# Effect of the interdiffusion at the polymer/polymer interface on the flexural properties of over-moulded short glass fibre/glass fabric reinforced PA6 composites

Eric Lafranche, Thierry Renault, Patricia Krawczak

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Eric Lafranche, Thierry Renault, Patricia Krawczak. Effect of the interdiffusion at the polymer/polymer interface on the flexural properties of over-moulded short glass fibre/glass fabric reinforced PA6 composites. 17th International ESAFORM Conference on Material Forming – ESAFORM 2014, May 2014, Espoo, Finland. pp.821-828, 10.4028/www.scientific.net/KEM.611-612.821. hal-01782064

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# Effect of the interdiffusion at the polymer/polymer interface on the flexural properties of over-moulded short glass fibre/glass fabric reinforced PA6 composites

Eric Lafranche¹,ᵃ*, Thierry Renault ²,ᵇ, Patricia Krawczak³,ᶜ

¹,³ Department of Polymers and Composites Technology & Mechanical Engineering, École des Mines de Douai, France

² Faurecia R&D center, Etampes, France

ᵃ* eric.lafranche@mines-douai.fr, ᵇ thierry.renault@faurecia.com,

ᶜ paticia.karwczak@mines-douai.fr

**Keywords:** Injection over-moulding, polyamide6, continuous glass fibre composite, molecular diffusion, mechanical properties,

**Abstract.** The injection over-moulding of 30wt% short glass fibre reinforced PA6 (SGF from Solvay Engineering Plastics) onto consolidated unbalanced (87/13) 70 wt% glass fabric reinforced PA6 (Continuous Fibre Reinforced Thermoplastic (CFRT) from Solvay Engineering Plastics) was investigated with an objective of optimising the flexural and interlaminar shearing of the consolidated structure. Among the processing parameters, the temperature of the fabric before injection and the over-moulded melt temperature associated to the mould temperature (cooling rate of the part) were revealed as the main parameters governing the mechanical properties of the part. Moreover, the flexural modulus and the apparent interlaminar shear strength fall down critically in the main direction (chain direction of the fabric) at a CFRT temperature of 150°C. The effect of the SGF/CFRT interface was quantified in term of quadratic distance of diffusion through the interface. First, the 1D cooling of the complex was simulated according to the heat transfer module of COMSOL Multiphysics® in order to determine the variation of the temperature field during the cooling stage. The calculations were achieved with an initial CFRT temperature of 23, 100, 150 and 200°C, the mould and SGF melt temperatures were kept constant. The diffusion theory has then been applied to calculate the variation of the auto-diffusion coefficient through the thickness during the complex cooling, the diffusion is supposed occurring only at a temperature above the PA6 crystallisation temperature (185°C). The calculation of the quadratic distance of diffusion through the thickness confirmed the mechanical results. Under a CFRT temperature of 150°C, the ability to the molecular diffusion at the interface becomes non-existent. The melt temperature of the SGF PA6 should be sufficient to melt the CFRT PA6 interface, the time of diffusion directed by both the CFRT and mould temperatures (cooling rate) has to be long enough to allow the molecular diffusion from one material to the other.

## Introduction

Because of their high specific properties and their high potentiality as light material compared to conventional materials, composite materials have ever growing application fields. In particular for high volume production of reinforced plastic parts, two main composite material families are now widely used: on the one hand, thermoset composites such as Sheet Moulding Compound (SMC) or Bulk Moulding compound (BMC), and on the other hand thermoplastic composites such as GMT (Glass Mat reinforced Thermoplastics) and chopped fibre reinforced pellets (short or long fibres). If chopped glass fibre reinforcement parts give the same specific strength as metal, their specific moduli does not reach the same levels. Continuous glass fibre composites associate high specific strength and high specific modulus but they can not be obtained in complex geometries and they can present a lack of ductility in crash conditions. During the recent years, the associations between continuous fibre reinforced thermoplastics (CFRT) and chopped fibre reinforced thermoplastics
have been investigated in order to reduce this limitation in structural crash applications and thus to
compete the metal in the automotive industry [1,2].

The first industrial investigations have been realized by BASF and Faurecia [2] for the weight reduction of the back seat and by BASF for seat panel of the Opel Astra. The polyamide light continuous glass fibre insert composed both of unidirectional polyamide impregnated rovings (UD tapes) and fabric reinforced prepregs were thermoformed to three-dimensional shapes [2,3]. An impact modified short glass fibre reinforced polyamide was the over-moulding to achieve the complex geometries of the parts.

In the association of two or more materials, the part performances depends on the own
properties of each material associated as well as their interface and/or interphase properties induced
during by the manufacturing process (thermal or chemical assembly...). In the case of the over-
moulding of two compatible thermoplastics, the assembly is mostly considered as chemical linking
depending of the macromolecular chemical nature of the two polymers and if there is the presence
of surface re-melting, the linking will be considered as a welding depending of the contact time, the
surface wettability and chain diffusions [4,5]

In most cases, more than one mechanism contributes to the adhesion of one polymer onto the
other. The mechanical adhesion model adapted for rough or porous surfaces [6]. According to this
model, the wettability between the two polymers is the preponderant factor to the adhesion. The
electrical theory developed by Deryagin where the substrate and the adhesive are compared to a
condenser created by the electrical loads at the surfaces increasing the energy of adhesion [6]. The
thermodynamic model, widely used today, where the adhesion results from the interatomic and
intermolecular forces at the interface, the amplitude of these forces can be connected to the
fundamental thermodynamic quantities as surface free energies of the two polymers [7] . The
chemical link theory where the chemical links are considered as primary links in opposition to
interactions (Van der Waals), the adhesion depends on the amount of chemical links at the surface
[7].The interfacial layer of low cohesion where the formation of an interfacial layer (interphase)
with their own properties or gradient of properties different from those of the two polymers is
considered [7]. According to this theory, the energy of adhesion is always equal to the cohesive
energy of the lowest cohesion layer. Finally, the adhesion by the diffusion theory is characterized
by the macromolecular chain entanglement. The chain entanglement is all the more favoured than
the two polymers are compatibles. This theory is often applied to the uncross-linked or low
crystalline polymers and above the glass transition temperature [8].

For the multilayer extrusion process, the mechanisms of adhesion between the layers of compatible and incompatible polymers have been widely studied for the common polymers used. The rheological tools coupled with mechanical analysis permits to understand the role of the interphase created by both the interfacial diffusion and chemical reactions for incompatible polymers [9,10]. The authors studies in particular the role of molecular weight and polydispersity index of the polymers on the diffusion process coupled to the effects of the temperature and time on similar amorphous polymer bilayer structures [11,12].

Regarding the over-moulding of two compatible polymers, the total melting of the surface is
necessary to the development of the interdiffusion mechanism [13]. A good adhesion is normally
obtained when the molecules of the two compatible polymers diffuse between them to create
molecular entanglement at the interface [14].

The aim of the present work was to investigate the injection over-moulding 30wt% short glass fibre reinforced PA6 onto consolidated unbalanced glass fabric reinforced PA6 with the objective to optimise the flexural and interlaminar shearing of the complex. The quality of the adhesion is approached in terms of interdiffusion at the interface.
# Experimental

## Material and processing conditions

The Continuous Fibre Reinforced Thermoplastic was an unbalanced (87/13) 70 wt% glass fabric reinforced polyamide 6 (CFRT from Solvay Engineering Plastics), the over-moulded material was a 30 wt% short glass fibre reinforced polyamide 6 (SGF) with high impact resistance (Technyl® PA6 from Solvay Engineering Plastics).

The over-moulding of a 300x120x3 mm plate part feeds at its centre by a direct sprue gate was processed on a 350 ton clamping force injection moulding machine (KM 350-1900, Krauss-Maffei, Germany). The optimum moulding conditions were previously determined by Tagushi DOE to achieve the highest flexural strength and the highest interlaminate shear stress. The processing conditions are reported in Table 1. The CFRT was previously heated in an Infrared oven (Matherm, France) at the desired temperature.

<table><caption>Table 1. Moulding conditions for the over-moulded PA6</caption><thead><tr><td>Processing parameters</td><td>Set up value</td></tr></thead><tbody><tr><td>Mould temperature (°C)</td><td>85</td></tr><tr><td>Melt temperature (°C)</td><td>250</td></tr><tr><td>Volume flow rate (cm³.s⁻¹)</td><td>385</td></tr><tr><td>Holding pressure (Bar)</td><td>650</td></tr><tr><td>Screw rotation speed (cm.s⁻¹)</td><td>25</td></tr></tbody></table>

## Rheological characterizations

The master curves at 240°C of polyamides 6 (CFRT and SGF matrix) were carried out by oscillatory shear rheology at 240°C, 250°C, 260°C and 270°C under nitrogen atmosphere on compression-moulded circular samples (40 mm diameter, 2 mm thick) on a rotational rheometer (ARES, Rheometric Scientific, USA) with parallel plates. Linear domains of the materials were identified from strain sweeps and a common strain of 1% was then selected for the different samples whatever the testing temperature. Frequency sweeps were run from 1.10⁻⁴ to 100 rad.s⁻¹ in order to record the storage (elastic) modulus G', the loss modulus G'' and the complex viscosity η *. The master curves are presented in figure 1.

## Mechanical testing

Bending test leading to the determination of the flexural properties (strength, defined as maximum stress, and modulus) were performed according to ISO 178 on a standard tensile machine (Model 1185 Instron, England) on six samples (dimensions 60 x 25 x 3 mm) in both longitudinal and transverse directions (Figure 2 a and b).

The apparent interlaminar shear strengths (ILSS) were performed according to ISO 14130 on a standard tensile machine (Model 1185 Instron, England) on sixteen samples (dimensions 30 x 15 x 3 mm) in both longitudinal and transverse directions (Figure 2 c and d).

The samples were first dried at 60°C during 72 hours and stored during 168 hours at 23°C, 50% relative humidity before testing to reach 0.4% RH.

## Morphological characterizations

Optical morphology analysis was performed by a polarizing microscope in reflection mode (Jenapol, CarlZeiss Jena) associated to a CCD camera coupled to a computer to get and record
suitable images. These images were then analysed by means of an image processing software package (Visilog® 6.9, Noesis). Morphological analysis was performed on cryo-fractured surfaces of injection-moulded samples. A thin layer of gold was sputter deposited onto the sample. Imaging of the samples was carried out under high vacuum with a Scanning Electron Microscope (S-4300SE/N, Hitachi, Japan) operating at 10 kV.

<figure><img src="image_2.png" /><figcaption>Fig. 1: Master curves of modulus and complex viscosity versus frequency at 240°C:</figcaption></figure>

a) Short glass fibre matrix, b) CFRT matrix

<figure><img src="image_3.png" /><figcaption>Fig. 2: Location of test samples</figcaption></figure>

## Results and discussion

### Mechanical results

Above the melt temperature of the CFRT PA6 matrix, the melt impregnation of the CFRT by the over-moulded short glass fibre PA6 matrix is obtained and the mechanical properties are at their highest levels (Figures 3a, 3b, 3c). A manufacture industrial process implies an over-moulding onto a complex shape performs, in consequence at a temperature below the melting or crystallization temperature, at least out of the interfacial regions. Beyond a CFRT temperature of 150°C, the mechanical properties of the parts are preserved (Figures 3a, 3b, 3c) with a more and less performance level of a fully impregnated insert. According to Williams Landel Ferry when the temperature is placed between the glass temperature (Tg) and Tg+100°C (152°C in the present case), the viscosity depends more on configurationally variations connected to the thermal activation than of the chemical nature or the structure of the polymer. It is thus in this interval that the phenomenon or interdiffusion or self-diffusion will be the most sensitive in the temperature.
<figure><img src="image_4.png" /><figcaption>Fig. 3: Influence of CFRT temperature on flexural modulus (a), flexural strength (b) and apparent interlaminar shear strengths (ILSS)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4: Influence of CFRT temperature on the SGF/CFRT interface: 100°C (a), 150°C (b)</figcaption></figure>

## Morphological Analysis

Morphological analyses have been carried out both by using optical microscopy on polished samples and by SEM on cryo-fractured samples. The optical microscopy observations presented on superimposition of the figure 3 show a disappearance of the interface parting line above a CFRT
temperature of 150°C. The SEM pictures (Figure 4) confirm the best adhesion with no particular transition between the two structures (injected SGF/CFRT) at high CFRT temperature.

### Calculation of the molecular interdiffusion at the SGF/CFRT interface

On the base of the works of G. de Gennes [15] and Edwards [16], Graesseley [17] proposed a self-diffusion coefficient as a function of viscoelastic parameters of the polymer:

$$D = \frac{G_N^0}{135} \left( \frac{\rho RT}{G_N^0} \right)^2 \left( \frac{R_g^2}{M_w} \right) \frac{M_c(T)}{M_w^2 \eta_{0,M_c}(T)} \quad (1)$$

Where $G_N^0$ is the plateau modulus, $\rho$ is the polymer density, $R$ is the virtual gas constant, $T$ is the absolute temperature, $Rg^2$ is the mean square end to end distance of the chain, $M_w$ is the molecular weight, $M_c(T)$ is the critical molecular weight at the temperature $T$, $\eta_{0,M_c}(T)$ is the viscosity at the critical molecular weight at the temperature $T$.

Generally the plateau modulus is obtained from dynamic rheology measurement. The storage modulus is noticed at the frequency where $\tan\delta = G'/G'$ reaches a minimum. For the crystalline polymers which crystallize below their melting point, the plateau modulus cannot be measured. Wu & al [8], purposed to determine $G_N^0$ with the following equation:

$$\log\left(\frac{G_N^0}{G_C}\right) = 0.380 + \frac{2.63(\log p)}{1 + 2.45(\log p)} \quad (2)$$

Where $G_c$ is the modulus obtained at the frequency where $G'=G''=G_c$ and $p$ is the polydispersity index of the crystalline polymer.

The critical molecular weight $M_c$ is linked to the molecular weight of entanglements $M_e$ by the following equation [18]:

$$M_c(T) = 2M_e(T) \quad \text{and} \quad M_e(T) = \frac{\rho RT}{G_N^0} \quad (3), (4) [19]$$

The mean end to end distance of chain is fixed to a common value found in the literature at $Rg=10^{-6}m$ [17].

The table 2 presents the data and the self-diffusion calculation of the Technyl® polyamide 6 at 240°C, as comparison Wu & al [17] established a $G_N^0$ value of 1.8 $10^6$ Pa for a PA6 [17]

<table><caption>Table 2: Data and self-diffusion calculation</caption><tr><td>p</td><td>2.38</td><td>Me(T) (g/mol)</td><td>3.72E+03</td></tr><tr><td>G<sub>c</sub> (Pa)</td><td>1.06E+05</td><td>Mc(T) (g/mol)</td><td>7.44E+03</td></tr><tr><td>G<sub>N</sub><sup>0</sup></td><td>1.33E+06</td><td>η<sub>0</sub>(T) (Pa.s)</td><td>950</td></tr><tr><td>ρ (g/m<sup>3</sup>)</td><td>1.14E+06</td><td>η<sub>0</sub>,Mc(T) (Pa.s)</td><td>294.239619</td></tr><tr><td>R (J/mol.K)</td><td>8.314</td><td>Rg (m)</td><td>1.00E-08</td></tr><tr><td>T (K)</td><td>523</td><td>Mw (g/mol)</td><td>10500</td></tr><tr><td colspan="4">D (m<sup>2</sup>/s) = 9.24E-17</td></tr></table>
The self-diffusion coefficient depends on the thermo-dependant factors as the critical molecular weight or the Newtonian viscosity. Coupling the calculation with the temperature variation during the cooling stage of the part, it is then possible to access to the quadratic distance of diffusion through the plate thickness. With this consideration, the 1D cooling stage calculation has been realized with COMSOL Multiphysic® software according to the schematic representation in the figure 5.

<figure><img src="image_6.png" /><figcaption>Fig.5: Schematic representation of the over-moulded cooled system</figcaption></figure>

By considering that the self-diffusion can occur only when the temperature is superior to the temperature of crystallization of the PA6 (185°C), the diffusion time can be calculated by the equation:

$$T_z(t_{\text{diff}}^z) = T_c \qquad (5)$$

Where: $T_z$: temperature to median plan

$t_{\text{diff}}^z$: diffusion time to median plan

$T_c$: temperature of crystallization

The quadratic distance of diffusion can be then calculated from the integration of the curves:

$$\langle l^2 \rangle_z = 2 \cdot \int_0^{t_{\text{diff}}^z} D(t) dt \qquad (6)$$

The self-diffusion can occur only at a quadratic distance of diffusion superior to the mean square end to end distance $Rg^2=10^{16}m$. Under a CFRT temperature of 150°C, the probability of self diffusion at the interfaces is non-existent (Figure 6). The calculated values are in accordance with the mechanical measurements.

<figure><img src="image_7.png" /><figcaption>Fig.6: Quadratic distance of diffusion through the thickness in function of CFRT temperature</figcaption></figure>

## Conclusion

The over-moulding 30wt% short glass fibre reinforced PA6 onto consolidated unbalanced glass fabric reinforced PA6 have shown:

* A decrease of the flexural properties and the apparent interlaminar shear strengths at a CFRT temperature below 150°C due to a bad interfacial adhesion revealed by microscopy.
* This temperature limit of the PA6 corresponding to Tg+100°C is the temperature where the phenomenon of self-diffusion or interdiffusion can occur.

* The mechanical experimental measurements have been explained in terms of quadratic distance of diffusion through the thickness. The probability of self diffusion at the interfaces becomes non-existent at a temperature below 150°C

## Acknowledgment

This work was carried out in the frame of the “LYCOS” project dedicated to the automotive seat lightweigth. The authors are indebted to the French Ministry of Economy, Finance and Industry, DGCIS Department, for its financial support (Contract FUI AAP 10 F1103075Q). Thanks are also Faurecia (France), leader of the Lycos project. The authors also gratefully acknowledge the support and contribution of CISIT (International Campus on Safety and Intermodality in Transportation), the Nord-Pas-de-Calais Region and the European Community (FEDER, European Funds for Regional Development) for funding of the equipments.

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