S.C. Cifuentes a,b , F.A. López a
, R. Benavente b , J. L. González-Carrasco a,c a. National Center for Metallurgical Research (CENIM-CSIC), Madrid, Spain b. Institute of Polymer Science and Technology (ICTP-CSIC) Madrid, Spain c. Biomedical Research Networking center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Spain e-mail:
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The development of PLLA/Mg composites responds to the need of overcoming the lack of bioactivity and low mechanical properties of current bioresorbable biomaterials for medical applications. Their manufacturing will likely require a high temperature step to mould the material by using a compression, extrusion or injection process.
Given that previous works have established that metal hydroxides/oxides reduce the thermal stability of PLLA, in order to move forward the PLLA/Mg composite processing and design, the study of the effects of pure Mg particles on PLLA thermal stability and melting behaviour, as well as its effect on the mechanical properties becomes an ineludible goal.
Screw
Die
Extruded solid
PLLA
0.5 %Mg +
1 %Mg
Materials: A poly-L-lactic acid matrix is reinforced with Mg particles of 50 um.
The PLLAXMg composites are compounded by extrusion and moulded by compression.
Characterization: The effect on the melting behaviour and thermal stability of
Mg on PLLA is studied by DSC and TGA, respectively. The mechanical studies are performed under compression tests on an universal machine.
Mg
1,0
PLLA
0,5
PLLA05Mg
0,0
PLLA1Mg
-0,5
PLLA3Mg
-1,0 PLLA5Mg
-1,5
PLLA7Mg
-2,0
-2,5
50 100 150
Temperature (ºC)
Fig. 1. DSC diagrams
200
Mg has a mild effect on the cold crystallization and melting temperatures. They are shifted towards lower values as Mg content increases. A content of
7% of Mg has a more evident effect on the melting peak, where the material starts to melt near
150ºC.
Compression Moulding
T melt=190ºC
P= 20 - 30 bar f=12 mm h= 2 mm
100
80
60
PLDA
40
20
0
PLLA
PLLA05Mg
PLLA1Mg
PLLA3Mg
PLLA5Mg
PLLA7Mg
PLDA02Mg
100 200 300 400 500 600
Temperature (ºC)
Fig. 2. Thermogravimetric curves
100
1 m
90
80
180ºC
PLLA
PLLA05Mg
PLLA1Mg
PLLA3Mg
PLLA5Mg
PLLA7Mg
70
1 mm
1 10 time (h)
Fig. 3. Isothermal thermogravimetric curves
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0 y0
A1 t1
Data: Derivadas_negativdalfadt
Model: ExpGro1
Equation: y = A1*exp(x/t1) + y0
Weighting: y No weighting
Chi^2/DoF = 0.00312
R^2 = 0.99662
0.06953
0.00202
1.02506
±0.03113
±0.00175
±0.12919
-1 0 1 2 3 4 5 6 7 8
%Mg
Fig. 4. Thermal degradation rate
1 mm for commercial processing (extrusion, injection) (~ <200ºC). Figs. 3 and 4 make more evident the effect of Mg on the thermal degradation of PLLA as, in isothermic conditions, PLLA thermal degradation rate has an exponential growth as the Mg content increases.
1 mm
600
500
400
300
200
100
0
PLDA1Mg
84
82
80
78
76
74
0 1 2 3 4 5 6 7 8
72
94
92
90
88
86
%Mg
Fig. 5 shows the effect of Mg on the mechanical properties under compression. There are the reinforcement effect as well as the thermal degradation effect. Mg improves the mechanical properties of the composite when the reinforcement effect is greater than the thermal degradation effect, this occurs until 5% of Mg content. When the material has a 7% of Mg the mechanical properties drop dramatically due to the higger effect of thermal degradation
Mg does not compromise the processability of the material when proper parameters for reinforcement are selected.
There is a specific volume fraction of Mg where the highest mechanical properties are achieved.
PLLA/Mg composites with improved mechanical properties can be manufactured by plastic s processing technologies.
Special thanks to: Project MAT2012-37736-C05-01, Spanish National Research
Council - CSIC and European Social Fund (Fondo Social Europeo) for JAE-I3P
Grant. CIBER-BBN is supported by the Ministerio de Salud Carlos III