溶胶黏度进行表征

CharacterizingMelt Viscosity of Rigid PolyVinyl Compounds using a Dynamic Strain Rheometer

Prakash Hatti, Swayajith Sahadevan, GE India Technology Centre,Bangalore

Keywords:thermoplastics, PVC, melt stability, viscosity, rotational rheometer, intrinsicviscosity, capillary rheometer

ABSTRACT

Injection molding big parts andcomplex shapes with rigid PVC is probably the most demanding PVC process interms of melt viscosities. PVC is a material with physical and chemicalcharacteristics that make it process slightly more difficult in comparison withother thermoplastics. The high viscosity contributes to the development of ahigh amount of heat when the material is subjected to the shear applied by thescrew during the plasticization. The thermal instability of the compoundresults from the breakdown either when it is heated at temperatures higher thanadmissible or when it remains for too long at high temperature. Hence evaluatingthe material behavior in terms of its thermal stability and melt viscosity isimportant for end-users of PVC compounds. This paper attempts to assistengineers who are faced with the task of screening compounds from varioussuppliers, prior to full-scale evaluation of the compound in productionmachines.

INTRODUCTION

Traditional tests for polymersfocus on the amount (mass) of material passing through a known diameter orificein a given time frame, usually 10 min. This type of test, known as Fluidity Indexor Melt Index (MI), provides a single point measurement of the flowability of amaterial. The standard measurement for Melt Flow of PVC compounds is generallynot done according to ASTM D 1238(1). An extension, ASTM D 3364 (2)is usedspecifically for flow rate measurements of PVC compounds while detecting andcontrolling various polymer instabilities associated with the flow rate. Inthis test, no control or knowledge of the material flow pattern exists, andthus the strain history is different for each material studied. This singlepoint value is a composite of both the viscosity and elasticity in the sample.This empirical value is not defined in terms of a particular deformation, butrepresents a single point value which typically serves to rank or screen materials.Due to the nature of the measurement, it is not possible to determine theeffect of the viscosity contribution independent of the elasticity. Many times,samples with the same MI value, process drastically different from each other.This method is best used as quality control(3) for the flow behavior of moltenthermoplastics. It does not provides a fundamental property measurement and mayor may not correlate with processing behavior.

Dynamic measurements, on the otherhand, obtained with Strain/Stress Controlled Rheometers provide a valuablemeans of characterizing information regarding the material’s thermal and flowbehavior. These instruments have the ability to rigorously determine both theelastic and viscous response of a sample in a single experiment. Thestrain-controlled instrument applies a controlled shear strain in the form ofdisplacement and measures the stress through a torque transducer to calculatethe modulus, viscosity etc.

The materials under investigationcomprised three rigid PVC compositions labeled as Sample A, B and C. Eachcomposition is markedly different in their composition with variations in thethermal stabilizer type and concentration, apart from differences in the typeof the base PVC resin.

SOLUTION

The complex viscosity is evaluatedusing the strain controlled Rheometer RDA III with a parallel plate geometry.The experiments were conducted in accordance with ASTM D 4440/ISO 6721-10(4,5). The samples for the test are compression molded into 25 mm circular discs,after appropriate pre-drying.

Linear visco-elastic behavior isdefined, for the purpose of the standard requirement, wherein the modulus isindependent of the applied strain. This assumption is necessary for thecomparison of the test data. Therefore the amplitude of oscillation is set suchthat the deformation of the specimen occurs within the linear-viscoelasticregion. Figure 1 shows a strain sweep performed on samples A from 1% to 100 %strain to define this linear viscoelastic region. All samples A, B, C showed alinearity onset at ~10% strain.

Figure 1: Strain Sweep @ 200°C of PVC sample A

A frequency sweep was done on thesamples A, B and C to compare the complex viscosity of each sample and to checkthe validity of Cox-Merz rule(6).

Sample A exhibits the lowestcomplex viscosity (1173.2 Pa s) at 100 rad/sec. The overlaid results of themeasured complex viscosity on the straincontrol rheometer for samples A, B, Care shown in figure 2.

Figure 2: Frequency Sweep @ 200°C of PVC sample A, B, C

A capillary rheometer(7) was usedto evaluate the apparent viscosity over the practical range of shear rate from100-10000 s-1. The test conditions were: -die with L/D of 1/30; - temperatureof 200 °C. The shear viscosity was recorded for all samples. As can be seenfrom the figure 3, sample A exhibited a lower viscosity profile (1131.12 Pa sat 100 s-1) across the shear rate range of 100-10000 s-1) compared to the sampleB and C.

Figure 3: High Shear Viscosity @ 200°C measured in a capillary rheometer

This observation(8) on thecapillary rheometer reflects similar trends as observed in the viscosity profileobtained from the RDAII as shown in table 1.

Table 1: Viscosity comparison of PVC samples A, B, C

A time sweep was performed toevaluate the melt stability under the following test conditions: Geometry 25 mmparallel plate; Gap 1.93 mm; Frequency 6.2832 rad/sec; Temperature 200ºC; Strainof 10%; Time 1200 seconds.

Dynamic viscosity values werecompared after ~ 215 seconds and shown in figure 4. Sample A exhibited adynamic viscosity of 3894.1 Pa s; Sample B of 7656.2 Pa s and Sample C of 11430Pa s. The complex viscosity for all three samples is slightly increasing withtime.

Figure 4: Time Sweep @ 200°C for sample A, B C

To further augment the observedviscosity profiles, the ‘K value’ or the ‘Viscosity Number’ defined by ISO/R174-1961 (9) for the PVC resins A, B and C were analyzed. Solution withdifferent concentrations of the PVC resins in cyclohexanone were prepared. Theflow times of the solvent and the solutions of resin were measured at 27°C byUbbelhode viscometry and the viscosity number was calculated by extrapolatingto zero concentration.

The viscosity number (K) as definedby ISO/R174-1961(9) is calculated as:

K= (t-t₀)/ t₀*C

where t = time of flow in secondsof the solution, t₀ = time of flow in seconds of the redistilledcyclohexanone and C = concentration in g of resin per ml of solution.

The viscosity number is reported tothe nearest whole number. Sample A shows a markedly low Kvalue of 53, sample Bexhibits a K-value of 56 and sample C of 59. This correlates qualitatively withthe differences in the viscosities observed on the strain controlled rheometer& the capillary rheometer.

CONCLUSION

Sample C exhibits a relativelylower viscosity profile compared to sample B and C. The viscosity trendobserved can be considered as an indirect measure of the material’s intrinsicmelt viscosity. This parameter defined by the solution techniques (K-Value) forlinear polymers, correlates with the polymer average molecular weight. TheStrain Controlled Rheometer thus provides a valuable, reliable and fast methodto evaluate the melt-stability compared to traditional test methods. The meltviscosity serves a useful purpose of characterizing new compounds, screeningmultiple formulations or providing protocols for quality control. Supplementarytechniques such as torque rheometer testing, static dynamic testing on the2-roll mills will provide information on the degradation rate, thermal instability,etc. A combination of these techniques generates multiple indicators on the PVCproces/ sability prior to full-scale production trails.

REFERENCES

1. American Society for Testing andMaterials, Standard D 1238-99, Standard Test Method for Melt Flow Rates ofThermoplastics by Extrusion Plastometer

2. American Society for Testing andMaterials, Standard D 3364-99, Standard Test Method for Flow Rates for PVC withMolecular Structural Implications

3. Nass, Leonard I., Testing RigidPVC Products, Second Edition of Encyclopedia of PVC

4. American Society for Testing andMaterials, Standard D 4440-01, Standard Test Method for Plastics: DynamicMechanical Properties: Melt Rheology

5. ISO 6721-1, 2001, Determinationof Dynamic Mechanical Properties, General Principles Part 1

6. (Cox Merz Rule)- A well-knownempiricism in the rheology of polymer melts is the Cox-Merz rule, which relatesthe linear dynamic moduli as functions of frequency to the steady shear flowviscosity curves. This relationship is very useful because it allows toestimate steady flow viscosity curves from the more readily obtainable fromdynamic mechanic rheological measurements.

7. American Society for Testing andMaterials, Standard D 3835-961, Standard Test Method for Determination of Propertiesfor Polymeric Materials by means of Capillary Rheometer

8. Cox, W.P, Merz, E.H.,Correlation of dynamic and steady flow viscosities, J. Polym. Sci., 28, 619-622(1959)

9. ISO/R 174-1961, Determination ofviscosity number of poly (vinyl chloride) resin in solution.

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