effectiveness of blowing agents in the cellular injection molding process · ·...

©Smithers Information Ltd. 2015

189Cellular Polymers, Vol. 34, No. 4, 2015

Effectiveness of Blowing Agents in the Cellular Injection Molding Process


Tomasz Garbacz and Pawel Palutkiewicz

Department of Mechanical Engineering, Lublin University of Technology, 36 Nadbystrzycka Str., 20-816 Lublin, Poland

Department of Polymer Processing, Czestochowa University of Technology, 19c Armii Krajowej Ave., 42-201 Czestochowa, Poland

Received: 17 December 2014, Accepted: 26 March 2015

SUMMARY

The study was undertaken to investigate the effect of the type and content of blowing agents in the polymeric materials being processed on the structure and selected physical and mechanical properties of the obtained injection molded parts. In the study of the cellular injection molding process, three standard and generally available thermoplastics were used. In the experiments, the blowing agent content (0-2.0% by mass, i.e. 0, 0.4, 0.8 and 2.0% (wt)) fed into the polymer being processed was adopted as a variable factor. In the present study, the blowing agents with the endothermic decomposition characteristics and exothermic decomposition characteristics, coming in a granulated form with a diameter ranging from 1.2 to 3 mm were used.

Based on the results of investigating porosity, porous structure image analysis as well as microscopic examination of the structure was shown. It has been found, that the favorable content of the blowing agent in the polymeric material should be of up to 0.8% wt.. With such content of the blowing agent in the polymeric material, favorable strength properties are retained in porous parts, the pore distribution is uniform and the pores have similar sizes.

Keywords: Thermoplastic, Cellular, Injection molding, Blowing agents, Mechanical properties

190 Cellular Polymers, Vol. 34, No. 4, 2015


INTRODUCTION

Cellular injection molding consists in feeding into a polymeric material a blowing agent which – under appropriate pressure and temperature conditions – decomposes to release gas. The gas is dissolved in the polymeric material.

Blowing agents especially include nitric compounds, sodium bicarbonate and mixtures of organic acids and hydrocarbons. The reaction may be endothermic or exothermic. What should be considered when using blowing agents during processing, is that typical (decomposition) temperature must be exceeded for a specified period of time, in order to obtain relevant amount of gas [1, 2, 3].

The solid blowing agents used in the foaming process can have either exothermic or endothermic decomposition characteristics (the so-called exothermic and endothermic blowing agents) [2, 4]. Depending on processing conditions and the method employed, the use of exothermic blowing agents can lead to local overheating and, in effect, non-uniform porous structure of products [5, 6]. Preceding the gas release, blowing agents undergo the same processes as polymeric materials, i.e. heating, compression, homogenization and conveying. Once the gas release is initiated (after the appropriate temperature has been reached), numerous micro bubbles occur and rapidly dissolve in the surrounding polymeric material [1, 7, 8]. For that reason, injection molded parts produced by such methods need to undergo intensive cooling in order to prevent strains and to retain their correct porous structure. In the case of blowing agents with the endothermic decomposition characteristics, gas expansion in the processing abruptly ends once the thermal energy flow has been stopped. The use of such blowing agents considerably shortens the cooling time of parts [2, 6, 7].

The decomposition products of the above blowing agents mainly include carbon dioxide, a minimal amount of water, nitrogen and totally harmless, even to food products, sodium bicarbonate. Carbon dioxide and other low-quantity products diffuse into the environment in the course of processing [5, 7, 8].

The properties of porous products depend on such factors as the type and amount of a blowing agent as well as additional ingredients, polymeric material type, size and number of pores created in the injection process or temperature distribution in the plasticizing system of the injection molder. Appropriately selected processing conditions allow for producing porous products with new, modified physical and technological properties, such as lower weight, enhanced damping and insulating properties, the possibility of product reutilization with the same or acceptable (by relevant norms) physical and technological properties of the porous product retained [2, 8, 9, 10].

The research in methods for producing porous injection molded products has been conducted in several research centers located both in Poland and



abroad. One problem that is rarely discussed and still inadequately investigated concerns the effect of various blowing agents (exothermic, endothermic, pellets or microspheres) on the effectiveness of polymer process [2, 4, 5, 7, 11]. The results reported in the studies demonstrate that the chemical properties of the blowing agents facilitate the polymer flow in the plasticizing unit and injection mold. They do not determine, however, if the blowing agents have any impact on the effectiveness and efficiency of the injection molding process.

Rachtanapun P., Selke S.E.M., Matuana L.M. and Matuana L.M., Park C.B., Balatinecz J.J. [5, 10] investigated the polymer process with regard to decomposition, number of cells, mechanical properties and structure of cellular extruded products made of HDPE/PP, PVC. The studies conducted by E. Bociaga [12, 13] define impact of different variables of processing conditions on the characteristics of the porous structure, mechanical properties and surface state (gloss and color of parts). Unfortunately, these studies have focused on only one type of HDPE modified by one type of blowing agent. The studies in methods for producing porous composites using nanocomposites [14, 15] are also of vital importance, as they present physical, mechanical, and thermo-mechanical properties of the produced porous nanocomposites. The authors of [16, 17] discuss the properties of injection molded parts produced from polypropylene, polyethylene-polypropylene, a polyolefin composite by cellular injection molding with the MuCell method at variable technological conditions. Using the DTA/DSC methods, A. Błędzki et al. [18] have found a significant effect of a blowing agent contained in the polycarbonate PC and in a porous product filled with wood fibres on its thermal properties. In [19, 20], possibilities of producing PP products filled with carbon black and blowing agents modified by microwave radiation are described. It was showed a significant impact of blowing agent content of the density, quantity and size of the pores, of obtained PP composites. The studies [3, 21] also describe the effectiveness of producing cellular products, yet only with regard to the material effectiveness at variable injection molding conditions or the use of hybrid solid-cellular co-injection molding process [22].

Despite the numerous publications on extrusion including cellular extrusion, there is, in fact, lack of information on the effectiveness of blowing agents in the cellular injection molding process performed using different polymers and blowing agents, at constant conditions of foaming injection process.

Part of a more comprehensive research project, the present study was undertaken to investigate the effect of the type and content of blowing agents in the polymeric materials being processed on the structure and selected mechanical properties of the obtained injection molded parts.



Materials

Three thermoplastics were used in the experiments. The first one was polypropylene PP, marketed under the trade name Moplen – EP440G. According to the data provided by the manufacturer, the thermoplastic has a density of 900 kg/m3, melt flow rate (MFR) of 1.3 g/10 min (230°C, 2.16 kg), and tensile stress at yield of 27 MPa. Another polymeric material used in the experiments was polyethylene LDPE, marketed under the trade name MALEN E, FABS 23-DO22, with a density of 925 kg/m3, MFR range 1.6-2.5 g/10 min (190oC, 2.16 kg), tensile stress at yield of 12 MPa. The third polymer used in the experiments was poly(vinyl) chloride PVC, marketed under the trade name ALFAVINYL - GFM/4-31-TR, with a density of 1330 kg/m3 and tensile stress at yield of 21 MPa, according to the data provided by the manufacturer.

In the experiments, three types of blowing agents were used: Hostatron P 1941 and Hydrocerol PLC 751, both manufactured by Clariant Masterbatches GmbH, and Adcol blow X1020, manufactured by Colex (Clariant Group).

The blowing agent, with their trade name P 1941, have the endothermic decomposition characteristics. They come in a granulated form with a diameter range from 1.2 to 1.8 mm and length from 2.3 to 2.5 mm, with polyethylene as a carrier material. The blowing agent in both P 1941 is a mixture of sodium acid carbonate and 2–hydroxypropane-tricarboxylic acid (citric acid).

Adcol-blow X1020 is an exothermically decomposing system that has the form of cylindrical pellets 2 to 3 mm in diameter. The blowing agent contained in this material system is a mixture of an appropriate ratio of chemical compounds, among others azodicarbonamide. According to the manufacturer’s recommendations, good foaming efficiency can be achieved at processing temperatures in the range of 145-180°C.

Hydrocerol PLC 751 is a blowing agent with an exothermic decomposition behaviour and nucleating properties. It has a granulated form with spherical grains whose diameter ranges from 2.4 to 2.8 mm. In order to obtain high foaming process efficiency, the processing temperature should range between 170-190°C. The active substances in this blowing agent constitute a mixture of appropriately proportioned chemical compounds, such as azodicarbonamide.

When, in the proper temperature, the process of gas emission is started, numerous pores which are then generated dissolve in the surrounding plastic material due to the operation of pressure and surface development. The emerging cells may be filled in with air or with any other gases, for instance CO2 and N2, yet later they are replaced with air as a result of diffusion.

Selected properties of the discussed blowing agents are listed in Table 1.



Table 1. Selected properties of blowing agents used in the cellular injection molding process

No. Characteristic quantity Hostatron P 1941

Adcol blowX1020

Hydrocerol PLC 751

1 Active agent content, % 50 50 50

2 Chemistry Endothermic Exothermic Exothermic

3 Decomposition initiation, ºC 160 145 170

4 Optimum processing temperature, ºC

180–210 170–210 170–190

5 Injection dosage, % 0.5–2.5 0.5–2.0 0.5–2.5

Based on the adopted research program, the polymeric material for cellular injection molding was modified in such a way that the blowing agents were fed into it during the mechanical mixing process.

Polymers were modified by introducing blowing agents into them in the Laboratory of the Department of Polymer Processes of the Lublin University of Technology. The mass of the individual components of the modified polymers, i.e., the contents of polymer and blowing agents, were measured with a WTE-2000 laboratory balance reading to 0.1 g.

Dosing of blowing agents were selected based on own research [4, 7, 9, 12] and the analysis of the state of knowledge. It was found, that for the plan of experiment, with three different types of plastics, three different types blowing agent. It was found that blowing agents should be dosed in the range of the minimum, average and maximum specified by the state of the art. The blowing agents used in the cellular injection molding process were fed into the polymers being processed in the following quantities: 0.4%, 0.8% and 2.0% by mass (w/w). In order to obtain high process efficiency for the above blowing agents, the processing temperature range was 180-210°C.

EXPERIMENTAL

The test stand consists of a screw injection molding machine, CS-88/63, equipped with an injection mold. The machine has a single-screw plasticizing system, with a screw diameter of 36 mm. To the moveable table of the machine a moveable subassembly of the injection mold (Figure 1a) is mounted, in which two mold cavities are located. The mold cavity has the shape and dimensions of a tensile specimen for static tension tests, the so–called specimen type II [26], shown in Figure 1a and b.



Figure 1. Test stand used in the experiments: a) injection mold unit and molded specimens, b) fragment of the molded part with a visible porous structure

The mold cavity dimensions are as follows: length 150 mm, width from 10 to 20 mm, thickness 4 mm. Directly in the mold cavities there are point ejector pins mounted, with a diameter of 6 mm. In the fixed subassembly of the mold, there are flow system channels which supply the polymeric material to the mold cavities; the channels have a direct contact with an injection nozzle of the plasticizing system.

The following parameters of the cellular injection molding process were adopted: injection and clamping time – 2 seconds; molded piece cooling time – 35 s, temperature of thermostated mold – 50±1oC.Injection molding machine CS-88/63 do not have possibility to direct control and readout of injection pressure and counter-pressure. The value specifying the injection pressure is the pressure in the hydraulic system, aimed at testing the value of - 10 MPa. In the experiments, the temperatures of the polymers being investigated (PP, LDPE, PVC) were set in the particular heating zones of the plasticizing system in the following way: in zone I 180oC, in zone II – 190oC, III – 200oC, IV – 210oC.

The specimens of the injection molded parts were then examined in structural properties in compliance with relevant recommendations and norms [23-25]. Their density and porosity, were investigated. In addition to that, their coating structure was analyzed based on the digital images made.

The method of determining standard density of plastic materials was applied during studies on density of sample products from cellular plastics [23]. It was the immersion method, which was used as a measuring liquid ethanol with density 810 kg/m3. Measurements were taken in accordance with recommendations of a relevant standard. The density of the test sample was performed by removing the solid layer with a microtome, and their respective grinding. Refined particles of sample, whose mass ranged from 1 to 5 g, were the object of study. Measurements were carried out using 5 repeats.



In order to determine selected mechanical properties of the injection molded parts, both hardness tests using Shore’s method and tests for tensile strength, yield point and elongation at break were conducted, in accordance with the relevant norms (ISO 868 (2003) and ISO 527-1 (2010) [ 24, 25].

The applied shape and dimensions of the specimens complied with the relevant norm. The specimen thickness corresponded to the injection molded part thickness and it was measured and registered each time together with the width of the measuring length right before the tests.

Strength properties of the injection molded parts under static tension were determined using a testing machine manufactured by Zwick Z010. The machine was equipped with 10 kN screw wedge chucks together with the accessories. The measurements were done at a tensile speed of 10 mm/min and under the measuring load range 0-500 N. The hardness tests by Shore’s method were conducted using an Affri-manufactured hardness tester, type ART13 – Shore’s method D. Such choice was due to the employed method for determining hardness of solid polymers. Measurement of the mechanical properties of the porous parts were carried out five times for each type of test samples.

The investigation and analysis of the porous structure of the produced injection molded parts were conducted using an confocal microscope, type Olympus FluoView FV1000 and copyright position of image analysis of porous structure and the author’s stand for porous structure image analysis. Microscope FV1000 equipped with 1.3 Mpix camera, enabling direct viewing of microscanning image on computer screen. Observation of specimens structure and its recording was made in reflected light with suitable magnification. Average area of the pores is the average individual pore size is measured using the image analysis software Pixel Fox.

The porous structure image analysis is used to determine geometric characteristics of pores based on the images of porous structure taken. The images are taken using an optical camera with an electronic amplifying system and then analyzed on a computer stand equipped with special graphics software.

RESULTS AND DISCUSSION

Tables 2-4 show the results of the measurements of the density of injection molded parts and its porosity and area of pores obtained at various contents of the blowing agent. Porosity, being a property determining the volume of gaseous phase in the porous product, at the same time determines the value



by which the density of the product is reduced. The results in the table are mean values of test results obtained from five measurements with standard deviation. The obtained product was a rod having porosity of 15-45%, density ranging from 925 to 555 kg/m3 (for PE), from 900 to 560 kg/m3 (for PP) and from 1330 to 735 kg/m3 (for PVC) respectively, depending on the blowing agent used.

Figure 2 shows an example of the image obtained in the Pixel Fox used to determine the average value of the area of the pores in the analyzed cross-sectional samples. Preparations for the analysis of the porous structure was obtained from dumbbell-shape samples, and were executed using microtome. Samples were cut in half of dumbbell, the cross section was 4x10 mm, length was 6 mm.

Figure 2. Sample image from Pixel-Fox program used for determining the areas of pores

The dependence of the density and porosity on the blowing agent content shown in Tables 2-4 proves that both the content of the blowing agent applied and its decomposition characteristics are of vital importance here. The application of the exothermic blowing agents (Adcol blow X1020, Hydrocerol PLC 751) resulted in obtaining a product with a higher number of pores and increased porosity compared to the product obtained using the endothermic agent (Hostatron P 1941).



Tab

le 2

. R

esu

lts

of

stu

die

s on

th

e d

en

sity

, poro

sity

and a

rea o

f pore

s of

part

of

poly

eth

ylene

No

.B

low

ing

ag

ent

typ

eB

low

ing

ag

ent

cont

ent,

%

Den

sity

of

inje

ctio

n m

old

ed p

arts

, kg

/m3

kg/m

3

SD

for

den

sity

Po

rosi

ty,

%S

Dfo

r p

oro

sity

Ave

rag

e ar

ea o

f p

ore

sm

m2

SD

for

aver

age

area

, %

10

925

00

00

0

2en

doth

erm

icbl

owin

g ag

ent

Hos

tatr

on P

1941

0.4

785

1515

20.

019

3.5

30.

872

021

223

0.01

23.

2

42.

061

029

343

0.01

13.

0

5ex

othe

rmic

blo

win

g ag

ent

Adc

ol b

low

X10

20

0.4

740

1820

20.

020

6.8

60.

864

529

303

0.01

23.

4

72.

057

032

384

0.01

03.

0

8ex

othe

rmic

blo

win

g ag

ent

Hyd

roce

rol P

LC 7

51

0.4

740

1720

20.

015

6.0

90.

862

528

323

0.01

22.

4

102.

055

533

404

0.01

11.

5



Tab

le 3

. R

esu

lts

of

stu

die

s on

th

e d

en

sity

, poro

sity

and a

rea o

f pore

s of

part

of

poly

pro

pyl

ene

No

.B

low

ing

ag

ent

typ

eB

low

ing

ag

ent

cont

ent,

%

Den

sity

of

inje

ctio

n m

old

ed p

arts

, kg

/m3

kg/m

3

SD

for

den

sity

Po

rosi

ty,

%S

Dfo

r p

oro

sity

Ave

rag

e ar

ea o

f p

ore

sm

m2

SD

for

aver

age

area

, %

10

900

00

00

0

2en

doth

erm

icbl

owin

g ag

ent

Hos

tatr

on P

1941

0.4

810

1510

10.

015

4.5

30.

870

028

223

0.01

24.

2

42.

063

024

303

0.01

04.

0

5ex

othe

rmic

blo

win

g ag

ent

Adc

ol b

low

X10

20

0.4

735

1718

20.

018

5.8

60.

868

518

242

0.01

33.

4

72.

061

528

323

0.00

93.

0

8ex

othe

rmic

blo

win

g ag

ent

Hyd

roce

rol P

LC 7

51

0.4

740

1618

20.

019

3.5

90.

870

025

222

0.01

53.

3

102.

056

028

384

0.01

42.

5



Tab

le 4

. R

esu

lts

of

stu

die

s on

th

e d

en

sity

, poro

sity

and a

rea o

f pore

s of

part

of

poly

(vin

yl c

hlo

ride)

No

.B

low

ing

ag

ent

typ

eB

low

ing

ag

ent

cont

ent,

%

Den

sity

of

inje

ctio

n m

old

ed p

arts

, kg

/m3

SD

for

den

sity

Po

rosi

ty,

%S

Dfo

r p

oro

sity

Ave

rag

e ar

ea

of

po

res

mm

2

SD

for

aver

age

area

,%

10

1330

00

00

0

2en

doth

erm

ic

blow

ing

agen

tH

osta

tron

P19

41

0.4

1130

2015

30.

019

5.5

30.

810

0018

253

0.01

43.

5

42.

074

519

443

0,01

11.

0

5ex

othe

rmic

blo

win

g ag

ent

Adc

ol b

low

X10

20

0.4

110

1718

30.

018

6.8

60.

898

520

262

0.01

14.

5

72.

077

520

423

0.00

83.

0

8ex

othe

rmic

blo

win

g ag

ent

Hyd

roce

rol P

LC 7

51

0.4

1065

1620

30.

020

2.2

90.

890

519

324

0.01

81.

8

102.

073

530

455

0.01

51.

0



Irrespective of the blowing agent type used (with an endothermic or exothermic decomposition behaviour), the decrease in the density of the injection molded parts led to increased porosity depending on the percentage of the blowing agent dosed. Polymer porosity can be most effectively increased if the blowing agent content in the polymer is 0.8 wt%. This is confirmed by the porosity obtained, which amounted to 22 and 32% respectively, with the injection molded parts continuity maintained in the whole cross section at the same time. In the case of the injection molded parts produced with 0.4% blowing agent content in the polymer, the obtained porosity value – 10 and 18%, respectively –is not sufficient given the desired polymer savings. This results that the porosity of the plastic increases most effectively at blowing agent contents in the range from 0.8–2.0%. This is borne out by the fact that at the obtained degree of porosity of 22-45% and the injection molded parts had a uniform pore distribution and a uniform pore size.

Amount of blowing agent dosage have influence also on size of pores, which is determined by average area of pores. Increasing content of exothermic blowing agent in plastics from 0.4 to 2.0% caused decrease in area of pores od 0.020 do 0.008 mm2. Its intensive decrease is observed with 0.8% content of blowing agent. Suitable dependence occur in of case of injection molded parts produced during containing exothermic blowing agent.

The results of hardness of the injection molded parts are graphically represented in Figures 3-5. The type of a blowing agent used, be it with the endothermic

Figure 3. Dependence of Shore D hardness of injection molded parts on the content of blowing agent Hostatron P 1941



Figure 4. Dependence of Shore D hardness of PVC injection molded parts on the content of blowing agents used

Figure 5. Dependence of Shore D hardness of PP injection molded parts on the content of blowing agents used



or exothermic decomposition characteristics, has no effect on the surface hardness of the injection molded parts. The changes in hardness illustrated in the figures predominantly depend on the type and properties of the polymeric materials used in the cellular process.

The results of determining selected strength properties of the injection molded parts, obtained at different contents of the blowing agent in the polymers being processed, are shown in Table 5 and Figures 6-8. The table presents the average values with standard deviation of test results yield point, elongation at break of the injection molded parts. Figures present results of tensile strength examinations of investigated materials.

With increase content of blowing agent in the material decreases the value of the yield point. It was observed, that along with the increased porosity of the product, yield point is reduced, but with diminishing intensity. The largest decrease in yield point occurs at the content of the 0.8% of blowing agent in the plastic. There is less intensity of the drop to 2% of the blowing agent. This relationship is similar for each of the materials and blowing agents.

Elongation at break is reduced regularly in a non-linear way within the whole range of increasing value of product’s of porosity. Intensity of stress reduction is higher when the blowing agent content ranges from 0.8 to 2.0% of the mass in relation to the mass of the plastic. Elongation is reduced more intensely with lower blowing agent content ranging from 0 to 0.4% of the mass.

It has been observed that increasing the blowing agent content in the product decreases the value of tensile strength in a non-linear manner in the whole content range of the blowing agent in the polymer (Figures 6-8). The distribution of the curves demonstrates that at high content of the blowing agent (over 0.8%) in the polymer, the intensity of the decrease in tensile strength is lower, irrespective of the type of the polymer being processed and the type of the blowing agent used.

The results of the tensile strength tests of the specimens made from the polymers modified by the blowing agents correspond to the results of the conducted hardness tests. The injection molded parts made from porous polymers have decreased mechanical properties, strength properties included.

Significant differences in mechanical strength of the porous injection molded parts can be observed. For example, the blowing agent P 1941, with the endothermic decomposition characteristics, dosed in the range from 0 to 2.0% causes a decrease in tensile strength by 27% on average in the case of polypropylene (Figure 6), by 23% when fed into PE, and by 35% in the case of PVC.

When the blowing agent with the exothermic decomposition characteristics is used (Hydrocerol PLC 751), the change in tensile strength is more considerable



Tab

le 5

. R

esu

lts

of

stu

die

s on

th

e s

tren

gth

pro

pert

ies

of

the inje

cti

on m

old

ed p

art

s

Yie

ld p

oin

t,M

Pa

Elo

ngat

ion

at

bre

ak,

%

Yie

ld p

oin

t,M

Pa

Elo

ngat

ion

at

bre

ak,

%

Yie

ld p

oin

t,M

Pa

Elo

ngat

ion

at

bre

ak,

%

End

oth

erm

ic b

low

ing

ag

ent

Ho

stat

ron

P19

41E

xoth

erm

ic b

low

ing

ag

ent

Ad

col

blo

w X

1020

Exo

ther

mic

blo

win

g a

gen

t H

ydro

cero

l P

LC 7

51

Blo

win

g a

gen

t co

nten

t,%

po

lyp

rop

ylen

e

0 1

9.0±

0.4

150

±4

19.

0±0.

4 1

50±

4 1

9.0±

0.4

150

±4

0.4

16.

5±0.

9 1

05±

5 1

7.5±

0.7

120

±10

17.

0±0.

7 1

05±

8

0.8

13.

5±0.

7 7

5±5

16.

5±0.

4 8

0±5

15.

5±0.

6 9

5±6

2.0

11.0

±0.

8 5

0±4

15.

0±0.

6 5

5±5

14.

5±0.

6 8

0±6

po

lyet

hyle

ne

0 8

.5±

0.6

105

±3

8.5

±0.

6 1

05±

3 8

.5±

0.6

105

±3

0.4

7.5

±1.

1 1

00±

9 7

.0±

0.6

95±

8 7

.5±

0.6

90±

8

0.8

7.0

±0.

8 8

0±8

6.5

±0.

6 8

0±6

7.0

±0.

5 8

0±5

2.0

6.5

±0.

9 5

5±4

6.0

±0.

6 5

0±4

6.5

±0.

4 5

0±4

po

ly(v

inyl

chl

ori

de)

0 8

.0±

0.4

220

±5

8.0

±0.

4 2

20±

5 8

.0±

0.4

220

± 5

0.4

7.0

±0.

6 2

00±

12 7

.5±

0.7

180

±11

7.0

±0.

6 2

10±

11

0.8

6.5

±0.

6 1

80±

12 6

.0±

0.4

170

±10

6.5

±0.

4 1

80±

10

2.0

5.0

±0.

4 1

50±

10 5

.5±

0.3

145

±10

5.5

±0.

4 1

45±

12



Figure 6. Dependence of tensile strength of the injection molded parts made from the injected polymers on the content of endothermic blowing agent Hostatron P 1941

Figure 7. Dependence of tensile strength of PP, PE and PVC injection molded parts on the content of exothermic blowing agent Hydrocerol PLC 751



and it has the following values: PP – 27%, PE – 28% , and in the case of PVC – 32%, as illustrated in Figure 7.

The change in tensile strength for the polypropylene molded parts, graphically represented in Figure 8, differs from the distributions for polyethylene and poly(vinyl) chloride. Significant differences in the value of change in mechanical strength of the injection molded parts can be observed. The blowing agent, dosed in the amount of 2%, irrespective of the characteristics of its effect, causes a substantial worsening of strength properties. The pore structure becomes finer and finer, the number of pores increases, while their size is decreased. Moreover, such a large content of the blowing agent is economically ineffective.

The strength properties, hardness, yield point and tensile strength discussed in the paper depend to a great extend also on the characteristics of the blowing agent used. This is largely due to thermal properties of the polymeric materials being used and the effect of the blowing agents on the polymeric materials used in the process. However, such dependence has not been thoroughly investigated yet and will therefore be a subject of further studies.

Figure 8. Dependence of tensile strength of PP, PE and PVC injection molded parts on the content of exothermic blowing agent Adcol blow X1020



The examination of the physical structure of the produced porous molded parts was conducted using on confocal microscope and the stand for porous structure image analysis. Specimens have form of thickness 3 mm and length x width: 10x4 mm. Specimens were cut by using the microtome type Erjung 37427 (Germany). Some examples of porous structure images are shown in Figures 9-12.

A discernible effect of the blowing agent used, its type and effect characteristics on the obtained morphology of porous molded pieces has been observed. Based on the analysis of the photographs taken, it has been found that the injection molded parts with 0.4% content of the blowing agent (Figure 9) have a clearly visible solid outer layer and the their pore distribution is non-uniform to the greatest extent.

In the case of the PVC molded parts with 0.4% content of the blowing agent, the obtained value of porosity is however insufficient. Such arrangement of pores can be caused by uneven dosing into the polymeric material such a small amount of the blowing agent as well as by the characteristics of the blowing system being employed (Figure 9c).

Figure 9. Cross section of porous structure of injection molded parts made from the examined polymers: a) PE + 0.4% P 1941, b) PVC + 0.4% X1020, c) PE + 0.4% PLC 751, d ) PVC + 0.4% PLC 751



In the molded parts with 0.8% content of the blowing agent, the pore distribution is more uniform, and the visible pores are of similar sizes (Figure 10). In the case of the molded parts with 2.0% content of the blowing agent (Figures 11, 12), there is a substantial concentration of pores of various sizes which, in extreme cases, can cause a discontinuity in the solid outer layer. For specific contents of the blowing agent in the polymeric material, the examined pore diameter increases as the measuring capacity changes towards the product core. The smallest pore diameter was observed for the solid surface.

The thickness of the outer surface depends largely on the cooling of the product injection. In the case of research molding cooling time was constant at 35 s cooling time resulted Founded form a porous solid parts of the outer shell. There was no noticeable change in the thickness of the solid layer depending on the type and content of blowing agents in the plastic [Figure 9c (PVC +0.4%), Figure 10a (PP +0.8%)]. Differences appearance of a solid layer shown in the figures are related to the type of polymer material used.

Figure 10. Cross section of porous structure of injection molded parts made from the examined polymers: a) PE + 0.8% PLC 751, b) PP + 0.8% X1020, c) PP + 0.8% P 1941, d) PVC + 0.8% P 1941



The closer to the core the solid outer layer is, the more the pore size increases. The increase was most intensive at the lowest content of the blowing agent in the polymer being processed. As the dose of the blowing agent increases, the increase in the pore size for particular contents of this agent was smaller and smaller.

The change in the number of pores and their surface quantity in the cross section of the molded parts can also depend on cooling intensity. Fast cooling hampers the occurrence and growth of the pores, especially of those located closer to the surface layer. The outer layer of the molded part undergoes direct and fastest cooling in the mold, which is why the size and surface quantity of the micro pores produced in this region are lower compared to the regions located closer to the core of the molded parts (Figures 11 b, 12 d).

In the case of the porous parts produced using the blowing agent with the endothermic decomposition characteristics, the gas release in the course of processing comes to an end once the energy supply is stopped (Hostatron P 1941). The obtained porous structure is uniform; the pores have a spherical or quasi spherical shape. The pores have similar sizes, irrespective of their location in the product.

Figure 11. Cross section of porous structure of injection molded parts: a) PE + 2.0% P 1941, b) PP + 2.0% X1020, c) PVC + 2.0% PLC751, d) PVC + 2.0% X1020



Figure 12. Cross section of porous structure of poly(vinyl) chloride injection molded parts: a) PE + 2.0% PLC 751, b) PVC + 0.4% X1020, c) PP + 2.0% P 1941, d) PVC + 0.8% PLC 751

Table 6 and Figures 13-15 presents the results of measurements of the thickness of the solid layer of porous materials.

At the 0.4% of blowing agent content, the solid layer thickness varies from 1.00 mm (for LDPE) to 1.38 mm (for PVC), however, the most uniform thickness of the solid layer were observed during foaming with exothermic blowing agent (Figure 13b), and most uniform during foaming with endothermic blowing agent. Foaming with a blowing agent with 0.8% of blowing agent content resulted in a significant reduction in the thickness of the solid layer (Figures 14a, b). Solid layer thickness ranged from 0.80 to 0.86 mm without significantly depending on the type of material and the type of blowing agent.

The result of the application of 2% of the blowing agent was a further reduction of the thickness of the surface layer from 0.44 to 0.49 mm. Solid layer was thickest for the porous PVC, and most dimensionally stable in the case of injecting LDPE and PP (Figure 15a, b), regardless of the type of blowing agent.



Tab

le 6

. R

esu

lts

of

stu

die

s on

th

e o

ute

r la

yer

of

the inje

cti

on m

old

ed p

art

s

No

Blo

win

g a

gen

t ty

pe

Blo

win

g

agen

t co

nten

t, %

Out

er la

yer

of

inje

ctio

n m

old

ed

par

ts, m

mkg

/m3

SD

for

out

er

laye

r

Out

er la

yer

of

inje

ctio

n m

old

ed

par

ts, m

mkg

/m3

SD

for

out

er

laye

r

Out

er la

yer

of

inje

ctio

n m

old

ed

par

ts, m

mkg

/m3

SD

for

out

er

laye

r

1M

ater

ials

poly

prop

ylen

epo

lyet

hyle

nepo

ly(v

inyl

chl

orid

e)

2en

doth

erm

ic

blow

ing

agen

tH

osta

tron

P19

41

0.4

1.30

0.25

1.15

0.25

1.38

0.30

30.

80.

840.

050.

860.

060.

800.

08

42.

00.

460.

050.

440.

080.

520.

09

5ex

othe

rmic

bl

owin

g ag

ent

Adc

ol b

low

X10

20

0.4

1.28

0.22

1.00

0.18

1.25

0.10

60.

80.

860.

050.

900.

080.

850.

09

72.

00.

520.

030.

480.

040.

500.

05

8ex

othe

rmic

bl

owin

g ag

ent

Hyd

roce

rol P

LC

751

0.4

1.04

0.15

1.10

0.10

1.14

0.12

90.

80.

800.

090.

770.

050.

840.

08

102.

00.

450.

050.

480.

080.

490.

07



Figure 13. Cross section of porous structure of injection molded parts from examined polymers: a) PP + 0.4% P 1941, b) PVC + 0.4% PLC 751

Figure 14. Cross section of porous structure of injection molded parts from examined polymers: a) PE + 0.8% X1020, b) PVC + 0.8% PLC 751

Figure 15. Cross section of porous structure of injection molded parts from examined polymers: a) PVC + 2.0% PLC 751, b) PP + 2.0% P 1941



CONCLUSIONS

The aim of the study, which are part of a broader research program was to determine the influence of the type, content of blowing agents in materials processed by cellular injection molding on the structure and selected physical and mechanical properties of the received porous parts. The resulting of porous parts have a solid outer surface and a porous core, under appropriate physical and mechanical properties.

Analyzing the results of density, porosity and optical examination of the physical structure of injection molded parts has been determined that the favourable content of the blowing agents in the polymer should amount to 0.4-0.8 wt%, which leads to density decrease by approximately 32%. The porosity determines the gaseous phase content in a cellular product, determining at the same time the density decrease value of this product.

The mechanical properties of cellular products, including their hardness, are affected by the macromolecular shape, orientation and bond. An increase in the blowing agent content causes an increase in the polymer specific volume as well as in the number and size of pores obtained in the cellular process. A lower number of cross-linkages in the cellular polymeric material means decreased strength properties of such product.

This study found a clear influence of the type and the content of blowing agent on the mechanical properties as characterized a yield point and tensile strength. Yield point is lower, depending on the type of plastic, in the range of 19-32% for dosage of exothermic blowing agent and decreased from 19 to 43% in the case of endothermic blowing agent. Yield point ranges from 5.0 is 15.0 MPa, whereas an ultimate elongation ranges from 50 to 200%, depending on the type of plastic and blowing agents of the type and content. Tensile strength is changed in the same way, and decreases from 30 to 38% of the exothermic blowing agent and from 19 to 38% of the endothermic blowing agent.

Owing to the application of the blowing agent with the exothermic decomposition characteristics (Adcol blow X1020, Hydrocerol PLC 751 system), a more porous part, with a higher number of pores is produced. The initiated decomposition of the exothermic agent happens in an uncontrolled manner, even when heat transfer needed to decomposition is stopped. For this reason, parts produced with such blowing agents often have a non-uniform porous structure.

In the case of porous parts produced using the blowing agents with the endothermic decomposition characteristics, the gas release in the course of processing ends once the heat transfer stopped (Hostatron P 1941). The obtained porous structure is uniform, the pores have a spherical or quasi



spherical shape. The number of pores seen under the microscope, occurring in specific regions of the cross section, increases in a directly proportional manner with increasing the content of the blowing agent in the polymeric material being processed.

The influence of the amount and type of blowing agents on thickness of the solid in the products obtained porous was presented. It was shown, that changing the dosage of blowing agent from 0.4 to 2.0% will reduce the thickness of the solid layer almost 2.5 times, on average from 1.30 to 0.50 mm. Type of polymer material and the type of blowing agent has a visible effect on the thickness of the solid layer.

The porous structure is an advantage of injection molded parts produced in the cellular injection molding process, as it results in a decreased amount of the polymeric material (even by 30%) needed in their production. Changing the injection molded part structure from solid to porous by using appropriate types and content of the blowing agents, contributes to obtain parts with new functional properties, without significant worsening mechanical properties, and in short injection cycles.

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