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Next generation UV stabilized, impact modified Polyacetal copolymer (POM) for automotive interior applications.

Polyoxymethylene (POM) is a semi-crystalline polymer that exhibits good stiffness and tensile strength, but has inherently less notch impact strength compared to more amorphous polymers. Traditionally, several different types of impact modifiers have been used to improve the impact strength of POM. The goal has been to obtain a polymer that has improved impact strength, but still retains most of the stiffness and tensile strength of standard POM. In addition, POM has proved to be difficult to modify due to its high level of crystallinity and the incompatibility of POM with most impact modifiers. Furthermore, colorability of impact modified POM can be more difficult due to increase light scatter which is discussed later. For automotive interior applications, UV stability of these conventional impact modified POM formulations has exhibited more color change than typical POM UV grades without impact modification. This has limited the use of UV stabilized POM inside the vehicle and in other demanding UV applications.

Impact Modificationz

New Blend Technology

To overcome some of the issues with conventional impact modified POM, it would be advantageous to improve the compatibility between the POM polymer matrix and the impact modifier. A new technology was developed that accomplishes this [1,2]. By the incorporation of functional groups like, for example, hydroxy groups into the POM chain, the material is able to react via multifunctional coupling agents with impact modifiers such as Thermoplastic Polyurethane elastomers (TPU) for example. This chemical coupling leads to a compatibilization between the POM phase and the impact modifier. The better phase adhesion leads to better mechanical properties of the blends, especially the impact properties and the weld line performance. In melt processing there are less phase separation effects in molding and, therefore, less mold deposit and delamination. Due to the better compatibility, even higher levels of impact modifiers can be used versus conventional impact modified POM. This new blend technology yields products that have improved mechanical properties and processing behavior compared to standard impact modified POM.

Impact Strength and Tensile Modulus

In general, when elastomeric type impact modifiers are added to most polymers, the stiffness (modulus) is inversely proportional to the concentration of impact modifier. In other words, as the impact strength in general increases, the stiffness or modulus of the resulting blend decreases. Furthermore, conventional blends with impact modifiers chosen to have less effect on stiffness, impact strength gains can be minimal, particularly at cold temperatures (-30[degrees]C for example). For some polymer/application profiles, this relationship is acceptable. However, since POM is generally chosen for its stiffness and high tensile strength, this relationship is not necessarily desirable.

The new polymer blend technology has facilitated the development of grades with an improved impact/stiffness relation compared to standard POM impact modified grades. Using typical levels of impact modifier, the new technology can achieve up to 75% higher Charpy notched impact strength than comparable standard POM impact modified grades, while still maintaining similar stiffness and tensile strength as can be seen in Figure 1. All of the physical property testing was conducted using ISO standard molding and testing conditions. The higher impact strength makes it ideal for applications that require improved impact strength, along with comparable stiffness and tensile strength to standard impact modified POM.

Extending this new technology further, grades with super high impact strength have been achieved by applying the new blend technology with POM backbone functionalization and coupling to higher loadings of impact modifier. Figure 2 shows the Charpy notched impact values for three levels of impact modifier (Blend 1 through 3) using the new blend technology compared to the impact strength of two such "extremely tough" super high impact versions (super high blend 1 and 2). As one can see, the impact for the super high blends is about a factor of 5 higher than for the other new blend technology materials which use conventional impact modifier levels. The new compatibilization technology is extremely effective for higher loadings of impact modifier.

Figure 3 shows a comparison of the property profile of the new Super High Impact Blend 1 versus a commercially available high impact modified acetal homopolymer and also versus a high impact modified polyamide 6,6. Super High Blend 1 exhibits a superior elongation, impact strength and weld line performance versus the modified POM homopolymer. In comparison to the modified PA 6,6, Super High Blend 1 shows a superior elongation and stiffness. For completeness, the physical property data for Super High Blends 1 and 2 are included in Table 1.

Weld Line Performance

The weld line performance for standard impact modified POM has generally been poor when using traditional impact modifiers. The incompatibility between the POM and the impact modifiers results in poor interaction between the two flow fronts at the weld line interface. The modified POM backbone allows for improved compatibility and interaction between the flow fronts resulting in up to a 290% improvement in weld line elongation at break compared to standard impact modified POM. A comparison between the impact strength and weld line elongation at break for New Blend Technology Blend 3 versus a conventional impact modified POM is included in Figure 4.

The morphology of the weld line interface has been evaluated by freeze fracturing a standard weld line bar in the direction of the flow and then examining the surface by SEM. The micrographs for standard impact modified POM exhibit a well defined interface at the weld line. The micrograph for the new blend technology shows a much more random flow pattern at the weld line, which leads to the improved weld line performance. The micrographs for Blend 3 and conventional impact modified POM are included in Figure 5.

Processing Improvements

The improved compatibility between the POM and the impact modifier of the new impact modified grades reduces the amount of cooling time required in the mold by as much as 30% for both the Blend 1 and Blend 2 versions compared to comparable conventional impact modified POM grades. This leads to reduced cycle time and increased productivity during injection molding.

New Blend Technology also leads to less mold deposit than comparable standard impact modified grades. A mold deposit study was conducted that consisted of molding 5000 shots of both the comparable standard impact modified POM and new blend technology Blend 1 and Blend 2 samples. Upon visual comparison, the amount of mold deposit on the surface of the mold was significantly less for the Blend 1 and 2 samples versus the standard impact modified POM.

Colorability

Before we can discuss the colorability of semicrystalline resins and impact modified versions in particular, it is important that the reader has an understanding of light scattering and its effect on the visual color perception process [3].

Scattering occurs when the incident light beam contacts particles or regions within the polymer system that have an index of refraction which is different from that of the base polymer. The change in refractive index that the light beam encounters causes the light to be redirected and, if the index of refraction is increasing, to slow down. The index of refraction is a physical property of the substance. It is determined by the equation:

sin [[theta].sub.1]/sin [[theta].sub.2] = [eta],

where [[theta].sub.1] and [[theta].sub.2] are the angles formed by the incident light beam and the refracted ray versus the normal as shown in Figure 6. If scattering occurs nearly equal at all wavelengths with no absorption, the object will appear white. That is how titanium dioxide appears to impart its white color to objects--by scattering virtually all of the incident light.

The amount of scattering, as you might expect, depends on the magnitude of difference in refractive index between the polymer and the scattering substance. The direction of difference does not matter, only the magnitude as depicted in Figure 7. Finally, there is an optimum particle size for scattering to occur that is dependent on the index of refraction of the substance and the medium, and the wavelength of light. This optimum particle size is generally in the range of 1/2 the wavelength of the incident light. Smaller or larger particle sizes will scatter less light compared to the optimum size.

It is important to understand the mechanism of scattering because of its significant impact on the coloring of polymers and blends, including impact modified POM. Increasing the amount of scattering in the resin system will increase the amount of diffuse reflection (white light) which is mixed with the reflected colored light generated by the pigment interactions. This mixing will dilute the color strength and the color of the object will appear lighter and less bright to the observer. The end result may be that there is so much light scattering of the resin/blend system that certain colors can no longer be achieved. Or if they can be achieved, other properties may be adversely affected such as impact strength and cost. In either case, the practical color gamut or palette that is obtainable with this particular resin system may be limited.

Impact modifiers cover a broad range of chemistries from diene core-shell to acrylic polymers to elastomeric polymers. Since these additives are generally polymeric in nature, diffuse reflection will occur at the polymer/modifier interfaces. Diffuse reflection can increase due to internal light reflection or scattering at phase interfaces if the impact modifiers are at least partially immiscible or their refractive indexes are significantly different than the polymer matrix. Impact modified resins including impact modified POM are generally more opaque than the neat polymer. This will result in colors that appear lighter and duller in the impact modified resin. Table 2 contains three examples of impact modified POM (Blend 3) colors compared to the neat resin without modifier. As expected, the impact modified colors are lighter and have lower chroma, as evidenced by the higher DL* and the negative DC* values respectively.

For automotive interior colors required for a UV stabilized formulation, the degree of light scattering imposed by the impact modified resin generally can be overcome without sacrifice in impact strength or cost. Today's colors are typically shades of grays, tans and browns where a reduction in the amount of white pigment can be used to compensate for the increased light scattering. If deep royal blues and maroons were to return to the automotive interior palette, colorability of impact modified POM may be more of a concern and a bigger challenge. Outside of automotive, chromatic yellows, oranges and reds are more difficult but not impossible to achieve with impact modified POM. Accessible color gamut is somewhat reduced in these areas based on pigment loading, impact strength, and cost factors.

UV Stabilization

A typical UV stabilization system for any polymer generally contains one or more of the following: an antioxidant, a UV absorber, and a hindered amine light stabilizer (HALS). POM is no different as these compounds help prevent thermal and photo-oxidation of the polymer. Complexity increases when considering impact modified POM grades including the modifier type and hard segment/soft segment chemistries (if elastomeric). Add to that other processing aids, additives/fillers, and colorants, and one has a complex total system which needs to be made UV stable.

Starting with the basics: acetal copolymer resin and optimized UV stabilizers, Table 3 shows the importance of using colorants that are UV stable to achieve the desired end state of a colorable, UV stable POM system. This data was generated using a Xenon arc weatherometer operated according to interior automotive test method SAE J2412. Color difference after exposure was calculated and reported, and is less than 0.5 units for the optimized system.

As with most polymers, the introduction of impact modifiers to the UV stabilized base resin system has an adverse effect on the performance. Table 4 shows an impact modified POM (Modifier 1) using two different UV systems in four colors exposed to only 1/2 the total UV energy as the data in Table 3. Color difference values for UV 1 are significantly above the DE* target of <3.0. UV2 provides some improvement, but only in dark colors such as dark gray. This type of performance has limited the use of impact modified POM in automotive interiors.

Newer automotive safety standards for head impact and the increased use of airbags inside the vehicle have increased the requirement for more ductile plastics. Plastic parts located on the front of the headliner (visor clips), on the instrument panel (trim bezels), on the steering wheel (trim bezels), and on doors (handles and other trim found in the side impact airbag zone) now require ductile rather than brittle failures. Conventional UV stabilized acetal copolymer POM (such as shown in Table 3) meets the highest levels of UV requirements, but can be too brittle to be used in parts in these regions depending on the specific part design. An impact modified, UV stabilized POM grade would be an important material to meet the UV requirements and the ductile failure requirements for parts in these impact zones.

To this end, work has been completed using a total systems approach to this complex problem. Various impact modifiers and UV stabilization systems were evaluated for UV performance using interior automotive test method PV1303, with exposure up to 2,800 kJ/[m.sup.2]. Samples were prepared in a light gray color deemed difficult to stabilize as the smallest color change would be visually apparent. A control sample from Table 4 was included in a dark gray color as well. Modifiers 1,2,3 combined with UV 2,3,4 all show poor performance with DEcmc values as high as 17 as presented in Table 5. However, Modifier 4/UV5 shows exceptional performance similar to neat UV POM of Table 3. This optimized formulation was molded into automotive clip parts and met all functional requirements. Therefore we have been successful at combining impact properties with optimized UV performance in developing a next generation UV stabilized, impact modified Polyacetal copolymer (POM) for automotive interior applications.

Conclusions

New blend technology impact modified grades exhibit improved impact strength and weld line performance, with improved stiffness and tensile strength compared to conventional impact modified POM. New blend technology also allows the extension of performance to extraordinarily high impact strength that may close the gap between impact modified POM and impact modified Polyamide.

For automotive interior applications, colorability has been shown to not be an issue with today's common color palette. More importantly, a breakthrough in the development of an impact modified, UV stabilized POM resin has been demonstrated for these demanding applications. The level of UV stability achieved is now similar to neat UV acetal copolymer resin.

Acknowledgements

We would like to acknowledge the Ticona analytical labs and molding and testing labs in Florence, KY and Hoechst, Germany for generating the analytical data.

Data Tables and Figures

Product performance and material data values included in this publication are either based on evaluating laboratory test specimens and represent data that fall within the normal range of properties or were compiled from various published sources. To the best of our knowledge, the information contained in this publication is accurate; however no representation is made as to its suitability in any specific application for establishing maximum, minimum, or ranges of values for specification purposes. Color data presented in the tables have been calculated under illuminant "D-65", 10[degrees] observer, specular included, expressed in CIELab units or CMC (1.3:1).

References

(1) L. Larson, U. Ziegler, "A New Generation of Impact Modified Polyoxymethylene", Society of Plastics Engineers, Proceedings, Eurotec [TM] 2011

(2) L. Larson, J. Lipke, "Improved Mechanical Properties and Product Performance for Impact Modified Polyoxymethylene Co-Polymers", Society of Plastics Engineers, Proceedings, ANTEC[R] 2010

(3) B. Mulholland, "Effect of Additives on the Color & Appearance of Plastics", Society of Plastics Engineers, Proceedings, ANTEC[R] 2007

To the best of available knowledge, the information contained in this publication is accurate, however the foregoing represents proof-of-concept data on small-scale replicas and are approximate in nature. Properties of molded parts can be influenced by a wide variety of factors including, but not limited to, material selection, formulations, part design, processing conditions and environmental exposure. Any determination of the suitability of a particular material or composite and part design for any use contemplated by the user is the sole responsibility of the user.

Bruce M. Mulholland

Ticona

Table 1: Physical Property Data for Super High Blends 1 & 2

Properties                                 Super High   Super High
                                            Blend 1      Blend 2

Tensile Modulus                 Mpa           950          1200
Yield Stregth                   MPa            30           35
Strain at yield                  %             30           25
HDT(A) 1.8 Mpa               [degrees]C        60           64
Charpy notched impact @
  23[degrees]C              kJ/[m.sup.2]      100          100
Charpy notched impact @
  -30[degrees]C             kJ/[m.sup.2]       15           15
Weldline Strain at break         %             17           7
Specific gravity            g/[cm.sup.3]      1.32         1.33

Table 2: Effect of Impact Modifier on Colorability of POM

Color                DL*        DC*

Gray                 4.5        -1.8
Bright Red           2.8        -2.4
Bright Yellow        3.6        -4.0

Table 3: Effect of Stabilization and Colorants on UV
Performance. Acetal Copolymer Resin--Tan Color
Formulation Exposed to SAE J2412, 1,240.8 kJ/[m.sup.2]

UV package              Colorants       DE *

None                    UV stable       23.30
UV                    Not UV stable     8.23
UV                      UV stable       0.46
Target Performance                      <3.0

Table 4: Effect of Impact Modifier on UV Performance
Impact Modified Acetal Copolymer Resin Exposed to
SAE J2412, 601.6 kJ/[m.sup.2]

Formula           Color Name     DL *    Da *    Db *    DE *

Modifier I/UV1    Light Beige    5.26    -1.05   -2.87   6.08
Modifier I/UVI    Medium Beige   5.64    -1.33   -3.71   6.88
Modifier I/UV1    Medium Gray    6.10    -1.28   -1.94   6.53
Modifier I/UV2    Dark Gray      1.71    -0.58   0.21    1.82

Table 5: Impact Modified Acetal Copolymer Resin
Exposed to PV1303, 2,800 kJ/[m.sup.2]

Formula           Color Name    DEcmc

Modifier 1/UV2    Dark Gray        5.3
Modifier 2/UV3    Light gray      17.0
Modifier 2/UV4    Light gray      11.6
Modifier 3/UV4    Light dray       3.1
Modifier 4/UV5    Light gray       0.4
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Title Annotation:Polyoxymethylene
Author:Mulholland, Bruce M.
Publication:Plastics Engineering
Geographic Code:1USA
Date:Apr 1, 2013
Words:3030
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