A new formulation for polymeric separator gels for ... · Blood serum separator gel significantly improves serum and plasma analyte stability, facilitating storage and transport,

Progress in Rubber, Plastics and Recycling Technology, Vol. 34, No. 1, 2018 35

A new formulation for polymeric separator gels for potential use in blood serum separator tubes

Bahareh Babakhani1, Saeed Ostad Movahed*,2, Saghar Ghazy1 and Ali Ahmadpour1

1Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran2Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran

Received: 5th November 2016, Accepted: 9th February 2017

ABSTRACT

Blood serum separator tubes (SST) are used for collecting blood samples for performing clinical chemistry assays. Some SST’s have separator gels inside the tube which enable them better separation of the blood serum from packed cells during centrifugation. The cost, weak performance and interaction with blood ingredients are the most concerns of the available commercial gels. A commercial and cheap silicone oil as a polymeric base of the gel was chosen and formulated without and with several fillers. Subsequently, the compounds were crosslinked through a free radical crosslinking mechanism using dicumyl peroxide (DCP). The crosslinking took place in both, an oven and as well as under microwave irradiation in normal and under pressure conditions. The FTIR spectrometer analysis showed that both chain ends of the used silicone oil were terminated with a vinyl group. It also revealed that blood serum separator gel can be produced from selected silicone oil type. Among of different curing apparatus, curing in an oven was preferred due to less curing time and electrical energy consumption. The curing in normal pressure showed better results when compared with curing under pressure. Increasing the filler and DCP with various amounts had positive effect on gel densities. Silica was the most efficient filler among of the studied fillers. The cured compound filled with 10 and 8 phr silica and DCP, respectively,

*Correspondence to E-mail: S. Ostad Movahed ([email protected])

©Smithers Information Ltd., 2018

Bahareh Babakhani, Saeed Ostad Movahed, Saghar Ghazy and Ali Ahmadpour

36 Progress in Rubber, Plastics and Recycling Technology, Vol. 34, No. 1, 2018

was chosen as appropriate gel for SST due to suitable density and thixotropy. The selected gel was cured in oven under normal pressure for 30 minutes at 160°C.

Keywords: Polymeric gel; blood collection tube; filler; crosslinking

1 INTRODUCTION

Blood collection, storage conditions and processing methods are the main stages for clinical chemistry assays. Proper blood collection including appropri-ate choose of blood collection tubes, are needed to ensure the accuracy of the test results [1]. In past, syringes were used for blood collection, but in recent years, vacuum blood collection tubes have been replaced by syringe [1]. The first evacuated tube was invented by Joseph Kleiner in 1949 and assigned to Becton Dickinson [2]. The current evacuated tube systems included vacuum tubes with colored stopper and holder [3]. The tubes are available for adults and children in various sizes and the color of their stopper determines the type of tube additives [4]. In addition, they have special chemicals which facilitate blood clot formation [5].

Plain blood collection tubes were associated with many limitations. Some of these limitations include inappropriate performance in prolonged storage of the blood samples and difficulties for suitable separation of the blood serum from red blood cells. Moreover, long contact between serum and blood cells may change the serum color from yellow to red [6]. In order to overcome these lim-itations and issues, the blood serum separator tubes (SST) was introduced to laboratories in 1976 by Becton Dickinson [7]. SST tubes contain spray-coated silica and a polymer gel for serum separation. They use for serum determina-tions in chemistry, serology, immunology and HIV tests [1]. Serum separator gel that is located at end of the tube plays the role of a stable chemical and physical barrier between serum and blood clot [7].

Analysis of the blood samples often requires separation of whole blood into a serum fraction and a cell-containing fraction so SST tubes were noted quickly [8]. Blood serum separator gel significantly improves serum and plasma analyte stability, facilitating storage and transport, creating a serum or plasma with higher performance and etc. [9]. The most important factors that affect position of the serum separator gel in blood collection tubes are gel viscosity and density, centrifugation speed, temperature, and storage conditions [1,10]. Thixotropy and density are two important features in all separator gels which facilitate the separation of serum or plasma from cells [11,12]. The viscosity of the thixotropic materials reduces with increasing shearing time and amount. This property is essential for a blood serum separator gel due to the viscosity of the gel should be reduced during centrifugation for proper placing of the gel between bood serum and cells [13].



The gel manufacturers often use a liquid polymer along with an organic or inorganic filler in gel formulation to achieve appropriate density of the gel. Emerson et al. [4] prepared an in-situ rigid gel in a blood collection tube for separation of cell-depleted phase and cell-enriched phase. In this inven-tion, a polymerizable composition was added to the tube and then it was sterilized by using gamma irradiation or heat. Lamont et al. [14] pro-posed an effective co-polyester barrier from a short-chain dibasic acid, a polymeric fatty acid and a branched-chain saturated aliphatic diol. In this invention, small amounts of silica as inert filler were particularly useful with the co-polyesters to improved flow characteristics of the gel during centrifugation.

T. Conway [15] invented a device and method for separating heavier and lighter fractions of a fluid sample. The device comprised a thixotropic gel material such as mixtures of silicon and hydrophobic silicon dioxide powders or a mixture of a liquid polymer and silicon dioxide powder. Anraku et al. [16] proposed a formulation for blood serum separation gel. The composition was a polycyclic hydrocarbon compound, a cyclopentadiene oligomer and a phthalic ester. Suto et al. [17] invented a serum or plasma separating mate-rial included a moisture curing component. The moisture curing component would be a reactive silicone-based compound, an a-cyanoacrylate-based compound, a one-component moisture curing polyurethane resin, a moisture curing epoxy resin and a moisture curing polysulfide resin. In another attempt, F. Emerson [18] provided a separator substance that was polymerized in a short time to a desired hardness. Their separator gel had a polyester back-bone. Bowen and co-workers [19] investigated the effect of gel, clot activa-tor and surfactant content of the commercial SST’s on Thyroxin and Cortisol content of the human serum samples. They concluded that aforementioned materials had not any adverse effect on these analytes. Zungun et al. [20] compared the effect of two commercial SST’s, named improvacuter and BD vacutainer with SST without gel for various hormones in the aspects of sta-bility and influence of gel separators. They reached to this point that all SST’s had more or less the same chemical test results with the exception for two hormones, Estradiol and Testosterone. Hadi [21] studied the effect of gel con-tent SST’s on vitamin A and D in blood serum by using a high performance liquid chromatography (HPLC). He suggested that gel had adverse effect on quantitative determination of the tested vitamins. Steuer et al. [22] investi-gated the interaction of various gels in commercial SST’s with several drugs in blood serum. Their results showed that hydrophobic drugs were affected by presence of gel in SST.

However, the most available commercial separator gels are expensive and they have several performance limitations including, instability for certain ana-lytes, instability of the polymeric gel under extreme temperature conditions, presence of a part of gel or an oily film in the serum, absorption of specific drugs and some steroid hormones into the gel [23].



The main goal of this study is to introduce a new formula for used gels in the blood serum separator tubes. The proposed formula reduces SST cost as well as minimizes the aforementioned issues of the available commercial separator gels. A silicone oil as viscous liquid polymer and a peroxide (DCP, dicumyl per-oxide) as crosslinking agent with several fillers including talk, aluminum oxide, titanium dioxide, calcium carbonate, silica, and nano silica were chosen and formulated. The effects of curing technique, peroxide content, kind and amount of fillers, curing temperature and time on the density of the produced gels as the most important physical property of the gel, were studied and results were discussed. Finally, a new formulation for the separator gel was introduced.

2 EXPERIMENTAL

2.1 Materials-Polymer, Fillers and Chemicals

Silicone oil (SL5300A, Dimethylpolysiloxane, Vinyl terminated, KCC company, Korea), Scheme 1 with the density, viscosity and vinyl content of 0.892 g/ml, 1000 cp and 0.104 mmol/g, respectively, with clear appearance, silica (filler, ISATIS silica, Iran), nano silica (filler, ULTRASIL VN 3, Evonik Industries AG, Germany), Aluminum oxide, Titanium dioxide, talk powder, Calcium carbonate (fillers, MERCK, Germany), Dicumyl peroxide (DCP, High temperature catalyst, SIGMA ALDRICH, USA), Methanol (solvent, MERCK, Germany).

2.2 FTIR Spectrometer Analysis

The samples were extracted with acetone for 48 hours and then with toluene to remove all low molecules and subsequently dried in a vacuum oven at 70°C for 24 h before FTIR (Avatar 370-FTIR spectrophotometer) analysis. The resi-due gel samples were identified using the KBr-disc method. FTIR spectrometer analysis was performed to determine and confirm terminal groups of the sili-cone oil. Figure 1 shows the FTIR spectrum for used silicone oil.

2.3 Preparation of the Gels

For gel formulation, specified amount of the silicone oil, filler (if any) and DCP were weighed and strongly mixed with the help of a stirrer on a hot plate up to

Scheme 1 Chemical formula for used silicone oil

CH

CH3

Si

CH3

CH2

CH3

CH3

CH3

CH3

O (Si O )n Si CH CH2



melting point of DCP (About 40°C). After that the mixture was placed in either a glassy vessel (for atmospheric or normal pressure curing) or in a metallic mold (Figure 2, for curing under pressure). Subsequently, the assembly was cured either in an oven or by a microwave apparatus. Oven curing was carried out in

Figure 1 The FTIR spectrum for used silicone oil

Figure 2 The special designed mold for curing the silicone oil under pressure



Table 1 Formulations and curing condition for cured gels in oven (No filler in formulations, cured in normal pressure

Compound no.

Silicone oil (phr)

DCP (phr)

Curing temperature

(°C)

Curing time (min)

Gel formation

Density (g/ml)

1 100 0.5 160 30 No –

2 100 1 160 30 No –

3 100 2 160 30 No –

4 100 4 160 30 No –

5 100 8 160 30 Yes 0.911

6 100 16 160 30 Yes 0.926

7 100 4 160 40 No –

8 100 4 160 50 No –

9 100 4 170 30 No –

10 100 4 180 30 No –

Table 2 Formulations and curing condition for cured gels in oven (No filler in formulations, cured under pressure

Compound no.

Silicone oil (phr)

DCP (phr)

Curing temperature

(°C)

Curing time (min)

Gel formation

Density (g/ml)

11 100 8 160 10 No –

12 100 8 160 30 No –

13 100 8 160 60 No –

14 100 8 160 75 Yes 0.826

15 100 8 160 120 Yes 0.874

16 100 8 160 180 Yes 0.885

a laboratory oven, Finetech, SFCN-301. Micrwave curing was carried out in a laboratory microwave apparatus (GMO-530, Gosonic) with an output power of 900 Watt, frequency of 2000 MHz, and internal capacity of 30 Liter. Tables 1–6 show gel formulations and curing conditions.

2.4 Measurement of the Density of the Prepared Gels

Density of the prepared gels (rgel) was measured by use of Archimedes princi-ple. Due to insolubility of the prepared gels in methanol, this solvent was chosen for density measurement using following equations:

w w Vr s methanol− = ×ρ (1)



Table 3 Formulations and curing condition for cured gels in microwave (No filler in formulations, cured in normal pressure)

Compound no.

Silicone oil (phr)

DCP (phr)

Curing temperature

(°C)

Curing time (min)

Gel formation

Density (g/ml)

17 100 2 120 40 No –

18 100 4 120 40 No –

19 100 8 120 40 No –

20 100 16 120 40 No –

21 100 2 120 80 No –

22 100 4 120 80 No –

23 100 8 120 80 No –

24 100 16 120 80 No –

25 100 2 120 100 No –

26 100 4 120 100 Yes 0.906

27 100 8 120 100 Yes 0.919

28 100 16 120 100 Yes 0.937

Table 4 Formulations and curing condition for cured gels in oven with several fillers (cured in normal pressure)

Compound no.

DCP (phr)

Filler type Filler (phr)

Curing temperature

(°C)

Curing time (min)

Gel formation

Density (g/ml)

29 8 Calcium carbonate

15 160 30 No –

30 8 Talk powder

15 160 30 No –

31 8 Titanium dioxide

15 160 30 No –

32 8 Aluminum oxide

15 160 30 Yes 1.096

33 8 Titanium dioxide

30 160 30 Yes 1.113

34 8 Aluminum oxide

30 160 30 Yes 1.142

35 8 Nano silica 10 160 30 Yes 0.992

36 8 Nano silica 15 160 30 Yes 1.015

37 8 Nano silica 20 160 30 Yes 1.088

38 8 Nano silica 30 160 30 Yes 1.089

39 8 Nano silica 40 160 30 Yes 1.090



Table 5 Formulations and curing condition for cured gels in oven with silica as filler (cured in normal pressure)

Compound no.

Silicone oil (phr)

Silica (phr)

DCP (phr)

Curing temperature

(°C)

Curing time (min)

Gel formation

Density (g/ml)

40 100 2 8 160 30 Yes 0.959

41 100 5 8 160 30 Yes 0.946

42 100 10 8 160 30 Yes 1.050

43 100 12 8 160 30 Yes 1.092

44 100 15 8 160 30 Yes 1.121

45 100 20 8 160 30 Yes 1.128

46 100 30 8 160 30 Yes 1.128

47 100 40 8 160 30 Yes 1.346

ρρgel

r

r s methanol

mgvg

w(w w )/

= =−

(2)

Where Wr is real weight of gel (g), Ws is weight of gel in methanol (g), V is volume of gel (ml) and rmethanol is density of methanol (0.777 g/ml). The density of the prepared gels were summarized in Tables 1–6.

3 RESULTS AND DISCUSSION

For an ideal separation, the thixotropic gel should have a stable composition in the absence of any substantial application of centrifugal force. Moreover, the gel should be chemically inert, and have a density which is intermediate that of the blood phases to be separated [24]. In this study, silicone oil with vinyl termi-nated groups was used as polymeric viscous liquid because it had desire prop-erties, i.e., physiological inertness, cheap, commercially available and blood compatibility [25]. Silicone oils can be crosslinked and cured (vulcanized) into solids by using a variety of curing systems and heating sources. The heating curing by using a heating source, i.e., microwave and or heating by an oven is a common curing method for silicone oil composites [26]. The curing by micro-wave irradiation has several advantages including, homogeneous heating due to deep penetration of the microwaves in the composite (polymer) matrix [27]. In health care applications, free radical (peroxide) curing system is preferred [28]. Free-radical cure systems use peroxides that are either vinyl-specific or vinyl-nonspecific in nature. Dicumyl peroxide (DCP) is one of the vinyl-specific peroxides and require the presence of vinyl species [29]. DCP is used as a high temperature catalyst in the rubber and plastic industries. The compounds containing DCP can be cured at temperatures above 149°C [30]. The specific



Tab

le 6

For

mul

atio

ns a

nd c

urin

g co

nditi

on fo

r cu

red

gel

s in

mic

row

ave

with

sev

eral

fille

rs (c

ured

in n

orm

al p

ress

ure)

Co

mp

oun

d n

o.

Sili

cone

oil

(phr

)D

CP

(phr

)F

iller

typ

e F

iller

(p

hr)

Irra

dia

tion

time

(min

)P

ow

er

(%)

Gel

fo

rmat

ion

Den

sity

(g

/ml)

4810

08

Cal

cium

car

bona

te15

120

100

Yes

1.30

9

4910

08

Talk

pow

der

1512

010

0N

o–

5010

08

Tita

nium

dio

xide

1512

010

0Ye

s 1

.243

5110

08

Alu

min

um o

xide

1512

010

0Ye

s 1

.443

5210

08

Silic

a15

120

100

Yes

1.4

80

5310

08

Nan

o si

lica

1012

040

No

–

5410

08

Nan

o si

lica

1012

060

No

–

5510

08

Nan

o si

lica

1012

080

Yes

0.8

90

5610

08

Nan

o si

lica

1012

010

0Ye

s0.

909

5710

08

Nan

o si

lica

1512

010

0Ye

s0.

961

5810

08

Nan

o si

lica

3012

010

0 Y

es0.

966

5910

08

Nan

o si

lica

4012

010

0Ye

s1.

064



gravity of the whole human blood is generally within the range of 1.048 to 1.066 g/ml. Human blood can be easily centrifuged for separation of the lighter serum portion having a specific gravity within the range of 1.026 to 1.031 g/ml and the heavier clot portion, consisting mainly of red blood cells, having a specific gravity within the range of 1.092 to 1.095 g/ml. In order to separate blood into two major portions, the specific gravity of thixotropic gel should be within the range of 1.03 to 1.09 g/ml (preferably 1.04 g/ml) [1,31].

3.1 Identification of the Vinyl Groups in Used Silicone Oil by FTIR Spectrometer Analysis

The FTIR is a device that shoots infrared radiation (energy), into a sample and measures the amount of IR radiation that is absorbed and transmitted by the chemical bonds between atoms in a molecule. Figure 1 shows the FTIR spectrums for used silicone oil (Scheme 1). As it observes, a peak appeared at 1598.8 cm-1 which can be attributed to C C bond in vinyl groups of the silicone oil. In addition, a peak at 864.97 cm-1 along with a small peak around 3100 cm-1 prove the presence of alkene C H bonds in formulation of the silicone oil. The peaks were appeared at 1412.68 and 2905.27 cm-1 belong to Alkane C H bonds in methyl groups that bonded to silicone from both sides (Scheme 1).

Scheme 2 Mechanism of the free radical crosslinking of used silicone oil with DCP



Figure 3 Appearance of the prepared gel containing (a) 8 phr DCP, compound 5 and (b) 16 phr DCP, compound 6

3.2 Effect of the Peroxide Content on Density and Appearance of the Prepared Gum (Without Filler) Gels

Due to existence of vinyl group in both ends of each macromolecule in silicone oil (Scheme 1), DCP crosslinks through these sites via free radical mechanism (Scheme 2). In these experiments, polymer curing (crosslinking) took place in normal or atmospheric pressure in an oven and or in a microwave apparatus. Here, the mixtures were placed in a glassy open vessel. For curing under pres-sure, the mixtures were placed in a special mold (Figure 2) and after that the mold was placed in oven for curing. Table 1 shows formulations and curing conditions for cured gels in oven with no filler in formulations, and in normal pressure. The curing temperature and time for Compounds 1–5 were 160°C and 30 minutes, respectively. The DCP values increased from 0.5 to 16 phr. As it observes, adding DCP up to 4 phr resulted in no gel formation. However, gels formed with 8 and 16 phr DCP with the densities of 0.9110 and 0.926 g/ml, respectively. Doubling the DCP values from 8 to 16 phr had only 1.6% increase on density which from economic view point, it is not acceptable. In addition, the appearance changed from a transparent to an opaque gel (Figure 3). Increas-ing curing time from 30 to 40 and 50 minutes (compare Compounds 4,7 and 8 in Table 1) and curing temperature from 160°C to 170 and 180°C (compare Compound 4, 9 and 10 in the same table) had not any effect on gel formation. It was concluded that Compound 5 with 8 phr DCP in formulation with curing temperature and time of 160°C and 30 minutes, respectively, can be chosen for further modification as a suitable candidate for SST gel.

Table 2 presents formulations and curing conditions for cured gels in oven under pressure and without filler in formulations. All compounds had 8 phr DCP



with the same curing temperature (160°C) with different curing times, 10, 30, 60, 75, 120 and 180 minutes. Up to 60 minutes, no gel was formed while increasing curing time to the values of 75, 120 and 180 minutes caused gel formation with densities, 0.826, 0.874 and 0.885 g/ml for Compounds 14 to 16, respectively. As observes, curing under pressure had adverse effect on gel formation through increasing curing time and reducing density when compared with the corresponding values for normal pressure curing. The reason refers to the capture of the bubbles of light and gaseous curing reaction byprod-ucts in gel structure. These bubbles could not escape from the reaction vessel because, reactions took placed under pressure in a closed mold (Figure 2). Table 3 depicts formulations and curing conditions for cured gels in microwave with no filler in formulations, and in normal pressure. The irradiation time for all compounds were 120 minutes with different amounts of DCP, 2, 4, 8 and 10 phr for three microwave powers, 40, 80 and 100%. For powers less than 100%, no gel formed regardless of the DCP value in compound’s formulation (Compounds 17 to 24). For 100% power, Compounds 26 to 28, gel formed with the exception of the Compound 25 (2 phr DCP) with the density values of 0.906, 0.919 and 0.937 g/ml, respectively. Although, these density values were completely comparable with those of the oven cured gels under atmospheric pressure, but relatively long irradiation time (120 minutes) was not desired economically.

3.3 Effect of the Filler Type and Content on Density and Appearance of the Prepared Gels

Six fillers, calcium carbonate, talk powder, titanium dioxide, aluminum oxide, nano silica and silica were chosen and Compounds 29 to 32 and 36 (Table 4) and Compound 44 (Table 5) were prepared. These compounds had 15 phr filler and 8 phr DCP and they were cured for 30 minutes at 160°C in oven under atmospheric pressure. No gel formed for loaded compounds with calcium car-bonate, talk powder and titanium dioxide (Compounds 29 to 31). However, gel formed when the value of the titanium dioxide increased from 15 phr to 30 phr (Compound 33) with the density of 1.113 g/ml. Compounds loaded with aluminum oxide, nano silica and silica (Compounds 32, 36 and 44) were cured properly and gel formed with the densities of 1.096, 1.015 and 1.121 g/ml, respectively. Increasing filler had positive effect on gel densities when com-pared with the density of the gum compound (Compound 5, Table 1) with the value of 0.911 g/ml. As it observes, silica loaded compound had the highest density. It was predictable because silicone oil and silica have silicone in their backbones and they are compatible with each other. Unexpectedly, the den-sity of nano silica loaded compound was less than that of the aluminum oxide loaded compound. Figure 4 compares the densities of the compounds loaded with different amounts of nano silica (Compounds 35 to 39, Table 4) and silica



Figure 4 Density versus filler loading for compounds loaded with various amounts of nano silica and silica

(Compounds 42 to 47, Table 5). The densities lie down in the range of 0.992 to 1.090 g/ml and 1.050 to 1.346 g/ml for nano silica and silica, respectively. Generally, the values for silica are extremely higher when compared with those values for nao silica. The reason may refer to nano size of the silica particles and their suitable dispersion in silicone oil.

Compounds 48 to 52 and 57 (Table 6) have 8 phr DCP and 15 phr various fillers, calcium carbonate, talk powder, titanium dioxide, aluminum oxide, silica and nano silica, with 120 minutes irradiation time at 100% microwave power. As it observes, with the exception of loaded compound with talk powder (Com-pound 49), the rest compounds were cured and gel formed properly. Unlike curing compounds in oven, microwave irradiation caused gel formation for cal-cium carbonate loaded compound (compare Compound 29 with Compound 48 in Tables 4 and 6). The corresponding densities for calcium carbonate, tita-nium dioxide, aluminum oxide, silica and nao silica were 1.309, 1.243, 1.443, 1.480 and 0.961 g/ml, respectively. The lowest value belongs to nano silica due to proper dispersion of this filler in polymer matrix.

To investigate the effect of microwave power on gel formation, four more compounds (Compounds 53 to 56, Table 6) were prepared and cured with 10 phr nano silica and 8 phr DCP. For two powers, 40 and 60%, no gel formed while for 80 and 100% power gel formed with corresponding densities of 0.890 and 0.909 g/ml, respectively. As a conclusion, microwave power was more effective on gel formation of the filled compounds when compared with gum (without filler) compounds (compare Compound 23 with 55 in Tables 3 and 6).



This is due to microwave irradiation is effective only on polar species and nano silica has a polar chemical structure. The effect of different amounts of nano silica loading for 100% irradiation power was studied (Compounds 56 to 59, Table 6). The corresponding densities for 10, 15, 30 and 40 phr nano silica were 0.909, 0.961, 0.966 and 1.064 g/ml, respectively. As expected, filler loading increased the densities sensitively.

3.4 Thixotropy and Performance of the Selected Gels During Blood Processing

Thixotropy is a time dependent shear thinning property of the materials, namely, polymers. Certain gels, polymers and or fluids that are thick (viscous) under static conditions will flow (become thin, less viscous) over time when shaken, centrifuged or otherwise stressed (time dependent viscosity) [32]. The complication of thixotropy arises because this reversible microstructural change takes time due to local spatial rearrangement of the components (Scheme 3) [33]. So many factors affect thixotropy of a polymeric material including pH [34], temperature [35], stress conditions and aging time [36]. As mentioned earlier, the density of gel in the range of 1.03 to 1.09 g/ml may be suitable for proper separation of the blood serum from clots. How-ever, proper selection of the gel density does not necessarily guarantee the appropriate performance of the gel. The rheological (thixotropy) properties of the gel has enough importance. Lacking of suitable thixotropy causes

Scheme 3 Breakdown of a 3D thixotropic structure [18]



Figure 5 The performance of the cured gels after centrifugation filled with titanium dioxide (a), aluminum oxide (b), with 40 phr nano silica(c), with 15 phr nano silica (d) and without filler with microwave curing (e) for compounds 33, 32, 38, 36 and 26, respectively

either complete or partially floatation or remaining of the gel in the bottom of the testing tubes. The ideal gel should completely place between blood clots and serum. Figure 5 shows the performance of the cured gels after centrifugation filled with titanium dioxide (a), aluminum oxide (b), with 40 phr nano silica(c), with 15 phr nano silica (d) and without filler with microwave curing (e) for Compounds 33, 32, 38, 36 and 26, respectively. All selected gels had desired densities with the exception of Compound 26 with the density of 0.906 g/ml. However, Figures 5 (a) to 5(d) clearly show although gels had densities in the appropriate range but they could not float after cen-trifugation and stayed in the bottom of the testing tubes. Compound 26 (Figure 5-e) completely floated on the surface of the serum. This compound had good thixotropy but due to its lower density, it is not suitable as a blood serum separator gel. Figure 6 depicts the performance of the cured gels after centrifugation filled with different amounts of silica, 5 phr (a), 10 phr (b) and 12 phr (c) for Compounds 41, 42 and 43 in Table 5, respectively. These compounds had densities with the values of 0.946, 1.050 and 1.092 g/ml. All aforementioned compounds had desired thixotropy and moved from bottom of the tubes during centrifugation.

For Compound 41 (Figure 6-a), the total gel was floated on the serum surface while for the left two compounds, gels placed exactly between blood clots and serum. As a conclusion, Compound 42 filled with 10 phr silica and 8 phr DCP was chosen as appropriate gel for blood serum separator gel (Figure 7).



Figure 7 Appearance of the selected gel for optimum blood serum separation with 10 and 8 phr silica and DCP in formulation, respectively (compound 42, Table 5)

Figure 6 The performance of the cured gels after centrifugation filled with different amounts of silica, 5 phr (a), 10 phr (b) and 12 phr (c) for compounds 41, 42 and 43 in Table 5, respectively



4 CONCLUSIONS

From this study, it can be concluded:

1 The FTIR spectrometer analysis showed that both chain ends of the used silicone oil were terminated with a vinyl group.

2 Blood serum separagor gel can be produced from selected silicone oil type using DCP as crosslink agent through a free radical crosslinking mechanism.

3 Increasing DCP up to 16 phr helped effectively the gel formation. 4 Among of different curing techniques, curing in an oven was preferred

due to less curing time and electrical energy consumption. The curing in normal pressure showed better results when compared with curing under pressure.

5 Increasing the filler amounts in gel formulation had positive effect on gel densities.

6 Silica was the most efficient filler among of the various studied fillers.7 The cured compound filled with 10 and 8 phr silica and DCP, respectively,

was chosen as appropriate gel for blood serum separator tube due to its suitable density and thixotropy The gel was cured in oven in normal pressure for 30 minutes at 160°C.

5 ACKNOWLEDGMENT

The authors sincerely thank the staffs of Dr. Moayed clinical laboratory for their cooperation for testing the performance of the cured gels.

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