Editor’s Note: This
paper was awarded the Pressure Sensitive Tape Council’s 2008 Carl A. Dahlquist
Award for Best Technical Paper.
Polypropylene is a polymorphic semi-crystalline polymer that can crystallize in
more than one crystal form. The most common crystal form is the alpha, or
monoclinic form, which melts at about 160
beta nucleant masterbatch, identified as MPM 1101, contains a proprietary
mixture of a beta-nucleating agent plus other additives, and was produced in a
pellet form by way of melt compounding. The carrier resin in this masterbatch
was a 12 MFR non-nucleated PP homopolymer resin. All of the beta nucleated
films and molded parts described in this article were prepared by blending this
masterbatch with pellets of a non-nucleated PP resin using the masterbatch at a
concentration of 2-3%. The PP resins used included homopolymer and impact
copolymer, and all were made using Zeigler-Natta-based catalyst systems.
resins were extruded through a flat die to produce sheets ranging in thickness
from 0.5 to 0.6 mm at melt temperatures of about 230°C. The molten sheets were
cooled by contacting a chrome-polished roll that was part of a three-roll
stack. Roll temperatures were 80-95°C. The sheet was drawn in the machine
direction by passing it over a series of heated rolls moving at different
speeds. The draw ratio, as measured by the speed differential between the slow
and fast rolls, was in the range of 5:1 to 6:1.
The beta-crystal content of the final part is dependent on both
the concentration of the beta nucleant masterbatch used to produce the part and
the thermal and processing history used during fabrication. DSC testing is
commonly used to assess beta nucleation; a typical DSC melting curve for a
beta-nucleated and a non-nucleated PP resin is illustrated in Figure 1. Here,
the samples were first heated up at 10°C per minute to 230°C to destroy any
prior thermal history, then cooled at a rate of 10°C per minute. The curves
illustrated in the figure were obtained on the second heat scan. The
beta-nucleated material shows two melting peaks, as discussed earlier. The
lower peak is the dominant peak, at 153°C, due to the beta crystal phase, and
the smaller peak at 166°C is due to the melting of the alpha crystals. Although
it is possible to measure the heats of fusion of the two melting peaks as a
method of quantifying the amount of beta crystallinity, this tends to
underestimate the beta content, since some of the polypropylene will
recrystallize into the alpha form after melting, thereby contributing to the
second melting peak for the alpha phase.
A more quantitative way of characterizing the beta content is to measure it
directly on an extruded sheet or injection-molded part using wide-angle X-ray
diffraction (WAXD). A typical WAXD scan of a sample containing both alpha and
beta crystals is illustrated in Figure 2.
The beta phase shows a particularly strong diffraction peak due to the (300)
plane, while the alpha phase shows three strong peaks due to the (110), (130)
and (040) planes. An empirical ratio of the intensity of these peaks known as
the “K” value and defined2
as follows is measured to
define the beta content:
Here, K will vary from 0 for a sample with no beta crystals to 1.0 for a sample
with only beta crystals. For most of the examples discussed, the K value for
beta-nucleated materials fell in the range of 0.8-0.9.
It should be noted that, in the case of the oriented film, the WAXD
measurements must be made on the extruded sheet before it has been oriented,
since the orientation process converts the beta crystal to alpha crystals. This
was done by using the edge trim from the extruded sheet for the X-ray
Physical Property Testing
tensile properties of the films were measured in both the machine (MD) and
transverse (TD) directions using an Instron tester according to ASTM D 886.
Density measurements were performed using either hydrostatic displacement or by
measuring the dimensions of a piece of film, and then dividing the mass of the
film by its volume.
Results and DiscussionsMOPP Films Made from
A series of monoaxially oriented films (MOPP) were produced from impact
copolymer PP (ICP). The precursor sheets were cast onto a heated cast roll set
at 90°C. The beta nucleated sheets contained 2.5% of the BETAPP masterbatch.
The starting sheet thickness was about 0.58 mm, and the final film thickness
was in the range of 0.12-0.135 mm. The X-ray “K” values of the precursor sheets
and the properties of the MOPP films are given in the table.
The table shows that films 1 and 6, which contained high levels of beta
crystallinity, had lower densities and improved tensile properties compared to
film 2. The elongation at break values for these two high-beta-content films
were somewhat lower than that of film 2. Films 1 and 6 were also more opaque
than that of film 2, due to the presence of the microvoids in the film.
Mopp Films Made from HPP and ICP
In a second series of runs, non-nucleated and beta-nucleated MOPP films were
produced using both an ICP and a homopolymer PP (HPP) resin. The beta-nucleated
versions of these films also contained the MPM 1101 masterbatch at a few different
levels; the cast roll temperature was set at about 90°C. The sheet thickness
was about 0.50 mm, and the final film thickness was about 0.080 mm.
The films containing the beta nucleant appeared more opaque
than the non-nucleated films, as illustrated in Figure 3. In the figure, each
of the films has been placed over two business cards, with the upper card being
in direct contact with the film and the lower card positioned about 2 cm below
the plane of the film, in order to illustrate the contact and see-through
clarity of the two films. The relative opacity data for impact copolymer and
homopolymer (HPP) films are illustrated in Figure 4.
these figures, we see that the opacity of the beta-nucleated ICP film was much
greater than that of the non-nucleated ICP film, and the opacity of both the
ICP and HPP films increased with increasing beta-crystal content, as measured
by the X-ray K value. The ICP film also exhibited a greater increase in opacity
following beta nucleation, compared to that of the HPP film. The densities of
the different film samples are illustrated in Figure 5, which shows that the
trend in film density correlates quite well with the trend seen in the opacity
The ultimate tensile-strength values of these films in the
machine direction (MD) are illustrated in Figure 6. None of these samples
showed any evidence of a yield point, and the maximum tensile strength occurred
at break. Here we see a decrease in tensile strength with increased beta crystallinity
for each resin type.
The break elongations in the MD exhibit a similar trend with
beta-crystal content (see Figure 7).
About 14 years ago, this author invented3
beta-nucleated PP film product that developed a very high level of microvoiding
when it was biaxially stretched on a biaxially oriented PP film (BOPP) line. By
incorporating certain other components in the formulation, we were able to
produce an open-celled structure where the microvoids interconnected with one
another. This film had the unique property of having very high levels of vapor
transmission (i.e., it was “breathable”), while still repelling liquid water.
The breathable/waterproof nature of the film allows it to be used in protective
clothing, where the wearer’s comfort is increased due to evaporation of
perspiration while liquids are kept out. In addition to blocking liquids, this
microporous film also blocks microorganisms (it passes ASTM F 1670 – Blood
Borne Pathogen & ASTM F 1671 – Viral Penetration), which allows it to be used
in medical protective gowns and drapes. This breathable film is also used in
house wrap and roofing membranes.
Summary and Conclusions
have shown that opaque/microvoided films can be produced by adding a unique
masterbatch containing a beta-nucleating agent to a non-nucleated polypropylene
resin, and then stretching an extruded sheet of that resin below the melting
point of the beta-crystalline phase. When the extruded sheet is made from an
impact copolymer resin, the degree of opacity and density reduction is much
greater than when the base resin is a polypropylene homopolymer. The degree of
opacity and density reduction also increases at higher levels of beta-crystal
content. The reduction in film density and the ability to produce a white film without
the use of white pigment permits a reduction in both raw materials and cost.
Breathable, waterproof films can also be produced using beta-nucleation
technology. The new Mayzo beta-nucleant masterbatch permits the film producer
to beta nucleate a conventional non-nucleated PP resin at the extruder hopper
in order to achieve these benefits and the associated cost savings.
author would like to acknowledge the support of Mayzo and Mr. Ben Milazzo in
conducting this research and developing this product.
For more information, visit www.mayzo.com.