Compound analysis, formula reconstruction.

The need to perform chemical analysis and formula reconstruction on rubber compounds originates from varied sources. Requests may originate with rubber mixers, fabricators and end users of rubber parts. These analyses can be employed as part of a quality assurance program, to identify mixing and compounding errors, or to determine the composition of a competitor's rubber product.

Analytical wet chemistry, electron microscopy, along with sophisticated instrumental test methods have given the chemist the necessary tools to get qualification and quantification information on virtually all ingredients in thermoset rubber, thermoplastic rubber and plastic materials. The data generated from these analytical methods can be used to reconstruct the compound formulation.

The objective of this article is to present a brief overview of the analytical techniques that were employed in a rubber compound analysis and formula reconstruction. The results of analyses on four different rubber compounds will be presented. These compounds were unknown to the chemical/ analytical laboratory at the time the analyses were performed. A comparison between the reconstructed formulations and the actual compounds, known only to the compounding and mixing department, are included.

Experimental procedures

The methods for qualitative and quantitative analysis of the composition of rubber products are detailed in ASTM D 297 "Rubber Products - Chemical Analysis" (ref. 1). In addition to these methods, several other techniques were employed in the identification of various rubber compounding ingredients.

The analysis of a rubber compound can be divided into the following steps:

* Extraction of rubber compound.

* Polymer identification and quantification.

* Carbon black percent and type.

* Identification and quantification of inorganic components, e.g. zinc oxide, silica.

* Analytical evaluation of the extract to identify and quantify such components as oils and plasticizers, antidegradants and curatives, etc.

* Determination of sulfur level.

Extraction of rubber compound

A weighed specimen of rubber, approximately two grams, was cut into small pieces and put into an extraction thimble which was in turn put into an extraction cup. The cup was suspended from a tin condenser in a flask containing 75 [cm.sup.3] of acetone.

The extraction process continued for 16 hours at reflux temperature.

The rubber sample was removed, dried for 16 hours at 70 [degrees] C and retained for polymer identification, carbon black and ash analyses.

The acetone was evaporated from the extract by heating on a hotplate under a gentle flow of nitrogen. The extract was then retained for further analysis.

Polymer identification and quantification

Polymer identification was performed using the techniques described in ASTM D 297 which references ASTM D 3677 "Rubber Identification By Infrared Spectrophotometry" (ref. 2). These techniques initially involve the determination of the presence of chlorine by the Beilstein and Congo Red methods.

The resultant rubber compound was boiled in orthodichlorobenzene to dissolve the polymer. This solution was filtered and a film was cast on a salt crystal. Infrared spectroscopy (FTIR) was run to determine the identity of the polymer(s).

An FTIR scan was printed out by the computer on the instrument. A typical scan contains numerous peaks that appear at different wavelength numbers on a log graph. Each scan was compared to the control polymer IR scans.

The determination of the rubber hydrocarbon content was performed using the indirect method as outlined in ASTM D 297. The calculation is as follows:

Rubber polymer, % = A (100-B)

where:

A = factor listed in ASTM D 297 table 1;

B = sum of percentages of total extract, combined sulfur, inorganic fillers and carbon black.

Table 1 - rubber compound analysis

Compound. ID: P-1 Compound. color: Black

A. Polymer identification
(Infra-red spectroscopy)
1. SBR
2. Polybutadiene rubber
3.

B. Ash content: 1.7%

C. Semi-quantitive ash analysis
(Atomic absorption)
  a. >10%         Zn
  b. 5-25%
  c. 1-10%
  d. 0.5-5%      Al
  e. 0.1-1%.
  f. 0.05-0.5%   Fe, Ca, Mg
  g. 0.01-0.1%   Cu
  i. Not detected   Pb, Ba, Si, Ti

D. Total hydrocarbon: 42.4%

E. Total sulfur 1.79%
(Leco sulfur determinator)

F. Specific gravity 1.131

G. Wax content

H. Extractables: 22.3% (ASTM D297)
   1. Color Amber
   2. Consistency semi-solid

I. Carbon black: 32.2%

J. Extract analysis
   1. Plasticizers; D2702
   a. Aromatic oil
   b.
   c.
   2. Antioxidants; D3156
   a. Santoflex 13
   b. Agerite resin D
   c.
   3. Accelerators (suggested)
   a. Santocure
   b.
   c.

K. Beilstein Negative
(Presence of halogens)

L. Shore-A Hardness 68

Percent and type of carbon black and inorganic materials

A portion of the previously extracted rubber compound was weighed and then pyrolyzed in a nitrogen atmosphere at 800 to 900 [degrees] C to remove all of the components except the carbon black and inorganics (metals). The specimen was weighed and placed in a muffle furnace at 800 [degrees] C to 900 [degrees] C until all traces of carbon black were burned off. The specimen was weighed a final time and a simple calculation was performed to determine carbon and ash content.

The resultant pyrolyzed carbon black from the previous step was examined under a transmission electron microscope (TEM) and representative areas transmitted to an image measurement through an on-line camera interface with the TEM. The individual carbon black particle sizes were measured. The data were transferred to an Excel spreadsheet of a Windows-based PC and a histogram developed. Based on ASTM D 1765 "Carbon Blacks Used In Rubber Products" (ref. 3) and the derived histogram of particle size distribution, the carbon black size was determined.

The final components to be identified and quantified are the components of the ash. The inorganics most commonly detected in rubber compounds are lead, zinc, calcium, magnesium, iron, silica, aluminum, barium, copper and titanium. In order to identify these materials, the ash was dissolved in hydrochloric acid. Atomic absorption spectrophotometry was then employed to identify the inorganics in the HCI solution.

Extract analysis

The extract, from the initial extraction, contains additional compounding ingredients that can be further analyzed. Generally these ingredients may be plasticizers and/or process oils, resins, accelerators or accelerator fragments, antidegradants, waxes, process aids and free sulfur, which must be separated and analyzed.

We used the following techniques for analysis of extract for these rubber chemicals:

* Accelerators - thin layer chromatography (TLC); gas chromatography/mass spectrophotometry (GC/MS); gas chromatography (GC); gas chromatography/mass spectrophotometry (GC/MS).

* Antidegradants - high performance liquid chromatography (HPLC); thin layer chromatography (TLC); gas chromatography/mass spectrophotometry (GC/MS).

* Plasticizers - infrared spectrophotometry.

High performance liquid chromatography (HPLC) and gas chromatography (GC) were used to identify and quantify antioxidants and accelerators. The procedures involved quantitatively extracting the samples in acetone followed by an analysis of the extract. The column type used for HPLC was a reverse phase C-18 and a capillary column for the GC. A 25 ml injection volume was used for HPLC, also the ultraviolet spectra and absorption maxima of the detected peaks with those of standard chemicals.

Quantification of the identified chemicals was done by external standard method. Several concentrations of standard solutions were prepared in chloroform using the standard material. These were analyzed by HPLC and/or GC to generate an area normalized integrated chromatograph.

The values of the peak area of the chemicals in each standard solution and its corresponding concentration were then plotted to give a calibration graph. The slope obtained was used to calculate the concentration of the chemicals in the samples.

Gas chromatography/mass spectrometry (GC/MS) was also employed to verify the results from GC and HPLC. GC/MS is particularly useful in identifying accelerator fragments at the parts per billion (ppb) level.

Total sulfur determination

Total sulfur content in the sample was determined by the use of a sulfur determinator. Specimen was weighed and pyrolyzed in the machine at 1,350 [degrees] C, emitting sulfur dioxide. A sensor in the instrument then measures the amount of [SO.sub.2] and automatically calculates the total sulfur in the specimen.

If a more detailed analysis is required, additional work including free sulfur content, halogen content, acrylonitrile content, wax content, resin identification and fiber identification can be performed.

Mixing and curing

Compounds representing radial passenger tire, radiator hose, oil pan seal and engine gasket identified as P-1, P-2, P-3 and P-4 were mixed by the compounding and mixing department. The actual formulation for these materials was known only to the personnel directly involved with weighing, mixing and curing. The actual recipes were given to the chemical laboratory for comparison, after the analyses and reconstructions were completed.

Rheometer testing was performed on these compounds using an oscillating disc rheometer (ODR) to determine the optimum cure times based on [TC.sub.90] data. One 6" x 6" x 0.075" test rubber sheet was cured from each compound, and submitted to the analytical laboratory for analysis and formula reconstruction.

Results and discussion

Compound P-1

Polymer identification

The polymer in compound P-1 was identified as a blend of styrene-butadiene rubber and polybutadiene. This identification was based on a comparison of the FTIR scan to a series of control curves representing various SBR/BR blend ratios. The FTIR spectrum showed the peak heights of polybutadiene at 720 Wave number, [cm.sup.-1] and styrene-butadiene rubber at 707 Wave number, [cm.sup.-1]. These peaks were measured and a ratio was determined. The ratio was compared to a previously prepared SBR/BR calibration curve and the concentration of the two polymers in the blend was determined. No attempt was made to determine if SBR was a solution or an emulsion SBR. We did not determine styrene content of SBR, we assumed it was a 23% styrene content SBR.

Carbon black type

An electron photomicrograph of the carbon black showed the particles to be more or less the same size, indicating the presence of only one type of black. A transmission electron microscope (TEM) histogram showed a single peak size distribution, confirming only one type of carbon black was present. Based on ASTM D 1765 and from the histogram analysis of the TEM results, the carbon black was identified as an N200 series black. Electron microscope surface area (EMSA) measurement indicated the black to be an N234 type. We further measured the particle size and surface area of an N234 carbon black control sample. These measurements confirm that the carbon black in compound P-1 is N234.

Extract analysis

GC, HPLC and FTIR were used to identify the accelerator as Santocure, the antioxidants as Santoflex 13 and Agerite Resin D, and the plasticizer an aromatic oil.

The quantification of the accelerator and antioxidant was done using HPLC and GC as explained earlier.

Identification and quantification of inorganic components

The ash content of P-1 was determined to be 1.7%. Atomic absorption spectrophotometry of the ash showed it to consist mostly of zinc, with other metals (Al, Fe, Ca, Mg, Cu, Pb, Ba, Si, Ti) to be present only in trace amounts. Therefore, only zinc, as zinc oxide, was included in the reconstructed formulation.

Using the analytical data from table 1 and a proprietary computer software program, the compound formula for P-1 was reconstructed and is given in table 2.

Table 2 - proposed compound formula

Compound. ID: P-1; Compound. color: Black

Ingredients              PHR    Extractables   Ash   Volume

Styrene-butadiene        40.0       0.4                42.6
Polybutadiene            60.0       0.6                65.2
N234 carbon black        75.3                          41.8
Zinc oxide                4.0                  4.0      0.7
Aromatic oil             43.8       43.8               46.1
Stearic acid              1.0       1.0                 1.2
Agerite resin D           2.0       2.0                 1.7
Santoflex 13              1.0       1.0                 0.8
Misc. extractables(*)     3.0       3.0                 3.3
Santocure                 1.0       0.2                 0.7
Sulfur                    2.0                           1.0
Total                   233.1       52.0       4.0    205.1

Theoretical ash content        1.7%
Theoretical extractables       22.3%
Theoretical carbon black       32.3%
Theoretical specific gravity    1.137

Comments: (*) It may contain wax, processing aids ... etc.

From table 2, note that the ash content, extractables and carbon black content of the reconstructed formulation match the data of the analyzed sample. The calculated specific gravity also comes very close to the data from the reconstructed compound.

After formula reconstruction was completed in table 2, the compounding department gave us the actual compound recipe. Reconstructed formula and actual recipe are compared in table 3. Upon examination of the above comparison, it is evident that the reconstruction is very close to the actual recipe.

Table 3 - comparison of reconstructed formulation with
actual recipe for compound P-1

Ingredient                    Reconstructed    Actual
                                formulation    recipe

Styrene butadiene rubber              40.00     45.00
Polybutadiene rubber                  60.00     55.00
N234 carbon black                     75.30     70.00
Zinc oxide                             4.00      3.00
Aromatic oil                          43.80     37.50
Stearic acid                           1.00      2.00
Agerite resin D                        2.00      2.00
Santoflex 13                           1.00      1.00
Miscellaneous extractables             3.00        --
Santocure                              1.00      1.00
Sulfur                                 2.00      1.75
Reogen                                   --      1.00
Sunolite 240                             --      3.00

Total                                236.10    222.25

It should be noted that the polymer/oil used in the actual recipe was SBR 1712, a masterbatch of SBR containing 37.5 parts of aromatic oil.

The miscellaneous extractables in the reconstructed formulation include waxes, process aids, etc. In the actual recipe this would be the Reogen and Sunolite 240.

The ratio of the polymer blend, SBR/BR in the reconstructed formulation, is a ratio of the significant peaks of the infrared spectrum. The differences between the actual and reconstructed formula can be attributed to the interpretation of this ratio.

Compound P-2

Polymer identification

The polymer in compound P-2 was identified as ethylenepropylene rubber due to the strong FTIR peak at wave number 1370 and also a medium peak at wave number 710.

Carbon black type

An electron photomicrograph of the carbon black in this compound shows the presence of only one type of black. The particle size distribution histogram indicated that the black is an N500 series material. Based on these data, an N550 type of carbon black was chosen for the formula reconstruction.

Tern panicle, size distribution

The carbon black in compound P-2 had a TEM particle size distribution average of 45.1. The size was 22.0 nm minimum and 99.0 nm maximum and the ASTM 500 size range was 40-48 nm.

Extract analysis

GC, HPLC and FTIR were used to identify and quantify naphthenic oil, TMTD, DPTT and ZDBC in this compound. No antioxidants were detected.

Ash analysis

Using atomic absorption spectrophotometry, zinc was found to be the only significant component of the 1.3% ash content in this compound. This was put into the reconstructed formulation as zinc oxide.

Discussion

A reconstruction of the formulation for compound P-2 is shown in table 4. Most of the levels of ingredients fall within acceptable limits when comparing the reconstructed formulation to the actual recipe.

Table 4 - comparison of reconstructed formulation
with actual recipe for compound P-2

Ingredient                    Reconstructed    Actual
                                formulation    recipe

Ethylene propylene rubber            100.00    100.00
N550 carbon black                    203.00    200.00
Zinc oxide                             5.90      5.00
Naphthenic oil                       147.80    140.00
Stearic acid                           1.00      1.00
Miscellaneous extractables             2.00        --
TMTD                                   3.00      1.50
TMTM                                     --      1.50
DPTT                                   1.00      1.00
ZDBC                                   1.00      1.50
Sulfur                                 0.50      0.50
Paraffin wax                             --      5.00

Total                                465.20    457.00

The paraffin wax in the actual recipe is included as miscellaneous extractables in the reconstruction.

We were unable to identify tetramethyl thiuram monosulfide (TMTM) in the analysis. This is most probably due to the fact that during vulcanization, the TMTM breaks down into fragments that are very similar to tetramethyl thiuram disulfide (TMTD) fragments. Thus, we reported only TMTD in the reconstructed formulation.

Compound P-3

Polymer identification

Since this particular polymer was solubilized during the acetone extraction process, it was necessary to treat the polymer identification somewhat differently than is usually done. The polymer was precipitated out of the extract and put into a methanol solution. A film of this solution was then cast on a salt crystal and an FTIR scan performed. The result of the FTIR testing was a scan that matched an acrylic ester copolymer control spectrum. Thus, polymer was identified as an acrylic ester.

Carbon black amount and type

An electron photomicrograph and related particle size histogram with single peak in particle size distribution indicated only one type of black was used in this compound. Based on these data and ASTM particle size range for 600 series black, an N600 series black was chosen in the formula reconstruction for compound P-3.

Tem particle, size distribution

The carbon black in compound P-3 had a TEM particle size distribution average of 51.9. The size was 20.4 nm minimum and 128.5 nm maximum and the ASTM 600 size range was 49-60 nm.

Extract analysis

GC, HPLC and FTIR were used to identify Agerite Stalite S as the antioxidant. GC/MS was used to identify Hytemp NPC 50 as the curative in the P-3 compound. No plasticizer was detected in this compound. Although it was not detected, published literature indicates that sodium stearate is frequently used as a cure additive in acrylic ester compounds, and was included in the formula reconstruction.

Ash analysis

The total ash content in P-3 was extremely low, only 0.6%. It was analyzed using atomic absorption spectrophotometry. However, the levels of inorganics were so low that they were considered to be trace amounts and were not included in the reconstructed formulation.

Discussion

Table 5 compares the reconstructed formulation and the actual recipe for compound P-3. The actual recipe contained N550 carbon black, whereas we identified the carbon black as falling within the N600 series. The average particle size of 51.9 mm, although on the low end, falls between the ASTM Standards of 49-60 nm for 600 series black. Since we successfully identified the black as an N500 series type in compound P-2, we can only suggest that the black used in P-3 might have been from a different supplier that did in fact run on the high particle size, more closely to a low end N600 type.

Table 5 - comparison of reconstructed formulation
and actual recipe for compound P-3

Ingredient                    Reconstructed    Actual
                                formulation    recipe

Acrylic exter copolymer              100.00    100.00
N550 carbon black                        --     65.00
N660 carbon black                     66.50        --
Stearic acid                           1.00      1.00
Agerite stalite S                      2.00      2.00
Miscellaneous extractables             2.00      2.00
Sodium stearate                        4.00      4.00
Hytemp                                 2.00      2.00

Total                                177.50    176.00

All of the other ingredients in the P-3 reconstruction very closely match the actual recipe.

Compound P4

The FTIR scan having peaks at 2220 Wavenumber [cm.sup.-1] and at approximately 700, Wave number [cm.sup.-1], identifies the polymer as 100% hydrogenated nitrile (HNBR).

Electron microscopy and the particle size distribution histogram indicates the carbon black in this compound is an N900 series black. Therefore, N990 carbon black was chosen for the formula reconstruction. The carbon black had an average particle size of 247.8. The ASTM Standards for 900 series is 201-500 nm.

Thin layer chromatography (TLC) and gas chromatography (GC) results indicated no accelerators are present. However, residual ([Alpha]-[Alpha]-bis (t-butylperoxyl) diisopropylbenzene was detected by GC. These findings, along with the very low total sulfur level, 0.36%, lead to the choice of Vulcup R as a curative in this compound.

The plasticizer, as well as the antioxidant, was chosen on the basis of FFIR and HPLC data generated as a result of testing on the extract.

It was not possible to identify the coagent in this compound. However, based on published literature, 7.5 phr were included in the formula reconstruction. The coagent most probably would be HVAg2 or a similar material (ref. 4).

Table 6 shows comparison of reconstructed formulation with actual recipe. Note that VC-60P is a 60% active peroxide curative, whereas Vulcup R is 96% active. This accounts for the apparent discrepancies in the curative level in this comparison.

Table 6 - comparison of reconstructed formulation
with actual recipe for compound P-4

Ingredient                Reconstructed    Actual
                            formulation    recipe

Hydrogenated nitrile             100.00    100.00
N990 carbon black                 29.90     30.00
Magnesium oxide                   10.80     10.00
TOTM                               4.70      5.00
Naugard 445                        2.00      2.00
Vanox ZMTI                          100      1.00
Miscellaneous extractables         1.00      1.00
Coagent                            7.50      7.50
Vulcup R                           2.00        --
VC-60P                               --      3.34

Total                            158.90    160.50

All of the other ingredients compare favorably.

Summary

Four unknown cured rubber compounds representing radial passenger tire tread, radiator hose, oil pan seal and engine gasket products were analyzed, using acetone extraction, pyrolysis, infrared spectrophotometry, high performance liquid chromatography, gas chromatography, Leco sulfur determination, atomic absorption spectrophotometry and gas chromatography/mass spectrophotometry, and electron microscopy.

We found the reconstructed formulations to be very similar to the actual recipes that were mixed.

Polymer identification was an exact match in all formulations. There was a discrepancy of 5 phr in the polymer ratio in compound P-1.

The antioxidant identifications and levels were an exact match in all of the compounds. The carbon black types were successfully identified except in compound P-3. The carbon black level in P-1 showed the greatest discrepancy, 5.3 phr, of all the compounds.

The oils in all compounds were successfully identified, with a 7.8 phr discrepancy in the level in P-2 being the greatest of all the compounds.

The curatives in all of the compounds except P-2 were successfully identified and quantified. The two thiurams, TMTM and TMTD, were not separated in the analysis of P-2. Ash analysis (inorganics) was very accurate on all compounds.

This article shows that, following the proper analytical techniques, rubber formulations can be successfully reconstructed from cured compound, giving an excellent tool to rubber mixers, fabricators and end users of rubber parts.

References

1,2,3. Annual Book of ASTM Standards, Volume 09.01 (1996). 4. Curing Systems For Zetpol, Zeon Chemicals, Technical Paper 25.1.22, (1992).