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Technical

Chlorinated polyethylene advances for thermoset elastomers

By Ray Laakso, Gary Marchand

Virginia Guffey and Rajan Vara

Dow Chemical Co.

Chlorinated polyethylene synthetic elastomers have been successfully used in thermoset and thermoplastic applica- tions since their commercial introduc- tion in the 1960s.1-4 Recent advances in CM technology have been developed that have the potential to expand the current range of performance of CM compounds, especially in the thermoset elastomer arena.

TECHNICAL NOTEBOOK Edited by Harold Herzlichh

A new CM-based elastomer system in- corporates improved cure technology us- ing thiadiazole derivatives and a novel CM product, which in combination yields the possibility of developing more highly- extended CM compounds with greater latitude in formulation ingredients, filler

Executive summary Chlorinated polyethylene, with its saturated backbone and polar halogen

groups, provides a combination of heat, ozone and oil resistance in crosslinked elastomer compounds. CM has been used in many industrial and automotive applications such as fuel- and oil-resistant hose covers, power steering hoses, oil-resistant air ducts and other demanding applications. CM can be crosslinked using peroxides or nucleophilic sulfur thiadiazole derivative sys- tems. While TD is preferred by many end users, its broader use has been limit- ed due to inconsistent cure and poor shelf life stability.

This paper discusses an improved TD cure system for use with CM elastomer compounds based on two technical innovations: improved cure consistency us- ing chelation technology and variation of the accelerators used to modify the cure rate and bin stability of the compound.

The improved cure technology eliminates a major deficiency of existing TD cure technology and results in more consistent cure performance and longer shelf-life of mixed compounds. The new cure technology will be illustrated by application to a new highly extendible CM product which has the ability to ac- cept high levels of plasticizers and fillers and yet provide good physical proper- ties and processing behavior.

try on the batches to monitor perform- ance. In Fig. 1, the acceptable QC “gates” are indicated by the curves with the cir- cular symbols. Some batches fall within the acceptable rheometer limits, but many others fall outside of that range. This erratic cure performance potentially leads to other problems during the pro- gression to the final cured article: extru- sion, curing and performance variations which in turn can create excess scrap and higher production costs.

The cause of this “short-term” thiadia- zole cure variation is typically attrib- uted to mixing differences, impurities and/or moisture. The authors conducted analyses on commercially mixed CM compounds in an effort to determine the effect of ingredient variations and con- taminants on cure performance. An ana- lytical protocol was designed to separate the formulation components into organ- ic, polymeric and inorganic categories. The results showed detrimental effects

and oil levels, and cure control. The thiadiazole curing system for

Fig. 1. Cure inconsistency observed in

rheometer cure curves.

chlorinated elastomers has been com- mercially available since the 1980s.5

Thiadiazole-based cure systems for CM provide certain advantages over perox- ide-cured systems: generation of fewer volatile byproducts, good mold release characteristics, the ability to use less ex- pensive compounding ingredients such as aromatic oils, and the ability to cure over a wider range of temperatures.

Despite these attributes, two problems remain that limit the usefulness of these thiadiazole-based systems.6,7 The first is inconsistency of the vulcanization rate

(batch-to-batch consistency). The second is premature vulcanization of the com- pound during storage (bin stability) or processing (scorch safety) prior to form- ing the vulcanized article. These issues have caused problems for commercial users of the thiadiazole cure system with CM and in many cases have caused them to abandon this cure option altogether.

The cure inconsistency is characterized by variations in the cure rheometer curves as shown in Fig. 1. Production compounding facilities will typically run quality control tests such as cure rheome-

on cure rates of even small levels of zinc contamination, and statistical analysis showed a strong correlation existed be- tween zinc contamination and the varia- tions in the Oscillating Disc Rheometer t50 cure times as shown in Fig. 2.

Although the zinc contamination level correlated well with t50 cure times, the level of zinc contamination in the sequen- tial batches did not decrease steadily as the batches progressed. As seen in Fig. 3, the zinc contamination level was observed to create large variations in the batch-to- batch consistency (t50 cure times) from the

Fig. 2. Correlation of zinc contamination in CM compounds with t50 cure time. Fig. 4. Typical bin stability changes with current thiadiazole system.

Fig. 3. Zinc contamination in sequentially mixed commercial production batches Fig. 5. Effect of zinc contamination on ODR t50 cure time with 1,10-phenanthroline and its effect on rheometer t50 cure times. as chelating agent.

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Technical first to last batch of the sequence, but there was also a significant spike in the zinc lev- el for the sixth batch.

One possibility for the sudden spike with the sixth batch is that a zinc-con- taining additive, for example zinc oxide, could have been used in the mixer for an earlier sequence of mixes with another polymer compound and remained in the mixer until it was dislodged and con- taminated the sixth CM batch. Perhaps the zinc contamination came from a weigh tote that was contaminated with a zinc-containing compound. Whatever the origin, the effect of zinc contamina- tion on a thiadiazole-cured CM com- pound is detrimental and significant.

The second deficiency with TD cured systems is that they are not very stable under typical storage conditions in high heat, high humidity environments. This is illustrated in Fig. 4 comparing the change in Mooney viscosity after one week of aging at 43°C and 90-percent relative humidity. Both samples, which vary only in the choice of accelerator, undergo substantial changes in viscosity during this time period.

One of the thiadiazole cure improve-

of zinc contamination on thiadiazole-cured CM compounds. The goals of the research to improve the thiadiazole cure system were to: a) develop technology to improve the batch-to-batch cure consistency, i.e., minimize/eliminate the effects of zinc con- tamination, and b) improve the shelf-life stability of the uncured CM/thiadiazole compound.

Experimental Ingredients The ingredients were used as received

from the suppliers as listed in Table I. Mixing of the compounds Compounds were mixed in a Farrel

Corp. Banbury BR style mixer using an upside-down mixing procedure and ap- proximately 0.75 load factor. In the up- side-down procedure, the dry ingredients are charged to the mixer first, followed by the liquid ingredients and finally the polymer. A slow mixing speed was used. The chute was “swept down” after fluxing and the mixture was dumped from the mixer at approximately 105°C. The dis- charged compound was cooled via work- ing on a 15.2 centimeters x 33 centime- ters two-roll mill. Dispersion was ensured

as it came off the mill and then reinserted through the rolls in a lengthwise direc- tion. The process was repeated 5-6 times and then this large blanket (about 1-1.5 centimeters thick) was removed from the mill and cooled on a flat surface. Approxi- mately one-half of the large blanket was reworked on the mill in a similar manner as the large blanket to produce a thinner blanket—about 3-4 mm in thickness. A blanket of this approximate thickness was used for sample preparation/testing.

Sample preparation and testing The 3-4 mm rubber blanket was used

for sample testing. Compression molded plaques about 2 mm thick were cured using rheometer t90+10-percent cure times at the specified test temperature.

Testing was conducted as per the pro- cedures listed in Table II. Uncured compounds were aged in a Tempera- ture/Humidity Chamber from Associat- ed Environmental Systems (Ayers, MA) Model LH-6. The conditions in the hu- midity chamber were chosen to simulate accelerated aging under warehousing or

Equation 2. Bin rate calculation.

shipping conditions (90-percent relative humidity and 43°C).

Zinc contamination effects were simu- lated by the addition of zinc oxide (85 wt percent on an EPR binder) on the roll mill. Typically, each compounded batch was split into two parts and the appro- priate amount of zinc oxide was added to one portion to yield the desired quantity of zinc in the final compound.

Shelf-life and processing safety calcu- lations

The Mooney scorch was used to esti- mate processing safety. Accelerated ag- ing was conducted in the humidity oven as previously described. The parameters t3, t5 and t10 refer to the time for the Mooney viscosity to rise by three, five and 10 units respectively. A scorch rate can be calculated by dividing two by the difference between t5 and t3. However, if the Mooney viscosity fails to change more than three to five units during the test, a scorch rate can be calculated by Equation 1.

See Elastomer, page 18

ments described in this paper was de- via a “cigar-rolling” technique in which MU(aged) - MU(original) signed to minimize the detrimental effects the blanket was rolled into a “cigar” shape Bin Rate (MU/hour) = ————————————————————————-

168 hours

Equation 1. Calculation of scorch rate.

(Mooney Viscosity @ 25 min - Mooney Minimum) Scorch Rate = ——————————————————————————————— (25 min - time @ Mooney Minimum)

Fig. 6. Effect of zinc contamination on ODR delta torque with 1,10-phenanthroline as

chelating agent.

Fig. 7. Comparison of ODR cure curve with and without Zn contamination and PEI.

Fig. 8. Comparison of the ODR cure curves with and without lithium citrate hydrate as

an accelerator.

Table I. Ingredients.

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Technical ing agent were tested using the recipes shown in Table III. In this example, 1,10-

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Elastomer Continued from page 17

A measure of the bin stability, (i.e., the safety of the compound to changes in vis- cosity during storage of the compound) can be obtained by examining the differ- ence between the Mooney minimum vis- cosity after storage for one week (168 hours) at 43°C and 90-percent relative humidity versus the original Mooney minimum viscosity measured directly af- ter the compound was made. Equation 2 shows the relevant measure of the rate of change in the Mooney viscosity during storage (increase in Mooney units “MU” per hour) indicated as Bin Rate.

For the ODR testing, ML and MH refer to the minimum and maximum torques

phenanthroline was used as the chelating agent. The level of zinc contamination was about 500 ppm in the compound.

The cure parameters for the Table III compounds are listed in Table IV. The ODR cure test was run for 25 minutes for all samples at 177°C. The data for Sam- ples 1 and 2 indicate that the cure rate has been markedly depressed by the ad- dition of zinc oxide. This is shown most clearly by the large increase in t50 cure time, which is the time to reach 50 per- cent of the change between the minimum (ML min) and the maximum (MH max) torque, and the t90 cure time, which is the time required for the sample to reach 90 percent of the torque difference.

The MH max in Sample 2 is also much less than in Sample 1, indicating that by

The authors Ray Laakso’s more than 25-year industrial career with Dow Chemical Co. and

DuPont Dow Elastomers has been focused primarily on thermoset elas- tomer applications in research and development, technical service and develop- ment, process research, wire and cable, and automotive.

Virginia Guffey has worked for Dow Chemical and DuPont Dow Elas- tomers since 1988 and has worked process research, product research, and technical service and development for plastics, thermoset elastomers and TPVs. She supports Dow’s Specialty Plastics and Elastomers portfolio of prod- ucts, providing assistance to customers.

Gary Marchand has worked for Dow Chemical and DuPont Dow Elas- tomers in research and development concentrating on process and product re- search for both thermoplastic elastomers and thermoset chlorinated elas- tomers.

Rajan Vara has more than 30 years of experience in research and develop- ment, technical service and process development in the rubber fabrication and polymer manufacturing industries.

measured during the test. The t2, t50 and t90 parameters are the times for the torque to change 2 percent, 50 percent and 90 percent of the difference between MH and ML. The maximum cure rate was obtained directly from the slope of the ODR curve by calculating the slope of the curve from point to point and taking the maximum value of the slope.

Results and discussion Batch-to-batch consistency impro-

vements. The addition of a compound-soluble

nitrogen-containing bidentate chelating agent to thiadiazole/CM formulations has been found to reduce the inconsis- tency of the cure associated with con- tamination of the composition with zinc oxide.7 The improvement in consistency of the cure can be accomplished with minimal changes in the vulcanization reaction or the resultant physical prop- erties of the vulcanizate.

The effects of zinc contamination on CM compounds with and without the addition of a nitrogen-containing bidentate chelat-

the end of the test, the sample was not fin- ished crosslinking. In contrast, Samples 3 and 4 have markedly reduced differences in the cure rate parameters. The addition of the chelating agent successfully reduced the effects of zinc contamination on cure parameters for these CM compounds.

The results of several of the cure pa- rameters are shown graphically in Figs. 5 and 6.

The physical properties of the vulcan- izates are shown in Table V. Tensile strength and elongation values were im- proved by the addition of the chelating agent. Levels of chelating agent should be adjusted for optimal performance.

Polymeric chelating agents also can be used to improve the batch-to-batch consistency. Polyethylenimine (PEI) with its nitrogen containing bidentate structure was found to be effective in minimizing the detrimental effects of approximately 500 ppm zinc in the com- pound. Compounds were tested using the recipes shown in Table VI. The cure parameters are listed in Table VII.

The ODR cure parameters in Table

VII indicate the detrimental effects of zinc contamination when no chelating agent is present; the t90 cure time in- creases from 8.96 minutes to 21.4 min- utes (about 135 percent increase). When the PEI chelating agent is incorporated, the t90 cure time is decreased vs. the con- trol (7.17 minutes vs. 8.96 minutes) and desirably there is minimal change created by the addition of the zinc contamination.

The t90 time changed <2 percent with the addition of the ~500 ppm zinc in the presence of the PEI (7.29 minutes vs. 7.17 minutes). The PEI has successfully minimized the detrimental effects of zinc on cure performance. Representative ex- amples of the effects of zinc contamina- tion on ODR curves are shown in Fig. 7.

Shelf-life / processing safety The second major focus of the re-

search with thiadiazole-cured CM is premature vulcanization of the com- pound during storage (bin stability) or processing (scorch safety) prior to form- ing the vulcanized article.

Previous work describes the basic inter- actions of the curative, accelerator and in- organic base in the thiadiazole crosslink-

by making novel changes to the accelera- tor system while leaving the dithiol cura- tive and the inorganic base unchanged.9

An improved combination of bin sta- bility, scorch safety and cure rate can be achieved by using an aromatic hetero- cyclic quaternary ammonium salt, such as pyridinium or imidazolinium salts as accelerator and by the proper selection of the number of carbon atoms present in the groups that are bonded to the ni- trogen atom(s) of the accelerator.

In the following examples, two alternate accelerator systems (AP—alkylpyridini- um salt and IMID—imidazolium salt) were compared to two “traditional” accel- erators (TBAB and 3,5-diethyl-1,2-dihy- dro-1-phenyl-2-propylpyridine (DDPP)) as shown in Table IX. All accelerators were added such that 0.0011 moles of the accel- erator were employed per 100 grams of the rubber used in the composition.

The advantages of the new accelerator systems are seen in the data in Table X. The Mooney Scorch measurements indi- cate better scorch safety for the new accel- erators by the higher t3 and t5 values. Af- ter aging for seven days at 43°C and 90

Table II. Test procedures. ing of CM.8 In the present study, it was percent relative humidity, the compounds

found that improvements in the shelf- with the new accelerators still exhibited life/processing safety can be accomplished See Elastomer, page 20

Table V. Physical properties for samples containing 1,10-phenanthroline.

Table III. Recipes using 1,10-phenanthroline as a chelating agent. Table VI. Recipes using polyethylenimine as a chelating agent.

Table IV. Cure parameters for samples containing 1,10-phenanthroline. Table VII. Cure parameters for samples containing PEI.

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Technical vantages over the traditional accelera- 8.3 dN-m/min and the magnesium sul- tors indicating possibilities to produce fate heptahydrate sample 12.2 dN-

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vantages for cure response: 1) When no zinc contamination is

Elastomer Continued from page 18 an advantage in terms of scorch and a smaller increase in minimum viscosity (~8-12 MU rise for the new accelerators vs. ~30-50 MU rise for the traditional ac- celerators). The ODR cure rates for the new accelerators were comparable to those of the traditional accelerators, meaning that scorch safety was obtained without significant change to the cure rate.

A valuable tool for comparison is to look at the ratio of the cure rate to that of processing, i.e., scorch rate or bin rates. It is desirable to have a high ratio of cure/scorch (CR/SR) and cure/bin (CR/BR). A particular compound can be optimized for both if the product of the two ratios, cure/scorch and cure/bin is ex- amined. Both the alkylpyridinium and the imidazolium accelerators provide ad-

compounds with improved scorch safety and better shelf-life stability, while main- taining sufficient rate and state of cure.

Investigation of hydrated salts on cure parameters

The use of hydrated salts with the thiadiazole cure system has been report- ed in the literature to improve the cure rate in TD based systems.10 Magnesium sulfate heptahydrate salt has typically been used or recommended for cure rate increases. We have determined that there are more effective hydrated salts than previously suggested.11

Several different hydrated salts were compounded using the recipes shown in Table XI. The cure parameters are list- ed in Table XII.

The lithium citrate hydrate provided the fastest cure rate (18.3 dN-m/min) of any of the samples tested. The control with no hydrated salt had a cure rate of

m/min. Relative to the control sample, the lithium citrate hydrate increased the cure rate by 121 percent compared to 47 percent for magnesium sulfate heptahydrate. Although the scorch rates are slightly higher for the hydrated salts vs. the control, the cure rate/scorch rate and the cure rate/bin rate ratios are de- sirably higher, especially for the lithium citrate hydrate sample.

Technologies combined for im- proved CM elastomer cure system

Several individual components have been described thus far: ● additives that offer improved resist-

ance to zinc contamination to improve batch-to-batch consistency, ● alternate cure accelerators that of-

fer improved shelf-life / processing safe- ty, and ● novel hydrated salts for improving

the cure rate. The components can be used individu-

ally, but in most cases a combination

present, the t90 cure times are reduced. 2) When zinc contamination is pres- ent, the t90 times are minimally affected by the presence of the zinc contamina- tion. For example, compare the control DDPP sample t90 times with and with- out zinc contamination. When no zinc is present, the t90 is 6.98 minutes.

The addition of 300 ppm zinc causes the t90 to undesirably increase to 19.32 minutes. However, when the combina- tion of PEI and lithium citrate is includ- ed, the t90 times are much less different: 3.31 minutes for the control vs. 3.63 min- utes when 300 ppm zinc is present. In this case of using DDPP, the presence of zinc undesirably increased the t90 time by 176 percent vs. a desirable <10 per- cent increase for the sample containing the PEI/lithium citrate combination.

The imidazolium accelerator also showed an improvement with the PEI/lithium citrate combination in the presence of 300 ppm zinc contamination:

Table VIII. Physical properties for samples containing PEI.

Table IX. Recipes for comparison of accelerator types.

provides the most desirable end result. Several combinations of accelerator, chelating agent (in this case PEI), and hydrated salt were prepared using the recipes shown in Table XIII. In the ex- periment, one of the typical accelerators (DDPP) was used at “normal” and “half” level since the PEI and hydrated salt can impact cure performance. The MDR results of the experiments are shown in Table XIV.

The combination of lithium citrate and chelating agent provides several ad-

50.7 percent increase in t90 with PEI/lithium citrate vs. 222 percent in- crease without. The improvement was also observed with the samples contain- ing reduced level of DDPP: 5.9 percent in- crease (with PEI / lithium citrate) vs. 110 percent increase (without PEI / lithium citrate) in the presence of 300 ppm zinc.

3) The maximum cure rate data also indicate the improvements when one uses the combination of PEI / lithium citrate—especially in the presence of zinc contamination.

Table XI. Recipes for hydrated salt studies.

Table X. Cure parameter comparison of accelerator types.

Table XII. Cure parameters for hydrated salt studies. Table XIII. Recipes for improved thiadiazole cure technology combinations: accelerator,

PEI and hydrated salt.

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Technical

When the PEI/lithium citrate is not present, the zinc contamination undesir- ably reduces the maximum cure rate by 80-90 percent vs. approximately 3-7 per- cent when lithium citrate and PEI are present.

New highly extendible CM product combined with improved TD cure system

The use of a highly extendible CM poly- mer (HECM)12 in conjunction with the im- proved TD cure technology provides a route to an improved CM system. To test the concept, recipes were mixed as shown in Table XV. The 420 phr loading is high for a CM-based compound, but it was cho- sen to illustrate the effects of extendibility differences vs. current CM grades. The CM 0836 and 135A samples are two of the

highest molecular weight grades available commercially. The CM 9934 represents the highly extendible CM resin.

Properties for the highly extended com- pounds crosslinked with the improved cure system are shown in Table XVI. The compound prepared with the CM 9934 ex- hibited about 40 percent to 65 percent higher tensile strength than any of the compounds prepared with the other CM grades.

In addition, the moduli values were higher while maintaining similar elonga- tion. The tear strength of the compound prepared with the CM 9934/Improved Thiadiazole Cure Technology was also much higher than compounds prepared with the other “traditional” CM grades by approximately 25 to 80 percent.

Summary Recent advances in CM technology

have been developed that have the po- tential to expand the current range of performance of CM compounds, espe- cially in the thermoset elastomer arena. A new CM-based elastomer system in- corporates two technical innovations: improved thiadiazole cure technology using thiadiazole derivatives and the development of an HECM grade.

These two innovations are comprised of several individual components that form an improved CM-based elastomer system: ● Batch-to-batch consistency additive.

● Zinc has been identified as a major cause of cure inconsistency with TD- cured CM. ● Additives that offer improved resist-

improved shelf-life and processing safe- ty for TD-cured CM. ● Novel hydrated salts can be employed

to improve the cure rate, especially when used with reduced levels of typical cure accelerators for the TD curing of CM. ● Polymer grade. ● A more highly extendible CM poly-

mer is capable of accepting higher filler and oil loadings.

The proper combination of these com- ponents yields the most desired balance of processing, performance and value in the end-use performance of the final cured article. These technical advances yield the possibility of developing more highly extended CM recipes using the TD cure system to provide greater lati- tude in formulation ingredients, filler and oil levels, and curing options.

Table XIV. Data for improved thiadiazole cure technology combinations: accelera- ance to zinc contamination have been

tor, PEI and hydrated salt. successfully identified and should im- prove batch-to-batch consistency. ● Shelf-life/processing safety improve- ments. ● Alternate cure accelerators, for ex-

ample those based on aromatic hetero- cyclic quaternary ammonium salts, offer

Acknowledgements The authors wish to acknowledge the

valuable discussions, input and labora- tory work provided by Sandra Watson, Sonja Delatte, Jeff Savoie, Melanie Pat- ton, Pam Kenny, and Pamela Landry of the Dow Chemical Co.

References.

Table XV. Recipes comparing highly extendible CM grade to standard grades.

Table XVI. Data comparing highly extendible CM grade to standard grades.