Carbon blacks

INTRODUCTION

Carbon black is the most important filler for rubbers.The reinforcing ability of carbon black was discovered decades back.Today around 50 types of black are produced.

Carbon black is made by either incomplete combustion of hydrocarbon by thermal decomposition of the hydrocarbons.

The different forms of production

Furnace process:

Most of the carbon black are produced by this method.In this partial combustion is done in horizontal furnaces.The raw material is liquid or gaseous aromatic hydrocarbons.A yellow flame is produced due to limited supply of air, this leads to partial combustion which causes soot to form,this carbon black which is present along with Co2 and water vapour in the gas produced.This is cooled by water spray and passed through a series of cyclone separators from which the carbon black is collected.This particle size is small and these particles have to be pelletized for easy handling.

Thermal Black:
It accounts for about 5% of the carbon black production.It is formed by the Thermal decomposition of the hydrocarbons.Raw material is natural gas or generator gas or liquid hydrocarbon. The production is through two stages in a pair of chambers lined with refractory material.The particle size is larger and difficult to pelletise.

Lamp Black:
This is obtained by particle combustion of liquid hydrocarbon in open pans,similar to the soot obtained by a lamp fuelled by burning paraffin.It is the oldest known carbon Black.

Channel Black:

Natural gas is burnt in that houses containing a large number of burners,with a limiter supply of air. The channels split the flame and receive the black which deposit on it.The channels are moved to and fro and scrapers will remove the deposited black which is sent for further processing. Channel black makes the rubber more electrically conductive.

Important characteristics of carbon black are particle size ,particle structure and interaction and reinforcing ability of the surface factors.

Carbon Blacks

 Classification Symbols of Carbon Black

Carbon black	Symbol
Channel	C
Furnace	F
Thermal	T

To this basic indication letters are added that indicate particular properties, as shown in Table 3.9.

Newly developed types of carbon black were difficult to characterise by the below letter code and. therefore several further classification systems have been proposed, of which only the ASTM D 1765 classification system has become widespread.

The ASTM D 1765 classification system uses a four-character code for the identification of carbon black grades, composed of one letter and three numbers. The letter N means that the rate of vulcanisation of the

rubber-loaded compound with this type of carbon black is normal, which is typical of furnace blacks. The letter S means that the vulcanisation is slower than normal.

Carbon Black Property Symbols

Types	Grade	Name
Channel	HPC	hard processing channel
	MPC	medium processing channel
	EPC	easy processing channel
Anthracene channel	EPC A	easy processing channel anthracene
Furnace	FEF	fast extrusion furnace
	HAF	high abrasion furnace
	ISAF	Intermediate super abrasion furnance
	SAF	super abrasion furnace
	GPF	general-purpose furnace
	SRF	semi-reinforcing furnace
	HMF	high-modulus furnace
	FF	fine furnace
Thermal	MT	medium thermal
	FT	fine thermal
Acetylene	Ac	acetylene
Lamp	La	Lamp

Further grades were introduced later, which required more detailed grading: HAF-LS high abrasion furnace black, with a low structure; the furnace black ISAF-LS is labeled similarly.

 Classification of Carbon Blacks according to ASTM-D 1765  Typical Properties

ASTM Designation		•   Target Values
	Particle Size (approx.)	Iodine Adorption No. D 1510 g/kg	DBP No., D 2414 cm3/100g	DBP No., Compressed Sample, D 3493, cm3/100g	CTAB, D 3765, m2/g	Nitrogen Adorption D 3037, m2/g	Tint Strength D 3265
N110	22	145	113	98	126	143	124
N121	–	121	132	112	121	132	121
S212	–	–	85	82	119	117	115
N220	28	121	114	100	111	119	115
N231	28	121	92	86	108	117	117
N234	28	120	125	100	119	126	124
N242	28	121	124	106	111	125	116
N293	–	145	100	92	114	130	117
N299	–	108	124	105	104	108	113
S315	32	–	79	75	95	88	–
N326	32	82	72	69	83	84	112
N330	32	82	102	88	83	83	103
N332	32	84	101	90	–	–	118
N339	–	90	120	101	95	96	110
N347	32	90	124	100	88	90	103
N351	–	68	120	97	74	73	100
N358	32	84	150	112	88	87	99

N375	–	90	114	97	98	100	115
N472	–	250	178	114	145	270	–
N539	47	43	111	84	41	41	–
N550	47	43	121	88	42	42	–
N630	–	36	78	62	38	38	–
N642	–	36	64	62	37	37	–
N650	–	36	122	87	38	38	–
N660	70	36	90	75	35	35	–
N683	70	35	133	–	39	37	–
N754	–	24	58	57	29	–	–
N762	83	27	65	57	29	28	–
N765	–	31	115	86	33	31	–
N774	83	29	72	62	29	29	–
N787	–	30	80	74	32	30	–
N907	300	–	34	–	–	11	–
N908	–	–	34	–	–	–	–
N990	300	–	43	40	9	9	–
N991	–	–	35	38	8	7	–

Moisture Adsorption Properties of Rubber Grade Carbon Black

Carbon Black when produced contains very little moisture, but during storage carbon black adsorb moisture from the atmosphere depending on the humidity in the atmosphere.

The adsorption of the moisture depends on the type of the black. Excessively adsorbed moisture can results in porosity during moulding, extrusion of the rubber compounds. It will also increase the mixing time of the compounds to achieve proper carbon black  dispersion.

The presence of the high moisture could influence the rate of vulcanisation in some rubber compound and the rate of the vulcanisation can be either accelerated or reduced by the presence of moisture, depending on the curing systems being used.

It is important to note that rate of moisture adsorption would be much slower in the FIBC bags compared to paper bags packing.

Moisture Adsorption Data:

GRADE	ASTM	Iodine Adsorption NO. g/Kg.	Equilibrium Moisture Adsorption (Wt.%) at Relative Humidity
			31%	55%	79%
N234	N234	121	2.0	3.6	4.7
ISAF	N220	121	1.0	2.3	3.3
N339	N339	90	1.7	2.95	4.05
HAF	N330	82	1.1	2.33	2.8
FEF	N550	43	0.5	1.22	1.6
GPF	N660	36	0.5	1.22	1.6
SRF	N774	29	0.3	0.6	0.8

  

Effects of Surface Area and “Structure” on Carbon Black Colloidal Properties and Performance on Rubber.

Colloidal properties	Increasing Surface area	Increasing “structure” of DBP absorption
Particle size	Decreases	No effect
Aggregate Size	Decreases	Increases
Iodine number	Increases	No effect
Elongation	Not significant	Decreases
Hardness	Increases	Increases
Impact resistance	Decreases	Not significant
Abrasion resistance	Increases	Increases
II Rubber properties A. Processing
Loading capacity	Decreases	Decreases
Mix  incorporation ime	Increases	Increases
Mooney Viscosity	Increases	Increases
Dimensional stability (green)	Not significant	Increases
Extrusion shrinkage (die swell)	Not significant	Decreases
B. Vulcanisation
Tensile Strength	Increases	Not significant
Modulus	Not significant	Increases