Study on Poly(Viny1 Chloride) Paste Blending Resins

LIUCHENG ZHANG, HUIWEN TAI, JINWEI ZHANG, and W D I LIU

Department of Chemical Engineering Hebei Institute of Technology

Tianjin, 3001 30, People’s Republic of China

Polyvinyl chloride paste blending resins were prepared by a suspension polymer- ization process, and the optimum ingredients and technological conditions were studied experimentally. The effect of dispersant, surfactant, and agitator design on the particle morphology was discussed. In order to improve the thermal stability of the blending resin, stabilizer was added to the polymerization system, and a special after-treatment was adopted. The effect of blending resins on the rheological and gelation characteristics of the plastisols formed and on mechanical properties, thermal stability, and foamability of the finished articles is demonstrated.

INTRODUCTION

n order to improve the viscosity properties of paste I dispersion and to reduce the cost of the paste for- mulation, it is the usual practice to blend paste poly- vinyl chloride resins (paste PVC) with coarse particles of vinyl chloride resins. These vinyl chloride resins in the form of coarse particles are generally called poly- vinyl chloride paste blending resins or polyvinyl chlo- ride blending resins (blending resins) ( 1, 2).

The blending resins should preferably be in the form of spherical single particles with smooth surfaces in order to increase the effect of reducing the viscosity of paste dispersions ( 3 ) . The blending resins can bring about a greater effect on viscosity reduction as their particle diameters are larger. On the other hand, larger particle diameters cause such disadvantages as the degradation of properties and the reduction of clarity owing to insufficient melting during process- ing, and the sedimentation of polymer particles in paste dispersions. To avoid this disadvantage, the blending resins should preferably have an average particle diameter of 10 to 80 microns, i.e., intermedi- ate between paste resins and general-purpose resins.

Blending resins affect the surface hardness and me- chanical properties of the material. With the same plasticizer content, the tear strength and break elon- gation are reduced. Normally, the use of blending res- ins makes it possible to prepare harder material, re- sulting in higher tear strengths and better abrasion resistance. The increases in the gelling temperature or time that are the result of using blending resins also allow the improvement of the mechanical properties of the finished articles.

Blending resins have been used in flooring, cover layers, synthetic leather, cast and dipped articles, and other applications.

The following general quality criteria are important in the processing of blending resins: optimum particle size and narrow particle size distribution for each paste PVC: ideal spherical form; good gelling proper- ties; good heat stability: low water absorption of the final films: low plasticizer absorption and low sedi- mentation tendency in paste; no detrimental effect on the foamability of foam recipes: and neutral test and physiological acceptability. The patent literature on blending resins dates back roughly 35 years. In 1966, Borden Chemical was successful in supplying the in- dustry with homopolymer blending resins ( 3 ) . These resins intended to partially replace the paste resin and thus reduce the cost and lower the viscosity of the paste formulation without detracting from any of the physical properties. Prior to this, BF Goodrich had supplied vinyl chloride /vinylidene chloride blending resins. Currently, vinyl chloride homopolymers and copolymers of vinyl chloride with vinyl acetate, vi- nylidene chloride, and maleic esters are commer- cially available. Most of the processes for preparing blending resins are proprietary, and most processes for manufacturing blending resins are suspension polymerization techniques (4-1 1). Systematic stud- ies of blending resins have rarely been reported in the literature. In the present work, the preparation process and properties of blending resins were stud- ied experimentally. The effect of dispersants, surfac- tants, and agitation on the morphology of particles is discussed. In order to improve the thermal stabil- ity of the blending resins, stabilizer was added to the polymerization system, and special after-treatment was adopted. The effect of blending resins on rheo- logical and gelation characteristics of the pastisols and on mechanical properties, thermal stability, and foamability of the finished articles are also dem- onstrated.

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Study on Polywinyl Chloride) Paste Blending Resins

EXPERIMENTAL DETAILS

Materials

Vinyl chloride (VCM1:purity > 99.9%, acetylene con- tent < 0.01%; deionized water: C1-content < 100 ppm; azobis(2,4-dimethyl Valeronitrile) (AIVN): industrial grade; Di(-ethylhexyl) peroxydicarbonate (EHP): in- dustrial grade; bis(oxyethy1 phenyl) peroxydicar- bonate (BPPD): industrial grade; NaOH: analytical reagent; polyvinyl alcohol (PVA): KH-20; hydroxy- propylmethyl cellulose (HPMC): chemically pure; gelatin: analytically pure; H,O,: chemically pure; dibutyltin dilaurate (DBTDL): industrial grade; am- monium sulfide, (NH,),S: chemically pure; acetone, cyclohexnone and bisphenol A: chemically pure: sil- icone fluid: industrial grade.

Synthesis of Blending Resins

A 30 liter stainless steel autoclave equipped with an agitator and a jacket for heating and cooling was charged with a known quantity of demineralized water and initiator, dispersant, surfactant, and buffers. The autoclave was sealed, pressure tested, and evacuated. The mixture was agitated for 15 min. After that, VCM was added to the reactor and agitated for another 30 min, and then the temperature was raised to the po- lymerization temperature. The agitation was contin- ued throughout the reaction until a pressure drop of 0.8 MPa in the reactor occurred, and then a certain amount of termination agent and stabilizer was in- jected into the reactor. When pressure in reactor had fallen to 0.5 MPa, the reaction was then stopped and the polymer slurry was removed from the reactor. A general recipe is shown in TabIe 1 .

Measurement

The gelation behavior of the blending resins was determined using a Brabender Plasticorder PLE-330. A given amount of paste resin and blending resin was mixed thoroughly with plasticizer, lubricant, and sta- bilizer. The heating rate was G"C/min, and the rota- tion speed was 30 rpm.

A rotational viscometer (NDJ- 1, Shanghai Balance factory, China) was employed to measure the paste viscosity. The particle diameter and the particle size distribution were measured by particle size distribu- tion analyzer, CAPA-300. K-value was measured us- ing Ubbelohde-viscometer (25 ? 0.05"C). The me- chanical properties of finished articles were measured by a tensile tester (100 mm/min). Bulk density was determined by a 100 rnL cylinder (diameter 45 mm),

Table 1. A General Recipe for a Blending Resin.

Material Amount

Water VCM AWN Gelatin -Surfactant

191 91 16.0 g 30.0 g 10 - 150 g

and plasticizer adsorption was determined by centri- fuge (model 800, China).

RESULTS AND DISCUSSION

Effect of Dispersant and Surface on Particle Size and Particle Morphology of the Blending Resin

The particle size was represented as having average diameter. The particle size distribution was defined by Hansion (12) follows:

Distribution width index

where D, and D, stand for number-average diameter and weight-average diameter, respectively, and N , and Di stand for the number and diameter of particle i, respectively.

Table 2 shows the influence of dispersants on the properties of the blending resin. The effects of dispers- ant amount on particle size and size distribution are shown in Fig. 1 . From Table 2 it can be seen that when gelatin was used as a dispersant, denser and more spherically shaped particles were formed. Figure 1 shows that the greater the amount of gelatin, the more narrow the distribution width. The experimental re- sults described above may be explained as follows. During the suspension polymerization process, the VCM droplets are converted gradually from a non- sticky liquid, through a mixed droplet system consist- ing of a PVC/VCM phase with some free VCM to PVC grains containing some VCM. In the intermediate stages of this polymerization, the particles are viscous and sticky and tend to agglomerate. In most agitation systems, this agglomeration process would occur to an uncontrolled extent, and large lumps of PVC would result. The presence of the dispersant may prevent this problem. A s a water-soluble polymer, the dispers- ant is capable of fully protecting the original droplets leading to PVC granules. That is, the dispersants tend to gravitate to the interface between the two phases, and stabilize the droplets by lowering the interfacial tension between the two phases and by steric stabili- zation. On the other hand, water-soluble polymers may cause an opposite effect of inducing instability in dispersed systems; this phenomenon is referred to as "bridging effect" (13). In addition, it has recently been proposed that there is a contribution to the attractive force between proximate colloidal particles suspended in a solution of macromolecules when the effective diameter of the macromolecule coils exceeds the dis-

Table 2. The Effect of Dispersant on the Particle Morphology and Bulk Density of Blending Resins.

Bulk DOP Sample Density Absorption Particle Number Dispersant g/ml g/g PVC Shape

PO02 gelatin 0.58 0.13 glass bead PO03 PVA 0.48 0.29 cotton ball PO05 HPMC 0.46 0.33 cotton ball

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Liucheng Zhang e t al.

zw

150

1w

50

2 4 c

-

-

-I 4

0 i -------- ------A 0 10 M 30 40 50 EU 70 50 50 1W

gelarin amount (g)

Fig. 1 . Dependence oJparticle diameter and its distribution on the gelation amount.

lance between the particles; this force can be termed as the “volume restriction” or “exclusion” force. Under certain conditions, this force can induce agglomera- tion (14). Therefore, the soluble polymer can stabilize or destabilize the dispersion systems. The final result depends on the competition between these two as- pects. Considerations described above can explain the experimental results shown in Fig. 1.

From T a b l e 3 and Fig. 2 it can be seen that oil- soluble surfactants increase the bulk density of blend- ing resin and form uniform particle shapes. On the other hand, water-soluble surfactants reduce the di- ameter of particles.

Experimental data (see T a b l e 4) show that as the amount of water-soluble surfactant is increased, the particle diameter is reduced and the distribution of particle diameters is widened. Generally, the amount of water-soluble surfactant used should not exceed the critical micelle concentration (CMC] of the water- soluble surfactant; otherwise, fine particles cannot be formed.

As the amount of oil-soluble surfactant increases, the bulk density increases and the oil absorption de- creases, but when the amount is in excess of 100 g, this tendency greatly decreases (see T a b l e 5). Experi- mental data indicate that when oil-soluble nonionic surfactants are employed in combination with certain water-soluble surfactants, denser and smaller diam- eter particles can be formed, as shown in T a b l e 6.

Table 3. The Effect of Surfactant on the Particle Morphology and Bulk Density of Blending Resins.

Particle Bulk DOP Sample Amount Diameter Density Absorption Number Surfactant* (9) (pm) (g/ml) (g/g PVC)

PO1 2 0 113 0.56 0.13 P158 OA-01 120 100 0.61 0.1 0 P161 OA-02 120 98 0.60 0.09 P174 ON-01 120 96 0.62 0.09 P179 ON-02 120 98 0.60 0.1 1 P186 WA-01 6 50 0.61 0.14 P190 WA-02 6 56 0.56 0.15

OA stands for oil-soluble anionic surfactant; ON stands for oil-soluble nonionic surfactant; WA stands for water-soluble anionic surfactant.

Influence of Agitator Design and Agitation Speed on Particle Size and Particle Morphology

Agitation has the function of producing the neces- sary droplet size, maintaining a suspension of these droplets and ultimately of PVC grains, and ensuring good heat transfer from the polymerizing mass to the autoclave walls. The agitator design and agitation speed have a great effect on the particle size and par- ticle morphology.

A paddle blade stirrer with three blades was used in this work. The influence of blade shape and the num- ber of layers of blades on particle size is shown in T a b l e 7.

In this work, the minimum and maximum critical agitation speeds are 120 rpm and 850 rpm, respec- tively. In this range of agitation speed, the relationship between agitation speed and particle diameter and plasticizer-absorption are shown in Figs. 3 and 4 , re- spectively.

Figures 3 and 4 show that between minimum and maximum agitation speed, as the agitation speed is increased, the particle size decreases, but the plasti- cizer-absorption increases, i.e., the bulk density de- creases.

In order to obtain a blending resin with smaller particle size and higher bulk density, it was proposed that higher agitator speed should be used in the initial stage of polymerization. Agitator speed was then low- ered as polymerization conversion proceeded. The de- pendence of particle diameter and plasticizer-absorp- lion on polymerization conversion at which the agitation speed was changed to a lower level is shown in Figs . 5 and 6.

Influence of Other Factors

Experimental results indicate that the addition of stabilizer during polymerization can increase the heat stability of the blending resin from 3.8 min to 7.3 min. The water/VCM ratio should be -2. The pH value of the polymerization system should be about 9- 1 1, a value that can be controlled by a buffer (NaOH, NaHCO,, and (NH,),S). A typical polymerization recipe is shown in T a b l e 8.

Influence of Blending Resin on the Properties of the Plastisol

The main advantage in the use of blending resins in processing of PVC pastes is the significant reduction in viscosity and dilation, as shown in Figs. 7 and 8. Figure 7 shows that the maximum viscosity reduction is achieved with about 50 wt% of blending resin. This is due to the larger particle size and the lower oil-absorption or larger bulk density of the blending resins (15).

Over a wide range of shear rates, the plastisol per- forms as a pseudo-plastic flow at low or high shear rates, and indicates shear thickening at intermediate shear rates (16). Willey (18) has shown that at a given volume fraction, interparticle forces are constant, and that Brownian motion governs the viscosity at low

32 JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1

Study on Polywinyl Chloride) Paste Blending Resins

Figure 2 (c ) P174 - 200X Figure 2(d) P186 - 200X Fig. 2. Collage of photomicrographs of blending resin particles [a) P102, magnification 100X; [b) P158, magnification 200X: (c) P174, magnification 200X: Id) P186, magnification 200x.

shear rates. Therefore, the pseudo-plastic behavior a t low shear rate may be explained as a competition between the randomizing effects of the Brownian mo- tion and the “orienting” effect of the shear force. At

higher shear rates, the Brownian effects become in- significant. The maximum in the flow curves at inter- mediate shear rates, the changeover of shear thicken- ing to pseudo-plastic behavior, has been explained

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Liucheng Zhang et al.

0.17

9 0.16

5 0.15

0.14

h

$5

H

Q

Table 4. The Effect of Water-Soluble Anionic Surfactant Amount on the Particle Size.

Amount Average Sample of WA-01 Diameter Number (9) (wm) Dispersity

-

-

-

- PO74 0 136 1.4386 P180 3.0 74 1.5637 P185 6.0 52 1.4892 P189 9.0 47 1.7645 P196 12.0 42 3.6542

Table 5. The Effect of Oil-Soluble Nonionic Surfactant Amount on the Particle Size and Bulk Density.

Amount Bulk Sample of ON-01 Density Oil-Absorption Number (9) (g/ml) (g/g PVC) Dispersity

PO1 4 0 0.56 0.14 1.8401 P170 50 0.59 0.1 2 1.7967 P171 100 0.60 0.1 2 1.8249 P175 150 0.60 0.1 1 1.7861

500 60 m 890 900

agbflm speed (rpm)

Fig. 3. Influence of agitation speed on the average particle diameter.

0.18 [ 1

e

a1 ‘ I 5M3 6 M ) m 800 900 loo0

agltatlon apeed(rpm)

Fig. 4. Influence of agitation speed on the speciJicplasticizer- absorption.

, ’

I C 5 l C 1 5 2 f l 2 5 3 0 3 5

pdymemabon cmvarwn (46)

Fig. 5. Dependence of particle average diameter on the con- version at which the agitation speed was changed to a lower level.

Table 6. Influence of Combination of Oil-Soluble Surfactant With Water-Soluble Surfactant on the Particle Size and Bulk Density.

Amount of Amount of Amount of Bulk Oil Average Sample OA-02 ON-01 WA-01 Density Absorption Diameter Number (9) (9) (9) (g/ml) (g/ml) (wm)

P163 120 0 P176 0 120

6 0.53 0.14 50 6 0.59 0.1 1 49

~ ~~ ~~

Table 7. Influence of Blade Shape and Number of Layers on the Particle Size.’

Sample Blade Number Layer Shape

Length Width Particle of Blade of Blade Diameter Dispersity of

(mm) (mm) (pm) Particle Size

PO05 mono-layer three blade 45 paddle 100 40 large lumps PO28 two-layer two blade 45 paddle 120 40 160 2.9769 PO39 three-layer the two lower layer with 45 paddle; upper layer 120 40 127 1.8946

PO76 three-layer the upper layer with level paddle; the two lower layers 120 40 112 1.4321 with level paddle

with 45 paddle

*Recipe of polymerization systems, see Table 1; agitation speed is 450 rpm; polymerization temperature 62°C.

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Study on Polywinyl Chloride) Paste Blending Resins

Table 8. Polymerization Recipe.*

Component Amount

VCM 91 H2O 19 I AWN 20 9 gelatin 50 g OB-01 120 g WA-01 6 g NaOH 4.5 g NaHCO, 5 9 (NH'l)*S 3 9 EDTA 3 9 DBTDL 3 9 bisphenol A 2 9 silicone fluid 10 rnl

* Polymerization temperature is 62"C, and agitation speed is 300-600 rpm.

0.16 I

- 0.14 ,

2 0.13 ij li 0.12

+

I 0 5 10 15 2C 25 33 35 40

pdymwaa:ron conversm ('A)

Fig. 6. Dependence ofplasticizer absorption on the conversion at which the agitation speed was charged to a lower level.

10 1

1

0 I . I 0 20 40 60 80 100

amounl of blmding resin(%) Fig. 7. Egect of amount of blending resin on the viscosity of PVC plastisol: Amount of DOP: ( 1 ) 40%; (2) 50%.

(19) as a competition between the number of particle- particle interactions (links) and the lifetime of such links. Thus with increasing shear rates, the number of links would increase, and the viscosity would rise. The

0.01 0.1 1 10 100 loo0

shear rate (s")

Opaste resin:blending resin:D0P= 100:0:100 o pasta r8sin:blending resin:D0P=70:30:100 u paste rwhblending resin;WP=50:%:100

Fig. 8. Dependence of viscosity on the shear ratefor various amounts of blending resin. Paste resin: blending resin: DOP is [a) 1OO:O:lOO; (bJ 70:30:100; [cJ 50:50:100.

" 60 To 80

ternpaawe ('C)

Fig. 9. Influence of blending resin on the gelling behavior of PVC plastisol; Paste resin:blending resin:DOP is ( 1 ) 1OO:O: 1 00; (21 80:20:100; (3J 50:50:100.

link lifetime will decrease with increasing shear rates, and when this overcomes the effect of the former, the viscosity will fall again. As shown in Fig. 8, adding blending resins with larger particles reduces the shear thickening behavior, and when the added amount of blending resin reaches about 50%, shear thickening behavior almost vanishes. These experimental data show that the addition of larger-particle blending resin reduces the interaction between particles.

This effect means that some pastes are made coat- able only when there is an addition of blending resins.

JOURNAL OF VINYL B ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 35

Liucheng Zhang et al.

Table 9. Influence of Blending Resin on the Storage Stability of the Plastisol.

Blending Resin: Paste Resin:

DOP

Initial Viscosity Viscosity of Plastisol of Plastisol After 10 Days

(Pa. S) (Pa. S)

Increment of Viscosity

(%I 0:100:100 50:50:100

6.70 2.00

1.16 2.60

78.1 30.0

Table 10. Influence of Blending Resin on the Mechanical and Physical Properties of Finished Articles.

Paste Break Tensile Vicat Water- Permeability D0P:Paste Resin: Viscosity Elongation Strength Shore Softening Absorption Coefficient

Blending Resin (Pa. S) (W (MPa) Hardness Point (“C) (“/I (cm3/cm2/s)

55:lOO:O 9.0 246 14.3 58.0 102 0.37 8.5 46:50:50 9.0 182 18.6 87.0 113 0.35 8.2 44:50:50 9.0 163 19.3 89.5 115 0.31 7.3

The reduction of viscosity in the low shear range means that the pastes are self-deaerating or can be deaerated more easily. In addition, the better flow be- hind the blade prevents spreading faults (e.g., blade stripes).

The reduction of viscosity achieved by blending res- ins in the rotational casting process means that the plastisol will flow into more complicated and narrower moldings. This viscosity reduction also works to the benefit of the dipping and casting process because the formation of bubbles is prevented in complicated ar- ticles and thinner wall thicknesses may be achieved.

Figure 9 shows that, as a result of using blending resin, the gelling temperature and gelling time was increased. This allows the improvement of the me- chanical properties of the finished articles.

Experimental data also show that the viscosity of the plastisol after aging is improved by addition of blending resin, as shown in Table 9.

The Effect of the Blending Resin on the Properties of Finished Articles

The use of blending resins affect, among other things, surface hardness and mechanical properties. With the same plasticizer content, the mechanical properties (break elongation and tensile strength) are reduced. On the other hand, the use of blending resins normally makes it possible to make harder material, thereby giving higher mechanical properties and bet- ter physical properties, as shown in Table 10. Table 1 0 also shows that the use of blending resins signifi- cantly reduces the water absorption of the finished articles.

CONCLUSION

Blending resins were synthesized by a suspension polymerization process. The optimum polymerization recipe was adopted, which includes using gelatin as the main dispersant and employing oil-soluble non-

ionic surfactant in combination with a certain amount of water-soluble surfactant. Thermal stabilizer was added during the polymerization process. A three- layer blade paddle stirrer was used in polymerization. In the initial stage of polymerization, a higher agita- tion speed was adopted, and then the agitation speed was changed to a lower level in a proper polymeriza- tion conversion. Blending resins with optimum parti- cle size and narrower size distribution, larger bulk density, and good heat stability were prepared. The blending resins reduce significantly paste viscosity and improve the rheological behavior of the plastisol and increase the gelling temperature or time. The me- chanical and physical properties of the finished arti- cles were also improved by use of the blending resins.

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