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Structure and Properties of Poly(Vinyl Chloride)-Triallyl Cyanurate Plastisols.

Triallyl cyanurate (TAC) has been used as a reactive plasticizer to promote the high-temperature creep resistance of poly(vinyl chloride) (PVC) plastisols. The resultant crosslinked structure is characterized using gel content and swell ratio measurements as well as Fourier transform infrared spectroscopy. The crosslinking reaction was initiated using peroxide. The effect on the network structure of using a free radical scavenger in the formulation has also been studied. The gel yield and crosslink density in the gel increase with increasing TAC concentration in the plastisol, while the grafted PVC fraction and the residual unsaturation of TAC behave in the opposite way. Introduction of TAC into the plastisol promotes creep resistance at high temperatures, and the logarithmic creep rate was found to decrease linearly with increasing crosslink density.


Poly(vinyl chloride) (PVC) plastisols are widely used in applications such as synthetic leathers, floor coverings and liners. They offer the advantages of low cost and ease of processing inherent in a low viscosity fluid system [1]. However, these materials have some limitations. They have only a moderate upper service temperature limit and a limited resistance to solvents. In processes such as sterilization and welding, products made of PVC plastisols are exposed to high temperature environments for short periods of time. Formulations must be heat-stabilized to avoid any degradation and excessive deformation. For some applications the products must be creep-resistant at high temperatures while retaining their flexibility at low temperatures. In a process such as in-place molding, which is widely used to manufacture products such as automobile air cleaner seals, pipe gaskets and cap liners, pumping and injection of low viscosity PVC plastisols are involved. When developing a PVC plastisol formulation for such a process, one must take into account its rheological characteristics, such as thixotropic behaviors and viscosity stability, in addition to its mechanical property requirements.

Owing to the high content of liquid plasticizers in a typical PVC plastisol formulation, the glass transition temperature for the fused plastisol is below room temperature. When exposed to a high temperature environment under stress, a fused plastisol incurs permanent deformation and its sealability is destroyed. To extend the application range, the use of either heat-resistant plasticizers, reactive plasticizers or fillers could be effective. A formulation that can be used without major processing equipment and condition changes is preferred. The addition of fillers such as calcium carbonate or heat-resistant plasticizers such as polymeric plasticizers is prohibited if the viscosity is a major concern. A reasonable alternative has been the use of polyfunctional monomers (PEM) as plasticizers, such as triallyl cyanurate (TAC) and trimethylolpropane trimethacrylate (TMPTMA). With the introduction of a permanent chemical network, it is possible to increase the maximum application temperature and to vary the me chanical properties of a plastisol to a great extent.

PVC by itself has a low crosslinking yield when irradiated or when a peroxide initiator is used. Measures to promote the crosslinking capability of PVC include grafting the PVC chains with polyfunctional nucleophiles or silanes, the addition of reactive plasticizers into a PVC formulation, and so on. In the presence of PFM molecules, PVC can be crosslinked using [gamma]-ray irradiation. Upon irradiation, homopolymerization of the PFM molecules, graft copolymerization of PFM molecules onto PVC and random crosslinking among PVC chains can occur in these PVC-PPM systems [2-5]. For rigid PVC formulations, the crosslinked structures enhance the high temperature properties significantly, while there are only minor changes in room temperature properties [6, 7]. Radiation crosslinking is not a very versatile process. Being a strong nucleophile and a weak base, a thiol group can be used to replace the chlorine atom on the PVC chain and crosslinks can thus be introduced [8-10]. These crosslinking systems suffer from t he disadvantages of poor viscosity stability and poor reaction rate control. Silane crosslinking could be an economical alternative [11]. Recently, Sundb0 et al. introduced a reactive glycidylmethacrylate group to the PVC backbone to serve as the site for anchoring the mercaptopropyl triethoxysilane crosslinking agent for a PVC plastisol [12]. A high gel yield could thus be obtained. However, the hydrolytic crosslinking process takes a long time to complete.

PVC resins are highly heat-sensitive. Peroxides are rarely used in PVC formulations to avoid the risks of degradation. The release of HC1 during PVC compound processing can cause premature peroxide decomposition [13]. The formation of carbonyl groups from the oxidative degradation of PVC under combined [gamma]-radiation and elevated temperature was attributed to a peroxide-mediated mechanism [14]. However, the use of plasticizers reduces the chemical degradation and release of HCl remarkably [15]. Studies on radiation-induced PVC degradation show that Ca-Zn stearate compound stabilizers have a pronounced effect in reducing dehydrochlorination and oxidative degradation [16]. Epoxy plasticizers were reported to be effective in stabilizing the irradiated PVC compounds [17]. They are known to have excellent heat- and light-stabilizing functions and the oxirane oxygen can act synergistically with metallic stabilizers based on Ba, Cd, Ca and Zn [18]. With a proper selection of peroxide and stabilizer systems, pero xide-initiated crosslinking in the presence of PFMs offers the advantages of low cost, versatility and ease of processing.

This work investigated the influence of a PFM, TAC, and a free radical scavenger, 2,2'-methylene-bis-(4-methyl-6-tertiary butyl phenol), on the network structure of PVC-TAC plastisols. Short-term tensile creep behaviors under high-temperature conditions were studied to explore the structure-property relationship for this reactive plastisol system.



The materials used along with their descriptions are listed in Table 1. All materials were used as received.

Sample Preparation and Analysis

The formulations were blended using a laboratory mixer at a mixing speed of 1750 rpm. All of the liquids in the formulations, with the exception of peroxide, were mixed first in a mixing chamber, followed by the gradual addition of PVC powder, and finally the peroxide was added. The compounding process took about 30 minutes and the plastisols were kept below a temperature of 40[degrees]C throughout this process. The compositions of all formulations are listed in Table 2.

The rate of fusion or crosslinking was evaluated using a Rheometric Scientific SR5 rheometer with a parallel plate fixture for the plastisols immediately after compounding. The test was undertaken at 150[degrees]C and at a frequency of 1 Hz so that proper fusion and crosslinking could be reached within a reasonable length of time. The gel content and swell ratio of the fused samples were determined with tetrahydrofuran, THF, reflux extraction. The unreacted plasticizer content was determined with diethyl ether, DEE, reflux extraction. A Nicolet FTIR system Magna-IR 560 was used to assess the residual TAC unsaturation. FTIR spectra were taken on both liquid plastisols and fused PVC films. The tensile creep properties were tested using a Perkin-Elmer DMA 7 under a stress level of [10.sup.5] Pa at 120[degrees]C. Dumbbell geometry test pieces with a gauge length of 10 mm were cut from 2-mm-thick sheets. Both the films for the FTIR test and the sheets for the creep test were compression-molded at a temperature of 200[degrees]C for 10 minutes.


Structure Characterizations

To understand the gel/sol partition phenomenon, the compound gel content and swell ratio were measured by dissolving the network in a good solvent. The detailed results of these experiments for each formulation are listed in Table 3. Figure 1 shows the relationship between the concentration in plastisol and gel content. The gel content increased monotonically with increasing TAC concentration. In each formulation, the gel content was higher than the TAC concentration in the plastisols. By comparing the extractable plasticizer content obtained from diethyl ether extraction experiments with the percent plasticizers excluding TAC in each formulation (Table 3), it can be concluded that a small amount of nonreactive plasticizer was chemically bound to the fused polymer. This also implies that almost all TAC monomers had reacted and were not extracted. The higher gel content values indicate that all TAC molecules and part of the PVC polymers were involved in the crosslinking reaction. TAC is capable of grafting at active H atoms on PVC as well as forming a TAC network. However, as shown in Fig. 1, the PVC fraction in gel decreased with increasing TAC loading. There was nearly 80% PVC in the PVC-TAC network for the formulation with the lowest TAC loading (formulation 8), while there is only 50% PVC in the network for the formulation with the highest TAC loading (formulation 2). That is, graft copolymerization predominates for plastisols with a low TAC concentration, whereas highly crosslinked semipenetrating polymer networks (or a structure combining features of both a crosslinked copolymer and an interpenetrating network) are observed for plastisols with a higher TAC concentration. Interestingly, the behavior of PVC incorporation into the TAC network is quite different from the case when TMPTMA is used as a reactive plasticizer. The PVC fraction in the gel increased with TMPTMA loading when a peroxide crosslinking initiator was used [19]. Nethsinghe et al. also reported that the PVC fraction did not vary significantly with the TMPTMA level at constant irradiation dose, but increased with increasing irradiation doses at constant TMPTMA levels [6, 7]. Compared with TMPTMA. TAC is especially effective in the formation of PVC-PFM graft chains in a network structure at a low concentration: PVC fraction is 0 for PVC-TMPTMA plastisols at a TMPTMA concentration of 5% [19].

The shelf life of a plastisol is considerably limited by the existence of reactive plasticizers and peroxides in the formulations. In industrial practice, plastisols are stored for some period of time before its uses. Commercial grade plastisols have a lifetime of 2-6 months [20]. The free radicals generated from peroxide decomposition at room temperature can cause a premature reaction in these reactive plasticizers and result in a dramatic increase of plastisol viscosity or the formation of gels. A plastisol alone without the presence of reactive plasticizers will be plasticized slowly through plasticizer diffusion into the PVC particles. The viscosity will thus increase because of the swell in PVC particles. A free radical scavenger can be used synergistically along with a viscosity depressant to effectively stabilize the viscosity. When a radical scavenger, 2,2'-methylenebis-(4-methyl-6-tertiary butyl phenol), was added into the formulations, the trend was similar to that found in formulations without a ra dical scavenger, as is shown in Fig. 1. The side effect from using a radical scavenger was a reduction in the crosslinking efficiency of TAO, as is evidenced by the lower gel content and the higher swell ratio in Table 3. The effect was not as obvious as in the PVC-TMPTMA plastisol [19]. The amount of PVC chains grafted into the crosslinking network was also reduced with the use of a radical scavenger.

The swell ratio value is related to the crosslink density using the Flory-Rehner equation (21, 22):

[v.sub.s] = - ln(1 - [v.sub.r]) + [v.sub.r] + [chi][[v.sup.2].sub.r]/[V.sub.1][[[v.sup.1/3].sub.r] - (2/f) [v.sub.r]] (1)

where [v.sub.s] is the effective network chains per unit volume of gel, [v.sub.r] the volume fraction of polymers in the swollen network (the inverse of swell ratio), [V.sub.1] the molar volume of THF (82.11 [cm.sup.3]/mole at 25.C), [chi] the coefficient of interaction between THF and PVC-TAC ([chi] = 0.515) and f the crosslinking functionality (f = 6 for TAC). [chi] is estimated using [23]

[chi] [approximate] 0.34 + [V.sub.1]/RT [([[delta].sub.P] - [[delta].sub.S]).sup.2] (2)

The value of the solubility parameter, [[delta].sub.S], of THF is 19.6 [J.sup.1/2]/[cm.sup.3/2]. The value of [[delta].sub.S] of crosslinked PVCTAC is taken as the average value of those of PVC (19.2 [J.sup.1/2]/[cm.sup.3/2]) and TAC (24.5 [J.sup.1/2]/[cm.sup.3/2]), which is 21.9 [J.sup.1/2]/[cm.sup.3/2]. The calculated crosslink density values for each formulation are also shown in Fig. 2. It can be seen that as the concentration of TAO in the formulation increases, the crosslinking efficiency of the peroxide increases. The crosslink density values obtained in this study were lower than those obtained for PVC-TMPTMA plastisols [6, 9, 20], where the values ranges from [10.sup.-5] to [l0.sup.-3] mole/[cm.sup.3]. This could be attributed to the difference in PVC fraction in gel: PVC fraction in gel is in the range of 0 to 40 wt% for PVC-TMPTMA plastisols, while it is in the range of 45 to 80 wt% for PVC-TAC plastisols.

The reactions involved in the crosslinking of the plastisol include the addition reaction of TAC molecules and the graft reaction of TAC onto the PVC chains. When the initiators decompose, two major types of chain radicals exist: PVC radicals that are formed by the loss of a hydrogen or chlorine atom, and TAC chain radicals that are formed by the opening of a [pi]-bond. The rate of the addition (propagation) reaction can be expressed by

[R.sub.P] = [k.sub.P][M*] [M] (3)

where [[kappa].sub.p] is the rate constant, [M] is the concentration of double bonds and [M*] is the total TAC chain radical concentration. The graft reaction rate for PVC can be expressed by

[R.sub.G] = [[kappa].sub.G][M*] [PVC*] (4)

where [[kappa].sub.G] is the rate constant and [PVC*] is the concentration of PVC chain radicals. The termination of the chain reaction occurs by combination, disproportionation and graft reaction. It is also possible that the chain addition reaction is terminated by the impurities existing in the plastisol. The crosslinking yield is low for radiation crosslinking of PVC in the absence of PFM. It is unlikely that seif-crosslinking of PVC would have any contribution to the gel formation. Since the peroxide concentrations were nearly equal in all formulations, the concentrations of primary radicals generated from decomposition of peroxide molecules were essentially equal. The concentration of TAC chain radicals was proportional to the concentration of TAC monomers. It has been reported that at the first stage of the reaction, only one double bond on TAC was involved and a chain containing pendant double bonds was formed [5]. Only after the depletion of most TAC monomers, could th e crosslinking and graft reaction took place. At a low TAC concentration, a graft reaction between PVC chain radicals and TAC chain radicals predominates and loose PVC-TAO networks are formed. At a higher TAC concentration, a higher [M] value makes the crosslinking reaction proceed faster over the graft reaction. Because of the high functionality of TAC, there was a higher probability that a chain or a gel molecule bore more than one radical and a high extent of intra-molecular addition occurred when the TAC concentration was high. The resultant PVC-TAC network was highly crosslinked. Note that the amount of PVC incorporated into the TAC network, also shown in Table 3, is almost constant in all of the formulations. This could be attributed to the constant concentration of PVC chain radicals since the amount of PVC and peroxide are constant in all of the formulations.

The amount of residual unsaturation was analyzed using the FTIR spectroscopy of the reactive PVC plastisols. The resonance peak at 3090 [cm.sup.-1] arises from = C-H stretching mode of allyl groups on TAC and the resonance peak at 698 [cm.sup.-1] arises from C-C1 stretching mode of PVC [2]. The latter was used as an internal reference in the assessment of residual unsaturation. The unsaturation was calculated using the ratio of the area under each peak in the absorbance spectra. The residual unsaturation was then determined using the ratio of unsaturation of the fused plastisol to that of the unfused liquid plastisol. The results are shown in Fig. 3. A higher level of unsaturation was observed for formulations with a lower TAC concentration. The addition of a radical scavenger also helped to prevent the consumption of unsaturation and a higher level of unsaturation was observed. Dakin considered the network formation of PVC-allylic PFM system as comprising three stages: formation of graft chains of PVC-PFM with pendant double bonds, transition to a three-dimensional network as a result of the joining of PVC-PFM chains through the reaction of those pendant double bonds, and reaction of residual double bonds inside the network [5]. As shown in Fig. 2, there is a difference in order of magnitude of the crosslink density for, say, formulations 2 and 8. A gel formed with a high concentration of TAC is highly crosslinked and is in the third stage of network formation; while that with a low concentration of TAC is more branched and less crosslinked and is still in the second stage.

The level of residual unsaturation for PVC-TAC plastisols is also considerably lower than that for PVC-TMPTMA plastisols (in the range of 5% to 15%) as reported previously [19], which also helps to explain the lower level of crosslink density for the PVC-TAC systems. Interestingly, while the levels of residual unsaturation and PVC fraction in the gel are lower with a peroxide-initiated crosslinking method than with an irradiation method for PVC-TMPTMA systems [6, 7, 19], the two methods yield similar results for PVC-TAC systems as one compares the results in this work with those reported by Dakin [5]. In irradiation methods, PVC and the PFM were fused at high temperatures first; and irradiation was carried out at low temperatures on the fused samples where the mobility of the PFM molecule was greatly reduced. The crosslinking between the PFM molecules was restricted and the grafting of PVC became the dominant process and higher values of the PVC fraction in the gel were obtained [2]. The rate of crosslinking between TAC molecules in the plastisol proceeds much slower than that between TMPTMA molecules (see next section). Even though the crosslinking was carried out at high temperatures using a peroxide method, the differences in levels of residual unsaturation and PVC fraction between the two crosslinking methods are not as prominent for the PVC-TAC systems.

Mechanical Properties

The rate of fusion is vital to formulating a PVC plastisol in that it dictates process conditions and throughput. Optimal material properties will not be accomplished if fusion is not complete. In the presence of a PFM, however, the level of crosslinking for the formulation becomes more important because it dictates the material properties. It is essential that fusion must precede polymerization and crosslinking in that premature polymerization and crosslinking cause phase separation between crosslinked TAC, PVC and other plasticizers. The resultant materials will be hard, brittle, and of little use. The growth of dynamic mechanical properties of a plastisol serves as a good indicator of the state of fusion or crosslinking. Shown in Fig. 4 is the isothermal development of complex shear moduli of formulations 1, 2 and 3 at a temperature of 150[degrees]C. It was found that a state of full fusion could be achieved in less than 5 minutes (formulation 1), while the time needed to achieve a complete crosslinking t ook more than 40 minutes (formulations 2 and 3). Fusion had been achieved to an appreciable extent before the reactive plastisols began to crosslink. This indicates that solid solutions were formed and the compounds crosslinked in the fused state. The crosslinking rates (the rates of growth of the shear modulus) and the times to reach full crosslinking of both formulations 2 and 3 seem to be comparable. It is also worth noting that the mechanical moduli of crosslinked formulations are much higher than that for uncrosslinked formulations. At 150[degrees]C, the moduli of formulations 1, 2 and 3 are about 0.05, 2.3 and 1.4 MPa, respectively. Crosslink density played an important role here. The higher crosslink density of formulation 2 resulted in a much higher shear modulus, as compared with formulation 3. However, formulation 3 is a much safer formulation when the shelf life is of major concern in that the low dosage of the released free radicals can be scavenged. Otherwise, the heat generated during a compound ing period or some temperature fluctuations in a shipping and storage period may trigger premature crosslinking reactions and the plastisol hardens or becomes too viscous to be meltprocessed. Compared with the PVC-TMPTMA plastisol, in which the crosslinking reaction could be completed within 10 minutes at 140[degrees]C, the crosslinking in PVC-TAO plastisols is far less efficient [19].

Products made of PVC plastisol are not intended for long-term high temperature applications. However, there are many occasions in which they are exposed to a short-term high-temperature environment under stress. The short-term creep behaviors of the fused specimens were assessed at [120[degrees]C for two hours in uniaxial extension creep experiments. Shown in Fig. 5 are the creep curves for specimens of all formulations. Strain is defined here as the change in length of the specimen divided by its original length. The shapes of all of the curves are similar. When a constant stress was applied, there was an instantaneous strain. The initial creep rate was high and decreased rapidly within a short duration. In the second part of the curves. the creep rate was essentially constant. Traditionally, the slope in the second part of the creep curves is taken as a measure of the creep rate of a material. The addition of a reactive plasticizer promoted creep resistance, as is evidenced by the lower magnitude of strain (and hence lower creep compliance). The use of a radical scavenger reduced the creep resistance consistently by reducing the amount of gel. The logarithmic creep rate for formulations 1 through 9 is plotted versus crosslink density in Fig. 6. The higher the crosslink density caused by higher TAC concentrations, the better the creep resistance at high temperatures. The logarithmic creep rate decreased linearly with increasing crosslink density. It is important that the grafting of PVC chains onto TAC networks played an important role in improving the creep resistance. For formulations 2 through 9, the materials that are not chemically connected to the crosslinked network make up a fraction of 45% to 80%. The creep is considered to be contributed mostly by these materials. Free PVC chains are tangled into the PVC-TAC network. They have to wriggle their way through the highly constrained environment in order to relax the imposed stress. This brings about a dramatic reduction in creep rate.


The use of TAG as a reactive plasticizer is very effective in promoting the gel yield in a PVC plastisol using a Peroxide-crosslinking method. Gel contents and crosslink densities increase with increasing TAC content in the plastisol. A large fraction of PVC is incorporated into the network structure. At a concentration of 5% TAG in the compound, the graft reaction between the PVC chain radicals and the TAC chain radicals predominates. Within the gel, a loose network comprising 25% TAC and 75% of PVC is formed. At a concentration of 25% TAC in the compound, the graft reaction and the homopolymerization of TAC monomers are of equal importance. A highly crosslinked network comprising 50% of each constituent is formed. The addition of a free radical scavenger into the plastisol reduces the gel content, the PVC fraction in the gel, and the crosslink density. The residual unsaturation of TAC decreases with increasing TAC content, which is consistent with our findings in gel content and crosslink densities. For th e formulations in this study, the rate of fusion was higher than the rate of crosslinking. However, depending on the TAC concentration, the fused plastisol can be transformed from a flexible material into a very rigid material. Introduction of TAO into the plastisol promotes the creep resistance at 120[degrees]C, and the higher the TAC concentration, the better the creep resistance. The logarithmic creep rate was found to decrease linearly with increasing crosslink density.


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Table 1. Materials.
Designation Description
PVC An emulsion grade PVC resin,
 trade name PR-F and P = 1600
Ca/Zn stabilizer A liquid calcium/zinc stabilizer, trade
 name CZ-10
DINP Diisononyl phthalate
ESBO Epoxidized soybean oil
TAC Triallyl cyanurate
BYK 5025 A liquid viscosity depressant
Peroxide Dicumyl peroxide
Radical scavenger 2,2'-methylene-bis-(4-methyl-6-tertiary
 butyl phenol), used as a free radical
Designation Source
PVC Formosa Plastics Corp.
Ca/Zn stabilizer
 Akzo Chemicals Inc.
DINP Union Petrochemical Corp.
ESBO Chang Chun Petrochemical Corp.
TAC Aldrich Chemicals
BYK 5025 BYK Chemie GmbH
Peroxide Coin Chemicals Corp.
Radical scavenger Aldrich Chemicals
Table 2. Plastisol Formulations.
Components Formulation
 1 2 3 4 5 6
PVC 100 100 100 100 100 100
DINP 60 20 20 30 30 40
ESBO 10 10 10 10 10 10
TAO -- 40 40 30 30 20
Peroxide -- 1 1 1 1 1
BYK5025 2 2 2 2 2 2
Ca/Zn stabilizer 2.5 2.5 2.5 2.5 2.5 2.5
Radical scavenger -- -- 0.1 -- 0.1 --
 7 8 9
PVC 100 100 100
DINP 40 50 50
ESBO 10 10 10
TAO 20 10 10
Peroxide 1 1 1
BYK5025 2 2 2
Ca/Zn stabilizer 2.5 2.5 2.5
Radical scavenger 0.1 -- 0.1
Table 3. Results From THF and DEE Reflux Extractions.
Formulation 1 2 3 4 5 6 7
Gel content (%) 0 45.6 42.3 38.2 37.9 32.2 30.2
TAC concentration in 0 22.9 22.9 17.2 17.2 11.5 11.5
 plastisol (weight%)
PVC fraction in gel (%) + 0 49.8 45.9 55.0 54.6 64.3 61.9
PVC incorporated into -- 39.9 34.1 36.9 36.3 36.3 32.8
 the TAC network (g) ++
Extractable plasticizer 42.4 18.1 19.3 23.5 25.6 30.3 30.3
 content (%) *
Percent plasticizer 42.7 19.8 19.8 25.5 25.5 31.2 31.2
 excluding TAC ***
Swell ratio -- 6.9 7.4 8.9 9.0 9.2 11.7
Formulation 8 9
Gel content (%) 24.6 20.8
TAC concentration in 5.7 5.7
 plastisol (weight%)
PVC fraction in gel (%) + 76.8 72.6
PVC incorporated into 33.2 26.5
 the TAC network (g) ++
Extractable plasticizer 35.0 35.4
 content (%) *
Percent plasticizer 37.0 37.0
 excluding TAC ***
Swell ratio 14.4 20
(+) 100 x [1-(TAC concentration)/(Gel content)].
(++) Total weight in Table 2 for each formulation
(g) x Gel content x PVC fraction in gel.
(*) Experimental values obtained from diethyl ether extraction.
(**) Calculated from listed formulations, Including DINP, ESBO, BYK
5025, Ca/Zn stabilizer and radical scavenger.

[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]
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Date:Jun 1, 2001
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