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Articles - Rubber Technical Papers



The term moulied goods applies to a wide variety of rubber products made by shaping uncured rubber compound in moulds under high pressure at elevated temperature. Many items such as M.C. Sheet, straps, automobile components, diaphrums bumpers etc. are in this group. A few items, which are manufacture in large scale in this country, are discussed below:

1. Micro Cellular Soling (M.C.)

Microcellular a sheet are very popular in footwear industry and has
replaced leather to a large extend. It has got following advantages.

" Lighthness and hence pleasant to wear and is available in pleasing colours
" Good flex properties
" Cheaper than leather soles

Microcellular sheets are manufactured mainly from a blend
Of natural rubber and SBR-1958. SBR 1958 is thermoplastic and hence gives better expansion and lower shrinkage along with high abrasion resistance and tear strength. Ground crumbs of mesh size 20-60 are also used. Highly reinforcing fillers (precipitated silica) helps in shrinkage reduction and also improve the abrasion resistance. In addition to this semi-reinforcing filler like calcium silicate, aluminum silicate and clay are also used for modifying the properties of the sheet and also to reduce its modifying the properties of the sheet and also to reduce its compound cost. DNPT type blowing agents are used for the production of the N.O. and havai sheets. A good dosage of stearic acid is also used for the decomposition of blowing agent. Typical compound formulations for M.C. sheet and havai sole sheet are given below:


Microcellular sheet compounds

Natural rubber 60 80
Synaprene 1958 40 20
Zinc oxide 75 5
Stearic acid 4 6
Light mineral oil 3 12
Coumarone Indene Resin 5 5
Calcium silicate 40 20
Precipitated Silica 10 --
Chinaclay 50 100
Micro Crumbs -- 30
Diethylene glycol 1.5 1.0
N.C.Wax 1 1.0
Titanium dioxide 10 10
Antioxidant Sp 1 1
Accelerator F 1.2 1.2
Sulfur 2.5 2.5
DNPT blowing agent 4 6.0

Press cure for 6 minutes at 145°C, post cure for 2-3 hrs. In bot air oven at 80-100°C.

The compound was prepared using a two roll-mixing mill and kept for maturation for 21 hrs. The compound is again, are warmed before moulding into sheet.

Micro Cellular sheets are best moulded in hydraulic press, as high pressure is required for getting fine uniform cells. Multi day light hydraulic pressure heated with steam or electricity are used. Temperature and pressure are very important in getting quality finished goods. For moulding microcallular products, a specific pressure of 18 to 20 kg/ cmª per PHR of DNPT blowing agent must be used. Such high pressure is required as te blowing agent on decomposition creates high back-pressure which tends to open the press. Hence the external pressure should be enough to prevent mould opening. The moulding gauge pressure can be calculated as follows:

DNPT blowing agent used - 5 phr.

Specific moulding pressure = 5 x 18 = 90 kg/ cmª

Mould size = 30ª x 20ª = 75 cm x 50 cm

Mould area = 75 x 50 = 3750 cm²

Ramdiameter = 18" = 45 cm.

Therefore Ram Area = Ti x d² = 3.14² x 45² = 1591 cm² = 1600

Now gauge pressure is calculated using the following equations.

Gauge pressure kg/ cm² = Specific Pressure X mould area
Ram area

= 90 X 3750 = 210 kg/ cm²

Hence for satisfactory moulding of micro sheet gauge pressure should be min. 210 kg/ cm²

The rubber compound is filled in the moulds to full length and breadth with 3½ by volume in excess. The flash formed in moulding will act as a seal between mould and press platen and this prevents the escape of gas, which is formed within the sheet during cure. The sheets are usually cured in the press for about 6 to 8 minutes at 145°C. During the vulcanization time large evolution of gas which is kept dissolved in the rubber and when the press is opened, there is sudden ralease of pressure and the sheet expand in length, breadth and thickness. The mould frames has got levelled edges tapering from the bottom outwards so that when the press is opened, the sheet expand and jump out of the frame without any distortion. The sheets are slightly undercured at this time and hance the sheets are again subjected to postcuring.

Typical vulcanisate properties and specifications of the M.C. sheet are given below:

Property M.C Sheet Havai Sheet
1.Relative Density(sp,gr) 0.45-0.5 0.3-0.5
2.Hardness, shore 50-60 35-45
3.Charge in hardness after aging at100oCfor
+5 +5
4. Compression set at
27oC for 24 hrs (max)
12 15
5. Split tear strength kg.(min) 6 4.5
6. heat shrinkageat 100oCn for 1 hrs. in both direction(max) 3 3
7. Water absorption % by mass(max) 2 2
8. Ross flex test
  (a)  Crack initiation cycle (min)
  (b)  600% crack growth cycle (min)





Trouble shooting in M.C. Sheet production

1. In consistant production (varying-sheet size and hardness)

The reason is the uneven degree of pressure which may be due to (a) uneven curing
Characteristics of the batches; (b) varying temperature in the platants (c) varying precure time given in the production.

(a) To maintain the curing characteristics at equal level the batches are blended. The batches are made with curatives and blowing agent and there store for around 24 hrs. On the next day, sheets from different batches are picked up for warming and the blanks are made. In this way the curing characteristics of the batches can be evened out to a certain extend.

(b) The temperature variations in the platens of a multiday light press can be due to faulty steam connections or water logging in a steam-heated press or due to faulty working of the temperature controllers on an electrically heated press.

(c) If the press opening on a hydraulic press can be automatically controlled by a timer variation in the press cure time can be avoided.

2.Non-uniform sheet (Edges non parallel)

This defect results if there is a loss of gas compare compound from one side of the mould or the degree of precure varies over the sheet.

(a) If the mould is warped or improperly designed and the seal along the edge is not perfect, when the internal pressure in the compound shoots up after the decomposition of the blowing agent oozing of the compound Starts. The sheet has lower expansion in the region when the loss of gas has occurred.

If the mould loading is the high the flash that develops at the sides is too thick. A
thick flash may not set up before the internal pressure builds up. When the blowing agent decomposes and the xxx internal pressure builds up the thick unvulcanised flash will be pushed out and the compound leakage can start.

On a hand press the closing pressures are not of high order. With higher mould loading thick flash is inevitable. Even with correct mould loading on a hand press the mould may not get properly closed along all sides and again compound leakage may start.

b) Sometimes on a steam heated platen the temperatures over the whole platen aree may not be constant due water logging towards the steam exit end. The resultant uneven degree of precure results in uneven expansion. On an electrically heated platen with multiple heaters the temperature can be varied over the platers surface.

Measures should be taken to avoid this temperature difference over the surface.

3.Curved sheets

This defect can arise when (a) the bottom and top platens around the mould have unequal temperature or (b) the blank in composed of sheets from batches with uneven curing characteristics/uneven content of blowing agent or (c) mould loading or high leading to thicker flash formation.

a) If the platen temperatures are different (which can easily happen on a multiday light steam heated press with platens joined in series), the side in contact with hot platen gets a higher degree of precure and hence expands less. The sheets on removal from the mould bents with the side in contact with hot platen on the inside.

b) When multicoloured sheets are to be manufacture the base composition (without colours) should be identical for the different coloured batches to ensure identical xxx precuring characteristics and identical expansion characteristic. Any deviation in this respect will lead to production of curved sheets.

c) Any excess compound squeezed out of the mould on closure should be immediately wiped out. The thin flash on the side of MC soling sheets is always curved to a greater extent and hence has less tendency to expand. When the sheets come out from the moulds, the flash should be immediately trimmed off. The hot sheets should be stacked one above other with a heavy weight on top to flatten the sheets.

4. Warped or Weavy sheets

This defect is due to the fact that on release of pressure the sheet is unable to pop out of the mould immediately. This may happened if (a) the opening of the press is not quick enough or (b) the sheet is wedged in the mould because of wedging of the flash in between the mould plates.

5. Bulging in the sheets.

i) The cutsection shows torn rough edged walls on the bubble. This is a case of undercure or too low degree of precure. The precure time should be lengthened.

ii) The cutsection shows smooth walls on the bubble. This is a case of air entrapment in between the layers of the blank. When the blank is built-up of layers it is preferable to keep the preforms stacked up and stored for few hours so that the air in between the layers diffuses out and the layers fuse together.

6. Hoses and pinholes in the finished sheet

This defect can arise due to (a) entrapped air in the blank or (b) undispersed grit in the compound and (c) undisparsed blowing agent particle.

a) If thick blanks are sheeted out directly from the mill it is very likely that the blank will contain air bubbles which enlarge on expansion of the sheet. In this case, it is advisable to allow the compound to run on the roll for xx sometime and then cut out the sheet.

b) Any filler/sand or grit particle will not expand along with the sheet and will form a pinhole in the sheet. On examination of the pinhole by a magnifying glass the filler particle can be seen and its nature can be sometimes judged by its appearance.

c) The problem can arise with improper xx types of DNPT type blowing agents. If the batches after mixing the blowing agent are stored for about 24 hours and then rewarmed this type of defect is mostly overcome. If the problem persists blowing agent should be changed or also addition of the blowing agent little earlier in the mixing cycle can be considered.

7. Sheets with cracked edges.

If the compound is highly loaded with inert mineral fillers especially in case of the compounds based on high quantity of emulsion SBR and reclaim the hot tear strength of the vulcanizate is quite low. On release of the moulding pressure the sheet expands, the thin flash tears out at places and sometimes the tear propagates to the sheet proper. Sometime the sheet immediately on removal is perfect but in a few minutes starts cracking and results in a scrap product. The remedy lies in changing the compound formulation. Part replacement of SR by natural rubber, reduction in the inert filler level along with the replacement of the level of by reinforcing fillers, increase in the level of plasticisers and resins help to overcome the problem.

8. Bubbles on the sheet during postcure

This is due to incomplete decomposition of the blowing agent during precure. The precure time can be increased (after reduction in accelerator dosage to keep the degree of precure at the same level). The decomposition of the blowing agent can be activated further by use of acidic substance eg: Benzoic

9. High Shrinkage during postcure/stabilization step

The degree of precure can be reduced by (a) lowering the accelerator dosage of (b) lowering the precuring temperature.

For production of consistently good quality of microcellular Soling it is not necessary to have elaborate production machinery and extensive laboratory facilities to keep control at every stage of production. By taking into consideration the basics of microcellular production and taking reasonable care at various stages of production even a small scale manufacturer will be in a position to Manufacture consistently good quality xx product.

Automobile Components

Rubber products are important to automobiles because they perform certain important functioning better then any other known material. Some of the important jobs done by alastomeric Products are shown in table I.

Function of Rubber Components

1 - Traction for power transmission
2 - Reduction or Insulation of Vibration
3 - Seal for Air and Fluids or against contaminants
4 - Electrical insulation
5 - Binder for Friction Material Adhesives, Sealers, gaskets
6 - Energy Absorption for safety padding
7 - Flexible Body panels.

The majority of rubber parts used in cars perform functions 2 and 3. Isolation of sound or other vibrations is accomplished by engine mountings, isolators' etc. Rubber weather-strips and brake boots prevent air, water or dust from entering the passenger compartment or vital components of vehicle engine seals, transmission seals as well as various types of hose function to seal fluids one type or another and contain them within a desired performance area. A relatively new application of elastomer product is their use in flexible front and rear body panels to provide a surface that will not be damaged by dumper movement during minor impact. Compound formulations for a few typical items are given below:

Compound formulations

1. Rubber bush   2.Rubber pad  
  p.b.w   p.b.w
Natural rubber 100,00 Natural rubber 100,00
Zinc oxide 5.00 Zinc oxide 5.00
Stearic acid 2.00 Stearic acid 2.00
A.O HSL 0.75 A.O HSL 0.75
HAF black 45.00 SRF black 60.00
Aromatic oil 5.00 DBP 10.00
Vulcafor F 1.20 CBS 1.20
TMTD 0.10 Sulfur 2.00
Sulfur 2.50    
Press cure 101 at 140oC Press cure 81 at 160oC



3. Rubber Spring   2.Rubber pad  
  p.b.w   p.b.w
Natural rubber 100,00 Natural rubber 95.00
Zinc oxide 5.00 Zinc oxide 5.00
Stearic acid 2.00 SBR 1502 5.00
A.O HSL 0.75 Stearic acid 2.00
SRF black 45.00 A.O HSL 0.75
DBP 12.00 SRF black 40.00
CBS 1.20 FEF blacj 20.00
Sulfur 1.50 DBP 10.00
    CBS 1.5
    TMTD 1.5
    Sulfur 0.4
Press cure 101 at 160oC Press cure 101 at 160oC



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