Short Notes #4

FOOTWEAR COMPOUNDING

 

 

Typical Mixing Lines

 

 


Typical Formulations

 

Banbury 1st Step Mixing                          Powder                                    PBPC

                                                               PHR

 

1.                       BR                                           65                                            65

2.                       IR/TTRL                                  25                                            24.570

3.                       NBR                                        10                                            10

4.                       Homogeniser                            2.5

5.                       Microcrystalline Wax                1.0

6.                       Antiox. BHT                             1.0

7.                       Processing Aids                        1 - 2

8.                       PEG 4000                                2.5

9.                       St. Acid                                    0.25 – 1.0

10.                   Processing Oil                          0 - 4

11.                   PPTD. Silica                             45

12.                   Silane Coupling Agent               0.7 – 2.0

13.                   Zinc Oxide                               5

14.                   Titanium Dioxide                       12 - 15

15.                   MBTS (DM)                            1.5 – 1.7

16.                   MBT (M)                                 0 – 0.5

17.                   TMTM(TS)                              0.05 – 0.15

18.                   Others Pigment                         As Required                             As Required

1st Step MB.                                           172.5 – 175.850                      170.520 – 173.070

 

Mixing Cycle       :                                   5` - 8`

 

 

Final Mix.                                               Batch Weight

2 Roll Mixing                                          (kg)

                                                               x 0.348

 

MB                                                         60.000                                     60.000

OTS-80%                                              0.653                                       -

(DM                                                       0.522)                                      -

(M                                                          0.087)                                      -

(TS                                                         0.020)                                      -

Sin RubtechTM ISE – 75                          -                                               0.696

MBTS – 80                                            -                                               0.522

MBT – 80                                              -                                               0.087

TMTM – 80                                           -                                               0.020

                                                               60.653                                     61.385

 

Mixing Cycle                                           10` - 22`                                  6` - 11`

 

 

COMMENTS

 

1.                       Mixing and compounding practises differ quite a bit in footwear manufacture.

 

2.                       Those who add all accelerator powders in 1st Step MB are prone to cure variation since the small amount of M & TS can be adsorbed on the silica surfaces or diminished due to “fly losses”.

 

3.                       Those who add all accelerator and IS powders on 2-roll mills tend to have the longest mixing cycle and still have dispersion problems without tight nipping.

 

4.                       Use of Zn stearate powdering or zinc stearate powder floating on cooling water tends to give uneven coating of zinc stearate and possible Zn Stearate bloom.

The best system is where Final Mix is pre-cooled before entering a zinc stearate dispersion/emulsion and the sheets passed through a drying/cooling conveyor.  No phasing out of zinc stearate dispersion/emulsion encountered.  Prudence suggests that where zinc stearate anti-tack is used, the stearic acid level should be £ 1.0.

 

5                         Where our PBPCS are used, all accelerators and ISE – 75 can be added on the 2 roll mill with productivity improvements of 10 – 100% possible.  Tight nipping can be reduced to 1 x or even eliminated.  Scorch, cure variation rejects & remills can be reduced to £ 0.50% .

 

6                         The absence of adequate brush-down (preferably with air jets) imposes a tight nipping operation even of 1st step MB in some factories.

 

7.                       The absence of a stockblender on 2-roll mills makes final mixing longer.

 

8.                       The absence of sheet stripping knives make take off a more laborious operation.

 

9.                       When using our EPDM bound chemicals, some adjustment in raw rubber weigh-ups can be made if cost savings can be realised.

 

10.                    The improvement in mixing efficiency coupled with reduction in rejects at Final Mixes and finished moulded soles are expected to reduce bottom-line costing on switching over to our PBPCS.

 

 

REFRACTIVE INDICATES

 

 

 

Water                                                     1.33

Acetone                                                  1.36

Silicas                                                    1.43 – 1.55

Fumed Amorphous Silica                        1.46

Benzene                                                  1.50

BR                                                          1.51

NR                                                         1.52

SBR                                                       1.53

ZnCO3                                                    1.618

CaCO3                                                   1.681

CaSiO4                                                   1.735

ZnO                                                        2.02

 

Look for silicas with as close a refractive index as NR/SR or better still lower RI.

 

 

TRANSPARENT SOLINGS OTHER THAN PU

 

 

PU transparent solings cost at least 20% more than NR/SR types.  For window-transparent solings besides PU, EPDM, EPDM/IR, IR/BR, IR/BR/SSBR all peroxide cured have been tried.  I am told EPDM peroxide cured gives adhesion problem similar to EU-P formulations.  Some R & D work on EPDM/EVA peroxide cured can be done or at worst use a NR/BR peroxide cured as the bridge compound.

 

EVA can increase polarity of EPDM especially those grades with ³ 28% VA content.

 

 

PU COLOUR STABILISATION

 

 

Best combination of UV absorber + hindered amines based on 2, 2, 6, 6, tetramethyl piperidinyl structures.

 

 

                        Ex. Cyasorb UV531

                                    +

                            Tinuvin 765

 

 

TREATED FUMED AMORPHOUS SILICA

 

 

To avoid storage hardening use treated fumed amorphous silica.

 

Ex. Cab-O-Sil TS530

 

Hexamethyldisilizane reacts with surface hydroxyl groups and-replaces these with the hyrophobic trimethylsily groups liberating NH3 which easily evaporates.

 

 

MILIABLE PU FOR ATHLETIC FOOTWEAR

 

1.                       Polyurethane rubbers PU are made by reacting a liquid “macrodiol”such as hydroxyl-terminated polyester or polyether polyols with a diisocyanate and these low molecular weight prepolymers chain extend by further reaction with simple microdiols to give the PU rubbers.

 

2.  Examples of macrodiols, microdiols and diisocyanates.

 

Macrodiols

 

Chemical Name

Type

Commercial

1. Poly (tetramethylene adipate) glycol

Polyester

--

2. Polyethylene glycol adipate

Polyester

Vulkollan Bayer

3. Ethylene glycol + ethanolamine + adipic acid

Polyester amide

Vulcaprene ICI

4. Polytetramethylene ether glycol

Polyether

--

5. Mixtures of above diols

--

--

 

Diisocyanates

 

Chemical Name

Type

Commercial

1. Toluene diisocyanate

Aromatic

TDI

2. Diphenyl methane diisocyanate

Aromatic

MDI

3. Methylene bis (4-cyclohexyl-isocyanate)

Cyclo-aliphatic

H12MDI

4. 1,6 Hexane diisocyanate

Open chain aliphatic

HDI

 

Microdiols

 

Chemical Name

Type

Commercial

1. Ethylene glycol         

Aliphatic

EG

2. 1,4 butanediol                      

Aliphatic

1,4 BDO

3. 1,4 bis (2-hydroxyethoxy) benzene

Aromatic-aliphatic

HQEE

4. Glycerol (monoallyl ether)

Adiprene

C (Du Pont)

5. Trimethylolpropane monoallyl ether

--

--

 

 

CODING BY ASTM

 

PU with polyester macrodiols --------- AU

PU with polyether macrodiols --------- EU

 

 

GRADES & CLASSIFICATION

 

 

Grade    

 

Polyester AU-P  Ester/MDI                    Peroxide                       Urepan 640 Bayer       

                                                                                                   Millathane 97 TSE

Polyester AU-S                                      Sulphur/Peroxide          Urepan 620 Bayer

Polyether EU-S   Ether/MDI                    Sulphur/Peroxide          Urepan 720 Bayer

Polyether EU-S   Ether/H12MDI              Sulphur/Peroxide          Millathane HT

                                                                                                   Adiprene E

 

 

SELECTION OF PU GRADE FOR FOOTWEAR

 

1.  Hydrolysis resistance

  2.  Colour Resistance (light stability)

  3.  Transparency

  4.  Wear Resistance

  5.  Storage Hardening

  6.  Hot Tear Resistance

  7.  Good dry & wet traction

  8.  Achievable hardness

  9.  Oil resistance

10.  Heat resistance

 

Early experience with AU grades showed up deficiencies in hydrolytic and colour resistance.  Storage hardening of AU & EU grades due to PU crystallisation resolved by selecting lower molecular weights macrodiols.  Typically macrodiols of MW 1000 – 5000 used.  For lower crystallisation PU, macrodiols of £ 2000 MW used.

 

Light stability is achieved by using the more stable aliphatic diisocyanates but these resulted in poorer hot tear strength.  This is compensated by use of fumed silica and a peroxide bi-functional coagent.

 

Sulphur cured EU gave adhesion problem to NR/SR blends with aqueous cements.  By using peroxide cured EU and peroxide cured NR/BR blends solved this problem.

 

 

STATE OF ART

 

Little work published on comparison of standard NR/SR blends with PU in athletic footwear.

The recent recommended formulations by TSE (Rubb. World. Vol 219, No. 3 Dec 1998) are :

 

                                       NR/SR                         EU

                                       PHR                             PHR

SMRL                            30                               -

BR                                  40                                -

SBR 1502                      30                                -

Millathane HT                 -                                   100

Stearic Acid                    1.0                               0.25

Fumed Silica                   -                                   20.0

Silane A172                    -                                   0.25

Sartomer SR231             -                                   3.0

UMB                              -                                   0.003

Lupersol 231                  -                                   0.50

Carbowax 3350              3.0                               -

Hisil 233                         45.0                             -

Zinc Oxide                      5.0                               -

Naph. Oil                        5.0                               -

Wax                               1.0                               -

Antiox. NS                     1.0                               -

MBTS                            1.0                               -

MBT                              0.5                               -

TMTD                            0.5                               -

TMTM                           0.2                               -

S                                    2.2                               -

Shore A                                                              69

T.S. MPa                                                            26.1

EB %                                                                  395.0

Tear kN/m                                                          29.7

Transparency                                                      Excellent

 

 

Compounding of EU for footwear begs for more R & D work.

 

1.  The adhesion problem has to be looked at from at least 2 angles :

 

          ·  Compatibility of NR/SR & EU

 

          ·  Cure rates

 

Solubility parameters of NR, SBR & BR are about 8.2 (cal.cm-3)1/2 and that of EU ³ 14.  Addition of some NBR or CR to NR/SR should help.  It is a well accepted fact that best bonding is achieved by a slow or delayed action accelerator.  Both solings of NR/SR and PU compounds use fast activated MBTS/MBT/S systems.  A non-discolouring sulphenamide like OTOS can be looked at as the tertiary accelerator.

 

2.      The use of fumed silica is not only expensive but this will definitely give rise to  storage hardening problem due to filler-filler interaction.  We need to look at pptd. silica’s of same refractive index of NR/SR and EU for transparency .  The bound water in pptd. silica may not be the reason for poor transparency.  There are also pretreated hydrophobic fumed silicas which may not storage harden.

 

The use of coupling agent in a fumed silica filled, EU compound is questionable as wear resistance is not a problem.  If too high Mooney is encountered, the use of PU plasticisers can be tried.  Acrylic coagents are also known to reduce Mooney viscosity but also scorch safety.  A good compromise co-agent is Saret 517.                                    

 

 

 

 

 

1st Step masterbatch

SHOE SOLINGS WHITE/ WHITE PHR

 

TRANSPARENT HONEY PHR

SMRL/IR

30.0

A

2.5

SBR 1502       

28.5

77.0

BR 150

40.0

20.0

SIN RUBTECHTM H21          

1.5

1.5

Stearic Acid    

1.0

B

1.0

Antiox. Bht/Cpl

1.0

1.0

Microcrystalline Wax

1.0

1.0

PEG 4000

3.0

4.5

A1891/SI – 69

1.0

C

1.0

Pptd. Silica      

25.0

25.0

TIO2   

18.0

-