FIELD OF THE INVENTION
The present invention relates to a rubber covered squeeze rolls used in acid cleaning section of the pickling and tandem cold rolling mill of steel sheets. More particularly it relates to develop formulation for these rubbers.
BACKGROUND OF THE INVENTION
Iron oxide scale on the surface of the low or ultra-low carbon steel sheet arising from previous hot rolling operation is removed through pickling of the steel sheet in hot acidic medium. The pickling liquor constitutes of 18% HCl at 70-800C. After acid pickling, the acidic liquid remaining on the sheet surface is removed in the primary and secondary cleaning sections of the pickling and tandem cold rolling mill. Effective cleaning of the sheet surface ensures chloride concentration on the sheet surface less than 60 ppm after cleaning. Ineffective cleaning causes higher concentration of chloride remaining on the surface leading to contamination in subsequent processes resulting in rejection of the coil due to surface stain. Effective cleaning of the steel surface after acid pickling is ensured by proper squeezing action of the rubber covered rolls in primary cleaning and DM water cleaning (secondary cleaning) sections.
The material being processed in the line has the following characteristics:
. Low or ultra low carbon steels after hot rolling
• Input sheet thickness varies from 2.00 - 6.50 mm
. Sheet width varies from 800 - 1500 mm
• Output of these lines are used mainly by auto, construction and white
goods sector

The processing conditions are described below:

Dimension of the rubber clad squeeze rolls at primary and secondary cleaning sections:

The usual service life of the commercially available squeeze rolls working in such severe condition is 10 to 30 days. The usual nature of failures observed are surface wear of roll, groove formation at roll edges, V-shaped cut at edges, axial & circumferential corrugation followed by blister formation and all the above defects finally lead to de-lamination between rubbers covering to metal core.
These squeeze rolls remain continuously immersed in the acidic liquid under high line tension, speed and pressure. Therefore, rubber cover of these rolls needs to be resistant to the chemical attack. Any change in surface characteristics like color, glossiness, smoothness due to reactions with hot acid affect the coefficient of friction resulting in slippage of the sheet on roll surface contributing to cutting of the cover rubber. Chemical attack to the rubber cover leads to volume swell or volume shrinkage due to absorption of the working fluid. Chemical reaction with the compounding ingredients of the rubber cover leads to micro porosity, leaching and volume change. Reaction with base rubber material leads to substitution or addition and chain degradation. It also causes the change in initial

physical properties of the rubber cover which enhances the chance of surface cut marks and further reduces the service life of these rolls.
Resistance of a rubber compound to chemical attack is measured through swelling studies as per ASTM D 471-98 standard. To study swelling resistance of a polymer, it is immersed in working medium (here 18% HCl) at 90oC in thermostatically temperature controlled oil bath for 7 days. The results are reported in terms of percentage volume loss and weight loss as defined below

W1 = initial weight in air, W2 = initial weight in water, W3 = weight of the treated rubber in air and W4 = weight of the treated rubber in water.
Test pad samples of the commercial acid cleaning section squeeze rolls made from neoprene based rubber formulations were tested in laboratory for swelling properties and results are listed below:



Inappropriate rubber composition which is not sufficiently resistant to hot acidic working solutions as revealed from high swelling index is the reason for 10-30 days service life of the commercial rolls. Service life of the commercial rolls is restricted to 10-30 days because of the following reasons:
• Cover rubber formulation react and degenerate faster in hot acidic working
solution leading to high swelling index
• Faster change in hardness of the rubber cover leading change in friction coefficient at surface causing slippage of the sheet while passing over the rolls leading to surface cut marks
• Slippage of the strip/sheet on roll surface lead to cut marks on the cover which further lowers the service life of these rolls.
An Indian patent with patent No. 235697 disclosed a modified neoprene based rubber formulation which shows better chemical resistance than the commercially available rubber compounds in hot HCl medium. It is reported that the modified neoprene formulation reduces the swelling index properties by optimizing the filler loading in the base rubber and the problems of edge cuts, surface blister formation was reduced to great extent.

Another Indian patent with patent No. 243830 disclosed a new acrylo nitrile butadiene (NBR) based rubber formulation which is reported to overcome the above mentioned surface defects on the rubber clad rolls. The NBR based formulation also reported to reduce the swelling index properties and this improve the service life in hot HCl working medium of PLTCM.
A hypalon based rubber formulation was proposed in another Indian patent with patent No. 251615. It is reported to overcome the above mentioned surface defects on the rubber clad rolls in hot chromic acid working medium of continuous galvanizing line. The hypalon based formation also reported to reduce the swelling index properties and this improve the service life of the rubber clad rolls.
Through prior arts it is evident that hypalon rubber based formulation provides superior resistance to acidic working environment (HCl and chromic acid) and rubber clad rolls made out of hypalon based rubber compounds give effective squeezing properties for longer duration of time in comparison to commercially available rubber clad rolls as mentioned in Table 1 and Table 2.
But in recent past, commercial availability of hypalon grade of synthetic rubber was a big concern and volatility in prices was also observed. Therefore, a need was felt to partly or completely replace hypalon based rubber formulations for application in the production lines of cold rolling operation.
OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, an object of the invention is to find a suitable alternative of existing hypalon based rubber clad rolls without much compromise to its characteristics of end application.

SUMMARY OF THE INVENTION
In one aspect, the invention provides a mixed rubber formulation for application as clad on squeeze rollers comprises (all in parts by 100 weight of rubber): Hypalon Polymer 50-90, Neoprene W Polymer 10-50, MgO 0-5, Litharge 0-10, Epoxy 0-10, Nickel dibutyl dithiocarbamate (NBC) 1, Furnace Black 20-30, Cumarone Indene (C.I) Resin- 3, Polyethylene glycol (PEG) 1, Tri hydro Quinoline (TQ)- 2, MBTS 0.5, Tetrone A-0.75, and Sulfur-0-1.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a mixed rubber formulation for application as clad on squeeze rollers, the mixed rubber formulation comprising (all in parts by 100 weight of rubber): Hypalon Polymer 50-90, Neoprene W Polymer 10-50, MgO 0-5, Litharge 0-10, Epoxy 0-10, Nickel dibutyl dithiocarbamate (NBC) 1, Furnace Black 20-30, Cumarone Indene (C.I) Resin- 3, Polyethylene glycol (PEG) 1, Tri hydro Quinoline (TQ)- 2, MBTS 0.5, Tetrone A-0.75, and Sulfur-0-1.
In accordance with an embodiment of the invention a mixed rubber formulation for application as clad on squeeze rollers is developed and thereby claimed. The claimed formulation is Hypalon Polymer 50-90 phr, Neoprene W Polymer 10-50 phr, MgO 0-5 phr, Litharge 0-10 phr, Epoxy 0-10 phr, Nickel dibutyl dithiocarbamate (NBC) 1 phr, Furnace Black 20-30 phr, Cumarone Indene (C.I) Resin- 3 phr, Polyethylene glycol (PEG) 1 phr, Tri hydro Quinoline (TQ)- 2 phr, MBTS-0.5 phr, Tetrone A-0.75 phr, and Sulfur-0-1 phr (phr : parts by 100 weight of rubber) (ref. H3, H4 and H5 of Table 3A).
The mixed rubber formulation is further processed to build clad rolls using calendaring and hot extrusion techniques.

Number of new formulations combining hypalon and neoprene rubber as base with varying contents was tried in laboratory. In these formulations hypalon content was gradually replaced with neoprene rubber and swelling index properties were checked first to find out its effect on chemical resistivity in the hot acidic working environment. Table 3A shows that hypalon content was gradually reduced from 100 to 20 parts by weight of rubber (phr) and neoprene rubber content was gradually increased from 0 to 80 phr.

Two different kinds of reinforcing agents like silica based (VN3 Silica, china clay) and carbon black (different forms of furnace extrusion products) were used in the different formulations (Ref. Table 3A and 3B).
Table 4 shows the corresponding swelling values and swelling index of the different compositions. Swelling properties and change in physical properties after exposing these developed formulations in simulated working environments were tested in details to achieve the optimum rubber compound composition (Ref. Table 5 and Table 6).



100 phr hypalon based formulation (H6) was used as a reference to understand the extent of reduction in chemical resistance with replacement of hypalon with neoprene rubber in the base formulation.
H5, H3 and H4 formulations did not show much reduction in chemical resistance where hypalon was replaced upto 50 phr by neoprene in base rubber. (Ref. Table 4)
Below 50 phr, compositions like H1, H9 started showing drastic drop in chemical resistance with substantial change in volume and/or weight swell. These formulations also showed higher degree of hardness change (5 Shore A) after soaking in acidic medium compared to H5, H3 or H4. (Ref. Table 4).
Interestingly formulations like H2 and H7 in spite of having hypalon content more or equal to 50 phr in base rubber showed higher swelling properties. It can be noted from Table 3A that in H2 and H7 formulations, silica based reinforcing agents were used instead of carbon black reinforcements and hence the reason for higher swelling properties.

Table 3A and Table 4 clearly show that carbon reinforcement based rubber formulations (like H4, H5) showed better chemical resistance than silica based formulations like (H2, H7) even if the hypalon-neoprene content remained same. Similar trend was also observed with complete hypalon based formulations also (H6 and H8 in Table 3A and Table 4).
The results confirmed that at most 50 phr hypalon can be replaced by 50 phr neoprene without much sacrifice in the chemical resistant properties. But below 50 phr, the properties start to degrade faster.
H5, H3 and H4 showed significantly improved results in terms of swell Index along with minimum change in hardness, tear and abrasion resistance after swelling test in laboratory. This criteria was adopted to ensure that the rubber clad maintain original properties and surface condition in actual service condition.
The optimum composition was further checked for higher scale process ability ((Ref. Table 7) to ensure the selected composition can be processed through calendaring/hot extrusion route to produce the rubber clad rolls for end application in pickling line.
Table 7: Rheograph Data for the formulations
Temperature: 1500C; Chart motor: 30min; Range Scale: 100; Arc+/-: 30



Viscosity and flow characteristics of the optimum formulations were tested to ensure the process-ability at higher scale of manufacturing required to build the actual rubber clad roll of 350mm diameter for use in pickling and tandem cold rolling mill. Table 7 above demonstrates the rheograph data for the developed optimum formulations obtained through Oscillating Disc Rheometer (Monsanto Rheometer; R-100S) complied with ASTM D 2084-95 standard.
Decrease in co-efficient of friction between metal and rubber surface due to high swelling properties like 4-10% change in volume/weight as reported with commercial rubber formulations (Table 1) causes change in surface condition and physical properties. As a result, differential speed of rotation happens between rubber rolls and steel sheet and starts slippage. The clad rolls become sluggish and move at a slower velocity than the speed of the sheet. It causes slippage between roll surface and moving sheet. The relative slippage along with the already prevailing differences in speed at high pressure and line tension between entry and processing section of the line further enhances the sheer force on the cover rubber resulting in increased abrasion, developing cut marks on the rubber surface. Hence, the formulation is also required to be highly abrasion and cut resistant along with resistance to chemical attack to minimize the change in physical properties at the severe working conditions.

The rubber is made resistant to chemical attack by proper selection of the synthetic rubber grade. The rubber cover was further made abrasion resistance through suitable reinforcing agent providing high modulus characteristics. In an embodiment of the current invention, furnace black (different varieties) and silica were used as re-enforcing agent. H2 and H7 formulations (Ref. Table 3A) were loaded with silica based reinforcements whereas rest of the formulations were loaded with different forms carbon black reinforcements. The silica reinforced rubber compounds show higher swelling properties compared to the others (Ref. Table 3A and Table 4 for H2 vs H5 and H4 vs H7 compositions). Different forms of carbon black reinforcements did not show any change in swelling properties and original physical propertied to start with.
The formulations were tested using applicable standards in the industry. Hardness of the samples was determined using Shore ‘A’ Durometer (Shore Instrument and Manufacturing Co, Jamaica, New York, USA) as per ASTM D 2240-98. Readings were taken after 15 seconds of the indentation when firm contact was established with the specimen. The swelling experiments were carried out following ASTM D 471-98. The developed formulations were immersed in acid at 90o C in thermostatically controlled oil bath for 7 days. Table 4 reports these properties of the different formulations. Physical properties were determined by taking dumbbell shaped specimens punched (Punch press, Model P/44, MS Instruments Company Inc., Stony Creek, New York) from the moulded sheets with ASTM D 412-80 Type C die along the grain direction for tensile testing and 90o angled specimens for tear testing across the grain direction. Tensile Properties like modulus, tensile strength and elongation at break of the rubber compounds were determined according to ASTM D 412-98 test method. The tests were carried out in a Zwick Universal Testing Machine (UTM) model 1445 (Zwick GmbH & Co, Ulm, Germany) at crosshead speed of 500 mm/min at 25 + 2o C. The modulus and tensile strength were reported in MPa and elongation at break in percentage. Tear strength of the vulcanized rubber was

determined on un-nicked 90° angle specimen (Die C) using Zwick UTM 1445 as per ASTM D 624-98 at room temperature. Physical properties before and after soaking in working medium are reported in Table 5 and Table 6.
Advantages:
The optimized formulation makes a unique balance between mechanical and chemical working conditions and hence, meeting the demand of life cycle for an extended period of time along with the dimensional characteristics of rubber clad rolls in particular. The flowability and curing characteristics of the formulation is designed such that rolls can be cladded with the developed formulation by using calendaring or hot extrusion technique.

WE CLAIM:
1. A mixed rubber formulation for application as clad on squeeze rollers,
the mixed rubber formulation comprising (all in parts by 100 weight of
rubber):
Hypalon Polymer 50-90, Neoprene W Polymer 10-50, MgO 0-5, Litharge 0-10, Epoxy 0-10, Nickel dibutyl dithiocarbamate (NBC) 1, Furnace Black 20-30, Cumarone Indene (C.I) Resin- 3, Polyethylene glycol (PEG) 1, Tri hydro Quinoline (TQ)- 2, MBTS 0.5, Tetrone A-0.75, and Sulfur-0-1.
2. The mixed rubber formulation as claimed in claim 1, wherein the furnace black is high abrasion furnace black (HAF).
3. The mixed rubber formulation as claimed in claim 1, wherein the furnace black is semi reinforcing furnace black (SRF).
4. The mixed rubber formulation as claimed in claim 1, wherein the furnace black is fast extruding furnace black (FEF).
5. The mixed rubber formulation as claimed in claim 1, wherein the furnace black is mineral thermal (MT) black.

6. The mixed rubber formulation as claimed in claims 1& 2, wherein
mixed rubber formulation comprises Hypalon Polymer- 50, Neoprene
W Polymer- 50, Epoxy-10, Nickel dibutyl dithiocarbamate (NBC)-1,
high abrasion furnace black(HAF)-20,cumarone Indene (CI) Resin-
high abrasion furnace black (HAF)-20 cumarone Indence (CI resin
3 Polyethylene glycol (PEG)-1, Tri hydro Quinoline (TQ)- 2, MBTS-0.5, Tetrone A-0.75, Sulfur-1 (all in parts by 100 weight of rubber). 7 The mixed rubber formulation as claimed in claims 1 & 2, wherein mixed rubber formulation comprises Hypalon Polymer- 60 Neoorene W Polymer- 40, MgO-5, Litharge-10, Epoxy-0, Nickel dibutyl dithiocarbamate (NBC)-1, high abrasion furnace black (HAF)- 20 Cumarone Indene (C.I) Resin- 3, Polyethylene glycol (PEG)- 1, Tri hydro Quinoline (TQ) 2. MBTS -0.5, Tetrone A-0.75 (all in parts by 100 weight of rubber). 8 The mixed rubber formulation as claimed in claims 1 & 5, wherein mixed rubber formulation comprises Hypalon Polymer- 70 Neoprene W Polymer- 30, Epoxy-10, Nickel dibutyl drthiocarbamate (NBC)-1, mineral thermal, (MT) black - 26, Cumarone Indene (Ci) resin-3 Polyethylene glycol (PEG)- 1, Tri hydro Quinoline (TQ)- 2 MBTS-0.5, Tetrone A-0.75, sulfur-1. (all in parts by 100 weight of rubber). 9 The mixed rubber formulation as claimed in claim 1, wherein ft. mixed rubber formulation is processed to build Clad rolls using calendaring and hot extrusion techniques.