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Surface Treatment of Mica

During processing of siliceous products, molecular bonds are broken. The unsaturated terminal silicon and oxygen atoms react with water molecules to form hydroxyl groups. Whereas other water molecules can be absorbed, these water molecules cannot be fully removed by compounding, even under vacuum conditions at high temperature over a long treatment time. Typically (except talc) minerals have high polarity and surface tension (i.e., surface-free energy). On the other hand, typical polymers are lower in surface tension and less polar. Mixing the two is - simplified expressed - like mixing water (the mineral) with oil (the polymer) resulting in a high interfacial tension (due to the mismatch of polar and dispersive parts of surface tensions) and finally in a separation into two phases with minimized surface area. The resulting effects - taking into account that the mineral is a solid and the polymer (when being processed/compounded) is more or less a liquid - are agglomeration of the mineral particles leading to an uneven distribution of the latter and high interfacial tension resulting in easy cleavage of the interface between the two phases (after solidification) being the weakest part in the compound. Even worse, sometimes entrapment of air between mineral and polymer can be observed. Those voids are the nuclei for stress propagation.

By surface treatment of mineral fillers with silanes or silane-based compounds, those interfering effects at the interfaces between the polymer and the filler system can be minimized by - at least - adjusting the surface tensions and polarities of the surfaces.

Silanes are bifunctional chemicals that consist of stable organofunctional and hydrolyzable reactive terminal groups. The hydrolyzable group combines with the inorganic filler surface, while the organofunctional groups harmonize with the organic binder (polymer) system.

An important advantage of this method of incorporating already surface-treated fillers directly into a polymer system over “in situ” treatment is that the condensation by-products already escape during coating of the filler and do not get into and remain in the polymer system, as they do in the case of in situ post-silane treatment. Those condensation by-products (usually humidity and alcohols) typically weaken the polymer matrix. Additionally pre-coated fillers are easier dispersed into a polymer than uncoated ones tending to show much less agglomeration. To achieve an optimum chemical bond between the polymer and the functional filler, a silane specially adjusted to the polymer system must be applied to the surface of the filler.

Comparing the tough fracture of neat mica compounded in a polyamide with a surface-treated mica void can be identified between the mica platelets and the polymer matrix when using the neat mineral. Figures 4 and 5 show tough fracture SEMs of the two compounds.

Another comprehensive comparison of surface-treated and non-surface-treated mineral reinforcing fillers in polyamide is given by Hilgers and Mohr (Thorsten Hilgers 2010). Table 6 shows the reinforcing mineral fillers in comparison.

Tables 7 and 8 show the mechanical and thermal data at 20% (by weight) filler load in polyamide 6.

Fig. 4 Scanning electron micrograph; micronized muscovite mica without surface treatment in polyamide after a tough fracture

Scanning electron micrograph; micronized muscovite mica with surface treatment in polyamide after a tough fracture

Fig. 5 Scanning electron micrograph; micronized muscovite mica with surface treatment in polyamide after a tough fracture

They conclude for mica:

The thermal and mechanical properties of polyamide compounds can be considerably improved by using platelet-shaped muscovite and phlogopite micas. In this way the following properties can be achieved:

  • • Improved isotropic shrinkage
  • • A clear reduction in the shrinkage
  • • An improvement in the distortion sensitivity
  • • An increase in tensile strength and in the module of elasticity
  • • An increase in the rigidity with retention of the tenacity

Coated fillers are more easily incorporated into a polymer in comparison to uncoated ones.

An optimal effect between a polymer and the high-performance filler is achieved by a coating agent that is matched to the polymer system.

Table 6 Summary of the filler modifications of wollastonite, mica, and kaolin tested in polyamide 6

Date sheet values

Description of the filler

Product name

d50 [pm]

d90 [pm]

Method

Phlogopite medium

TREFIL 1232400

11

26

Sedigraph

Phlogopite medium silanized

TREFIL 1232-400 AST

11

26

Sedigraph

Phlogopite fine

TREFIL 1232500

11

18

Sedigraph

Phlogopite fine silanized

TREFIL

1232-500

AST/1

11

18

Sedigraph

Muscovite coarse

MICA TG

40

90

Air jet screen

Muscovite coarse silanized

TREMICA 1305-003 AST

40

90

Air jet screen

Muscovite medium

MICA SG

7.5

15

Sedigraph

Muscovite medium silanized

TREMICA 1155-006 AST

7.5

15

Sedigraph

Muscovite fine

MICA SFG 20

5

10

Sedigraph

Muscovite fine silanized

TREMICA 1155-010 AST

5

10

Sedigraph

Wollastonite, short-needled, medium

TREMIN

283-400

6

16

Cilas laser granulometer

Wollastonite, short-needled, medium, silanized

TREMIN 283-400 AST

6

16

Cilas laser granulometer

Wollastonite, short-needled, fine

TREMIN

283-600

3.5

9

Cilas laser granulometer

Wollastonite, short-needled, fine, silanized

TREMIN 283-600 AST

3.5

9

Cilas laser granulometer

Wollastonite, long-needled, medium

TREMIN

939-400

25

67

Needle length/ image analysis

Wollastonite, long-needled, medium, silanized

TREMIN 939-300 AST

30

69

Needle length/ image analysis

Kaolin, pronounced platelet shape, fine

Kaolin TEC 110

1.1

3.5

Sedigraph

Kaolin, pronounced platelet shape, fine, silanized

Kaolin TEC 110 AST

1.1

3.5

Sedigraph

Glass fiber

Glass fiber 4 mm dl0pm

-

-

-

Kaolin, calcinated, silanized

Calcinated kaolin sil.

1.1

5.2

Sedigraph

Filler

Concentration of filler [%]

Tensile test

Pendulum impact tests IZOD

DIN EN ISO 527-1

ISO 180 IZOD

Tensile strength [MPa]

Elongation at break [%]

Modulus

[MPa]

Impact

Notched

impact

Resistance

[kJ/nr]

Resistance

[kJ/nr]

TREFIL 1232-400

20.2

90

4.3

6,330

29

2.9

TREFIL 1232-400 AST

20.3

92

4.5

6,270

30

2.8

TREFIL 1232-500

20.2

88

3.2

5,930

33

3.6

TREFIL 1232-500 AST/1

19.0

89

4.4

5,930

34

3.7

MICA TG

20.3

86

3.9

6,550

24

2.9

TREMICA 1305-003 AST

19.7

89

2.6

6,010

30

3.3

MICA SG

19.5

90

4.9

5,580

38

3.5

TREMICA 1155-006 AST

19.4

94

4.5

5,320

51

3.6

MICA SFG 20

19.6

91

5.5

5,390

47

3.4

TREMICA 1155-010 AST

19.9

93

4.7

5,170

54

3.7

TRE MIN 283-400

20.0

81

3.9

4,190

47

3.6

TRE MIN 283-400 AST

19.9

84

5.2

4,170

58

4.0

TRE MIN 283-600

19.3

82

4.5

4,170

55

3.8

TRE MIN 283-600 AST

19.8

85

3.9

4,000

65

3.9

TRE MIN 939-400

19.8

96

4.6

6,130

42

3.1

TRE MIN 939-300 AST

19.3

93

7.0

5,290

56

3.1

Kaolin TEC 110

19.7

92

4.3

5,780

40

3.2

Kaolin TEC 110 AST

19.1

90

5.0

5,410

51

3.4

Glass fiber 4 mm, d 10pm

20.0

144

5.7

7,110

39

6.3

Calcinated kaolin, silanized

18.5

88

9.3

4,280

N.B.

3.9

Polyamide

0.0

86

8.4

3,210

107

2.5

Table 8 HDT and shrinkage data of different reinforcing mineral fillers at around 20% (by weight) filler load in polyamide 6

Filler

Concentration of filler [%]

Heat distortion temperature

Shrinkage

ISO 75

ISO 294-4

HDT A [°C]

Parallel

[%]

Perpendicular

[%]

Delta

[%]

TREFIL 1232400

20.2

135

0.9

1.0

0.09

TREFIL 1232400 AST

20.3

136

0.9

1.0

0.09

TREFIL 1232500

20.2

123

0.9

1.0

0.07

TREFIL 1232500 AST/1

19.0

122

0.9

1.0

0.07

MICA TG

20.3

146

0.9

1.0

0.08

TREMICA 1305-003 AST

19.7

140

0.9

1.0

0.08

MICA SG

19.5

110

0.9

1.0

0.06

TREMICA 1155-006 AST

19.4

108

0.9

1.0

0.06

MICA SFG 20

19.6

107

1.0

1.1

0.09

TREMICA 1155-010 AST

19.9

104

1.0

1.1

0.09

TREMIN

283-400

20.0

88

1.3

1.3

0.02

TREMIN 283-400 AST

19.9

86

1.3

1.3

0.03

TREMIN

283-600

19.3

85

1.3

1.4

0.01

TREMIN 283-600 AST

19.8

84

1.3

1.4

0.02

TREMIN

939-400

19.8

129

0.9

1.1

0.20

TREMIN 939-300 AST

19.3

114

0.9

1.1

0.20

Kaolin TEC 110

19.7

132

1.0

1.2

0.20

Kaolin TEC 110 AST

19.1

128

1.1

1.3

0.21

Glass fiber 4 mm, dl0nm

20.0

198

0.4

1.0

0.55

Calcinated kaolin silanized

18.5

79

1.4

1.4

0.06

Polyamide

0.0

72

1.6

1.6

0.06

Due to this the system, improving properties of the filler are optimally utilized in most cases. In particular tenacity can be improved by the use of silanized fillers.

 
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