The role of solubility and critical temperatures for the efficiency of sorbitol clarifiers in polypropylene

Article abstract of DOI:10.1039/c4ra01917b

The optical properties of polypropylene (PP) were modified by nine different sorbitol type clarifiers available commercially or synthesized in the study. The solubility of the clarifiers in PP was estimated by thermodynamic model calculations. The results

Full text of DOI:10.1039/c4ra01917b

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The role of solubility and critical temperatures for
the efficiency of sorbitol clarifiers in polypropylene
Cite this: RSC Adv., 2014, 4, 19737
ab
*
Zsuzsanna Horvath,
Benjamin Gyarmati,c Alfred Menyhard,ab Petar Doshev,d
´
´ ´ ´
Markus Gahleitner,d Jozsef Vargaab and Bela Pukanszkyab
´
´
´
The optical properties of polypropylene (PP) were modified by nine different sorbitol type clarifiers available
commercially or synthesized in the study. The solubility of the clarifiers in PP was estimated by
thermodynamic model calculations. The results showed that the solubility of these additives in PP is
small, a few 1000 ppm at most. Solubility is determined by the chemical structure of the sorbitol, and
the heat of fusion of the latter changes solubility by at least one order of magnitude. Solubility can be
estimated reasonably by the Flory–Huggins lattice theory. The morphology of most sorbitols transforms
at
a temperature much below their melting point upon heating. This transformation, which is
accompanied by crystal perfection, seems to influence melting and solubility. A fibrillar structure forms
upon the cooling of molten sorbitols, but the diameter of the fibrils is much larger than those forming in
the polymer melt. The nucleating effect of the clarifier depends on solubility, but also on processing
conditions. Nucleus density is related to the amount of dissolved clarifier. A close correlation was found
between the Flory–Huggins interaction parameter of sorbitols and the smallest achievable haze, which
can be explained by the effect of solubility and nucleus density.
Received 5th March 2014
Accepted 11th April 2014
DOI: 10.1039/c4ra01917b
www.rsc.org/advances
already quite some time ago to determine correlations among
polymer type, nucleating agent content, processing conditions
and the optical properties of polymer products.3,4 The results
showed that both polymer properties, including melt ow rate
(MFR) and chain structure, and processing conditions strongly
inuence transparency and other optical characteristics.
1. Introduction
Transparency and optical clarity are basic requirements for a
number of products, especially in the packaging area,1 and the
demand for them increases continuously. Previously, mostly
amorphous polymers were used for this purpose, since light is
scattered on different structural units in semicrystalline poly-
mers making the products opaque. In polyolens incident light
is scattered on the crystallites, spherulites and also on the
interface between the amorphous and crystalline phases having
different refractive indices.2 Crystalline units are oen large
enough to interfere with visible light resulting in considerable
haze. The latter quantity is oen used for the characterization of
the optical appearance of plastic products. Haze is the ux of
light scattered within the angular range between 2.5 and 90ꢀ
normalized to the total transmitted ux.2
Recently the use of polypropylene for the production of
transparent products became industrial practice due to the low
haze achieved by the proper design of polymer structure and the
appropriate selection of additives, but also due to the accept-
able price of this polymer.1,5,6 Low haze materials are usually
prepared from random copolymers and the desired optical
properties are achieved by the use of clariers, which are mostly
different sorbitol derivatives,7–13 even if 1,3,5-benzenetrisamides
have appeared as an alternative in recent years.14,15 It is an
accepted fact now that these additives dissolve in the polymer
and generate a microcrystalline structure during cooling thus
improving haze.16,17 Quite a few papers deal with the structure of
the clarier aer its crystallization, the mechanism of nucle-
ation and clarication, and the optical properties of PP.16–20
Smith et al.16 for example, carried out a fundamental and
very thorough study on the binary system of polypropylene and
1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS). They
determined the solubility of the additive by optical microscopy
(OM) and differential scanning calorimetry (DSC) and created a
non-equilibrium phase diagram describing brilliantly the
various phases forming upon cooling or heating and their
structure in the entire composition and in a wide temperature
Lately, considerable effort has been spent on the improve-
ment of the optical properties of crystalline polymers including
polypropylene (PP). Extensive experiments were carried out
aLaboratory of Plastics and Rubber Technology, Department of Physical Chemistry and
Materials Science, Budapest University of Technology and Economics, P.O. Box 91,
H-1521 Budapest, Hungary. E-mail: zshorvath@mail.bme.hu
bInstitute of Materials Science and Environmental Chemistry, Research Center for
Natural Sciences, Hungarian Academy of Sciences, P.O. Box 286, H-1519 Budapest,
Hungary
cSo Matters Group, Department of Physical Chemistry and Materials Science,
´
Budapest University of Technology and Economics, Budafoki ut 8, H-1111 Budapest,
Hungary
dBorealis Polyolene GmbH, St.-Peter-Strasse 25, A-4021 Linz, Austria
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range. Later they used the same principles and approach for the were synthesized in our laboratory. The chemical name, source
study of the solubility of a new clarier 1,2,3-tridesoxy-4,6:5,7- and abbreviation of the clariers used are compiled in Table 1
bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN)17 in PP. and their structure is presented in Fig. 1.
However, these excellent works raised at least as many ques-
The synthesis of DBS is described here as an example. This
tions as they solved. Sorbitols are polar compounds which product corresponds to the commercial grade Millad 3905. 91.1
cannot dissolve in PP in large quantities. Dissolution is further g (0.5 mol) sorbitol was dissolved in 200 ml water and 118.9 g
hindered by the fact that the melting temperature of DMDBS is (0.625 mol) p-toluenesulfonic acid hydrate was added to the
very high, 273.7 ꢀC. Processing temperatures never reached this solution. Aer the dissolution of the solid components 91.5 ml
temperature in the study cited above,16 thus the solubility limits (0.9 mol) benzaldehyde was introduced during vigorous stir-
determined for the two clariers, i.e. 2.5 and 5.0 wt%, respec- ring. Moderate heat generated in the reaction mixture and
tively, are rather surprising. In practice an order of magnitude precipitation of the crystalline material began. Aer 75 min,
smaller amount is used efficiently and haze usually deteriorates additional 100 ml of water was added to the viscous mixture to
already above a few thousand ppm. The contradiction might be make stirring possible, followed by further 100 ml aer 3 and
explained by the fact that the detection level of optical micros- 4.25 hours, respectively. Aer 5.5 hours reaction time the
copy is in the range of 0.1 mm and DSC signals are also size mixture was neutralized with the aqueous solution of 30 g
dependent,21–23 while the crystalline brils formed from sorbi- NaOH until a slightly basic solution was obtained. The nice,
tols are claimed to be of nanometer size.24,25 The papers do not white, crystalline material was ltered and then a suspension
explain the role of the network formed by the clarier during was prepared in water and ltered again. 104.7 g product was
nucleation and the authors did not study the effect of clarier obtained aer drying. The product was dispersed in 750 ml
structure on solubility and efficiency. Finally, the trans- water containing 7 g hydroxyl amine hydrochloride and 5.3 g
formation of the clarier at considerably lower temperatures sodium carbonate and stirred at 80 ꢀC in order to remove
than their melting point has not been mentioned in these two, unreacted benzaldehyde. Stirring was continued until the
or in fact in any other papers published.
strong odor of the solution disappeared. The next day the
Considering the published information and the questions material was ltered, washed with distilled water and dried. The
mentioned above, the goal of our study was to investigate the product was characterized by NMR, FTIR and DSC. The
solubility of a number of commercial and specially synthesized following chemical shis were obtained for DBS (400 MHz,
sorbitol clariers in PP, check at least magnitudes by model DMSO-d6, d): 3.279–4.130 (m, 8H, saccharide CH and CH2);
calculations and compare the predicted values to measured 4.364 (t, 1H, CH2OH); 4.813 (d, 1H, CHOH); 5.600 (s, 2H, ArCH);
ones. We show that sorbitol crystals may transform upon 7.354 (m, 10H, ArH). Yield was about 50%, purity 90%, and
heating, discuss the possible effect of this transformation on melting point 216.3 ꢀC. The procedure was practically the same
solubility and efficiency, and relate these latter to the chemical for D4ClBS and D4NBS, and only the solvent was different for
structure of the sorbitol.
D3MoBS and D4MoBS (25 ml methanol and 200 ml cyclohexane
was used instead of water).
The stabilizers (1000 ppm Irganox 1010 and 1000 ppm
Irganox 168) and the clariers were homogenized with the
polymer in a Brabender W 50 EH internal mixer at 210 ꢀC for 10
2. Experimental
The polypropylene used in the study was the non-stabilized base
powder of the RE 420 MO grade produced and supplied by min. The melt was transferred to a Fontijne SRA 100 compres-
Borealis Polyolene GmbH. The melt ow rate (MFR, ISO 1133) sion molding machine to produce plates of 1 mm thickness.
ꢀ
of the polymer at 230 ꢀC and 2.16 kg is 12 g/10 min and its Compression molding was done at 190 C for 5 min. Samples
ethylene content is 3.5 wt%. The polymer is a reactor grade and specimens cut from the plates were used for further study.
which is based on a 4th generation Ziegler–Natta catalyst and it The melting and crystallization characteristics of the samples
has a weight average molecular weight, Mw, of 195 kg molꢁ1 and
were determined by differential scanning calorimetry (DSC)
using a Perkin Elmer DSC 7 equipment. 3–5 mg samples were
heated to 220 C at 10 C min heating rate, kept there for 5
a polydispersity, Mw/Mn, of 5. Four sorbitol type commercial
clariers were obtained from Milliken, USA, and another ve
ꢁ1
ꢀ
ꢀ
Table 1 Chemical name, source and abbreviation of the sorbitols used in the study
Abbreviation
Chemical name
Source
R1
R2
M (g molꢁ1
)
D4NBS
D4MoBS
D3MoBS
D4ClBS
DBS
DBS
MDBS
DMDBS
DOPMN
1,3:2,4-bis(p-nitrobenzylidene)sorbitol
1,3:2,4-bis(p-methoxybenzylidene)sorbitol
1,3:2,4-bis(m-methoxybenzylidene)sorbitol
1,3:2,4-bis(p-chlorobenzylidene)sorbitol
1,3:2,4-dibenzylidenesorbitol
Laboratory
Laboratory
Laboratory
Laboratory
Laboratory
Millad 3905
Millad 3940
Millad 3988
Millad NX 8000
NO2
OCH3
H
Cl
H
H
CH3
CH3
H
H
OCH3
H
H
H
H
CH3
448
418
418
428
358
358
386
414
484
1,3:2,4-dibenzylidenesorbitol
1,3:2,4-bis(p-methylbenzylidene)sorbitol
1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol
1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]nonitol
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Fig. 1 Chemical structure of the sorbitol clarifiers used in the study.
min to erase thermal history and then cooled down to 50 ꢀC
with the same rate to record crystallization characteristics. Aer
1 min holdꢁin1g time the samples were heated again to 200 ꢀC at
10 ꢀC min rate to determine melting temperature and the
heat of fusion. The optical behavior of the polymers was char-
acterized by haze measurements carried out on the 1 mm thick
compression molded plates according to the ASTM D-1003-95
standard using a Hunterlab ColorQuest XE equipment. Crys-
tallization and crystal transformation were studied by polari-
zation optical microscopy (OM) using a Zeiss Axioscop equipped
with a Leica DMC 320 digital camera and a Mettler FP82 type
hot stage. The micrographs were recorded with the Leica IM50
soware. The crystal structure of the original and the trans-
formed sorbitols was studied by XRD using a Phillips PW 1830/
PW 1050 equipment with CuKa radiation at 40 kV and 35 mA
anode excitation between 3 and 35ꢀ 2q angles.
3. Results and discussion
Fig. 2 Effect of clarifier content on the peak temperature of crystal-
lization of PP containing the DMDBS (Millad 3988) clarifier. Determi-
nation of the threshold concentration of nucleation.
First solubility limits and the effect of sorbitol crystallinity on it
are checked by model calculations and then we show the
transformation of the crystalline structure of the sorbitol upon
heating and determine characteristic temperatures. The effect
of solubility, transformation, and the chemical structure of the
additive on clarication and nucleation efficiency is discussed
in the nal section of the paper.
nucleation begins above this concentration and a plateau value
is reached at a few thousand ppm additive concentration. The
correlation presented in Fig. 2 allows us the determination of an
experimental solubility value (ce) at the temperature of
solidication.
3.1 Solubility
One would expect very small solubility of the rather polar
As mentioned above, the limited solubility of sorbitol clariers sorbitol molecules in PP, because their interaction with each
is an accepted fact and their high efficiency is explained by the other must be much stronger than with the polymer which is
26,27
¨
dissolution of the compound in PP at the high temperature of capable to interact only by weak dispersive forces. Foldes
processing and its crystallization prior that of the polymer showed experimentally that only about 0.1 wt% of phenolic
during cooling.20 The solubility limit at the temperature of the antioxidants of similar polarity and molecular weight as our
solidication of PP determines the threshold concentration of clariers dissolve in PE, which explains the low threshold level
nucleation. The phenomenon is demonstrated well by Fig. 2 indicated by Fig. 2. Solubility is hindered further by the crys-
showing the dependence of the peak temperature of crystalli- tallinity of the additive; the heat of fusion must be compensated
zation on the concentration of the clarier. Below certain, by interactions or by melting, the former being rather weak as
relatively small clarier concentration crystallization tempera- described above.
ture remains constant that is explained by the existence of a
The dissolution of an additive in a polymer is determined by
eutectic temperature at low clarier contents.16 Efficient the free energy of mixing (DGm):
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and
DGm ¼ DHm ꢁ TDSm
(1)
DHm ¼ RTc4142 + DHmelt41
(5)
which must be negative for spontaneous dissolution. DHm is the
enthalpy and DSm the entropy change of mixing in eqn (1). The
thermodynamic condition of equilibrium is dened as the
equality of chemical potential in the two phases:
where 41 is the volume fraction of the sorbitol, 42 is that of the
polymer, R is the universal gas constant, Np is the degree of
polymerization of PP, T is absolute temperature, c is the Flory–
Huggins interaction parameter and DHmelt is the heat of fusion
of the sorbitol. Eqn (5) shows that the crystalline nature of the
additive further limits solubility. The only unknown in the
equations is the Flory–Huggins interaction parameter, which
was calculated from the Hildebrand solubility parameters of the
components.30
Dms1 ¼ Dms2
(2)
where Dms1 and Dms2 are the changes of chemical potential
during the dissolution of the sorbitol, corresponding to the
polymer-lean and polymer-rich phases, respectively.
According to the condition of spontaneous dissolution (DG <
0) and the condition of thermodynamic equilibrium (eqn (2)),
solubility can be estimated by eqn (3).
2
Vsðds ꢁ dPP
Þ
c ¼
(6)
RT
dDGm
¼ DGm0 ¼ 0
(3)
where Vs is a reference segment volume, while ds and dPP are the
solubility parameters of the sorbitol and PP, respectively. The
estimated van der Waals volume of PP repeating units was
chosen as reference volume, which was calculated from group
contributions;31,32 the Vs value of 31 cm3 molꢁ1 corresponding to
dc
In order to estimate solubility we need a model which
denes the terms of eqn (1); we used the Flory–Huggins lattice
model28 for this purpose in the present study. Although the
model has limitations, it is relatively simple and can be used for
comparative purposes. The lattice used is presented in Fig. 3
and shows that the sorbitol molecule is quite large compared to
the repeating unit of PP considerably limiting the entropy
change of mixing. The arrangement of the sorbitol molecule in
the lattice was estimated by using the model calculations of
Smith et al.29 According to the Flory–Huggins model the
enthalpy and entropy terms of the free energy of mixing can be
expressed as
3
˚
51 A per repeating unit was used in the calculations as a result.
Accordingly, a sorbitol molecule occupies approximately 10
cells in the lattice, which agrees well with the prediction of
Smith et al.29 showing that sorbitol molecules are one order of
3
˚
magnitude larger (400–600 A per sorbitol molecule) than a PP
repeating unit (see Fig. 3). Recently a slightly modied formula
was proposed by Miller-Chou and Koenig,33 but its use practi-
cally did not modify the results of our calculations. Solubility
parameters were calculated from the group contributions of
Van Krevelen and Hozyer.31 This latter approach means a
further limitation and simplication, but suffices for compar-
ative purposes, for the estimation of the inuence of the
chemical structure of the sorbitol on solubility and nucleating
efficiency.
ꢀ
ꢁ
42
DSm ¼ ꢁR 41 ln 41 þ
ln 42
(4)
Np
The dependence of the free energy of mixing (DGm) and that
of its rst derivative (DGm0 ) on sorbitol content is presented in
ꢀ
Fig. 4 at 127 C for MDBS (Millad 3940). We can see that the
predicted solubility is very small at this temperature, namely
below 4000 ppm. It is interesting to note that this value is much
smaller than the ones determined by Smith et al. (2.5 and 5 wt%
for Millad 3988 and NX 8000, respectively) using optical
microscopy but agrees well with everyday practice at least in its
magnitude. The temperature dependence of predicted solu-
bility (cc) determined in the way indicated in Fig. 4 is presented
in Fig. 5 for the same clarier. Solubility was calculated both for
the amorphous and the crystalline additive, and the difference
is striking. The solubility of the crystalline additive is at least
one order of magnitude smaller than that of the amorphous
compound. This difference explains well the necessity of pro-
cessing sorbitol containing compounds at relatively high
temperatures in order to achieve any kind of efficiency in
nucleation and clarication.34,35 Accordingly, the second term in
eqn (5) cannot be neglected and crystallinity must be always
considered in the estimation of the solubility of sorbitols.
Solubility values determined experimentally in the way
shown in Fig. 2 and estimated by the approach presented above
Fig. 3 Lattice model for the estimation of the solubility of sorbitols in
PP; full circles: sorbitol; empty circles: PP.
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Fig. 6 Loose correlation between experimental (ce) and predicted (cc)
solubility of the clarifiers listed in Table 1 in PP at the temperature of
solidification. The broken line is to guide the eye and indicates the
trend of the correlation.
Fig. 4 Calculated free energy of mixing and its derivative plotted as a
function of clarifier content in the concentration range of interest;
MDBS (Millad 3940), 127 ꢀC; determination of estimated solubility (cc).
3.2 Sorbitol structure, characteristic temperatures
As the analysis of the product aer synthesis shows, sorbitols
are mainly crystalline materials. However, they are obtained in
the form of a ne powder, which is rather difficult to disperse in
the polymer, a common problem of sorbitol clariers. The OM
micrographs of a clarier (MDBS, Millad 3940) are presented in
Fig. 7 before (7a) and aer (7b) thermally induced trans-
formation. For this transformation the sample was rst heated
fast (50 ꢀC minꢁ1) to a temperature somewhat below the
expected transformation temperature identied by preliminary
experiments and then slowly (2 ꢀC minꢁ1) further to determine
this characteristic value as precisely as possible. The micro-
graph shows clearly that the original ne powder (Fig. 7a)
transforms during heating and forms very ne needle like
crystals (Fig. 7b). All sorbitols except D4NBS underwent such a
transformation. According to our knowledge this trans-
formation has never been shown in the literature before and its
possible impact on nucleation has consequently not been
considered either.
Fig. 5 Effect of sorbitol crystallinity and temperature on the solubility
of the clarifier MDBS in PP.
Two questions may be raised in relation with the trans-
formation: the process behind, and its effect on nucleation
efficiency. The original and the transformed powders were
studied by XRD. A representative example is shown in Fig. 8.
The transformed powder was annealed under the same condi-
tions as the samples in the OM experiments. The two patterns
are very similar to each other. Both powders are crystalline, and
the crystal modication does not change upon heat treatment.
As a consequence we may conclude that transformation results
in crystal perfection, an increase in crystallinity, in crystals with
different shape and larger perfection. Increased crystallinity
might hinder solubility, but the larger surface area of the needle
like crystals should improve and accelerate it considerably.
One might speculate that not solubility, but transformation and
the needle like crystals initiate nucleation and result in
(see Fig. 4) are plotted against each other in Fig. 6. A tendency
can be seen, but the correlation is rather loose with a consid-
erable deviation of MDBS (Millad 3940) from the general
tendency. A possible reason for the deviation is the strong effect
of kinetics as pointed out also by Kristiansen et al.16 in their rst
paper. The time dependence of phase separation, nucleation
and crystallization must inuence the value of ce considerably.
Nevertheless, the model calculations presented above prove
that sorbitols are soluble in PP, but only in very small amounts.
Thus, any factor inuencing dissolution, like crystalline struc-
ture and processing temperature, will affect also the efficiency
of the clarier.
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clarication, but the size of the crystals is in the micron range,
thus they should scatter light and deteriorate optical properties.
Therefore solubility must be important and play a role in the
effect of sorbitol clariers. Further increase of temperature
results in the melting of the crystals, while subsequent cooling
in the formation of brils and a network structure as shown
earlier.11,19 The brils formed in this way are also large, micron-
sized, and cannot be related to nucleation and clarication. The
small amounts soluble in PP must form the nanometer sized
brils predicted and shown earlier,24,25 which are not visible
even in the light microscope.
The transformation temperature was determined for all
sorbitols included in this study and the values are presented in
Table 2 together with melting temperatures determined by DSC.
The predicted solubility is also listed in Table 2 for comparison.
According to this table, transformation temperatures are always
signicantly lower than melting temperatures indicating that
transformation must play a role in dissolution and nucleation.
This assumption is strongly supported by the fact and explains
that oen processing temperatures much lower than the
melting point of the sorbitol are sufficient to achieve good
clarity. A good example is the DMDBS (Millad 3988) clarier and
the work of Kristiansen et al.16 The highest temperature used for
ꢀ
processing by the authors was 260 C and different processing
ꢀ
temperatures between 230 and 260 C were used for the prep-
aration of samples for their various measurements, which did
not seem to inuence results and conclusions. We must
emphasize here that the melting temperature of the product is
273.7 ꢀC determined by DSC. On the other hand, its trans-
formation or recrystallization temperature is 226.0 ꢀC, which is
below all temperatures used, thus transformation must play a
role in dissolution and nucleation.
Fig. 7 Transformation of the crystalline structure of MDBS (Millad
3940) powder upon heating. Micrographs were recorded at 30 ꢀC (a)
and 210 ꢀC (b), respectively.
3.3 Effect and efficiency
The effect of the chemical structure on the efficiency of the
clariers studied is shown in Fig. 9 where haze is plotted as a
function of clarier content for the sorbitols listed in Table 1.
The stepwise decrease in haze with increasing clarier content
indicates the solubility of the additive in the polymer and is in
complete agreement with the effect shown already in Fig. 1 for
Table 2 Predicted solubility and critical temperatures of the sorbitol
clarifiers used in the study
b
Clarier
cc (ppm)
Tta (ꢀC)
Tm (ꢀC)
D4MoBS
D3MoBS
D4NBS
D4ClBS
DBS
DBS
MDBS
DMDBS
DOPMN
1410
1830
—
3630
2080
1410
3280
2370
1830
177.0
169.0
—
208.0
157.5
150.0
147.5
226.0
176.0
192.2
202.2
269.6
236.5
216.3
227.1
225.8
273.7
245.7
a
b
Fig. 8 Effect of heat treatment on the crystalline structure of DBS;
crystal perfection.
Transformation temperature.
by DSC.
Melting temperature determined
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Fig. 9 Haze plotted against clarifier content. Effect of chemical
structure on the efficiency of sorbitols as clarifiers. Symbols: D4NBS;
D3MoBS; D4MoBS; DBS; DBS(M3905); MDBS; DMDBS;
D4ClBS; DOPMN.
Fig. 10 Effect of processing temperature on the efficiency of DMDBS
sorbitol as clarifier.
the peak temperature of crystallinity. The height of the step
depends on the efficiency of the clarier, i.e. on its solubility in
the polymer.6,20 Large differences are seen in effect depending
on the chemical structure of the clarier. Only three correla-
tions are drawn to guide the eye, but both the tendency and the
differences are clear. Most of the haze values determined at
5000 ppm are above the smallest value measured, because of
limited solubility resulting in larger sorbitol crystals scattering
light.
The results presented in the previous section and in Table 2
indicate the importance of characteristic temperatures. In order
to check the inuence of processing on clarifying efficiency, the
composition dependence of haze was determined at various
temperatures. Some results are presented in Fig. 10 for DMDBS,
the Millad 3988 clarier. PP containing the nucleating agent
was processed at 190, 200, 210, 220 and 240 ꢀC. Only three series
are plotted in Fig. 10 to facilitate viewing. A large jump in haze is
Fig. 11 Correlation between the Flory–Huggins interaction parameter
of and the smallest haze achieved by the various clarifiers studied.
ꢀ
observed above 190 C and a practically constant effect above
220 ꢀC. We must emphasize here the fact that the trans-
formation temperature of the sorbitol is 226 ꢀC and its melting
point is 273.7 ꢀC. The same applies to other sorbitols as well. PP increasing solubility. Naturally, the correlation must be
containing DBS, for example, was processed between 170 and treated with care, since several other factors, like synthesis
ꢀ
230 C, and no difference in effect was seen in this range (not conditions, transformation and kinetic effects all inuence
shown), because the transformation temperature of this solubility, nucleation and clarication. The dissimilar
compound is 157.5 ꢀC, while its melting point is 216.3 ꢀC. behavior of D4ClBS needs further study and considerations.
Obviously, rather crystal transformation temperature and not The correlation, however, agrees well with the results pre-
melting determines the effect and efficiency of sorbitol sented above and can be explained reasonably. As shown in
clariers.
Fig. 5 and as expected, solubility increases with temperature,
Finally, in order to see the inuence of chemical structure thus larger solubility at the temperature of solidication
and solubility on the efficiency of the sorbitols, the smallest means larger dissolved amounts at higher temperatures as
haze achieved with each clarier was plotted against the well. As a consequence, more clarier will precipitate and
Flory–Huggins interaction parameter in Fig. 11. Apart from crystallize upon cooling resulting in larger number of brils,
the effect of D4ClBS, the correlation is clear, efficiency larger nucleus density and efficiency. A close correlation was
increases with decreasing interaction parameter, i.e. shown between nucleus density and clarity before.16,20
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Naturally this tentative explanation cannot take into account
the size of the brils formed during the crystallization of the
clarier or the structure and role of the sorbitol network
formed. These might cause the dissimilar effect of D4ClBS
and as mentioned, the phenomenon needs further study and
explanation.
In order to check the assumption presented above, nucleus
density was calculated from the non-isothermal DSC traces
used for the estimation of ce by an approach developed
recently.36 Changes in the volume of crystalline material are
calculated as a function of temperature from crystallization
traces using the density of the crystalline phase (0.936 g cmꢁ3).2
Accordingly, the time dependence of nucleus density can be
given as
4. Conclusions
Model calculations carried out for the estimation of the solu-
bility of a considerable number of sorbitol clariers showed that
solubility in PP is small, a few 1000 ppm at most, in agreement
with industrial experience. Solubility is determined by the
chemical structure of the sorbitol and the heat of fusion of the
latter changes solubility by at least one order of magnitude.
Solubility can be estimated reasonably by the Flory–Huggins
lattice theory. The morphology of most sorbitols transforms at a
temperature much below their melting point upon heating. The
transformation, which is accompanied by crystal perfection,
seems to inuence melting and solubility which indicates that
polypropylenes containing a sorbitol clarier must not neces-
sarily be heated above the melting temperature of the nucle-
ating agent in order to achieve the desired effect, the decrease of
haze. A brillar structure forms upon cooling of molten sorbi-
tols, but the diameter of the brils is much larger than those
3DVcr
2ꢁAx
Nt ¼
ꢁ Ntꢁt
(7)
i
3
4pðGstiÞ
ꢀ
ꢁð1ꢁx Þd
t
1
Ax ¼
(8) forming in the polymer melt. The nucleation effect of the clar-
ier depends on its solubility, but also on processing condi-
tions, mostly on the temperature of the polypropylene melt.
Solubility decreases with increasing polarity, substituents con-
taining heteroatoms (alkoxy, halogen, nitro) seem to be less
advantageous than those consisting of only alkyl groups.
Nucleus density is related to the amount of dissolved clarier. A
rather good correlation was found between the Flory–Huggins
interaction parameter of sorbitols and the minimum haze
achieved, which was explained with the effect of solubility and
nucleus density.
1 ꢁ xt
where Vcr is the volume of crystalline material, N is nucleus
density, G is crystal growth rate and t is time. Ax is an internal
empirical function, which takes into account the closing of free
growing surfaces during crystallization and xt is the conversion.
Constant d depends on the dimensionality of growth and its
value is three in our case (spherulitic growth). Final nucleus
density is obtained as the sum of Nt from the start to the end of
the crystallization process. The smallest haze achieved is
plotted against nucleus density in Fig. 12. As shown by the
gure, the correlation is very close for all clariers except
D3MoBS. The deviation might have the same reason as for that
of D4ClBS in the previous gure, but it needs further study.
Nevertheless, we can conclude with rather large certainty that
larger solubility results in larger nucleus density and better (i.e.
lower) haze.
Acknowledgements
´
´
´
´
´
´
´
The help and advice of Bela Turcsanyi and Gabor Pal Penzes in
the synthesis of the sorbitols and that of Istvan Sajo in the XRD
measurements are highly appreciated. We acknowledge the
´
assistance of Attila Domjan in the NMR characterization of the
sorbitols synthesized. The authors also acknowledge the
nancial support of the National Scientic Research Fund of
Hungary (OTKA Grant no. K 101124 and PD 109346) for the
project on structure–property correlations of polymeric mate-
´
´
rials. Another author (Alfred Menyhard) would like to express
´
his gratitude to the Janos Bolyai Research Scholarship of the
Hungarian Academy of Sciences for its nancial support.
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