Processing of expandable polystyrene using microwave energy

Processing of expandable polystyrene using microwave energy

Journal of Materials Processing Technology, 29 (1992) 341-350 Elsevier 341 Processing of expandable polystyrene using microwave energy H.Y. Yeong an...

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Journal of Materials Processing Technology, 29 (1992) 341-350 Elsevier


Processing of expandable polystyrene using microwave energy H.Y. Yeong an d S.W. Lye School o[ Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 2263 {Received May 31, 1991; accepted June 14, 1991)

Industrial Summary Expandable polystyrene (EPS) commonlyknown as styrofoam, is used widelyin the packaging industry for the protection of goods. The styrofoam cushion is produced commerciallyby a steam injection process from pre-expanded EPS beads. This process, which requires the cyclic heating of the aluminium mould for expansion of the EPS and the subsequent coolingof the same mould before the ejection of the product, is highly inefficient in terms of energy utilization. In the presently reported investigation, microwave energy was used to process the EPS. Experimentation was carried out in three phases comprising: (a) pre-expansion of the EPS; (b) moulding using pre-expanded beads; and (c) direct moulding from EPS raw material, i.e. by-passing the preexpansion stage. Various properties of the pre-expanded beads and of the mouldedproducts produced under various settings of the processparameters wereexplored,includingthe determination of the compressivestrength of the mouldedproduct to assess its potential as a protective cushion. The results for the three phases have been evaluated and discussed, the conclusion from which is that microwaveenergy can offer a good alternative to steam in the processingof EPS.

1. Introduction Expandable polystyrene ( E P S ) is a white polymeric material used commonly in the packing industry primarily for the purpose of protecting finished goods such as household appliances, computers, etc., from damage due to impact shock arising from being dropped or otherwise roughly t r e a t e d during t r a n s p o r t a t i o n or material handling. Various properties of E P S make it an excellent packaging material, among which are its lightness (low weight-tovolume ratio), low t h e r m a l conductance, high moisture resistance and good shock absorption. To produce the customized moulded product for protecting the finished goods, two processing stages are required [1,2 ]. T h e raw material, E P S , initially in t i ny beads of appr oxi m at el y 1 m m diameter, is first p r e - e x p a n d e d into larger beads, the final sizes being d e p e n d e n t on the absolute density required for t he moulded cushion. After these p r e - e x p a n d e d beads have been aged (stabilized

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in an ordinary atmosphere over a period of about 6 to 24 hours), these beads can be used for final moulding using a process quite similar to that of thermoplastic injection moulding. In this injection moulding process, pre-expanded beads are injected into custom-made moulds, followed by the injection of steam into the mould chamber to provide the heating medium for the fusion of the beads. During this heating phase the beads expand further and also, being enclosed in a confined space, fuse together to form the final moulded product or cushion. Before the cushion is ejected, it has to be cooled to 40 °C to allow the cushion to stabilize and cease expansion. After the cushion is ejected from the mould it undergoes a drying process to remove the moisture. Since every production cycle requires that the mould be heated for fusion of the cushion and subsequently cooled for ejection of the cushion, this process leads to poor energy utilization and efficiency. Arising from this inefficient usage of energy, some research work was carried out to explore the use of alternative sources of energy. Ohuchi and Ohuchi [3 ] conducted experiments using microwave irradiation. To promote the fusion of the EPS beads a binding agent, carboxylmethylcellulose (CMC), was added. Three parameters were investigated: the quantity of material; the amount of water added; and the duration of heating, subjective evaluation of the final products being based on fusion and surface finish. Nazar et al. [4] performed trials with microwave energy but used brine (sodium chloride solution) instead of CMC as a coating on the EPS beads. Although both of these tests demonstrated that microwave energy can be used as an alternative source of energy, the introduction of an additional substance in the final moulded product is not acceptable in the packaging industry, as problems such as contamination and corrosion of the finished goods may occur with the presence of these substances. In this investigation, moulding of the EPS was conducted using microwave energy and water as the medium to promote fusion. Various processing parameters and the properties of the cushion were studied. 2. Experimental w o r k

As the effects of variation of each parameter have not been established in previous work using microwave energy, the approach adopted in this investigation was to change the parameters one at a time to determine the general relationships between the parameters, rather than to try to seek optimization of the process. The experimental work was carried out in three phases. The first phase was to explore the effects of variation in various process parameters on the preexpansion of the EPS beads. The second phase was to explore the possibility of moulding the pre-expanded beads from phase one. Lastly, in phase three, direct moulding was attempted using raw EPS material, thereby by-passing the pre-expansion stage.


All the experiments were conducted using a commercially available Goldstar microwave oven, model ER-761ME, with a maximum power output of 700 W. A Mettler PC 440 electronic weighing machine was employed to determine the quantity of material used in the experiments. Two types of raw E P S materials were used, these being P351 from BASF and Grade 712 from Dyno Chemical. 2.1. Phase one: Pre-expansion In this phase, experiments were carried out using a glass container, the water being added to the EPS material by means of a burette. A Mitutoyo Toolmakers' microscope model TM-201 was used to measure the size of the pre-expanded beads. 2.2. Phase two: Moulding using pre-expanded beads In the conventional moulding of EPS, aluminium is used for the mould material as it offers the advantages of high heat conductivity, good corrosion resistance and relatively low cost. In processing using microwave energy, the use of metals as mould materials is not possible and an electrical insulator must be used. Certain properties must be met in selecting the mould material, amongst which are the two fundamental requirements of being able to withstand a temperature of at least 120 ° C and an expansion pressure of 3 bars [5 ]. The mould used in this phase of experimentation was machined from phenolic plastic, see Fig. 1. 2.3. Phase three: Direct moulding from raw E P S beads Based on the findings in phase two, the experiments were repeated using P351 raw-material beads in place of the pre-expanded beads used previously, All. D l r ~ e n s l o n s In I~i, .




Fig. 1. Phenolic mould






the same mould being used for the experiments. To test the compressive strength of the moulded product, a Shimadzu Servopulser EHF-EA20 and Controller 4880 were used. 3. R e s u l t s and d i s c u s s i o n s

In the design of the EPS packaging, the moulded cushion must be able to provide a cushioning effect to the packaged product. One important parameter that is used to determine the suitability of the moulded cushion for packaging is its actual (absolute) density. Unlike low thermal conductivity and water absorption, which are inherent properties of the material used, the cushion density depends greatly on process parameters, such as the expansion time (the time of exposure to microwave energy) and the energy input.

3.1. Determination of the actual density of pre-expanded beads Initial batches of pre-expanded beads obtained under phase one of the experimental work were used for two density measurements. The actual density was obtained from direct measurement of the diameter of each bead to determine the volume of the beads in each sample, whilst the bulk, or apparent density was obtained by measuring the bulk weight and bulk volume of the beads. From the measurements a ratio of the actual density to the bulk density was determined. Four batches of pre-expanded beads were processed using the same values of the process variables excepting for the expansion duration. The diameters of 50 beads from each batch were measured and the mean diameter of each batch of beads was used to calculate the actual volume. To determine that the sample size was representative of the population of the batch, a statistical test was done for each batch based on a 95% confidence level and an error of 5% [6 ]: calculations showed that a sample size of 50 beads was sufficient. The relationship between the bulk density and the actual density is shown in Fig. 2, the ratio of the actual density to the bulk density being found to be 1.53. Since the actual density is cumbersome to measure, its value can be determined from the more easily measured bulk density multiplied by the predetermined ratio. 3.2. Effect of expansion time on bulk density A series of experiments was carried out using the same quantity of P351 material, microwave power input and amount of water, but with different expansion times. The resulting bulk densities for the various expansion times are shown in Fig. 3. Initially, when the expansion time is less than 1 minute, little expansion of the EPS material is noticed, as the energy is being absorbed by the water and the beads. As the water changes to steam and the bead temperature rises, the beads expand rapidly through the expansion of the pentane gas impreg-

345 100


o i~



~ 20 IZl I 20


I 40

I 60


I 100

I 120

I 140


Actual density (kg/eubic m) Fig. 2. Relationship between actual density and bulk density.

..~ 700, .~ 6O0 500 400 300 "0


100 0








Expansion time (rain) Fig. 3. Bulk density versus expansion time.

nated in the beads. After about two minutes, the rate of expansion reduces rapidly as the beads reach almost maximum sizes, further increasing of the expansion time producing minimal change in the bulk densities.

3.3. Effect of quantity of water on the bulk density The bulk densities obtained by varying the quantity of water added whilst keeping other process variables constant are shown in Fig. 4. The material used was P351 and the amount was kept constant at 10 g. With the same expansion time, the bulk density decreased rapidly with added water up to about 20 g of added water. Beyond 20 g, further increase in the amount of added water produced a negligible decrease in bulk density. From the results, it appears that a material/water weight ratio of about 1: 2 is sufficient for near complete preexpansion of the beads.

346 60





45 I


4.f 3~ 'o o



I 0

I 10


I 15

I 20

I 25


I 30

I 35

I 40


I 45

I 50


Qty of water added (ml)

Fig. 4. Bulk density versus quantity of water added.

.o 4O0 .a o



200 100

=_~_=-__o--~ 0







Expansion time (rain) -e-420 W

-a- 560 W

-I- 700 W

Fig. 5. Bulk density versus expansion time.

3.4. Effect of power input on the bulk density The experiments were repeated using the material Grade 712 instead of Grade P351. For this series of experiments, the changes in bulk density with expansion time were noted for three levels of power input, i.e. 420, 560 and 700 W, the results being shown in Fig. 5. These experiment results showed that preexpansion of EPS materials can be carried out successfully using microwave energy and water as a medium. 3.5. Moulding with pre-expanded beads The material used in the moulding was pre-expanded Grade 712 beads, the bulk density of these pre-expanded beads being 100.83 k g / m 3. The beads were made to fill the cavity of the mould. Two mouldings were obtained with differ-


ent moulding times of 1.4 and 2 minutes. Various measurements were obtained from the specimen sections of each moulding.

3.5.1. Uni[ormity of the bulk density One important property of a moulded cushion is the need for uniformity of its density in order to be able to provide uniform protection of the packaged goods. Any variation in density will cause different shock/impact force absorption, thus subjecting the packaged product to possible damage. The bulk densities of three sections taken from the top, the centre and bottom of the moulding were compared with the bulk density of the whole moulding, as shown in Table 1. About 1% to 3% variation in the bulk densities was noted, this variation being regardable as negligible. 3.5.2. Dimensional accuracy To check the dimensional accuracy of the mouldings against the mould dimensions, dimensions at four positions, as indicated in Fig. 6, were taken from the mouldings, the two sets of measurements and the mould dimensions being shown in Table 2. Measurements of the internal depth (C) of the mouldings showed a variation of about 4%. The variations of the other dimensions were TABLE1 Density at various locations S/No.

Density (kg/m 3) Top



Complete moulding

98.40 101.61

100.95 102.05

101.25 102.13

101.01 102.06

1 2

A (62.0ram)



Fig. 6. Dimensions of the mould.


348 TABLE 2 Dimensions of the mouldings S/No.

1 2

Dimensions (mm) A (62.0) a

B {21.0)

C (30.0)

D (50.0)

61.0 61.6

21.2 21.4

31.0 28.8

52.3 52.8

~Dimensions of the mould. TABLE 3 Density at various locations S/No.

1 2 3

Density (kg/m 3) Top



Complete moulding

94.92 90.84 88.53

94.21 90.66 88.04

93.96 90.15 87.88

94.81 90.73 88.47

within 1% of the mould dimensions, which is generally acceptable for EPS packaging.

3.6. Moulding directly from raw material In this series of mouldings, the raw P351 EPS beads replaced the pre-expanded beads as the input material for moulding. This effectively eliminated the need for pre-expansion of the raw material beads as is presently done in conventional moulding of the EPS packaging foam. The same series of measurements was carried out as for Sections 3.5.1. and 3.5.2., the results being shown in Tables 3 and 4. The similarity of these measurements with those of Section 3.5., indicates that the pre-expansion stage can be eliminated when moulding EPS with microwave energy.

3.6.1. Compressive strength of the mouldings The compressive strength of the EPS mouldings is one of the fundamental considerations used in the design of the packaging cushion. As an approximation, the design value used should not exceed two-thirds of the measured value of the compressive strength, which also varies with the actual density of the cushion [ 7 ]. Preliminary studies were made of the compressive strength of the P351

349 TABLE 4 Dimensions of the mouldings S/No.

Dimensions (mm)

1 2 3

A (62.0) ~

B (21.0)

C (30.0)

D (50.0)

61.8 61.9 61.8

21.0 21.0 20.8

30.7 30.5 30.0

49.8 49.7 49.7

aDimensions of the mould. A







"N 4




0 Compression (ram) Fig. 7. Typical variation of compressive load with compression: A--initial compression; B--elastic range; C--irreversible deformation (1 kgf=9.81 N).

mouldings. Quadrants of 15 m m height were taken at different locations of the mouldings and tested, Fig. 7 showing a typical variation of compressive load with compression depth. The pattern observed follows closely the compressive stress versus strain curves measured for conventionally moulded cushions [8], excepting for the initial compression, which would not have been present had the surface of the test sample been flat and even. Further tests need to be conducted to determine the compressive strength at various densities and to compare the values with those from conventionally moulded cushions. As a guide, the compressive strength of an EPS cushion is indicated by the measured value at 10% compression. 4. C o n c l u s i o n s

A series of experiments was carried out on two grades of EPS materials using microwave energy. Initially, pre-expansion of the raw material was carried out

350 using water as the medium. Results have shown t h a t the bulk density varies with the a m o u n t of water added and the duration of microwave exposure. Further moulding using pre-expanded beads was carried out successfully. The dimensional accuracy and bulk-density uniformity were found to be less t h a n 4%. Moulding was carried out using raw material beads instead of pre-expanded beads, the mouldings obtained achieving results similar to those of mouldings made with pre-expanded beads. This provides a good indication t h a t the preexpansion stage used presently in industry can be eliminated when moulding is carried out using microwave energy.

Acknowledgements The authors acknowledge the contributions made by Ms Loo Siew Yin in this investigation.

References 1 Technical Information Bulletins on Styropor, Pre-expansion of Styropor, TI-054/2e, BASF, Ludwigshafen,Germany, 1978. 2 TechnicalInformation Bulletinson Styropor,Expansion of Styropor P501, TI-068/4e, BASF, Ludwigshafen,Germany, 1975. 3 A. Ohuchi and K. Ohuchi, Moldingof polystyrenefoam by microwaveirradiation, Patent JP 7318943, Japan, 1973 (in Japanese). 4 S.E. Nazar, J. Leidner and F.M. Svirklys, Polystyrene bead expansion process, US Patent 4,765,934A, 1988. 5 K. Stoeckhert, Mold-Making Handbook for the Plastics Engineer, Hanser Publishers, Munich, Germany, 1983. 6 D.L.Harnett, Statistical Methods, Addison-Wesley,Reading,MA, 3rd edn., 1982. 7 Information for Technical Personnel, Design of Stackable Styropor Packs, No. 129E, BASF, Ludwigshafen,Germany, 1984. 8 Technical Information Bulletins on Styropor, Compressive Strength of Expanded Styropor, TI-062/le, BASF, Ludwigshafen,Germany, 1974.