Various Applications of Atomic Absorption Spectrometric

2. Principal Chemical and Analytical Methods Used in Reverse Engineering

2.5 Atomic Absorption Spectroscopy (AAS)

2.5.3 Various Applications of Atomic Absorption Spectrometric

2.5.3.1 Analysis of Natural Rubber, Synthetic Rubbers, and Recycled Materials

Trace amounts of transition metal ions that undergo one electron transfer reactions, like copper (Cu), iron (Fe), manganese (Mn), cobalt (Co) ions, etc., have been reported to be rubber poisoning metals. These metals catalyze the aging reaction, and as a result, life of

TABLE 2.17

Detection Limit (Microgram per Liter) of Various Atomic Spectrometric Techniques Elements

Flame AAS (microgram

per liter)

Hg/Hydride-AAS (microgram per

liter)

GF-AAS (microgram

per liter)

ICP Emission (microgram per liter)

ICP-MS (microgram per

liter)

Al 45.00 — 0.30 6.00 0.0060

As 150.00 0.030 0.50 30.00 0.0060

Ba 15.00 — 0.90 0.15 0.0020

Ca 1.50 — 0.03 0.15 2.0 (44Ca)

Cd 0.80 — 0.02 1.50 0.003

Co 9.00 — 0.40 3.00 0.0009

Cr 3.00 — 0.08 3.00 0.0200

Cu 1.50 — 0.25 1.50 0.0030

Fe 5.00 — 0.30 1.50 0.4 (54Fe)

Hg 300.00 0.009 1.50 30.00 0.0040

K 3.00 — 0.02 75.00 1.0000

Mg 0.15 — 0.01 0.15 0.0070

Mn 1.50 — 0.09 0.60 0.0020

Mo 45.00 — 0.20 7.50 0.0030

Na 0.30 — 0.05 6.00 0.0500

Ni 6.00 — 0.80 6.00 0.005 (60Ni)

Pb 15.00 — 0.15 30.00 0.0010

Si 90.00 — 2.50 5.00 0.7000

Sn 150.00 0.2 0.50 60.00 0.0020

Zn 1.50 — 0.30 1.50 0.0030

Source: Courtesy of PerkinElmer, Waltham, Massachusetts.

a product decreases. The metal impurities may come in rubber compound from different sources including rubber chemicals, synthetic rubbers that are polymerized by using some catalysts, and natural rubber that comes from nature. The metal catalyst is reported as an active hydroperoxide decomposer in both its higher and lower oxidation states.

In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alk- oxy radicals:

+ → ⋅+ + ⋅

+ → ⋅+ +⋅

→ ⋅ ⋅

+ +

ROOH M ROO M H

ROOH M RO M OH

2ROOH ROO +RO + H O

n (n-1)

(n-1) n

Copper has an effect on a gum compound of oil extended styrene-butadiene rubber, and cobalt affects the stability of divinyl rubber. The metal poisoning is more destructive in polymers containing an unsaturated backbone. Copper, as a poisoning metal, drastically reduces the effective service life in NR-based formulations.

The raw natural or synthetic rubbers are kept in a muffle furnace at a temperature of 550°C. The inorganic residue (ash) thus obtained is dissolved in either hydrochloric acid or nitric acid and is quantitatively transferred to a volumetric flask. The solution is now ready for AAS analysis.

Microwave digestion is also used for sample solution preparation because of the advan- tages including the following: contamination is minimized, volatiles are retained, microwave heat goes to the core of the sample, the result is reproducible, and the sample is super heated. Various decomposition reagents are used for microwave digestion. Nitric acid (HNO₃) is used for the oxidation of organic matter, and it also makes metal in soluble format. It is used in combination with hydrogen peroxide (H₂O₂), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and hydrofluoric acid (HF). H₂SO₄ is used for dehydration and charring of organic material in combination with H₂O₂, HNO₃, perchloric acid (HClO₄), and HF. HF is particularly used for dissolution of silicates; it is also used in combination with HNO₃, HCl, phosphoric acid (H₃PO₄), fluoro-boric acid (HBF₄), and boric acid (H₃BO₃).

The solutions prepared above are now ready for analysis by using atomic spectrophotom- eters. The AAS analysis conditions used for some typical metals, which are known as poisoning metals, are given in Table 2.18.

The poisoning metal analysis is a very important test parameter for refined or recycled materials. Analysis results of metal content of virgin reclaim, crumb rubber, and virgin oil and recovered oils determined through AAS-3300 equipment (from PerkinElmer, Waltham, Massachusetts) are given in Table 2.19. The upper limit of recycled material in a rubber compound formulation is also obtained.

TABLE 2.18

Typical AAS Analysis Condition of Some Metals in Rubber Industry

Parameters Cu Mn Fe Co

Wavelength (nm) 324.8 279.5 248.3 240.7

Slit width (nm) 0.7 0.2 0.2 0.2

Flame used Air-acetylene

oxidizing Air-acetylene

oxidizing Air-acetylene oxidizing

Technique Background

corrected (deuterium arc)

Background corrected (deuterium arc)

2.5.3.2 Steel Wire Characterization

In the case of tires, with the advancement of radial technology, the multi-filament brass- plated steel cord is being used as an important reinforcing material for tire carcass and belt reinforcement. The steel cord is coated with brass for better bonding with rubber. Sulfur vulcanizable rubbers during vulcanization form a direct bond to brass, zinc (Zn), palla- dium (Pd), nickel (Ni), and nickel/zinc or zinc/cobalt (Co). Brass plating on the surface of steel cord reacts with the sulfur in rubber compounds during the curing process of tire, forming a copper sulfide (Cu_xS) film of brass. The desired chemical reaction taking place at rubber/brass-plated steel cord interphase may be considered as mixed ionic and covalent chemical reactions.

A steel wire sample of 0.2 to 0.4 g is accurately weighed, treated with chloroform toluene mixture (50:50) to remove any organic contaminants, and dried in oven at 105°C. After drying, the sample is cooled in desiccators, and the coating is stripped with 5 mL concentrated nitric acid to 200 mL volumetric flask and made up to mark. The solution is now ready for analysis by AAS. The typical AAS analysis condition and characterization data of steel wire samples are shown in Table 2.20. The typical analysis data of two steel wire samples used in the tire industry are given in Table 2.21.

The bead wire of 0.965 mm and 1.60 mm is widely used in the tire industries. The func- tion of bead wires is to hold the tire with the rim of the vehicle. Bead wire is made of steel, and the wire is normally coated with copper and tin. The bead wire generally contains 97 to 99% copper and 1 to 3% tin. The coating weight is typically found in the range 0.15 to 0.65 g/kg of sample. If a 10 g sample is used for analysis and stripped coating is made up to

TABLE 2.19

Comparative Analysis Data of Various Virgin and Recycled Materials in AAS

Sample ID Cu (ppm) Mn (ppm) Fe (ppm)

Regular ISNR-20 3.0 2.9 5.0

40-mesh crumb rubber 8.1 5.9 960.5

80-mesh crumb rubber 13.0 1.3 282.0

100-mesh crumb rubber 8.8 25.2 3250.0

Super-fine reclaim rubber 86.7 64.5 1300.0

White reclaim rubber 60.3 51.2 5700.0

Regular aromatic oil 1.13 0.08 2.7

Recycled dust seal oil

(centrifuged for 24 h) 33.9 3.8 4.5

TABLE 2.20

Typical AAS Analysis Condition

Parameters Copper Zinc

Wavelength 324.8 nm 213.9

Slit width 0.7 nm 0.7

Flame type Air-acetylene oxidizing Air-acetylene oxidizing Technique Background corrected Background corrected

(deuterium arc) (deuterium arc)

Read time 5 s 5 s (read delay 2 s)

100 mL, only 0.15 to 2 mg/L tin will be available in solution. The low level of tin is analyzed by GF-AAS, AAS-FIAS or ICP-AES and ICP-MS.

The coating analysis method described in this study is briefly described here. A 10 g bead wire sample is treated with concentrated HNO₃, and the coating is stripped out to 100 mL volumetric flask. Diluted sample is analyzed for copper in AAS, and sample solution is diluted with FIAS carrier solution (saturated boric acid with 1% HCl) and analyzed with FIAS-AAS. Characterization with the AAS-FIAS technique provides good repeatability and reproducibility of bead wire characterization. The AAS-FIAS analysis condition used in the above study and characterization data of bead wire samples are shown in Tables 2.22 and 2.23, respectively.

2.5.3.3 Analysis of Rubber Compounding Ingredients

The estimation of metals is essential for process control, product acceptance, and research activities. The sample is dissolved in mineral acid and analyzed by AAS equipment.

The AAS analysis condition and most commonly found metal impurities in various rubber compounding ingredients are provided in Tables 2.24 and 2.25, respectively.

TABLE 2.21

Analysis of Coating of Steel Wires Used in Tire Industries

Sample ID Parameters Cu (%) Zinc (%)

2 + 2 × 0.25 style steel cord Median

Range 63.4

61.3–68.1 36.6 31.9–38.7 2 × 0.30 style steel cord Median

Range 63.9

62.8–64.7 36.1 35.1–37.2 Note: Data are on the basis of 10 samples.

TABLE 2.22

AAS-FIAS Instrumental Analysis Parameters of AAS

Parameters Copper Tin

Wavelength 324.8 nm 286.3 nm

Slit width 0.7 nm 0.7 nm

Technique Background-corrected atomic absorption (impact bead also used)

Atomic absorption

Data processing Time average Peak height with 37 point smoothening

Cell temperature — 900

Read time 5 s 15 s

Flame type C2H2:Air —

TABLE 2.23

Repeatability of Bead Wire Characterization by FIAS-AAS

Parameters Tin (%) Copper (%) Plating Weight (g/kg)

Mean (five samples) 1.1 98.9 0.48

Standard deviation (five samples) 0.07 0.07 0.003

The values shown in parentheses are medians of 10, 20, 5, and 15 of different clay, ZnO, stearic acid, and VP latex samples, respectively.

2.5.3.4 Reverse Engineering/Benchmarking

AAS is widely used for the analysis of rubber vulcanizate. This is especially used to find out the type of vulcanizing system, amount of inorganic fillers, and amount of special additives used in an unknown compound.

For example, in a sulfur cure system, zinc oxide is used as an activator. In a metal oxide curing system, ZnO is used as a crosslinking agent along with magnesium oxide which acts as an acid scavenger. Aluminum hydroxide, aluminum silicate, antimony trioxide, barium ferrite, barium sulfate, calcium carbonate, calcium oxide, calcium hydroxide, calcium sulfate, lead monoxide, lithopone, magnesium carbonate, magnesium oxide, titanium oxide, and zinc carbonate are used in rubber compounding to achieve various properties for finished products.

An analysis summary of some rubber-based products by AAS is described in Tables 2.26 and 2.27. It would be useful in the reverse engineering of any rubber product formulation.

TABLE 2.24

Typical AAS Analysis Condition

Parameters Mn Cd Pb Fe Ni

Wavelength 279.5 228.8 283.3 248.3 232.0

Slit width 0.2 0.7 0.7 0.2 0.2

Flame type Air-acetylene

oxidizing Air-acetylene

oxidizing Air-acetylene oxidizing Technique Background

corrected (deuterium arc)

Background corrected (deuterium arc)

TABLE 2.25

Commonly Found Metal Impurities in Rubber Compounding Ingredients Clay Mn

(ppm)

ZnO (Rubber Grade) Stearic Acid VP Latex Cd (ppm) Pb (ppm) Fe (ppm) Fe (ppm) Ni (ppm)

≤40 (3.65) ≤170 (10.00) ≤1500 (210) ≤110 (5.67) ≤23.98 (8.3) ≤10.04 (1.17)

TABLE 2.26

Typical AAS Analysis Condition

Parameters Zn Ca Mg Al

Wavelength 213.9 422.7 285.2 309.3

Slit width 0.7 0.7 0.7 0.7

Flame type Air-acetylene

oxidizing Air-acetylene

oxidizing Nitrous oxide-acetylene reducing Technique Background

corrected (deuterium arc)

Background corrected (deuterium arc)

Metal content analysis also may be helpful in the analysis of root causes of failure of mar- ket-returned products.

2.6 Microscopy and Image Analysis

Dalam dokumen Reverse Engineering of Rubber Products (Halaman 134-139)