Joint mix on the line...

March - April 2003

Much is understood about polymeric materials but what happens where they are joined? The weld line properties of polymeric mixtures come under scrutiny.

Sam Rostami is an experienced scientist with interests in physical chemistry of polymers, polymer interfaces and adhesion. He worked in ICI Advanced Materials and Acrylics R&D Departments for fifteen years. He gained experience in fundamental research, new product development and characterisation of multi-component materials. He is co-editor and author of a book on 'Multi-component Polymeric Systems', author of five chapters in technical books, over fifty technical papers and seven patents. He joined TWI in 2001.

In polymers, the weld line strength is generally perceived to be weaker than the bulk. In particular, it is necessary to reduce or eliminate the effect of weld line on polymers that are used in engineering applications. Generally speaking the molecular weight, molecular weight distributions, molecular architectures and orientations influence the weld line properties of polymers. As Sam Rostami reports the sensitivity to the loss in strength at the weld line increases from homopolymers to multi-component to liquid crystalline polymers. In addition, fillers can also affect the weld line performance of polymers.

In this work, the weld line behaviour of unfilled, toughened and mineral filled blends of Nylon 66 with poly (ether sulphone), PES, was investigated as a model system. The effects of material properties and processing conditions were also investigated. The blends of Nylon 66 with PES are described elsewhere. These blends are incompatible with a fine dispersion of one phase into another, (see Fig.1).

Description of the weld-line

Two types of weld lines are commonly observed in an injection moulding process; a cold weld line, where two melt fronts meet head-on and a hot weld line, where the flows go around a pin and join together again. The cold weld is also known as a static, and the hot weld as a dynamic, weld line. This report is concerned with the static weld lines only. For this purpose, single and double-gated tensile bars were used to study the weld line performance of the materials. Tensile properties were measured to quantify the effects. Figure 2 shows a schematic drawing of the double-gated tensile mould and the runner system used.

Materials

An injection moulding grade of Nylon 66 and a low viscosity grade of PES were used. Nylon 66 is a semi-crystalline polymer with a crystalline melting temperature of about 262°C, whereas PES is an amorphous polymer with a glass transition temperature of about 220°C. Table 1 contains the relevant properties of the Nylon 66 and PES homopolymers.

Table 1 Properties of Nylon 66 and PES

Experimental

Injection Moulding Machine

A BOY15S Dipronic injection-moulding machine was used. It consisted of a hydraulic clamping unit, a three heated-zone barrel and a shut-off nozzle for processing low viscosity polymer melts, such as Nylon 66. A single-gated (not producing a weld line) or double-gated (producing a weld line) mould was used. The design of the double-gated mould used did not allow sufficient venting. The trapped air inside the mould is pushed forward by the melt fronts, which could potentially cause a V-notch at the weld zone, as shown schematically in Fig.3. The mould temperature was controlled to within 2°C at 90°C, 120°C and 150°C. The injection cycle used is shown schematically in Fig.4. A total cycle of 36 seconds, an injection pressure of 1270 bar, holding pressure of 800 bar and injection speed of 50 mm/s were used. These conditions were established in a prior study. In addition, to obtain an idea about the actual melt temperature, velocity and the shear rate profile across the weld line a computer simulation study was undertaken. For example, from the simulated temperature profile, as shown in Fig.5, a skin-core structure with the skin thickness of 90 microns is predicted for Nylon 66 at a processing temperature of 300°C and mould temperature of 90°C.

In addition, the simulation shows the melt velocity to be higher and the shear rate to be lower in the centre of the melt front.

Tensile property measurements

All injection moulded specimens were stored in a dry condition at the room temperature for five days prior to tensile tests. The tensile measurements were performed according to BS2782. At least five specimens were tested in eachcase.

Results

Hydrolytic degradation of Nylon 66

Nylons are susceptible to hydrolytic degradation, particularly in their molten state and when their moisture content is high. However, when their moisture content is below an 'equilibrium' value, they undergo further polymerisation.The level of moisture content in Nylon 66 governs the competition between hydrolysis and polymerisation via the reversible hydrolysis reaction shown below:

The effect of moisture on the hydrolysis or the polymerisation of Nylon 66 was monitored by measuring its moisture content prior to use. A Karl-Fischer titration method is used for this purpose. A number of trials were carried outto study the effect of moisture on the weld line properties of Nylon 66. These are described below.

Nylon 66 was moulded at set barrel temperatures of 280°C and 300°C and a mould temperature of 90°C. These temperatures were selected to correspond with the processing temperature for Nylon 66: PES blends. Nylon 66with different levels of moisture content was used. The tensile strength and percentage elongation to break obtained are reported in Tables 2 and 3 respectively. As shown, providing the moisture content of Nylon 66 is below 0.26wt% and its melt temperature is lower than 300°C, its tensile properties remain unaffected. This level of moisture content was therefore maintained throughout the subsequent work.

Table 2 Tensile strength of Nylon 66 with and without weld lines

*Retained strength is the ratio between tensile strength of the sample with weld line to corresponding sample without.

Table 3 Percentage elongation to break of Nylon 66 with or without the weld lines

*Retained strength is the ratio between tensile strength of the sample with weld line to corresponding sample without.

Weld lines in Nylon 66:PES Blends

Three different blends of Nylon 66 with PES were prepared and injection moulded using either a single-gated mould (without weld line) or double-gated mould (with the weld line). A set melt temperature of 300°C and mould temperatures of 90°C, 120°C and 150°C were used. The tensile strength and percentage elongation at break were measured as described before. The results obtained are shown in Tables 4 and 5.

Table 4 Tensile strength of Nylon 66:PES blends with and without weld line

Table 5 Percentage elongation to break for Nylon 66:PES blends with and without weld line

As shown, the tensile strength of 70:30 blend increases with increasing mould temperature due to an enhanced melt flow at higher temperatures. However, the tensile strength of the samples with the weld line decreases, presumably dueto presence of compressed air at the weld region in the unvented mould used.

For the 30:70 blend, where the high viscosity PES becomes the major phase, only around 40 to 44% of the original properties were retained. This is due to the reduction in melt flow and interfacial diffusion in the weld region. A comparison between mould filling of 70:30 and 30:70 blends at a mould temperature of 150°C is shown in Fig.6. As shown in this figure, the high viscosity blend travels less, under the same moulding condition. Even when the two melt fronts meet, the interfacial diffusion is expected to be lower for the high viscosity blend. The inverse relationship between melt viscosity, η, and the diffusion coefficient,

, is shown below:

In this equation, G⁰, <R _e >² and ^τ rep, represent three molecular characteristics of the polymer chain, namely plateau modulus, average end-to-end distance and the longest reptation relaxation time. For example, at a temperature of 290°C and a shear rate of 1000 s ^-1, the PES viscosity is 30 times higher than Nylon 66. Assuming a similar value for the product of G⁰ <R _e >² for both polymers, the diffusion rate becomes about 30 times slower for PES compared with Nylon 66. The theoretical weld time, _Tweld is expected to increase as the chain diffusion decreases since;

However, the diffusion coefficients of the blends are different, but related, to that of parent homo-polymers by, eg:

D _blend =(Φ _PES /D _PES +Φ _Nylon66 /D _Nylon66 )(δΔμ _Nylon66 /δΦ _PES )(Φ _PES Φ _Nylon66 )/(RT)

δΔμ _Nylon66 /δΦ _PES

is the change in chemical potential of Nylon 66 and Φ _PES the volume fraction of PES in the mixtures. A rough estimation of the diffusion coefficient from this equation shows that the 70:30 w% blend of Nylon 66:PES should diffuse twice as fast as the 30:70 w% blend.

Furthermore, when the two melt fronts meet, the slow diffusion (or high viscosity) combined with the presence of entrapped gaseous products accumulated at the melt fronts, can create a weakness in the form of V-notches at the welded region. Table 6 shows the measured depth and width of V-notches for the samples in Tables 3 and 4 using a 'Stylus Surface Profile' apparatus.

Table 6 The depth and width of V-notch of the Nylon 66:PES blends at the weld line

Effect of nylon toughening agent on the weld lines

Maleic anhydride functionalised ethylene propylene copolymer (MEP) is an effective toughening agent for Nylon 66. It is particularly suitable for amine-ended polymers as the amine groups react rapidly with the anhydride functional groups to enhance the compatibility between Nylon 66 and MEP. The adverse side of this reaction is the increase in the melt viscosity of Nylon 66, which affects its weld line properties. For example, 5wt% MEP was added to a 70:30 Nylon 66: PES blend. Lower weld line strengths, as expected from the higher viscosity blends, were obtained, as shown in Table 7.

Table 7 Tensile strength of toughened blends with and without weld line

Effect of mineral fillers on the weld lines

A 70:30 blend of Nylon: PES was prepared as before with either 20wt% talc or glass fibres added to it. The effects of the talc and glass fibre on the weld line strength of the sample were measured as before. The results showed a systematic loss of the weld line strength with the addition of talc or glass fibres, see Table 8. The elongation to break was similarly affected, Table 9. The presence of the talc or glass fibre in the weld line region is clearly responsible for the loss of strength and elongation to break. The orientation of the glass fibres parallel to the weld line further reduces the weld strength.

Table 8. Tensile strength of mineral filled blend with and without weld line

Table 9 Percentage elongation to break of mineral filled with and without weld line

Conclusions

The presence of a high viscosity polymeric second phase and mineral fillers reduces the weld line properties of polymers. Higher viscosity reduces the inter-diffusion of the two melt fronts causing a loss of weld line strength. As the melt diffusion reduces, the required weld time increases. Higher melt and mould temperatures should improve the weld line strength. However, this was not observed in our case probably due to insufficient venting at the weld region. These results highlight the importance of venting at the weld zone. In special cases, moulds with an overflow cavity, as shown in Fig.7, can be used to minimise the effect of the weld line.