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 Compounding of Polypropylene |
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Following the trends that have been established, mica is the next step up in the stiffness/HDT ladder. Likewise, it results in further decreased impact. Mica is found in a platelet form rather than in the more particular form of CaCO3. As the fillers pass from the particulate form through the platelet form on their way to a fibrous-type form, the changes in physical properties correspond. From another viewpoint, as the aspect ratio, or length-to-diameter (or thickness) ratio of the filler increases, strength tends to increase and impact to decrease. Mica offers outstanding stiffness as well as increased HDT. These properties are further enhanced by the addition of a coupling agent. Table (3a) summarizes data on some mica-filled PP. |
| Table 3 a : TYPICAL PRODUCT PROPERTIES OF MICA FILLED -PP |
Property |
Unfilled |
40% |
40% Coupled |
50% |
50% Coupled |
Melt flow rate, condition L,g/10 min. |
4 |
2 |
2 |
1.5 |
2 |
Density, g/cc |
0.903 |
1.23 |
1.23 |
1.36 |
1.36 |
Tensile strength (yield) MPa. |
35 |
43 |
46 |
45 |
50 |
Flexural modulus(1% secant), MPa |
1656 |
5796 |
6555 |
7245 |
8211 |
Rockwell R hardness |
99 |
88 |
82 |
85 |
887 |
Heat deflection temperature (66 psi), deg.C |
88 |
136 |
138 |
138 |
138 |
Heat deflection temperature (264 psi), deg.C |
96 |
111 |
114 |
118 |
118 |
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Mica-PP composites can be as stiff as steel sheet, but weight only 45% as such. Mica filled PP products are considered inexpensive substitute for glass fibre filled PP (Table 3b). An interesting use of mica is in PP-PE coopolymer foam for loud speakers and musical instruments due to excellent acoustic properties of mica mineral. The higher speed of sound in mica allows for a more compact speaker cone. |
Table 3 b : Mechanical Properties of Mica & Glass - PP |
Property
|
Unfilled
|
20%
|
40%
|
Tensile strength,
MPa |
34
|
42
|
43
|
Flexural modulus,
MPa |
1311
|
6417
|
7176
|
Izod impact,
J/m
Notched at 220 deg. C
Unnotched at 22 deg. C |
24
No Break |
42
501 |
35
235 |
heat deflection
temperature
deg. C at 264 psi |
58
|
125
|
108
|
Mould shrinkage,
% lengthwise |
2.0
|
0.3
|
0.8
|
|
Glass -Fibre Reinforced PP
Glass fibre reinforced PP is a high growth segment of the market. These products have high tensile strength and HDT. Applications include those areas requiring the chemical resistance of PP with the strength of engineering resins. A drawback of these materials had been their tendency to distort in the final product. Recent advances have resulted in easy-flow grades that significantly reduce this tendency to warp. Other developments include the production of higher impact grades. These improvements have opened new applications in the appliance and industrial markets.
Typical fibre lengths of glass are1/8 to 3/16 inch, although longer fibres are available for specialty applications. The standard glass diameter for PP applications is 13 microns. The factors influencing properties are the base resin, the starting glass-fibre geometry, the compounding and processing techniques, the presence or absence and type of a coupling agent. Table (4) lists typical properties of glass filled PP. Table (5) illustrates the effect of polymer type and melt flow rate of physical properties using standard 3/16 inch starting fibre length and a coupling agent. As can be seen the property balances available compare favourably with many other engineering materials.
Table 4 : Typical Product Properties of Glass filled-PP
Property |
Units |
Base - PP
Homo
polymer |
20% glass filled PP |
30% glass filled PP |
40% glass filled PP |
Density |
g/cc |
0.90 |
1.06-1.08 |
1.15-1.17 |
1.19-1.21 |
Tensile strength at yield |
MPa |
35 |
35.5 |
36.0 |
37.0 |
Tensile strength at break |
MPa |
23 |
32.5 |
35.0 |
36.0 |
Elongation at break |
% |
60 |
30 |
20 |
30 |
Flexural strength |
MPa |
330 |
440 |
450 |
450 |
HDT 66 psi |
deg.C. |
75 |
90 |
97 |
112 |
Table 5 : Typical Product Properties of Glass Reinforced, Coupled Material
Property |
Homopolymer |
Copolymer |
20% |
40% |
20% |
40% |
20% |
40% |
|
Melt flow rate, condition L, g/10 min. |
3 |
2 |
18 |
12 |
1.8 |
1.5 |
Density, g/cc |
1.04 |
1.22 |
1.04 |
1.22 |
1.05 |
1.14 |
Tensile Strength (yield) MPa |
83 |
103 |
77 |
99 |
61 |
86 |
Flexural modulus (1% secant), MPa |
4209 |
6831 |
4071 |
7590 |
3450 |
4830 |
Heat Deflection temperature (66 psi ) deg. C. |
141 |
144 |
144 |
145 |
141 |
143 |
Heat deflection temperature |
234 |
136 |
134 |
137 |
141 |
143 |
Notched Izod (23 deg. C) J/m |
85 |
112 |
75 |
96 |
149 |
192 |
Applications of glass filled PP are in fan blades, head lamp housing, chemical process equipment, washing machine tanks etc.
Table (6) demonstrates the effect of the type of coupling agent on PP. As can be seen, the coupling agent provides a considerable increase in tensile strength. In addition, other properties, such as creep resistance are also improved. The mechanism of the coupling agent is to form a bond between the sizing agent on the glass fibre and the specially treated PP resin.
Table- 6 : Effect Coupling Agent on a 30% Glass-Reinforced PP
Property |
No. of Coupling Agent |
Coupling Agent A |
Coupling Agent B |
Melt flow rate, condition L, g/10 min. |
15 |
12 |
12 |
tensile Strength (yield)MPa |
71 |
88 |
100 |
Yield Elongation (%) |
1.8 |
2.1 |
2..9 |
Flexural modulus(1% secant), MPa |
6141 |
6279 |
6348 |
Heat deflection temperature (264 psi) deg. C. |
143 |
146 |
146 |
Notched Izod (23deg. C) J/m |
69 |
85 |
96 |
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Table (7) shows a comparison of properties of filled PP homopolymer
Property |
Units |
Base PP Homopolymer |
20% CaCO3 filled |
20% talc filled |
20% glass filled |
20% glass reinforced |
MFI |
g/10 min. |
4 |
3 |
3.5 |
3.5 |
3.5 |
Density |
g/cc |
0.9 |
1.06-1.08 |
1.06-1.08 |
1.06-1.08 |
1.06-1.08 |
Mould Shrinkage |
% |
2.1-2.3 |
1.6-1.8 |
1.5-1.7 |
0.8-1.0 |
0.8-1.0 |
Tensile strength at break |
MPa |
23 |
18 |
25 |
32.5 |
80.0 |
Elongation at break |
% |
60 |
70 |
50 |
30 |
- |
Flexural strength |
MPa |
33 |
35 |
47 |
44 |
61 |
|
deg.C |
75 |
75 |
99 |
90 |
141 |
Other fillers and Reinforcements
Many other fillers and reinforcements also can be used with PP, including wood flour, ground corn stalks and other cellulose containing substances. The cellulosic based products provide low-cost opportunities for achieving high stiffness, but their applications is limited owing to their tendency to char and generate water at processing temperatures.
For the achievement of conductivity and/or static dissipation properties, metal powders, silver coated glass spheres, metal wires and conductive carbons have been used. TO reduce the coefficient of friction and to improve wear characteristics. TeflonR and Silicone are used. These systems are highly specialized and designed for particular applications.
Colour Systems
The production of precoloured PP can be accomplished through the use of previously selected pigment systems that have been distributed uniformly in the polymer. For successful colour matching, the viscosity of the resin, the ease of pigment dispersion, the compounding equipment used and the selection of dispersing aids must be considered. In order to ensure adequate dispersion, pigment systems are often predispersed in a masterbatch. When utilizing a masterbatch, the carrier must be compatible with PP matrix.
In addition, the possible nucleating effect of certain pigments, heat stability of pigments which could affect processing and physical properties of PP respectively must also be considered.
Flame Retardant PP
Materials used as flame retardants can be broadly classified as inorganic fillers and organic compounds. However, for making polypropylene flame retardant most of the conventional inorganic fillers cannot be used. This is because of high processing temperatures of polypropylene. At times, to achieve desired level of flame retardance substantial quantities of inorganic fillers are required to be used. This affects the properties of end product drastically. Hence, polypropylene is made flame retardant by using organic flame retardants.
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