Foam extrusion of PP-based wood plastic composites with chemical blowing agents and the Celuka technique

Abstract

Wood plastic composites have gained relevance in recent years as an alternative to wood boards. However, because the cavities in wood fibres are compressed by high processing pressure during the extrusion of wood plastic composites, the product densities show a range of up to 1.5 g/cm³ depending on wood content and base material. Particularly in large-sized products, this may be disadvantageous for processors and end users. Foaming of the plastic matrix is a promising approach to reduce the density of wood plastic composites products. This article discusses the foam extrusion of PP-based wood plastic composites with chemical blowing agents in combination with the Celuka technique. Integral wood plastic composites foam with a rigid and plain outer layer was produced using a parallel, counter rotating twin screw extruder. The profiles obtained were analysed with respect to foam structure and mechanical properties. It was possible to achieve a density reduction of up to 0.7 g/cm³ in the foamed wood plastic composites profiles. Furthermore, we demonstrate that wood fibre length and type of chemical blowing agent have a strong effect on the resulting foam morphology.

Keywords

Wood plastic composites chemical blowing agent foam extrusion Celuka technique

Introduction

Because of their greater sustainability and environmental compatibility as well as the option of more resource-efficient use of fossil fuels, wood plastic composites (WPC) have recorded significant growth for a number of years by comparison with pure plastics. In many applications, WPC products can replace plastic and pure wood items, as defined property profiles are set with the wood fibres used. Particularly in terms of WPC quality, there have been significant developments in recent years.^1–3

Both injection moulding and extrusion are suited for the production of moulded components in WPC. The latter procedure particularly has become commercially popular for WPC, as the material used is highly viscous due to the wood fibres used as a filler or reinforcing material.^4,5 However, in the extrusion of WPC profiles, the hollows in the wood fibres are compressed by the high processing pressures, such that densities of up to 1.5 g/cm³ are obtained on the product, depending on wood content and matrix material.^4–6 Particularly in case of large-size components, this is a great disadvantage for processors and end customers. Foaming of the polymer matrix with chemical blowing agents (CBA) represents a promising approach here.

Initial investigations showed that the foaming of WPC in the extrusion process is basically possible.^6–18 WPC foams are generally produced in two stages. In the first stage, a WPC compound is prepared, which is then foamed by the addition of blowing agents in a single or twin screw extruder.^{6–11,13,14,16–18} Chemical^{6,7,9,10,12,14,17} and physical^{6,8,11,13,16,18} blowing agents can be used here. In the work performed previously, the WPC extrudate was produced in the free foaming process.^6–18 An alternative technique that has been used virtually never before for WPC is the so-called Celuka technique, with which the molten mass begins to foam inwardly only during calibration and an integral foam is formed by immediate cooling the surface.^19,20

Experimental section

Materials

WPC formulation

The principal constituents of WPC are plastic and wood as well as process- and long-term additives. Because of the limited thermal stability of wood, only polymers with a permanent melt temperature below 200℃ in processing can be used as a matrix material for the production of WPC. To round off the application profile and guarantee good processing, a variety of additives can be added.^10,21–23

Two PP types – PP type A (MFR 2 g/10 min, Homopolymer) and type B (MFR 2.4 g/10 min, high melt strength) – have been used as the matrix material and three wood fibre types mainly comprising spruce wood with different fibre lengths – wood fibre type 1 (short), type 2 (medium) and type 3 (long) – were used for the investigations. Furthermore, each formulation includes a maleic anhydride grafted PP as the bonding agent (MAH-g-PP, 1.4% MAH content), a standard temperature stabiliser and an external stearate-based lubricant. The WPC formulations investigated are summarised in Table 1.

Table 1.

WPC formulations investigated.

Matrix material	Wood fibre	Wood fibre content (wt.%)	Bonding agent (wt.%)	Stabiliser (wt.%)	Lubricant (wt.%)
A	1	40	2	0.2	1
A	1	50	2	0.2	1
A	1	60	2	0.2	1
A	2	40	2	0.2	1
A	3	40	2	0.2	1
B	1	40	2	0.2	1

The WPC compounds were produced on a parallel, intermeshing twin screw extruder ZSK 26 Mcc from Coperion with a screw diameter of 26 mm and a length–diameter ratio of L/D = 44. The moisture content of the wood fibres was compensated by an accordingly greater content.

CBA

Three different blowing agent systems were used for foaming the WPC compounds produced: blowing agent 1 (azodicarbonamide, exothermic), blowing agent 2 (bicarbonate, endothermic) and blowing agent 3 (alkane-filled microspheres). The blowing agent content used for foaming WPC is shown in Table 2.

Table 2.

Content of blowing agents investigated.

Blowing agent	Blowing agent content (wt.%)
1	2	3	4
2	2	3
3	2	2.5	3

Foam extrusion

Experimental setup

To conduct the foaming tests, an intermeshing, counter rotating twin screw extruder DS 7.22 from Weber Maschinenfabrik GmbH with a screw diameter of 70 mm and a length–diameter ratio of L/D = 22 was used. To remove the residual moisture during extrusion, a vacuum degassing system was also connected. A calibration and cooling line, a caterpillar drive and a saw were used as subsequent equipment (Figure 1). The material was fed into the main feeding hopper as a homogenous WPC/blowing agent premix. The extruder has four temperature zones, whereas the feed zone was always tempered at 150℃. The remaining three zones were tempered mostly at 185℃. All three zones of the adapter were also tempered mostly at 185℃. The zone temperatures were varied by plus or minus 10℃. The aim was to achieve an even melt quality with a mean melt temperature of about 180℃. To investigate the effect of the machine on the foaming behaviour, the significant process parameters were varied: die temperature in the stages 190, 220 and 240℃; and screw speed in the increments 10, 12 and 14 r/min.

Celuka technique

Foaming using the Celuka technique (Figure 2) is already used successfully in PVC processing. Here, in the die, the molten mass containing the blowing agent is divided up with a mandrel, thus maintaining the melt pressure. This keeps the inert gas resulting from the blowing agent in solution. At the transition from die to calibration, the pressure drops rapidly as a result of the absence of the mandrel, which allows the gas to expand and facilitates inward foaming in the calibration. The profile surface is cooled immediately after leaving the nozzle, which results in an integral foam with a hard, smooth shell.^19,20

The Celuka die used for the test series allows the production of foamed solid rectangular profiles with the dimensions 120 × 25 mm. The dimensions of the mandrel are chosen so that 40% of the profile surface cross section was available for foaming of the WPC molten mass. Furthermore, the option to apply an additional plastic layer to the foamed WPC profile by means of coextrusion is available.

Foam analysis and mechanical testing

After reaching a stable test point, foam profiles were taken for the subsequent analyses from which the samples required for the respective tests were prepared with a circular saw.

The density was determined on the basis of DIN EN ISO 1183-1 procedure A with the aid of scales of type XS 105 from Mettler Toledo. The 10 × 10 × 10 mm³ samples were taken from the centre of the WPC foam profiles without edge layer. To determine the Charpy impact strength on the basis of DIN EN ISO 179-1, samples with the dimensions 120 × 25 × 4 mm³ were taken from the profiles and tested ‘edgewise’. To perform the test, a pendulum impact tester 5102.100/00 from Zwick/Roell AG with a 4 J impact pendulum was used. The width of the sample supports was 75 mm. The bending properties of the WPC foam profiles were determined on the basis of DIN EN 310 by means of the three-point bending test on 600 mm long profiles. The maximum breaking load and deflection at a load of 500 N were recorded here. The test was performed on a universal testing machine Z250 from Zwick/Roell. The testing speed here was 20 mm/min and the support distance was 500 mm. Figure 3 shows the tests described with the corresponding samples.

For microscopic examination of the foam structure, rupture surfaces of selected samples were coated with gold using an E5100 coating unit from Polaron Equipment then viewed with a scanning electron microscope (SEM) SUPRA™ 40VP from Carl Zeiss NTS GmbH. With the aid of computer tomography, the homogeneity of the WPC foam was examined three dimensionally. A CT scanner of type exaCT®S50HR from Wenzel Volumetrik was used here.

Results and discussion

Behaviour of different CBA

Within the scope of this work, the effect of CBA based on azodicarbonamide and bicarbonate and of blowing agents with alkane-filled microspheres on the foaming behaviour of WPC with PP type A and 40 wt.% wood fibres of type 1 was investigated.

Blowing agent 1 was added in proportions of 2 to 4 wt.% for the investigations. No foaming was achieved here with increasing blowing agent content, as the exothermic decomposition of the blowing agent started early in the extrusion process. The generated nitrogen during the exothermic decomposition of the blowing agent was diffused by the relatively porous WPC molten mass and lost for foaming at the degassing vent, which could not be closed completely (Figure 4, left). In the extrusion tests with blowing agent 2, it was also impossible to achieve foaming by varying the blowing agent content, as the endothermically formed CO₂ behaves in the same way as the nitrogen from blowing agent 1. Blowing agents 1 and 2 can therefore be considered critical for foaming of WPC by means of the Celuka technique with an extruder that is not gastight.

With blowing agent 3, inward foaming of the hollow chamber profile was possible in all proportions investigated (Figure 4, right), as the microspheres used do not release low-molecular gas but rather simply expand due to the isopentane and isooctane encapsulated in polyacrylonitrile.

Process parameters

To investigate the effect of the process parameters on the foaming behaviour, the die temperature of the Celuka die was varied in the stages 190, 220 and 240℃ at a screw speed of 12 r/min and the screw speed in the increments 10, 12 and 14 r/min at a die temperature of 220℃. To do this, the WPC with PP type A and 40 wt.% wood fibres of type 1 with 2.5 wt.% blowing agent was used. Because of the short residence time, the melt temperature was below 200℃ even at high die temperatures.

With die temperatures of 190 and 220℃, it was possible to produce completely foamed profiles (Figure 5, left), whereas the profiles produced with a die temperature of 220℃ showed the lowest medium density of 0.673 ± 0.023 g/cm³. The WPC foam profiles produced with low and high die temperatures achieved slightly higher density values. By visual assessment, slightly better homogeneity was detectable with a die temperature of 220℃, whereas the profiles displayed sink marks and inhomogeneities at a die temperature of 240℃ due to the premature foaming of blowing agent 3 in the extruder cylinder.

By increasing the speed from 10 to 12 r/min, it was possible to fill the profiles completely due to the resulting higher flow rate (Figure 5, right). However, the increased drive speed due to the higher flow rate caused stretching of the profiles at a screw speed of 14 r/min, which resulted in reduced foaming of the profiles. The profiles with a screw speed of 12 r/min showed the lowest medium density and therefore the best foaming behaviour, whereas the densities of the profiles with 10 and 14 r/min were slightly higher.

The production of homogenously foamed WPC profiles by means of the Celuka technique requires the co-ordinated interaction of die temperature and screw speed. In the tests conducted, the middle settings investigated (220℃, 12 r/min) displayed the best foaming results.

Effect of wood fibre content on foaming behaviour

To investigate the effect of the wood fibre content on foaming behaviour, WPC formulations with 40, 50 and 60 wt.% wood fibres of type 1 were used, respectively, in PP type A and 2.5 wt.% blowing agent 3. The die temperature here was 220℃ and the screw speed 12 r/min. For the formulations with 50 and 60 wt.% wood fibres, it was necessary to increase the cylinder temperatures after the degassing vent. The falling mass temperature observed here is the result of the insulating effect of the wood fibres in conjunction with the varying thermal conductivity of the molten polymer. The resulting lower mass temperature particularly with a wood fibre content of 60 wt.% resulted in incomplete fusing of some WPC granules, although the microspheres expanded (Figure 6).

As the wood fibre content increases, less matrix material is available, which can foam with the aid of blowing agent. The wood fibre content in the formulation likewise has a significant effect on the rheological properties of the WPC molten mass. To analyse this effect, the viscosity changes depending on shear rate were investigated by means of a high pressure capillary rheometer 6000 from Göttfert in accordance with DIN EN ISO 11443 at a temperature of 185℃ and a shear rate from 200 to 2000 s^-1. This is approximately equivalent to the thermal and rheological loads during extrusion. The rheometer was fitted with a slit capillary; the capillary length was 100 mm, the measuring length 50 mm and the channel height 1 mm. From the measured values, the real shear velocities and viscosities were calculated by means of Weissenberg-Rabinowitsch correction (Figure 7).

The analyses showed that as the wood fibre content rises the viscosity increases. The viscosities of the WPC compounds with 40 and 50 wt.% are in a similar range, whereas the viscosity of the compound with 60 wt.% wood fibre content is significantly higher and determinable only for low-shear velocities.

Effect of wood fibre length on foaming behaviour

To determine the effect of the wood fibre length on the foaming behaviour of WPC, extrusion tests were conducted with three different wood fibre lengths (short, medium and long) with content of 40 wt.% in the WPC on the basis of PP type A. The content of blowing agent 3 was 2.5 wt.%, the die temperature 220℃ and the screw speed 12 r/min. To process the long wood fibres of type 3, it was necessary to increase the cylinder temperatures after the degassing vent in order to plasticise the material, as more energy had to be fed in from outside to heat the more voluminous, long wood fibres due to the insulating properties. Despite this temperature increase, some WPC granules did not completely fuse, whereas blowing agent 3 expanded. The result was inhomogenously foamed WPC profiles (Figure 8). The density of the WPC foam profiles decreases as the wood fibre length increases. By foaming, a density reduction of 20% with short fibres, 25% with medium fibres and 36% with long fibres compared to unfoamed WPC and identical formulations can be achieved due to the inhomogeneities described here.

In the SEM pictures (Figure 9), the short wood fibres of type 1 can be identified only with difficulty in the fracture pattern, whereas a good distribution of the wood fibres is recognisable with the medium wood fibres of type 2. In the pictures of the WPC foam profile with long wood fibres of type 3, the agglomerates of expanded microspheres from blowing agent 3 are clearly visible besides the wood fibres. Good foam homogeneity was therefore achievable particularly with short and medium fibres. The average diameter of the foam pores in the homogenous foam with wood fibres of types 1 and 2 is approx. 100 µm.

Effect of PP type on foaming behaviour of WPC

The strength and viscosity of the molten mass display a significant effect on the formation of the foam structure. Because of this, it was investigated whether a PP with higher melt strength (type B) has a significant effect on the foaming behaviour of WPC compared with the base polymer (type A). The investigations into this were conducted with 40 wt.% wood fibres of type 1 at die temperature 220℃ and screw speed 12 r/min.

Figure 7 showed that the viscosities of both WPC compounds are heavily dependent on shear velocity; the viscosities drop as the shear rate increases. The highest total viscosity for WPC is reached with type A as the matrix material, while type B shows significantly reduced viscosity of the WPC despite a similar MFR value, due to its branched molecular structure.

The density measurements of the foamed WPC profiles each with 2.5 wt.% blowing agent 3 showed that the WPC with material matrix type B displayed 23% lower density due to the improved foaming as a result of lower viscosity and higher melt strength compared with material matrix type A.

Water absorption of WPC foam

Wood fibres and foam structures are essentially critical with respect to water absorption. Therefore, the water absorption of selected WPC foam profiles was investigated by determining the change in mass on the basis of DIN EN ISO 62 (storage under water at 23℃). The investigations were conducted on three 128 mm long profile segments each, which were dried at 80℃ to a constant weight prior to the tests. The water absorption of the WPC formulations foamed with 2.5 wt.% blowing agent 3 was investigated, respectively, with 40 wt.% short and medium wood fibres and PP type A. In addition, the effect of a coex layer on water absorption was investigated with the use of short wood fibres. The cut edges of the profiles were not sealed here.

Immersion in water demonstrated that complete saturation did not occur even after 2000 h, due to protracted diffusion processes in the foam (Figure 10). Because of the closed-cell foam structure created by the microspheres in blowing agent 3, the WPC foam profiles absorbed less water than were assumed for a WPC foam. The water is absorbed only through the exposed wood fibres on the surface. The difference in water absorption with a change in mass of only approx. 3% for medium and almost 9% for short wood fibres can be explained by eased diffusion in the fibre network and poorer foam homogeneity when short wood fibres are used. Equally, this theory is supported by the lower percentage of fibre content on the profile surface with medium fibres. The low standard deviation for foamed profiles with 40 wt.% medium wood fibres and PP type A without coex layer is not apparent in the chart due to the consistent values. Furthermore, the water absorption for foam profiles with short wood fibres can be reduced by more than half with the application of a coex layer.

Mechanical properties of WPC foam

As an example, the effects of wood fibre length and PP type on the mechanical properties were investigated on the basis of the bending properties and Charpy impact strengths of the WPC foam profiles.

Bending properties

The results of the bending tests (Figure 11) show that the foamed profiles with 40 wt.% short and medium wood fibres and densities of 0.673 and 0.690 g/cm³, respectively, display similarly high breaking loads, whereas the profiles with 40 wt.% long wood fibres and a density of 0.599 g/cm³ achieved the lowest breaking loads due to inhomogeneities. The reinforcing effect of the wood fibres is interrupted by the flaws arising. The greater deflection of the profiles with long wood fibres arises from areas with a locally greater degree of foaming, which yields better to the applied test pressure than the homogenously foamed profiles. The maximum breaking force and the deflection of the WPC profiles with matrix material B are similar to those of matrix material A when short wood fibres are used.

Charpy impact strength

The profiles with 40 wt.% short and medium wood fibres display impact strengths between 4.2 and 5.3 kJ/m², whereas the values of the profiles with 40 wt.% long wood fibres emerge as significantly lower, in line with the investigations of the bending properties (Figure 12). The impact strengths of the profiles with matrix material B and short wood fibres are likewise similar to those of matrix material A.

Computer tomographical analysis

For three-dimensional examination of the foam morphology, a profile with 40 wt.% of short wood fibres, PP type A and 2.5 wt.% blowing agent 3 was analysed computer tomographically as an example. Using the evaluation software, air pockets were specifically detected here, which were displayed in colour depending on volume. The air pockets detected here correspond to the gas-filled foam cells (Figure 13). Essentially, a homogenous distribution of the foam pores and therefore good homogeneity of the WPC foam with blowing agent 3 can be detected. By comparing with the real WPC profile, it was possible to determine that the areas with no detectable pores are unfoamed WPC.

Conclusion

The investigations conducted in this work demonstrate that WPC based on PP can be foamed by means of extrusion in the Celuka technique. It was possible to produce homogenous WPC foam profiles here with densities from 0.7 to 0.8 g/cm³. By means of extrusion tests, it was moreover shown that no satisfactory foaming of the WPC is possible using this technique with CBA based on azodicarbonamide and bicarbonate. However, good foaming of the WPC was achieved with the isopentane-filled microspheres. In order to produce homogenously foamed WPC profiles in the Celuka technique, the interaction of die temperature and screw speed is required. The processing window for the extrusion of WPC foam is very small as plasticisation of the WPC must be guaranteed and premature reaction of the blowing agent must be prevented. With respect to the wood fibre content, it was determined that foaming of WPC based on PP is possible with a wood content of up to 50 wt.% using the available die. By using varying wood fibre lengths, it was determined that good homogeneity of the foam is achieved primarily with medium fibre length, which equally realises a positive effect on the mechanical properties. The PP type of the matrix material displayed only limited effect on the foaming behaviour of WPC. Because of the closed-cell foam structure created by the microspheres in blowing agent 3, the WPC foam profiles absorb less water than was assumed for a WPC foam. The water is absorbed only through the exposed wood fibres on the surface.

Footnotes

Acknowledgements

The administrative assistance of Forschungszentrum Jülich is gratefully acknowledged. Moreover, the authors thank REHAU AG + Co, Rehau, Germany, for the excellent cooperation.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The financial support of the Bavarian Ministry of Economic Affairs and Media, Energy and Technology, Munich, within the project ‘Entwicklung eines Extrusionsverfahrens für geschäumte, holzgefüllte Polypropylenprofile’ (VIII/7-3665g/946/2-NW-1005-0012).

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