controlling crosslink density of coreactive polymers in an extruder reactor

Controlling Crosslink Density of Coreactive

Polymers in an Extruder Reactor

John Curiy and Paul Andersen Werner & Pjleiderer Corporation

Ramsey, New Jersey 07446

ABSTRACT A common technique for the manufacture of polymer alloys is melt blending of polymers with spec@c chemical interactions. For instance, an amido ester crosslink forms when oxazoline-gm@d PS is mixed with acid-grafted PE. The crosslink builds melt viscosity and stabilizes product morphology. Reaction control is critical since overreacted alloys form gels and underreacted alloys have gross composition gradients andlor tend to phase coalesce so that product properties are variable. Polymer reactions in plasticating compounders are prutiCurCrrry dimult control problems since several unit operations involving plastication, heat exchange, mixing, and separattng of by-products must be sequenced properly to control a reaction. Reactive polystyrene (OPS) and reactive polyethylene (APE) with oxazoline and carboxylic acid functionulily, respectively, were melt blended in a ZSK compounding extrudkr under a variety of conditions. The extent of crosslink was monitored by the strong shear viscosity increase of the reactedproduct, the infrared absorption by the crosslink, and reactant depletion in the melt. The phase morphology was chamcterized by optical phase contrast microscopy. A response suflace of product shear viscosity vs. the primaq operational parameters of extrusion rate and extruder RPM is interpreted in terms of characteristic reaction time and development of reactive interfaces during the mixing process. Response sugaces result from storing coordinates and responses while exercising a new feedback loop control system.

INTRODUCTION cost effective than developing a new polymer chemis- try-

For a combination of two or more materials to form an effective blend, the components must be thermody- namically miscible or chemically compatibilized. A key effect of compatibilization is drastic reduction in the in-

Development of new materials to meet market de- mands for expanded performance of polymers is being pursued through the combination of existing polymer systems as either blends or alloys. This approach is more

Advances in Polymer Technology, Vol. 11, No. 1, 3-10 (1991/1992) 0 1992 by John Wiley & Sons, Inc. CCC 0730-6679/92/010003-08$04.00

CONTROLLING CROSSLINK DESTINY

terfacial tension so that fine dispersions are more easily formed by melt blending. Plochocki et al.' have shown that domain size is reduced in a melt-blended mixture with the addition of a small amount of copolymer com- patibilizer. The associated histograms from that system (Fig. 1) show distribution of domain sizes is significantly narrower for the compatibilized alloy than the mechanical blend.

Property controlling morphology is characterized by disperse phase size, shape, and texture. The morphology is influenced by component selection, concentration, and process history. Compared with compatibilized mixtures, simple blends are susceptible to larger disperse domain size, broader size distribution, supradomain texture, and coalescence of the dispersed domain, all of which are a result of high interfacial tension. Morphology control is important, for example, to achieve the proper balance of impact strength, tensile and flexural modulus, and hard- ness. In a rubber-toughened polymer matrix, one needs a specific volume percent of well-distributed impact mod- ifier with a narrow distribution about an optimum dispersed phase sizeZ in an appropriate matrix. Unless the two polymers are compatible or compatibilized, fine morphologies are not achievable.

To obtain the compatibilization needed for improved dispersion, stable morphologies, and the corresponding system properties, one can:

1. Use a third ingredient with miscibility in both component phases as a compatibilizing agent (e.g., an interfacially active block copolymer).

2. Chemically modify the matrix for compatibility with the dispersed phase.

3. Chemically modify the dispersed phase for compatibility with the matrix phase.

4. Or, where there is no modification that will create compatibilization, coreact melt-blended materials that have had reactive groups grafted to them.

The coreactive systems are unique. Even though the components of the first three techniques may have been modified to achieve compatibilization, miscibility is usu- ally achieved by amorphous phase associations. These systems have fairly broad processing windows. Coreac- tive systems depend on chemical bonding between the two components. To obtain the desired compatibilization without making an intractable product, it is critical to control the level of bonding (x-linking).

Effective control of a commercial interfacial crosslinking process requires an online indication of crosslink density. Lacking a convenient online analytical sensor, a push-down correlation can be used, which, in the case of a coreactive material, could be x-link density + shear viscosity + machine-level operating signal.

4

Domain diameter

Domain diameter

FIGURE 1 Dispersed phase size distribution in a polyblend and a corresponding alloy prepared in the same mixing process. (Upper blend of PWPS = a1 and lower alloy of PWPS/block copolymer = 2/1/.17.) Alloy shows finer phase size with narrow deviation. After Plochocki et al.'

This work is an exploratory investigation of the reaction behavior of a model coreactive system processed on a compounding extruder and the development of these push-down correlations. Specifically, it was our goal to correlate the chemical evidence of crosslinking sensed by IR absorption with the more easily measured shear viscosity, and then use the viscosity signal to build a model from machine responses like die pressure, actual rate, and melt temperature, which are sensitive to crosslink density.

MATERIAL SYSTEM

An ideal coreactive system for reactive process inves- tigations would have an analytically measurable inter- action and a rate-determining mechanism with nonam- biguous process dependencies. For instance, high reaction activation energy and a lack of reaction by-products are desirable qualities. Among the coreactive alloys suitable for melt compounding is the mixture of oxazoline-functionalized polystyrene (OPS) and acrylic-acid-functionalized polyethylene (APE). The batch preparation, torque rheometry, and compatibility of the blends, as well as

VOL. 11, NO. 1

CONTROLLJNG CROSSLINK DESTINY

.... CH2-CH ........ CH2-CH ........ CH2-CH ........ CH2-CH ....

I “=P COOH

I .... CH2-CH ........ CH2-CH ........ CH2-CH ........ CH2-CH ....

flGURE 2 Chemical structure of reactants and product. After Baker and Sa le~rn .~

coupling kinetics and crosslink monitoring, were studied by Baker and Saleem.’~~ According to their work, the crosslink species was verified via Fourier transform infrared (FTIR) as being an amido-ester crosslink (Fig. 2). The more extensively reacted samples produced a higher reaction torque on their Haake mixer. This system is particularly attractive for extrusion study because there is no reaction by-product and because the reaction is limited by mass transfer considerations (degree of mixing) rather than chemical kinetics. It is therefore sensitive to mixture quality and less sensitive to shear dissipative heating. This is known because products manufactured at different temperatures are identically compatibilitzed as shown by differential scanning calorimetry.

MIXING SYSTEM

The mixing of polymer alloys and blends can be done effectively by continuous extrusion. Key machine and material parameters for reactive extrusion processes are shown in Table I.

The melt compounder must accommodate mixing conditions, residence time, preferred melt temperature, and by-product removal required by the interphase reaction.

TABLE I Key Parameters for Reactive Extrusion

Mixing Conditions-Rate, RPM, Configuration, Addition Sequence Residence Time-Rate, RF’M Melt Reaction Temperature Reaction By-Product Removal Material Properties-Viscosity Ratio, Interfacial Tension, Elasticity Ratio, Material Diffusivity Reaction Kinetics-Mole Ratio, Characteristic Reaction Time, Temperature

The dispersed phase shape and size depend upon extrusion conditions and certain material parameters, particularly, viscosity ratio and interfacial tension. In experiments where RPM and rate were varied, the average dispersed size of certain blends correlate to mechanical- specific energy. It is well known that extruder configuration (selection and placement of mixing elements) and length contribute to specific energy and presumably prop- agation of interfaces. These parameters (rate, RPM, and configuration) are important control considerations in production of coreacted alloys.

EXPERIMENTAL

An intimate pellet blend of the two lots (XUS40056.01 and XUS40056.00) of Dow’s OPS was prepared to yield a 1.2 wt % functionality. APE (Dow’s Primacor 1430) had 9 wt % acrylic acid functionality. Polystyrene and polyethylene grades were also available for dilution of the functionalized polymers.

For formulation testing and IR spectrometer calibration, polymers were melt blended on the Haake Rheomix 600. Torque and temperature during the mixing process were recorded. Films were pressed from the melt mixture for FTIR analysis using a Perkin-Elmer Model 1760 FTIR with globar IR source and DTGS pyroelectric detector.

The continuous compounding extruder was a ZSK-40 corotating intermeshing extruder. The compounder was fed by two loss-in-weight feeders, one for each of the reactants. The extrusion die was fitted with on-line Gott- fert capillary rheograph and an Automatik IROS 100 on- line transmission FTIR with ZNSE windows, spaced at 100 pm and using a DTGS detector. The entire compounding cell was controlled by a Bailey Network 90 distributed control system. A flow diagram of the compounding line is shown as Figure 3.

Photomicrographs were prepared on a Nikon Optiphot microscope with Nomarski differential interference contrast optics and green interference filter. Samples were microtomed from quenched unconstrained strand samples from compounder die.

The study was divided by two mixing techniques, batch and continuous. Batch experiments were used to determine the viability of the basic relationships that would then be run on the continuous system.

RESULTS AND DISCUSSION

Batch System

To determine a x-link density calibration curve, batch- mixed samples were scanned by FTIR. The samples con- tained the same mole ratios of OPS to APE but were

ADVANCES IN POLYMER TECHNOLOGY 5


FIGURE 3 Continuous reactive processing featuring closed-loop quality control. 1. Twin-Screw extruder (ZSK 40); 2. microcomputer controller; 3. material feeder-reactive polystyrene (OPS); 4. material feeder-xreactive polyethylene (APE); 5. strand die; 6. on-line IR spectrometer (IROS/100); 7. signal processor; 8. on-line rheometer (Bypass-Rheograph); 9. signal processor.

Sample: Mixture

1400 1300 1200 1100 1000 900 CM-1 800 Wave number

FIGURE 4 FTlR of batch mixtures with increasing diluent showing decrease of APE reactant consumption. a. Primacor Acid G O stretch feature; b. Primacor acid OH bend feature; 1. 60/40/0 OPS (l%)/APE/PS; 2. 45130125 OPS (l%)/APUPS; 3. 30/20/50 OPS (l%)/APE/PS. Courtesy D. Shelley.

0.8 0.7

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FIGURE 5 Calibration curve generated from batch samples that is used to relate IR absorbance (at 1236 cm-') to acid group concentration in reacted mixtures. Courtesy D. Shelley.

6 VOL. 11, NO. 1


TABLE I1 Degree of Crosslinking in Batch Blends Using Acid Reactant Depletion Method from FTIR Data

Acid Oxazolie Mixer Reacted Available Crosslink Extent of Torque

OPS Concentration Reaction @ 12 min Temp. Sample (1.6%) APWPS (groups/cm3 x lo-", (wt (%) (m-& ("C)

~~~

6 60/4010 0.58 0.56 1.2 1 0 0 1325 24 1 7 45130125 0.29 0.42 0.83 69 835 237 8 30120150 0.09 0.28 0.57 32 690 235

TABLE III Operating Parameters vs. Shear Viscosity

~ ~~~~~ ~ ~

Rate Shear Viscosity" Sample RPM (pph) (Pa - s) ~ _ _ _ _ ~ ~ ~

35 300 90 862 38 300 30 Intractable 36 200 50 898 3 9 200 50 922 34 100 90 777 37 100 30 893

'L = 60 scc-', 200°C. %SO OPSIAPE. AU orhers 45/55.

each more heavily diluted with crystal PS (Dow 685D) with similar shear viscosity to OPS. Since the extent of reaction was previously found4 to be limited by mass transfer, reactant dilution should be a convenient way to retard reaction. Since OPE is more highly functionalized than OPS, the reaction environment is acid rich at the selected mole ratio.

When the batch mixer torque reached a steady state, it was assumed that OPS was totally reacted in the pres- ence of the excess acid. Because amido-ester bonds ab- sorb IR in a region of gross interference, a loss of reactant scheme was used to describe extent of reaction. The C=N stretch feature of oxazoline absorbs weakly in a region of gross interference. The C-0 feature of the acid absorbs too intensely, but either the acid C-0 stretch/OH bend feature at 1236 cm-' or the acid 0-H out of plane bend at 94W cm are useful for quantifi- cation. Figure 4 shows transmission spectra for a group of mixtures with increasing levels of diluent that shows a decreasing relative absorbance of acid functionality remaining in the blend. Once apparent absorbance at 1236 cm-' is corrected for film thickness and the absorbance values are compared to spectra of APE stan- dards with known acid content, an acid concentration absorbance correlation can be found (Fig. 5).

To check this calibration curve and the mass transfer criterion for reaction, a second test was run with increasing levels of diluent. These mixtures had the same basic

ADVANCES IN POLYMER TECHNOLOGY

recipe as before with the exception of substituting 1.6 for 1.0% OPS. The mixer was run for 12 min after plastication. Even after 12 min, diluted mixtures were still developing torque. The residual acid concentration was then measured via IR absorption-acid concentration correlation. The amount of reacted acid was determined by the difference of residual coflcentraton from the known initial concentration. The results (Table 11) show that with OPS dilution the reaction is less efficient and the cal- culated x-link density correlates to increasing mixer torque. This is further evidence that mixing limits extent of reaction and that crosslink density is reflected by shear viscosity.

Continuous Systems

A control system was fitted with a feed backloop from the on-line viscometer to extruder speed and component feed rate. To eliminate the lag of viscometer sampling, the system was set up to control continuously on a mod- eled viscosity based upon die pressure, melt temperature, and total rate. This viscosity model was updated every 8 min based upon time-stamped information from the viscometer. The on-line FTIR was not run often enough to give statistically significant correlation between shear viscosity and crosslink density, but did show some qualitative effects.

To avoid undesirable gel in the strands or other bad effects such as dangerous torque or pressure, samples were produced within an experimentally determined operating window, using RPM and rate as manipulated variables. The 60/40 OPS/APE composition of the batch system produced nervy product on the compounder so the composition was changed to 45/55, which reduces the possible ultimate crosslink density. Table 111 shows data from some of the samples run. It shows that intractable material that plugs the on-line rheometer can be produced by the extruder and that increasing RPM or decreasing rate causes a melt viscosity increase in the product. Effects of process condition and formulation upon morphology and rheology of samples collected near

7


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6 1000

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500

OSAMPLE 34 OSAMPLE 39

5 1 0 100

APPAPENT SHEAR RATE(SEC- 1)

(4 FIGURE 6 Morphological and rheological properties of samples produced at the limits of the continuous compounder’s operating window. (a, b) Optical phase contrast microscopy at 500 x magnification from extruded strand sectioned along its length. (c) Apparent shear viscosity from on-line capillary viscorneter with 3 mm diameter and 100 mm length. Samples are identified in Table 111.

the window limits are shown as Figure 6. Samples mixed at low RPM/feed rate ratios and formulated for lower potential reactivity possess coarser morphologies and lower shear viscosity. The morphologies of poorly reacted samples are characteristically coarse, display gross-scale in- homogeneity, and are susceptible to orientation in flow fields. By contrast, the morphologies of highly reacted samples are finely textured and nondirectional. An increase in shear viscosity, particularly at low deformation rate, is another consequence of the interfacial reaction. This is expected since the higher concentration of network chains in the highly reacted material causes flow resistance. However, the shear viscosity of both materials at high deformation rate is probably dependent upon char- acteristics of the common matrix material and therefore not an indication of the crosslinking reaction.

It has been shown that dispersion quality is responsive to RPM/feed rate ratio’ and to specific mechanical energy.’ The material structure becomes finer and more uniform as both the ratio and specific energy increase. This suggests the possibility of torque feedback control strategies.

The few on-line FTIR runs gave qualitative support to the viscosity results. Figure 7 shows, for instance, that two samples produced at different RPM/rate ratios have virtually superimposable absorbance spectra except in the region of depleted acid functionality, where the sample with higher RPM/rate ratio shows lower absorbance, in- dicating lower residual acid content. This is corroborated by viscosity data (Table 111) that show the more extensively reacted sample (36) to have a higher viscosity.

Using a multivariant feedback loop, control data were collected and product shear viscosity was mapped against RPM and rate in pph (Fig. 8). Note that this response surface has two maxima, one near the origin and a second at low-rate and high RPM. The origin localized maximum corresponds to a long residence time environment where diffusion of reactants to the interface is significant. The high RPM maximum illustrates the effectiveness of convected flows in micromixing.

CONCLUSION

The coreactive alloy made from OPE-APE was selected as a model system to investigate reaction kinetics in the operability range of the ZSK-40.

A quantitative method that relates extent of crosslink reaction to features of the FTIR spectra has been devel- oped. The extent of reaction for laboratory batch samples was measured and found to correlate to mixer torque. The on-line FTIR confirmed a correlation between extent of reaction and shear viscosity of the melt as acquired by an on-line viscometer. The melt criterion was used

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0.26

0.1 0

5826/35 - 90 pph, 300 rpm 5826/36 - 50 pph, 200 rpm

I I I I I I I

2000 1800 1600 1400 1200 1000 800 500

Wave number cm -1

FIGURE 7 IR spectra from on-line FTlR showing differences in energy absorption of features related to residual APE in the product for two sample conditions. Samples are identified in Table 111.

OPS/APE = 45:55

Apparent viscosity Pa-s

@ 41 sed' and 2OOOC

RPM = 90-300 6 p p h Rate 50-125

RPM = 90-300 6 p p h Rate 50-125

FIGURE 8 Shear viscosity response surface for OPS-APE reaction on ZSK-40.

ADVANCES IN POLYMER TECHNOLOGY 9


to quantify the reactivity dependency with respect to RPM and rate.

REFERENCES

1. A Plochocki, S. Dagli, J. Curry, and J. Starita, Polym. Engng. Sci., 29(10), 617 (1989). 2. B. Epstein, U.S. Pat. 4,174,358 (1978). 3. W. Baker and M. Saleem, Polym. Engng. Sci. 27(20), 1634 (1987). 4. W. Baker and M. Sateem, Polymer, 28(12), 2057 (1987). 5. J. Curry and P. Andersen, Proc. 48th SPE AhTEC 1938,

ACKNOWLEDGMENT

All FTIR scans and data interpretation are courtesy of Prof. D. Shelly, Stevens Institute of Technology (cur- rently Northeast Texas Tech). (19%

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controlling crosslink density of coreactive polymers in an extruder reactor

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