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Talc in Plastics
Technical Bulletin 1301
CONTENTS
INTRODUCTION
2
BENEFITS OF TALC IN POLYPROPYLENE COMPOUNDS
3
1. STIFFNESS (E-MODULUS)
3
2. THERMAL CONDUCTIVITY
3
3. NUCLEATION
3
4. IMPACT STRENGTH
4
5. DEFLECTION TEMPERATURE
4
6. CREEP RESISTANCE
5
7. BARRIER PROPERTIES
6
8. CHEMICAL RESISTANCE
7
NEW MARKETS FOR TALC-FILLED POLYMERS
8
INTRODUCTION
Pure talc, the softest of all minerals with a Mohs hardness of 1, is an organophilic, water repellent
and chemically inert mineral. It is characterised as a hydrated magnesium sheet silicate with the
formula Mg3 Si4O10 (OH)2. Talc consists of a layer or sheet of brucite (Mg(OH)2) sandwiched between
two sheets of silica (SiO2) (see Figure 1).
Weak Van der Waal’s forces bond the crystal lattice of talc.
Thus, talc undergoes cleavage very readily, is very soft and
has a soapy feel.
Talc is usually lamellar (platy), but the aspect ratio can vary
considerably. Its high aspect ratio is the most important
property for its use in plastics.
The term “talc” covers a wide range of natural products.
Impurities commonly encountered include magnesite
(magnesium carbonate), calcite, quartz and chlorite (a
mix of Mg- Al- and Fe-silicate/Mg(OH)2). Among the
­different modifications of talc, mostly pure and lamellar
talc grades are used in the plastic industry.
Talc is a functional component in paper, paints, plastics,
rubbers, ceramics, fertilizers, animal feed, cosmetics, pharma­
ceuticals and other applications.
In plastics, it is used to stiffen thermoplastics, mainly polypropylene but also polyethylene and polyamide (nylon).
Main applications are automotive parts, household appliances and engineering plastics.
Si
O
OH
Mg
Figure 1: Talc crystal structure
Technical Bulletin 1301 – Talc in Plastics
2
1. STIFFNESS (E-MODULUS)
The main reason for incorporating talc in plastics is to
increase the stiffness (E-modulus). The degree of rigidity
depends on the filling level, aspect ratio and fineness of
the talc (Figure 2).
Tensile modulus (MPa)
BENEFITS OF TALC IN POLYPROPYLENE COMPOUNDS
4000
3500
3000
High aspect ratio talc
Medium aspect ratio talc
Calcium carbonate
2500
2000
1500
1000
10
20
30
Mineral loading (wt %)
40
Figure 2: Stiffness of a PP compound with high aspect ratio talc,
a mineral with medium aspect ratio, and calcium carbonate
Because of talc’s significantly higher thermal conductivity
(compared to the polymer), the heat introduced and
generated during processing is transmitted through the
mixture more quickly (Figure 3). The heat is also transported
out of the compound faster during cooling.
Incorporating talc in a compound increases the thermal
conductivity, resulting in faster production rates. Experience with filled polymers is that conductivity depends ­
only on the filler content, within reasonable tolerances.
Thermal conductivity (W/m°K)
2. THERMAL CONDUCTIVITY
0.55
0.50
0.45
F ine medium aspect ratio talc
Medium lamellar talc
Fine lamellar talc
0.40
0.35
0.30
0.25
10
20
30
Mineral loading (wt %)
40
Figure 3: Thermal conductivity of PP compounded with talc
3. NUCLEATION
The crystallisation of polypropylene is promoted by small amounts of preferably fine talc, which acts
as a nucleating agent. Crystallisation starts at a higher temperature in the presence of talc, compared
to unfilled PP. The impact strength is improved (Figure 4) but this is primarily due to an increase in
the crystallisation of the PP and not the mechanical properties of the talc itself. There is also a change
in modulus (Figure 5) as a result of the change in crystallinity.
Nucleation of PP: Impact Strength
45
40
35
30
25
20
15
10
5
0
Nucleation of PP: E-modulus
2000
1
Loading (wt %)
Flexural modulus (N/mm2)
Charpy impact strength (kJ/m2)
0.5
Talc
Sodium benzoate
Ca-carbonate
1800
1600
1400
1200
1000
1.5
2
0.5
1
Loading (wt %)
1.5
2
Figures 4 and 5: Impact and rigidity of nucleated PP
Technical Bulletin 1301 – Talc in Plastics
3
4. IMPACT STRENGTH
2000
70
60
1900
50
40
1600
30
20
1300
10
1000
Impact strength (kJ/m2)
Flexural modulus (MPa)
Addition of mineral fillers will not generally improve impact
strength. There are exceptions, for example the use of fine
talc in PP compounds for car bumpers. In the latter case, ­
5 to 10 % of fine talc is added. Impact strength decreases
at higher loadings (Figure 6).
0
5
10
15
Talc loading (wt %)
20
Figure 6: Influence of fine talc on high impact PP
5. DEFLECTION TEMPERATURE
In many applications such as in plastic parts for cars or packaging, rigidity is
­required at elevated temperatures. The heat distortion temperature (HDT) can
be used to demonstrate how a mineral influences the stiffness of a plastic
compound at elevated temperatures. Lamellar talc with high aspect ratio improves the deflection temperature of polyolefins to a greater extent than talc
with a lower aspect ratio (Figure 7).
Deflection temperature (°C)
160
140
140
120
121
100
80
109
97
60
40
20
0
PP
PP + 20 %
MEDIUM TALC I
PP + 20 %
MEDIUM TALC II
PP + 40 %
MEDIUM TALC II
Figure 7: Deflection temperature of compounds with medium aspect ratio talc (I), high aspect ratio talc (II) and unfilled PP
Technical Bulletin 1301 – Talc in Plastics
4
6. CREEP RESISTANCE
Substantial reduction of creep is achieved with filled polymers in comparison to unfilled ones. Best results in our creep
tests were obtained with fine platy talc. Various fillers and
filler combinations reduced creep as follows:
High aspect ratio talc >
medium aspect ratio talc >
blend of talc and carbonate >
calcium carbonate >
unfilled polypropylene (Figure 8).
Information obtained from short-term tests of PP can be
extrapolated to predict properties over a longer period of
time at a constant temperature. The conventional short-term
modulus is replaced in formulas by the creep modulus. The
creep modulus, which is important for expected service
life under load, can be calculated from creep tests. The
figure below applies to a five-year period (Figure 9). Typical
products where creep has to be taken into consideration
are buried plastic pipes (e. g. for sewage water).
1.4
1.2
Strain (%)
1
0.8
0.6
PP
PP+20 % Calcium carbonate
PP+20 % Lamellar talc
PP+40 % Medium aspect
ratio talc
PP+30 % Lamellar talc
0.4
0.2
0
2
4
Time (years)
Figure 8: Creep of PP and filled polypropylene
900
Creep modulus (N/mm2)
800
700
600
500
400
300
200
100
0
PP
PP + 20 %
PP + 20 %
CALCIUM CARBONATE TALC / CARBONATE (1:2)
PP + 20 %
TALC
PP + 30 %
TALC
Figure 9: Creep modulus (for five years)
Technical Bulletin 1301 – Talc in Plastics
5
7. BARRIER PROPERTIES
Water vapor transmission g/(m2 x 24 h)
Water vapor and oxygen transmission are important factors to control
in food packaging. They directly influence the shelf life of the food
contained inside. Talc provides the opportunity to reduce transmission
rates for water vapor (Figure 10) and oxygen (Figure 11). The lamellar
talc particles are mostly orientated in films and will constrain the
water vapor and oxygen on its way through the packaging.
0.6
0.5
0.4
0.3
0.2
0.1
0
PPH
PPH + 30 %TALC
D50=3.0μ
PPH + 30 %TALC
D50=2.1μ
PPH + 30 %
CA-CARBONATE
PPH + 30 %TALC
D50=2.1μ
PPH + 30 %
CA-CARBONATE
Oxygen transmission cm3/(m2 x 24 h)
Figure 10: Reduced water vapor transmission in polyolefin food packaging by talc
450
400
350
300
250
200
150
100
50
0
PPH
PPH + 30 %TALC
D50=3.0μ
Figure 11: Reduced oxygen transmission in polyolefin food packaging by talc
Technical Bulletin 1301 – Talc in Plastics
6
8. CHEMICAL RESISTANCE
Talc is water repellent and chemically inert. This is very important for the direct contact of mineral filled packaging
material with food-stuffs. Migration tests are done with
different simulants (distilled water, 3 % acetic acid, 10 %
ethanol and rectified olive oil).
Even with 3 % acetic acid, overall migration requirements
can be fulfilled (< 10mg/dm2 sample). (Figure 12)
EN 1186-5:
Test methods for overall migration from plastics into
aqueous food simulants by cell.
Simulant 3 % acetic acid
Test conditions 10 days, 40 °C
The overall migration limit is 10 mg/dm2
SAMPLE
OVERALL MIGRATION
mg/dm2 SAMPLE
PP homopolymer + 30 %
Ca-carbonate (EXH1 SP)
79 – 128
PP homopolymer + 30 %
Talc d50 = 3.0 μm
0.4 – 1.0
Figure 12: Overall migration of PP/Talc, simulant 3 % acetic acid
Technical Bulletin 1301 – Talc in Plastics
7
NEW MARKETS FOR TALC-FILLED POLYMERS
The automotive and domestic appliances markets are still
the dominating users of talc-filled compounds, but new
markets are being developed. Their growth depends partly
on the extent to which end-users actively seek alternative
materials to PVC and PS. Markets of interest here include
profiles, pipes and food packaging.
In replacement of PVC for plastic pipes, there is a need to
compensate for the lower ring stiffness of polyolefins, but
also to reduce undesirable long-term properties of unfilled
polypropylene and polyethylene, such as their tendency to
creep (deform under long-term strain). Talc is the preferred
additive in this application, as it imparts high stiffness, which
allows a reduction in wall thickness. Impact resistance at
sub-zero temperatures is unimpaired.
Talc-filled polypropylene is also finding new markets in
food packaging applications. Migration requirements
­according to EN 1186-5 can be met, and higher rigidity and
barrier properties (e. g. reduction of oxygen permeability)
are imparted. Talc improves output in extrusion and shortens
cycle times in thermoforming, due to crystallisation and
better heat transfer.
These benefits make talc compounds very competitive for
food packaging, so there is considerable potential in this
application.
MONDO MINERALS B.V.
Kajuitweg 8 • 1041 AR Amsterdam • The Netherlands • E-mail: [email protected]
www.mondominerals.com
The information contained in this Technical Bulletin relates only to the specific tests designated herein and does not relate to the use of our products in combination with any other material or in any
process. The information provided herein is based on technical data that Mondo Minerals believes to be reliable, however Mondo Minerals makes no representation or warranty as to the completeness
or accuracy thereof and Mondo Minerals assumes no liability resulting from its use for any claims, losses, or damages of any third party. Recipients using this information must exercise their own
judgement as to the appropriateness of its use, and it is the user’s responsibility to assess the materials suitability (including safety) for a particular purpose prior to such use.